Particle Flow and PUPPI in the Level-1 Trigger at CMS for the HL-LHC Benjamin Kreis1;∗ for the CMS Collaboration 1Fermi National Accelerator Laboratory, Batavia, IL 60510, USA Abstract. With the planned addition of tracking information to the Compact Muon Solenoid (CMS) Level-1 trigger for the High-Luminosity Large Hadron Collider (HL-LHC), the trigger algorithms can be completely reconceptualized. We explore the feasibility of using particle flow-like reconstruction and pileup per particle identification (PUPPI) pileup mitigation at the hardware trigger level. This represents a new type of multi-subdetector pattern recognition chal- lenge for the HL-LHC. We present proof-of-principle studies on both physics and hardware-resource performance of a prototype algorithm for use by CMS in the HL-LHC era. 1 Introduction In the mid-2020s, the High-Luminosity Large Hadron Collider (HL-LHC) at CERN will begin colliding protons with an instantaneous luminosity leveled to 5 × 1034 cm−2s−1 with the goal of delivering approximately 300 fb−1 per year for ten years [1]. This increase in luminosity will enable the general-purpose experiments at the HL-LHC, CMS and ATLAS, to maintain progress in their rich physics programs of precision measurements of the standard model and searches for physics beyond the standard model, such as supersymmetry and dark matter. The challenge for the HL-LHC experiments will be to maintain detection efficiency for interesting physics processes at the electroweak energy scale occurring in the same bunch crossings as additional inelastic interactions, known as pileup. Pileup produces extra hits in the tracking subdetectors and extra energy depositions in the calorimeter subdetectors that must be separated from the process of interest. In addition, the subdetectors must be able to withstand the radiation damage caused by the particles produced by these many proton interactions. At the HL-LHC, the average number of pileup interactions is expected to reach 200. The CMS experiment [2] will undergo a comprehensive upgrade to maintain performance in the high-luminosity conditions of the HL-LHC. In this paper, we focus on upgrades to the Level-1 trigger, the first stage of data processing that has a coarse, global view of the detector and determines which collision events to fully read out for further analysis. We explore the feasibility of using particle-level reconstruction and pileup mitigation, the best performing algorithms in current offline processing, in the upgraded trigger. In Section 2, we describe the inputs to the upgraded Level-1 trigger and its architecture. The inputs will ∗e-mail: [email protected] Particle Flow and PUPPI in the Level-1 Trigger at CMS for the HL-LHC Benjamin Kreis, Fermi National Accelerator Laboratory, Batavia, IL for the CMS Collaboration [email protected] Abstract include tracks forWith the the first planned time, and addition the trigger of the will tracking have a correlatorinformationthat in formsthe Level the global1 trigger in CMS for the HL-LHC, the algorithms for Level 1 trigger can view of each collisionbe completely event. Particle-level reconceptualized. reconstruction Following and the pileup example mitigation for offline algorithms reconstruction in CMS to use complementary subsystem information and for the correlator are described in Section 3. In Section 4, results on physics performance and mitigate pileup, we explore the feasibility of using Particle Flow-like and pileup per particle identification (PUPPI) techniques at the uses lead, copper, and copper-tungsten alloy as absorbers while copper and steel are used for the hardware resource requirementshadronic part. Siliconbased sensors are used in on the region a with proof-of-principle the highest level of radiation, whilst study are presented. hardwareplastic trigger scintillator with on-tile level. silicon photomultipliers This are usedrepresents for the part of the CE-H where thea new type of multi-subdetector pattern recognition challenge for the HL-LHC. We present proof-of- radiation level is the lowest. The key parameters of the HGCAL are summarised in figure 1. Surfaces of about 600 m2 of silicon sensors and 500 m2 of scintillator tiles are used. The silicon principle sensorsstudies are finely segmented on into 0.both5 and 1 cm2 cells physics with active thicknesses and of 300 µm, 200 resourceµm usage performance of a prototype algorithm for use by CMS in the HL-LHC era. or 120 µm depending on the region of the detector. These cell parameters are chosen such that 2 Level-1 Trigger forthe signal-over-noise HL-LHC ratio remains high enough for the measurement of MIP signals over the full lifetime of the HGCAL. Scintillator cells of 1° and 1.25° are used, arranged in a ⌘-φ grid. 2018 JINST 13 C02043 CMS Detector Upgrades for HL-LHC Chapter 1. The CMS Phase-2 muon detector Particle Flow Algorithm for Level-1 Trigger 18 2.3. Overview of the upgraded tracker concept 21 Upgrades to the CMS detector for HL-LHC, described in detailη elsewhere0.1 0.2 0.3 0.4 0.5 0.6 0.7 [3,0.8 0.9 4],1.0 1.1 will bring θ° 84.3° 78.6° 73.1° 67.7° 62.5° 57.5° 52.8° 48.4° 44.3° 40.4° 36.8° η θ° 8 ) 1.2 33.5° DTs Hexagonal modules based on Si sensors m CSCs standalone muons R ( MB4 RPCs charged hadrons radiation regions of CE-H RB4 1.3 30.5° 7 trigger primitivesGEMs linked new and improved inputs to the Level-1 trigger. The inputs, referredWheelto 1 as , “Cassettes”: multiple modules mounted on Wheel 0 Wheel 2 22 Chapter 2. Overview of the Phase-2 Tracker Upgrade iRPCs ME0 muons cooling plates with electronics and absorbers MB3 1.4 27.7° CMS-TDR-17-001 6 RB3 T(EM) readout in ME1/3 muon/track RE1/3 RE2/3 RE3/3 RE4/3 1.5 25.2° (track) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 T electrons neutral hadrons 1.6 MB2 track are formed by trigger primitive generators (TPGs). For the first time5 at CMS,RB2 theME2/2 Level-1ME3/2 ME4/2 1.6 22.8° trig- T 1200 linking 1.7 20.7° MB1 (closest in pT) (track), or p 1.8 RB1 T r [mm] 1000 ~2.3m 4 1.8 18.8° RE1/2 RE2/2 RE2/2 RE3/2 RE4/2 unlinked tracks 2.0 1.9 17.0° 800 ME1/2 ger will receive trigger primitives representing tracks with transverseSolenoid magnet momentum pT2.0 15.4°> 2-3 tracks photons 2.2 3 2.1 14.0° 2.2 12.6° (EM) ~ p 600 T (track) 2.4 2.3 11.5° p ME2/1 ME3/1 ME4/1 T 2.6 HCAL RE3/1 RE4/1 2.4 10.4° T(EM) Figure 2.4: Illustration of the module concept. (a) Correlation of signals in closely-spaced GE2/1 significantly larger than p 400 pT 2.8 2 2.5 9.4° 3.0 GE1/1 due to electron+photon sensors enables rejection of low-p particles; the channels shown in green represent the selec- ME1/1 GeV from a new outer trackingT subdetector. The TPGs of a new endcapECAL calorimeter will pro- 2.8 7.0° 200 tion window to define an accepted stub. (b) The same transverse momentum corresponds to a Steel 3.0 5.7° (calo) significantly 4.0 1 HGCAL T larger distance between the two signals at large radii for a given sensor spacing. (c) For the end- Silicon ME0 tracker 0 4.0 2.1° cap discs, a larger spacing between the sensors is needed to achieve the same discriminating CMS-TDR-016 (track) due to linked, or unlinked low p unlinked calorimeter, 0 500 1000 1500 2000 2500 z [mm] 5.0 0.77° electromagnetic/ no track, Tor p or p 13 C02043 0 larger than p vide energy depositionpower as in the barrel atinformation the same radius. at improved~2m granularity. New0 1 TPGs2 3 4 in5 6 the7 8 barrel9 10 11 12 electro- Figure 2.3: Sketch of one quarter of the tracker layout in r-z view. In the Inner Tracker the z (m) significantly larger J.-B. Sauvan 2018 JINST green lines correspond to pixel modules made of two readout chips and the yellow lines to track linking than p 473 old depends on the acceptance window, which can be tuned to a certain level by programming Figure 1.4: An R-z cross section of a quadrant of the CMS detector, including the Phase-2 up- electromagnetic electron+photon pixel modules with four readout chips.474 Inthe the respective Outer Tracker setting in the the blue readout and chip. red Forlines the representpT modules a few different values of sensor grades (RE3/1, RE4/1, GE1/1, GE2/1, ME0). The acronym iRPCs in the legend refers to the (closest in ΔR) new improved RPC chambers RE3/1 and RE4/1. The interaction point is at the lower left cor- calorimeter/track mageticthe two types ofcalorimeter modules described in the475 text.spacing are used, will optimized to also achieve the desired improvepT filtering in different regions granularity, of the de- while the TPGs of the hadron calorimeter unlinked tracks ner. The locations of the various muon stations are shown in color (MB = DT = Drift Tubes, ME calorimeter All new silicon476 tector (Fig. 2.4 (b) and (c)).tracker For a pitch of about 100 µ m betweenH silicon i gstrips (or h macro-pixels, g r a n u l a r i t y New Cathode Strip 477 as detailed below) in the transverse plane, sufficient pT resolution can be achieved down to a = CSC = Cathode Strip Chambers, RB and RE = RPC = Resistive Plate Chambers, GE and ME0 linking 478 radius of about 200 mm in a barrelFigure geometry, 1.