1. 1 Electron Neutrinos in the Pi-Zero Detector of the T2K Experiment Melanie Day University of Rochester 12/6/12

2. 2 Overview ● Brief History of Neutrinos ● Purpose of T2K ● Beam ● Near detectors ● SuperKamiokande ● Intrinsic v e & Cross Section ● Particle Detection and Reconstruction ● v e Analyses

3. 3 A Brief History of Neutrinos ● First hypothesized by Wolfgang Pauli to explain continuous energy spectrum of electrons in beta decay ● Relativistic arguments seemed to demand that the neutrino be massless for this reason ● Glashow-Weinberg-Salam model unified the electroweak forces in 1970s with a left handed neutrino, the electron, the photon, and the W ±, Z 0 and Higgs bosons ● Confirmed theory of parity violation by Lee and Yang in 1956. Maximal violation creates requirement that all neutrinos have same helicity, which experiment proved to be left handed

4. 4 The Solar Neutrino Problem ● In the late 1960s, Ray Davis did an experiment at Homestake Mine to detect neutrinos from the sun ● Used the conversion of chlorine into argon which could be counted by bubbling helium through the tank ● The result was that the number of interactions recorded were about 1/3 of Bahcall's solar model ● Various explanations were proposed regarding improper modelling of the solar temperature, pressure etc.

5. 5 Neutrino Oscillations ● Hypothesized by Bruno Pontecorvo around 1957 ● Basically neutrino ”flavor” state (detected neutrinos) made up of several ”mass” states ● As waves propogate, measured flavor state can change ● ● Comparable to constructive and destructive interference in light ● Requires that neutrinos have a small, non-zero mass flavor state mass state Two flavor oscillation approximation v μ = v 2 + v 3 v τ = v 2 - v 3

6. 6 Neutrino Oscillations cont. ● Homestake Experiment measured only one type of neutrino ● ● SNO experiment measured three types of neutrino interactions 1) Charged Current - Only electron neutrinos, same as Homestake 2) Neutral Current-All three neutrino types 3) Elastic Scattering-All three neutrino types with excess of electron neutrinos ● In 2001, SNO confirmed the total number of neutrinos coming from the sun agreed with Bahcall's original prediction ● Electron neutrino fraction was only ~35%, in good agreement with the Homestake measurement and the oscillation theory

7. 7 The Neutrino Mixing Matrix ● Mixing between flavor and mass states can be written mathematically as: ● Where U αi is the unitary PMNS mixing matrix described below, with c ij and s ij the sine and cosine of the three mixing angles θ 23, θ 13 and θ 12 : ● θ 23 and θ 12 have been measured by several experiments(SNO, KAMLand, Super- KamiokaNDE, MINOS, MiniBooNE,K2K etc.) ● δ parameter, which is related to the amount of CP violation in the neutrino sector, measurable if sin 2 2θ 13 > ~0.001 ● Currently measurements of θ 13 underway by Double Chooz, NOvA, RENO, Daya Bay, MINOS and T2K, with Daya Bay measuring sin 2 2θ 13 ≈ 0.089 ± 0.010 ± 0.005 from a greater than 5σ deficit

8. 8 Long Baseline Neutrino Experiments ● Probability of neutrino oscillation is related to the ratio of the distance L to the neutrino energy E ● In some cases, simple two neutrino approximation is valid: ● ● Long baseline experiments produce neutrinos within a certain energy spectrum and then build detectors at the point where the oscillation from one type to another is maximal vμvμ vτvτ veve

9. 9 The T2K Experiment ● The main goal of T2K is to precisely measure muon neutrino to electron neutrino oscillation, which has sensitivity to θ 13 through the following equation: ● ● T2K was designed to do better than original CHOOZ θ 13 limit by an order of magnitude for the known value of Δm 2 23 ~ 2.3 x 10 -3 eV 2 ● Major backgrounds to this measurement are neutral current π 0 production and the intrinsic electron neutrino component of the beam ● Complementary to reactor neutrino experiments, which can only measure anti-neutrino disappearance

10. 10 Current Neutrino Physics Goals ● Constrain δ parameter ● Full v μ to v e oscillation probability is dependent on δ if θ 13 large ● Combination of CP violation in neutrinos and Majorana sterile neutrinos - Leptogenesis ● Could explain baryon asymmetry ● Determine neutrino mass hierarchy

11. 11 Tokai to Kamioka(T2K) ● Beam from Tokai, Japan, J-PARC facility ● Have near detector 280m from target ● Far detector ~295 km away, maximizes oscillation of 500-700 MeV v with known Δm 2 23 ● Super-Kamiokande water Cherenkov in Kamioka, Japan ● Used for solar, atmospheric and long baseline(K2K) neutrino experiments since 1985

12. 12 T2K Beam ● Beam is accelerated using RF cavities ● Naturally form ”bunches” based on the field frequency ● Three acceleration phases, Linac, Rapid Cycling Synchrotron, Main Ring Synchrotron ● Ultimately produce 30 GeV protons ● 8 bunches extracted in 5 μs spills

13. 13 T2K Beam cont. ● Three magnetic focusing horns ● Improve neutrino flux by placing target inside the first horn - cylindrical for stability ● Two subsequent parabolic horns - Better to focus the beam of pions ● Neutrinos are produced primarily from pion decay process ● π + μ + + v μ

14. 14 T2K Beam cont. ● In the direction of the beam, the neutrino energy is proportional to the pion momentum ● Can choose an angle such that all pion momenta produce neutrinos with the same energy ● Set beam 2.5° angle from the direction of far detector ● Result is narrower beam energy spectrum with a peak around 500-700 MeV

15. 15 T2K Beam Content ● v e background of about 0.5% overall, 1% at peak energy ● v e flux uncertainty of ~15 - 20% at oscillation max ● More uncertain at large energies due to uncertainty in kaon production ● Important to measure v e and other background at the near detectors E peak ν e Parents

16. 16 Measuring the T2K Beam ● On-axis measurement of beam content done by: ● Beam monitors ● Muon monitor ● INGRID ● Off-axis measurement of beam neutrino interactions done by: ● ND280

17. 17 ND280 P0D - Most upstream, scintillator and active material, water target can be emptied Three gaseous argon TPCs with two FGDs (Fine grain detectors), one scintillator only, one with water targets ECal is used for veto, similar in composition to FGD with lead layers SMRD - scintillator in magnet gaps TPC FGD POD TPC FGD TPC ECal SMRD 7.6 m Bea m

18. 18 Event Displays P0DTPC1TPC3TPC2FGD ECal -P0D has large fiducial mass that stops many particles -Tracks that pass through multiple detectors are likely to be muons P0DTPC1TPC3TPC2FGD ECal Hadronic shower candidate

19. 19 SuperKamiokande ● Located 1 km deep within Mt. Ikenoyama ● Water Cherenkov detector with 22.5 kton fiducial volume ● Has an inner detector and an outer detector veto contained in a large cylindrical cavern ● Uses roughly 13,000 PMT tubes

20. 20 Event Display muon-like event electron- like event

21. 21 v e Oscillation Analysis

22. 22 v e Interactions ● Electron neutrinos make up about 1% of the beam at peak energy ● One of the dominant background to the oscillation measurement at the far detector ● Significant uncertainties exist in the charged current quasi-elastic cross section ● Possible variations of the axial mass ● Contributions from form factor uncertainties, meson exchange currents, modeling of complex nucleons Excess at low Q 2 compared to RFG with M A = 1.03

23. 23 The Pi-Zero Detector(P0D) ● Optimized for detecting electromagnetic showers ● Need to distinguish between photon ( ) and electron showers ● Form ”tracks” from particle ● interactions ● Particle ID based on track features P0D TPC1 Electromagnetic shower

24. 24 MPPC ● MPPCs(Multi-pixel photon counter) detect photons from particle interactions on scintillator ● Hundreds of pixels on each MPPC, each containing an avalanching photo-diode ● ``Avalanching” increases gain to detectable levels(factor of ~1 x 10 5 ) ● Single pixel activation -> Single photoelectron measured ● MPPCs are small, low cross talk, non-magnetic

25. 25 MPPC Crosstalk Measurement ● Number of thermal electrons excited in pedestal mode is Poisson distributed ● Crosstalk in MPPCs happens when avalanching carriers from one pixel excite an adjacent pixel ● Measure crosstalk in 480 ns integration window from thermal electron interactions from ratio of events in pedestal to one photoelectron peak and pedestal to two photoelectron peak 0 1 2

26. 26 Particle Identification - Track Seeds ● Particle interactions produce energy deposits in the detector ● Have X and Y position and charge of each energy deposit ● Linear deposits of energy are found with a Hough Transform ● The identified hits are fitted using Principal Component Analysis (PCA) and hits near the end of the new line are added (road following) to create tracks ”seeds” ● Get charge deposit and directional information from each X and Y layer from Kalman filter Deposited Energy Hough TransformPCA + Road Following

27. 27 Particle Identification - Track Merging ● Electromagnetic interactions do not deposit energy linearly ● Single track can sometimes be reconstructed as multiple tracks ● Want to merge tracks for electron case but not pi- zero case ● From particle gun studies, choose to merge tracks within 0.2 rad and within 100 mm from side of most upstream track Electro n Pi- zero

28. 28 Particle Identification - Expand Seeds ● Energy deposited too far from identified tracks will not be incorporated in seeds ● To increase the reconstruction efficiency of particle charge deposition, want to ”expand” seeds ● Optimal distance for adding hits with minimal increase in noise is 40 mm from track sides, no further than 200 mm downstream of most upstream point

29. 29 v e Analysis Cuts-Fiducial and 1 3D Track Cut ● Fiducial Cut - Only accept events within certain region ● Choose 250 mm from edges and inside water target ● Eliminates backgrounds from neutrinos interacting in the sand and cosmic rays ● After track recombination methods, expect only one identified track

30. 30 v e Analysis Cuts-Width Cut ● Muons are produced in large numbers by interactions of v μ from the beam ● Distribution of deposited energy is wider for electrons than for muons ● Calculate energy weighted width of every track ● Tracks that are less than 1 mm wide are eliminated

31. 31 v e Analysis Cuts-Kinematic Cuts ● Electrons from CCQE interactions are predominately forward going ● Cut events with track angle > 45° ● Use CCQE approximation to calculate neutrino energy of track ● Current reconstruction below 1.5 GeV is poor, kaons dominate at high energy, so events with E v <1.5 GeV are eliminated

32. 32 v e Analysis Cuts-Secondary Vertex Cut ● Event is required to have only one 3D track for every identified vertex ● Can have multiple identified vertices in an event ● Vertices with high energy deposits at large angle with respect to the candidate track are likely background ● Sum the product of the energy and angle of each layer ● Keep events with p T <100 MeV rad

33. 33 Systematics ● In an ideal world, data and Monte Carlo are exactly the same ● Unfortunately not so, there are discrepancies ● Try to determine size of all likely discrepancies ● Vary components of Monte Carlo simulation by size of discrepancy and see the effect on the analysis ● The variation in the analysis result is the systematic uncertainty Track Width, Data vs. Monte Carlo Due to mismodeled noise, width is higher in Data than in Monte Carlo

34. 34 Systematics ● EM Scale-Time variation, noise, photon detection efficiency, saturation, material uncertainty ● Cross Sections- Uncertainty in signal and background modeling ● Rate Normalization- Uncertainty in flux normalization from TPC analysis or beam systematics ● Fiducial Volume- Uncertainty from varying fiducial definition ● Muon Rejection- Difference in muon width in Data vs. MC ● Mass Uncertainty- Normalization uncertainty from water, active material mass uncertainty

35. 35 P0D v e Rate Analysis ● Intrinsic v e are a a major background for the oscillation analysis ● Check on kaon production to eliminate possible source of multiple ring excess ● Ratio of background subtracted data with signal to compare data with model ● Result consistent with Monte Carlo within ~30% estimated error ● R = (D-B)/S = 1.19 ● ±0.15(stat.)±0.26(det.) Passing event

36. 36 P0D v e Q 2 Analysis ● Many uncertainties in CCQE cross section rate ● Q 2 analysis has only CCQE-like v e as signal (no visible energy from other particles) ● Compare signal with background subtracted data ● Result over full Q 2 range is consistent with one ● Also see discrepancy at low Q 2 ● Not enough statistics for a definitive statement ● R = (D-B)/S = 0.97 ● ±0.17(stat.)±0.40(syst)

37. 37 Summary ● Exciting mysteries exist in neutrino physics - sterile neutrinos, mass hierarchy, leptogenesis ● T2K measures parameters of mixing matrix to help solve these mysteries ● Measuring the intrinsic v e of the T2K beam constrains errors on the oscillation measurement ● Distribution of CCQE events gives shape constraint for cross section

38. 38 Backup Slides

39. 39 Full v μ to v e Oscillation Probability ● δ terms will not be discernable if θ 13 is zero since they are ~sinθ 13

40. 40 Scintillator Light Production ● Scintillator coated in reflective TiO 2 to retain light ● Triangular scintillator bars give better position resolution ● Alternate vertical (Y) and horizontal(X) layers for 3D reconstruction

41. 41 Scintillator Light Attenuation ● MPPC measures light on only one end ● Further away -> Less light measured ● Use radioactive source to scan response at every point ● Fit with function ● Where