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Beam Parameters

The settings on this page relate to the beam identities and energies, to a beam momentum spread and to a beam interaction spot. As always, momenta and energies are to be given in units of GeV, and of space and time in mm.

Incoming beams

The identities and energies of the two incoming beam particles should be specified using the settings in this section. Note that, if nothing is done, you will default to LHC at 14 TeV.

Currently the beam particles must be either a hadron pair, a lepton pair, a photon pair, a lepton and a hadron or a photon and a hadron. In the first category p p and pbar p combinations dominate, but it is also possible to combine with pi^+, pi^- and pi^0. In the second e^+ e^- and mu^+ mu^- would be the most useful combinations, but also others should work if combined with an appropriate hard process.

Beams:idA   (default = 2212)
The PDG id code for the first incoming particle. Allowed codes include
2212 = p, -2212 = pbar,
2112 = n, -2112 = nbar,
211 = pi^+, -211 = pi^-, 111 = pi^0,
990 = Pomeron (used in diffractive machinery; here mainly for debug purposes),
22 = gamma (for gamma-gamma and gamma-hadron interactions, more info ";?>here),
11 = e^-, -11 = e^+,
13 = mu^-, -13 = mu^+,
and a few more leptons/neutrinos in a few combinations.

Beams:idB   (default = 2212)
The PDG id code for the second incoming particle.

Beams:frameType   (default = 1; minimum = 1; maximum = 5)
Choice of frame for the two colliding particles. For options 1 - 3 the beam identities are specified above, while they are obtained by the Les Houches information for options 4 and 5.
1 : the beams are colliding in their CM frame, and therefore only the CM energy needs to be provided, see Beams:eCM below.
2 : the beams are back-to-back, but with different energies, see Beams:eA and Beams:eB below. This option could also be used for fixed-target configurations.
3 : the beams are not back-to-back, and therefore the three-momentum of each incoming particle needs to be specified, see Beams:pxA through Beams:pzB below.
4 : the beam and event information is stored in a Les Houches Event File, see Beams:LHEF below.
5 : the beam and event information is obtained by a pointer to an LHAup class instance.


Beams:eCM   (default = 14000.; minimum = 10.)
Collision CM energy, to be set if Beams:frameType = 1.

Beams:eA   (default = 7000.; minimum = 0.)
The energy of the first incoming particle, moving in the +z direction, to be set if Beams:frameType = 2. If the particle energy is smaller than its mass it is assumed to be at rest.

Beams:eB   (default = 7000.; minimum = 0.)
The energy of the second incoming particle, moving in the -z direction, to be set if Beams:frameType = 2. If the particle energy is smaller than its mass it is assumed to be at rest.

Beams:pxA   (default = 0.)
The p_x component of the first incoming particle, to be set if Beams:frameType = 3.

Beams:pyA   (default = 0.)
The p_y component of the first incoming particle, to be set if Beams:frameType = 3.

Beams:pzA   (default = 7000.)
The p_z component of the first incoming particle, to be set if Beams:frameType = 3.

Beams:pxB   (default = 0.)
The p_x component of the second incoming particle, to be set if Beams:frameType = 3.

Beams:pyB   (default = 0.)
The p_y component of the second incoming particle, to be set if Beams:frameType = 3.

Beams:pzB   (default = -7000.)
The p_z component of the second incoming particle, to be set if Beams:frameType = 3.

Beams:LHEF   (default = void)
The name of a Les Houches Event File, to be set if Beams:frameType = 4.

Beams:LHEFheader   (default = void)
As some information in a Les Houches Event File init block is only known at the end of generation, some programs choose to output this as a separate file. If Beams:LHEFheader is given, information up till the end of the init block is read from this file, with the events themselves read as usual from the file given in Beams:LHEF.

Beams:newLHEFsameInit On Off   (default = off)
Allow to begin reading events from a new LHEF or or a new LHAup instance without a completely new initialization. Only useful when Beams:frameType = 4 or 5.

Beams:readLHEFheaders On Off   (default = on)
Read in LHEF header blocks and store them in the ";?>Info class. See also ";?>LHAupLHEF for more information.

mode  Beams:nSkipLHEFatInit   (default = 0)
Skip the first nSkip events of the input stream (cf. the LHAup::skipEvent(nSkip) method). Only used when Beams:frameType = 4 or 5.

Beams:strictLHEFscale On Off   (default = off)
Always use the SCALUP value read from LHEF as production scale for particles, also including particles coming from resonance decays. By default, the production scale of resonance decay products (W-, Z-, H-bosons, top quarks) will be set to M/2, where M is the mass of the resonance.

Beams:setProductionScalesFromLHEF On Off   (default = off)
If enabled, production scales for all particles in an input Les Houches event will be set by using auxiliary information provided in the input event. Depending on which LHEF version is used, different information will be read. For LHEF 1.0, scale information can be transferred by including a line starting with the hashtag (#) character at the end of the event. The hashtag should be followed by the sequential scale values of the particles with status 1. For LHEF 3.0, the standardised <scales> tag is used instead. In this case, please ensure that production scale of the final state particle with position i in the input event will be transferred as the value of a scales tag attribute which contains i at the end of the attribute name, separated from the rest of the name by an underscore (_). An example <scales> tag would be <scales muf="100" pt_start_3="100" pt_start_4="50"> </scales>. This could be used to set the production scales of particles three and four to 100 GeV and 50 GeV, respectively, provided that these particles have status 1.

Beam momentum spread

This framework currently is intended for a modest beam spread, such as experienced at hadron colliders. Thus it can be safely assumed that the physics does not change over the CM energy range probed, so that the parameters of the physics initialization at the nominal energy can be used as is. Currently it can not be used for the more extensive energy spread expected at linear e^+ e^- colliders. Also, any attempt to combine it with external Les Houches input of parton-level events is at own risk.

On this page you can set the momentum spread according to a simple Gaussian distribution. If you instead want a more sophisticated parametrization, you can write and link your own ";?>BeamShape class.

Beams:allowMomentumSpread On Off   (default = off)
Allow the beam momenta to be smeared around their initialization nominal values.

Beams:sigmaPxA   (default = 0.; minimum = 0.)
The width of a Gaussian distribution of the p_x spread of the first incoming particle.

Beams:sigmaPyA   (default = 0.; minimum = 0.)
The width of a Gaussian distribution of the p_y spread of the first incoming particle.

Beams:sigmaPzA   (default = 0.; minimum = 0.)
The width of a Gaussian distribution of the p_z spread of the first incoming particle.

Beams:maxDevA   (default = 5.; minimum = 0.)
The triply Gaussian distribution (p_x, p_y, p_z) is restricted to a maximal total deviation from the nominal values (p_x0, p_y0, p_z0) for the first incoming particle, like
(p_x - p_x0)^2/sigma_px^2 + (p_y - p_y0)^2/sigma_py^2 + (p_z - p_z0)^2/sigma_pz^2 < maxDev^2
(Note the absence of a factor 2 in the denominator, unlike the Gaussians used to pick (p_x, p_y, p_z).)

Beams:sigmaPxB   (default = 0.; minimum = 0.)
The width of a Gaussian distribution of the p_x spread of the second incoming particle.

Beams:sigmaPyB   (default = 0.; minimum = 0.)
The width of a Gaussian distribution of the p_y spread of the second incoming particle.

Beams:sigmaPzB   (default = 0.; minimum = 0.)
The width of a Gaussian distribution of the p_z spread of the second incoming particle.

Beams:maxDevB   (default = 5.; minimum = 0.)
The triply Gaussian distribution (p_x, p_y, p_z) is restricted to a maximal total deviation from the nominal values (p_x0, p_y0, p_z0), for the second incoming particle, like
(p_x - p_x0)^2/sigma_px^2 + (p_y - p_y0)^2/sigma_py^2 + (p_z - p_z0)^2/sigma_pz^2 < maxDev^2
(Note the absence of a factor 2 in the denominator, unlike the Gaussians used to pick (p_x, p_y, p_z).)

Beam interaction vertex

On this page you can set the spread of the interaction vertex according to a simple Gaussian distribution. If you instead want a more sophisticated parametrization, you can write and link your own ";?>BeamShape class.

Beams:allowVertexSpread On Off   (default = off)
Allow the interaction vertex of the two colliding beams to be smeared. If off, then the vertex is set to be the origin.

Beams:sigmaVertexX   (default = 0.; minimum = 0.)
The width of a Gaussian distribution of the x location of the interaction vertex.

Beams:sigmaVertexY   (default = 0.; minimum = 0.)
The width of a Gaussian distribution of the y location of the interaction vertex.

Beams:sigmaVertexZ   (default = 0.; minimum = 0.)
The width of a Gaussian distribution of the z location of the interaction vertex.

Beams:maxDevVertex   (default = 5.; minimum = 0.)
The triply Gaussian distribution of interaction vertex position (x, y, z) is restricted to a maximal total deviation from the origin, like
x^2/sigma_x^2 + y^2/sigma_y^2 + z^2/sigma_z^2 < maxDevVertex^2
(Note the absence of a factor 2 in the denominator, unlike the Gaussians used to pick (x, y, z).)

Beams:sigmaTime   (default = 0.; minimum = 0.)
The width of a Gaussian distribution of the collision time (in units of mm/c). Note that, if the above space parametrization is viewed as the effect of two incoming beams along the +-z axis, with each beam having a Gaussian spread, then the spread of the time would also become a Gaussian with the same width as the z one (times the velocity of the beams, which we expect is close to unity). For flexibility we have not enforced any such relation, however.

Beams:maxDevTime   (default = 5.; minimum = 0.)
The collision time is restricted to be in the range |t| < sigma_t * maxDevTime.

The distributions above are all centered at the origin. It is also possible to shift the above distributions to be centered around another nominal position. You must have Beams:allowVertexSpread = on to use this possibility.

Beams:offsetVertexX   (default = 0.)
The x location of the interaction vertex is centered at this value.

Beams:offsetVertexY   (default = 0.)
The y location of the interaction vertex is centered at this value.

Beams:offsetVertexZ   (default = 0.)
The z location of the interaction vertex is centered at this value.

Beams:offsetTime   (default = 0.)
The time t of the interaction vertex is centered at this value. "?>