Metodología de la investigación

The T2K neutrino beam [57] is produced at J-PARC by colliding 30 GeV protons with a graphite target. Magnetic horns focus hadrons produced with the desired charge (and defocus wrong-sign hadrons), which then decay in flight to produce a predominantly muon neutrino beam. An overview of the J-PARC site is shown in Figure 3.2.

The J-PARC accelerator, which accelerates the protons, is described in Section 3.1.1.1.

Both the primary neutrino beamline, where the protons are directed for T2K, and the secondary beamline, where the protons strike the target and the secondary beam is focused, are described in Section 3.1.1.2. T2K is an off-axis experiment, meaning that the beam is not directed directly towards the far detector, rather 2.5^◦ off-axis, the motivation for which is discussed in Section3.1.1.3.

3.1.1.1 J-PARC accelerator

Three accelerators are used to produce the 30 GeV proton beam. First, a linear ac-celerator accelerates H⁻ ions up to kinetic energies of 400 MeV. Charge stripping foil then removes the electrons from the protons before the second stage, a rapid-cycling synchrotron (RCS), which accelerates the beam up to 3 GeV kinetic energy. Finally, the main ring (MR) synchrotron accelerates the beam up to 30 GeV. Only ∼5% of bunches from the RCS are supplied to the MR; the rest are supplied to other facilities and beamlines on the J-PARC site. The MR can be extracted in two ways, slow and fast

Figure 3.2: Overview of the J-PARC accelerator complex. This figure has been reproduced from Reference [56].

extraction, but only the latter is used by T2K. In fast extraction mode, all 8 bunches in the MR are delivered in a single spill to the neutrino beamline.

3.1.1.2 T2K beamline

The T2K neutrino beamline is shown in Figure 3.3. The primary beamline, consisting of the preparation, arc and final focusing sections, is primarily used to bend the beam towards Kamioka and slightly downwards. Various beam monitors are used to ensure a stable beam and to minimise beam loss. These are described in References [55,57].

The secondary beamline consists of the target station (TS), decay volume, beam dump and muon monitor, as shown in Figure 3.4. The entire TS is located within a helium gas filled vessel. Protons enter the TS from the left of Figure 3.4, passing through a titanium alloy window which separates the vacuum of the primary beamline from the helium of the TS. An upstream collimator (the baffle) protects the magnetic horns from stray protons. Then the optical transition radiation monitor [231] is used, along with information from monitors in the final focusing section of the primary beamline, to guide the beam onto the target.

The target itself is a 91.4 cm long (1.9 interaction lengths), 2.6 cm diameter and 1.8 g/cm³ graphite rod located within the first magnetic horn. The target is encased in titanium and cooled by helium gas.

Figure 3.3: Overview of the T2K neutrino beamline. This figure has been reproduced from Figure 2 of Reference [55].

Each of the three magnetic horns is pulsed with 250–300 kA to produce a toroidal magnetic field in time with the proton beam arrival on the target. This results in the collection and focusing of positively (negatively) charged secondary particles to enhance the neutrino (antineutrino) flux [232]. Wrong sign secondaries are defocused by the horns, so running the horns with different polarities enhances either the neutrino or the antineutrino component of the beam.

The focused secondary beam is then allowed to decay in a ∼96 m long decay volume, which has a cross section of 1.4 m wide × 1.7 m high (3.0 m wide × 5.0 m high) at the upstream (downstream) end. At the downstream end of the decay volume is a 75 t graphite beam dump measuring 3.174 m long × 1.94 m wide × 4.69 m high, with an additional 2.4 m thick series of iron plates at the downstream end. The beam dump stops all hadrons and most muons below 5.0 GeV.

A muon monitor [233] is located downstream of the beam dump to measure the remaining muons in order to monitor the neutrino beam direction to a precision of 0.25 mrad on a bunch by bunch basis, and to monitor the neutrino beam intensity.

3.1.1.3 Off-axis approach

The off-axis approach exploits the kinematics of the two body pion decay used to produce the majority of neutrinos in accelerator experiments π^± → µ^±+ ν⁽–⁾

µ. Neglecting the neutrino mass, the neutrino energy in the pion centre of mass frame, E_ν^CM, can be

Figure 3.4: The secondary beamline viewed from the side, protons travel from left to right in this view. The inset shows the target station in more detail. The beam passes through a collimator (the baffle) and the OTR monitor which guides it onto the target. The resulting secondary particles (π and K) are focused by the magnetic horns and allowed to decay in the decay volume to produce the neutrino beam. Remaining hadrons and lower energy muons are absorbed by the beam dump. Penetrating muons are measured by the muon monitor to monitor the beam direction and intensity on a bunch by bunch basis. This figure has been reproduced from Figure 6 of Reference [55].

expressed as [234]

E_ν^CM= m²_π− m²_µ 2mπ

= 29.8 MeV, (3.1)

where mπ and mµ are the pion and muon masses. Therefore there is a maximum transverse component to the neutrino momentum with respect to the beam axis. From this it can be inferred that the maximum neutrino energy decreases with off-axis angle, and that an off-axis beam has a much narrower peak in energy, as can be seen in Figure3.5. It is clear that small changes in the off-axis angle affect the energy spectrum, so the beam direction must be tightly controlled for this approach to be useful in an oscillation experiment.