Zhu, Q Q

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29th International Cosmic Ray Conference Pune (2005) 4, 97100Searching for Point Source Signals through MuonsQ.Q Zhu , X.H. Ma and Y.Q. Ma for the L3+C collaboration(a) Institute of High energy Physics, CAS, P.O.Box 918, Beijing, 100049, ChinaPresenter: Q.Q. Zhu (zhuqq@mail.ihep.ac.cn), chn-zhu-Q-abs1-og22-posterThe results from an analysis of muon data on cosmic ray source signals collected with the L3+C experiment atLEP CERN are presented. A general-purpose sky survey method for any detectors with anisotropic acceptanceis described.1. MotivationThe energy spectrum of charged cosmic rays spans a very broad range, from sub-GeV to energies larger thanGeV. The origin of Ultra-High Energy (UHE) cosmic rays have been studied for about 50 years. Since1989 several Very High Energy (VHE) gamma ray sources have been discovered by means of different ex-perimental techniques, the Imaging Atmospheric Cherenkov Telescope (e.g. Whipple, HEGRA, HESS, CON-GAROO) and the EAS array (e.g. MILAGRO,AS  ,KASCADE, ... etc). The spectrum of gamma rays can beexplained by ”lepton models” (synchrotron radiation from accelerated electrons, inverse self-Compton effect).However the origin of nuclei sources of UHE cosmic rays is still unclear, though various theoretical modelsexist [1, 2].The unique properties of the L3+C muon spectrometer [3, 4], as well as its location at a shallow depth (lowmuon energy threshold), offered a new opportunity to search for large flares from known as well as yet un-known sources at relatively low energies,  100-1000 GeV. New data on high energy gammas are expected toconstrain models of particle acceleration at the source. However, the sensitivity of a muon tracking detector issignificantly smaller for the detection of cosmic 	 -rays due to the very small cross-section for photo-productionof charged pions. This disadvantage is partially offset by the fact that the muon tracking detector can have alower energy threshold (larger gamma fluxes !), even lower than the presently operational Cherenkov tele-scopes.2. The L3+C muon spectrometer and the dataThe L3 detector (Figure 1) [3, 4] was designed to measure accurately muons, electrons, gammas and hadronsat the large electron positron collider LEP, CERN. It consisted of a huge solenoidal magnet of 1000  volumewith a field of 0.5 T and a set of high precision drift chambers.The detector is located under only 30 m of overburden, at an altitude of 450 m above sea level, and the energythreshold for atmospheric muons is  15 GeV. The coordinates of the L3+C site are 6.02  E, 46.25  N.The addition of timing scintillators on the top of the magnet, as well as a trigger and readout system differentfrom the L3 experiment, enabled the acquisition of cosmic ray muon data, independently of the running of L3and LEP. The arrival time of an event was recorded with an accuracy of 1  sec (GPS timing). The chamber,scintillator, and trigger efficiencies have been discussed in detail elsewhere [5].The data were taken during two periods, mid-July to November 1999, and April to November 2000. Overall triggers have been recorded, during an effective live-time of 312 days.Concerning the detection properties important for this discussion: The angular resolution is less than 2  for98 Q.Q. Zhu, X.H. Ma and Y.Q. Mamuon energies above 20 GeV and for muons crossing at least three drift chambers. The acceptance of L3+Cdepends on the zenith and azimuthal direction, and is slightly different for positive and negative muons.ﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬀﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁ ﬁL3 innerdetectorsmagnet drift chambers11 msurfacescintillators3 m3 mmolasseµ30 mFigure 1. Schematic view of the experimental setup3. Search for steady cosmic muon sourcesA particular sky survey has been made in the following way. The sky field covered by L3+C has been sub-divided into 40 by 90 cells, with a cell width ﬂﬃ of 2  in declination and ﬂ! = "# in right ascension. Thetrajectory of each cell across the sky has itself been subdivided into 40 zenith angle segments from zenith angle$= % towards the south east, to 60  towards northwest, each segment having a width of ﬂ'&)(+*,$-=. /.All muon events were distributed in a 3 dimensional frame ( "103210"bins). 45&) 76ﬃ86$-is the numberof muon events in each bin. The background for a given zenith angle bin (which is fixed with respect to aparticular detection direction of the detector - with its particular acceptance and efficiency ) has been measuredfrom the content of the given declination band swept from  = 0 to 24 h across the particular bin. For eachof the 90 bins in  , with given ﬃ and$, the event distributions have been found to be quite uniform. Figure 2shows two typical examples.In a given declination band of ﬃ:9 , for the cell of  <; at zenith angle$>=, the significance of excess eventsabove the background 4@?BADC is calculated according to [6]E=GF;IHKJML4ON?BP=QF;SRQTVUSW3X=GF;X=QF;&4N?P=GF;4N?P=QF;W4?ADC=GF;-<YW4?BADC=QF;ZRQTVU&SW3X=GF;-&4?ADC=QF;4N?P=GF;W4?BADC=QF;-[Y]\_^`6 (1)Searching for Point Source Signals through Muons 99where NN?BP=QF;is the number of events in the ( ﬃ86a ; 6$ =)-bin; b is the index of the zenith angle bin and runs from 1to 40 for given ﬃ and  ; c the index of the right ascension bin, ranging from 1 to 90 for given ﬃ and$.X=GF; isthe live-time ratio:X=QF;dH e&) [;>6$8=gfﬃ-hjik+lFkml;e&Q k6$ = fﬃ-(2)heree&) 76$!fﬃ-is the exposure time corresponding to N(  S6$nfﬃ ), andNN?P=GF;= N(  <;>6$8=]fﬃ ), N ?BADC=QF;Hh ikDlFkml;45&) k6$8=ofﬃ-.Decl.= 31 o, Zenith= 30 o(SE),  Mean Muon= 26634   Mean Exposed Time= 283750 sCounting Rate Changes with R.A.Count Rate (/sec.)Decl.= 31 o, Zenith= 27 o(SE),  Mean Muon= 30354.4   Mean Exposed Time= 322735 sRight AscensionFigure 2. Two examples of the muon rate distributionfrom various right ascension cells with p = ( q>rts3r ) u, v = qw u (towards the South East) and p = ( q>r]sxr ) u, v = yz u respectively. The arrow indicates the bin ofR.A. = ( rrD{SsVy ) u .anumber of bins bnumber of cellscnumber of cells dprobabilityFigure 3. a.) | -distribution from each ( }o~p~v ) -bin.The inserted dotted line is the | -distribution for the celllocated at }'rr{ u and p5qr u , with an apparent ex-cess of events. b.) Distribution of |. , averaged overall zenith bins for each } cell. c.) and d.)  -distribution.The maximum one is ve orders of magnitude higherthan the background.Figure 3a shows that the observed U=GF; are distributed as a Gaussian around a mean  = 0 with a variance of 1.In order to estimate the significance of this ”excess” in this particular (  ; 6ﬃ )-cell, the Student’s  was cal-culated by taking U=QF; from all zenith bins of the cell and taking the corresponding distribution width intoconsideration,100 Q.Q. Zhu, X.H. Ma and Y.Q. Ma; HE;Ł 6 (3)whereE;IHﬀ, S` HD .HereTis the number of zenith angle bins withXtN?BADClarger than 15 for each &Q S6aﬃ--cell. It ranges from 25 to40, depending on the upper culmination of each declination band.TheEdistribution shown with dotted line in Figure 3a represents an unusual distribution for two cells (  =9 2  and ﬃ = 31 9 1  , or alternatively  =29 2  and ﬃ =-31 9 1  in case of up going muons). Themean is positive and the distribution is narrow (E; = 0.691, = 0.1038  t ;H 6.66, see Figure 3c).The normalized  -distribution, represented as a probability is plotted in Figure 3d, suggesting that the observedeffect may be due to an excess. The chance to be a statistical fluctuation of the background is+ , with atotal number of trials for this excess being 40090 = 3600. Therefore no significant excess can be claimed forthe flux from this particular direction. 90 % CL upper limits can be placed on the flux of muons from variousdirections which are in the range, 1 to 10i(B`,at a muon energy threshold of 20 GeV.4. Discussions and ConclusionsA general-purpose sky survey method for any detector with anisotropic acceptance has been proposed and ap-plied for a cosmic ray point source search with the high resolution muon spectrometer of the L3+C experiment.Data collected for 312 days over the periods July-November 1999 and April-November 2000 have shown noevidence for a significant excess of events. Low gamma flux values and too low muon detection efficiency arethe reason.AcknowledgmentsThe L3+C group would like to express their thanks to CERN for their continuous support.References[1] L.O’C., Drury et al., Astro. Astrophys., 287 (1994) 959.[2] P. L. Bierman et al., Astron. Astrophys, 369 (2000) 269.[3] B. Adeva et al., Nucl.Instr. Meth., A 289 (1990) 35.[4] O. Adriani et al., Nucl.Inst. Meth., A 488 (2002) 209.[5] O. Adriani et al., Phys. Lett. B 598 (2004) 15.[6] T.P. Li and Y.Q. Ma, Astrophys. J., 272 (1983) 317.

Searching for Point Source Signals through Muons

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