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\subsubsection{Minimum Bias and Underlying Event}
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Minimum Bias (MB) collisions at the LHC are generic inelastic
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proton-proton interactions, including hard scattering and diffractive
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components. Studying the features of the MB at the LHC and cross-checking them with the predictions of the Monte Carlo generators is
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particularly important in order to describe the unavoidable background
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of pile-up at high luminosity. The Underlying Event (UE), instead,
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is the softer component of a single proton-proton collision
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accompanying the hard scattering, and accounting for a large fraction
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of the activity in terms of multiplicity and momentum of the observed
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particles.
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The phenomenology of MB and UE is not the same, but the experimental
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methodology to study them are similar, mostly relying on the
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reconstruction of charged tracks.
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For what concerns the UE, one can use the topological structure of
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hadron-hadron collisions to study this activity looking only at the
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outgoing charged particles.
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% REFERENZA CDF.
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Jets are constructed from charged particles using a simple
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clustering algorithm and then the direction of the leading charged
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particle jet is used to isolate regions of eta-phi space that are
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sensitive to the UE. The transverse region to the charged particle jet
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direction, is almost perpendicular to the plane of the hard 2-to-2
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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 ($Z \rightarrow \mu \mu$)
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\end{itemize}
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For each kind of events a filter was created that defines the output
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stream based on the RECOSIM format. In detail the selection criteria
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that 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 ($Z \rightarrow \mu \mu$): 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 Tier-1 (FNAL and CNAF) and the output was skimmed to the subscribing Tier-2 (Pisa and Florida). The MB skim output was transferred to the Pisa Tier-2 while the output of the Hadronic Jet sand the Drell-Yan skims were transferred to the Florida Tier-2.
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The analysis step was performed using CRAB. Two different analyses for
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the MB and UE observables were run at Pisa and Florida. Starting from
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a total sample of $3\times 10^{6}$ MB events, $1.2 \times 10^{6}$
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Hadronic Jets and $0.5 \times 10^{6}$ DY, we finally performed the
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analysis on $2 \times 10^{5}$ MB, $10^{5}$ QCD Jet and $10^{5}$ DY
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events.
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The analysis code is committed in CMSSW under the packages
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AnalysisExamples/MinimumBiasUnderlyingEvent. The modules using the
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reconstructed and generator level quantities provide directly the
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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} |
