Production and fragmentation of the D10 meson in e+e− annihilations at √s=29 GeV

Neutral D¹ meson production in e⁺e⁻ annihilation at √s=29 GeV has been studied using the high resolution spectrometer. The decay of D¹⁰ into D⁰γ, where the D⁰ decays into K^−π+, has been observed. The production cross section in units of the point cross section is 0.63±0.22 for fractional energy Z⩾0.5. The fragmentation function is compared with that of the D¹⁺ meson measured in the same experiment.     

Measurement of η' → π+π−γ in γγ collisions

The production of the pseudoscalar meson η'(958) is observed in the reaction e⁺e−→ e⁺e⁻η′→e⁺e⁻π⁺π⁻γ with the ARGUS detector at DESY. We measure the product Γ_γγ(η')Br(η′ →ϱγ) to be 1.13±0.04±0.13 keV. Using the known branching ratio s, we calculate Γ_γγ(η′) to be 3.76±0.13±0.47 keV and Γ_η' to be 203±32 keV.     

Electroproduction final states as ω → 1

The successful model of Barbour and Moorhouse for pion photoproduction is extended to electroproduction and combined with recent ideas on configuration mixing or SU(6) breaking in deep inelastic electroproduction as ω → 1. We find that two consequences obtain in the current fragmentation region as ω → 1: (a) the missing mass spectrum in ep → eπ^?X and en → eπ⁺X will be featureless (phase space); (b) in ep → eM^+,0X and en → eM^0,?X for any non-strange meson M the missing hadron system will contain only octets. Hence, in particular, ep → eπΔ will vanish relative to ep → eπ⁺n as ω → 1. For K production only Λ and no Σ final states will be produced.     

Coherent π Electroproduction on the Deuteron

A general analysis of polarization phenomena for coherent pion electroproduction on deuterons is presented. The spin and isospin structures of the γ*+d→d+P⁰ amplitudes (where γ* is a virtual photon) are established and relationships between meson electroproduction on deuterons and on nucleons are given in the framework of the impulse approximation. The reaction e+d→e+d+π⁰ is investigated in detail, for a relatively large value of momentum transfer, 0.5 GeV²≤−k²≤2 GeV², both at threshold and in the region of Δ-isobar excitation. Special attention is devoted to the sensitivity of different contributions to the exclusive cross section for d(e, eπ⁰)d, as the γ*πω form factor or the NN-potential. The predicted k²-dependence of the cross section agrees well with the available experimental data.     

Evidence for the τ → νϱπ decay mode

Annihilation of e⁺e^? into final states with a single electron has been studied with the PLUTO detector at the DORIS storage ring at CMS energies from 3.6 to 5 GeV. In the sample of 4-prong events without any detected photon we observe 21 events which we assign to the reaction e⁺e^? → τ⁺τ^? → ννe + ν?⁰π. We obtain a branching ratio for τ⁺ → ν?⁰π⁺ of 0.050 ± 0.015 with an overall systematic uncertainty of 30%. The data are consistent with the ?π coming from an A₁ meson.     

The process e + e − → ωπ0 near the Φ resonance

The reaction e ⁺ e ^?→ωπ⁰ near a Φ resonance was studied with a spherical neutral detector at VEPP-2M e ⁺ e ^? collider. Both main modes of decay of a ω meson were investigated: ω→π⁺π^?π⁰ and ω→π ⁰γ. The probability of decay Φ→ωπ⁰ was obtained from the magnitude of the interference wave in the cross section for the reaction e ⁺ e ^?→ωπ⁰→ π⁺π^?π⁰π⁰: B(Φ→ωπ⁰) = (5.5 _?1.4 ^+1.6 ±0.3) ×10^?5. The ratio of the partial widths of the ω meson was obtained from the ratio of the cross sections for the two modes: Γ(ω→π⁰γ )/Γ(ω→π⁺π^?π⁰) = 0.0994 ± 0.0036 ± 0.0038.     

On the possible existence of a vector meson ϱ' (1250)

By combining new results obtained at C.M. energies of 1.2 and 1.3 GeV with previous data obtained at lower energies from the e⁺e^? annihilation process e⁺e^? → π⁺π^?π^oπ^o, we get an indication in favour of the existence of a new vector meson of the ? type, ?' (1250), the first daughter of the ? in the predictions of the Veneziano model. Further results on the annihilation process e⁺e^? → π (1600) → π⁺π^?π⁺π^? are also presented.     

Do B meson decays exclude a light Higgs?

In the standard model with three generations a light Higgs boson will be produced in a quarter of all B meson decays. A novel calculation of the b → sH matrix element is given. Existing data on B meson decays excludes a light Higgs with mass less than 300 MeV or between 2 and 3.7 GeV. Because of theoretical uncertainties in the branching fractions for Higgs production and decay, a window between 300 MeV and 2 GeV is still allowed. Such a Higgs could be discovered at the ϒ (4S) in inclusive π⁺π⁻ or K⁺ K⁻ invariant mass plots, and in events with six kaons.     

Observation of charmless B meson decays

Using the ARGUS detector at the e⁺e⁻ storage ring DORIS II, we have observed charmless decays of B mesons into the final states pp̄π^± and pp̄π⁺π⁻. The significance of the signal corresponds to more than five standard deviations. The branching ratios are (5.2±1.4±1.9)×10⁻⁴ for the three-body and (6.0±2.0±2.2)×10⁻⁴ for the four-body final state. These decays cannot proceed via the dominant b→c transitions, and we show that they are not the result of penguin-type processes. Thus, the observed decays must represent b→u quark transitions. Consequently, the Kobayashi-Maskawa matrix element V_ub is non-zero.     

Pion pair production in photon-photon collisions at DCI

This paper presents experimental results on π⁺π⁻ production threshold from the collision of quasi-real photons. The data, obtained at the e⁺e⁻ collider DCI, are a combination of the results from the DM1 and DM2 experiments. Using the e⁺e⁻ and π⁺π⁻ production for normalization and cross-checks, we observe a pion pair yield at low invariant mass (W < 500 MeV/c²) which is approximately twice the one expected from Born terms.     

Extended sources,final state interactions and Bose-Einstein correlations

The isospin 2 ππ interaction is repulsive, thes-wave phase shift reaching ～?20° at the mass of the ρ meson. Theπ ^± π ^±correlation function shows a strong Bose-Einstein enhancement but no clear sign of this dynamical repulsion. I present the theory of final state interactions appropriate to particle production from an extended source and demonstrate that the data are entirely consistent with the known features of the ππI=2 interaction. The results are relevant to multiple pion production ine ⁺ e ^? annihilation, hadronic reactions and heavy ion collisions.     

The parton model,duality and deep-inelastic single-particle distributions

Within the quark parton model the two-component duality idea is generalized to single hadron inclusive distributions in deep-inelastic scattering. The discussion is limited to the current fragmentation region. The number of independent distribution functions is reduced. Relations as well as inequalities among the structure functions are obtained. In particular, the excess of π⁺ over π^? in the electroproduction off a proton comes out as a consequence and the differential cross section for e⁺e^? → π+ anything is fully determined by the electroproduction of pions on nucleons.     

Inclusive π 0 and KS 0 production in two-photon collisions at LEP

The reactions e⁺e⁻→e⁺e⁻ π ⁰ X and e⁺e⁻→e⁺e⁻K_S ⁰ X are studied using data collected at LEP with the L3 detector at centre-of-mass energies between 189 and 202 GeV. Inclusive differential cross sections are measured as a function of the particle transverse momentum p _t and the pseudo-rapidity. For p _t ⩽1.5 GeV, the π ⁰ and K_S ⁰ differential cross sections are described by an exponential, typical of soft hadronic processes. For p _t ⩾1.5 GeV, the cross sections show the presence of perturbative QCD processes, described by a power-law. The data are compared to Monte Carlo predictions and to NLO QCD calculations.     

The production of neutral vector mesons by bremsstrahlung in electron-positron colliding beams

We study the bremsstrahlung production of the ? meson in the reaction e⁺e^? → e⁺e^?? (→ e⁺e^?π⁺π^?). This reaction gives a C = ?1 background which complicates the study of C = +1 two-photon processes at the new colliding-beam facilities. The cross section for the reaction rises from approximately 0.3 nb to 0.7 nb as the beam energy increases from 2 GeV to 15 GeV. From a study of the distributions of the final leptons and pions, we find a suitable choice of cuts which will reduce the event rate down to a small fraction of R. It is not possible to attribute the three-prong events seen at DELCO and PLUTO to this particular production mechanism.     

Radiative Kaon decays and CP violation in chiral perturbation theory

Chiral perturbation theory is a very useful framework for testing the standard model in processes where long-distance effects are expected to play an essential rôle. We analyze the rare K decays K⁰→γℓ⁺ℓ⁻, K⁺→π⁺γ3 and K_L→π⁰ℓ⁺ℓ⁻ in the effective chiral formulation of the standard model. These processes, like the decays K⁰→γγ, K⁺→π⁺ℓ⁺ℓ⁻, K_S→π⁰ℓ⁺ℓ⁻ and K⁰→π⁰γγ discussed in previous work, have the property that the corresponding amplitudes vanish to lowest order in chiral perturbation theory. Precise predictions for decay rates and spectra are made in terms of a few coupling constants not restricted by softly broken chiral symmetry alone. Special consideration is given to various possible tests of CP noninvariance in these decays, in particular to effects due to intrinsic CP violation in the amplitudes. We find that chiral perturbation theory correlates different CP-violating observables such as the charge asymmetries in K^±→π^±γγ and K^±→π^±ℓ⁺ℓ⁻, the one-photon exchange contribution to K_L→π⁰e⁺e⁻ and the transverse polarization in K_L →π⁰μ⁺μ⁻. Detailed numerical results are shown.     

First observation of γγ → ωω

The reaction γγ → 2π⁺ 2π⁻ 2π⁰ has been studied using the ARGUS detector at the e⁺e⁻ storage ring DORIS II at DESY. Production of ω mesons is observed and, in particular, the reaction γγ → ωω is seen for the first time. The cross section for γγ → ωω has an enhancement at ∼ 1.9 GeV/c² of about 10 nb. The cross sections for γγ → 2π⁺ 2π⁻ 2π⁰ and γγ → ωπ⁺ π⁻ π⁰ are also given.     

Broadening of ρ(→ e+e−) meson produced coherently in the photonuclear reaction

The e ⁺ e ^? invariant mass distribution spectra are calculated to estimate the hadron parameters of the ρ meson produced coherently in the photonuclear reaction. The elementary reaction occurring in the nucleus is considered to proceed as γN → ρ ⁰ N; ρ ⁰ → e ⁺ e ^?. We describe the elementary ρ meson photoproduction by the experimentally determined reaction amplitude $f_{\gamma {\rm N} \to \rho ^0 {\rm N}}$ . The ρ meson propagator is presented by the eikonal form, and the ρ meson nucleus optical potential V _Oρ appearing in it is estimated using the “t?” approximation. The ρ meson nucleon scattering amplitude f _ρN extracted from the measurements is used to generate this potential. The calculated e ⁺ e ^? invariant mass distribution spectra are compared with those measured at Jefferson Laboratory. The calculated results for the transparency ratio are also presented.     

An improved upper limit on the ντ-mass from the decay π−→π−π−π−π+π+ντ

Using the ARGUS detector at the e⁺e⁻ storage ring DORIS II, we have observed the decay τ⁻→π⁻π⁻π⁻π⁺π⁺ν_τ in tau-pair events produced at center-or-mass energies between 9.4 and 10.6 GeV. From the 5π invariant mass distribution we derive an upper limit of m(ν_τ)<35 MeV/c² at the 95% confidence level. The branching ratio for this decay channel is found to be (0.064±0.023±0.01)%.     

Medium effects on meson couplings and dilepton spectra

The πωπ coupling constant is calculated in a relativistic hadronic framework incorporating nucleons and Δ(1232) isobars. Medium modifications are included. The vertex is analyzed in context of the ω → π ⁰e⁺e^? and ρ ^a → π ^a e⁺e^? Dalitz decays in nuclear matter. A sizeable increase of the widths for these decays is found for invariant mass of dileptons in the range 0.2–0.5 GeV. This increase may help, among other medium effects, to explain the problem of the low-mass dilepton enhancement seen in relativistic heavy-ion collisions.