This Conceptual Design Report describes LUXE (Laser Und XFEL Experiment), an experimental campaign that aims to combine the high-quality and high-energy electron beam of the European XFEL with a powerful laser to explore the uncharted terrain of quantum electrodynamics characterised by both high energy and high intensity. We will reach this hitherto inaccessible regime of quantum physics by analysing high-energy electron-photon and photon-photon interactions in the extreme environment provided by an intense laser focus. The physics background and its relevance are presented in the science case which in turn leads to, and justifies, the ensuing plan for all aspects of the experiment: Our choice of experimental parameters allows (i) field strengths to be probed where the coupling to charges becomes non-perturbative and (ii) a precision to be achieved that permits a detailed comparison of the measured data with calculations. In addition, the high photon flux predicted will enable a sensitive search for new physics beyond the Standard Model. The initial phase of the experiment will employ an existing 40 TW laser, whereas the second phase will utilise an upgraded laser power of 350 TW. All expectations regarding the performance of the experimental set-up as well as the expected physics results are based on detailed numerical simulations throughout.
We present a search for the e + e − decay of a hypothetical dark photon, also names U vector boson, in inclusive dielectron spectra measured by HADES in the p (3.5 GeV) + p, Nb reactions, as well as the Ar (1.756 GeV/u ) + KCl reaction. An upper limit on the kinetic mixing parameter squared ǫ 2 at 90% CL has been obtained for the mass range MU = 0.02 − 0.55 GeV/c 2 and is compared with the present world data set. For masses 0.03 -0.1 GeV/c 2 , the limit has been lowered with respect to previous results, allowing now to exclude a large part of the parameter region favoured by the muon g − 2 anomaly. Furthermore, an improved upper limit on the branching ratio of 2.3 × 10 −6 has been set on the helicity-suppressed direct decay of the eta meson, η → e + e − , at 90% CL.
We consider the intermediate mass continuum of dileptons (between φ and J/ψ) in ultrarelativistic heavy-ion collisions. The thermal signal depends essentially on thermodynamic state parameters of the hottest parton stage as (τ i λ q i T 3 i ) 2 convoluted with an involved detector acceptance function. A refined analysis of the transverse pair momentum spectrum at fixed dilepton transverse mass can reveal the maximum temperature of parton matter.
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