Beam Remnants
Introduction
The BeamParticle
class contains information on all partons
extracted from a beam (so far). As each consecutive multiple interaction
defines its respective incoming parton to the hard scattering a
new slot is added to the list. This information is modified when
the backwards evolution of the spacelike shower defines a new
initiator parton. It is used, both for the multiple interactions
and the spacelike showers, to define rescaled parton densities based
on the x and flavours already extracted, and to distinguish
between valence, sea and companion quarks. Once the perturbative
evolution is finished, further beam remnants are added to obtain a
consistent set of flavours. The current physics framework is further
described in [1].
Much of the above information is stored in a vector of
ResolvedParton
objects, which each contains flavour and
momentum information, as well as valence/companion information and more.
The BeamParticle
method list()
shows the contents of
this vector, mainly for debug purposes.
The BeamRemnants
class takes over for the final step of adding
primordial kT to the initiators and remnants, assigning the
relative longitudinal momentum sharing among the remnants, and
constructing the overall kinematics. This step couples the two sides
of an event, and could therefore not be covered in the
BeamParticle
class, which only considers one beam at a time.
Neither of the methods of these classes are intended for general use,
and so are not described here.
Main variables
Currently there are no crucial parameters to consider. The choice of
parton densities is made in the Pythia
class, see the
Generic page. Then pointers to the pdf's are handed on to
BeamParticle
at initialization, for all subsequent usage.
Further variables
mode name="Beams:maxValQuark" default="3" min="0" max="5"
The maximum valence quark kind allowed in acceptable incoming beams,
for which multiple interactions are simulated. Default is that hadrons
may contain u, d and s quarks,
but not c and b ones, since sensible
kinematics has not really been worked out for the latter.
mode name="Beams:companionPower" default="4" min="0" max="4"
When a sea quark has been found, a companion antisea quark ought to be
nearby in x. The shape of this distribution can be derived
from the gluon mother distribution convoluted with the
g -> q qbar splitting kernel. In practice, simple solutions
are only feasible if the gluon shape is assumed to be of the form
g(x) ~ (1 - x)^p / x, where p is an integer power,
the parameter above. Allowed values correspond to the cases programmed.
Since the whole framework is approximate anyway, this should be good
enough. Note that companions typically are found at small Q^2,
if at all, so the form is supposed to represent g(x) at small
Q^2 scales, close to the lower cutoff for multiple interactions.
parameter name="Beams:primordialKTwidth" default="1." min="0."
The width of Gaussian distributions in p_x and p_y
separately that is assigned as a primordial kT to initiators
and beam remnants.
When assigning relative momentum fractions to beam-remnant partons,
valence quarks are chosen according to a distribution like
(1 - x)^power / sqrt(x). This power is given below
for quarks in mesons, and separately for u and d
quarks in the proton, based on the approximate shape of low-Q^2
parton densities. The power for other baryons is derived from the
proton ones, by an appropriate mixing. The x of a diquark
is chosen as the sum of its two constituent x values, and can
thus be above unity. (A common rescaling of all remnant partons and
particles will fix that.) An additional enhancement of the diquark
momentum is obtained by its x value being rescaled by the
valenceDiqEnhance
factor.
parameter name="Beams:valencePowerMeson" default="0.8" min="0."
The abovementioned power for valence quarks in mesons.
parameter name="Beams:valencePowerUinP" default="3.5" min="0."
The abovementioned power for valence u quarks in protons.
parameter name="Beams:valencePowerDinP" default="2.0" min="0."
The abovementioned power for valence d quarks in protons.
parameter name="Beams:valenceDiqEnhance" default="2.0" min="0.5"
max="10."
Enhancement factor for valence diqaurks in baryons, relative to the
simple sum of the two constituent quarks.
flag name="Beams:allowJunction" default="on"
The off
option is intended for debug purposes only, as follows.
When more than one valence quark is kicked out of a baryon beam,
as part of the multiple interactions scenario, the subsequent
hadronization is described in terms of a junction string topology.
This description involves a number of technical complications that
may make the program more unstable. As an alternative, by switching
this option off, junction configurations are rejected, and the
multiple interactions and their showers are redone until a
junction-free topology is found.
Diffractive system
When an incoming hadron beam is diffractively excited, it is moddeled
as if either a valence quark or a gluon is kicked out from the hadron.
In the former case this produces a simple strong to the leftover
remnant, in the latter it gives a hairpin arrangement where a string
is stretched from one quark in the remnant, via the gluon, back to the
rest of the remnant. The latter ought to dominate at higher mass of
the diffractive system. Therefore an approximate behaviour like
P_q / P_g = N / m^p
is assumed.
parameter name="Beams:pickQuarkNorm" default="5.0" min="0."
The abovementioned normalization N for the relative quark
rate in diffractive systems.
parameter name="Beams:pickQuarkPower" default="1.0" min="0."
The abovementioned mass-dependence power p for the relative
quark rate in diffractive systems.
When a gluon is kicked out from the hadron, the longitudinal momentum
sharing between the the two remnant partons is determined by the
same parameters as above. It is plausible that the primordial
kT may be lower than in perturbative processes, however:
parameter name="Beams:diffPrimKTwidth" default="0.5" min="0."
The width of Gaussian distributions in p_x and p_y
separately that is assigned as a primordial kT to the two
beam remnants when a gluon is kicked out of a diffractive system.
parameter name="Beams:diffLargeMassSuppress" default="2." min="0."
The choice of longitudinal and transverse structure of a diffractive
beam remnant implies a remnant mass m_rem distribution that
knows no bounds. A suppression like (1 - m_rem^2 / m_diff^2)^p
is therefore introduced, where p is the
diffLargeMassSuppress
parameter.
References
- T. Sjostrand and P.Z. Skands, JHEP 03 (2004) 053