The OPERA neutrino oscillation experiment Paolo Strolin University

The OPERA neutrino oscillation experiment Paolo Strolin University and INFN, Napoli

What is “Neutrino oscillation” Pontecorvo, 1957 Maki, Nakagawa and Sakata, 1962 Pontecorvo and Gribov, 1969 • A phenomenon induced by m > 0 m = 0 in “Standard Model” of elementary particles • The observed e and are “flavour eigenstates”, related by a mixing matrix to the invisible “mass eigenstates” 1 2 and 3 • At a large distance from the source, it implies an apparent “metamorphosis” of a flavour into a different one (e. g. ) • A very sensitive method to measure m i 2

Atmospheric neutrino oscillations disappearance (interpreted as → ) observed by Super-Kamiokande best fit: m 2 = 2. 4 10 -3 e. V 2, sin 2 2 = 1 Independently observed by MACRO Confirmed by K 2 K and MINOS still missing: direct observation (“appearance”) of oscillated

OPERA ● Main aim: unambiguous evidence for oscillation in the parameter region indicated by the atmospheric neutrino disappearance ● Search for appearance in a pure beam at a long baseline (CNGS beam, = 17 Ge. V, L = 732 km) ● Detection of CC interactions with the unstable lepton identified by the direct observation of its decay ● Hybrid set-up (nuclear emulsions + electronic detectors) needed to observe decays

The CNGS beam LNGS 730 km CERN Beam main features L 730 km 17 Ge. V L/ 43 km/Ge. V ( e+ e)/ 0. 87% / 2. 1% prompt negligible “off peak” Limiting for → e searches Event rate for OPERA (~1. 3 kton) with 4. 5· 1019 pot (200 days/year) ~ 6200 (CC + NC) / year ~ 25 CC/year for ∆m 2 = 2. 4· 10 -3 e. V 2

Laboratori Nazionali del Gran Sasso - INFN OPERA Roma • Neutrino physics (mass, oscillations, stellar physics) • Dark matter • Nuclear reactions of astrophysics interest • Gravitational waves • Geophysics • Biology • 1400 m rock coverage • cosmic µ reduction factor = 10– 6 → 1/m 2 h • underground area: 18 000 m 2 • 756 scientists from 25 countries

To identify leptons, “see” their decay “kink” decay kink ~ m m 0. 6 The challenge rare process massive target AND see the decay m resolution Lead – nuclear emulsion sandwich “Emulsion Cloud Chamber” ( ECC)

The Emulsion Cloud Chamber (ECC) 1 mm • Emulsions for tracking, passive material as target < m space res. mass – Established technique • charmed “X-particle” first observed in cosmic rays (1971) • DONUT/FNAL beam-dump experiment: observed (2000) Pb Emulsion layers • m 2 = O (10 -3 e. V 2 ) Mtarget ~ O (kton) – modular structure (“bricks”): basic performance is preserved – large detector sensitivity, complexity – required: “industrial” emulsions, fast automatic scanning • The dense lead-emulsion sandwich allows – electron identification by showering – momentum measurement by Multiple Coulomb Scattering track segments

The OPERA Collaboration IPNL, IPHC, LAPP Bern Neuchatel ETH Zurich Brussels Hamburg, Münster, Rostock Zagreb Sofia INR ITEP, MSU, LPI JINR, Obninsk Tunis Aichi, Toho Kobe, Nagoya Utsunomiya L’Aquila, Bari, Bologna, Napoli, Padova, Roma, Salerno, LNF, LNGS Technion Haifa Gyeongsang University METU Ankara 36 INSTITUTIONS, ~160 PHYSICISTS

The OPERA ”hybrid” detector Target • lead/emulsion ECC bricks • emulsion tracking resolution: x< 1μm 1 mrad • 154, 750 “bricks” ~ 1. 3 kton • planes of scintillator strips SM 1 SM 2 10 m Emulsion layers 1 mm 10 m 20 m Muon Spectrometers Pb 1 brick: 8. 3 kg • Magnetized iron slabs. • Vertical drift tubes for momentum measurement from deflection through 24 magnet instrumented with 22 RPC planes • ∆p/p~20% up to 50 Ge. V

Lead – Emulsion target Brick (basic module) 56 Pb 1 mm thick plates 57 emulsions films 10 X 0 8. 3 kg Changeable Sheet (CS) Doublet of emulsion films with very low background, attached to the brick, as interface to electronic detectors Emulsion film (Fuji-Nagoya) 10. 2 cm 12. 5 cm emulsion layers (43 m thick) on both sides of a 200 m thick plastic base Bricks assembled in walls Total target mass : 1300 t wall 52 x 64 bricks

Brick Assembly Machine (BAM) at LNGS Robots produce bricks at a rate of 700 bricks / day

Brick Manipulator System (BMS) Robot for brick insertion (target filling) and removal (during run) Ventouse Vehicle Carousel mechanism Brick

An event in OPERA Target Trackers μ spectrometer Pb/Em. target Extract selected brick with CS Pb/Em. brick Basic “cell” Electronic detectors select ν interaction brick and CS μ ID, charge and p 8 cm Pb Emulsion ντ 1 mm Emulsion analysis vertex search decay search e/μ ID, kinematics

Automated Emulsion Scanning Started with the pioneering work in Nagoya ~ 30 bricks will be daily extracted from the target and analyzed quasi-online using high-speed automatic systems in Several labs in Europe and Japan European Scanning System S-UTS (Japan) High speed CCD Camera (3 k. Hz) Piezo-controlled objective lens Synchronization of objective lens and stage Constant speed stage Customized commercial optics and mechanics + asynchronous DAQ software Hardware-coded algorithms Continuous movement of emulsion

Microscope field of view 43 m emulsion sheet 16 tomographic images 3 D reconstruction of particle tracks 200 m Passing-through tracks rejection Track segments found in 8 consecutive films Automatic microscopy: from a microscope field of view to a reconstructed vertex Vertex reconstruction

Search for oscillation: expected number of events decay channels Signal m 2 = 2. 4 x 10 -3 e. V 2 m 2 = 3. 0 x 10 -3 e. V 2 Background µ 2. 9 4. 2 0. 17 e 3. 5 5. 0 0. 17 h 3. 1 4. 4 0. 24 3 h 0. 9 1. 3 0. 17 ALL 10. 4 15. 90 0. 76 Main background sources: - charm production and decays full mixing, 5 years run @ 4. 5 x 1019 pot / year - hadron re-interactions in lead - large-angle muon scattering in lead

OPERA sensitivity Discovery probability (%) Discovery probability vs m 2 4 -s evidence 3 -s evidence Exclusion plot at 90% C. L

First OPERA Physics Run Sept. 24 - Oct. 22, 2007 • A short run: 0. 0824*1019 protons on target with 1. 8*1013 p/extraction 3. 6 effective days at nominal proton intensity 4. 5*1013 • On October 3 rd observed the first event in the target • 365 events in the whole detector selected analyzed 331 events passed through the analysis cuts (303 expected) • 38 events (31. 5 expected) in the neutrino target Their study is ongoing

Time Selection of Beam Events Beam GPS Tflight = 2. 44 ms TCERN SPS extraction TOPERA Event Time Cosmic ray background TOPERA - TCERN resolution ~ 100 ns

In target events in the electronic detectors (top: display in the whole detector; bottom: display in the target) MIAMI 2007 22

EVENT 179673325 in el. det. and ECC quasi-elastic-like topology p (by ionization) ECC IP = 0. 33 m Slope. X=-0. 040 Slope. Y=0. 014 IPh = 4. 50 m Slope. X=-0. 380 Slope. Y=-0. 602

EVENT 178969961 in el. det. and ECC Track a ECC b IP ( m) A -0. 028, 0. 095 6 57 5. 2 B -0. 114 , 0. 143 (7? ) 57 0. 3 0. 091, 0. 009 57 6. 2 -0. 166, -0. 342 32 21. 4 E 0. 127, 0. 075 35 16. 4 -0. 370, 0. 430 28 15. 3 Shower 0. 019, -0. 006 38 - c C e f Stop film F CS track D d Slope 6 prongs 2, 8

EVENT 180400976 in el. det. and ECC Track Slope. X Slope. Y Length IP 1 0. 026 -0. 097 30 -33 5. 7 2( ) -0. 024 0. 019 30 -56 1. 2 3 0. 045 0. 197 30 -52 1. 7 4 0. 024 0. 203 30 -52 2. 6 5 0. 100 0. 151 30 -51 7. 9 6 -0. 047 0. 199 31 -45 2. 9 7 0. 073 0. 152 31 -45 8. 8 8 0. 085 0. 254 31 -43 10. 3 9 0. 074 0. 201 30 -33 4. 6 Track 1: kink between plates 33 and 34 Flight length of parent track 5 mm ECC Kink attributed to hadronic interaction

Conclusions § Main aim: prove the → interpretation of the atmospheric disappearance, by observing → appearance § First physics run 24 Sept. - 22 Oct. 2007 § The whole chain from data acquisition to event analysis has been tested § The studies on the acquired events are ongoing § Wait for the 2008 physics run and the 1 st appearance !

“Neutrino oscillations” Pontecorvo, 1957 Maki, Nakagawa and Sakata, 1962 Pontecorvo and Gribov, 1969 • A phenomenon induced by m > 0 • Implies the apparent “metamorphosis” of a into a different one (e. g. ) • A very sensitive method to measure m 2 The PMNS matrix Atmospheric CP violation phase Solar, reactor analysis Terms probed by next experiments

September 24 th to October 22 nd 2007: first OPERA Physics Run CNGS run parameters On October 3 rd OPERA observed the first event in the target During the run we have accumulated 0. 0824*1019 pot onto the target with a mean value of 1. 8*1013 proton per extraction: this represents 3. 6 effective days for the run! SPS super-cycle with 3 CNGS cycles every 39. 6 s

Data analysis and event selection Data are extracted from the DAQ data base every 12 hours and processed with the OPERA reconstruction package. In time events are selected and stored on a dedicated file. Then in time events are scanned visually and stored by categories: • magnet interaction • rock muon • in target CC and NC like (according to the muon id) The interactions in the material surrounding OPERA target are analysed separately and are used for the monitoring on the CNGS beam and of the OPERA detector

Beam events CC event originated upstream of the detector (BOREXINO, rocks) CC event originated in the first magnet

Data analysis and event selection Main physical distribution for the selected events The Vertex location is defined as the first hit of the 3 D reconstructed track The vertical angle of the track. Fit: 3. 4± 0. 2 degree The Energy of the muon as measured with the RPC of the spectrometer

In target events analysis We have observed 38 events (31. 5 expected) in the emulsion target area • 22 charged current • 9 neutral current compatible with the relative cross sections The events are analysed through the brick finding package: • a neural network for the wall finding • the hadronic shower parametrization • the exact configuration and alignment of the target at the time of the event taking into account the wall elongation as a function of the filling • the probability estimate for the vertex location in each brick

Target Tracker 1. 7 m 7 m 6. 9 m 7 m Plastic scintillator strips (AMCRYS-H, 6. 7 m x 2. 6 cm x 1 cm) readout by Kuraray WLS fibres + Hamamatsu PMT’s (64 channels) Target Tracker tasks ● Trigger: > 99% ● Initiate muon tagging MIAMI 2007 31 walls Alberto Marotta - INFN Napoli 35

Events Time Structure OPERA • Beam events are selected correlating events GPS time information with CERN beam spill time • 319 events on time selected Cosmic ray background Extraction length = 10. 5 sec

Angular distribution of all events Angle w. r. t. horizontal Cosmic µ: mountain rock structure asymmetry coherently with previous MACRO observation Zoom on Beam events Coming 3. 5° from below Azimuth angle