arXiv subjects

Zhen-Xing Zhao

Publications and source records attributed to Zhen-Xing Zhao.

33 records · Page 2Linked to original sources

arXiv2020

The semi-leptonic form factors of $Λ_{b}\toΛ_{c}$ and $Ξ_{b}\toΞ_{c}$ in QCD sum rules

In this work, the full leading order results of the form factors for $Ξ_{b}\toΞ_{c}$ and $Λ_{b}\toΛ_{c}$ are obtained in QCD sum rules. Contributions from up to dim-5 have been considered. For completeness, we also study the two-point correlation function to obtain the pole residues of $Ξ_{Q}$ and $Λ_{Q}$, and higher accuracy is achieved. For the three-point correlation function, since stable Borel regions can not be found, about $20\%$ uncertainties are introduced for the form factors of $Ξ_{b}\toΞ_{c}$ and $Λ_{b}\toΛ_{c}$. Our results for the form factors are consistent with those of the Lattice QCD within errors.

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arXiv2020

Novel method to reliably determine the photon helicity in $b\to sγ$

A sizable right-handed photon polarization in $b\to sγ$ is a clear signal for new physics. In this work we point out that the photon helicity in $b\to sγ$ can be unambiguously extracted by combining the measurements in $B\to K_1γ$ and the Cabibbo-favored $D\to K_1 e^+ν$ decay. We propose a ratio of up-down asymmetries in $D\to K_1 e^+ν$ to quantify the hadronic effects. { A method for measuring in experiment the involved partial decay widths in the ratio is discussed, and experimental facilities like BESIII, Belle-II and LHCb are likely to measure this ratio. We also give the angular distribution that is useful to extract the photon polarization in the presence of different kaon resonances. }

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arXiv2020

QCD Sum Rules Analysis of Weak Decays of Doubly-Heavy Baryons

We calculate the weak decay form factors of doubly-heavy baryons using three-point QCD sum rules. The Cutkosky rules are used to derive the double dispersion relations. We include perturbative contributions and condensation contributions up to dimension five, and point out that the perturbative contributions and condensates with lowest dimensions dominate. An estimate of part of gluon-gluon condensates show that it plays a less important role. With these form factors at hand, we present a phenomenological study of semileptonic decays. The future experimental facilities can test these predictions, and deepen our understanding of the dynamics in decays of doubly-heavy baryons.

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arXiv2020

The SVZ sum rules and the heavy quark limit for $Λ_{Q}$

In this work, we evaluate the accuracy of the leading order results in Shifman-Vainshtein-Zakharov (SVZ) sum rules and the leading power results in the heavy quark limit for the mass of $Λ_{Q}$. Up to dim-5 condensate contributions are considered. By comparing with the experimental results, we find that the leading order results in SVZ sum rules can reach about 5\% accuracy both for $Λ_{b}$ and $Λ_{c}$, and it seems that better results can be obtained from the sum rules of the heavy quark limit both for $Λ_{b}$ and $Λ_{c}$. We re-check that the SVZ sum rules in the heavy quark limit coincide with the HQET sum rules. As a byproduct, we also have an exploratory discussion on the $ud$ diquark in $Λ_{Q}$.

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arXiv2020

Towards a Heavy Diquark Effective Theory for Weak Decays of Doubly Heavy Baryons

We construct a leading-order effective field theory for both scalar and axial-vector heavy diquarks, and consider its power expansion in the heavy diquark limit. By assuming the transition from QCD to diquark effective theory, we derive the most general form for the effective diquark transition currents based on the heavy diquark symmetry. The short-distance coefficients between QCD and heavy diquark effective field theory are also obtained by a tree level matching. With the effective currents in the heavy diquark limit, we perform a reduction of the form factors for semi-leptonic decays of doubly heavy baryons, and find that only one nonperturbative function is remaining. It is shown that this soft function can be related to the Isgur-Wise function in heavy meson transitions. As a phenomenological application, we take a single pole structure for the reduced form factor, and use it to calculate the semi-leptonic decay widths of doubly heavy baryons. The obtained results are consistent with others given in the literature, and can be tested in the future.

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arXiv2019

Study of $s\to dν\barν$ rare hyperon decays within the Standard Model and new physics

FCNC processes offer important tools to test the Standard Model (SM), and to search for possible new physics. In this work, we investigate the $s\to dν\barν$ rare hyperon decays in SM and beyond. We find that in SM the branching ratios for these rare hyperon decays range from $10^{-14}$ to $10^{-11}$. When all the errors in the form factors are included, we find that the final branching fractions for most decay modes have an uncertainty of about $5\%$ to $10\%$. After taking into account the contribution from new physics, the generalized SUSY extension of SM and the minimal 331 model, the decay widths for these channels can be enhanced by a factor of $2 \sim 7$.

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arXiv2019

Light-Cone Sum Rules Analysis of $Ξ_{QQ^{\prime}q}\toΛ_{Q^{\prime}}$ Weak Decays

We analyze the weak decay of doubly-heavy baryon decays into anti-triplets $Λ_Q$ with light-cone sum rules. To calculate the decay form factors, both bottom and charmed anti-triplets $Λ_b$ and $Λ_c$ are described by the same set of leading twist light-cone distribution functions. With the obtained form factors, we perform a phenomenology study on the corresponding semi-leptonic decays. The decay widths are calculated and the branching ratios given in this work are expected to be tested by future experimental data, which will help us to understand the underlying dynamics in doubly-heavy baryon decays.

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arXiv2018

Weak decays of doubly heavy baryons: "decay constants"

Inspired by the recent observation of the $Ξ_{cc}^{++}$ by LHCb collaboration, we explore the "decay constants" of doubly heavy baryons in the framework of QCD sum rules. With the $Ξ_{cc}, Ξ_{bc}, Ξ_{bb}$, and $Ω_{cc}, Ω_{bc}, Ω_{bb}$ baryons interpolated by three-quark operators, we calculate the correlation functions using the operator product expansion and include the contribution from operators up to dimension six. On the hadron side, we consider both contributions from the lowest-lying states with $J^P=1/2^+$ and from negative parity baryons with $J^P=1/2^-$. We find that the results are stable and the contaminations from negative parity baryons are not severe. These results are ingredients for the QCD study of weak decays and other properties of doubly-heavy baryons.

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arXiv2018

Weak decays of doubly heavy baryons: the FCNC processes

The discovery of doubly heavy baryon provides us with a new platform for precisely testing Standard Model and searching for new physics. As a continuation of our previous works, we investigate the FCNC processes of doubly heavy baryons. Light-front approach is adopted to extract the form factors, in which the two spectator quarks are viewed as a diquark. Results for form factors are then used to predict some phenomenological observables, such as the decay width and the forward-backward asymmetry. We find that most of the branching ratios for $b\to s$ processes are $10^{-8}\sim10^{-7}$ and those for $b\to d$ processes are $10^{-9}\sim10^{-8}$. The flavor SU(3) symmetry and symmetry breaking effects are explored. Parametric uncertainties are also investigated.

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arXiv2018

Weak Decays of Heavy Baryons in Light-Front Approach

In this work, we perform a analysis of semi-leptonic and nonleptonic weak decays of heavy baryons: $Λ_{b},Ξ_{b},Ω_{b}$ and $Λ_{c},Ξ_{c},Ω_{c}$. For nonleptonic decay modes, we study only the factorizable channels induced by the external W-emission. The two spectator quarks in baryonic transitions are treated as a diquark and form factors are calculated in the light-front approach. Using the results for form factors, we also calculate some corresponding semi-leptonic and nonleptonic decay widths. We find that our results are comparable with the available experimental data and other theoretical predictions. Decay branching fractions for many channels are found to reach the level $10^{-3}\sim10^{-2}$, which are promising to be discovered in the future measurements at BESIII, LHCb and BelleII. The SU(3) symmetry in semi-leptonic decays is examined and sources of symmetry breaking are discussed.

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arXiv2018

Weak decays of doubly heavy baryons: the $1/2\to3/2$ case

As a continuation of our previous works, we investigate the weak decays of doubly heavy baryons into a spin-$3/2$ singly or doubly heavy baryon. Light-front approach is adopted to handle the dynamics in the transitions, in which the two spectator quarks are approximated as a diquark. Results for form factors are then used to calculate decay widths of semi-leptonic and nonleptonic processes. The flavor SU(3) symmetry and symmetry breaking effects in semi-leptonic decays modes are explored, and we point out that in charm sector, there are sizable symmetry breaking effects. For nonleptonic decay modes, we study only the factorizable channels induced by the external W-emission. We find that branching fractions for most 1/2 to 3/2 transitions are approximately one order of magnitude smaller than the corresponding ones for the 1/2 to 1/2 transitions. Parametric uncertainties are also investigated in detail. This work, together with our previous works, are beneficial to the experimental studies of doubly heavy baryons at LHC and other experiments.

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arXiv2018

Discovery Potentials of Doubly Charmed Baryons

The existence of doubly heavy flavor baryons has not been well established experimentally so far. In this Letter we systematically investigate the weak decays of the doubly charmed baryons, $Ξ_{cc}^{++}$ and $Ξ_{cc}^{+}$, which would be helpful for experimental searches for these particles. The branching fractions are studied under the factorization hypothesis for the short-distance contributions and considering the rescattering effect for the long-distance contributions which are significantly enhanced. Comparing all the decay modes, we recommend the processes of $Ξ_{cc}^{++}\to Λ_c^+K^-π^+π^+$ and $Ξ_c^+π^+$ as the first priority for experiments to search for the doubly heavy baryons.

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arXiv2017

Weak Decays of Doubly Heavy Baryons: the $1/2\to 1/2$ case

Very recently, the LHCb collaboration has observed in the final state $Λ_c^+ K^-π^+π^+$ a resonant structure that is identified as the doubly-charmed baryon $Ξ_{cc}^{++}$. Inspired by this observation, we investigate the weak decays of doubly heavy baryons $Ξ_{cc}^{++}$, $Ξ_{cc}^{+}$, $Ω_{cc}^{+}$, $Ξ_{bc}^{(\prime)+}$, $Ξ_{bc}^{(\prime)0}$, $Ω_{bc}^{(\prime)0}$, $Ξ_{bb}^{0}$, $Ξ_{bb}^{-}$ and $Ω_{bb}^{-}$ and focus on the decays into spin $1/2$ baryons in this paper. At the quark level these decay processes are induced by the $c\to d/s$ or $b\to u/c$ transitions, and the two spectator quarks can be viewed as a scalar or axial vector diquark. We first derive the hadronic form factors for these transitions in the light-front approach and then apply them to predict the partial widths for the semi-leptonic and non-leptonic decays of doubly heavy baryons. We find that a number of decay channels are sizable and can be examined in future measurements at experimental facilities like LHC, Belle II and CEPC.

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arXiv2016

$B_c\to B_{sJ}$ form factors and $B_c$ decays into $B_{sJ}$ in covariant light-front approach

We suggest to study the $B_{s}$ and its excitations $B_{sJ}$ in the $B_c$ decays. We calculate the $B_c\to B_{sJ}$ and $B_c\to B_{J}$ form factors within the covariant light-front quark model, where the $B_{sJ}$ and $B_{J}$ denote an s-wave or p-wave $\bar bs$ and $\bar bd$ meson, respectively. The form factors at $q^2=0$ are directly computed while their $q^2$-distributions are obtained by the extrapolation. The derived form factors are then used to study semileptonic $B_c\to (B_{sJ},B_{J})\bar\ellν$ decays, and nonleptonic $B_c\to B_{sJ}π$. Branching fractions and polarizations are predicted in the standard model. We find that the branching fractions are sizable and might be accessible at the LHC experiment and future high-energy $e^+e^-$ colliders with a high luminosity at the $Z$-pole. The future experimental measurements are helpful to study the nonperturbative QCD dynamics in the presence of a heavy spectator and also of great value for the spectroscopy study.

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arXiv2015

$a_1(1260), a_1(1420)$ and the production in heavy meson decays

The $a_1(1420)$ with $I^G(J^{PC})= 1^-(1^{++})$ observed in the $π^+ f_0(980)$ final state in the $π^-p\to π^+π^-π^- p$ process by the COMPASS collaboration seems unlikely to be an ordinary $\bar qq$ mesonic state. Available theoretical explanations include tetraquark or rescattering effects due to $a_1(1260)$ decays. If the $a_1(1420)$ were induced by the rescattering, its production rates are completely determined by those of the $a_1(1260)$. In this work, we propose to explore the ratios of branching fractions of heavy meson weak decays into the $a_1(1420)$ and $a_1(1260)$, and testing the universality of these ratios would be a straightforward way to validate/invalidate the rescattering explanation. The decay modes include in the charm sector the $D^0\to a_1^-\ell^+ν$ and $D^0\to π^\pm a_1^\mp$, and in the bottom sector $\overline B^0\to a_1^+ \ell^- \barν$, $B\to D a_1, π^\pm a_1^\mp$, $B_c\to J/ψa_1$ and $Λ_b\to Λ_c a_1$. We calculate the branching ratios for various decay modes into the $a_1(1260)$. The numerical results indicate that there is a promising prospect to study these decays on experiments including BES-III, LHCb, Babar, Belle and CLEO-c, the forthcoming Super-KEKB factory and the under-design Circular Electron-Positron Collider. Experimental analyses in future will lead to a deeper understanding of the nature of the $a_1(1420)$.

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