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 [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). 

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]. 

-- MinkyooChoi - 26 Mar 2009

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Topic revision: r1 - 2009-03-26 - MinkyooChoi
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