1 <chapter name="Beam Parameters">
3 <h2>Beam Parameters</h2>
5 The settings on this page relate to the beam identities and energies,
6 to a beam momentum spread and to a beam interaction spot.
7 As always, momenta and energies are to be given in units of GeV,
8 and of space and time in mm.
10 <h3>Incoming beams</h3>
12 There are two ways to set the identities and energies of the two incoming
13 beam particles. One is to use the <code>init()</code> method with no
14 arguments. Then the settings variables below will be read and used. The
15 alternative is to call <code>init(...)</code> with arguments that provide
16 this information. Then you need not use the variables below (although it
17 would still be possible). Note that, if nothing is done, you will default
18 to LHC at the nominal energy.
20 <modeopen name="Beams:idA" default="2212">
21 The PDG <code>id</code> code for the first incoming particle.
24 <modeopen name="Beams:idB" default="2212">
25 The PDG <code>id</code> code for the second incoming particle.
28 <modepick name="Beams:frameType" default="1" min="1" max="4">
29 Choice of frame for the two colliding particles.
30 <option value="1">the beams are colliding in their cm frame,
31 and therefore only the cm energy needs to be provided, see
32 <code>Beams:eCM</code> below.
34 <option value="2">the beams are back-to-back, but with different energies,
35 see <code>Beams:eA</code> and <code>Beams:eB</code> below.
36 This option could also be used for fixed-target configurations.
38 <option value="3">the beams are not back-to-back, and therefore the
39 three-momentum of each incoming particle needs to be specified, see
40 <code>Beams:pxA</code> through <code>Beams:pzB</code> below.
42 <option value="4">the beam and event information is stored in a
43 Les Houches Event File, see <code>Beams:LHEF</code> below.
47 <parm name="Beams:eCM" default="14000." min="10.">
48 Collision CM energy, to be set if <code>Beams:frameType</code> = 1.
51 <parm name="Beams:eA" default="7000." min="0.">
52 The energy of the first incoming particle, moving in the
53 <ei>+z </ei>direction, to be set if <code>Beams:frameType</code> = 2.
54 If the particle energy is smaller than its mass
55 it is assumed to be at rest.
58 <parm name="Beams:eB" default="7000." min="0.">
59 The energy of the second incoming particle, moving in the
60 <ei>-z</ei> direction, to be set if <code>Beams:frameType</code> = 2.
61 If the particle energy is smaller than its mass
62 it is assumed to be at rest.
65 <parm name="Beams:pxA" default="0.">
66 The <ei>p_x</ei> component of the first incoming particle,
67 to be set if <code>Beams:frameType</code> = 3.
70 <parm name="Beams:pyA" default="0.">
71 The <ei>p_y</ei> component of the first incoming particle,
72 to be set if <code>Beams:frameType</code> = 3.
75 <parm name="Beams:pzA" default="7000.">
76 The <ei>p_z</ei> component of the first incoming particle,
77 to be set if <code>Beams:frameType</code> = 3.
80 <parm name="Beams:pxB" default="0.">
81 The <ei>p_x</ei> component of the second incoming particle,
82 to be set if <code>Beams:frameType</code> = 3.
85 <parm name="Beams:pyB" default="0.">
86 The <ei>p_y</ei> component of the second incoming particle,
87 to be set if <code>Beams:frameType</code> = 3.
90 <parm name="Beams:pzB" default="-7000.">
91 The <ei>p_z</ei> component of the second incoming particle,
92 to be set if <code>Beams:frameType</code> = 3.
95 <word name="Beams:LHEF" default="void">
96 The name of a Les Houches Event File,
97 to be set if <code>Beams:frameType</code> = 4.
100 <h3>Beam momentum spread</h3>
102 This framework currently is intended for a modest beam spread, such as
103 experienced at hadron colliders. Thus it can be safely assumed that the
104 physics does not change over the CM energy range probed, so that the
105 parameters of the physics initialization at the nominal energy can be
106 used as is. Currently it can <b>not</b> be used for the more extensive
107 energy spread expected at linear <ei>e^+ e^-</ei> colliders. Also,
108 any attempt to combine it with external Les Houches input of
109 parton-level events is at own risk.
112 On this page you can set the momentum spread according to a simple
113 Gaussian distribution. If you instead want a more sophisticated
114 parametrization, you can write and link your own
115 <aloc href="BeamShape"><code>BeamShape</code></aloc> class.
117 <flag name="Beams:allowMomentumSpread" default="off">
118 Allow the beam momenta to be smeared around their initialization
122 <parm name="Beams:sigmaPxA" default="0." min="0.">
123 The width of a Gaussian distribution of the <ei>p_x</ei> spread of the
124 first incoming particle.
127 <parm name="Beams:sigmaPyA" default="0." min="0.">
128 The width of a Gaussian distribution of the <ei>p_y</ei> spread of the
129 first incoming particle.
132 <parm name="Beams:sigmaPzA" default="0." min="0.">
133 The width of a Gaussian distribution of the <ei>p_z</ei> spread of the
134 first incoming particle.
137 <parm name="Beams:maxDevA" default="5." min="0.">
138 The triply Gaussian distribution <ei>(p_x, p_y, p_z)</ei> is restricted to
139 a maximal total deviation from the nominal values <ei>(p_x0, p_y0, p_z0)</ei>
140 for the first incoming particle, like
142 (p_x - p_x0)^2/sigma_px^2 + (p_y - p_y0)^2/sigma_py^2 +
143 (p_z - p_z0)^2/sigma_pz^2 < maxDev^2
145 (Note the absence of a factor 2 in the denominator, unlike the Gaussians
146 used to pick <ei>(p_x, p_y, p_z)</ei>.)
149 <parm name="Beams:sigmaPxB" default="0." min="0.">
150 The width of a Gaussian distribution of the <ei>p_x</ei> spread of the
151 second incoming particle.
154 <parm name="Beams:sigmaPyB" default="0." min="0.">
155 The width of a Gaussian distribution of the <ei>p_y</ei> spread of the
156 second incoming particle.
159 <parm name="Beams:sigmaPzB" default="0." min="0.">
160 The width of a Gaussian distribution of the <ei>p_z</ei> spread of the
161 second incoming particle.
164 <parm name="Beams:maxDevB" default="5." min="0.">
165 The triply Gaussian distribution <ei>(p_x, p_y, p_z)</ei> is restricted to
166 a maximal total deviation from the nominal values <ei>(p_x0, p_y0, p_z0)</ei>,
167 for the second incoming particle, like
169 (p_x - p_x0)^2/sigma_px^2 + (p_y - p_y0)^2/sigma_py^2 +
170 (p_z - p_z0)^2/sigma_pz^2 < maxDev^2
172 (Note the absence of a factor 2 in the denominator, unlike the Gaussians
173 used to pick <ei>(p_x, p_y, p_z)</ei>.)
176 <h3>Beam interaction vertex</h3>
178 On this page you can set the spread of the interaction vertex according to
179 a simple Gaussian distribution. If you instead want a more sophisticated
180 parametrization, you can write and link your own
181 <aloc href="BeamShape"><code>BeamShape</code></aloc> class.
183 <flag name="Beams:allowVertexSpread" default="off">
184 Allow the interaction vertex of the two colliding beams to be smeared.
185 If off, then the vertex is set to be the origin.
188 <parm name="Beams:sigmaVertexX" default="0." min="0.">
189 The width of a Gaussian distribution of the <ei>x</ei> location of the
193 <parm name="Beams:sigmaVertexY" default="0." min="0.">
194 The width of a Gaussian distribution of the <ei>y</ei> location of the
198 <parm name="Beams:sigmaVertexZ" default="0." min="0.">
199 The width of a Gaussian distribution of the <ei>z</ei> location of the
203 <parm name="Beams:maxDevVertex" default="5." min="0.">
204 The triply Gaussian distribution of interaction vertex position
205 <ei>(x, y, z)</ei> is restricted to a maximal total deviation from the
208 x^2/sigma_x^2 + y^2/sigma_y^2 + z^2/sigma_z^2 < maxDevVertex^2
210 (Note the absence of a factor 2 in the denominator, unlike the Gaussians
211 used to pick <ei>(x, y, z)</ei>.)
214 <parm name="Beams:sigmaTime" default="0." min="0.">
215 The width of a Gaussian distribution of the collision time (in units of
216 mm/c). Note that, if the above space parametrization is viewed as the
217 effect of two incoming beams along the <ei>+-z</ei> axis, with each beam
218 having a Gaussian spread, then the spread of the time would also become
219 a Gaussian with the same width as the <ei>z</ei> one (times the
220 velocity of the beams, which we expect is close to unity). For flexibility
221 we have not enforced any such relation, however.
224 <parm name="Beams:maxDevTime" default="5." min="0.">
225 The collision time is restricted to be in the range
226 <ei>|t| < sigma_t * maxDevTime</ei>.
230 The distributions above are all centered at the origin. It is also
231 possible to shift the above distributions to be centered around another
232 nominal position. You must have <code>Beams:allowVertexSpread = on</code>
233 to use this possibility.
235 <parm name="Beams:offsetVertexX" default="0.">
236 The <ei>x</ei> location of the interaction vertex is centered at this value.
239 <parm name="Beams:offsetVertexY" default="0.">
240 The <ei>y</ei> location of the interaction vertex is centered at this value.
243 <parm name="Beams:offsetVertexZ" default="0.">
244 The <ei>z</ei> location of the interaction vertex is centered at this value.
247 <parm name="Beams:offsetTime" default="0.">
248 The time <ei>t</ei> of the interaction vertex is centered at this value.
253 <!-- Copyright (C) 2008 Torbjorn Sjostrand -->