Showing papers on "Up quark published in 2012"

QCD quark condensate in external magnetic fields

Abstract: We present a comprehensive analysis of the light condensates in QCD with $1+1+1$ sea quark flavors (with mass-degenerate light quarks of different electric charges) at zero and nonzero temperatures of up to 190 MeV and external magnetic fields $Bl1\text{ }\text{ }{\mathrm{GeV}}^{2}/e$. We employ stout smeared staggered fermions with physical quark masses and extrapolate the results to the continuum limit. At low temperatures we confirm the magnetic catalysis scenario predicted by many model calculations while around the crossover the condensate develops a complex dependence on the external magnetic field, resulting in a decrease of the transition temperature.

Strange quark contributions to nucleon mass and spin from lattice QCD

Abstract: Contributions of strange quarks to the mass and spin of the nucleon, charac- terized by the observables fTs ands, respectively, are investigated within lattice QCD. The calculation employs a 2+1-flavor mixed-action lattice scheme, thus treating the strange quark degrees of freedom in dynamical fashion. Numerical results are obtained at three pion masses, mπ = 495MeV, 356MeV, and 293MeV, renormalized, and chirally extrapolated to the physical pion mass. The value ex- tracted fors at the physical pion mass in the MS scheme at a scale of 2GeV iss = −0.031(17), whereas the strange quark contribution to the nucleon mass amounts to fTs = 0.046(11). In the employed mixed-action scheme, the nucleon valence quarks as well as the strange quarks entering the nucleon matrix elements which determine fTs ands are realized as domain wall fermions, propagators of which are evaluated in MILC 2+1-flavor dynamical asqtad quark ensembles. The use of domain wall fermions leads to mild renormalization behavior which proves especially advantageous in the extraction of fTs.

The strange and light quark contributions to the nucleon mass from Lattice QCD

Abstract: We determine the strangeness and light quark fractions of the nucleon mass by computing the quark line connected and disconnected contributions to the matrix elements m_q in lattice QCD, using the non-perturbatively improved Sheikholeslami-Wohlert Wilson Fermionic action We simulate n_F=2 mass degenerate sea quarks with a pion mass of about 285 MeV and a lattice spacing a approx 0073 fm The renormalization of the matrix elements involves mixing between contributions from different quark flavours The pion-nucleon sigma-term is extrapolated to physical quark masses exploiting the sea quark mass dependence of the nucleon mass We obtain the renormalized values \sigma_{\pi N} = 38(12) MeV at the physical point and f_{T_s}=\sigma_s/m_N= 0012(14)^{+10}_{-3} for the strangeness contribution at our larger than physical sea quark mass

Repressing anarchy in neutrino mass textures

Abstract: The recent results that θ 13 is relatively large, of the order of the previous upper bound, and the indications of a sizable deviation of θ 23 from the maximal value are in agreement with the predictions of Anarchy in the lepton sector. The quark and charged lepton hierarchies can then be reproduced in a SU(5) GUT context by attributing non-vanishing U(1)FN charges, different for each family, only to the SU(5) tenplet states. The fact that the observed mass hierarchies are stronger for up quarks than for down quarks and charged leptons supports this idea. As discussed in the past, in the flexible context of SU(5) ⊗ U(1)FN, different patterns of charges can be adopted going from Anarchy to various types of hierarchy. We revisit this approach by also considering new models and we compare all versions to the present data. As a result we confirm that, by relaxing the ansatz of equal U(1)FN charges for all SU(5) pentaplets and singlets, better agreement with the data than for Anarchy is obtained without increasing the model complexity. We also present the distributions obtained in the different models for the Dirac CP-violating phase. Finally we discuss the relative merits of these simple models.

s s ¯ sea pair contribution to electromagnetic observables of the proton in the unquenched quark model

Abstract: We present an unquenched quark model calculation of the $s\overline{s}$ sea pairs' contribution to the magnetic moment of the proton (the strange magnetic moment) and its charge radius (the strange radius). In our approach the effects of the $s\overline{s}$ pairs are taken explicitly into account through a microscopic, QCD-inspired, quark-antiquark pair creation mechanism. Our results for the two ``strangeness'' observables are compatible with the latest experimental results and recent lattice calculations.

The influence of strong magnetic fields on proto-quark stars

Abstract: We analyze different stages of magnetized quark star evolution incorporating baryon number conservation and using an anisotropic energy momentum tensor. The first stages of the evolution are simulated through the inclusion of trapped neutrinos and fixed entropy per particle, while in the last stage the star is taken to be deleptonized and cold. We find that, although strong magnetic fields modify quark star masses, the evolution of isolated stars needs to be constrained by fixed baryon number, which necessarily lowers the possible star masses. Moreover, magnetic field effects, measured by the difference between the parallel and perpendicular pressures, are more pronounced in the beginning of the star evolution, when there is a larger number of charged leptons and up quarks. We also show that having a spatially varying magnetic field allows for larger magnetic fields to be supported.

Hybrid protoneutron stars with the Dyson-Schwinger quark model

Abstract: We study the hadron-quark phase transition at finite temperature in the interior of protoneutron stars, combining the Dyson-Schwinger model for quark matter with the Brueckner-Hartree-Fock approach for hadronic matter. We discuss the dependence of the results on different nuclear three-body forces and on details of the quark model. A maximum mass exceeding two solar masses can only be obtained with a strong three-body force and suitable parameter values in the Dyson-Schwinger model. With a hybrid configuration, the maximum mass of protoneutron stars is larger than that of cold neutron stars, such that a delayed collapse might be possible in principle.

Search for Pair Production of a New b ′ Quark that Decays into a Z Boson and a Bottom Quark with the ATLAS Detector

Abstract: A search is reported for the pair production of a new quark b' with at least one b' decaying to a Z boson and a bottom quark. The data, corresponding to 2.0 fb(-1) of integrated luminosity, were co ...

Quark and strange quark matter in f(R) gravity for Bianchi type I and V space-times

Abstract: Behaviors of quark matter and strange quark matter which exist in the first seconds of the early Universe in f(R) gravity are studied for Bianchi I and V universes. In this respect, we obtain exact solutions of the modified Einstein field equations by using anisotropy feature of Bianchi I and V space-times. In particular, we investigate exact f(R) functions for Bianchi I as the contribution of strange quark and quark matter. Also, we have concluded that quark matter may contribute to the early acceleration of the universe since quark matter behaves like phantom-type dark energy. Furthermore, obtained f(R) solutions represents early eras of the Universe since f(R) solutions for quark matter coincide with f(R) equations for inflation. From this point, we can reach the conclusion that quarks may be source of the early dark energy of the universe or source of little inflation due to their repulsive force.

Magnetized strange quark matter in a quasiparticle description

Abstract: The quasiparticle model is extended to investigate the properties of strange quark matter in a strong magnetic field at finite densities. For the density-dependent quark mass, self-consistent thermodynamic treatment is obtained with an additional effective bag parameter, which depends not only on the density but also on the magnetic field strength. The magnetic field makes strange quark matter more stable energetically when the magnetic field strength is less than a critical value of the order 10(7) Gauss depending on the QCD scale Lambda. Instead of being a monotonic function of the density for the QCD scale parameter Lambda > 126 MeV, the effective bag function has a maximum near 0.3-0.4 fm(-3). The influence of the magnetic field and the QCD scale parameter on the stiffness of the equation of state of the magnetized strange quark matter and the possible maximum mass of strange stars are discussed.

Quark core impact on hybrid star cooling

Abstract: In this paper we investigate the thermal evolution of hybrid stars, objects composed of a quark matter core, enveloped by ordinary hadronic matter. Our purpose is to investigate how important the microscopic properties of the quark core are to the thermal evolution of the star. In order to do that we use a simple Massachusetts Institute of Technology (MIT) bag model for the quark core and a relativistic-mean-field model for the hadronic envelope. By choosing different values for the microscopic parameters (bag constant, strange quark mass, strong coupling constant), we obtain hybrid stars with different quark core properties. We also consider the possibility of color superconductivity in the quark core. With this simple approach, we have found a set of microscopic parameters that lead to a good agreement with those of observed cooling neutron stars. Our results can be used to obtain clues regarding the properties of the quark core in hybrid stars and to refine more sophisticated models for the equation of state of quark matter.

_{u-d} from lattice QCD at nearly physical quark masses

Abstract: We determine the second Mellin moment of the isovector quark parton distribution function _{u-d} from lattice QCD with N_f=2 sea quark flavours, employing the non-perturbatively improved Wilson-Sheikholeslami-Wohlert action at a pseudoscalar mass of 157(6) MeV. The result is converted non-perturbatively to the RI'-MOM scheme and then perturbatively to the MSbar scheme at a scale mu = 2 GeV. As the quark mass is reduced we find the lattice prediction to approach the value extracted from experiments.

Quark matter symmetry energy and quark stars

Abstract: We extend the confined-density-dependent-mass (CDDM) model to include isospin dependence of the equivalent quark mass. Within the confined-isospin-density-dependent-mass (CIDDM) model, we study the quark matter symmetry energy, the stability of strange quark matter, and the properties of quark stars. We find that including isospin dependence of the equivalent quark mass can significantly influence the quark matter symmetry energy as well as the properties of strange quark matter and quark stars. While the recently discovered large mass pulsars PSR J1614-2230 and PSR J0348+0432 with masses around $2M_{\odot}$ cannot be quark stars within the CDDM model, they can be well described by quark stars in the CIDDM model. In particular, our results indicate that the two-flavor $u$-$d$ quark matter symmetry energy should be at least about twice that of a free quark gas or normal quark matter within conventional Nambu-Jona-Lasinio model in order to describe the PSR J1614-2230 and PSR J0348+0432 as quark stars.

Anisotropic pressure in strange quark matter under the presence of a strong magnetic field

Abstract: Thermodynamic properties of strange quark matter in strong magnetic fields $H$ up to $10^{20}$ G are considered within the MIT bag model at zero temperature implying the constraints of total baryon number conservation, charge neutrality and chemical equilibrium The pressure anisotropy, exhibiting in the difference between the pressures along and perpendicular to the field direction, becomes essential at $H>H_{th}$, with the estimate $10^{17}

Lambda-to-kaon ratio enhancement in heavy ion collisions at several TeV.

Abstract: We introduced recently a new theoretical scheme which accounts for hydrodynamically expanding bulk matter, jets, and the interaction between the two. Important for the particle production at intermediate values of transverse momentum (p(t)) are jet hadrons produced inside the fluid. They pick up quarks and antiquarks (or diquarks) from the thermal matter rather than creating them via the Schwinger mechanism-the usual mechanism of hadron production from string fragmentation. These hadrons carry plasma properties (flavor, flow) but also the large momentum of the transversely moving string segment connecting quark and antiquark (or diquark). They therefore show up at quite large values of p(t), not polluted by soft particle production. We will show that this mechanism leads to a pronounced peak in the Lambda-to-kaon ratio at intermediate p(t). The effect increases substantially with centrality, which reflects the increasing transverse size with centrality.

Effective Potential Approach to Quark Ferromagnetization in High Density Quark Matter

Abstract: A possibility of spontaneous magnetization in high density symmetric quark matter is investigated using the NJL type effective model of QCD by means of an effective potential with respect to an auxiliary field It is shown that the quark ferromagnetic condensate has non-vanishing value at high baryon density due to the tensor-type four-point interaction between quarks Subject Index: 230

Thermodynamic Instabilities in Dynamical Quark Models with Complex Conjugate Mass Poles

Abstract: We show that the Cornwall-Jackiw-Tomboulis thermodynamic potential of dynamical quark models with a quark propagator represented by complex conjugate mass poles inevitably exhibits thermodynamic instabilities. We find that the minimal coupling of the quark sector to a Polyakov loop potential can strongly suppress but not completely remove such instabilities. This general effect is explicitly demonstrated in the framework of a covariant, chirally symmetric, effective quark model.

Light baryons and their electromagnetic interactions in the covariant constituent quark model

Abstract: We extend the confined covariant constituent quark model that was previously developed by us for mesons to the baryon sector. In our numerical calculation we use the same values for the constituent quark masses and the infrared cutoff as have been previously used in the meson sector. In a first application we describe the static properties of the proton and neutron, and the $\ensuremath{\Lambda}$ hyperon (magnetic moments and charge radii) and the behavior of the nucleon form factors at low momentum transfers. We discuss in some detail the conservation of gauge invariance of the electromagnetic transition matrix elements in the presence of a nonlocal coupling of the baryons to the three constituent quark fields.

Improving the top quark forward-backward asymmetry measurement at the LHC

Abstract: At the LHC, top quark pairs are dominantly produced from gluons, making it difficult to measure the top quark forward-backward asymmetry. To improve the asymmetry measurement, we study variables that can distinguish between top quarks produced from quarks and those from gluons: the invariant mass of the top pair, the rapidity of the top-antitop system in the lab frame, the rapidity of the top quark in the top-antitop rest frame, the top quark polarization and the top-antitop spin correlation. We combine all the variables in a likelihood discriminant method to separate quark-initiated events from gluon-initiated events. We apply our method on models including G-prime's and W-prime's motivated by the recent observation of a large top quark forward-backward asymmetry at the Tevatron. We have found that the significance of the asymmetry measurement can be improved by 10% to 30%. At the same time, the central values of the asymmetry increase by 40% to 100%. We have also analytically derived the best spin quantization axes for studying top quark polarization as well as spin-correlation for the new physics models.

Higher Dimensional Cosmological Model with Quark and Strange Quark Matter

Abstract: We study higher dimensional homogeneous cosmological model in the presence of quark and strange quark matter. The dynamical behavior of the model for the strange quark matter equation of state of the form \(p= \frac{1}{3} (\rho- 4 B_{c})\) are studied.

Quark masses and mixings in the RS1 model with a condensing 4th generation

Abstract: We study the hierarchy of quark masses and mixings in a model based on a 5-dimensional spacetime with constant curvature of Randall-Sundrum type with two branes, where the Electroweak Symmetry Breaking is caused dynamically by the conden- sation of a 4th generation of quarks, due to underlying physics from the 5D bulk and the first KK gluons. We first study the hierarchy of quark masses and mixings that can be obtained from purely adjusting the profile localizations, finding that realistic masses are not reproduced unless non trivial hierarchies of underlying 4-fermion interactions from the bulk are included. Then we study global U(1) symmetries that can be imposed in order to obtain non-symmetric modified Fritzsch-like textures in the mass matrices that reproduce reasonably well quark masses and CKM mixings.

The Energy Loss of a Heavy Quark Moving in a Viscous Fluid

Abstract: To study the rate of energy and momentum loss of a heavy quark in QGP, specifically in the hydrodynamic regime, we use fluid/gravity duality and construct a perturbative procedure to find the string solution in gravity side. We show that by this construction the drag force exerted on the quark can be computed perturbatively, order by order in a boundary derivative expansion. At ideal order, our result is just the drag force exerted on a moving quark in thermal plasma with thermodynamics variables promoted to become local functions of space and time. Furthermore, we apply this procedure to a transverse quark in Bjorken flow and compute the first-derivative corrections, namely the viscous corrections, to the drag force.

One-gluon-exchange effect on the properties of quark matter and strange stars

Abstract: We present a modified version of quark mass scaling via considering the important one-gluonexchange interaction between quarks in the quark mass density-dependent model. The properties of strange quark matter and the structure of strange stars are then studied with the new scaling and a self-consist...

Core collapse supernovae in the QCD phase diagram

Abstract: We compare two classes of hybrid equations of state with a hadron-to-quark matter phase transition in their application to core collapse supernova simulations. The first one uses the quark bag model and describes the transition to three-flavor quark matter at low critical densities. The second one employs a Polyakov-loop extended Nambu-Jona-Lasinio (PNJL) model with parameters describing a phase transition to two-flavor quark matter at higher critical densities. These models possess a distinctly different temperature dependence of their transition densities which turns out to be crucial for the possible appearance of quark matter in supernova cores. During the early post-bounce accretion phase quark matter is found only if the phase transition takes place at sufficiently low densities as in the study based on the bag model. The increase critical density with increasing temperature, as obtained for our PNJL parametrization, prevents the formation of quark matter. The further evolution of the core collapse supernova as obtained applying the quark bag model leads to a structural reconfiguration of the central protoneutron star where, in addition to a massive pure quark matter core, a strong hydrodynamic shock wave forms and a second neutrino burst is released during the shock propagation across the neutrinospheres. We discuss the severe constraints in the freedom of choice of quark matter models and their parametrization due to the recently observed 2M⊙ pulsar and their implications for further studies of core collapse supernovae in the QCD phase diagram.

Geometry of Quark and Strange Quark Matter in Higher Dimensional General Relativity

Abstract: In this paper we study quark matter and strange quark matter in higher-dimensional spherical symmetric space-times. We analyze strange quark matter for the different equations of state and obtain the space-time geometry of quark and strange quark matter. We also discuss the features of the obtained solutions in the context of higher-dimensional general theory of relativity.

Thermal mass and dispersion relations of quarks in the deconfined phase of quenched QCD

Abstract: Temporal quark correlation functions are analyzed in quenched lattice QCD for two values of temperature above the critical temperature (${T}_{c}$) for deconfinement, $T=1.5{T}_{c}$ and $3{T}_{c}$. A two-pole ansatz for the quark spectral function is used to determine the bare quark mass and the momentum dependence of excitation spectra on large lattices of size up to ${128}^{3}\ifmmode\times\else\texttimes\fi{}16$. The dependence of the quark correlator on these parameters as well as the finite volume dependence of the excitation energies are analyzed in detail in order to examine the reliability of our analysis. Our results suggest the existence of quasiparticle peaks in the quark spectrum. We furthermore find evidence that the dispersion relation of the plasmino mode has a minimum at nonzero momentum even in the nonperturbative region near ${T}_{c}$. We also elaborate on the enhancement of the quark correlator near the chiral limit which is observed at $T=1.5{T}_{c}$ on about half of the gauge configurations. We attribute this to the presence of near zero-modes of the fermion matrix that are associated with nontrivial topology of the gauge configurations.

Confinement in G(2) gauge theories using the thick center vortex model and domain structures

Abstract: The last candidate for the fundamental theory of hadronic force is the quark model. The interaction between quarks is described by the exchange of the non-abelian gauge fields called gluons. For the first time, the quark model was introduced by Gellman and Zweig in 1964. Besides the successfulness of the quark model, no free quark has been observed yet. However, the particles in the Lagrangian of any theory must exist in the physical spectrum. Now that the quarks appear in the QCD Lagrangian, the main question is: where are the quarks? Are they permanently confined? In fact, these are not the quarks which are not found in the nature but it is the color which is confined and quarks are confined because of their colors. In other words, particles with the degrees of freedom of colors are confined. Therefore, all the free particles are colorless and colored particles can not be found in the spectrum of the particles. In addition to the quarks, gluons which are the particles in the adjoint representation of the gauge group