LMA Solution Research Articles

Implications of the Dark-LMA solution for neutrino mass matrices

In this work we have re-investigated two different kinds of texture zero ansatz of the low energy neutrino mass matrix in view of the Dark-Large-Mixing-Angle (DLMA) solution of the solar neutrino problem which can arise in the presence of non-standard interactions. In particular we revisit the cases of (i) one zero mass matrices when the lowest neutrino mass is zero and (ii) one zero texture with a vanishing minor. In our study we find that for most of the cases, the texture zero conditions which are allowed for the LMA solution, are also allowed for the DLMA solution. However, we found two textures belonging to the case of one zero texture with a vanishing minor where LMA solution does not give a viable solution whereas DLMA solution does. We analyze all the possible texture zero cases belonging to these two kinds of texture zero structures in detail and present correlations between different parameters. We also present the predictions for the effective neutrino mass governing neutrino-less double beta decay for the allowed textures.

On resolving the dark LMA solution at neutrino oscillation experiments

In presence of non standard interactions (NSI), the solar neutrino data is consistent with two solutions, one close to the standard LMA solution with sin2θ12 ≃ 0.31 and another with {sin}^2{theta}_{12}^Dsimeq 0.69left(=1-{sin}^2{theta}_{12}right) . The latter has been called the Dark LMA (DLMA) solution in the literature and essentially brings an octant degeneracy in the measurement of the mixing angle θ12. This θ12 octant degeneracy is hard to resolve via oscillations because of the existence of the so-called “generalised mass hierarchy degeneracy” of the neutrino mass matrix in presence of NSI. One might think that if the mass hierarchy is independently determined in a non-oscillation experiment such as neutrino-less double beta decay, one might be able to break the θ12 octant degeneracy. In this paper we study this in detail in the context of long-baseline experiments (Pμμ channel) as well as reactor experiments (Pee channel) and show that if we combine information from both long-baseline and reactor experiments we can find the correct octant and hence value of θ12. We elaborate the reasons for it and study the prospects of determining the θ12 octant using T2HK, DUNE and JUNO experiments. Of course, one would need information on the neutrino mass hierarchy as well.

A model for large non-standard interactions of neutrinos leading to the LMA-Dark solution

It is well-known that in addition to the standard LMA solution to solar anomaly, there is another solution called LMA-Dark which requires Non-Standard Interactions (NSI) with effective couplings as large as the Fermi coupling. Although this solution satisfies all the bounds from various neutrino oscillation observations and even provides a better fit to low energy solar neutrino spectrum, it is not as popular as the LMA solution mainly because no model compatible with the existing bounds has been so far constructed to give rise to this solution. We introduce a model that provides a foundation for such large NSI with strength and flavor structure required for the LMA-Dark solution. This model is based on a new U(1)′ gauge interaction with a gauge boson of mass ∼10 MeV under which quarks as well as the second and third generations of leptons are charged. We show that observable effects can appear in the spectrum of supernova and high energy cosmic neutrinos. Our model predicts a new contribution to the muon magnetic dipole moment and new rare meson decay modes.

Solar Neutrinos and the Borexino experiment

Solar neutrinos are produced in the core of the Sun in different nuclear reactions all based on the conversion of hydrogen into helium, releasing energy and making the Sun shine. Until now, the observation of solar neutrinos has demonstrated: a) the nuclear origin of the Sun's energy; b) that the ve produced were undergoing lepton flavor transformation into vμ or vτ, the neutrino oscillation mechanism. In the recent years, the Borexino experiment, in the Gran Sasso underground laboratory, has made significant contributions to the solar neutrino spectroscopy: first observation and precision measurement of the 7Be neutrinos, first observation of the pep reaction, stringent limit on CNO neutrinos, observation of 8B neutrinos with a 3 MeV threshold. These measurements reinforce the so-called LMA solution of the neutrino oscillation explaining the solar ve survival probability as a function of energy.

Novel results on low energy neutrino physics

Many progresses have been achieved in the study of low energy neutrinos from Sun and Earth. In the solar neutrinos the flux from 7Be has been measured with a total error <5% (introducing strong constraints also on the pp flux), while the day/night effect in that energy region has been determined at 1%. The 8B neutrinos have been detected with a threshold down to 3 MeV, while the solar neutrinos flux from pep reaction has been measured together with a stringent limit on CNO. These results give the experimental proof of the neutrino oscillation in vacuum and the validation of the MSW-LMA model in that region, while the day/night allows the isolation of the LMA solution by means of the solar neutrinos only, without the assumption of CPT symmetry. The evidence of the antineutrinos produced within the Earth by radioactive decays is now very robust, but more statistics is needed to clearly estimate the radiogenic contribution to the terrestrial caloric energy.

Experimental evidence of electron neutrino oscillations and validation of MSW–LMA model with Borexino

We report the real time measurements of 7Be and 8B solar neutrino fluxes performed with the Borexino experiment at the Laboratori Nazionali del Gran Sasso. The achievement of these measurements was possible thanks to the excellent levels of the radiopurity reached. The measurement of the 7Be in real time is the first direct measurements of the survival probability for solar electron neutrinos in the vacuum region. For 8B we reached a threshold energy of 3MeV which is the lowest achieved so far in real time. For the first time, the same apparatus can measure two different oscillation regions (vacuum-driven and matter-enhanced) predicted by the MSW–LMA model. Borexino also quotes the ratio between the survival probabilities, corresponding to 1.93 ± 0.75, and validates the presence of the transition region between the two oscillation regimes, according to the MSW–LMA solution. In addition, a preliminary result on the Day–Night Asymmetry (ADN) for the 7Be neutrino flux is presented and corresponds to 0.007 ± 0.073. This measurement makes Borexino able to give once more an independent confirmation of the MSW–LMA solution.

SNO+: predictions from standard solar models and resonant spin flavour precession

Time variability of the solar neutrino flux especially in the low and intermediate energy sector remains an open question and, if it exists, it is likely to be originated from the magnetic moment transition from active to light sterile neutrinos at times of intense solar activity and magnetic field. We examine the prospects for the SNO+ experiment to address this important issue and to distinguish between the two classes of solar models which are currently identified as corresponding to a high (SSM I) and a low (SSM II) heavy element abundance. We also evaluate the predictions from these two models for the Chlorine experiment event rate in the standard LMA and LMA+Resonant Spin Flavour Precession (RSFP) scenarios. It is found that after three years of SNO+ data taking, the pep flux measurement will be able to discriminate between the standard LMA and LMA+RSFP scenarios, independently of which is the correct solar model. If the LMA rate is measured, RSFP with B0 ∼ 280kG can be excluded at more than 4σ. A low rate would signal new physics, excluding all the 90% allowed range of the standard LMA solution at 3σ, and a time variability would be a strong signature of the RSFP model. The CNO fluxes are the ones for which the two SSM predictions exhibit the largest differences, so their measurement at SNO+ will be important to favour one or the other. The distinction will be clearer after LMA or RSFP are confirmed with pep, but still, a CNO measurement at the level of SSM I/LMA will disfavour SSM II at about 3 σ. We conclude that consistency between future pep and CNO flux measurements at SNO+ and Chlorine would either favour an LMA+RSFP scenario or favour SSM II over SSM I.

Results from Solar and Reactor Neutrino Experiments

The evidence for neutrino flavour change and for the LMA solution for the neutrino mixing parameters is briefly summarized. Recent results and current research from SNO and SK are then described along with the potential for KamLAND, Borexino and planned new experiments to extend our knowledge by studying low energy solar neutrinos. The sensitivity of reactor experiments in determining θ 13 is discussed and the detection technique and principal backgrounds are explained. Finally the status of proposed and funded reactor θ 13 experiments is presented.

TESTING THE STABILITY OF THE SOLAR NEUTRINO LMA SOLUTION WITH A BAYESIAN ANALYSIS

We analyze with the Bayesian method the solar and KamLAND neutrino data in terms of neutrino oscillations. We show that Bayesian credible regions with a flat prior in the [Formula: see text] plane strongly support the LMA solution, in agreement with the usual chi-square analysis. Other reasonable priors are considered in order to test the stability of the LMA solution. We show that priors which favor small or large values of the mixing angle lead to minor changes of the allowed LMA region, affecting mainly its large- tan 2θ12 part.

KamLAND: Updated results

KamLAND has accumulated 766.3 ton-year of data with extended period and enlarged fiducial volume. No oscillation predicts 365.2±23.7 reactor neutrino events and 17.8±7.3 background events in the data, while only 258 events were observed. This deficit is consistent with the previous evidence for reactor antineutrino disappearance and strengthened the confidence level to 99.998%. Increased statistics allowed us to examine spectral distortion and the observed spectrum was inconsistent with no-oscillation shape at 99.6% C.L. Combining these deficit and spectral distortion, no-oscillation is excluded at 99.999995% C.L. The observed distortion is fully consistent with the neutrino oscillation hypothesis with the oscillation parameter of the LMA solution for the solar neutrino problem and the distortion disagrees with the other flavor conversion hypothesis such as neutrino decay and decoherence. If one plots the survival probability as a function of L 0 / E where L 0 is chosen at typical reactor distance of 180 km, one sees clear oscillatory behavior. This is a stand-alone evidence of neutrino oscillation. This clear signal of oscillation provided precise determination of Δ m 2 to be 7.9 −0.5 +0.6 × 10 −5 eV 2 when analyzed in two neutrino oscillation scenario.

How does helioseismology constrain solar neutrino properties?

This review summarizes the advances in solar modelling due to helioseismic measurements. One objective is the correct determination of the different neutrino fluxes and the establishment of the first neutrino properties. The two solar probes (acoustic modes and detected neutrinos) are today in excellent agreement after introduction of the LMA solution for the neutrino oscillations. This consistency is impressive and confirms the interest to look to neutrinos of the Universe. We note that the introduction of recent improvements on essential ingredients of solar modelling put in evidence the limitation of the “Solar Standard Model”. Discrepancies between its results and solar observations are rich of physics beyond this framework, with evident perspectives to look for other neutrino properties, in pursuing the investigation of different objects of the Universe including the Sun.

A NEUTRINO MASS MODEL WITH REFLECTION SYMMETRY

We demonstrate that an SU (2)L× U (1)Y model with the same particle content as Standard Model (SM) and discrete reflection symmetry between second and third generations of leptons gives rise to charged lepton and neutrino mass matrices which can accommodate the present solar, atmospheric, WMAP neutrino experimental results. The model predicts the value of |U13| which could be tested in neutrino factories and the effective Majorana neutrino mass &lt;mee&gt; which is at the lower end of the present experimental value. Neutrino masses are generated through dim=5 operators and the scale of which are constrained by the value of &lt;mee&gt;. If, in future neutrinoless double beta decay experiments namely, MOON, EXO, GENIUS shift the lower bound on &lt;mee&gt; by one order, the present model will fail to accommodate the solar neutrino mixing angle due to LMA solution.

Supersymmetric threshold corrections to Δm2⊙

For nearly degenerate neutrinos, quantum corrections can modify the tree-level masses via low energy supersymmetric threshold corrections comparable to the solar oscillation mass scale. We numerically calculate corrections to neutrino masses in minimal supergravity (mSugra) and Gauge Mediated Supersymmetry Breaking (GMSB) scenarios and identify parameter spaces in the high energy regime for which the solar neutrino mass splitting becomes too large compared to the LMA solution. We show that such considerations can give bounds on GMSB and mSugra models which can be useful. On the contrary, if we start from degenerate mass eigenvalues at the tree level, these threshold corrections being generation dependent, can also produce the required mass splitting at solar scale for regions of parameter space.

Random matter density perturbations and LMA

There are reasons to believe that mechanisms exist in the solar interior which lead to random density perturbations in the resonant region of the Large Mixing Angle solution to the solar neutrino problem. We find that, in the presence of these density perturbations, the best fit point in the ( $\sin^2 2\theta, \Delta m^2$ ) parameter space moves to smaller values, compared with the values obtained for the standard LMA solution. Combining solar data with KamLAND results, we find a new compatibility region, which we call VERY-LOW LMA, where $\sin^2 2\theta \approx 0.6$ and Δ m2≈ 2× 10–5 eV2, for random density fluctuations of order $5\% < \xi< 8\%$ . We argue that such values of density fluctuations are still allowed by helioseismological observations at small scales of order 10 - 1000 km deep inside the solar core. PACS: 26.65 – 90.60J – 96.60.H

Large mixing angle solution to the solar neutrino problem and random matter density perturbations

There are reasons to believe that mechanisms exist in the solar interior which lead to random density perturbations in the resonant region of the Large Mixing Angle solution to the solar neutrino problem. We find that, in the presence of these density perturbations, the best fit point in the (sin22θ,Δm2) parameter space moves to smaller values, compared with the values obtained for the standard LMA solution. Combining solar data with KamLAND results, we find a new compatibility region, which we call very-low LMA, where sin22θ≈0.6 and Δm2≈2×10−5 eV2, for random density fluctuations of order 5%<ξ<8%. We argue that such values of density fluctuations are still allowed by helioseismological observations at small scales of order 10–1000 km deep inside the solar core.

Updates to the dualized standard model on fermion masses and mixings

The Dualized Standard Model has scored a number of successes in explaining the fermion mass hierarchy and mixing pattern. This note contains updates to those results including (a) an improved treatment of neutrino oscillation free from previous assumptions on neutrino masses, and hence admitting now the preferred LMA solution to solar neutrinos, (b) an understanding of the limitation of the 1-loop calculation so far performed, thus explaining the two previous discrepancies with data, and (c) an analytic derivation and confirmation of the numerical results previously obtained.

Global analysis of solar neutrinos

The recent data from SNO are analysed with the global data from the other solar neutrino experiments, first in a model independent way and then assuming two-flavour oscillations. Neutrino oscillations to active flavours can best explain the global data with the LMA solution emerging as the best-fit. However, transitions to ‘mixed’ states with up to 30% sterile mixture are still allowed at 1σ.

Implications of partially degenerate neutrinos at a high scale in light of KamLAND and WMAP results

Electroweak radiative corrections can generate the neutrino (mass)$^2$ difference required for the large mixing angle solution (LMA) to the solar neutrino problem if two of the neutrinos are assumed degenerate at high energy. We test this possibility with the existing experimental knowledge of the low energy neutrino mass and mixing parameters. We derive restrictions on ranges of the high scale mixing matrix elements and obtain predictions for the low energy parameters required in order to get the LMA solution of the solar neutrino problem picked out by KamLAND. We find that in the case of standard model this is achieved only when the (degenerate) neutrino masses lie in the range $(0.7-2) \eV$ which is at odds with the cosmological limit $m_{\nu}<0.23 \eV$ (at $95 % C.L$) established recently using WMAP results. Thus SM radiative corrections cannot easily generate the LMA solution in this scenario. However, the LMA solution is possible in case of the MSSM electroweak corrections with (almost) degenerate spectrum or with inverted mass hierarchy for limited ranges in the high scale parameters.

Combining the first KamLAND results with solar neutrino data

We consider the impact of the recent KamLAND data on neutrino oscillations, the first terrestrial neutrino experiment that can probe the solar neutrino anomaly. By combining the first 145.1 days of KamLAND data with the full sample of the latest solar neutrino data we find an enhanced rejection against non-large-mixing-angle (LMA) oscillations, allowed only at more than $4\ensuremath{\sigma}$ with respect to the LMA solution. Furthermore, the new data have a strong impact in narrowing down the allowed range of $\ensuremath{\Delta}{m}^{2}$ inside the LMA region. In contrast, our global analysis indicates that the new data have little impact on the location of the best fit point. In particular, the solar neutrino mixing remains significantly nonmaximal $(3\ensuremath{\sigma}).$

Minimal scenarios for leptogenesis andCPviolation

The relation between leptogenesis and CP violation at low energies is analyzed in detail in the framework of the minimal seesaw mechanism. Working, without loss of generality, in a weak basis where both the charged lepton and the right-handed Majorana mass matrices are diagonal and real, we consider a convenient generic parametrization of the Dirac neutrino Yukawa coupling matrix and identify the necessary condition which has to be satisfied in order to establish a direct link between leptogenesis and CP violation at low energies. In the context of the LMA solution of the solar neutrino problem, we present minimal scenarios which allow for the full determination of the cosmological baryon asymmetry and the strength of CP violation in neutrino oscillations. Some specific realizations of these minimal scenarios are considered. The question of the relative sign between the baryon asymmetry and CP violation at low energies is also discussed.