Overview of Charm Production -- J/ψ's and Open Charm

Michael J. Leitch

Los Alamos National Laboratory

P-25 MS H846, Los Alamos, NM 87544 USA

**Abstract.** Heavy quark production provides a sensitive window for the
study of the gluon structure of nucleons and its modification in nuclei.
It also provides a very important means of studying the hot-dense
conditions created in high-energy collisions of heavy nuclei and a
critical probe to look for deconfinement in this hot-dense matter. I
will review, from an experimental point of view, the physics issues as
seen in current experimental results for charm production and point out
the remaining puzzles.

Keywords: Heavy quarks, J/ψ, ψ' and , nuclear effects, shadowing.

PACS: 13.85.Ni, 14.65.Dw, 24.85.+p, 25.75.-q, 25.75.Dw

J/ψ, ψ' and 's are produced primarily from gluons in the projectile and
target. Open charm or beauty production shares this sensitivity to the
gluons as well as to other initial state effects in nuclei such as
initial-state gluon energy loss and multiple scattering causing p~T~
broadening. However, only the bound heavy-quark states suffer absorption
in the final state. A longstanding problem in J/ψ production is that
models that produced singlet $c\overline{c}$state predicted cross
sections that were several orders of magnitude smaller than those
observed in CDF[^1]. Although color-octet production (COM) is able to
reproduce these cross sections the matrix elements determined are not
universal and don't work for photo production of J/ψ's. A serious
problem is that the COM predicts transverse polarization at high p~T~,
but all measurements (CDF, E866/NuSea[^2]) so far see no substantial
polarization. One exception to this is in the sector, where the ~1S~,
like the J/ψ, has no polarization whereas the ~2S+3S~ has maximal
polarization[^3]. It is possible this is because the higher states have
little feed-down from higher mass states, while both the J/ψ and ~1S~
have substantial feed-down (\~30% for the J/ψ[^4]) which tends to
destroy any polarization. Clearly a measurement of the ψ' polarization
would be of interest since it does not suffer from feed-down. For single
charm (D meson) production, recent results from RHIC at √S = 200 GeV are
presently confused by a factor of two (but only 3-sigma) disagreement
between results from STAR and PHENIX[^5].

![](media/image1.png){width="1.5791666666666666in"
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height="1.9951388888888888in"}![](media/image3.jpeg){width="1.9520833333333334in"
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**Figure 1.** a) First measurements of the J/ψ cross section for √s =
200 GeV pp collisions from PHENIX. b) Polarization of production at 800
GeV showing none for the ~1S~ and maximal for the ~2S+3s.~ c) Nuclear
dependence versus x~F~ for J/ψ and ψ' production at 800 GeV.

Effects of the nuclear medium on production of heavy quarks include
shadowing (depletion at small x) of the gluon distributions in a
nucleus, energy loss and multiple scattering of the gluons before the
hard interaction, and, for onia, final-state absorption. Shadowing is
thought to involved coherence effects that effectively "shadow" the
partons inside a nucleus or can also be thought of as a saturation
effect in a nucleus, where at small enough x the gluons from different
nucleons overlap and interact with each other -- causing a promotion
through gg→g interactions to higher x and a corresponding reduction in
their populations at small x, e.g. as in a recent model called the
color-glass condensate (CGC)[^6].

The E866 nuclear dependence versus x~F~ is shown in Fig. 1c, where =1
would correspond to no change in the per nucleon cross section. Near
x~F~ = 0 the suppression of the resonances and lack of suppression of
open-charm is thought to be due to absorption of the resonances. At
these small x~F~ values the $c\overline{c}$'s are moving slowly out of
the nucleus after their creation should be starting to hadronize in the
nucleus and the larger suppression of the ψ' relative to the J/ψ would
be due to the larger size and looser binding of the ψ' compared to the
J/ψ. While at larger x~F~ they traverse the nucleus as a $c\overline{c}$
pair, only hadronizing way outside, and therefore experience the same
effects. The strong increase of the suppression as x~F~ increases is
thought to be due to a combination of shadowing (large x~F~ is small x
in the nuclear target) and also energy loss of the gluon in the initial
state[^7]. As shown in Fig. 2b, new data from PHENIX at √s = 200 GeV
also shows similar features, but so far with much worse statistical
precision. However, if one looks at the suppression seen at three
different energies (√s = 19 GeV (NA3), 39 GeV (E866) and 200 GeV
(PHENIX)) the suppression does not scale with x~2~ but does, as shown in
Fig. 2c, with x~F~. This appears to indicate that shadowing (which
should scale with x~2~), is not the dominant physics in the large x~F~
behavior of the J/ψ. The reason for the apparent scaling with x~F~
remains a puzzle, although I suppose it conceivable that energy loss
provides the bulk of the high-x~F~ suppression at the lower energies but
that it vanishes at higher energies (RHIC) where only a weak shadowing
is seen. Early studies at RHIC also indicate, that open-charm is not
modified in either d-Au or Au-Au collisions.

![](media/image4.png){width="1.59375in"
height="2.0694444444444446in"}![](media/image5.png){width="1.6618055555555555in"
height="2.0493055555555557in"}![](media/image6.png){width="1.2722222222222221in"
height="2.1902777777777778in"}

**Figure 2.** a) Nuclear dependence of J/ψ production versus rapidity at
√s = 200 GeV. b) Same versus x~F~. c) p~T~ broadening for J/ψ production
at √s = 39 GeV.

In Fig. 2c, one also sees that J/ψ production experiences p~T~
broadening, generally thought to come from multiple-scattering in the
initial state.

In high-energy nucleus-nucleus collisions heavy quarks and the onia are
thought to provide an important tool for the detection and study of the
strongly interacting deconfined matter that may be made. In particular
the color-screening in a deconfined media would heavily suppress J/ψ
production[^8]. On the other hand, recent models driven by the large
amount of charm created in these collisions also predict an enhancement
from recombination of that charm[^9]. At CERN when the observed
suppression in Pb-Pb collisions was compared to simple estimates using a
fixed nuclear cross section for the normal suppression (as determined
from p-nucleus studies)[^10]; an anomalous suppression at high energy
density (small impact parameter collisions) was observed and, by some,
is cited as evidence of creation of a quark-gluon plasma. However, to
many the estimate of the normal nuclear suppression seems inadequate to
clearly determine if the new de-confined state of matter was actually
seen. So far similar studies for the higher-energy collisions at RHIC
have been inconclusive due the to low luminosity at RHIC so far and the
corresponding low statistics for J/ψ in Au-Au collisions[^11]. We are
looking forward to more substantial results in the near future as the
analysis is completed of the much higher RHIC Au-Au run just completed.

[^1]: Beneke, Kramer, Phys. Rev. D55, 5269 (1997).

[^2]: T. Affolder et al. (CDF) Phys. Rev. Lett. 85, 2886 (2000) & T.
    Chang et al. (E866), Phys. Rev. Lett. 91, 211801 (2003).

[^3]: C.N. Brown et al. (E866/NuSea), Phys. Rev. Lett. 86, 2529 (2001).

[^4]: L. Antoniazzi et al. (E705), Phys. Rev. Lett. 70, 383 (1993) &
    preliminary results from HERA-B.

[^5]: J. Adams, et al. (STAR) nucl-ex/0407006; PHENIX preliminary, Quark
    Matter 2004.

[^6]: L.D. McLerran & R. Venugopalan, Phys. Rev. D49, 2233 (1994) ;
    Phys. Rev. D49, 3352 (1994).

[^7]: B. Kopeliovich et al., Nucl. Phys. A696, 669 (2001); R. Vogt,
    hep-ph/9907317.

[^8]: T. Matsui, H. Satz, Phys. Lett. B 178, 416 (1986).

[^9]: R.L. Thews et al., Phys. Rev. C63, 054905 (2001); L. Grandchamp et
    al. hep-ph/0403204.

[^10]: M.C. Abreu et al. (NA50), Phys. Lett. B477, 28 (2000) & Phys.
    Lett. B 521, 195 (2001).

[^11]: S.S. Adler et al. (PHENIX), Phys. Rev. C69, 014901 (2004).