April 2012

The LHC just saw its first collisions at 8 TeV, and all seems well. This should be an exciting year for the accelerator, and a film crew is documenting the action as part of a project called CERNPeople. It consists of a YouTube channel and a Google+ page, worth checking out. Throughout the year they’ll be putting up short videos in which they talk to the scientists and technicians about this and that. Here they ask about a perennial topic: competition between the experiments.

Via JenLuc Piquant’s twitter feed, here’s one time I’m not going to stick up for my colleagues in the social sciences: a misguided attempt to cast the search for empirical support as “physics envy.” It’s a New York Times Op-Ed by Kevin Clarke and David Primo, political scientists at the University of Rochester.

There is something rightly labeled “physics envy,” and it is a temptation justly to be resisted: the preference for reducing everything to simple and clean quantitative models whether or not they provide accurate representations of the phenomena under study. The great thing about physics is that we study systems that are so simple that it’s quite useful to invoke highly idealized models, from which fairly accurate quantitative predictions can be extracted. The messy real world of the social sciences doesn’t always give us that luxury. The envy becomes pernicious when we attack a social-science problem by picking a few simple assumptions, and then acting like those assumptions are reality just because the model is so pretty.

However, that’s not what Clarke and Primo are warning against. Their aim is at something altogether different: the idea that theories should be tested empirically! They write,

Many social scientists contend that science has a method, and if you want to be scientific, you should adopt it. The method requires you to devise a theoretical model, deduce a testable hypothesis from the model and then test the hypothesis against the world…

But we believe that this way of thinking is badly mistaken and detrimental to social research. For the sake of everyone who stands to gain from a better knowledge of politics, economics and society, the social sciences need to overcome their inferiority complex, reject hypothetico-deductivism and embrace the fact that they are mature disciplines with no need to emulate other sciences…

Unfortunately, the belief that every theory must have its empirical support (and vice versa) now constrains the kinds of social science projects that are undertaken, alters the trajectory of academic careers and drives graduate training. Rather than attempt to imitate the hard sciences, social scientists would be better off doing what they do best: thinking deeply about what prompts human beings to behave the way they do.

Sorry, but “thinking deeply” doesn’t cut it. People are not especially logical creatures, and we’re just not smart enough to gain true knowledge about the world by the power of reason alone. That’s why empiricism was invented in the first place, and why it’s been so spectacularly successful over the last few centuries.

Clarke and Primo seem to confuse “the need for empirical testing” with “the need for every model proposed to be backed up by data before it gets published.” If they had stuck to rejecting the latter narrow idea, they would have had a decent case. Certainly we physicists don’t require that every model be supported by data before it is published — otherwise my CV (and those of most of my friends) would be a lot shorter! But we all agree that the ultimate test of an idea is a confrontation with data, even if a theory might be too immature for that confrontation to take place just yet.

Zooming around the LHC, colliding at unprecedentedly high energies: 8 trillion electron volts total, in comparison with last year’s 7 TeV. The ultimate goal is to reach an amazing 14 TeV, although that won’t happen soon — the plan is to shut down for quite a while after the end of this year’s run, tighten the gaskets and so forth, and then resume the march to higher and higher energies.

This year’s run is all about luminosity, i.e. getting as many collisions in the can as they can. Last year they reached about 5 inverse femtobarns, while this year they’re shooting for 15 inverse femtobarns. Yes, those are the goofiest units in all of physics. Think of it this way: imagine the protons entering a detector are shooting at a tiny target with some fixed size, measured in units of area. Then we can measure the luminosity by counting the number of protons passing through that area in a fixed moment of time: i.e., the number of protons per square centimeter per second. That’s at any one moment; if we integrate up over the course of a year, the “per second” disappears and leaves us with the total number of protons that have passed through the target area, i.e. a certain number of protons per square centimeter. But that number would be enormously huge, so rather than using square centimeters, particle physicists like to use “barns,” defined as 10^-24 cm². (Broad side of a barn, get it?) But even measuring the luminosity in inverse barns would be really big, so they go for inverse femtobarns (1 fb = 10^-39 cm²). Long story short: 10 inverse femtobarns is equivalent to 10⁴⁰ protons passing through a 1 cm target area. (That’s much larger than the number of collisions — to get the number of collisions for any particular process, you need to multiply by the cross-section for that process, which is often quite tiny. That’s why particle physics is hard! Still, there will be a buttload of collisions.)

Anyway, I’m pretty sure the LHC is back to colliding protons after this year’s winter shutdown, and they’re smashing together at 8 TeV. But to be honest my only hard evidence is from Twitter, where the ATLAS collaboration has tweeted this image.

Meanwhile, results are still coming out from last year’s run. Sadly, they’re doing a great job at constraining possible new physics, but no convincing discoveries as yet. Here’s a recent result from LHCb, the experiment that looks at decays of mesons containing b quarks. This plot is from David Straub, from a talk at Moriond, based on this paper.

Horizontal axis is the fraction of time (the branching ratio) bottom/strange mesons decay into two muons, while the vertical axis is the fraction of time bottom/down mesons do the same thing. These numbers have specific predictions within the good old Standard Model, but it’s very easy for new physics such as supersymmetry to enhance the numbers quite a bit. LHCb has put an upper limit on both quantities, which rules out all the gray area of the plot, leaving only the colorful part at the bottom left. The colors correspond to possible predictions in different versions of supersymmetry. As you see, it would have been very easy to have detected a substantial deviation from the Standard Model by now, but no such luck. This doesn’t mean some other version of supersymmetry isn’t right, just that we’ll have to try harder. No question that a proper update of our likelihood functions will have to decrease the chance that we expect fo find SUSY at the LHC compared to what we would have thought a few years ago, however. This is why the march to higher energies will be so important.

If you want to ask some detailed questions about the accelerator and the experiment, the CMS and ATLAS collaborations are having a Google+ hangout this Wednesday that you are welcome to join. It starts at 7 am Los Angeles time, so I’m unlikely to make it, but let us know if anyone here participates.