arXiv subjects

Paul Hoyer

Publications and source records attributed to Paul Hoyer.

71 records · Page 4Linked to original sources

arXiv1998

The Dynamics of Quarkonium Production

Quarkonium production is a sensitive probe of the dynamics of hard scattering, which can `measure' the environment of the heavy quark pair after it is created in a hard process. Quarkonium hadroproduction data indicates that the produced pair experiences a secondary, hard interaction and then ceases to interact at a stage when the pair is still compact (compared to the size of the quarkonium wave function). These features differ from scenarios mostly discussed so far. An approach which relies on an early, perturbative rescattering of the pair with a comoving color field can explain many observed features, including the polarization, the $χ_1/χ_2$ ratio and the nuclear target $A$-dependence, which are difficult to understand otherwise.

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arXiv1997

Rescattering Effects in Quarkonium Production

We study eta_c and J/psi hadroproduction induced by multiple scattering off fixed centres in the target. We determine the minimum number of hard scatterings required and show that additional soft scatterings may be factorized, at the level of the production amplitude for the eta_c and of the cross section for the J/psi. The J/psi provides an interesting example of soft rescattering effects occurring inside a hard vertex. We also explain the qualitative difference between the transverse momentum broadening of the J/psi and of the Upsilon observed in collisions on nuclei. We point out that rescattering from spectators produced by beam and target parton evolution may have important effects in J/psi production.

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arXiv1997

A Perturbative Gluon Condensate

There is considerable freedom in setting boundary conditions to perturbation theory at $t=\pm\infty$. The standard PQED and PQCD expansions are based on the (empty) perturbative vacuum. Since the true QCD ground state is expected to have a condensate of low momentum gluons, I propose studying expansions around states containing free particles. A specific boundary condition (the `Perturbative Gluon Condensate'), which implies a freezing of low momentum gluons, could be a candidate for describing confinement physics. The modified perturbative series has a number of new features, although it is formally equivalent to the standard one and a priori equally justified.

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arXiv1997

The Intrinsic Gluon Component of the Nucleon

Using an intrinsic parton model we estimate the rough shape and size of the intrinsic gluon component of the nucleon, corresponding to an energy scale $Q$ of the order $\lqcd$. It is nearly as hard in shape as the valence quark, while its size accounts for a quarter of the nucleon momentum. Both are in qualitative agreement with the input leading twist gluon distribution assumed by Glück, Reya and Vogt at this scale in order to reproduce the observed distributions at $Q^2 \gsim 1$ GeV$^2$ via perturbative QCD evolution.

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arXiv1997

Physics at ELFE

I review some central physics opportunities at the 15 ... 30 GeV continuous beam electron accelerator ELFE, proposed to be built in conjunction with the DESY linear collider. Our present detailed knowledge of single parton distributions in hadrons and nuclei needs to be supplemented by measurements of compact valence quark configurations, accessible through hard exclusive scattering, and of compact multiparton subsystems which contribute to semi-inclusive processes. Cumulative (x>1, x_F>1) processes in nuclei measure short-range correlations between partons belonging to different nucleons in the same nucleus. The same configurations may give rise to subthreshold production of light hadrons and charm. The challenges of understanding high energy charmonium production indicate that charmonium will be a sensitive probe of color dynamics at ELFE. At low energies, charmonium forms inside the target nucleus, allowing a determination of $c\bar c$ bound state interactions in nuclear matter.

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arXiv1997

Charmonium Production at ELFE Energies

I discuss issues related to charmonium production, in view of physics possibilities at a 15 ... 30 GeV continuous beam electron facility. High energy photo- and hadroproduction of heavy quarkonia presents several challenges to QCD models concerning cross sections, polarization and nuclear target dependence. Theoretical approaches based on color evaporation as well as on color singlet and color octet mechanisms have met with both successes and failures, indicating that charmonium production is a sensitive probe of color dynamics. Experiments close to charm kinematic threshold will be sensitive also to target substructure since only unusual, compact target configurations contribute. In particular, subthreshold production on nuclei should identify nuclear hot spots of high energy density. At low energies, charmonium will form inside the target nucleus, allowing a determination of c cbar bound state interactions in nuclear matter.

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arXiv1996

Physics Opportunities at ELFE

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arXiv1996

Rapidity gaps in perturbative QCD

We analyze diffractive deep inelastic scattering within perturbative QCD by studying lepton scattering on a heavy quark target. Simple explicit expressions are derived in impact parameter space for the photon wave function and the scattering cross sections corresponding to single and double Coulomb gluon exchange. At limited momentum transfers to the target, the results agree with the general features of the ``aligned jet model''. The color--singlet exchange cross section receives a leading twist contribution only from the aligned jet region, where the transverse size of the photon wave function remains finite in the Bjorken scaling limit. In contrast to inclusive DIS, in diffractive events there is no leading twist contribution to $σ_L/σ_T$ from the lowest order $(q\bar q)$ photon Fock state, and the cross section for heavy quarks is power suppressed in the quark mass. There are also important contributions with large momentum transfer to the target, which corresponds to events having high transverse momentum production in both the projectile and target rapidity regions, separated by a rapidity gap.

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arXiv1996

A Perturbative Gluon Condensate?

I propose that the properties of QCD perturbation theory should be investigated when the boundary state (`perturbative vacuum') at $t= \pm\infty$ includes gluons. Any boundary state that has an overlap with the true QCD ground state generates a perturbative series that (when summed to all orders) is formally exact. Through an analogy with the boundary condition corresponding to a fermion condensate, I propose an explicit form for a `perturbative gluon condensate' that suppresses low momentum gluon production, thus generating an effective mass gap. Standard perturbative calculations are modified only through a change in the $iε$ prescription of low momentum $(|\pv| \lsim Λ_{QCD})$ gluon propagators. Gauge invariance is expected to be preserved since this modification is equivalent to adding on-shell external particles. Renormalizability is unaffected since only low-momentum propagators are modified. Due to the asymptotic low momentum gluons boost invariance is not explicit. Lorentz invariance should be restored in the sum to all orders in analogy to standard bound state calculations.

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arXiv1995

Nuclear Effects in Charm Production

I briefly review our understanding of the nuclear target dependence of charm and charmonium production, in view of charm physics at Elfe.

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arXiv1995

Interactions on Nuclei

I review hard photon initiated processes on nuclei. The space-time development of the DIS reaction as viewed in the target rest frame qualitatively describes the nuclear shadowing of quark and gluon distributions, although it may be difficult to understand the very weak $Q^2$ dependence of the low $x$ data. The current jet hadron energy distribution at large $ν$ is accurately independent of the target size even at very small energy fractions $z \simeq 0.05$. Color transparency is verified for vector meson $(J/ψ,\ ρ)$ production, but remains enigmatic in quasiexclusive proton knockout processes. I emphasize the importance of understanding short-range correlations in nuclei, as manifested by subthreshold production and cumulative $x>1$ DIS processes.

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arXiv1995

The Interpretation of Rapidity Gaps at Hera

In leading twist deep inelastic $ep$ scattering, the virtual photon interaction is fast compared to the time scale of soft color rearrangement. We compare the Pomeron exchange model, in which a neutral cluster is preformed, with a gluon exchange model, in which color is exchanged after the hard interaction. We find several features of the DIS data and of data on exclusive hard processes that favor a gluon exchange scenario. If correct, the postulate of soft color interactions between the produced ($q\bar q$) system and the target has important implications for other processes. In particular, this may explain the puzzles of charmonium hadroproduction.

hep-ph↗

arXiv1995

Systematics of Heavy Quark Production at HERA

We discuss heavy quark and quarkonium production in various kinematic regions at the HERA ep collider. In contrast to fixed target experiments, collider kinematics allows the possibility of detailed measurements of particle production in the proton fragmentation region. One thus can study parton correlations in the proton Fock states materialized by the virtual photon probe. We discuss various configurations of inelastic electron-proton scattering, including peripheral, diffractive, and deep inelastic processes. In particular, we show that intrinsic heavy quark Fock states can be identified by the observation of quarkonium production at large $x_F$ and a low mean transverse momentum which is insensitive to the virtuality $Q^2$ of the photon.

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arXiv1993

QCD Effects in Particle Production on Nuclei

I discuss some questions related to hard scattering processes in nuclei and corrections to the leading twist approximation. The QCD factorization theorem requires that high energy partons do not lose energy while traversing the nucleus. I explain the physical reason for this. The theorem also states that spectator partons, not involved in the hard collision, have no influence on the inclusive cross section. Important spectator effects are, however, seen in the data for certain reactions and in some kinematical regions. I discuss the reasons from a phenomenological and theoretical point of view. Finally, I mention some methods for analyzing hard processes in regions of very large $x$, where the leading twist terms are not dominant.

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arXiv1993

The Schwinger Model and Perturbative Charge Screening

\noindent The well-known exact solution of the massless Schwinger model can be simply obtained by perturbing around a vacuum without the Dirac sea of filled negative energy states. The unusual vacuum structure changes the sign of the $iε$ prescription at the negative energy pole of the free fermion propagator. Consequently, all convergent fermion loop integrals vanish. The logarithmically divergent two-point loop gives a mass $e/\sqrtπ$ to the photon, which turns into the free pointlike boson of the Schwinger model. We discuss the possiblity of using corresponding expansions to describe charge screening effects in the massive Schwinger model and the confinement phenomenon of QCD.

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arXiv1992

New QCD Effects at Large x

In heavy quark production at large Feynman $x$ there are two hardness scales, one given by the heavy quark pair mass $\M^2$\ and the other by $Λ_{QCD}^2/(1-x)$. When these two scales are comparable, the twist expansion of Perturbative QCD breaks down. We discuss the dynamics in this new QCD limit, where $μ^2=\M^2(1-x)$ is held fixed as $\M^2\to \infty$. New diagrams are found to contribute, which can enhance the cross section above that expected for leading twist. The heavy quarks are produced by a peripheral scattering on the target of hardness $μ^2$. This leads, in particular, to a nuclear target dependence of $A^{2/3}$ at small $μ$. Qualitatively, the dynamics in the new limit agrees with earlier phenomenological models of ``intrinsic'' heavy quark production.

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arXiv1992

A Bound on the Energy Loss of Partons in Nuclei

We derive a quantum mechanical upper bound on the amount of radiative energy loss suffered by high energy quarks and gluons in nuclear matter. The bound shows that the nuclear suppression observed in quarkonium production at high $x_F$ cannot be explained in terms of energy loss of the initial or final parton states. We also argue that no nuclear suppression is expected in the photoproduction of light hadrons at large $x_F$.

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