Results from Electron-Positron Collisions at BESIII

Abstract

A selection of results from electron-positron collisions at BESIII are reviewed. The results presented here illustrate the wide range of physics topics that can be studied using the Beijing Electron Positron Collider (BEPC). At low collision energies, the cross section for e + e − → π + π − provides much-needed input into theoretical calculations of the anomalous magnetic moment of the muon, and the reaction e + e − → pp provides access to the electromagnetic form factors of the proton. In the charmonium region, a large sample of ψ decays can be used to measure new decay modes of charmonium states. And at higher energies, BESIII is uniquely situated to explore questions concerning the still-unexplained XY Z states. PoS(ICHEP2016)600Results from BESIII Ryan E. Mitchell BESIII has collected a variety of data sets for e + e − collisions with center-of-mass energies between 2.0 and 4.6 GeV [1]. A few of the highlights, from low to high energy, include: a scan of the region between 2.0 and 3.0 GeV; 1.3 billion J/ψ decays; 450 million ψ decays; 2.9 fb −1 of data at the ψ(3770) mass; about 3 fb −1 at 4.18 GeV (primarily for studies of the D s meson); 0.8 fb −1 in a scan of the region between 3.85 and 4.59 GeV (spread over 104 points); and over 4 fb −1 collected between 3.81 and 4.60 GeV (in sets ranging from 50 pb −1 to 1.1 fb −1 ) for studies of the XY Z states. In addition to these fixed energies, one can also study e + e − collisions at any lower center-of-mass energy using the Initial State Radiation (ISR) technique, where photons are radiated from the primary e + or e − before the collision. The following represents a small selection of the recent results that have been derived from these data sets. Anomalous Magnetic Moment of the MuonThe difference between the Standard Model (SM) and the experimental (E821) values for the anomalous magnetic moment of the muon, a µ ≡ (g µ − 2)/2, is currently larger than 3σ :The error in the SM calculation is dominated by the Hadronic Vacuum Polarization (HVP) contribution, which is estimated using experimental input from e + e − collisions to hadrons. The cross section for e + e − collisions to hadrons, in turn, is dominated by the reaction e + e → π + π − in the region of the ρ meson, corresponding to collision energies between 600 and 900 MeV. But here there are experimental differences between BaBar and KLOE, as shown in Figure 1b, on the order of several sigma. If only the BaBar measurement is used in a SM µ , the difference between SM and experiment drops below 3σ . It is thus crucial to provide more experimental input. Letters B 753 (2016) 629-638 635 ed due to this efmoved by applying lgorithm. Here, the a MC sample that ter τ is determined rections for the raof 2%, one obtains is found to be ry and agree with as systematic unmethod (1) is used rection thin the Phokhara ry precise descrip-.5% [16]. polarization effects dressed cross secis adjusted for the and dressed cross (7) n the investigated ces are: ncertainty is student sources. Firstly, e data samples are n ...