Of Particular Significance

A Tale of 2.1 Cities

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 03/15/2012

2.1 = 0.1 (Great Barrington) + 1 (Cambridge/Boston) + 1 (Geneva, Switzerland)   The LHC is about to turn on again! news on that below…

Last week I spent a couple of days at my undergraduate college, Simon’s Rock (a very small and little-known school, in the rural town of Great Barrington in Western Massachusetts.) On Thursday I gave a lecture there for a general audience on the Large Hadron Collider [LHC], similar to the one I gave at the Secret Science Club (from which video clips are available here.) Part of what I love about this little school is that classes are small and discussion-oriented.  There are few if any lectures where the professor talks and the students just listen. Also, the students have to write a lot of papers. As a result, they spend a lot of time thinking critically and learn to ask really good questions. I found this to be true not only after my talk but at lunch the following day, when I spent almost three hours in conversation with a good number of them — none of whom are planning to go into particle physics per se, but all of whom had interesting futures to talk about. Another benefit of their small classes and small community is that they’re unafraid of talking to faculty; they understand us for what we are — older students with a love of learning. As far as I am concerned, it’s a terrific educational environment, much better, I’m afraid, than the ones at which I’ve been teaching.  (Oh, and by the way, you can start there after 10th grade; so if you know a kid who hates high school…)

Then I spent the early part of this week visiting Harvard University, in Cambridge, Massachusetts. Even though it is spring break there and a lot of people were away, I found it very stimulating, as always. In addition to it being a great place to think about physics that might lie beyond the Standard Model, there are several experts there on aspects of the Standard Model itself [the equations we use to describe the known particles and forces of nature] , especially the complicated physics of quarks and gluons.

A year of effort at the LHC, as we have learned from the La Thuile and Moriond conferences, has so far turned up nothing obviously unexpected.  The only new things in the data are the surprise from LHCb, which might not be that exciting in the end, and the still-shifting-but-quite-interesting hints of a possible (some would say probable, some would say certain) Higgs particle with a mass of around 125 GeV/c2. (The December hints are described here; the  January CMS advance here; the most recent claims from Tevatron, and the recently reduced excess from ATLAS, are discussed here and here; the new slightly reduced but interestingly shifted excess at CMS is discussed here.)  Obviously a big goal of the coming year will be to firm up those hints into strong evidence, or show them to be ephemeral.

But the other big goal has to be not just to gather more data but to dig deeper into it, using methods that are more subtle, more challenging, more powerful and more precise. To do this requires using more of the tools that particle theorists and experimentalists have at their disposal. That can only be done if experimentalists can distinguish what they observe from what they expected to observe, which they can only do if they know with high precision what they expected to observe, which they can only do if theorists and experimentalists collectively are very skilled at both calculating and measuring. Well, theorists like me can help by improving our calculations and proposing new measurement techniques… and in both of these areas we’re going to need to push the envelope in order to ensure that the maximum amount of information is squeezed from LHC data.

Speaking of LHC data, it’s about to get busy in Geneva again! The LHC is turning on! Beams of protons are circulating in the LHC now, and 2012 collisions and data-taking are starting, apparently, tomorrow. The first item of business is presumably to get the machine running properly at 8 TeV of energy per proton-proton collision (you may recall that last year’s data was taken at 7 TeV; I discussed the increase here.) That may take a little while. As always, the early part of the year will have a relatively low data rate, and the majority of the data will be taken as the year closes. Last year, the first three months of the eight-month “year” produced only about 20% of the full year’s data (here’s a plot from ATLAS showing this). It may not be so skewed in 2012, but we will still see a similar effect. It is hoped that about four times as many collisions will be produced in 2012 as were produced in 2011. Since the energy per collision is also higher, the number of Higgs particles produced will be increased by slightly more — by a bit more than five times.

The one tricky thing is that the conditions under which the measurements are going to be made are going to be more difficult than last year. This has to do with pile-up, and the trigger, which I described and discussed in some detail here. The increase in the total number of proton-proton collisions per second, and the number of simultaneous collisions that occur once every 50 billionths of a second, will force the requirements that determine whether to permanently store the data from a collision (i.e., whether to trigger on it, in LHC-speak) to become increasingly severe.

This means that although the LHC will produce five times as many Higgs particles as in 2011, it is not obvious that the LHC experiments ATLAS and CMS will store five times as many as they did in 2011. It is inevitable (and expected) that they will lose many of them; how a particular Higgs particle decays determines how easy it is for the automated trigger system to recognize its decay debris as possibly interesting and worth storing. The concern is that the losses may be quite a bit worse this year than last. The ATLAS and CMS experimentalists are well aware of this and working very, very hard to keep as many of these Higgs particles as possible… but it is not easy. They have similar challenges for certain other types of physics too. Making the trigger as efficient as possible at keeping collisions that might come from Higgs particles, supersymmetry, quirks, friends of dark matter, or something altogether unexpected is going to be one of the big stories, and big headaches, of 2012.

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6 Responses

  1. I love your blog, but I am deeply annoyed by the black “follow” block that appears to the lower right with no exit icon, making it impossible to read the pages you referenence in the CERN newsletter. I’m already a follower, dammit!

  2. One thing one hears is that something new “must” be seen at the LHC because the SM becomes non-unitary at those energies. I’ve always assumed this referred to a hypothetical Higgs-free SM in which the massive vector fields would cause unitarity problems…does that sound right? Anyway, since almost all the Higgs space is ruled out, and there’s only a tiny hint in the allowed window, should we not be running up on this non-unitarity issue already, if it’s really an issue? I guess another way to ask this question is, how are theorists calculating the cross sections that would be “expected” if there were no Higgs at, say, 125 GeV? Do the calculations then assume a Higgs of some other mass, or are they made within a Higgs-free version of the theory?

    1. Importantly, your statement “since almost all of the Higgs space is ruled out” is not correct.

      The correct statement is “almost all of the Standard Model Higgs space is ruled out.”

      The difference is absolutely crucial. The Standard Model Higgs is the simplest possible Higgs particle. If this is ruled out, the number of possibilities explodes, and it will take us most of a decade to rule out “all of the Higgs space”.

      So NO, we are not even close to running into the non-unitarity issue. It’s many years off.

      You may want to read http://blogs.discovermagazine.com/cosmicvariance/2011/12/06/guest-post-matt-strassler-on-hunting-for-the-higgs/

      where I explain the implications of this essential distinction, and


      which though out of date will give you some idea of the complexity that can arise if/when the Standard Model Higgs is excluded.

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