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Giovanni Ossola

Publications and source records attributed to Giovanni Ossola.

30 records · Page 2Linked to original sources
arXiv2009

NLO QED Corrections to Hard-Bremsstrahlung Emission in Bhabha Scattering

In this paper we present a numerical implementation of the one-loop QED corrections to the hard-bremsstrahlung process e- e+ \to e- e+ gamma. These corrections can be included in the Monte Carlo event generators employed for simulating Bhabha scattering events at low-energy high-luminosity electron-positron colliders. The calculation is performed by employing the reduction method developed by Ossola, Papadopoulos and Pittau. Our results are implemented in a modular code for the numerical evaluation of the scattering amplitudes for any given phase-space point. In a similar way, we also evaluate the one-loop QED corrections to e- e+ \to mu- mu+ gamma, which represents an interesting application of the method in the presence of two different mass scales in the loops.

hep-ph↗
arXiv2008

CutTools: a program implementing the OPP reduction method to compute one-loop amplitudes

We present a program that implements the OPP reduction method to extract the coefficients of the one-loop scalar integrals from a user defined (sub)-amplitude or Feynman Diagram, as well as the rational terms coming from the 4-dimensional part of the numerator. The rational pieces coming from the epsilon-dimensional part of the numerator are treated as an external input, and can be computed with the help of dedicated tree-level like Feynman rules. Possible numerical instabilities are dealt with the help of arbitrary precision routines, that activate only when needed.

hep-ph↗
arXiv2008

On the Rational Terms of the one-loop amplitudes

The various sources of Rational Terms contributing to the one-loop amplitudes are critically discussed. We show that the terms originating from the generic (n-4)-dimensional structure of the numerator of the one-loop amplitude can be derived by using appropriate Feynman rules within a tree-like computation. For the terms that originate from the reduction of the 4-dimensional part of the numerator, we present two different strategies and explicit algorithms to compute them.

hep-ph↗
arXiv2007

A new method for the numerical evaluation of one-loop amplitudes

We recently presented a new method for the evaluation of one-loop amplitude of arbitrary scattering processes, in which the reduction to scalar integrals is performed at the integrand level. In this talk, we review the main features of the method and briefly summarize the results of the first calculations performed using it.

hep-ph↗
arXiv2007

Numerical Evaluation of Six-Photon Amplitudes

We apply the recently proposed amplitude reduction at the integrand level method, to the computation of the scattering process 2 photons -> 4 photons, including the case of a massive fermion loop. We also present several improvements of the method, including a general strategy to reconstruct the rational part of any one-loop amplitude and the treatment of vanishing Gram-determinants.

hep-ph↗
arXiv2007

Dynamic critical behavior of the Chayes-Machta-Swendsen-Wang algorithm

We study the dynamic critical behavior of the Chayes-Machta dynamics for the Fortuin-Kasteleyn random-cluster model, which generalizes the Swendsen-Wang dynamics for the q-state Potts model to noninteger q, in two and three spatial dimensions, by Monte Carlo simulation. We show that the Li-Sokal bound z \ge α/νis close to but probably not sharp in d=2, and is far from sharp in d=3, for all q. The conjecture z \ge β/νis false (for some values of q) in both d=2 and d=3.

cond-mat.stat-mech↗
arXiv2006

Reducing full one-loop amplitudes to scalar integrals at the integrand level

We show how to extract the coefficients of the 4-, 3-, 2- and 1-point one-loop scalar integrals from the full one-loop amplitude of arbitrary scattering processes. In a similar fashion, also the rational terms can be derived. Basically no information on the analytical structure of the amplitude is required, making our method appealing for an efficient numerical implementation

hep-ph↗
arXiv2006

Hadronic mass and q^2 moments in B->Xu l nu

We present OPE predictions for the hadronic mass and q^2 moments in inclusive semileptonic charmless B decays, with a lower cut on the charged lepton energy and an upper cut on the hadronic invariant mass. We include non-perturbative corrections through O(1/m_b^3) and perturbative contributions through O(alpha_s^2 beta_0). We also investigate the range of the cut on the hadronic mass for which the local OPE can be considered valid and give estimates of the residual theoretical uncertainty. The study of these moments is important to constrain the effect of the Weak Annihilation (WA) contributions and the shape of the distribution function, providing a more precise inclusive determination of V_ub.

hep-ph↗
arXiv2004

Systematic errors due to linear congruential random-number generators with the Swendsen-Wang algorithm: A warning

We show that linear congruential pseudo-random-number generators can cause systematic errors in Monte Carlo simulations using the Swendsen-Wang algorithm, if the lattice size is a multiple of a very large power of 2 and one random number is used per bond. These systematic errors arise from correlations within a single bond-update half-sweep. The errors can be eliminated (or at least radically reduced) by updating the bonds in a random order or in an aperiodic manner. It also helps to use a generator of large modulus (e.g. 60 or more bits).

hep-lat↗
arXiv2004

Dynamic critical behavior of the Swendsen--Wang Algorithm for the three-dimensional Ising model

We have performed a high-precision Monte Carlo study of the dynamic critical behavior of the Swendsen-Wang algorithm for the three-dimensional Ising model at the critical point. For the dynamic critical exponents associated to the integrated autocorrelation times of the "energy-like" observables, we find z_{int,N} = z_{int,E} = z_{int,E'} = 0.459 +- 0.005 +- 0.025, where the first error bar represents statistical error (68% confidence interval) and the second error bar represents possible systematic error due to corrections to scaling (68% subjective confidence interval). For the "susceptibility-like" observables, we find z_{int,M^2} = z_{int,S_2} = 0.443 +- 0.005 +- 0.030. For the dynamic critical exponent associated to the exponential autocorrelation time, we find z_{exp} \approx 0.481. Our data are consistent with the Coddington-Baillie conjecture z_{SW} = β/ν\approx 0.5183, especially if it is interpreted as referring to z_{exp}.

hep-lat↗
arXiv2003

Considerations Concerning the Contributions of Fundamental Particles to the Vacuum Energy Density

The covariant regularization of the contributions of fundamental particles to the vacuum energy density is implemented in the Pauli-Villars, dimensional regularization, and Feynman regulator frameworks. Rules of correspondence between dimensional regularization and cutoff calculations are discussed. Invoking the scale invariance of free field theories in the massless limit, as well as consistency with the rules of correspondence, it is argued that quartic divergencies are absent in the case of free fields, while it is shown that they arise when interactions are present.

hep-ph↗