A low-mass top in single top events ? November 20, 2006Posted by dorigo in news, physics, science.
I am by nature a very gullible person in my private life, but when it comes to physics I am utterly skeptical. It helps in my job… So when Tony Smith (http://valdostamuseum.org/hamsmith/) contacted me to discuss the possibility of low-mass top quark events in the Tevatron data, and his hypothesis of several states, one lighter and one heavier than the regular one found at 172 GeV, I automatically frowned.
However hard to believe, the idea has some interesting aspects. It entails believing that the 172 GeV top is related to the Higgs as a ttbar condensate, and the 140 GeV top is related to other quarks and therefore to single top things such as t-bbar. If that is the case, the single top should show up at the Tevatron at 140 GeV rather than at 172.
The idea is not new… And it has some basis in an old diatriba that went on in the early days before top discovery, in Run I at CDF. I do not remember the details correctly any more, but I seem to recall that a CDF collaborator had analyzed together with two outsiders (non-CDF particle physicists) the early data and claimed not authorized that the top quark could be there, at a mass of 140 GeV or so.
Later, there were some speculations about the dileptonic top candidates used to show a lower top mass than the other events. But frankly, in Run II the data is such in beautiful agreement to Monte Carlo predictions, that I would be tempted to shrug my shoulders…
However, CDF has now two public analyses of single top searches out, and while one sees no signal after constructing a global likelihood, the other does measure a meaningful cross section – but a part of the excess of signal events clusters at low mass, indeed.
With a single top event, one cannot really run a kinematical fitter very effectively to extract the most probable top quark mass. So one usually looks at things such as the mass of lepton,neutrino, and b-tagged jet, which has a sizable probability of being the triplet of objects emitted in top decay. If one looks at the distribution shown below, one does observe an excess of candidates with respect to non-single-top processes, but indeed part of the excess clusters at low mass (if the legend does not read well, orange and red are the contributions from t-channel and s-channel single top production in the selected sample).
To be fair, this is only one of many distributions that show beautiful agreement with Standard Model expectations. See, for instance, the signed product of lepton charge by rapidity of the b-jet, a variable that shows asymmetry in the s-channel production process:
As always, time will tell… For now, the result of this analysis are not unequivocal but beautifully in line with the SM, with a single top cross section of 2.72+1.46-1.30 pb.
And by the way, in June I was asked by Michael Schmitt to make a prediction on what exciting new result CDF might produce before the end of 2006. I said single top observation, and the above plots show I was right… Almost. No observation yet…. But more to the point, CDF has discovered several other exciting things in the last few months! Exclusive diphoton production (see a post I wrote here a couple days ago), new Sigma_b baryons… Our experiment is not a boring place!