The collective behavior of K$^0_\mathrm{S}$ and $\Lambda/\bar{\Lambda}$ strange hadrons is studied by measuring the elliptic azimuthal anisotropy ($v_2$) using the scalar-product and multiparticle correlation methods. Proton-lead (pPb) collisions at a nucleon-nucleon center-of-mass energy $\sqrt{s_\mathrm{NN}}$ = 8.16 TeV and lead-lead (PbPb) collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV collected by the CMS experiment at the LHC are investigated. Nonflow effects in the pPb collisions are studied by using a subevent cumulant analysis and by excluding events where a jet with transverse momentum greater than 20\GeV is present. The strange hadron $v_2$ values extracted in \pPb collisions via the four- and six-particle correlation method are found to be nearly identical, suggesting the collective behavior. Comparisons of the pPb and PbPb results for both strange hadrons and charged particles illustrate how event-by-event flow fluctuations depend on the system size.

Using 1.8 fb-1 of pp collisions at a center-of-mass energy of 7 TeV recorded by the ATLAS detector at the Large Hadron Collider, we present measurements of the production cross sections of Upsilon(1S,2S,3S) mesons. Upsilon mesons are reconstructed using the di-muon decay mode. Total production cross sections for p_T<70 GeV and in the rapidity interval |Upsilon|<2.25 are measured to be 8.01+-0.02+-0.36+-0.31 nb, 2.05+-0.01+-0.12+-0.08 nb, 0.92+-0.01+-0.07+-0.04 nb respectively, with uncertainties separated into statistical, systematic, and luminosity measurement effects. In addition, differential cross section times di-muon branching fractions for Upsilon(1S), Upsilon(2S), and Upsilon(3S) as a function of Upsilon transverse momentum p_T and rapidity are presented. These cross sections are obtained assuming unpolarized production. If the production polarization is fully transverse or longitudinal with no azimuthal dependence in the helicity frame the cross section may vary by approximately +-20%. If a non-trivial azimuthal dependence is considered, integrated cross sections may be significantly enhanced by a factor of two or more. We compare our results to several theoretical models of Upsilon meson production, finding that none provide an accurate description of our data over the full range of Upsilon transverse momenta accessible with this dataset.

Azimuthal decorrelations between the two central jets with the largest transverse momenta are sensitive to the dynamics of events with multiple jets. We present a measurement of the normalized differential cross section based on the full dataset (L=36/pb) acquired by the ATLAS detector during the 2010 sqrt(s)=7 TeV proton-proton run of the LHC. The measured distributions include jets with transverse momenta up to 1.3 TeV, probing perturbative QCD in a high energy regime.

The first search for narrow resonances decaying to three well-separated hadronic jets is presented. The search uses proton-proton collision data corresponding to an integrated luminosity of 138 fb$^{-1}$ at $\sqrt{s}$ = 13 TeV, collected at the CERN LHC. No significant deviations from the background predictions are observed between 1.75-9.00 TeV. The results provide the first mass limits on a right-handed boson Z$_{\mathrm{R}}$ decaying to three gluons, an excited quark decaying via a vector boson to three quarks, as well as updated limits on a Kaluza-Klein gluon decaying via a radion to three gluons.

A search is presented for fractionally charged particles with charge below 1$e$, using their small energy loss in the tracking detector as a key variable to observe a signal. The analyzed data set corresponds to an integrated luminosity of 138 fb$^{-1}$ of proton-proton collisions collected at $\sqrt{s}$ = 13 TeV in 2016-2018 at the CERN LHC. This is the first search at the LHC for new particles with charges between $e/$3 and $e$. Masses up to 640 GeV and charges as low as $e/$3 are excluded at 95% confidence level. These are the most stringent limits to date for the considered Drell-Yan-like production mode.

The first search for scalar leptoquarks produced in $\tau$-lepton-quark collisions is presented. It is based on a set of proton-proton collision data recorded with the CMS detector at the LHC at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 138 fb$^{-1}$. The reconstructed final state consists of a jet, significant missing transverse momentum, and a $\tau$ lepton reconstructed through its hadronic or leptonic decays. Limits are set on the product of the leptoquark production cross section and branching fraction and interpreted as exclusions in the plane of the leptoquark mass and the leptoquark-$\tau$-quark coupling strength.

The prompt and non-prompt production cross-sections for the chi_c1 and chi_c2 charmonium states are measured in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector at the LHC using 4.5 fb^-1 of integrated luminosity. The chi_c states are reconstructed through the radiative decay chi_c -> J/psi gamma (with J/psi -> mu+mu-) where photons are reconstructed from gamma -> e+e- conversions. The production rate of the chi_c2 state relative to the chi_c1 state is measured for prompt and non-prompt chi_c as a function of J/psi transverse momentum. The prompt chi_c cross-sections are combined with existing measurements of prompt J/psi production to derive the fraction of prompt J/psi produced in feed-down from chi_c decays. The fractions of chi_c1 and chi_c2 produced in b-hadron decays are also measured.

This Letter presents a search for magnetic monopoles with the ATLAS detector at the CERN Large Hadron Collider using an integrated luminosity of 2.0 fb^-1 of pp collisions recorded at a center-of-mass energy of sqrt(s)=7 TeV. No event is found in the signal region, leading to an upper limit on the production cross section at 95% confidence level of 1.6/epsilon fb for Dirac magnetic monopoles with the minimum unit magnetic charge and with mass between 200 GeV and 1500 GeV, where epsilon is the monopole reconstruction efficiency. The efficiency epsilon is high and uniform in the fiducial region given by pseudorapidity |eta|<1.37 and transverse kinetic energy 600-700<E^kin sin(theta)<1400 GeV. The minimum value of 700 GeV is for monopoles of mass 200 GeV, whereas the minimum value of 600 GeV is applicable for higher mass monopoles. Therefore, the upper limit on the production cross section at 95% confidence level is 2 fb in this fiducial region. Assuming the kinematic distributions from Drell-Yan pair production of spin-1/2 Dirac magnetic monopoles, the efficiency is in the range 1%-10%, leading to an upper limit on the cross section at 95% confidence level that varies from 145 fb to 16 fb for monopoles with mass between 200 GeV and 1200 GeV. This limit is weaker than the fiducial limit because most of these monopoles lie outside the fiducial region.

This letter describes the observation of the light-by-light scattering process, $\gamma\gamma\rightarrow\gamma\gamma$, in Pb+Pb collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV. The analysis is conducted using a data sample corresponding to an integrated luminosity of 1.73 nb$^{-1}$, collected in November 2018 by the ATLAS experiment at the LHC. Light-by-light scattering candidates are selected in events with two photons produced exclusively, each with transverse energy $E_{\textrm{T}}^{\gamma} > 3$ GeV and pseudorapidity $|\eta_{\gamma}| < 2.37$, diphoton invariant mass above 6 GeV, and small diphoton transverse momentum and acoplanarity. After applying all selection criteria, 59 candidate events are observed for a background expectation of 12 $\pm$ 3 events. The observed excess of events over the expected background has a significance of 8.2 standard deviations. The measured fiducial cross section is 78 $\pm$ 13 (stat.) $\pm$ 7 (syst.) $\pm$ 3 (lumi.) nb.

Results are presented from a search for CP violation in top quark pair production, using proton-proton collisions at a center-of-mass energy of 13 TeV. The data used for this analysis consist of final states with two charged leptons collected by the CMS experiment, and correspond to an integrated luminosity of 35.9 fb$^{-1}$. The search uses two observables, $\mathcal{O}_1$ and $\mathcal{O}_3$, which are Lorentz scalars. The observable $\mathcal{O}_1$ is constructed from the four-momenta of the charged leptons and the reconstructed top quarks, while $\mathcal{O}_3$ consists of the four-momenta of the charged leptons and the b quarks originating from the top quarks. Asymmetries in these observables are sensitive to CP violation, and their measurement is used to determine the chromoelectric dipole moment of the top quark. The results are consistent with the expectation from the standard model.

A search for the production of three massive vector bosons in proton--proton collisions is performed using data at $\sqrt{s}=13\,TeV$ recorded with the ATLAS detector at the Large Hadron Collider in the years 2015--2017, corresponding to an integrated luminosity of $79.8\,\text{fb}^{-1}$. Events with two same-sign leptons $\ell$ (electrons or muons) and at least two reconstructed jets are selected to search for $WWW\to\ell\nu\ell\nu qq$. Events with three leptons without any same-flavour opposite-sign lepton pairs are used to search for $WWW\to\ell\nu\ell\nu\ell\nu$, while events with three leptons and at least one same-flavour opposite-sign lepton pair and one or more reconstructed jets are used to search for $WWZ\to\ell\nu qq \ell\ell$. Finally, events with four leptons are analysed to search for $WWZ\to\ell\nu\ell\nu\ell\ell$ and $WZZ\to qq \ell\ell\ell\ell$. Evidence for the joint production of three massive vector bosons is observed with a significance of 4.0 standard deviations, where the expectation is 3.1 standard deviations.

The production of Z bosons associated with jets is measured in pp collisions at $\sqrt{s}$ = 13 TeV with data recorded with the CMS experiment at the LHC corresponding to an integrated luminosity of 36.3 fb$^{-1}$. The multiplicity of jets with transverse momentum $p_\mathrm{T}$$\gt$ 30 GeV is measured for different regions of the Z boson's $p_\mathrm{T}$(Z), from lower than 10 GeV to higher than 100 GeV. The azimuthal correlation $\Delta \phi$ between the Z boson and the leading jet, as well as the correlations between the two leading jets are measured in three regions of $p_\mathrm{T}$(Z). The measurements are compared with several predictions at leading and next-to-leading orders, interfaced with parton showers. Predictions based on transverse-momentum dependent parton distributions and corresponding parton showers give a good description of the measurement in the regions where multiple parton interactions and higher jet multiplicities are not important. The effects of multiple parton interactions are shown to be important to correctly describe the measured spectra in the low $p_\mathrm{T}$(Z) regions.

The observation of the production of four top quarks in proton-proton collisions is reported, based on a data sample collected by the CMS experiment at a center-of-mass energy of 13 TeV in 2016-2018 at the CERN LHC and corresponding to an integrated luminosity of 138 fb$^{-1}$. Events with two same-sign, three, or four charged leptons (electrons and muons) and additional jets are analyzed. Compared to previous results in these channels, updated identification techniques for charged leptons and jets originating from the hadronization of b quarks, as well as a revised multivariate analysis strategy to distinguish the signal process from the main backgrounds, lead to an improved expected signal significance of 4.9 standard deviations above the background-only hypothesis. Four top quark production is observed with a significance of 5.6 standard deviations, and its cross section is measured to be 17.7 $^{+3.7}_{-3.5}$ (stat) $^{+2.3}_{-1.9}$ (syst) fb, in agreement with the available standard model predictions.

The production of four top quarks ($\mathrm{t\bar{t}t\bar{t}}$) is studied with LHC proton-proton collision data samples collected by the CMS experiment at a center-of-mass energy of 13 TeV, and corresponding to integrated luminosities of up to 138 fb$^{-1}$. Events that have no leptons (all-hadronic), one lepton, or two opposite-sign leptons (where lepton refers only to prompt electrons or prompt muons) are considered. This is the first $\mathrm{t\bar{t}t\bar{t}}$ measurement that includes the all-hadronic final state. The observed significance of the $\mathrm{t\bar{t}t\bar{t}}$ signal in these final states of 3.9 standard deviations (1.5 expected) provides evidence for $\mathrm{t\bar{t}t\bar{t}}$ production, with a measured cross section of 36 $^{+12}_{-11}$ fb. Combined with earlier CMS results in other final states, the signal significance is 4.0 standard deviations (3.2 expected). The combination returns an observed cross section of 17 $\pm$ 4 (stat) $\pm$ 3 (syst) fb, which is consistent with the standard model prediction.

The path-length dependent parton energy loss within the dense partonic medium created in lead-lead collisions at a nucleon-nucleon center-of-mass energy of $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV is studied by determining the azimuthal anisotropies for dijets with high transverse momentum. The data were collected by the CMS experiment in 2018 and correspond to an integrated luminosity of 1.69 nb$^{-1}$. For events containing back-to-back jets, correlations in relative azimuthal angle and pseudorapidity ($\eta$) between jets and hadrons, and between two hadrons, are constructed. The anisotropies are expressed as the Fourier expansion coefficients $v_n$, $n = $ 2-4 of these azimuthal distributions. The dijet $v_n$ values are extracted from long-range (1.5 $\lt$$\vert\Delta\eta\vert$$\lt$ 2.5) components of these correlations, which suppresses the background contributions from jet fragmentation processes. Positive dijet $v_2$ values are observed which increase from central to more peripheral events, while the $v_3$ and $v_4$ values are consistent with zero within experimental uncertainties.

An inclusive search for a new-physics signature of lepton-jet resonances has been performed by the ATLAS experiment. Scalar leptoquarks, pair-produced in $pp$ collisions at $\sqrt{s}$ = 13 TeV at the Large Hadron Collider, have been considered. An integrated luminosity of 3.2 fb$^{-1}$, corresponding to the full 2015 dataset was used. First (second) generation leptoquarks were sought in events with two electrons (muons) and two or more jets. The observed event yield in each channel is consistent with Standard Model background expectations. The observed (expected) lower limits on the leptoquark mass at 95% confidence level are 1100 GeV and 1050 GeV (1160 GeV and 1040 GeV) for first and second generation leptoquarks, respectively, assuming a branching ratio into a charged lepton and a quark of 100%. Upper limits on the aforementioned branching ratio are also given as a function of leptoquark mass. Compared with the results of earlier ATLAS searches, the sensitivity is increased for leptoquark masses above 860 GeV, and the observed exclusion limits confirm and extend the published results.

Protons consist of three valence quarks, two up-quarks and one down-quark, held together by gluons and a sea of quark-antiquark pairs. Collectively, quarks and gluons are referred to as partons. In a proton-proton collision, typically only one parton of each proton undergoes a hard scattering - referred to as single-parton scattering - leaving the remainder of each proton only slightly disturbed. Here, we report the study of double- and triple-parton scatterings through the simultaneous production of three J/$\psi$ mesons, which consist of a charm quark-antiquark pair, in proton-proton collisions recorded with the CMS experiment at the Large Hadron Collider. We observed this process - reconstructed through the decays of J/$\psi$ mesons into pairs of oppositely charged muons - with a statistical significance above five standard deviations. We measured the inclusive fiducial cross section to be 272 $^{+141}_{-104}$ (stat) $\pm$ 17 (syst) fb, and compared it to theoretical expectations for triple-J/$\psi$ meson production in single-, double- and triple-parton scattering scenarios. Assuming factorization of multiple hard-scattering probabilities in terms of single-parton scattering cross sections, double- and triple-parton scattering are the dominant contributions for the measured process.

Measurements of jet substructure describing the composition of quark- and gluon-initiated jets are presented. Proton-proton (pp) collision data at $\sqrt{s}$ =13 TeV collected with the CMS detector are used, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. Generalized angularities are measured that characterize the jet substructure and distinguish quark- and gluon-initiated jets. These observables are sensitive to the distributions of transverse momenta and angular distances within a jet. The analysis is performed using a data sample of dijet events enriched in gluon-initiated jets, and, for the first time, a Z+jet event sample enriched in quark-initiated jets. The observables are measured in bins of jet transverse momentum, and as a function of the jet radius parameter. Each measurement is repeated applying a "soft drop" grooming procedure that removes soft and large angle radiation from the jet. Using these measurements, the ability of various models to describe jet substructure is assessed, showing a clear need for improvements in Monte Carlo generators.

A precise measurement of the invisible width of the Z boson produced in proton-proton collisions at a center-of-mass energy of 13 TeV is presented using data recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 36.3 fb$^{-1}$. The result is obtained from a simultaneous fit to kinematic distributions for two data samples of Z boson plus jets: one dominated by Z boson decays to invisible particles and the other by Z boson decays to muon and electron pairs. The invisible width is measured to be 523 $\pm$ 3 (stat) $\pm$ 16 (syst) MeV. This result is the first precise measurement of the invisible width of the Z boson at a hadron collider, and is the single most precise direct measurement to date, competitive with the combined result of the direct measurements from the LEP experiments.

The first collider search for dark matter arising from a strongly coupled hidden sector is presented and uses a data sample corresponding to 138 fb$^{-1}$, collected with the CMS detector at the CERN LHC, at $\sqrt{s} =$ 13 TeV. The hidden sector is hypothesized to couple to the standard model (SM) via a heavy leptophobic Z' mediator produced as a resonance in proton-proton collisions. The mediator decay results in two "semivisible" jets, containing both visible matter and invisible dark matter. The final state therefore includes moderate missing energy aligned with one of the jets, a signature ignored by most dark matter searches. No structure in the dijet transverse mass spectra compatible with the signal is observed. Assuming the Z' has a universal coupling of 0.25 to the SM quarks, an inclusive search, relevant to any model that exhibits this kinematic behavior, excludes mediator masses of 1.5-4.0 TeV at 95% confidence level, depending on the other signal model parameters. To enhance the sensitivity of the search for this particular class of hidden sector models, a boosted decision tree (BDT) is trained using jet substructure variables to distinguish between semivisible jets and SM jets from background processes. When the BDT is employed to identify each jet in the dijet system as semivisible, the mediator mass exclusion increases to 5.1 TeV, for wider ranges of the other signal model parameters. These limits exclude a wide range of strongly coupled hidden sector models for the first time.

Diboson production in association with jets is studied in the fully leptonic final states, pp $\to$ (Z$\gamma^*$)(Z/$\gamma^*$)+jets $\to$ 2$\ell$2$\ell'$+jets, ($\ell,\ell'$ = e or $\mu$) in proton-proton collisions at a center-of-mass energy of 13 TeV. The data sample corresponds to an integrated luminosity of 138 fb$^{-1}$ collected with the CMS detector at the LHC. Differential distributions and normalized differential cross sections are measured as a function of jet multiplicity, transverse momentum $p_\mathrm{T}$, pseudorapidity $\eta$, invariant mass and $\Delta\eta$ of the highest-$p_\mathrm{T}$ and second-highest-$p_\mathrm{T}$ jets, and as a function of invariant mass of the four-lepton system for events with various jet multiplicities. These differential cross sections are compared with theoretical predictions that mostly agree with the experimental data. However, in a few regions we observe discrepancies between the predicted and measured values. Further improvement of the predictions is required to describe the ZZ+jets production in the whole phase space.

A study of WZ production in proton-proton collisions at sqrt(s) = 7 TeV is presented using data corresponding to an integrated luminosity of 4.6 fb^-1 collected with the ATLAS detector at the Large Hadron Collider in 2011. In total, 317 candidates, with a background expectation of 68+/-10 events, are observed in double-leptonic decay final states with electrons, muons and missing transverse momentum. The total cross-section is determined to be sigma_WZ(tot) = 19.0+1.4/-1.3(stat.)+/-0.9(syst.)+/-0.4(lumi.) pb, consistent with the Standard Model expectation of 17.6+1.1/-1.0 pb. Limits on anomalous triple gauge boson couplings are derived using the transverse momentum spectrum of Z bosons in the selected events. The cross section is also presented as a function of Z boson transverse momentum and diboson invariant mass.

A measurement of event-plane correlations involving two or three event planes of different order is presented as a function of centrality for 7 ub-1 Pb+Pb collision data at sqrt(s_NN)=2.76 TeV, recorded by the ATLAS experiment at the LHC. Fourteen correlators are measured using a standard event-plane method and a scalar-product method, and the latter method is found to give a systematically larger correlation signal. Several different trends in the centrality dependence of these correlators are observed. These trends are not reproduced by predictions based on the Glauber model, which includes only the correlations from the collision geometry in the initial state. Calculations that include the final-state collective dynamics are able to describe qualitatively, and in some cases also quantitatively, the centrality dependence of the measured correlators. These observations suggest that both the fluctuations in the initial geometry and non-linear mixing between different harmonics in the final state are important for creating these correlations in momentum space.

We present a measurement of two-particle angular correlations in proton-proton collisions at sqrt(s) = 900 GeV and 7 TeV. The collision events were collected during 2009 and 2010 with the ATLAS detector at the Large Hadron Collider using a single-arm minimum bias trigger. Correlations are measured for charged particles produced in the kinematic range of transverse momentum pT > 100 MeV and pseudorapidity |eta| < 2.5. A complex structure in pseudorapidity and azimuth is observed at both collision energies. Results are compared to Pythia 8 and Herwig++ as well as to the AMBT2B, DW and Perugia 2011 tunes of Pythia 6. The data are not satisfactorily described by any of these models.

A $6.8 \ {\rm nb^{-1}}$ sample of $pp$ collision data collected under low-luminosity conditions at $\sqrt{s} = 7$ TeV by the ATLAS detector at the Large Hadron Collider is used to study diffractive dijet production. Events containing at least two jets with $p_\mathrm{T} > 20$ GeV are selected and analysed in terms of variables which discriminate between diffractive and non-diffractive processes. Cross sections are measured differentially in $\Delta\eta^F$, the size of the observable forward region of pseudorapidity which is devoid of hadronic activity, and in an estimator, $\tilde{\xi}$, of the fractional momentum loss of the proton assuming single diffractive dissociation ($pp \rightarrow pX$). Model comparisons indicate a dominant non-diffractive contribution up to moderately large $\Delta\eta^F$ and small $\tilde{\xi}$, with a diffractive contribution which is significant at the highest $\Delta\eta^F$ and the lowest $\tilde{\xi}$. The rapidity-gap survival probability is estimated from comparisons of the data in this latter region with predictions based on diffractive parton distribution functions.