In this paper, we give a short overview of neutrino oscillation experiments with emphasis on current European programmes of interest for INFN and on mid-term perspectives. In particular, we discuss the results that strengthen the standard three-family interpretation of leptonic mixing and the tension originating from the persistent LSND-Miniboone anomaly together with updated reactor data

Terranova, F.

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DOI 10.1393/ncc/i2011-11041-6Colloquia: IFAE 2011IL NUOVO CIMENTO Vol. 34 C, N. 6 Novembre-Dicembre 2011Neutrino oscillation experimentsF. TerranovaINFN, Laboratori Nazionali di Frascati - Frascati (Rome), Italy(ricevuto il 29 Luglio 2011; pubblicato online il 4 Novembre 2011)Summary. — In this paper, we give a short overview of neutrino oscillation exper-iments with emphasis on current European programmes of interest for INFN andon mid-term perspectives. In particular, we discuss the results that strengthen thestandard three-family interpretation of leptonic mixing and the tension originatingfrom the persistent LSND-Miniboone anomaly together with updated reactor data.PACS 14.60.Pq – Neutrino mass and mixing.PACS 01.50.Pa – Laboratory experiments and apparatus.1. – The precision era of neutrino oscillation physicsExperimental neutrino physics has witnessed a revolution between 1998 and 2003:in those years the phenomenon of flavor oscillation has been tested by a wealth ofexperimental data and the standard three-family paradigm emerged [1]. The experi-mental results obtained so far point to two very distinct mass differences among theneutrino mass eigenstates, Δm2sol = Δm221 ≡ m22 −m21 = 7.65+0.23−0.20 × 10−5 eV2 [2] and|Δm2atm| = |Δm232| ≡ |m23 −m22|  |m23 −m21| = 2.32+0.12−0.08 × 10−3 eV2 [3]. Δm221 is oftencalled the “solar mass scale” because it drives oscillation of solar neutrinos, but, if theenergy of the neutrino and the source-to-detector distance are properly tuned, it can alsobe measured employing artificial neutrinos, e.g., reactor neutrinos located about 100 kmfrom the detector (KAMLAND). Similarly, atmospheric neutrinos mainly oscillate at afrequency that depends on Δm232 (“atmospheric scale”). Again, experiments with arti-ficial neutrinos were able to see oscillations at the atmospheric scale using neutrinos ofenergy O(1) GeV and baselines of a few hundreds of km (K2K and, later on, MINOS).In the last few years, this paradigm has been further supported by solar data thanks tothe Borexino experiment, which is running since 2007. The main goal of Borexino isthe measurement of 7Be solar neutrinos (0.862MeV) by means of neutrino-elastic scatter-ing. Due to the kinematic of the neutrino-electron interaction, the signal of 7Be neutrinosis identified through a Compton-like shoulder at about 0.66MeV. Since 2008, the Borex-ino Collaboration has been able to exploit a detector whose performance significantlyexceeded the expectations in terms of intrinsic radioactive background. The results fromc© Societa` Italiana di Fisica 111112 F. TERRANOVA7Be have been recently updated [4] and yield 46.0± 1.5(stat.)+1.6−1.5(syst.) counts/day/100tons; they, hence, represent the first direct measurement of a sub-MeV solar neutrinorate with an accuracy better than 5%. These results falsify the hypothesis of no oscilla-tion for 7Be solar neutrinos at 4.9σ CL. Similarly, the null day-night asymmetry resultreported in [5] confirms the Large-Mixing-Angle solution of the solar neutrino puzzlealready obtained in 2003 (Δm2sol > 10−5 eV2 and large values of θ12); such confirmation,however, is now achieved without resorting to the assumption of CPT conservation whencombining reactor with solar data, as it was done before with the SNO and KAMLANDdata. In addition, Borexino provided the first standalone test of the Mikheyev-Smirnov-Wolfenstein mechanism, measuring simultaneously both neutrinos from 7Be and from8B; in fact, the survival probability of the former is marginally affected by matter effects,while the oscillation of 8B neutrinos is completely matter-dominated [6].2. – Appearance of new flavorsThe appearance of new flavors, different from the initial flavor of the source, has alwaysbeen considered the most direct proof of the oscillation of neutrinos. Unfortunately, allsources we have at our disposal to observe oscillations at the solar scale (solar and reactorneutrinos) produce νe (or νe) with energy well below the kinematic threshold for muonproduction. As a consequence, it is impossible to test in a straightforward manner theoccurrence of νe → νμ or νe → ντ transitions through the observation of muons or tausproduced by charged-current (CC) neutrino interactions with matter.At the atmospheric scale (atmospheric and multi-GeV artificial neutrinos from thedecay in flight of pions), νμ → νe transitions can be observed in appearance mode. How-ever, the smallness of the mixing angle θ13 between the first and third family suppressesthis transition at least by one order of magnitude [1]. Therefore, an appearance mea-surement that is aimed at observing a large neutrino transition probability must resortto νμ → ντ . A possible exception could be due to a “large” value of θ13, i.e. a valueclose to current limits from CHOOZ. In this case, an excess of νe CC events could beobserved from artificial νμ sources. Such possibility is hinted by the very recent resultsof the T2K experiment [7].Seeking for νμ → ντ , i.e. observing final state ντ CC interactions, is a major experi-mental challenge. The source must produce neutrinos above the kinematic threshold fortau production, 3.5GeV for scattering in nuclei. In addition, the detector must be ableto select an enriched sample of tau leptons in the bulk of muons and hadrons producedby νμ CC and NC interactions. It comes as no surprise that the most direct test of theoscillation phenomenon through the observation of tau appearance still deserves a con-clusive evidence. In 2010, however, important milestones have been achieved, especiallyby the OPERA experiment.Inclusive analyses, as the one published by Superkamiokande in 2006 [8] and recentlyupdated in [9], try to distill a tau-enriched sample in the bulk of νμ and νe interactionsand take advantage of the large statistics and of the peculiar kinematics of tau decays.Exclusive measurements are even more ambitious since they aim at observing the ap-pearance of tau leptons on an event-by-event basis. They need a detector with highspatial resolution, such as to observe the decay in flight of the tau and, at the same time,a high-intensity source with an energy well exceeding the kinematic threshold for tauproduction. The only facility that is able to fulfill simultaneously these requirements isthe CNGS facility in Europe. The CNGS beam is a pure νμ beam with a mean energyof 17GeV produced at CERN and pointing to the Gran Sasso Laboratories of INFNNEUTRINO OSCILLATION EXPERIMENTS 113in Italy (LNGS), 730 km away from the source. The intrinsic ντ contamination, mainlyoriginating from the decay of Ds, is negligible (< 10−6). The beam is also contaminatedat the 0.8% by νe, resulting from the decay in flight of muons along the decay tunnel andfrom Ke3 decays. Since the tau lepton is identified from its decay topology, backgroundfrom prompt νe is immaterial.Performing an event-by-event appearance measurement of the νμ → ντ transition isthe main goal of the OPERA experiment [10], which has been built from 2004 to 2008in the Hall C of LNGS as a far detector for CNGS. In OPERA, the ντ appearance signalis detected through the measurement of the decay daughter particles of the τ leptonproduced in CC ντ interactions. Since the short-lived τ particle has an average decaylength of about 1 mm at the CNGS beam energy, a micrometric detection resolutionis needed. In OPERA, neutrinos interact in a large mass target made of lead platesinterspaced with nuclear emulsion films acting as high-accuracy tracking devices. Thiskind of detector is historically called an Emulsion Cloud Chamber (ECC) and it has beensuccessfully applied by the DONUT experiment to perform the first direct observationof ντ charged-current interactions in a ντ -enriched beam at Fermilab.In 2010 [11] the OPERA Collaboration reported the observation of a first candidateντ CC interaction in the detector. The primary neutrino interaction consisted of 7 tracksof which one exhibits a visible kink. Two electromagnetic showers due to γ-rays havebeen located; they are clearly associated with the event and were produced at the de-cay vertex. Figure 1 shows a display of the event, which was identified in a samplecorresponding to 1.89× 1019 p.o.t. in the CNGS νμ beam. The total transverse momen-tum PT of the daughter particles with respect to the parent track is (0.47+0.24−0.12)GeV,above the lower selection cut-off at 0.3GeV. The missing transverse momentum PmissTat the primary vertex is (0.57+0.32−0.17)GeV. This is lower than the upper selection cut-offat 1GeV. The angle Φ between the parent track and the rest of the hadronic showerin the transverse plane is equal to (3.01 ± 0.03) rad, largely above the lower selectioncut-off fixed at π/2. The invariant mass of γ-rays is (120± 20(stat.)± 35(syst.))MeV2,supporting the hypothesis that they originate from a π0 decay. Similarly the invari-ant mass of the charged decay product assumed to be a π− and of the two γ-rays is(640+125−80 (stat.)+100−90 (syst.))MeV, which is compatible with the ρ(770) mass. The branch-ing ratio of the decay mode τ → ρ−ντ is about 25%. The observation of one possibletau candidate in the decay channel h−(π0)ντ has a significance of 2.36σ of not being abackground fluctuation from a background of 0.018 ± 0.007. If one considers all decaymodes included in the search, corresponding to 0.54±0.13 expected taus, the significanceof the observation becomes 2.01σ from the total predicted background of 0.045± 0.023.3. – Challenging the three-family interpretationDuring the last decade, most of the experimental efforts have been focused on ground-ing the oscillation evidence at the solar and atmospheric scales and, simultaneously, onperforming precision measurements of the leading oscillation parameters. An early dis-covery of a non-null value of θ13 would soon open a new phase of this precision oscillationera [12]. If θ13 is non-zero, the subdominant νμ → νe amplitude and its T or CP con-jugates encode a wealth of information. In particular, it allows for the determination ofθ13 and the Dirac complex phase of the leptonic mixing matrix. The νμ → νe transitionprobability is also perturbed by matter effects if the path of the neutrinos through theEarth is sufficiently large. The perturbation depends on the sign of Δm231 and, therefore,it can be used to determine the hierarchy among the neutrino masses. These consid-114 F. TERRANOVA1235678daughterγ1γ21000 μm1234 parent568daughterγ1100 μm6714532 8 daughterCSγ1 γ22mm10mmFig. 1. – (Colour on-line) Display of the OPERA τ candidate event. Top left: view transverseto the neutrino direction. Top right: same view zoomed on the vertices. Bottom: longitudinalview.erations explain the enormous interest toward novel neutrino sources operating at theatmospheric scale as, for instance, the Neutrino Factories or the Beta Beams [13].On the other hand, experimental data collected outside the solar and atmosphericscale [14] show a level of precision and control of systematics errors that is often substan-dard compared with the previous ones. It is therefore unclear whether a few persistentanomalies, mostly pointing toward a new Δm2  Δm232 are hinting toward a breakdownof the standard three-family interpretation or they are mere experimental artifacts. Thepersistent LSND anomaly, possibly confirmed by the antineutrino Miniboone data, re-mains by far the strongest from the experimental point of view. Still, none of thesemeasurements have been done with the usual near-versus-far detector comparison and,in addition, any simple explanation is ruled out by the puzzling Miniboone data in neu-trino mode. A vigorous programme is carried out at Fermilab and, possibly, at SNS toestablish the existence of a new source of flavor conversion at a scale Δm2  Δm232.NEUTRINO OSCILLATION EXPERIMENTS 115CERN could also contribute exploiting a dedicated neutrino beamline from the CERN-PS [15]. Three other anomalies currently challenge the standard scenario: a systematicdeficit of events from short-baseline reactor experiments resulting from new flux calcu-lations [16], a deficit from the calibration with radioactive sources (“Gallium anomaly”)of the GALLEX and SAGE solar neutrino experiments and, finally, a slight discrepancybetween the leading oscillation parameters θ23 and Δm232 as measured by MINOS inneutrino and antineutrino disappearance mode. None of these anomalies are compelling,especially if one accounts for full systematic uncertainties. However, major improvementscould be obtained by dedicated runs of facilities that have been built to test the standardthree-family model. We mention in particular, the proposal of running MINOS in par-allel with NOνA (MINOS+) and the possibility of calibrating Borexino with radioactivesources at LNGS [14].REFERENCES[1] Nakamura K. and Petcov S. T., Neutrino mass, mixing, and oscillations, in NakamuraK. et al., J. Phys. G, 37 (2010) 075021.[2] Schwetz T., Tortola M. A. and Valle J. W. F., New J. Phys., 10 (2008) 113011.[3] Adamson P. et al. (MINOS Collaboration), Phys. Rev. Lett., 106 (2011) 181801.[4] Bellini G. et al. (Borexino Collaboration), Precision measurement of the 7Be solarneutrino interaction rate in Borexino, arXiv:1104.1816 [hep-ex] preprint (2011).[5] Bellini G. et al. (Borexino Collaboration), Absence of day-night asymmetry of862 keV 7Be solar neutrino rate in Borexino and MSW oscillation parameters, arXiv:1104.2150 [hep-ex] preprint (2011).[6] Bellini G. et al. (Borexino Collaboration), Phys. Rev. D, 82 (2010) 033006.[7] Abe K. et al. (T2K Collaboration), Indication of Electron Neutrino Appearance froman Accelerator-produced Off-axis Muon Neutrino Beam, arXiv:1106.2822 [hep-ex] preprint(2011).[8] Abe K. et al. (Super-Kamiokande Collaboration), Phys. Rev. Lett., 97 (2006) 171801.[9] Wendell R., Talk at 11th International Workshop on Next generation Nucleon Decay andNeutrino Detectors, December 13-16, 2010, Toyama, Japan.[10] Guler M. et al. (OPERA Collaboration), An appearance experiment to search forνμ → ντ oscillations in the CNGS beam: experimental proposal, CERN-SPSC-2000-028,LNGS P25/2000.[11] Agafonova N. et al. (OPERA Collaboration), Phys. Lett. B, 691 (2010) 138.[12] Battiston R., Mezzetto M., Migliozzi P. and Terranova F., Riv. Nuovo Cimento,033 (2010) 313.[13] Bandyopadhyay A. et al. (ISS Physics Working Group Collaboration), Rep. Prog.Phys., 72 (2009) 106201.[14] For a review see e.g. Workshop on Beyond Three Family Neutrino Oscillations, LNGS,May 3-4, 2011, http://beyond3nu.lngs.infn.it/.[15] Baibussinov B. et al., A New search for anomalous neutrino oscillations at the CERN-PS,arXiv:0909.0355 [hep-ex] preprint (2009).[16] Mention G., Fechner M., Lasserre T., Mueller T. A., Lhuillier D., Cribier M.and Letourneau A., Phys. Rev. D, 83 (2011) 073006.

Neutrino oscillation experiments

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Neutrino oscillation experiments

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