Hunting for Neutrinos and Dark Matter
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Research
Research in our group focuses on investigation of fundamental properties of neutrinos and direct detection of dark matter particles. Neutrinos are among the most elusive and least known elementary particles. It's been known for a long time that neutrinos are very light particles,
perhaps even massless particles that travel at the speed of light. But there's never been a compelling reason _why_
they should be massless, unlike the vast majority of subatomic particles, and yet no evidence that they actually had mass.
Before the discovery of neutrino oscillation, the lightest particles
known to have mass was the electron, about 2000 times lighter than
the proton or neutron. Neutrino oscillation showed that neutrinos
do have some mass (without mass, they don't oscillate), and while the
oscillation gives mass differences, the results make clear that
neutrinos are much lighter than even electrons; perhaps a million times
lighter. It is still a puzzle how neutrinos got a mass that is so close
to zero, yet not quite zero. Neutrino mass may provide answers to some of the most fundamental questions of the physics today: from the contribution to the missing mass of the universe to the explanation of the dominance of matter over antimatter in the universe.
Nowadays we can speak with confidence about features of neutrino oscillations among the three neutrino types: electron, muon and tau and we have measured oscillation lengths (described by their masses) and amount of mixing among types (given by oscillation angles).
However, there have been hints that there may exist a fourth, very massive sterile neutrino with oscillation length of just a couple of meters and experiments are underway to investigate it.
In recent years, several complementary cosmological evidence have been collected that point to the existance of dark matter - new form of matter that differs from the Standard Model particles and the visible universe we observe. As a matter of fact, dark matter and dark energy compose about 96% of the Universe mass-energy, while the visible Universe that we know most about represents just 4%! A promising candidate for this new type of matter are Weakly Interating Massive Particles (WIMPs): relatively heavy, stable, chargeless particles, interacting via gravitational and weak force only. Astrophysical data indicate thar even our own galaxy is immersed in the dark matter halo. While we have strong indication of dark matter, direct detection of WIMPs or any other kind of dark matter has not been achieved yet. But the direct detection is the only way forward in understanding its true nature and learning more about this dominant component of the Universe.
Our group has bee involved in the KamLAND experiment in Japan for several years. KamLAND made a high precision measurement of neutrino oscillation parameters with antineutrinos coming from nuclear reactors around Japan and beyond. KamLAND also made the first ever experimental detection of antineutrinos of geological origin coming from beta decays in radioactive decay chains of uranium and thorium inside the Earth. In 1904, Ernest Rutherford hypothesized that radioactive decays of uranium and thorium provide a source of the internal heat of the Earth. KamLAND measurement proved that indeed large fraction of the Earth's internal heat comes from radioactivity.
The next big goal for KamLAND may be the hunt for the fourth neutrino flavor and this plan is part of the CeLAND project: a PBq 144Ce antineutrino source put either on the side or in the heart of KamLAND in search of neutrino oscillations at a just few meters oscillation length.
We live in a matter dominated universe, although the birth of our universe was characterized by production of equal amounts of matter and antimatter in the Big Bang explosion. At some point, in early universe, matter won, and became dominant in the cooling, expanding universe due to a CP violation effect or a slightly different behavior exhibited between matter and antimatter. Although CP violation has been observed in quarks, the effect is insufficient to account for the amount of matter in the universe today. This raises a question if such an effect may exist in neutrinos. But to measure the CP-violation in neutrinos, one must first establish the size of the neutrino mixing angle Theta 13 that always appears as a multiplication factor along with the CP-violation phase. Recent measurements of Theta13 show that it is relatively large, and place a high demand for the precise measurement of the size of Theta 13. This knowledge will directly influence our ability to measure CP-violation in neutrinos and its role in the dominance of matter over antimatter in the universe!
The Double Chooz experiment is currently taking data in France got the measurement of the neutrino mixing angle Theta13 with antineutrinos coming from the Chooz nuclear reactor plant. Large size of Theta13 confirmed independently by three neutrino experiments observing neutrinos from nuclear reactors set a stage for the future Long Baseline Neutrino Experiment (LBNE) designed to measure CP-violation in neutrinos.
DarkSide detector is currently under constraction in the Gran Sasso underground laboratory in Italy and it is set to conduct one of the most sensitive direct dark matter searches to date using low radioactivity argon gas acquired from underground sources. Once operating, DarkSide will conduct practicaly background free search for WIMPs with unpresedented sensitivity.
Last modified on December 20, 2012 by jelena@phys.hawaii.edu