The main goal of the IceCube Deep Core Array is to search for neutrinos of
astrophysical origins. Atmospheric neutrinos are commonly considered as a
background for these searches. We show here that cascade measurements in the
Ice Cube Deep Core Array can provide strong evidence for tau neutrino
appearance in atmospheric neutrino oscillations. A careful study of these tau
neutrinos is crucial, since they constitute an irreducible background for
astrophysical neutrino detection.Comment: 5 pages, 3 figure

Gerardo Giordano

Irina Mocioiu

O. Schulz

Olga Mena

English

arXiv

The main goal of the IceCube Deep Core Array is to search for neutrinos of astrophysical origins. Atmospheric neutrinos are commonly considered as a background for these searches. We show here that cascade measurements in the Ice Cube Deep Core Array can provide strong evidence for tau neutrino appearance in atmospheric neutrino oscillations. Controlling systematic uncertainties will be the limiting factor in the analysis. A careful study of these tau neutrinos is crucial, since they constitute an irreducible background for astrophysical neutrino detection

Giordano, Gerardo

Mena Requejo, Olga

Mocioiu, Irina

Repositori d'Objectes Digitals per a l'Ensenyament la Recerca i la Cultura

arXiv:1004.3519v1  [hep-ph]  20 Apr 2010Atmospheric neutrino oscillations and tau neutrinos in iceGerardo Giordano1, Olga Mena2, Irina Mocioiu11Department of Physics, Pennsylvania State University, University Park, PA 16802, USA and2 Instituto de F´ısica Corpuscular, IFIC, CSIC and Universidad de Valencia, Spain(Dated: April 21, 2010)The main goal of the IceCube Deep Core Array is to search for neutrinos of astrophysical origins.Atmospheric neutrinos are commonly considered as a background for these searches. We show herethat cascade measurements in the Ice Cube Deep Core Array can provide strong evidence for tauneutrino appearance in atmospheric neutrino oscillations. A careful study of these tau neutrinos iscrucial, since they constitute an irreducible background for astrophysical neutrino detection.PACS numbers: 14.60.PqI. INTRODUCTIONOver the last decade, a large number of experiments ofdifferent types have provided strong evidence for neutrinooscillations and thus for physics beyond the StandardModel, see Ref. [1] and references therein.Cosmic ray interactions in the atmosphere give a nat-ural beam of neutrinos. These atmospheric neutrinos inthe GeV range have been used by the Super-Kamiokande(SK) detector to provide evidence for neutrino oscilla-tions [2]. The large size of neutrino telescopes such asAMANDA, IceCube and KM3NeTmakes possible the de-tection of a large number of atmospheric neutrino eventswith a higher energy threshold, ∼ 100 GeV, even thoughthe neutrino flux decreases rapidly with energy (∼ E−3ν ).Built to detect neutrinos from astrophysical sources, orfrom the decay of Weakly Interacting Massive Particles(WIMPs) annihilations [3], at the high detection thresh-old energies of these ice/water Cherenkov detectors, neu-trino oscillation effects would be small.Recently, a low energy extension of the IceCube de-tector, the Ice Cube Deep Core array (ICDC) has beenproposed and deployed [4]. It consists of six densely in-strumented strings (7 m spacing among optical modules)located in the deep center region of the IceCube detec-tor plus the seven nearest standard IceCube strings. Itsgoal is to significantly improve the atmospheric muonrejection and to extend the IceCube neutrino detectioncapabilities in the low energy domain, down to muon orcascade energies as low as 5 GeV. The instrumented vol-ume is 15 Mton. Such a low threshold array buried deepinside IceCube will open up a new energy window onthe universe. It will search for neutrinos from sources inthe Southern hemisphere, in particular, from the galac-tic center region, as well as for neutrinos from WIMPannihilation, as originally motivated. In [5] we have pro-posed neutrino oscillation physics as a further motiva-tion for building such an array. In particular, we haveanalyzed the sensitivity of ICDC to the neutrino masshierarchy. ICDC can detect up to 100, 000 atmosphericneutrino events per year, orders of magnitude beyond thepresent data sample, providing rich opportunities for de-tailed oscillation studies. In the same spirit of Ref. [5],we concentrate here on the neutrino oscillation analysis inICDC, focusing now on the cascade signal. By exploitingthe cascade channel, ICDC could firstly provide strongevidence for tau neutrino appearance from oscillationsof atmospheric neutrinos, greatly improving previous SKresults on tau neutrino appearance evidence [6].Section II reviews briefly our present knowledge of theneutrino oscillation parameters as well as their expectederrors from near future facilities. We then describe thecascade analysis details, presenting our main results insection III.II. NEUTRINO OSCILLATIONS ANDCASCADES IN ICDCNeutrino data from solar, atmospheric, reactor andaccelerator experiments is well understood in terms ofthree-flavor neutrino oscillations. Two ∆m2 values andtwo (large) mixing angles are well determined, while thethird mixing angle is limited to be very small. The CP-violating phase (δ) is completely unconstrained. In addi-tion, the sign of ∆m231 is also unknown.The best fit oscillation parameter values obtained frompresent data are [7]:|∆m231| = 2.5× 10−3eV2 (1)∆m221 = 7.6× 10−5eV2 (2)sin2 2θ23 = 1 (3)tan2 θ12 = 0.47 (4)and sin2 2θ13 ≤ 0.15 for ∆m231 = 2.5 × 10−3eV2. No-tice that an extra unknown in the neutrino oscillationscenario is the octant in which θ23 lies, if sin2 2θ23 6= 1.This has been dubbed in the literature as the θ23 octantambiguity.In the near future, long baseline experiments like MI-NOS [8] will improve the current precision on ∆m231and possibly discover a non-zero value of θ13, if this isclose to the present upper limit. In a few years, reac-tor experiments like DoubleChooz [9], RENO [10] andDayaBay [11] will provide improved sensitivity to θ13.This information can be used as input in our analysis,reducing some of the parameter uncertainties.25 10 15 20 25 30 35 40 45 50Eν [GeV]00.20.40.60.81Oscillation probabilitiesνµ − νµνµ − ντFIG. 1: νµ survival probability and νµ → ντ oscillation proba-bility as a function of the neutrino energy (in GeV), assumingupward going neutrinos and sin2 2θ13 = 0.1.In the past, atmospheric neutrinos in the SK detectorhave provided evidence for neutrino oscillations and thefirst measurements of |∆m231| and sin2 2θ23 [2], thereforeproviding compelling evidence for neutrino oscillationsversus other more exotic phenomena [12]. While facingmore systematics than accelerator/reactor experimentsdue to the uncertainties in the (natural source) neutrinofluxes, atmospheric neutrinos provide great opportunitiesfor exploring oscillation physics due to the large range ofenergies and pathlengths that they span. The ICDC de-tector will collect a data sample which is a few orders ofmagnitude larger than the SK experiment and can alsomeasure energy and directional information, such thatmany of the systematic errors associated with unknownnormalizations of fluxes, cross-sections, etc. can be muchbetter understood by using the data itself.The IceCube detector and its Deep Core extension areoptimized for detecting muon tracks from the chargedcurrent interactions of νµ. It is, however, possible toalso detect cascades [13]. While these type of events nolonger provide directional information, their energy canbe measured quite precisely. We analyze here the cascadeevents in the ICDC array. There are several contributionsto the cascade signal: charged current interactions of νe,neutral current interactions of all neutrino flavors andelectromagnetic and hadronic decays of tau leptons pro-duced in charged current interactions of ντ . There areseveral important observations which suggest that the ντsignal can be significant and can provide evidence for ντappearance from oscillations of νµ:• First, the atmospheric electron neutrino and electronantineutrino fluxes at the relevant energies are signifi-cantly lower than the muon neutrino flux, such that theνe charged current interactions do not completely over-whelm the event rate.• In addition, the energy range covered by ICDC corre-sponds to a maximum of νµ → ντ oscillations (minimumof νµ survival), as can be noticed from Fig. 1.The large flux of atmospheric muon neutrinos can thuslead to a large flux of tau neutrinos. This has alreadybeen noted in [5], where it was shown that νµ → ντ →τ → µ provides a non-negligible contribution to the muontrack rate. It is also important to note that, unlike for theSK detector, which is sensitive at much lower energies,tau threshold production effects are relatively small, onlyaffecting the lowest energy events detected by ICDC.We investigate the neutrino energy range between10 GeV and 100 GeV, assuming bins of 5 GeV widthin the observable energy. Cascades have very little di-rectional information, especially at these low energies, sowe integrate over all upward going directions [19]. Thedownward going neutrinos are largely unaffected by os-cillations, so they can be used for determining the atmo-spheric neutrino flux and thus the contribution of the νecharged current interactions to the overall cascade rate.In our numerical calculations we have taken into accountfull three flavor oscillations. It is however straightforwardto see that solar parameters do not play an important rolein the analysis due to the rather high energy thresholdof ICDC. Also, θ13 effects, while in principle observablefor values of θ13 close to the present bound, do not affectany conclusions regarding ντ rates, which are determinedby the (maximal) atmospheric mixing angle θ23.The spectrum of νe induced events as a function ofobservable energy is given by:dNedEobs= 2pinT t∫d cos θ V σCC(νe)(Eν)(dφνe (θ, Eν)dEνdΩPνe→νe(Eν , θ) +dφνµ (θ, Eν)dEνdΩPνµ→νe(Eν , θ))+ (ν → ν¯) , (5)where nT is the number density of targets, V is the vol-ume of the detector, θ is the zenith angle direction of theneutrino and t is the observation time. The detector isa cylinder of 250 m diameter and 350 m height, i.e. thetotal physical mass is of around 15 Mton. The low en-ergy events however are mostly single-string events andwe consider the effective volume in this case to be sixcylinders of 40 m radius centered around the six denselyinstrumented ICDC strings.The second term of Eq. (5), which contains the con-tribution from oscillations of νµ → νe, is negligible inpractice. This contribution is very small even for the3maximum current allowed value of sin2 2θ13 and afterconsidering enhancements in the oscillation probabilitiesdue to matter effects inside the Earth. It could, however,become relevant in the presence of non-standard neutrinointeractions.For charged current electron neutrino interactions, theentire energy of the incident neutrinos is transferred tothe cascade and thus measured.For neutral current interactions, only a fraction y of theinitial neutrino interaction is transferred to the cascade.Due to the steep energy dependence of the atmosphericneutrino fluxes, their contribution is thus expected to besmaller. The spectrum for neutral current (NC) inter-actions as a function of the observable energy is givenby:dNNCdEobs= 2pinT t∫d cos θ V∫∞EobsdEνdφνidEνdΩ1EνPνi→νj (Eν , θ)dσNCjdy(y, Eν)∣∣∣y=Eobs/Eν+ (ν → ν¯) . (6)For tau neutrinos, charged current (CC) interactionslead to two types of contributions to cascade events: elec-tromagnetic and hadronic, depending on the tau decaymode. The rate for hadronic events is given by:dNhadτdEobs= 2pinT t∫d cos θ V∫∞EobsdEν∫ EνEobsdEτdφνµdEνdΩPνµ→ντ (Eν , θ)dσCCτdEτ(Eτ , Eν)dnhaddEτ(Eτ ) + (ν → ν¯) . (7)For our numerical calculations, we use differential neu-trino cross-sections as given by [14] with CTEQ6 par-ton distribution functions. In terms of dimensionlessvariables typically used for these cross-sections we have:dσCCτ /dEτ ∼ 1/Eν dσCCτ /dy with y = 1 − Eτ/Eν anddnhad/dEτ ∼ E2ons/Eτ dnhad/dz with z = 1 − Eobs/Eτ .It is important to note that for tau neutrinos, tau thresh-old suppression is still important in the lower energyrange discussed here. We have included the thresholdcorrections following Ref. [15]. The 3.5 GeV tau leptonproduction threshold energy has made the tau neutrinodetection very difficult up to now, since the atmosphericneutrino flux for energies above tau lepton productionthreshold is very low for existing detectors. For instance,for the SK experiment, and assuming maximal mixing inthe νµ − ντ sector, only one CC tau neutrino event isexpected per kton-year of exposure. The ICDC exper-iment, which benefits from a much larger instrumentedvolumen than the SK experiment, is in an unique posi-tion to detect tau neutrino interactions, as it has goodsensitivity in an energy range where neutrino oscillationslead to a large number of tau neutrinos and, at the sametime, is high enough to no longer be strongly affected bytau threshold suppression.The rate for electromagnetic cascade events is given byan expression similar to Eq. (7), the only difference beingin the decay rate dn/dEτ .5 10 15 20 25 30 35 40 45 50Eobs [GeV]10100100010000dN/dE obsνe CCνe NCνµ NCFIG. 2: Electron CC and all flavors NC events for one yearICDC exposure, see text for details in the calculation.III. RESULTSFigure 2 shows the cascade rates coming from νecharged current interactions and neutral current inter-actions of νe and νµ. These events are indistinguish-able from the ντ cascade events that are of interest tous, so they constitute an irreducible background for theντ search. Figure 3 illustrates the cascade rates fromcharged current ντ interactions followed by hadronic orelectromagnetic tau decays, as well as ντ neutral currentinteractions.While the background is significant, the number of410 20 30 40 50E obs [GeV]1101001000dN/d E obselectromagmnetic tau decayshadronic tau decaysντ neutral current interactionsFIG. 3: Tau cascade events for one year of ICDC exposure.events is very high. Neglecting systematics errors, thestatistical significance could be defined asS =ns√ns + nb, (8)ns being the number of ντ events and nb the numberof background events (from νe changed current interac-tions and νe,µ neutral current interactions). A statisti-cally significant (3σ) ντ appearance signal could be ob-tained in only a few months of observation. However,systematic uncertainties will limit the analysis. One ofthe biggest uncertainties is the energy threshold for cas-cade identification. At the lowest energies consideredhere, muon track events would be very short [20] andtherefore, cascade events and muon track events wouldbe indistinguishable. If cascade events can not be dis-tinguished from muon tracks, the background to the ντsignal becomes significantly higher (by about an orderof magnitude) due to the contribution from the muonevent tracks. Even in the case where muon track eventsand cascade events can not be distinguished at low en-ergies, the ντ signal can become statistically significantin a year of exposure. Good muon track reconstructionwould be extremely useful for reducing this systematicuncertainty and might be achievable with the plannedadditional strings in the center of the detector. Thereare other systematic uncertainties affecting the analysis,as the knowledge of the interaction cross-sections, atmo-spheric neutrino fluxes, and other neutrino oscillation pa-rameters. However, these systematic errors are expectedto be under control in the next few years, exploiting datafrom future reactor and accelerator experiments as wellas atmospheric neutrino data from the ICDC experimentin different angular and energy ranges than the ones usedfor the ντ analysis. Appearance of tau neutrinos from os-cillations of atmospheric νµ is thus likely to be detectedin the near future.IV. OUTLOOKThe IceCube detector and its Deep Core array providea great opportunity for studies of atmospheric neutrinos.Being the largest existing neutrino detector, it will ac-cumulate a huge number of atmospheric neutrino eventsover an enormous energy range, thus allowing for detailedstudies of oscillation physics, Earth density, atmosphericneutrino fluxes and new physics [16]. In order to extractall this information it is necessary to use energy and an-gular distribution information, as well as flavor compo-sition, all possible to obtain with the IceCube detector.Qualitatively, there are three main energy intervals andthree main angular regions which are sensitive to differ-ent types of physics.At very high energies, above 10 TeV, neutrino inter-action cross-sections become high enough that neutrinosgoing through the Earth start getting attenuated [17].This effect is sensitive to neutrino interaction cross-sections and to the density profile of the Earth.The “intermediate” energy region, between 50 GeVand 1 TeV can provide good information about the at-mospheric neutrino flux, which can be used to improvethe uncertainties in the simulated atmospheric neutrinofluxes [18].In our paper we concentrated on the “low” energy re-gion, below about 40 GeV, where neutrino oscillation ef-fects can be significant. Although the IceCube detectorhas higher energy thresholds, its low energy extension,the Ice Cube Deep Core array (ICDC), extends the Ice-Cube neutrino detection capabilities in the low energy do-main, down to muon or cascade energies as low as 5 GeV.We have shown that tau neutrinos from oscillations of at-mospheric muon neutrinos could be detected in the nextfew years with a high significance level, providing there-fore the first sizable sample of tau neutrinos. This largenumber of tau neutrinos could allow for measurements ofντ interaction cross-sections and studies of non-standardneutrino interactions, which are largely unconstrained inthe tau sector. In summary, ICDC offers a unique win-dow towards a better understanding of neutrino proper-ties due to its very high atmospheric neutrino statistics.Careful studies of the expected atmospheric neutrino os-cillation signals in ICDC, as the one carried out herefor the ντ appearance signal, are mandatory, since atmo-spheric neutrinos constitute an irreducible background toastrophysical neutrino searches.AcknowledgmentsThis work was supported in part by the NSF grantPHY-0855529. I. M. would like to thank the Aspen Cen-ter for Physics where part of this work was completed.O. M. work is supported by the MICINN (Spain) Ramo´ny Cajal contract, AYA2008-03531 and CSD2007-00060.5[1] M. C. Gonzalez-Garcia and M. Maltoni, Phys. Rept. 460,1 (2008).[2] Y. Ashie et al. [Super-Kamiokande Collaboration], Phys.Rev. D 71 (2005) 112005.[3] For a recent overview, see:IceCube Collaboration, arXiv:0711.0353 [astro-ph]; andT. A. Collaboration, arXiv:1002.0701 [astro-ph.HE].[4] O. Schulz [IceCube Collaboration], AIP Conf. Proc.1085, 783 (2009); C. Wiebusch and f. t. I. Collabora-tion, arXiv:0907.2263 [astro-ph.IM].[5] O. Mena, I. Mocioiu and S. Razzaque, Phys. Rev. D 78,093003 (2008) [arXiv:0803.3044 [hep-ph]].[6] K. Abe et al. [Super-Kamiokande Collaboration], Phys.Rev. Lett. 97, 171801 (2006).[7] M. C. Gonzalez-Garcia, M. Maltoni and J. Salvado,arXiv:1001.4524 [hep-ph].[8] P. Adamson et al. [MINOS Collaboration], Phys. Rev.Lett. 103 (2009) 261802.[9] F. Ardellier et al. [Double Chooz Collaboration],arXiv:hep-ex/0606025.[10] J. K. Ahn [RENO Collaboration], arXiv:1003.1391 [hep-ex].[11] X. Guo et al. [Daya-Bay Collaboration],arXiv:hep-ex/0701029.[12] M. C. Gonzalez-Garcia and M. Maltoni, Phys. Lett. B663, 405 (2008) [arXiv:0802.3699 [hep-ph]].[13] M. D’Agostino, arXiv:0910.2555 [astro-ph.HE] andarXiv:0910.0215 [astro-ph.HE].[14] R. Gandhi, C. Quigg, M. H. Reno and I. Sarcevic,Astropart. Phys. 5, 81 (1996); R. Gandhi, C. Quigg,M. H. Reno and I. Sarcevic, Phys. Rev. D 58, 093009(1998).[15] S. Kretzer and M. H. Reno, Phys. Rev. D 66, 113007(2002). [arXiv:hep-ph/0208187].[16] M. C. Gonzalez-Garcia, F. Halzen and M. Maltoni, Phys.Rev. D 71, 093010 (2005) [arXiv:hep-ph/0502223].[17] M. C. Gonzalez-Garcia, F. Halzen, M. Maltoni andH. K. M. Tanaka, Phys. Rev. Lett. 100, 061802 (2008).[18] M. C. Gonzalez-Garcia, M. Maltoni and J. Rojo, JHEP0610, 075 (2006).[19] We have integrated over the zenith angle direction θ ofthe incident neutrinos, from cν = −1 (vertically upwardgoing) up to cν = 0 (horizontally incident), being cν thecosine of θ.[20] A 5 GeV muon will have a track in ice of ∼ 25 m.

Atmospheric neutrino oscillations and tau neutrinos in ice

Crossref

Physical Review D

The main goal of the IceCube Deep Core Array is to search for neutrinos of astrophysical origins. Atmospheric neutrinos are commonly considered as a background for these searches. We show here that cascade measurements in the Ice Cube Deep Core Array can provide strong evidence for tau neutrino appearance in atmospheric neutrino oscillations. Controlling systematic uncertainties will be the limiting factor in the analysis. A careful study of these tau neutrinos is crucial, since they constitute an irreducible background for astrophysical neutrino detection.This work was supported in part by the NSF grant
PHY-0855529. I. M. would like to thank the Aspen Center for Physics where part of this work was completed.
O. M. work is supported by the MICINN (Spain) Ramón
y Cajal contract, AYA2008-03531 and CSD2007-00060.Peer reviewe

Mena, Olga

Mocioiu, I.

Digital.CSIC

Atmospheric neutrino oscillations and tau neutrinos in ice

Abstract

Similar works

Full text

Available Versions

Repositori d'Objectes Digitals per a l'Ensenyament la Recerca i la Cultura

Crossref

Digital.CSIC