I’m currently in the middle of two important transitions, one logistical, the other scientific.
The logistical transition is my move to CERN. I got stuck in Cambridge for an extra two weeks because of a delay in getting my long-stay visa for France. With help from people on both sides of the Atlantic, I learned that the problem was that the person in charge of these visas at the French Ministry of Foreign Affairs had quit without leaving instructions for her replacement. After a week of uncertainty, on Friday afternoon I received my visa and rebooked my flights. In the meanwhile, I’m staying with patient, generous friends and living out of the suitcases I packed for my move.
The other transition is that I am wrapping up one measurement, with a paper about to clear (I hope!) the last stages of the ATLAS internal approval process, and am about to begin work on a new one. I’m excited because I’m starting to work on the search for the Higgs boson in the H → WW → lνlν channel. The W is the massive gauge boson that mediates the weak force, the l represents an electron or muon, and the ν represents a neutrino. This is a key search channel for exploring the still-allowed mass values for the Standard Model Higgs, since for all possible Higgs boson masses greater than 120 GeV, the Higgs decays to a WW in at least 10% of events, as shown in the image to the right, taken from the LHC Higgs Cross Section Working Group. For a sense of scale, masses below 115 GeV are excluded by direct searches at the LEP collider (the electron-positron collider that was the original inhabitant of the LHC tunnel). Each W can decay to either a quark-antiquark pair, or a charged lepton and a neutrino. Requiring each W to decay to either an eν or μν pair includes only about 5% of WW decays, but channels with leptons potentially yield a much clearer signal, because there is less background. That is, if you choose an event with the characteristic features of a WW → lνlν candidate, odds are pretty good that you actually have a WW event, and not an event from a different source with features that mimic a WW event.
In the results shown at EPS, both the ATLAS and CMS Higgs searches reported limits that were not as good as expected for low masses (130 GeV < m(H) < 160 GeV or so). The excess of events driving the degradation in the limits was mainly in the WW → lνlν final state. In the update shown at the Lepton-Photon conference last week in Mumbai, India, the excess has become less significant (See the CERN post for a nice summary). But this will remain a hot topic for the next year, since whether the excess stays or goes away, we will have enough data to confirm or exclude the Standard Model Higgs Boson by the end of the year.
But it’s kind of a funny choice of projects for me, because I always swore I would never work on a Higgs search. My reasoning has been that even if a Standard-Model-like Higgs Boson does exist, it can’t possibly be the whole story, and the analysis has already got too many people working on it anyways.
I still firmly believe the former, since there are a number of questions, such as the nature of dark matter and the disparate strengths of the different fundamental forces, which the Higgs boson does nothing to answer. But those questions will likely still be around in a year, and there seems to be a puzzle in the WW dataset right now. We have the opportunity in the coming months to discover an elusive particle, or make a definitive statement about its absence. Also right at this moment, I have the chance to devote almost 100% of my attention to some measurement, uninterrupted, for the next several months. This sort of opportunity is likely to be increasingly rare as my career progresses, and the draw of the WW puzzle is powerful.
As for excuse number two, well, it’s just an excuse. It’s true that there are good people already working on this measurement, but that just means that progress can be fast. Working with lots of good people also means that I should learn a lot, one of my goals for any project. I worry a little about what I’ll be able to contribute, but I’ve worked with similar signatures (top-antitop → WbWb → lνb lνb and plain old W → lν) in the past, so I’m hoping that I can help in spite of being a bit late to the party.
Friday night I head to CERN. Higgs or no Higgs, it ought to be an interesting year.
