Particle Astrophysics Laboratory
Neutrino and dark matter physics: an introduction
We are often told that there are four known forces governing the Universe and everything in it: electromagnetism, gravity, the strong interaction, and the weak interaction. We experience gravity every day; all everyday materials are made of atoms, which are largely assembled and governed by electromagnetism (as is the light, sound, and touch with which we observe them). Though we can't trivially "experience" it, we know that nuclei exist and are held together by the strong force. That's all either familiar or reasonably easy to grasp. By comparison, the weak force sounds like an afterthought, poking around during nuclear beta decay but otherwise scarce.It turns out that weak interactions are tremendously important in shaping the Universe. All of the Universe's non-hydrogen nuclei (helium, carbon, oxygen, iron, etc.---what astronomers call "metals") arose in weak-interaction-rich environments, both ordinary stellar burning and explosive nucleosynthesis. Supernovae are thought to explode with a weak-interaction-driven neutrino wind---a vital step in distributing newly-fused metal atoms to future stars and planets. It's possible that the baryon asymmetry---the fact that the Universe is made of matter, rather than half matter and half antimatter---arose in one of the weak interaction's asymmetries. And, equally importantly, the numbers and sizes of galaxies in the modern Universe depended on the nonrelativistic mass density of the young Universe---and that number hinged crucially on the nature of "dark matter", whose formation and survival may have been governed entirely by the weak interaction or one of its yet-to-be-discovered relatives.
This is very interesting stuff, which fortunately lends itself to really exciting experiments. This is what we do.
Neutrinos
We plan to build a group at UCSB to work on measuring the neutrino mass via ultra-precise beta decay kinematics. The primary technique is to look at the range of electron energies which can occur in tritium (3H) beta decay. Each decay emits an electron, a 3He nucleus, and an neutrino; every decay releases the same total amount of energy. However, this energy is divided up among several daughters: the 3He nucleus gets its 3 GeV rest-mass-energy and a (small) variable amount of kinetic energy; the electron gets its 511 keV rest-mass-energy and a (large) variable amount of kinetic energy. The neutrino gets a large, variable amount of kinetic energy and a small as yet unknown rest mass energy. When the neutrino's rest mass takes up this slice of the total energy pie, less energy is available for the kinetic energies of the electron and nucleus. This deficit may be measurable in a large, ultraprecise electron energy spectrometer.The major group working on this measurement is the KATRIN collaboration, in which Prof. Monreal has been involved since 2005.
Despite its complexity, KATRIN is at heart a traditional nuclear-physics spectrometer---it uses classical electro- and magnetostatic forces to steer electrons towards or away from a detector in an energy-dependent fashion. At UCSB, we hope to demonstrating an entirely new electron energy measurement technique, perhaps more akin to the FTICR spectrometers familiar to biochemists. We refer to this effort as "Project 8".
Dark matter
The early Universe started off hot, then cooled down and expanded. From a particle-physics perspective, this had remarkable consequences: think of any of the high-energy phenomenon that we struggle to produce at particle colliders like the Tevatron or the LHC. That phenomenon happened naturally, everywhere, if you go back to the appropriately-hot stage of the Big Bang---but it stopped happening a fraction of a second later because the typical thermal energy dropped below some threshhold. Several of these threshholds left behind "relics"---stable particles that could be manufactured copiously on the hotter side of the threshhold, but couldn't be destroyed again (and didn't decay) when the Universe cooled. There are a series of examples:- When the Universe was at about 300,000 years old and 3000 degrees K, it was hot enough to both emit and reabsorb ultraviolet light. When the Universe cooled a bit further---over about 20,000 years---it lost this ability; the last batch of photons emitted was never reabsorbed, and survives to this day as the Cosmic Microwave Background.
- When the Universe was about 3 minutes old, it was hot enough (a few MeV energy) to cause fusion and fission of light nuclei (deuterium, helium, lithium). When it cooled below the photofission temperatures, the last batch of light nuclei survived; this was called Big Bang Nucleosynthesis and is the source of most of the deuterium on Earth today.
- At temperatures of about 100 GeV, the Universe similarly readily created and reabsorbed neutrinos; the last batch of neutrinos should still survive (though they are as yet undetected) as a Cosmic Neutrino Background.
- Think in the opposite direction: we know how physics works at 3000 K, at 1 MeV, and at 100 GeV (we've studied these energies in the lab) so we can predict the existence of relic photons, nuclei, and neutrinos---collections of stable particles that have been hanging around since the Big Bang. Meanwhile, we have also observed dark matter, which (as far as we can tell) looks exactly like a heavy, stable particle that's been hanging around since the Big Bang. Suppose that at 1000 GeV, or 100,000 GeV---an energy we have not studied in the lab---there's a similar transition, leaving behind a relic population just like the familiar ones, only of a particle not yet seen in colliders. This seems to be a perfectly reasonable explanation for dark matter.
These WIMPS would be expected to cruise around the Milky Way (including through Earth) at speeds of about 300 km/s ... and to occasionally collide with nuclei and impart a detectable kick. The search for these kicks is a major component of "direct" dark matter experiments like DEAP, MiniCLEAN, DMTPC, or CDMS. At UCSB, Prof. Monreal is beginning work on detector development and construction for direct detection experiments.
WIMPS should occasionally collide with one another, also---the models actually constrain this process much better than they constrain WIMP-nucleus collisions. There are various diverse ideas for how to detect WIMP-WIMP collisions and their debris; this is a fertile area for both theory/phenomenology and experiment.