arXiv subjects

Wojciech Broniowski

Publications and source records attributed to Wojciech Broniowski.

At least 127 records · Page 7Linked to original sources

arXiv2010

Two-body nucleon-nucleon correlations in Glauber-like models

We investigate the role of the central two-body nucleon-nucleon correlations on typical quantities observed in relativistic heavy-ion collisions. Basic correlation measures, such as the fluctuations of the participant eccentricity, initial size fluctuations, or the fluctuations of the number of sources producing particles, are sensitive at the level 10-20% to the inclusion of the two-body correlations. However, the realistic correlation function gives virtually indistinguishable results from the {\em hard-core repulsion} with the expulsion distance set to $\sim 0.9$ fm. In the second part of the talk we compare the spherical and Gaussian wounding profiles and find that the latter, which is more realistic, leads to reduced eccentricity and fluctuations. This has significance for precision studies of the elliptic flow.

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arXiv2010

Scalar-isoscalar states, gravitational form factors, and dimension-2 condensates in a large-Nc Regge approach

Scalar-isoscalar states are analyzed within the large-Nc Regge approach. We find that the lightest f0(600) scalar-isoscalar state fits very well into the pattern of the radial Regge trajectory. We confirm the obtained mass values from an analysis of the pion and nucleon spin-0 gravitational form factors, recently measured on the lattice. We find that a simple two-state model suggests a meson nature of f0(600), and a glueball nature of f0(980), which naturally explains the ratios of various coupling constants. Finally, matching to the OPE requires a fine-tuned mass condition of the vanishing dimension-2 condensate in the Regge approach with infinitely many scalar-isoscalar states.

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arXiv2010

Transversity relations, chiral and holographic models, and pion wave functions from lattice QCD

We analyze the equal-time Bethe-Salpeter quark wave functions of the pion in various models. We discuss how the quenched lattice QCD results with delocalized pion interpolators can be identified with the coarse grained wave functions, typical of low-energy effective models. Actually, we find that one-loop chiral quark models predict that pseudoscalar and tensor wave functions have the same shape, while the axial component is more extended. These facts are accurately confirmed by the lattice. We also show how the transversity information, relevant for the light-cone physics, can be straightforwardly obtained from the equal-time rest-frame lattice calculations. This remarkable relation provides a way to extract, for instance, the equal-time holographic wave functions and compare them, quite favorably, to the lattice calculations.

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arXiv2010

Pion transition form factor in the Regge approach

We explore the BaBar puzzle within the Regge approach. After reviewing the chiral quark models in applications to PDF and PDA of the pion, we argue that variants of these models, fulfilling the chiral anomaly, may in fact violate the second Terazawa-West unitarity bound, which is based on unverified assumptions for the real part of the amplitude. Consequently, the transition form factor need not vanish at large values of the photon virtuality. Then we show that the experimental data may be properly explained with incomplete vector-meson dominance in a simple model with one state, as well as in more sophisticated radial Regge models including infinitely many states. We also consider the experimental constraint from the rare Z -> pi0 gamma decay, which is comfortably satisfied in our approach. Finally, we point out that the photon momentum asymmetry parameter may noticeably influence the precision fits to the data.

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arXiv2010

Transversity form factors of the pion in chiral quark models

The transversity form factors of the pion, involving matrix elements of bilocal tensor currents, are evaluated in chiral quark models, both in the local Nambu--Jona-Lasinio with the Pauli-Villars regularization, as well as in nonlocal models involving momentum-dependent quark mass. After suitable QCD evolution the agreement with recent lattice calculations is very good, in accordance to the fact that the spontaneously broken chiral symmetry governs the dynamics of the pion. Meson dominance of form factors with expected meson masses also works properly, conforming to the parton-hadron duality in the considered process.

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arXiv2010

Pion transition form factor in the Regge approach and incomplete vector-meson dominance

The concept of incomplete vector-meson dominance and Regge models is applied to the transition form factor of the pion. First, we argue that variants of the chiral quark model fulfilling the chiral anomaly may violate the Terazawa-West unitarity bounds, as these bounds are based on unverified assumptions for the real parts of the amplitudes, precluding a possible presence of polynomial terms. A direct consequence is that the transition form factor need not necessarily vanish at large values of the photon virtuality. Moreover, in the range of the BaBar experiment, the Terazawa-West bound is an order of magnitude above the data, thus is of formal rather than practical interest. Then we demonstrate how the experimental data may be properly explained with incomplete vector-meson dominance in a simple model with one state, as well as in more sophisticated Regge models. Generalizations of the simple Regge model along the lines of Dominguez result in a proper description of the data, where one may adjust the parameters in such a way that the Terazawa-West bound is satisfied or violated. We also impose the experimental constraint from the Z -> pi0 gamma decay. Finally, we point out that the photon momentum asymmetry parameter may noticeably influence the precision analysis.

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arXiv2010

Two-body nucleon-nucleon correlations in Glauber models of relativistic heavy-ion collisions

We investigate the influence of the central two-body NN correlations on several quantities observed in relativistic heavy-ion collisions. It is demonstrated with explicit Monte Carlo simulations, that the basic correlation measures, such as the fluctuations of participant eccentricity, initial size fluctuations, or the fluctuations of the number of sources producing particles, are all sensitive to the inclusion of the two-body correlations. The effect is at the level of about 10-20%. Moreover, the realistic (Gaussian) correlation function gives indistinguishable results from the hard-core repulsion, with the expulsion distance set to 0.9 fm. Thus, for investigations of the considered correlation measures, it is sufficient to use the Monte Carlo generators accounting for the hard-core repulsion.

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arXiv2010

Scalar-isoscalar states in the large-Nc Regge approach

Scalar-isoscalar states (0++) are investigated within the large-Nc Regge approach. We elaborate on the consequences of including the lightest f0(600) scalar-isoscalar state into such an analysis, where the position of f0(600) fits very well into the pattern of the radial Regge trajectory. Furthermore, we point out that the pion and nucleon spin-0 gravitational form factors, recently measured on the lattice, provide valuable information on the low-mass spectrum of the scalar-isoscalar states on the basis of the scalar-meson dominance in the spin-0 channel. Through the fits to these data we find m_sigma= 450-600 MeV. We compare the predictions of various fits and methods. An analysis of the QCD condensates in the two-point correlators provides further constraints on the parameters of the scalar-isoscalar sector. We find that a simple two-state model suggests a meson nature of f0(600), and a glueball nature of f0(980), which naturally explains the ratios of various coupling constants. Finally, we note that the fine-tuned condition of the vanishing dimension-2 condensate in the Regge approach with infinitely many scalar-isoscalar states yields a reasonable value for the mass of the lighest glueball state.

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arXiv2009

Pion wave function from lattice QCD vs. chiral quark models

We analyze the equal-time Bethe-Salpeter quark wave function of the pion obtained from a quenched lattice QCD calculation with delocalized quark interpolators. We find that the result agrees remarkably well with the predictions of the Nambu--Jona-Lasinio model in all channels. We choose the quenched lattice QCD, since it is closer to the large-Nc limit of the Nambu--Jona-Lasinio model. We also show how transversity information, relevant for the light-cone physics, can be obtained from our equal-time rest-frame lattice calculations.

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arXiv2009

Size fluctuations of the initial source and the event-by-event transverse momentum fluctuations in relativistic heavy-ion collisions

We show that the event-by-event fluctuations of the transverse size of the initial source, which follow directly from the Glauber treatment of the earliest stage of relativistic heavy-ion collisions, cause, after hydrodynamic evolution, fluctuations of the transverse flow velocity at hadronic freeze-out. This in turn leads to event-by-event fluctuations of the average transverse momentum, p_T. Simulations with GLISSANDO for the Glauber phase, followed by a realistic hydrodynamic evolution and statistical hadronization carried out with THERMINATOR, lead to agreement with the RHIC data. In particular, the magnitude of the effect, its centrality dependence, and the weak dependence on the incident energy are properly reproduced. Our results show that bulk of the observed event-by-event p_T fluctuations may be explained by the fluctuations of the size of the initial source.

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arXiv2009

Fluctuations in relativistic heavy-ion collisions from the Glauber models

In the first part of the talk we discuss the role of the two-body nucleon-nucleon correlations on signatures of the heavy-ion collisions which are a priori expected to be sensitive to these effects. We find that while the fluctuations of the number of produced particles are indeed affected, other quantities (v_2 fluctuations, size fluctuations) are insensitive to the presence of the NN correlations in the nucleon distributions. In the second part we show that the fluctuations of the transverse size of the initial source cause, after a suitable hydrodynamic evolution, fluctuations of the transverse flow velocity at hadronic freeze-out. This in turn yields the event-by-event fluctuations of the transverse momentum of the produced particles, p_T. Our results demonstrate that practically all of the observed event-by-event p_T fluctuations may be explained this way.

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arXiv2009

Gravitational, electromagnetic, and transition form factors of the pion

Results of the Spectral Quark Model for the gravitational, electromagnetic, and transition form factors of the pion are discussed. In this model both the parton distribution amplitude and the parton distribution function are flat, in agreement with the transverse lattice calculations at low renormalization scales. The model predictions for the gravitational form factor are compared to the lattice data, with good agreement. We also find a remarkable relation between the three form factors, holding within our model, which besides reproducing the anomaly, provides a relation between radii which is reasonably well fulfilled. Comparison with the CELLO, CLEO, and BaBar data for the transition form factor is also considered.

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arXiv2009

Pion quadrupole polarizabilities in the Nambu--Jona-Lasinio model

We present the results for the pion electromagnetic quadrupole polarizabilities, calculated within the Nambu--Jona-Lasinio model. We obtain the sign and magnitude in agreement with the respective experimental analysis based on the Dispersion Sum Rules. At the same time the dipole polarizabilities are well reproduced. Comparison is also made with the results from the Chiral Perturbation Theory.

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arXiv2009

Application of chiral quarks to high-energy processes and lattice QCD

Results of the chiral quark models for the soft matrix elements involving pions and photons, relevant for high-energy processes, are reviewed. We discuss quantities related to the generalized parton distributions of the pion: the parton distribution functions, the parton distribution amplitudes, and the generalized form factors. The model predictions are compared to the data or lattice simulations, with good agreement. The QCD evolution from the low quark model scale up to the experimental scales is a crucial ingredient of the approach.

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arXiv2009

Quadrupole polarizabilities of the pion in the Nambu--Jona-Lasinio model

The electromagnetic dipole and quadrupole polarizabilities of the neutral and charged pions are calculated in the Nambu--Jona-Lasinio model. Our results agree with the recent experimental analysis of these quantities based on Dispersion Sum Rules. Comparison is made with the results from the Chiral Perturbation Theory.

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arXiv2009

Photon interactions and chiral dynamics

Twist-2 components of the real and virtual photon distribution amplitudes are evaluated in several chiral quark models. The results, obtained at the quark model scale, are then evolved to higher scales, probed in experiments or in lattice QCD. We also analyze the related form factors and coupling constants. Our results are a genuine dynamical prediction, following from the chiral dynamics.

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arXiv2009

From chiral quark models to high-energy processes

We show the results of low-energy chiral quark models for soft matrix elements involving pions and photons. Such soft elements, upon convolution with the hard matrix elements, are relevant in various high-energy processes. We focus on quantities related to the generalized parton distributions of the pion: the parton distribution functions, the parton distribution amplitudes, and the generalized form factors. Wherever possible, the model predictions are confronted with the data or lattice simulations, where surprisingly good agreement is achieved. The QCD evolution from the low quark model scale up to the scale of the data is crucial for this agreement.

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arXiv2009

A note on the QCD evolution of generalized form factors

Generalized form factors of hadrons are objects appearing in moments of the generalized parton distributions. Their leading-order DGLAP-ERBL QCD evolution is exceedingly simple and the solution is given in terms of matrix triangular structures of linear equations where the coefficients are the evolution ratios. We point out that this solution has a practical importance in analyses where the generalized form factors are basic objects, e.g., the lattice-gauge studies or models. It also displays general features of their evolution.

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