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.
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),
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.
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.
flag Beams:strictLHEFscale
(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.
flag Beams:setProductionScalesFromLHEF
(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.
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.