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\begin{document}

\title
      [Infandum, regina, iubes renovare dolorem]
      {Measurement of Open Heavy Flavor Production in the STAR experiment at RHIC}

\classification{13.20.Fc, 13.25.Ft, 24.85.+p, 25.75.Cj}
\keywords{STAR, open heavy flavor mesons, non-photonic electrons,
cross section}

\author{Xin Li, for STAR Collarboration}{
  address={Purdue University, West Lafayette, IN 47907, USA}
  }


\begin{abstract}
We present the STAR results of non-photonic electron measurements
and direct reconstruction of charm mesons at $\sqrt{s_{NN}} = 200 $ GeV.
We also briefly outline the future perspectives of studying
open heavy flavor production at RHIC.}
\end{abstract}


\date{\today}


\maketitle

\section{Introduction}

Heavy quarks are unique probes to study the strongly-coupled
Quark-Gluon Plasma created at RHIC. Unlike light quarks, heavy
quark masses come mostly from spontaneous symmetry breaking, which
makes them ideal for studying the medium's QCD properties. Due to
their large masses, they are produced early in the collisions and
are expected to interact with the medium quite differently than
light quarks.  Detailed studies of the open heavy flavor meson
productions in heavy-ion collisions and the baseline p+p and d+A
collisions provide crucial information in understanding the
medium's properties.

In STAR experiment, we can study open heavy flavor mesons
indirectly through measuring non-photonic electron production, and
directly through reconstructing charm mesons via their hadronic
decay channels. The non-photonic electron, with the help of online
triggers, allows studying  heavy quark production at high $p_T$,
while the direct measurements allow direct access of heavy quark
kinematics and avoid the complication of disentangling charm and
bottom quark components.  In this paper, we report recent results
from both fronts.


\section{ Measurements of  Non-photonic Electron Production}

The STAR experiment has measured open charm production through its
semi-leptonic decay ($D,B \to l v_{l} X$) \cite{refa}. The
dominant backgrounds are photonic electrons from photon conversion
in the detector material and Dalitz decay of $\pi^{0}$ and $\eta$
mesons. They can be partially identified  through the
reconstructed invariant mass distributions of electron-positron
pairs. Not all photonic electrons can be identified this way since
one of the electrons in a pair may escape  the detector, or has a
momentum too low to be reconstructed. This inefficiency in finding
photonic electrons is estimated through simulation and is used to
estimate the total photonic electron background.

A long standing puzzle was that the measured high $p_{T}$
non-photonic electron invariant yield in both $p+p$ and Au+Au
collisions differ by about a factor of two between STAR and
PHENIX. This was resolved by uncovering a mistake, similar in all
of our published $p+p$, {\it d}+Au and Au+Au results, in applying
the background finding efficiency when subtracting the photonic
electron background \cite{refa}.  Figure 1 shows the nuclear
modification factor, $R_{AA}$, for $d$+Au and Au+Au collisions.
Since the mistake has similar effects in all collision specifies,
the corrected measurements are quite close to the original results
in central values but with larger statistical errors.  The results
indicate substantial energy loss of heavy quarks in dense matter
created at RHIC.

\begin{figure}
  \includegraphics[height=.3\textheight]{fig1}
  \caption{(Color online) The nuclear modification factor, $R_{AA}$, for $d$+Au and Au+Au collisions at
  $\sqrt{s_{NN}} = 200$ GeV, together with different theoretical model predictions. Look at Ref. [2] for details.}
\end{figure}

Electrons from bottom and charm meson decays are the two dominant
components of the non-photonic electrons. The relative
contribution to the non-photonic electrons from bottom and charm
meson decays can be disentangled utilizing their different decay
kinematics. In STAR, this is done though measuring the azimuthal
correlation between non-photonic electrons and charged hadrons as
well as the correlation between non-photonic electrons and $D^0$
\cite{refb}. The distribution of the azimuthal angle between
non-photonic electrons and charged hadrons from bottom meson
decays is much wider than that from charm meson decays. Through
fitting the data using a function combining the two different
distributions, we obtained the contribution of B-decay electron to
the non-photonic electron yield $(e_B/(e_B+e_D))$. By using the
measured $e_B/(e_B+e_D)$, together with the measured non-photonic
electron cross section \cite{refc} with the electrons from
$J/\psi$, $\gamma$ decay and Drell-Yan processes subtracted, one
can disentangle these two components. Figure 2 shows the invariant
cross section of non-photonic electrons from bottom and charm
mesons as a function of $p_{T}$ \cite{refc} and the corresponding
Fixed Order plus Next-to-Leading Logarithms  predictions (FONLL)
\cite{refd}, along with the ratio of each measurement to the FONLL
calculations. One can see the measurements are consistent with the
FONLL prediction within its theoretical uncertainties. The
integrated cross section of electrons at $3$ GeV/$c < p_T < 10$
GeV/$c$ from bottom and charm meson decays are determined as

\begin{displaymath}
\frac{d\sigma_{(B \to e) + (B \to D \to
e)}}{dy_{e}}\vert_{y_{e}=0} = 4.0 \pm 0.5(stat.) \pm 1.1(syst.)nb
\end{displaymath}

\begin{displaymath}
\frac{d\sigma_{(D \to e)}}{dy_{e}}\vert_{y_{e}=0} = 6.2 \pm
0.7(stat.) \pm 1.5(syst.)nb
\end{displaymath}


\begin{figure}
  \includegraphics[height=.4\textheight]{fig2}
  \caption{(Color online) Invariant cross section of electrons from bottom (upper-left) and charm meson (upper-right) decay,
   together with the ratio of the corresponding measurements to the FONLL predictions for bottom (lower-left) and charm electrons (lower-right),
   from Ref. \cite{refc}.}
\end{figure}


\section{Direct Reconstruction of Open Charm Mesons }

A direct reconstruction of the D meson was performed in Run2009
$p+p$ collisions at $\sqrt{s} = 200$ GeV. $D^0$ and $\overline
{D^0}$ are reconstructed through the decay channel $D^0$
($\overline {D^0}$) $\to K^{\mp}\pi^{\pm}$ with a branching ratio
(BR) of 3.89\%. $D^{*\pm}$ mesons are reconstructed through the
decay channel $D^{*\pm} \to D^{0}\pi^{\pm}$ ($BR = 67.7\%$), $D^0
\to K^{-}\pi^{+}$ and its charge conjugate. Figure 3 shows the
invariant mass distributions for the $D^0$ (left panel) and $D^*$
meson (right panel) after background subtraction. Significant
signal of directly reconstructed $D^0$ and $D^*$ meson are
observed.


\begin{figure}
  \includegraphics[height=.3\textheight]{fig3}
  \caption{The invariant mass distributions after subtracting backgrounds.
   $M_{K\pi}$ for $D^0$, $M_{K\pi\pi} - M_{K\pi}$ for $D^{*\pm}$, respectively}
\end{figure}



\section{Summary and outlook}

In summary, we present the STAR non-photonic electron measurements
at $\sqrt{s_{NN}} = 200$ GeV and report the status of the direct
reconstruction of D mesons in 200 GeV $p+p$ collisions. The
measurements of non-photonic electron production in $p+p$
collisions are consistent with the pQCD predictions. The $R_{AA}$
in Au+Au collisions indicate substantial energy loss of heavy
quark in the dense matter created at RHIC. In the future, with the
STAR heavy flavor tracker (HFT) and Muon telescope detector (MTD)
installed, the physics reach of STAR heavy flavor program will be
significantly extended. HFT will determine precisely the secondary
decay vertex of heavy flavor hadrons and improve the
signal-to-background ratio of all heavy flavor measurements. MTD
will allow studying heavy quark through muon decay channels. These
detector upgrades, together with the RHIC-II luminosity upgrade,
will open an opportunity for us to conduct the high precision
measurements of heavy flavor hadrons at RHIC.


\begin{thebibliography}{99}

\bibitem{refa}J. Adams et al. [STAR Collaboration], \emph{Phys. Rev. Lett.} 94,
062301 (2005); B. I. Abelev et al. [STAR Collaboration],
\emph{Phys. Rev. Lett.} 98, 192301 (2007); B. I. Abelev et al.
[STAR Collaboration], \emph{Phys. Rev. Lett.} 106, 159902 (2011).

\bibitem{refb}M. M. Aggarwal et al. [STAR Collaboration], \emph{Phys. Rev.
Lett.} 105, 202301 (2010).

\bibitem{refc}H. Agakishiev et al. [STAR Collaboration],  \emph{Phys. Rev. D}
83, 52006 (2011).

\bibitem{refd}M. Cacciari, \emph{Nucl. Phys. A} 783, 189 (2007); M. Cacciari, P.
Nason and R. Vogt, \emph{Phys. Rev. Lett.} 95, 122001 (2005); R.
Vogt, private communication.

\end{thebibliography}


\end{document}