Author: Matt Strassler

This week I’m at CERN, home of the Large Hadron Collider [LHC] (and have been assigned the office of none other than John Ellis, who perhaps surprisingly is out of town.) Tomorrow, some new results on the Higgs particle search at ATLAS and CMS, the two general-purpose experiments at the LHC, will be presented.  In today’s post, I’ll first summarize the  situation, and then I’ll explain what I’ll be paying closest attention to in tomorrow’s presentation.

First, a summary of basics, with links.

• The Higgs field gives mass to the known elementary particles; if it hadn’t done so, by becoming non-zero throughout the universe, atoms (and we) wouldn’t exist.
• We know almost nothing about this field. (Elementary or composite?  One or more?  Why non-zero?)
• The Higgs particle is a ripple in the Higgs field; finding and studying the Higgs particle would give us insights into the Higgs field.
• The Standard Model Higgs particle is the simplest possible form of the Higgs particle, a ripple in the simplest possible Higgs field.
• Currently the LHC is in Phase 1 of the Higgs search, whose goal is to find, or rule out, the Standard Model Higgs (SM Higgs for short.)
• What is Phase 2 of the search? That depends on whether we find something that looks like a SM Higgs, or whether we exclude that possibility.
• If an SM-like Higgs is found, we will have to study it in great detail to make sure it is really what it appears to be.
• If no SM-like Higgs is found, we will have to look for a much broader list of possible types of Higgs particles, and for other phenomena.
• More general details on the Higgs can be found here (in the Higgs FAQ, and in this summary page on the SM Higgs.)
• A lot of specific details on the Standard Model Higgs particle can be found in the Standard Model Higgs Trilogy, with its three articles on SM Higgs production, SM Higgs decays, and the search for (and study of) the SM Higgs.
• What if there is a Higgs field but no observable Higgs particle? That could certainly be the case — a number of speculative theories dating back decades suggest it — but
• It would be about 10 years before we’d know this with confidence, and
• It can only happen if there are new phenomena in nature that the LHC should be able to start exploring.

Now, about the presentations on Tuesday.  Despite various rumors, I do not expect (and most experts I talk to do not expect, and CERN itself warns us not to expect) any evidence presented tomorrow to be convincing yet; here’s why.  But we might (or might not) see some evidence that is moderately compelling.  How should we evaluate it?  What should we be watching for?

[My reasoning here may be opaque to you unless you’ve read through the Standard Model Higgs trilogy (at least the first half of part 3, and also the older article it links to) or you’re already somewhat knowledgeable about the subject.]

First, how compelling are the hints of the Higgs particle shown by ATLAS and CMS, treating the experiments separately?

• More precisely, how much of the signal for each experiment comes from the clean searches for Higgs –> two-photons and Higgs –> ZZ –> two lepton-antilepton pairs, which have low systematic and theoretical errors and give a sharp measurement of the Higgs mass, and how much comes from the messy WW –> lepton/anti-neutrino anti-lepton/neutrino signal, which has much higher systematic and theoretical errors and gives a vague estimate of the Higgs mass?
• In particular, if you were to drop the Higgs –> WW measurement entirely, are the signals of the Higgs in two-photons and in two lepton-antilepton pairs convincing by themselves? (This tests whether the result’s significance relies most heavily on a tricky and controversial measurement or on the simpler non-controversial ones.)
• If Higgs –> two lepton-antilepton pairs plays an important role, this will be because of one or two collisions that suggest the same Higgs mass as a signal in Higgs –> two photons. How much less convincing would the result become if you were to remove one of those collisions? (This checks sensitivity of the result to an individual chance event.)
• How much of the region near to the signal has actually been excluded? Should we believe evidence in favor of, say, a 125 GeV Higgs if a Higgs at 128-135 or 118-122 has not yet been excluded? (This helps clarify whether the search is really close to completion.)
• If the WW measurement plays a large role in the evidence, how confident do the theory experts on Higgs –> WW, and on the various backgrounds to this search, seem to be with the theoretical errors stated by the experiments? Do the experiments have detailed evidence that they understand their systematic errors on this measurement? How well under control is their ability to measure the “missing momentum” signal of the neutrinos with sufficient accuracy and precision? (Until we have convincing answers to these questions, I personally think we should be very cautious trusting the WW contribution to the significance of the result.)

Next, we should view the experiments together.  Are they really seeing evidence for the same thing?

• Are the ATLAS and CMS signals for a Higgs particle really sitting at the same Higgs mass? or do the preferred masses differ by more than 2%, which would be inconsistent with a Higgs particle’s signal?
• Does the evidence really become much more compelling when you combine the evidence from the two experiments (which we’ll have to do by eye, since the statistically valid, officially authorized combination of the two will probably not be carried out for quite some time)?

I’ll try to get most or all these questions answered for you, to the best of my ability, by tomorrow evening or Wednesday morning. Whether I’ll be able to get you any information during and/or right after the presentation tomorrow may depend on how much of a zoo it is at CERN tomorrow, and how well the wireless network can handle the inevitable massive flood of data going in and out of CERN!