Restructured the post: My preliminary discussion is first, the updates from the talks are now at the end. The take-away message from the LHC talks:
The most interesting point here is that ATLAS and CMS [crudely] taken together exclude all regions for the Standard Model Higgs particle except
- below 145 GeV,
- the range 288-296 GeV, and
- above 464 GeV.
Since precision measurements of other quantities in nature indirectly make the region above 464 GeV very unlikely, and even that small window around 290 is probably excluded when the two experimental results are combined properly, we are probably down to the region 115 — 145 GeV for the Standard Model Higgs.
Not much change in the Higgs hint at 140… some details have shifted, but they went by too fast. It needs a closer look. But don’t expect any real news. [Update: the significance of the hint has gone down a bit --- but is this due just to the new data or have the analyses changed? Also, from the past month's data, CMS saw no additional two-lepton/two-antilepton events in that mass range (at Grenoble they reported two events at 142 GeV.)]
8/22/11 10:30 Mumbai time
The search for the Higgs particle — a ripple in the Higgs field, the field which is on average non-zero and provides the mass for the known elementary particles — continues at the Tevatron and Large Hadron Collider. We will probably get some new information within an hour or so.
Right now, the focus is on the search for the simplest possible Higgs particle — the one that goes by the name of Standard Model Higgs and arises from the simplest possible type of Higgs field. There are searches also going on for somewhat more complicated possibilities — Higgs particles that decay in unusual ways, or several types of Higgs particles, etc. But the first task of the experiments is to find the Standard Model Higgs particle, if it is there in nature, or rule it out, forcing us to look for something more complex.
Last month, we heard (a) that the Standard Model Higgs is probably not heavy (its mass-energy [E=m c-squared] probably does not lie in the range above 150 GeV and below about 450, though there are still some gaps in the coverage), and (b) there is a hint of a Standard Model Higgs particle around 140 GeV, although this could also be a more complicated non-standard Higgs particle at masses somewhat lower (perhaps 115-130 GeV.)
But as I emphasized in this detailed article, there are two types of searches for the Standard Model Higgs particle: some simple ones, easy to interpret, that take a while, and difficult ones that require less data but are subject to more subtleties. That hint at 140 relies on a very difficult technique, and so there is some question as to whether it might be an artifact of some type.
We will be watching today at the conference in Mumbai to see the updates on both the easy and the difficult searches. If the experiments really do update their results from the Grenoble conference by including more data, we should see (1) a firmer exclusion of a heavy Standard Model Higgs, except — if it is actually there! or if there is a statistical fluke — at one or two points where there will be a slight hint of a Higgs; (2) an update of the easier search strategies, which could be quite interesting indeed, as there may be enough data now for them to be useful; and (3) probably very little new on the more difficult search strategy, because there are probably internal studies being done to check the methodology, and they may not be complete. The hints seen in the more difficult strategies will still be there, but I suspect they will continue for now to rest on somewhat uncertain ground.
8/22/11 10:45 Mumbai time
[Right now we have an introductory theory talk by Abdelhak Djouadi, from France. Why the theorist gets 35 minutes and the ATLAS and CMS experimental talks --- the real data! --- get only 20 minutes each is beyond me. The ATLAS talk will be given by Aleandro Nisati from Italy (with whom I had the pleasure to work briefly), and the CMS talk by Vivek Sharma from the U.S.A. And the Tevatron talk, combining CDF and D0 data, will be given by Marco Verzocchi, also from the U.S.A.]
Update 1: 11:39 ATLAS talk — Excellent! the full data set is being used for the two-lepton/two-antilepton search! [Nothing special in two-photon signature.] 11:53 EXACTLY as was predicted above… nice improvement in exclusion in heavy mass region, except for a few gaps; no real change in the Higgs hint below 150. I think at 95%: excluded 146-232, 256-282, 296-464 GeV. Striking: 99% exclusion of Standard Model Higgs between 160-220 and 300-420 GeV.
Update 2: 12:07 CMS talk (not quite all the recorded data I think) [in two-photon signature, some excess at 140 GeV, but I think ATLAS did not see one there; presumably just statistical fluctuation] 12:20 And AGAIN, exactly as predicted from CMS.
The most interesting point here is that ATLAS and CMS [crudely] taken together exclude all regions except below 145 GeV, 288-296 GeV, and above 464 GeV.
12:51 The Tevatron (CDF and D0 experiments) reports exclusion of the Standard Model Higgs in these ranges
- Observed: 100-109 and 156-177 GeV
- Expected: 100-108 and 148-181 GeV
Since observed is weaker than expected at the low end of the upper range, that means that there is a tiny hint of a Higgs below 150 or so, so this result isn’t in conflict with the LHC hint at around 140 GeV. But we wouldn’t expect Tevatron to be as sensitive as the LHC experiments to a 140 GeV Higgs, if there is one.
Most interesting result from Tevatron: both CDF and D0 see an excess of events consistent with a supersymmetric Higgs particle in the mass range 120-150 GeV, in a production mechanism that is absent in the Standard Model. However this is a tough measurement and it is hard to know whether to trust this… and the excesses are not that large. (Experts: this is b-quark plus Higgs, where Higgs decays to b-quark and b-antiquark. Note b-quark-plus-Higgs where Higgs decays to tau-lepton and tau-antilepton is not observed at the corresponding expected rate, so this Higgs would have to be exotic in at least one other way if this were a real signal.)