Two silicon strip detectors, the Trigger Tracker(TT) and the Inner Tracker(IT) will be constructed for the LHCb experiment. Transverse momentum information extracted from the TT will be used in the Level 1 trigger. The IT is part of the main tracking system behind the magnet. Both silicon detectors will be read out using a custom-developed chip by the ASIC lab in Heidelberg. The signal-over-noise behavior and performance of various geometrical designs of the silicon sensors, in conjunction with the Beetle read-out chip, have been extensively studied in test beam experiments. Results from those experiments are presented, and have been used in the final choice of sensor geometry

Adeva, B.

Agari, M.

Bauer, C.

Baumeister, D.

Bay, A.

Bernet, R.

Bernhardt, R. P.

Blouw, J.

Carron, B.

Ermoline, Y.

Esperante, D.

Frei, R.

Gassner, J.

Hofmann, W.

Jimenez-Otero, S.

Knoepfle, K. T.

Koestner, S.

Lehner, F.

Loechner, S.

Lois, C.

Needham, M.

Pugatch, V.

Schmelling, M.

Siegler, M.

Steinkamp, O.

Straumann, U.

Tran, M. T.

Vazquez, P.

Vollhardt, A.

Volyanskyy, D.

Voss, H.

Knoepfle, K.

Tran, M.

Bernhardt, R.

MPG.PuRe

LHCb 2004-051The LHCb Silicon Tracker ProjectM. Agaria, C. Bauera, D. Baumeistera, J. Blouwa∗, W. Hofmanna, K.T. Knöpflea, S. Löchnera,M. Schmellinga, V. Pugatchb, A. Bayc, B. Carronc, R. Freic, S. Jiminez-Oteroc, M.-T. Tranc, H. Vossc,B. Adevad, D. Esperanted, C. Loisd, P. Vasquezd, R.P. Bernharde, R. Bernete, Y. Ermolinee,J. Gassnere, S. Köstnere, F. Lehnere, M. Needhame, M. Sieglere, O. Steinkampe, U. Straumanne,A. Vollhardte, D. VolyanskyyeaMax Planck Institute for Nuclear PhysicsbKiev Institute for Nuclear ResearchcLaboratoire de Physique des Hautes Energies, Ecole Polytechnique de LausannedUniversidad de Santiago de CompostelaePhysik-Insitut Universität ZürichTwo silicon strip detectors, the Trigger Tracker(TT) and the Inner Tracker(IT) will be constructed for theLHCb experiment. Transverse momentum information extracted from the TT will be used in the Level 1 trigger.The IT is part of the main tracking system behind the magnet. Both silicon detectors will be read out using acustom-developed chip by the ASIC lab in Heidelberg. The signal-over-noise behavior and performance of variousgeometrical designs of the silicon sensors, in conjunction with the Beetle read-out chip, have been extensivelystudied in test beam experiments. Results from those experiments are presented, and have been used in the finalchoice of sensor geometry.1. IntroductionThe LHCb experiment [1] aims at measuringCP-violation in the B-meson sector and rare de-cays. In pp̄ collisions, B-meson pairs will emergepredominantly parallel to the incoming protonbeams. Therefore, a single-arm forward spec-trometer has been designed. Tracking informa-tion will be supplied by a vertex detector, theTrigger Tracker, an outer tracker and the corre-sponding Inner Tracker. Particle identificationis given by ring imaging Čerenkov detectors infront of, and behind the 4 Tm dipole magnet,a hadronic and an electromagnetic calorimeter.Muon tracking chambers are used for the muontrigger, and track reconstruction.∗Corresponding author,email: Johan.Blouw@mpi-hd.mpg.de2. Silicon Tracker ProjectThe Silicon Tracker project comprises the de-sign and construction of two silicon strip detec-tors. The Trigger Tracker will be placed in thefringe field, in front of the dipole magnet to de-termine the transverse momentum of charged par-ticles. In the off-line tracking, the TT will be em-ployed to reconstruct particles, e.g. K0s, whichdecay outside the vertex detector.The Inner Tracker will be used to cover thatpart of the acceptance around the beam pipe be-hind the magnet where the particle flux densityis around 105 cm−2s−1. The granularity of theIT is chosen such that an occupancy of less ∼ 2%is expected, thus leading to efficient track recon-struction.The design of the silicon sensors is a compro-mise between tracking efficiency on one hand, andmaintaining a low material budget and a limitedfinancial budget on the other hand. From thepoint of view of multiple scattering and track res-1LHCb 2004-051Figure 1. A sketch of the layout of a 0◦ plane forthe second TT station. Here, respectively fourand three sensors are combined in one read-outunit.olution, one would minimize the thickness of thesensors, but from the point of view of signal-over-noise performance, i.e. detection efficiency, onewould like to employ ’thicker’ silicon sensors.2.1. Trigger TrackerThe information provided by the TriggerTracker will be used in the L1 trigger to rejectbackground events. The detector will consist oftwo stations of two planes each in a 0◦ and a5◦ stereo-angle configuration. The TT will coverthe complete acceptance of 300 mrad in the ver-tical plane, and 250 mrad in the horizontal plane.As a consequence, multiple sensors will be joinedin a single detector element. At the position(z = 232 cm and z = 262 cm, respectively) wherethe TT stations will be located, this constitutesan area of ∼ 1.44 × 1.17 m2 for the first station,and ∼ 1.60 × 1.32 m2 for the second station. Asketch of the layout of the 0◦ planes is shown inFigure 1.Fourteen sensors will be mounted on two paral-21.841.452.9125.636.35 36.3519.8Figure 2. A sketch of the layout of a 0◦ plane forthe Inner Tracker. The measures are in centime-ters. Each of the three station will cover a similaracceptance.lel rails, forming a single ladder. One ladder willconsist of 2× 2 read-out units, of four- and threesensors respectively. The three-sensor unit willbe bonded to a 100 µm thick Kapton cable, inorder to connect the inner units to the front-endread-out hybrids located outside the acceptance,at the bottom and on top of the TT planes.The two planes of each station are mounted inan air-tight box. The frames of the box will becooled by liquid C6F14, such that the ambienttemperature remains at 5◦ Celsius. This way, thesilicon sensors are expected to maintain their re-quired performance for the duration of the exper-iment. The box will be flushed with dry nitrogento prevent condensation on the cold surfaces.2.2. Inner TrackerA cross-shaped silicon strip detector, depictedin Fig. 2, has been designed and will be installedbehind the magnet at the position of the corre-sponding Outer Tracker detector. Three stationsof four planes each, two 0◦-planes and two stereo-angle planes, at ±5◦, have been designed. Here,single- and two-sensor ladders are used.Although the IT covers only a small fraction(1.5%) of the acceptance, more than 20% of theparticles will traverse this part of the spectrom-eter. By using sensors with a ∼ 200 µm pitch,2LHCb 2004-051track position0 0.2 0.4 0.6 0.8 1S/N (4 strips)05101520(a) 500 µm thicktrack position0 0.2 0.4 0.6 0.8 1S/N (4 strips)05101520(b) 410 µm thicktrack position0 0.2 0.4 0.6 0.8 1S/N (4 strips)05101520(c) 320 µm thickFigure 3. Signal-over-noise performance for the three sensors. The signal is shown as a function of thefractional position between two read-out strips.a planar resolution of better than 60 µm is ex-pected. Correspondingly, this will result in a mo-mentum resolution of δp/p ≈ 0.4%. In case of anexclusive reconstruction of the Bs → DsK decaychannel, the contribution of the tracking resolu-tion to the Bs mass resolution is about equal tothe contribution from the angular resolution atthe Bs vertex.Each IT station consists of four boxes, arrangedaround the beam pipe. The diameter of the holein the center of the respective IT stations followsthe 10 mrad conical shape of the beam pipe. Thefront-end chip will be located in the acceptanceof the spectrometer, consequently the amount ofdead material needed for cooling has to be mini-mized.2.3. Front-endThe front-end chip, called Beetle [2], has beendesigned at the ASIC laboratory in Heidelberg.This is a chip-design laboratory founded by theMax Planck Institute for Nuclear Physics and theRuprecht-Karls-Universität.The 128-channel read-out chip, which employsa 40 MHz sampling rate, has been developed forthe LHCb experiment. It will be used in the inthe TT, IT, the LHCb vertex detector and forthe pile-up veto. The Beetle chip was designedusing 0.25 µm CMOS technology, and has beenextensively tested. In an irradiation test, the chiphas been proven to be fully operational, even aftera dose of 45 Mrad. With a fast rise time of 14 ns,the signals can be uniquely associated to theirrespective bunch crossings.3. Test Beam ResultsIn order to arrive at a judicious choice of sen-sor geometry and thickness, various geometricalconfigurations of strip design, number of sensorsper ladder and sensor thicknesses have been in-vestigated. These tests have been performed atTable 1Sensor properties used in the test beam. All lad-ders listed consisted of three sensors.Ladder strip implant pitch thicknesslength widthnr. (cm) (µm) (mm) (mm)1 32.4 60 198 3202 26.3 56 228 4103 28.9 46 183 500the X7 test beam facility at CERN, using versions1.1 and 1.2 of the Beetle front-end chip. An ex-tensive description of these tests and their resultsis given in Refs. [3,4].In Table 1, a list is given of the various sensor3LHCb 2004-051track position0 0.2 0.4 0.6 0.8 1efficiency0.960.970.980.991(a) 500 µm thicktrack position0 0.2 0.4 0.6 0.8 1efficiency0.960.970.980.991(b) 410 µm thicktrack position0 0.2 0.4 0.6 0.8 1efficiency0.850.90.951(c) 320 µm thickFigure 4. Efficiency performance plots for the three sensors. The signal threshold was chosen such thatthe noise rate was less than 6 × 10−4.thicknesses and designs of strip geometry. Theseladders have been operated in a 120 GeVπ−beam. Triggering was provided by four scintil-lators positioned in front of and behind the lad-ders. The tracks were reconstructed using a beamtelescope, consisting of four double-sided siliconstrip sensors [5], provided by the HERA-B vertexgroup. With a track resolution of ∼ 14 µm at theposition of the ladders, a precise study could bemade of the performance of the various ladders.In particular, the inter-strip dependence of thesignal-over-noise ratio (S/N) has been extensivelystudied. With pitches as large as 228 µm and sen-sors as thin as 320 µm, it was unclear whether theresulting efficiency would be homogeneous acrossthe inter-strip region.In Figure 3, the results of such a study for thethree cases mentioned in Table 1 are shown. Asubstantial drop in S/N can be observed, inde-pendent of the substrate thickness. It can beconcluded that the 320 µm thick sensors exhibita limited signal-over-noise performance. Whenthese data are used to calculate tracking efficien-cies, a drop of up to 9% is observed for the 320 µmsensors, as can be seen in Figure 4. For valuesof S/N < 9, the threshold selection criterion im-posed on the data leads to a drop in efficiency, seeFig. 4(c). In addition, it can be observed that athigher values of signal-over-noise, the effect of theS/N drop in between two strips on the trackingefficiency is negligible. The process of charge col-lection within the sensors has been simulated [6].The observed charge loss could be reproduced,and is caused by charges trapped at the surfaceof the silicon ladders in the low field region be-tween the strips.4. ConclusionsTwo silicon strip detectors have been designedfor LHCb. The Beetle front-end, designed foroperation in the harsh environment imposed byLHC will be used in the read-out electronics. TheBeetle chip and various designs of the sensorsand ladders have been tested. From these testsit was concluded that for the TT, sensors of atleast 500µm thickness are needed, while for theIT 320 µm thick sensors for the single-sensor lad-ders are sufficient, and 410 µm thick sensors willbe used for the two-sensor ladders.REFERENCES1. LHCb Reoptimized Detector Design and Per-formance TDR, CERN-LHCC 2003-30.2. U. Trunk et al., CERN-LHCC 2003-055.3. M. Agari et al., LHCb Note 2002-058.4. M. Agari et al., LHCb Note 2003-082.5. C. Bauer et al., NIM A447, 2000, p. 61.6. S. Heule and F. Lehner, LHCb Note 2003-159.4

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The LHCb Silicon Tracker Project

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The LHCb Silicon Tracker Project

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