# Presentation: 742920

## Study of Charged Hadrons in Au-Au Collisions at                    with the PHENIX Time Expansion Chamber Dmitri Kotchetkov for the PHENIX CollaborationDepartment of Physics, University of California at RiversideRiverside, California 92521 U.S.A.Electronic address: kotchet@citrus.ucr.edu

**                                      ****Introduction**
-           In the summer of 2000 the PHENIX detector recorded data from
- relativistic Au+Au collisions at the center of mass energy of 130 GeV.
- Study of transverse momentum (pt) spectra of created charged hadrons
- provides insight on state conditions after the collisions. The PHENIX
- Collaboration reported results of pt spectra of charged hadrons suggesting
- suppression of hadrons with large transverse momentum in central
- Au+Au collisions [1]. In the performed analysis the PHENIX Drift
- Chamber (DC) was used as a primary device for track reconstruction
- and momentum measurements. The Time Expansion Chamber (TEC) is
- another PHENIX subsystem capable of providing tracking and transverse
- momentum information. During the RHIC physics run in 2000 the TEC
- recorded data sufficient enough to perform independent analysis of pt
- spectra of charged hadrons. It is reasonable to obtain results of pt
- distributions employing tracks reconstructed only in the TEC and
- ignoring tracking information from the DC. By doing this, one can check
- correctness of track reconstruction in the DC and pt spectra of the
- PHENIX published results from the physics run of 2000. On the other
- hand, one can verify how successfully the TEC functioned as a tracking
- device in respect to DC performance.
**               ****The PHENIX Time Expansion Chamber**
-           The Time Expansion Chamber is a sub-detector located in the East
- Central Arm of the PHENIX at the distance from the collision vertex
- approximately R = 410-457 cm. It covers 900 of the PHENIX azimuthal
- angle and 0.35 units of pseudorapidity.
-             Figure 1. Passage of a charged track from the collision vertex
-                  through the PHENIX East Central Arm (view from top).
-           The TEC is arranged in 6-plane 4 sectors and has 64,080 wires and
- 20,480 readout channels making it an excellent tracking device. In 2000
- the TEC had only two middle sectors with 4 planes instrumented
- electronically. One of the sectors is used in analysis of charged hadrons.
- The TEC is capable of measuring only transverse momenta of charged
- hadrons. For track reconstruction and pt measurement the following
- algorithm is applied. On all 4 planes hits of charged particles are found.
- Two angles are measured: azimuthal angle  and inclination angle . Hit
- points of the same track have the same  and . A peak in -. space
- indicates the track. The transverse momentum is computed as a function
- of as
-                                               pt=1/(21.5).
-                           Figure 2. Track reconstruction in the TEC.
-                              Figure 3. TEC configuration in 2000.

- Sector 2
- (4 planes)

- Sector 1
- (4 planes)

- Data taken

- Data in analysis

**                                          ****Analysis**
-           The Time Expansion Chamber provides tracking information and
- the value of transverse momentum of a charged particle. The TEC tracks
- need to be associated with particles’ hits in some other subsystems
- located in the East Arm of the PHENIX detector: the Pad Chamber 1
- (PC1), the Pad Chamber 3 (PC3), the Time of Flight Hodoscope (TOF),
- and the Lead Glass Electromagnetic Calorimeter (EMC).
-             Figure 4. The PHENIX East Arm. The TEC and subsystems
-                       that provide complimentary tracking information.
-           Positive and negative hadrons are analyzed with the following
- centrality selections: 1) 0-5%; 2) 5-15%; 3) 15-30%; 4) 30-60%;
- 5) 60-80%; 6) 80-92%; and 7) 0-10%. The available statistics is as
- follows:
**                           ****Centrality         Events         Tracks**
-                              Min bias          1.3M             10.6M
-                              0-5%                81K              2.6M
-                              5-15%              147K            3.2M
-                              15-30%            202K            2.6M
-                              30-60%            400K            2.0M
-                              60-80%            301K            275K
-                              80-92%            148K            40K
-                              0-10%              155K            4.3M
-           The analysis procedure includes:
**A) Track association cuts. **Closest distances between TEC tracks and
- associated hits in PC1, PC3, TOF, and EMC are computed. Also, the
- distances between the projections of tracks on the plane perpendicular to
- the beam line and the collision vertex are found. 3- cuts are applied
- to resulting distributions to select tracks. (2- cut is used to estimate
- systematic errors due to track association).
**B) Acceptance.** Subsystems’ dead areas that result in losing tracking
- information are found, as well as the range of the azimuthal angle and
- pseudorapidity. (Changing this range allow to estimate systematic
- errors due to acceptance correction).
**C) “Unphysical”** background subtraction. The background due to
- accidental TEC track matching with other subsystems’ hits (“ghost
- tracks”) is determined by changing the sign of the Z coordinates (which
- are the coordinates along the beam line) of the subsystem’s hits and
- performing search for closest distances between the TEC tracks and the
- “flipped” hits. The resulting distributions are subtracted from track
- association distributions.
**D) Study of track splitting and merging.** These effects are estimated by
- comparing the angular differences between two TEC tracks from
- mixed events and two TEC tracks from the same events. It is found
- that effects of track splitting and merging are negligible in the TEC.
-           The following corrections are applied to the resulting pt spectra:
**1) Tracking efficiency.** This efficiency is found through the procedure of
- merging one simulated track into data. Then, the efficiency is the ratio of
- the number of reconstructed later merged tracks to the total number of
- merged tracks. The efficiency is a function of the number of tracks in the
- TEC and changes from approximately 90% for 3-5 tracks to 35-40% for
- 60-80 tracks.
**2) Association cut efficiency**. Association cuts are applied on all
- subsystems used in the analysis except for the cut on the vertex. Then,
- the distribution of the vertex association is found (“cut distribution”).
- The same distribution is made when there are no association cuts applied
- to other subsystems (“uncut distribution”). The ratio of “cut distribution”
- to “uncut distribution” gives the association cut efficiency.
**3) Correction due to finite momentum resolution.** In 2000 TEC
- momentum resolution was 4.8% + 5.5%pt . To make the correction, one
- has to fit the measured pt distribution with exponential function, then
- smear it with 4.8% + 5.5%pt , and fit it again to find the correction
- function.
**4) Surviving probability.** The correction to decay processes is
- approximately 97% at pt = 0.5 GeV and becomes negligible with increase
- of transverse momentum.
**5) Physical background.** The PHENIX Simulation Group produced 100K
- Hijing minimum bias events. The correction due to physical background
- is computed by finding ratio of the number of primary simulated particles
- found in the TEC to the total number of simulated particles found in the
- TEC. The systematic error due to physical background is estimated
- through the ratio of simulated particles to data particles for each
- momentum bin.

**                                      ****Final results**
-           After applying all necessary corrections, final results of transverse
- momentum distributions of charged hadrons are obtained. Figure 5
- shows pt  spectra for all centralities studied in the analysis. Only
- centrality-dependent systematic errors are included in error bars. Figures
- 6 and 7 show pt spectra obtained in the TEC in comparison with the
- PHENIX published results from the Drift Chamber charged tracks.
- Statistical errors only are included in error bars of pt  spectra from the
- Drift Chamber and the Time Expansion Chamber tracks. Systematic error
- bands are shown for the  TEC data. The results of transverse momentum
- spectra from two PHENIX subsystems are in good agreement.
-            Figure 5. Spectra of charged hadrons reconstructed from
-            TEC tracks for different centralities. Statistical and centrality-
-            dependent systematic errors are shown.
-            Figure 6. Spectra of charged hadrons for 0-10% central
-            events reconstructed from TEC tracks in comparison with the
-            Drift Chamber data. Systematic error bands are shown for
-            the TEC data.
-            Figure 7. Spectra of charged hadrons for 60-80% peripheral
-            events reconstructed from TEC tracks in comparison with the
-            Drift Chamber data. Systematic error bands are shown for
-            the TEC data.
**	****         ****Acknowledgements**
-           Author would like to thank Alexander Lebedev and Marzia Rosati
- from Iowa State University (Ames, IA, USA) for valuable suggestions
- and discussions, as well as for letting use some of results of their own
- study. Author also would like to thank Richard Seto from University of
- California at Riverside (Riverside, CA, USA) for guidance and
- advisement through this analysis.
- 	                     **Reference**
- 1. K. Adcox et al., “Suppression of Hadrons with Large Transverse Momentum in Central
- Au + Au Collisions at the C.M. Energy of 130 AGeV“, Phys. Rev. Lett., 88, 022301 (2002),
- nucl-ex/ 0109003.

- PC1

- TEC

- PC3

- TOF

- EMC

- 0-5%
- 5-15%
- 15-30%
- 30-60%
- 60-80%
- 80-92%

- PHENIX Preliminary

- Drift chamber
- (nucl-ex/0109003)
- Time Expansion Chamber

- 0-10% central events

- PHENIX Preliminary

- Drift chamber
- (nucl-ex/0109003)
- Time Expansion Chamber

- 60-80% peripheral events

- PHENIX Preliminary

- (h++h-)/2

- (h++h-)/2

- (h++h-)/2