The BABAR Muon System

Luca Lista

INFN Sezione di Napoli

**Abstract.** The BABAR muon detector is described. The physics
requirements, the design and the performance measured in the first year
of data taking are presented.

# INTRODUCTION

The BABAR\[1\] Muon detector, also known as IFR (Instrumented Flux
Return) \[2\], consists of the magnet yoke for the flux return of the
superconductive magnet instrumented with Resistive Plate Chambers (RPCs)
\[3\]. The system is designed to detect muons as well as K~L~, which are
important to reconstruct B decays relevant to CP violation measurements,
like B J/ K~L~.

# DETECTOR DESCRIPTION

The IFR has an hexagonal configuration (fig. 1), and is divided in a
barrel and two end caps. Each end cap is divided in two half-doors in
order to permit the access to the inner detector; each half door is
divided vertically in three sectors. The iron is segmented in a graded
way with plates of thickness that varies from 2cm to 10cm, increasing
with the distance from the interaction point. The motivation for the
graded segmentation is mainly to improve the performance of K~L~
detection. Each gap between iron plates is filled with a plane of RPC
detectors. The barrel and the two end caps are instrumented with 19 and
18 planes of RPC respectively. The system is complemented with two
layers of cylindrical RPCs inside the coil.

The total area covered by the active detectors is larger than 1500 m^2^.
The total iron thickness is 65 cm in the barrel and 60cm in the end
caps. The angular width of the uncovered regions is 300 mrad in the
forward direction, and 400 mrad in the backward.

The overall system dimensions are 635 cm (length) 584 cm (height) 675 cm
(width), and the iron weight is 312 tons for the barrel and 225 tons for
each of the end caps, excluding the supports.

![](media/image1.wmf){width="3.120833333333333in"
height="3.2416666666666667in"}**Figure 1.** Geometrical structure of the
Instrumented Flux Return

# THE ACTIVE DETECTORS

Resistive Plate Chambers (fig. 2) consist of two high resistive
(10^11^cm) bakelite electrode plates painted with graphite on one side.
The 2 mm gap between the electrodes is filled with a gas mixture of
Argon, C~2~H~2~F~4~ and Isobutane. Aluminum pick up strips run on both
sides of the detector and provide a two dimensional readout. The
detector size and strip pitches vary according to the geometry of the
detector. The cylindrical detector has orthogonal strips that measure
*r* and *z* as well as stereo strips with a tilt angle of 30 that allow
ambiguities reduction.

![](media/image2.png){width="3.6in"
height="2.520138888888889in"}**Figure 2.** Section of a Resistive Plate
Chamber

Each layer is obtained laminating three adjacent planar modules in the
barrel; each sector of the end caps is filled with chamber made with two
modules. The lamination of strip planes is used to join modules into
chambers. One of the two strip planes runs over the full detector, in
such a way to permit to treat the layer as a single logical detector
unit.

# READOUT Electronics

Approximately 3000 Front End Cards (FECs) \[4\] are connected directly
to readout strips; one board services 16 strips. A snapshot of the 16
strips digital signal is taken upon receiving a trigger. The serial
information is sent to the IFR FIFO boards \[5\] used for the
electronics readout. The readout is interlaced (even strips and odd
strips are read by different FECs), in order to reduce the inefficiency
in case of electronics failure, since the average strip multiplicity per
hit is larger than one.

#  MUON IDENTIFICATION

Several informations are used to identify muons \[6\], exploiting the
different interactions of muons and pions in the iron. The measured
number of nuclear interaction lengths is compared to the value expected
from the extrapolation of an ideal muon track, considering the average
energy loss. The average and r. m. s. of the strip multiplicity pattern
are used to reject pions that may produce a shower in the IFR. Finally,
the hits in the IFR are compared to the extrapolation of the track
measured in the inner tacking devices, and a "track match" ^2^ is
computed. The IFR track is modeled with a polynomial, and the ^2^ of the
fit to this model, together with the track match ^2^, are used as
discriminating variables. The IFR information is complemented with the
energy loss measured in the electromagnetic calorimeter. Five selections
are defined to provide different levels of efficiency and purity needed
for analysis purposes. Figure 3 shows the performance of the IFR in
terms of muon identification efficiency and pion misidentification
probability for what is defined as the \"*tight*\" selection.

The combination of the different discriminating variables in a
likelihood ratio technique has been proven to provide a further
improvement to muon identification performances.

# K~L~ identification

Neutral hadrons are identified in the IFR and the Electromagnetic
Calorimeter. The IFR information is reconstructed as a three-dimensional
cluster, requiring at least two layers in depth.

In order to evaluate K~L~ identification efficiency, events ee with
K~S~K~L~ were selected so that the K~L~ direction and energy can be
determined from the kinematics of the reconstructed photon and the K~S~.

![](media/image3.png){width="5.9in" height="2.9340277777777777in"}

**Figure 3.** Muon identification efficiency and pion misidentification
probability.

# Acknowledgments

Help from Dr. Marcello Piccolo is gratefully acknowledged.

# References

1\. The BaBar Physics Book, **SLAC-R-504** (1999).

2\. F.Anulli *et al.* *Nucl. Instr. Methods* **A409** (1998).

3.  R.Santonico, R.Cardarelli, *Nucl. Instr. Methods* **A187** (1981).

4.  N.Cavallo *et al*., *Nucl. Instr. Methods* **A409** (1998).

5.  G.Crosetti *et al.*, *Proceedings of the "International Workshop on
    Resistive Plate Chamber and Related Detectors",* October 28-29 1999,
    Bari, Italy.

6.  L.Lista, *Proceedings of "CHEP 2000 - Computing in High Energy and
    Nuclear Physics"conference,* February 7-11 2000, Padova, Italy.