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\subsubsection{Minimum Bias and Underlying Event}
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The Minimum Bias (MB) at the LHC is the generic inelastic proton-proton interaction, including hard scattering and diffractive components. Studying the features of the MB at the LHC and cross checking them with the predictions of the Monte Carlo generators, is particularly important in order to describe the unavoidable background of the pile-up at high luminosity. The Underlying Event (UE), instead, is the softer component of a single proton-proton collision accompanying the hard scattering, and accounting for a large fraction of the activity in terms of multiplicity and momentum of the observed particles.
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MB and UE are not the same thing but the experimental methodology to study them are similar, mostly relying on the reconstruction of charged tracks.
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For what concerns the UE, one can use the topological structure of hadron-hadron collisions to study this activity looking only at the outgoing charged particles REFERENZA CDF.
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Jets are constructed from the charged particles using a simple clustering algorithm and then the direction of the leading charged particle jet is used to isolate regions of eta-phi space that are sensitive to the UE. The transverse region to the charged particle jet direction, is almost perpendicular to the plane of the hard 2-to-2 scattering and is therefore very sensitive to the UE.
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The Underlying Event (UE) analysis is based on the selection of the following events:
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\begin{itemize}
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\item MinimumBias
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\item Hadronic Jet
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\item Drell-Yan ($Z \leftarrow \mu \mu$)
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\end{itemize}
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For each kind of events a filter was created that defines the output stream based on the RECOSIM format. In detail the selection criteria that we have been adopted are:
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\begin{itemize}
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\item Minimum Bias: a Minimum Bias trigger strategy is not yet defined, as it is not defined higher level selection criteria for this kind of events. A random selection is applied in order to select the events without introducing any additional bias.
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\item Hadronic Jets: the selection is performed requiring at least one calorimetric jet with a Transverse momentum greater than 20 GeV/c.
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\item Drell-Yan ($Z \leftarrow \mu \mu$): the stream is defined requiring at least two muons in the central region ( $|\eta| < 2.5$ ) having a minimum transverse momentum of 3 GeV/c and invariant mass above 15 GeV/c.
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\end{itemize}
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Of course the first filter is also used in the Minimum Bias analysis.
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These three filters have been run at T1 (FNAL and CNAF) and the output was skimmed to the subscribing T2 (Pisa and Florida). The MB skim output was transferred to the Pisa T2 while the output of the Hadronic Jet sand the Drell-Yan skims were transferred to the Florida T2.
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The analysis step was performed using CRAB. Two different analyses for the MB and UE observables were run at Pisa and Florida. Starting from a total sample of $3\circ 10^{6}$ MB events, $1.2 \circ 10^{6}$ Hadronic Jets and $0.5 \circ 10^{6}$ DY, we finally performed the analysis on $2 \circ 10^{5}$ MB, $10^{5}$ QCD Jet and $10^{5}$ DY events.
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The analysis code is committed in CMSSW under the packages AnalysisExamples/MinimumBiasUnderlyingEvent. The modules using the reconstructed and generator level quantities provide directly the relevant plots of the analysis.
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In the MB analysis we are particularly interested on the event track multiplicity and
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transverse momentum distribution Figure~\ref{fig:minbias_variables}.
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\begin{figure}
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\centering
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\includegraphics[scale=0.3]{figs/dN_vs_pt_MB}
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\includegraphics[scale=0.3]{figs/dN_vs_eta_MB}
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\caption{(right) Pt distribution of reconstructed track (left) multiplicity of
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reconstructed track as function of $\eta$}
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\label{fig:minbias_variables}
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\end{figure}
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In the UE analysis for the hadronic topologies, instead, we concentrate on the observables
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in the activity in different regions with respect to the charged jets, studying in particular the energy
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density ($\delta P_{Tsum}/\delta \eta \delta \phi $) and the charge density ($\delta N_{chg}/\delta \eta \delta \phi$ ).
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Figure~\ref{fig:ue_minbias} and Figure~\ref{fig:ue_jets} report the energy and charge density against the distance with respect
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to the leading charged jets, for Minimum Bias and Hadronic Jet events respectively.
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\begin{figure}
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\centering
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\includegraphics[scale=0.3]{figs/dPt_vs_dphi_MB}
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\includegraphics[scale=0.3]{figs/dN_vs_dphi_MB}
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\caption{UE variables evaluated in Minimum Bias events, (right) energy density, (left) charge density.}
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\label{fig:ue_minbias}
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\end{figure}
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\begin{figure}
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\centering
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\includegraphics[scale=0.3]{figs/dPt_vs_dphi_Jet}
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\includegraphics[scale=0.3]{figs/dN_vs_dphi_Jet}
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\caption{UE variables evaluated in Hadronic jet events, (right) energy density, (left) charge density.}
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\label{fig:ue_jets}
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\end{figure} |
