References - Bogoliubov Laboratory of Theoretical Physics - JINR

Figure 2: Compilation of the world data on
the longitudinal spin-transfer coefficient DΛ According to Eq. 1 the protons are preferentially
emitted along the spin direction of their
parent Λ which provides an opportunity to measure
Λ polarization by measuring the ”forwardbackward”
asymmetry of the decay products in
the Λ rest frame. Λ events were selected by requiring
the presence of at least two hadron candidates
of opposite charge. In the event selection
the fact has been used that at HERMES kinematics
the detected proton (or anti-proton) is
always the leading particle. The combinatorial
background has been mainly suppressed using
this kinematical criterion and a restriction imposed
on the distance between the primary (Λ
LL
production) and secondary (Λ decay) vertices. on xF .
The spin transfer from the longitudinally polarized beam to the Λ hyperon was studied
in semi-inclusive deep-inelastic scattering (DIS) eN → e ′� ΛX (Fig.1), where the scattered
0.6
0.5
HERMES preliminary
E704
STAR
0.4
0.3
0.2
0.1
0.0
-0.1
0.5 1 2 4 8
t(
GeV)
16 32
Figure 3: Compilation of the world data on
spin transfer from a polarized nucleon to the
Λ(forHERMESKΛ LL )vs√ positrons were detected in coincidence with Λ
events. The requirements Q
t variable.
2 > 0.8 GeV2and W > 2 GeV, where −Q2 = q2 is the fourmomentum
transfer squared of the exchanged
virtual photon and W is the invariant mass of
the photon-nucleon system, were imposed on the
positron kinematics to ensure that the events
originated from the DIS domain. In addition,
the requirement y =1− E ′ /E < 0.85 was imposed
to exclude the large contribution of radiative
corrections, where E(E ′ ) is the energy of the
positron before (after) the scattering process.
The component of the polarization transferred
along the L ′ direction from the virtual
photon to the produced Λ hyperon is given by
P Λ L ′ = PbD(y)D Λ LL ′,wherePbis the longitudinal
beam polarization, D(y) � [1 − (1 − y) 2 ]/[1 + (1 − y) 2 ] is the depolarization factor and L
is the primary quantization axis, directed along the virtual photon momentum, and it is
assumed that the spin of the virtual photon is directed along its momentum.
The spin transfer coefficient DΛ LL ′ describes the probability that the polarization of
the virtual photon is transferred via the struck quark to the Λ hyperon along a secondary
quantization axis L ′ . In this analysis, the quantization axis L ′ is chosen along the virtual
photon direction L, i.e. L ′ = L. It is important to remember that the direction L is taken
in the Λ rest frame.
The extraction of D Λ LL
Spin tran sfer
from the data is based on the moments method applied to a
beam helicity balanced data set [2]. The data have also been analyzed with the maximum
likelihood method. The results has been found to be very close to those obtained by the
moments method.
In order to estimate the systematic uncertainty of the obtained results an identical
263