%
% paper.tex, <first name, patronymic name, and last name of the corresponding author>, e-mail
%
\documentclass[
%aps,%
fleqn
12pt,%
final,%
notitlepage,%
oneside,%
onecolumn,%
nobibnotes,%
nofootinbib,% In the current version of REVTeX, when this option is on,
%             footnotes in tables do not work.
superscriptaddress,%
noshowpacs,%
centertags]%
{revtex4}
%\usepackage[dvips]{graphicx} 
%\usepackage{vmargin} 
%\usepackage{epsfig}
%\usepackage[T1]{fontenc} 
 

\begin{document}
%\selectlanguage{russian} % For a paper in Russian
\selectlanguage{english} % For a paper in English
%
\title{Jet reconstruction performance in the ALICE experiment}
%
\author{\firstname{Magali} \surname{Estienne} (for the ALICE Collaboration)}
%\email[]{magali.estienne@ires.in2p3.fr}
%\homepage[]{Your web page}
%\thanks{}
%\altaffiliation{(for the ALICE Collaboration)}
%\affiliation{Institut Pluridisciplinaire Hubert Curien, \\
%23 rue du Loess, BP 28, 67037 Strasbourg - FRANCE}
\affiliation{magali.estienne@ires.in2p3.fr}
%
%\author{\firstname{...} \surname{...}}
%\email[]{Your e-mail address}
%\homepage[]{Your web page}
%\thanks{}
%\altaffiliation{}
%\noaffiliation
%
\begin{abstract}
The study of jets in the ALICE experiment is motivated. Some of their
reconstruction performances in p+p and Pb+Pb collisions are
summarized.
\end{abstract}
%
\maketitle
%
%\section{Answer to question}

Under extreme conditions of pressure or temperature, nuclear matter is
expected to undergo a phase transition to a deconfined medium, the
quark gluon plasma (QGP). Such an atypic state of matter can be
defined as a very high density medium in which free partons strongly
interact. Experimentally, it is possible to re-create the conditions
necessary to its formation by colliding heavy ions at
ultra-relativistic velocity. It is part of the experimental program
which will be performed at the Large Hadron Collider at CERN. ALICE,
one of the four LHC experiments, is dedicated to the study of the QGP
properties.

In QCD, jets are defined as cascades of consecutive emissions of
partons initiated by partons from an initial hard scattering. This
process is followed by the fragmentation of partons into clusters of
particles emitted in some opposite collimated directions. In heavy ion
collisions (HIC), the scene of parton fragmentation is modified from
vacuum to a dense medium. The hard process first occurs, and after
comes the QGP formation. Thus, the partons in the jet will first
travel through a dense color medium. Before to fragment, they are
expected to lose energy through collisional energy loss and medium
induced gluon radiation, dominant process in a QGP, also called "jet
quenching"~\cite{1}. Direct consequences of this quenching effect are
a modification of the jet shape as well as of the parton fragmentation
fonction associated~\cite{2}. The ALICE experiment is planning to
measure these modifications thanks to its excellent tracking
capabilities down to momenta of order of $\Lambda_{QCD}$ and up to 100
GeV/c and thanks to its large PID capabilities~\cite{3}.

It has hereby been necessary to study the capabilities of the ALICE
apparatus to reconstruct such objects in the high multiplicity
environment of HIC. Different types of jet finding (cone and k$_T$)
are under study for both p+p and Pb+Pb collisions~\cite{4,5}. As
regards the heavy ion environment, because of the really large
background in a cone of radius ($R = \sqrt{\Delta \eta^{2}+\Delta
\phi^{2}}$) equal to unity, one will have to deal with at the LHC
energies, as well as because of its large fluctuations, some studies
have been performed in order, first, to reduce their contributions in
each event, and then to subtract their remaining contributions event
by event in the process of jet finding. For the former, HIJING
simulations have shown that the application of a p$_T$ cut of 1 or 2
GeV/c on charged particles and the reduction of the jet cone size to R
= 0.3-0.4 have to be applied. As regards the later, three methods of
subtraction (statistic, cone and ratio) have been tested and
compared~\cite{3,6}.

With one of the jet finders which have been studied (iterative jet cone
finder based on the UA1 cone algorithm) a mean cone energy
($E_{mean}$) of 45 Gev and a resolution of $\sim $40\% ($RMS/E_{mean}$)
using only charged particles in the reconstruction are obtained for
the reconstruction of 100 GeV mono-energetic jets in p+p collisions
from full detector simulation. The inclusion of neutral particles
measured in the ALICE calorimeter~\cite{7} allows to increase
$E_{mean}$ to 70 GeV and improves the resolution up to $\sim$ 30\% with a
cone radius of R = 0.4. A deeper study of background subtraction in
Pb+Pb collisions has shown that the jet reconstruction algorithms
bias the mean background energy contained in the jet cone toward
values higher than the ones found in the same area outside the
jet. This bias leads to an over estimation of the reconstructed
energy~\cite{8}. We obtain a preliminary $E_{mean}$ of 75 GeV and a
resolution of 30-35\% for the case charged + neutral particles for 100
GeV mono-energetic jets in Minbias Pb+Pb collisions.

The full jet spectrum has also been simulated and studied in p+p and Pb+Pb
collisions. Due to the steeply falling jet production spectrum and the
background and signal fluctuations of the reconstructed jet energy, we
observe a smearing of the reconstructed spectra in p+p. This effect is
increased in Pb+Pb due to the background fluctuations and its bias in
the jet finding process.

Jets clearly present some advantages to study the jet quenching effect
in Pb+Pb collisions at LHC compared to the physics of the leading
particles which is trigger and surface biased. With its reconstruction
capabilities, ALICE will be able to reconstruct jets with enough
resolution for Pb+Pb studies eventhough a smearing of the
reconstructed jet spectrum is observed, due to the steeply falling
shape of the input spectrum. This effect will have to be well under
control, both in elementary and Pb+Pb collisions.

\begin{thebibliography}{99}

\bibitem{1}
C. A. Salgado, these proceedings (2008).

\bibitem{2}
N. Borghini \& U. Wiedemann, arXiv:hep-ph/0506218.

\bibitem{3}
ALICE Collaboration, ALICE Physics Performance Report (PPR), Volume II,
J. Phys. G {\bf 32}, 1295 (2006).

\bibitem{4}
G. Arnison {\it et al.} (UA1 Collaboration), Phys. Lett. B {\bf 132}, 214 (1983).

\bibitem{5}
G. Salam \& M. Cacciari, Phys. Lett. B {\bf 641}, 57-61 (2006).

\bibitem{6}
S.L. Blyth {\it et al.}, arXiv:nucl-ex/0609023.

\bibitem{7}
ALICE Collaboration, Electromagnetic Calorimeter, Addendum to the Technical Proposal, CERN-LHCC-2006-014 (2006).

\bibitem{8}
A. Abrahantes, R. Diaz, M. Lopez Noriega, E. Lopez \& A. Morsch, ALICE-INT-2008-005.

\end{thebibliography}
%
\end{document}
