Study of Neutrino Oscillation in the OPERA Experiment

Tomokazu Matsuo (Toho University)on behalf of the OPERA collaboration*

*) 140 physicists, 28 institutions in 11 countries

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Motivation and ConceptFirst detection of neutrino oscillation in appearance mode using νμ→ντ by tau identification to confirm disappearance results of SK1998.

SK 1998

Tau ID by DONuT 2001

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-

decay “kink”

~1 mm

oscillation

-, h-, e-

plus 3-prong decay modes

• Event by event identification ‐ ‐of tau neutrino interaction

• Detection of τ decay topology to identify ντ

• High spatial resolution required

Requirements:• long baseline• high energy neutrino• large mass• τ identification 　 capability

The OPERA experimentA νμ beam was generated at CERN CNGS beam line and detection of neutrino oscillation was performed at LNGS in Italy, at a distance of 730 km.

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beam compositions

SM1 SM2Brick walls + Target Tracker(TT)

Muon Spectrometer RPC+ Drift Tubes (HPT)

Muon SpectrometerMuon ID, momentum and charge measurement

Target area

OPERA detector

730km

νμ beam

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Total 1.25 kton, ~150000 ECC bricks

Target area Muon Spectrometer

Emulsion film

Lead plate

Target Tracker(TT)

~20m

Location of neutrino interactions in ECC brick

1. Find the brick containing the neutrino interaction using TT hits2. Check CS to confirm the brick3. Follow up tracks to interaction vertex in the ECC brick4. Scan 10 films around interaction vertex (blue box)

TTCS

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3m 4m 5m

Event viewer ofOPERA target area

τ daughter from ντ(MC)

NC+CC νμ events (MC),

NC+CC νμ events (Data)

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Pick up track chains Search vertex topologyScan 1cm×1cm around vertex

1cm

Detection of decay topologies

IP

IP > 10μm + 0.01Δz

Search track which have large Impact Parameter (IP) IP distribution

Δz: distance to the vertextrack segment

Beam exposure and analysis status

Beam: 5year (965days) 17.97×1019 p.o.t.

Overall 80% of the proposal value (22.5×1019 p.o.t.)

Completed: 2008, 20092010-12 on going with optimised strategy

Located: 6067, Decay search: 4969~60% of expected value done 7

‘08 ‘09 ‘10 ‘11 ‘12

date

p.o.

t.

Run 2008 → 2012

date

even

t

Located6067

CS found10585

ECC scanned9629

CS scanned14737

DS 4969

1st / 2nd ντ candidate events1st ντ candidate event　 Reported in May 2010

Decay channel: τ → 1h (τ → ρ ντ , ρ →π0 π, π0 → 2γ)

2nd ντ candidate event　 Reported in June 2012

Decay channel: τ → 3h

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A new ντ candidate event (3rd)

Decay channel: τ → μ

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Reported first in March 2013

The 3rd candidate event

Topology of the τ→μ decay event

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• All variables passed the kinematical cuts of OPERA

• γ is attached to the primary vertex

muon

primary track

4τ

candidate

376 μm

e-pair

film 38 film 39 film 40 film 41 film 42

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3

2

The 3rd candidate event

pτ

ph total

pri. h

e-pair

φ=154.5°

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TT hits of new ντ candidate event (3rd)TOP SIDE

zoom

The 3rd candidate event

Negative μ: 5.6σ significance

Charge sign of μ from curvature

XB

Target Tracker hits

RPC hits

P2<0 → negative charge5.6 σ significanceR ~ 85 cm

P-value = 0.063% (probability to reconstruct a m+ stopping in the 7th iron layer with p2 < -0.00389 cm-1)

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The 3rd candidate event

Muon momentum by range in the electronic detector: 2.8±0.2 GeV/cMCS in the brick consistent 3.1 [2.6,4.0] GeV/c

First charge measurable event

The oscillated neutrino is ντ (not ντ)

←μ+

μ- →

π-π-

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Background sources• Charm:

• Charm hadron decays in νμ CC interactions

• μ is not identified

• Hadron re-interaction:• Hadron interacts in a lead

plate or an emulsion film close to the primary vertex

• nuclear fragments are not observed

Real charm events in OPERA

• Muon scattering:Muon interacts near the vertex and exhibits a kink topology with a large angle

Test experiment

Background estimation (hadron)

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10GeV/c

4GeV/c2GeV/cMultiplicity

Kink angle (1-prong)

nuclear fragments in emulsion

4GeV/c2GeV/c

Multiplicity

Emission angle(cos q)

10GeV/c

MC: b < 0.7

ForwardBackward Forward

Backward

ForwardBackward

We confirmed MC simulation estimation of hadron backgrounds by using the data analysis of ECC bricks exposed to 2GeV/c , 4 GeV/c and 10GeV/c pion beams and reduced systematic uncertainty.

Error bars : Experimental dataHistogram : Simulated data

Secondary track emission: 30%

nuclear fragments:10%

Nuclear fragment associated probability

2 10GeV/c4

Hadronic background was reduced by “40%” by requiring no association of large angle nuclear fragments.

100%

0%

0 5 0 5 0 8

0 0.6 0 0.6 0 0.6 180° 0° 180° 0°180° 0°

0 15 0 15 0 15

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・ data- MC

New technique for background reduction

They argee well Well produced by MC

Statistical considerationsExtended sample to muonic interactions

Extended sampleSignal Background Charm μ scattering had int

τ → h 0.66 0.045 0.029 0.016 τ → 3h 0.51 0.090 0.087 0.003 τ → μ 0.56 0.026 0.0084 0.018 τ → e 0.49 0.065 0.065total 2.22 0.226 0.19 0.018 0.019

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3 observed events in the τ → h(1st), τ → 3h(2nd) and τ → μ(3rd) channelsPvalue = P0 = 1.125 x 10-4 Probability to be explained by background = 7.29×10-4

This corresponds to 3.2 σ significance of non-null observation by simple counting method.At a first likelihood approach, a 3.5σ level significance has been calculated.

New developmentsLarge angle scan High speed scan

Speed: 0.6m2/h (under construction)→ speed up, increase location efficiencyScan angle: |tan θ| < 4.0 → reduce background

efficiency of large angle MIP particle

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|tanθ|=3.080%90%

Angle limit of normal scan

Extended limit more than 6x

1 2 3

ν from the bottom

possibility of off-timing cosmic ray event analysis, such as atmospheric neutrino

oscillation

Future prospect:

Conclusions

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• 3 ντ candidate events have been found.• The 3rd event is τ- →μ- decay channel.• Background have been evaluated more

precisely with test experiment analysis. • With a first likelihood approach, a 3.5σ level

has been calculated.• 4σ observation is within reach.