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1.
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 ωπ 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.  相似文献   

2.
We investigate the production of pions in heavyion collisions in the energy range of 1–2 GeV/A. The dynamics of the nucleus-nucleus collisions is described by a set of coupled transport equations of the Boltzmann-Uehling-Uhlenbeck type for baryons and mesons. Besides theN(938) and theΔ(1232) we also take into account nucleon resonances up to masses of 1.95 GeV/c2 as well asπ-,η- andρ-mesons. We study in detail the influence of the higher baryonic resonances and the 2π-production channels (NN→NNππ) on the pion spectra in comparison toπ ? data fromAr+KCl collisions at 1.8 GeV/A andπ 0-data forAu+Au at 1.0GeV/A. We, furthermore, present a detailed comparison of differential pion angular distributions with the BEVALAC data forAr+KCl at 1.8 GeV/A. The general agreement obtained indicates that the overall reaction dynamics is well described by our novel transport approach.  相似文献   

3.
A current algebra calculation of e+e-→4π± cross section is carried out using the axial current matrix element obtained from τ→π?ν. Assuming ?′→?+2π(I=0, l=0), the following results are obtained: for the A1 resonance, 1.05?mA ?1.15 GeV; for ?′, Γ?′(total)≈0.23 GeV, M?′≈1.5 GeV, Γ(?′→e+e-)≈0.4 keV×B?1(?′→?′ →?π+π).  相似文献   

4.
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 φρω = 113.5±1.3±1.7°.  相似文献   

5.
A thorough study of lepton-pair production from two-photon annihilation in p?)p collisions is presented. The differential cross section is calculated over a large range of energies (27?√s?800 GeV as a function of the dilepton mass M as well as the dilepton transverse momentum QT and the Feynman variable xF. No kinematical approximations (such as the equivalent photon approximation) are made. For QT ≈ 0 the two-photon mechanism represents an important fraction of the pp→e+e?X cross section already at ISR energies, whereas at ISABELLE energies it dramatically dominates in the interval 0?QT?1 GeV. At ISR energies these conclusions follow from a direct comparison of the two-photon contribution with pp→e+e?X data. For the ISABELLE energy range the expected O(αs) QCD contribution to pp→?+??X, corrected for soft gluon radiation to all orders (in leading bilogarithmic approximation), was taken as a reference. At larger QT and ISR energies the γγ contribution is negligible, whereas at √s = 800 GeV γγ/QCD? 10–20% almost everywhere. Furthermore, two-photon candidate events from the ISR are shown to be in reasonable agreement with theory. A decomposition of the γγ cross section into contributions from both proton vertices being elastic, inelastic and of mixed configuration is given. The results provide important clues for a future isolation of the two-photon mechanism.  相似文献   

6.
7.
The reaction γ + Be → e+e? + X was studied with a bremsstrahlung beam of 7.2 GeV maximum energy. For invariant pair masses 1.8 ? Me+e? 2.6 GeV, no evidence for narrow resonance of width smaller than 40 MeV was found. For the mass interval 2.1 ? Me+e? ? 2.6 GeV, we obtained an upper limit of σ · Be+e? < 2 × 10?35 cm2/nucleon (90% confidence level), assuming a production mechanism similar to the θ (1019) meson.  相似文献   

8.
A method is developed for discriminating between e+e?e+e? and e+e? → π+π? events in the center-of-mass (cms) energy range of 0.5 to 1.0 GeV by energy deposition in the electromagnetic calorimeter of the SND detector using a machine learning algorithm. Identification efficiencies for e+e?e+e? and e+e? → π+π? events are 99.6 and 99.8%, respectively.  相似文献   

9.
Two examples of charmed baryon production by neutrinos have been observed in BEBC filled with hydrogen. Both events fit uniquely the reaction νp → μ?pK?π+π+ and thus apparently violate the ΔS = ΔQ rule. None of the appropriate mass combinations is consistent with the mass values of the D0 and D+ mesons. However, for the mass combinations of the pK?π+ systems, values of (2.285 ± 0.005) GeV and (2.280 ± 0.003) GeV are found for event 1 and 2, respectively. These values agree with the mass of (2.285 ± 0.006) GeV for the Λc+ charmed baryon determined recently in e+e? collisions at SLAC.  相似文献   

10.
In the SKAT bubble chamber neutrino experiment production of a short-living neutral particle with mass (1.4?m?2.5) GeV/c2 and lifetime τ≈6×10?12 s has been detected. The event may be interpreted as production and decay of the heavy lepton M0μ?+e++νe with mass (1.4?mM0?2.1) GeV/c2. It might also be the production of a charmed particle D0→e++τ?+νe (D0→e++π?+νe) with mass 1.4?mD0?2.5) GeV/c2 in a non-diagonal neutral current. The probability to interpret the event as other possible processes is very low.  相似文献   

11.
A spin-parity 1+?π enhancement is observed for the 3π mass spectrum in the reaction K?p → Σ?π+π+π? where events with a small (K?Σ?) momentum transfer are selected. The mass (1040 MeV) and width (230 MeV) of this enhancement are reminiscent of the A1 bump which has mainly been observed in the diffractive-like processes. The cross section for this enhancement at 4.15 GeV/c incident K? momentum is (3.6 ± 0.5) μb.  相似文献   

12.
The invariant mass spectrum of neutral meson states from π?p interactions at 40 GeV/c incident momentum has been investigated in a high statistics experiment performed at the 70 GeV IHEP accelerator. To detect the high energy photons coming from the produced neutral states, a hodoscope spectrometer with a computer on-line was used. A clear structure on the mass spectrum of dipions produced in the reaction π?p→π°π°n is observed at 2 GeV. The decay angular distributions show in this mass region the variation with mass typical of a state with a spin J = 4. The mass of the observed meson is found to be M = (2020±30)MeV and the estimate of the full width is (180±60) MeV.  相似文献   

13.
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?πoπo, 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.  相似文献   

14.
15.
The reaction K?p → K?π?π+p has been measured at 25 and 40 GeV/c at the Serpukhov Proton Accelerator. The production cross section at 25 and 40 GeV/c as a function of momentum transfer and Kππ mass is presented, and results of the partial-wave analysis of the Kππ system yielding information about Q(1300), K1(1400) and L(1770) mesons are discussed.  相似文献   

16.
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.  相似文献   

17.
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.  相似文献   

18.
Measurements of the inclusive pion electroproduction reaction e?+p → e?+π±+X made at the Wilson Synchrotron Laboratory are reported. Data were taken at points with nominal (Q2, W) values of (0.6, 2.67), (1.2, 2.67), (2.0, 2.67) and (1.2, 2.15) in (GeV2, GeV). A more complete angular distribution was taken at (1.2, 2.67). The virtual photoproduction cross section for π+ + missing mass is studied as a function of missing mass, Feynman scaling variables, Q2 and W. The invariant cross section show a simple behavior as a function of Q2 and W. Its dependenc on P2, x′ and φ is determined.  相似文献   

19.
We present measurements from a spark chamber experiment of the differential cross-sections for pp → π?π+, K?K+ at 20 momenta in the range 0.8–2.4 GeV/c (c.m. energy 2.02 to 2.57 GeV). The c.m. angular range was ?0.95 < cosθ1 < 0.95. There are about 2000π?π+ events and about 300 K?K+ events at each momentum.  相似文献   

20.
Inequality involving σ(e+e? → nπ), n = even > 2 is derived in the 1.6 GeV region and is used to get an upper bound on the 2π branching ratio of ?′ (1600).  相似文献   

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