[en] A theory with such a mathematical beauty cannot be wrong: this was one of the main arguments in favor of string theory, which unifies all known physical theories of fundamental interactions in a single coherent description of the universe. But no one has ever observed strings, not even indirectly, neither the space of extra dimensions where they live. However, there is a hope that the 'hidden' dimensions of string theory are much larger than what we thought in the past and they become within experimental reach in the near future, together with the strings themselves

[en] The linear polarization P (approximately perpendicular minus parallel to the scattering plane) of large-Psub(T) direct photons from unpolarized hadrons is considered. Contrary to standard QCD, where to 0(1) P vanishes, and to 0(αsub(s)) P is very small and changes sign at angle thetasub(cm)=90⁰, it is shown that supersymmetric theories (involving s quarks and light gluinos), already to 0(1), imply a substantial and positive P through a wide range of angles including thetasub(cm)=90⁰. For antipp→γ+X at collider energy and Psub(T)> or approximately 30 GeV, with s quark mass 20 GeV we find P approximately 10% - 5% decreasing with Psub(T). We offer a qualitative understanding and discuss the significance of our results

[en] In contrast to the old heterotic string case, the (weakly coupled) type I brane framework allows to have all, part or none of the standard model gauge group factors propagating in large extra-dimensions of TeV ^-1 size. We investigate the main experimental signatures of these possibilities, related to the production of Kaluza-Klein excitations of gluons and electroweak gauge bosons. A discovery through direct observation of resonances is possible only for compactification scales below 6 TeV. However effects due to exchange of virtual Kaluza-Klein excitations could be observed for higher scales. We find that LHC can probe compactification scales as high as 20 TeV for excitations of gluons and 8-15 TeV for excitations of electroweak gauge bosons. Finally, in the case where no gauge boson feels the extra-dimension, we find that effective contact interactions due to massive string mode oscillations dominate those due to the exchange of Kaluza-Klein excitations of gravitons and could be used to obtain bounds on the string scale

No abstract available

[en] We study supersymmetry breaking by Scherk-Schwarz compactifications in type I string theory. While in the gravitational sector all mass splittings are proportional to a (large) compactification radius, supersymmetry remains unbroken for the massless excitations of D-branes orthogonal to the large dimension. In this sector, supersymmetry breaking can then be mediated by gravitational interactions alone, that are expected to be suppressed by powers of the Planck mass. The mechanism is non-perturbative from the heterotic viewpoint and requires a compactification radius at intermediate energies of order 10¹²-10¹⁴ GeV This can also explain the value of Newton's constant if the string scale is close to the unification scale, of order 10¹⁶ GeV

[en] We study total and partial supersymmetry breaking by freely acting orbifolds, or equivalently by Scherk-Schwarz compactifications, in type I string theory. In particular, we describe a four-dimensional chiral compactification with spontaneously broken N = 1 supersymmetry, some models with partial N = 4 → N = 2 and N = 4 → N = 1 supersymmetry breaking and their heterotic and M-theory duals. A generic feature of these models is that in the gravitational sector and in the spectrum of D-branes parallel to the breaking coordinate, all mass splittings are proportional to the compactification scale, while global (extended) supersymmetry remains unbroken at tree level for the massless excitations of D-branes transverse to the breaking direction

[en] We compute threshold effects to gauge couplings in four-dimensional Z_N orientifold models of type I strings with GN = 2 and GN = 1 supersymmetry, and study their dependence on the geometric moduli. We also compute the tree-level (disk) couplings of the open sector gauge fields to the twisted closed string moduli of the orbifold in various models and study their effects and that of the one-loop threshold corrections on gauge coupling unification. We interpret the results from the (supergravity) effective theory point of view and comment on the conjectured heterotic-type I duality

[en] We study open descendants of non-supersymmetric type IIB asymmetric (freely acting) obifolds with zero cosmological constant. A generic feature of these models is that supersymmetry remains unbroken on the brane at all mass levels, while it is broken in the bulk in a way that preserves Fermi-Bose degeneracy in both the massless and massive (closed string) spectrum. This property remains valid in the heterotic dual of the type 11 model but only for the massless excitations. A possible application of these constructions concerns scenarios of low-energy supersymmetry breaking with large dimensions

[en] We study the sparticle spectroscopy and electroweak breaking of theories where supersymmetry is broken by compactification (Scherk-Schwarz mechanism) at a TeV The evolution of the soft terms above the compactification scale and the resulting sparticle spectrum are very different from those of the usual MSSM and gauge-mediated theories. This is traced to the softness of the Scherk-Schwarz mechanism which leads to scalar sparticle masses that are only logarithmically sensitive to the cutoff starting at two loops. As a result, the mass-squareds of the squarks and sleptons are a loop factor smaller than those of the gauginos. In addition, the mechanism is very predictive and the sparticle spectrum depends on just two new parameters. A significant advantage of this mechanism relative to gauge mediation is that a Higgsino mass μ ∼ M_susy is automatically generated when supersymmetry is broken. Our analysis applies equally well to theories where the cutoff is near a TeV or M _Pl or some intermediate scale. We also use these observations to show how we may obtain compactification radii which are hierarchically larger than the fundamental cutoff scale

[en] We derive a universal thermal effective potential, which describes all possible high-temperature instabilities of the known N = 4 superstrings, using the properties of gauged N = 4 supergravity. These instabilities are due to three non-perturbative thermal dyonic modes, which become tachyonic in a region of the thermal moduli space. The latter is described by three moduli, s, t, u, which are common to all non-perturbative dual-equivalent strings with N = 4 supersymmetry in five dimensions: the heterotic on T⁴ x S¹, the type IIB on K₃ x S¹, and the type I on T⁴ x S¹. The non-perturbative instabilities are analyzed. These strings undergo a high-temperature transition to a new phase in which five-branes condense. This phase is described in detail, using both the effective supergravity and non-critical string theory in six dimensions. In the new phase, supersymmetry is perturbatively restored but broken at the non-perturbative level. In the infinite-temperature limit the theory is topological with an N = 2 supersymmetry based on a topologically non-trivial hyper-Kaehler manifold