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 [Sjo04].

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.

parm 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.

parm name="Beams:valencePowerMeson" default="0.8" min="0."
The abovementioned power for valence quarks in mesons.

parm name="Beams:valencePowerUinP" default="3.5" min="0."
The abovementioned power for valence u quarks in protons.

parm name="Beams:valencePowerDinP" default="2.0" min="0."
The abovementioned power for valence d quarks in protons.

parm 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.

parm name="Beams:pickQuarkNorm" default="5.0" min="0."
The abovementioned normalization N for the relative quark rate in diffractive systems.

parm 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:

parm 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.

parm 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.