A new precise top mass measurement with jets January 7, 2007Posted by dorigo in news, personal, physics, science.
CDF has blessed last Thursday a new precise measurement of the top quark mass, obtained from the decay of top quark pairs to six hadronic jets. In this post I will provide some background information, while in the next one I will give the measurement and discuss more technical details and additional information about it.
The “6-jet”, or “all-hadronic” decay mode of top quark pairs arises when both W bosons emitted in the disintegration of top and antitop produce a pair of quarks (see diagram on the left). Each of the six quarks (the two additional ones are b-quarks from the top decays) fragments in a stream of hadrons, which we see in our detector as a localized energy deposit, a cluster.
The six clusters are not easy to measure well, and a lot of instrumental and physical effects make a precise reconstruction of the decay kinematics quite hard to achieve from that information alone.
Moreover, this decay mode of the top pair is made tough to observe by the huge rate of QCD processes that can mimic the same final state, but which did not involve the decay of top quarks. To give you an idea, a sample of ten thousand events with six jets is expected to contain just two or three top pair decays before a careful selection is operated.
It is because of these problems that when the top quark was first sought in CDF and D0 during Run I, people focused on the golden signatures involving one or two charged leptons: the W boson decay can indeed produce a lepton-neutrino pair – 11% of the times for each lepton species. So people was then looking for events with a high energy electron or muon, a few additional hadronic jets (from the other W and the two b quarks) and missing energy from the neutrino (the single-lepton signature) or events with two charged leptons, missing energy, and just two jets.
Indeed, the top quark was discovered in those golden signatures in 1995, but two years afterwards a paper appeared in PRL: “First observation of the all-hadronic decay of top quark pairs” (a copy is available at http://www-cdf.fnal.gov/physics/preprints/cdf4025_top_allhad_prl.ps.gz), which was in part the product of my undergraduate studies – my laurea thesis. I am proud to say that most of the technology still used today to extract top quarks from the all-hadronic mode was invented by my group in the early nineties, when people frowned at us because they thought ours was just a lost cause. But we did find a signal and measure a cross section and mass!
In Run II people have realized that with the improved b-tagging capabilities of CDF and D0, and refined selection criteria based on neural networks (but still founded on our old kinematical analysis) the all-hadronic final state can indeed contribute sizably to the overall knowledge of the top quark.
This week’s result by CDF, mainly the effort of Jim Lungu and his colleagues of the University of Florida, is only the last of a string of measurements based on that decay topology. I contributed to the first few Run II results but then left that line of research, concentrating on other issues, but I still salute with pride a new precise measurement of the top quark mass, which will contribute to the world average when added to the other determinations.