Measurement of electron neutrino CCQE-like cross-section in MINERvA

2.9 MB·English

by  Jeremy Wolcott

Measurement of electron neutrino CCQE-like
cross-section in MINERvA
5
1
0
2
Jeremy Wolcott∗, for the MINERvA collaboration
n
a UniversityOfRochester,Rochester,NewYork14610USA
J E-mail: [email protected]

1
2

Theelectron-neutrinocharged-currentquasi-elastic(CCQE)cross-sectiononnucleiisanimpor-
]
x tant input parameter to appearance-type neutrino oscillation experiments. Current experiments
e
- typically work from the muon neutrino cross-section and apply corrections from theoretical ar-
p
gumentstoobtainapredictionfortheelectronneutrinocross-section,buttodatetherehasbeen
e
h no experimental verification of the estimates for this channel at an energy scale appropriate to
[
such experiments. We present a preliminary result from the MINERvA experiment on the first

1
measurement of an exclusive reaction in few-GeV electron neutrino interactions, namely, the
v
4 cross-sectionforaCCQE-likeprocess. Theresultisgivenbothasdifferentialcross-sectionsvs.
1
theelectronenergy,electronangle,andQ2,aswellasatotalcross-sectionvs. neutrinoenergy.
2
5
0
.
1
0
5
1
:
v
i
X
r
a
16thInternationalWorkshoponNeutrinoFactoriesandFutureNeutrinoBeamFacilities
25-30August,2014
UniversityofGlasgow,UnitedKingdom
∗Speaker.
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
MeasurementofelectronneutrinoCCQE-likecross-sectioninMINERvA JeremyWolcott
1. Introduction
Current terrestrial neutrino oscillation experiments searching for fundamental information in
the neutrino sector, such as the neutrino mass hierarchy and whether CP violation occurs for lep-
tons,usuallyemployexperimentaldesignswhichrelyonthepartialoscillationofabeamofmuon
neutrinosintoelectronneutrinos.[1,2]Theseexperimentsbuildlargedetectorsofheavymaterials
tomaximizetherateofneutrinointeractions,andthenexaminetheenergydistributionoftheneu-
trinos that do interact with the detector, comparing the observed spectrum with predictions based
onhypothesesofnooscillationoroscillationwithgivenparameters.
Correct prediction of the observed energy spectrum for electron neutrino interactions—on
whichtheseoscillationresultsdepend—requiresanaccuratemodeloftheratesandoutgoingparti-
clekinematics. This,inessence,boilsdowntoaneedforpreciseν cross-sectionsonthedetector
e
materials in use. And yet, because of the difficulties associated with producing few-GeV electron
neutrino beams, even when including very recent results, only two such cross-section measure-
ments exist[3, 4]. Furthermore, the small statistics and inclusive nature of both of these measure-
ments make their use as model discriminators challenging. Instead, most simulations begin from
thewealthofhigh-precisioncross-sectiondataavailableformuonneutrinosandapplycorrections
suchasthosediscussedinref. [5]toobtainapredictionforν .
e
Weofferhereapreliminaryresultinanefforttoproduceahigher-statisticscross-sectionfora
quasi-elastic-likeelectronneutrinoprocess,whichisamongthedominantreactionmechanismsat
mostenergiesofinteresttooscillationexperiments. WeusetheMINERνAdetector,whichconsists
of a central sampling scintillator region, built from strips of fluoror-doped scintillator glued into
sheets, then stacked transverse to the beam axis; both barrel-style and downstream longitudinal
electromagnetic and hadronic sampling calorimeters; and a collection of upstream passive targets
oflead,iron,graphite,water,andliquidhelium. Thedetectordesignandperformancearediscussed
infulldetailelsewhere.[6]MINERνAoccupiesspaceintheNuMIν beam,whereitwasexposed
µ
toafluxof∼99%ν and∼1%ν mostlybetween3-5GeVforthisdataset. Wealsocomparethe
µ e
resultforν toasimilar,previousMINERνAresultforν toevaluatehowsimilartheyare.
e µ
2. Signaldefinition
In traditional charged-current quasi-elastic neutrino scattering, CCQE, the neutrino is con-
verted to a charged lepton via exchange of a W boson with a nucleon, resulting in the following
reaction: ν n→l−p. (Antineutrinoscatteringreversestheleptonnumberandisospin: ν¯ p→l+n.)
l l
BecausetheMINERνAdetectorisnotmagnetized,wecannotdifferentiatebetweenelectronsand
positrons on an event-by-event basis. Moreover, hadrons exiting the nucleus after the interaction
canre-interactandchangeidentityorejectotherhadrons[7];furthermore,pairsofnucleonscorre-
latedwithintheinitialstatemaycausemultiplenucleonstobeejectedbyasingleinteraction[8,9].
Therefore,wedefinethesignalprocess“phenomenologically,”byitsfinal-stateparticles: wesearch
for events with either an electron or positron, no other leptons or photons, any number of nucle-
ons, and no other hadrons. We call this type of event “CCQE-like.” We also demand that events
originatefroma5.57-tonvolumefiducialvolumeinthecentralscintillatorregionofMINERνA.
2
MeasurementofelectronneutrinoCCQE-likecross-sectioninMINERvA JeremyWolcott
3. Eventselectionandbackgrounds
Candidate events are selected from the data based on three major criteria. First, a candidate
must contain a reconstructed cone object of angle 7.5◦, originating in the fiducial volume, which
is identified as a candidate electromagnetic cascade by a multivariate PID algorithm. The latter
combines details of the energy deposition pattern both longitudinally (mean dE/dx, fraction of
energy at downstream end of cone) and transverse to the axis of the cone (mean shower width)
usingak-nearest-neighbors(kNN)algorithm. Secondly,weseparateelectronsandpositronsfrom
photonsbycuttingeventsinwhichtheenergydepositionattheupstreamendoftheconeisconsis-
tentwithtwoparticlesratherthanone(sincephotonstypicallyinteractinMINERνAbyproducing
an electron-positron pair). At this point, the cone object becomes the electron candidate. Our fi-
nalcriterionisanattempttoselectCCQE-likeinteractionsusingaclassifierwecall“extraenergy
fraction,”Ψ,which,whenanevent’svisibleenergynotinsidetheelectroncandidateorasphereof
radius30cmcenteredaroundtheconevertexisdenoted“extraenergy,”isdefinedas:
E
extra
Ψ= (3.1)
E
electron
Ourcutisafunctionofthetotalvisibleenergyoftheevent. Thecutatthemostprobabletotalvisi-
bleenergy,E =0.4GeV,isillustratedinfig. 1. Finally,weretainonlyeventswithreconstructed
vis
electronenergyintherange1GeV≤E ≤10GeV.Herethelowerboundexcludesaregionwhere
e
the background estimate is still under further study, and the upper bound restricts the sample to
events where the uncertainties on flux prediction are tolerable. The distribution of events selected
bythissequenceisshowninfig. 2.
Figure1: SamplecutonΨ(definedinthetext)atthemostprobableeventvisibleenergy,E =0.4
vis
GeV.
3
MeasurementofelectronneutrinoCCQE-likecross-sectioninMINERvA JeremyWolcott
Figure2: Eventsampleafterallselectioncuts.
Asfig. 2shows,evenafterthefinalselection,asignificantfractionofthesampleispredicted
to be from background processes. We attempt to constrain the background model by examining
sidebands in two of the variables already mentioned. The first of these is in the dE/dx measured
atthefrontoftheelectroncandidate; bychoosingasampleatlargervalues, wecanobtainaside-
band rich in photon background events. The second sideband is in the extra energy fraction Ψ;
a sample of events at larger Ψ constitutes a sideband rich in inelastic background. We use these
sidebands together to fit the normalizations of the three major backgrounds: ν non CCQE-like,
e
non-ν coherentpion,andotherinelasticevents. Thethreebackgroundclasses’normalizationsare
e
fittedsimultaneously,usingdistributionsinbothreconstructedcandidateelectronangleandenergy,
acrossthetwosidebands,toobtainasinglescalefactorthatrepresentsthebestestimateofthetotal
normalization of the background as compared to the prediction from GENIE. We obtain a scale
factor of 0.69; this overall reduction is a similar trend to that observed when similar procedures
were performed on other MINERνA analyses. Subsequent to the constraint, we scale the back-
groundsinthesignalregionandsubtractthemfromthedata. Wethencompareittothesimulated
predictionofthesignalprocess.
4. Cross-sectionresult
We calculate three differential cross-sections in electron angle, electron energy, and four-
momentum transfered from neutrino to nucleus Q2, as well as the total cross-section vs. neu-
trino energy. For neutrino energy and Q2, we employ the commonly-used CCQE approximations
(assumingastationary targetnucleon)whichallow ustocomputethemfrom justtheleptonkine-
4
MeasurementofelectronneutrinoCCQE-likecross-sectioninMINERvA JeremyWolcott
matics:
m2−(m −E )2−m2+2(m −E E )
EQE = n p b e p b e (4.1)
ν 2(m −E −E +p cosθ )
p b e e e
Q2 =2EQE(E −p cosθ )−m2 (4.2)
QE ν e e e e
Differentialcross-sectionsarecalculatedinbinsiaccordingtothefollowingruleforsamplevari-
able ξ, with ε representing signal acceptance, Φ the flux integrated over the energy range of the
measurement,T thenumberoftargets(CHmolecules)inthefiducialregion,∆ thewidthofbini,
n i
andU amatrixcorrectingfordetectorsmearinginthevariableofinterest:
ij
(cid:18) (cid:19)
dσ 1 (cid:16) (cid:17)
= ×∑U Ndata−Nbkndpred (4.3)
dξ εΦT (∆) ij j j
i i n i j
(The formula for the total cross-section differs only in that the flux is integrated only over the
energyofbini,ratherthanthewholeenergyrange,andthatwethereforedonotneedtodivideby
thebinwidth∆.)
i
WeperformunfoldinginthesefourvariablesusingaBayesiantechnique[10]withasingleit-
eration. TheunfoldingmatricesU neededasinputarepredictedbyoursimulation. Ourprediction
ij
for the neutrino flux Φ by which we then divide is derived from a GEANT4-based simulation of
theNuMIbeamline(describedfurtherinref. [11]). Inaddition,weuseaninsituMINERνAmea-
surementbasedonelasticscatteringofneutrinosfromatomicelectrons[13]toprovideadata-based
constraintforthefluxestimate.
The cross-section vs. electron angle obtained from this procedure is given in fig. 3. We note
thatthesimulationpredictssubstantiallymoreeventsinthemostforwardbinsthanweobservein
our data. In addition, the measured cross-section vs. Q2 , shown in fig. 4a, appears to exhibit
QE
a noticeable migration from low to high Q2 as compared to the prediction. This is in contrast to
the analogous cross-section measured in MINERνA using muon neutrinos, fig. 4b, which agrees
muchbetter—ifnotperfectly—withGENIE’smodel.
5. Conclusions
Thoughtheν cross-sectionisvitallyimportantforneutrinooscillationsearches,experimental
e
challengeshavepreventedextensivemeasurementofthisquantityuntilrecently. Inthispreliminary
resultfromMINERνA,weobserveadiscrepancyatlowanglesbetweenthemodelinGENIE2.6.2
and our data in dσ/dθ . Furthermore, we find that the Q2 spectrum we observe appears to be
e QE
harderforν CCQEthanitisforν CCQE,incontrasttothepredictionofGENIE.Ifsubstantiated
e µ
byfurtherstudy,theseobservationswillnecessitatemodificationstothemodelscurrentlyinusein
neutrinogeneratorssoastoensuretheycorrectlysimulatetheelectronneutrinokinematics. Work
isstillongoingtocharacterizethebackgroundsintheE <1GeVregion,andafullresultwillbe
e
publishedsoon.
References
[1] K.Abeetal.(T2Kcollaboration),TheT2KExperiment,NIMA659106
[physics.ins-det/1106.1238].
5
MeasurementofelectronneutrinoCCQE-likecross-sectioninMINERvA JeremyWolcott
5
H) MINERνA Preliminary
C Absolutely normalized (3.34× 1020 P.O.T.)

e / 4 Data
e Simulation
r
g
e 3
d

/

2
m 2
c

9
3
0 1
1
(

σ e
θ
dd 0
0 5 10 15 20 25 30 35 40 45
θ (deg)
e
Figure3: Differentialcross-sectionvs. electronangle. Innererrorsarestatistical;outerarestatisti-
caladdedinquadraturewithsystematic.
CH) 120 Absolutely normMaIlNizEeRd ν(A3 .3P4r×e 1l0i2m0 iPn.Oa.rTy.) ×10 36 MINERνA Preliminary • ν Tracker → CCQE
2 / GeV 100 DSiamtaulation eutrons) 0.01.21 DMaotnate Carlo
2 / m 80 2/6 nV 0.08
39 c10 4600 2/Gecm 00..0046
σd (2QQE20 2dQ (QE 0.02
d σ/d 00 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
0 Q2 (GeV2)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 QE
Q2 (GeV2) (b)ν (measurementfromref. [12])
QE µ
(a)ν
e
Figure4: Differentialcross-sectionvs. Q2 (definedinthetext)forelectronandmuonneutrinos.
QE
Innererrorsarestatistical;outerarestatisticaladdedinquadraturewithsystematic.
[2] D.S.Ayresetal.(NOvAcollaboration),TheNOvATechnicalDesignReport,
FERMILAB-DESIGN-2007-01.
[3] J.Blietschauetal.(Gargamellecollaboration),Totalcrosssectionsforν andν¯ interactionsand
e e
searchforneutrinooscillationsanddecay,Nucl.Phys.B133205.
[4] K.Abeetal.(T2Kcollaboration),MeasurementoftheInclusiveElectronNeutrinoChargedCurrent
CrossSectiononCarbonwiththeT2KNearDetector,PRL113241803[hep-ex/1407.7389].
6
MeasurementofelectronneutrinoCCQE-likecross-sectioninMINERvA JeremyWolcott
[5] M.DayandK.S.McFarland,DifferencesinQuasi-ElasticCross-SectionsofMuonandElectron
Neutrinos,PRD86053003[hep-ph/1206.6745].
[6] L.Aliagaetal.(MINERνAcollaboration),Design,Calibration,andPerformanceoftheMINERvA
Detector,NIMA743130[physics.ins-det/1305.5199].
[7] O.Lalakulich,U.Mosel,andK.Gallmeister,Energyreconstructioninquasielasticscatteringinthe
MiniBooNEandT2Kexperiments,PRC86054606[nucl-th/1208.3678].
[8] M.MartiniandM.Ericson,Quasielasticandmultinucleonexcitationsinantineutrino-nucleus
interactions,PRC87065501[nucl-th/1303.7199].
[9] J.Nieves,M.Valverde,andM.J.VicenteVacas,InclusiveNucleonEmissionInducedby
Quasi–ElasticNeutrino–NucleusInteractions,PRC73025504[hep-ph/0511204].
[10] G.D’Agostini,AmultidimensionalunfoldingmethodbasedonBayes’theorem,NIMA362487.
[11] L.Fieldsetal.(MINERvAcollaboration),MeasurementofMuonAntineutrinoQuasi-Elastic
ScatteringonaHydrocarbonTargetatE ∼3.5GeV,PRL111022501[hep-ex/1305.2234].
ν
[12] G.A.Fiorentinietal.(MINERvAcollaboration),MeasurementofMuonNeutrinoQuasi-Elastic
ScatteringonaHydrocarbonTargetatE ∼3.5GeV,PRL111022502[hep-ex/1305.2243].
ν
[13] J.Park,DirectMeasurementoftheNuMIFluxwithNeutrino-ElectronScatteringinMINERνA,
FNALJETPseminar,20Dec2013.
7