We consider a new approach towards constructing approximate holographic duals of QCD from experimental hadron properties. This framework allows us to derive a gravity dual which reproduces the empirically found linear square-mass trajectories of universal slope for radially and orbitally excited hadrons. Conformal symmetry breaking in the bulk is exclusively due to infrared deformations of the anti-de Sitter metric and governed by one free mass scale proportional to Λ QCD . The resulting background geometry exhibits dual signatures of confinement and provides the first examples of holographically generated linear trajectories in the baryon sector. The predictions for the light hadron spectrum include new relations between trajectory slopes and ground state masses and are in good overall agreement with experiment.

We construct a new solution of five-dimensional gravity coupled to a dilaton which encodes essential features of holographic QCD backgrounds dynamically. In particular, it implements linear confinement, i.e. the area law behavior of the Wilson loop, by means of a dynamically deformed anti-de Sitter metric. The predicted square masses of the light-flavored natural-parity mesons and their excitations lie on linear trajectories of approximately universal slope with respect to both radial and spin quantum numbers and are in satisfactory agreement with experimental data.Over the past decade a qualitatively new perspective on strong-interaction physics emerged from gauge/string dualities [1] and the underlying holographic principle. These dualities map (i) string theory spectra on asymptotically AdS ×X spacetimes (i.e. AAdS ×X, where X is a compact space) into gauge invariant, local operators of the dual field theory, (ii) the fields parameterizing the boundary conditions into sources for the dual operators, and (iii) the string theory partition function (or its lowenergy gravity limit) into the generating functional of the field-theory correlators. As a consequence, the notoriously complex strong-coupling regime of large-N c gauge theories can be approximated (in low-curvature regions) by weakly coupled and hence analytically treatable classical gravities. The gauge/gravity correspondence thereby supplies new analytical tools for the study of hadronic observables in the non-perturbative regime of the strong force.Applications of gauge/gravity dualities to "QCD-like" gauge theories either start from specific D-brane setups in ten-(or five-) dimensional supergravity and derive the corresponding gauge theory properties, or try to guess a suitable background and to improve it in bottom-up fashion by comparing the predictions to QCD data. Even the simplest and oldest bottom-up (or "AdS/QCD") model, the hard wall [2], reproduces a surprising amount of hadron phenomenology [3]. The conformal invariance of AdS 5 in the UV reproduces, in particular, the counting rules which govern the scaling behavior of hard QCD scattering amplitudes, while an infrared cutoff on the fifth dimension at the QCD scale Λ QCD implements the mass gap and discrete hadron spectra.The hard-wall predictions for the squared masses of light-flavor hadrons depend quadratically on the principal and spin excitation quantum numbers [3], however, in contrast to the theoretically expected and approximately observed linear Regge behavior [4]. A straightforward way to correct this shortcoming was suggested in Ref. [5] where the AdS 5 geometry is kept intact while an additional dilaton background field with quadratic dependence on the extra dimension is exclusively responsible for conformal symmetry breaking. This dilaton soft-wall model indeed generates linear Regge trajectories m 2 n,S ∼ n + S for light-flavor mesons of spin S and radial excitation level n. (Regge behavior can alternatively be encoded via IR deformations of the AdS 5 metric [6,7].) However, the...

Among the light nuclear clusters the α-particle is by far the strongest bound system and therefore expected to play a significant role in the dynamics of nuclei and the phases of nuclear matter. To systematically study the properties of the α-particle we have derived an effective four-body equation of the Alt-Grassberger-Sandhas (AGS) type that includes the dominant medium effects, i.e. self energy corrections and Pauli-blocking in a consistent way. The equation is solved utilizing the energy dependent pole expansion for the subsystem amplitudes. We find that the Mott transition of an α-particle at rest differs from that expected from perturbation theory and occurs at approximately 1/10 of nuclear matter densities.

We investigate the effect of different forms of relativistic spin coupling of constituent quarks in the nucleon electromagnetic form factors. The fourdimensional integrations in the two-loop Feynman diagram are reduced to the null-plane, such that the light-front wave function is introduced in the computation of the form factors. The neutron charge form factor is very sensitive to different choices of spin coupling schemes, once its magnetic moment is fitted to the experimental value. The scalar coupling between two quarks is preferred by the neutron data, when a reasonable fit of the proton magnetic momentum is found.

The three-nucleon scattering problem in a nuclear medium is considered within the Faddeev technique. In particular the deuteron break-up cross section that governs the formation and the break-up reactions of deuterons (N N N ↔ N d) in a nuclear environment is calculated at finite temperatures and densities. A significant enhancement of the in-medium break-up cross section with increasing density has been found.Formation of light clusters such as deuterons, helium and alpha particles is an important aspect of heavy ion collisions at intermediate energies, see e.g. [1]. Empirical evidence, including recent experimental data on cluster formation, see refs. [2,3], indicate that a large fraction of deuterons can be formed in heavy-ion collision of energiesDuring the expansion of the system the density can drop below the Mott-density of deuteron dissociation [4][5][6]. In this region the deuteron abundances will be determined by deuteron formation, N N N → dN , and break-up, dN → N N N , reactions. Since the deuteron formation rate can be expressed through the break-up probability, the final outcome of the reaction will be essentially controlled by the deuteron break-up cross section. Previous studies of the kinetics of deuteron production have utilized the impulse approximation to calculate the reaction cross section at energies above 200 MeV/A [7]. For lower energies, viz. E/A ≤ 200 MeV, the impulse approximation fails and a full three-body treatment of the scattering problem is necessary. Furthermore, a consistent treatment of cluster formation in expanding hot and dense matter requires the inclusion of medium effects into the respective reaction cross sections.The essentials of the three-body problem in the vacuum case are well known, see e.g. ref.[8]. In the following we utilize the AGS formalism [9] suitably modified, in order to treat the three-body problem taking into account the nuclear medium in the quasi-particle approximation. To this end, we will rely on a separable representation of the nucleon-nucleon potential. This choice simplifies the problem considerably. A systematic investigation of separable parameterizations of "realistic" potentials has been pursued e.g. by Plessas and collaborators [10]. We note that solutions of the three-body problem using "realistic" N N potentials have been achieved e.g. by the Bochum group [11], and the Bonn group in the framework of the W -matrix approach [12].Recently, separable approximations have been used to solve the pion deuteron scattering problem [13] and successfully applied to coherent photo-and electro-production of pions on the deuteron [14]. A detailed discussion of the numerical method used in these cases has been given in [15]. In the present work we have modified this pion deuteron code [16] to solve the nucleon-deuteron scattering problem. Our results for the dominant three-body transition matrix elements agree within ≃ 5% with the previous calculations by Doleschall [17].The AGS equations for the transition matrix U αβ (z) are given in a compact notatio...