Event generators at BESⅢ

We present a brief remark and introduction to event generators for tau-charm physics currently used at BESⅢ, including KKMC, BesEvtGen, Bhlumi, Bhwide, Babayaga and inclusive Monte-Carlo event generators. This paper provides basic information on event gen

Chinese Physics C(HEP&NP)Vol.32,No.8,Aug.,2008 Event generators at BES *

PING Rong-Gang( )1)

(Institute of High Energy Physics,Chinese Academy of Sciences,Beijing100049,China) Abstract We present a brief remark and introduction to event generators for tau-charm physics currently used at BES ,including KKMC,BesEvtGen,Bhlumi,Bhwide,Babayaga and inclusive Monte-Carlo event generators.This paper provides basic information on event generators for BES users.

Key words event generators,BES ,Monte-Carlo simulation

PACS02.70.Uu,13.20.Gd

1Introduction

The precision of measurements will become one of the central issues or challenges for tau-charm physics analysis at BES .The measurement for the reso-nance parameters,say charmonia and light hadron states,is expected to reach the statistical accuracy of about1%or less.For measurements of the branching fractions for charmonium transitions or exclusive de-cays,the precision will be highly improved by one or-der or more of magnitude compared with the current PDG values.For tau-mass measurement,the statis-tical error is expected to reduce toδmτ∼0.09MeV, and the precision of measuring Michel parameter is expected to improve by a factor of2or4.These re-quirements call for high precision Monte-Carlo(MC) event generators.

Event generators with high quality or precision are essential to do a reliable MC simulation to re-move the systematic uncertainty as much as possi-ble.In experiment,MC simulations are used to de-termine detection efficiencies or study backgrounds. For high precision measurements,one expects that the MC generators could simulate the processes un-der study as really as possible.Hence,the genera-tors only with kinematic information(e.g.pure phase space)do not meet this requirement.Recently,high precision generators for QED processes are developed based on the technique of Yennie-Frautchi-Suura ex-ponentiation to describe the process e+e−→ff(f: fermion).The official precision tags of these genera-tors are about1%or less,e.g.KKMC,Bhlumi and so on.The generators with dynamical information for hadron decays have also been developed,such as Evt-Gen,for BaBar and CLEO collaborations to study B physics.These provide us with a large room to make a choice among the existent generators to simulate tau-charm physics processes.

However,most of these generators are originally designed for high energy physics above the tau-charm energy scale.Generally speaking,the MC generator is physics or model dependent.There are a few MC generators covering the full energy scale of high en-ergy physics.So at tau-charm energy scale,the immi-gration of the MC generators,originally designed for the high energy scale physics,needs afine tuning of parameters concerned,and further comparison with data is necessary.In order to generate exclusive char-monium decays,more models are needed to construct in the EvtGen framework.

In this paper,we present a general description about the BES generator framework,and give brief introduction to the BES event generators,such as KKMC,BesEvtGen,some QED generators and some inclusive generators.For details,the users are sug-gested to refer to the generator guides or concerned publications.

2Generator framework

The generator framework of BES was approved to use KKMC+BesEvtGen to generate charmo-nium decay events.For charmonia produced from the e+e−annihilation,the process can be described

We present a brief remark and introduction to event generators for tau-charm physics currently used at BESⅢ, including KKMC, BesEvtGen, Bhlumi, Bhwide, Babayaga and inclusive Monte-Carlo event generators. This paper provides basic information on event gen

600Chinese Physics C(HEP&NP)Vol.32

1)The z-axis is defined along the beam direction in the laboratory system.

2)Currently,the McTruths of these generators are not available in the simulation

3)The version is V00-11-07

We present a brief remark and introduction to event generators for tau-charm physics currently used at BESⅢ, including KKMC, BesEvtGen, Bhlumi, Bhwide, Babayaga and inclusive Monte-Carlo event generators. This paper provides basic information on event gen

No.8PING Rong-Gang Event generators at BES 601

Tr Dρ0

λρ,λ ρ

Dρ0

λρ,λ ρ

Aρ0→π+π−

λρ Aρ0→π+π−

λ ρ ∗.

(3)

To decay theπ0with the full correlations between all kinematic variables in the decay,the EvtGen in-terface automatically calculates the backward spin-density matrix by

˜Dρ0

λρ,λ ρ=Aρ0→π+π−

λρ Aρ0→π+π−

λ ρ ∗,(4)

then the spin-density matrix for theπ0is

Dπ0=

λψ,λρ,λ ρ˜Dρ0

λρ,λ ρ

M J/ψ→ρ0π0

λψ,λρ M J/ψ→ρ0π0

λψ,λ ρ ∗,(5)

this is a constant for a spin-0particle.So theπ0decay

is selected by the probability

Pπ0=

1

1)They correspond to generate events with1-D diagram,Dalitz plot and to generate3-body decays with a Dalitz plot plus two angular distribution plots.

We present a brief remark and introduction to event generators for tau-charm physics currently used at BESⅢ, including KKMC, BesEvtGen, Bhlumi, Bhwide, Babayaga and inclusive Monte-Carlo event generators. This paper provides basic information on event gen

602Chinese Physics C(HEP&NP)Vol.32

References

1Jadach S,Ward B F L,Was http://mu.,2000, 130:260;Jadach S,Ward B F L,Was Z.Phys.Rev.D, 2001,63:113009

2http://jadach.web.cern.ch/jadach/

3http:///DocDB/0000/000018/001/guide.

pdf

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