Fragmentation
Fragmentation in PYTHIA is based on the Lund string model
[And83, Sjo84]. Several different aspects are involved in
the physics description, which here therefore is split accordingly.
This also, at least partly, reflect the set of classes involved in
the fragmentation machinery.
The variables collected here have a very wide span of usefulness.
Some would be central in any hadronization tuning exercise, others
should not be touched except by experts.
The fragmentation flavour-choice machinery is also used in a few
other places of the program, notably particle decays, and is thus
described on the separate Flavour
Selection page.
Fragmentation functions
The StringZ class handles the choice of longitudinal
lightcone fraction z according to one of two possible
shape sets.
The Lund symmetric fragmentation function [And83] is the
only alternative for light quarks. It is of the form
f(z) = (1/z) * (1-z)^a * exp(-b m_T^2 / z)
with the two main free parameters a and b to be
tuned to data. They are stored in
The a parameter of the Lund symmetric fragmentation function.
The b parameter of the Lund symmetric fragmentation function.
In principle, each flavour can have a different a. Then,
for going from an old flavour i to a new j one
the shape is
f(z) = (1/z) * z^{a_i} * ((1-z)/z)^{a_j} * exp(-b * m_T^2 / z)
This is only implemented for diquarks relative to normal quarks:
allows a larger a for diquarks, with total
a = aLund + aExtraDiquark.
Finally, the Bowler modification [Bow81] introduces an extra
factor
1/z^{r_Q * b * m_Q^2}
for heavy quarks. To keep some flexibility, a multiplicative factor
r_Q is introduced, which ought to be unity (provided that
quark masses were uniquely defined) but can be set in
r_c, i.e. the above parameter for c quarks.
r_b, i.e. the above parameter for b quarks.
r_h, i.e. the above parameter for heavier hypothetical quarks,
or in general any new coloured particle long-lived enough to hadronize.
As an alternative, it is possible to switch over to the
Peterson/SLAC formula [Pet83]
f(z) = 1 / ( z * (1 - 1/z - epsilon/(1-z))^2 )
for charm, bottom and heavier (defined as above) by the three flags
use Peterson for c quarks.
use Peterson for b quarks.
use Peterson for hypothetical heavier quarks.
When switched on, the corresponding epsilon values are chosen to be
epsilon_c, i.e. the above parameter for c quarks.
epsilon_b, i.e. the above parameter for b quarks.
epsilon_h, i.e. the above parameter for hypothetical heavier
quarks, normalized to the case where m_h = m_b. The actually
used parameter is then epsilon = epsilon_h * (m_b^2 / m_h^2).
This allows a sensible scaling to a particle with an unknown higher
mass without the need for a user intervention.
Fragmentation pT
The StringPT class handles the choice of fragmentation
pT. At each string breaking the quark and antiquark of the pair are
supposed to receive opposite and compensating pT kicks according
to a Gaussian distribution in p_x and p_y separately.
Call sigma_q the width of the p_x and p_y
distributions separately, i.e.
d(Prob) = exp( -(p_x^2 + p_y^2) / 2 sigma_q^2).
Then the total squared width is
<pT^2> = <p_x^2> + <p_y^2> = 2 sigma_q^2 = sigma^2.
It is this latter number that is stored in
the width sigma in the fragmentation process.
Since a normal hadron receives pT contributions for two string
breakings, it has a <p_x^2>_had = <p_y^2>_had = sigma^2,
and thus <pT^2>_had = 2 sigma^2.
Some studies on isolated particles at LEP has indicated the need for
a slightly enhanced rate in the high-pT tail of the above
distribution. This would have to be reviewed in the context of a
complete retune of parton showers and hadronization, but for the
moment we stay with the current recipe, to boost the above pT
by a factor enhancedWidth for a small fraction
enhancedFraction of the breakups, where
enhancedFraction,the fraction of string breaks with enhanced
width.
enhancedWidth,the enhancement of the width in this fraction.
Jet joining procedure
String fragmentation is carried out iteratively from both string ends
inwards, which means that the two chains of hadrons have to be joined up
somewhere in the middle of the event. This joining is described by
parameters that in principle follows from the standard fragmentation
parameters, but in a way too complicated to parametrize. The dependence
is rather mild, however, so for a sensible range of variation the
parameters in this section should not be touched.
Is used to define a W_min = m_q1 + m_q2 + stopMass,
where m_q1 and m_q2 are the masses of the two
current endpoint quarks or diquarks.
Add to W_min an amount stopNewFlav * m_q_last,
where q_last is the last q qbar pair produced
between the final two hadrons.
The W_min above is then smeared uniformly in the range
W_min_smeared = W_min * [ 1 - stopSmear, 1 + stopSmear ].
This W_min_smeared is then compared with the current remaining
W_transverse to determine if there is energy left for further
particle production. If not, i.e. if
W_transverse < W_min_smeared, the final two particles are
produced from what is currently left, if possible. (If not, the
fragmentation process is started over.)
Simplifying systems
There are a few situations when it is meaningful to simplify the
original task, one way or another.
Decides whether a partonic system should be considered as a normal
string or a ministring, the latter only producing one or two primary
hadrons. The system mass should be above mStringMin plus the
sum of quark/diquark constituent masses for a normal string description,
else the ministring scenario is used.
When two colour-connected partons are very nearby, with at least
one being a gluon, they can be joined into one, to avoid technical
problems of very small string regions. The requirement for joining is
that the invariant mass of the pair is below mJoin, where a
gluon only counts with half its momentum, i.e. with its contribution
to the string region under consideration. (Note that, for technical
reasons, the 0.2 GeV lower limit is de facto hardcoded.)
When the invariant mass of two of the quarks in a three-quark junction
string system becomes too small, the system is simplified to a
quark-diquark simple string. The requirement for this simplification
is that the diquark mass, minus the two quark masses, falls below
mJoinJunction. Gluons on the string between the junction and
the respective quark, if any, are counted as part of the quark
four-momentum. Those on the two combined legs are clustered with the
diquark when it is formed.
Ministrings
The MiniStringFragmentation machinery is only used when a
string system has so small invariant mass that normal string fragmentation
is difficult/impossible. Instead one or two particles are produced,
in the former case shuffling energy-momentum relative to another
colour singlet system in the event, while preserving the invariant
mass of that system. With one exception parameters are the same as
defined for normal string fragmentation, to the extent that they are
at all applicable in this case.
A discussion of the relevant physics is found in [Nor00].
The current implementation does not completely abide to the scheme
presented there, however, but has in part been simplified. (In part
for greater clarity, in part since the class is not quite finished yet.)
Whenever the machinery is called, first this many attempts are made
to pick two hadrons that the system fragments to. If the hadrons are
too massive the attempt will fail, but a new subsequent try could
involve other flavour and hadrons and thus still succeed.
After nTry attempts, instead an attempt is made to produce a
single hadron from the system. Should also this fail, some further
attempts at obtaining two hadrons will be made before eventually
giving up.
Junction treatment
A junction topology corresponds to an Y arrangement of strings
i.e. where three string pieces have to be joined up in a junction.
Such topologies can arise if several valence quarks are kicked out
from a proton beam, or in baryon-number-violating SUSY decays.
Special attention is necessary to handle the region just around
the junction, where the baryon number topologically is located.
The junction fragmentation scheme is described in [Sjo03].
The parameters in this section should not be touched except by experts.
Used to find the effective rest frame of the junction, which is
complicated when the three string legs may contain additional
gluons between the junction and the endpoint. To this end,
a pull is defined as a weighed sum of the momenta on each leg,
where the weight is exp(- eSum / eNormJunction), with
eSum the summed energy of all partons closer to the junction
than the currently considered one (in the junction rest frame).
Should in principle be (close to) sqrt((1 + a) / b), with
a and b the parameters of the Lund symmetric
fragmentation function.
Retry (up to 10 times) when the first two considered strings in to a
junction both have a remaining energy (in the junction rest frame)
above this number.
Retry (up to 10 times) when the first two considered strings in to a
junction has a highest remaining energy (in the junction rest frame)
above a random energy evenly distributed between
eBothLeftJunction and
eBothLeftJunction + eMaxLeftJunction
(drawn anew for each test).
Retry (up to 10 times) when the invariant mass-squared of the final leg
and the leftover momentum of the first two treated legs falls below
eMinLeftJunction times the energy of the final leg (in the
junction rest frame).