What Neutrino Experiments Need to Know

1. What Neutrino ExperimentsNeed to Know Kevin McFarland University of Rochester ECT* 16 May 2012

2. Outline • Neutrino scattering vs. electron scattering • Goals of neutrino oscillation experiments • “Narrow” and Broad Beam Experiments • What Needs to be Modeled • Current Practices • Possible Paths to Progress K. McFarland, Needs for Neutrinos

3. Neutrinos vs. Electrons K. McFarland, Needs for Neutrinos

4. Neutrino Dictionary for Parity Violators Physics Concept Electron s |APC|2+2ReAPC*APV+negligible APV ~10-6 a limiting systematic Polbeam a number you choose • Ebeam Target Detector K. McFarland, Needs for Neutrinos

5. Neutrino Dictionary for Parity Violators Physics Concept Electron Neutrino s |APC|2+2ReAPC*APV+negligible negligible APV 1 ~10-6 1-mn2/En2 a limiting systematic Polbeam a number you choose a distribution you barely know • Ebeam Target Detector (see “Target”) K. McFarland, Needs for Neutrinos

6. Neutrino Facts of Life • Neutrino experiments require massive targets to carry out goals • Few 104 or 105 kg of target material of current and “near future” experiments • We only know what we see in the final state • Targets are large nuclei • Carbon, Oxygen, Argon, Iron are all being used in current or near future experiments • Detectors have severe limitations • Need to measure interactions throughout target • Must balance expense vs. capability K. McFarland, Needs for Neutrinos

7. Neutrino Oscillation Goals(at lightening speed) K. McFarland, Needs for Neutrinos

8. Neutrino Flavor • Neutrinos were discovered by • the final state positron is no accident! • we’ve seen neutrinosproduce all threecharged leptons in weak interactions • The Z boson decays into three (and only three) neutrino states K. McFarland, Needs for Neutrinos

9. Neutrino Flavor Mixing • The defining question of the field today turns out to be from an unusual conjecture (Pontecorvo) • Are these neutrinos “of definite flavor”the eigenstatesof the neutrino mass matrix • Or are we looking at neutrino puree? K. McFarland, Needs for Neutrinos

10. Neutrino Flavor Mixing (cont’d) • If neutrinos mass states mixto form flavors • and the masses are different… • flavors of neutrinos can change in flight • Explains Davis’ “solar neutrino puzzle” • since only electron flavor neutrinos aredetected ν+n→p+e- K. McFarland, Needs for Neutrinos

11. Neutrino Flavor Oscillation • Each neutrino wavefunctionhas a time-varying phase in its rest frame, • Now, imagine you produce a neutrino of definite momentum but is a mixture of two masses, m1, m2 • so pick up a phase difference in lab frame K. McFarland, Needs for Neutrinos

12. Neutrino Oscillation (cont’d) • Phase difference leads to interference effect, just like with sound waves of two frequencies • frequency difference sets period of “beats” ν2 νμ ν3 K. McFarland, Needs for Neutrinos

13. Neutrino Oscillation (cont’d) • Phase difference • Analog of “volume disappearing” in beats is original neutrino flavor disappearing • and appearance of a new flavor • more generally, mixing need not be maximal only two generations for now! K. McFarland, Needs for Neutrinos

14. e- density Neutrino Oscillation (cont’d) appropriate units give the usual numerical factor1.27 GeV/km-eV2 • For two generations… • Oscillations require mass differences • Oscillation parameters are mass-squared differences, dm2, and mixing angles, q. • One correction to this is matter… changes q, L dep. Wolfenstein, PRD (1978) K. McFarland, Needs for Neutrinos

15. Solar Neutrinos: SNO • D2O target uniquely observed: • charged-current • neutral-current • The former is onlyobserved for ne(lepton mass) • The latter for all types • Solar flux is consistentwith models • but not all ne at earth K. McFarland, Needs for Neutrinos

16. KAMLAND • Sources wereJapanesereactors • 150-200 kmfor most offlux. Rate uncertainty ~6% • 1 kTonscint. detector inold Kamiokande cavern • overwhelming confirmationthat neutrinos change flavorin the sun via mattereffects K. McFarland, Needs for Neutrinos

17. Atmospheric Neutrinos • Neutrino energy: few 100 MeV – few GeV • Flavor ratio robustly predicted • Distance in flight: ~20km (down) to 12700 km (up) K. McFarland, Needs for Neutrinos

18. Super-Kamiokande • Super-Kdetector hasexcellent e/mseparation • Up / down difference: L/E • Muons distorted, electrons not; so mostly 2004 Super-K analysis old, but good data! K. McFarland, Needs for Neutrinos

19. MINOS 735km baseline 5.4kton Far Det. 1 kton Near Det. Running since early 2005 Precise measurement of nmdisappearance energy gives dm223 K. McFarland, Needs for Neutrinos

20. t n 1 mm Pb Emulsion layers 1.8kTon fiugres courtesy A. Bueno figures courtesy D. Autiero CNGS • Goal: ntappearance • 0.15 MWatt source • high energy nmbeam & 732 km baseline • handfuls of events/yr e-, 9.5 GeV, pT=0.47 GeV/c  interaction, E=19 GeV 3kton K. McFarland, Needs for Neutrinos

21. Two Mass Splitings: Three Generations figures courtesy B. Kayser • Oscillations have told us the splittings in m2, but nothing about the hierarchy • The electron neutrino potential (matter effects) can resolve this in oscillations, however. dmsol2 dm122≈8x10-5eV2dmatm2 dm232≈2.5x10-3eV2 K. McFarland, Needs for Neutrinos

22. Three Generation Mixing slide courtesy D. Harris • Note the new mixing in middle, and the phase, d K. McFarland, Needs for Neutrinos

23. LARGE SMALL LARGE SMALL Are Two Paths Open to Us? • If “reactor” mixing, q13, is small, but not too small, there is an interesting possibility • At atmospheric L/E, dm232, q13 ne nm dm122, q12 K. McFarland, Needs for Neutrinos

24. Implication of two paths • Two amplitudes • If both small,but not too small,both can contribute ~ equally • Relative phase, d, between them can lead toCP violation (neutrinos and anti-neutrinos differ) in oscillations! dm232, q13 ne nm dm122, q12 K. McFarland, Needs for Neutrinos

25. q13in 2011 • T2K, an accelerator experiment, showed a signal of 6 events • 1.5 expected if q13=0 • Consistent, but less significant, indication from MINOS shortly after K. McFarland, Needs for Neutrinos

26. q13in 2012 • Two reactor experiments recently showed overwhelming evidence for large q13. • Both place detectors near and far (~1km) from reactors • Look for a smallrate differencebetween twolocations K. McFarland, Needs for Neutrinos

27. q13in 2012: Daya Bay Figures from K. Heeger K. McFarland, Needs for Neutrinos

28. q13in 2012: RENO Figures from S.B. Kim K. McFarland, Needs for Neutrinos

29. Implications of Large q13 • If q13is large, then one of the two pathsis larger than the other. • This implies large signals, but small CP asymmetries dm232, q13 ne nm dm122, q12 K. McFarland, Needs for Neutrinos

30. Implications of Large q13 • Quantitative analysis to illustrate this expected behavior • Fractional asymmetry decreases as q13 increases • We live here • Statistics are (relatively) high, so the challenge will be controlling systematic uncertainties. K. McFarland, Needs for Neutrinos

31. Current and Future Experiments K. McFarland, Needs for Neutrinos

32. Narrow Band Beam • “CP violation” (interference term) and matter effects lead to a complicated mix… • Simplest case:first oscillationmaximum, neutrinos andanti-neutrinos • CP violation gives ellipsebut matter effects shiftthe ellipse in along-baseline acceleratorexperiment… Minakata & Nunokawa JHEP 2001 K. McFarland, Needs for Neutrinos

33. Broadband Beam • See different mixture of solar/interference “CP” term,matter effects at different oscillation maxima • This shows E. Recall argument of vacuum oscillation term is ~L/E FNAL-DUSEL L=1500km K. McFarland, Needs for Neutrinos

34. E Beam Design Options • All experiments will want to see first oscillation maximum, L/E ~ 400 km/GeV • Then one has a choice… Broad Band Beam Covering Multiple Oscillation Peaks Narrow Band Beam at First Oscillation Peak • Because there are many parameters, need neutrino and anti-neutrino measurements (minimally) • Perhaps multiple baselines • In principle, can measure everything with one experiment! • However require much larger L/E and L • Also need good energy resolution at low neutrino energies K. McFarland, Neutrinos at Accelerators

35. Narrow Band Beam:Off-axis Techinque • First Suggested by BNL-889 proposal • Take advantage of Lorentz Boost and 2-body kinematics • Concentrate nm fluxat one energy • Backgrounds lower: • NC or other feed-downfrom highlow energy • ne (3-body decays) • Generally optimal if onlyaccessing “first maximum” figure courtesy D. Harris K. McFarland, Neutrinos at Accelerators

36. Narrow: T2K • Tunable off-axis beam from J-PARC to Super-K detector • beam and nm backgrounds are kept below 1% for ne signal • ~2200 nm events/yr (w/o osc.) d=0, no matter effects K. McFarland, Neutrinos at Accelerators figures courtesy T. Kobayashi

37. Narrow: NOnA figure courtesy M. Messier • Use Existing NuMI beamline • Build new 15kTon Scintillator Detector • 820km baseline--compromise between reach in q13 and matter effects Goal: neappearance In nmbeam Assuming Dm2=2.5x10-3eV2 figures courtesy J. Cooper ne+A→p p+p- e- K. McFarland, Neutrinos at Accelerators

38. Broad(er): LBNE figures courtesy M. Diwan 33 kTon (fiducial) Liquid Argon TPC K. McFarland, Needs for Neutrinos

39. Needs for Modeling K. McFarland, Needs for Neutrinos

40. Illustration: T2K • Backgrounds are significant • Primarily from neutral current neutral pion production • Neutrino energy is a powerful background discriminant, but has little other information about oscillations • No official plot yet, but I can guarantee you that the backgrounds in neutrino and anti-neutrino beams are different.  K. McFarland, Needs for Neutrinos

41. Illustration: LBNE • Maximum CP effect is range of red-blue curve • Backgrounds are significant, vary with energy and are different between neutrino and anti-neutrino beams • Pileup of backgrounds at lower energy makes 2nd maximum only marginally useful in optimized design • Spectral information plays a role • CP effect may show up primarily as a rate decrease in one beam and a spectral shift in the other K. McFarland, Needs for Neutrinos

42. Generic Features • Physics goals require comparing neutrino and anti-neutrino transition probabilities • Backgrounds are significant and different • Reconstructing the neutrino energy is key • For T2K, this is quasi-elastic states • For NOvA, LBNE, need to reconstruct neutrino energy for inelastic final states K. McFarland, Needs for Neutrinos

43. Challenges K. McFarland, Needs for Neutrinos

44. Energy Reconstruction: Quasi-Elastic • Sam and Juan covered this extensively in the context of MiniBooNE data. • Inferred neutrino energy changes if target is multinucleon. Lalakulich, Gallmeister, Mosel,1203.2935 ex: Mosel/Lalakulich 1204.2269, Martini et al. 1202.4745, Lalakulichet al. 1203.2935, Leitner/Mosel PRC81, 064614 (2010) K. McFarland, Needs for Neutrinos

45. Energy Reconstruction: Inelastic • Here the problem is actually worse • Detector energy response varies • Neutrons often exit without interacting • Proton and alpha ionization saturates • π-capture on nuclei at rest, π+ decay, π0 decay to photons and leave their rest mass in detector • Any detector, even liquid argon, will only correctly identify a fraction of the final state • Need to know details of final state in four vector and particle content to correct for response K. McFarland, Needs for Neutrinos

46. p0 backgroundfrom En>peak Modeling Backgrounds signal • νeappearance is very sensitive • signal rate is low so even rare backgrounds contribute! • Current approach is to measurethe process elsewhere and scale to the oscillation detector • But data constraints on neutral current from neutrino scattering can’t tell us the cross-section as a function of energy (missing final state neutrino) • So there is always an unknown correction that comes… from a model, of course. K. McFarland, Needs for Neutrinos

47. Current Practices K. McFarland, Needs for Neutrinos

48. The Essential Tension • Ulrich Mosel’s brilliant observation at NuINT11: • Theorist’s paradigm: “A good generator does not have to fit the data, provided [its model] is right” • Experimentalist’s paradigm: “A good generator does not have to be right, provided it fits the data” • Most of the generators currently used by oscillation experiments (NUANCE, GENIE, NEUT) are written and tuned by experimentalists • See above! Our generators are wrong. WRONG! • Models do not fit (all) the data, although they provide insight into features of this data K. McFarland, Needs for Neutrinos

49. Neutrino Generators • GENIE, NUANCE, NEUT are the generators currently used in neutrino oscillation and cross-section experiments • Share same approach, with minor variations • Relativistic Fermi Gas in Initial State • Free nucleon cross-sections • LlewllynSmith formalism for quasi-elastic scattering • Rein-Sehgalcalcluation/fit for resonance production • Duality based models for deep inelastic scattering • Cascade models for final state interactions • Roughly, propagate final state particles through nucleus and allow them to interact. Constrained by πN, NN measurements. K. McFarland, Needs for Neutrinos

50. What is Useful about Generators? • This approach gives a set of four vectors for every particle leaving the nucleus • Essential for oscillation experiments where limited detectors have responses that vary wildly depending on final state particle • Many tunable parameters, and it is always easy to add more • Why? Initial model isn’t self-consistent anyway, so experimenters just tune knobs to make data agree • Which of course only applies to data we have and may or may not be predictive for the future. K. McFarland, Needs for Neutrinos