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Giorgio’s top mass result now public! October 12, 2006

Posted by dorigo in news, physics, science.
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Giorgio Cortiana, formerly a PhD student working with me, and now a research associate at the University of Padova, blessed a new measurement of the top quark mass just an hour ago, at the CDF top meeting.

The measurement is not very precise, and cannot compete with the best determinations in the lepton plus jets channel. But the data sample it is based upon had never been used before for a top mass measurement. It is 311 pb-1 of data containing as a top-antitop signature just missing transverse energy (signalling the presence of an energetic neutrino from leptonic W decay) plus hadronic jets. That is, the data belongs to the “single lepton” category, where one top decays to hadrons (three jets) and the other to a b-jet, a lepton and a neutrino, but the charged lepton is not identified.

The analysis actually vetoes events with an identified lepton, and is thus orthogonal to all other lepton plus jets datasets. This means Giorgio selects events discarded otherwise… Basically it is one more top-rich sample for free. And in fact, before his cross section measurements (obtained last year, and published in PRL 96, 202002 (2006): http://www-cdf.fnal.gov/physics/preprints/cdf7963_metjets_ttbar_xsec_3.ps ) nobody had thought of collecting top events by just tagging the neutrino and jets. You can read about that analysis in a summary I wrote last year in the Quantum Diaries blog: http://qd.typepad.com/6/2005/07/ok_so_i_promise.html

To fit for a top mass in events where not only the neutrino, but also the charged lepton went undetected, use is made of the Ht – the sum of jet energies plus missing Et in every event. That quantity is correlated with the top mass, and well discriminates the backgrounds from top production. In the plot above, the blue histogram is the top contribution and the red histogram represents the background. The black points are well fit to a top mass template of mass around 170 GeV.

Giorgio’s result is Mt = 172.3 +10.8-9.6 +-10.8 GeV. A large uncertainty, but with five times more data already on tape and some analysis improvements (a better background modeling -for the time being extracted from the data but probably extractable from Monte Carlo with higher accuracy- and maybe a non-dimensional mass estimator) it is possible to reduce it significantly in the future.

This particular analysis will not be determinant in reducing the world average measurement of the top quark mass, but adding this small bit of knowledge to our investigations on the top quark is still valuable science. The events selected by Giorgio may in the future be used for studies of tau decays of the top quark – an elusive signature in other datasets. 

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