Performance of the Aleph upgraded silicon vertex detector

L,. " - L~=AR PHYSIC c

PROCEEDINGS SUPPLEMENTS ELSEVIER

Nuclear Physics B (Proc. Suppl.) 54B (1997) 317-322

Performance of the Aleph Upgraded Silicon Vertex Detector Presented by G. Rizzo D. Creanza, M. de Palma, M. Girone, G. Maggi, G. Selvaggi, L. Silvestris, G. Raso, P. Tempesta ~ M. Burns, P. Coyle, C. Engster, M. Frank, L. Moneta, M. Waehnik, A. Wagner, J. Zaslavsky b E. Focardi, G. Sguaz~oni, G. Parilni, E. Searlini c . A. Halley, V. O'Shea, C. R a i n e d . G. Barber, W. Cameron, P. Dornan, D. Gentry, N. Konstantinidis, A. Moutoussi, J. Nash, D. Price, A. Staeey, L.W. Toudup e . M.I. Williams f . M. Billault, A. Bonissent, G. Bujosa, D. Calvet, J. Cart, C. Diaconu, P. E. Blanc, J. J. Destelle, P. Karst, P. Payre, D. Rousseau, M. Thulasidas. g . H. Dietl, H.-G. Moser, R. Settles, H. Seywerd, G. Waltermann h . S. Bettailni, F. Bosi, R. Dell'Orso, A. Messineo, A. Profeti, G. Rizzo, P.G. Verdini, J. Walsh i . j.p. Bizzell, P.D. Maley, J.C. Thompson, A.E. Wright J . S. Black, H.Y. Kim, k . L. Bosisio 1 . j. Putz, J. Rothberg, S. Wasserbaeeh m . p. Elmer n Dipartimento di Fisiea, INFN Sezione di Bail, 70126 Bari, Italy b European Laboratory for Particle Physics (CERN), 1211 Geneva 23, Switzerland c Dipartimento di Fisiea, Universith di Firenze, INFN Sezione di Firenze, 50125 Firenze, Italy d Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ,United Kingdom Department of Physics, Imperial College, London SW7 2BZ, United Kingdom f Department of Physics, University of Lancaster, Lancaster LA1 4YB, United Kingdom g Centre de Physique des Partieules, Faeult~ des Sciences de Luminy, IN~P3-CNRS, 13288 Marseille, France h Max-Planck-Institut f'fir Physik, Werner-Heisenberg-Institut, 80805 Miinchen, Fed. Rep. of Germany i Dipartimento di Fisica dell'Universith e INFN Sezione di Pisa, 56010 Pisa, Italy J Particle Physics Dept., Rutherford Appleton Laboratory, Chilton, Didcot, Oxon O X l l OQX, United Kingdom

k Institute for Particle Physics, University of California at Santa Cruz, Santa Cruz, C A 95064, U S A I Dipartimcnto di Fisica, Universit/t di Trieste e I N F N Sezione di Trieste, 34127 Trieste, Italy m Experimental Elementary Particle Physics, University of Washington, W A

98195 Seattle, U.S.A.

n Department of Physics, University of Wisconsin, Madison, W I 53706, U S A The ALEPH Vertex Detector (VDET) has been upgraded for the second phase of LEP running. The new version still uses double sided silicon strip detectors, fabricated with the same technology as the previous one, but the upgraded one is twice as long and has about half passive material in the tracking volume. Furthermore the readout electronics is now radiation hard (MX7-tLH chips). An almost complete version of the upgraded VDET was installed in ALEPH during a three week LEP technical stop and took data in November 1995 during the LEP run at 130 GeV. The new detector worked well showing high signal over noise ratio and good efficiency. The point resolution measuzed dmq_ng this run, using high momentum muons, 13 /~m in the r - ~b view and 21 t~m in the r - z view, is domi-ated by the alignment precision, due to the low statistics available for this short LEP run. This result is however acceptable, since for lower momentum charged particle, the multiple scattering gives a significant contribution to the final impact parameter resolution. A better resolution has been achieved in the next run, when an initial period at the Z peak has been foreseen to calibrate and align the whole detector. 0920-5632/97/$17.00 © 1997 Elsevier Science B.'~ All rights reserved. PII: S0920-5632(97)00129-1

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1. I n t r o d u c t i o n Upgrade

and

Motivation

for t h e

The Silicon Vertex Detector (VDET) has been fully operational in ALEPH since 1991, for the data taking of LEP at the Z peak [1]. The apparatus consisted of two layers of silicon strip detectors with double sided readout, the first one operating in colliding beam environment. The main goal of such a detector was to provide high precision measurements of the trajectory of charged particles near the interaction point, to reconstruct the decay topologies of short lived particles. All the measurements performed by ALEPH on heavy flavour physics rely heavily on the optimal performance of the vertex detector. One of the most useful tool, developed thanks to the better track resolution achieved with VDET, has been a lifetime tag algorithm, based on the track impact parameter measurement [2], to identify b events in a very efficient and effective way. The presence of a vertex detector to tag b quarks is even more fundamental for the physics program of LEP at higher energies, in particular it is crucial for Higgs boson searches. For example, for a SM Higgs boson, the decay H --, bb, with a branching ratio ..- 85%,is one of the most promising channel to look for, but a background reduction to an adequate level is possible only with an efficient b-tagging algorithm. A similar statement holds for the pair production of SUSY Higgs bosons h and A in which the four jet topology, leading to a pure b final state, occurs in --. 80% of the cases [3]. For a given luminosity the sensitivity of the apparatus to the Higgs boson mass is strictly dependent on the b-tagging performance of the vertex detector. For this reason, for operation at LEP2, the previous VDET presented some limitations: • a restricted solid angle coverage, since only particle with angle with respect to the beam axis between 45 ° and 135 ° cross both layers of the detector; • some of the components are not radiation hard (particularly readout preamplifiers and AC-eoupling capacitors) and during the run at the Z peak several radiation

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Figure 1. Monte Carlo simulation showing the improvement in b-tagging performance (b efficiency vs. b purity in hadronic Z decays) with the upgraded VDET (1996 geometry) with respect to the old one (1995 geometry).

accidents damaged the detector causing inefficient regions; * the readout electronics for the z strips runs down the length of the detector and place large amount of material (~_ 4% X0) in the central angular region. Purthermore this non-uniformly distributed material leads to a tail in track impact parameter distribution which degrades the b-tagging performance. The vertex detector has been upgraded to overcome these limitations for more efficient btagging. The new VDET has been designed extending the solid angle coverage of the outer layer to match that of the inner layer, and reducing the amount of passive material in the central region routing the readout strips on the z side at the end of the deteetor, where also the readout for the r - ~ strips is placed. The upgraded detector is also more robust to the radiation, since the new readout electronics are radiation-hard and the AC-capacitors, that couple the strips to the readout chips, have been protected by diodes.

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319

The silicon detector have been fabricated with the same technology as for the old V D E T , since their performance is insensitive to the radiation in the dose range expected at LEP2. A Monte Carlo study was performed to estim a t e the improvement in b-tagging efficiency with the upgraded version of the detector. The results in Fig. 1 show a 25% improvement in b efficiency at 80% b purity. To give an idea of the impact of this improvement, one can consider the reduction in the minimum luminosity needed for Higgs discovery: in the search for SM tiiggs in the channel e + e - ~ H Z ---* bb q q , one can expect to reduce the minimum luminosity needed by approximately 35%, for rnH = 90 GeV. 2. Overall design a n d Construction The new V D E T consists of double sided silicon rnicrostrip detectors arranged in two nearly cylindrical layers, at radii 6.3 and 11.0 cm, both with an active length of 39 cm, twice that of the old detector. With this geometry, particles with angles with respect to the b e a m axis 0 > 29 o have two points measured in both the r - ¢ and z views. In the outer layer there are 15 mechanically independent units, called faces;the inner layer has 9 faces. A schematic drawing of the mechanical support with one face mounted is shown in Fig. 2. Each face is m a d e out of two independent electricaJ units, modules, which are glued, to form a rigid mechanical structure, on a Kevlar support (omega beam). Each module consists of three identical double sided silicon detectors of 5.26 x 6.54 c m 2. These detectors have been fabricated with the same techonology and biasing scheme as adopted for the previous V D E T : the p+ strips on the junction (r - ¢) side are biased via the guard ring, surrounding the active detector area, using the punch through effect between two close p+ implants. On the ohmic (z) side the n + strips are biased by taking advantage of the electron accumulation layer present at silicon-silicon oxide interface [4]. This technology is intrinsically quite robust to radiation d a m a g e at the dose expected during LEP2 operation (50-100 Krad). T h e readout strip pitch is 50 # m in the r - ¢ view and 100 # m in the z view. In the r -

Figure 2. Mechanical assembly of the V D E T .

view the strips on the three wafers in a module are daisy-chained and then read out via the same electronics channel, while in the z view they are multiplexed using an etched Upilex circuit glued on the z side of the detectors. In this way the 1920 z readout strips can be read out into 960 channels and the readout electronics can be placed outside of the active region, reducing the average a m o u n t of material to _~ 1.4%X0. Each module is equipped with one electronics h y b r i d for each view, which holds the eight MX7 radiation hard chips. These chips are custom charge-integrating amplifiers that shift out the signals from 128 channels onto a single analog output. The 1024 channels for each view are sequentially readout in a single line. During the construction each component has been carefully tested. For the silicon detector the leakage current and the isolation of each strip has been measured to identify possible sources of noise. During the hybrid assembly all the amplifier channels have been tested to look for dead or noisy channels. After the module assembly

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and the microbonding of the silicon strips to the readout electronics a complete test of the readout channels has been performed using an infrared spot to illuminate every silicon strip. With this test, and the following interventions needed, it has been possible to end up with modules with a fraction of dead or noisy channels less than 1.5% per view. Before the instanation of the detector in the experiment the signal-to-noise ratio for both views of all modules has been measured using a X°eRu source and the same data acquisition system as used in the experiment. The average S/N measured has been 18.? in z and 29.5 in r - ~b; this difference between the two sides was expected due to the higher noise on z, coming from the higher total capacitance and the lower bias resistance for the strips in this view, and since in the z side the thin Upilex fanout, capacitively coupled to the silicon, causes a 15% loss of signal.

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3. R u n n i n g P e r f o r m a n c e a n d R e s u l t s The upgraded V D E T has been installed in ALEPH in October 1995, during a three week LEP technical stop, before the first short running at high energy. As the complete set of 24 faces was not available, 19 faces were installed and the remaining five slots in the mechanical support were equipped with d u m m y glass or silicon faces, to provide azimuthal symmetry in the passive material. After an initial short run at the Z peak, 3000 events Z ---, qff, L E P delivered an integrated luminosity of 6 pb -x at x/~ = 130 and 136 GeV. During this run the detector worked well and there were only two hardware problems: one module was not powered up, since during the installation one microbonding was damaged, and one MX7 chip in another module showed unstable performance. The rest of the detector had very stable performance with a good efficiency (_> 97%). In Fig. 3 Is shown the mean efficiency by module, defined as the probabiity to associate a hit to a track reconstructed in the outer tracking chambers. The gaps in the distribution correspond to the d u m m y modules. Offiine reconstruction of the V D E T data is or-

ganized into two steps; first the hit position reconstruction is done, then these precise hits are added to improve the track already reconstructed in the outer A L E P H tracking. To benefit from the V D E T intrinsic point resolution, the position of the wafers relative to the outer tracking chambers must be known up to a few microns. T h e alignment procedure is performed with data, using the tracks reconstructed in the outer tracking. The discrepancy between hits on V D E T and the extrapolated impact point of the track, usually called residual, is minimized with respect to the face positions, considered as a rigid body. Since the intrinsic hit resolution of V D E T ( ~ ¢ "~ 7~zm and ~r, ~_ 13pm) is a factor o f t e n better than that of the outer tracking it is necessary to make extensive use of self consistency constraints (e.g. the overlap between wafers adjacent in ~b) to reach the required alignment precision. The residual distribution after the alignment is centred on zero and its width has contribution from the point intrinsic resolution of V D E T and the final alignment precision. A first alignment of the detector has been per-

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same alignment parameters obtained for the initim Z run. Furthermore the effect was compatible with a time dependent face "bowing". Further investigation of the distortion was carried out with laboratory tests and the conclusion was that the kevlar omega beam, that supports the face, suffers a bowing which depends on the humidity changes during the run. Two solution have been adopted to fix the problem. At the end of the run the faces were reinforced adding a thin carbon fibre beam to the kevlar omega beam; laboratory tests showed that with the new omega beam reinforced the distortion with the humidity variation was negligible. The humidity around V D E T is now controlled.

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formed with the 3000 hadronic events collected at the Z peak, and the final residual distribution is shown in Fig. 4. In both views the resolution measured cr,¢ ~_ 13#m and ~, _~ 21#m is dominated by the alignment precision, due to the low statistic available. This result is however acceptable, since, for low momentum tracks, the multiple scattering gives a significant contribution to the final impact parameter resolution. With the alignment precision achieved the performance of the b-tagging algorithm is not degraded, as has been checked with a Monte Carlo simulation. Furthermore better resolution has been achieved in the next run of LEP at higher energy since an initial longer period at the Z peak has been foreseen to calibrate and align the whole detector. Looking at the high energy data, collected after the short run at the Z, a mechanical instability of the faces was discovered. The first evidence of face distortion was a poorer than expected spatial resolution observed in these data, using the

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The software solution to cure the collected data was to repeat the alignment procedure on the high energy runs including a parameter to take into account a time dependent distorsion of the

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D Creanza et al./Nuclear Physics B (Proc. Suppl.) 54B (1997) 317-322

faces. In this way the alignment precision on the data collected at high energy improved and the final resolution obtained was comparable to the one achieved during the initial run at the Z peak. The final residual distribution for the high energy run is shown in FIG. 5. The resolution measured ~,~b --~ 17/zm and ~z ~- 22/~m is dominated by the alignment precision, due to the low statistic available. 4. C o n c l u s i o n s An almost complete version of the upgraded silicon vertex detector has been succeffully installed in A L E P H in October 1995 and took data during the first run of LEP at high energy. The new detector worked well showing high efficiency and signal over noise ratio. The point resolution achieved during this short run was dominated by the alignment precision, due to the low statistics available, but acceptable since the performance of the b-tagging algorithm axe not degraded with this results. Furthermore a better resolution has been achieved in the next run of LEP at higher energy, since an initial period at the Z peak has been forseen to calibrate and align the whole detector. A mechanical instability of the faces was observed due to humidity variation in the cavern during the d a t a taking, and the problem has been succesfully solved for the next run reinforcing the omega beam with a thin carbon fibre support and controlling the humidity around the detector. The upgraded V D E T has been completed and installed in A L E P H in April 1996.

REFERENCES 1.

H. Seywerd et M. , "The Design, Construction and Performance of the ALEPH Silicon Vertex Detector", Submitted to NIM. C E R N

PPE/96-041. 2.

D. Buskufic et al. (ALEPH Collaboration). "A precise measurement of r Z ~ b ~ / r Z ~ h ~ d , ~ , " . P h y s . L e t t . B 3 1 3

3.

G. Altaxelli e t a / . , "Higgs Physics" in "Physics at LEP2", C E R N R e p o r t 96-01

(1993) 535-548.

(199e) Vol.1

4.

G. Batignani et ence with a large con strip detector Nucl. Instr. and

al. , "Operational experidetector system using siliwith double sided readout" M e t h . & 3 2 6 ( 1 9 9 3 ) 183.