A Quantum Diaries Survivor

 A comment by some anonymous reader just made me realize I never posted the updated Tevatron combination on Higgs boson cross section limits, recently updated and shown at winter conferences. It is shown below.

As you can see, the observed limit at 160 GeV is at a mere 1.09 times the Standard Model Higgs cross section: the Tevatron got this far with 2.4 inverse femtobarns of data because of a lucky negative fluctuation in the number of observed events with two W boson candidates -which may arise from decays but whose main source is diboson production, . If observed and expected events had been the same, the limit would have been 1.62 times the SM, as in the hatched black line.

One can also observe in the plot that CDF is doing slightly better with its analyses than D0 at low mass, while high-mass searches have the two experiments speak with a single voice. At low mass, CDF appears to have worked a bit harder - I think the difference is due to a larger lepton acceptance in CDF and better dijet mass resolution, since b-tagging sees D0 slightly ahead instead. Both experiments, in any case, see unlucky positive excesses at low mass, so that the observed limit at 115 GeV is 5.09 times the SM cross section, while the expected limit was x3.27. A unlucky excess in CDF and D0 which sits right where LEP II also saw an excess at 1.7 standard deviations. Not yet anything to get excited about - the Higgs contribution in the search datasets at 115 GeV would be still negligible- but maybe still suggestive: after all, a 3-sigma downward fluke of backgrounds would have already allowed the Tevatron to exclude the 115 GeV point!

The talk given by my colleague Tom Junk -a devout Higgs hunter- at last week’s CDF Collaboration Meeting contained an insight into the matter of extracting 95% confidence-level limits on the Higgs boson production cross section at the Tevatron, and two plots that I had not seen before - two plots that made me really smile with satisfaction.

CDF and D0 have really been doing their utmost to squeeze as much information on the existence of the Higgs boson from their data since the onset of Run II, six years ago. We all know that - but seeing it on a graph is different than trusting the surprising progress of the analyses.

Here are two plots that should make everybody who are used to frown at projected sensitivity plots ponder. They show, as a function of integrated luminosity per experiment, the expected limit on Higgs production cross section (in the usual “times-SM” units - where, for a given mass of the Higgs particle, the limit is divided by the SM expectation) obtained by the Tevatron combination in 2005, 2006, 2007, and 2008. This is done for two representative Higgs masses: the “LEP-II-like” 115 GeV (top), and the “WW-resonant” 160 GeV mass (bottom). Check them out below.

Through each expected 95% CL limit point (the colored dots, each sitting at the luminosity value on which it is based) passes a curve which represents the believed scaling of the limit with luminosity - accounting for statistical power given luminosity and known background fractions in the various search channels. Also shown is a yellow band which is considered to be the “optimal” result achievable by the experiments once they will have optimized the optimizable, and added all possible search channels -even those contributing peanuts to the combination.

By looking at the progression of points from 2005 onwards, I get an exhilarating effect. I have always maintained, in this blog and elsewhere, that credit should be given to CDF and D0 for consistently out-doing themselves, through ingenuity and inventiveness. It happened with top quark mass measurements, it happened with W mass measurements, and it is exactly what one sees here: every year, the increase in sensitivity is to be ascribed just as much to the improvements in the analyses as to the increase in luminosity! 

Now think of what you’d get from the plots if you were only shown one of the points, its luminosity scaling curve, and the yellow band lying below: you would be tempted to think that somebody is trying to sell you a unlikely, almost magical, increase in sensitivity. You would conclude that the line through the point is what is going to happen, and the yellow band below it is wishful thinking at best: what the experiments want to sell to their funding agencies. Not so!

By looking at these plots, I have little doubt that, given enough time, the Tevatron experiments would one day find the Higgs boson, if it is there. What a wonderful, success-studded laboratory is Fermilab! I really hope it will never be closed.