arXiv subjects

M. Benayoun

Publications and source records attributed to M. Benayoun.

140 records · Page 8Linked to original sources

arXiv2009

A Global Treatment Of VMD Physics Up To The $ϕ$: I. $e^+e^-$ Annihilations, Anomalies And Vector Meson Partial Widths

The HLS Model, equipped with a mechanism providing the breaking of U(3)/SU(3) symmetry and an isospin symmetry breaking leading naturally to vector meson mixing, has been recently shown to successfully account for $e^+ e^- \ra π^+π^-$ cross section and for the dipion spectrum in $τ$ decay. The present study shows that the full anomalous sector of the HLS model can be considered and is validated by the experimental data. Indeed, this extended model provides a successful simultaneous fit to the $e^+ e^- \ra π^+π^-$ data together with the available data on $e^+ e^- \ra π^0γ$, $e^+ e^- \ra ηγ$ and $e^+ e^- \ra π^0 π^+π^-$ cross sections. It is shown that the fit of these data sets also predicts an accurate description of the $η/η^\prime \ra π^+ π^- γ$ decays fully consistent with the reported information on their branching fractions and spectra. Finally, one also derives from our global fits products of widths of the form $Γ(V \ra f_1)Γ(V \ra e^+ e^-) $ and ratios of the form $Γ(V \ra f_1)/Γ(V \ra f_2)$ describing decays of vector mesons to several non--leptonic final states.

hep-ph↗

arXiv2008

The Dipion Mass Spectrum In e+e- Annihilation and tau Decay: A Dynamical (rho0, omega, phi) Mixing Approach

We readdress the problem of finding a simultaneous description of the pion form factor data in e+e- annihilations and in tau decays. For this purpose, we work in the framework of the Hidden Local Symmetry (HLS) Lagrangian and modify the vector meson mass term by including the pion and kaon loop contributions. This leads us to define the physical rho, omega and phi fields as linear combinations of their ideal partners, with coefficients being meromorphic functions of s, the square of the 4--momentum flowing into the vector meson lines. This allows us to define a dynamical, i.e. s-dependent, vector meson mixing scheme. The model is overconstrained by extending the framework in order to include the description of all meson radiative (V P gamma and P gamma gamma couplings) and leptonic (Ve+e- couplings) decays and also the isospin breaking (omega/ phi --> pi+ pi-) decay modes. The model provides a simultaneous, consistent and good description of the e+e- and tau dipion spectra. The expression for pion form factor in the latter case is derived from those in the former case by switching off the isospin breaking effects specific to e+e- and switching on those for tau decays. Besides, the model also provides a good account of all decay modes of the form V P gamma, Pgamma gamma as well as the isospin breaking decay modes. It leads us to propose new reference values for the rho^0 --> e+ e- and omega --> pi+ pi- partial widths which are part of our description of the pion form factor. Other topics (phi --> K anti K, the rho meson mass and width parameters) are briefly discussed. Therefore, we confirm the 3.3 sigma discrepancy between the theoretical estimate of a_mu based on e+e- and its direct BNL measurement.

hep-ph↗

arXiv2003

Anomalous $η/η^\prime$ Decays: The Triangle and Box Anomalies

We examine the decay modes $η/\etp\ra π^+ π^- γ$ within the context of the Hidden Local Symmetry (HLS) Model. Using numerical information derived in previous fits to $VPγ$ and $Ve^+e^-$ decay modes in isolation and the $ρ$ lineshape determined in a previous fit to the pion form factor, we show that all aspects of these decays can be predicted with fair accuracy. Freeing some parameters does not improve the picture. This is interpreted as a strong evidence in favor of the box anomaly in the $η/\etp$ decays, which occurs at precisely the level expected. We also construct the set of equations defining the amplitudes for $η/\etp\ra π^+ π^- γ$ and $ η/\etp \ra \ggam $ at the chiral limit, as predicted from the anomalous HLS Lagrangian appropriately broken. This provides a set of four equations depending on only one parameter, instead of three for the traditional set. This is also shown to match the (two--angle, two--decay--constant) $η-\etp$ mixing scheme recently proposed and is also fairly well fulfilled by the data. The information returned from fits also matches expectations from previously published fits to the $VPγ$ decay modes in isolation.

nucl-th↗

arXiv2003

The Pion Form Factor Within the Hidden Local Symmetry Model

We analyze a pion form factor formulation which fulfills the Analyticity requirement within the Hidden Local Symmetry (HLS) Model. We show that it implies an $s$--dependent dressing of the $ρ-γ$ VMD coupling and an account of several coupled channels. The corresponding function $F_π(s)$ provides nice fits of the pion form factor data from $s=-0.25$ to $s=1$ GeV$^2$. It is shown that the coupling to $K \bar{K}$ has little effects, while $\omg π^0$ improves significantly the fit quality below the $ϕ$ mass. All parameters, except for the subtraction polynomial coefficients are fixed from the rest of the HLS phenomenology. The fits show consistency with the expected behaviour of $F_π(s)$ at $s=0$ up to ${\cal O} (s^2)$ and with the phase shift data on $δ_1^1(s)$ from threshold to somewhat above the $ϕ$ mass. The $\omg$ sector is also examined in relation with recent data from CMD--2.

nucl-th↗

arXiv2002

Isospin Symmetry Breaking within the HLS Model: A Full ($ρ, ω, ϕ$) Mixing Scheme

We study the way isospin symmetry violation can be generated within the Hidden Local Symmetry (HLS) Model. We show that isospin symmetry breaking effects on pseudoscalar mesons naturally induces correspondingly effects within the physics of vector mesons, through kaon loops. In this way, one recovers all features traditionally expected from $ρ-\omg$ mixing and one finds support for the Orsay phase modelling of the $e^+e^- \ra π^+ π^-$ amplitude. We then examine an effective procedure which generates mixing in the whole $ρ$, $\omg$, $ϕ$ sector of the HLS Model. The corresponding model allows us to account for all two body decays of light mesons accessible to the HLS model in modulus and phase, leaving aside the $ρ\ra ππ$ and $K^* \ra K π$ modes only, which raise a specific problem. Comparison with experimental data is performed and covers modulus and phase information; this represents 26 physics quantities successfully described with very good fit quality within a constrained model which accounts for SU(3) breaking, nonet symmetry breaking in the pseudoscalar sector and, now, isospin symmetry breaking.

nucl-th↗

arXiv2001

VMD, the WZW Lagrangian and ChPT: The Third Mixing Angle

We show that the Hidden Local Symmetry Model, supplemented with well-known procedures for breaking flavor SU(3) and nonet symmetry, provides all the information contained in the standard Chiral Perturbation Theory (ChPT) Lagrangian ${\cal L}^{(0)}+{\cal L}^{(1)}$. This allows to rely on radiative decays of light mesons ($VPγ$ and $P γγ$) in order to extract some numerical information of relevance to ChPT: a value for $Λ_1=0.20 \pm 0.04$, a quark mass ratio of $\simeq 21.2 \pm 2.4$, and a negligible departure from the Gell-Mann--Okubo mass formula. The mixing angles are $θ_8=-20.40^\circ \pm 0.96^\circ$ and $θ_0=-0.05^\circ \pm 0.99^\circ$. We also give the values of all decay constants. It is shown that the common mixing pattern with one mixing angle $θ_P$ is actually quite appropriate and algebraically related to the $η/η'$ mixing pattern presently preferred by the ChPT community. For instance the traditional $θ_P$ is functionally related to the ChPT $θ_8$ and fulfills $θ_P \simeq θ_8/2$. The vanishing of $θ_0$, supported by all data on radiative decays, gives a novel relation between mixing angles and the violation of nonet symmetry in the pseudoscalar sector. Finally, it is shown that the interplay of nonet symmetry breaking through U(3) $\ra$ SU(3)$\times$ U(1) satisfies all requirements of the physics of radiative decays without any need for additional glueballs.

hep-ph↗

arXiv2000

An Effective Approach to VMD at One Loop Order and the Departures from Ideal Mixing for Vector Mesons

We examine the mechanisms producing departures from ideal mixing for vector mesons within the context of the Hidden Local Symmetry (HLS) model. We show that kaon loop transitions between the ideal combinations of the $ω$ and $ϕ$ mesons necessitate a field transformation in order to get the mass eigenstates. It is shown that this transformation is close to a rotation for processes involving, like meson decays, on--shell $ω$ and $ϕ$ mesons. The HLS model predicts a momentum dependent, slowly varying mixing angle between the ideal states. We examine numerically the consequences of this for radiative and leptonic decays of light mesons. The mean $ω-ϕ$ mixing angle is found smaller than its ideal value; this is exhibited separately in radiative and in leptonic decays. Effects of nonet symmetry breaking in the vector sector are compared to those produced by the field rotation implied by the HLS model.

nucl-th↗

arXiv1999

Spectroscopy at B-factories Using Hard Photon Emission

The process of hard photon emission by initial electrons (positrons) at B-factories is discussed. It is shown that studies of the bottomonium spectroscopy will be feasible for the planned integrated luminosity of the B-factory experiments.

hep-ph↗

arXiv1999

A VMD Based, Nonet and SU(3) Symmetry Broken Model For Radiative Decays of Light Mesons

We present a VMD based model aiming to describe all radiative decays of light mesons. We show that the SU(3) breaking mechanism proposed by Bando, Kugo and Yamawaki (BKY), supplemented by nonet symmetry breaking in the pseudoscalar sector are sufficient to provide a nice description of all data, except the charged K* radiative width. It is also shown that nonet symmetry breaking has effects which cannot be disantangled from those produced by coupling of glue to the eta' meson. Coupling of glue to $η$ is not found to be required by the data. Asssuming the charged K* radiative width is indeed at its presently accepted value necessitates to supplement the BKY breaking in a way which finally preserves an equivalence statement between the VMD approach to radiative decays and the Wess-Zumino-Witten Lagrangian.

hep-ph↗

arXiv1999

Radiative Decays, Nonet Symmetry and SU(3) Breaking

We re-examine the problem of simultaneously describing in a consistent way all radiative and leptonic decays of light mesons (V -> P gamma, P -> V gamma, P -> gamma gamma, V -> e^+ e^-). For this purpose, we rely on the Hidden Local Symmetry model in both its anomalous and non--anomalous sectors. We show that the SU(3) symmetry breaking scheme proposed by Bando, Kugo and Yamawaki, supplemented with nonet symmetry breaking in the pseudoscalar sector, allows one to reach a nice agreement with all data, except for the K^{*+/-} radiative decay. An extension of this breaking pattern allows one to account for this particular decay mode too. Considered together, the whole set of radiative decays provides a pseudoscalar mixing angle theta_P ~ -11^o and a value for theta_V which is ~ 3^o from that of ideal mixing. We also show that it is impossible, in a practical sense, to disentangle the effects of nonet symmetry breaking and those of glue inside the eta', using only light meson decays.

hep-ph↗

arXiv1999

Vector meson dominance and the rho meson

We discuss the properties of vector mesons, in particular the rho^0, in the context of the Hidden Local Symmetry (HLS) model. This provides a unified framework to study several aspects of the low energy QCD sector. Firstly, we show that in the HLS model the physical photon is massless, without requiring off field diagonalization. We then demonstrate the equivalence of HLS and the two existing representations of vector meson dominance, VMD1 and VMD2, at both tree level and one loop order. Finally the S matrix pole position is shown to provide a model and process independent means of specifying the rho mass and width, in contrast to the real axis prescription currently used in the Particle Data Group tables.

hep-ph↗

arXiv1998

SU(3) breaking and Hidden Local Symmetry

We study the various existing implementations of SU(3) breaking in the Hidden Local Symmetry model for the low energy hadronic sector following a mechanism originally proposed by Bando, Kugo and Yamawaki (BKY). We pay particular attention to hermiticity and current conservation. Following this, we present a new method for including symmetry breaking effects which preserves the BKY mas s relation among vector mesons. Symmetry breaking (SB) necessarily requires a transformation of the pseudoscalar fields, which, following BKY, we refer to a s field renormalization. We examine the consequences of propagating this throug h all Lagrangian terms including the anomalous ones. We thus explore the consequences of these various SB schemes for both charged and neutral pseudoscalar decay constants as measured in weak and anomalous decays respectively.

hep-ph↗

arXiv1998

Reconstruction and Particle Identification for a DIRC System

We study the reconstruction and particle identification (PID) problem for Ring Imaging devices providing a good knowledge of the direction of the Cerenkov photons, as the DIRC system, on which we specialize. We advocate first the use of the stereographic projection as a tool allowing a suitable representation of the photon data, as it allows to represent the Cerenkov cone always as a circle. We set up an algorithm able to perform reliably a fit of circle arcs of small angular opening, by minimising a true Chi2 expression. The system we develop for PID relies on this algorithm and on a procedure able to remove background photons with a high efficiency. We thus show that, even when the background is large, it is possible to perform an efficient PID by means of a fit algorithm which finally provides all the circle parameters; these are connected with the charged track direction and its Cerenkov angle. It is shown that background effects can be dealt without spoiling significantly the reconstruction probability distributions.

hep-ex↗

arXiv1998

New results in rho^0 meson physics

We compare the predictions of a range of existing models based on the Vector Meson Dominance hypothesis with data on e^+ e^- -> pi^+ pi^$ and e^+ e^- -> mu^+ mu^- cross-sections and the phase and near-threshold behavior of the timelike pion form factor, with the aim of determining which (if any) of these models is capable of providing an accurate representation of the full range of experimental data. We find that, of the models considered, only that proposed by Bando et al. is able to consistently account for all information, provided one allows its parameter "a" to vary from the usual value of 2 to 2.4. Our fit with this model gives a point-like coupling (gamma pi^+ π^-) of magnitude ~ -e/6, while the common formulation of VMD excludes such a term. The resulting values for the rho mass and pi^+ pi^- and e^+e^- partial widths as well as the branching ratio for the decay omega -> pi^+ pi^- obtained within the context of this model are consistent with previous results.

hep-ph↗