Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userZ-induced FCNC decays of top, beauty and strange quarks
60
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Anomalous b --> s transitions from LHCb data may suggest a new massive gauge boson Z that couples to the left-handed b --> s current, which in turn implies a coupling to the t --> c current. In this paper, we study flavor-changing neutral current (FCNC) decays of the top quark induced by a Z boson, namely t --> c Z, based on a model of the gauged L_mu - L_tau symmetry (the difference between the muon and tauon numbers) with vector-like quarks, which was introduced to explain the anomalous LHCb data. We illustrate that searching for t --> c Z via Z --> mu^+ mu^- with LHC Run 1 data can already probe a parameter region which is unexplored by B physics for the Z mass around O(10) GeV or more. We further extend the model to very light Z with mass below 400 MeV, which is motivated by the muon g-2 anomaly. Taking rare B and K meson decay data into account, we give upper limits on the t --> c Z branching ratio for the light Z case, and discuss about its observability at the LHC. We also scrutinize the possibility that the decay K_L --> pi^0 Z with Z --> nu nubar may lead to apparent violation of the usual Grossman-Nir bound of B(K_L --> pi^0 nu nubar) < 1.4 x 10^-9.
rate research
Read More
We present a comprehensive analysis of the loop induced top quark FCNC signals at the LHC within one class of the simplified model. The loop level FCNC interactions are well motivated to avoid the hierarchy of the top quark couplings from the new physics and standard model. Such a theory will posit a Majorana dark matter candidate and could be tested through dark matter relic density, direct detection experiments (the scattering between dark matter and heavy nuclei), and the collider signals at the LHC. We find that the spin-independent (SI) scattering between Majorana dark matter and nuclei will vanish at the leading order, while the next-to-leading order correction to the SI scattering becomes significance to constrain the parameter space of the model. A detailed comparison from direct detection experiments and LHC searches is also discussed and both of them are very important to full constrain the model.
The ATLAS experiment sensitivity to top quark Flavour Changing Neutral Current (FCNC) decays was studied at LHC using ttbar events. While one of the top quarks is expected to follow the dominant Standard Model decay t->bW, the other decays through a FCNC channel, i.e. t-> Z u(c), t-> gamma u(c) or t-> g u(c). Different types of analyses, applied to each FCNC decay mode, were compared. The FCNC branching ratio sensitivity (assuming a 5sigma signal significance) and 95% confidence level limits on the branching ratios (in the hypothesis of signal absence) were obtained.
In this work, we calculate the form factors for $J/psito bar{D}^{(*)0}$ induced by the flavor changing neutral currents (FCNC) in terms of the QCD sum rules. Making use of these form factors, we further calculate the branching fractions of semileptonic decays $J/psi to bar{D}^{(*)0} l ^+ l^-$ ($l=e, mu$). In particular, we formulate the matrix element $<J/psi|T_{mu u}| bar{D}^{*0}>$ with $T_{mu u}$ being a tensor current, which was not fully discussed in previous literature. Our analysis indicates that if only the standard model (SM) applies, the production of single charmed mesons at the present electron-positron colliders is too small to be observed even the resonance effects are included, therefore if an anomalous production rates are observed, it would be a hint of new physics beyond SM. Even though the predicted branching ratios are beyond the reach of present facilities which can be seen from a rough order estimate, the more accurate formulation of the three point correlation function derived in this work has theoretical significance and the technique can also be applied to other places. In analog to some complicated theoretical derivations which do not have immediate phenomenological application yet, if the future experiments can provide sufficient luminosity and accuracy, the results would be helpful.
We present results for the differential distributions of jets from non-leptonic decays of polarized top quarks within the Standard Model, including QCD radiative corrections. Our work extends existing results which are only available for semileptonic top quark decays at the parton level. For $t(uparrow)$ -> b-jet + 2 light jets we compute in particular the QCD-corrected top-spin analysing power of the b-quark jet and the least energetic light jet. The dependence of the results on the choice of the jet recombination scheme is found to be small. In addition we compute the spin analysing power of the thrust axis. Our results constitute a so far missing ingredient to analyse top quark production and subsequent non-leptonic decay at next-to-leading order in alpha_s, keeping the full information on the top quark polarization.
Precision tests of the Kobayashi-Maskawa model of CP violation are discussed, pointing out possible signatures for other sources of CP violation and for new flavor-changing operators. The current status of the most accurate tests is summarized.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
