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Xiao-Gang He

Publications and source records attributed to Xiao-Gang He.

At least 253 records · Page 14Linked to original sources
arXiv2001

CP Violation in Hyperon Decays

In this talk I review theoretical predictions for CP violation in non-leptonic hyperon decays in the Standard Model and models beyond. In the Standard Model the CP violating observable $A$ in the polarization asymmetries of $Λ\to p π^-$ and $Ξ^- \to Λπ^-$ decays are predicted to be in the ranges $(-0.61\sim 6.8)\times 10^{-5}$ and $-(0.1\sim 1)\times 10^{-5}$, respectively. These ranges are below the sensitivity of $1.4\times 10^{-4}$ for $A(Λ^0_-)+A(Ξ^0_-)$ for E871 experiment at Fermilab. When going beyond the SM, such as, Supersymmetric and Left-Right symmetric models, $A$ can be as large as $10^{-3}$ and a few times of $10^{-4}$, respectively. Studies of hyperon decays can provide important information about CP violation.

hep-ph↗
arXiv2001

Constraints on SUSY Gluonic Dipole Interaction from B\to KπDecays

In low energy SUSY theories exchange of gluino and squark with left-right mixing can produce a large gluonic dipole interaction. In this paper we study the effects of this interaction on $B\to K π$ using QCD improved factorization method. The Standard Model predicts a smaller branching ratio for $B^0 \to \bar K^0 π^0$ than experimental measured one. We find that within the parameter space allowed from $B\to γX_s$ constraint, the SUSY dipole interaction can enhance this branching ratio to agree with the experimental measurement. Combining recent data for all the four $\bar B^0 \to K^- π^+, \bar K^0 π^0$ and $B^- \to K^- π^0, \bar K^0 π^-$ decay modes, we find that the allowed parameter space is reduced significantly compared with that using $B\to X_s γ$ data alone. It is found that even with these constraints, the predictions for CP violation in these modes can be dramatically different from those of the SM predictions.

hep-ph↗
arXiv2001

A new parametrization of the neutrino mixing matrix for neutrino oscillations

In this paper we study three active neutrino oscillations, favored by recent data from SuperK and SNO, using a new parametrization of the lepton mixing matrix $V$ constructed from a linear combination of the unit matrix $I$, and a hermitian unitary matrix $U$, that is, $V = \cosθI + i\sin θU$. There are only three real parameters in $V$ including the parameter $θ$. It is interesting to find that experimental data on atmospheric neutrino dictates the angle $θ$ to be $π/4$ such that the $ν_μ$ and $ν_τ$ mixing is maximal. The solar neutrino problem is solved via the MSW effect with a small mixing angle, with $U$ depending on one small parameter $ε$. The resulting mixing matrix with just two parameters ($θ$ and $ε$) predicts that the oscillating probabilities for $ν_e\to ν_μ$ and $ν_e \to ν_τ$ to be equal and of the order $2ε^2 = (0.25\sim 2.5)\times 10^{-3}$. The measurement of CP asymmetries at the proposed Neutrino Factories would also provide a test of our parametrization.

hep-ph↗
arXiv2001

Accelerating Universe and Event Horizon

It has been argued in the literature that if a universe is expanding with an accelerating rate indefinitely, it presents a challenge to string theories due to the existence of event horizons. We study the fate of a currently accelerating universe. We show that the universe will continue to accelerate indefinitely if the parameter $ω= p/ρ$ of the equation of state is a constant, no matter how many different types of energy (matter, radiation, quintessence, cosmological constant and etc) are contained in the universe. This type of universe will always exhibit an event horizon indicating that such a universe may not be derived from string theories. We also comment on some related issues.

astro-ph↗
arXiv2001

Signatures of Non-commutative QED at Photon Colliders

In this paper we study non-commutative (NC) QED signatures at photon colliders through pair production of charged leptons $(\ell^+ \ell^-)$ and charged scalars $(H^+ H^-)$. The NC corrections for the fermion pair production can be easily obtained since NC QED with fermions has been extensively studied in the literature. NC QED with scalars is less studied. To obtain the cross section for $H^+H^-$ productions, we first investigate the structure of NC QED with scalars, and then study the corrections due to the NC geometry to the ordinary QED cross sections. Finally by folding in the photon spectra for a $γγ$ collider with laser back-scattered photons from the $e^+ e^-$ machine, we obtain 95% CL lower bound on the NC scale using the above two processes. We find that, with $\sqrt{s} = 0.5, 1.0$, and $ 1.5$ TeV and integrated luminosity $L = 500(fb^{-1})$, the NC scale up to 0.7, 1.2, and 1.6 TeV can be probed, respectively, while, for monochromatic photon beams, these numbers become 1.1, 1.7, 2.6 TeV, respectively.

hep-ph↗
arXiv2000

Semi-Inclusive B\to K(K^*) X Decays with Initial Bound State Effects

The effects of initial $b$ quark bound state for the semi-inclusive decays $B\to K(K^*) X$ are studied using light cone expansion and heavy quark effective theory methods. We find that the initial bound state effects on the branching ratios and CP asymmetries are small. In the light cone expansion approach, the CP-averaged branching ratios are increased by about 2% with respect to the free $b$-quark decay. For $\bar B^0 \to K^- (K^{*-}) X$, the CP-averaged branching ratios are sensitive to the phase $γ$ and the CP asymmetry can be as large as 7% (14%), whereas for $B^-\to \bar K^0 (\bar K^{*0})X$ the CP-averaged branching ratios are not sensitive to $γ$ and the CP asymmetries are small ($< 1%$). The CP-averaged branching ratios are predicted to be in the ranges $(0.53 \sim 1.5)\times 10^{-4}$ [$(0.25 \sim 2.0)\times 10^{-4}$] for $\bar B^0 \to K^- (K^{*-})X$ and $(0.77 \sim 0.84)\times 10^{-4}$ [$(0.67 \sim 0.74)\times 10^{-4}$] for $B^-\to \bar K^0 (\bar K^{*0}) X$, depending on the value of the CP violating phase $γ$. In the heavy quark effective theory approach, we find that the branching ratios are decreased by about 10% and the CP asymmetries are not affected. These predictions can be tested in the near future.

hep-ph↗
arXiv2000

The Topological AC and HMW Effects, and the Dual Current in 2+1 Dimensions

We study the Aharonov-Casher (AC) effect and the related He-McKellar-Wilkens (HMW) effect in 2+1 dimensions. In this restricted space these effects are the result of the interaction of the electromagnetic field tensor with the dual of a current. Transferring the dual operation from the current to the field tensor shows that this interaction may be reinterpreted as due to the interaction of an effective vector potential and a current, and the AC and HMW effects follow immediately. A general proof of this for particles with an arbitrary spin is provided. The restriction to 2+1 dimensions, with this interpretation, provides a unified way of treating the AC and HMW effects for an arbitrary spin. Perhaps more interestingly the treatment shows that a spin-0 particle can show AC and HMW effects, although it has no magnetic or electric dipole moment in the usual sense.

quant-ph↗
arXiv2000

Ruling out the Weinberg Model of Spontaneous CP Violation

There have been many declarations of the death of the Weinberg model of spontaneous CP violation. Previous studies, before the recent measurements of $ε'/ε$, indicated that the model could not accommodate the experimental values on $ε$ in $K^0-\bar K^0$ mixing, the neutron electric dipole moment (EDM), the branching ratio of $b\to s γ$ and the upper limit on $ε'/ε$. We point out that these studies were based on optimistic estimates of the uncertainties in the calculations and that when more realistic estimates of these errors are used the Weinberg model cannot be conclusively ruled out from these considerations alone. Here we use these realistic error estimates to analyze the present situation of the Weinberg model. The latest results from Belle and BaBar on $\sin 2 β$ allow the small values of this parameter which occur naturally in the Weinberg model. However, in this model, the recently measured value of $\Re(ε'/ε) = (1.92\pm 0.25)\times 10^{-3}$ cannot be made compatible with the branching ratio $B(b\to s γ) = (3.15\pm0.54)\times 10^{-4}$. As a result we conclude that the Weinberg model is now confidently and conservatively ruled out.

hep-ph↗
arXiv2000

$B\to ϕK$ and $B\to ϕX_s$ in the Heavy Quark Limit

We study $B\to ϕK$ and $B\to ϕX_s$ decays in the heavy quark limit using perturbative QCD. The next leading order corrections introduce substantial modifications to the naive factorization results (more than 50%). The branching ratio $Br(B\to ϕK)$ is predicted to be in the range $(F^{B\to K}_1(m^2_ϕ)/0.33)^2(3.5\sim 4.2) \times 10^{-6}$ which is within the one $σ$ allowed region from the central value of $6.2\times 10^{-6}$ measured by CLEO, but outside the one $σ$ allowed region from the central value of $17.2\times 10^{-6}$ measured by BELLE for reasonable $F_1^{B\to K}$. For the semi-inclusive decay $B\to ϕX_s$ we also include initial bound state effect in the heavy quark limit which decreases the branching ratio by about 10%. $Br(B\to ϕX_s)$ is predicted to be in the range $(5.1\sim 6.3)\times 10^{-5}$.

hep-ph↗
arXiv2000

Electroweak Model Independent Tests for SU(3) Symmetry in Hadronic B Decays

We study effects of new physics beyond the Standard Model on SU(3) symmetry in charmless hadronic two body B decays. It is found that several equalities for some of the decay amplitudes, such as $A(B_d (B_u) \to π^+π^-,π^+ K^- (π^- \bar K^0)) =A(B_s \to K^+ π^-, K^- K^+ (K^0 \bar K^0))$, $A(B_d \to π^+ρ^-, π^- ρ^+, K^-ρ^+, π^+ K^{*-}) = A(B_s \to K^+ ρ^-, π^- K^{*+}, K^- K^{*+}, K^+ K^{*-})$, $A(B_d (B_u) \to ρ^+ρ^-, ρ^+ K^{*-}(ρ^- \bar K^{*0})) =A(B_s \to K^{*+} ρ^-, K^{*-} K^{*+} (K^{*0} \bar K^{*0}))$, predicted by SU(3) symmetry in the SM are not affected by new physics. These relations provide important electroweak model independent tests for SU(3) symmetry in B decays.

hep-ph↗
arXiv2000

CP Violation in the SM and Beyond in Hadronic B Decays

Three different methods, using $B_d\to J/ψK_S$, $J/ψK_S π^0$, $B_d\to K^-π^+, π^+π^-$ and $B_u\to K^- π^0, \bar K^0 π^-, π^-π^0$, to extract hadronic model independent information about new physics are discussed in this talk.

hep-ph↗
arXiv2000

SU(3) Flavor Symmetry and CP Violating Rate Differences for Charmless $B\to P V$ Decays

We derive several relations between CP violating rate differences $Δ(B\to PV) = Γ(B\to PV) - Γ(\bar B \to \bar P \bar V)$ for charmless $B\to P V$ decays in the Standard Model using SU(3) flavor symmetry. It is found that although the relations between branching ratios of $ΔS = 0$ and $ΔS = -1$ processes are complicated, there are simple relations independent of hadronic models between some of the $ΔS =0$ and $ΔS = -1$ rate differences due to the unitarity property of the Kobayashi-Maskawa matrix, such as $ Δ(B\to π^+ ρ^-) = -Δ(B\to π^+ K^{*-})$, $Δ(B\to π^- ρ^+) = - Δ(K^- ρ^+)$. SU(3) breaking effects are also estimated using factorization approximation. These relations can be tested at B factories in the near future.

hep-ph↗
arXiv2000

Neutrino mass induced radiatively by supersymmetric leptoquarks

We show how nonzero Majorana neutrino masses can be radiatively generated by extending the MSSM with leptoquark chiral multiplets without violating R-parity. It is found that, with these particles, the R-parity conservation does not imply lepton number conservation. Neutrino masses generated at a one-loop level are closely related to the down quark mass matrix. The ratio of neutrino mass-squared splittings $Δm^2_{ν_2-ν_1}/Δm^2_{ν_3-ν_2}$ obtained is naturally close to $Δm^2_{s-d}/Δm^2_{b-s}\sim 10^{-3}$ which is in the right region required to explain both the atmospheric neutrino data and the MSW solutions for the solar neutrino data.

hep-ph↗
arXiv1999

Constraints on the phase $γ$ and new physics from $B\to Kπ$ Decays

Recent results from CLEO on $B\to Kπ$ indicate that the phase $γ$ may be substantially different from that obtained from other fit to the KM matrix elements in the Standard Model. We show that $γ$ extracted using $B\to Kπ, ππ$ is sensitive to new physics occurring at loop level. It provides a powerful method to probe new physics in electroweak penguin interactions. Using effects due to anomalous gauge couplings as an example, we show that within the allowed ranges for these couplings information about $γ$ obtained from $B\to K π, ππ$ can be very different from the Standard Model prediction.

hep-ph↗
arXiv1999

CP Violation in Hyperon Decays from Supersymmetry

It was pointed out recently that supersymmetry can generate flavor-changing gluonic dipole operators with sufficiently large coefficients to dominate the observed value of epsilon'/epsilon. We point out that the same operators contribute to direct CP violation in hyperon decay and can generate a CP violating asymmetry A(Λ^0_-) in the range probed by the current E871 experiment. Interestingly, models that naturally reproduce the relation lambda = sqrt{m_d/m_s} do not generate epsilon'/epsilon but could lead to an A(Λ^0_-) of O(10^{-3}).

hep-ph↗