arXiv subjects

X. Y. Pham

Publications and source records attributed to X. Y. Pham.

At least 19 recordsLinked to original sources

arXiv2015

Glueball Physics in QCD

The Abelian decomposition of QCD which decomposes the gluons to the color neutral binding gluons and the colored valence gluons shows that QCD can be viewed as the restricted QCD (RCD) made of the binding gluons which has the valence gluons as colored source, and simplifies the QCD dynamics greatly. In particular, it tells that the gauge covariant valence gluons can be treated as the constituents of hadrons, and generalizes the quark model to the quark and valence gluon model. So it provides a comprehensive picture of glueballs and their mixing with quarkoniums, and predicts new hybrid hadrons made of quarks and valence gluons. We discuss how these predictions could be confirmed experimentally. In particular we present a systematic search for the ground state glueballs and their mixing with quarkoniums below 2 GeV in $0^{++}$, $2^{++}$, and $0^{-+}$ channels within the framework of QCD, and predict the relative branching ratios of the radiative decay of $ψ$ to the physical states.

hep-ph↗

arXiv2011

Chiral Anomaly Effects and the BaBar Measurements of the $γγ^{*}\to π^{0}$ Transition Form Factor

The recent BaBar measurements of the $γγ^{*}\to π^{0}$ transition form factor show spectacular deviation from perturbative QCD prediction for large space-like $Q^{2}$ up to $34\,\rm GeV^{2}$. When plotted against $Q^{2}$, $Q^{2}F(Q^{2})$ shows steady increase with $Q^{2}$ in contrast with the flat $Q^{2}$ behavior predicted by perturbative QCD, and at $34\,\rm GeV^{2}$ is more than 50% larger than the QCD prediction. Stimulated by the BaBar measurements, we revisit our previous paper on the cancellation of anomaly effects in high energy processes $Z^{0}\to π^{0}γ$, $e^{+}e^{-}\to π^{0}γ$ and apply our results to the $γ^{*}γ\to π^{0}$ transition form factor measured in the $e^{+}e^{-}\to e^{+}e^{-}π^{0}$ process with one highly virtual photon. We find that, the transition form factor $F(Q^{2})$ behaves as $(\frac{m^{2}}{Q^{2}})\times (\ln(Q^{2}/m^{2}))^{2}$ and produces a striking agreement with the BaBar data for $Q^{2}F(Q^{2})$ with $m=132\,\rm MeV$ which also reproduces very well the CLEO data at lower $Q^{2}$.

hep-ph↗

arXiv2011

Chiral anomaly, triangle loop and the the $γγ^{*}\to π^{0}$ form factor

The recent BaBar measurements of the $γγ^{*}\to π^{0}$ form factor show spectacular deviation from perturbative QCD computations for large space-like $Q^{2}$. At $34\,\rm GeV^{2}$ the data are more than 50% larger than theoretical predictions. Stimulated by these new experimental results, we revisit our previous paper on triangle loop effects related to chiral anomaly, and apply our method to the $γ+ γ^* \to π^0$ form factor measured in the single tag mode $e^{+} + e^{-}\to e^{+} + e^{-} + π^{0}$ with one highly virtual photon. The resultant form factor $F(Q^{2})$ - which depends on only one parameter (the mass $m$ of up, down quark circulating in the triangle loop) behaves like $(\frac{m^{2}}{Q^{2}})\times (\ln(Q^{2}/m^{2}))^{2}$ - shows a striking agreement with BaBar data for $m \approx 132\,\rm MeV$. The rising logarithm squared form factor, surprisingly unnoticed in the literature, is in sharp contrast with the rather flat ones derived from perturbative QCD approaches.

hep-ph↗

arXiv2000

Superheavy Majorana neutrino effects in the lepton-number violating e- + e- -> mu- + mu- process

In the minimal extension of the standard electroweak theory with ultra massive Majorana neutrinos, the process e- + e- -> mu- + mu- could be observable, in sharp contrast with the reaction e- + e- -> W- + W- which is entirely controlled by neutrinoless double beta decay $ββ_{0 ν}$ data. Our result provides the process background that must be confronted "new physics" models which postulate doubly charged particles, such as the gauge bilepton Y^{-} in the $SU(3)_{\rm c} \times SU(3)_{\rm L} \times U(1)$ model, the left-right $SU(2)_{\rm L} \times SU(2)_{\rm R} \times U(1)_{\rm B-L}$ one, and its supersymmetric version with doubly charged Higgs multiplets.

hep-ph↗

arXiv1999

On the Dynamical Symmetry Breaking of the Electroweak Interactions by the Top Quark

We discuss the electroweak gauge symmetry breaking triggered by a new strong attractive interaction to condensate fermion-antifermion, and topcolor is a prototype. To deal with the fermion pairing, a general method based on the Hubbard-Stratonovich transformation in the functional integral approach is used. We derive a formula which relates the $W^\pm$, $Z^0$ weak boson masses to that of the condensated fermion, thus generalizing the Pagels-Stokar formula obtained in QCD. The custodial SU(2) electroweak symmetry turns out to be systematically violated, the deviation of $ρ\equiv M_W^2/(M_Z^2\cos^2θ_W) $ from unity is related to the new physics scale $Λ$. Some phenomenological consequences of the top-pair condensation models are discussed. Distinctive signatures of the $\bar{t}t$ scalar bound state, a Higgs boson like denoted by $H_t$, are the dominant decay modes $H_t\to Υ+γ,$ $H_t\to Υ+Z^0$, and $H_t \to B^* +\bar{B}^*$.

hep-ph↗

arXiv1999

The decays "neutrino{heavy} -> neutrino{light} + photon" and "neutrino{heavy} -> neutrino{light} e+ e-" of massive neutrinos

If, as recently reported by the Super-Kamiokande collaboration, the neutrinos are massive, the heaviest one would not be stable and, though chargeless, could in particular decay into a lighter neutrino and a photon by quantum loop effects. The corresponding rate is computed in the standard model with massive Dirac neutrinos as a function of the neutrino masses and mixing angles. The lifetime of the decaying neutrino is estimated to be approximately 10^44 years for a mass 5 10^{-2} eV. If kinematically possible, the decay of a heavy neutrino into a lighter one plus an e+ e- pair occurs at tree level and its one-loop radiative corrections get enhanced by a large logarithm of the electron mass acting as an infrared cutoff. It then largely dominates the photonic mode by several orders of magnitude, corresponding to a lifetime approximately equal to 10^{-2} year for a mass 1.1 MeV.

hep-ph↗

arXiv1999

One-loop flavor changing electromagnetic transitions

We discuss the effect of the external fermion masses in the flavor-changing radiative transitions of a heavy fermion (quark or lepton) to a lighter fermion at the one-loop level, and point out an often overlooked crucial difference in the sign of a charge factor between transitions of the down type $s\to dγ$ and the up type $c\to uγ$. We give formulas for the $F\to fγ$ effective vertex in various approximations and the exact formula for $t\to cγ$ and $τ\to μγ$.

hep-ph↗

arXiv1999

Massive Neutrino Decays

Neutrino physics is used as an illustrative example for an elementary introduction to the computational method of Feynman loop-diagrams and decay rates. If the neutrinos are as massive as recently reported by the Super-Kamiokande results, the heaviest neutrino $ν_H$ would not be stable. Although chargeless, it could decay -- by quantum loop effect -- into a lighter neutrino $ ν_L$ by emitting a photon: $ν_H \to ν_L + γ$. If kinematically possible, the $ν_H \to ν_L$ + e$^+$ + e$^-$ mode could occur at the tree diagram level and furthermore get enhanced, at one-loop radiative corrections, by a large logarithm of the electron mass acting as an infrared cutoff.

hep-ph↗

arXiv1998

Lepton flavor changing in neutrinoless $τ$ decays

Neutrino oscillations, as recently reported by the Super-Kamiokande collaboration, imply that lepton numbers could be violated, and $τ^{\pm}\to μ^{\pm}+\ell^{+}+\ell^{-},τ^{\pm}\toμ^{\pm}+ρ^0$ are some typical examples. We point out that in these neutrinoless modes, the GIM cancelation is much milder with only a logarithmic behavior $\log (m_j /m_k)$ where $m_{j, k}$ are the neutrino masses. This is in sharp contrast with the vanishingly small amplitude $τ^{\pm}\to μ^{\pm}+γ$ strongly suppressed by the quadratic power $(m_j^2-m_k^2)/ M_{\rm W}^2$. In comparison with the hopelessly small branching ratio B$(τ^{\pm}\to μ^{\pm}+γ)\approx 10^{-40}$, the B$(τ^{\pm}\toμ^{\pm}+\ell^{+}+\ell^{-})$ could be larger than $10^{-14}$. The latter mode, if measurable, could give one more constraint to the lepton mixing angle $\sin 2θ_{jk}$ and the neutrino mass ratio $m_j/m_k$, and therefore is complementary to neutrino oscillation experiments.

hep-ph↗

arXiv1998

Is the lepton flavour changing observable in $Z \to μ^\pm + τ^\mp$ decay?

Neutrino oscillations as suggested by the recent Super-Kamiokande result imply that lepton numbers could be violated, and Z$\to μ^{\mp} +τ^{\pm}$ is a typical manifestation. We point out that for this mode, the GIM cancelation is much milder with only a logarithmic behavior $\log (m_3 /m_2)$ where $m_j$ is the neutrino mass. The ratio $Γ$(Z $\to μ^{\mp} + τ^{\pm})/Γ($Z $ \to μ^{-} + μ^{+})$ could be about $10^{-5}-10^{-6}$, in sharp contrast with the vanishingly small rate $τ^{\pm}\to μ^{\pm} + γ$ strongly suppressed by a quadratic power $(m_3^2-m_2^2)/ M_{\rm W}^2$. Being complementary to neutrino oscillation experiments, measurements of Z$\to μ^{\mp} +τ^{\pm}$ -- which are at hand with the present colliders -- would give one more constraint to the lepton mixing angle $\sin 2θ_{jk}$ and the neutrino mass ratio $m_j/m_k$.

hep-ph↗

arXiv1996

A new CP violation mechanism generated by the standard neutral Higgs boson : the $η\to π+ π$ case

Strictly within the standard electro-weak interaction, CP violation in the flavour conserving process $η\rightarrow π+ π$ could originate from the mixing of the $η$ meson with the virtual scalar Higgs boson $ H^{0}$ via $W$ and top quark exchange. The parity-violation carried by weak gauge bosons makes the mixing possible by quantum effect at two-loop level. Nowhere the Kobayashi-Maskawa (KM) phase mechanism is needed. The phenomenon reveals an unexpected new role of the Higgs boson in the CP symmetry breaking. For the Higgs mass between 100-600 GeV, the $η\rightarrow π+π$ branching ratio is found to be $3.6 \cdot 10^{-26} - 2.4 \cdot 10^{-29}$ , hence CP violation mechanisms beyond the Standard Model are the only ones that could give rise to its observation at existing or near future $η$ factories, unless the Higgs mass is improbably as light as 550 $MeV$.

hep-ph↗

arXiv1995

CP violation in $η\to ππ$ by Higgs - $η$ mixing through two-loop quantum effects

Strictly within the standard electro-weak interaction, CP violation in the flavour conserving process $η\ra π+ π$ could originate from the mixing of the $η$ meson with the virtual scalar Higgs $ H^{0}$ via $ W^{+}+W^{-}$ and $Z^{0}+Z^{0}$ exchange. The parity-violation carried by these weak gauge bosons makes the mixing possible at two-loop level. Nowhere the Kobayashi-Maskawa (KM) phase mechanism is needed. For the Higgs mass between 100-600 GeV, the $η\ra π+π$ branching ratio is found to be $3\cdot 10^{-26} - 2\cdot 10^{-29}$, hence unconventional CP violation mechanisms are the only ones that could give rise to its observation at the existing or near future $η$ factories, unless the Higgs mass is improbably as light as 10 MeV.

hep-ph↗

arXiv1995

A Model for the decay of the $D_s^{+}$ meson into three pions,

We propose a phenomenological two component model describing the decay amplitude of the process $D_s^{+} \ra 3 π$, whose rate has been found surprisingly large. The first component is a constant background $F_{NR}$, and the second one is a Breit-Wigner type amplitude associated to a quasi two body $f_0(980)$ $π^{+}$ state. We show that it is possible to reproduce the observed rate for $D_s^{+} \ra π^{+}π^{+}π^{-}$ as well as the two other measured branching ratios for the non resonant part and the resonant $f_0π^{+}$ one, with a common parameter $F_{NR}$. Predictions are given for the $D_s^{+} \ra π^{0}π^{0}π^{+}$ rates, as well as for the various $π^{+}$ and $π^{-}$, or $π^{0}$ and $π^{+}$ energy distributions for the two possible final states.

hep-ph↗

arXiv1995

SU(2) HEAVY FLAVOUR SYMMETRY FOR B -> K(K^*) HADRONIC FORM FACTORS

We show that the factorization assumption in colour-suppressed $B$ meson decays is not ruled out by experimental data on $B \ra K(K^*) + J/Ψ(Ψ^{'})$. The problem previously pointed out might be due to an inadequate choice of hadronic form factors. Within the Isgur-Wise SU(2) heavy flavour symmetry framework, we search for possible $q^2$-dependences of form factors that satisfy both the large longitudinal polarization $ρ_L$ observed in $B \ra K^* + J/Ψ$ and the relatively small ratio of rates $R_{J/Ψ} = Γ(B \ra K^* + J/Ψ)/Γ(B \ra K + J/Ψ)$. We find out that the puzzle could be essentially understood if the $A_1(q^2)$ form factor is frankly decreasing (instead of being almost constant or increasing as commonly assumed). Of course, the possibility of understanding experimental data is not necessarily a proof of factorization.

hep-ph↗

arXiv1995

Factorization and SU(2) Heavy Flavor Symmetry for B-Meson Decays Producing Charmonium

We show that the factorization assumption in color-suppressed $B$ meson decays is not ruled out by experimental data on $B \ra K(K^*) + J/Ψ(Ψ^{'})$. The problem previously pointed out might be due to an inadequate choice of hadronic form factors. Within the Isgur-Wise SU(2) heavy flavor symmetry framework, we search for possible $q^2$ dependence of form factors that are capable of explaining simultaneously the large longitudinal polarization $ρ_L$ observed in $B \ra K^* + J/Ψ$ and the relatively small ratio of rates $R_{J/Ψ} = Γ(B \ra K^* + J/Ψ)/Γ(B \ra K + J/Ψ)$. We find out that the puzzle could be essentially understood if the $A_1(q^2)$ form factor is frankly decreasing, instead of being almost constant or increasing as commonly assumed. Of course, the possibility of understanding experimental data is not necessarily a proof of factorization.

hep-ph↗

arXiv1994

Testing factorization in colour-suppressed beauty decays with the B -> η_c + K(K^*) modes

The large discrepancy recently observed between $B \ra J/Ψ+ K (K^*)$ data and commonly used hadronic form factor models might be due to a breakdown of the factorization assumption. By showing that the $B \ra η_c + K (K^*)$ rates can be calculated without relying on any form factor model, we propose a test of the factorization hypothesis. This test is free of possible uncertainties caused by final state interactions and $W$-exchange graph contributions. If factorization holds, the measurable ratio $Γ(B \ra η_c + K^*)/Γ(B \ra η_c + K )$ is predicted to be in the range from 0.18 to 0.95 when the CLEO II data on $B \ra J/Ψ+ K (K^*)$ are used.

hep-ph↗

arXiv1994

Implications of factorization for the determination of hadronic form factors in $D_s^+ \ra ϕ$ transition

Using factorization we determine the allowed domains of the ratios of form factors, $x = A_2(0)/A_1(0)$ and $y = V(0)/A_1(0)$, from the experimentally measured ratio $R_h \equiv Γ(D_s^+ \ra ϕρ^+)/Γ(D_s^+ \ra ϕπ^+)$ assuming three different scenarios for the $q^2$-dependence of the form factors. We find that the allowed domains overlap with those obtained by using the experimentally measured ratio $R_{s\ell} = Γ(D^+_s \ra ϕ\ell^+ ν_{\ell})/Γ(D^+_s \ra ϕπ^+)$ provided that the phenomenological parameter $a_1$ is $1.23$. Such a comparison presents a genuine test of factorization. We calculate the longitudinal polarization fraction, $Γ_L/Γ\equiv Γ(D_s^+ \ra ϕ_L ρ^+_L)/Γ(D_s^+ \ra ϕρ^+)$, in the three scenarios for the $q^2$-dependence of the form factors and emphasize the importance of measuring $Γ_L/Γ$. Finally we discuss the $q^2$-distribution of the semileptonic decay and find that it is rather insensitive to the scenarios for the $q^2$-dependence of the form factors, and unless very accurate data can be obtained it is unlikely to discriminate between the different scenarios. Useful information on the value of $x$ might be obtained by the magnitude of the $q^2$-distribution near $q^2 = 0$. However the most precise information on $x$ and $y$ would come from the knowledge of the longitudinal and left-right transverse polarizations of the final vector mesons in hadronic and/or semileptonic decays.

hep-ph↗

arXiv1994

On the Determination of $a_1$ and $a_2$ from Hadronic Two Body $B$ Decays

{}From Class I decays : $B^o \ra π^+ ( ρ^+ ) + D^- ( {D^*}^- )$, we determine $a_1$, and from Class III decays : $B^+ \ra π^+ ( ρ^+ ) + \ol{D}^o ( {\ol{D}^*}^o )$, we determine an allowed domain in the $( a_1, a_2 )$ plane. We find that within one standard deviation errors, the allowed band of $a_1$ from Class I decays is at least three standard deviations removed from the allowed domain $( a_1, a_2 )$ from Class III decays.If we expand the experimental errors to two standard deviations we do find a small intersection between the $a_1$ band and the allowed $( a_1, a_2 )$ domain. The results usually quoted in the literature lie in this intersection. We suggest : (1) an independent measurement of the branching ratio for the Class III decay, $B^+ \ra ρ^+ \ol{D}^o $, (2) a high-statistics measurement of the branching ratio of the Class I decay, $B \ra \ol{D} ( \ol{D}^* ) + D_s ( D_s^* )$ in both charged states, and (3) a measurement of the longitudinal polarization fraction in the Class III decay $B^+ \ra ρ^+ {\ol{D}^*}^o $ to shed more light on the questions involved .

hep-ph↗