Megaton Water Cherenkov Detector Research Articles

MEMPHYS: A next generation megaton water Cherenkov detector in Europe

MEMPHYS (MEgaton Mass PHYSics) is a proposed large-scale water Cherenkov detector to be installed deep underground. It is dedicated to nucleon decay searches, neutrinos from supernovae, solar and atmospheric neutrinos, as well as neutrinos from a future Super Beam or Beta-Beam to measure the CP violating phase in the leptonic sector and the mass hierarchy with atmospheric neutrinos. This proceeding will show the status of the design studies concerning in particular the optical layout and the dedicated test-bench (MEMPHYNO). In addition, some results in terms of physics performances obtained by simulations will be shown.

Fast time variations of supernova neutrino fluxes and their detectability

In the delayed explosion scenario of core-collapse supernovae, the accretion phase shows pronounced convective overturns and a low-multipole hydrodynamic instability, the standing accretion shock instability. These effects imprint detectable fast time variations on the emerging neutrino flux. Among existing detectors, IceCube is best suited to this task, providing an event rate of $\ensuremath{\sim}1000\text{ }\text{ }{\mathrm{ms}}^{\ensuremath{-}1}$ during the accretion phase for a fiducial SN distance of 10 kpc, comparable to what could be achieved with a megaton water Cherenkov detector. If the standing accretion shock instability activity lasts for several hundred ms, a Fourier component with an amplitude of 1% of the average signal clearly sticks out from the shot noise. We analyze in detail the output of axially symmetric hydrodynamical simulations that predict much larger amplitudes up to frequencies of a few hundred Hz. If these models are roughly representative for realistic SNe, fast time variations of the neutrino signal are easily detectable in IceCube or future megaton-class instruments. We also discuss the information that could be deduced from such a measurement about the physics in the SN core and the explosion mechanism of the SN.

Probing nonstandard neutrino interactions with supernova neutrinos

We analyze the possibility of probing nonstandard neutrino interactions (NSI, for short) through the detection of neutrinos produced in a future galactic supernova (SN). We consider the effect of NSI on the neutrino propagation through the SN envelope within a three-neutrino framework, paying special attention to the inclusion of NSI-induced resonant conversions, which may take place in the most deleptonized inner layers. We study the possibility of detecting NSI effects in a Megaton water Cherenkov detector, either through modulation effects in the ${\overline{\ensuremath{\nu}}}_{e}$ spectrum due to (i) the passage of shock waves through the SN envelope, (ii) the time dependence of the electron fraction, and (iii) the Earth matter effects; or, finally, through the possible detectability of the neutronization ${\ensuremath{\nu}}_{e}$ burst. We find that the ${\overline{\ensuremath{\nu}}}_{e}$ spectrum can exhibit dramatic features due to the internal NSI-induced resonant conversion. This occurs for nonuniversal NSI strengths of a few %, and for very small flavor-changing NSI above a $\mathrm{\text{few}}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$.

The diffuse supernova neutrino flux, supernova rate and SN1987A

I calculate the diffuse flux of electron antineutrinos from all supernovae using the information on the neutrino spectrum from SN1987A and the information on the rate of supernovae from direct supernova observations. The interval of flux allowed at 99% confidence level is ∼0.05–0.35 cm −2 s −1 above the SuperKamiokande (SK) energy cut of 19.3 MeV. This result is at least a factor of ∼4 smaller than the current SK upper limit of 1.2 cm −2 s −1, thus motivating the experimental efforts to lower the detection energy threshold or to upgrade to higher volumes. A Megaton water Cherenkov detector with ∼90% efficiency would record ∼2–44 inverse beta decay events a year depending on the energy cut.

Resolving parameter degeneracies in long-baseline experiments by atmospheric neutrino data

In this work we show that the physics reach of a long-baseline (LBL) neutrino oscillation experiment based on a superbeam and a megaton water Cherenkov detector can be significantly increased if the LBL data are combined with data from atmospheric neutrinos (ATM) provided by the same detector. ATM data are sensitive to the octant of ${\ensuremath{\theta}}_{23}$ and to the type of the neutrino mass hierarchy, mainly through three-flavor effects in $e$-like events. This allows to resolve the so-called ${\ensuremath{\theta}}_{23}$- and $\mathrm{sign}(\ensuremath{\Delta}{m}_{31}^{2})$-parameter degeneracies in LBL data. As a consequence it becomes possible to distinguish the normal from the inverted neutrino mass ordering at $2\ensuremath{\sigma}$ C.L. from a combined $\mathrm{LBL}+\mathrm{ATM}$ analysis if ${sin}^{2}2{\ensuremath{\theta}}_{13}\ensuremath{\gtrsim}0.02$. The potential to identify the true values of ${sin}^{2}2{\ensuremath{\theta}}_{13}$ and the CP-phase ${\ensuremath{\delta}}_{\mathrm{CP}}$ is significantly increased through the lifting of the degeneracies. These claims are supported by a detailed simulation of the T2K (phase II) LBL experiment combined with a full three-flavor analysis of ATM data in the HyperKamiokande detector.

Exploiting the neutronization burst of a galactic supernova

One of the robust features found in simulations of core-collapse supernovae (SNe) is the prompt neutronization burst, i.e. the first $\sim 25$ milliseconds after bounce when the SN emits with very high luminosity mainly $\nu_e$ neutrinos. We examine the dependence of this burst on variations in the input of current SN models and find that recent improvements of the electron capture rates as well as uncertainties in the nuclear equation of state or a variation of the progenitor mass have only little effect on the signature of the neutronization peak in a megaton water Cherenkov detector for different neutrino mixing schemes. We show that exploiting the time-structure of the neutronization peak allows one to identify the case of a normal mass hierarchy and large 13-mixing angle $\theta_{13}$, where the peak is absent. The robustness of the predicted total event number in the neutronization burst makes a measurement of the distance to the SN feasible with a precision of about 5%, even in the likely case that the SN is optically obscured.

Signatures of supernova neutrino oscillations in the Earth mantle and core

The Earth matter effects on supernova (SN) neutrinos can be identifiedat a single detector through peaks in the Fourier transform of their`inverse-energy' spectrum. The positions of these peaks areindependent of the SN models and therefore the peaks can be used as arobust signature of the Earth matter effects, which in turn candistinguish between different neutrino mixing scenarios. Whereas onlyone genuine peak is observable when the neutrinos traverse only theEarth mantle, traversing also the core gives rise to multiple peaks.We calculate the strengths and positions of these peaks analyticallyand explore their features at a large scintillation detector as wellas at a megaton water Cherenkov detector through Monte Carlosimulations. We propose a simple algorithm to identify the peaks inthe actual data and quantify the chances of a peak identification as afunction of the location of the SN in the sky.