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><aloc href="ProgramFlow">init(...)</aloc></code>
16 with arguments that provide this information. Then you need not use the
17 variables below (although it would still be possible). Note that, if nothing
18 is done, you will default to LHC at 14 TeV.
21 Currently the beam particles must be either a hadron pair or a lepton
22 pair. In the former category <ei>p p</ei> and <ei>pbar p</ei>
23 combinations dominate, but it is also possible to combine with
24 <ei>pi^+</ei>, <ei>pi^-</ei> and <ei>pi^0</ei>. In the latter
25 <ei>e^+ e^-</ei> and <ei>mu^+ mu^-</ei> would be the most useful
26 combinations, but also others should work if combined with an
27 appropriate hard process.
29 <modeopen name="Beams:idA" default="2212">
30 The PDG <code>id</code> code for the first incoming particle.
32 <br/><ei>2212 = p</ei>, <ei>-2212 = pbar</ei>,
33 <br/><ei>211 = pi^+</ei>, <ei>-211 = pi^-</ei>, <ei>111 = pi^0</ei>,
34 <br/><ei>990 = Pomeron</ei> (used in diffractive machinery;
35 here mainly for debug purposes),
36 <br/><ei>11 = e^-</ei>, <ei>-11 = e^+</ei>,
37 <br/><ei>13 = mu^-</ei>, <ei>-13 = mu^+</ei>,
38 <br/>and a few more leptons/neutrinos in a few combinations.
41 <modeopen name="Beams:idB" default="2212">
42 The PDG <code>id</code> code for the second incoming particle.
45 <modepick name="Beams:frameType" default="1" min="1" max="5">
46 Choice of frame for the two colliding particles. For options
47 1 - 3 the beam identities are specified above, while they are
48 obtained by the Les Houches information for options 4 and 5.
49 <option value="1">the beams are colliding in their CM frame,
50 and therefore only the CM energy needs to be provided, see
51 <code>Beams:eCM</code> below.
53 <option value="2">the beams are back-to-back, but with different energies,
54 see <code>Beams:eA</code> and <code>Beams:eB</code> below.
55 This option could also be used for fixed-target configurations.
57 <option value="3">the beams are not back-to-back, and therefore the
58 three-momentum of each incoming particle needs to be specified, see
59 <code>Beams:pxA</code> through <code>Beams:pzB</code> below.
61 <option value="4">the beam and event information is stored in a
62 <aloc href="LesHouchesAccord">Les Houches Event File</aloc>,
63 see <code>Beams:LHEF</code> below.
65 <option value="5">the beam and event information is obtained by a
66 pointer to an <code><aloc href="LesHouchesAccord">LHAup</aloc></code>
71 <parm name="Beams:eCM" default="14000." min="10.">
72 Collision CM energy, to be set if <code>Beams:frameType</code> = 1.
75 <parm name="Beams:eA" default="7000." min="0.">
76 The energy of the first incoming particle, moving in the
77 <ei>+z </ei>direction, to be set if <code>Beams:frameType</code> = 2.
78 If the particle energy is smaller than its mass
79 it is assumed to be at rest.
82 <parm name="Beams:eB" default="7000." min="0.">
83 The energy of the second incoming particle, moving in the
84 <ei>-z</ei> direction, to be set if <code>Beams:frameType</code> = 2.
85 If the particle energy is smaller than its mass
86 it is assumed to be at rest.
89 <parm name="Beams:pxA" default="0.">
90 The <ei>p_x</ei> component of the first incoming particle,
91 to be set if <code>Beams:frameType</code> = 3.
94 <parm name="Beams:pyA" default="0.">
95 The <ei>p_y</ei> component of the first incoming particle,
96 to be set if <code>Beams:frameType</code> = 3.
99 <parm name="Beams:pzA" default="7000.">
100 The <ei>p_z</ei> component of the first incoming particle,
101 to be set if <code>Beams:frameType</code> = 3.
104 <parm name="Beams:pxB" default="0.">
105 The <ei>p_x</ei> component of the second incoming particle,
106 to be set if <code>Beams:frameType</code> = 3.
109 <parm name="Beams:pyB" default="0.">
110 The <ei>p_y</ei> component of the second incoming particle,
111 to be set if <code>Beams:frameType</code> = 3.
114 <parm name="Beams:pzB" default="-7000.">
115 The <ei>p_z</ei> component of the second incoming particle,
116 to be set if <code>Beams:frameType</code> = 3.
119 <word name="Beams:LHEF" default="void">
120 The name of a Les Houches Event File,
121 to be set if <code>Beams:frameType</code> = 4.
124 <word name="Beams:LHEFheader" default="void">
125 As some information in a Les Houches Event File init block is only known
126 at the end of generation, some programs choose to output this as a
127 separate file. If <code>Beams:LHEFheader</code> is given, information up
128 till the end of the init block is read from this file, with
129 the events themselves read as usual from the file given in
130 <code>Beams:LHEF</code>.
133 <flag name="Beams:newLHEFsameInit" default="off">
134 Allow to begin reading events from a new LHEF or or a new
135 <code>LHAup</code> instance without a completely new initialization.
136 Only useful when <code>Beams:frameType</code> = 4 or 5.
139 <flag name="Beams:readLHEFheaders" default="on">
140 Read in LHEF header blocks and store them in the
141 <aloc href="EventInformation">Info</aloc> class. See also
142 <aloc href="LesHouchesAccord">LHAupLHEF</aloc> for more information.
145 <mode name="Beams:nSkipLHEFatInit" default="0">
146 Skip the first <ei>nSkip</ei> events of the input stream
147 (cf. the <code>LHAup::skipEvent(nSkip)</code> method).
148 Only used when <code>Beams:frameType</code> = 4 or 5.
151 <h3>Beam momentum spread</h3>
153 This framework currently is intended for a modest beam spread, such as
154 experienced at hadron colliders. Thus it can be safely assumed that the
155 physics does not change over the CM energy range probed, so that the
156 parameters of the physics initialization at the nominal energy can be
157 used as is. Currently it can <b>not</b> be used for the more extensive
158 energy spread expected at linear <ei>e^+ e^-</ei> colliders. Also,
159 any attempt to combine it with external Les Houches input of
160 parton-level events is at own risk.
163 On this page you can set the momentum spread according to a simple
164 Gaussian distribution. If you instead want a more sophisticated
165 parametrization, you can write and link your own
166 <code><aloc href="BeamShape">BeamShape</aloc></code> class.
168 <flag name="Beams:allowMomentumSpread" default="off">
169 Allow the beam momenta to be smeared around their initialization
173 <parm name="Beams:sigmaPxA" default="0." min="0.">
174 The width of a Gaussian distribution of the <ei>p_x</ei> spread of the
175 first incoming particle.
178 <parm name="Beams:sigmaPyA" default="0." min="0.">
179 The width of a Gaussian distribution of the <ei>p_y</ei> spread of the
180 first incoming particle.
183 <parm name="Beams:sigmaPzA" default="0." min="0.">
184 The width of a Gaussian distribution of the <ei>p_z</ei> spread of the
185 first incoming particle.
188 <parm name="Beams:maxDevA" default="5." min="0.">
189 The triply Gaussian distribution <ei>(p_x, p_y, p_z)</ei> is restricted to
190 a maximal total deviation from the nominal values <ei>(p_x0, p_y0, p_z0)</ei>
191 for the first incoming particle, like
193 (p_x - p_x0)^2/sigma_px^2 + (p_y - p_y0)^2/sigma_py^2 +
194 (p_z - p_z0)^2/sigma_pz^2 < maxDev^2
196 (Note the absence of a factor 2 in the denominator, unlike the Gaussians
197 used to pick <ei>(p_x, p_y, p_z)</ei>.)
200 <parm name="Beams:sigmaPxB" default="0." min="0.">
201 The width of a Gaussian distribution of the <ei>p_x</ei> spread of the
202 second incoming particle.
205 <parm name="Beams:sigmaPyB" default="0." min="0.">
206 The width of a Gaussian distribution of the <ei>p_y</ei> spread of the
207 second incoming particle.
210 <parm name="Beams:sigmaPzB" default="0." min="0.">
211 The width of a Gaussian distribution of the <ei>p_z</ei> spread of the
212 second incoming particle.
215 <parm name="Beams:maxDevB" default="5." min="0.">
216 The triply Gaussian distribution <ei>(p_x, p_y, p_z)</ei> is restricted to
217 a maximal total deviation from the nominal values <ei>(p_x0, p_y0, p_z0)</ei>,
218 for the second incoming particle, like
220 (p_x - p_x0)^2/sigma_px^2 + (p_y - p_y0)^2/sigma_py^2 +
221 (p_z - p_z0)^2/sigma_pz^2 < maxDev^2
223 (Note the absence of a factor 2 in the denominator, unlike the Gaussians
224 used to pick <ei>(p_x, p_y, p_z)</ei>.)
227 <h3>Beam interaction vertex</h3>
229 On this page you can set the spread of the interaction vertex according to
230 a simple Gaussian distribution. If you instead want a more sophisticated
231 parametrization, you can write and link your own
232 <code><aloc href="BeamShape">BeamShape</aloc></code> class.
234 <flag name="Beams:allowVertexSpread" default="off">
235 Allow the interaction vertex of the two colliding beams to be smeared.
236 If off, then the vertex is set to be the origin.
239 <parm name="Beams:sigmaVertexX" default="0." min="0.">
240 The width of a Gaussian distribution of the <ei>x</ei> location of the
244 <parm name="Beams:sigmaVertexY" default="0." min="0.">
245 The width of a Gaussian distribution of the <ei>y</ei> location of the
249 <parm name="Beams:sigmaVertexZ" default="0." min="0.">
250 The width of a Gaussian distribution of the <ei>z</ei> location of the
254 <parm name="Beams:maxDevVertex" default="5." min="0.">
255 The triply Gaussian distribution of interaction vertex position
256 <ei>(x, y, z)</ei> is restricted to a maximal total deviation from the
259 x^2/sigma_x^2 + y^2/sigma_y^2 + z^2/sigma_z^2 < maxDevVertex^2
261 (Note the absence of a factor 2 in the denominator, unlike the Gaussians
262 used to pick <ei>(x, y, z)</ei>.)
265 <parm name="Beams:sigmaTime" default="0." min="0.">
266 The width of a Gaussian distribution of the collision time (in units of
267 mm/c). Note that, if the above space parametrization is viewed as the
268 effect of two incoming beams along the <ei>+-z</ei> axis, with each beam
269 having a Gaussian spread, then the spread of the time would also become
270 a Gaussian with the same width as the <ei>z</ei> one (times the
271 velocity of the beams, which we expect is close to unity). For flexibility
272 we have not enforced any such relation, however.
275 <parm name="Beams:maxDevTime" default="5." min="0.">
276 The collision time is restricted to be in the range
277 <ei>|t| < sigma_t * maxDevTime</ei>.
281 The distributions above are all centered at the origin. It is also
282 possible to shift the above distributions to be centered around another
283 nominal position. You must have <code>Beams:allowVertexSpread = on</code>
284 to use this possibility.
286 <parm name="Beams:offsetVertexX" default="0.">
287 The <ei>x</ei> location of the interaction vertex is centered at this value.
290 <parm name="Beams:offsetVertexY" default="0.">
291 The <ei>y</ei> location of the interaction vertex is centered at this value.
294 <parm name="Beams:offsetVertexZ" default="0.">
295 The <ei>z</ei> location of the interaction vertex is centered at this value.
298 <parm name="Beams:offsetTime" default="0.">
299 The time <ei>t</ei> of the interaction vertex is centered at this value.
304 <!-- Copyright (C) 2012 Torbjorn Sjostrand -->