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\begin{document}
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\bf

\begin{poster}

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\begin{minipage}[c][10cm][c]{0.76\textwidth}
  \begin{center}
    \hspace{3.0cm}\Huge {\color[rgb]{0.5,0,0} Double hump structure observed in
    azimuthal correlations in pp collision ?}
  \end{center}
  \begin{center}
    \hspace{3.0cm}\Large Alejandro Ayala, Eleazar Cuautle, Isabel
    Dom\'inguez, Antonio Ortiz, Guy Pai\'c\\
      \end{center}
  \begin{center}
    \hspace{3.0cm}\large{\textit{$^{1}$Instituto de Ciencias
        Nucleares, Universidad Nacional Auton\'oma de M\'exico, 04510
        M\'exico}}
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\vspace{0.5cm}

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\Large {\color[rgb]{0,0,0.5}Introduction}
\end{center}
}

\vspace{0.5cm}

The azimuthal correlations allow the study of important parameters
in pp collisions like the $J_{t}$ and $k_{t}$ they are important
for comparison with similar parameters in heavy ion collisions.
Considerable attention has been focused on the so called "double
hump structure" observed in the away side azimuthal correlations
\cite{star} in central Au-Au collisions, accompanied by the claim
that no such structure has been observed  in pp collisions. In the
present work we analyze the pp case using a sample of Pythia
generated minimum bias events and the Event Shape Analysis
(ESA)\cite{ortiz} to isolate specific event topologies. The
topological analysis in terms of the transverse thrust ($T$)
variable reveals that indeed, as one would expect from QCD, there
is a finite probability for three prong events were three jets are
emitted a 120 degrees from each other.

%In the present work we investigate the shape of the away-side
%correlation peak in collisions at $\sqrt{s_{NN}}=200$ GeV using an
%approach based on the Event Shape Analysis (ESA).
  We focus our attention on the following questions:

\begin{enumerate}
\item[1.-] Is it possible to identify and isolate events with
  topologies that resemble the double hump structure, using ESA in
  proton-proton collisions at 200 GeV?
\item[2.-] What is the effect of the event structure on the width of
  the away peak in the case of dijets?
 \end{enumerate}

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{
\begin{center}
\Large {\color[rgb]{0,0,0.5}Event Shape Analysis Basis}
\end{center}
}

\vspace{0.5cm}

The shape analysis consists in constructing a two dimensional
distribution with the variables: thrust  ($T$) and recoil ($R$).
By definition these variables are infrared and collinear safe
\cite{salam}. $T$ is defined as:

\begin{equation}
   T_{\perp}\equiv \underbrace{max}_{\overrightarrow{n}_T}
   \frac{\sum_{i}|\overrightarrow{q}_{\perp,i}\cdot\overrightarrow{n}_{T}|}
        {\sum_{i}|\overrightarrow{q}_{\perp,i}|},
\end{equation}

where the sum runs over all particles in the final state within
the acceptance, $\overrightarrow{q}_{\perp,i}$ is the transverse
momentum of the particle $i$; $\overrightarrow{n}_{T}$ is the
vector that maximizes the ratio. $T$ (the sphericity in the
transverse plane, $1-T$, closely related to $T$) will be equal to
1 (0) for a jet detected within the acceptance. On the other hand
a fully isotropic event will result in a sphericity of 0.5. $R$ is
defined as:

\begin{equation}
R_{\perp}\equiv \frac{|\sum_{i}\overrightarrow{q}_{\perp
    i}|}{\sum_{i}|\overrightarrow{q}_{\perp i}|},
\end{equation}
where again, the sum runs over all particles in the final state
within the acceptance. The necessity for this variable stems from
the incomplete acceptance we are faced with in the present
application. As defined above, the maximum value of the recoil
will be for a jet event where only one jet is detected within the
acceptance.

In this section, we used 500000  minimum bias events:
proton-proton collisions at 200 GeV in the c. m., such events were
simulated using the Pythia event generator version 6.3. We
restrict our analysis to the acceptance: $|\eta|\leq1$. In order
to perform the computation we demanded at least one particle with
$p_{t}\geq0.8$ GeV/c. After that the participants to get the shape
variables are primary charged particles with $p_{t}\geq0.5$ GeV.
In the figure 1 there is the so called thrust map for such sample.



We limit ourselves in the present work to the three simplest
configurations labelled A, B and C:
\begin{itemize}
{\color[rgb]{1,0,0} \large{ \item[]The low $R$ and $1-T$ region
which is known to correspond to
  dijets.}}
{\color[rgb]{0,0,1} \large{\item[]The high $R$ and low $1-T$
region which is known to correspond to
  single jets (with the second jet being out of the acceptance).}}
{\color[rgb]{0,0.8,0} \large{\item[]The high $1-T$ region which is
known to correspond to the most isotropic events.}}
\end{itemize}
%======================================================================
\begin{center}
\epsfig{file=thrustmap500mil.eps,width=0.5\linewidth}\mycaption{Thrust
map}
\end{center}
%======================================================================



In the figure 2 is shown the correlations $\Delta \phi$ vs.
$p_{t}$ obtained  for the above topologies (we performed a
rotation to place the leading particle at $\Delta \phi=\pi/2$
rad).

%======================================================================
%\epsfig{file=figshape/thrustmap.eps,width=0.5\linewidth}
%\epsfig{file=figshape/distribucionazimutalABC.eps,width=0.5\linewidth}
\begin{center}
\epsfig{file=deltaphivspt500mil.eps,width=0.7\linewidth}\mycaption{$\Delta
\phi$ vs. $p_{t}$, for the different topologies.}
\end{center}
%======================================================================

 In order to see the evolution of the away side peak, we plot in the figure 3 the $\Delta \phi$ distribution (left plot).


 In the right plot we compare the shape of the away peak obtained from the topology when only two
 jets are identify with the shape obtained for the same peak in the case where the whole sample is taken.
 One easily sees that there are notable differences in the width of the peak. This does not seem to be surprising
 because we see that the contribution of the three prong events does contribute to the width of the away peak.
 There are also other configurations present in the Thrust map that play a similar role of broadening the away peak.
 Finally, we demonstrate that the event shape analysis may be applied even at low energy  yielding the possibility to
 disentangle many specific topologies.

%======================================================================
\epsfig{file=deltaphinew500mil.eps,width=0.5\linewidth}
\epsfig{file=awaysidepeak500mil.eps,width=0.5\linewidth}
%======================================================================

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\begin{center}
\Large {\color[rgb]{0,0,0.5} Conclusions}
\end{center}
}

\vspace{0.5cm}

We have presented the Event Shape Analysis and we applied it to
proton-proton collisions at 200 GeV. We demonstrated that it is
possible to isolate and identify events with a double hump
structure in the away side of the azimuthal correlation.


%%% References
\begin{thebibliography}{9}

\bibitem{qpythia} N.~Armesto, L.~Cunqueiro and C.~A.~Salgado
{\bf arXiv:0809.4433[hep-ph] }.

\bibitem{ortiz} In simulations done at 10 TeV in the framework of ALICE where
  visualization tools are available we clearly identify the presence of three
  jets. See A.~Ortiz and G.~Paic, ALICE note, to be published.

\bibitem{star} (STAR Collaboration) {\it Phys. Rev. Lett.} 91, 072304
(2003).

\bibitem{salam}  A. Banfi, G. Salam, G. Zanderighi, JHEP {\bf 0503}
(2005). 073.

\end{thebibliography}

\vspace{-0.5cm}

\Large{Acknowledgments}

\vspace{0.5cm}

\large{CONACyT projects 52162-F, 44268-F, PAPIIT project IN107105
and
  PAEP project, as well as the HELEN project for the support}

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\end{poster}
\end{document}
