Thursday 25 June 2009

Angles and Demons

Since writing down the standard model back in the summer of love the only progress in particle theory has been the discovery that neutrinos have masses. This fact makes the leptons similar in spirit to the quarks in the sense that transitions between different flavors are possible. In both cases the flavor eigenstates, that is the states to which the W bosons couple, are not the same as the mass eigenstates but linear combinations thereof. This fact opens the door to a fascinating endeavor of measuring the angles in the unitary matrix that relates the flavor and the mass eigenstates.

The angles in the quark sector have been measured from every angle. The final conclusion is that what they taught at school was right: the sum of the angles in a (unitarity) triangle is equal to 180 degrees. In the lepton sector, experiment is lagging behind: so far we know only two of the angles. The one known as the atmospheric or $\theta_{23}$ angle (responsible in particular for the transitions of atmospheric muon neutrinos into tau ones) is close to 45 degrees. The solar or $\theta_{12}$ angle (responsible for the vanishing of electron neutrinos produced in the Sun) turned out to be a bit smaller, about 30 degrees. For the last angle, at the moment we have only an upper bound from the CHOOZ reactor experiment: $\theta_{13} < 11$ degrees at 90 percent C.L.

Now what have demons to do with it? I recently came across a talk from the MINOS collaboration a few month ago in Fermilab. MINOS, when on leave from Hades, studies the muon neutrino beam sent over the distance of 735 kilometers from Fermilab to a far detector located in the Soudan mine in Minnesota. One reason to bother (just imagine what it takes to dig such a long tunnel to send the neutrino beam over several states) is a precise measurement of the angle $\theta_{23}$ which controls the fraction of muon neutrinos that disappear on the way. But there may be more fun than that. Most of the muon neutrinos that vanish turn into tau neutrinos who escape detection. However, if $\theta_{13}$ is non-zero then a small fraction of the muon neutrinos should turn into electron neutrinos, and those receive a warm welcome in Soudan. Thus, MINOS is one in a long queue of experiments trying to pinpoint $\theta_{13}$.

In February this year MÌNOS announced their first results concerning the electron neutrino appearance. They see 35 electron event, roughly 1.5 sigma above the expected background of $27$ events. Not too significant, but already tantalizing. Moreover, if the MINOS data are combined with all available neutrino data the hint for a non-zero $\theta_{13}$ is strengthened to 2 sigma. The central value for $\theta_{13}$ inferred from the overall fit is 8 degrees (plus minus 4) - just below the CHOOZ limit.

If the current hints converge to a full-fledged measurement of $\theta_{13}$ in the 5-10 degrees ballpark then there are some far reaching consequences. First of all, measuring the $\theta_{13}$ angle paves the way to measuring yet another angle (isn't particle physics exciting?), more precisely the CP violating phase in the neutrino mixing matrix. Secondly, it would appear that the mixing angles in the lepton sector are pretty random numbers with no structure, in stark contrast to the quark sector where the CKM matrix displays a very hierarchical structure. In other words, neutrinos would prove to be anarchic. That would mean that anarchy is at rule, at least in the lepton sector, for the first time since Barcelona'36.

Friday 12 June 2009

Boston Tea Party

Last weekend I made a trip to Boston and had a privilege to see bits of the SUSY'09 conference. Backreaction and Quantum Diaries have already run their stories but in my opinion they did not fully capture the grandeur of the event. 9 days in a row, including foreplay. Over 400 participants, not counting squatters. 42 plenary speakers, most of whom witnessed the glorious days when supersymmetry was conceived. Seven parallel parallel sessions to cover every aspect of supersymmetry that has not yet been covered thoroughly enough. Royal coffee break menu fully adequate to the royal conference fee. And so on and on since 16 years and into the future.

Meanwhile, there is no single hint from experiment that supersymmetry is realized in nature... but that should not upset anyone. As my fellow blogger skillfully put it, supersymmetry is the "shining beacon", the "raison d’etre" and for this reason "the conundrum is how it will be discovered, not if". That's why every year we come together to enjoy old familiar faces and old familiar talks. The point is, while waiting for the inevitable, to maintain that kind of spirit that David Lodge praised in his books.

On the picture below, the photographer about to make a photograph of the SUSY'09 participants.

Tuesday 9 June 2009

Life After FERMI

I mean, after FERMI's first electron data that shed new light on the currently hottest topic in astroparticle physics - the origin of the cosmic-ray positron excess measured by PAMELA. The PAMELA anomaly trumpeted last summer, combined with the data from ATIC (who claims a spectacular excess of electrons at few hundred GeV), prompted zillions of publications that speculate of its dark matter origin. A few weeks ago FERMI revealed their first measurement of the electron+positron cosmic ray spectrum up to 1 TeV. Here is my summary of what that implies for the models of dark matter.

Let's begin with a handful of facts:
  • Thanks to FERMI and HESS we now have a pretty accurate picture of the cosmic-ray electron+positron spectrum. Up to 1 TeV, the spectrum is well approximated by power law, $\sim E^{-3}$, while above TeV it becomes softer (larger power).
  • ATIC screwed: there is no clear feature in the electron spectrum. In principle, one should describe the present situation as "two experiments giving inconsistent results", given that ATIC's brand new data continue displaying a distinct bump with even smaller errorbars. But, somehow, the public opinion prefers a fancy high-tech satellite over a dirty leaking balloon somewhere in the cold Antarctica. More seriously, FERMI simply beats ATIC with a hundred times more statistics.
  • The electron spectrum near TeV is above the background predicted by conventional cosmic-ray propagation models (which prefer a larger power, $\sim E^{-3.3}$) in which the electrons are produced by scattering of cosmic ray protons. But that alone is hardly an anomaly, and the spectrum can be easily fitted by cranking up the propagation model.
  • However, combining FERMI with the PAMELA positron data strongly indicates the presence of a new primary component of electrons/positrons (as opposed to the secondary production by protons) .
  • The primary component could be injected by nearby pulsars, or by another so far unidentified astrophysical process, or by dark matter annihilating or decaying in our galaxy. We have to wait for the jury to reach the verdict.
  • But, light (less than 1 TeV) dark matter particle as a source of the PAMELA and FERMI excess is now strongly disfavored. The reason is that a light particle would lead to sharp spectral features at energies comparable to its mass, whereas FERMI sees none of that. If dark matter is the cause of the excess, it has to be relatively heavy, 1 TeV or more.

In fact, the new FERMI data did not really lead to a slaughter of the dark matter models or its authors. New papers keep appearing in which the excess is fitted with axions, neutralinos, winos, KK particles, F-theory or three little pigs. But Occam is waving his razor menacingly, and we are reaching the point where boring astrophysics becomes the simplest explanation of all available data. The dark matter models, although still viable, have to be intelligently designed to yield observable signals in the positron and electron channels, but none in antiprotons, gamma rays or neutrinos. Incidentally, the electron channel is the one where the astrophysical background is most difficult to control...

The cool thing is that by the end of this summer we may further disfavor dark matter or return it to grace. The new crucial piece of information will be FERMI's measurement of the diffuse gamma ray spectrum that will extend its previous measurement to larger energies. If the electrons and positrons observed by PAMELA and FERMI originate from dark matter it means that they are produced all over the galaxy. Once produced, the electrons lose their energy by scattering on starlight and on the CMB or by synchrotron radiation in the galactic magnetic fields, which leads to a diffuse flux of photons at energies of a few hundred GeV. Given the number of electrons needed to explain PAMELA and FERMI, the diffuse signal should be detectable by 1-year FERMI data. The important thing is that boring astrophysics cannot easily fake that signal. On the other hand, the absence of features in diffuse gamma would be a huge setback for the dark matter interpretation. The experimental data are expected on August 12, so just a little patience please...

For more details and plots see the slides of Alessandro Strumia's talk at Planck'09; check also for the connection between dark matter and dialectic materialism.