-   Results reported at the recent QM05 conference on the nuclear
    modification factor R~AA~ indicate that the heavy-quark energy loss
    in central Au+Au collisions is similar to that of the light-quarks
    (u, d, s). Since there is no directly reconstructed heavy-quark
    hadron distributions in experiments at RHIC, the electron R~AA~ was
    used for the heavy-quark study for the electron transverse momentum
    range: 4 \< p~T~ \< 10 GeV/c. This experimental observation
    contradicts our early understanding of the pQCD interactions of
    energetic partons in hot and dense medium where much less energy
    loss is expected for heavy-quarks compared with the light ones
    \[Kharzeev1,Gyulassy1,Armesto1\]. When analysis the data with only
    electron p~T~ distributions, the issue is further complicated by the
    unknown mixture of charm and beauty contributions in heavy-ion
    collisions at RHIC energies. Reconstructed charmed-hadron
    distributions and angular correlations are absolutely needed in
    order to detangle the important problem at RHIC.

-   ![](media/image1.png){width="4.557638888888889in"
    height="3.5944444444444446in"}

-   

-   **[Figure 1:]{.underline}** D-meson correlation functions from 200
    GeV p+p collisions. Default parameters in the Pythia model \[1\]
    were used in the calculations. A clear back-to-back correlation in
    the charmed meson production is observed. Solid-line and diamonds
    represent the results with angular smearing for σ~φ~ = π/4 and π/2,
    respectively.

-   without energy loss, respectively.

-   [Heavy-quark angular correlation:]{.underline} Similar to jets
    production, heavy-quark production requires large momentum transfer
    therefore one expects a distinct back-to-back topology. Again
    because their heavy mass, the charm- (or beauty-) hadrons are also
    formed with the back-to-back structure in elementary collisions as
    shown in Figure 1 by the open-circles. In this calculation, the
    Pythia (v6.2) was used with the default sets of parameters. As one
    can see in the figure, there is a clear back-to-back correlation for
    the D-mesons. Here we propose to utilized the distingue correlation
    to study the charm-quark energy loss in high-energy nuclear
    collisions. In the following, we will discuss three observables:

-   

(i) When a charm-quark interacts with the medium, it will not only loss
    its energy, the original angular correlation pattern as shown in the
    figure will also be modified. In Fig. 1, the tests with angular
    smearing of \<σ~φ~ \> = π/4 and π/2 shown as solid-line and diamonds
    are also plotted. In addition, the change of the angular correlation
    depends on the nature of the interaction. Most of the (semi)elastic
    scatterings are forwarded \[Tai1,Rapp1\], while the inelastic
    scatterings, such as the gluon radiative energy loss \[Gyulassy1\],
    would lead to a much wider smearing in the final correlation. In the
    inelastic scattering scenario \[Gyulassy1,Armenstro1\], the
    energy-loss occurs in deep plasma and the final correlation function
    reflects the hot/dense properties of the medium. On the other hand,
    the resonant scattering happens at near T~c~ \[Rapp1\]. Although
    both scenarios lead to sizable energy-loss \[Gyulassy1,Rapp1\], the
    angular correlations should allow us to distinguish these two
    different mechanisms in high-energy nuclear collisions. In order to
    perform the measurement, a large acceptance with the reconstructed
    charm-hadrons are essential. The proposed HFT, plus STAR TPC and
    TOF, will be necessary for this study.

(ii) Recently, the measurements on charm production by the Bell
     Collaboration \[Bell\] showed a surprising large cross section for
     J/ψ in sqrt(s)=10.6 GeV e+e collisions. Even more surprising, more
     than half of the observed J/ψ's were accompanied with ccbar pairs.
     This result contracts our current understanding for J/ψ production
     in the pQCD framework, such as those discussed in Refs. \[Pythia,
     Cho, Baek, Yuan\], and implies a different production mechanism for
     heavy-quarks in elementary collisions \[Kharzee2\]. As proposed in
     \[Kharzeev2\], the gluon fragmentation will become increasingly
     important for collisions at higher bombarding energies. In
     elementary collisions, the main difference between the new and
     conventional processes lies in the angular correlation of the
     produced charm-hadrons. With the proposed HFT and STAR EMC
     (Electro-Magnetic Calorimeter), we will study the correlation to
     further the pQCD in p+p collisions at RHIC energies. In addition,
     in high-energy nuclear collisions, gluon density is high
     \[Gyulassy2, Kharzeev4\] which might enhance the effect observed in
     elementary collisions. Such correlation study will certainly shed
     light on the production mechanisms for charm and charmonium at
     RHIC.

(iii) As discussed above that heavy-quarks production leads to the
      correlation between particle and its anti-particles. It also
      reflected in their decayed products, such as the electron pairs.
      In this case, it causes the background at the intermediate mass
      region 1 \< m~ee~ \< 3 GeV/c^2^ \[Rapp2\]. Here m~ee~ is the
      invariant mass of the electron pair. Analyzing the correlated
      electron pairs with the proposed HFT, the charm background will be
      greatly reduced. More discussion on this will be presented in
      Section 2.5.

-   [References:]{.underline}

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-   

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