arXiv subjects

J. T. Londergan

Publications and source records attributed to J. T. Londergan.

At least 37 records · Page 2Linked to original sources

arXiv2006

Exclusive electroproduction and the quark structure of the nucleon

The natural interpretation of deep inelastic scattering is in terms of hard scattering on QCD constituents of the target. We examine the relation between amplitudes measured in exclusive lepto-production and the quark content of the nucleon. We show that in the Bjorken limit, the natural interpretation of amplitudes measured in these hard exclusive processes is in terms of the quark content of the meson cloud and not the target itself. Therefore the most efficient representation of these exclusive processes is in terms of leading Regge amplitudes.

hep-ph↗

arXiv2006

Parton Charge Symmetry Violation: Electromagnetic Effects and W Production Asymmetries

Recent phenomenological work has examined two different ways of including charge symmetry violation in parton distribution functions. First, a global phenomenological fit to high energy data has included charge symmetry breaking terms, leading to limits on the magnitude of parton charge symmetry breaking. In a second approach, two groups have included the coupling of partons to photons in the QCD evolution equations. One possible experiment that could search for isospin violation in parton distributions is a measurement of the asymmetry in W production at a collider. In this work we include both of the postulated sources of parton charge symmetry violation. We show that, given charge symmetry violation of a magnitude consistent with existing high energy data, the expected W production asymmetries would be quite small, generally less than one percent.

hep-ph↗

arXiv2005

Experimental Tests of Charge Symmetry Violation in Parton Distributions

Recently, a global phenomenological fit to high energy data has included charge symmetry breaking terms, leading to limits on the allowed magnitude of such effects. We discuss two possible experiments that could search for isospin violation in valence parton distributions. We show that, given the magnitude of charge symmetry violation consistent with existing global data, such experiments might expect to see effects at a level of several percent. Alternatively, such experiments could significantly decrease the upper limits on isospin violation in parton distributions.

hep-ph↗

arXiv2005

Final State Interactions in Decays of the Exotic $π_{1}$ Meson

We analyze the role of final state interactions in decay of the lighest exotic meson, $pi_1$ with $J^{PC}=1^{-+}. We use the relativistic Lippmann-Schwinger equation for two coupled $πb_{1}$ and $πρ$ channels. The first one is the predicted dominant decay mode of the $π_{1}$, whereas in the other a narrow $π_1(1600)$ exotic signal has been reported by the E852 collaboration. The FSI potential is constructed, based on the $ω$ meson exchange between the two channels. We find that this process introduces corrections to the $π_{1}$ widths of the order of only a few MeV. Therefore, we conclude that a substantial $πρ$ mode cannot be generated through level mixing.

hep-ph↗

arXiv2004

Possible Explanations for the NuTeV Weinberg Angle Measurement

The NuTeV collaboration has made an independent determination of the Weinberg angle by measuring charged and neutral-current cross sections from neutrino and antineutrino DIS on iron. Their value differs by 3 standard deviations from that obtained from measurements at the Z pole. We review this experiment and assess various possible explanations for this result, both within the Standard Model ("old physics") and outside the Standard Model ("new physics").

hep-ph↗

arXiv2004

Towards a Relativistic Description of Exotic Meson Decays

This work analyses hadronic decays of exotic mesons, with a focus on the lightest one, the $J^{PC}=1^{-+}$ $π_{1}$, in a fully relativistic formalism, and makes comparisons with non-relativistic results. We also discuss Coulomb gauge decays of normal mesons that proceed through their hybrid components. The relativistic spin wave functions of mesons and hybrids are constructed based on unitary representations of the Lorentz group. The radial wave functions are obtained from phenomenological considerations of the mass operator. Fully relativistic results (with Wigner rotations) differ significantly from non-relativistic ones. We also find that the decay channels $π_{1}\toπb_{1}, πf_{1}, KK_{1}$ are favored, in agreement with results obtained using other models.

hep-ph↗

arXiv2004

Constraints on Parton Charge Symmetry and Implications for Neutrino Reactions

For the first time, charge symmetry breaking terms in parton distribution functions have been included in a global fit to high energy data. We review the results obtained for both valence and sea quark charge symmetry violation, and we compare these results with the most stringent experimental upper limits on charge symmetry violation for parton distribution functions, and with theoretical estimates of charge symmetry violation. The limits allowed in the global fit would tolerate a rather large violation of charge symmetry. We discuss the implications of this for the extraction of the Weinberg angle in neutrino DIS by the NuTeV collaboration.

hep-ph↗

arXiv2003

Charge Symmetry Violation Corrections to Determination of the Weinberg Angle in Neutrino Reactions

We show that the correction to the Paschos-Wolfenstein relation associated with charge symmetry violation in the valence quark distributions is essentially model independent. It is proportional to a ratio of quark momenta that is independent of Q^2. This result provides a natural explanation of the surprisingly good agreement found between our earlier estimates within several different models. When applied to the recent NuTeV measurement, this effect significantly reduces the discrepancy with other determinations of the Weinberg angle.

hep-ph↗

arXiv2003

Separation of Soft and Hard Physics in Deeply Virtual Compton Scattering

A model for deeply virtual Compton scattering, based on analytical light-cone hadron wave functions is presented and studied at energies currently accessible at Jefferson Laboratory, DESY, and beyond. It is shown that perpendicular vector components play an important role at Q^2 < 10 GeV^2 and that the meson-exchange diagrams are important at all energies. This could significantly impact the physical interpretation of the underlying hadron amplitudes.

hep-ph↗

arXiv2003

Charge Symmetry Violating Contributions to Neutrino Reactions

The NuTeV group has measured charged and neutral current reactions for neutrinos on iron targets. Ratios of these cross sections provide an independent measurement of the Weinberg angle. The NuTeV value for sin^2 theta_W is three standard deviations larger than the value measured in other electroweak processes. By reviewing theoretical estimates of parton charge symmetry violation (CSV), we study CSV contributions to the NuTeV measurement. We conclude that charge symmetry violating effects should remove roughly 30% of the discrepancy between the NuTeV result and other determinations of sin^2 theta_W.

hep-ph↗

arXiv2002

Separation of Soft and Hard Physics in DVCS

A model for deeply virtual Compton scattering, based on analytical light-cone hadron wave functions is presented and studied at energies currently accessible at Jefferson Laboratory and DESY. It is shown that poles and perpendicular vector components play an important role at Q^2 < 10 GeV^2. A Q^2 suppressed diagram has to be included at these low energies, but becomes negligible above 10 GeV^2. Future prospects and developments of this model are discussed.

hep-ph↗

arXiv2001

Relativistic unitary description of pi pi scattering

A unitary framework based on the Bakamjian-Thomas construction of relativistic quantum mechanics is used to describe two-pion scattering from threshold to 1400 MeV. The framework properly includes unitarity cuts for one-, two- and three-hadron states and provides an excellent description of the available data for pi pi phase shifts and inelasticities. The role and importance of three-hadron cuts are calculated and discussed.

nucl-th↗

arXiv2000

Lambda Polarization in Polarized Proton-Proton Collisions at RHIC

We discuss Lambda polarization in semi-inclusive proton-proton collisions, with one of the protons longitudinally polarized. The hyperfine interaction responsible for the $Δ$-$N$ and $Σ$-$Λ$ mass splittings gives rise to flavor asymmetric fragmentation functions and to sizable polarized non-strange fragmentation functions. We predict large positive Lambda polarization in polarized proton-proton collisions at large rapidities of the produced Lambda, while other models, based on SU(3) flavor symmetric fragmentation functions, predict zero or negative Lambda polarization. The effect of $Σ^0$ and $Σ^*$ decays is also discussed. Forthcoming experiments at RHIC will be able to differentiate between these predictions.

hep-ph↗

arXiv1999

On the $ξ$-Distribution of Inclusively Produced Particles in $e^+e^-$ Annihilation

We discuss the momentum distributions of inclusively produced particles in $e^+e^-$ annihilation. We show that the dependence of the position of the maxima of the $ξ=\ln(1/z)$ spectra on the mass of the produced particles follows naturally from the general definition of fragmentation functions when energy-momentum conservation is correctly incorporated.

hep-ph↗

arXiv1999

Structure and Production of Lambda Baryons

We discuss the quark parton structure of the $Λ$ baryon and the fragmentation of quarks into $Λ$ baryons. We show that the hyperfine interaction, responsible for the $Δ$-$N$ and $Σ^0$-$Λ$ mass splittings, leads not only to sizeable SU(3) and SU(6) symmetry breaking in the quark distributions of the $Λ$, but also to significant polarized non-strange quark distributions. The same arguments suggest flavor asymmetric quark fragmentation functions and non-zero polarized non-strange quark fragmentation functions. The calculated fragmentation functions give a good description of all measured observables. We predict significant positive $Λ$ polarization in semi-inclusive DIS experiments while models based on SU(3) flavor symmetry predict zero or negative $Λ$ polarization. Our approach also provides a natural explanation for the dependence of the maximum of the $ξ=\ln(1/z)$ spectrum on the mass of the particles produced in $e^+e^-$ annihilation.

hep-ph↗

arXiv1999

A New Analysis of Charge Symmetry Violation in Parton Distributions

To date, the strongest indication of charge symmetry violation in parton distributions has been obtained by comparing the $F_2$ structure functions from CCFR neutrino data and NMC muon data. We show that in order to make precise tests of charge symmetry with the neutrino data, two conditions must be satisfied. First, the nuclear shadowing calculations must be made explicitly for neutrinos, not simply taken from muon data on nuclei. Second, the contribution of strange and charm quarks should be calculated explicitly using next-to-leading order [NLO] QCD, and the ``slow rescaling'' charm threshold correction should not be applied to the neutrino data. When these criteria are satisfied, the comparison is consistent with charge symmetry within the experimental errors and the present uncertainty in the strange quark distribution of the nucleon.

hep-ph↗

arXiv1998

Evidence for Charge Symmetry Violation in Parton Distributions

By comparing structure functions measured in neutrino and charged lepton deep inelastic scattering, one can test the validity of parton charge symmetry. New experiments allow us to make such tests, which set rather tight upper limits on parton charge symmetry violation [CSV] for intermediate Bjorken $x$, but which appear to show sizable CSV effects at small $x$. We show that neither nuclear shadowing nor contributions from strange and antistrange quark distributions can account for the experimentally observed difference between the two structure functions. We are therefore forced to consider the possibility of a large CSV effect in the nucleon sea quark distributions. We discuss the consequences of this effect for other observables, and we propose an experiment which could detect a large CSV component in the nucleon sea.

hep-ph↗

arXiv1998

Comparison of Neutrino and Muon Structure Functions, Shadowing Corrections and Charge Symmetry Violation

Comparison of structure functions measured in neutrino and charged lepton deep inelastic scattering can be used to test basic properties of parton distribution functions such as the validity of charge symmetry. Recent experiments indicate a substantial discrepancy between $F_2^ν$ and $F_2^μ$ in the region of small Bjorken-x. We discuss nuclear corrections and strange and anti strange quark effects and show that none of them can account for the observed discrepancy. These results suggest surprisingly large CSV effects in nucleon sea distributions.

hep-ph↗