1

LHCb 98-054, ECAL 07-JUL-1998

A new method of light collecting in the preshower.

V.Baskov, L.Gorbov, B.Govorkov, V.Kim, V.Polianski, A.Verdi Lebedev Physical Institute, Moscow, Russia

Abstract

A new method of light collecting with preshower is suggested. The method consists in combining the preshower and the ECAL into a single whole and applying the «shashlik» technique for collecting light both from all scintillators to determine the electron (photon) total energy and from a part of them working as a preshower. Calculations show that the best enrichment of the electron sample by the factor of 8 can be obtained when the preshower thickness is 3.2Xo and the detection efficiency is 0.9. In case of both electron and photon registration at the L0-trigger level the best results for can be achieved when the preshower thickness is 4 – 5 Xo.

Introduction

It is well known that nuclear cascades limit the level of pion rejection in an electromagnetic calorimeter because almost all the energy of charged pions is transferred to neutral pions. If such a cascade occurs within the detector pions and electrons (photons) with the same energies are indistinguishable with L0-trigger. In this case a preshower is a means for suppressing the background. At high energies the achievable enrichment factor for the electron sample is inversely proportional to the preshower thickness or, more precisely, is proportional to the ratio of the electromagnetic detector thickness to the preshower thickness (for example, when the total detector 2 thickness is 25 Xo and the preshower thickness is 2.5Xo the enrichment factor increases by the factor of 10 at the L0-trigger level.

A usual preshower configuration consists of a single layer and a single plastic scintillator layer (for example, 14 mm (2.5Xo) Pb and 10 mm scintillator). An attempt to find the best compromise between pion rejection and ECAL energy resolution was made in the work [1] and it was shown that the enrichment factor for the electron sample can be as high as 10 at least while the energy resolution of the ECAL remains quite tolerable. In the this work a method, which breaks the link between the preshower thickness and the energy resolution of the ECAL and permits us to optimize the “preshower” preserving the “ideal” resolution of the ECAL, is suggested. This method allows us to optimize the preshower from the point of view of both e-/e+ and γ registration.

The method consists in combining the preshower and the ECAL into a single whole and applying the “shashlik” technique for collecting light both from the scintillators of the preshower and from all ECAL scintillators.

Two ways of light collecting from the conventional scintillator preshower (2.5Xo + 1 cm scintillator) are known [2]. Both consist in winding the sides of a scintillator plates either with a single WLS-fiber (spiral winding) or with bands of WLS-fiber, thus connecting the scintillator plate with quick photodetectors. Both methods have some advantages, but also some disadvantages:

• the spatial division of the preshower and the ECAL;

• incomplete light-tightness of the detector set;

• relatively small light output, which puts some difficulty to use APD for light registration .

We suggest a third way of light-collecting with a preshower which is free of the disadvantages named above and is a logical extension of the shashlik ideology, presented in the TP

LHCb. 3

The main features of the method

The preshower and the ECAL are the single whole. Data about electron (photon) energy are taken from a sandwich (Pb-scintillator, 70 cells, 2 mm Pb and 4 mm scintillator each) with WLS fibers

[2]. There are also additional WLS-fibers which take information about the energy deposited in the preshower and which are only sensitive at the definite part of ECAL. For example, if we use the (0

–3.2)Xo section, it corresponds approximately to the preshower of the TP. The methods allows us to change the limits and the width of a chosen interval. For instance, it is possible to widen the interval of the preshower to (0 – 6)Xo. Further we will give the results of calculations of pion rejection factor for various of the cascade curve and energies of electrons, photons and pions.

The purely illustrative scheme of the light-collecting experiment is shown in Fig. 1. New

WLS-fibers are added to the conventional "shashlik" setup. These WLS-fibers are shorter than the detector length and therefore take information from a part of the detector. They are connected to a photodetector with light-guides. At least two ways of this connection are possible:

1. Transparent quartz fibers are used as the light-guides and shifter contacts with quartz

through air. It should be note that quartz is known to have good radiation tolerance and

the large light absorption length;

2. WLS-fibers with light-sensitive and purely light conducting parts are used.

Though there is no industrial production of such WLS-fibers now, we have no doubts that it will begin in the nearest future.

Besides the task to find a method for connecting the shifter and the lightguide it is necessary to increase (in this case, to double) the numbers of holes on the ECAL. Is it dangerous? WLS-fibers with the diameter of about 1 mm spaces at 5-10 mm are usually used in “shahslik”-type calorimeters. The total area of fibers cross-sections are about 1-2% of the ECAL cross-section. 4

Calculation results for electron, gamma and pion spectra

A longitudinal “shahslik” taken from the TP with the cross section 100 mm x 100 mm was used in the calculations . Monte-Carlo simulation using the GEANT package was carried out . Particles impinged on the “shashlik” normally and in the center. There is 1 MeV cut for both electrons

(photons) and hadrons.

Fig.’s 2 show the spectra of energy losses for electrons, photons and pions of various energies for the interval of WLS-fibers sensitivity (the “preshower” thickness): (0 –3.2)Xo.

The rejection factor for electrons, photons and pions for various incident energis 2, 10, 50,

100, and 200 GeV are given in Tables 1-5 for Ethr = 32 MeV and in Tables 6-10 for Ethr = 64 MeV

(electromagnetic shower fluctuations are taken into account). Fig.’s 3 illustrate how the rejection factor Re(γ) depends on the incident energy. The tables show that the best results for electrons are achieved at registered energies 2 to 200 GeV and the preshower thickness 3 – 4Xo (the enrichment factor is 7-8 with 0.9-0.99 efficiency of electron detection). When both electrons and photons are registered at the L0-trigger level it is desirable to increase the preshower thickness to 4.5 – 5Xo (the efficiency of electron detection is 0.96-0.999 for all energies from 2 to 200 GeV and the efficiency of gamma detection is 0.74-0.94 for the same energy range). Substantial increasing of electron and especially photon detection efficiency leads to some decreasing of the enrichment factor for electrons to about 6. It should be note that the proposed method allows us, if the number of the electronic channels is increased, to measure the whole cascade curve and to improve substantially the enrichment factor for the electron sample (up to 102).

In this work we restricted ourselves to consideration of the preshower-ECAL system. It should be note that when the thickness of the “preshower” is increased, i.e. the registered part of the cascade curve is widened, from 0 to 4 – 5Xo the light output is substantially increased which permits APD photodetectors to be used for both the WLS-preshower and the ECAL. 5

Conclusion

The “shahslik” –type systems are well studied in general and by the participants of this experiment in particular. The main features of such systems are homogenious sensitivity (98-99%), good time resolution (about 1 ns), high energy resolution. They permit us to estimate the direction of the shower propagation by measuring transverse distributions at two different depths (as in our case).

The last feature is important for photon registration. All these properties are preserved when the proposed method of light collecting is used. Besides, the cost of the electromagnetic detector is slightly cheaper in this case. Therefore we suggest to carry out detailed experimental and Monte-

Carlo study of this method in the frame of R&D and are ready to take part in it.

References

[1] Adinalfi M., Optimization of the LHCB preshower LHCb 98-015, ECAL, 03-FEB-1998.

[2] LHCb Technical Proposal CERN/LHCC 98-4, LHCC/P4, 20 February 1998. 6

Energy of treshould Ethr = 32 MeV.

1.8 Xo

Ein particles, επ εe Re=εe/επ εγ Rγ=εγ/επ GeV 2 0.091 0.206 2.260 0.086 0.945 10 0.10 0.601 6.00 0.255 2.550 50 0.128 0.908 7.107 0.412 3.226 100 0.146 0.953 6.546 0.435 2.991 200 0.166 0.990 5.976 0.544 3.283 2.5 Xo

Ein particles, επ εe Re=εe/επ εγ Rγ=εγ/επ GeV 2 0.124 0.709 5,702 0.363 2,920 10 0.136 0.960 7,074 0.560 4,127 50 0.177 0.997 5,648 0.662 3,752 100 0.188 0.999 5,333 0.683 3,643 200 0.222 0.999 4,511 0.757 3,413 3.2 Xo

Ein particles, επ εe Re=εe/επ εγ Rγ=εγ/επ GeV 2 0.157 0.940 5,985 0.619 3,945 10 0.170 0.996 5,851 0.752 4,419 50 0.208 0.999 4,804 0.815 3,919 100 0.234 0.999 4,283 0.790 3,850 200 0.258 0.999 3,882 0.864 3,354

3.9 Xo

Ein particles, επ εe Re=εe/επ εγ Rγ=εγ/επ GeV 2 0.188 0.986 5,235 0.776 4,118 10 0.205 0.999 4,881 0.848 4,144 50 0.249 0.999 4,031 0.888 3,569 100 0.269 0.999 3,713 0.888 3,297 200 0.296 0.999 3,383 0.935 3,163 4.6 Xo

Ein particles, επ εe Re=εe/επ εγ Rγ=εγ/επ GeV 2 0.224 0.996 4,445 0.865 3,861 10 0.239 0.999 4,188 0.913 3,824 50 0.282 0.999 3,544 0.932 3,301 100 0.300 0.999 3,338 0.933 3,116 200 0.333 0.999 2,997 0.964 2,888 7

Energy of treshould Ethr = 64 MeV.

1.8 Xo

Ein particles, επ εe Re=εe/επ εγ Rγ=εγ/επ GeV 2 0.043 0.010 0.241 0.005 0.116 10 0.059 0.124 0.209 0.048 0.814 50 0.077 0.519 6.743 0.191 2.486 100 0.087 0.681 7.816 0.255 2.928 200 0.102 0.854 8.434 0.383 3.750 2.5 Xo

Ein particles, επ εe Re=εe/επ εγ Rγ=εγ/επ GeV 2 0.068 0.171 2.494 0.079 1.159 10 0.088 0.705 7.968 0.334 3.772 50 0.116 0.962 8.326 0.534 4.625 100 0.127 0.989 7.797 0.567 4.473 200 0.140 0.998 7.129 0.669 4.776 3.2 Xo

Ein particles, επ εe Re=εe/επ εγ Rγ=εγ/επ GeV 2 0.098 0.586 5.964 0.310 3.157 10 0.118 0.965 8.215 0.611 5.200 50 0.149 0.999 6.710 0.733 4.925 100 0.160 0.999 6.232 0.723 4.508 200 0.190 0.999 5.247 0.816 4.286

3.9 Xo

Ein particles, επ εe Re=εe/επ εγ Rγ=εγ/επ GeV 2 0.129 0.862 6.670 0.555 4.296 10 0.148 0.992 6.722 0.781 5.291 50 0.181 0.999 5.518 0.844 4.660 100 0.194 0.999 5.159 0.832 4.295 200 0.221 0.999 4.533 0.906 4.107 4.6 Xo

Ein particles, επ εe Re=εe/επ εγ Rγ=εγ/επ GeV 2 0.157 0.959 6.091 0.738 4.689 10 0.178 0.997 5.601 0.865 4.858 50 0.207 0.999 4.830 0.907 4.381 100 0.225 0.999 4.445 0.910 4.046 200 0.249 0.999 4.022 0.941 3.784 8

dE/Eodt 10

e, 30 GeV

5

0 5 10 15 20 t, Xo Preshower light guide

dE (0-nXo) ------

Etot WLS-fibres

Fig.1. Main scheme of experimental setup. 9

π e

π γ

Fig.2a 10

π e

π γ

Fig.2b 11

π e

π γ

Fig.2c 12

π e

π γ

Fig.2d 13

π e

π γ

Fig.2e

Fig.2a-e. The spectra of energy loss for electrons, photons, and pions at 3.2 Xo of thickness «preshower». Origin energy of particles: 3a- 2 GeV; 3b-10 GeV ; 3c- 50 GeV; 3d- 100 GeV; 3e-200 GeV respectively. 14

0 50 100 150 200 2.5Xo 7,5 3.2Xo7,5

7,0 3.9Xo7,0 4.6Xo 6,5 Ethr=32 MeV 6,5

6,0 6,0

π 5,5 5,5 ε / e

ε 5,0 5,0 = e 4,5 4,5 R

4,0 4,0

3,5 3,5

3,0 3,0

0 50 100 150 200

Energy of particles Ein, GeV Fig.3a.

0 50 100 150 200 2.5Xo 4,6 3.2Xo4,6

4,4 3.9Xo4,4 4.6Xo E =32 MeV 4,2 thr 4,2

4,0 4,0 π

ε 3,8 3,8 / γ ε 3,6 3,6 = γ R 3,4 3,4

3,2 3,2

3,0 3,0

2,8 2,8 0 50 100 150 200

Energy of particles Ein, GeV Fig.3b

Fig.3a(b). Scatter plots for Re(γ) – Ein dependence from the tables data (partly) at different thicknesses of «preshower». Ethr = 32 MeV 15

0 50 100 150 200 2.5Xo 9 3.2Xo9 3.9Xo 8 4.6Xo8 Etrh=64 MeV

7 7 π

ε 6 6 / e ε 5 5 = = e R 4 4

3 3

2 2 0 50 100 150 200

Energy of particles Ein, GeV Fig.3c.

2.5Xo 0 50 100 150 200 3.2Xo 3.9Xo 5 E =64 MeV 5 trh 4.6Xo

4 4 π ε / γ 3 3 ε = = γ R 2 2

1 1

0 50 100 150 200

Energy of particles Ein, GeV Fig.3d.

Fig.3c(d). Scatter plots for Re(γ) - Ein dependence from the tables data (partly) at different tnickntsses of «preshower». Ethr = 64 MeV.