Study of the process e + e − → π+π− in the energy region 400 < \sqrt s < 1000 MeV^\P

The cross section of the process e ⁺ e ^? → π⁺π^? was measured in the spherical neutral detector experiment at the VEPP-2M collider in the energy region $400 < \sqrt s < 1000 MeV$ . This measurement was based on about 12.4 × 10⁶ selected collinear events, which include 7.4 × 10⁶ e ⁺ e ^? → e ⁺ e ^?, 4.5 × 10⁶ e ⁺ e ^? → π⁺π^?, and 0.5 × 10⁶ e ⁺ e ^? → μ⁺μ^? selected events. The systematic uncertainty of cross section determination is 1.3%. The ρ-meson parameters were determined as m _ρ = 774.9±0.4±0.5 MeV, Γ _ρ = 146.5±0.8±1.5 MeV, and σ(ρ → π⁺π^?) = 1220 ± 7 ± 16 nb and the parameters of the G-parity suppressed decay ω → π⁺π^? as σ(ω → π⁺π^?) = 29.9 ± 1.4 ± 1.0 nb and φ_ρω = 113.5±1.3±1.7°.     

Fourth-generation signatures in D0-;D̄0 mixing and rare D decays

We study the effects of a fourth generation on D⁰-D̄⁰ mixing and the rare decays D⁰→μ⁺μ⁻, D→ℓ⁺ℓ ⁻X (where X is nonstrange), D→πℓ⁺ℓ⁻ and D→ϱγ (orωγ). D⁰-D̄⁰ mixing can be as large as the experimental limit of 0.5%. The fourth generation can enhance the branching ratios for the rare decays by several orders of magnitude: it can be as large as 10⁻⁹ for the decays D⁰→μ⁺μ⁻, D→ℓ⁺ℓ⁻X and D→πℓ⁺ℓ⁻. The long-distance contribution to the radiative decays is estimated to be ∼ 10⁻⁸ and is shown to be dominant in the three-generation model. The fourth generation can bring this branching ratio up to ∼ 10⁻⁶.     

Search for rare φ decays in π+π−γ final state

A search for φ radiative decays has been performed using a data sample of about 2.0 million φ decays collected by the CMD-2 detector at VEPP-2M collider in Novosibirsk. From the selected e⁺e⁻→π⁺π⁻γ events the following results were obtained:B(φ→f₀(980)γ)<1×10⁻⁴ for destructive andB(φ→f₀(980)γ)<7×10⁻⁴ for constructive interference with the Bremsstrahlung process respectively,B(φ→γ→π⁺π⁻γ)<3×10⁻⁵ for E_γ>20 MeV,B(φ→ργ)<7×10⁻⁴. From the selected e⁺e⁻→μ⁺μ⁻γ eventsB(φ→μ⁺μ⁻γ)=(2.3±1.0)×10⁻⁵ has been obtained for E_γ>20 MeV. The upper limit on the P,CP-violating decay η→π⁺π⁻ has also been placed:B(η→π⁺π⁻)<9×10⁻⁴. All upper limits are at 90% C.L.     

The evaluation of the branching ratios of the decays φ(1020)→2π<Superscript>+</Superscript>2π<Superscript>−</Superscript>π<Superscript>0</Superscript>, π<Superscript>+</Superscript>π<Superscript>−</Superscript>3π<Superscript>0</Superscript>

Combining the Okubo-Zweig-Iizuka rule in the decay φ→ρπ→π⁺π^?π⁰ with the ρ→4π decay amplitudes, we calculate the φ→2π⁺2π^?π⁰ and φ→π⁺π^?3π⁰ ones. The partial widths of the above φ decays are evaluated, and the excitation curves in e⁺e^? annihilation are obtained, assuming reasonable particular relations among the parameters characterizing the anomalous terms of the HLS Lagrangian. The evaluated branching ratios B_φ→π⁺_π^?_3π⁰ ≈ 2 × 10^?7 and B_φ→2π⁺_2π^?_π⁰ ≈ 7 × 10^?7 are such that, with the luminosity L=500 pb^?1 attained at DAΦNE φ factory, one may already possess about 1685 events of the decays φ→5π.     

The e + e −→π0π0γ process below 1.0 GeV

The e ⁺ e ^?→π⁰π⁰ γreaction was studied in the energy range 0.36–0.97 GeV on a VEPP-2M e ⁺ e ^? collider with an SND detector. The decay probabilities were found to be B(ω→ π⁰π⁰γ)= (7.8 ±2.7±2.0)×10^?5 and B(π→ π⁰π⁰γ)= (4.8 _?1.8 ^+3.4 ±0.2)×10^?5.     

Observation of the π0 → e+e− decay

We have observed for the first time the very rare π⁰ → e⁺e^? decay. The tagged π⁰'s come from K⁺ → π⁺π⁰ decays in flight. We find the branching ratio to be (2.23_?1.1^+2.4) × 10^?7 (90% C.L.). This value is well above the QED unitary limit and comes in rough agreement with a second-order electromagnetic process mediated by vector mesons.     

Study of some rare decays of the K+ meson

In a search for very rare decays of the K⁺ meson we observed the K⁺→μ⁺νe⁺e^? and K⁺→e⁺ν e⁺e^? modes with branching ratios of (10±3) ×10^?7 and (2⁺²_?2)×10^?7 respectively. We also put new limits on a decay allowed by the alternate scheme of lepton conservation K⁺→π^?μ⁺e⁺, on a decay with double charged weak current K⁺→π^? e⁺e⁺ and on other forbidden decays K⁺→μ^?νe⁺e⁺ and K⁺→π⁺μ^±e^±.     

Effects of additional Higgses in supersymmetric grand unified theories

We consider modifications of the minimal supersymmetric SU(5) grand unified model. With the additional Higgses 10 +$\text{10}$, the theory may be modified so that the proton decays dominantly via the conventional mode p→e⁺π with the lifetime π_p=(0.04-710)×10³⁰yr, and sin²θ_W=0.222±0.003.     

Limits on the couplings of ϱ′ (1300 Me) and rhov;″ (1700 MeV) to the photon and to the (ππ) system

The analysis of the reaction e⁺e^?→π⁺π^? measured at the e⁺e^? colliding beam machine ADONE shows that, if ?′ and ?″ exist, the cross sections compare as follows (taking the ? as the reference point): $\text{σ(}\text{e}{}^{\mathrm{+}}\text{e}{}^{\mathrm{?}}\text{→ ? → π}{}^{\mathrm{+}}\text{π}{}^{\mathrm{?}}\text{)}$: σ(e⁺e^? → ?′ → π⁺π^?): σ(e⁺e^? → ?″ → π⁺π^?) = 1: (7 ± 4) × 10^?3: (1 ± 5) × 10^?4. The square of the product of their couplings to the photon (γ_?) and the γγ system (g_?ππ) are derived.     

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.     

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.     

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.     

Can B→ppπ(π) be misidentified b→c transitions?

Recently reported decays of the B→pp̄π and pp̄ππ have been interpreted as evidence for the b→u transition. It is shown that a certain class of b→c transitions could be misidentified and interpreted as B→pp̄π(π).     

Measurement of the relative probability of the decay φ → η′γ in the channel η′ → ηπ0π0, η → γγ

The process e ⁺ e ^? → η′γ, η′ → η π⁰π⁰, η → γγ is investigated by an SND detector in experiment on a VEPP-2M equipment with colliding electron-positron beams. The analysis of experimental data yields a value of B(? → η′γ)=(6.7 _?4.2 ^+5.0 ±1.5)×10^?5 for the decay probability. Taking into account a previous measurement of this quantity by an SND detector in the decay channel η′ → π⁺π^?η, one finally obtains B(?→η ′γ)=(6.7 _?2.4 ^+2.8 ±0.8)×10^?5.     

Limits for short-lived neutral particles emitted in μ+ or π+ decay

In the data taken for the μ⁺ → e⁺e⁺e^- and π⁺ → e⁺νe⁺e^- experiments a search was made for neutral particles φ produced in the decay μ⁺ → e⁺φ or π⁺ → e⁺νφ with subsequent decay φ → e⁺e^-. No candidates were found, Upper limits for the branching ratios are given as a function of their masses and lifetimes. For lifetimes below 10⁻¹⁰ s limits down to 2 × 10⁻¹² are obtained.     

Study of conversion decays φ→ηe+e− and η→γe+e− in the experiment with SND detector at VEPP-2M collider

The conversion decays φ→ηe⁺e⁻ and η→γe⁺e⁻ were studied by the SND detector using φ-meson production in e⁺e⁻-annihilation at VEPP-2M collider. The branching ratios of these decays were measured: Br(φ→ηe⁺e⁻)=(1.19±0.19±0.07)×10⁻⁴ and Br(η→γe⁺e⁻)=(5.15±0.62±0.39)×10⁻³. The e⁺e⁻-pair mass spectra and transition form factors were also studied.     

Evidence for exclusive ηc production in γγ interactions

We observe for evidence for γγ → η_c production in the reaction e⁺e⁻ → e⁺e⁻K_S⁰K^±π^∓. The product Γ_γγ(η_c)·B(η_c → K_S⁰K^±π^∓) is measured to be 0.5_−0.15^+0.2±0.1 keV.     

Measurement of the slope of the π0-electromagnetic form factor

Using a magnetic spectrometer, we have studied 30 000 π⁰ → e⁺e^?γ events coming from K_π2⁺ decays in flight. We find a positive value for the slope of the neutral-pion electromagnetic form factor: a = +0.10 ± 0.03 (or a = +0.05 ± 0.03 without radiative corrections). This value is higher than the predictions of vector meson models.     

Isospin violation in the standard model and the decay F+ → π+π0

The decay F⁺ → π⁺π⁰ can proceed via isospin violating mechanisms similarly to $\text{ψ′ →}\text{J}\text{ψπ}{}^0$. I estimate that π⁰-η-′ mixing together with symmetry breaking in the amplitude lead to Γ(F⁺ → π⁺π⁰)/Γ(F⁺ → π⁺η) = (2.2 ± 0.7) × 10^?3 or (1.0 ± 0.3) × 10^?3, if annihilation processes dominate nonleptonic charmed meson decays. These values correspond to different assumptions for two-meson production in F decays.     

Determination of the radiative widths of the η′ and A2 from two-photon exchange production

From the two-photon exchange processes e⁺e^? → e⁺e^?η' (958) → e⁺e^??⁰γ and e⁺e^? → e⁺e^?A₂ (1310) → e⁺e^?π⁺π^?π⁰ observed using the CELLO detector at PETRA the radiative widths of the η' and A₂ have been determined with the results: Γ_γγ(η') = 5.4 ± 1.0[stat.] ± 0.7[syst.] keV; Γ_γγ(A₂) = 0.59 ± 0.14[stat.] _?0.08^+0.31[syst.] keV.