IFSM representation of Brownian motion with applications to simulation

Several methods are currently available to simulate paths of the Brownian motion. In particular, paths of the BM can be simulated using the properties of the increments of the process like in the Euler scheme, or as the limit of a random walk or via L2 decomposition like the Kac-Siegert/Karnounen-Loeve series. In this paper we first propose a IFSM (Iterated Function Systems with Maps) operator whose fixed point is the trajectory of the BM. We then use this representation of the process to simulate its trajectories. The resulting simulated trajectories are self-affine, continuous and fractal by construction. This fact produces more realistic trajectories than other schemes in the sense that their geometry is closer to the one of the true BM's trajectories. The IFSM trajectory of the BM can then be used to generate more realistic solutions of stochastic differential equations

Transverse-momentum resummation for heavy-quark hadroproduction

We consider the production of a pair of heavy quarks ($Q{\bar Q}$) in hadronic collisions. When the transverse momentum $q_T$ of the heavy-quark pair is much smaller than its invariant mass, the QCD perturbative expansion is affected by large logarithmic terms that must be resummed to all-orders. This behavior is well known from the simpler case of hadroproduction of colourless high-mass systems, such as vector or Higgs boson(s). In the case of $Q{\bar Q}$ production, the final-state heavy quarks carry colour charge and are responsible for additional soft radiation (through direct emission and interferences with initial-state radiation) that complicates the evaluation of the logarithmically-enhanced terms in the small-$q_T$ region. We present the all-order resummation structure of the logarithmic contributions, which includes colour flow evolution factors due to soft wide-angle radiation. Resummation is performed at the completely differential level with respect to the kinematical variables of the produced heavy quarks. Soft-parton radiation produces azimuthal correlations that are fully taken into account by the resummation formalism. These azimuthal correlations are entangled with those that are produced by initial-state collinear radiation. We present explicit analytical results up to next-to-leading order and next-to-next-to-leading logarithmic accuracy.Comment: Some comments expanded and references added. Version published on NP

Lie symmetries of Einstein's vacuum equations in N dimensions

We investigate Lie symmetries of Einstein's vacuum equations in N dimensions, with a cosmological term. For this purpose, we first write down the second prolongation of the symmetry generating vector fields, and compute its action on Einstein's equations. Instead of setting to zero the coefficients of all independent partial derivatives (which involves a very complicated substitution of Einstein's equations), we set to zero the coefficients of derivatives that do not appear in Einstein's equations. This considerably constrains the coefficients of symmetry generating vector fields. Using the Lie algebra property of generators of symmetries and the fact that general coordinate transformations are symmetries of Einstein's equations, we are then able to obtain all the Lie symmetries. The method we have used can likely be applied to other types of equations

Possible Contribution to Electron and Positron Fluxes from Pulsars and their Nebulae

The AMS-02 experiment confirms the excess of positrons in cosmic rays (CRs) for energy above 10 GeV with respect to the secondary production of positrons in the interstellar medium. This is interpreted as evidence of the existence of a primary source of these particles. Possible candidates are dark matter or astrophysical sources. In this work we discuss the possible contribution due to pulsars and their nebulae. Our key assumption is that the primary spectrum of electrons and positrons at the source is the same of the well known photon spectrum observed from gamma-rays telescopes. Using a diffusion model in the Galaxy we propagate the source spectra up to the Solar System. We compare our results with the recent experiments and with the LIS modelComment: To appear in the Proceedings of the 14th ICATPP Conference, Villa Olmo 23-27 September 201

Relational quality: A dynamic framework for assessing the role of trust in strategic alliances

The organizational literature has always posited that «trust» plays a key role in economic exchanges, particularly when one or another party is subject to the risk of opportunistic behaviour, incomplete monitoring, or when moral hazard problems arise. These conditions are almost always present in the case of alliances and joint ventures between independent parties. This paper explores the concept of «relational quality» in one such inter-organizational form ­dyadic alliances­ where past experience and the shadow of the future play an important role. Relational quality is important, as it affects the extent to which partners substitute reliance on trust for more formal control mechanisms. Building on theory, case studies and survey data, we develop a framework for thinking about trust in dynamic and practical terms. We define three elements affecting relational quality in alliances: the initial conditions surrounding the exchange, the cumulative experiences of the parties with each other's behaviours as they interact, and the impact that external events have on perceptions of behaviour and attitudes of the parties about each other's trustworthiness. We use data on a sample of alliances with one Spanish partner to explore the relative impact of these elements and develop a more precise set of propositions from this framework. The paper should guide further work towards quantifying the role of trust as a control mechanism in the performance of strategic alliances.Alliances; economic exchanges; joint ventures;

Roles played by relational trust in strategic alliances

In this paper we develop the concept of relational quality as a proxy for relational trust. Exploration of data from 67 alliances confirms that relational quality is composed of three elements: initial conditions, partner interactions, and external events. We offer propositions on the relative importance of each of these elements depending on the different roles relational trust may play in strategic alliances: the role of a control mechanism, of a governance mechanism, or as an enabler of high risk initiativesManagement; Strategy

Relational quality: Managing trust in corporate alliances

The management literature has often argued that "trust" plays a key role in economic exchanges, particularly when one or another party is subject to the risk of opportunistic behaviour and incomplete monitoring, or when problems due to moral hazard or asymmetric information arise. These conditions are almost always present in the case of corporate alliances and joint ventures. We propose that one aspect of trust, what we call "relational quality", is fundamental to the maintenance of good working conditions in two-party alliances where past experience and the shadow of the future play important roles. Relying on a growing body of theory and a number of case studies, we develop a framework for thinking about trust in dynamic and practical terms. We conclude that a reservoir of relational quality exists in any such relationship, and that the level of trust implied in such a reservoir will not only influence whether and how future conflicts are resolved, but also is itself affected by the positive (or negative) resolution of such conflicts. Finally, we identify three elements that contribute to the relational quality reservoir in alliances: 1) the initial conditions surrounding the alliance formation; 2) the cumulative experience of the parties with each others' behaviours as the alliance unfolds; and 3) the impact that external events or behaviours outside the alliance's context have on the perceptions and attitudes the parties have about each other's trustworthiness. We conclude with some recommendations for more effective management of corporate alliances.corporate alliances; economic exchanges; monitoring;

Pulsar Wind Nebulae as a source of the observed electron and positron excess at high energy: the case of Vela-X

We investigate, in terms of production from pulsars and their nebulae, the cosmic ray positron and electron fluxes above $\sim10$ GeV, observed by the AMS-02 experiment up to 1 TeV. We concentrate on the Vela-X case. Starting from the gamma-ray photon spectrum of the source, generated via synchrotron and inverse Compton processes, we estimated the electron and positron injection spectra. Several features are fixed from observations of Vela-X and unknown parameters are borrowed from the Crab nebula. The particle spectra produced in the pulsar wind nebula are then propagated up to the Solar System, using a diffusion model. Differently from previous works, the omnidirectional intensity excess for electrons and positrons is obtained as a difference between the AMS-02 data and the corresponding local interstellar spectrum. An equal amount of electron and positron excess is observed and we interpreted this excess (above $\sim$100 GeV in the AMS-02 data) as a supply coming from Vela-X. The particle contribution is consistent with models predicting the gamma-ray emission at the source. The input of a few more young pulsars is also allowed, while below $\sim$100 GeV more aged pulsars could be the main contributors.Comment: Accepted for publication in Journal of High Energy Astrophysics (2015