How many neutrinos should exist ?

The SM assumes three types of neutrinos. However, these authors analyzed data to demonstrate the need of two more types [http://www.nature.com/nature/journal/v478/n7369/full/478328a.html?WT.ec_id=NATURE-20111020].

Neutrino oscillations, observed through the transmutation of neutrinos of one type into neutrinos of another type, occur if there is mixing between neutrino types and if individual neutrino types consist of a linear combination of different neutrino masses. (At present, the masses and mixings of the fundamental quarks and leptons can be measured but are not fully understood.) In the case of two-neutrino mixing — for example, mixing between the muon neutrino and the electron neutrino — the probability (P) that a muon neutrino (ν_μ) will oscillate into an electron neutrino (ν_e) is given by P(ν_μ ν_e) = sin²(2θ)sin²(1.27Δm²L/E). Here, θ, in radians, describes the mixing between the muon neutrino and electron neutrino; Δm² is the difference of the squares of the masses of the two neutrinos in square electronvolts; L is the distance travelled by the muon neutrino in kilometres; and E is the muon-neutrino energy in gigaelectronvolts.

In general, the number of different neutrino masses equals the number of neutrino types, so that three-neutrino mixing involves three neutrino masses and two independent Δm² values, whereas five-neutrino mixing involves five neutrino masses and four independent Δm² values. Neutrino oscillations have been observed at a Δm² of about 7 × 10⁻⁵ eV² by detectors that measure the flow of neutrinos from the Sun and experiments that detect neutrinos at a long distance from nuclear reactors. The oscillations have been detected at a Δm² of around 2 × 10⁻³ eV² by detectors that measure the flow of neutrinos from the atmosphere and by experiments in which neutrinos are measured at a long distance from particle accelerators. In addition to these confirmed observations of neutrino oscillations, there is also evidence for oscillations at a Δm² of about 1 eV² from short-distance accelerator and reactor neutrino experiments^{2, 3, 4}. However, it is not possible to explain this third Δm² value with only three neutrino masses. Therefore, additional neutrino masses are required.

In their study, Kopp et al.¹ tried fitting the world neutrino-oscillation data to theoretical models involving four different neutrino masses (three active neutrinos plus one sterile neutrino) and then five different neutrino masses (three active plus two sterile neutrinos; Fig. 1). They found that one sterile neutrino was insufficient to explain the world data, but two gave a satisfactory global fit. (Similar fits are discussed elsewhere^{5, 6, 7, 8, 9, 10}.) One other feature of the authors' two-sterile-neutrino fit is that it allows for violation in leptons of the charge–parity (CP) symmetry — according to which particles and antiparticles behave like mirror images of each other — or for a difference between neutrino oscillations and antineutrino oscillations. Such CP violation might help to explain the r-process, in which heavy elements are produced through nuclear reactions involving rapid neutron capture (hence the 'r'), and the production of heavy elements in neutrino bursts from stellar explosions known as supernovae. It might also help to explain why the Universe is dominated by matter and not by an equal amount of matter and antimatter.

Neutrino not that fast !!

I like this blog entry by Zz. He highlighted a recent article that did not see data indicating superfast!

High energy neutrinos undetected in IceCube

High energy neutrinos are expected to come out of gamma ray bursts. On earth there is a IceCube, which is designed to catch these neutrinos. However, after 13 months of run, they find null results. [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.141101]

GRBs result from a giant star explosion or the collision of star remnants. These cosmic cataclysms produce—in addition to gamma rays—high-speed protons that are thought to account for the highest energy cosmic rays observed on Earth. Near the source, these protons may run into photons and end up generating neutrinos with energies far above 1 TeV. In the past, neutrino detectors on Earth have not been large enough to capture one of these high-energy neutrinos with any likelihood.

IceCube, which was completed in December 2010, is a kilometer-cubed array of photodetectors that have been drilled down into the Antarctic ice cap. Neutrinos typically fly through the array without leaving a trace, but occasionally one will collide with a nucleus and create a charged particle that emits light as it moves through the ice. The IceCube team compared 13 months of their data (collected when the array was half finished) to observations of 117 GRBs measured independently over the same time period. Contrary to expectations, no high-energy neutrinos were detected within a half-hour of each GRB. Theorists may need to rethink their models of GRBs, as well as look for other possible sources for the highest energy cosmic rays. – Michael Schirber

Superfluid Exists in the core of a neutron star

This is a wonderful discovery. A neutron star 11,000 light years far from us was observed cooling down extrodinarily fast (4% in the past 10 years), which has been deciphered as a signature of the existence of such superfluid. The pressure exerted by gravity on the core of a neutron star is huge and neutrons, which are fermions, were predicted to form boson-like pairs under such high pressure circumstances. Such pairs condense and move coherently and make a superfluid, which is frictionless. More neutrinoes shall be released, and hence more energy shall be taken away, resulting in the faster cooling rate, explain these authors. [http://www.wired.com/wiredscience/2011/02/superfluid-neutron-star/]

New experiment alluding to the matter antimatter asymmetry

In a recent post, I mentioned an experiment measuring the production rates of muons and anti-muons in nuclear collisions and the results shows CP violation. Here is a new one on neutrino and anti-neutrino, which is also argued to display this violation.

Since the 1960s, physicists have been gathering evidence that neutrinos can switch, or oscillate, between three different flavors — muon, electron and tau, each of which has a different mass. However, they have not yet been able to rule out the possibility that more types of neutrino might exist.

In an effort to help nail down the number of neutrinos, MiniBooNE physicists send beams of neutrinos or antineutrinos down a 500-meter tunnel, at the end of which sits a 250,000-gallon tank of mineral oil. When neutrinos or antineutrinos collide with a carbon atom in the mineral oil, the energy traces left behind allow physicists to identify what flavor of neutrino took part in the collision. Neutrinos, which have no charge, rarely interact with other matter, so such collisions are rare.

MiniBooNE was set up in 2002 to confirm or refute a controversial finding from an experiment at the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Laboratory. In 1990, the LSND reported that a higher-than-expected number of antineutrinos appeared to be oscillating over relatively short distances, which suggests the existence of a fourth type of neutrino, known as a “sterile” neutrino.

In 2007, MiniBooNE researchers announced that their neutrino experiments did not produce oscillations similar to those seen at LSND. At the time, they assumed the same would hold true for antineutrinos. “In 2007, I would have told you that you can pretty much rule out LSND,” says MIT physics professor Janet Conrad, a member of the MiniBooNE collaboration and an author of the new paper.

MiniBooNE then switched to antineutrino mode and collected data for the next three years. The research team didn’t look at all of the data until earlier this year, when they were shocked to find more oscillations than would be expected from only three neutrino flavors — the same result as LSND.

Already, theoretical physicists are posting papers online with theories to account for the new results. However, “there’s no clear and immediate explanation,” says Karsten Heeger, a neutrino physicist at the University of Wisconsin. “To nail it down, we need more data from MiniBooNE, and then we need to experimentally test it in a different way.”

The MiniBooNE team plans to collect antineutrino data for another 18 months. Conrad also hopes to launch a new experiment that would use a cyclotron, a type of particle accelerator in which particles travel in a circle instead of a straight line, to help confirm or refute the MiniBooNE results.

Upper bound on neutrino mass ; friction

(1) This new bound is now estimated to be of 0.28ev, summed over all three neutrino types.
The estimate is based on the data of cosmic microwave background. The reasoning should be very interesting (I have not gone through the original paper yet).

(2)As this study suggests, the sources of friction are still an open question.

Neutrino helps shielding seismic waves

This experiment is said to be a significant step toward a better understanding of the chemical makeup inside the earth. Glad to know that, knowledge of neutrino can be useful even in predicting seismic events, which recently happened frequently.