Hadron Spectrum Research Articles

Archeology and evolution of QCD

These are excerpts from the closing talk at the "XIIth Conference on Quark Confinement and the Hadron Spectrum", which took place last Summer in Thessaloniki --an excellent place to enjoy an interest in archeology. A more complete personal view of the early days of QCD and the rest of the Standard Model is given in [1]. Here I discuss a few of the points which --to my judgement-- illustrate well the QCD evolution (in time), both from a scientific and a sociological point of view.

Development of the Baldin approach for the relativistic nuclear interactions

This article presents the development of the A.M.Baldin approach using a description of relativistic nuclear interactions in the four velocity space. The modification of this approach taking into account the energy dependence of inclusive spectra of hadron has allowed us to describe the differential cross section as a function of the transverse momentum and a satisfactory description of the slopes of the hadron spectra as functions of the energy, has been obtained. The hadron spectra as functions of the energy, has been obtained. Results of our calculations have coincided with experimental data in a wide energy range with high precision.

Investigation of EAS cores

The development of nuclear-electromagnetic cascade models in air in the late forties have shown informational content of the study of cores of extensive air showers (EAS). These investigations were the main goal in different experiments which were carried out over many years by a variety of methods. Outcomes of such investigations obtained in the HADRON experiment using an X-ray emulsion chamber (XREC) as a core detector are considered. The N e spectrum of EAS associated with γ-ray families, spectra of γ-rays (hadrons) in EAS cores and the N e dependence of the muon number, ⟨N μ ⟩, in EAS with γ-ray families are obtained for the first time at energies of 1015 –1017 eV with this method. A number of new effects were observed, namely, an abnormal scaling violation in hadron spectra which are fundamentally different from model predictions, an excess of muon number in EAS associated with γ-ray families, and the penetrating component in EAS cores. It is supposed that the abnormal behavior of γ-ray spectra and N e dependence of the muon number are explained by the emergence of a penetrating component in the 1st PCR spectrum ‘knee’ range. Nuclear and astrophysical explanations of the origin of the penetrating component are discussed. The necessity of considering the contribution of a single close cosmic-ray source to explain the PCR spectrum in the knee range is noted.

Supersymmetry across the Hadronic Spectrum

Semiclassical light-front bound-state equations for hadrons are presented and compared with experiment. The essential dynamical feature is the holographic approach; that is, the hadronic equations in four-dimensional Minkowski space are derived as holograms of classical equations in a 5-dimensional anti-de Sitter space. The form of the equations is constrained by the imposed superconformal algebra, which fixes the form of the light-front potential. If conformal symmetry is strongly broken by heavy quark masses, the combination of supersymmetry and the classical action in the 5-dimensional space still fixes the form of the potential. By heavy quark symmetry, the strength of the potential is related to the heavy quark mass. The contribution is based on several recent papers in collaboration with Stan Brodsky and Guy de Téramond.

Rescuing the nonjet (NJ) azimuth quadrupole from the flow narrative

According to the flow narrative commonly applied to high-energy nuclear collisions a cylindrical-quadrupole component of 1D azimuth angular correlations is conventionally denoted by quantity $v_2$ and interpreted to represent elliptic flow. Jet angular correlations may also contribute to $v_2$ data as "nonflow" depending on the method used to calculate $v_2$, but 2D graphical methods are available to insure accurate separation. The nonjet (NJ) quadrupole has various properties inconsistent with a flow interpretation, including the observation that NJ quadrupole centrality variation in A-A collisions has no relation to strongly-varying jet modification ("jet quenching") in those collisions commonly attributed to jet interaction with a flowing dense medium. In this presentation I describe isolation of quadrupole spectra from pt-differential $v_2(p_t)$ data from the RHIC and LHC. I demonstrate that quadrupole spectra have characteristics very different from the single-particle spectra for most hadrons, that quadrupole spectra indicate a common boosted hadron source for a small minority of hadrons that "carry" the NJ quadrupole structure, that the narrow source-boost distribution is characteristic of an expanding thin cylindrical shell (strongly contradicting hydro descriptions), and that in the boost frame a single universal quadrupole spectrum (L\'evy distribution) on transverse mass $m_t$ accurately describes data for several hadron species scaled according to their statistical-model abundances. The quadrupole spectrum shape changes very little from RHIC to LHC energies. Taken in combination those characteristics strongly suggest a unique {\em nonflow} (and nonjet) QCD mechanism for the NJ quadrupole conventionally represented by $v_2$.

Quark model description of baryon properties

The goal of this project was a systematic calculation, consistent with the mass spectrum, of electroweak properties of baryons, as well as of the hadronic two-body decays of baryon resonances. The basis of the calculations was a relativistically covariant quark model for mesons and baryons, which allows for the calculation of (momentum dependent) hadronic form factors and coupling vertices in the relevant photon and hadron induced reactions and simultaneously describes the hadronic mass spectrum up to 3 GeV.

Neutral meson production measurements with the ALICE at the LHC

Identified hadron spectra are considered to be sensitive to the transport properties of strongly interacting matter produced in high-energy nucleus-nucleus collisions. π0 and η mesons in ALICE are identified via their two-photon decays by using calorimeters and the central tracking system. In the latter, photons are measured via their conversion to electron-positron pairs in the material of the inner ALICE barrel tracking detectors. The measured production spectra in pp, p–Pb and Pb–Pb collisions at mid–rapidity and over a wide pT range will be presented in the available Large Hadron Collider (LHC) energies of Run I. The resulting nuclear modification factor RAA at different centrality classes shows a clear pattern of strong suppression in the hot QCD medium with respect to pp collisions. Comparison of the ALICE results on neutral mesons with lower-energy experiments is also discussed.

Recent results from the NA61/SHINE strong interaction physics programme

The main physics goals of the NA61/SHINE programme on strong interactions are the study of the properties of the onset of deconfinement and the search for signatures of the critical point of strongly interacting matter. For this purpose a scan of the two dimensional phase diagram ($T$-$\mu_{B}$) is being performed at the SPS by measurements of hadron production in nucleus-nucleus collisions as a function of collision energy and system size. This contribution presents intriguing results on the energy dependence of hadron spectra and yields in inelastic p+p and centrality selected Be+Be and Ar+Sc collisions. In particular, the energy dependence of the signals of deconfinement, the "horn", "step" and "kink", and new results on fluctuations and correlations are shown and compared with the corresponding data of other experiments and model predictions.

Strangeness at finite temperature from Lattice QCD

The precision reached by recent lattice QCD results allows for the first time to investigate whether the measured hadronic spectrum is missing some additional strange states, which are predicted by the Quark Model but have not yet been detected. This can be done by comparing some sensitive thermodynamic observables from lattice QCD to the predictions of the Hadron Resonance Gas model (with the inclusion of decays [3]). We propose a set of observables, defined as linear combinations of conserved charge fluctuations, where one can separate baryons by their strange quark content. These observables can isolate the multiplicity fluctuations of kaons from lattice QCD, which can then be compared with experimental results.

Chiral symmetry restoration in static-light mesons: A chiral restoration theorem, the quark running mass m(k) , and chiral restoration signals in the lattice QCD spectra

Chiral symmetry restoration high in the hadron spectra is expected but it remains to be confirmed both in lattice QCD computations and in experiments. Recently, a theorem was derived, relating chiral symmetry restoration high in the hadron spectra to the spontaneous generation of the dynamical quark mass in QCD. We refine the theorem in the case of static-light mesons. Utilizing chiral quark model computations and lattice QCD results for the spectrum of mesons composed by a static antiquark and a light quark, we explore chiral symmetry restoration in the spectrum and the quark running mass m(k).

Constraining high energy interaction mechanisms by studying forward hadron production at the LHC

We demonstrate that underlying assumptions concerning the structure of constituent parton Fock states in hadrons make a strong impact on the predictions of hadronic interaction models for forward hadron spectra and for long-range correlations between central and forward hadron production. Our analysis shows that combined studies of proton-proton collisions at the Large Hadron Collider by central and forward-looking detectors have a rich potential for discriminating between the main model approaches.

Scale Independent Workable Model of Final Unification

We show that, Schwarzschild radius of Planck mass plays a vital role in electroweak and strong interactions. With reference to the observed large proportionality ratio, , it seems appropriate to consider a large nuclear gravitational constant, . Qualitatively this idea is in agreement with 'Strong gravity' concept proposed by Abdus Salam and C.Sivaram [Mod. Phys. Lett., A8(4), 321- 326. (1993)]. We would like to suggest that, by replacing the Newtonian gravitational constant with the proposed nuclear gravitational constant, predicted high energy levels of String theory can be brought down to the current hadronic scale. Based on this idea, we defined the nuclear Planck mass, and proposed a quantized model mechanism for understanding the hadronic mass spectrum.

Thermalization of dense hadronic matter in Au + Au collisions at energies available at the Facility for Antiproton and Ion Research

The conditions of local thermodynamic equilibrium of baryons (nonstrange, strange) and mesons (strange) are presented for central Au + Au collisions at Facility for Antiproton and Ion Research (FAIR) energies using the microscopic transport model UrQMD. The net particle density, longitudinal-to-transverse pressure anisotropy, and inverse slope parameters of the energy spectra of nonstrange and strange hadrons are calculated inside a cell in the central region within rapidity window $|y|l1.0$ at different time steps after the collisions. We observed that the strangeness content is dominated by baryons at all energies; however, contributions from mesons become significant at higher energies. The time scale obtained from local pressure (momentum) isotropization and thermalization of energy spectra are nearly equal and found to decrease with increase in laboratory energy. The equilibrium thermodynamic properties of the system are obtained with a statistical thermal model. The time evolution of the entropy densities at FAIR energies are found to be very similar to the ideal hydrodynamic behavior at top Relativistic Heavy Ion Collider (RHIC) energy.

Decay constants in soft wall AdS/QCD revisited

Phenomenological AdS/QCD models, like hard wall and soft wall, provide hadronic mass spectra in reasonable consistency with experimental and (or) lattice results. These simple models are inspired in the AdS/CFT correspondence and assume that gauge/ gravity duality holds in a scenario where conformal invariance is broken through the introduction of an energy scale. Another important property of hadrons: the decay constant, can also be obtained from these models. However, a consistent formulation of an AdS/QCD model that reproduces the observed behavior of decay constants of vector meson excited states is still lacking. In particular: for radially excited states of heavy vector mesons, the experimental data lead to decay constants that decrease with the radial excitation level. We show here that a modified framework of soft wall AdS/QCD involving an additional dimensionfull parameter, associated with an ultraviolet energy scale, provides decay constants decreasing with radial excitation level. In this version of the soft wall model the two point function of gauge theory operators is calculated at a finite position of the anti-de Sitter space radial coordinate.

The sensitivity of RpA to colour recombination effects

In hadronization models with colour recombination, partons are allowed to regroup into colour singlet structures that are different from those determined by the perturbative parton shower. This aims at modeling the possibility that soft interactions of partons with the underlying event can change colour connections. If such an effect is at play in proton-proton collisions, it may be expected to be enhanced in proton-nucleus collisions due to the higher colour charge density in the underlying event. Here, we provide a qualitative argument that colour recombination effects could lead to a multiplicity dependent hardening of single inclusive hadron spectra that dies out very weakly (∝1/p⊥) with increasing transverse momentum. We present results of a (conservative) model implementation in the cluster hadronization model of the SHERPA event generator. In this model, we find that colour recombination effects harden indeed the single inclusive hadron spectra without affecting the jet spectra, but that this effect does not depend significantly on underlying event activity. We explain this model feature and we argue why, in general, data on proton-nucleus collisions can help to constrain hadronization models used in proton-proton event generators.

QCD test with the momentum spectra and the second binomial moment R2 using e+e− data

Experimental data on the shape of hadronic momentum spectra are compared to the context of calculations in modified leading log approximation (MLLA), under the assumption of local parton hadron duality (LPHD). Considered are the inclusive momentum spectra and charged particle distributions in the reaction e+e− → hadrons, using the experimental as well as the Monte Carlo data at center of mass energies up to 200 GeV. Values of the second binomial moment R2 obtained from the multiplicity distributions are also studied. Our results are compared with both experimental data from high and low energy e+e−, e−p, and pp experiments and QCD calculations in various theoretical approaches. In general, good agreement is found between the measurements and the corresponding QCD predictions. A quantitative check of LPHD + MLLA has been performed by extracting a value of the strong coupling constant from the corresponding distributions. Possible explanations for all these features will be presented in this paper.

Massive Photons: An Infrared Regularization Scheme for Lattice QCD+QED.

Standard methods for including electromagnetic interactions in lattice quantum chromodynamics calculations result in power-law finite-volume corrections to physical quantities. Removing these by extrapolation requires costly computations at multiple volumes. We introduce a photon mass to alternatively regulate the infrared, and rely on effective field theory to remove its unphysical effects. Electromagnetic modifications to the hadron spectrum are reliably estimated with a precision and cost comparable to conventional approaches that utilize multiple larger volumes. A significant overall cost advantage emerges when accounting for ensemble generation. The proposed method may benefit lattice calculations involving multiple charged hadrons, as well as quantum many-body computations with long-range Coulomb interactions.

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: [Formula: see text] The dynamics of the superpartner hadrons also match; for example, the power-law fall-off of the form factors are the same for the mesonic and baryonic superpartners, in agreement with twist counting rules. An effective supersymmetric light-front Hamiltonian for hadrons composed of light quarks can be constructed by embedding superconformal quantum mechanics into AdS space. This procedure also generates a spin–spin interaction between the hadronic constituents. A specific breaking of conformal symmetry inside the graded algebra determines a unique quark-confining light-front potential for light hadrons in agreement with the soft-wall AdS/QCD approach and light-front holography. Only one mass parameter [Formula: see text] 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 length scale which underlies their structure. The mass for the pion eigenstate vanishes in the chiral limit. When one includes the constituent quark masses using the Feynman–Hellman theorem, the predictions are consistent with the empirical features of the light-quark hadronic spectra. Our analysis can be consistently applied to the excitation spectra of the [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] meson families as well as to the [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] baryons. We also predict the existence of tetraquarks which are degenerate in mass with baryons with the same angular momentum. The mass-squared of the light hadrons can be expressed in a universal and frame-independent decomposition of contributions from the constituent kinetic energy, the confinement potential, and spin–spin contributions. We also predict features of hadron dynamics, including hadronic light-front wave functions, distribution amplitudes, form factors, valence structure functions and vector meson electroproduction phenomenology. The mass scale [Formula: see text] can be connected to the parameter [Formula: see text] in the QCD running coupling by matching the nonperturbative dynamics, as described by the light-front holographic approach to the perturbative QCD regime. The result is an effective coupling defined at all momenta. The matching of the high and low momentum-transfer regimes determines a scale [Formula: see text] proportional to [Formula: see text] which sets the interface between perturbative and nonperturbative hadron dynamics. The use of [Formula: see text] to resolve the factorization scale uncertainty for structure functions and distribution amplitudes, in combination with the scheme-independent Principle of Maximal Conformality (PMC) procedure for setting renormalization scales, can greatly improve the precision of perturbative QCD predictions.

Meson Electro-/Photo-Production from QCD

I present the calculation of the $\pi^+\gamma^\star\to\pi^+\pi^0$ transition amplitude from quantum chromodynamics performed by the Hadron Spectrum Collaboration. The amplitude is determined for a range of values of the photon virtuality and the final state energy. One observes a clear dynamical enhancement due to the presence of the $\rho$ resonance. By fitting the transition amplitude and analytically continuing it onto the $\rho$-pole, the $\rho\to\pi\gamma^\star$ form factor is obtained. This exploratory calculation, performed using lattice quantum chromodynamics, constitutes the very first determination of an electroweak decay of a hadronic resonance directly from the fundamental theory of quarks and gluons. In this talk, I highlight some of the necessary steps that made this calculation possible, placing emphasis on recently developed formalism. Finally, I discuss the status and outlook of the field for the study of $N\gamma^\star\to N^\star\to N\pi$ transitions.

Nuclear modification of jet and hadron spectra with CMS

The Compact Muon Solenoid (CMS) experiment measures various stages of nuclear collisions utilizing the different types of beams from the Large Hadron Collider (LHC). PbPb collisions can provide remarkable insight into the final state effects, such as jet quenching. However, recent studies of pPb collisions shed light on initial state effects and complement a chronological picture of the nuclear interaction. CMS results of jet and hadron measurements in such collisions are presented.