The Sciences

The LHC, the Tevatron, and the Higgs Boson

Cosmic VarianceBy John ConwayJul 27, 2011 6:37 PM


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A few weeks back I wrote about the remarkable milestones passed by the Tevatron and LHC, and prognosticated that if there was ever a time when new discoveries could come out rapidly, this was it, especially for the LHC experiments analyzing a data sample 30 times larger than the previous one. The result? Nature is being coy - in basically every new particle search for new particles and phenomena conducted by the CMS and ATLAS experiments at the LHC, we see naught but eerie agreement with the predictions for ordinary standard model background. A huge raft of results has been presented at two large international conferences: the annual European Physical Society meeting on high energy physics in Grenoble, France, and the Particles and Nuclei International Conference (PANIC11) at MIT in Cambridge, MA. I presented the CMS results on the searches for the Higgs boson at the latter on Tuesday...more on that below. There is a trove of material available online for these two conferences and at the ATLAS and CMS public physics result web sites. Let’s just look at a couple examples, though. The theme of the PANIC conference was the centennial of Rutherford’s discovery of the nucleus, and what better way to celebrate it than to essentially perform his experiment with a million times more energy, and peer inside quarks to see if...well, to see if there is an inside, to see if they have substructure. Naturally, to do the experiment we smash quarks together and see if we see any hint that there is something smaller inside, which would manifest itself as an excess of particle jets coming out sideways to the beam, much as Rutherford’s students Geiger and Marsden saw alpha particles deflected from their gold foil at angles far too large to explain.

But in the first graph here all we see is a smooth spectrum, agreeing exceedingly well with the predictions, extending out to huge bumps, no excess in the tails, and no excess of jets coming out sideways. In one fell swoop we’ve extended the limit on the size of quarks down by a factor of three or four. As far as we can tell, quarks are pointlike. It’s the subject of the first paper from CMS using 1 fb-1. Another huge effort went into searching for evidence that there may be supersymmetric partners for the known fermions and bosons - a search that has been underway for the past three decades. If supersymmetry is present in nature it would help solve a theoretical riddle as to why the calculated mass of the much-touted Higgs boson can remain stable in the face of enormous quantum mechanical correction factors. Supersymmertry would provide a mechanism to largely cancel these corrections.

Supersymmetry could show up in a variety of ways at the Tevatron and LHC, but with three and a half times more energy than the Tevatron, the LHC has a huge advantage in this search, and already with last year’s sample of data the LHC experiments blew past all of the Tevatron exclusion limits. And now with thirty times more data, in search channel after search channel, the story from CMS and ATLAS is the same: no hint of supersymmetry is evident anywhere. You shouldn‘t take this statement to mean that supersymmetry cannot exist. All we can say at this point is that if it does exist, in a generic, simple version of supersymmetry called mSUGRA, the masses of the partners of the quarks appear to be very heavy, over 1 TeV. The heavier they are, the less effective their power is to cancel the Higgs boson corrections. And theorists are very inventive, and are thinking about supersymmetry models that might not show up so easily in our experiments [see the comment below by Matt Strassler]. And what of the Higgs boson? Here, I must say, the story is becoming very interesting. The LHC experiments have a big advantage over the Tevatron, and the bottom line is that for high mass Higgs bosons, in the mass range above 150 GeV or so, the LHC has totally eclipsed the Tevatron, basically ruling out a Higgs boson with mass anywhere from about 150 GeV to 450 GeV. This is directly a result of the huge increase in the size of the data sample, and combining a half dozen search channels. Both CMS and ATLAS obtain similar results in this mass range, and despite a slight excess around 230 GeV in the ATLAS experiment, I think I can say with confidence that the Higgs boson will not be discovered in that regime. But I personally never thought that this was likely. The sum total of the world’s data on precise measurements of the W and Z boson masses and properties, and the mass of the top quark, when taken together, tend to suggest a very light Higgs boson, much nearer 100 GeV. In fact the best predicted value for the Higgs boson mass is a good deal less than 80 GeV, but the LEP 2 experiments excluded a standard model Higgs boson with mass less than 114.4 GeV. This defines the low end of the present search window, which now extends to 150 GeV or so, and the precision data favor the low end of this range. In a nutshell, what is happening is that the Tevatron experiments are bringing down a curtain on the Higgs boson, but the curtain is lower on the low mass end. The LHC is bringing down the curtain, too, but from the high mass end. So the two machines are in a race to achieve sensitivity to a standard model Higgs boson in the low mass range near 120 GeV. The Tevatron experiments will collect their last data in two months, but the LHC experiments will keep collecting data, probably quadrupling the present sample by the end of the year. So time is a factor here as well, and the Tevatron experiments now truly have one last chance to cover the interesting low mass range. But what to we mean by “cover” the range? If there is simply no Higgs boson to be discovered, then my prediction is that the Tevatron experiments can exclude it with 95% confidence up to a mass of around 120 GeV with the final data sample. If the Higgs boson truly is in that mass range, however, the experiments should not be able to exclude it! The LHC will continue to press on, the experimenters will continue to improve and refine the analyses, and by he end of the year, I predict, if all goes well at the LHC we will either exclude the Higgs’ existence all the way down to the Tevatron limit or begin to see an excess. In fact, the LHC data from both CMS and ATLAS are showing an excess in a broad range at low masses. Now this could be a systematic underestimate of the backgrounds, a statistical fluctuation in the observed spectra, or it might, just might be due to the presence of a low mass Higgs boson. It is not surprisin g that the excess is n a broad mass range, because one of the most sensitive channels, in which the Higgs decays to WW, has little or no mass resolution. This excess is why the press has recently picked up on this excitement - it’s quite in line with what one would expect to see if there is a Higgs boson signal just beginning to show itself. To truly discover the Higgs boson will take a LOT more data, which we will get from the LHC in 2012. Now, we usually reserve the word “discover” for the situation where we have a “5 sigma” excess, by which we mean that there is less than one chance in over 3 million that a statistical fluctuation of the background alone could give us what we observe, or more. This is a stringent criterion, and not at all easy to establish, taking into account all the various experimental uncertainties. If the Higgs boson mass is near 120 GeV, can we get a 5 sigma discovery by the end of 2012? It may take combining the data from the LHC experiments with the data from the Tevatron, a radical concept at present I have to say, but technically possible. So despite the coyness of Mother Nature as to the nature of any new physics beyond the standard model, it’s nevertheless a very exciting time in the field, and who knows: maybe if we hold our mouths just right, cock our heads and squint just so, we might soon see something we hadn’t quite thought of before.

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