Saturday, 31 October 2015

Next Generation Collider in China

I'm a little behind on this, but I did move in to my new apartment today.  The Guardian has reported that Chinese state media has announced that the proposed 100 TeV experiment will begin construction within 5 years.
China will begin work on the world’s largest supercollider in 2020, a mega-machine aimed at increasing understanding of the elusive Higgs boson, state-run media has reported.

 The facility, designed to smash subatomic particles together at enormous speed, will reportedly be at least twice the size of Europe’s physics lab, the Swiss-based Cern, where the Higgs boson was discovered.
The final concept design for the project is on track for completion by the end of 2016, Wang Yifang, director of the Institute of High Energy Physics at the China Academy of Sciences, told the China Daily.
This is hardly a surprise.  This idea has been talked about for several years now, and while the Chinese proposal is not the only one it has seemed the most likely to get the necessary funding.  This statement is notable in that if the Chinese press is reporting on it, the government must be moving towards confirming it.

This is good news for those of us in the field.  Even theorists like myself rely on a healthy experimental program for our work (and for our jobs).  And even though the LHC will run for another decade or so, we have to be thinking about the next machine now because design and construction are such lengthy processes. 

It's worth noting that the Chinese machine won't be the next major experimental advance; the International Linear Collider, which will almost certainly be built in Japan, should come online sooner.  However, the ILC will operate at lower energies than the LHC and focus on highly accurate studies of the Higgs.  The LHC and the Chinese machine (mostly) collide protons, while the ILC will collide electrons and positrons.  Without going into details, the former allow you to get to higher energies while the latter allow you to make more precise measurements.

But the Chinese machine will probably be needed to answer certain basic questions.  One of these is to definitively test naturalness of the Higgs.  Naturalness is roughly the idea that input parameters should not be much larger than output parameters.  The Higgs mass has two inputs; a fundamental, currently uncalculable one and a quantum correction.  Already, the absence of new particles to control the quantum correction suggests that these two unrelated terms should be equal and opposite to a 1% accuracy, which is why most people expected the LHC to have found something before now.  However, there are other examples in nature where a cancellation to 0.1% has happened.  The LHC and new machine together will be able to go down to a 0.01% cancellation, at which point it is fair to say that Higgs naturalness has failed (and alternative ideas such as the multiverse must be considered).

Another major question is dark matter.  If DM is made up of a single heavy particle whose abundance today is set by thermal processes in the early universe, a 100 TeV machine can almost certainly discover it.  Some exotic cases might survive, but commonly-studied examples like the scalar singlet, pure Wino or pure Higgsino will be found or excluded.  For the latter two, this is the only absolutely certain way to exclude them.

Finally, these two possibilities are focused on the LHC not finding anything new.  If instead we are lucky and we do get a discovery in the next couple of years, a new machine is even more valuable.  Our most popular theories of new physics propose a lot of new particles, be it all the superpartners in SUSY or all the top partners and resonances in composite Higgses.  But it is now clear that, if these ideas are correct, the LHC will only discover a few of the lightest states.  A full test of these models would demand a higher energy machine.

So the science case for this experiment is pretty solid.  The problem is that the cost is also pretty high and the world economy is not that good.  We've already had one major experiment cancelled in the US, and it's not clear that the EU could afford another LHC (though there are some efforts in that direction).  Evidence that the next generation experiment is a step closer to becoming reality is therefore quite reassuring, and the second best news I had this week after finding my new place.

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