arXiv subjects

Xiao-Gang He

Publications and source records attributed to Xiao-Gang He.

At least 217 records · Page 12Linked to original sources

arXiv2006

Some properties of the newly observed X(1835) state at BES

Recently the BES collaboration has announced observation of a resonant state in the $π^+π^- η'$ spectrum in $J/ψ\to γπ^+π^-η'$ decay. Fitting the data with a $0^{-+}$ state, the mass is determined to be 1833.7 MeV with $7.7σ$ statistic significance. This state is consistent with the one extracted from previously reported $p \bar p$ threshold enhancement data in $J/ψ\to γp \bar p$. We study the properties of this state using QCD anomaly and QCD sum rules assuming X(1835) to be a pseudoscalar and show that it is consistent with data. We find that this state has a sizeable matrix element $<0|G\tilde G|G_p>$ leading to branching ratios of $(2.61\sim 7.37)\times 10^{-3}$ and $(2.21\sim 10.61)\times 10^{-2}$ for $J/ψ\to γG_p$ and for $G_p \to π^+π^- η'$, respectively. Combining the calculated branching ratio of $J/ψ\to γG_p$ and data on threshold enhancement in $J/ψ\to γp \bar p$, we determine the coupling for $G_p- p-\bar p$ interaction. We finally study branching ratios of other $J/ψ\to γ+ {three mesons}$ decay modes. We find that $J/ψ\to γG_p \to γ(π^+π^- η, K K π^0)$ can provide useful tests for the mechanism proposed.

hep-ph↗

arXiv2006

Minimal Modification To The Tri-bimaximal Neutrino Mixing

Current experimental data on neutrino oscillations are consistent with the tri-bimaximal mixing. If future experimental data will determine a non-zero $V_{e3}$ and/or find CP violations in neutrino oscillations, there is the need to modify the mixing pattern. We find that a simple neutrino mass matrix, resulting from $A_4$ family symmetry breaking with residual $Z_3$ and $Z_2$ discrete symmetries respectively for the Higgs sectors generating the charged lepton and neutrino mass matrices, can satisfy the required modifications. The neutrino mass matrix is minimally modified with just one additional complex parameter compared with the one producing the tri-bimaximal mixing. In this case, the CP violating Jarlskog factor $J$ has a simple form ($|J|=|V_{e1}V_{e3}|/2\sqrt{3}$ for real neutrino mass matrix), and also $V_{μi} = 1/\sqrt{3}$. We also discuss how this mixing matrix can be tested experimentally.

hep-ph↗

arXiv2006

PQCD Calculation for $Λ_{b}\to Λγ$ in the Standard Model

We calculate the branching ratio of $Λ_b \to Λγ$ in the standard model using the PQCD method. The predicted branching ratio $B(Λ_b \to Λγ)$ is about $(4.3\sim 8.6)\times 10^{-8}$, with reasonable parameter ranges in the heavy baryon distribution amplitude. This branching ratio is much smaller than those obtained in other hadronic model calculations. Future experimental data can provide important information on applicability of the PQCD method to heavy baryon radiative decay.

hep-ph↗

arXiv2006

Probe Noncommutative Space-Time Scale Using $γγ\to Z$ At ILC

In the standard model production of on-shell Z boson at a photon collider (or Z decays into $γγ$) is strictly forbidden by angular momentum conservation and Bose statistics (the Yang's Theorem). In the standard model with noncommutative space-time this process can occur. Therefore this process provides an important probe for the noncommutative space-time. The $γγ$ collision at the ILC by laser backscattering of the electron and positron beams offers an ideal place to carry out such a study. Assuming an integrated luminosity of 500 fb$^{-1}$, we show that the constraint which can be achieved on $Γ(Z\to γγ)$ is three to four orders of magnitude better than the current bound of $5.2\times 10^{-5}$ GeV. The noncommutative scale can be probed up to a few TeVs.

hep-ph↗

arXiv2006

$B_s-\bar B_s$ Mixing constraints on FCNC and a non-universal $Z^\prime$

$B_s - \bar B_s$ mixing has been measured recently by D0 and CDF. The range predicted in the standard model is consistent with data. However, the standard model central values for $ΔM_{B_s}$ and also $ΔM_ {B_d}$ are away from the data which may be indications of new physics. Using the observed values of $ΔM_{B_s}$ and $ΔM_{B_d}$ we study general constraints on flavor changing $Z^\prime$ interactions. In models with non-universal $Z^\prime$ couplings we find that significant enhancements over the standard model are still possible in the rare decay modes $B\to X_s τ^+ τ^-$, $B\to X_s ν\bar ν$ and $B_s\to τ^+ τ^-$. Tree level $Z^\prime$ contributions to $K\to πν\bar ν$ are now constrained to be very small, but one loop effects can still enhance the standard model rate by a factor of two.

hep-ph↗

arXiv2006

X(1812) in Quarkonia-Glueball-Hybrid Mixing Scheme

Recently a $J^{PC}=0^{++}$ (X(1812)) state with a mass near the threshold of $ω$ and $ϕ$ has been observed by the BES collaboration in $J/ψ\to γωϕ$ decay. It has been suggested that it is a $I^G = 0^+$ state. If it is true, this state fits in a mixing scheme based on quarkonia, glueball and hybrid (QGH) very nicely where five physical states are predicted. Together with the known $f_0(1370)$, $f_0(1500)$, $f_0(1710)$, and $f_0(1790)$ states, X(1812) completes the five members in this family. Using known experimental data on these particles we determine the ranges of the mixing parameters and predict decay properties for X(1812). We also discuss some features which may be able to distinguish between four-quark and hybrid mixing schemes.

hep-ph↗

arXiv2006

A_4 flavour symmetry breaking scheme for understanding quark and neutrino mixing angles

We propose a spontaneous A_4 flavour symmetry breaking scheme to understand the observed pattern of quark and neutrino mixing. The fermion mass eigenvalues are arbitrary, but the mixing angles are constrained in such a way that the overall patterns are explained while also leaving sufficient freedom to fit the detailed features of the observed values, including CP violating phases. The scheme realises the proposal of Low and Volkas to generate zero quark mixing and tribimaximal neutrino mixing at tree-level, with deviations from both arising from small corrections after spontaneous A_4 breaking. In the neutrino sector, the breaking is A_4 --> Z_2, while in the quark and charged-lepton sectors it is A_4 --> Z_3 = C_3. The full theory has A_4 completely broken, but the two different unbroken subgroups in the two sectors force the dominant mixing patterns to be as stated above. Radiative effects within each sector are shown to deviate neutrino mixing from tribimaximal, while maintaining zero quark mixing. Interactions between the two sectors -- "cross-talk" -- induce nonzero quark mixing, and additional deviation from tribimaximal neutrino mixing. We discuss the vacuum alignment challenge the scenario faces, and suggest three generic ways to approach the problem. We follow up one of those ways by sketching how an explicit model realising the symmetry breaking structure may be constructed.

hep-ph↗

arXiv2006

New Members in the $0^+$ ($0^{++})$ Family

Recent BES data on $J/ψ\to ϕππ$ indicate that there is a possible new spin-0 state ($f_0(1790)$) with a mass of $m=1790^{+40}_{-30} \mathrm{MeV}/\mathrm{c}^2$. Assuming it to be an iso-singlet $0^+(0^{++})$, we propose a new mixing scheme to describe this and the other three known iso-singlet $f_0(1370)$, $f_0(1500)$, $f_0(1710)$ states by adding iso-singlet hybrid states to the usual basis of two iso-singlet quarkonia and one glueball. Since there are two iso-singlet hybrid states, $(u\bar u+d\bar d)g/\sqrt 2$ and $s\bar sg$, this new basis implies existence of another iso-singlet state $X$. Using known data, we estimate the ranges of the mixing parameters. We find two sets of solutions with X mass predicted to be about 1820 MeV and 1760 MeV, respectively. We also study implications on the decay properties of these new states.

hep-ph↗

arXiv2005

The Decay Sigma^+ -> p l^+ l^- within the Standard Model

The HyperCP collaboration has recently reported the observation of three events for the decay Sigma^+ -> p mu^+ mu^-. They have suggested that new physics may be required to understand the implied decay rate and the observed M_{mumu} distribution. Motivated by this result, we re-examine this mode within the standard model, considering both the short-distance and long-distance contributions. The long-distance part depends on four complex form-factors. We determine their imaginary parts from unitarity, fix two of the real parts from the Sigma^+ -> p gamma measurements, and estimate the other two with vector-meson-dominance models. Taking into account constraints from Sigma^+ -> p e^+ e^-, we find that Sigma^+ -> p mu^+ mu^- is long-distance dominated and its rate falls within the range suggested by the HyperCP measurement.

hep-ph↗

arXiv2005

Mixing of Pentaquark and Molecular States

There are experimental evidences for the existence of narrow states $Θ^+$ and $Θ_c$ with the same quantum numbers of $uudd\bar s$ and $uudd\bar c$ pentaquarks and also $NK^{(*)}$ and $ND^{(*)}$ molecular states. Their masses deviate from many theoretical estimates of the pure pentaquark and molecular states. In this work we study the possibility that the observed $Θ^+$ and $Θ_c$ are mixtures of pure pentaquark and molecular states. The mixing parameters are in general related to non-perturbative QCD which are not calculable at present. We determine them by fitting data from known states and then generalize the mechanism to $Θ_b$ to predict its mass and width. The mixing mechanism can also naturally explain the narrow width for $Θ^+$ and $Θ_c$ through destructive interferences, even if the pure pentaquark and molecular states have much larger decay widths. We also briefly discuss the properties of the partner eigenstates of $Θ^+$ and $Θ_c$ and the possibility of experimentally observe them. Moreover, probable consequences of multi-state mixing are also addressed.

hep-ph↗

arXiv2005

Implications of a new particle from the HyperCP data on Σ^+ \to p μ^+ μ^-

The HyperCP collaboration has recently reported the observation of three events for the decay Σ^+ \to p mu^+ mu^- with an invariant mass m_{μ^+μ^-} for a muon and an anti-muon pair of \sim 214 MeV. They suggest that a new particle state X may be needed to explain the observed m_{μ^+μ^_} distribution. Motivated by this result, we study the properties of such a hypothetical particle. We first use K^+ \to pi^+ μ^+μ^- data to conclude that X cannot be a scalar or a vector particle. We then collect existing constraints on a pseudo-scalar or axial vector X and find that these possibilities are still allowed as explanations for the HyperCP data. Finally we assume that the HyperCP data is indeed explained by a new pseudo-scalar or axial vector particle and use this to predict enhanced rates for K_L \to π^+π^- X \to π^+π^-μ^+μ^- and Ω^- to Ξ^- X \to Ξ^- μ^+μ^-.

hep-ph↗

arXiv2005

Model of Geometric Neutrino Mixing

Current neutrino oscillation data from solar, atmospheric, and reactor experiments are consistent with the neutrino mixing matrix elements taking values sin^2θ_{12} = 1/3, sin^2θ_{23} = 1/2, and sin^2θ_{13}=0. We present a class of renormalizable gauge models which realize such a geometric mixing pattern naturally. These models, which are based on the non--Abelian discrete symmetry A_4, place significant restrictions on the neutrino mass spectrum, which we analyze. It is shown that baryogenesis via leptogenesis occurs quite naturally, with a single phase (determined from neutrino oscillation data) appearing in leptonic asymmetry and in neutrinoless double beta decay. Such predicted correlations would provide further tests of this class of models.

hep-ph↗

arXiv2005

Hadron decay amplitudes from $B\to K π$ and $B\to ππ$ decays

One can analyze the hadronic decay amplitudes for $B\to PP$ decays using flavor SU(3) symmetry in different ways, such as the algebraic and quark diagram approaches. We derive specific relations between these two sets of amplitudes. In the Standard Model the leading hadronic decay amplitudes depend on only five independent parameters which can be determined using recent experimental data on the branching ratios and CP violating asymmetries of $B\to K π$ and $B \to ππ$. We find, however, that the leading amplitudes provide a best fit solution with a large $χ^2$, which cannot therefore be regarded as a good fit. Keeping sub-leading terms, makes it possible to have a reasonable minimal $χ^2$. We also find that in general the color suppressed decay amplitude is comparable with the color allowed amplitude, contrary to expectations.

hep-ph↗

arXiv2005

Bi-Large Neutrino Mixing See-Saw Mass Matrix with Texture Zeros and Leptogenesis

We study constraints on neutrino properties from texture zeros in bi-large mixing See-Saw mass matrix and also from leptogenesis. Texture zeros may occur in the light (class a)) or in the heavy (class b)) neutrino mass matrices. Each of these two classes has 5 different forms which can produce non-trivial three generation mixing with at least one texture zero. We find that two types of texture zero mass matrices in both class a) and class b) can be consistent with present data on neutrino masses, mixing and produce the observed baryon asymmetry of the universe. None of the neutrinos can have zero masses with the lightest of the light neutrinos having a mass larger than about 0.039 eV for class a) and 0.002 eV for class b). In these models although CKM CP violating phase vanishes, non-zero Majorana phases, however, can exist and play an important role in producing the observed baryon asymmetry in our universe through leptogenesis mechanism. The requirement of producing the observed baryon asymmetry can further distinguish different models and also restrict the See-Saw scale to be in the range $10^{12}\sim 10^{15}$ GeV.

hep-ph↗

arXiv2005

Berry Phase in Neutrino Oscillations

We study the Berry phase in neutrino oscillations for both Dirac and Majorana neutrinos. In order to have a Berry phase, the neutrino oscillations must occur in a varying medium, the neutrino-background interactions must depend on at least two independent densities, and also there must be CP violation if the neutrino interactions with matter are mediated only by the standard model W and Z boson exchanges which implies that there must be at least three generations of neutrinos. The CP violating Majorana phases do not play a role in generating a Berry phase. We show that a natural way to satisfy the conditions for the generation of a Berry phase is to have sterile neutrinos with active-sterile neutrino mixing, in which case at least two active and one sterile neutrinos are required. If there are additional new CP violating flavor changing interactions, it is also possible to have a non-zero Berry phase with just two generations.

hep-ph↗

arXiv2005

Does The $\triangle I ={1/2}$ Rule Hold In D And B $\to ππ$ Decays?

Although two pion decays of $K$, $D$ and $B$ have similar isospin structures, there are dramatic differences in the ratios $R_K$ and $R_{D,B}$ of amplitudes from $ΔI =3/2$ and $ΔI =1/2$ interactions. In $K\to ππ$ decays there is the famous $ΔI =1/2$ rule with $R_K \approx 1/22$, whereas in $B(D) \to ππ$ decays the ratios $R_{D,B}$ are of order one and therefore there is no such a rule. In this work we study decay amplitudes in $B(D)\to ππ$ using QCD factorization calculations paying particular attention to discrepancies between data and theoretical estimates. Since isospin does not play a special role in factorization calculations, no $ΔI =1/2$ rule is expected. We find that theoretical results on the size of the amplitudes are in qualitative agreement with data. However the phases for the amplitudes are very different. We show that the effects of re-scattering between the two pions in the final state can play a crucial rule in understanding the differences between $B(D)\to ππ$ and $K\to ππ$ decays. We also comment on the role of isospin analysis which applies to the study of CP violation in $B\to ππ$ decays.

hep-ph↗

arXiv2005

Flavor Physics in Kaon Decays

In this talk I review some topics related to flavor physics in kaon decays with emphasis on the roles which kaon decays can play in the determinations of hadronic matrix elements, the CKM parameters and new physics beyond the Standard Model.

hep-ph↗

arXiv2004

Parity, Charge Conjugation and SU(3) Constraints on Threshold Enhancement in $J/ψ$ decays into $γp\bar p$ and $K p \barΛ$

We study the threshold enhancement effects of baryon-anti-baryon systems in $J/ψ$ decays at BES using parity $P$, charge conjugation $C$, and flavor SU(3) symmetries. The $P$ and $C$ symmetries restrict the $p\bar p$ in $J/ψ\to γp\bar p$ to be in a state with $C=+1$, while the $p\bar p$ in $J/ψ\to π^0 p\bar p$ to be with $C=-1$. Combining the $C$ and $P$ symmetries with flavor SU(3) symmetry, i.e. the CPS symmetry, we find that the $Λ\bar p$ system cannot be in $0^+$ and $0^-$ states in $J/ψ\to K^+ Λ\bar p$. We provide a consistent explanation of observation and non-observation of near threshold enhancements in $J/ψ\to K^+ Λ\bar p$ and $J/ψ\to π^0 p \bar p$, respectively. We also find that near baryon pair threshold enhancement can happen in several channels for $J/ψ$ decays and can be several times larger than that observed in $J/ψ\to K^+ Λ\bar p$ in some channels.

hep-ph↗