[u/mrichter/AliRoot.git] / TEvtGen / EvtGenModels / EvtBTo3pi.F

C--------------------------------------------------------------------------
C
C Environment:
C      This software is part of the EvtGen package developed jointly
C      for the BaBar and CLEO collaborations.  If you use all or part
C      of it, please give an appropriate acknowledgement.
C
C Copyright Information: See EvtGen/COPYRIGHT
C      Copyright (C) 1998      Caltech, UCSB
C
C Module: EvtBTo3pi.F
C
C Description:
C
C Modification history:
C
C    DJL/RYD     August 11, 1998         Module created
C
C------------------------------------------------------------------------
C===================================================================
C This package is providing a B -->-- 3pions decay generator
C Its is composed of the following subroutines:
C 
C [*] EVTHowToUse  
C                 This is an How To Use routine where one may find the
C                 implementation of the time dependance: That is to	  
C                 say that it shows how the output of the routine is	
C                 supposed to be used in the mind of the authors.
C
C===================================================================
C [0] EVT3pions
C                 The routine to be called. Note that on the first call
C                 some initialization will be made, in particular the
C                 computation of a normalization factor designed to help
C                 the subsequent time-dependent generation of events.
C                 The normalisation done inside EVT3pions is such that
C                 at the level of the time implementation, the maximum
C                 time-dependant weight is unity : nothing is to be 
C                 computed to generate unity-weight events. The exact
C                 meaning of the above is made explicit in the HowToUse
C                 routine.
C [1] first_step
C                 Generation of the kinematics of the 3 pions 
C                 It uses the function evtranf which is a random number 
C                 generator providing an uniform distribution 
C                 of Real*4 random number between 0 and 1	
C [2] compute
C                 return the amplitudes of the B0     -->-- 3pions
C                                   and of the B0_bar -->-- 3pions
C                 corrected for the generation mechanism
C                 The notations used are the ones of the paper of
C                 A. Snyder and H. Quinn [Phys.Rev.D48 (1993) 2139] 
C [3] BreitWigner
C                 compute the Breit-Wigner of the contributing rho s
C                 taking into account the cosine term linked to the
C                 zero-helicity of the rho s. There is three forms of
C                 Breit-Wigners available. The first one is the simple
C                 non-relativistic form, while the second and third
C                 ones are more involved relativistic expressions which
C                 in addition incorporate the contributions of three
C                 rho resonances [rho(770:1450:1700)]. The parameters
C                 used are the ones resulting from the ALEPH analysis
C                 which might be found in CERN-PPE:97013 (submitted to
Commit	Line	Data
da0e9ce3	1	C--------------------------------------------------------------------------
	2	C
	3	C Environment:
	4	C This software is part of the EvtGen package developed jointly
	5	C for the BaBar and CLEO collaborations. If you use all or part
	6	C of it, please give an appropriate acknowledgement.
	7	C
	8	C Copyright Information: See EvtGen/COPYRIGHT
	9	C Copyright (C) 1998 Caltech, UCSB
	10	C
	11	C Module: EvtBTo3pi.F
	12	C
	13	C Description:
	14	C
	15	C Modification history:
	16	C
	17	C DJL/RYD August 11, 1998 Module created
	18	C
	19	C------------------------------------------------------------------------
	20	C===================================================================
	21	C This package is providing a B -->-- 3pions decay generator
	22	C Its is composed of the following subroutines:
	23	C
	24	C [*] EVTHowToUse
	25	C This is an How To Use routine where one may find the
	26	C implementation of the time dependance: That is to
	27	C say that it shows how the output of the routine is
	28	C supposed to be used in the mind of the authors.
	29	C
	30	C===================================================================
	31	C [0] EVT3pions
	32	C The routine to be called. Note that on the first call
	33	C some initialization will be made, in particular the
	34	C computation of a normalization factor designed to help
	35	C the subsequent time-dependent generation of events.
	36	C The normalisation done inside EVT3pions is such that
	37	C at the level of the time implementation, the maximum
	38	C time-dependant weight is unity : nothing is to be
	39	C computed to generate unity-weight events. The exact
	40	C meaning of the above is made explicit in the HowToUse
	41	C routine.
	42	C [1] first_step
	43	C Generation of the kinematics of the 3 pions
	44	C It uses the function evtranf which is a random number
	45	C generator providing an uniform distribution
	46	C of Real*4 random number between 0 and 1
	47	C [2] compute
	48	C return the amplitudes of the B0 -->-- 3pions
	49	C and of the B0_bar -->-- 3pions
	50	C corrected for the generation mechanism
	51	C The notations used are the ones of the paper of
	52	C A. Snyder and H. Quinn [Phys.Rev.D48 (1993) 2139]
	53	C [3] BreitWigner
	54	C compute the Breit-Wigner of the contributing rho s
	55	C taking into account the cosine term linked to the
	56	C zero-helicity of the rho s. There is three forms of
	57	C Breit-Wigners available. The first one is the simple
	58	C non-relativistic form, while the second and third
	59	C ones are more involved relativistic expressions which
	60	C in addition incorporate the contributions of three
	61	C rho resonances [rho(770:1450:1700)]. The parameters
	62	C used are the ones resulting from the ALEPH analysis
	63	C which might be found in CERN-PPE:97013 (submitted to
	64