2,350 research outputs found
Drell-Yan process at forward rapidity at the LHC
We analyze the Drell-Yan lepton pair production at forward rapidity at the
Large Hadron Collider. Using the dipole framework for the computation of the
cross section we find a significant suppression in comparison to the collinear
factorization formula due to saturation effects in the dipole cross section. We
develop a twist expansion in powers of Q_s^2/M^2 where Q_s is the saturation
scale and M the invariant mass of the produced lepton pair. For the nominal LHC
energy the leading twist description is sufficient down to masses of 6 GeV.
Below that value the higher twist terms give a significant contribution.Comment: 13 pages, 7 figure
Geometric scaling for the total gamma^* p cross section in the low x region
We observe that the saturation model of deep inelastic scattering, which
successfully describes inclusive and diffractive data at small x, predicts a
geometric scaling of the total gamma^* p cross section in the region of small
Bjorken variable x. The geometric scaling in this case means that the cross
section is a function of only one dimensionless variable tau = Q^2 R_0^2(x),
where the function R_0(x) (called saturation radius) decreases with decreasing
x. We show that the experimental data from HERA in the region x<0.01 confirm
the expectations of this scaling over a very broad region of Q^2. We suggest
that the geometric scaling is more general than the saturation model.Comment: 12 pages, 4 figures, LaTeX, revised version to appear in journal. 1
new figure, several new references added, extended discussion on saturation
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Confinement, quark mass functions, and spontaneous chiral symmetry breaking in Minkowski space
We formulate the covariant equations for quark-antiquark bound states in
Minkowski space in the framework of the Covariant Spectator Theory. The quark
propagators are dressed with the same kernel that describes the interaction
between different quarks. We show that these equations are charge-conjugation
invariant, and that in the chiral limit of vanishing bare quark mass, a
massless pseudoscalar bound state is produced in a Nambu-Jona-Lasinio (NJL)
mechanism, which is associated with the Goldstone boson of spontaneous chiral
symmetry breaking. In this introductory paper, we test the formalism by using a
simplified kernel consisting of a momentum-space delta-function with a vector
Lorentz structure, to which one adds a mixed scalar and vector confining
interaction. The scalar part of the confining interaction is not chirally
invariant by itself, but decouples from the equations in the chiral limit and
therefore allows the NJL mechanism to work. With this model we calculate the
quark mass function, and we compare our Minkowski-space results to lattice QCD
data obtained in Euclidean space. In a companion paper, we apply this formalism
to a calculation of the pion form factor.Comment: 17 pages, 12 figures, version published in Phys. Rev.
Pion electromagnetic form factor in the Covariant Spectator Theory
The pion electromagnetic form factor at spacelike momentum transfer is
calculated in relativistic impulse approximation using the Covariant Spectator
Theory. The same dressed quark mass function and the equation for the pion
bound-state vertex function as discussed in the companion paper are used for
the calculation, together with a dressed quark current that satisfies the
Ward-Takahashi identity. The results obtained for the pion form factor are in
agreement with experimental data, they exhibit the typical monopole behavior at
high-momentum transfer, and they satisfy some remarkable scaling relations.Comment: 11 pages, 8 figures, version published in Phys. Rev.
Application of the Covariant Spectator Theory to the study of heavy and heavy-light mesons
As an application of the Covariant Spectator Theory (CST) we calculate the
spectrum of heavy-light and heavy-heavy mesons using covariant versions of a
linear confining potential, a one- gluon exchange, and a constant interaction.
The CST equations possess the correct one-body limit and are therefore
well-suited to describe mesons in which one quark is much heavier than the
other. We find a good fit to the mass spectrum of heavy-light and heavy-heavy
mesons with just three parameters (apart from the quark masses). Remarkably,
the fit parameters are nearly unchanged when we fit to experimental
pseudoscalar states only or to the whole spectrum. Because pseudoscalar states
are insensitive to spin-orbit interactions and do not determine spin-spin
interactions separately from central interactions, this result suggests that it
is the covariance of the kernel that correctly predicts the spin-dependent
quark-antiquark interaction
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