Heavy flavours in AA collisions: production, transport and final spectra

A multi-step setup for heavy-flavour studies in high-energy nucleus-nucleus (AA) collisions --- addressing within a comprehensive framework the initial Q-Qbar production, the propagation in the hot medium until decoupling and the final hadronization and decays --- is presented. The initial hard production of Q-Qbar pairs is simulated using the POWHEG pQCD event generator, interfaced with the PYTHIA parton shower. Outcomes of the calculations are compared to experimental data in pp collisions and are used as a validated benchmark for the study of medium effects. In the AA case, the propagation of the heavy quarks in the medium is described in a framework provided by the relativistic Langevin equation. For the latter, different choices of transport coefficients are explored (either provided by a perturbative calculation or extracted from lattice-QCD simulations) and the corresponding numerical results are compared to experimental data from RHIC and the LHC. In particular, outcomes for the nuclear modification factor R_AA and for the elliptic flow v_2 of D/B mesons, heavy-flavour electrons and non-prompt J/\psi's are displayed

Extending a previous analysis on a possible modulation effect in WIMP direct search

In this note we present an extension of our previous analysis of some preliminary experimental results of the DAMA/NaI Collaboration which might be indicative of a yearly modulation effect. Here we present a direct way for obtaining from the experimental data the relevant cosmological implications for relic neutralinos. We find that some of the configurations singled out by the DAMA/NaI results would have cosmological properties compatible with a neutralino as a dominant component of cold dark matter (on the average in the Universe and in our galactic halo)

Supersymmetric candidates for dark matter

Direct and indirect detection rates of relic neutralinos are reviewed in the framework of the Minimal Supersymmetric Standard Model. The theoretical estimates are compared with the most recent experimental limits from low-background detectors and neutrino telescopes. The properties of neutralino under the hypothesis that preliminary experimental results of the DAMA/NaI Collaboration may be indicative of a yearly modulation effect are examined

Interference effects in heavy Higgs production via gluon fusion in the singlet extension of the Standard Model

The measurements of the properties of the Higgs boson still leave room for a non minimal scalar sector. Extensions of the Standard Model typically involve multiple neutral Higgs fields which can interfere among themselves. We show that these interference effects can be substantial taking as example the one Higgs Singlet Model, the simplest renormalizable addition to the SM

Doorway States in the Random-Phase Approximation

By coupling a doorway state to a see of random background states, we develop the theory of doorway states in the framework of the random-phase approximation (RPA). Because of the symmetry of the RPA equations, that theory is radically different from the standard description of doorway states in the shell model. We derive the Pastur equation in the limit of large matrix dimension and show that the results agree with those of matrix diagonalization in large spaces. The complexity of the Pastur equation does not allow for an analytical approach that would approximately describe the doorway state. Our numerical results display unexpected features: The coupling of the doorway state with states of opposite energy leads to strong mutual attraction

Parity-violating longitudinal response

The parity-violating quasielastic electron scattering response is explored within the context of a model that builds antisymmetrized random phase approximation and Hartree-Fock correlations on a relativistic Fermi gas basis. Particular emphasis is put on the weak-neutral longitudinal response function, since this observable displays a strong sensitivity to isospin correlations: specifically, it is shown how, through a diagrammatic cancellation/filtration mechanism, this response acts as a magnifier of pionic correlations in the nuclear medium. The parity-violating longitudinal response function also displays appreciable sensitivity to the electric strangeness content of the nucleon, thus making quasielastic electron scattering a possible candidate to measure the nucleon electric strange form factor at relatively high momentum transfers. Finally, we discuss how observables, related to the asymmetry, can be constructed to disentangle the nuclear and nucleonic effects

A simple solution for marginal deformations in open string field theory

We derive a new open string field theory solution for boundary marginal deformations generated by chiral currents with singular self-OPE. The solution is algebraically identical to the Kiermaier-Okawa-Soler solution and it is gauge equivalent to the TakahashiTanimoto identity-based solution. It is wedge-based and we can analytically evaluate the Ellwood invariant and the action, reproducing the expected results from BCFT. By studying the isomorphism between the states of the initial and final background a dual derivation of the Ellwood invariant is also obtained

Analyzing high-energy factorization beyond next-to-leading logarithmic accuracy

We provide a complete and detailed study of the high-energy limit of four-parton scattering amplitudes in QCD, giving explicit results at two loops and higher orders, and going beyond next-to-leading logarithmic (NLL) accuracy. Building upon recent results, we use the techniques of infrared factorization to investigate the failure of the simplest form of Regge factorization, starting at next-to-next-to-leading logarithmic accuracy (NNLL) in ln( s/ | t |). We provide detailed accounts and explicit expressions for the terms responsible for this breaking in the case of two-loop and three-loop quark and gluon amplitudes in QCD; in particular, we recover and explain a known non-logarithmic double-pole contribution at two-loops, and we compute all non-factorizing single-logarithmic singular contributions at three loops. Conversely, we use high-energy factorization to show that the hard functions of infrared factorization vanish in d = 4 to all orders in the coupling, up to NLL accuracy in ln( s/ | t |). This provides clear evidence for the infrared origin of high-energy logarithms. Finally, we extend earlier studies to t -channel exchanges of color representations beyond the octet, which enables us to give predictions based on the dipole formula for single-pole NLL contributions at three and four loops

Correlators of arbitrary untwisted operators and excited twist operators for N branes at angles

We compute the generic correlator with L untwisted operators and N (excited) twist fields for branes at angles on T2 and show that it is given by a generalization of the Wick theorem. We give also the recipe to compute efficiently the generic OPE between an untwisted operator and an excited twisted state

A different kind of string

In U(1) lattice gauge theory in three spacetime dimensions, the problem of confinement can be studied analytically in a semi-classical approach, in terms of a gas of monopoles with Coulomb-like interactions. In addition, this theory can be mapped to a spin model via an exact duality transformation, which allows one to perform high-precision numerical studies of the confining potential. Taking advantage of these properties, we carried out an accurate investigation of the effective string describing the low-energy properties of flux tubes in this confining gauge theory. We found striking deviations from the expected Nambu-Goto-like behavior, and, for the first time, evidence for contributions that can be described by a term proportional to the extrinsic curvature of the effective string worldsheet. Such term is allowed by Lorentz invariance, and its presence in the infrared regime of the U(1) model was indeed predicted by Polyakov several years ago. Our results show that this term scales as expected according to Polyakov’s solution, and becomes the dominant contribution to the effective string action in the continuum limit. We also demonstrate analytically that the corrections to the confining potential induced by the extrinsic curvature term can be related to the partition function of the massive perturbation of a c = 1 bosonic conformal field theory. The implications of our results for SU( N ) Yang-Mills theories in three and in four spacetime dimensions are discussed