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Corrinne Mills | Harvard University | USA

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Quick link: The Fantastic LHC Machine

Monday, October 17th, 2011

On the occasion of the LHC having delivered 5 inverse femtobarns of integrated luminosity, I want to put in a plug for my favorite source of interesting LHC facts:

LHC: the guide by the CERN communications group.

Since it’s from 2009, a few things in it are out of date. Most notably, the total beam energy is quoted as 7 TeV per beam, but we’re currently running at 3.5. On the whole, though, it’s a very nice introduction to the technology of the LHC and the questions we hope to address with the LHC experiments. I’ll readily admit that I’ve pulled material from this brochure to use in my talks! I hope they will consider making an updated version soon.

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Science is Happening!

Monday, September 26th, 2011

I’ve been following the discussion over the CNGS neutrino velocity measurement from the OPERA collaboration with great interest.  A lot of excellent stuff has already been written on this blog on the subject.  A couple of my favorite posts are this early take by Kathy Copic, and Michael Schmitt’s post, which has some interesting links.  I’m not going to even attempt to duplicate their efforts.  Instead, I just want to share a few of my impressions from the last several days.

The very short version is that the OPERA collaboration has measured the travel time (a little under 2.5 milliseconds) of neutrinos from CERN, where they are produced, to their detector in the Gran Sasso laboratory in Italy, with an uncertainty of 5 parts per million. They also measure the distance traveled (730 km) with better than a part per million accuracy. They divide and find a that the neutrinos seem to arrive 60 ns faster than expected, corresponding to a velocity greater than the speed of light by a part in 10,000.

Yeah, exactly.

It’s a startling result. The collaboration was right to submit their work (http://arxiv.org/abs/1109.4897) for public review at this point, I think, and they’ve done a good job of not overstating their claims. There has been a huge response from the particle physics community and the tone is skeptical but collegial. I tried to go to the seminar here at CERN on Friday but it was already standing room only, so I fled back to my office to watch the webcast (see Aidan’s liveblog of the seminar). There was nearly an hour of questions afterwards, many of which were quite good.

The questions at first centered on the distance measurement, which may seem at first blush to be the weak link in the measurement, until you realize that the distance is measured rather more precisely than the time of flight (see the numbers in my first paragraph). The speaker explained that the distance measurement is based on well-established geodesy techniques and confirmed that the precision of the velocity measurement is really determined by the precision of the time-of-flight measurement.

The rest of the questions that I thought were good revolved around one key fact: when OPERA measures the time of flight of the neutrinos, they don’t measure the time of flight of an individual neutrino. Rather, a batch of neutrinos are produced in some short window of time (10.5 μs) by a bunch of protons hitting a target. Then, the time-of-flight measurement is based off of fitting the recorded times of the events with a template based off of the measured time structure of the proton bunch created at CERN.

One question asked at the talk is whether the time structure of the neutrino bunch is somehow modified between departure and arrival, possibly by correlations between the position in time within the bunch and the way that the beam spreads out as it travels to Grand Sasso.  It’s an open question, as far as I can tell, and it strikes me as a potentially important one.

The majority of the community’s attention right now seems to be on the statistical analysis of the data as a possible source of unaccounted-for systematic uncertainty in the determination of the travel time. Because they are measuring the travel time of a bunch of neutrinos, they are essentially measuring the timing of the leading and trailing edge of the bunch at CERN and then again at Gran Sasso. But there’s a lot packed into that “essentially”, which people are now unpacking. Related to the above concern, the seminar speaker said in response to a question that the bunch length — the distance between the leading edge and trailing edge — is fixed in the analysis so that it’s not possible to account for the bunch somehow stretching or shrinking between CERN and Gran Sasso. The speaker at the seminar pointed to the good chisquared (chisquared is a standard measure of goodness of fit) as evidence for the quality of the fit (i.e. how well the model matches the shape of the data). It was pointed out by a questioner that the chisquared is not a useful measure of goodness of fit in this case, since the information it contains is diluted by the good fit to the points in the middle which don’t actually contribute that much information on where the edges are.   My variation on this theme is to wonder if you get a different answer by only fitting the leading half or only fitting the trailing half of the distribution.

It’s going to be interesting to see how this shakes out over time. In the meanwhile, I think that so far it’s a pretty great example of science working the way it’s supposed to.

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In Transit

Sunday, August 28th, 2011

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.

Higgs Branching Ratios 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.

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Hello World

Monday, July 25th, 2011

I confess to dreading the day when they have wireless on airplanes. I’ve spent a lot of time traveling in the last year, and have come to value the time in the air as an opportunity for focused thinking. Taking away email and the perennial distraction of the internet makes it possible to write, to think, to really read papers, to develop coherent presentations. Yes, even in coach class.

So here is my first US LHC blog post, which I am writing this from the plane from Dallas to Boston. I spent 10 days in Madison, WI for the CTEQ summer school and workshop. It was a great experience, and I’ll write more about it in a later post. From Madison I traveled directly to Dallas to meet up with my brother and sister-in-law at my parents’ house. I had a few work obligations to follow through on while I was there, but it was nice to mostly unplug for a while and spend time with family before I shift my home base to Europe for the indefinite future.

At the end of August, after four years of traveling back and forth (by choice), I’ll be moving from Boston to Geneva. I’ll still be employed by Harvard as a postdoc, but for the first time I’ll be spending most of my time at CERN. CERN is in Switzerland, right on the Swiss-French border. Like many CERN visitors, I’ll be living in France, staying at our group’s apartment just a little ways up the hill in Thoiry. When I get back to Boston, I’ll need to immediately start the process of acquiring visas for both countries. Of the logistical details I need to settle, that seems likely to be the most challenging.

It should be a great year to be at CERN. Data are pouring in, and many of the measurements forming the broad LHC physics program have at least been started. If the intriguing results shown EPS turn out to be the first glimpses of new particles, it could be a very exciting year as those signals, and perhaps others, become unambiguous.

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