The events collected with the lower part of the MACRO detector at the Gran Sasso Laboratory have been analyzed looking for neutrino-induced muons. Upward throughgoing events have been measured and the result is compared with Monte Carlo prediction. The first results concerning upward stopping muons and partially contained events are also reported. A search has been made for astrophysical point

sources of neutrinos. No point sources have been observed

Spurio, M.

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IL NUOVO CIMENTO VOL. 20 C, N. 6 Novembre-Dicembre 1997Physics and astrophysics with high energy  in MACRO()M. SPURIOfor the MACRO CollaborationDipartimento di Fisica dell’Universita` and INFN - Bologna, Italy(ricevuto il 5 Dicembre 1996; approvato il 18 Marzo 1997)Summary. — The events collected with the lower part of the MACRO detector at theGran Sasso Laboratory have been analyzed looking for neutrino-induced muons. Up-ward throughgoing events have been measured and the result is compared with MonteCarlo prediction. The first results concerning upward stopping muons and partiallycontained events are also reported. A search has been made for astrophysical pointsources of neutrinos. No point sources have been observed.PACS 96.40 – Cosmic rays.PACS 01.30.Cc – Conference proceedings.1. – IntroductionLarge underground experiments can detect neutrinos produced by various sourceswith different energy spectra. For energies up to about 15 MeV, the majority comes fromnuclear fusion processes in the Sun. Very rarely, slightly higher-energy neutrinos and an-tineutrinos of all flavors come in a burst of about 10 s duration from a nearby supernovaexplosion. For these low energy neutrinos, positive results exist: the detection of solareand the detection of the SN1987A eburst. At higher energy, atmospheric neutrinosdominate. These ’s are the results of the decays of  and k produced by the inter-actions of cosmic rays in the upper atmosphere. From these processes, we expect that:  2eand e=e += .Recently, considerable interest has been generated in precise measurements of theflux of atmospheric neutrinos due to the apparent anomaly in the ratio of contained muonneutrino to electron neutrino interactions as seen by the Kamiokande [1] and IMB de-tectors [2]. The Frejus [3] and NUSEX [4] detectors, however, have observed the ex-pected number of contained events with smaller statistics. The Soudan II Collaboration() Paper presented at the VII Cosmic Physics National Conference, Rimini, October 26-28, 1994.G Societa` Italiana di Fisica 977978 M. SPURIOhas reported preliminary results which are in agreement with the IMB and Kamiokandeobservations [5].Finally, several models exist which predict a flux of VHE and UHE ’s from astrophys-ical sources (galactic, like neutron stars, black holes and young supernovae remnants; ex-tragalactic, like Active Galactic Nuclei). Also if these neutrinos travel through very longdistances, the probability of their being either deviated or stopped is very low, given thevery small cross-section and the low interstellar medium density.The MACRO underground detector is instrumented for the detection of neutrino burstfrom supernovae explosion, using the large amount of liquid scintillator (600 tons). How-ever, in this paper we focus mainly on the detection of atmospheric ’s in different energyintervals, and the search for possible cosmic point sources of neutrinos.2. – The MACRO Experiment as a  detectorMACRO [6] (Monopole, Astrophysics and Cosmic Ray Observatory) is a large-areaunderground detector, with a mean rock overburden of  3700 hg cm 2, located at theLaboratori Nazionali del Gran Sasso in Italy. It consists of 6 supermodules (SM) withoverall dimensions of 72 m 12 m 9 m. Each SM has a lower part of 4.8 m height andan upper part (Attico) of 4.2 m. The lower detector consists of a large rectangular boxwith an outer shell of liquid scintillator counters and 6 layers of streamer tubes; it has10 horizontal layers of plastic streamer tubes distributed inside the box. The eight innerstreamer tube planes are separated by passive absorbers (60 g cm 2 of iron and rock) inorder to set a minimum threshold of about 1 GeV for vertical muons crossing the detector.The streamer tubes are used for tracking, and are equipped with readout electronics onboth the wires (3 cm cells) and on cathode strips, placed at a stereo angle of 27 withrespect to the wires. The intrinsic angular resolution (studied using down-going muons[6]) for reconstructed tracks in the detector with at least four planes of streamer tubeshit is 0:14 for wire tracks and 0:29 for strip tracks. The scintillator system for the lowerpart consists of 192 horizontal boxes of liquid scintillator (11:2 m 0:73 m 0:19 m) and98 vertical boxes (11:1 m 0:23 m 0:43 m). After calibration, the time resolution formuons in a scintillator box is about 500 ps which corresponds to a position resolutionalong the length of the box (determined using the difference in time at the two box ends)of about 11 cm.To run as a neutrino detector, MACRO needs both the streamer tubes and scintillatorsubsystem working with high efficiency. The data we present here refers to three differentrunning periods without Attico. The first (Period I) used one SM and lasted from March1989 until November 1991, with a live time of about 1.4 y. The second (Period II) wasbetween March 1992 to December 1992, when the detector was under construction. Thethird (Period III) is from December 1992 until June 1993, using 6 SM with a live time of0.42 y.Neutrino interactions can be detected via the production of a charged lepton throughthe process N ! X . Referring to fig. 1, we can thus distinguish four different eventtopologies: 1) upward throughgoing muons; 2) upward going stopping muons ; 3) partiallycontained upward going muons; 4) partially contained downward going muons.For topologies 1) and 2) the interaction happens in the rock below the apparatus,while in 3) and 4) it happens in the 5 kton of the passive absorber inside the apparatus.The neutrino average energy for topologies 2) and 3) is in a region of particular inter-est for the atmospheric neutrino problem. It must be noted that MACRO is not able toPHYSICS AND ASTROPHYSICS WITH HIGH ENERGY  IN MACRO 979Fig. 1. – MACRO cross-section. The different neutrino-induced muon topologies are reported (seetext). The stars indicate a scintillator hit.distinguish between upward going stopping muons (2) and partially contained downwardgoing muons (4). Partially contained upward going muons (3) can be collected only withthe Attico part of the apparatus, which is taking data from Summer 1994. A small amountof eand einteractions inside the detector can also produce a detectable signal. Thepossibility of distinguishing between muon neutrino and electron neutrino interactions isat present under study.For each topology of neutrino-induced events, using a Monte Carlo (MC) simulation weevaluated the neutrino energy spectra which produce a detectable muon in MACRO. TheMC consists of four parts: i) an atmospheric neutrino flux; the so-called Bartol (low andhigh energy) flux was used [7]; ii) the c:c:totneutrino cross-section; iii) the propagation ofthe produced muon; this has been done using the energy loss calculation in standard rockof Lohmann et al. [8]; iv) a full simulation of the detector response, using a GEANT [9]based simulation program.The charged current (c.c.) cross-section for the  interactions has been calculated980 M. SPURIOFig. 2. – Neutrino energy spectra which produce a detected muon in MACRO. Energy in GeV.as [10]c:c:tot= q:e:+ 1+ D:I:S::(1)q:e:= c.c. quasi-elastic scattering (N ! lN 0), which take into account also the leptonmass, the Fermi momentum, the Pauli exclusion principle and nuclear binding; 1= c.c.at the -resonance (N ! lN 0; W  1:4 GeV); D:I:S:= c.c. deep inelastic scattering,calculated using the Morfin and Tung parton distribution set S1 [11].In fig. 2 the distribution of the neutrino energies which produce a detected muon inMACRO for the different event topologies (partially contained events have been added) ispresented. Due to the systematic uncertainty on the  flux, on cross-section and apparatusacceptance, the statistical sample in fig. 2 is for 40 to 50 years of data taking with the fullapparatus. In column 2 and 3 of table I are reported the  energy intervals in which 90%of the signal is contained and the  mean energy for each event topology. The 5% of theTABLE I. – Column 2: neutrino energy interval for 90% of the neutrino-induced muons detected inMACRO (see text). Column 3: mean neutrino energy in the interval. Column 4: accessible valuein the m2 region for neutrino oscillations.Event topology 90% of  signal hEi m2 hEi=hLiGeV GeV eV21 4 < E< 500 80 10 22 0:85 < E< 35 5.0 10 33 0:65 < E< 22 3.7 10 34 0:50 < E< 15 2.3 10 1PHYSICS AND ASTROPHYSICS WITH HIGH ENERGY  IN MACRO 981Fig. 3. – Distribution of cos (zenith) for the upgoing muon flux for the combined data of Period I andIII. The extensions to the statistical error bars are the point-by-point estimates of the systematicerror. The Monte Carlo expectation using the Bartol flux is shown in the shaded regions with a15% systematic error range.lowest and the 5% of the highest energy events were excluded.Neutrinos which give rise to upward going muons (topology from 1 to 3) have a typicalpath length hLi  104 km from the production point to the detector, while hLi  10 km forpartially contained downward going muons. The mean distance and the neutrino energyset the scale for the region in m2 accessible to each topology in the expression for theoscillation probability (column 4 of table I).3. – Neutrino-induced muon measurements3.1. Upward going muons. – This measurement was done using the time of flight be-tween two layers of scintillation counters. Data from Period I and Period III were used.A rejection factor of at least 105 is necessary to separate the upgoing neutrino-inducedmuons from the ordinary downgoing atmospheric muons. The path length D betweenscintillator boxes is calculated using the streamer tube tracks. The velocity of each parti-cle is then simply c = D=(Tb Tu), where Tb(Tu) is the mean time measured in the lower(upper) scintillator plane. Using this convention, up throughgoing  have a 1= =  1.Figure 3 shows the zenith angle distribution of upgoing muon events for the combineddata set of Period I and III, compared to the Monte Carlo expectation. The total numberof detected and expected events are also shown in fig. 3. The systematic error on the datareflects the uncertainty in the acceptance, efficiency factor and background subtractionfor each point.The integrated number of detected events in all zenith bins agrees with Monte Carlopredictions to within 1:1. The 2-test applied to the experimental zenith distributioncompared to the Monte Carlo gives a 2 = 8:0. For 5 D.o.F (no oscillation hypothesis) the982 M. SPURIOTABLE II. – Number of detected up stopping muons in the period III data sample vs. the crossednumber of streamer planes and crossed detector thickness.Number of planes g cm 2 Number of events2  40 183  100 94  160 35  220 36  280 2probability of this 2 is 16%.3.2. Stopping and partially contained muons. – Upward going stopping and partiallycontained muons (hereafter up stopping muons) have the same topology in the apparatus,and can be detected if the muons cross a scintillator counter in the bottom plane and atleast two streamer tube planes. The search for up stopping muons was done using onlydata of Period III, and it is still in progress. The analysis takes into account several de-tector’s effects (gaps, inefficiencies, etc.) that could simulate a stopping muon. In table IIthe number of detected events vs. the number of crossed streamer tube planes is reported(i.e. different energy threshold for the muon).A possible source of background for these events is the large-angle pion productiondue to downward going muon interaction in the rock below the detector. An example of adownward going muon plus a backward going  (identified studying the crossing time onthe scintillator counters) is presented in fig. 4. During the considered data taking period38 event of this kind were detected. A background event can happens when the downwardparticle does non intersect the detector. Using a phenomenological Monte Carlo, a roughestimate of this background was evaluated as 42 background events, and affects mainlythe configuration of the first raw of table II.Fig. 4. – Downward going muon with a backward pion. The time measurement in the scintillatorcounters suggests a flight direction as indicated by the arrows.PHYSICS AND ASTROPHYSICS WITH HIGH ENERGY  IN MACRO 9834. – Search for astrophysical point sources of A search has been made for upgoing muons produced by neutrinos coming from celes-tial objects using the whole data sample. The very good angular resolution of MACROallows for tight angular bins around possible sources. The angular difference between thedetected  direction and the parent  depends on the source spectral index, the multiplescattering of the muon in the crossed rock and the detector intrinsic angular resolution.We calculated via Monte Carlo that, given a spectral index between 2.0 and 2.2 [12], asearch cone of 3 (half-width) around the direction of a source will include 90% of themuons observed in MACRO from that source. The expected background (due, in thiscase, to atmospheric neutrinos and calculated using the data itself) inside of each searchcone is roughly 0:1 event for the considered data taking period.We have investigated 13 different sources (CygX-3, HerX-1, Crab, Geminga, SS433,ScoXr-1, Galactic Center, VelaXR-1, Vela Pulsar, CenXR-1, LMCX-4, SN1987A, LMCX-2). No events were found within the 3 search bin centered on each of the sources listed.Flux upper limits (90% C.L.) from these sources range from 6  10 13 cm 2s 1 (CygX-3)to 2:4  10 14 cm 2s 1 (LMC).In addition, we have made a search for clusters of events independent of whether theylie near some known object. No statistically significant cluster of events has been found.REFERENCES[1] HIRATA K. S. et al., Phys. Lett. B, 280 (1992) 146.[2] CASPER D. et al., Phys. Rev. Lett., 66 (1991) 2561.[3] BERGER CH. et al., Phys. Lett. B, 245 (1990) 305; 227 (1989) 489.[4] AGLIETTA M. et al., Europhys. Lett., 8 (1989) 611.[5] ALLISON W. et al., ANL-HEP-CP-93-34.[6] AHLEN S. P. et al., Nucl. Instrum. Methods A, 324 (1993) 337.[7] FRATI W. et al., Phys. Rev. D, 48 (1993) 1140; GAISSER T. K., private communication.[8] LOHMANN W. et al., CERN-EP/85-03, Mar, 1985.[9] BRUN R. et al., CERN report DD/EE84-1, 1987.[10] LIPARI P., LUSIGNOLI M. and SARTOGO F., Phys. Rev. Lett., 74 (1995) 4384.[11] MORFIN J. G. and TUNG W. K., Z. Phys. C, 52 (1991) 13.[12] STANEV T., Nucl. Phys. B, 35 (1994) 185.

Physics and astrophysics with high energy v in MACRO

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Physics and astrophysics with high energy v in MACRO

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