Form factors and branching ratio for the B→lνγ decay

Form factors parameterizing radiative leptonic decays of heavy mesons (B⁺→γl⁺ν_l) against photon energy are computed in the language of dispersion relations. The states contributing to the absorptive part in the dispersion relation are the multiparticle continuum, estimated by the quark triangle graph, and resonances with quantum numbers 1^- and 1⁺, which includes B^* and B_A ^* and their radial excitations, which model the higher state contributions. Constraints provided by the asymptotic behavior of the structure dependent amplitude, Ward identities and gauge invariance are used to provide useful information for the parameters needed. The couplings g_BB*γ and f_BBA*γ are predicted as we restrict ourselves to the first radial excitation; otherwise using these as an input the radiative decay coupling constants for the radial excitations are predicted. The value of the branching ratio for the process B⁺→γμ⁺ν_μ is found to be in the range 0.5×10^-6. A detailed comparison is given with other approaches.     

On the high-statistics investigation of the <Emphasis Type="Italic">a</Emphasis><Subscript>0</Subscript>(980) resonance in the γγ → π<Superscript>0</Superscript>η reaction

High-statistics data on the γγ → π⁰η reaction will make it possible to conclude whether the K ⁺ K ⁻-loop rescattering mechanism, γγ → K ⁺ K ⁻ → a ₀(980) → π⁰η, is the main mechanism of the production of a ₀(980) isovector resonance. This mechanism provides a reasonable value of 20–30 nb at the maximum for the manifestation of a ₀(980) in the γγ → π⁰η cross section. It also gives rise to a noticeable narrowing of the a ₀(980) peak to its effective (observed) width ≈20–30 MeV in the γγ → π⁰η channel. The decay width averaged over the resonance mass distribution is 〈Γ_{α 0} → K ⁺ K ⁻ → γγ〉_πη ≈ 0.13 keV. The experimental confirmation of this scenario would be important evidence in favor of the q ² $ \bar q $ \bar q ² nature of light scalar mesons.     

Study of triple-gauge-boson couplings ZZZ, ZZγ and Zγγ at LEP

Neutral triple-gauge-boson couplings ZZZ, ZZγ and Zγγ have been studied with the DELPHI detector using data at energies between 183 and 208 GeV. Limits are derived on these couplings from an analysis of the reactions e⁺e^-→Zγ, using data from the final states γff̄, with f=q or ν, from e⁺e^-→ZZ, using data from the four-fermion final states qq̄qq̄, qq̄μ⁺μ^-, qq̄e⁺e^-, qq̄νν̄, μ⁺μ^-νν̄ and e⁺e^-νν̄, and from e⁺e^-→Zγ^*, in which the final state γ is off mass-shell, using data from the four-fermion final states qq̄e⁺e^- and qq̄μ⁺μ^-. No evidence for the presence of such couplings is observed, in agreement with the predictions of the Standard Model.     

Study of double-tagged γγ events at LEPII

Double-tagged interactions of photons with virtualities Q² between 10 GeV² and 200 GeV² are studied with the data collected by DELPHI at LEPII from 1998 to 2000, corresponding to an integrated luminosity of 550 pb^-1. The γ^*γ^*→μ⁺μ^- data agree with QED predictions. The cross-section of the reaction γ^*γ^*→hadrons is measured and compared to the LO and NLO BFKL calculations.     

A determination of the centre-of-mass energy at LEP2 using radiative two-fermion events

Using e⁺e^-→μ⁺μ^-(γ) and events radiative to the Z pole, DELPHI has determined the centre-of-mass energy, , using energy and momentum constraint methods. The results are expressed as deviations from the nominal LEP centre-of-mass energy, measured using other techniques. The results are found to be compatible with the LEP Energy Working Group estimates for a combination of the 1997 to 2000 data sets.     

A massive photon in a strong magnetic field: Generation and decay channels

The conditions for the formation of an effective photon mass in a strong magnetic field are considered. It is shown that massless photons are sterile. The channels γ* → e⁺e⁻ and γ* → v_iv_i for the decay of a massive photon and their possible contribution to the mechanism of energy losses by collapsing astrophysical objects are discussed. Moscow State Industrial University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 35–39, February, 1997.     

Two-photon decays of pseudotensor mesons <Emphasis Type="Italic">π</Emphasis><Subscript>2</Subscript>(1670), <Emphasis Type="Italic">π</Emphasis><Subscript>2</Subscript>(2250), and <Emphasis Type="Italic">π</Emphasis><Subscript>2</Subscript>(2510)

In terms of relativistic and gauge-invariant spectral-integral technique, we calculate amplitude for the π ₂ → γγ decay as a three-step transition π ₂ → q $ \bar q $ \bar q (2⁻⁺) → γ + q $ \bar q $ \bar q (1⁻⁻) → γγ. Using wave functions of the q $ \bar q $ \bar q components of photon (hep-ph/0406320) and those of pseudotensor mesons π ₂(1670), π ₂(2000), π ₂(2250), and π ₂(2510) (hep-ph/0511109), we obtain the following values for partial widths: Γ _{π2(1670)→γγ} = 0.072 ± 0.007 keV, Γ _{π2(2000)→γγ} = 0.037 ± 0.004 keV, Γ _{π2(2250)→γγ} = 0.027 ± 0.003 keV, and Γ _{π2(2510)→γγ} = 0.022 ± 0.002 keV. In the calculations, we use the momentum operator expansion, and, in this connection, discuss the problem of nilpotent operators inherent in the considered amplitude. It should be emphasized that measurement of two-photon decays of the π ₂ states can give valuable information about the long-range quark-antiquark forces which, as one may guess, are related to the quark confinement.     

Study of K<Superscript>*</Superscript><Subscript>0</Subscript>(1430) and a<Subscript>0</Subscript>(980) from B→K<Superscript>*</Superscript><Subscript>0</Subscript>(1430)π and B→a<Subscript>0</Subscript>(980)K decays

We use the decay modes B→K^* ₀(1430)π and B→a₀(980)K to study the scalar mesons K^* ₀(1430) and a₀(980) within the framework of perturbative QCD. For B→K^* ₀(1430)π, we perform our calculation in two scenarios for the scalar meson spectrum. The results indicate that scenario II is more favored by the experimental data than scenario I. The important contribution from annihilation diagrams can enhance the branching ratios by about 50% in scenario I and about 30% in scenario II. The direct CP-asymmetries in B→K^* ₀(1430)π are small, which is consistent with the present experiments. The predicted branching ratio of B→a₀(980)K in scenario I differs from the experiments by a factor of 2, which indicates that a₀(980) cannot be interpreted as q̄q.     

B→K1?+?? decays in a family non-universal Z′ model

The implications of the family non-universal Z′ model in the B→K ₁(1270,1400)ℓ ⁺ ℓ ⁻(ℓ=e ,μ ,τ) decays are explored, where the mass eigenstates K ₁(1270, 1400) are the mixtures of ¹ P ₁ and ³ P ₁ states with the mixing angle θ. In this work, considering the Z′ boson and setting the mixing angle θ=(−34±13)^○, we analyze the branching ratio, the dilepton invariant mass spectrum, the normalized forward–backward asymmetry and lepton polarization asymmetries of each decay mode. We find that all observables of B→K ₁(1270)μ ⁺ μ ⁻ are sensitive to the Z′ contribution. Moreover, the observables of B→K ₁(1400)μ ⁺ μ ⁻ have a relatively strong θ-dependence; thus, the Z′ contribution will be buried by the uncertainty of the mixing angle θ. Furthermore, the zero crossing position in the FBA spectrum of B→K ₁(1270)μ ⁺ μ ⁻ at low dilepton mass will move to the positive direction with Z′ contribution. For the tau modes, the effects of Z′ are not remarkable due to the small phase space. These results could be tested in the running LHC-b experiment and Super-B factory.     

Mechanism of the production of the <Emphasis Type="Italic">a</Emphasis><Subscript>0</Subscript>(980) resonance in the γγ → π<Superscript>0</Superscript>η reaction

High-statistics Belle data on the γγ → π⁰η reaction have been analyzed in order to reveal the mechanism of two-photon production and the nature of the a ₀(980) resonance. The solution obtained for the γγ → π⁰η amplitude is in agreement with the predictions of the chiral theory for the πη-scattering length; with the strong coupling of the a ₀(980) resonance with the πη, K $ \bar K $ \bar K , and πη′ channels; and with the key role of the a ₀(980) → (K $ \bar K $ \bar K + π⁰η + π⁰η′) → γγ rescattering mechanisms in the a ₀(980) → γγ decay. This picture is much in favor of the q ² $ \bar q $ \bar q ² nature of a ₀(980) resonance and is consistent with the properties of its partners, σ₀(600) and f ₀(980) resonances, in particular, with those manifested in the γγ → ππ reactions. The important role of vector exchanges in the formation of the nonresonant background in the γγ → π⁰η reaction has been revealed. Preliminary information on the π⁰η → π⁰η reaction has been obtained.     

K0πand K*0Σ+ photoproduction off the proton

The exclusive reactions γp → K ^*0Σ⁺(1189) and γp → K ⁰π⁰Σ⁺(1189) , leading to the p 4π⁰ final state, have been measured with a tagged photon beam for incident energies from threshold up to 2.5GeV. The experiment has been performed at the tagged photon facility of the ELSA accelerator (Bonn). The Crystal Barrel and TAPS detectors were combined to a photon detector system of almost 4π geometrical acceptance. Differential and total cross-sections are reported. At energies close to the threshold, a flat angular distribution has been observed for the reaction γp → K ⁰π⁰Σ⁺ suggesting dominant s -channel production. Σ^*(1385) and higher-lying hyperon states have been observed. An enhancement in the forward direction in the angular distributions of the reaction γp → K ^*0Σ⁺ indicates a t -channel exchange contribution to the reaction mechanism. The experimental data are in reasonable agreement with recent theoretical predictions.     

On mass corrections to the decay <Emphasis Type="Italic">P</Emphasis> → <Emphasis Type="Italic">l</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">l</Emphasis><Superscript>−</Superscript>

The Mellin-Barnes representation is used to improve the theoretical estimate of mass corrections to the width of a light pseudoscalar meson decay into a lepton pair, P → l ⁺ l ⁻. The full resummation of the terms ln(m _l ²/Λ²)(m _l ²/Λ²) ⁿ and (m _l ²/Λ²) ⁿ to the decay amplitude is performed, where m _l is the lepton mass and Λ ≈ m _ρ is the characteristic scale of the P → γ*γ* form factor. The total effect of the mass corrections for the e ⁺ e ⁻ channel is negligible and, for the μ⁺μ⁻channel, its order is of a few percent. The text was submitted by the authors in English.     

Measurements of the absolute branching fractions for D→K̄πe+νe, D→K̄*e+νe and determination of Γ(D+→K̄*0e+νe)/Γ(D+→K̄0e+νe)

Using the data of about 33 pb^-1 collected at and around 3.773 GeV with the BES-II detector at the BEPC collider, we have studied the exclusive semileptonic decays D⁺→K^-π⁺e⁺ν_e, D⁰→K̄⁰π^-e⁺ν_e, D⁺→K̄^*0e⁺ν_e and D⁰→K^*-e⁺ν_e. The absolute branching fractions for the decays are measured to be BF(D⁺→K^-π⁺e⁺ν_e)=(3.50±0.75±0.27)%, BF(D⁰→K̄⁰π^-e⁺ν_e)=(2.61±1.04±0.28)%, BF(D⁺→K̄^*0e⁺ν_e)=(5.06±1.21±0.40)% and BF(D⁰→K^*-e⁺ν_e)=(2.87±1.48±0.39)%. The ratio of the vector to pseudoscalar semileptonic decay rates Γ(D⁺→K̄^*0e⁺ν_e)/Γ(D⁺→K̄⁰e⁺ν_e) is determined to be 0.57±0.17±0.02.     

Diffractive photoproduction of D*±(2010) at HERA

Diffractive photoproduction of D^*±(2010) mesons was measured with the ZEUS detector at the ep collider HERA, using an integrated luminosity of 78.6 pb^-1. The D^* mesons were reconstructed in the kinematic range: transverse momentum p_T(D^*) > 1.9 GeV and pseudorapidity |η(D^*)|<1.6, using the decay D^*+→D⁰π⁺ _s followed by D⁰→K^-π⁺(+c.c.). Diffractive events were identified by a large gap in pseudorapidity between the produced hadronic state and the outgoing proton. Cross sections are reported for photon–proton centre-of-mass energies in the range 130 < W < 300 GeV and for photon virtualities Q² < 1 GeV², in two ranges of the Pomeron fractional momentum x_IP<0.035 and x_IP<0.01. The relative contribution of diffractive events to the inclusive D^*±(2010) photoproduction cross section is about 6%. The data are in agreement with perturbative QCD calculations based on various parameterisations of diffractive parton distribution functions. The results are consistent with diffractive QCD factorisation.     

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

Recently the BES collaboration has announced the observation of a resonant state in the π⁺π^-η’ spectrum in J/ψ→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 pp̄ threshold enhancement data in J/ψ→γpp̄. We study the properties of this state using QCD anomaly and QCD sum rules, assuming X(1835) to be a pseudoscalar, and we show that it is consistent with the data. We find that this state has a sizeable matrix element , leading to branching ratios of (2.61–7.37)×10^-3 and (2.21–10.61)×10^-2 for J/ψ→γG_p and for G_p→π⁺π^-η^′, respectively. Combining the calculated branching ratio of J/ψ→γG_p and data on threshold enhancement in J/ψ→γpp̄, we determine the coupling for the G_p–p–p̄ interaction. We finally study the branching ratios of the other J/ψ→γ+three mesons decay modes. We find that J/ψ→γG_p→γ(π⁺π^-η,KKπ⁰) can provide useful tests for the mechanism proposed. PACS 11.55.Hx; 12.39.Fe; 12.39.Mk; 13.25.Gv     

Higgs and Z<Superscript>′</Superscript> phenomenology in B–L extension of the standard model at LHC

The phenomenology of the low scale U(1)_B–L extension of the standard model and its implications at LHC energies is presented. In this model, an extra gauge boson corresponding to B–L gauge symmetry and an extra SM singlet scalar (heavy Higgs boson) are predicted. We show a detailed analysis of both heavy and light Higgs bosons decay and production in addition to the possible decay channels of the new gauge boson. We find that the cross sections of the SM-like Higgs production are reduced by ∼20–30%, while its decay branching ratios remain intact. The extra Higgs boson has relatively small cross sections and the branching ratios of Z^′→l⁺l^- are of order ∼20% to be compared to ∼3% of the SM results. Hence, the search for Z^′ is accessible via a clean dilepton signal at LHC.     

η and η′ Physics at MAMI

Decays of the η and the η′ represent a significant portion of the scientific program with the Crystal Ball at MAMI. This review summarizes the current status of the program and discusses some recent results, such as the slope parameter of the η → 3π⁰ Dalitz plot, the matrix element of η → π⁰γγ and the invariant mass of e ⁺ e ⁻γ from the η → e ⁺ e ⁻γ Dalitz decay. Possible future measurements are discussed.     

Confinement singularities in two-pion decays of mesons

We consider as an example the two-pion decay of the ρ meson, the ³ S ₁ q $ \bar q $ \bar q state of the constituent quarks—the decay being determined by the transition q $ \bar q $ \bar q → ππ contains information about confinement interactions. One can specify in this decay two types of transitions: (i) the bremsstrahlung radiation of a pion q → q + π (or $ \bar q $ \bar q → $ \bar q $ \bar q + π) with a subsequent fusion q $ \bar q $ \bar q → π, and (ii) the direct transition q $ \bar q $ \bar q → ππ. We demonstrate how in the amplitudes of the corresponding transitions the quark singularities have to disappear, i.e., what is the way the quark confinement at relatively short distances can be realized. We calculate and estimate the contributions of processes with bremsstrahlung radiation of the pion and of the direct transition q $ \bar q $ \bar q → ππ. The estimates demonstrate that the processes involving the direct transition q $ \bar q $ \bar q → ππ are necessary, but they cannot be determined unambiguously by the decay ρ(775) → ππ. We conclude that for the determination of the q $ \bar q $ \bar q → ππ transition more complete data on the resonance decays into the ππ channels are needed than those available at the moment.