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\begin{document}
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\title{Heavy Flavor Tracker (HFT) :  A new Silicon Tracker for STAR experiment at RHIC%
\thanks{Presented at Excited QCD 2010}%
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}
\author{Spiros Margetis \\ for the star collaboration
\address{Kent State University, Ohio, USA}
%\and
%the Name(s) of other Author(s)
%\address{and their affiliation}
}
\maketitle
\begin{abstract}
The HFT is a new central silicon upgrade for the STAR experiment at RHIC. It is replacing the decommissioned 
silicon drift detector with active pixel technology in order to achieve about an order of magnitude
 better track pointing (DCA) resolution. This will allow for a direct and full topological 
 reconstruction of charmed meson decays (e.g.\ D$^0$ etc.)
and a better determination of B - meson spectra. Key measurements are D$^0$ elliptic 
flow determination, especially in the lower transverse momenta (p$_T$) region and detailed identified 
heavy quark suppression studies at high p$_T$ (R$_{CP}$/R$_{AA}$).

\end{abstract}
\PACS{13.20.Ft, 13.20.Hw, 12.38.-t, 25.75.-q, 25.75.Cj, 12.38.Mh}
  
\section{Introduction}
Due to their large masses, heavy flavor ($c$ and $b$) quarks are produced in the early stages of heavy ion 
collisions where the full initial energy is available for particle production \cite{ref1}. 
Radiative energy loss in dense partonic matter is thought to be inversely proportional
to the quark mass. Early measurements of heavy flavor energy loss at RHIC using the 
decay-electron spectra of D and B mesons showed a suppression similar to that of light quarks \cite{jaro}.
This puzzling result lead theorists to re-speculate the cause of this effect. Experimentally 
it is difficult to separate the charm and bottom contributions in the electron spectra. 
The two major experiments at RHIC, PHENIX and STAR both decided to upgrade their silicon vertex 
detectors in order to be able to improve their measuring capabilities.
The STAR approach and goal is to obtain
a precise measurement of heavy flavor production by identifying the decay 
of charmed mesons using direct topological reconstruction and thus disentangling the $c$ 
and $b$ contributions. 


The Heavy Flavor Tracker (HFT) is a state-of-the-art micro-vertex detector utilizing active pixel sensors and silicon strip technology. The HFT will significantly extend the physics reach of the STAR experiment for precision measurements of the yields and spectra of particles containing heavy quarks. This will be accomplished through topological identification of mesons and baryons containing charm quarks, such as D$^0$ and $\Lambda _C$, by the reconstruction of their displaced decay vertices with a precision of approximately 50 $\mu$m in p+p, d+A, and A+A collisions. The combined measurements of directly identified charm hadrons and of the total non-photonic electrons will enable us to identify the bottom production at RHIC, including the bottom production cross section and R$_{AA}$ and v$_2$ of the decay electrons.
The HFT consists of 4 layers of silicon detectors grouped into three subsystems with different technologies, guaranteeing increasing resolution when tracking from the TPC towards the vertex of the collision. The Silicon Strip Detector (SSD) is an existing detector in double-sided strip technology. It forms the outermost layer of the HFT. The Intermediate Silicon Tracker (IST), consisting of a layer of single-sided strip-pixel detectors, is located inside the SSD. Two layers of silicon pixel detector (PXL) are inside the IST. The pixel detectors have the resolution necessary for a precision measurement of the displaced vertex. With the HFT, the Time-of-Flight detector and the TPC we will study the physics of mid-rapidity charm and bottom production.
The pixel detector will use CMOS Active Pixel Sensors (APS), an innovative technology never used before in a collider experiment \cite{ref2}. The APS sensors are only 50 $\mu$m thick with the first layer at
a distance of only 2.5 cm from the interaction point. This opens up a new realm of possibilities for physics measurements. In particular, a thin detector (0.4 -- 0.5\% of a radiation length per layer) in STAR makes it possible to do the direct topological reconstruction of open charm hadrons down to very low transverse momentum by the identification of the charged daughters of the hadronic decay.



%The HFT has the necessary resolution for such a measurement that 
%requires high precision. It is the assembly of the existing Silicon Strip Detector (SSD) and 2 
%new detecting devices: the Intermediate Silicon Tracker (IST) and the PIXEL detector. 
%The PIXEL is composed by 2 layers of monolithic CMOS Active Pixel sensors \cite{ref2} 
%which measure with great accuracy the position of a particle within a few centimeters of 
%the interaction region. These very thin layers 
%minimize the multiple coulomb scaterring.
%The intermediate tracking system is made by the IST and the existing SSD (Table \ref{tab:fit-res}). 
%The purpose of the IST and the SSD is to link tracks found in the STAR Time Projection 
%Chamber (TPC) to the PIXEL detector. 

\begin{table}[!ht]
\caption{Characteristics of each silicon layer of the HFT\label{tab:fit-res}}
\begin{center}
\item[]\begin{tabular}{c c c c c c}
\hline
Detector&Radius&Technology&Silicon &Hit resolution& Thickness \\
                &             &                     &     thickness                         &R/$\phi-Z$                     & in $X_\mathrm{0}$  \\
                &(cm)&                     &      ($\mu$m)       &  ($\mu$m - $\mu$m)       &                  \\
\hline
SSD&23&2-side strips&300&30 - 857&1.0\%\\
IST&14&strip-pads&300&170 -1700&1.2\% \\
PIXEL&2.5, 8&Active Pixels& 50&10 - 10&0.4\%\\
\hline
\end{tabular}
\end{center}
\end{table}

\section{Physics performance}
Simulations presented in this proceedings were performed using the full STAR 
geometry package with 10k AuAu HIJING central events at $\sqrt{s_{NN}}$ = 200 GeV 
embedded with $D^\mathrm{0}$ and $\Lambda_\mathrm{c}$ particles, forced to decay 
to their hadronic channels ($D^\mathrm{0}$$\rightarrow$$K^\mathrm{-}$$\pi^\mathrm{+}$,  
$\Lambda_\mathrm{c}$$\rightarrow$$K^\mathrm{-}$$\pi^\mathrm{+}p$). Their reconstruction 
efficiencies are based on particle identification of daughter particles provided by the TPC 
and extended to higher $p_\mathrm{T}$ with the Time of Flight detector (TOF) : $K$-$\pi$ 
and ($K$+$\pi$)-$p$ separations were done up to $p_\mathrm{T}$ $\leq$ 1.6 GeV/c 
and $p_\mathrm{T}$ $\leq$ 3 GeV/c, respectively. Topological cuts have been also 
applied to the $D^\mathrm{0}$ candidates. The effect of {\it out of time} events is 
included in the PIXEL simulation at a rate corresponding to RHIC-II luminosity.
%\begin{figure}[!htpb]
%\begin{center}
%\includegraphics[scale=0.20]{allplot_new.eps}
%\caption{Simulations showing the statistical error projections for key measurements with the HFT\label%{fig:keytest}}
%\end{center}
%\end{figure} 
\begin{figure}[!h]
\begin{center}
%\epsfig{figure=D0_v2.eps}
%\epsfig{D0_Rcp.eps}
%\epsfig{updated_Lc_over_D0.eps}
\includegraphics[width=0.49\textwidth]{eqcd-fig1.eps}
\includegraphics[width=0.49\textwidth]{D0_v2.eps}
%\includegraphics[width=0.28\textwidth]{updated_Lc_over_D0.eps}
\end{center}
\caption{Projections of key measurements with HFT\label{fig:keytest}}
\end{figure} 

\section{Estimated $D^\mathrm{0}$ and $\Lambda_\mathrm{c}$ reconstruction performances}

Figure \ref{fig:keytest} shows the statistical error projections for the key measurements 
with the HFT in 500 M minimum bias AuAu collisions. Fig. \ref{fig:keytest}(left) is the
 flow parameter $v_{2}$, shown for two extreme scenarios [charm quark flow equal
  to light quark flow (red circles) and charm quark does not flow (green triangles)]. 
\begin{figure}[!h]
\begin{center}
%\epsfig{figure=D0_v2.eps}
%\epsfig{D0_Rcp.eps}
%\epsfig{updated_Lc_over_D0.eps}
%\includegraphics[width=0.28\textwidth]{D0_v2.eps}
\includegraphics[width=0.60\textwidth]{D0_Rcp.eps}
%\includegraphics[width=0.28\textwidth]{updated_Lc_over_D0.eps}
\end{center}
\caption{Projections of key measurements with HFT\label{fig:keytest}}
\end{figure} The HFT will be able to distinguish with great accuracy these 2 cases. 
Figure \ref{fig:keytest}(middle) shows the suppression factor $R_\mathrm{CP}$ 
of $D^\mathrm{0}$: the HFT will be able to measure it directly 
for $p_\mathrm{T}$ $\leq$ 10 GeV/c via the hadronic channel thus 
avoiding the indirect method using non-photonic electrons. A measurement 
of $\Lambda_\mathrm{c}$ is important to perform since 
the $\Lambda_\mathrm{c}$/$D^\mathrm{0}$ ratio may be enhanced, 
indicating a similar pattern to the baryon/meson ratio involving light 
quarks in the intermediate $p_\mathrm{T}$ region \cite{ref3}. 
\begin{figure}[!h]
\begin{center}
%\epsfig{figure=D0_v2.eps}
%\epsfig{D0_Rcp.eps}
%\epsfig{updated_Lc_over_D0.eps}
%\includegraphics[width=0.28\textwidth]{D0_v2.eps}
%\includegraphics[width=0.28\textwidth]{D0_Rcp.eps}
\includegraphics[width=0.60\textwidth]{updated_Lc_over_D0.eps}
\end{center}
\caption{Projections of key measurements with HFT\label{fig:keytest}}
\end{figure} Two scenarios are investigated for the $\Lambda_\mathrm{c}$/$D^\mathrm{0}$ 
ratio \cite{ref4} : no enhancement and same enhancement as $\Lambda$/$K_\mathrm{s}^\mathrm{0}$.
We see from Fig. \ref{fig:keytest}(right) that the statistical errors are 
sufficiently small, making a measurement of baryon/meson ratio in
 charm sector with good precision in heavy ion collisions.
 \section{Summary}
The HFT, by using low mass CMOS sensors, will be able to directly 
reconstruct charm hadrons over a large momentum range and, thus, 
study flow and energy loss of heavy flavor particles. Several physics 
capabilities such as baryon/meson ratio in the charm sector have been studied. 
\section{Next section}
The text...
\subsection{Subsection}
The text...

\begin{thebibliography}{00} % do not change 
\bibitem{ref1}Z. Lin and M. Gyulassy, {\it Phys. Rev. C} {\bf77} (1996) 1222.
\bibitem{jaro}J. Bielcik, {\it these proceedings} 
\bibitem{ref2}E. Anderssen et al., {\it A Heavy Flavor Tracker for 
STAR} (http://www.osti.gov/bridge/servlets/purl/939892-be12Up/939892.pdf).
\bibitem{ref3}S. H. Lee et al., {\it Phys.\ Rev.\ Lett.} {\bf 100} (2008) 222301.
\bibitem{ref4}J. Kapitan, {\it Eur. Phys. J. C} {\bf62} (2009) 217-221.
\end{thebibliography}

\end{document}

