1 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 on the right is an illustration of double layer STAR BEMC SMD. Two independent wire layers, separated by aluminium extrusion, image electromagnetic showers in the  and  directions on corresponding pad layers. BSMD BEMC module

2 Some SMD QA pictures from AuAu 2004 Fig.1 Number of hits in eta plane vs number of hits in phi plane (sum).Last year’s problem was fixed, now we have 1.5 sigma cut, no fixed energy cuts anymore. Fig.2 Energy sum of eta plane vs energy sum of phi plane. Eta plane is higher than Phi by factor of 1.2. That’s because of some material (aluminium) located in front of phi plane. Fig.3 Energy sum corellation between BEMC and SMD planes. Eta plane is off by the factor of 0.81, and Phi plane is off by the factor of So we need to correct SMD calibration! Fig.4 Overall pedestal position distribution. This year we do not have two distinctive peaks, so even if we shift pedestals +1 like last year, we will not have such hit losses as last year.

3 e-e- SMD plane h There is no hit directly under the track in most cases Electromagnetic shower VS hadronic shower in SMD SMD plane The hadronic shower is observed as a number of fragments scattered within the corresponded plane. These fragments might be as narrow as electrons, so they can be easily misidentified as an electromagnetic. So, there is no distinctive hadronic shower in SMD. It makes e/h separation based on shower shape somewhat difficult. The electromagnetic shower has the gaussian shape(separate planes) or it could be fitted with two exponents. 80% of shower energy is deposited in 2-3 strips MC profile of the shower pedestal

4 Number of Hits in Eta, Phi SMD planes cut of N hits >= 2 in both planes (+200MeV minimum energy deposition) gives us R f =14, keeping ~80% efficiency (Pt dependant, pseudorapidity dependant) Fig.5Fig.6

5 e-e- SMD plane h pedestal Electromagnetic shower VS hadronic shower in SMD after nHits cut After nHits cut we see that electromagnetic shower and hadronic shower look quite similar after the cut. Assuming 100% error on energy deposition, situation looks even worse. Anyway, shower shape separation is possible, but one needs to do the following studies: - pseudorapidity dependence of shower shape, whether it is simple RMS or gaussian fit [SMD strips have different width for pseudorapidity region and ] - Pt dependence. With the growing Pt, electromagnetic showers become narrower and narrower. Estimated amount of electrons needed to build the profile is around 10k electrons for each Pt bin. That is not available from dAu run, should be done from AuAu dataset as early as possible. This is a study of single pT bin ( 2.5 GeV < pT < 3.5 GeV) from dAu dataset. We fitted 5 SMD strips in both planes under the track with gaussian, taking gaussian sigma as a discrimination parameter. On average, hadrons are wider than electrons. On the fig.7 you can see the relative electron-finding efficiency for this gaussian sigma cut versus rejection factor gained. This method provides nice rejection factor if efficiency is not an issue. Fig.7 Shower position: Shower shape:

6 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. 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 (this pictures are from HT-triggered data, in pure Minimum Bias everything looks worse). 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. Fig.9 Fig.8 Fig.10

7 Figure 11. 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 12. 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. SMD contribution to electron identification

8 Electron analysis: another way to correct for conversion electrons Energy-sharing assymetry from MC as prediction vs measured asymmetry from tagged conversion pairs. Agrees quite well with embedding data, conversion electrons from measured Pi0 spectra next in line… Fig.15 SVT image from tagged conversions (around 2k) Fig.16 SVT image from MC conversions, for comparison 75k conversion pairs Fig.13Fig.14

9 Fig.19 SVT image from tagged conversions (around 2k) Fig. 20 SVT image from MC conversions, for comparison 75k conversion pairs Fig.17 Conversion angle, degrees Electron analysis: more plots Fig.18

10 Parallel electron analysis : Barrel & Barrel Fresh results and TODO lists Fig.20 Black triangles are D&J electrons from dAu (MinBias only!), red dots are current Alex Suaide’s electrons from dAu (different from QM!) Electron cross-section changed with new production. Now both cross-sections (Barrel vs. Barrel) agree quite well. PPL-STAR group TODO list:  finish calculation of conversion electrons from PHENIX   (or STAR, when available)  check Trigger Bias studies again and compare it to Alex when his new points will be available  finish systematic errors studies ASAP. SMD-related TODO list. We need around 1M AuAu HT triggered data to finish SMD behavior studies:  SMD calibration issues. Equalization and final calibration check is needed.  Profiles for electrons, especially for high Pt’s. This should increase e/h rejection power by a factor 1.5 – 3.0 (making overall BEMC rejection power ~ !)

11 Preliminary conversions using PHENIX pi0s [ Alternative to existing methods ] Matching between measured conversion spectra and pi0 conversion electrons is good!