Excited Quarks Research Articles

Negative parity baryons in the1/Ncexpansion: The three towers of states revisited

We discuss the compatibility between the quark-shell picture and the meson-nucleon scattering picture in large $N_c$ QCD for mixed symmetric $\ell$ = 1 states previously analyzed by using a simple Hamiltonian including operators up to order $\mathcal{O}(N^0_c)$ defined in the standard ground state symmetric core + excited quark procedure. Here we introduce a Hamiltonian of order $\mathcal{O}(N^0_c)$ defined in a new approach where the separation of the system into two parts is not required. Three degenerate sets of states (towers) with the same quantum numbers as in the scattering picture and in the standard procedure are obtained. Thus the 0 is equally achieved. The eigenvalues of the presently chosen Hamiltonian also have simple analytic expressions, depending linearly on the three dynamical coefficients entering the Hamiltonian. This reinforces the validity of the new approach which had already 0 described excited negative parity baryons in a large energy range.

Search for new physics in dijet mass and angular distributions in pp collisions at ${\boldsymbol{\sqrt{\boldsymbol{s}}} \boldsymbol{=} {\bf 7}}$ TeV measured with the ATLAS detector

We present a search for physics beyond the Standard Model in proton–proton collisions at a centre-of-mass energy of $\sqrt{s} = {7}\;{\rm{TeV}}$ , performed with the ATLAS detector at the Large Hadron Collider (LHC). No evidence for new physics is found in dijet mass and angular distributions and stringent limits are set on a variety of models of new physics, including excited quarks, quark contact interactions, axigluons, and quantum black holes.

Search for new physics in the dijet mass distribution using 1 fb−1 of pp collision data at [formula omitted] collected by the ATLAS detector

Invariant mass distributions of jet pairs (dijets) produced in LHC proton–proton collisions at a centre-of-mass energy s=7 TeV have been studied using a data set corresponding to an integrated luminosity of 1.0 fb-1 recorded in 2011 by ATLAS. Dijet masses up to ∼4 TeV are observed in the data, and no evidence of resonance production over background is found. Limits are set at 95% C.L. for several new physics hypotheses: excited quarks are excluded for masses below 299 TeV, axigluons are excluded for masses below 3.32 TeV, and colour octet scalar resonances are excluded for masses below 1.92 TeV.

Quarkyonic Chiral Spirals in a Magnetic Field

Quarkyonic matter (QyM) is a large Nc phase of cold dense quark matter recently suggested by McLerran and Pisarski [1]. The main feature of QyM is the existence of asymptotically free quarks deep in the Fermi sea and confined excitations at the Fermi surface. The quarks lying deep in the Fermi sea are weakly interacting because they are hard to be excited due to Pauli blocking. Their interactions are hence very energetic and the confining part of the interaction does not play any role [2]. On the other hand, excitations of quarks within a shell of width ΛQCD from the Fermi surface interact through infrared singular gluons at large Nc and hence are confined. In has been recently argued [3, 4] that chiral symmetry can be broken in QyM through the formation of a translational non-invariant condensate that arises from the pairing between a quark with momentum p, and the hole formed by removing a quark with opposite momentum p from the Fermi surface. The inhomogeneous condensate that forms in QyM is a linear combination of the chiral condensate 〈ψψ〉, and a spin-one, isosinglet odd-parity condensate of 〈ψσψ〉. Here z is the direction of motion of the wave. At each given patch of the Fermi surface, z is the direction perpendicular to the surface. The authors of Ref. [3] called this combination of two inhomogeneous condensates a Quarkyonic Chiral Spiral (QyCS). The 〈ψσψ〉 component corresponds to the condensation of an electric dipole moment. The QyCS is then characterized by a spatial oscillation between chiral and electric dipole condensates that breaks parity and gives rise to an inhomogeneous electric field. On the other hand, a common feature of heavy-ion collisions is the generation of strong magnetic fields that are produced in peripheral collisions by the positively charged ions moving at almost the speed of light. For the Au-Au collisions at the Relativistic Heavy Ion Collider (RHIC) at BNL the field produced is estimated to be ∼ 10G [5]. Even though this magnetic field decays quickly, it only decays to a tenth of the original value for a time scale of order of the inverse of the saturation scale at RHIC [5], hence it may influence the properties of the QCD phases probed by the experiment. Strong magnetic fields will likely be also generated in future experiments planned at the Facility for Antiproton and Ion Research (FAIR) at GSI, the Nuclotron-Ion Collider Facility (NICA) at JINR, and the Japan Proton Accelerator Research Complex (JPARK) at JAERI, all of which intend to complement the experiments at RHIC by reaching regions of even higher densities and intermediate to low temperatures in the QCD phase map. In this paper we discuss the effects of an external magnetic field on QyM. As will be seen below, the presence of a magnetic field of strength comparable to the square of the QCD scale gives rise to the formation of a second chiral spiral, given by a spatial oscillation between a pion condensate 〈ψγ5ψ〉 and a spin-one condensate 〈ψγγψ〉. The spin-one condensate corresponds to an inhomogeneous magnetic moment in the direction of the field.

Effect of spin-3/2 top quark excitation on $ t\overline t $ production at the LHC

A spin-3/2 top quark excitation originating in string realisations of the RS scenario has an effect on the production of top-antitop pairs in hadron-hadron collisions. We study the additional contribution of this state to the cross section for the LHC. We comment on the prospect of discovery or exclusion for a range of masses and couplings at both 7 and 14 TeV and show that the reach extends significantly with increasing centre-of-mass energy. Results are compared with the effect of a hypothetical spin-1/2 excited top as well as other BSM scenarios which predict the presence of higher dimension effective operators. We also calculate distributions for several observables which can be used to distinguish between the different scenarios in the presence of a deviation from the SM prediction for the ttbar cross section.

Production and correlations of b quarks at the LHC energies

Processes leading to b-quark production at the LHC energies are simulated by using the Pythia generator. For proton-proton interactions at 7 and 14 TeV, the cross sections for b-quark production are evaluated for three mechanisms that are taken into account in the Pythia generator. These are quark-pair production, the excitation of sea quarks, and gluon splitting. Correlations of azimuthal angles of b-quark emission and properties of accompanying particles in events induced by the various mechanisms of b-quark production are calculated. The results are compared with data from the CDF experiment.

Negative parity baryons in the1/Ncexpansion: The quark excitation versus the meson-nucleon resonance picture

In order to better understand the fundamental issue regarding the compatibility between the quark-shell picture and that of resonances in meson-nucleon scattering in large $N_c$ QCD we extend the work of Cohen and Lebed on mixed symmetric $\ell $ = 1 baryons to analyze excited states with $\ell $ = 3. We give an explicit proof on the degeneracy of mass eigenvalues of a simple Hamiltonian including operators up to order $\mathcal{O}(N^0_c)$ \emph{i. e.} neglecting $1/N_c$ corrections in the quark-shell picture in the large $N_c$ limit. We obtain three degenerate multiplets formed of $\ell = 3$ states, as in the case of $\ell $ = 1 baryons. The compatibility between this picture and that of resonances in meson-nucleon scattering is discussed in the light of the present results.

Search for resonances in the dijet mass spectrum from 7 TeV pp collisions at CMS

A search for narrow resonances with a mass of at least 1 TeV in the dijet mass spectrum is performed using pp collisions at s=7 TeV corresponding to an integrated luminosity of 1 fb−1, collected by the CMS experiment at the LHC. No resonances are observed. Upper limits at the 95% confidence level are presented on the product of the resonance cross section, branching fraction into dijets, and acceptance, separately for decays into quark–quark, quark–gluon, and gluon–gluon pairs. The data exclude new particles predicted in the following models at the 95% confidence level: string resonances with mass less than 4.00 TeV, E6 diquarks with mass less than 3.52 TeV, excited quarks with mass less than 2.49 TeV, axigluons and colorons with mass less than 2.47 TeV, and W′ bosons with mass less than 1.51 TeV. These results extend previous exclusions from the dijet mass search technique.

A search for new physics in dijet mass and angular distributions in pp collisions at TeV measured with the ATLAS detector

A search for new interactions and resonances produced in LHC proton–proton (pp) collisions at a centre-of-mass energy was performed with the ATLAS detector. Using a dataset with an integrated luminosity of 36 pb− 1, dijet mass and angular distributions were measured up to dijet masses of ∼3.5 TeV and were found to be in good agreement with Standard Model predictions. This analysis sets limits at 95% CL on various models for new physics: an excited quark is excluded for mass between 0.60 and 2.64 TeV, an axigluon hypothesis is excluded for axigluon masses between 0.60 and 2.10 TeV and quantum black holes are excluded in models with six extra space–time dimensions for quantum gravity scales between 0.75 and 3.67 TeV. Production cross section limits as a function of dijet mass are set using a simplified Gaussian signal model to facilitate comparisons with other hypotheses. Analysis of the dijet angular distribution using a novel technique simultaneously employing the dijet mass excludes quark contact interactions with a compositeness scale Λ below 9.5 TeV.

ϒ production as a probe for early state dynamics in high energy nuclear collisions at RHIC

ϒ production in heavy ion collisions at RHIC energy is investigated. While the transverse momentum spectra of the ground state ϒ(1s) are controlled by the initial state Cronin effect, the excited bb¯ states are characterized by the competition between the cold and hot nuclear matter effects and sensitive to the dissociation temperatures determined by the heavy quark potential. We emphasize that it is necessary to measure the excited heavy quark states in order to extract the early stage information in high energy nuclear collisions at RHIC.

Publisher’s Note: Search for Dijet Resonances in 7 TeVppCollisions at CMS [Phys. Rev. Lett.105, 211801 (2010)

A search for narrow resonances in the dijet mass spectrum is performed using data corresponding to an integrated luminosity of 2.9 inverse pb collected by the CMS experiment at the LHC. Upper limits at the 95% confidence level (CL) are presented on the product of the resonance cross section, branching fraction into dijets, and acceptance, separately for decays into quark-quark, quark-gluon, or gluon-gluon pairs. The data exclude new particles predicted in the following models at the 95% CL: string resonances, with mass less than 2.50 TeV, excited quarks, with mass less than 1.58 TeV, and axigluons, colorons, and E_6 diquarks, in specific mass intervals. This extends previously published limits on these models.

Search for Dijet Resonances in 7 TeVppCollisions at CMS

A search for narrow resonances in the dijet mass spectrum is performed using data corresponding to an integrated luminosity of 2.9 pb⁻¹ collected by the CMS experiment at the Large Hadron Collider. Upper limits at the 95% confidence level are presented on the product of the resonance cross section, branching fraction into dijets, and acceptance, separately for decays into quark-quark, quark-gluon, or gluon-gluon pairs. The data exclude new particles predicted in the following models at the 95% confidence level: string resonances, with mass less than 2.50 TeV, excited quarks, with mass less than 1.58 TeV, and axigluons, colorons, and E6 diquarks, in specific mass intervals. This extends previously published limits on these models.

STRING CORRECTIONS TO QCD

In this paper we are considering coorections to strong interactions which are due to colored string Regge excitations. In case of a low string scale within the TeV region these higher spin excitations of quarks and gluons will lead to spectacular, universal dijet signatures at the LHC, which are true for a large class of models in the string landscape.

Group theoretical study of nonstrange and strange mixed symmetric baryon states[Nc−1,1]in the1/Ncexpansion

Using group theory arguments we extend and complete our previous work by deriving all SU(6) exact wave functions associated with the spectrum of mixed symmetric baryon states $[{N}_{c}\ensuremath{-}1,1]$ in the $1/{N}_{c}$ expansion. The extension to SU(6) enables us to study the mass spectra of both strange and nonstrange baryons, while previous work was restricted to nonstrange baryons described by SU(4). The wave functions are specially written in a form to allow a comparison with the approximate, customarily used wave functions, where the system is separated into a ground state core and an excited quark. We show that the matrix elements of the flavor operator calculated with the exact wave functions acquire the same asymptotic form at large ${N}_{c}$, irrespective of the spin-flavor multiplet contained in $[{N}_{c}\ensuremath{-}1,1]$, while with the approximate wave function one cannot obtain a similar behavior. The isoscalar factors of the permutation group of ${N}_{c}$ particles derived here can be used in any problem where a given fermion system is described by the partition $[{N}_{c}\ensuremath{-}1,1]$, and one fermion has to be separated from the rest.

Phase diagram evolution at finite coupling in strong coupling lattice QCD

We investigate the chiral phase transition in the strong coupling lattice QCD at finite temperature and density with finite coupling effects. We adopt one species of staggered fermion, and develop an analytic formulation based on strong coupling and cluster expansions. We derive the effective potential as a function of two order parameters, the chiral condensate $\ensuremath{\sigma}$ and the quark number density ${\ensuremath{\rho}}_{q}$, in a self-consistent treatment of the next-to-leading order (NLO) effective action terms. NLO contributions lead to modifications of quark mass, chemical potential, and the quark wave function renormalization factor. While the ratio ${\ensuremath{\mu}}_{c}(T=0)/{T}_{c}(\ensuremath{\mu}=0)$ is too small in the strong coupling limit, it is found to increase as $\ensuremath{\beta}=2{N}_{c}/{g}^{2}$ increases. The critical point is found to move in the lower $T$ direction as $\ensuremath{\beta}$ increases. Since the vector interaction induced by ${\ensuremath{\rho}}_{q}$ is shown to grow as $\ensuremath{\beta}$, the present trend is consistent with the results in Nambu-Jona-Lasinio models. The interplay between two order parameters leads to the existence of partially chiral restored matter, where effective chemical potential is automatically adjusted to the quark excitation energy.

Quark excitations through the prism of direct photon plus jet at the LHC

The quest to know the structure of matter has resulted in various theoretical speculations wherein additional colored fermions are postulated. Arising either as Kaluza-Klein excitations of ordinary quarks, or as excited states in scenarios wherein the quarks themselves are composites, or even in theories with extended gauge symmetry, the presence of such fermions (${q}^{*}$) can potentially be manifested in $\ensuremath{\gamma}+\mathrm{\text{jet}}$ final states at the LHC. Using unitarized amplitudes and the CMS setup, we demonstrate that in the initial phase of LHC operation (with an integrated luminosity of $200\text{ }\text{ }{\mathrm{pb}}^{\ensuremath{-}1}$) one can discover such states for a mass up to 2.0 TeV. The discovery of a ${q}^{*}$ with a mass as large as $\ensuremath{\sim}5\text{ }\text{ }\mathrm{TeV}$ can be achieved for an integrated luminosity of $\ensuremath{\sim}140\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$. We also comment on the feasibility of mass determination.

Search for excited quarks in ep collisions at HERA

A search for excited quarks is performed using the full e±p data sample collected by the H1 experiment at HERA, corresponding to a total integrated luminosity of 475 pb−1. The electroweak decays of excited quarks q∗→qγ, q∗→qZ and q∗→qW with subsequent hadronic or leptonic decays of the W and Z bosons are considered. No evidence for first generation excited quark production is found. Mass dependent exclusion limits on q∗ production cross sections and on the ratio f/Λ of the coupling to the compositeness scale are derived within gauge mediated models. These limits extend the excluded region compared to previous excited quark searches.

Search for new particles decaying into dijets in proton-antiproton collisions ats=1.96 TeV

We present a search for new particles whose decays produce two jets (dijets) using proton-antiproton collision data corresponding to an integrated luminosity of 1.13 fb-1 collected with the CDF II detector. The measured dijet mass spectrum is found to be consistent with next-to-leading-order perturbative QCD predictions, and no significant evidence of new particles is found. We set upper limits at the 95% confidence level on cross sections times the branching fraction for the production of new particles decaying into dijets with both jets having a rapidity magnitude |y| < 1. These limits are used to determine the mass exclusions for the excited quark, axigluon, flavor-universal coloron, E6 diquark, color-octet technirho, W', and Z'.

About the possibility of five-dimensional effective theories for low-energy QCD

The AdS/QCD models suggest the interesting idea that the effective theory of low-energy QCD may be formulated as a 5-dimensional field theory in the weak coupling regime in which the fifth coordinate plays a role of inverse energy scale. Taking the point of view that this is just an efficient parameterization of the non-perturbative dynamics of strong interactions, we discuss on a qualitative level an alternative possibility for a simpler 5-dimensional parameterization of the main phenomena in the low-energy QCD. We propose to interpret the effect of chiral symmetry breaking as the effective appearance of a compactified extra dimension with the radius of the order of the inverse scale of chiral symmetry breaking. Following some heuristic arguments two dual scenarios for the emergence of the excited light mesons are introduced. In the first scenario, the meson resonances are interpreted as the effects of Kaluza–Klein excitations of quarks inside mesons, in the second one as the formation of gluon strings wound around the compactified dimension an appropriate number of times. Matching of these scenarios permits us to express the slope of radial Regge trajectories through the order parameters of the chiral symmetry breaking, with the compactification radius being excluded. This example shows qualitatively that the extra dimension may play an auxiliary role providing a short way for deriving new relations.

Quark mass dependence of thermal excitations in QCD in one-loop approximation

A comprehensive determination of the quark mass dependence in the dispersion relations of thermal excitations of gluons and quarks in non-Abelian gauge theory (QCD) is presented for the one-loop approximation in the Feynman gauge. Larger values of the coupling are admitted, and some aspects of the gauge dependence are discussed. In a Dyson–Schwinger type approach, the effect of higher orders is estimated for asymptotic thermal masses.