11institutetext: Department of Physics, Duke University, Durham, NC 27708, USA
22institutetext: Department of Physics and Astronomy, Wayne State University,
Detroit MI 48201, USA. 33institutetext: Akita International University, Yuwa,
Akita-city 010-1292, Japan.

# Photon-triggered jets as probes of multi-stage jet modification

Chathuranga Sirimanna 1122<EMAIL_ADDRESS>Yasuki Tachibana for
the JETSCAPE Collaboration 33

###### Abstract

Prompt photons are created in the early stages of heavy ion collisions and
traverse the QGP medium without any interaction. Therefore, photon-triggered
jets can be used to study the jet quenching in the QGP medium. In this work,
photon-triggered jets are studied through different jet and jet substructure
observables for different collision systems and energies using the JETSCAPE
framework. Since the multistage evolution used in the JETSCAPE framework is
adequate to describe a wide range of experimental observables simultaneously
using the same parameter tune, we use the same parameters tuned for jet and
leading hadron studies. The same isolation criteria used in the experimental
analysis are used to identify prompt photons for better comparison. For the
first time, high-accuracy JETSCAPE results are compared with multi-energy LHC
and RHIC measurements to better understand the deviations observed in prior
studies. This study highlights the importance of multistage evolution for the
simultaneous description of experimental observables through different
collision systems and energies using a single parameter tune.

##  1 Introduction

Heavy ion collisions offer a means to explore the early stages of the
universe, particularly the Quark Gluon Plasma (QGP) formed microseconds after
the Big Bang. Among the various probes employed to study QGP in heavy ion
collisions, prompt photons are crucial Wang:1996yh , as they enable the
estimation of energy and direction for the initiating parton on the away side
jet before energy loss occurs. In recent years, there has been a growing
interest in investigating observables related to prompt photons within both
theoretical and experimental communities. The experimental analysis of
identifying prompt photons from a large number of shower and fragmentation
photons poses significant challenges, and a common practice involves applying
isolation criteria. Given that photons do not interact with the QGP medium,
these isolated photons predominantly consist of prompt photons.

The scarcity of events featuring prompt photons is a consequence of the low
cross-section in hard scattering processes leading to their production. In
theoretical simulations, generating modified events with prompt photons
requires adjusting the cross-section to significantly reduce computation time.
However, it’s important to note that this simulation approach differs from
experimental analysis, which relies on isolated direct photons.

In this study, we compare various observables using both modified prompt
photon events and full events generated by JETSCAPE 3.X (utilizing
PP19JETSCAPE:2019udz  and AA22JETSCAPE:2022jer  tunes) against available
experimental results. Additionally, for the first time, we explore isolated
photon and di-jet correlation, groomed jet substructure associated with
$\gamma$-triggered jets, and $\gamma$-jet asymmetry, using new JETSCAPE
results featuring improved statistics Sirimanna:2020ggn .

##  2 Simulating photon triggered jets using JETSCAPE framework

JETSCAPE is the first framework that supports multistage evolution, where
multiple modules can be employed to simulate the partonic shower within a QGP
medium, depending on the virtuality of each shower parton. This multistage
evolution capability enables the JETSCAPE framework to simultaneously describe
numerous experimental observables across different center-of-mass energies and
centralities using a single parameter tune.

In the simulation of p-p collisions, partons produced in the initial hard
scattering in PYTHIA are fed into the MATTER energy loss module for vacuum
partonic showers. Subsequently, all final state partons from MATTER are
transferred back to PYTHIA for string hadronization. For Pb-Pb collisions,
pre-generated event-by-event hydro profiles with initial conditions are
utilized. Similar to p-p collisions, partons produced in the initial hard
scattering in PYTHIA are transferred into MATTER and LBT energy loss modules
based on their virtuality. Final state hadrons are generated using PYTHIA’s
string hadronization, following the partonic shower. The same set of
parameters used in previous p-pJETSCAPE:2019udz  and Pb-PbJETSCAPE:2022jer
studies is applied in this study without further parameter tuning.

In this study, trigger photons are identified using the same isolation
criterion employed in experimental analysis for both modified prompt-photon
events and full events ATLAS:2018dgb ; CMS:2017ehl . The same sets of
generated events at $\sqrt{S_{NN}}=5.02\leavevmode\nobreak\ \text{TeV}$ are
used to analyze a wide variety of observables and compare them with available
experimental results. After identifying the isolated photon in an event, jets
are clustered using the anti-$k_{T}$ jet clustering algorithm within FastJet.

##  3 Results and Discussion

The $\gamma$-jet asymmetry was investigated for both p-p and Pb-Pb collisions
using modified prompt-photon and full events, and the findings were compared
with those of ATLAS and CMS. Figures 1 and 2 illustrate comparisons of
$X_{j\gamma}$ distributions for four different $p_{T}^{\gamma}$ intervals with
ATLAS results for p-p and Pb-Pb, respectively. Given that ATLAS results are
unfolded, unmodified JETSCAPE results were used in this comparison. Despite
the considerably large uncertainties in the results from full events, due to
the rarity of events with prompt photons, a better agreement can be observed
in both p-p and Pb-Pb results.

Figure 1: $\gamma$-jet asymmetry for p-p collisions using prompt-photon events
and full events generated by JETSCAPE compared with ATLAS results. Here four
$p_{T\gamma}$ regions from $63.1GeV$ to $200GeV$ are used.

Figure 2: $\gamma$-jet asymmetry for Pb-Pb collisions using prompt-photon
events and full events generated by JETSCAPE compared with ATLAS results. Same
$p_{T\gamma}$ regions as Figure 1 is used here.

Since CMS results for both p-p and Pb-Pb are smeared, the same smearing
function is applied for a proper comparison, as illustrated in Figure 3.
Similarly, in this case, results from full events exhibit better agreement
with the experimental results. Although the isolated photons mainly consist of
prompt photons, these findings suggest a significant contribution to the
transverse momentum imbalance from other photons, including those produced in
the partonic shower and fragmentation photons.

Figure 3: $\gamma$-jet asymmetry for p-p and Pb-Pb (0-10%) collisions using
prompt-photon events and full events generated by JETSCAPE compared with CMS
results. Here both p-p and Pb-Pb distributions are smeared according to the
smearing function for 0-10% centrality.

Figure 4 displays the $z_{g}$ distribution, which measures the energy
imbalance of the hardest split in a $\gamma$-triggered jet. The ratio between
Pb-Pb and p-p illustrates the modification from the QGP medium. As observed in
Figure 4, the $z_{g}$ distribution does not exhibit a significant dependence
on the transverse momentum imbalance, $X_{j\gamma}$. Although there are no
experimental results available yet to compare with these groomed jet
substructure observables using photon-triggered jets, ongoing experimental
studies are being conducted.

Figure 4: $z_{g}$ distribution using photon-triggered jets calculated for four
different $X_{J\gamma}$ regions by using prompt-photon events generated by
JETSCAPE.

The isolated photon and multi-jet correlation is another recent experimental
measurement by ATLAS collaborationATLAS-CONF-2023-008 . In Figure 5, the
$x_{JJ\gamma}=(\vec{p}_{1}+\vec{p}_{2})_{T}/p_{T,\gamma}$ distribution using
JETSCAPE for p-p, Pb-Pb, and the ratio between them is compared to ATLAS
preliminary data. Although JETSCAPE shows excellent agreement with the ratio,
a multiplication factor of 1.4 is required to accurately describe both p-p and
Pb-Pb results. Since no model can explain these results for isolated photon
and di-jet correlation without utilizing such a multiplication factor,
additional theoretical studies are necessary to comprehend these observables.

Figure 5: Isolated photon and di-jet correlation, $x_{JJ\gamma}$ calculated
using prompt-photon events generated by JETSCAPE compared with ATLAS
preliminary results ATLAS-CONF-2023-008 .

JETSCAPE results for the majority of photon-triggered jet observables provide
an excellent description of the experimental results without requiring
additional parameter tuning. While current theoretical knowledge is
insufficient to fully understand certain observables like isolated photon and
multi-jet correlation, JETSCAPE, with its multistage evolution, offers an
excellent description of all stages of jet evolution â a significant
improvement over single-stage jet evolution. This analysis was conducted
without further modification of the previous PP19 and AA22 parameter tunes.
Therefore, this study serves as a parameter-free verification of multistage
evolution.

## Acknowledgments

These proceedings are supported in part by the National Science Foundation
(NSF) within the framework of the JETSCAPE collaboration, under grant numbers
ACI-1550300 OAC-2004571 (CSSI:X-SCAPE) and in part by the U.S. Department of
Energy (DOE) under grant number DE-SC0013460.

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