wiki:InputEx
Last modified 5 years ago Last modified on 05/02/12 17:27:01

Basic Syntax Rules

  • Use a ">" to separate the initial state particle(s) from the final state ones.
  • Initial state can be one (decay) or two (scattering) particles.
  • Use " x x > z > y y y " form to require particle z to appear in a s-channel intermediate state (not recommended due to gauge invariance).
  • Use " x x > y y y $ z" form to forbid particle z from appearing in a s-channel intermediate state (not recommended due to gauge invariance).
  • Use " x x > y y y / z" form to exclude particle z to appear as internal particle in any diagram.
  • Use comma to specify decay chains. For instance "x x > Z W, Z >y y, W > k k" means "x x > Z W" where Z then decays to yy and W to kk.
  • Coupling orders are automatically detected to generate the leading order processes. To explicitly specify coupling orders, add them after the process on the same line:
    x x > y y y / z QCD=0
    x x > Z W QED=2, Z > y y, W > k k
    Note that if a coupling order is omitted, it defaults to infinity (or, for restricted couplings such as HIG in the HEFT model, to the restricted value).
  • Use the multi-label "p" to indicate a proton or an anti-proton.
    The symbol "p~" does not exist. In fact the type of initial state (pp or ppbar or parton-parton fixed energy) is specified later, during the run, in the run_card.dat.
    The multi-lable "j" suggests a final-state jet.
  • Note that the syntax for MadGraph 5 differs slightly from previous versions: a space (" ") is required between particle names, and the decay chain syntax has been upgraded to a more powerful syntax which allows better control of the different processes.

Process Examples

PROCESS Sample Diagram Notes
t > b e+ ve Decay of the top. Particle one is decayed, particles 2,3,4 are the decay products
u u~ > d d~ QED=2 Both the QCD and QED are set to 2. This entails that all the possible contributions (W,gluon,photon,Z) appear. In total 4 diagrams appear.
u u~ > d d~ No coupling orders are specified, so only the dominating (QCD) contribution is included. The QED contributions (W,photon,Z) are now off.
u u~ > d d~ /w+ QCD=0 The maximum number of QCD couplings is set to zero, and the contribution from an internal W+ (or W-) are eliminated. As a result only the contributions from the exchange of a photon and a Z appear.
u u~ > z > d d~ a QCD=0 The maximum number of QCD couplings is set to zero, and the presence of a Z in an s-channel diagram is requested. As a result possible contributions from the exchange of a W in the t-channel or a photon in the s-channel are excluded.
g g > t t~, (t >b w+, w+ > e+ ve), (t~ > b~ w-, w- > e- ve~) This is t tbar production with semileptonic decays for both tops. No coupling orders are specified, so he production process will have only the dominating QCD contribution. The requirement of having the two top on-shell avoids the generation of non resonant contributions such as gg>bb~ZZ with the subsequent decays of the Z's, giving in total 203 diagrams instead of just 3.

Process Input Examples using Protons and Jets

PROCESS_DEFINITION Directories Subprocesses Notes
p p > e+ ve P0_qq_lvl d~ u -> e+ ve
u d~ -> e+ ve
s~ c -> e+ ve
c s~ -> e+ ve
The special codes p and j, sum over gluon, and n light quark flavors and their anti_particles, as specified by the option. MadGraph is able to identify and combine explicitly identical subprocesses, and combine processes with same type of particles in the initial and final state (q/g/l/vl) into single directories
p p > W+ j j, W+ > l+ vl P0_qq_lvlqq
P0_gq_lvlgq
P0_gg_lvlqq
u~ u -> e+ ve u~ d, g g -> mu+ vm s c~, ...
u~ u -> mu+ vm u~ d, g g -> e+ ve s c~, ...
In this example, W+ + jets production is requested, with the subsequent leptonic decay of the W+. If l=e,mu is chosen in the options, then both possibilities are included. MadGraph is able to identify and combine explicitly identical subprocesses, and combine processes with same type of particles in the initial and final state (q/g/l/vl) into common directories.

-- Main.JohanAlwall - 2011-09-24

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