Measurements of the spectral location of the structured target resonance for ultrarelativistic electrons

K.K Andersen,S.L Andersen,H Knudsen,R.E Mikkelsen,H.D Thomsen,U.I Uggerhøj,T.N Wistisen,J Esberg,P Sona,A Mangiarotti,T.J Ketel

doi:10.1016/j.physletb.2014.03.055

Abstract

When an ultrarelativistic electron traverses two closely spaced foils, a radiation spectrum ‘resonance’ appears, arising from the photon formation length extending from one foil, across the gap and into the second foil. Several theoretical approaches yield quite different answers to the spectral location of this ‘resonance’, and we have therefore in the CERN NA63 collaboration addressed the question experimentally with a 178 GeV electron beam. The experimental technique used – where a variable gap separates two thin gold foils – allows for a direct measurement of a length that is closely related to the distance over which the photon formation takes place. These are the first measurements to observe the gap dependence of the energy of the ‘resonance’ in the radiation spectrum from a structured target on a truly macroscopic scale up to 0.5 mm. The results are compared with the theory of Baier and Katkov, with both the modified and unmodified theories of Blankenbecler as well as with a naïve, straightforward expectation. Surprisingly, the experiment shows a clear preference for the two latter, comparatively unsophisticated, approaches.

Highlights

The low-energy radiation emitted by an ultrarelativistic electron traversing a dense medium is suppressed due to the well-known Landau–Pomeranchuk–Migdal (LPM) effect
Whereas the coherence appears over many crystalline planes in coherent bremsstrahlung, due to the averaging mentioned above for the structured target, one can argue that the resonance should disappear when the number of foils in the target is increased
The measured spectra are generally in good agreement with simulations. It is worth noting how the radiation spectrum changes with lg

Summary

Introduction

The low-energy radiation emitted by an ultrarelativistic electron traversing a dense medium is suppressed due to the well-known Landau–Pomeranchuk–Migdal (LPM) effect. An analogy can be made between the radiation emitted by an electron traversing such an above described structured target, where foils are seen as ‘macroscopic crystalline planes’, and the well-known coherent bremsstrahlung Whereas the coherence appears over many crystalline planes in coherent bremsstrahlung, due to the averaging mentioned above for the structured target, one can argue that the resonance should disappear when the number of foils in the target is increased. Initial investigations of such many foil targets have been performed and showed no sign of a resonance in the radiation spectrum [3]. It could be expected that the measured location of the spectral position of these structured target ‘resonances’ –

Objectives

Results

Conclusion