No Arabic abstract
Combining the $bto smu^+mu^-$ anomaly and dark matter observables, we study the capability of LHC to test dark matter, $Z^{prime}$, and vector-like quark. We focus on a local $U(1)_{L_mu-L_tau}$ model with a vector-like $SU(2)_L$ doublet quark $Q$ and a complex singlet scalar whose lightest component $X_I$ is a candidate of dark matter. After imposing relevant constraints, we find that the $bto smu^+mu^-$ anomaly and the relic abundance of dark matter favor $m_{X_I}< 350$ GeV and $m_{Z^{prime}}< 450$ GeV for $m_Q<$ 2 TeV and $m_{X_R}<$ 2 TeV (the heavy partner of $m_{X_I}$). The current searches for jets and missing transverse momentum at the LHC sizably reduce the mass ranges of the vector-like quark, and $m_Q$ is required to be larger than 1.7 TeV. Finally, we discuss the possibility of probing these new particles at the high luminosity LHC via the QCD process $pp to Dbar{D}$ or $pp to Ubar{U}$ followed by the decay $Dto s (b) ZX_I$ or $U to u (c) Z X_I$ and then $Ztomu^+mu^-$. Taking a benchmark point of $m_Q$=1.93 TeV, $m_{Z^prime}=170$ GeV, and $m_{X_I}=$ 145 GeV, we perform a detailed Monte Carlo simulation, and find that such benchmark point can be accessible at the 14 TeV LHC with an integrated luminosity 3000 fb$^{-1}$.
Vector-like quarks (VLQs) that are partners of the heavy top and bottom quarks are predicted in many extensions of the Standard Model (SM). We explore the possibility that these states could explain not only the longstanding anomaly in the forward-backward asymmetry in $b$-quark production at LEP, $A_{rm FB}^b $, but also the more recent $sim 2sigma$ deviation of the cross section for the associated Higgs production with top quark pairs at the LHC, $sigma(ppto tbar t H)$. Introducing three illustrative models for VLQs with different representations under the SM gauge group, we show that the two anomalies can be resolved while satisfying all other theoretical and experimental constraints. In this case, the three different models predict VLQ states in the $1-2$ TeV mass range that can be soon probed at the LHC. In a second step, we discuss the sensitivity on the VLQ masses and couplings that could be obtained by means of a percent level accuracy in the measurement of ratios of partial Higgs decay widths, in particular $Gamma(H ! to! gammagamma)/Gamma(H ! to! ZZ^*)$ and $Gamma(H ! to ! bbar b)/Gamma(H ! to ! WW^*)$. We show that top and bottom VL partners with masses up to $sim 5$ TeV and exotic VLQs with masses in the $10$ TeV range can be probed at the high-luminosity LHC.
The existence of new vector-like quarks is often predicted by models of new physics beyond the Standard Model, and the development of discovery strategies at colliders is the object of an intense effort from the high-energy community. Our analysis aims at identifying the constraints on and peculiar signatures of simplified scenarios containing textit{two} vector-like quark doublets mixing with textit{any} of the SM quark generations. This scenario is a necessary ingredient of a broad class of theoretically motivated constructions. We focus on the two charge $2/3$ states $t_{1,2}^prime$ that, due to their peculiar mixing patterns, feature new production and decay modes that are not searched for at the LHC: single production of the heavier state can dominate over the light one, while pair production via electroweak interactions overcomes the QCD one for masses at the TeV scale.
We present a novel study of the non-abelian vector dark matter candidate $W^prime$ with a MeV$-$GeV low mass range, accompanied by a dark photon $A^prime$ and a dark $Z^prime$ of similar masses in the context of a simplified gauged two-Higgs-doublet model. The model is scrutinized by taking into account various experimental constraints including dark photon searches, electroweak precision data, relic density of dark matter together with its direct and indirect searches, mono-jet and Higgs collider physics from LHC. The viable parameter space of the model consistent with all experimental and theoretical constraints is exhibited. While a dark $Z^prime$ can be the dominant contribution in the relic density due to resonant annihilation of dark matter, a dark photon is crucial to dark matter direct detection. We demonstrate that the parameter space can be further probed in the near future by sub-GeV dark matter experiments like CDEX, NEWS-G and SuperCDMS.
The vector boson fusion (VBF) event topology at the Large Hadron Collider (LHC) allows efficient suppression of dijet backgrounds and is therefore a promising target for new physics searches. We consider dark matter models which interact with the Standard Model through the electroweak sector: either through new scalar and pseudoscalar mediators which can be embedded into the Higgs sector, or via effective operators suppressed by some higher scale, and therefore have significant VBF production cross-sections. Using realistic simulations of the ATLAS and CMS analysis chain, including estimates of major error sources, we project the discovery and exclusion potential of the LHC for these models over the next decade.
There are lots of new physics models which predict an extra neutral gauge boson, referred as Z-boson. In a certain class of these new physics models, the Z-boson has flavor-dependent couplings with the fermions in the Standard Model (SM). Based on a simple model in which couplings of the SM fermions in the third generation with the Z-boson are different from those of the corresponding fermions in the first two generations, we study the signatures of Z-boson at the Large Hadron Collider (LHC) and the International Linear Collider (ILC). We show that at the LHC, the Z-boson with mass around 1 TeV can be produced through the Drell-Yan processes and its dilepton decay modes provide us clean signatures not only for the resonant production of Z-boson but also for flavor-dependences of the production cross sections. We also study fermion pair productions at the ILC involving the virtual Z-boson exchange. Even though the center-of-energy of the ILC is much lower than a Z-boson mass, the angular distributions and the forward-backward asymmetries of fermion pair productions show not only sizable deviations from the SM predictions but also significant flavor-dependences.