The ALICE Collaboration at the CERN LHC has measured the inclusive production cross section of isolated photons at midrapidity as a function of the photon transverse momentum ($p_{\rm T}^{\gamma}$), in Pb$-$Pb collisions in different centrality intervals, and in pp collisions, at centre-of-momentum energy per nucleon pair of $\sqrt{s_{\rm NN}}=5.02$ TeV. The photon transverse momentum range is between 10-14 and 40-140 GeV/$c$, depending on the collision system and on the Pb$-$Pb centrality class. The result extends to lower $p_{\rm T}^{\gamma}$ than previously published results by the ATLAS and CMS experiments at the same collision energy. The covered pseudorapidity range is $|\eta^{\gamma}| <0.67$. The isolation selection is based on a charged particle isolation momentum threshold $p_{\rm T}^{\rm iso,~ch} = 1.5$ GeV/$c$ within a cone of radii $R=0.2$ and $0.4$. The nuclear modification factor is calculated and found to be consistent with unity in all centrality classes, and also consistent with the HG-PYTHIA model, which describes the event selection and geometry biases that affect the centrality determination in peripheral Pb$-$Pb collisions. The measurement is compared to next-to-leading order perturbative QCD calculations and to the measurements of isolated photons and Z$^0$ bosons from the CMS experiment, which are all found to be in agreement.
The jet axis decorrelation in inclusive jets is studied using lead-lead (PbPb) collisions at a center-of-mass energy per nucleon pair of 5.02 TeV. The jet axis decorrelation is defined as the angular difference between two definitions of the jet axis. It is obtained by applying two recombination schemes on all the constituents of a given jet reconstructed by the anti-$k_\mathrm{T}$ sequential algorithm with a distance parameter of $R$ = 0.4. The data set, corresponding to an integrated luminosity of 0.66 nb$^{-1}$, was collected in 2018 with the CMS detector at the CERN LHC. The jet axis decorrelations are examined across collision centrality selections and intervals of jet transverse momentum. A centrality dependent evolution of the measured distributions is observed, with a progressive narrowing seen in more central events. This narrowing could result from medium-induced modification of the internal jet structure or reflect color charge effects in energy loss. This new measurement probes jet substructure in previously unexplored kinematic domains and show great promise for providing new insights on the color charge dependence of energy loss to jet-quenching models.
This Letter presents a constraint on the total width of the Higgs boson ($\Gamma_H$) using a combined measurement of on-shell Higgs boson production and the production of four top quarks, which involves contributions from off-shell Higgs boson-mediated processes. This method relies on the assumption that the tree-level Higgs-top Yukawa coupling strength is the same for on-shell and off-shell Higgs boson production processes, thereby avoiding any assumptions about the relationship between on-shell and off-shell gluon fusion Higgs production rates, which were central to previous measurements. The result is based on up to 140 fb$^{-1}$ of proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV collected with the ATLAS detector at the Large Hadron Collider. The observed (expected) 95% confidence level upper limit on $\Gamma_H$ is 450 MeV (75 MeV). Additionally, considering the constraint on the Higgs-top Yukawa coupling from loop-induced Higgs boson production and decay processes further yields an observed (expected) upper limit of 160 MeV (55 MeV).
A search for charged-lepton-flavour violating $\mu\tau qt$ ($q=u,c$) interactions is presented, considering both top-quark production and decay. The data analysed correspond to 140 $\textrm{fb}^{-1}$ of proton-proton collisions at a centre-of-mass energy of $\sqrt{s}= $13 TeV recorded with the ATLAS detector at the Large Hadron Collider. The analysis targets events containing two muons with the same electric charge, a hadronically decaying $\tau$-lepton and at least one jet, with exactly one $b$-tagged jet, produced by a $\mu\tau qt$ interaction. Agreement with the Standard Model expectation within $1.6\sigma$ is observed, and limits are set at the 95% CL on the charged-lepton-flavour violation branching ratio of $\mathcal{B}(t \to \mu\tau q) < 8.7 \times 10^{-7}$. An Effective Field Theory interpretation is performed yielding 95% CL limits on Wilson coefficients, dependent on the flavour of the associated light quark and the Lorentz structure of the coupling. These range from $|c_{\mathsf{lequ}}^{3(2313)}| / \Lambda^{2} < 0.10\textrm{ TeV}^{-2}$ for $\mu\tau ut$ to $|c_{\mathsf{ lequ}}^{1(2323)}| / \Lambda^{2} < 1.8\textrm{ TeV}^{-2}$ for $\mu\tau ct$. An additional interpretation is performed for scalar leptoquark production inducing charged lepton flavour violation, with fixed inter-generational couplings. Upper limits on leptoquark coupling strengths are set at the 95% CL, ranging from $\lambda^{\textrm{LQ}} = $1.3 to $\lambda^{\textrm{LQ}} = $3.7 for leptoquark masses between 0.5 and 2.0 TeV.
This paper presents for the first time a precise measurement of the production properties of the Z boson in the full phase space of the decay leptons. This is in contrast to the many previous precise unfolded measurements performed in the fiducial phase space of the decay leptons. The measurement is obtained from proton-proton collision data collected by the ATLAS experiment in 2012 at $\sqrt s$ = 8 TeV at the LHC and corresponding to an integrated luminosity of 20.2 fb$^{-1}$. The results, based on a total of 15.3 million Z-boson decays to electron and muon pairs, extend and improve a previous measurement of the full set of angular coefficients describing Z-boson decay. The double-differential cross-section distributions in Z-boson transverse momentum p$_T$ and rapidity y are measured in the pole region, defined as 80 $<$ m $<$ 100 GeV, over the range $|y| <$ 3.6. The total uncertainty of the normalised cross-section measurements in the peak region of the p$_T$ distribution is dominated by statistical uncertainties over the full range and increases as a function of rapidity from 0.5-1.0% for $|y| <$ 2.0 to 2-7% at higher rapidities. The results for the rapidity-dependent transverse momentum distributions are compared to state-of-the-art QCD predictions, which combine in the best cases approximate N$^4$LL resummation with N$^3$LO fixed-order perturbative calculations. The differential rapidity distributions integrated over p$_T$ are even more precise, with accuracies from 0.2-0.3% for $|y| <$ 2.0 to 0.4-0.9% at higher rapidities, and are compared to fixed-order QCD predictions using the most recent parton distribution functions. The agreement between data and predictions is quite good in most cases.
A search for a heavy pseudoscalar Higgs boson, A, decaying to a 125 GeV Higgs boson h and a Z boson is presented. The h boson is identified via its decay to a pair of tau leptons, while the Z boson is identified via its decay to a pair of electrons or muons. The search targets the production of the A boson via the gluon-gluon fusion process, gg $\to$ A, and in association with bottom quarks, $\mathrm{b\bar{b}}$A. The analysis uses a data sample corresponding to an integrated luminosity of 138 fb$^{-1}$ collected with the CMS detector at the CERN LHC in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV. Constraints are set on the product of the cross sections of the A production mechanisms and the A $\to$ Zh decay branching fraction. The observed (expected) upper limit at 95% confidence level ranges from 0.049 (0.060) pb to 1.02 (0.79) pb for the gg $\to$ A process and from 0.053 (0.059) pb to 0.79 (0.61) pb for the $\text{b}\bar{\text{b}}$A process in the probed range of the A boson mass, $m_\text{A}$, from 225 GeV to 1 TeV. The results of the search are used to constrain parameters within the ${\text{M}_{\text{h,EFT}}^{\text{125}}}$ benchmark scenario of the minimal supersymmetric extension of the standard model. Values of $\tan\beta$ below 2.2 are excluded in this scenario at 95% confidence level for all $m_\text{A}$ values in the range from 225 to 350 GeV.
The measurements of the Higgs boson (H) production cross sections performed by the CMS Collaboration in the four-lepton (4$\ell$, $\ell$ = e, $\mu$) final state at a center-of-mass energy $\sqrt{s}$ = 13.6 TeV are presented. These measurements are based on data collected with the CMS detector at the CERN LHC in 2022, corresponding to an integrated luminosity of 34.7 fb$^{-1}$. Cross sections are measured in a fiducial region closely matching the experimental acceptance, both inclusively and differentially, as a function of the transverse momentum and the absolute value of the rapidity of the four-lepton system. The H $\to$ ZZ $\to$ 4$\ell$ inclusive fiducial cross section is measured to be 2.89 $^{+0.53}_{-0.49}$ (stat) $^{+0.29}_{-0.21}$ (syst) fb, in agreement with the standard model expectation of 3.09 $^{+0.27}_{-0.24}$ fb.
A pioneering machine-learning-based flavor-tagging algorithm combining same-side and opposite-side tagging is used to obtain the equivalent of 27$\,$000 tagged B$^0_\mathrm{s}$$\to$ J/$\psi\, \phi$(1020) decays from pp collisions at $\sqrt{s} =$ 13 TeV, collected by the CMS experiment and corresponding to an integrated luminosity of 96.5 fb$^{-1}$. A time- and flavor-dependent angular analysis of the $\mu^+\mu^-$K$^+$K$^-$ final state is used to measure parameters of the $\mathrm{B}^0_\mathrm{s}$-$\overline{\mathrm{B}}^0_\mathrm{s}$ system. The weak phase is measured to be $\phi_\mathrm{s}$ = $-$73 $\pm$ 23 (stat) $\pm$ 7 (syst) mrad, which, combined with a $\sqrt{s}$ = 8 TeV CMS result, gives $\phi_\mathrm{s}$ = $-$74 $\pm$ 23 mrad. This value differs from zero by 3.2 standard deviations, providing evidence for $CP$ violation in B$^0_\mathrm{s}$$\to$ J/$\psi\,\phi$(1020) decays. All measured physics parameters are found to agree with standard model predictions where available.
A search for heavy neutral gauge bosons (Z') decaying into a pair of tau leptons is performed in proton-proton collisions at $\sqrt{s}$ = 13 TeV at the CERN LHC. The data were collected with the CMS detector and correspond to an integrated luminosity of 138 fb$^{-1}$. The observations are found to be in agreement with the expectation from standard model processes. Limits at 95% confidence level are set on the product of the Z' production cross section and its branching fraction to tau lepton pairs for a range of Z' boson masses. For a narrow resonance in the sequential standard model scenario, a Z' boson with a mass below 3.5 TeV is excluded. This is the most stringent limit to date from this type of search.
Inclusive cross-sections for top-quark pair production in association with charm quarks are measured with proton-proton collision data at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 140 fb$^{-1}$, collected with the ATLAS experiment at the LHC between 2015 and 2018. The measurements are performed by requiring one or two charged leptons (electrons and muons), two $b$-tagged jets, and at least one additional jet in the final state. A custom flavor-tagging algorithm is employed for the simultaneous identification of $b$-jets and $c$-jets. In a fiducial phase space that replicates the acceptance of the ATLAS detector, the cross-sections for $t\bar{t}+ {\geq} 2c$ and $t\bar{t}+1c$ production are measured to be $1.28^{+0.27}_{-0.24}\;\text{pb}$ and $6.4^{+1.0}_{-0.9}\;\text{pb}$, respectively. The measurements are primarily limited by uncertainties in the modeling of inclusive $t\bar{t}$ and $t\bar{t}+b\bar{b}$ production, in the calibration of the flavor-tagging algorithm, and by data statistics. Cross-section predictions from various $t\bar{t}$ simulations are largely consistent with the measured cross-section values, though all underpredict the observed values by 0.5 to 2.0 standard deviations. In a phase-space volume without requirements on the $t\bar{t}$ decay products and the jet multiplicity, the cross-section ratios of $t\bar{t}+ {\geq} 2c$ and $t\bar{t}+1c$ to total $t\bar{t}+\text{jets}$ production are determined to be $(1.23 \pm 0.25) \%$ and $(8.8 \pm 1.3) \%$.
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