QCD Spectrum Research Articles

Stable pentaquarks from strange chiral multiplets

The assumption of strong diquark correlations in the QCD spectrum suggests flavor multiplets of hadrons that are degenerate in the chiral limit. Generally it would be unnatural for there to be degeneracy in the hadron spectrum that is not protected by a QCD symmetry. Here we show---for pentaquarks constructed from diquarks---that these degeneracies can be naturally protected by the full chiral symmetry of QCD. The resulting chiral multiplet structure recovers the ideally mixed pentaquark mass spectrum of the diquark model, and interestingly, requires that the axial couplings of the pentaquarks to states outside the degenerate multiplets vanish in the chiral limit. This result suggests that if these hadrons exist, they are stable in the chiral limit and therefore have widths that scale as the fourth power of the kaon mass over the chiral-symmetry breaking scale. Natural-size widths are of order a few MeV.

Quark coalescence for charmed mesons in ultrarelativistic heavy-ion collisions

We investigate effects of charm-quark interactions in a quark–gluon plasma on the production of D and J/ψ mesons in high-energy heavy-ion collisions. Employing a previously constructed coalescence model that successfully reproduces the transverse momentum (pT) spectra and elliptic flow (v2(pT)) of light hadrons at RHIC from underlying light-quark distributions at the phase transition temperature Tc, D-meson and J/ψpT-spectra are evaluated. For the charm-quark distributions, we consider two limiting scenarios: (i) no rescattering, corresponding to perturbative QCD (pQCD) spectra and (ii) complete thermalization including transverse expansion. We find that D-mesons acquire a minimal v2 inherited from their light-quark content and corresponding semileptonic decay spectra of single electrons practically preserve the v2 of the parent particles, exhibiting marked differences between the pQCD and thermal scenarios for pT⩾1 GeV. Likewise, the pT-spectra and yields of J/ψ's differ appreciably in the two scenarios.

Fragmentation functions in supersymmetric QCD and ultrahigh energy cosmic ray spectra produced in top-down models

We present results from two different methods for the calculation of hadron spectra in QCD and supersymmetric QCD with large primary energies $\sqrt{s}$ up to ${10}^{16}\mathrm{GeV}.$ The two methods considered are a Monte Carlo (MC) simulation and the evolution of fragmentation functions described by the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) equations. We find that the pion, nucleon, and all-hadron spectra calculated with the two methods agree well. The MC simulation is performed with new hadronization functions (in comparison with our previous work), motivated by low energy $(\sqrt{s}<{M}_{Z})$ data and the DGLAP equation. The hadron spectra calculated with both sets of hadronization functions agree well, which indicates that our method for calculating the hadronization function works successfully. The small difference in the calculated hadron spectra characterizes the uncertainties of this method. We calculate also the spectra of photons, neutrinos, and nucleons and compare them with other published results. The agreement is good for all x from $\ensuremath{\sim}{10}^{\ensuremath{-}5}$ up to $x<~0.3.$ The consistency of the spectra calculated by different methods allows us to consider the spectral shape as a signature of models with decays or annihilations of superheavy particles, such as topological defects or superheavy dark matter. The ultrahigh energy cosmic ray spectra from these sources are calculated.

Light hadron spectrum in three-flavor QCD with O(a)-improved Wilson quark action

Abstract We report on a calculation of the light hadron spectrum and quark masses in three-flavor dynamical QCD using the non-perturbatively O(a)-improved Wilson quark action and a renormalization-group improved gauge action. Simulations are carried out on a 163 × 32 lattice at β = 1.9, where a−1 ≅ 2 GeV, with 6 ud quark masses corresponding to m π m ϱ ≅ 0.64–0.77 and 2 s quark masses close to the physical value. We observe that the inclusion of dynamical strange quark brings the lattice QCD meson spectrum to good agreement with experiment. Dynamical strange quarks also lead to a reduction of the uds quark masses by about 15%.

The QCD spectrum: Mixing, strong decays, and the role of sea quarks

The light hadron spectrum as computed in nonperturbative QCD is reviewed and compared to lattice data and experiment. The mixing of mesons, hybrids and glueballs is calculated in the Field Correlator Method. The strong decay mechanisms are found out in the method and compared to the known phenomenological models. The role of sea quarks (unquenched approximation) is studied analytically using radially excited mesons as an example, and compared to experiment.

Ds spectrum and leptonic decays with Fermilab heavy quarks and improved staggered light quarks

We present preliminary results for the D_s meson spectrum and decay constants in unquenched lattice QCD. Simulations are carried out with 2+1 dynamical quarks using gauge configurations generated by the MILC collaboration. We use the ``asqtad'' a^2 improved staggered action for the light quarks, and the clover heavy quark action with the Fermilab interpretation. We compare our spectrum results with the newly discovered 0+ and 1+ states in the D_s system.

New near-threshold mesons

We show that under a number of rather plausible assumptions QCD spectrum may contain a number of mesons which have not been predicted or observed. Such states will have the quantum numbers of two existing mesons and masses very close to the dissociation threshold into the two mesons. Moreover, at least one of the two mesonic constituents itself must be very close to its dissociation threshold. In particular, one might expect the existence of loosely bound systems of D and D∗sJ(2317); similarly, K and f0(980), K̄ and f0(980), K and a0(980) and K̄ and a0(980) can be bound. The mechanism for binding in these cases is the S-wave kaon exchange. The nearness of one of the constituents to its decay threshold into a kaon plus a remainder, implies that the range of the kaon exchange force becomes abnormally long—significantly longer than 1/mK which greatly aids the binding.

Low-lying meson spectrum of largeNCstrongly coupled lattice QCD

We compute the low energy mass spectrum of lattice QCD in the large $N_C$ limit. Expanding around a gauge-invariant ground state, which spontaneously breaks the discrete chiral symmetry, we derive an improved strong-coupling expansion and evaluate, for any value of $N_C$, the masses of the low-lying states in the unflavored meson spectrum. We then take the 't Hooft limit by rescaling $g^2 N_C\to g^2$; the 't Hooft limit is smooth and no arbitrary parameters are needed. We find, already at the fourth order of the strong coupling perturbation theory, a very good agreement between the results of our lattice computation and the known continuum values.

Pseudo Goldstone spectrum of 2-colorQCDat finite density

We examine the spectrum of two-color lattice QCD with one staggered quark field (four flavors) at a finite chemical potential $(\ensuremath{\mu})$ for quark number, on a ${12}^{3}\ifmmode\times\else\texttimes\fi{}24$ lattice. First we present evidence that the system undergoes a transition to a state with a diquark condensate, which spontaneously breaks quark number at $\ensuremath{\mu}{=m}_{\ensuremath{\pi}}/2,$ and that this transition is mean field in nature. We then examine the three states that would be Goldstone bosons at $\ensuremath{\mu}=0$ for zero Dirac and Majorana quark masses. The predictions of chiral effective Lagrangians give a good description of the behavior of these masses for $\ensuremath{\mu}<{m}_{\ensuremath{\pi}}/2.$ Except for the heaviest of these states, these predictions diverge from our measurements, once $\ensuremath{\mu}$ is significantly greater than ${m}_{\ensuremath{\pi}}/2.$ However, the qualitative behavior of these masses indicates that the physics is very similar to that predicted by these effective Lagrangians, and there is some indication that at least part of these discrepancies is due to saturation, a lattice artifact.

Light hadron spectroscopy with two flavors ofO(a)-improved dynamical quarks

We present a high statistics study of the light hadron spectrum and quark masses in QCD with two flavors of dynamical quarks. Numerical simulations are carried out using the plaquette gauge action and the O(a)-improved Wilson quark action at $\ensuremath{\beta}=5.2,$ where the lattice spacing is found to be $a=0.0887(11)\mathrm{fm}$ from the $\ensuremath{\rho}$ meson mass, on a ${20}^{3}\ifmmode\times\else\texttimes\fi{}48$ lattice. At each of five sea quark masses corresponding to ${m}_{\mathrm{PS}}{/m}_{\mathrm{V}}\ensuremath{\simeq}0.8--0.6,$ we generate 12 000 trajectories using a symmetrically preconditioned Hybrid Monte Carlo algorithm. Finite spatial volume effects are investigated employing ${12}^{3}\ifmmode\times\else\texttimes\fi{}48,$ ${16}^{3}\ifmmode\times\else\texttimes\fi{}48$ lattices. We also perform a set of simulations in quenched QCD with the same lattice actions at a similar lattice spacing to those for the full QCD runs. In the meson sector we find clear evidence of sea quark effects. The J parameter increases for lighter sea quark masses, and the full QCD meson masses are systematically closer to experiment than in quenched QCD. Careful finite-size studies are made to ascertain that these are not due to finite-size effects. Evidence of sea quark effects is less clear in the baryon sector due to larger finite-size effects. We also calculate light quark masses and find ${m}_{\mathrm{ud}}^{\overline{\mathrm{MS}}}(2\mathrm{}\mathrm{GeV}{)=3.223(}_{\ensuremath{-}0.069}^{+0.046})\mathrm{MeV}$ and ${m}_{s}^{\overline{\mathrm{MS}}}(2\mathrm{}\mathrm{GeV}{)=84.5(}_{\ensuremath{-}1.7}^{+12.0})\mathrm{MeV}$ which are about 20% smaller than in quenched QCD.

QCD meson spectrum in the large Nc limit

A calculation of the low energy mass spectrum of QCD in the large N c limit is presented. The low-lying states in the meson spectrum up to the fourth order in the strong coupling perturbative expansion are explicitly computed. The 't Hooft limit is smooth and the meson masses are in very good agreement with experiment.

Scattering and resonances in QCD 2

Extending previous works on the spectrum of QCD 2, we now investigate the 2D analogue of meson-baryon scattering. We use semi-classical methods, perturbing around classical soliton solutions. In the case of one flavor, we find that the effective potential is reflectionless. In the case of several flavors, the method yields a potential which depends on the momentum of the incoming particle. In this case there is both transmission and reflection. In both cases no resonances appear.

The solution of a chiral random matrix model with complex eigenvalues

We describe in detail the solution of the extension of the chiral Gaussian Unitary Ensemble (chGUE) into the complex plane. The correlation functions of the model are first calculated for a finite number of N complex eigenvalues, where we exploit the existence of orthogonal Laguerre polynomials in the complex plane. When taking the large-N limit we derive new correlation functions in the case of weak and strong non-Hermiticity, thus describing the transition from the chGUE to a generalized Ginibre ensemble. Applications to the Dirac operator eigenvalue spectrum in QCD with non-vanishing chemical potential are briefly discussed. This is an extended version of arXiv:hep-th/0204068.

Potential nonrelativistic QCD and heavy quarkonium spectrum in next-to-next-to-next-to-leading order

In this talk I review the calculation of the third order corrections to the heavy quarkonium spectrum in the nonrelativistic effective theory framework and its application to the phenomenology of top quark threshold production.

Spectrum and Regge trajectories in QCD

Starting in the 1960s, an active group of physicists under the guidance of Prof. K.A. Ter-Martirosyan began creating the theory of high-energy processes in QCD. From the beginning, the key element of this theory is the notion of Regge trajectories and, in particular, of the Pomeron trajectory, which have been introduced phenomenologically. In this talk, I review the problem of the spectrum and Regge trajectories as it can be derived from nonperturbative QCD dynamics.

Scattering and resonances in QCD 2

Extending previous works on the spectrum of QCD 2, we now investigate the 2D analogue of meson–baryon scattering. We use semi-classical methods, perturbing around classical soliton solutions. We start with the Abelian case, corresponding to one flavor, and find that in this case the effective potential is reflectionless. We obtain an explicit expression for the forward phase shift. In the non-Abelian case of several flavors, the method yields a potential which depends on the momentum of the incoming particle. In this case there is both transmission and reflection. In both cases no resonances appear. As a byproduct, we derive the general conditions for a 2D scalar quantum field theoretical action to yield a reflectionless effective potential when one expands in small fluctuations about the classical solution.

On the spectrum of(1+1)-dimensional QCD withSU(Nc)currents

Extending previous work, we calculate the fermionic spectrum of two-dimensional QCD $({\mathrm{QCD}}_{2})$ in the formulation with ${SU(N}_{c})$ currents. Together with the results in the bosonic sector this allows us to address the as yet unresolved task of finding the single-particle states of this theory as a function of the ratio of the numbers of flavors and colors, $\ensuremath{\lambda}{=N}_{f}{/N}_{c},$ anew. We construct the Hamiltonian matrix in the DLCQ formulation as an algebraic function of the harmonic resolution K and the continuous parameter \ensuremath{\lambda} in the Veneziano limit. We find that the fermion momentum is a function of \ensuremath{\lambda} in the discrete approach. A universality, existing only in two dimensions, dictates that the well-known 't Hooft and large ${N}_{f}$ spectra be reproduced in the limits $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\lambda}}0$ and \ensuremath{\infty}, which we confirm. We identify their single-particle content which is surprisingly the same as in the bosonic sectors. All multiparticle states are classified in terms of their constituents. These findings allow for an identification of the lowest single particles of the adjoint theory. While we do not succeed in interpreting this spectrum completely, evidence is presented for the conjecture that adjoint ${\mathrm{QCD}}_{2}$ has a bosonic and an independent fermionic Regge trajectory of single-particle states.

QCD spectrum with three quark flavors

We present results from a lattice hadron spectrum calculation using three flavors of dynamical quarks --- two light and one strange---and quenched simulations for comparison. These simulations were done using a one-loop Symanzik improved gauge action and an improved Kogut-Susskind quark action. The lattice spacings, and hence also the physical volumes, were tuned to be the same in all the runs to better expose differences due to flavor number. Lattice spacings were tuned using the static quark potential, so as a by-product we obtain updated results for the effect of sea quarks on the static quark potential. We find indications that the full QCD meson spectrum is in better agreement with experiment than the quenched spectrum. For the ${0}^{++}$ ${(a}_{0})$ meson we see a coupling to two pseudoscalar mesons, or a meson decay on the lattice.

Light hadron spectrum and light quark masses from lattice QCD

We present recent results on the light hadron spectrum and light quark masses in lattice QCD, both in the quenched approximation and with two flavors of dynamical quarks. Simulations are made with a renormalization-group improved gauge action and a tadpole-improved clover quark action. Our new results in the quenched simulation are in good agreement with our previous calculation using the standard lattice action. A comparison between the results in full and quenched QCD clearly shows the existence of sea quark effects in meson and quark masses.

Domain walls in supersymmetric QCD: The taming of the zoo

We provide a unified picture of the domain wall spectrum in supersymmetric QCD with Nc colors and Nf flavors of quarks in the (anti-) fundamental representation. Within the framework of the Veneziano-Yankielowicz-Taylor effective Lagrangian, we consider domain walls connecting chiral symmetry breaking vacua, and we take the quark masses to be degenerate. For Nf/Nc<1/2, there is one BPS saturated domain wall for any value of the quark mass m. For 1/2=<Nf/Nc<1 there are two critical masses, m* and m**, which depend on the number of colors and flavors only through the ratio Nf/Nc; if m<m*, there are two BPS walls, if m*<m<m** there is one non-BPS wall, and if m>m** there is no domain wall. We numerically determine m* and m** as a function of Nf/Nc, and we find that m** approaches a constant value in the limit that this ratio goes to one.