Dvofotonska fizika u MAMI

The structure of the nucleon can be described by the electromagnetic form factors GE and GM. These form factors can be determined by elastic electron-proton scattering with the well-known Rosenbluth technique. Alternatively, the ratio of the electric and the magnetic form factors GE/GM can be determined by the socalled polarization-transfer technique. The measurements reveal a large discrepancy between the two methods at large momentum transfers. There is evidence that twophoton exchange contributions to the elastic scattering have been underestimated. While the two-photon exchange amplitude is difficult to compute from theory due to the excited intermediate hadronic states, its imaginary part can be accessed experimentally by measuring the asymmetry A⊥ in the cross section of elastic ep scattering where the electrons are transversely polarized parallel or antiparallel to the normal of the scattering plane, respectively. The PVA4 experiment at MAMI is currently performing such measurements covering a large range of momentum transfers.Struktura nukleona opisuje se elektromagnetskim faktorima oblika GE i GM. Ti se faktori oblika određuju elastičnim raspršenjem elektrona na protonima poznatom Rosenbluthovom metodom. Druga metoda je određivanje omjera električnog i magnetskog faktora oblika GE/GM primjenom takozvane metode prijenosa polarizacije. Mjerenja pokazuju velike razlike ove dvije metode pri velikim prijenosima impulsa. Neki podaci ukazuju da su doprinosi dvofotonske izmjene elastičnom raspršenju bili podcijenjeni. Teorijsko određivanje amplitude dvofotonske izmjene je vrlo teško zbog uzbudnih hadronskih međustanja. Međutim, imaginarni dio amplitude može se odrediti mjerenjem asimetrije A⊥ udarnog presjeka elastičnog ep raspršenja, u kojemu su elektroni poprečno polarizirani u smjeru odnosno suprotno u odnosu na okomicu na ravninu raspršenja. Eksperiment PVA4 u MAMI provodi ta mjerenja za široko područje prijenosa impulsa

Isospin breaking in the vector current of the nucleon

Extraction of the nucleon's strange form factors from experimental data requires a quantitative understanding of the unavoidable contamination from isospin violation. A number of authors have addressed this issue during the past decade, and their work is reviewed here. The predictions from early models are largely consistent with recent results that rely as much as possible on input from QCD symmetries and related experimental data. The resulting bounds on isospin violation are sufficiently precise to be of value to on-going experimental and theoretical studies of the nucleon's strange form factors.Comment: 5 pages, 3 figures. Presented at the International Workshop "From Parity Violation to Hadronic Structure and more...", Milos, Greece, 16-20 May 2006. Version 2 is only to update Refs. [21] and [25

Future Directions in Parity Violation: From Quarks to the Cosmos

I discuss the prospects for future studies of parity-violating (PV) interactions at low energies and the insights they might provide about open questions in the Standard Model as well as physics that lies beyond it. I cover four types of parity-violating observables: PV electron scattering; PV hadronic interactions; PV correlations in weak decays; and searches for the permanent electric dipole moments of quantum systems.Comment: Talk given at PAVI 06 workshop on parity-violating interactions, Milos, Greece (May, 2006); 10 page

A4 Instrument

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

Results from the forward G0 experiment

The G0 experiment is dedicated to the determination of the strange quark contribution to the electric and magnetic nucleon form factors for a large range of momentum transfers between 0.1 to 1(GeV/c)2 . This information is provided by the asymmetries of cross-sections measured with longitudinally polarized electrons in elastic electron-proton scattering and quasi-elastic electron-deuteron scattering. A set of measurements at two different Q2 will allow the complete separation of the electric and magnetic weak, as well as axial nucleon form factors. This report summarizes the physics case, gives details about the dedicated set-up used, and shows the results of the combination of the strange quark contribution in the electric and magnetic form factors of the protons. The experiment was performed at the Jefferson Laboratory, during years 2003 and 2004, and will be completed after backward-angle measurements in 2006, 2007