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T: Teilchenphysik
T 403: Diffraktion I
T 403.2: Group Report
Wednesday, March 25, 1998, 08:55–09:20, HS C
Diffractive vector meson production at HERMES — •Falk Meissner — DESY Zeuthen, Platanenallee 6, 15738 Zeuthen
The electro-production of ρ0,φ, ω and J/ψ vector
mesons has been studied at the HERMES experiment at the HERA accelerator
at DESY. Longitudinally polarised positrons of 27.5 GeV are scattered
on different polarised and unpolarised internal gas targets.
In 1997 the third year of data taking was successfully completed.
The measured cross sections cover a Q2 range of 0.2 < Q2 < 5 GeV2
and thus span over a region where pQCD is expected to become valid.
Additionally they might resolve a possible discrepancy between existing
data from NMC and E665.
The measurement of the polar angular distributions of the ρ0–decay
gives access to the ρ0 longitudinal polarisation and provides a test
of s-channel helicity conservation.
Similar analyses are in progress for the φ and ω mesons,
where smaller event samples were collected.
The comparison of ρ0 production on 1H , 2H, 3He and
14N targets allows a determination of nuclear transparency.
The J/ψ–meson is studied within the charm program of HERMES.
The production of charmed mesons gives a handle on the gluon content
of the nucleon, however different models of J/ψ–formation have
to be distinguished. In the HERMES kinematics diffractive J/ψ
production is expected to be dominant.