Parrilla de Programación

The 30 GeV protons are fast extracted from the MR and bent tightly inside the ring into the neutrino beamline, as shown in Figure 2.4. The fast extraction pro- cess involves the extraction of all eight proton bunches within a single cycle of the MR. The neutrino beamline consists of two sections, the primary and secondary beamlines. The primary beamline starts with the kicker magnets used to extract the proton beam from the MR. The beam is bent round the tight arc section by a series of combined function superconducting magnets and passed through a series

Figure 2.4: A schematic diagram of the MR and Neutrino Beamline.

of monitors to check the beam profile and other properties. The magnets are known as combined function magnets because they both steer and focus the proton beam. The proton beam then passes through a graphite baffle with a 30 mm beam hole to remove the beam halo before colliding with the graphite target. The beam halo refers to the small number of protons on the edge of the beam that can escape and induce radioactivity in the components of the accelerator. The baffle is therefore used to protect the target region from radiation damage. The target is 91.4 cm

long, 2.6 cm in diameter, has a density of 1.8 gcm−3 _{and is housed within the first}

magnetic horn. The low density of the target helps to protect it from the high tem- peratures, expected to reach up to 700◦_{C, generated by the protons incident on the} target. Figure 2.5 shows the total number of protons on target (POT) delivered by the MR over the 2010-2011 run period. The beam power steadily increased during the two running periods and reached 145 kW before the 2011 Japan Earthquake.

The secondary beamline, shown in Figure 2.6, starts with the three magnetic focusing horns used to focus the positively charged mesons from the proton-target collision into the 96 m decay pipe. The three horns run with an operating current of about 250 kA. The mesons travel along the decay pipe until they decay to produce neutrinos. Some negative mesons that are highly boosted along the beam direction will not be removed by the magnetic horns, causing a contamination of antineutrinos in the beamline. Muon neutrinos are mostly produced by the following meson decays:

Figure 2.5: The integrated number of protons on target (POT) delivered by the accelerator from January 2010 until the shutdown caused by the major earthquake in March 2011. The flat region in the centre of the plot corresponds to the summer shutdown period. Figure from [59].

Figure 2.6: A diagram showing the components of the secondary beamline with a zoomed in view of the target station. Figure from [58]

π+→µ+νµ (2.1)

K+_→µ+νµ (2.2)

and electron neutrinos by

K+_→e+_ν

eπ0 (2.3)

K_L0 →e+νeπ− (2.4)

µ+_→e+νeν¯µ (2.5)

The charge conjugates of these processes, while suppressed by the focusing of positive mesons, occur to produce the corresponding antineutrinos. Figure 2.7 shows the relative fluxes of the different neutrinos and antineutrinos seen at Super-K from the beam simulation. The antineutrinos have approximately 10% of the flux of their corresponding neutrino and the νe component of the beam is approximately 1%.

The decaying muons shown in Equation 2.5 are not produced in the proton-target collisions but in processes such as those given in Equations 2.1 and 2.2. The beam dump, positioned downstream of the decay pipe, 109 m from the target, consists of 2.4 m of iron and 3.17 m of graphite in thickness. It was designed to stop all particles

other than neutrinos and muons with momenta greater than 5.0 GeV/c. These high

energy muons pass into the muon monitor, a detector used to measure the beam intensity and direction. The neutrinos pass through the muon monitor and travel inland towards INGRID, ND280 and Super-K.

Neutrino Flux Simulation

The neutrino flux simulation is very important as it provides the base neutrino flux used to generate the simulations for all of the detectors, as shown for the Super-K flux in Figure 2.7. The flux simulation models the interaction of the proton beam with the graphite target, and predicts the spectrum of neutrinos coming from the decay of the mesons created in the interactions. The simulation uses GEANT[60] to model the entire secondary beamline and the 30 GeV proton-graphite interactions are modelled by FLUKA[61]. The cross-sections controlling the production of pions and kaons in the target were tuned using measurements from the NA61/SHINE[62] experiment at CERN. NA61/SHINE had a dedicated run where a 30 GeV beam of protons was fired onto a replica T2K graphite target to measure the meson pro- duction cross-sections for the proton momentum used in the T2K beam. In phase space regions not covered by NA61/SHINE for kaon production, additional data

Figure 2.7: The predicted neutrino flux from simulation at Super-K broken down by (anti)neutrino type.

from proton-beryllium experiments in the 1970s (Allaby et al[63] and Eichten et al[64]) were used to tune the cross-section values.