1. New results from the OPERA experiment
Tomoko Ariga
AEC-LHEP, University of Bern
On behalf of the OPERA Collaboration

2. 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)

3. Physics results from OPERA
ντ appearance in νμ beam
Observation 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 E)
νμ→ νe oscillations (Talk by S. Zemskova in SESSION E)
Cosmic-ray physics
TeV atmospheric muon charge ratio, published in Eur. Phys. J. C (2014) 74:2933

4. 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)

5. Key technology: Emulsion particle detectors (unbeatable position resolution)
AgBr crystal
Cross-sectional view
OPERA film
Emulsion layer (44 m)
Plastic base (200 m)
Emulsion layer (44 m)
200 nm
20 m
10 GeV/c 
beam
Sensitivity 36 grains/100 m
3D tracking device
Intrinsic resolution 50 nm

6. The CNGS νμ beam (CERN Neutrinos to Gran Sasso)
LNGS
732 km
produced by the SPS 400 GeV proton beam
The OPERA detector at LNGS exposed from
A total x 1019 protons on target (20% less than the design value) resulted in ν interactions

7. 2 identical super modules
The OPERA detector
2 identical super modules
cs doublet 1/10000
Target section
(~75000 units)
Target section
(~75000 units)
μ spectrometer
μ spectrometer
Total target mass 1.2 kton

8. Analysis of neutrino interactions
(Animation)
Volume scanning
Tracks found in
the interface films
Vertex reconstruction
Track follow-up
Emulsion gives 3D vector data with vertex position resolution of a few μm
Location efficiency
JHEP 11 (2013) 036
5 mm
location efficiency : CC 74%, NC 48% (2 brick analysis strategy)
τ decay search
Impact parameter
Kink search
Parent search
(Eur. Phys. J. C (2014) 74:2986)

9. Charm decays: control sample for the τ decay search
Charm and τ decays have similar topologies
data sample : CC events collected in 2008 – 2010 runs
Good agreement between data and expectations
Eur. Phys. J. C (2014) 74:2986

10. ντ analysis: kinematical selection
Cuts fixed since the beginning of the experiment
p2ryT
p2ry m
Transverse plane
pmissT : vectorial sum of the transverse momenta of primaries (except the parent) and daughters wrt beam direction
p2ryT : transverse momentum of the daughter wrt the parent direction
pmissT : vectorial sum of the transverse momenta of primaries
(except the parent) and daughters wrt beam direction
ϕlH : angle between the parent and the vectorial sum of the other primaries in the transverse plane
p2ryT : transverse momentum of the daughter wrt the parent direction
P2ry : scalar sum of the daughters’ momentum
θkink : average 3D angle between the parent and its daughters
m : invariant mass of the daughters
θkink τ ντ τ
ϕlH
pmissT
zdec
Emulsion film

11. 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
mom cut at 15 GeV/c ->
loss of muonic tau decays only 4%, reduce numuCC 33%, charm 28%.
5408 events analyzed and 5 events observed

12. The 1st candidate: τ→1h The 2nd candidate: τ→3h
Phys. Lett. B 691 (2010)
The 2nd candidate: τ→3h
JHEP 11 (2013) 036

13. The 3rd candidate: τ-→μ-
Physical Review D 89, (R) (2014)
The 4th candidate: τ→1h
PTEP (2014) 101C01
The charge measurement is performed using the bending of the track in the magnetized iron given by the four available RPC ( resistive plate chambers) hits -> negative charge (5.6 sigma significance).

14. The 5th candidate: τ→1h recently reported
New
Transverse plane P1
90 mrad kink with a 960 μm flight length

15. 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 channel
Grain density proportional to energy deposition

16. The 5th candidate: measured parameters
Fully consistent with the expected distributions for signal

17. Summary of all candidate events
psum : the scalar sum of the momenta of all particles measured in the emulsion films
Fully consistent with the expected distribution

18. Background sources
CC interactions with charm production Hadronic interactions Large angle μ scattering
if the primary lepton is not identified and the daughter charge is not measured
Monte Carlo simulation tuned on CHORUS data
Reduced by multi-brick tracking μ- νμ D+
FLUKA and test beam data
Reduced by nuclear fragment search and large angle scanning νμ h
LAS: 6% -> negligible
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/ ) μ- νμ

19. Significance 5 observed events with 0.25 background events expected
Expected signal and background events for the analyzed data sample
5 observed events with 0.25 background events expected
Probability to be explained by background
Fisher 1.1x10-7
Profile likelihood 1.1x10-7
Corresponding to 5.1 σ exclusion of the background-only hypothesis
Systematic uncertainty of the expected signal 20%
- nu_tau cross section 10%
- detection efficiency : stat err of the charm control sample 15%
- atmospheric osci param : dm2 about 8%, sin 6% -> 10%
- neutrino flux 10% but normalized by the detected nu_mu CC -> negligible
Charm background 20%
- cross section 8%, fraction 10%, stat err of the control sample 15%
Probability of observing ≥5 candidates (2.9 S+B events expected): 17%
Frequency of configurations being less probable than the observed one: 6.4%

20. Estimation of Δm223 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
Consistent with more precise measurements by other experiments

21. Conclusions Discovery of ντ appearance
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
multivariate analysis -> 12 signal + 2 bg expected
arxiv.org/abs/
(accepted for publication in Phys. Rev. Lett.)

22. The OPERA collaboration
140 physicists from 26 institutions in 11 countries
Belgium
IIHE-ULB Brussels
Italy
Bari
Bologna
Frascati
LNGS
Naples
Padova
Rome
Salerno
Russia
INR RAS Moscow
LPI RAS Moscow
SINP MSU Moscow
JINR Dubna
Croatia
IRB Zagreb
France
LAPP Annecy
IPHC Strasbourg
Japan
Aichi
Toho
Kobe
Nagoya
Nihon
Germany
Hamburg
Switzerland
Bern
Israel
Technion Haifa
Turkey
METU, Ankara
Korea
Jinju

23. Thank you for your attention

24. Backup slides

25. Measurement of the TeV atmospheric muon charge ratio
Eur. Phys. J. C (2014) 74:2933
The muon charge ratio measured as a function of the vertical surface energy.
Our data are fitted together with the L3+C data (the continuous line).
The dashed, dotted and dash-dot lines are the fit results with the different models for prompt muon production in the atmosphere.
Vertical surface energy

26. Validation of the hadronic background by test beams
Interaction rate
Kink angle (1-prong events)
Dots with error bars : data
Histogram : simulation
Black : data
Red : simulation (FLUKA)
Probability of associated nuclear fragments

27. Muon/hadron separation by “track follow-down"
(primary tracks are followed until the stopping point)
: track length
: range in lead of a muon
: average density along the path
: lead density L
R(p)
<ρ> ρ
A track is classified as a muon
if DTFD is above 0.8
The lower cut for muon identification set at 660 g/cm^2.
To separate muons from hadrons, momentum-range correlations are characterized by a discriminating variable D_TFD.