The CMS Particle Flow algorithm in CMS Boris
The CMS Particle Flow algorithm in CMS Boris Mangano (ETH Zürich) on behalf of the CMS collaboration 1
Reconstruct & identify all stable particles in the event in a optimal way Tracker ECAL HCAL Magnet Muon 2
m neutral hadron photon charged hadrons 3
From particles to PF particles Detector measurements “True” or generated particles m neutral hadron photon P Int artic era le c t De tec ion & tio n charged hadrons PF particles m neutral hadron photon Analysis as if it is done on generator level particles Flow e l c i t Par ion t c u r t s recon charged hadrons Boris Mangano Latsis Symposium 2013 4
Particle flow past and present Boris Mangano Latsis Symposium 2013 5
Particle flow and jets Transverse view (x-y plane) Boris Mangano Latsis Symposium 2013 page 6
Calorimeter resolution: Can Tracker help Calorimeter also in this? Calorimeter (Eecal + Ehad) resolution to hadrons: For CMS, stochastic term a ≈ 110 -120% Why is it so large and how can be reduced? Let’s consider for a moment a toy model for a calorimeter Calorimeter response R is: R=1 for E=20 Ge. V R<1 for E<20 Ge. V R>1 for E>20 Ge. V Boris Mangano Latsis Symposium 2013 7
measured energy calorimeter 55 Ge. V 20 Ge. V Boris Mangano 35 Ge. V 20 Ge. V 5 Ge. V 40 Ge. V 20 Ge. V 30 Ge. V 20 Ge. V “true” energy fragmentation/h adronization Calorimeter resolution & response: fragmentation 45 Ge. V 20 Ge. V 10 Ge. V 20 Ge. V 50 Ge. V parton Latsis Symposium 2013 50 Ge. V parton 8
measured energy calorimeter 20 Ge. V 5 Ge. V 20 Ge. V “true” energy fragmentation/h adronization Calorimeter resolution & response: fragmentation 45 Ge. V Boris Mangano 20 Ge. V 10 Ge. V Measured jet energy depends on how the parton fragment 50 Ge. V parton Latsis Symposium 2013 9
Calorimeter resolution: intrinsic fluctuations single particle calorimeter 20 Ge. V hadron 21 Ge. V 20 Ge. V 19 Ge. V 20 Ge. V Single particle energy measurement depends on intrinsic fluctuations of: - calorimeter sampling - showering -. . Boris Mangano Latsis Symposium 2013 10
reconstructed tracks calorimeter clusters Tracker+Calorimeter: Jet. Plus. Track 20 Ge. V 5 Ge. V 20 Ge. V Option 1: subtract from calorimeter measurements the expected average energy deposit caused by the pointing tracks 20 Ge. V 10 Ge. V - reduces effect of parton fragmentation - measurement is still sensitive to intrinsic calorimeter resolution “Jet. Plus. Track” or Energy. Flow approach Boris Mangano Latsis Symposium 2013 11
20 Ge. V Boris Mangano Option 2: replace observed calorimeter cluster energy with the energy of the pointing/matched tracks 20 Ge. V 5 Ge. V 20 Ge. V reconstructed particle reconstructed tracks calorimeter clusters Tracker+Calorimeter: Particle. Flow 20 Ge. V 10 Ge. V - reduce effect of parton fragmentation - effectively replace calorimeter energy resolution with tracker momentum resolution for charged hadrons - neutral hadrons reconstruction still dominated by calorimeter resolution 50 Ge. V parton Particle Flow approach Latsis Symposium 2013 12
A real case: CMS detector • Calorimeter jet: – E = EHCAL + EECAL – σ(E) ~ calo resolution to hadron energy: 120 % / √E – direction biased (B = 3. 8 T) • Particle flow jet: – charged hadrons • σ(p. T)/p. T ~ 1% • direction measured at vertex – photons/electrons • σ(E)/E ~ 1% / √E • good direction resolution – neutral hadrons • σ(E)/E ~ 120 % / √E Boris Mangano Still poor resolution, but neutral hadrons are the smallest component of the jet/event particles: - 70% charged hadrons - 20% photons - less than 10% neutral hadrons Latsis Symposium 2013 13
Jet energy resolution Particle Flow converges to a calorimetric measurement at high p. T when: - calorimetric clusters corresponding to different particles cannot be separated - calorimetric resolution is comparable or better than tracker one Boris Mangano Latsis Symposium 2013 14
Jet energy response PF Jets Calo Jets PF jet response almost independent from the flavour of the jet-initiating parton Boris Mangano Latsis Symposium 2013 15
Tau reconstruction p 0 p+ SIMULATION pp+ Barrel particle flow calorimeter-based t Particle flow is at its best in the reconstruction of taus: neutral hadron component (the component that is worst measured) is minimal Boris Mangano Latsis Symposium 2013 16
MET resolution Z p. T > 100 Ge. V Boris Mangano Latsis Symposium 2013 17
Electron reconstruction and Isolation Boris Mangano Latsis Symposium 2013 18
CONCLUSION The CMS Particle Flow: • Improves the reconstruction of basically all physics objects (resolution improvement up to a factor 2 X for Jets and MET) • Makes analysis of data as if it is done on generator level particles • Performs in data as expected from simulation Most analyses in CMS are now using Particle Flow Boris Mangano Latsis Symposium 2013 19
Why particle flow ? The whole is greater than the sum of its parts (Aristotle) Boris Mangano Latsis Symposium 2013 20
Backup slides Boris Mangano Latsis Symposium 2013 21
Backup slides on cluster-track linking Boris Mangano Latsis Symposium 2013 22
Linking – ECAL view • Track impact within cluster boundaries track & cluster linked Boris Mangano Latsis Symposium 2013 23
Linking – HCAL view • Track impact within cluster boundaries track & cluster linked • Clusters overlapping clusters linked Boris Mangano Latsis Symposium 2013 24
Links and blocks • Links: – Track-ECAL – Track-HCAL – ECAL-HCAL – Track-track – ECAL-preshower ECAL Boris Mangano Track • The block building rule: HCAL ECAL Track Latsis Symposium 2013 – 2 linked PF elements are put in the same blocks 3 typical blocks 25
Charged hadrons, overlapping neutrals • For each HCAL cluster, compare: – Sum of track momenta p – Calorimeter energy E • Linked to the tracks • Calibrated for hadrons • E and p compatible – Charged hadrons • E > p + 120% √p – Charged hadrons + – Photon / neutral hadron • E<<p – Need attention … – Rare: muon, fake track Boris Mangano Latsis Symposium 2013 26
Charged+neutrals: E ≈ p • Charged hadron energy from a fit of pi and E – i = 1, . . , Ntracks – Calorimeter and track resolution accounted for • Makes the best use of the tracker and calorimeters – Tracker measurement at low p. T – Converges to calorimeter measurement at high E Boris Mangano Latsis Symposium 2013 27
Charged+neutrals: E > p • Significant excess of energy in the calorimeters: E > p + 120% √E • Charged hadrons [ pi ] • Neutrals: – E from ECAL or HCAL only: • HCAL h 0 • ECAL γ [E–p] [ EECAL – p/b ] – E from ECAL and HCAL: • E-p > EECAL ? – γ – h 0 [ EECAL ] with the rest – γ [ (E – p) / b ] • Else: Boris Mangano Latsis Symposium 2013 Always give precedence to photons 28
Backup slides on tracker/tracking Boris Mangano Latsis Symposium 2013 29
Tracking system • Huge silicon tracker • Hermetic • Highly efficient TIB Boris Mangano TOB Latsis Symposium 2013 30
Tracking system • Huge silicon tracker • Hermetic • Highly efficient • But up to 1. 8 X 0 – Nuclear interactions – g conversions – e- brems Boris Mangano Latsis Symposium 2013 31
Tracking • Efficient also for secondary tracks • Secondary tracks used in PF: Nuclear interaction vertices – Charged hadrons from nuclear interactions Displaced beam pipe! • No double-counting of the primary track momentum – Conversion electrons • Converted brems from electrons Boris Mangano Latsis Symposium 2013 32
Backup slides on PF clustering Boris Mangano Latsis Symposium 2013 33
PF Clustering • Used in: – ECAL, HCAL, preshower • Iterative, energy sharing – Gaussian shower profile with fixed σ • Seed thresholds – ECAL : E > 0. 23 Ge. V – HCAL : E > 0. 8 Ge. V Boris Mangano Latsis Symposium 2013 34
PF Clustering • Used in: – ECAL, HCAL, preshower • Iterative, energy sharing – Gaussian shower profile with fixed σ • Seed thresholds – ECAL : E > 0. 23 Ge. V – HCAL : E > 0. 8 Ge. V Boris Mangano Latsis Symposium 2013 35
Other Backup slides Boris Mangano Latsis Symposium 2013 36
MET response Boris Mangano Latsis Symposium 2013 37
Jet energy resolution (MC) Factor 2 improvement at low p. T Boris Mangano Particle Flow converges to a calorimetric measurement at high p. T when calorimetric clusters corresponding to different particles cannot be separated Latsis Symposium 2013 38
• ϕ 1 HCAL tower • η Jets : η and ϕ Resolution Boris Mangano Latsis Symposium 2013 39
Recipe for a good particle flow • Separate neutrals from charged hadrons – Field integral (Bx. R) – Calorimeter granularity • Efficient tracking • Minimize material before calorimeters • Clever algorithm to compensate for detector imperfections Boris Mangano PF Jet, p. T = 140 Ge. V/c Data Latsis Symposium 2013 40
Recipe for a good particle flow • Strong magnetic field: 3. 8 T • ECAL radius 1. 29 m • Bx. R = 4. 9 T. m – ALEPH: – ATLAS: – CDF: – DO: Boris Mangano PF Jet, p. T = 140 Ge. V/c Data Latsis Symposium 2013 1. 5 x 1. 8 = 2. 7 T. m 2. 0 x 1. 2 = 2. 4 T. m 1. 5 x 1. 5 = 2. 25 T. m 2. 0 x 0. 8 = 1. 6 T. m 41
Neutral/charged separation (1) ECAL granularity • A typical jet – p. T = 50 Ge. V/c • Cell size: – 0. 017 x 0. 017 Good! Boris Mangano Latsis Symposium 2013 42
Neutral/charged separation (2) HCAL granularity • A typical jet – p. T = 50 Ge. V/c • Cell size: – 0. 085 x 0. 085 – 5 ECAL crystals Bad… Boris Mangano Latsis Symposium 2013 43
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