arXiv subjects

Cédric Lorcé

Publications and source records attributed to Cédric Lorcé.

At least 55 records · Page 3Linked to original sources

arXiv2019

Revisiting the mechanical properties of the nucleon

We discuss in detail the distributions of energy, radial pressure and tangential pressure inside the nucleon. In particular, this discussion is carried on in both the instant form and the front form of dynamics. Moreover we show for the first time how these mechanical concepts can be defined when the average nucleon momentum does not vanish. We express the conditions of hydrostatic equilibrium and stability in terms of these two and three-dimensional energy and pressure distributions. We briefly discuss the phenomenological relevance of our findings with a simple yet realistic model. In the light of this exhaustive mechanical description of the nucleon, we also present several possible connections between hadronic physics and compact stars, like e.g. the study of the equation of state for matter under extreme conditions and stability constraints.

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arXiv2018

The origin of the nucleon mass

It is often claimed that 98% of the nucleon mass is generated by quantum chromodynamics. The decomposition of the nucleon mass based on the trace of the energy-momentum tensor suggests that gluons play by far a dominant role. About 25 years ago, Ji proposed another decomposition based on the energy component of the energy-momentum tensor, leading to a quite different picture. Recently, we critically revisited these decompositions and argued that both overlooked pressure effects. In particular we showed that Ji's decomposition, although mathematically correct, makes little sense from a physical point of view. We identify the proper mass decomposition along with a balance equation for the pressure forces.

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arXiv2018

Gluon density fluctuations in dilute hadrons

Motivated by the relation existing between the gluon density in a hadron and the multiplicity of the particles measured in the final state of hadron-nucleus collisions, we study systematically the fluctuations of the gluon density in onia, which are the simplest dilute hadrons, of different sizes and at various rapidities. We argue that the small and the large-multiplicity tails of the gluon distributions present universal features, which should translate into properties of the multiplicity of the particles measured in the final state of high-energy proton-nucleus collisions, or of deep-inelastic scattering at a future electron-ion collider. We propose simple physical pictures of the rare events populating the tails of the multiplicity distribution that allow us to derive analytical formulas describing these universal behaviors, and we compare them to the results of Monte Carlo simulations.

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arXiv2018

The relativistic center of mass in field theory with spin

In order to unravel the origin of the nucleon spin, one has to study in detail the question of orbital angular momentum, and in particular the reference point about which it is defined. With this in mind, we review the concept of relativistic center of mass, generalize the discussion to the case of asymmetric energy-momentum tensors, and establish the link with the light-front formalism. We find that the $p$-wave in the Dirac plane-wave solutions arises from a relativistic quantum-mechanical effect which forces the canonical reference point to depend on the observer. This explains why longitudinal spin is much simpler to study than transverse spin. It is also the reason behind the observation of induced shifts and distortions in the parton distributions defined within the light-front formalism.

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arXiv2018

Twist-3 GPDs in Deeply-Virtual Compton Scattering

The sub-leading power of the scattering amplitude for deeply-virtual Compton scattering (DVCS) off the nucleon contains leading-twist and twist-3 generalized parton distributions (GPDs). We point out that in DVCS, at twist-3 accuracy, one cannot address any individual twist-3 GPD. This complication appears on top of the deconvolution issues familiar from the twist-2 DVCS amplitude. Accessible are exclusively linear combinations involving both vector and axial-vector twist-3 GPDs. This implies, in particular, that the (kinetic) orbital angular momentum of quarks can hardly be constrained by twist-3 DVCS observables. Moreover, using the quark-target model, we find that twist-3 GPDs can be discontinuous. The discontinuities however cancel in the DVCS amplitude, which further supports the hypothesis of factorization at twist-3 accuracy.

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arXiv2017

New explicit expressions for Dirac bilinears

We derive new explicit expressions for the Dirac bilinears based on a generic representation of the massive Dirac spinors with canonical polarization. These bilinears depend on a direction $n$ in Minkowski space which specifies the form of dynamics. We argue that such a dependence is unavoidable in a relativistic theory with spin, since it originates from Wigner rotation effects. Contrary to most of the expressions found in the literature, our ones are valid for all momenta and canonical polarizations of the spinors. As a by-product, we also obtain a generic explicit expression for the covariant spin vector.

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arXiv2017

On the hadron mass decomposition

We argue that the standard decompositions of the hadron mass overlook pressure effects, and hence should be interpreted with great care. Based on the semiclassical picture, we propose a new decomposition that properly accounts for these pressure effects. Because of Lorentz covariance, we stress that the hadron mass decomposition automatically comes along with a stability constraint, which we discuss for the first time. We show also that if a hadron is seen as made of quarks and gluons, one cannot decompose its mass into more than two contributions without running into trouble with the consistency of the physical interpretation. In particular, the so-called quark mass and trace anomaly contributions appear to be purely conventional. Based on the current phenomenological values, we find that in average quarks exert a repulsive force inside nucleons, balanced exactly by the gluon attractive force.

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arXiv2017

Distribution of the number of particles in the final state of hadron-nucleus collisions

Recently, Liou, Mueller and Munier have argued that proton-nucleus collisions at the LHC may give access to the full statistics of the event-by-event fluctuations of the gluon density in the proton. Indeed, the number of particles produced in an event in rapidity slices in the fragmentation region of the proton may, under some well-defined assumptions, be directly related to the number of gluons which have a transverse momentum larger than the nuclear saturation scale present in the proton at the time of the interaction with the nucleus. A first calculation of the probability distribution of the number of gluons in a hadron was performed, using the color dipole model. In this talk, we review this proposal, and present preliminary numerical calculations which support the analytical results obtained so far.

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arXiv2017

Spatial distribution of angular momentum inside the nucleon

We discuss in detail the spatial distribution of angular momentum inside the nucleon. We show that the discrepancies between different definitions originate from terms that integrate to zero. Even though these terms can safely be dropped at the integrated level, they have to be taken into account at the density level. Using the scalar diquark model, we illustrate our results and, for the first time, check explicitly that the equivalence between kinetic and canonical orbital angular momentum persists at the density level, as expected in a system without gauge degrees of freedom.

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arXiv2017

Quark transverse spin-orbit correlations

We extend the study of quark spin-orbit correlations in the nucleon to the case of transverse polarization. At the leading-twist level, this completes the spin structure of the quark kinetic energy-momentum tensor. In particular, we revisit the transversity decomposition of angular momentum proposed a decade ago by Burkardt and introduce a new transverse correlation, namely between quark transversity and orbital angular momentum. We also provide for the first time the Wandzura-Wilczek expression for the second Mellin moment of twist-3 transversity generalized parton distributions, along with a new sum rule. Based on lattice calculation results, we conclude that the quark transverse spin-orbit correlation is negative for both up and down flavors, just like in the longitudinal case.

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arXiv2016

Transverse phase space and its multipole decomposition

Relativistic phase space distributions are very interesting objects as they allow one to gather the information extracted from various types of experiments into a single coherent picture. Focusing on the four-dimensional transverse phase space, we identified all the possible angular correlations providing at the same time a clear physical interpretation of all the leading-twist generalized and transverse-momentum dependent parton distributions. We also developed a convenient representation of this four-dimensional space.

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arXiv2016

Universal Effective Hadron Dynamics from Superconformal Algebra

An effective supersymmetric QCD light-front Hamiltonian for hadrons composed of light quarks, which includes a spin-spin interaction between the hadronic constituents, is constructed by embedding superconformal quantum mechanics into AdS space. A specific breaking of conformal symmetry inside the graded algebra determines a unique effective quark-confining potential for light hadrons, as well as remarkable connections between the meson and baryon spectra. The results are consistent with the empirical features of the light-quark hadron spectra, including a universal mass scale for the slopes of the meson and baryon Regge trajectories and a zero-mass pion in the limit of massless quarks. Our analysis is consistently applied to the excitation spectra of the $π, ρ, K, K^*$ and $ϕ$ meson families as well as to the $N, Δ, Λ, Σ, Σ^*, Ξ$ and $Ξ^*$ in the baryon sector. We also predict the existence of tetraquarks which are degenerate in mass with baryons with the same angular momentum. The mass of light hadrons is expressed in a universal and frame-independent decomposition in the semiclassical approximation described here.

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arXiv2016

Meson/Baryon/Tetraquark Supersymmetry from Superconformal Algebra and Light-Front Holography

Superconformal algebra leads to remarkable connections between the masses of mesons and baryons of the same parity -- supersymmetric relations between the bosonic and fermionic bound states of QCD. Supercharges connect the mesonic eigenstates to their baryonic superpartners, where the mesons have internal angular momentum one unit higher than the baryons. We also predict the existence of tetraquarks which are degenerate in mass with baryons with the same angular momentum. An effective supersymmetric light-front Hamiltonian for hadrons composed of light quarks can be constructed by embedding superconformal quantum mechanics into AdS space. The breaking of conformal symmetry determines a unique quark-confining light-front potential for light hadrons including spin-spin interactions in agreement with the soft-wall AdS/QCD approach and light-front holography. The mass-squared of the light hadrons can be expressed as a frame-independent decomposition of contributions from the constituent kinetic energy, the confinement potential, and spin-spin contributions. The mass of the pion eigenstate vanishes in the chiral limit. Only one mass parameter appears; it sets the confinement mass scale, a universal value for the slope of all Regge trajectories, the nonzero mass of the proton and other hadrons in the chiral limit, as well as the mass parameter of the pQCD running coupling. The result is an effective coupling defined at all momenta. The matching of the high and low momentum-transfer regimes determines a scale $Q_0$ which sets the interface between perturbative and nonperturbative hadron dynamics. as well as the factorization scale for structure functions and distribution amplitudes. This procedure, in combination with the scheme-independent PMC procedure for setting renormalization scales, can greatly improve the precision of QCD predictions.

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arXiv2016

Proper definition and evolution of generalized transverse momentum distributions

We consider one of the most fundamental sets of hadronic matrix elements, namely the generalized transverse momentum distributions (GTMDs), and argue that their existing definitions lack proper evolution properties. By exploiting the similarity of GTMDs with the much better understood transverse momentum distributions, we argue that the existing definitions of GTMDs have to include an additional dependence on soft gluon radiation in order to render them properly defined. With this, we manage to obtain the evolution kernel of all (un)polarized quark and gluon GTMDs, which turns out to be spin independent. As a byproduct, all large logarithms can be resummed up to next-to-next-to-leading-logarithmic accuracy with the currently known perturbative ingredients.

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arXiv2016

Quark and gluon orbital angular momentum: Where are we?

The orbital angular momentum of quarks and gluons contributes significantly to the proton spin budget and attracted a lot of attention in the recent years, both theoretically and experimentally. We summarize the various definitions of parton orbital angular momentum together with their relations with parton distributions functions. In particular, we highlight current theoretical puzzles and give some prospects.

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arXiv2016

The gauge-invariant canonical energy-momentum tensor

The canonical energy-momentum tensor is often considered as a purely academic object because of its gauge dependence. However, it has recently been realized that canonical quantities can in fact be defined in a gauge-invariant way provided that strict locality is abandoned, the non-local aspect being dictacted in high-energy physics by the factorization theorems. Using the general techniques for the parametrization of non-local parton correlators, we provide for the first time a complete parametrization of the energy-momentum tensor (generalizing the purely local parametrizations of Ji and Bakker-Leader-Trueman used for the kinetic energy-momentum tensor) and identify explicitly the parts accessible from measurable two-parton distribution functions (TMDs and GPDs). As by-products, we confirm the absence of model-independent relations between TMDs and parton orbital angular momentum, recover in a much simpler way the Burkardt sum rule and derive three similar new sum rules expressing the conservation of transverse momentum.

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arXiv2015

The light-front energy-momentum tensor

We present the first complete parametrization for the matrix elements of the generic light-front gauge-invariant energy-momentum tensor, derive the expressions giving separately the spin and orbital angular momentum of quarks and gluons as probed in high-energy scattering experiments, and discuss the relations with two-parton generalized and transverse-momentum dependent distributions. As a by-product, we recovered the Burkardt sum rule, clarified its physical meaning and obtained similar new sum rules for higher-twist distributions.

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arXiv2015

The gluon Sivers distribution: status and future prospects

This is a review of what is currently known about the gluon Sivers distribution and of what are the opportunities to learn more about it. Because single transverse spin asymmetries in $p^\uparrow \, p \to π\, X$ provide only indirect information about the gluon Sivers function through the relation with the quark-gluon and tri-gluon Qiu-Sterman functions, current data from hadronic collisions at RHIC have not yet been translated into a solid constraint on the gluon Sivers function. SIDIS data, including the COMPASS deuteron data, allow for a gluon Sivers contribution that is of the natural size expected from large $N_c$ arguments, which is ${\cal O}(1/N_c)$ times the nonsinglet quark Sivers contribution. Several very promising processes to measure the gluon Sivers effect directly have been put forward, which apart from ongoing and future investigations at RHIC, would strongly favor experiments at AFTER@LHC and a possible future Electron-Ion Collider. Due to the inherent process dependence of TMDs, the gluon Sivers TMD probed in the various processes are not necessarily the same, but rather can be different linear combinations of two universal gluon Sivers functions that have different behavior under charge conjugation and that therefore satisfy different theoretical constraints. For this reason both hadronic and DIS type of collisions are essential in the study of the role of gluons in transversely polarized protons.

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