Physics Models

Deuteron, unpolarized, proton/neutron tagged, inclusive scattering

Spectator tagging in inclusive DIS on the unpolarized deuteron e + De'+p+X is used for (a) precision measurements of the free neutron structure functions F2n and FLn, by measuring the dependence on the recoil proton momentum at low momenta pR < 100 MeV (rest frame) and extrapolating to the on-shell point; (b) exploration of the bound neutron structure functions, by measuring the recoil momentum dependence at high momenta pR > 100 MeV. The physics models aim to describe effects of nuclear binding in the impulse approximation, using the deuteron light-front spectral function; and final-state interactions between the produced in the deep-inelastic scattering process and the spectator nucleon. An important goal is to implement the correct analytic properties of the tagged cross section in the recoil momentum for the on-shell extrapolation. Presently available physics models are:

  • TAG: Package implementing the basic e + De'+N+X cross section with application routines (tabulation, counting rate estimates). Minimal model using light-front impulse approximation, analytic deuteron wave function, and empirical QCD-based parametrizations of the free nucleon structure functions. No final-state interactions (may be added). Allows user-defined deuteron wave function and nucleon structure functions. FORTRAN 77 code, modular structure, customizable. Intended mainly for process simulations and studies of on-shell extrapolation. Maintained by C. Weiss. [Documentation, Code]
  • Full physics model of the e + De'+N+X cross section with application routines. Implements impulse approximation with realistic deuteron wave functions (including high-momentum components), parametrizations of the free nucleon structure functions, and an empirical model of final-state interactions (work in progress). Pre-defined deuteron wave functions and nucleon structure functions. C++ code, requires standard unix tools for compilation and execution. Intended for full physics simulations at moderate and large recoil momenta. Maintained by W. Cosyn. [Documentation and download instructions]

 

Deuteron, unpolarized, proton/neutron tagged, small-x shadowing

Coherent nuclear effects influence the interpretation of spectator tagging experiments at small x and are an interesting field of study in their own right. At x < 0.01 there is a significant probability for DIS on a single nucleon to leave the nucleon intact and recoiling with a momentum of ~few 100 MeV in the final state (diffractive scattering). In DIS on the deuteron this channel enables quantum-mechanical interference between scattering on the proton and the neutron and causes a deviation from the impulse approximation known as nuclear shadowing. In DIS with specator tagging e + De'+p+X the same mechanism distorts the recoil proton momentum spectrum. It also enables strong final-state interactions between the proton and neutron recoiling at small relative momenta. Available physics models:

  • Shadowing correction to e + De'+N+X cross section at small x. Recoil momentum-dependent shadowing correction calculated using empirical diffractive nucleon structure functions and deuteron light-front wave function [Frankfurt, Guzey, Strikman, Phys. Rept. 512 (2012) 255, INSPIRE ]. No explicit low-energy final-state interactions (to be added). Code provides numerical value of shadowing correction through grid interpolation. Shadowing correction available in the form of "ratio to" and "difference from" impulse approximation. FORTRAN 77 code. Maintained by V. Guzey. [Documentation, Code]

 

Deuteron, longitudinally polarized, proton/neutron tagged, inclusive scattering

DIS on the longitdinally polarized deuteron with spectator proton tagging, e + De'+p+X, represents a powerful method for measuring the neutron spin structure functions g1n and g2n. On-shell extrapolation in the recoil proton momentum eliminates nuclear modifications of the neutron structure function as well as the polarization uncertainty due to the deuteron's D-wave. Tagging can also be used to study the nuclear modifications of spin structure functions at higher recoil momenta pR > 100 MeV. Physics models aim to describe the deuteron spin structure functions in the impulse approximation, using the spin-dependent light-front spectral function, and to include final-state interactions and shadowing effects. Presently available:

  • TAG: Implements double-polarized e + De'+N+X cross section in EIC kinematics. Electron and deuteron longitudinally polarized along respective beam directions. Deuteron cross section calculated in light-front impulse approximation, no final-state interactions or shadowing. Minimal approximation to polarized deuteron structure: no D-wave, no relativistic spin-orbit effects (to be added). User-defined nucleon spin structure functions. FORTRAN 77 code, modular structure, customizable. Intended for process simulations and studies of on-shell extrapolation. Maintained by C. Weiss. [Documentation, Code]
  • Full physics model for polarized e + De'+N+X is being developed. Includes realistic wave functions, D-wave, and relativistic spin-orbit effects.

 

Helium-3, unpolarized, inclusive scattering

Spectator tagging in DIS on A > 2 nuclei offers many interesting possibilities (breakup channels, configurations), but the theoretical interpretation of the recoil momentum dependence is generally much more challenging than for A = 2. The high-energy scattering process can happen in a variety of initial-state nuclear configurations, and the recoil momentum spectrum is strongly affected by low-energy final-state interactions. R&D focuses on identifying exceptional channels/configurations in which a simple theoretical interpretation becomes possible. Examples are the the deuteron breakup channel in Helium-3, e + 3He → e'+D+X, or the double proton breakup channel e + 3He → e'+ (pp) + X, or nucleon tagging at large recoil momenta pR > 100 MeV. The development of physics models for these processes is at a very early stage. Available models are:

  • Preliminary implementation of unpolarized e + 3He → e'+N + N +X. Impulse approximation, no final-state interactions. 3He spectral function derived from realistic 3-body wave function. C++ code, requires standard unix tools for compilation and execution. Intended for process simulations and rate estimates. Maintained by W. Cosyn. [Documentation and download instructions]

 

 

Notice: The physics model codes posted here include copies of freely available scientific computer codes from other authors, which implement results published in the literature and provide specific input to the physics models (e.g., parametrizations of the nucleon structure functions). These codes are redistributed in accordance with standard practice in scientific applications, to allow users to compile the physics model codes from a single source. Users are advised to reference the appropriate journal articles whenever publishing results based on a particular input code. References are given in the instructions to the individual physics model codes.