Analysis results of the rst combined test of the LArgon and. TILECAL barrel calorimeter prototypes. M. Cavalli- Sforza, I. Efthymiopoulos, F.

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1 ATLAS Internal Note TILECAL-No-67 December 1995 Analysis results of the rst combined test of the LArgon and TILECAL barrel calorimeter prototypes M. Cobal, M. Nessi CERN, Geneva, Switzerland M. Cavalli- Sforza, I. Efthymiopoulos, F. Teubert Institut de Fisica d'altes Energies, Universitat Autonoma de Barcelona, Spain S. Nemecek Institute of Physics of ASCR, Praha, Czech Republic A. Gomes, A. Henriques LIP and Univ. of Lisbon, Portugal D. Costanzo, B. Di Girolamo, E. Mazzoni Pisa University and INFN, Pisa, Italy I. Vichou LAL, Orsay, France B. Lund-Jensen Stockholm University, Sweden Abstract The combined electromagnetic and hadronic prototype calorimeters response to pions of various energies (20, 50, 100, 150, 200 and 300 GeV) at an incident angle of 11:3 0 has been investigated. As a rst step, the energy released in the prototype has been reconstructed using a minimal set of corrections introduced to take into account various detector eects. The resulting e/ ratio has been studied. Then, to improve the performance, a more sophisticated correction algorithm has been developed, based on dierent weights assigned to the calorimeter longitudinal samplings. This procedure is similar to what has been done for the analysis of the standalone data [2]. Detector energy resolution and linearity have been calculated and the results obtained with the two techniques are compared. The transverse and longitudinal pion shower developments have been examined, as well as the longitudinal leakage. Finally, the response to muons has been studied, and the noise has been evaluated for both the detectors.

2 Contents 1 Test Beam Setup 2 2 Pion response Data Sample : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : Energy reconstruction : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : Energy reconstruction using a simple approach as a benchmark. : : : : : : : : : : Energy lost in the cryostat : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : Energy dependence of the pion response : : : : : : : : : : : : : : : : : : : : : : : : Energy reconstruction using a more sophisticated approach. The weighting technique. : : 8 3 Lateral Shower Proles 14 4 Longitudinal Shower Proles 14 5 Muon Response at 300 GeV Muon Response in the electromagnetic calorimeter : : : : : : : : : : : : : : : : : : : : : : Muon Response in the Hadron calorimeter : : : : : : : : : : : : : : : : : : : : : : : : : : : Combined response : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 21 6 Noise evaluation 23 7 Conclusions 32 1

3 1 Test Beam Setup The ATLAS detector [1] will include a barrel liquid argon (LAr) electromagnetic calorimeter, using the accordion geometry and a large scintillating tile hadronic barrel calorimeter, based on a sampling technique using steel absorber material and scintillating plates read out by wavelength shifting bers. The hadron calorimeter [2, 3, 4] will be segmented into three layers, approximately 1.5, 4.2 and 1.9 at =0. This calorimeter layout has been already successfully proven in beam tests as part of the RD34 project [2]. The Tile prototype consists of ve 1 m long modules, each one spanning 2=64 in azimuth, with a front face of cm 2. The radial depth is 180 cm, going from an inner radius of 200 cm to an outer radius of 380 cm; it corresponds to about 9 interaction lengths. The calorimeter is radially segmented into four depth segments by grouping bers from dierent tiles. The segmentation is ' 0:1 0:1. For the rst time, in September 1994, this prototype has been tested together with the prototype of the ATLAS barrel em calorimeter. The Tile calorimeter has been placed on a xed table, just behind the LAr Accordion cryostat. Both the cryostat and the tile calorimeter have been tilted with respect to the beam axis by 11:3 o to ensure full containment at least in the electromagnetic part (see Fig.1). For the electromagnetic (em) section the large 2 m prototype [5] of RD3 has been used, located Cryostat EM Accordion Tilecal 20 GeV 300 GeV µ, e, π Θ = 11.3 S3-4 BC m Figure 1: test beam setup for the combined LAr and tile calorimeter combined run close to the back of the cryostat to reduce the distance between the two active devices. In front of the Accordion, inside the cryostat, in the liquid, a tube of extruded material has been placed to minimize as far as possible the amount of interactions in front of the em calorimeter. The electromagnetic calorimeter prototype consists of a stack of three azimuthal modules covering 27 in. The segmentation is ' 0:018 0:02 for the rst two compartments (9 X o in depth each) and ' 0:036 in the last one (7 X o ). The calorimeter, constructed with a fully projective geometry, is equipped with dierent types of preampliers (Si, GaAs and 0T), followed by shaping ampliers (t p () ' 20 ns), Track&Hold circuits, and 12-bit ADCs. For this layout the eective distance between the two active parts of the detector is of the order of 50 cm, instead of the 25 cm as foreseen in the ATLAS setup. The amount of material has been quantied to be about 2X o in between the two calorimeters. This value is similar to the ATLAS design value, but the material type is dierent: steel instead of aluminum for the cryostat. Figure 2 shows the material between the two calorimeters. 2

4 Figure 2: Material in between the em and hadronic calorimeters during the combined run test beam. The total depth corresponds to about 10.1, to be compared with the 9.6 foreseen in the ATLAS setup [1]. The eective calorimeter depth for pions travelling at a angle of 11.3 o is of A large scintillator wall (\muon wall") covering about 1 m 2 of surface has been placed on the back of the calorimeter to quantify leakage. For this run, in the em section, only a matrix of cells around the nominal beam position was read out, covering a total surface of about 25 cm 25 cm. This choice, as will be shown later, was limiting the transversal containment of the hadronic showers. Beam elements and beam chambers in front of the cryostat are used to dene the beam and its quality. The energy of the beam (pions, electrons and muons) has been varied from 20 to 300 GeV. 3

5 2 Pion response 2.1 Data Sample Pion events of dierent energies (20, 50, 100, 150, 300 GeV) are used in this analysis. On the average about 10 5 events are available for each energy value. The runs have been taken as close as possible in time to avoid changes in the setup or time variations. Analysis cuts are applied in order to reduce the number of events which had an interaction in the material before the electromagnetic calorimeter. To this purpose the beam elements installed in front of the cryostat (3 beam x-y chambers) and the information from the LArgon preshower are used. The cuts applied depend in value from the beam energy (since the beam is not stable). Here we report the selection applied in the case of the 300 GeV runs: The trigger bit for physics events is required: rndm=0.and.tiltrig=1 The signal in the beam scintillator should be compatible with that of a MIP: s2.gt.250.and.s2.lt.500 Tracks with large angles with respect to the beam direction are removed: (bc2_x-bc3_x).gt.-1.and.(bc2_x-bc3_x).lt.4 (bc2_y-bc3_y).gt.-2.and.(bc2_y-bc3_y).lt.2 bc3_x.gt.-20.and.bc1_x.gt.-50 The energy in the preshower is required to be compatible with a MIP: E_pscls1.gt.-0.1.AND.E_pscls1.lt.0.1 E_pscls2.gt AND.E_pscls2.lt In gure 3 the beam spot in the preshower and in the rst calorimeter sampling is shown before and after the cuts, as well as the energy in the two preshower layers. In gure 4 the raw energy from the two calorimeters is shown for the 300 GeV pion beam after all the cuts have been applied. To reject the muon signal in the pion runs, a cut in the total energy was used and the muon wall energy deposition was checked too. 2.2 Energy reconstruction In the case of the combined test beam data, two peculiar eects contribute to degrade the energy resolution: 1. The presence of the cryostat between the electromagnetic and hadronic calorimeters. The cryostat is made by passive material in which a certain amount of energy is lost. 2. The non-compensating nature of both the electromagnetic and hadronic calorimeters, which makes the e/ ratio dierent from 1. To properly correct the measured energy, we developed two dierent techniques. The rst one consists of a simple approach which aims to use a minimal set of corrections, related to the specic detector eects. The second one is more sophisticated. 4

6 Figure 3: The beam spot in the preshower (up) and in the rst calorimeter sampling (middle), before (left) and after (right) applying the cuts. In the two bottom plots, the energy in the two preshower layers is shown. The lines de ne the selected area after the cuts Energy reconstruction using a simple approach as a benchmark. We followed the steps listed below: 1. Find an intercalibration constant a between the electromagnetic and the hadronic calorimeter by minimizing the resolution for the total energy, which is expressed as: 0 = a Ehad + Eem Etot (1) 2. Determine the energy lost in the cryostat (Ecryo ). The correction consists of a constant c multiplied by the geometrical mean between the energy released in the last electromagnetic and the rst hadronic samplings. p Ecryo = c Ehad1 Eem3 5 (2)

7 Figure 4: The raw electromagnetic calorimeter energy (in GeV) vs the raw hadron calorimeter energy (in pcb) for the case of 300 GeV pions, after all the analysis cuts have been applied. The muon events are clearly seen in the bottom left side of the plot. This particular expression was chosen after Montecarlo studies. The value of the constant c is determined such as to minimize the resolution of the total energy, now expressed as: E 00 tot = E 0 tot + c E cryo (3) 3. Introduce a quadratic correction for the em section to get the response independent from the energy released in this compartment: Finally, the total reconstructed energy is expressed as: E 000 tot = E 00 tot + b E 2 em (4) E tot = a E had + E em + b E 2 em + c p a Ehad1 E em3 (5) with a = 0:17, b =?0:00038, c = 0:44. As a rst approach these 3 parameters are taken as energy independent, calculated for the 300 GeV case. Fig. 5 shows a distribution of the total energy versus the electromagnetic one for the various correction steps, in the case of 300 GeV pions Energy lost in the cryostat Fig. 6a shows the distribution of the estimated energy lost in the 0.1 depth of the cryostat, normalized to the nominal pion beam energy as a function of the fraction of energy released in 6

8 Figure 5: Total energy vs electromagnetic energy for the various steps of the total energy reconstruction. the electromagnetic compartment. The shape of the proles for the two extreme values of energy is similar and both peak at around 20 %. Fig. 6b shows the percentage of the total energy lost in the cryostat for the various energies under study. The value is about 4 %, almost constant for dierent incident beam energies Energy dependence of the pion response The reconstructed energy distributions for all the investigated energies are shown in Fig. 7. Peak and values are extracted with a Gaussian t over a 2 range. Low energy tails appear, mostly due to events which suer from an incomplete lateral or longitudinal containment of the shower. Fig. 8 shows how the low energy tails at 300 GeV are reduced once all the events which leave a signal in the muon wall, are rejected. This shows that after 10.3 of calorimeter length there is still longitudinal leakage. More detailed punch-trough studies have been performed in Chapter 5. In the 20 GeV pions case, some low energy tail survives. This eect is mostly due to pathological events, beam contamination or material in front of the prototype. Table 1 summarizes the mean, sigma, and energy resolution for the various beam energies. The dierence between the corrected mean energy value (E meas ) and the nominal beam energies (E beam ) is plotted in gure 11 as a function of the energy. The t to the distribution intercepts the y axis at 1.9 GeV (black dots). This oset can be considered as an additional correction parameter independent from energy. To calculate the energy resolution it has been summed to the mean energy values of table 1. It can be seen from table 1 or gure 7 that the benchmark reconstructed energy is about 10% lower than the beam energy. This is due to the fact that, as shown in gure 5, the hadronic and electromagnetic calorimeters have been intercalibrated by taking as a reference point the case where the measured energy is released entirely in the hadronic compartment. There is about a 10% dierence in the measured energy between this case, and the case where the energy is entirely released in the electromagnetic compartment. This dierence reect the e/ value. The e/ ratio as a function of energy is given in Fig. 9. For this calculation electrons of the 7

9 Figure 6: (a) Distribution of the estimated energy lost in the cryostat normalized to the nominal pion beam energy as a function of the fraction of energy released in the electromagnetic compartment. (b) Percentage of the total energy lost in the cryostat for the various energies under study. Energy (GeV) (GeV) (GeV) +of f 20 16:1 0:1 3:3 0:1 18:3 0: :0 0:1 4:8 0:1 10:9 0: :9 0:2 6:7 0:1 7:5 0: :6 0:6 8:5 0:6 6:4 0: :4 0:2 10:2 0:1 5:66 0: :6 0:1 13:2 0:1 4:93 0:04 Table 1: Mean energy, and energy resolution for the various beam energy, using the benchmark approach. The energy resolution has been calculated taking into account the mean energy oset of 1.9 GeV. same energies were available. The mean values listed in table 1 have been used, not taking into account the oset correction. The ratios are in the range of 1.25 to Energy reconstruction using a more sophisticated approach. The weighting technique. As previously said, a more sophisticated approach to correct the energy response of our prototype has been developed. Experience from previous calorimeter [2, 9] suggest to try sampling corrections in order to improve both aspects of the performance (linearity and resolution). The correction strategy chosen here is to compensate for local 0 uctuations by adjusting downwards the response of readout cells with large signals, by means of a minimal set of parameters. A similar method has been already successfully applied to the Tilecal standalone data [2]. Weighting parameters are introduced for each longitudinal sampling and the charge measured for each cell is corrected according to the formula: Q corr ij = Q ij (1? W j Q j Q ij ) (6) where Q ij is the signal (charge) in each readout cell, with index i for the transverse segmentation and index j for the longitudinal segmentation, Q j is the sum of the signals in all the cells in a 8

10 Figure 7: Pion energy spectra at dierent incident energies. longitudinal sampling and W j is the weight to be found for each sampling. In total 8 parameters have to be determined: one for each detector sampling plus an additional intercalibration parameter between the two calorimeters. The method chosen to determine these parameters is the following: 1. The 4 parameters of the hadronic compartment are determined by selecting events which have an em energy deposit corresponding to about one mip. The total energy (taking only the Tilecal read-out) is then calculated using these weights. The mean energy is then xed at the beam energy value multiplying it by an intercalibration constant f (used as the intercalibration constant a in the benchmark energy reconstruction). 2. The cryostat energy, parameterized in the form explained above and with the same value for c, is then added. 3. The parameters for each electromagnetic calorimeter sampling are determined by relaxing the mip requirement, in an iterative way. To reconstruct the energies, we did not use the exact weights found by using the minimization technique, but rather the values coming from a parameterization of the weights as a function of energy. The rst 4 plots of Fig. 10 show the energy dependence of the weights assigned to the four hadronic longitudinal samplings. The next 3 are related to the em samplings, and the last one to the em-hadronic calorimeters intercalibration. Table 2 summarizes the results for all the beam energies. Again, in order to calculate the energy resolution an oset from the nominal beam value was added to the measured energy. Figure 11 shows that this oset is of GeV. 9

11 Figure 8: Low energy tails in the 300 GeV pions line shape before (upper plot) and after (lower plot) the rejection of the events which leave a signal in the muon chambers). Energy (GeV) (GeV) (GeV) +of f 20 17:0 0:1 3:4 0:1 20:1 0: :0 0:1 4:6 0:1 10:2 0: :6 0:2 6:0 0:1 6:6 0: :2 0:6 7:8 0:7 5:7 0: :2 0:2 9:3 0:1 5:02 0: :6 0:1 10:9 0:1 4:03 0:04 Table 2: Mean energy, and energy resolution for the various beam energy using the sampling correction approach. The energy resolution has been calculated taking into account the mean energy oset of GeV. The energy resolutions (/E) vs 1/ p E for the corrected (weighted) and uncorrected (benchmark) combined data are given in Fig. 12. The energy resolution for the corrected combined data is similar to what obtained for the Tilecal standalone case (shown in the same gure with stars) for energy values above 100 GeV. However, it degrades at low energies mainly due to instrumental eects not yet entirely under control. The energy resolution distribution is tted by the function (49:3 %= p E + 1:1 %) 3:1=E (overlayed to the data in Fig. 12). It is well known that an e/ 6= 1 causes deviation from linearity in the hadronic response vs energy, besides broadening the energy resolution. Fig. 13 compares the linearity for the weighted (or \corrected pions") data and for the data where the minimal set of correction have been applied (\uncorrected pions"). The E/E beam ratios are normalized to the value at 100 GeV. We can conclude that the weighting technique which has been developed gives a satisfactory result, by improving the energy resolution (apart from the point at 20 GeV), and by showing only a small energy dependence of the weighting parameters. Nevertheless, we are still working to improve. 10

12 Figure 9: Distribution of the e/ ratio vs the beam energy. Figure 10: Parameterization of the weight for each longitudinal sampling and of the intercalibration parameter as a function of the energy. 11

13 Figure 11: Oset of the measured mean energy from the nominal beam value as a function of the beam energy. The dark points refer to the case of the simple energy correction (benchmarks), the empty ones to the more sophisticated approach of weighting. The two sets of point have been tted with linear functions. Figure 12: Energy resolution before and after corrections. The combined data are compared with the results from the 94 Tilecal standalone test beam (stars). 12

14 Figure 13: Linearity plot. All the points are normalized to the point at 100 GeV. 13

15 Figure 14: (a) Energy fraction/ interval in the rst em longitudinal sampling for pions of 300 GeV (solid line), 20 GeV (dashed line), and 20 GeV which left less than 10 GeV of total energy (dotted line). Each distribution sums to 1. Sampling tot Em samp Em samp Em samp Cryostat Eh samp Eh samp Eh samp Eh samp Table 3: Longitudinal segmentation of the two calorimeters at =0. The value for the cryostat is put just for display purposes. 3 Lateral Shower Proles We studied the lateral containment of pions of dierent energies. Figure 14 shows the lateral prole as seen in the rst em sampling for pions of 300 GeV (solid line), compared with pions of 20 GeV (dashed line). The lateral prole for those 20 GeV pions which loose less than 10 GeV in the two calorimeters is plotted too (dotted line). From here one can see that for the 20 GeV pions which contribute to the low energy tail in the energy distribution (see Fig. 7) there is no total lateral containment. 4 Longitudinal Shower Proles The longitudinal (benchmark) energy density prole is the distribution of the fraction of the total energy released in one sampling per interaction length (E i ) vs the interaction length. This fractional energy is therefore dened as: E i = < E samp(i) > = < E tot > i (7) where i is the longitudinal sampling number, i is the number of interaction lengths which characterize sampling i, and E tot is the total energy released in the calorimeters. The number of interaction lengths for each detector sampling and for the cryostat are given in table 3. The upper plot of Fig. 15 shows the longitudinal prole for 300 GeV pions together with the Montecarlo 14

16 prediction. The two curves superimposed represent the analytical expression, proposed by Bock et al. [10] and calculated with the input parameters appropriate to our case, which describes the pion shower in the electromagnetic calorimeter (rst 3 longitudinal samplings). The dashed line describes a shower which starts at the beginning of the calorimeter, while the dotted line describes a shower starting after 0.2. The calculation has not been extended to include the shower development prediction in the samplings of the hadron compartment because further study are necessary to extend the Bock parameterization to an hybrid calorimeter. Figure 15: (a) Longitudinal energy density prole for 300 GeV pions for data and Montecarlo. The two curves superimposed represent the analytical expression which describes the pion shower in the electromagnetic calorimeter. The dashed line describes a shower which starts at the beginning of the calorimeter, the dotted line describes a shower starting after 0.2. (b) Longitudinal energy density prole for pions of dierent energies. In analogy to what has been done in the standalone analysis, punchthroughs studies have been performed for the combined run, using pions with energies of 50, 100, 200 and 300 GeV. The calorimeter length, including the hadronic compartment girder, crossed by the particles which enter at an angle of is of about 11. Punchthrough particles can be muons from and K decays in an hadronic cascade, or charged particles (mainly soft electrons and hadrons) from showers not fully contained in the calorimeter. For the shower leakage measurement a punchthrough detector consisting of 10 scintillators was located behind the calorimeters. The 2 cm thick scintillators were grouped in an array covering approximately 73 cm in the vertical 15

17 Figure 16: (a) Punchthrough probability for pions (combined LAr/tile). Results from RD5 and from CCFR (extrapolated to 1.85 m of iron) are also plotted. The dashed line shows the expectation for the ATLAS conguration. (b) Energy loss for events with longitudinal leakage (combined LAr/tile). and 96 cm in the horizontal direction. The beam was almost centering this scintillator wall in the vertical direction, while it was displaced in the horizontal direction. The acceptance has been extrapolated from that of the standalone hadron calorimeter setup and of the muon wall centered behind [7]. Only punchthrough particles of energy corresponding to at least one mip in any of the 10 scintillators has been taken into account. Mip energy in the scintillator have been evaluated as in the standalone setup when possible, assuming a hit in the counter i if ADC i > (< ADC > i mip?3(i) mip) with the average and values extracted from muon data. For the scintillators for which there is no muon data available, the \minimum" between the pedestal peak and one mip peak has been taken for hit threshold. With this approach, very soft particles have not been considered as punch-throughs. Fig. 16a shows the probability of pion induced shower longitudinal leakage as a function of the beam energy, compared with results obtained by the RD5 [11] and CCFR [12] collaborations. The estimated iron equivalent length was about 1.85 m, corresponding to TILE- CAL plus electromagnetic calorimeter and other materials. There is good agreement with RD5 values, but CCFR values give a higher punchthrough probability. The dashed line shows the expectation for the ATLAS conguration (10.6 at =0). Fig. 16b shows the energy loss as a function of the beam energy, where the energy loss is dened as the peak-to-peak dierence between the energy distributions of events with and without leakage. From here one sees that the energy loss for events with longitudinal leakage is about 2-3 %. 16

18 5 Muon Response at 300 GeV Together with the pions at various energies, during the September 1994 test beam period a large amount of muon data at the highest available energy of 300 GeV was taken, in order to investigate the calorimeter response to muons. In our study for the energy reconstruction, these data are very useful as a cross check of the detector intercalibration. The reconstruction of the muon signal in both calorimeters is not trivial since the signal is small, close to the noise level. The algorithms used as well as the results obtained in each case are explained below. These algorithms are not fully optimized to get the best possible muon signal (especially in the case of LAr) since this was not the purpose of this note. The same energy scale as in the benchmark energy reconstruction algorithm was used. The scales of all distributions are in GeV. 5.1 Muon Response in the electromagnetic calorimeter In order to reconstruct the muon signal in the electromagnetic calorimeter one has to be very careful in the selection of the cells to be summed up. This because the noise level of MeV per cell can very well mask the expected signal of about MeV per longitudinal sampling, when a large amount of cells is used to calculate the total energy. In addition, the pedestal for each readout cell have to be very well evaluated and their stability in time has to be veried. The muon signal is reconstructed in each sampling by using rst the cell with the maximum energy deposit (cell max) in the 5x5 window around the nominal beam position. Then, between the 8 adjacent cells to the cell max, the one with the maximum signal is added to obtain the sampling energy, which we call E 2max. To avoid random selections due to cell noise and to reconstruct the particle path, the position of the cell max is left to vary within one cell in both the and the directions in the three calorimeter samplings. In order to estimate the noise contribution to the muon signal random trigger events taken at the same time with the physics events are used. Since in the evaluation of E 2max we take a pair of channels which dier for each event, to have a good estimate of the noise level one should calculate the energy deposit from the random trigger events keeping the proportion that each pair of cells contributes to the E 2max energy distribution. In Fig. 17 the separation of the signal distribution from that of the random triggers is shown for the three calorimeter samplings and for their sum. 5.2 Muon Response in the Hadron calorimeter The muon signal in the hadron calorimeter is reconstructed as it was done for the standalone case. To reduce the inuence of noisy channels only the central modules (modules 2-4) are used. A careful pedestal subtraction has to be done to reduce the inuence of the noise to the muon signal measurements. The direct sum of all the PMTs in each longitudinal sampling is made for both physics and random trigger events. In Fig. 18 we see the separation between the random trigger distribution and the muon signal for each sampling and for the whole calorimeter. As we see, due to the very small signal, the separation is not very good especially for the rst calorimeter sampling. To further reduce the inuence of some noisy channels in the signal, the sum of only 9 readout cells (18 PMTs) was made. The nonet was selected for each sampling around the cell were the beam was supposed to enter (cell 3 in the rst sampling, and cell 4 for the others). In Fig. 19 the separation between the muon signal and the random trigger distribution is shown for the case of the nonet. The better separation of the signal from the pedestal distribution is evident (g 19). However, since these muons are very energetic we might have energy losses by summing only few cells. Therefore there is always an interplay between the better energy measurement which requires more cells to be added and the noise level which in this case increases. 17

19 Figure 17: The muon signal distributions from physics events and for random trigger events in the em calorimeter to show their separation. The t is done using the Moyal function. 18

20 Figure 18: The distribution from the random trigger events and the muon signal for the Hadron calorimeter. 19

21 Figure 19: The distribution from the random trigger events and the muon signal for the hadron calorimeter using only 9 cells. 20

22 Moyal peak 60% Trmean Em samp (0.094 ) Em Samp (0.12 ) Em Samp (0.11) Em total (0.38) EH Samp (0.31 ) (0.31) EH Samp (0.48) (0.47) EH Samp (0.62) (0.61 ) EH Samp (0.80) (0.78 5) EH total (2.7 ) (2.6 ) Table 4: The muon signal for the two calorimeters using a Moyal t and the truncated mean method. The numbers in parentheses refer to the case when only the E max energy is used for the em calorimeter and when only 9 cells per sampling are used for TileCal. 5.3 Combined response The muon signal is extracted from the energy distributions of the two calorimeters, using both a Moyal [6] t and the truncated mean at 60% truncation point. The results are listed in table 4. In Fig. 20 the reconstructed muon signal per radiation length is shown for all the calorimeter samplings and for the total per calorimeter. As can be seen in the case of the TileCal the total energy is much higher than the sum of the sampling energies, indicating some inuence of the coherent noise which needs further study outside the scope of this note. The variation of the muon peak along the calorimeter samplings is due to the fact that high energy muons can create relatively large showers which might not be fully contained within the cells we sum for the energy reconstruction. This was investigated in detail for the case of TileCal [13] and is certainly valid for the LArgon as well. The larger value of the signal for the third EM sampling can be due to the broad pedestal distribution. 21

23 Figure 20: The reconstructed muon signal in GeV of the electromagnetic (left) and the hadronic (right) calorimeters for each sampling and the whole calorimeter (last points) is shown in the upper two plots. In the bottom plots, the signal per radiation length for each calorimeter sampling and for the total is shown as well. 22

24 6 Noise evaluation In order to evaluate the eect of the electronic noise on the energy resolution, a detailed study was performed. The electronic noise scales as 1/E and aects the /E ratio at low energies. To measure the noise level in both calorimeters the pedestal events (random triggers) taken at the extension of the SPS burst along with the physics data were used. The total noise ( tot ) in the system readout channels has contributions from two sources: 1. the incoherent part, or the purely random noise ( incoh ) from the electronics, and 2. the coherent part ( coh ), which can come from various sources, like electronic cross-talk or pickup noise from external sources. The incoherent noise scales as p N ch, with N ch being the number of the readout channels considered, while the coherent one is proportional to N ch, which makes its presence very undesirable because small values per channel can degrade the resolution signicantly if a large number of readout channels is involved. To evaluate the two noise contributions two sets of distributions were used: 1. the odd distribution, where the pedestal values for each channel were added using the formula: X E odd = (?1) i E cell (i) (8) i=1;n of course for even number of channels and, 2. the even distribution using: E even = X i=1;n E cell (i) (9) In the rst sum, only the incoherent noise contributes since global shifts cancel out, while in the second both the coherent and the incoherent terms contribute quadratically. Doing a simple algebra and taking the of the odd and even distributions, one has for the coherent and incoherent noise per readout cell: incoh = odd = p N ch (10) co = q 2 even? 2 odd =N ch (11) In our calorimeters, and using the 11x11 window of the electromagnetic part for the analysis, we have in total 497 channels: 297 from the LArgon and 200 from the TileCal. In table 5 the denition of the 11x11 window for the LArgon calorimeter is shown using cell number units. The denitions of the areas corresponding to the dierent sums for the EM calorimeter are shown in Fig. 21. The 3x3,5x5,7x7 etc windows are always dened with respect to the dened center per sampling (table 5), and are inclusive. For example the cells used for the 3x3 sum are included in the 5x5 sum and so forth. In Fig. 22 the distributions for E even and E odd the random trigger events and for dierent cell sums are shown for the EM calorimeter. Since for the EM calorimeter the readout is done using dierent types of electronics, as seen in g 21, the noise level contribution from each of them might be dierent, but for the purpose of this analysis this was not taken into consideration. For the hadron calorimeter, inclusive sums per sampling per module and for the whole calorimeter were used (g. 23). Using all the above distributions the noise per cell for the coherent and the incoherent part can be extracted as explained above. The results are listed in tables 6 and 7. 23

25 Window Center Min cell Max cell Sampl Sampl Sampl 3 12(23,24) 10(19,20) 14(27,28) Table 5: The LArgon 11x11 cell window used for the analysis and its dimensions for the three calorimeter samplings. The numbers here are in cell units with the (1,1) cell being in the lower left corner of the EM prototype as seen by the beam. The cells in the 3 rd EM sampling are double in width in the direction. Figure 21: View of the EM calorimeter with the dierent areas for the sums 24

26 Figure 22: The E even and E odd distributions for the whole calorimeter sampling and for dierent sums. 25

27 Figure 23: The E even and E odd distributions for the whole HD calorimeter sampling and for dierent sums. 26

28 Figure 24: The noise variation for the total (solid line) and the non-coherent part (dashed line) for the 3 calorimeter cells and their sum. The points represent the various cell sums (3x3,5x5,7x7,9x9, 11x11). 27

29 sampl Esum incoh incoh coh coh 1 3x x x x win x x x x win x x x x win total 3x x x x win Table 6: The results for the noise per cell for dierent sums in the EM calorimeter. In all the above plots, a few very noisy cells in the EM calorimeter have been removed. The coherent noise level per cell in the EM calorimeter is about 4-5 MeV, and the incoherent about MeV per cell, compatible with previous measurements. For the hadron calorimeter the coherent noise per cell is at the level of 4-6 MeV, while the incoherent one is at the level of the MeV per cell. In Fig. 24, 25 the variation of the total and the incoherent part of the noise for each calorimeter sampling and for the dierent sums are plotted for both calorimeters. The total noise contribution is about 1.5 GeV with the coherent noise contributing about 0.9 GeV (g 26). These results are well reproduced when dierent runs at dierent times are analyzed. From this analysis a noise term of at least 1.5 GeV/E is fully justied. 28

30 module Esum nc nc co co 1 samp samp samp samp samp samp samp samp samp samp samp samp samp samp samp samp samp samp samp samp samp samp samp samp samp samp samp samp Table 7: The results for the noise per cell for dierent sums in the HD calorimeter. 29

31 Figure 25: The noise variation for the total (solid line) and the non-coherent part (dashed line) for the hadron calorimeter. 30

32 Figure 26: The total energy for the two calorimeters for the `odd' and `even' sums. 31

33 7 Conclusions The results from the combined test beam data analysis were presented in this note. The combined prototype performance meets the ATLAS calorimetry requirements, although more work needs to be done to understand this complex hybrid environment. Other algorithms to reconstruct the total energy are being developed. Over the next few years more data will be taken in the combined mode, presumably with a more "Atlas-like" setup. It is also foreseen to take data and to study the behaviour of the calorimeters in the very low energy region (a few hundred MeV up to a few GeV) to gain experience on the energy scale problem. The people who signed this note are the authors of the nal analysis on the combined test beam data. However, many other people contributed directly to this work in the past, so that they cannot be acknowledged individually. Our thanks go to the whole Atlas LAr, Tilecal and DAQ subsystems for all the help given to the authors as well as for the eorts put during the test beam data taking period. A publication will be extracted from this note. Such a publication will also appear as a CALO note, with a properly extended list of authors. 32

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