Baikal-GVD: first cluster Dubna
A.D. Avrorin, A.V. Avrorin, V.M. Aynutdinov, R. Bannash, I.A. Belolaptikov, D.Yu. Bogorodsky, V.B. Brudanin, N.M. Budnev, I.A. Danilchenko, S.V. Demidov, G.V. Domogatsky, A.A. Doroshenko, A.N. Dyachok, Zh.-A.M. Dzhilkibaev, S.V. Fialkovsky, A.R. Gafarov, O.N. Gaponenko, K.V. Golubkov, T.I. Gress, Z. Honz, K.G. Kebkal, O.G. Kebkal, K.V. Konischev, A.V. Korobchenko, A.P. Koshechkin, F.K. Koshel, A.V. Kozhin, V.F. Kulepov, D.A. Kuleshov, V.I. Ljashuk, M.B. Milenin, R.A. Mirgazov, E.R. Osipova, A.I. Panfilov, L.V. Pan'kov, E.N. Pliskovsky, M.I. Rozanov, E.V. Rjabov, B.A. Shaybonov, A.A. Sheifler, M.D. Shelepov, A.V. Skurihin, A.A. Smagina, O.V. Suvorova, V.A. Tabolenko, B.A. Tarashansky, S.A. Yakovlev, A.V. Zagorodnikov, V.A. Zhukov, V.L. Zurbanov
aa r X i v : . [ phy s i c s . i n s - d e t ] N ov Baikal-GVD: first cluster Dubna
A.D. Avrorin a , A.V. Avrorin a , V.M. Aynutdinov a , R. Bannash g , I.A. Belolaptikov b ,D.Yu. Bogorodsky c , V.B. Brudanin b , N.M. Budnev c , I.A. Danilchenko a , S.V.Demidov a , G.V. Domogatsky a , A.A. Doroshenko a , A.N. Dyachok c , Zh.-A.M.Dzhilkibaev a , S.V. Fialkovsky e , A.R. Gafarov c , O.N. Gaponenko a , K.V. Golubkov a , T.I.Gress c , Z. Honz b , K.G. Kebkal g , O.G. Kebkal g , K.V. Konischev b , A.V. Korobchenko b ,A.P. Koshechkin a , F.K. Koshel a , A.V. Kozhin d , V.F. Kulepov e , D.A. Kuleshov a , V.I.Ljashuk a , M.B. Milenin e , R.A. Mirgazov c , E.R. Osipova d , A.I. Panfilov a , L.V. Pan’kov c ,E.N. Pliskovsky b , M.I. Rozanov f , E.V. Rjabov c , B.A. Shaybonov b , A.A. Sheifler a , M.D.Shelepov a , A.V. Skurihin d , A.A. Smagina b , O.V. Suvorova ∗ a , V.A. Tabolenko c , B.A.Tarashansky c , S.A. Yakovlev g , A.V. Zagorodnikov c , V.A. Zhukov a , and V.L. Zurbanov c a Institute for Nuclear Research, Moscow, 117312 Russia b Joint Institute for Nuclear Research, Dubna, 141980 Russia c Irkutsk State University, Irkutsk, 664003 Russia d Institute of Nuclear Physics, Moscow State University, Moscow, 119991 Russia e Nizhni Novgorod State Technical University, Nizhni Novgorod, 603950 Russia f St. Petersburg State Marine Technical University, St. Petersburg, 190008 Russia g EvoLogics, GermanyE-mail: [email protected]
In April 2015 the demonstration cluster "Dubna" was deployed and started to take data in LakeBaikal. This array is the first cluster of the cubic kilometer scale Gigaton Volume Detector(Baikal-GVD), which is constructed in Lake Baikal. In this contribution we will review the designand status of the array.
The European Physical Society Conference on High Energy Physics22–29 July 2015Vienna, Austria ∗ Speaker. c (cid:13) Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/ aikal-GVD
O.V. Suvorova
1. Introduction
The neutrino telescope Baikal Gigaton Volume Detector (GVD) is currently under constructionin Lake Baikal [1], which is a deep underwater Cherenkov detector of next generation. Main streamof neutrino studies with Baikal-GVD is search for astrophysical neutrinos incoming from lowerhemisphere with energies higher a few tens of TeV. Potential astrophysical targets for neutrinodetection with GVD are celestial objects which are visible in GeV-TeV gamma-rays (SNRs, AGNs,GRBs and so on) or invisible dark matter in the halo of the Milky Way, the Galactic Center (GC),dwarfs, and also in the center of the Sun and the Earth. At 51 ◦ North latitude of Baikal sitevisibility time is more than 65% of observation time for directions on Southern sky from wherea most part of first candidates on cosmic neutrinos has been detected by IceCube [2]. They havenot been identified with known sources because of 10-15 degree uncertainties in reconstruction ofneutrino induced cascades in polar ice. Unique optical properties of the Baikal deepwater give anadvantadge in angular resolution down to 4 degrees or less in detection of Cherenkov photons fromelectro-magnetic or hadron showers produced by high energy neutrinos in Lake Baikal.The site chosen for the experiment is in the southern basin of Lake Baikal. Here, the combina-tion of hydrological, hydro-physical, and landscape factors is studied to be optimal for deploymentand operation of the neutrino telescope. The water depth is about 1360 m at distances beginningfrom about of three kilometers from the shore. The water transparency is characterized by anabsorption length of about 20–25 m and a scattering length of 30–50 m [1]. The water chemilu-minescence is moderate at the detector site. Baikal-GVD is a three-dimensional lattice of opticalmodules (OMs), that is photomultiplier tubes housed in transparent pressure spheres, arranged atvertical load-carrying cables to form strings. The telescope has a modular structure and consists ofclusters of strings, functionally independent sub-arrays, which are connected to shore by individualelectro-optical cables as shown in Fig. 1 (left). Each cluster has a central string identical to sevenothers distant at radius of 60 meters in baseline configuration.
2. First demonstration cluster - Dubna
In April 2015 the first Baikal-GVD cluster named
Dubna was deployed and started operationin Lake Baikal following timetable of the project [1, 3]. The Dubna cluster encloses 1.7 Megatonsof fresh Baikal water. The cluster comprises a total of 192 optical modules arranged at eight 345m long strings, as well as an acoustic positioning system. There is also an instrumentation stringwith equipment for array calibration and monitoring of environmental parameters. Artistic viewof the Dubna cluster is shown in Fig. 1 (right). Each string comprises 24 OMs spaced by 15 m atdepths of 900 m to 1250 m below the surface. In 2015 seven side strings have been located at areduced radius of 40 m around a central one. The reason is to increase the sensitivity to low-energyatmospheric muons and neutrinos which are used for array calibration. In the next year, strings willbe moved to the baseline distances.A section is the basic detection unit of the GVD neutrino telescope. It comprises 12 opticalmodules (OM) and a central module (CeM). Among the basic functions of OMs are detection ofthe particle radiation; shaping of the output analog pulse for signal transmission to the ADC board;control of the PMT operation modes; calibration and monitoring of the parameters of OM electronic2 aikal-GVD
O.V. Suvorova
Figure 1:
Artistic views. Left: Layout of the GVD. In inner box the one cluster is shown. Right: The firstGVD cluster Dubna. components. The block diagram of a section and OM view are published elsewere [3, 4, 5]. Eachoptical module consists of a pressure-resistant glass sphere of 43.2 cm diameter which holds theOM electronics and the PMT which is surrounded by a high permittivity alloy cage for shieldingit against the Earth magnetic field. A large photomultiplier tube Hamamatsu R7081-100 with a10-inch hemispherical photocathode and quantum efficiency up to 35% has been selected as lightsensor. Besides the PMT, an OM comprises a high voltage power supply unit (HV), a fast two-channel preamplifier, and a controller. For time and amplitude calibration of the measuring channel,two LEDs are installed in the optical module. The OM controller is intended for HV control andmonitoring for PMT noise measurements and for time and amplitude calibration [4].The PMT signals from all OMs are transmitted to the CeM via 90 meters of coaxial cables,where they are digitized by custom-made 12-channel ADC boards with 200 MHz sampling rate.The slow-control board located in the CeM provides data communication between OM and CeMvia an underwater RS-485 bus. Also, this unit is intended for OM power control (to switch poweron/off for each optical module independently). The ADC board provides trigger logic, data readoutand digital processing, and connection via local Ethernet to the cluster DAQ center, control of thesection operation and the section trigger logic. A request analyzer forms the section trigger request(local trigger) on the basis of channel requests L (low channel threshold, 0.3 p.e.) and H (highthreshold, 3 p.e.) from 12 ADC channels. This unit contains a programmable coincidence matrix(12Hx12L), which provides a simple way to generate the section trigger request. There are twobasic trigger modes: (A) coincidences of > N L-requests within a selectable time window, or (B)coincidences of L and H requests from any neighbouring OMs within a section. A request of thesection trigger is transferred from the Master board through a string communication module (CoM)3 aikal-GVD
O.V. Suvorova to the cluster DAQ-center, where a global trigger for all sections is generated. Data from the stringsare transferred through DSL-modem Ethernet channel to the cluster center. The data transmissionbetween the cluster DAQ-center and shore station is provided through optical fiber lines extendedat about 6 km. yy/mm/dd E v e n t s , Master statistics
Figure 2:
Left: Integrated number of recorded events since April 2015. Right: The time difference betweensubsequent events is shown for one run.
In 2015 the Dubna cluster is operating in several testing and data taking modes. Since Apriletill June about 1.7 · events have been recorded. Stability and efficiency of the cluster operationduring 2015 are illustrated in Fig. 2 (left) by integral rate of general trigger. Quality of data isseen in Fig. 2 (right) with distribution of time difference between subsequent events for one run.Obtained exponential behavior is consistent with expectation for randomly distributed experimentalevents.
3. Performance and sensitivity
The first cluster of Baikal-GVD in its baseline configuration will have the potential to detectastrophysical neutrinos with a flux value measured recently by IceCube [2]. The search for high-energy neutrinos is based on the selection of cascade events generated by neutrino interactions inthe sensitive volume of array. After applying an iterative procedure of vertex reconstruction fol-lowed by the rejection of hits contradicting the cascade hypothesis on each iteration stage, eventswith a final multiplicity of hit OMs N hit >
20 are selected as high-energy neutrino events. Showereffective volumes for two GVD configurations are shown in Fig.3 (left). Shower effective vol-umes (11/3 condition - at least 11 hit OMs on at least 3 strings) for GVD*4 are about of 0.4–2.4km above 10 TeV. The accuracy of shower energy reconstruction with GVD configuration of10368 OMs is about of 20–35% depending on shower energy, while directional resolution (medianvalue) is 4 ◦ , which is substantially better than the 10–15 degrees accuracy for IceCube [2]. Theexpected number of background events from atmospheric neutrinos is strongly suppressed for ener-gies higher than 100 TeV. We expect about one event per year with E sh >
100 TeV from an all-flavorastrophysical flux in GVD-cluster with the normalization E × Flux = 3.6 · − GeV cm − s − sr − ,compared to about 10 events in IceCube. Preliminary estimate of the cluster sensitivity to one fla-vor neutrino flux with an E − spectrum and flavor ratio 1:1:1 for all–flavor flux as function of the4 aikal-GVD O.V. Suvorova observation years is shown in Fig.3 (right), with no systematics accounts. Three year expositionallows sensitivity at a level of flux value measured by IceCube.
Figure 3:
Sensitivities of GVD configurations to neutrino induced showers. Left: Effective volumes ofcascades detection. The curves labeled by GVD ∗ − spectrum asfunction of the observation years. The long-dashed line indicates the one flavor neutrino flux value obtainedby IceCube. A e ff n , m E n or m DM , GeVb ‘ bW + W - t + t - n‘nm + m - total 10 -26 -25 -24 -23 -22 -21 -20 -19 -18 -17 -16
10 100 1000 10000 < s A v > , c m s - m DM , GeV t + t - Natural scale P A M E L A F E R M I + H . E . S . S . FERMI dSphsMAGIC, Segue1HESS, dSphsIC79, GCIC59, VirgoANTARES 2007-2012, GCGVD 1 yr, sensitivity
Figure 4:
Left: Neutrino effective area of single GVD cluster (total, black) and averaged over neutrinospectra effective areas for different channels (color). Right: Sensitivity of GVD to tau-channel of darkmatter annihilations in the Galactic Center in comparison with other experiments.
We studied the GVD sensitivity with 12 clusters to neutrinos from dark matter annihilationsin the Galactic Center for 1 year of livetime observation. For this study we apply a muon trig-ger formed by requirements to select events with at least 6 fired OMs on at least 3 strings (3/6).The neutrino effective area for selection (3/6) as a function of neutrino energy for one cluster ispresented in Fig.4 (left) by black line. We choose a search region as a cone around the direction For case of DM decays in the GC and for more details see Ref. [6] and references therein. aikal-GVD O.V. Suvorova towards the GC with half angle y . The expected number of signal events in the search region forthe livetime T is estimated as follow N ( y ) = T h s A v i R r local p m DM J DW Z dE · S ( E ) dN n dE . (3.1)Here S ( E ) is neutrino effective area of the telescope. In Fig. 4 (left) along with effective area of onecluster (black) are shown effective areas averaged over neutrino spectrum dN n dE in given annihilationchannel.We consider b ¯ b , t + t − , m + m − , W + W − and n ¯ n channels, where in the latter case we assumeflavor symmetric annihilation. Neutrino spectra from dark matter annihilation have been takenfrom [7]. The astrophysical factor J DW here is an average value over the search region. The ex-pected upper bounds on dark matter annihilation cross section have been obtained from this equa-tion. There are several theoretical uncertainties in the number of signal events related to neutrinooscillation parameters, neutrino-nucleon cross section etc. However, the most important of them isthe uncertainty related to lack of knowledge of dark matter density profile near the GC. Our studyshows that 1 year GVD sensitivity with incorporated realistic efficiency and systematic uncertain-ties achieves values 5 · − cm s − for dark matter annihilation cross section and 2 . · s forDM lifetime in the most energetic n ¯ n channel.To summarize, since April 2015 the data taking with the first full-completed cluster Dubna ofthe Baikal Gigaton Volume Detector has been started. The array comprises 192 optical modules.The modules are arranged at depths down to 1,300 m. Over its next stages of construction, the tele-scope will be stepwise extended by deploying new clusters. By 2020, it is planned to be consistedof 10-12 clusters with a total volume of about 0.4 cubic kilometers.The work of S.V. Demidov and O.V. Suvorova was supported by the RSCF grant 14-12-01430. References [1] V.Aynutdinov et al., The prototype string for the km3-scale Baikal neutrino telescope, NIM A602 227.[2] M.G.Aartsen et al., Observation of high-energy astrophysical neutrinos in three years if IceCube data,Phys. Rev. Lett. 113 101101. Science 342 1242856.[3] A.D.Avrorin et al., The first construction phase of the Baikal-GVD neutrino telescope, ICRC, 2015.[4] A.Avrorin et al., Data acquisition system of the NT1000 Baikal neutrino telescope, Instr. and Exp.Tech. 57 262.[5] A.D.Avrorin et al., The optical module of the Baikal-GVD neutrino telescope, ICRC, 2015.[6] A.D.Avrorin et al., Sensitivity of Baikal giga volume telescope NT1000 to neutrino emission towardthe center of Galactic dark matter halo, JETP Lett. 101 (2015) 5, 289-294; arXiv:1412.3672.[7] P. Baratella et al., PPPC 4 DM n : a Poor Particle Physicist Cookbook for Neutrinos from Dark Matterannihilations in the Sun, JCAP (2014) 053; arXiv:1312.6408.(2014) 053; arXiv:1312.6408.