ISSN 0253-2778

CN 34-1054/N

2016 Vol. 46, No. 6

Research Articles:Physics
γγ physics analyses at BESⅢ
REDMER C. F. (the BESⅢ collaboration)
2016, 46(6): 449-455. doi: 10.3969/j.issn.0253-2778.2016.06.001
The BESⅢ Collaboration has recently embarked on a two-photon physics program. Its main motivation is given by the large uncertainty of the contribution of hadronic light-by-light scattering to the anomalous magnetic moment of the muon αμ, and the need of electromagnetic transition form factors(TFF) as experimental input to improve the calculations. Data acquired with the BESⅢ detector at center-of-mass energies from 3.77 to 4.6 GeV allow for the determination of TFFs of light pseudoscalar mesons. The measurements are performed with a single-tag technique and result in unprecedented accuracy at momentum transfers below 2 GeV2, the region of highest importance for the calculations of αμ. Employing the same approach, the first double-tagged measurement of the pion transition form factor has been started. It is the first step towards a model independent parameterization of the TFF of the π0. Additionally, measurements of multi-meson final states have been engaged.
Hadronic contribution from light by light processes in (g-2) of muon in nonlocal quark model
2016, 46(6): 456-461. doi: 10.3969/j.issn.0253-2778.2016.06.002
The hadronic corrections to the muon anomalous magnetic moment αμ, due to the full gauge-invariant set of diagrams with dynamical quark loop and intermediate pseudoscalar and scalar states light-by-light scattering insertions, are calculated in the framework of the nonlocal chiral quark model. These diagrams correspond to all hadronic light-by-light scattering contributions to αμ in the leading order of the 1/Nc expansion in quark model. The result of the quark loop contribution is μHLbL,Loop=(11.0±0.9) ·10-10, and the total result is αμHLbL,NχQM=(16.8±1.2) ·10-10.
η transition form factor: A combined analysis of space- and time-like experimental data through rational approximants
2016, 46(6): 462-469. doi: 10.3969/j.issn.0253-2778.2016.06.003
A combined analysis of the space- and time-like experimental data for the η transition form factor is performed in a model-independent way by means of rational approximants. The recent measurement of the e+e- invariant mass spectrum of the η→e+e--γ decay provided by the A2 Collaboration allowed us to extract the most precise and up-to-date slope and curvature parameters of the form factor. The impact of this new analysis on the ηη′ mixing parameters and the VPγ couplings is also discussed.
sin2θW theory and new physics
LEE Hye-Sung
2016, 46(6): 470-475. doi: 10.3969/j.issn.0253-2778.2016.06.004
After briefly discussing the importance of the precise measurement of the weak mixing angle, the implication of the dark Z on the low-Q2 parity tests is discussed. The dark Z is a very light (roughly, MeV-GeV scale) gauge boson, which couples to the electromagnetic current as well as the weak neutral current.
Perspectives of a precise measurement of the charge asymmetry in muon pair production at Belle Ⅱ
FERBER T., SHWARTZ B. (for the Belle/BelleⅡ collaboration)
2016, 46(6): 476-480. doi: 10.3969/j.issn.0253-2778.2016.06.005
Forward-backward asymmetry in the muon pairs production in electron-positron annihilation is caused by the interference of the photon and Z-boson already at leading order. A high precise measurement of the value of this asymmetry, AFB, at the Super B-factory will provide stringent limitations on the New Physics effects. Even though AFB≈0.01 at 10 GeV center-of-mass energy of B-factory operation, a huge statistics expected at the BelleⅡ experiment will provide an opportunity to obtain high precision. This report briefly describes perspectives as well as obstacles on the way to achieve the precise results.
Measuring the weak mixing angle with the P2 experiment at MESA
2016, 46(6): 481-487. doi: 10.3969/j.issn.0253-2778.2016.06.006
The P2 experiment in Mainz aims to measure the weak mixing angle sin2θW in electron- proton scattering to a precision of 0.13%. In order to suppress uncertainties due to proton structure and contributions from box graphs, both a low average momentum transfer Q2 of 4.5×10-3GeV2/c2 and a low beam energy of 155 MeV are chosen. In order to collect the enormous statistics required for this measurement, the new Mainz energy recovery superconducting accelerator(MESA) is being constructed. These proceedings describe the motivation for the measurement, the experimental and accelerator challenges and plans to tackle them.
Dalitz decay studies at BEPCⅢ
WANG Dayong (for BEPCⅢ Collaboration)
2016, 46(6): 488-493. doi: 10.3969/j.issn.0253-2778.2016.06.007
EM Dalitz decays could be sensitive and provide very rich information about meson structures, and they play an important role in constraining the uncertainties to (g-2)μ. BEPCⅢ has performed and published several Dalitz decays,including η′→γe+e-, J/ψ→Pe+e-(P=η′,η /π0), η′→ωe+e- etc. These processes are all observed and measured for the first time, furthering our knowledge about meson structures and meson interactions. Many more related work is ongoing and there will be more results to come, which will further test the theory against the measurements and could be sensitvie to the new physics beyond the standard model.
Combined study of the η and η′ mesons: Phenomenology, chiral extrapolation of lattice QCD and effective field theory
GUO Zhihui
2016, 46(6): 494-501. doi: 10.3969/j.issn.0253-2778.2016.06.008
A comprehensive phenomenological study is carried out of the decay processes with the η or η′ in the initial/final states within the effective field theory approach. Two primary types of processes are analyzed: The ones only with lightflavor hadrons and those involving the J/ψ. The couplings from the effective Lagrangian, together with the ηη′ mixing parameters from the two-mixing-angle scheme, are fitted to a large number of experimental data, including the various decay widths and the form factors. With the phenomenological mixing parameters and the lattice simulation data of the masses and decay constants of the light pseudoscalar mesons as inputs, a next-to-next-to-leading order study is performed of the ηη′ mixing system in U(3) chiral perturbation theory. Updated values of the relevant low energy constants are obtained.
Study of e+e-→K+K- η process with the CMD-3 detector at VEPP-2000 collider
2016, 46(6): 502-506. doi: 10.3969/j.issn.0253-2778.2016.06.009
The results of the study of e+e--→Ф(1020)η process with the CMD-3 detector are reported. The analysis is based on an integrated luminosity of 22 pb-1 collected by the CMD-3 in 2011-2012. It was established that only Ф(1020)η intermediate state can be recognized at the current level of CMD-3 statistics. The cross section of e+e--→Ф(1020)η process was measured at 30 center-of-mass energy points in the range from 1.59 up to 2.0 GeV. The η meson was treated as a recoil particle and all the modes of η decay were used. A total of 1454±48 events of signal process were selected. The measured cross section was approximated according to vector meson dominance model as a sum of Ф(1680) and nonresonant amplitudes, and the preliminary results for Ф(1680) meson parameters have been obtained.
Measurement of e+e-→ηπ+π- and e+e-→ωπ+π- cross sections with the CMD-3 detector at the VEPP-2000 collider
2016, 46(6): 514-522. doi: doi:10.3969/j.issn.0253-2778.2016.06.011
The processes +e-→ηπ+π-→γγπ+π-,e+e-→ηπ+π-→π0π+π-π+π- and e+e-→ωπ+π-→π0π+π-π+π- have been studied with the CMD-3 detector at the VEPP2000 collider. For analysis data collected in the center-of-mass energy range from 1.2 to 2.0 GeV is used. Studied data corresponding to an integrated luminosity of 3×104 nb-1 were recorded in 2011 and 2012. The Born cross section of e+e-→ηπ+π- has been measured in the η→γγ channel and is in good agreement with results obtained in other experiments. There are also preliminary results for the e+e--→ηπ+π- and e+e-→ωπ+π- Born cross sections in the η→π+π-π0 and ω→π+π-π0 final states, respectively. The e+e-→ηπ+π- Born cross section data have been used to determine the τ-→ηπ-π0ντ decay branching fraction.
Study of the e+e-→VP reactions at the VEPP-2000 e+e- collider with the SND detector
BOTOV A. A.(for the SND collaboration)
2016, 46(6): 523-527. doi: 10.3969/j.issn.0253-2778.2016.06.012
The reactions e+e-→ρπ, ωπ,ρη, ωη have been studied in the energy region 1.05-2.00 GeV. The experiment has been carried out at the VEPP-2000 e+e- collider with the SND detector. The measured cross sections have been fitted in the vector meson dominance model.
Search for the decays η'→e+e- and η→e+e- at the VEPP-2000 e+e- collider
KARDAPOLTSEV L.V. (for the SND Collaboration)
2016, 46(6): 528-532. doi: 10.3969/j.issn.0253-2778.2016.06.013
A search for the rare decay η′→e+e- has been performed with the SND detector at the VEPP-2000 e+e- collider. The inverse reaction e+e-→η′ and η′ five decay chains have been used for this search. The upper limit Γη′→e+e-<0.002 eV at the 90% confidence level has been set. A sensitivity of SND in a search for η→e+e- decay has been studied. For this purpose a data sample with an integrated luminosity of 108 nb-1 collected in the centerofmass energy range 520580 MeV was analyzed. No background events for the reaction e+e-→η with decay η→π0π0π0 have been found. In the absence of background, a sensitivity to B(η→e+e-) of 10-6 can be reached during two weeks of VEPP-2000 operation.
Research Reviews: Physics
Study of the process e+e-→KK in the center-of-mass energy range 1004-1060 MeV with the CMD-3 detector at e+e- VEPP-2000 collider
2016, 46(6): 507-513. doi: 10.3969/j.issn.0253-2778.2016.06.010
The e+e-→KS0+KL0 and e+e-→K-K+ cross sections have been measured in the center-of-mass energy range 1004-1060 MeV for 25 energy points with about 2%3% systematic uncertainties. The analysis is based on 5.5 pb-1 of integrated luminosity collected with the CMD-3 detector at the VEPP-2000 e+e- collider. The measured cross section is approximated according to vector meson dominance model as a sum of Ф, ω, ρ like amplitudes and their excitations, and Ф(1020) meson parameters have been obtained.