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An impressive march forwards March 18, 2008

Posted 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.



1. Kea - March 18, 2008

Never be closed?! Cute. These efforts to rule out the fairy field are really remarkable. Congratulations once again to the whole team!

2. dorigo - March 18, 2008

Hello Kea, thank you. Well, never… Let’s say until all protons have decayed, would that be a more realistic wish ?


3. Anonymous - March 18, 2008

Is this Tevatron combined or just CDF? Looking at the plots below, CDF is already 1.5x and 5x above SM for 160 and 115 GeV respectively, so I think it’s just CDF. Tevatron combined should be even better than those plots. 🙂

4. dorigo - March 18, 2008


no, the points are Tevatron combination results. Expected limits: the actual ones are x5.09 at 115 and x1.09 at 160 GeV; the expected ones (which are a more accurate model of the sensitivity of the searches) are instead x3.27 and x1.62, as in the plots.


5. Anonymous - March 18, 2008

Ahh, ok

6. Anonymous - March 18, 2008

Actually — not a big deal but — IMHO the projection should use expected for the future portion of the data, but actual for the data already collected. Note that if there’s a 2 sigma downward fluctuation at 160 GeV in the first 2 fb-1, that’s not going to change.

7. dorigo - March 18, 2008

Interesting comment, but quite debatable point. A downward fluctuation remains if it was statistical, increases its discrepancy if it was systematic, and is reabsorbed with added data if due to a previous mistake in some background evaluation 🙂
So, what to use… I agree. In absence of a notitia criminis, one should assume the fluke was a fluke and take it for what it is: start from the -2sigma and work out expected increase in lumi from there.


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