References - Bogoliubov Laboratory of Theoretical Physics - JINR

LAMBDA PHYSICS AT HERMES
S. Belostotski, Yu. Naryshkin and D. Veretennikov
(on behalf of the HERMES collaboration)
Petersburg Nuclear Physics Institute
† E-mail: naryshk@mail.desy.de
Abstract
Results for Λ and ¯ Λ hyperon polarization measured by the HERMES experiment
using the 27.6 GeV longitudinally polarized positron beam incident on polarized
or unpolarized gas targets are reported. The longitudinal spin transfer from the
beam to the hyperon DΛ LL has been measured at Q2 > 0.8 GeV2predominantly at positive xF and values of DΛ LL =0.102 ± 0.056(stat) ± 0.020(syst) forΛand
DΛ LL =0.152±0.139(stat)±0.030(syst) for¯ Λ have been obtained. The spin transfer
from the longitudinally polarized target to the hyperon has been measured in
KΛ LL
the quasi-real photoproduction regime (Q2 ≈ 0GeV2 )withanaveragephoton
energy of � 15 GeV, resulting in KΛ LL =0.024 ± 0.008(stat) ± 0.003(syst) forΛ
and KΛ LL =0.002 ± 0.019(stat) ± 0.008(syst) for¯ Λ, respectively. The spontaneous
transverse Λ and ¯ Λ polarization (not related to beam or target polarization) has
also been studied in a wide range of nuclear targets.
The HERMES experiment offers a good opportunity to study Λ polarization both
in semi-inclusive, eN → e ′� ΛX, and inclusive, eN → � ΛX reactions, using the 27.6 GeV
polarized positron beam of the HERA accelerator incident on internal polarized or unpolarized
gas targets. The HERMES detector, described in detail in [1], is a forward
magnetic spectrometer with angular acceptance ±(40–140) mrad in the vertical direction
and ±170 mrad in the horizontal direction. During the data taking period the typical
beam polarization was 0.4-0.5. The beam helicity was reversed about once per month.
HERMES used polarized hydrogen and deuterium
targets with typical polarization of 85%
and unpolarized targets 1H, 2D, 3He, 4He, 14N, 20 84 132 Ne, Kr and Xe in a wide range of atomic
numbers.
The Λ hyperon polarization P Λ was measured
through its pπ− decay channel. In this
parity-violating decay, the angular distribution
of the proton has a form
P
e
γ
∗
q
e’
q
hadronization
dN
dΩp
= Figure 1: The single-quark scattering mechanism
leading to Λ production in the current
fragmentation region of polarized deep inelastic
electron scattering.
dN0
(1 + αP
dΩp
Λ · cosθp), (1)
where θp is the angle between the proton
momentum and the direction of the Λ
polarization in the Λ rest frame, and α = 0.642 ± 0.013 is the analyzing
power of the weak decay. The symbols dN/dΩp and dN0/dΩp denote the distributions
for the decay of polarized and unpolarized Λ samples, respectively.
262
Λ
X