Pomita Ghoshal

Mass Hierarchy Determination via future Atmospheric Neutrino Detectors

We study the problem of determination of the sign of {delta}m{sub 31}{sup 2}, or the neutrino mass hierarchy, through observations of atmospheric neutrinos in future detectors. We consider two proposed detector types: (a) Megaton sized water Cerenkov detectors, which can measure the event rates of {nu}{sub {mu}}+{nu}{sub {mu}} and {nu}{sub e}+{nu}{sub e} and (b) 100 kton sized magnetized iron detectors, which can measure the event rates of {nu}{sub {mu}} and {nu}{sub {mu}}. For energies and path lengths relevant to atmospheric neutrinos, these rates obtain significant matter contributions from P{sub {mu}}{sub e}, P{sub {mu}}{sub {mu}} and P{sub ee}, leading to an appreciable sensitivity to the hierarchy. We do a binned {chi}{sup 2} analysis of simulated data in these two types of detectors which includes the effect of smearing in neutrino energy and direction and incorporates detector efficiencies and relevant statistical, theoretical and systematic errors. We also marginalize the {chi}{sup 2} over the allowed ranges of neutrino parameters in order to accurately account for their uncertainties. Finally, we compare the performance of both types of detectors vis a vis the hierarchy determination.

Neutrino Mass Hierarchy and Octant Determination with Atmospheric Neutrinos

The recent discovery by the Daya-Bay and RENO experiments, that θ(13) is nonzero and relatively large, significantly impacts existing experiments and the planning of future facilities. In many scenarios, the nonzero value of θ(13) implies that θ(23) is likely to be different from π/4. Additionally, large detectors will be sensitive to matter effects on the oscillations of atmospheric neutrinos, making it possible to determine the neutrino mass hierarchy and the octant of θ(23). We show that a 50 kT magnetized liquid argon neutrino detector can ascertain the mass hierarchy with a significance larger than 4σ with moderate exposure times, and the octant at the level of 2-3σ with greater exposure.

Can atmospheric neutrino experiments provide the first hint of leptonic CP violation

The measurement of a non-zero value of the 1-3 mixing angle has paved the way for the determination of leptonic CP violation. However the current generation long-baseline experiments T2K and NOvA have limited sensitivity to delta_{CP}. In this paper we show for the first time, the significance of that atmospheric neutrino experiments in providing the first hint of CP violation in conjunction with T2K and NOvA. In particular, we find that adding atmospheric neutrino data from the ICAL detector at the India-based Neutrino Observatory (INO) to T2K and NOvA results in a two-fold increase in the range of delta_{CP} values for which a 2 sigma hint of CP violation can be obtained. In fact in the parameter region unfavorable for the latter experiments, the first signature of CP violation may well come from the inclusion of atmospheric neutrino data.

New look at the degeneracies in the neutrino oscillation parameters, and their resolution by T2K, NO nu A and ICAL

The three major unknown neutrino oscillation parameters at the present juncture are the mass hierarchy, the octant of the mixing angle theta(23) and the CP phase delta(CP). It is well known that the presence of hierarchy-delta(CP) and octant degeneracies affects the unambiguous determination of these parameters. In this paper, we show that a comprehensive way to study the remaining parameter degeneracies is in the form of a generalized hierarchy -theta(23) - delta(CP) degeneracy. This is best depicted as contours in the test (theta(23) - delta(CP)) plane for different representative true values of parameters. We show that the wrong-hierarchy and/or wrong-octant solutions can be further classified into eight different solutions depending on whether they occur with the wrong or right value of delta(CP). These eight solutions are different from the original eightfold degenerate solutions and can exist, in principle, even if theta(13) is known. These multiple solutions, apart from affecting the determination of the true hierarchy and octant, also affect the accurate estimation of delta(CP). We identify which of these eight different degenerate solutions can occur in the test (theta(23) - delta(CP)) parameter space, taking the long-baseline experiment NO nu A running in the neutrino mode as an example. The inclusion of the NO nu A antineutrino run removes the wrong-octant solutions appearing with both right and wrong hierarchy. Adding T2K data to this resolves the wrong hierarchy-right octant solutions to a large extent. The remaining wrong-hierarchy solutions can be removed by combining NO nu A + T2K with atmospheric neutrino data. We demonstrate this using ICAL@INO as the prototype atmospheric neutrino detector. We find that the degeneracies can be resolved at the 2 sigma level by the combined data set, for the true parameter space considered in the study.

Large matter effects in nu(mu) ---> nu(tau) oscillations

We show that matter effects change the numu-->nutau oscillation probability by as much as 70% for certain ranges of energies and path lengths. Consequently, the numu-->nutau survival probability also undergoes large changes. A proper understanding of numu survival rates must consider matter effects in Pmutau as well as Pmue. We comment on (a) how these matter effects may be observed and the sign of Delta31 determined in atmospheric neutrino measurements and at neutrino factories, and (b) how they lead to heightened sensitivity for small theta13.

Atmospheric neutrinos as a probe of eV

The down-going atmospheric \nu_{\mu} and {\bar{\nu_{\mu}}} fluxes can be significantly altered due to the presence of eV^2-scale active-sterile oscillations. We study the sensitivity of a large Liquid Argon detector and a large magnetized iron detector (like the proposed ICAL at INO) to these oscillations. Such oscillations are indicated by results from LSND, and more recently, from MiniBooNE and from reanalyses of reactor experiments following recent recalculations of reactor fluxes. There are other tentative indications of the presence of sterile states in both the \nu and {\bar{\nu}} sectors as well. Using the allowed sterile parameter ranges in a 3+1 mixing framework in order to test these results, we perform a fit assuming active-sterile oscillations in both the muon neutrino and antineutrino sectors, and compute oscillation exclusion limits using atmospheric down-going muon neutrino and anti-neutrino events. We find that (for both \nu_{\mu} and {\bar{\nu_{\mu}}}) a Liquid Argon detector, an ICAL-like detector or a combined analysis of both detectors with an exposure of 1 Mt yr provides significant sensitivity to regions of parameter space in the range 0.1 < \Delta m^2 < 5 eV^2 for \sin^2 2\Theta_{\mu\mu}\geq 0.08. Thus atmospheric neutrino experiments can provide complementary coverage in these regions, improving sensitivity limits in combination with bounds from other experiments on these parameters. We also analyse the bounds using muon antineutrino events only and compare them with the results from MiniBooNE.

^2

The octant of the atmospheric mixing angle

-scale active-sterile oscillations

\theta_{23}

Octant of

is still undetermined by neutrino oscillation experiments. Long-baseline experiments like NO

\theta_{23}

\nu