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Study of Final State Radiation in H -> ZZ to 4l Events: Implications on Mass Measurement

This study examines final state photon emission in Z decay & its impact on mass measurement in Higgs to four-lepton events. Findings suggest minimal bias on mass reconstruction but background contamination necessitates post-event FSR recovery. Tools like Pythia and Sherpa generate similar results, enabling analysis of ZZ continuum FSR characteristics. A simple FSR selection is applied based on energy and angular criteria, showing high purity and correct assignment rates. The analysis on Zee vs. Zmumu reveals expectations for ZZ to 4e and Z to ee processes. Isolation cuts are employed with minimal efficiency loss, while overlaps between electron and muon signatures are addressed. Finally, results from the new baseline selection up to 4.0/fb are presented, highlighting the significance of FSR recovery algorithms in identifying candidate events.

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Study of Final State Radiation in H -> ZZ to 4l Events: Implications on Mass Measurement

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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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