The STAR experiment reports new, high-precision measurements of the transverse single-spin asymmetries for $π^{\pm}$ within jets, namely the Collins asymmetries, from transversely polarized ${p^{\uparrow}p}$ collisions at $\sqrt{s}$ = 510 GeV. The energy-scaled distribution of jet transverse momentum, $x_{\mathrm{T}} = 2p_{\mathrm{T,jet}}/\sqrt s$, shows a remarkable consistency for Collins asymmetries of $π^{\pm}$ in jets between $\sqrt{s}$ = 200 GeV and 510 GeV. This indicates that the Collins asymmetries are nearly energy independent with, at most, a very weak scale dependence in $p^{\uparrow}p$ collisions. These results extend to high-momentum scales ($Q^2 \leq 3400$ GeV$^2$) and enable unique tests of evolution and universality in the transverse-momentum-dependent formalism, thus providing important constraints for the Collins fragmentation functions.
Collins asymmetries, $A_{\mathrm{UT}}^{\sin(\phi_S - \phi_H)}$, as a function of jet $x_{\mathrm{T}}$ ($\equiv \frac{2p_{\mathrm{T,jet}}}{\sqrt{s}}$) for $\pi^{+}$ in $p^{\uparrow}p$ collisions at $\sqrt{s} = 510$ GeV. Vertical bars show the statistical uncertainties; boxes show the systematic uncertainties in $x_{\mathrm{T}}$ and $A_{\mathrm{UT}}$
Collins asymmetries, $A_{\mathrm{UT}}^{\sin(\phi_S - \phi_H)}$, as a function of jet $x_{\mathrm{T}}$ ($\equiv \frac{2p_{\mathrm{T,jet}}}{\sqrt{s}}$) for $\pi^{-}$ in $p^{\uparrow}p$ collisions at $\sqrt{s} = 510$ GeV. Vertical bars show the statistical uncertainties; boxes show the systematic uncertainties in $x_{\mathrm{T}}$ and $A_{\mathrm{UT}}
Collins asymmetries, $A_{\mathrm{UT}}^{\sin(\phi_S - \phi_H)}$, as a function of $\pi^{+}$ momentum fraction longitudinal momentum fraction $z$ in $p^{\uparrow}p$ collisions at $\sqrt{s} = 510$ GeV. Vertical bars show the statistical uncertainties; boxes show the systematic uncertainties.
We present the first measurements of the forward and midrapidity $η$-meson cross sections from $p$$+$$p$ collisions at $\sqrt{s}=500$ and $510$~GeV, respectively. We also report the midrapidity $η/π^0$ ratio at 510 GeV. The forward cross section is measured differentially in $η$-meson transverse momentum ($p_T$) from 1.0 to 6.5~GeV/$c$ for pseudorapidity $3.0<|η|<3.8$. The midrapidity cross section is measured from 3.5 to 44 GeV/$c$ for pseudorapidity $|η|<0.35$. Both cross sections serve as critical inputs to an updated global analysis of the $η$-meson fragmentation functions.
The invariant differential cross section of $\eta$ mesons at forward rapidity in pp collisions at center-of-mass energy 500 GeV.
The invariant differential cross section of $\eta$ mesons at central rapidity in pp collisions at center-of-mass energy 510 GeV.
The ratio of $\eta$ to $\pi^0$ cross sections at central rapidity in pp collisions at center-of-mass energy 510 GeV.
The jet cross-section and jet-substructure observables in $p$$+$$p$ collisions at $\sqrt{s}=200$ GeV were measured by the PHENIX Collaboration at the Relativistic Heavy Ion Collider (RHIC). Jets are reconstructed from charged-particle tracks and electromagnetic-calorimeter clusters using the anti-$k_{t}$ algorithm with a jet radius $R=0.3$ for jets with transverse momentum within $8.0<p_T<40.0$ GeV/$c$ and pseudorapidity $|η|<0.15$. Measurements include the jet cross section, as well as distributions of SoftDrop-groomed momentum fraction ($z_g$), charged-particle transverse momentum with respect to jet axis ($j_T$), and radial distributions of charged particles within jets ($r$). Also meaureed was the distribution of $ξ=-ln(z)$, where $z$ is the fraction of the jet momentum carried by the charged particle. The measurements are compared to theoretical next-to and next-to-next-to-leading-order calculatios, PYTHIA event generator, and to other existing experimental results. Indicated from these meaurements is a lower particle multiplicity in jets at RHIC energies when compared to models. Also noted are implications for future jet measurements with sPHENIX at RHIC as well as at the future Electron-Ion Collider.
The jet differential cross section as a function of jet $p_T$. Statistical uncertainties are typically smaller than the data points while systematic uncertainties are shown with boxes. An overall normalization systematic of 7% is not included in the point-by-point systematic uncertainties.
Distribution of the SoftDrop groomed momentum fraction $z_g$ for different jet $p_T$ bins. Standard SoftDrop parameters were used ($z_{cut}<0.1$ and $\beta=0$).
$\xi$ distributions for different jet $p_T$ bins.
Ultrarelativistic heavy-ion collisions produce a state of hot and dense strongly interacting QCD matter called quark--gluon plasma (QGP). On an event-by-event basis, the volume of the QGP in ultracentral collisions is mostly constant, while its total entropy can vary significantly due to quantum fluctuations, leading to variations in the temperature of the system. Exploiting this unique feature of ultracentral collisions allows for the interpretation of the correlation of the mean transverse momentum of produced charged hadrons and the number of charged hadrons as a measure for the speed of sound. It is determined by fitting the relative increase in transverse momentum with respect to the relative change in the average charged-particle density measured at midrapidity. This study reports the event-average transverse momentum of charged particles as well as the self-normalized variance, skewness, and kurtosis of the event-by-event transverse momentum distribution in ultracentral Pb-Pb collisions at a center-of-mass energy of 5.02 TeV per nucleon pair using the ALICE detector. Different centrality estimators based on charged-particle multiplicity or the transverse energy of the event are used to select ultracentral collisions. By ensuring a pseudorapidity gap between the region used to define the centrality and the region used to perform the measurement, the influence of biases on the rise of the mean transverse momentum is tested. The measured values are found to strongly depend on the exploited centrality estimator. The variance shows a steep decrease towards ultracentral collisions, while the skewness variables show a maximum, followed by a fast decrease. These non-Gaussian features are understood in terms of the vanishing of the impact-parameter fluctuations contributing to the event-to-event transverse momentum distribution.
Average number of participating nucleons ($\langle N_{\mathrm{part}} \rangle$) as a function of centrality percentile in $\mathrm{Pb}-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02~\mathrm{TeV}$. Data points are shown for centrality estimators based on $N_{\mathrm{ch}}$, ${N_{\mathrm{tracklets}}}$, and $E_{\mathrm{T}}$ within $|\eta|\leq 0.8$.
Average number of participating nucleons ($\langle N_{\mathrm{part}} \rangle$) as a function of centrality percentile in $\mathrm{Pb}-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02~\mathrm{TeV}$. Data points are shown for centrality estimator based on $N_{\mathrm{ch}} \in$ $-3.7<\eta<-1.7$ and $2.8 < \eta <5.1$.
Normalized $p_{\mathrm{T}}$-spectrum ratio as a function as a function of centrality in $\mathrm{Pb}-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02~\mathrm{TeV}$. Data points are shown for centrality estimator based on $N_{\mathrm{ch}} \in$ $0.5 \leq |\eta|\leq 0.8$.
The STAR Collaboration reports measurements of acoplanarity using semi-inclusive distributions of charged-particle jets recoiling from direct photon and $\pi^{0}$ triggers, in central Au-Au and pp collisions at $\sqrt{s_{\rm NN}}=200$ GeV. Significant medium-induced acoplanarity broadening is observed for large but not small recoil jet resolution parameter, corresponding to recoil jet yield enhancement up to a factor of $\approx20$ for trigger-recoil azimuthal separation far from $\pi$. This phenomenology is indicative of the response of the Quark-Gluon Plasma to excitation, but not the scattering of jets off of its quasiparticles. The measurements are not well-described by current theoretical models which incorporate jet quenching.
Corrected Yield R=0.2 pi0+jet 10-15 pp at sqrt{s_{NN}}=200 GeV
Corrected Yield R=0.2 pi0+jet 15-20 pp at sqrt{s_{NN}}=200 GeV
Corrected Yield R=0.5 pi0+jet 10-15 pp at sqrt{s_{NN}}=200 GeV
The polarization of Lambda, Anti-Lambda, Xi, and Anti-Xi hyperons along the angular momentum of the system has been measured in isobar collisions of Ru+Ru and Zr+Zr at $\sqrt{s_{NN}}$ = 200 GeV with the STAR detector at RHIC. The polarization dependence on collision centrality is explored and found to show an increasing trend in more peripheral collisions. Dependencies on transverse momentum and pseudorapidity are investigated for Lambda and Anti-Lambda hyperons, but no significant dependence has been observed. The polarization measurements for Lambda and Anti-Lambda are consistent with each other, indicating little contribution of the spin-magnetic coupling in the observed polarization. The results for Lambda hyperons measurements are qualitatively consistent with hydrodynamic calculations incorporating effects from shear-induced polarization and thermal vorticity, and show no obvious system size dependence in comparison with previous results in Au+Au collisions. For the first time, the dependence of the polarization on the hyperon's emission azimuthal angle with respect to the second harmonic event plane is extracted and shows stronger polarization for the in-plane emitted hyperons at the level of 2.4$\sigma$ significance in 20-50% centrality. The measurements of Xi hyperons polarization via the polarization transfer analysis exhibit a finite positive polarization, 2.9$\sigma$ significance in 20-50% centrality, slightly enhanced compared to the inclusive Lambda polarization.
Resolutions of the first-order event plane from the ZDC-SMDs as a function of collision centrality in Ru+Ru and Zr+Zr collisions at $\sqrt{s_{NN}}$ = 200 GeV.
Resolutions of the second-order event plane from the TPC as a function of collision centrality in Ru+Ru and Zr+Zr collisions at $\sqrt{s_{NN}}$ = 200 GeV.
Global polarization of $\Lambda$ and $\bar\Lambda$ and their difference as a function of centrality in combined Ru+Ru and Zr+Zr collisions at $\sqrt{s_{NN}}$ = 200 GeV.
This work reports femtoscopic correlations of p$-$p ($\bar{\rm p}-\bar{\rm p}$) and p$-$d ($\bar{\rm p}-\bar{\rm d}$) pairs measured in Pb$-$Pb collisions at center-of-mass energy per nucleon $\sqrt{s_{\rm NN}}$ = 5.02 TeV in the ALICE Collaboration. A fit to the measured proton-proton correlation functions allows one to extract the dependence of the nucleon femtoscopic radius of the particle-emitting source on the pair transverse mass ($m_\text{T}$) and on the average charge particle multiplicity $\langle\text{dN}_\text{ch}/\text{d}η\rangle^{1/3}$ for three centrality intervals (0$-$10$\%$, 10$-$30$\%$, 30$-$50$\%$). In both cases, the expected power-law and linear scalings are observed, respectively. The measured p$-$d correlations can be described by both two- and three-body calculations, indicating that the femtoscopy observable is not sensitive to the short-distance features of the dynamics of the p$-$(p$-$n) system, due to the large inter-particle distances in Pb$-$Pb collisions at the LHC. Indeed, in this study, the minimum measured femtoscopic source sizes for protons and deuterons have a minimum value at $2.73^{+0.05}_{-0.05}$ and $3.10^{+1.04}_{-0.86}$ fm, respectively, for the 30$-$50$\%$ centrality collisions. Moreover, the $m_{\rm{T}}$-scaling obtained for the p$-$p and p$-$d systems is compatible within 1$σ$ of the uncertainties. These findings provide new input for fundamental studies on the production of light (anti)nuclei under extreme conditions.
proton-proton (same charge) correlation function for centrality 0-10% from Pb-Pb collisions at 5020 GeV
proton-proton (same charge) correlation function for centrality 10-30% from Pb-Pb collisions at 5020 GeV
proton-proton (same charge) correlation function for centrality 30-50% from Pb-Pb collisions at 5020 GeV
This Letter presents measurements of long-range transverse-momentum correlations using a new observable, $v_{0}(p_\mathrm{T})$, which serves as a probe of radial flow and medium properties in heavy-ion collisions. Results are reported for inclusive charged particles, pions, kaons, and protons across various centrality intervals in Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV, recorded by the ALICE detector. A pseudorapidity-gap technique, similar to that used in anisotropic-flow studies, is employed to suppress short-range correlations. At low $p_\mathrm{T}$, a characteristic mass ordering consistent with hydrodynamic collective flow is observed. At higher $p_\mathrm{T}$ ($> 3$ GeV/$c$), protons exhibit larger $v_{0}(p_\mathrm{T})$ than pions and kaons, in agreement with expectations from quark-recombination models. These results are sensitive to the bulk viscosity and the equation of state of the QCD medium formed in heavy-ion collisions.
The $p_\mathrm{T}$ dependence of $v_{0}(p_\mathrm{T})$ for inclusive charged particles is measured in Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV within the 10$–$20% centrality interval, using a two-particle correlation method with a pseudorapidity gap of $\Delta\eta = 0.4$.
The $p_\mathrm{T}$ dependence of $v_{0}(p_\mathrm{T})$ for inclusive charged particles is measured in Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV within the 30$–$40% centrality interval, using a two-particle correlation method with a pseudorapidity gap of $\Delta\eta = 0.4$.
The $p_\mathrm{T}$ dependence of $v_{0}(p_\mathrm{T})$ for inclusive charged particles is measured in Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV within the 60$–$70% centrality interval, using a two-particle correlation method with a pseudorapidity gap of $\Delta\eta = 0.4$.
The formation of light (anti)nuclei with mass number A of a few units (e.g., d, $^3$He, and $^4$He) in high-energy hadronic collisions presents a longstanding mystery in nuclear physics [1,2]. It is not clear how nuclei bound by a few MeV can emerge in environments characterized by temperatures above 100 MeV [3-5], about 100,000 times hotter than the center of the Sun. Despite extensive studies, this question remained unanswered. The ALICE Collaboration now addresses it with a novel approach using deuteron-pion momentum correlations in proton-proton (pp) collisions at the Large Hadron Collider (LHC). Our results provide model-independent evidence that about 80% of the observed (anti)deuterons are produced in nuclear fusion reactions [6] following the decay of short-lived resonances, such as the $\Delta (1232)$. These findings resolve a crucial gap in our understanding of nucleosynthesis in hadronic collisions. Beyond answering the fundamental question on how nuclei are formed in hadronic collisions, the results can be employed in the modeling of the production of light and heavy nuclei in cosmic rays [7] and dark matter decays [8,9].
Measured $\pi^{+}$–d$\oplus\pi^{-}$–$\overline{\mathrm{d}}$ (left panel) correlation function.
Measured $\pi^{-}$–d$\oplus\pi^{+}$–$\overline{\mathrm{d}}$ (right panel) correlation function.
The extracted kinetic decoupling temperature is derived from $\pi^{+}$–d correlation functions.
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 non-perturbative QCD on jet formation and structure. The angular differences between various jet axes, $ΔR_{\rm axis}$, 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 $\sqrt{s}=5.02$ TeV by the ALICE experiment at the LHC is presented for jets with transverse momentum $p_{\rm T}^{\rm ch~jet} \geq 10$${\rm GeV}/c$ and D$^0$ mesons with $p_{\rm T}^{\rm D^{0}} \geq 5$${\rm 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, $ΔR_{\mathrm{axis-D^0}}$, are reported, along with the angle, $ΔR_{\mathrm{axis}}$, 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.
$\Delta R_{\rm axis}$ distribution for $\rm STD-D^0$ for $\rm D^0$-tagged jets of $R=0.4$, in the intervals $10<p_{\rm T}^{\rm ch \ jet}<20 \ {\rm GeV}/c$ and $5<p_{\rm T}^{\rm D^0}<20 \ {\rm GeV}/c$.
$\Delta R_{\rm axis}$ distribution for $\rm WTA-D^0$ for $\rm D^0$-tagged jets of $R=0.4$, in the intervals $10<p_{\rm T}^{\rm ch \ jet}<20 \ {\rm GeV}/c$ and $5<p_{\rm T}^{\rm D^0}<20 \ {\rm GeV}/c$.
$\Delta R_{\rm axis}$ distribution for $\rm STD-WTA$ for $\rm D^0$-tagged jets of $R=0.4$, in the intervals $10<p_{\rm T}^{\rm ch \ jet}<20 \ {\rm GeV}/c$ and $5<p_{\rm T}^{\rm D^0}<20 \ {\rm GeV}/c$.