3 <title>Beam Parameters</title>
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9 <h2>Beam Parameters</h2>
11 The settings on this page relate to the beam identities and energies,
12 to a beam momentum spread and to a beam interaction spot.
13 As always, momenta and energies are to be given in units of GeV,
14 and of space and time in mm.
16 <h3>Incoming beams</h3>
18 There are two ways to set the identities and energies of the two incoming
19 beam particles. One is to use the <code>init()</code> method with no
20 arguments. Then the settings variables below will be read and used. The
21 alternative is to call <code><a href="ProgramFlow.html" target="page">init(...)</a></code>
22 with arguments that provide this information. Then you need not use the
23 variables below (although it would still be possible). Note that, if nothing
24 is done, you will default to LHC at 14 TeV.
27 Currently the beam particles must be either a hadron pair or a lepton
28 pair. In the former category <i>p p</i> and <i>pbar p</i>
29 combinations dominate, but it is also possible to combine with
30 <i>pi^+</i>, <i>pi^-</i> and <i>pi^0</i>. In the latter
31 <i>e^+ e^-</i> and <i>mu^+ mu^-</i> would be the most useful
32 combinations, but also others should work if combined with an
33 appropriate hard process.
35 <p/><code>mode </code><strong> Beams:idA </strong>
36 (<code>default = <strong>2212</strong></code>)<br/>
37 The PDG <code>id</code> code for the first incoming particle.
39 <br/><i>2212 = p</i>, <i>-2212 = pbar</i>,
40 <br/><i>211 = pi^+</i>, <i>-211 = pi^-</i>, <i>111 = pi^0</i>,
41 <br/><i>990 = Pomeron</i> (used in diffractive machinery;
42 here mainly for debug purposes),
43 <br/><i>11 = e^-</i>, <i>-11 = e^+</i>,
44 <br/><i>13 = mu^-</i>, <i>-13 = mu^+</i>,
45 <br/>and a few more leptons/neutrinos in a few combinations.
48 <p/><code>mode </code><strong> Beams:idB </strong>
49 (<code>default = <strong>2212</strong></code>)<br/>
50 The PDG <code>id</code> code for the second incoming particle.
53 <p/><code>mode </code><strong> Beams:frameType </strong>
54 (<code>default = <strong>1</strong></code>; <code>minimum = 1</code>; <code>maximum = 5</code>)<br/>
55 Choice of frame for the two colliding particles. For options
56 1 - 3 the beam identities are specified above, while they are
57 obtained by the Les Houches information for options 4 and 5.
58 <br/><code>option </code><strong> 1</strong> : the beams are colliding in their CM frame,
59 and therefore only the CM energy needs to be provided, see
60 <code>Beams:eCM</code> below.
62 <br/><code>option </code><strong> 2</strong> : the beams are back-to-back, but with different energies,
63 see <code>Beams:eA</code> and <code>Beams:eB</code> below.
64 This option could also be used for fixed-target configurations.
66 <br/><code>option </code><strong> 3</strong> : the beams are not back-to-back, and therefore the
67 three-momentum of each incoming particle needs to be specified, see
68 <code>Beams:pxA</code> through <code>Beams:pzB</code> below.
70 <br/><code>option </code><strong> 4</strong> : the beam and event information is stored in a
71 <a href="LesHouchesAccord.html" target="page">Les Houches Event File</a>,
72 see <code>Beams:LHEF</code> below.
74 <br/><code>option </code><strong> 5</strong> : the beam and event information is obtained by a
75 pointer to an <code><a href="LesHouchesAccord.html" target="page">LHAup</a></code>
80 <p/><code>parm </code><strong> Beams:eCM </strong>
81 (<code>default = <strong>14000.</strong></code>; <code>minimum = 10.</code>)<br/>
82 Collision CM energy, to be set if <code>Beams:frameType</code> = 1.
85 <p/><code>parm </code><strong> Beams:eA </strong>
86 (<code>default = <strong>7000.</strong></code>; <code>minimum = 0.</code>)<br/>
87 The energy of the first incoming particle, moving in the
88 <i>+z </i>direction, to be set if <code>Beams:frameType</code> = 2.
89 If the particle energy is smaller than its mass
90 it is assumed to be at rest.
93 <p/><code>parm </code><strong> Beams:eB </strong>
94 (<code>default = <strong>7000.</strong></code>; <code>minimum = 0.</code>)<br/>
95 The energy of the second incoming particle, moving in the
96 <i>-z</i> direction, to be set if <code>Beams:frameType</code> = 2.
97 If the particle energy is smaller than its mass
98 it is assumed to be at rest.
101 <p/><code>parm </code><strong> Beams:pxA </strong>
102 (<code>default = <strong>0.</strong></code>)<br/>
103 The <i>p_x</i> component of the first incoming particle,
104 to be set if <code>Beams:frameType</code> = 3.
107 <p/><code>parm </code><strong> Beams:pyA </strong>
108 (<code>default = <strong>0.</strong></code>)<br/>
109 The <i>p_y</i> component of the first incoming particle,
110 to be set if <code>Beams:frameType</code> = 3.
113 <p/><code>parm </code><strong> Beams:pzA </strong>
114 (<code>default = <strong>7000.</strong></code>)<br/>
115 The <i>p_z</i> component of the first incoming particle,
116 to be set if <code>Beams:frameType</code> = 3.
119 <p/><code>parm </code><strong> Beams:pxB </strong>
120 (<code>default = <strong>0.</strong></code>)<br/>
121 The <i>p_x</i> component of the second incoming particle,
122 to be set if <code>Beams:frameType</code> = 3.
125 <p/><code>parm </code><strong> Beams:pyB </strong>
126 (<code>default = <strong>0.</strong></code>)<br/>
127 The <i>p_y</i> component of the second incoming particle,
128 to be set if <code>Beams:frameType</code> = 3.
131 <p/><code>parm </code><strong> Beams:pzB </strong>
132 (<code>default = <strong>-7000.</strong></code>)<br/>
133 The <i>p_z</i> component of the second incoming particle,
134 to be set if <code>Beams:frameType</code> = 3.
137 <p/><code>word </code><strong> Beams:LHEF </strong>
138 (<code>default = <strong>void</strong></code>)<br/>
139 The name of a Les Houches Event File,
140 to be set if <code>Beams:frameType</code> = 4.
143 <p/><code>word </code><strong> Beams:LHEFheader </strong>
144 (<code>default = <strong>void</strong></code>)<br/>
145 As some information in a Les Houches Event File init block is only known
146 at the end of generation, some programs choose to output this as a
147 separate file. If <code>Beams:LHEFheader</code> is given, information up
148 till the end of the init block is read from this file, with
149 the events themselves read as usual from the file given in
150 <code>Beams:LHEF</code>.
153 <p/><code>flag </code><strong> Beams:newLHEFsameInit </strong>
154 (<code>default = <strong>off</strong></code>)<br/>
155 Allow to begin reading events from a new LHEF or or a new
156 <code>LHAup</code> instance without a completely new initialization.
157 Only useful when <code>Beams:frameType</code> = 4 or 5.
160 <p/><code>flag </code><strong> Beams:readLHEFheaders </strong>
161 (<code>default = <strong>on</strong></code>)<br/>
162 Read in LHEF header blocks and store them in the
163 <a href="EventInformation.html" target="page">Info</a> class. See also
164 <a href="LesHouchesAccord.html" target="page">LHAupLHEF</a> for more information.
167 <p/><code>mode </code><strong> Beams:nSkipLHEFatInit </strong>
168 (<code>default = <strong>0</strong></code>)<br/>
169 Skip the first <i>nSkip</i> events of the input stream
170 (cf. the <code>LHAup::skipEvent(nSkip)</code> method).
171 Only used when <code>Beams:frameType</code> = 4 or 5.
174 <h3>Beam momentum spread</h3>
176 This framework currently is intended for a modest beam spread, such as
177 experienced at hadron colliders. Thus it can be safely assumed that the
178 physics does not change over the CM energy range probed, so that the
179 parameters of the physics initialization at the nominal energy can be
180 used as is. Currently it can <b>not</b> be used for the more extensive
181 energy spread expected at linear <i>e^+ e^-</i> colliders. Also,
182 any attempt to combine it with external Les Houches input of
183 parton-level events is at own risk.
186 On this page you can set the momentum spread according to a simple
187 Gaussian distribution. If you instead want a more sophisticated
188 parametrization, you can write and link your own
189 <code><a href="BeamShape.html" target="page">BeamShape</a></code> class.
191 <p/><code>flag </code><strong> Beams:allowMomentumSpread </strong>
192 (<code>default = <strong>off</strong></code>)<br/>
193 Allow the beam momenta to be smeared around their initialization
197 <p/><code>parm </code><strong> Beams:sigmaPxA </strong>
198 (<code>default = <strong>0.</strong></code>; <code>minimum = 0.</code>)<br/>
199 The width of a Gaussian distribution of the <i>p_x</i> spread of the
200 first incoming particle.
203 <p/><code>parm </code><strong> Beams:sigmaPyA </strong>
204 (<code>default = <strong>0.</strong></code>; <code>minimum = 0.</code>)<br/>
205 The width of a Gaussian distribution of the <i>p_y</i> spread of the
206 first incoming particle.
209 <p/><code>parm </code><strong> Beams:sigmaPzA </strong>
210 (<code>default = <strong>0.</strong></code>; <code>minimum = 0.</code>)<br/>
211 The width of a Gaussian distribution of the <i>p_z</i> spread of the
212 first incoming particle.
215 <p/><code>parm </code><strong> Beams:maxDevA </strong>
216 (<code>default = <strong>5.</strong></code>; <code>minimum = 0.</code>)<br/>
217 The triply Gaussian distribution <i>(p_x, p_y, p_z)</i> is restricted to
218 a maximal total deviation from the nominal values <i>(p_x0, p_y0, p_z0)</i>
219 for the first incoming particle, like
221 (p_x - p_x0)^2/sigma_px^2 + (p_y - p_y0)^2/sigma_py^2 +
222 (p_z - p_z0)^2/sigma_pz^2 < maxDev^2
224 (Note the absence of a factor 2 in the denominator, unlike the Gaussians
225 used to pick <i>(p_x, p_y, p_z)</i>.)
228 <p/><code>parm </code><strong> Beams:sigmaPxB </strong>
229 (<code>default = <strong>0.</strong></code>; <code>minimum = 0.</code>)<br/>
230 The width of a Gaussian distribution of the <i>p_x</i> spread of the
231 second incoming particle.
234 <p/><code>parm </code><strong> Beams:sigmaPyB </strong>
235 (<code>default = <strong>0.</strong></code>; <code>minimum = 0.</code>)<br/>
236 The width of a Gaussian distribution of the <i>p_y</i> spread of the
237 second incoming particle.
240 <p/><code>parm </code><strong> Beams:sigmaPzB </strong>
241 (<code>default = <strong>0.</strong></code>; <code>minimum = 0.</code>)<br/>
242 The width of a Gaussian distribution of the <i>p_z</i> spread of the
243 second incoming particle.
246 <p/><code>parm </code><strong> Beams:maxDevB </strong>
247 (<code>default = <strong>5.</strong></code>; <code>minimum = 0.</code>)<br/>
248 The triply Gaussian distribution <i>(p_x, p_y, p_z)</i> is restricted to
249 a maximal total deviation from the nominal values <i>(p_x0, p_y0, p_z0)</i>,
250 for the second incoming particle, like
252 (p_x - p_x0)^2/sigma_px^2 + (p_y - p_y0)^2/sigma_py^2 +
253 (p_z - p_z0)^2/sigma_pz^2 < maxDev^2
255 (Note the absence of a factor 2 in the denominator, unlike the Gaussians
256 used to pick <i>(p_x, p_y, p_z)</i>.)
259 <h3>Beam interaction vertex</h3>
261 On this page you can set the spread of the interaction vertex according to
262 a simple Gaussian distribution. If you instead want a more sophisticated
263 parametrization, you can write and link your own
264 <code><a href="BeamShape.html" target="page">BeamShape</a></code> class.
266 <p/><code>flag </code><strong> Beams:allowVertexSpread </strong>
267 (<code>default = <strong>off</strong></code>)<br/>
268 Allow the interaction vertex of the two colliding beams to be smeared.
269 If off, then the vertex is set to be the origin.
272 <p/><code>parm </code><strong> Beams:sigmaVertexX </strong>
273 (<code>default = <strong>0.</strong></code>; <code>minimum = 0.</code>)<br/>
274 The width of a Gaussian distribution of the <i>x</i> location of the
278 <p/><code>parm </code><strong> Beams:sigmaVertexY </strong>
279 (<code>default = <strong>0.</strong></code>; <code>minimum = 0.</code>)<br/>
280 The width of a Gaussian distribution of the <i>y</i> location of the
284 <p/><code>parm </code><strong> Beams:sigmaVertexZ </strong>
285 (<code>default = <strong>0.</strong></code>; <code>minimum = 0.</code>)<br/>
286 The width of a Gaussian distribution of the <i>z</i> location of the
290 <p/><code>parm </code><strong> Beams:maxDevVertex </strong>
291 (<code>default = <strong>5.</strong></code>; <code>minimum = 0.</code>)<br/>
292 The triply Gaussian distribution of interaction vertex position
293 <i>(x, y, z)</i> is restricted to a maximal total deviation from the
296 x^2/sigma_x^2 + y^2/sigma_y^2 + z^2/sigma_z^2 < maxDevVertex^2
298 (Note the absence of a factor 2 in the denominator, unlike the Gaussians
299 used to pick <i>(x, y, z)</i>.)
302 <p/><code>parm </code><strong> Beams:sigmaTime </strong>
303 (<code>default = <strong>0.</strong></code>; <code>minimum = 0.</code>)<br/>
304 The width of a Gaussian distribution of the collision time (in units of
305 mm/c). Note that, if the above space parametrization is viewed as the
306 effect of two incoming beams along the <i>+-z</i> axis, with each beam
307 having a Gaussian spread, then the spread of the time would also become
308 a Gaussian with the same width as the <i>z</i> one (times the
309 velocity of the beams, which we expect is close to unity). For flexibility
310 we have not enforced any such relation, however.
313 <p/><code>parm </code><strong> Beams:maxDevTime </strong>
314 (<code>default = <strong>5.</strong></code>; <code>minimum = 0.</code>)<br/>
315 The collision time is restricted to be in the range
316 <i>|t| < sigma_t * maxDevTime</i>.
320 The distributions above are all centered at the origin. It is also
321 possible to shift the above distributions to be centered around another
322 nominal position. You must have <code>Beams:allowVertexSpread = on</code>
323 to use this possibility.
325 <p/><code>parm </code><strong> Beams:offsetVertexX </strong>
326 (<code>default = <strong>0.</strong></code>)<br/>
327 The <i>x</i> location of the interaction vertex is centered at this value.
330 <p/><code>parm </code><strong> Beams:offsetVertexY </strong>
331 (<code>default = <strong>0.</strong></code>)<br/>
332 The <i>y</i> location of the interaction vertex is centered at this value.
335 <p/><code>parm </code><strong> Beams:offsetVertexZ </strong>
336 (<code>default = <strong>0.</strong></code>)<br/>
337 The <i>z</i> location of the interaction vertex is centered at this value.
340 <p/><code>parm </code><strong> Beams:offsetTime </strong>
341 (<code>default = <strong>0.</strong></code>)<br/>
342 The time <i>t</i> of the interaction vertex is centered at this value.
348 <!-- Copyright (C) 2012 Torbjorn Sjostrand -->