This paper presents the first observation of top-quark pair production in association with two photons ($t\bar{t}\gamma\gamma$). The measurement is performed in the single-lepton decay channel using proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider. The data correspond to an integrated luminosity of 140 fb$^{-1}$ recorded during Run 2 at a centre-of-mass energy of 13 TeV. The $t\bar{t}\gamma\gamma$ production cross section, measured in a fiducial phase space based on particle-level kinematic criteria for the lepton, photons, and jets, is found to be $2.42^{+0.58}_{-0.53}\, \text{fb}$, corresponding to an observed significance of 5.2 standard deviations. Additionally, the ratio of the production cross section of $t\bar{t}\gamma\gamma$ to top-quark pair production in association with one photon is determined, yielding $(3.30^{+0.70}_{-0.65})\times 10^{-3}$.
Measured $t\bar{t}\gamma\gamma$ production fiducial inclusive cross-section in single-lepton decay channel.
Measured ratio of production cross sections of $t\bar{t}\gamma\gamma$ to $t\bar{t}\gamma$ in single-lepton decay channel.
Summary of the relative impact of all the systematic uncertainties, in percentage, on the $t\bar{t}\gamma\gamma$ fiducial inclusive cross section and $R_{t\bar{t}\gamma\gamma/t\bar{t}\gamma}$ grouped into different categories. The category ‘Jet’ corresponds to the effect of JES, jet resolution and JVT uncertainties, ‘Photon’ and ‘Leptons’ include all experimental uncertainties related to photons and leptons (including trigger uncertainties), respectively.
The vacuum is now understood to possess a rich and complex structure, characterized by fluctuating energy fields and a condensate of virtual quark-antiquark pairs. The spontaneous breaking of the approximate chiral symmetry, signaled by the nonvanishing quark condensate $\langle q\bar{q}\rangle$, is dynamically generated through topologically nontrivial gauge configurations such as instantons. The precise mechanism linking the chiral symmetry breaking to the mass generation associated with quark confinement remains a profound open question in Quantum Chromodynamics (QCD) - the fundamental theory of strong interaction. High energy proton-proton collisions could liberate virtual quark-antiquark pairs from the vacuum that subsequently undergo confinement to form hadrons, whose properties could serve as probes into QCD confinement and the quark condensate. Here, we report evidence of spin correlations in $Λ\barΛ$ hyperon pairs inherited from spin-correlated strange quark-antiquark virtual pairs. Measurements by the STAR experiment at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory reveal a relative polarization signal of $(18 \pm 4)\%$ that links the virtual spin-correlated quark pairs from the QCD vacuum to their final-state hadron counterparts. Crucially, this correlation vanishes when the hyperon pairs are widely separated in angle, consistent with the decoherence of the quantum system. Our findings provide a new experimental paradigm for exploring the dynamics and interplay of quark confinement and entanglement.
$\mathrm{d}N/\mathrm{d}\cos\theta^\star$ distributions of decay (anti-)protons for short-range pairs ($|\Delta y| < 0.5$ and $|\Delta \phi| < \pi/3$) $\Lambda\bar{\Lambda}$, $\Lambda\Lambda$, and $\bar{\Lambda}\bar{\Lambda}$ hyperon pairs measured at mid-rapidity ($|y| < 1$).
$\mathrm{d}N/\mathrm{d}\cos\theta^\star$ distributions of decay (anti-)protons for long-range pairs ($0.5 < |\Delta y| < 2.0$ or $\pi/3 <|\Delta \phi| < \pi$) $\Lambda\bar{\Lambda}$, $\Lambda\Lambda$, and $\bar{\Lambda}\bar{\Lambda}$ hyperon pairs measured at mid-rapidity ($|y| < 1$).
Spin correlation $P_\mathrm{\Lambda_1\Lambda_2}$ of short-range $\Lambda\bar{\Lambda}$ (1), $\Lambda\Lambda$ (2), and $\bar{\Lambda}\bar{\Lambda}$ (3) hyperon pars and $K^0_\mathrm{S}K^0_\mathrm{S}$ (4) meson pairs.
While dual-phase xenon time projection chambers (TPCs) have driven the sensitivity towards weakly interacting massive particles (WIMPs) at the GeV/c^2 to TeV/c^2 mass scale, the scope for sub-GeV/c^2 dark matter particles is hindered by a limited nuclear recoil energy detection threshold. One approach to probe for lighter candidates is to consider cases where they have been boosted by collisions with cosmic rays in the Milky Way, such that the additional kinetic energy lifts their induced signatures above the nominal threshold. In this Letter, we report first results of a search for cosmic ray-boosted dark matter (CRDM) with a combined 4.2 tonne-year exposure from the LUX-ZEPLIN (LZ) experiment. We observe no excess above the expected backgrounds and establish world-leading constraints on the spin-independent CRDM-nucleon cross section as small as 3.9 * 10^{-33} cm^2 at 90% confidence level for sub-GeV/c^2 masses.
90% CL CRDM-nucleon cross sections
Measurements of $W^+W^-\rightarrow e^\pm νμ^\mp ν$ production cross-sections are presented, providing a test of the predictions of perturbative quantum chromodynamics and the electroweak theory. The measurements are based on data from $pp$ collisions at $\sqrt{s}=13$ TeV recorded by the ATLAS detector at the Large Hadron Collider in 2015-2018, corresponding to an integrated luminosity of 140 fb$^{-1}$. The number of events due to top-quark pair production, the largest background, is reduced by rejecting events containing jets with $b$-hadron decays. An improved methodology for estimating the remaining top-quark background enables a precise measurement of $W^+W^-$ cross-sections with no additional requirements on jets. The fiducial $W^+W^-$ cross-section is determined in a maximum-likelihood fit with an uncertainty of 3.1%. The measurement is extrapolated to the full phase space, resulting in a total $W^+W^-$ cross-section of $127\pm4$ pb. Differential cross-sections are measured as a function of twelve observables that comprehensively describe the kinematics of $W^+W^-$ events. The measurements are compared with state-of-the-art theory calculations and excellent agreement with predictions is observed. A charge asymmetry in the lepton rapidity is observed as a function of the dilepton invariant mass, in agreement with the Standard Model expectation. A CP-odd observable is measured to be consistent with no CP violation. Limits on Standard Model effective field theory Wilson coefficients in the Warsaw basis are obtained from the differential cross-sections.
Measured fiducial cross-section compared with theoretical predictions from MiNNLO+Pythia8, Geneva+Pythia8, Sherpa2.2.12, and MATRIX2.1. The predictions are based on the NNPDF3.0 (red squares) and NNPDF3.1 luxQED (blue dots) PDF sets. The nNNLO predictions include photon-induced contributions (always using NNPDF3.1 luxQED) and NLO QCD corrections to the gluon-gluon initial state. The $q\bar{q}\rightarrow WW$ predictions from MiNNLO, Geneva, and Sherpa2.2.12 are combined with a Sherpa2.2.2 prediction of gluon-induced production, scaled by an inclusive NLO K-factor of 1.7. Inner (outer) error bars on the theory predictions correspond to PDF (the combination of scale and PDF) uncertainties. The MATRIX nNNLO QCD $\otimes$ NLO EW prediction using NNPDF3.1 luxQED, the best available prediction of the integrated fiducial cross-section, is in good agreement with the measurement.
Fiducial differential cross-sections as a function of $p_{\mathrm{T}}^{\mathrm{lead.\,lep.}}$. The measured cross-section values are shown as points with error bars giving the statistical uncertainty and solid bands indicating the size of the total uncertainty. The right-hand-side axis indicates the integrated cross-section of the rightmost bin. The results are compared to fixed-order nNNLO QCD + NLO EW predictions of Matrix 2.1, with the NNLO + PS predictions from Powheg MiNNLO + Pythia8 and Geneva + Pythia8, as well as Sherpa2.2.12 NLO + PS predictions. The last three predictions are combined with Sherpa 2.2.2 for the $gg$ initial state and Sherpa 2.2.12 for electroweak $WWjj$ production. These contributions are modelled at LO but a NLO QCD $k$-factor of 1.7 is applied for gluon induced production. Theoretical predictions are indicated as markers with vertical lines denoting PDF, scale and parton shower uncertainties. Markers are staggered for better visibility.
Correlation matrix of the statistical uncertainties in the measured fiducial cross section for the observable $p_{\mathrm{T}}^{\mathrm{lead.\,lep.}}$.
The STAR Collaboration reports measurements of acoplanarity using semi--inclusive distributions of charged--particle jets recoiling from direct photon and $π^{0}$ triggers, in central Au+Au and $pp$ collisions at $\sqrt{s_\mathrm{NN}}=200$ 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 $π$. 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.
Partonic collectivity is one of the necessary signatures for the formation of quark-gluon plasma in high-energy nuclear collisions. Number of constituent quarks (NCQ) scaling has been observed for hadron elliptic flow $v_2$ in top energy nuclear collisions at the Relativistic Heavy Ion Collider and the LHC, and this has been theoretically suggested as strong evidence for partonic collectivity. In this Letter, a systematic analysis of $v_2$ of $π^{\pm}$, $K^{\pm}$, $K^{0}_{S}$, $p$, and $Λ$ in Au+Au collisions at ${\sqrt{s_{_{\rm{NN}}}}}$ = 3.2, 3.5, 3.9, and 4.5 GeV, with the STAR experiment at the Relativistic Heavy Ion Collider, is presented. NCQ scaling is markedly violated at 3.2 GeV, consistent with a hadronic-interaction dominated equation of state. However, as the collision energy increases, a gradual evolution to NCQ scaling is observed. This beam-energy dependence of $v_2$ for all hadrons studied provides evidence for the onset of dominant partonic interactions by ${\sqrt{s_{_{\rm{NN}}}}}$ = 4.5 GeV.
$p_{T}$ dependence of $v_{2}$ for $\pi^{+}$ in Au+Au collisions at 3 GeV
$p_{T}$ dependence of $v_{2}$ for $\pi^{+}$ in Au+Au collisions at 3.2 GeV
$p_{T}$ dependence of $v_{2}$ for $\pi^{+}$ in Au+Au collisions at 3.5 GeV
We report precision measurements on cumulants ($C_{n}$) and factorial cumulants ($\kappa_{n}$) of (net-)proton number distributions up to fourth-order in Au+Au collisions from phase II of the Beam Energy Scan program at RHIC. (Anti-)protons are selected at midrapidity ($|y|<0.5$) within a transverse momentum range of $0.4 < p_T < 2.0$ GeV/$c$. The collision energy and centrality dependence of these cumulants are studied over center-of-mass energies $\sqrt{s_{NN}}$ = 7.7 -- 27 GeV. Relative to various non-critical-point model calculations and peripheral collision 70-80% data, the net-proton $C_4/C_2$ measurement in 0-5% collisions shows a minimum around 19.6 GeV for significance of deviation at $\sim2$--$5\sigma$. In addition, deviations from non-critical baselines around the same collision energy region are also seen in proton factorial cumulant ratios, especially in $\kappa_2/\kappa_1$ and $\kappa_3/\kappa_1$. Dynamical model calculations including a critical point are called for in order to understand these precision measurements.
Reference multiplicity distributions (RefMult3 and RefMult3X) for Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6 GeV along with MC Glauber model fits.
Collision centrality dependence of net-proton cumulants in Au+Au collisions for $\sqrt{s_{NN}}$ = 7.7 - 27 GeV obtained with RefMult3 and RefMult3X centrality definition.
Collision energy dependence of net-proton cumulant ratios and proton factorial cumulant ratios in Au+Au collisions from $\sqrt{s_{NN}}$ = 7.7 - 200 GeV for 0-5$\%$ and 70-80$\%$ centrality. Data from $\sqrt{s_{NN}}$ = 7.7 - 27 GeV are from BES-II (obtained with RefMult3X except 27 GeV where RefMult3 is used). Data from $\sqrt{s_{NN}}$ = 39 - 200 GeV are from BES-I (RefMult3).
The mass of the top quark is measured using top-antitop-quark pair events with high transverse momentum top quarks. The dataset, collected with the ATLAS detector in proton--proton collisions at $\sqrt{s}=13$ TeV delivered by the Large Hadron Collider, corresponds to an integrated luminosity of 140 fb$^{-1}$. The analysis targets events in the lepton-plus-jets decay channel, with an electron or muon from a semi-leptonically decaying top quark and a hadronically decaying top quark that is sufficiently energetic to be reconstructed as a single large-radius jet. The mean of the invariant mass of the reconstructed large-radius jet provides the sensitivity to the top quark mass and is simultaneously fitted with two additional observables to reduce the impact of the systematic uncertainties. The top quark mass is measured to be $m_t = 172.95 \pm 0.53$ GeV, which is the most precise ATLAS measurement from a single channel.
Values and uncertainties for the parameters of interest in the profile likelihood fit to $\overline{m_J}$, $m_{jj}$, and $m_{tj}$ using data. The parameters of interest are the top quark mass, $m_t$, and the ratio of the measured cross-section to the Standard Model expectation of the $t\bar{t}$ cross-section, $\mu$.
Post-fit central values and uncertaintes for the nuisance parameters (including MC stat uncertainty terms) used in the profile likelihood fit to $\overline{m_J}$, $m_{jj}$, and $m_{tj}$ using data.
Covariance matrix for the profile likelihood fit to $\overline{m_J}$, $m_{jj}$, and $m_{tj}$ using data.
Hard-scattered partons ejected from high-energy proton-proton collisions undergo parton shower and hadronization, resulting in collimated collections of particles that are clustered into jets. A substructure observable that highlights the transition between the perturbative and non-perturbative regimes of jet evolution in terms of the angle between two particles is the two-point energy correlator (EEC). In this letter, the first measurement of the EEC at RHIC is presented, using data taken from 200 GeV $p$+$p$ collisions by the STAR experiment. The EEC is measured both for all the pairs of particles in jets and separately for pairs with like and opposite electric charges. These measurements demonstrate that the transition between perturbative and non-perturbative effects occurs within an angular region that is consistent with expectations of a universal hadronization regime that scales with jet momentum. Additionally, a deviation from Monte-Carlo predictions at small angles in the charge-selected sample could result from mechanics of hadronization not fully captured by current models.
Corrected distributions of the normalized EEC differential in $R_{L}$ for $R_{\rm jet}=$ 0.6, with jet transverse momentum selections 15 $< p_{\rm T, jet} <$ 20 GeV/c and 30 $< p_{\rm T, jet} <$ 50 GeV/c
Corrected distributions of the normalized EEC within jets, differential in $ \left\langle p_{\rm T,jet} \right\rangle R_{L} $ at $R_{\rm jet} =$ 0.6 for one $p_{\rm T, jet}$ selection. Each distribution is normalized to integrate to one in $R_{L}$ prior to shifting.
Corrected distributions of the normalized EEC within jets, differential in $ \left\langle p_{\rm T,jet} \right\rangle R_{L} $ at $R_{\rm jet} =$ 0.6 for one $p_{\rm T, jet}$ selection. Each distribution is normalized to integrate to one in $R_{L}$ prior to shifting.
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