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\title{ Photon-Photon and Photon-Nucleus Collisions in ALICE at the LHC }

\author{Joakim Nystrand\footnote{Department of Physics and Technology, University of Bergen, Bergen, Norway}}

\date{January 22, 2007}

\maketitle

ALICE is a general purpose heavy-ion experiment at the Large Hadron Collider 
(LHC) at CERN. Its primary aim is to study central nucleus-nucleus 
collisions (Pb+Pb) in order to investigate the properties of highly 
excited nuclear matter and the quark-gluon plasma\cite{Alessandro:2006yt}. 

This talk deals with electromagnetic (photon-induced) interactions in 
ultra-peripheral collisions in which there is no overlap between the 
colliding ions\cite{Bertulani:2005ru}. These events will have a very 
different topology compared 
with the central, hadronic interactions. They will therefore require different 
trigger and analysis techniques but will also broaden the physics potential 
of ALICE. 

ALICE is designed to handle charged particle multiplicities up to 
$d n_{ch} / d \eta =$ 8000, far above what is expected for ultra-peripheral 
collisions. Reconstructing the ultra-peripheral events should thus 
not be a problem. 
The major challenge will instead be triggering without a prohibitively high 
background rate. 

The Time-of-Flight and Si-pixel detectors can provide a 
low-multiplicity trigger at mid-rapidity. The foreseen ultra-peripheral 
trigger will combine the information from these detectors with a requirement 
of no signal 
in the V0 and T0 detectors. These are scintillator (V0) and 
cherenkov counters (T0) located at forward rapidities on both sides 
of the collisions point with full azimuthal coverage for  
pseudo-rapidities $\approx 2 \leq | \eta | \leq 5$. 

The expected rates for several interesting ultra-peripheral interactions 
within the ALICE acceptance are high. For coherent
production of the heavy vector mesons $J / \Psi$ and $\Upsilon$, rates of 
150,000 and 600, respectively, in the $e^+ e^-$ decay channel can be 
expected within one ALICE year (corresponding to $10^6$~s). 
%For $\rho^0$ mesons, the corresponding number is $200 \cdot 10^6$. 
The rates for photoproduction of heavy-quarks and jets will also be 
high, and the yield from these processes within the ALICE acceptance 
should be significant. 


\begin{thebibliography}{99}

%\cite{Alessandro:2006yt}
\bibitem{Alessandro:2006yt}
B.~Alessandro {\it et al.}  [ALICE Collaboration],
%``ALICE: Physics performance report, volume II,''
J.\ Phys.\ G {\bf 32} (2006) 1295.
%%CITATION = JPHGB,G32,1295;%%

%\cite{Bertulani:2005ru}
\bibitem{Bertulani:2005ru}
C.~A.~Bertulani, S.~R.~Klein and J.~Nystrand,
%``Physics of ultra-peripheral nuclear collisions,''
Ann.\ Rev.\ Nucl.\ Part.\ Sci.\  {\bf 55} (2005) 271
[arXiv:nucl-ex/0502005].
%%CITATION = NUCL-EX 0502005;%%

\end{thebibliography}

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