The spin structure of the nucleon Aidala, Christine A.; Bass, Steven D.; Hasch, Delia ...
Reviews of modern physics,
06/2013, Letnik:
85, Številka:
2
Journal Article
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This article reviews our present understanding of QCD spin physics: the proton spin puzzle and new developments aimed at understanding the transverse structure of the nucleon. Present experimental ...investigations of the nucleon's internal spin structure, the theoretical interpretation of the different measurements, and the open questions and challenges for future investigation are discussed.
The coherent flow of electrons through a two-dimensional electron gas (2DEG) offers promising approaches for spintronics and quantum information processing. Cryogenic scanning probe microscopes ...(SPMs) are a valuable tool for imaging electron motion, but have been limited by their inability to follow such motion through an open structure under an applied magnetic field. Here we report a way to visualize the flow of electron waves from one point to another by using the SPM tip to create a lens in the 2DEG below. The lens deflects electrons and casts a shadow downstream. We use this technique to image magnetic focusing in a GaAs 2DEG. Magnetic focusing occurs when electrons flowing from one quantum point contact (QPC) rejoin at a second QPC a number of cyclotron diameters away. Our images show semicircular trajectories as the electrons bounce along the boundary, as well as fringes created by the interference of multiple paths, demonstrating that the flow is coherent. Remarkable agreement between experiment and theory demonstrates our ability to visualize electron trajectories in a magnetic field, and to make a new type of imaging electron interferometer.
The super Pioneering High Energy Nuclear Interaction eXperiment (sPHENIX) at the Relativistic Heavy Ion Collider will perform high-precision measurements of jets and heavy flavor observables for a ...wide selection of nuclear collision systems, elucidating the microscopic nature of strongly interacting matter ranging from nucleons to the strongly coupled quark-gluon plasma. A prototype of the sPHENIX calorimeter system was tested at the Fermilab Test Beam Facility as experiment T-1044 in the spring of 2016. The electromagnetic calorimeter (EMCal) prototype is composed of scintillating fibers embedded in a mixture of tungsten powder and epoxy. The hadronic calorimeter (HCal) prototype is composed of tilted steel plates alternating with the plastic scintillator. Results of the test beam reveal the energy resolution for electrons in the EMCal is <inline-formula> <tex-math notation="LaTeX">2.8\%\oplus 15.5\%/\sqrt {E} </tex-math></inline-formula> and the energy resolution for hadrons in the combined EMCal plus HCal system is <inline-formula> <tex-math notation="LaTeX">13.5\%\oplus 64.9\%/\sqrt {E} </tex-math></inline-formula>. These results demonstrate that the performance of the proposed calorimeter system satisfies the sPHENIX specifications.
sPHENIX is a new experiment under construction for the Relativistic Heavy Ion Collider at Brookhaven National Laboratory which will study the quark-gluon plasma to further the understanding of ...quantum chromodynamics (QCP) matter and interactions. A prototype of the sPHENIX electromagnetic calorimeter (EMCal) was tested at the Fermilab Test Beam Facility in Spring 2018 as experiment T-1044. The EMCal prototype corresponds to a solid angle of <inline-formula> <tex-math notation="LaTeX">\Delta \eta \times \Delta \phi = 0.2 \times 0.2 </tex-math></inline-formula> centered at pseudo-rapidity <inline-formula> <tex-math notation="LaTeX">\eta = 1 </tex-math></inline-formula>. The prototype consists of scintillating fibers embedded in a mix of tungsten powder and epoxy. The fibers project back approximately to the center of the sPHENIX detector, giving 2-D projectivity. The energy response of the EMCal prototype was studied as a function of position and input energy. The energy resolution of the EMCal prototype was obtained after applying a position-dependent energy correction and a beam profile correction. Two separate position-dependent corrections were considered. The EMCal energy resolution was found to be <inline-formula> <tex-math notation="LaTeX">\sigma (E)/\langle E\rangle = 3.5(0.1) \oplus 13.3(0.2)/\sqrt {E} </tex-math></inline-formula> based on the hodoscope position-dependent correction, and <inline-formula> <tex-math notation="LaTeX">\sigma (E)/\langle E\rangle = 3.0(0.1) \oplus 15.4(0.3)/\sqrt {E} </tex-math></inline-formula> based on the cluster position-dependent correction. These energy resolution results meet the requirements of the sPHENIX physics program.
The fundamental building blocks of the proton-quarks and gluons-have been known for decades. However, we still have an incomplete theoretical and experimental understanding of how these particles and ...their dynamics give rise to the quantum bound state of the proton and its physical properties, such as its spin
. The two up quarks and the single down quark that comprise the proton in the simplest picture account only for a few per cent of the proton mass, the bulk of which is in the form of quark kinetic and potential energy and gluon energy from the strong force
. An essential feature of this force, as described by quantum chromodynamics, is its ability to create matter-antimatter quark pairs inside the proton that exist only for a very short time. Their fleeting existence makes the antimatter quarks within protons difficult to study, but their existence is discernible in reactions in which a matter-antimatter quark pair annihilates. In this picture of quark-antiquark creation by the strong force, the probability distributions as a function of momentum for the presence of up and down antimatter quarks should be nearly identical, given that their masses are very similar and small compared to the mass of the proton
. Here we provide evidence from muon pair production measurements that these distributions are considerably different, with more abundant down antimatter quarks than up antimatter quarks over a wide range of momenta. These results are expected to revive interest in several proposed mechanisms for the origin of this antimatter asymmetry in the proton that had been disfavoured by previous results
, and point to future measurements that can distinguish between these mechanisms.
A structure is observed in the Formula omitted mass spectrum in a sample of proton-proton collisions at centre-of-mass energies of 7, 8, and 13 TeV, collected with the LHCb detector and corresponding ...to a total integrated luminosity of 9 Formula omitted. The structure is interpreted as the result of overlapping excited Formula omitted states. With high significance, a two-peak hypothesis provides a better description of the data than a single resonance. Under this hypothesis the masses and widths of the two states, assuming they decay directly to Formula omitted, are determined to be m1=6063.5±1.2(stat)±0.8(syst)Me,GAMMA1=26±4(stat)±4(syst)Me,m2=6114±3(stat)±5(syst)Me,GAMMA2=66±18(stat)±21(syst)Me.Alternative values assuming a decay through Formula omitted, with a missing photon from the Formula omitted decay, which are shifted by approximately 45 Formula omittedV, are also determined. The possibility of a single state decaying in both channels is also considered. The ratio of the total production cross-section times branching fraction of the new states relative to the previously observed Formula omitted state is determined to be Formula omitted.
A branching fraction measurement of the Formula omitted decay is presented using proton-proton collision data collected with the LHCb experiment, corresponding to an integrated luminosity of Formula ...omitted. The branching fraction is found to be Formula omitted Formula omitted, where the first uncertainty is statistical, the second systematic and the third is due to the uncertainty on the Formula omitted, Formula omitted and Formula omitted branching fractions. This is the most precise single measurement of this quantity to date. As this decay proceeds through a single amplitude involving a Formula omitted charged-current transition, the result provides information on non-factorisable strong interaction effects and the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element Formula omitted. Additionally, the collision energy dependence of the hadronisation-fraction ratio Formula omitted is measured through Formula omitted and Formula omitted decays.
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