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Register a new userSearch for an invisibly decaying $Z^{prime}$ boson at Belle II in $e^+ e^- to mu^+ mu^- (e^{pm} mu^{mp})$ plus missing energy final states
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Theories beyond the standard model often predict the existence of an additional neutral boson, the $Z^{prime}$. Using data collected by the Belle II experiment during 2018 at the SuperKEKB collider, we perform the first searches for the invisible decay of a $Z^{prime}$ in the process $e^+ e^- to mu^+ mu^- Z^{prime}$ and of a lepton-flavor-violating $Z^{prime}$ in $e^+ e^- to e^{pm} mu^{mp} Z^{prime}$. We do not find any excess of events and set 90% credibility level upper limits on the cross sections of these processes. We translate the former, in the framework of an $L_{mu}-L_{tau}$ theory, into upper limits on the $Z^{prime}$ coupling constant at the level of $5 times 10^{-2}$ -- $1$ $M_{Z^prime}leq 6$ GeV/$c^2$.
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Long-lived particles decaying to $e^pm mu^mp u$, with masses between 7 and $50$ GeV/c$^2$ and lifetimes between 2 and $50$ ps, are searched for by looking at displaced vertices containing electrons and muons of opposite charges. The search is performed using $5.4$ fb$^{-1}$ of $pp$ collisions collected with the LHCb detector at a centre-of-mass energy of $sqrt{s} = 13$ TeV. Three mechanisms of production of long-lived particles are considered: the direct pair production from quark interactions, the pair production from the decay of a Standard-Model-like Higgs boson with a mass of $125$ GeV/c$^2$, and the charged current production from an on-shell $W$ boson with an additional lepton. No evidence of these long-lived states is obtained and upper limits on the production cross-section times branching fraction are set on the different production modes.
An overview is presented of a method to search for $D^0to{}e^{pm}mu^{mp}$ with LHCb data. In order to reduce combinatorial backgrounds, tagged $D^0$ candidates from the decay $D^{ast+}to{}D^0pi^+$ are used. This measurement is performed with respect to $mathcal{B}left(D^0to{}pi^+pi^-right)$, which cancels uncertainties in the luminosity and $D^{ast+}$ production cross-section. It is estimated that using $3,mathrm{fb}^{-1}$ of LHCb data an upper limit can be attained of $mathcal{O}left(10^{-7}right)$ at a $90%$ confidence level.
A search for the lepton-flavour violating decays $B^0_s to e^{pm} mu^{mp}$ and $B^0 to e^{pm} mu^{mp}$ is performed with a data sample, corresponding to an integrated luminosity of 1.0 fb$^{-1}$ of $pp$ collisions at $sqrt{s} = 7$ TeV, collected by the LHCb experiment. The observed number of $B^0_s to e^{pm} mu^{mp}$ and $B^0 to e^{pm} mu^{mp}$ candidates is consistent with background expectations. Upper limits on the branching fractions of both decays are determined to be $BR(B^0_s to e^{pm} mu^{mp}) < 1.1 ,(1.4) times 10^{-8}$ and $BR (B^0 to e^{pm} mu^{mp}) < 2.8 ,(3.7) times 10^{-9}$ at 90% (95%) confidence level (C.L.). These limits are a factor of twenty lower than those set by previous experiments. Lower bounds on the Pati-Salam leptoquark masses are also calculated, $M_{rm LQ} (B^0_s to e^{pm} mu^{mp}) > 107$ TeV/c$^2$ and $M_{rm LQ} (B^0 to e^{pm} mu^{mp}) > 126$ TeV/c$^2$ at 95% C.L., and are a factor of two higher than the previous bounds.
We present a search for the dark photon $A^{prime}$ in the $B^0 to A^{prime} A^{prime}$ decays, where $A^{prime}$ subsequently decays to $e^+ e^-$, $mu^+ mu^-$, and $pi^+ pi^-$. The search is performed by analyzing $772 times 10^6$ $Boverline{B}$ events collected by the Belle detector at the KEKB $e^+ e^-$ energy-asymmetric collider at the $Upsilon (4S)$ resonance. No signal is found in the dark photon mass range $0.01~mathrm{GeV}/c^2 le m_{A^{prime}} le 2.62~mathrm{GeV}/c^2$, and we set upper limits of the branching fraction of $B^0 to A^{prime} A^{prime}$ at the 90% confidence level. The products of branching fractions, $mathcal{B}(B^0 to A^{prime} A^{prime}) times mathcal{B}(A^{prime} to e^+ e^-)^2$ and $mathcal{B}(B^0 to A^{prime} A^{prime}) times mathcal{B}(A^{prime} to mu^+ mu^-)^2$, have limits of the order of $10^{-8}$ depending on the $A^{prime}$ mass. Furthermore, considering $A^{prime}$ decay rate to each pair of charged particles, the upper limits of $mathcal{B}(B^0 to A^{prime} A^{prime})$ are of the order of $10^{-8}$-$10^{-5}$. From the upper limits of $mathcal{B}(B^0 to A^{prime} A^{prime})$, we obtain the Higgs portal coupling for each assumed dark photon and dark Higgs mass. The Higgs portal couplings are of the order of $10^{-2}$-$10^{-1}$ at $m_{h} simeq m_{B^0} pm 40~mathrm{MeV}/c^2$ and $10^{-1}$-$1$ at $m_{h} simeq m_{B^0} pm 3~mathrm{GeV}/c^2$.
We searched for evidence of a Higgsstrahlung process in a secluded sector, leading to a final state with a dark photon U and a dark Higgs boson h, with the KLOE detector at DAFNE. We investigated the case of h lighter than U, with U decaying into a muon pair and h producing a missing energy signature. We found no evidence of the process and set upper limits to its parameters in the range 2m_mu<m_U<1000 MeV, m_h<m_U.
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