arXiv subjects

Zhen-Jun Xiao

Publications and source records attributed to Zhen-Jun Xiao.

At least 73 records · Page 4Linked to original sources

arXiv2017

Nonleptonic charmless decays of $B_c\to TP, TV$ in the perturbative QCD approach

Two-body charmless hadronic $B_c$ decays involving a light $1^3\!P_2$-tensor($T$) meson are investigated for the first time within the framework of perturbative QCD(pQCD) at leading order, in which the other meson is the lightest pseudoscalar($P$) or vector($V$) state. The concerned processes can only occur through the pure weak annihilation topology in the standard model. We predict the CP-averaged branching ratios and polarization fractions of those considered decays in Cabibbo-Kobayashi-Maskawa(CKM) favored and suppressed modes. Phenomenologically, several modes$-$such as the $B_c \to K_2^*(1430) K$ and the CKM-favored $B_c \to TV-$have large decay rates of $10^{-6}$, which are expected to be detected at Large Hadron Collider experiments in the near future. Moreover, all of the $B_c \to TV$ modes are governed by the longitudinal amplitudes in the pQCD calculations and the corresponding fractions vary around $78\%$-$98\%$. A confirmation of these results could prove the reliability of the pQCD approach used here and further shed some light on the annihilation decay mechanism.

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arXiv2017

Branching ratios, $CP$ asymmetries and polarizations of $B\rightarrow ψ(2S) V$ decays

We analyzed the nonleptonic decays $B/B_s\to ψ(2S) V $ with $V=(ρ, ω, K^{*}, ϕ)$ by employing the perturbative QCD (PQCD) factorization approach. Here the branching ratios, the $CP$ asymmetries and the complete set of polarization observables are investigated systematically. Besides the traditional contributions from the factorizable and nonfactorizable diagrams at the leading order, the next-to-leading order (NLO) vertex corrections could also provide considerable contributions. The PQCD predictions for the branching ratios of the $B_{(s)}\to ψ(2S)K^{*}, ψ(2S) ϕ$ decays are consistent with the measured values within errors. As for $B\to ψ(2S) ρ, ψ(2S) ω$ decays, the branching ratios can reach the order of $10^{-5}$ and could be measured in the LHCb and Belle-II experiments. The numerical results show that the direct $CP$ asymmetries of the considered decays are very small. Thus the observation of any large direct $CP$ asymmetry for these decays will be a signal for new physics. The mixing induced $CP$ asymmetries in the neutral modes are very close to $\sin 2β_{(s)}$, which suggests that these channels can give a cross-check on the measurement of the Cabbibo-Kobayashi-Maskawa (CKM) angle $β$ and $β_s$. We found that the longitudinal polarization fractions $f_0$ are suppressed to $\sim 50\%$ due to the large nonfactorizable contributions. The magnitudes and phases of the two transverse amplitudes $\mathcal {A}_{\parallel}$ and $\mathcal {A}_{\perp}$ are roughly equal, which is an indication for the approximate light quark helicity conservation in these decays. The overall polarization observables of $B\to ψ(2S) K^{*0}$ and $B_s\to ψ(2S) ϕ$ channels are also in good agreement with the experimental measurements as reported by LHCb and BaBar. Other results can also be tested by the LHCb and Belle-II experiments.

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arXiv2017

$S$-wave resonance contributions to the $B^0_{(s)}\to η_c{(2S)}π^+π^-$ in the perturbative QCD factorization approach

By employing the perturbative QCD (PQCD) factorization approach, we study the quasi-two-body $B^0_{(s)}\to η_c{(2S)}π^+π^-$ decays, where the pion pair comes from the $S$-wave resonance $f_0(X)$. The Breit$-$Wigner formula for the $f_0(500)$ and $f_0(1500)$ resonances, and the Flatté model for the $f_0(980)$ resonance are adopted to parameterize the time-like scalar form factors in the two-pion distribution amplitudes. As a comparison, Bugg's model is also used for the wide $f_0(500)$ in this work. For decay rates, we found the following PQCD predictions: (a) $ {\cal B}(B^0_s\to η_c(2S) f_0(X)[π^+π^-]_s )=\left ( 2.67^{+1.78}_{-1.08} \right )\times 10^{-5}$ when the contributions from $f_0(980)$ and $f_0(1500)$ are all taken into account; (b) ${\cal B}(B^0\to η_c(2S) f_0(500)[π^+π^-]_s)= \left ( 1.40 ^{+0.92}_{-0.56} \right ) \times 10^{-6}$ in the Breit-Wigner model and $ \left ( 1.53 ^{+0.97}_{-0.61} \right ) \times 10^{-6}$ in the Bugg's model.

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arXiv2017

Relativistic corrections to the form factors of $B_c$ into $S$-wave Charmonium

We investigate the form factors of $B_c$ meson into $S$-wave charmonium within the nonrelativistic QCD effective theory and obtain the next-to-leading order relativistic corrections to the form factors, where both the $B_c$ meson and the charmonium are treated as the nonrelativistic bound states. Treating the charm quark as a light quark in the limit \ $m_c/m_b\to 0$, some form factors are identical at the maximum recoil point, which are consistent with the predictions in the heavy-quark effective theory and the large-energy effective theory. Considering that the branching ratios of $B^+_c \to J/ψD_s^+$ and $B^+_c \to J/ψD_s^{*+}$ have been measured by the LHCb and ATLAS Collaborations recently, we employ the form factors of $B_c$ meson into $S$-wave charmonium at the next-to-leading order accuracy to these two decay channels and obtain more precise predictions of their decay rates. Numerical results indicate that the factorizable diagrams dominate the contribution in these two channels, while the colour-suppressed and the annihilation diagrams contribute less than 10 percent. Our results are consistent with the LHCb and ATLAS data.

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arXiv2017

Quasi-two-body decays $B_{(s)}\to Pρ\to Pππ$ in perturbative QCD approach

In this work, we calculate the $CP$-averaged branching ratios and the direct $CP$-violating asymmetries of the quasi-two-body decays $B_{(s)} \to P (ρ\to) ππ$ by employing the perturbative QCD (PQCD) approach (here $P$ stands for a light pseudoscalar meson $π, K, η$ or $η^{\prime}$). The vector current time-like form factor $F_π$, which contains the final state interactions between the pion pair in the resonant region associated with the $P$-wave states $ρ(770)$ along with the two-pion distribution amplitudes, are employed to describe the interactions between the $ρ$ and the pion pair under the hypothesis of the conserved vector current. We found that (a) the PQCD predictions for the branching ratios and the direct $CP$-violating asymmetries for most considered $B_{(s)} \to P (ρ\to) ππ$ decays agree with currently available data within errors; (b) for ${\cal B}(B \to π^0 ρ^0 \to π^0(π^+ π^-)$, the PQCD prediction is much smaller than the measured one; and (c) for $B^+ \to π^+(ρ^0\to)π^+ π^-$ decay mode, we found a negative $CP$ asymmetry $(-27.5^{+3.0}_{-3.7})\%$, which agrees with other theoretical predictions but different in sign from those as reported by BaBar and LHCb Collaboration.

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arXiv2016

Nonleptonic decays of $B \to ( f_1(1285),f_1(1420) ) V$ in the perturbative QCD approach

We investigate the branching ratios, the polarization fractions, the direct CP-violating asymmetries, and the relative phases in 20 nonleptonic decay modes of $B \to f_1 V$ within the framework of perturbative QCD approach at leading order with $f_1$ including two $^3\!P_1$-axial-vector states $f_1(1285)$ and $f_1(1420)$. Here, $B$ denotes $B^+$, $B^0$, and $B_s^0$ mesons and $V$ stands for the lightest vector mesons $ρ$, $K^*$, $ω$, and $ϕ$ , respectively. The $B_s^0 \to f_1 V$ decays are studied theoretically for the first time in the literature. Together with the angle $ϕ_{f_1} \approx (24^{+3.2}_{-2.7})^\circ$ extracted from the measurement through $B_{d/s} \to J/ψf_1(1285)$ modes for the $f_1(1285)-f_1(1420)$ mixing system, it is of great interest to find phenomenologically some modes such as the tree-dominated $B^+ \to f_1 ρ^+$ and the penguin-dominated $B^{+,0} \to f_1 K^{*+,0}, B_s^0 \to f_1 ϕ$ with large branching ratios around ${\cal O}(10^{-6})$ or even ${\cal O}(10^{-5})$, which are expected measurable at the LHCb and/or the Belle-II experiments in the near future. The good agreement [sharp contrast] of branching ratios and decay pattern for $B^+ \to f_1 ρ^+, B^{+,0} \to f_1(1285) K^{*+,0} [B^{+,0} \to f_1(1420) K^{*+,0}]$ decays between QCD factorization and perturbative QCD factorization predictions can help us to distinguish these two rather different factorization approaches via precision measurements, which would also be helpful for us to explore the annihilation decay mechanism through its important roles for the considered $B \to f_1 V$ decays.

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arXiv2016

The $S$-wave resonance contributions to the three-body decays $B^0_{(s)}\to η_c f_0(X)\to η_cπ^+π^-$ in perturbative QCD approach

In this paper, we study the three-body decays $B^0/B^0_s \to η_c f_0(X)\to η_c π^+π^-$ by employing the perturbative QCD (PQCD) factorization approach. We evaluate the $S$-wave resonance contributions by using the two-pion distribution amplitude $Φ_{ππ}^{\rm S}$. The Breit-Wigner formula for the $f_0(500)$, $f_0(1500)$, and $f_0(1790)$ resonances and the Flatté model for the $f_0(980)$ resonance are adopted to parameterize the time-like scalar form factors $F_{s}(ω^2)$. We also use the D.~V.~Bugg model to parameterize the $f_0(500)$ and compare the relevant theoretical predictions from different models. We found the following results: (a) the PQCD predictions for the branching ratios are ${\cal B}(B^0\to η_c f_0(500)[π^+π^-])= \left ( 1.53 ^{+0.76}_{-0.35} \right ) \times 10^{-6}$ for Breit-Wigner model and ${\cal B}(B^0\to η_c f_0(500)[π^+π^-])= \left ( 2.31 ^{+0.96}_{-0.48} \right ) \times 10^{-6}$ for D.~V.~Bugg model; (b) $ {\cal B}(B_s\to η_c f_0(X)[π^+π^-] ) =\left ( 5.02^{+1.49}_{-1.08} \right )\times 10^{-5}$ when the contributions from $f_0(X)=(f_0(980),f_0(1500),f_0(1790))$ are all taken into account; and (c) The considered decays could be measured at the ongoing LHCb experiment, consequently, the formalism of two-hadron distribution amplitudes could also be tested by such experiments.

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arXiv2016

Production and decays of a light $ϕ^0$ in the LRTH model under the LHC Higgs data

In this paper we study the production and decays of a light pseudoscalar boson $ϕ^0$ with $m_{ϕ^0} \leq m_h/2$ appeared in the left-right twin Higgs (LRTH) model, and explore its phenomenological consequences when the latest LHC Higgs data are taken into account. We found that (a) the decay rate $Br(h\to ϕ^0ϕ^0)$ can be as large as $80\%$ and can suppress significantly the visible $γγ$ signal rate, but the latest LHC Higgs data put a strong constraint on it: $Br(h \to ϕ^0ϕ^0) \leq 30\%$ at $3σ$ level; (b) the $p$-value of the LRTH model is around $0.6$, smaller than that of the SM in most of the parameter space and approaches the SM value $0.8$ for a sufficiently large $f$ parameter; (c) the neutral pseudoscalar $ϕ^0$ dominantly decay into $b\bar{b}$ and the decay rate $Br(ϕ^0\to b\bar{b})$ can be larger than $80\%$ for $m_{ϕ^0}\leq 60$ GeV, and the second main decay mode is $ϕ^0\to τ^{+}τ^{-}$ with a branching ratio about $14\%$; and (d) at the future $e^-e^+$ collider with $\sqrt{s}=250$ GeV, the processes $e^{+}e^{-}\to Zh\to Z(ϕ^0ϕ^0)\to Z(4b,2b2τ)$ are promising for discovering such a light pseudoscalar $ϕ^0$.

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arXiv2016

Searches for the FCNC couplings from top-Higgs associated production signal with $h\to γγ$ at the LHC

In this paper, we study the observability of the top-Higgs flavor changing neutral current (FCNC) $tqh$ coupling through the process $pp\to qg\to t(\to \ell^{+}bν)h(\to γγ)$ at the Large Hadron Collider~(LHC), where $\ell=e, μ$. Our numerical results show that, in some parameter regions, the LHC may observe the above signals at the $5σ$ level. Otherwise, the branching ratios $Br(t\to uh)$ and $Br(t\to ch)$ can be respectively probed to $0.036\%$ and $0.13\%$ at $3σ$ level at 14 TeV LHC with the high integrated luminosity of 3000 fb$^{-1}$. On the other hand, studying the charge ratio for the lepton in top quark decay can be not only used to discriminate between signal and backgrounds, but also used to discriminate between $tuh$ and $tch$ couplings, for which anomalous single top production comes from the up initiated channel and charm initiated channel.

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arXiv2016

Searches for top-Higgs FCNC couplings via the Whj signal with $h\to γγ$ at the LHC

In this paper, we investigated the process $pp \to W^{-}hj$ induced by the top-Higgs flavor changing neutral current (FCNC) couplings at the LHC. We found that the cross section of $pp \to W^{-}hj$ mainly comes from the resonant process $pp\to W^{-}t\to W^{-}hq$ due to the anomalous $tqH$ couplings (where $q$ denotes up and charm quarks). We further studied the observability of the top-Higgs FCNC couplings through the process $pp \to W^{-}(\to \ell^{-} \barν_{\ell}) h( \to γγ) j$ and found that the branching ratios $Br(t\to qh)$ can be probed to $0.16\%$ at $3σ$ level at 14 TeV LHC with an integrated luminosity of 3000 fb$^{-1}$.

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arXiv2016

Large non-factorizable contributions in $B \to a_0 a_0$ decays

We investigate three tree-dominated $B \to a_0 a_0$ decays for the first time in the perturbative QCD(pQCD) approach at leading order in the standard model, with $a_0$ standing for the light scalar $a_0(980)$ state, which is assumed as a meson based on the model of conventional two-quark$(q\bar q)$ structure. All the topologies of the Feynman diagrams such as the non-factorizable spectator ones and the annihilation ones are calculated in the pQCD approach. It is of great interest to find that, contrary to the known $B \to ππ$ decays, the $B \to a_0 a_0$ decays are governed by the large non-factorizable contributions, which give rise to the large $B \to a_0 a_0$ decay rates in the order of $10^{-6} \sim 10^{-5}$, although the $a_0$ meson has an extremely small vector decay constant $f_{a_0}$. Also observed are large direct CP-violating asymmetries around $15\%$ and $30\%$ for the $B^0 \to a_0^0 a_0^0$ and $a_0^+ a_0^-$ modes. These sizable predictions could be easily examined at the running Large Hadron Collider and the near future Super-B/Belle-II experiments. The future precision measurements combined with these pQCD predictions might be helpful to explore the complicated QCD dynamics and the inner structure of the light scalar $a_0$, as well as to complementarily constrain the unitary angle $α$.

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arXiv2016

Resolving the $B \to K π$ puzzle by Glauber-gluon effects

We extend the perturbative QCD formalism including the Glauber gluons, which has been shown to accommodate the measured $B \to ππ$ and $B^0\toρ^0 ρ^0$ branching ratios simultaneously, to the analysis of the $B \to K π$ and $K \bar K$ decays. It is observed that the convolution of the universal Glauber phase factors with the transverse-momentum-dependent kaon wave function reveals weaker (stronger) Glauber effects than in the pion ($ρ$ meson) case as expected. Our predictions for the branching ratios and the direct CP asymmetries of the $B \to K π$ and $K \bar K$ modes at next-to-leading-order accuracy agree well with data. In particular, the predicted difference of the $B^\pm \to K^\pm π^0$ and $B^0 \to K^\pm π^\mp$ direct CP asymmetries, $ΔA_{Kπ}\equiv A_{\rm CP}^{\rm dir}(K^\pm π^0)[0.021 \pm 0.016] - A_{\rm CP}^{\rm dir}(K^\pm π^\mp)[-0.081 \pm 0.017] = 0.102 \pm 0.023$, is consistent with the measured $ΔA_{Kπ} = 0.119 \pm 0.022$ within uncertainties, and the known $B \to Kπ$ puzzle is resolved. The above $B \to ππ$, $Kπ$ and $K \bar K$ studies confirm that the Glauber gluons associated with pseudo-Nambu-Goldstone bosons enhance the color-suppressed tree amplitude significantly, but have a small impact on other topological amplitudes.

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arXiv2015

The $B\to D^{(*)} lν_l$ decays in the pQCD approach with the Lattice QCD input

In this paper, we studied the semileptonic decays $B \to D^{(*)} l^- \barν_l$ by using the "pQCD+Lattice QCD" method. We made the extrapolation for the six relevant form factors by using the input values obtained from the pQCD factorization approach in the low $q^2$ region of $0\leq q^2 \leq m_τ^2$, and the Lattice QCD input at the end-point $q^2=q^2_{\rm max}$. We then calculated the ratios $R(D)$ and $R(D^*)$ of the branching ratios ${\cal B}(B \to D^{(*)} l^- \barν_l)$, and found numerically that (1) the "pQCD+Lattice QCD" predictions for the branching ratios ${\cal B}(B \to D^{(*)} l^-\barν_l)$ agree well with the measured values within one standard deviation, and (2) the "pQCD+Lattice QCD" predictions for the ratios $R(D^{(*)})$ are $ R(D)=0.337^{+0.038}_{-0.037}$ and $ R(D^*)=0.269^{+0.021}_{-0.020}$, they agree with the data within $2σ$ deviation, in other words, one can explain the "$R(D^{(*)})$-puzzle" in the framework of the standard model.

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arXiv2015

The $ργ^* \to π(ρ)$ transition form factors in the Perturbative QCD factorization approach

In this paper, we studied the $ργ^* \to π$ and $ργ^*\to ρ$ transition processes and made the calculations for the $ρπ$ transition form factor $Q^4 F_{ρπ}(Q^2)$ and the $ρ$ meson electromagnetic form factors, $F_{\rm LL, LT,TT}(Q^2)$ and $F_{1,2,3}(Q^2)$, by employing the perturbative QCD (PQCD) factorization approach. For the $ργ^* \to π$ transition, we found that the contribution to form factor $Q^4 F_{ρπ}(Q)$ from the term proportional to the distribution amplitude combination $ϕ^T_ρ(x_1)ϕ^P_π(x_2)$ is absolutely dominant, and the PQCD predictions for both the size and the $Q^2$-dependence of this form factor $Q^4 F_{ρπ}(Q^2)$ agree well with those from the extended ADS/QCD models or the light-cone QCD sum rule. For the $ργ^* \to ρ$ transition and in the region of $Q^2\geq 3$ GeV$^2$, further more, we found that the PQCD predictions for the magnitude and their $Q^2$-dependence of the $F_1(Q^2)$ and $F_2(Q^2)$ form factors agree well with those from the QCD sum rule, while the PQCD prediction for $F_3(Q^2)$ is much larger than the one from the QCD sum rule.

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arXiv2015

Transverse-momentum-dependent wave functions with Glauber gluons in $B \to ππ$, $ρρ$ decays

We investigate the Glauber-gluon effect on the $B \to ππ$ and $ρρ$ decays, which is introduced via a convolution of a universal Glauber phase factor with transverse-momentum-dependent(TMD) meson wave functions in the $k_T$ factorization theorem. For an appropriate parametrization of the Glauber phase, it is observed that a TMD wave function for the pion ($ρ$ meson) with a weak (strong) falloff in parton transverse momentum $k_T$ leads to significant (moderate) modification of the $B^0 \to π^0 π^0$ ($B^0 \to ρ^0 ρ^0$) branching ratio: the former (latter) is enhanced (reduced) by about a factor of 2 (15\%). This observation is consistent with the dual role of the pion as a massless Nambu-Goldstone boson and as a $q \bar q$ bound state, which requires a tighter spatial distribution of its leading Fock state relative to higher Fock states. The agreement between the theoretical predictions and the data for all the $B \to ππ$ and $ρ^0 ρ^0$ branching ratios is then improved simultaneously, and it is possible to resolve the $B \to ππ$ puzzle.

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arXiv2015

Time-like pion electromagnetic form factors in $k_{T}$ factorization with the Next-to-leading-order twist-3 contribution

We calculate the time-like pion electromagnetic form factor in the $k_T$ factorization formalism with the inclusion of the next-to-leading-order(NLO) corrections to the leading-twist and sub-leading-twist contributions. It's found that the total NLO correction can enhance (reduce) the magnitude (strong phase) of the leading order form factor by $20\% - 30\%$ ( $< 15^o$) in the considered invariant mass squared $q^2 > 5$ GeV$^2$, and the NLO twist-3 correction play the key role to narrow the gap between the pQCD predictions and the measured values for the time-like pion electromagnetic form factor.

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arXiv2015

Single Higgs boson production at the ILC in the left-right twin Higgs model

In this work, we analyze three dominant single SM-like Higgs boson production processes in the left-right twin Higgs model (LRTHM): the Higgs-strahlung (HS) process $e^{+}e^{-}\rightarrow Zh$, the vector boson fusion (VBF) process $e^{+}e^{-}\to ν\barνh$ and the associate production with top pair process $e^{+}e^{-}\to t\bar{t}h$ for three possible energy stages of the International Linear Collider (ILC), and compared our results with the expected experimental accuracies for various accessible Higgs decay channels. The following observations have been obtained: (i) In the reasonable parameter space, the LRTHM can generate moderate contributions to theses processes with polarized beams; (ii) Among various Higgs boson decay channels, the $b\bar{b}$ signal strength is most sensitive to the LRTHM due to the high expected precision. For the $t\bar{t}h$ production process, the absolute value of $μ_{b\bar{b}}$ may deviate from the SM prediction by over $8.7\%$ and thus may be detectable at the future ILC with $\sqrt{s}=1$ TeV; (iii) The future ILC experiments may give strong limit on the scale parameter $f$: for the case of ILC-250 GeV, for example, the lower limit for parameter $f$ of the LRTHM is $f > 1150$ GeV at the $2σ$ level.

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arXiv2015

The NLO contributions to the scalar pion form factors and the ${\cal O}(α_s^2)$ annihilation corrections to the $B\to ππ$ decays

In this paper, by employing the $k_{T}$ factorization theorem, we made the first calculation for the space-like scalar pion form factor $Q^2 F(Q^2)$ at the leading order (LO) and the next-to-leading order (NLO) level, and then found the time-like scalar pion form factor $F'^{(1)}_{\rm a,I}$ by analytic continuation from the space-like one. From the analytical evaluations and the numerical results, we found the following points: (a) the NLO correction to the space-like scalar pion form factor has an opposite sign with the LO one but is very small in magnitude, can produce at most $10\%$ decrease to LO result in the considered $Q^2$ region; (b) the NLO time-like scalar pion form factor $F'^{(1)}_{\rm a,I}$ describes the ${\cal O}(α_s^2)$ contribution to the factorizable annihilation diagrams of the considered $B \to ππ$ decays, i.e. the NLO annihilation correction; (c) the NLO part of the form factor $F'^{(1)}_{\rm a,I}$ is very small in size, and is almost independent with the variation of cutoff scale $μ_0$, but this form factor has a large strong phase around $-55^\circ$ and may play an important role in producing large CP violation for $B\to ππ$ decays; and (d) for $B^0 \to π^+π^-$ and $ π^0π^0$ decays, the newly known NLO annihilation correction can produce only a very small enhancement to their branching ratios, less than $3\%$ in magnitude, and therefore we could not interpret the well-known $ππ$-puzzle by the inclusion of this NLO correction to the factorizable annihilation diagrams.

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