Of Particular Significance

Why so quiet at LHC?

July 6, 2011

If you’ve been waiting breathlessly for news on the Large Hadron Collider, you are probably wondering why it’s so quiet.  Very little news in the press.  Is something wrong?

On the contrary!  The LHC is working extraordinarily well right now.  In fact, the clouds are gathering for a torrential deluge of new results!   The National Particle Service has issued a Flash Flood Watch for late July.

Yes, the LHC is operating at half the design energy — which is to say that each collision between two protons has half of the maximum energy that the machine was designed to provide.  But energy is not everything!

There are three ingredients to a successful collider: high energy, a high collision rate, and high reliability. Among these three, one can make some compromises.  What value would a machine be that could operate at the highest energy but could only manufacture one collision a day, or which was down for repairs 99% of the time?  We would all gladly accept somewhat lower energy if it meant a machine operating and producing many collisions.

The original plan for the LHC was to go to the highest energies right away and slowly bump up the collision rate.  Instead, because of the accident back in 2008 and the ensuing discovery of a couple of design flaws that need eventual repair, the decision was made to stay at lower energies, in order to protect the machine, and try to crank up the collision rate.  Though this was a backup strategy, it has worked out very well indeed.

For many measurements, including the search for the Higgs particle, losing half the energy means we need a collision rate about three-to-five times higher than we would have at full energy, in order to make enough Higgs particles (if they’re there) to unambiguously detect them.  Fortunately, the machine is delivering at a pace that few people anticipated a year ago.  Already in 2011 the LHC has produced about 40 times as many collisions as were produced all of last year!   It is well on track to get well more than 100 times as much data as we saw in 2010.

What has already been gathered is enough to do a great many first-of-their-kind measurements.  Dozens.  Maybe hundreds.  My experimental colleagues are drowning in data, and working full-tilt, analyzing it in as many ways as they can think of and they have time for.  They look harried, excited, exhausted, driven, all at once.  They are engaged in extraordinarily complex and challenging work.  Theirs is not nice clean data that comes packaged in a nice clean spreadsheet.  Analysis of this data is not a matter of writing a simple computer program, pressing “enter”, and waiting ten seconds for your Nobel-Prize-winning graph to appear on the screen.   Despite the fact that there are thousands of people working on each of the two main experiments (ATLAS and CMS), it isn’t nearly as many people as you would ideally want for such a huge undertaking.  (Stay tuned for posts that explain how LHC research is really done.)

Ok, fine, things are great — but then, why so quiet?  Because the experimentalists are gearing up for two big conferences, one toward the end of July in France, one toward the end of August in India.  They are rushing to finish all their measurements in time to present them publicly at those conferences.  It is there that we will see many new results, to be quickly followed, or perhaps slightly preceded, by pre-publication plots, papers, press releases, posts on blogs, and lots of other results of particular significance.

There are a few things we can be sure of.  The search for the Higgs particle will move forward a step.   Not a giant step wildly outdistancing the achievements of the Tevatron experiments, but a good-sized one that will at least bring the LHC and the Tevatron neck-and-neck, and perhaps already move the LHC beyond its older and weaker cousin.  Meanwhile, the searches for many classes of new phenomena — from supersymmetric particles to signs of extra dimensions to hints of hidden valleys and lepton-jets to weird particles with names like “leptoquark” and “axigluon” — will make some very big steps forward. (All those fancy names deserve an explanation — but not now.  I’ll get back to them another time.)

Will there be actual discoveries?  It is possible, certainly, but not as likely as hints.  There will surely be hints of new phenomena.  Whenever you look at this much data for the first time, you will always find flukes, and weird bumps in your plots, and bizarre individual collisions that look extraordinary.  And as always, most of these hints will turn out, with more data or more careful consideration, to be statistical accidents, subtle errors, or artifacts of how the data is gathered or analyzed.  But maybe one or two of the dozen strange hints we’re likely to see will turn out to be of something real.  To figure that out for sure, we’ll need yet more data.  Fortunately, barring a technical problem, the end of 2012 should bring us as much as ten times as much data as we have now.

All’s quiet on the LHC front.  But not for long.

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A decay of a Higgs boson, as reconstructed by the CMS experiment at the LHC