3.4 Experimental apparatus
3.4.3 Particle Identification detectors
x=−ρcos (qBzt/mγ) y=ρsin (qBzt/mγ) z=pLt/mγ
, (3.31)
withqthe charge of the particle,Bz the magnitude of the axial field andpL the longitudinal momentum. Figure3.15represents the helix in the (x, y) projection and along the zaxis in the five tracker stations.
(x0,y0) x
y
ρ
xz y
•
•
•
•
• •
•
• •
•
Figure 3.15:Two-dimensional projections of the helix in the transverse plane (left) and along the beam axis (right). The blue lines represent the five tracker stations where the helix is sampled.
The radius of the circle in the transverse plane is proportional to the transverse momentum of the particle,pT, through
ρ= pT
qBz. (3.32)
The gradient of the arc-length of the track,s, with respect to the longitudinal position,z, is a function of the longitudinal momentum,pL, through
ds dz =p
1 + (pT/pL)2. (3.33) Non-negligible transverse momentum is required to reconstruct longitudi-nal momentum. The irregular spacing between the station allows one to remove ambiguities about the number of turns between stations [177]. Simu-lations yield a resolution of 1.264 MeV/c in transverse and 3.974 MeV/c in longitudinal momentum.
3.4.3 Particle Identification detectors
Particle identification is achieved with a time-of-flight (TOF) system, Cherenkov threshold counters (Ckovs) and downstream calorimetry. The
upstream PID provides pion and electron rejection while the downstream section tags muons that decayed inside the cooling channel.
Three TOF stations are positioned along the cooling section to provide accurate timing and a reliable upstream velocity-based particle identification variable [178, 179]. TOF0 is located at the end of the second quadrupole triplet, Q4–6, while TOF1 and TOF2 are positioned at the entrance and at the exit of the magnetic channel, respectively. All three stations consist of two perpendicular layers of scintillator bars that provide precise timing information and a rough space point reconstruction. TOF1 is composed of two planes of seven 6 cm wide and 2.5 cm thick bars of polyvinyl toluene (PVT), as represented to the left of figure3.16. TOF1 supplies the trigger for the experiment in coincidence with the ISIS clock. Individual stations achieve a resolution of∼50 ps. The time-of-flight between TOF0 and TOF1 is represented for a 300 MeV/c nominal momentum pionic beam to the right of figure3.16. The leftmost peak corresponds to positrons travelling at the speed of light. At equal momentum, pions travel slower than muons.
Figure 3.16: (Left) CAD model of the TOF1 detector. (Right) Time-of-flight distribution of a 300 MeV/cnominal momentum pionic beam between the first two hodoscopes, TOF0 and TOF1.
At momenta higher than 300 MeV/c, the time-of-flight difference between muons and pions becomes small over a distance of 10 m with respect to detector resolution. Two Cherenkov counters are used to provide a sufficiently good pion/muon separation in that regime [180]. The active radiator is a high density silica aerogel plate that produces Cherenkov light read out by four 8" EMI 9356 KA photomultipliers, as depicted on the left of figure3.17.
The association of the Ckovs and the first two TOF station allows one to achieve a muon purity upstream the magnetic channel of 99.98% [181]. The refraction indices are measured at 1.069±0.003 in Ckov-a and 1.112±.004 in
Ckov-b and yield radiation thresholds of 272±3 MeV/c and 213±4 MeV/c, respectively. The turn-on curve is represented in figure3.17for Ckov-b [182].
Figure 3.17:(Left) Schematics of a Ckov threshold counter. (Right) Average photo-electron yield in Ckov-b as a function of the momentum of impinging muons.
The electron background rejection at the end of the cooling channel is based on the Electron Muon calorimeter (EMcal) station. The electrons shower in a preshower sampling calorimeter, KLOE-Light (KL), while muons penetrate it. The showers and the muon tracks are detected downstream in a fully-active scintillator tracking calorimeter, the Electron-Muon Ranger (EMR), extensively described in chapter4. The geometry of the hits in the EMR allows for the efficient tagging of muon decay in flight [183].
The KL consists of 21 cells of grooved lead layers interwoven with 1 mm scintillating polystyrene fibres inserted and glued in the gaps in a 2:1 volume ratio, as depicted in figure 3.18. It is ∼4 cm thick, which corresponds to about 2.5 radiation lengths. The KL relative energy resolution isσE/E= 7 %/p
E(GeV) and its time resolution isσt= 70 ps/p
E(GeV) [184]. The right side of figure3.18shows the total charge reconstructed in the KL for muons of various impinging momenta.
1.35 mm
0.98mm
Figure 3.18:(Left) Cross section showing the substructure of a KL cell. (Right) Total charge reconstructed in the KL for muons of various impinging momenta.
CHAPTER 4
Electron-Muon Ranger
The Muon Ionization Cooling Experiment requires a strong particle iden-tification system. Simulations have shown that TOF2 alone cannot ensure the rejection of electrons produced by the decay in flight of muons inside the cooling channel [185]. A detector able to accurately discriminate the electrons and positrons from the signal muons is required to achieve the specified systematic level of precision [22]. A downstream detector system has been devised for muon tagging at the end of the channel. It consists of a sampling preshower calorimeter, the KL, coupled to a fully-active tracking calorimeter, the Electron-Muon Ranger (EMR). The construction of the EMR, its role in MICE and its performance are presented in this chapter.
4.1 Function
The MICE collaboration is attempting to measure a clear ionization cooling signal of muon beams of tunable emittance and energy. Other particle species constitute a background to this measurement. Robust particle identification upstream and downstream of the cooling channel is a fundamental task to lower the level of systematic uncertainty to match the precision requirements.
The muon beam line uses a proton beam to drive the production of muons from pion decays. The pions that remain in the beam at the entrance of the cooling channel are a source of uncertainty. The two dipoles select the particles such that they have a momentum distribution centred around the same value. The time-of-flight system installed upstream and downstream
the cooling channel allows for the efficient discrimination of pions from muons to achieve an excellent muon beam purity [186].
Another source of background is the dark current produced by the RF cavities operating in high electric and magnetic fields. Electrons are ripped off the surface of the cavities and accelerated along the cooling channel, causing bremsstrahlung photon emission and background noise in the trackers. This phenomenon has been thoroughly studied and understood [172,187].
The final source of uncertainty stems from the muons that decay in flight inside the cooling channel. Figure4.1 shows the momentum distributions of the muons and electrons present downstream of the second spectrometer for a negative 300 MeV/c central momentum beam [188]. Kinematic cuts can reject about 80 % of decay electrons, which is not sufficient to avoid non-negligible systematic errors on the cooling measurement. A detector capable of identifying electrons of any energy in any momentum configuration is critical as they span a wide range of momentum.
Figure 4.1: Momentum distribution of muons (blue lozenge) and electrons (red squares) downstream the MICE cooling channel for a 300 MeV/cbeam [188].
Several solutions based on a calorimeter system were proposed and their performance in terms of electron/muon separation efficiency were studied with G4MICE simulations [185]. The proposed designs included various combinations of TOF2, the KL and a fully-active plastic scintillator detector, SW1. The left panel of figure4.2shows the background identification efficiency as a function of muon acceptance for three alternative designs and a 140±14 MeV/c muon beam. The configuration that includes the SW is the only one that achieves the required level of purity. The right panel of figure 4.2 shows background identification efficiency for different nominal momentum and configuration. The design composed of TOF2 and the SW yields the best possible performance and is the sole configuration that meets
1The original proposed design of the EMR, called SandWich (SW), was composed of 10 modules of plastic scintillator with different thickness. The detector design was changed mainly due to cost reduction and simplification of the manufacturing.
the background rejection requirements for the four beam central momenta to be studied by MICE.
Figure 4.2:(Left) Background identification efficiency as a function muon acceptance for a 140±14 MeV/cbeam. The solid black line corresponds to SW+TOF, the dash-dotted red line to KL, the dashed red and black lines to KL and SW without TOF, respectively, and the purple solid line to TOF only. (Right) Background rejection efficiency at 99.9% muon acceptance for four different beam central momenta and five designs of downstream apparatus [185].
The downstream background rejection approach is to distinguish electrons from muons using the longitudinal profile of their energy deposition at the end of the cooling section. A high-Z material is used to initiate the electron shower but let the muons through. It is combined with a low-Z fully-active calorimeter that measures the energy deposition. Showers exhibit scarce energy deposition in the calorimeter while muons lose energy throughout.