A Quantum Diaries Survivor

This morning I gave my seminar at PPC08, and I was able to record it on my camera. So, rather than giving a transcript, I could in principle get away easily by pasting here a simple link to my presentation in .mpg format. However, I will be able to do that only as I go back home next week, since transferring 500mbytes via wireless is not something I want to entertain myself with. I am thus going to put here a few of the slides, commenting them as I did during my talk, and I will update the post next week to include the link to the video file. For now, you can get a pdf file with all the slides here.

Note: This post is dedicated to Louise Riofrio, who kindly mentions my talk in her wonderful blog today…

I started with the usual mention of the experimental apparata: the first is the Tevatron collider (see slide below), which has delivered 4 inverse femtobarns of 2-TeV proton-antiproton interactions to the CDF and D0 detectors so far. The inverse femtobarn is a unit of measure of integrated luminosity , which tells you how many events are produced for a process with a given cross section : since the total proton-antiproton cross section is about 60 millibarns, 4 inverse femtobarns correspond to , or a total of 240 trillion proton-antiproton collisions. I mentioned that the Tevatron expects to double the delivered luminosity if it will be allowed to run through year 2010, and I promised to show what that means for the precision of some critical measurements and searches.

Next I discussed the CDF detector (see slide below). I pointed out that its original design dates back to the year 1980, and that it was constructed to discover the top quark - something it achieved in 1995, but it has produced an enormous amount of excellent measurements in addition. CDF is a magnetic spectrometer where a inner tracking system made of 7 layers of silicon microstrip sensors is embedded in a large drift chamber, and the two are contained within a 1.4 Tesla solenoid. Outside the magnet are electromagnetic and hadronic calorimeters, surrounded by muon chambers.

I discussed only a few results on top physics. The top quark is a remarkable particle: the heaviest of all known elementary bodies, it decays before having time to hadronize since its natural width is an order of magnitude larger than the scale of quantum chromodynamical interactions. So we can study its properties free from the hassle of non-calculable soft QCD effects. The large mass of the top begs the question: why is it so large ? Why is its yukawa coupling very close to unity ? Is some form of new physics hiding in the phenomenology of top production and decay ?

Production of top quark pairs at the Tevatron occurs mainly by quark-antiquark annihilation, the diagram shown on the left in the slide below. Since each top almost always decays to a W boson and a b-quark, one can classify the final states according to the decay products of the two W bosons. In the upper right graph one can see how the possible final states break down in terms of relative rates: the most probable -and most background-ridden- is the all-hadronic final state, where both W bosons decay to quark pairs, and you get six hadronic jets from the top pairs. I also discussed single top production mechanisms, shown in the diagrams at the bottom: these have a comparable production rate, but they are much harder to extract from the data due to larger backgrounds.

I then showed a summary of top pair cross section measurements, mentioning that there are by now dozens of different determinations. The average has a uncertainty of 12%, and its agreement with NNLO predictions shows that the technology of perturbative QCD calculations is in good shape.

After discussing one measurement of top cross section in detail, I went on with mass determinations. The technology of these measurements has improved greatly in the past few years, and by now the top mass is measured by CDF with a 1.1% uncertainty. The average with D0 has been carried out on results obtained with 2/fb of data, and it produces , which is a 0.8% uncertainty. The top quark mass is important for the building of models of new physics beyond the standard model, and of course it provides a stringent constraint to electroweak fits of standard model observables. It is foreseen that the full Run II dataset in 2010 will allow the Tevatron to reach a 1 GeV precision on the top mass, or even slightly better than that.

I next showed the combined measurement of single top production cross section. A very complicated and advanced technique using evolved neural networks and genetic algorithms allows to optimize the measurement, and a 3.7-sigma evidence for the signal is obtained by CDF. This is a unlucky chance, since the expected sensitivity exceeded 5-sigma. But we have already collected enough data to grant the canonical “observation-level” significance in the near future, as the data is analyzed. I also mentioned that the future measurements will allow to determine the matrix element with a precision of 7%.

As far as new results on B physics are concerned, I only showed a couple. One is about the discovery of the exhilarating baryons, which are seen through the exclusive decay chain to J/psi mesons and baryons, with the latter in turn producing two pions and a proton with two separate vertices. The other is the new CDF limit on the branching fraction of mesons to muon pairs, a process heavily suppressed in the standard model, which is enhanced in SUSY models. I discussed the latter result elsewhere recently.

I then presented two searches for supersymmetry recently performed in CDF: one for chargino-neutralino production in events with three leptons and missing transverse energy, and another for squarks and gluino signatures in jets and missing transverse energy. I also discussed these results recently in this blog, in a recent series of posts about dark matter searches at colliders.

I showed results on W mass and width measurements, on which CDF has the best measurements in the world. In particular, the W mass measurement has been produced with only 200 inverse picobarns of data, which is a twentieth of the data we have collected this far: CDF may reach below 20 MeV on the accuracy of W mass determination.

Diboson production has been observed in all its manifestations by CDF: the latest ones were WZ and ZZ production, which may give rise to spectacularly clean events (see the event display shown in the slides below: a perfect WZ candidate). The measurements of cross section are in excellent agreement with SM predictions.

Finally, I discussed the current limits on Higgs boson production. I think I have discussed this particular topic frequently enough in this blog to allow myself to skip a description of my slides here. I concluded my talk mentioning the string of successes of CDF in the recent past, and the prospects for precision SM measurements and reach of Higgs searches. I pointed out that CDF is the longest lasting physics experiment ever (yeah yeah, if we exclude the pitch drop experiment)…

There were several questions by the audience, most of them centered on Higgs boson limits and searches. I was of course happy to answer them, in particular to show that the results have kept improving more than the increase of luminosity they relied upon. In conclusion, it is always a great pleasure to present CDF results… A remarkable experiment indeed!

Just a note while I prepare my own summary of today’s talks: Mandeep Gill, a astrophysicist from Ohio State University, is also blogging on the talks we are listening at PPC08 this week. Please check his notes here….

Still many interesting talks in the afternoon session at PPC08 today, and I continued the unprecedented record of NOT dozing during the talks. Maybe I am growing old.

Alexander Kusenko was the first to speak, and he discussed “The dark side of light fermions: sterile neutrinos as dark matter“. I would love to be able to report in detail the contents of his talk, which was enlightening, but I would do a very poor job because I tried to understand it by following it with undivided attention rather than taking notes (core duo upgrade to grey matter, anyone?). I failed miserably, but what I did get and took notes about was, however, the fact that sterile neutrinos can explain the velocity distribution of pulsars.

Before I get to that, however, let me tell you how I found myself grinning during Krusenko’s talk. He discussed at some point the fact that making the Majorana mass large one can observe small masses with a Yukawa coupling of order unity. He pointed out that all quarks have y<<1 except the top quark: if these couplings come from string theory, they have a reason to be all of order unity; while from extra dimension kinds of models these would typically be exponentially small (as a function of the scale of the extra dimension). So, concluded Alexander, “both options are equally likely”. I found that sentence a daring stretch (and attached a “UN” before “likely” in my notes)! He appeared to be inferring some freedom in the phenomenology of neutrinos from the fact that string theory and large extra dimension models make conflicting predictions… Tsk tsk.

Anyway, about pulsars. Pulsars are rapidly rotating, magnetized neutron stars produced in supernova explosions. They have been known to have a velocity spectrum which far exceeds that of other stars - by about an order of magnitude. Proposed explanations of this phenomenon do not work: for example, the explosion itself does not seem to have enough asymmetry. There is a lot of energy in the explosion, but 99% of it escapes with neutrinos. If these escape anisotropically they could propel the pulsar.  Through the reactions with polarized electrons one can easily get 10% asymmetries in the neutrino production in the core of imploding supernovas,  while one needs just 1% to explain the velocity distribution. The asymmetry, however, is lost as they rescatter inside the star. If instead one has sterile neutrinos, they interact much more weakly than ordinary ones, they escape asymmetrically, and a potential 10% asymmetry in produced neutrinos, 10% of which could be going to sterile neutrinos, would explain it.

Alexander pointed out that sterile neutrinos must have lifetimes larger than age of universe, but they can annihilate, and produce photons which have energy m/2. So concetrations of sterile neutrino dark matter emits x-rays.

In summary, we have introduced additional degrees of freedom in our particle model when we discovered neutrino masses. This implies the need of right-handed neutrinos. Usually one sets a large Majorana mass and hides these right-handed states to a very high mass scale, but one neutrino remains at low mass, and it could provide a good dark matter candidate. It could play a role in baryon asymmetry of the universe, and explain pulsar velocities. X-ray telescopes could discover it, and if discovered, the line from relic sterile neutrino annihilation can be used to map out the redishift distribution of DM. This could potentially be used in an optimistic future to study the structure and the expansion history of the Universe.

After Krusenko’s talk, A. Riess discussed the “implications for dark energy of supernovae with redshift larger than one“. With the Hubble space telescope they made a systematic search for supernovae in a small region of sky, and found 50 in three years, 25 of them with z>1. These allowed an improved constraint on w=1.06+-0.10. A paper is in preparation. I am quite sorry to have no chance of reporting about the very interesting discussion Riess made of distance indicators like cepheids, keplerian motion of masers around the central black hole in NGC4258 -which anchors the distance of this galaxy and allows relative distances to become absolute measurements- and parallax measurements made on cepheids. His talk was very instructive, but my brain does not work well tonight…

Dragan Huterer discussed “A decision tree for dark energy“. In two words, he proposes to start from the simple lambda_CDM model and try more complex constructions incrementally, starting with the extensions that have more predictive power. He also discussed a generic figure of merit of measurements - basically defined as , or the inverse of the area of 95% CL measurements of these two parameters. Apologies here too for having to cut this summary short…

Mark Trodden concluded the afternoon talks by discussing “Cosmic Acceleration and Modified Gravity“. Mark is a blogger and so I feel no shame in saying he should report about his talk rather than having me do it. However, I found very interesting a note he made on extrapolations of newtonian mechanics working or not working. When observed perturbations of the Uranus orbit pointed to the existence of an outer planet, Neptune, the prediction was on solid ground: As an effective theory, you expect newtonian mechanics to work well at higher radii. On the other hand, when Mercury’s precession was hypotesized to come from perturbing effects from an inner planet, the answer turned out to be wrong -there, newtonian mechanics did break down, and general relativity showed its effect.  The lesson for modified gravity models is clear.

The session was concluded by a panel led by Krusenko, where some of the speakers were asked a few questions. The first was: “If you had a billion dollars how would you spend them for measuring cosmological parameters ?”

A.Riess said he would do the easiest things first. Before launching to space, one has to look at things that are easy and less expensive. He sees a lot of room. Space has advantages (stability of conditions) but it is hard to get there. We want measurements to the few percent level on a few cosmological parameters: maybe can we do that now from the ground, or from instruments that are
already in space now. He stated the necessity of being more creative at using the resources we already have. All of the small but significant improvements that are possible matter.

I gave my own answer by grabbing the microphone after a few of the speakers had their say.  I said we do not need billions of dollars: there is, in fact, a lot we can do with much smaller amounts of money on the ground, without spending huge amounts of money for space experiments. CP violation experiments can be improved with limited budgets, and also low energy hadronic cross sections and nuclear cross sections are quite important for the understanding of the early universe. The LHC did cost a few billion dollars, and it will maybe give us an answer about dark matter; but in general, particle physics -even direct DM searches- require smaller budgets, and the payoff may be large.