1 Indications of Electron Neutrino Appearance at T2K Melanie Day University of Rochester On Behalf of the T2K Collaboration 9/20/11

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 Many experiments currently trying to get a measurement of this elusive mixing angle, including Double CHOOZ, RENO, MINOS, NOvA, Daya Bay and T2K

7 The T2K Experiment The main goal of T2K is to measure the muon neutrino to electron neutrino oscillation described by the following equation: By searching for this oscillation, the parameter θ 13 can be measured Want to choose energy range 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 Sensitivity Predictions Until recently, previous best measurement of θ 13 was done by the CHOOZ collaboration CHOOZ was able to eliminate a large area of parameter space to 90% confidence Shows θ 13 is small, compared to other two angles which are large T2K was designed to do better by an order of magnitude, especially for the known value of Δm 2 23 ~ 2.3 x eV 2

9 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

10 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 145 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

11 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

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

13 SHINE/NA61 Experiment at CERN with several goals, including measuring hadron production in hadron-nucleus interactions for neutrino experiments Uses T2K like target(graphite) at same energy as T2K(30 GeV protons) Currently used to better understand pion production, but will be used for kaon tuning also by comparing FLUKA predictions to NA61 data

14 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 peak ν e Parents

15 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

16 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

17 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 photodiode array

18 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

19 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 Beam -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

20 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 P0D TPC1

21 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 and muons, but may move to using other detectors to veto events

22 TPC 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

23 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

24 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

25 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 Too thin High angle energy deposit

26 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

27 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

28 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

29 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) Selected events(black dots) seem to be grouped on upstream side of detector Beam

30 Event Display muon-like eventelectron- like event

31 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

32 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

33 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

34 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

35 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

36 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 six candidates

37 Selected Events

38 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

39 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

40 Result The predicted number of events is: 1.5 ± 0.3 if sin 2 2θ 13 = 0 5.5±1.0 if sin 2 2θ 13 = 0.1 An observation of six events is inconsistent with θ 13 =0 with a 2.5 σ significance Construct confidence interval following the unified ordering prescription of Feldman and Cousins At 90% confidence interval the data are consistent with 0.03(0.04) < sin 2 2θ 13 < 0.28(0.34) with δcp = 0 for normal(inverted) hierarchy

41 Earthquake and Future On March 11, 2011 Japan was hit with a magnitude 9.0 earthquake No one at J-PARC was injured J-PARC was 260 km from the epicenter and 100km from the Fukushima power plant The area was temporarily at a higher level of radiation due to the problems, but has returned to normal Parts of the beam and detectors were damaged Currently J-PARC plans to begin operating again in December of 2011 T2K data taking will restart as soon as possible

42 Backup

43 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

44 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% )