Here we present analysis results from the MINERvA experiment

for scattering of neutrinos on nucleus in an energy region of few GeV. These results cover a plethora of processes important for high precision neutrino oscillation measurements in which recent results have suggested that the currently used models are insufficient

Carneiro, M. F.

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DOI 10.1393/ncc/i2015-15124-0Colloquia: LaThuile15IL NUOVO CIMENTO 38 C (2015) 124Neutrino interactions on nuclei at MINERvAM. F. Carneiro on behalf of the MINERvA CollaborationCBPF - Centro Brasileiro de Pesquisas F´ısicas - Rua Dr. Xavier Sigaud 150,Urca, Rio de Janeiro, RJ, 22290-180, Brazilreceived 2 October 2015Summary. — Here we present analysis results from the MINERvA experimentfor scattering of neutrinos on nucleus in an energy region of few GeV. These resultscover a plethora of processes important for high precision neutrino oscillation mea-surements in which recent results have suggested that the currently used models areinsufficient.PACS 13.15.+g – Neutrino interactions.PACS 25.80.Hp – Pion-induced reactions.PACS 25.30.-c – Lepton-induced reactions.1. – DescriptionThe MINERvA experiment is a fully active, high resolution detector designed to studyneutrino-nucleus scattering in the few-GeV region and provide inputs for neutrino oscilla-tion experiments. The experiment also examines nuclear effects and parton distributionfunctions (PDFs) using a variety of targets materials. Precision neutrino measurementsaiming to determine mass hierarchy, probe CP violation, or looking for new physics,require precise knowledge of cross sections, final states, and nuclear effects in neutrinoscattering. These experiments need models that will correctly predict the rate of eventsfor neutrino interactions, especially using medium-heavy materials such as steel, argon,carbon and oxygen. The true neutrino energy relation with the final state particles is alsoa key information for neutrino oscillation physics since the flavor oscillation probabilitydepends on neutrino energy.The detector is situated in Fermilab’s NuMI beamline [1] along with the MINOS andNOvA experiments. During the period of 2010 through the Spring of 2012 the MINERvAdetector took data in the “low energy” mode, in which the peak for the neutrino energywas around 3GeV. Since then, the NuMI beamline is working in “medium energy” modewith the neutrino energy peak at 6GeV.MINERvA [2] is comprised of 120 hexagonal modules stacked along the beamline. Thedetector is segmented transversely into: the inner detector, with planes of solid scintillatorand passive nuclear target regions of carbon, iron, lead and water; a region of pureCreative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 M. F. CARNEIRO on behalf of the MINERvA COLLABORATIONscintillator strips; downstream electromagnetic calorimeter and hadronic calorimeters;and an outer detector composed of a frame of steel with embedded scintillator, whichalso serves as the supporting structure. The scintillator strips have a triangular shapethat permits 3mm of position resolution and are placed in adjacent planes offset by60 degrees from each other, enabling a three-dimensional track reconstruction. TheMINOS near detector [3] is situated two meters downstream of the MINERvA detectorand serves as a magnetized muon spectrometer.The calculation of cross section σi for Nj events in T interaction targets comparedto Bj background predictions in bins of neutrino energy i and considering the detectorefficiency of i is given by(1a) σi =Uij(Nj −Bj)ΦTi,where Φ is the integrated neutrino flux in bin i and Uij is an unfolding function thatconverts from reconstructed bins j to true bins i.2. – Charged-current quasielastic scatteringCharged-current quasielastic (CCQE) scattering νl(ν¯l) + n(p)→ l−(l+) + p(n) is thesimplest process between a neutrino and a nucleon mediated by a W boson. We havegood knowledge about neutrino-nucleon interactions from hydrogen and deuterium [4,5]bubble chamber measurements. Neutrino event generators consider the nucleon to bequasi-free in a Relativistic Fermi Gas (RFG) [6] in order to correct for nuclear medium.Recent data, however, do not agree with the relativistic Fermi gas prediction motivatingthe MINERvA CCQE analysis presented here.MINERvA’s first CCQE results [7,8] select candidate events with vertex in the plasticscintillator portion of the detector, a muon in the final state with momentum and chargereconstructed in the MINOS near detector, and low hadronic recoil in the MINERvAdetector to reduce deep inelastic scattering (DIS) and resonance backgrounds. Assum-ing that the scattering takes place in one single, at-rest, nucleon, we can use the muon(antimuon) to calculate Q2 and produce a flux-integrated cross section dσ/dQ2. Figure 1)2 (GeVQE2Q-210 -110 1Ratio to GENIE0.60.811.21.41.61.8data=0.99AGENIE RFG M=0.99ANuWro RFG M=0.99ANuWro RPA M=1.35ANuWro RFG M=0.99 + TEMANuWro RFG M=0.99ANuWro SF M CCQE→ Tracker ν  •A νMINERArea Normalized < 10 GeVν1.5 < E)2 (GeVQE2Q-210 -110 1Ratio to GENIE0.60.811.21.41.61.8data=0.99AGENIE RFG M=0.99ANuWro RFG M=0.99ANuWro RPA M=1.35ANuWro RFG M=0.99 + TEMANuWro RFG M=0.99ANuWro SF M CCQE→ Tracker ν  •A νMINERArea Normalized < 10 GeVν1.5 < EFig. 1. – MINERvA νμ CCQE-like data shown as a function of Q2 calculated from the muonkinematics as well as models from the NuWro generator [9] in a ratio to the GENIE eventgenerator prediction. Neutrino data on the left and antineutrino on the right. In both casesthe model predictions were weighted to the integral of the data producing a shape comparisonwhere flux uncertainties largely cancel.NEUTRINO INTERACTIONS ON NUCLEI AT MINERvA 3 )2 ( GeVQE,p2Q0 0.5 1 1.5 2Ratio to GENIE0.811.21.41.61.8Data GENIE RFGNuWro RFG NuWro RFG+RPANuWro RFG+TEM NuWro RFG+RPA+NievesAνMINER • p -μ → Tracker μνShape ComparisonsFig. 2. – MINERvA νμ CCQE data shown as a function of Q2 calculated from the protonkinematics as well as models from the NuWro generator in a ratio to the GENIE event generatorprediction. The model predictions were weighted to the integral of the data producing a shapecomparison were flux uncertainties largely cancel.shows the results for antineutrino and neutrino analysis, with a number of model pre-dictions. Each prediction has been scaled to match the integral of the data so we canhave a shape comparison with reduced flux uncertainties, and then divided by the pre-diction of the GENIE generator [10]. As seen in other experimental results, the Fermigas model from GENIE does not correctly predict the data. The better agreement comesfrom the prediction of the transverse enhancement model (TEM) [11], which includesparametrization of nucleon-nucleon correlations based on electron scattering data.The second CCQE analysis [12] presented here uses a sample of events where not justthe muon was identified, but also a proton. This analysis uses only the neutrino datasetand uses the proton kinematics to reconstruct Q2. This excludes the need for the muonto enter the MINOS detector, for example, an event with a muon by exiting the sideof MINERvA would be selected, which increase angular acceptance. As well as quasi-elastic events, this includes resonant or DIS events that undergo final-state interactions,leaving only nucleons in the final state. Quasi-elastics could produce more than oneproton if the initial proton re-interacts in FSI and produces another, if we scatter from acorrelated pair of nucleons. The result, shown in fig. 2 in the same format as fig. 1, showsa discrepancy with the GENIE prediction which is not reproduced by any of the modelsat high Q2, demonstrating the need for improved modeling of the hadronic system inCCQE interactions.3. – Charged-current single charged pion productionThe delta resonance νμ + p → μ− + Δ++ → μ− + π+ + p is the main process forpion production by neutrino scattering. Usually a resonance such as Δ(1232) is createdand decays to a pion and nucleon. Final state interactions (FSI) can absorb the pionin interactions with nuclear targets, mimicking the quasi-elastic signal and making pionproduction a major background for detectors that use CCQE as their signal. On the otherhand FSI can produce pions contaminating the quasi-elastic signal. The MINERvA pionproduction measurement [13] aims to examine the modeling of final state interactionsby comparing distributions of pion kinematics to those predicted by models. The eventselection applied the requirement for a muon that had reached the MINOS near detector,4 M. F. CARNEIRO on behalf of the MINERvA COLLABORATIONPion Kinetic Energy (MeV)0 50 100 150 200 250 300 350 400/MeV/nucleon)2  (cmπ/dTσd0510152025-4210×DataGENIE 2.6.2 hA FSIGENIE 2.6.2 No FSINEUT 5.3.1 (CH)NuWroACS (CH)GiBUU (CH) X  (W < 1.4 GeV)±π 1-μ → Tracker μνPOT Normalized Pion Angle wrt Beam (deg)0 20 40 60 80 100 120 140 160 180/degree/nucleon)2  (cmπθ/dσd 01020304050607080-4210×DataGENIE 2.6.2 hA FSIGENIE 2.6.2 No FSINEUT 5.3.1 (CH)NuWroACS (CH) X  (W < 1.4 GeV)±π 1-μ → Tracker μνPOT Normalized Fig. 3. – Flux-integrated differential cross section for muon neutrino events producing a singlecharged pion in MINERvA shown in function of pion energy and angle. Data is presented withseveral model predictions that were scaled to match the integral of the data.and a candidate pion reconstructed inside the MINERvA detector without creating ahadron shower. Pions can be identified in MINERvA by their dE/dx and the presenceof a Michel electron from the pion to muon to electron decay chain. Figure 3 shows theresulting pion kinematics, after a data-driven background subtraction constrained by theW > 1.4GeV sideband along with several model predictions, including the GENIE eventgenerator with and without the final state interactions implemented. The results showa strong preference for models that include a full treatment of FSI.4. – Charged-current single neutral pion production by antineutrinoAs in the last presented result, the delta resonance ν¯μ+p→ μ++Δ0 → μ++π0+n isthe main process studied by this analysis. As the π0 decays into two photons it can mimican electron neutrino signal making this a important channel for oscillation experiments.This analysis [14] uses events with muons in the final state matched with MINOS dataand antineutrino events that happened in the tracker region. Figure 4 shows the resultsfor the pion kinematics and the data has a better agreement when final state interactionsare included. This is the first measurement, at these energies, of the differential crosssections vs. π0 kinematics for this pion production channel. These cross sections can beused as a benchmark to evaluate neutrino event generator performance for π0 productionby antineutrinos for current and future oscillation experiments.5. – Charged-current scattering on different nucleiThe MINERvA detector upstream region has five passive targets of different materialsas lead, iron and graphite, separated by between four and sixteen planes of scintillator.This configuration permits a study of the neutrino cross section dependence with nuclearmass. Also by looking into ratios of cross section measurements on each different passivematerial target to the same measurement in the tracker, which consists of CH scintillator,the flux uncertainty is largely canceled. These measurements [15] aim to address thenecessity from oscillation experiments for accurate predictions for the neutrino crosssection in specific materials as well as to investigate the EMC effect using neutrino-nucleusscattering.NEUTRINO INTERACTIONS ON NUCLEI AT MINERvA 5 (GeV/c)0πp0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4/nucleon/(GeV/c))2 cm-40 (100πdpσd510152025303540DataGENIE w/ FSIGENIE w/o FSIAνMINERX (X has no mesons)0π1+μ →Tracker μνArea normalized2.01e+20 POT (deg.)0πθ0 20 40 60 80 100 120 140 160 180/nucleon/(deg.))2 cm-42 (100πθdσd510152025DataGENIE w/ FSIGENIE w/o FSIAνMINERX (X has no mesons)0π1+μ →Tracker μνArea normalized2.01e+20 POTFig. 4. – Flux-integrated differential cross section for antineutrino events producing a singleneutral pion shown in function of pion energy and angle. MINERvA π0 data is presented withGENIE event generator predictions with and without final state interaction implemented whereboth predictions were scaled to match the integral of the data.Fig. 5. – Ratios of charged current interaction cross sections on carbon, iron and lead to thecross section on plastic scintillator. The top row shows the ratio as a function of neutrino energy,while the bottom row shows the ratio as a function of Bjorken x.We select neutrino events happening in the upstream part of the detector, inside thepassive targets, and where the final state muon is matched to a muon event in MINOS.This last requirement constraints our angular acceptance to values less than 17 degreeswith respect to the beam, and an energy above 2GeV. Using a similar selection we canselect events that happened in the tracker region and have results in the ratios betweenthese two samples. Figure 5 shows this result as functions of Eν and Bjorken x. Thedata are consistent with the prediction of GENIE in energy, but there’s a discrepancyat high x, which increases with mass number of the nucleus. Around 60% of the eventswith values of x > 0.7 consist of CCQE events, so this is an important effect that needsto be understood by neutrino oscillation experiments.6. – ConclusionThe MINERvA collaboration is looking into a large range of important neutrino-nucleus cross section measurements which aim to understand, test and improve the6 M. F. CARNEIRO on behalf of the MINERvA COLLABORATIONmodel of these processes, and thus to reduce systematics in oscillation experiments. Theexperiment is currently taking a new dataset in the Medium Energy beam configurationthat will not only provide higher statistics for these analyses, but will also provide theability to measure these processes on different nuclei.∗ ∗ ∗This work was supported by the Fermi National Accelerator Laboratory under USDepartment of Energy contract No. DE-AC02-07CH11359 which included the MIN-ERvA construction project. Construction support also was granted by the UnitedStates National Science Foundation under Award PHY-0619727 and by the Universityof Rochester. Support for participating scientists was provided by NSF and DOE (USA)by CAPES and CNPq (Brazil), by CoNaCyT (Mexico), by CONICYT (Chile), by CON-CYTEC, DGI-PUCP and IDI/IGI-UNI (Peru), by Latin American Center for Physics(CLAF) and by RAS and the Russian Ministry of Education and Science (Russia). Wethank the MINOS Collaboration for use of its near detector data. Finally, we thank thestaff of Fermilab for support of the beamline and the detector.REFERENCES[1] Anderson K., Bernstein B., Boehnlein D., Bourkland K. R., Childress S. et al.,Fermilab-Design-1998-01 Technical Report (1998).[2] The MINERvA Collaboration (Aliaga L. et al.), Nucl. Instrum. Methods A, 743(2014) 130.[3] MINOS Collaboration (Michael D. G. et al.), Nucl. Instrum. Methods A, 596 (2008)190.[4] Bodek A., Avvakumov S., Bradford R. and Budd H. S., J. Phys.: Conf. Ser., 110(2008) 082004.[5] Kuzmin K. S., Lyubushkin V. V. and Naumov V. A., Eur. Phys. J. C, 54 (2008) 517.[6] Smith R. and Moniz E., Nucl. Phys. B, 43 (1972) 605.[7] The MINERvA Collaboration (Fiorentini G. A., Schmitz D. et al.), Phys. Rev.Lett., 111 (2013) 022502.[8] The MINERvA Collaboration (Fields L., Chvojka J. et al.), Phys. Rev. Lett., 111(2013) 022501.[9] Graczyk K. M. and Sobczyk J. T., Eur. Phys. J. C, 31 (2003) 177.[10] Andreopoulos C. et al., Nucl. Instrum. Methods A, 288 (2010) 87.[11] Bodek A., Budd H. and Christy M., Eur. Phys. J. C, 71 (2011) 1726.[12] The MINERvA Collaboration (Walton T., Betancourt M. et al.), Phys. Rev. D,91 (2015) 071301.[13] The MINERvA Collaboration (Eberly B. et al.), arXiv:1406.6415 [hep-ex].[14] The MINERvA Collaboration (Le T. et al.), arXiv:1503.02107 [hep-ex].[15] The MINERvA Collaboration (Tice B. et al.), Phys. Rev. Lett., 112 (2014) 231801.

Neutrino interactions on nuclei at MINERvA

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Neutrino interactions on nuclei at MINERvA

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