An impressive march forwards March 18, 2008Posted by dorigo in news, physics, science.
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.