[Comphep-common] [Fwd: Re: [Comphep-devel] [Fwd: uncertainty on 2l invariant mass distributions of ZZ production]]

[Alexander Kryukov]

-------- Forwarded Message --------
From: Mikhail Dubinin <dubinin at theory.sinp.msu.ru>
To: Alexander Kryukov <kryukov at theory.sinp.msu.ru>
Cc: comphep-devel <comphep-devel at theory.sinp.msu.ru>
Subject: Re: [Comphep-devel] [Fwd: [Comphep-common] uncertainty on 2l
invariant mass distributions of ZZ production]
Date: Mon, 2 Jun 2008 18:04:01 +0400 (MSD)


Dear Pedro,

(1) yes, CompHEP includes Z^*-Z^, Z-gamma^* and 
gamma^*-gamma^* interferences since it calculates
complete set of twelve 2 -> 4 diagrams. See recent
detalization for pp -> 4 muons on the page

https://twiki.cern.ch/twiki/bin/view/CMS/Regularization

( and also https://twiki.cern.ch/twiki/bin/view/CMS/CompHEPInterface )

(2) main sources of uncertainties are in PDF's, especially at
small invariant masses. The PDF factorization
scale at m_Z, usually taken in this case, is an approximation. One can
also try to take not CTEQ*L (leadind log QCD evolution in PDF's) but
other choices to feel the QCD uncertainties.
I remember that Alexander Sherstnev studied recently pp->ZZ case
together with Florida group.
Probably he can give you refernces to his detailed publications.

Best wishes               Mikhail Dubinin







On Mon, 2 Jun 2008, Alexander Kryukov wrote:

> Hi,
>
> I get the mail (see below).
>
>              -- A.K>
>
> -------- Forwarded Message --------
> From: Pedro Ribeiro <ribeiro at lip.pt>
> To: comphep-common at theory.sinp.msu.ru
> Cc: Michele Gallinaro <michgall at lip.pt>, Malgorzata Kazana
> <kazana at lip.pt>
> Subject: [Comphep-common] uncertainty on 2l invariant mass distributions
> of ZZ production
> Date: Mon, 02 Jun 2008 11:57:50 +0100
>
> Dear CompHEP team
>
>
> We are studying UED signal in 4 lepton final state for which the main
> background is ZZ->4l. We have been using ZZ->4l samples produced with
> Comphep.
>
> In order to discriminate signal from background we have been looking at
> the distribution of dilepton invariant mass, which has a triangular
> shape and a endpoint between 20-60 GeV (depending on the model)  for the
> signal due to the presence of a "neutralino" like particle. As expected,
> the same distribution for the ZZ background shows a peak at the Z mass.
>
> We wish to use the Z peak of the dilepton invariant mass distribution to
> normalize the distribution when data is available. Then we would rely on
> the shape as given my MC to extrapolate the event yield in our signal
> region which is located at lower mass scale (10-60GeV) .
>
> Two questions:
>
> - Does compHep includes interference between Z and virtual gamma ?
>
> - Which are the most relevant sources of uncertainty in the shape of
> dilepton invariant mass distribution between 10 and 60 GeV, that is,
> which parameters in CompHEP should we vary to get a estimate of the
> uncertainty ?
>
> Best Regards
>
> Pedro
>
>
> _______________________________________________
> Comphep-common mailing list
> Comphep-common at theory.sinp.msu.ru
> http://theory.sinp.msu.ru/mailman/listinfo/comphep-common
>

-- 
шБ
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Alexander Kryukov <kryukov at theory.sinp.msu.ru>
SINP MSU, Moscow