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SUMMARY:First Monte Carlo Global QCD Analysis of Pion Parton Distributions
DTSTART;VALUE=DATE-TIME:20180515T182000Z
DTEND;VALUE=DATE-TIME:20180515T184000Z
DTSTAMP;VALUE=DATE-TIME:20211128T061631Z
UID:indico-contribution-3215@indico.jlab.org
DESCRIPTION:Speakers: Patrick Barry (North Carolina State University)\nPio
ns have long been associated with being the lightest bound state made of q
uarks\, antiquarks and gluons (partons)\, as well as being the Goldstone b
oson produced from the breaking of effective chiral symmetry. In previous
works\, its parton distribution function (PDF) has been determined only b
y $\\pi A$ Drell-Yan (DY) experiments such as E615 and NA10 done at Fermil
ab and CERN\, respectively. In the DY process\, two hadrons collide\, wit
h one donating a quark and the other donating an antiquark\, eventually pr
oducing a dimuon pair. Since the initial annihilation provides enough ene
rgy to create a heavy lepton pair\, the quark/antiquark must have high mom
entum. Thus\, the DY process can only resolve the high momentum fraction
($x_\\pi \\gt 0.2$) region of the pion’s parton distribution\, which con
strains only the valence PDFs. To more accurately determine the pion PDF
at lower $x_\\pi$\, we must include data from leading neutron (LN) electro
production experiments such as H1 and Zeus done at HERA at DESY. Here\, a
target proton struck with an electron transforms into a detected neutron\
, a process which at forward angles\, is dominated by the one-pion exchang
e model. Inherently\, the theory has some model dependence (albeit relati
vely weak) coming from the splitting function’s regularization prescript
ion\, of which we test five. This process probes much smaller momentum fr
action as $x_\\pi \\sim 10^{-3}$\, where the sea and gluon dominate\, prov
iding an unseen constraint on the sea quark and gluon PDFs. In practice\,
we fit the parton distributions to all existing DY and LN datasets using
a Monte Carlo (MC) method based on nested sampling. The method uniquely c
alculates the likelihood function at various positions in vector space to
nest around the global maximum likelihood value. The MC nature allows us
to rigorously quantify the uncertainty in the PDFs. We successfully have
determined the pion PDFs at both the high-$x_\\pi$ and low-$x_\\pi$\, and
in doing so\, we obtain a $\\chi^2$ of $0.96 – 1.04$ over the five model
s. The PDFs we obtain reveal that at the input scale\, the gluon carries
a significant fraction of the pion momentum $\\sim 30\\%$\, which is highe
r than predicted with only the DY data\, and the sea carries $\\sim 15\\%$
of the momentum fraction of the pion. We can also describe the $\\bar{d}
-\\bar{u}$ asymmetry in the proton by using the determined pion PDF and th
e same pion splitting functions as used in the LN process.\n\nhttps://indi
co.jlab.org/event/252/contributions/3215/
LOCATION:Jefferson Lab - CEBAF Center Auditorium
URL:https://indico.jlab.org/event/252/contributions/3215/
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