The two-photon exchange experiment at DESY

Abstract

We propose a new measurement of the ratio of positron-proton to electron-proton elastic scattering at DESY. The purpose is to determine the contributions beyond single-photon exchange, which are essential for the Quantum Electrodynamic (QED) description of the most fundamental process in hadronic physics. By utilizing a 20 cm long liquid hydrogen target in conjunction with the extracted beam from the DESY synchrotron, we can achieve an average luminosity of

$$2.12\times 10^{35}$$

2.12 ×

10 35

cm

$$^{-2}\cdot $$

- 2

·

s

$$^{-1}$$

- 1

(

$$\approx 200$$

≈ 200

times the luminosity achieved by OLYMPUS). The proposed two-photon exchange experiment (TPEX) entails a commissioning run at a beam energy of 2 GeV, followed by measurements at 3 GeV, thereby providing new data up to

$$Q^2=4.6$$

Q 2

= 4.6

(GeV/c)

$$^2$$

2

(twice the range of current measurements). We present and discuss the proposed experimental setup, run plan, and expectations.