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?
- From the now-closed Tevatron, the CDF and DZero experiments presented their results, separately and combined, mainly focusing on the search for one Standard Model Higgs process, described in Tuesday’s post.
- From the LHC, CMS had already presented its main results from 2011 data on the Standard Model Higgs search, so there wasn’t much new from them. But there are three searches which CMS had presented in December that ATLAS had not (at least not with the full 2011 data). Today, ATLAS presented results on them all.
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:
- Higgs decaying to photons
- 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:
- Higgs decaying to a lepton, an anti-lepton, a neutrino and an anti-neutrino
- Higgs decaying to a tau lepton/anti-lepton pair
- 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.