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NuclearPhysicsB Proceedings Supplement NuclearPhysicsBProceedingsSupplement00(2015)1–3 Super-PINGU for measuring CP violation SoeburRazzaquea,,A.Yu. Smirnovb,c, aDepartmentofPhysics,UniversityofJohannesburg,POBox524,AucklandPark2006,SouthAfrica bMax-Planck-InstituteforNuclearPhysics,Saupfercheckweg1,D-69117Heidelberg,Germany cInternationalCentreforTheoreticalPhysics,StradaCostiera11,I-34100Trieste,Italy 5 1 0 2 n Abstract a J We propose to measure leptonic CP phase, after neutrino mass hierarchy is established, with an upgrade of the 3 PINGUdetectorandusingatmosphericneutrinoflux. Theupgrade,calledsuper-PINGU,willrequireafewmegaton 1 effective volume at 0.5-1 GeV range to distinguish δ in the range π/2-3π/2 from 0 after 4 years of operation. The distinguishability(similartosignificance)ofmeasuringδdependscruciallyonvariousflux, cross-section, eventre- ] construction (energy and angle) and flavor identification uncertainties. We explore effects of these uncertainties on h p thedistinguishabilityofmeasuringCPphaseandsuggestpossiblewaystominimizetheirimpact. - p Keywords: e Neutrinooscillation,CPviolation,atmosphericneutrino h [ 1 1. Introduction 2. MethodologyandResults v 5 EstablishingCPviolationintheleptonicsectorisan TomeasureCPphasewithatmosphericneutrinos,we 4 outstanding problem in particle physics. Atmospheric calculateneutrinoeventsinSuper-PINGUbyvaryingδ 1 neutrinofluxmeasurementsinlargewater/icedetectors and compare with δ = 0, keeping all other oscillation 3 0 canbeusedtodeterminetheDiracCPphaseδ.Informa- parametersfixed. Weuseaneffectivemass(bothforνµ . tionondifferentδvaluesisencodedintheneutrinoos- andν )parameterizedas 1 e 50 c(millaatttieornepffreocbta).bilAitiesyssateftmeratpircopshaigfattoiofnthinesipdreobthaebieliatireths ρVeff(Eν)=2.6(cid:2)log(Eν/GeV)+1(cid:3)1.32 Mt, (1) 1 withincreasingδinawide,∼0.2–2GeV,energyrange which can be realized for a total of 126 strings and 60 v: iskeytomeasureδwithatmosphericneutrinoflux[1]. DOMsperstring. Thisgivesaneffectivemassof∼ 2.8 i IthasbeenfoundrecentlythatthePINGUandORCA Mtat∼ 1-2GeV,whichis4timeslargerthanPINGU. X detectorwith∼ 3GeVthresholdwillhavegoodsensi- The number of neutrino events for a particular flavor ar tivitytodeterminingtheneutrinomasshierarchy[2,3, α = e,µ with energies and zenith angles in small bins 4,5]. However, measurementofδwouldrequireade- ∆(E )and∆(cosθ )markedbysubscriptijcanbecal- ν z tector with larger effective volume and improved char- culatedas acteristics in the < 2 GeV range and in this context a (cid:90) future upgrade of PINGU (and also of ORCA), called Nij,α = 2πNAρT dcosθz Super-PINGU, was proposed in Ref. [1] with detailed (cid:90) ∆icosθz estimateofsensitivity. Herewepresenthighlightsfrom dEν Veff,α(Eν)dα(Eν,θz). (2) thatwork. ∆jEν HereT istheexposuretime,N istheAvogadro’snum- A ber. The density of events of type α, d (the number Emailaddresses:[email protected](SoeburRazzaque), α [email protected](A.Yu.Smirnov) of events per unit time per target nucleon), is given by /NuclearPhysicsBProceedingsSupplement00(2015)1–3 2 Figure1:DistributionsofSijforνµ+ν¯µeventsincaseofdifferentδ Figure2:ThesameasFig.1butforνe+ν¯eevents. valuesandδ=0intheEν–cosθzplane. Eνandθzarereconstructed neutrinoenergyanddirectionaftersmearingofthetrueenerg√yanddi- rectionusingPINGUreconstructionfunctionswithafactor 3better Thetotaldistinguishability resolutions.Normalmasshierarchyisassumed. (cid:115) (cid:88) S = S2 (4) σ ij d (E ,θ ) = σ Φ + σ¯ Φ¯ in terms of fluxes at the ij α ν z α α α α detector, and Φα = Φ0µPµα + Φ0ePeα with correspond- isaquickmeasureofsignificance. ingoscillationprobabilities Pµα and Peα . Theoriginal Figure 1 shows the distinguishability Sij for νµ +ν¯µ muonandelectronneutrinofluxesattheproductionare events with f = 0 for different δ values and using 1- Φ0µ =Φ0µ(Eν,θz)andΦ0e =Φ0e(Eν,θz). year of Super-PINGU data. Normal mass hierarchy is We have computed the distributions of νµ + ν¯µ and assumed. Theshapeofthedistributions,speciallytheir νe +ν¯e events for δ = 0 and δ (cid:44) 0 and take difference domain structures, is largely explained as due to grids betweenthedistributionsintheEν–cosθzplanetostudy ofsolar,atmosphericandinterferencemagiclinesinthe their properties. Since there are errors associated with E –cosθ plane. The oscillation probabilities are inde- ν z reconstructing the true neutrino energy and directions, pendent of δ along these lines, thus separating regions wesmeartheidealdistributionswiththeenergyandan- ofsamesigndistinguishability. Figure2showsS dis- ij gular resolution functions of the detector to mimic the tributionsforν +ν¯ events. e e realsituation. Toestimatethesensitivityofmeasuringa The uncertainties associated with atmospheric neu- CPphasedifferentfromzeroweemployadistinguisha- trinoflux,νNcrosssection,effectivevolume,etc.affect bilityparameterdefinedas neutrinoeventdistributionsacrossbinsintheE –cosθ ν z S (f)=(Nδ −N0)/σ (f), (3) plane. Weincludeeffectsofthesecorrelateduncertain- ij ij ij ij ties in our calculation with analogy to the pull method where Nδ and N0 are the reconstructed number of inχ2 analysis. Inparticularweminimizethefollowing ij ij events in the ij-th bin in the E –cosθ plane for δ and distinguishabilityparameter ν z δ=0,respectively,andσ2(f)= N0+(fN0)2isthetotal  ecrorrorreliantetdhesyisjt-ethmbatiinc.ePrraoriarjsm[e2t]eranfidjiws aetmaikejeasufr=e o2f.5u%n-. Sσtot(ξk) = (cid:88)(cid:88)[Niδj,l(ξk)σ−2Ni0j,l(ξkst)]2 l=e,µ ij ij,l /NuclearPhysicsBProceedingsSupplement00(2015)1–3 3 +(cid:88)(ξk−σ2ξkst)21/2, (5) k k whereξ arethepullvariablesandξst aretheirstandard k k values. Theeventdistributionswithvaryingξ arecal- k culatedas Nδ (ξ ) = αz(E/2GeV)η ij,l k l ×[1+β(0.5+cosθ )]Nδ (ξst), (6) z ij,l k whereαistheoverallnormalizationfactorwiththeer- ror σ = 0.2, z is the flux (flavor) ratio uncertainty α l (z ≡ 1forν events),withtheerrorσ = 0.05;ηisthe e e z energytiltparameterwithσ =0.1;βisthezenithangle η tiltwithσ = 0.04. Figure3showstheStot minimized β σ over(ξ )fordifferentcorrelateduncertaintiesaswellas k for no correlated uncertainties. A threshold energy of 0.5GeVhasbeenassumed. Notethatthecontributions ofν andν channelstoStot arecomparable. e µ σ 3. Discussion Weestimatethatafter4yearsofoperationand2.5% systematics,Super-PINGUwith0.5GeVthresholdwill be able to distinguish δ = π/4,π/2,π,3π/2 from zero withStot(π/4) = (1−3),Stot(π/2) = (3−8),Stot(π) = σ σ σ (6−14), Stot(3π/2) = (3−8). The ranges depend on σ effects of different correlated systematics. These val- uesareafactor4–6improvementoverthesensitivityof PINGUtoδwith3GeVthreshold. The sensitivity of Super-PINGU to δ can be further improvedwithfollowingpossibilities: Figure3: DistinguishabilityofmeasuringCPphaseintheνµandνe channels,witheffectsofvariouscorrelateduncertainties.2.5%uncor- • Decreaseofenergythresholdto0.2GeVfrom0.5 relateduncertaintyhasbeenassumedinallcases. GeVwithadenserarray. Thismayincreasesensi- tivityby30%. References • Stringent kinematical cut can be used to create a [1] S. Razzaque and A. Yu. Smirnov, JHEP (submitted) high-quality event sample with better reconstruc- [arXiv:1406.1407[hep-ph]]. tionoftheneutrinoenergy,directionandflavor. [2] E.K.Akhmedov,S.RazzaqueandA.Yu.Smirnov,JHEP1302 (2013)082[Erratum-ibid.1307(2013)026][arXiv:1205.7071 • An increased exposure time wi√ll also increase the [hep-ph]]. sensitivitytoCPbyafactor∝ t. [3] M.RibordyandA.Yu.Smirnov, Phys.Rev.D87(2013)11, 113007[arXiv:1303.0758[hep-ph]]. • Improvedflavoridentificationatlowenergies. [4] M. G. Aartsen et al. [IceCube-PINGU Collaboration], arXiv:1401.2046[physics.ins-det]. • Increase the density of DOMs or photocathode [5] U.F.Katz[KM3NeTCollaboration],[arXiv:1402.1022[astro- coverage. ph.IM]]. • Statisticalseparationbetweentheneutrinoandan- tineutrinoevents. Our results show that Super-PINGU can be compet- itive to other proposals for measuring the leptonic CP phaseassociatedwithlongbase-line(LBL)accelerator experiments.