Azimuthal asymmetry
using energy flow method

Azimuthal angle distribution at Q2>100 GeV2

Energy flow method

Experimental results

LO and NLO predictions

Comparison DATA with predictions

Summary

DIS 2005

A.Ukleja on behalf of the ZEUS Collaboration

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Azimuthal asymmetry comes from:

Two-body processes (BGF and QCDC)

Boson polarization

Longitudinally polarized electron beam

Parity violating weak interactions

Final hadron polarization

Intrinsic parton momentum in the proton

Future:

asymmetry can be measured for

Longitudinally polarized lepton beam

CC events

Azimuthal angle definition for the ep →ehX process

Experimental methods

Fitted function

Moments of distributions of f trigonometrical functions – means

-4<hHCM<-3.5

Means:

The 1st moment:

Previous ZEUS measurements

Distribution of the azimuthal angle – ZEUS paper 2002

Breit frame

Jet analysis

See Oscar Gonzalez thesis and

DESY 02-171, Phys.Lett. B551 (2003) 226-240

Fitted to experimental data

Small Large

Distribution of the azimuthal angle – ZEUS paper 2000

HCM frame

z, pT method

Multiplicity method

Charged hadrons

See Eduardo Rodriguez thesis and

DESY 00-040, Phys.Lett. B481 (2000) 199-212

For hadrons with pT>pc

pc

New method of analysis
Energy Flow

Energy flow method in the laboratory frame

Each particle direction is weighted with its energy

Energy flow objects EFO

EFO used as pseudohadrons

pQCD

infrared and collinear

singularities out

Peccei, Rückl (1978)

Look into pseudorapidity

If integrated over

the dominant contribution:

from QCDC

to

from BGF

to

Comments on the energy flow method and pseudorapidity

charged and neutral hadrons included

hard partons (E*T larger) provides a larger contribution

hadrons nearby in the HCM frame → nearby in LAB

sensitive to parton fragmentation → no dependence on jet algorithms

multiplicity method with charged hadrons → sensitive to hadronization

calorimeter energy scale is canceled

no hadrons but clusters of energy

the quantities like z=Pph/Pq are not well measured

Global selection criteria

27.5 GeV

e

q

forward

p

820 GeV

rear

Azimuthal asymmetry energy flow method

Monte Carlo Models

LEPTO 6.5.1 – matrix element and parton shower

ARIADNE 4.12 – colour dipole model (LO)

DISENT – NLO dipole factorization formulae and

subtraction technique

Azimuthal asymmetry energy flow method

Uncertainties for NLO due to factorization

and renormalization scales were calculated by changing

Azimuthal asymmetry energy flow method

Azimuthal Asymmetry
as a function of ET

Azimuthal Asymmetry as ETHCM

-5.<HCM<-2.5

-2.5<HCM<-1.

-1.<HCM<0.

-5.<HCM<-2.5

-2.5<HCM<-1.

-1.<HCM<0.

Summary and conclusions

A novel approach to azimuthal asymmetry is proposed which

provides precise measurements and small statistical errors

in the wider interval of phase space

The method permits to:

include charged and neutral hadrons

enhance contributions of hard partons by weighting with energy, i.e. energy flow

investigate contributions of BGF w.r.t QCDC

compare these results with the previous ZEUS measurements

The main results are:

the NLO effects give non negligible contribution

they provide better agreement with experimental data

some small discrepancies are visible which cannot be explained by experimental errors

Summary and conclusions

2004-03-18T10:13:25