After a brief presentation of the MACRO detector we discuss the data on
atmospheric neutrinos and neutrino oscillations, on high energy (E> 1 GeV)
neutrino astronomy, on indirect searches for WIMPs and low energy (E >7 MeV)
stellar collapse neutrinos.Comment: 8 pages, 7 figures. Invited paper at the Chacaltaya Meeting on Cosmic
  Ray Physics, La Paz, 23-27 July, 200

Giacomelli, G.

Margiotta, A.

English

arXiv

After a brief presentation of the MACRO detector we discuss the data on atmospheric neutrinos and neutrino oscillations, on high energy (Eν > 1 GeV) neutrino astronomy, on indirect searches for WIMPs and low energy (Eν � 7 MeV) stellar collapse neutrinos

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IL NUOVO CIMENTO Vol. 24 C, N. 4-5 Luglio-Ottobre 2001Neutrino physics and astrophysics with the MACRO detector(∗)G. Giacomelli and A. MargiottaDipartimento di Fisica dell’Universita` di Bologna and INFN - I-40127 Bologna, Italy(ricevuto il 13 Ottobre 2000; approvato il 12 Febbraio 2001)Summary.—After a brief presentation of the MACRO detector we discuss the dataon atmospheric neutrinos and neutrino oscillations, on high energy (Eν > 1 GeV)neutrino astronomy, on indirect searches for WIMPs and low energy (Eν  7 MeV)stellar collapse neutrinos.PACS 13.10 – Weak and electromagnetic interactions of leptons.PACS 13.15 – Neutrino interactions.PACS 14.60.Pq – Neutrino mass and mixing.PACS 01.30.Cc – Conference proceedings.1. – IntroductionMACRO is a multipurpose underground detector designed to search for rare eventsin the cosmic radiation. It was optimized to search for the supermassive magnetic mono-poles (MMs) predicted by Grand Unified Theories (GUT) of the electroweak and stronginteractions. MACRO performs measurements in areas of astrophysics, nuclear, particleand cosmic ray physics. In this paper we shall discuss atmospheric neutrinos and neu-trino oscillations, high energy (Eν  1 GeV) neutrino astronomy, indirect searches forWIMPs and searches for low energy (Eν  7 MeV) stellar collapse neutrinos. Thedetector has global dimensions of 12×9.3×76.5 m3 and provides a total acceptance to anisotropic flux of particles of ∼ 10000m2 sr. The total mass is  5300 t. A cross-sectionof the detector is shown in fig. 1 [1]. It has three sub-detectors: liquid scintillation coun-ters, limited streamer tubes and nuclear track detectors. The mean rock depth of theoverburden is  3700 m.w.e.; the minimum is 3150 m.w.e. This defines the minimumdowngoing muon energy at the surface as ∼ 1.3TeV in order to reach the undergroundlab. The average residual energy and the muon flux at the lab depth are ∼ 310 GeVand ∼ 1 m−2 h−1, respectively.(∗) Paper presented at the Chacaltaya Meeting on Cosmic Ray Physics, La Paz, Bolivia,July 23-27, 2000.c© Societa` Italiana di Fisica 761762 G. GIACOMELLI and A. MARGIOTTAFig. 1. – Event topologies induced by νµ interactions in and around MACRO. The black boxesare scintillator hits. The track direction and versus are measured by the streamer tubes andtime of flight (ToF).2. – Atmospheric ν. Neutrino oscillationsNeutrino oscillations. For massive neutrinos one has to consider the weak flavoureigenstates (νe, νµ, ντ , which are relevant for π → µνµ, K→ µνµ and µ→ eνeνµ decaysand for ν interactions) and mass eigenstates (ν1, ν2, ν3, relevant for propagation). Theflavour eigenstates are linear combinations of the mass eigenstates, νl =∑3m=1 Ulmνm.For two flavours{νµ = ν2 cos θ + ν3 sin θ ,ντ = −ν2 sin θ + ν3 cos θ ,(1)where θ is the mixing angle. The rate of νµ disappearance is given byP (νµ → νµ)  1− sin2 2θ sin2(1.27∆m2L/Eν) ,(2)∆m2 = m2ν3 −m2ν2 , L = distance (in m) from the decay point to the interaction point.Only disappearance experiments have been performed until now; the final proof ofν-oscillations will come from appearance experiments, P (νµ → ντ ) = 1 − P (νµ → νµ),in which one observes neutrinos which have not been produced in the π, K decays.Atmospheric neutrinos. Primary cosmic rays produce in the upper atmosphere pionsand kaons, which decay, π → µν, K → µν; and µ± →(−)ν µ + (−)ν e +e±. Neutrinos areproduced in a spherical surface, ∼ 10 km from the Earth, and move towards the Earth.Neutrino-induced upward-going muons. They are identified using the streamer tubesystem (for tracking) and the scintillator system (for time-of-flight (ToF) measurement).A rejection factor of at least 105 is needed in order to separate the up-going muons fromNEUTRINO PHYSICS AND ASTROPHYSICS WITH THE MACRO ETC. 7638765Upwardgoing µ flux (10–13 cm–2  s–1  sr–1 )4321– 1 – 0.8 – 0.6 – 0.4 – 0.2 00cos θFig. 2. – The black points are the measured fluxes of the up throughgoing muons with Eµ >1 GeV plotted vs. zenith angle θ. The dashed line is the expectation for no oscillations; it has a17 % scale uncertainty. The solid line is the fit to an oscillated muon flux, obtaining maximummixing and ∆m2 = 0.0025 eV2.the large background coming from down-going muons [2]. Figure 1 shows three differenttopologies of neutrino events: up throughgoing muons, semicontained upgoing muons(IU) and up stopping muons+semicontained downgoing muons (UGS + ID) [2]. Theparent νµ energy spectra for the three event topologies were computed with Monte Carlomethods. The number of events measured and expected for the three topologies is givenin table I. Sources of background and systematic effects were studied in detail in [3] andfound to be negligible.High energy data. The up throughgoing muons come from νµ’s interacting in the rockbelow the detector; the νµ’s have a median energy Eν ∼ 50 GeV. The muons withEµ > 1 GeV cross the whole detector; the ToF information provided by the scintillationcounters allows the determination of their direction (versus).Figure 2 shows the zenith angle distribution of the measured flux of up throughgoingmuons with energy  1 GeV; the Monte Carlo expectation for no oscillations is shown asa dashed line, and for a νµ → ντ oscillated flux with sin2 2θ = 1 and ∆m2 = 0.0025 eV2,as a solid line. The data have been compared with Monte Carlo simulations, usingthe neutrino flux computed by the Bartol group and the cross-sections for neutrinointeractions calculated using the GRV94 parton distribution; the propagation of muonsto the detector was done using the energy loss calculation by Lohmann et al. [2]. Thetotal theoretical uncertainty on the expected muon flux, adding in quadrature the errorsfrom neutrino flux, cross-section and muon propagation, is 17 %; it is mainly a scale errorthat does not change the shape of the angular distribution. The ratio of the observednumber of events to the expectation without oscillations is given in table I.The independent probabilities for obtaining the number of events and the angulardistribution observed have been calculated for various parameter values. The value of764 G. GIACOMELLI and A. MARGIOTTA1.52.02.510-4 10-3 10-210-5 0.1∆m2 (eV)2-1.0<cos(theta)<-0.6-0.6<cos(theta)<0R=ratio nsterileratio ntaumeasured valueFig. 3. – The flux ratio vertical/horizontal for upthroughgoing muons. The measured point isplotted at a ∆m2 around the minimum of χ2 for νµ → ντ oscillations.∆m2 obtained from the shape of the angular distribution is equal to the value neededto obtain the observed reduction in the number of events; for νµ → ντ oscillations themaximum probability is 57 %, with best parameters ∆m2 = 0.0025 eV2, sin2 2θ = 1.The probability for no-oscillations is 0.4 %. The probability for νµ → νsterile oscillationsis 0.15%; combining these probabilities with the measured ratio of horizontal/verticalmuons (fig. 3) we conclude that νµ → νsterile oscillations are disfavoured compared toνµ → ντ . Figure 4 shows the 90 % CL regions for νµ → ντ , computed according toref. [4].Low energy data. The upgoing semicontained muons come from νµ interactions insidethe lower apparatus. Since two scintillation counters are intercepted, the ToF is appliedto identify the upward going muons (fig. 1). The average parent neutrino energy forthese events is 4.2 GeV.If the atmospheric neutrino anomalies are the results of νµ → ντ oscillations withmaximum mixing and ∆m2 between 10−3 and 10−2 eV2 one expects a reduction ofabout a factor of two in the flux of these events, without any distortion in the shape ofthe angular distribution. This is what is observed in fig. 5.The up stopping muons are due to external νµ interactions yielding upgoing muontracks stopping in the detector; the semicontained downgoing muons are due to νµ in-duced downgoing tracks with vertices in the lower MACRO (fig. 1). The events wereselected by means of topological criteria; the lack of time information prevents to dis-tinguish the two sub samples, for which an almost equal number of events is expected;the average neutrino energy for these events is  3.5GeV. In case of oscillations withthe quoted parameters, a reduction in the flux of the up stopping events as the semicon-tained upgoing muons is expected. No reduction is instead expected for the semicontaineddowngoing events (coming from neutrinos with path lengths of ∼ 10 km).The MC simulation for the low energy data uses the Bartol neutrino flux and theneutrino low energy cross-sections of ref. [5]. The number of events and the angulardistributions are compared with the predictions in fig. 5 and table I. The measured lowenergy data show a uniform deficit with respect to the predictions; there is good agree-NEUTRINO PHYSICS AND ASTROPHYSICS WITH THE MACRO ETC. 76510-110-210-310-410-50∆m2(eV2)10.2 0.4 0.6 0.8sin2 2θLow energiesUpthrough-goingFig. 4. – Confidence regions for νµ → ντ oscillations at the 90 % CL calculated according to [4],obtained from MACRO high energy and low energy data.ment with the predictions based on neutrino oscillations using the parameters obtainedfrom up throughgoing muons. The allowed region in the ∆m2, sin2 2θ plane is shown infig. 4.Using the double ratio R = (Data/MC)IU/(Data/MC)ID+UGS between data and MCof the two low energy data sets, the theoretical uncertainties on neutrino flux and cross-sections almost disappear (a residual 5 % uncertainty remains due to the small differencesbetween the energy spectra of the two samples). The average value of the double ratioover the measured zenith angle distribution is R  0.59± 0.07 stat. R = 0.75 is expectedin case of no oscillations, R = 0.58 in case of νµ → ντ oscillations.Table I. – Event summary for the MACRO atmospheric neutrino flux analyses. The ratiosR = Data/MC are relative to MC expectations assuming no oscillations (column 3).Events Predictions (Bartol flux) R = Data/MCselected No. oscillationsUp 768 989 0.731± 0.028st ± 0.044sys ± 0.124ththroughgoingInternal Up 135 202 0.55± 0.04st ± 0.06sys ± 0.14thUp Stop + 229 273 0.70± 0.04st ± 0.07sys ± 0.18thIn Down766 G. GIACOMELLI and A. MARGIOTTA120100806040200 1  0.8  0.6  0.4  0.2 0cos θ120100806040200 1  0.8  0.6  0.4  0.2 0cos θNumberofevents/binNumberofevents/binUGS + IDIUFig. 5. – Measured and expected number of low energy muon neutrino events vs. zenith angle.Top graph: up stopping plus down semicontained; bottom graph: up semicontained. The solidlines are the predictions without oscillations; the dotted lines are the predictions assumingneutrino oscillations with the parameters obtained from the up throughgoing sample.Table II. – High energy neutrino astronomy: muon and neutrino flux limits (90 % CL) forselected sources calculated according to the prescription in [4]. Previous best limits (B is forBaksan, I is for IMB) were computed using the classical Poissonian method.Source δ Data Backg µ-Flux Prev. best ν-Flux3◦ 3◦ limit µ limit limit10−14cm−2s−1 10−14cm−2s−1 10−6cm−2s−1SN1987A −69.3◦ 0 2.1 0.14 1.15 B 0.29Vela P −45.2◦ 1 1.7 0.45 0.78 I 0.84SN1006 −41.7◦ 1 1.5 0.50 - 0.92Gal. Cen. −28.9◦ 0 1.0 0.30 0.95 B 0.57Kep1604 −21.5◦ 2 1.0 1.02 - 1.92ScoXR-1 −15.6◦ 1 1.0 0.77 1.5 B 1.45Geminga 18.3◦ 0 0.5 1.04 3.1 I 1.95Crab 22.0◦ 1 0.5 2.30 2.6 B 4.30Her X-1 35.4◦ 0 0.2 3.05 4.3 I 6.45NEUTRINO PHYSICS AND ASTROPHYSICS WITH THE MACRO ETC. 767Fig. 6. – High energy neutrino astronomy. Upgoing muon event distribution in equatorial coor-dinates (1197 events).3. – High energy muon neutrino astronomyThe excellent angular resolution of the detector allows a sensitive search for up-goingmuons produced by neutrinos coming from celestial sources; in this case the atmosphericneutrinos are the main background. The pointing capability of MACRO was demon-strated by the observed “shadows” of the Moon and of the Sun, which produce a “shield”to the cosmic rays. We used a sample of 45× 106 downgoing muons, looking at the bidi-mensional density of the events around the center of the Moon and of the Sun [6]. Thedensity of events shows a depletion with a statistical significance of 5.5 σ for the Moonand of 4.2 σ for the Sun. The slight displacements of the maximum deficits are consistentwith the deflection of the primary protons due to the geomagnetic field for the Moonand to the combined effect of the magnetic field of the Sun and of the geomagnetic field.The angular resolution, corresponding to a cone angle containing 68 % of the events,is 0.9◦ ± 0.3◦; it is obtained from double muon events. An excess of events over theatmospheric ν background was searched for around the positions of known sources in 3◦(half width) angular bins. This value was chosen so as to take into account the angularsmearing produced by the multiple scattering in the rock below the detector and by theenergy-integrated angular distribution of the scattered muon, with respect to the neu-trino direction. A total of 1197 events was used in this search, see fig. 6. No excess wasobserved; the 90% CL limits on the neutrino fluxes from specific celestial sources are atthe level of ∼ 10−6 cm−2 s−1, see table II [7].We also searched for time coincidences of our upgoing muons with γ-ray bursts asgiven in the BATSE 3B and 4B catalogues, for the period April 91–October 99 [7]. Nostatistically significant time correlation was found.768 G. GIACOMELLI and A. MARGIOTTA10-1110-1210-1310-1410-1510-1610-170 50 100 150 200 250 300mχ (GeV)(µ>0)φEarth(cm-2s-1)µFig. 7. – Upward-going muon flux vs. neutralino mass mχ for Eµth = 1 GeV from the Earth.Each dot is obtained varying model parameters. The solid line is the MACRO flux limit(90% CL); the line for the no-oscillation hypothesis is essentially indistinguishable in the logscale from the one for the νµ → ντ oscillation hypothesis (it could be about two times lower).The open circles indicate models excluded by direct measurements (particularly the DAMA/NaIexperiment) and assume a local dark matter density of 0.5 GeV cm−3.4. – Indirect searches for WIMPsWeakly Interacting Massive Particles (WIMPs) could be part of the galactic darkmatter; they could be intercepted by celestial bodies, slowed down and trapped in theircenters. WIMPs and anti-WIMPs could annihilate and yield pions, kaons and then neu-trinos of GeV or TeV energy coming from a small angular window from the celestial bodycenters. The 90% CL limit for the flux from the Earth center is ∼ 0.8 · 10−14 cm−2 s−1for a 10◦ cone around the vertical. The limit from the Sun is ∼ 1.4×10−14 cm−2 s−1. Ifthe WIMPs are identified with the lowest mass neutralinos, the MACRO limit constrainsthe stable neutralino mass, following the models of Bottino et al., see fig. 7 [8].5. – Neutrinos from stellar gravitational collapsesA stellar gravitational collapse (GC) is expected to produce a large burst of all typesof neutrinos and antineutrinos with energies of 7–30 MeV for a duration of ∼ 10 s. Theν¯e’s can be detected via the reaction ν¯e + p→ n + e+ in the liquid scintillator. About100–150 ν¯e events would be detected in our 580 t of scintillator for a stellar collapseat the center of our Galaxy. We use two electronic systems for detecting ν¯e’s fromstellar gravitational collapses [9]. Both systems have an energy threshold of ∼ 7 MeVand record pulse shape, charge and timing information. Immediately after a trigger,NEUTRINO PHYSICS AND ASTROPHYSICS WITH THE MACRO ETC. 769the dedicated system lowers its threshold to about 1 MeV, for a duration of 800 µ s,in order to detect (with  25% efficiency) the 2.2 MeV γ released in the reactionn + p → d + γ2.2 MeV induced by the neutron produced in the primary process. Aredundant supernova alarm system is in operation: we have defined a general procedureto alert the physics and astrophysics communities in case of an interesting alarm [9].A procedure to link the various supernovae observatories around the world (MACRO,SuperK, LVD, SNO, etc.) was set up. Our lifetime fraction in the last four years was 97.5%. No stellar gravitational collapses were observed in our Galaxy since 1989.6. – ConclusionsWe presented new MACRO data on upgoing muons of low and high energy. Thesedata are relevant for the study of neutrino oscillations, in particular νµ → ντ [10].We discussed the limits obtained from the searches for astrophysical sources of H.E.neutrinos, for WIMPs and for low energy neutrinos from gravitational stellar collapses.REFERENCES[1] De Marzo C. et al., Nuovo Cimento C, 9 (1986)281; Calicchio M. et al., Nucl. Instrum.Methods A, 264 (1988) 18; Ahlen S. et al., Nucl. Instrum. Methods A, 324 (1993) 337.[2] Ambrosio M. et al., Phys. Lett. B, 434 (1998) 451; 357 (1995) 481; 478 (2000) 5.[3] Ambrosio M. et al., Astropart. Phys., 9 (1998) 105.[4] Feldman G. and Cousins R., Phys. Rev. D, 57 (1998) 3873.[5] Lipari L. et al., Phys. Rev. Lett., 74 (1995) 384.[6] Ambrosio M. et al., Phys. Rev. D, 59 (1999) 012003.[7] Ambrosio M. et al., Neutrino astronomy with the MACRO detector, astro-ph/0002492.[8] Ambrosio M. et al., Phys. Rev. D, 60 (1999) 082002.[9] Ambrosio M. et al., Astropart. Phys., 8 (1998) 123.[10] Giacomelli G., Closing lecture at the 1999 S. Miniato Workshop, hep-ex/0001008.

Neutrino physics and astrophysics with the MACRO detector

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Neutrino physics and astrophysics with the MACRO detector

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