References - Bogoliubov Laboratory of Theoretical Physics - JINR

Strange quark distributions. The strange quark momentum, S(x) =s(x) +¯s(x),
and helicity, ΔS(x) =Δs(x) +Δ¯s(x), distributions were measured [12] using data of
semi-inclusive production of charged kaons off deuterium target, an isoscalar target. The
analysis is based on the assumption of charge conjugation invariance in fragmentation
and isospin symmetry between proton and neutron.
The multiplicity of charged kaons produced off deuterium target can be written at leading
order in the following form:
dN K (x)
dN DIS (x) = Q(x) � DK Q (z)dz + S(x) � DK S (z)dz
, (3)
5Q(x)+2S(x)
where Q(x) =u(x) +ū(x) +d(x) + ¯ d(x) is the non-strange parton distribution, DK Q (z)
and DK S (z) are the fragmentation functions of non-strange and strange quarks into kaons,
respectively. Using parameterization of Q(x) from CTEQ6L one may extract the momentum
distribution S(x). The results are presented in Fig. 7 (left panel), together
with parameterizations of xS(x) andx(ū(x)+ ¯ d(x)) from CTEQ6L. The shape of xS(x)
measured by HERMES is incompatible with xS(x) fromCTEQ6Laswellaswiththe
assumption it corresponds to the average distribution of the non-strange sea.
In the extraction of the helicity distribution ΔS(x), only the double-spin asymmetry
AK �,d (x, Q2 ) for all charged kaons, irrespective of charge, and the inclusive asymmetry
A�,d(x, Q2 ) are used. They can be expressed in terms of the non-strange quark helicity
distribution ΔQ(x) =Δu(x) +Δū(x) +Δd(x) +Δ¯ d(x) and the strange quark helicity
distribution ΔS(x):
A�,d(x) d2 N DIS (x)
dxdQ 2 = KLL(x, Q 2 )[5ΔQ(x)+2ΔS(x)] ,
A K �,d (x)d2 N K (x)
dxdQ 2 = KLL(x, Q 2 )
�
ΔQ(x)
�
D K Q (z)dz +ΔS(x)
�
D K S (z)dz
�
. (4)
Here, KLL is a kinematic factor. These equations permit the simultaneous extraction of
the helicity distribution ΔQ(x) and the strange helicity distribution ΔS(x). The results
are presented in Fig. 7 (right panel). The strange helicity distribution is consistent with
zero over the measured range.
xS(x)
0.4
0.2
0
Fit
CTEQ6L
x(u –
(x)+d –
(x))
0.2
0.1
0
0.2
xΔQ(x)
xΔS(x)
Leader et al., PRD73, 034023 (2006)
-0.1
0.02 0.1 0.6
0.02 0.1 0.6
x
X
Figure 7: Left panel: The strange quark distribution xS(x) atQ2 =2.5 GeV2 . The solid curve is a fit of
the data, the dashed curve shows the result from CTEQ6L, and the dot-dash curve is the average of light
antiquark distributions from CTEQ6L. Right panel: nonstrange and strange quark helicity distributions
at Q2 =2.5 GeV2 . The error bars are statistical, and the bands represent the systematic uncertainties.
The curves are the LO results of world data analysis.
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0.1
0