Nuclear Instruments and Methods in Physics Research A 623 (2010) 168–170

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Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

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journal homepage: www.elsevier.com/locate/nima

The upgrade of the CMS tracker for Super-LHC

G. Hall �

Blackett Laboratory, Imperial College, London SW7 2AZ, UK

For the CMS Collaboration

a r t i c l e i n f o

Available online 1 March 2010

Keywords:

Pixels

Silicon microstrips

SLHC

Trigger

Detector upgrade

02/$ - see front matter & 2010 Elsevier B.V. A

016/j.nima.2010.02.184

: +44 207 594 7800; fax: +44 207 823 8830.

ail address: g.hall@imperial.ac.uk

a b s t r a c t

The CMS experiment at LHC is planning a major upgrade to adapt to increases in luminosity. It will be

achieved in two stages, with a long shutdown about ten years after start-up. The new tracker should

cope with several hundred interactions per bunch crossing and fluxes of thousands of charged particles

emerging from 40 MHz collisions. CMS has identified a novel requirement, to utilise tracker data in the

first level trigger. The motivations for the upgrade and recent progress are described.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

The LHC peak luminosity of 1034 cm�2 s�1 will eventuallydeliver about 50 fb�1/yr [1,2]. CMS was designed for 10 yearsoperation, but only after experiments have run for sometime willit be understood whether longer term operation might be possibleand worthwhile. Upgrade of the LHC accelerator to Super-LHC foresees operating at 1035 cm�2 s�1 luminosity to provideincreased statistics. Under these conditions most of CMS shouldsurvive predicted irradiation levels and perform well with fewchanges. Trigger and data acquisition systems would take advant-age of technology evolution to cope with SLHC data volumesand rates.

1.1. Present tracking system

The Tracker surrounds the interaction point and providesprecise, efficient measurement of charged particle trajectories andsecondary vertices. It comprises pixels in three barrel layers atradii 4.4–10.2 cm and ten barrel layers of silicon microstrips to aradius of 1.1 m. It includes two endcap disks in the pixel detectorand nine in the strip tracker on each side, plus inner barrel disks,extending acceptance to |Z| of 2.5. With about 200 m2 of activearea the CMS system is the largest silicon tracker ever built [3].

Its construction required new production and quality controlmethods. The microstrip detector took cosmic ray data on thesurface prior to installation and was inserted inside CMS inDecember 2007 followed by pixels in July 2008. The pixel systemis quickly removable, in case of beam-pipe bake-outs; its

ll rights reserved.

installation and connection required only a few days, which is amajor advantage for an upgrade. Inner layer replacement wasforeseen after several years of high luminosity operation assensors reach irradiation limits.

2. Upgrades

Most CMS sub-detectors will not change much for SLHC. It isimportant to maintain compatibility and retain the Level 1 triggerrate limit of 100 kHz. Trigger latency can increase from �3.2 to6.4ms, limited by electromagnetic calorimeter pipelines.

The notable exception is the tracking system, whose perfor-mance will eventually be degraded by radiation damage causedby immense particle fluxes. Greater radiation tolerance will berequired, especially for sensors. In contrast, ASIC electronicsshould withstand SLHC radiation levels but the 0.25mm CMOStechnology pioneered by CMS will be superseded by moreadvanced processes [4].

In the congested SLHC environment of 300–400 events perbeam crossing, with thousands of particles emerging from inter-actions, higher granularity is required. CMS also requires to usetracker data in the first level trigger decision.

2.1. Physics requirements

The motivation for a luminosity upgrade is to improve physicsstudies [5]. Detector performance must remain similar to that atLHC, but with higher particle fluxes, detector occupancies, triggerrates, and radiation damage. Thus, the remarkable achievementsin LHC detector development must be surpassed in this even morechallenging environment.

G. Hall / Nuclear Instruments and Methods in Physics Research A 623 (2010) 168–170 169

Examples of the SLHC physics potential include improvementsin Standard Model parameters, e.g. Higgs couplings, and newphysics hopefully awaiting discovery, such as SUSY spectroscopy.There will be higher mass searches for composite quarks,new gauge bosons, multi-TeV squarks and gluinos, and extradimensions.

While a successful search for the Standard Model Higgsmight produce a few thousand events, depending on mass, with300 fb�1 of data more events will be needed to confirm itsproperties and verify its origin. Simulations of HH productionpredict only a few hundred signal events with significant back-ground even with 3000 fb�1 of data [6].

With such small numbers of events, the detector must be evenbetter than the present one to cope with SLHC particle fluxes andevent pileup. If no Higgs exists, deeper investigations of rarechannels will be required, also with significant backgrounds.A new detector should adapt to unexpected discoveries.

2.2. Phase I

The first phase of the machine upgrade might be aboutfive years after LHC start-up, to reach a peak luminosity of2–3�1034 cm�2 s�1. Inner focusing magnets will be replaced,larger aperture collimators installed and the proton linac replacedto reach the ultimate LHC current. Around the same time theinner layer of the pixel system should be replaced. It lookspossible to rebuild the pixel system to achieve improvedperformance by reducing material [7,8].

The new pixel system will have 4 barrel layers at radii from4 to 16 cm and 3 disks in both endcaps. The monophase liquidcooling will be replaced by an evaporative CO2 system with ultra-light mechanics. Data would be transmitted from modules onmicro-twisted pair cables to move material on the presentbulkhead out of the tracking volume.

The ROC chip would undergo minor changes to increase bufferdepths and transmit binary data at 160 MHz. The present 40 MHzanalogue links, which transmit pulse height information andencode digital data, will be modified to send 320 Mbps digitaldata. A new Token Bit Manager chip will multiplex ROC data at320 MHz, and modules to digitise analogue data replaced withdeserialisers. These are minor changes to a well proven systemwhich should allow its replacement to be rapidly integratedinto CMS.

Although the number of detector modules will roughly double,the system can use the same power cables and needs minorpower supply system changes. The benefit could be a factor 3reduction in material budget, by weight, of the pixel tracker [8].

2.3. A better tracker for Phase II?

High quality tracking and vertexing performance, which atpresent look excellent, must certainly be maintained in thecongested SLHC environment. From simulations of heavy ionevents in the present tracker, with similar track density to SLHC,an extra pixel layer would restore track seeding losses. A newlayout can be optimised for track finding and jet reconstruction.Granularity must increase because of leakage currents as well astrack recognition.

For tracking, multiple scattering, photon conversions, brems-strahlung and hadronic interactions are undesirable, and dependon limiting material. However cables, cooling and mechanicalsupport grow with power dissipated. A major constraint for a newsystem is that cooling pipes, power cables and optical fibresfollow complex, congested routes and installation was timeconsuming and difficult. It is unlikely they can be replaced.

2.4. Tracker input to the Level-1 trigger

Single m, electron and jet Level 1 trigger rates at SLHC willgreatly exceed 100 kHz and cannot be reduced sufficiently byincreasing pT thresholds. Tracking information in the present HighLevel Trigger (HLT) provides additional rejection power whichmotivates future use of Tracker data at L1. One proposal usescluster width information to eliminate low pT tracks up to Z¼1.7[9]. An alternative concept deploys closely spaced layers andcompares hit patterns [10]. The pT cut is set by the angle of a trackin the layer, and the logic might be relatively simple.

Such methods should improve muon momentum measure-ments and eliminate fake candidates; a hit in a limited ~Zfwindow should discriminate between ambiguous candidates andimprove pT.

In the HLT an energetic calorimeter shower direction isextrapolated to a pixel layer to search for an isolated hit. Thiscould be exploited in the electron trigger but because of highoccupancy will not work well at SLHC, so alternatives are understudy [11], using coarse pixels at intermediate radius to selecthigh pT hits which would also be useful for jets.

The major difficulty implementing tracking triggers at Level 1is that the data volume is too high to transfer all hits off-detectorfor decision logic. On-detector data reduction is therefore essen-tial. Selective readout, guided by an outer sub-detector, is onepossible new feature. It is vital not to degrade performance byintroducing more material. Pixellated trigger layers will be morepower hungry than microstrip layers, so the challenge is obvious.

2.5. The Phase-II Tracker project

The second phase of the LHC machine upgrade to reach1035 cm�2 s�1 might be about five years after Phase I. Fromexperience, building a tracker of this size requires at least tenyears and the trigger requirement makes this even morechallenging.

The Phase II design is in progress, by means of simulationsguided by a more elementary ‘‘layout tool’’ to quickly evaluateoptions. Reliability depends on assumptions so it is essential tomodel power and material accurately, which implies under-standing module design and on-detector trigger processing.

Flexible ‘‘straw-man’’ layouts have been built and are startingto produce results for comparisons.

3. Conclusions

CMS is systematically using simulations to define a new Trackerlayout and achieve similar performance with reduced materialbudget. The largest challenges are power and Level 1 triggering.

Acknowledgments

Many colleagues in CMS developed the outstanding trackingdetector which has now been completed and are contributing topreparations to improve it for an even more demanding future.The space available for this paper does not allow to do justice toall work in progress.

References

[1] The CMS Collaboration, CMS Tracker Technical Design Report, CERN/LHCC98-6 (1998) and Addendum CERN/LHCC 2000-016 (2000).

[2] The CMS estimate of integrated luminosity assumes 107 s operation annuallyat nominal luminosity with a 50% operational efficiency factor.

G. Hall / Nuclear Instruments and Methods in Physics Research A 623 (2010) 168–170170

[3] The CMS Collaboration The CMS experiment at the CERN LHC 2008 JINST 3S08004 [doi:10.1088/1748-0221/3/08/S08004].

[4] G. Hall, Nucl. Instr. and Meth. Phys. Res. A 541 (2005) 248.[5] F. Gianotti, M. Mangano, T. Virdee, et al., Eur. Phys. J. C39 (2004) 293,

doi:10.1140/epjc/s2004-02061-6.[6] The CMS Collaboration, CMS Technical Design Report, Volume II: Physics

Performance. J. Phys. G: Nucl. Part. Phys. 34 (2007) [doi:10.1088/0954-3899/34/6/E01].

[7] G. Hall, The upgrade programme of the CMS Tracker at SLHC, 17thInternational Workshop on Vertex detectors, Vertex 2008, To be published

in Proceedings of Science July 2000 /http://pos.sissa.it/cgi-bin/reader/conf.cgi?confid=68S.

[8] R. Horisberger, Private communication and internal CMS presentations.[9] F. Palla, 2007, JINST 2 P02002.

[10] J. Jones, G. Hall, C. Foudas, A. Rose, A Pixel Detector for Level-1 Triggering atSLHC, arXiv:physics/0510228. CERN Report CERN-2005-011 (2005) pp. 130–134.

[11] M. Pesaresi, Private communication and internal CMS presentations.