arXiv subjects

A. Mirizzi

Publications and source records attributed to A. Mirizzi.

27 records · Page 2Linked to original sources

arXiv2006

Earth matter effects in supernova neutrinos: Optimal detector locations

A model-independent experimental signature for flavor oscillations in the neutrino signal from the next Galactic supernova (SN) would be the observation of Earth matter effects. We calculate the probability for observing a Galactic SN shadowed by the Earth as a function of the detector's geographic latitude. This probability depends only mildly on details of the Galactic SN distribution. A location at the North Pole would be optimal with a shadowing probability of about 60%, but a far-northern location such as Pyhasalmi in Finland, the proposed site for a large-volume scintillator detector, is almost equivalent (58%). We also consider several pairs of detector locations and calculate the probability that only one of them is shadowed, allowing a comparison between a shadowed and a direct signal. For the South Pole combined with Kamioka this probability is almost 75%, for the South Pole combined with Pyhasalmi it is almost 90%. One particular scenario consists of a large-volume scintillator detector located in Pyhasalmi to measure the geo-neutrino flux in a continental location and another such detector in Hawaii to measure it in an oceanic location. The probability that only one of them is shadowed exceeds 50% whereas the probability that at least one is shadowed is about 80%. We provide an online tool to calculate different shadowing probabilities for the one- and two-detector cases.

astro-ph↗

arXiv2005

New analysis of the SN 1987A neutrinos with a flexible spectral shape

We analyze the neutrino events from the supernova (SN) 1987A detected by the Kamiokande II (KII) and Irvine-Michigan-Brookhaven (IMB) experiments. For the time-integrated flux we assume a quasi-thermal spectrum of the form $(E/E_0)^αe^{-(α+1)E/E_0}$ where $α$ plays the role of a spectral index. This simple representation not only allows one to fit the total energy $E_{\rm tot}$ emitted in $\barν_e$ and the average energy $ $, but also accommodates a wide range of shapes, notably anti-pinched spectra that are broader than a thermal distribution. We find that the pile-up of low-energy events near threshold in KII forces the best-fit value for $α$ to the lowest value of any assumed prior range. This applies to the KII events alone as well as to a common analysis of the two data sets. The preference of the data for an ``unphysical'' spectral shape implies that one can extract meaningful values for $ $ and $E_{\rm tot}$ only if one fixes a prior value for $α$. The tension between the KII and IMB data sets and theoretical expectations for $ $ is not resolved by an anti-pinched spectrum.

astro-ph↗

arXiv2005

Photon-axion conversion as a mechanism for supernova dimming: Limits from CMB spectral distortion

Axion-photon conversion induced by intergalactic magnetic fields has been proposed as an explanation for the dimming of distant supernovae of type Ia SNe Ia) without cosmic acceleration. The effect depends on the intergalactic electron density $n_e$ as well as the $B$-field strength and domain size. We show that for $n_e\alt 10^{-9} {\rm cm}^{-3}$ the same mechanism would cause excessive spectral distortion of the cosmic microwave background (CMB). This small-$n_e$ parameter region had been left open by the most restrictive previous constraints based on the dispersion of quasar (QSO) spectra. The combination of CMB and QSO limits suggests that the photon-axion conversion mechanism can only play a subleading role for SN Ia dimming. A combined analysis of all the observables affected by the photon-axion oscillations would be required to give a final verdict on the viability of this model.

astro-ph↗

arXiv2005

New cosmological mass limit on thermal relic axions

Observations of the cosmological large-scale structure provide well-established neutrino mass limits. We extend this argument to thermal relic axions. We calculate the axion thermal freeze-out temperature and thus their cosmological abundance on the basis of their interaction with pions. For hadronic axions we find a new mass limit $m_a<1.05$ eV (95% CL), corresponding to a limit on the axion decay constant of $f_a>5.7\times 10^6$ GeV. For other models this constraint is significantly weakened only if the axion-pion coupling is strongly suppressed. For comparison we note that the same approach leads to $\sum m_ν<0.65$ eV (95% CL) for neutrinos.

hep-ph↗

arXiv2005

Probing supernova shock waves and neutrino flavor transitions in next-generation water-Cherenkov detectors

Several current projects aim at building a large water-Cherenkov detector, with a fiducial volume about 20 times larger than in the current Super-Kamiokande experiment. These projects include the Underground nucleon decay and Neutrino Observatory (UNO) in the Henderson Mine (Colorado), the Hyper-Kamiokande (HK) detector in the Tochibora Mine (Japan), and the MEgaton class PHYSics (MEMPHYS) detector in the Frejus site (Europe). We study the physics potential of a reference next-generation detector (0.4 Mton of fiducial mass) in providing information on supernova neutrino flavor transitions with unprecedented statistics. After discussing the ingredients of our calculations, we compute neutrino event rates from inverse beta decay ($\barν_e p\to e^+ n $), elastic scattering on electrons, and scattering on oxygen, with emphasis on their time spectra, which may encode combined information on neutrino oscillation parameters and on supernova forward (and possibly reverse) shock waves. In particular, we show that an appropriate ratio of low-to-high energy events can faithfully monitor the time evolution of the neutrino crossing probability along the shock-wave profile. We also discuss some background issues related to the detection of supernova relic neutrinos, with and without the addition of gadolinium.

hep-ph↗

arXiv2004

Decays of supernova relic neutrinos

We propose that future observations of supernova relic neutrinos (SRN) can be used to probe neutrino decay models. We focus on invisible (e.g. Majoron) decays, and work out the general solution of SRN kinetic equations in the presence of oscillations plus decay. We then apply the general solution to specific decay scenario, and show that the predicted SRN event rate can span the whole range below the current experimental bounds. Therefore, future SRN observations will surely have an impact on the neutrino decay parameter space.

hep-ph↗

arXiv2004

Three-generation flavor transitions and decays of supernova relic neutrinos

If neutrinos have mass, they can also decay. Decay lifetimes of cosmological interest can be probed, in principle, through the detection of the redshifted, diffuse neutrino flux produced by all past supernovae--the so-called supernova relic neutrino (SRN) flux. In this work, we solve the SRN kinetic equations in the general case of three-generation flavor transitions followed by invisible (nonradiative) two-body decays. We then use the general solution to calculate observable SRN spectra in some representative decay scenarios. It is shown that, in the presence of decay, the SRN event rate can basically span the whole range below the current experimental upper bound--a range accessible to future experimental projects. Radiative SRN decays are also briefly discussed.

hep-ph↗

arXiv2003

Analysis of energy- and time-dependence of supernova shock effects on neutrino crossing probabilities

It has recently been realized that supernova neutrino signals may be affected by shock propagation over a time interval of a few seconds after bounce. In the standard three-neutrino oscillation scenario, such effects crucially depend on the neutrino level crossing probability P_H in the 1-3 sector. By using a simplified parametrization of the time-dependent supernova radial density profile, we explicitly show that simple analytical expressions for P_H accurately reproduce the phase-averaged results of numerical calculations in the relevant parameter space. Such expressions are then used to study the structure of P_H as a function of energy and time, with particular attention to cases involving multiple crossing along the shock profile. Illustrative applications are given in terms of positron spectra generated by supernova electron antineutrinos through inverse beta decay.

hep-ph↗

arXiv2002

Reexamining nonstandard interaction effects on supernova neutrino flavor oscillations

Several extensions of the standard electroweak model allow new four-fermion interactions (nu_a nu_b * ff) with strength eps_ab*G_F, where (a,b) are flavor indices. We revisit their effects on flavor oscillations of massive (anti)neutrinos in supernovae, in order to achieve, in the region above the protoneutron star, an analytical treatment valid for generic values of the neutrino mixing angles (omega,phi,psi)=(theta_12,theta_13,theta_23). Assuming that eps_ab<<1, we find that the leading effects on the flavor transitions occurring at high (H) and low (L) density along the supernova matter profile can be simply embedded through the replacements phi-->phi+eps_H and omega-->omega+eps_L, respectively, where eps_H and eps_L are specific linear combinations of the eps_ab's. Similar replacements hold for eventual oscillations in the Earth matter. From a phenomenological point of view, the most relevant consequence is a possible uncontrolled bias (phi-->phi+eps_H) in the value of the mixing angle phi inferred by inversion of supernova neutrino data. Such a drawback, however, does not preclude the discrimination of the neutrino mass spectrum hierarchy (direct or inverse) through supernova neutrino oscillations.

hep-ph↗