ICARUS-T600 at the INFN-LNGS Gran Sasso Laboratory is the

first underground large-mass Liquid-Argon TPC: exposed to the CNGS neutrino beam from CERN, it has been taking data since May 2010. Thanks to its excellent resolution and 3D imaging, it allows an unprecedented event visualization quality combined with a good calorimetric reconstruction and the electronic event processing. After a first commissioning phase, it has started an interesting physics program, ranging from νμ → ντ oscillation search in appearance to matter stability study, thus demonstrating the feasibility and effectiveness of the Liquid-Argon TPC technique. Furthermore, ICARUS-T600 represents a major milestone towards the realization of much larger Liquid-Argon detectors for future rare events physics. The idea to

use a LAr-TPC experiment at a refurbished CERN-PS neutrino beam is presented as a possible solution to the sterile neutrino puzzle

Fava, A.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2012-11143-7Colloquia: LaThuile11IL NUOVO CIMENTO Vol. 35 C, N. 1 Gennaio-Febbraio 2012Icarus and status of Liquid-Argon technologyA. Fava for the ICARUS CollaborationINFN, Sezione di Padova - Via Marzolo 8, I-35131, Padova, Italy(ricevuto il 29 Settembre 2011; pubblicato online il 25 Gennaio 2012)Summary. — ICARUS-T600 at the INFN-LNGS Gran Sasso Laboratory is thefirst underground large-mass Liquid-Argon TPC: exposed to the CNGS neutrinobeam from CERN, it has been taking data since May 2010. Thanks to its excellentresolution and 3D imaging, it allows an unprecedented event visualization qualitycombined with a good calorimetric reconstruction and the electronic event process-ing. After a first commissioning phase, it has started an interesting physics program,ranging from νμ → ντ oscillation search in appearance to matter stability study, thusdemonstrating the feasibility and effectiveness of the Liquid-Argon TPC technique.Furthermore, ICARUS-T600 represents a major milestone towards the realizationof much larger Liquid-Argon detectors for future rare events physics. The idea touse a LAr-TPC experiment at a refurbished CERN-PS neutrino beam is presentedas a possible solution to the sterile neutrino puzzle.PACS 95.55.Vj – Neutrino, muon, pion, and other elementary particle detectors;cosmic ray detectors.PACS 13.15.+g – Neutrino interactions.PACS 07.05.Fb – Design of experiments.1. – LAr TPC technique: working principle and performanceThe idea of a Liquid-Argon Time Projection Chamber (LAr-TPC) was first proposedby C. Rubbia in 1977 [1] as a powerful detection technique to provide a 3D imaging of anyionizing event. Nowadays LAr-TPC can be considered the heir of the bubble chamberdetector, because of the high granularity and excellent spatial resolution and calorimetricproperties; but it has some interesting additional features, being continuously sensitiveand self-triggering detector, and the advantage of being scalable to bigger masses.The working principle of the LAr-TPC (fig. 1) is based on the two processes bywhich charged particles loose energy in liquid argon, i.e. scintillation and ionization.Scintillation light, emitted in the infrared at 128 nm wavelength with 5000 γ/mm yeld,provides a prompt signal made by a fast (∼ 6 ns) and a slow (∼ 1.6μs) component. Bymeans of ionization, instead, each charged particle produces ∼ 6000 electrons per mm,which are drifted by a uniform and intense electric field towards several (transparent)c© Societa` Italiana di Fisica 447448 A. FAVA for the ICARUS COLLABORATIONFig. 1. – Simple scheme of the LAr-TPC working principle: from the interaction of the incomingparticle with LAr to the readout system.wire arrays where the signal is recorded in a nondestructive way, ensuring a redundanttridimensional track reconstruction. A key feature for LAr-TPC experiments is the levelof purity of liquid argon, since electro-negative molecules (mainly O2, H2O and CO2)could eventually capture the drifting electrons before they reach the anode plane.The high resolution and granularity of this detection technique (less than 1mm3)allows a precise reconstruction of events topology and the recognition of the particlesproduced in interactions in LAr. Calorimetric measurement is also possible over a verywide energy range, from MeV to several tens of GeV, allowing particle identification viadE/dx ionization signal.For long muon tracks escaping the detector, momentum is determined exploiting theirmultiple scattering by a Kalman filter algorithm, i.e. studying the track displacementswith respect to a straight line; this procedure, validated on cosmic rays stopping muons,allows a resolution Δp/p down to 10%, depending mainly on the track length [2]. Elec-trons, instead, are identified with full efficiency by the characteristic electromagneticshowering, well separated from π0 combining γ reconstruction, dE/dx signal comparisonand π0 invariant-mass measurement at the level of 10−3 [3]. This feature guarantees a90% efficiency identification of the leading electron in νe CC interactions, while rejectingNC interactions to a negligible level.The electromagnetic energy resolution σ(E)/E = 0.03/√E(GeV)⊕0.01 is estimatedin agreement with the π0 → γγ invariant mass measurements in the sub-GeV energyrange. The measurement of the Michel electron spectrum from muon decays, wherebremsstrahlung photons emission is taken into account [4], provided the energy resolutionbelow critical energy (∼ 30MeV), σ(E)/E = 0.11/√E(MeV)⊕ 0.02. At higher energiesthe estimated resolution for hadronic showers is σ(E)/E = 0.30/√E(GeV). However theLAr-TPC detector allows to identify and measure, track by track, each hadron producedin interactions, through ionization and range, leading to a much better energy resolution.2. – ICARUS-T600 experimentThe ICARUS T600 LAr-TPC detector, presently taking data in the Hall B of theINFN Gran Sasso underground National Laboratory (LNGS), is the largest Liquid-ArgonTPC ever built, with the cryostats containing more than 600 tons of LAr. Its detectiontechnique offers the possibility to collect “bubble chamber like” events to address a widephysics program: the main goal is the observation of the νμ → ντ oscillation in the CNGSneutrino beam from CERN to Gran Sasso, but this detector can also be used to studysolar and atmospheric neutrino events and to improve the limit on proton decay in somebackground free channels.ICARUS AND STATUS OF LIQUID-ARGON TECHNOLOGY 449Fig. 2. – The ICARUS T600 detector in Hall B at the LNGS underground laboratory (left) anda simple sketch of the inner TPCs structure (right).2.1. Detector overview . – The ICARUS-T600 detector [5] (see fig. 2) consists of alarge cryostat split into two identical, adjacent and independet half-modules, with anoverall volume of about 760 tons of ultra-pure liquid Argon at 89K temperature. Eachhalf-module, with internal dimensions 3.6 × 3.9 × 19.6m3, houses two Time ProjectionChambers (TPC) separated by a common cathode. The anode of each TPC is madeof three parallel wire planes, 3mm apart, oriented at 0◦ and ±60◦ w.r.t. the horizontaldirection: in all 53248 wires, with length up to 9m, are installed. By appropriate voltagebiasing, the first two planes (Induction-1 and Induction-2 planes) are transparent to driftelectrons and measure them in a non-destructive way, whereas the ionization charge isfinally collected by the last one (Collection plane). The application of an electric fieldED = 500V/cm, kept uniform by appropriate field shaping electrodes, ensures that the1.5m maximum drift distance is covered in 1ms. The signals coming from each wire arecontinuously read and digitized at 25MHz (thus 1 t-sample ∼ 400 ns) and recorded inmulti-event circular buffers.A prompt detection of the scintillation light is also necessary to determine the absolutetime of the ionizing events. For this purpose arrays of Photo Multiplier Tubes (PMTs),operating at the LAr cryogenic temperature [6] and made sensible to VUV scintillationlight (= 128 nm) by applying a wavelength shifter layer (TPB), are installed behind thewire planes.2.2. The Liquid-Argon purity . – The main technological challenge in the develop-ment of a large mass LAr-TPC is the capability to ensure and maintain a high LArpurity level. In ICARUS-T600 detector an elaborate cryogenic plant, comprehensive ofOxysorb/Hydrosorb filters, performs both gas and liquid recirculation to reduce and keepat an exceptionally low level the electro-negative impurities, especially water and oxygen,in order to obtain free electron lifetime of several milliseconds.The electron lifetime is continuously monitored studying the attenuation of the chargesignal as a function of the drift time along “clean” through-going muon tracks in Collec-tion view, i.e. straight tracks without clear δ-rays and associated γ’s; the negative signalinduced by the PMTs on the wires (fig. 3) marks the time at which the track entered thedetector. About 50 muon tracks are sufficient to measure day-by-day the electron chargeattenuation within a 3% precision, dominated by residual Landau charge fluctuations.450 A. FAVA for the ICARUS COLLABORATIONFig. 3. – A muon track candidate for the purity measurement (right) and three different wiresignals (left) in Collection view. The t = 0 signal, induced by PMTs on the wires, are circled.With the liquid recirculation turned on, the LAr purity steadily increased, reachingvalues of free electron lifetime (τe) exceeding 6ms in both half-modules after few monthsof operation (fig. 4). This corresponds to 0.05 ppb O2 equivalent impurity concentration,producing a maximum 16% charge attenuation, at the maximum 1.5m drift distance.2.3. Physics programme. – ICARUS-T600 is the major milestone towards the real-ization of a multikiloton LAr-TPC detector [7], but it can also address some interestingphysics in itself, thanks to its high resolution, ∼mm3 granularity, information redun-dancy and particle identification capability [8].The main goal is the search for νμ → ντ oscillation in the CNGS beam, i.e. a beamalmost pure in νμ with average energy Eν ∼ 17.4GeV, traveling over 732 km from CERNto Gran Sasso. ICARUS-T600 looks for ντ appearance in the electron decay channelτ → eνν of the τ produced by ντCC interaction in LAr: in 2011–2012 run almost 3000νμCC interactions are expected (1.1 · 1020 pot), leading to 3 τ → e events over 7 νeCCFig. 4. – Free electron lifetime monitoring in 2010 run, for both cryostats.ICARUS AND STATUS OF LIQUID-ARGON TECHNOLOGY 451background events due to the intrinsic beam contamination. The background can berejected with kinematical selection criteria based on missing transverse momentum, thuseventually allowing to detect 1-2 ντ CNGS events in next 2 years. On the same beamthe search for sterile neutrinos in LNSD parameter space can be also performed, lookingat an excess of νe CC events.ICARUS-T600 is studying also neutrinos from natural sources: ∼ 80 unbiased CCatmospheric neutrino interactions are expected per year, and solar electron neutrinointeractions with energy greater than 8MeV can be detected.Finally, thanks to the powerful background rejection and its 3 × 1032 nucleons,ICARUS-T600 can play a role in the long sought for proton decay search, in particularin interesting exotic channels not accessible to Cˇerenkov detectors. With an exposureof a few years its sensitivity on some “super-symmetric favored” nucleon decay channelswill exceed the present known limits.2.4. 2010 physics run. – ICARUS-T600 started its operation in May 2010, after a longR&D and installation phase, collecting right from the beginning both cosmic rays andCNGS neutrino events. The trigger system relies on the scintillation light signals, witha starting layout based, for each of the four TPC chambers, on the analog sum of signalfrom PMTs with a 100 photo-electron discrimination threshold. The trigger for cosmicrays exploits the coincidence of the PMTs sum signals of the two adjacent chambers in thesame half-module, relying on the 50% transparency of the cathode mechanical structure:this allows an efficient reduction of the spurious signals maximizing the detection oflow energy events. An overall acquisition rate of 25mHz has been achieved well belowthe maximum allowed DAQ rate, resulting in about 83 cosmic events per hour. ForCNGS neutrino events the proton extraction time information is also available, sincean “early warning” signal is sent from CERN to LNGS 80ms before the first protonspill extraction. Thus, accounting for the CNGS SPS cycle structure, i.e. two spills50ms apart and lasting 10.5μs each, a dedicated trigger strategy has been chosen for theCNGS neutrino interactions, based on the presence of the PMT signal within a ∼ 50μsgate opened in correspondence to the predicted extraction times delayed by the neutrinotime of flight (2.44ms) from CERN to LNGS. A trigger rate of about 1mHz is obtained,including neutrino interactions inside the detector and muons from neutrino interactionsin the upstream rocks.The CNGS run started in stable conditions on October 1st and continued till thebeam shutdown, on November 22nd; in this period 5.8 ·1018 pot were collected out of the8 ·1018 delivered by CERN, with a detector lifetime up to 90% since November 1st (fig. 5,top). The 78% of the whole collected sample of events, corresponding to 4.52 · 1018, hasbeen preliminarly analyzed: 94 νμCC and 32 NC events have been identified by means ofvisual scanning into a 434 tons fiducial volume, while 6 events need for further analysisto be classified (being at edges, with μ track too short do be visually recognized); thisresult is in full agreement with the number of interactions predicted in the whole energyrange up to 100GeV ((2.6 νμ CC + 0.86 ν NC) ·10−17/pot), to be corrected for fiducialvolume and DAQ dead-time.The analysis of the time distributions of this event sample, compared with the CNGSproton extraction time, allows to reconstruct the 10.5μs spill duration (fig. 5, bottom),suggesting an excellent precision in the events timestamp.The neutrino interaction events are then fully 3D reconstructed: muons, pions, pro-tons and kaons are identified by studying the event topology and the energy depositionper track length unit as a function of the particle range (dE/dx versus range) with452 A. FAVA for the ICARUS COLLABORATIONFig. 5. – Top: number of pot collected by ICARUS-T600 in the Oct.1st–Nov.22nd run comparedwith the beam intensity delivered by CERN. Bottom: distribution of the difference between theneutrino interaction timestamp and the corresponding CNGS proton extraction time.a dedicated reconstruction program based on the polygonal line algorithm [9] and onneural network. Electrons are recognized by the characteristic electromagnetic shower-ing. Momentum of long muon tracks escaping the detector momentum is determined bymultiple scattering. An example of event reconstruction is reported in fig. 6.3. – A two LAr-TPC experiment at a CERN-PS neutrino beam to solve thesterile neutrino puzzleRecently a sterile neutrino puzzle is growing, catching the interest of the particlephysics community, due to the increasing number of experimental anomalies. On oneside the 3.8 σνe excess signal in a νμ beam first observed by LSND has been confirmedby the MiniBooNE experiment, suggesting a possible νμ → νe oscillation with largeΔm2 (0.2 < Δm2 < 2.0 eV2, sin2 2θ < 10−3) beyond the 3ν flavour oscillation schemeFig. 6. – Example of a fully reconstructed CNGS νμCC event, collected in 2010 run.ICARUS AND STATUS OF LIQUID-ARGON TECHNOLOGY 453Fig. 7. – Top: sensitivity of the CERN-PS experiment to the νe appearance signal in 2 years(left) and 4 years (right) data taking with in neutrino and antineutrino beam mode, respectively.Bottom: sensitivity to the disappearance reactor anomaly at the far detector.as observed in solar/atmospheric neutrino experiments. On the other side, a recent re-evaluation of the νe reactor spectra (∼ 3% flux increase) brought out a νe deficit at manyshort-baseline reactor experiments and revived the SAGE/GALLEX νe deficit from theMegaCurie radioactive source, hinting at a fast disappearance rate (Δm2 > 1.5 eV2,0.02 < sin2 2θ < 0.23 at 99.7% C.L.). Finally, the latest WMAP data seem not toexclude, or even to prefer, a scenario with more than 3 neutrinos.To clarify the situation a definitive experiment is envisaged: the proposal to use2 strictly identical LAr-TPC detectors on a refurbished neutrino beam at the CERN-PS represents a possible solution [10]. The neutrino beam would be a low energy νμbeam produced by 19.2GeV protons of at least 1.25 · 1020 pot/y intensity. The fardetector, located at ∼ 850m from the target, could be ICARUS-T600 itself, while thenear detector, at ∼ 127m, would be a ∼ 150 tons active mass LAr-TPC (possibly a cloneof a ICARUS-T600 semi-module with length reduced by a factor 2). The LAr-TPCtechnique appears the ideal detector for the study of low-energy neutrino events thanksto its very high νe detection efficiency combined with an extremely high rejection ofassociated NC background events. Moreover the usage of two identical detectors togetherwith the very similar intrinsic νe spectra in the two positions, ensure the canceling out454 A. FAVA for the ICARUS COLLABORATIONof experimental and cross-section biases. It would thus be possible to perform the searchfor both νμ → νe LSND appearance signal and νe → νx reactor disappearance anomaly,with promising sensitivity in only 2(4) years data taking in ν(ν) mode (fig. 7).4. – ConclusionsThe ICARUS-T600 detector, installed underground at the LNGS laboratory, hasstarted data taking during 2010 after a long R&D and installation phase. The successfulassembly and operation of this LAr-TPC is the experimental proof that this techniqueis mature. It has demonstrated to have unique imaging capability, spatial and calori-metric resolutions and the possibility to efficiently distinguish electron from π0 signals,thus allowing to reconstruct and identify events in a new way with respect to the otherneutrino experiments.After a short commissioning phase this experiment is ready for the 2011–2012 run,addressing a wide physics programme. The main goal is to collect events from the CNGSneutrinos beam from CERN-SPS to search for the νμ → ντ oscillation and LSND-like νeexcess, but also to study solar and atmospheric neutrino and explore in a new way thenucleon stability in particular channels beyond the present limits.Furthermore ICARUS-T600 is so far the major milestone towards the realization ofa much more massive LAr detector. Actually the employment of this technique at arefurbished CERN-PS ν beam has been proposed after the ICARUS-T600 exploitationat LNGS to definitely solve the sterile neutrino puzzle.REFERENCES[1] Rubbia C., The Liquid-Argon Time Projection Chamber: A new Concept for NeutrinoDetector, CERN-EP/77-08 (1977).[2] Ankowski A. et al. (ICARUS Collaboration), E. Phys. J. C, 48 (2006) 667.[3] Ankowski A. et al. (ICARUS Collaboration), Acta Phys. Polon. B, 41 (2010) 103.[4] Amoruso S. et al. (ICARUS Collaboration), E. Phys. J. C, 33 (2004) 233.[5] Amerio S. et al. (ICARUS Collaboration), Nucl. Instrum. Methods A, 527 (2004)329.[6] Ankowski A. et al.(ICARUS Collaboration), Nucl. Instrum. Methods A, 556 (2005)146.[7] Baibussinov B. et al., Astropart. Phys., 29 (2008) 174.[8] Arneodo F. et al. (ICARUS Collaboration), ICARUS initial physics program, LNGSP28/01, LNGS-EXP 13/89 add. 1/01.[9] Kegl B., Krzyzak A., Linder T. and Zeger K., IEEE Trans. Pattern Anal. MachineIntell., 22 (2000) 281.[10] Baibussionov B. et al., A new search for anomalous neutrino oscillations at the CERN-PS, arXiv:0909.0355; Rubbia C. (ICARUS/CNGS2-P323 Collaboration), PhysicsProgramme for ICARUS after 2012, CERN-SPSC-2011-012, SPSC-M-773 (2011).

Icarus and status of Liquid-Argon technology

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Icarus and status of Liquid-Argon technology

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