Quark Antiquark Research Articles

Pp elastic scattering at LHC in near forward direction

We predict pp elastic differential cross section at LHC at the c.m. energy s=14 TeV and momentum transfer range |t|=0–10 GeV2, which is planned to be measured by the TOTEM group. The field theory model underlying our phenomenological investigation describes the nucleon as a composite object with an outer cloud of quark–antiquark condensate, an inner core of topological baryonic charge, and a still smaller quark-bag of valence quarks. The model satisfactorily describes the asymptotic behavior of σtot(s) and ρ(s) as well as the measured p¯p elastic dσ/dt at s=546 GeV, 630 GeV, and 1.8 TeV. The large |t| elastic amplitude of the model incorporates the BFKL pomeron in next to leading order approximation, the perturbative dimensional counting behavior, and the confinement of valence quarks in a small region within the nucleon.

γ*γ* SCATTERING VIA SECONDARY REGGEON EXCHANGE IN QCD

We summarize the results on the high energy behavior of quark–antiquark exchange in γ*γ* elastic scattering. The ladder diagrams, summed in the double logarithmic approximation, provide a perturbative QCD model for secondary reggeon exchange.

Mass spectra and leptonic decay widths of heavy quarkonia by using psi function

In this study, a non-relativistic two-body wave equation is used to describe the properties of heavy quark–antiquark systems with a potential proportional to the ψ-function. The wave equation is transformed into a true eigenvalue equation and solved numerically. Both the resonance masses and the leptonic decay widths of and mesons are calculated. The obtained results showed that the quark–antiquark interaction in these systems could be described adequately by using this simple potential form which contains one adjustable parameter besides the quark masses.

Dirac–Schrödinger equation for quark–antiquark bound states and derivation of its interaction kernel

The four-dimensional Dirac–Schrödinger equation satisfied by quark–antiquark bound states is derived from quantum chromodynamics. Different from the Bethe–Salpeter equation, the equation derived is a kind of first-order differential equation of Schrödinger-type in the position space. Especially, the interaction kernel in the equation is given by two different closed expressions. One expression which contains only a few types of Green's functions is derived with the aid of the equations of motion satisfied by some kinds of Green's functions. Another expression, which is represented in terms of the quark, antiquark and gluon propagators and some kinds of proper vertices, is derived by means of the technique of irreducible decomposition of Green's functions. The kernel derived not only can easily be calculated by the perturbation method, but also provides a suitable basis for nonperturbative investigations. Furthermore, it is shown that the four-dimensional Dirac–Schrödinger equation and its kernel can be directly reduced to rigorous three-dimensional forms in the equal-time Lorentz frame and the Dirac–Schrödinger equation can be reduced to an equivalent Pauli–Schrödinger equation which is represented in the Pauli spinor space. To show the applicability of the closed expressions derived and to demonstrate the equivalence between the two different expressions of the kernel, the t-channel and s-channel one gluon exchange kernels are chosen as an example to show how they are derived from the closed expressions. In addition, the connection of the Dirac–Schrödinger equation with the Bethe–Salpeter equation is discussed.

High energy pA collisions in the color glass condensate approach II: quark pair production

We compute the production of quark–antiquark pairs in high energy collisions between a small and a large projectile, as in proton–nucleus collisions, in the framework of the color glass condensate. We derive a general expression for quark pair-production, which is not k ⊥ -factorizable. However, k ⊥ -factorization is recovered in the limit of large mass pairs or large quark–antiquark momenta. Our results are amenable to a simple interpretation and suggest how multi-parton correlations at small x can be quantified in high-energy proton/deuteron–nucleus collisions.

The instanton molecule liquid and ‘sticky molasses’ above Tc

The main objective of this work is to explore the evolution in the structure of the quark–antiquark bound states in going down in the chirally restored phase from the so-called zero binding points Tzb to the QCD critical temperature Tc at which the Nambu–Goldstone and Wigner–Weyl modes meet. In doing this, we adopt the idea recently introduced by Shuryak and Zahed for charmed , light-quark mesons π, σ, ρ, A1 and gluons that at Tzb, the quark–antiquark scattering length goes through ∞ at which conformal invariance is restored, thereby transforming the matter into a near-perfect fluid behaving hydrodynamically, as found at RHIC. We show that the binding of these states is accomplished by the combination of (i) the colour Coulomb interaction, (ii) the relativistic effects and (iii) the interaction induced by the instanton–anti-instanton molecules. The spin–spin forces turned out to be small. While near Tzb all mesons are large-size non-relativistic objects bound by the Coulomb attraction, near Tc they get much more tightly bound, with many-body collective interactions becoming important and making the σ and π masses approach zero (in the chiral limit). The wavefunction at the origin grows strongly with binding, and the near-local four-Fermi interactions induced by the instanton molecules play an increasingly more important role as the temperature moves downward towards Tc.

In-medium modification of J/$psi; break-up cross sections

The in-medium modification of J/ψ break-up cross sections due to the hadronic Mott effect for light (π,ρ) and open-charmed (D,D∗) quark–antiquark bound states at the chiral/deconfinement phase transition is discussed. The spectral broadening of these mesons lowers the threshold for J/ψ break-up reactions leading to a step-like enhancement in the reaction rate. This mechanism for enhanced J/ψ dissociation is suggested as the physical mechanism underlying the anomalous J/ψ suppression observed by the NA50 experiment.

Charm quark–antiquark correlations in photon–proton scattering

Correlation of charm-quark–charm-antiquark in γp scattering are calculated in the kt-factorization approach. We apply different unintegrated gluon distributions (uGDF) used in the literature. The results of our calculations are compared with very recent experimental results from the FOCUS Collaboration. The CCFM uGDF developed recently by Kwieciński et al. gives a good description of the data. New observables are suggested for future studies. Predictions and perspectives for the HERA energies are presented.

Contribution of asymmetric strange–antistrange sea to the Paschos–Wolfenstein relation

The NuTeV Collaboration reported a value of sin2θw measured in neutrino–nucleon deep inelastic scattering, and found that the value is three standard deviations from the standard model prediction. This result is obtained under the assumption that the strange quark–antiquark sea of nucleons are symmetric, and that the up and down quark distributions are symmetric with the simultaneous interchange of u↔d and p↔n. We discuss the contribution of asymmetric strange–antistrange sea to the Paschos–Wolfenstein relation in the extraction of weak mixing angle sin2θw. We also point out that the contribution of asymmetric strange–antistrange sea should remove roughly 30–80% of the discrepancy between the NuTeV result and other determinations of sin2θw when using the light-cone meson–baryon model to calculate the contribution of the strange–antistrange sea.

In-medium modification of J/ ψ break-up cross sections

The in-medium modification of J/ ψ break-up cross sections due to the hadronic Mott effect for light ( π, ρ) and open-charmed (D,D ∗) quark–antiquark bound states at the chiral/deconfinement phase transition is discussed. The spectral broadening of these mesons lowers the threshold for J/ ψ break-up reactions leading to a step-like enhancement in the reaction rate. This mechanism for enhanced J/ ψ dissociation is suggested as the physical mechanism underlying the anomalous J/ ψ suppression observed by the NA50 experiment.

QCD corrections to γγ→ZZ at small scattering angles

QCD corrections to the electroweak cross section of γγ→ZZ at high energies and small scattering angles have been calculated. The dominant contributions are due to t-channel gluon exchange, i.e., photons dissociate into quark–antiquark pairs giving rise to two colour dipoles which interact through gluons. Corrections resulting from the leading log BFKL amplitude are of the order of a few percent close to the forward region already at the 1 TeV energy range and are rising with the scattering energy. We also considered the helicity non-conserving cases in which the QCD corrections in comparison to the electroweak part of the amplitude strongly grow with energy. The helicity non-conserving scattering process is of particular interest since it is sensitive to the Higgs sector.

Balance functions in coalescence model

It is shown that the quark–antiquark coalescence mechanism for pion production allows to explain the small pseudorapidity width of the balance function observed for central collisions of heavy ions, provided effects of the finite acceptance region and of the transverse flow are taken into account. In contrast, the standard hadronic cluster model is not compatible with this data.

R-parity violating decays of the gluino

Assuming the lightest supersymmetric particle is the gluino, we treat the decays g̃→qq̄ν and g̃→gν. Such couplings can be induced by the R-parity violating quark–squark–lepton interaction which can also be responsible for neutrino masses and mixings. These R-parity violating gluino decays have the same final state structure (jets plus missing energy) as previously considered decays into quark–antiquark–photino and gluon–gravitino but with significantly different gluino lifetimes.

BFKL dynamics in jet evolution

We calculate e+e−→QQ̄(k)+anything in a certain momentum, k, region for heavy quark–antiquark (QQ̄) production. In our chosen region we find that the number of heavy quark pairs produced is determined by BFKL dynamics and the energy dependence of the number of pairs is given by αP−1 the hard pomeron intercept.

Intermediate mass dileptons from a chemically non-equilibrated quark–gluon plasma

We have studied the evolution and dilepton production of a chemically equilibrating quark-gluon plasma considering finite baryon density based on Jüttner distribution of partons, and found that due to the increasing of the quark phase life-time with increasing initial quark chemical potential, high initial temperature, large gluon density of the system as well as large gluon fusion gg→c c ̄ cross section in the intermediate mass region, quark–antiquark annihilation q q ̄ →l l ̄ , especially, thermal charmed quarks from the gluon fusion gg→c c ̄ and quark–antiquark annihilation q q ̄ →c c ̄ provide a dominant contribution to dileptons, resulting in a significant enhancement of intermediate mass dileptons.

From deep inelastic scattering to proton-nucleus collisions in the color glass condensate model

We show that particle production in proton-nucleus (pA) collisions in the Color Glass Condensate model can be related to Deep Inelastic Scattering of leptons on protons/nuclei (DIS). The common building block is the quark antiquark (or gluon-gluon) dipole cross section which is present in both DIS and pA processes. This correspondence in a sense generalizes the standard leading twist approach to pA collisions based on collinear factorization and perturbative QCD, and allows one to express the pA cross sections in terms of a universal quantity (dipole cross section) which, in principle, can be measured in DIS or other processes. Therefore, using the parameterization of dipole cross section at HERA, one can calculate particle production cross sections in proton-nucleus collisions at high energies. Alternatively, one could use proton-nucleus experiments to further constrain models of the dipole cross-section. We show that the McLerran-Venugopalan model predicts enhancement of cross sections at large p_t (Cronin effect) and suppression of cross sections at low p_t. The cross over depends on rapidity and moves to higher p_t as one goes to more forward rapidities.

Pp ELASTIC SCATTERING AT LHC AND NUCLEON STRUCTURE

High energy elastic pp scattering at the Large Hadron Collider (LHC) at c.m. energy 14 TeV is predicted using the asymptotic behavior of σ tot (s) and ρ(s) known from dispersion relation calculations and the measured elastic [Formula: see text] differential cross-section at [Formula: see text]. The effective field theory model underlying the phenomenological analysis describes the nucleon as having an outer cloud of quark–antiquark condensed ground state, an inner core of topological baryonic charge of radius ≃ 0.44 F and a still smaller valence quark-bag of radius ≲ 0.1 F. The LHC experiment TOTEM (Total and Elastic Measurement), if carried out with sufficient precision from |t| = 0 to |t| > 10 GeV 2, will be able to test this structure of the nucleon.

The quark–antiquark asymmetry of the strange sea of the nucleon

The strange sea of the proton is generally assumed to have quark–antiquark symmetry. However, it has been known for some time that non-perturbative processes involving the meson cloud of the proton may break this symmetry. Recently this has been of interest as it affects the analysis of the so-called ‘NuTeV anomaly’, and could explain the large discrepancy between the NuTeV measurement of sin2θW and the currently accepted value. In this Letter we re-examine strange–anti-strange asymmetry using the meson cloud model. We calculate contributions to the strange sea arising from fluctuations in the proton wavefunction to states containing either Lambda or Sigma hyperons together with either Kaons or pseudovector K∗ mesons. We find that we should not ignore fluctuations involving K∗ mesons in this picture. The strange sea asymmetry is found to be small, and is unlikely to affect the analysis of the Llewellyn–Smith cross section ratios or the Paschos–Wolfenstein relationship.

The role of produced hadrons in J/ψ suppression

Hadrons produced in high-energy heavy-ion collisions can suppress the production of J/ψ and other charmonium states by charmonium spontaneous dissociation as the hot hadronic environment alters the interaction between the charm quark and charm antiquark. Furthermore, hadrons can thermalize a charmonium to excite it to higher charmonium states which subsequently dissociate spontaneously. They can also collide with a charmonium to lead to its prompt dissociation into an open charm pair.

Variationally improved perturbation theory and the potential −α/r + βr

Variationally improved perturbation theory is applied to the potential −αr + βr,which is relevant to heavy quark–antiquark systems. The problem is advocated asan instructive ‘laboratory’ for undergraduate exploration.