125593487
NEW DUALITY CONSTRAINTS ON MESON AMPLITUDES
120719039
Duality constraints for mesons
Abstract A new method of imposing semi-local duality constraints is discussed, which satisfies the theoretical requirements of crossing symmetry and self-consistency. The duality relations involve only physical on-shell particles, and give many conditions on their masses and couplings. All constraints from PP→PP and PV→PP amplitudes with ordinary and strange particles give compatible results and are in good agreement with experiment. In particular, the near linearity of Regge trajectories is explained. The internal consistency of constraints with charmed particles is not as good, and implies stronger contributions from daughter resonances.
35241283
Mass limits on particles from pulsed sources: How reliable are they?
It is widely believed that the arrival at Earth of pulsed emission from a cosmic accelerator such as Cygnus X-3 puts mass limits on the particles carrying the radiation of order 1 GeV. We show that these simple kinematic estimates are unreliable and can often be avoided. The actual mass limit depends critically on the detailed structure of the pulse, i.e., on (i) the shape of the flux as a function of energy and (ii) the minimum and maximum energies. We illustrate this by showing that masses of 100 GeV or more for the ''cygnets'' responsible for the unexplained pulsed muon signal from Cygnus X-3 can be made consistent with present experiments. These masses are large enough to accommodate the 3/sup 0/ angular spread of the muon pulses and to exclude the production of cygnets in accelerator experiments.
10635169
Nuclear dependence of structure functions in coordinate space
The momentum distributions of partons in bound nucleons are known to depend significantly on the size of the nucleus. The Fourier transform of the momentum (xBj) distribution measures the overlap between Fock components of the nucleon wave function which differ by a displacement of one parton along the light cone. The magnitude of the overlap thus determines the average range of mobility of the parton in the nucleon. By comparing the Fourier transforms of structure functions for several nuclei we study the dependence of quark mobility on nuclear size. We find a surprisingly small nuclear dependence (< 2% for He, C and Ca) for displacements t = z ≲ 2.5 fm, after which a nuclear suppression due to shadowing sets in. The nuclear effects observed in momentum space for xBj ≲ 0.4 can be understood as a reflection of only the large distance shadowing in coordinate space.
125123165
Systematics of charm production in hadronic collisions
Charmed particles produced in hadron collisions have a harder momentum spectrum than predicted by QCD at leading twist. This and related features of the data can be understood as resulting from coalescence of the charm quark with co‐moving light quarks, and a contribution of intrinsic charm at large values of xF.
120494867
Parton evolution as a branching process
Abstract The evolution of parton distributions in high-energy scattering is described using the methods of branching processes. Exclusive, inclusive and multiplicity distributions can be simply obtained from a functional form of the evolution equations. Previous results derived from leading logarithmic terms in perturbation theory are verified. As a new application, we calculate the multiplicity distribution of partons with momenta above a given cutoff.
117547375
Filling Perturbative Ground States
I discuss a degree of freedom in formulating perturbation theory that is often neglected: the in- and out-states need not be empty. The inclusion of (free) particles in the asymptotic states modifies the on-shell prescription of the free propagator. This affects loop contributions -- but the modified expansion is {\it a priori} as justified as the standard one with Feynman prescription. 
It is possible to dress the quark propagator to all orders with zero-momentum gluons from the perturbative ground state. The dressed quark has no pole and thus cannot appear as an external particle in the S-matrix. Chiral symmetry may be spontaneously broken, but Lorentz and gauge symmetry is exact. Adding loop corrections to this ``dressed tree approximation'' gives a formally exact PQCD expansion.
119307830
Bound states at lowest order in hbar
Bound states poles in scattering amplitudes are generated by the divergence of the perturbative series due to enhanced Coulomb scattering near thresholds. This suggests to organize bound state calculations according to an expansion in hbar, i.e., in the number of loops. I study QED and QCD bound states at lowest order in hbar, which are analogous to Born amplitudes. The absence of loops allows the use of retarded boundary conditions where particles only propagate forward in time, making a hamiltonian approach feasible. The instantaneous A^0 field is determined by the equations of motion separately for each Fock component of the bound state. The field equations are compatible with a linear A^0 potential as a homogeneous, non-perturbative solution. Stationarity of the action determines the direction of the ensuing constant electric field in each Fock state. Applying this approach to relativistic qqbar and qqq states in QCD results in a bound state equation which provides a reasonable description of the spectrum, including linear Regge trajectories. The equal-time wave functions have unique Lorentz transformation properties, which ensure the correct dependence of the bound state energy on the center-of-mass momentum. The qqq potential is gauge covariant and confines the three quarks in a symmetric way.
118891192
Rescattering Effects on DIS Parton Distributions
Rescattering of the struck quark in Deep Inelastic Scattering implies that measured parton distributions are not directly related to the Fock state probabilities of the target wave function. The production amplitudes acquire dynamical phases, which gives rise to shadowing and diffraction in DIS.
18881983
QCD factorization at fixed Q^2(1-x)
Amplitudes of hard {\it exclusive} processes such as \gamma^*(Q^2) N \to \gamma Y, where Y=N (DVCS) or any other state with a limited mass (M_Y^2 << Q^2), factorize into a hard subprocess amplitude and a target (transition) GPD. The corresponding {\it inclusive} cross section, summed over all states Y of a given (limited) mass, is then given by the discontinuity of a forward multiparton distribution. An application to the Drell-Yan process \pi^+ N \to \gamma^*(x_F,Q^2)+Y allows to explain the observed longitudinal polarization of the virtual photon at high x_F.
119327590
Semi-exclusive DVCS
We consider Semi-Exclusive Deeply Virtual Compton Scattering, gamma* p \to gamma Y (SECS), where Y is an inclusive state of intermediate mass, \Lambda_QCD<<m_Y<<Q. When the photon is produced with a large transverse momentum k_T \sim m_Y the subprocess is hard and the struck quark fragments independently of the target spectators. Using completeness this allows to express the SECS cross section in terms of ordinary parton distributions. Apart from direct comparisons with data (yet to come) new information on Bloom-Gilman duality may be obtained through comparisons of resonance production via DVCS (gamma* p \to gamma N*) with the SECS scaling distribution in m_Y.