# A Momentum transfer and energy scale

1. Oct 17, 2016

### Josh1079

Hi, I am recently working on a project involving simulations and I'm pretty confused about the parameter energy scale Q (contained in TruthEvent pdfInfo). I tried to figure out what that parameter means and from some sources online I think Q is the absolute value of momentum transfer q. Therefore, I'm a bit confused about how the value of Q is determined. There are a lot of interactions in one event and I guess that gives a different q, but as far as I know the Q is for per event, so how do they determine Q?

I tried to find information from the class reference (http://hep.uchicago.edu/~kkrizka/rootcoreapis/d7/d81/classxAOD_1_1TruthEvent__v1.html) but can't really find much.

Thanks!

2. Oct 17, 2016

### ChrisVer

I am not sure, since I don't understand what that class stands for... but don't you have a primary vertex?

3. Oct 17, 2016

### Josh1079

Ah! You mean it comes from the pp collision vertex? This makes perfect sense! Thanks!!!

4. Oct 17, 2016

### ChrisVer

I don't know, in general all the other vertices are considered as pileup and so may not contain the process that you want to measure [because I read some cross section methods in the class]...

5. Oct 17, 2016

### RGevo

It sounds like this q2 (pdfinfo) is likely to correspond to the scale at which the PDF is evaluated For the hard process.

This scale is generally set as a scale typical of the hard process (the primary interaction of an 'event').

If you are producing a W boson, the mass of the w boson might be a good choice. When you have multiple vertices, it is a little less obvious and you try to pick a scale which is representative of many events you are looking at. For example the transverse mass of the w boson (which could be produced in association with other qcd particles for example).

Finally, the probability of finding an incoming Parton inside the proton is provided by a Parton distribution function which describes this probability as a function of Q and x (momentum fraction of the proton that it carries)