Quark States Research Articles

Color Screening by Pions

Lattice QCD is used to calculate the potential between a static quark and antiquark in the presence of a finite density of pi+'s. Correlation functions of multiple pi+'s are used in conjunction with Wilson-loop correlators to determine the difference between the QQ potential in free space and in the presence of a pion condensate. The modifications to the potential are found to have significant dependence on the QQ separation over the range r < or approximately 1 fm explored in this work. Our results are consistent with the pion-condensate behaving as a (nonlinear) chromodielectric.

Exploration of a physical picture for the QCD scalar channel

A generalized linear sigma model is employed to study the quark structure of low lying scalar as well as pseudoscalar states. The model allows the possible mixing of quark anti-quark states with others made of two quarks and two antiquarks but no a priori assumption is made about the quark contents of the predicted physical states. Effects of SU(3) symmetry breaking are included. The lighter conventional pseudoscalars turn out to be primarily of two quark type whereas the lighter scalars have very large four quark admixtures.

Construction and tests of the MRPC detectors for TOF in ALICE

CERN-LHC (Large Hadron Collider) accelerator facility will provide heavy ions (Pb–Pb) collisions with a center-of-mass (CM) energy of about 5.5 TeV per nucleon pair. In the extreme conditions of temperature and energy density created in such collisions, a transition from hadronic matter towards a deconfined state of quarks and gluons is predicted by Quantum Chromodynamics (QCD) calculations on the lattice. The Time Of Flight (TOF) detector system of the ALICE (A Large Ion Collider Experiment) apparatus, is presently progressing in the assembling process at LHC at CERN. The TOF, in combination with the other central tracking detectors of ALICE provides an excellent Particle IDentification (PID) in the momentum range 0.2 – 2.5 GeV / c for K / π and up to 4 GeV/ c for K/p. The ALICE TOF is a barrel detector consisting of double-stack Multigap Resistive Plate Chamber (MRPC) strips, equipped with readout pads. The MRPC is characterized by an intrinsic time resolution below 50 ps and an efficiency over 99%. The assembling procedures, the tests of mechanics, cooling system and electronics of the 8 m long TOF “supermodules”, together with the performance tests before installation in the experimental area, will be presented.

Heavy-quark free energies, internal-energy and entropy contributions

We present lattice QCD results on heavy-quark free energies, extract from its temperature dependence the entropy and internal-energy contributions, and discuss the onset of medium effects that lead to screening of static quark–antiquark sources in a thermal medium. The detailed analysis of the temperature and distance dependence of the different contributions indicate the complex non-perturbative nature of strongly interacting matter. We shall discuss the necessity to include those effects in studies on the behavior of heavy quarks, heavy-quark bound states and their dissociation in the quark–gluon plasma phase.

Consistent scenario forB→PSdecays

We consider B{yields}PS decays where P stands for pseudoscalar and S for a heavy (1500 MeV) scalar meson. We achieve agreement with available experimental data, which includes two orders of magnitude hierarchy, assuming the scalars mesons are two quark states. The contribution of the dipolar penguin operator O{sub 11} is quantified.

Sigma meson in pole-dominated QCD sum rules

The properties of the $\ensuremath{\sigma}$ meson are studied using the QCD sum rules for tetraquark operators. In the SU(3) chiral limit, we separately investigate SU(3) singlet and octet tetraquark states as constituents of the $\ensuremath{\sigma}$ meson and discuss their roles for the classification of the light scalar nonets $\ensuremath{\sigma}$, ${f}_{0}$, ${a}_{0}$, and $\ensuremath{\kappa}$ as candidates of tetraquark states. All of our analyses are performed in the suitable Borel window which is indispensable to avoid the pseudopeak artifacts outside of the Borel window. We can set up acceptably wide Borel windows after preparing favorable linear combinations of operators and including the dimension 12 terms in the operator-product expansion. Taking into account the possible large widths, we evaluate masses for singlet and octet states as 700--850 and 600--750 MeV, respectively, although the octet operators have a smaller overlap with the tetraquark states than the singlet case, which requires careful interpretations. The splitting of the singlet and octet states emerges from the number of the $\overline{q}q$ annihilation diagrams, which include the color singlet annihilation processes $qq\overline{q}\overline{q}\ensuremath{\rightarrow}(q\overline{q}{)}_{1}$ and the color octet annihilation processes $qq\overline{q}\overline{q}\ensuremath{\rightarrow}G(q\overline{q}{)}_{8}$. The mass of the $\ensuremath{\sigma}$ meson is evaluated as 600--800 MeV, which is much closer to the experimental value $\ensuremath{\sim}500\text{ }\text{ }\mathrm{MeV}$ than the mass evaluated by 2-quark correlator analyses, $\ensuremath{\sim}1.0\text{ }\text{ }\mathrm{GeV}$. This indicates that the tetraquark state shares a larger fraction in the $\ensuremath{\sigma}$ meson than ordinary two quark meson states.

Twistor-inspired construction of massive quark amplitudes

The analog of the Cachazo-Svrcek-Witten rules for scattering amplitudes with massive quarks is derived following an approach previously employed for amplitudes with massive scalars. A prescription for the external wave functions is given that leads to a one-to-one relation between fields in the action and spin states of massive quarks. Several examples for the application of the rules are given and the structure of some all-multiplicity amplitudes with a pair of massive quarks is discussed. The rules make supersymmetric relations to amplitudes with massive scalars manifest at the level of the action. The formalism is extended to several quark flavors with different masses.

Bound states of multiple top quarks due to Higgs exchange

Froggatt, Nielsen et al. [C. D. Froggatt, L. V. Laperashvili, R. B. Nevzorov, H. B. Nielsen, and C. R. Das, arXiv:0804.4506; C. D. Froggatt, H. B. Nielsen, and L. V. Laperashvili, Int. J. Mod. Phys. A 20, 1268 (2005); C. D. Froggatt and H. B. Nielsen, Surv. High Energy Phys. 18, 55 (2003).] suggested that the Higgs boson exchange between top quarks produces enough attraction to generate their multiple bound states. Furthermore they claimed that the system of 6 top and 6 antitop quarks is bound so strongly that the binding energy nearly compensates the masses of $t$ quarks, making it very light. We calculated the energy of such states more accurately, variationally and by self-consistent mean-field approximation, and found that these states are weakly bound for a massless Higgs boson and unbound taking into account its mass.

Strong and electroweak NLO corrections to Higgs-boson production in vector-boson fusion at the LHC

We present results on the strong and electroweak NLO corrections to the production of a Higgs boson plus two hard jets via weak interactions at the LHC. The calculation includes all weak-boson fusion and quark-antiquark annihilation diagrams as well as all related interferences. We discuss corrections of different origin (QCD corrections of vector-boson-fusion type and interferences, electroweak corrections induced by quark or photonic initial states, heavy-Higgs-boson effects, etc.) and give some new results for distributions for a Higgs-boson mass of 200 GeV. The electroweak corrections are of the same size as the QCD corrections, viz. typically at the level of 5-10% for a Higgs-boson mass up to \sim 700 GeV. In general, they do not simply rescale differential distributions, but induce distortions at the level of 10%. The discussed corrections have been implemented in a flexible Monte Carlo event generator.

Light meson spectroscopy results from CLEO-c

Decays of charmonium and charmed mesons provide a suitable environment in which to study the properties of light quark states. Using the approximately 25 million ψ ( 2 S ) decays collected with the CLEO-c detector at the Cornell Electron Storage Ring we present a precision measurement of the η mass and branching fractions and studies of spectroscopy in hadronic χ c decay. Additionally we use 572 pb−1 of data collected at the ψ ( 3770 ) to perform an analysis of the resonant substructure in the D + → K − π + π + decay channel.

Searching high and low for bottomonium

The BABAR collaboration at SLAC has observed the radiative decay of an excited state of bottomonium (the bound state of a bottom quark and its antiparticle) to its ground state . Observing this long-sought ground state should enable better tests of quantum chromodynamic calculations of quark interactions and the computational approach called lattice quantum chromodynamics.

Origin of resonances in the chiral unitary approach

We study the origin of the resonances associated with pole singularities of the scattering amplitude in the chiral unitary approach. We propose a ``natural renormalization'' scheme using the low-energy interaction and the general principle of the scattering theory. We develop a method to distinguish dynamically generated resonances from genuine quark states [Castillejo-Dalitz-Dyson (CDD) poles] using the natural renormalization scheme and phenomenological fitting. Analyzing physical meson-baryon scatterings, we find that the $\ensuremath{\Lambda}(1405)$ resonance is largely dominated by the meson-baryon molecule component. In contrast, the $N(1535)$ resonance requires a sizable CDD pole contribution, while the effect of the meson-baryon dynamics is also important.

Relation between chiral symmetry breaking and confinement in YM-theories

Spectral sums of the Dirac-Wilson operator and their relation to the Polyakov loop are thoroughly investigated. The approach by Gattringer is generalized to mode sums which reconstruct the Polyakov loop locally. This opens the possibility to study the mode sum approximation to the Polyakov loop correlator. The approach is rederived for the ab initio continuum formulation of Yang-Mills theories, and the convergence of the mode sum is studied in detail. The mode sums are then explicitly calculated for the Schwinger model and SU(2) gauge theory in a homogeneous background field. Using SU(2) lattice gauge theory, the IR dominated mode sums are considered and the mode sum approximation to the static quark antiquark potential is obtained numerically. We find a good agreement between the mode sum approximation and the static potential at large distances for the confinement and the high temperature plasma phase.

Chiral dynamics and meson with non-commutative dipole field in gauge/gravity dual

Apply the T-duality and smeared twist to the D3-brane solution one can construct the supergravity backgrounds which may dual to supersymmetric or non-supersymmetric non-commutative dipole field theory. We introduce D7-brane probe into the dual supergravity background to study the chiral dynamics and meson spectrum therein. We first find that the non-commutative dipole field does not induce the chiral symmetry breaking even if the supersymmetry was completely broken, contrast to the conventional believing that the chiral symmetry will be broken in the non-supersymmetric theory. Next, we find that the dipole field does not modify the meson spectrum in the supersymmetric theory while it will reduce the meson bound-state energy in the non-supersymmetric theory. We also evaluate the static quark–anti-quark potential and see that the dipole field has an effect to produce attractive force between the quark and anti-quark.

Transition form factors of theN*(1535)as a dynamically generated resonance

We discuss how electromagnetic properties provide useful tests of the nature of resonances, and we study these properties for the ${N}^{*}(1535)$ that appears dynamically generated from the strong interaction of mesons and baryons. Within this coupled-channels chiral unitary approach, we evaluate the ${A}_{1/2}$ and ${S}_{1/2}$ helicity amplitudes as a function of ${Q}^{2}$ for the electromagnetic ${N}^{*}(1535)\ensuremath{\rightarrow}{\ensuremath{\gamma}}^{*}N$ transition. Within the same formalism we evaluate the cross section for the reactions $\ensuremath{\gamma}N\ensuremath{\rightarrow}\ensuremath{\eta}N$. We find a fair agreement for the absolute values of the transition amplitudes, as well as for the ${Q}^{2}$ dependence of the amplitudes, within theoretical and experimental uncertainties discussed in the article. The ratios obtained between the ${S}_{1/2}$ and ${A}_{1/2}$ for the neutron or proton states of the ${N}^{*}(1535)$ are in qualitative agreement with experiment and there is agreement on the signs. The same occurs for the ratio of cross sections for the $\ensuremath{\eta}$ photoproduction on neutron and proton targets in the vicinity of the ${N}^{*}(1535)$ energy. The global results support the idea of this resonance as being dynamically generated, hence, largely built up from meson baryon components. However, the details of the model indicate that an admixture with a genuine quark state is also demanded that could help obtain a better agreement with experimental data.

Acceleration, energy loss and screening in strongly-coupled gauge theories

We explore various aspects of the motion of heavy quarks in strongly-coupled gauge theories, employing the AdS/CFT correspondence. Building on earlier work by Mikhailov, we study the dispersion relation and energy loss of an accelerating finite-mass quark in N=4 super-Yang-Mills, both in vacuum and in the presence of a thermal plasma. In the former case, we notice that the application of an external force modifies the dispersion relation. In the latter case, we find in particular that when a static heavy quark is accelerated by an external force, its rate of energy loss is initially insensitive to the plasma, and there is a delay before this rate approaches the value derived previously from the analysis of stationary or late-time configurations. Following up on work by Herzog et al., we also consider the evolution of a quark and antiquark as they separate from one another after formation, learning how the AdS/CFT setup distinguishes between the singlet and adjoint configurations, and locating the transition to the stage where the deceleration of each particle is properly accounted for by a constant friction coefficient. Additionally, we examine the way in which the energy of a quark-antiquark pair moving jointly through the plasma scales with the quark mass. We find that the velocity-dependence of the screening length is drastically modified in the ultra-relativistic region, and is comparable with that of the transition distance mentioned above.

Non-perturbative renormalisation of ΔF= 2 four-fermion operators in two-flavour QCD

Using Schrodinger Functional methods, we compute the non-perturbative renormalisation and renormalisation group running of several four-fermion operators, in the framework of lattice simulations with two dynamical Wilson quarks. Two classes of operators have been targeted: (i) those with left-left current structure and four propagating quark fields; (ii) all operators containing two static quarks. In both cases, only the parity-odd contributions have been considered, being the ones that renormalise multiplicatively. Our results, once combined with future simulations of the corresponding lattice hadronic matrix elements, may be used for the computation of phenomenological quantities of interest, such as BK and BB (the latter also in the static limit).

The Fluid Nature of Quark-Gluon Plasma

Collisions of heavy nuclei at very high energies offer the exciting possibility of experimentally exploring the phase transformation from hadronic to partonic degrees of freedom which is predicted to occur at several times normal nuclear density and/or for temperatures in excess of ∼ 170 MeV . Such a state, often referred to as a quark-gluon plasma, is thought to have been the dominant form of matter in the universe in the first few microseconds after the Big Bang. Data from the first five years of heavy ion collisions of Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) clearly demonstrate that these very high temperatures and densities have been achieved. While there are strong suggestions of the role of quark degrees of freedom in determining the final-state distributions of the produced matter, there is also compelling evidence that the matter does not behave as a quasi-ideal state of free quarks and gluons. Rather, its behavior is that of a dense fluid with very low kinematic viscosity exhibiting strong hydrodynamic flow and nearly complete absorption of high momentum probes. The current status of the RHIC experimental studies is presented, with a special emphasis on the fluid properties of the created matter, which may in fact be the most perfect fluid ever studied in the laboratory.

Hadron loops: General theorems and application to charmonium

In this paper, we develop a formalism for incorporating hadron loops into the quark model. We derive expressions for mass shifts, continuum components, and mixing amplitudes of ``quenched'' quark model states due to hadron loops, as perturbation series in the valence-continuum coupling Hamiltonian. We prove three general theorems regarding the effects of hadron loops, which show that given certain constraints on the external ``bare'' quark model states, the valence-continuum coupling, and the hadrons summed in the loops, the following results hold: (1) The loop mass shifts are identical for all states within a given $N,L$ multiplet. (2) These states have the same total open-flavor decay widths. (3) Loop-induced valence configuration mixing vanishes provided that ${L}_{i}\ensuremath{\ne}{L}_{f}$ or ${S}_{i}\ensuremath{\ne}{S}_{f}$. The charmonium system is used as a numerical case study, with the ${}^{3}{P}_{0}$ decay model providing the valence-continuum coupling. We evaluate the mass shifts and continuum mixing numerically for all $1S,1P$, and $2S$ charmonium valence states due to loops of $D,{D}^{*},{D}_{s}$, and ${D}_{s}^{*}$ meson pairs. We find that the mass shifts are quite large but numerically similar for all the low-lying charmonium states, as suggested by the first theorem. Thus, loop mass shifts may have been ``hidden'' in the valence quark model by a change of parameters. The two-meson continuum components of the physical charmonium states are also found to be large, creating challenges for the interpretation of the constituent quark model.

Hadronic physics

A selection of studies highlighting different manifestations of the strong interaction are presented. Many new results have become available this summer in the regimes of discovery, systematic survey, and precision measurements of bound quark states.