A Multimessenger Neutrino Point Source Search with IceCube Mădălina Chera FLC Group Meeting 04.10.2010 Mădălina Chera
Overview 1 Introduction to UHE Cosmic Rays and Neutrino Astrophysics; 2 Motivation for a multimessenger study; 3 The detectors: IceCube, HiRes and PAO; 4 Data samples; 5 Binned analysis method; 6 Preliminary results; 7 Outlook & conclusions.
Ultra High Energy Cosmic Rays and Neutrinos Astrophysics Introduction (I) Cosmic rays: an almost one century old discovery; Extensive research performed ever since the discovery has revealed important aspects regarding the CR: the nature of the CR and the energy spectrum.
Ultra High Energy Cosmic Rays and Neutrinos Astrophysics Introduction (II) 1. Composition of the cosmic rays: 4 charged nuclei; 4 γ rays, e±, antiprotons, neutrinos; 4 neutrons.
Ultra High Energy Cosmic Rays and Neutrinos Astrophysics Introduction (III) 2. Energy spectrum:
Astrophysical Particle Acceleration Scenarios For energies ranging from 10 15 ev up to 1020 ev there are 2 scenarios: í TOP-DOWN SCENARIOS: supermassive relic particles; å disfavored by recently observed flux limits. í BOTTOM-UP SCENARIOS: Fermi acceleration ã Key concepts: energy gain ( E = ξ E0 ); escape probability Pesc. ã Leads to the observed power-law spectrum: N 1 Pesc E E0 α ;
Astrophysical UHE Neutrino Production Neutrinos as secondaries produced in the vicinity of the source high matter density at source region 0 π p+p π+ +2p +n + p low matter densities (sources larger than the stellar scale) + π +n p + γ + π 0 +p in both situations, the pions decay resulting: π+ π π0 µ+ + ν µ µ + νµ e + + νe + ν µ + νµ e + ν e + νµ + ν µ γγ ratio of neutrino flavours: νe : νµ : ντ = νe : νµ : ντ = 1 : 2 : 0
The Motivation and Concept of the Multimessenger Study Motivation ã verifying the predictions of the hadronic acceleration models; WHY NEUTRINOS? ã neutrinos are not influenced by intergalactic magnetic fields; ã they interact weakly; ã they are distinguishable from the atmospheric neutrino background! Concept correlate significant excesses in the neutrino observations with incoming directions of other cosmic messengers: charged cosmic rays or gamma rays;
The IceCube Detector located in Antarctica; consists of 5200 PMTs placed in a lattice structure; 86 strings: each string holds 60 DOMs; energy threshold of 100 GeV; max. sensitivity at 10 TeV; detector coordinates: zenith and azimuth. cosmic ray coordinates: declination=zenith-90 and right ascension.
The Pierre Auger Observatory and the HiRes Experiment The Pierre Auger Observatory ã situated in Malargue, Argentina; ã consists of: a surface array of Cherenkov detectors; 24 fluorescence telescopes. The HiRes Experiment ã located in the vicinity of Salt Lake City, USA; ã consists of two detection stations; ã HiRes-I: 21 mirrors, HiRes-II: 42 mirrors.
Data samples: IceCube Data IceCube40 real data set ã acquired with a detector configuration of 40 strings; ã recorded over an uptime period of 375.5 days (from 5.04.2008 until 20.05.2009); ã data covers the full sky (from -85 to +85 in declination); ã event selection leads to 36900 events in the final sample; ã angular resolution: 0.6 in the Southern sky and 0.8 in the Northern sky. Simulated neutrino sample ã contains 5 106 primary neutrinos spread on the full sky; ã all generated neutrinos interact in the detector s volume; ã the proabability of each interaction is stored.
Data samples: UHECR events from PAO and HiRes The PAO data sample: ã 27 events with energies above 6 1019 ev ; ã data obtained over a runtime period of 3.5 years; ã events spread from -60 to +15 declination; ã angular resolution 0.9. The HiRes data sample: ã 13 events with energies above 6 1019 ev ; ã data obtained before 2006; ã events spread from -30 to +60 declination; ã angular resolution 0.8. The two data sets are handled as one single sample.
The Binned Analysis Method ã Concept: search for any significant excess in the IceCube data sample focused around the arrival directions of the UHECR events provided by the HiRes and PAO. consider a circular region around each UHECR arrival direction, i.e. a bin; count the number of events found in each bin; null hypothesis: no correlation = background only data; alternative hypothesis: correlation exists = signal detected over background data; test statistic: sum of the number of events found in each bin around each source direction.
Method Optimization: Obtaining the data - Background only Simulating the background: (100000 maps) å IC 40 data: (full sky) 36900 events recorded over a runtime of 375 days; å scramble right ascension data for each event; å leave declination data intact.
Method Optimization: Obtaining the data - Background + Signal Simulating the signal + background: (10000 maps per each flux strength) å simulate magnetic deflection: randomly shift source position within a region of angular distance of 3 ; å inject signal events around sources: 1 2 3 4 divide skymap in 3 declination bands; calculate average neutrino number in each declination band for a certain flux strength; calculate the number of neutrinos to be injected in the region of each source by randomly sampling a Poisson distribution centered around the previously calculated average and by taking into account the chosen source strength (flux); add the computed number of neutrinos over scrambled background while randomly removing an equal number of background events in order to keep the statistics unbiased.
Distributions of the Test Statistic We have considered 21 different bin sizes (angular distances): from 1 to 5 in steps of 0.2 plus a bin size of 2.5 ; We have assumed a constant flux identical for all sources. We have used 17 different flux strengths for each set of 10000 generated maps. The strengths range from 0.1 10 8 GeV/cm2 s to 1.7 10 8 GeV/cm2 s.
Preliminary results - The Discovery Potential Discovery potential [Φ0 ] = the neutrino flux strength necessary for a 5σ discovery in 50% of the trials. no energy cut: Φ0 = 0.91 10 8 GeV/cm2 s with energy cut: Φ0 = 0.36 10 8 GeV/cm2 s
Preliminary results - Bin Optimization Chosen optimal bin size: 2.5.
Preliminary results - The Sensitivity The sensitivity = the average upper limit on the flux calculated from a sample of background only data. It was calculated with the Feldman-Cousins method. Chosen optimal bin size: 2.5.
Comparison to IC22 results Discovery potential: Φ0 = 1 10 8 GeV/cm2 s; Optimal bin size: 3.
Future prospects Use maximum likelihood method including the stacking of the sources and the magnetic deflection in the signal term. ã Improvements with respect to the binned method: 3 solves the information reduction problem; 3 solves the optimization problem. THE BINNED METHOD AWAITS THE UNBLINDING!
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