1. Evidence of Electron Neutrino Appearance at T2K
Melanie Day
University of Rochester
On Behalf of the T2K Collaboration
9/20/12

2. Overview Brief History of Neutrinos Purpose of T2K Beam Near detectors
SuperKamiokande
ve measurement

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±, Z0 and Higgs bosons
Confirmed theory of parity violation by Lee and Yang in Maximal violation creates requirement that all neutrinos have same helicity, which experiment proved to be left handed

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. Neutrino Oscillations
As early as 1957 Bruno Pontecorvo had hypothesized neutrino oscillation
Neutrino flavor state could be a mix of various neutrino mass states which oscillate from one flavor to another such that the measured neutrino varies over time
Requires that neutrinos have a small, non-zero mass
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

6. The Neutrino Mixing Matrix
Mixing between flavor and mass states can be written mathematically as:
Where Uαi is the unitary PMNS mixing matrix described below, with cij and sij the sine and cosine of the three mixing angles θ23 , θ13 and θ12 :
θ23 and θ12 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 sin22θ13 > ~.001
Currently measurements of θ13 underway by Double Chooz, NOvA, RENO, Daya Bay, MINOS and T2K, with Daya Bay measuring sin22θ13 ≈ ± ± from a greater than 5σ deficit

7. The T2K Experiment
The main goal of T2K is to precisely measure muon neutrino to electron neutrino oscillation, which has sensitivity to θ13 through the following equation:
T2K was designed to do better than original CHOOZ θ13 limit by an order of magnitude for the known value of Δm223 ~ 2.3 x 10-3 eV2
Choose energy peak and distance ratio L/E that maximizes oscillation
Major backgrounds to this measurement are neutral current π0 production and the intrinsic electron neutrino component of the beam

8. T2K and Reactor Neutrinos
T2K measures both muon neutrino disappearance and electron neutrino appearance
Reactor neutrinos can only measure electron anti-neutrino disappearance
T2K is sensitive to θ23, and can be used to constrain δ
Reactor neutrino results provide tight limits on θ13 that improve T2K sensitivity to δ
Results are complementary

9. 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 MeV v with known Δm223
Super-Kamiokande water Cherenkov in Kamioka, Japan
Used for solar, atmospheric and long baseline(K2K) neutrino experiments since 1985
Currently three running periods, Run 1&2 before and Run 3 after earthquake

10. T2K Beam
30 GeV protons with a cycle of 0.3 Hz, designed for up to 50 GeV
8 bunches extracted in 5 μs spills
Three magnetic focusing horns
Designed for proton beam power of 750 kW but currently highest power achieved is 200 kW
Set beam 2.5° angle from the direction of far detector
Result is narrower beam energy spectrum with a peak around MeV

11. T2K Beam Content
ve background of about 0.5% overall, 1% at peak energy
ve flux uncertainty of ~ % at oscillation max
More uncertain at large energies due to uncertainty in kaon production
Important to measure ve and other background at the near detectors
Epeak
νe Parents

12. SHINE/NA61 Experiment at CERN
A Main Goal: Hadron production uncertainty measurements
T2K replica target, T2K energy protons
SHINE/NA61 K0
Pions
Kaons

13. 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

14. INGRID 280 m from target Measure beam direction, intensity and profile
Scintillator and iron layers, MPPC photodiodes
Scintillator only proton module for CCQE (vμ + n p + μ-) ID

15. ND280
TPC
FGD
POD
ECal
SMRD
7.6 m
Beam
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

16. Event Displays Hadronic shower candidate
P0D
TPC1
TPC3
TPC2
FGD
ECal
Hadronic shower candidate
P0D
TPC1
TPC3
TPC2
FGD
ECal
-P0D has large fiducial mass that stops many particles
-Tracks that pass through multiple detectors are likely to be muons

17. TPC Particle Identification Analysis
-Electrons and muons both negative, single track
-Use TPC1 and TPC2 to measure dE/dx
-Muon ID:Tracks in all three TPCs, negatively charged, muon dE/dX in TPC3
-Muon sample reconstructed momentum range is MeV
-Study result: Energy resolution is (7.8 ± .2)%, mean energy loss 1.3 keV/cm
-Most particles fall in ”muon” range, with some outliers

18. TPC vμ Result 1.08X1020 POT FGD1 vertex
Negative track with ionization compatible with a muon is selected
Veto events with track in TPC1
CCQE- Single track, no Michels
All samples used with beam data, cross section studies to constrain uncertainties

19. Data/MC ratio f(νe)= 0.85±0.15(stat.)±0.11(syst.)
TPC ve Analysis
Constrain intrinsic ve in the beam
TPC excels at discriminating electrons from muons
Analysis Requires:
Interactions in both FGDs
TPC dE/dx compatible with the electron hypothesis
ECal shower particle identification
Positive analysis to constrain the γ background
Data/MC ratio f(νe)= 0.85±0.15(stat.)±0.11(syst.)

20. P0D Analyses P0D - π0 detector
Optimized for detecting electromagnetic showers
Need to distinguish between photon ( ) and electron showers
Form ”tracks” from particle
interactions
Particle ID based on track features
π 0 →γγ
P0D
TPC1
Electromagnetic shower

21. P0D ve Results Electron particle identification:
Single fiducial track, neutrino energy above 1.5 GeV, <45° from beam direction
Wide energy deposit
No energy deposits at high angle or distance from track candidate
Result consistent with Monte Carlo within ~30% estimated error
High angle
energy deposit
Too thin
R = (D-B)/S = 0.91
±0.13(stat.)±0.18(det.)
±0.13(flux × xsec.)
Passing event

22. 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

23. Timing
T2K GPS provides ~50 ns synchronization between SuperK and JPARC beam trigger
Signal is required to be within expected beam window
Require no events in 100 μs before trigger

24. Fiducial Require no vertex in outer detector
Beam
Require no vertex in outer detector
Require vertex within dotted blue lines
Pictures show events after all cuts except fiducial
Black dots - Run 1 + Run 2
Pink - Run 3
Previously events clustered near edge, but recent data is more evenly distributed

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

26. Ring Finding Try to find most energetic ring
Determine vertex where time residual from all PMTs is a minimum
Place where second derivative of charge vs. angle from vertex distribution is zero is ring edge
Iterate to obtain maximum goodness of fit
Use log likelihood method to count rings based on five parameters:
Single vs. multi sample charge
Average charge of multiple sample
Difference between outer and innermost ring in multiple
Difference between average of multiple outer rings and innermost ring
Charge residual for multiple case

27. Particle ID
Distinguish between electron and muon events by shape and angle
Electrons have diffuse ring and muon rings are sharp
Electrons have a Cherenkov angle of about 42° and muons have a smaller angle at low energy
Also have log likelihood based on expected distribution of charge for muon and electron case

28. Momentum and Energy Calculation
Use information from ring finding, ring counting and PID as well as detector energy calibration to reconstruct momentum
Once ring is found, can generate expected charge distribution
Fit charge normalization for each ring
If multiple rings, separate charge based on expected charge ratio
Calculate electron neutrino energy using CCQE approximation
E<100 MeV
Cut events with E<100 MeV or reconstructed neutrino energy > 1250 MeV

29. Michel Electrons
Background from muons and charged pions decaying to muons
Muons decay to produce two neutrinos and an electron
Can spot a muon decay by the detection of an electron soon after
Events with associated decay electrons are vetoed
Event failing due to decay electron

30. π0 Background π0 decays into two photons
Photons produce rings that are similar to electron rings
Expect in π0 case there will be two rings
Energy of two rings should peak at the π0 mass
Force two rings, cut out calculated mass > 105 MeV/c2

31. Cut Summary

32. Selected Events

33. Cross Section Uncertainties
Any measurement of neutrino interactions is constrained by the understanding of the cross sections
NEUT, previously used by K2K, is used to generate neutrino events from cross section predictions
Used information from recent MiniBooNE and SciBooNE papers to estimate uncertainty on various cross section parameters

34. Total Systematics
Study systematic uncertainty in SK using cosmic rays, electrons from muon decays and atmospheric neutrino interactions
Detector systematic is combination of:
Fiducial volume
Energy scale
Delayed electron tagging efficiency
π0 rejection efficiency
One ring e-like acceptance
Muon rejection
Invariant mass calculation uncertainties
Other systematics come from previously mentioned studies

35. Results
An observation of eleven events is inconsistent with θ13 =0 with a 3.2 σ significance
Construct confidence interval for normal and inverted hierarchy
Precise measurements of θ13 combined with T2K measurement improves sensitivity to δ

36. Conclusions and Future Work
Despite setback of earthquake, T2K is making progress, almost doubling the Run 1&2 data set since data taking resumed
With 2.56 x 1020 POT T2K sees a 3.2 σ excess of electron neutrino events at the far detector
Get sin22θ13 ≈ for normal hierarchy
Result is consistent with recent reactor neutrino results, and future data taking, especially combined with NOvA results, will allow constraints on the CP violating parameter δ

37. Backup

38. T2K Data Taking

39. T2K Beam Monitors
Five current transformers (CT), which are toroids used to measure proton beam intensity and timing to 10 ns
21 electrostatic monitors(ESM) measure the position of the beam and are composed of four segmented cyclindrical electrodes
19 segmented secondary emission monitors(SSEM) measure the beam profile including center, width, and divergence and are only used during beam tuning
1 Optical Transition Radiation(OTR) Monitor is made of titanium alloy foil placed at 45º from the beam direction, producing transition radiation as the beam passes through, which is used to produce an image of the proton beam profile

40. Muon Monitors
Kaons in the beam generally decay to either pions or muon and muon neutrino
Pions in the beam generally decay into muons and muon neutrinos
Measuring muons gives some information about these decays in the beam
Muon monitors consist of two detectors: ionization chambers with Argon or Helium gas and silicon PIN photodiodes
Can measure beam direction within mRad
Monitors stability of beam intensity within ~3%
ionization chamber
photodiode
array

41. MPPC
Most of the scintillator based near detectors use MPPCs(Multi-pixel photon counter)
Hundreds of pixels on each MPPC, each containing an avalanching photo-diode
Because of running in geiger mode, single incident photon can cause electron ”avalanche”
Increases gain to detectable levels(factor of ~10e5)
Activation of a pixel registers a single photoelectron measurement
MPPCs are small and non-magnetic

42. Misidentified Muons
-Look at likelihood of misidentifying a muon as an electron
-Sample of events IDed as muons in TPC3 with a maximum of one negative track in each TPC
- Between 200 and Mev/c a 1σ electron ID cut will give a muon fake rate of .19%
- A 2σ electron ID cut will give a fake rate of .72%
1σ(.19%)
2σ(.72%)

44. P0D NC 1π0 Analysis
Pre-selection– Require event to be within beam spill
Fiducial– Require vertex in water target
No μ-like– Reject CC events
2 EM-like– π0→γγ
No μ-decay – No delayed hit clusters
π0 Direction– Require π0 in forward direction(cosθ < 0.6)
EM Charge– Apply additional PID to EM shower
EM Separation– Require decay γs to be separated by 50 mm
Data/MC ratio: 0.84±0.16(stat.)±0.18(syst.)

46. Flux Error Using ND280 Data