I will discuss the impact of the LHC luminosity upgrade on CMS detector. While most of the CMS can possibly cope with the increased luminosity, the Tracker must undergo a major redesign in technology both in terms of detector substrates as well as in the data transfer links. I will show the impact on CMS of reduced bunch length and machine elements close to the interaction point

Palla, Fabrizio

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CMS Upgrades for SLHCFabrizio Palla, on behalf of the CMS Collaboration ∗ INFN Pisa, I-56127 Pisa, ItalyAbstractI will discuss the impact of the LHC luminosity upgradeon CMS detector. While most of the CMS can possiblycope with the increased luminosity, the Tracker mustundergo a major redesign in technology both in terms ofdetector substrates as well as in the data transfer links. Iwill show the impact on CMS of reduced bunch length andmachine elements close to the interaction point.INTRODUCTIONThe CMS detector at the LHC is a general purpose Col-lider detector. The momentum of charged particles is mea-sured using a silicon tracker detector in a 4T magneticfield and their energies using electromagnetic (ECAL) andhadronic (HCAL) calorimeters. A muon system surroundsthe detector. The design of the different subdetectors ofCMS has been carefully studied in order to match the LHCrunning conditions, both in terms of the collision frequencyas well regarding the harsh radiation environment, TheR&D studies for the several detector components have beencarried out in the time scale of a decade before starting thefinal construction and assembly.The foreseen LHC upgrade (also known as SLHC),which is meant to provide an order of magnitude increase inthe delivered luminosity, implies a general revision of theCMS detector to cope with the increased radiation levelsand the (possible) increased frequency of the collisions.The CMS Collaboration has carried out three workshopson the subject in the past two years[1]. The main pointwhich has emerged from the discussions is that most of theCMS systems will survive and continue to operate with-out changes to inaccessible electronic systems, even if thatwill have constraints on the future machine operation. Themajor exceptions is represented by the Tracker which isexpected to be entirely replaced for SLHC, due to the in-creased radiation, and the Trigger that has to rebuild theLevel-1 processors. In addition there is a strong belief thatthe Tracker information must be taken into account alreadyat the Level-1 trigger decision.The paper is organised as follow. In Section I willbriefely explain the implications of the increased radiationlevels for the different CMS subdetectors. In Section I willaddress the impact of possible machine operations choiceson CMS functioning at SLHC.∗Fabrizio.Palla@cern.chRADIATION ENVIRONMENTThe harsh radiation environment of SLHC is shown inTable 1 which illustrates the expected dose and fluenciesfor the current CMS Tracking detectors at an integrated lu-minosity of 2500 fb−1.The current pixel detector can sustain about 3 ×1015cm−2 fluencies. In addition, the increased track multiplic-ity per bunch crossing would increase by an order of mag-nitude, resulting in an unacceptable hit occupancy in thecurrent detectors, compromising the tracking capability ofCMS at SLHC. Also the cumulative damage (such as TIDand NIEL) would increase by a factor of five.It is therefore needed to replace the current Tracker ofCMS with a more radiation tolerant and higher granularitysystem. The R& D of new sensor material technology isthe goal of the RD50 Collaboration [2].Table 1: Fluxes and doses at SLHC for 2500 fb−1 inte-grated luminosity at different radii, corresponding to thecurrent positions of the pixel and silicon strip tracker detec-tors. Figures from M. Huhtinen talk at CMS SLHC work-shop February 2004.Pixel 4 cm fast hadrons 1.6×1016 cm−2Dose 4.2 MGyPixel 11 cm fast hadrons 2.3×1015 cm−2Dose 0.94 MGyStrip 22 cm fast hadrons 8 ×1014 cm−2Dose 350 kGyStrips 75 cm fast hadrons 1.5×1014 cm−2Dose 35 kGyStrips 115 cm fast hadrons 1 ×1014 cm−2Dose 9.3 kGyThe description of a possible new Tracker layout goesbeyond the scope of this report and is not given here. How-ever is worth mentioning that CMS considers that the newTracker will play an important role at the lowest Triggerlevel, participating actively to select, already at Level 1, in-teresting events from the QCD background. The new CMSTracker will therefore be build according the above require-ments.The large radiation environment will also affect the runof the Electromagnetic Calorimeter (ECAL) in two re-spects. The first is due to the activation of the supercrystalelectronics which are located in the end caps. The doserate at pseudorapidity η = 1.48 (outer ECAL radius) is 0.2LHC-LUMI-05 PROCEEDINGS7mSv/h, while the one at η = 3 (inner ECAL radius) is 5mSv/h. The high activation levels will therefore restrict theaccess time for human intervention on the electronics incase of needs. The second aspect of the large radiation in-crease at SLHC compared to LHC is the formation of colorcentres in the crystals, which will reduce the collected lightyield. The dose in the ECAL end caps will vary from 0.1 to1 MGy for 2500 fb−1 integrated luminosity. It is thereforeunder study whether the reduced light yield would need theECAL end caps replacement with a different technology.Also the very forward Hadron Calorimeter which ex-tends up to η = 5 will need to have revised.The Muon system of CMS will also suffer from the hos-tile cavern background, especially for the outermost muonchambers, where a better shielding is needed in order tooperate at SLHC.Finally, part of the Trigger Electronics located on thesubdetectors the radiation damage, and eventually be re-placed.IMPACT OF THE MACHINE LAYOUT ONCMS RUNNINGApart from the above mentioned subdetector upgrades orshielding, the impact of the SLHC on the CMS running ismainly visible in the Electronics, the Trigger and the DataAcquisition Systems.The increased occupancy will degrade the performanceof the algorithms, as well as increasing the event size tobe read out. The First Level Trigger rates output shouldnot exceed the 100KHz limit in order to avoid rebuildingthe front end electronics where possible. It is believed thatin order to match the above mentioned requirements CMSshould develop a Trackeer Level 1 Trigger, and possibly ex-port some of the Higher Level Trigger algorithms to Level1.The DAQ system will be a multi-Terabit/s network con-gestion free and scalable.I will discuss now the impact of different machine lay-outs on CMS running.Beam Pipe and forward shieldingsThe current Beryllium beam pipe at the interaction point,whose diameter is 58 mm is not foreseen to be replaced.However the forward beam pipe diameter could increaseup to 400 mm after the Hadron Forward Calorimeter andits shadow, since the TOTEM and CASTOR calorimeterswill not be present.In view of a better shielding in the very forward region,where the TAS is located, it will be needed to reinforcethe rotating system which is near the limits of mechani-cal strengths. A new concept or supplementary system isneeded.Bunch spacingA SLHC bunch spacing not multiple of 12.5 ns wouldhave severe consequences for CMS Electronics. In fact at12.5 ns many of the ECAL, HCAL and Muon front endelectronics would remain; however for a 10 or 15 ns spac-ing CMS will need to rebuild most of this electronics. Acost review must be put in place to have the correct figure,but it is envisaged that the cost could easily exceed severalhundreds Million Euros.It is therefore highly beneficial for CMS to define at thisstage the bunch spacing with one or two possible scenar-ios, which would offer a solid starting point for R&D andcost/benefits estimate.Shorter L*Any elements of the machine inside CMS will have con-seqences in the forward detectors and need to be studiedand evaluated once having in hand a proposal from the ma-chine group. CMS would be able to accomodate some ele-ments, but would clearly depend on the layout proposed.Apossible location at around 20 meters from the interactionpoint might fit well with CMS. A re-design of the forwardshielding will be needed and due to a probable increasedsensitivity to beam accidents a super Beam Control Moni-tor will be needed. Of course it would be extremely bene-ficial the experience gained in running the LHC.MaintenanceThe increased activation will seriously affect the main-tenance of the detector. In particular, as shown above, anyworker working at the Tracker end-flange should not inte-grate more than 10 hours, while working at the ECAL endcaps he/she will receive in one hour the one year alloweddose of 5mSv.In addition, all those subdetectors which will not be re-placed for SLHC must be very reliable by then, and is notexcluded that some systems will reveal themselves be lessradiation tolerant than expected and therefore need eithermaintenance or replacement. All interventions needed toaccess the detector will result in a integrated luminosityloss.Finally, not all changes can be made in a single shut-down. Installing the new shielding might be in conflict withsome new detector installation. It is envisaged that one ormore extended shutdowns will be needed.Additional delay will result from the commissioning ofthe new detectors. At the same time, the machine itself willprobably need some understanding and would therefore notresult in a total loss of time.ACKNOWLEDGEMENTSIt is a pleasure to thank Francesco Ruggiero and Wal-ter Scandale for their warm hospitality and enjoiable work-shop. I would also like to thank A. Ball, W. Smith and G.Hall for their help in preparing this contribution.LHC-LUMI-05 PROCEEDINGS8REFERENCES[1] Feb 2004 http://agenda.cern.ch/fullAgenda.php?ida=a036368on the General startup; July 2004http://agenda.cern.ch/fullAgenda.php?ida=a041379 mostlyaddressing the Tracker, Trigger and Electronics; July 2005http://agenda.cern.ch/fullAgenda.php?ida=a053123 focusingon Trigger and Tracker upgrades[2] RD50 Collaboration, rd50.web.cern.ch/rd50/LHC-LUMI-05 PROCEEDINGS9

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