## 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

There are two ways to set the identities and energies of the two incoming beam particles. One is to use the `init()` method with no arguments. Then the settings variables below will be read and used. The alternative is to call `init(...)` with arguments that provide this information. Then you need not use the variables below (although it would still be possible). Note that, if nothing is done, you will default to LHC at 14 TeV.

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

`mode` 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),
11 = e^-, -11 = e^+,
13 = mu^-, -13 = mu^+,
and a few more leptons/neutrinos in a few combinations.

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

`mode` 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.
`option` 1 : the beams are colliding in their CM frame, and therefore only the CM energy needs to be provided, see `Beams:eCM` below.
`option` 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.
`option` 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.
`option` 4 : the beam and event information is stored in a Les Houches Event File, see `Beams:LHEF` below.
`option` 5 : the beam and event information is obtained by a pointer to an `LHAup` class instance.

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

`parm` 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.

`parm` 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.

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

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

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

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

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

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

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

`word` 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`.

`flag` Beams:newLHEFsameInit (`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.

`flag` Beams:readLHEFheaders (`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.

### 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.

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

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

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

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

`parm` 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).)

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

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

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

`parm` 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.

`flag` Beams:allowVertexSpread (`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.

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

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

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

`parm` 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).)

`parm` 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.

`parm` 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.

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

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

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

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