Have they found that elusive goddamn particle?

Well, maybe…  As was the case with Tevatron’s false alarm, it’s far too soon to say that the search for “that damned elusive [Higgs] particle” is over.  But there is a difference this time.

I thought something was up when I read the Guardian’s tame particle physicist Jon Butterworth’s post on Thursday.

I’m writing this on the plane home from CERN. I would have loved to post more about ATLAS preparations for the European Physical Society meeting in Grenoble, but I haven’t had time and anyway half of it was secret until now. (The Higgs results are still not quite public, watch this space.)

As it turns out, he had already written his next post on the latest results from the ATLAS experiment at the Large Hadron Collider. From that post.

We have excluded at 95% confidence a Standard Model Higgs with a mass between 155 GeV and 190 GeV, and similarly between 285 GeV and 450 GeV. Some of this range was already excluded by the Tevatron experiments, but this is still a big advance. In fact around 290 GeV we are doing “better” than expected, which could of course just be to do with fluctuations in the data.

But it gets even more interesting about 155 GeV. Below here, we are not doing as well as we should.

This means one of three things:

(1) We were unlucky, and an upward random fluctuation in the background messed up our sensitivity.

(2) We did something wrong and failed to find it yet (these are still preliminary results, though they have been reviewed by the ATLAS collaboration).

(3) A Higgs boson, or something very much like it, is lurking in there somewhere below 155 GeV and is starting to emerge, blinking, into the light.

I am writing this now because I want to say what I think about our own data before I see what the Tevatron experiments, or our friends across the LHC, CMS, are going to say. By the time you read this, I will probably know. But here you get my judgement of what our stuff means, independent of any bias from other experiments.

If they show exclusion limits going down below 155 GeV, this means ATLAS was unlucky (or wrong) and the Higgs has less room to breath.

If they show exclusion limits which they didn’t expect to go below 155 GeV, this means they are not sensitive enough to say anything about the ATLAS result in this region.

If they show exclusion limits which, like us, they expected to go below 155 GeV but which don’t, then the chances that this is the first sign of the Higgs boson emerging into the light are increased.

It’s not decisive, but from the ATLAS data alone the odds have shifted in favour of the Higgs being around. When CMS and the Tevatron show their data, the odds will either shift back again, or shift further in favour of the Higgs. I don’t know yet. But in contrast to some previous occasions, it’s now getting really interesting.

As I said at the beginning, it’s far too early to say that the search for that elusive God goddamn particle  is over.  But what’s different this time is that the CMS team have independently reported two ‘bumps’ in their data in the same region.

Quantum Field Theorist, Matt Strassler, summarizes the results here

The Standard Model Higgs particle is the simplest possible Higgs particle. The Higgs particle may be more complicated, and/or there may be multiple types of Higgs particles.  But right now, at the Tevatron and LHC, the focus is on the following question: is the Higgs particle the one predicted by the Standard Model, and if so, what is its mass?

For years we’ve known that the Standard Model Higgs particle can only have a mass above 115 GeV (the limit comes from a previous collider called LEP II) and below about 800 GeV (the limit comes from a breakdown in the mathematics of the Standard Model.) 

The Tevatron experiments CDF and DZero had managed to rule out a chunk of the Higgs mass range around 160 GeV, and updated that result this morning with some improvements, excluding about 155-175.  Then came the ATLAS and CMS talks.  I thought ATLAS and CMS would do a bit better than the Tevatron experiments.   But I was completely wrong.

Starting late this morning and ending by mid-afternoon, I watched in some astonishment as step by step the LHC experiments ruled out most [and probably all– we’ll find out in a month at most when they combine their results] of the Higgs mass region between about 150 GeV and 450 GeV.  This represents a giant leap forward, a spectacular end of the Tevatron’s reign, and very big news.  The entire intermediate range for the Higgs mass may be gone, eliminated from our collective discussion in one short hour.

But that wasn’t the end of it, by a long shot.  When ATLAS showed that they had an excess of events in their search for a Higgs decaying to 2 W particles, of a type not inconsistent with a Standard Model Higgs with mass in the 120-150 GeV range, I sat up a little straighter in my chair.  We all waited to find out what CMS would say.  And then CMS showed almost the same thing, though with smaller statistical significance.   It could have been the reverse, as it has been so many times before; so often when one experiment sees something unusual, its sibling sees something normal, and we go back to our regular business.   Not this time.

It was no ordinary day.

Read the whole thing.  As he notes at the end

In a few months the experimentalists at ATLAS and CMS should have enough data to be able to straighten this out.  I expect by somewhere between December and March we will have a definitive answer as to whether today’s events in Grenoble were truly historic, or merely a major milestone.

A last word of caution.  It should still be viewed as unlikely that we have observed the first sign of the Higgs particle.  Remember, most hints of new phenomena disappear with more data.  But still, while it is true that few hints are followed by discoveries, it is also true that every discovery begins with a hint.

Indeed.  In the meantime, here’s a good introduction to the Standard Model of Particle Physics from Cern News – it’s the first in a series of videos.

Of course, there may be further wrinkles ahead…

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