251 1.1. The concept of "antigravity" 223 5.3. Morrison's "antigravity" analysis 253 1.2. The arguments against "antigravity" 225 5.4. 3ravivector field 253 1.3. The antiproton gravity experiment 226 5.5. Graviscalar field 253 1.4. Gravity and CPT 226 5.6. Graviscalar field coupled to electromagnetism 254 2. Theoretical ideas on gravity and antimatter 227 5.7. Newtonian gravity using an apparent paradox 254 2.1. Quantum gravity 227 6. The Schiff argument 255 2.2. Quantum gravity and antimatter 230 6.1. QED vacuum polarization and the principle ofequiva-3. Particle and antiparticle gravity experiments 232 lence 255 3.1. The Fairbank experiments 232 6.2. QCD vacuum polarization 257 3.2. The antiproton gravity experiment 234 7. The Good argument 258 3.3. Other antimatter gravity experiments 236 7.1. The argument using absolute potentials 258 3.4. Laboratory tests of gravity on neutrons and photons 237 7.2. The argument independent of absolute potentials 259 4. Other experiments relevant to antimatter gravity 239 8. Conclusions 262 4.1. Airy experiments 239 8.1. Experimental restrictions on anomalous antimatter 4.2. Tests of the inverse-square law 241 gravity 262 4.3. Tests of the principle of equivalence 245 8.2. Critical experiments for longer-ranged forces 267 4.4. Astrophysics experiments 248 8.3. Antiproton gravity and the principle of equivalence 269 5. The Morrison argument 250 References 270 5.1. Invariance principles 250 * This review is dedicated to the memory of William Fairbank. His pioneering attempt to measure the gravitational acceleration of the positron was crucial to this field. Fairbank had strongly urged us to write this article.

Experimental data indicate small spin-orbit splittings in hadrons. For heavy-light mesons we identify a relativistic symmetry that suppresses these splittings. We suggest an experimental test in electron-positron annihilation. Furthermore, we argue that the dynamics necessary for this symmetry are possible in QCD.

We examine the conventional picture that gluons carry about half of the nucleon momentum in the asymptotic limit. We show that this large fraction is due to an unsuitable definition of the gluon momentum in an interacting theory. If defined in a gauge-invariant and consistent way, the asymptotic gluon momentum fraction is computed to be only about one-fifth. This result suggests that the asymptotic limit of the nucleon spin structure should also be reexamined. A possible experimental test of our finding is discussed in terms of novel parton distribution functions.

We study the relative contribution of partonic sub-processes to D meson production and D mesontriggered inclusive di-hadrons to lowest order in perturbative QCD. While gluon fusion dominates the creation of large angle DD pairs, charm on light parton scattering determines the yield of single inclusive D mesons. The distinctly different non-perturbative fragmentation of c quarks into D mesons versus the fragmentation of quarks and gluons into light hadrons results in a strong transverse momentum dependence of anticharm content of the away-side charm-triggered jet.In p+A reactions, we calculate and resum the coherent nuclear-enhanced power corrections from the final-state partonic scattering in the medium. We find that single and double inclusive open charm production can be suppressed as much as the yield of neutral pions from dynamical high-twist shadowing. Effects of energy loss in p+A collisions are also investigated phenomenologically and may lead to significantly weaker transverse momentum dependence of the nuclear attenuation.

We show how, by considering the cumulative effect of tiny quantum gravitational fluctuations over very large distances, it may be possible to: (a) reconcile nucleosynthesis bounds on the density parameter of the Universe with the predictions of inflationary cosmology, and (b) reproduce the inferred variation of the density parameter with distance. Our calculation can be interpreted as a computation of the contribution of quantum gravitational degrees of freedom to the (local) energy density of the Universe.