CalcHEP

external off-shell particles

Asked by andrew williams

When generating matrix elements for numerical evaluation is it possible to allow for external particles to be off-shell. In particular I am generating 1->2 and 2->1 matrix elements in C for evaluation externally.

At the moment the square of the external 4-momenta are replaced with the masses symbolically during generation. Is it possible to turn this off so that the squared 4-momenta are evaluated using the supplied kinematics.

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Alexander Pukhov (pukhov) said : 		#1

Sorry, we have not off-shell calculations in CalcHEP. The reason is possible problems with broken gauge invariance in case of off-shell calculations. But If you use batch calculations then applying Breit-Wigner CalcHEP replaces on-she particles on virtual ones.
Best
   Alexander Pukhov

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Alexander Belyaev (alexander.belyaev) said : 		#2

Hi Andrew,

as Sasha Pukhov said,
you CAN put an external particles off-the mass shell using batch session --
e.g

when you connect production X,X->Y and decay Y->Z,Z
the particle Y will be put off-the mass shell according to its with
with the respective Breight-Wigner probability.

Is it what you have been asking?

Cheers,
     Sasha

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