# P1: Accomplishments of ﬁrst funding period

• Establish an ISR programme at BESIII; Flagship analysis: $$e^+e^-\to \pi^+ \pi^- \gamma$$ channel
At BESIII an ISR program has been successfully launched by Mainz researchers. Before, no such analyses were carried out by the collaboration. Although originally foreseen for the second funding period, we immediately embarked on the analysis of the most relevant hadronic channel for $$(g-2) \mu$$, namely the measurement of the pion form factor $$e^+e^-\to \pi^+ \pi^- \gamma$$. A publication around the ρ resonance in the energy range between 600 and 900 MeV is expected for the ﬁrst half of 2015. We expect a measurement with a systematic uncertainty of approximately 1 %, which allows to be competititve with recent measurements from KLOE and BABAR.
• ISR channels $$e^+e^-\to \pi^+ \pi^- \pi^0 \gamma$$ and $$e^+e^-\to\pi^+ \pi^- \pi^0 \pi^0 \gamma$$
We are also working on the next two important hadronic channels in view of the $$(g-2) \mu$$ motivation: the three pion and the partly neutral four pion channels. While the latter is very complicated due to the presence of two neutral pions, the analysis of the tree pion channel is in an advanced state and we also expect a publication within 2015.
• A side product of the two-pion analysis:  $$e^+e^-\to \mu^+ \mu^- \gamma$$
The measurement of the pion form factor requires a very good pion - muon separation as the process $$e^+e^- \to \mu^+ \mu^- \gamma$$  comes out to be the relevant background process. As a consequence, a high-statistics and well-understood radiative muon sample is at our disposal, which allows to extract interesting information in itself. Two aspects are currently under investigation: (i) the electronic width of $$J/\psi$$, an important quantity for precision physics, can be measured with the world’s best accuracy; (ii) a dark photon search can be carried out in the mass range above approximately 1 GeV. Both analyses are in an advanced state.
• Search for non-vector resonances in $$e^+e^-$$ collision
The production of particles with quantum numbers different from JP,C = 1−− is forbidden at leading order in $$e^+e^-$$ annihilation. Indeed, never in the history of $$e^+e^-$$ physics, has a production of a non-vector resonance been observed. However, due to the high luminosity of the BEPC-II collider, a production via a two-photon processes becomes indeed realistic. We have worked out a proposal for a search for the $$\chi^{c1}$$ charmonium resonance (JP,C = 1++) and the collaboration has decided to spend three weeks of beam time in 2015 for this project. In parallel, we also published a paper in collaboration with theoreticians to estimate the cross section for a two-photon production of the famous X(3872) resonance, which is a good candidate for an exotic resonance. Furthermore, we also searched for the X(3872) particle via ISR production in the existing data set above 4 GeV. No signal has been observed, but a two orders of magnitude improved upper limit for the electronic width of the X(3872) has been measured. We shortly will publish this analysis.
• Dark photon search at A1/MAMI
The successful program of dark photon searches with the A1 high resolution spectrometers has continued within the course of the ﬁrst funding period. Two beam times have been carried out to cover the mass range above 50 MeV for this search. The parameter range tested with this data is especially interesting as it covers a large part of the parameter range, in which a dark photon could explain the deviation presently seen between the direct measurement of $$(g-2) \mu$$ and the Standard Model prediction. No signiﬁcant signal for the dark photon was found and the analysis was published in Phys. Rev. Lett. A detailed study of the electromagnetic background used to determine the exclusion limits was also published.
• Design of a high-resolution spectrometer setup for MESA
The design of a high resolution double-arm spectrometer (dubbed MAGIX) for the internal target experiment at MESA is continuing. Initially thought as a spectrometer exclusively for dark photon research, we now plan a multiple-purpose detector, which allows to cover a broad range of physics topics. A magnet-optical design for the dipole-spectrometer is ﬁnished and currently students and postdocs are working on the design of the target as well as the focal plane detector, which will be based on GEM technology.
• Lattice QCD activities within project P1
see separate summary of all lattice QCD activities within CRC1044