Excitement is reaching fever pitch as the presentation at CERN on the Higgs search approaches! What’s important is that they measure By now the experiments have actually finished their analyses, and the press is reporting (harmless) rumors that both of the big Large Hadron Collider experiments, ATLAS and CMS, are seeing something. Presumably, assuming the rumors aren’t wrong, what they are observing, using this year’s data, is signs of a Higgs-like particle with a mass of 125 GeV/c2 that are roughly similar to what they reported in December, January and March using last year’s data. The details will be fascinating to see!
UPDATE: The CDF and DZero experiments at the now-closed Tevatron just updated their Higgs searches, and they find some amount of evidence in favor of a Higgs particle being produced through its interaction with W and Z particles and decaying to bottom quark/anti-quark pairs. Resonaances has a summary. This process is interesting because neither this production mechanism nor this decay mode are yet accessible to ATLAS and CMS; they don’t have enough data collected yet. One should be a bit cautious about such difficult analyses, but taken at face value these measurements are complementary to what ATLAS and CMS can tell us on Wednesday.
What’s all this fuss about? For the non-technical reader (and/or for your non-technical friends) I’ve written an article trying to put this historic moment in perspective, using language that is as non-technical and unfrightening as I can manage. If you agree that this is a good article for non-experts, please forward the link to others!
For the reader who wants to follow more details: Let me remind you of the sharpest questions that we’ll be asking on Wednesday, many of which are the same or almost the same as those we asked last December. (You can review my various background articles on the Higgs if you want to understand why these are the right questions):
For both ATLAS and CMS, we will ask:
- Is there a new particle apparent, as a bump in a plot, in the search for a Higgs decaying to two photons?
- Is there supporting evidence of a bump, at around the same mass as for the photon search, in the search for a Higgs decaying to two lepton/anti-lepton pairs?
- Is there supporting evidence for an excess of some sort in the searches for a Higgs decaying (a) to a tau lepton/anti-lepton pair, (b) to a bottom quark/anti-quark pair, and (c) to a lepton, anti-lepton, neutrino and anti-neutrino?
These are just questions about what the data actually shows; there are some additional sub-questions, but we’ll get back to them after the data actually appears. Then,
- Does evidence in favor of a new particle support the idea that this is a Higgs particle?
The easiest way to get a positive answer would be to see evidence that the particle decays to two lepton/anti-lepton pairs, or to a lepton, anti-lepton, neutrino and anti-neutrino — because a Higgs particle is a ripple in a Higgs field, and a Higgs field, by definition, gives mass to the W and Z particles, and consequently the Higgs particle must be able to decay, pretty often, to two W or two Z particles (more precisely, for a lightweight Higgs particle, to a real W or Z particle and a virtual W or Z particle). If the answer is yes, then we ask:
- Does evidence in favor of this particle suggest that it is a Higgs, but not a Higgs of the simplest type (i.e., that it is not a Standard Model Higgs)?
The easiest way to get a positive answer would be to see evidence that rates for the various modes of production of the Higgs, or the probabilities for various ways that the Higgs can decay, or both, are very different from those predicted for a simplest Higgs of mass of about 125 GeV/c2. Most likely we’ll get an ambiguous answer this week, and the answer to this question may not become clear for several years — but we could get lucky even now, if the Higgs in nature is dramatically distinct from the simplest case.
Then the next questions will be:
- Is what CMS observes largely consistent with what ATLAS observes? In short, do the results for one experiment largely confirm the results for the other experiment?
- Are there any notable discrepancies between the two experiments that might undermine confidence in their results?
Obviously, consistency between the two experiments will greatly increase confidence in the results!
Onward to Geneva, and IndependHiggs Day!!!