1 Evidence of Electron Neutrino Appearance at T2K Melanie Day University of Rochester On Behalf of the T2K Collaboration 7/12

2 Overview ● Brief History of Neutrinos ● Neutrino Oscillation ● Purpose of T2K ● Beam ● Near detectors ● SuperKamiokande ● Reconstruction and analysis cuts ● v e analysis result

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 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 the predictions by John Bahcall ● 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 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.) to ~10% for sin 2 2θ 23 and ~3% for θ 12 ● δ parameter, which is related to the amount of CP violation in the neutrino sector, only exists if θ 13 is non-zero, measurable if sin 2 2θ 13 > ~1 x ● Currently measurements of θ 13 are planned by Double Chooz, NOvA, RENO, Daya Bay, MINOS and T2K, with Daya Bay measuring sin 2 2θ 13 ≈ ± ± from a greater than 5σ deficit

7 The T2K Experiment ● The main goal of T2K is to precisely measure θ 13 through the muon neutrino to electron neutrino oscillation described by the following equation: ● Can measure both muon and electron neutrino, unlike reactor experiments ● T2K was designed to do better than original CHOOZ measurement by an order of magnitude, especially for the known value of Δm 2 23 ~ 2.3 x eV 2 ● Choose energy peak and distance ratio L/E that maximizes oscillation ● Major backgrounds to this measurement are neutral current π 0 production and the intrinsic electron neutrino component of the beam

8 Tokai to Kamioka(T2K) ● Beam produced in Tokai, Japan, at the J-PARC facility and was constructed for the experiment ● Have near detector 280m from target to monitor beam before oscillation ● Far detector is located ~295 km away, giving maximum oscillation for energies between MeV based on measurements of Δm 2 23 ● Use Super-Kamiokande water Cherenkov detector located in Kamioka, Japan which has been previously used for solar, atmospheric and long baseline(K2K) neutrino experiments since 1985

9 T2K Beam ● Accelerator provides 30 GeV protons with a cycle of 0.3 Hz, though was designed for up to 50 GeV ● Bunch structure with 8 bunches extracted in 5 μs spills ● Have three magnetic focusing horns ● Designed for proton beam power of 750 kW but currently highest power achieved is 200 kW ● Center of beam is set at an angle of 2.5° from the direction of the far detector ● This gives a narrower beam with a peak around MeV

10 Beam Modelling ● Need to model beam behavior to estimate flux at the various detectors ● FLUKA is chosen to simulate proton interactions and hadronic chains in the target because the predictions were found to be closest to studies on a similar target ● Particles exiting the target are simulated by JNUBEAM, a Monte Carlo generated from GEANT3 by the T2K collaboration ● Hadron interactions outside the target region are simulated by GCALOR ● Use measurements from NA61/SHINE and various beam monitors and near detectors to tune simulations

11 Modeling Uncertainties ● When protons strike target, produce kaons and pions which then decay primarily to muons ● and muon neutrinos ● Beam production uncertainty dominated by uncertainties in particle production ● Otherwise uncertainty is caused by uncertainty in beam shape from various ● components ● Need to constantly monitor beam and horns to keep uncertainties low

12 SHINE/NA61 ● Experiment at CERN with several goals, including measuring hadron production in hadron-nucleus interactions for neutrino experiments ● Now have data with T2K replica target and operates at same energy as T2K(30 GeV protons) ● T2K uses pion and kaon production data from NA61 compared to Monte Carlo predictions to improve flux uncertainties

13 T2K Beam Content ● Use discussed models to predict number of neutrinos at the far detector ● Predict electron neutrino background of about 0.5% overall, and 1% at peak energy ● Electron neutrino flux uncertainty of ~15-20% at oscillation max ● More uncertain at large energies due to uncertainty in kaon production ● Important to measure electron neutrinos and other background at the near detectors E pea k ν e Parents

14 Measuring the T2K Beam ● On-axis measurement of beam content done by: ● Beam monitors ● Beam monitors- Located in the target station, monitor various beam properties ● Muon monitor ● Muon monitor-Directly after decay pipe, measures muon content of the beam ● INGRID ● INGRID: Measures beam axis direction at 280m from the production target ● Off-axis measurement of beam neutrino interactions done by: ● ND280 ● ND280: 280m and 2.5º from the beam, made up of the P0D, SMRD, TPC, FGD, and ECal

15 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

16 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.25 mRad ● Monitors stability of beam intensity within ~3% ionization chamber photodiod e array

17 INGRID ● On axis detector located 280 m from the target ● Can monitor beam direction with precision.4 mRad as well as intensity and beam profile ● Has a scintillator only proton module and a main detector made of scintillator and iron layers surrounded by veto planes ● Read out information about interactions using MPPC, a kind of silicon photodiode

18 ND280 TPC FGD POD TPC FGD TPC m from target - P0D is most upstream detector and has the largest fiducial mass -Has triangular scintillator bars and water target that can be filled and emptied for cross section measurement -ND280 has three TPC detectors with FGD detectors between them -FGD contain segmented scintillator bars with water in one of the two for cross section measurement Bea m -ECal is made of scintillator and lead calorimetry and the SMRD of scintillator instrumenting gaps in the magnet. Since picture, surrounding ECal region has also been installed. - ECal has similar capabilities to P0D and FGD in measuring events ECal SMRD

19 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 -Electromagnetic shower candidate P0 D TPC 1

20 TPC Muon Neutrino Analysis - The TPC uses a track based analysis and information from magnet interactions - Muons may be negatively charged and single tracked – Electrons are similar, and are therefore a major background – Use energy loss in detector to discriminate between electrons and muons ● Current analysis uses TPC dE/dx and momentum to discriminate between electrons, only using FGD information as a veto, but better reconstruction tools are being created

21 Muon Identification Analysis -Study energy loss by measuring dE/dx in TPC1 and TPC2 to discriminate between electrons, muons and other backgrounds -Muon sample is mostly events that have tracks in all three TPCs and that are identified as being negatively charged and in the correct dE/dX region for TPC3(i.e events that are IDed as muons in TPC3) -Muon sample reconstructed momentum range is MeV -Study result: deposited energy resolution is (7.8 ±.2)% with mean energy loss of 1.3 keV/cm -See that most particles fall in ”muon” range, with some outliers

22 TPC v μ Result ● Use to constrain event rate at the far detector ● Uses 2.88 x p.o.t(about a third of total data) ● Most energetic negative track with ionization compatible with a muon is selected ● Veto events with track in TPC1 ● See agreement with Monte Carlo predictions within uncertainties over full energy range

23 TPC v e Analysis ● Constrain intrinsic v e in the beam ● TPC expected to excel at discriminating electrons from backgrounds ● Analysis Requires: ● Interactions in both FGDs ● TPC dE/dx compatible with the electron hypothesis ● ECal shower particle identification ● Positive analysis to constraint the γ background

24 TPC v e Analysis cont. ● Large background from photons at low momentum ● Get Data/MC ratio f(ν e )= 0.845±0.146(stat.)±0.107(syst.)

25 P0D Analyses ● P0D stands for π 0 detector, and the main goal is to measure this background ● P0D is optimized for detecting electromagnetic showers, using scintillator and high Z materials like brass and lead ● Biggest challenge is to distinguish between photon showers ( ) and electron showers ● Of all ND280 detectors, P0D has largest fiducial mass (about 13 tons) and therefore has the highest number of interactions ● This is an advantage in studying v e interactions in the first few years when TPC statistics are low

26 P0D NC 1π 0 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– π 0 →γγ ● No μ-decay – No delayed hit clusters ● π 0 Direction– Require π 0 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

27 P0D NC 1π 0 Analysis cont. ● Data/MC ratio: 0.84±0.16(stat.)±0.18(syst.)

28 Current P0D v e Results ● Analysis aims for clearest electron neutrino signal ● Single fiducial track, neutrino energy above 1.5 GeV, <45° from beam direction ● Wide median energy deposit ● No energy deposits at high angle or distance from track candidate ● Result is consistent with Monte Carlo within 30% estimated error ● R = (D-B)/S = 0.91±0.13(stat.)±0.18(det.)±0.1 3(flux × xsec.) Too thin High angle energy deposit

29 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

30 Reconstructing v e Events ● Get information from PMT timing, charge and position ● Reconstruct ● Vertex ● Number of Cherenkov rings ● Direction ● Particle ID ● Momentum ● Use timing and momentum information to veto muon and pi-zero backgrounds ● Veto high energy electron neutrino candidates also to reduce intrinsic electron neutrino background

31 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

32 Fiducial ● Require no vertex in outer detector ● Require vertex within certain distance from walls and top and bottom of detector ● Pictures show events after all cuts except fiducial(cross vertex is vetoed), where black dots are from Run 1 + Run 2 and pink dots are from the Run 3 analysis ● Previously criticized clustering of events near edge, but recent data is more evenly distributed Bea m

33 Event Display muon-like event electron- like event

34 Ring Finding ● Try to find most energetic ring ● Determine vertex where time residual from all PMTs is a minimum ● Make distribution of charge vs. angle from vertex ● Place where second derivative of distribution is zero is location of ring ● Iterate process 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

35 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

36 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 1250 MeV

37 Michel Electrons ● 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

38 π 0 Background ● π 0 decays into two photons ● Photons produce rings that are similar to electron rings ● Expect in π 0 case there will be two rings ● Energy of two rings should peak at the π 0 mass ● Force two rings, cut out calculated mass > 105 MeV/c 2

39 Cut Summary ● No activity in outer detector or 100 μs before trigger time ● More than 30 MeV electron equivalent energy in inner detector ● Vertex inside inner detector ● Single e-like ring ● Visible energy > 100 MeV ● No delayed electron signal ● Invariant mass for two ring less than 105 MeV/c 2 ● CCQE neutrino energy < 1250 MeV ● After these cuts, have eleven candidates

40 Selected Events

41 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 cross section predictions ● Used information from recent MiniBooNE and SciBooNE papers to estimate uncertainty on various cross sections

42 Total Systematics ● Study systematic uncertainty in SK using cosmic rays, electrons from muon decays and atmospheric neutrino interactions ● Total systematic is combination of: ● Fiducial volume ● Energy scale ● Delayed electron tagging efficiency ● π 0 rejection efficiency ● One ring e-like acceptance ● Muon rejection ● Invariant mass calculation uncertainties ● Other systematics come from previously mentioned studies

43 Results ● An observation of eleven events is inconsistent with θ 13 =0 with a 3.2 σ significance ● Construct confidence interval following the unified ordering prescription of Feldman and Cousins ● At 90% confidence interval the 2011 Run 1&2 data are consistent with 0.03(0.04) < sin 2 2θ 13 < 0.28(0.34) with δcp = 0 for normal(inverted) hierarchy ● Newest result including Run 3 provides tighter constraint

44 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 POT T2K sees a 3.2 σ excess of electron neutrino events at the far detector ● Get sin 2 2θ 13 ≈ for normal hierarchy ● Result is consistent with recent Daya Bay and RENO results, and future data taking will allow constraints on the CP violating parameter δ

45 Backup

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47 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

48 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 800 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% )

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51 Flux Error Using ND280 Data