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Using data samples collected with the BESIII detector at the BEPCII collider, we measure the Born cross section of e þ e − → pp at 12 center-of-mass energies from 2232.4 to 3671.0 MeV. The corresponding effective electromagnetic form factor of the proton is deduced under the assumption that the electric and magnetic form factors are equal ðjG E j ¼ jG M jÞ. In addition, the ratio of electric to magnetic form factors, jG E =G M j, and jG M j are extracted by fitting the polar angle distribution of the proton for the data samples with larger statistics, namely at ffiffi ffi s p ¼ 2232.4 and 2400.0 MeV and a combined sample at ffiffi ffi s p ¼ 3050.0, 3060.0 and 3080.0 MeV, respectively. The measured cross sections are in agreement with recent results from BABAR, improving the overall uncertainty by about 30%. The jG E =G M j ratios are close to unity and consistent with BABAR results in the same q 2 region, which indicates the data are consistent with the assumption that jG E j ¼ jG M j within uncertainties.

The process e þ e − → ΛΛ is studied using data samples at ffiffi ffi s p ¼ 2.2324, 2.400, 2.800 and 3.080 GeV collected with the BESIII detector operating at the BEPCII collider. The Born cross section is measured at ffiffi ffi s p ¼ 2.2324 GeV, which is 1.0 MeV above the ΛΛ mass threshold, to be 305 AE 45 þ66 −36 pb, where the first uncertainty is statistical and the second systematic. The cross section near threshold is larger than that expected from theory, which predicts the cross section to vanish at threshold. The Born cross sections at ffiffi ffi s p ¼ 2.400, 2.800 and 3.080 GeV are measured and found to be consistent with previous experimental results, but with improved precision. Finally, the corresponding effective electromagnetic form factors of Λ are deduced.

Using a data set of 2.93 fb −1 taken at a center-of-mass energy sqrt(s)=3.773 GeV with the BESIII detector at the BEPCII collider, we perform a search for an extra U(1) gauge boson, also denoted as a dark photon. We examine the initial state radiation reactions e+e−→e+e−γISR and e+e−→μ+μ−γISR for this search, where the dark photon would appear as an enhancement in the invariant mass distribution of the leptonic pairs. We observe no obvious enhancement in the mass range between 1.5 and 3.4 GeV/ c 2 and set a 90% confidence level upper limit on the mixing strength of the dark photon and the Standard Model photon. We obtain a competitive limit in the tested mass range