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Indications of Electron Neutrino Appearance at T2K

Melanie Day

University of Rochester

On Behalf of the T2K Collaboration

9/20/11

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Overview

Brief History of Neutrinos Neutrino Oscillation Purpose of T2K Beam Near detectors SuperKamiokande Reconstruction and analysis cuts v

e analysis result

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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 1956. Maximal violation creates requirement that all neutrinos have same helicity, which experiment proved to be left handed

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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.

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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

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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.) to ~10% for sin22θ23 and ~3% for θ12

δ parameter, which is related to the amount of CP violation in the neutrino sector, only exists if θ13 is non-zero, measurable if sin22θ13 > ~1 x 10-3

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

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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 θ13 can be measured

Want to choose energy range 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

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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 θ13 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 Δm2

23 ~ 2.3

x 10-3 eV2

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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 500-700 MeV based on measurements of Δm2

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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

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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 500-700 MeV

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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

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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

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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

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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

Epeak

νe Parents

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Measuring the T2K Beam

On-axis measurement of beam content done by:

Beam monitorsBeam monitors- Located in the target station, monitor various beam properties

Muon monitorMuon monitor-Directly after decay pipe, measures muon content of the beam

INGRIDINGRID: Measures beam axis direction at 280m from the production target

Off-axis measurement of beam neutrino interactions done by:

ND280ND280: 280m and 2.5º from the beam, made up of the P0D, SMRD, TPC, FGD, and ECal

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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

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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

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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

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ND280

TPCFGD

POD

TPC

FGD

TPC

- 280 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

SM

RD

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Event Displays

P0D TPC1 TPC3TPC2FGD FGD ECal

-P0D has large fiducial mass that stops many particles-Tracks that pass through multiple detectors are likely to be muons

P0D TPC1 TPC3TPC2FGD FGD ECal

-Hadronic shower candidate

-Electromagnetic shower candidate

P0D TPC1

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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

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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 400-500 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

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TPC vμ Result

Use to constrain event rate at the far detector

Uses 2.88 x 1019 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

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P0D Analyses

P0D stands for π0 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 ve interactions in the first few

years when TPC statistics are low

0

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Current P0D ve 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 angleenergy deposit

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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

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Reconstructing ve 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

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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

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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

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Event Displaymuon-like event electron- like event

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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

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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

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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

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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

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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

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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/c2

CCQE neutrino energy < 1250 MeV

After these cuts, have six candidates

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Selected Events

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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

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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 π0 rejection efficiency One ring e-like acceptance Muon rejection Invariant mass calculation

uncertainties Other systematics come from previously

mentioned studies

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Result

The predicted number of events is:

1.5 ± 0.3 if sin22θ13

= 0

5.5±1.0 if sin22θ13

= 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) < sin22θ

13 <

0.28(0.34) with δcp = 0 for normal(inverted) hierarchy

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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

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Backup

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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

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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%)