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%                  Quark Matter 2009                                 %
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\journal{Nuclear Physics A} % do not change
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%% QM09Author: please enter your  
%% Title, author and address info here; please do not use footnotes

% Your Title - please modify
\title{The Transition Radiation Detector for ALICE at LHC}

% Principle author, and co-authors - please modify
\author{MinJung Kweon$^{a}$ for the ALICE TRD Collaboration}

% Address - please modify
% note that if you have authors from several institutions, we recommend
% labelling these [a], [b], [c] etc.
\address[a]{Physikalisches Institut, Universitaet Heidelberg, % label [a]
Heidelberg, 69120, Germany}

\begin{abstract} % do not change
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The Transition Radiation Detector (TRD) for the ALICE experiment at the
Large Hadron Collider (LHC) identifies electrons and performs online
tracking in the challenging high multiplicity environment of heavy-ion
collisions within 6 microsecond after the interaction and thus requires
excellent position resolution and pion rejection capability.
The TRD consists of 540 Xe gas-filled pad readout drift chambers with
radiators arranged in 18 super-modules in barrel geometry in the central
part of the ALICE detector. The large active area of roughly 700 m$^{2}$ is
covered by almost 1.2 million readout channels.
Presently, three of in total 18 TRD super-modules are installed in the
ALICE central barrel and commissioning of the detector using tracks from
cosmic radiation coacting with other ALICE sub-detectors was successfully
performed.
For a period of six months, four installed super-modules of the detector
were commissioned with cosmic radiation including a cosmic trigger
generated by the TRD at level 1. We will report on the performance and
current understanding of the detector based on these data.

This proceeding focuses XXX.

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\section{Introduction}

ALICE (A Large Ion Collider Experiment) is a general-purpose heavy-ion experiment 
designed to study the physics of strongly interacting matter and the quark-gluon plasma
in nucleus-nucleus collisions at the LHC\cite{alice_ppr_vol2}. It studies medium property 
with the collision products such as heavy quarkonia, open charm and beauty, light vector meson, direct-$\gamma$, 
jets and high-$p_{T}$ hadrons. One of the most powerful probe of the created QCD medium is heavy quarkonia 
and the suppression or enhancement\cite{SHM_model} 
of it will tell us the role of screening of color charge and statistical recombination.
Thus measurements of electrons are indispensable.

An important task of the Transition Radiation Detector (TRD) for the ALICE experiment is to supplement
the Time Projection Chamber (TPC) electron/pion identification by a pion rejection factor of the order of 100 at momenta
in axcess of 1 GeV/c. In addition, by measuremet of energy loss, the TRD improves the identification
of other charged particle\cite{alice_ppr_vol2}.

The TRD also contributes global central barrel tracking together with the Inner Tracking System (ITS) and TPC. 
It improves overall momentum resolution, especially at higher momenta. The resolution is as good as ~ 3.5\% at 100 GeV\cite{alice_ppr_vol2}.

Additionally, TRD provides a fast trigger for single/pairs of electrons and cluster of high-$p_{T}$ tracks,
 thus it allows us to study rare probes such as high-$p_{T}$ $\textit{J}$/$\psi$, $\Upsilon$ and high-$E_{T}$ jets.

\section{Working Principle of the TRD}

% Transition radiation (TR) x rays are produced when a highly relativistic particle crosses a refractive index interface.

The aimed pion rejection by a factor 100 at high-$p_{T}$ is obtained by production of transition radiation
which is unique for electrons within the relevant momentum range.
The relativistic electron ($\gamma$$\ge$1000) radiate photons in the X-ray range
when it tranverse a boundary between media of different diffractive indices. This transition radiation (TR) photons
($<$ 30 keV) are absorbed by high-$Z$ gas mixture(Xe,CO$_{2}$) of high photoabsorption cross section. The recognition of
this TR by characteristic peak at large drift time allows us to separate electron from pions.

The TRD surrounds the TPC in the central barrel($|$$\eta $ $<$ 0.9$|$) of ALICE with 540 chambers.
The chambers are arranged in 18 super modules containing 5 stacks along the longitudinal and 6 layers along the radial direction.
It has 694 m$^{2}$ active area and 28 m$^{2}$ gas volume of Xe/CO$_{2}$. 
The total radiation length is $\sim$ 24\% and the weight is $\sim$ 30 tons.
% leading to incorrect momentum determination or loss of electrons and to reduce phton conversions resulting in
% increased occupancey as well as incorrect matching.

Each chamber consists of a carbon fibre laminated Rohacell/polypropylene fibre sandwich radiator(48 mm), 
drift(30 mm) and multi-wire proportional(7 mm) section with cathode readout pad.
While very thin in radiation lengths to reduce Bremsstrahlung, the panel and the radiator provide 
enough mechanical rigidity of the chamber to keep gain uniformity better than 20\%.
%The entire readout electronics is directly mounted on the back panel of the detector.
Cross-sectional views of one TRD chamber together with average signals are shown in Fig. .

%The active area of 694$m^{2}$ is covered by ~1 million readout channels. 

%ALICE TRD is composed with drift chambers with cathode pad
%readout at 10MHz combined with a fiber/form sandwich radiator in front. 

% A charged particle tranversing a boundary between media of different diffractive indices can radiate a photon.
% This relativistic effect strongly depends on $\gamma$=E/m and is only relevant for $\gamma$$\ge$1000. 
% Such a high relativisticity occurs only for electrons within the relevant momentum range. 
% The photons produced are predominantly in the X-ray range. The detection and recognition of this transition radiation
% is used for the identification of an electron.

%Due to the low probability of transition radiation emission many boundaries are needed. This is achieved by the use of
%a special radiator consisting of a combination of polypropylene fibres and form. The thickness of the radiator is limited
%by the necessity of intermediate tracking devices - needed for track matching at high multiplicities. This determines 
%the structure of the TRD consisting of six layers, each comprising a radiator and a tracking chamber. 
%For the detection of the transition radiation the tracking chamber is used as well, which requires the efficient detection
%of X-ray photons. 

%Each detector element consists of a carbon fibre laminated Rohacell/polypropylene fibre sandwich radiator of 48 mm thickness, 
%a drift section of 30 mm thickness, and a multi-wire porportional chamber section (7 mm) with pad readout. The pad planes are
%supported by a honeycomb carbonfibre sandwich back panel (22 mm). While very light, the panel and the radiator provide enough
%mechanical rigidity of the chamber to cope with overpressure up to 1 mbar to ensure a deformation of less than 1 mm. 
%The entire readout electronics is directly mounted on the back panel of the detector. Including the water cooling system
%the total thickness of a single detector layer is 125 mm. In the bending plane ($r\phi$) each pad row consists of 144 pads. 
%The central chambers consist of 12, all others of 16 pad rows. This leads to an overall channel count of 1.18x$10^{6}$. 
%The total active area subtended by the pads is 716 $m^{2}$. 

%Cross-sectional views of one TRD chamber together with average signals are shown in Fig. . Ionizing radiation produces electrons
%in the counting gas (Xe/$CO_{2}$ (85:15)). Particles exceeding the threshold for transition radiation production ($gamma$>- 1000) will
%in addition produce about 1.45 X-ray photons in the energy range of 1 to 30 keV. X-rays in this energy resime are efficiently
%converted by the high-Z counting gas with the largest conversion probability at the very beginning of the drift region. 
%All electrons from ionization energy loss and X-ray conversions will dirft wowards the anode wires. After gas amplification in the 
%vicinity of the anode wires the signal is induced on the readout pads. Signals of a typical track are shown in the inset of the
%central panel of Fig . The inclination of the track in the bending direction is a direct measure of its transverse momentum. 
%For particles with a momentum of 2 GeV/c the average amplitude of the cathode pad signal versus drift time is shown in the right panel. 
%Two effects lead to an efficient discrimination between electrons and pions. i) the increased specific energy loss of electrons 
%compared to pions at this momentum (difference between triangles and squares)  and ii) the absorption of transition radiation 
%generated by electrons (circles) predominantly at the beginning of the drift section corresponding to large drift times.  

The TRD front-end electronics (FEE) is directly mounted on the back panel of the chamber. 
Groups of 18 pads are connected by short cables to a Multi-Chip Module (MCM) which comprises
the Pre-Amplifier and Shaper Amplifier (PASA) and the Tracklet Processor (TRAP).
The PASA has a 120 ns shaping time, a gain of 12.4 mV/pC, and a equivalent noise charge of 850 electrons
at 25 pC of input capacitance. 
The TRAP chip comprises 21 channels 10 MHz 10 bits ADCs,
four stages of digital filters, event buffers, readout interface and local tracking unit 
(Preprocessor and four 120 MHz CPUs)
which allow to calculate the inclination of a track in the bending direction as well as the total charge
deposited along the track.  %mentioning electron pid?
% This feature allows to identify high-$p_{T}$ particles at the trigger level. 
% Evaluation of the deposited energy will furthermore allow to tag possible electron candidates at the trigger level.
One Read-Out Board (ROB) consists of 17 or 18 MCMs and 6 or 8 ROBs are mounted on one chamber. 
Each chamber has one Linux based Detector Control System (DCS) board and two optical readout interface modules (ORI)
for data shipping.

The local track segments (tracklets) and the raw data acquired by TRAP are sent to the global tracking unit (GTU)
via 2.5 Gbps ORI. Based on the tracklets merged, GTU performs transverse momentum reconstruction and electron identification.
After finding high-$p_{T}$ tracks and identifying electrons, various trigger schemes are applied for di-electron decays and jets.
Then this trigger contribution is sent to the ALICE central trigger processor (CTP) within 6.1 $\mu s$ to drive the Level-1 trigger decision.
Such a fast processing is done thanks to the massively parallel hardware architecture of the GTU whose core is FPGA-based. 
It is capable of processing up to 20k tracklets within 2 $\mu s$.
GTU also forward raw data to DAQ when Level-2 accepted.



\begin{figure}[ht]
\begin{minipage}[b]{0.5\linewidth}
\centering
\includegraphics[scale=1]{plots/004.design_principle1.eps}
\caption{default}
\label{fig:figure1}
\end{minipage}
\hspace{0.5cm}
\begin{minipage}[b]{0.5\linewidth}
\centering
\includegraphics[scale=0.8]{plots/pulse_height_prototype.eps}
\caption{default}
\label{fig:figure2}
\end{minipage}
\end{figure}

%The entire trigger timing sequence involving the LTU and the GTU is depicted in Fig. .

%The TRD trigger function is performed in two stages as shown in Fig. ; one is at the TRAP and the other is at the GTU.
%The tracklets calculated by the TRAPs on detector are sent to GTU by 1080 ORIs. The GTU processes up to 20k tracklets per event
%in about 1.8 $\mu s$ by means of massive parallelism. 90 large FPGA combine tracks and search for high-$p_{T}$ tracks. Then the trigger
%decision is sent to the ALICE central trigger processor (CTP) at ~ 6 $\mu s$.

%The FEE is controlled by a detector control system (DCS) board attached on one of the ROBs. The DCS board has an Ethernet interface,
%ARM-based CPU with Linux as operating system, trigger and clock receiver, control interface for TRAPs, and various peripherals.


%It consists of 6 or 8 readout boards (ROB) per chamber
%depending on the chamber size, and sub-components. One ROB consists of 17 or 18 multi-chip sub-components. 
%One ROB consists of 17 or 18 multi-chip modules (MCM), depending on the ROB type, see Fig. . 
%Each MCM consists of a printed circuit board with ball grid array implementation, and two chips which are an 18 channels 
%charge sensitive pre-amplifier and shaper amplifier (PASA), and a tracklet processor (TRAP). The PASA has a 120 ns shaping time,
%a gain of 12.4 mV/pC, and an equivalent noise charge of 850 electrons at 25 pC of input capacitance. The PASA output 
%is directly bondwired to the input of the TRAP where ADCs are integrated. The TRAP chip is an ASIC containing 21 channels 
%10 MHz 10 bits ADCs, four stages of digital filters, tracking processors, four 120 MHz CPUs, and data readout interface.
%The track information (tracklets) and the raw data acquired by TRAP are sent to the global tracking unit (GTU) via 2.5 Gbps 
%optical readout interface modules (ORI). Two ORIs are prepared for each chamber.

%Groups of 18 pads within a row are connected by short cables to a Multi-Chip Module (MCM) on the back-side of the chamber.
%This module comprises the Pre-Amplifier and Shaper Amplifier (PASA) and the Tracklet Processor (TRAP). 
%The pad charges are registered on the PASA individually. The PASA contains several amplification stages and converts
%the input signal to a voltage on the pedestal value. Additionally, the signal is shaped to cancel the long ion tail.
%The PASA output voltages are converted to digital values by ADCs in the TRAP. The sampling rate is 10 MHz and the signal is 
%digitized with 10 bits resolution. In addition to the PASA channles from the same MCM one or two channels from the PASA
%in the left or right MCM, respectively, are included. This sharing, leading to 21 ADC channels per MCM, is needed for the
%on-chip tracklet calculation.
%The digitized data is processed in a series of digital filters. This is necessary at this stage since the trigger calculation 
%is based on this information.




\begin{figure}[ht]
\centering

%%
%% here is an example syntax if you want to have a single eps file in the figure
%\includegraphics[scale=0.19]{integrated_charm_with_FONLL.eps}         
%% - then you need to use commands a la those shown in the included 'mkpaper'
%% script: e.g. by doing 'source mkpaper'
%%
%% next is an example for including 2 non-eps files in the same figure
%\includegraphics[width=0.4\textwidth]{integrated_charm_with_FONLL.jpg}
\includegraphics[width=0.95\textwidth]{001.cosmic_eventdisplay.eps}
%% in the example here we give slightly more horiz. space for the 2nd figure 
%% - then you can just use 'pdflatex example.tex', and get a pdf directly
%%
\caption[]{Cosmic-ray event display triggered with TRD Level-1}
\label{cosmic_eventdisplay}
\end{figure}


\section{Integration, Installation and Commissioning}


The assembly of a supermodule starts with the assembly of the supermodule hull including distribution of low voltage and cooling water. 
Afterwards, the readout chambers are installed layer by layer. Each layer undergoes tests of electronics, gas tightness, high-voltage
distribution and cooling. The integration of a supermodule is concluded by several days of cosmics data taking to produce a calibration data set. 
%The first supermodule was built at Heidelberg in 2008 and rest of them are being constructed in Muenster. 
Then the completed supermodules are transported to CERN for their final installation. Before its insertion in the ALICE setup, 
extensive testing is done on the surface to ensure that the detector is working.  

After installation, noise level was checked. We archived average 1.1 ADC noise level which is close to design goal and dead channles are less than 0.1\%. 

Four supermodules were installed in the ALICE in 2008 and participated in the cosmic-ray data taking together with other detectors. 
Besides the proof of combined operation, these runs are used to gather reference data for alignment and calibration from tranversing cosmic particles.
The cosmic trigger decision was based on coincident hits in the Time Of Flight (TOF) detector and GTU Level-1 trigger.
It was first time to use the full chain of trigger sequence for TRD within ALICE setup and the GTU Level-1 trigger was the first running Level-1 trigger in ALICE.
Level-1 rejection from Level-0 was by a factor 20 and Level-1 rate was $\sim$0.05 Hz with the purity of larger than 85\%.
Total 55k tracks were recorded in the TRD under difficult constraints for cosmic-ray which should require tracks close to horizontal at 60 m below the surface. 
A display of a cosmic-ray event is shown in Fig. .

The TRD Detector Control System (DCS) which ensures safe and stable operation and monitoring of the detector has been also commissioned during this runs within the ALICE experiment.

Before the LHC injection test in 2008, we has progressed towards readiness for data taking. 

%The running of a complex system as the TRD is facilitated by an efficient and user-friendly detector control system (DCS)
%which ensures safe and stable operation and monitoring of the detector. It is built on a SCADA system, allowing modelling,
%supervising and operating the detector via finite state machines, which also provide easy scalability. 

%The pretrigger system was successfully commissioned during several data taking periods with ALICE using cosmic events and a first circulating proton beam
%at LHC injection energy in September 2008.

The signal as a function of time averaged over all detectors is showing inf Fig. .

%During the cosmic-ray data taking, the TRD was operated with an Ar-CO$_{2}$ gas mixture to replace the more expensive nominal(Xe-based) mixture. 


%During data taking, the TRD require a trigger signal within 500 ns after the interaction.
%This is 1 $\mu$s before the first trigger decision by the ALICE experiment is made. 
%The design of the TRD pretrigger, using 88 analogue input from the V0 and T0 detector
%and interfacing an additional 576 digital input from TOF via an external unit, has been
%developed and is now installed in the L3-magnet of the ALIC experiment. All input information
%from the fast V0 and T0 detectors is processed by 27 FPGAs, which are mounted at a distance of
%1.5 m to 4.5 m around the interaction point. The pretrigger system was successfully commissioned 
%during several data taking periods with ALICE using cosmic events and a first circulating proton beam 
%at LHC injection energy in September 2008. Trigger signals have been provided to the Central
%Trigger Processor of ALICE. The system is highly flexible to account for different physics conditions
%in p+p and Pb+Pb collisions. 
%The completion of the pretrigger system, realized in summer 2008, allowed for a readout of the 
%complete detector signal and enabled for the first time the track reconstruction in real operation.
%The signal as a function of time averaged over all detectors is showing inf Fig. . During the cosmic-ray
%data taking in ALICE in the fall of 2008 a total of about 50k tracks were recorded in the TRD. This was
%made possible due to the successful operation of the TRD Level 1 trigger, allowing a fast and efficient
%track decision a the GTU level. 

The tracking and PID algorithms of the TRD rely on the knowledge of several calibration constant depending
on temperature and pressure, the gas composition and the chamber geometry. These are the drift velocity of the
electrons, the time-offset of the signal, the gas gain and the width of the Pad Response Function. They will
be calibrated using the raw signal of the detector or the signal from reconstructed tracks in p+p and Pb+Pb 
collisions. During the data taking, a first calibration is performed on the online systems, the Data Acquisition (DAQ)
and High Level Trigger (HLT) and later also done in offline. 
With the assumption that the cosmic rays are uniformly distributed over the detector,
a gas gain variation of about 16\% was found over the chambers. Fig.  shows the gain factors as function of
the detector number for the four supermodules operated in ALICE in 2008. 

The drift velocity and time offset are determined with the average signal as function of time shown in Fig. . 
The results obtained onlie were consistent with those obtained offline after the tracking in a second pass calibration.
The extracted drift velocity values had a variation of 3.3\% over the chambers. 



\begin{figure}[ht]
\begin{minipage}[b]{0.5\linewidth}
\centering
\includegraphics[scale=1]{plots/002.pulse_height.eps}
\caption{default}
\label{fig:figure1}
\end{minipage}
\hspace{0.5cm}
\begin{minipage}[b]{0.5\linewidth}
\centering
\includegraphics[scale=0.8]{plots/006.trd_gain_driftvelocity.eps}
\caption{default}
\label{fig:figure2}
\end{minipage}
\end{figure}


%It models and realizes the complex TRD FEE communication architecture in the supervisory control layer. 
%The TRD DCS is realized as a large distributed system scattered over ten computres. It integrated about a quarter million embedded
%processors mounted on the detector chambers that implemet complex on-detector controls with massive use of ethernet
%for both interprocess communication and device control. The DCS monitors about three thousand paraemters read out by the FEE.
%The TRD low and high voltage systems implement all together more than twelve hundred channels that are controlled and monitored 
%independently. All parameters relevant to offline analysis are archived by means of dedicated mechanisms implemented in the
%TRD DCS. The data read from the detector is archived on an ORACLE database cluster. A special emphasis has been put on making the
%TRD DCS graphical user interfaces intuitive and user-friendly for non-export operation. An example is shown in Fig. . 

%The TRD DCS has been commissioned during runs within the ALICE experiment in the cosmic-ray data taking in 2008.

The result for the spatial resolution obtained are summarized in Fig. .


As a part of commissioning, at 2004 and 2007 there were test beam measurement at CERN PS with electron and pion beam.
The likelihood distributions for six layers, based on the total energy deposit in one layer are shown in Fig.  for the
momentum of 2 GeV/c. Cuts of given electron efficiency are imposed on the likelihood value and the pion efficiency, $\pi_{eff}$, 
is calculated. The momentum dependance of pion efficiency calculated with different likelihood method is shown in Fig. . 
It demonstrate that we exceed design goal of factor 100 pion rejection for isolated tracks.


The resulting pion rejection capability was determined as a function of particle momentum.


\begin{figure}[ht]
\begin{minipage}[b]{0.5\linewidth}
\centering
\includegraphics[scale=0.8]{plots/liklihood.eps}
\caption{default}
\label{fig:figure1}
\end{minipage}
\hspace{0.5cm}
\begin{minipage}[b]{0.5\linewidth}
\centering
\includegraphics[scale=0.8]{plots/pion_efficiency.eps}
\caption{default}
\label{fig:figure2}
\end{minipage}
\end{figure}



Eight out of the total 18 supermodules of the TRD will be ready in the ALICE setup when cosmic-ray data taking resumes July 2009
and will contribute to physics results with beams expected to start in fall 2009. The TRD will be completed during the next 
shutdown period.

%% end of main text

\section*{Acknowledgments} % please check/modify, comment out or delete if not needed
This is where one places acknowledgments for funding bodies etc., if needed.
For the large collaborations, this is listed once and for all, together with 
the author lists etc. in the proceedings back-material.


\begin{thebibliography}{00} % do not change 
   
\bibitem{alice_ppr_vol2} Alice Physics Performance Report VolumeII, ALICE Collaboration et al., {\it J. Phys. G: Nucl. Part. Phys.} {\bf 32} (2006)
1295-2040
 
\bibitem{SHM_model} A. Andronic, P. Braun-Munzinger, K. Redlich, J. Stachel, [arXiv:0805.4781v1] 

\bibitem{star_elec} B. I. Abelev et al., {\it Phys. Rev. Lett.} {\bf 98} (2007)
192301.

\bibitem{c_to_b} A. Adare et al., [arXiv:0903.4851]


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