We measure the spin-density matrix elements (SDMEs) of the $\Delta^{++}(1232)$ in the photoproduction reaction $\gamma p \to \pi^-\Delta^{++}(1232)$ with the GlueX experiment in Hall D at Jefferson Lab. The measurement uses a linearly--polarized photon beam with energies from $8.2$ to $8.8$~GeV and the statistical precision of the SDMEs exceeds the previous measurement by three orders of magnitude for the momentum transfer squared region below $1.4$ GeV$^2$. The data are sensitive to the previously undetermined relative sign between couplings in existing Regge-exchange models. Linear combinations of the extracted SDMEs allow for a decomposition into natural and unnatural--exchange amplitudes. We find that the unnatural exchange plays an important role in the low momentum transfer region.
A search is presented for rare decays of the Z and Higgs bosons to a photon and a J/$\psi$ or a $\psi$(2S) meson, with the charmonium state subsequentially decaying to a pair of muons. The data set corresponds to an integrated luminosity of 123 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 13 TeV collected with the CMS detector at the LHC. No evidence for branching fractions of these rare decay channels larger than predicted in the standard model is observed. Upper limits at 95% confidence level are set: $\mathcal{B}$(H $\to$ J/$\psi \gamma$) $\lt$ 2.6 $\times$ 10$^{-4}$, $\mathcal{B}$(H $\to$ $\psi$(2S)$\gamma$) $\lt$ 9.9 $\times$ 10$^{-4}$, $\mathcal{B}$(Z $\to$ J/$\psi \gamma$) $\lt$ 0.6 $\times$ 10$^{-6}$, and $\mathcal{B}$(Z $\to$ $\psi$(2S)$\gamma$) $\lt$ 1.3 $\times$ 10$^{-6}$. The ratio of the Higgs boson coupling modifiers $\kappa_\mathrm{c} / \kappa_\gamma$ is constrained to be in the interval ($-$157, $+$199) at 95% confidence level. Assuming $\kappa_\gamma = 1$, this interval becomes ($-$166, $+$208).
A search for the rare $\eta$$\to$$\mu^+\mu^-\mu^+\mu^-$ double-Dalitz decay is performed using a sample of proton-proton collisions, collected by the CMS experiment at the CERN LHC with high-rate muon triggers in 2017-2018 and corresponding to an integrated luminosity of 101 fb$^{-1}$. A signal having a statistical significance well in excess of 5 standard deviations is observed. Using the $\eta$$\to$$\mu^+ \mu^-$ decay as normalization, the branching fraction $\mathcal{B}(\eta$$\to$$\mu^+\mu^-\mu^+\mu^-)$ = [5.0 $\pm$ 0.8 (stat) $\pm$ 0.7 (syst) $\pm$ 0.7 ($\mathcal{B}_{2\mu}$)] $\times$ 10$^{-9}$ is measured, where the last term is the uncertainty in the normalization channel branching fraction. This work achieves an improved precision of over five orders of magnitude compared to previous results, leading to the first measurement of this branching fraction, which is found to agree with theoretical predictions.
A search for the production of pairs of heavy Majorana neutrinos (N$_\ell$) from the decays of Z' bosons is performed using the CMS detector at the LHC. The data were collected in proton-proton collisions at a center-of-mass energy of $\sqrt{s}$ = 13 TeV, with an integrated luminosity of 138 fb$^{-1}$. The signature for the search is an excess in the invariant mass distribution of the final-state objects, two same-flavor leptons (e or $\mu$) and at least two jets. No significant excess of events beyond the expected background is observed. Upper limits at 95% confidence level are set on the product of the Z' production cross section and its branching fraction to a pair of N$_\ell$, as functions of N$_\ell$ and Z' boson masses ($m_{\mathrm{N}_\ell}$ and $m_\mathrm{Z'}$, respectively) for $m_\mathrm{Z'}$ from 0.4 to 4.6 TeV and $m_{\mathrm{N}_\ell}$ from 0.1 TeV to $m_\mathrm{Z'}$/2. In the theoretical framework of a left-right symmetric model, exclusion bounds in the $m_{\mathrm{N}_\ell}-m_\mathrm{Z'}$ plane are presented in both the electron and muon channels. The observed upper limit on $m_\mathrm{Z'}$ reaches up to 4.42 TeV. These are the most restrictive limits to date on the mass of N$_\ell$ as a function of the Z' boson mass.
The central exclusive production of charged-hadron pairs in pp collisions at a centre-of-mass energy of 13 TeV is examined, based on data collected in a special high-$\beta^*$ run of the LHC. The nonresonant continuum processes are studied with the invariant mass of the centrally produced two-pion system in the resonance-free region, $m_{\pi^+\pi^-}$$\lt$ 0.7 GeV or $m_{\pi^+\pi^-}$$\gt$ 1.8 GeV. Differential cross sections as functions of the azimuthal angle between the surviving protons, squared exchanged four-momenta, and $m_{\pi^+\pi^-}$ are measured in a wide region of scattered proton transverse momenta, between 0.2 and 0.8 GeV, and for pion rapidities $\lvert y\rvert$$\lt$ 2. A rich structure of interactions related to double-pomeron exchange is observed. A parabolic minimum in the distribution of the two-proton azimuthal angle is observed for the first time. It can be interpreted as an effect of additional pomeron exchanges between the protons from the interference between the bare and the rescattered amplitudes. After model tuning, various physical quantities are determined that are related to the pomeron cross section, proton-pomeron and meson-pomeron form factors, pomeron trajectory and intercept, and coefficients of diffractive eigenstates of the proton.
The first measurement of the inclusive and normalised differential cross sections of single top quark production in association with a W boson in proton-proton collisions at a centre-of-mass energy of 13.6 TeV is presented. The data were recorded with the CMS detector at the LHC in 2022, and correspond to an integrated luminosity of 34.7 fb$^{-1}$. The analysed events contain one muon and one electron in the final state. For the inclusive measurement, multivariate discriminants exploiting the kinematic properties of the events are used to separate the signal from the dominant top quark-antiquark production background. A cross section of 82.3 $\pm$ 2.1 (stat) ${}^{+9.9}_{-9.7}$ (syst) $\pm$ 3.3 (lumi) pb is obtained, consistent with the predictions of the standard model. A fiducial region is defined according to the detector acceptance to perform the differential measurements. The resulting differential distributions are unfolded to particle level and show good agreement with the predictions at next-to-leading order in perturbative quantum chromodynamics.
In this Letter, the first evidence of the ${}^4_{\bar{\Lambda}}\overline{\mathrm{He}}$ antihypernucleus is presented, along with the first measurement at the LHC of the production of (anti)hypernuclei with mass number $A=4$, specifically (anti)${}^4_{\Lambda}\mathrm{H}$ and (anti)${}^4_{\Lambda}\mathrm{He}$. In addition, the antiparticle-to-particle ratios for both hypernuclei (${}^4_{\bar{\Lambda}}\overline{\mathrm{H}}$ / ${}^4_{\Lambda}\mathrm{H}$~and ${}^4_{\bar{\Lambda}}\overline{\mathrm{He}}$ / ${}^4_{\Lambda}\mathrm{He}$) are shown, which are sensitive to the baryochemical potential of the strongly-interacting matter created in heavy-ion collisions. The results are obtained from a data sample of central Pb--Pb collisions, collected during the 2018 LHC data-taking at a center-of-mass energy per nucleon pair of $\sqrt{s_{\mathrm{NN}}} = $ 5.02 TeV. The yields measured for the average of the charge-conjugated states are found to be $[0.78 \; \pm \; 0.19 \; \mathrm{(stat.)} \; \pm \; 0.17 \; \mathrm{(syst.)}] \times 10^{-6}$ for the (anti)${}^4_{\Lambda}\mathrm{H}$ and $[1.08 \; \pm \; 0.34 \; \mathrm{(stat.)} \; \pm \; 0.20 \; \mathrm{(syst.)}] \times 10^{-6}$ for the (anti)${}^4_{\Lambda}\mathrm{He}$, and the measured antiparticle-to-particle ratios are in agreement with unity. The presence of (anti)${}^4_{\Lambda}\mathrm{H}$ and (anti)${}^4_{\Lambda}\mathrm{He}$ excited states is expected to strongly enhance the production yield of these hypernuclei. The yield values exhibit a combined deviation of 3.3$\sigma$ from the theoretical ground-state-only expectation, while the inclusion of the excited states in the calculations leads to an agreement within 0.6$\sigma$ with the present measurements. Additionally, the measured (anti)${}^4_{\Lambda}\mathrm{H}$ and (anti)${}^4_{\Lambda}\mathrm{He}$ masses are compatible with the world-average values within the uncertainties.
A search for nonresonant Higgs boson pair production in the $b\bar{b}b\bar{b}$ final state is presented. The analysis uses 126 fb$^{-1}$ of $pp$ collision data at $\sqrt{s}={13}$ TeV collected with the ATLAS detector at the Large Hadron Collider, and targets both the gluon-gluon fusion and vector-boson fusion production modes. No evidence of the signal is found and the observed (expected) upper limit on the cross-section for nonresonant Higgs boson pair production is determined to be 5.4 (8.1) times the Standard Model predicted cross-section at 95% confidence level. Constraints are placed on modifiers to the $HHH$ and $HHVV$ couplings. The observed (expected) $2\sigma$ constraints on the $HHH$ coupling modifier, $\kappa_\lambda$, are determined to be $[-3.5, 11.3]$ ($[-5.4, 11.4]$), while the corresponding constraints for the $HHVV$ coupling modifier, $\kappa_{2V}$, are $[-0.0, 2.1]$ ($[-0.1, 2.1]$). In addition, constraints on relevant coefficients are derived in the context of the Standard Model effective field theory and Higgs effective field theory, and upper limits on the $HH$ production cross-section are placed in seven Higgs effective field theory benchmark scenarios.
A search is conducted for a new scalar boson $S$, with a mass distinct from that of the Higgs boson, decaying into four leptons ($\ell =$$e$, $\mu$) via an intermediate state containing two on-shell, promptly decaying new spin-1 bosons $Z_\text{d}$: $S \rightarrow Z_\text{d}Z_\text{d} \rightarrow 4\ell$, where the $Z_\text{d}$ boson has a mass between 15 and 300 GeV, and the $S$ boson has a mass between either 30 and 115 GeV or 130 and 800 GeV. The search uses proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider with an integrated luminosity of 139 fb$^{-1}$ at a centre-of-mass energy of $\sqrt{s}=13$ TeV. No significant excess above the Standard Model background expectation is observed. Upper limits at 95% confidence level are set on the production cross-section times branching ratio, $\sigma(gg \to S) \times \mathcal{B}(S\rightarrow Z_\text{d}Z_\text{d} \rightarrow 4\ell)$, as a function of the mass of both particles, $m_S$ and $m_{Z\text{d}}$.
A measurement of the Higgs boson mass and width via its decay to two Z bosons is presented. Proton-proton collision data collected by the CMS experiment, corresponding to an integrated luminosity of 138 fb$^{-1}$ at a center-of-mass energy of 13 TeV is used. The invariant mass distribution of four leptons in the on-shell Higgs boson decay is used to measure its mass and constrain its width. This yields the most precise single measurement of the Higgs boson mass to date, 125.04 $\pm$ 0.12 GeV, and an upper limit on the width $\Gamma_\mathrm{H}$$\lt$ 330 MeV at 95% confidence level. A combination of the on- and off-shell Higgs boson production decaying to four leptons is used to determine the Higgs boson width, assuming that no new virtual particles affect the production, a premise that is tested by adding new heavy particles in the gluon fusion loop model. This result is combined with a previous CMS analysis of the off-shell Higgs boson production with decay to two leptons and two neutrinos, giving a measured Higgs boson width of 3.0 $^{+2.0}_{-1.5}$ MeV, in agreement with the standard model prediction of 4.1 MeV. The strength of the off-shell Higgs boson production is also reported. The scenario of no off-shell Higgs boson production is excluded at a confidence level corresponding to 3.8 standard deviations.
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