Neutrino Oscillation Detection at the T2K Experiment

Transcription

1 School of Physics and Astronomy Queen Mary University of London Neutrino Oscillation Detection at the T2K Experiment Sean Cooper ( ) 1 April 2016 Supervisor: Dr Teppei Katori SPA6913 Physics Review Project 15 Credit Units Submitted in partial fulfilment of the requirements for the degree of MSci Physics from Queen Mary University of London

2 Declaration I hereby certify that this project report, which is approximately eight thousand words in length, has been written by me at the School of Physics and Astronomy, Queen Mary University of London, that all material in this dissertation which is not my own work has been properly acknowledged, and that it has not been submitted in any previous application for a degree. Sean Cooper ( ) ii

3 Abstract The purpose of the T2K experiment is to precisely determine the θ 13 mixing angle. It is measured by observing ν µ ν e oscillations, resulting in sin 2 2θ 13 = (for normal mass hierarchy and fixed oscillation parameters). θ 13 is the last of the lepton sector mixing angles, governing the rates of neutrino mixing, to be determined. The T2K s experimental setup is explained, and the theory of neutrino oscillations reviewed. The proposed Hyper-Kamiokande experiment is also briefly discussed. iii

4 Contents List of Figures List of Tables vi vii 1 Introduction 1 2 Neutrino Mixing Theory Oscillation Probabilities in a Vacuum The Two Neutrino Approximation Three Neutrinos in a Vacuum Oscillations in Matter Observing Oscillations Measuring a Probability Determining the θ 13 Mixing Angle The T2K Experiment Overview Beam Production Proton Acceleration (J-PARC) Primary Beamline Pion Production Secondary Beamline Beam Performance Monitoring (INGRID) Pre-Oscillation Measurements ND280 Overview The UA1 Magnetic Yoke The Pi-Zero Detector (PØD) Time Projection Chambers (TPCs) Fine Grained Detectors (FGDs) The Electromagnetic Calorimeter (ECal) iv

5 3.3.7 The Side Muon Range Detector (SMRD) Post-Oscillation Measurements The Super-Kamiokande Far Detector ν µ and ν e Event Reconstruction The T2K s θ 13 Measurements Eliminating Unwanted Backgrounds The θ 13 Results The Hyper-Kamiokande Overview Beam Production Pre-Oscillation Measurements Post-Oscillation Measurements Conclusion 36 Bibliography 37 v

6 List of Figures 2.1 The probabilities of measuring a muon neutrino as a different flavour, over some relatively short range The neutral-current interaction ν e,µ,τ e ν e,µ,τ e that applies to all neutrino and antineutrino flavours The charged-current interactions of ν e and ν e with the electrons present in matter Passage of the muon neutrino beam from J-PARC to Super-K. [15] A comparison of how off-axis positioning influences muon neutrino disappearance, and beam profile, 295 km from the source. [16] A rudimentary diagram of a drift tube LINAC. The arrows denote the particles relative velocities A schematic showing the primary and secondary beamlines at J- PARC. [15] A side view of the secondary beamline. A close-up of the target station is also shown. [16] A cross section of the magnetic focusing horn containing the graphite target. [15] The predicted neutrino flux at the Super-K for different magnetic horn currents. [16] The INGRID on-axis detector. [21] An exploded cross section of the ND280. [15] A schematic of the PØD, the beam enters from the left side of the figure. [22] A simplified cut-away drawing of a TPC. [24] An example event inside the ND280 s tracking section. A neutrino appears to have undergone deep inelastic scattering inside the FGD1. A product of an interaction outside of the tracking section can be seen in the top-left. A more typical neutrino event would involve fewer particle tracks than seen here. [24] vi

7 3.13 A diagram of the Super-Kamiokande. [15] A cone of Cherenkov radiation (blue) emitted by a charged particle (red) Two example event displays from the Super-K. [15] These plots show the possible values of sin 2 2θ 13 for all possible values of δ CP. The top plot (a) shows the values assuming normal mass hierarchy, and the bottom plot (b) assumes an inverted mass hierarchy. The 68% and 90% confidence regions are clearly shown, along with several best-fit lines. [14] A schematic view of the proposed Hyper-K detector. [8] A cross section of one of the Hyper-K s two cylinders. [8] List of Tables 3.1 The fraction of neutrino flux sourced from each type of meson. The left columns show percentages relative to their specific flavour, and the right columns show percentages relative to the total neutrino flux. [16] The oscillation parameters used to achieve the plots seen in figure 4.1. [14] vii

8 1 Introduction In 1930, Wolfgang Pauli postulated the existence of a small, neutral particle. This particle, later dubbed the neutrino, was proposed to explain the missing momenta seen in beta decay experiments. These neutral leptons became part of the Standard Model of particle physics, and were assumed to be massless. In a paper first published in 1957, Bruno Pontecorvo postulated the idea of neutrino mixing. [1] This mixing was proposed as a parallel to Kaon mixing, a process in which charge-parity violation is observed. This theory of neutrino oscillations allows neutrinos to change their leptonic charge, being observed as one flavour at one time, and another flavour at a later time. The theory arises when the weakly interacting neutrinos are rewritten as superpositions of mass eigenstates. This requires that massive neutrino states exist, something which ran counter to the Standard Model. In the late 1960 s, the Homestake Experiment measured the neutrino flux coming from the Sun. [2] These experiments measured a deficit in the number of electron neutrinos they had expected, sparking what became known as the Solar Neutrino Problem. Later experiments confirmed that this solar neutrino deficit was a direct result of neutrino oscillation. Neutrino oscillation probabilities are governed by parameters called mixing angles. The purpose of the T2K experiment is to precisely measure θ 13, the last unknown lepton sector mixing angle. [3] This is done by looking for the appearance of electron neutrinos in a muon neutrino beam. 1

9 2 Neutrino Mixing 2.1 Theory Oscillation Probabilities in a Vacuum Let us consider the weakly interacting neutrinos of the Standard Model: ν e, ν µ and ν τ. Each of these neutrinos, along with its charged counterpart, forms one of three generations: electronic, muonic or tauonic. As such, each family is attributed a distinct lepton number. Under weak interactions these three lepton numbers are conserved independently, however neutrino oscillation offers the possibility of forgoing this conservation law. Instead it considers the probability of a neutrino, of some definite flavour, to be later measured as a different flavour altogether. In the following equations, the Dirac notation ν will represent the quantum mechanical state of a neutrino. Since this state exists over some vector space, we can decompose it into a linear combination of the basis elements spanning that space. As such, we will construct a basis from the mass eigenstates of the flavourful neutrinos: [4] ν l = Uli ν i (2.1) i Here, l = e, µ, τ labels our definite flavour neutrinos, and i = 1, 2, 3 labels the definite mass eigenstates from which they are comprised. The key observation, is that we can write each of our three Standard model neutrinos as a superposition of the same mass eigenbasis. The coefficients relating the flavourful neutrinos to their mass eigenstates are said to be elements of a unitary matrix, U, known as the Pontecorvo-Maki-Nakagawa-Sakata mixing matrix. 1 However, whilst U determines the fraction of each eigenstate present at some instantaneous time, the ratios of these eigenstates at a later time are not necessarily constant. In natural units, c = = 1, 1 Some extensions of the Standard Model, such as the seesaw mechanism, do not preclude the possibility of a non-unitary mixing matrix. However, for the purposes of this report the writer will only consider unitary mixing matrices. 2

10 the time evolved mass eigenstates can be expressed as: ν i (t) = e iĥt ν i (2.2) When applied to the eigenstate ν i, the Hamiltonian, Ĥ, yields the corresponding energy eigenvalue E i. Upper bounds on the neutrino masses have been calculated, with them expected to be of the order of 1 ev/c 2 or lower. [5] Since these masses are very small, and lowest currently detectable neutrinos are orders of magnitude more energetic, we can employ an ultrarelativistic approximation: E i = p 2 + m 2 i (2.3) Here, p denotes the neutrino s momentum and m i labels the mass of each eigenstate. Since p >> m i we shall consider the Taylor series of E i about m i = 0. Discarding terms of order m 4 i or higher we find: E i p + m2 i 2p E + m2 i 2E (2.4) By combining this result with (2.2), we can write the time evolved mass eigenstate in terms of m i and the neutrino s total energy E: ν i (t) = e iel e im2 i L/2E ν i (2.5) Since these neutrinos are ultrarelativistic, and we are in natural units, the parameter t has been replaced by the distance travelled L. Finally, consider the probability that, at some time t, a neutrino known to initially known to be of flavour l is measured as flavour l : P l l = ν l ν l (t) 2 2 = U l iulie im2 i L/2E i (2.6) Thus we have derived the oscillation probability, of some flavourful neutrino, in terms of the eigenstate masses and the mixing matrix The Two Neutrino Approximation The equations in were found without reference to the number of neutrinos that exist. Back in 1957, when Bruno Pontecorvo first postulated the idea of neutrino oscillation, there were only two known neutrino flavours. Today s Standard Model of 3

11 course includes three neutrinos, however a two flavour approximation often proves a reasonably accurate one. For example, Figure 2.1 sees that an initially muon neutrino is much more likely to be detected as either a muon-type or tau-type neutrino than it is to be detected as an electron neutrino. Atmospheric neutrinos created by cosmic rays entering our atmosphere are predominantly muonic. Whilst the suppression of electron neutrinos seen in this plot is only valid over a relatively short range, it is valid over the distance that the atmospheric neutrinos travel. In this case, a two neutrino approximation that ignores electron neutrinos altogether can be a reliable one. 1.0 Detection Probability Ν e Ν Μ Ν Τ Figure 2.1: The probabilities of measuring a muon neutrino as a different flavour, over some relatively short range. Since in this approximation we only have two neutrino flavours, it is sufficient to consider just two mass eigenstates. As such, the neutrino mixing matrix is a 2x2 unitary matrix. One key feature of unitary matrices is that they preserve norms, thus we can conveniently consider our matrix to take the form of a rotation operator: U = [ cos θ sin θ ] sin θ cos θ (2.7) Now the 2x2 mixing matrix is a function of just one parameter, θ, the mixing angle. By substituting the elements from this matrix into (2.6) we arrive at the probability of our flavour l neutrino oscillating to the second flavour l : P l l = ] sin θ cos θ [e im2 1 L/2E e im2 2 L/2E 2 (2.8) 4

12 With some manipulation this equation can be reduced to the more simplified form: ( ) m P l l = sin 2 2θ sin 2 2 L 4E (2.9) Here, the new parameter m 2 m 2 2 m 2 1 is known as the squared mass difference. One can observe that θ determines the maximum probability amplitudes, and m 2 the period of oscillations. It is also trivial to notice that maximal mixing would occur with sin 2 2θ = 1, i.e. θ = π/4. Let us now return to the atmospheric neutrino example. If we were to only consider muon and tau neutrinos, then in the case of maximal mixing our flavour states could be written as: ν µ = 1 2 ( ν 1 + ν 2 ) ν τ = 1 2 ( ν 1 ν 2 ) (2.10) Experimental measurements of the atmospheric mixing angle have found sin 2 2θ > [6] As such, the use of a maximal mixing two neutrino approximation is sufficient for many calculations Three Neutrinos in a Vacuum The Standard Model of particle physics contains three generations of neutrino; naturally then the most precise formalism of neutrino mixing is one which describes all three of these flavours. Consequently, a 3x3 mixing matrix is required. In a 2x2 mixing matrix was conveniently expressed as a rotation about the angle θ. A higher order PMNS matrix requires more parameters, nonetheless it can still be written in terms of rotation operators. needed: [7] In this case, three distinct rotations are c 12 s 12 0 c 13 0 s 13 e iδ CP R 12 = s 12 c 12 0 R 13 = R 23 = 0 c 23 s s 13 e iδ CP 0 c 13 0 s 23 c 23 (2.11) Here, c ij cos θ ij and s ij sin θ ij. We see that three mixing angles, θ 12, θ 13, and θ 23, are required, as well as the phase angle δ CP. This phase, δ CP, parametrises the contribution due to charge-parity violation. If there is no CP violation then we would observe δ CP = 0, with all δ CP 0 corresponding to at least some CP violation 5

13 in the lepton sector. Current experiments are unable to directly measure δ CP, however proposed projects such as the Hyper-Kamiokande hope to determine this CP phase. [8] A large δ CP could explain the universe s matter/antimatter asymmetry. The rotations (2.11) can then be combined to give the PMNS matrix: c 12 c 13 s 12 c 13 s 13 e iδ CP U = R 12 R 13 R 23 = s 12 c 23 c 12 s 23 s 13 e iδ CP c 12 c 23 s 12 s 23 s 13 e iδ CP s 23 c 13 s 12 s 23 c 12 c 23 s 13 e iδ CP c 12 s 23 s 12 c 23 s 13 e iδ CP c 23 c 13 (2.12) It is encouraging that this matrix appears to be the leptonic analogue of the Cabibbo- Kobayashi-Maskawa quark mixing matrix. However, we must consider the possibility that neutrinos are Majorana particles. If this is indeed the case, the PMNS matrix will differ from the CKM matrix by including two additional parameters. These parameters, α 1 and α 2, are the Majorana phases, which can be applied to the mixing matrix as so: [9] e iα 1/2 0 0 U = U 0 e iα 2/2 0 (2.13) Whilst these Majorana phases would have an effect on some processes, including neutrinoless double beta decay, they do not make any contribution to neutrino oscillation. As such, when considering neutrino mixing there is no advantage to using the corrected matrix, U, over the previously derived one. At present it is still unknown whether neutrinos behave as Majorana particles or not. If they are found to be Majorana particles then the neutrino be would identical to its antimatter counterpart. In the oscillation probability was written in terms of the squared mass difference m 2. By further simplifying (2.6) the probabilities become: P l l = δ ll 4 ( m 2 ) Re[UliU l iu lj Ul j] sin 2 ij L 4E i>j + 2 ( m 2 ) (2.14) Im[UliU l iu lj Ul ij L j] sin 4E i>j Here, m 2 ij m 2 i m 2 j. By comparing this to the mixing matrix found in (2.12), we see that all Im[Uli U l iu lj Ul j ] = 0 if there is no CP violation (i.e. δ CP = 0). This equation can be expanded in terms of the mixing angles, in a similar manner to (2.8), however the result is somewhat unwieldy. 6

14 2.1.4 Oscillations in Matter Thus far we have considered the process of neutrino oscillation in vacua. Let us not forget, however, that oscillation is not the only process that neutrinos undergo; they are free to weakly interact with other particles. The way these matter effects modify neutrino oscillations was first considered by Lincoln Wolfenstein in [10] The formalisation of the propagation of neutrinos through matter is now known as the Mikheyev-Smirnov-Wolfenstein effect. ν e,µ,τ ν e,µ,τ Z 0 e e Figure 2.2: The neutral-current interaction ν e,µ,τ e ν e,µ,τ e that applies to all neutrino and antineutrino flavours. When considering matter s effect on neutrino oscillation, we are primarily interested in the processes which preserve our neutrinos i.e., scattering processes, particularly those that result in coherent forward scattering. Figure 2.2 depicts the neutralcurrent interaction neutrinos undergo in matter, occurring with electrons and nucleons alike. Importantly, this process is the same for all flavours of neutrino and antineutrino. This causes a phenomenon analogous to the optical refraction of light. e ν e ν e e W ± W ν e e e ν e (a) ν e e ν e e (b) ν e e ν e e Figure 2.3: The charged-current interactions of ν e and ν e with the electrons present in matter. Besides neutral-current interactions, neutrinos will also undergo charged-current 7

15 interactions when passing through matter. The leptons of which stable matter is comprised are predominantly electrons. As such, we need only consider the chargedcurrent processes in which neutrinos exchange W bosons with electrons. Figures 2.3a and 2.3b depict the relevant interactions. Crucially, these processes only scatter electron neutrinos (and electron antineutrinos). The unequal numbers of particles and antiparticles in ordinary matter can cause a matter-induced CPT violation. [11] Experiments searching for CP violation intrinsic to the oscillation process will have to account for these fake violations. We can introduce these weak interactions into our neutrino mixing model by considering them as a potential acting on our flavourful neutrinos. This allows us to write the system s Hamiltonian, Ĥ = Ĥ0 + V, with V being a potential perturbing our vacuum Hamiltonian H 0. [12] The potential V can be thought of as a combination of a neutral-current potential and a charged-current potential. As such, this potential acting on some flavour l neutrino is V l = δ le V cc + V nc with the Kronecker delta denoting that only electronic neutrinos feel the charged-current contribution. Since our neutral-current contribution effects all neutrino flavours equally, it merely introduces a phase shift to our system, and is of no physical importance. [10] We can therefore ignore these contributions, simplifying our potential to one that only acts on the electron neutrino: 2 [13] V e = 2G F n e (2.15) Here, G F is the Fermi coupling constant, and n e is the electron density of the medium. It can be shown, using an ultrarelativistic approximation similar to (2.4), that Ĥ0 written in terms of the mixing matrix is: [12] Ĥ 0 = 1 2E UM2 diagu (2.16) With M 2 diag diag(m2 1, m 2 2, m 2 3). For simplicity, we will now consider the two neutrino approximation seen in In this approximation H 0 simplifies to: Ĥ 0 = m2 4E [ cos 2θ sin 2θ ] sin 2θ cos 2θ (2.17) With this vacuum Hamiltonian now established we can write the Hamiltonian in 2 The potential acting on an electron antineutrino is negative instead of positive. It is of the same magnitude as the potential seen in (2.15). 8

16 matter: Ĥ = m2 4E [ cos 2θ sin 2θ ] [ ] sin 2θ V e 0 + cos 2θ 0 0 (2.18) We are free to add any multiple of the identity to the Hamiltonian without changing its physical meaning. Hence: Ĥ = m2 4E [ cos 2θ sin 2θ ] sin 2θ + cos 2θ [ V e /2 0 0 V e /2 ] (2.19) In this form it is difficult to gain any intuitive sense of what difference this matter potential has made to the neutrino mixing. However, in the case where the matter density is constant, we can restructure this Hamiltonian as: Ĥ = m2 m 4E [ cos 2θ m sin 2θ m sin 2θ m cos 2θ m ] (2.20) This is reminiscent of the vacuum Hamiltonian except with two new parameters: m 2 m, the effective squared mass difference, and θ m, the effective mixing angle in matter. With clearly: Ĥ cast in a form analogous to (2.17), the oscillation probability is P l l = sin 2 2θ m ( ) m sin 2 2 m L 4E (2.21) By equating (2.19) and (2.20) we are able to determine these matter parameters in terms of the vacuum parameters: [12] sin 2 2θ m = 1 R sin2 2θ and m 2 m = R m 2 (2.22) Where R is the resonance factor: R ( cos 2θ 2V ) 2 ee + sin 2 2θ (2.23) m 2 In a vacuum the electron density is zero. In this limit V e 0, therefore R 1 and the vacuum equations are recovered. Interestingly, maximal mixing occurs in cases where 2V e E/ m 2 = cos 2θ, resulting in sin 2 2θ m = 1. [13] This resonant condition maximises the oscillation amplitudes, even in cases where vacuum mixing would be very small. 9

17 2.2 Observing Oscillations Measuring a Probability At first, a probability can seem like a very abstract quantity to measure. Let s consider a simple example in which a fair coin is flipped. If the coin is flipped just a few times the results appear unpredictable. However, if the coin is flipped many times we would see that it lands heads up approximately 50% of the time. By using a large sample size the probability becomes an easily predicted statistical effect. Let s now consider a neutrino beam that is generated as just one flavour, l. At some later time the beam hits a target, allowing us to measure its flavour composition. If many neutrinos are measured, the probability of a neutrino having oscillated to flavour l is: P l l = Number of l detected Total number of neutrinos (2.24) However, neutrinos do not interact very abundantly with matter. This means detecting a large sample size is difficult, and so the random nature of the oscillations introduces uncertainty in the calculated probabilities Determining the θ 13 Mixing Angle The T2K experiment looks for the appearance of electron neutrinos in a muon neutrino beam. This rate of appearance is then used to infer the ν µ ν e oscillation probability, from which the θ 13 mixing angle can be determined. The full equation for this oscillation probability, including the terms describing matter effects, is given by: [14] 1 P νµ νe = (A 1) 2 sin2 2θ 13 sin 2 θ 23 sin 2 [(A 1) ] α A(1 A) cos θ 13 sin 2θ 12 sin 2θ 23 sin 2θ 13 sin δ CP sin sin A sin[(1 A) ] α + A(1 A) cos θ 13 sin 2θ 12 sin 2θ 23 sin 2θ 13 cos δ CP cos sin A sin[(1 A) ] + α2 A 2 cos2 θ 23 sin 2 2θ 12 sin 2 A (2.25) 10

18 Here: α = m2 21 1, = m2 m 2 32 L, and A = 2 E 2G 32 4E F n e. This equation is difficult m 2 32 to interpret, however over the L/E range covered by the T2K it can be simplified to: [14] P νµ νe sin 2 2θ 13 sin 2 θ 23 sin 2 m2 32L 4E (2.26) We see that the rate of ν e appearance in the beam can be used to calculate θ 13 providing that we know the parameters: θ 23, m 2 32, L, and E. Using this method to calculate θ 13 requires the squared mass difference m This means that the experimental value of θ 13 is different depending on whether the neutrino mass hierarchy is normal or inverted. The neutrino mass hierarchy refers the orderings of the mass eigenstate masses. In the normal hierarchy m 2 1 < m 2 2 < m 2 3, whereas in the inverted hierarchy m 2 3 < m 2 1 < m 2 2. It is still unknown whether the mass hierarchy is normal or inverted, so the sign of m 2 32 could be positive or negative. This means that different values of θ 13 will be calculated depending on which hierarchy is assumed. 11

19 3 The T2K Experiment 3.1 Overview The T2K experiment looks for neutrino oscillation in a beam of muon neutrinos produced at the J-PARC facility in Tokai, Japan. The premise is that by measuring the beam twice, shortly after production and then again at the Super-Kamiokande observatory, the neutrinos can be characterised both before and after oscillation. 1 The difference in these two measurements then allows for the muon neutrinos oscillation probabilities to be calculated. Since neutrinos seldom interact with matter, the beam conveniently passes through the Earth s crust with no need of a pipe to contain it. Figure 3.1 shows the beam s path from J-PARC to the Super-K. Clearly we can divide the experiment into three sections: The J-PARC facility where the beam is produced. The near detector (ND280), 280 m from the source. This is where the preoscillation measurements are made. The far detector (Super-K), 295 km from the source. It is here that the postoscillation measurements are made. Figure 3.1: Passage of the muon neutrino beam from J-PARC to Super-K. [15] 1 The Super-K is based near the Kamioka section of Hida City, hence the experiment s namesake: Tokai-to-Kamioka. 12

20 The T2K s primary goal is to precisely determine the θ 13 mixing angle, which governs the rate of ν µ ν e oscillations. However, as seen in figure 2.1, the appearance of electron neutrinos is very scarce over these ranges. To maximise these oscillations, the detectors are positioned off-axis from the beam s centre. Figure 3.2 shows that setting the detector 2.5 off-axis results in a narrow-band of energies centred around 0.6 GeV. This setup is clearly optimal, maximising the conversion of muon neutrinos to other flavours. Figure 3.2: A comparison of how off-axis positioning influences muon neutrino disappearance, and beam profile, 295 km from the source. [16] 3.2 Beam Production Proton Acceleration (J-PARC) The production of the T2K s neutrino beam begins with the acceleration of protons. These protons then collide with a target, producing a shower of pions and kaons. The muon neutrinos are then generated during the decay of these mesons. The beam initially consists of hydrogen anions produced by a negative ion source. These H ions are then accelerated up to 400 MeV by a 330 m long linear accelerator (LINAC). 2 [17] The majority of the H acceleration takes place in between the 2 Unless otherwise stated, the energy values given from here on are kinetic energies. 13

21 accelerator s drift tubes. As illustrated by figure 3.3, there are electric fields of alternating polarity in each drift tube. A radio frequency driven voltage is used to generate these electric fields. As each particle exits a tube, the oscillating voltage flips the electric fields. This means that the H ions always see a positive electric field in front of them and a negative one behind them. As such, the particles are only ever accelerated and not decelerated. This method requires the ions to spend the same amount of time in each drift tube. Consequently, as the beam speeds up the tubes must increase in length. Due to this constraint, LINACs are not ideal for accelerating particles to high energies. Figure 3.3: A rudimentary diagram of a drift tube LINAC. The arrows denote the particles relative velocities. After exiting the LINAC, the beam is injected into a rapid-cycling synchrotron (RCS). As they are injected, the H ions pass through a charge-stripping foil. This foil removes the unwanted electrons, leaving just a proton beam behind. The beam s path into the RCS is guided by a magnetic field. As such, any unstripped particles will follow a different trajectory and not make it into the synchrotron. Not only does stripping the beam leave us with the desired protons, but it also means that the same voltage used to accelerate the beam towards the foil will accelerate it away. Synchrotrons overcome the size constraint of LINACs by directing their beam along a closed path. As the electric fields accelerate the protons, the magnetic field used to guide them must increase. Since the magnetic field is time-dependant, the protons are sent through in bunches instead of as a continuum. Just two of these bunches are accelerated by the RCS per cycle. [3] The RCS accelerates the beam up to 3 GeV. Approximately 5% of the proton bunches are directed into the main ring (MR) synchrotron, the remainder are supplied to the Material and Life Science Facility to be used in other experiments. [3] The MR has two extraction points: one used for hadron beamline experiments, and the other to produce the T2K s neutrino beamline. The proton bunches headed to the neutrino beamline circle the MR just once before extraction. The MR accelerates these protons up to 30 GeV, however it has the capacity to increase this energy to 14

22 50 GeV in the future. [3] Primary Beamline After the accelerated proton bunches leave the MR they form what is known as the primary beamline. As seen in figure 3.4, the primary beamline can be split into three sections: the preparation section, the arc section, and the final focusing section. The primary beamline is where any fine tunings, and final adjustments, are made to the beam before it hits the target. Figure 3.4: A schematic showing the primary and secondary beamlines at J-PARC. [15] During the preparation section the extracted proton beam is tuned by 11 normal conducting magnets. [3] Of these magnets there are 4 steering magnets, 2 dipole magnets, and 5 quadrupole magnets. The steering and dipole magnets are used to guide the beam as it travels from the extraction point to the arc section. Quadrupole magnets create a field that is at it s minimum at the beam s centre, but increases rapidly with radial distance. For this reason, the quadrupole magnets are used to focus the beam. As the proton beam travels through the arc section it is bent through a total angle of 80.7, resulting in a 104 m radius of curvature. [3] This bending aims the beam in the direction of Kamioka (westwards across Japan). Superconducting magnets are 15

23 used to bend the beam through this dramatic angle. The final focusing section marks the last segment of the primary beamline, after which the protons will hit the graphite target. 10 normal conducting magnets are used: 4 steering magnets, 2 dipole magnets, and 4 quadrupole magnets. These magnets focus and guide the beam onto the target, which is just 2.6 cm in diameter. [3] Focusing the beam is required to maximise the number of interactions with the target. The magnets in this section are also used to angle the beam downwards with respect to the horizontal. [3] The proton bunches are monitored throughout the primary beamline. These readings allow the beam s tuning to be optimised, and therefore minimise the beam loss. Various properties of the beam are measured including: beam intensity and beam position. Since the monitors are numerous, it is possible to reconstruct each bunch s beam profile at many stages though the primary beamline Pion Production The 30 GeV protons from the primary beamline bombard a stationary graphite target producing a variety of hadrons. The target is 2.6 cm in diameter and 91.4 cm long, which corresponds to approximately 1.9 interaction lengths. [3] As the beam hits the target a large amount of energy is deposited, this results in an almost instantaneous increase in temperature. [18] Graphite was chosen as a target material since its melting point is relatively high. It is also stable, which makes it an easy substance to work with. Whilst a higher density might allow for more interactions, substances denser than graphite are unlikely to withstand the heat load. [19] When the incident protons interact with the target nucleons hadrons are produced. These produced hadrons, the large majority of which are pions or other mesons, then either decay or undergo secondary interactions with the target. The lack of knowledge about what hadrons are produced here is the largest source of uncertainty on the T2K s initial neutrino flux measurements. [20] The NA61/SHINE experiment at the CERN SPS replicates the proton-graphite interactions that take place in the neutrino beamline, cataloguing the hadrons produced by these collisions. It is hoped that NA61/SHINE can help reduce the total systematic uncertainty on the neutrino flux to less than 5%. [20] The NA61/SHINE collides 31 GeV protons with two different targets. In one experiment, a thin 2 cm target is used. [20] Here, the secondary hadrons produced from the primary proton-nucleon interactions are measured. These secondary hadrons account for approximately 60% of the T2K neutrino flux. [20] The remaining 40% 16

24 comes from the decay of tertiary hadrons produced in successive interactions. This is measured in the second experiment, which utilises a replica of the T2K target. The data from NA61/SHINE is then used to build and improve Monte Carlo simulations of the T2K neutrino beamline. Table 3.1 shows Monte Carlo data of the predicted neutrino flux from the different hadron decays. The data shows that whilst small, the ν e production is measurable. Any of this ν e flux should be accounted for to minimise errors in the T2K s results. Flux of each neutrino flavour (%) Flux of all neutrino flavours (%) Parent ν µ ν µ ν e ν e ν µ ν µ ν e ν e Secondary π ± K ± KL Tertiary π ± K ± KL Table 3.1: The fraction of neutrino flux sourced from each type of meson. The left columns show percentages relative to their specific flavour, and the right columns show percentages relative to the total neutrino flux. [16] The majority of the muon neutrino beam is sourced from pion decays, with some fraction from kaons. Leptons produced during these decays may then also decay, producing neutrinos of various flavours. The π + meson s primary decay mode is π + µ + ν µ. In the Standard Model, the neutrinos produced in these π + decays must always be left-handed. For helicity to be conserved the emitted antilepton should also be left-handed. In the limit where the electron s mass is negligible, we see that an emitted positron must be right-handed. This would defy helicity conservation. Of course the electron is not massless, however it is much less massive than the muon. This means that electronic decay mode is helicity suppressed, with the π + meson much more likely to decay to a muon neutrino. The antimuon produced from this process may later go on to decay as so: µ + e + ν µ ν e. 17

25 3.2.4 Secondary Beamline The secondary beamline defines the final section of the neutrino beamline at J- PARC. It follows the proton-target interaction products as they decay into the final neutrino beam. This secondary beamline is comprised of three parts: the target station, the decay volume, and the beam dump. Figure 3.5 shows a cross section of the beamline, detailing each of these three sections. Figure 3.5: A side view of the secondary beamline. A close-up of the target station is also shown. [16] The secondary beamline is water cooled and filled with helium gas. Helium gas reduces the production of nitrogen oxides and radioactive materials; it also serves to reduce the number of pions that are absorbed before they decay. [18] The target station s beam window separates this gas filled environment from the primary beamline s near vacuum. It is made from a helium-cooled titanium-alloy, and is 0.3 mm thick. [3] This allows the high energy protons to pass through, but prevents the gas from escaping. Once the protons enter the target station they travel through a 1.7 m long graphite baffle. The proton beam passes through a 3 mm diameter hole in this baffle. [3] Its purpose is to remove any off-course protons, preventing them from damaging any of the first magnetic horn s components. After exiting the baffle the beam is measured by an optical transistor radiation monitor (OTR). This OTR has a thin titaniumalloy foil angled at 45 to the proton beam. [3] As the beam passes through the foil a narrow cone of visible light is produced. This transition radiation can then be measured to attain a two-dimensional profile of the beam. The OTR has an eightposition carousel that allows the foil to be switched for different beam energies. After passing through the OTR, the beam hits the target (as described in 3.2.3). 18

26 There are three magnetic focusing horns in the target station. These magnetic horns generate toroidal magnetic fields that allow the positive mesons to be focused independently of the negative ones. This selective focusing keeps π + mesons on-axis, but removes π mesons, increasing the ν µ to ν µ ratio. Neutral particles, such as π 0 mesons, cannot be focused. Instead Monte Carlo simulations must be used to estimate their trajectories. Figure 3.6: A cross section of the magnetic focusing horn containing the graphite target. [15] Figure 3.7: The predicted neutrino flux at the Super-K for different magnetic horn currents. [16] Figure 3.6 shows a cross section of the first magnetic horn, with the graphite target contained within it. The graphite target, light green in the image, is fitted inside the horn. The target must be fitted internally to ensure that all of the hadrons are focused, regardless where along the target they are produced. The three magnetic focusing horns are essential for a well-directed neutrino beam. A focused, on-axis, secondary beamline corresponds to a narrow neutrino beam. This can be seen in 19

27 figure 3.7, where the neutrino flux at the Super-K is compared with and without the magnetic horns activated. After the interaction products leave the target station they travel through the decay volume. As the name suggests, this is where the majority of the mesons decay and the neutrinos are produced. Any particles that do not decay during this 96 m long decay volume will reach the beam dump. The beam dump s core is made of 75 tons of graphite, and is contained within a helium vessel. The iron plates surrounding this vessel add an additional 2.4 m of thickness. Any hadrons, or tertiary muons below 5 GeV/c, that reach this point will be stopped. [3] Only muons above 5 GeV/c can pass through the beam dump, and into the muon monitor. 3 [3] The majority of these muons would have been produced during the π + decays. This means that data from the muon monitor can be used to characterise the neutrinos also produced during these decays. As previously seen in table 3.1, this corresponds to the majority of the neutrinos in the T2K s beam. As such, the muon monitor is able to measure the neutrino beam s direction to a precision better than 0.25 mrad, and its intensity to better than 3%. [3] Due to this experiment s long baseline, high precision measurements are important to accurately calculate the neutrino flux. A second detector, just downstream from the muon monitor, measures the flux and momentum distribution of the remaining muons. [3] Beam Performance Monitoring (INGRID) The Interactive Neutrino GRID (INGRID) is an on-axis neutrino detector 280 m downstream from the proton beam s target. [21] INGRID is comprised of 16 identical modules, their arrangement can be seen in figure 3.8. The centre of INGRID s cross sits directly inline with the direction of the primary proton beamline. [3] This 0 point is the only part of the detector covered by two modules. INGRID s main purpose is to monitor the neutrino beam direction to a precision better than 1 mrad. [21] This is achieved by comparing the number of neutrino events observed in each module, providing a measurement of the beam s centre. Each of INGRID s modules consists of nine iron target plates and 11 tracking scintillator planes, arranged in alternating layers. [21] The iron plates serve as an interaction medium for the incident neutrinos. Any charged particles produced should then be detected in the scintillating planes downstream of the interaction. Since the neutrinos are not directly detectable, their interaction products will begin midway through the module. Particles detected at the start of the module are 3 The majority of the particles detected by the muon monitor are actually antimuons. 20

28 disregarded, as it is not possible to tell whether they were sourced from a neutrino interaction. The modules are surrounded by veto scintillator planes. These veto planes identify particles entering from the side of the module, allowing them to be disregarded. In addition to INGRID s standard modules, there is unique proton module located at the cross s centre. The purpose of this module is to determine through which channel the neutrinos have interacted. It uses scintillating bars to track the protons, pions, and muons, produced during neutrino interactions [21] Figure 3.8: The INGRID on-axis detector. [21] 3.3 Pre-Oscillation Measurements ND280 Overview The ND280 takes measurements of the neutrino beam, 280 m from its source. This detector s goal is to take pre-oscillation observations of the beam, at the same offaxis angle as the Super-K. The beam s flavour composition, energy spectrum, and interaction rates are measured. [3] Figure 3.9 shows a breakdown of the ND280 s components. These components are actually a selection of different subdetectors, all surrounded by the UA1 Magnetic Yoke. The ND280 s core consists of the Pi- Zero Detector, followed by a series of TPCs and FGDs. These central detectors are surrounded by a series of Electromagnetic Calorimeters. There is also a Side Muon Range Detector located inside of the UA1 s yoke. Despite housing all of these 21

29 detectors, the internal volume enclosed by the UA1 magnet is only 7.0 m x 3.5 m x 3.6 m. [3] Figure 3.9: An exploded cross section of the ND280. [15] The UA1 Magnetic Yoke The UA1 magnet consists of 16 C-shaped flux return yokes, eight of which can be seen in figure 3.9. [3] The remaining eight sit directly opposite them, forming a ring covering the four sides parallel to the beam s direction. The UA1 magnet was repurposed for this experiment after its previous usage during the UA1 and NOMAD experiments at CERN. It generates a horizontally orientated dipole field with a field strength of 0.2 T. [3] This magnetic field allows the detectors to distinguish between the positive and negatively charge particles generated during the neutrino interactions. It also allows the momenta of these particles to be easily determined The Pi-Zero Detector (PØD) The first ND280 s subdetector that the beam encounters is the PØD. The PØD s primary goal is to measure the neutral current process ν µ +N ν µ +N +π 0 +X on a water (H 2 O) target. [22] These single π 0 producing neutral current interactions form a significant background in the ν e events observed at the Super-K. [23] In the Super-K, electron neutrinos undergo charged current quasi-elastic (CCQE) interactions. High 22

30 energy electrons generated by these interactions will then trigger electromagnetic showers. The aforementioned π 0 mesons decay into two photons, each of which can also cause an electromagnetic shower. Generally, the Super-K can identify these photon sourced showers since they come in pairs, whereas the CCQE induced showers are sourced from single particles. However, if only one of the photon showers is observed, the signal becomes indistinguishable from an electron sourced one. This means that νµ neutral current events can be falsely observed as νe events. The number of false νe events due to this background can be estimated by using the PØD s measurements of the neutral current process s cross section. Figure 3.10: A schematic of the PØD, the beam enters from the left side of the figure. [22] The PØD contains a total of 40 scintillator modules, or PØDules, which are assembled into four super-pødules. [22] These super-pødules can be seen in figure The Upstream Ecal and Central ECal super-pødules consist of seven PØDules alternating with seven steel-clad lead sheets. [22] Any electromagnetic showers that occur here will be contained by these lead sheets. These ECal super-pødules allow any signals caused by interactions before or after the water target to be rejected. Instead of a lead containment sheet, the Water Target super-pødules have a brass 23

31 sheet and water bladder between each scintillating PØDule. Each of these Water Target super-pødules contains 13 scintillating PØDules. The PØD contains over 1900 kg of water for the π 0 producing interactions to occur within, emulating the water filled Super-K. [3] Each PØDule is a plane formed from 260 scintillating bars. [22] Each plane consists of one layer of horizontal bars and one layer of vertical bars. [3] A wavelength-shifting fibre is threaded through each bar. A scintillator fluoresces when ionising radiation passes through it. Luminescent materials in the scintillator are excited by the incoming particle. This excitation energy is then re-emitted in the form of photons. An optical fibre then guides this light to a photodetector at the edge of the PØDule. These fibres have been doped with a wavelength shifter. This wavelength shifter absorbs any photons that are too energetic and re-emits multiple lower energy ones. Each bar is given a reflective coating to increase the probability that emitted light hits an optical fibre. The PØD s scintillating bars provide sufficient segmentation to reconstruct both charged particle tracks and electromagnetic showers. [3] Time Projection Chambers (TPCs) Just downstream of the PØD is the tracking section of the ND280. This tracking section consists of three TPCs and two FGDs. The FGDs are positioned either side of the middle TPC, as seen in figure 3.9. The TPCs perform three key functions: [3] The excellent three-dimensional tracking provided by the TPCs allows the number of incident charged particles, and their orientations, to be determined. By measuring the momenta of these charged particles, the event rate inside the ND280 can be found as a function of neutrino energy. The amount of ionisation done by these charged particles is recorded. Particles can then be easily identified by combining this ionisation data with their measured momenta. The TPCs do not contain their own target material, instead they measure particles created elsewhere in the ND280. Figure 3.11 shows a simplified illustration of a TPC. The inner box is filled with an Ar : CF 4 : i-c 4 H 10 (95 : 3 : 2) gas mixture. [24] As a charged particle passes through the TPC it will ionise this gas producing electrons. The cathode s electric field will cause these electrons to drift towards the micromegas detectors where they are then observed. Due to argon s zero electronegativity, it unlikely that a drift electron will 24

32 be absorbed before it reaches a detector. By combining the pattern of signals in the detectors with the electron arrival time, a 3D image of the incident particle s trajectory and be produced. [3] The space between the inner and outer walls is filled with CO 2 gas. This acts to insulate the inner box from the grounded outer box. [24] Figure 3.11: A simplified cut-away drawing of a TPC. [24] Acceleration due to the TPC s electric field will be in the x-direction, whereas acceleration due to the ND280 s magnetic field will not. This allows both the charge and momenta of the incident particles to be calculated. A calibration system uses the photoelectric effect to produce electrons at known points on the cathode. The trajectories these photoelectrons can be used calculate and inhomogeneities in the electric and magnetic fields. [24] Fine Grained Detectors (FGDs) The main purpose of the FGDs is to provide target mass for the neutrino beam to interact with. The first FGD (FGD1) is 86.1% carbon (by mass) and is comprised solely of scintillating bars. [25] Whilst acting as the FGD1 s target mass, the scintillators also allow the produced charged particles to be tracked right from the interaction vertex scintillating bars are arranged in alternating horizontal and vertical planes of 192 bars, providing a total of 1.1 tons of target material. [3] Figure 3.12 shows an example of an event inside ND280 s tracking section. The interaction point inside the FGD1 can be clearly seen, with the process s products being tracked in each stage of the tracking section. 25

33 Figure 3.12: An example event inside the ND280 s tracking section. A neutrino appears to have undergone deep inelastic scattering inside the FGD1. A product of an interaction outside of the tracking section can be seen in the top-left. A more typical neutrino event would involve fewer particle tracks than seen here. [24] The FGD furthest downstream (FGD2) uses both water and scintillating bars as its target material. [26] The FGD2 s scintillators can be split into seven pairs of horizontal and vertical planes. Water targets are placed alternating with these pairs of planes, for a total of six water targets. Corrugated polycarbonate walls are used to contain each sheet of water. In total the FGD2 contains 2688 scintillator bars, and 15 cm thick of water in the beam direction. [3] The FGD2 allows the interaction rates of neutrinos with water to be observed, which can be used to predict the rates of processes occurring in the Super-K. [26] The Electromagnetic Calorimeter (ECal) The ECal fills the space between the inner detectors (PØD, TPCs, FGDs) and the UA1 magnet. It s purpose is to aid these inner detectors in reconstructing events. The ECal measures the energy, and direction, of photons and charged particles. [3] As seen in figure 3.9, it consists of three sections: the PØD-ECal, the barrel-ecal, and the Ds-ECal (Downstream ECal). [27] The PØD-ECal is comprised of six ECal modules that surround the PØD, two above it, two below it, and one either side. These modules are attached directly to the UA1 magnet, two top and two bottom modules are required so that the magnet s halves can properly separate. The barrel- ECal is also made up of six modules; these are also attached to the UA1 magnet and surround the tracker volume in the same manner. The Ds-ECal consists of just one module, which sits behind the final TPC, orientated to be orthogonal to the beam. 26

34 Each module consists of layers of scintillating bars, with lead containment sheets between each layer. In the same manner as the inner detectors scintillators, the layers are assembled with bar orientation alternating at 90. [3] Not only does the lead contain an electromagnetic showers, it also acts as a target target mass and radiator for each layer. [27] The ECal is constrained by the ND280 s internal dimensions, consequently the modules are not all the same size. This means the PØD-ECal s modules are much thinner than the those in the barrel-ecal, containing fewer scintillator-lead layers The Side Muon Range Detector (SMRD) The SMRD consists of 440 scintillating modules that fill the 1.7 cm air gaps between the 4.8 cm thick steel plates that make up the UA1 magnet. [3] It has three purposes: To measure the momenta of muons escaping the inner detector at high angles with respect to the beam direction. [28] To identify background generated by neutrino interactions occurring in the magnetic yoke and the ND280 s surrounding walls. [28] To detect any cosmic ray muons that reach the ND280. [3] 3.4 Post-Oscillation Measurements The Super-Kamiokande Far Detector The Super-Kamiokande neutrino observatory acts as the T2K s far detector, and is located 295 km west of the beam s source. [3] It provides post-oscillation measurements of the neutrinos, looking for ν e appearance and ν µ disappearance in the beam. The detector cavity, clearly illustrated in figure 3.13, lies 1 km beneath the peak of Mt. Ikenoyama. [29] This cylindrical cavity is filled with 50 kton of pure water, which acts as a target material for the neutrino beam to interact with. The Super-K s inner walls are covered with a total of 11,129 inward facing photomultiplier tubes (PMTs). [3] These PMTs make up the inner detector (ID), and detect any Cherenkov radiation emitted by the interaction products. Surrounding the ID is an outer detector (OD), comprised of 1,885 outward facing PMTs. [3] Light-proof plastic sheets are used to optically separate the ID and OD. The outer side of this separating layer is coated with a reflective material to improve the light collection capability of the OD s PMT array. Despite being deep under the 27

35 Figure 3.13: A diagram of the Super-Kamiokande. [15] mountain, some cosmic rays still reach the Super-K. The OD allows these rays to be vetoed, providing almost 100% rejection of this cosmic background. [3] The Super-K is currently in its fourth running period: SK-IV. Maintenance and upgrades are done during the downtime between each running period. In November 2001, during the upgrade from SK-I to SK-II, an accident caused about 60% of the detectors PMTs to be destroyed. [30] This occurred when a single PMT imploded, causing a shockwave that destroyed adjacent PMTs, creating a chain reaction. Precautions have since been put in place to prevent such a chain reaction from reoccurring ν µ and ν e Event Reconstruction The flavour composition of the oscillated neutrino beam is inferred by counting the CCQE interactions undergone by muon and electron neutrinos. [3] The charged leptons produced in these CCQE interactions are of the same flavour as the neutrinos that caused them. These charged leptons, providing that they are of high enough energy, will produce a forward-going cone of Cherenkov radiation. When these Cherenkov photons reach the ID s PMTs they produce a ring-shaped pattern that can be used to identify the source particle. [3] Cherenkov radiation is emitted when a charged particle travels through a medium faster than the phase velocity of light in that medium. As seen in figure 3.14, the 28

36 Figure 3.14: A cone of Cherenkov radiation (blue) emitted by a charged particle (red). emission of this radiation as the particle propagates results in a cone of light. The emission angle, θ can be calculated as: [31] cos θ = 1 nβ (3.1) Where n is the refractive index of the medium, and β is the charged particle s speed as a fraction of the speed of light. An angle of θ = 0 corresponds to no radiation emission. As such, we see that in water the minimum speed for a particle to produce Cherenkov radiation is β [31] As the produced electrons propagate through the Super-K they generate electromagnetic showers. An electromagnetic shower begins when an electron decelerates and emits Bremsstrahlung radiation, which then undergoes pair production to produce an electron-positron pair. These leptons then produce successive Bremsstrahlung radiation, which then pair produces more leptons. This cycle continues until either the leptons or the Bremsstrahlung photons no longer have enough energy to continue the shower. As such, one electron can result in many charged particles, each of which may produce Cherenkov radiation. This corresponds to a very fuzzy, smeared out, ring pattern detected by the PMTs. [3] An example of one these electron-type events can be seen in figure 3.15a. 29

37 (a) An electron-type event. (b) A muon-type event. Figure 3.15: Two example event displays from the Super-K. [15] The higher mass of the muons makes them more resilient to changes in their momentum. [3] Hence, they are much less likely to undergo this showering process. This means that the muon-type events produce much sharper rings of radiation. Figure 3.15b shows an example event display of one of these muon-type events. 30

38 4 The T2K s θ 13 Measurements 4.1 Eliminating Unwanted Backgrounds For the Super-K s results to be useful, the true signal needs to be distinguished from the unwanted background. There are two main sources of background in the Super-K s ν e signal: The beam produced at J-PARC is not solely composed of muon neutrinos, it also contains a small fraction of electron neutrinos. Over the T2K s baseline these electron neutrinos are unlikely to have oscillated, and so will account for some of the signal detected at the Super-K. Pre-oscillation measurements of the beam are used to measure this initial ν e flux. As detailed in 3.3.3, π 0 mesons sourced from ν µ interactions can be mistaken for ν e events. Simulations can be done to estimate the two-ring patterns that the π 0 decays result in, and predict the percentage of decays that will result in just one electron-like ring. The measurements of interaction cross sections made at the ND280 can be used to create accurate Monte Carlo simulations. These simulations are useful for properly determining these background signals. The true events can then be found by subtracting the background from the measured signal. 4.2 The θ 13 Results Figure 4.1 shows the T2K s measurements of sin 2 2θ 13 from As previously described in 2.2.2, the value of θ 13 depends on whether the neutrino mass hierarchy is normal or inverted. The ν e appearance at the Super-K corresponds to a best-fit value of sin 2 2θ 13 = for a normal mass hierarchy. [14] This best-fit value is given to a 68% confidence level, the other oscillation parameters are fixed at the values specified in table

39 Parameter Value m ev 2 m ev 2 sin 2 θ sin 2 2θ δ CP 0 ν travel length 295 km Earth matter density 2.6 g/cm 3 Table 4.1: The oscillation parameters used to achieve the plots seen in figure 4.1. [14] Figure 4.1: These plots show the possible values of sin 2 2θ 13 for all possible values of δ CP. The top plot (a) shows the values assuming normal mass hierarchy, and the bottom plot (b) assumes an inverted mass hierarchy. The 68% and 90% confidence regions are clearly shown, along with several best-fit lines. [14] 32

40 5 The Hyper-Kamiokande 5.1 Overview The Hyper-Kamiokande (Hyper-K) is a proposed next generation neutrino experiment. It would involve the construction of a new water Cherenkov detector, to act as the far detector in a long baseline experiment similar to the T2K. Its primary goal is to determine the leptonic CP violation phase, δcp, to better than 19. [8] The Hyper-K will either be positioned 8 km south of the Super-K, or under Mt. Ikenoyama where the Super-K is located. The J-PARC s design means that both of these sites are 295 km from the beam s source, at an off-axis angle of 2.5. As such, the Hyper-K will be able to operate over the same baseline as the T2K experiment has, using the existing J-PARC facility. Figure 5.1 shows the proposed layout of the Hyper-K detector. Figure 5.1: A schematic view of the proposed Hyper-K detector. [8] 33