113 research outputs found
Gluon shadowing in heavy flavor production off nuclei
No full textGluon shadowing which is the main source of nuclear effects for production of heavy flavored hadrons, remains unknown. We develop a light-cone dipole approach aiming at simplifying the calculations of nuclear shadowing for heavy flavor production, as well as the cross section which does not need next-to- leading and higher order corrections. A substantial process dependence of gluon shadowing is found at the scale of charm mass manifesting a deviation from QCD factorization. The magnitude of the shadowing effect correlates with the symmetry properties and color state of the produced cc pair. It is about twice as large as in DIS [B.Z. Kopeliovich et al,. Phys. Rev. D 62 (2000) 054022], but smaller than for charmonium production [B.Z. Kopeliovich et al., Nucl. Phys. A 696 (2001) 669]. The higher twist shadowing correction related to a nonzero size of the cc pair is not negligible and steeply rises with energy. We predict an appreciable suppression by shadowing for charm production in heavy ion collisions at RHIC and a stronger effect at LHC. At the same time, we expect no visible difference between nuclear effects for minimal bias and central collisions, as is suggested by recent data from the PHENIX experiment [PHENIX Collaboration, K. Adcox et al., nucl- ex/0202002]. We also demonstrate that at medium high energies when no shadowing is possible, final state interaction may cause a rather strong absorption of heavy flavored hadrons produced at large x(F). (C) 2002 Elsevier Science B.V. All rights reserved
Transparent Nuclei and Deuteron-Gold Collisions at Relativistic Energies
No full textThe current normalization of the cross section of inclusive high- pT particle production in deuteron-gold collisions measured at RHIC relies on Glauber model calculations for the inelastic d -Au cross section. These calculations should be corrected for diffraction. Moreover, they miss Gribov's inelastic shadowing which makes nuclei more transparent (color transparency) and reduces the inelastic cross section. The magnitude of this effect rises with energy and one may anticipate it to affect dramatically the normalization of the RHIC data. We evaluate the inelastic shadowing corrections employing the light-cone dipole formalism which effectively sums up multiple interactions in all orders. We found a rather modest correction factor for the current normalization of data for high- pT hadron production in d -Au collisions. The results of experiments insensitive to diffraction (PHENIX, PHOBOS) should be renormalized by about 20% down, while those which include diffraction (STAR), by only 10% . In spite of smallness of the correction it eliminates the Cronin enhancement in the PHENIX data for pions. The largest theoretical uncertainty comes from the part of inelastic shadowing which is related to diffractive gluon radiation or gluon shadowing. Our estimate is adjusted to data for the triple-Pomeron coupling and is small, however, other models do not have such a restriction and predict much stronger gluon shadowing. Thus, one arrives at quite diverse predictions for the correction factor which may be even as small as K = 0.65 . Therefore, one should admit that the current data for high- pT hadron production in d -Au collisions at RHIC cannot exclude in a model independent way a possibility of initial state suppression proposed by Kharzeev-Levin-McLerran. To settle this uncertainty one should directly measure the inelastic d -Au cross sections at RHIC. Also, collisions with a tagged spectator nucleon may serve as a sensitive probe for nuclear transparency and inelastic shadowing. We found an illuminating quantum-mechanical effect: the nucleus acts like a lens focusing spectators into a very narrow cone
Charmonium production off nuclei: from SPS to RHIC
No full textThe physics of charmonium suppression in nuclear collisions drastically changes between the energies of SPS and RHIC. Mechanisms suppressing charmonia at the SPS are reviewed, neither of which is important at RHIC. On the other hand, coherence, or shadowing of c quarks and gluons barely seen at SPS, becomes a dominant effect at RHIC providing a much stronger suppression. The onset of coherence at Fermilab energies explains why the observed cross section ratio falls steeply at large Feynman x(F). In nuclear collisions the variation of charmonium suppression with x(F) suggests a sensitive probe to look for the QGP
Small gluonic spots in the nucleon: searching for signatures in data
No full textNuclear shadowing and colour glass condensate need sufficiently small x to provide an overlap of parton clouds in the longitudinal direction. Another condition vital for these effects is overlap of partons in impact parameters, which is not easy to fulfil for gluons, even in heavy nuclei. This is because of smallness of gluonic spots related to the weakness of diffractive gluon radiation appearing in data. The predicted weak gluon shadowing and colour glass condensate effect are now appearing in data for J/Ψ production and Cronin effect in d–Au collisions at RHIC. Smallness of gluonic spots leads to a rather small value of α'P, the slope of the Pomeron trajectory, indeed appearing in elastic photoproduction of J/Ψ. At the same time, unitarity saturation in elastic pp collisions leads to a substantial increase of α'P in good agreement with data
Azimuthal asymmetry of neutrons produced by polarized protons
No full textThe transverse single-spin asymmetry AN(t), for inclusive leading neutron production in polarised pp collisions is calculated in the energy range of RHIC. Absorptive corrections to the pion pole generating a relative phase between the spin-flip and non-flip amplitudes, are found to be insufficient to explain the magnitude of AN observed recently in the PHENIX experiment. A larger contribution,
comes from the interference of pion and the effective a-Reggeon, which includes the a1 pole and the (dominant) πρ Regge cut. Assuming that this state saturates the spectral function of the axial current we determined its coupling to the nucleons applying the PCAC and the 2d Weinberg sum rule. The results of the parameter-free calculation of AN are in excellent agreement with the PHENIX data
Diffractive gauge bosons production beyond QCD factorization
No full textWe discuss single diffractive gauge boson d (gamma*, W+/-, Z) production in proton-proton collisions at different (Large Hadron Collider and Relativistic Heavy Ion Collider) energies within the color dipole approach. The calculations are performed for gauge bosons produced at forward rapidities. The diffractive cross section is predicted as a function of the fractional momentum and invariant mass of the lepton pair. We found a dramatic breakdown of the diffractive QCD factorization caused by an interplay of hard and soft interactions. Data from the CDF experiment on diffractive production of W and Z are well explained in a parameter-free way. DOI: 10.1103/PhysRevD.86.11403
Unconventional mechanisms of heavy quark fragmentation
No full textHeavy and light quarks produced in high- partonic collisions radiate
differently. Heavy quarks regenerate their color field, stripped-off in the
hard reaction, much faster than the light ones and radiate a significantly
smaller fraction of the initial quark energy. This peculiar feature of
heavy-quark jets leads to a specific shape of the fragmentation functions
observed in annihilation. Differently from light flavors, the heavy
quark fragmentation function strongly peaks at large fractional momentum ,
i.e. the produced heavy-light mesons, or , carry the main fraction of
the jet momentum. This is a clear evidence of the dead-cone effect, and of a
short production time of a heavy-light mesons. Contrary to propagation of a
small dipole, which survives in the medium due to color transparency,
a heavy-light dipole promptly expands to a large size. Such a big
dipole has no chance to remain intact in a dense medium produced in
relativistic heavy ion collisions. On the other hand, a breakup of such a
dipole does not affect much the production rate of mesons,
differently from the case of light meson production.Comment: 9 pages, submitted to UNIVERCE (MDPI
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