tag:theconversation.com,2011:/uk/topics/supersymmetry-15503/articlesSupersymmetry – The Conversation2018-11-05T13:07:27Ztag:theconversation.com,2011:article/1062602018-11-05T13:07:27Z2018-11-05T13:07:27ZMystery particle spotted? Discovery would require physics so weird that nobody has even thought of it<figure><img src="https://images.theconversation.com/files/243858/original/file-20181105-83635-194cuwl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">CMS detector.</span> <span class="attribution"><span class="source">Laura Gilchrist/Flickr</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p>There was a huge amount of excitement when the Higgs boson was first spotted back in 2012 – a discovery that bagged the <a href="https://theconversation.com/could-the-higgs-nobel-be-the-end-of-particle-physics-18978">Nobel Prize for Physics in 2013</a>. The particle completed the so-called <a href="https://home.cern/about/physics/standard-model">standard model</a>, our current best theory of understanding nature at the level of particles.</p>
<p>Now scientists at the Large Hadron Collider (LHC) at Cern think they may have <a href="https://www.theguardian.com/science/2018/oct/31/has-new-ghost-subatomic-particle-manifested-at-large-hadron-collider">seen another particle</a>, detected as a peak at a certain energy in the data, although the finding is yet to be confirmed. Again there’s a lot of excitement among particle physicists, but this time it is mixed with a sense of anxiety. Unlike the Higgs particle, which confirmed our understanding of physical reality, this new particle seems to threaten it. </p>
<p>The new result – consisting of a mysterious bump in the data at 28 GeV (a unit of energy) – <a href="https://arxiv.org/abs/1808.01890">has been published</a> as a preprint on ArXiv. It is not yet in a peer-reviewed journal – but that’s not a big issue. The LHC collaborations have very tight internal review procedures, and we can be confident that the authors have done the sums correctly when they report a “4.2 standard deviation significance”. That means that the probability of getting a peak this big by chance – created by random noise in the data rather than a real particle – is only 0.0013%. That’s tiny – 13 in a million. So it seems like it must a real event rather than random noise – but nobody’s opening the champagne yet.</p>
<h2>What the data says</h2>
<p>Many LHC experiments, which smash beams of protons (particles in the atomic nucleus) together, find evidence for new and exotic particles by looking for an unusual build up of known particles, such as photons (particles of light) or electrons. That’s because heavy and “invisible” particles such as the Higgs are often unstable and tend to fall apart (decay) into lighter particles that are easier to detect. We can therefore look for these particles in experimental data to work out whether they are the result of a heavier particle decay. The LHC <a href="https://home.cern/about/updates/2018/09/lhcb-experiment-discovers-two-perhaps-three-new-particles">has found many new particles</a> by such techniques, and they have all fitted into the standard model.</p>
<p>The new finding comes from an experiment involving the <a href="https://home.cern/about/experiments/cms">CMS detector</a>, which recorded a number of pairs of muons – <a href="https://theconversation.com/particle-physicists-discover-mysterious-structure-in-great-pyramid-heres-how-they-did-it-86783">well known and easily identified particles</a> that are similar to electrons, but heavier. It analysed their energies and directions and asked: if this pair came from the decay of a single parent particle, what would the mass of that parent be?</p>
<p>In most cases, pairs of muons come from different sources – originating from two different events rather than the decay of one particle. If you try to calculate a parent mass in such cases it would therefore spread out over a wide range of energies rather than creating a narrow peak specifically at 28GeV (or some other energy) in the data. But in this case it certainly looks like there’s a peak. Perhaps. You can look at the figure and you can judge for yourself.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/243860/original/file-20181105-83648-1qhdfby.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/243860/original/file-20181105-83648-1qhdfby.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=576&fit=crop&dpr=1 600w, https://images.theconversation.com/files/243860/original/file-20181105-83648-1qhdfby.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=576&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/243860/original/file-20181105-83648-1qhdfby.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=576&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/243860/original/file-20181105-83648-1qhdfby.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=724&fit=crop&dpr=1 754w, https://images.theconversation.com/files/243860/original/file-20181105-83648-1qhdfby.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=724&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/243860/original/file-20181105-83648-1qhdfby.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=724&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">New data.</span>
<span class="attribution"><span class="source">CMS Collaboration</span></span>
</figcaption>
</figure>
<p>Is this a real peak or is it just a statistical fluctuation due to the random scatter of the points about the background (the dashed curve)? If it’s real that means that a few of these muon pairs did indeed come from just a large parent particle that decayed by emitting muons – and no such 28 GeV particle has ever been seen before.</p>
<p>So it is all looking rather intriguing, but, history has taught us caution. Effects this significant have appeared in the past, only to vanish when more data is taken. The Digamma(750) anomaly is a <a href="https://en.wikipedia.org/wiki/750_GeV_diphoton_excess">recent example</a> from a long succession of false alarms – spurious “discoveries” due to equipment glitches, over-enthusiastic analysis or just bad luck. </p>
<p>This is partly due to something called the <a href="https://en.wikipedia.org/wiki/Look-elsewhere_effect">“look elsewhere effect</a>”: although the probability of random noise producing a peak if you look specifically at a value of 28 GeV may be 13 in a million, such noise could give a peak somewhere else in the plot, maybe at 29GeV or 16GeV. The probabilities of these being due to chance are also tiny when considered respectively, but the sum of these tiny probabilities is not so tiny (though still pretty small). That means it is not impossible for a peak to be created by random noise.</p>
<p>And there are some puzzling aspects. For example, the bump appeared in one LHC run but not in another, when the energy was doubled. One would expect any new phenomena to get bigger when the energy is higher. It may be that there are reasons for this, but at the moment it’s an uncomfortable fact.</p>
<h2>New physical reality?</h2>
<p>The theory is even more incongruous. Just as experimental particle physicists spend their time looking for new particles, theorists spend their time thinking of new particles that it would make sense to look for: particles that would fill in the missing pieces of the standard model, or explain <a href="https://theconversation.com/from-machos-to-wimps-meet-the-top-five-candidates-for-dark-matter-51516">dark matter</a> (a type of invisible matter), or both. But no one has suggested anything like this.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/243651/original/file-20181102-83654-1f24iz4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/243651/original/file-20181102-83654-1f24iz4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=553&fit=crop&dpr=1 600w, https://images.theconversation.com/files/243651/original/file-20181102-83654-1f24iz4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=553&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/243651/original/file-20181102-83654-1f24iz4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=553&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/243651/original/file-20181102-83654-1f24iz4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=695&fit=crop&dpr=1 754w, https://images.theconversation.com/files/243651/original/file-20181102-83654-1f24iz4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=695&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/243651/original/file-20181102-83654-1f24iz4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=695&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">CMS model of a Higgs boson decaying into two jets of hadrons and two electrons.</span>
<span class="attribution"><span class="source">Lucas Taylor/CERN</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>For example, theorists suggest we could find a lighter version of the Higgs particle. But anything of that ilk would not decay to muons. A light <a href="https://home.cern/about/physics/z-boson">Z boson</a> or a <a href="https://epp.slac.stanford.edu/research/heavy-photon-search">heavy photon</a> have also been talked about, but they would interact with electrons. That means we should have probably discovered them already as electrons are easy to detect. The potential new particle does not match the properties of any of those proposed.</p>
<p>If this particle really exists, then it is not just outside the standard model but outside it in a way that nobody anticipated. Just as Newtonian gravity gave way to Einstein’s general relativity, the standard model will be superseded. But the replacement will not be any of the favoured candidates that has already been proposed to extend standard model: including <a href="https://home.cern/about/physics/supersymmetry">supersymmetry</a>, extra dimensions and <a href="https://theconversation.com/stephen-hawking-had-pinned-his-hopes-on-m-theory-to-fully-explain-the-universe-heres-what-it-is-93440">grand unification theories</a>. These all propose new particles, but none with properties like the one we might have just seen. It will have to be something so weird that nobody has suggested it yet. </p>
<p>Luckily the other big LHC experiment, ATLAS, has similar data from their experiments The team is still analysing it, and will report in due course. Cynical experience says that they will report a null signal, and this result will join the gallery of statistical fluctuations. But maybe – just maybe – they will see something. And then life for experimentalists and theorists will suddenly get very busy and very interesting.</p><img src="https://counter.theconversation.com/content/106260/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Roger Barlow is a member of the Liberal Democrats</span></em></p>Scientists at Cern’s Large Hadron Collider have seen something that may force us to abandon everything we thought we knew about the world on the level of particles.Roger Barlow, Research Professor and Director of the International Institute for Accelerator Applications, University of HuddersfieldLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/934402018-03-15T15:36:07Z2018-03-15T15:36:07ZStephen Hawking had pinned his hopes on ‘M-theory’ to fully explain the universe – here’s what it is<figure><img src="https://images.theconversation.com/files/210604/original/file-20180315-104671-1v6k6ba.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Stephen Hawking.</span> <span class="attribution"><span class="source">Lwp Kommunikáció/Flickr</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Rumour has it that Albert Einstein spent his last few hours on Earth <a href="http://www.bbc.co.uk/sn/tvradio/programmes/horizon/einstein_symphony_prog_summary.shtml">scribbling something</a> on a piece of paper in a last attempt to formulate a theory of everything. Some 60 years later, another legendary figure in theoretical physics, Stephen Hawking, may have <a href="https://theconversation.com/stephen-hawking-martin-rees-looks-back-on-colleagues-spectacular-success-against-all-odds-93379">passed away</a> with similar thoughts. We know Hawking thought something called “M-theory” is <a href="https://arstechnica.com/science/2012/07/steven-hawking-on-time-travel-m-theory-and-extra-terrestrial-life/">our best bet</a> for a complete theory of the universe. But what is it?</p>
<p>Since the formulation of Einstein’s <a href="https://theconversation.com/how-einsteins-general-theory-of-relativity-killed-off-common-sense-physics-50042">theory of general relativity</a> in 1915, every theoretical physicist has been dreaming of reconciling our understanding of the infinitely small world of atoms and particles with that of the infinitely large scale of the cosmos. While the latter is effectively described by Einstein’s equations, the former is predicted with extraordinary accuracy by the so-called <a href="https://theconversation.com/explainer-standard-model-of-particle-physics-2539">Standard Model</a> of fundamental interactions.</p>
<p>Our current understanding is that the interaction between physical objects is described by four <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Forces/funfor.html">fundamental forces</a>. Two of them – gravity and electromagnetism – are relevant for us on a macroscopic level, we deal with them in our everyday life. The other two, dubbed strong and weak interactions, act on a very small scale and become relevant only when dealing with subatomic processes.</p>
<p>The standard model of fundamental interactions provides a unified framework for three of these forces, but gravity cannot be consistently included in this picture. Despite its accurate description of large scale phenomena such as a planet’s orbit or galaxy dynamics, general relativity breaks down at very short distances. According to the standard model, all forces are mediated by specific particles. For gravity, a particle called the graviton does the job. But when trying to calculate how these gravitons interact, nonsensical infinities appear.</p>
<p>A consistent theory of gravity should be valid at any scale and should take into account the quantum nature of fundamental particles. This would accommodate gravity in a unified framework with the other three fundamental interactions, thus providing the celebrated theory of everything. Of course, since Einstein’s death in 1955, a lot of progress has been made and nowadays our best candidate goes under the name of M-theory.</p>
<h2>String revolution</h2>
<p>To understand the basic idea of M-theory, one has to go back to the 1970s when scientists realised that, rather than describing the universe based on point like particles, you could describe it in terms of tiny oscillating strings (tubes of energy). This new way of thinking about the fundamental constituents of nature turned out to solve many theoretical problems. Above all, a particular oscillation of the string could be interpreted as a graviton. And unlike the standard theory of gravity, string theory can describe its interactions mathematically without getting strange infinities. Thus, gravity was finally included in a unified framework. </p>
<p>After this exciting discovery, theoretical physicists devoted a lot of effort to understanding the consequences of this seminal idea. However, as often happens with scientific research, the history of string theory is characterised by ups and downs. At first, people were puzzled because it predicted the existence of a particle which travels faster than the speed of light, dubbed a “tachyon”. This prediction was in contrast with all the experimental observations and cast serious doubt on string theory. </p>
<p>Nevertheless, this issue was solved in the early 1980s by the introduction of something called “supersymmetry” in string theory. This predicts that every particle has a superpartner and, by an extraordinary coincidence, the same condition actually eliminates the tachyon. This first success is commonly known as “<a href="http://theory.caltech.edu/people/jhs/strings/str133.html">the first string revolution</a>”. </p>
<p>Another striking feature is that string theory requires the existence of ten spacetime dimensions. Currently, we only know of four: depth, height, width and time. Although this might seem a major obstacle, several solutions have been proposed and nowadays it is considered as a notable feature, rather than a problem. </p>
<p>For example, we could somehow be forced to live in a four dimensional world without any access to the extra dimensions. Or the extra dimensions could be “compactified” on such a small scale we wouldn’t notice them. However, different compactifications would lead to different values of the physical constants and, therefore, different physics laws. A possible solution is that our universe is just one of many in <a href="https://theconversation.com/the-theory-of-parallel-universes-is-not-just-maths-it-is-science-that-can-be-tested-46497">an infinite “multiverse”</a>, governed by different physics laws. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/210611/original/file-20180315-104639-6zqo8a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/210611/original/file-20180315-104639-6zqo8a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=424&fit=crop&dpr=1 600w, https://images.theconversation.com/files/210611/original/file-20180315-104639-6zqo8a.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=424&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/210611/original/file-20180315-104639-6zqo8a.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=424&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/210611/original/file-20180315-104639-6zqo8a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=533&fit=crop&dpr=1 754w, https://images.theconversation.com/files/210611/original/file-20180315-104639-6zqo8a.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=533&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/210611/original/file-20180315-104639-6zqo8a.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=533&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Are there other universes?</span>
<span class="attribution"><span class="source">Pixabay.</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>This may seem odd, but a lot of theoretical physicists are coming around to this idea. If you are not convinced you may try to read the novel <a href="http://www.geom.uiuc.edu/%7Ebanchoff/Flatland/">Flatland: a romance of many dimensions</a> by Edwin Abbott, in which the characters are forced to live in two space dimensions and are unable to realise there is a third one.</p>
<h2>M-theory</h2>
<p>But there was one remaining pressing issue that was bothering string theorists at the time. A thorough classification showed the existence of five different consistent string theories, and it was unclear why nature would pick one out of five.</p>
<p>This is when M-theory entered the game. During the <a href="http://discovermagazine.com/2008/dec/13-the-man-who-led-the-second-superstring-revolution">second string revolution</a>, in 1995, physicists proposed that the five consistent string theories are actually only different faces of a unique theory which lives in eleven spacetime dimensions and is known as M-theory. It includes each of the string theories in different physical contexts, <a href="https://www.quantamagazine.org/why-is-m-theory-the-leading-candidate-for-theory-of-everything-20171218/">but is still valid for all of them</a>. This extremely fascinating picture has led most theoretical physicists to believe in M-theory as the theory of everything – it is also more mathematically consistent than other candidate theories.</p>
<p>Nevertheless, so far M-theory has struggled in producing predictions that can be tested by experiments. Supersymmetry is <a href="https://theconversation.com/large-hadron-collider-sees-tantalising-hints-of-a-new-particle-that-could-revolutionise-physics-52457">currently being tested</a> at the Large Hadron Collider. If scientists do find evidence of superpartners, that would ultimately strengthen M-theory. But it still remains a challenge for current theoretical physicists to produce testable predictions and for experimental physicists to set up experiments to test them. </p>
<p>Most great physicists and cosmologists are driven by a passion to find that beautiful, simple description of the world that can explain everything. And although we are not quite there yet, we wouldn’t have a chance without the sharp, creative minds of people like Hawking.</p><img src="https://counter.theconversation.com/content/93440/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Lorenzo Bianchi receives funding from the European Union’s Horizon 2020 research and innovation
programme under the Marie Sklodowska-Curie grant agreement No 749909.</span></em></p>Stephen Hawking thought a form of string theory could be our best bet for a ‘theory of everything’.Lorenzo Bianchi, Marie Curie Fellow in Theoretical Physics, Queen Mary University of LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/515162015-12-14T13:33:11Z2015-12-14T13:33:11ZFrom MACHOs to WIMPs: meet the top five candidates for ‘dark matter’<figure><img src="https://images.theconversation.com/files/103721/original/image-20151130-10288-mprcay.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Astronomers believe that the dark blue ring in this image must be mysterious dark matter.</span> <span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Dark_matter#/media/File:CL0024%2B17.jpg">NASA/ESA/wikimedia</a></span></figcaption></figure><p>When we look out at the universe – even with the most powerful of telescopes – we can only see a <a href="https://theconversation.com/how-we-plan-to-bring-dark-matter-to-light-46989">fraction of the matter</a> we know must be there. In fact, for every gram’s worth of atoms in the universe, there is at least <a href="https://theconversation.com/shedding-new-light-on-the-search-for-the-invisible-dark-matter-40083">five times more invisible material</a> called “dark matter”. So far scientists have failed to detect it, despite spending decades searching. </p>
<p>The reason we know it exists is because of the gravitational pull of galaxy clusters and other phenomena we observe. The matter we can see in a cluster isn’t enough to hold it together by gravity alone, meaning some additional invisible or obscure matter must be present. But we have no idea what it is – it could be made up of new, yet undiscovered particles. </p>
<p>There are four fundamental forces that a dark matter particle could interact with. There is the <a href="http://www.livescience.com/48575-strong-force.html">strong force</a> that binds together the atomic nucleus; the <a href="http://www.livescience.com/49254-weak-force.html">weak force</a> which governs the decay of particles such as radioactivity; an <a href="http://emandpplabs.nscee.edu/cool/temporary/doors/forces/electromagforce/electromagnetic.htm">electromagnetic force</a> that mediates the force between charged particles; and the <a href="http://www.universetoday.com/75321/gravitational-force/">gravitational force</a> which governs gravitational interaction. To observe matter in space we need it to interact via the electromagnetic force, as this involves the release of light or other electromagnetic radiation that a telescope can register.</p>
<p>There are quite a few candidates already – each with its own particular way of interacting. However, some theories are more likely to be successful than others. Here are the five candidates for particles that I think have the best chance.</p>
<h2>1. The WIMP</h2>
<p>The <a href="http://www.universetoday.com/41878/wimps/">weakly interacting massive particle</a>, or WIMP, is a hypothetical particle that looks promising. It would be completely different from the type of matter we know and would interact via the electromagnetic force, which would explain why they are largely invisible in space. Roughly 100,000 of these would pass through every square centimetre of the Earth each second, interacting only via the weak force and gravity with surrounding matter. </p>
<p>If WIMPs exist, mathematical modelling shows there must be about five times more of these than normal matter, which coincides with the abundance of dark matter that we observe in the universe. This means we should be able to detect them through their collisions as this would cause the charged particles on Earth to recoil, <a href="https://theconversation.com/the-search-for-dark-matter-and-dark-energy-just-got-interesting-46422">producing light that we can observe</a> in experiments such as <a href="http://xenon.astro.columbia.edu/XENON100_Experiment/">XENON100</a>.</p>
<p>WIMPs have been the subject of a lot of extensive research, especially beyond the <a href="https://theconversation.com/explainer-what-are-fundamental-particles-38339">Standard Model of physics</a>, which independently predicted that such a particle must exist – a coincidence dubbed the “<a href="http://news.sciencemag.org/physics/2014/10/dark-matter-out-wimps-simps">WIMP miracle</a>”. </p>
<h2>2. The axion</h2>
<p><a href="http://arstechnica.com/science/2015/01/if-dark-matter-is-really-axions-we-could-find-out-soon/">Axions</a> are low-mass, slow-moving particles that don’t have a charge and only interact weakly with other matter which makes them difficult – but but not impossible – to detect. Only axions of a specific mass would be able to explain the invisible nature of dark matter – if they are any lighter or heavier we would be able to see them. And if axions do exist they would be able to decay into a pair of light particle (photons), which means we could detect them by looking for such pairs. Experiments including the <a href="http://depts.washington.edu/admx/index.shtml">Axion Dark Matter Experiment</a> is currently looking for axions in this way.</p>
<h2>3. The MACHO</h2>
<p>MACHO stands for “<a href="http://www.redorbit.com/reference/massive_compact_halo_object_macho/">massive astrophysical compact halo object</a>” and was one of the first proposed candidates for dark matter. These objects, including neutron stars, and brown and white dwarfs, are composed of ordinary matter. So how could they be invisible? The reason is that they emit very little to no light. </p>
<p>One way to observe them is by monitoring the brightness of distant stars. As light rays bend when they pass close to a massive object, light from a distant source may be focused by a closer object to produce a sudden brightening of the distant object. This effect, known as <a href="http://www.cfhtlens.org/public/what-gravitational-lensing">gravitational lensing</a>, depends on how much matter, both normal and dark, is in a galaxy – we can use it to calculate the amount of matter lurking around. However, we now know it is unlikely that <a href="http://www.astro.caltech.edu/%7Egeorge/ay20/eaa-wimps-machos.pdf">enough of these dark bodies</a> could accumulate to make up the vast amount of dark matter that exists.</p>
<h2>4. The Kaluza-Klein particle</h2>
<p>The <a href="http://www.livescience.com/50465-most-wanted-particles-lhc.html">Kaluza-Klein theory</a> is built around the existence of an invisible “fifth dimension” curled up in space, in addition to the three spatial dimensions we know (height, width, depth), and time. This theory, a precursor to <a href="https://theconversation.com/explainer-string-theory-2983">string theory</a>, predicts the existence of a particle that could be a dark matter particle, which would have the same mass <a href="http://news.nationalgeographic.com/news/2008/11/081119-dark-matter-antarctica_2.html">as 550 to 650 protons</a> (these make up the atomic nucleus together with neutrons). </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/105115/original/image-20151209-15575-1yf2g3k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/105115/original/image-20151209-15575-1yf2g3k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=392&fit=crop&dpr=1 600w, https://images.theconversation.com/files/105115/original/image-20151209-15575-1yf2g3k.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=392&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/105115/original/image-20151209-15575-1yf2g3k.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=392&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/105115/original/image-20151209-15575-1yf2g3k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=492&fit=crop&dpr=1 754w, https://images.theconversation.com/files/105115/original/image-20151209-15575-1yf2g3k.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=492&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/105115/original/image-20151209-15575-1yf2g3k.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=492&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">No sign of Kaluza yet.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/11304375@N07/2046228644">Image Editor/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>This kind of particle could interact both via electromagnetism and gravity. However, as it is curled up in a dimension we can’t see, we wouldn’t observe it by just by looking at the sky. Luckily, the particle should be is easy to look for in experiments as it should <a href="http://news.yahoo.com/giant-atom-smasher-revs-physicists-reveal-theyre-looking-193433263.html">decay into particles we can measure</a> – into neutrinos and photons. However, powerful particle accelerators like the Large Hadron Collider are yet to detect it. </p>
<h2>5. The gravitino</h2>
<p>Theories combining general relativity and “supersymmetry” predict the existence of a particle called the <a href="http://www.dailygalaxy.com/my_weblog/2013/01/gravitinos-will-they-unlock-the-mystery-of-dark-matter-in-the-universe.html">gravitino</a>. Supersymmetry, which is a successful theory explaining a lot of observations in physics, states that all <a href="https://theconversation.com/explainer-how-does-an-experiment-at-the-large-hadron-collider-work-42846">“boson” particles</a> – such as the photon (light particle)– have a “superpartner”, the photino, with a property called “spin” (a type of angular momentum) that differs by a half-integer. The gravitino would be the superpartner of the hypothetical “graviton”, thought to mediate the force of gravitation. And in some models of supergravity where the gravitino is very light, it could account for dark matter.</p><img src="https://counter.theconversation.com/content/51516/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Johar Ashfaque does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>While we know that dark matter exists, we have no idea what it is. Luckily, there is no shortage of suggestions.Johar Ashfaque, PhD student in string phenomenology, University of LiverpoolLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/428462015-06-05T15:34:31Z2015-06-05T15:34:31ZExplainer: how does an experiment at the Large Hadron Collider work?<figure><img src="https://images.theconversation.com/files/84113/original/image-20150605-8677-1ykfc31.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Supersize symmetry</span> <span class="attribution"><span class="source">Maximilien Brice/CERN</span></span></figcaption></figure><p>It’s not every day my Twitter feed is full of people talking about flat-tops, squeezing and injections, but then Wednesday 3 June was not an average day for the Large Hadron Collider.</p>
<p>The LHC is the world’s largest particle accelerator and lies in a tunnel below <a href="http://home.web.cern.ch">CERN</a>, the European physics lab just outside Geneva. And on Wednesday it was restarted after two year break for repairs and upgrades, ready to push our understanding of the universe to new limits. </p>
<p>As my fellow physicists crowded into the control rooms and waited for things to get underway, I was at a workshop in France. But I was able to follow the <a href="http://run2-13tev.web.cern.ch">switch-on online</a>. Here’s how things went down.</p>
<p><strong>8.09am. Injection: Billions of protons are loaded into the LHC.</strong></p>
<p>The LHC is a ring roughly 28km around that accelerates protons almost to the speed of light before colliding them head on. Protons are particles found in the atomic nucleus, roughly one thousand-million-millionth of a metre in size.</p>
<p>They are easiest to get from hydrogen, the simplest atom with just one electron orbiting one proton. The LHC starts with a bottle of hydrogen gas, which is sent through an electric field to strip away the electrons, leaving just the protons. Electric and magnetic fields are the key to a particle accelerator: because protons are positively charged, they accelerate when in an electric field and bend in a circle in a magnetic field. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/84109/original/image-20150605-8725-9xav1g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/84109/original/image-20150605-8725-9xav1g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/84109/original/image-20150605-8725-9xav1g.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/84109/original/image-20150605-8725-9xav1g.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/84109/original/image-20150605-8725-9xav1g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/84109/original/image-20150605-8725-9xav1g.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/84109/original/image-20150605-8725-9xav1g.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Big data.</span>
<span class="attribution"><span class="source">M.Brice/CERN</span></span>
</figcaption>
</figure>
<p><strong>9.45am. Ramp: Once the LHC is fully loaded, its two proton beams are slowly accelerated up to collision energy, now a world-record 6.5TeV per beam.</strong></p>
<p>Accelerating billions of protons to close to the speed of light, directing them all the way around the LHC, and then colliding them head-on, is a delicate balancing act performed by high voltage equipment and giant magnets. This is an amazing technical achievement. Indeed one of the main applications of particle physics research is in the industrial applications of the technology it develops along the way, from proton therapy cancer treatment to the <a href="http://home.web.cern.ch/topics/birth-web">world wide web</a>.</p>
<p>But for me, the excitement is in the science: the LHC is exploring the universe at the smallest scales. Everything we have learned so far is formulated in the <a href="http://home.web.cern.ch/about/physics/standard-model">Standard Model</a>, a theory which describes the universe made of tiny particles, and gives the rules for how these particles behave. By smashing some of these particles together at high energy, we are able to test these rules and make new discoveries.</p>
<p>The LHC “Run 1” (2010-2013) provided enough data to test the Standard Model to new levels of precision and discover the <a href="https://theconversation.com/definitely-maybe-evidence-grows-for-positive-id-of-higgs-boson-12790">Higgs boson</a>. This particle was predicted in the 1960s and plays a central role in the Standard Model. But it was almost 50 years before we had a machine powerful enough to discover it. As well as high energy, it needed lots of data: the Higgs boson is a rare thing, and fewer than one in a billion collisions at the LHC produce one.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/84110/original/image-20150605-8697-1094eps.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/84110/original/image-20150605-8697-1094eps.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/84110/original/image-20150605-8697-1094eps.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/84110/original/image-20150605-8697-1094eps.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/84110/original/image-20150605-8697-1094eps.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/84110/original/image-20150605-8697-1094eps.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/84110/original/image-20150605-8697-1094eps.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Tense moments.</span>
<span class="attribution"><span class="source">Laurent Egli/CERN</span></span>
</figcaption>
</figure>
<p><strong>10.12am. Flat top: Beam energy levels off after reaching the target.</strong></p>
<p>These were tense moments for the CERN team on Wednesday. The LHC was operating at the <a href="https://theconversation.com/large-hadron-collider-is-back-to-change-our-understanding-of-the-universe-again-42775">highest energy ever</a> achieved in a particle accelerator. “Run 2” will collide protons at 60% higher energies than Run 1 by pushing the magnets and accelerators to the limit. We hope this extra reach will allow us to tackle some of the big questions in particle physics.</p>
<p>One of the main topics is <a href="http://home.web.cern.ch/about/physics/dark-matter">dark matter</a>. This seems to be a new type of particle spread through the entire universe. And with the LHC Run 2 we hope to make it in the lab for the first time. But if the Higgs boson is rare, dark matter is even rarer, and we will need to sort through a lot of collisions before having a hope of finding it.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/84111/original/image-20150605-8725-szj84l.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/84111/original/image-20150605-8725-szj84l.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=309&fit=crop&dpr=1 600w, https://images.theconversation.com/files/84111/original/image-20150605-8725-szj84l.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=309&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/84111/original/image-20150605-8725-szj84l.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=309&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/84111/original/image-20150605-8725-szj84l.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=388&fit=crop&dpr=1 754w, https://images.theconversation.com/files/84111/original/image-20150605-8725-szj84l.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=388&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/84111/original/image-20150605-8725-szj84l.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=388&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Worlds collide.</span>
<span class="attribution"><span class="source">CMS/CERN</span></span>
</figcaption>
</figure>
<p><strong>10.17am. Squeeze: The beams are fine-tuned, and focused at the four points around the LHC where they cross, and the experiments will record the collisions</strong></p>
<p>Almost there. The experiments now need to wait for the all-clear before they can start recording, and we begin studying things that have never been seen before. Still, many of the collisions will not be interesting, as the protons just smash apart without doing anything exciting.</p>
<p>To make matters worse, the rare new particles we are looking for also tend to be very unstable, and decay too quickly to be seen directly. So the job of the experiments is to measure whatever particles do come out of a collision and try to reconstruct what happened, looking for evidence of something unusual.</p>
<p>As well as dark matter, there are many other ideas to test, such as <a href="http://home.web.cern.ch/about/physics/supersymmetry">supersymmetry</a>, new gauge bosons, quantum black holes and heavy neutrinos, all of which we could reconstruct from the LHC collisions. Part of the joy and pain of science is that a <a href="https://theconversation.com/is-this-the-end-of-particle-physics-as-we-know-it-lets-hope-not-42849">new discovery</a> could come in a matter of days, or a matter of years.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/84107/original/image-20150605-8674-1xscz2w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/84107/original/image-20150605-8674-1xscz2w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/84107/original/image-20150605-8674-1xscz2w.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/84107/original/image-20150605-8674-1xscz2w.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/84107/original/image-20150605-8674-1xscz2w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/84107/original/image-20150605-8674-1xscz2w.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/84107/original/image-20150605-8674-1xscz2w.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Champagne flowing.</span>
<span class="attribution"><span class="source">Mike Struik/CERN</span></span>
</figcaption>
</figure>
<p><strong>10.43am. Stable beams: The LHC is now running smoothly, the beams are behaving as expected, and the experiments can start recording data.</strong></p>
<p>Run 2 has begun! Champagne is flowing at CERN. Now the attention moves to analysing the new data, and it’s time for the rest of us to get back to work.</p><img src="https://counter.theconversation.com/content/42846/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Gavin Hesketh receives funding from The Royal Society and STFC.</span></em></p>Running the world’s largest particle accelerator requires a lot of energy, but it could reveal the secrets of the universe.Gavin Hesketh, Lecturer in Particle Physics, UCLLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/383392015-03-20T06:32:22Z2015-03-20T06:32:22ZExplainer: what are fundamental particles?<figure><img src="https://images.theconversation.com/files/75089/original/image-20150317-22277-fpy7cs.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The epoch of the leptons existed for nine seconds after the Big Bang. </span> <span class="attribution"><span class="source">Big Bang by Shutterstock</span></span></figcaption></figure><p>It is often <a href="http://phys.org/news/2013-10-ancient-greece-nobel-prize-higgs.html">claimed that the Ancient Greeks</a> were the first to identify objects that have no size, yet are able to build up the world around us through their interactions. And as we are able to observe the world in tinier and tinier detail through <a href="https://theconversation.com/nobel-prize-in-chemistry-beating-natures-limits-to-build-super-microscopes-32444">microscopes of increasing power</a>, it is natural to wonder what these objects are made of. </p>
<p>We believe we have found some of these objects: subatomic particles, or fundamental particles, which having no size can have no substructure. We are now seeking to explain the properties of these particles and working to show how these can be used to explain the contents of the universe. </p>
<p>There are two types of fundamental particles: matter particles, some of which combine to produce the world about us, and force particles – one of which, the photon, is responsible for electromagnetic radiation. These are classified in <a href="http://home.web.cern.ch/about/physics/standard-model">the standard model of particle physics</a>, which theorises how the basic building blocks of matter interact, governed by fundamental forces. Matter particles are fermions while force particles are bosons.</p>
<h2>Matter particles: quarks and leptons</h2>
<p>Matter particles are split into two groups: quarks and leptons – there are six of these, each with a corresponding partner. </p>
<p>Leptons are divided into three pairs. Each pair has an elementary particle with a charge and one with no charge – one that is much lighter and extremely difficult to detect. The lightest of these pairs is the electron and electron-neutrino. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/75083/original/image-20150317-22294-qb87ml.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/75083/original/image-20150317-22294-qb87ml.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/75083/original/image-20150317-22294-qb87ml.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=750&fit=crop&dpr=1 600w, https://images.theconversation.com/files/75083/original/image-20150317-22294-qb87ml.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=750&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/75083/original/image-20150317-22294-qb87ml.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=750&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/75083/original/image-20150317-22294-qb87ml.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=942&fit=crop&dpr=1 754w, https://images.theconversation.com/files/75083/original/image-20150317-22294-qb87ml.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=942&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/75083/original/image-20150317-22294-qb87ml.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=942&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">And then some.</span>
<span class="attribution"><a class="source" href="http://www.flickr.com/photos/51324729@N06/8521699366/in/photolist-dZ2U5E-4QfW8W-dZ2SkS-dZ2Wk9-dYWf14-dYWapc-dZ2PyU-fHXxg3-dYWare-dYWbv8-dYWdWZ-dZ2Vub-dYWeSi-dYW8Yz-dYWfqM-dZ2WJ1-dYWeTc-dYWfhD-dYWf3k-dYWdRg-dZ2VHQ-dYWvqV-dYWq3i-dZ2Wxy-dYWf8K-dZ2T5Y-dYW9tg-dZ2mGC-dZ2ooS-bmSrT4-dyhNrD-3fCzD8-aueBhN-wTpD-dYVETe-eiwixk-boDzCD-eiwkS6-8tbk55-dYWekV-dZ2jNG-eiC5SU-ijUZn-A7Tvu-7sR4E3-6qTSH-8eodca-b7w3ZR-82Ur7X-82Uqgk">James Childs</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>The charged electron is responsible for electric currents. Its uncharged partner, known as the electron-neutrino, is produced copiously in the sun and these interact so weakly with their surroundings that they pass unhindered through the Earth. A million of them pass through every square centimetre of your body every second, day and night. </p>
<p>Electron-neutrinos are produced in unimaginable numbers <a href="http://hep.bu.edu/%7Esuperk/gc.html">during supernova explosions</a> and it is these particles that disperse elements produced by nuclear burning into the universe. These elements include the carbon from which we are made, the oxygen we breathe, and almost everything else on earth. Therefore, in spite of the reluctance of neutrinos to interact with other fundamental particles, they are vital for our existence. The other two neutrino pairs (called muon and muon neutrino, tau and tau neutrino) appear to be just heavier versions of the electron. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/75082/original/image-20150317-22297-1qfduuz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/75082/original/image-20150317-22297-1qfduuz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/75082/original/image-20150317-22297-1qfduuz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/75082/original/image-20150317-22297-1qfduuz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/75082/original/image-20150317-22297-1qfduuz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/75082/original/image-20150317-22297-1qfduuz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/75082/original/image-20150317-22297-1qfduuz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/75082/original/image-20150317-22297-1qfduuz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">J J Thomson’s 1897 cathode ray tube with magnet coils – used to discover the electron.</span>
<span class="attribution"><a class="source" href="http://www.flickr.com/photos/sciencemuseum/9663807404/in/photolist-fHXxg3-dZ2U5E-4QfW8W-dZ2SkS-dZ2Wk9-dYWf14-dYWapc-dZ2PyU-dYWare-dYWbv8-dYWdWZ-dZ2Vub-dYWeSi-dYW8Yz-dYWfqM-dZ2WJ1-dYWeTc-dYWfhD-dYWf3k-dYWdRg-dZ2mGC-dyhNrD-3fCzD8-aueBhN-wTpD-eiwixk-boDzCD-eiwkS6-8tbk55-eiC5SU-ijUZn-6qTSH-8eodca-b7w3ZR-82Uqgk-dZ2VHQ-dYWvqV-dYWq3i-dZ2Wxy-dYWf8K-dZ2T5Y-dYW9tg-dZ2ooS-bmSrT4-dYVETe-dYWekV-dZ2jNG-A7Tvu-7sR4E3-82Ur7X">Science Museum London</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
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</figure>
<p>Since normal matter does not contain these particles it may seem that they are an unnecessary complication. However <a href="http://historyoftheuniverse.com/index.php?p=leptonEpoch.htm">during the first one to ten seconds</a> of the universe following the Big Bang, they had a crucial role to play in establishing the structure of the universe in which we live – known as the Lepton Epoch.</p>
<p>The six quarks are also split into three pairs with whimsical names: “up” with “down”, “charm” with “strange”, and “top” with “bottom” (previously called “truth” and “beauty” though regrettably changed). The up and down quarks stick together to form the protons and neutrons which lie at the heart of every atom. Again only the lightest pair of quarks are found in normal matter, the charm/strange and top/bottom pairs seem to play no role in the universe as it now exists, but, like the heavier leptons, played a role in the early moments of the universe and helped to create one that is amenable to our existence.</p>
<h2>Force particles</h2>
<p>There are six force particles in the standard model, <a href="http://www.particleadventure.org/unseen.html">which create the interactions</a> between matter particles. They are divided into <a href="https://theconversation.com/what-will-we-find-next-inside-the-large-hadron-collider-38664">four fundamental forces</a>: gravitational, electromagnetic, strong and weak forces.</p>
<p>A photon is a particle of light and <a href="https://van.physics.illinois.edu/qa/listing.php?id=414">is responsible for electric and magnetic fields</a>, created by the exchange of photons from one charged object to another. </p>
<p>The gluon produces the force responsible for holding quarks together to form protons and neutrons, and for holding those protons and neutrons together to form heavier nuclei. </p>
<p>Three particles named the “W plus”, the “W minus” and the “Z zero” – referred to as intermediate vector bosons – are responsible for the process of radioactive decay and for <a href="http://www.damtp.cam.ac.uk/user/db275/concepts/Particles.pdf">the processes in the sun which cause it to shine</a>. A sixth force particle, the graviton, is believed to be responsible for gravitation, but <a href="http://io9.com/what-are-gravitons-and-why-cant-we-see-them-1643904640">has not yet been observed</a>.</p>
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<h2>Anti-matter: the science fiction reality</h2>
<p>We also know of the existence of anti-matter. This is a concept much beloved by science fiction writers, but it really does exist. Anti-matter particles have been frequently observed. For example, the positron (the anti-particle of the electron) is used in medicine to map our internal organs <a href="https://theconversation.com/the-science-of-medical-imaging-spect-and-pet-14086">using positron emission tomography</a> (PET). Famously when a particle meets its anti-particle they both annihilate each other and a burst of energy is produced. A PET scanner is used to detect this. </p>
<p>Each of the matter particles above has a partner particle which has the same mass, but opposite electric charge, so we can double the number of matter particles (six quarks and six leptons) to arrive at a final number of 24.</p>
<p>We give matter quarks a number of +1 and anti-matter quarks a value of -1. If we add up the number of matter quarks plus the number of anti-matter quarks then we get the net number of quarks in the universe, this never varies. If we have enough energy we can create any of the matter quarks as long as we create an anti-matter quark at the same time. In the early moments of the universe these particles were being created continuously – now they are only created in the collisions of cosmic rays with the atmosphere of planets and stars.</p>
<h2>The famous Higgs boson</h2>
<p>There is a final particle which completes the roll call of particles in what is referred as the standard model of particle physics so far described. It is the Higgs, predicted by Peter Higgs 50 years ago, and whose <a href="http://home.web.cern.ch/topics/higgs-boson">discovery at CERN</a> in 2012 led to a Nobel Prize for Higgs and Francois Englert. </p>
<p>The Higgs boson is an odd particle: it is the second heaviest of the standard model particles and it resists a simple explanation. It is often said to be the origin of mass, which is true, but misleading. It gives mass to the quarks, and quarks make up the protons and neutrons, but only 2% of the mass of protons and neutrons is provided by the quarks, and the rest is from the energy in the gluons.</p>
<p>At this point we have accounted for all the particles required by the standard model: six force particles, 24 matter particles and one Higgs particle – a total of 31 fundamental particles. Despite what we know about them, their properties have not been measured well enough to allow us to say definitively that these particles are all that is needed to build the universe we see around us, and we certainly <a href="https://theconversation.com/beyond-the-higgs-boson-five-reasons-physics-is-still-interesting-20380">don’t have all the answers</a>. The next run of the <a href="http://home.web.cern.ch/topics/large-hadron-collider">Large Hadron Collider</a> will allow us to refine our measurements of some of these properties – but there is something else. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/75091/original/image-20150317-22288-1mp6qat.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/75091/original/image-20150317-22288-1mp6qat.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=392&fit=crop&dpr=1 600w, https://images.theconversation.com/files/75091/original/image-20150317-22288-1mp6qat.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=392&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/75091/original/image-20150317-22288-1mp6qat.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=392&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/75091/original/image-20150317-22288-1mp6qat.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=492&fit=crop&dpr=1 754w, https://images.theconversation.com/files/75091/original/image-20150317-22288-1mp6qat.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=492&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/75091/original/image-20150317-22288-1mp6qat.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=492&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">The great collider.</span>
<span class="attribution"><a class="source" href="http://www.flickr.com/photos/11304375@N07/2046228644/in/photolist-5pdeiQ-5m4QNL-4qZaVM-4roqPb-4ropM1-5s3JdT-47Psud-fZeRQ5-815XEw-812Vo6-812NhM-815Xib-815XQ1-815WVo-815Xy7-815WNo-812PAF-812P3F-A7Tuj-4rjnLp">Image Editor</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Yet the theory is still wrong</h2>
<p>The beautiful theory, the standard model, has been tested and re-tested over two decades and more; and we have not yet made a measurement that is in contradiction with our predictions. But we know that the standard model must be wrong. When we collide two fundamental particles together a number of outcomes are possible. Our theory allows us to calculate the probability that any particular outcome can occur, but at energies beyond which we have so far achieved it predicts that some of these outcomes occur with a probability of greater than 100% – clearly nonsense. </p>
<p>Theoretical physicists have spent much effort in trying to construct a theory which gives sensible answers at all energies, while giving the same answer as the standard model in every circumstance in which the standard model has been tested. </p>
<p>The most common modification implies that there are very heavy undiscovered particles. The fact they are heavy means lots of energy will be needed to produce them. The properties of these extra particles can be chosen to make sure that the resulting theory gives sensible answers at all energies, but they have no effect on the measurements that agree so well with the standard model. </p>
<p>The number of these undiscovered and as-yet-unseen particles depends on which theory you choose to believe. The most popular class of these theories are called <a href="http://home.web.cern.ch/about/physics/supersymmetry">supersymmetric</a> theories and they imply that all the particles which we have seen have a much heavier counterpart. However, if they are too heavy, problems will arise at energies we can produce before these particles are found. But the energies that will be reached in <a href="https://theconversation.com/what-will-we-find-next-inside-the-large-hadron-collider-38664">the next run of the LHC</a> are high enough that an absence of new particles will be a blow to all supersymmetric theories.</p><img src="https://counter.theconversation.com/content/38339/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Paul Kyberd works on Compact Muon Solenoid, an experiment at the LHC collider at CERN</span></em></p>Subatomic particles have shaped and continue to shape our universe but despite perfect predictions by physicists, the theory about unseen particles is still wrong.Paul Kyberd, Senior Lecturer in Particle Physics Informatics, Brunel University LondonLicensed as Creative Commons – attribution, no derivatives.