arXiv subjects

Guohuai Zhu

Publications and source records attributed to Guohuai Zhu.

At least 37 records · Page 2Linked to original sources

arXiv2011

Gamma-rays from Nearby Clusters: Constraints on Selected Decaying Dark Matter Models

Recently, the Fermi-LAT collaboration reported upper limits on the GeV gamma-ray flux from nearby clusters of galaxies. Motivated by these limits, we study corresponding constraints on gamma-ray emissions from two specific decaying dark matter models, one via grand unification scale suppressed operators and the other via R-parity violating operators. Both can account for the PAMELA and Fermi-LAT excesses of e^\pm. For GUT decaying dark matter, the gamma-rays from the M49 and Fornax clusters, with energy in the range of 1 to 10 GeV, lead to the most stringent constraints to date. As a result, this dark matter is disfavored with conventional model of e^\pm background. In addition, it is likely that some tension exists between the Fermi-LAT e^\pm excess and the gamma ray constraints for any decaying dark matter model, provided conventional model of e^\pm background is adopted. Nevertheless, the GUT decaying dark matter can still solely account for the PAMELA positron fraction excess without violating the gamma-ray constraints. For the gravitino dark matter model with R-parity violation, cluster observations do not give tight constraints. This is because a different e^\pm background has been adopted which leads to relatively light dark matter mass around 200 GeV.

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arXiv2010

Bd \to π^- K^{(*)+} and Bs \to π^+(ρ^+) K^- decays with QCD factorization and flavor symmetry

The QCD factorization (QCDF) method usually contains infrared divergences which introduce large model dependence to its predictions on charmless B decays. The amplitudes of charmless B decays can be decomposed into "tree" and "penguin" parts which are conventionally defined, not from the topology of the dominant diagrams, but through their associated CKM factors V_{ub}^* V_{uq} and V_{tb}^* V_{tq}, respectively, with q=d,s. We find that for B_{d,s} \to π^+ K^- decays, the "tree" amplitude can be well estimated in QCDF with small errors, as the endpoint singularities have been canceled to a large extent. With this as the only input from QCDF and combined with flavor symmetry, the branching ratio of B_s \to π^+ K^- are estimated to be significantly larger than the CDF measurement. This contradiction could be solved if the form factor F^{B_s K} is smaller than the light cone sum rules estimation or the "tree" amplitude has been over estimated in QCDF. The latter possibility could happen if charming penguins are nonperturbative and not small, as argued in soft collinear effective theory. To differentiate between these two possibilities, we examine the similar B_s \to ρ^+ K^- decay with the same technique. It is found that a large part of the uncertainties are canceled in the ratio B(B_s \to ρ^+ K^-)/B(B_s \to π^+ K^-). It is predicted to be 2.5 \pm 0.2 in QCDF which is independent on the form factor. However if charming penguins are important, this ratio could be very different from the QCDF prediction. Therefore the ratio of these two branching ratios could be an interesting indicator of the role of charming penguins in charmless B decays.

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arXiv2010

Decaying Dark Matter in Supersymmetric SU(5) Models

Motivated by recent observations from Pamela, Fermi and H.E.S.S., we consider dark matter decays in the framework of supersymmetric SU(5) grand unification theories. An SU(5) singlet S is assumed to be the main component of dark matters, which decays into visible particles through dimension six operators suppressed by the grand unification scale. Under certain conditions, S decays dominantly into a pair of sleptons with universal coupling for all generations. Subsequently, electrons and positrons are produced from cascade decays of these sleptons. These cascade decay chains smooth the electron/positron spectrum, which permit naturally a good fit to the Fermi LAT data. The observed positron fraction upturn by PAMELA can be reproduced simultaneously. We have also calculated diffuse gamma-ray spectra due to the electron/positron excesses and compared them with the preliminary Fermi LAT data from 0.1 GeV to 10 GeV in the region 0<l <360, 10<|b|<20. The photon spectrum of energy above 100 GeV, mainly from final state radiations, may be checked in the near future.

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arXiv2010

Invisible Higgs decay with B\to Kν\barν constraint

If the Higgs boson were the only particle within the LHC accessible range, precision measurement of the Higgs's properties would play a unique role in studying electroweak symmetry breaking as well as possible new physics. We try to use low energy experiments such as rare B decay to constrain a challenging decay mode of Higgs, in which a Higgs decays to a pair of light (\approx 1 \sim 2 GeV) SM singlet S and becomes invisible. By using the current experimental bound of rare decay B\to Kν\barν and computing the contribution of B\to K SS to (the) B\to K+\cancel{E}, we obtain an upper bound on the Higgs coupling to such light singlet. It is interesting that the partial width of the invisible decay mode h\to SS by taking the upper bound value of coupling is at a comparable level with h\to WW/ZZ or WW^(*) decay modes, making the Higgs identifiable but with a different predicted decay BR from the standard model Higgs decay. It will then have an impact on precision measurement of the Higgs's properties. We also study the implication for cosmology from such a light singlet and propose a solution to the potential problem.

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arXiv2008

Dielectron and Diphoton channels in 2HDM

It was observed recently by Randall and Wise that, any particle in the Standard Model cannot decay to $e^+e^-$ and $γγ$ final states with comparably measurable branching ratios. This is also true for most extensions of the Standard Model, with the Randall-Sundrum model as an outstanding exception. In this paper, we show that two-Higgs-Doublet-Models (2HDM) yield another possible exception if certain parameters are properly chosen. In addition, we have checked that this model survives the tests of low energy processes, including the anomalous magnetic moment and electric dipole moment of leptons, lepton-flavor-violating decays $μ^- \to e^- γ$ and $e^- e^+ e^-$.

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arXiv2008

Some Phenomenologies of Unparticle Physics

Fermionic unparticles are introduced and their basic properties are discussed. Some phenomenologies related are exploited, such as their effects on charged Higgs boson decays and anomalous magnetic moments of leptons. Also, it has been found that measurements of $B^0-\bar B^0$ mixing could yield interesting constraints on couplings between unparticle operators and standard model fields.

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arXiv2008

Unparticle Physics and A_{FB}^b on the Z pole

An attempt has been made to address the 3σanomaly of the forward-backward asymmetry of b quark in LEP data via an unparticle sector. For most part of the parameter space except certain particular regions, the anomaly could not be explained away plausibly, when constraints from other LEP observables are taken into account.

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arXiv2007

Unitarity Constraints on effective interaction in $πN$ scattering

Positivity constraints are derived on pion-nucleon scattering amplitudes and their even-order derivatives inside the Mandelstam Triangle with the help of dispersion relations. Fairly interesting constraints are obtained on some of the low energy constants, by a combination of the chiral perturbation theory for heavy baryons and existing fitting results from available pion-nucleon phase shifts at intermediate energies.

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arXiv2006

$B \to K^* \ell^+\ell^-$ in soft-collinear effective theory

We study the rare B decay $B \to K^* \ell^+ \ell^-$ using soft-collinear effective theory (SCET). At leading power in $1/m_b$, a factorization formula is obtained valid to all orders in $α_s$. For phenomenological application, we calculate the decay amplitude including order $α_s$ corrections, and resum the logarithms by evolving the matching coefficients from the hard scale ${\cal O}(m_b)$ down to the scale $\sqrt{m_b Λ_h}$. The branching ratio for $B \to K^* \ell^+ \ell^-$ is uncertain due to the imprecise knowledge of the soft form factors $ζ_\perp (q^2)$ and $ζ_\parallel (q^2)$. Constraining the soft form factor $ζ_\perp (q^2=0)$ from data on $B \to K^* γ$ yields $ζ_\perp (q^2=0)=0.32 \pm 0.02$. Using this input, together with the light-cone sum rules to determine the $q^2$dependence of $ζ_\perp (q^2)$ and the other soft form factor $ζ_\parallel (q^2)$, we eastimate the partially integrated branching ratio in the range $1~{GeV}^2 \le q^2 \le 7~{GeV}^2$ to be $(2.92^{+0.67}_{-0.61}) \times 10^{-7}$. We discuss how to reduce the form factor related uncertainty by combining data on $B \to ρ(\to ππ) \ell ν_\ell$ and $B\to K^* (\to Kπ) \ell^+\ell^-$. The forward-backward asymmetry is less sensitive to the input parameters. In particular, for the zero-point of the forward backward asymmetry in the standard model, we get $q_0^2=(4.07^{+0.13}_{-0.12})~{GeV}^2$. The scale dependence of $q_0^2$ is discussed in detail.

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arXiv2005

$B \to χ_{c0,2} K$ decays: a model estimation

In this paper, we investigate the vertex corrections and spectator hard scattering contributions to $B \to χ_{c0,2}K$ decays, which has no leading contribution from naive factorization scheme. A non-zero binding energy $b=2m_c-M$ is introduced to regularize the infrared divergence of the vertex part. The spectator diagrams also contain logarithmic and linear infrared divergences, for which we adopt a model dependent parametrization. If we neglect possible strong phases in the hard spectator contributions, we obtain a too small branching ratio for $χ_{c0}K$ while too large one for $χ_{c2}K$, as can be seen from the ratio of the branching ratio of $B^+ \to χ_{c2}K^+$ to that of $B^+ \to χ_{c0} K^+$, which is predicted to be $2.15^{+0.63}_{-0.76}$ in our model, while experimentally it should be about 0.1 or even smaller. But a closer examination shows that, assuming large strong phases difference between the twist-2 and twist-3 spectator terms, together with a slightly larger spectator infrared cutoff parameter $Λ_h$, it is possible to accommodate the experimental data. This shows that, for $B\to χ_{c0,2}K$ decays with no factorizable contributions, QCDF seems capable of producing decay rates close to experiments, in contrast to the $B\to J/ψK$ decay which is dominated by the factorizable contributions.

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arXiv2004

Leading power SCET analysis of $e^+ e^- \to J/ψg g$

Recently, Belle and BaBar Collaborations observed surprising suppression in the endpoint $J/ψ$ spectrum, which stimulates us to examine the endpoint behaviors of the $e^+ e^- \to J/ψgg$ production. We calculate the $J/ψ$ momentum and angular distributions for this process within the framework of the soft-collinear effective theory (SCET). The decreasing spectrum in the endpoint region is obtained by summing the Sudakov logarithms. We also find a large discrepancy between the NRQCD and SCET spectrum in the endpoint region even before the large logarithms are summed, which is probably due to the fact that only the scalar structure of the two-gluon system is picked out in the leading power expansion. A comparison with the process $Υ\to γgg$ is made.

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arXiv2004

Power Corrections in Charmless B Decays

In this paper, we focus on the role of power corrections in QCD factorization(QCDF) method in charmless two-body nonleptonic $B$ meson decays. We use the ratio of the branching fraction of $B^+ \to π^+ K^{\ast 0}$ to that of $B^0 \to π^- ρ^+$, for which the theoretical uncertainties are greatly reduced, to show clearly that the power corrections in charmless B decays are probably large. With other similar ratios considered, for example, for the $B^0 \to K^- ρ^+$ decay, it is very likely that, among various sources of power corrections, annihilation topology plays an indispensable role at least for penguin dominated $\rm PV$ channels. We also consider some selective ratios of direct CP asymmetries. Among these, we find that, if power corrections other than the chirally enhanced power corrections and annihilation topology were negligible, QCDF would predict the direct CP asymmetry of $B \to π^+ π^-$ to be about 3 times larger than that of $B \to π^\pm K^\mp$, with opposite sign. Experimentally any significant deviation from this prediction would suggest either new physics or possibly the importance of long-distance rescattering effects.

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arXiv2004

CP Asymmetries of $B \to ϕK$ and $B \to η^{(\prime)} K$ Decays Using a Global Fit in QCD Factorization

We analyze the CP asymmetries of $B\to ϕK$ and $B\to η^{(\prime)} K$ modes in the QCD improved factorization framework. For our calculation we use the phenomenological parameters predetermined from the global fit for the available $B\to PP$ and $VP$ modes (without the quark-level subprocess $b\to s\bar s s$). We show that the large negative $\sin (2ϕ_1)_{ϕK_s}$ and the large branching ratio for $B^{\pm}\to η' K^{\pm}$ can be simultaneously explained in the context of supersymmetry (SUSY). The R-parity conserving SUGRA models are used and their parameter space is constrained with the observed dark matter relic density along with other experimental constraints. The R-parity violating SUSY models are also used to show that they can provide solutions. We calculate the CP asymmetries for different $B^{\pm (0)} \to ϕK^{\pm (0)}$ and $B^{\pm (0)} \to η^{(')} K^{\pm (0)}$ modes and show that the SUSY model predictions are consistent with the available experimental data.

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arXiv2004

An Analysis of $B \to η' K$ Decays Using a Global Fit in QCD Factorization

In the framework of QCD factorization, we study $B^{+(0)} \to η' K^{+(0)}$ decays. In order to more reliably determine the phenomenological parameters $X_H$ and $X_A$ arising from end-point divergences in the hard spectator scattering and weak annihilation contributions, we use the global analysis for twelve $B \to PP$ and $VP$ decay modes, such as $B \to ππ$, $πK$, $ρπ$, $ρK$, etc, but excluding the modes whose (dominant) internal quark-level process is $b \to s \bar s s$. Based on the global analysis, we critically investigate possible magnitudes of $X_{H,A}$ and find that both large and small $X_{H,A}$ terms are allowed by the global fit. In the case of the large $X_{H,A}$ effects, the standard model (SM) prediction of the branching ratios (BRs) for $B^{+(0)} \to η' K^{+(0)}$ is large and well consistent with the experimental results. In contrast, in the case of the small $X_{H,A}$ effects, the SM prediction for these BRs is smaller than the experimental data. Motivated by the recent Belle measurement of $\sin (2ϕ_1)$ through $B^0 \to ϕK_s$, if we take into account possible new physics effects on the quark-level process $b \to s \bar s s$, we can explicitly show that these large BRs can be understood even in the small $X_{H,A}$ case. Specifically, we present two new physics scenarios: R-parity violating SUSY and R-parity conserving SUSY.

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arXiv2003

Charmless two-body B decays: A global analysis with QCD factorization

In this paper, we perform a global analysis of $B \to PP$ and $PV$ decays with the QCD factorization approach. It is encouraging to observe that the predictions of QCD factorization are in good agreement with experiment. The best fit $γ$ is around $79^\circ$. The penguin-to-tree ratio $|P_{ππ}/T_{ππ}|$ of $π^+ π^-$ decays is preferred to be larger than 0.3. We also show the confidence levels for some interesting channels: $B^0 \to π^0 π^0$, $K^+ K^-$ and $B^+ \to ωπ^+$, $ωK^+$. For $B \to πK^\ast$ decays, they are expected to have smaller branching ratios with more precise measurements.

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arXiv2002

Phenomenological Analysis of Charmless Decays Bs->PP,PV with QCD Factorization

We calculated the CP averaged branching ratios and CP-violating asymmetries of two-body charmless hadronic Bs->PP,PV decays with the QCDF approach, including the contributions from the chirally enhanced power corrections and weak annihilations. Only several decay modes, such as Bs -> K^(*)K, K^(*{\pm})π^{\mp}, K^{\pm}ρ^{\mp}, η^(')η^('), have large branching ratios, which may be observed in the near future. The penguin-to-tree ratio P_{ππ}/T_{ππ} and a bound on angle $γ$ are given.

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arXiv2002

Phenomenological Analysis of Charmless Decays B-->PV with QCD Factorization

We study the hadronic charmless two-body B decays to final states involving pseudoscalar and vector mesons with QCD factorization approach, including the contributions from the chirally enhanced power corrections and weak annihilations. The CP-averaged branching ratios and CP-violating asymmetries are given. Most of our results are in agreement with the present measurements, but the branching ratios of some decay channels are only marginally consistent with the experimental observations. Considering the large uncertainties, great advances in both experiment and theory in the near future are strongly expected.

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arXiv2002

Phenomenological Analysis of B->PP Decays with QCD Factorization

In this paper, we study nonleptonic charmless B decays to two light pseudoscalar mesons within the frame of QCD factorization, including the contributions from the chirally enhanced power corrections and weak annihilation. Predictions for the CP-averaged branching ratios and CP-violating asymmetries are given. Within the reasonable range of the parameters, we find that our predictions for the branching ratios of B -> PP are consistent with the present experimental data. But because of the logarithmic divergences at the endpoints in the hard spectator scatterings and weak annihilation, there are still large uncertainties in these predictions.

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