Single spin asymmetries in inclusive high energy hadron-hadron collision processes

TL;DR

Single-spin asymmetries in high-energy hadron-hadron collisions are reviewed, focusing on large left-right asymmetries $A_N$ that challenge perturbative QCD predictions. The paper contrasts perturbative QCD hard-scattering models with a nonperturbative Berliner orbiting-valence-quark model, emphasizing orbital motion and hadronic surface effects as sources of spin phenomena. It surveys data characteristics across pions, kaons, hyperons, and lepton pairs, and outlines experimental tests to discriminate between competing mechanisms. It also links $A_N$ to hyperon polarization and proposes DIS and photon-dissociation probes to study photon structure and test the underlying dynamics of spin effects.

Abstract

It has been realized for quite a long time that single-spin experiments, in which one of the colliding objects is transversely polarized, can be helpful in studying the properties of strong interaction in general and in testing Quantum Chromodynamics (QCD) in particular. Striking effects have been observed in the past few years which deviate drastically from the expectation of the perturbative QCD parton model. These effects have received much attention. New experiments of the similar type are underway and/or planned. Different theoretical attempts have been made to understand these effects. In this review, the special role played by singly polarized high-energy hadron-hadron collisions in High Energy Spin Physics is emphasized. Characteristics of the available data for inclusive hadron productions are briefly summarized. Different theoretical approaches for such processes are reviewed with special attention to a non-perturbative model which explicitly takes the orbital motion of the valence quarks and hadronic surface effects into account. The connection between such asymmetries and hyperon polarization in unpolarized reactions is discussed. An example of the possible application of such experimental results in other processes is given.

Figures (17)

• Figure 1: Data for left-right asymmetries $A_N$'s from FNAL E704 Collaboration.
• Figure 2: Schematic illustration of factorization theorem in $A+B\to C+X$. The square in the center of the figure represents the elementary process $a+b\to c+d$ which can be calculated using perturbative QCD.
• Figure 3: The non-direct-formation parts of the inclusive pion production cross section for $p(0)+p(0)\to \pi^+ +X$ and $p(0)+p(0)\to \pi^- +X$ are shown as function of $x_F$. This figure is taken from [\ref{['LM94']}].
• Figure 4: The measured $x_F$-distribution for $p(0)+p(0)\to\pi^++X$ and that for $p(0)+p(0)\to\pi^-+X$ are shown as the sum of the following two parts: (1) the isospin-independent non-direct-formation part (parameterization mentioned in Fig.2 shown as dashed curves), (2) the corresponding isospin-dependent parts $\kappa _\pi u_v(x_F)\bar{q}_s(x_0/x_F) x_F$ for $\pi ^+$ and $\kappa _\pi d_v(x_F)\bar{q}_s(x_0/x_F) x_F$ for $\pi ^-$ (shown as dash-dotted curves). This figure is taken from [\ref{['LM94']}].
• Figure 5: Inclusive invariant cross section $Ed^3\sigma/d^3p$ for $p(0)+p(0)\to K^++X$ as a function of $x_F$ the ISR energies is shown as the sum of the following two parts: (1) the isospin-independent non-direct-formation part which is taken as the same as $Ed^3\sigma/d^3p$ for $p(0)+p(0)\to K^-+X$ [parameterized as $N(1-x_F)^3exp(-10x_F^3)$, shown by the dashed curve], (2) the corresponding flavor-dependent direct formation part $\kappa _K u_v(x_F)\bar{s}_s(x_0/x_F) x_F$ (shown by the dashed-dotted curve). This figure is taken from [\ref{['BLM96']}].