.

New Experimental Method for a Precise Measurement of the Proton Charge Radius at the S-DALINAC * I. Pysmentska, P. von Neumann-Cosel, S. Rathi, A. Richter, G. Schrieder and A. Shevchenko Institut für Kernphysik, TU Darmstadt *Supported by the DFG under contract SFB 634

Introduction Proton charge radius – one of the fundamental quantities in physics! Its precise determination is very important - to understand its structure in terms of quark and gluon degrees of freedom of Quantum Chromodynamics. for high-precision tests of Quantum Electrodynamics using Lamb shift measurements .

Proton charge radius – the data

Differential cross-section H(e,e)p ì æ 2 + t 2 ö ü ï G G q ï q 2 ç ÷ F ( q 2 ) = í E M + 2 t G 2 tan 2 ý , with t º ç ÷ ï 1 + t M 2 ï 4 M 2 î è ø þ At low , q 0

Experimental method e- E Ep Previous New Spectrometer qe qp Si Detectors Ep qp qe Drp/rp < 1%

S-DALINAC

Set-up Si-Detectors QCLAM Spectrometer DW = 35 msr

Setup 472 mm e 28 Detectors

Experimental energy spectrum and background

A typical spectrum with pulsed beam Energy (Channel no.) 65° p e- 50 ns e- p Linearised spectrum Time (Channel no.)

Projected spectra p e- Time Resolution ~ 5 ns

Measured energy spectra 0.22 0.31 0.38 0.44 q (fm-1) 1/E2 fit

Outlook Proton charge radius Measurements at higher beam energies !

Rosenbluth Separation Separation of el. and mag. contributions: Same q, different kinematics (Ee,q) e-p Scattering

Spectrometer spectra

.

Test Experiment

Effect of beam spot on the angle defination of the detector: x target 90-q q r dq a c d Defining detector aperture q` Beam dia. = a = 2mm Detector Distance= r = 20cm tan dq = c/ r = a*cos()/r

Si Detectors Reichweite qp° Ep(MeV) Dicke (mm) Si – Detectors 2.5 x 2.5 cm2 qp° Ep(MeV) Reichweite (mm) Dicke 20° 8.90 570 800 30° 7.57 440 600 40° 5.92 290 380 50° 4.16 160 250 60° 2.52 70 100 70° 1.18 22 50 80° 0.3 3 12