NINPHA

National Initiative on the Physics of Hadrons

Abstract

The NINPHA project is dedicated to understanding how hadron structure and phenomenology emerge from the underlying QCD dynamics of confined quarks and gluons. Shedding light on the microscopic mechanisms which lead to the observed masses and spins of hadrons, in fact, will pave the way for a better understanding of confinement and hadronization. To achieve this, NINPHA is developing state-of-the-art phenomenological extractions of maps of partons in momentum space (Transverse Momentum Dependent distributions – TMDs) and in position space (Generalized Parton Distributions – GPDs), including related electromagnetic and gravitational form factors, also exploring advanced approaches to expose the complete information contained in the Wigner distributions (of which TMDs and GPDs are suitable projections).

NINPHA members are at the forefront of extensive phenomenological studies of quark TMDs and GPDs in nucleons and nuclei, and are working to validate proper factorization theorems, evolution equations, and universality issues related to them. Over the next three years, NINPHA plans to extend and refine our understanding of the 3D hadron structure by matching TMD-based formulations at small transverse momenta (compared to the hard scale of the process) to highly accurate perturbative collinear calculations at large transverse momenta, and by improving our knowledge of TMDs and GPDs through robust global fits of fixed-target data (JLab, Compass) and collider data (RHIC, LHC, SLAC, KEK, BEPC). NINPHA will also continue to explore gluon TMDs through single and associated quarkonium production, the orbital angular momentum of partons through GPDs and Wigner distributions, and the dynamics at small parton momenta.

NINPHA activities are also dedicated to the study of the excited hadron spectrum and decays, especially to multiquark states, by means of effective field theories (EFTs), continuum QCD and quark models. Dispersion theory and EFTs applied to data analyses can improve the results extracted from experiments and offer new insight on the nature of exotic hadrons. The calculation of nuclear matrix elements and transport equations will support the theory activities for the NUMEN experiment.

A nonperturbative, relativistic description of bound systems is mandatory in the EIC era, given the expected accuracy in the 3D tomography of light nuclei and hadrons. To this end, the Light-front Hamiltonian dynamics framework, with the most refined nuclear interactions, will be adopted for nuclei, while both Minkowskian continuum-QCD and holographic approaches will be used for hadrons. Numerical studies stemming from previous investigations will also be explored with quantum computing tools.

NINPHA scientists are recognized world experts in their field and have a longstanding tradition of fruitful cooperation with several experimental communities, like JLab and RHIC in the U.S., SPS and LHC at CERN, Belle in Japan and BESIII in China. Moreover, they will continue playing a pivotal role in providing theoretical support to facilities currently under way, like the Electron-Ion Collider (EIC) and the LHCspin project, where the above scientific topics are the pillars of the corresponding physics programs.

NINPHA

National Initiative on Physics of Hadrons

Scientific activities of the various Research Units

In the last few year, the NINPHA team members have been working together to achieve a complete description of the internal hadron structure in terms of quarks and gluons, in a 3D momentum and coordinate space. This is an innovative way to look at the nucleon, which opens our understanding of its structure to new dimensions and reveals properties otherwise inaccessible. Moreover, it offers an original methodology to study the problem of nucleon spin and orbital angular momentum. NINPHA researchers have now reached the necessary maturity to make a definitive step forward in the quality of their achievements. All NINPHA nodes will play a fundamental role in their area of expertise; the tasks of each Unit are described in what follows.

INFN Unit: Cagliari

The Cagliari Unit extends and deepens both the formal and the phenomenological study of the 3D structure of (polarized) hadrons within the transverse momentum dependent (TMD) theoretical approach.

With the approval of the Electron Ion Collider (EIC) in the USA and the proposal of a polarized fixed target experiment at the LHC, great attention is devoted to the computation of state-of-the-art predictions and estimates for unpolarized cross sections and azimuthal and spin asymmetries in the kinematical configurations of these forthcoming experiments, based on accurate global phenomenological analyses of combined information coming from running experiments at RHIC, JLab, CERN, SLAC and KEK, among others.

More in detail, we consider the role of the poorly known TMD gluon distributions in quarkonium production and polarization in proton-proton collisions and in semi-inclusive deep inelastic scattering (SIDIS), within the TMD and NRQCD approaches.

The puzzling transverse Lambda hyperon polarization, observed in unpolarized proton-proton collisions and in annihilations is studied, together with its possible explanation in terms of TMD polarizing fragmentation functions.

The production and the azimuthal distribution of (polarized) leading hadrons inside jets in proton-proton collisions, e⁺ e annihilations and SIDIS are analyzed, as well as single and double spin asymmetries in meson production in polarized proton-proton collisions at RHIC and the forthcoming NICA experiment.

Our research program will benefit, and we plan to further strengthen, our traditional collaborations with members of the Torino and Pavia units, while profiting also of the expertise of the Genova and Perugia units, respectively on quarkonium spectroscopy and nuclear effects in polarized hadron collisions. We also plan to maintain and strengthen our international collaborations with leading experimental and theoretical groups in the field. We will put particular attention to the training role of our activity for young PhD students and post- doc researchers. We are members of the EIC user group and of the PAX collaboration, and collaborate with the NLOAccess project.

INFN Unit: Genova

Hadron spectroscopy will be studied by the Genova Unit, from both a theoretical and a phenomenological point of view, developing also amplitude analysis tools. Tetraquarks, Pentaquarks and Hybrids, and their manifestations in decays or cross sections, will be addressed in order to shed light on the true nature of exotics. These studies will also provide theoretical support for experiments at JLab and LHCb.

The formalism for 3-body decays, recently reviewed by the JPAC Collaboration, will be applied to several reactions of interest for spectroscopy. The effort to develop the 3- body dispersive formalism, taking final state interactions into account, can be exploited for spectroscopy as well as heavy flavor physics. In particular, the latter helps improving sensitivity to CP-violating phases in 3-body decays, e.g. in . This is of great interest now that small CP violations have been measured in the charm sector.

The Genova Unit has predicted before detection the new states recently seen by LHCb, in accordance with the experimental data both for masses and widths new 2019 LHCb pentaquark states too. We will profit of the previous experience: on the one hand, heavy hadron spectroscopy and decays will be studied in a systematic way; on the other hand, exotic hadrons will be addressed in collaboration with research groups based in Europe, China, Japan, USA and Latin America. The Genova and Perugia nodes will complete a relativistic description of 3-body bound systems, that will be used to study form factors and for modeling TMDs and GPDs for the EIC. A spectroscopy program will be developed for the future EIC.

Nuclear matrix elements will be calculated for the INFN NUMEN experiment at LNS on heavy ion Double Charge Exchange (DCE), as theoretical support. Transport equations will be developed as well. The Genoa group has recently published the first theory article on DCE showing the relation with neutrino-less double-beta decay. Finally, we will profit of the effort done for NUMEN, to calculate nuclear matrix elements and cross sections also for the INFN Gran Sasso experiments.

INFN Unit: Pavia

In view of the recent approval of the EIC project, of the running JLab12 program and of the proposal for a polarized fixed target experiment at the LHC, the Pavia node extends its studies on the 3D structure of (polarized) hadrons along different and complementary lines. The transition to precision physics for unpolarized quark TMDs is fully exploited by extending the top perturbative accuracy reached in our fitting framework "NangaParbat" to a global fitting strategy, including SIDIS data, and by continuing the benchmark with LHC EW Working Group codes, to deepen the exploration of the impact of non-perturbative effects on Standard Model parameters (such as the W mass).

This framework will be extended to polarized quark TMDs, in particular to the EIC golden channel of the Sivers effect, by improving our knowledge of the sea quark contribution. We will study in detail the inclusive jet production and hadron-in-jet production in view of the predicted capability of the EIC to abundantly produce jets. The accuracy of our global fit of semi-inclusive di-hadron production data (SIDIS, e+e- annihilations, hadronic collisions) will be improved to achieve the first extraction of the chiral-odd transversity distribution at NLO in a collinear framework, resulting in a more precise determination of the nucleon tensor charge, the so-called silver measurement of the EIC.

The spectator model for gluon TMDs will be improved by including naive T-odd quantities and by studying in more detail the low-x phenomenology in order to explore the transition from the DGLAP to the BFKL evolution regime. We will study a parametrization of the Light-Front Wave Function (LFWF), both for the 3-quark Fock state of the proton and for the pion, using model-independent relations with distribution amplitudes (DAs) input from lattice. This will open the way to a comprehensive study of PDFs, TMDs, GPDs, and to phenomenological applications like the pion-induced Drell-Yan measurement planned in the COMPASS++/AMBER program.

New processes giving access to the Wigner function will be studied, like the di-jet production in ultra-peripheral p-A collisions, that is sensitive to the orbital angular momentum of gluons both at small and moderate x. We will investigate the renormalization of the energy-momentum tensor inducing the proton mass sum rule, and study the scheme dependence of the various terms, giving numerical results up to three loops in the strong coupling. A new subtracted dispersion relation framework will be developed to reduce the theoretical uncertainties in the extraction of proton generalized polarizabilities from Virtual Compton Scattering at JLab12.

INFN Unit: Perugia

For the new generation of experiments at high energy and high luminosity facilities, such as JLab12, BESIII, the High-Luminosity-LHC (HL-LHC) and, in particular, for those planned at the future EIC, the relativistic treatment of the dynamics of hadrons and hadronic matter, the main interest of the Perugia team, will acquire an increasing importance. On a fundamental level, the underlying non-perturbative structure of hadrons will be studied within a continuum QCD framework, implementing a phenomenological tool based on the Bethe-Salpeter equation coupled to the particle and quanta gap equations, with the aim of extending the existing Euclidean investigations to the Minkowski space. The ultimate goal is the study of real systems, with constituent fermions, such as nucleons and light nuclei. Beyond the ground state properties, in a more phenomenological framework, investigations will be undertaken of the mixing between glueballs and scalar meson spectra, as well as of the strong dynamics underlying charmonia decays into baryon-anti-baryon pairs, using effective Lagrangians, looking for novel phenomena likely accessible at new facilities.

A challenging description, at the same time relativistic and realistic, of light nuclei, used as beams at the EIC, is one of the goals of the collaboration. This will be approached implementing Poincaré covariance in the study of their three-dimensional structure, in coordinate and in momentum space, using the Light-Front form of relativistic Dynamics.

This activity will lead, through the calculation of momentum distributions and spectral functions up to an unprecedented accuracy, to the suggestion and planning of novel measurements in semi-inclusive and exclusive reactions, such as deeply virtual Compton scattering, at the EIC and at existing facilities with electromagnetic probes.

The complementary possibilities offered by hadron beams will eventually be scrutinized. Successful directions developed in recent years to study relativistic multiparton dynamics, such as the extraction of new information on the proton structure from double parton distributions, entering the description of hard double parton scattering in proton-proton collisions, and the study of color fluctuations in diffractive proton-nucleus scattering, are promising tools towards the analysis of the HL-LHC program of measurements.

INFN Unit: Torino

The Torino group focuses its activity on TMD analyses with maximal pQCD input of unpolarized SIDIS cross section. As a further step in the effort to understand hadronic transverse momentum dependent phenomena, we are performing an analysis on unpolarized semi-inclusive deeply inelastic scattering data which will include maximal constraints from perturbative QCD. This will bring a significant improvement in both the quality of the description of current data sets and the predictive power of the extracted TMD functions. We are currently calculating the perturbative corrections of the intermediate transverse momentum region, which will complete the ingredients to perform an analysis at order .

Universality breaking effects due to process-dependent soft factors included in the definition of TMDs are investigated, with special focus on annihilations in one and two hadrons, within a TMD factorization approach as well as in the collinear approximation. We have recently proposed a working frame in which the TMDs can be defined by neatly separating the soft and collinear (non-perturbative) terms from the contributions that can be calculated perturbatively, properly reabsorbing the rapidity divergences. This scheme allows us to restore the possibility to perform global phenomenological studies of TMD physics, simultaneously analysing data from different hadronic processes.

We will focus mainly on unpolarized multiplicites and cross sections: by using experimental data from SIDIS, Drell-Yan and e⁺e^- annihilation processes we will perform global fits for valence and sea quarks, to refine the extraction of the unpolarized TMD distribution and fragmentation functions, with special attention to the region of transition between pertubative and non-perturative regimes, and the corresponding critical value of q_T. This kind of approach will then be extended to the study of polarized TMDs, like the transversity, Collins, and Sivers functions.

Universality and universality-breaking effects will also be studied in relation to hadronic processes, like semi-inclusive pion production in proton-proton scattering.

In collaboration with the Cagliari node, we have developed the necessary know-how to apply a reweighting procedure to try and overcome the present limitations in the description of such TMD effects.