Agafonova, N Y

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29th International Cosmic Ray Conference Pune (2005) 5, 307310Triggering on Neutrino Bursts in LVDN. Agafonova , M.Aglietta, P.Antonioli, G.Bari, V.Boyarkin , G.Bruno, E.Dobrynina ,W.Fulgione, P.Galeotti, M.Garbini, P.L.Ghia, P.Giusti, F. E.Kemp, V.Kuznetsov ,A.S.Malguin , H.Menghetti, A.Porta, A.Pesci, I.A.Pless, V.G.Ryasny ,O.G.Ryazhskaya , O.Saavedra, G.Sartorelli, M.Selvi, C.Vigorito, L.Votano,V.F.Yakushev , G.T.Zatsepin and A.Zichichi1 IFSI-INAF, Torino, University of Torino and INFN-Torino, Italy2 University of Bologna and INFN-Bologna, Italy3 University of Campinas, Campinas, Brazil4 Institute for Nuclear Research, Russian Academy of Sciences, Moscow, Russia5 Massachusetts Institute of Technology, Cambridge, USA6 INFN-LNF, Frascati, ItalyPresenter: W. Fulgione (fulgione@to.infn.it), ita-fulgione-W-abs1-og27-oralThe capability of a supernova neutrino detector to identify clusters of signals is discussed. A definition ofthe detector sensitivity in terms of maximum detectable distance is proposed. Different trigger strategies,implemented in the running LVD detector, running in the INFN Gran Sasso National Laboratory, are comparedand discussed.1. IntroductionThe detection of neutrinos from SN1987A marked the beginning of a new phase of neutrino astrophysics [1].In spite of the lack of a “standard” model of the gravitational collapse of a massive star, the correlated neutrinoemission appears to be well established.However since this first  observation was guided by the optical one, the detector capabilities of identifyinga  burst in the absence of an ”external trigger” should be demonstrated very carefully. In the presence of anoptical counterpart, on the other hand, the prompt identification of the neutrino signal could alert the worldwidenetwork of observatories allowing to study the rare event in all the different aspects since its onset.LVD, located in Hall A of the INFN Gran Sasso National Laboratory (Italy), at the depth of 3500 m w.e., isa 1 kt liquid scintillator detector consisting of 840 identical counters whose major purpose is monitoring theGalaxy to study neutrino bursts from Gravitational Stellar Collapses (GSC) [2]. Its modularity and rock over-burden together with the trigger strategy make this detector particularly suited to on line disentangle, from thebackground, a neutrino signal. We will show in this paper the criteria for neutrino burst identification (section2), consisting in two steps: the filters applied to the data and the selection algorithms. In section 3 we willdiscuss the detector sensitivity in terms of physical parameters such as the fluence or the source distance.2. Event Selection2.1 Data FiltersThere are two different filtering levels: the first is applied on the data, to reject signals not considered perti-nent (e.g., atmospheric muons); the second is applied on the counters (to reject poorly performing ones) anddetermines the LVD active mass (	). The following filters are applied to the data:1. the energy corresponding to detected pulses must be ﬁﬀﬃﬂ !MeV ( #"%$MeV);308 N. Agafonova et al.2. coincidences among 2 or more counters in a time window &('"ﬃ)*!ns are rejected (muon rejection).For the counters selection we reject:1. counters that, during the previous run and after the energy cut, have a counting rate +-,/.10ﬂ 3234s265;2. counters which participate to a cluster with a multiplicity 798 which correspond to :<;=3>?@A79BDCFE ﬂ0ﬂG2IH, where 7 is the cluster multiplicity and Cthe number of active counters (topological cut).2.2 Selection Algorithms2.2.1 Time Sequence AlgorithmIn the time sequence algorithm the searched signal is a cluster of pulses detected inside a time window of adefined duration, &(' . All other signal characteristics like detailed time structure of the clustered pulses, energyspectrum and flavor signatures, are left to a subsequent independent analysis. The signal is simply characterizedby its multiplicity 7 , i.e., the number of pulses detected during &(' , and by &(' itself.Each time period of data taking J is scanned by an on-line algorithm, called Supernova Monitor, through a“sliding window” that has been chosen of duration &('"K)Ls: J will then be divided into C"K)0NMOQPﬂintervals, each one starting at the middle of the previous one. The background imitation frequency for a cluster(m, 20 s) will be: RS8T>U@A7WVYX*Z[;\V)LE"^]L_*`\0:S;(=I>U@)L0badcAeYf\gEevent 0 day2h5Knowing the detector background rate, X!Z[; , to any imitation frequency corresponds a minimum multiplicity7jidk of the detectable cluster (such that : ;(=I> @A7jilkE,nm o7pidk [3], to take into account fluctuations).For LVD running in stand-alone mode the on-line alarm threshold is set at the level of R 8T>"qﬂ0ﬂ r2Is0 y265,corresponding to 7jidk/,)Gﬂ. On the other hand, running in coincidence with other detectors (e.g., in theSNEWS project [4]) naturally reduces the amount of fake alarms: in this case the LVD alarm threshold is setto R 8T>"tﬂ0 week2h5, corresponding to 7uilkv,ﬂ _.2.2.2 The Inverse Beta Decay SignatureIndependently on the unknown neutrino properties and in the frame of ”standard” Supernova models, it isreasonable to state that the inverse beta decay (ibd) reaction ( w ﬀSxpy{z}|(~{xW ) is responsible for at leastoL\of the total number of interactions due to a GSC in a Hydrogen-based detector. Since LVD is able to recognizethe ibd reaction with a known efficiency Drm _, we consider, in each cluster, only “signed” pulses, i.e., thoseaccompanied by a delayed one (IBD-1). In the background imitation frequency formula, X Z[; is substituted byXZ;n  , with X Z;n "XZ[;@E0w:SZ[;"%)Gm ]0ﬂ G2303Łl52h5( w: Z; being the average probability for a pulseto be followed by a delayed one due to background).Additionally, we can relax the condition on the ibd signature requiring that only part of the pulses in the clusterare accompanied by delayed ones (IBD-2).In this case we reject all the clusters which have a number of ”signed” pulsesﬃ, such: d;dk:@Vl7WVyE:k , where :@Vl7WV yEis the binomial probability to have r signed pulses in a cluster of multiplicity 7 . Wechoose : k"rmTﬂas an example (the case : k"corresponds to the pure time sequence algorithm).Both algorithms are of course more sensitive to lower multiplicities with respect to the pure time sequence one(see table 1). However, because of the  -capture efficiency  , the IBD-1 method is less effective, while theIBD-2, even if more efficient than IBD-1, still is not powerful enough to justify the loss in model independence.Triggering on Neutrino Bursts in LVD 309Table 1.RS8T>"ﬂ0  |(|2h5RS8T>"ﬂ0ﬂ 2Is02h5strategy 7j>N8 skk'  ¡£¢~<¤ ¥F¦Y§(¨ ?ª©¡{« 7j>N8 skk'  ¡£¢~<¤ ¥F¦d§(¨ ?©¡{«time seq.$ ﬂ _ )rmﬂ0ﬂ r2I4D5 )rﬂ )Gm ]0ﬂG2I4D5IBD-1$ $ )Gm _0ﬂ r2I4D5 ﬂ.m `0ﬂG2I4D5IBD-2$ ﬂ `@¬E ﬂLm o0ﬂ r2I4D518(7))Gm `0ﬂ r2I4D5time seq.ﬂ  $ ﬂ!m.10ﬂ r2I4D5 ﬂ ﬂLm­$0ﬂG2I4D5IBD-1ﬂ  ` ﬂLm ]0ﬂ r2I4D5 _ )Gm `0ﬂG2I4D5IBD-2ﬂ  _@) E ﬂLm®)0ﬂ  2I4D59(4)ﬂLm _0ﬂ  2I4D5Burst detection efficiency for different strategies and for two values of imitation frequency R 8> , for¯"-ﬂ L!ton. is expressed in MeV; ' in seconds. 7 >8  represents the minimum detectable clustermultiplicity (90% c.l.) (between parenthesis the minimum number of double pulses requested in the cluster).3. ° Burst SensitivityWe discuss here the detector sensitivity in term of physical parameters, like for example the minimum de-tectable fluence (time integrated  flux at the detector) or the maximum detectable distance.The cluster multiplicity, 7  , due to the signal, detected during a time interval &(' , in the case of pulsed wL±emission, is:C±[²"ﬃ¯³Y´[µ0C#¶10 ·0Ok' ¡I¸¹~k¤ ¥ŁﬀlºF¦Y§\»¼½¡»¼½«@v¾¿½Elv¾¿½where:  is the detector efficiency,À³d´[µ[t] is the detector active mass, CÁ¶ is the number of free protons perscintillator ton, « @ ¾¿ ½Eis the neutrino interaction cross section, andF¦d§»¼½¡»¼½ is the differential w(± flux at thedetector.In the absence of any hypothesis on the source emission spectra and on  oscillation parameters, we can ex-press the detector burst sensitivity in terms of 90% c.l. minimum flux at Earth 0 cross-section, integrated over&(' :'F¦Y§¡«">NÂÄÃ2alceOŁ¸ÅﬁÆ ÇÈÆ É . These are shown in Table 1, for the different selection criteria, and forRS8T>"-ﬂ0ﬂ r2Is0 y265and RS8T>"-ﬂ0 week2h5.To infer the LVD sensitivity, we compare such values of'F¦d§¡« with the one measured by KamiokandeII on the occasion of the SN1987A (at ¬)kpc), ranging between_rm. and$Gm®$0ﬂ234ds. 1 Under the hypothesisof isotropic  emission, this value can be scaled to different distances: e.g., at 10 kpc, it would correspondtoﬂLm o0ﬂ r2I4k and`nm­$0ﬂ r2I4D5at 20 kpc. Comparing these with the results of Table 1, it follows that, withany of the selection algorithms applied to LVD in the present background conditions, we get a burst selectionsensitivity well suited to cover the entire Galaxy. Based on such definition of sensitivity to neutrino bursts,taking into account that the LVD active mass is dynamical, it is straightforward to derive that the detector issensitive to the entire Galaxy ( Ê/)Lkpc) if,Ë¬Lt for R 8T>"-ﬂ0ﬂG2Is0 y2h5.ÌThe range is determined by the value of the detected number of events: either 11, if all the events are considered as ÍÎ(Ï interactions, or9, when taking into account the possible contribution due to background contamination and elastic scattering on electrons [5].310 N. Agafonova et al.4. ConclusionsWe have shown that the LVD neutrino observatory is able to identify on line neutrino bursts from GravitationalStellar Collapses occurring in the whole Galaxy. In the occurrence of the next galactic core collapse supernova,even in the absence of an optical counterpart, LVD will be able to trigger other observations. This capabilityis attained, besides through its large mass and deep location, by an optimization of the event selection chain aswe have described in the paper.References[1] R.M.Bionta et al., Phys. Rev. Lett. 58 (1987) 1494;K.Hirata et al., Phys. Rev. Lett. 58 (1987) 1490;E.N.Alekseev, L.N.Alekseeva, V.I.Volchenko and I.V.Krivosheina, JETP Lett. 45 (1987) 589 andPhys.Lett.B 205 (1988) 209;V.L.Dadykin et al., JETP Lett. 45 (1987) 593 [Prisma Zh. Eksp. Teor. Fiz. 45 (1987) 464].[2] LVD Collaboration, Il Nuovo Cimento A105 (1992) 1793[3] Montanet, L., et al., 1994, Phys. Rev., D, 50, 1173[4] http://hep.bu.edu/  snnet/P.Antonioli et al., New Jurnal of Physics 6 (2004) 114[5] M.L.Costantini, A.Ianni and F.Vissani astro-ph/0403436

Triggering on Neutrino Bursts in LVD

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