Michela Chiosso (Università di Torino)
AMBER is a newly proposed fixed-target experiment at the M2 beam line of the SPS, devoted to various fundamental QCD measurements, with a Proposal very recently approved by the CERN Research Board for a Phase-1 program and a Letter of Intent made public for a longer term program.
Such an unrivaled installation would make the experimental hall EHN2 the site for a great variety of measurements to address fundamental issues of strong interactions in the medium and long-term future.
The elastic muon-proton scattering process, using high-energy muons, is proposed as a novel approach to the long-standing puzzle of the proton charge radius. Such a measurement constitutes a highly-welcomed complementary approach in this area of world-wide activity.
Operating with protons, the largely unknown antiproton production cross section can be measured, which constitutes important input for the upcoming activities in the Search for Dark Matter.
Especially the world-unique SPS M2 beam line, when operated with high-energy pions, can be used to shed light to the emergence of hadron masses. How can we explain the emergence of the proton mass and the nearly masslessness of the pion? The origin of hadron masses is deeply connected to the parton dynamics, and how it differs in protons or mesons. A determination of the valence-quark PDF of the pion, through Drell-Yan production, would provide the needed sensitivity to the mechanism(s) responsible for the emergence of mass in QCD. While the nucleon parton distributions are known with good precision already, the pion ones are very poorly known, the last measurements being more than 30 years old and suffering from very large uncertainties. The kaon parton dynamics, on the other hand, was never measured.
For a longer-term program an upgrade of the M2 beam line with radio-frequency separation of kaon and antiproton beams of high energy and high intensity would allow for further unique opportunity to shed new light to the light-meson structure, by exploring both kaon-induced Drell-Yan and direct photon production.
The rich physics program planned at the AMBER experiment will be presented. World competition and possible timelines will be discussed.