115 GeV Higgs: is evidence piling up ? March 29, 2008Posted by dorigo in news, physics, science.
Ever since the Tevatron Run II experiments have started to produce results of the search for a Standard Model Higgs boson, we have had a chance to compare observed and expected limits, and indulge in vacuous but entertaining speculations.
Now, however, the datasets amassed by CDF and D0 start to be large enough that a Higgs signal could -with a good dose of luck- start to cause observable effects. In other words, the game of limit-setting has been the default for seven years, but at some point one has to decide that rather than setting a limit one can start to quote a significance. When a significance is a number significantly smaller than two or three, the exercise is of dubious utility, but still, it gives some indication. If experiments are confident in their evaluation of systematic uncertainties, there really is nothing wrong in estimating the significance of a departure from background of the accumulated data in some Higgs-sensitive final state.
Of course, there is a boundary. Experimentalists in search for a new particle usually have to follow some kind of moral prescription. One usually starts trying to establish evidence for a particle, and if the data do not grant any, then one sets a limit. It is the case, for instance, of the search for a t’ quark which I described a few days ago.
In the case of Higgs searches, however, the fact that the expected signal is very, very small has changed that. Rather than first seeking evidence and then setting limits, experimentalists in CDF and D0 have so far relied on the expected reach -obtained through pseudo-experiments based on the analysis strategy and the dataset size- to decide what direction to take: determining a significance or quoting a limit. And the latter has always been the outcome.
As I mentioned above, despite the insufficient sensitivity, there is nothing wrong with estimating a significance. Even better than a significance, however, is a quantity cooked up by statisticians, which is the ratio between the likelihood that the data is signal+background-like and the likelihood that it is only background-like. Such a quantity has recently been computed by Wade Fisher – one of the experimentalists in D0 in charge of computing combined results of Higgs searches.
Below you can see what Fisher finds, in a very informative plot. The curves you should concentrate your attention on are the red and black hatched lines describing the relative likelihood of signal+background and background alone, plus the black line which represent the value assumed by the data, as a function of the Higgs mass (green and yellow areas represent the and range of variation of the background-alone hypothesis).
Three things to take home from this plot:
the signal+background and background alone curves already are quite separated at 160 GeV: a signal there would start to have an impact in CDF and D0 data – at about 1.5 standard deviations, as you can gather by observing that the red hatched line lies in the middle of the yellow band ( to ). That fact can be deduced also by the limit set by the Tevatron experiments at 95% CL limit on the 160 GeV Higgs cross section (<1.09 times the SM value): in practice, they have already “excluded” a 160 GeV Higgs at 85 to 90% CL (but these are my own guesstimates, so please do not trust them too much: they are not official). It was possible to reach the <1.09 point because of a negative fluctuation of backgrounds, as also shown by the full black line.
At lower masses, the two hatched curves are much closer together: there, the sensitivity is still wanting.
Between 115 and 130 GeV, however, the data is marginally more in line with the hypothesis that both signal and background are contributing. In other words, the 1.7-sigma significance obtained by LEP II experiments for a 115 GeV Higgs boson is starting to receive a small confirmation by the Tevatron data. Quite insufficient to get excited yet, but you know how these things work: discoveries may come in the matter of a night, or take months to consolidate.
It remains to say that despite having half a foot in CDF and a foot and a half in CMS, I am still rooting for CDF to discover the Higgs! As I mentioned in an interview three years ago (see video here), it would be fantastic if we found the Higgs in CDF. Go Tevatron!