Process Properties
Here is collected some possibilities to affect the generation of
all internally implemented processes in one go, or some coherent
subset thereof. Phase space cuts appear on a special page.
Incoming partons
InFlux
is base class for the combination of allowed incoming
partons in a given process, and keeps track of parton densities and
common weight factors (such as charge) required to give the process
cross section.
There is one useful degree of freedom in this class:
mode
InFlux:nQuark
(default = 5
; minimum = 0
; maximum = 5
)
Number of allowed incoming quark flavours in the beams; a change
to 4 would thus exclude b and bbar as incoming
partons, etc.
There is also the possibility to obtain some documentation.
flag
InFlux:showChannels
(default = off
)
At initialization show which incoming flavours and flavour combinations
have beeen set up, process by process, along with initial weight
(e.g. charges or CKM factors) assigned to each of them.
Generic cross sections
SigmaProcess
is base class for all hard processes
implemented in PYTHIA 8. For 2 -> 1 processes it
should give sigmaHat(sHat), for 2 -> 2 ones
d(sigmaHat(sHat, tHat))/d(tHat). The matrix-element
coding is also used by the multiple-interactions machinery,
but with a separate choice of alpha_strong(M_Z^2) value
and running, and separate scale choices. Also, in 2 -> 2
processes where resonances are produced, their couplings and thereby
their Breit-Wigner shapes are always evaluated with the resonance mass
as scale, irrespective of the choices below.
The size of QCD cross sections is mainly determined by
parm
SigmaProcess:alphaSvalue
(default = 0.1265
; minimum = 0.06
; maximum = 0.25
)
The alpha_strong value at scale M_Z^2.
The actual value is then regulated by the running to the Q^2
renormalization scale, at which alpha_strong is evaluated
mode
SigmaProcess:alphaSorder
(default = 1
; minimum = 0
; maximum = 2
)
Order at which alpha_strong runs,
option
0 : zeroth order, i.e. alpha_strong is kept
fixed.
option
1 : first order, which is the normal value.
option
2 : second order. Since other parts of the code do
not go to second order there is no strong reason to use this option,
but there is also nothing wrong with it.
QED interactions are regulated by the alpha_electromagnetic
value at the Q^2 renormalization scale of an interaction.
mode
SigmaProcess:alphaEMorder
(default = 1
; minimum = -1
; maximum = 1
)
The running of alpha_em used in hard processes.
option
1 : first-order running, constrained to agree with
StandardModel:alphaEMmZ
at the Z^0 mass.
option
0 : zeroth order, i.e. alpha_em is kept
fixed at its value at vanishing momentum transfer.
option
-1 : zeroth order, i.e. alpha_em is kept
fixed, but at StandardModel:alphaEMmZ
, i.e. its value
at the Z^0 mass.
The Q^2 renormalization scale is sHat for
2 -> 1 processes while there is a choice for 2 -> 2
ones:
mode
SigmaProcess:renormScale
(default = 0
; minimum = 0
; maximum = 1
)
The Q^2 renormalization scale for 2 -> 2 processes.
option
0 : the product of the transverse masses of the two
outgoing particles, i.e. sqrt((pT^2 + m3^2) * (pT2 + m4^2)).
option
1 : the average of the squared transverse masses,
i.e. pT^2 + 0.5 * (m3^2 + m4^2). Useful for comparisons
with PYTHIA 6.
It is also possible to shift the choice by a global factor.
parm
SigmaProcess:renormMult
(default = 1.
; minimum = 0.1
; maximum = 10.
)
The Q^2 renormalization scale for 2 -> 1 and
2 -> 2 processes is multiplied by this factor relative to the
scale described above. Should be use sparingly for 2 -> 1
processes.
Corresponding options exist for the Q^2 factorization scale
used as argument in PDF's. Again sHat is scale for 2 -> 1
processes while there is a choice for 2 -> 2 ones:
mode
SigmaProcess:factorScale
(default = 0
; minimum = 0
; maximum = 1
)
The Q^2 factorization scale for 2 -> 2 processes.
option
0 : the smaller of the squared transverse masses of the two
outgoing particles, i.e. pT^2 + min(m3^2 + m4^2).
option
1 : the average of the squared transverse masses,
i.e. pT^2 + 0.5 * (m3^2 + m4^2). Useful for comparisons
with PYTHIA 6.
It is also possible to shift the choice by a global factor.
parm
SigmaProcess:factorMult
(default = 1.
; minimum = 0.1
; maximum = 10.
)
The Q^2 factorization scale for 2 -> 1 and
2 -> 2 processes is multiplied by this factor relative to the
scale described above. Should be use sparingly for 2 -> 1
processes.
Special cross sections
Here settings that affect some special group of processes, but not all.
mode
SigmaProcess:nQuark
(default = 3
; minimum = 0
; maximum = 5
)
Number of allowed outgoing new quark flavours in
q qbar -> q' qbar' and g g-> q qbar processes,
where quarks are treated as massless in the matrix-element expressions
(but correctly in the phase space). It is thus assumed that c cbar
and b bbar are added separately with masses taken into account.
A change to 4 would also include c cbar in the massless
approximation, etc.
mode
SigmaProcess:nQuarkInLoop
(default = 5
; minimum = 3
; maximum = 6
)
Number of quark flavours included in the box graphs resposible for
g g -> g gamma and g g-> gamma gamma processes.
Owing to the complexity if the massive expressions, quarks are treated
as massless. The default value should be applicable in the range of
transverse momenta above the b mass but below the t one.
mode
SigmaProcess:gmZmode
(default = 0
; minimum = 0
; maximum = 2
)
Choice of full gamma^*/Z^0 structure or not in relevant
processes.
option
0 : full gamma^*/Z^0 structure,
with interference included.
option
1 : only pure gamma^* contribution.
option
2 : only pure Z^0 contribution.