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Study of FSR in H to ZZ to 4l events

Study of FSR in H to ZZ to 4l events. Higgs meeting 11/11/2011 C. Charlot & D. Sabes, for the H->4l group. Introduction. Final state photon emission from the Z (*) decay can potentialy affect the mass measurement

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Study of FSR in H to ZZ to 4l events

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  1. Study of FSR in H to ZZ to 4l events Higgs meeting 11/11/2011 C. Charlot & D. Sabes, for the H->4l group

  2. Introduction • Final state photon emission from the Z(*) decay can potentialy affect the mass measurement • Small effect on the Z mass cut acceptance thanks to the large window used (50<m<120) • FSR in general little affects the Z mass reconstruction, only rare cases of hard emission can bias the mass measurement significantly compared to the instrumental resolution • A large part is already recovered by the SuperClustering in the ECAL (60% of FSR within the SC area) • In recovering such events, one has to care about background contamination that leads to mass migrations, hence we currently apply FSR recovery at the end of the event selection • Less background • Event interpretation at the end of the analysis

  3. Generators tools for FSR Pythia Sherpa In the following, Pythia MC is used Pythia gives similar results than photos https://indico.cern.ch/getFile.py/access?contribId=2&resId=0&materialId=slides&confId=88501 Pythia and Sherpa gives similar results for the ZZ continuum

  4. FSR caracteristics h No ET cut ET>5 GeV ETg DR(l, g) No ET cut

  5. FSR caracteristics AN2011-044 AN2011-044 AN2011-044 Results found similar to the ones obtaind in previous CMS studies AN2011-044 (DY measurement , Zmmg) also show small pileup contribution for ET>5 GeV as well as excellent data/MC agreement

  6. Effect on invariant masses Generated spectrum=before FSR Fraction of (ee) loss with 60GeV cut because of FSR : 1.41% Fraction of (ee) loss with 70GeV cut because of FSR : 3.29% Fraction of (ee) loss with 80GeV cut because of FSR : 6.76% Fraction of (ee) loss with 86GeV cut because of FSR : 10.73% pT(l)>7 GeV

  7. FSR selection Based on the previous studies, a simple FSR selection is used FSR selection • FSR candidates are SC or reconstructed photons satisfying: • ET>5(10) GeV for the Z1(Z2), as FSR photons have harder spectrum than backgd photons • min DR(l, g)<0.7, as FSR are emitted mostly in the lepton direction • For the pair containg the closest lepton, |m(llg)-91.2|<|m(ll)-91.2|, to further reduce backgd contamination • Don’t try other combination Other CMS studies: • AN2011-044 (DY measurement, Zmmg): • « Close » photons: DR<0.5, ET>3 GeV • « Wide » photons: DR>0.5, ET>3 GeV, Iso<0.5 • AN2011-088 (photon calibration, Zmmg): • ET>10 GeV, DR<0.8 • 40<mmm <80 GeV, 70<mmmg <100 GeV or 87.2<mmmg <95.2 GeV

  8. FSR selection: performances • Purity results: ET>5, DR<0.7 • After ET cut only: 50.5% • After ET cut and DR cut: 85.7% • After all cuts: 94.9% Fraction of correct assignment when attaching the FSR to the closest lepton: Overal90.31% DR<0.199.80% 0.1<DR<0.3 94.33% 0.3<DR<0.779.76% • Purity results: ET>5, 0.3<DR<0.7 • After all cuts: 69.3%

  9. Zee vs Zmumu Up to now expectations presented for ZZ->4e and Z->ee • Zeeg/Zmmg ~ 2 in the range ET=[5-30] GeV • But the excess in the electron case is mostly constituted by colinear emission • Well within the SC collection area • Hence expects nearly the same rate in Zee and Zmm DR(l, g) ETg

  10. FSR and isolation • The HZZ4l analysis uses an isolation cut on the sum of the two least isolated leptons • CombIso3+CombIso4<0.35 • The combined isolation makes use of the ECAL measurement (cone DR=0.3), hence expect FSR events (~74% within such cone size) to induce an efficiency loss due to the isolation cut • In HZZ4l selection the isolation cut is loose enough so to expect a small efficiency loss • Moreover for the case of electrons, a large fraction of FSR (~60%) is within the Supercluster area, and an additional fracrion falls in the jurassic veto area • In the end the efficiency loss is estimated to a few %, this needs to be checked and possibly the ECAL isolation could be relaxed for the muon case

  11. e/mu overlap FSR in Zmumu also leads to well known e/mu ambiguities • A photon SC nearly colinear with the muon track seeds a GSF track using the same hits as the muon track • ET>4 GeV is required to seed an electron track • Removed in the analysis by assigning the candidate to a muon when the tracks are fully shared (a cone of dR=0.05 is used)

  12. Results Results for the new baseline selection up to 4.0/fb • Applying the FSR recovery algorithm on data we found 3 candidate events with FSR(4 candidates FSR) passing the baseline selection, out of the 61 HZZ4l candidate events (122 Zll pairs) • Event M (4mu): • photon of ET=13 GeV, DR=0.59 from closest lepton (muon, Z1 pair) • mZ1=77.8 mZ1=91.3 GeV, m4l = 119.0 GeV  m4lg = 131.9 GeV • Event RF (2e2mu): • photon of ET=18.8 GeV, DR=0.66 from closest lepton (electron, Z1 pair) and photon of ET=11.0 GeV, DR=0.68 from closest lepton (muon, Z2 pair) • mZ1=75.3 mZ1=93.0 GeV, mZ2=12.9 mZ2=17.4 GeV, m4l = 129.9 GeV  m4lg = 162.9 GeV • Event AY (4mu): • SC of ET=7.6 GeV, DR=0.34 from closest lepton (muon, Z1 pair) • mZ1=83.4 mZ1=94.2 GeV, m4l = 114.8 GeV -> m4lg = 126.0 GeV

  13. Results • New event BK in last 0.7/fb (still be checked with the standard code) • 1 extra photon of ET=19.0 GeV, well isolated • Adding the photon to the closest lepton (muon, DR=0.31), which belongs to the Z2 pair increases its invariant mass from 68.6 to 91.4 GeV • With this photon added to the 4l, the new mass becomes 191.2 GeV instead of 170.5 GeV

  14. Results Observed vs expected for the new baseline selection up to 4.0/fb • Expected fraction of events with FSR passing the selection, 50<mZ1<120, 12<mZ2<120 • 4e case: 2.1%; 4mu case: 2.2% • For 2e2mu case, assume A(2e2mu)=(A(4e)+A(4mu))/2 • Observed/expected restricted to 0.3<DR<0.7 • No observed events for DR<0.3 61 HZZ4l candidates

  15. Event candidate M • 4mu event • One extra electron in the forward region (pT=11.7 GeV), well away from the leptons and not well isolated • 3 other tracks pT>5 GeV, one being the additonal electron • pfMET: 21.8 GeV • 6 vertices, all leptons coming from the same primary vertex

  16. Event candidate M • Extra photon of ET=13 GeV, well isolated, passing the FSR selection • Adding it to the closest muon (DR=0.55), the one with pT=21 GeV which belongs to the Z1 pair increases its invariant mass from 77.8 to 91.3 GeV • With this photon added to the 4l, the new mass becomes 131.9 GeV instead of 119.0 GeV

  17. Event candidate RF • 2e2mu event • No extra electron nor muon • 2 extra photons • 3 extra tracks pT>5 GeV • 2 extra jets ET>10 GeV • pfMET: 13.7 GeV • 4 vertices, all leptons coming from the same primary vertex

  18. Event candidate RF • 2 extra photons of ET=18.8 and 11.0 GeV, well isolated and passing the FSR selection • Adding the leading to the closest lepton (electron, DR=0.66), which belongs to the Z1 pair increases its invariant mass from 75.3 to 93.0 GeV • Adding the subleading to the closest lepton (muon, DR=0.66), which belongs to the Z2 pair increases its invariant mass from 12.9 to 17.4 GeV • With these photons added to the 4l, the new mass becomes 162.9 GeV instead of 129.9 GeV

  19. Event candidate AY • 4mu event • 1 extra electron, pT=6.7 GeV, within a jet • 1 extra isolated SC • 6 extra tracks pT>5 (one is the extra electron) • 18 extra jets ET>10 GeV (leading one with ~27 GeV) • pfMET: 11.6 GeV • 25 vertices (all leptons coming from the same primary vertex)

  20. Event candidate AY • SC of ET=7.6 GeV isolated and passing the FSR selection • Adding the leading to the closest lepton (muon, DR=0.34), which belongs to the Z1 pair increases its invariant mass from 83.4 to 94.2 GeV • With this photon added to the 4l, the new mass becomes 126.0 GeV instead of 114.8 GeV

  21. Conclusions • A simple algorithm is used to recover wide angle hard FSR photon from Z(*) decay • ET>5 (10) for 50<mZ<120 (12<mZ<50) • dR<0.7 • |m(llg)-mZ0|<|m(ll)-mZ0| with mZ0=91.2 GeV • The algorithm is applied a posteriori at the end of the analysis • Benefits from low background • Allow for further event interpretation • Up to 4.0/fb a total of 3 events out of the 61 events passing the baseline selection is found with photon candidates passing the FSR selection • Within expectation given the current stat • Applying or not the FSR recovery is a matter of event interpretation • In average it improves if the purity is >50% but with low stat it can also wash out a Higgs signal.. • A 2D plot m4lg vs m4l can be a nice way to keep open the event interpretation

  22. Next steps • Full integration in the standard software (ongoing) • Better corrections and calibration for photons (ongoing) • Up to now using superclusters, moving to photons (ET>10 GeV) and SC (5<ET<10 GeV) • Need to adapt photon corrections for the low ET regime • Data driven control (underway, ~2 weeks needed) • Purity measurement using Z->ll at step 1 of the analysis, using Z->ll as tag and photon passing part of the FSR selection to probe e.g. the purity of the DmZ cut • Further optimisation of the purity working point • Data driven control of pileup effect • Control of efficiency on data • Associated systematics (to be started)

  23. Backup

  24. Event candidate M • 4mu event • One extra electron in the forward region (pT=11.7 GeV), well away from the leptons and not isolated • 3 other tracks pT>5 GeV, one being the additonal electron • pfMET: 21.8 GeV • 6 vertices, all leptons coming from the same primary vertex • Extra photon of ET=13 GeV, well isolated, passing the FSR selection • Adding it to the closest muon (DR=0.55), the one with pT=21 GeV which belongs to the Z1 pair increases its invariant mass from 77.8 to 91.3 GeV • With this photon added to the 4l, the new mass becomes 131.9 GeV instead of 119.0 GeV

  25. Event candidate RF • 4e event • No extra electron nor muon • 2 extra photons • 3 extra tracks pT>5 GeV • 2 extra jets ET>10 GeV • pfMET: 13.7 GeV • 4 vertices, all leptons coming from the same primary vertex • Presence of 2 extra photons of ET=18.8 and 11.0 GeV, well isolated and passing the FSR selection • Adding the leading to the closest lepton (electron, DR=0.66), which belongs to the Z1 pair increases its invariant mass from 75.3 to 93.0 GeV • Adding the subleading to the closest lepton (muon, DR=0.68), which belongs to the Z2 pair increases its invariant mass from 12.9 to 17.4 GeV • With these photons added to the 4l, the new mass becomes 162.9 GeV instead of 129.9 GeV

  26. Event candidate BK • 2e2mu event • 2 extra electrons forming a nice fully reconstructed conversion • 1 extra photon, well isolated • 10 extra tracks pT>5 (clustered in 2 jets) • 3 extra jets Et>10 • pfMET: 9.1 GeV • 10 vertices, all leptons coming from the same primary vertex • 1 extra photon of ET=19.0 GeV, well isolated • Adding the photon to the closest lepton (muon, DR=0.31), which belongs to the Z2 pair increases its invariant mass from 68.6 to 91.4 GeV • With this photon added to the 4l, the new mass becomes 191.2 GeV instead of 170.5 GeV

  27. Results Expectation: Higgs review numbers • Expected fraction of events with FSR passing the selection, 50<mZ1<120, 12<mZ2<120 • Was estimated 1.8% for the 4e case • now is 2.1% (more stat, new baseline cuts used) • Was estimated x2 lower for 4mu case • Given that all the 4e additonal yield is colinear it is entirely absorbed in the SC Higgs review 61 HZZ4l candidates New estimates

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