[u/mrichter/AliRoot.git] / PYTHIA8 / pythia8140 / xmldoc / TotalCrossSections.xml

<chapter name="Total Cross Sections">

<h2>Total Cross Sections</h2>

The <code>SigmaTotal</code> class returns the total, elastic, diffractive 
and nondiffractive cross sections in hadronic collisions, and also the
slopes of the <ei>d(sigma)/dt</ei> distributions. The parametrizations 
used are from <ref>Sch94, Sch97</ref> which borrows some of the total cross 
sections from <ref>Don92</ref>.

<p/>
There are strong indications that the currently implemented diffractive 
cross section parametrizations, which should be in reasonable agreement 
with data at lower energies, overestimate the diffractive rate at larger 
values. If you wish to explore this (or other) aspect, it is possible to 
override the cross section values in two different ways. The first offers 
(almost) complete freedom, but needs to be defined separately for each 
CM energy, while the second introduces a simpler parametrized damping. 
The two cannot be combined. Furthermore the Coulomb term for elastic
scattering, which by default is off, can be switched on.

<p/>
The allowed combinations of incoming particles are <ei>p + p</ei>, 
<ei>pbar + p</ei>, <ei>pi+ + p</ei>, <ei>pi- + p</ei>, 
<ei>pi0/rho0 + p</ei>, <ei>phi + p</ei>, <ei>J/psi + p</ei>, 
<ei>rho + rho</ei>, <ei>rho + phi</ei>, <ei>rho + J/psi</ei>, 
<ei>phi + phi</ei>, <ei>phi + J/psi</ei>, <ei>J/psi + J/psi</ei>.   
The strong emphasis on vector mesons is related to the description
of <ei>gamma + p</ei> and <ei>gamma + gamma</ei> interactions in a 
Vector Dominance Model framework (which will not be available for some 
time to come, so this is a bit of overkill). Nevertheless, the sections
below, with allowed variations, are mainly intended to make sense for
<ei>p + p</ei>.

<h3>Set cross sections</h3>

<flag name="SigmaTotal:setOwn" default="off">
Allow a user to set own cross sections by hand; on/off = true/false.
</flag>

<p/>
When <code>SigmaTotal:setOwn = on</code>, the user is expected to set 
values for the corresponding cross sections:

<parm name="SigmaTotal:sigmaTot" default="80." min="0.">
Total cross section in mb.
</parm>

<parm name="SigmaTotal:sigmaEl" default="20." min="0.">
Elastic cross section in mb.
</parm>

<parm name="SigmaTotal:sigmaXB" default="8." min="0.">
Single Diffractive cross section <ei>A + B -> X + B</ei> in mb.
</parm>

<parm name="SigmaTotal:sigmaAX" default="8." min="0.">
Single Diffractive cross section <ei>A + B -> A + X</ei> in mb.
</parm>

<parm name="SigmaTotal:sigmaXX" default="4." min="0.">
Double Diffractive cross section <ei>A + B -> X_1 + X_2</ei> in mb.
</parm>

<p/>
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. 

<h3>Dampen diffractive cross sections</h3>

<flag name="SigmaDiffractive:dampen" default="no">
Allow a user to dampen diffractive cross sections; on/off = true/false.
</flag>

<p/>
When <code>SigmaDiffractive:dampen = on</code>, the three diffractive 
cross sections are damped so that they never can exceed the respective 
values below. Specifically, if the standard parametrization gives 
the cross section <ei>sigma_old(s)</ei> and a fixed <ei>sigma_max</ei> 
is set, the actual cross section becomes <ei>sigma_new(s) 
= sigma_old(s) * sigma_max / (sigma_old(s) + sigma_max)</ei>. 
This reduces to <ei>sigma_old(s)</ei> at low energies and to 
<ei>sigma_max</ei> at high ones. Note that the asymptotic value 
is approached quite slowly, however.

<parm name="SigmaDiffractive:maxXB" default="15." min="0.">
The above <ei>sigma_max</ei> for <ei>A + B -> X + B</ei> in mb.
</parm>

<parm name="SigmaDiffractive:maxAX" default="15." min="0.">
The above <ei>sigma_max</ei> for <ei>A + B -> X + B</ei> in mb.
</parm>

<parm name="SigmaDiffractive:maxXX" default="15." min="0.">
The above <ei>sigma_max</ei> for <ei>A + B -> X + B</ei> in mb.
</parm>

<p/>
As above, a reduced diffractive cross section automatically translates
into an increased nondiffractive one, such that the total (and elastic)
cross section remains fixed. 

 
<h3>Set elastic cross section</h3>

<p/>
In the above option the <ei>t</ei> 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
<code>SigmaTotal:setOwn</code> you can further switch on a machinery 
to include the Coulomb term, including interference with the conventional
strong-interaction Pomeron one <ref>Ber87</ref>. Then the elastic cross 
section is no longer taken from <code>SigmaTotal:sigmaEl</code> but 
derived from the parameters below and <code>SigmaTotal:sigmaTot</code>, 
using the optical theorem. The machinery is only intended to be used for
<ei>p p</ei> and <ei>pbar p</ei> collisions. The description of 
diffractive events, and especially their slopes, remains unchanged. 

<flag name="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.
</flag>

<parm name="SigmaElastic:bSlope" default="18." min="0.">
the slope <ei>b</ei> of the strong-interaction term <ei>exp(bt)</ei>,
in units of GeV^-2. 
</parm>

<parm name="SigmaElastic:rho" default="0.13" min="-1." max="1.">
the ratio of the real to the imaginary parts of the nuclear scattering
amplitude.
</parm>

<parm name="SigmaElastic:lambda" default="0.71" min="0.1" max="2.">
the main parameter of the electric form factor
<ei>G(t) = lambda^2 / (lambda + |t|)^2</ei>, in units of GeV^2.
</parm>

<parm name="SigmaElastic:tAbsMin" default="5e-5" min="1e-10">
since the Coulomb contribution is infinite a lower limit on 
<ei>|t|</ei> must be set to regularize the divergence, 
in units of GeV^2.
</parm>

<parm name="SigmaElastic:phaseConst" default="0.577">
The Coulomb term is taken to contain a phase factor 
<ei>exp(+- i alpha phi(t))</ei>, with + for <ei>p p</ei> and - for 
<ei>pbar p</ei>, where <ei>phi(t) = - phaseConst - ln(-B t/2)</ei>.
This constant is model dependent <ref>Cah82</ref>.
</parm>

</chapter>

<!-- Copyright (C) 2010 Torbjorn Sjostrand -->
Commit	Line	Data
b584e2f5	1	<chapter name="Total Cross Sections">
	2
	3	<h2>Total Cross Sections</h2>
	4
	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>Sch94, Sch97</ref> which borrows some of the total cross
	9	sections from <ref>Don92</ref>.
	10
	11	<p/>
	12	There are strong indications that the currently implemented diffractive
	13	cross section parametrizations, which should be in reasonable agreement
	14	with data at lower energies, overestimate the diffractive rate at larger
	15	values. If you wish to explore this (or other) aspect, it is possible to
	16	override the cross section values in two different ways. The first offers
	17	(almost) complete freedom, but needs to be defined separately for each
	18	CM energy, while the second introduces a simpler parametrized damping.
	19	The two cannot be combined. Furthermore the Coulomb term for elastic
	20	scattering, which by default is off, can be switched on.
	21
	22	<p/>
	23	The allowed combinations of incoming particles are <ei>p + p</ei>,
	24	<ei>pbar + p</ei>, <ei>pi+ + p</ei>, <ei>pi- + p</ei>,
	25	<ei>pi0/rho0 + p</ei>, <ei>phi + p</ei>, <ei>J/psi + p</ei>,
	26	<ei>rho + rho</ei>, <ei>rho + phi</ei>, <ei>rho + J/psi</ei>,
	27	<ei>phi + phi</ei>, <ei>phi + J/psi</ei>, <ei>J/psi + J/psi</ei>.
	28	The strong emphasis on vector mesons is related to the description
	29	of <ei>gamma + p</ei> and <ei>gamma + gamma</ei> interactions in a
	30	Vector Dominance Model framework (which will not be available for some
	31	time to come, so this is a bit of overkill). Nevertheless, the sections
	32	below, with allowed variations, are mainly intended to make sense for
	33	<ei>p + p</ei>.
	34
	35	<h3>Set cross sections</h3>
	36
	37	<flag name="SigmaTotal:setOwn" default="off">
	38	Allow a user to set own cross sections by hand; on/off = true/false.
	39	</flag>
	40
	41	<p/>
	42	When <code>SigmaTotal:setOwn = on</code>, the user is expected to set
	43	values for the corresponding cross sections:
	44
	45	<parm name="SigmaTotal:sigmaTot" default="80." min="0.">
	46	Total cross section in mb.
	47	</parm>
	48
	49	<parm name="SigmaTotal:sigmaEl" default="20." min="0.">
	50	Elastic cross section in mb.
	51	</parm>
	52
	53	<parm name="SigmaTotal:sigmaXB" default="8." min="0.">
	54	Single Diffractive cross section <ei>A + B -> X + B</ei> in mb.
	55	</parm>
	56
	57	<parm name="SigmaTotal:sigmaAX" default="8." min="0.">
	58	Single Diffractive cross section <ei>A + B -> A + X</ei> in mb.
	59	</parm>
	60
	61	<parm name="SigmaTotal:sigmaXX" default="4." min="0.">
	62	Double Diffractive cross section <ei>A + B -> X_1 + X_2</ei> in mb.
	63	</parm>
	64
65	<p/>
66	Note that the total cross section subtracted by the elastic and various
67	diffractive ones gives the inelastic nondiffractive cross section,
68	which therefore is not set separately. If this cross section evaluates
69	to be negative the internal parametrizations are used instead of the
70	ones here. However, since the nondiffractive inelastic cross section
71	is what makes up the minimum-bias event class, and plays a major role
72	in the description of multiple interactions, it is important that a
73	consistent set is used.
74
75	<h3>Dampen diffractive cross sections</h3>
76
77	<flag name="SigmaDiffractive:dampen" default="no">
78	Allow a user to dampen diffractive cross sections; on/off = true/false.
79	</flag>
80
81	<p/>
82	When <code>SigmaDiffractive:dampen = on</code>, the three diffractive
83	cross sections are damped so that they never can exceed the respective
84	values below. Specifically, if the standard parametrization gives
85	the cross section <ei>sigma_old(s)</ei> and a fixed <ei>sigma_max</ei>
86	is set, the actual cross section becomes <ei>sigma_new(s)
87	= sigma_old(s) * sigma_max / (sigma_old(s) + sigma_max)</ei>.
88	This reduces to <ei>sigma_old(s)</ei> at low energies and to
89	<ei>sigma_max</ei> at high ones. Note that the asymptotic value
90	is approached quite slowly, however.
91
92	<parm name="SigmaDiffractive:maxXB" default="15." min="0.">
93	The above <ei>sigma_max</ei> for <ei>A + B -> X + B</ei> in mb.
94	</parm>
95
96	<parm name="SigmaDiffractive:maxAX" default="15." min="0.">
97	The above <ei>sigma_max</ei> for <ei>A + B -> X + B</ei> in mb.
98	</parm>
99
100	<parm name="SigmaDiffractive:maxXX" default="15." min="0.">
101	The above <ei>sigma_max</ei> for <ei>A + B -> X + B</ei> in mb.
102	</parm>
103
104	<p/>
105	As above, a reduced diffractive cross section automatically translates
106	into an increased nondiffractive one, such that the total (and elastic)
107	cross section remains fixed.
108
109
110	<h3>Set elastic cross section</h3>
111
112	<p/>
113	In the above option the <ei>t</ei> slopes are based on the internal
114	parametrizations. In addition there is no Coulomb-term contribution
115	to the elastic (or total) cross section, which of course becomes
116	infinite if this contribution is included. If you have switched on
117	<code>SigmaTotal:setOwn</code> you can further switch on a machinery
118	to include the Coulomb term, including interference with the conventional
119	strong-interaction Pomeron one <ref>Ber87</ref>. Then the elastic cross
120	section is no longer taken from <code>SigmaTotal:sigmaEl</code> but
121	derived from the parameters below and <code>SigmaTotal:sigmaTot</code>,
122	using the optical theorem. The machinery is only intended to be used for
123	<ei>p p</ei> and <ei>pbar p</ei> collisions. The description of
124	diffractive events, and especially their slopes, remains unchanged.
125
126	<flag name="SigmaElastic:setOwn" default="no">
127	Allow a user to set parameters for the normalization and shape of the
128	elastic cross section the by hand; yes/no = true/false.
129	</flag>
130
131	<parm name="SigmaElastic:bSlope" default="18." min="0.">
132	the slope <ei>b</ei> of the strong-interaction term <ei>exp(bt)</ei>,
133	in units of GeV^-2.
134	</parm>
135
136	<parm name="SigmaElastic:rho" default="0.13" min="-1." max="1.">
137	the ratio of the real to the imaginary parts of the nuclear scattering
138	amplitude.
139	</parm>
140
141	<parm name="SigmaElastic:lambda" default="0.71" min="0.1" max="2.">
142	the main parameter of the electric form factor
143	<ei>G(t) = lambda^2 / (lambda + \|t\|)^2</ei>, in units of GeV^2.
144	</parm>
145
146	<parm name="SigmaElastic:tAbsMin" default="5e-5" min="1e-10">
147	since the Coulomb contribution is infinite a lower limit on
148	<ei>\|t\|</ei> must be set to regularize the divergence,
149	in units of GeV^2.
150	</parm>
151
152	<parm name="SigmaElastic:phaseConst" default="0.577">
153	The Coulomb term is taken to contain a phase factor
154	<ei>exp(+- i alpha phi(t))</ei>, with + for <ei>p p</ei> and - for
155	<ei>pbar p</ei>, where <ei>phi(t) = - phaseConst - ln(-B t/2)</ei>.
156	This constant is model dependent <ref>Cah82</ref>.
157	</parm>
158
159	</chapter>
160
161	<!-- Copyright (C) 2010 Torbjorn Sjostrand -->