Principles of general final-state resummation and automated implementation

Next-to-leading logarithmic final-state resummed predictions have traditionally been calculated, manually, separately for each observable. In this article we derive NLL resummed results for generic observables. We highlight and discuss the conditions that the observable should satisfy for the approach to be valid, in particular continuous globalness and recursive infrared and collinear safety. The resulting resummation formula is expressed in terms of certain well-defined characteristics of the observable. We have written a computer program, CAESAR, which, given a subroutine for an arbitrary observable, determines those characteristics, enabling full automation of a large class of final-state resummations, in a range of processes.Comment: 111 pages. 6 figures, JHEP class included. Section 1 contains a guide to reading the article; results obtained with CAESAR are available at http://qcd-caesar.org; v2 includes substantial new explanatory material (expansion of section 2, new appendices D & E), additional references, and corrects misprint

A new procedure to analyze RNA non-branching structures

RNA structure prediction and structural motifs analysis are challenging tasks in the investigation of RNA function. We propose a novel procedure to detect structural motifs shared between two RNAs (a reference and a target). In particular, we developed two core modules: (i) nbRSSP_extractor, to assign a unique structure to the reference RNA encoded by a set of non-branching structures; (ii) SSD_finder, to detect structural motifs that the target RNA shares with the reference, by means of a new score function that rewards the relative distance of the target non-branching structures compared to the reference ones. We integrated these algorithms with already existing software to reach a coherent pipeline able to perform the following two main tasks: prediction of RNA structures (integration of RNALfold and nbRSSP_extractor) and search for chains of matches (integration of Structator and SSD_finder)

The nature and strength of inter-layer binding in graphite

We computed the inter-layer bonding properties of graphite using an ab-initio many body theory. We carried out variational and diffusion quantum Monte Carlo calculations and found an equilibrium inter-layer binding energy in good agreement with most recent experiments. We also analyzed the behavior of the total energy as a function of interlayer separation at large distances comparing the results with the predictions of the random phase approximation.Comment: 5 pages; to appear in Phys. Rev. Let

Dielectric Response of Periodic Systems from Quantum Monte Carlo Calculations

We present a novel approach that allows to calculate the dielectric response of periodic systems in the quantum Monte Carlo formalism. We employ a many-body generalization for the electric enthalpy functional, where the coupling with the field is expressed via the Berry-phase formulation for the macroscopic polarization. A self-consistent local Hamiltonian then determines the ground-state wavefunction, allowing for accurate diffusion quantum Monte Carlo calculations where the polarization's fixed point is estimated from the average on an iterative sequence, sampled via forward-walking. This approach has been validated for the case of an isolated hydrogen atom, and then applied to a periodic system, to calculate the dielectric susceptibility of molecular-hydrogen chains. The results found are in excellent agreement with the best estimates obtained from the extrapolation of quantum-chemistry calculations.Comment: 5 page 2figure

How bright is the proton? A precise determination of the photon parton distribution function

It has become apparent in recent years that it is important, notably for a range of physics studies at the Large Hadron Collider, to have accurate knowledge on the distribution of photons in the proton. We show how the photon parton distribution function (PDF) can be determined in a model-independent manner, using electron-proton ($ep$) scattering data, in effect viewing the $ep\to e+X$ process as an electron scattering off the photon field of the proton. To this end, we consider an imaginary, beyond Standard Model process with a flavour changing photon-lepton vertex. We write its cross section in two ways, one in terms of proton structure functions, the other in terms of a photon distribution. Requiring their equivalence yields the photon distribution as an integral over proton structure functions. As a result of the good precision of $ep$ data, we constrain the photon PDF at the level of 1-2% over a wide range of momentum fractions.Comment: 6 pages, 5 figures; v3 includes small textual changes and an updated CT14 comparison in Fig. 4, as published. LUXqed_PDF4LHC15_nnlo_100 set available from LHAPDF, further information at http://cern.ch/luxqed