Tuesday, March 12, 2013

2013 Neutrino Telescope Blog

A conference blog is covering the latest news from this month's 2013 Neutrino Telescope conference in Venice (NeuTel3), mostly written by Tommaso Dorigo of the Quantum Diaries Survivor blog, is full of the latest news is neutrino physics.  So far, there have been ten meaty posts at the blog today covering all of the presentations so far except RENO.  There are three more days of conference presentations after today.  The Conference website is here.

As Peter Woit notes "if as is looking all too possible, the LHC finds no new physics besides the Higgs, neutrino experiments may be where attention focuses in the future as the best hope for this."

The big stories are (1) that PMNS parameters are receiving increasingly precise measurements; (2) that SuperK data combined with global fits tends to favor a 1 pi value of the CP violation parameter in the PMNS matrix rather than a zero value, and (3) that 3 sigma to 3.8 sigma signals at LSDN and MiniBooNE that seemed to suggest a light sterile neutrino can only be consistent with many other experments seeing no such signal is the sterile neutrino has a mass of about 0.7 eV and an extremely tiny mixing angle with the other neutrino types - a type of sterile neutrino that is disfavored by the cosmology data.

Detailed highlights of each of the presentation based on the blog posts and conference materials appear below the jump.  The RENO presentation materials were not available at the time this post was written.

Meanwhile, a presentation at Moriond today rules out BSM physics of certain kinds due to B meson decay results at the LHC.  Deviations from SM predictions on the order of more than 50% are largely ruled out. MSSM with a light pseudoscalar Higgs boson and a high tan beta is ruled out.  And, flavor changing Z penguins which are typical for models with partial compositeness (composite Higgs, RS, etc.) are now starting to become possible to probe and distinguish experimentally. 

Other BSM phenomena excluded to higher energy scales are summarized here.  Contact forces of various kinds are excluded in various scenarios at the 7 TeV to 14 TeV energy scale.  Extra dimensions, composite quarks and leptons, excited fermions, W' and Z' bosons, vector-like quarks, and quantum black holes are excluded to the TeV to 5 TeV scale depending on the particular exotic particle or phenomena searched for at the LHC.  Fourth generation fermions, scalar lepto-quarks, techni-hadrons, and certain kinds of SUSY Higgs bosons (e.g. charged Higgs bosons) are excluded at levels ranging from about 375 GeV to 675 GeV (see also this SUSY Higgs search).  Lightest SUSY particle exclusions and exclusions of particles with irregular electromagnetic charges are excluded at masses in the low hundreds of GeVs.  The exclusions are for direct particle searches at LHC only and don't account for other more indirect evidence that also disfavor these exotic phenomena (e.g. the evidence against Technicolor theories flowing from the existence of a 125 GeV-ish Higgs boson).


Fundamental Neutrino Property Measurements

A consensus and new level of precision is emerging in the measurements of the parameters of the PMNS matrix and mass differences of the neutrino types.  Progress is being made towards pinning down a CP violation parameter in the PMNS matrix (that data seem to favor a non-zero value slightly) and towards determining if the neutrino mass hierarchy is normal or inverted (which is necessary to pin down the value of theta23 between two possibilities).

Daya Bay: "Daya Bay is a reactor neutrino experiment designed to measure the oscillation parameter sin^2 2theta_13 to a precision of 0.01 at 90% confidence level. . . . The result now is of 0.089 +- 0.010 +- 0.005 for sin^2 2 theta_13."

Thus, the one sigma confidence interval for sin 2 theta 13 is a bit narrower than 0.074-0.104.

Double Chooz: "Their result is sin^2(2 theta_13) = 0.109 +-0.040. Systematics are not the dominant source of error (0.03 stat, 0.025 syst) . . . With the near detector in three years they can shrink the measurement errors by a factor of three. This assumes that the error on the detection goes down from 1 to 0.2%, and that the background rejection can reduce the related systematics by a half. He mentioned that for theta_13 determinations reactor experiments will dominate the proceedings for a long time, mainly because of the multi-detector technology."

Thus, the one sigma confidence inteval for sin 2 theta 12 is 0.069-0.149, which is a bit wider than the Daya Bay result.  The central value of the Double Choose measurement is a bit less than 1.3 sigma away from the Daya Bay central result - so given the uncertainty in the results, the results are consistent with each other. 

KamLand: "By combining solar results information and results of theta_13 experiments, they get a measurement of dm_21^2 with a 2.3% precision: (7.53+-0.18) x 10^-5 eV^2). Without external constraints from theta_13 the result does not vary much: 7.50 +- 0.20 x 10^-5 eV^2."

We may not know the absolute mass of the three neutrino mass eigenstates, but we know the magnitude of the difference between the first and the second state with great precision.

This experiment has a variety of projects going on at once. It has also confirmed leading models of neutrino generation from radioactive elements inside the Earth and is engaged in a neutrinoless double beta decay search.

Borexino: "Borexino is a detector in the Gran Sasso mine built to study the low-energy spectrum of neutrinos from the Sun, up to the 7Be and pep neutrinos, and 8B neutrnios up to 3 MeV energy. . . . from an astrophysical point of view one wants to test the models for high- or low-metallicity options of solar models. The second reason is connected to neutrino oscillations: for solar neutrinos, the parametrization is via the 1-2 mixing, for which there is a squared mass difference of 7.54+-0.26 eV^2. There is a shape of the neutrino survival probability as a function of energy which can be investigated in the MeV range. . . . They are also studying geo-neutrinos. . . . The talk then focused on the evidence of a seasonal modulation of neutrinos from 7Be line. . . . there is a 5-sigma evidence of the oscillation."

The geoneutrino measurements like those of KamLand confirm leading models of neutrino generation from radioactive elements inside the Earth.

MINOS: Mass gap from 2 to 3 is 2.41 +0.09 -0.10 * 10^-3 eV^2.  Sin 2theta is 0.950 +0.035-0.036,  consistent with Super-K and T2K.  The delta mass is about 0.09 * 10^-3 ev^2 higher and sin2theta is about 0.02 higher in a four parameter fit for anti-neutrinos only with bigger error bars that produce a result consistent with no actual difference between the neutrino and anti-neutrino values.  The mass difference discrepency is less than 0.5 sigma.  Reactor data is sin2theta13 of 0.098 +/- 0.013.  MINOS+ can almost rule out the allowed low mass LSND direct dark matter detection region.  The theta23 value of 0.4 or 0.6 depending on choice of hierarchy is the best fit.

The global fit for sin^2(2theta13) from all available neutrino experiment data is 0.0982 +/- 0.009.  Expressed as a one sigma confidence interval that is 0.089-0.107.

Electron Neutrinos: "In the last decade we have learned that the electron neutrino is a superposition of at least three massive states. The coefficients of this superposition (to the best of our current knowledge) are 0.82, 0.55, -0.16. The energy level splittings are proportional to 7.5 E-5 eV^2 (the states 1 and 2) and 2.4 E-3 eV^2 (state 3 with respect to the others). . . .
We can learn a lot from electron neutrinos, but we must ask ourselves what else we may learn with muon neutrinos too. With only electron-neutrino oscillations, one cannot probe complex coefficients, nor the distribution of muon and tau neutrino flavours, such as the angle theta_23. So one must do a global neutrino data analysis, where one sees the interplay between the different oscillation channels. It is useful to show a progression of constraints (see arxiv:1205.5254) by incorporating first the long-baseline plus solar and KamLand results, then adding short-baseline reactor experiments, and finally adding the atmospheric neutrino information.
Current data in the plane sin^2 theta_23 versus sin^2 theta_13, once all other parameters are marginalized away, show to slightly favor non-maximal mixing, with values of sin^2 theta_23 at 0.4 and 0.6. This both for the normal and inverted hierarchy. Solar and KamLand data prefer a value of sin^2 theta_13 of 0.02, so this is unable to lift the degeneracy. Adding short-baseline reactor constraints, the lower value of sin^2 theta_23 becomes slightly favoured: the first octant solution. This especially in the scenario of a normal hierarchy.
Finally if we add the atmospheric neutrino data, we end up with a marked preference for the first octant. As for the CP violating phase, delta, it is basically unconstrained. If one includes short-baseline results still delta remains free to vary. With atmospheric nu data, we get a marked preference for delta of about pi, since this helps fitting sub-GeV electron-like excess seen in SuperKamiokande.
So if we make an overall synopsis of the oscillation parameters, one sees that the less-well-constrained parameter is of course delta; as for sim^2_23, the first octant is preferred but both are allowed at three-sigma. There are however no hints for a preference in the hierarchy. As for the fractional 1-sigma accuracy in the parameters, the dm_12^2 parameter has 2.6% accuracy, while sin^2 theta_23 has a 14% accuracy. The electron neutrino can thus be described as a superposition of three states with the mentioned coefficients.

Assuming that m_2b is non-zero (Klapdor et al.), and the recent claim from the south-pole collaboration that the sum of neutrino masses is in the 0.1-0.54 eV plane, then there is a narrow region of compatibility with present data. Not excluded yet by KamLand-Zen and EXO. If we believe it, then the mass of each neutrino is expected to be slightly below 0.2 eV. This is in the reach of Planck, and of double-beta decay experiments ! So this scenario, while not totally crazy, could be proven this year. Very interestingly, since the allowed region is very small, one can really tell if the Majorana phase is positive in this scenario."
This is one of the only papers of which I am aware that suggests that the experimental evidence favors a non-zero CP violation parameter for the PMNS matrix.  Note that Klapdor's work is an outlier in neutrinoless double beta decay physics whose results are not considered credible by many others in this field.

Direct Sterile Neutrino Searches

While not entirely ruled out, the possible signal of a sterile neutrino seen at LSND and MiniBooNE has been limited to a tiny parameter space with a very low oscillation frequency and a roughly 0.7 eV mass that is basically inconsistent with cosmology requirements for dark matter particles.  This would be hot dark matter which was ruled out experimentally decades ago.

Sterile Neutrino Search with Icarus:  "Sterile neutrinos were hypothesized in a paper by Pontecorvo in 1957, as particles not interacting in any way except gravitationally. They of course are extremely hard to detect. They could be contributing to dark matter in the universe. Sterile neutrinos may also mix with ordinary neutrinos with a mass term. Evidence may be building up by anomalies observed by several experiments. The speaker showed a table of several results from LSND, MiniBooNE, Gallium experiments, and ones from reactors, which are all in the 1 to 3 standard deviation ballpark.
ICARUS, which searched for electron neutrinos in the 10-30 GeV region from the CNGS muon neutrino beam.  . . . Data taking went on in the period October 2010- December 2012. . . . they have so far collected 1091 events, and the rate is consistent within 6% with Monte Carlo expectations. The total data corresponds to a total of 0.86E20 POT. . . . The energy cut is E<30 GeV to optimize signal to noise ratio. . . . In the analyzed data they expected from normal beam contaminations 3.0+-0.4 events, plus 1.3+-0.3 from theta_13 oscillations (for sin^2(theta_13)=0.0242+-0.0026), and 0.7 from nu_mu->nu_tau oscillations. So the total is of 5.0+-0.6 events. Backgrounds are expected to amount to 3.7+-0.6 events. As for the signal selection efficiency, they estimate it at 0.74+-0.05, and confirm this measurement with alternative methods.
In the data, two candidates are observed. This observation is presently compatible with the absence of a LSND anomaly. The limits on number of events are 3.41 and 7.13 at 90% and 99% CL respectively. ICARUS should have observed as many as 30 events if the LSND signal were true. The surviving area in the plane of tan^2(theta) vs dm^2 is a tiny dot located at values of (dm^2, sin^2(2theta)) = (0.5 eV^2, 0.005). In that spot there is overall agreement within 90% CL between the limit of ICARUS, the limits of KARMEN, and the positive signals of LSND and MiniBooNE."
Thus, the ICARUS data show no evidence of sterile neutrinos, tend to contradict an apparent positive direct dark matter detection signal from the LSND experiment, and confirm the non-detection of dark matter by the KARMEN experiment.

There is apparently a small space in the parameter space which could be consistent with all of the data including a positive signal at MiniBoonNE, however.  This deserves a closer look at the presentation as that doesn't make a lot of sense to me in the summary form reported in the blog post without a larger context.

OPERA: the surviving area in the dm^2, sin^2(2theta) plane of just a dot at about 0.5 ev^2, 0.005 that is consistent with the LSDN/MiniBooNE signal and the limits of ICARUS, KARMEN and OPERA is essentially the same in as in ICARUS except tht the limitation on sin^2(2theta) is more a bit more strict and must be perhaps 0.001 to 0.002 lower than in ICARUS (eyeballing the chart).  The 90% upper limit of sin^2(2thetanew) is 0.0072.  Both ICARUS and OPERA are looking at the delta mass and theta of a new fourth kind of neutrino (e.g. a sterile neutrino that oscillates with regular neutrinos), rather than ordinary PMNS parameters.  A LSDN/MiniBooNE signal should have produced 9.4 +/- 1.3 events, but only 6 events were observed in the time period from 2008-2012, which is consistent with no signal and is 2.6 sigma below the expected LSDN/MiniBooNe signal expectation.  OPERA has for the first time ever directly observed a muon neutrino to tau neutrino oscillation (2 events from 2008-2012).  There were 19 muon neutrino to electron neutrino oscillation events at OPERA.    If there really is a valid LSND/MiniBooNE anomaly (which is present at the 3 to 3.8 sigma level) one assumes a normal hierarchy and a mass of the electron neutrino less than delta mass between the first and second neutrino mass eigenvalues, you end up with m1=almost zero, m2=7.5*10^-5 eV, m3=2.5*10^-3 and mNEW=0.7 eV, with a really tiny oscillation frequency. 

My own analysis is that basically, ICARUS and OPERA come close to contradicting LSND and MiniBooNE almost entirely.

A 0.7 eV sterile neutrino would also require that Neff be four and change rather than three and change (the current data is almost midway between them) and would produce a non-Standard Model neutrino that does not fit neatly in the three generations of fermion found in the Standard Model despite the fact that an overall four generations of fermion model is disfavored overall.  Yet, a sterile neutrino with just 0.7 eV of mass would be hot dark matter rather than warm dark matter or cold dark matter, and hence would be inconsistent with the amount of large scale structure observed in the universe.    It would also softly contradict cosmology evidence regarding the sum of the masses of all of the neutrino species combined (a much heavier sterile neutrino isn't necessarily a neutrino from a cosmology perspective).  The cosmology evidence favors a dark matter sterile neutrino that is on the order of a hundred to a thousand times as heavy of the only mass range still permitted by the combined neutrino experiment data.  A sterile neutrino signal in the appropriate warm or cold dark matter mass range, given other things we know about dark matter, cannot be squared with the combined experimental data.

So, in the showdown between ICARUS, OPERA and KARMEN on one hand that have not seen sterile neutrino signals, and LSDN/MiniBooNE that have, which is a few years of data away from being irreconcilable entirely, the non-observations are favored over the experiments that have allegedly seen signals.

Proton Decay Rate Measurements

Experimental lower bounds on proton decay rates are gradually rising and the more this happens, them more this disfavors many GUT models.

SuperKamiokande: "[O]ne of the open questions is that we must find the octant where the parameter theta_23 lies, and the mass hierarchy (the sign of dm_13^2). . . . For solar neutrinos, they have a dataset of 3904 days of operation: 57574.1 events (don’t ask me what the .1 means!!) . . . For the flux measurement of solar neutrino, they got down to a total error of 1.7%. They measure the day/night asymmetry, due to regeneration of electron neutrinos in the earth, one expects a 2-3% effect. The spectrum variation depends on dm_12. Their result is consistent with the dm_12 value and with the model. . . .  For proton decay searches, they got to 1.3E34 years in the positron-pizero decay mode, and several other limits in other final states. . . . after the discovery of nu oscillations SuperKamiokande made great progress in the study of neutrino phenomenology. After 17 years of operation, SK is stilll quite active, and will continue data taking for at least 10 more years, to study the mass hierarchy, CPV, relic neutrinos, and to look for neutrino bursts from supernovae and search for proton decay."

The Standard Model predicts that protons are stable and do not decay.  A large class of grand unified theories (GUTs), however, generically predict proton decay and the 1.3*10^34 year experimental minimum length of the half-life of a proton greatly contrains these models, for example, ruling out the simplest possible SU(5) GUT that can contain all of the particles of the Standard Model.

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