References - Bogoliubov Laboratory of Theoretical Physics - JINR

angle between the direction of the hyperon decay daughter baryon (proton for Λ and
antiproton for ¯ Λ) and the direction of the virtual photon, and α =+(−)0.642 ± 0.013 is
the Λ( ¯ Λ) decay parameter. In the present analysis we used both target cells despite of
their polarization vector, which results in averaging of target cells polarization.
It’s convenient to use the spin transfer coefficient D Λ(¯ Λ)
LL , which is defined as the part
of beam polarization Pb that is transferred to the hyperon:
PL = D Λ(¯ Λ)
LL Pb D(y),
where D(y) = 1−(1−y)2
1+(1−y) 2 is the virtual photon depolarization factor.
The dependence of DΛ LL and D ¯ Λ
LL on Bjorken scaling variable x is shown in Fig. 1a.
The spin transfer values for Λ and ¯ Λ are not equal to each other. DΛ LL is close to zero
over the whole x range, whereas D ¯ Λ
LL reaches 40-50%.
LL
D
1
0.8
0.6
0.4
0.2
0
−0.2
−0.4
−0.6
COMPASS
Λ
Λ
−2
10
10
x
−1
LL
D
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
COMPASS
Λ
0 0.1 0.2 0.3 0.4 0.5 0.6
xF
(a) (b)
Figure 1: (a): DLL dependence for Λ and ¯ ΛonBjorkenx. Systematic errors are shown at
the bottom, black color for Λ, light gray for ¯ Λ. Solid line is theoretical calculation of [7]
for Λ, dashed line – ¯ Λ. The hyperon spin model was SU(6). (b): DLL dependence of ¯ Λ
on xF , calculated in [7] for parton distributions GRV98LO (dashed line), CTEQ5L (solid
line) and CTEQ5L with turned off spin transfer from ¯s-quark for SU(6) (dash-dotted line)
and BJ [11] (dotted line) hyperon spin models.
The average kinematic values for Λ are: ¯xF =0.22, Bjorken ¯x =0.03, fractional
energy ¯z =0.27, ¯y =0.46, Q 2 =3.7 (GeV/c) 2 . In the selected xF region ¯ Λ kinematic
distributions are almost identical to Λ distributions. Spin transfer values averaged over
all kinematic range are:
D Λ LL = −0.012 ± 0.047stat ± 0.024syst,
D ¯ Λ
LL = 0.249 ± 0.056stat ± 0.049syst.
The resulting DLL values are compared with the data from previous experiments in
Fig. 2. The present results are in good agreement with others and for ¯ Λ they are the
most precise DLL measurements available today. The xF dependence of DLL for ¯ Λisin
good agreement with the NOMAD [3] result of DLL =0.23 ± 0.15 ± 0.08 at ¯xF =0.18.
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