The first measurements of the production of muons and electrons from heavy-flavour hadron decays in Xe-Xe collisions at $\sqrt{s_{\rm NN}}$ = 5.44 TeV, using the ALICE detector at the LHC, are reported. The measurement of the nuclear modification factor $R_{\rm AA}$ is performed as a function of transverse momentum $p_{\rm T}$ in several centrality classes at forward rapidity ($2.5 < y <4$) and midrapidity ($\vert y \vert < 0.8$) for muons and electrons from heavy-flavour hadron decays, respectively. A suppression by a factor up to about 2.5 compared to the binary-scaled pp reference is observed in central collisions at both central and forward rapidities. The $R_{\rm AA}$ of muons from heavy-flavour hadron decays is compared to previous measurements in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV. When the nuclear modification factors are compared in the centrality classes 0-10% for Xe-Xe collisions and 10-20% for Pb-Pb collisions, which have similar charged-particle multiplicity density, a similar suppression, with $R_{\rm AA} \sim 0.4$ in the $p_{\rm T}$ interval $4 < p_{\rm T} < 8$ GeV/$c$, is observed. The comparison of the measured $R_{\rm AA}$ values in the two collision systems brings new insights on the properties of the quark-gluon plasma by investigating the system-size and path-length dependence of medium-induced parton energy loss. The results of muons and electrons from heavy-flavour hadron decays provide new constraints to model calculations.
Transverse momentum differential production of muons from heavy-flavour hadron decays at forward rapidity in Xe-Xe collisions at 5.44 TeV.
Transverse momentum differential production of muons from heavy-flavour hadron decays at forward rapidity in Xe-Xe collisions at 5.44 TeV.
Transverse momentum differential production of muons from heavy-flavour hadron decays at forward rapidity in Xe-Xe collisions at 5.44 TeV.
A search for a heavy neutral Higgs boson, $A$, decaying into a $Z$ boson and another heavy Higgs boson, $H$, is performed using a data sample corresponding to an integrated luminosity of 139 fb$^{-1}$ from proton-proton collisions at $\sqrt{s}$ = 13 TeV recorded by the ATLAS detector at the LHC. The search considers the $Z$ boson decaying into electrons or muons and the $H$ boson into a pair of $b$-quarks or $W$ bosons. The mass range considered is 230-800 GeV for the $A$ boson and 130-700 GeV for the $H$ boson. The data are in good agreement with the background predicted by the Standard Model, and therefore 95% confidence-level upper limits for $\sigma \times B(A\rightarrow ZH) \times B(H\rightarrow bb$ or $H\rightarrow WW)$ are set. The upper limits are in the range 0.0062-0.380 pb for the $H\rightarrow bb$ channel and in the range 0.023-8.9 pb for the $H\rightarrow WW$ channel. An interpretation of the results in the context of two-Higgs-Doublet models is also given.
The mass distribution of the bb system before any mbb window cuts for the 2 tag category in b-associated production. The signal distribution for (mA, mH) = (600, 300) GeV is also shown, and is normalised such that the production cross-section times the branching ratios B(A->ZH)xB(H->bb) corresponds to 1 pb. Background components are displayed separately.
The mass distribution of the bb system before any mbb window cuts for the 3 tag category in b-associated production. The signal distribution for (mA, mH) = (600, 300) GeV is also shown, and is normalised such that the production cross-section times the branching ratios B(A->ZH)xB(H->bb) corresponds to 1 pb. Background components are displayed separately.
The mllbb mass distribution for the mbb window defined for (mA, mH)=(600, 300) GeV in the 2 tag category with gluon-gluon fusion production is shown. The number of entries shown in each bin is the number of events in that bin divided by the width of the bin. The signal distribution for (mA, mH) = (600, 300) GeV is also shown, and is normalised such that the production cross-section times the branching ratios B(A->ZH)xB(H->bb) corresponds to 1 pb. Background components are displayed separately.
A search is performed for the rare decay W$^\pm\to\pi^\pm\gamma$ in proton-proton collisions at $\sqrt{s} =$ 13 TeV. Data corresponding to an on W integrated luminosity of 137 fb$^{-1}$ were collected during 2016 to 2018 with the CMS detector. This analysis exploits a novel search strategy based on W boson production in top quark pair events. An inclusive search for the W$^\pm\to\pi^\pm\gamma$ decay is not optimal at the LHC because of the high trigger thresholds. Instead, a trigger selection is exploited in which the W boson originating from one of the top quarks is used to tag the event in a leptonic decay. The W boson emerging from the other top quark is used to search for the W$^\pm\to\pi^\pm\gamma$ signature. Such decays are characterized by an isolated track pointing to a large energy deposit, and by an isolated photon of large transverse momentum. The presence of b quark jets reduces the background from the hadronization of light-flavor quarks and gluons. The W$^\pm\to\pi^\pm\gamma$ decay is not observed. An upper exclusion limit is set to this branching fraction, corresponding to 1.50 $\times$ 10$^{-5}$ at 95% confidence level, whereas the expected upper limit exclusion limit is 0.85 $^{+0.52}_{-0.29}$ $\times$ 10$^{-5}$.
The product of signal efficiency and acceptance per year and per lepton channel (muon or electron).
Expected and observed upper exclusion limits on the branching fraction of the decay of a W boson into a pion and a photon, using 2016 to 2018 data.
Jet fragmentation transverse momentum ($j_{\rm T}$) distributions are measured in proton-proton (pp) and proton-lead (p-Pb) collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV with the ALICE experiment at the LHC. Jets are reconstructed with the ALICE tracking detectors and electromagnetic calorimeter using the anti-$k_{\rm T}$ algorithm with resolution parameter $R=0.4$ in the pseudorapidity range $|\eta|<0.25$. The $j_{\rm T}$ values are calculated for charged particles inside a fixed cone with a radius $R = 0.4$ around the reconstructed jet axis. The measured $j_{\rm T}$ distributions are compared with a variety of parton-shower models. Herwig and PYTHIA 8 based models describe the data well for the higher $j_{\rm T}$ region, while they underestimate the lower $j_{\rm T}$ region. The $j_{\rm T}$ distributions are further characterised by fitting them with a function composed of an inverse gamma function for higher $j_{\rm T}$ values (called the "wide component"), related to the perturbative component of the fragmentation process, and with a Gaussian for lower $j_{\rm T}$ values (called the "narrow component"), predominantly connected to the hadronisation process. The width of the Gaussian has only a weak dependence on jet transverse momentum, while that of the inverse gamma function increases with increasing jet transverse momentum. For the narrow component, the measured trends are successfully described by all models except for Herwig. For the wide component, Herwig and PYTHIA 8 based models slightly underestimate the data for the higher jet transverse momentum region. These measurements set constraints on models of jet fragmentation and hadronisation.
$j_\mathrm{T}$ distribution in pp collisions at $\sqrt{s}$ = 5.02 TeV in $40 < p_\mathrm{T, jet}$ < 60 GeV/$c$.
$j_\mathrm{T}$ distributions in p--Pb collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV in $40 < p_\mathrm{T, jet}$ < 150 GeV/$c$.
$\sqrt{\langle j_\mathrm{T}^2 \rangle }$ values for the narrow and wide components in pp collisions at $\sqrt{s}$ = 5.02 TeV in $40 < p_\mathrm{T, jet}$ < 60 GeV/$c$.
The prompt production of the charm baryon $\rm \Lambda_{c}^{+}$ and the $\rm \Lambda_{c}^{+}/\mathrm {D^0}$ production ratios were measured at midrapidity with the ALICE detector in pp and p-Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$TeV. These new measurements show a clear decrease of the $\rm \Lambda_{c}^{+}/\mathrm {D^0}$ ratio with increasing transverse momentum ($p_{\rm T}$) in both collision systems in the range $2
Prompt $\Lambda_{\rm {c}}^{+}$ baryon $p_{\rm {T}}$-differential cross section in pp collisions at $\sqrt{s} = 5.02$ TeV in the rapidity interval $|y|<0.5$.
Prompt $\Lambda_{\rm {c}}^{+}$ baryon $p_{\rm {T}}$-differential cross section in p-Pb collisions at $\sqrt{s_{\rm {NN}}} = 5.02$ TeV in the rapidity interval $-0.96 \lt y \lt 0.04$.
The nuclear modification factor $R_\mathrm{pPb}$ of prompt $\Lambda_{\rm {c}}^{+}$ baryons in p-Pb collisions at $\sqrt{s_{\rm {NN}}} = 5.02$ TeV in the rapidity interval $ -0.96\lt y \lt 0.04$.
The first measurement of the dependence of $\gamma\gamma$$\to$$\mu^{+}\mu^{-}$ production on the multiplicity of neutrons emitted very close to the beam direction in ultraperipheral heavy ion collisions is reported. Data for lead-lead interactions at $\sqrt{s_\mathrm{NN}} =$ 5.02 TeV, with an integrated luminosity of approximately 1.5 nb$^{-1}$, were collected using the CMS detector at the LHC. The azimuthal correlations between the two muons in the invariant mass region 8 $\lt$$m_{\mu\mu}$$\lt$ 60 GeV are extracted for events including 0, 1, or at least 2 neutrons detected in the forward pseudorapidity range $|\eta|$$\gt$ 8.3. The back-to-back correlation structure from leading-order photon-photon scattering is found to be significantly broader for events with a larger number of emitted neutrons from each nucleus, corresponding to interactions with a smaller impact parameter. This observation provides a data-driven demonstration that the average transverse momentum of photons emitted from relativistic heavy ions has an impact parameter dependence. These results provide new constraints on models of photon-induced interactions in ultraperipheral collisions. They also provide a baseline to search for possible final-state effects on lepton pairs caused by traversing a quark-gluon plasma produced in hadronic heavy ion collisions.
Neutron multiplicity dependence of acoplanarity ($\alpha$) from process $\gamma\gamma$ to $\mu^+\mu^-$ in ultraperipheral PbPb at $\sqrt{s_{NN}}=5.02$ TeV.
Neutron multiplicity dependence of acoplanarity ($\alpha$) from process $\gamma\gamma$ to $\mu^+\mu^-$ in ultraperipheral PbPb at $\sqrt{s_{NN}}=5.02$ TeV.
Neutron multiplicity dependence of acoplanarity ($\alpha$) from process $\gamma\gamma$ to $\mu^+\mu^-$ in ultraperipheral PbPb at $\sqrt{s_{NN}}=5.02$ TeV.
A search for dark matter is conducted in final states containing a photon and missing transverse momentum in proton$-$proton collisions at $\sqrt{s}$ = 13 TeV. The data, collected during 2015$-$2018 by the ATLAS experiment at the CERN LHC, correspond to an integrated luminosity of 139 fb$^{-1}$. No deviations from the predictions of the Standard Model are observed and 95% confidence-level upper limits between 2.45 fb and 0.5 fb are set on the visible cross section for contributions from physics beyond the Standard Model, in different ranges of the missing transverse momentum. The results are interpreted as 95% confidence-level limits in models where weakly interacting dark-matter candidates are pair-produced via an s-channel axial-vector or vector mediator. Dark-matter candidates with masses up to 415 (580) GeV are excluded for axial-vector (vector) mediators, while the maximum excluded mass of the mediator is 1460 (1470) GeV. In addition, the results are expressed in terms of 95% confidence-level limits on the parameters of a model with an axion-like particle produced in association with a photon, and are used to constrain the coupling $g_{aZ\gamma}$ of an axion-like particle to the electroweak gauge bosons.
Distribution of $E^{miss}_T$ in data and for the expected SM background in the SRs after performing the 'simplified shape fit'. Overflows are included in the fourth bin of each distribution. The error bars are statistical, and the dashed band includes statistical and systematic uncertainties determined by the fit. The expectations for the simplified model for two different values of $m_{\chi}$ and $m_{med}$, and with $g_{q}=0.25$ and $g_{\chi}=1.0$ and for the ALP model are also shown. The lower panel shows the ratio of data to expected background event yields.
Distribution of $E^{miss}_T$ in data and for the expected SM background in the Single-Muon CR after performing the 'simplified shape fit'. Overflows are included in the fourth bin of each distribution. The $E^{miss}_T$ calculation in this CR does not include the muon contribution. The error bars are statistical, and the dashed band includes statistical and systematic uncertainties determined by the fit. The lower panel shows the ratio of data to expected background event yields.
Distribution of $E^{miss}_T$ in data and for the expected SM background in the Two-Muon CR after performing the 'simplified shape fit'. Overflows are included in the fourth bin of each distribution. The $E^{miss}_T$ calculation in this CR does not include the muon contribution. The error bars are statistical, and the dashed band includes statistical and systematic uncertainties determined by the fit. The lower panel shows the ratio of data to expected background event yields.
The rate for Higgs (H) bosons production in association with either one (tH) or two ($\mathrm{t\bar{t}}$H) top quarks is measured in final states containing multiple electrons, muons, or tau leptons decaying to hadrons and a neutrino, using proton-proton collisions recorded at a center-of-mass energy of 13 TeV by the CMS experiment. The analyzed data correspond to an integrated luminosity of 137 fb$^{-1}$. The analysis is aimed at events that contain H $\to$ WW, H $\to$$\tau\tau$, or H $\to$ ZZ decays and each of the top quark(s) decays either to lepton+jets or all-jet channels. Sensitivity to signal is maximized by including ten signatures in the analysis, depending on the lepton multiplicity. The separation among the tH, the $\mathrm{t\bar{t}}$H, and the backgrounds is enhanced through machine-learning techniques and matrix-element methods. The measured production rates for the $\mathrm{t\bar{t}}$H and tH signals correspond to 0.92 $\pm$ 0.19 (stat) $^{+0.17}_{-0.13}$ (syst) and 5.7 $\pm$ 2.7 (stat) $\pm$ 3.0 (syst) of their respective standard model (SM) expectations. The corresponding observed (expected) significance amounts to 4.7 (5.2) standard deviations for $\mathrm{t\bar{t}}$H, and to 1.4 (0.3) for tH production. Assuming that the Higgs boson coupling to the tau lepton is equal in strength to its expectation in the SM, the coupling $y_{\mathrm{t}}$ of the Higgs boson to the top quark divided by its SM expectation, $\kappa_\mathrm{t}$ = $y_\mathrm{t} / y_\mathrm{t}^\mathrm{SM}$, is constrained to be within $-$0.9 $\lt$$\kappa_\mathrm{t}$$\lt$$-$0.7 or 0.7 $\lt$$\kappa_\mathrm{t}$$\lt$ 1.1, at 95% confidence level. This result is the most sensitive measurement of the $\mathrm{t\bar{t}}$H production rate to date.
Number of events selected in the $3\ell$- and $4\ell$-CRs and in the CR for the ttW background, compared to the event yields expected from different types of background and from the ttH and tH signals, after the fit to data is performed. Uncertainties shown include all systematic components.
Summary of the sources of systematic and statistical uncertainties and their impact on the measurement of the ttH and tH signal rates, and the measured value of the unconstrained nuisance parameters. The quantity $\Delta r_{x}/r_{x}$ corresponds to the change in uncertainty when fixing the nuisance parameters associated with that uncertainty in the fit. Under the label "MC and sideband statistical uncertainty" are the uncertainties associated with the limited number of simulated MC events and the amount of data events in the application region of the MP method.
Number of events selected in the 2$\ell$ss+0$\tau_{\mathrm{h}}$, 3$\ell$+0$\tau_{\mathrm{h}}$, and 2$\ell$ss+1$\tau_{\mathrm{h}}$ analysis channels compared to the event yields expected from the $ tH$ and $ H$ signals and from background processes. The expected event yields are computed for the values of nuisance parameters and of the POI obtained from the ML fit. The best fit values of the POI amount to $\hat{r_{ttH}} = 0.92$ and $\hat{r_{tH}} = 5.7$. Quoted uncertainties represent the sum of statistical and systematic components.
Charged particle multiplicity distributions in positron-proton deep inelastic scattering at a centre-of-mass energy $\sqrt{s}=319$ GeV are measured. The data are collected with the H1 detector at HERA corresponding to an integrated luminosity of $136$ pb${}^{-1}$. Charged particle multiplicities are measured as a function of photon virtuality $Q^2$, inelasticity $y$ and pseudorapidity $\eta$ in the laboratory and the hadronic centre-of-mass frames. Predictions from different Monte Carlo models are compared to the data. The first and second moments of the multiplicity distributions are determined and the KNO scaling behaviour is investigated. The multiplicity distributions as a function of $Q^2$ and the Bjorken variable $x_{\rm Bj}$ are converted to the hadron entropy $S_{\rm hadron}$, and predictions from a quantum entanglement model are tested.
Charged particle multiplicity distribution $P(N)$ measured as a function of the number of charged particles $N$ in 4x4 kinematic bins of $Q^2$ and $y$.
Charged particle multiplicity distribution $P(N)$ measured as a function of the number of charged particles $N$ in three overlapping pseudorapidity ranges $-1.2<\eta_{lab}\vert<0.2$, $-0.5<\eta_{lab}\vert<0.9$ and $0.2<\eta_{lab}\vert<1.6$, subdivided into 4x4 kinematic bins of $Q^2$ and $y$.
Charged particle multiplicity distribution $P(N)$ measured as a function of the number of charged particles $N$ with the additional restriction to select only particles from the current region of the Breit frame $0<\eta^{*}<4$, in 4x4 kinematic bins of $Q^2$ and $y$.
A search for the supersymmetric partners of quarks and gluons (squarks and gluinos) in final states containing jets and missing transverse momentum, but no electrons or muons, is presented. The data used in this search were recorded by the ATLAS experiment in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb$^{-1}$. The results are interpreted in the context of various $R$-parity-conserving models where squarks and gluinos are produced in pairs or in association and a neutralino is the lightest supersymmetric particle. An exclusion limit at the 95% confidence level on the mass of the gluino is set at 2.30 TeV for a simplified model containing only a gluino and the lightest neutralino, assuming the latter is massless. For a simplified model involving the strong production of mass-degenerate first- and second-generation squarks, squark masses below 1.85 TeV are excluded if the lightest neutralino is massless. These limits extend substantially beyond the region of supersymmetric parameter space excluded previously by similar searches with the ATLAS detector.
Observed $m_{\mathrm{eff}}$ distributions in signal regions MB-SSd. The histograms show the MC background predictions normalised by the background-only fit. The hatched (red) error bands indicate experimental and MC statistical uncertainties. Expected distributions for benchmark signal model points, normalised using the approximate NNLO+NNLL cross-section times integrated luminosity, are also shown for comparison (masses in GeV).
Observed metSig distributions in signal regions MB-SSd. The histograms show the MC background predictions normalised by the background-only fit. The hatched (red) error bands indicate experimental and MC statistical uncertainties. Expected distributions for benchmark signal model points, normalised using the approximate NNLO+NNLL cross-section times integrated luminosity, are also shown for comparison (masses in GeV).
Observed $m_{\mathrm{eff}}$ distributions in signal regions MB-GGd. The histograms show the MC background predictions normalised by the background-only fit. The hatched (red) error bands indicate experimental and MC statistical uncertainties. Expected distributions for benchmark signal model points, normalised using the approximate NNLO+NNLL cross-section times integrated luminosity, are also shown for comparison (masses in GeV).
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