The conference has been extremely interesting thanks to the numerous experimental talks with news and hints on different topics. Among the others, however, probably the most discussed one has been the “revival” of sterile neutrinos. Indeed in the past edition of this conference, the MiniBooNE results seemed to disagree with the LSND anomaly. After that something happened (more statistics, better analyses, new values of the reactor fluxes) and suddenly the hypothesis of the sterile neutrinos finds several supporters.

HOWEVER… How strong are the indications?

Let’s start discussing the MiniBooNE results. A strong indication  of a new oscillation is in the peak of signal in the anti-ν_e appearance. BUT…: there are several points at low energy that have not been identified yet. This generates some doubts and therefore it is necessary to clarify this issue.

Furthermore, moving to the new measurements of the reactor fluxes, he systematic errors have not been discussed and they could be even larger than the estimated!!! Remember that these new values had a relevant impact on the results presented during the conference.

Taking in consideration the Gallium Anomaly, it should be remembered that it depends on the assumed cross section! Moreover, it does not really agree with the reactor angle: while the oscillation frequencies are very close to each other, the angle does not.

Not only terrestrial experiments open the possibility at sterile neutrinos, but they are allowed (and also supported) by Cosmology and the strong constrain come from BBN, N_s < 1.2 (@95% CL).

ON THE OTHER HAND… what does theory say? They are not necessary at all. Moreover, the related phenomenology makes the description of the lepton sector more complicated. But, maybe, they could be a remnant of some hidden sector and in this case it would be a great discovery!!

In this conference, we had the opportunity to listen to the results of two independent analyses, by Giunti and Schwetz. Both of them agree on the fact that 3+1 scheme is disfavoured (no CPV and tension between appearance and disappearance). The 3+2 scheme and the 3+1+NSI could be slightly better. In the 3+2 case, the would be two eV neutrinos.

The scenario arising from experiments and phenomenological fit is quite complicated and deserves a better analysis and further investigations. Carlo Rubbia presented a proposal for a new experiment at CERN, that would be “a dream experiment for sterile neutrinos” BUT… do we really need new experiments? Maybe no, or at least now they are badly needed. Indeed MiniBooNE will present new results in the summer.

Continuing discussing this topic, it has been opened the possibility that CPV could be responsible for some of the tensions, BUT… the difference between ν’s and anti-ν’s reported by MINOS could probably go away! On the contrary, it is not supported by SK, that gets equal neutrino and antineutrino mixing favored by the data.

By the way, discussing SK, it is impressive the precision that they reached in the last years (τ/B > 1.9×10^33)

=========> the 3ν standard scenario continues to be the reference framework.

In this scenario, there are still several open questions. In primes, the determination of the reactor angle: nowadays it seems that a positive value is suggested, but it depends on the treatment of the SBL reactor data. Thus we need to do some further progress in this direction.

André Rubbia presented the first T2K results at this conference and they appear extremely interesting and encouraging, but the T2K running was interrupted by the earthquake. “We hope it can be repaired fast and that the situation is Japan will improve very soon for all the population”.

However, in 3 years there should be a great improvement on the reactor angle

More questions, however, need attention:
_the absolute scale of neutrino mass
_shift from maximality of the atmospheric angle
_the sign of the atmospheric mass squared difference
_the CPV in the neutrino oscillations
_the Majorana/Dirac nature of neutrinos

A dedicated comment should be reserved to this last topic and to the 0ν2β-decay. A lot of effort is devoted to find a signal of 0ν2β (and a lot of it in the Italian territory) and a detection would be a proof of L non conservation.

This is of course linked to the very promising leptogenesis, that could give informations on the high energy parameters. While an equally important non conservation, that one of the leptonic flavour, is under investigation with very nice results at MEG. Also from this side, we expect interesting and decisive results very soon.

LFV would be a strong evidences of new physic and a complementary indication could come from Dark Matter searches at LHC

Up to now, there are important lessons on neutrino masses and mixing:
_neutrinos are massive (at least two)
_probably the masses are small  because they are Majorana
_their masses are inversely proportional to the large scale of Lepton number violation
_L violation scale could be linked to the mass of the RH neutrinos, close to 10^(14-15) GeV, suggesting SUSY GUT scenarios
_decays of RH neutrinos with CP&L violation can produce B-L asymmetry and therefore can explain baryogenesis
_detecting 0ν2β would prove the Majorana nature of neutrinos and L non conservation.

BUT… neutrinos are not a significant component of DM
BUT… different classes of models are still possible

It is an interesting remark that the Cabibbo angle is a common parameter arising both in the quark and in the lepton sectors. Where? Well, the ratio among the solar and the atmospheric mass squared differences is defined as the parameter r∼1/30 and its square root is indeed very close to the Cabbibo angle:
for a hierarchical spectrum

This suggests the same hierarchy parameters for quarks, leptons and neutrinos. This is connected with the possible approaches, that have been implemented in the last 10 years:
-Tri-Bimaximal mixing, or similar mixing patterns, as a well-defined starting point
-Lepton-Quark complementarity as indication of a common origin among lepton and quark mixings.

There are two neutrino mixing patterns that well approximate the experimental values of the mixings: the first one is the very well-known Tri-Bimaximal scheme and the second is a new income, the Golden Ration pattern.

There has been a lot of activity in trying to reproduce these neutrino patterns in the context of flavour models, in which global discrete non-Abelian symmetries are added to the gauge group of the SM.
There is a vast literature dealing with the TB patter, but only few papers on the GR one. The common feature of models, dealing with these patterns, is that the corrections that usually arise are quite small, let’s say Cabibbo^2. The general result is that the reactor angle is given of this order of magnitude and therefore very small, below the future expected sensitivity.

The alternative approach, considering quark-lepton complementary, suggested instead that a very large value of the solar angle, even if in completely disagreement with the data, could be a reasonable starting point if the corrections are large, let’s say Cabibbo. This scenario corresponds to a third interesting mixing pattern called Bimaximal scheme. In the few models constructed with this pattern, the reactor angle arises very close to its present upper bound and therefore it will be tested very soon in the experiments.

A third possibility could be the correct description of nature: maybe all these “discrete” mixing patters are accidents and then anarchical approach, lopsided model, continuous flavour symmetries, could provide better descriptions.

In all this discussion, however, we cannot forget quarks. Indeed while neutrinos seem to prefer a discrete symmetry, quarks can be easily and elegantly described by a continuous symmetry. This is a non-trivial discussion, but any interesting answer or even any illuminating strategy has appeared.

Let’s summarize:
_neutrino mixing angles are large except the reactor, that is small
_the experimental values are compatible with “discrete” schemes as Tri-Bimaximal, Golden Ratio or Bimaximal patterns.
_Maybe this points to discrete symmetries.
_In principle, there is no contradiction between large neutrino mixing angles and small quark ones
_Quarks, however, seem to not support discrete flavory symmetries
_Natural GUT’s describing quarks and leptons with “discrete” schemes are difficult to construct, in particulate for SO(10)

I conclude reporting the last slide of the talk and I add my personal acknowledgements to all the other organizers of the conference.

(posted by Luca Merlo)

The collaboration is composed by 150 scientists and engineers from 31 institutes in 7 European countries. The detector is located 40 km south of the Mediterranean coast of southern France, off Toulon. At that location they have an eccellent view of the center of our galaxy by looking at upward-going neutrinos.

In the sea, the angular resolution is better than in ice, and can reach 0.3 to 0.1 degrees for the direction of a shower. The detector consists of 12 lines of photomultiplier tubes, 885 in each. The last line of PMT detectors was installed in May 2008. Recently a new junction box was connected to the infrastructure: this is dedicated to sea science projects.

At this point Coyle made a sort of Freudian slip when he showed a graph with green and black or red dots, each representing a different PMT. He said “90% of them are STILL… 90% of them are giving data”. It begged the question,  what is lifetime of PMTs in Antares ? However unfortunately I could not ask it, since there was very little time for question after the talk.

The speaker then explained that a difficulty for the experiment with respect to ice arrays is that of course the detector is not stable: it moves around in sea currents; since however they need to know where it is with high accuracy to avoid spoiling the angular resolution,  they have a positioning system which every 2 minutes measures inclination and rotation of hte arrays.

The device can measure attenuation length precisely with multi-wavelength beacons that fire signals which the arrays record.

Track reconstruction is performed with a maximum likelihood fit to the muon hypothesis. Neutrinos and muons are distinguished by a “quality parameter” of the fit. One isolates the upward going neutrinos by reconstructing the cos(theta) of the track arrival.

Coyle then showed a nice histogram of counts acquired by looking in the direction of the Moon. Our satellite of course shadows downward-going muons, since cosmics are stopped by it. The graph showed a nice dip, underlying that their angular resolution is indeed what they claim. Unfortunately his talk is not online yet, so I cannot attach the relevant plot… Similarly, the angular resolution for upward going neutrinos is of about 0.5 degrees.

He then showed a nice sky map from a live time of 295 days. Events plotted were constituted for 60% by atmospheric neutrinos, for 40% by downgoing muons reconstructed as upgoing.

They did a candidate list search, with a list of 24 likely places in the sky from where one might observe neutrinos. They calculated the probability of the observed events from those sources. The most significant candidate is a galactic microquasar, whose probability of being generated by background sources alone is 6.8%, so not significant. Still interesting to watch.

Antares does not see any significant sources, so they set a limit on the flux at 10^-7 GeV cm^-2 s^-1, integrated in solid angle. However, they have not used an energy estimator in the analysis yet. Significant improvements in limit will be obtained once that is used.

They are also doing GRB-triggered searches: alerts from SWIFT satellite and from Fermi. >1300 have been recorded to Jan 2011. No signal is observed.

They can also do the opposite: if they see two neutrinos within 15 minutes and 3 degrees they send an alert within 10 seconds, to telescopes which look at 1.9×1.9 degrees of sky around the potential source. They use two 25 cm “Tarot” telescopes in France, plus another system called Rotse, four 45cm telescopes.

Coyle concluded his presentation by remarking that Antares is a major step in the direction of constructing a multi-km^3 array in deep sea. Let us hope we will see that built soon!

• Opportunities of Gallium experiments with artificial neutrino sources for investigation of transition to sterile states (Valery Gorbachev)
• From paired super-radiance to neutrino mass spectroscopy using atoms (N. Sasao)
• LUCIFER: A Scintillating Bolometer Array for the Search of Neutrinoless Double Beta Decay (Laura Cardani )
• Improving the KM3NeT sensitivity (Oleg Kalekin)
• Uncovering Multiple CP-Nonconserving Mechanisms of Neutrinoless Double Beta Decay (Aurora Meroni)
• SNO+ experiment as a Neutrino Telescope (Takashi Iida)
• Leptogenesis in a scenario inspired to SO(10) with a compact spectrum for the right-handed neutrinos (Franco Buccella)
• Effects of Resonant Spin-Flavor Conversion of Supernova Neutrinos in Supernova Neutrino Event (Takashi Ioshida)
• Remarks on the forces generated by two-neutrino exchange (Silvano Petrarca)
• A road to reach higher precision in Borexino: the detector calibration campaigns (Alessandra Carlotta Re)
• A T’ flavour model for fermions and its phenomenology (Luca Merlo)
• Beta spectroscopy with superconducting calorimeters for the direct measurement of the neutrino mass (Daniela Bagliani)
• Monte Carlo simulation study of the muon-induced neutron flux in LNGS (Rino Persiani)
• Neutrino Mass Hierarchy Determination using Reactor Antineutrinos (Pomita Ghoshal)
• The 14N(p,g)15O reaction and the metallicity of the Sun (Antonio Caciolli)
• A new design for the CERN-Frejus neutrino Super Beam (Andrea Longhin)
• The MEMPHYS Project (Alessandra Tonazzo)
• Discovery potential of a future very large underwater neutrino telescope in the Mediterranean Sea (Apostolos Tsirigotis)
• Measurements of neutron spectra characteristics using NaCl in LVD (Irina Shakiryanova)
• Analysis of the temperature variations of neutrons generated by muons in the detector LVD (Olga Ryazhskaya)
• The T2K TPC calibration tools and relation to physics results (Panagiotis Stamoulis)
• Status and performance of ND280, the T2K near detectors (Jeffrey Wilkes)
• Efforts towards a measurement of charged current quasi-elastic neutrino interactions in ND280 (Lorena Escudero)
• Study of Neutrino Interactions Using the Electronic Detectors and Emulsion-Lead Targets of the OPERA Experiment (Kose Umut)
• The Muon Ionization Cooling Experiment (Pierrick Hanlet)

We’ll see about that. I am personally not an expert of neutrino physics, and my field of research is a different one (I work for the CMS experiment at CERN). My colleague bloggers here are more into the matter, and it will be their decision what to do with this successful but young endeavour.

Anyway, there is a lot more blogging to do yet. Let me list here the talks we will hear today here at NEUTEL:

• Physics with Neutrino Telescopes (Teresa Montaruli)
• Predicting Galactic Neutrino Fluxes from Gamma Ray Data (Felix Aharonian)
• Diffuse Neutrino Fluxes (Juergen Brunner)
• Cosmogenic Neutrinos (Subir Sarkar)
• IceCube: Status and Developments (Tom Gaisser)
• What do we know about the Origin of Cosmic Rays? (Paolo Lipari)
• Results from the AUGER and other UEHCR experiments (Sergio Navas)
• Antares (Paschal Coyle)
• KM3NeT  (Piera Sapienza)
• Perspectives in Neutrino Physics (Guido Altarelli)

So stay tuned!

The existing data between 1 and 20 GeV is poorly understood. It is mainly coming from bubble chamber data, wide band neutrino beams, low statistics samples, with large systematic errors from large uncertainties in the fluxes.

We care about cross sections for precise neutrino oscillation determinations. In oscillation experiments you need dm^2 L/E_beam = 1 to maximize the oscillation effect. With delta m^2 in the range of 2.4E-3 eV^2 and L of hundreds of kilometers, the beam energy comes out in the few GeV range. So we need precise cross sections there.

Disappearance oscillation measurements look at a distortion in the neutrino energy spectrum for E<5 GeV. The experiments however measure the visible energy, which depends on the flux, the cross section, and the detector response. A dependence is also clear on the particle multiplicities and particle type produced. The final state interactions do matter for these measurements, too.

For appearance oscillation measurements you try to measure theta_13, looking for electron neutrinos in muon neutrino beams.There are backgrounds from neutral pions arising in neutral current interactions; the decays of neutral pions produce two gammas, and one of them can easily mimic the electron. There is an intrinsic nu_e component  in the beam to be aware of, too. It is thus critical to measure these background processes using the same nuclear targets.

MINERVA is positioned to resolve discrepancies between different experiments. NOMAD and MiniBoone show a disagreement in their cross section as a function of energy, measured in different energy range, as shown in the figure below.

The MINERVA collaboration consists of about 100 nuclear and particle physicists from 22 institutions in 7 countries. The detector is an active segmented scintillator with nuclear targets of carbon, iron and lead (helium and water targets are coming soon). Measuring the interactions of neutrinos with all targets in the same detector reduces systematic errors between them. There are 120 modules of four types: nuclear target, tracker, and E and Had calorimeters. Detectors are not magnetized, so muons are penetrating into MINOS to determine the sign of the muon.

The targets have parts made of carbon, lead, and iron in exagonal patterns. Five patterns are used. They have side “ears” with which they can be mounted on rails for easy assembly. Below are shown the different kinds of exagons making up the metal target.

The tracker modules are composed of an inner detector made of two layers of scintillator bars. Surrounding them there’s a “side” electromagnetic calorimeter, sampling the energy as it exits the detector.

The scintillating elements contain WLS fibers reading them out from within a central hole. These allow to get a very nice position resolution of 2.65 mm. There are 40 thousand such bars.

The detector has good tracking resolution, calorimetry, timing information within few nanoseconds, containment of events except muon measured in Minos. Particle ID is there, too.

Understanding the flux is extremely important. The beamline has a lot of instrumentation. To maximize the flux the target and the horns are movable. This can be used to tune the neutrino energy. They can thus select neutrino or antineutrino beams of various energies.

They started data taking in 11/2009 with 0.8E20 POT at low energy and collecting antineutrinos, when only 55% of the detector was built. Then, in 2/2010 they installed the rest of the detector, and then ran a low energy neutrino beam, gathering interactions from 1.2E20 POTs. Now they are running since November 2010 in low-energy antineutrino mode, so far 1.2E20 POTs again. In the spring 2011 they will start again with neutrinos. In the summer of 2012 the Fermialb accelerator will be shut down, and switch to medium energy.

Cross section errors: statistical errors are not a problem, the absolute cross sections are  dominated by systematics. There are typical 30% errors on these fluxes, but the goal of Minerva is a 7% erro in the flux shape, and 10% in its normalization. They will normalize to the narrow band beam at CCFR at their high energy end. The event spectrum will be measured with quasi-elastic events.

The comparison of muon anti-neutrino data in Q^2 with Monte Carlo predictions (GENIE and GEANT) shows that the data undershoot the prediction (see right). They are trying to understand it in more detail now.

In conclusion, they are studying in precision the neutrino interactions in the low energy range (1-20 GeV), using a fine grained high resolution detector at the high-flux NuMI beam. They plan to improve our knowledge of neutrino cross sections at low Q^2, and their target dependence. This will reduce the systematics of many oscillation experiments.

The basic concept is that the baryon asymmetry in the universe can be originated through a dynamical mechanism: a lepton asymmetry is first generated and after translated into baryon asymmetry through non-perturbative effects.

The minimal scenario is the type I See-Saw in which three RH neutrinos are responsible for the smallness of the light neutrino masses. In the standard type I See-Saw, the mass of these RH neutrinos should be very large, more than 10^9 GeV. It is obvious that such energies cannot be reproduce at our experiments and therefore direct detection of RH neutrinos is excluded. However, it could be possible to get indirect bounds. Let’s investigate on this possibility.

The neutrino Lagrangian is given by

and we are interested in the possibility of constraining mD and M. In this 3×3 matrices there are some non-physical parameters and therefore it is convenient to move to a basis with only physical parameters: the Casas-Ibarra parametrization helps,

Counting the number of parameters, we have 2×3 masses for the light and heavy neutrinos and 2×6 angles and phases for the matrices U and Ω.
On the other hand, neutrino experiments can give infos only on 9 parameters, the low energy ones.
—> Leptogenesis is important to get infos on the high energy parameters

Let’s enter more in details. The simplest description – Vanilla Leptogenesis – is based in a series of approximations:

1) neglecting in the decay of the RH neutrinos the flavour composition of final leptons, but just considering different rates for decays in leptons and in anti-leptons.

The difference among the two Γ’s is proportional to the parameter ε, that controls the total CP asymmetry. If ε is non-vanishing then lepton asymmetry is generated and partly converted into baryon asymmetry, ηB, by sphaleron processes, if the reheating temperature is higher than 100 GeV.
The final result depends also on the number of RH neutrinos that are out-of equilibrium at the moment of the decay. The out-of-equilibrium condition is necessary otherwise any contribution would get cancelled.

Concentrating on the origin of the ε parameter, it is computed by the interference of tree level and loops:

Notice that in the analytical expression appears the product mD^† mD, in green, that does not depend on the PMNS matrix. This is the link between the leptogenesis and the high energy parameters of the type I See-Saw.

2) There is a strong hierarchy among the RH neutrinos

3) The heaviest RH neutrino does not interfere with the decays of N2, the second heaviest state. In this case, the CP asymmetry of the lightest RH neutrino is dominant wrt the other ε and therefore we are discussing a N1-dominanted scenario.

4) Fine-tuned mass cancellation are avoided. This translates in an upper bound on ε1 in terms of the light neutrino masses and M1

5) The classical kinetic equations are integrated on momenta.

Under all these assumptions, it is possible to identify:
——–> upper bound on the light LH neutrino mass
——–> lower bound on the lighter heavy RH neutrino

An interesting observation is related to the wash-out: simply speaking, the wash-out removes all the pre-existing asymmetries and therefore it removes the dependence from any, even strange, physics at higher temperatures that could contribute to the baryon asymmetry, in addition to leptogenesis.

This simplified scenario can be improved in several different ways:

Pasquale concentrated only on few of these possibilities. Regarding the first approximation, that one on the flavour of the final leptons in the RH neutrino decays, it is well justified only for RH neutrino masses larger than 10^12 GeV. Otherwise, the τ-Yukawa interactions are fast enough to break the coherent evolution of the lepton states (this means that the evolving lepton states become a mixture of a τ and of μ+e). If we go further below in energy, if M1<10^9 GeV then also the μ-Yukawas are in equilibrium and therefore we get a 3-flavour regime.

In this way, the contribution to the baryon asymmetry accounts for three different contributions, one for each final lepton flavour.

The consequences of considering flavour are in the bounds of the Vanilla Leptogenesis: they get relaxed and now they DO depend on the PMNS phases.

It is possible to distinguish 10 different RH neutrino mass patterns, related to the energy at which the different lepton Yukawas enter the equilibrium. Pasquale continued with a detailed description on a heavy flavored scenario, with a lot of very nice plots and discussions. I encourage you to take a look of the talk in the webpage of the conference.

I just conclude this brief summary with the main message of the talk:
Leptogenesis is a complementary tool to low energy neutrino experiments in order to investigate the neutrino parameter space.
However, this interplay is not sufficient to over-constrain the See-Saw parameter space and it is necessary to either look for additional phenomenologies, such as LFV, or restrict the parameter space with additional assumptions, such as in BSM frameworks like GUT’s.

Today is almost over, but tomorrow I will be here again for the last day of the conference. Stay tuned!

(posted by Luca Merlo)

Moving backwards prom parties to physics, and trying to draw a partial conclusion from what we heard up to now, I would say that the most longed-for experimental results (in primis XENON100 but also MEG) didn’t come, but one issue definitely emerged, i.e. the lack of knowledge of neutrino interactions on short-baselines both at reactors and accelerators. Actually in these days many anomalous effects have been presented, by T. Lasserre and G. Mills, possibly hinting at a 4th sterile neutrino state (T. Schwetz and C. Giunti), so that the idea of a definite experiment, presented this morning by C. Rubbia, is really attractive.

As already mentioned in a previous post, the idea is to put 2 identical LAr-TPC detectors at a refurbised CERN-PS muon neutrino beam.

Layout of near and far detectors for a future sterile neutrino search experiments at a refurbished nu-mu beam at CERN-PS

The idea behind this proposal is based on the search for spectral differences of electron like specific signatures in two identical LAr-TPC detectors but at two different distances: a 150t detector, to be build anew, 127m far from target and a 600t detector, possibly ICARUST600 moved from LNGS to CERN after the end ofoperations, 850m far from target.

LAr-TPC technique has demonstrated to work properly with ICARUS first operations; the other key-feature for the success of such an experiment is the identity of the nu-e spectra in Near and Far positions (due to the fact that electrons are produced exclusively by the K-decays with a much wider angular distribution). Sensitivities of such an experiment are very promising in the look for both the disappearance “reactor” anomaly and in the appearance LSND/MiniBooNE signal.

Sensitivities of the proposed experiment at CERN-PS: to disappearance anomalies at 90% C.L. (left) in 2 years data taking; to the appearance signal in 2 years data taking with neutrino beam (middle) and 4 years data taking with anti-neutrino beam (right).

Rubbia ended up by underlying the need for “fresh manpower”: a new Collaboration should be built up in order to start an experiment like that.

This is going to be a change of paradigma, and it’s one of the highlights of
the conference.

Furthermore the model with two sterile neutrinos is definitely better than the model with just one. More steriles or other exotic ingredients like non standard neutrino interactions or CPT violation seem to be no more necessary.

What happened to generate such a change? The main ingredient has been provided by Thierry Lasserre and his collaborators (see the dedicated topic). With an impressive effort of some years they re-computed from scratch the detailed flux of reactor antineutrinos. They ended up with an estimated antineutrino flux about 3% higher than previous estimates. This new computation transformed the several reactor antineutrino oscillation experiments at short baselines, performed in the 80′s, from evidences against steriles (no deficit observed of the expected antineutrino flux) to evidences in favor of sterile neutrinos (3% deficit of the expected flux).

Piece by piece the evidences in favor of sterile neutrinos are not that great (they are considered anomalies rather than evidences), but the overall picture now favors sterile neutrinos. These ideas are also further elaborated by C. Giunti in a talk of  Thursday morning (see transcript by L.Merlo below). Future experimental  programs needed to provide better information on sterile neutrinos have been discussed by Geoff Mills and Carlo Rubbia again on Thursday morning. Reports on those talks are coming soon.

(Posted by M.Mezzetto)

To read about standard 3-ν oscillations just look at http://neutel11.wordpress.com/2011/03/15/overview-on-neutrino-phenomenology/ while here I am entering the most exotic topic of extra sterile neutrinos.

Sterile neutrinos have been theorized by Bruno Pontecorvo long time ago (as fairly remembered by Carlo Rubbia). Only more recently we thought that maybe he could be right and precisely after LSND data. LSND looks at anti-ν_μ -> anti-ν_e conversion and it could be understood in terms of a third oscillation frequency: Δm^2_LSND ≥ 0.2 eV^2 (that is extremely larger than the other two).

MiniBooNE experiment was expected to say something on this anomaly, working at larger distance and energy. There is an agreement between the two sets of data:

The scenario that arises at this point consists in the oscillation between the 3 active neutrinos and new states, where the mixings with these new states are very very small:

It is not the first time that we think at more than 3 neutrinos: the type I See-Saw mechanism indeed introduces several RH neutrinos. However, these states are extremely heavy (in general more than 10^10 GeV) and cannot be our new neutrinos.

What is defined as STERILE neutrino is a LIGHT ANTI-RH neutrino (notice that it is a LH state), with no standard model interactions. Active neutrinos can oscillate into sterile neutrinos and what should be looked for are 1) Disappearance of active neutrinos and 2) Indirect evidence through combined fit of data.

The subsequent question is how many sterile neutrinos are present?
We know from LEP that the Z decay into invisible tells information only on active neutrinos. On the other side, Cosmology can put constraints only on thermalized sterile neutrinos and therefore it depends on the type of interactions that were active at the time of the decoupling. From CMB and LSS there is a bound on the number of thermalized sterile neutrinos and on the sum of their masses:

At 95% CL, N_s=1.61+- 0.92 with a bound on the masses of m < 0.70 eV.

Assuming a scheme with 3 active neutrinos and only 1 sterile neutrino (3+1 scheme) it is possible to discuss SBL oscillation probabilities. It is interesting to note that in this scenario NO CP VIOLATION is allowed. This is interesting because of a tension between LSND+KARMEN+ MiniBooNE anti-ν_μ -> anti-ν_e data and MiniBooNE ν_μ -> ν_e data, suggesting CP Violation.

On the other hand, in schemes with 3 active neutrinos and either 2 sterile ones (3+2 scheme) or 1 sterile neutrino and non standard interactions (3+1+NSI), CPV is allowed (See the talk by Thomas Schwetz).

Furthermore, considering experiments on ν_e disappearance, ν_μ disappearance and ν_μ -> ν_e conversion, it is possible to put strong constraints on sin^2(2θ_eμ): the green line below!!

This underline a strong tension between anti-ν_μ -> anti-ν_e appearance and disappearance limits: that with the new reactor fluxes appears as

This tension, however is reduced when considering 3+2 or 3+1+NSI schemes.

An alternative is that maybe this is an indication that CPT is violated. To read about a brief exchange of ideas on it between Altarelli and Rubbia, just click on http://neutel11.wordpress.com/2011/03/16/titans-cross-swords-on-cpt-violation/

This CPT violation has already been proposed also during the presentation of MINOS results and by the yesterday paper by CDF collaboration in arXiv:1103.2782 [hep-ex].

At this point, one can think at considering ONLY the data from antineutrino oscillations (because without CPT these should be independent from neutrino oscillation data) and perform a fit of the data on the 3+1 scheme. The results are interesting, predicting a large SBL anti-ν_μ disappearance at 0.1<Δm^2<1eV.

An other complementary indication to the anomaly in LSND could come from the Gallium Anomaly (SBL ν_e disappearance) that shows a tension with the Reactor data (anti-ν_e disappearance). A combined fit is in the picture:

In this scenario there are two very welcome predictions for β-decay and 0ν2β-decay:
mβ∼0.14-1.63 eV
mββ∼0.02-0.35 eV

A work in progress is the interesting combined analysis on LSND and MiniBooNE + Gallium and Reactor Anomalies, thus stay tuned (to arXiv)…

(posted by Luca Merlo)

The workshop will be held at Fermilab from May 12th to May 14th. There is an indico agenda page already set up here.

The program includes:

• Experimental short baseline neutrino data
• Theoretical interpretation of short-baseline neutrino data
• Future neutrino facilities
• Future short-baseline experiments.

So mark your agendas if you plan to participate!

UPDATE: the link above does not work from here, strangely enough (it is correct). Here are alternative instructions to reach the site: start from http://www.aps.org/units/dpb/news/shortbaseline.cfm and then follow the link there to indico.fnal.gov.