Nuclear Physics B (Proc. Suppl.) 168 (2007) 173–175www.elsevierphysics.comThe OPERA experimentMaximiliano Sioli a for the OPERA Collaborationa Dipartimento di Fisica dell’Università di Bologna and INFN-Sezione di Bologna,V.le C. Berti Pichat 6/2, I-40127 Bologna, ItalyOPERA is a long baseline neutrino oscillation experiment running in appearance mode. It was built to unambiguouslysettle the atmospheric neutrino anomaly observing ν τ emerging from the CNGS ν µ beam. It is a hybridemulsion/electronic apparatus currently installed in the Hall C of the underground Gran Sasso Laboratories. Wediscuss the main aspects of the experiment and provide some glimpse of the August 2006 run.1. INTRODUCTIONThe widely accepted interpretation of the atmosphericneutrino anomaly - namely the neutrinodeficit as a function of the zenith angle andenergy - is in terms of neutrino flavor oscillations[1–4]. The ν µ → ν e oscillation channel has beenexcluded to be the dominant one [5,6] and also theν µ → ν sterile hypothesis was rejected at 99% C.L.[7,8]. Terrestrial experiments confirmed the oscillationinterpretation of ν µ disappearance [9,10]in the same parameter region indicated by atmosphericneutrino experiments.The final settling of the oscillation hypothesisin the atmospheric sector will be accomplishedby studying the ν µ → ν τ channel in appearancemode. The OPERA experiment  was designedto directly observe the decays of the τ leptonsproduced in ν τ CC interactions. The detector isplaced on the path of the CNGS neutrino beam, 730 km away from the source at CERN 1 .The beam was optimized in order to maximizethe number of ν τ CC interactions at the LNGSsite, keeping the energy constraint to be above theτ production threshold. The result is a wide bandneutrino beam with an average energy of ∼17GeV, sitting in a L/E ν region quite far from theoscillation maximum. The ¯ν µ contamination isabout 4%, the ν e +¯ν e is below 1% and the promptν τ at production is negligible. A nominal beamintensity of 4.5 × 10 19 p.o.t. per year is expected1 For details about the CNGS beam see A. Guglielmi’s contributionin these proceedings.for five year run operations. Accordingly, we expectabout 31000 neutrino induced events in theOPERA target.2. THE OPERA DETECTOR AND ITSPHYSICS PERFORMANCESThe detector is composed of two Super-Modules (SM1 and SM2), each one formed bya target section and a muon spectrometer. Thetarget section is a series of 31 walls followed bytwo scintillator planes (named TT, Target Trackers),with an effective granularity of 2.6 cm oneach transversal coordinate. The main goal ofthe TT planes is to provide a trigger for the neutrinointeractions and to localize the brick wherethe event occurred.Each wall is filled with 3328 bricks, which arethe elemental basic units of the detector. A brickis a pile of 57 emulsion sheets interleaved with 1mm thick lead plates, for a total length of 7.5cm along the beam direction and 10.2 × 12.7cm 2 on the transversal sides. An additional doubletof emulsion sheets (Changeable Sheets, CS)is placed downward each brick to guide the TTpredictions inside the brick itself.An emulsion sheet is a 205 µm plastic basedouble-coated with two 44 µm emulsion films.The passage of a m.i.p. in an emulsion film producesa photographical damage which, under aproper chemical manipulation, may be eventuallyamplified and visualized as a set of alignedsmall grains in the view of an optical microscope.0920-5632/$ – see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.nuclphysbps.2007.02.074
174M. Sioli / Nuclear Physics B (Proc. Suppl.) 168 (2007) 173–175This procedure allows to reconstruct track segmentswith a sub-micrometric position resolutionand an angular resolution of the order of 1 mrad.This is enough for the detection and reconstructionof τ decay topologies. The brick productionstarted in October 2006 with the aim of completingthe full target for the high intensity run of2007.The OPERA spectrometers consist of activedetectors and a dipolar magnet made of two ironwalls interleaved by pairs of high precision trackers(PT). Each wall is made of 12 iron plates5 cm thick, alternating with RPC planes. Themagnetic flux density in the tracking region is1.55 T with vertical field lines of opposite directionsin the two magnet walls. Each spectrometeris equipped with 6 fourfold layers of drifttubes; each tube is 8 m long with an outer diameterof 38 mm. They offer a spatial resolutionof the order of ∼300 µm and a momentum resolution∆p/p ≤ 0.25 for momenta up to p =25GeV/c; the charge misidentification is expectedto be 0.1% − 0.3%. Each of the two drift-tubeplanes of the PT upstream of the dipole magnetis complemented by an RPC plane with two 42.6 ◦crossed strip-layers called XPC’s. Finally, a vetosystem made of glass RPC’s, is placed in front ofthe first SM.The detection efficiency of τ decays was studiedby Monte Carlo simulations. The τ decaysinside the bricks are classified in two categories:long and short decays. Short decays correspondto the case where the τ decays in the same leadplate where the neutrino interaction occurred. Inthis case the τ candidates are selected on the basisof the impact parameter (IP) of the τ daughtertrack with respect to the interaction vertex,which in turn could be reliably determined onlyfor multi-prong deeply inelastic scattering. In thelong τ decays, the decay occurs in the first or seconddownstream lead plate. τ candidates are selectedon the basis of the detection of a reasonablylarge kink angle between the τ and the daughtertrack, both for deeply inelastic and quasi elasticneutrino interactions.For a beam intensity of 4.5 × 10 19 pot/year,the expected number of τ events detected in theemulsions is 12.8 for ∆m 2 =2.4 × 10 −3 eV 2 (or19.9 for ∆m 2 =3× 10 −3 eV 2 ). The backgroundis expected to be less than one event.Due to the good electron identification capability,OPERA will also perform a ν µ → ν e analysis.In this case the limiting factor is the statisticalfluctuation of the background events, whichare dominated by the intrinsic ν e of the beam.If the angle θ 13 is not so far from the CHOOZlimit (sin 2 2θ 13< ∼ 0.14 for ∆m 2 =2.5 × 10 −3 eV 2 ),OPERA has the potential to observe the appearanceof ν e . In the case no excess will be observed,a limit as low as sin 2 2θ 13 ≤ 0.06 may be set.3. THE FIRST RUN WITH CNGS NEU-TRINOSA low intensity run with neutrinos took placefrom 18 to 30 August 2006 with a total integratedintensity of 7.6 × 10 17 pot . The beam wasactive for a time equivalent to about five daysof CNGS running with nominal intensity. TheCNGS-OPERA synchronization is fulfilled by aGPS-based timing system, whose intrinsic accuracyhas been measured to be
M. Sioli / Nuclear Physics B (Proc. Suppl.) 168 (2007) 173–175 175Figure 1. Time distribution of events collectedin the first neutrino run. In the right panel isreported the difference between each event andthe time of the closest extraction.Figure 2. Angular distribution of single-trackevents collected during the first neutrino run(black points). For comparison we superimposedthe prediction of the Monte Carlo simulation.CC interactions in the iron spectrometer.Fig. 2 shows the angular distribution with respectto the horizontal axis selecting single-trackevents with a minimum number of 6 layers of firedRPCs in each spectrometer. In the same figure issuperimposed the simulation of cosmic ray muonswithout beam events. The comparison shows aclear peak of beam-related events around the horizontaldirection. A gaussian fit to the angle ofthe event on-time with the beam (shown in theinset of Fig. 2) yielded a mean muon angle of3.4 ± 0.3 ◦ in agreement with the expected value.During the August run, it was also proved thecapability of the CS doublets to link the cm scaleof the electronic detectors to the µm scaleofthenuclear emulsion detectors.REFERENCES1. K. S. Hirata et al. [KAMIOKANDE-II Collaboration],Phys. Lett. B 205 (1998) 416.2. Y. Fukuda et al. [Super-Kamiokande Collaboration],Phys. Rev. Lett. 81 (1998) 1562;J. Hosaka et al., Phys. Rev. D 74 (2006)032002; K. Abe et al., arXiv:hep-ex/0607059,submitted to Phys. Rev. Lett.3. S. P. Ahlen et al. [MACRO Collaboration],Phys. Lett. B 357 (1995) 481; M. Ambrosioet al., Phys. Lett. B 434 (1998) 451; M. Ambrosioet al., Eur. Phys. J. C 36 (2004) 323.4. W. W. M. Allison et al. [SOUDAN2 Collaboration],Phys. Lett. B 449 (1999) 137;W. W. M. Allison et al., Phys. Rev. D 72(2005) 052005.5. M. Apollonio et al. [CHOOZ Collaboration],Eur. Phys. J. C 27 (2003) 331.6. F. Boehm et al. [Palo Verde Collaboration],Phys. Rev. D 64 (2001) 112001.7. M. Ambrosio et al. [MACRO Collaboration],Phys. Lett. B 517 (2001) 59.8. S. Fukuda et al. [Super-Kamiokande Collaboration],Phys. Rev. Lett. 85 (2000) 3999.9. M. H. Ahn et al. [K2K Collaboration], Phys.Rev. D 74 (2006) 072003.10. D.G. Michael et al. [MINOS Collaboration],arXiv:hep-ex/0607088, submitted to Phys.Rev. Lett.11. M. Guler et al. [OPERA Collaboration],CERN-SPSC-2000-028; Y. Declais et al.,CERN-SPSC-2002-029 SPSC-059.12. CNGS project:http://proj-cngs.web.cern.ch/proj-cngs/.13. R. Acquafredda et al. [OPERA Collaboration],New J. Phys. 8 (2006) 303.