Giorgio’s top mass result now public! October 12, 2006
Posted by dorigo in news, physics, science.add a comment
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.
Match drawn after regular games October 12, 2006
Posted by dorigo in chess, games, news.add a comment
The World Chess Championship match between Kramnik and Topalov will be decided by four active-chess games, where each player has 25 minutes plus a small move-by-move increment to complete the game.
Today the 12th game was played. It was another tense struggle, but the balance, if a dynamical one, was never upset. It was also a quite instructive game, of a classical nature: a Slav defence where after a few minor piece exchanges white was left with a fianchettoed light-squared bishop against a well-centralized knight, and where white conducted a minority attack on the queenside – which as per any elementary book was well countered with a kingside king hunt. A perpetual check ended properly in a position where any other decision would have caused great risks for both players.
Tomorrow will be the final day of the match. If the score is still even after the four rapid games, there will be four blitz games, and if the score is still tied after those, a dreaded Armageddon game.
The Armageddon game is one where white is given 5 minutes against black’s 4, and black in exchange gets draw odds: if white does not manage to win despite the move and time advantage, he loses the match. A bloodthirsty situation!
Tomorrow I will get some popcorn, this might get really ugly!
655 thousand missing in Iraq October 12, 2006
Posted by dorigo in news, politics, science.2 comments
A study by a group of researchers of the John Hopkins University in Baltimore, which will be published by the british medical magazine “The Lancet” tomorrow, has produced a disturbing evidence for the aftermath (unfortunately a current-math) of the war in Iraq.
According to the study, which was based on sampling the mortality rate in 1849 Iraqi households before and after March 2003, the number of deaths attributable to causes not present before the US bombing and invasion of the country is a staggering 655,000.
The figure confirms -better say worsens- a former result of the same team, who had evaluated at 112 thousand the casualties in October 2004.
George Bush – who had estimated at 30 thousand the casualties in Iraq in the past- was quick to label the result as “not credible”, and the same did the Iraqi government, through Ali Debbagh, who claimed the results “do not reflect reality” and asked researchers to adopt more precise and clear methods to estimate these sensitive figures.
I must say I have my doubts that the research was conducted in a statistically meaningful and unbiased way – and especially the method used to collect the data appears questionable: if I understood correctly, the mortality rate is computed by polling households for the number of deaths in two periods, pre- and post- March 2003.
However, I will abstain from making further comments on the credibility of the study: I want to read the article first. Besides, if the figures quoted above are correct (or even overestimated by a factor of two or three) they confirm what I have already known since before 2003: that the US action in Iraq, besides being motivated by private interests of oil companies and not by the alleged presence of WMD or ties with Al-Qaeda, was going to be a humanitarian tragedy of huge scale. One that the US and its allies (alas, Italy included) will dearly pay for in the future, in terms of chronicization of the new cold war of the third millennium: that opposing Islamic republics to the West.
