The 2011 run of the Large Hadron Collider is now over. It stopped on October 31st, and will resume in March. In the last two months, the LHC has far exceeded expectations. None of the data collected since August has yet been analyzed. The analysis is now underway.
In July, I posted two diaries about the Higgs Boson. The first, Higgs Boson Announcement Coming Soon? gave a very detailed introduction to the Higgs Boson and why it has been the "holy grail" of particle physics for forty years. The second, Higgs Boson Announcement v2.0 gave the results presented at the Europhysics meeting in July. I was surprised and gratified when both diaries shot up to the top of the rec list and received lots of comments.
I'm writing this diary to update interested Kossacks on the situation. Next week, the LHC will present results at the Hadron Collider Physics meeting in Paris, but they are unlikely to present any of the post-August results. As a result, there will be minor tweaks on the current situation. The full analysis from each of the two experiments on the entire 2011 dataset might be ready in mid-December, but are unlikely to be released without a lot of checking---the most likely target is a major conference the first week of March in France. Note: you will not learn what the Higgs boson is in this diary. If you're interested, please read the first diary listed above.
Remember that there is no "smoking gun" for a Higgs boson. Every signature of its existence can also be produced via non-Higgs effects. Rather, one must have a lot of data and look for an excess in Higgs-related signatures. It is very much like having a theory that a coin will give heads 53% of the time. If you flip it 100 times and get 53 heads, that proves nothing since a normal coin will often do the same. If you flip it 100 times and get 48 heads, that also proves nothing since a normal coin will often give 45. You need 1000's of coin flips to really test the theory.
As noted in earlier diaries, the properties of the Higgs boson are completely understood, except its mass. In the Standard Model, if you tell me the mass of the Higgs, I can tell you all of its properties (including the signatures at the LHC). The mass is measured in GeV (a proton has a mass of 0.938 GeV).
Before July, 2011
Earlier searches for the Higgs show that it must have a mass greater than 114 GeV. A small region of 158-172 GeV was excluded as well by the Tevatron at Fermilab. For technical reasons, it can't be heavier than 600 GeV or so. This left two "windows": between 114 GeV and 158 GeV, and 172 GeV to 600 GeV. For decades, particle physicists have strongly preferred the lighter window. There are two reasons:
1. Very high precision measurements of other Standard Model properties are weakly sensitive to the Higgs mass. They indicate that the Higgs is most likely below 145 GeV.
2. Many particle physicists like a theory called supersymmetry. The Standard Model of Supersymmetry explicitly predicts that the Higgs must be below 135 GeV.
Thus, the lower window is the most interesting. Unfortunately, it becomes harder and harder to see the Higgs as one moves down in mass from 158 to 114. So one would expect the upper part of the window to be probed first. The LHC has a good chance of completely closing that window (if the Higgs isn't there) with data collected in the 2011 run.
July, 2011
As noted in the earlier diary, in July, the LHC, based on a collected data sample of 1.0 inverse femtobarn (the unit used for the amount of data collected), reported that the region from 145 GeV to 500 GeV had been completely excluded, except for a small window in the 250 GeV range. They had actually hoped to exclude the 130-145 GeV region as well, but they saw excess events, and this caused great excitement. There seemed to be evidence of a Higgs around 140 GeV, but there wasn't enough data for any official announcement. They had no possibility of constraining the 115-130 GeV region.
August, 2011
At the "Lepton-Photon" Conference in late August, the data sample reported was 1.6-2.0 inverse femtobarns. The small window in the 250 GeV range was completely closed, and they thus could exclude a Higgs from 145 GeV to approximately 600 GeV, leaving only the 115-145 GeV window left. The evidence for a Higgs around 140 GeV also got weaker, causing much more disappointment in the media than in the field, since the original evidence wasn't that solid. They still saw excess events, now in the 120-140 range, but again, not enough data. They then began a very long and productive run, which ended near the end of October. The total data collected at that point was over 5.0 inverse femtobarns. If the Higgs isn't in the window, they should be able to exclude it. If it is the upper part (130-145) of the window, they should be able to discover it. If it is the lower part of the window, they will see strong evidence of it, but might not quite have enough to formally claim "discovery" (for stats people: they will get 3-sigma evidence, but 5-sigma is the standard for discovery).
Near future
At the conference next week, CERN is expected to merge the results from the two detectors for data collected through August. This should not give surprises, since the individual results have been out for a while, and many have already unofficially combined them. So I would not expect anything exciting. The experiments will then continue to analyze their data. There is a chance of an announcement in December, but the most likely time is the first week of March. With luck, then I will hopefully submit the final version of these diaries.