Violation in Mixing

2.2 The BABAR detector. 63
2.2.4 The Čerenkov-based detector �ÁÊ�.
The particle identification system is crucial for BABAR since the �È violation analysis requires the ability to
fully reconstruct one of the B meson and to tag the flavour of the other B decay: the momenta of the kaons
used for flavour tagging extend up to about ��� with most of them below ���. On the other hand,
pions and kaons from the rare two-body decays � � � � and � � Ã � must be well separated:
they have momenta between �� and �� ��� with a strong momentum-polar angle correlation of the
tracks (higher momenta occur at more forward angles because of the c.m. system boost). So the particle
identification system should be:
¯ thin and uniform in term of radiation lengths to minimize degradation of the calorimeter energy
resolution,
¯ small in the radial dimension to reduce the volume (cost) of the calorimeter,
¯ with fast signal response,
¯ able to tolerate high background.
�ÁÊ� stands for Detection of Internally ReflectedČerenkov light and it refers to a new kind of ring-imaging
Čerenkov detector which meets the above requirements. The particle identification in the �ÁÊ� is based
on the Čerenkov radiation produced by charged particles crossing a material with a speed higher than light
speed in that material. The angular opening of theČerenkov radiation cone depends on the particle speed:
Ó× � � Ò¬
where � is the Čerenkov cone opening angle, Ò is the refractive index of the material and ¬ is the particle
velocity over . The principle of the detection is based on the fact that the magnitudes of angles are
maintained upon reflection from a flat surface.
Since particles are produced mainly forward in the detector because of the boost, the �ÁÊ� photon detector
is placed at the backward end: the principal components of the �ÁÊ� are shown in fig. (2-11). The
�ÁÊ� is placed in the barrel region and consists of �� long, straight bars arranged in a -sided polygonal
barrel. The bars are �� Ñ-thick, �� Ñ-wide and ��� Ñ-long: they are placed into hermetically sealed
containers, called bar boxes, made of very thin aluminum-hexcel panels. Within a single bar box, bars
are optically isolated by a � � �Ñ air gap enforced by custom shims made from aluminum foil.
The radiator material used for the bars is synthetic fused silica: the bars serve both as radiators and as light
pipes for the portion of the light trapped in the radiator by total internal reflection. Synthetic silica has been
chosen because of its resistance to ionizing radiation, its long attenuation length, its large index of refraction,
its low chromatic dispersion within its wavelength acceptance.
The Čerenkov radiation is produced within these bars and is brought, through successive total internal
reflections, in the backward direction outside the tracking and magnetic volumes: only the backward end
THE BABAR EXPERIMENT