SEMINARIS EXTERNS

       Esteban Calzetta (Universidad de Buenos Aires)

       Data: Dijous, 17 de juny, 14:00h

       Lloc: Aula 505

RESUM: The tunneling probability for a system modelling macroscopic quantum tunneling is computed. We consider an open quantum system with one degree of freedom consisting of a particle trapped in a cubic potential interacting with an environment characterized by dissipative and a diffusion parameters. A representation based on the energy eigenfunctions of the free system is used to write the dynamical equation of the reduced Wigner function for the open quantum system. This equation becomes very simple in that representation. The use of WKB approximation for the eigenfunctions suggests an approximation which allows an analytic computation of the tunneling rate assuming that the system is initially trapped in the false ground state. In the weak dissipation limit we find that the decoherence produced by the environment suppresses tunneling in agreement with results in other macroscopic quantum systems with different potentials.

       Daniel Z. Freedman (MIT)

       Data: Dimarts, 15 de juny, 14:00h

       Lloc: Aula 505

RESUM: The rather simple AdS_4 sliced dilaton domain wall in Type IIB supergravity found by Bak et al (hep-th/0304129) is singularity-free and breaks supersymmetry completely. In this seminar we present an extension of the Witten positive energy argument which gives strong evidence that the solution is stable. (See hep-th/0312055), We also discuss new results on a proposed gauge theory dual of this solution which is an interface conformal field theory. Certain quantities can be precisely matched between gauge theory and gravity.

       Itzhak Bars (University of Southern California)

       Data: Dijous, 14 de juny, 14:00h

       Lloc: Aula 505

RESUM: MSFT is an alternative formulation of Witten's cubic open string field theory (SFT). The main novelty is that string interaction is formulated in terms of the simple Moyal product that describes a quantum mechanics type non-commutative structure induced by string joining. The advantage of MSFT is that it is much simpler for practical computations in SFT, while fully agreeing with former approaches, such as the oscillator formulation or conformal field theory formulation of SFT. In this lecture I will describe the main ideas of MSFT, new results for off-shell string amplitudes that were not obtained in other approaches in string theory, and an analytic approach to the computation of the nonperturbative vacuum of SFT. I will also describe generalizations of MSFT to curved space, and spacetime supersymmetry using pure spinors.

       Eduard de Rafael (Marseille, CPT)

       Data: Dijous, 3 de juny, 14:00h

       Lloc: Aula 505

RESUM: The correlation function of a V-A current with a V+A current is discussed within the framework of QCD in the Large-Nc limit. Applications to the determination of weak matrix elements will also be discussed.

       Mauro Francaviglia, Turin University

       Data: Dimarts, 1 de juny, 14:00h

       Lloc: Aula 505

RESUM: Thermodynamical properties of gravitational systems have been known since the '70s. In particular black holes are known to emit radiation as a black body at a temperature T which depends on quantities related to the horizon and black hole entropy S has been argued to be directly related to the horizon area. More recently, modern geometrical tools have been used to investigate the thermodynamical properties of more general solutions in covariant theories and their relation to the problem of information loss (particles, fields and geodesics). This seminar will be both devoted to introduce black hole entropy and to present the new geometric insights. It is well known that in a large class of black-holes Hawking-Bekenstein entropy equals one quarter of the area of the outer horizon. It is also well known that in such class of space-times the entropy, besides having an obvious statistical meaning, is generated by a variational prescription in terms of the physical parameters of the hole. Preliminary but incomplete results (Wald, Wald & Iyer) have expressed this geometrical origin in a (non-covariant) Hamiltonian setting, based on particular ad-hoc prescriptions. It has been recently realized that a fully covariant, totally unrestricted and fairly general geometrical scheme exists, which generates the entropy of a much larger class of exact solutions with singularities (e.g., black-holes) and in a much larger class of Lagrangian field theories (including topological fields and gauge fields). The general formula (which in known cases not covered by the one-quarter-law reproduces exactly the entropy calculated by statistical methods via the appropriate partition function) is an output of Noether's theory of conservation laws; the entropy is a space-time integral of an appropriate (and covariant) symplectic form, related to Komar's contribution to the Hamiltonian. It is generally formed by various terms, which reflect homological properties around the singularities (horizons, strings, etc..) and of the asymptotic boundary of space-time. Komar's superpotential turns out to have the general interpretation as the generator of the finite quantity TS, while (geometrical) entropy turns out to be a measure of the impossibility of foliating spacetime into a family of global Cauchy surfaces.

       Petr Lavrov (Tomsk)

       Data: Dijous, 27 de maig, 14:00h

       Lloc: Aula 505

RESUM: Even (odd) symplectic supermanifolds equipped with a connection respecting this symplectic structure are studied. Such supermanifolds can be considered as generalization of Fedosov manifolds to the supersymmetric case. Jacobi and Bianchi identities for symplectic curvature tensor are derived. Properties of Ricci and scalar tensors on arbitrary Fedosov supermanifolds are investigated.

       Jose L. F. Barbon (CERN)

       Data: Dijous, 20 de maig, 14:00h

       Lloc: Aula 505

RESUM: I review the basic properties of an AdS/CFT model that simulates chiral physics in QCD. It is shown how this model provides a string realization of the Witten-Veneziano scenario for the eta' mass.

       Alessandro Papa (INFN Cosenza)

       Data: Dijous, 13 de maig, 14:00h

       Lloc: Aula 505

RESUM: The talk is organized in two parts. The first one is based on the role played by universality in the setup of methods and in the predictions of numerical results in finite temperature SU(N) pure gauge theories. In particular, the cases of the numerical determination of the critical exponent $\nu$ of the correlation length at the confinement/deconfinement phase transition and of screening masses in the deconfined phase near criticality are considered.

       Pedro Silva

       Data: Dijous,6 maig, 14:00h

       Lloc: Aula 505

RESUM: We present 1/4 bps configurations in AdS_5 corresponding to composite Giant gravitons systems. The dual CFT operators are conjecture to be the product of chiral primaries and descendant of chiral primaries. These states are the first examples of 1/4 supersymetric states at large R-charge.

       José A. Cembranos (UCM)

       Data: Dijous,1 d'abril, 14:00h

       Lloc: Aula 505

RESUM: The afterglows of gamma-ray bursts (GRBs) can serve as probes of the gas and dust both local to the progenitor and in the intergalactic medium. The most exciting utility, in this respect, will be in the detection of a population of GRBs beyond redshift z ~ 8; such bursts would offer a rare glimpse of the universe before it was fully reionized, mark locations of protogalaxies, and constrain formation scenarios of the first stars. Detection, however, requires rapid, sensitive, and comprehensive multicolor imaging at infrared wavelengths. I will discuss an initiative at Mt. Hopkins, called PAIRITEL, to automate the observations of new GRBs from the Swift satellite at infrared wavelengths, optimized for the discovery of high redshift bursts.

       Joshua Bloom (Harvard)

       Data: Dijous, 18 de març, 14:30h

       Lloc: Aula 505

RESUM: The afterglows of gamma-ray bursts (GRBs) can serve as probes of the gas and dust both local to the progenitor and in the intergalactic medium. The most exciting utility, in this respect, will be in the detection of a population of GRBs beyond redshift z ~ 8; such bursts would offer a rare glimpse of the universe before it was fully reionized, mark locations of protogalaxies, and constrain formation scenarios of the first stars. Detection, however, requires rapid, sensitive, and comprehensive multicolor imaging at infrared wavelengths. I will discuss an initiative at Mt. Hopkins, called PAIRITEL, to automate the observations of new GRBs from the Swift satellite at infrared wavelengths, optimized for the discovery of high redshift bursts.

       Frederic Schuller (Perimeter Institute)

       Data: Dijous, 11 de març, 14:30h

       Lloc: Aula 505

RESUM: Any candidate theory of quantum gravity must address the breakdown of the classical smooth manifold picture of space-time at distances comparable to the Planck length. String theory, in contrast, is formulated on conventional space-time. However, we show that in the low energy limit, the dynamics of generally curved Dirichlet p-branes
possess an extended local isometry group, which can be absorbed into the brane geometry as an almost product structure. The induced kinematics encode two invariant scales, namely a minimal length and a maximal speed, without breaking general covariance. Phenomenological consequences for physics on a D-brane at low energy are then seen to manifest themselves by the kinematical effects of a maximal acceleration. In particular we find a correction to the Thomas precession in atomic physics, a Pauli-Villars regularization of quantum field theory, and whole class of ghost-free gravity theories whose static spherically symmetric solutions are free of spacelike singularities.

       Enrico Robutti (INFN Genova)

       Data: Dijous, 4 de març, 14:30h

       Lloc: Aula 505

RESUM: Charmonium states are produced copiously at B-factories through a variety of processes. In the last few years, thanks to the large amount of data accumulated, BaBar, Belle and Cleo have been able to bring a substantial contribution to the study of spectroscopy, production and decay of charmonia. The field has been revitalised by the
discovery of new resonances and the unveiling of puzzling phenomena related to production mechanisms. An overview of recent results will be presented, with a look at their phenomenological implications.

       José Edelstein

       Data: Divendres, 20 de febrer, 14:00h

       Lloc: Aula 505

RESUM: I will present non-supersymmetric solutions of supergravity admitting the interpretation of a D-brane intersecting with and S-brane. To this end, I will firstly generalize the construction of partially localized D-brane solutions to the near extremal case. An intersection rule for these configurations is derived. I will discuss some aspects of this system that might be relevant for cosmological setups in string theory.

       Cedric Deffayet (Paris)

       Data: Dijous, 19 de febrer , 14:00h

       Lloc: Aula 505

RESUM: I will discuss aspects of 4D theories obtained by discretizing one dimension in 5D general relativity (GR) in the spirit of the deconstruction program. Using an ADM type of splitting of the metric, I will show that one can explicitly construct out of the metric the analog of the link fields that have been used in gauge theories. I will then discuss the spectrum of the theories considered, as well as contrast exact solutions of the discretized theory with exact solutions of 5D GR. I will also address the relations of the theories considered, with theories of "massive gravity", and shortly review some motivations and problems of the latter.

       Carlos Muñoz (UAM)

       Data: Dijous, 12 de febrer , 14:00h

       Lloc: Aula 505

RESUM: Recently, several experiments for the direct detection of dark matter have reported results. One of the collaborations actually claims that the first evidence for dark matter has already been observed, in particular in the form of Weakly Interacting Massive Particles (WIMPs). This issue will be discussed in this talk. After an introduction to dark matter, its direct detection through elastic scattering with nuclei in a material will be analyzed. Finally, whether or not the existing theoretical models supersymmetry, superstrings) give rise to cross sections in the range where current dark matter detectors are sensitive, will be discussed in some detail.

       Eduard Gorbar (Bogolyubov Inst, Kiev)

       Data: Dijous, 29 de gener , 14:00h

       Lloc: Aula 505

RESUM: The renormalization group equations and decoupling of massive fields in curved spacetime are studied and the cases of massive scalar, fermion, and vector fields are considered. In the higher derivative sector we arrive at the standard form of decoupling and obtain the renormalization group $\beta$-functions in both UV and IR regimes as the limits of general expressions. For the cosmologogical and Newton constants the corresponding $beta$-functions turned out to be unaccessible in the considered perturbative regime and, in particular, the form of decoupling remains unclear.

       José Antonio Oller (U. Murcia)

       Data: Dilluns, 26 de Gener, 14:00h

       Lloc: Aula 505

RESUM: Employing results from a recent determination of the scalar K\pi form factor F_0^{K\pi} within a coupled channel dispersion relation analysis \cite{jop01}, in this work we calculate the slope and curvature of F_0^{K\pi}(t) at zero momentum transfer. Knowledge of the slope and curvature of the scalar K\pi form factor, together with a recently calculated expression for F_0^{K\pi}(t) in chiral perturbation theory at order p6, enable to estimate the \cO(p6) chiral constants C_{12}^r=(0.5 \pm 5.4)\cdot 10^{-7} and (C_{12}^r+C_{34}^r)=(3.1 \pm 1.4)\cdot 10^{-6}. Our findings also allow to estimate the contribution coming from the C_i to the vector form factor F_+^{K\pi}(0) which is crucial for a precise determination of |V_{us}| from K_{l3} decays. Our result F_+^{K\pi}(0)|_{C_i^r}=-\,0.018 \pm 0.008 is in perfect agreement with a previous estimate given in the pioneering work by Leutwyler and Roos.

       Henriette Elvang (University of California in Santa Barbara)

       Data: Dilluns, 19 de gener, 14:00h

       Lloc: Aula 505

RESUM: In higher dimensional asymptotically flat spacetimes, static black holes are not much different from 4d static black holes. However, allowing for other asymptotics -- as in Kaluza-Klein (KK) theory where the asymptotic metric is flat space times a circle -- gravity opens up for a richer set of exact black hole vacuum solutions. Examples in 5d KK theory are black holes on a "bubble of nothing": a bubble keeps two black holes of arbitrary size apart in a static configuration. In 6d KK theory, a bubble can balance a black ring. In this talk I'll discuss black hole-bubble solutions in 5d and 6d Kaluza-Klein gravity; KK bubbles will be introduced. I'll discuss the thermodynamics of these black holes, and how they fit into a recently proposed phase diagram of black holes in KK theory.

       Marcello Ortaggio (Praga)

       Data: Dilluns, 19 de gener, 14:00h

       Lloc: Aula 505

RESUM: In classical Maxwell's theory, the radiative component of the electromagnetic field of an isolated source is defined as the leading (1/r) term ``at large distances''. In order to ``see'' radiation, one has to approach infinity along light rays (null geodesics). Penrose has generalized such a picture to describe asymptotic properties of arbitrary massless fields in curved spacetimes. His ``conformal'' procedure enables one to characterize a field ``near infinity'' in a purely geometric (covariant) way: a typical fall off behaviour (``peeling'') of the field as it proceeds along a null geodesic naturally singles out a ``1/r'' term, which defines the radiation field. However, as remarked by Penrose himself several times, accordi= ng to this definition ``the radiation field concept is less invariant when the conformal infinity is spacelike or timelike'' (for example in the deSitter and anti--deSitter spacetimes) ``than when it is null''. Here, we employ the general Penrose approach for a specific study of gravitational and electromagnetic radiation generated by uniformly accelerated charged black holes in the anti--deSitter universe. We find and interpret the pattern of radiation which characterizes the dependence of the fields on the null direction from which the conform al infinity is approached. This directional characteristic supplements he peeling property and provides an explicit demonstration of the ``non-invariance'' of the concept of radiation.

       José María Román, Instituto de Física Teórica (CSIC-UAM), Madrid

       Data: 8 de gener, 14:00h

       Lloc: Aula 505

RESUM: We compute the finite size effects in the ground state energy,
equivalently the effective central charge $c_{\rm eff}$, based on
S-matrix theories recently conjectured to describe a cyclic regime of
the Kosterlitz-Thouless renormalization group flows. The effective
central charge has periodic properties consistent with renormalization
group predictions. Whereas $c_{\rm eff}$ for the massive case has a
singularity in the very deep ultra-violet, we argue that the massless
version is non-singular and periodic on all length scales.

       Manel Martinez (IFAE)

       Data: 11 de desembre, 14:00h

       Lloc: Sala de Graus. Facultat de Física

RESUM: The MAGIC telescope is the largest IACT (Imaging Air Cherenkov Telescope) ever built and is presently in its commissioning phase. Its first target is the exploration of the universe with gamma rays of energies above 30 GeV with an unprecedented sensitivity. These characteristics should allow the discovery and detailed study of a plethora of new astrophysical sources, out of which many are expected to
be at cosmological distances. The prospects for using MAGIC for measurements which may have a direct impact on our present cosmological understanding of the universe, such as the DARK ENERGY and the DARK MATTER, along with other fundamental questions such a the QUANTUM NATURE OF GRAVITY, will be discussed.

 

       Prof. Karlheinz Langanke ( Institute of Physics and Astronomy, University of             Århus, Denmark)

       Data: 10 de desembre, 12:00h

       Lloc: Sala de Graus. Facultat de Física
 

       Prof. Karlheinz Langanke ( Institute of Physics and Astronomy, University of             Århus, Denmark)

       Data: 9-12 de desembre, 15:00h

       Lloc: Aula 222. Facultat de Física
 

CONTINGUTS:

1. Stellar evolution: Hydrostatic burning phases
2. Stellar evolution: Final stages and Core collapse supernovae
3. Neutron capture nucleosynthesis: s-process and r-process
4. Binary systems: explosive hydrogen burning in novae and x-ray bursters; type Ia supernovae


 

       Carlos Herdeiro

       Data: Dilluns, 4 de desembre, 14:00h

       Lloc: Aula 505.
 

RESUM: Closed Timelike Curves are present in a number of solutions of Classical General Relativity and are, in general, regarded as pathological. It was suggested by Hawking in the early 90s that the `Chronology Protection Mechanism' should involve quantum effects. It becomes therefore interesting to explore the nature of CTC's in a framework of quantum gravity like string theory. The recent discovery of supersymmetric Godel type universes in string theory which are T-dual to pp-waves and the existence of supersymmetric rotating black holes (which have Closed Timelike Curves) possessing a CFT description motivated several suggestions on how string theory might cope with Closed Timelike Curves. In this seminar I will review some of these recent suggestions.




 

       Patricio Salgado, Univ. de Concepcion, Chile

       Data: Divendres, 21 de novembre, 14:00 h

       Lloc: Aula 505
 

RESUM: The most general Lagrangian for gravity in n dimensions built up on the same principles as General Relativity (general covariance, second order equations for the metric, and no explicit torsion) is a polynomial of degree n/2 in the curvature known as the Lanczos-Lovelock Lagrangian. Lanczos-Lovelock (LL) theories have the same fiels, symmetries and local degrees of freedom as ordinary gravity. It is shown that a LL gravity theory invariant Both under local Lorentz rotations, and under local Anti de Sitter boosts can be constructed. It is also shown that such a construction is possible for odd and even dimensions.

 

       Paolo Merlatti (Nordita)

       Data: Dijous, 20 de novembre, 14:00 h

       Lloc: Aula 505
 

RESUM: In this talk I use the latest developed techniques (related to the generalized Konishi anomaly) to give a rather general description of the quantum properties of N=1 Super Yang-Mills theories. Both perturbative and non-perturbative physics is discussed, examining in particular problems related to the phenomenon of gaugino condensation.

       Georg Weiglein (IPPP Durham)

       Data: dijous, 30 d'octubre, 16:00h

       Lloc: Sala de Graus de Fisica
 

RESUM:The present status of electroweak precision tests of the electroweak Standard Model (SM) is summarized and the resulting indirect constraints in the mass of the SM Higgs boson are discussed. In the Minimal Supersymmetric extension of the Standard Model (MSSM) the mass of the lightest Higgs boson is not a free parameter as in the SM, and a firm upper bound can be established. The phenomenology of Higgs physics in the MSSM at the next generation of colliders is discussed. The possible interplay between the LHC and a future Linear Collider in analysing the mechanism of electroweak symmetry breaking and the underlying structure of Supersymmetric models is investigated.

       Rubén Portugués (DAMTP, Cambridge)

       Data: dijous, 23 d'octubre, 16:00h

       Lloc: Aula 505
 

RESUM: Gravity/gauge theory dualities continue to provide insight into the nonperturbative behaviour of certain gauge theories. The duals of N=1 theories have been constructed using both fractional and wrapped branes, which I will quickly review. We consider a distinct but related setting, M theory on a manifold of G2 holonomy, and study the realisation of chiral symmetry breaking in the gravity picture. We argue that the chiral symmetry is dual to an angle which corresponds to a special non-isometric direction in the Gm2 metric. We call such a direction a "massive isometry". The anomaly is then dual to spontaneous symmetry breaking in M theory. We go on to further develop the concept of massive isometry.

       V.F. Mukhanov, Universitat de Munic, Alemanya

       Data: dijous, 2 d'octubre, 16:00h

       Lloc: Sala de Graus de Fisica
 

RESUM: I derive analitically the spectrum of the CMB fluctuations and discuss how it depends on the various cosmological parameters. The predictions of inflation are compared with the recent observational results.

 

       Matthias Steinhauser (DESY)

       Data: dijous, 25 de setembre, 14:00h

       Lloc: Aula 131 (Aulari de Física)
 

RESUM: The masses of the heavy quarks are fundamental parameters of the Standard Model and thus should be determined with high accuracy. In this talk I will concentrate on the recent developments achieved in the determination of the charm and bottom quark masses using sum rules and the determination of the bottom and top quark masses using high-order calculations within the framework of no n-relativistic QCD.

       Gianluca Cavoto (Princeton Univ./Universita' di Roma)

       Data: dilluns, 15 de setembre, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM: In its first three years of operations, the BaBar and Belle detectors at the SLAC and KEK e^+e^- asymmetric B factories have recorded more than 100 million psilon(4s)->BBbar decays each. This large data sample allows to test the Standard Model CKM picture of CP violation by measuring the angles of the Unitarity Triangle and search for for possible New Physics effects. In this seminar I will review some of the recent results presented at the summer conferences by the two experiments with special emphasis on the experimental techniques.

       Tanmay Vachaspati (CWRU, Cleveland)

       Data: dijous, 17 de juliol, 12:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM: A remarkable correspondence is shown to exist between the spectrum of magnetic monopoles in an SU(5) model and the known standard model fermions. There is evidence suggesting that the family structure of the standard model can be understood in terms of replication of magnetic monopoles.

       Pawel O. Mazur, U. South Carolina

       Data: dijous, 17 de juliol, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM: I show that the problem of gravitation is best understood as quantum many body problem. To this end I will introduce my 1996 constituent model of gravitation. I find that a zero temperature gravitating mass $M(N)$ behaves like a system of $N$ Einstein condensed bosons with a mass lower than a Planck mass. At slightly higher temperatures this system behaves like a superfluid, i.e., soft bosonic excitations dominate a specific heat of a gravitational mass-energy. Among other things I will describe
the 1996 prediction, a small positive vacuum energy density. It follows from my constituent model of gravitation that:
1) gravitational collapse of matter leads to a formation of objects called quasi-black hole objects (QBHOs) that are ultra-cold and thermally stable.
2) there are no semiclassical ultra-hot Hawking exploding black holes and no Hawking effect. Instead there are thermally stable QBHOs.
3) thermally stable gravitating object, a QBHO, of an arbitrarily large gravitational mass $M$ at zero temperature is necessarily characterized by a positive and nonvanishing `vacuum energy density' of an order $M^{-2}$.
4) largest thermally stable gravitating object is the observable Universe itself and hence the so-called vacuum energy or cosmological constant must be positive and nonvanishing. The total number $N_{_{U}}\sim 10^{123}$ of heavy constituent bosons was estimated from the then known bounds on the cosmological constant. This was also the prediction of `dark energy' that in the Planck units is $1/N_{_{U}}$.
5) Harrison-Zel'dovich spectrum of density fluctuations is derived with the correct amplitude on the scale of the horizon. Galactic rotation is studied.

       Antonio Vairo (CERN)

       Data: dijous, 10 de juliol, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM: I will discuss Poincare' invariance in the context of non-relativistic
effective field theories of QCD. We show, in the specific cases of
HQET/NRQCD and pNRQCD, that the algebra of the generators of the Poincare'
transformations imposes precise constraints on the form of the Lagrangian.
In the case of HQET they are the relations formerly obtained by
reparametrization invariance, in the case of pNRQCD they are new.

       Beatrix Hiesmayr (UAB)

       Data: dijous, 26 de juny, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM: Particle physics has become an interesting testing ground for fundamental questions of quantum mechanics (QM). The massive entangled neutral kaon system is specially suitable as it offers a unique laboratory to test various aspects of particle physics (CP violation, CPT violation, ...) but also to test the foundations of QM (local realistic theories versus QM, Bell inequalities, decoherence effects,...).

In this talk I focus on decoherence effects due to the interaction of the system with the environment. I derive an upper bound on the strength of the decoherence effect via data of the CPLEAR experiment performed at CERN. Further I discuss this effect in the light of different measures of entanglement and give a simple geometrical interpretation.

       Esteban Calzetta (Buenos Aires)

       Data: dilluns, 19 de juny, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM: We analyze the phenomena of Bose Novae, as described by Donley et al [Nature 412, 295 (2001)], by focusing on the behavior of excitations or fluctuations above the condensate, as driven by the dynamics of the condensate (rather than the dynamics of the condensate alone or the kinetics of the atoms). The dynamics of the condensate squeezes and amplifies the quantum excitations, mixing the positive and negative frequency components of their wave functions thereby creating particles which appear as bursts and jets. By analyzing the changing amplitude and particle content of these excitations, our simple physical picture (based on a test field approximation) explains well the overall features of the Bose Novae phenomena and provide excellent quantitative fits with experimental data on several aspects, such as the scaling behavior of
the collapse time and the amount of particles in the jet. The predictions of the bursts at this level of approximation is less than satisfactory but may be improved on by including the backreaction of the excitations on the condensate. The mechanism behind the dominant effect -- parametric amplification of vacuum fluctuations and freezing of modes
outside of horizon -- is similar to that of cosmological particle creation and structure formation in a rapid quench (which is fundamentally different from Hawking radiation in black holes). This shows that BEC dynamics is a promising venue for doing `laboratory
cosmology'.

       Matteo Cacciari

       Data: dilluns, 12 de juny, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM: Updated predictions for B and D meson production at the Tevatron are given. The proper inclusion of perturbative and non-perturbative heavy quark fragmentation effects is shown to lead to a good description of the data, which for B mesons were previously at variance with theoretical predictions. On a more theoretical note, we describe how a renormalon model can be employed to produce a description of the non-perturbative component in terms of a power correction series.

        Herbert Dreiner (Bonn)

       Data: dilluns, 2 de juny, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM: For non-universal gaugino masses, collider experiments do not provide any lower bound on the mass of the lightest neutralino. We review the supersymmetric parameter space which leads to light neutralinos, $M_\lsp \lsim {\cal O}(1\gev)$, and find that such neutralinos are almost pure bino. In light of this, we examine the neutralino lower mass bound obtained from supernova 1987A (SN1987A). We consider the production of binos in both electron-positron annihilation and nucleon-nucleon binostrahlung. For electron-positron annihilation, we take into account the radial and temporal dependence of the temperature and degeneracy of the supernova core. We also separately consider the Raffelt criterion and show that the two lead to consistent results. For the case of bino production in $NN$ collisions, we use the Raffelt criterion and incorporate recent advances in the understanding of the strong-interaction part of the calculation in order to estimate the impact of bino radiation on the SN1987A neutrino signal. Considering these two bino production channels allows us to determine separate and combined limits on the neutralino mass as a function of the selectron and squark masses. For $M_\lsp \sim 100 \mev$ values of the selectron mass between 300 and 900 GeV are inconsistent with the supernova neutrino signal. On the other hand, in contrast to previous works, we find that SN1987A provides almost no bound on the squark masses: only a small window of values around 300 GeV can be excluded, and even then this window closes once $M_\lsp \gsim 20 \mev$.

        David Kosower (Saclay)

       Data: dijous, 29 de maig, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM: I will give a brief review of recent years' progress in the ongoing program of NNLO corrections to jet physics. I will then discuss recent work on "antenna" functions which collect and summarize the infrared-singular behavior of QCD amplitudes. I will show how to apply these functions to the development of a numerically-oriented formalism for leading-log resummation in arbitrary-multiplicity jet quantities.

        Daniel Chung (CERN)

       Data: dijous, 22 de maig, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM:We discuss the minimal theoretical uncertainty in inflationary curvature fluctuations due to the nonuniqueness of the vacuum, even in the absence of any trans-Planckian uncertainties or effective field theory cutoff related effects.

        Thomas Mannel (Karlsruhe)

       Data: dijous, 15 de maig, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM:After some short introduction into the theory of inclusive B decsya I shall consider spectra of such decays discussed in the framework of the 1/m expansion. In certain regions of phase space this expansion breaks down and needs to be replaced by a twist expansion. I will discuss some recent developments in this field, including a brief discussion on soft collinear effective theory (SCET).

 

        Stephan Recksiegel (Munich)

       Data: dijous, 8 de maig, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM:We compare the perturbatively calculated QCD potential to that obtained
from lattice calculations in the theory without light quark flavours.
We examine E_tot(r) = 2 m_pole + V_QCD(r) by re-expressing it in the MSbar
mass m = m^MSbar(m^MSbar) and by choosing specific prescriptions for
fixing the scale mu (dependent on r and m). By adjusting m so as to
maximise the range of convergence, we show that perturbative and lattice
calculations agree up to 3 r_0 ~ 7.5 GeV^-1 (r_0 is the Sommer scale)
within the perturbative uncertainty of order Lambda^3 r^2.
A purely QCD based, model independent calculation of the charmonium and
bottomonium spectra that makes use of this potential is presented.

        Miguel Costa (Lisboa)

       Data: dijous, 24 d'abril, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM:We analyze the classical stability of string cosmologies driven by the
dynamics of orientifold planes. These models are related to time-dependent
orbifolds, and resolve the orbifold singularities which are otherwise
problematic by introducing orientifold planes. In particular, we show that
the instability discussed by Horowitz and Polchinski for pure orbifold
models is resolved by the presence of the orientifolds. Moreover, we
discuss the issue of stability of the cosmological Cauchy horizon, and we
show that it is stable to small perturbations due to in-falling matter.

       Renaud Parentani (Tours)

       Data: dijous, 10 d'abril, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM: In the inflationary scenario, the large scale structure of the
universe results from the amplification of vacuum fluctuations by pair
creation processes. by an appropriate use of wave packets, we reveal the
space-time structure of the correlations amongst the two particles in each
pair. we show that every pair emerges from vacuum configurations which are
torn apart so as to give rise to two semi-classical currents: that carried
by the particle and that of its `partner'. during inflation, the partner's
current lives behind the hubble horizon centered around the particle.
hence any measurement performed within a hubble patch would correspond to
an uncorrelated density matrix, as for hawking radiation. however, when
inflation stops, the hubble radius grows and eventually encompasses the
partner. when this is realized the coherence is recovered within a patch.
we relate this quantum coherence to the so-called acoustic peaks which are
now well observed in the cmb.

        Raúl Rabadán

       Data: dilluns, 7 d'abril, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM:Compactifying a higher-dimensional theory defined in R^{1,3+n} on an n-dimensional manifold {\cal M} results in a spectrum of four-dimensional (bosonic) fields with masses m^2_i = \lambda_i, where - \lambda_i are the eigenvalues of the Laplacian on the compact manifold. The question we address in this paper is the inverse: given the masses of the Kaluza-Klein fields in four dimensions, what can we say about the size and shape (i.e. the topology and the metric) of the compact manifold? We present some examples of isospectral manifolds (i.e., different manifolds which give rise to the same Kaluza-Klein mass spectrum). Some of these examples are Ricci-flat, complex and K\"{a}hler and so they are isospectral backgrounds for string theory. Utilizing results from finite spectral geometry, we also discuss the accuracy of reconstructing the properties of the compact manifold (e.g., its dimension, volume, and curvature etc) from measuring the masses of only a finite number of Kaluza-Klein modes

       Daniele Binosi (Valencia)

       Data: dijous, 3 d'abril, 14:00h

       Lloc: Aula 123, planta cinquena (UB).
 

RESUM: The Pinch Technique (PT) is a diagrammatic method which exploits
the underlying symmetries encoded in a physical amplitude (such as an
S-matrix element) in order to construct effective Green's functions with
special properties. After reviewing PT formulation at one loop, I will
discuss the basic ideas which have lead recently to its all-order
generalization, its connection with other formalisms (the background field
method and the Batalin-Vilkovisky formalism), and some possible
applications.

       Yolanda Lozano (Oviedo)

       Data: dijous, 27 de març, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM: From a non-Abelian worldvolume action for systems of coincident gravitons
we provide a microscopical description of giant graviton configurations in
AdS_m x S^n backgrounds in terms of the polarisation of the gravitons into
higher dimensional fuzzy surfaces. We show the perfect agreement, for
large number of gravitons, with the macroscopical description given in the
literature.

       Rodolfo Sassot

       Data: dilluns, 24 de març, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

       Jose Edelstein (Lisboa)

       Data: dijous, 20 de març, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM: I will review some aspects of the gravity dual of ${\cal N}=1$ supersymmetric gauge theory in four dimensions, arising in the noncompact directions of D6-branes wrapping a special Lagrangian three-cycle in a Calabi-Yau threefold, within the framework of eight dimensional gauged supergravity. It amounts to a purely geometrical background of M-theory corresponding to a 7d real manifold with $G_2$ holonomy. I will construct all complete $G_2$ holonomy metrics of cohomogeneity one and $S^3 \times S^3$ principal orbits. The approach rests on a generalization of the twisting procedure used in this framework. I will then obtain a unified picture for the conifold singularity resolution and argue that gauged supergravity, through this novel prescription for the twisting, provides an appropriate framework to smooth out singularities in the context of gravity/gauge theory duality.

 

       Vladimir Smirnov (Moscow)

       Data: dijous, 17 de març, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM: The strategy of expansion by regions turns out to be a universal method for expanding Feynman integrals in various limits of momenta and masses. It can be applied not only to typically Euclidean limits, where other methods are also available, but also to limits typical of Minkowski space. This strategy is reviewed and illustrated through numerous examples, with special attention to expansions near threshold and in the Sudakov limit.

 

       Mariano Quiros (ICREA/IFAE)

       Data: dijous, 13 de març, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM: If the inverse compactification radius of a higher-dimensional theory is hierarchically smaller (little hierarchy) than the scale of the underlying (string) theory the mass of the extra-dimensional components of gauge fields (Higgses) can be protected against quadratic divergences by the higher-dimensional gauge invariance. The Higgs mass is then insensitive to the cutoff thus providing a non-supersymmetric solution to the hierarchy problem. However this scenario can be jeopardized by brane effects.

 

       David Wands (Portsmouth)

       Data: dijous, 6 de març, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM: I discuss different models of the early universe that seek to describe
the origin of primordial density perturbations from initial vacuum
fluctuations. I will contrast the predictions of conventional models of
an inflationary expansion with pre big bang or ekpyrotic models based
upon an early collapsing phase.

 

       John Donoghue (U.MASS.)

       Data: dijous, 20 de febrer, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM: At ordinary energies, general relativity can be quantized and produces
well behaved quantum preditions when treated as an effective field
theory. I will describe the structure of the effective field theory and
will illustrate it with a series of calculations.

 

       Manuel Masip (Granada)

       Data: dijous, 6 de febrer, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM: Ultrahigh energy neutrinos can be used to explore the physics at the TeV scale. I discuss the neutrino-nucleon cross section in models with extra dimensions and the fundamental scale at the TeV. In particular, I discuss de production of string resonances, the elastic contributions mediated by higher dimensional gravitons,
and the production of microscopic black holes.

 

       Fernando Quevedo (Cambridge)

       Data: dijous, 23 de gener, 14:00h

       Lloc: Aula 505, planta cinquena (UB).
 

RESUM: The interaction of branes and antibranes provides a good
scenario for the realisation of cosmological inflation with explicitly
computable potentials from string theory. The ending of inflation is also
provided by a stringy field: the open string tachyon. An overview is
presented of these ideas including a discussion on the bound state of
branes and antibranes (the `branonium') that has some surprisng
properties..