[Rivet] Z boson mass taken from ZFinder is pre-FSR or post-FSR

[Frank Siegert]

Hi Yu-hsiang,

the ZFinder object will (by default) use "dressed" leptons, which
means that photon radiation within a cone of dR=0.1 is included. This
is a generator independent and less QED-FSR-dependent definition,
which is neither pre-FSR (often called "Born" level leptons) nor
post-FSR (often called "bare" leptons). It does sum parts of the FSR
but not all of it (namely not the wide angle emissions). If you want
to use "bare" leptons for your comparison you would have to set dR=0.0
in the ZFinder constructor.

If on the other hand you are writing a Rivet plugin for an actual
experimental measurement the question is what is the truth level to
which your analysis has been unfolded ("dressed", "Born" or "bare")?

Cheers,
Frank


On 9 June 2015 at 12:14, Yu-hsiang Chang 張祐祥 <index0192 at yahoo.com.tw> wrote:
> Hi all,
>
> I have question for the FinalState and the ZFinder in Rivet.
>
> I want to know when I take the Z boson mass from the ZFinder, it is pre-FSR
> or post-FSR? (undergoes final state radiation(FSR) or not. In other words,
> does the 4-momentum of Z boson includes the energy and momentum of the FSR
> photon? )
>
> For example, my Rivet plugin SMP_13_003.cc has these content :
>
> ---------------------------------------------------
>     void init() {
>
>       // Full final state
>        const FinalState fs(-5.0, 5.0);
>        addProjection(fs, "FS");
>
>        // Z finders for muons
>        const ZFinder zfe(fs, -2.5, 2.5, 0*GeV, PID::ELECTRON, 15*GeV,
> 1500*GeV);
>        addProjection(zfe, "ZFE");
>
> ......
>     }
>
>
>     void analyze(const Event& event) {
>
>       const ParticleVector& z = zfe.bosons();
>       const double mass_z = z[0].mass();
>
> ......
>     }
> ---------------------------------------------------
>
> So I want to know the mass taken from the Z boson is pre-FSR or post-FSR?
>
> Since I use the result of my rivet plugin (from YODA and then transform it
> to ROOT format),
> to compare with the pre-FSR and post-FSR mass of Z boson (generated by
> Powheg),
> and I find my rivet result is consistent to the pre-FSR instead of post-FSR.
> (previous one tell me that the FinalState object has already undergone the
> FSR,
> but since now I see inconsistent so I want to ask the part of ZFinder)
>
> I put my rivet plugin (SMP_13_003.cc), and the comparison (snap shot ) in
> attachment.
> In comparison, the red point is the result of my rivet plugin, and the blue
> line is the pre-FSR or post-FSR.
>
> Thanks.
>
> Yu-hsiang
>
>
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