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Xiao-Gang He

Publications and source records attributed to Xiao-Gang He.

At least 289 records · Page 16Linked to original sources
arXiv1997

CP Violation

In this lecture I review the present status of CP violation in the Standard Model and some of its extensions and discuss ways to distinguish different models. Contents 1. Introduction 2. CP violation in the Standard Model 3. Test the Standard Model in B decays 4. Models for CP violation 5. The KM unitarity triangle and new physics 6. Direct CP violation in neutral Kaon system 7. The electric dipole moment 8. Partial rate asymmetry 9. Test of CP violation involving polarization measurement 10. Baryon number asymmetry 11. Conclusion

hep-ph↗
arXiv1997

A New Method Of Distinguishing Models For The High-$Q^2$ Events At HERA

Many explanations for the excess high-Q^2 $e^+p \to e^+X$ events from H1 and ZEUS at HERA have been proposed each with criticisms. We propose a new method to distinguish different models by looking at a new distribution which is insensitive to parton distribution function but sensitive to new physics.

hep-ph↗
arXiv1997

Delta I=3/2 and Delta S =2 Hyperon Decays in Chiral Perturbation Theory

We study the Delta I =3/2 and Delta S =2 amplitudes for hyperon decays of the form (B to B(prime) pion) at lowest order in chiral perturbation theory. At this order, the Delta I=3/2 amplitudes depend on only one constant. We extract the value of this constant from experiment and find a reasonable description of these processes within experimental errors. The same constant determines the Delta S =2 transitions which, in the standard model, are too small to be observed. We find that new physics with parity odd Delta S=2 interactions can produce observable rates in hyperon decays while evading the bounds from K0 - K0(bar) mixing.

hep-ph↗
arXiv1997

Contact Interaction Explanation of HERA Events and $SU(3)_C\times SU(2)_L\times U(1)_Y$ Invariance

We reexamine the contact interaction hypothesis for excess high-$Q^2$ events seen at HERA in $e^+ p \to e^+ X$ scattering. We consider the most general structure of contact interactions insisting on $SU(3)_C\times SU(2)_L \times U(1)_Y$ invariance. Apart from constraints for $e^+ e^- \to q\bar q$ , Drell-Yan and atomic parity violation experiments, we find stringent constraints from neutrino deep inelastic data. A possible choice of contact terms that escape present bounds is still possible, althought new data from FERMILAB and LEP should further constrain such a possibility.

hep-ph↗
arXiv1996

Color-Octet Contribution and Direct CP Violation in $B\to ψ(ψ') X$

We study $c \bar c$ color-octet contribution to $B\to ψ(ψ') X$. When this contribution is included, the theoretical predictions for the branching ratios become in much better agreement with the experiment. This mechanism also enhances the partial rate asymmetries by about a factor of five. The inclusive $ψ(ψ')$ resulting from $b\to d+{}$gluon can have asymmetry around a few percent whereas those from $b\to s+{}$gluon has it around $4\times 10^{-4}$. The asymmetry in the former modes should be observable, to a significance of $3σ$, with about $(1-10)\times 10^8B$ mesons.

hep-ph↗
arXiv1996

Single Isospin Decay Amplitude and CP Violation

We study partial rate asymmetry in decays with single isospin final state. For K meson or hyperon decays, the partial rate asymmetries are always zero if the final states are single isospin states. In B decays the situation is dramatically different and partial rate asymmetries can be non-zero even if the final states are single isospin states. We calculated partial rate asymmetries for several B decays with single isospin amplitude in the final states using factorization approximation. We find that in some cases the asymmetries can be large.

hep-ph↗
arXiv1996

Constraints on CP violating four-fermion interactions

It has been shown that CP violating electron-nucleon and nucleon-nucleon interactions can induce atomic electric dipole moments and are therefore constrained from experimental data. We show that using the experimental upper bounds on neutron and electron electric dipole moments, one can also obtain constraints, in some cases better ones, on these interactions. In addition stringent constraints can also be obtained for muon-quark and tauon-quark four-fermion CP violating interactions, which cannot be constrained from atomic electric dipole moment experiments.

hep-ph↗
arXiv1996

Remarks on a Technique of Measuring CP phase $α$

We present a method to measure the CKM phase $α$ and the tree and penguin (strong and electroweak) amplitudes in $B \rightarrow ππ$ decays, based on isospin consideration and the weak assumption that all tree amplitudes have a common strong phase and all penguin amplitudes have a different common phase. The method needs only the time-independent measurements of the relevant decay rates in $B \rightarrow ππ$. We also propose a method to experimentally examine the validity of the assumption that all penguin amplitudes have the same strong phases, and to extract detailed informations about the hadronic matrix elements.

hep-ph↗
arXiv1996

B decays and models for CP violation

The decay modes $B$ to $ππ$, $ψK_S$, $K^- D$, $πK$ and $ηK$ are promising channels to study the unitarity triangle of the CP violating CKM matrix. In this paper I study the consequences of these measurements in the Weinberg model. I show that using the same set of measurements, the following different mechanisms for CP violation can be distinguished: 1) CP is violated in the CKM sector only; 2) CP is violated spontaneously in the Higgs sector only; And 3) CP is violated in both the CKM and Higgs sectors.

hep-ph↗
arXiv1995

CP Asymmetry In Neutral $B$ System At Symmetric Colliders

Contrary to the conventional belief, time integrated asymmetry are measurable in selected final states in the neutral $B$ system at symmetric $e^+e^-$ colliders. They occur due to the interplay of weak and strong phases of two different amplitudes in addition to the $B^0 - \bar B^0$ mixing. Observation of these asymmetries would be evidence for direct CP violation in the decay amplitudes.

hep-ph↗
arXiv1995

A Study of Some Methods for Measuring CKM CP Violating Phases

We study the influence of penguin (especially, electroweak penguin) effects on some methods of measuring the angles $α$, $β$, and $γ$ in the CKM unitarity triangle. We use next-to-leading order effective Hamiltonian, and present numerical estimates based on the factorization approximation. We find that some techniques suggested in the literature, especially for $α$ determination, are not workable in light of the electroweak penguin effects. Nevertheless, there are methods that would work for each angle determination. For angle $β$ we consider $B \rightarrow D^{+} D^{-}$ mode and estimate the penguin contamination. For angle $γ$ we consider a method based on SU(3) symmetry and carefully consider SU(3) breaking effects. We point out regions in the parameter space where this method could be used reliably.

hep-ph↗
arXiv1995

Importance of Dipole Penguin Operator in $B$ Decays

The importance of dipole penguin operator, $O_{11} = (g_s/8π^2)m_b \bar s σ_{μν}RT^a b G^{μν}_a$, on B decays is demonstrated. It is shown that branching ratios for the inclusive decay $b\to s s\bar s$, the semi-inclusive decay $b\to sϕ$, and the exclusive decay $B\to πK$ are enhanced by 20% to 30%. A useful consequence of this calculation is the inclusive $b\to ss\bar s$ spectrum which will be essential in isolating pure penguin process.

hep-ph↗
arXiv1995

CP Violation in $Lambda \to p π^-$ Beyond the Standard Model

The $\cp$ violating asymmetry $A(Λ^0_-)$ has been estimated to occur at the level of a few times $10^{-5}$ within the minimal standard model. The experiment E871 expects to reach a sensitivity of $10^{-4}$ to the asymmetry $A(Λ^0_-)+A(Ξ^-_-)$. In this paper we study some of the implications of such a measurement for $\cp$ violation beyond the minimal standard model. We find that it is possible to have $A(Λ^0_-)$ at the few times $10^{-4}$ level while satisfying the constraints imposed by the measurements of $\cp$ violation in kaon decays.

hep-ph↗
arXiv1995

A method for determining CP violating phase $γ$

A new way of determining the phases of weak amplitudes in charged $B$ decays based on SU(3) symmetry is proposed. The CP violating phase $γ$ can now be determined without the previous difficulty associated with electroweak penguins.

hep-ph↗
arXiv1995

Energy Distribution of $ϕ$ in Pure Penguin Induced $B$ Decays

We study the energy distribution of $ϕ$ in pure penguin induced $B\rightarrow X_s ϕ$ taking into account the fermi motion of b inside $B$ meson for $b\rightarrow sϕ$ and also modification due to gluon bremsstrahlung process $b\rightarrow sϕg$. We find that the contribution to $B\rightarrow X_sϕ$ from $b\rightarrow sϕg$ is less than 3\%. This study provides a criterion for including most of the $ϕ$'s produced in a penguin process.

hep-ph↗
arXiv1995

TEST OF CP VIOLATION IN NON-LEPTONIC HYPERON DECAYS

In this talk I discuss CP violation in hyperon decays in Left-Right symmetric models. I show that the asymmetry in polarization in $Λ\to pπ^-$ can be as large as $6\times 10^{-4}$ in these models, which is an order of magnitude larger than the Standard Model prediction.

hep-ph↗