1,160 research outputs found
The Gerasimov-Drell-Hearn Sum Rule and the Spin Structure of the Nucleon
The Gerasimov-Drell-Hearn sum rule is one of several dispersive sum rules
that connect the Compton scattering amplitudes to the inclusive photoproduction
cross sections of the target under investigation. Being based on such universal
principles as causality, unitarity, and gauge invariance, these sum rules
provide a unique testing ground to study the internal degrees of freedom that
hold the system together. The present article reviews these sum rules for the
spin-dependent cross sections of the nucleon by presenting an overview of
recent experiments and theoretical approaches. The generalization from real to
virtual photons provides a microscope of variable resolution: At small
virtuality of the photon, the data sample information about the long range
phenomena, which are described by effective degrees of freedom (Goldstone
bosons and collective resonances), whereas the primary degrees of freedom
(quarks and gluons) become visible at the larger virtualities. Through a rich
body of new data and several theoretical developments, a unified picture of
virtual Compton scattering emerges, which ranges from coherent to incoherent
processes, and from the generalized spin polarizabilities on the low-energy
side to higher twist effects in deep inelastic lepton scattering.Comment: 32 pages, 19 figures, review articl
The {\eta}'-carbon potential at low meson momenta
The production of mesons in coincidence with forward-going
protons has been studied in photon-induced reactions on C and on a
liquid hydrogen (LH) target for incoming photon energies of 1.3-2.6 GeV at
the electron accelerator ELSA. The mesons have been identified
via the decay
registered with the CBELSA/TAPS detector system. Coincident protons have been
identified in the MiniTAPS BaF array at polar angles of . Under these kinematic constraints the
mesons are produced with relatively low kinetic energy (
150 MeV) since the coincident protons take over most of the momentum of the
incident-photon beam. For the C-target this allows the determination of the
real part of the -carbon potential at low meson momenta by
comparing with collision model calculations of the kinetic energy
distribution and excitation function. Fitting the latter data for
mesons going backwards in the center-of-mass system yields a potential depth of
V = (44 16(stat)15(syst)) MeV, consistent with earlier
determinations of the potential depth in inclusive measurements for average
momenta of 1.1 GeV/. Within the experimental
uncertainties, there is no indication of a momentum dependence of the
-carbon potential. The LH data, taken as a reference to check
the data analysis and the model calculations, provide differential and integral
cross sections in good agreement with previous results for
photoproduction off the free proton.Comment: 9 pages, 13 figures. arXiv admin note: text overlap with
arXiv:1608.0607
Experimental constraints on the -nucleus real potential
In a search for mesic states, the production of -mesons in
coincidence with forward going protons has been studied in photon induced
reactions on C for incident photon energies of 1250 - 3100 MeV. The
pairs from decays of bound or quasi-free -mesons have
been measured with the CBELSA/TAPS detector system in coincidence with protons
registered in the MiniTAPS forward array. Structures in the total energy
distribution of the pairs, which would indicate the population
and decay of bound B states, are not observed. The
cross section of 0.3 nb/MeV/sr observed in the bound state energy regime
between -100 and 0 MeV may be accounted for by yield leaking into the bound
state regime because of the large in-medium width of the -meson. A
comparison of the measured total energy distribution with calculations suggests
the real part of the B potential to be small and only
weakly attractive with 35(stat) 20(syst) MeV
in contrast to some theoretical predictions of attractive potentials with a
depth of 100 - 150 MeV.Comment: 13 pages, 8 figure
Search for the decay at Belle
We report a search for the rare charmless decay
using a data sample of
pairs collected at the resonance with
the Belle detector at the KEKB asymmetric-energy collider. No
statistically significant signal is found and a 90% confidence-level upper
limit is set on the decay branching fraction as .Comment: 8 pages, 3 figures, submitted to PRD(RC
Measurements of the masses and widths of the and baryons
We present measurements of the masses and decay widths of the baryonic states
and using a data sample
corresponding to an integrated luminosity of 711 fb collected with the
Belle detector at the KEKB asymmetric-energy collider operating at
the resonance. We report the mass differences with respect to
the baryon MeV/, MeV/,
MeV/, MeV/, and the decay widths
MeV/,
MeV/,
MeV/,
MeV/,
where the first uncertainties are statistical and the second are systematic.
The isospin mass splittings are measured to be
MeV/ and
MeV/. These results are the most precise to date.Comment: 13 pages, 4 figures, Submitted to PRD(RC
Evidence for Isospin Violation and Measurement of Asymmetries in
We report the first evidence for isospin violation in and
the first measurement of difference of asymmetries between and . This analysis is based on the data sample
containing pairs that was collected with the Belle
detector at the KEKB energy-asymmetric collider. We find evidence for
the isospin violation with a significance of 3.1, \%, where
the third uncertainty is due to the uncertainty on the fraction of to
production in decays. The measured value is
consistent with predictions of the SM. The result for the difference of
asymmetries is \%, consistent with zero. The measured branching fractions and
asymmetries for charged and neutral meson decays are the most precise to
date. We also calculate the ratio of branching fractions of to .Comment: 11 pages, 7 figures. shown at FPCP2017. accepted by PR
Search for decays with semileptonic tagging at Belle
We present the results of a search for the rare decays , where stands for and . The results are
obtained with pairs collected with the Belle
detector at the KEKB collider. We reconstruct one meson in a
semileptonic decay and require a single meson but nothing else on the
signal side. We observe no significant signal and set upper limits on the
branching fractions. The limits set on the , , , ,
, and channels
are the world's most stringent.Comment: Submitted to PR
Photoproduction of eta mesons from the neutron: cross sections and double polarization observable E
Photoproduction of mesons from neutrons} \abstract{Results from
measurements of the photoproduction of mesons from quasifree protons and
neutrons are summarized. The experiments were performed with the CBELSA/TAPS
detector at the electron accelerator ELSA in Bonn using the
decay. A liquid deuterium target was used for the
measurement of total cross sections and angular distributions. The results
confirm earlier measurements from Bonn and the MAMI facility in Mainz about the
existence of a narrow structure in the excitation function of . The current angular distributions show a forward-backward
asymmetry, which was previously not seen, but was predicted by model
calculations including an additional narrow state. Furthermore, data
obtained with a longitudinally polarized, deuterated butanol target and a
circularly polarized photon beam were analyzed to determine the double
polarization observable . Both data sets together were also used to extract
the helicity dependent cross sections and . The
narrow structure in the excitation function of
appears associated with the helicity-1/2 component of the reaction
Measurement of the Branching Fraction of the Decay in Fully Reconstructed Events at Belle
We present an analysis of the exclusive
decay, where represents an
electron or a muon, with the assumption of charge-conjugation symmetry and
lepton universality. The analysis uses the full data sample
collected by the Belle detector, corresponding to 711 fb of integrated
luminosity. We select the events by fully reconstructing one meson in
hadronic decay modes, subsequently determining the properties of the other
meson. We extract the signal yields using a binned maximum-likelihood fit to
the missing-mass squared distribution in bins of the invariant mass of the two
pions or the momentum transfer squared. We measure a total branching fraction
of , where the
uncertainties are statistical and systematic, respectively. This result is the
first reported measurement of this decay.Comment: 23 pages, 19 figure
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