Nuclear Instruments and Methods in Physics Research A279 (1989) 223-235

223North-Holland, Amsterdam

TRACKING AT THE SSC/LHC

H.F.-W. SADROZINSKI, A. SEIDEN and A.J. WEINSTEINSanta Cruz Institute for Particle Physics, University of California, Santa Cruz, CA 95064, USA

We present the design of a tracking system for the SSC/LHC based on silicon microstrip technology with excellent momentumresolution and good multitrack resolution . The parameters of the tracking device are based on an analysis of the requirements of 1TeV physics . The performance of the silicon tracker has been investigated in a Monte Carlo study . An ongoing R&D program todevelop radiation-hard fast readout chips for the silicon microstrips is described.

1. Introduction

In the past [1,2] the discussion of detector capabili-ties at the next generation large hadron collider(SSC/LHC) has been biased towards calorimetry. Inmost physics studies, information from calorimeters hasbeen used to define the topology of the event (numberof jets, missing E� etc.) and to measure the energy ofthe electrons and the mass of jets . In contrast, trackinghas been reduced to a minimal role as a backup for thecalorimeter, to detect detached vertices and identifymultiple interactions within one beam crossing . Only amodest momentum resolution was required . Inspired bythe work of the Compact Detector Subgroup of the1987 SSC Detector Workshop [3j, we propose to extend

Charged Event Multiplicity n~hFig. 1 . Charged multiplicity n~h in jets recoiling against Wbosons in pp collisions (PYTHIA 4.8) : (a) W+W events froma Higgs with 1 TeV mass, (b) W+QCD jets events with 1 TeV

mass.

0168-9002/89/$03.50 CC Elsevier Science Publishers B.V .(North-Holland Physics Publishing Division)

the function of the tracking to include resolving tracksinside dense jets and measuring momenta of isolatedhigh energy leptons with precision comparable to thecalorimeter in order to momentum-analyze both elec-trons and muons with the same resolution .

2. Physics motivation for high precision tracking

On can measure the performance of general purposeSSC/LHC detectors on their ability to discover theHiggs boson, if its mass is in the 100 GeV to 1 TeVrange, and to study its decays . Depending on its mass,the Higgs search will focus on different signatures andthus will require different detector capabilities. This isshown in table 1 [3], where as a function of Higgs mass

60 80 100 120 60 80 100 120Jet Mass [GeV]

Jet Mass [GeV]

Fig. 2 . Mass of jets recoiling against W bosons (solid histo-gram : W+W events from a Higgs with 1 TeV mass, dottedhistogram : W+QCD jets events with 1 TeV mass, both nor-malized to one SSC year), (a) all events, (b) events with

n,h< 40.

V. TRACKING DEVICES

8000 800

(a) (b)

6000 600

s.rvr4000 400

mG

2000 200

0

224

Table 1H ° detection in a compact solenoid detector

1 TeV

H.F. -W Sadrozinski et al. / Tracking at the SSC/LHC

W+Wt

the detector requirements are shown. For the light Higgsdecaying into bb one must have excellent vertex recon-struction to tag b decays [4] . For the intermediateHiggs, which decays to ZZ pairs leading to a 4-leptonfinal state, good pair mass resolution in both electronsand muons is necessary to increase the rate by a factorof four over the electron-only case [5] . At large Higgsmasses, where in the WW decay one of the W's isdetected in its hadronic decay, the ability to trackefficiently charged particles of the underlying eventallows one to discriminate between the signal (WW)and the major background (W + QCDjet) . To quantifythis, we show in fig . 1 the charged event multiplicitiesn,h from PYTHIA 4.8 [6] for the two cases (only trackswith rapidity JYJ < 2.5 and transverse momentum pt >0.5 GeV in events containing a jet with the mass of theW are counted) . Fig. la is the charged multiplicity forH --> (W --i Iv) + (W -jets) with a mean multiplicity of

Precise lepton measure-ment in TeV range

n ch = 32, and fig . lb shows nch for the background(W Iv) + QCDjets with a mean of noh =75 . A cut atn,h < 40 retains 78% of the signal while rejecting 92% ofthe background . Fig. 2 shows the effect of this multipl-icity cut on the mass distribution of the jets recoilingagainst the leptonically decaying W. Fig. 2a shows thesignal around 84 GeV and the background before thecut (signal/background = 1 : 7), and fig. 2b shows theresult of the multiplicity cut (signal/background =1 :0 .7).A precision tracking device which allows the de-

termination of the charged multiplicity down to pt = 0.5GeV thus provides crucial information complementaryto the calorimeter to suppress the background suffi-ciently . This is possible because the tracking device haspotentially more sensitivity to the soft component of theevent (tracking is possible down to 250 MeV) and hasmuch better spatial resolving power.

MHo(GeV/c 2) Signature

Advantages of CompactSolenoid Detector

DetectedRates Per Year(1040 cm 2)

60

H0 - yy Good 7r°/-y separation 500in microconverter

100

F_ tv - Vertex detection (b tag)H'W Good N/e ID in Jets 400L- bb Good missing pt

150

H0 - ZZ' - Good Z mass reconstruction2MZ in both up and ee

300 140 (Z -" e only)560 (Z - tc, e)

H0 --> ZZ - 4t+ Good Z mass measurement- ZZ - t+t-vv in both ee and PW up 1 TeV- WW - evty Good missing pt

500 29 (Z -> e only)116 (Z -+ p, e)

r ev, py,TVH'-WW - Best jet capability:

700 L-. qq - Calorimetry 5000Precise, hi resolution

t+t- analysis of charged 1000tracks and neutrals

L- qq inside jets

3 . Design of a 1 TeV tracking system

The environment at the SSC/LHC will be a chal-lenge to a tracking device. It can be characterized bylarge multiplicities, dense jets, stiff tracks and a highcollision rate . Jets with an energy of 1 TeV will have acharged multiplicity of n .h = 50-100 and a mean angu-lar track separation in the core of (B) = 0.5 mrad (i .e .,500 wm at 1 m radius!) [5] . On the other hand, isolatedleptons from Z or W decays will have transversemomenta of the order up to p, = 1 TeV . Additionally, ifthe readout is not accomplished within the time be-tween beam crossings (16 ns at the SSC) so-calledminimum-bias events will be added to the interestingevents, increasing the occupancy . This calls for a detec-tor which has small cells commensurate to the trackseparation, good intrinsic position resolution, and fastcollection time .A tracker consisting of Si microstrips with 25 Wm

pitch fits these requirements . It was shown [7] that adouble hit resolution of 100 P m and an intrinsic resolu-tion of better than 5 wm can be achieved . The collectiontime can be made as small as 10 ns .A model for the organization of such detectors is the

vector tracking chamber, made of jet cells . The jet cellgeometry was introduced by the JADE collaboration .The first of such chambers incorporating stereo wires,and smaller jet cells to aid in pattern recognition, wasused by the MARK III detector. Subsequent examplesare the trackers for the MARK II, CDF, SLD andZEUS. Note also, that the use of a vector trackingchamber allows a crude local calculation of the trans-verse momentum, using track segments ; this allows thereconstruction of high transverse momentum tracks only,in an efficient manner. This would vastly shrink thecomputer time needed for track reconstruction for eventsin which only high p, tracks are desired .

3.1 . Parameters of the tracking chamber

The parameters of the tracker are as follows (see fig .3) :

(1) It is made of 16 layers of silicon strips (25 wmpitch) organized into 8 pairs (superlayers) of detectors .The detectors are assumed to be double sided (one sidewith axial strips, the other with small angle stereo) sothe total number of measurements is 32. Thus thedetector can be thought of as 8 axial and 8 stereosuperlayers, although the axial and stereo measurementsare grouped together locally. This can be compared totypical vector drift chambers, such as the new MARKII chamber, which has 6 superlayers of each type .

(2) The radius varies from 8 to 50 cm . The innerradius is determined by the amount of radiation damagethat can be tolerated . For the SSC operating at aluminosity of 1033 CM-2 s-1 , this inner radius corre-

H.F.-W. Sadrozinski et al. / Tracking at the SSC/LHC

¢~+++f iÍfl

ili \\\\

titiy¢

ti++~ ¢+~ +++ +~' yv',,,í ~~\

'k 'k

0 10 20 30 40 50

Fig. 3 . Beam view of silicon tracking system. The outer radiusis 50 cm .

sponds to about 0.6 Mrad per year [8] . Clearly, thedesign of sufficiently radiation-resistant electronics iscritically important. Note that for high momentumtracks, the impact parameter error is still - 5 [Lm, evenstarting 8 cm from the origin, since the long lever armof the tracking device provides a very well measuredangle and curvature .

(3) We assume each layer is 150 [Lm thick and hasdouble sided readout, a technological developmentwhich seems to be realistic [9] . This gives 2.5% of aradiation length of material for the silicon and probablyabout 4-5% Xo in total if we include a support struc-ture . It is of great importance to keep the amount ofmaterial in this range to avoid creating many softelectron-positron pairs from photon conversions (seesection 5) . In addition, B meson vertex reconstructionfor the light Higgs scenario involves lower momentumtracks and is sensitive to multiple scattering errors . Forour design the multiple scattering contribution to theimpact parameter error is about 80 [tm/p with p inGeV . Requiring an error < 20 ~tm implies B taggingusing tracks with p > 4 GeV . For the light Higgs searchdiscussed above, we find that the efficiency for recon-structing a 100 GeV Higgs -> bb with at least twocharged tracks falling within the central f 2 units ofrapidity is reduced by only 14% if we do B taggingrequiring two tracks with p > 4 GeV.

3.2 . Pattern recognition requirements

225

We show in fig. 4 two pairs of detector layers . Thedetector arrangement is specified by the distances 8 andA, which are the separation between pairs in a super-layer and between superlayers, respectively . The con-flicting demands on 6 and A are as follows :(a) would like 6 small enough to make matching of

points on a track simple ;

V . TRACKING DEVICES

22 6

Em < ri(B) .

Fig. 4 . A track passing through two adjacent layer pairs .

(b) would like 8 large enough that the track angle(tangent vector) is well measured ;

(c) would like 4 big enough to fill a given space; and(d) would like d small enough to allow frequent sam-

pling.For the pattern recognition we require that :

(1) different tracks give distinct hits ;(2) hits in the paired layers, separated by S, can be

locally associated into correct vector segments ; and(3) vector segments from different pairs can be cor-

rectly linked into tracks.We define :

Ptrack = track's radius of curvature;(0) = typical track separation angle;a�,

=position resolution of measuring element;E�,

=two track resolving power in space, locally, for ameasuring element;

5

=separation of elements used to measure trackvectors ;

rm

= outer radius of device ; andri=mean radius of the i th pair .

From the physics considerations discussed earlier,(0) = 0.5 mrad . For a silicon strip detector with 25 ~tmpitch, am= 5 Wm and E,. = 100 ttm. Suitable signalprocessing could distinguish tracks entering adjacentmicrostrips, giving Em = 25 Wm at the expense of aslightly degraded position resolution .

We now quantify the relation between parameters sothat the three pattern recognition requirements can bemet:

(1) To see distinct hits from different tracks in layeri we need :

This is a very stringent requirement, which even for the

H.F.-W. Sadrozinski et al. / Tracking at the SSC/LHC

excellent double track resolution of the silicon striptracker is fulfilled for only the outer 5 superlayers (outof 8 total) with radii ri >_ 20 cm . Thus the device shouldbe able to yield a sufficient number of distinct hits fornearly all of the tracks.

(2) To match points into a segment we choose thesimplest algorithm illustrated by the dashed line in fig .4. To each point in the inner layer of a pair we associatethe point with the closest (p angle in the outer memberof the pair. For a curving track the deviation in spacefrom a perfect match, using this algorithm, is given by :

2Ptrack[

Is .However, in the jet core we can expect nearby hits fromvery stiff tracks with a mean spacing of (B)ri . Thesecan cause incorrect associations in segment formation.Thus, the simple algorithm for matching points intovectored segments will work provided :

riS < (B)ri,

2 Ptrackor

S2(9) < Ptrack

This relation needs to be valid for all momenta withinthe jet core and provides the motivation for a small S.

(3) The basis for linking vector segments [10] isillustrated in fig . 5 . The angle between the tangentvectors and a chord connecting the segments is the samefor both vector segments belonging to the same circle .Each angle Bt or 02 is measured with a precision givenby C2 a.18. Linking can be done if this accuracy isadequate. We show in fig . 6 the trajectories of two veryhigh momentum tracks separated by a production angleof (0). An incorrect link yields the dashed track whose

Fig . 5 . Relation for vector segments lying on one track .

Fig. 6. Angular relations for a spurious track (dashed) createdfrom hits from two real tracks.

angles do not matchby an amount ( B ). Thus, the goalfor the linking to work is :

S < (6) .This provides the motivation for a large S.

For am = 5 wm and S =1 cm, we get am/8 = 0.5mrad, which is - (0). We will see below, using a fullMonte Carlo simulation, that S =1 cm is indeed closeto optimum.

In the above we have assumed that tracks come fromthe origin when we match the two hits into a vectorsegment (but not in the vector segment linking) . Weshow in fig. 7 what happens for a track with an impactparameter b.

The displacement of the second hit in a closelyspaced pair of detectors at radius ri for a very stiff trackwith impact parameter b, with respect to a straight linethrough the origin, is d= bS/ri . For the decay of D or

H.F.-W. Sadrozinski et al. / Tracking at the SSC/LHC

B t = A Z +

Fig. 7 . Relation of points to be associated into a vectorsegment for a stiff track with impact parameter b.

Fig. 8. High pt ISAJET event in a 3 T field .

B mesons, b ::5 300 gym. To avoid errors in associationfor pairs, we require as before :

d= b8 < r,

or

b< r2

3.3. Monte Carlo studies

22 7

Taking S = 1 cm, and rt > 8 cm implies that there is noproblem for b < 320 pm even for the innermost layer.There is a problem Ks, which might have to be pickedup in a second pass, if they are close to the jet axis .

In ref . [5], we have looked in more detail at theability to do tracking in 1 TeV jets with a silicontracker. The program ISAJET [111 was used to generateevents with two quark or gluon jets, each with pt >_ 1TeV. The mean multiplicity within JYJ < 2 in theseevents is 330, of which 118 have pt > 1 GeV. Fig. 8shows a high pt event in a 3 T magnetic field. A regionnear the core of the tight 1.5 TeV jet in the lower leftquadrant of the event, at a radius of 26 cm, has beenblown up as fig . 9. The scale is matched to the size of asingle strip detector containing 1024 25-~tm elements.(Note the strips have been tilted to compensate for theLorentz angle due to the magnetic field.) At this scale,which is the one relevant to track finding, the stifftracks are not confusing.

To quantify this further, we have performed the firsttwo steps of the pattern recognition for events with 1TeV jets as outlined in section 3.2 . Details can be foundin ref. [5] ; we report here only the results . For trackswith p t > 1 GeV, we evaluate the efficiency vs pt forfinding tracks if we require at least NV EC = 4, 6, or 8

V. TRACKING DEVICES

228

K=-dS5/dr

~1 + (r do/dr)2

op = 4> + árcslnKi .

H.F.-W. Sadrozinski et al. / Tracking at the SSC/LHC

correctly found vector segments to call the track found.The results for S chosen equal to 1 cm are shown in fig .10, where the last bin includes overflows. The efficiencyapproaches 100% for high momentum tracks. In con-trast to conventional drift chamber trackers, which aremost efficient for lowp, tracks which separate fromjets, the silicon tracker we describe is most efficient forthe high-p t tracks in the core of jets . We find that aS = 2 cm is noticeably worse than S = 1 cm. ChoosingS = 5 mm would give about a 10% increase in efficiencyif all 8 segments are required and little change for thecase of 6 or 4 segments found.

Once vector segments are found, they must be unam-biguously linked together to form tracks . One way to dothis is to use a technique employed successfully with theMark II jet-cell central drift chamber at the SLC [12] .The idea is to calculate the curvature (K = 1/2ptrack)and azimuth at the origin $o of a track passing throughthe origin and tangent to the vector segment. A vectorsegment provides a measure, at average radius r, of thetrack azimuth q) and rate of change of azimuth do/dr.We thus calculate

This will produce a mapping in which all vectorsegments found for a given track should he near onepoint in the K-$o plane. The vector segments found forthe event shown in fig . 8 have been plotted in fig . 11a.Up to eight vector segments are plotted for each track.

Fig. 9 . Blowup of high p, event showing the jet core .

Fig . 10. Tracking efficiency vs p t as a function of number ofsegments reconstructed (8 =1 cm). The error bars are 95%confidence level limits, from the statistics of the Monte Carlo

run.

0.4

0.2

0 .0

-0 .2

-0 .4

Fig. 11 . Plot of u vs $0 for vector segments found in the eventin fig. 8 . Up to 8 vector segments per track, and vectorsegments found incorrectly are also shown. (a) The entireevent, 8 = 5 mm. (b) Blow-up of one of the 1.5 TeV jets, 8 = 5

mm, (c) same as (b), but 8 =10 mm.

One can immediately see the two jets in the event. Theeffect of resolution in curvature is also seen from thespread in points for each track. For this plot we haveused am = 5 l.Lm and 8=5 nun.

Fig. llb is a blowup of the region around the jet at(po = 3.9 rad. The effect of resolution is now moreobvious, and it is also clear that the error in ic isstrongly correlated with the error in $o . The improvedvector segment resolution obtained by increasing 8 from5 to 10 mm results in clearly separated tracks within thejet, as shown in fig . llc. Note the spread in measured$o values is Fam/8, the parameter introduced earlierto quantify the ability to link vector segments. Afterlooking at plots for many events with 1 TeV jets, itbecomes clear that 1 cm is a good choice for 8 and willusually allow vector segment linking for the tracks inthe jet core .

3.4. z measurement

In ref . [5] we have looked at the use of stereomeasurements to determine the z coordinate . Since eachstrip detector is double sided, we orient the strips on

H. F.-W. Sadrozinski et al. / Tracking at the SSC/LHC 229

one side of the detector at a small angle (the stereoangle a) with respect to the axial direction. For detec-tors of length 1=10 cm, a stereo angle of a = 5 mradyields a "stereo displacement" s = la = 500 lim. Weassume each pair of strips is organized as shown in fig .12, with u stereo strips on one detector of a pair and vstereo on the other (a� = -a�).

The goal for the pattern recognition is to measure zcoordinates locally within each pair . Each of the 8 pairsmaking up the full detector would then have spacecoordinates and tangent vectors assigned locally to eachtrack. These would then have to be matched from layerto layer, as discussed above.

Tracks separated by more than the stereo displace-ment s in the axial projection do not interfere with eachother at all . A value of s = 500 wm is probably smallenough to guarantee that most stereo hits can be associ-ated by selecting from a small number of alternativeassignments .

The matching problem in z locally can come formtwo tracks which can be confused or from many trackscausing accidental matches.

In general, we can minimize the z matching confu-sion by taking a very small stereo angle a, trading offresolution in z. This does not significantly degrade themomentum or mass resolution for a solenoidal field . Inaddition, we only need 1 mm resolution in z at thevertex to separate most multiple interactions . The verygood 5 gm resolution of the strips, giving oz = 1.4 mmfor a = 5 mrad, will yield a resolution in z at the vertexof 835 pm.

The confusion from many tracks can be quantifiedin terms of the detector and physics parameters . Recallthat within a jet, the mean displacement between tracksat a radius r is r(B) . If a pair of axial strips have beensuccessfully associated within the jet, all tracks withinthe stereo displacement region s (defined above) willleave stereo hits which can confuse the z matching . Thenumber of tracks entering this region is Ns = s/(r(B )) .All of these tracks will leave stereo hits in the u planewhich could accidentally associate with another hit inthe v plane falling within a region of full width tam,

Axial stripson top

u, small angle stereostrips on bottom

v, small angle stereostrips on top

Axial stripson bottom

Fig. 12. Orientation of strips for a closely spaced pair ofdetectors.

V. TRACKING DEVICES

0 2 4 800 (a) 5 mm spacing

0 .05

0.00i

Y-0.05

-0.10 3.84 3.88 3.88 3.9 3.9200 (b) 5 mm spacing

0 .05

0.00

Y-0 .05 C

-0 .19 3.84 3.88 3.88 3.9 3.92. � . .o (c)10 mm spacing

23 0

" 2 -,TT-~------

1 .2

10

G ____ 00 10 20 30 40 50 60 70 80

0 10 20 30 40 50 60 70 80Pr (GeV/c)

P, (GeV/c)

Fig. 13 . Efficiency vs p, for correctly matching the z strip s ina vector segment found correctly in 4p . (a) a = 5 mrad . (b)

a = 20 mrad .

which is the resolution characterizing the correct choice .The probability of there being such a hit in the v planeis therefore P= NS 2 am/s, and the number of potentialincorrect z pairings is Ni.,-, = NP. Thus we arrive atthe scaling rule for stereo association to work well

2amsNinc-z

= Ns22Qm

=

z < 1 .e

For the detector considered above, Ninc- = 0.5 at aradius of 20 cm. Thus, at larger radii, it maybe possibleto use longer detectors (e .g ., bonding two or three 10 cmlong detectors together) or larger pitch in order tominimize the number of electronics channels required .

In order to test our scaling rule for z matching, weuse the same events containing 1 TeV jets as were usedin section 3.3 above, where the finite beam length withvzo = 5 cm is included. We apply the z matching al-gorithm above to vector segments that were found cor-rectly in the axial projection. The efficiency for makingthe correct z match, as a function of p, of the track, isshown in figs . 13a and 13b for the cases a = 5 mrad and20 mrad, respectively. The efficiency in the former caseis > 95% for all tracks, falling at high p, where tracksare in the center of jets . It is clear that the efficiencybegins to degrade significantly when the stereo angle isincreased to 20 mrad, as predicted by our scaling rule,and we have verified that the effect continues at largerstereo angles .

We conclude that choosing S = 1 cm, a = 5 mradprovides a good match to the physics demands at 1TeV.

4. Effect of the magnetic field and momentum resolution

We assume the presence of an axial magnetic field tomeasure the momentum of the tracks. Because themomentum resolution varies inversely with the mag-netic field, it is desirable to have the largest fieldpossible . However, the magnetic field also has an im-pact on the pattern recognition efficiency. The fieldmay improve the reconstruction efficiency by separating

H.F.-W. Sadrozinski et al. / Tracking at the SSC/LHC

closely spaced tracks in the outer layers, or degrade theefficiency by causing tracks to cross in the outer layers,leading to confusion.

It appears that whether the tracks are relativelyisolated, in the periphery, or in the core of a jet, thedominant effect of the magnetic field is to cause tracksto cross, leading to confusion and inefficiency . It istherefore desirable to have the lowest magnetic fieldpossible to obtain the needed momentum resolution .With a field of 2.5 T, the inefficiency for vector segmentfinding in the inner layers due to the close spacing oftracks is approximately the same as the one in the outerlayers due to track crossings [5] .

The magnetic field can also degrade the double trackresolution by spreading the charge (electrons and holes)over several strips due to the Lorentz drift of thecharges in the magnetic field. This effect can be reducedby orienting the detector elements so that their normalvector makes an angle 0, with respect to the xy-radiusvector . To minimize the problem, we can orient thedetectors at an angle which is the average of the Lorentzangles for each charge (electron and holes), and choosea magnetic field which is low enough so that the resid-ual Lorentz drift spreads the charge over a strip widthor less . A detector of 150 Wm thickness in a fieldB= 2.5 T oriented at tan BL = (0 e+A, h ) B/2 will spreadthe charge by 22 .7 gm on either side of the detector, justless than one strip width; thus the impact on the doubletrack separation will be small. Larger magnetic fieldsworsen the problem.

We look at the expected momentum resolution forthe silicon tracker discussed above. We will assume avertex constrained fit as would be appropriate for thehigh momentum leptons from the heavy Higgs decay.Assuming that the four measurements in a superlayergive an effective measurement with an error Q, and thatthe eight superlayers are approximately equally spacedout to a maximum radius rm gives [5]QP, 5.5apt 0 .3Br�Taking a = 3 wm, after averaging over the four mea-surements in a superlayer, rm = 0.5 m and a field of 2.5T yieldsapt

=8 .8%p, (TeVPt

For the intermediate mass Higgs search we wish toreconstruct Z° masses using leptons with a resolutionbetter than the natural Z ° width limit which impliesthat we wantapt

< 2% at about 200 GeV.Pt

Thus, the resolution found above is adequate and, again,we find that a field of B = 2.5 T is sufficient . Since we

U

50

40

30

20

10

-

Silicon_ Microstrip Tracker

10°

10 20 30 40 50 60 70 80 90 100

Z

(cm)

Fig. 14 . Plan of silicon tracking system, with azimuthal symme-try.

want to minimize the tracking inefficiency and theLorentz angle for electron motion in the silicon, 2.5 T isprobably an optimum choice for the field.

For such good resolution the multiple scatteringcontribution to the resolution will dominate up to ratherhigh momentum. Assuming 5% of a radiation lengthand a 50 cm path length the multiple scattering contri-bution is approximately ap/p1 = 0.8% for B =2.5 T.This number is comparable to the number typical ofpresent day central drift chamber systems.

Fig. 14 shows a possible arrangement for the track-ing detector in r vs z, having azimuthal symmetry . It

Table 2Detector parameters and scaling rules

Physics parameters for:

H.F.-W. Sadrozinski et al. / Tracking at the SSC/LHC

1 TeVjets

0 150 ~-

0 . 125

y 0.075

R. 0.050

e0.025

k

104010

50 GeVjets

231

0

0.5

1

1 .5RAPIDITY Y

Fig. 15. Momentum resolution vs rapidity for silicon trackingsystem.

contains about 40 m2 of silicon detectors. Going out in0 this arrangement gives:(a) tracking with

>_ 6 superlayers for 170 ° > 0 > 10 °(IYI -< 2.3);

(b) tracking with 8 superlayers for 160 ° _> 0 _> 20 ° (IYI< 1.75) ;

V. TRACKING DEVICES

Drift chamber Mark II/SLC

2.5 0.566 661 0.84 4160 15050 20> 100 2303 mm 4 mm50 74100 1002.8 1.90.26 1.00.26 0

6.0 0.450.15 0.0067.1 0.4

16 .0 0.2

Track angular separation (0) (mrad)Jet angular width 812 (mrad)Track pt in jet core (GeV)

Detector parameters

0.51050

Si tracker

Magnetic field B (T) 2.5Radius of curvature p =3.3 p, /B (m) 66Radial detector spacing 8 (cm) 1Superlayer thickness 8SL (cm) 1Maximum radius rm (cm) 50Inner radius rl (cm) 8Detector length l (cm) 10Double track separation em 100 JimStereo angle a (mrad) 5Position resolution am (ILm) 5z position resolution a, = Y2Qm/a (mm) 1.4Momentum resolution up /Pt

2 (1/TeV) 0.09Occupancy from min bias (TeV-1) 0.003

Scaling rule requirementsResolve hits: em/r( 0)

23 2

(c) tracking over a fixed radius of 50 cm for 154 ° >_B>_ 26' (JYJ 5 1 .44) ; and

(d) tracking with axial strips for 143 ° >_ 0 >_ 37 ° (1 YJ 51.10) .Assuming ap /p t = 8% pt (TeV-1) at 90' and ignor-

ing multiple scattering, fig . 15 shows UP/p2 vs rapidity,for p in TeV.

The use of silicon, combined with a moderately largemagnetic field, allows very good resolution . It is essen-tial, however, to align the silicon to an accuracy of a fewmicrons in order to achieve this resolution in practice .A comparison of the parameters of the proposed Si

tracker with the scaling rules (see table 2) shows thatthe SI tracking chamber is well suited to the require-ments of physics at 1 TeV. On the other hand, a trackerbased on conventional technologies like the one pro-posed for the large solenoid [13] will not be suited forthe TeV scale due to problems with occupancy (seebelow) and double track resolution e . For comparison,the performance of the MARK II chamber at the SLCis shown, which is designed for physics at 100 GeV andvery low duty cycle .

5. Occupancy of the tracking chamber

The occupancy of the tracking chamber is de-teriorated by (a) pile-up of minimum-bias events, (b)trapped tracks, and (c) photon conversions .

If the collection and readout time of the trackingdevice is larger than the time between beam crossings,several events will pile up in the detector, thus in-creasing the occupancy correspondingly. The presenceof a large magnetic field tends to trap particles in thefield increasing the occupancy and radiation damage .

Fig. 16 shows the transverse momentum spectrumfrom ISAJET for minimum bias events . A track will be

PT (JYI s 2, Minbias Events)

H.F.-W. Sadrozinski et ad. / Tracking at the SSC/LHC

Fig. 16 . Transverse momentum spectrum for minimum biasevents. The ranges indicated by arrows are described in the

text .

trapped in the field if the diameter of curvature is lessor equal to the outer radius of the chamber: 2Ptrack _< rm .Thus, for example, trapping will occur for: pt < 190MeV for the compact solenoid [3] with rm = 50 cm,B = 2.5 ; or p<

the 20% occupancy for the large solenoid detector, withXo = 8% [13], will increase to 26% due to photon con-versions .

6. R&D program

The tracking system based on silicon microstripsdescribed above poses many challenging problems forresearch. Questions concerning alignment and powerdissipation have to be solved soon .

The other area of needed research is the develop-ment of fast, low power and radiation hard readoutelectronics . It is important to note that these character-istics are already met by today's silicon microstrip de-tectors, which have collection times of the order 10-20ns and are essentially radiation hard . The only degrada-tion observed is an increased dark current Id in thevolume V after irradiation with a fluence F

Id = aFV,

where a = 3 X 10-17 A/cm for 12 GeV protons, anda = 1.6 X 10 -16 A/cm for thermal neutrons . (Differentexperiments [14] agree on these numbers within a factorof 2. A potentially large annealing effect is observed[15] for realistic dose rates, which could lower thedamage factor a considerably.) The fluctuations of thisincreased dark current leads to an increase in noise andlimits the ultimate fluence F or useful lifetime of thedetector for fixed signal/noise ratio R. The fluencelimit F, for a detector of area A and thickness w isgiven by

while the signal/noise ratio R is given by the ratio ofsignal electrons from a minimum ionizing particle

Table 3Expected lifetime of silicon microstrip detectors and pixel devices

H.F.-W. Sadrozinski et al. / Tracking at the SSC/LHC

(25 000 in 300 ~tm Si) divided by the fluctuation on thenoise charge Id A t collected in the shaping time At,

R25000

w=

>0.03

Id, At/1 .6 X 10 -19

with w in im and Id, in A. This leads to the fluencelimit F, at which the signal/noise ratio R is dominatedby the dark current

I wF, =1 .1 X 10 -

Rz At AaThe fluence limit can be increased in different ways :pixel devices decreases the area A of an active elementdrastically, but require a large signal/noise ratio R andlonger shaping time At ; microstrip detectors have rela-tively large area but can operate with smaller signal/noise ratio and shaping time. The fact that the fluencelimit can be raised by decreasing the shaping time of thereadout was pointed out by Kondo et al. [16].

In table 3, the fluence limit of both protons andneutrons for silicon microstrip detectors and for pixeldevices [17,18] are shown. Also shown is the expectedyearly fluence [8] at the projected closest radial distancefrom the beam (r = 3 cm for pixel, r = 8 cm for Si Wstrips), and corresponding expected lifetimes for thedifferent devices. Lifetimes of more than 10 years for alldevices are expected, making a combination of siliconmicrostrip tracker with a pixel device inside possible . Asa caveat, it should be pointed out that the compositionof the calorimeter and its size can increase the expectedyearly neutron fluence by factors of 3 or 4 [8] . On theother hand, as mentioned earlier, annealing effects arenot taken into account in the damage factors .

The requirements of radiation hardness, speed, andlow power consumption are not necessarily met bypresent day readout electrons [19-21] and have to bethe focus of intensive R&D efforts . Because of thedensity of strips, the readout has to be in VLSI, directlycoupled to the detectors . For the high rate future col-

233

Si 1, stripdetector

Pixel devicesRef. [17] Ref. [18]

Area A (NM)2 25X105 100 X 100 24x24Thickness w ([,m) 150 300 200Shaping time 0 t (ns) 16 100 160Signal/noise R 20 100 20Fluencelimit F,(p)(protons/cm2) 3X 10 14 1X101s 2x10 17Yearly fluence (protons/cmz) [8] 2 X 10 13 10 14

1014

Lifetime (protons) (years) 15 10 2000Fluence limit F,(n) (neutrons/cmz) 6 X 10 13 2X1014 3.7X106Yearly fluence (neutrons/cmz) [8] 2.4 X 10 12 2.4 X10 12 2.4 X 10 12Lifetime (neutrons) (years) 24 80 15000

V. TRACKING DEVICES

234

H.F.-W. Sadrozinski ei al.

bus

Fig. 17 . Block diagram of the fast readout system for Simicrostrip detectors : AACC = Analog Amplifier-Comparator

Chip, DTSC = Digital Time Slice Chip .

liders, the readout has to include a pipeline of about 1lts to store the data before the first-level trigger arrives,and preferably a second-level pipeline until readout intothe microcomputer.

We have designed a readout system shown schemati-cally in fig . 17 which consists of an analogamplifier-discriminator chip (ÀADC) and a digital timeslice chip (DTSC) . A more detailed description of thesystem is given in ref . [22] . The analog chip is designedin dielectric isolated bipolar technology for low noiseand potential radiation hardness, which we will testsoon. The main design aim is low power consumption.The pipelining of the events is done digitally in theDTSC (shown in fig. 18), which is 64 levels deep in thelevel 1 buffer and 16 deep in the level 2 buffer. To savepower, the buffer is an SRAM and the chip is beingbuilt in CMOS.

We are building first a scaled up (100 ltm instead of25 lim pitch) and slower (10 MHz instead of 60 MHz)version in 2 lim CMOS and plan to test the principle ofoperation of this prototype in the Leading Proton Spec-trometer (LPS) of the ZEUS detector at HERA. Anothervery important development will be tested there: the

Increment Write Pointer

0000

b Write Pointer b Write Pointer

/ Tracking at the SSC/LHC

radiation hardening of the chips. We have started acollaboration with both a rad-hard foundry in the U.S .and with Los Alamos National Laboratories to test andevaluate rad-hard processes and the final rad-hardproduct. Initial data are very promising, because radia-tion resistance of up to many Mrad have been achieved[23] .

7. Conclusions

We propose a tracking device consisting of siliconmicrostrip detectors with 25 ltm pitch to do 1 TeVphysics at the SSC/LHC. The parameters have beenoptimized for high efficiency for transverse momentabetween pt = 0.5 GeV and 1 TeV. Excellent doubletrack resolution (100 ltm) and momentum resolution(ap /pt = 8% at 1 TeV) can be achieved . The perfor-mance of the tracking chamber has been checked usingMonte Carlo events containing 1 TeV jets and a realis-tic, albeit simple, tracking algorithm. The effects ofevent pileup, trapped tracks and photon conversionshave been investigated for the proposed silicon trackingchamber, where they have minimal effects and alterna-tively for a conventional tracker based on drift tubes,where they produce significant increase in occupancy.Finally, our R&D program to develop radiation hard,fast and low-power readout chips is outlined .

References

Proc. 1984 Summer Study on the Design and Utilizationof the SSC, Snowmass, 1984, eds. R. Donaldson and J.G.Morfin (Batavia, Illinois, Fermilab, 1985);Proc . 1986 Summer Study on the Physics of the SSC,Snowmass, 1986, eds. R. Donaldson and J. Marx (Ameri-can Institute of Physics, New York, 1988) .Proc . Workshop on Physics at Future Accelerators, LaThuile and Geneva, CERN 87-07 (1987).J. Kirkby et al ., Report of the Compact Detector Sub-group, Proc . Workshop on Experiments, Detectors, andExperimental Areas for the Supercollider, Berkeley, 1987 .J.E. Brau et al ., University of Tennessee HEP PreprintUTHEP-88-0601 (1988) .H.F.-W. Sadrozinski, A. Seiden and A.J . Weinstein, Proc .

[2]

[3]

[41

[51pads Level I Level 2

Buffer Buffer

Topical Seminar on Perspectives for Experimental Ap-paratus, San Miniato, 1988, Nucl. Instr . and Meth. A277(1989) 92 .

[61 a: H.-U. Bengtsson and T. Sjoestrand, PYTHIA 4.8,UCLA-87-001 (1987) ;b: Also Proc UCLA Workshop on Observable StandardModel Physics at the SSC : Monte Carlo Simulation and

b Read Pointer b Read Pointer Detector Capabilities, eds. H.-U . Bengtsson, C. Buchanan,IncrementRead Pointer Off Chip T. Gottschalk and A. Soni (World Scientific, Singapore,

1986) p. 228.Fig. 18 . Block diagram of the Digital Time Slice Chip (DTSC) . [71 C. Adolphsen et al ., Nucl. Instr . and Meth . A253 (1987)

The chip has 64 parallel channels . 444.

[8] Radiation Levels in the SSC Interaction Regions, ed. D.E .Groom, SSC-SR-1033, Berkeley (1988) .K. Yamamoto, Hamamatsu Corp ., private communica-tion.

[10] W . Atwood, SLAC SLD experiment, Internal Note #135(1985) .

[11] F.E. Paige and S.D. Protopopescu, ISAJET 5.34 (1986) ;Also ref. [6b], p . 213 .

[12] J. Perl et al., Nucl . Instr . and Meth. A252 (1986) 616 .[13] G.G. Hanson et al ., Report of the Large Solenoid Detec-

tor Group, Proc . Workshop on Experiments, Detectors,and Experimental Areas for the Supercollider, Berkeley,1987 .

[14] T. Ohsugi et al., Nucl. Instr . and Meth . A265 (1988) 105and references cited therein .

[15] T. Kondo, Proceedings of the 1988 Summer Study onHigh Energy Physics in the 1990s, Snowmass, CO, 1988.

[16] T. Ohsugi, H. Ikeda and T . Kondo, private communica-tion .

H.F.-W. Sadrozinski et al. / Tracking at the SSC/LHC 235

[17] D.R. Nygren, Proc . 1988 Summer Study on High EnergyPhysics in the 1990s, Snowmass, CO, 1988 .

[18] S.L. Shapiro, Proc. 1988 Summer Study on High EnergyPhysics in the 1990s, Snowmass, CO, 1988.

[19] P . Dauncy et al ., Proc . IEEE Nuclear Science Symp ., SanFrancisco, CA, 1987 .

[20] G.R . Stephenson and H . Schoenbacher in : The Feasibilityof Experiments at High Luminosity at the LHC, ed . D.H.Mulvey, CERN 88-02 (1988) .

[21] J.E . Gover and J.R. Stour, Basic Radiation Effects inNuclear Power Electronics Technology, SAND 85-0776,Albuquerque, NM, 1986 .

[22] D.E . Dorfan, these Proceedings (Int . Conf . on AdvancedTechnology and Particle Physics, Como, Italy, 1988) Nucl .Instr. and Meth. A279 (1989) 186 .

[23] R. Woodruff (United Technologies MicroelectronicsCenter), private communication.

V . TRACKING DEVICES