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Experimental Prospects at the LHC: Particle Detection and Reconstruction

This lecture series provides an introduction to the ATLAS and CMS experiments at the LHC, discussing historical perspective, physics goals, expected performances of detectors, and experiences with cosmic and collision data. It also explores the challenges of physics at the LHC, including the unprecedented scope and timescales for simulations, material effects on particle detection and reconstruction, and design choices made for the ATLAS and CMS experiments.

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Experimental Prospects at the LHC: Particle Detection and Reconstruction

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  1. Honorabilis et amplissimus rector, laudati conlegae Experimental prospects at the LHC

  2. Particle detection and reconstruction at the LHC (and Tevatron) Lecture 1 • Introduction to ATLAS/CMS experiments at the LHC: historical perspective and physics goals • Experimental environment and main design choices Lecture 2 • Expected performances of ATLAS and CMS detectors Lecture 3 • Experience collected with cosmics (2008-2009), first data with collisions (end 2009) and expectations for early physics

  3. Interactions every 25 ns … In 25 ns particles travel 7.5 m Physics at the LHC: the environment 22 m Weight: 7000 t 44 m Time-of-flight Cable length ~100 meters … • In 25 ns signals travel 5 m

  4. Physics at the LHC: the challenge Unprecedented scope and timescales for simulations Many examples from ATLAS/CMS (and also ALICE/LHCb): •  30 million volumes simulated in GEANT • Tbytes (hundreds of millions) of simulated events over 10 years • Full reconstruction of all benchmark Higgs-boson decays • Unprecedented amount of material in Inner Detectors leads to significant losses of e.g. charged-pion tracks (up to 20% at 1 GeV) and to significant degradation of EM calo intrinsic performance (mostly for electrons but for photons too) • b-tagging at low luminosity, for e.g. H  bb searches, yields performance similar to that which has been achieved at LEP

  5. Main specific design choices of ATLAS/CMS • Size of ATLAS/CMS directly related to energies of particles produced: need to absorb energy of 1 TeV electrons (30 X0 or 18 cm of Pb), of 1 TeV pions (11 l or 2 m Fe) and to measure momenta of 1 TeV muons outside calorimeters (BL2 is key factor to optimise) • Choice of magnet system has shaped the experiments in a major way • Magnet required to measure momenta and directions of charged particles near vertex (solenoid provides bend in plane transverse to beams) • Magnet also required to measure muon momenta (muons are the only charged particles not absorbed in calorimeter absorbers) • ATLAS choice: separate magnet systems (“small” 2 T solenoid for tracker and huge toroids with large BL2 for muon spectrometer) • Pros: large acceptance in polar angle for muons and excellent muon momentum resolution without using inner tracker • Cons: very expensive and large-scale toroid magnet system • CMS choice: one large 4 T solenoid with instrumented return yoke • Pros: excellent momentum resolution using inner tracker and more compact experiment • Cons: limited performance for stand-alone muon measurements (and trigger) and limited space for calorimeter inside coil

  6. Main specific design choices of ATLAS/CMS • At the LHC, which is essentially a gluon-gluon collider, the unambiguous identification and precise measurement of leptons is the key to many areas of physics: • electrons are relatively easy to measure precisely in EM calorimeters but very hard to identify (imagine jet  leading p- with p-  leading p0 very early in shower) • muons in contrast are relatively easy to identify behind calorimeters but very hard to measure accurately at high energies  This has also shaped to a large extent the global design and technology choices of the two experiments • EM calorimetry of ATLAS and CMS is based on very different technologies • ATLAS uses LAr sampling calorimeter with good energy resolution and excellent lateral and longitudinal segmentation (e/g identification) • CMS use PbWO4 scintillating crystals with excellent energy resolution and lateral segmentation but no longitudinal segmentation • Broadly speaking, signals from H  gg or H ZZ* 4e should appear as narrow peaks (intrinsically much narrower in CMS) above essentially pure background from same final state (intrinsically background from fakes smaller in ATLAS)

  7. ATLAS/CMS: from design to reality 20.5 kA All magnets have reached their design currents: a major technical milestone!

  8. ATLAS/CMS: from design to reality Amount of material in ATLAS and CMS inner trackers Weight: 4.5 tons Weight: 3.7 tons LEP detectors Active sensors and mechanics account each only for ~ 10% of material budget Need to bring 70 kW power into tracker and to remove similar amount of heat Very distributed set of heat sources and power-hungry electronics inside volume: this has led to complex layout of services, most of which were not at all understood at the time of the TDRs

  9. ATLAS/CMS: from design to reality 1.90 Material increased by ~ factor 2-2.5 from 1994 (approval) to now (end constr.) Electrons lose between 25% and 70% of their energy before reaching EM calo Between 20% and 65% of photons convert into e+e- pair before EM calo Need to know material to ~ 1% X0 for precision measurement of mW (< 10 MeV)!

  10. ATLAS/CMS: from design to reality Performance of CMS tracker is undoubtedly superior to that of ATLAS in terms of momentum resolution. Vertexing and b-tagging performances are similar.However, impact of material and B-field already visible on efficiencies.

  11. Remember that tracking at the LHC is a risky business! ATLAS pixels, September 2006 CMS silicon strips • 200 m2 Si, 9.6 million channels • 99.8% fully operational • Signal/noise ~ 25/1 • 20% cosmics test under way • Inst. in CMS: August 2007 • All modules and services integrated and tested • 80 million channels ! • 10%-scale system test with cosmics done at CERN • Inst. in ATLAS: June 2007 CMS Tracker Inner Barrel, November 2006

  12. Remember that tracking at the LHC is a risky business! ATLAS pixel beam tests: intrinsic resolution in bending plane before and after irradiation to a fluence of 1015 neutronsequ per cm2 Pixel size is 50 mm x 400 mm in Rf x z CMS pixel beam tests in 3T field: extrapolate by simulation to expected behaviour versus incidence angle, voltage bias and total neutron fluence collected in 4T field Pixel size is 150 mm x 150 mm in Rf x z But ATLAS/CMS tracking specs do not marry well with detailed particle-ID

  13. What are the limitations of ATLAS/CMS tracking detectors in terms of particle-ID? Look at ALICE/LHCb ALICE TPC (Time Projection Chamber) • Measure many samples of dE/dx per track (need >> 25 ns!!) • At low momenta, non-relativistic particles can be separated from each other through precise dE/dx measurements:Bethe-Bloch:-<dE/dx> = k 1/2 ( 0.5 Log(2mec222Tmax/I2) - 2-/2)

  14. What are the limitations of ATLAS/CMS tracking detectors in terms of particle-ID? Look at ALICE/LHCb Overall particle-ID in ALICE for heavy-ion physics

  15. What are the limitations of ATLAS/CMS tracking detectors in terms of particle-ID? Look at ALICE/LHCb LHC-b RICH detectors RICH1: larger solid angle,lower part of momentum spectrum • Aerogel (hygroscopic…) -n=1.03  (=1)=242 mrad -thickness=5 cm -nb detected photons=~7/ring (=1) • C4F10 p=1013 mb at –1.9C -n=1.0014 /260 nm  (=1)=53 mrad -thickness=85cm -nb photons=~30/ring RICH2: • CF4 -n=1.0005 /260 nm  (=1)=32 mrad -thickness=180cm -nb photons=~30/ring

  16. What are the limitations of ATLAS/CMS tracking detectors in terms of particle-ID? Look at ALICE/LHCb With RICH LHC-b RICH detectors

  17. ATLAS/CMS: from design to reality R&D and construction for 15 years  excellent EM calo intrinsic performance Stand-alone performance measured in beams with electrons from 10 to 250 GeV

  18. ATLAS/CMS: from design to reality ATLAS All g ATLAS Actual performance expected in real detector quite different!! Photons at 100 GeVATLAS: 1-1.5% energy resol. (all g)CMS: 0.8% energy resol. (eg ~ 70%) Electrons at 50 GeVATLAS: 1.5-2.5% energy resol. (use EM calo only)CMS: ~ 2.0% energy resol. (combine EM calo and tracker)

  19. ATLAS/CMS: from design to reality Huge effort in test-beams to measure performance of overall calorimetry with single particles and tune MC tools: not completed!

  20. ATLAS/CMS: from design to reality One word about neutrinos in hadron colliders: • since most of the energy of the colliding protons escapes down the beam pipe, one can only use the energy-momentum balance in the transverse plane  concepts such as ETmiss, missing transverse momentum and mass are often used (only missing component is Ezmiss)  reconstruct “fully” certain topologies with neutrinos, e.g. W  ln and even better H  tt  lnlnt hnt • the detector must therefore be quite hermetic transverse energy flow fully measured with reasonable accuracy  no neutrino escapes undetected no human enters without major effort (fast access to some parts of ATLAS/CMS quite difficult)

  21. ATLAS/CMS: from design to reality CMS 11l

  22. ATLAS/CMS: from design to reality ATLAS 11l

  23. ATLAS/CMS: from design to reality ATLAS For an integrated luminosity of ~ 100 pb-1, expect a few events like this? This is apparent ETmiss occurring in fiducial region of detector!

  24. ATLAS/CMS: from design to reality ATLAS Curve: 0.55 √SET ATLAS Biggest difference in performance perhaps for hadronic calo Jets at 1000 GeVATLAS ~ 3% energy resolutionCMS ~ 5% energy resolution, (but expect sizable improvement using tracks at lower energies) Curve: 0.57 √SET ETmiss at SET = 2000 GeV ATLAS: s ~ 25 GeV CMS: s ~ 40 GeV This may be important for high mass H/A to tt

  25. ATLAS/CMS: from design to reality CMS Biggest difference in performance perhaps for hadronic calo: how much can be recovered using energy-flow algorithms? Jets in 20-100 GeV range are particularly important for searches (e.g. H  bb) For ET ~ 50 GeV in barrel: ATLAS: ~ 10% energy resolution CMS: ~ 19% energy resolution (with calo only), ~ 14% energy resolution (with calo + tracks) Some words of caution though: danger from hadronic interactions in tracker material non-Gaussian tails in response gains smaller at large h (material) and at high energy linearity of response at low energy!

  26. ATLAS/CMS: from design to reality CMS CMS muon spectrometer • Superior combined momentum resolution in central region • Limited stand-alone resolution and trigger (at very high luminosities) due to multiple scattering in iron • Degraded overall resolution in the forward regions (|h| > 2.0) where solenoid bending power becomes insufficient

  27. ATLAS/CMS: from design to reality ATLAS ATLAS muon spectrometer • Excellent stand-alone capabilities and coverage in open geometry • Complicated geometry and field configuration (large fluctuations in acceptance and performance over full potential h x f coverage (|h| < 2.7)

  28. ATLAS/CMS: from design to reality 2(+4) CMS muon performance driven by tracker: better than ATLAS at h ~ 0 ATLAS muon stand-alone performance excellent over whole h range

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