PoS(NuFact2017)053 https://pos.sissa.it/ significance. This number increases σ ) significance for 57% (resp. 74%) of the possible true values σ using a 187kton fiducial mass detector. We then discuss the im- 23 θ (resp. 3 σ ∗† , and determine the mass hierarchy with more than 3 significance if a second detector is added close to the first one in a staged approach, and to σ CP if this additional detector is instead located in Korea, giving a longer baseline for the acceler- [email protected] δ σ Speaker. On behalf of the Hyper-Kamiokande proto-collaboration In this paper,Kamiokande we for present the study the ofiment neutrino potential for oscillations. of the We exclusion first the of presenthierarchy the next the and conservation sensitivity of generation of the of CP octant the neutrino of symmetry, exper- andprovements experiment obtained the with Hyper- determination a of second the detector of mass With a identical single fiducial detector, mass and in combining different the configurations. atmospheric analysis neutrinos, of Hyper-Kamiokande accelerator can neutrinos exclude the (295km conservation baseline)trino of and oscillations CP with symmetry in 5 neu- of to 4 6 ator neutrinos. † ∗ Copyright owned by the author(s) under the terms of the Creative Commons c Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). The 19th International Workshop on Neutrinos25-30 from September, Accelerators-NUFACT2017 2017 Uppsala University, Uppsala, Sweden Christophe Bronner Kamioka Observatory, Institute for Cosmic RayE-mail: Research, The University of Tokyo Physics potential of Hyper-Kamiokande for neutrino oscillation measurements PoS(NuFact2017)053 angle with the ◦ Christophe Bronner ] builds on the successful strategies 1 ]. . Those questions can be addressed by precise 3 23 1 θ ] were used to study the physics potential of this project. In this 5 ]. An intense muon neutrino/anti-neutrino beam is produced from 4 ], and the construction of an additional intermediate water Cerenkov detector (E61). In 2 Since the experimental apparatus of the T2HK project is similar to the T2K one, tools devel- As the candidate site for the first Hyper-Kamiokande detector is located 8km south of the Neutrino oscillations have been extensively studied since their discovery at the end of the 20th The next generation experiment Hyper-Kamiokande [ part, we consider one 187 fiducial volumethe detector, using current scaled Super-K simulation detector and (in reconstruction particular from improvements coming from the better photo-sensors are oped to study the T2K sensitivity [ accelerated protons hitting a graphiteover target 295km in before J-PARC reaching on the the east far cost detector of located Japan, in and a propagates direction making a 2.5 2.2 Sensitivity study method this paper, we will presentwith the only potential one detector, of and this then project with for the different the options study considered of for2. neutrino the second oscillations, Long detector. first baseline oscillations: Tokai to Hyper-Kamiokande (T2HK) 2.1 Experimental apparatus current Super-K detector, the accelerator neutrino partof of the the current experiment T2K is experiment very [ similar to the setup axis of the beam. Aneutrino complex beam, of and near to detectors study located it 280mT2K to from control setup the the is target systematic the uncertainties. allows presence to The700m of monitor main and an the difference 2km with additional the from intermediate the waterby target, Cerenkov sampling detector which the located will beam between allow at further different reduction off-axis angles of [ the systematic uncertainties century, and the phenomenon isance and now the well disappearance established of after neutrinososcillation model of the have a observations been certain of measured, flavor. leaving bothlations Most three respect of the main CP the open appear- symmetry, parameters questions: a of symmetry whetherneutrino the between neutrino matter 3 mass oscil- and flavor eigenstates anti-matter, is, what the and ordering the of octant the of Hyper-K physics potential 1. The Hyper-Kamiokande project measurements of the values of somerespond of to the the model observation parameters. ofdone However, more those so subtle measurements far, variations cor- and in will thestatistics. require oscillation both pattern precise than understanding what of has the been systematic uncertainties, and large used to study neutrinoimprovements oscillations designed in to the study the Super-Kamiokande,improved three K2K photsensors questions and will presented T2K be above. used experiments,height, First, 74m to with diameter larger increase cylindric detectors detectors statistics. each with correspondingtwo to The detectors a will baseline fiducial be volume design built of either 190 includes in ktons.neutrino two a beam The staging 60m part approach, of or in the differentcontrol experiment, locations. a of For more the the intense accelerator systematic beamdetectors uncertainties will [ be will used be to improved increase by statistics, and an upgrade of the current T2K near PoS(NuFact2017)053 ) CP σ ) = δ and and 13 ) 4 . This θ /c 23 =90° =0° 2 2 θ CP ( ( CP CP δ 2 (resp. 3 2 δ δ ) as can be eV 8 10 ◦ 3 σ − 90 10 = Christophe Bronner × CP 4 . δ 2 = (true 2 32 ◦ m s , and to measure the value of 7 ∆ CP Running time (year) δ ) to 23 ◦ 8704, 0 . -like candidate events at the far detec- 1.3MW beam 1year = 10 0 e =

0 2 4 6

ν 0

/ 50 40 30 20 10 60 δ of Error e (degree) ) = CP ν δ 12 θ 2 ( 2 (true 2 -like candidate events), more than 10000 events in ◦ µ ν by the reactor experiments (assumed to be sin 5, sin as a function of exposure (right). . ) 0 13 and CP θ δ 150 µ 2 ) = ν ( [degree]

2 interaction interaction 23 CP θ δ 100 ( 2 HK 2tank staging 10 years 50 1, sin . . The systematic uncertainties are estimated based on the T2K experience . 0 4 1 0 /c 5σ 3σ 2 ) = eV 13 θ 5 -50 2 Systematic uncertainties assumed for the accelerator neutrino sensitivity studies. − . ( 1 2 appearanceappearance 3.0% 3.2% 0.5% 1.5% 0.7% 3.2% 1.5% 3.9% 10 disappearance 3.3% 0.9% 1.0% 3.6% disappearance 3.3% 0.9% 1.1% 3.6% -100 × 6 =0.1 Ability of T2HK to exclude conservation of CP symmetry as a function of the true value of . =0.5 13 23 7 Table 1: 2θ θ 2 2 -150 = sin sin . Using the measurement of sin Normal mass hierarchy For the appearance samples, corresponding to the For the disappearance samples (

ν ν ν ν Mode sample Flux + ND constrained ND independent Far detector Total

2 32 2 21 8 6 4 2 0

10 σ=√χ

m m 2 this parameter with a precision ranging from 7 tor, more than 1000 events are expectedof in events each and running mode, reconstructed with energy clear spectra differences predicted in the for number the different possiblesignificance values for of 57% (resp. 74%) of the possible trueseen values on of figure Figure 1: ∆ each running mode are expected, allowing a precise measurement of the parameters sin 2.3 Expected sensitivities will allow to exclude the conservation of CP symmetry in neutrino oscillations with 5 (left) and precision of the measurement of and the expected improvements comingthe from larger the atmospheric new neutrino near control and sampleare intermediate for summarized detectors, the in far table as detector. well The as size of those uncertainties Hyper-K physics potential not considered). A runningbetween time neutrino and of anti-neutrino 10to running years be modes. with known a and Importantly, 1.3MW the tosidered beam be mass are is sin normal hierarchy in assumed, is this with assumed ∆ part, a and 1:3 the ratio true values of the oscillation parameters con- PoS(NuFact2017)053 , 23 can . θ ) 23 23 θ θ ( 2 ). 2 (fig ) 45) to 0.017 (true Christophe Bronner . 23 0 θ ( 2 . The octant of around the Earth, with 4 ) = e /c ν 23 23 2 θ θ ( 2 eV 2 0.6 and 5 if the hierarchy is normal. The e − sin ν ) , 10 µ 23 θ ν × ( , 2 4 . µ ν significance can still be reached for the as a function of the true value of sin is 1 σ 23 2 32 θ m ∆ 3 ], and the sensitivity was studied using scaled simulation ranges from 0.006 (true sin 6 ) Normal Mass Hierarchy 23 θ ( 2 0.4 0.45 0.5 0.55

3 2 1 0 9 8 7 6 5 4

10 significance for the most favorable values of sin

wrong octant rejection octant wrong σ . Atmospheric neutrinos also have some sensitivity to the octant of σ ) 23 θ ( 2 5), while the expected precision on . Ability of T2HK to determine the octant of 0 ) = 005), the precision on sin . 23 0 θ Atmospheric neutrino oscillations can be studied in Hyper-Kamiokande in a similar way to Using atmospheric neutrinos only, Hyper-Kamiokande can determine the mass hierarchy with Atmospheric neutrinos are produced by the collisions of cosmic rays with particles in the at- ( significance for a number of possible true values of sin ± Figure 2: 2 1 σ . largest values of sin what is done in the Super-K experiment [ sin a wide range of energies (fromon the 100 Earth MeV surface to after several propagating TeV),This over and different makes which distances, will those from reach 10km neutrinos a tomass detector very more hierarchy. located than The interesting propagation 10000km. through to matterpared study modifies to the neutrino vacuum neutrino oscillation, oscillation oscillations, probability in and com- wayscombination that particularly of depend neutrino the of energy the and mass propagationoscillation probability hierarchy. length from there In the is particular electron a to for themass matter a muon hierarchy induced certain flavor, is which resonance normal, happens in and only the for only neutrinos for if anti-neutrinos the if it is3.1 inverted. Expected Sensitivities with atmospheric neutrinos only sensitivity is lower if the hierarchy is inverted, but 3 3 and analysis tools fromcompared Super-K. to In the particular current no Super-K10 improvements analysis years are of of assumed. running the with systematic Theassumed a uncertainties exposure to 187 be considered kton known fiducial corresponds in volume to this detector, part. and the mass hierarchy is no longer mosphere. Those collisions produce an isotropic flux of Hyper-K physics potential 0 also be determined with 3 3. Atmospheric neutrino oscillations PoS(NuFact2017)053 . CP ) δ 23 23 (resp. (right) θ θ ( 2 2 σ 23 sin 0.6 θ . CP δ 0.55 Christophe Bronner 0.5 0.45 significance for the largest values of , in both cases the sensitivity is highly σ 3 0.4

significance for all true values of sin

1 7 6 5 4 3 2 0

Wrong Octant Rejection Octant Wrong χ Δ 2 σ 4 , and with 5 ) , the mass hierarchy can then be determined at better , which is not well constrained by the study of the os- 4 23 ) 23 θ θ 23 ( 2 0.6 2 θ . sin ( ) 2 23 θ ( 2 0.55 for which conservation of CP symmetry can be excluded with 5 CP 0.5 δ 0.45 regardless of whether the true hierarchy is normal or inverted. This combination is also Ability of Hyper-Kamiokande to determine the mass hierarchy (left) and the octant of significance for all values of sin ) σ 23 0.4 θ The first option for the second detector is to follow a staged approach, where a second similar Atmospheric neutrinos are sensitive to the mass hierarchy, but the precision of the measure- for certain combinations of true values of the two. In those cases, the atmospheric neutrinos 1 0 6 5 4 3 2 (

) significance would reach 62% (resp. 78%). In combination with atmospheric neutrinos, the 2 Wrong Hierarchy Rejection Hierarchy Wrong χ Δ 2 σ CP 3 mass hierarchy could be determined at better than 4 allow to break the degeneracy, and increase the ability to reject the4. conservation of Second CP detector symmetry. 4.1 Staged approach detector would be built closeThis to would the allow first to one, increaseviolation and of start the CP taking statistics, symmetry data improving and 6possible the the years true mass ability values after hierarchy. of of the Using the first beam experiment detector. neutrinos to only, the study fraction the of the sin interesting for the search for thecomes violation from of beam CP neutrinos, symmetry: there although canδ most be of degeneracy the between sensitivity the to mass hierarchy and the value of cillations of atmospheric neutrinos alone. Onaccelerator the allow other to hand, measure the precisely this beamhierarchy, parameter, neutrinos mainly but from do due the not J-PARC to have the muchanalysis too sensitivity of to short the the baseline oscillations mass between of the J-PARCthe two and mass types Kamioka. hierarchy. of As neutrinos, With can the a be experiment combined than seen becomes on more 3 figure sensitive to ment depends on the uncertainty on sin using atmospheric neutrinos only. In both(inverted), figures, and blue the (red) width corresponds of to the the bands true corresponds hierarchy to being the normal uncertainty on the true value of 3.2 Expected Sensitivities with beam and atmospheric neutrinos combined Figure 3: Hyper-K physics potential but less than the beam neutrinos.dependent As on can the be true seen value of on sin figure PoS(NuFact2017)053 CP δ Christophe Bronner ], as early as possible. 7 10 9 8 . With a second detector in Korea, the Running (Years) Time 5 7 significance for all possible true values of σ 6 5 5 =0.4 =0.5 =0.6 4 =0.4 =0.5 =0.6 23 23 23 23 23 23 θ θ θ θ θ θ 2 2 2 2 2 2 3 can be seen on figure 2 Atm + Beam Normal) (True sin Atm + Beam Normal) (True sin Atm + Beam Normal) (True sin Atm + Beam Inverted) (True sin Atm + Beam Inverted) (True sin Atm + Beam Inverted) (True sin 23 θ 1

1 6 5 4 3 2 0

Wrong Hierarchy Rejection Hierarchy Wrong χ Δ 2 , similarly to what can be done with atmospheric neutrinos, but with beam CP δ Ability of Hyper-Kamiokande to determine the mass hierarchy using both atmospheric and ac- . For the octant determination, the sensitivity is mainly increased by having a second ) 23 θ The analysis used to study the sensitivity of the experiment with a second detector in Korea The other option studied is to build the second detector in Korea [ ( 2 mass hierarchy can be determined at more than 6 sin detector, regardless of whether ita is second located detector in in Korea or Koreaand allows in the to Japan. value break For of the the potential study degeneracy of between CP the symmetry, mass hierarchy is slightly different for thea beam result neutrino part sensitivities than for the similarhere one configurations is used based might for on look the a different resultson more in reported the recent so this T2K uncertainties far, part. analysis likely as (2016), tosensitivity The with affect of a the model the systematic study different used configurations uncertainty of of model CP thehierarchy focused violation experiment and in obtained with a the this large analysis octant statistics for of experiment. the mass The This would increase the statistics fordetector atmospheric in neutrinos in Kamioka a would very do, similarAs way but the to opens baseline what new a from possibilities second J-PARCcompared for to to the Korea accelerator 295km), is neutrino the much program. their longer beam oscillations than neutrinos would the would become one propagatebaseline sensitive to is through to Kamioka that more the (1000-1100km the expected matter, mass numberthe and of hierarchy. study events therefore of is The CP smaller, but symmetry: disadvantagemaximum, this for of the configuration which would Korean a the also detector differences longer benefit would betweenare the be larger predicted than located spectra for close for the to different firstdifferences true the oscillation would values second maximum make of studied oscillation the with experiment aratio less detector of sensitive in neutrino to Kamioka. and systematic anti-neutrino Those uncertainties events. larger on the predicted Figure 4: celerator neutrinos as a function of(inverted). running time. Blue (red) corresponds to the true hierarchy being normal 4.2 Detector in Korea Hyper-K physics potential PoS(NuFact2017)053 σ sig- . JD 23 θ CP σ 2 δ sin 0.6 significance for 0.55 Christophe Bronner σ 2) 1) × × 2. 0.5 / Atm+Beam (JD+KD) Beam (JD+KD) Atm+Beam (JD Atm+Beam (JD Atm (JD+KD) π ± 0.45 ,106 (2011) (2015) 043C01 0.4 A659 ,

8 6 4 2 0

16 14 12 10

Wrong Octant Rejection Octant Wrong χ Δ 2 6 23 θ 2 sin and determine the mass hierarchy with more than 3 0.6 Nucl. Instrum. Meth. Prog. Theor. Exp. Phys. CP δ 0.55 CERN SPSC-EOI-015 (2017) detector in Korea. ≡ if the true value of this parameter is close to et al. 0.5 CP δ , Proposal for the NuPRISM Experiment in the J-PARC Neutrino Beamline, (Hyper-Kamiokande Proto-Collaboration), Hyper-Kamiokande Design Report, (T2K collaboration), (T2K collaboration), (Super-Kamiokande Collaboration), arXiv:1710.09126 [hep-ex] (Hyper-Kamiokande Proto-Collaboration), arXiv:1611.06118 [hep-ex] 2) 1) . With a 187 kton fiducial mass detector and combining atmospheric and beam × × (right). The width of the bands corresponds to the uncertainty on the true value of 0.45 23 et al. θ 23 et al. et al. et al. et al. et al. θ Sensitivity of different configurations of the Hyper-K experiment to the mass hierarchy (left) and Atm+Beam (JD+KD) Beam (JD+KD) Atm+Beam (JD Atm+Beam (JD Atm (JD+KD) 0.4 KEK-PREPRINT-2016-21, ICRR-REPORT-701-2016-1 http://j-parc.jp/researcher/Hadron/en/pac_1607/pdf/P61_2016-17.pdf The next generation neutrino experiment Hyper-Kamiokande will allow to study precisely

8 6 4 2 0

14 12 10

Wrong Hierarchy Rejection Hierarchy Wrong Δ

χ detector in Japan and KD 2 [1] K. Abe [2] A. Blondel, M. Zito [4] K. Abe [5] K. Abe [3] S. Bhadra [6] K. Abe [7] K. Abe nificance. A second detector ofa identical 295km fiducial mass, baseline located for either acceleratorneutrinos, close neutrinos) to around or the 1000km) first in would one Korea (giving located increase (giving in a the Korea, the longer sensitivity. experiment baseline would be In forsignificance. able particular, accelerator to determine with the a mass hierarchy second with detector more than 6 References Figure 5: the octant of 5. Summary whether CP symmetry is conserved orthe not in octant neutrino of oscillations, the neutrino mass hierarchy and Hyper-K physics potential neutrinos alone. Also a detectorof in the Korea measurement at a smaller off-axis angle, would increase the precision ≡ neutrinos, the experiment can exclude57% the of conservation the of possible CP true symmetry values with of 5