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1. $D^0$ -meson-tagged jet axes difference in proton-proton collisions at $\sqrt{s}=5.02\mathrm{TeV}$

   https://doi.org/10.1103/nt4q-7t77  

   Acharya, S; Agarwal, A; Aglieri_Rinella, G; Aglietta, L; Agnello, M; Agrawal, N; Ahammed, Z; Ahmad, S; Ahn, S U; Ahuja, I; et al (November 2025, Physical Review D)

   Heavy-flavor quarks produced in proton-proton (pp) collisions provide a unique opportunity to investigate the evolution of quark-initiated parton showers from initial hard scatterings to final-state hadrons. By examining jets that contain heavy-flavor hadrons, this study explores the effects of both perturbative and nonperturbative QCD on jet formation and structure. The angular differences between various jet axes, $\mathrm{\Delta }{R}_{\mathrm{axis}}$, offer insight into the radiation patterns and fragmentation of charm quarks. The first measurement of $D^0$-tagged jet axes differences in pp collisions at $\sqrt{s}=5.02\mathrm{TeV}$by the ALICE experiment at the LHC is presented for jets with transverse momentum $p_{\mathrm{T}}^{\text{ch jet}}\ge 10\mathrm{GeV}/c$and $D^0$mesons with $p_{\mathrm{T}}^{{\mathrm{D}}^0}\ge 5\mathrm{GeV}/c$. In this $D^0$-meson-tagged jet measurement, three jet axis definitions, each with different sensitivities to soft, wide-angle radiation, are used: the standard axis, soft drop groomed axis, and winner-takes-all axis. Measurements of the radial distributions of $D^0$mesons with respect to the jet axes, $\mathrm{\Delta }{R}_{\mathrm{axis}\text{−}{\mathrm{D}}^0}$, are reported, along with the angle, $\mathrm{\Delta }{R}_{\mathrm{axis}}$, between the three jet axes. The $D^0$meson emerges as the leading particle in these jets, closely aligning with the winner-takes-all axis and diverging from the standard jet axis. The results also examine how varying the sensitivity to soft radiation with grooming influences the orientation of the soft drop jet axis and uncover that charm-jet structure is more likely to survive grooming when the soft drop axis is further from the $D^0$direction, providing further evidence of the dead-cone effect recently measured by ALICE. 

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2. Search for Quasiparticle Scattering in the Quark-Gluon Plasma with Jet Splittings in $pp$ and Pb-Pb Collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$

   https://doi.org/10.1103/PhysRevLett.135.031901  

   Acharya, S; Agarwal, A; Aglieri_Rinella, G; Aglietta, L; Agnello, M; Agrawal, N; Ahammed, Z; Ahmad, S; Ahn, S U; Ahuja, I; et al (July 2025, Physical Review Letters)

   The ALICE Collaboration reports measurements of the large relative transverse momentum ( $k_T$) component of jet substructure in $pp$and Pb-Pb collisions at center-of-mass energy per nucleon pair $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. Enhancement in the yield of such large- $k_{\mathrm{T}}$emissions in head-on Pb-Pb collisions is predicted to arise from partonic scattering with quasiparticles of the quark-gluon plasma. The analysis utilizes charged-particle jets reconstructed by the anti- $k_{\mathrm{T}}$algorithm with resolution parameter $R=0.2$in the transverse-momentum interval $60<p_{\mathrm{T},\mathrm{ch},\mathrm{jet}}<80\mathrm{GeV}/c$. The soft drop and dynamical grooming algorithms are used to identify high transverse momentum splittings in the jet shower. Comparison of measurements in Pb-Pb and $pp$collisions shows medium-induced narrowing, corresponding to yield suppression of high- $k_T$splittings, in contrast to the expectation of yield enhancement due to quasiparticle scattering. The measurements are compared to theoretical model calculations incorporating jet modification due to jet-medium interactions (“jet quenching”), both with and without quasiparticle scattering effects. These measurements provide new insight into the underlying mechanisms and theoretical modeling of jet quenching. 

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3. Observation of Medium-Induced Yield Enhancement and Acoplanarity Broadening of Low- $p_{\mathrm{T}}$ Jets from Measurements in $pp$ and Central Pb-Pb Collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$

   https://doi.org/10.1103/PhysRevLett.133.022301  

   Acharya, S; Adamová, D; Aglieri_Rinella, G; Agnello, M; Agrawal, N; Ahammed, Z; Ahmad, S; Ahn, S U; Ahuja, I; Akindinov, A; et al (July 2024, Physical Review Letters)

   The ALICE Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high transverse momentum (high $p_{\mathrm{T}}$) hadron trigger in proton-proton and central Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. A data-driven statistical method is used to mitigate the large uncorrelated background in central Pb-Pb collisions. Recoil jet distributions are reported for jet resolution parameter $R=0.2$, 0.4, and 0.5 in the range $7<p_{\mathrm{T},\text{jet}}<140\mathrm{GeV}/c$and trigger-recoil jet azimuthal separation $\pi /2<\mathrm{\Delta }\phi <\pi$. The measurements exhibit a marked medium-induced jet yield enhancement at low $p_{\mathrm{T}}$and at large azimuthal deviation from $\mathrm{\Delta }\phi \sim \pi$. The enhancement is characterized by its dependence on $\mathrm{\Delta }\phi$, which has a slope that differs from zero by $4.7\sigma$. Comparisons to model calculations incorporating different formulations of jet quenching are reported. These comparisons indicate that the observed yield enhancement arises from the response of the QGP medium to jet propagation. © 2024 CERN, for the ALICE Collaboration2024CERN 

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4. Observation of Enhanced Long-Range Elliptic Anisotropies Inside High-Multiplicity Jets in $pp$ Collisions at $\sqrt{s}=13\mathrm{TeV}$

   https://doi.org/10.1103/PhysRevLett.133.142301  

   Hayrapetyan, A; Tumasyan, A; Adam, W; Andrejkovic, J W; Bergauer, T; Chatterjee, S; Damanakis, K; Dragicevic, M; Hussain, P S; Jeitler, M; et al (September 2024, Physical Review Letters)

   A search for collective effects inside jets produced in proton-proton collisions is performed via correlation measurements of charged particles using the CMS detector at the CERN LHC. The analysis uses data collected at a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$, corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$. Jets are reconstructed with the anti- $k_{\mathrm{T}}$algorithm with a distance parameter of 0.8 and are required to have transverse momentum greater than 550 GeV and pseudorapidity $\left|{\eta }^{\text{jet}}\right|<1.6$. Two-particle correlations among the charged particles within the jets are studied as functions of the particles’ azimuthal angle and pseudorapidity separations ( $\mathrm{\Delta }{\varphi }^{*}$and $\mathrm{\Delta }{\eta }^{*}$) in a jet coordinate basis, where particles’ ${\eta }^{*}$, ${\varphi }^{*}$are defined relative to the direction of the jet. The correlation functions are studied in classes of in-jet charged-particle multiplicity up to $N_{\mathrm{ch}}^{\mathrm{j}}\approx 100$. Fourier harmonics are extracted from long-range azimuthal correlation functions to characterize azimuthal anisotropy for $\left|\mathrm{\Delta }{\eta }^{*}\right|>2$. For low- $N_{\mathrm{ch}}^{\mathrm{j}}$jets, the long-range elliptic anisotropic harmonic, $v_2^{*}$, is observed to decrease with $N_{\mathrm{ch}}^{\mathrm{j}}$. This trend is well described by Monte Carlo event generators. However, a rising trend for $v_2^{*}$emerges at $N_{\mathrm{ch}}^{\mathrm{j}}\gtrsim 80$, hinting at a possible onset of collective behavior, which is not reproduced by the models tested. This observation yields new insights into the dynamics of jet evolution in the vacuum. © 2024 CERN, for the CMS Collaboration2024CERN 

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5. Particle production as a function of charged-particle flattenicity in $pp$ collisions at $\sqrt{s}=13\mathrm{TeV}$

   https://doi.org/10.1103/PhysRevD.111.012010  

   Acharya, S; Adamová, D; Agarwal, A; Aglieri_Rinella, G; Aglietta, L; Agnello, M; Agrawal, N; Ahammed, Z; Ahmad, S; Ahn, S U; et al (January 2025, Physical Review D)

   This paper reports the first measurement of the transverse momentum ( $p_{\mathrm{T}}$) spectra of primary charged pions, kaons, (anti)protons, and unidentified particles as a function of the charged-particle flattenicity in pp collisions at $\sqrt{s}=13\mathrm{TeV}$. Flattenicity is a novel event shape observable that is measured in the pseudorapidity intervals covered by the V0 detector, $2.8<\eta <5.1$and $-3.7<\eta <-1.7$. According to QCD-inspired phenomenological models, it shows sensitivity to multiparton interactions and is less affected by biases toward larger $p_{\mathrm{T}}$due to local multiplicity fluctuations in the V0 acceptance than multiplicity. The analysis is performed in minimum-bias (MB) as well as in high-multiplicity events up to $p_{\mathrm{T}}=20\mathrm{GeV}/c$. The event selection requires at least one charged particle produced in the pseudorapidity interval $\left|\eta \right|<1$. The measured $p_{\mathrm{T}}$distributions, average $p_{\mathrm{T}}$, kaon-to-pion and proton-to-pion particle ratios, presented in this paper, are compared to model calculations using 8 based on color strings and EPOS LHC. The modification of the $p_{\mathrm{T}}$-spectral shapes in low-flattenicity events that have large event activity with respect to those measured in MB events develops a pronounced peak at intermediate $p_{\mathrm{T}}$( $2<p_{\mathrm{T}}<8\mathrm{GeV}/c$), and approaches the vicinity of unity at higher $p_{\mathrm{T}}$. The results are qualitatively described by , and they show different behavior than those measured as a function of charged-particle multiplicity based on the V0M estimator. © 2025 CERN, for the ALICE Collaboration2025CERN 

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