Particle Astrophysics Laboratory
Introduction to neutrino physics
(Video: The KATRIN experiment moving from factory to lab---with inches to spare.)
At first glance, from a particle physics perspectice, the neutrino doesn't look like anything special---just an electron (or a muon, or a tau lepton) with no electrical charge. Other than charge-related behaviors, like emitting photons, anything that that an electron can do, a neutrino can do too: interact with W and Z bosons, have a 1/2-integer spin, have a magnetic moment. But it turns out the details are extremely interesting.
Unlike the electron, muon, and tau lepton, the three types of neutrinos have very small masses, very close together. This seems to be a hole in our understanding of why anything has mass at all. (Imagine going to a foreign country and seeing their currency for the first time. You wouldn't be surprised to learn that they have bills valued at $1, $5, $10, $50, $100, and $500; you'd be very surprised to learn that they also issue bills for $0.00005, and $0.00006, and $0.00008.)
These masses are so small, we can't yet measure them directly. The KATRIN experiment is an attempt to measure the electron neutrino mass by observing a small amount of missing energy (or, more correctly, missing phase space) in the beta decay of tritium (3H).
Like the electron, muon, and tau lepton, the neutrinos have in three different masses. But these masses do not correspond to the lepton flavors. We don't have a low-mass neutrino that couples to the electron, a medium-mass neutrino that couples to the muon, and a heavy neutrino that couples to the tau. We have a low-mass neutrino that couples to a particular quantum-mechanical mixture of electron, muon, and tau; each of the neutrino masses couples to a different such mixture. This issue, in reverse, has a dramatic experimental consequence: all of our laboratory and astrophysical neutrino sources start off in a quantum-entangled mix of flavors. This has the weird experimental effect of "neutrino oscillations", one of the most exciting particle physics discoveries of the 1990s and 2000s.
The neutrino may have played an important role in the early Universe, participating in some of the various symmetry breakings that occurred as the Universe cooled. It may also play a role in the Universe today, since huge numbers of "relic neutrinos" are thought to have survived the Big Bang, and may tweak the kinetic energy/mass energy balance of galaxies even today.
The KATRIN experiment
The KATRIN experiment is poised to use one of the world's largest vacuum chambers (certainly the largest ultra-high-vacuum chamber) as an energy-filter: it will take a large supply of tritium gas (1 curie in the measurement volume at a time, many megacuries more in circulation) and sort its decay electrons into two classes: those above a sharply-defined energy threshhold and those below. With the threshhold set within a few volts of 18.757 kV, only a tiny fraction of the decays will emit an above-threshhold electron---these will be counted. The behavior of this count rate, as a function of the exact threshhold setting, tells us the shape of the tritium decay phase space, and thus the neutrino mass. The KATRIN system, from left to right, contains: a pipe filled with low-pressure T2 gas in a magnetic field; a series of complex pumps so that tritium gas cannot (but beta-decay electrons can) flow down the pipe to the right; a large spectrometer tank in which the beta-electron beam is steered and filtered; and finally an electron detector which counts anything that passes the filter.KATRIN is located in Karlsruhe, Germany, on the east bank of the Rhine and the northern edge of the Black Forest. KATRIN's main spectrometer vacuum tank was manufactured in Deggendorf, only 400km away---but on the Danube. The spectrometer was deemed too big to ship overland, so it took the 9000km scenic barge route via the Danube, Black Sea, Marmara, Aegean, Mediterranean, North Atlantic, and finally up the Rhine to Karlsruhe. Only the final 5km between the port and the lab were routed overland, and that with great difficulty (but attracting 30,000 spectators) as you can see in the video at the top of the page.
See the central KATRIN page to learn more about the spectrometer.
At UCSB we are working on KATRIN's rear wall and other calibration systems.