Eur. Phys. J. C (2016) 76:695
DOI 10.1140/epjc/s10052-016-4534-6
Regular Article - Theoretical Physics
Simulating V+jet processes in heavy ion collisions with JEWEL
Raghav Kunnawalkam Elayavalli
1,4,a
, Korinna Christine Zapp
2,3,4,b
1
Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854, USA
2
CENTRA, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
3
Laboratório de Instrumentação e Física Experimental de Partículas (LIP), Av. Elias Garcia 14-1, 1000-149 Lisboa, Portugal
4
Theoretical Physics Department, CERN, 1211 Geneve 23, Switzerland
Received: 19 August 2016 / Accepted: 23 November 2016 / Published online: 18 December 2016
© The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract Processes in which a jet recoils against an elec-
troweak boson complement studies of jet quenching in heavy
ion collisions at the LHC. As the boson does not interact
strongly it escapes the dense medium unmodified and thus
provides a more direct access to the hard scattering kinemat-
ics than can be obtained in di-jet events. First measurements
of jet modification in these processes are now available from
the LHC experiments and will improve greatly with better
statistics in the future. We present an extension of Jewel to
boson–jet processes. Jewel is a dynamical framework for jet
evolution in a dense background based on perturbative QCD
that is in agreement with a large variety of jet observables.
We also obtain a good description of the CMS and ATLAS
data for γ +jet and Z +jet processes at 2.76 and 5.02 TeV.
1 Introduction
During Run I of the LHC, the modifications of jets due to
re-scattering in the dense medium created in heavy ion colli-
sions have been studied mostly in single-inclusive jet observ-
ables and di-jet events. They are dominated by pure QCD
production processes, which have by far the largest cross
sections. However, in these events it is practically impossi-
ble 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 pro-
duction 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 pro-
duction in Pb + Pb collisions at the LHC [1–6], which show
that the observed rates are consistent with binary scaling
and nuclear PDFs. The boson thus allows us to experimen-
a
e-mail: raghav.k.e@cern.ch
b
e-mail: korinna.zapp@cern.ch
tally 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 p
⊥
ratio fluctuates considerably from one
event to another (cf. Fig. 2). Nevertheless, since the initial
parton p
⊥
is known on average, boson–jet processes provide
valuable information that is complementary to pure QCD
processes. First measurements [7–11] are still limited by
statistics, but this will improve in future LHC running. There
have also been attempts to study γ –hadron correlations at
RHIC [12,13], but these are much more sensitive to poorly
constrained hadronisation effects as opposed to jets.
The theoretical description of jet quenching in boson–jet
events is the same as in pure jet events, in some approaches
boson–jet [14–21]orγ –hadron [22–24] observables have
been discussed specifically. Jet quenching calculations still
struggle to describe all jet quenching observables at the same
time. Boson+jet processes provide an important test for the
predictions of jet quenching frameworks that have already
been constrained on other jet quenching data.
We here present an extension of Jewel to boson–jet pro-
cesses.
1
After a summary of the new features, we compare
Jewel to boson–jet data from LHC Run I and II.
2 Simulating V +jet processes with JEWEL
Jewel is a fully dynamical perturbative framework for jet
quenching. It describes the simultaneous scale evolution
of hard partons giving rise to jets and re-scattering in the
medium. The former is implemented in the form of a virtu-
ality ordered parton shower. All partons in the shower in
addition to the jet evolution, undergo re-scattering in the
background. These interactions are described by 2 → 2 per-
turbative QCD matrix elements supplemented with parton
showers and can thus be elastic or inelastic, where the two
1
The code is available at http://jewel.hepforge.org.
123