Dear TWIST people:

     Multiple scattering has been taken into account in 
the Kalman filter for straight track reconstruction.
 
     The improvement is summaried in the following table.
The study was based on the Monte Carlo data with the 
multiple scattering flag (MULS) turned on. Positrons were
generated at an orientation of theta = 20 degree and phi 
= 30 degree. The efficiency of reconstruction is 100% at
these angles. Numbers in parentheses are when multiple
scattering is NOT taken care of in the Kalman filter. The
number of event analyzed per run is 250.

================================================================     
 positron   mean of the    and its      mean of the    and its   
 momentum   residuals of   r.m.s.       residuals of   r.m.s.
 in MeV/c   reconstructed               reconstructed  
            theta's                     phi's        
----------------------------------------------------------------
    10    -0.01 (-0.03)    1.8 (2.0)    0.07 (-0.12)   4.4 (4.4)      
    15     0.03 ( 0.06)    1.4 (1.5)    0.24 ( 0.24)   3.2 (3.2)
    20    -0.11 ( 0.01)    0.9 (1.0)    0.35 ( 0.31)   2.5 (2.5)
    30     0.00 ( 0.05)    0.7 (0.8)    0.32 ( 0.29)   2.0 (2.0)
    40    -0.06 (-0.02)    0.7 (0.6)    0.08 ( 0.05)   1.6 (1.8)         
================================================================

     It looks that there are improvements in theta  
determination for positrons of low momenta (< ~30 MeV/c).     
The number of iterations for the Kalman filter to converge
to a chi-squared is about the same (~15). At high momenta,
there seems to be no improvement compared with the case of 
no multiple scattering in the filter. The picture can be 
understood since the multiple scattering angle is inversely 
proportional to the particle momentum. The radiation length 
used in calculating the multiple scattering angle is taken
to be 300.5 meter for our chamber composite. 

Sun-Chong

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