arXiv subjects

C. Weiss

Publications and source records attributed to C. Weiss.

At least 37 records · Page 2Linked to original sources

arXiv2018

Nucleon and nuclear structure through dilepton production

Transverse momentum distributions and generalized parton distributions provide a comprehensive framework for the three-dimensional imaging of the nucleon and the nucleus experimentally using deeply virtual semi-exclusive and exclusive processes. The advent of combined high luminosity facilities and large acceptance detector capabilities enables experimental investigation of the partonic structure of hadrons with time-like virtual probes, in complement to the rich on-going space-like virtual probe program. The merits and benefits of the dilepton production channel for nuclear structure studies are discussed within the context of the International Workshop on Nucleon and Nuclear Structure through Dilepton Production taking place at the European Center for Theoretical Studies in Nuclear Physics and Related Areas (ECT$^{\star}$) of Trento. Particularly, the double deeply virtual Compton scattering, the time-like Compton scattering, the deeply virtual meson production, and the Drell-Yan processes are reviewed and a strategy for high impact experimental measurements is proposed.

nucl-ex↗

arXiv2018

Accurate nucleon electromagnetic form factors from dispersively improved chiral effective field theory

We present a theoretical parametrization of the nucleon electromagnetic form factors (FFs) based on a combination of chiral effective field theory and dispersion analysis. The isovector spectral functions on the two-pion cut are computed using elastic unitarity, chiral pion-nucleon amplitudes, and timelike pion FF data. Higher-mass isovector and isoscalar t-channel states are described by effective poles, whose strength is fixed by sum rules (charges, radii). Excellent agreement with the spacelike proton and neutron FF data is achieved up to Q^2 \sim 1 GeV^2. Our parametrization provides proper analyticity and theoretical uncertainty estimates and can be used for low-Q^2 FF studies and proton radius extraction.

hep-ph↗

arXiv2017

Nucleon form factors in dispersively improved Chiral Effective Field Theory II: Electromagnetic form factors

We study the nucleon electromagnetic form factors (EM FFs) using a recently developed method combining Chiral Effective Field Theory ($χ$EFT) and dispersion analysis. The spectral functions on the two-pion cut at $t > 4 M_π^2$ are constructed using the elastic unitarity relation and an $N/D$ representation. $χ$EFT is used to calculate the real functions $J_\pm^1 (t) = f_\pm^1(t)/F_π(t)$ (ratios of the complex $ππ\rightarrow N \bar N$ partial-wave amplitudes and the timelike pion FF), which are free of $ππ$ rescattering. Rescattering effects are included through the empirical timelike pion FF $|F_π(t)|^2$. The method allows us to compute the isovector EM spectral functions up to $t \sim 1$ GeV$^2$ with controlled accuracy (LO, NLO, and partial N2LO). With the spectral functions we calculate the isovector nucleon EM FFs and their derivatives at $t = 0$ (EM radii, moments) using subtracted dispersion relations. We predict the values of higher FF derivatives with minimal uncertainties and explain their collective behavior. We estimate the individual proton and neutron FFs by adding an empirical parametrization of the isoscalar sector. Excellent agreement with the present low-$Q^2$ FF data is achieved up to $\sim$0.5 GeV$^2$ for $G_E$, and up to $\sim$0.2 GeV$^2$ for $G_M$. Our results can be used to guide the analysis of low-$Q^2$ elastic scattering data and the extraction of the proton charge radius.

hep-ph↗

arXiv2017

Nucleon form factors in dispersively improved Chiral Effective Field Theory I: Scalar form factor

We propose a method for calculating the nucleon form factors (FFs) of $G$-parity-even operators by combining Chiral Effective Field Theory ($χ$EFT) and dispersion analysis. The FFs are expressed as dispersive integrals over the two-pion cut at $t > 4 M_π^2$. The spectral functions are obtained from the elastic unitarity condition and expressed as products of the complex $ππ\rightarrow N\bar N$ partial-wave amplitudes and the timelike pion FF. $χ$EFT is used to calculate the ratio of the partial-wave amplitudes and the pion FF, which is real and free of $ππ$ rescattering in the $t$-channel ($N/D$ method). The rescattering effects are then incorporated by multiplying with the squared modulus of the empirical pion FF. The procedure results in a marked improvement compared to conventional $χ$EFT calculations of the spectral functions. We apply the method to the nucleon scalar FF and compute the scalar spectral function, the scalar radius, the $t$-dependent FF, and the Cheng-Dashen discrepancy. Higher-order chiral corrections are estimated through the $πN$ low-energy constants. Results are in excellent agreement with dispersion-theoretical calculations. We elaborate several other interesting aspects of our method. The results show proper scaling behavior in the large-$N_c$ limit of QCD because the $χ$EFT calculation includes $N$ and $Δ$ intermediate states. The squared modulus of the timelike pion FF required by our method can be extracted from Lattice QCD calculations of vacuum correlation functions of the operator at large Euclidean distances. Our method can be applied to the nucleon FFs of other operators of interest, such as the isovector-vector current, the energy-momentum tensor, and twist-2 QCD operators (moments of generalized parton distributions).

hep-ph↗

arXiv2017

Electron-deuteron deep-inelastic scattering with spectator nucleon tagging and final-state interactions at intermediate x

We consider electron-deuteron deep-inelastic scattering (DIS) with detection of a proton in the nuclear fragmentation region ("spectator tagging") as a method for extracting the free neutron structure functions and studying their nuclear modifications. Such measurements could be performed at a future Electron-Ion Collider (EIC) with suitable forward detectors. The measured proton recoil momentum (~< 100 MeV in the deuteron rest frame) specifies the deuteron configuration during the high-energy process and permits a controlled theoretical treatment of nuclear effects. Nuclear and nucleonic structure are separated using methods of light-front quantum mechanics. The impulse approximation (IA) to the tagged DIS cross section contains the free neutron pole, which can be reached by on-shell extrapolation in the recoil momentum. Final-state interactions (FSI) distort the recoil momentum distribution away from the pole. In the intermediate-x region 0.1 < x < 0.5 FSI arise predominantly from interactions of the spectator proton with slow hadrons produced in the DIS process on the neutron (rest frame momenta ~< 1 GeV, target fragmentation region). We construct a schematic model describing this effect, using final-state hadron distributions measured in nucleon DIS experiments and low-energy hadron scattering amplitudes. We investigate the magnitude of FSI, their dependence on the recoil momentum (angular dependence, forward/backward regions), their analytic properties, and their effect on the on-shell extrapolation. We comment on the prospects for neutron structure extraction in tagged DIS with EIC. We discuss possible extensions of the FSI model to other kinematic regions (large/small x). In tagged DIS at x << 0.1 FSI resulting from diffractive scattering on the nucleons become important and require separate treatment.

hep-ph↗

arXiv2017

Peripheral transverse densities of the baryon octet from chiral effective field theory and dispersion analysis

The baryon electromagnetic form factors are expressed in terms of two-dimensional densities describing the distribution of charge and magnetization in transverse space at fixed light-front time. We calculate the transverse densities of the spin-1/2 flavor-octet baryons at peripheral distances b = O(M_π^{-1}) using methods of relativistic chiral effective field theory (χEFT) and dispersion analysis. The densities are represented as dispersive integrals over the imaginary parts of the form factors in the timelike region (spectral functions). The isovector spectral functions on the two-pion cut t > 4 M_π^2 are calculated using relativistic χEFT including octet and decuplet baryons. The χEFT calculations are extended into the ρmeson mass region using an N/D method that incorporates the pion electromagnetic form factor data. The isoscalar spectral functions are modeled by vector meson poles. We compute the peripheral charge and magnetization densities in the octet baryon states, estimate the uncertainties, and determine the quark flavor decomposition. The approach can be extended to baryon form factors of other operators and the moments of generalized parton distributions.

hep-ph↗

arXiv2017

Transverse densities of octet baryons from chiral effective field theory

Transverse densities describe the distribution of charge and current at fixed light-front time and provide a frame-independent spatial representation of hadrons as relativistic systems. We calculate the transverse densities of the octet baryons at peripheral distances b = O(M_pi^{-1}) in an approach combining chiral effective field theory (ChEFT) and dispersion analysis. The densities are represented as dispersive integrals of the imaginary parts of the baryon electromagnetic form factors in the timelike region (spectral functions). The spectral functions on the two-pion cut at t > 4 M_pi^2 are computed using relativistic ChEFT with octet and decuplet baryons in the EOMS renormalization scheme. The calculations are extended into the rho-meson mass region, using a dispersive method that incorporates the timelike pion form-factor data. The approach allows us to construct densities at distances b > 1 fm with controlled uncertainties. Our results provide insight into the peripheral structure of nucleons and hyperons and can be compared with empirical densities and lattice-QCD calculations.

hep-ph↗

arXiv2016

Heavy quark production at an Electron-Ion Collider

An Electron-Ion Collider (EIC) with center-of-mass energies sqrt(s_{eN}) ~ 20-100 GeV and luminosity L ~ 10^{34} cm^{-2} s^{-1} would offer new opportunities to study heavy quark production in high-energy electron or photon scattering on protons and nuclei. We report about an R&D project exploring the feasibility of direct measurements of nuclear gluon densities at large x (gluonic EMC effect, antishadowing) using open charm production at EIC. We describe the charm production rates and angle-momentum distributions at large x and discuss methods of charm reconstruction using next-generation detector capabilities (pi/K identification, vertex reconstruction). The results can be used also for other physics applications of heavy quark production at EIC (fragmentation functions, jets, heavy quark propagation in nuclei).

hep-ex↗

arXiv2016

Tagged spectator DIS on a polarized spin-1 target

We discuss the process of deep-inelastic electron scattering (DIS) on the polarized deuteron with detection of a nucleon in the nuclear fragmentation region ("spectator tagging"). We cover (a) the general structure of the semi-inclusive DIS cross section on a spin-1 target; (b) the tagged structure functions in the impulse approximation, where deuteron structure is described by the $NN$ light-front wave function; (c) the extraction of free neutron structure through on-shell extrapolation in the recoil proton momentum. As an application we consider the extraction of the neutron spin structure function $g_{1n}$ through polarized electron scattering on the longitudinally polarized deuteron with proton tagging and on-shell extrapolation. Such measurements would be possible at an Electron-Ion Collider (EIC) with polarized deuteron beams and forward proton detectors.

hep-ph↗

arXiv2016

Probing nuclear gluons with heavy quarks at EIC

We explore the feasibility of direct measurements of nuclear gluon densities using heavy-quark production (open charm, beauty) at a future Electron-Ion Collider (EIC). We focus on the regions x > 0.3 (EMC effect) and x ~ 0.05-0.1 (antishadowing), where the nuclear modifications of the gluon density offer insight into non-nucleonic degrees of freedom and the QCD structure of nucleon-nucleon interactions. We describe the charm production rates and momentum distributions in nuclear deep-inelastic scattering (DIS) at large x_B, and comment on the possible methods for charm reconstruction using next-generation detectors at the EIC (pi/K identification, tracking, vertex detection).

hep-ph↗

arXiv2016

Spin-flavor structure of chiral-odd GPDs in the large-N_c limit

We study the spin-flavor structure of the nucleon's chiral-odd generalized parton distributions (transversity GPDs) in the large-N_c limit of QCD. In contrast to the chiral-even case, only three combinations of the four chiral-odd GPDs are non-zero in the leading order of the 1/N_c expansion: E_T-bar = E_T + 2 H_T-tilde, H_T and E_T-tilde. The degeneracy is explained by the absence of spin-orbit interactions correlating the transverse momentum transfer with the transverse quark spin. It can also be deduced from the natural N_c-scaling of the quark-nucleon helicity amplitudes associated with the GPDs. In the GPD E_T-bar the flavor-singlet component u + d is leading in the 1/N_c expansion, while in H_T and E_T-tilde it is the flavor-nonsinglet components u - d. The large-N_c relations are consistent with the spin-flavor structure extracted from hard exclusive pi0 and eta electroproduction data, if it is assumed that the processes are mediated by twist-3 amplitudes involving the chiral-odd GPDs and the chiral-odd pseudoscalar meson distribution amplitudes.

hep-ph↗

arXiv2016

Light-front representation of chiral dynamics with Delta isobar and large-N_c relations

Transverse densities describe the spatial distribution of electromagnetic current in the nucleon at fixed light-front time. At peripheral distances b = O(M_pi^{-1}) the densities are governed by chiral dynamics and can be calculated model-independently using chiral effective field theory (EFT). Recent work has shown that the EFT results can be represented in first-quantized form, as overlap integrals of chiral light-front wave functions describing the transition of the nucleon to soft-pion-nucleon intermediate states, resulting in a quantum-mechanical picture of the peripheral transverse densities. We now extend this representation to include intermediate states with Delta isobars and implement relations based on the large-N_c limit of QCD. We derive the wave function overlap formulas for the Delta contributions to the peripheral transverse densities by way of a three-dimensional reduction of relativistic chiral EFT expressions. Our procedure effectively maintains rotational invariance and avoids the ambiguities with higher-spin particles in the light-front time-ordered approach. We study the interplay of pi-N and pi-Delta intermediate states in the quantum-mechanical picture of the densities in a transversely polarized nucleon. We show that the correct N_c-scaling of the charge and magnetization densities emerges as the result of the particular combination of currents generated by intermediate states with degenerate N and Delta. The off-shell behavior of the chiral EFT is summarized in contact terms and can be studied easily. The methods developed here can be applied to other peripheral densities and to moments of the nucleon's generalized parton distributions.

hep-ph↗

arXiv2016

QCD NLO with Powheg matching and top threshold matching in WHIZARD

We present the status of the automation of NLO processes within the event generator WHIZARD. The program provides an automated FKS subtraction and phase space integration over the FKS regions, while the (QCD) NLO matrix element is accessed via the Binoth Les Houches Interface from an externally linked one-loop program. Massless and massive test cases and validation are shown for several e+e- processes. Furthermore, we discuss work in progress and future plans. The second part covers the matching of the NRQCD prediction with NLL threshold resummation to the NLO continuum top pair production at lepton colliders. Both the S-wave and P-wave production of the top pair are taken into account in the resummation. The inclusion in WHIZARD allows to study more exclusive observables than just the total cross section and automatically accounts for important electroweak and relativistic corrections in the threshold region.

hep-ph↗

arXiv2016

Top Physics in WHIZARD

In this talk we summarize the top physics setup in the event generator WHIZARD with a main focus on lepton colliders. This includes full six-, eight- and ten-fermion processes, factorized processes and spin correlations. For lepton colliders, QCD NLO processes for top quark physics are available and will be discussed. A special focus is on the top-quark pair threshold, where a special implementation combines a non-relativistic effective field theory calculation augmented by a next-to-leading threshold logarithm resummation with a continuum relativistic fixed-order QCD NLO simulation.

hep-ph↗

arXiv2016

Automation of NLO processes and decays and POWHEG matching in WHIZARD

We give a status report on the automation of next-to-leading order processes within the Monte Carlo event generator WHIZARD, using GoSam and OpenLoops as provider for one-loop matrix elements. To deal with divergences, WHIZARD uses automated FKS subtraction, and the phase space for singular regions is generated automatically. NLO examples for both scattering and decay processes with a focus on e+e- processes are shown. Also, first NLO-studies of observables for collisions of polarized leptons beams, e.g. at the ILC, will be presented. Furthermore, the automatic matching of the fixed-order NLO amplitudes with emissions from the parton shower within the POWHEG formalism inside WHIZARD will be discussed. We also present results for top pairs at threshold in lepton collisions, including matching between a resummed threshold calculation and fixed-order NLO. This allows the investigation of more exclusive differential observables.

hep-ph↗

arXiv2016

Electron-deuteron DIS with spectator tagging at EIC: Development of theoretical framework

An Electron-Ion Collider (EIC) would enable next-generation measurements of deep-inelastic scattering (DIS) on the deuteron with detection of a forward-moving nucleon (p, n) and measurement of its recoil momentum ("spectator tagging"). Such experiments offer full control of the nuclear configuration during the high-energy process and can be used for precision studies of the neutron's partonic structure and its spin dependence, nuclear modifications of partonic structure, and nuclear shadowing at small x. We review the theoretical description of spectator tagging at EIC energies (light-front nuclear structure, on-shell extrapolation in the recoil nucleon momentum, final-state interactions, diffractive effects at small x) and report about on-going developments.

hep-ph↗

arXiv2016

Chiral-odd GPDs in large-N_c QCD

We study the flavor structure of the nucleon's chiral-odd generalized parton distributions (transversity GPDs) in the large-N_c limit of QCD. It is found that in the distributions H_T and E_T-tilde the flavor-nonsinglet component u - d is leading in the 1/N_c expansion, while in E_T and H_T-tilde it is the flavor-singlet component u + d. This pattern is consistent with the flavor structure extracted from hard exclusive pi^0 and eta electroproduction data, assuming that the processes are dominated by the twist-3 mechanism involving the chiral-odd pseudoscalar meson distribution amplitudes.

hep-ph↗

arXiv2015

Automated NLO QCD Corrections with WHIZARD

We briefly discuss the current status of NLO QCD automation in the Monte Carlo event generator WHIZARD. The functionality is presented for the explicit study of off-shell top quark production with associated backgrounds at a lepton collider.

hep-ph↗