Searcharxiv⌕ Search

arXiv subjects

Zong-Guo Si

Publications and source records attributed to Zong-Guo Si.

At least 73 records · Page 4Linked to original sources
arXiv2015

Top quark spin correlations and polarization at the LHC: standard model predictions and effects of anomalous top chromo moments

A number of top-spin observables are computed within the Standard Model (SM), at next-to-leading order in the strong and weak gauge couplings for hadronic top-quark anti-quark (ttbar) production and decay at the LHC for center-of-mass energies 7 and 8 TeV. For dileptonic final states we consider the azimuthal angle correlation, the helicity correlation, and the opening angle distribution; for lepton plus jets final states we determine distributions and asymmetries that trace a longitudinal and transverse polarization, respectively, of the t and t-bar samples. In addition, we investigate the effects of a non-zero chromo-magnetic and chromo-electric dipole moment of the top quark on these and other top-spin observables and associated asymmetries. These observables allow to disentangle the contributions from the real and imaginary parts of these moments.

hep-ph↗
arXiv2014

$B\to π\ell^{+}\ell^{-}$ decays revisited in the standard model

A new estimate is presented of the dileptonic $B$ decays $B\toπ\ell^+\ell^-(\ell=e,μ,τ)$ in naive factorization within the standard-model (SM) framework. Using a combination of several approaches, we investigate the behavior of the $B\to π$ form factors in the entire region of the momentum transfer squared $q^2$. For the vector and scalar form factors, we employ the light cone sum rule (LCSR) with a chiral current correlator to estimate, at twist-2 next-to-leading order (NLO) accuracy, their shapes in small and intermediate kinematical region. Then a simultaneous fit to a Bourrely-Caprini-Lellouch (BCL) parametrization is performed of the sum rule predictions and the corresponding lattice QCD (LQCD) results available at some high $q^2$'s. The same approach is applied for the tensor form factor, except that at large $q^2$ we use as input the LQCD data on the corresponding $B\to K$ form factor in combination with a $SU_F(3)$ symmetry breaking ansatz. Employing the fitted BCL parameterizations, we evaluate, as an illustrative example, several of the observables of the charged decay modes $B^-\toπ^-\ell^+\ell^-$, including the dilepton invariant mass distribution and branching ratio. For the dielectron and dimuon modes, the branching ratios are estimated at $\mathcal{B}(B^-\toπ^-e^+e^-)=(2.263^{+0.227}_{-0.192})\times10^{-8}$ and $\mathcal{B}(B^-\toπ^-μ^+μ^-)=(2.259^{+0.226}_{-0.191}) \times10^{-8}$. The latter shows an excellent agreement with the recent experimental measurement at LHCb and hence puts a stringent constraint on the contribution from possible new physics. We arrive at, for the ditau mode, $\mathcal{B}(B^-\toπ^-τ^+τ^-)=(1.017^{+0.118}_{-0.139})\times10^{-8}$, which is one order of magnitude larger than the existing theoretical predictions.

hep-ph↗
arXiv2014

Application of the Principle of Maximum Conformality to the Top-Quark Charge Asymmetry at the LHC

The Principle of Maximum Conformality (PMC) provides a systematic and process-independent method to derive renormalization scheme- and scale- independent fixed-order pQCD predictions. In Ref.\cite{pmc3}, we studied the top-quark charge asymmetry at the Tevatron. By applying the PMC, we have shown that the large discrepancies for the top-quark charge asymmetry between the Standard Model estimate and the CDF and D0 data are greatly reduced. In the present paper, with the help of the Bernreuther-Si program, we present a detailed PMC analysis on the top-quark pair production up to next-to-next-to-leading order level at the LHC. After applying PMC scale setting, the pQCD prediction for the top-quark charge asymmetry at the LHC has very small scale uncertainty; e.g., $A_{\rm C}|_{\rm 7 TeV;PMC} =\left(1.15^{+0.01}_{-0.03}\right)\%$, $A_{\rm C}|_{\rm 8 TeV;PMC} =\left(1.03^{+0.01}_{+0.00}\right)\%$, and $A_{\rm C}|_{\rm 14 TeV;PMC} =\left(0.62^{+0.00}_{-0.02}\right)\%$. The corresponding predictions using conventional scale setting are: $A_{\rm C}|_{\rm 7 TeV;Conv.} =\left(1.23^{+0.14}_{-0.14}\right)\%$, $A_{\rm C}|_{\rm 8 TeV;Conv.} =\left(1.11^{+0.17}_{-0.13}\right)\%$, and $A_{\rm C}|_{\rm 14 TeV;Conv.} =\left(0.67^{+0.05}_{-0.05}\right)\%$. In these predictions, the scale errors are predicted by varying the initial renormalization and factorization scales in the ranges $μ^{\rm init}_r\in[m_t/2,2m_t]$ and $μ_f\in[m_t/2,2m_t]$. The PMC predictions are also in better agreement with the available ATLAS and CMS data. In addition, we have calculated the top-quark charge asymmetry assuming several typical cuts on the top-pair invariant mass $M_{t\bar{t}}$. For example, assuming $M_{t\bar{t}}>0.5 ~ {\rm TeV}$ and $μ_f=μ^{\rm init}_r =m_t$, we obtain $A_{\rm C}|_{\rm 7 TeV;PMC}=2.67\%$, $A_{\rm C}|_{\rm 8 TeV;PMC}=2.39\%$, and $A_{\rm C}|_{\rm 14 TeV;PMC}=1.28\%$.

hep-ph↗
arXiv2014

Study Majorana Neutrino Contribution to B-meson Semi-leptonic Rare Decays

B meson semi-leptonic rare decays are sensitive to new physics beyond standard model. We study the $B^{-}\to π^{-}μ^{+}μ^{-}$ process and investigate the Majorana neutrino contribution to its decay width. The constraints on the Majorana neutrino mass and mixing parameter are obtained from this decay channel with the latest LHCb data. Utilizing the best fit for the parameters, we study the lepton number violating decay $B^{-}\to π^{+}μ^{-}μ^{-}$, and find its branching ratio is about $6.4\times10^{-10}$, which is consistent with the LHCb data reported recently.

hep-ph↗
arXiv2014

Determination of charged Higgs couplings at the LHC

We review the study of the charged Higgs and top quark associated production at the LHC with the presence of an additional scalar doublet. Top quark spin effects are related to the Higgs fermion couplings through this process. The angular distributions with respect to top quark spin turn out to be distinctive observables to study the $Htb$ interaction in different models.

hep-ph↗
arXiv2014

Light Top Squark in Precision Top Quark Sample

The uncertainty of $t\bar{t}$ production cross section measurement at LHC is at a-few-percent level which still allows the stop pair production $\tilde{t}\tilde{t}^{*}$ with identical final states $2b+\ell+nj+{E}_{T}$. In this paper, we attempt to use the existing measurement of $W$-polarization in top quark decay to improve the distinction between stop and top quark states. We apply the ATLAS $χ^2$ method of $W$-polarization measurement in semi-leptonic $t\bar{t}$ final state to semi-leptonic stop pair samples and study its prediction. We find that the faked top events from stop mostly contribute to the left-handed polarized $W$ due to the reconstruction. The benchmark point with maximal contribution to top events only changes $F_{L}$ by 1%. After comparing with the current experiments, we conclude that the current measurement of $W$-polarization in $t$ decay cannot exclude the light stop scenario.

hep-ph↗
arXiv2014

Search for $W^\prime$ signal via $tW^\prime$ associated production at LHC

A variety of new physics models predict the existence of extra charged gauge bosons ($W^\prime$). It is verified that $W^\prime$ and top quark associated production is a promising process to search for $W^\prime$ signal at the LHC. We study the collider signatures of multi-jets$+$lepton$+\met$ by reconstructing the $tW'$ intermediate state through the decay modes of $W^\prime \to tb$, $W^\prime \to q \bar q'$ and $W^\prime \to l ν$ respectively. An angular distribution related to charged lepton and top quark moving direction is provided to distinguish alternative left-handed or right-handed chiral couplings of $W^\prime$ to quarks in the hadronic decay modes. The superiority on the search of $W^\prime$ is demonstrated in the leptonic decay channel for the left-handed type interaction, which is forbidden for the right-handed type interaction in the most theories. To be more realistic, the relevant standard model backgrounds are simulated. We adopt various kinematic cuts to suppress the backgrounds and collect the integral luminosity needed for the corresponding process detected at the LHC with $3σ$ sensitivity. We also provide a forward-backward asymmetry which is related to the chirality of $W^\prime$. The successive studies can shed light on the potential searching for $W^\prime$ signal as well as distinguishing typical new physics models.

hep-ph↗
arXiv2014

Search for doubly charmed hadron at B factory

The doubly charmed hadron production at B factories is of special importance for the study of the hadron structure and the color connections before hadronization. To suppress the combination background fluctuations of the reconstructed hadron mass spectra, we suggest a three-jet event shape trigger. After these three jets are identified by their energy and angular distributions, it is found that: 1) The background process $e^+ e^- \to c\bar {c} \to h's$ in consideration of the final hadron system $Λ_c^+K^-π^+ +X$ are significantly suppressed. 2) For the selected events, about half of the particles, $Λ_c^+$, $K^-$, $π^+$, which obviously can not belong to the decay products of doubly charmed hadron, can be vetoed. The relevant hadronization is investigated.

hep-ph↗
arXiv2014

Colour connections of four quark $Q\bar{Q}Q'\bar{Q}'$ system and doubly heavy baryon production in $e^{+}e^{-}$ annihilation

The hadronization effects induced by various colour connections of the four quark system in $e^{+}e^{-}$ annihilation are briefly reviewed. A special colour connection of four heavy quark system without colour-separation favours the production of doubly heavy baryons. For the related three-jet case, the corresponding hadronization has not been considered. We argue that it can be effectively described as two string fragmentation besides the leading heavy diquark fragmentation. The production rate and the properties of final state hadron systems are discussed. Emphasis is on the string effect as a finger print of this hadronization procedure.

hep-ph↗
arXiv2013

Fourth generation Majorana neutrino, dark matter and Higgs physics

We consider extensions of the standard model with fourth generation fermions (SM4) in which extra symmetries are introduced such that the transitions between the fourth generation fermions and the ones in the first three generations are forbidden. In these models, the stringent lower bounds on the masses of fourth generation quarks from direct searches are relaxed, and the lightest fourth neutrino is allowed to be stable and light enough to trigger the Higgs boson invisible decay. In addition, the fourth Majorana neutrino can be a subdominant but highly detectable dark matter component. We perform a global analysis of the current LHC data on the Higgs production and decay in this type of SM4. The results show that the mass of the lightest fourth Majorana neutrino is confined in the range $\sim 41-59$ GeV. Within the allowed parameter space, the predicted effective cross-section for spin-independent DM-nucleus scattering is $\sim 3\times 10^{-48}-6\times 10^{-46} \text{cm}^{2}$, which is close to the current Xenon100 upper limit and is within the reach of the Xenon1T experiment in the near future. The predicted spin-dependent cross sections can also reach $\sim 8\times 10^{-40}\text{cm}^{2}$.

hep-ph↗
arXiv2013

Search for $Z'$ via $ZH$ associated production at LHC

Many new physics models predict the existence of extra neutral gauge bosons ($Z'$). Inspired by the recent development on Higgs search, we study the properties of $Z'$ via the Higgs and $Z$ boson associated production. The couplings of $Z'$ to quarks can be investigated through $pp \to Z'\to ZH\to l^+l^-b \bar b$ process, which also provides extraordinary signal for understanding the properties of $Z'ZH$ interaction. The standard model background processes can be significantly suppressed by adopting appropriate kinematic cuts. The charged lepton angular distributions are related to the ratio of chiral couplings of $Z'$ to quarks via $Z'ZH$ interaction.

hep-ph↗
arXiv2013

Search for Majorana Neutrino Signal in $B_c$ Meson Rare Decay

We study the $B_C$ meson rare decay in order to search for the Majorana neutrino signal. It is found that the the corresponding decay rate is sensitive to the Majorana neutrino mass and mixing angles. The signal of $B_C^\pm\to l_1^\pm l_2^\pm M^\mp $ induced by the Majorana neutrino within the mass region $m_π<m_n<m_B$ may be observed at LHCb.

hep-ph↗
arXiv2012

Top quark and leptonic charge asymmetries for the Tevatron and LHC

We compute, for $t \bar t$ production at the LHC and at the Tevatron, several charge asymmetries to NLO QCD, including also the electromagnetic and weak-interaction corrections. We calculate these asymmetries both inclusively and with additional kinematic cuts and compare our results, where possible, with recent experimental results and with SM predictions. The $t \bar t$ asymmetries induce also corresponding asymmetries for the charged leptons from semileptonic top-quark decay. Although these asymmetries are, in the SM, smaller than the corresponding ones for top quarks, they are expected to be measurable quite precisely. In fact, measurement of a lepton asymmetry in $\ell$ + jets events was reported by the D$\emptyset$ and CDF experiments. We analyze and compute to NLO in the gauge couplings leptonic charge asymmetries for dileptonic and semileptonic $\ttbar$ events, with and without acceptance cuts, at the Tevatron and the LHC.

hep-ph↗
arXiv2012

The Mass Difference of $F-\bar{F}$ with SO(3) Family Gauge Symmetry

As a simple extension, a non-Abelian family gauge symmetry SO(3), as well as three family Majorana neutrinos, was introduced to explain the tri-bimaximal mixing matrix of neutrinos. We discuss the effect of the possible SO(3) family gauge interaction to the mass differences of $K-\bar{K}$, $B_d-\bar{B}_d$, $B_s-\bar{B}_s$ and $D-\bar{D}$, and get the constrains to the new gauge bosons.

hep-ph↗
arXiv2011

Identify Charged Higgs Boson in $W^\pm H^\mp$ Associated Production at LHC

We investigate the possibility to discover the charged Higgs via $pp\to W^{\pm}H^{\mp}\to l+\met+b\bar{b}jj$ process at LHC, which suffers from large QCD backgrounds. We optimize the kinematic cuts to suppress the backgrounds, so that the reconstruction of the charged Higgs through hadronic decay is possible. The angular distribution of the b-jet from $H^{\pm}$ decay is investigated as a way to identify the charged scalar from vector bosons.

hep-ph↗
arXiv2011

Probing $W^\prime_L WH$ and $W^\prime_R W H$ Interaction at LHC

Many new physics models predict the existence of TeV-scale charged gauge boson $W^\prime$ together with Higgs boson(s). We study the $W^\prime WH$ interaction and explore the angular distribution of charged lepton to distinguish $W_R^\prime WH$ from $W_L^\prime WH$ in $pp\to HW\to b \bar b l ν$ process at the LHC. It is found that a new type forward-backward asymmetry($A_{FB}$) relating to the angle between the direction of the charged lepton in $W$ rest frame and that of the reconstructed $W^\prime$ in laboratory frame is useful to investigate the properties of $W^\prime W H$ interaction. We analyze the Standard Model backgrounds and develop a set of cuts to highlight the signal and suppress the backgrounds at LHC. We find that $A_{FB}$ can reach 0.03(-0.07) for $W_R^\prime$($W_L^\prime$) production at $\sqrt{S}=14$ TeV.

hep-ph↗
arXiv2010

Unitarity and Entropy Change in Exclusive Quark Combination Models

Entropy change in exclusive quark combination models is not an isolated problem. Contrary to adding and tuning some parameters to the relevant model(s) to fix the entropy, we show that it relates to the most general principles. Unitarity of the combination model is demonstrated to play the central rôle that guarantees the non-decrease of the entropy in the exclusive combination process.

hep-ph↗