-\@writefile{lof}{\contentsline {figure}{\numberline {2}{\ignorespaces \textsc {Jewel+Pythia} results for $x_{J,\gamma }$ for the most central ($0-30\%$), peripheral ($30-100\%$) and p+p events compared to CMS\nobreakspace {}\cite {cmsypjet}. The plots are for the higher $p_{T}^{\gamma } > 80$ GeV bins. Both data and MC are normalised to unity. Since CMS results are not unfolded, we smear our jet $p_{\perp }$ with the function given in\nobreakspace {}\cite {Chatrchyan:2012gt} to match the detector resolution. Data error bars correspond to statistical errors only. The bottom panels in each of the plots show the ratio with data and the yellow bands represent the statistical uncertainty in the data.}}{3}}
-\newlabel{fig:ypjet_xjy}{{2}{3}}
\citation{atlasypjet}
\citation{atlaszpjet}
\citation{atlaszpjet}
\citation{atlaszpjet}
-\@writefile{lof}{\contentsline {figure}{\numberline {3}{\ignorespaces Average value of the $<x_{J, \gamma }>$ shown as a function of the photon's transverse momenta compared to CMS\nobreakspace {}\cite {cmsypjet} for central (top), peripheral (middle) and pp(bottom) events. \textsc {Jewel+pythia} results are shown in red lines while data are plotted with black markers. Data error bars correspond to statistical errors only.}}{4}}
-\@writefile{lof}{\contentsline {figure}{\numberline {4}{\ignorespaces JEWEL+PYTHIA results at \unit [2.76]{TeV} for the $x_{J,Z}$ for R=0.4 jets in central (top) and periperal (bottom) events compared to ATLAS\nobreakspace {}\cite {atlaszpjet}. Data error bars correspond to statistical errors only.}}{4}}
+\@writefile{lof}{\contentsline {figure}{\numberline {5}{\ignorespaces \textsc {Jewel+Pythia} results at \unit [2.76]{TeV} for $x_{JZ}$ for $R=0.4$ jets in central events compared to ATLAS\nobreakspace {}\cite {atlaszpjet}. Data error bars correspond to statistical errors only.}}{3}}
+\newlabel{fig:zpjet_atlas_r4}{{5}{3}}
\citation{cmszpjet}
-\@writefile{lof}{\contentsline {figure}{\numberline {5}{\ignorespaces \textsc {Jewel+Pythia} results at 5.02 TeV for the $x_{J,Z}$ with R=0.3 jets and $p_{T}^{Z}>60$[GeV/c] (top) and average value of $<x_{J,Z}>$ (bottom) compared to the latest results from CMS\nobreakspace {}\cite {cmszpjet}.}}{5}}
-\newlabel{fig:zpjet_cms_r3}{{5}{5}}
-\@writefile{lof}{\contentsline {figure}{\numberline {6}{\ignorespaces $R_{AA}$ comparing $Z+$Jet (blue) with $\gamma +$Jet (red) events (color online). We see a slight increase in the Z+Jet $R_{AA}$ at low jet $p_{T}$ but no difference at high $p_{T}$.}}{5}}
-\newlabel{fig:zyjet_raa}{{6}{5}}
+\newlabel{fig:ypjet_average}{{3.1}{4}}
+\@writefile{lof}{\contentsline {figure}{\numberline {3}{\ignorespaces Average value of the $\delimiter "426830A x_{J, \gamma }\delimiter "526930B $ shown as a function of the photon's transverse momenta compared to CMS\nobreakspace {}\cite {cmsypjet} p+p (left) and central Pb+Pb events (right). \textsc {Jewel+pythia} results are shown in red lines while data are plotted with black markers. Data error bars correspond to statistical errors only.}}{4}}
+\@writefile{lof}{\contentsline {figure}{\numberline {9}{\ignorespaces Top plot shows the $\Delta \phi $ distribution for R=0.4 jets with the leading leptons and generated Ws in the event. We see a good correlation with a high lepton pT threshold. Bottom plot shows the $x_{J,\mu }$ distributions with the muons and we see a shift in the PbPb to the left when compared with our pp events.}}{6}}
+\newlabel{fig:wpjet_r4}{{9}{6}}
+\@writefile{lof}{\contentsline {figure}{\numberline {8}{\ignorespaces $R_{AA}$ comparing $Z+$Jet (blue) with $\gamma +$Jet (red) events (color online). We see a slight increase in the Z+Jet $R_{AA}$ at low jet $p_{T}$ but no difference at high $p_{T}$.}}{6}}
+\newlabel{fig:zyjet_raa}{{8}{6}}
\bibcite{Casalderrey-Solana:2015vaa}{14}
\bibcite{Chien:2015hda}{15}
\bibcite{Nguyen:2010wb}{16}
\bibcite{Hamed:2008yz}{17}
\bibcite{Wang:2010yz}{18}
\bibcite{Renk:2009ur}{19}
\bibcite{Zapp:2012ak}{20}
\bibcite{Zapp:2013vla}{21}
\bibcite{Zapp:2011ya}{22}
\bibcite{Zapp:2013zya}{23}
\bibcite{Shen:2012vn}{24}
-\bibcite{Shen:privcom}{25}
+\bibcite{Shen:2014vra}{25}
\bibcite{Sjostrand:2006za}{26}
-\@writefile{lof}{\contentsline {figure}{\numberline {7}{\ignorespaces Top plot shows the $\Delta \phi $ distribution for R=0.4 jets with the leading leptons and generated Ws in the event. We see a good correlation with a high lepton pT threshold. Bottom plot shows the $x_{J,\mu }$ distributions with the muons and we see a shift in the PbPb to the left when compared with our pp events.}}{6}}
%about the article that should go on the front page should be
%placed here. General acknowledgments should be placed at the end of the article.
\thankstext{e1}{e-mail: raghav.k.e@cern.ch}
\thankstext{e2}{e-mail: korinna.zapp@cern.ch}
%\authorrunning{Short form of author list} % if too long for running head
\institute{Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA \label{addr1}
\and CENTRA, Instituto Superior T\'ecnico, Universidade de Lisboa, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal \label{addr2}
- \and
- Physics Department, Theory Unit, CERN, CH-1211 Gen\`eve 23, Switzerland \label{addr3}
+ \and
+ Laborat\'{o}rio de Instrumenta\c{c}\~{a}o e F\'{\i}sica Experimental de Part\'{\i}culas (LIP), Av. Elias Garcia 14-1,
+1000-149 Lisboa, Portugal\label{addr3}
+ \and
+ Theory Department, CERN, CH-1211 Gen\`eve 23, Switzerland \label{addr4}
}
\date{Received: date / Accepted: date}
% The correct dates will be entered by the editor
\maketitle
\begin{abstract}
Studies of jet quenching at the LHC have thrown light upon several interesting characteristics of the QGP. We look at the special class of events which have a photon or vector boson scattering of a quark or gluon leading to the observable of a jet with the reconstructed boson in the final state. These bosons interact only via the electro-weak force and hence leave the strongly interacting plasma uninhibited. We calculate the transverse momentum imbalance ($x_{J,V}$) of the recoiled jet to that of the boson in the event. These results are compared with CMS and ATLAS measurements at $2.76$ and $5.02$ TeV.
During Run I of the LHC, the modifications of jets due to re-scattering in the dense medium created in heavy ion collisions have been studied mostly in single-inclusive jet observables and di-jet events. They are dominated by the pure QCD production processes, which have by far the largest cross sections. However, in these events it is practically impossible to determine the hard scattering kinematics, as all jets undergo quenching in the medium. This is different in $V$+jet processes, where a hard jet recoils against an electroweak gauge boson. The bosons -- and in the cases of $Z$ and $W$ production the leptonic decay products -- do not interact strongly and thus escape unmodified from the medium. This has been confirmed by measurements of inclusive vector boson production in Pb+Pb collisions at the LHC~\cite{Aad:2015lcb,Aad:2012ew,Aad:2014bha,Chatrchyan:2012vq,Chatrchyan:2014csa,Chatrchyan:2012nt}, which show that the observed rates are consistent with binary scaling and nuclear PDFs. The boson thus allows us to experimentally access the hard scattering kinematics. However, due to QCD corrections, in particular initial state radiation, the boson's and the parton's transverse momentum do not match exactly and the $\pt$ ratio fluctuates considerably from one event to another (cf. Fig.~\ref{fig:ypjet_xjy}). Nevertheless, since the initial parton $\pt$ is known on average, boson-jet processes provide valuable information that is complementary to the pure QCD processes. First measurements~\cite{Chatrchyan:2012gt,atlasypjet,atlaszpjet,cmsypjet,cmszpjet} are still limited by statistics, but this will improve in future LHC running. There have also been attempts to study $\gamma$-hadron correlations at RHIC~\cite{Nguyen:2010wb,Hamed:2008yz}, but these are much more sensitive to poorly constrained hadronization effects than jets.
The theoretical description of jet quenching in boson-jet events is the same as in pure jet events, in some approaches boson-jet~\cite{Wang:1996pe,Casalderrey-Solana:2015vaa,Chien:2015hda} or $\gamma$-hadron~\cite{Wang:2010yz,Renk:2009ur,Zhang:2009fg} observables have been discussed specifically. We here present \textsc{Jewel}\,2.1.0\footnote{The code is available at \texttt{http://jewel.hepforge.org}.}, which is an extension of \textsc{Jewel}\,2.0.2 to boson-jet processes. After a summary of the new features, we compare \textsc{Jewel} to boson-jet data from LHC Run I and II.
\section{Simulating V+jet processes with JEWEL}
\label{sec:jewel}
\textsc{Jewel}~\cite{Zapp:2012ak} is a fully dynamical perturbative framework for jet quenching. It describes the simultaneous scale evolution of hard partons giving rise to jets and res-scattering in the medium. Partons belonging to the jet undergo collisions with partons from the thermal background. These interactions are described by $2\to 2$ matrix elements and parton showers and can thus be elastic or inelastic, where the two types of interactions occur with the (leading log) correct relative rates. Whenever there are competing sources of radiation, the emission with the shorter formation time is realised. This means that only sufficiently hard re-scatterings can perturb the scale evolution of the hard jet production process and induce additional radiation. In this way hard structures inside jets remain unchanged. All scattering processes within the formation time of a medium-induced emission act coherently, which means that only the vectorial sum of the momentum transfers matters for the gluon emission. This is the QCD analogue of the Landau-Pomerantchuk-Migdal effect, which is also implemented in \textsc{Jewel}~\cite{Zapp:2011ya}.
For jet evolution in vacuum \textsc{Jewel} reduces to a standard virtuality ordered final state parton shower. Initial state parton showers, hard jet production matrix elements, hadronization and hadron decays are generated by \textsc{Pythia}\,6.4~\cite{Sjostrand:2006za}.
\smallskip
In the new version we have included processes with a jet recoiling against a
vector boson. The corresponding diagrams are shown in
Fig.~\ref{fig:production}. These correspond to either a quark scattering off a
gluon (Compton scattering) or a quark anti-quark pair annihilating to produce a
boson and a gluon. For photons, the box diagram $gg\to \gamma g$ is also
included. The leptonic decays of the heavy boson $Z$ and $W$ are simulated as
well.
\begin{figure}[h!] % figure placement: here, top, bottom, or page
\caption{Feynman diagrams for V+Jet processes included in \textsc{Jewel} 2.1.0}
\label{fig:production}
\end{figure}
Hard photons can also be radiated off quarks during jet evolution. These
fragmentation photons are typically accompanied by hadronic activity and are
suppressed by requiring the photon to be isolated. However, it is still
possible that fragmentation photons pass the isolation criterion. The probility
for this to happen obviously is small and depends on the cuts. It has to the
best of our knowledge not been quantified in a heavy ion environment in he
presence of jet quenching. In the current \textsc{Jewel} version the
fragmentation photons are also not included as with the cuts used in the
present analyses the fragmentation component is expected to be small.
\subsection{The new parameters and Switches}
We have expanded the parameter set listed in~\cite{Zapp:2013vla} as follows
(default values are given in parantheses).
\begin{description}
\item[PROCESS (`PPJJ'):] process that is to be simulated by matrix element,
available options are
\begin{description}
\item[`EEJJ':] di-jet production in $e^+$+$e^-$ collisions
\item[`PPJJ':] di-jet production in p+p collisions
\item[`PPYJ':] all $\gamma$+jet processes in p+p
\item[`PPYQ':] only $\gamma$+quark production in p+p
\item[`PPYG':] only $\gamma$+gluon production in p+p
\item[`PPZJ':] all $Z$+jet processes in p+p
\item[`PPZQ':] only $Z$+quark production in p+p
\item[`PPZG':] only $Z$+gluon production in p+p
\item[`PPWJ':] all $W^\pm$+jet processes in p+p
\item[`PPWQ':] only $W^\pm$+quark production in p+p
\item[`PPWG':] only $W^\pm$+gluon production in p+p
\end{description}
Note that hard jet production processes in p+p collisions form the basis also
for A+A simulations.
\item[CHANNEL (`MUON'):] decay channel for the heavy $W$ and $Z$ bosons,
available are `ELEC' and `MUON' for the decay to electrons/positrons and muons,
respectively
\end{description}
\section{Comparisons to data}
-We generate events in the standard setup~\cite{Zapp:2013vla} at $\sqrt{s_\text{NN}} = \unit[2.76]{TeV}$ and $\sqrt{s_\text{NN}} = \unit[5.02]{TeV}$ with the simple parametrisation of the background discussed in detail in~\cite{Zapp:2013zya}. The initial conditions for the background model are initial time $\tau_\text{i}=\unit[0.6]{fm}$ and temperature $T_\text{i}=\unit[485]{MeV}$ for $\sqrt{s_\text{NN}} = \unit[2.76]{TeV}$~\cite{Shen:2012vn} and $\tau_\text{i}=\unit[0.4]{fm}$ and $T_\text{i}=\unit[590]{MeV}$ for $\sqrt{s_\text{NN}} = \unit[5.02]{TeV}$~\cite{Shen:privcom}. The proton PDF set is \textsc{Cteq6LL}~\cite{Pumplin:2002vw} and for the Pb+Pb sample the \textsc{Eps09}~\cite{Eskola:2009uj} nuclear PDF set is used in addition, both are provided by \textsc{Lhapdf}~\cite{Whalley:2005nh}.
+We generate events in the standard setup~\cite{Zapp:2013vla} at $\sqrt{s_\text{NN}} = \unit[2.76]{TeV}$ and $\sqrt{s_\text{NN}} = \unit[5.02]{TeV}$ with the simple parametrisation of the background discussed in detail in~\cite{Zapp:2013zya}. The initial conditions for the background model are initial time $\tau_\text{i}=\unit[0.6]{fm}$ and temperature $T_\text{i}=\unit[485]{MeV}$ for $\sqrt{s_\text{NN}} = \unit[2.76]{TeV}$~\cite{Shen:2012vn} and $\tau_\text{i}=\unit[0.4]{fm}$ and $T_\text{i}=\unit[590]{MeV}$ for $\sqrt{s_\text{NN}} = \unit[5.02]{TeV}$~\cite{Shen:2014vra}. The proton PDF set is \textsc{Cteq6LL}~\cite{Pumplin:2002vw} and for the Pb+Pb sample the \textsc{Eps09}~\cite{Eskola:2009uj} nuclear PDF set is used in addition, both are provided by \textsc{Lhapdf}~\cite{Whalley:2005nh}.
We use the \textsc{Rivet} analysis framework~\cite{Buckley:2010ar} for all our
studies. Jets are reconstructed using the same jet algorithm as the experiments
(anti-$k_\perp$~\cite{Cacciari:2008gp}) from the \textsc{FastJet}
+ \caption{\textsc{Jewel+Pythia} results at \unit[2.76]{TeV} for $x_{JZ}$ for $R=0.4$
+jets in central events compared to
ATLAS~\cite{atlaszpjet}. Data error bars correspond to statistical errors only.}
\label{fig:zpjet_atlas_r4}
\end{figure}
In the case of the Z and W, we utilize the muon decay channel in our simulations due to it being experimentally very clean. The Z candidate's momentum is reconstructed from the dimuon pairs and we require its mass in the window $70 < M_{Z} < 110$ [GeV/$c^2$] and $\pt>60$ [GeV/c]. The Jets are reconstructed with the same anti $k_{T}$ algorithm with resolution parameter $R=0.4$, with the kinematic cut on its $\pt>25$ [GeV/c] and it is required to be found in the barrel region $|\eta|<2.1$. Similar to the $\gamma+$Jets, we impose a $\Delta \phi_{Z, Jets} > 7\pi/8$ to select the back to back recoiled pairs.
Fig:~\ref{fig:zpjet_atlas_r4} shows the ATLAS~\cite{atlaszpjet} preliminary result for the $\pt$ imbalance in the black circle markers, our vacuum simulation in \textsc{Jewel+Pythia} in blue dotted lines and most central (0-20\%) events in the red line. As before, the uncertainty shown in the data are statistical.
- \caption{\textsc{Jewel+Pythia} results at 5.02 TeV for the $x_{J,Z}$ with R=0.3 jets and $p_{T}^{Z}>60$[GeV/c] (top) and average value of $<x_{J,Z}>$ (bottom) compared to the latest results from CMS~\cite{cmszpjet}.}
+% \caption{\textsc{Jewel+Pythia} results at 5.02 TeV for the $x_{J,Z}$ with R=0.3 jets and $p_{T}^{Z}>60$[GeV/c] (top) and average value of $<x_{J,Z}>$ (bottom) compared to the latest results from CMS~\cite{cmszpjet}.}
+ \caption{...}%\textsc{Jewel+Pythia} results at 5.02 TeV for the $x_{J,Z}$ with R=0.3 jets and $p_{T}^{Z}>60$[GeV/c] (top) and average value of $<x_{J,Z}>$ (bottom) compared to the latest results from CMS~\cite{cmszpjet}.}
+ \caption{...}%\textsc{Jewel+Pythia} results at 5.02 TeV for the $x_{J,Z}$ with R=0.3 jets and $p_{T}^{Z}>60$[GeV/c] (top) and average value of $<x_{J,Z}>$ (bottom) compared to the latest results from CMS~\cite{cmszpjet}.}
+ \label{fig:zpjet_cms_avcjZ}
+\end{figure*}
Fig:~\ref{fig:zpjet_cms_r3} shows the latest CMS Z+Jet~\cite{cmszpjet} result at $\sqrt{s_{NN}}=5.02$ TeV with R=0.3 jets and $p^{Z}_{T}>60$ [GeV/c] for central events ($0-30\%$). The CMS jets are not unfolded so we perform a smearing based on the jet resolution in the respective kinematic and event selection bins.
It is also informative to look at the nuclear modification factors ($R_{AA}$) of jets in the events recoiling off a $\gamma,Z$. Due to the large mass of the $Z$ boson, the jet spectra in JEWEL is harder when compared with those jets recoiling off a $\gamma$. This influences the $R_{AA}$ for $Z+$jets to be less suppressed at the low pT range as shown in Fig:~\ref{fig:zyjet_raa}.
\caption{$R_{AA}$ comparing $Z+$Jet (blue) with $\gamma+$Jet (red) events (color online). We see a slight increase in the Z+Jet $R_{AA}$ at low jet $p_{T}$ but no difference at high $p_{T}$.}
\label{fig:zyjet_raa}
\end{figure}
Reconstructing a W boson candidate in the heavy ion environment is poor due to the ambiguous nature of the missing ET in the event. Due to in-medium energy loss, the MET in such events do not accurately represent the neutrino, required to get the W. Top of Fig:~\ref{fig:wpjet_r4} shows the $\Delta \phi$ distributions for the reconstructed jets (anti $k_{T}$, R=0.4) with the generator level $W^{\pm}$ in the red line and with the leading lepton in the event ($\mu$) in the blue dotted line (color online). These distributions are plotted with a significantly high lepton transverse momentum cut off $p^{\mu}_{T}>60$ [GeV/c] to select only events where the lepton decay product carries a larger fraction of the $W^{\pm}$'s transverse momentum. We also impost a $\Delta R_{J, \mu}>0.6$ to ensure no overlap between our reconstructed jet and lepton collections. We see the $\Delta \phi$ distribution are similar for the $W^{\pm}$ and leading lepton and we construct the transverse momentum imbalance for the jets with the leptons (after applying a veto on Z events). This is shown in the bottom of Fig:~\ref{fig:wpjet_r4} with PbPb (red solid curves) showing a clear shift when compared with pp (blue dotted curves).
\caption{Top plot shows the $\Delta \phi$ distribution for R=0.4 jets with the leading leptons and generated Ws in the event. We see a good correlation with a high lepton pT threshold. Bottom plot shows the $x_{J,\mu}$ distributions with the muons and we see a shift in the PbPb to the left when compared with our pp events.}
\label{fig:wpjet_r4}
-\end{figure}
+\end{figure*}
\section{Conclusions}
We announce the release of JEWEL 2.1.0 with the additional capability of simulating V+Jets events. Upon comparing with LHC Run1 and Run2 data we find good agreement for $\gamma/Z+$Jet events. This showcases confidence in the jet quenching algorithm implemented in JEWEL and its usability for performing predictions of hard scattered jet observables and for comparisons with data.
\begin{acknowledgements}
-This work was supported by Funda\c{c}\~{a}o para a Ci\^{e}ncia e a Tecnologia (Portugal) under postdoctoral fellowship SFRH/BPD/102844/2014 (KCZ) and by the European Union as part of the FP7 Marie Curie Initial Training Network MCnetITN (PITN-GA-2012-315877) (RKE).
+This work was supported by Funda\c{c}\~{a}o para a Ci\^{e}ncia e a Tecnologia (Portugal) under project CERN/FIS-NUC/0049/2015 and postdoctoral fellowship SFRH/BPD/102844/2014 (KCZ) and by the European Union as part of the FP7 Marie Curie Initial Training Network MCnetITN (PITN-GA-2012-315877) (RKE).
We also like to thank Dr. Chun Shen for providing the initial hydrodynamics parameters for our event generation at 5 TeV.
%\bibliographystyle{spmpsci} % mathematics and physical sciences
%\bibliographystyle{spphys} % APS-like style for physics
%\bibliography{V+jet.bib} % name your BibTeX data base
% Non-BibTeX users please use
\begin{thebibliography}{}
%\cite{Aad:2015lcb}
\bibitem{Aad:2015lcb}
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Phys.\ Rev.\ C {\bf 93} (2016) no.3, 034914
doi:10.1103/PhysRevC.93.034914
[arXiv:1506.08552 [hep-ex]].
%%CITATION = doi:10.1103/PhysRevC.93.034914;%%
%11 citations counted in INSPIRE as of 31 May 2016
%\cite{Aad:2012ew}
\bibitem{Aad:2012ew}
G.~Aad {\it et al.} [ATLAS Collaboration],
%``Measurement of $Z$ boson Production in Pb+Pb Collisions at $\sqrt{s_{NN}}=2.76$ TeV with the ATLAS Detector,''
%60 citations counted in INSPIRE as of 31 May 2016
%\cite{Aad:2014bha}
\bibitem{Aad:2014bha}
G.~Aad {\it et al.} [ATLAS Collaboration],
%``Measurement of the production and lepton charge asymmetry of $W$ bosons in Pb+Pb collisions at $\mathbf {\sqrt{\mathbf {s}_{\mathrm {\mathbf {NN}}}}=2.76\;TeV}$ with the ATLAS detector,''