Electron Identification in the Compressed Baryonic Matter ( CBM) experiment

One of the key physics programs of CBM is to study the spectroscopy of both low and high mass vector mesons (namely ,!, , J/ , ') through their di-electron decay channels. The first step towards this is the identification of electrons. In the current CBM set up, electron identificatio n involves the Ring Imaging CHerenkov counter (RICH), the Transition Radiation Detector (TRD) and the Time Of Flight (TOF) detector. The information from RICH and TRD are used to identify the electrons and those from TOF help to suppress the pions in the identified electron sam- ple. A software package has been developed to perform the electron identification. The methods and performance of electron identification after full event reconstruction will be presented in this report. In the event reconstruction, particles are first tracked by the tracking detectors placed inside a dipole magnetic field providing the momentum of the tracks. The informa- tion from RICH, TRD and TOF is then associated to these tracks to form a global track. T h e p e r f o r m a n c e o f t h e d e t e c t o r s a s w e l l a s t h e e l e c - t r o n i d e n t i fi c a t i o n s o f t w a r e w e r e s t u d i e d w i t h c e n t r a l A u + A u c o l l i s i o n s a t 2 5 A G e V b e a m e n e r g y , g e n e r a t e d b y U r Q M D ( 3 ) . T h e s e e v e n t s w e r e t r a c k e d t h r o u g h t h e C B M d e t e c t o r s y s t e m u s i n g G E A N T 3 ( 4 ) a n d r e c o n s t r u c t e d a f t e r - w a r d s . T h e e l e c t r o n i d e n t i fi c a t i o n e f fi c i e n c y a s w e l l a s t h e - s u p p r e s s i o n f a c t o r a s f u n c t i o n o f m o m e n t u m a r e s h o w n i n F i g . 1 . W i t h t h e c o m b i n e d i n f o r m a t i o n f r o m a l l d e t e c t o r s w e a c h i e v e a n e f fi c i e n c y o f 7 0 % f o r m o m e n t a > 2 G e V / c . T h e d r o p i n e f fi c i e n c y t o w a r d s l o w m o m e n t u m r e s u l t s f r o m t r a c k i n g , r i n g fi n d i n g e f fi c i e n c i e s a n d T R D a n d T O F a c c e p - t a n c e l o s s e s c o m p a r e d t o R I C H. U s i n g t h e R I C H a l o n e , a - s u p p r e s s i o n f a c t o r o f 5 0 0 i s a c h i e v e d w h e r e a s a f t e r c o m - b i n i n g i n f o r m a t i o n f r o m T R D a n d T O F , t h e - s u p p r e s s i o n f a c t o r r e a c h e s u p t o ∼ 1 0 5 f o r l o w m o m e n t u m . F r o m t h e s e r e s u l t s i t i s c l e a r t h a t t h e u s e o f T R D h e l p s s i g n i fi - c a n t l y t o d i s c a r d t h e w r o n g l y i d e n t i fi e d p i o n s a s e l e c t r o n s f r o m t h e i d e n t i fi e d e l e c t r o n s a m p l e a n d i n a d d i t i o n t o t h a t t h e T O F b r i n g s d o w n t h e - s u p p r e s s i o n f a c t o r b y a n o r d e r o f m a g n i t u d e f o r m o m e n t a < 2 G e V / c . T h e s e t o f c u t s f o r d i f f e r e n t d e t e c t o r s a r e b e i n g o p t i m i z e d t o o b t a i n t h e b e s t v a l u e s o f t h e s e t w o q u a n t i t i e s . T h e e l e c t r o n i d e n t i fi c a t i o n i s e x p e c t e d t o b e f u r t h e r i m p r o v e d b y a n o p t i m i z e d c o m b i - n a t i o n o f v a r i o u s d e t e c t o r i n f o r m a t i o n .