Flavor asymmetries in the proton and semi-inclusive processes Page: 3 of 15
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In particular, whether one corrects for Fermi motion only, or in addition for
binding and nucleon off-shell effects, the extracted neutron structure functions
for x > 0.7 can differ dramatically 6. The question is therefore how to avoid
uncertainties in the extraction procedure introduced by nuclear effects ,8.
One possibility is to measure the relative yields of 7r+ and 7r- mesons
in semi-inclusive scattering in the current fragmentation region. At large z
the u quark fragments primarily into a r+, while a d fragments into a 7r-,
so that at large x and z one has a direct measure of d/u. (Although one
should not be too close to z = 1, as the fragmentation process there may
no longer be incoherent, or factorizable into a partonic cross section and a
target-independent fragmentation function.)
The HERMES Collaboration has previously extracted the d/u ratio from
the 7r+-7r- difference using both proton and bound neutron targets 1. The
advantage of using both p and n is that all dependence on fragmentation
functions cancels, removing any uncertainty that might be introduced by in-
complete knowledge of the hadronization process. For a proton target one has
(dropping the explicit Q2 dependence):
N + ~ 4u(x) D(z) + d(x) D(z), (2)
NP - 4u(x) D(z) + d(x) D(z), (3)
where D(z) = DW+ = Dd is the leading fragmentation function (assuming
isospin symmetry), and D(z) = D2 + DJ is the non-leading fragmentation
function. With the corresponding expression for a neutron, one can construct
R _N"+- NJ 4d(x)/u() -1
RP(x, z) = NP+ - N- 4 - d(x)/u(x) (4)
which is then independent of the fragmentation function, and is a function of
The disadvantage of extracting d/u from this ratio is that one must still
account for nuclear effects in obtaining NnJ+ from the proton and deuteron
data. The HERMES Collaboration 1 assumed that N,j = NdJ} - N7}. Be-
yond x ~ 0.7, however, the difference between the ratios with NnJ corrected
for nuclear effects and that which is not is quite dramatic 9. Consequently a
d/u ratio obtained from such a measurement without nuclear corrections could
potentially give misleading results.
One can avoid the problem of nuclear corrections altogether by comparing
data for 7r+ and ir- production on protons alone. From the ratio of the 7r- to
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Melnitchouk, W. Flavor asymmetries in the proton and semi-inclusive processes, article, April 1, 1999; Newport News, Virginia. (digital.library.unt.edu/ark:/67531/metadc711764/m1/3/: accessed October 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.