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\journal{Nuclear Physics A} 

\begin{document}

\begin{frontmatter} 

% Your Title - please insert
\title{Measurement of Isolated Direct Photons in Lead-Lead Collisions at $\sqrt{s_{NN}}=2.76$ TeV with the ATLAS Detector}


%% Single author (and collaboration) - please insert
\author{Iwona Grabowska-Bold (for the ATLAS\fnref{col1} Collaboration)}
\fntext[col1] {A list of members of the ATLAS Collaboration and acknowledgements can be found at the end of this issue.}
\address{AGH University of Science and Technology, Al. A. Mickiewicza 30, Krak\'ow, Poland}

%% Multiple authors
%\author[auth2]{Marcus Junius Brutus}
%\address[auth1]{Somewhere, Rome}
%\address[auth2]{Somewhere else, Rome}


\begin{abstract} 
   Direct photons are a powerful tool to study heavy ion
   collisions. Their production rates provide access to the
   parton distribution functions, which are expected to be modified by nuclear
   effects. They also provide a mean to calibrate the energy
   of jets that are produced in the medium, and thus are a tool to
   probe the physics of jet quenching more precisely both through jet
   rates and fragmentation properties. The ATLAS detector measures
   photons with its hermetic, longitudinally segmented calorimeter,
   which gives excellent spatial and energy resolution, and detailed
   information about the shower shape of each measured photon. This
   gives powerful rejection against backgrounds from
   neutral pions produced in jets. Rejection against jet fragmentation
   products is further enhanced by isolation criteria, which can be
   based on energy flow in the calorimeter. 
   First results on the rates of isolated direct photons from
   approximately $0.13\;\rm{n b^{-1}}$ of lead-lead data will be shown, as a
   function of transverse momentum, pseudorapidity and centrality.
\end{abstract} 

\end{frontmatter} % do not change

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\section{Introduction}

While jet quenching in heavy ion~(HI) collisions at the Large Hadron Collider (LHC) was demonstrated
using di-jet asymmetry distributions~\cite{atlas_jet_asymmetry}, the detailed physics
mechanism of this phenomenon needs further understanding. One of the necessary factors in
understanding jet quenching is having a proper calibration of the
initial jet energy. Di-jet measurements are biased because it is not
known whether or not the highest \pt\ jet was itself
quenched. On the other hand single jet measurements~\cite{atlas_single_jets1,atlas_single_jets2}
 at a given measured transverse momentum integrate over a
range of initial unquenched jet energies. Therefore, another class of
measurements which replaces one of the jets with an electroweak boson ($\gamma$, W or
Z), opens the possibility of calibrating the energy of the accompanying
jet~\cite{wang}. The ATLAS and CMS collaborations have reported
on the first correlation measurements of electroweak bosons with jets in
Ref.~\cite{atlas_photons_jets, atlas_z_jets, cms_photons_jets}.

As an important prerequisite of photon-jet measurements, it is
essential to measure photon rates in lead-lead collisions. 
 At high energy prompt photons are expected to have two primary sources;
the first is direct emission, which proceeds via quark-gluon
Compton scattering $qg\rightarrow q\gamma$  or quark-antiquark annihilation $q\bar{q}\rightarrow g\gamma$,
the second is the production of a single hard photon during parton
fragmentation, called fragmentation contribution. 
%At leading order, there is a meaningful distinction between the two
%processes, but at next-to-leading-order (NLO) the two cannot be
%unambiguously separated. In order to remove QCD background events from
%di-jet processes, as well as fragmentation photons, an “isolation”
%criterion is typically applied within a cone of a well-defined radius
%relative to the photon direction. That can be applied consistently to a QCD
%calculation such that prompt photon rates can be calculated. At high
%$p_T$, photon rates are expected to be modified by nuclear effects on the
%nucleon PDFs, as well as by jet-medium interactions.

In this proceedings, a measurement of isolated prompt photons in lead-lead
collisions at $\sqrt{s_{NN}}=2.76$~TeV with the ATLAS detector~\cite{atlas_detector} is
presented and discussed. Yields per event are measured averaged over $|\eta| < 1.3$
and as a function of event centrality and photon transverse momentum
 $p_T$ ranging from 45 to 200 GeV.

\section{Event Selection}

The sample of events used in this analysis was collected in the 2011 HI
run using the first level electromagnetic calorimeter trigger~\cite{atlas_trigger},
which requires a cluster of cells with transverse energy exceeding $E_{T}$ = 16 GeV.
 This trigger is found to be  100\% efficient for photons with $E_{T}> 20$~GeV.
Analyzed events are divided into centrality classes, which are defined
based on a total sum of $E_T$ in the forward calorimeter~\cite{atlas_centrality}.
%Centrality reflects the overlap volume of the two colliding
%nuclei. Collisions with a small (large) impact parameter
%are referred to as central (peripheral). 

Photons are reconstructed using information from 
the ATLAS calorimeter after underlying event~(UE) 
background subtraction. A sliding window algorithm is seeded on 
the second layer of the electromagnetic calorimeter, 
which typically samples over 50\% of the deposited photon energy.
The energy measurement is made using all three layers of the
electromagnetic calorimeter and the presampler. The photon conversion 
recovery procedure is not performed, due to the overwhelming number
of combinatorial pairs in more central collisions in the high track multiplicity
environment of the HI collision. Therefore, a
substantial fraction of converted photons is still reconstructed
by the photon algorithm. Following the reconstruction requirements, 
further photon identification cuts are made.  The fine-grained,
longitudinally segmented calorimeter allows detailed characterization of the
shape of each photon shower, providing tools to reject jets and
hadrons, while maintaining the high efficiency for the photons themselves.
Nine shower shape variables are used in this
analysis~\cite{atlas_direct_photons}. Along with the kinematic cuts they constitute
the photon selection. After application of the photon cuts to the
0-80\% centrality class, but before applying isolation requirements, 
there are 6435 photon candidates.

\begin{figure}[htbp!]
\begin{center}
\includegraphics[width=0.31\textwidth]{../fig_02a.pdf}
\includegraphics[width=0.31\textwidth]{../fig_02b.pdf}
\includegraphics[width=0.31\textwidth]{../fig_02c.pdf}
\end{center}
\caption{ Comparisons of three photon identification variables for
  data (black points) with results from simulation for 0-10\% central
  events~\cite{atlas_direct_photons}. $w_{s,3}$
  is the RMS width of the three cells including and surrounding the
  cluster maximum in the first calorimeter layer. $w_{\eta,2}$ is the RMS width of the
  energy distribution of the cluster in the second layer in the $\eta$
  direction and $R_{had1}$ is the ratio of transverse energy measured in the first
  layer of the hadronic calorimeter to the transverse energy of the
  photon cluster. Error bars represent statistical uncertainties.
  The simulation is shown both fully integrated (yellow) and
  broken out into contributions from unconverted photons (red
  histogram) and converted photons (blue histogram). }
\label{fig:shower_shapes}
\end{figure}

Figure~\ref{fig:shower_shapes} shows three shower 
shape variables for data from the 0-10\% centrality bin, compared to the
Monte Carlo~(MC) simulation. In all cases, electromagnetic clusters 
 with reconstructed transverse energy greater than 40 GeV 
and $|\eta| < 1.3$ are shown. 
The simulation also allows to inspect the contributions from converted and
unconverted photons. It is found that the broad features of the
distributions are reproduced well by the simulation. 

In order to reduce the contribution of di-jet background events, 
as well as events containing fragmentation photons, an “isolation”
criterion is applied within a cone aligned with the photon direction.
%In order to further reject clusters arising from hadronic fragments of
%jets, particularly neutral mesons, the calorimeter is also used to 
%measure an isolation energy for each photon candidate. 
The isolation transverse energy, $E_{T}(R_{iso})$, defined as the sum
of transverse energies in the calorimeter cells in a cone 
$\sqrt{ \Delta \eta^2 + \Delta \phi^2} < R_{iso}=0.3$ 
around the photon axis after the UE background subtraction,
is required to be less than 6 GeV.

\section{Results}
\begin{figure}
\begin{center}
\includegraphics[width=0.45\textwidth]{../fig_11.pdf}
\includegraphics[width=0.45\textwidth]{../fig_12.pdf}
\end{center}
\caption{(left) Efficiency corrected yields of prompt photons in
$|\eta|<1.3$ using photon cuts, isolation cone radius $R_{iso} =
0.3$ and isolation transverse energy of 6 GeV. 
(right) Centrality dependence of the photon yield per event in each 
$p_{T}$ bin, scaled by the average nuclear thickness function 
$\langle T_{AA} \rangle$ for that centrality interval.
The horizontal axis is the average number of participants $\langle
N_{part} \rangle$ 
for each selected centrality interval. Statistical errors are shown by
the error bars.  Systematic uncertainties on the photon yields are
combined and shown by the yellow bands~\cite{atlas_direct_photons}.  }
\label{fig:photon_yields}
\end{figure}


The double sideband technique~\cite{atlas_direct_photons} is applied
to extract the photon yield per minimum bias event
 $(1/N_{evt})dN_{\gamma}/dp_{T}(p_{T},c)$, divided by the mean
nuclear thickness $\langle T_{AA} \rangle = \langle N_{coll} \rangle/\sigma^{pp}_{tot}$ which scales as the number of binary
collisions $\langle N_{coll} \rangle$, where $\sigma^{pp}_{tot}$ is the
total inelastic $pp$ cross section. The photon yield is defined as:
\begin{equation}
\frac{1}{N_{evt}}\frac{dN_{\gamma}}{dp_{T}}(p_{T},c)=\frac{N^{sig}}{\epsilon_{tot}\times
N_{evt}\times \Delta p_{T}},\nonumber
\end{equation}
where $N^{sig}$ is the background-subtracted yield of direct photons,
$\epsilon_{tot}$ is the total photon reconstruction efficiency defined
as the probability that a photon would be reconstructed, identified and isolated,
$N_{evt}$ is the number of events recorded with a minimum bias 
trigger~\cite{atlas_mb_trigger} in the centrality bin
$c$ and $\Delta p_{T}$ is the width of the transverse 
momentum interval. 
%The photon yields are calculated for each $p_{T}$ and centrality interval for photons 
%measured in $|\eta| < 1.3$. 
The photon yields divided by $\langle T_{AA} \rangle$ are shown in the
left panel of Figure~\ref{fig:photon_yields} for different bins of centrality.
The results are scaled in centrality intervals, except for the 40-80\%
bin, as labeled in the figure for visibility reasons.
The systematic uncertainties are indicated by yellow bands with the
biggest contributions from the photon selection cuts
and the  isolation criterion at the level of 20\% each. Also CMS
data~\cite{cms_direct_photons} for 0-10\% central HI collisions and
 $|\eta| \!< \! 1.44$, which is a 10\% larger interval in $\eta$
comparing to ATLAS, and a tighter isolation condition $E_T(R_{iso}=0.4)<5$~GeV
are superimposed on the most central bin.
CMS $pp$ data~\cite{cms_direct_photons} from 
$\sqrt{s} = 2.76$~TeV, superimposed on the most peripheral bin, are
also shown. The two sets of measurements agree in shape and normalization.


The right panel of Figure~\ref{fig:photon_yields} shows the photon
yield divided by $\langle T_{AA} \rangle$ in each $p_T$ interval as a function of the
number of participants $\langle N_{part} \rangle$. It can be seen that
there is no significant
centrality dependence in any of the measured $p_{T}$ intervals. This
indicates that photon production rates in HI collisions scale linearly
with the mean nuclear thickness, or equivalently with the average
number of binary collisions.


\section{Summary}

In this proceedings, measurements of isolated prompt photon yields in lead-lead 
collisions are shown as a function of collision centrality
in a kinematic range of $45\!<\! p_{T}\! <\!200$~GeV and $|\eta|\!
<\! 1.3$. Photons have been reconstructed using the large-acceptance, 
longitudinally fine segmented ATLAS electromagnetic calorimeter, 
after event-by-event subtraction of the underlying event background. 
After scaling the yields by the mean nuclear thickness $\langle T_{AA}
\rangle$, photon production rates are observed to be constant as a function of centrality
within experimental uncertainties. This observation implies a linear scaling
with the number of binary collisions indicating that photons are
a useful tool for calibrating the initial momentum transfer of the
hard process, which will be important for future jet quenching measurements.

%\section*{References}

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\bibitem{wang} X. -N. Wang, Z. Huang and I. Sarcevic, 
Phys. Rev. Lett. 77, 231 (1996).
\bibitem{atlas_photons_jets} ATLAS Collaboration, ``Measurement of the correlation of jets with high $p_{T}$ isolated prompt photons in lead-lead collisions at $\sqrt{s_{NN}}$ = 2.76 TeV with the ATLAS detector at the LHC'',
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\bibitem{cms_photons_jets} CMS Collaboration, ``Studies of jet quenching using isolated-photon+jet correlations in PbPb and pp collisions at  $\sqrt{s_{NN}}$ = 2.76~TeV'',
CMS-HIN-11-010, http://arxiv.org/abs/1205.0206.
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\bibitem{atlas_direct_photons} ATLAS Collaboration, ``Measurement of
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\end{thebibliography}
\end{document}
\bye
