arXiv subjects

Yasuki Tachibana

Publications and source records attributed to Yasuki Tachibana.

At least 19 recordsLinked to original sources

arXiv2025

Interplay of prompt and non-prompt photons in photon-triggered jet observables

Prompt photons are important yet challenging to observe in relativistic heavy-ion collisions, as they are produced in the early stages and traverse almost the entire QGP medium without interaction. Experimental analyses typically employ isolation cuts, in the hope to identify prompt photons. Most theoretical studies consider only events with actual prompt photons, assuming no contribution from isolated non-prompt photons to reduce computational cost. For the first time, we present a study that compares simulation results generated using inclusive (bremsstrahlung) and prompt-photon events with multiple experimental observables for both $p-p$ and $Pb-Pb$ collisions at $5.02$ TeV. Simulations are carried out using the multi-stage JETSCAPE framework tuned to describe the quenching of jets and hadrons. Isolated non-prompt photons are generated in hard photon bremsstrahlung, where the photon is radiated at a sufficient angle to the jet. Several photon triggered jet and jet substructure observables show significant contributions from inclusive photons, yielding an improvement in comparison with experimental data. Novel photon triggered jet substructure observables are also expected to show new structures, yet to be detected in experiment. This effort examines the significance of isolated non-prompt photons using parameters tuned for a simultaneous description of the leading hadron and jet spectrum, and thus provides an independent verification of the multistage evolution framework.

hep-ph↗

arXiv2025

Effects of hadronic reinteraction on jet fragmentation from small to large systems

We investigate the impact of the hadronic phase on jet quenching in nuclear collider experiments, an open question in heavy-ion physics. Previous studies in a simplified setup suggest that hadronic interactions could have significant effects, but a systematic analysis is needed. Using the X-SCAPE event generator with the SMASH afterburner, we study the role of hadronic rescattering on jet fragmentation hadrons. Applying this framework to $e^++e^-$ collisions, we demonstrate that even in small systems with limited particle production, hadronic interactions lead to measurable modifications in final-state hadronic and jet observables by comparing scenarios with and without afterburner rescattering.

hep-ph↗

arXiv2025

Rapidity scan with DCCI at LHC energy

We extend the dynamical core-corona initialization (DCCI2) model to include the baryon number evolution in the entire system created in high-energy heavy-ion collisions. Introducing the source term for the baryon number, we describe the early-stage equilibration and later-stage hydrodynamic evolution of the baryon number throughout the system from midrapidity to forward rapidity. Through numerical simulations with this extended model, we show that extremely large baryon chemical potentials are realized in forward rapidity regions at the LHC energy and are comparable to those of the BES energies. Moreover, we show that fluctuations of baryon chemical potentials are large and, consequently, negative baryon chemical potential regions appear at midrapidity.

nucl-th↗

arXiv2025

Modified Coherence and the Transverse Extent of Jets

We present a study of the transverse size of parton showers and their diminishing interaction with the medium in the high virtuality stage of jet evolution. We consider the process of a hard quark produced in deep inelastic scattering off a large nucleus. Single gluon radiation from this quark, in the absence of scattering, is re-derived using wave-packets. This allows for a derivation of the quantum uncertainty size of the hard quark, at the point of splitting. This uncertainty size is then incorporated within a Monte-Carlo shower routine yielding transverse shower sizes noticeably larger than the classical antenna size of the shower. No clear relation is found between the full uncertainty size of the shower and the virtuality of the originating parton. The single gluon emission from the hard quark is then re-analysed for the case of single rescattering off the remainder of the nucleus. A relation is derived between the jet transport coefficient $\hat{q}$ and the gluon Transverse Momentum Dependent Parton Distribution Function (gTMDPDF). Solving this relation, for a simple case, clearly demonstrates the weakening of $\hat{q}$ with the virtuality of the hard splitting parton.

hep-ph↗

arXiv2024

Jet Quenching: From Theory to Simulation

With the explosion of data on jet based observables in relativistic heavy-ion collisions at the Large Hadron Collider and the Relativistic Heavy-Ion Collider, perturbative QCD based simulations of these processes, often interacting with an expanding viscous fluid dynamical background, have taken center stage. This review is meant to bridge the gap between theory, simulation and phenomenology of jet modification in a dense medium. We will demonstrate how the existence of such end-to-end event generators with semi-realistic or even fully realistic final states allows for the most rigorous comparisons between pQCD based jet modification theory and experiment. State-of-the-art calculations of several jet based observables are presented. Extensions of this theory to jets in the small systems of $p$-$A$ and $e$-$A$ collisions is discussed.

hep-ph↗

arXiv2024

Hadronisation in event generators from small to large systems

The results of the dynamical core-corona initialisation framework in p+p and Pb+Pb collisions at the LHC energies are presented. We extract the fractions of final hadron yields originating from equilibrated and non-equilibrated matter as functions of multiplicity. We show that the contribution from non-equilibrated matter is non-negligible even in intermediate and central Pb+Pb collisions. The particle production from non-equilibrated matter behaves as a correction on $c_2\{4\}$ that is purely obtained from the equilibrated matter. The result poses a warning on Bayesian parameter estimation with conventional hydrodynamic models. The observed flow coefficients might need a reinterpretation with new dynamical models which incorporate the particle production from non-equilibrated matter.

nucl-th↗

arXiv2023

3D Multi-system Bayesian Calibration with Energy Conservation to Study Rapidity-dependent Dynamics of Nuclear Collisions

Considerable information about the early-stage dynamics of heavy-ion collisions is encoded in the rapidity dependence of measurements. To leverage the large amount of experimental data, we perform a systematic analysis using three-dimensional hydrodynamic simulations of multiple collision systems -- large and small, symmetric and asymmetric. Specifically, we perform fully 3D multi-stage hydrodynamic simulations initialized by a parameterized model for rapidity-dependent energy deposition, which we calibrate on the hadron multiplicity and anisotropic flow coefficients. We utilize Bayesian inference to constrain properties of the early- and late- time dynamics of the system, and highlight the impact of enforcing global energy conservation in our 3D model.

nucl-th↗

arXiv2023

Hybrid Hadronization of Jet Showers from $e^++e^-$ to $A+A$ with JETSCAPE

In this talk we review jet production in a large variety of collision systems using the JETSCAPE event generator and Hybrid Hadronization. Hybrid Hadronization combines quark recombination, applicable when distances between partons in phase space are small, and string fragmentation appropriate for dilute parton systems. It can therefore smoothly describe the transition from very dilute parton systems like $e^++e^-$ to full $A+A$ collisions. We test this picture by using JETSCAPE to generate jets in various systems. Comparison to experimental data in $e^++e^-$ and $p+p$ collisions allows for a precise tuning of vacuum baseline parameters in JETSCAPE and Hybrid Hadronization. Proceeding to systems with jets embedded in a medium, we study in-medium hadronization for jet showers. We quantify the effects of an ambient medium, focusing in particular on the dependence on the collective flow and size of the medium. Our results clarify the effects we expect from in-medium hadronization of jets on observables like fragmentation functions, hadron chemistry and jet shape.

hep-ph↗

arXiv2023

A multistage framework for studying the evolution of jets and high-$p_T$ probes in small collision systems

Understanding the modification of jets and high-$p_T$ probes in small systems requires the integration of soft and hard physics. We present recent developments in extending the JETSCAPE framework to build an event generator, which includes correlations between soft and hard partons, to study jet observables in small systems. The multi-scale physics of the collision is separated into different stages. Hard scatterings are first sampled at binary collision positions provided by the Glauber geometry. They are then propagated backward in space-time following an initial-state shower to obtain the initiating partons' energies and momenta before the collision. These energies and momenta are then subtracted from the incoming colliding nucleons for soft-particle production, modeled by the 3D-Glauber + hydrodynamics + hadronic transport framework. This new hybrid approach (X-SCAPE) includes non-trivial correlations between jet and soft particle productions in small systems. We calibrate this framework with the final state hadrons' $p_T$-spectra from low to high $p_T$ in $p$-$p$, and and then compare with the spectra in $p$-$Pb$ collisions from the LHC. We also present results for additional observables such as the distributions of event activity as a function of the hardest jet $p_T$ in forward and mid-rapidity for both $p$-$p$ and $p$-$Pb$ collisions.

hep-ph↗

arXiv2022

Interplay between core and corona from small to large systems

We present new results in $p$+$p$ and Pb+Pb collisions at the LHC energies from the updated dynamical core--corona initialization framework (DCCI2). The fractions of final hadron yields originating from equilibrated and non-equilibrated components are extracted as functions of multiplicity. We find that the contributions from non-equilibrated components are non-negligible even in Pb+Pb collisions and affect $p_T$-integrated multi-particle correlations. These suggest the importance of non-equilibrated components for the sophisticated extraction of properties of the quark gluon plasma from comparisons between dynamical frameworks and experimental data.

nucl-th↗

arXiv2022

Bulk medium evolution has considerable effects on jet observables

We consider the case, in QCD, of a single jet propagating within a strongly interacting fluid, of finite extent. Interactions lead to the appearance of a source of energy-momentum within the fluid. The remnant jet that escapes the container is analyzed along with portions of the medium excited by the jet. We study the effect of a static versus a semi-realistic expanding medium, with jets traveling inward versus outward. We consider the medium response via recoils in partonic scatterings based on a weakly-coupled description and its combination with hydrodynamical medium response based on a strongly-coupled description, followed by incorporation into a jet. The effects of these limits on the reconstructed energy, momentum and mass of the jet, as a function of the angle away from the original parton direction are studied. It is demonstrated that different flow velocity configurations in the medium produce considerable differences in jet observables. This work highlights the importance of accurate dynamical modeling of the soft medium as a foundation on which to calculate jet modification, and casts skepticism on results obtained without such modeling.

nucl-th↗

arXiv2022

Non-equilibrium components in very low transverse momentum region in high-energy nuclear collisions

We analyze Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV with a novel framework based on the dynamical core--corona picture that describes particle productions from both equilibrium and non-equilibrium components. We remark the possibility of the contribution from non-equilibrium components at very low transverse momentum ($p_{T}$) region and show that such contributions significantly affect $p_{T}$-integrated four-particle cumulants. These results strongly suggest the necessity of non-equilibrium components when one extracts properties of the quark-gluon plasma from experimental data using sophisticated dynamical models based on relativistic hydrodynamics.

nucl-th↗

arXiv2021

Interplay between core and corona components in high-energy nuclear collisions

We establish the updated version of dynamical core--corona initialization framework (DCCI2) as a unified description from small to large colliding systems and from low to high transverse momentum ($p_T$) regions. Using DCCI2, we investigate effects of interplay between locally equilibrated and non-equilibrated systems, in other words, core and corona components in high-energy nuclear collisions. Given experimental multiplicity distributions and yield ratios of $Ω$ baryons to charged pions as inputs, we extract the fraction of core and corona components in p+p collisions at $\sqrt{s}=7$ TeV and Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV. We find core contribution overtakes corona contribution as increasing multiplicity above $\langle dN_{\mathrm{ch}}/dη\rangle_{|η|<0.5} \sim 18$ regardless of the collision system or energy. We also see that the core contribution exceeds the corona contribution only in 0.0-0.95\% multiplicity class in p+p collisions. Notably, there is a small enhancement of corona contribution with $\sim20$\% below $p_T\sim 1$ GeV even in minimum bias Pb+Pb collisions. We find that the corona contribution at low $p_T$ gives $\sim 15$-$38$ $\%$ correction on $v_2\{2\}$ at $N_{\mathrm{ch}}\lesssim 370$. This raises a problem in conventional hydrodynamic analyses in which low $p_T$ soft hadrons originate solely from core components. We finally scrutinize the roles of string fragmentation and the longitudinal expansion in the transverse energy per unit rapidity, which is crucial in initial conditions for hydrodynamics from event generators based on string models.

nucl-th↗

arXiv2020

Unified description of hadron yield ratios from dynamical core-corona initialization

We develop the dynamical core-corona initialization framework as a phenomenological description of the formation of quark gluon plasma (QGP) fluids in high-energy nuclear collisions. Using this framework, we investigate the fraction of the fluidized energy to the total energy and strange hadron yield ratios as functions of multiplicity and scrutinize the multiplicity scaling of hadron yield ratios recently reported by the ALICE Collaboration. Our results strongly indicate that the QGP fluids are partly formed even at the averaged multiplicity for nonsingle diffractive p+p events.

nucl-th↗

arXiv2019

Nuclear modification of jet shape for inclusive jets and $γ$-jets at the LHC energies

With our coupled jet-fluid model, we study the nuclear modifications of full jets and jet structures for single inclusive jets and $γ$-jets in Pb+Pb collisions at $5.02$~ATeV and $2.76$~ATeV. The in-medium evolution of full jet shower is described by a set of coupled transport equations including the effects of collisional energy loss, transverse momentum broadening and medium-induced splitting process. The dynamical evolution of bulk medium is simulated by solving relativistic hydrodynamic equation with source term which accounts for the energy and momentum deposited by hard jet shower to soft medium. Our study demonstrates that the hydrodynamic medium response to jet propagation significantly enhances the broadening of jet shape at large angles and is essential for the cone-size dependence of jet energy loss and nuclear modification factor of inclusive jet production. It is also found that the nuclear modification pattern of jet shape is sensitive to jet energy but has weak dependence on the flavor of the parton that initiates the jet. Our result can naturally explain the different nuclear modification patterns of jet shape functions for single inclusive jet and $γ$-jet events as observed by the CMS Collaboration, and can be tested in the future by measuring the jet shape function over a wider range of jet energies in heavy-ion collisions.

nucl-th↗

arXiv2018

Enhancement of strange baryons in high-multiplicity proton-proton and proton-nucleus collisions

We investigate the enhancement of yields of strange and multi-strange baryons in proton-proton (p+p), proton-lead (p+Pb) and lead-lead (Pb+Pb) collisions at the Large Hadron Collider (LHC) energies from a dynamical core-corona initialization model. We first generate partons just after the collisions by using event generators. These partons dynamically generate the quark gluon plasma (QGP) fluids through the source terms in the hydrodynamic equations. According to the core-corona picture, this process tends to happen where the density of generated partons is high and their transverse momentum is low. Some partons do not fully participate in this process when they are in dilute regions or their transverse momentum is high and subsequently fragment into hadrons through string fragmentation. In this framework, the final hadrons come from either chemically equilibrated fluids as in the conventional hydrodynamic models or string fragmentation. We calculate the ratio of strange baryons to charged pions as a function of multiplicity and find that it monotonically increases up to $dN_{\mathrm{ch}}/dη\sim 100$ and then saturates above. This suggests that the QGP fluids are \textit{partly} created and that their fraction increases with multiplicity in p+p and p+Pb collisions at LHC energies.

nucl-th↗

arXiv2018

Nuclear modification of full jets and jet structure in relativistic nuclear collisions

With our coupled jet-fluid model, we study the nuclear modifications of full jets and jet structures for single inclusive jet and $γ$-jet events in relativistic heavy-ion collisions. The evolution of full jet showers is studied via a set of coupled transport equations including the effects of collisional energy loss, transverse momentum broadening and medium-induced splitting. The dynamics of the jet energy and momentum deposited into the medium is described by hydrodynamic equations with source terms. Our detailed analysis indicates that collisional absorption (energy loss) tends to narrow the jet shape function while transverse momentum kicks and medium-induced radiations broaden the jet transverse profile. Also, jet-induced flow plays a significant contribution to jet shape function and dominates at large angles away from the jet axis. The final nuclear modification pattern for the jet shape function is a combined effect from various jet-medium interaction mechanisms. Our detailed studies for single inclusive jets and $γ$-jets for various kinematics indicate that the nuclear modification of jet shape has strong dependence on jet energy and collision energy, and weak dependence on jet flavor (quark or gluon).

nucl-th↗

arXiv2017

New approach to initializing hydrodynamic fields and mini-jet propagation in quark-gluon fluids

We propose a new approach to initialize the hydrodynamic fields such as energy density distributions and four flow velocity fields in hydrodynamic modeling of high-energy nuclear collisions at the collider energies. Instead of matching the energy-momentum tensor or putting the initial conditions of quark-gluon fluids at a fixed initial time, we utilize a framework of relativistic hydrodynamic equations with source terms to describe the initial stage. Putting the energy and momentum loss rate of the initial partons into the source terms, we obtain hydrodynamic initial conditions dynamically. The resultant initial profile of the quark-gluon fluid looks highly bumpy as seen in the conventional event-by-event initial conditions. In addition, initial random flow velocity fields also are generated as a consequence of momentum deposition from the initial partons. We regard the partons that survive after the dynamical initialization process as the mini-jets and find sizable effects of both mini-jet propagation in the quark-gluon fluids and initial random transverse flow on the final momentum spectra and anisotropic flow observables. We perform event-by-event $(3+1)$-dimensional ideal hydrodynamic simulations with this new framework that enables us to describe the hydrodynamic bulk collectivity, parton energy loss, and interplay among them in a unified manner.

nucl-th↗