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{\bf {\Large Determining the Nuclear Gluon Distribution in UPCs}}\\

{\bf \large R. Vogt

Nuclear Science Division, LBNL, Berkeley, CA, USA

and

Physics Department, University of California, Davis, CA, USA}
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In this talk, we discuss photoproduction of hard processes, accessible at
RHIC and the LHC.  The high photon-nucleon center of mass energy, 
$\sqrt{S_{\gamma N}}$, and the large luminosities result in high rates for
$Q \overline Q$ (charm and bottom) \cite{KNV,SVW} and jet$+$jet \cite{SVW} 
final states.  
In particular, at the LHC, the high energies and large rapidity coverage make
the nuclear gluon distribution accessible in the interesting low $x$ and 
perturbative $Q^2$ regime.  In $AA$ collisions, both ions serve as photon
emitters and the effective photoproduction process is $\gamma A$. The
photon flux from a nucleus is enhanced by a factor of $Z^2$ relative to the
proton.  Thus in $pA$ interactions, the $\gamma A$ luminosity, where the photon
comes from the proton, is negligible relative to the $\gamma p$ luminosity
where the the photon comes from the nucleus.

We consider both direct photoproduction,
where a photon from the
virtual photon field surrounding the nucleus interacts with a parton in the
opposite nucleus, and resolved production, where the virtual photon fluctuates
into states with multiple gluons and $q \overline q$ pairs, opening more
channels but increasing the average $x$ probed.  The nuclear gluon distribution
is more cleanly probed in direct photoproduction.  
The possible effects of nuclear modifications of the parton densities on the 
$\gamma A$ interactions, most important at low $x$ and moderate $Q^2$ 
\cite{KNV}.  We compared two parameterizations of nuclear shadowing which
differ strongly for gluons, EKS98 \cite{EKS} and FGS \cite{FGS}, to show the 
range of the measurable effect.  It is precisely this modification that these
LHC measurements can study.  The average $x$ values for the nucleon parton
momentum fraction are shown as a function of heavy quark $p_T$ (integrated
over rapidity) and rapidity ($p_T$ integrated) and jet $p_T$ (rapidity 
integrated).  We have also shown the $b \overline b$ and jet rates as a
histogram in $x$ and $p_T$ in the ATLAS acceptance \cite{SVW}.

Heavy flavor production is especially useful since, in direct photoproduction,
the only contribution is from $\gamma g \rightarrow Q \overline Q$.  Of course
heavy flavor measurements are rather difficult and jet measurements may be
simpler.  Jet production proceeds dominantly by $\gamma g \rightarrow q 
\overline q$ at low $p_T$ but includes a contribution from the QCD Compton
process, $\gamma q (\overline q) \rightarrow g q (\overline q)$ which, although
small at moderate $p_T$, increases with $p_T$.  The heavy flavor results are
shown as a function of quark $p_T$ and rapidity as well as $Q \overline Q$ pair
mass.  The jet results are shown as a function of jet $p_T$.  Final state
pion, kaon and proton $p_T$ distributions are also shown -- if jets are
not directly measurable, these leading hadrons may be.  

The results show that the rates are sufficiently high for these processes
to provide clean measurements of the nuclear gluon distribution for $\gamma A$
in $AA$ and the proton parton distribution $\gamma p$ in $pA$ interactions.


\begin{thebibliography}{100}
\bibitem{KNV}
  S.~R.~Klein, J.~Nystrand and R.~Vogt,
  %``Heavy quark photoproduction in ultra-peripheral heavy ion collisions,''
  Phys.\ Rev.\ C {\bf 66} (2002) 044906.

\bibitem{SVW} M. Strikman, R. Vogt and S. White, Phys. Rev. Lett. {\bf 96} 
(2006) 082001.

\bibitem{EKS}
K.~J.~Eskola, V.~J.~Kolhinen and P.~V.~Ruuskanen,
Nucl.\ Phys.\ B {\bf 535} (1998) 351;
K.~J.~Eskola, V.~J.~Kolhinen and C.~A.~Salgado, Eur.\ Phys.\ J.\ C {\bf 9}
(1999) 61.

%\cite{Frankfurt:2003zd}
\bibitem{FGS}
L.~Frankfurt, V.~Guzey and M.~Strikman,
%``Leading twist nuclear shadowing: A user's guide,''
Phys. Rev. D {\bf 71} (2005) 054001.

\end{thebibliography}

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