[Rivet] Analyses with partonic centre of mass energy?

[Andy Buckley]

Hi Adam,

There isn't a convenient mechanism for this, because a) it's unobservable,
and b) it depends on what you consider to be the hard-scattering process.

To elaborate on the second of those, let's say you define s_hat from the
4-vectors of the quarks entering the hard-scattering vertex in the event
record. Now use that definition on two events with the same kinematics: a
ttj event in which the extra jet comes from the matrix element, and an
equivalent event in which the jet comes from the initial-state parton
shower. You'd see a different value even though the physics is the same (in
this idealised example), just because the calculation has been broken up
differently: that would clearly not be a good physical definition. I'm not
sure if a definition that used the partons "emerging" from the
initial-state protons would be any less problematic.

Rivet's main aim is to emphasise well-defined observables rather than
intermediate quantities in calculations, so this kind of quantity is not
"automatically" available -- you need to manually access the event graph to
compute whatever definition you choose. You can get to that via the
Event::genEvent() function.

Andy


On 4 December 2017 at 12:39, Adam Makda <adam.makda at durham.ac.uk> wrote:

> Hello,
>
> I'm a student relatively new to Rivet and C++ in general.
>
> I'm currently using Rivet to analyse pp -> t tbar (stable top quark pair
> production at LHC13). I want to be able to project out the partons' (within
> the proton) centre of mass energy. This is usually denoted as
> $\sqrt{\hat{s}}$ in LaTeX.
>
> I'm aware of a projector called 'Beams' but this only projects out
> $\sqrt{s}$ ie. in my simulations, 13 TeV.
>
> I was wondering if there is an example within the existing analyses I
> could use as a template in order to project out the initial state partons
> in an event and extract their energies?
>
> Any help will be appreciated. If this isn't the purpose of this contact
> email address, is there anyone who could help?
>
> Thanks
>
> Adam Makda
>
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-- 
Dr Andy Buckley, Lecturer / Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow
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