aa r X i v : . [ h e p - e x ] N ov The MINERvA Neutrino Interaction Experiment
Richard Gran
University of Minnesota Duluth
Abstract.
The MINERvA neutrino interaction experiment in the NuMI beam at Fermilab will measure several aspectsof neutrino interactions in the few GeV energy region. We will make cross section and form factor measurements usinga fine-grained fully active scintillator (CH) target, and also investigate nuclear effects on neutrino interactions as wellas hadron rescattering using integral nuclear targets made of helium, carbon, iron, and lead. We will improve or add toexisting measurements and address some specific questions that are important for current and upcoming neutrino oscillationexperiments. This paper reports on the progress toward the design, construction, and calibration of the detector, which weexpect will get its first neutrino beam in 2009.
Keywords:
Neutrino Interactions
PACS:
OVERVIEW OF THE MINERVAEXPERIMENT
MINERvA has a broad physics measurement program.We will measure neutrino interaction cross sections bothinclusive and exclusive, and with very high statistics,using the NuMI neutrino beam at Fermilab. In addition,we will simultaneously study nuclear effects on neutrinointeractions using several integral nuclear targets.The two most important pieces of the MINERvA ex-periment are the fully active, fine-grained inner trackerregion and the upstream nuclear target region. Thetracker layers are made of triangular shaped bars of ex-truded scintillator with 3.3 cm base and 1.7 cm height,the latter sets the plane thickness. By analyzing the en-ergy sharing between these interlocking triangles, wewill have 3 mm position resolution for reconstructedtracks. The scintillator planes are arranged in UXVX se-quences to give stereo 3D reconstruction of multi-trackevents. Each plane is a 2.2 meter wide hexagon; 120planes make this inner tracker region and another 76planes are described in the next two paragraphs.The same tracking layers continue upstream of theinner tracker to the nuclear target region where layersof carbon, iron, and lead are interspersed. Because ofthe nearly identical tracking layers, we will have a directcomparison of interaction rates and hadron productionand rescattering between these nuclear targets. Thoughthe tracking environment will not be identical, we willalso analyze data from a liquid Helium target region justupstream from the solid nuclear targets.Other features of the detector include a downstreamelectromagnetic calorimeter (ECAL) region with 2 mmthick lead sheets and a hadron calorimeter (HCAL) withone inch steel plates between scintillator planes. Behindthis is the existing MINOS near detector which we will use as a muon spectrometer. There are also electromag-netic and hadron calorimeter regions along the sides ofthe detector to measure particles leaving the neutrinovertex or scattered at large angles, making the detectornearly hermetic. Taken together, the MINERvA detectorhas excellent vertex, tracking, and calorimetric coveragefor most products produced by neutrino interactions.The fully active region will have approximately threetons fiducial volume of plastic scintillator (CH) in theinner tracker and another 6.2 tons of material in the nu-clear target region, when the plastic, carbon, iron, andlead are combined. For the expected one year runningin the current NuMI low energy beam (4 x 10 protonson target) and three years running in the medium energybeam (12 x 10 POT), we will accumulate over ninemillion charged-current events in the fully active fidu-cial volume. Within this we expect to have a sample of800,000 quasielastic and 1.6 million resonance events,with another 2 million in the resonance-DIS transitionregion and over 4 million in the deep inelastic scatter-ing region. Total event samples from the nuclear targetswill range from 400,000 to 2.5 million, giving very highstatistics for those comparisons.The collaboration successfully completed a full mod-ule (two scintillator planes) prototype and full DAQ sys-tem tests during Spring and Summer 2007. The MIN-ERvA experiment also completed its Department of En-ergy Critical Decision Three (CD3) review, and approvalwas granted in November 2007.
PHYSICS CAPABILITIES
Because the MINERvA measurement program is sobroad, I will only highlight a few of the physics capa-bilities. More complete details on all the measurementshat have been considered quantitatively can be found inchapter two of the MINERvA Technical Design Report,available on the web [1]. Topics not covered here includecoherent pion production [2], deep inelastic scattering,structure functions, and quark-hadron duality.
Flux
Several absolute cross section measurements dependon an accurate model of the neutrino flux through theMINERvA detector. This flux model will be based ona combination of measurements. Following the MINOSexperience, we will have neutrino data from multiplebeam configurations [3], which allow some ability to dis-entangle the flux model from the cross section model.There are results coming out from the first run of Fermi-lab E-907 (MIPP) [4] with the hadron spectrum off of theMINOS target. There is also a proposed upgrade programfor MIPP [5] which will provide additional improve-ments to their hadron production measurements. Finally,there is an initiative to improve information from themuon monitors to more directly gain information fromrate of pion decays to muons and neutrinos [6]. Whenthis information is incorporated into the analysis, we ex-pect to know the flux to the 5% level for most of the 1 to20 GeV region, with a little less constraint at the edgesof the beam’s focusing peak.
Test beam calibrations
A beam test of the MINERvA detector design andcomponents is being prepared. We will expose a 40 planemini-MINERvA detector where each plane is 107 x 107cm square, which is 63 strips wide instead of the full 128strips. These planes will have the same UXVX sequenceand tracking capabilities as the full MINERvA detector.To test the tracker, electromagnetic, and hadroncalorimetry regions, the lead and steel absorber will beremovable. This will allow us to test all the basic config-urations and combinations. In addition to all ECAL andall HCAL configurations, two examples of combinationswe are likely to test are: 10 tracker + 20 ECAL + 10HCAL layers or 20 ECAL + 20 HCAL.The top priority for the test beam effort is to exposethe detector to hadrons in as similar a momentum rangeas possible and obtain a calibration of the calorimetricquantities such as total visible energy deposit and itsfluctuations. For quasielastic and resonance interactions,the pion momentum distribution is dominated by protonsand pions at and below 500 MeV/c. At around this mo-mentum, it is already likely that the hadron will undergoan inelastic interaction rather than range out. On the other hand, the spectrum from the low hadron invariant massinteractions has a tail going up to a few GeV while highinvariant mass “deep inelastic scattering” events have amuch higher momentum spectrum.The beamline that will host this measurement is Fer-milab’s Meson Test Beam Facility. This beam has re-cently been upgraded to provide a usable pion rate downto momenta of 1 GeV/c. The design of a tertiary beamand beam selection components is underway; we expectit will give pions to 300 MeV/c. The beam will also de-liver protons, electrons, and muons for other tests andcalibrations. We are scheduled to run in Fall 2008.
Quasielastic interactions
Quasielastic interactions represent one of the mostimportant subsamples in the MINERvA data. Becauseof the simple two-body kinematics and the MINERvAtracker design, we expect to have full kinematic recon-struction down to very low Q . The quasielastic crosssection is the anchor for all cross sections around 1 GeV,where the single pion production cross section starts toturn on, but well below where the inclusive cross sectionis dominated by multi-hadron production and well mea-sured above 50 GeV. Except possibly for the tau appear-ance signature at CNGS, all other recent, current, andupcoming neutrino oscillation experiments will dependon an accurate model or constraint of this cross sectionfor their measurement.Because of the expected knowledge of the flux, MIN-ERvA will be able to provide a measurement of this ex-clusive cross section based on the observed rate. In ad-dition, the tracking abilities and high statistics will al-low for a study of the axial vector form factor, includingboth the extraction of the M A parameter as well as inves-tigation into deviations from the simple dipole form. InMINERvA, such studies will be limited by the eventualsystematic errors, including the knowledge of the fluxspectrum, the muon range calibration, and the hadron en-ergy calibrations mentioned above. We will also studythe more abundant but more complicated single pion fi-nal states, including resonance production.At this time, there appears to be a few puzzles aboutthe quasielastic interaction, which MINERvA will be ina good position to address. The extracted value of M A from K2K [7] and MiniBooNE [8] appears to be slightlyhigher than expected based on previous deuterium bub-ble chamber measurements. At this time, it is not cer-tain whether one or more nuclear effects for oxygen orcarbon are mismodeled, or if there is some other fun-damental change in the underlying form factor. It mayalso simply be some systematics inherent in how thesemeasurement techniques differ from those used with thexquisite kinematic information available from the bub-ble chamber data.There is also some concern about whether the param-eter M A is adequate to reproduce both the absolute crosssection as well as the shape of the Q distribution simul-taneously, especially across the range of nuclei and ener-gies of interest today.A second puzzle is the very low Q < . region. Separate from the bulk of the Q distribution, themodels currently in use do not do a good job reproduc-ing the data. Recently, MiniBooNE has presented a newmodel parameter that allows them to reproduce this fea-ture of the Q spectrum for their oscillation analysis[8].The MINERvA measurements will benefit from thecombination of enormous statistics, improved constrainton the flux, excellent tracking down to very low momen-tum, and good energy reconstruction. MINERvA coversa range of energies that overlap with the K2K and Mini-BooNE measurements on the low end, which include orare just above the region where NOvA and T2K will ex-pect their oscillation signatures. This energy range con-tinues up to tens of GeV where some kinds of nuclear ef-fects should become negligible. Finally, the integral nu-clear targets will further allow us to isolate and identifypotential effects due to the nuclear environment. Nuclear reinteractions
Another topic MINERvA will address is nuclear rein-teractions. Hadrons produced in the final state of any ofthese interactions with a single nucleus must escape thenuclear environment before they will be seen in the de-tector. Protons will rescatter, occasionally producing an-other pion or kicking out a neutron. More importantly,pions can undergo a variety of interactions, from simplescattering to charge exchange to absorption.Some experiments, such as MINOS, are dependent onhadron calorimetry in order to reconstruct and observedistortions in the neutrino energy spectrum. Therefore, amis-modeling of the visible hadron energy is an impor-tant systematic effect. Other experiments, such as thosewhich use the water Cerenkov technique, depend on thekinematic reconstruction of the event and their abilityto select and model quasielastic events. In this case, theidentification of the recoil proton from other protons orpions coming from non quasielastic events in oxygen nu-clei is important. Finally, the upcoming searches for elec-tron appearance will have to contend with a significantbackground from neutral pions, which are produced di-rectly and also indirectly by these reinteractions.MINERvA will combine the excellent tracking andhigh statistics of the inner tracker made of scintillator(CH) with the event rate and track multiplicity observed in the tracking layers downstream from the integral nu-clear targets. Downstream from the inner tracker is anelectromagnetic calorimeter which will force the conver-sion of and provide good information about the gammarays from neutral pion decays.
OUTLOOK
During Spring 2008 and into the summer we will con-struct a multi-plane tracking prototype which will in-clude inner tracker as well as electromagnetic calorime-ter sections.The full detector installation will happen in 2009, fol-lowed by operation in the low energy NuMI beam alongwith the end of the MINOS run. We expect our initialresults from this run. A changeover to the medium en-ergy beam required for the NOvA experiment is cur-rently expected to happen in 2011-2012, followed by an-other three years of operation.
ACKNOWLEDGMENTS
The MINERvA experiment is supported by the U.S. De-partment of Energy, the U.S. National Science Founda-tion, the Office of Special Accounts for Research Grantsof the University of Athens, Greece, DAI-PUCP andCONCYTEC from Peru, the CNPq and CAPES fromBrazil, CONCYTEG and CONACYT from Mexico andApoyo a la Investigacion Cientifica from the Universidadde Guanajuato.
REFERENCES
1. The MINERvA Collaboration,
Tech-nical Design Report available at http://minerva-docdb.fnal.gov/cgi-bin/ShowDocument?docid=700 .2. H. Gallagher and D. Harris, to appear in
Proc. NuInt07 (2007).3. MINOS Collaboration, submitted to
Phys. Rev. D arXiv:0711.0769.4. J. Paley, these proceedings (2007).5. MIPP Upgrade Collaboration arXiv:hep-ex/0609057.6. S. Kopp, to appear in
Proc. NuInt07 arXiv:0709.2737.7. R. Gran, E.J. Jeon, et al. (K2K Collaboration)