Model-dependent Contributions Research Articles

Revisiting transverse momentum broadening in dense QCD media

We reconsider the problem of transverse momentum broadening of a highly-energetic parton suffering multiple scatterings in dense colored media, such as the thermal Quark-Gluon plasma or large nuclei. In the framework of Moli\`ere's theory of multiple scattering we re-derive a simple analytic formula, to be used in jet quenching phenomenology, that accounts for both the multiple soft and hard Rutherford scattering regimes. Further, we discuss the sensitivity of momentum broadening to modeling of the non-perturbative infrared sector by presenting a detailed analytic and numerical comparison between the two widely used models in phenomenology: the Hard Thermal Loop and the Gyulassy-Wang potentials. We show that for the relevant values of the parameters the non-universal, model dependent contributions are negligible, at LHC, RHIC and EIC energies thus consolidating the predictive power of jet quenching theory.

Jet tagging in the Lund plane with graph networks

The identification of boosted heavy particles such as top quarks or vector bosons is one of the key problems arising in experimental studies at the Large Hadron Collider. In this article, we introduce LundNet, a novel jet tagging method which relies on graph neural networks and an efficient description of the radiation patterns within a jet to optimally disentangle signatures of boosted objects from background events. We apply this framework to a number of different benchmarks, showing significantly improved performance for top tagging compared to existing state-of-the-art algorithms. We study the robustness of the LundNet taggers to non-perturbative and detector effects, and show how kinematic cuts in the Lund plane can mitigate overfitting of the neural network to model-dependent contributions. Finally, we consider the computational complexity of this method and its scaling as a function of kinematic Lund plane cuts, showing an order of magnitude improvement in speed over previous graph-based taggers.

W *γ* π form factors in the τ¯→ π¯ℓ +ℓ¯ ντ decays

We study the disintegration τ- →π-ℓ+ℓ-ντ, where l denotes either an electron or a muon. This process requires knowledge about the W*γ*π interaction, which is modeled using Resonance Chiral Theory. Within this framework we study the sensitivity of the observables of the process, such as the branching ratios and the dilepton mass spectra, to the model dependent contributions. The information of the hadronization of the quark currents is encoded in form factors which are evaluated using the lightest vector and axial-vector multiplets as degrees of freedom. We determine the couplings by matching the asymptotic QCD behavior of the form factors in the limit of an infinite number of colours. The branching ratios we obtain show that the process with l = e should be measured soon in B factories.

Transit-time aspects of ENA production models for the inner heliosheath

It is still a major challenge to understand the data of energetic neutral atoms (ENAs) that arrive from the outer heliosphere. These data are provided by the IBEX mission. With the most recent data covering three years of dynamical evolution, it is now possible to study temporal correlations with the solar cycle. We present in this paper for the first time an extended study dedicated to the transit-time delay between the plasma that is ejected from the Sun towards the outer heliosphere and the ENA fluxes that reach the detector after the solar wind protons undergo charge exchange processes in their distant production regions in the inner heliosheath. We derive transit-time delays caused by both general and non-trivial model-dependent contributions for arbitrary points in the inner heliosheath. In addition, we also calculate the average transit-time delay along the line-of-sight in selected directions, allowing to estimate how strongly an extended emission region can reduce the resolution that is possible with transit-time studies. We are able to demonstrate that ENAs generated beyond a certain characteristic streamline distance are mostly removed by charge exchange, and thus do not contribute significantly to the keV-ENA fluxes observed by IBEX. For this reason, transit-time delays do provide a viable tool for studying the impact of the solar cycle on ENA fluxes.

Structure-dependent radiative corrections toϕ→K+K−/KLKSdecays

Current predictions for the ratio of $\ensuremath{\phi}\ensuremath{\rightarrow}{K}^{+}{K}^{\ensuremath{-}}/{K}_{L}{K}_{S}$ decay rates exceed the corresponding experimental value in about 5 standard deviations. By far, the dominant sources of isospin breaking to this ratio are the phase-space (52%) and the electromagnetic radiative (4.3%, computed within scalar QED) corrections. Here we estimate the effects of the electromagnetic structure of kaons and other model-dependent contributions into the radiative corrections.

Radiative two-pion decay of the tau lepton

We consider the bremsstrahlung and model-dependent contributions to the radiative decay $\tau^- \to \pi^-\pi^0\nu_{\tau}\gamma$ in the context of a meson dominance model. We focus on several observables related to this decay, including the branching ratio and the photon and di-pion spectra. Particular attention is paid to the sensitivity of different observables upon the effects of model-dependent contributions and of the magnetic dipole moment of the $\rho^-(770)$ vector meson. Important numerical differences are found with respect to results obtained in the framework of chiral perturbation theory.

Beyond the soft photon approximation in radiative productionand decay of charged vector mesons

We study the effects of model-dependent contributions and the electricquadrupole moment of vector mesons in the decays V-→P-P0γ and τ-→νV-γ. Theirinterference with the amplitude originating from the radiation due toelectric charges vanishes for photons emitted collinearly to the chargedparticle in the final state. This brings further support to our claim inprevious works, that measurements of the photon energy spectrum fornearly collinear photons in those decays are suitable for a firstmeasurement of the magnetic dipole moment of charged vector mesons.

Gauge invariance and finite width effects in radiative two-pionτlepton decay

The contribution of the ${\ensuremath{\rho}}^{\ifmmode\pm\else\textpm\fi{}}$ vector meson to the $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\tau}}\ensuremath{\pi}\ensuremath{\pi}\ensuremath{\nu}\ensuremath{\gamma}$ decay is considered as a potential source for the determination of the magnetic dipole moment of this light vector meson. In order to keep the gauge invariance of the whole decay amplitude, a procedure similar to the fermion-loop scheme for charged gauge bosons is implemented to incorporate the finite width effects of the ${\ensuremath{\rho}}^{\ifmmode\pm\else\textpm\fi{}}$ vector meson. The absorptive pieces of the one-loop corrections to the propagators and electromagnetic vertices of the ${\ensuremath{\rho}}^{\ifmmode\pm\else\textpm\fi{}}$ meson and ${W}^{\ifmmode\pm\else\textpm\fi{}}$ gauge boson have identical forms in the limit of massless particles in the loops, suggesting this to be a universal feature of spin-one unstable particles. Model-dependent contributions to the $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\tau}}\ensuremath{\pi}\ensuremath{\pi}\ensuremath{\nu}\ensuremath{\gamma}$ decay are suppressed by fixing the two-pion invariant mass distribution at the rho meson mass value. The resulting photon energy and angular distribution is relatively sensitive to the effects of the $\ensuremath{\rho}$ magnetic dipole moment.

Vector-meson magnetic dipole moment effects in radiativeτdecays

We study the possibility that the magnetic dipole moment of light charged vector mesons could be measured from their effects in ${\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{V}^{\ensuremath{-}}{\ensuremath{\nu}}_{\ensuremath{\tau}}\ensuremath{\gamma}$ decays. We conclude that the energy spectrum and angular distribution of photons emitted at small angles with respect to vector mesons is sensitive to the effects of the magnetic dipole moment. Model-dependent contributions and photon radiation off other electromagnetic multipoles are small in this region. We also compute the effects of the magnetic dipole moment on the integrated rates and photon energy spectrum of these $\ensuremath{\tau}$ lepton decays.

Modular invariance and nonrenormalizable interactions

We examine the modular properties on nonrenormalizable superpotential terms in the free fermionic formulation of string theory and show that the requirement of modular invariance necessitates the nonvanishing of certain Nth order nonrenormalizable terms. In a class of models we explicitly verify that the nontrivial structure imposed by the modular invariance is indeed present, and further explore the universal and model-dependent contributions to higher-order nonrenormalizable terms in the superpotential. We also discuss the phenomenological implications of the above observations.

Some general properties of theO(α)corrections to parity violation in atoms

The model-dependent contributions to the parity-violating electron-nucleon interaction arising from $\ensuremath{\gamma}Z$ box diagrams are discussed on the basis of a formulation in which the hadronic parts of the amplitude are treated exactly ab initio. This leads to a derivation of general properties and more refined estimates of these effects. A method to take into account the large-distance part of the hadronic contributions to the $\ensuremath{\gamma}Z$ mixing diagrams using experimental information on ${e}^{+}+{e}^{\ensuremath{-}}$\ensuremath{\rightarrow}hadrons is briefly discussed. Predictions for the ${C}_{\mathrm{iN}} (i=1,2, N=p,n)$ are updated taking into account the more detailed estimates of this paper.

Two-pion exchange contribution to the nucleon-nucleon potential

The low-energy scattering of two nucleons is studied within the framework of dispersion theory, using the Mandelstam representation. Certain ϱ-functions occur in the treatment of this problem and these ϱ-functions are expressed in terms of the helicity amplitudes for the process 2π → N N for any value of the angular momentum J. These ϱ functions are evaluated using the BCL model and using the “realistic” helicity amplitudes for the S- and P-waves for the process 2π → N N , as supplied by Professor Hamilton and his co-workers. Next, going over to the potential approximation, the two-pion contribution to the nucleon-nucleon potential was evaluated. In the nucleon-nucleon potential, as calculated by Cottingham and Vinh Mau, we replace the BCL S- and P-wave model-dependent contributions by the corresponding S- and P-wave contributions calculated using the realistic helicity amplitude data for the process 2π → N N . The nucleon-nucleon potentials so obtained in the various cases, such as the central potential, the spin-orbit potential, etc., are described graphically by curves for both the triplet and singlet states. The various ϱ-functions obtained are also illustrated graphically and a critical discussion of the work is included. In conclusion, we observe that the nucleon-nucleon potential we calculate agrees with that calculated by Cottingham and Vinh Mau and with the Yale curves for the potential, except in special cases where the ω meson is expected to contribute strongly.