Forward Muon Stations (ME1/1)
JINR and the Institutes of
Belarus and Bulgaria bear responsibility for the design and construction
of the forward muon stations (ME1/1).
The station ME1/1 plays a key
role in the experiment because it matches the tracks of the muon
system to the tracks of the inner tracker. The simulation had shown
[3] that its resolution should be significantly better than the one of
the endcap muon system. The required resolution is 75
 m. The track matching improves the
momentum resolution with respect to the one achieved in the measurements
with standalone muon system from 15% to 2% at  =2
and pT=100 GeV, while
mass resolution for
->mu mu
decay, improves an order of magnitude. Above that, for efficient
identification of the bunch crossing, the timing resolution of ME1/1
should be a few nanoseconds. The above stringent requirements clash with
the experimental conditions. The station is located inside the magnet
solenoid in front of the iron yoke. Therefore, it operates in a strong
axial magnetic field of about 3 T. At the location of the station, the
maximum of the background rate up to 1 kHz/cm is reached. At last, there
are stringent geometric restrictions on the placement of the station.
There have been no operating detectors until now to match this challenge.
In 1993, the method of wire gas chambers with the cathode readout was
suggested for the endcap muon chamber
[1]. The method is developed in Dubna.
For the first time, a large cathode strip chamber of the dimensions
3x1.5 m was designed at Dubna in 1979 under the research and development
program of the NA4 experiment
[5]. However, a wide application of this
method was restricted by the requirement of availability of a large number
of precision channels of analog electronics. The progress of the last decade
in the microchip technology made it possible to use this promising technology
in the modern experimental physics. The multilayer cathode strip chambers
provide precise measurements of the azimuthal coordinate of the track
sagitta in a magnetic field by measuring the distribution of the charges
induced on a few cathode strips. The precision of the radial coordinate
measurement is bounded by anode wire groups. The timing resolution is high
because of the short drift time and the availability of the measurement of
the second signal out of six layers.
Years of the studies of the above method resulted in the knowledge of the
influence of the strong magnetic field on the spatial resolution, and of
the ways to compensate this influence. The number of the layers of the
chamber has been optimised to achieve high efficiency of the track
reconstruction of the hard muons with their radiative secondaries taken
into account. In collaboration with the Minsk group, dedicated front-end
anode and cathode microchips have been designed. They are matched with
the parameters of the chambers and have high rate capability. Studies on
a number of prototypes have shown that the performance and the rate
capability of the cathode strip chambers (see Fig. 5) satisfy the requirements
of the CMS experiments. The chambers with the electronics designed by
RDMS and manufactured in Minsk can operate at low gas gain, namely,
in the range of (5-7)x104, keeping the sufficient
track efficiency, spatial and timing resolution. Working at low gas
amplification is important to exclude the aging effects.
The deterioration of the chamber performance at the background expected
at LHC (up to 100 kHz per strip) is quite small.
The ME1/1 station is
located in the interface system of the endcap hadron calorimeter.
All the cables and the services of the endcap detectors pass through
this zone over the location of ME1/1. Access to the electronics of the
ME1/1 chambers is provided by the design of the interface system of
the hadron calorimeter. Namely, all the communications of the preshower
detector, electromagnetic and hadron calorimeters are placed in
Z- shaped cable trays on the brackets of the interface system.
The layout of the ME1/1 zone is optimised to have enough room for
the RPC trigger detectors, and to guarantee efficient shielding
against the background neutrons. The layout of the electronics was
under a deep scrutiny because of the presence of severe constraints
on the dead zones related to the necessity to guarantee the hermiticity
of the set-up. Full-size mock-up of the chamber and the ME1/1 station
has been constructed to check and confirm the integration of the station.
The mock-up involves a detailed representation of the electronics, connectors,
cables, cooling system, etc.
The successful technical design review of the ME1/1 chambers
[5]
initiated the mass-production of the chambers. Production of the panels
and preparation of the technical equipment is carried out starting
from 2000. In November 2000, a radiation test of the anode electronics
has been performed successfully. Production of the anode electronics
is under way in Minsk. Production of the high-voltage and low-voltage
power supply systems has been started in Bulgaria. By the updated CMS
detector construction schedule that optimises the occupation of the CMS
assembly hall, the production of the chambers had started in Dubna in
the middle of 2001. The production should be completed in 2003 -half
a year earlier than by the previous schedule.
Fig. 5:
a) Performance of the cathode
strip chambers of the muon station ME1/1. The spatial resolution of
a layer, the efficiency of the track reconstruction, and the timing
resolution of the station were measured with P3 and P4 the prototypes
instrumented with Minsk electronics at the muon beam.
b) Rate capability of ME1/1 at the background up to 500 kHz per
strip, and at the expected background of 100 kHz per strip.
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