3 <title>Total Cross Sections</title>
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30 <h2>Total Cross Sections</h2>
32 The <code>SigmaTotal</code> class returns the total, elastic, diffractive
33 and nondiffractive cross sections in hadronic collisions, and also the
34 slopes of the <i>d(sigma)/dt</i> distributions. The parametrizations
35 used are from [<a href="Bibliography.php" target="page">Sch94, Sch97</a>] which borrows some of the total cross
36 sections from [<a href="Bibliography.php" target="page">Don92</a>].
39 There are strong indications that the currently implemented diffractive
40 cross section parametrizations, which should be in reasonable agreement
41 with data at lower energies, overestimate the diffractive rate at larger
42 values. If you wish to explore this (or other) aspect, it is possible to
43 override the cross section values in two different ways. The first offers
44 (almost) complete freedom, but needs to be defined separately for each
45 CM energy, while the second introduces a simpler parametrized damping.
46 The two cannot be combined. Furthermore the Coulomb term for elastic
47 scattering, which by default is off, can be switched on.
50 The allowed combinations of incoming particles are <i>p + p</i>,
51 <i>pbar + p</i>, <i>pi+ + p</i>, <i>pi- + p</i>,
52 <i>pi0/rho0 + p</i>, <i>phi + p</i>, <i>J/psi + p</i>,
53 <i>rho + rho</i>, <i>rho + phi</i>, <i>rho + J/psi</i>,
54 <i>phi + phi</i>, <i>phi + J/psi</i>, <i>J/psi + J/psi</i>.
55 The strong emphasis on vector mesons is related to the description
56 of <i>gamma + p</i> and <i>gamma + gamma</i> interactions in a
57 Vector Dominance Model framework (which will not be available for some
58 time to come, so this is a bit of overkill). Nevertheless, the sections
59 below, with allowed variations, are mainly intended to make sense for
62 <h3>Set cross sections</h3>
64 <br/><br/><strong>SigmaTotal:setOwn</strong> <input type="radio" name="1" value="on"><strong>On</strong>
65 <input type="radio" name="1" value="off" checked="checked"><strong>Off</strong>
66 (<code>default = <strong>off</strong></code>)<br/>
67 Allow a user to set own cross sections by hand; on/off = true/false.
71 When <code>SigmaTotal:setOwn = on</code>, the user is expected to set
72 values for the corresponding cross sections:
74 <br/><br/><table><tr><td><strong>SigmaTotal:sigmaTot </td><td></td><td> <input type="text" name="2" value="80." size="20"/> (<code>default = <strong>80.</strong></code>; <code>minimum = 0.</code>)</td></tr></table>
75 Total cross section in mb.
78 <br/><br/><table><tr><td><strong>SigmaTotal:sigmaEl </td><td></td><td> <input type="text" name="3" value="20." size="20"/> (<code>default = <strong>20.</strong></code>; <code>minimum = 0.</code>)</td></tr></table>
79 Elastic cross section in mb.
82 <br/><br/><table><tr><td><strong>SigmaTotal:sigmaXB </td><td></td><td> <input type="text" name="4" value="8." size="20"/> (<code>default = <strong>8.</strong></code>; <code>minimum = 0.</code>)</td></tr></table>
83 Single Diffractive cross section <i>A + B -> X + B</i> in mb.
86 <br/><br/><table><tr><td><strong>SigmaTotal:sigmaAX </td><td></td><td> <input type="text" name="5" value="8." size="20"/> (<code>default = <strong>8.</strong></code>; <code>minimum = 0.</code>)</td></tr></table>
87 Single Diffractive cross section <i>A + B -> A + X</i> in mb.
90 <br/><br/><table><tr><td><strong>SigmaTotal:sigmaXX </td><td></td><td> <input type="text" name="6" value="4." size="20"/> (<code>default = <strong>4.</strong></code>; <code>minimum = 0.</code>)</td></tr></table>
91 Double Diffractive cross section <i>A + B -> X_1 + X_2</i> in mb.
95 Note that the total cross section subtracted by the elastic and various
96 diffractive ones gives the inelastic nondiffractive cross section,
97 which therefore is not set separately. If this cross section evaluates
98 to be negative the internal parametrizations are used instead of the
99 ones here. However, since the nondiffractive inelastic cross section
100 is what makes up the minimum-bias event class, and plays a major role
101 in the description of multiple interactions, it is important that a
102 consistent set is used.
104 <h3>Dampen diffractive cross sections</h3>
106 <br/><br/><strong>SigmaDiffractive:dampen</strong> <input type="radio" name="7" value="on"><strong>On</strong>
107 <input type="radio" name="7" value="off"><strong>Off</strong>
108 (<code>default = <strong>no</strong></code>)<br/>
109 Allow a user to dampen diffractive cross sections; on/off = true/false.
113 When <code>SigmaDiffractive:dampen = on</code>, the three diffractive
114 cross sections are damped so that they never can exceed the respective
115 values below. Specifically, if the standard parametrization gives
116 the cross section <i>sigma_old(s)</i> and a fixed <i>sigma_max</i>
117 is set, the actual cross section becomes <i>sigma_new(s)
118 = sigma_old(s) * sigma_max / (sigma_old(s) + sigma_max)</i>.
119 This reduces to <i>sigma_old(s)</i> at low energies and to
120 <i>sigma_max</i> at high ones. Note that the asymptotic value
121 is approached quite slowly, however.
123 <br/><br/><table><tr><td><strong>SigmaDiffractive:maxXB </td><td></td><td> <input type="text" name="8" value="15." size="20"/> (<code>default = <strong>15.</strong></code>; <code>minimum = 0.</code>)</td></tr></table>
124 The above <i>sigma_max</i> for <i>A + B -> X + B</i> in mb.
127 <br/><br/><table><tr><td><strong>SigmaDiffractive:maxAX </td><td></td><td> <input type="text" name="9" value="15." size="20"/> (<code>default = <strong>15.</strong></code>; <code>minimum = 0.</code>)</td></tr></table>
128 The above <i>sigma_max</i> for <i>A + B -> X + B</i> in mb.
131 <br/><br/><table><tr><td><strong>SigmaDiffractive:maxXX </td><td></td><td> <input type="text" name="10" value="15." size="20"/> (<code>default = <strong>15.</strong></code>; <code>minimum = 0.</code>)</td></tr></table>
132 The above <i>sigma_max</i> for <i>A + B -> X + B</i> in mb.
136 As above, a reduced diffractive cross section automatically translates
137 into an increased nondiffractive one, such that the total (and elastic)
138 cross section remains fixed.
141 <h3>Set elastic cross section</h3>
144 In the above option the <i>t</i> slopes are based on the internal
145 parametrizations. In addition there is no Coulomb-term contribution
146 to the elastic (or total) cross section, which of course becomes
147 infinite if this contribution is included. If you have switched on
148 <code>SigmaTotal:setOwn</code> you can further switch on a machinery
149 to include the Coulomb term, including interference with the conventional
150 strong-interaction Pomeron one [<a href="Bibliography.php" target="page">Ber87</a>]. Then the elastic cross
151 section is no longer taken from <code>SigmaTotal:sigmaEl</code> but
152 derived from the parameters below and <code>SigmaTotal:sigmaTot</code>,
153 using the optical theorem. The machinery is only intended to be used for
154 <i>p p</i> and <i>pbar p</i> collisions. The description of
155 diffractive events, and especially their slopes, remains unchanged.
157 <br/><br/><strong>SigmaElastic:setOwn</strong> <input type="radio" name="11" value="on"><strong>On</strong>
158 <input type="radio" name="11" value="off"><strong>Off</strong>
159 (<code>default = <strong>no</strong></code>)<br/>
160 Allow a user to set parameters for the normalization and shape of the
161 elastic cross section the by hand; yes/no = true/false.
164 <br/><br/><table><tr><td><strong>SigmaElastic:bSlope </td><td></td><td> <input type="text" name="12" value="18." size="20"/> (<code>default = <strong>18.</strong></code>; <code>minimum = 0.</code>)</td></tr></table>
165 the slope <i>b</i> of the strong-interaction term <i>exp(bt)</i>,
169 <br/><br/><table><tr><td><strong>SigmaElastic:rho </td><td></td><td> <input type="text" name="13" value="0.13" size="20"/> (<code>default = <strong>0.13</strong></code>; <code>minimum = -1.</code>; <code>maximum = 1.</code>)</td></tr></table>
170 the ratio of the real to the imaginary parts of the nuclear scattering
174 <br/><br/><table><tr><td><strong>SigmaElastic:lambda </td><td></td><td> <input type="text" name="14" value="0.71" size="20"/> (<code>default = <strong>0.71</strong></code>; <code>minimum = 0.1</code>; <code>maximum = 2.</code>)</td></tr></table>
175 the main parameter of the electric form factor
176 <i>G(t) = lambda^2 / (lambda + |t|)^2</i>, in units of GeV^2.
179 <br/><br/><table><tr><td><strong>SigmaElastic:tAbsMin </td><td></td><td> <input type="text" name="15" value="5e-5" size="20"/> (<code>default = <strong>5e-5</strong></code>; <code>minimum = 1e-10</code>)</td></tr></table>
180 since the Coulomb contribution is infinite a lower limit on
181 <i>|t|</i> must be set to regularize the divergence,
185 <br/><br/><table><tr><td><strong>SigmaElastic:phaseConst </td><td></td><td> <input type="text" name="16" value="0.577" size="20"/> (<code>default = <strong>0.577</strong></code>)</td></tr></table>
186 The Coulomb term is taken to contain a phase factor
187 <i>exp(+- i alpha phi(t))</i>, with + for <i>p p</i> and - for
188 <i>pbar p</i>, where <i>phi(t) = - phaseConst - ln(-B t/2)</i>.
189 This constant is model dependent [<a href="Bibliography.php" target="page">Cah82</a>].
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