Tevatron excludes chunk of Higgs masses! March 13, 2009Posted by dorigo in news, physics, science.
Tags: CDF, DZERO, Higgs boson, Tevatron
This just in – the Fermilab site has the news on the new exclusion in a range of Higgs masses. At 95% C.L., the Higgs boson cannot have a mass in the 160-170 GeV range, as shown in the graph below. The new limit is shown by the orange band.
This is the first real exclusion range on the Higgs boson mass from CDF and DZERO. I will have more to say about this great new result during the weekend.
UPDATE: maybe the most interesting thing is not the limit shown above, but the information contained in the graph shown below. It shows how the combination of CDF and DZERO searches for the Higgs bosons end up agreeing with the background-only hypothesis (black hatched curve) or the background plus signal hypothesis (red curve), as a function of the unknown value of the Higgs boson mass. The full black line seems to favor the signal plus background hypothesis, although only marginally and at just the 1-sigma level, at around 130 GeV of mass:
However, they say that if you like sausages and if you follow laws, you should not ask how these things are made. The same goes with global limits, to some extent. In this case it is not a criticism of the limit by itself, but rather of the interpretation that one might be led to give to it. In fact, the width of the green band should put you en garde against wild speculations: It would be extremely suspicious if the black line did not venture outside of the green band somewhere, even in case the Higgs boson does not exist!
That is because the band shows the expected range of 1-sigma fluctuations -due to statistical effects, and not to systematic ones such as the real presence of a signal!- and since the black curve is extracted from the data by combining many datasets and each individual point of the line (in, say, 5-GeV intervals) has little correlation with the others, it is entirely appropriate for the curve to not be fully contained in the green area! So, the fact that the black curve overlaps with the signal plus background hypothesis at 130 GeV really -really!- means very, very little.
What does mean something is that the hatched black and red curves appear separated by about one-sigma (the width of the green band surrounding the background-only black hatched curve) over a wide range of Higgs masses. This says that the two Tevatron experiments have by now reached a sensitivity of about 1-sigma to the signal with the data they have analyzed so far. Beware: they are already sitting on about twice as much data (most analyses rely on about 2.5/fb of collisions, but the Tevatron has already delivered to the experiments over 5/fb). So they expect new results, significantly improved, by this summer.
It does seem that at last, the game of Higgs hunting is starting to get exciting again, after a hiatus of about 7 years following the tentative signal seen by the LEP II experiments!