“LSND Anomaly” with the ICARUS Detector in the CNGS Neutrino Beam

Eur. Phys. J. C manuscript No. (will be inserted by the editor)

Experimental search for the “LSND anomaly” with the ICARUS detector in the CNGS neutrino beam

M. Antonello1, B. Baibussinov2, P. Benetti3, E. Calligarich3, N. Canci1, S. Centro2, A. Cesana4, K. Cie´slik5, D.B. Cline6, A.G. Cocco7, A. Dabrowska5, D. Dequal2, A. Dermenev8, R. Dolfini3, C. Farnese2, A. Fava2, A. Ferrari9, G. Fiorillo7, D. Gibin2, S. Gninenko8, A. Guglielmi2, M. Haranczyk5, J. Holeczek10, A. Ivashkin10, M. Kirsanov8, J. Kisiel10, I. Kochanek10, J. Lagoda11, S. Mania10, A. Menegolli3, G. Meng2, C. Montanari3, S. Otwinowski6, A. Piazzoli3, P. Picchi12, F. Pietropaolo2, P. Plonski13, A. Rappoldi3, G. L. Raselli3, M. Rossella3, C. Rubbia1,9, P. R. Sala4, E. Scantamburlo1, A. Scaramelli4, E. Segreto1, F. Sergiampietri14, D. Stefan1, J. Stepaniak11, R. Sulej11,1, M. Szarska5, M. Terrani4, F. Varanini2, S. Ventura2, C. Vignoli1, H.G. Wang6, X. Yang6, A. Zalewska5, K. Zaremba13 1 INFN - Laboratori Nazionali del Gran Sasso, Assergi, Italy 2Universitàdi Padova e INFN, Padova, Italy 3Universitàdi Pavia e INFN, Pavia, Italy 4Politecnico di Milano e INFN, Milano, Italy 5H.NiewodniczańskiInstitute of Nuclear Physics, Kraków,Poland 6Department of Physics, UCLA, Los Angeles, USA 7UniversitàFederico II di Napoli e INFN, Napoli, Italy 8Institute for Nuclear Research of the Russian Academy of Sciences, Moscow, Russia 9CERN, Geneva, Switzerland 10A.So ltanInstitute for Nuclear Studies, Warszawa, Poland 11Institute of Physics, University of Silesia, Katowice, Poland 12INFN Laboratori Nazionali di Frascati, Frascati, Italy 13Institute for Radioelectronics, Warsaw Univ. of Technology, Warsaw, Poland 14Universitàdi Pisa e INFN, Pisa, Italy Received: date / Revised version: date

Abstract We report an early result from the ICARUS are set respectively. The result strongly limits the win- experiment on the search for a νµ → νe signal due to dow of open options for the LSND anomaly to a narrow 2 2 2 the LSND anomaly. The search was performed with the region around ∆m , sin (2θ) new = (0.5 eV , 0.005), ICARUS T600 detector located at the Gran Sasso Lab- where there is an overall agreement (90% CL) between oratory, receiving CNGS neutrinos from CERN at an the present ICARUS limit, the published limits of KAR- average energy of about 20 GeV, after a flight path of MEN and the published positive signals of LSND and ∼730 km. The LSND anomaly would manifest as an ex- MiniBooNE Collaborations. arXiv:1209.0122v4 [hep-ex] 19 Feb 2013 cess of νe events, characterized by a fast energy oscilla- 2 2 PACS Neutrino Ocillations · Sterile neutrinos tion averaging approximately to sin (1.27∆mnewL/Eν ) 2 ≈ 1/2 with probability Pνµ→νe = 1/2 sin (2θnew). The present analysis is based on 1091 neutrino events, which 1 Introduction are about 50% of the ICARUS data collected in 2010- 2011. Two clear νe events have been found, compared The possible presence of neutrino oscillations into ster- with the expectation of 3.7 ± 0.6 events from conven- ile states has been proposed by B. Pontecorvo [1]. An tional sources. Within the range of our observations, experimental search for an anomalousν ¯e production at this result is compatible with the absence of a LSND short distances has been performed by the LSND ex- anomaly. At 90% and 99% confidence levels the limits of periment [2] at the Los Alamos 800 MeV proton ac- 3.4 and 7.3 events corresponding to oscillation probabil- celerator, which reported an anomalous excess ofν ¯e −3 −2 ities Pν →ν ≤ 5.4×10 and Pν →ν ≤ 1.1×10 µ e µ e fromν ¯µ originated by muons from pions at rest with 2 hEν i ≈ 30 MeV and L ≈ 30 m. It is well known that According to detailed Monte Carlo (MC) calcula- anti-neutrino oscillations at such a small distance from tions of the neutrino beam [10], about 2850 charged the source should imply the presence of additional mass- current (CC) events/kt/year are expected at LNGS for squared differences, largely in excess of the three neu- a nominal proton beam intensity of 4.5 × 1019 pot/year trino mixing standard model values. The LSND signal with a spectral contamination from anti-neutrino of −3 Pν¯µ→ν¯e = (2.64 ± 0.67 ± 0.45) × 10 corresponds to about 2% and an electron component of slightly less a rate of (87.9±22.4±6.0) events, namely a 3.8 σ effect than 1%. The neutrino flux and spectra expectations at L/Eν ∼ 0.5 − 1.0 m/MeV. are obtained with a complete simulation of all the beam A recent result from MiniBooNe [3], performed with line elements based on the FLUKA Monte Carlo code [11, neutrinos from the 8 GeV FNAL-Booster in a similar 12], and are available to all experiments on the CNGS L/Eν range has confirmed in both the neutrino and beam [13]. The hadron interaction models in FLUKA antineutrino channels a combined 3.8 σ LSND-like os- have been benchmarked on several sets of experimen- cillation signal. With the formula tal data, among which the data from the old NA20, NA56 and the present NA49 hadron production exper- 1.27∆m2 (eV2)L(m) P = sin2 (2θ ) sin2 new (1) iments [14,15,16,17]. Conservatively a 10 % systemat- νµ→νe new E (MeV) ν ics, introduced by the hadron production model in the these results correspond to a new signal somewhere computed fluxes, can be assessed when averaging over 2 2 within a wide interval ∆mnew ≈ 0.01 to 1.0 eV and a the angular acceptance of ≈ 30 mrad of the beam optics. 2 corresponding associated value of sin (2θnew). This level of agreement is demonstrated, for example, In addition, an apparent ν orν ¯ disappearance e e by the comparison with pt-integrated pion production anomaly has been recently detected from (a) nearby data of NA49 as reported in Figure 7 in [12]. nuclear reactors [4] and (b) from Mega-Curie k-capture This conclusion is corroborated by the absolute com- calibration sources [5,6], originally developed for the parison of the horizontal and vertical distributions of Gallium experiments to detect solar νe. Also these ef- the signals of the CNGS muon pit detectors with the 2 fects seem to occur for a ∆mnew value much higher full beam line simulation, have shown an agreement than the experimentally measured ones for the three within few percents in the first pit and ranging from neutrino oscillation scenario, in the order of magni- few percents to 10% in the second one [18]. tude of the LSND anomaly. These anomalies may in- According to the full neutrino beam calculation, deed represent an unified approach, in which one or 75% of muon neutrinos are coming from decays of pion more ∆m2 may have a common origin, with the val- new produced at the target, the rest is due to kaons, (6%) ues of sin2 (2θ ) for different channels reflecting the new and tertiary decays (19%). Electron neutrinos are orig- so far unknown structure of the U matrix, with (j,k) inated by pions through the subsequent muon decay j, k = number of ordinary and sterile neutrinos. (37%) as well as by kaons (43%), the remaining 20% is With the help of a novel development of a large due to tertiary decays. Hence, due to correlations be- mass “Gargamelle class” LAr-TPC imaging detector, tween the ν and ν common origins, significant cancel- the ICARUS experiment [7,8] at the Gran Sasso under- µ e lations occur in the systematics of the ν /ν ratio. As a ground laboratory (LNGS) is hereby visually searching e µ result, the integral error on the ν /ν ratio is estimated for the signature of such a signal due to a LSND-like e µ to be better than 7 %. anomaly in the CERN to Gran Sasso neutrino beam The ICARUS experiment is operated at L/E ≈ (CNGS). ν 36.5 m/MeV, a value much larger than the one of the experiments where anomalies appeared. In first approx- 2 The experimental setup imation, a hypothetical νµ → νe LSND anomaly will produce very fast oscillations as a function of the neu- 2 2 The CNGS facility [9] provides a neutrino beam com- trino energy Eν , averaging to sin 1.27∆mnewL/Eν ≈ 2 posed mainly of muon neutrinos peaked in the range 1/2 and Pνµ→νe = 1/2 sin (2θnew). This signal will 10 ≤ Eν ≤ 30 GeV. The CERN-SPS 400 GeV proton have to be compared with the small, but significant, beam with about 2 × 1013 protons on target (pot) per backgrounds due to other and more conventional neu- spill is sent to a segmented carbon target followed by trino sources. a magnetic horn and a reflector, focusing charged sec- It is well known and widely described in [8], that ondary mesons into a 1 km long decay tunnel. Produced the TPC developed by the ICARUS group provides, neutrinos are pointing down with a 52 mrad slope to- in a massive liquid Argon (LAr) volume, a completely ward the Gran Sasso laboratory (LNGS) located at a uniform imaging with accuracy, density and interac- distance of 730 km. tion lengths comparable to the ones of, for instance, 3 a heavy Freon bubble chamber. This innovative detec- along the drift direction [7], [20]. A PMT threshold, set tion technique allows observing the actual “image” of at 100 photoelectrons, allows full detection efficiency for 3 each charged track with a resolution of few mm , thus events with energy deposition (Edep) as low as few hun- extending in a liquid the method originally proposed by dreds MeV. Indeed, a trigger efficiency exceeding 99% Charpak et al. [19] in a gas. for Edep > 500 MeV has been measured on a large event The ICARUS-T600 detector, smoothly operated over sample (1.1 × 106 spills, 1.7 × 1019 p.o.t.) collected trig- the last three years in the underground Hall B of the gering only on the CNGS extraction signal. The trigger LNGS laboratory, has a mass in excess of 600 ton of ul- efficiency at very low energies depends on the topology tra high purity LAr, out of which 476 are instrumented and localisation of the event. Monte Carlo simulations and 447 are defined as fiducial volume for the selec- indicate a 100% efficiency for CNGS charged current tion of neutrino events. A detailed description of the (CC) events and a >90% efficiency for neutral current detector design, construction and test can be found in (NC) events. dedicated articles [7,8]. It allows identification and mea- A CNGS trigger rate of about 1 mHz was obtained surement of the ionisation image of all tracks produced including neutrino interactions inside the detector and within the fiducial volume to which a 500 V/cm uni- muons from neutrino interactions in the upstream rock. form electric field is applied (for maximum drift path of 1.5 m). Sensing and recording of the signals induced 3 Data selection by the drifted electrons (drift velocity ≈ 1.6 mm/µs) is provided by a set of three parallel planes of wires, 3 mm Empty events inside the recorded CNGS sample are apart, 3 mm pitch, facing the drift volume. Wires on rejected through a dedicated automatic filter based on ◦ ◦ each plane are oriented at a different angle (0 , +60 , charge deposition, whose efficiency close to 100% has ◦ -60 ) with respect to the horizontal direction. By appro- been checked on a sample of few thousands visually priate voltage biasing, the first two planes (Induction- scanned events. A few neutrino interactions/day with 1 and Induction-2) provide signals in non-destructive vertex in the fiducial volume are recorded, as expected. way, whereas the ionisation charge is finally collected The identification of the primary vertex and of 2D by the last one (Collection). This provides three pro- objects, like tracks and showers, is performed visually. jective views of the same event simultaneously, allowing The obtained clusters and reference points are fed to the both space point reconstruction and precise calorimet- three dimensional reconstruction algorithm described ric measurement of the collected charges. in detail in [21]. The collected charge is calculated for In order to ensure in LAr the visibility of tracks each “hit” (a point in the wire-drift projection) in the drifting over several meters, an equivalent Oxygen electro- Collection view after automatic hit finding and hit fit- negative content smaller than a few tens of ppt (parts ting [8], [21]. Each hit is corrected for the signal atten- per trillion) is required. During the present experiment, uation along the drift, according to the purity value as the free electron lifetime has been maintained most of continuously monitored with cosmic muons. Stopping the time in excess of 5 ms corresponding to a maximum tracks are processed for particle identification through 18% signal correction for the longest 1.5 m drift path specific ionisation [21]. The total deposited energy is of the LAr-TPC [7]. obtained by calibrated sum of hit charges in the region Electronics is designed to allow continuous read- spanned by the event, with an average correction fac- out, digitization and independent waveform recording tor for signal quenching in LAr. Muon neutrino charged of signals from each of the wires of the TPC. A 10-bit current events are identified with the requirement of a ADC digitization at 400 ns sampling provides a dy- track exiting the primary vertex and travelling at least namic range of up to about 100 minimum ionising par- 250 cm in the detector. ticles. The average electronic noise is typically of about In order to reproduce the signals from the actual 1500 electrons r.m.s., to be compared with ∼15000 free events, a sophisticated simulation package dedicated to electrons signal recorded for a 3 mm minimum ionising the ICARUS T600 detector has been developed. Neu- particle (S/N∼10). trino events are generated according to the expected A total of 74 photomultipliers (PMT) of 8” diame- spectra with uniform vertex position within the T600 ter sensitive to the 128 nm LAr UV-light, located be- sensitive volume. The adopted neutrino event gener- hind the transparent wire planes, are used to detect ator [22] includes quasi-elastic, resonant and deep in- the prompt scintillation light produced in LAr simul- elastic processes and is embedded in the nuclear reac- taneously with ionisation. They are used to trigger the tion model of FLUKA. Therefore it accounts for the presence of the neutrino signal within a CNGS related effects of Fermi motion, Pauli principle, and other ini- 60 µs gate and define the precise location of the event tial and final state effects such as, for instance, rein- 4

3 x10 Data 16 MC (pair production 16 Data 14 and Compton) 14 MC MC Compton only 12 12 10 8

10 Counts (A) 8 6 Counts 4 6 2 4 0 0 1 2 3 4 5 6 7 8 9 10 2 dE/dx (MeV/cm) 0 Fig. 2 Average ionisation in the ﬁrst 8 wire hits for sub- 0 1 2 3 4 5 6 7 GeV photons in the T600 data (full squares), compared to dE/dx (MeV/cm) Monte Carlo expectations (solid line) normalised to the same number of events. In MC case, the Compton contribution is shown also separately (dotted line).

(B)

Fig. 3 Typical Monte Carlo generated νµ → νe event from Fig. 1 (A) Energy deposition density distribution for muons the ICARUS full simulation program [11,12,22] with Ee = in CNGS CC interactions, compared with Monte Carlo, nor- 11 GeV and pT = 1.0 GeV/c. The close similarity of the MC malised to the same number of entries. Each entry corre- simulation with actual ICARUS events (see Figure 4 (A) and sponds to one wire hit. (B) Experimental raw energy distribu- (B)) is apparent. tion Edep for muon neutrinos and antineutrinos CC interaction in the ICARUS T600 detector (blue symbols) compared with the Monte Carlo expectations (red solid histogram), nor- from the real data. Such a procedure results in a re- malised to the same number of entries. markably close similarity between real and simulated events. teractions of the reaction products inside the target The good agreement between the observed and pre- Argon nucleus [23]. All reaction products are trans- dicted wire signals is shown in Figure 1A for CNGS ported in the T600 volume, with detailed simulation muon tracks recorded in CC events. Those tracks are of energy losses by ionisation, delta ray production, distributed over all the detector volume, and have been electromagnetic and hadronic interactions. Ionisation recorded during several months of operation, thus this charge along the track is subject to the experimen- plot includes all possible effects due to spatial or tem- tally observed recombination effects [24]. Energy de- poral non-uniformity. The agreement on the average positions are registered in grid structures that repro- value is at the level of 2.5%. Similar comparisons have duce the actual wire orientation and spacing, with a been performed on single tracks from long stopping fine granularity (0.2 mm) in the drift direction. The muons, whose energy can be measured. The distribution resulting charge is convoluted with the readout chan- of dE/dx for each track has been fitted with the convo- nel response (including wire signal induction and elec- lution of a Landau function with a gaussian. The fitted tronics response), including noise parameters extracted value of the most probable dE/dx agrees at 2% level 5 with Monte Carlo expectations, and the fitted gaus- All Monte Carlo predictions have been normalized sian σ is about 10%, reflecting the expected hit charge to the experimental total number of observed CNGS signal/noise ratio ∼ 10. Similar agreement is obtained neutrino events before any cut. for protons and pions [21]. Figure 1B shows the exper- The radiation length of LAr is 14 cm (≈ 45 read- imental raw energy distribution Edep for the observed out wires), corresponding to a γ-conversion length of νµ +ν ¯µ CC interactions compared with the MC expec- 18 cm. The ionisation information of the early part – tations [25]. The average value of the energy deposited before the showering of the e.m. track has occurred – is in the detector is reproduced within 2.5% and its rms examined wire by wire in order to tag the presence of within 10%. an initial electron emitted in the neutrino interaction,

The search for νµ → νe events due to a LSND as a powerful eliminator of γ-converting pairs, which anomaly has been performed as follows. The ICARUS are generally separated from the vertex and generate experimental sample has been based on 168 neutrino double minimum ionising tracks. The rejection factor events collected in 2010 (5.8 × 1018 pot) and 923 events based on ionisation increases dramatically with increas- collected in 2011 (2.7 × 1019 pot out of the 4.4 × 1019 ing photon energies, while the electron identification ef- collected in 2011), leading to a total of 1091 observed ficiency is almost constant. Indeed, the possible photon neutrino events, in good agreement, within 6%, with misidentification is essentially due to photons undergo- the Monte Carlo expectation. To this initial sample, a ing Compton scattering, whose cross section becomes minimal fiducial volume cut has been applied to collect negligible with respect to the pair production above as much statistics as possible: the interaction vertex a few hundreds MeV. Monte Carlo studies indicate a is required to be at a distance of at least 5 cm from residual contamination of about 0.18% for the energy each side of the active volume and at least 50 cm from spectrum of photons from pion decays in CNGS events, its downstream wall. These cuts allow for the identi- rising to a few % in the sub-GeV energy region. The fication of electron showers, but are neither stringent loss in efficiency for electron showers is only 10%. First enough for the reconstruction of neutrino energies, nor results from an ongoing study on low energy showers 0 for the identification of νµCC vs NC events. Further- from isolated secondary π ’s in the T600 CNGS data more, only events with a deposited energy smaller than confirm the MC expectation (see Figure 2). The plot 30 GeV have been included in the analysis, in order to shows a good agreement between data and simulations, optimize the signal over background ratio. Indeed, the including the low ionisation tail due to Compton inter- oscillated events are expected to have energies in the actions. 10-30 GeV range, like the bulk of the muon neutrino In the present analysis, the “electron signature” has spectrum, while the beam νe contamination extends to been defined by the following requirements: higher energies. (a) vertex of the event inside the fiducial volume; The estimation of the fraction of background and (b) visible event energy smaller than 30 GeV, in order oscillated events falling in the required energy cuts has to reduce the beam νe background; been performed on large samples (order of 10000 for (c) the presence of a charged track starting directly each neutrino specie) of simulated events, where the from the vertex, fully consistent over at least 8 wire spectrum of the oscillated events has been assumed to hits with a minimum ionising relativistic particle, be equal to the νµ CC spectrum (mass effects on the i.e. the average dE/dx must be lower than 3.1 MeV/cm cross sections are assumed to be negligible in this energy after removal of visible delta rays (see Figure 1A), range). Since the agreement of the simulations with the and subsequently building up into a shower; deposited energy spectrum is very good and the en- (d) visible spatial separation from other ionising tracks ergy cut concerns only about 15% of the events, the within 150 mrad in the immediate vicinity of the cut on visible energy introduces a negligible systematic vertex in at least one of the two transverse views error on the signal expectation. The same is true for (±60◦), except for short proton like recoils due to all background sources, except the νe beam component nuclear interactions. whose energy spectrum extends to higher energies. In this case, any uncertainty in the deposited energy spec- In order to determine the electron signature selec- trum is reflected in an equal uncertainty on the effect of tion efficiency η, νe events have been generated with the energy cut. On the basis of the comparisons shown MC according to the νµ CC spectrum. A simulated in in Figure 1 and described previously, we assumed a event is shown in Figure 3. Out of an initial sample conservative 10% systematics on the effect of the energy of 171 νµ → νe MC reconstructed events, 146 events cut on the beam νe background, to be added to the one have a visible energy smaller than 30 GeV, 122 of which on the prediction of the νe/νµ ratio. satisfy the fiducial volume cuts (a). These events have 6

(A) Eele= 7.5 ± 0.3 GeV (B)

pt = 1.3 ± 0.18 GeV/c e.m. shower e.m. shower 10.5 cm buildup buildup single mip track 18.9 cm single mip track

incoming incoming 80 cm neutrino Eele= 10 ± 1.8 GeV 37 cm neutrino

pt = 1.8 ± 0.4 GeV/c

100 cm 50 cm >10 MeV/cm Single wire signal 10 10 (C)

9 9

8 δ -ray 8

7 B B 7 B 6 6 B B 5 5 B 2 m.i.p. B 4 B B B 4 B dE/dx (MeV/cm) dE/dx (MeV/cm) 3 3 B B B B B B B B 2 B 1 m.i.p. 2 B B 1 B 1 B 0 0 14 16 18 20 22 24 26 28 30 32 34 36 38 .0 1.0 Wire number of track direction dN/dE (a.u.) Fig. 4 Experimental picture of the two observed events (A) and (B) with a clearly identified electron signature out of the total sample of 1091 neutrino interactions. Event in (A) has a total energy of 11.5 ± 1.8 GeV, and a transverse electron momentum of 1.8 ± 0.4 GeV/c. Event in (B) has a visible energy of ∼17 GeV and a transverse momentum of 1.3 ± 0.18 GeV/c. In both events the single electron shower in the transverse plane is clearly opposite to the remaining of the event. (C): display of the actual dE/dx along individual wires of the electron shower shown in Figure 4A, in the region (≥ 4.5 cm from primary vertex) where the track is well separated from other tracks and heavily ionising nuclear prongs. As a reference, the expected dE/dx distribution for single and double minimum ionising tracks (see Figure 1A), are also displayed. The dE/dx evolution from single ionising electron to shower is also shown. been visually and independently scanned by three dif- algorithm. All CNGS beam original and oscillated neu- ferent people in different locations. An excellent agree- trino flavors have been taken into account. Automatic ment has been found with differences in less than 3% cuts mimicking the data cuts have been applied to the of the sample. As a result, the average number of posi- simulated events. After the fiducial and deposited en- tively identified electron-like neutrino events is 90, cor- ergy cuts (C1 in Table 1) , background neutral current responding to a selection efficiency η = 0.74 ± 0.05. In and charged current events have been retained as “elec- a good approximation η is independent of the details of tron” candidates if no muon-like track could be inden- the energy spectrum. The systematic error on η induced tified, and at least one energetic photon (at least 100 by the dE/dx cut is bound to be smaller than 1% from MeV) pointing to the primary vertex was present (C2). the already discussed agreement to better than 2.5% The requirements for the shower isolation and for a con- between the measured and the predicted scale of the version distance smaller than 1 cm were then applied dE/dx for muons in νµCC (see Figure 1A). (C3). Finally, the discrimination based on the specific ionisation was applied as an average factor (C4). The A similar scan of 800 MC neutral current events has effect of the various cuts is summarised in Table 1. With shown no presence of apparent νµ → νe events, consis- this method, that is not fully equivalent to the visual tent for our sample with an estimated upper limit of 0.3 scan, the estimated background from misidentified NC events (including possibly misidentified νµ CC events). and νµ CC events amounts to 0.09 events, and the the Moreover, an independent estimation of the background simulated efficiency on νe CC events (after the fiducial rejection efficiency has been performed on a much larger and energy cuts) is found to be 74% in agreement with MC sample with a fast simulation and reconstruction 7 the scanning method. The contribution from a 2.5% un- Figure 4C displays the actual dE/dx along individ- certainty on the dE/dx scale would modify this back- ual wires of the electron shower shown in Figure 4A, in ground estimate by less than 10%. the region (≥ 4.5 cm from primary vertex), where the track is well separated from other tracks and heavily ionising nuclear prongs. As a reference, the expected Table 1 Fraction of Monte Carlo events surviving the auto- dE/dx distribution for single and double minimum ion- matic selection cuts, defined as follows. C1: Edep < 30 GeV; C2: no identified muon, at least one shower; C3: one shower ising tracks (see Figure 1A), are also displayed. The with initial point (conversion point in case of a photon) at a dE/dx evolution from single ionising electron to shower distance smaller than 1 cm from the neutrino interaction ver- is also shown. tex, separated from other tracks; C4: single ionisation in the first 8 samples. All event categories are reduced to 0.93 after the cut on fiducial volume. The signal selection efficiency (after the fiducial and energy cuts) results to be 0.6/0.81=0.74, 4 Results and discussion in agreement with the visual scanning method.

Sel. νe CC νe CC ντ CC NC νµ CC νe CC cut beam θ13 signal 68% CL 90% CL C1 0.47 0.92 0.93 0.89 0.89 0.81 95% CL 99% CL

C2 0.47 0.92 0.17 0.66 0.19 0.81 MiniBooNE 33σ CL C3 0.33 0.79 0.14 0.10 0.03 0.66 KARMEN2 90% CL C4 0.30 0.71 0.13 0.0002 0.00005 0.60 LSND 90% CL LSND 99% CL NuTeV (90%) CCFR (90%) NOMAD (90%) ) 2

(eV Excluded at 90% CL The expected number of νe events due to conven- 2 tional sources in the energy range and ﬁducial volumes Excluded at 99% CL

Δ m ICARUS deﬁned in (a) and (b) are as follows:

– 3.0 ± 0.4 events due to the estimated νe beam contamination; – 1.3 ± 0.3 νe events due to the presence of θ13 oscil- 1 2 lations from sin (θ13) = 0.0242 ± 0.0026 [26]; – 0.7 ± 0.05 ντ with τ → e from the three neutrino mixing standard model predictions [27], giving a total of 5.0±0.6 expected events, where the un- sin2(2θ) certainty on the NC and CC contaminations has been Fig. 5 Two-dimensional plot of ∆m2 vs sin2 (2θ ) for included. The expected visible background is then 3.7± new the main published experiments sensitive to the νµ → 0.6 (syst. error only) events after the selection efficiency νe anomaly [2,3,29,30,31,32] and the present ICARUS re- η = 0.74 ± 0.05 reduction has been applied. Given the sult. The ICARUS limits to the oscillation probability are P ≤ 5.4 × 10−3 and P ≤ 1.1 × 10−2 , cor- smallness of the number of electron like signal expected νµ→νe νµ→νe 2 −2 2 in absence of LSND anomaly, the estimated systematic responding to sin (2θnew) ≤ 1.1 × 10 and sin (2θnew) ≤ 2.2 × 10−2 respectively at 90% and 99% CL. Limits corre- uncertainty on the predicted number is clearly negligi- spond to 3.41 and to 7.13 events. ble w.r.t. its statistical fluctuation. In the recorded experimental sample, two events in which a νe signature have been identified, to be com- Within the range of our observations, our result is pared with the above expectation of 3.7 events for con- compatible with the absence of a LSND anomaly. Fol- ventional sources. The event in Figure 4A has a total lowing Ref. [28], at statistical confidence levels of 90% energy of 11.5 ± 2.0 GeV and an electron of 10 ± 1.8 and 99% and taking into account the detection effi- GeV taking into account a partially missing component ciency η, the limits due to the LSND anomaly are re- of the e.m. shower. The event in Figure 4B has 17 GeV spectively 3.4 and 7.1 events. According to the above of visible energy and an electron of 7.5 ± 0.3 GeV. In described experimental sample and the number of recorded both events the single electron shower in the transverse events, the corresponding limits on the oscillation prob- −3 plane is opposite to the remaining of the event, with ability are Pνµ→νe = 5.4×10 and Pνµ→νe = 1.1× the electron transverse momentum of 1.8 ± 0.4 GeV/c 10−2 respectively. The exclusion area of the ICARUS and 1.3 ± 0.18 GeV/c respectively. experiment is shown in Figure 5 in terms of the two- 8

1 # sin2(2θ) Δm2(eV2) 1 1 0.04 2 0.3 0.07 2 & 3 3 0.1 0.11 4 5 4 3.0E-2 0.21 5 1.0E-2 0.37 6 9 6 5.0E-3 0.50 8 7 3.3E-3 0.8 7 8 2.0E-3 1.5 9 2.5E-3 3.5

Fig. 6 Observed values of the LSND and MiniBooNE results are given with Pνµ→νe as a function of the distance L/Eν . The lines are examples of oscillation patterns with sets of parameters chosen within the MiniBoone allowed region [3]. In particular, line 1 corresponds to the MiniBoone best ﬁt in the combined 3+1 model[3]. All lines are consistent with data at low L/Eν values. Solid lines, labeled from 6 to 9, are also compatible with the present ICARUS result. Instead, parameter sets indicated by 1-5 (dashed lines), are driven by the additional signal recorded by MiniBooNE for L/Eν > 1 m/MeV, but they are entirely ruled out by the present result because they would imply an excessive oscillation probability at the large L/Eν values investigated 2 2 2 by ICARUS. Line 6 shows the “best value” including ICARUS results, with (∆m , sin (2θ))new = (0.5 eV , 0.005).

2 2 dimensional plot of sin (2θnew) and ∆mnew. In most of L/Eν ≥ 1 m/MeV (Figure 6), corresponding to a sig- the area covered by ICARUS and allowed by LSND and nificant signal peak at smaller values of Eν . The actual MiniBooNE, the oscillation averages approximately to a origin of the excess may need further clarification, as 2 2 half of its highest value, sin 1.27∆mnewL/Eν ≈ 1/2. already pointed out by the MiniBooNE Collaboration 2 For lower values of ∆mnew, the longer baseline strongly and for instance by Giunti and Laveder [33]. In the enhances the oscillation probability with respect to the low mass peak region the dominant signal is due to νµ one of the short baseline experiments. In ICARUS and misidentified background adding to the observed LNSD 2 2 2 for instance with ∆m , sin (2θ) new = (0.11 eV , 0.10) signal. as many as 30 anomalous νµ → νe events should have As already mentioned, the present experiment ex- been present with Eν ≤ 30 GeV in the analysed sample. plores much larger values of L/Eν , but the ICARUS The present result strongly limits the window of op- results exclude also a substantial fraction of the Mini- 2 2 tions from the MiniBooNE experiment. Using a likelihood- BooNE ∆m , sin (2θ) new curves shown in Figure 6, ratio technique [3], CP conservation and the same os- in particular the ones labeled from 1 to 5. cillation probability for neutrinos and antineutrinos, a A detailed comparison among the various results best MiniBooNE fit for Quasi Elastic (QE) events in the on different oscillation phenomena, between different QE energy range 200 MeV < Eν < 3000 MeV has been pairs of neutrino flavours, each having specific mix- 2 2 2 2 given at ∆m , sin (2θ) new = 0.037 eV , 1.00 . This ing angles and ∆m is shown in Figure 7 [27]. Even is clearly excluded by the ICARUS result. A 3+2 joint if disappearance and appearance results should not be QE 2 oscillation fit as a function of Eν in both neutrino referred to a single effective θ and ∆m , the plot al- and antineutrino modes has also been reported [3] with lows situating the residual ”LSND anomaly” in the 2 2 2 2 best fit values ∆m41 = 0.082 eV , ∆m51 = 0.476 eV , framework of the present neutrino oscillation results. 2 2 2 2 2 2 |Ue,4| |Uµ,4| = 0.1844, |Ue,5| |Uµ,5| = 0.00547. Also While for ∆mnew >> 1 eV there is already disagree- 2 in this case the MiniBooNE value of ∆m41 is clearly in- ment between the allowed regions from the published 2 2 compatible with the present ICARUS result. experiments, for ∆mnew ≤ 1 eV the ICARUS result The oscillation probabilities from LSND are in the now allows to define a much smaller, narrower region 2 2 2 L/Eν ≤ 1 m/MeV region. The MiniBooNE result has centered around ∆m , sin (2θ) new= (0.5 eV , 0.005) extended the data to additional values in the region in which there is 90% CL agreement between (1) the 9

2 10 CDHSW

CHORUS 101

KARMEN2 NOMAD NOMAD νe↔νX NOMAD ν ↔ν 0 CHORUS μ τ ] 10 2 νe↔ντ ν ↔ν LSND 90 / 99 % e μ [eV –1 2 10 All limits are at 90%CL ICARUS Bugey MiniBooNE

Δ m unless otherwise noted –2 MINOS 10 SuperK 90 / 99 % “LSND anomaly” 10–3 surviving area 90% CL (this work) K2K

10–4 10–2 100 102 tan2(θ) Fig. 7 Regions in the (∆m2, tan2 (θ)) plane excluded by the ICARUS experiment compared with the published results [27]. 2 2 While for ∆mnew 1eV there is already disagreement for νµ → νe between the allowed regions from the published 2 2 experiments, for ∆mnew ≤ 1eV the ICARUS result now allows to define a much smaller, narrower allowed region centered 2 2 2 around (∆m , sin (2θ))new = (0.5eV , 0.005) in which there is a 90% C.L. overall agreement. present ICARUS limit, (2) the limits of KARMEN and References (3) the positive signals of LSND and MiniBooNE collaborations. This is the area in which the expectations 1. B. Pontecorvo, Zh. Eksp. Teor. Fiz. 53, 1717 (1967) [Sov. Phys. JETP 26, 984 (1968)]. from cosmology suggest a substantial contribution to 2. A. Aguilar et al. Phys. Rev. D 64, 112007 (2001). the dark mass signal. 3. A. A. Aguilar-Arevalo et al.,arXiv:1207.4809v1 [hep-ex] 19 This region will be better explored by the proposed Jul 2012 and references therein. 4. G. Mention et al., Phys.Rev. D83 (2011) 073006 and ref- ICARUS/NESSiE dual detector experiment [34] to be erences therein. performed at CERN at much shorter distances (∼300 m 5. J. N. Abdurashitov et al., Phys. Rev. C 80, 015807 (2009). and ∼1.6 km) and lower neutrino energies, which in- 6. F. Kaether, W. Hampel, G. Heusser, J. Kiko, and T. crease the event rate, reduce the overall multiplicity of Kirsten, Phys. Lett. B 685, 47 (2010) and references therein. 7. C. Rubbia et al., JINST 6 P07011 (2011) and references the events, enlarge the angular range and therefore im- therein. prove substantially the νe selection efficiency. 8. S. Amerio et al., Nucl. Instr. and Meth. A527, 329 (2004). 9. G. Aquistapace et al. CERN 98-02, INFN/AE-89-05 (1998); R. Bailey et al. CERN-SL/99-034 (DI), INFN/AE- 99/05 Addendum (1999); E. Gschwendtner et al., CERN- ATS-2010-153 (2010). Acknowledgements 10. A. Ferrari et al., CERN-AB-Note-2006-038 (2006). 11. A. Ferrari et al., CERN-2005-10, INFN/TC-05/11, (2005). The ICARUS Collaboration acknowledges the funda- 12. G. Battistoni et al., AIP Conf. Proc. 896, 31-49, (2007). mental contribution to the construction and operation 13. http://www.mi.infn.it/∼psala/Icarus/cngs.html of the experiment given by INFN and, in particular, 14. H. W. Atherton et al., CERN Yellow Report 80-07 (1980). 15. G. Ambrosini et al., Eur. Phys. J. C10 605 (1999). by the LNGS Laboratory and its Director. The Polish 16. G. Collazuol et al., Nucl. Instr. and Meth. A449, 609 groups acknowledge the support of the Ministry of Sci- (2000). ence and Higher Education , and of National Science 17. C. Alt et al., Eur. Phys. J. C 49, 897, (2007). Centre, Poland. Finally, we thank CERN, in particu- 18. A. Ferrari et al, Nucl. Phys. B (Proc. Suppl.) 168, 169 (2007). lar the CNGS staff, for the successful operation of the 19. G. Charpak et al., Nucl. Instrum. and Meth. 80, 13 neutrino beam facility. (1970). 10

20. M. Antonello et al., Phys. Lett. B 713, 17 (2012). 21. M. Antonello et al, arXiv:1210.5089, accepted for publi- cation in Advances in High Energy Physics (2013) 22. G. Battistoni et al., Proceedings of the 12th Interna- tional conference on nuclear reaction mechanisms, Varenna (Italy), June 15-19, 2009, p.307. 23. F. Arneodo et al., Phys. Rev. D74, 112001 (2006). 24. S. Amoruso et al, Nucl. Instr. Meth. A523, 275 (2004). 25. A. Antonello et al., Phys. Lett. B 711, 270 (2012). 26. G. L. Fogli et al. arXiv:1205.5254 [hep-ph] (2012); 27. J Beringer et al. (Particle Data Group), Phys. Rev. D86, 010001 (2012). 28. G.J. Feldman and R. D. Cousins, Phys. Rev. D 57 (1988) 3873. 29. B. Armbruster et al.,Phys. Rev. D 65, 112001 (2002). 30. P. Astier et al.,Phys. Lett. B 570 19 (2003). 31. A. Romosan et al., Phys. Rev. Lett. 78 2912 (1997). 32. S. Avvakumov et al., Phys. Rev. Lett. 89 (2002) 011804. 33. C. Giunti and M. Laveder, Phys. Rev. D 82, 053005 (2010). 34. M. Antonello et al., SPSC-P-347 (2012); C. Rubbia et al., SPSC-P-345 (2011).