The Radiative Return: A Review of Experimental Results

The radiative return is a new method for hadronic cross section measurements at electron-positron colliders, which are operated at a fixed center-of-mass energy. In order to lower the effective hadronic mass M_hadr only such events are taken, in which one of the electrons or positrons has emitted an initial state radiation photon. We present precision measurements of the pion form factor from the Frascati phi-factory DAPHNE with the KLOE experiment and measurements of higher particle multiplicities as well as a measurement of the timelike proton-antiproton form factor from the BaBar experiment at the B-factory PEP-II. These radiative return measurements are compared to results, which are obtained by means of an energy scan, i.e. by means of a systematic variation of the beam energy of the collider. We also report on the impact of these measurements on the hadronic contribution of the anomalous magnetic moment of the muon, which is obtained via a dispersion integral using hadronic cross section data as input.Comment: Talk given at the International Workshop " e+e- Collisions from phi to psi ", BINP Novosibirsk(Russia), Feb. 27 - March 2, 2006; 9 pages, 10 figure

Measurement of the hadronic cross section at DAPHNE with the KLOE detector

We have measured the cross section sigma(e+e- \to pi+pi-gamma) as a function of the pi+pi- invariant mass, M_pipi, with the KLOE detector at DAPHNE (W=m_phi=1.02GeV). The photon in the above process is due to Initial State Radiation. Dividing by a theoretical radiator function we obtain the cross section sigma(e+e- \to pi+pi-) for the mass range 0.37<M_pipi^2<0.93GeV^2. We extract the pion form factor and the hadronic contribution to the muon anomaly, a_mu.Comment: Proceedings for HADRON2003, Aschaffenburg/Germany, Aug.31-Sept.6 2003, 6 pages, 2figure

Measurement of the Pion Formfactor with KLOE and Study of the Reaction f0(980) to pi+pi-

At the Frascati phi-factory DAPHNE the pion formfactor is measured by means of the 'radiative return', i.e. by using events in which one of the collider electrons (positrons) has radiated an initial state radiation photon, lowering in such a way the invariant mass M_pipi of the two-pion-system. In a recent publication of the KLOE collaboration the initial state radiation photon had been required to be at small polar angles with respect to the beam axis. We are presenting results from a new and complementary analysis in which the photon is tagged at large polar angles. Only like this the threshold region M_pipi^2<0.35 GeV^2 becomes accessible. Moreover, the final state pi+pi-gamma allows to study the phi radiative decay into the scalar particle f_0(980) with f_0(980) \to pi+pi-. For the first time the two-pion mass spectrum could be fitted with different theoretical models for the description of this phi radiative decay.Comment: Proceedings of the 41st Rencontres de Moriond, QCD and Hadronic Interactions; La Thuile, March 18-25, 2006; 6 pages; 4 figure

Nucleon Electromagnetic Form Factors in the Timelike Region

The electromagnetic form factors of the proton and the neutron in the timelike region are reviewed. In view of the forthcoming experimental projects devoted to investigate these observables, we present the current status of the field and we emphasize the relevant role, that accurate measurements, refined phenomenological analyses, as well as microscopic models will play with the goal of achieving deeper insights into the structure of the nucleon and its inner dynamics.Comment: 60 pages, 35 figures. To be published in Progress in Particle and Nuclear Physics 201

KLOE Perspectives for R-Measurements at DAFNE2

As a future upgrade of the Frascati phi factory DAPHNE an increase of the center-of-mass energy of the accelerator up to W=2GeV has been proposed (DAFNE2). In this case the hadronic cross section in the energy range between 1-2 GeV can be measured with the KLOE detector. The feasibility of these measurements and the impact on the hadronic contribution to the anomalous magnetic moment of the muon are discussed. The possibilities for an energy scan are compared with the radiative return technique, in which the accelerator is running at a fixed center-of-mass energy and ISR-events are taken to lower the invariant mass of the hadronic system.Comment: Invited talk at the Workshop on 'e+e- in the 1-2 GeV range', Alghero/Sardinia, Sept. 10-13, 2003 (eConf C030910.1), 4 pages, 3 figure

Measurement of the e+e- Multihadronic Cross Sections below 4.5 GeV with BaBar

We present a summary of the hadronic cross section measurements performed with BaBar at the PEP-II collider via radiative return. BaBar has performed measurements of exclusive final states containing 3, 4 and 6 hadrons via this complementary method, as well as a measurement of the proton form factor.Comment: Talk given at the International Workshop on Chiral Dynamcis 2006, Chapel Hill, NC (USA), Sept. 18-22, 2006; 4 pages, 4 figure

Measurements of the Hadronic Cross Section and of Meson Transition Form Factors at BESIII for an improved Standard Model Prediction of (g-2)mu

The BESIII experiment at the Beijing tau-charm factory BEPCII recently has embarked on a series of form factor measurements with the goal to improve the hadronic vacuum polarization as well as the hadronic light-by-light contributions to (g-2)mu. The status of the flagship measurements and preliminary results are presentedComment: 5 pages, 5 figures; Proceedings of 13th International Workshop on Tau Lepton Physics (Tau14), Aachen, Germany, 15-19 September, 201

Measurement of the hadronic cross section at KLOE using the radiative return

We report on the measurement of the hadronic cross section below 1GeV at the electron-positron-collider DAPHNE, using the multiple purpose detector KLOE. The radiative return, which is due to initial state radiation (e+ e- \to gamma + hadrons), allows us to obtain the cross section for variable center-of-mass-energies of the hadronic system from the 2m_pi threshold up to 1.02GeV. This measurement can be performed while DAPHNE is running with a fixed accelerator energy on the phi mass (1.02 GeV). For the exclusive process e+ e- \to pi+ pi- gamma, the status of the analysis and first preliminary results of the invariant mass spectrum of the two-pion-state are presented.Comment: Invited talk at the workshop "e+e- Physics at Intermediate Energies", SLAC, April 30 - May 2, 2001 (T07

Search for light dark matter with the MESA accelerator

At the Institute for Nuclear Physics of the Johannes Gutenberg University in Mainz, the MESA facility is currently being constructed. At its core there is a new superconducting energy-recovery linac which will provide intense electron beams for precision experiments in subatomic physics. An important part of the MESA physics program consists in the search for a "dark sector" which is a candidate explanation for the longstanding dark matter problem. This report will highlight the MESA dark sector program, and in particular two experiments will be described. The first one, MAGIX, is a two-spectrometer setup employing an internal gas-jet target installed on a recirculation arc of MESA. The second one is a beam-dump experiment for directly detecting dark matter particles. The experiments are in the R\&D phase and the current status and future prospects will be presented.Comment: 10 pages, 3 figures, CIPANP18 Conference Proceeding

Dark Matter at the Intensity Frontier: the new MESA electron accelerator facility

Dark Matter is being searched with a variety of methods, each of which tackles this challenge focusing on different kinds of particles, masses and couplings. Here we describe Dark Matter searches conducted with accelerators and fixed targets. In particular, we highlight the contribution of the experiments being built at the new Mainz Energy-recovery Superconducting Accelerator (MESA) facility. MESA will provide intense electron beams for hadron and nuclear physics, as well as for Dark Matter searches with competitive sensitivities.Comment: Proceedings of ALPS 2019, April 22 - 27, 2019, Obergurgl, Austria. arXiv admin note: substantial text overlap with arXiv:1809.0716