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M. Vänttinen

Publications and source records attributed to M. Vänttinen.

15 recordsLinked to original sources
arXiv2000

Nuclear Shadowing and the Optics of Hadronic Fluctuations

A coordinate space description of shadowing in deep-inelastic lepton-nucleus scattering is presented. The picture in the laboratory frame is that of quark-gluon fluctuations of the high-energy virtual photon, propagating coherently over large light-cone distances in the nuclear medium. We discuss the detailed dependence of the coherence effects on the invariant mass of the fluctuation. We comment on the issue of possible saturation in the shadowing effects at very small Bjorken-$x$.

nucl-th↗
arXiv2000

NLO Corrections to Deeply-Virtual Compton Scattering

We have calculated the NLO corrections to the twist-2 part of the deeply-virtual Compton scattering amplitude. Our results for the transverse and antisymmetric parts agree with recent calculations by Ji and Osborne and by Belitsky and Müller. In addition we present NLO results for the longitudinal part of the amplitude.

hep-ph↗
arXiv1998

Exclusive $J/ψ$ photoproduction and gluon polarization

In exclusive $J/ψ$ production by polarized photons incident on polarized protons, a finite polarization asymmetry arises because of $c\bar c$ Fermi-motion and binding-energy effects. The asymmetry depends on the polarized nonforward gluon distribution of the proton and thus gives information on gluon polarization in the proton. The analyzing power, however, is rather small.

hep-ph↗
arXiv1998

Nuclear Quark and Gluon Distributions in Coordinate Space

In coordinate space, quark and gluon distributions of the nucleon are defined as correlation functions involving two field operators separated by a light-cone distance $y^+ = 2l$. We study the nuclear modifications of these distributions. The largest effect is a strong depletion of parton distributions (shadowing) at large longitudinal distances, which starts for all parton species at $l=2$ fm, i.e. at the average nucleon-nucleon separation in nuclei. On the other hand, the nuclear radius does not play a significant role. At $l \lsim 1$ fm, nuclear modifications of parton distributions are small. The intrinsic structure of individual nucleons is evidently not very much affected by nuclear binding. In particular, there is no evidence for a significant increase of the quark or gluon correlation length in bound nucleons.

hep-ph↗
arXiv1998

Hard exclusive $J/ψ$ leptoproduction on polarized targets

We consider the exclusive production of $J/ψ$ mesons in polarized virtual-photon--proton collisions. We derive helicity amplitudes for the dominant subprocess $γ^* g \to J/ψg$ and show that polarization asymmetries vanish in the case of collinear scattering. Thus, contrary to what has been suggested earlier, this process is not a good probe of the polarized gluon distribution $Δg(x,Q^2)$ of the proton.

hep-ph↗
arXiv1997

$χ_{c J}$ production in $e^+e^-$ annihilation

Inclusive production of $χ_{c J}$ in $e^+e^-$ annihilation is an excellent probe of the role of higher Fock states in the production of heavy quarkonia. Within the non-relativistic QCD approach, contributions from the short-distance production of colour-octet $c\bar{c}$ pairs are significantly larger than those from colour-singlet production. At the same time, $χ_{c J}$ production rates are significantly smaller than expected in the colour evaporation approach. Measurements of $χ_{\c J}$ production at CLEO and future B-factories will thus constitute a major test of theoretical approaches to the production of heavy quarkonia.

hep-ph↗
arXiv1997

Inelastic photoproduction of polarised $J/ψ$

The comparison of $J/ψ$ photoproduction data in the inelastic region with theoretical predictions based on the NRQCD approach has remained somewhat ambiguous and controversial, in particular at large values of the inelasticity variable $z$. We study the polar and azimuthal decay angular distribution of $J/ψ$ mesons as functions of $z$ and transverse momentum $p_t$. Future measurements of decay angular distributions at the HERA $ep$ collider will provide a new test of theoretical approaches to factorisation between perturbation theory and quarkonium bound-state dynamics and shed light on the colour-octet production fraction in various regions of $z$ and $p_t$.

hep-ph↗
arXiv1996

Nuclear Dependence of Structure Functions in Coordinate Space

The momentum distributions of partons in bound nucleons are known to depend significantly on the size of the nucleus. The Fourier transform of the momentum ($\xbj$) distribution measures the overlap between Fock components of the nucleon wave function which differ by a displacement of one parton along the light cone. The magnitude of the overlap thus determines the average range of mobility of the parton in the nucleon. By comparing the Fourier transforms of structure functions for several nuclei we study the dependence of quark mobility on nuclear size. We find a surprisingly small nuclear dependence ($<2\%$ for He, C and Ca) for displacements $t=z \lsim 2.5$ fm, after which a nuclear suppression due to shadowing sets in. The nuclear effects observed in momentum space for \mbox{$\xbj \lsim 0.4$} can be understood as a reflection of only the large distance shadowing in coordinate space.

hep-ph↗
arXiv1996

Color-Octet $J/ψ$ Production at Low $p_\perp$

We study contributions from color-octet quarkonium formation mechanisms to $J/ψ$ hadroproduction at low $p_\perp$. We include transitions of color-octet $c\bar{c}$ states into ``direct'' $J/ψ$ and into $χ_{1,2}$ which decay radiatively into a $J/ψ$. Together with earlier work, this calculation constitutes a complete analysis of $p_\perp$-integrated $J/ψ$ production at leading twist. We find that the leading-twist contribution is not sufficient to reproduce the observed production rates and polarization of the $J/ψ$ and $χ_{1,2}$. Hence there must exist other important quarkonium production mechanisms at low $p_\perp$.

hep-ph↗
arXiv1995

Color-Octet $ψ'$ Production at Low $p_\perp$

We study contributions from color-octet quarkonium formation mechanisms to $p_\perp$-integrated $ψ'$ production cross sections in pion-nucleon reactions. The observed polarization of the $ψ'$ is not reproduced by the lowest-order leading-twist color-singlet and color-octet mechanisms. This suggests that there are important quarkonium production mechanisms beyond leading twist.

hep-ph↗
arXiv1995

Hadroproduction of Charmonium at Large $x_F$

We present a higher-twist production mechanism for $η_c$ and $χ_J$ charmonium at large momentum fraction $x_F$ in pion-nucleon collisions. The higher-twist contribution is essentially independent of $x_F$ and therefore becomes dominant at large $x_F$, where the leading-twist contribution falls off as $(1-x_F)^3$. We show that the higher-twist mechanism produces longitudinally polarized $χ_1$ and $χ_2$. For the $χ_2$, this is clearly different from the leading-twist prediction of transverse polarization. For the $χ_1$, the polarization of the leading- and higher-twist contributions is qualitatively similar.

hep-ph↗
arXiv1994

Hadroproduction and Polarization of Charmonium

In the limit of heavy quark mass, the production cross section and polarization of quarkonia can be calculated in perturbative QCD. We study the $p_\perp$-averaged production of charmonium states in $πN$ collisions at fixed target energies. The data on the relative production rates of $\jp$ and $χ_J$ is found to disagree with leading twist QCD. The polarization of the $\jp$ indicates that the discrepancy is not due to poorly known parton distributions nor to the size of higher order effects ($K$-factors). Rather, the disagreement suggests important higher twist corrections, as has been surmised earlier from the nuclear target $A$-dependence of the production cross section.

hep-ph↗
arXiv1994

Polarization as a Probe to the Production Mechanisms of Charmonium in $πN$ Collisions

Measurements of the polarization of $\jp$ produced in pion-nucleus collisions are in disagreement with leading twist QCD prediction where $\jp$ is observed to have negligible polarization whereas theory predicts substantial polarization. We argue that this discrepancy cannot be due to poorly known structure functions nor the relative production rates of $\jp$ and $χ_J$. The disagreement between theory and experiment suggests important higher twist corrections, as has earlier been surmised from the anomalous non-factorized nuclear $A$-dependence of the $\jp$ cross section.

hep-ph↗
arXiv1994

Higher Twist Effects in the Drell-Yan Angular Distribution

We study the Drell-Yan process $πN \rightarrow μ^+ μ^- X$ at large $x_F$ using perturbative QCD. A higher-twist mechanism suggested by Berger and Brodsky is known to qualitatively explain the observed $x_F$ dependence of the muon angular distribution, but the predicted large $x_F$ behavior differs quantitatively from observations. We have repeated the model calculation taking into account the effects of nonasymptotic kinematics. At fixed-target energies we find important corrections which improve the agreement with data. The asymptotic result of Berger and Brodsky is recovered only at much higher energies. We discuss the generic reasons for the large corrections at high $x_F$. A proper understanding of the $x_F \to 1$ data would give important information on the pion distribution amplitude and exclusive form factor.

hep-ph↗