Obtaining Fluence Upper Limit for Electron Antineutrinos Associated with Gamma Ray Bursts

1. Obtaining Fluence Upper Limit for Electron Antineutrinos Associated with Gamma Ray Bursts Maria Toropova MEPhI — NRC Kurchatov Institute Borexino General Meeting December 17, 2013

2. History of GRB Neutrinos Study The mechanism of GRBs is yet not well-known. All proposed theoretical models need to be confirmed by experiment. Search for neutrino events correlated with GRBs previously had been performed by First approach to study GRBs in Borexino was performed by V.Kobychev: Search for time correlation between antineutrino candidates and GRBs (internal note, Aug 27, 2010). Model-independent limits for the fluence of low-energy (below 7 MeV) electron antineutrinos were set. • Antares • Baikal • Baksan • IceCube • SNO • SuperKamiokande

3. Data Selection Borexino FADC data were used. Data-taking period: from December 2009 to September 2013. Data from Swift and Fermi satellites were used to build the GRB list. More than 1000 GRBs were taken for analysis.

4. Event Selection Electron antineutrinos are detected via the inverse beta decay reaction: Ethr = 1.8 MeV Analysis cuts: Energy of the prompt event: 1-50 MeV Energy of the delayed event: 1.3-3 MeV Time between events less than 1.5 ms Pulse shape cut to avoid noise events

5. Background Sources Background sources are: Geo-neutrinos (0.011±0.003 ev/day) Antineutrinos from nuclear power plants (0.023±0.005 ev/day) Cosmogenic Li-He (0.102±0.017 ev/day) Background was estimated according to the last geo-neutrino paper: 1.57*10-6 ev/s

6. Method used Method proposed by SNO collaboration was used (arxiv:1309.0910v1 [asrto-ph]) Two time windows chosen: ±1000 and ±5000 seconds around burst time. To show up the possible excess of signal over known background the maximum likelihood method was used.

7. Maximum Likelihood Method Assumption: extra events in data are directly related to the burst intensity expected number of detected events correlated with each GRB Ij is GRB intensity (given by Swift and Fermi data in units erg/cm2), wj is weighting factor, rB is background rate. We expect actual number of events to form a Poisson distribution. (kj is observed number of events) Likelihood for all bursts

8. Minimization Minuit minimization of the L function with respect to α. Time window = ±1000 sec αfit= 175.5 -ln(L(αfit))=38.791 The 90% upper limit on α: =>α90 =440 Time window = ±5000 sec αfit= 503 -ln(L(αfit))=154.351 α90 = 934

9. Obtaining Fluence Limit As far as neutrino energy spectrum is unknown, the limits on neutrino fluence are usually expressed in terms of ''Green's function'' fluence for mono-energetic neutrinos: Ni = 1.34*1031 is the number of free protons σi is the cross section for antineutrinos εi = 0.84 is the detection efficiency (geo-neutrino paper) N90 is the 90-percent confidence limit on the number of burst-associated antineutrinos

10. Results For the time window ±1000 s (±5000 s) 3 (17) antineutrino candidates found. Consistent with the number of accidental coincedences found. Two interesting events in correlation with GRBs were found: 1. Run16168, FADC ev. 1917, BX ev. 254560, Eprompt = 18 MeV, 1.3 s after muon, 3237 s before GRB. 2. Run17888, FADC ev. 176, BX ev. 12596, Eprompt = 8.5 MeV, 9 s after muon, 4276 s after GRB.

11. Results • In the energy range of 2-7 MeV Borexino limits stronger than SNO data. • For E > 7 MeV SuperK limits are stronger.

12. Conclusion Fluence upper limits for electron antineutrinos from GRBs in the energy range of 2-50 MeV were obtained. In the energy range of 2-7 MeV Borexino limits are stronger than those obtained by SNO.