Available on CMS information server CMS NOTE 1996/005 The Compact Muon Solenoid Experiment CMS Note Mailing address: CMS CERN, CH-1211 GENEVA 23, Switzerland Performance of the Silicon Detectors for the CMS Barrel Tracker L. Silvestris Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Via Amendola 173, 70126 Bari, Italy Abstract The Compact Muon Solenoid is a detector designed for the Large Hadron Collider. High particle rates combined with a magnetic field of 4 T make particle tracking a challenge. The baseline is to use silicon pixel and microstrip detectors and microstrip gas chambers. This talk focuses on the barrel part of the silicon tracker. It is made of 816 modules (368 single-sided and 448 double-sided) arranged in three layers. The basic module consists of four 300m thick silicon microstrip detectors, glued together to obtain a total active area of 51:2 250mm 2. The choice of silicon strip detector is based primarily on three elements: AC-coupling integrated on the detector substrates; polysilicon resistors used as bias elements; p-stop isolation to control the interstrip resistance of the ohmic side in double-sided detectors. A review of beam test results is described for different parts of the CMS Barrel Tracker. \conference{presented at {\em Position-Sensitive Detectors}, The University of Manchester, 9-13 September 1996} Submitted to Elsevier Preprint
1 Introduction Figure 1: Layout of the Tracking System of CMS The Large Hadron Collider (LHC) will produce high energy collisions of protons at a center-of-mass (CM) energy of 14 T ev and luminosity up to 10 34 cm?2 s?1. The potential physics at LHC will include the discovery or exclusion of the Standard Model Higgs Boson at masses above the maximum reach at LEP (order of 100 GeV) to 1 TeV; the study of WW,WZ,ZZ scattering at large CM energies, the discovery of Higgs Bosons in the Minimum Supersymmetric Standard Model (MSSM), the search for composite structures in quarks, gluons and weak bosons, the study of CP-Violationin the b-quark sector. The Compact Muon Solenoid (CMS) experiment is built around a large, 13 m long, 6 m diameter, high-field superconducting solenoid leading to a compact design for the muon spectrometer. The electromagnetic and hadronic calorimeters are located inside the 4 T field produced by the coil, while a sophisticated tracking system performs track reconstruction, momentum measurement and pattern recognition [1]. 2 The CMS Central Tracking The goal of the CMS central tracking system is to reconstruct isolated high P T muons and electrons with a momentum resolution better than P T = P T 0:15P T (where P T in TeV) at high efficiencies over a rapidity range of jj < 2:6. These values were defined using as a benchmark process the detection of an intermediate mass Higgs boson which decays in H! ZZ! 4l [1, 2]. The high rate of interactions at the full luminosity determines a challenging environment for an advanced tracking system. The main problem for the central tracker will be the pattern recognition. At a luminosity of 10 34 cm?2 s?1, interesting events will be superimposed to a background of about 500 soft charged tracks within the rapidity range considered from 15 minimum bias events occurring in the same bunch crossing. Their vertices are distributed along the beam direction (z-axis) with a r.m.s. of 5.3 cm. To solve the pattern recognition problem at high luminosity detectors with small cell sizes are required. In CMS silicon and gas microstrip detectors provide the required granularity and precision. Strip lengths of the order of 10 cm are needed to maintain cell occupancy below 1%. This leads to a large number of channels ( 10 7 ) [3]. Another important effect is the presence of the very high radiation level in the collision region. This is due to primary interactions and to the presence of neutrons evaporated from nuclear interactions in the material of the electromagnetic calorimeter. Therefore, radiation resistance is required both for detectors and read-out electronics. The requirements in terms of pattern recognition, radiation hardness and tracking resolution lead to a tracking system based on silicon pixel, microstrip detectors and microstrip gas chamber (MSGC). Fig. 1 shows how the detector planes are distributed in the cylindrical tracking volume of CMS with dimensions jzj < 3:0 m, R < 1:3 m. The entire tracker is subdivided into barrel and forward regions meeting at jj 1:8. The detailed design of the tracker is still evolving. At the moment a track in the barrel region encounters first two layers of pixel detectors (125 125m 2 ) providing a measurement accuracy of 15m in both coordinates, then three layers of microstrip silicon detectors of 50m read-out pitch providing in r? high precision points of 15m, followed by seven layers of 200m MSGC giving a point resolution around 40m [4]. 3 The Silicon Microstrip Detector The silicon tracker is required to have a powerful vertex finding capability in the transverse plane over a large momentum range for b-tagging and heavy quark physics and must be able to distinguish different interaction vertices at full luminosity. The barrel part covers the radial region 20 cm < r < 40 cm is divided into 9 wheels. Each barrel wheel (Fig. 2) is instrumented with detector modules arranged in 16 spirals. In the 5 central wheels each arm contains seven modules. The first two and the last two modules contain double-sided detectors, while the intermediate three use single-sided 1
Figure 2: Layout of the Silicon Barrel Wheel read-out unit carbon fibre rail front-end chips silicon detectors control chip connector positioning pin cooling pipe support Figure 3: The Basic Microstrip Detector Module detectors. In the barrel region the strips oriented along the beam direction are read-out with a 50m pitch in (r; ). The stereo strips make an angle of 60 mrad with the corresponding electrodes on the p-side. The read-out pitch is 200m. Double-sided and single-sided detectors have identical p-side design. Each strip is AC-coupled to external amplifiers by means of integrated capacitors grown on the wafer via a deposition of thin dielectric layers. A polysilicon resistor provides the bias to the strips via the guard-ring structure. A similar structure is produced on the ohmic side of the double-sided devices with the addition of an isolation p-stop box surrounding each electrode. In the devices featuring the double-metal technology the ohmic electrodes are connected, through a small contact, with a set of metal electrodes deposited on top of a thick insulator layer.([5]). 4 The Silicon Barrel Module The basic microstrip detector module for the barrel (Fig. 3) consists of four 300m thick silicon wafers glued together to obtain a total length of 25 cm with 1024 strips that run parallel to the beam. Two detectors are daisychained and connected to the readout electronics. The support of the readout unit, containing front-end amplifiers, control chips and connectors, is made out of a low mass, high thermal conductivity carbon fibre. This acts as a heat 2
45 40 35 30 25 20 15 10 5 0-0.02-0.015-0.01-0.005 0 0.005 0.01 0.015 0.02 Residual 0.02 0.015 0.01 0.005 0-0.005-0.01-0.015-0.02-1 -0.8-0.6-0.4-0.2 0 0.2 0.4 0.6 0.8 1 residual along the detector Figure 4: Residual Distribution in the r? plane for the full size module a) and Residual Distribution along the detector b) bridge for the power dissipated by the electronics [6]. 5 Beam Test Results The results described in this paper were obtained in two beam tests performed at the SPS at Cern in 1995: one in July in the H2 beam line with 300 GeV/c muons and the second one in September in the X7 beam with 50 GeV/c pions. In both cases the detectors under test were mounted on an optical bench and two external reference systems were used for particle tracking. Both systems used silicon detectors: details can be found in [7, 8]. Using a single-sided full size detector (1024 strips with a pitch of 50m) and defining the signal-to-noise ratio as the ratio between the most probable cluster charge and the mean of the cluster noise a value of 25:1 was measured. This value stayed almost constant when a voltage bias scan was performed. This means that no worsening of the module behaviour was found. In Fig. 4 the residual distributionfor the same detector is shown. If the effects of the track extrapolation error and of the multiple scattering due to the material in front of the detectors are taken into account we find a detector intrinsic resolution in r? better than 15m for perpendicular tracks. The lower part of the same figure shows that there is a good uniformity in the position resolution along the readout strips. In Tab. 1 the r? resolution versus the tilting angle is quoted. This study was made using double-sided detectors. The resolution improves slightly Table 1: Resolution and Number of Strips per Cluster versus Tilt Angle angle(degree) 0. 5. 14. 20 25. r? resolution 14.7 14.1 13.1 14.2 19.3 nb. strips per cluster 1.3 1.4 1.6 2.1 2.4 with the angle, as one may expect, when the average cluster size increases from the value of 1.3 strips to around 2. There is a worsening of the resolution when the cluster size exceeds two strips. The same behaviour is observed for the r? z residuals. The space resolution in the r? z view is evaluated looking at the detector response in double-sided modules on the n-side. A study for different read-out pitches (100 m and 200 m) was made. The resolution values obtained for the r? z coordinate, reconstructed from n-side strips at the stereo angle of 100mrad, is 325 m for 100 m and 375 m for 200 m respectively. Comparing these two values one can conclude that it s possible to save the cost of half the electronics channels preserving a good performance. 6 Conclusions The baseline layout of the CMS silicon tracker is defined. Many details of the design are still in evolution; infact a lot of beam-tests and montecarlo studies are foreseen. The optimisation process is expected to converge for the end of 1997 (Technical Design Report). 3
References [1] The Compact Muon Solenoid, Technical Proposal, CERN/LHCC 94-38 LHCCC/P1, 15 December 1994 [2] T. S. Virdee, PP Physics at the LHC, CMS TN/95-168 [3] A. Khanov and N. Stepanov, The CMS Tracker performance estimations with CMSIM100, CMS TN/95-199 [4] F. Angelini et al., N.I.M. A343 (1994) 441 [5] P. Weiss et al., Wafer-Scale Technology for Double-Sided Silicon Microstrip Particle Detectors, The 7 th International Conference on Solid-State Sensors and Actuators [6] RD20 Collaboration: RD20 Status Report 1995, CERN-LHCC/96-2 [7] L. Celano et al., A High Resolution Beam Telescope Built with Double Sided Silicon Strip Detectors,CERN PPE/95-106, Submitted to N.I.M. [8] C. Albajar et al, N.I.M. A 364 (1995) 473-487 4