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Ken Bloom | USLHC | USA

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Don’t let the black dots fool you….

(With apologies to Michael Turner….)

Here, some rapid post-game analysis for all of you. First, a big thanks to fellow US LHC bloggers Aidan Randle-Conde and Seth Zenz for their tweeting during today’s seminar, which made it easier for all of us in Nebraska to follow what was going on. Based on the social media, press releases and so forth that I’m following, this is a very big day for the field of particle physics, and it’s fun to be a part of it.

Let’s remember that the LHC only ended the 2011 proton run about six weeks ago, and in that short time since CMS and ATLAS have analyzed all the data recorded. You almost never see a turnaround as fast as that, given the data processing required and the careful validation that needs to be done of the data and then the analyses themselves. Congratulations are very much in order to the computing teams for the experiments, and all the people who are checking data quality, and all the people who stayed up late making the plots that were shown in today’s presentations.

The boilerplate summary has already been said, but I haven’t had the opportunity to say it yet: today’s results are certainly tantalizing, but it’s impossible to know what they will amount to in the long term. We’ve seen signals of this significance disappear before. Perhaps the more solid thing to talk about is the fact that the window of possibility for the standard-model Higgs is slowly but surely closing, as both experiments have now excluded a wide range of the possible Higgs masses. (The caveat here is the phrase “standard-model Higgs”; I noticed the other night that two teams of theorists — on the same day — posted articles saying that the addition of just one more particle to our theories could change all of these conclusions.) I’m not sure that when the LHC started up two years ago we would have imagined that we’d be able to make such strides so quickly.

In short, there is reason to be excited — but we don’t know what the reason is yet! We might be close to discovering a Higgs boson, or we might be close to excluding it. In either case, 2012 will be a decisive year for particle physics as we have understood it for the past thirty or forty years.

Now, on to the perhaps controversial part of the post. As I was trying to follow the talks today, I started to wonder — this experiment sees a peak here, this one sees one there. Is anyone being lucky? Face it, CMS and ATLAS got to record one set of data. If we were to record the same amount of integrated luminosity once more, we’d have a different set of events, and maybe we’d get some interesting events again, or maybe not. You can’t know. However, if we were to do the experiment again, we’d have the same detectors, and the same analysis techniques. The data are just some form of luck, one roll of the dice. The real figure of merit for how well the experiments are doing in the Higgs search is not the result you get from this one dataset, but how well you would expect to do for any given dataset of this size.

Fortunately the experiments tell you how well they expect to do — it’s encoded in the “expected limit” lines on the result plots. Here are those plots for the low-mass Higgs region of the search for CMS and ATLAS. Now, try not to look too hard at the black dots:

Low-mass Higgs search limit plot from CMS
Low-mass Higgs search limit plot from ATLAS

Just eyeballing things, I’d say that CMS expected an exclusion limit (in the absence of a Higgs) of 117 GeV, and ATLAS about 124 GeV. Obviously there are uncertainty bands on this…if we take the two standard deviation line from the expected limit and call that the worst-case scenario (or at least a worse-case scenario) then CMS would expect to exclude to about 133 GeV and ATLAS about 137 GeV. In the spirit of comity, I’ll declare this a tie. As an experimentalist, I would claim that the burning question for 2012 is not what the mass of the Higgs boson is, or whether it exists at all. Instead, we should be asking how quickly the experiments can push that “expected” line down such that there is the potential of excluding the Higgs with the data in hand. It will be done with some combination of more data and more cleverness.

Maybe today we’ve been lucky enough to see the first hint of a Higgs boson. Or maybe not! My experience in life is that you do have to be a bit lucky to get ahead…but before you can be lucky, you have to be good. What we have seen today is that both experiments are downright excellent! Next year we’ll find out if we’re lucky, too.