A long trip February 25, 2007
Posted by dorigo in personal, physics, travel.6 comments
By the time of this foward-timestamped post, I will be on a flight from Madrid to Mexico City, together with Mariarosa, Filippo, and Ilaria. The torture -er, flight- time is about 12 hours! I have never taken so long a flight, and the prospect of spending the time with the kids in a constrained space (and I hear Iberia does not excel in leg room) is a nightmarish one.
I think we will survive… Somehow. I have tried to think in advance at all the nuisances that make the difference between a normal and a good trip, but it is not easy to foresee everything. So, for instance, I do have a laptop with a few DVDs and 12 hours of battery, and we are bringing food for the kids (who usually will repel the airplane meals), but I have been unable to reserve neighboring seats in advance (Iberia has NO call center, damn it!).
I should be able to post erratically next week, since the house we are going to spend our time in has wireless connection. I plan to write about the “consistency” of the Z->bb data with the H->tau tau “signal” found by CDF and the excess of events of a D0 analysis of Z->bb; but I will also try and post some pictures of the places we will visit.
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.
A few pictures of the CMX muon system February 22, 2007
Posted by dorigo in personal, physics, science.2 comments
As more forgotten pictures emerge from my hard drive, I am brought to remember my years as a Harvard post-doc, when I worked at the commissioning of the upgrade of the Central Muon eXtension (CMX) for the CDF II detector, in 1999 and 2000.
The CMX is a system of detectors designed to detect muon tracks as they exit the central detector in the pseudorapidity region [-1.0,-0.6] and [0.6,1.0]. That just means muons with trajectories making an angle with the beam between 40 and 60 degrees, very roughly speaking. They are made of eight layers of single-wire drift chambers measuring 6″x1″ (inches) in section and about two meters in length. Each layer of chambers is staggered with respect to the others, so that all muons hitting the detector are bound to leave a signal in at least four layers despite the uninstrumented regions between the chambers. The 8 layers are sandwiched by scintillator planes, read by phototubes at each end. The system has a nice redundancy, which allows for a high detection efficiency.
All of the 2135 CMX drift tubes existed well before I started working at it: already in Run I CDF had four arches of CMX sections, covering 120 degrees of azimuth each - 240 degrees covering the positive rapidity region [0.6,1.0] and 240 the negative rapidity region [-1.0,-0.6]. My job was to assemble, test, and install the 35% of chambers left behind (on the shelves in the Harvard HUHEPL building), which were necessary to cover one of the two 30 degrees gap at the top of the detector (with a “keystone” module) and two 90 degree gaps at the bottom (with two “miniskirts”).
Below you see the two modules making the “keystone”, assembled and installed on the ceiling of the CDF collision hall. A railing system allows them to be taken apart as they appear here.
Below you see one of the north 120 degree arches, set just a few inches away from its working position. At the top there is the juncture with the keystone. In the background, the yellow iron of the forward toroid, with a double ring of extrusion called “snout” made with 24″ thick steel, to stop stray particles from the beam from hitting the muon chambers.
About the snout, it was designed by none other than Melissa Franklin herself. I think I will take the time to discuss the problem here for a second, since it illustrates very nicely some issues with the design of particle detectors.
Muons are the only particles we can detect which traverse thick slabs of matter undeflected, so we place “muon chambers” outside the massive cores of our particle detectors to measure their ionization and reconstruct their momentum. Very few light hadrons are able to punch through our calorimeters, and their rate is manageable and they constitute a tolerable background to real muon signals. However, when in Run II the position of the forward toroids (of which you see one in the picture above) had to be moved closer to the central detector to satisfy the needs of the improved design, a problem arose: particles exiting the interaction region at small angle could hit the structure, bouncing off towards the muon chambers without traversing enough matter to stop them, and producing a large background rate, as pictured in the following graph:
Bringing the yellow structure closer means that the path of green tracks (background) to the CMX arches is much shorter than in Run I, and thus their arrival time is much more similar to that of real muons which had to cross the central detector. The two scintillators of which the CMX is endowed were able to reject the late signal of backgrounds in Run I by electronic modules which perform the average detection time of the two light pulses: this improves the timing resolution and allows to reject the “green tracks” of the picture above. Below you see a cartoon describing the issue: on the left you see the two scintillating planes (in black), and the two light signals (blue arrows) resulting from the passage of a track (in red). On the right, the arrival time of the signal (in red) and the arrival time of the background (in green) are separated by an electronic “gate”, a requirement that the signal is synchronous with a certain time window.
In Run II, the time difference is insufficient due to the shorter path of background tracks, and thus the need of the two “snouts” shown in black in the first graph above.
Now for the other pictures. The keystone is the first module I worked at, in the winter of 1999. After assembly and installment on the ceiling, it required electrical work. I remember hanging up there, 30 feet above ground on a flimsy cherry-picker… It was fun! below, you can see some detail of the wiring, the preamplifier boards, the gas system tubing (transparent), and the high voltage cables (in red). The thick black cables are those carrying out the amplified signals. Below and above the pack of drift chambers you can also see the scintillators and their curved light guides.
Bear in mind that these detectors had been designed in the early eighties, and produced in the late eighties for CDF Run I…. Nowadays the muon chambers look much fancier than that!
Finally, a picture of myself besides my beloved chambers. On my right, you can again see the steel of the “snout”.
Quarks, BJ, and me February 21, 2007
Posted by dorigo in humor, personal, physics, science, social life, travel.13 comments
The Nobel prize winner James Bjorken is pictured in this rare archive photograph with the famous physicist Tommaso Dorigo, co-discoverer of the heaviest quark:
The date of the picture is unknown, but it looks like BJ is considering Tommaso’s ideas about assigning partons a variable fraction of the proton momentum through a continuous variable x.
Jokes aside, this was taken in 1996 at a SLAC summer school in Stanford, CA. We were really discussing quarks, in truth: the six-jet decay mode of the top pairs recently discovered at the Tevatron - my paper on that signature had not been published yet.
I stumbled in this picture, which I long thought lost, while putting some order in my messy hard drive…
Probing the Hbb vertex at the LHC February 21, 2007
Posted by dorigo in Blogroll, internet, news, physics, politics, science.4 comments
In a comment to a recent post about non-mainstream searches for the Higgs boson at the LHC, Jester asked me with what precision the CMS and ATLAS experiments are expected to measure the Higgs-bb coupling.
That made me think… In fact, the LHC experiments are not putting great trust in the H->bb decay to spot that elusive particle. The reason is that the signature of two b-quark jets is simply not distinctive enough to allow a reasonable signal-to-noise ratio -indeed, the observation of Z to bb decays is already quite tough, and its rate is a thousand times larger than the H->bb process. Therefore, to study the Hbb vertex one has to rely on the production of a Higgs boson together with other bodies, whose characteristics allow a significant background reduction. The candidates are:
1) associated production with a vector boson:
- WH -> l nu b anti-b;
- ZH -> l l b anti-b.
These processes are not so favorable at the LHC as they are at the Tevatron, and indeed backgrounds from QCD production of b-jets associated with the vector boson prevent a significant measurement of the rate of these processes, from which the Hbb vertex coupling could in principle be extracted.
2) associated production with a top-antitop pair: in this case the signature is quite distinctive, but the rate is small enough to make the measurement of the Hbb vertex quite hard. Moreover, what one would be probing in a ttbb final state would be the combination of top and b-quark vertices, and thus the Hbb coupling would require a separate measurement of Htt coupling to be extracted.
A study of the chances of measuring the various couplings of the Higgs boson at the LHC has been done three years ago by M.Duhrssen, S.Heinemeyer, H.Logan, D.Rainwater, G.Weiglein and D.Zeppenfeld. They put together a global likelihood fit that included information from all the observable Higgs decay channels by the CMS and ATLAS experiments, in two scenarios: 30/fb (which the LHC should deliver in the first few years of running), and 300/fb (which represents a much longer time scale).
The fit considers the size of the expected signals obtained by the experiments as a function of a large parameter space of the different couplings, to extract the sensitivity of a global analysis. Some of the results are summarized in the plot below.
One sees that for 30/fb of data (a similar plot is available from page 11 of their preprint, available here ) the Hbb coupling can be constrained to about 40% if the Higgs boson mass is smaller than 130 GeV. For larger values of the mass, the decay cannot be probed meaningfully, due to its rapidly falling probability in favor of decays to vector boson pairs (WW, ZZ).
It is not too interesting to ask what could be done at the Tevatron, if the Higgs were found there. For even in the rosiest of scenarios -say, 8/fb of collected data per experiment, a Higgs mass of 125 GeV, and all the suggested analysis improvements working like a charm- CDF and D0 would be unable to see much more than a faint evidence of only a couple of final states: the associated production of WH and ZH yielding leptons plus a bb pair (at a 3-sigma level, once every subprocess were thrown in the mix), and maybe the direct production of H decays to W boson pairs (at maybe 1.5-sigma of significance). A fit of these two “excesses” could prove far-fetched.
Instead, the question of what could be done at the ILC (if, and that is a big if, it were constructed) might be more interesting to answer. A recent paper addresses that issue: a linear collider producing 500/fb of electron-positron collisions with a reasonably achievable polarization of the beams could study all Higgs boson couplings quite well, and in particular the Hbb vertex could be probed to 1% accuracy by determining the heavy flavor composition of jet pairs produced in association with two leptons from Z decay in the e+e- –> ZH production process. Interestingly, even the Hcc vertex could be probed with that technique, to about 10% accuracy.
It remains to be seen if the ILC will be constructed after all. Recently, the nobel prize winner Burton Richter has joined the group of those of us who gloomily believe the LHC has to find something really new in order to motivate the large investment for the linear collider - and gloomy is the feeling, since chances that no new physics is behind the next door are fat.
One more family pic February 21, 2007
Posted by dorigo in personal.2 comments
In order to be politically correct, after yesterday’s picture of Ilaria I have better post a picture of Filippo too… My excuses to those die-hard physicists reading this blog who are unconcerned with the personal aspects of their colleagues’ life!
Nothing new from the dielectron spectrum February 20, 2007
Posted by dorigo in news, physics, science.7 comments
CDF released a few days ago a new result in the search of high-mass resonances decaying to electron-positron pairs, based on 1.3/fb of Run II data. There are quite a few models of new physics that may yield the searched signature: lots of Z’ models, and Randall-Sundrum gravitons.
The search is quite straightforward from an experimental standpoint, and indeed you can see in the plot below that the mass spectrum is simply understood as the sum of the dominant Drell-Yan production - with the outstanding Z boson at 91 GeV- and fake electron processes, which contaminate the sample at a minimum level.
The agreement of data with Standard Model processes is quite nice, or disturbing, depending on what side you are rooting for. Indeed, no fluctuation in the data appears anywhere in the spectrum, as quantitatively described by the following plot:
In it, you can see that the minimum agreement of the data with expectation occurs at 370 GeV, where the probability of observing as many events as the data is of the order of 1%. Strange ? Absolutely not: quite in line, in fact, with the expected range of minimum observed probability (the region boxed in blue), given that we are checking a wide spectrum.
So from the agreement Z’ models - which allow the computation of an expected cross section for these resonances, as a function of their unknown mass - can be ruled out: you can see it in the following plot.
You can see that the black curve, which is the limit on the cross section of a Z’ decay to a electron-positron pair (the y axis) obtained from the data, is lower than the expected cross section for all the Z’ models up to 730 GeV (for the model which predicts the smallest cross section).
Also interesting is to compare the prediction for a Randall-Sundrum graviton with the cross section limit. It is done in the following plot:
Here, to plot the data several choices were made on the ratio k/Mp, a parameter of the model. K describes the ”scale of warping of the extra dimension”, and Mp is the effective Planck scale. You can see that more warping means a less observable graviton - so we can exclude a RS graviton up to 820 GeV if k/Mp=0.1, but much less so if k is smaller.
All in all, just another nice agreement with SM and Tevatron data. The 1000 dollars I bet on the lack of new Physics at the LHC feel comfortable in my safe.
Ilaria turns four February 20, 2007
Posted by dorigo in games, personal, social life.3 comments
Today Ilaria turns 4. For my experience with kids, four is a turning point - they are no longer babies, but individuals who can surprise you with their independent thought. They become easier to manage, but harder to command.
We had a party on Sunday with a few schoolmates of Ilaria, plus a few of Filippo’s friends. Ten kids all in all, but it was as usual very hard to keep them quiet. But I had prepared for that. I put together a crossword on a big board, and bought prizes. The kids had to guess the words from simple definitions (I had two sets, one for the 4-year-olds, and one for the 8-year-old friends of Filippo). It went surprisingly well, since they participated in the game and were quiet for more than a half hour.
Below Ilaria is dressed up and waiting for her friends next to the party table:
