Physical Object

GSI-2024-00486

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png A4 Instrument

A4 Collaboration (Corresponding author)

2024
GSI Darmstadt

Please use a persistent id in citations: doi:10.15120/GSI-2024-00486

Abstract: The atomic nucleus is a bound state of protons and neutrons (so called nucleons). The A4collaboration is investigating the structure of these nucleons. In a simple picture, nucleons are made upof three elementary constituents, two up- and one down-quark. Today's view of the nucleon is morecomplex: Beside the valence quarks also gluons, the force carrier of QCD, and quark-antiquark pairsknown as sea quarks contribute to the properties of proton and neutron. In the quark sea also flavorsoccur which are not present in the valence quarks. The A4 collaboration is aiming to measure suchflavor contribution, especially those of the strange quarks since they are the lightest non-valencequarks. In the experiment, polarized electrons are scattered off unpolarized nucleons and detected in alead fluoride calorimeter. Depending on the polarization state there are tiny differences in the interactionstrength due to the parity violation in the weak interaction. Consequently, the number of scatteredelectrons vary for the two polarization states. These numbers can be determined by the measurementof the parity violation asymmetry. This asymmetry reveals the distribution of strange quarks within thenucleon. The A4 collaboration measures small asymmetries in the cross section of elastic scattering ofpolarized electrons off an unpolarized target, basically hydrogen or deuterium. The momentumtransfers achieved either in forward angle- or backward angle-configuration of the detector varybetween 0.02 (GeV/c)^{2} and 2.2 (GeV/c)^{2}. There are two main physics goals: (i) Parity violatingelectron scattering asymmetries are measured with a longitudinally polarized electron beam. Usinginput from the Standard Model the contribution of strange sea quarks to the vector form factors of thenucleon are determined. The combination of measurements on hydrogen and deuterium allow for anadditional determination of the axial form factor of the proton referring to the nuclear anapole moment.(ii) Using a transversely polarized electron beam, the observed asymmetries arise at leading order fromthe interference of the one- and the two-photon-exchange amplitude. These asymmetries are sensitiveto excited intermediate states of the nucleon. The imaginary part of the two-photon exchange amplitudecan be determined. A high power liquid hydrogen target of 10 cm or 20 cm length and an electronbeam of I = 20 muA lead to luminosities in the order of L = 10^{38} cm^{-2}s^{-1}. The scatteredelectrons are measured by a total absorbing, segmented lead fluoride calorimeter which deals withevent rated of about 100 MHz. The degree of polarization of the electron beam is measured by a laserCompton backscatter polarimeter simultanously to the main experiment. With this apparatus, theinternal cavity concept is realized for the first time.

Note: Years active: 1999-2012

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Contributing Institute(s):

1. HIM / MAM (MAS)

Research Program(s):

1. 899 - ohne Topic (POF4-899) (POF4-899)

Appears in the scientific report 2024

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