
%% abbreviations

\newcommand{\sqrtsnn}{\sqrt{s_{_{NN}}}}
\providecommand{\ups}{\Upsilon}
\providecommand{\upsp}{\Upsilon^{'}}
\providecommand{\upspp}{\Upsilon^{''}}

\providecommand{\str}{{\sc starlight }}
\providecommand{\gaga}{\gamma\,\gamma}
\providecommand{\gp}{\gamma\,p}
\providecommand{\gA}{\gamma\,A}
\providecommand{\gpb}{\gamma\,Pb}

\providecommand{\ee}{e^+e^-}
\providecommand{\mumu}{\mu^+\mu^-}
\providecommand{\lele}{l^{+}\,l^{-}}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\title{Low-x QCD via electromagnetic PbPb collisions at 5.5 TeV in CMS}

David~d'Enterria$^1$, Aurelien Hees$^{1,2}$ for the CMS collaboration

$^1$CERN, Geneva, Switzerland
$^2$Universit\'e Catholique de Louvain, Belgium

Photoproduction of heavy-vector mesons in electromagnetic interactions (aka.
Ultra-Peripheral Collisions, UPCs) of heavy-ions at very high energies provides
direct information on the parton distribution function (PDF) in the nucleus
at low values of Bjorken-$x$ (see e.g.~\cite{dde_qm05} and refs. therein). 
The capabilities for the measurement of $\ups$ produced in ultraperipheral 
PbPb collisions at $\sqrtsnn$ = 5.5 TeV in the Compact Muon Solenoid 
(CMS) experiment at LHC~\cite{PTDR1} are studied in the $\ee$ and $\mumu$ 
decay modes. The measurement uses the CMS tracker, the ECAL and the muon 
chambers ($|\eta|<$ 2.5) tagging the ultra-peripheral PbPb events with forward 
neutrons detected in the Zero Degree Calorimeters (ZDC)~\cite{Grachov:2006ke}.
The \str event generator~\cite{starlight} is used as input Monte Carlo for the $\ups$ 
photoproduction differential cross-sections as well as for the most significant source of 
physical background: coherent production of high-mass lepton pairs in $\gaga\rightarrow\lele$ 
processes. The mass, $p_T$ and rapidity spectra of the $\ups$ obtained after background 
subtraction with a full simulation and reconstruction analysis are presented.
The excellent bottomonia mass resolution of CMS muon chambers will allow one to
measure separately the different vector states ($\ups,\ups',\ups''$). The dedicated 
L1 and high-level triggers proposed for the UPC measurement are also discussed. 
The final expected rates ($\sim$400 $\ups$'s) for a nominal run with integrated 
luminosity of 0.5 nb$^{-1}$ will allow for a detailed differential study of the 
rapidity dependence of $\ups$ photo-production which is expected to constrain 
the underlying gluon densities in the Pb nucleus in a kinematic regime 
($x\approx$10$^{-3}$, $Q^2\approx$ 40 GeV$^2$) completely unexplored 
so far~\cite{d'Enterria:2007}.

\begin{thebibliography}
\bibitem{d'Enterria:2006ep}D.~d'Enterria [PHENIX Collaboration], QM'05 proceeds., nucl-ex/0601001.%%CITATION = NUCL-EX 0601001;%%
\bibitem{PTDR1}CMS Collaboration, ``CMS Physics Technical Design Report, Volume 1", CERN-LHCC-2006-001, CMS-TDR-008-1.
\bibitem{Grachov:2006ke}O.~A.~Grachov {\it et al.} [CMS Collaboration], AIP Conf.\ Proc.\  {\bf 867} (2006) 258%%CITATION = NUCL-EX 0608052;%%
\bibitem{starlight}J. Nystrand, Nucl. Phys. A 752(2005)470c;  S.~Klein, J.~Nystrand \PRL 92 (2004) 142003;
A.J. Baltz, S.R. Klein, J. Nystrand, \PRL 89(2002)012301; S.R. Klein, J. Nystrand, \PRC 60(1999)014903.
\bibitem{d'Enterria:2007}D.~d'Enterria [CMS collaboration], QM'06 proceeds.,  J. Phys~{\bf G}: Nucl. Part. Phys} to be submitted.%%CITATION = NUCL-EX 0601001;%%
\end{thebibliography}
