Γγ Couplings Research Articles

Light-by-light forward scattering amplitudes in Lattice QCD

We present our preliminary results on the calculation of hadronic light-by-light forward scattering amplitudes using vector four-point correlation functions computed on the lattice. Using a dispersive approach, forward scattering amplitudes can be described by γ*γ* → hadrons fusion cross sections and then compared with phenomenology. We show that only a few states are needed to reproduce our data. In particular, the sum rules considered in this study imply relations between meson–γγ couplings and provide valuable information about individual form factors which are often used to estimate the meson-pole contributions to the hadronic light-by-light contribution to the (g – 2) of the muon.

Higgs couplings at the end of 2012

Abstract Performing a fit to all publicly available data, we analyze the extent to which the latest results from the LHC and Tevatron constrain the couplings of the Higgs boson like state at ~ 125 GeV. To this end we assume that only Standard Model (SM) particles appear in the Higgs decays, but tree-level Higgs couplings to the up-quarks, down-quarks and vector bosons, relative to the SM are free parameters. We also assume that the leptonic couplings relative to the SM are the same as for the down-quark, and a custodial symmetry for the V = W, Z couplings. In the simplest approach, the effective Higgs couplings to gluons and photons are computed in terms of the previous parameters. This approach is also applied to Two-Higgs-Doublet Models of Type I and Type II. However, we also explore the possibility that the net Higgs to gg and γγ couplings have extra loop contributions coming from Beyond-the-Standard Model physics. We find that the SM p- value ~ 0.5 is more than 2σ away from fits in which: a) there is some non-SM contribution to the γγ coupling of the Higgs; or b) the sign of the top quark coupling to the Higgs is opposite that of the W coupling. In both these cases p-values ~ 0.9 can be achieved. Since option b) is difficult to realize in realistic models, it would seem that new physics contributions to the effective couplings of the Higgs are preferred.

LOCATION, CORRELATION, RADIATION: WHERE IS THE σ, WHAT IS ITS STRUCTURE, AND WHAT IS ITS COUPLING TO PHOTONS?

Scalar mesons are a key expression of the infrared regime of QCD. The lightest of these is the σ. Now that its pole in the complex energy plane has been precisely located, we can ask whether this state is transiently [Formula: see text] or [Formula: see text] or a multi-meson molecule or largely glue? The two-photon decay of the σ can, in principle, discriminate between these possibilities. We review here how the γγ → π+π-, π0π0 cross-sections can be accurately computed. The result not only agrees with experiment, but definitively fixes the radiative coupling of the σ. This equates to a two-photon width of (4.1 ± 0.3) keV, which accords with the simple nonrelativistic quark model expectation for a [Formula: see text], [Formula: see text] scalar. Nevertheless, robust predictions from relativistic strong coupling QCD are required for each of the possible compositions before we can be sure which one really delivers the determined γγ coupling.

The scalar sector of the Randall–Sundrum model

We consider the scalar sector of the Randall–Sundrum model. We derive the effective potential for the Standard Model Higgs-boson sector interacting with Kaluza–Klein excitations of the graviton ( h μ νn ) and the radion ( φ) and show that only the Standard Model vacuum solution of ∂V( h)/ ∂h=0 ( h is the Higgs field) is allowed. We then turn to our main focus: the consequences of the curvature-scalar mixing ξR H † H (where H is a Higgs doublet field on the visible brane), which causes the physical mass eigenstates h and φ to be mixtures of the original Higgs and radion fields. First, we discuss the theoretical constraints on the allowed parameter space. Next, we give precise procedures for computing the h and φ couplings given the physical eigenstate masses, m h and m φ , ξ and the new physics scales of the model. Relations among these new-physics scales are discussed and a set of values not far above the smallest values required by precision electroweak constraints and RunI data is chosen. A simple result for the sum of the ZZh and ZZφ squared couplings relative to the ZZh SM squared coupling is derived. We demonstrate that this sum rule in combination with LEP/LEP2 data implies that not both the h and φ can be light. We present explicit results for the still allowed region in the ( m h , m φ ) plane that remains after imposing the appropriate LEP/LEP2 upper limits coming from the Higgs-strahlung channel. In the remaining allowed region of parameter space, we examine numerically the couplings and branching ratios of the h and φ for several cases with m h =120 GeV and m φ ⩽300 GeV. The resulting prospects for detection of the h and φ at the LHC, a future LC and a γγ collider are reviewed. For moderate | ξ|, both the anomalous h→ gg coupling and (when m h >2 m φ ) the non-standard decay channel h→ φφ can substantially impact h discovery. Presence of the latter is a direct signature for non-zero ξ. We find that BR( h→ φφ) as large as 30–40% is possible when | ξ| is large. Conversely, if m φ >2 m h then BR( φ→ hh) is generally large. Sensitivity to the model-dependent magnitude of the cubic radion potential term is discussed. We find that detection of a light φ might require the LC. Detection of a heavy φ might need to take into account the φ→ hh channel. The feasibility of experimentally measuring the anomalous gg and γγ couplings of the h and φ is examined.

Study of anomalous ZZ γ and Z γγ couplings at LEP

We search for anomalous ZZ γ and Z γγ couplings with the L3 detector at LEP. The analysis is based on the study of the process e +e +→Z γ at center-of-mass energies in the range 161 GeV < s <183 GeV . No evidence for anomalous effects is found. Limits at the 95% confidence level are set on the values of the eight possible anomalous couplings. Depending on the type of coupling new physics scales below 213 GeV to 1083 GeV are excluded.

Evidence for spin-one resonance production in the reaction gamma gamma *--> pi + pi - pi 0 pi 0.

Using data from the TPC/Two-Gamma experiment at the SLAC e+e− storage ring PEP, a C=+1 resonance has been observed in the π+π−π0γ final state resulting from the fusion of one nearly real and one quite virtual photon. The actual decay channel is probably π+π−π0π0, where one final-state photon is not detected, and the mass of the fully reconstructed state would be approximately 1525 MeV. A four-pion decay mode in turn implies that the resonance has even isospin. The nonobservation of this R(1525) when both initial-state photons are nearly real suggests a spin-1 assignment. Since the large measured value of the product of the branching ratio into π+π−π0π0 and the γγ coupling makes it unlikely that this state is the mostly s¯s f1(1510), its interpretation may lie outside of conventional meson spectroscopy. There is a second, less-significant enhancement observed in the same reaction at a four-pion mass centered around 2020 MeV.Received 15 March 1993DOI:https://doi.org/10.1103/PhysRevD.48.3976©1993 American Physical Society

F 1 (1285) formation in photon-photon fusion reactions

We have observed formation of the f 1 (1285) in the reaction e +e -→e +e - π + π - η( η→ γγ). Its γγ ∗ width is determined in several Q 2 bins. The γγ coupling parameter for the f 1 (1285) is found to be 2.4±0.5±0.5 keV. This value is compared to that for the X (1420), another J=1 state formed in γγ fusion reactions, which may belong to the same meson nonet.

Bethe-Salpeter dynamics for two-photon processes

Recent experimental data on single hadron production by two-photon beams inpetra andpep have provided a unique opportunity for testing specific models of confinement through a study of one of their cleanest predictionsviz the γγ →H amplitudes. Motivated by this new facility, aqcd-oriented Bethe-Salpeter model of harmonic confinement, which has already been found to describe rather well several classes of hadronic data (from mass spectra to electromagnetic and pionic couplings), is now employed for a detailed comparison of its predictions onP → γγ andT → γγ couplings with the data. The agreement is quite good for all cases except one (η → γγ).

Kaon Partial-Conservation-of-Axial-Vector-Current Anomaly andKL→γγDecay

The low-energy theorem is derived which relates the invariant vertex function of KL→γγ to the corresponding function of 〈2γ|[Q56, Lw]|0〉, where it is assumed that Schwinger and seagull terms, if present, cancel against each other. A chiral Lagrangian model is studied which satisfies PCAC and which has an octet weak Lagrangian with [Q56, Lw]=0. In this model, the effective KL→γγ coupling constant produced by certain baryon loop graphs is found to remain finite as the momentum of the kaon goes to zero. This contradicts the low-energy theorem and indicates the failure of seagull and Schwinger terms to cancel. The effective coupling produced by this anomaly is estimated to give a rate roughly comparable to the experimental one. Implications for mesonic effective-Lagrangian studies of K decays are discussed.

Canonical anomalies and broken scale invariance

Canonical behavior of strong interactions at short distances causes an anomaly in the trace identity involving two electromagnetic currents and the energy-momentum tensor. The anomaly is connected with the high energy behavior of e +e − → γ → hadrons and the ϵ(700)γγ coupling constant.

Theoretical investigation of the reaction γγ → ππ and the ϵ → γγ and f → γγ decay widths

The γγ → ππ cross section and the ϵ → γγ decays width are calculated on the basis of partial-wave dispersion relations. The left hand cut integral is approximated by vector meson exchange diagrams. On the right hand cut elastic unitarity is taken into account which establishes a close correspondence between the cross section and the ππ phase shifts. The I = 0, J = 2 phase shift is parametrized in terms of the f(1260) and for the corresponding J = 0 phase shift a “down” and an “up” solution (ϵ Breit-Wigner) is considered. The cross section is found to depend sensitively on the form of the ππ final state interaction. The obtained ϵ → γγ and f → γγ couplings are compared with other estimates.

Finite-energy sum rules and the reactions ee → eeϵ (750) and ee → eef (1260)

The ϵγγ and f γγ coupling constants are calculated from finite-energy sum rules for γπ → πγ. The cross sections for ϵ and f production in colliding electron beam reactions are found to be significant. There is reasonable agreement with the tensor meson dominance estimates for the f γγ couplings.