Showing papers on "Proton spin crisis published in 2003"

Characterization of 1H−1H Distances in a Uniformly 2H,15N-Labeled SH3 Domain by MAS Solid-State NMR Spectroscopy§

Abstract: In this communication, we demonstrate the feasibility of obtaining long-range 1H−1H distance information by MAS solid-state NMR for a microcrystalline, uniformly 2H,15N-labeled sample of a SH3 domain of chicken α-spectrin. The experiments yield NOESY-type spectra and rely on the favorable dispersion of the 15N chemical shifts of the protein backbone. Perdeuteration of nonexchangeable sites is employed to simplify proton spin systems and to obtain multiple structural information. Two mixing schemes, 1H−1H double quantum filtered Post-C7 and 1H spin diffusion, are implemented to obtain quantitative 1H−1H distance information. Post-C7 and spin diffusion cross-peak buildup rates are discussed for initial-rate fitting and in the framework of n = 0 rotational resonance (rotor driven spin diffusion), respectively. Different deuteration schemes were tested to find conditions where short-range 1H−1H interactions are truncated (e.g., between HN and Hα), but long-range interactions are retained (e.g., between HN and...

Quantitative measurements of proton spin‐lattice (T1) and spin–spin (T2) relaxation times in the mouse brain at 7.0 T

Abstract: The goal of this work is to provide regional T1 and T2 values at a field strength of 7 T for the normal mouse brain at 6 weeks and 1 year old. A novel segmented snapshot FLASH sequence was used to measure T1 in the hippocampus, corpus callosum, and the retrosplenial granular (RSG) cortex; T2 measurements were made in the same regions using a single spin echo sequence repeated at six separate echo times. Both T1 and T2 measurements were validated with phantom measurements. Magn Reson Med 49:576–580, 2003. © 2003 Wiley-Liss, Inc.

Spin structure functions

Abstract: ▪ Abstract We review the study of the internal spin structure of the proton and neutron. High-energy scattering of polarized leptons by polarized protons, neutrons, and deuterons provides a measurement of the nucleon spin structure functions. These structure functions give information on the polarized quark contributions to the spin of the proton and the neutron and allow tests of the quark-parton model and quantum chromodynamics. We discuss the formalism of deep inelastic scattering of polarized leptons on polarized nucleons, the past decade of experimental progress, and future programs to measure the polarized gluon contribution to the proton spin.

Evidence for quark-hadron duality in the proton spin asymmetry A(1)

Abstract: Spin-dependent lepton-nucleon scattering data have been used to investigate the validity of the concept of quark-hadron duality for the spin asymmetry A_1. Longitudinally polarized positrons were scattered off a longitudinally polarized hydrogen target for values of Q^2 between 1.2 and 12 GeV^2 and values of W^2 between 1 and 4 GeV^2. The average double-spin asymmetry in the nucleon resonance region is found to agree with that measured in deep-inelastic scattering at the same values of the Bjorken scaling variable x. This finding implies that the description of A_1 in terms of quark degrees of freedom is valid also in the nucleon resonance region for values of Q^2 above 1.6 GeV^2.

Proton spin structure and valence quarks

Abstract: The spin structure of a system of quasifree fermions having a total angular momentum $J=1/2$ is studied in a consistently covariant approach. Within this model the relations between the spin functions are obtained. Their particular cases are the sum rules of Wanzura and Wilczek, Efremov, Leader, and Teryaev, and Burkhardt and Cottingham and also the expression for the Wanzura-Wilczek twist 2 term ${g}_{2}^{\mathrm{WW}}.$ With the use of the proton valence quark distributions as input, the corresponding spin functions are obtained. The resulting structure functions ${g}_{1}$ and ${g}_{2}$ are quite compatible with the experimental data.

The 'Proton spin crisis' : A Quantum query

Abstract: The "proton spin crisis" was introduced in the late 1980s, when the EMC-experiment revealed that little or nothing of a proton's spin seemed to be carried by its quarks. The main objective of this paper is to point out that it is wrong to assume that the proton spin, measured by completely different experimental setups, should be the same in all circumstances.

Fixed Poles, Polarized Glue and Nucleon Spin Structure

Abstract: We review the theory and present status of the proton spin problem with emphasis on possible gluonic and sea contributions. We discuss the possibility of a J=1 fixed pole correction to the Ellis-Jaffe sum rule for polarized deep inelastic scattering. Fixed poles in the real part of the forward Compton scattering amplitude have the potential to induce subtraction constant corrections to sum rules for photon nucleon scattering.

Fixed poles, polarized glue and nucleon spin structure

Abstract: We review the theory and present status of the proton spin problem with emphasis on possible gluonic and sea contributions. We discuss the possibility of a J=1 fixed pole correction to the Ellis-Jaffe sum rule for polarized deep inelastic scattering. Fixed poles in the real part of the forward Compton scattering amplitude have the potential to induce subtraction constant corrections to sum rules for photon nucleon scattering.

Some Structural Features of HCOOH-H2O Mixtures According to Proton Spin-Spin Relaxation Data

Abstract: The structure of the formic acid-water system was studied by proton spin-spin relaxation at 298 K. The simplest compositions of the heterosolvates were determined.

NMR proton spin dynamics in thermotropic liquid crystals subject to multipulse excitation

Abstract: Previous experiments of NMR spin-lattice relaxation times as a function of the Larmor frequency, as measured with the field-cycling technique (FC), were shown to be very useful to disentangle the various molecular motions, both local and collective, that dominate the relaxation in different time scales in liquid crystals. However, there are many examples where the known theoretical models that represent the molecular relaxation mechanisms cannot be fitted to the experimental trend in the region of low fields, making it difficult to obtain reliable values for the spectral densities involved, especially for the cooperative motions which dominate at low frequencies. In some cases, these anomalies are loosely ascribed to "local-field" effects but, to our knowledge, there is not a detailed explanation about the origin of these problems nor the range of frequencies where they should be expected. With the aim of isolating the dipolar effects from the influence of molecular dynamics, and taking into account the previous results in solids, in this work we investigate the response of the proton spin system of thermotropic liquid crystals 4-pentyl-4'-cyanobiphenyl (5CB) and 4-octyl-4'-cyanobiphenyl (8CB) in nematic and smectic A phases, due to the NMR multipulse sequence 90( composite function )y-(tau-thetax-tau)N. The nuclear magnetization presents an early transient period characterized by strong oscillations, after which a quasistationary state is attained. Subsequently, this state relaxes towards internal equilibrium over a time much longer than the transverse relaxation time T2. As occurs in solids, the decay time of the quasistationary state T2e presents a minimum when the pulse width thetax and the offset of the radiofrequency are set to satisfy resonance conditions (spin-lock). When measured as a function of the pulse spacing tau in "on-resonance" experiments, T2e shows the behavior expected for cross relaxation between the effective Zeeman and dipolar reservoirs, in accordance with the thermodynamic theory previously developed for solids. Particularly, for values of tau comparable with T2, the relaxation rate follows a power law T2e proportional to tau(-2), in all the observed cases, for the resonance conditions thetax=pi/3 and equivalent frequency omegae=pi/3tau. When tau is similar to or greater than typical dipolar periods, the relaxation rate becomes constant and for tau much shorter than T2, the thermodynamic reservoirs get decoupled. These experiments confirm that the thermodynamic picture is valid also in liquid crystals and the cross relaxation between the reservoirs can be detected without interference with spin-lattice relaxation effects. Accordingly, this technique can be used to estimate the frequency range, where cross-relaxation effects can be expected when Zeeman and dipolar reservoirs are put in thermal contact with each other and with the lattice, as in FC experiments. In particular, the present results allow us to associate the anomalies observed in low-field spin-lattice relaxation with nonadiabatic energy exchange between the reservoirs.

Spin Physics at RHIC

Abstract: The physics goals that will be addressed by colliding polarized protons at the Relativistic Heavy Ion Collider (RHIC) are described. The RHIC spin program provides a new generation of experiments that will unfold the quark, anti‐quark and gluon contributions to the proton’s spin. In addition to these longer term goals, this paper describes what was learned from the first polarized proton collisions at s=200 GeV. These collisions took place in a five‐week run during the second year of RHIC operation.

The "proton spin crisis" - a quantum query

Abstract: The "proton spin crisis" was introduced in the late 1980s, when the EMC-experiment revealed that little or nothing of a proton's spin seemed to be carried by its quarks. The main objective of this paper is to point out that it is wrong to assume that the proton spin, measured by completely different experimental setups, should be the same in all circumstances.

Polarized parton distributions from charged-current deep-inelastic scattering

Abstract: We investigate the capabilities of a neutrino factory in the determination of polarized parton distributions from charged-current deep-inelastic scattering experiments, with special attention to the accuracy of this kind of measurements. We show that a neutrino factory would allow to distinguish between different theoretical scenarios for the proton spin structure.

What do we know about the proton spin structure

Abstract: A brief summary of the theoretical and experimental knowledge of the spin structure of the proton is presented. The helicity distributions of quark and gluons are discussed, together with their related sum rules. The transversity distribution is also introduced with possible strategies for its measurement. Novel spin dependent and $\bfk_\perp$ unintegrated distribution and fragmentation functions are discussed, in connection with a new and rich phenomenology of transverse single spin asymmetries.

Acceleration of Polarized Protons at RHIC

Abstract: Relativistic Heavy Ion Collider (RHIC) ended its second year of operation in January 2002 with five weeks of polarized proton collisions. Polarized protons were successfully injected in both RHIC rings and maintained polarization during acceleration up to 100 GeV per ring using two Siberian snakes in each ring. This is the first time that polarized protons have been accelerated to 100 GeV. The machine performance and accomplishments during the polarized proton run will be reviewed. The plans for the next polarized proton run will be outlined.

Baryon resonances and strong QCD

Abstract: Light-baryon resonances (with u,d, and s quarks in the SU(3) classification) fall on Regge trajectories. When their squared masses are plotted against the intrinsic orbital angular momenta {\rm L}, $\Delta^*$'s with even and odd parity can be described by the same Regge trajectory. For a given {\rm L}, nucleon resonances with spin {\rm S}=3/2 are approximately degenerate in mass with $\Delta$ resonances. To which total angular momentum {\rm L} and {\rm S} couple has no significant impact on the baryon mass. Nucleons with spin 1/2 are shifted in mass; the shift is - in units of squared masses - proportional to the component in the wave function which is antisymmetric in spin and flavor. Based on these observations, a new baryon mass formula is proposed which reproduces nearly all known baryon masses. It is shown that the masses are compatible with a quark-diquark picture while the richness of the experimentally known states require three particles to participate in the dynamics. This conflict is resolved by proposing that quarks polarize the QCD condensates and are surrounded by a polarization cloud shielding the color. A new interpretation of constituent quarks as colored quark clusters emerges; their interaction is responsible for the mass spectrum. Fast flavor exchange between the colored quark clusters exhausts the dynamical richness of the three-particle dynamics. The colored-quark-cluster model provides a mechanism in which the linear confinement potential can be traced to the increase of the volume in which the condensates are polarized. The quark-spin magnetic moment induces currents in the polarized condensates which absorb the quark-spin angular momentum: the proton spin is not carried by quark spins. The model provides a new picture of hybrids and glueballs.

Pion and kaon polarizabilities at CERN COMPASS

Abstract: Objective: The electric ( ¯ ) and magnetic ( ¯ ) pion Compton polarizabilities characterize the pion’s deformation in the electromagnetic field of the during Compton scattering. They depend on the rigidity of the pion’s internal structure as a composite particle. The polarizabilities deduced by Antipov et al. in their low statistics Primakoff experiment ( 7000 events) were ¯ = ¯ = 6:8 1:4 1:2, in units of 10 43 cm 3 . This value, ignoring the large error bars, is about three times larger than the chiral perturbation theory (PT) prediction. Taking into account the very high beam intensity, fast data acquisition, high acceptance and good resolution of the CERN COMPASS experiment, one can expect from COMPASS statistics a factor 6000 higher, a data sample that includes many tests to control systematic errors, and a significantly reduced total measurement uncertainty for ¯ , of order 0.4. Methodology: CERN COMPASS studies of pion-photon interactions, to achieve a unique Primakoff physics program centered on pion polarizability studies. We use 100-200 GeV pion beams and a virtual photon target, and magnetic spectrometers and calorimeters to measure the complete kinematics of pion-photon reactions. COMPASS was set up during 2000/01, including a successful Primakoff test run, and then began data taking with a muon beam for the proton spin physics component of its program. COMPASS will next run its spin physics program and Primakoff program preparations, followed by its pion beam physics program, including pion polarizability. For pion polarizability, scattering will be measured via radiative pion scattering (pion Bremsstrahlung) in the nuclear Coulomb field: + Z ! 0 + + Z: A virtual photon from the Coulomb field of the target nucleus is scattered from the pion and emerges as a real photon accompanying the pion at small forward angles in the laboratory frame, while the target nucleus (in the ground state) recoils with a small transverse momentum kick pt. The radiative pion scattering reaction is equivalent to + ! + scattering for laboratory ’s of order 1 GeV incident on a target at rest. The pion polarizabilities are determined by their effect on the shape of the measured Compton scattering angular distribution. Significance:

On Spin Content of the Proton

Abstract: It is shown, in the frame of the model proposed by author, that spin of the proton could be described by the orbital momentum of quark and qluon condensate circulating around valence quarks.

Spin-Dependent Forces and Current Quark Masses

Abstract: The J = (3/2) Δ, J = 1/2 Nucleon mass difference shows the quark energies can be spin dependent. It is natural to expect that the quark wave functions also depend on spin. A spin-dependent quark force is fitted to the proton and neutron magnetic moments, axial charge, and spin content using a (1/2+)3 configuration for the quarks and assuming only zero mass u and d quarks are in the nucleon. In the octet, such spin-dependent forces lead to different wave functions for quarks with spin parallel or antiparallel to the nucleon spin. The eigen-energy of this potential is 0.15 GeV higher for quark spin parallel than for the quark spin antiparallel to the proton spin. This potential predicts a single quark energy of 0.37 GeV for mass-less quarks in the Delta. Assuming the quark forces are flavor independent, this potential predicts magnetic moments of a bound strange quark to be very close to those determined empirically from the octet magnetic moments.

Anomalous Temperature Dependence of Proton Spin-Lattice Relaxation in Tribromoacetamide (TBAA)

Abstract: The temperature dependence of proton spin–lattice relaxation rate was measured in a molecular crystal, tribromoacetamide (TBAA). An anomalous narrow peak of the relaxation rate, which may be related to the critical phenomena accompanying phase transition, has been found in the vicinity of phase transition temperatures. Another peak has also been observed, which is an ordinary broad one characteristic of the relaxation caused by the fluctuation of the dipolar interaction.