The Sciences

Making the (Higgs) Sausage

Cosmic VarianceBy John ConwayDec 8, 2011 10:44 PM


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For the past year, physicists at the LHC experiments CMS and ATLAS have been analyzing ever–increasing data samples from the huge machine. Rumors are now circulating about what the experiments might announce at next week's presentations at CERN regarding the search for the Higgs boson. Next Tuesday there will be a joint seminar from the two experiments at CERN in which the latest results are shown. And though I cannot tell you everything that we will say next week (and nothing about the ATLAS results, which I have not seen), from the public statements made by the CERN Director General you already know that an unambiguous discovery is not yet in the offing. But, following on Matt Strassler's excellent post about the physics, I thought it might be interesting to tell you what it's been like this past year getting to this stage in this search. As you probably know, each of the two big experiments has over 3000 physicists participating, from all over the world. Many, but by no means the majority, are resident at CERN; most are at their home institutions in Europe, North America, and Asia and elsewhere. The main thing that allows us to collaborate on a global scale like this is video conferencing. We used a system called EVO, developed at Caltech, which allows us to schedule meetings and connect to them from a laptop or desktop computer, or even dial in by phone. Sometimes it's clear that people are connected by phone from the oddest places: once I heard the clear sounds of someone participating in the meeting from a train ride in Italy, oftentimes you hear people speak while they are driving the (hopefully with a hands-free device), and often one hears the sounds of children in the background (including my own). The issue is that meetings can be at any time of day for different people in different continents. Fortunately the experiments have gravitated toward having meetings in the late afternoon, Europe time, which makes it early morning for people like me in California. A good thing about our videoconferencing system is that you actually have a choice whether or not to transmit or receive the video part of the meeting, which tends to be less useful than looking at material under discussion, which is usually in the form of PowerPoint slides. That makes it even easier to participate early in the morning! No one has to see you in your pajamas, and they probably don't want to. I did it this morning, in fact, at 6 am. So what are all these meetings? In CMS, our whole system of producing physics results has a sort of pyramidal structure. Each experiment has a number of physics analysis groups which meet a weekly or biweekly, typically, and have two “conveners" who set the agenda and run the meetings. These convener positions are typically held by senior people in the collaboration such as professors or senior lab scientists, for two years at a stretch, one convener changing out each year. They report to an overall physics coordinator and his or her deputies. Within the physics analysis groups are subgroups devoted to sets of analyses which share common themes, common tools, or similar approaches. Each of these subgroups in turn is led by a pair of conveners who establish the ongoing analyses and guide them to eventual approval within physics analysis group. We have what I think is a pretty impressive internal website devoted to tracking the progress of each physics analysis. From a single website you can drill down into a particular physics group find the analysis you want get links to all the documentation, and follow what's happening. In parallel, there is a web system for recording the material presented at every meeting. The goal of every analysis is to be approved by its physics group, so it can be shown in public at conferences and seminars. This requires having complete documentation including internal notes with full details of the analysis, and a “public analysis summary” which is available to the public, and which often serves as the basis for a peer–reviewed paper which soon follows. Every analysis is assigned an analysis review committee of three to five people with experience in the topic, who act as a sort of hit squad, keeping the analyzers on their toes with questions and comments at every stage of the analysis, both on the actual analysis details and on the documentation. After all, if we are not our own worst critics, someone else will gladly fill the role! The process from initially recording data from proton–proton collisions to ultimate physics results can take months. By now the basic algorithms which are run on every collision in order to reconstruct what happened are well-established. But during the year the running conditions of the accelerator changed with ever–increasing rates of proton–proton collisions happening. In every 25 nanosecond “bunch crossing”, by the end of running this year we were recording an average of up to 10 proton–proton collisions. Typically only one of these is of interest and the rest are “minimum bias” events in which the protons strike glancing blows. Nevertheless, these additional interactions caused us a lot of trouble this year because they result in additional energy recorded by the detectors, additional charged tracks, and skew various quantities which we are trying to measure in each collision. This was one of the major challenges of 2011. In parallel with processing the data that we record, we run full simulations of well–known standard model collision processes which represent our background when we are doing searches for new particles. There is a big organizational challenge in doing these simulations, which run on a worldwide grid of computers devoted to CMS data analysis. We make use of the Open Science Grid for this in the US, the EuroGrid in Europe, and other clusters scattered all around the world, comprising tens of thousands of computing nodes. The basic idea of any new particle search is simple: you make a selection which retains as many collision events potentially coming from the new particle, while retaining as few background events as possible. Then you predict the number of background events from well-known processes, and see if any excess remains. Almost all analyses these days use the distribution of some final quantity (such as the estimated mass of the new particle) to look for these excesses. At this point one can then use statistical techniques to estimate the largest contribution that could be possible given the observed spectrum, or if there is an access, calculate the probability that the background alone could give rise to such an excess. This is how we quote the statistical significance of the results. The details, though, boggle the mind. The graphic here shows the complexity of the statistical procedure for correctly keeping track of all the little correlations that can occur among and between the search channels. (This graphic is courtesy Kyle Cranmer of NYU, one of the main Higgs combiners in the ATLAS experiment.)

A great deal of our meetings is devoted to studying the level of agreement between our observed spectra and predicted spectra based on full simulation or clever techniques using the actual observed data to predict the background. In the case of the search for the Higgs boson, there are a couple dozen “channels” in which to search, which reflect how the Higgs is produced and how it decays. The results from these individual channels are then combined into one final statistical analysis which essentially answers the question: is there evidence of Higgs boson production if the Higgs masses thus-and-such a value? What will be presented next week at CERN is in fact the result of that analysis and as much detail as possible about the results feeding into the answer. This year has seen a dramatic leap in our knowledge about where the Higgs boson isn't, and as of a few weeks ago the combined results from CMS And ATLAS left open only a small mass window from 115–140 GeV where it could exist. As luck would have it, the remaining mass region is the most difficult to explore with the LHC, but it has been clear for some time that with the data set anticipated this year, and now recorded and analyzed, the LHC could close the window even further, and perhaps all the way after combining the results from both experiments and with the data from the Tevatron. But the window will only close completely if the Higgs is not there. For a while, earlier this fall, there was rampant speculation in the science media about the possibility that “there is no Higgs boson” but as the allowed mass window has shrunk, it's shrunk down right down to the region where we would expect the Higgs boson to exist, if it does. So we shouldn't give up yet! We've known it will take a lot more data to establish the existence of the Higgs boson at the golden five sigma level and begin to measure its mass, etc., but by next summer I think the it should be clear one way or the other. My own role in this whole process started years ago when I worked with my students and postdoc to create a new algorithms for identifying tau lepton decays (the tau is the heaviest partner of the electron), and helped develop new methods for calculating the Higgs boson mass from its decays to pairs of taus. By last year, before we had an appreciably large sample of physics data, we had established within the physics analysis groups the methods we wanted to deploy in this search. We teamed up with groups from other institutions and, a year ago, another professor (Sridhara Dasu from University of Wisconsin-Madison) and I led a team of about 10 students and postdocs in getting the first version of this analysis through the full process. It took months, but we eventually published the results in is a Physical Review Letter in the spring, as the LHC started to deliver much higher luminosity. With new data in hand, we “turned the crank” on the same analysis, more or less, for the summer conferences adding a few embellishments, and then improved it again this fall. This pattern was repeated in parallel by a dozen other teams in the Higgs search. I would reckon there are at least 200 people involved in the search in a serious way in CMS. It's been more of a marathon than a sprint for all concerned, and now my former student is now a postdoc at Wisconsin and my former postdoc is now a scientist at a Ecole Polytechnique in France. Our analysis group, I can tell you, has some of the most talented physicists with whom I've ever had the privilege to work. For me, that's one of the great joys of being in this field: you are surrounded by really smart people. It will be interesting to see how the media spin the results that emerge next week. Physicists still smart from the sting of an article in the New York Times back in 1992 with the title "300 Physicists Fail to Find Supersymmetry" and have become much more media-savvy. If you believe the rumors, then perhaps a more apt metaphor is that of a tiny, growing new plant, two leaves reaching above the soil. With more water and light, it will grow. And grow. And grow.

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