Friday 29 January 2010

Farewell to the Noughties - Experiment

Year beginnings are always lazy in both theory and experiment (except for most important decisions being taken, but others write about it). So it's a perfect moment for writing all sorts of summaries. Like, for example, summarizing the entire decade. Here I would like to give the list of the most important experiments in the last decade from the point of view of a particle physicist. The other day I named the noughties the most depressing decade ever, and experiment is one of the main reasons. But not for the lack of trying. The pain is that all these beautiful experiments kept confirming the old truths rather than showing new horizons.

Cosmic rays tickle our imagination because of the huge energies involved: the most energetic beasts reach the stunning energies of order $10^{8}$ TeV in the Earth rest frame. (this translates to hundreds of TeV in the center of mass frame of the collision, still much more than what we can achieve in present colliders). Some earlier experiments suggested that some of these cosmic rays are TOO energetic. Above the energy threshold known as the GZK cutoff a cosmic particle should quickly lose its energy due to interactions with the cosmic microwave background. Observation of cosmic ray events above the GZK cutoff would defy the foundations of physics, maybe pointing to modifications of such fundamental symmetries as the Lorentz symmetry. Auger killed these reveries. The cosmic ray spectrum displays the superboring GZK cutoff at $5x10^{7}$ TeV, more or less where it should be. What a disappointment.

Neutrinos are the only subfield of particle physics that enjoyed experimental progress in the last decade. Of course, the really groundbreaking discovery of atmospheric neutrino oscillations falls into the previous decade. The SNO experiment only swiped the floor in the early noughties, by obtaining a solid proof that the solar neutrinos also oscillate. They demonstrated that the total number of neutrinos arriving from the Sun is more or less what we expect from our solar models, but that some of the neutrinos change the identity from electron to muon ones.

The true legacy of this experiment is still unclear at the moment of writing. One certain thing is that it turned out very influential, pushing particle theorists in a new direction. PAMELA has made precise measurements of cosmic ray protons, electrons, and their antiparticles, at energies extending to hundreds of GeV. The experiment reached the celebrity status after announcing that the positron spectrum displays a completely different shape than that predicted by standard models of our galaxy. Dark matter particles floating in our galaxy and annihilating into light SM particles provide one tantalizing explanation of that discrepancy. But huge astrophysical uncertainties involved in theoretical predictions make any strong conclusions impossible. It might be that in the future PAMELA will be promoted to the first harbinger of new physics. But more likely, downgraded to yet another false lead.

Generally, flavor physics is best suited for botanists. Yet new physics hunters cannot afford the comfort of ignoring it. Because of approximate symmetries of the SM that suppress certain transitions between the generations of quarks, flavor physics is very sensitive to contributions from new hypothetical heavy particles. BaBar and its twin sister Belle produced kilograms of upsilon mesons (the ones made of a b-quark and a b-antiquark), which allowed them to precisely measure their properties. The results showed no major deviations from the predictions of the standard model, apart from a few glitches here and there that, maliciously, occur in observables under poor theoretical control. These results provide a strong hint that, apart from the Higgs boson, there is no new particles in the near energy reach. Scaring.

Everybody hates them now: theorists for playing such a cruel game on them, while other experimentalists for getting too much attention. Yet they have been the leader in the field of dark matter direct detection for most of the decade. Depressingly, their leadership consisted in setting more and more stringent limits on the dark matter-nucleon cross section. One solid fact that has been established is that the dark matter particle is not a WIMP in its simplest form. That is to say, it cannot be a weak scale particle interacting via Z boson exchange with the weak coupling strength. But many other options are still wide open, so the hunt continues.

Tevatron Run-2
Great expectations, beautiful performance, gazillion events, hundreds of clever physicists devising clever tricks to extract tiny signals from the data. And nothing that would raise an eyebrow. Precise measurement of the W mass, or discovering omega bee baryons, is not the kind of story our grandchildren will want to listen. But the most depressing must be the that thousands of Higgs bosons have probably been produced at the Tevatron, maybe hundreds have been written on tape, but we just could not see it in all this hadronic mess. After LEP and B-factories, the Tevatron gave us yet another hint that the physics of electroweak symmetry breaking might be less rich than we hoped for.

For more than 7 years WMAP has been making precise measurements of the anisotropies in the Cosmic Microwave Background. The only surprise was that our shaky theoretical models describe the data so well. The experiment turned cosmology into precision physics, bringing it dangerously close to Lord Kelvin's nightmare. But there is a glimmer of hope. WMAP's greatest achievement is a precise determination of the amount of various forms of matter in the universe. In particular, it solidly established that dark matter does exist. Which is the most tangible proof we have that the current standard model of particle physics is not the whole story. Maybe this decade we'll find out what's beyond.

Saturday 16 January 2010

Eyes on Xenon

Not the Higgs but dark matter is the true Holy Grail of high energy physics, given that only the purest can hope to discover it. For most of the past decade the leader of the quest to detect the dark matter particle has been the CDMS collaboration. Unfortunately, this amounted to setting better and better limits on the interaction strength of dark matter with nucleons, apart from this shadow of a hint of a possibility of two events announced last year. Although CDMS stays in the game and will continue taking data as super-CDMS, it is bound to lose the yellow shirt soon. For the moment, the primary contender is Xenon100 - a scaled up version of the Xenon10 detector that was in operation in 2006-2007 in Gran Sasso, Italy.

Xenon experiments use a completely different detection technology than solid state detectors such as CDMS. The detector is filled with xenon in the dual liquid/gas phase. When a xenon atom gets hit, it reports this fact to experimenters in two different ways. Photons produced when the atom returns from the excited state is promptly registered by the phototubes located around the detector volume. Besides, the electrons ionized from the atom drift slowly in the applied electric field, and they are registered after some delay. It turns out that the ratio of the scintillation (S1) and the ionization (S2) signals is different for nuclear recoils (that are due to WIMPs, once the experiment is shielded from neutrons) and electron recoils (that are due to ubiquitous backgrounds like photons).

Thus, by measuring the S1/S2 ratio xenon experiments are able reject most of the background. Furthermore, from the two signals and their relative delay it is possible to reconstruct where in the detector volume the hit occurred. Obviously, background events are more likely to occur near the walls of the tank. Therefore making a larger experiment not only increases the probability of registering a dark matter recoil event , but also decreases the background in the central volume - the property referred to as self-shielding. Add to this the good radioactive purity and relative availability of xenon (you just have to smash a lot of lightbulbs), and you understand why big xenon detectors are taking over the field of direct detection.

The prototype Xenon10 detector was not only a proof-of-principle but also a great success story. For some time, Xenon10 was providing the best constraint on the spin-independent WIMP-nucleon cross-section. In fact, it still sets the best limit for the WIMP masses in the 10-50 GeV range, while for larger masses it was later outraced by CDMS. After so much success, the group decided that things are going too smoothly, and set up a huge pillow fight to ease the tension. As a result, Xenon bifurcated into two rival experiments called Xenon and LUX; the latter was banished from sunny Italy into bottomless pits of South Dakota.

The two groups continued, each on its own, scaling up the same technology, each facing an orthogonal set of problems. Apparently, Xenon was the first to pull together. Last year calibrations were made and the physics run is due any time now. According to the official Xenon propaganda, just 40 live days is enough to push the limit on WIMP-nucleon cross section down to 6x$10^{-9}$ picobarns for a 100 GeV WIMP, almost a factor of 10 better than the current CDMS limit of 4x$10^{-8}$. If either of the two events reported by CDMS is really due to dark matter, by this summer we might have a discovery of the century. If not, the quest will continue, with more and more experiments joining in the race. One-ton monster versions of xenon experiments whose sensitivity should reach $10^{-11}$ picobarns are expected in the second half of this decade.

So tons of excitement ahead. As soon as first rumors appear, you know where to look ;-)

Monday 11 January 2010


Welcome back after winter holidays! In the meantime the year 2009 has gone to past along with the whole damn decade. Nobody here is going to shed a tear for the noughties - definitely the most depressing decade in the history of particle physics. It closes the balance with *zero* major experimental discoveries, while particle theory has also produce little to write down in history books. The optimistic conclusion is that from this point things can only get better :-)

So what good do I expect in 2010? This year is going to be very special, in that we have two particle accelerators at the high energy frontier. Such a situation occurs for the first time in my life, I mean life as a physicist. Hopefully not for the last time...

All eyes are of course are turned toward the LHC. After the Baby Hadron Collider (BHC) phase last year, following the Aborted Hadron Collider (AHC) in 2008, this year the machine enters the difficult Coming-of-age Hadron Collider (CHC) phase. Even though discoveries are highly unlikely at this stage, we will be following with mouths wide open each step toward becoming the full-fledged LHC: first 7 TeV collisions, first inverse picobarns acquired, first W and Z bosons, and finally first top quarks on the European soil. Meanwhile, the Tevatron does not rust yet. The most fascinating is of course its quest for the Higgs: what mass range will they exclude, will they see a bump somewhere. And, one never knows, one of its many new physics searches may finally bring exciting results.

However, as we already got used to in this century, discoveries are much more likely to literally fall from the sky. End of last of year, the CDMS collaboration decided to go down in flames and announced a detection of statistically insignificant but thought-provoking two scattering events that could be triggered by dark matter particles. This year a much more sensitive dark matter detector called Xenon100 begins taking data. If any of the two CDMS events was really due to dark matter, Xenon100 should grab a discovery by this summer. That is definitely the most awaited result of the year.

Up in the sky, the Fermi gamma-ray telescope is still alive and taking data. This year should bring an answer if the haze - a population of energetic electrons and positrons in the center of the galaxy that is difficult to account for by astrophysical sources - really exists. Moreover, Fermi is continuing its search for subhalos - small satellite galaxies made entirely of dark matter that may glow in gamma rays due to dark matter annihilation. Deeper in space, the Planck satellite is sitting at the Lagrange point L2 and making precise measurements of the Cosmic Microwave Background since September last year. If all goes well we should have the first results this year, and we eagerly expect Planck's measurement of the CMB polarization that should greatly surpass in precision the polarization data of its predecessor WMAP. As usual, astrophysics will probably not bring a clear cut fundamental discovery, but may give us something to think about.

So, lots of things to get excited about, lots of rumors to spread. Even if the year 2010 will not turn very fruitful, at least it should not be boring.