научная статья по теме SENSITIVITY OF BAIKAL-GVD NEUTRINO TELESCOPE TO NEUTRINO EMISSION TOWARD THE CENTER OF GALACTIC DARK MATTER HALO Физика

Текст научной статьи на тему «SENSITIVITY OF BAIKAL-GVD NEUTRINO TELESCOPE TO NEUTRINO EMISSION TOWARD THE CENTER OF GALACTIC DARK MATTER HALO»

Pis'ma v ZhETF, vol. 101, iss. 5, pp. 315-320

© 2015 March 10

Sensitivity of Baikal-GVD neutrino telescope to neutrino emission toward the center of Galactic dark matter halo

A. D. Avrorina, A. V. Avrorina, V. M. AynutdinoV1, R. Bannasch9, I. A. Belolaptikovb, D. Yu. Bogorodskyb, V. B. Brudaninb, N. M. BudneV2, I. A. Danilchenkoa, S. V. Demidoval\ G. V. Domogatskya, A. A. Doroshenkoa, A. N. Dyachokc, Zh.-A. M. DzhilkibaeV1, S. V. Fialkovskf2, A. R. Gafarovc, O. N. Gaponenkoa, K. V. GolubkoV1, T. I. Gressc, Z. Honzb, K. G. Kebkal9, O. G. Kebkal9, K. V. Konischevb, E. N. Konstantinov*2, A. V. Korobchenko0, A. P. Koshechkiif, F. K. KosheP, A. V. Kozhind, V. F. KulepoV2, D. A. Kuleshova, V. I. Ljashuka, M. B. Milenine, R. A. Mirgazov0, E. R. Osipov.ad, A. I. PanGloV\ L. V. Pan'kov0, A. A. Perevalov0, E. N. PliskovskM. I. Rozanovf, V. Yu. RubtzoV2, E. V. Rjabovc, B. A. Shaybonovb, A. A. Sheifiera, A. V. Skurihind, A. A. Smaginab, O. V. Suvorova" l\ B. A. Tarashanskf2, S. A. Yakovlev9, A. V. ZagorodnikoV2, V. A. ZhukoV1, V. L. ZurbanoV2

aInstitute for Nuclear Research RAS, 117312 Moscow, Russia

b Joint Institute for Nuclear Research, 141980 Dubna, Russia

cIrkutsk State University, 664003 Irkutsk, Russia

d Skobeltsyn Institute of Nuclear Physics MSU, 119991 Moscow, Russia

eNizhni Novgorod State Technical University, 603005 Nizhni Novgorod, Russia

■f St.Petersburg State Marine University, 199034 St.Petersburg, Russia

9 EvoLogics GmbH, 13355 Berlin, Germany

Submitted 16 December 2014

We analyse sensitivity of the gigaton volume telescope Baikal-GVD for detection of neutrino signal from dark matter annihilations or decays in the Galactic Center. Expected bounds on dark matter annihilation cross section and its lifetime are found for several annihilation/decay channels.

DOI: 10.7868/S0370274X1505001X

There is a lot of evidence on existence of dark matter (DM). Cosmological observations indicate that DM contributes about 27% in total energy density within ACDM model. Weakly Interacting Massive Particles (WIMP) are among the most interesting candidates for the dark matter. Currently, tremendous efforts are put in the searches for DM at colliders, in direct detection experiments and, finally, in indirect searches for a signal in products of WIMP annihilations/decays in astro-physical observations. The Galactic Center (GC) is a very promising region to look for such a signal. Recent analysis of gamma telescope FERMI-LAT dataset performed by few groups for several years of observation indicates on the existence of a diffuse gamma-ray excess from the center of our Galaxy at energies 10-20 GeV and a gamma-line feature about 133 GeV with a significance about 3a [1]. There are many systematic uncertainties here due to gamma-ray background from galactic diffuse emission and from close around local astrophysical

-^e-mal: demidov@ms2.inr.ac.ru; suvorova@cpc.inr.ac.ru

sources like pulsars or supernova remnants. Another attractive possibility is to look for neutrino signal from DM and it has been put forward by neutrino telescope collaborations. Recently the IceCube collaboration announced four dozen candidates [2] for neutrinos of astrophysical origin with energies above hundreds TeV. However there is no any significant clustering of events in any direction.

New Baikal-GVD project aimed on installation of gigaton volume detector in lake Baikal [3-5] is now in progress. The main goal of GVD telescope is a search for neutrinos of astrophysical origin. Here, we estimate one year sensitivity to look for possible neutrino signal from dark matter in the center of our galaxy for the GVD telescope in planned configuration of 12 clusters composed by 2304 photodetectors per 96 strings. The telescope GVD is building in a place of previous telescope NT200 [6] in the south basin of the Lake Baikal, at a distance 3.5 km off the shore. Local coordinates are 51.83° N and 104.33° E. Position provides average visibility for the GC almost 75 % per day.

Fig. 1. Layout of the GVD. In inner box the one cluster is shown

Actual position of the center of Galaxy to be used is taken with right ascension « 266.42° and declination « -29.01°.

The Baikal water optical properties have been studied for a long time and are characterized by absorption length 20—24 m at 480 nm and a scattering length 30—70 m depending on season. The total trigger rate is expected to be approximately 100 Hz, dominated by downgoing atmospheric muons. The detection of rela-tivistic particles crossing effective volume of a deep underwater telescope implies collection of their Cherenkov radiation by optical modules (OMs) synchronized in time and calibrated in pulses. Events arriving from down hemisphere are considered as candidates on those originated from neutrino scatterings off nucleons in surrounding water or rock. The main challenge is to suppress atmospheric muon background from upper hemi-

sphere exceeding upward going neutrino flux by factor 106.

Stages of the GVD telescope design, its commissioning and deployment of the first prototypes are successfully completed. Presently there are five strings of the first demonstration cluster operating since April 2014 [7, 8]. The first cluster with eight strings will be fully deployed next April 2015. The Baikal-GVD is designed as 3D arrays of photomultiplier tubes (PMTs) each enclosed in an optical module. Each optical module consists of a pressure-resistant glass sphere with 43.2 cm diameter which holds OM electronics and PMT surrounded by a high permittivity alloy cage for shielding it against the Earths magnetic field. Large photomultiplier tube Hamamatsu R7081-100 is selected as light sensor of OM. The tube gain adjusted to about 107 and factor 10 by the first channel of the preamplifier results

in a spectrometric channel linearity range up to about 100 photoelectrons (see [7] for details). The OMs are arranged on vertical load-carrying cables to form strings. Clusters of strings form functionally independent sub-arrays connected to shore by individual electro-optical cables as shown in Fig. 1. Each cluster has a central string identical to seven others distant at radius of 60 meters. The OMs are spaced by 15 m along each string and are faced downward. They are combined in sections on each string. In current design there are two sections on cluster. The distance between the central strings of neighboring clusters is 300 m. Details of data acquisition, basic controls, methods of calibrations, hard- and soft-ware triggers can be found in [4, 5, 7]. For the present study we apply a muon trigger formed by requirements to select events with at least 6 fired OMs on at least 3 strings within 500 ns. Here we make a conservative estimate and suppose the worse situation with angular resolution 4.5° for track events. The neutrino effective area at trigger level selection (3/6) as a function of energy for one cluster for neutrinos from GC is presented in Fig. 2 by black line.

10000

EV or mDM (GeV)

Fig. 2. Neutrino effective area of single Baikal-GVD cluster (black) and averaged over neutrino spectra effective axe as for different annihilation channels (color)

rescaled by the distance from GC to the Solar system Ro and by the local DM density piOCai as follows

J2(V) =

dl Ro

p2 (Vño - 2rR0 cos V> + r2)

/'focal

(2)

where ip is the angular distance from GC to the direction of observation and the integration in (2) goes to I max which is much larger than the size of the Galaxy. There are several models for the DM density profile in the galaxies and, in particular, in the Milky Way2'. Here we consider Navarro-Frenk-White (NFW) [10, 11], the Ivravtsov et al. [12], the Moore et al. [13], and Burkert [14] profiles. The last model is currently favored by the observational data [15]. The profiles can be parametrized by

p(r) =

P o

(<5 + r/rs)^[l + (r/rs)a](/3-7)/a'

(3)

where numerical quantities are presented in Table.

Parameters of DM density profiles

Model a ß 7 rs, kpc po, GeV/cm3

NFW 1 3 1 0 20 0.3

Kravt.sov 2 3 0.4 0 10 0.37

Moore 1.5 3 1.5 0 28 0.27

Burkert 2 3 1 1 9.26 1.88

In the case of dark matter decay in the Galaxy expected neutrino flux is

d,(f) v ~dE

1

T"DM

JiW

RoPh

ocal

dNv

47T';??-dm d,E

(4)

where tdm is DM particle lifetime and Ji(tp) is the following integral

JiW

¿I p ^v7Ro - 2rR0 cos ^ + r2

Ro piocal

(5)

Expected neutrino flux from DM annihilations in the Galaxy has the following form

d<f>v ~dE

^ J2(V0 ñopLal

4lrmdm

dNv ~dE

(1)

Here, (ffAf) is annihilation cross section averaged over DM velocity distribution, d,Nv/d,E is neutrino (and antineutrino) spectrum per act of annihilation. The dimen-sionless quantity J2W is the square of the DM density in MW, p2(r), integrated along the line of sight and

Neutrino spectra from dark matter annihilation/decay have been taken from [16]. We consider 56, t+t~, pT'F+TF-, and vv channels, where in the latter case we assume flavor symmetric annihilation. Note that the authors of [16] artificially modified the neutrino spectra for vv to be able to solve their evolution equations which resulted to a large smearing of the monochromatic line. We change these spectra back to their physical width conserving the absolute

2'We do not take into account DM substructures which in gen-

eral could modify the results [9].

norm3'. For calculation of muon-neutrino energy spectra at the Earth we use probabilities for long-baseline oscillations. As neutrino oscillation parameters we use [17] the following values: Am^ = 7.6 • 10~5eV2, Am§! = 2.48 • l(T3eV2, dCP = 0, sin2 012 = 0.323, sin2 $23 = 0.567, sin2 = 0.0234. Neutrino spectra at the Earth level are presented in Fig. 3 for

100 80 60

Ü I 40

20

bb tV-nV-

W+W~ vV~ ■

/

/

У-*

\

\ \

s \

>--------

0 100 200 300 400 500 K, (GeV)

above. The average total number of background events coming from low hemisphere for one year is expected to be 4300. Distribution of background events in angular distan

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