Progress On Neutrino-Proton Neutral-Current Scattering In MicroBooNE

0.2 MB·

by  Stephen Pate

Progress On Neutrino-Proton Neutral-Current
Scattering In MicroBooNE
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1 Stephen Pate∗†
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2 (fortheMicroBooNECollaboration)
PhysicsDepartment,NewMexicoStateUniversity,LasCrucesNM88003,USA
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a E-mail: [email protected]
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1 The MicroBooNE Experiment at the Fermi National Accelerator Laboratory, an 89-ton active
mass liquid argon time projection chamber, affords a unique opportunity to observe low-Q2
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x neutral-current neutrino-proton scattering events. Neutral-current neutrino-proton scattering at
e
- Q2<1GeV2 isdominatedbytheproton’saxialformfactor,whichcanbewrittenasacombina-
l
c tionofcontributionsfromtheup,down,andstrangequarks: G (Q2)= 1[−Gu(Q2)+Gd(Q2)+
u A 2 A A
Gs(Q2)]. Thecontributionfromupanddownquarkshasbeenestablishedinpastcharged-current
n A
[ measurements. The contribution from strange quarks at low Q2 remains unmeasured; this is of

1 greatinterestsincethestrangequarkcontributiontotheprotonspincanbedeterminedfromthe
v low-Q2 behavior: ∆S =Gs(Q2 =0). MicroBooNE began operating in the Booster Neutrino
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8 BeaminOctober2015. IwillpresentthestatusinobservingisolatedprotontracksintheMicro-
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BooNEdetectorasasignatureforneutral-currentneutrino-protonevents. Thesensitivityofthe
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0 MicroBooNEexperimentformeasuringthestrangequarkcontributiontotheprotonspinwillbe
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1 discussed.
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The26thInternationalNuclearPhysicsConference
11-16September,2016
Adelaide,Australia
∗Speaker.
†ThisresearchsupportedbytheUSDepartmentofEnergy,OfficeofScience,MediumEnergyNuclearPhysics.
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
ProgressOnNeutrino-ProtonNeutral-CurrentScatteringInMicroBooNE StephenPate
1. Motivation
Thecrosssectionforneutrino-protonneutral-currentscattering[12],νp→νp,isdetermined
by the standard model behavior of neutrinos and the structure of the proton as expressed in the
electric,magnetic,andaxialformfactorsoftheproton;Gp(Q2),Gp(Q2),andGp(Q2)respectively.
E M A
Theelectricandmagneticformfactorsarewell-determinedformomentumtransfersQ2<1GeV2
fromelectron-nucleonelasticscatteringdata[4]. Thecontributiontotheaxialformfactorfromup
and down quarks is also well-determined for Q2 <1 GeV2 by neutrino-deuteron charged-current
reaction data [7], and at Q2 = 0 GeV2 from neutron decay [8]. The dominant unknown is the
strangequarkcontributiontotheaxialformfactor: Gs(Q2). Ameasurementofthisformfactorat
A
low Q2 can determine the total strange quark contribution to the proton spin, ∆S≡∆s+∆s¯, from
anextrapolationtoQ2=0: ∆S=Gs(Q2=0)[21].
A
Thephysicsinterestinthestrangequarkcontributiontothenucleonspinislong-standingand
widespread. In addition to being a missing piece of the proton spin puzzle, it is also vital for
theinterpretationofsearchesforheavydark-matterparticles[10]. Three-dimensionalsimulations
of supernovae [17] are sensitive to the value of ∆S, as are atomic parity-violation experiments
on hydrogen [13]. Recent lattice QCD calculations [11, 5] give small values (less than 0.003 in
magnitude)for∆S;thisrequiresexperimentalverification.
Thefirstexperimentaldataon∆Scamefrommeasurementsoftheinclusivedeep-inelasticscat-
tering(DIS)ofpolarizedmuonsfrompolarizedhydrogen,intheEMCexperimentinthe1980s[3],
and indicated a negative value for ∆S. This has been confirmed in all subsequent inclusive mea-
surements at SMC, SLAC, HERMES, COMPASS, and JLab. The analysis of polarized inclusive
DIS data always assumes SU(3) flavor symmetry, combining the extrapolated integral of the DIS
measurementswiththetripletandoctetaxialchargesdeterminedfromhyperonβ-decay.
Later,itbecamepossibletoobservesemi-inclusivedeep-inelasticscattering,wheretheleading
hadron(apionorkaon)servedto“tag”thestruckquark. ThesemeasurementsbySMC,HERMES,
and COMPASS [3] have consistently implied that ∆S is consistent with zero, in contradiction to
the inclusive measurements. The analysis of these data differs strongly from that of the inclusive
data, not using SU(3) flavor symmetry but instead relying on an understanding of quark→hadron
fragmentationfunctions.
Thisdichotomybetweentheresultsoftheinclusiveandsemi-inclusivemeasurementscontin-
uestothepresentday. Globalanalyses[9,18,16,14,6]ofleptonicDISandpolarized ppcollision
datashowthisdiscrepancyinthedeterminationof∆S.
Analternativemethodtodetermine∆Sisavailablefromameasurementoftheaxialformfactor
of the proton in elastic neutrino-proton scattering. Cross sections for elastic νp scattering (from
withincarbonnuclei)existfromtheBNL-E734experiment[2]andMiniBooNE[1],butneitherof
thesemeasurementsextendsbelowQ2 =0.45GeV2 andthereforecannotbereliablyextrapolated
to Q2 =0 for a precise determination of ∆S. An analysis of currently available data was explored
indetailin[22]and[19]. Ameasurementofneutrino-protonelasticscatteringatlowQ2 willmake
possibleamoreprecisedeterminationof∆S.
Such a measurement is a difficult pursuit. The observable part of the final state in elastic
νp→νp is a single isolated proton track. For Q2 ≈0.1 GeV2, the kinetic energy of the proton
willbeapproximately50MeV.Inaliquidorsoliddetector,suchaprotonmighttravelonlyafew
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ProgressOnNeutrino-ProtonNeutral-CurrentScatteringInMicroBooNE StephenPate
centimeters. The development of large-scale liquid argon time projection chambers, however, has
madetheefficientdetectionofsuchevents,withgoodstatistics,arealisticpossibility.
Another difficulty arises from the use of a nuclear target, in our case the argon-40 nucleus.
In particular, the proton in the final state may re-scatter with other nucleons before escaping the
nucleus. Tomitigatethisandothereffects,wehavechosenasourobservabletheneutral-currentto
charged-currentyieldratio(seeSection3),
N(νp→νp)
R = .
NC/CC N(νn→µp)
Wewillneedinputfromnucleartheorytoestablishthereliabilityofthis(oranyother)approachto
theeffectsthenucleuswillhaveontheneutral-currentandcharged-currentyields.
2. TheMicroBooNEExperiment
MicroBooNE[15]isanaccelerator-basedneutrinoexperimentatFermiNationalAccelerator
Laboratory,centeredaroundaliquidargontimeprojectionchamber(LArTPC)withanactivemass
of 89 tons of liquid argon in a field cage of dimensions (10 m)×(2.3 m)×(2.6 m). The TPC was
placed into the experimental hall in the summer of 2014, installed and commissioned, observed
first cosmic rays in the summer of 2015, and began to take data with the Booster Neutrino Beam
(BNB)onOctober15,2015. Sincethattime,in-beamdatahasbeentakenusing3.4×1020protons-
on-target.
AnalysisoftheTPCwiredataproducesimagesofchargevs. timeintheTPCvolume,reveal-
ingtracksfromcharged-particlesthattransitedtheactivevolume. Withthreewireplanes,thereare
three images of each event, making possible a three-dimensional reconstruction of the ionization
tracks. In addition to the charged-track images, a light collection system (composed of 32 8-inch
photomultipliertubes)recordsflashesoflightfromthescintillationoftheliquidargon. Therewill
bemillionsoftheseimagesandassociatedlightflashesrecordedoverthecourseoftheexperiment,
sotraditional“visualscanning”ofsuchdataisoutofthequestion. Automatedsoftwaremustsort
throughtheevents;forlarge-scaleLArTPCdatathiswillbethefirsttimesuchautomatedsorting
hasbeenattempted.
For finding the isolated proton tracks of interest here, Katherine Woodruff of NMSU has led
an effort to utilize a “boosted decision tree” technique. A boosted decision tree is a series of
if/then/elsequestionsthatistunedusing“trainingdata.” Inourcase,thequestionswillbedirected
at a variety of track features; geometry (track length and orientation), calorimetry (total charge,
total light), and matching between the track location and flash location. The result of the tree, for
eachtrack,istheassignmentofprobabilitiesthatthetrackisamemberoffivedifferentclasses: a
proton,amuon,apion,anelectromagneticshower,oracosmic-generatedtrack.
Using a full simulation of the detector, including all detector effects (true geometry, realistic
noise,missingwires,etc.),wecantrainthetreetooptimizethetrackclassification. Then,usinga
secondsetofsimulatedevents,wedeterminetheefficiencyofthetreeforeachclass. Thenweare
abletousetheboosteddecisiontreeonrealin-beamdata.
Figure1showsacandidateforanisolatedprotontrackthatwasselectedbyaboosteddecision
tree. The proton probability assigned to this track was 87%. This view of the track is from just
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ProgressOnNeutrino-ProtonNeutral-CurrentScatteringInMicroBooNE StephenPate
Figure 1: A proton candidate track in MicroBooNE data selected by a trained boosted decision tree. The
wirespacingis3mm. Thepixelcolorisproportionaltoionizationchargedensity.
onewireplane. Usingallthreeplanes,thereconstructedthree-dimensionaltracklengthis5.9cm,
corresponding to a proton kinetic energy of approximately 82 MeV. If this were in fact a neutral-
currentscatteringevent,themomentumtransferwouldbeQ2≈0.15GeV2.
3. PotentialImpact
What will be the impact on our knowledge of ∆S due to the new neutral-current elastic scat-
teringdatafromMicroBooNE?Thisquestionhasbeenstudiedinsomedetailandreportedalready
in [20]; a review of that work is given here. The central feature of this study is a global fit of
elasticelectron-nucleonandneutrino-electronscatteringdatatoextractthestrangequarkcontribu-
tion to the electric, magnetic and axial form factors of the proton; Gs(Q2), Gs (Q2), and Gs(Q2)
E M A
respectively. Asimplefunctionalmodelisusedforthosethreeformfactorsinthisglobalfit:
∆S+S Q2
Gs =ρ τ Gs =µ Gs = A
E s M s A (1+Q2/Λ2)2
A
where ρ is the strangeness radius, τ =Q2/4M2, µ is the strangeness contribution to the proton
s N s
magnetic moment, and S and Λ are shape parameters that were needed to obtain a good fit
A A
to the existing data. The result (see Table 1) is that Gs(Q2) and Gs (Q2) are well constrained
E M
throughouttherange0<Q2<1.0GeV2byexistingdata(andareconsistentwithzero),butGs(Q2)
A
isunconstrainedatlowQ2 duetothelackofneutrino-protonelasticdatabelowQ2=0.45GeV2.
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ProgressOnNeutrino-ProtonNeutral-CurrentScatteringInMicroBooNE StephenPate
Table1: Preliminaryresultsforour5-parameterfittothe49elasticneutrino-andparity-violatingelectron-
scatteringdatapointsfromBNLE734,HAPPEx,SAMPLE,G0,andPVA4.Thefirstuncertaintyarisesfrom
theexperimentaldatauncertainties;theseconduncertaintyarisesfromuncertaintiesinradiativecorrections.
Note: thesevaluesdifferfromreference[20]duetotheinclusionofnewdatafromPVA4onscatteringfrom
deuterium.
Parameter Fitvalue
ρ −0.10±0.09±0.03
s
µ 0.056±0.029±0.022
s
∆S −0.29±0.42±0.19
Λ 1.1±1.0±1.1
A
S 0.4±0.5±0.2
A
Taking that fit as a starting point, a simulation1 was performed to generate a set of neutral
current (NC) νp→νp and charged-current (CC) νn→ µp events corresponding to a sample of
MicroBooNE data using 2×1020 protons-on-target, that is about 1/3 of the data sample that Mi-
croBooNE plans to collect in our initial run. These simulated events were used to estimate the
statisticaluncertaintyintheNC/CCyieldratio,
N(νp→νp)
R = .
NC/CC N(νn→µp)
Thisratioischosenasourexperimentalobservablenotonlybecausemanyexperimentaluncertain-
tiesapproximatelycancelinthisratio(e.g. protondetectionefficiencies,targetmass,neutrinoflux),
butalsobecausesometheoreticaluncertainties(e.g. nuclearfinalstateeffectsontheprotonyield)
maycancelaswell. Thissampledoesnotrepresentthefullstatisticsweexpectintheexperiment,
butontheotherhanditdoesnotincludetheeffectofreconstructionefficienciesandbackgrounds
fromneutronsandcosmicrays. Ourintentionhereistoqualitativelyshowthepotentialimpactof
newdatainthelow-Q2 region.
The simulatedresults forR are thentreated as actual dataand the global fitis repeated.
NC/CC
ThechangeintheuncertaintiesofthefitparametersareshowninTable2. Thefittedmeanvalues
fortheformfactorsdonotshiftbecausetheywerethebasisoftheMicroBooNEeventsimulation,
so these are not shown in the table; however the uncertainties in those form factors are reduced.
Theuncertaintiesintheelectricandmagneticformfactorsareimproved,butnotverysignificantly.
However, the uncertainty in the parameters for the axial form factor (∆S, S , and Λ ) are greatly
A A
reduced, by approximately a factor of 10. This illustrates the very significant impact that Micro-
BooNE data on neutral current elastic scattering can potentially have on our knowledge of the
strangenesscontributiontotheaxialformfactorandtotheprotonspin.
4. Summary
TheMicroBooNEExperimenthastakendataforoneyearwiththeBoosterNeutrinoBeamat
Fermilab, the start of a multi-year data program. We expect to have a dataset on elastic neutrino-
proton interactions in argon that will allow us to determine the strangeness contribution to the
1ThankstoB.Fleming,J.Spitz,andV.Papavassiliouforprovidingthissimulation.
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ProgressOnNeutrino-ProtonNeutral-CurrentScatteringInMicroBooNE StephenPate
Table 2: Improvement in uncertainties in global fit parameters, when simulated MicroBooNE data are
includedinthefit.
Parameter UsingExistingData IncludingMicroBooNE
ρ ±0.09±0.03 ±0.08±0.02
s
µ ±0.029±0.022 ±0.023±0.017
s
∆S ±0.42±0.19 ±0.036±0.003
Λ ±1.0±1.1 ±0.42±0.03
A
S ±0.5±0.2 ±0.05±0.02
A
proton axial form factor and thus provide a value for the strangeness contribution to the proton
spin. Thiswillbethefirstsuchmeasurementinaliquidargonneutrinodetector.
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