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Explainer: the Z’ (hypothetical) particle

Is the “Z-prime” lurking within a “jungle” of particles at the LHC? Argonne National Laboratory

There’s nothing like an unexpected result to get physicists excited.

So in 2008, when some strange behaviour was detected from a rarely-produced particle known as the “top quark”, there was much interest and speculation.

Now, with the publication of an intriguing new study by University of Michigan researchers we may be closer to explaining the top quark’s behaviour.

The Standard Model of Particle Physics describes the fundamental particles (and their interactions) that combine to form everything we see in the universe.

But this model is not complete.

There are already hints from high-energy experiments that suggest the existence of so-called missing elements – particles that have yet to be seen. The properties of these particles might be able to solve some important riddles.

Let’s take stock for a moment. Everyday matter (including you and I) is made up of just a few ingredients: relatively light “up” and “down” quarks stuck together with “gluons” (think “glue”) to form “protons” and “neutrons”.

Along with “electrons”, these last two particles make up the atoms from which all matter is constructed.

In studying the results from high-energy systems such as those produced by Europe’s Large Hadron Collider (LHC), we’ve found there are a handful of other interesting particles out there.

And importantly, there are four more types of quarks, the heaviest of which is named “top”.

So why the current excitement? Strap yourself in, reader; this is where things get complicated.

The 2008 discovery, made by physicists at the Tevatron (a particle collider near Illinois), uncovered a strange asymmetry in the direction that top quarks exit a collision between protons and anti-protons.

What does this mean? In short, the top quarks preferred to move in a particular direction rather than being uniformly distributed.

This result, on this scale, was not predicted by the Standard Model, and as a result has caused a bit of a stir amongst physicists (who nit-pick such predictions with a fine-toothed comb).

As is customarily the case in particle physics, theories abound for a “new” particle that can explain the discrepancy. This “new” particle could be any number of different things.

Well, it could be a particle that comes from a pool of mysterious “supersymmetric” or dark matter particles whose existence we are yet to confirm. It could also originate from an as-yet unseen force.

Unfortunately, such theories tend to raise more questions than they answer. Luckily, some very elegant options remain, including the solutions described by the University of Michigan researchers.

According to their study, the Tevatron results could point towards the existence of the hypothetical Z’ (pronounced “Z-prime”) particle.

Beautiful as this theory might be, it is mere speculation unless the Z’ particle can be found.

One way to tell if the Z’ actually exists is to look for it in the jungle of particles created in collisions at the LHC, which operates at a much higher energy level than the Tevatron.

With any luck, the LHC will be able to create the Z’ within a couple of months, assuming the particle has a mass lower than a particular limit.

So what happens if we do detect the Z’?

Well, at the moment, the existence of Z’ is simply one theory competing with several other theories, so finding it would be an unexpected result for a great number of physicists.

And physicists love unexpected results.

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