Search results for "Quark"
showing 10 items of 2905 documents
Next-to-next-to-leading orderO(α2αs2)results for top quark pair production in photon-photon collisions: The one-loop squared contributions
2006
We calculate the one-loop squared contributions to the next-to-next-to-leading order ${\cal O}(\alpha^2\alpha_s^2)$ radiative QCD corrections for the production of heavy quark pairs in the collisions of unpolarized on--shell photons. In particular, we present analytical results for the squared matrix elements that correspond to the product of the one--loop amplitudes. All results of the perturbative calculation are given in the dimensional regularization scheme. These results represent the Abelian part of the corresponding gluon--induced next-to-next-to-leading order cross section for heavy quark pair hadroproduction.
Off-forward Matrix Elements in Light-front Hamiltonian QCD
2002
We investigate the off-forward matrix element of the light cone vector operator for a dressed quark state in light-front Hamiltonian perturbation theory. We obtain the corresponding splitting functions in a straightforward way. We show that the end point singularity is canceled by the contribution from the normalization of state. Considering mixing with the gluon operator, we verify the helicity sum rule in perturbation theory. We show that the quark mass effects are suppressed in the plus component of the matrix element but in the transverse component, they are not suppressed. We emphasize that this is a particularity of the off-forward matrix element and is absent in the forward case.
Skyrmions at high density
2017
The phase diagram of quantum chromodynamics is conjectured to have a rich structure containing at least three forms of matter: hadronic nuclear matter, quarkyonic matter and quark–gluon plasma. We justify the origin of the quarkyonic phase transition in a chiral-quark model and describe its formulation in terms of Skyrme crystals.
Degrees of freedom and the phase transitions of two-flavor QCD
2008
We study two effective models for QCD, the Nambu-Jona-Lasinio -model and the linear sigma model extended by including a Polyakov loop potential, which is fitted to reproduce the pure gauge theory thermodynamics, and a coupling between the chiral fields and the Polyakov loop. Thus the resulting models have as relevant degrees of freedom the Polyakov loop and chiral fields. By comparing the extended models with the bare chiral models we can conclude that the addition of the Polyakov loop is necessary in order to obtain both qualitative and quantitative agreement with known results at finite temperatures. These results are extended to finite net-quark densities, several thermodynamical quantit…
Massive color-octet bosons and the charge asymmetries of top quarks at hadron colliders
2008
Several models predict the existence of heavy colored resonances decaying to top quarks in the TeV energy range that might be discovered at the LHC. In some of those models, moreover, a sizable charge asymmetry of top versus antitop quarks might be generated. The detection of these exotic resonances, however, requires selecting data samples where the top and the antitop quarks are highly boosted, which is experimentally very challenging. We asses that the measurement of the top quark charge asymmetry at the LHC is very sensitive to the existence of excited states of the gluon with axial-vector couplings to quarks. We use a toy model with general flavour independent couplings, and show that …
Stability of multiquarks in an improved flip-flop model of confinement
2012
We review some recent studies on the string model of confinement inspired by the strong-coupling regime of QCD and its application to exotic multiquark configurations. This includes two quarks and two antiquarks, four quarks and one antiquark, six quarks, and three quarks and three antiquarks with a careful treatment of the corresponding few-body problem.
Measurement of Dijet Angular Distributions ats=1.96 TeVand Searches for Quark Compositeness and Extra Spatial Dimensions
2009
We present the first measurement of dijet angular distributions in ppbar collisions at sqrt{s}=1.96TeV at the Fermilab Tevatron Collider. The measurement is based on a dataset corresponding to an integrated luminosity of up to 0.7fb-1 collected with the D0 detector. Dijet angular distributions have been measured over a range of dijet masses, from 0.25TeV to above 1.1TeV. The data are in good agreement with the predictions of perturbative QCD and are used to constrain new physics models including quark compositeness, large extra dimensions, and TeV-1 scale extra dimensions. For all models we set the most stringent direct limits to date.
Elliptic Flow of Electrons from Beauty-Hadron Decays in Pb-Pb Collisions at sNN=5.02 TeV
2021
The elliptic flow of electrons from beauty hadron decays at midrapidity ($|y|$ $<$ 0.8) is measured in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV with the ALICE detector at the LHC. The azimuthal distribution of the particles produced in the collisions can be parameterized with a Fourier expansion, in which the second harmonic coefficient represents the elliptic flow, $v_{\rm 2}$. The $v_{\rm 2}$ coefficient is measured for the first time in transverse momentum ($p_{\rm{T}}$) range 1.3-6 GeV/$c$ in the centrality class 30-50%. The measurement of electrons from beauty-hadron decays exploits their larger mean proper decay length $c\tau \approx$ 500 $\mu$m compared to that of charm had…
Quarks in nuclei
1992
We review some properties of Quantum Chromodynamics, the theory of the hadronic interactions, which serve as guidelines to introduce low energy models of hadron structure. Among these we shall center our attention in the non relativistic quark model and the topological bag model. We present some of their applications to actual problems in experimental and theoretical nuclear physics. In particular we discuss exotic nuclei, quark matter, deep inelastic scattering, proton spin,... and their relation to such phenomena as quark Pauli blocking, strangeness enhancement, nuclear structure functions, bosonization,...
Quarks in Few Hadron Systems
1994
We make use of QCD-based quark models to analyze the observability of quark effects in few hadron systems. A hadron is described by a two phase picture. The dynamics of the interior (perturbative) phase consists of a mechanism to confine the elementary degrees of freedom and an asymptotic QGD type interaction. The dynamics of the exterior (non perturbative) phase is approximated by an effective mesonic lagrangian.