In ultra-relativistic heavy ion collisions at the LHC, each nucleus acts a sources of high-energy real photons that can scatter off the opposing nucleus in ultra-peripheral photonuclear ($\gamma+A$) collisions. Hard scattering processes initiated by the photons in such collisions provide a novel method for probing nuclear parton distributions in a kinematic region not easily accessible to other measurements. ATLAS has measured production of dijet and multi-jet final states in ultra-peripheral Pb+Pb collisions at $\sqrt{s_{\text{NN}}} = 5.02$ TeV using a data set recorded in 2018 with an integrated luminosity of 1.72 $\text{nb}^{-1}$. Photonuclear final states are selected by requiring a rapidity gap in the photon direction; this selects events where one of the outgoing nuclei remains intact. Jets are reconstructed using the anti-$k_\text{t}$ algorithm with radius parameter, $R = 0.4$. Triple-differential cross-sections, unfolded for detector response, are measured and presented using two sets of kinematic variables. The first set consists of the total transverse momentum ($H_\text{T}$),rapidity, and mass of the jet system. The second set uses $H_\text{T}$ and particle-level nuclear and photon parton momentum fractions, $x_\text{A}$ and $z_{\gamma}$, respectively. The results are compared with leading-order (LO) perturbative QCD calculations of photonuclear jet production cross-sections, where all LO predictions using existing fits fall below the data in the shadowing region. More detailed theoretical comparisons will allow these results to strongly constrain nuclear parton distributions, and these data provide results from the LHC directly comparable to early physics results at the planned Electron-Ion Collider.
The fraction of photonuclear jet events passing the fiducial requirements in which the photon-emitting nucleus does not break up as a function of \zg. The systematic uncertainties are not symmetrized, and correlations in uncertainties are neglected for both the total systematic uncertainty and statistical uncertainty.
Fully unfolded triple-differential cross-sections as a function of $H_\text{T}$, $y_\text{jets}$, and $m_\text{jets}$. Systematic uncertainties are decomposed into symmetrized nuisance parameters, where parameters labelled "Corr" are fully correlated bin-to-bin, while parameters labelled "Uncorr" should be treated as un-correlated bin-to-bin. These cross-sections are not corrected for the effects of additional nuclear break-up. Values for the total fiducial cross-section in each bin are reported with full statistical and systematic uncertainties. Fractions of the total bin volume occupied by the fiducial region, fractions of the total cross-section in that bin satisfying fiducial requirements, and mean bin values for each axis variable are derived from Pythia 8 Monte Carlo and reported as well. For more details on these quantities, see Appendix B.
Fully unfolded triple-differential cross-sections as a function of $H_\text{T}$, $x_\text{A}$, and $z_{\gamma}$. Systematic uncertainties are decomposed into symmetrized nuisance parameters, where parameters labelled "Corr" are fully correlated bin-to-bin, while parameters labelled "Uncorr" should be treated as un-correlated bin-to-bin. These cross-sections are not corrected for the effects of additional nuclear break-up. Values for the total fiducial cross-section in each bin are reported with full statistical and systematic uncertainties. Fractions of the total bin volume occupied by the fiducial region, fractions of the total cross-section in that bin satisfying fiducial requirements, and mean bin values for each axis variable are derived from Pythia 8 Monte Carlo and reported as well. For more details on these quantities, see Appendix B.
This Letter presents a search for highly ionizing magnetic monopoles in 262$~\mu$b$^{-1}$ of ultraperipheral Pb+Pb collision data at $\sqrt{s_{_\textrm{NN}}}=5.36$ TeV collected by the ATLAS detector at the LHC. A new methodology that exploits the properties of clusters of hits reconstructed in the innermost silicon detector layers is introduced to study highly ionizing particles in heavy-ion data. No significant excess above the background, which is estimated using a data-driven technique, is observed. Using a nonperturbative semiclassical model, upper limits at 95% confidence level are set on the cross-section for pair production of monopoles with a single Dirac magnetic charge in the mass range of 20-150 GeV. The search significantly improves on the previous cross-section limits for production of low-mass monopoles in ultraperipheral Pb+Pb collisions.
Expected and observed cross-section upper limits computed using the CL$_{s}$ method for $|q_{m}| = 1 g_{\textrm{D}}$ and assuming FPA model
We report multi-differential measurements of strange hadron production ranging from mid- to target-rapidity in Au+Au collisions at a center-of-momentum energy per nucleon pair of $\sqrt{s_{\rm NN}}=3$ GeV with the STAR experiment at RHIC. $K^0_S$ meson and $\Lambda$ hyperon yields are measured via their weak decay channels. Collision centrality and rapidity dependences of the transverse momentum spectra and particle ratios are presented. Particle mass and centrality dependence of the average transverse momenta of $\Lambda$ and $K^0_S$ are compared with other strange particles, providing evidence of the development of hadronic rescattering in such collisions. The 4$\pi$ yields of each of these strange hadrons show a consistent centrality dependence. Discussions on radial flow, the strange hadron production mechanism, and properties of the medium created in such collisions are presented together with results from hadronic transport and thermal model calculations.
dN/dy of lambda for different centrality bins.
dN/dy of Ks0 for different centrality bins.
Rapidity dependence of Lambda/p for different centrality bins.
High-energy nuclear collisions create a quark-gluon plasma, whose initial condition and subsequent expansion vary from event to event, impacting the distribution of the event-wise average transverse momentum ($P([p_{\mathrm{T}}])$). Distinguishing between contributions from fluctuations in the size of the nuclear overlap area (geometrical component) and other sources at fixed size (intrinsic component) presents a challenge. Here, these two components are distinguished by measuring the mean, variance, and skewness of $P([p_{\mathrm{T}}])$ in $^{208}$Pb+$^{208}$Pb and $^{129}$Xe+$^{129}$Xe collisions at $\sqrt{s_{{\mathrm{NN}}}} = 5.02$ and 5.44 TeV, respectively, using the ATLAS detector at the LHC. All observables show distinct changes in behavior in ultra-central collisions, where the geometrical variations are suppressed as the overlap area reaches its maximum. These results demonstrate a new technique to disentangle geometrical and intrinsic fluctuations, enabling constraints on initial condition and properties of the quark-gluon plasma, such as the speed of sound.
Data from Figure 1, panel a, $\left\langle[p_{T}]\right\rangle$ vs $N_{ch}$ for Pb+Pb collisions, 0.5 $ <p_{T}< $ 5 GeV/c, $|\eta|< $ 2.5
Data from Figure 1, panel b, $\left\langle[p_{T}]\right\rangle$ vs $N_{ch}$ for Pb+Pb collisions, 0.5 $ <p_{T}< $ 5 GeV/c, $|\eta|< $ 2.5
Data from Figure 1, panel b, $\left\langle[p_{T}]\right\rangle$ vs $N_{ch}$ for Xe+Xe collisions, 0.5 $ <p_{T}< $ 5 GeV/c, $|\eta|< $ 2.5
In heavy-ion collision experiments, the global collectivity of final-state particles can be quantified by anisotropic flow coefficients ($v_n$). The first-order flow coefficient, also referred to as the directed flow ($v_{1}$), describes the collective sideward motion of produced particles and nuclear fragments in heavy-ion collisions. It carries information on the very early stage of the collision, especially at large pseudorapidity ($\eta$), where it is believed to be generated during the nuclear passage time. Directed flow therefore probes the onset of bulk collective dynamics during thermalization, providing valuable experimental guidance to models of the pre-equilibrium stage. In 2018, the Event Plane Detector (EPD) was installed in STAR and used for the Beam Energy Scan phase-II (BES-II) data taking. The combination of EPD ($2.1 <|\eta|< 5.1$) and high-statistics BES-II data enables us to extend the $v_{1}$ measurement to the forward and backward $\eta$ regions. In this paper, we present the measurement of $v_{1}$ over a wide $\eta$ range in Au+Au collisions at $\sqrt{s_{NN}}=$ 19.6 and 27 GeV using the STAR EPD. The results of the analysis at $\sqrt{s_{NN}}=$19.6 GeV exhibit excellent consistency with the previous PHOBOS measurement, while elevating the precision of the overall measurement. The increased precision of the measurement also revealed finer structures in heavy-ion collisions, including a potential observation of the first-order event-plane decorrelation. Multiple physics models were compared to the experimental results. Only a transport model and a three-fluid hybrid model can reproduce a sizable $v_{1}$ at large $\eta$ as was observed experimentally. The model comparison also indicates $v_{1}$ at large $\eta$ might be sensitive to the QGP phase transition.
Directed flow vs pseudorapidity.
Directed flow vs pseudorapidity.
Directed flow vs pseudorapidity.
This paper reports the observation of top-quark pair production in proton-lead collisions in the ATLAS experiment at the Large Hadron Collider. The measurement is performed using 165 nb$^{-1}$ of $p$+Pb data collected at $\sqrt{s_\mathrm{NN}}=8.16$ TeV in 2016. Events are categorised in two analysis channels, consisting of either events with exactly one lepton (electron or muon) and at least four jets, or events with two opposite-charge leptons and at least two jets. In both channels at least one $b$-tagged jet is also required. Top-quark pair production is observed with a significance over five standard deviations in each channel. The top-quark pair production cross-section is measured to be $\sigma_{t\bar{t}}= 58.1\pm 2.0\;\mathrm{(stat.)\;^{+4.8}_{-4.4} \;\mathrm{(syst.)}}\;\mathrm{nb}$, with a total uncertainty of 9%. In addition, the nuclear modification factor is measured to be $R_{p\mathrm{A}} = 1.090\pm0.039\;(\mathrm{stat.})\;^{+0.094}_{-0.087}\;(\mathrm{syst.})$. The measurements are found to be in good agreement with theory predictions involving nuclear parton distribution functions.
The figure shows the pre-fit distribution of events as a function of $H_{\mathrm{T}}^{\ell,j} = \sum_{\ell,j} p_{T}^{\ell,j}$, scalar sum of $p_T$ for all jets and leptons in the $\ell+$jets channel, in proton-lead (p+Pb) collisions at a center-of-mass energy of $\sqrt{s_{\mathrm{NN}}} = 8.16$ TeV, with an integrated luminosity of 165 nb$^{-1}$. The data correspond to the $1\ell 1b$ $e$+jets channel in a pre-fit configuration. The stacked histograms represent different processes contributing to the event yield, including top quark pair production ($t\bar{t}$), single top, $W$ boson production with $b$, $c$, and light quarks, $Z$ boson production with $b$, $c$, and light quarks, diboson, and fake lepton backgrounds.
The figure shows the post-fit distribution of events as a function of $H_{\mathrm{T}}^{\ell,j} = \sum_{\ell,j} p_{T}^{\ell,j}$, scalar sum of $p_T$ for all jets and leptons in the $\ell+$jets channel, in proton-lead (p+Pb) collisions at a center-of-mass energy of $\sqrt{s_{\mathrm{NN}}} = 8.16$ TeV, with an integrated luminosity of 165 nb$^{-1}$. The data correspond to the $1\ell 1b$ $e$+jets channel in a pre-fit configuration. The stacked histograms represent different processes contributing to the event yield, including top quark pair production ($t\bar{t}$), single top, $W$ boson production with $b$, $c$, and light quarks, $Z$ boson production with $b$, $c$, and light quarks, diboson, and fake lepton backgrounds.
The figure shows the pre-fit distribution of events as a function of $H_{\mathrm{T}}^{\ell,j} = \sum_{\ell,j} p_{T}^{\ell,j}$, scalar sum of $p_T$ for all jets and leptons in the $\ell+$jets channel, in proton-lead (p+Pb) collisions at a center-of-mass energy of $\sqrt{s_{\mathrm{NN}}} = 8.16$ TeV, with an integrated luminosity of 165 nb$^{-1}$. The data correspond to the $1\ell 1b$ $\mu$+jets channel in a pre-fit configuration. The stacked histograms represent different processes contributing to the event yield, including top quark pair production ($t\bar{t}$), single top, $W$ boson production with $b$, $c$, and light quarks, $Z$ boson production with $b$, $c$, and light quarks, diboson, and fake lepton backgrounds.
We report results on an elastic cross section measurement in proton-proton collisions at a center-of-mass energy $\sqrt{s}=510$ GeV, obtained with the Roman Pot setup of the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The elastic differential cross section is measured in the four-momentum transfer squared range $0.23 \leq -t \leq 0.67$ GeV$^2$. We find that a constant slope $B$ does not fit the data in the aforementioned $t$ range, and we obtain a much better fit using a second-order polynomial for $B(t)$. The $t$ dependence of $B$ is determined using six subintervals of $t$ in the STAR measured $t$ range, and is in good agreement with the phenomenological models. The measured elastic differential cross section $\mathrm{d}\sigma/\mathrm{dt}$ agrees well with the results obtained at $\sqrt{s} = 546$ GeV for proton--antiproton collisions by the UA4 experiment. We also determine that the integrated elastic cross section within the STAR $t$-range is $\sigma^\mathrm{fid}_\mathrm{el} = 462.1 \pm 0.9 (\mathrm{stat.}) \pm 1.1 (\mathrm {syst.}) \pm 11.6 (\mathrm {scale})$~$\mu\mathrm{b}$.
Top panel: The $pp$ elastic differential cross section $d\sigma/dt$ fitted with an exponential $A e^{-B(t)|t|}$. Bottom panel: Residuals (Data - Fit)/Error. Uncertainties on the data points are smaller than the symbol size. The vertical scale uncertainty of 2.5% is not included in in the full error.
Results of the exponential function $A e^{-B(t)|t|}$ fit to the elastic differential cross section data as well as the integrated fiducial cross section are listed. Also listed are the corresponding values of the statistical and systematic uncertainties. The scale (luminosity and trigger efficiency) uncertainty of 2.5% applicable to the fit parameter $A$ and fiducial cross section $\sigma^\mathrm{fid}_\mathrm{el}$ is not included in the full error.
The measurement of hard scatterings in proton-nucleus collisions has resulted in a greater understanding of both the proton and nuclear structure. ATLAS measured the centrality dependence of the dijet yield using 165 nb$^{-1}$ of $p$+Pb data collected at $\sqrt{s_{_\text{NN}}}$ = 8.16 TeV in 2016. The event centrality, which reflects the $p$+Pb impact parameter, is characterized by the total transverse energy registered in the Pb-going side of the forward calorimeter. The central-to-peripheral ratio of the scaled dijet yields, $R_\mathrm{CP}$, is evaluated, and the results are presented as a function of variables that reflect the kinematics of the initial hard parton scattering process. The $R_\mathrm{CP}$ shows a scaling with the Bjorken-$x$ of the parton originating from the proton, $x_p$, while no such trend is observed as a function of $x_\mathrm{Pb}$. This analysis provides unique input to understanding the role of small proton spatial configurations in $p$+Pb collisions by covering parton momentum fractions from the valence region down to $x_p \sim 10^{-3}$ and $x_\mathrm{Pb}\sim 4\cdot10^{-4}$.
$R_\text{CP}$ plotted as a function of approximated $x_p$ for $-3.0 < y_b < -2.0$ and $0.0 < y^* < 1.0$, constructed using $\langle y_{\text{b}} \rangle$ and $\langle y^{*} \rangle$. The proton-going direction is defined by $y_{\text{b}} > 0$.
$R_\text{CP}$ plotted as a function of approximated $x_p$ for $-2.0 < y_b < -1.0$ and $0.0 < y^* < 1.0$, constructed using $\langle y_{\text{b}} \rangle$ and $\langle y^{*} \rangle$. The proton-going direction is defined by $y_{\text{b}} > 0$.
$R_\text{CP}$ plotted as a function of approximated $x_p$ for $-2.0 < y_b < -1.0$ and $1.0 < y^* < 2.0$, constructed using $\langle y_{\text{b}} \rangle$ and $\langle y^{*} \rangle$. The proton-going direction is defined by $y_{\text{b}} > 0$.
In relativistic heavy-ion collisions, a global spin polarization, $P_\mathrm{H}$, of $\Lambda$ and $\bar{\Lambda}$ hyperons along the direction of the system angular momentum was discovered and measured across a broad range of collision energies and demonstrated a trend of increasing $P_\mathrm{H}$ with decreasing $\sqrt{s_{NN}}$. A splitting between $\Lambda$ and $\bar{\Lambda}$ polarization may be possible due to their different magnetic moments in a late-stage magnetic field sustained by the quark-gluon plasma which is formed in the collision. The results presented in this study find no significant splitting at the collision energies of $\sqrt{s_{NN}}=19.6$ and $27$ GeV in the RHIC Beam Energy Scan Phase II using the STAR detector, with an upper limit of $P_{\bar{\Lambda}}-P_{\Lambda}<0.24$% and $P_{\bar{\Lambda}}-P_{\Lambda}<0.35$%, respectively, at a 95% confidence level. We derive an upper limit on the na\"ive extraction of the late-stage magnetic field of $B<9.4\cdot10^{12}$ T and $B<1.4\cdot10^{13}$ T at $\sqrt{s_{NN}}=19.6$ and $27$ GeV, respectively, although more thorough derivations are needed. Differential measurements of $P_\mathrm{H}$ were performed with respect to collision centrality, transverse momentum, and rapidity. With our current acceptance of $|y|<1$ and uncertainties, we observe no dependence on transverse momentum and rapidity in this analysis. These results challenge multiple existing model calculations following a variety of different assumptions which have each predicted a strong dependence on rapidity in this collision-energy range.
The first-order event-plane resolution determined by the STAR EPD as a function of collision centrality is roughly doubled in comparison to previous analyses using the STAR BBC. We see $R_{\rm EP}^{(1)}$ peak for mid-central collisions.
The mid-central $P_{\rm H}$ measurements reported in this work are shown alongside previous measurements in the upper panel, and are consistent with previous measurements at the energies studied here. The difference between integrated $P_{\bar{\Lambda}}$ and $P_{\Lambda}$ is shown at $\sqrt{s_{\rm{NN}}}$=19.6 and 27 GeV alongside previous measurements in the lower panel. The splittings observed with these high-statistics data sets are consistent with zero. Statistical uncertainties are represented as lines while systematic uncertainties are represented as boxes. The previous $P_{\bar{\Lambda}}-P_{\Lambda}$ result at $\sqrt{s_{\rm NN}}=7.7$ GeV is outside the axis range, but is consistent with zero within $2\sigma$.
$P_{\rm H}$ measurements are shown as a function of collision centrality at $\sqrt{s_{\rm NN}}$=19.6 and 27 GeV. Statistical uncertainties are represented as lines while systematic uncertainties are represented as boxes. $P_{\rm H}$ increases with collision centrality at $\sqrt{s_{\rm NN}}$=19.6 and 27 GeV, as expected from an angular-momentum-driven phenomenon.
Parton energy loss in the quark-gluon plasma (QGP) is studied with a measurement of photon-tagged jet production in 1.7 nb$^{-1}$ of Pb+Pb data and 260 pb$^{-1}$ of $pp$ data, both at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV, with the ATLAS detector. The process $pp \to \gamma$+jet+$X$ and its analogue in Pb+Pb collisions is measured in events containing an isolated photon with transverse momentum ($p_\mathrm{T}$) above $50$ GeV and reported as a function of jet $p_\mathrm{T}$. This selection results in a sample of jets with a steeply falling $p_\mathrm{T}$ distribution that are mostly initiated by the showering of quarks. The $pp$ and Pb+Pb measurements are used to report the nuclear modification factor, $R_\mathrm{AA}$, and the fractional energy loss, $S_\mathrm{loss}$, for photon-tagged jets. In addition, the results are compared with the analogous ones for inclusive jets, which have a significantly smaller quark-initiated fraction. The $R_\mathrm{AA}$ and $S_\mathrm{loss}$ values are found to be significantly different between those for photon-tagged jets and inclusive jets, demonstrating that energy loss in the QGP is sensitive to the colour-charge of the initiating parton. The results are also compared with a variety of theoretical models of colour-charge-dependent energy loss.
The differential cross-section of photon-tagged jets as a function of jet $p_{\mathrm{T}}$ in pp collisions.
The yields of photon-tagged jets as a function of jet $p_{\mathrm{T}}$ in Pb+Pb collisions for different centrality intervals.
The nuclear modification factor of photon-tagged jets as a function of jet $p_{\mathrm{T}}$ for different centrality intervals.
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