Flavor Changing Neutral Current Interactions Research Articles

Top quark rare decays via loop-induced FCNC interactions in extended mirror fermion model

Flavor changing neutral current (FCNC) interactions for a top quark t decays into Xq with X represents a neutral gauge or Higgs boson, and q a up- or charm-quark are highly suppressed in the Standard Model (SM) due to the Glashow–Iliopoulos–Miami mechanism. Whilst current limits on the branching ratios of these processes have been established at the order of 10−4 from the Large Hadron Collider experiments, SM predictions are at least nine orders of magnitude below. In this work, we study some of these FCNC processes in the context of an extended mirror fermion model, originally proposed to implement the electroweak scale seesaw mechanism for non-sterile right-handed neutrinos. We show that one can probe the process t→Zc for a wide range of parameter space with branching ratios varying from 10−6 to 10−8, comparable with various new physics models including the general two Higgs doublet model with or without flavor violations at tree level, minimal supersymmetric standard model with or without R-parity, and extra dimension model.

Bc→(J/Ψ,ηc)τν semileptonic decays within the standard model and beyond

Deviations from the standard model prediction have been observed not only in $b \to c$ charged current interactions but also in $b \to s$ flavor changing neutral current interactions. In particular, the deviation observed in the measured ratio of branching fractions $R_D = \mathcal B(B \to D\tau\nu)/\mathcal B(B \to D\,l\,\nu)$ and $R_{D^{\ast}} =\mathcal B(B \to D^{\ast}\tau\nu)/\mathcal B(B \to D^{\ast}\,l\,\nu)$, where $l = (e,\,\mu)$, is more pronounced and the combined excess currently stands at $3.9\sigma$ level. If it persists and confirmed by future experiments, it would be a definite hint of new physics. In this context, we consider $B_c \to \eta_c\,l\,\nu$ and $B_c \to J/\Psi\,l\,\nu$ decays mediated via $b \to c\,l\,\nu$ charged current interactions and employ the most general effective Lagrangian in the presence of new physics to give prediction on various observables such as ratio of branching ratio, tau polarization fraction, and forward backward asymmetry for these decay modes.

Probing the Anomalous FCNC tqγ Couplings at Large Hadron Electron Collider

We investigate the anomalous flavour changing neutral current (FCNC) interactions of top quark through the process e-p→e-W±q+X. We calculate the signal and background cross sections in electron-proton collisions at Large Hadron electron Collider (LHeC) with a 7 TeV proton beam from the LHC and a new 60 GeV electron beam from energy recovery linac (ERL). We study the relevant background processes including one electron and three jets in the final state. The distributions of the invariant mass of two jets and an additional jet tagged as b-jet are used to account for signal and background events after the analysis cuts. We find upper bounds on anomalous FCNC couplings λ of the order of 10-2 at LHeC for a luminosity projection of 500 fb-1 together with the fast simulation of detector effects. As a matter of interest, we analyze the sensitivity to the couplings (λu, λc) and find an enhanced sensitivity to λc at the LHeC when compared to the results from the HERA.

Constraints and implications on Higgs FCNC couplings from precision measurement of Bs→μ+μ− decay

We study constraints and implications of the recent LHCb measurement of ${\cal B}(B_s \to \mu^+\mu^-)$ for tree-level Higgs-mediated flavor-changing neutral current (FCNC) interactions. Combined with experimental data on $B_s$ mass difference $\Delta m_s$, the $h \to \mu \tau$, and the $h \to \tau^+\tau^-$ decay branching ratios from the LHC, we find that the Higgs FCNC couplings are severely constrained. The allowed regions for $B_s \to \mu \tau$, $\tau\tau$ and $h \to sb$ decays are obtained. Current data allow large CP violation in the $h \to \tau^+ \tau^-$ decay. Consequences of the Cheng-Sher ansatz for the Higgs Yukawa couplings are discussed in some detail.

Top quark FCNC couplings at future circular hadron electron colliders

A study of single top quark production via flavor changing neutral current interactions at $tq\gamma$ vertices is performed at future circular hadron electron collider. The signal cross sections for the processes $e^{-}p\to e^{-}W^{\pm}q+X$ and $e^{-}p\to e^{-}W^{\pm}bq+X$ in the collision of electron beam with energy $E_e=$ 60 GeV and proton beam with energy $E_p=$ 50 TeV are calculated. In the analysis, the invariant mass distributions of three jets reconstructing top quark mass, requiring one b-tagged jet and other two jets reconstructing the $W$ mass are used to count signal and background events after all selection cuts. The upper limits on the anomalous flavor changing neutral current $tq\gamma$ couplings are found to be $\lambda_q < $ 0.01 at future circular hadron electron collider for $L_{int}=100$ fb$^{-1}$ with the fast simulation of detector effects. Signal significance depending on the couplings $\lambda_q$ is analyzed and an enhanced sensitivity is found to the branching ratio BR($t\to q\gamma$) at the future circular hadron electron collider when compared to the current experimental results.

Search for associated production of a Z boson with a single top quark and for tZ flavour-changing interactions in pp collisions at sqrt{s}=8 TeV

A search for the production of a single top quark in association with a Z boson is presented, both to identify the expected standard model process and to search for flavour-changing neutral current interactions. The data sample corresponds to an integrated luminosity of 19.7 fb−1 recorded by the CMS experiment at the LHC in proton-proton collisions at sqrt{s}=8 TeV. Final states with three leptons (electrons or muons) and at least one jet are investigated. An events yield compatible with tZq standard model production is observed, and the corresponding cross section is measured to be σ(pp → tZq → ℓνbℓ+ℓ−q) = 10− 7+ 8 fb with a significance of 2.4 standard deviations. No presence of flavour-changing neutral current production of tZq is observed. Exclusion limits at 95% confidence level on the branching fractions of a top quark decaying to a Z boson and an up or a charm quark are found to be ℬ(t → Zu) < 0.022% and ℬ(t → Zc) < 0.049%.

Anomalous tqg and tqH couplings effects in a Top-Higgs Final State

We study the anomalous production of a single top quark in association with a Higgs boson at the LHC originating from flavor-changing neutral current interactions in tqg and tqH vertices. We derive the discovery potentials and 68% C.L. upper limits considering leptonic decay of the top quark and the Higgs boson decay into a bb‾ pair with 10 fb−1 integrated luminosity of data at 14 TeV. We propose a charge ratio for the lepton in top quark decay in terms of lepton pT and η as a strong tool to observe the signal.

Search for Top Quark Flavor-Changing Neutral Currents at CMS

We present searches for the top quark flavor-changing neutral current (FCNC) interactions by the CMS experiment. The FCNC searches have been performed in the vertices of tqZ, tqg, and tqγ in decay and production processes. The results are based on the data collected from proton proton collisions at the LHC at centre-of-mass energies of 7 and 8 TeV.

Search for single top-quark production via flavour-changing neutral currents at 8TeV with the ATLAS detector.

A search for single top-quark production via flavour-changing neutral current processes from gluon plus up- or charm-quark initial states in proton–proton collisions at the LHC is presented. Data collected with the ATLAS detector in 2012 at a centre-of-mass energy of 8 TeV and corresponding to an integrated luminosity of 20.3 fb^{-1} are used. Candidate events for a top quark decaying into a lepton, a neutrino and a jet are selected and classified into signal- and background-like candidates using a neural network. No signal is observed and an upper limit on the production cross-section multiplied by the t rightarrow Wb branching fraction is set. The observed 95 % CL limit is sigma _{qg rightarrow t} {,times ,} mathcal {B}(t rightarrow Wb)< {3.4},mathrm{pb} and the expected 95 % CL limit is sigma _{qg rightarrow t} times mathcal {B}(t rightarrow Wb)< {2.9},mathrm{pb}. The observed limit can be interpreted as upper limits on the coupling constants of the flavour-changing neutral current interactions divided by the scale of new physics kappa _{ugt}/Lambda < 5.8 times 10^{-3}, mathrm{TeV}^{-1} and kappa _{cgt}/Lambda < 13 times 10^{-3}, mathrm{TeV} and on the branching fractions mathcal {B}(t rightarrow ug) < {4.0 times 10^{-5}} and mathcal {B}(t rightarrow cg) < {20 times 10^{-5}}.

Search for top quark flavor changing neutral currents in same-sign top quark production

The presence of the anomalous top quark flavour changing neutral current (FCNC) interactions leads to the production of same-sign top quarks in proton-proton collisions. The results of a search for events with same-sign dileptons and b-jets conducted by CMS collaboration with 10.5 $fb^{-1}$ of data collected in pp colisions at $\sqrt{s}=8$ TeV are used to obtain the constraints on the strength of top quark FCNC interactions. The 95$%$ confidence level upper limits on the branching ratios of top quark decays to a light quark $q=u,c$ and a gauge or a Higgs boson are set to be ${\cal B}R(t\rightarrow u \gamma) < 1.27%$, ${\cal B}R(t\rightarrow u Z) < 0.8% $, ${\cal B}R(t\rightarrow u g) < 1.02% $ and ${\cal B}R(t\rightarrow u H) < 4.21% $. The sensitivity of future searches in the same-sign top quark channel is also presented.

Probing the anomalous FCNC interactions in a top-Higgs boson final state and the charge ratio approach

We study the anomalous production of a single top quark in association with a Higgs boson at the LHC originating from flavor changing neutral current (FCNC) interactions in $tqg$ and $tqH$ vertices. We derive the discovery potentials and $68\%$ C.L. upper limits considering leptonic decay of the top quark and the Higgs boson decay into a $b\bar{b}$ pair with 10 fb$^{-1}$ integrated luminosity of data in proton-proton collisions at the center-of-mass energy of 14 TeV. We propose a charge ratio for the lepton in top quark decay in terms of lepton $p_{T}$ and $\eta$ as a strong tool to observe the signal. In particular, we show that the charge ratio increases significantly at large $p_{T}$ of the charged lepton. While the main background from $t\bar{t}$ is nearly charge symmetric and $W+jets$ background has much smaller charge ratio with respect to the signal. We show that this feature can also be used in the probe of anomalous single top production with a $Z-$boson or a photon which are under the attention of the experimental collaborations.

OnD→Xul+l−within the Standard Model and frameworks like the littlest Higgs model withTParity

The $D\ensuremath{\rightarrow}{X}_{u}{l}^{+}{l}^{\ensuremath{-}}$ transitions---branching ratios, forward-backward (FB) asymmetry ${A}_{\mathrm{FB}}^{c}$, the $CP$ asymmetry ${A}_{CP}^{c}$, and the $CP$ asymmetry in the forward-backward asymmetry ${A}_{\mathrm{FB}}^{CP}$---have two sources: for ${D}^{\ifmmode\pm\else\textpm\fi{}}$ they represent a pure $\ensuremath{\Delta}C=1$ $\ensuremath{\Delta}Q=0$ current interaction whereas neutral $D$ mesons can also communicate via their antihadron. Standard model (SM) contributions to $\mathrm{BR}(D\ensuremath{\rightarrow}{X}_{u}{l}^{+}{l}^{\ensuremath{-}})$ come primarily from long distance dynamics, which overshadow short distance contributions by several orders of magnitude; still they fall much below the present upper experimental bounds. Even the SM contributions to ${A}_{\mathrm{FB}}^{c}$, ${A}_{CP}^{c}$, and ${A}_{\mathrm{FB}}^{CP}$ are tiny, quite unlike in beauty hadrons. The branching ratios are hardly dented by contributions from the littlest Higgs models with $T$ parity (LHT) even in the short distance regime, let alone in the SM long distances dynamics. Yet the asymmetries ${A}_{\mathrm{FB}}^{c}$, ${A}_{CP}^{c}$, and ${A}_{\mathrm{FB}}^{CP}$ in these new physics models can be enhanced over SM predictions, as they arise purely from short distance dynamics; this can occur, in particular, for ${A}_{\mathrm{FB}}^{c}$ and ${A}_{\mathrm{FB}}^{CP}$, which get enhanced by orders of magnitudes. Even such enhancements hardly reach absolute sizes for observable experimental effects for ${A}_{\mathrm{FB}}^{c}$ and ${A}_{CP}^{c}$. However, LHT contributions to ${A}_{\mathrm{FB}}^{CP}$ could be measured in experiments like the LHCb and the SuperB Collaboration. These results lead us to draw further conclusions on flavor-changing neutral-current interactions within LHT-like models through some simple scaling arguments that encapsulate the essence of flavor dynamics in and beyond the standard model.

Phenomenology in the Zee model with theA4symmetry

The Zee model generates neutrino masses at the one-loop level by adding charged SU(2)_L-singlet and extra SU(2)_L-doublet scalars to the standard model of particle physics. As the origin of the nontrivial structure of the lepton flavor mixing, we introduce the softly broken A_4 symmetry to the Zee model. This model is compatible with the tribimaximal mixing which agrees well with neutrino oscillation measurements. Then, a sum rule m_1 e^{i alpha_12} + 2 m_2 + 3 m_3 e^{i alpha_32} = 0 is obtained and it results in Delta m^2_31 < 0 and m_3 > 1.8*10^{-2}eV. The effective mass |(M_nu)_{ee}| for the neutrinoless double beta decay is predicted as | (M_\nu)_{ee} | > 1.7*10^{-2}eV. The characteristic particles in this model are SU(2)_L-singlet charged Higgs bosons s^+_alpha (alpha=xi,eta,zeta) which are made from a 3-representation of A_4. Contributions of s^+_alpha to the lepton flavor violating decays of charged leptons are almost forbidden by an approximately remaining Z_3 symmetry; only BR(tau to ebar mu mu) can be sizable by the flavor changing neutral current interaction with SU(2)_L-doublet scalars. Therefore, s^+_alpha can be easily light enough to be discovered at the LHC with satisfying current constraints. The flavor structures of BR(s^-_alpha to ell nu) are also discussed.

D0 same-charge dimuon asymmetry and possible newCPviolation sources in theBs−B¯ssystem

Recently, the D0 collaboration reported a large CP violation in the same-sign dimuon charge asymmetry which has the $3.2 \sigma$ deviation from the value estimated in the Standard Model. In this paper, several new physics models are considered: the MSSM, two Higgs doublet model, the recent dodeca model, and a new $Z'$ model. Generally, it is hard to achieve such a large CP violation consistently with other experimental constraints. We find that a scheme with extra non-anomalous U(1)$'$ gauge symmetry is barely consistent. In general, the extra $Z'$ gauge boson induces the flavor changing neutral current interactions at tree level, which is the basic reason allowing a large new physics CP violation. To preserve the U(1)$'$ symmetry at high energy, SU(2)$_L$ singlet exotic heavy quarks of mass above 1 TeV and the Standard Model gauge singlet scalars are introduced.

Impact of flavor-changing neutral current top quark interactions onBR(t→bW)

We study the effect that flavor-changing neutral current interactions of the top quark will have on the branching ratio of charged decays of the top quark. We have performed an integrated analysis using Tevatron and $B$-factories data and with just the further assumption that the Cabibbo-Kobayashi-Maskawa matrix is unitary, we can obtain very restrictive bounds on the strong and electroweak flavor-changing neutral current branching ratios $\mathrm{Br}(t\ensuremath{\rightarrow}qX)&lt;4.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, where $X$ is any vector boson and a sum in $q=u$, $c$ is implied.

Spontaneous CP violating phase as the CKM matrix phase

We propose that the CP violating phase in the CKM mixing matrix is identical to the CP phases responsible for the spontaneous CP violation in the Higgs potential. A multi-Higgs model with Peccei–Quinn (PQ) symmetry is constructed to realize this idea. The CP violating phase does not vanish when all Higgs masses become large. In general, here are flavor changing neutral current (FCNC) interactions mediated by neutral Higgs bosons at the tree level. However, unlike general multi-Higgs models, the FCNC Yukawa couplings are fixed in terms of the quark masses and CKM mixing angles. Implications for meson–anti-meson mixing, including recent data on D–D mixing, and the electric dipole moment (EDM) of the neutron are studied. We find that the neutral Higgs boson masses can be at the order of one hundred GeV. The neutron EDM can be close to the present experimental upper bound.

SCALAR DARK MATTER EFFECTS IN HIGGS AND TOP QUARK DECAYS

We study possible observational effects of scalar dark matter, the darkon D, in Higgs h and top quark t decay processes, h → DD and t → cDD in the minimal Standard Model (SM) and its two Higgs doublet model (THDM) extension supplemented with a SM singlet darkon scalar field D. We find that the darkon D can have a mass in the range of sub-GeV to several tens of GeV, interesting for LHC and ILC colliders, to produce the required dark matter relic density. In the SM with a darkon, t → cDD only occurs at loop level giving a very small rate, while the rate for Higgs decay h → DD can be large. In THDM III with a darkon, where tree level flavor changing neutral current (FCNC) interaction exists, a sizable rate for t → cDD is also possible.

SUSY Higgs boson flavor-changing neutral currents at the LHC

We compute and analyze the Flavor-Changing Neutral Current (FCNC) interactions of Minimal Supersymmetric Standard Model Higgs bosons (h = h0, H0, A0) with heavy quarks (top and bottom), focusing on the strongly-interacting sector. We correlate the Higgs bosons production cross-section at the LHC with the FCNC decay branching ratios and find the maximum allowed values of the production rates, sigma(pp -> h -> qq') = sigma(pp -> h) x B(h -> qq') (qq'= tc or bs) after taking into account limits from low energy data on flavor-changing interactions. We single out the top channel, with a maximum production rate of sigma^max(pp -> h -> tc) =~ 10^-3 - 10^-2 pb, as the most promising FCNC channel to be detected at the LHC.

Single top-quark production by direct supersymmetric flavor-changing neutral-current interactions at the LHC

Production of (electrically neutral) heavy-quark pairs, such as t{\bar c} and {\bar t}c, is extremely suppressed in the SM. In supersymmetric (SUSY) theories, such as the MSSM, the number of these events can be significantly enhanced thanks (mainly) to the FCNC couplings of gluinos. We compute the efficiency of this mechanism for FCNC production of heavy quarks at the LHC. We find that \sigma (pp\to t\bar{c}+\bar{t}c) can reach 1 pb, and therefore one can expect up to 10^{5} events per 100 fb^{-1} of integrated luminosity (with no counterpart in the SM). Their detection would be instant evidence of new physics, and could be a strong indication of underlying SUSY dynamics.

On quadratic divergences in supergravity, vacuum energy and the supersymmetric flavor problem

We examine the phenomenological consequences of quadratically divergent contributions to the scalar potential in supergravity effective Lagrangians. We focus specifically on the effect of these corrections on the vacuum configuration of scalar fields in softly-broken supersymmetric theories and the role these corrections play in generating nondiagonal soft scalar masses. Both effects can only be properly studied when the divergences are regulated in a manifestly supersymmetric manner—something which has thus far been neglected in past treatments. We show how a supersymmetric regularization can impact past conclusions about both types of phenomena and discuss what types of high-energy theories are likely to be safe from unwanted flavor-changing neutral current interactions in the context of supergravity theories derived from heterotic string compactifications.