1. Abstract Abdul-Salam 5, D. Arkhipkin 2, S. Chatopadhyay 4, T.M. Cormier 5, W. Dong 1, S. Guertin 1, M.M. de Moura 3, A. Pavlinov 5, A. Stolpovsky 5, A. A. P. Suaide 3, D. Thein 1, S. Trentalange 1, O. Tsai 1, C. Whitten 1, Y. Zoulkharneeva 2, for the STAR Collaboration The STAR Barrel electromagnetic calorimeter was installed at the 50% level in the 2003 d + Au and p+p RHIC run at √SNN = 200GeV. The acceptance of the calorimeter was 0 <  < 1.0 with full azimuthal coverage. For these runs, the instrumented detector consisted of 2400 Pb/Scintillator sampling calorimeter towers each spanning  x  =0.05 x 0.05 and two layers of gaseous shower maximum detector (SMD) located at a depth of ~ 5 radiation lengths within the sampling layers. Strip readout of the SMD with 18,000 channels in the  and  directions provided the location and spatial distribution of showers with high resolution. Several million minimum bias and high tower triggered events have been analyzed using TPC tracking and dE/dx identification to create samples of electron and hadron candidates that have been used to study the calorimeter performance. In particular, in the present work, we focus on a comparison of the measured and simulated characteristics of electromagnetic and hadronic showers in the SMD and calorimeter towers with particular emphasis on the contribution of the SMD to electron-hadron discrimination, photon and  o reconstruction. 1. University of California, Los Angeles, 2. Particle Physics Laboratory, JINR, Dubna, 3. University of Sao Paulo, 4. Variable Energy Cyclotron Centre, Kolkata, 5. Wayne State University Performance of the STAR Barrel Electromagnetic Calorimeter Electromagnetic Calorimeter  0 reconstruction SMD contribution to electron identification To evaluate SMD contribution to electron identification we used following cuts to get an initial electron and hadron candidate samples: * Electron candidates ● track momentum p > 1.5 GeV ● DCA to the primary vertex < 1.5 cm ● track fit points > 20 ● Zvertex < 50 cm ● -1.65 < nSigmaElectron < 1.65 * Hadron candidates ● track momentum p > 1.5 GeV ● DCA to the primary vertex < 1.5 cm ● track fit points > 20 ● Zvertex < 50 cm ● -2.0 < nSigmaPiMinus < 2.0 Applying SMD cuts to the sample of hadron candidates allows us to evaluate SMD hadron rejection power. Figure 21. On this plot you can see relative SMD electron registration efficiency. This was done by demanding at least 2 SMD hits in both SMD planes directly under the projected track. Also deposition of at least 200 Mev was requred. SMD efficiency was studied on a part of electron sample selected using a narrow cut on dEdx in a “safe” region (TPC) + BEMC cut applied. Figure 22. Black dots show us TPC electron/hadron rejection power versus electron registration efficiency. It is easy to notice that BEMC+SMD combined power is a very good addition to TPC e/h rejection power. It adds factor of ~200 to basic TPC e/h rejection, which is very important at high Pt, where dEdx rejection power is getting smaller. Figure 23. This is a purity of obtained electron sample versus transverse momentum (after all TPC+BEMC+SMD cuts). Purity plotted on this figure is the percentage of electrons in electron- candidate sample after all cuts. It is very important characteristic for semileptonic decays of charm & bottom studies. BEMC + SMD usage allows us to get very pure electron samples. Figures 24-25-26. Typical example of electron-candidate sample “evolution” during analysis. On the first plot one can see dEdx plot of 2 GeV tracks (no EMC cuts). Electron peak is clearly seen, but it is obvious that it is heavily contaminated by hadrons. On the second plot we applied BEMC cut of 0.5 < E/p < 1.5, keeping 95% efficiency. Hadronic peak lost one order of magnitude, electron peak stays the same in height, but it is seen now much more clearly. Third plot is the result of SMD cuts work. Now we see that hadronic background is small (comparing to electron peak) and electrons can be clearly indentified. From our MC studies we found that we have following SMD characteristics as a powerful e/h rejection tools : ● Number of SMD hits directly under the projected track (both planes) ● Sigma Region / sigma Shower ratio ( is used to discard wide showers / noisy / problematic areas) Overall rejection factor obtained from MC is ~200 (Pt dependent). We studied 11M dAuCombined STAR data with EMC present, and found that real rejection power is consistent with MC. Rf real = 206! BEMC High Tower triggers In order to enhance high-pt range of the spectra, the EMC provides the a high-pt level 0 trigger in STAR. Trigger signals generated at level 0 are : ● Patch (patch consists of 4x4 towers): this corresponds to the sum of all towers in a 0.2x0.2 ( ,  ) patch ● High tower: this gives the digitized signals for the towers with largest energy deposition in the patch. ● Jet patch : 1.0 unit in  and 1.0 unit in  directions respectively. Figure 11. This plot shows the effect of HighTower-1 trigger on electron yield in dAu collisions. Factor of ~100 improvement at 2.5+Gev/c is clearly indicated. Figure 12. This picture shows the increase of electron yield for HighTower-2 trigger (ratio between HT2/HT1) in dAu collisions. HT2 trigger increases electron yield at 4.5+ GeV/c by the factor of ~100-1000. Figure 13. Plot above shows the overall EMC cluster energy yield for different trigger types in dAu collisions. High-pt triggers enrich EMC energy points by the factor of 20 – 40. BEMC calibration and status 1. Tower calibration ( MIP calibration ) 2. Electron-based gain correction for dAu data (Y2003) The tower calibration is based on single tower MIP spectrum fits. It was analysed 3.5 M dAuCombined events. The MIP candidates are selected if they satisfy following conditions: ● P > 1.2 GeV/c ● Number of TPC space points > 20 ● Projection in the inner and outer calorimeter radius is in the same tower ● Only one projected track in tower ● Track is isolated in 3x3 tower patch Mip peak is fitted with two gaussians (one for peak, other for background). Here you can see typical MIP spectra. After the new MIP peak calibration was done it was possible to do a GLOBAL electron calibration to correct the global tower gain due to systematics on MIP calibration. This figure shows the electron energy as a function of TPC momentum. One can see that the detector response is very linear up to 8 GeV/c (Figure 6). The global gain correction extracted from a linear fit from this plot is 1.064. To double check the gain correction we plotted the p/E peak position as a function of a distance to the center of a tower (figure 7). Geant simulations of EMC for electrons in this momentum range are also shown (red line). One can see that agreement is quite good. In order to acquire the desired quality on EMC physics results it is very important to calibrate the detector properly. The sensitivity and precision of the measurement depends on the way the calibration is done. We used following method to make Barrel EMC calibration : ● Minimum Ionisation Particle (MIP) response ● Electron gain correction The use of the MIP spectra for the EMC calibration comes from the fact that many charged hadrons are produced at every collision at RHIC. When striking the EMC, a significant fraction (~30-40%) of high energy charged hadrons do not deposit a large amount of energy via nuclear interactions, instead depositing ~250-350 MeV of electron equivalent energy in the calorimeter due largely to electromagnetic ionization. MIP deposited energy varies because the depth of the towers change with  (towers are projectile to the center of STAR). On Figure 5 you can see the momentum distribution of the electron-candidate versus EMC energy (single tower measurement). Electron peak position is a function of distance to the center of tower and electron Pt (due to the energy leakage to neighbouring towers).  0 The STAR EMC was constructed to allow reconstruction of moderate to high Pt  0 's over a solid angle, corresponding to full TPC tracking.  0 Figure 8. Invariant mass plot for Pt [1.5..2.5]. 10M dAuCombined events processed.  0 peak is clearly seen. Figure 9. Same plot, additional requirement of phi/eta width > 0 added. Overview BEMC ● 0 <  < 1.0 ● full asimutal coverage ● 60 modules – ( ,  ) module ~ (1.0, 0.1) – 40 towers/module ●21 X 0 ●( ,  ) tower ~ (0.05, 0.05) SMD ● 60 modules ● 150 strips in  and 150 strips in  per module ● (   ) ~ (0.007, 0.007) What was installed during last run : which provides a broad capabilities for search and investigation of QGP (gammas, e +, e -, and neutral systems) Plays the crucial part when polarized program of the STAR experiment will be implementing ( direct photons, W- and Z-bosons, et all Barrel EMC is one of the extremely important components of the STAR detector: The Barrel Electromagnetic Calorimeter, (EMC), is located inside the aluminium coil of the STAR solenoid and covers |  | ≤ 1.0 and 2  in azimuth, thus matching the acceptance for full TPC tracking. The front face of the calorimeter is at radius of  220 cm from and parallel to beam axis. The design for the Barrel Electromagnetic Calorimeter includes a total of 120 calorimeter modules, each subtending 6 o in  (~0.1 radian) and 1.0 unit in  The calorimeter is a sampling calorimeter, and the core of each module consists of a lead-scintillator stack and shower maximum detectors situated approximately 5 radiation lengths from the front of the stack. There are 20 layers of 5 mm thick lead, 19 layers of 5 mm thick scintillator and 2 layers of 6 mm thick scintillator (used in preshower portion of the detector). Figure 1 - Figure 2 – Cross sectional view of the STAR detector Figure 3 – Side view of the STAR EMC module SMD performance overview Figure 15. This figure shows the example of SMD pedestal position vs time (dAu run). Most of the strips are stable within 1 adc count. Figure 16. This picture shows average number of hits generated by SMD for dAu collisions (3xsigma pedestal cut). Red curve is a number of hits distibution for the phi plane, black curve is a distibution for the eta plane. Figure 17. Here is the number of hits corellation between two SMD planes. This plot proves corellation between  and  plane, which is a good sign of working SMD. It means that both planes generate roughly equal number of hits for each event, without systematic deviations. Figures 18-19. These two plots show us SMD uniformity for eta and phi plane correspondingly. Figure 20. This plot is the first check for SMD calibration (SMD ETA plane example). SMD is not intended for absolute calibration, but first results show that our calibration guess is correct. Ratio of electron energy deposition versus electron- candidate track momentum shows a good peak at 1.0. It will be studied in details during AuAu run, Pt and eta dependence will be checked.. Summary : SMD is fully ready to use. No significant hardware problems or noise was found. SMD calibration and gain studies will be continued using electron sample from full dAu statistics. Shower maximum detector (SMD) is a wire proportional counter – strip readout detector using gas amplification. SMD is used to provide a spatial resolution in a calorimeter which has segmentation (towers) significantly larger than an electromagnetic shower size. While the BEMC towers provide precise energy measurements for isolated electromagnetic showers, the high spatial resolution provided by the SMD is essential for  o reconstruction, direct  identification, and electron identification. Information on shower position, shape, and from the signal amplitude the electromagnetic shower longitudal development, are provided. Figure 14 on the left is an illustration of double layer STAR BEMC SMD to the left. Two independent wire layers, separated by aluminium extrusion, image electromagnetic showers in the  and  directions on corresponding pad layers. We describe SMD characteristics were studied for the Y2003 run. Electron statistics from dAu run was not enough to perform complete studies of SMD equalization and gain in different eta bins and at different Pt, so work is still in progress. But at this point no dramatic difference from expected results was found. Figure 14 - Schematic illustration of double layer STAR SMD. Figure 7. Figure 5. Figure 6. Figure 4. Typical MIP spectrum Current BEMC resolution achieved is : 16/  E Figure 10 – The relative neutral pion yield for d- Au Collisions at  s nn =200 GeV measured by STAR at RHIC. In figure 10 the relative neutral pion yield for d-Au Collisions at   s nn =200 GeV measured by STAR at RHIC is shown. The red points are our previously measured charged hadron cross section (h+ + h-)/2. The neutral pion per event yield has been scaled to the charged hadron to show the agreement in pT dependence between the two data sets. The pion data was taken with three different triggers: minBias (same trigger as for the TPC data), High Tower 1 (Emc energy threshold of 2.4 GeV), High Tower 2 (threshold 4.8 Gev). Corrections were applied for each data set, by simulating pion decay and reconstruction, including realistic detector parameters and electronic effects such as photomultiplier tube gain variations, trigger threshold variations and position of collision vertex. STAR preliminary