Proceedings Series


Vol. 7 (2014), No. 3, pp. 409 – 645

International Meeting Excited QCD

Bjelašnica Mountain, Sarajevo, Bosnia and Herzegovina; February 2–8, 2014

Hadron Dynamics with Vector Mesons: Matching Theory and Experiment to Identify New Resonances

abstract

In this paper, we discuss the \(\gamma p \to K^0 {\mit \Sigma }^+\) and \(\gamma n \to K^0 {\mit \Sigma }^0\) reactions, with a peculiar behavior around the \(K^{*0} {\mit \Lambda }\) threshold, due to a \(1/2^-\) resonance around 2035 MeV. Similarly, we discuss a BES experiment, \(J/\psi \to \eta K^{*0} \bar K^{*0}\) decay, which supports the existence of a new \(h_1\) resonance around 1830 MeV that was predicted from the vector–vector interaction. A short discussion is then made about recent advances in the charm and beauty sectors.


Recent KLOE Results on Hadron Physics

abstract

Recent KLOE results concerning the \(\eta \to 3\pi \) decay, the Dalitz decays \(\phi \to \eta e^+ e^-\) and \(\phi \to \pi ^0 e^+ e^-\), and the \(\eta \) and \(\pi ^0\pi ^0\) production in \(\gamma \gamma \) collisions are reported. Prospects for the KLOE-2 data-taking are also presented.


The \({\mit \Sigma }_{\pi N}\) Term, Chiral Multiplet Mixing and Hidden Strangeness in the Nucleon

abstract

We calculate the \({\mit \Sigma }_{\pi N}\) term in the chiral mixing approach to baryons, i.e. , with SU\(_{\rm L}(3) \times {\rm SU_R}(3)\) chiral multiplets \([({6},{3})\oplus ({3},{6})]\), \([{3},\bar {{3}})\oplus (\bar {{3}},{3})]\) and \([(\bar {{3}},{3})\oplus ({3},\bar {{3}})]\), admixed in the baryons, using known constraints on the current quark masses \(m_{u}^{0}, m_{d}^{0}\). We show that the \([({6},{3})\oplus ({3},{6})]\) multiplet’s contribution is enhanced by a factor of \(\frac {57}{9}\simeq 6.33\), due to SU\(_{\rm L}(2) \times {\rm SU_R}(2)\) algebra, that leads to \({\mit \Sigma }_{\pi N} \geq \left (1 + \frac {48}{9} \sin ^2 \theta \right ){3 \over 2} \left (m_{u}^{0} + m_{d}^{0}\right ) = 60\) MeV, in general accord with “experimental” values of \({\mit \Sigma }_{\pi N}\). The chiral mixing angle \(\theta \) is given by \(\sin ^2\theta = \frac {3}{8}\left (g_{\rm A}^{(0)} + g_{\rm A}^{(3)}\right )\), where \(g_{\rm A}^{(0)} = 0.33 \pm 0.08\), or \(0.28 \pm 0.16\) is the flavor-singlet axial coupling, and \(g_{\rm A}^{(3)} = 1.267\), is the third component of the octet one. These results show that there is no need for \(q^4 {\bar q}\) components, and in particular, no need for an \(s \bar s\) component in the nucleon.


Recent Results from HERA on the Proton Structure

abstract

The most recent results on the proton structure obtained from the H1 and ZEUS experiments at HERA are presented. The new preliminary result on parton density functions extracted from the leading-order QCD fit to the preliminary combined H1 and ZEUS data is presented. Electro-weak precision measurements of the double-differential inclusive neutral-current cross section, measured with the ZEUS detector at a centre-of-mass energy of 319 GeV in \(e^+ p\) and \(e^- p\) collisions are shown. They span the kinematic domain of high momentum transfer squared, \(Q^2\), and high Bjorken \(x\), up to \(x \simeq 1\). Using the HERA data collected at three different centre-of-mass energies, the H1 Collaboration has updated their measurement of the longitudinal structure function, \(F_{\rm L}\), with new results in high \(Q^2\) region.


Observation of a Dynamical QCD String

abstract

Mesons constructed from the quark propagators without the lowest-lying eigenmodes of the Dirac operator reveal not only restored SU\((2)_{\rm L} \times {\rm SU}(2)_{\rm R}\) chiral and U(1)\(_{\rm A}\) symmetries, but actually a higher symmetry. All possible chiral and U(1)\(_{\rm A}\) multiplets for the states of the same spin are degenerate, i.e. , the energy of the observed quantum levels does not depend on the spin orientation of quarks in the system and their parities. The quark-spin independence of the energy levels implies absence of the magnetic interactions in the system. The ultrarelativistic quark–antiquark system with only the color-electric interactions can be interpreted (or defined) as a dynamical QCD string.


Dense Hadronic Matter in Neutron Stars

abstract

The existence of stars with masses up to \(2 M_{\odot }\) and the hints of the existence of stars with radii smaller than \(\sim 11\) km seem to require, at the same time, a stiff and a soft hadronic equation of state at large densities. We argue that these two, apparently contradicting, constraints are actually an indication of the existence of two families of compact stars: hadronic stars which could be very compact and quark stars which could be vary massive. In this respect, a crucial role is played, in the hadronic equation of state, by the delta isobars whose early appearance shifts to large densities the formation of hyperons. We also discuss how recent experimental information on the symmetry energy of nuclear matter at saturation indicates, indirectly, an early appearance of delta isobars in neutron star matter.


Vortices and Chiral Symmetry Breaking

abstract

We analyze the creation of near-zero modes from would-be zero modes of various topological charge contributions from classical center vortices in SU(2) lattice gauge theory. We show that colorful spherical vortex and instanton congurations have very similar Dirac eigenmodes and give rise to a finite density of near-zero modes, leading to chiral symmetry breaking via the Banks–Casher formula. We discuss the influence of magnetic vortex fluxes on quarks and how center vortices may break chiral symmetry.


Exotic Hadrons as Feshbach Resonances

abstract

Since ten years ago a host of exotic resonances have challenged the usual quarkonium picture. A number of ideas have been put forward to explain these new states, but a comprehensive framework is still missing. We review the estimates for prompt production cross sections at hadron collider, and show that the interpretation of exotic states in terms of hadron molecules is not favored. We show a recently proposed model to solve this paradox and to explain the nature of above-threshold states in terms of Feshbach resonances.


The Scalar–Isovector Sector in the Extended Linear Sigma Model

abstract

We study basic properties of scalar hadronic resonances within the so-called extended Linear Sigma Model (eLSM), which is an effective model of QCD based on chiral symmetry and dilatation invariance. In particular, we focus on the mass and decay width of the scalar isovector state \(a_{0}(1450)\) and perform a numerical study of the propagator pole(s) on the unphysical Riemann sheets. In this work, this meson is understood as a seed state explicitly included in the eLSM. Our results show that the inclusion of hadronic loops does not modify much the previously obtained tree-level results. Moreover, the \(a_{0}(980)\) cannot be found as a propagator pole generated by hadronic loop contributions.


Dynamic Holographic QCD

abstract

I introduce holographic techniques for modelling strongly coupled gauge theories and AdS/QCD. Dynamic AdS/QCD is a variant in which the formation of the chiral condensate is dynamically determined. As an example, I use the model, based on perturbative computations of the running of the anomalous dimension of the quark mass \(\gamma \), to study SU(\(N_{\mathrm {c}}\)) gauge theory with \(N_{\mathrm {f}}\) quark flavours.


Scalar Photoproduction on the Proton at CLAS and GlueX Energies

abstract

In this work, we present the results of a theoretical analysis of the data on photoproduction of \(f_0(980)\) meson in the laboratory photon energy between \(3.0\) GeV and \(3.8\) GeV. A comparison is done to the measurements performed by the CLAS Collaboration at JLab accelerator for the exclusive reaction \(\gamma p \to p f_0(980)\). The differential and integrated total cross sections are also computed for the cases of the mesons \(f_0(1500)\), and \(f_0(1710)\), focusing on the GlueX energy regime with photon energy \(E_\gamma = 9\) GeV.


Meson Production in Proton–Proton Scattering Within an Extended Linear Sigma Model

abstract

We study the production of non-strange mesons in proton–proton scattering reactions within an effective model of mesons and baryons with global chiral symmetry. The model includes the nucleon (\(N\)) and its chiral partner (\(N^{\ast }\)) and their couplings to the \(N_{\rm f}=2\) multiplets of (pseudo)scalar and (axial-)vector mesons. Results for the production of \(\omega \), \(\eta \) mesons are presented.


Low-\(x\) Evolution Equation for Proton Green Function

abstract

In the next-to-leading order, we discuss the low-\(x\) evolution equation for the baryon Wilson loop operator, which is a natural model for the Green function describing proton scattering in the Regge limit.


Supersymmetric Yang–Mills Plasma

abstract

The \({\cal N} =4\) super Yang–Mills plasma is studied in the regime of weak coupling. Collective excitations and collisional processes are discussed and compared to those of QCD plasma. The two systems are concluded to be very similar to each other with the differences mostly reflecting different numbers of degrees of freedom.


Non-equilibrium Ghosts

abstract

We discuss how to introduce Faddeev–Popov ghosts to the Keldysh–Schwinger formalism describing equilibrium and non-equilibrium statistical systems of quantum fields such as the quark–gluon plasma which is considered.


New Results on Charm Production at HERA

abstract

The copious production of charm quarks at HERA yielded measurements of the charm contribution to the proton structure and of the charm mass and fragmentation parameters of charmed hadrons. Several measurements of charm production in deep inelastic scattering from the H1 and ZEUS collaborations as well as combined data are presented. They provide a powerful vindication of the form of the gluon density in the proton derived from scaling violations of inclusive deep inelastic scattering data. A QCD fit to the charm data leads to a measurement of the charm mass and provides precise predictions e.g. for \(W\) and \(Z\) production at the LHC. In addition, charm fragmentation fractions in photoproduction to ground state charm hadrons are compared to \(e^+ e^-\) data and support the hypothesis of fragmentation independence of the production process.


Recent Results on Diffraction at HERA

abstract

Data in which a proton is tagged along the beampipe after the primary interaction represents the cleanest way to measure diffraction. Data from both H1 and ZEUS collaborations on inclusive diffractive deep inelastic scattering, with a tagged proton, have been combined. Inclusive diffractive deep inelastic scattering can be considered as the exchange of a colourless object and is amenable to QCD fits in which the structure of this object, sometimes called a Pomeron, is determined. Given such processes are expected to factorise, such a Pomeron structure can be used to predict the rates of jet production in e.g. diffractive photoproduction. The comparison of jet data with the predictions of next-to-leading order QCD are discussed in the context of factorising breaking. Another way to study diffraction is through the production of vector mesons which is also characterised by a colourless exchange with the proton which in lowest order QCD is realised as a colourless gluon pair. Therefore, consideration of the dependence of the \(J/\psi \) cross section on the photon–proton centre-of-mass energy is related to the rise of the square of the gluon density towards low values of Bjorken \(x\). Comparisons of HERA data with such theories and other data are presented.


AdS/CFT and the Axial Sector of Large-\(N\) Yang–Mills Theory

abstract

In holographic models of large-\(N\) gauge theories, the pure-glue axial sector is described in terms of a massless pseudoscalar field, dual to the topological density operator Tr\( F^{\mu \nu } \tilde {F}_{\mu \nu }.\) I will outline how the duality can be used to compute observables such as axial glueball masses, as well as correlation functions and transport coefficients in the axial sector. I will consider 5-dimensional phenomenological holographic models for pure Yang–Mills (YM) theory, and focus on the particular set of CP-odd observables connected to the topological density operator. This provides a simple case study of how the holographic correspondence in association with other techniques can provide quantitative results and (possibly) predictions.


Holographic Glueball Decay

abstract

We announce new results on glueball decay rates in the Sakai–Sugimoto model, a realization of holographic QCD from first principles, that has only one coupling constant and an overall mass scale as free parameters. We extend a previous investigation by Hashimoto, Tan and Terashima who have considered the lowest scalar glueball which arises from a somewhat exotic polarization of supergravity modes and whose mass is uncomfortably small in comparison with lattice results. On the other hand, the scalar glueball dual to the dilaton turns out to have a mass of about twice the mass of the rho meson (1487 MeV), very close to the scalar meson \(f_0(1500)\) that is frequently interpreted as predominantly glue. Calculating the decay rate into two pions, we find a surprisingly good agreement with experimental data for the \(f_0(1500)\). We have also obtained decay widths for tensor and excited scalar glueballs, indicating universal narrowness.


The Non-ordinary Regge Behavior of the \(f_0(500)\) Meson

abstract

We review how the Regge trajectory of an elastic resonance can be obtained just from its pole position and coupling, by means of a dispersive formalism. This allows to deal correctly with the finite widths of resonances in Regge trajectories. For the \(\rho (770)\) meson, this method leads to the ordinary linear Regge trajectory with a universal slope. In contrast, for the \(f_0(500)\) meson, the resulting Regge trajectory is non-linear and with much smaller slope. This is another strong indication of the non-ordinary nature of the lightest scalar meson.


Why \(f_0(500)\) Must Be Narrower?

abstract

Proof of correctness of the results obtained in the recent analysis of the \(\pi \pi \) interactions using new dispersion relations with imposed crossing symmetry condition is presented. The proof concerns position of the \(f_0(500)\) (former \(\sigma \)) pole and is based on a purely mathematical relations and properties of analytic functions. It is shown that the mere analysis of amplitudes expressed by the trigonometric functions and their derivatives clearly define the area in which mass of the \(\sigma \) and its width must be located. These results require also a knowledge of integrals of amplitudes over the physical region.


Studying and Removing Effects of Fixed Topology

abstract

At small lattice spacing or when using overlap fermions, lattice QCD simulations tend to become stuck in a single topological sector. Physical observables, e.g. hadron masses, then differ from their full QCD counterparts by \(1/V\) corrections, where \(V\) is the spacetime volume. These corrections can be calculated order by order using the saddle point method. We calculate all corrections proportional to \(1/V^2\) and \(1/V^3\), and test the resulting equations for several models: a quantum mechanical particle on a circle, the Schwinger model and SU(2) Yang–Mills theory.


SU(2) Landau Gluon Propagator Around Criticality

abstract

We present a status report of our study of Landau-gauge gluon propagators for SU(2) gauge theory at finite temperature.


Approaching the Chiral Point in Two-flavour Lattice Simulations

abstract

We investigate the behaviour of the pion decay constant and the pion mass in two-flavour lattice QCD, with the physical and chiral points as ultimate goal. Measurements come from the ensembles generated by the CLS initiative using the \(\mathcal {O}(a)\)-improved Wilson formulation, with lattice spacing down to about 0.05 fermi and pion masses as low as 190 MeV. The applicability of SU(2) chiral perturbation theory is investigated, and various functional forms, and their range of validity, are compared. The physical scale is set through the kaon decay constant, whose measurement is enabled by inserting a third, heavier valence strange quark.


The Role of Hadronization Processes in Determination of Fragmentation Functions

abstract

We present the results of a global fit to data from different hadronization processes such as single-inclusive electron–positron annihilation and semi-inclusive deep inelastic scattering to calculate pion and kaon fragmentation functions. We perform an improvement to the pion and kaon fragmentation functions at next-to-leading order (NLO), by including recent single-inclusive electron–positron annihilation data from BaBar and Belle at \(Q = 10.54\) GeV and \(Q = 10.52\) GeV, respectively. Our main purpose is to show how much imposing of these new data in our analysis improves the fragmentation functions of pion and kaon at NLO.


Probing Anomalous Top Quark Couplings in Diffractive Events at the LHC

abstract

We investigated the impact of anomalous chromomagnetic and chromoelectric dipole moments on the top pair production in diffractive events at the LHC. The exclusive diffractive production of top quarks provides clean environment due to having one proton intact. We found that the effect of these corrections is remarkable in \(pp \rightarrow p\gamma p \rightarrow pt\bar {t} X\) processes.


The Effect of Polarized DIS Hadronization Process on the Determination of Polarized Parton Distributions

abstract

We present our new determination of polarized PDFs of the nucleon at NLO accuracy performing a QCD fit on the global set of newest combined inclusive deep inelastic scattering (DIS) and the semi-inclusive polarized deep inelastic scattering (SIDIS) data. Considering SIDIS data, which comes from the hadronization of polarized DIS process, helps us to break SU(2) and SU(3) symmetry and light sea-quark decomposition happens. Our results are compared with the most precise theoretical models obtained by recent analyses.


Nonperturbative Effects of Divergent Ghost Loops

abstract

We report on a recently unveiled connection at the nonperturbative level between the masslessness of the ghost, the precise form of the gluon propagator in the deep infrared, and the divergences observed in certain kinematic limits of the three-gluon vertex.


Probing Nucleons with Photons at the Quark Level

abstract

Describing electromagnetic interactions with hadrons from the quark level requires knowledge of the underlying quark–gluon ingredients. I discuss some properties of the quark–photon vertex and quark Compton vertex, and the role of electromagnetic gauge invariance and vector-meson dominance. A simple parametrization for the quark–photon vertex is given.


Non-perturbative Features of the Three-gluon Vertex in Landau Gauge

abstract

We present a calculation of the three-gluon vertex from its Dyson–Schwinger equation in the Landau-gauge Yang–Mills theory. All tensor structures are considered and back-coupled self-consistently. Within the chosen truncation, two-loop diagrams as well as diagrams containing Green’s functions beyond the primitively divergent ones are neglected. Only the three-gluon vertex is chosen to be dynamical; the other propagators and vertex functions are provided as separate solutions of their Dyson–Schwinger equations or by Ansatz. For both scaling- and decoupling-type solutions we observe, in agreement with other studies, a sign change in the tree-level tensor dressing at a non-perturbative scale.


Infrared Behaviour of Propagators and Running Coupling in the Conformal Window of QCD

abstract

Using the Dyson–Schwinger approach we investigate Landau gauge QCD with a relatively large number of chiral quark flavours. A self-consis-tent treatment on the propagator level enables us to study unquenching effects via the quark loop diagram in the gluon equation. Above the critical number of fermion flavours, the non-perturbative running coupling develops a plateau over a wide momentum range. Correspondingly, the propagators follow a power law behaviour in this momentum range indicating conformal behaviour. Our value \(N_{\rm f}^{\rm crit}=4.5\) is strongly sensitive to the details of the quark–gluon vertex calling for more detailed investigations in future studies.


Understanding Confinement via Instanton-monopoles

abstract

We discuss the role and importance of instanton-monopoles in QCD-like theories with and without matter. Most particularly we focus on SU(2) super QCD with heavy flavors.


Intriguing Relations Between the LECs of Wilson \(\chi \)-PT and Spectra of the Wilson Dirac Operator

abstract

We discuss the behavior of the spectral densities of the non-Hermitian Wilson Dirac operator. Moreover, we derive compact relations between the leading order LECs of Wilson \(\chi \)-PT and observables that can be measured by a lattice simulation. These relations can be used to determine the LECs from lattice simulations.


Model-independent Analysis of Scenarios with Vector-like Quarks

abstract

The phenomenology at the LHC of scenarios containing new heavy vector-like quarks is discussed. Focus will be given to their main production and decay channels, and a new method to analyse scenarios of new physics with the presence of multiple vector-like quarks with generic hypotheses about their couplings to SM quarks will be introduced.


Higgs Discovery Status from ATLAS

abstract

An overview of the latest results of the Higgs boson search with the ATLAS experiment at the LHC, following its discovery, is presented. The various decay channels are discussed in the context of the Standard Model and Beyond Standard Model searches. Results on the properties of the boson are also provided, with focus on the mass measurement, spin, parity and production mechanisms.


ERRATUM for Acta Phys. Pol. B Proc. Suppl. 6, 917 (2013)

Restoration of Axial Symmetry and Its Possible Relation with Restoration of Chiral Symmetry and Deconfinement at Finite Temperature


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