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MiniBooNE : Nuclear Simulation and Neutrino Interaction Measurements

GSA lecture series 5/5/11. MiniBooNE : Nuclear Simulation and Neutrino Interaction Measurements. Joe Grange University of Florida. Outline. Introduction to MiniBooNE MiniBooNE nuclear simulation and surprises in data

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MiniBooNE : Nuclear Simulation and Neutrino Interaction Measurements

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  1. GSA lecture series 5/5/11 MiniBooNE: Nuclear Simulationand Neutrino Interaction Measurements Joe Grange University of Florida

  2. Outline • Introduction to MiniBooNE • MiniBooNE nuclear simulation and surprises in data • A possible reconciliation supported in electron scattering data • Final state interactions • Conclusions

  3. Introduction to MiniBooNE • MiniBooNE nuclear simulation and surprises in data • A possible reconciliation supported in electron scattering data • Final state interactions • Conclusions

  4. MiniBooNEMini Booster Neutrino Experiment Booster Ring (8 GeV protons extracted) MiniBooNE detector hall Fermilab Batavia, IL Particle beam

  5. MiniBooNEMini Booster Neutrino Experiment Bison Booster Ring (8 GeV protons extracted) MiniBooNE detector hall Fermilab Batavia, IL Particle beam

  6. MiniBooNEMini Booster Neutrino Experiment Malevolent geese Bison Booster Ring (8 GeV protons extracted) MiniBooNE detector hall Fermilab Batavia, IL Particle beam

  7. Booster Target Hall decay region absorber target and horn detector FNAL Booster dirt Booster primary beam secondary beam tertiary beam (protons) (mesons) (neutrinos) Booster Neutrino Beam 8.9 GeV/c momentum protons extracted from Booster, steered toward a Beryllium target in bunches of 5 × 1012 at a maximum rate of 5 Hz

  8. decay region absorber target and horn detector FNAL Booster dirt nm Booster primary beam secondary beam tertiary beam (protons) (mesons) (neutrinos) Booster Neutrino Beam Magnetic horn with reversible polarity focuses either neutrino or anti-neutrino parent mesons (“neutrino” vs “anti-neutrino” mode) p- p+ p+ p-

  9. MiniBooNE Flux • Flux prediction based exclusively on external data - no in situ tuning MiniBooNE collaboration, Phys. Rev. D79, 072002 (2009) • Dedicated pion production data taken by HARP experiment to predict neutrino flux at MiniBooNE • A spline fit to these data brings flux uncertainty to ~9% HARP collaboration, Eur. Phys. J. C52 29 (2007)

  10. MiniBooNE Detector • 6.1m radius sphere houses 800 tons of pure mineral oil. • Oil serves as both the nuclear target (CH2) and medium for particle tracking, particle ID • 1520 Photo Multiplier Tubes uniformly dispersed in 2 regions of tank: - 240 in outer veto region - 1280 in signal volume

  11. Introduction to MiniBooNE • MiniBooNE nuclear simulation and surprises in data • A possible reconciliation supported in electron scattering data • Final state interactions • Conclusions

  12. Nuclear Simulation • MiniBooNE uses the Relativistic Fermi Gas model (RFG) Nucl. Phys. B43, 605 (1972) • Models nucleons as independent, quasi-free particles bound by a constant EB • All struck (outgoing) nucleons subject to Pauli blocking. This is enforced by a global Fermi momentum kF

  13. Determining EB, kF • Electron scattering data on 12C as a function of energy transfer informs both parameters: • Peak of energy transfer distribution represents scattering off nucleons at rest. This position is shifted from the same position in free nucleon by the binding energy appropriate to (e,e’) neutral current scattering • Fermi momentum kF set by the width of the distribution 12C(e,e’) Ee=500MeV q=600 kF = 221 MeV/c EB = 25 MeV Nucl. Phys. A402, 515 (1983) w=Ee-Ee’ (MeV)

  14. E Determining EB, kF • EB for neutrino charged-current (n + N -> l± + N’) interactions distinct from neutral-current (e + N -> e + N) EB, as separation energy between final, initial states are different Common initial state (12C ground) ± 12C Excited State 12N Excited State Electron Scattering to the Continuum n,l- Quasi-elastic Scattering Different final states

  15. Determining EB, kF • How much different? The splitting can be estimated by the symmetry term in the semi-empirical mass formula: • ES = 9 MeV for A = 12, Z = 7 (for CC interactions with n -> p ) • EB = 25 + 9 MeV = 34 MeV Es=28(A-2Z)2/A MeV symmetry splitting (e,e’) data

  16. That’s it! • Specifying EB, kF fully describes the RFG model - it combines bare nucleon physics with a potential energy well and Pauli blocking. • A quick calculation: • Nuclear density approximately constant: • The mean separation distance between nucleons is • The nucleon diameter is 1.25 fm. • Naïve to assume nucleon independence

  17. Another way of saying • The nucleus very likely has a rich structure the RFG falls short of approximating well • We’ve seen evidence of this in MiniBooNE data

  18. The “golden channel” • Many experiments use the interaction nl + n -> l- + p (Charged-current Quasi-Elastic, or CCQE) to search for neutrino oscillations due to it’s simple multiplicity • Also can reconstruct initial neutrino energy and squared four-momentum transfer based solely on observing the outgoing lepton (dominantly m in MiniBooNE): • MiniBooNE: Cherenkov detector designed with this in mind - CCQE is the most prevalent interaction at MiniBooNE energy

  19. The “golden channel” • Critical for MiniBooNE oscillation analysis - acts as the “near detector” • Looking for ~0.26% transmutation nm -> ne • Measure nm’s, have a robust prediction for various oscillation signals • Constrains amount of intrinsic ne from p -> m + nm, m -> e + ne + nm • Assuming lepton universality, nm measurement reduces error on ne prediction

  20. The “golden channel” • The model we use to predict the CCQE cross section comes from Smith & Moniz Nucl. Phys. B43, 605 (1972) • A, B, C functions of vector and axial form factors • Using conserved vector current we use form factors extracted from electron scattering for the vector contribution • In this model, this leaves neutrino experiments one and only one parameter to measure, the axial mass MA

  21. The “golden channel” • Many neutrino experiments measured MA by shooting high energy neutrino beams typically at bubble chamber detectors housing mostly light nuclear targets • Measurements converged around MA = 1.0 GeV Previous world average: MA = 1.02 ± 0.01 GeV J. Phys.: Conf. Ser. 110 082004 (2008)

  22. Early Days of MiniBooNE • Subsequent to understanding detector response and verifying event reconstruction algorithms on calibration data, MiniBooNE found surprises in this CCQE golden channel • Around 30% discrepancy between predicted, measured event rate • Muon scattering angle shape wrong

  23. Early Days of MiniBooNE • Subsequent to understanding detector response and verifying event reconstruction algorithms on calibration data, MiniBooNE found surprises in this CCQE golden channel • Around 30% discrepancy between predicted, measured event rate • Muon scattering angle shape wrong In principle, this could be due to either flux or cross section mismodeling

  24. Early Days of MiniBooNE • Subsequent to understanding detector response and verifying event reconstruction algorithms on calibration data, MiniBooNE found surprises in this CCQE golden channel • Around 30% discrepancy between predicted, measured event rate • Muon scattering angle shape wrong In principle, this could be due to either flux or cross section mismodeling Implies cross section is the likely culprit

  25. HARP MiniBooNE Flux MiniBooNE flux prediction comes from dedicated meson production data (HARP experiment at CERN) - Same target (but thinner) and proton beam momentum Total flux error with these data is ~9% Eur. Phys. J. C52 29 (2007)

  26. Outgoing m phase space • With plenty of statistics taken (more than all events from all previous CCQE scattering experiments combined!), able to strongly comment on the muon scattering shape problem data/prediction • s(Q2) • F(En) • Lines of data/prediction discrepancy generally follow lines of constant Q2 , not En

  27. Outgoing m phase space • With plenty of statistics taken (more than all events from all previous CCQE scattering experiments combined!), able to strongly comment on the muon scattering shape problem The interaction cross section is wrong!! data/prediction • s(Q2) • F(En) • Lines of data/prediction discrepancy generally follow lines of constant Q2 , not En

  28. Looking for alternatives • With the admission the RFG is inadequate, we look to more modern models… • Find general theory consensus that the absolute interaction cross section with the RFG and MA = 1 GeV is about right

  29. Looking for alternatives • With the admission the RFG is inadequate, we look to more modern models… • Find general theory consensus that the absolute interaction cross section with the RFG and MA = 1 GeV is about right

  30. Finding a “solution” within the RFG Using the RFG nuclear model, tuning the axial mass MA and an empirical Pauli blocking fits the data! MA = 1.35 ± 0.17 GeV This simultaneously fixed the muon scattering shape problem and provided agreement with measured event rates

  31. MA tension • MA = 1.35 ± 0.17 GeV clearly disagrees with the measurements from light target data • However…

  32. MA tension arxiv: 1007.2195 • More recent measurements have also observed higher values of MA • These measurements mostly from fitting Q2 shapes • MA is important for overall normalization as well

  33. Total MiniBooNE cross section • MiniBooNE CCQE cross section ~40% higher than most modern models (!) • One model has come forward that can account for the observed enhancement while maintaining consistency with previous MA measurements

  34. Total MiniBooNE cross section • MiniBooNE CCQE cross section ~40% higher than most modern models (!) • One model has come forward that can account for the observed enhancement while maintaining consistency with previous MA measurements

  35. Martini model • Possible MA reconciliation: nuclear correlation effects in 12C result in an “extra” (nm + [n+p] -> m- + p + p) part of the CCQE cross section not present in light target experiments and indistinguishable from “true CCQE” (nm + n -> m- + p) in MiniBooNE • MiniBooNE “blind” to outgoing nucleons PRC 80, 065001 (2009)

  36. Comparison to NOMAD • NOMAD one of the few other MA extraction measurements with absolute flux knowledge • Normalized from Deep Inelastic Scattering (nm + 12C -> m- + X) events • Same nuclear target as MiniBooNE (12C), much higher energy, and absolute cross section measurement consistent with MA = 1.0 GeV (!) • Martini et al. double-nucleon knockout mechanism may explain the discrepancy

  37. Comparison to NOMAD • NOMAD one of the few other MA extraction measurements with absolute flux knowledge • Normalized from Deep Inelastic Scattering (nm + 12C -> m- + X) events • Same nuclear target as MiniBooNE (12C), much higher energy, and absolute cross section measurement consistent with MA = 1.0 GeV (!) Be careful what you define as “CCQE”! • Martini et al. double-nucleon knockout mechanism may explain the discrepancy

  38. However, comparing to s(En) not sufficient - kinematics very important • Main result from MiniBooNE CCQE analysis is the model-independent double-differential cross section as a function of outgoing muon energy, angle • So far, varying degrees of compatibility with dbl-nucleon knockout model Martini, Elba X

  39. Enhancement in electron scattering data • “Super Scaling”: For A ≥ 12, nuclear density approximately constant - does a simple scaling describe results from one nucleus to another? • Scales approximately linearly for different nuclear targets and y < 0 • Divergent for y > 0 Phys.  Rev.  C38,  1801 (1988), Phys.  Rev.  C60,  065502 (1999)  

  40. Enhancement in electron scattering data • (e,e’) scattering data decades old shows the transverse part of the (e,e’) cross section scales with momentum transfer • CCQE cross section enhancement due to increasing MA also grows with momentum transfer Phys.  Rev.  C60,  065502  (1999)

  41. Enhancement in electron scattering data • Transverse current significantly greater than longitudinal in (e,e’) data • In RFG, fL(y) = fT(y)(!!) • Does not seem crazy that the total n cross section on 12C could exceed that for 6 free neutrons • No rigorous connection between n and (e,e’) cross section yet • Axial enhancement?

  42. More electron scattering support for NN correlations • Recent Jefferson Lab (USA) experiment: • scatter electron beam on 12C foil, observe final state electron only in a special kinematic region: xB = Q2/2mw = 1.2; xB is Bjorken scaling variable, “the fraction of nucleon momentum carried by struck quark”. • xB > 1 means struck quark carries more momentum than the entire nucleon, implying NN correlation n 12C e’ e 19.5o 40.1o p

  43. More electron scattering support for NN correlations • Results: ~20% of nucleons in correlated states, mostly n-p pairs, which for nm CCQE interactions lead to p-p in final state Science 320, 1476 (2008)

  44. Anti-neutrinos • Martini group predicts enhancement from NN pairs to be less for anti-neutrinos compared to neutrinos due difference in axial-vector cross-section interference.

  45. Anti-neutrinos • nm data: data/MC ratio:1.05 ± 0.08 • MC: MA = 1.35 GeV

  46. Anti-neutrinos • Anti-nm data: data/MC ratio: 1.21 ± 0.12 • MC: MA = 1.35 GeV • In conflict with Martini group prediction

  47. Future experimental tests • Smaller enhancement predicted for anti-neutrinos • MiniBooNE • Tracking detectors sensitive to multi-proton final states • ArgoNeut, MINERnA, NOMAD, T2K ND MINERvA ArgoNeut

  48. Final state interactions • Final state interactions greatly complicate the experimental picture - what we measure is not necessarily a product of the n-nucleus interaction • Attempts to correct data for these effects subject to model dependence and large uncertainty

  49. Final state interactions • Example: for En ~ 1GeV, 12C nucleus (MiniBooNE), estimates for p0 FSI: • Lose: 20% (p0 + X -> X’) • Gain: 10% (p± + X -> p0 + X’) • Large uncertainty (30-50%) based on model comparisons with sparse external p + A data • An experimental mess

  50. Intermediate solution:measure “observable” cross sections • 12C a typical nuclear target in present and future n osc experiments, so better not to confuse results by introducing model-dependent corrections • MiniBooNE model-independent cross section measurements: • NCp0, CCp0, CCp+, CCQE

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