научная статья по теме MAY HEAVY NEUTRINOS SOLVE UNDERGROUND AND COSMIC-RAY PUZZLES? Физика

Текст научной статьи на тему «MAY HEAVY NEUTRINOS SOLVE UNDERGROUND AND COSMIC-RAY PUZZLES?»

ЯДЕРНАЯ ФИЗИКА, 2008, том 71, № 1, с. 148-162

ЭЛЕМЕНТАРНЫЕ ЧАСТИЦЫ И ПОЛЯ

MAY HEAVY NEUTRINOS SOLVE UNDERGROUND AND COSMIC-RAY PUZZLES?

© 2008 K. M. Belotsky1)*, D. Fargion2)**, M. Yu. Khlopov1)***, R. V. Konoplich3)****

Received October 10, 2006; in final form, May 25, 2007

Primordial heavy neutrinos of the 4th generation might explain different astrophysical puzzles. The simplest 4th-neutrino scenario is consistent with known 4th-neutrino physics, cosmic ray antimatter, cosmic gamma fluxes, and positive signals in underground detectors for a very narrow neutrino mass window (46—47 GeV). However, the account for constraint of underground experiment CDMS prohibits solution of cosmic-ray puzzles in this scenario. We have analyzed extended heavy-neutrino models related to the clumpiness of neutrino density, new interactions in heavy-neutrino annihilation, neutrino asymmetry, neutrino decay. We found that in these models the cosmic-ray imprint may fit the positive underground signals in DAMA/NaI experiment in all mass range 46—70 GeV allowed from uncertainties of electroweak parameters, while satisfaction of CDMS constraint reduces mass range to around 50 GeV, where all data can come to consent in the framework of the considered hypothesis.

PACS: 14.60.St

1. INTRODUCTION

The problem of dark matter (DM) of the Universe was revealed about 70 years ago. Several possible physical candidates were suggested since that time and several approaches to probe these candidates appeared. However, observed phenomena, which have unclear nature yet, could not be decisively matched with expected effects caused by existing candidates.

An important step in exploration of DM problem was a development of direct searches for Weakly Interacting Massive Particles (WIMPs). Underground detectors were created in which effects of nucleus recoil induced by its interaction with cosmic WIMP were searched for. A positive result at 6.3a C.L. was obtained in the DAMA/NaI underground setup at the Gran Sasso National Laboratory of INFN by exploiting the distinctive WIMP annual modulation signature [1]. Being model independent, this positive

1)Moscow Engineering Physics Institute; Center for Cos-moparticle Physics "Cosmion" of Keldysh Institute of Applied Mathematics, Moscow, Russia.

2)Universita' di Roma "La Sapienza" and INFN, Italy; INFN, Rome1, Italy.

3)New York University, USA; Manhattan College, Riverdale, New York, USA.

E-mail: k-belotsky@yandex.ru

E-mail: daniele.fargion@roma1.infn.it

E-mail: maxim.khlopov@roma1.infn.it; http://maxim.

khlopov.free.fr/

E-mail: rk60@nyu.edu

result cannot be directly compared with the single-model-dependent negative results of other groups, which have also used different target nuclei, different experimental strategies, different setups, and all assumptions fixed at a single set [1]. Moreover, it can be shown [1] that these negative results are actually not incompatible with the positive signal by DAMA/NaI.

On the other hand, an indirect probing of DM can be based on data for cosmic rays (CR). The presence of DM in the Galaxy in form of WIMPs can cause an appearance of cosmic particles of high energy due to an annihilation or decay of the WIMPs. An implication of these annihilation (decay) sources of CR could remove possible contradiction between observed CR fluxes and their predictions on the base of standard CR model.

The result of DAMA/NaI is a challenge for DM studies, expected to shed light on the origin of DM. In fact, the existing physical candidates of dominant DM can hardly or not at all provide an explanation of the result of DAMA/NaI. WIMP candidates such as neutralino, axion, gravitino, sterile neutrino, axino, mirror (shadow) matter are able to compose all the required missing mass of the Universe, however, all these candidates, except neutralino, are virtually sterile particles in respect to their interactions with an ordinary matter. Therefore, it looks like the measurements of DAMA/NaI as well as anomalies observed in CR spectra require a nonsterile DM which, in particular, could be a nondominant DM component

in the form of heavy neutrinos of the 4th generation [2,

3].

A possibility to explain the DAMA/NaI result within the framework of Standard Model (SM) extended to the 4th generation of fermions, revealed in [2], is the subject of current consideration.

The heavy neutrino (N) is supposed to be a neutral fermion of the new 4th generation possessing the standard weak interaction. According to recent analysis of precision electroweak data [4], where possible virtual contributions of 4th-generation particles were taken into account, a fit is compatible with the 4th neutrino, being Dirac and (meta)stable, in a mass range about 50 GeV (47—50 GeV is 1a interval, 46.3—75 GeV is 2a interval) [4] and other 4th-generation particles satisfying their direct experimental constraints (above 100—300 GeV). In the following we will assume that the 4th-neutrino mass m is about 50 GeV.

The fourth generation can find theoretical ground in framework of heterotic string theory [5]. Together with the 4th generation another important consequence of this theory is a prediction of new symmetries) in addition to those of SM. In [6] the new U(1) symmetry connected with strictly conserved charge ("y charge"), absent for known particles, was ascribed to only new, 4th generation of quarks and leptons. It provides a physical reason for a decoupling of 4th-generation particles from known three generations and, sequently, a stability of the lightest from them (neutrino). Lightest quark of the 4th generation in such framework can decay due to GUT-type interaction and can be long living enough to be of cosmological interest [7]. Possible role of a gauge y charge in heavy-neutrino effects is considered in Section 5.2. Also note, that there are phenomenolog-ical models built on the base of the 4th generation with heavy (Dirac) neutrino almost decoupled from three known neutrinos with a view to explain masses of light neutrinos and mixing between them [8] as well as a model of the 4th family with the stable 4th neutrino, following from 5U(4) symmetry [9].

If the 4th neutrino is sufficiently long living or absolutely stable, its primordial gas from the early Universe can survive to the present time. Heavy neutrinos are frozen out (annihilate) at the temperature T & m/30 ~ 2 GeV and during successive hot (RD) stage of the Universe they have no significant influence on dynamics of the Universe expansion and on primordial nucleosynthesis (the same conclusion turns out to be true for a gas of y quanta, if they exist). In the modern Universe primordial heavy neutrinos will contribute to Cold Dark Matter (CDM) and

concentrate in the Galaxy. However, in the case of charge symmetry of 4th-generation particles the 4th neutrinos are not able to account for a bulk of missing mass. In the mass range about 50—80 GeV they can make up 10_5 —10_2 of total density of the Universe, being a nondominant DM component. This leads to a scenario of multicomponent DM consisting of a subdominant heavy-neutrino component and a dominant component, which we assume sterile. A complex analysis of astrophysical effects induced by a presence of a subdominant heavy-neutrino (nonsterile) DM component is the purpose of the present paper.

It is worth noting that the 4th generation of quarks and leptons considered here and neutralino, which is widely considered as the candidate for WIMPs, are naturally incorporated in the framework of heterotic string phenomenology4). It appeals to future multi-component DM analysis of the results of direct and indirect WIMP searches. However, the astrophysical uncertainties revealed below even for the model of the 4th-generation neutrino, which is the simplest physical model and implies the minimal amount of parameters, demonstrate all the complications to be expected in such multicomponent DM approach.

We should remark that even if, in principle, the ultrahigh-energy (UHE) neutrino of the 4th family might produce a very exciting scattering with relic heavy neutrino (a heavy-Z-boson burst), nevertheless the cosmic relic heavy neutrinos are too diluted and poor in number to be anyway competitive with the much more abundant (by 13 order of magnitude) and effective light-neutrino [11] scattering. So there is little consequence of any heavy-Z-boson-burst model interactions. The eventuality for a UHE neutrino (produced, for instance, in top—down models as [12]) to be a source of amplified resonant interaction with electron or quark (as for SUSY UHE neutralino scattering into selectron and squark channels [13]) is absent: there are no s-channel interaction able to overcome the electroweak cross sections but only much less effective ¿-channel processes. Finally, the UHE heavy neutrinos created by top—down models may nevertheless induce charged and neutral current interactions in neutrino detectors (SK, UNO, ICECUBE) almost undistinguishable from lighter UHE neutrino scattering in matter. This effect has just the ability to increase by a small factor (~25%) the event rate in ICECUBE or EUSO neutrino-induced events if top—down mechanism is the main source of UHE CR.

4)Mirror (shadow) matter also naturally follows from heterotic string phenomenology. It is usually considered as sterile but its WIMP effects may play a role [10].

2. ESTIMATION OF LOCAL HEAVY-NEUTRINO DENSITY FROM DAMA/NaI EXPERIMENT

A contribution of heavy neutrinos plocN to the total local density ploc is given by a ratio

t PlocN

sloe = -•

Ploc

Approximately this parameter can be estimated as

Cloc ~ ^N/^CDM (!)

by assuming a dominance of CDM in the Galaxy and by choosing the local fraction of relic heavy neutrinos N equal to their contribution to the cosmological density of CDM.

The results of DAMA/NaI, based on measurements of an "active" DM component, i.e., in our assumptions on cosmic heavy neutrinos, give the fraction Cloc of heavy neu

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