A search for the $K^{+}\rightarrow\pi^{+}X$ decay, where $X$ is a long-lived feebly interacting particle, is performed through an interpretation of the $K^{+}\rightarrow\pi^{+}\nu\bar{\nu}$ analysis of data collected in 2017 by the NA62 experiment at CERN. Two ranges of $X$ masses, $0$-$110\,\text{MeV}/c^{2}$ and $154$-$260\,\text{MeV}/c^{2}$, and lifetimes above $100\,\text{ps}$ are considered. The limits set on the branching ratio, $\text{BR}(K^{+}\rightarrow\pi^{+}X)$, are competitive with previously reported searches in the first mass range, and improve on current limits in the second mass range by more than an order of magnitude.
Observed and expected upper limits on branching ratio \(K^{+}\rightarrow\pi^{+}X\) at 90% CL.
Observed upper limits on branching ratio \(K^{+}\rightarrow\pi^{+}X\) at 90% CL as functions of X mass and lifetime.
Exclusion region limits on coupling strength \(sin^{2}\theta\) at 90% CL as a function of X mass, for visible X decays.
The NA62 experiment reports the branching ratio measurement BR$(K^+ \rightarrow \pi^+ \nu\bar{\nu}) = (10.6^{+4.0}_{-3.4} |_{\rm stat} \pm 0.9_{\rm syst}) \times 10 ^{-11}$ at 68% CL, based on the observation of 20 signal candidates with an expected background of 7.0 events from the total data sample collected at the CERN SPS during 2016-2018. This provides evidence for the very rare $K^+ \rightarrow \pi^+ \nu\bar{\nu}$ decay, observed with a significance of 3.4$\sigma$. The experiment achieves a single event sensitivity of $(0.839\pm 0.054)\times 10^{-11}$, corresponding to 10.0 events assuming the Standard Model branching ratio of $(8.4\pm1.0)\times10^{-11}$. This measurement is also used to set limits on BR($K^+ \to \pi^+ X$), where $X$ is a scalar or pseudo-scalar particle. Details are given of the analysis of the 2018 data sample, which corresponds to about 80% of the total data sample.
Observed and expected upper limits on branching ratio \(K^{+}\rightarrow\pi^{+}X\) at 90% CL.
Observed upper limits on branching ratio \(K^{+}\rightarrow\pi^{+}X\) at 90% CL as functions of X mass and lifetime.
Exclusion region limits on coupling strength \(sin^{2}\theta\) at 90% CL as a function of X mass, for visible X decays.
A search for long-lived particles decaying into muon pairs is performed using proton-proton collisions at a center-of-mass energy of 13 TeV, collected by the CMS experiment at the LHC in 2017 and 2018, corresponding to an integrated luminosity of 101 fb$^{-1}$. The data sets used in this search were collected with a dedicated dimuon trigger stream with low transverse momentum thresholds, recorded at high rate by retaining a reduced amount of information, in order to explore otherwise inaccessible phase space at low dimuon mass and nonzero displacement from the primary interaction vertex. No significant excess of events beyond the standard model expectation is found. Upper limits on branching fractions at 95% confidence level are set on a wide range of mass and lifetime hypotheses in beyond the standard model frameworks with the Higgs boson decaying into a pair of long-lived dark photons, or with a long-lived scalar resonance arising from a decay of a b hadron. The limits are the most stringent to date for substantial regions of the parameter space. These results can be also used to constrain models of displaced dimuons that are not explicitly considered in this paper.
Expected and observed limits at 95% CL on the branching fraction B(hb --> PHI X) . B(PHI --> mu+mu-) as a function of signal mass for signal lifetime of 1 mm
Expected and observed limits at 95% CL on the branching fraction B(hb --> PHI X) . B(PHI --> mu+mu-) as a function of signal mass for signal lifetime of 100 mm
Expected and observed limits at 95% CL on the branching fraction B(H --> ZD ZD) . B(ZD --> mu+mu-) as a function of signal mass for signal lifetime of 1 mm
A search is reported for pairs of light Higgs bosons (H$_1$) produced in supersymmetric cascade decays in final states with small missing transverse momentum. A data set of LHC pp collisions collected with the CMS detector at $\sqrt{s}$ = 13 TeV and corresponding to an integrated luminosity of 138 fb$^{-1}$ is used. The search targets events where both H$_1$ bosons decay into $\mathrm{b\bar{b}}$ pairs that are reconstructed as large-radius jets using substructure techniques. No evidence is found for an excess of events beyond the background expectations of the standard model (SM). Results from the search are interpreted in the next-to-minimal supersymmetric extension of the SM, where a "singlino" of small mass leads to squark and gluino cascade decays that can predominantly end in a highly Lorentz-boosted singlet-like H$_1$ and a singlino-like neutralino of small transverse momentum. Upper limits are set on the product of the squark or gluino pair production cross section and the square of the $\mathrm{b\bar{b}}$ branching fraction of the H$_1$ in a benchmark model containing almost mass-degenerate gluinos and light-flavour squarks. Under the assumption of an SM-like H$_1$$\to$$\mathrm{b\bar{b}}$ branching fraction, H$_1$ bosons with masses in the range 40-120 GeV arising from the decays of squarks or gluinos with a mass of 1200 to 2500 GeV are excluded at 95% confidence level.
Reference acceptance times efficiency values for the kinematic selection and $H_T>3500\;\mathrm{GeV}$ requirements ($A_{\mathrm{kin}}$) for the benchmark signal model with different values of $m_{\mathrm{SUSY}}$. These values are independent of $m_{\mathrm{H_1}}$ within 2% in the range $30 \le m_{\mathrm{H_1}} \le 125\;\mathrm{GeV}$.
Upper limits at 95% CL on $\sigma\times\mathcal{B}^2(\mathrm{H}_1\rightarrow b\bar{b}) \times A_{\mathrm{kin}}$ as a function of $m_{\mathrm{H_1}}$. The results are independent of $m_{\mathrm{SUSY}}$ within 10% in the range $1600<m_{\mathrm{SUSY}}<2800\;\mathrm{GeV}$.
Upper limits at 95% CL on $\sigma\times\mathcal{B}^2(\mathrm{H}_1\rightarrow b\bar{b})$ as a function of $m_{\mathrm{H_1}}$ for $m_{\mathrm{SUSY}}=1200\;\mathrm{GeV}$.
The NA62 experiment at CERN, designed to study the ultra-rare decay $K^+ \to \pi^+\nu\overline{\nu}$, has also collected data in beam-dump mode. In this configuration, dark photons may be produced by protons dumped on an absorber and reach a decay volume beginning 80 m downstream. A search for dark photons decaying in flight to $\mu^+\mu^-$ pairs is reported, based on a sample of $1.4 \times 10^{17}$ protons on dump collected in 2021. No evidence for a dark photon signal is observed. A region of the parameter space is excluded at 90% CL, improving on previous experimental limits for dark photon masses between 215 and 550 MeV$/c^2$.
90% CL upper limit in dark photon coupling vs mass parameter space.
90% CL upper limit in \(BR(B \rightarrow K a, a \rightarrow \mu^+ \mu^-)\) vs lifetime \(\tau \) parameter space.
The first search for ultra-rare $K^+$ decays into the $\pi^+e^+e^-e^+e^-$ final state is reported, using a dataset collected by the NA62 experiment at CERN in 2017-2018. An upper limit of $1.4\times 10^{-8}$ at 90% CL is obtained for the branching ratio of the $K^+\to\pi^+e^+e^-e^+e^-$ decay, predicted in the Standard Model to be $(7.2\pm0.7)\times 10^{-11}$. Upper limits at 90% CL are obtained at the level of $10^{-9}$ for the branching ratios of two prompt decay chains involving pair-production of hidden-sector mediators: $K^+\to\pi^+aa$, $a\to e^+e^-$ and $K^+\to\pi^+S$, $S\to A^\prime A^\prime$, $A^\prime\to e^+e^-$.
See caption of Fig 4.
This paper describes a search for dark photons ($\gamma_d$) in Higgs boson decay ($H \to \gamma\gamma_d$) produced in proton-proton collisions through the $ZH$ production mode at the Large Hadron Collider at $\sqrt{s}=13$ TeV. The transverse mass of the photon and the missing transverse momentum from the non-interacting $\gamma_d$ would present a distinctive signature at the Higgs boson mass resonance. The results presented use the total Run-2 integrated luminosity of 139 fb$^{-1}$, recorded by the ATLAS detector at the LHC . The dominant reducible background processes have been estimated using data-driven techniques. A Boosted Decision Tree (BDT) technique was adopted to enhance the sensitivity of the search. Given that no excess is observed with respect to the Standard Model predictions, an observed (expected) upper limit on the branching ratio BR$(H\to \gamma\gamma_d)$ of 2.28$\%$ (2.82$^{+1.33}_{-0.84}\%$) is set at 95$\%$ CL for massless $\gamma_d$. For higher dark photons masses up to 40 GeV, the observed (expected) upper limits at 95$\%$ CL are found to be within the [2.19-2.52]$\%$ ([2.71-3.11]$\%$) range.
Distribution of the BDT classifier response for data and for the expected SM background before the background-only fit. The expectations for the signal are also shown for the massless dark photon and for dark photon mass values of 20 GeV and 40 GeV, assuming BR(H$\to\gamma\gamma_d$) = 5%. Uncertainties shown are statistical for data, while for backgrounds include statistical and systematic sources.
Distribution of the BDT classifier response for data and for the expected SM background after the background-only fit. The expectations for the signal are also shown for the massless dark photon and for dark photon mass values of 20 GeV and 40 GeV, assuming BR(H$\to\gamma\gamma_d$) = 5%. Uncertainties shown are statistical for data, while for backgrounds include statistical and systematic sources determined by the multiple-bin fit.
Background, data and signal yields in bins of BDT, in SR and VV$\gamma$ CR, after the background-only fit. The expectations for the signal are shown for the massless dark photon and for dark photon mass values of 20 GeV and 40 GeV, assuming BR(H$\to\gamma\gamma_d$) = 5%. Uncertainties are statistical for data, while for backgrounds include statistical and systematic sources.
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