References - Bogoliubov Laboratory of Theoretical Physics - JINR

analysis was carried out reconstructing h + h − hadron pairs produced in invariant-mass
windows below and above the Λ ( ¯ Λ) mass peak. As an additional check of a possible false
polarization, the decay K0 s → π+ π− was studied. These analyzes resulted in Dh+ h− LL =
−0.002 ± 0.011 and D K0 s
LL = −0.016 ± 0.035.
Λ
0.2
Λ
0.1
0
−0.1
1.0
−
Λ
Λ −
The HERMES data for D
0.5
Λ LL [3] are presented
in Fig. 2 as a function of xF together with
data of the NOMAD experiment at CERN [4],
the E665 experiment at Fermilab [5] and the
COMPASS experiment at CERN [6]. As seen
from Fig. 2, the NOMAD, COMPASS and HER-
MES results are compatible in the kinematic
region of overlap −0.1 < xF < 0.3. The
HERMES results, averaged over kinematics are
=0.102 ± 0.056(stat) ± 0.020(syst) forΛ
P n
(GeV)
0
0 0.1 0.2 0.3 0.4 0.5 0.6
ζ
DΛ LL
and D ¯ Λ
LL =0.152 ± 0.139(stat) ± 0.030(syst) for
¯Λ, respectively.
The spin transfer from the longitudinally polarized
1H and 2D targets to the hyperon, KΛ LL ,
Figure 4: Transverse polarizations PΛ and
P¯Λ (upper panel) and mean 〈pT〉 (lower panel) was measured in the quasi-real photoproduction
as functions of ζ =(EΛ + pzΛ)/(Ee + pe). regime. The KΛ LL extraction procedure is similar
to the DΛ LL analysis [7]. The kinematical dependence of KΛ √ � LL as a function of
t = −(pΛ − pp) 2 , where pΛ (pp) is the four-momentum of the Λ hyperon (target
nucleon), is presented in Fig. 3 together with results obtained by the E704 (p↑p → � ΛX,
pbeam = 200 GeV/c) [8] and STAR (�pp → � ΛX, √ s = 200 GeV) [9] collaborations. The
HERMES data confirm the trend of KΛ LL increasing with decreasing √ t previously observed
by the E704 collaboration. Averaged over kinematics the HERMES result for the
=0.024 ± 0.008(stat) ± 0.003(syst) for the Λ hyperon, while for the
spin transfer is KΛ LL
¯Λ hyperon it is compatible with zero: K ¯ Λ
LL =0.002 ± 0.019(stat) ± 0.008(syst).
P Λ
0.2
0.1
0
-0.1
1 H
2 D
0 < ζ < 1
HERMES PRELIMINARY
3 He
4 He
14 N
20 Ne
132 Xe
84 Kr
1 10 10
A
2
Figure 5: Transverse Λ polarization vs
atomic number of nuclei.
Transverse Λ polarization was observed and
investigated in many high-energy scattering experiments,
with a wide variety of hadron beams
and kinematic settings [10, 11]. Because of
the parity-conserving nature of the electromagnetic
and the strong interaction, any final-state
hadron (hyperon) polarization in an inclusive reaction
with unpolarized beam and target can
only point along the normal ˆn = �pe×�pΛ
|�pe×�pΛ| of
the production plane, where �pe is the (positron)
beam momentum and �pΛ is the Λ momentum,
respectively. Possible mechanisms for the origin
of this polarization were discussed for example
in Refs. [12], [13], [14] [15] [16], [17].
The formalism of the extraction of transverse Λ polarization is based upon the up/down
symmetry of the HERMES detector and the moments method [2, 18]. Averaged over the
experimental kinematics, the net Λ polarization extracted from the data collected in the
years 1996-2005 with various targets was found to be signficantly positive while the net
264