Couplings and Scales
 
  - Couplings and K factor
- Renormalization scales
- Factorization scales
Here is collected some possibilities to modify the scale choices 
of couplings and parton densities for all internally implemented 
hard processes. This is based on them all being derived from theSigmaProcess base class. The matrix-element coding is 
also used by the multiparton-interactions machinery, but there with a 
separate choice of alpha_strong(M_Z^2) value and running, 
and separate PDF scale choices. Also, in 2 → 2 and 
2 → 3 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. 
 
 
We stress that couplings and scales are set separately from the 
values on this page for 
multiparton interactions, 
timelike showers, and 
spacelike showers. 
This allows a bigger flexibility, but also requires a bit more work 
e.g. if you insist on using the same alpha_s everywhere. 
 
 
Couplings and K factor
 
 
The size of QCD cross sections is mainly determined by 
parm   SigmaProcess:alphaSvalue   
 (default = 0.13; 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 = 3)
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.   
option  3 : third order, with the same comment as for second 
order. The expression in the 2006 RPP is used here.   
   
 
 
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. 
   
   
 
 
In addition there is the possibility of a global rescaling of 
cross sections (which could not easily be accommodated by a 
changed alpha_strong, since alpha_strong runs) 
parm   SigmaProcess:Kfactor   
 (default = 1.0; minimum = 0.5; maximum = 4.0)
Multiply almost all cross sections by this common fix factor. Excluded 
are only unresolved processes, where cross sections are better 
set directly, and 
multiparton interactions, which have a separate K factor 
of their own. 
This degree of freedom is primarily intended for hadron colliders, and 
should not normally be used for e^+e^- annihilation processes. 
   
 
 
Renormalization scales
 
 
The Q^2 renormalization scale can be chosen among a few different 
alternatives, separately for 2 → 1, 2 → 2 and two 
different kinds of 2 → 3 processes. In addition a common 
multiplicative factor may be imposed. 
 
mode   SigmaProcess:renormScale1   
 (default = 1; minimum = 1; maximum = 2)
The Q^2 renormalization scale for 2 → 1 processes. 
The same options also apply for those 2 → 2 and 
2 → 3 processes that have been specially marked as 
proceeding only through an s-channel resonance, by the 
isSChannel() virtual method of SigmaProcess. 
option  1 : the squared invariant mass, i.e. sHat. 
   
option  2 : fix scale set in SigmaProcess:renormFixScale 
below. 
   
   
 
mode   SigmaProcess:renormScale2   
 (default = 2; minimum = 1; maximum = 6)
The Q^2 renormalization scale for 2 → 2 processes. 
option  1 : the smaller of the squared transverse masses of the two 
outgoing particles, i.e. min(mT_3^2, mT_4^2) = 
pT^2 + min(m_3^2, m_4^2). 
   
option  2 : the geometric mean of the squared transverse masses of 
the two outgoing particles, i.e. mT_3 * mT_4 = 
sqrt((pT^2 + m_3^2) * (pT^2 + m_4^2)). 
   
option  3 : the arithmetic mean of the squared transverse masses of 
the two outgoing particles, i.e. (mT_3^2 + mT_4^2) / 2 = 
pT^2 + 0.5 * (m_3^2 + m_4^2). Useful for comparisons 
with PYTHIA 6, where this is the default. 
   
option  4 : squared invariant mass of the system, 
i.e. sHat. Useful for processes dominated by 
s-channel exchange. 
   
option  5 : fix scale set in SigmaProcess:renormFixScale 
below. 
   
option  6 : Use squared invariant momentum transfer -tHat. 
This is a common choice for lepton-hadron scattering processes. In that 
case -tHat=Q^2. 
   
   
 
mode   SigmaProcess:renormScale3   
 (default = 3; minimum = 1; maximum = 6)
The Q^2 renormalization scale for "normal" 2 → 3 
processes, i.e excepting the vector-boson-fusion processes below. 
Here it is assumed that particle masses in the final state either match 
or are heavier than that of any t-channel propagator particle. 
(Currently only g g / q qbar → H^0 Q Qbar processes are 
implemented, where the "match" criterion holds.) 
option  1 : the smaller of the squared transverse masses of the three 
outgoing particles, i.e. min(mT_3^2, mT_4^2, mT_5^2). 
   
option  2 : the geometric mean of the two smallest squared transverse 
masses of the three outgoing particles, i.e. 
sqrt( mT_3^2 * mT_4^2 * mT_5^2 / max(mT_3^2, mT_4^2, mT_5^2) ). 
   
option  3 : the geometric mean of the squared transverse masses of the 
three outgoing particles, i.e. (mT_3^2 * mT_4^2 * mT_5^2)^(1/3). 
   
option  4 : the arithmetic mean of the squared transverse masses of 
the three outgoing particles, i.e. (mT_3^2 + mT_4^2 + mT_5^2)/3. 
   
option  5 : squared invariant mass of the system, 
i.e. sHat. 
   
option  6 : fix scale set in SigmaProcess:renormFixScale 
below. 
   
   
 
mode   SigmaProcess:renormScale3VV   
 (default = 3; minimum = 1; maximum = 6)
The Q^2 renormalization scale for 2 → 3 
vector-boson-fusion processes, i.e. f_1 f_2 → H^0 f_3 f_4 
with Z^0 or W^+-  t-channel propagators. 
Here the transverse masses of the outgoing fermions do not reflect the 
virtualities of the exchanged bosons. A better estimate is obtained 
by replacing the final-state fermion masses by the vector-boson ones 
in the definition of transverse masses. We denote these combinations 
mT_Vi^2 = m_V^2 + pT_i^2. 
option  1 : the squared mass m_V^2 of the exchanged 
vector boson. 
   
option  2 : the geometric mean of the two propagator virtuality 
estimates, i.e. sqrt(mT_V3^2 * mT_V4^2). 
   
option  3 : the geometric mean of the three relevant squared 
transverse masses, i.e. (mT_V3^2 * mT_V4^2 * mT_H^2)^(1/3). 
   
option  4 : the arithmetic mean of the three relevant squared 
transverse masses, i.e. (mT_V3^2 + mT_V4^2 + mT_H^2)/3. 
   
option  5 : squared invariant mass of the system, 
i.e. sHat. 
   
option  6 : fix scale set in SigmaProcess:renormFixScale 
below. 
   
   
 
parm   SigmaProcess:renormMultFac   
 (default = 1.; minimum = 0.1; maximum = 10.)
The Q^2 renormalization scale for 2 → 1, 
2 → 2 and 2 → 3 processes is multiplied by 
this factor relative to the scale described above (except for the options 
with a fix scale). Should be use sparingly for 2 → 1 processes. 
   
 
parm   SigmaProcess:renormFixScale   
 (default = 10000.; minimum = 1.)
A fix Q^2 value used as renormalization scale for 
2 → 1, 2 → 2 and 2 → 3 processes 
in some of the options above. 
   
 
 
Factorization scales
 
 
Corresponding options exist for the Q^2 factorization scale 
used as argument in PDF's. Again there is a choice of form for 
2 → 1, 2 → 2 and 2 → 3 processes 
separately. For simplicity we have let the numbering of options agree, 
for each event class separately, between normalization and factorization 
scales, and the description has therefore been slightly shortened. The 
default values are not necessarily the same, however. 
 
mode   SigmaProcess:factorScale1   
 (default = 1; minimum = 1; maximum = 2)
The Q^2 factorization scale for 2 → 1 processes. 
The same options also apply for those 2 → 2 and 
2 → 3 processes that have been specially marked as 
proceeding only through an s-channel resonance. 
option  1 : the squared invariant mass, i.e. sHat. 
   
option  2 : fix scale set in SigmaProcess:factorFixScale 
below. 
   
   
 
mode   SigmaProcess:factorScale2   
 (default = 1; minimum = 1; maximum = 6)
The Q^2 factorization scale for 2 → 2 processes. 
option  1 : the smaller of the squared transverse masses of the two 
outgoing particles. 
   
option  2 : the geometric mean of the squared transverse masses of 
the two outgoing particles. 
   
option  3 : the arithmetic mean of the squared transverse masses of 
the two outgoing particles. Useful for comparisons with PYTHIA 6, where 
this is the default. 
   
option  4 : squared invariant mass of the system, 
i.e. sHat. Useful for processes dominated by 
s-channel exchange. 
   
option  5 : fix scale set in SigmaProcess:factorFixScale 
below. 
   
option  6 : Use squared invariant momentum transfer -tHat. 
This is a common choice for lepton-hadron scattering processes. In that 
case -tHat=Q^2. 
   
   
 
mode   SigmaProcess:factorScale3   
 (default = 2; minimum = 1; maximum = 6)
The Q^2 factorization scale for "normal" 2 → 3 
processes, i.e excepting the vector-boson-fusion processes below. 
option  1 : the smaller of the squared transverse masses of the three 
outgoing particles. 
   
option  2 : the geometric mean of the two smallest squared transverse 
masses of the three outgoing particles. 
   
option  3 : the geometric mean of the squared transverse masses of the 
three outgoing particles. 
   
option  4 : the arithmetic mean of the squared transverse masses of 
the three outgoing particles. 
   
option  5 : squared invariant mass of the system, 
i.e. sHat. 
   
option  6 : fix scale set in SigmaProcess:factorFixScale 
below. 
   
   
 
mode   SigmaProcess:factorScale3VV   
 (default = 2; minimum = 1; maximum = 6)
The Q^2 factorization scale for 2 → 3 
vector-boson-fusion processes, i.e. f_1 f_2 → H^0 f_3 f_4 
with Z^0 or W^+-  t-channel propagators. 
Here we again introduce the combinations mT_Vi^2 = m_V^2 + pT_i^2 
as replacements for the normal squared transverse masses of the two 
outgoing quarks. 
option  1 : the squared mass m_V^2 of the exchanged 
vector boson. 
   
option  2 : the geometric mean of the two propagator virtuality 
estimates. 
   
option  3 : the geometric mean of the three relevant squared 
transverse masses. 
   
option  4 : the arithmetic mean of the three relevant squared 
transverse masses. 
   
option  5 : squared invariant mass of the system, 
i.e. sHat. 
   
option  6 : fix scale set in SigmaProcess:factorFixScale 
below. 
   
   
 
parm   SigmaProcess:factorMultFac   
 (default = 1.; minimum = 0.1; maximum = 10.)
The Q^2 factorization scale for 2 → 1, 
2 → 2 and 2 → 3 processes is multiplied by 
this factor relative to the scale described above (except for the options 
with a fix scale). Should be use sparingly for 2 → 1 processes. 
   
 
parm   SigmaProcess:factorFixScale   
 (default = 10000.; minimum = 1.)
A fix Q^2 value used as factorization scale for 2 → 1, 
2 → 2 and 2 → 3 processes in some of the options 
above.