This manual describes how to use the NLO di-photon production cross section program gamma2MC, by giving line-by-line instructions for the input file "gamma2MC.indat".
It should be fairly easy to use the input file,
"gamma2MC.indat", just by modifying the example
that came with the source distribution. However, there are a few
general formatting requirements to be aware of.
The input file must have exactly 19 lines. Although some lines are ignored
for certain choices of processes or cuts, the line must still be there in
order for the program to function properly.
In the following explanations
of the inputs, a list {a, b, c} indicates a choice of string inputs
on a line, while an ordered list [x y z] indicates multiple numerical
parameters input on a single line. Possible inputs are colored
red.
If the input to a line
is a string, then it MUST have trailing white spaces after the string
(NOT TABS!). It is easiest to do this by keeping the explanatory
comment after the input string (i.e, white spaces followed by #x Comment)
from the example file.
Any input which is a string must be spelled exactly as given below,
including capitalization and without quotation marks, or else the program will
complain.
The program ignores anything in a line that comes after the expected input.
HIGGS : | Through the Higgs resonance: g g --> H X, followed by H --> gamma gamma. The production cross section of the Higgs boson is through gluon-gluon fusion (with all associated channels at NLO) in the heavy top quark limit (top quark mass dependence is included only in the LO prefactor). The branching ratio to gamma gamma is not included. It can be obtained elsewhere, such as from the program HDECAY. |
GG : | Through the (formally NNLO) process g g --> gamma gamma X, which occurs at one loop. This box contribution is then treated as the LO part of a NLO calculation, where only gluon-gluon initial-states are considered. |
QQ : | Through q qbar --> gamma gamma X , as well as all associated channels through NLO. This process at NLO has a collinear singularity when the photon and a final-state quark are collinear. This singularity has been MSbar-subtracted, so that the calculation should be finite for any isolation cuts. However, no fragmentation processes are included, so only a calculation with an IR-safe isolation (such as Frixione's smooth isolation cut) has any physical meaning at NLO. (I.e, the calculation with the IR-safe isolation cut does not depend on the subtraction prescription and does not get any contribution from photon fragmentation processes.) |
QQTOTAL : | Include both quark-antiquark and quark-gluon initiated contributions at LO or NLO. |
QQONLY : | Include only the quark-antiquark initiated contributions at LO or NLO. Note that these contributions have no collinear singularity with the final-state photons. |
QGONLY : | Include only the quark-gluon and antiquark-gluon initated contributions. These only occur at NLO. They contain collinear contributions with the final-state photons, which have been MSbar-subtracted. |
none : | As the name implies, no cuts are performed. |
hcut : | The two-photons are reconstructed into a single particle, and a cut is performed on the rapidity yh of this particle. The event is cut if |yh|> yhcut. This is useful for calculations of Higgs production, without regard to the Higgs decay. |
noIsolation : | Minimal photon cuts only. |
standard : | Minimal photon cuts plus the following isolation cut. The event is cut if the amount of transverse hadronic energy in a cone of radius Rcut around either photon is greater than Etcut. |
frixione : | Minimal photon cuts plus the following smooth isolation cut, suggested by Frixione. The event is cut if the amount of transverse hadronic energy in ANY cone of radius r<Rcut around either photon is greater than [pt_gamma*epsilon*(1-cos(r))/(1-cos(Rcut))]. |
D0 : | Minimal photon cuts plus the following isolation cut, used by the D0 collaboration. The event is cut if the amount of transverse hadronic energy in a cone of radius Rcut around either photon is greater than [epsilon*pt_gamma]. |
ptcut : | Minimal photon cuts plus standard isolation cuts plus an observed jet cut. The event is cut if the jet does not have transverse energy greater than Etjet. |
jetveto : | Minimal photon cuts plus standard isolation cuts plus an additional cut on jets in a larger cone around the photon. The event is cut if there is a jet with transverse energy greater than Etjet within a cone of radius Rjet around either photon. It is assumed that Etjet>Etcut and Rjet>Rcut. Since at NLO, there is at most one colored parton in the final-state, there can be no dependence on the jet cone size in this calculation. (This would have to be corrected at NNLO and beyond.) |
annulus : | Minimal photon cuts plus standard isolation cuts plus a cut to include at least one jet in an annulus around either photon. The event is cut if there is no jet with transverse energy greater than Etjet in an annulus of inner radius Rcut and outer radius Rjet around either photon. Note that if we remove the events passing the annulus cut from those passing the standard cut, this is equivalent to applying the jetveto cut. |
userdefined : | Minimal photon cuts plus user-defined isolation cuts. The user can define a new set of isolation (or other) cuts by modifying the class function "cutUser::cutIsolation(p1,p2,p3)" in the file "Cuts.C". In this function p1 and p2 are the final-state photon four-vectors, while p3 is the final-state (colored) parton four-vector. Note that this set of cuts is always in addition to the minimal photon cuts. The new isolation cuts can depend on up to four variables (input in line #11), which are "cutUser::P1", "cutUser::P2", "cutUser::P3", and "cutUser::P4". (See the class definition in file "Cuts.h".) After modifying the class function, the program must be recompiled by typing "make gamma2MC" on the command line. (If this class function is not modified, then this set of cuts is the same as the standard isolation cuts by default.) |
ycut : | Require |y_gam|<ycut for each photon. |
pt2 : | Require smaller photon pt satisfy pt>pt2 (in GeV). |
pt1 : | Require larger photon pt satisfy pt>pt1 (in GeV). |
none : | Line is ignored. |
hcut : | [yhcut]. For example, 2.5 |
noIsolation : | Line is ignored. |
standard : | [Rcut Etcut] with Etcut in GeV. For example, 0.4 15.0 |
frixione : | [Rcut epsilon]. For example, 1.0 1.0 |
D0 : | [Rcut epsilon]. For example, 0.4 0.07 |
ptcut : | [Rcut Etcut Etjet] with Etcut, Etjet in GeV. For example, 0.4 15.0 30.0 |
jetveto : | [Rcut Etcut Rjet Etjet] with Etcut, Etjet in GeV. For example, 0.4 5.0 2.0 15.0 |
annulus : | [Rcut Etcut Rjet Etjet] with Etcut, Etjet in GeV. For example, 0.4 5.0 2.0 15.0 |
userdefined : | [P1 P2 P3 P4] as defined by user. For example, 1.0 2.0 3.0 4.0 |
Renormalization : | mu_R=chiR*M_gamgam. |
Factorization : | mu_F=chiF*M_gamgam. |
mu_fr=chifr*M_gamgam. |
COSTHETASTAR : | Center-of-mass scattering angle of the photons. |
YSTAR : | y^star = |y_gam1 - y_gam2| |
TANHYSTAR : | tanh(y^star) |
YGG : | Let q = p_gam1 + p_gam2. Then y_gg = rapidity of q. |
PHIGG : | phi_gg = |phi_gam1 - phi_gam2| (defined in interval [0,PI]). |
QT : | Let q = p_gam1 + p_gam2. Then q_T = transverse momentum of q. |
MGG : | Let q = p_gam1 + p_gam2. Then M_gamgam= invariant mass of q. |
USERDEFINED : | The user can define a new distribution to histogram by modifying the class function "distUser::Param(p1,p2)" in the file "Distribution.C". In this function p1 and p2 are the final-state photon four-vectors. (See the class definition in file "Distribution.h" for information.) After modifying the class function, the program must be recompiled by typing "make gamma2MC" on the command line. (If this class function is not modified, then this distribution is the same as MGG by default.) |