Just Blessed!!!!!!! February 23, 2007
Posted by dorigo in news, personal, physics, science, travel.11 comments
Julien blessed our new result of a Z->bb signal from Run II CDF data.
We measure a signal of 5674+1540-725 events, and a b-Jet energy scale factor of 0.974+0.020-0.018. The latter is the most precise and only determination of this quantity so far. It basically tells us the amount of miscalibration of our jet energy corrections for b-jets.
Using the b-JES for the reduction of the top quark mass systematic uncertainty will require more work. But today’s step is a long and important one. We expect that most of CDF top mass determinations will benefit from it.
In the plot below, hot from the press, you see as blue points with tiny error bars the dijet mass distribution of experimental data after a tight selection aimed at increasing as much as possible the elusive Z signal. The black curve is the fit, the fitted Z signal is in red, and the QCD background is in green. In the inset you see the excess of data over the background alone, compared to the fitted Z signal.
Tomorrow I am on a flight to Madrid, Mexico City, and Cancun. I will arrive in Cancun on Sunday at 11AM, and from then, I will relax for 10 days on a nice villa in the mayan riviera. Having the blessing of the Z signal behind my shouders will make it an even sweeter vacation!
Mercy to the rapist soldiers February 23, 2007
Posted by dorigo in news, politics.11 comments
I read in the news today that after James Barker, recently sentenced with 90 years of imprisonment, now a second soldier, sergeant Paul Cortez, will do 100 years of time for the horrendous crime they committed in Iraq, where they raped and killed a 14 years old girl, after killing the three members of her family.
Three other soldiers, Steven Green, Jesse Spielman and Bryan Howard, are still on trial. The first, being no longer in the military, will appear in front of a federal court, where he will face charges punishable with the death penalty.
These five men committed a horrible act. I am against the death penalty, but I have no objection to a long time sentence in cases such as this one. However, by reading the story, I cannot help feeling that the soldiers themselves are, in some way, victims. Sacrificial ones, to be clear.
The US waged war to Iraq motivating it with the false claim that the Iraqi regime possessed weapons of mass destruction - a fact that, even if proven true by itself would have meant nothing, given that no less than nine countries own nuclear weapons, US included. The United States are responsible for a horrible civil war (yes, we can say civil war now: many thanks to the Bush administration for allowing us to call it for what it is) which is killing tens of thousands of innocent civilians.
In the face of the astonishing level of death, destruction, and grief that the Bush administration has caused, the horrendous act of the five american soliders may well be put in perspective. Of course, it is tough to justify with the harshness of their job conditions the amount of violence they unleashed. But today, with a cold mind, one can say that if the five soldiers face 100-year charges, then those who sent those soldiers and tens of thousand more to bomb, destroy, kill, occupy and rule Iraq, should be charged with much more than that.
My job today is to think positively - the hard punishment of these soldiers is an act of democracy and justice. In the back of my mind, though, I can’t repel the thought that these sentences stink of a clumsy way to attempt to pacify our souls by showing us that the United States are a civil, democratic country, and that their army soldiers are indeed punished if they commit crimes abroad because they deserve it, and not for propaganda.
It still aches to remember the Cermis tragedy…
Blessing the Z to bb signal today! February 23, 2007
Posted by dorigo in news, personal, physics, science.add a comment
Today, if all goes as expected, the exactly 11 years of work I devoted to the search for Z->bb decays will finally produce their first fruit.
I discussed earlier the difficulty to extract the decay signal of a Z boson in two b-quarks from hadronic collisions. So, if the four LEP experiments detected millions of these decays, why bother extracting only a few thousands of them at the Tevatron ?
Indeed, the Z boson is one of the best known animals in the particle zoo. In fact, just because it is so well known, it makes a perfect calibration line. Any experiment measuring the Z mass with whatever tool they have, have a chance to calibrate their tool to a part in hundred thousand - that is the precision with which we know the Z mass.
That is what I have worked for so long to demonstrate was doable: calibrate the jet energy response of our detector using the Z. Of course, I was not alone in this endeavour: but I was the first to work at it, and I alone saw a signal of Z->bb decays in Run I with CDF, which motivated others to study the feasibility of jet calibration in Run II.
Now, five years into Run II, the Z->bb working group includes nine physicists: two from the University of Chicago (Mel Shochet, Shawn Kwang), three from the University of Padova (Julien Donini, TD, Mia Tosi - not shown today!), two from Rockefeller University (Ken-ichi Hatakeyama, Tomonobu Tomura), and two from the University of Pennsylvania (Christopher Neu, Daniel Whiteson). The pictures of our american colleagues are shown here in the order I cited them.
The measurement of the Z peak allows to determine the b-jet energy scale with a 2% accuracy. This translates in a 2 GeV systematic uncertainties in top quark measurements based on the very clean dilepton final state - which only includes b-jets in the signature. Compare with the best measurements so far obtained by CDF in that final state:
the current matrix-element measurement of top mass from dilepton events with 955/pb of data (incidentally, almost twice as much data as that on which we base our Z-driven 2% scale measurement) obtains Mtop=167.3+-4.6(stat)+-3.8(syst) GeV, where 3.3 GeV is the jet energy scale systematics alone. Including our result this should decrease roughly by a factor of two.
a different measurement, using a kinematic method, finds Mtop=168.1+-5.6(stat)+-4.0(syst) GeV, again 3.3 GeV being the systematics due to the b-jet energy scale alone.
All other top mass measurements in CDF will also benefit greatly from the new scale measurement. However, more work will be required to understand the uncertainties due to extrapolating a result obtained with b-tagged jets reconstructed with a cone of 0.7 radians to generic jets reconstructed with a narrower 0.4 radians cone. Also, the energy spectrum of jets from top quark decay is not coincident (although pretty close) to that of jets from Z decay, and that will also be the subject of investigation.
Of course, whenever one reaches a goal, many other goals appear on the horizon - the most obvious of which is publishing the result. Nonetheless, now it is the time to cheer at the result.
It is therefore with a lot of pride that I will sit in our conference room in my University tonight, connected by video with the CDF Theatre, hearing Julien’s blessing talk.
