WHIZARD

How to set ISR photon energy

Asked by Jing-Hang Fu

Hi,

We have some questions regarding the electron beam PDFs in Whizard.

Typically, there should be a parameter to implement a cutoff that prevents divergence in ISR emission. However, we're unsure which specific variable serves this purpose effectively. For example, we can set an energy cutoff for the final state particles at the gen-level. Additionally, we've come across another parameter, isr_q_max, which seems to influence ISR productions. So, which parameter should we use to ensure the correct generation of ISR events?

Could you please clarify what isr_q_max represents? Is it the maximum energy of ISR photons or the minimum remaining energy after ISR photon emission?

Could I ask you for some help please ? Thanks.

Cheers
Jing-Hang Fu

Question information

Language:
English Edit question
Status:
Answered
For:
WHIZARD Edit question
Assignee:
Krzysztof Mekala Edit question
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Last reply:
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Krzysztof Mekala (krzysztofmekala) said : 		#1

Hi Jing-Hang,
the variable which should be set is "isr_mass" which corresponds to the mass of the beam particle. However, the default value of this variable is taken from the model in use which means that if your model contains massive electrons, it will be handled automatically by Whizard. On the other hand, some models do assume electrons to be massless and then, this variable can be set by hand to allow Whizard to generate ISR.

Let me comment on the two other things you have mentioned:
1) In general, the "cuts" statement does not affect ISR photons. For example, if you want to generate any pT for ISR photons (see Sec. 10.4 of the manual), it is done at the event-generation level, after the integration. The way to control ISR kinematics is to define "selection" criteria.

2) "isr_q_max" sets the upper cutoff for ISR photons and as default, it is the collision energy. Typically, you do not have to change the value. However, if you are interested in studying hard, exclusive photon emissions, it is recommended to generate them at the Matrix Element level (i.e. by including them explicitly in your process definition) and this variable can be used for matching between the "soft" and "hard" regimes.

Cheers,
Krzysztof

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