35371897
Semi-exclusive production of photons at HERA
Abstract. We study the feasibility of measuring the semi-exclusive photon production $\gamma + p \to \gamma + Y$ at HERA. The cross section of photons produced at large transverse momenta, recoiling off an inclusive system Y of limited mass, can without photon isolation cuts simply be expressed in terms of hard PQCD subprocesses and standard target parton distributions. With the help of event generators we identify the kinematic region where quark and gluon fragmentation processes can be neglected. The cross section in this semi-exclusive region is large enough to be measured with an upgraded HERA luminosity of ${\cal L} = 100 {\mathrm{pb}}^{-1}$. The subprocesses of lowest order in $\alpha_{\mathrm{s}}$ are suppressed at low recoil masses $M_Y$, compared to higher order gluon exchange, i.e. BFKL contributions. The distinct $M_Y$-dependence makes it possible to determine experimentally the kinematic range where the higher order processes dominate.
120890782
Mesons and baryons from QCD
Abstract We study a gauge covariant method for quark bound states in 3 + 1 dimensions, based on a stationary phase approximation of the gluon path integral. Assuming thet lim | x |→∞ F μν a ( x ) F a μν ( x ) ≠ 0 we find meson (q q ) and baryon (qqq) states bound by a Coulomb + linear potential. The quark wave function is confined, as in the bag model. The high-spin states lie on asymptotically linear Regge trajectories of universal slope.
119081579
Perturbative QCD with quark and gluon condensates
The quark 〈 ψ ¯ ψ 〉 and gluon ( F μν F μν ) vacuum expectation values are non-vanishing at low orders in α s when the perturbative ground state includes quark and gluon pairs. This offers possibilities of studying the effects of a non-trivial vacuum using formally exact perturbative methods. The QCD Feynman rules are modified only for the free, on-shell quark and gluon propagators at zero four-momentum. Gauge and lorentz invariance is maintained, while chiral symmetry is spontaneously broken by the ground state.
125593487
NEW DUALITY CONSTRAINTS ON MESON AMPLITUDES
121596214
Test of duality in baryon--antibaryon amplitudes from Reggeon--particle scattering
Abstract By studying the backward production data for meson resonances we investigate the dual properties of baryon-Reggeised antibaryon elastic scattering. We find strong support for the duality prediction that meson resonances in the baryon antibaryon channel should be dual to an exotic trajectory.
120877951
Sister trajectories in the dual resonance model
Abstract The existence of a Regge tranjectory γ = α /3 − 1 in the eight-point amplitude of the Dual Resonance Model set of demonstrated. It is suggested that the three presently known trajectories α, β and γ are part of an infinite set of “sister” trajectories, the expected properties of which are discussed.
119388879
Comments on the Relativity of Shape
In this talk I address three topics related to the shape of hadrons:1. The Lorentz contraction of bound states. Few dedicated studies of this exist — I describe a recent calculation for ordinary atoms (positronium).2. Does the A‐dependence of nuclear structure functions indicate a change of proton shape in the nuclear environment? (My short answer is no.)3. The size of Fock states contributing to processes involving large momentum transfers. End‐point configurations can be transversally extended and yet sufficiently short‐lived to contribute coherently to hard scattering.
18528565
Factorization at fixed Q^2(1-x)
We consider QCD factorization between hard and soft subprocesses in inclusive reactions where the momentum fraction x of one parton approaches unity as the hard scale Q^2 -> \infty, such that Q^2(1-x) is fixed. In this ``BB limit'' the entire (multi-parton) Fock state containing the high x parton is coherent with the hard subprocess. The soft contribution is given by a forward multiparton matrix element. The BB limit corresponds to a fixed (large or small) missing mass and is thus closely connected to exclusive production. We analyze the Drell-Yan process h + N -> \gamma^* + X in detail, explaining why the virtual photon is longitudinally polarized for h = \pi and transversely polarized for h = p. The BB limit may be relevant also for other phenomena observed at high x, such as the large single spin asymmetries of p p -> \Lambda^\uparrow X and in p p^\uparrow -> \pi X.
119250582
Inclusive Perspectives
I discuss the relation between inclusive and exclusive dynamics suggested by Bloom-Gilman duality. Duality implies the simultaneous applicability of two distinct limits, the standard DIS limit of hard inclusive processes taken at fixed xB and a limit where the hadronic mass is held fixed. I review experimental evidence for the relevance of the fixed mass limit in inclusive processes at high xF. Semi-local duality suggests that inclusive and exclusive processes occur on the same target Fock states. DIS scaling then implies that the Fock states contributing to hard exclusive processes have a large transverse size, i.e., that the hard scattering occurs off a single parton which carries a large fraction of the hadron momentum.
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.
118260296
Introduction to QCD - A Bound State Perspective
These lecture notes focus on the bound state sector of QCD. Motivated by data which suggests that the strong coupling \alpha_s(Q) freezes at low Q, and by similarities between the spectra of hadrons and atoms, I discuss if and how QCD bound states may be treated perturbatively. I recall the basic principles of perturbative gauge theory bound states at lowest order in the \hbar expansion. Born level amplitudes are insensitive to the i\epsilon prescription of propagators, which allows to eliminate the Z-diagrams of relativistic, time-ordered Coulomb interactions. The Dirac wave function thus describes a single electron which propagates forward in time only, even though the bound state has any number of pair constituents when Feynman propagators are used. In the absence of an external potential, states that are bound by the Coulomb attraction of their constituents can be analogously described using only their valence degrees of freedom. The instantaneous A^0 field is determined by Gauss' law for each wave function component, i.e., for each position of the valence constituents. Solutions for A^0 obtained with a boundary condition that imposes an asymptotically constant energy density give rise to a linear potential for color singlet q\bar q and qqq states. The strength of the linear potential is determined by the boundary condition and is of lower order in \alpha_s than the gluon exchange interaction, which may then be treated as a higher order perturbative correction. Bound states evaluated to a given order in \alpha_s and \hbar must have the full symmetry of the exact theory, including the dynamic boost invariance of states quantized at equal time. The wave functions are indeed found to have such a hidden invariance, which ensures the correct dependence of the energy eigenvalues on the center of mass momentum. Thus relativistic bound states can be studied using perturbative methods.