Marek Karliner

Chiral symmetry and the spin of the proton

Abstract Recent EMC data on the spin-dependent proton structure function suggest that very little of the proton spin is due to the helicity of its constituent quarks. We argue that, at leading order in the 1 N c expansion, none of the proton spin would be carried by quarks in the chiral limit where mq=0. This result is derived in the Skyrme model, which is also used to estimate quark contribution to the proton spin when chiral symmetry and SU(3) are broken: this contribution turns out to be small. Therefore, even in the real world most of the proton spin is due to gluons and/or orbital angular momentum, as suggested by the EMC. We mention other experiments to test this suggestion.

An analysis of the angular momentum of the proton

Abstract We discuss the contributions to the proton helicity in the infinite-momentum frame from polarized quarks (Δq), polarized gluons (ΔG), and orbital angular momentum ( L c ). We first examine the behaviour of the polarized structure function g p 1 ( x ) at small x , arguing that it is consistent with dominance by the a 1 (1270) and related Regge trajectories. This supports the EMC estimate of ∫ 1 0 d xg p 1 ( x ), and hence the conclusions that Δ s Δ u +Δ d +Δ s ⋍0 . These conclusions are also supported by recent data on elastic ( ν ) p→ ( ν ) p scattering. Next we argue that in the Skyrme model with only pseudoscalar mesons, Δ G=0 and L z = 1 2 . These results still hold when the chiral lagrangian is made consistent with the scale transformation properties of QCD by incorporating a scalar gluonium field. Finally, we argue on general grounds and by explicit example that if the parameters of the chiral lagrangian are adjusted so that gluons carry ∼ 50% of the proton momentum, most of the orbital angular momentum L z is carried by quarks.

Determination of αs and the nucleon spin decomposition using recent polarized structure function data

New data on polarized µ−p and e−p scattering permit a first determination ofs using the Bjorken sum rule, as well as higher precision in determining the nucleon spin decomposition. Using perturbative QCD calculations to O( �s/�) 4 � for the non-singlet combination of structure functions, we finds(2.5 GeV 2 ) = 0.375 +0.062 0.081 , corresponding to �s(M 2 Z ) = 0.122 +0.005 0.009 , and using calculations to O( �s/�) 3 � for the singlet combination we findu = 0.83 ±0.03, �d = −0.43 ±0.03, �s = −0.10 ±0.03, �� ≡ �u+�d+�s = 0.31 ±0.07, at a renormalization scale Q 2 = 10 GeV 2 . Perturbative QCD corrections play an essential role in reconciling the interpretations of data taken using different targets. We discuss higher-twist uncertainties in these determinations. Theq determinations are used to update predictions for the couplings of massive Cold Dark Matter particles and axions to nucleons.

The OZI rule does not apply to baryons

Abstract Determinations of the π-N σ-term and the EMC measurement of deep inelastic polarized μp scattering and various theoretical arguments indicate that baryons contain a significant number of strange quark-antiquark pairs. Their presence gives rise to additional connected diagrams which evade the Okubo-Zweig-Iizuka (OZI) rule for meson-baryon couplings, and explain the apparent violation of the OZI rule in the reaction p p →φπ + π − as well as the process pp→ppφ+pions, and the backward peak seen in p p → K + K − . we catalogue predictions, based on moments of deep inelastic structure functions, for the OZI-evading couplings to baryons of 0−+, 0++, and 2++ and other mesons. Processes where these predictions can be tested include backward meson production in π-N scattering as well as p p annihilation and pp scattering.

Analysis of data on polarized lepton-nucleon scattering

Abstract We re-analyze data on deep inelastic polarized lepton-nucleon scattering, with particular attention to testing the Bjorken sum rule and estimating the quark contributions to the nucleon spin. Since only structure function data at fixed Q2 can be used to test sum rules, we use E142 asymmetry measurements and unpolarized structure function data to extract gn1 at fixed Q2 = 2 GeV2. When higher-twist effects, which are important at low Q2, are included, both the E142 and SMC data are compatible with the Bjorken sum rule within one standard deviation. Assuming validity of the Bjorken sum rule, we estimate the quark contributions to the nucleon spin, finding that their total net contribution is small, with the strange quark contribution non-zero and negative. The quark spin content of the nucleon spin is in agreement with Skyrme model.

SIZE AND SHAPE OF STRINGS

We study numerically and analytically spatial properties of the ground state of a fundamental string in the light-cone gauge. We find that strings are smooth and have divergent average size. Their properties are very different from what is expected from particles in a conventional field theory.

Abundant φ-meson production in pp annihilation at rest and strangeness in the nucleon

Abstract A large apparent violation of the OZI rule has recently been found in many channels in p p annihilation at LEAR. An interpretation of these data in terms of the “shake-out” and “rearrangement” of an intrinsic s s component of the nucleon wave function is proposed. This gives a channel-dependent, non-universal modification of the naive OZI prediction. Within this approach, we interpret the strong excess of φ production in S-wave p p annihilations in terms of the polarization of the nucleons s s component indicated by deep inelastic lepton-nucleon scattering experiments. This interpretation could be tested by measurements of s s meson production ratios in p p annihilation at rest and by experiments with polarized beams and polarized targets. We also propose a test of the intrinsic strangeness hypothesis in φ production in high-momentum transfer processes, via a difference in constituent counting rules from gluon-mediated production.

Baryons with two heavy quarks: Masses, production, decays, and detection

The large number of B c mesons observed by LHCb suggests a sizable cross section for producing doubly heavy baryons in the same experiment. Motivated by this, we estimate masses of the doubly heavy J = 1 / 2 baryons Ξ c c , Ξ b b , and Ξ b c , and their J = 3 / 2 hyperfine partners, using a method which accurately predicts the masses of ground-state baryons with a single heavy quark. We obtain M ( Ξ c c ) = 3627 ± 12 MeV , M ( Ξ c c * ) = 3690 ± 12 MeV , M ( Ξ b b ) = 10162 ± 12 MeV , M ( Ξ b b * ) = 10184 ± 12 MeV , M ( Ξ b c ) = 6914 ± 13 MeV , M ( Ξ ′ b c ) = 6933 ± 12 MeV , and M ( Ξ b c * ) = 6969 ± 14 MeV . As a byproduct, we estimate the hyperfine splitting between B c * and B c mesons to be 68 ± 8 MeV . We discuss P-wave excitations, production mechanisms, decay modes, lifetimes, and prospects for detection of the doubly heavy baryons.

Padé approximants, Borel transforms and renormalons: the Bjorken sum rule as a case study

We prove that Pade approximants yield increasingly accurate predictions of higher-order coefficients in QCD perturbation series whose high-order behavior is governed by a renormalon. We also prove that this convergence is accelerated if the perturbative series is Borel transformed. We apply Pade approximants and Borel transforms to the known perturbative coefficients for the Bjorken sum rule. The pade approximants reduce considerably the renormalization-scale dependence of the perturbative correction to the Bjorken sum rule. We argue that the known perturbative series is already dominated by an infra-red renormalon, whose residue we extract and compare with QCD sum-rule estimates by an higher-twist effects. We use the experimental data on the Bjorken sum rule to extract \({\alpha _s}\left( {M_Z^2} \right) = 0.116_{ - 0.006}^{ + 0.004}\) including theoretical errors due to the finite order of available perturbative QCD calculations, renormalization-scale dependence and higher-twist effects.

Quark solitons as constituents of hadrons

Abstract We exhibit static solutions of multi-flavour QCD in two dimensions that have the quantum numbers of baryons and mesons, constructed out of quark and anti-quark solitons. In isolation the latter solitons have infinite energy, corresponding to the presence of a string carrying the non-singlet colour flux off to spatial infinity. When Nc solitons of this type are combined, a static, finite-energy, colour singlet solution is formed, corresponding to a baryon. Similarly, static meson solutions are formed out of a soliton and an anti-soliton of different flavours. The stability of the mesons against annihilation is ensured by flavour conservation. The static solutions exist only when the fundamental fields of the bosonized lagrangian belong to U(Nc × Nf) rather than to SU(Nc) × U(Nf). Discussion of flavour-symmetry breaking requires a careful treatment of the normal-ordering ambiguity. Our results can be viewed as a derivation of the constituent quark model in QCD2, allowing a detailed study of constituent mass generation and of the heavy-quark symmetry.