Pions in primary cosmic rays of ultrahigh energies

In the framework of the Coleman-Glashow hypothesis of an extremely weak violation of Lorentz invariance, neutral and charged pions can be stable for energies above 10¹⁹ eV and enter into the composition of primary cosmic rays of ultrahigh energies. The kinematic exclusion of reactions of pions with relic photons is particularly important, because it allows the Greisen-Zatsepin-Kuzmin paradox to be resolved. The parameters of extensive air showers induced by primary pions calculated within the model of quark-gluon strings with allowance for the Landau-Pomeranchuk-Migdal effect and interactions of neutral pions of ultrahigh energies are not contradictory to the available data of observations. It has been shown that observations of production heights of muons with energies above 10GeV will make it possible to distinguish between primary nuclei, protons, and pions; to verify Lorentz invariance for energies above 10²⁰ eV; and to obtain a new limit on the difference between the maximum possible velocities of muons and pions (c_µ?c_π)<4×10^?26.     

Isospin symmetry violating effects and scattering length extraction from kaon decays

The isospin symmetry breaking effects in the charged kaons decays to two or three pions are considered. In semileptonic decay K ^± → π ⁺ π ^?e^± ν (called K _e4) these effects turn out to be crucial for correct extraction of ππ scattering lengths. Taking in account electromagnetic interaction between the pions in the final state and isospin symmetry breaking due to different masses of charged and neutral pions allows to adjust the values of scattering lengths obtained from experimental data on K _e4 decay and predictions of Chiral Perturbation Theory (ChPT). Final state interactions of pions in the decay K ^± → π ^± π ⁰ π ⁰ leading to the anomaly (cusp) in the π ⁰ π ⁰ invariant mass distribution in the vicinity of charged pions’ threshold are discussed and recent results of accounting of the electromagnetic interaction among charged pions leading to π ⁺ π ^? bound states (pioniumatom) just under the charged pions’ threshold are presented.     

Leading role pseudovector and vector mesons in quasielastic pion electroproduction on a nucleon by longitudinal photons at comparatively high virtual momenta in the range 0.4 ≤ |k| ≤ 1 GeV/c

It is shown that, under kinematical conditions of quasielastic knockout, the differential cross section for the exclusive process p(e, e′π ⁺)n induced by longitudinal virtual photons γ* _L changes its physical nature as the recoil momentum |k| of the spectator nucleon grows from values in the range |k| ～ 0.1–0.4 GeV/c, which are typical in studying the pion cloud of the nucleon, to values around |k| ～ 1 GeV/c. Calculations within the ³ P ₀ quark microscopic model reveal that the amplitudes for the deexcitation of vector and pseudovector mesons via the processes ρ ⁺ + γ* _L → π ⁺ and b ₁ ⁺ + γ* _L → π ⁺ become dominant here. Concurrently, the effective momentum distributions develop a substantial angular anisotropy of about 50% with respect to the angle between the momentum of the photon γ* _L and the recoil momentum of the spectator nucleon, the longitudinal differential cross sections undergoing respective changes. In this region, the energy of knock-on pions is expected to be about 5 GeV under conditions of quasilastic kinematics.     

Impact parameter analysis of the reaction π−p→pπ+π−π− at 9 and 12 GeV/c with very fast secondary proton

The results on the impact parameter for the reactions π^?p→pπ⁺π^?π^?, pK⁺K^?π^?, with a fast secondary forward proton are given at 9 and 12 GeV/c. For the reaction π^?p→pπ⁺π^?π^? our results corresponding to the small |u| region (|u| < 1 GeV²) are compared to the existing data for the small |t| region. We show that small |u| events are more central than small |t| events.     

Measurement of np elastic scattering at high energies and very small momentum transfers

The np elastic differential cross section has been measured for incident neutron momenta 100–400 GeV/c in the |t| range 6 · 10^?6 ? 5 · 10^?1 (GeV/c)². The np data of this experiment provide a first direct measurement of the hadronic amplitude for |t| < 10^?2 (GeV/c)², which is consistent with the extrapolations from higher |t| values. Our data for |t| < 10^?4 (GeV/c)² are consistent with a rise which can be attributed to Schwinger scattering, caused by the interaction of the neutron magnetic moment with the proton.     

Measurement of the KS0 → π+π−γ decay rate

From a study of 70 000 K° decays in the CERN 2m hydrogen bubble chamber, we have estimated the rate for K_S⁰ → π⁺π^?γ for different cuts in the γ momentum. The results are in good agreement with inner bremsstrahlung on K_S⁰ → π⁺π^?. For γ momentum in the K° c.m. greater than 50 MeV/c, we obtain the branching ratio: Γ(K_S → π⁺π^?γ, k>50 MeV/c)/Γ(K_S→π⁺π^?)=(2.8. ± 0.6)×10^?3. This leads to an estimate for the direct γ emission process: Γ(K_S→π⁺π^?γ direct)/Γ(K_S → π⁺π^?)=(0.3 ± 0.6) × 10^?3.     

Photoproduction of ϱ0 and ω on hydrogen at photon energies of 20 to 70 GeV

We present results on photoproduction of ?⁰ and ω in the reactions γp→π⁺π^?p and γp→π⁺π^?π⁰p by tagged photons in the energy ranges 20 to 70 GeV and 20 to 45 GeV, respectively. The production of the ?⁰ shows dominantly the characteristics of a diffractive process with respect to the E_γ and t dependence of the cross section and the spin density matrix. The ?⁰ photoproduction yields on average over the photon energy range a total cross section of σ(γp→?⁰p) = 9.4±0.1 μb with an additional systematic error of ±1 μb, and average slope parameters of the t distribution dσ/dt≈exp(?b|t| + ct²), of b=9.1±0.1 GeV^?2 and c = 3.1 ±±0.2 GeV^?4. The shape of the ?⁰ peak in the π⁺π^? invariant spectra shows a skewing similar to that observed at lower energies. The photoproduction of ω is also consistent with a diffractive process and has a cross section of σ(γp→ωp) = 1.2± 0.1 μb with an additional systematic error of ±0.2 μb. The average slope parameters of the t distribution are b=8.3 ± 1.3 GeV^?2 and c = 3.4±2.6 GeV^?4.     

On new predictions of the vector-meson dominance model for the reactions πN → ϱ0N and γN → πN

It is shown that the assumption of pole vector-meson dominance for the Ball invariant amplitude having a π-meson pole is sufficient for reproducing the longitudinal amplitude of ?⁰-production by the photoproduction amplitude at |t| < 0.1 (GeV/c)². The strong k²-dependence of the Ball invariant amplitude without the π-meson pole has been found by the deviations of theoretical predictions obtained in the VDM framework for the longitudinal ?⁰-production amplitude from the experimental data in the region |t| > 0.1 (GeV/c)². A natural explanation of the k²-dependence in the one-pion exchange model with absorption is given. The problem of A₂-exchange is discussed.     

169Tm NMR study of the high-Tc superconductor TmBa2Cu3O6.92

The ground state of Tm³⁺ ion (4t²,³H₆) in a crystal electric field (CEF) of TmBaCuO is a singlet, separated from the lowest excited states by an energy gap of 100÷130 cm^?1. The pulsed NMR of¹⁶⁹Tm nuclei (spinI=1/2) in a magnetically-oriented TmBa₂Cu₃O_6.92 power is studied at temperatures 1.5÷4.2 K. Two sorts of Tm nuclei are observed: rapidly relaxing Tm1 (≥80% of total amount, having relaxation timesT ₁ ⁽¹⁾ ≈35 ms,T ₂ ⁽¹⁾ =80÷250 μs) and slowly relaxing Tm2 nuclei (≦20%,T ₁ ⁽²⁾ =300÷1000 μs). The¹⁶⁹Tm NMR spectra are described by the spin-Hamiltonian ?₁ = ??Σγ _i H _i I _i (i = x,y,z), where |γ _x ⁽¹⁾/2π| = 5.3(1), |γ _y ⁽¹⁾/2π| = 6.6(1), |γ _x ⁽²⁾/2π| = |γ _y ⁽²⁾/2π| = 5.1(1), |γ _z ⁽¹⁾/2π| = |γ _z ⁽²⁾/2π| = 2.3(1) kHz/Oe, and axesx, y, z coincide with the crystal axesa, b, c. The Tm1 and Tm2 nuclei are identified as those belonging to orthorhombic (CEF of D_2h symmetry) and tetragonal (D_4h) phases of the TmBaCuO compound, respectively. Two-exponential and nearly temperature-independent nuclear relaxation is observed at temperatures 1.5÷4.2 K. The smallestT ₂ ⁽¹⁾ -values (≈80 μs) found for the orthorhombic (superconducting) phase in an external field H perpendicular to the crystalc-axis agree fairly well with those one could expect due to the dipole-dipole interaction of thulium nuclei. However, the slowing down of this rapid relaxation of the nuclear transverse magnetization in a field H‖c as well as the origin of unusually largeT ₂ ⁽²⁾ -values remain unclear.     

Investigation of the inclusive spectra of secondary protons and pions of low kinetic energies and of their wide pair correlations in π − A interactions at 43 GeV/c

The inclusive invariant cross sections for protons produced at angles of θ=90° and 60° and for positively and negatively charged pions produced at an angle of θ=90° are presented for π ^?Be, π ^?Al, and π ^?Cu interactions induced by 43-GeV/c incident π ^? mesons. The shape of the inclusive spectra of secondary hadrons, the A dependence of their cross sections, and the correlation functions for pairs of likely charged secondary hadrons at large angles of their divergence are studied. The kinematical region explored in the present article corresponded to kinetic energies of T≈0.16–0.70 GeV and T≈0.20–0.76 GeV for secondary protons and secondary pions, respectively. The angles of divergence of hadrons forming a pair, ψ, satisfied the condition cos ψ相似文献   

Soft π−p and pp elastic scattering in the energy range 30 to 345 GeV

Differential cross sections for π^?p and pp elastic scattering have been measured at incident momenta ranging from 30 to 345 GeV and in the t range 0.002 (GeV/c)² ? |t| ? 0.04 (GeV/c)². From the analysis of the data, the ratio ? (t = 0) of the real to the imaginary parts of the forward scattering amplitude was determined together with the logarithmic slope b of the diffraction cone.The results on the real parts confirm the validity of the forward dispersion relations at high energies. Using the dispersion relations, it was shown that the experimental data on ?_{π?p(t = 0)} require a continuous rise of the total πp cross sections, at least up to the energy of 2000 GeV, thus revealing a close similarity in high-energy behaviour of πp and pp interactions.The results on the slope parameters from this experiment together with the analysis of the available world data demonstrate that the existing experimental data are consistent with the hypothesis of a universal shrinkage of the hadronic diffraction cone at high energies. The value of the asymptotic shrinkage parameter α_p^′ was found to be independent of the kind of the incident hadron and of the momentum transfer in the t range |t| ? 0.2 (GeV/c)² : 2α_{p^′ = ± 3 (GeV/c)^?2}.     

Photoproduction of charged pions on deuterium in the first resonance region

Photoproduction of π⁺ and π^? on deuterium has been measured in the photon energy range from 240 to 400 MeV and for pion c.m. angles between 15° and 180°. The pions were analysed in angle and momentum by a magnetic spectrometer. From the measured π^?/π⁺ ratio, corrected for Coulomb interactions in the final state, differential cross sections of the reaction γ+n→π^?+p were calculated. Together with the π⁺ photoproduction our data show no isotensor contribution. Comparison of our data with the recent experiments done on the inverse reaction shows no evidence of a violation of time reversal invariance. With the measured π⁺ photoproduction on deuterium, a test of the spectator model has been made. Using the closure-approximation of Chew and Lewis our data agree within a range of ±10%.     

Models for the weak parity-violating vector meson exchange potential and the circular polarization in n + p → d + γ

We propose three models which lead to a p.v. nucleon-nucleon interaction mediated by charged and neutral vector mesons. Besides the ad hoc hypothesis of octet dominance, we consider two quark models (the Bose quark and colour Fermi quark model), which give a dynamical explanation of the $\text{ΔI =}\text{1}\text{2}$ rule in strangeness changing hyperon decays. They lead to p.v. potentials with different isospin dependence. We also derive the weak NN?_ρ° coupling from a SU(2) × U(1) gauge theory with neutral currents. The circular polarization P_γ of the γ-quanta in the capture of thermal neutrons on protons is calculated for these different models. The Reid hard-core (HC) and soft-core (SC) potentials have been chosen to take into account the strong interaction. Then the naively factorized Cabbibo current-current interaction with charged rho exchange only gives P_γ^HC = ?2.7 × 10^?8 and P_γ^SC = ?2.1 × 10^?8. The strong octet dominance and Bose quark model lead to a vanishing circular polarization |P_γ| ≈ (1–4) × 10^?9. The colour Fermi quark model enhances the circular polarization and gives P_γ ≈ ?5 × 10^?8.     

Momentum analysis of kaon and pion pairs produced from time-like photons at 1.6 GeV energy

We present results for the total cross section of e⁺e^? annihilation into two hadrons at 1.6 GeV: σ_ππ = σ_KK = (1.8 ± 1.1) × 10^-33 cm².From these values we obtain the time-like electromagnetic form factors these mesons: |F_π|² = 0.24 ± 0.14 and |F_K|² = 0.46 ± 0.26.     

Measurement of high momentum transfer π−p → π0n at 5.9 GeV/c

The differential cross section for π^?p → π⁰n has been measured in the t range 1.8 ? |t| ? 8.2 (GeV/c)² by a counter-spark chamber experiment detecting the neutron and both π⁰ decay photons. A broad minimum was found, centered at |t| = 5.2 (GeV/c)².     

Searches for rare and forbidden neutral decays of η mesons at the GAMS-4π facility

Searches for rare and forbidden neutral decays of η mesons are performed at the GAMS-4π facility. A charge-exchange reaction at an initial momentum of 32.5 GeV/c was used as a source of 3.7 × 10⁶ η mesons. The following upper limits are established at a 90% confidence level: Br(η → 3γ) < 1.6 × 10^?4, Br(η → 4γ) < 2.8 × 10^?4, Br(η → π⁰π⁰) < 3.5 × 10^?4, Br(η → π⁰π⁰γ) < 1.7 × 10^?3, Br(η → π⁰π⁰γγ) < 4.0 × 10^?3, Br(η → 3π⁰γ) < 2.4 × 10^?4, and Br(η → 4π⁰) < 2.0 × 10^?5.     

Study of π−π− scattering in π−n interactions at high energies

We have measured the inclusive cross section for the reaction π^?n→pX^?? at 21, 205 and 360 GeV/c in the kinematic region |t_np|<1.0 (GeV/c)². The differential cross sections can be described qualitatively by the reggeized one-pion-exchange amplitude at all momenta studied; however, there is evidence for the contribution of other amplitudes. The extracted π^?π^? total cross section for 4?s_ππ?340 GeV² is of the order of 10–12 mb. The average number of charged particles and dispersion of the charged particle multiplicity distributions for various s_ππ intervals are compatible with results observed in hadron-proton interactions.     

η → 3π decay and πη scattering in chiral SU(2) × SU(2) symmetry

The problem of η → 3π decay in chiral SU(2) × SU(2) symmetry is investigated. We argue that the well-known difficulties of the conventional pole model originate from underestimating the role of the η-meson pole graph. This model is shown to be suitable for describing η → 3π decay if the strong ηηππ vertex in the η-pole graph is determined by an SU(2) × SU(2)-invariant interaction. The π^o ? η transition constant g_πη is evaluated from U-spin invariance with the help of a current-mixing model: | g_πη | ≈ 0.05. It turns out that the experimental data on both the decay widths and the energy spectrum can be satisfied by fitting the value of the ηηππ coupling constant as a parameter. As a result, predictions for the s- and p-wave πη scattering lengths are obtained: |a_πη⁰| ≈ 0.2m_π^?1, |a_πη¹| ≈ 0.1m_π^?3.     

A determination of the CKM-matrix element ratio |V ts |/|V cb | from the rareB-decaysB→K *+γ andB→X s +γ

Implications of the recent CLEO observation [1] of the rare decay modeB→K ^*+γ having a combined branching ratio BR(B→K ^*+γ)=(4.5±1.5±0.9)×10^?5 and an improved upper limit on the inclusive branching ratio BR(B→X _s +γ)<5.4×10^?4 (95% C.L.) [2] are discussed in the context of the Standard Model (SM). Using the unitarity of the CKM-matrix and taking into account QCD radiative corrections in the decay rate and the inclusive photon energy spectrum we obtain an improved upper limit on the inclusive branching ratio BR(B→X _s +γ)<4.8×10^?4 (95% C.L.). This can be used to constrain possible non-SM contributions to the inclusive branching ratio, giving BR(B→X _s +γ) (non-SM)<3.0×10^?4 form _t >-108 GeV. Within the SM, we show that the resulting experimental upper limit can be interpreted as a corresponding limit on the CKM-matrix element ratio yielding |V _ts |/|V _cb |<1.67, with the top quark mass assumed to weigh less than 200 GeV. We calculate the relative exclusive to inclusive branching ratioR(K ^*/X _s )≡Γ(B→K ^*+γ)/Γ(B→X _s +γ), based on the inclusive hadronic invariant mass distribution inB→X _s +γ. Estimating theK ^*-contribution from this distribution in the threshold region (m _K +m _π)≤m _{X s} ≤0.97 GeV and using experimental measurements from the semileptonicD-decaysD→K+π+?v _? in the same mass interval, we obtainR(K ^*/X _s )=0.13±0.03. This enables us to put a lower bound on the ratio |V _ts |/|V _cb | from the 95% C.L. lower limit on the branching ratio BR(B→K ^*+γ)>1.6×10^?5 [1,2]. Combining the exclusive and inclusive decay rates, we determine 0.50≤|V _ts |/|V _cb |≤1.67 (at 95% C.L.).     

The OZI rule violating radiative decays of the heavy pseudoscalars

In lowest order of QCD, the rates for radiative transitions violating the OZI rule of heavy pseudoscalars are found to be extremely small. The η_c→ γ + ?, ω, ? width is predicted two orders of magnitude below present experimental bounds on X(2.8) → γ+π⁺π^?. As a by product, we get for the 4_γ parapositronium decay width Γ=m_eα⁷ · 1.352 X 10^?2.