1 <chapter name="Total Cross Sections">
3 <h2>Total Cross Sections</h2>
5 The <code>SigmaTotal</code> class returns the total, elastic, diffractive
6 and nondiffractive cross sections in hadronic collisions, and also the
7 slopes of the <ei>d(sigma)/dt</ei> distributions. The parametrizations
8 used are from <ref>Sch97</ref> which borrows some of the total cross
9 sections from <ref>Don92</ref>.
12 The allowed combinations of incoming particles are <ei>p + p</ei>,
13 <ei>pbar + p</ei>, <ei>pi+ + p</ei>, <ei>pi- + p</ei>,
14 <ei>pi0/rho0 + p</ei>, <ei>phi + p</ei>, <ei>J/psi + p</ei>,
15 <ei>rho + rho</ei>, <ei>rho + phi</ei>, <ei>rho + J/psi</ei>,
16 <ei>phi + phi</ei>, <ei>phi + J/psi</ei>, <ei>J/psi + J/psi</ei>.
17 The strong emphasis on vector mesons is related to the description
18 of <ei>gamma + p</ei> and <ei>gamma + gamma</ei> interactions in a
19 Vector Dominance Model framework (which will not be available for some
20 time to come, so this is a bit of overkill).
24 If the internally implemented cross section parametrizations are not
25 satisfactory, it is possible to override the cross section values
28 <flag name="SigmaTotal:setOwn" default="no">
29 Allow a user to set own cross sections by hand; yes/no = true/false.
33 When <code>SigmaTotal:setOwn = yes</code>, the user is expected to set
34 values for the corresponding cross sections:
36 <parm name="SigmaTotal:sigmaTot" default="80." min="0.">
37 Total cross section in mb.
40 <parm name="SigmaTotal:sigmaEl" default="20." min="0.">
41 Elastic cross section in mb.
44 <parm name="SigmaTotal:sigmaXB" default="8." min="0.">
45 Single Diffractive cross section <ei>A + B -> X + B</ei> in mb.
48 <parm name="SigmaTotal:sigmaAX" default="8." min="0.">
49 Single Diffractive cross section <ei>A + B -> A + X</ei> in mb.
52 <parm name="SigmaTotal:sigmaXX" default="4." min="0.">
53 Double Diffractive cross section <ei>A + B -> X_1 + X_2</ei> in mb.
57 Note that the total cross section subtracted by the elastic and various
58 diffractive ones gives the inelastic nondiffractive cross section,
59 which therefore is not set separately. If this cross section evaluates
60 to be negative the internal parametrizations are used instead of the
61 ones here. However, since the nondiffractive inelastic cross section
62 is what makes up the minimum-bias event class, and plays a major role
63 in the description of multiple interactions, it is important that a
64 consistent set is used.
67 In the above option the <ei>t</ei> slopes are based on the internal
68 parametrizations. In addition there is no Coulomb-term contribution
69 to the elastic (or total) cross section, which of course becomes
70 infinite if this contribution is included. If you have switched on
71 <code>SigmaTotal:setOwn</code> you can further switch on a machinery
72 to include the Coulomb term, including interference with the conventional
73 strong-interaction Pomeron one <ref>Ber87</ref>. Then the elastic cross
74 section is no longer taken from <code>SigmaTotal:sigmaEl</code> but
75 derived from the parameters below and <code>SigmaTotal:sigmaTot</code>,
76 using the optical theorem. The machinery is only intended to be used for
77 <ei>p p</ei> and <ei>pbar p</ei> collisions. The description of
78 diffractive events, and especially their slopes, remains unchanged.
80 <flag name="SigmaElastic:setOwn" default="no">
81 Allow a user to set parameters for the normalization and shape of the
82 elastic cross section the by hand; yes/no = true/false.
85 <parm name="SigmaElastic:bSlope" default="18." min="0.">
86 the slope <ei>b</ei> of the strong-interaction term <ei>exp(bt)</ei>,
90 <parm name="SigmaElastic:rho" default="0.13" min="-1." max="1.">
91 the ratio of the real to the imaginary parts of the nuclear scattering
95 <parm name="SigmaElastic:lambda" default="0.71" min="0.1" max="2.">
96 the main parameter of the electric form factor
97 <ei>G(t) = lambda^2 / (lambda + |t|)^2</ei>, in units of GeV^2.
100 <parm name="SigmaElastic:tAbsMin" default="5e-5" min="1e-10">
101 since the Coulomb contribution is infinite a lower limit on
102 <ei>|t|</ei> must be set to regularize the divergence,
106 <parm name="SigmaElastic:phaseConst" default="0.577">
107 The Coulomb term is taken to contain a phase factor
108 <ei>exp(+- i alpha phi(t))</ei>, with + for <ei>p p</ei> and - for
109 <ei>pbar p</ei>, where <ei>phi(t) = - phaseConst - ln(-B t/2)</ei>.
110 This constant is model dependent <ref>Cah82</ref>.
115 <!-- Copyright (C) 2008 Torbjorn Sjostrand -->