Review of Current and Future Neutrino Cross-Section ...lss.fnal.gov/archive/2009/conf/fermilab-conf-09-711-e.pdf · neutrino oscillation experiments where precise understanding of

Review of Current and Future Neutrino Cross-SectionExperiments

D. Schmitz

Fermi National Accelerator Laboratory, Batavia, IL, USA

Abstract. There has been a surge of progress and published results in neutrino cross-section physics in recent years. Inmany cases, absolute differential cross-sections are being measured for the first time and can be compared to interactionmodels first developed decades ago. These measurements are important input for the next generation of accelerator-basedneutrino oscillation experiments where precise understanding of both signal and background channels will be critical to theobservation of sub-dominant oscillation effects. This paper discusses recent results from several experiments and describesnew experiments currently under construction dedicated to making these measurements with unprecedented precision.

Keywords: Neutrino, scattering experiments, cross-sectionsPACS: 13.15.+g, 14.60.Lm

INTRODUCTION

The discovery of neutrino mass and mixing has ledto an ambitious worldwide program of long baseline,accelerator-based neutrino oscillation experiments. Inparticular, next generation experiments such as NOvA[1], T2K [2], and LBNE [3] endeavor to make precisionmeasurements of dominant oscillation channels, searchfor evidence of sub-dominant νµ → νe oscillations, anddiscover possible CP violation within the lepton sector.

These experiments rely on intense neutrino beams toreduce statistical uncertainties forcing a more carefulconsideration of sources of systematic errors, many ofwhich may have been negligible in previous long base-line oscillation experiments.

For example, our understanding of basic neutrino-nucleus interactions is incomplete and this directly af-fects how well one can separate signal from backgroundsand measure oscillation parameters. Further, long base-line experiments use heavier nuclear targets to increaseinteraction rates but the impacts of nuclear effects withinheavy nuclei on final state particle kinematics are sig-nificant. This, in turn, impacts one’s reconstruction ofthe incoming neutrino energy. These systematic effectsmust be well understood to maximize the sensitivity ofthe next generation of oscillation experiments.

Fortunately, a diverse program of modern experi-ments aims to make precision measurements of neutrino-nucleus interactions for neutrino energies up to a fewGeV. These experiments will measure absolute cross-sections for a variety of inclusive and exclusive fi-nal states and for a range of nuclear target materials.This should create the possibility of unfolding neutrino-nucleon scattering rates from complicated nuclear ef-fects.

Neutrino Energies and Target Materials

Figure 1 (top) shows the regions of neutrino energyand target materials relevant for current and future longbaseline oscillation experiments. NOvA uses a liquidscintillator detector (carbon) and sits off-axis from theNuMI (Neutrinos at the Main Injector) [4] beam at Fer-milab creating a relatively narrow neutrino flux around2 GeV. The T2K beam is lower energy (0.5-1 GeV) anduses a water target (oxygen). The LBNE (Long BaselineNeutrino Experiment) collaboration is currently planninga low energy wideband beam generated at Fermilab [5]and pointing to either a large water or liquid argon detec-tor at a far site. The MINOS [6] detector, running nowat Fermilab and Soudan, is constructed of iron and sitson-axis of the NuMI beam. Finally, the CNGS (CERNto Gran Sasso) [7] beam is significantly higher energyallowing for tau lepton production in lead within theOPERA [8] detector at Gran Sasso.

Figure 1 (bottom) shows the coverage provided bymodern experiments which include a program of neu-trino cross-section measurements. As defined here, “cur-rent” experiments are listed in the left column; “future”experiments are in the right column. By comparing thetwo panels, one sees that the coverage provided by theseexperiments relative to the LBL experiments’ needs isnearly complete. This is important given the difficultythat often arises in extrapolation of neutrino interactionmodels to different energies or nuclei. Further, the over-lap within the cross-section experiments can be lever-aged to untangle complicated systematics such as fluxpredictions and detector efficiencies from the underlyingphysics of the neutrino-nucleus interactions.

52

Downloaded 11 Jun 2010 to 131.225.103.35. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp

FERMILAB-CONF-09-711-E

FIGURE 1. Relevant neutrino energies and detector targetmaterials for current and next generation long baseline (LBL)neutrino oscillation experiments (top). Coverage of the cur-rent and future neutrino cross-section experiments in this same(Eν ,A) phase space (bottom).

Interaction Channels

Figure 2 shows the current cross-section data for νµ

charged current interactions between 100 MeV and 400GeV. The approximate energy range of the neutrinofluxes for various accelerator-based oscillation experi-ments is represented by the colored blocks. These exper-iments are done in a range where quasi-elastic scatter-ing and single pion production dominate the interactioncross-section.

In particular, next generation long baseline experi-ments, T2K, NOvA, and LBNE/DUSEL, will each relyon charged current quasi-elastic (CCQE) scattering asa signal channel in making precision measurements ofmuon neutrino disappearance, searching for electronneutrino appearance, or both.

Single pion production channels create importantbackgrounds for these measurements. Charged currentπ+ production (CCπ+) can mimic a CCQE interaction

FIGURE 2. Existing νµ charged current cross-section data(excludes any measurements discussed in this paper from Mini-BooNE, SciBooNE, K2K, or MINOS).

if the pion is absorbed within the target nucleus. Recon-structing such an event under a CCQE hypothesis resultsin a systematic shift to lower reconstructed neutrino en-ergy. This is an irreducible background event-by-event,making the ratio of CCQE to CCπ+ interaction rates animportant input in making a precision measurement ofthe energy dependence of νµ disappearance.

Neutral-current π0 production (NCπ0) is the largestcontributer to backgrounds in a search for electron neu-trino appearance in a muon neutrino beam. π0→ γγ de-cays can be mis-identified as a final state electron from aνe charged current interaction if one of the gammas is im-properly identified. This is not an irreducible backgroundand the mis-ID rate will depend on the detector technol-ogy employed, but the cross-section for NC π0 produc-tion is very poorly known. Analysis of data from cur-rent oscillation experiments and sensitivity estimates forfuture experiments would greatly benefit from improvedmeasurements of NC π0 production rates.

CURRENT EXPERIMENTS

Next, we discuss recent cross-section results from theMiniBooNE, SciBooNE, and MINOS experiments atFermilab as well as the NOMAD experiment conductedat CERN. Later, we introduce future cross-section exper-iments, in particular the MINERvA experiment at Fermi-lab.

Charged Current Quasi-Elastic Scattering

CCQE event samples are often analyzed using thecross-section formalism of Llewellyn Smith [9]. Therelativistic Fermi gas (RFG) model of Smith and Moniz

53


FIGURE 3. Charged current quasi-elastic (CCQE) per nu-cleon cross-section as measured by three experiments, Mini-BooNE [13, 14], SciBooNE [13], and NOMAD [15].

[10] is typically employed to expand this to includescattering off nucleons bound in a nucleus. The leastcertain parameter in this model is the axial-vector formfactor, given by

FA(Q2) = FA(0)(

1+Q2

M2A

)−2

, (1)

where the form factor at Q2 = 0, FA(0) = −1.267, iswell known from neutron beta decay experiments [11].MA, however, must be determined from neutrino scatter-ing data and the weighted average from available mea-surements (excluding any new measurements mentionedhere) is MA = 1.03 GeV [12].

Several experiments have recently presented new mea-surements of CCQE interaction rates, including Mini-BooNE [13, 14], SciBooNE [13], NOMAD [15], and MI-NOS [16]. Three of the four recent experimental resultsare compared in Figure 3. The two curves correspond tocross-section models with different values of the axialmass parameter, MA.

NOMAD measured the absolute charged currentquasi-elastic cross-section in the neutrino energy inter-val 3-100 GeV by normalizing to the total νµ chargedcurrent cross-section. As evidenced in Figure 3, theirresult is consistent with a cross-section model using theaverage axial mass value of 1.03 GeV. NOMAD extractsthis parameter from their absolute cross-section data tobe MA = 1.05± 0.02(stat)± 0.06(syst) GeV, and statesit is consistent with a pure Q2 shape analysis of their νµ

quasi-elastic 2-track events.MiniBooNE and SciBooNE have measured the abso-

lute CCQE cross-section in the neutrino energy rangebelow 2 GeV. Absolute normalization of the results isachieved through “dead-reckoning” of the flux from theBooster Neutrino Beam at Fermilab by incorporatinghadron production data from the HARP experiment [17]

into a sophisticated Geant4 Monte Carlo of the Boosterneutrino beamline [18]. These results indicate a quasi-elastic cross-section ∼ 30% higher than the RFG modelwith MA = 1.03 GeV. This is shown in Figure 3 wherethe MiniBooNE and SciBooNE data are compared to theNOMAD results. MiniBooNE extracts a value, MA =1.35± 0.17 GeV, not from the absolute cross-sectiondata, but from a shape only analysis of the Q2 distribu-tion of the quasi-elastic sample [14]. Nonetheless, excel-lent agreement in the absolute rate is achieved by thismodification to the model.

The MINOS experiment has not released absolutecross-section numbers, but a preliminary neutrino energyand Q2 analysis has been presented. MINOS extractsMA = 1.19+0.09

−0.10(fit)+0.12−0.14(syst) from a Q2 shape only fit

to their data [16].One can see that a potential inconsistency between

results of modern experiments is developing and this isnot currently understood. This has motivated tremendousrecent effort to advance the models describing quasi-elastic scattering in heavier nuclei [19], but additionaldata will be needed to unravel this mystery.

Charged Current π+ Production

Two experiments, K2K and MiniBooNE, have pre-sented recent measurements of the CCπ+ to CCQEcross-section ratio at neutrino energies below 3 GeV[20, 21] . Figure 4 shows their results compared to that ofthe Argonne 12-ft bubble chamber experiment publishedin 1982 [22].

Charged current charged pion production is often re-ported as a ratio to the CCQE event rate, reducing un-certainties arising from knowledge of the neutrino flux.As stated above, this ratio is an important component tounderstanding the level of background in a CCQE sig-nal channel of an oscillation experiment. To properly testmodels of CCπ+ production, however, requires differ-ential cross-section data describing the kinematic dis-tributions of the final state muon and pion. The ANLdata were used to study pion angular distributions and,more recently, MiniBooNE has measured absolute cross-section values as a function of both muon and pion angleand momentum [23].

Neutral Current π0 Production

Few measurements of absolute NCπ0 production areavailable in the literature [25, 26]. The MiniBooNE col-laboration has very recently completed the first absolutecross-section measurement of NC π0 production for bothneutrino and antineutrino scattering. Since the neutrino

54


FIGURE 4. CCπ+ to CCQE cross-section ratio as measuredby three experiments, MiniBooNE [21], K2K [20], and ANL[22].

FIGURE 5. Flux averaged differential cross-sections for νµ

and νµ induced NC single π0 production on CH2 measured byMiniBooNE [24].

energy cannot be reconstructed in NC π0 events, the re-sults are averaged over the incident neutrino flux, butcross-sections as a function of the π0 momentum andπ0 angle with respect to the interacting neutrino are pre-sented. These results are shown in Figure 5 and com-pared to their default Monte Carlo simulation which isbased on the model of Rein and Sehgal [27, 28].

The way the data have been presented in MiniBooNE’spublication illustrates an important point in how neutrinocross-section data will be used by future neutrino os-cillation experiments or to improve theoretical modelsof neutrino-nucleus scattering. NC π0 production cre-ates the single largest misidentification background inνµ → νe oscillation searches. These experiments, per-formed on heavier nuclear targets, are concerned onlywith the inclusive rate for π0’s leaving the nucleus andentering the detector medium. Charge exchange and ab-sorption within the nucleus are complicated effects andcan alter the bare neutrino-nucleon event rates. These

nuclear effects are a part of this challenging theoreticalproblem, so providing experimental data to the commu-nity as rates of inclusive final states, as MiniBooNE hasdone, is important.

FUTURE EXPERIMENTS

As MiniBooNE and the K2K experiment have exten-sively demonstrated, neutrino detectors constructed tomake oscillation measurements are often used to makeimportant cross-section measurements as well. Futureexperiments, such as T2K [29] and MicroBooNE [30],are similar in that their primary physics motivation comefrom elsewhere, but each experiment will include sig-nificant programs in neutrino cross-section physics. Onefuture experiment, MINERνA at Fermilab, is designedspecifically for a cross-section physics program aimed atprecision measurements of neutrino interactions in nucleifor energies up to a few GeV.

MINERvA makes use of a fine-grained, fully activedetector design and a range of nuclear target materials(helium, carbon, iron, and lead). The presence of mul-tiple nuclear targets in the same detector medium andplaced in the same neutrino beam provides a significantadvantage for unraveling the effects of heavier nuclei onneutrino interactions. Figure 6 shows a schematic dia-gram of the MINERνA detector which is comprised ofmultiple nuclear target modules and a fully active scin-tillator tracking region surrounded by hadronic and elec-tromagnetic calorimeters. The experiment is described inmuch greater detail elsewhere in these proceedings [31].

MINERνA collected first neutrino data in the NuMIbeam with a prototype detector from April-June, 2009.The prototype was built from full-scale detector modulesbut was ∼20% of the full length of MINERνA, beingcomprised of 24 total modules. Figure 7 shows two sam-ple events from the prototype run. The first is a candidatequasi-elastic event. The top track (actually to the left inthe horizontal plane of the detector) appears to be a muonexiting the back of the detector which can be matched toa track in the MINOS detector which is located 2 metersdownstream from MINERνA (not shown). The lowertrack ranges out but the increase in the energy depositedper plane (dE/dx) as it loses energy is clear. This is con-sistent with the track being a proton. The right panelshows a charged current π0 production candidate, whereagain there is an exiting muon track and two electromag-netic showers which appear consistent with photon con-versions and which point back to the primary vertex.

Since this paper was presented, the MINERνA proto-type detector has been removed and installation of thefinal MINERνA detector has begun. As of November,2009, 55% of the full detector modules are in place andcollecting data. The NuMI neutrino beam is running in

55


ν

SideHCAL

SideHCAL

Hetarget

FullyActiveScintillatorRegion

FIGURE 6. Schematic diagram of the MINERνA detector.The neutrino beam enters from the left, passing the nucleartarget modules at the upstream end. The fully active trackingand calorimeter regions are labeled. 2 meters downstream ofMINERνA is the MINOS near detector which functions as amuon range detector for charge current events in MINERνAwith an escaping forward muon.

photoelectrons photoelectrons

FIGURE 7. Two sample neutrino candidate events from theMINERVνA prototype detector run from April-June, 2009.The left panel shows a quasi-elastic candidate event; the rightshows a charged current π0 candidate.

antineutrino mode (reversed focusing horn currents). Thefull MINERνA detector is on schedule to be installed byMarch, 2010.

ACKNOWLEDGMENTS

I would like to thank Dan Kaplan and the NuFact ’09organizing committee for an excellent workshop and forthe opportunity to present this paper. I also thank SamZeller and Jorge Morfin for many useful conversationsregarding recent neutrino cross-section experimental re-sults. This work is supported [in part] by Fermi ResearchAlliance, LLC under Contract No. DEAC02-07CH11359with the United States Department of Energy.

REFERENCES

1. D. A. Harris [MINOS and NOvA Collaborations], Nucl.Phys. Proc. Suppl. 149, 150 (2005).

2. I. Kato [T2K Collaboration], J. Phys. Conf. Ser. 136,022018 (2008).

3. [LBNE Collaboration], http://lbne.fnal.gov/.4. http://www-numi.fnal.gov/numwork/

numiwk.html.5. http://projectx.fnal.gov/.6. P. L. Vahle [MINOS Collaboration], arXiv:

0810.3627.7. http://proj-cngs.web.cern.ch/

proj-cngs/.8. I. E. Kreslo [OPERA Collaboration], J. Phys. Conf. Ser.

171, 012066 (2009).9. C. H. Llewellyn Smith, Phys. Rept. 3, 261 (1972).10. R. A. Smith and E. J. Moniz, Nucl. Phys. B43, 605 (1972)

[Erratum-ibid. B101, 547 (1975)].11. W.-M.Yao et al. (Particle Data Group), J. Phys. G33, 1

(2006) and 2007 partial update for the 2008 edition.12. V. Bernard, L. Elouadrhiri and U. G. Meissner, J. Phys.

G28, R1 (2002), hep-ph/0107088.13. R. Tayloe, these Proceedings.14. T. Katori [MiniBooNE Collaboration], arXiv:

0909.1996[hep-ex].15. V. Lyubushkin et al. [NOMAD Collaboration], Eur. Phys.

J. C63, 355 (2009), arXiv:0812.4543[hep-ex].16. R. Gran, these Proceedings.17. M. G. Catanesi et al. [HARP Collaboration], Eur. Phys. J.

C52, 29 (2007), hep-ex/0702024.18. A. A. Aguilar-Arevalo et al. [MiniBooNE

Collaboration], Phys. Rev. D79, 072002 (2009)arXiv:0806.1449[hep-ex].

19. J. E. Amaro et al., Phys. Rev. C71, 015501(2005);Phys. Rev. C75, 034613 (2007); T. Leitner et al., Phys.Rev. C73, 065502 (2006); Phys. Rev. C79, 065502(2006); O. Benhar et al., Phys. Rev. D72, 053005 (2005);arXiv:0903.2329[hep-ph]; A. Butkevich et al.,Phys. Rev. C72, 025501 (2005); Phys. Rev. C 76, 045502(2007); Phys. Rev. C 80, 014610 (2009); S.K. Singh etal., arXiv:0808.2103[nucl-th]; J. Nieves et al.,Phys. Rev. C73, 025504 (2006); N. Jachowicz et al., Phys.Rev. C73, 024607 (2006); A.M. Ankowski et al., Phys.Rev. C77, 044311 (2008).

20. A. Rodriguez et al. [K2K Collaboration], Phys. Rev. D78,032003 (2008), arXiv:0805.0186[hep-ex].

21. A. A. Aguilar-Arevalo et al. [MiniBooNE Collaboration],arXiv:0904.3159v1[hep-ex].

22. G. M. Radecky et al., Phys. Rev. D25, 1161 (1982)[Erratum-ibid. D26, 3297 (1982)].

23. M. J. Wilking, FERMILAB-THESIS-2009-27.24. A. A. Aguilar-Arevalo et al. [The MiniBooNE

Collaboration], arXiv:0911.2063[hep-ex].25. H. Faissner et al., Phys. Lett. B125, 230 (1983).26. E. Hawker, Proc. NuInt 02, http://www.ps.uci.

edu/~nuint/proceedings/hawker.pdf.27. D. Rein and L. M. Sehgal, Nucl. Phys. B223, 29 (1983).28. D. Rein and L. M. Sehgal, Ann. Phys. 133, 79 (1981).29. S. Boyd. WIN 09 presentation http://indico.

cern.ch/contributionDisplay.py?contribId=167&sessionId=18&confId=54503.

30. M. Antonello, these Proceedings.31. B. Eberly, these Proceedings.

56


Documents

Review of Current and Future Neutrino Cross-Section ...lss.fnal.gov/archive/2009/conf/fermilab-conf-09-711-e.pdf · neutrino oscillation experiments where precise understanding of