I will discuss recent achievements in the study of neutrino oscillations obtained with the T2K, OPERA and ICARUS long-baseline neutrino experiments

Longhin, A.

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DOI 10.1393/ncc/i2015-15025-2Colloquia: IFAE 2014IL NUOVO CIMENTO 38 C (2015) 25Overview on neutrino oscillationsA. LonghinINFN, Laboratori Nazionali di Frascati - Frascati (RM), Italyreceived 7 January 2015Summary. — I will discuss recent achievements in the study of neutrino oscillationsobtained with the T2K, OPERA and ICARUS long-baseline neutrino experiments.PACS 14.60.Pq – Neutrino mass and mixing.1. – IntroductionAn increasing number of observations on flavour transitions in neutrino propagation(neutrino oscillations) has been performed since the late nineties when the phenomenonwas first measured using atmospheric neutrinos. A rather rich and diversified data setis well described assuming that three mass eigenstates (ν1, ν2, ν3) are mixed to flavoureigenstates (νe, νμ, ντ ) through a unitary mixing matrix (UPMNS, Pontecorvo-Maki-Nakagawa-Sakata).The measurement of the θ13 angle at reactors and accelerator experiments in spring2012 has been a remarkable jump forward in our knowledge of UPMNS. The first mea-surement by Daya-Bay is based on an energy-dependent disappearance of ν¯e neutrinosproduced from a nuclear reactors. The large observed value of θ13 = (8.5+0.20−0.21)◦ [1],opens the door to the measurement of the δ phase whose value is of particular interestsince it rules the possible existence of CP violation in the leptonic sector.In addition, the last two years have been marked by the first evidence of neutrinotransitions in which the charged-current interactions of oscillated species (νe and ντ )could be measured directly (in the so-called “appearance” mode) with a high signal-to-noise ratio in the J-PARC and CNGS accelerator νμ beams. The νμ → ντ transition hasalso been measured by SuperKamiokande (SK) in the atmospheric neutrino fluxes toughwith a lower purity [2].Several factors can explain the challenge of a direct experimental evidence of neutrinoappearance. The traditional solar and reactor neutrino sources have energy spectrawell below the τ -lepton production threshold (3.5GeV) such that both for νμ and ντonly neutral current interactions occur. On the other hand, atmospheric or accelerator-based νμ sources overcome this limitation. Nevertheless the task is still demanding dueto the experimental signature of the τ -lepton requiring either a micrometric trackingCreative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 A. LONGHINFig. 1. – Left: Energy spectrum of νe candidates at SK. Right: constraints in the sin2 2θ13-δCPplane from T2K and reactor experiments.accuracy over a large volume or a precise characterisation of the final state kinematicswith a large statistical sample. The first approach was followed by OPERA while thesecond by SK (and ICARUS). It must also be noted that the suppression of the crosssection due to mτ requires using relatively high beam energies, well beyond the νμ → ντoscillation maximum, unless using baselines of several thousands km. The νμ → νepath to appearance requires a “light” detector with good e/μ separation power but withless stringent requirements w.r.t to τ tagging. No kinematical suppression is present inthis case. This channel was for long not considered as the most favourable to due theprejudice that θ13 could be very small if not even null. In the end even if the mixing isnot maximal it is still large enough to allow large samples without the complication of τtagging and the kinematical suppression(1).2. – Results from T2K with the J-PARC beamThe experiment operates over a baseline of 295 km from the J-PARC 30GeV protonsynchrotron along the Pacific coast to the SK detector in the Japanese alps. The neutrinobeam, which is produced by a system of three magnetic horns, is the first exploiting theoff-axis tecnique. The angle between the protons direction and the far detector direction(2.5◦) allows to obtain a narrow νμ beam with a reduced νe contamination peaked atthe energy of maximal mixing for the “atmospheric” oscillation pattern (about 0.6GeV).The near site at 280m from the target is composed of an on-axis detector (INGRID) andan off-axis detector (ND280).2.1. Measurement of νμ → νe appearance. – The analysis [3] is performed on adata sample of 6.57 × 1020 protons on target (pot) accumulated during the period2010–2013. The energy spectrum of the 28 νe candidates CC interactions selected inthe SK fiducial volume is shown in fig. 1. The expected number of νμ → νe events is(20.4 ± 1.8) assuming sin2 2θ13 = 0.1, sin2 2θ23 = 1, δCP = 0 and the normal mass hi-erarchy (NH). The overall background contribution is estimated as (4.64 ± 0.53) eventsgiving a signal-to-background ratio 4-5. As it can be seen from the stacked histograms offig. 1 this sample is dominated by the intrinsic νe contamination in the νμ beam (69%)followed by a component of misidentified π0 from neutral current interactions (19%)(1) The peak energies oth the CNGS and J-PARC are 17 and 0.6GeV, respectively.OVERVIEW ON NEUTRINO OSCILLATIONS 3Fig. 2. – Left: Energy spectrum of νCCμ candidates at the far detector. Right: allowed regionsin the Δm232- sin2 θ23 plane.the remaining part being due to the so-called solar term in the νμ → νe oscillation prob-ability and ν¯ interactions. Two analyses have been performed, the first relying on theenergy spectrum of the reconstructed electron and the second also considering its ex-pected angular distribuion (p-θ analysis). The null hypothesis (θ13 = 0) can be excludedat the 7.5 σ level (5.5 expected). The allowed bands in the θ13-δ plane are shown infig. 1 (right) for NH or inverted mass hierarchy (IH) together with the constraint givenby reactor experiments (vertical straight bands) which are insensitive to the value of δ.By comparing the two measeurements a mild preference for δCP  −π/2 is observed(2).The confidence regions at 90% CL for sin2 2θ13 are: [0.097, 0.218] (best fit 0.150) and[0.118, 0.261] (best fit 0.182) for the NH and IH, respectively.2.2. Measurement of νμ → νμ disappearance. – The J-PARC beam was designed hav-ing a precise knowledge of the atmospheric mass splitting. The occurrence of maximaldisappearance exactly at the flux peak yields a macroscopic (∼ ×4) suppression of νCCμinteractions at the far site where the beam is a relatively pure (sub-threshold) ντ beam. Asreported in [4], a total of 446± 22.5 (sys.) events are expected in the no-disappearancehypothesis while 110 are actually observed (based on 6.57× 1020 pot). The energy spec-trum of these events at SK is shown in the upper plot of fig. 2 (left). The lower plot showsthe ratio to the unoscillated spectra clearly evidencing the energy-dependent oscillationpattern. These data allow to measure sin2 θ23 = 0.514(0.511) ± 0.055 (World leading,11% relative error) and Δm232 = 2.51(2.48) ± 0.10 × 10−3 (starting to be competitivewith the MINOS result) for NI (IH). The result is expressed in terms of sin2 θ23 to allowfor not-octant symmetric effects modulated by θ13 which are present in the oscillationprobability(3) in the proper 3-flavour treatment.J-PARC has operated stably at about 220 kW for most of the latest runs accumulating6.63× 1020 pot corresponding to about 8% of the design sample. Anti-neutrino data col-lection has also started in 2014 with the goal of improving the sensitivity to the δCP phase.3. – Results from OPERA and ICARUS with the CNGS beamThe CNGS beam has operated from 2006 to 2012 yielding about 1.8× 1020 pot (80%of the design) and providing OPERA and ICARUS with a 17GeV mean-energy νμ beam.(2) δ = −π/2 is the value which enhances the νμ → νe transition probability.(3) P (νμ → νμ)  1− 4 cos2 θ13 sin2 θ23[1− cos2 θ13 sin2 θ23] sin2(1.267Δm2L/Eν).4 A. LONGHINFig. 3. – Left: Display of the fourth ντ candidate. Right: Distribution of the scalar sum of themomenta of tracks in the emulsion detectors. Red lines show the central values measured forthe four ντ candidates.3.1. Measurement of νμ → ντ appearance. – The number of oscillated τ is only a smallfraction of the total flux occurring mostly at low energy of about 1.5GeV where the fluxis low. The oscillated component at higher energy where the bulk of the flux sits is muchreduced but it can be detected having a large-enough interaction cross section (thresholdat 3.5GeV) and a detectable average decay length for the produced τ lepton.OPERA is essentially a largely modular high-granularity “vertex detector” composedof emulsion detectors (bricks) interspersed with electronic tracking planes and a muonspectrometer system. Electronic detectors determine the position of the bricks containingthe neutrino interaction and reduce the background exploiting muon charge determina-tion. The target active mass is about 1.2 kt for a total of about 140000 bricks. Themicrometric tracking available over such a large active mass allows the detection on anevent-by-event basis (S/N ∼ 10) of the rare νCCτ interactions. The experimental signa-ture is given by a kink or displaced vertex topology (decay lengths of about 600μm) orby the occurrence of a high-impact parameter track from a τ decay.The dominant background is charm production in νCCμ events with an undetectedprimary muon and to a lesser extent hadronic interactions and large-angle Coulombscattering of νCCμ muons (LAS), for the τ → μ channel. The charm background is bench-marked on the sample of fully reconstructed charm νCCμ events (50 observed with 54± 4expected.) and the CHORUS charm sample. The hadronic background is tuned on realdata from test-beam exposures while the LAS is currently constrained by simulationsand scattering data in the literature.Bricks are ranked according to the probability of containing the neutrino interactions.All highest-probability bricks have been fully analysed while 2nd bricks in the probabilityranking have been analysed only for the 2008-09 run. The expected number [5] of detectedντ interactions with the presently analysed sample and the kinematical selection definedalready at the level of the experiment proposal is (2.1± 0.42) events while 4 candidateshave been observed with the following final states: one μ, one 3h and two 1h. The latestτ → h candidate [6] has been announced on 25 March 2014 and it is shown in the leftplot of fig. 3. Given the expected background level of (0.233±0.041) events, it is possibleto reject the no-appearance hypothesis at the 4.2σ level using Poisson statistics and theinformation on the observed decay channels.OVERVIEW ON NEUTRINO OSCILLATIONS 53.2. Measurement of νμ → νe. – Both ICARUS and OPERA presented measurementscharacterising the νe component of the CNGS beam thanks to their remarkable electronreconstruction capabilities. On the full data-set ICARUS observes 4 candidate eventswith Eν < 30GeV [7, 8] having an expectation of 6.4 ± 0.9. The major background isthe instrinsic νe in the beam with a contribution from νμ → νe and νμ → ντ . Usinghalf of the available sample, OPERA [9] observes 19 νe candidates (6 of which below20GeV) with an expected background of 19.8± 2.8 events. Parametrizing the transitionprobability with an effective 2-flavour formula these observations translate into a 90%CL upper limit on sin2 2θeff at 6.8 (7.2)× 10−3 for ICARUS (OPERA).4. – ConclusionsThe last two years have been rich in results involving neutrino appearance channelsboth at the CNGS (νμ → ντ,e) and J-PARC (νμ → νe) beams providing importantachievements to the collaborations involved in these demanding programs. Appearance isa key-point in the framework of neutrino oscillations and will continue to play such a rolewithin future high-statistics programs for the measurement of δCP , the mass hierarchyand eventually exotic or unexpected effects.∗ ∗ ∗I would like to thank the organizers for the opportunity of giving this presentationand getting to know the GSSI institute and its students in L’Aquila.REFERENCES[1] Gonzalez Garcia M. C. et al., JHEP, 11 (2014) 052 arXiv:1409.5439.[2] Abe K. et al. (Super-Kamiokande Collaboration), Phys. Rev. Lett., 110 (2013) 181802.[3] Abe K. et al. (T2K Collaboration), Phys. Rev. Lett., 112 (2014) 061802.[4] Abe K. et al. (T2K Collaboration), Phys. Rev. Lett., 112 (2014) 181801.[5] Agafonova N. et al. (The Opera Collaboration), JHEP, 11 (2013) 036.[6] Agafonova N. et al. (The Opera Collaboration), Prog. Theor. Exp. Phys., 2014 (2014)101C01 arXiv:1407.3513.[7] Antonello M. et al., Eur. Phys. J. C, 73 (2013) 2345.[8] Antonello M. et al., Eur. Phys. J. C, 73 (2013) 2599.[9] Agafonova N. et al. (The Opera Collaboration), JHEP, 07 (2013) 004.

Overview on neutrino oscillations

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Overview on neutrino oscillations

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