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Register a new userSearch for dark photons produced in 13 TeV $pp$ collisions
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Searches are performed for both prompt-like and long-lived dark photons, $A^{prime}$, produced in proton-proton collisions at a center-of-mass energy of 13 TeV, using $A^{prime}tomu^+mu^-$ decays and a data sample corresponding to an integrated luminosity of 1.6 fb$^{-1}$ collected with the LHCb detector. The prompt-like $A^{prime}$ search covers the mass range from near the dimuon threshold up to 70 GeV, while the long-lived $A^{prime}$ search is restricted to the low-mass region $214<m(A^{prime})<350$ MeV. No evidence for a signal is found, and 90% confidence level exclusion limits are placed on the $gamma$-$A^{prime}$ kinetic-mixing strength. The constraints placed on prompt-like dark photons are the most stringent to date for the mass range $10.6 < m(A^{prime}) < 70$ GeV, and are comparable to the best existing limits for $m(A^{prime}) < 0.5$ GeV. The search for long-lived dark photons is the first to achieve sensitivity using a displaced-vertex signature.
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A search is presented for a Higgs boson that is produced in association with a Z boson and that decays to an undetected particle together with an isolated photon. The search is performed by the CMS Collaboration at the Large Hadron Collider using a data set corresponding to an integrated luminosity of 137 fb$^{-1}$ recorded at a center-of-mass energy of 13 TeV. No significant excess of events above the expectation from the standard model background is found. The results are interpreted in the context of a theoretical model in which the undetected particle is a massless dark photon. An upper limit is set on the product of the cross section for associated Higgs and Z boson production and the branching fraction for such a Higgs boson decay, as a function of the Higgs boson mass. For a mass of 125 GeV, assuming the standard model production cross section, this corresponds to an observed (expected) upper limit on this branching fraction of 4.6 (3.6)% at 95% confidence level. These are the first limits on Higgs boson decays to final states that include an undetected massless dark photon.
The MoEDAL trapping detector, consists of approximately 800 kg of aluminium volumes. It was exposed during Run-2 of the LHC program to 6.46 fb^-1 of 13 TeV proton-proton collisions at the LHCb interaction point. Evidence for dyons (particles with electric and magnetic charge) captured in the trapping detector was sought by passing the aluminium volumes comprising the detector through a SQUID magnetometer. The presence of a trapped dyon would be signalled by a persistent current induced in the SQUID magnetometer. On the basis of a Drell-Yan production model, we exclude dyons with a magnetic charge ranging up to 5 Dirac charges, and an electric charge up to 200 times the fundamental electric charge for mass limits in the range 790 - 3130 GeV.
The production fractions of $overline{B}_s^0$ and $Lambda_b^0$ hadrons, normalized to the sum of $B^-$ and $overline{B}^0$ fractions, are measured in 13 TeV pp collisions using data collected by the LHCb experiment, corresponding to an integrated luminosity of 1.67/fb. These ratios, averaged over the $b$-hadron transverse momenta from 4 to 25 GeV and pseudorapidity from 2 to 5, are $0.122 pm 0.006$ for $overline{B}_s^0$, and $0.259 pm 0.018$ for $Lambda_b^0$, where the uncertainties arise from both statistical and systematic sources. The $Lambda_b^0$ ratio depends strongly on transverse momentum, while the $overline{B}_s^0$ ratio shows a mild dependence. Neither ratio shows variations with pseudorapidity. The measurements are made using semileptonic decays to minimize theoretical uncertainties. In addition, the ratio of $D^+$ to $D^0$ mesons produced in the sum of $overline{B}^0$ and $B^-$ semileptonic decays is determined as $0.359pm0.006pm 0.009$, where the uncertainties are statistical and systematic.
A search for flavour-changing neutral currents (FCNC) processes in proton-proton ($pp$) collisions at a centre-of-mass energy of 13 TeV with the ATLAS detector at the CERN Large Hadron Collider (LHC) is presented. The analysed data collected during the years of 2015 and 2016 corresponds to an integrated luminosity of 36.1 fb$^{-1}$. A search considering top-quark pair-production events is performed, with one top-quark decaying through the dominant Standard Model (SM) mode $t$ $to$ $Wb$, and the other through the $t$ $to$ $qZ$ ($q$=$u$,$c$) FCNC channel. The data are consistent with the SM expectation and the observed and expected upper limits on the branching ratio of $t$ $to$ $uZ$ and $t$ $to$ $cZ$ are set at 95% confidence level representing an improvement of about a factor 3 compared with the Run-1 data results from the ATLAS Collaboration.
The production cross-sections of $Upsilon(1S)$, $Upsilon(2S)$ and $Upsilon(3S)$ mesons in proton-proton collisions at $sqrt{s}$= 13 TeV are measured with a data sample corresponding to an integrated luminosity of $277 pm 11$ $rm pb^{-1}$ recorded by the LHCb experiment in 2015. The $Upsilon$ mesons are reconstructed in the decay mode $Upsilontomu^{+}mu^{-}$. The differential production cross-sections times the dimuon branching fractions are measured as a function of the $Upsilon$ transverse momentum, $p_{rm T}$, and rapidity, $y$, over the range $0 < p_{rm T}< 30$ GeV/c and $2.0 < y < 4.5$. The ratios of the cross-sections with respect to the LHCb measurement at $sqrt{s}$= 8 TeV are also determined. The measurements are compared with theoretical predictions based on NRQCD.
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