3-1-4 Sideway tracking detector
Particles emitted with larger than 30o in laboratory angles are measured by the side tracker.
The side tracker composes of silicon vertex detector surrounding the target and a time-projection
chamber (TPC). In addition to silicon vertex detectors, the transverse momentum resolution is
improved by about 20% for particles with above 0.6 GeV/c momenta. Decay products from +, ,
Ks, K*, etc., are detected mainly by the side tracker and the invariant mass resolution is important.
The requirement for the momentum resolution is better than 1.5%. The TPC has the benefit of less
material (a volume filled by gas only) and particle identification through dE/dx for low momentum
particles. The length of the TPC is 75 cm, the inner radius is 7.5 cm, and the outer radius is 50 cm.
The outer radius is determined to achieve better momentum resolution than 1.5% for high
momentum particles. The use of a low-Z gas is important for minimizing the multiple scattering
contribution to the momentum resolution. Ne/CH4 [90/10] is used as a chamber gas. The radiation
length is 354 m in contrast with 164 m for Ar/CH4 [90/10]). The helium-based gas has difficulty to
perform the particle identification with the energy deposit. The drift velocity of Ne/CH4 [90/10] gas
is about 3 cm/s with 200V/cm electric field. The cathode pads are placed on the end cap of the
upstream side. The end cap in the downstream side is made by a mylar sheet with 50 m thickness
reducing the multiple scattering contribution to the forward tracker. The cathode pad size is
0.75-cm-width and 1.5-cm-height. The pads are placed radially and the total layer is 28. The total
number of channels is about 7000. The azimuthal (rcoordinate is measured by the pad information
and the z coordinate is measured by the drift time. The Gas Electric Multiplier (GEM) is used as a
charge avalanche amplifier at the cathode plane. The expected position resolution is 150 m and 2
mm in the r and the z direction, respectively. In the current LEPS beam line, we use the TPC, which
is the smaller volume but the same pad size, and obtain the 150 m position resolution. We have the
data-taking system and these will be used for the LEPS2-TPC by adding some more same modules.
Figures show the total momentum resolutions estimated by the Monte Carlo simulation taking into
account detector resolution, multiple scattering and energy loss. The better resolution than 1.5% is
achieved for pions and kaons up to 1GeV/c. Protons scattering into the sideway have mostly more
than 0.6GeV/c momentum and the 1.5% momentum resolution is achieved for these protons.
Figure Total momentum resolution vs. polar angle for pions (left) and kaons (right).
3-1-5 Particle identification (PID) –sideway dE/dx
The task of PID in the sideway is a Kaon/pion separation up to 1GeV/c momentum. PID is
performed by TPC and range stack counters. Decay products coming into the sideway region could
have a few hundreds MeV. It is difficult to accept kaon’s far from the target due to its decay-in-flight.
PID is performed by using information on energy deposit in TPC for low momentum particles. The
left figure shows PID in TPC. The filled gas is Ne/CH4 [90/10] and the 5% resolution for the energy
deposition measurement is taken into account.
Particles coming out from the TPC are detected and identified by the range stack (RS). The RS
was used for the E949 experiment in order to separate two decay modes, K+  +0 and K+ 
+() ( separation) and good performance had been obtained. The range stack (RS) is a
cylindrical array of layers of plastic scintillators (BC408) segmented azimuthally into 24 sectors.
Each range-stack sector consists of 15 layers of 1.82-m-long x 19.05-mm-thick counters. The 20
layers were used for E949 experiment but, we use 15 layers only because the tracker radius is 50 cm
and our purpose is a K/p separation in the LEPS2 detector and 15 layers are enough to separate them.
The signals are read by PMT at each end through light guide. The average attenuation length is 2 m.
The RS fills the radial region between 55 cm and 89.6 cm. The most inner counters are used to
provide trigger signals and most outer layer is used to provide the TOF stop signal which is helpful
for particle identification of high momentum particles. Two layers of range-stacks multi-wire
proportional chambers (MWPC) are located within scintillators at 63.5 cm and 72.0 cm in order to
obtain tracking information in the RS and obtain the precise range of particles. The MWPCs measure
the azimuthal (r) and axial (z) positions. The right-top figure shows mass spectrum reconstructed
by momentum measured by tracker and the full kinetic energy deposit in the RS for protons, kaons
and pions below 0.7GeV/c, 0.5GeV/c and 0.2GeV/c, respectively. The right-bottom figure shows
PID for high momentum particles going through the RS. Kaons can be separated with pions with 3%
contamination with N(K)/N() = 103.
Figure Left: energy deposit vs. momentum measured with TPC. Right-top: Mass spectrum
reconstructed by RS. Right-bottom: energy deposit vs. momentum with RS. Blue green and red plots
are protons, kaons and pions, respectively.