arXiv subjects
Publications and source records attributed to Xiao-Gang He.
The gluon dipole operator, $O_{11}$, has received much attention recently because it can have a large coefficient in some SUSY extensions of the standard model. We find that the commonly used matrix element of $O_{11}$ of Bertolini, Eeg and Fabbrichesi is in rough (but accidental) numerical agreement with an estimate based on dimensional analysis.
We study gravitational lensing and the bending of light in low energy scale (M_S) gravity theories with extra space-time dimensions n. We find that due to the presence of spin-2 Kaluza-Klein states from compactification, a correction to the deflection angle with a strong quadratic dependence on the photon energy is introduced. No deviation from the Einstein General Relativity prediction for the deflection angle for photons grazing the Sun in the visible band with 15% accuracy (90% c.l.) implies that the scale M_S has to be larger than 1.4(2/(n-2))^{1/4} TeV and approximately 4 TeV for n=2. This lower bound is comparable with that from collider physics constraints. Gravitational lensing experiments with higher energy photons can provide stronger constraints.
The quark-level process $b \to s γ$ has been used extensively to place constraints on new interactions. These same interactions can be further constrained from the enhancement they induce in the quark-level $s \to d γ$ transition, to the extent that the short distance contributions can be separated from the long distance contributions. We parameterize what is known about the long distance amplitudes and subtract it from the data in radiative hyperon and kaon decays to constrain new interactions.
Surveying known hadronic rare B decays, we find that the factorization approximation can give a coherent account of $Kπ$, $ππ$ and $ρ^0π^+$ data and give predictions for $ω^0π^+$, $ρπ$ and $K^*π$ modes, {\it if ${Re}V_{ub}$ is taken as negative} (in standard phase convention) rather than positive. As further confirmation, we expect a lower $\sin2β$ value at B Factories as compared to current fits, and $B_s$ mixing close to LEP bounds at SLD and CDF.
We study the unitarity constraint on the two body Higgs boson elastic scattering in the presence of extra dimensions. The contributions from exchange of spin-2 and spin-0 Kaluza-Klein states can have large effect on the partial wave amplitude. Unitarity condition restrict the maximal allowed value for the ratio $r$ of the center of mass energy to the gravity scale to be less than one. Although the constraint on the standard Higgs boson mass for $r$ of order one is considerably relaxed, for small $r$ the constraint is similar to that in the Standard Model. The resulting bound on the Higgs boson mass is not dramatically altered if perturbative calculations are required to be valid up to the maximal allowed value for $r$.
We show that new physics effects due to extra dimensions can dramatically affect Higgs pair production at photon colliders. We find that the cross section due to extra dimensions with the scale $M_S$ of new physics around 1.5 TeV, the cross section can be as large as 0.11 pb (1.5pb) for monochromatic photon collision, $γγ\to HH$, with the collider energy $\sqrt{s} = 0.5 (1)$ TeV for Higgs mass of 100 (350) GeV. The cross section can be 3 fb (2.7 fb) for the same parameters for collisions using photon beams from electron or positron back scattered by laser. These cross sections are much larger than those predicted in the Standard Model. Higgs pair production at photon colliders can provide useful tests for new physics due to extra dimensions.
We study $B \to η' X_s$ within the framework of the Standard Model. Several mechanisms such as $b \to η' s g$ through the QCD anomaly, and $b \to η's $ and $B\to η' s \bar q$ arising from four-quark operators are treated simultaneously. Using QCD equations of motion, we relate the effective Hamiltonian for the first mechanism to that for the latter two. By incorporating next-to-leading-logarithmic(NLL) contributions, the first mechanism is shown to give a significant branching ratio for $B\to η' X_s$, while the other two mechanisms account for about 15% of the experimental value. The Standard Model prediction for $B\to η' X_s$ is consistent with the CLEO data.
Recently KTev collaboration has measured $Re(ε'/ε) = (28\pm 4.1) \times 10^{-4}$ which is in agreement with early measurement from NA31. The Standard Model prediction for $ε'/ε$ is on the lower end of the experimentally allowed range depending on models for hadronic matrix elements. In this paper we study the contributions from anomalous gauge couplings. We find that the contributions from anomalous couplings can be significant and can ehance $ε'/ε$ to have a value closer to data.
Supersymmetric models with nonuniversal down type squark masses can enrich the chiral structure and CP violating phenomena in $b\to sγ$ decays. Direct CP violation in $b\to s γ$, mixing induced CP violation in radiative $B_{d,s}$ decays (such as $B_s\to ϕγ$ and $B_d \to K^*_{1,2}γ$), and $Λ$ polarization in $Λ_b \to Λγ$ can be substantially different from the Standard Model. Future experiements at $e^+ e^-$ and hadronic B factories will give important information on the underlying theory for radiative $b$ decays.
The recently observed $B^-\to K^- π^0$, $\bar K^0 π^-$ and $\bar B^0 \to K^- π^+$ decay modes appear to have nearly equal branching ratios. This suggests that tree and electroweak penguins play an important role, and inclusion of the latter improves agreement between factorization calculation and experimental data. The value $γ$ in the range $90^\circle - 130^\circle$ and $220^\circle - 260^\circle$ is favored, while the $\bar B^0 \to \bar K^0 π^0$ rate is suppressed. Direct CP violation for $B\to K π$ modes can be large if final state interaction phases are large.
In this talk I discuss two topics related to B decays. I will first discuss the Standard Model prediction for $B\to η' X_s$, and then discuss how to use experimental information from $B\to J/ψK^*$ to test CP violation beyond the SM.
We study $B \to η' X_s$ within the framework of the Standard Model. Several mechanisms such as $b \to η' s g$ from the QCD anomaly, and $b \to η's $ and $B\to η' s \bar q$ from four-quark operators are treated simultaneously. We find that the first mechanism give a significant contribution to the branching ratio for $B\to η' X_s$, while the other two mechanisms account for about 15% of the experimental value. The Standard Model prediction for $B\to η' X_s$ is consistent with the CLEO data.
Within the Standard Model, CP rate asymmetries for $B\to K^-π^{+,0}$ could reach 10%. With strong final state phases, they could go up to 20--30%, even for $\bar K^0π^-$ mode which would have opposite sign. We can account for $K^-π^{+}$, $\bar K^0π^-$ and $ϕK$ rate data with new physics enhanced color dipole coupling and destructive interference. Asymmetries could reach 40--60% for $Kπ$ and $ϕK$ modes and are all of the same sign. We are unable to account for $K^-π^0$ rate. Our inclusive study supports our exclusive results.
The light quarks inside $B$ mesons are usually treated as spectators and do not affect the decay rates which are assumed to be purely due to b quark decays. In this paper we calculate the non-spectator contributions to inclusive charmless $B$ decays due to the spectator effects. We find that the non-spectator contributions to the branching ratio for $\bar B^0$ are small ($<2\times 10^{-4}$), but the contributions to $ΔS = 0$ and $ΔS = -1$, $B^-$ decay branching ratios can be as large as $-7.5\times 10^{-4}$ and $2\times 10^{-3}$, respectively. These contributions may play an important role in rare charmless $B$ decays.
Several methods proposed to measure the angle $γ$ in the KM unitarity triangle assumed that the tree contribution to $B^-\to π^-\bar K^0 $ is purely due to annihilation contributions and is negligibly small. This assumption has to be tested in order to have a correct interpretation of the experimental data. In this paper we show that using SU(3) symmetry, the smallness of the tree contribution can be tested in a dynamic model independent way. We also derive several relations between CP violating rate differences for $B\to P P$ decays without assuming the smallness of the annihilation contributions. These relations provide important tests for the Standard Model of CP violation.
We show that the study of certain observables in the angular distribution in $B\to J/ψK^*$ provide clear test for CP vioaltion beyond the Standard Model. These observables vanish in SM, but in models beyond SM some of them can be large enough to be measured at B factories.
The small P wave compoment of $B\to J/ψK^*$ measured by CLEO makes it practical to measure $\sin2β$ using $B^0\to J/ψK^{*0}\to J/ψK_S π^0$, independent from $B^0\to J/ψK_S$. Because these modes are color suppressed, new physics enhanced color dipole $bsg$ coupling, as hinted from the persistent ${\cal B}_{s.l.}$ and $n_C$ problems as well as the newly observed large semi-inclusive $B\to η^\prime + X_s$ decay, may have significant impact. We show that it may lead to a difference in the $\sin 2β$ measurements between $B^0\to J/ψK_S$ and $J/ψK_S π^0$ measureable at the B Factories in the near future.
The unexpectedly large branching ratios for $B\to η' K (η' X_s)$ decays could be of gluonic origin. We study the implications for $B\to ηK (ηX_s)$ and $P K (PX_s)$, where $P$ is the pseudoscalar glueball. In the mechanism proposed by Fritzsch, large branching ratios are predicted for these modes. The $B\to ηK$ rate is barely within the experimental limit, and $B\to P K$, $PX_s$ could be at the 0.1% and 1% level, respectively. Smaller but less definite results are found for the mechanism of $g^* \to η' g$ via the gluon anomaly.