Total Cross Sections
The SigmaTotal class returns the total, elastic, diffractive
and nondiffractive cross sections in hadronic collisions, and also the
slopes of the d(sigma)/dt distributions. The parametrizations
used are from [Sch94, Sch97] which borrows some of the total cross
sections from [Don92].
The allowed combinations of incoming particles are p + p,
pbar + p, pi+ + p, pi- + p,
pi0/rho0 + p, phi + p, J/psi + p,
rho + rho, rho + phi, rho + J/psi,
phi + phi, phi + J/psi, J/psi + J/psi.
The strong emphasis on vector mesons is related to the description
of gamma + p and gamma + gamma interactions in a
Vector Dominance Model framework (which will not be available for some
time to come, so this is a bit of overkill).
Variables
If the internally implemented cross section parametrizations are not
satisfactory, it is possible to override the cross section values
with
flag SigmaTotal:setOwn
(default = no)
Allow a user to set own cross sections by hand; yes/no = true/false.
When SigmaTotal:setOwn = yes, the user is expected to set
values for the corresponding cross sections:
parm SigmaTotal:sigmaTot
(default = 80.; minimum = 0.)
Total cross section in mb.
parm SigmaTotal:sigmaEl
(default = 20.; minimum = 0.)
Elastic cross section in mb.
parm SigmaTotal:sigmaXB
(default = 8.; minimum = 0.)
Single Diffractive cross section A + B -> X + B in mb.
parm SigmaTotal:sigmaAX
(default = 8.; minimum = 0.)
Single Diffractive cross section A + B -> A + X in mb.
parm SigmaTotal:sigmaXX
(default = 4.; minimum = 0.)
Double Diffractive cross section A + B -> X_1 + X_2 in mb.
Note that the total cross section subtracted by the elastic and various
diffractive ones gives the inelastic nondiffractive cross section,
which therefore is not set separately. If this cross section evaluates
to be negative the internal parametrizations are used instead of the
ones here. However, since the nondiffractive inelastic cross section
is what makes up the minimum-bias event class, and plays a major role
in the description of multiple interactions, it is important that a
consistent set is used.
In the above option the t slopes are based on the internal
parametrizations. In addition there is no Coulomb-term contribution
to the elastic (or total) cross section, which of course becomes
infinite if this contribution is included. If you have switched on
SigmaTotal:setOwn you can further switch on a machinery
to include the Coulomb term, including interference with the conventional
strong-interaction Pomeron one [Ber87]. Then the elastic cross
section is no longer taken from SigmaTotal:sigmaEl but
derived from the parameters below and SigmaTotal:sigmaTot,
using the optical theorem. The machinery is only intended to be used for
p p and pbar p collisions. The description of
diffractive events, and especially their slopes, remains unchanged.
flag SigmaElastic:setOwn
(default = no)
Allow a user to set parameters for the normalization and shape of the
elastic cross section the by hand; yes/no = true/false.
parm SigmaElastic:bSlope
(default = 18.; minimum = 0.)
the slope b of the strong-interaction term exp(bt),
in units of GeV^-2.
parm SigmaElastic:rho
(default = 0.13; minimum = -1.; maximum = 1.)
the ratio of the real to the imaginary parts of the nuclear scattering
amplitude.
parm SigmaElastic:lambda
(default = 0.71; minimum = 0.1; maximum = 2.)
the main parameter of the electric form factor
G(t) = lambda^2 / (lambda + |t|)^2, in units of GeV^2.
parm SigmaElastic:tAbsMin
(default = 5e-5; minimum = 1e-10)
since the Coulomb contribution is infinite a lower limit on
|t| must be set to regularize the divergence,
in units of GeV^2.
parm SigmaElastic:phaseConst
(default = 0.577)
The Coulomb term is taken to contain a phase factor
exp(+- i alpha phi(t)), with + for p p and - for
pbar p, where phi(t) = - phaseConst - ln(-B t/2).
This constant is model dependent [Cah82].