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The International Linear Collider is coming – but why do we need it?

While the world’s most powerful particle accelerator - the Large Hadron Collider (LHC) - is on a two-year hiatus for repairs and upgrade, engineers are getting ready to construct the next generation of…

At 31 kilometres long, the International Linear Collider is tipped to be one of the most exciting scientific instruments ever built. But why? ILC GDE

While the world’s most powerful particle accelerator - the Large Hadron Collider (LHC) - is on a two-year hiatus for repairs and upgrade, engineers are getting ready to construct the next generation of particle accelerator: the International Linear Collider (ILC).

On Wednesday, a team of around 2,000 scientists and engineers from three hundred universities around the world (the Linear Collider Collaboration) published the ILC Technical Design Report. The report describes the physics case to build the new collider, as well as the accelerator’s design and the sophisticated experiments it’s tipped to run.

When constructed, the 31km-long ILC will be the world’s longest linear accelerator. That record is currently held by the Stanford Linear Accelerator in the US, which is 3.2km long.

Both the Large Hadron Collider and the proposed International Linear Collider work by smashing particles at high speeds.

But, you may ask, why do we need a new collider when the Large Hadron Collider will only be out of action for two years?

The structure of the International Linear Collider

As its name suggests, the International Linear Collider will mainly consist of two linear accelerators, facing each other. Electrons in one accelerator and positrons in the other will travel at very high speeds (approaching the speed of light) before colliding at the centre of the 31km-long machine.

When fully operational, there will be approximately 7,000 electron-positron collisions per second in the new machine. The acceleration will be provided by 7,400 superconducting radio-frequency cavities placed along the length of the structure, and, like the LHC, will operate at temperatures near absolute zero.

The electrons and positrons will travel in bunches, with each bunch containing 20 billion electrons or positrons concentrated into an area much smaller than that of a human hair. The number of collisions per second will be very high and this is crucial to produce and study particles which rarely occur in nature.

Bird’s eye view of the ILC. Rey.Hori/KEK

LHC and ILC: different, but complementary

While collisions at the Large Hadron Collider offers a broad view that can reveal new particles and new processes, collisions at the International Linear Collider can reveal specific properties and detailed information. The two types of collider rely on each other. We need both if we are to get to the bottom of the fundamental questions in nature.

In simple terms, if the Large Hadron Collider is considered the discovery machine, the International Linear Collider will be the precision machine. Both are required to complement each other’s findings.

The LHC smashes complex particles (protons) - which are made up of many smaller particles - resulting in very messy collisions with lots of particles spraying everywhere. In contrast, the ILC will smash indivisible particles - electrons and positrons - which will produce very clean collisions. The video below explains how the LHC and ILC collisions will differ.

The new machine will (hopefully!) shed more light on the recently observed Higgs boson and make important contributions to the search for particles associated with dark matter.

It’s also likely to make precise measurements of top quark mass - the heaviest particle of the Standard Model of Physics - in a way not possible at circular hadron colliders such as LHC.

To capture the outcomes of the particle collisions, the ILC will use two sophisticated detectors, namely:

Artist’s impression of the ILC. Rey.Hori/KEK

  • SiD (Silicon Detector)
  • ILD (International Large Detector)

The proposed two-detector design is essential to provide the corroboration of findings in an independent way. These technologically advanced multipurpose detectors will share “one" interaction region using a push-pull approach under which only one detector would record data at a time.

At regular intervals, the detectors would switch positions so the other could then record data. This is in contrast to LHC, which hosts four major detectors placed at four interaction regions on its 27km ring.

So now what?

The Technical Design Report is a result of approximately two decades of hard work, and clearly emphasises the importance of building the new machine.

The next steps are to choose a suitable site to build the International Linear Collider - most probably Japan or the US - which then will be followed up with any necessary site-dependent design details. Construction should start within two to five years.

So even though it’s still relatively early days, one of the most exciting scientific projects ever conceived is poised to begin.

Join the conversation

16 Comments sorted by

  1. Ricky Buchanan

    logged in via Twitter

    Thank you so much for writing this - you've answered exactly what I was wondering! I've just finished reading "Present At Creation" which I felt did a stunningly good job of explaining LHC to a non-scientific audience - it was really interesting to read, and a great motivation to do my first year physics homework!

    Am I right in thinking, then, that the difference between the two is not really related to the linear versus circular design but instead about the detectors used, or is there some reason I'm unaware of that it's hard to accelerate electrons and positrons through curves? Or I guess the question is what is the advantage of a linear design to a particle accelerator? It's pretty obvious to see the advantage to a circular design where the beam can be accelerated multiple times by the same equipment, but less obvious to my "interested but uneducated" level of knowledge what the advantages of a linear design are ..

    1. Nitesh Soni

      ARC Research Associate (Centre of Excellence for Particle Physics at Tera Scale) at University of Adelaide

      In reply to Ricky Buchanan

      Just to mention that LHC and ILC will provide the different collisions e.g. LHC collides protons (made up of up, down quarks and gluons) while ILC collides electrons-positrons (point-like object) which are much lighter compare to protons. Whenever the charged particles accelerate in the circular accelerator, they suffer energy loss from so called the synchrotron radiation as i/(mass)^3 or 1/(radius of accelerator)^2. While its relatively easy to control protons in circular accelerator compare to electrons as electrons being lighter hence scientist decided to go for linear accelerator technology to accelerate electron upto very high energy where they don't suffer from synchrotron radiations. Having said that in the past electron-positron have also been accelerated in circular accelerator but the energy of collisions was very small.

  2. Peter Dawson

    Gap Decade

    I remember years ago listening to a talk given by a visiting maverick not-very-highly-qualified physicist chap. The topic was quantum physics, and his main point was that colliding particles together at great speed was directly comparable to colliding two motor vehicles together at top speed, and studying the pieces of wreckage left behind in an attempt to determine precisely how motor vehicles are constructed, and precisely how the identified parts function.

    Is there any reason that this analogy is completely inapplicable to the field of particle physics? Because for the complete layperson, it certainly does have at least a superficial ring of truth to it.

    1. Nitesh Soni

      ARC Research Associate (Centre of Excellence for Particle Physics at Tera Scale) at University of Adelaide

      In reply to Riddley Walker

      Oh yes, rightly so we want it.... but if you go to funding agency then the first thing they would ask why do you need this machine?

    2. Riddley Walker


      In reply to Nitesh Soni

      I do so love it when someone explains a little joke for the benefit of everybody else. So kind.

  3. Stephen Ralph

    carer at n/a

    It would be nice if for 5 years there could be a moratorium on spending money on anything that doesn't directly "help" in the answers to solving the world's immediate problems.

    Sure the ILC is a wonderful must have piece of do -dad, but if the earth goes down the tube much more, space will still be there, but the earth won't - or in pretty bad shape anyway.

    The billions this machine will cost could be used to deliver so much more that is practical and necessary.

    Argue all you want that we need to explore these issues and find the answer to how the universe was created, or the answer to "everything", but surely more pressing issues are at stake.

    1. Grendelus Malleolus

      Senior Nerd

      In reply to Stephen Ralph

      Stephen, any great work of science such as the LHC always has a multitude of practical spin-offs that do directly help solve the world's immediate problems.The very conversation we are engaged in occurs on the World Wide Web - developed by Tim Berners-Lee, a British scientist at CERN to ensure that the large amount of information that needed to be shared could be shared effectively.

      Some of the development of the LHC has seen spinoffs to medical technology - such as chips used in particle detection that have an application in medical imaging and allow doctors to use far lower doses of radiation.

      No scientific endeavor of this scale produces outcomes isolated to a narrow field. The sheer scale of the project and the range of disciplines engaged in the undertaking create a hotbed of innovation that flows on to practical outcomes.

  4. Caroline Copley


    Hmm it is interesting that people comment on the need for spending money on more pressing problems. There is the situation in this country right now where we are taking the money from universities because there is a more pressing problem apparently- school education (exemption to universities in Sth Aust that seem to just have received a windfall!).
    I have been stating endlessly on this website that there is a hugely pressing problem in the global arena which is in order to solve climate change…

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    1. Stephen Ralph

      carer at n/a

      In reply to Caroline Copley

      I am sceptical that your claim is that particle physics will solve everything.

      As I said we may not have the time to unlock the mystery of the universe.

      All I suggested was a short term moratorium to allow all those great minds to look at terra firma and the REAL problems at hands - god can wait.

    2. Caroline Copley


      In reply to Stephen Ralph

      I was not trying to say particle physics will solve everything, and also covered your other call by stressing the need for an Earthbound BIOTRON to investigate systems and processes involved in Earth's Biosphere.
      Doubt about the standard model and is nothing compared to the shortcomings in our understanding of life and its interactions with Earth. For example there are so many BILLION microbes per square metre, of which we only know thousands. So how to determine soil carbon without knowing…

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