Physics requirements to the Muon System
The need to measure muons in PANDA with wide momentum range, from sub-GeV
up to 10 GeV (J/ψ, charmonium, Drell-Yan, etc.), requires good muon
identification. For example, if we take DY muon pairs as the benchmark process
to evaluate the performance of PANDA detector the following features
matter - average energy about a few GeV and maximum reachable around
10 GeV as well as polar angle of muons approaching 150 degrees (in lab
system, for the 15GeV/c beam momentum). The Drell-Yan process is natural
choice for optimization of PANDA muon system as its muon pairs have
maximal momentum spread which covers the spectra of other processes of
interest with muons in final state, like those originating from J/ψ and
D-mesons (fig. 1). This figure shows the full and transverse momentum
(left and right columns, respectively) distributions for the single
muons in angular interval (at production point) up to 20 degrees,
which corresponds to the acceptance of the Forward Spectrometer of PANDA setup.
Generally saying the processes of interest having muons in final states have small cross sections
as compared with backgrounds. This supposes very good muon identification to maximize signal
to background ratio. Muon identification may be done by different detector systems and
methods, for example:
- muon tracker (good clear track sample visibility even in presence of hadronic shower)
- calorimetry, both electromagnetic and hadronic (dE/dx energy deposition corresponding to MIP signal);
- scintillator counters (time-of-flight measurement);
- Cerenkov counters.
The first system may be regarded as the main one for reliable muon identification,
the others - as important but complimentary. Correlating the signals from all independent
systems should provide desirable level of signal purity for muons. These correlations
become more important with the lower beam energy.
As usual, the main task of the muon system is a pattern recognition (muon identification)
and matching to the track segment inside the magnets. The precise muon momentum measurement
is being performed by inner trackers of the magnetic spectrometers.
Range System as a Muon System
The most suitable technology for detecting the muons in PANDA is a Range System (RS) with
proper granularity close to muon's straggling in the iron absorber (iron is used as a
magnet yoke also). The PANDA Collaboration in December 2008 has finally accepted the
proposal to use RS as a muon system.
The RS structure is a well known solution for both, detecting the muons
stopped by the absorber and those crossing the iron. In first case, one may even roughly
estimate the energy of muon. The stopping power of iron is about 1.5 GeV per metre
of absorber for the relativistic muons with dE/dx = 2 MeV/g. Keeping in mind the
specific dependence of muon momenta on the polar angle at PANDA kinematics we arrive
to the muon system shown on fig.2. The Target Spectrometer (TS) has two parts: the
Barrel (B) and the End Cap (EC). The Muon Filter (MF) has practically the same
mechanical structure as TS_EC and it serves for two purposes: first, it increases
the depth of absorber for intermediate angles for better detection of muons and,
second, it is an additional magnetic screen between the solenoid of TS and the
dipole of Forward Spectrometer (FS).
The cross section of TS and MF is given on fig.3. In the barrel part the granularity
of the iron absorber (sampling) is 3 cm. It is selected at such a minimal thickness
because of quite small energies of muons entering the barrel (below or about 1 GeV);
in fact, the thickness corresponds to the straggling of muons in iron. In the EC
and MF the 6 cm sampling is selected for better detection of muons with higher energies.