Physics Letters B 779 (2018) 409–413
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Sensitivity of jet substructure to jet-induced medium response
Guilherme Milhano
a,b,∗
, Urs Achim Wiedemann
a
, Korinna Christine Zapp
a,b
a
Laboratório de Instrumentação e Física Experimental de Partículas (LIP), Av . Prof. Gama Pinto, 2, 1649-003 Lisboa, Portugal
b
CERN, Theoretical Physics Department, CH-1211, Geneva 23, Switzerland
a r t i c l e i n f o a b s t r a c t
Article history:
Received
30 September 2017
Received
in revised form 19 December 2017
Accepted
10 January 2018
Available
online 19 January 2018
Editor:
J.-P. Blaizot
Jet quenching in heavy ion collisions is expected to be accompanied by recoil effects, but unambiguous
signals for the induced medium response have been difficult to identify so far. Here, we argue that mod-
ern
jet substructure measurements can improve this situation qualitatively since they are sensitive to the
momentum distribution inside the jet. We show that the groomed subjet shared momentum fraction z
g
,
and the girth of leading and subleading subjets signal recoil effects with dependencies that are absent
in a recoilless baseline. We find that recoil effects can explain most of the medium modifications to the
z
g
distribution observed in data. Furthermore, for jets passing the Soft Drop Condition, recoil effects in-
duce
in the differential distribution of subjet separation R
12
a characteristic increase with R
12
, and
they introduce a characteristic enhancement of the girth of the subleading subjet with decreasing z
g
.
We explain why these qualitatively novel features, that we establish in Jewel+Pythia simulations, reflect
generic physical properties of recoil effects that should therefore be searched for as telltale signatures of
jet-induced medium response.
© 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
High momentum transfer processes with hadronic final states
are generically and strongly modified when occurring within the
dense environment produced in nucleus–nucleus collisions. This
jet quenching phenomenon is being studied systematically at the
LHC for jet p
⊥
-spectra, dijet asymmetries, jet fragmentation func-
tions,
jet shapes and, most recently, for a large class of increas-
ingly
refined jet substructure observables. Jet quenching implies
jet-medium interactions. If the medium is close to a perfect liquid,
medium recoil propagates in the form of hydrodynamic excitations
[1], but it is expected to show signs of large angle scattering if jet-
medium
interactions were to resolve partonic degrees of freedom
in the medium [2–4]. Beyond confirming the assumed dynam-
ics
of jet-medium interactions, the observation of recoil distribu-
tions
is thus of great interest for characterizing the nature of the
medium.
However,
the characterization of jet recoil distributions has re-
mained
elusive so far for several reasons. In particular, recoil ef-
fects
are expected to contribute mainly to the soft large-angle
hadronic activity, but there are experimental and theoretical uncer-
*
Corresponding author.
E-mail
address: gmilhano@lip.pt (G. Milhano).
tainties in establishing soft recoil remnants on top of a large and
fluctuating background that need to be controlled. Also, many of
the measurements used to characterize jet quenching are remark-
ably
insensitive to soft large-angle activity. For instance, quenched
hadron spectra are by construction insensitive to how the lost en-
ergy
is distributed, and traditional jet quenching observables con-
structed
from jet p
⊥
and jet axis (such as the jet nuclear modifica-
tion
factor) are dominated by hard contributions. One may expect
that jet shape observables are more sensitive to the jet medium
response since they are sensitive to momentum distributions in-
side
the jet. Our main result will be to establish a first example for
a combination of jet substructure observables — namely measure-
ments
of the subjet shared momentum fraction z
g
and of girth —
that allow for the separation of recoil effects from alternative in-
terpretations
with both characteristic quantitative and qualitative
features in the data.
In
contrast to jet quenching models that parametrize (e.g., in
terms of the quenching parameters
ˆ
q and
ˆ
e) the recoil carried
away from the jet, fully dynamical event generators of jet quench-
ing
are better suited to study recoil effects as they can propagate
them into final state particle distributions. To exploit the result-
ing
phenomenological opportunities, however, one needs a robust
prescription for separating medium recoils from the initial thermal
https://doi.org/10.1016/j.physletb.2018.01.029
0370-2693/
© 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.