Current Knowledge of Neutrino Cross-Sections and Future Prospects D. Casper University of California, Irvine

Outline Why do we care? What do we know from past experiments? What are current experiments telling us? Outlook for future experiments

Neutrino Oscillation: Goals Probe fundamental parameters of Standard Model: Precision measurement of |  m 2 23 | Precision measurement of  23 Measurement of unknown angle  13 Determination of mass hierarchy sgn(  m 2 23 ) Search for leptonic CP violating phase  Measurements involve probabilities Need number of oscillated neutrinos and number of starting neutrinos Next generation experiments, starting with MINOS, will have impressive statistical power from high luminosity beams and massive detectors Systematic uncertainties must be controlled at a level comparable to the statistical errors to take full advantage “Normal”“Inverted” Expt.Dates Beam Power (MW) Det. Mass (kt) K2K MINOS J-PARC I NO A ?0.4~35

Neutrino Oscillation: Requirements Neutrino energy resolution for CC interactions K2K/T2K: Quasi-elastic channel MINOS/NO A: Calorimetry (E vis  E ) Control energy-scale systematics for high E -resolution sample at few percent level Efficiency Contamination Control near/far beam flux and energy spectrum differences at few percent level Control background for e appearance signature at few per-mille level Beam e, CC, NC contributions Control neutrino/anti-neutrino systematics at percent level for mass hierarchy and CP studies C. Walter, NUINT02 K2K NO A (  13 near CHOOZ bound) Normal Hierarchy Inverted Hierarchy

Neutrino Oscillation: Realities Only well-known neutrino interaction cross-section is for electron scattering Unfortunately useless for oscillation experiments with accelerators Available data for few-GeV reactions: Old (Early ’70s to mid 80’s) Normalized to quasi-elastic measurements, using obsolete form factor parameters, and introducing complicated correlated errors Beam spectrum and flux based on dubious hadron production models Undocumented and inconsistent corrections for nuclear targets Sparse Energy, hadronic mass reach limited Nuclear targets not applicable to common detector materials Almost no data on > 1  exclusive channels Anti-neutrino data even worse Low-statistics Neutral current 1  data based on a few dozens of events Mutually inconsistent

What About a Near Detector? Near detectors are important, but not a panacea  flux and spectrum differs far vs. near Not identical even without oscillation, due to extended source CC backgrounds and contamination extrapolate differently than NC+beam, due to oscillation Optimal sensitivity dictates that far detector is at oscillation maximum, making the difference as large as it can be Near/Far detectors usually not identical Far detector must be large, coarse-grained If identical, near detector has similar resolution and not suited to measure cross-sections K2K/T2K solution: two near detectors… Maximum physics reach requires: Near detector similar in composition, performance and resolution to far detector Good model/measurements of parent hadron beam Good understanding of exclusive neutrino cross- sections NO A P(    ) Nuance

K2K Near Detectors Extruded scintillator (15t) Multi-anode PMT (64ch) Wave-length shifting fiber EM calorimeter 1.7m 3m SciBar SciFi E (GeV)

MiniBooNE (H. Tanaka, WG2)

CC Quasi-Elastic Scattering Dominant reaction up to ~1 GeV energy Essential for E measurement in K2K/T2K The “well-measured” reaction Uncertain to “only” 20% or so for neutrinos Worse in important threshold region and for anti-neutrinos Axial form-factor not accessible to electron scattering Essential to modeling q 2 distribution Recoil proton reconstruction requires fine-grained design - impractical for oscillation detectors Recent work focuses on non-dipole form-factors, non-zero G n E measurements

K2K and MiniBooNE CCQE Rates K2K and MiniBooNE rates agree with MC for CCQE Only shape is measured, not absolute cross-sections Same data is used to measure the neutrino flux! K2K SciBar 2-ring QE (70% purity) MiniBooNE (88% purity)

CC Resonant Single-Pion Production Existing data inconsistent (factor 2 variations) Treatment of nuclear effects unclear Renewed theoretical interest with JLAB data Sato et al. Dynamical Model

K2K/MiniBooNE CC Pion Production 1-kton: Study of  0 proton decay background MiniBooNE: 85% purity for CC  ± sample (no results, in progress) 1-kton  0 candidates Normalized to total events

NC Single-Pion Production Historical samples of NC single pion production: ANL p  n  + (7 events) n  n  0 (7 events) Gargamelle p  p  0 (240 evts) n  n  0 (31 evts) Crucial background for e appearance searches!

K2K/MiniBooNE NC  0 Production K2K (Preliminary) MiniBooNE: Shape comparison only

Deep-Inelastic Scattering One area with lots of data and a clear theoretical framework, but uncertainties remain: Nuclear effects? Low-q 2 regime Connection/overlap with resonant production

Quark/Hadron Duality Recent JLAB data have revived interest in quark/hadron duality Bodek and Yang have shown that DIS cross- sections can be extended into the resonance regime, and match the “average” of the resonant cross-section Bodek and Yang

Nuclear Effects (QE/Resonant) 1kton SciFi SciBar MiniBooNE All currently running detectors see anomalous suppression at low-Q 2 One anomaly or two?

Nuclear Effects (DIS) EMC NMC E139 E665 shadowing original EMC finding Fermi motion x sea quarkvalence quark -- L H on L H off ++ Hadron formation length effects E = 5 GeV (NEUGEN) Bound nucleon structure functions

Final-State Interactions Renewed theoretical interest, plus new data from JLAB NEUT MC

“Hope is on the way…” HARP data (next talk) Hadron production measured with K2K, MiniBooNE targets at CERN Will provide essential data for neutrino fluxes, aid absolute cross-section measurements MINER A at NuMI (2006?) T2K Near Detectors (2009?) 280m: Fine-grained (design not finalized) 2km: Water Cherenkov + fine-grained (not yet approved) Potential to measure cross-sections around few GeV, if independent flux prediction available

MINER A at a Glance Scintillating strip design leverages DZERO, K2K, MINOS experience Modular construction, good spatial resolution, 3d-tracking, fast timing and dE/dx measurement at attractive cost Fully-active central volume surrounded by magnetized “calorimeters” Inner fiducial mass > 3 tons Iron + Lead planes upstream to vary nuclear target Parasitic operation within current NuMI/MINOS run-plan yields 1.25M neutrino events/ton Broad neutrino energy reach MINOS near detector can substitute for downstream muon ranger MIPP will measure NuMI hadron production within ~few percent, allowing precision absolute cross- section measurements

MINER A CCQE Measurements Full simulated analysis, including realistic detector simulation and reconstruction

MINER A Resonant Pion Production Errors statistical only, assuming 50% efficiency

Coherent Pion Production Neutral-current reaction important background for e appearance search Realistically, 100% uncertainties in rate for oscillation experiments Theoretical models vary significantly CC reaction (easier to measure) closely related to NC No data on light nuclei in energy region of relevance K2K and MiniBooNE may help at low energies, but NC background to e appearance feeds down from high energies Easier to distinguish with proton reconstruction and (for CC reaction) dE/dx MINER A Simulated Measurements

Conclusions Few-GeV neutrino interaction physics is (finally) leaving the “Dark Ages” Multiple, synergistic lines of attack are beginning to peel back our ignorance: Data from K2K and MiniBooNE Electron scattering data HEP/Nuclear collaboration (NUINT workshops) Revived theoretical attention to different questions MINER A and T2K can provide another quantum leap Independent knowledge of fluxes are vital for absolute measurements Continued progress on cross-sections will be invaluable for future oscillation experiments