New results from the OPERA experiment

Tomoko Ariga

AEC-LHEP, University of Bern

On behalf of the OPERA Collaboration

Motivation of the OPERA experiment

Neutrino oscillations have been studied in disappearance mode• Atmospheric sector : Super-Kamiokande, MACRO, MINOS, K2K,

T2K, ..

Long way to appearance• Super-Kamiokande : ντ appearance in atmospheric neutrino data

(statistical separation in background-dominated sample)• T2K : νe appearance in νμ beam

Main goal of OPERA•Verify νμ→ ντ oscillations in appearance mode (S/N ~ 10)

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Physics results from OPERA

•ντ appearance in νμ beamObservation of ντ appearance (4.2 σ), PTEP (2014) 101C01 In this talk, new results from the analysis of an enlarged data sample

Other topics• Sterile neutrino analysis (Talk by M. Tenti in SESSION 24.08. E)

• νμ→ νe oscillations (Talk by S. Zemskova in SESSION 24.08. E)

• Cosmic-ray physicsTeV atmospheric muon charge ratio, published in Eur. Phys. J. C (2014) 74:2933

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Principle of the experiment

•Detection of τ leptons in ντ charged-current interactions

Experimental requirements• Large target mass•Micrometric spatial resolution to detect short-lived τ lepton decays

τ’s life time 2.9x10-13 s (cτ 87 μm)

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Key technology: Emulsion particle detectors (unbeatable position resolution)

Cross-sectional view

Plastic base (200 m)

Emulsion layer (44 m)

Emulsion layer (44 m)

AgBr crystal

200 nm

OPERA film

20 m

10 GeV/c beam

Sensitivity 36 grains/100 m

3D tracking deviceIntrinsic resolution 50 nm

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The CNGS νμ beam (CERN Neutrinos to Gran Sasso)

The OPERA detector at LNGS exposed from 2008-2012

A total 17.97 x 1019 protons on target (20% less than the design value) resulted in 19505 ν interactions

CERN

LNGS

732 km

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produced by the SPS 400 GeV proton beam

Target section(~75000 units)

μ spectrometer Target section(~75000 units)

μ spectrometer

The OPERA detector

Total target mass 1.2 kton

2 identical super modules

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Analysis of neutrino interactions

• τ decay search• Impact parameter• Kink search• Parent search(Eur. Phys. J. C (2014) 74:2986)

5 mm

Emulsion gives 3D vector data with vertex position resolution of a few μm

Location efficiencyJHEP 11 (2013) 036

Tracks found in the interface films

Track follow-up

Volume scanningVertex reconstruction

(Animation)

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Charm decays: control sample for the τ decay searchCharm and τ decays have similar topologies

Good agreement between data and expectationsEur. Phys. J. C (2014) 74:2986

ντ analysis: kinematical selection

p2ryT

p2ry m

Cuts fixed since the beginning of the experiment

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τντ

zdec

θkink

pmissT

τ

ϕlH

Emulsion film

Transverse plane pmissT : vectorial sum of the

transverse momenta of primaries (except the parent) and daughters wrt beam directionp2ry

T : transverse momentum of the daughter wrt the parent direction

ντ analysis: data sample

•The 1st and 2nd most probable bricks for all runs• 15% larger than the previous one reported in PTEP (2014) 101C01

Number of events used in the analysis

5408 events analyzed and 5 events observed

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The 1st candidate: τ→1hPhys. Lett. B 691 (2010) 138-145

The 2nd candidate: τ→3hJHEP 11 (2013) 036

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The 4th candidate: τ→1hPTEP (2014) 101C01

The 3rd candidate: τ-→μ-

Physical Review D 89, 051102(R) (2014)

The 5th candidate: τ→1h recently reported

P1

Transverse plane

90 mrad kink with a 960 μm flight length

New

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The 5th candidate: particle identification

• Primary particle: hadronic interaction in the downstream brick

(measurement of dE/dx, consistent with proton hypothesis)charm background hypothesis discarded

(charm can be a background if the primary μ is not identified)

• Daughter: hadronic interaction in the 1st brick

Hadronic decay channel15

The 5th candidate: measured parameters

Fully consistent with the expected distributions for signal 16

Summary of all candidate events

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psum : the scalar sum of the momenta of all particles measured in the emulsion films

Fully consistent with the expected distribution

CC interactions with charm production

Hadronic interactions

Large angle μ scattering

Background sources

μ-

νμ

D+

νμνμ

h

μ-νμ

Monte Carlo simulation tuned on CHORUS data

Reduced by multi-brick tracking

FLUKA and test beam data

Reduced by nuclear fragment search and large angle scanning

New estimate by implementing a proper form factor for Lead : (1.2±0.1(stat.)±0.6(sys.))x10-7/νμCC well below the conservative value considered so far (arxiv.org/abs/1506.08759)

if the primary lepton is not identified and the daughter charge is not measured

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Significance

Expected signal and background events for the analyzed data sample

5 observed events with 0.25 background events expected

Probability to be explained by backgroundFisher 1.1x10-7

Profile likelihood 1.1x10-7

Corresponding to 5.1 σ exclusion of the background-only hypothesis

Probability of observing ≥5 candidates (2.9 S+B events expected): 17%Frequency of configurations being less probable than the observed one: 6.4%

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Estimation of Δm223

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90% confidence interval[2.0, 5.0] x 10-3 eV2

(assuming full mixing)

using profile likelihood, Feldman-Cousins, Bayesian

statistics, the difference among the

methods negligible

Conclusions

•Data sample including the first and second most probable bricks for all runs analyzed

• 5 ντ candidate events observed with 0.25 background events expected 5.1 σ significance for the exclusion of the null hypothesis

Discovery of ντ appearance

arxiv.org/abs/1507.01417 (accepted for publication in Phys. Rev. Lett.)

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The OPERA collaboration

BelgiumIIHE-ULB

Brussels

CroatiaIRB Zagreb

FranceLAPP Annecy

IPHC Strasbourg

GermanyHamburg

IsraelTechnion Haifa

ItalyBari

BolognaFrascati

LNGSNaplesPadova

RomeSalerno

JapanAichiTohoKobe

NagoyaNihon

KoreaJinju

RussiaINR RAS Moscow

LPI RAS Moscow

SINP MSU Moscow

JINR Dubna

SwitzerlandBern

TurkeyMETU, Ankara

140 physicists from 26 institutions in 11 countries

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Thank you for your attention

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Backup slides

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Measurement of the TeV atmospheric muon charge ratio

25Vertical surface energy

Eur. Phys. J. C (2014) 74:2933

Validation of the hadronic background by test beams

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Interaction rate

Dots with error bars : dataHistogram : simulation

Kink angle (1-prong events)

Black : dataRed : simulation (FLUKA)

Probability of associated nuclear fragments

Muon/hadron separation by “track follow-down"

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(primary tracks are followed until the stopping point)

: track length: range in lead of a muon: average density along the path: lead density

LR(p)<ρ>ρ

A track is classified as a muon if DTFD is above 0.8