Search

This paper presents a comprehensive review of both the theory and experimental successes of Quantum Chromodynamics, starting with its emergence as a well defined theory in 1972-73 and following developments and results up to the present day. Topics include a review of the earliest theoretical and experimental foundations; the fundamental constants of QCD; an introductory discussion of lattice QCD, the only known method for obtaining exact predictions from QCD; methods for approximating QCD, with special focus on effective field theories; QCD under extreme conditions; measurements and predictions of meson and baryon states; a special discussion of the structure of the nucleon; techniques for study of QCD at high energy, including treatment of jets and showers; measurements at colliders; weak decays and quark mixing; and a section on the future, which discusses new experimental facilities or upgrades currently funded. The paper is intended to provide a broad background for Ph.D. students and postdocs starting their career. Some contributions include personal accounts of how the ideas or experiments were developed., Comment: Invited volume for the EJPC; 567 pages if text/figures and 4783 references occupying about 160 additional pages. arXiv abstract abridged, for the complete abstract please see the full text

The scalar glueball is observed in a coupled-channel analysis of the $S$-wave amplitude from BESIII data on radiative $J/\psi$ decays and further data. Ten scalar isoscalar resonances were required to fit the data. Five of them were interpreted as mainly-singlet, five as mainly-octet resonances in SU(3). The yield of resonances showed a striking peak with properties expected from a scalar glueball. The $D$ wave amplitude in the BESIII data on radiative $J/\psi$ decays reveales a high-mass structure which can be described by a single Breit-Wigner or by the sum of three $\phi\phi$ resonances interpreted as tensor glueballs a long time ago. The structure - and further tensor resonances observed in radiative $J/\psi$ decays - are tentatively interpreted as tensor glueball. In $J/\psi$ decays into $\gamma\pi^0\pi^0\eta'$ several resonances are reported. The possibility is discussed that the pseudoscalar glueball might be hidden in these data., Comment: Contribution to XVth Quark confinement and the Hadron spectrum conference, August 1st - 6th, 2022, University of Stavanger, Norway

This is a contribution to the review "50 Years of Quantum Chromdynamics" edited by F. Gross and E. Klempt [arXiv:2212.11107], to be published in EPJC. The contribution reviews the new baryon resonances derived from photoproduction experiments. Implications of the new results for the interpretation of baryons are discussed., Comment: 11 pages, 7 figures, 2 tables

This is a contribution to the review "50 Years of Quantum Chromdynamics" edited by F. Gross and E. Klempt [arXiv:2212.11107], to be published in EPJC. The contribution remembers the early searches and explains how to find a glueball, based on its properties. The results of a coupled-channel analysis are presented that provides evidence for the scalar glueball and first hints for the tensor glueball. Data on radiative decays of $\psi(2S)$ and $\Upsilon(1S)$ show scalar intensity that is likely due to glueball production., Comment: 9 pages, 12 figures

This is a contribution to the review "50 Years of Quantum Chromdynamics" edited by F. Gross and E. Klempt [arXiv:2212.11107], to be published in EPJC. The contribution reviews the properties of baryons with one heavy flavor: the lifetimes of ground states and the spectrum of excited states. The importance of symmetries to understand the excitation spectrum is underlined. An overview of searches for pentaquarks is given., Comment: 10 pages, 8 tables, 7 figures

The mixing angles between scalar isoscalar resonances and a scalar glueball are determined from their decays into two pseudoscalar mesons. For $f_0(1370)$ and $f_0(1500)$, at most a small glueball component is admitted by the data. The decay modes of $f_0(1710)$, $f_0(1770)$, $f_0(2020)$, and $f_0(2100)$ require significant glueball fractions. Above this mass, the errors in the decay frequencies become too large to extract a glueball component. The summation of all observed glueball fractions up to 2100\,MeV yields (78\er18)\%. The glueball fractions as function of the mass are consistent with a scalar glueball at 1965\,MeV and a width of 370\,MeV as suggested by a measurement of the yield of scalar isoscalar mesons in radiative $J/\psi$ decays.

A coupled-channel analysis has been performed to identify the spectrum of scalar mesons. The data include BESIII data on radiative $J/\psi$ decays into $\pi^0\pi^0$,$K_SK_S$, $\eta\eta$, and $\omega\phi$, 15 Dalitz plots from $\bar pN$ annihilation at rest at LEAR, the CERN-Munich multipoles for $\pi\pi$ elastic scattering, the $S$-wave from BNL data on $\pi\pi$ scattering into $K_SK_S$, from GAMS data on $\pi\pi\to \pi^0\pi^0, \eta\eta$, and $\eta\eta'$, and NA48/2 data on low-mass $\pi\pi$ interactions from $K^\pm\to\pi\pi e^\pm\nu$ decays. The analysis reveals the existence of ten scalar isoscalar resonances. The resonances can be grouped into two classes: resonances with a large SU(3) singlet component and those with a large octet component. The production of isoscalar resonances with a large octet component should be suppressed in radiative $J/\psi$ decays. However, in a limited mass range centered at 1900\,MeV, these mesons are produced abundantly. Mainly-singlet scalar resonances are produced over the full mass range but with larger intensity at 1900\,MeV. The total scalar isoscalar yield in radiative decays into scalar mesons shows a clear peak which is interpreted as the scalar glueball of lowest mass., Comment: Contribution to Quark Confinement and the Hadron Spectrum 2021. 6 pages, 5 figures, 2 tables

The center-of-gravity rule is tested for heavy and light-quark mesons. In the heavy-meson sector, the rule is excellently satisfied. In the light-quark sector, the rule suggests that the $a_0(980)$ could be the spin-partner of $a_2(1320)$, $a_1(1260)$, and $b_1(1235)$; $f_0(500)$ the spin-partner of $f_2(1270)$, $f_1(1285)$, and $h_1(1170)$; and $f_0(980)$ the spin-partner of $f_2'(1525)$, $f_1(1420)$, and $h_1(1415)$. From the decay and the production of light scalar mesons we find a consistent mixing angle $\theta^{\rm s}=(14\pm4)^\circ$. We conclude that $f_0(980)$ is likely "octet-like" in SU(3) with a slightly larger $s\bar s$ content and $f_0(500)$ is SU(3) "singlet-like" with a larger $n\bar n$ component. The $a_0(1450)$, $K^*_0(1430)$, $f_0(1500)$ and $f_0(1370)$ are suggested as nonet of radial excitations. The scalar glueball is discussed as part of the wave function of scalar isoscalar mesons and not as additional "intruder". It seems not to cause supernumerosity., Comment: 10 pages, 2 figures

A coupled-channel analysis of BESIII data on radiative $J/\psi$ decays into $\pi\pi$, $K\bar K$, $\eta\eta$ and $\omega\phi$ has been performed. The partial-wave amplitude is constrained by a large number of further data. The analysis finds ten isoscalar scalar mesons. Their masses, widths and decay modes are determined. The scalar mesons are interpreted as mainly SU(3)-singlet and mainly octet states. Octet isoscalar scalar states are observed with significant yields only in the 1500-2100\,MeV mass region. Singlet scalar mesons are produced over a wide mass range but their yield peaks in the same mass region. The peak is interpreted as scalar glueball. Its mass and width are determined to $M=1865$\er25$^{+10}_{-30}$ {\rm MeV} and $\Gamma= 370$\er$50^{+30}_{-20}$ {\rm MeV}, its yield in radiative $J/\psi$ decays to ($5.8\pm 1.0)\,10^{-3}$., Comment: 11 pages, 4 figures

We highlight topical issues in hyperon spectroscopy that will be accessible when a secondary beam of neutral kaons is created in Hall D at Jefferson Lab as proposed to the JLab Program Advisory Committee. The new beam will have a flux of $10^4$ $K_L$/s. The reaction products will be analyzed in the GlueX experimental setup. We point out which physical questions need to be answered and suggest a number of novel experiments. In particular we suggest to measure the presently disputed SU(3) structure of the $\Lambda(1405)$, to search for Pentaquarks mit exotic quantum numbers and to search for baryons belonging to the SU(6) 20-plet representation., Comment: 10 pages, 6 figures

We discuss three possible scenarios for the interpretation of mesons containing a heavy quark and its antiquark near and above the first threshold for a decay into a pair of heavy mesons in a relative $S$--wave. View I assumes that these thresholds force the quark potential to flatten which implies that while in these energy ranges molecular states may be formed there should not be any quark--anti-quark states above these thresholds. View II assumes that the main part of the interaction between two mesons is due to the poles which originate from the $Q\bar Q$ interaction. The properties of the $Q\bar Q$ mesons are strongly influenced by opening thresholds but the number of states is given by the quark model. In View III, both types of mesons are admitted also near and above the open flavor thresholds: $Q\bar Q$ mesons and dynamically generated mesons. Experimental consequences of these different views are discussed., Comment: 15 pages, 5 figures

We analyze the current status of three P-wave baryon states $N(1710){1/ 2}^+$, $N(1900){3/2}^+$, and $\Delta(1600){3/2}^+$ as given in the Review of Particles Physics (RPP). Since the evidence for a particle's existence is linked to its RPP "star" rating, we discuss its subjective present definition. We also present the accumulating evidence supporting these states and give our new "star" rating recommendations., Comment: 6 pages, 2 figures

The proton mass arises from spontaneous breaking of chiral symmetry and the formation of constituent quarks. Their dynamics cannot be tested by proton tomography but only by studying excited baryons. However, the number of excited baryons is much smaller than expected within quark models; even worse, the existence of many known states has been challenged in a recent analysis which includes - compared to older analyses - high-precision data from meson factories. Hence $\pi N$ elastic scattering data do not provide a well-founded starting point of any phenomenological analysis of the baryon excitation spectrum. Photoproduction experiments now start to fill in this hole. Often, they confirm the old findings and even suggest a few new states. These results encourage attempts to compare the pattern of observed baryon resonances with predictions from quark models, from models generating baryons dynamically from meson-nucleon scattering amplitudes, from models based on gravitational theories, and with the conjecture that chiral symmetry may be restored at high excitation energies. Best agreement is found with a simple mass formula derived within AdS/QCD. Consequences for our understanding of QCD are discussed as well as experiments which may help to decide on the validity of models., Comment: Hadron 2009 invited talk, 8 pages, 6 figures, 4 tables

About 120 baryons and baryon resonances are known, from the abundant nucleon with $u$ and $d$ light-quark constituents up to the recently discovered $\Omega_b^-=bss$, and the $\Xi_b^-=bsd$ which contains one quark of each generation. In spite of this impressively large number of states, the underlying mechanisms leading to the excitation spectrum are not yet understood. Heavy-quark baryons suffer from a lack of known spin-parities. In the light-quark sector, quark-model calculations have met with considerable success in explaining the low-mass excitations spectrum but some important aspects like the mass degeneracy of positive-parity and negative-parity baryon excitations are not yet satisfactorily understood. At high masses, above 1.8 GeV, quark models predict a very high density of resonances per mass interval which is not observed. In this review, issues are identified discriminating between different views of the resonance spectrum; prospects are discussed how open questions in baryon spectroscopy may find answers from photo- and electro-production experiments which are presently carried out in various laboratories., Comment: Review article, 53 pages, 40 figures, 23 Tables. Review of Modern Physics (accepted for publication)

We introduce an improved mass formula for the nucleon and delta resonances and show how it emerges from AdS/QCD in a straightforward extension of the 'metric soft wall' gravity dual. The resulting spectrum depends on just one adjustable parameter, characterizing confinement-induced IR deformations of the anti-de Sitter (AdS) metric, and on the fraction of 'good' (i.e. maximally attractive) diquarks in the baryon's quark model wave function. Despite its simplicity, the predicted spectrum describes the masses of all 48 observed light-quark baryon states and their linear trajectory structure with unprecedented accuracy., Comment: 4 pages

The evidence for the existence of mesons with exotic quantum numbers and of hybrid candidates with non-exotic quantum numbers is critically reviewed, including candidates with hidden charm. Aims and methods of future searches for hybrid mesons are briefly discussed., Comment: Pleanry talk at Hadron07, Frascati, Italy, October 8-12, 2007

The spectroscopy of light and heavy mesons is reviewed with emphasis on glueballs, hybrids, and tetraquarks., Comment: 266 pages, 117 figures, 39 tables. to be published in Physics Reports

Glueballs and hybrids are predicted to exist but searches for them have failed to provide conclusive evidence. One--gluon exchange is not an important part of strong interactions in this energy regime. Instead, quarks seem to interact indirectly, via changes of the QCD vacuum. Strong interactions seem to be governed by instanton--induced interactions; the chiral soliton model gives a more suitable interpretation of the $\Theta^+(1540)$ than models based on the dynamics of four quarks and one antiquark., Comment: 10 pages, 4 figures, presented at MENU04, Beijing, 2004

The Particle Data Group decided to split the $\eta(1440)$ into two states, called $\eta_L$ and $\eta_H$. The $\eta(1295)$ and the $\eta_H$ are supposed to be the radial excitations of the $\eta$ and $\eta'$, respectively. The $\eta_L$ state cannot be accomodated in a quark model; it cannot be a $q\bar q$ state, however, it might be a glueball. In this contribution it is shown that that the $\eta(1295)$ does not have the properties which must be expected for a radially excited state. The splitting of the $\eta(1440)$ is traced to a node in the wave function of a radial excitation. Hence the two peaks, $\eta_L$ and $\eta_H$, originate from one resonance which is interpreted here as first radial excitation of the $\eta$., Comment: 8 pages, 3 figures, contribution to ICHEP04, Beijing, 2004

Hadron spectroscopy has received revitalised interest due to the discovery of states with unexpected properties. Discussed are the recent findings at BABAR, CLEO, BELLE and BES. A detailed report is given on the many experiments reporting evidence (or counter--evidence) for the $\Theta^+(1540)$ and other pentaquarks. In conventional meson and in baryon spectroscopy, evidence is emerging that one--gluon--exchange does not provide the appropiate means to understand low--energy QCD; instanton--induced interactions yield much more insight. In particular the rich spectrum of baryon resonances is very well suited to test dynamical quark models using constituent quarks, a confinement potential plus some residual interactions. A still controversial issue is the question if glueballs and hybrids exist. There is the possibility that two $q\bar q$ scalar states and a glueball form three observed resonances by mixing. However, there is also rather conclusive evidence against this interpretation. Mesons with exotic quantum numbers have been reported but there is no reason why they should be hybrids: a four--quark interpretation is enforced for the $\pi_1(1400)$ and not ruled out in the other cases., Comment: 139 pages, 91 figures, lectures at 18th Annual Hampton University Graduate Studies, Jefferson Lab, Newport News, Virginia, June 2-20, 2003

We discuss the mass spectrum of highly-excited nucleon and $\Delta^*$ resonances. The spectrum exhibits parity doublets, pairs of resonances of identical total angular momentum J but of opposite parity. It has been proposed that the parity doublets evidence restoration of chiral symmetry at large baryon excitation energies. We compare this conjecture with the possibility that high-mass states are organized into $(L,S)$-multiplets with defined intrinsic quark spins and orbital angular momenta. Indeed, the latter interpretation results in a better description of the data. There is however a small and statisticially not very significant trend of the data which might indicate that chiral symmetry is not yet restored but does influence the masses of highly excited states., Comment: 10 pages, 1 fig

Light-baryon resonances with u,d, and s quarks only can be classified using the non-relativistic quark model. When we assign to baryon resonances with total angular momenta J intrinsic orbital angular momenta L and spin S we make the following observations: plotting the squared masses of the light-baryon resonances against these intrinsic orbital angular momenta L, Delta's with even and odd parity can be described by the same Regge trajectory. For a given L, nucleon resonances with spin S=3/2 are approximately degenerate in mass with Delta resonances of same total orbital momentum L. To which total angular momentum L and S couple has no significant impact on the baryon mass. Nucleons with spin 1/2 are shifted in mass; the shift is - in units of squared masses - proportional to the component in the wave function which is antisymmetric in spin and flavor. Sequential resonances in the same partial wave are separated in mass square by the same spacing as observed in orbital angular momentum excitations. Based on these observations, a new baryon mass formula is proposed which reproduces nearly all known baryon masses., Comment: 4 pages, 1 figure