Z.G. Zhao

Heavy quarkonium physics

This report is the result of the collaboration and research effort of the Quarkonium Working Group over the last three years. It provides a comprehensive overview of the state of the art in heavy-quarkonium theory and experiment, covering quarkonium spectroscopy, decay, and production, the determination of QCD parameters from quarkonium observables, quarkonia in media, and the effects on quarkonia of physics beyond the Standard Model. An introduction to common theoretical and experimental tools is included. Future opportunities for research in quarkonium physics are also discussed.

Determination of the πN scattering lengths from pionic hydrogen

Abstract The final results from the pionic hydrogen X-ray experiment at PSI are presented. With this experiment the strong interaction energy level shift e 1s and the total decay width Γ 1s of the 1s state of pionic hydrogen were obtained from a precise measurement of the 3p–1s X-ray line: e 1s =−7.108±0.013(stat.)±0.034(syst.) eV (attractive) and Γ 1s = 0.868±0.040(stat.)±0.038(syst.) eV. The corresponding hadronic π N s-wave scattering lengths for elastic scattering and single charge exchange are: a π − p→π − p =0.0883±0.0008 m −1 π and a π − p→π 0 n =−0.128±0.006 m −1 π . Combining the results of the pionic hydrogen and deuterium measurements, the isoscalar ( b 0 ) and isovector ( b 1 ) scattering lengths, corresponding to an isospin symmetric interaction, can be calculated: b 0 =0.0016±0.0013 m −1 π and b 1 =−0.0868±0.0014 m −1 π .

KARMEN time anomaly: search for a neutral particle of mass 33.9 MeV in pion decay

We have searched for the pion decay pi(+)-->&mgr;+X, where X is a neutral particle of mass 33.905 MeV. This process was suggested by the KARMEN Collaboration to explain an anomaly in their observed time distribution of neutrino induced reactions. Having measured the muon momentum spectrum of charged pions decaying in flight, we find no evidence for this process and place an upper limit on the branching fraction eta</=6.0x10(-10) of such a decay at a 95% confidence level.

X-ray spectroscopy of the pionic deuterium atom

Abstract The low energy X-rays of the pionic deuterium 3P-1S transition were measured using a high resolution crystal spectrometer, together with a cyclotron trap (a magnetic device to increase the pion stopping density) and a CCD (charge-coupled device) detector system. The spectrometer resolution was 0.65 eV FWHM for a measured energy of approximately 3075 eV. This energy was measured with a precision of 0.1 eV. Compared to conventional methods, the cyclotron trap allowed for a gain in stopping density of about an order of magnitude. The CCDs had excellent spatial and energy resolutions. Non-X-ray background could therefore be almost completely eliminated. The 1S strong interaction shift ϵ1S and total decay width Γ1S were determined from the position and line shape of the X-ray peak. They are ϵ 1S ( shift ) = 2.43 ± 0.10 eV ( repulsive ), Γ 1S ( width ) = 1.02 ± 0.21 eV , where the statistical and systematic errors were added linearly. The total (complex) pionic deuterium S-wave scattering length aπ−d was deduced: a π − d = −0.0259(±0.0011) + i 0.0054(±0.0011)m π −1 . From the real part of aπ−d a constraint in terms of the isoscalar and isovector πN′ scattering lengths b0 and b1 was deduced. From Im aπ−d we determined the isoscalar coupling constant for π− absorption: |g0| = (2.6 ± 0.3) 10−2mπ−2. The experiments of the pionic hydrogen and deuterium S-wave scattering lengths were analyzed within the framework of a search for i isospin symmetry violation. The data are still compatible with isospin conservation. The scattering lengths deduced from the Karlsruhe-Helsinki phase shift analysis disagree with the present results.

Experimental determination of the kinetic energy distribution of π−p atoms in liquid hydrogen

Abstract We have investigated the kinetic energy distribution of π − p atoms in liquid hydrogen by measuring the Doppler broadening of neutron time-of-flight spectra from the reaction π − p → π 0 n . The existence of “high energy” (⪢1eV) components, containing about half of the π − p atoms, is confirmed and evidence for discrete peaks with energies up to ⋍200eV is found.

Search for a neutral particle of mass 33.9 MeV in pion decay

Abstract We have measured the muon momentum distribution in charged pion decay in flight in order to search for a small branching fraction η of pion decays π + → μ + + X , in which a heavy neutral particle X with a mass of 33.9 MeV would be emitted. The upper limit of η at a confidence level of 95% is found to be 2.6×10 −8 .

A high resolution reflecting crystal spectrometer to measure 3 keV pionic hydrogen and deuterium X-rays

Abstract A reflecting crystal spectrometer consisting of three cylindrically bent quartz (110) crystals is described, it was designed to measure the 3 keV Kβ X-rays from pionic hydrogen and deuterium. Charge coupled devices (CCDs) were used as X-ray detectors. Projecting the reflexes of all three crystals on one common focus, an instrumental energy resolution below 1 eV was obtained at an energy of 2.9 keV.

Kinetic energy of π−p-atoms in liquid and gaseous hydrogen

We have measured the Doppler broadening of neutron time-of-flight spectra from the reaction π−p → π0 + n in atomic states. From the data, we infer that the kinetic energy distribution of π−p-atoms in liquid and gaseous hydrogen contains discrete ‘high-energy’ components with energies as high as 200 eV attributed to Coulomb de-excitation. In liquid hydrogen, evidence for Coulomb de-excitation transitions with Δn = 2 has been found.

Destructive interference of s and p waves in 180°C π−p elastic scattering

Abstract The differential cross section for π−p elastic scattering shows a pronounced dip at 180° and incident pion laboratory energies around 57 MeV. This is due to the cancellation of the real parts of the s- and p-wave hadronic scattering amplitudes. The first observation of this dip is reported and the potential of exploiting the destructive interference phenomenon is discussed.

Final R-value results from 2–5 GeV from BES and QCD test with R scan data

Final results of the measurement of R = σ( e + e − → hadrons )/σ( e + e − → μ + μ − ) in the energy region from 2 to 5 GeV with the upgraded Beijing Spectrometer (BESII) at the Beijing Electron Positron Collider (BEPC) are presented. Preliminary results of the inclusive momentum spectra, multiplicity distribution, and second binomial moment measured with the R scan data at 2.2, 2.6, 3.0, 3.2, 4.6 and 4.8 GeV are reported.