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\Title{Measurement of the $Z/\gamma^*$ transverse momentum spectrum
  with ATLAS}

\bigskip\bigskip

%+\addtocontents{toc}{{\it Klemens M\"uller (On behalf of the ATLAS collaboration)}}
%+\label{MullerStart}

\begin{raggedright}  

{\it Klemens M\"uller, On behalf of the ATLAS
  collaboration\index{M\"uller, K. (On behalf of the ATLAS collaboration)}\\
University of Bonn, Physikalisches Institut\\
D-53115 Bonn, GERMANY}
\bigskip\bigskip
\end{raggedright}



A measurement of the $Z/\gamma^*$ transverse momentum ($p_{\rm{}T}^Z$)
in $\sqrt{s}=7\mathrm{\ Te\kern -0.1em V}$ proton-proton collisions at
the LHC is presented. The measurement extends the range in
$p_{\rm{}T}^Z$ up to $350\mathrm{\ Ge\kern -0.1em V}$ using $Z
\rightarrow e^+e^-$ and $Z \rightarrow \mu^+\mu^-$ decays collected
with the ATLAS detector in data sets of $35\,\rm{}pb^{-1}$ and
$40\,\rm{}pb^{-1}$, respectively.

\section{Introduction}
The study of the Z boson\index{Z boson} transverse momentum, relative to the beam axis,
 is sensitive to initial state quark and gluon radiation. 
It allows to test QCD\index{QCD} predictions and phenomenological models. 
In addition the description of vector boson transverse momentum is crucial
for measurement of the W mass.
The approach to calculate the differential cross section in $p_{\rm{}T}^Z$
depends on the energy scale, given by $p_{\rm{}T}^Z$. 
In the region of large $p_{\rm{}T}^Z \sim M_{Z}$, perturbative QCD calculations are expected to
 be reliable; however, these fixed order predictions diverge for
 vanishing $p_{\rm{}T}^Z$. In the region of $p_{\rm{}T}^Z \ll M_{Z}$, the leading
 contribution to the cross section comes from multiple soft gluon
 emissions, which is treated by resummation\index{resummation} of leading logarithms up
 to all orders in $\alpha_s$.
An alternate way to describe the low $p_{\rm{}T}^Z$ region is given by parton
shower generators\index{generators}. 

\section{Selection of $Z \rightarrow \ell^+\ell^-$ Candidates}
The measurement uses $Z \rightarrow e^+e^-$ and $Z \rightarrow \mu^+\mu^-$ decays in pp collisions at
$\sqrt{s} = 7\mathrm{\ Te\kern -0.1em V}$ collected with the ATLAS
detector in data  sets
with integrated luminosity of $35\,\rm{}pb^{-1}$ and $40\,\rm{}pb^{-1}$~\cite{Aad:2011gj}, respectively. Events were
triggered requiring the presence of a single electron/muon with
$p_{\rm{}T}>15/18\mathrm{\ Ge\kern -0.1em V}$.  A preselection of events required at least one
primary vertex to be reconstructed using at least 3 tracks. 
Z decays were selected
by choosing events which contained a pair of oppositely charged
electrons or muons with an invariant mass of
$66<m_{ll}<116\mathrm{\ Ge\kern -0.1em V}$. A common fiducial acceptance was chosen for electrons and muons
given by $p_T>20\mathrm{\ Ge\kern -0.1em V}$ and $|\eta|<2.4$.
Electron identification was based upon the shape of electromagnetic showers in
the calorimeter and track quality criteria.
Muons were reconstructed by identifying matching tracks from the inner detector and
the muon spectrometer and combining the measurements. Muon tracks were
required to originate from the primary
vertex and to be isolated to suppress background from
heavy flavour production. 
The selection yielded 8923 $Z \rightarrow e^+e^-$ and
15060 $Z \rightarrow \mu^+\mu^-$ candidate events~\cite{Aad:2011gj}. 


\section{Results and Conclusions}
The differential cross section in $p_{\rm{}T}^Z$ was measured in the $ee$ and
$\mu\mu$ channels up to $350\mathrm{\ Ge\kern -0.1em V}$. 
The observed data were corrected for detector effects and QED final
state radiation using correction factors per $p_{\rm{}T}^Z$ bin which were
determined from \textsc{Pythia} signal MC. 
The differential cross section was normalized to the inclusive cross
section  in the fiducial volume, cancelling  the luminosity
uncertainty and uncertainties on total signal efficiencies. 
The $Z \rightarrow e^+e^-$ and $Z \rightarrow \mu^+\mu^-$ channels
were combined with $\chi^2$ minimization, taking into account
correlated systematic errors, giving excellent agreement, with
$\chi^2/\mathrm{dof}=17.0/19$~\cite{Aad:2011gj}.


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\caption{Ratios of the combined data and various predictions over the \textsc{Resbos}
prediction for the normalized differential cross section as a function
of $p_{\rm{}T}^Z$. Left: \textsc{Fewz} predictions at $\mathcal{O}(\alpha_s)$ and $\mathcal{O}(\alpha_s^2)$;
right: predictions from the generators \textsc{Pythia}, \textsc{Mc$@$nlo},
\textsc{Powheg}, \textsc{Alpgen}, and \textsc{Sherpa}. 
The \textsc{Fewz} predictions are shown with combined scale,
$\alpha_s$, and PDF uncertainties. The data points are shown with
combined statistical and systematic uncertainty. At low $p_{\rm{}T}^Z$ the
$\mathcal{O}(\alpha_s)$ and $\mathcal{O}(\alpha_s^2)$ predictions of
\textsc{Fewz} diverge and are omitted.} %~\cite{Aad:2011gj}}
\label{fig:comparison}
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The observed normalized differential cross section was compared with
the various types of predictions. The ratio of the data and the
predictions over the \textsc{Resbos} prediction is shown in Fig.~\ref{fig:comparison}.
Fixed order perturbative QCD predictions, as provided by \textsc{Fewz} at
$\mathcal{O}(\alpha_s)$ and $\mathcal{O}(\alpha_s^2)$, describe the
data for $p_{\rm{}T}^Z > 18\mathrm{\ Ge\kern -0.1em V}$ within the scale uncertainty. 
Resummation of leading logarithms matched to fixed order pQCD
calculations as implemented in \textsc{Resbos} give good agreement over
the entire $p_{\rm{}T}^Z$ range. This shows the importance of resummation at
relatively high $p_{\rm{}T}^Z$. 
Various event generators are available that use fixed order matrix
element calculations for the hard process matched with parton showers. 
\textsc{Alpgen} and \textsc{Sherpa} implement tree level matrix
elements for Z boson production  in association with up to 5 hard
partons, matched to  parton showers. Both give good description of the
data over the entire spectrum.
The generators \textsc{Mc$@$nlo} and \textsc{Powheg} deviate from data at low
and high $p_{\rm{}T}^Z$, while Pythia describes well the measurement over the
entire range.  


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%\cite{Aad:2011gj}
\bibitem{Aad:2011gj}
  The ATLAS Collaboration,
  ``Measurement of the transverse momentum distribution of $Z/\gamma^*$ bosons in proton-proton collisions at $\sqrt{s}=7 \mathrm{\ Te\kern -0.1em V}$ with the ATLAS detector'', 
 Phys.\ Lett.\ B\ {\bf 705} 415-434 (2011)
% [arXiv:1107.2381 [hep-ex]].


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