CMS Phase II Upgrade Scope Document

Transcription

1 CERN-LHCC LHCC-G September 205 CMS Phase II Upgrade Scope Document CMS Collaboration Submitted to the CERN LHC Committee and the CERN Experiments Resource Review Board September 205 CERN-LHCC / LHCC-G-65 26/09/205 The High-Luminosity LHC (HL-LHC) has been identified as the highest priority program in High Energy Physics by both the European Strategy Group and the US Particle Physics Project Prioritization Panel. To fulfil the full potential of this program, which includes the study of the nature of the Higgs boson, the investigation of the properties of any newly discovered particles in the upcoming LHC runs, and the extension of the mass reach for further discoveries, an integrated luminosity of 3000 fb will have to be accumulated by the end of the program. In preparation for operation at the HL-LHC, CMS has documented the necessary upgrades and their expected costs in a Technical Proposal submitted to the CERN LHC Committee (LHCC) in mid-205. The Scope Document provides additional information to assist the LHCC and the CERN Resource Review Board (RRB) in their review of the CMS upgrade. The document commences with a summary of the process followed to develop the scope of the reference design described in the Technical Proposal. The upgrades of reduced scope that have been explored, along with two representative detector configurations that lower the cost, from the estimate of 265 MCHF for the reference design to 242 MCHF and 208 MCHF, are then presented. The performance of all three configurations is compared, along with the capability of the reference design to operate effectively at a potentially increased instantaneous luminosity, as recently introduced in projections for the HL-LHC. It is shown that the CMS reference upgrade will ensure the success of the full scientific program at the HL-LHC, providing also the opportunity to exploit the highest luminosity potential of the accelerator. An alternate configuration with limited reduction of scope should sustain good performance, but would limit the ability to profit from the highest luminosities for some fundamental and difficult measurements. Large scope reductions, as considered in the third configuration, will irrevocably have adverse effect on major parts of the physics program.

2 ii Contents Purpose and outline of the Scope Document The HL-LHC physics program and experimental challenges The physics opportunities at the HL-LHC HL-LHC beam conditions Overview of the CMS Phase-II upgrades Performance considerations for the Phase II upgrades Design optimization of the upgrade elements Upgrade cost estimates Upgrade configurations of reduced cost General considerations Options for cost reductions Upgrade configuration of 242 MCHF cost Upgrade configuration of 208 MCHF cost Comparative Performance studies Performance implications of reduced cost configurations Combined performance studies Phase II detector performance at pileup 40 and pileup Project organization and planning Phase II organization in CMS Project timeline R&D program Cost profile Concluding remarks A Project planning and cost estimates A. Tracker planning and cost estimate A.2 Barrel calorimeter planning and cost estimate A.3 Endcap calorimeter planning and cost estimate A.4 Muon systems planning and cost estimates A.5 Beam radiation instrumentation and luminosity planning and cost estimate A.6 Trigger/DAQ planning and cost estimate A.7 Infrastructure upgrades and logistic of work cost estimate

3 Purpose and outline of the Scope Document The Technical Proposal [] for the Phase II upgrade of the CMS detector identifies the subsystems that will either not survive the harsh radiation environment of the HL-LHC or not function efficiently because of the increased data rates. It also presents conceptual designs and technical solutions of upgrades that will address these issues in order to ensure that CMS fully exploits the physics potential of the HL-LHC. In defining the scope and extent of these upgrades, the design choices were made based on considerations of both performance and cost. Nevertheless, the design and cost optimization process will continue with the preparation and documentation of technical designs on a time scale of two years. By the end of this period, substantial progress will have been made to finalize the techniques and narrow down the uncertainties in the cost estimates. Further adjustment of the detector configurations will also be possible, based on new performance studies as well as on the first physics results obtained at 3 TeV in Run II. The estimated CERN CORE cost for the Phase II reference upgrades is 265 MCHF. As agreed with CERN and the Resource Review Board, the CMS Phase II Upgrade Scope Document evaluates two detector upgrades of reduced scope targeting lower costs in the range of 235 MCHF and 200 MCHF, respectively. Representative configurations for these two scenarios are derived from descoping or downgrading of upgrade elements of the reference design, in an attempt to preserve to the greatest possible extent the physics reach of the experiment. The first part of the Scope Document reviews the scientific motivation of the HL-LHC research program and discusses the requirements that the physics processes impose on several subsystems of the experiment. This is illustrated with a few representative performance studies that are presented in the Technical Proposal. Although it is not discussed in this document, it should be noted that the Phase II upgrades would also greatly benefit a program of Heavy Ion physics during the HL-LHC era. The accelerator high-luminosity beam conditions that will enable CMS to achieve the physics goals in the next two decades are then presented. The second part of the document summarizes the experimental challenges that arise from this operating environment. All the resulting upgrades are briefly described, outlining the criteria used to develop performant and cost-effective designs. A discussion of the costing methodology and a presentation of the cost estimates for each subproject of the upgrade conclude this part. The third part of the document presents the considerations used to define the reduced-scope detector configurations that correspond to the two scenarios of lower total cost. The cost reductions resulting from each descope are then estimated. The comparison of the performance of the two configurations with that of the reference design is carried out both at the level of physics objects reconstruction as well as for the full physics reach, as illustrated with some representative benchmark signals. The performance is presented for the baseline HL-LHC luminosity of cm 2 s. Also presented is the ability of the reference design to exploit cm 2 s, which is now believed to be the ultimate luminosity that the LHC can achieve. The document ends with a discussion of the project organization and planning, and concluding remarks.

4 2 2 The HL-LHC physics program and experimental challenges 2 The HL-LHC physics program and experimental challenges 2. The physics opportunities at the HL-LHC The goal of the CMS experiment at the Large Hadron Collider (LHC) is to answer fundamental questions in particle physics. What is the origin of elementary particle masses? What is the nature of the dark matter we observe in the Universe? Are the fundamental forces unified? How does QCD behave under extreme conditions? What physics causes the dominance of matter over antimatter? In the first major physics run in 20 and 202, at center-of-mass energies of 7 and 8 TeV, the collider reached a peak luminosity of cm 2 s, more than 75% of its design luminosity, and delivered an integrated luminosity of 25 fb to each of its two general purpose experiments, ATLAS and CMS. These data have yielded a vast number of physics results, summarized by the CMS collaboration in more than 400 publications. The major achievement of the run, and a milestone in humankind s understanding of nature, has been the observation in 202 of a new particle of mass 25 GeV by the ATLAS and CMS collaborations [2, 3]. In addition to discovering the new particle, CMS was able to show that it behaved like a standard model (SM) Higgs boson. Studies of the properties of this new particle decay have provided compelling evidence that it is indeed of spin and parity 0 + establishing it as a Higgs boson [4]. By using a combination of theory predictions for the decays and production, the couplings of the new boson to the known particles have been determined and are shown in Figure to follow a mass dependence characteristic of the Higgs field. The SM does not provide answers to the remaining questions. Those require new physics. Although the 25 GeV Higgs boson behaves like a SM Higgs boson, measurement of its properties are still not very precise. Deviations from perfect SM behavior because of its interaction with other forms of matter, including dark matter, could be a signature of this new physics. The detailed study of the 25 GeV Higgs boson is a scientific imperative that must be pursued to a much higher level of statistical precision than is available today. Many searches have been undertaken with the data taken in 20 and 202, but they have not yet revealed evidence of new physics beyond the standard model (BSM). The theory known as supersymmetry (SUSY) contains a partner for every SM particle, including a candidate for dark matter. Since no superpartners have yet been observed, if they exist, they would have to have very specific, hard-to-detect decay chains or higher masses than have been accessible so far at the LHC. With the present LHC results, in simplified models the SUSY partners of the gluons, the gluinos, and the partners of the two lighest generations of the quarks, the squarks, with masses below about TeV are excluded, while scenarios with 3 rd generation squarks, the sbottoms and the stops, with masses below TeV are still compatible with the data. SUSY also predicts several more Higgs-type particles. Searches for these have also been undertaken, but so far, no additional Higgs bosons have been found. The direct search for exotic processes and particles is another approach to discovering new physics. An example is the direct search for dark matter in a final state with two weakly interacting massive particles (WIMPS), characterized by large missing transverse energy, ET miss. The sensitivity of CMS in certain ranges of the dark matter particle mass, for example at very low mass, and cross section surpasses that of other search techniques, such as direct searches for interactions with bulk matter and indirect evidence from WIMP annihilation with accumulations of darkmatter in the sky. Other examples are searches for new gauge bosons with SM couplings, leptoquarks, and quarks with vector-like couplings. Searches for these particles have been carried out with data from the 20 and 202 LHC run but none has so far been observed.

5 2. The physics opportunities at the HL-LHC 3 New particles are expected at the TeV scale but have not yet been seen. This could mean that they exist at masses above the current level of sensitivity. It could also mean that they could be present at lower masses but their cross sections are lower than expected or their experimental signatures are especially difficult to observe. In either case, the sensitivity for searches of new particles grows with increased luminosity. The CMS physics program at the HL-LHC will build on the experience acquired and the results obtained from more than 300 fb of integrated luminosity accumulated in the first phase of the LHC operation. Independent of potential discoveries in this period, the physics program will continue the quest to answer fundamental questions in particle physics, on one hand with precision measurements and, on the other, by direct searches for new physics. In the rest of this section, a few examples of important physics goals achievable with 3000 fb accumulated at the end of the HL-LHC program are presented. The results are based on projections or simulations that take into account the improvements to the detector planned to preserve its capabilities at the Phase II high luminosity. More complete and detailed descriptions are given in the Technical Proposal. The study of the Higgs boson will continue to be central to the program and provides a powerful argument for higher luminosity. It will include precise measurements of its couplings to other particles, determining if it has a tensor structure, and the search for rare SM and BSM decays. The enormous dataset will give access to nearly all of the production processes and decays of the Higgs boson. Figure shows the current CMS results and a projection for the measurement of Higgs boson couplings in a dataset of 3000 fb at 4 TeV center-of-mass energy as a function of the boson or fermion masses [5, 6]. Compared to a precision of about 20% on Higgs boson couplings today, percent-level precision can be reached for most coupling measurements fb (8 TeV) + 5. fb (7 TeV) fb (4 TeV) /2 /2v) or (g λ f V CMS 68% CL 95% CL SM Higgs b τ W Z t /2 or (g/2v) λ CMS Projection 68% CL τ b W Z t µ (M, ε) fit 68% CL 95% CL Particle mass (GeV) µ mass (GeV) Figure : Higgs boson couplings as a function of boson or fermion masses from (left) data from Run I and (right) projections for 3000 f b at the HL-LHC. The couplings of fermions and m weak vector bosons are parametrized to be κ f f v (λ) and m κ V V v ((gv/2v) /2 ), respectively, to preserve a linear mass dependence. The dashed lines indicate the predicted dependence on the particle mass for the SM Higgs boson. In order to achieve the full benefit of the HL-LHC, CMS must continue to be able to reconstruct at the much higher luminosity all the standard physics analysis objects with high efficiency, low fake rate, and high resolution. Excellent electron, photon, and muon reconstruction is needed for Higgs decays to γγ, ZZ and WW and to observe Higgs decays to µ + µ. The dominant

6 4 2 The HL-LHC physics program and experimental challenges decay mode is H b b, which requires b-quark tagging and, consequently, continued precision reconstruction of primary and secondary vertices. The reconstruction of τ leptons also requires tracking of charged hadrons and the measurement of electromagnetic energy as well as muon and electron reconstruction. The goal of seeing Higgs bosons produced in association with a t t pair requires jet reconstruction and b-quark tagging. Because of its relatively low mass, the Higgs decay products have also low energy or transverse momentum. For efficient event selection of all processes, it is therefore mandatory that the thresholds on these variables are as low as possible at the first level of the event selection (hardware trigger). This also applies to several other SM or new physics processes. For instance, sub-dominant production mechanisms of the Higgs boson are more easily triggerable or produce event samples with better signal-tobackground ratios than the dominant gluon-gluon production mode. This is especially true for H b b, which is hard to trigger on alone, but can be produced in association with W or Z boson or top quarks, all of which are much easier to trigger on. All these requirements explain why the full capabilities of the original CMS design must be retained. Higgs boson production through Vector Boson Fusion (VBF) is a rare process in which the proton-proton collision is transformed into the collision of a pair of massive vector bosons. These collisions are characterized by two tagging jets travelling in opposite directions at relatively small angles with respect to the colliding beams, with the signal products going into the more central regions of the detector. Most of the tagging jets emit significant amounts of energy at low angles beyond the present tracker acceptance. Due to the harsher conditions at high luminosities, an extension of the tracker coverage to higher η is needed to efficiently tag these jets. This also applies to the similar vector boson scattering processes, which are important measurement of the role of the Higgs boson in the electroweak symmetry breaking. These measurements could also be sensitive to new physics through the triple-gauge couplings (TGCs) and quartic-gauge couplings (QCGs). In general, precision measurements of electroweak observables have played a key role in validating the SM and in putting constraints on BSM physics. Higgs boson coupling to charged leptons is a crucial measurement. The coupling to electrons is too small to measure even at the HL-LHC, but the coupling to τ s will be well-measured by the end of Phase II. This decay in the VBF production of a Higgs boson is one of the benchmark measurements used to evaluate the performance of the CMS upgrades. The coupling to the second-generation fermions will be probed for the first time by measuring the Higgs boson decay exclusively to two muons. The branching fraction in the SM of only 0 4, can be measured with a precision of about 5% with 3000 fb. This depends on the improvement in mass resolution (40%), because the upgraded detector has less mass in the tracking region, and in efficiency (20%), because of the extended η coverage, achieved with the upgraded detector as shown in Figure 2. Measurements of the di-higgs production with a very low cross section estimated to about 40 fb will allow the study of the Higgs boson self-coupling. This measurement is a unique way to fully establish the Higgs field potential. Figure 3 shows the simulated mass distribution measurement with the CMS upgrade for the HH bbγγ final state including the background. The ability to measure this process in the most promising final states will largely depend on the identification and momentum resolution performance for b-jet, photons and τ. In a major class of SUSY models, the lightest SUSY particle will be stable and interact very weakly with ordinary matter. This will result in events with large missing transverse energy, ET mis, which is taken as one of the main experimental signatures of SUSY. Search strategies have been refined to be more sensitive to hard-to-identify configurations. For example, attention is

7 2. The physics opportunities at the HL-LHC 5 Figure 2: Di-muon mass distributions for Higgs boson events simulated with the Phase I (nominal and after radiation damage from exposure to 000 fb of luminosity) and Phase II detectors. The distributions are normalized to take the relative selection efficiency of different detectors and conditions into account. being paid to stealth SUSY, in which new particles look very similar to SM backgrounds, and to compressed SUSY, where particles have very similar masses, making their decay configurations very hard to observe. Generic approaches to the searches have been developed that are somewhat insensitive to the details of specific production mechanisms or decay patterns. The sensitivity of CMS to SUSY and many other new physics signals improves with increasing luminosity and are important goals for the future. Figure 4 shows the reach for selected searches for supersymmetry for a dataset of 300 fb and 3000 fb at 4 TeV. The W ± H+ET miss measurement is another benchmark selected to assess the CMS upgrade physics reach. It should be noted that improvements in analysis and large samples of data will likely open the possibility for searches for rare signals with exceptionally low background that could outperform these projections. Another approach to discovering new physics is to make precision measurements of rare decays that are well-predicted in the SM. If new physics is present it might either enhance or suppress the rate of these decays. An example of such a precision measurement made by CMS is the first observation of the very rare decay B s µ + µ. This decay is very highly suppressed in the SM, but it can receive additional contributions from new physics. The observed branching fraction is , consistent with the expectations of the SM. The companion decay, B d µ + µ, is predicted to have a branching fraction that is a factor of 30 lower than the B s. At the HL-LHC, the decay B s µ + µ will become a precision measurement, the decay B d µ + µ will be established and its branching fraction will be measured with reasonable accuracy. The projected mass distribution is shown in Fig. 5. The significance of the B d, predicted to be 2.2σ after 300 fb, will improve to 6.8 σ with 3000 fb. This measurement is only possible with the new trigger capabilities provided by the tracker upgrade. The large luminosity sample collected at the HL-LHC will extend the reach of the search for

8 6 2 The HL-LHC physics program and experimental challenges s=4 TeV, PU=40 Number of Events CMS Simulation Toy data Combined fit HH->bbγγ Resonant bkg Non-resonant bkg [GeV/c ] Figure 3: Di-photon mass distribution for the estimated signal and background contributions in the HH bbγγ measurement. The data points show the result of a pseudo-experiment. M γγ Summary of CMS SUSY Projections with SMS - 5σ discovery: 4 TeV, 3000 fb χ ± χ WZχ χ 2-5σ discovery: 4 TeV, 300 fb 95% CL limits: 8 TeV χ ± χ WHχ χ 2 ~ g ~ g ttttχ 0 χ 0 ~ g ~ g qqqqχ 0 χ 0 Probe *up to* the quoted mass Mass scales [GeV] Figure 4: Reach of searches for supersymmetry. new heavy gauge bosons, to 6 TeV or more for SM couplings or, in the case of very narrow width resonances, probe regions of 0.5- TeV. Similarly searches for extra dimensions, compositeness, leptoquarks etc. can be extended in range by a few TeV. Many signatures for exotic phenomena include the production of heavy semi-stable particles that will either traverse or decay in the CMS detector. For these searches it is also imperative to keep the detector capabilities at least at the present quality level. An exciting possibility for new physics that could be within the reach of the LHC is the discovery of an elementary particle that explains the existence of dark matter. If it is caused by a particle, it is definitely not a member of our present standard model catalogue. The discovery

9 2. The physics opportunities at the HL-LHC 7 Figure 5: B s,d µ + µ with integrated luminosity of 300 fb (left) and 3000 fb (right). of supersymmetric particles that respected R-parity would likely be a big step forward in our understanding of dark matter. However several more generic techniques for looking for dark matter have been developed over the last few years. These look for an excess of events with large missing E T, accompanied by a single SM object such as a jet, photon, or vector boson, which could come from initial state radiation and also provides a trigger. These searches turn out to be competitive with the direct search experiments in certain regions of comparison, and projections indicate that the high luminosity upgrade of the LHC can pursue this search below the level of neutrino coherent scattering, which will be a concern, and possibly a limit, for the direct experiments [7]. For this program to be successful in CMS, it is particularly essential that the quality of the missing E T measurement is kept at a similar level as for the present data. Another exciting possibility is to use the Higgs boson as a search tool for dark matter. The Higgs boson may well be a portal connecting the standard model with other new physics sectors, such as the dark sector. In that case, and if the dark matter particle is relatively light, the search for dark matter in the decay of Higgs particles, via the so called invisible decay channel will be an important channel. A new channel proposed for a dark matter search is mono-higgs production [8] similar to e.g. the mono-jet signature, except that the Higgs is emitted in the final state from the produced dark matter particles. For this channel the high luminosity of the HL-LHC will be essential. In the event of a discovery during the first phase of the LHC, the large dataset of the HL-LHC will be critical to unveil the nature of the observed new particles. This will require precise measurement of their properties, such as production cross sections, masses, and spin-parity. It will also be essential to extend the searches of other related new physics signals. In parallel to the searches for new physics and in support of these discovery topics, many measurements of SM phenomena will be made at the HL-LHC. In addition to high statistics measurements that can provide insight into these processes, they will also help define SM backgrounds that must be known and well-modelled to carry out the discovery portion of the program. For example, parton distribution functions (PDFs) of the proton are crucial ingredients of measurements at the LHC. Future Higgs boson coupling measurements will be limited by PDF uncertainties unless significant progress is made. Other precision measurements, like the measurement of the W boson mass, the effective lepton mixing angle, and the strong coupling constant α S, have large uncertainties from PDFs. If new physics phenomena are discovered, their characterization will also suffer from PDF uncertainties, e.g. for gluino or squark produc-

10 8 2 The HL-LHC physics program and experimental challenges tion in the few TeV range, uncertainties can be as large as 00% since they probe PDFs at very large values of the parton momentum fraction, x. Improvements are needed from experimental data, theoretical calculations, and methodological framework. With the high luminosity data, CMS will contribute to this program by precision measurements of inclusive, differential, and double-differential cross sections of events with jets, photons, W and Z bosons, and top quarks. This requires excellent trigger and pileup mitigation capabilities. The charm and strange PDFs can be constrained by measurements of charm-tagged jets in events with electroweak bosons. This will also require excellent vertex reconstruction capabilities. The search for new physics builds on our knowledge of SM physics. 2.2 HL-LHC beam conditions Figure 6: LHC schedule for long shutdowns and luminosity projections through HL-LHC. To achieve the physics program CERN began planning an increase in the instantaneous and integrated luminosities of the LHC above the original design even before the machine went into operation. Major revisions to the machine or the experiments require access to the accelerator tunnels and the experimental areas that can only be accomplished efficiently during long shutdown periods. The current plan calls for a series of long periods of data-taking, referred to as Run I, Run II, Run III etc., interleaved with long shutdowns, designated LS, LS2 and LS3. Run I is the completed data-taking period in 20 and 202. During the first long shutdown, LS, in 203 and 204, modifications were made to the LHC to enable it to operate safely at a center-of-mass energy of 3 TeV for Run II. The bunch spacing has been reduced from 50 ns in Run I to 25 ns for all future runs. The original performance goal for the LHC, to operate at an instantaneous luminosity of 0 34 cm 2 s with 25 ns bunch spacing, is likely to be achieved relatively soon after the start of Run II. Under these conditions, CMS will experience an average of about 25 inelastic interactions per bunch crossing, referred to as pileup in the rest of this document. This is the operating scenario for which the CMS experiment was originally designed. A new scheme to form the bunch trains in the Proton Synchrotron (PS) should allow the luminosity to exceed the original design before the second long shutdown, LS2, planned for In LS2, the injector chain will be further improved and upgraded to deliver very bright bunches (high intensity and low emittance). It is anticipated that the peak luminosity could reach cm 2 s in Run 3, providing an integrated luminosity of over 300 fb by

11 2.2 HL-LHC beam conditions To maintain its present performance in this period, the CMS detector will undergo an initial series of staged upgrades in the period through LS2. This program, known as the CMS Phase I Upgrade, has been documented in a Phase I Technical Proposal [9] and three Technical Design reports (TDRs) describing the upgrades of the Pixel detector, the Hadron calorimeter and the hardware trigger [0 2]. By 2024, the quadrupole magnets that focus the beams at the ATLAS and CMS collision regions are expected to be close to the end of their lives due to radiation exposure. There will be another long shutdown, LS3, to replace them with new quadrupole triplets of larger aperture and higher field, and new insertion magnets will also be installed in the section preceding this region. With these changes, the focus of the beams at the interaction point will be substantially increased with a β parameter as low as 0 cm to 5 cm. This will allow very high peak luminosities that can be tuned (leveled) to lower values along the beam fills. In addition, crabcavities will be added to compensate the crossing angle of the beams, therefore extending the luminous region and reducing the density of the p-p collisions along the beam axis. The schedule of beam operations and long shutdowns, together with projections of the peak and integrated luminosities, is shown in Fig. 6. The high luminosity period that follows LS3 with the upgraded LHC is referred to here as HL-LHC or Phase II. The baseline operating scenario is to level the instantaneous luminosity at cm 2 s from a potential peak value of cm 2 s at the beginning of fills. By design of the LHC upgrades, the leveled luminosity could, however, be tuned to the ultimate value of cm 2 s with slightly shorter fills. In this case, the integrated luminosity would be increased by 30% for the same operating time, but CMS will see an increased pileup of 200 instead of 40. The ultimate instantaneous luminosity projection sets the particle occupancies, trigger requirements, and data rates that the experiments must be prepared to handle to fully exploit the potential of the accelerator. In this latter condition, an integrated luminosity of 4000 fb could be delivered by the end of Phase II, compared to 3000 fb at the baseline luminosity. This defines the requirement for the radiation tolerance margin of the detectors.

12 0 3 Overview of the CMS Phase-II upgrades 3 Overview of the CMS Phase-II upgrades Because of the constraints imposed by the physics program (see Section 2.), a primary goal of the Phase II upgrade is to maintain the excellent performance of the Phase I detector throughout the extended operation of HL-LHC. Under the harsh operating conditions at the HL-LHC, the main challenges that must be overcome are the radiation damage to the CMS detector from the large integrated luminosity and the huge pileup that comes from the high instantaneous luminosity. In developing the upgrade scope, CMS has made a major effort of simulation to understand these effects on the current detector and to identify the mandatory upgrades they require. Details of the issues for each of the CMS subsystems and the proposed remedies are given in the sub-detector chapters of the Technical Proposal. Here, only the main implications and features of the resulting upgrades are reviewed. 3. Performance considerations for the Phase II upgrades In order to design a detector that will continue to perform well as the integrated luminosity approaches fb, predictions of the dose rate and particle fluence for each type of particle are needed. Simulations are used to predict the magnitude and composition of radiation as a function of luminosity. The information on the performance of the current detector under irradiation is obtained from test beam measurements, special radiation exposures, and the beginnings of any radiation damage observed in Run I. All these measurements are used to benchmark the simulations of the radiation damage anticipated at the HL-LHC doses. From these studies, performed for all CMS sub-systems, it is very clear that the tracker and the endcap calorimeters must be entirely replaced for Phase II. With the replacement of these detectors, the performance issues associated with high pileup, that are also the most pronounced in the inner and forward detector regions can be addressed. Pileup effects refer to hits or energy deposits from the additional pp collisions in the current bunch-crossing other than that from the collision containing the hard scatter of interest. Pileup is the largest source of hits in the tracking system, it increases the combinatorial and the complexity of the track reconstruction and can increase the rate of fake tracks. It also adds extra energy to the calorimeter measurements, such as jet energies, associated with the collision that contained a hard scatter. Pileup confuses the trigger and also the offline reconstruction and interpretation of events. It increases the amount of data that has to be read out in each BX that contains a hard scatter and, in fact, at the HL-LHC, most of the data read out will be associated with the pileup collisions rather than the collision containing hard scatters. It also increases the execution time for the reconstruction of events in the High Level Trigger and the offline analysis. Pileup can be observed in a single bunch-crossing by the many collision vertices that are reconstructed by the tracking system. The new tracking system can be designed with enough segmentation to associate charged particles with the correct interaction vertices (up to some efficiency and accompanied by some fake rate). The present calorimeters in CMS however do not have capability to directly associate showers with particular vertices. This is likely to be possible with the new endcap calorimeters in which accurate timing, and finer lateral and longitudinal segmentation will be present. This can further improve pileup mitigation, particularly for neutral particles. In addition to the dominant in-time pileup, mentioned above, there is out-of-time pileup (OOT), which refers to energy left in calorimeters in the crossing of interest by particles in the previous or later bunch crossings The degree of OOT depends on the intrinsic time spread and

13 3. Performance considerations for the Phase II upgrades jitter of the pulses produced by particles in the detector, and by shaping times and other characteristics of the readout electronics. Using timing and pulse shape information, it is possible to correct the energy deposition associated with the OOT pileup. With upgrades of the calorimeter readout electronics required for trigger or other purposes, proper design of the new frontend chips will also allow implementing improved timing measurements to mitigate both the OOT and the in-time pileup effects (see below). To allow operation at the ultimate luminosity, the upgrades of all the readout electronics are designed for efficient data taking up to an average pileup of 200. With this possibility, CMS has also intensified a program of R&D into the use of precision timing to help solve the problem of vertex association for neutral particles. The colliding bunches at the LHC have an RMS length of about 5 cm along the beamline and collisions are spread out by the time it takes for the two colliding bunches to completely pass through one another. This results in a time distribution for the individual collisions within a bunch with an RMS of about 50 ps. With proper design of the barrel and endcap readout electronics shower energy deposits can be timed with a precision substantially lower than this spread, this will allow CMS to reduce the impact of pileup by selecting only those energy deposits consistent with occurring at the same time. A further step in the use of timing would be the addition to CMS of a dedicated timing layer sensitive to minimum ionizing particles (MIPS). This layer should be able to achieve a resolution of ps to measure the timing of the interaction vertex and in combination with the calorimeter measurement would allow an hermetic and full determination of the neutral energy associated with this vertex. The investigation of a timing layer is still at an early stage and is not included in the current scope of the CMS upgrades. However, the studies performed at 200 pileup presented in section 5.3 indicate that such a system could enhance the benefit of operating at the ultimate luminosity of the HL-LHC. The ability to ensure efficient event selection for data acquisition is a key prerequisite to fully benefit from increased luminosity. To achieve the required low transverse momentum, p T, or energy trigger thresholds, the hardware trigger must be upgraded. A sufficient reduction in trigger rate can only be accomplished by improving p T resolution to obtain lower rates without loss of efficiency, and by mitigating the effect of the combinatorial backgrounds arising from pileup. A new approach is therefore being developed that introduces tracking information in the hardware trigger, providing capabilities similar to the current online software trigger (High Level Trigger or HLT). This is an integral part of the design of the Phase II tracker and it also requires a new hardware architecture to incorporate tracker information throughout the trigger. While the addition of track information in the trigger provides mandatory gains in rate reduction with good efficiency, it is also necessary to increase the trigger acceptance rate in order to maintain the required acceptance for all of the important physics channels. This is particularly the case for triggers involving hadrons and photons, for which the sensitivity to pileup is higher and/or the track trigger is somewhat less efficient. The measurement of rare processes is a major goal of the HL-LHC physics program. This requires specific upgrades in the forward regions of the detector to maximize the physics acceptance over the largest possible solid angle. To ensure proper trigger performance within the present coverage, the muon system will be augmented with the addition of new chambers. The new endcap calorimeter configuration also offers the opportunity to extend the muon coverage with a tagging station up to η 3 or more, with significant acceptance gain for multi-muon final states. To also mitigate pileup effects in jet identification and energy measurement, the tracker will be extended up to η 4, thereby also covering the peak production region of jets accompanying Vector Boson Fusion (VBF) and Vector Boson Scattering (VBS) processes. With this extension, the measurements of total and missing energies, which are critical to new

14 2 3 Overview of the CMS Phase-II upgrades physics studies, will be greatly improved. The b-tagging acceptance will also be increased. 3.2 Design optimization of the upgrade elements Tracker system: to ensure adequate track reconstruction performance at the much higher pileup levels of the HL-LHC, the granularities of the outer tracker and of the pixel detector are respectively increased by roughly a factor 4 and 6 compared to the present systems. This produces similar level of occupancies as at a luminosity of 0 34 cm 2 s. In the outer tracker, this is achieved by shortening the lengths of the silicon sensor strips without changing the pitch significantly. The overall configuration of the detector has been optimized with a standalone simulation program that allowed the comparison of several configurations with a performance benchmark established by the current detector. The system that was chosen has barrel layers to cover the central rapidity region and disks in the endcaps, just like the current system. The number of layers in the new configuration has been reduced from 0 to 6 in the barrel and from 9 to 5 disks in the endcaps. This is possible thanks to the implementation of a fourth pixel layer (a solution already adopted for the Phase I upgrade) compared to the original design with three layers. It has to be noted that some of the configurations investigated were dropped for cost reasons. In section 2. it is shown that improved measurement resolution has significant benefits on important physics channels such as the H µµ, however the strip pitch in the outer layers of the detector has only been reduced to 90 µm. This value was identified as the threshold to a more expensive frontend hybrid technology. On the contrary, increasing the pitch by 20%, leading to a similar value as in the current detector, would result in a marginal cost-saving of less than 0.5% of the total tracker cost since it will only reduce the number of frontend chips. To implement track information at the hardware trigger level, a specific module concept has been invented. Each module is made of two sensors, and the on-detector readout electronics measure the direction of the tracks, bent by the high magnetic field of the solenoid. This allows the selection of only those hits associated with tracks of transverse momentum greater than 2 GeV to be sent to the trigger at the bunch crossing rate of 40 MHz. The spacings between the sensors have been optimized with the standalone software to ensure that this selection remains similar as the radius (bending) increases. In the backend electronic boards, tracks will be fully reconstructed and fitted before being sent to the central trigger. With this scheme, CMS has a strong plan to ensure powerful background rejection at the earliest stage of the event selection. To measure the z-coordinate of the tracks, one of the sensors in the modules of the three inner layers has ministrips that are only.5 mm long. Similarly to the strip pitch of the outer layers, the length of the ministrips is chosen to allow lower power consumption and less expensive technology for connection between the frontend chips and the sensors. This limits the resolution in the reconstruction of tracks for the trigger. It has to be noted that since the chips cover the full area of these sensors the number of wafers needed (which drives the cost) is independent of the number of channels. In the optimization process, the minimization of the material in the tracker has been a major goal. Thanks to configuration improvements, special module design, and new techniques developed for the cooling and power distribution systems, the mass in the tracking volume is greatly reduced, resulting in a rate of photon conversions that is lower by a factor 2 in the central region and by a factor up to 6 in the forward regions compared to the existing detector. To reach the high radiation tolerance required, the sensors will be thinner and produced in the planar n-in-p technology. The R&D program is now focusing on working with potential sensor vendors to develop final specifications that will minimize costs.

15 3.2 Design optimization of the upgrade elements 3 The pixel system will implement smaller pixels and thinner sensors. Different configurations described in the Technical Proposal are still being investigated. With up to 0 pixel disks in each of the forward regions, the system coverage will be extended to η 3.8, to cover as much as possible the calorimeter range. Barrel electromagnetic calorimeter: the lead tungstate crystals of the barrel electromagnetic calorimeter (ECAL) will remain performant for the entirety of the Phase II running period and will not be replaced. However, a substantial upgrade to the front-end electronics is required. This is mandatory in order to satisfy the upgraded hardware trigger requirements, to maintain the ability to trigger efficiently on electrons and photons via the improved rejection of anomalous signals in the photodetectors, and to mitigate radiation-induced effects that would degrade the energy resolution. The barrel supermodules will be removed from CMS during LS3 and the front-end electronics will be replaced. Following the upgrade, the data will be transferred off-detector at 40 MHz, simultaneously overcoming present limitations in trigger latency and acceptance rate. The full ECAL granularity will be made available to the hardware trigger, allowing more performant algorithms with improved pileup rejection to be developed. The supermodules will be operated at a lower temperature during Phase II. The predicted aging-induced noise increase in the avalanche photodiodes (APD), which would otherwise dominate the electron and photon energy resolution, will be significantly reduced by cooling the APDs from 8 C to 8 C. A new front-end chip will be designed with a shorter shaping time to further mitigate the APD noise and to provide better OOT pileup rejection. In addition, the design of the front-end chip will incorporate significantly improved rejection of anomalous signals in the APDs. The capability for precision timing is also considered in the specifications and design of the new front-end electronics. Calorimeter endcaps: the detector that will replace the current endcap calorimeters is called the High Granularity Calorimeter (HGCAL). It has electromagnetic and hadronic sections with excellent transverse and longitudinal segmentation for 3D measurement of shower topologies. The electromagnetic section consists of 28 tungsten and copper plates interleaved with silicon sensors as the active material. This provides 25 radiation lengths and.5 interaction lengths (λ). The hadronic part has a front section of 2 brass and copper plates interleaved with silicon sensors for a depth of 3.5 λ. This section measures the hadronic shower maximum measurement. With this design, the fine granularity will also allow precise timing measurements, that will further help mitigating the pileup effects (see section 5..2). The silicon sensor technology is n-in-p, as for the outer tracker, with three different active thicknesses depending on radius/radiation doses. Each sensor has a pad of 0.5 cm 2 or.0 cm 2 depending on its thickness. The choice of the pad size is mostly driven by the input capacitance at the entry of the frontend chip, to ensure sufficient signal to noise ratio for a Minimum Ionizing Particle (MIP) measurement. This allows proper calibration of the system, and at the same time, this range of transverse size is well suited to eliminate pileup energy along the longitudinal development of the shower. Similarly to the tracker, and particularly because the pitch and number of channels per chip is relatively small, reducing the transverse granularity would have little impact on the overall cost of the detector ( 3% of the total endcap calorimetry cost for a pad size increase by a factor 2). To cover the large dynamic range required, the frontend ASIC chip will feature a Time over Threshold TDC that provides the energy measurement and a time measurement with a precision of 50 ps for each pad when the energy deposit exceeds 60 fc ( 30 MIPs). This latter feature will enable the precise timing of showers. The design of the HGCAL draws upon the ILC/CALICE[3] concept. It has been optimized with a standalone simulation benchmarked to the ILC/CALICE performance predictions and

16 4 3 Overview of the CMS Phase-II upgrades incorporating pileup interactions. The optimization process is progressing with the implementation of an entirely new reconstruction software needed in the framework of the CMS particle flow reconstruction. Several developments for the HGCAL have synergies with, and benefit from, those required for the tracker or other systems. Particularly, this includes the approach to the sensor radiation tolerance and the commercial discussions with potential vendors. The R&D focuses on engineering of the complex frontend ASIC chip, of the module and absorber mechanics, and of the cooling system and services. The HGCAL part of the endcap calorimeter is followed by a new backing hadron calorimeter of similar design to the current HE detector, with brass plates interleaved with plastic scintillating tiles readout with a wavelength shifting fiber (WLS), to provide an overall depth of 0λ for the full calorimeter. The required radiation tolerance will be achieved with a new design of the tiles reducing the light path to the WLS and use of new scintillating material. The transverse granularity is also slightly increased to match the HGCAL geometry. Muon system: the various muon systems in CMS, Drift Tubes (DT) and Resistive Plate Chambers (RPC) in the barrel, and Cathode Strip Chambers (CSC) and RPCs in the endcaps, are expected to tolerate the increased radiation levels during Phase II without major degradation. Therefore there is no plan to replace these detectors, but further measurements are underway to confirm their radiation tolerance margins. The one exception is the readout electronics of the DTs, which will suffer radiation damage and will therefore be replaced. This change will also remove the current trigger rate limitation of the system at 300 khz. It has to be noted that also because of the new trigger specifications, some CSC chambers readout electronics will be upgraded, as explained in the trigger paragraph below. In addition to possible radiation-induced failures, the chambers will suffer failures from normal aging because of the long time (>30 years) during which they will operate. The long term rate of these failures has been estimated based on current operational experience. Although it may be possible to repair some failed units during annual shutdowns, it is likely that some will not be recoverable. This risk has been considered in defining the scope of the upgrade, in addition to other performance requirements. In particular, the muon system in the region.5 η 2.4 currently consists of four stations of CSCs. It is the only region of the muon detector that lacks redundant coverage despite the fact that it is a challenging region for muons in terms of backgrounds and momentum resolution. To maintain good efficiency for the muon trigger in this region, these four stations are complemented with additional chambers that make use of new detector technologies with higher rate capability. The first two stations (named GE and GE2) are in a region where the magnetic field is still reasonably high and so will use Gas Electron Multiplier (GEM) chambers for their high granularity and good position resolution. The two last stations (named RE3 and RE4) will use low-resistivity RPCs with lower granularity but good timing resolution to mitigate background effects and complete the redundancy of the system. This upgrade is along the lines of what was planned in the original design of CMS. The configurations of the detectors have been selected to provide adequate position resolution according to the level of magnetic field and of multiple scattering specific to the location of each station. Since the GEM technology is mature and ready for large scale production and because the first station will have substantial benefits to the muon trigger in Run III, it is planned to install it during LS2. More details are given in a dedicated Technical Design Report recently endorsed by the LHCC [4].