Session FB: Heavy Ions and Jets III

Studying hadronization at LHCb

The LHCb experiment at the Large Hadron Collider (LHC) is suited for studying how hadrons are formed from scattered quarks and gluons, collectively referred to as partons, in energetic proton-proton collisions. The hadronization process is a non-perturbative phenomenon, unlike hard scattering of partons and their shower processes and therefore can be learned from measurements, such as those involving jet sub-structure. Equipped with a forward spectrometer, the LHCb experiment achieves a transverse momentum resolution of $\frac{\Delta p_T}{p_T}< $1 % up to 200 GeV/c for charged tracks and a jet $p_T$ resolution of $<$15 %. This along with excellent particle identification capabilities offers a unique opportunity to measure with great precision hadronization variables $j_T$, z and r, defined to characterize multi-dimensional hadronization processes and their flavor dependence within jets. This talk will present recently published results for measurements of nonidentified hadrons within light quark-initiated jets as well as the status of other ongoing hadronization measurements at LHCb.   

Measurements of jet substructure in Pb-Pb and pp collisions at 5.02 TeV with ALICE

Jets are excellent probes of the QGP produced in heavy-ion (HI) collisions because the partons inside jets interact with the medium, leading to jet energy loss and substructure modification - a phenomenon called jet quenching. Recent measurements of groomed jet substructure in Pb—Pb and pp collisions at $\sqrt{s}_{NN} =$ 5.02 TeV using the ALICE detector will be shown. Specifically, Soft Drop (SD) grooming techniques are used to remove the soft radiation products inside a jet to access the jet’s hard emissions. Groomed substructure variables could be sensitive to medium-induced signals such as coherence, multiple soft-radiation, and single hard emissions. They are also directly calculable in pQCD. Previous measurements of jet splittings show evidence of these effects but have not been unfolded due to the large contribution of background splittings in the response. These new results benefit from an improved background subtraction technique and are measured in a phase space that suppresses background splittings by using semi-central (30-50%) collisions and stronger grooming conditions. This allows these variables to be unfolded for detector effects and background fluctuations, such that direct comparisons can be made theoretical calculations to help constrain jet quenching models.   

Jet substructure in p+p and p+Au collisions at $\sqrt{s_{NN}} = 200$ GeV at STAR

In order to attribute the partonic energy loss experienced by jets (jet quenching) observed in A+A collisions to the traversal of partons through the hot QCD medium, it is necessary to examine the cold nuclear matter (CNM) effects on the corresponding jets. Such an examination has historically been done using p+A collisions. In this talk, we present fully corrected measurements of jet substructure -- with a focus on jet mass -- in p+Au collisions at STAR at $\sqrt{s_{\rm{NN}}}=200$ GeV as a function of the event activity (EA) to increase or decrease the magnitude of CNM effects. EA is determined in the backward (Au-going) rapidity ($-5.0<\eta<-3.3$) by the STAR Beam-Beam Counter to minimize auto-correlation with jets measured at mid-rapidity. By differentiating the measured jets by rapidity, we explore potential Bjorken-$x$ dependence of CNM effects. Finally, we compare the results in p+Au collisions to model calculations, and to fully corrected corresponding measurements in p+p collisions and vacuum Monte Carlo models to isolate these CNM effects in anticipation of an upcoming jet mass measurement in Au+Au collisions.   

Measuring the Groomed Shared Momentum Fraction (Zg) in Au+Au Collisions at STAR Using a Semi-Inclusive Approach

Jet quenching is one of the main signals used to investigate the properties of a quark-gluon plasma (QGP). Besides energy loss, jet quenching can also manifest in the modification of jet substructure. This work focuses on measuring the substructure observable $z_{\mathrm{g}}$, a result of SoftDrop grooming, which probes the physics of the first hard splitting of a hard-scattered parton. This analysis employs a semi-inclusive approach, selecting candidate jets found within the recoil region of a high transverse momentum trigger particle. Requiring a high transverse momentum trigger object induces a surface bias on the event selection, potentially causing selected candidate jets in the recoil region to be biased towards having a longer path length within the medium. Consequently, these jets are expected to be more quenched and thus are good candidates to probe for modification of $z_{\mathrm{g}}$ at RHIC energies. In this analysis contribution from combinatorial jets, arising from the large fluctuating background in heavy-ion collisions, is subtracted from the signal at the ensemble level, using a mixed events technique. In this talk we will present the techniques used and the current preliminary results of $z_{\mathrm{g}}$ in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}} = 200$ GeV.   

Monte Carlo Simulations of Jet Acoplanarity Broadening in High Multiplicity pp Collisions

While it has traditionally been assumed that small systems (pp, pA) do not generate the QGP, recent searches for jet modification may challenge this assumption. One such study performed by ALICE has examined hadron-jet correlations in pp collisions. Broadening of recoil jet acoplanarity was preliminarily observed in high multiplicity collisions relative to minimum bias data. Interestingly, this result can be replicated by PYTHIA 8, which does not model the QGP. Efforts to understand simulated results have therefore been critical to interpreting this phenomenon. One possible explanation relates to the spatial distribution of leading hadrons and the extent to which they align with their associated jets. Another possibility relates to a selection bias owing to the definition of event activity. Simulations suggest that events tagged as high multiplicity using forward detectors are biased toward multi-jet events, skewing azimuthal distributions when forward jets are neglected. Analyses of PYTHIA 8 simulations will be presented to quantify the extent to which near-side broadening and multi-jet events contribute to acoplanarity broadening. Additionally, changes in the method of event classification will be proposed to minimize the multi-jet bias.   

Predictions for Modern Jet Quenching Observables at RHIC Energies with JEWEL

In 2023, the Relativistic Heavy Ion Collider will begin to deliver high-statistics Au+Au collision data to the new sPHENIX and existing STAR detectors. New experimental capabilities at these detectors will allow for a better understanding of how hard scattered partons propagate and lose energy in the quark-gluon plasma. In this talk, we use the latest version of JEWEL to study jet quenching observables which can be measured for the first time at RHIC. JEWEL is a Monte Carlo event generator that simulates jet evolution in a perturbative framework in both proton-proton and nucleus-nucleus collisions and has been mostly used for making predictions at LHC energies. JEWEL studies at RHIC energies may help guide the physics program and analysis techniques in the mid-2020's. In particular, JEWEL now includes photon-jet processes which are important to the sPHENIX physics program, as well as an improved medium response. We present results for photon-jet and jet substructure observables which can be measured with the high-statistics Au+Au data-taking at RHIC in 2023.   

First Results from Hybrid Hadronization in Small and Large Systems

Hybrid Hadronization is a new Monte Carlo package to hadronize systems of partons. It smoothly combines quark recombination applicable when distances between partons in phase space are small, and string fragmentation appropriate for dilute parton systems, following the picture outlined by Han et al. [PRC 93, 045207 (2016)]. Hybrid Hadronization integrates with PYTHIA 8 and can be applied to a variety of systems from $e^++e^-$ to A+A collisions. It takes systems of partons and their color flow information, for example from a Monte Carlo parton shower generator, as input. In addition, if for A+A collisions a thermal background medium is provided, the package allows to sample thermal partons that contribute to hadronization. Hybrid Hadronization is available for use as a standalone code and is also part of JETSCAPE since the 2.0 release. In this presentation we review the physics concepts underlying Hybrid Hadronization. We present calculations of hadron chemistry and fragmentation functions in small and large systems when Hybrid Hadronization is combined with parton shower Monte Carlos MATTER and LBT. In particular, we discuss observable effects of the recombination of shower partons with thermal partons.   

Evidence for top quark production in nucleus-nucleus collisions

Droplets of quark-gluon plasma (QGP), an exotic state of strongly interacting quantum chromodynamics matter, are routinely produced in heavy nuclei high-energy collisions. Although the experimental signatures marked a paradigm shift away from expectations of a weakly coupled QGP, a challenge remains as to how the locally deconfined state with a lifetime of a few fm can be resolved. A colored particle that decays mostly within the QGP is the top quark. Here we demonstrate, for the first time, that top quark decay products are identified, irrespective of whether interacting with the medium (bottom quarks) or not (leptonically decaying W bosons). Using lead-lead collision data recorded by the CMS detector, we report evidence for top quark pair production. Dilepton final states are selected, and the cross section is measured from a likelihood fit to a multivariate discriminator using lepton kinematic variables. The measurement is additionally performed considering the jets originating from the hadronization of bottom quarks, which improve the sensitivity to the signal process. The measurements, consistent with each other and the expectations from scaled proton-proton data as well as perturbative QCD, constitute the first step towards using the top quark as a novel tool to probe QGP.   

Measurement of Mid-rapidity Inclusive Jet Cross Section in $pp$ Collisions at $\sqrt{s}=200$~GeV

Jets provide the primary tool to study the partons inside protons. At RHIC, the dominant processes of the hard partonic interactions in proton-proton collisions is quark-gluon and, at high jet transverse momentum $p_T$, quark-quark scatterings. The cross section of inclusive jet production is one of the main observables to study the hard scattering. It is well described by pQCD in the collinear factorization framework. The STAR detector at a center-of-mass energy $\sqrt{s} = 200~\text{GeV}$ provides measurements at $x_T \equiv \frac{2p_T}{\sqrt{s}}$ as high as $\sim 0.4$. At this energy and in this kinematics region, the direct scattering on gluons inside the colliding protons contributes a sizeable fraction of the total events. Thus, measuring the inclusive jet cross section at RHIC, together with the past DIS measurements, can provide at high $x$ further constraints on the gluon Parton Distribution Function. An update towards a new measurement of inclusive jet cross section at mid-rapidity at STAR using the $\sqrt{s} = 200~\text{GeV}$ data from 2012 will be presented. Compared to the previous measurement from 2006, improvements include: employing the anti-$k_T$ jet reconstruction, a full barrel and endcap electromagnetic calorimeters acceptance with full unfolding of the detec   

Z Boson Jet Momentum Imbalance in pp Collisions at STAR

Proton-proton collisions at RHIC with center of mass energy 500 GeV produce Z bosons that recoil off jets. In this analysis, Z boson transverse momentum (pT) is compared to away-side jet pT, and the ratio, xjet,Z = pT,jet/pT,Z is measured. Utilizing data from the STAR experiment, the invariant mass method is used to identify Z bosons from their lepton pair decays. Candidate Z bosons are selected with a narrow window around the mass peak near 90 GeV where the back- ground contribution is significantly suppressed. Jets can be identified using the anti-kT algorithm from the Fastjet package. We find that results using STAR 2017 data can be compared with those simulated using the Pythia 8 event generator.