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The J-PARC (Japan Proton Accelerator Research Complex) facility is located in Tokai, Ibaraki, Japan. It is a joint project between the High Energy Accelerator Research Or- ganisation and the Japan Atomic Energy Agency. The particle accelerators based here are used to generate the neutrino beam, which is directed at a 2.5 ° angle to the ND280 and Super-K detectors [25]. An aerial view of the accelerator complex is shown in Fig- ure2.4.

The neutrino beam is created by first accelerating protons in the LINAC up to 400 MeV, then passing these into the Rapid Cycling Synchrotron (RCS) up to 3 GeV and the final stage of acceleration is in the Main Ring Synchrotron (MRS) which can accel- erate the H-ions. up to 30 GeV. Once the protons have reached maximum energy they are extracted using a magnetic guiding system. Each proton beam ‘spill’ is split into eight bunches and these are directed towards a graphite block, which then produces

2.3. J-PARC Accelerator Complex 43

FIGURE2.4: An aerial view of the J-PARC facility in Tokai. Outlined on the image are different stages of the accelerator used in the T2K experi- ment. The off-axis neutrinos from this accelerator have an energy range

of 600 MeV to 800 MeV [68].

secondary hadrons, predominantly pions and kaons. These pions are preferentially se- lected and focused by a series of magnetic horns and subsequently decay to produce neutrinos [25]. An example of this is shown in Figure2.5.

FIGURE2.5: A Feynman diagram for pion decay, producing anti-muons

and muon neutrinos [69].

The charge of the pion determines whether aνµorν¯µis produced in its decay. By changing the horn polarity, aπ− can be selected, resulting in a ν¯µ beam. This can be of great benefit when investigating CP violation, as explained in Chapter1. The T2K experiment will be spending a significant portion of its running time in the near future running inν¯µmode. The charged (of chosen polarity) meson beam then enters a long helium filled decay volume. The decay volume is constructed of thick steel walls sur- rounded by concrete shielding. Water cooling keeps the steel and concrete below 100 ° C. Most of the charged meson beam decays within this volume, predominantly via:

π+→µ++νµ (2.1) or

π−→µ−+ ¯νµ (2.2)

in the case ofν¯µbeam production.

At the end of the decay volume lies the beam dump. This is a water cooled block of graphite, which weighs 75 tons, along with 15 iron plates with a combined depth of 2.40 m. Only muons with an energy greater than approximately 5 GeV are able to penetrate this [70]. As the number of protons hitting the target is directly correlated to the number of charged-pions produced, and this in turn is directly correlated to the number of neutrinos produced, we can see the direct proportionality between Protons On Target (P.O.T) and total number of neutrinos produced.

2.3.1 Neutrino Beam Energy

The maximum sensitivity to the oscillation parameters is achieved by tuning the neu- trino beam energy to the oscillation maximum for the T2K baseline. The oscillation maximum occurs at a neutrino energy (Eν) less than 1 GeV for the 295 km baseline with

∆m2

23 ∼3×10−3 eV2. The narrower band flux at the off-axis angle and the neutrino

survival probability is shown in Figure2.6.

The energy of the neutrino beam produced using the aforementioned methods is shown in Eq. 2.3.

Eν =

m2_π−m2_µ

2(Eπ−pπcosθ)

(2.3)

where mπand mµare the masses of the pion and muon, Eπ, pπandθare the energy, momentum, and angle of the pion relative to the muon direction, respectively.

Figure2.6illustrates that by choosing a 2.5 ° off-axis angle, it is ensured that a nar- row band beam is produced, at the energies required, calculated using2.3. This can be

2.3. J-PARC Accelerator Complex 45

FIGURE2.6: A plot illustrating the narrow spread of energies at an off- axis angle of 2.5 °. It also shows the minimum survival probability forνµ

at that off-axis angle and T2K energy range. This represents a maximal chance of oscillation fromνµtoνe[71].

tuned to the oscillation maxiumum such that those neutrinos in our energy range of in- terest have a high probability of oscillation whereas the long high energy tail represents those which have very little chance of oscillation.

2.3.2 Beam Contamination and Helicity Suppression

As stated, there is a small contribution of kaons to the the charged meson beam, pro- duced when protons collide with the target. These will decay via either of the following, producing on average higher energy neutrinos than their pion counterparts:

K+→µ++νµ+π0 (2.5) Additionally there is a small background contamination from :

K+→e++νe+π0 (2.6)

µ+→e++νe+ ¯νµ (2.7)

There is also a background from the following :

π+→e++νe (2.8)

which occurs in<1.3×10−4of cases. This mode could be expected to be the main decay mode of the pion, but it is heavily suppressed due to spin and helicity consider- ations.

A charged (positive in this case) pion has spin zero, therefore the anti-lepton and neutrino must be emitted with opposite spins to preserve net zero spin and conserve angular momentum. However, because the weak interaction is sensitive only to the left chirality component of fields, the neutrino always has left chirality, which means it is left-handed, since for relativistic particles the helicity is the same as chirality. This means that the anti-lepton must be emitted with spin in the direction of its linear mo- mentum (i.e. also left-handed). However, considering the case if the anti-leptons were massless, they would only interact with the pion via the weak force in the right-handed form (because for relativistic particles helicity is opposite to chirality) and this decay mode would be prohibited. The anti-leptons are not always relativistic, so it is not re- quired for the helicity to be exactly opposite. Therefore, suppression of thee+ _decay

channel comes from the fact that its mass is much smaller than theµ+. Thee+ is rela- tively massless compared to the muon, and thus thee+mode is highly suppressed.