Feasibility studies for the measurement of time-like proton electromagnetic form factors from p¯ p→ μ+μ- at P ¯ ANDA at FAIR

This paper reports on Monte Carlo simulation results for future measurements of the moduli of time-like proton electromagnetic form factors, | GE| and | GM| , using the p¯ p→ μ+μ- reaction at P ¯ ANDA (FAIR). The electromagnetic form factors are fundamental quantities parameterizing the electric and magnetic structure of hadrons. This work estimates the statistical and total accuracy with which the form factors can be measured at P ¯ ANDA , using an analysis of simulated data within the PandaRoot software framework. The most crucial background channel is p¯ p→ π+π-, due to the very similar behavior of muons and pions in the detector. The suppression factors are evaluated for this and all other relevant background channels at different values of antiproton beam momentum. The signal/background separation is based on a multivariate analysis, using the Boosted Decision Trees method. An expected background subtraction is included in this study, based on realistic angular distributions of the background contribution. Systematic uncertainties are considered and the relative total uncertainties of the form factor measurements are presented

Study of doubly strange systems using stored antiprotons

Bound nuclear systems with two units of strangeness are still poorly known despite their importance for many strong interaction phenomena. Stored antiprotons beams in the GeV range represent an unparalleled factory for various hyperon-antihyperon pairs. Their outstanding large production probability in antiproton collisions will open the floodgates for a series of new studies of systems which contain two or even more units of strangeness at the P‾ANDA experiment at FAIR. For the first time, high resolution γ-spectroscopy of doubly strange ΛΛ-hypernuclei will be performed, thus complementing measurements of ground state decays of ΛΛ-hypernuclei at J-PARC or possible decays of particle unstable hypernuclei in heavy ion reactions. High resolution spectroscopy of multistrange Ξ−-atoms will be feasible and even the production of Ω−-atoms will be within reach. The latter might open the door to the |S|=3 world in strangeness nuclear physics, by the study of the hadronic Ω−-nucleus interaction. For the first time it will be possible to study the behavior of Ξ‾+ in nuclear systems under well controlled conditions

Measurement of the absolute branching fraction for Λc+→Λμ+νμ

We report the first measurement of the absolute branching fraction for Λc+→Λμ+νμ. This measurement is based on a sample of e+e− annihilation data produced at a center-of-mass energy s=4.6 GeV, collected with the BESIII detector at the BEPCII storage rings. The sample corresponds to an integrated luminosity of 567 pb−1. The branching fraction is determined to be B(Λc+→Λμ+νμ)=(3.49±0.46(stat)±0.27(syst))%. In addition, we calculate the ratio B(Λc+→Λμ+νμ)/B(Λc+→Λe+νe) to be 0.96±0.16(stat)±0.04(syst)

Measurements of cross section of e+e- --> ppbar pi0 at center-of-mass energies between 4.008 and 4.600 GeV

Based on e+e- annihilation data samples collected with the BESIII detector at the BEPCII collider at 13 center-of-mass energies from 4.008 to 4.600 GeV, measurements of the Born cross section of e+e- --> ppbar pi0 are performed. No significant resonant structure is observed in the measured energy dependence of the cross section. The upper limit on the Born cross section of e+e- --> Y(4260) --> ppbar pi0 at the 90% C.L. is determined to be 0.01 pb. The upper limit on the ratio of the branching fractions B(Y(4260) --> ppbar pi0 )/ B(Y(4260) --> pi+pi-Jpsi) at the 90% C.L. is determined to be 0.02%

Precise Measurement of the e+e- →π+π-J /ψ Cross Section at Center-of-Mass Energies from 3.77 to 4.60 GeV

The cross section for the process e+e-→π+π-J/ψ is measured precisely at center-of-mass energies from 3.77 to 4.60 GeV using 9 fb-1 of data collected with the BESIII detector operating at the BEPCII storage ring. Two resonant structures are observed in a fit to the cross section. The first resonance has a mass of (4222.0±3.1±1.4) MeV/c2 and a width of (44.1±4.3±2.0) MeV, while the second one has a mass of (4320.0±10.4±7.0) MeV/c2 and a width of (101.4-19.7+25.3±10.2) MeV, where the first errors are statistical and second ones are systematic. The first resonance agrees with the Y(4260) resonance reported by previous experiments. The precision of its resonant parameters is improved significantly. The second resonance is observed in e+e-→π+π-J/ψ for the first time. The statistical significance of this resonance is estimated to be larger than 7.6σ. The mass and width of the second resonance agree with the Y(4360) resonance reported by the BABAR and Belle experiments within errors. Finally, the Y(4008) resonance previously observed by the Belle experiment is not confirmed in the description of the BESIII data

Measurement of cross sections of the interactions e + e − → ϕϕω and e + e − → ϕϕϕ at center-of-mass energies from 4.008 to 4.600 GeV

Using data samples collected with the BESIII detector at the BEPCII collider at six center-of-mass energies between 4.008 and 4.600 GeV, we observe the processes e+e− → φφω and e+e− → φφφ. The Born cross sections are measured and the ratio of the cross sections σ (e+e− → φφω)/σ (e+e− → φφφ) is estimated to be 1.75±0.22±0.19 averaged over six energy points, where the first uncertainty is statistical and the second is systematic. The results represent first measurements of these interactions

Study of J/ψ and ψ(3686) → Σ(1385)0Σ¯(1385)0 and Ξ0Ξ¯0

We study the decays of J/ψ and ψ(3686) to the final states Σ(1385)0Σ¯(1385)0 and Ξ0Ξ¯0 based on a single baryon tag method using data samples of (1310.6±7.0)×106 J/ψ and (447.9±2.9)×106 ψ(3686) events collected with the BESIII detector at the BEPCII collider. The decays to Σ(1385)0Σ¯(1385)0 are observed for the first time. The measured branching fractions of J/ψ and ψ(3686)→Ξ0Ξ¯0 are in good agreement with, and much more precise, than the previously published results. The angular parameters for these decays are also measured for the first time. The measured angular decay parameter for J/ψ→Σ(1385)0Σ¯(1385)0, α=−0.64±0.03±0.10, is found to be negative, different to the other decay processes in this measurement. In addition, the "12\% rule" and isospin symmetry in the J/ψ and ψ(3686)→ΞΞ¯ and Σ(1385)Σ¯(1385) systems are tested

Evidence of Two Resonant Structures in e+e- →π+π-hc

The cross sections of e+e-→π+π-hc at center-of-mass energies from 3.896 to 4.600 GeV are measured using data samples collected with the BESIII detector operating at the Beijing Electron Positron Collider. The cross sections are found to be of the same order of magnitude as those of e+e-→π+π-J/ψ and e+e-→π+π-ψ(2S), but the line shape is inconsistent with the Y states observed in the latter two modes. Two structures are observed in the e+e-→π+π-hc cross sections around 4.22 and 4.39 GeV/c2, which we call Y(4220) and Y(4390), respectively. A fit with a coherent sum of two Breit-Wigner functions results in a mass of (4218.4-4.5+5.5±0.9) MeV/c2 and a width of (66.0-8.3+12.3±0.4) MeV for the Y(4220), and a mass of (4391.5-6.8+6.3±1.0) MeV/c2 and a width of (139.5-20.6+16.2±0.6) MeV for the Y(4390), where the first uncertainties are statistical and the second ones systematic. The statistical significance of Y(4220) and Y(4390) is 10σ over one structure assumption

Evidence for the singly Cabibbo suppressed decay Lambda_c+ --> p eta and search for Lambda_c+ --> p pi0

We study the singly Cabibbo suppressed decays Lambda_c+ --> p eta and Lambda_c+ → pπ0 using Lambda+Lambda_c- pairs produced by e+e- collisions at a center-of-mass energy of sqrt(s) =4.6 GeV. The data sample was collected by the BESIII detector at the BEPCII collider and corresponds to an integrated luminosity of 567 pb-1. We find the first evidence for the decay Lambda_c+ --> p eta with a statistical significance of 4.2 sigma and measure its branching fraction to be B( Lambda_c+ --> p eta)=(1.24±0.28(stat)±0.10(syst))10-3. No significant Lambda_c+ --> p pi0 signal is observed. We set an upper limit on its branching fraction B( Lambda_c+ --> p pi0)<2.7x10-4 at the 90% confidence level

Observation of an Anomalous Line Shape of the η′π+π- Mass Spectrum near the p p Mass Threshold in J /ψ →γη′π+π-

Using 1.09×109 J/ψ events collected by the BESIII experiment in 2012, we study the J/ψ→γη′π+π- process and observe a significant abrupt change in the slope of the η′π+π- invariant mass distribution at the proton-antiproton (pp) mass threshold. We use two models to characterize the η′π+π- line shape around 1.85 GeV/c2: one that explicitly incorporates the opening of a decay threshold in the mass spectrum (Flatté formula), and another that is the coherent sum of two resonant amplitudes. Both fits show almost equally good agreement with data, and suggest the existence of either a broad state around 1.85 GeV/c2 with strong couplings to the pp final states or a narrow state just below the pp mass threshold. Although we cannot distinguish between the fits, either one supports the existence of a pp moleculelike state or bound state with greater than 7σ significance