PoS(NOW2016)003. T2K oscillation results. Lorenzo Magaletti. INFN Sezione di Bari

Transcription

1 INFN Sezione di Bari The TK (Tokai-to-Kamioka) experiment is a second generation long baseline neutrino oscillation experiment that probes physics beyond the Standard Model. An off-axis neutrino beam with a peak energy of. GeV is produced at the J-PARC accelerator facility, with the flavour content dominated by either muon neutrinos or muon anti-neutrinos, depending on the choice of the polarity of the magnetic focusing horns. The neutrino beam is detected first in the near detector ND8, where the flavour composition of the incoming neutrino flux is not expected to be affected by oscillation, and then travels 9 km to the far detector Super-Kamiokande, where oscillation significantly affects the flavour composition. We report the results of a joint analysis of neutrino and antineutrino oscillations at TK with the ( ) disappearance and ( ) ( ) appearance channels, obtained by collecting a total statistic of 7.7 protons-ontarget in ν-mode and 7. in ν-mode. The results in the disappearance channel show that our data continue to prefer maximal θ mixing (sin θ =..8 +., in case of normal mass hierarchy) and no CPT violation, while in the appearance channel, we observed a large appearance and a low appearance with respect to the expectations. These results favour a δ CP π/, with a 9% confidence interval of [.,.9] in normal mass hierarchy and [.9,.7] in inverted mass hierarchy. We comment briefly on the future prospects for TK, including a proposal for extended running to accumulate protons-on-target, nearly three times the currently approved amount by, to gain substantial sensitivity to CP violating effects in ( ) ( ) oscillations if parameters are favorable. Neutrino Oscillation Workshop - September, Otranto (Lecce, Italy) Speaker. on behalf of the TK collaboration. c Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives. International License (CC BY-NC-ND.).

2 . The TK experiment The Tokai-to-Kamioka (TK) experiment [] uses a GeV proton beam from the J-PARC accelerator facility in Tokai (Ibaraki prefecture, Japan), to produce a high purity muon neutrino beam, which is detected first at 8 m from the neutrino production point in the near detector complex (composed by ND8 and INGRID detectors), where the flavour composition of the incoming neutrino flux is not expected to be affected by oscillation, and then travels 9 km to the far detector Super-Kamiokande [] (Gifu prefecture, Japan), where oscillation significantly affects the flavour composition. The near detector ND8 and the far detector Super-Kamiokande (SK) are placed. off-axis with respect to the neutrino beam centre, resulting in a quasi-monochromatic neutrino energy spectrum that is sharply peaked around. GeV in order to enhance the effect of neutrino oscillations at 9 km. The neutrino flux is obtained by hitting accelerated protons on a graphite target. A set of three pulsed electromagnets ( horn ) focuses either positive pions into a heliumfilled decay region to produce a beam primarily composed of (ν-mode), or negative pions to produce a ( ν-mode) enhanced beam. TK can investigate two neutrino oscillation channels and two anti-neutrino oscillation channels: ( ) disappearance and ( ) appearance. In the disappearance channel, the oscillation probability P( ), is sensitive to θ and m. In the appearance channel, the oscillation probability P( ) is sensitive to θ and the octant of θ in the leading term, and is sensitive to sinδ CP and matter effect in the sub-leading terms. At TK, the CP-violating phase δ CP has an effect up to %, while the matter effect has a lower impact ( %). This will result in asymmetries in the probabilities for the CP-conjugate channels and if sinδ CP or ±π, with negative (positive) values of sinδ CP enhancing (suppressing) oscillations and suppressing (enhancing) oscillations.. Predicted neutrino flux The neutrino flux from the TK beam line is predicted from a data-driven simulation based on Geant-based Monte Carlo, incorporating data from the NA/SHINE experiment [, ], which has provided critical measurements of the hadron production cross sections using a thin carbon and a TK replica target []. In addition, beam monitor data and the beam direction, profile and stability, measured by the on-axis near detector INGRID [], are incorporated into the flux prediction and its systematic errors, which are reduced at the level of % [7]. The predicted neutrino fluxes in both ν and ν modes at SK are shown in Fig... Near detector analysis The off-axis near detector ND8 consists of a number of sub-detectors installed inside the refurbished UA/NOMAD magnet, which provides a. T field to identify the charge of the particle passing through ND8. and Charged Current (CC) events are selected in the tracker region of ND8, which consists of three time projection chambers (TPC,, ) [8], interleaved with two fine-grained detectors (FGD, ) [9]. The upstream FGD detector consists of scintillator modules, while the downstream FGD contains seven scintillator modules alternating with six water modules. The observed energy loss in the tracker is used for particle identification which,

3 p.o.t) /MeV/ Flux (/cm Neutrino Mode Flux at SK (GeV) E ν p.o.t) /MeV/ Flux (/cm Antineutrino Mode Flux at SK (GeV) Figure : Predicted flux of neutrinos and antineutrinos by species at the SK detector in the absence of oscillation effects for ν-mode (left) and ν-mode (right). when combined with particle charge information, allows a precise separation and measurement of the (right-sign) and (wrong-sign) in the ν-mode. In the ν-mode case, muon neutrino induced CC interactions are selected in the FGDs Fiducial Volume (FV). Than the CC candidates are divided into three subsamples, according to the number of identified pions in the event: CC-π, CC-π + and CC-Other, which are dominated by quasi-elastic (CCQE), CC resonant pion production, and deep inelastic scattering interactions, respectively []. For the ν-mode case, both and CC interactions are selected and then divided into two subsamples, defined by the number of reconstructed tracks crossing the TPCs: ( ) CC--track, dominated by CCQE interactions and ( ) CC-N-tracks (N>), a mixture of resonant production and deep inelastic scattering []. The major update in the near detector analysis is the use of interactions in FGD. By including both FGD and FGD samples, resulting in a total of six samples in ν-mode and eight samples in ν-mode, the interaction properties on water can be effectively isolated, reducing the uncertainties related to extrapolating across differing nuclear targets in the near and far detectors. The ND8 analysis performs a simultaneous fit of the fourteen samples in muon momentum and polar angle distributions, in order to constrain parameters representing the systematic uncertainties in the neutrino flux and interaction models. The muon momentum distributions of CC-π sample (ν-mode) and CC--track sample ( ν-mode) in FGD after the ND8 fit, are shown in Fig.. The goodness of fit was found to be 8.%. For the parameters that ND8 can constrain, the fit reduces their effect on the uncertainty on the expected number of events at SK from -% to -%. E ν. Far detector analysis The far detector SK is a -kt (.-kt fiducial mass) water Cherenkov detector, where the volume is divided into an outer detector (OD) with 88 outward-facing -cm-diameter PMTs and an inner detector (ID) with 9 inward-facing -cm-diameter PMTs. The events arriving at SK from the J-PARC beam spill are synchronized with a GPS within ns precision. Candidate neutrino interactions must be fully contained in the fiducial volume. To enhance the quasi-elastic

4 Events/( MeV/c) ν CCQE ν CC p-h ν CC Res π ν CC Coh π Events/( MeV/c) ν CCQE ν non-ccqe ν CC Other ν NC modes ν modes ν CCQE ν non-ccqe PRELIMINARY Muon momentum (MeV/c) PRELIMINARY Muon momentum (MeV/c) Figure : Predicted (histogram) and observed (data point) muon momentum distributions after the near detector fit for CC-π events in ν-mode (left) and CC--track events in ν-mode (right). purity of the samples, the selected events must have a single identified Cherenkov ring associated to the outgoing lepton. The and samples are defined by the one ring muon-like candidates (Rµ), that implies a Cherenkov ring pattern consistent with a muon with a momentum of at least MeV/c, and at most one decay electron. The and samples are defined by the one ring electron-like candidates (Re) that implies an electron-like ring with a visible energy greater than MeV with no decay electrons that may signal the presence of pions in the event. The neutrino energy (E ν ) is reconstructed from momentum and direction of the lepton (assuming a two body interaction) and must be lower than MeV. Moreover the Re events are passed through a further likelihood discriminator that separates out events containing a π. The resulting four events distributions are shown in Fig... Oscillation results The results presented here are obtained with the whole dataset of 7.7 protons-ontarget (POT) collected in ν-mode and 7. POT collected in ν-mode, for a total amount of. POT. A likelihood-ratio formed with the four samples collected at SK (Re/Rµ in ν/ ν-mode), is used as test statistic ( log L ) where the nuisance parameters (flux, cross-section, detector and oscillation) are marginalised and the marginal likelihood is maximised as a function of the oscillation parameters of interest. Since the and disappearance probabilities are identical at leading order sin θ (neglecting the small matter effects for TK baseline), inconsistent measurements in the oscillation parameters between and disappearance analyses could be an indication of CPT violation or non-standard neutrino interactions. and candidate Rµ events are observed in ν-mode and ν-mode, respectively. We found a good agreement between between and disappearance results in both hierarchy cases. Therefore, no hint of CPT violation or non-standard neutrino interactions is observed. The left plot on Fig. shows the allowed regions in sin θ and m assuming normal hierarchy, where the TK data (black) is compared with other recent experimental results. The TK fit obtains sin θ =.+..8 (.. +. ) and m = ( ) ev with normal (inverted) mass hierarchy hypothesis. The TK measurement continues to be consistent with maximal mixing and is currently the

5 Ratio Ratio 7. TK Run -7c preliminary TK Run -7c preliminary Ratio. TK Run -7c preliminary Bottom: corresponding distributions for ν-mode. cted neutrino Reconstructed energy (GeV) neutrino energy (GeV) Ratio 7. TK Run -7c preliminary Figure : Top: reconstructed energy distributions for (left) and (right) candidates observed in ν-mode Table : Number of expected electron candidates in SK for the ν-mode and ν-mode data for various values of δ CP and the mass ordering, compared to the observation. The assumed value of other parameters are: sin θ =.7, sin θ =.8, m =.9 ev /c, sin θ =.8 and m = 7. ev /c. δ CP = π/ δ CP = δ CP = +π/ δ CP = ±π Observed ν-mode NH IH ν-mode NH IH world-leading measurement of sin θ. The CP violating phase δ CP is studied with the full joint fit analysis. As can be seen from Tab. in ν-mode, the number of observed Re events are larger than the expectation for appearance candidate events, while in ν-mode the observed Re events are lower than the expected appear-

6 ance events. This behavior indicates that our data prefer the value of δ CP inducing the largest ν- ν asymmetry: π/. The TK results are consistent with reactor ones, with the preference on the maximally CP violated region at δ CP = π/. The right plot in Fig. shows the logl as a function of δ CP and mass hierarchy, with the reactor constraint on sin θ [,, ]. The 9% confidence levels are built with the Feldman-Cousins method [], resulting in interval of [-., -.9] in normal hierarchy and [-.9, -.7] in inverted hierarchy. The TK measurement rejects CP conservation in neutrino oscillations for both mass hierarchy hypotheses with 9% confidence level. From a toy Monte Carlo study we find that, if the true value of δ CP = π/, then the probability for excluding δ CP = or ±π, at 9% confidence level is 9.% and 7.% respectively. ) /c ev - ( m %CL 9%CL TK best-fit Normal Hierarchy IceCube NOvA () MINOS+ TK Run-7c preliminary Super-K sin θ -lnl TK Run-7c preliminary Normal Hierarchy Inverted Hierarchy δ CP (radians) Figure : Left: Allowed regions in m and sin θ from the TK joint analysis (black) compared to other recent measurements. Right: 9% confidence level intervals in δ CP.. Prospects The TK experiment aims to reach the number of approved POT (7.8 ) around. However the current approved statistic is not enough to measure the CP violation with a sensitivity of σ for the current favoured values of δ CP. For this reason, the TK collaboration proposes a further extension of the TK data taking up to, called TK-II, in order to collect POT []. To achieve this goal, the accelerator and MR beam power need to be upgraded to reach. MW from the current beam power of kw. Moreover, various aspects of the TK experiment need to be upgraded accordingly as well: upgrade of the magnetic horn current, from present ka to ka to increase the effective POT by %, near detector upgrade and some analysis improvements to increase the statistic and reduce the systematic uncertainties. Considering all these improvements, TK-II can reach a σ sensitivity for the current favoured values of δ CP and measure θ with a precision of.7 as well. 7. Conclusions For the first time we present the results of a fully joint analysis across all four neutrino oscillation modes observed at TK ( / disappearance and / appearance), obtained with the full statistic collected so far (. POT split equally in ν-mode and ν-mode). Our data prefer

7 maximal mixing in θ, while the relatively large signal and small signal with respect to the expectations, favour the scenario of δ CP = π/ and normal hierarchy. Thanks to rapid increases in beam power and projected upgrades, beam power should continue to rise in the near future. The TK Collaboration proposes further extension of the TK run (TK-II) up to in order to collect POT, nearly three time the current approved 7.8 POT. Such extension of TK is needed in order to measure the CP violating effects with σ sensitivity, if the true value of δ CP is π/ and the mass hierarchy is normal, and to solve the octant of θ with a precision of.7. In order to achieve these goals, upgrades of the beamline to support beam power up to. MW, improved detectors, and refined analyses are required. A first stage of beamline upgrade is foreseen in 8. References [] K. Abe et al. (TK Collaboration), Nucl. Instrum. Methods A9, (). [] K. Abe et al., Nucl. Instrum. Meth., A9, -,. [] N. Abgrall et al. (NA/SHINE Collaboration), Phys. Rev. C 8, (). [] N. Abgrall et al. (NA/SHINE Collaboration), Phys. Rev. C 8, (). [] N. Abgrall et al. (NA/SHINE Collaboration), Eur. Phys. J. C7, 8 (). [] K. Abe et al. (TK Collaboration), Nucl. Instrum. Methods A9, (). [7] TK Collaboration (K. Abe (Tokyo U., ICRR) et al.), Phys.Rev. D87 () no., [8] N. Abgrall et al. (TK ND8 TPC Collaboration), Nucl. Instrum. Methods A7, (). [9] P. Amaudruz et al. (TK ND8 FGD Collaboration), Nucl. Instrum. Methods A9, (). [] K. Abe et al. (TK Collaboration), Phys. Rev. D9, 7 (). [] TK Collaboration (Ko Abe (Kamioka Observ.) et al.), Phys. Rev. Lett. () no.8, 88. [] F. P. An et al. Phys. Rev. Lett., 8, 78,. [] J. K. Ahn et al. Phys. Rev. Lett., 8, 98,. [] Y. Abe et al. Phys. Rev. Lett., 8, 8,. [] G. J. Feldman and R. D. Cousins, Phys. Rev. D 7, 87 (998). [] Ko Abe et al. e-print: arxiv:9..