A Quantum Diaries Survivor

Alexander Kusenko on sterile neutrinos, and the other afternoon talks May 20, 2008

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.