Tag Archives: hints

The Quiet Higgs Quake at CMS

Posted on March 13, 2012 | 33 Comments

Posts have been a bit rare due to overwork and travel, but I have a few things to say about the search for the Standard Model Higgs particle (the simplest possible form of the Higgs) at CMS, one of the two major Large Hadron Collider [LHC] experiments.   Last week I focused on the big news from the Tevatron and from ATLAS (the other major LHC experiment involved in the Higgs search), because the changes in their results were much larger than those from CMS, partly because CMS had already analyzed all their data for all of the different types of Higgs decays, and also the changes at CMS were, on the face of it, rather small.

However, the results at CMS have a significant effect on the overall picture, both negative and positive, and so deserve comment.

The main change from the December and January CMS two-photon Higgs search is that a more powerful and sophisticated technique has been applied to the same data. The results are roughly consistent with the previous search, but show important differences (which tells you how sensitive the current evidence is to how you slice the data.) Here are the consequences of the new result compared to the old, illustrated in the two figures late in the post: Continue reading →

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Taking Stock: Where is the Higgs Search Now?

Posted on March 7, 2012 | 28 Comments

Today, we got new information at the Moriond conference on the search for the Higgs particle (in particular, Phase 1 of the search, which involves the search for the simplest possible Higgs particle, called the “Standard Model Higgs”) from the Tevatron and the Large Hadron Collider [LHC], the Tevatron’s successor.  With those results in hand, and having had a little time to mull them over, let me give you a short summary.  If you want more details, read today’s earlier post and yesterday’s preparatory post.

Before I do that, let me make a remark.  There is a big difference between healthy skepticism and political denialism.  I get the impression that some people who are writing or reading other blogs misinterpret my caution with regard to experimental results as being somehow a political and unreasonable bias against the Higgs particle being present, either at a mass of 125 GeV/c2 or at all.  That’s ridiculous.  All that is going on is that I simply am not convinced yet by the data.  I’m a careful scientist… period.  And you’ll see that I’m consistent; later in this post I will advise you not to over-react negatively to what ATLAS didn’t see.

What happened today at the Moriond conference?

What did we learn?

The Tevatron experiments see a combined 2.2 standard deviation [2.2 ``sigma''] excess in their search, consistent with a Standard Model Higgs particle with a mass anywhere in the range of 115 to 135 GeV/c2.  This is not inconsistent with the Higgs hints that we saw in December from the LHC experiments.  Here I am being perhaps overly careful in not saying, more positively, “it is consistent with the Higgs hints…” only because this measurement is intrinsically too crude to allow us to narrow in on 124-126 GeV, where ATLAS and CMS see their hints.  In short, the Tevatron measurement could, in the end, turn out to indicate a Higgs at a different mass than the one indicated by the current ATLAS and CMS hints.  Anyway, it’s a minor and mostly a semantic point.

The results from ATLAS were a bit of a shock.  In all three processes on which ATLAS reported, CMS has presented results already, and in each case CMS saw a small excess (1 standard deviation [1"sigma"], which is  small indeed.)  But ATLAS reported today that it sees essentially no excess in any of the three, and even a deficit in one of them for low mass.  This has a big effect.

  • First, it allows ATLAS to exclude a Standard Model Higgs all the way up to 122 GeV/c2 (except for a little window 1 GeV/c2 wide centered at 118) and down to 129 GeV/c2.  The only large window left for the Standard Model Higgs particle is 122-129, more or less centered around the hint at 126 GeV/c2 that they saw in December.
  • But second, the significance of the December hint, when combined with the new data that shows no excesses in these three new processes, drops by about a full standard deviation.  That’s a pretty big drop.

What does it all mean?

I think it basically means, roughly, status quo.  We got some positive information and some negative information today, and none of it is that easy to interpret.  So I think we are roughly where we were before, except that we probably no longer have to worry about any Standard Model Higgs below 122 GeV/c2.  Before today we had a decent hint of a Standard Model-like Higgs particle with a mass around 125 GeV/c2; we still have it.  Let me explain what I mean.

There are easy (relatively!) searches for the Higgs, and there are hard ones.  The easy searches are the ones where the backgrounds are relatively simple and the signal is a narrow peak on a plot.  There are two:

  1. Higgs decaying to  photons
  2. Higgs decaying to two lepton/anti-lepton pairs (often called “four leptons” for short)

Results on these were presented by both ATLAS and CMS back in December.  The hard searches are the ones where the backgrounds are rather complicated and the signal is quite broad, so that a mistake in estimating a background can either create a fake signal or hide a real one.    There are three (mainly) for a lightweight Higgs:

  1. Higgs decaying to a lepton, an anti-lepton, a neutrino and an anti-neutrino
  2. Higgs decaying to a tau lepton/anti-lepton pair
  3. Higgs decaying to a bottom quark/anti-quark pair

These are the three that ATLAS reported on today (where they saw no sign of a Higgs signal), and that CMS presented back in December (and saw a small excess in all three.)  [ATLAS presented a result on the first one in December, but only using part of their data; it showed a small excess at the time, but not now.]  The third process is the main one in which CDF and DZero reported an excess today, though the first one also plays a role in interpreting that excess.

In other words, everything we learned today had to do with the difficult searches — the ones that are hard to perform, hard to interpret, and hard to check.  And everything we learned was 1 or 2 sigma information; not very compelling even statistically.

For this reason,

  • I would not conclude that the new Tevatron results make the 125 GeV Higgs case much stronger
  • I would not conclude that the new ATLAS results make the 125 GeV Higgs case much weaker

For the same reason, when I explained why I was skeptical of the evidence back in December, I told you that in my view the CMS excesses in the difficult searches did not make the case for a 125 GeV Higgs much more compelling.  Since the easy searches at CMS do not show as large excesses as ATLAS’s do, I wasn’t really comfortable with the whole case from CMS.   Their case improved in January, when they added a bit more information from their easy search for two photons.

If, like me, you discount the difficult Higgs searches somewhat relative to the easy Higgs ones, then almost nothing has changed, as far as the current Higgs hints, after today’s up and down information.  The excess in the two easy searches at ATLAS is still there, and there are excesses at CMS at least in the two-photon search.  Even from the beginning, I gave you good reasons to think the ATLAS’s easy-search excesses were a bit larger than they should be, probably due to an upward statistical fluctuation in the background.    Conversely I think now that one should not overstate how bad today’s ATLAS news is for the Higgs hints.  It’s still quite reasonable to think there may be a Standard Model Higgs there at 125 GeV/c2.  There’s some evidence in its favor, and it’s certainly not ruled out at this point. (Whereas now, almost all other masses are.)

So as usual I advise patience and calm and no hyperventilating; the 2012 data will settle the issue.  Either there is a Standard Model Higgs with a mass within a few percent of 125 GeV/c2 , or we’ll soon be fanning out in Phase 2 of the Higgs search, looking for all the other types of Higgs particles that might be out there.

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Posted in Uncategorized, LHC News, Other Collider News, Higgs

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Awaiting Higgs News from the Tevatron Experiments

Posted on March 6, 2012 | 12 Comments

The search for the Higgs particle has been dominated recently by the new kids on the block, the ATLAS and CMS experiments at the Large Hadron Collider [LHC], who benefit from the LHC’s record high energy per collision. But at its predecessor, the now-closed Tevatron, the CDF and DZero experiments still have a few tricks up their sleeves. Though the energy per collision in recent years at the Tevatron was 3.5 times smaller than was the LHC’s  in 2011,  CDF and DZero have twice as much data as do ATLAS and CMS right now. And there’s one more thing going for them. In contrast to the LHC, where protons collide with protons, at the Tevatron protons collided with antiprotons. That gives the Tevatron a little edge in one particular search mode for the Higgs. It won’t be enough to beat the LHC at the game for which it was designed, but it’s enough that the Tevatron experiments can at least play. And we’ll see results from the two experiments tomorrow (Wednesday) — with a preview already publicly available, as you’ll see below. Continue reading →

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Some Higgs News NOW

Posted on February 7, 2012 | 34 Comments

We’ve been expecting this: the ATLAS and CMS experiments at the Large Hadron Collider [LHC] are going to be making public shortly have made public the publishable form of their papers (preprints, which one submits to a journal for the purpose of peer review) on the search for the Standard Model Higgs particle (the simplest possible type of Higgs particle.)  These papers are based on the preliminary results that we heard about on December 13th, which I discussed in some detail here.    Here is a page where you can get all the ATLAS and CMS papers.

I’ll report after reading them.  There will probably be I see no major changes from December 13th, though there will be are interesting minor ones.  That’s because the measurements are very sensitive and precise.  [UPDATE 1: The two-photon measurement from CMS includes something new: they separate the small number of events that have two additional jets out from the rest. This subset of the data is sensitive to both the g g --> Higgs and q q --> q q Higgs production processes, and it shows an excess at 124 GeV/c2, consistent with the existence of a Standard Model-like Higgs at that mass.] [UPDATE 2: See below]

Continue reading →

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Reviewing the Search for the Higgs

Posted on January 24, 2012 | 16 Comments

Since we’re now approaching the time when the preliminary results from December on the search for the Higgs particle at the Large Hadron Collider (LHC) will be presented in final form, possibly with small but important adjustments, and since there will be additional results based on the fall’s data in the next few weeks, it would be good to do a little review of where things stand and where they’re going.  I won’t do this all in one post but let’s get started.

Of course there is a lot of material on this website already and I’ll point you to it.  Perhaps my most concise and least technical discussion of the search for the Higgs appeared as a guest post on Cosmic Variance (thank you to Sean Carroll for the invitation.)  In that post I emphasized that the LHC’s Higgs program has two phases (broadly speaking.)  The first phase is to search for and either find or exclude the simplest possible form of Higgs particle, known as the Standard Model Higgs particle (or SM Higgs for short).  This is a finite task, so at a certain point (presumably this year) this phase comes to an end.  The second phase, which will take a better part of a decade, and (as I’ll describe over time) begins this year, depends on whether a particle that resembles an SM Higgs is discovered or not.  To explain the basic logic, I presented a figure in the Cosmic Variance post, which I am reproducing here:

Fig. 1: From my guest post in Cosmic Variance: The basic logic of the Higgs search, showing Phase 1 in which the simplest form of Higgs particle (the Standard Model Higgs) is found or excluded, and the logical questions that will follow in Phase 2. See Figure 3 for a more complete version.

If you’ve been noticing how slowly the discovery of the Higgs takes place — first there are hints, and only months later can one know whether those hints are real or not — then it will not surprise you to learn that the line between Phase 1 and Phase 2 isn’t sharp. The scientists at the LHC experiments ATLAS and CMS will be doing both of them this year, because they can, and because they should.

  • The search for the SM Higgs in 2011 was so successful that only a few possibilities remain (see Figure 2): a Higgs between about 115 and 128 GeV/c2 or above 600 GeV/c2 (though the latter is disfavored by precision measurements.)
  • We’ve got clear-enough hints for a Higgs particle with a mass of about 125 GeV/c2 (where c is the speed of light and GeV is defined here) to take seriously the possibility that it’s been found.
  • If it’s been found, it resembles (only roughly so far, but that’s perhaps because there’s not enough data yet) an SM Higgs particle.
  • All that remains to be done in Phase 1 is to close the window between 115 and 127 GeV/c2 , to confirm or refute the hints around 125, and push up the limits at 600 up as far as they can go, perhaps 800 and beyond.

Fig. 2: Phase 1 of the Higgs search now leaves only a tiny window at small masses and a window at large masses that is disfavored by two decades of precision measurements. Notice how much progress was made in 2011 alone!

So we can start looking ahead to Phase 2 — indeed we must.    In particular, if there is indeed a new particle with a mass of 125 GeV/c2 that resembles an SM Higgs, then (from Figure 1) we now have to verify whether it is or isn’t exactly what the Standard Model predicts.  Any deviation whatsoever from the precise predictions of the Standard Model would be a historic, game-changing, and Nobel Prize-deserving discovery!  So the stakes are very high.

Now, Figure 1 was actually a simplified version of the game plan for the Higgs search.  A more complete version, which was too elaborate for the short and sweet Cosmic Variance article, is shown in Figure 3.  I’ll go through this figure carefully (which itself isn’t entirely complete) over the coming days and weeks.  But suffice it to say that 2012 will involve a lot of work on the three research programs located at the far right of the figure:

Fig. 3: As Phase 1 of the Higgs search comes to a close, Phase 2 begins, with a series of logical questions and corresponding measurements. Even while the hints of a Higgs particle at 125 GeV are being checked with more data, studies of whether this might or might not be a Standard Model Higgs particle will be underway.

As you can see from the figure, these very large research programs needs to be carried out whether or not the current hints of a Higgs turn out to be ephemeral.  In other words, the experimenters can be working already on Phase 2 no matter what the outcome of Phase 1 turns out to be!  Very convenient, of course…  but it means we need to do some planning for Phase 2 now!  [And not of all of the most urgent issues have been addressed yet, which is why I've been very busy... more on that later...]

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About the NY Times article from 8/02/11

Posted on August 4, 2011 | Leave a comment

On Tuesday, the New York Times had an article on the Higgs particle search.  Not bad, and does quote relevant people, but just a little bit thin on content.  If you want some actual content, try my article on the hints of the Higgs particle.

Also, the article falls into the common mistake of not distinguishing “The Higgs particle” from “The Standard Model Higgs particle.”   One paragraph says

The Higgs boson is the keystone and last undiscovered piece of the so-called Standard Model, a suite of equations that agrees with all the experiments physicists have been able to do so far in the laboratory.  If the Higgs boson does not exist, theorists will have to go back to their blackboards.

If you conflate the general with the specific, you will at some point down the line end up very confused about what the LHC is trying to do and what its results do and don’t mean.  It would be good for the public if the NY Times reporter would read the Higgs FAQ before phrasing a statement like this.  A more correct statement is:

The Standard Model Higgs boson is the keystone and last undiscovered piece of the so-called Standard Model, a suite of equations that agrees with all the experiments physicists have been able to do so far in the laboratory. If the Standard Model Higgs boson does not exist, this will be very exciting for particle physicists, as it will imply (as has been known for decades) that there must be other particles — perhaps multiple Higgs bosons, and/or perhaps other types of particles — that physicists have not yet discovered, but should be able to discover with the LHC.

The spin is pretty different.

And then there’s the generic paragraph about supersymmetry — more or less right, but again with some big missing points.  Well, stay tuned at this website for an article or two on supersymmetry, and what we have and haven’t learned about it from the LHC so far.

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