Status report on Baikal and AMANDA

PROCEEDINGS SUPPLEMENTS ELSEVIER

Nuclear PhysicsB (Proe. Suppl.) 66 (1998) 243-246

Status report on BAIKAL and AMANDA E. Kajfasz a ~Centre de Physique des Particules de Marseille, Case 907, 163 Avenue de Luminy, 13288 Marseille Cedex 9, France A status report is given onthe achievements of the Bailml and AMANDA collaborations in their efforts to detectect High Energy Cosmic Neutrinos.

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1. B A I K A L [1] 1.1. G e n e r a ] r e m a r k s The Baikal detector is installed 1350m deep in the Baikal lake (51°50~N-104°20'E). It is deployed from the stable ice platform that covers the lake several months a year. The goal of the collaboration is to complete the so-called NT-200 detector consisting of 192 optical modules (OMs) total, arranged in 8 strings holding 12 OM pairs each (see fig. 1). OMs of a pair are wired in coincidence (0.3 photo-electron (p.e.) threshold and 15ns time window) to form a channel. There is an electronic module (svjaska) for 2 channels. Channels of a string are connected to a string electronics module equipped with TDCs to digitize time on 11 bits ([-1000,+800]ns dynamic range, lns resolution) and amplitude on 10 bits ([1,1 000]p.e. dynamic range). Calibrations are performed every other day to check the stability of the digitization. Time calibrations are done by means of a nitrogen laser (300ps pulse) and optical fibers going separately to each OM pair. The muon trigger is built in the array electronics module by requiring _~ 3(4) local triggers within a 500ns time window. The chronology of the detector evolutions and improvements is the following: •

•

Apr'93-Mar'95: NT-36, a 3-string array with 6 0 M pairs on each string, is in operation. '95-'96:NT-72 (36 OM pairs) is in operation.

• '96:NT-72 is replaced by NT-96, a 4-string array with 12 OM pairs on each string. 0920-5632/98/$19.00 © 1998 ElsevierScience B.V. All rightsreserved. PII S0920-5632(98)00046-2

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With these detectors, 320 million muon events were recorded on the whole, corresponding to 700 days of data taking. • Apr'97: NT-144, a 6-string array equipped with 72 OM pairs, is in operation. • ~98:NT-200 will be completed. For NT-36 and NT-72, half of the OMs are looking upwards (downwards). However, because of sedimentation problems (60% decrease in sensitivity per year for an up-looking OM), only 2 pairs out of 12 on each string are up-looking in NT-96. 1.2. W a t e r p r o p e r t i e s Water luminescence in Baikal lake shows strong seasonal variations with typical values of 50kHz/OM. However these variations do not affect the muon trigger rate. The absorption length

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E. Kajfasz/Nuclear Physics B (Proc: Suppl.) 66 (1998) 243-246

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Figure 2. Vertical muon flux vs. water depth. Figure 3. Upgoing neutrino induced muon candidate. is Lab, ~ 20m for A G [470,500]nm, with <_ 20% seasonal variations. Light scattering is strongly anisotropic (< cos0 > ~ 0.95) and the scattering length is typically L,,~t ~ 15m. The effective scattering length is defined as:

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so L~y] ,-~ 300m. 1.3. P h y s i c s r e s u l t s Muon tracks are reconstructed by minimizing:

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where T~(0, ~, u0, v0, to) is the thne expected for a givent set of track parameters, ti is the measured time and o't~, is the timing error. The OM signal amplitudes are only used to calculate ~r~. A t m o s p h e r i c m u o n s : the reconstruction procedure is based on causality w.r.t, propagation of a naked muon, a X2 minimization with time information only, and quality criteria such as probabilities for a (non-)hit channel (not) to respond to a naked muon. 28% of the down-going

muons, but almost none of the muon bundles, survive after such a procedure which results in a 1.8 0 median angular ndsmatch. A zenith angle distribution ofmuon intensity has been measured, from which a distribution of the vertical muon flux as a function of water depth has been calculated. The Baikal results are in good agreement with other measurements and expectations (see fig 2). S e p e r a t i o n o f u p w a r d - g o i n g m u o n s : because it has too short a lever arm, NT-36 cannot discriminate between up- and down-going mnons: its signal to noise ratio ~(NT-36) is of order !45" Using Monte Carlo extrapolations, a -~(NT96)_> 1 and a ~(NT-200)_> 4 are expected. With NT-96, 3 neutrino induced muon candidates have been found in 12.9 days for 2.3 expected. Fig 3 shows one of the candidates. From NT-36 data, out of 83.3 million events, 2 nearly vertically upgoing muons have been found for 1.2 expected. The probability for these events to be fake is estimated to be between 0.01 and 0.1. From this result, a limit on the flux of up-going neutrino in-

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E. Kajfasz/Nuclear Physics B (Proc. Suppl.) 66 (1998) 243-246

Table 1 Effective surfaces for different detector configurations and E/~ thresholds. S~!! vertical up-going v-induced ~t's 1 TeV v-induced # ' s t,-induced p ' s (> 20GeV) NT-36 50m z 150m ~ 270m 2 NT-96 645m 2 NT-200 1400m ~ 2 300m 2

duced muons resulting from neutraiino annihilations in the center of the Earth (Eg > 6GeV) has been set to ~(90~oC.L.) _< 1.3 10-13/~ • crn - 2 . s -1 in a 150 cone around the nadir direction.

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1.4. T h e f u t u r e The collaboration has a proposal for an array with 2 500 OMs. It should have an effective surface S~f! E [3 l0 s, 3 10S]m 2 for energies in the range [0.1,100]TeV. 2. A M A N D A

[2]

2.1. G e n e r a l r e m a r k s AMANDA is a detector located at the South Pole in a 3kin-thick ice sheet. Holes are drilled with hot water and OMs are frozen into ice. 2km deep holes can be drilled and fully equipped in 6 days. Each OM is connected to the surface by its own cable. As ice is a sterile medium devoid of radio-activity, the background eountings rates are particularly low (a few 100Hz), so that offthe-shelf electronics can be used. Fig. 4 shows all the OM strings deployed up to ~97. 2.2. S e q u e n c e o f A M A N D A d e t e c t o r s In what follows, a chronology of the different detector configurations installed and to be installed at the South Pole is given. AMANDA-A/~9$-'94/0.8-1km: 4 strings were deployed. 73 out of 80 OMs survived refreeze. The noise rate is -.~ 1500Hz at a gain of 10 s. Labs ~ 280m at 410nm and 800m deep. Labs decreases with depth because of an increase in dust concentration. L,cat ~ 10-20cm because of air bubbles trapped in ice at relatively small depths. There in no way to reconstruct muon tracks with AMANDA-A b u t because of its large Lab, it can be used for calorimetry (trigger for

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AMANDA-B/195J96/1.55-1.95km: No air bubbles are present at this depth. Let! ~ 25m with < cos0 >--, 0.8 independant of wavelength. Lab, "~ 100m for A E [337, 450]nm and depth in the range [1650,1850]m. 4 strings were deployed. 79 out of 86 OMs survived refreeze. The noise rate is ~ 300Hz at a gain of 109. Timing resolution is ~ 3ns. The event rate for > 8 0 M s hit is 25Hz. A new SN/GRB trigger was installed: no candidates have been seen yet. AMANDA A / B coincidences

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E. Kajfasz/Nuclear Physics B (Proc. Suppl.) 66 (1998) 243-246

occur at a rate of --~ 0.2Hz, one of which is shown in fig. 5. AMANDA-B/~96J97/1.55-1.95km: 6 new

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with 4 strings and 10000m ~ (resp. 2.5 °) for the full AMANDA-B. AMANDA-H - towards a km a detector: 3 or 4 holes will be drilled at a depth of 2 600m in /97J98. Ice properties will be measured for depths in the range [1400,2 600]m. The holes will be instrumented with 1kin long strings equipped with -~ 80 OMs each. In ~98J99, 10 holes drilled on a 100m-radlus circle should be fully equipped aiming at Se/! -~100 000m 2 and a pointing error 1°. In t00-t04 the construction of a k m s detector extending the AMANDA-II configuration could be performed. R E F E R E N C E S

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. I. Sokalski, XXIInd Rencontres de Moriond, Les Arcs, France, Jan. 1997. The Baikal Underwater Neutrino Telescope DESY 97-033, Feb. 1997. . B. Price, XXIInd Rencontres de Moriond, Les Arcs, France, Jan. 1997. S.W. Barwick, contribution to the OECD Workshop on "A Deep-Sea Neutrino Observatory", Taormina, Italy, May 1997. F. Halzen, Workshop on High Energy Cosmic Neutrinos, Maxseille, l~ance, June 1997.

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strings with 36 O M s each were added. The full array has 290 operating OMs. A few candidates for up-going neutrinos induced muons were seen in the 4-string A M A N D A - B configuration. 2 nearly independant" codes have been developped for reconstruction and simulation. They agree with each other and with data of the 4string AMANDA-B configuration. The only cut used is that Ndirect, the number of hits with 6t--thit - t~n,catte~ed < 24ns, should be > 5. Photons with 5t > 24ns have substantially scattered, so they are not good for track measurements but stillcan be used to infer the energy of the muon. For a downward rejection ratio > l0 s, Se/! (resp. the median pointing error) should be 100m 2 (resp. 4.5 °) for AMANDA-B