Searches for New Phenomena in Events with Multiple Leptons with the ATLAS Detector

Transcription

1 Searches for New Phenomena in Events with Multiple Leptons with the ALAS Detector ÜLİN VAROL University of Massachusetts, Amherst On Behalf of the ALAS Collaboration

2 OULINE Introduction Excited e/μ Search rilepton Search ype-iii SeeSaw Heavy Fermions Search Conclusion

3 INRODUCION Multi-lepton final states are predicted by many extensions to the Standard Model wo leptons: Excited e/μ searches with two same flavour leptons with additional photon contribution in the final state ( ± )( )( γ) hree leptons: A generic search with three charged leptons in the final state ( )( )( or τ) Four leptons: A search for heavy neutrinos with four leptons in the final state (Z )(`±)(`±)! `+` 3

4 Excited e/μ: Introduction Search for lepton compositeness Motivation: Mass hierarchy and the generational structure of quarks and leptons SM leptons: Ground states Excited state ℓ*: Consequence of lepton compositeness Effective heory ` produced via contact interactions Effective model holds for m` < Λ Unknown parameters Compositeness scale Λ Mass of excited leptons m` Studied Channel: pp! ``! `` 4

5 Excited e/μ: Background Determination Event Selection wo lepton candidates and a photon candidate m`` > GeV, suppressing Z+γ Signal Regions m`` > m` + 5 GeV, if m` < 9 GeV m`` > 5 GeV, if m` 9 GeV Z+γ: dominant irreducible Z+jets: reducible W+γ+jets: for electron channel only Diboson, ttbar: small contributions / GeV ee 4 3 ALAS L dt = 3 fb s = 8 ev Data Z + Z + jets, diboson, tt, W+ +jets Bkg. uncertainty (m, ) = (., ) ev e* (m (m, ) = (.5, ) ev e*, ) = (.8, ) ev e* / GeV µµ 3 ALAS L dt = 3 fb s = 8 ev Data Z + Z + jets, diboson, tt Bkg. uncertainty (m, ) = (., ) ev µ* (m (m, ) = (.5, ) ev µ*, ) = (.8, ) ev µ* m` m ee m µµ 5

6 Excited e/μ: Results [ev] ALAS L dt = 3 fb s = 8 ev Observed limit Expected limit Expected ± m e* > [ev] ALAS L dt = 3 fb s = 8 ev Observed limit Expected limit Expected ± m µ* > ALAS fb, CMS 5 fb, s = 7 ev s = 7 ev ALAS fb, CMS 5 fb, s = 7 ev s = 7 ev m e* [ev] m µ* [ev] No significant excess is observed in the signal region % upper limits are set on (pp! `` ) B(`! ` ) For m` = Λ, m` <. ev excluded 6

7 hree Charged Leptons: Introduction A model-independent search for new phenomena with 3 charged leptons Rare SM events Dominated by WZ/ZZ production Any excess would be interesting Complementary search for previous analysis Avoiding optimization for specific models Creating signal regions for various possible signatures Presenting results as upper limits on event yields in a fiducial volume 7

8 hree Charged Leptons: Signal Regions Define four mutually exclusive signal channels On-Z 3 e/μ On-Z e/μ + τ Off-Z 3 e/μ Off-Z e/μ + τ On-Z: m`` Off-Z: m`` m Z < GeV m Z > GeV Minimum p, isolation, hit quality and fiducial volume requirements are applied to leptons Variable H leptons H jets E m e Definition Sum of 3 lepton p Sum of jet p Missing transverse energy H leptons + H jets + E 94 signal regions in total 8

9 hree Charged Leptons: Backgrounds Irreducible backgrounds: Prompt leptons produced in the hard interaction: WZ, ZZ, ttbar+w/z processes Z+γ ( γ e ) Reducible backgrounds: Events with up to prompt leptons with at least non-prompt lepton Non-isolated, fake lepton candidates Z+jets, W+jets, tt, single top, multijets Estimated by using data-driven fake factor method Events / GeV e/µ off-z s= 8 ev Ldt =.3 fb Data VV(V) Reducible tt+v(v) Syst. Unc. Events / GeV 4 3 e/µ + off-z τ s= 8 ev 5 Data Ldt =.3 fb Reducible VV(V) tt+v(v) Syst. Unc. Data/Bkg leptons H Data/Bkg leptons H leptons H Distributions are shown in terms of H leptons, H jets, m e and E No significant deviation from the expected background is observed 9

10 he % C.L upper limits set on the number of events from non-sm sources (N) CLs method is used hree Charged Leptons: Results s=8 ev Ldt =.3 fb Observed Expected Exp ± σ Exp ± σ he N limits are converted into limits on the visible cross section vis = R N Ldt Inclusive 3 e/µ : on-z 5 8 leptons H e/µ + τ Inclusive : on-z 5 8 leptons H No significant deviation is observed in any signal region under study 3 e/µ : off-z Inclusive 5 8 leptons H e/µ + τ : off-z Inclusive 5 8 leptons H s= 8 ev Ldt =.3 fb (Observed Limit) - (Expected Limit) [σ] All All All e/µ off-z e/µ + τ off-z 3 e/µ on-z e/µ + τ on-z All leptons lep H Min. p b-tags m m m E Inclusive eff E eff > GeV m W eff > GeV jets H 5 GeV E jets H <5 GeV

11 hree Charged Leptons: Model esting Allow to set limits on specific models by using model independent limits given in terms of Define a fiducial volume at the particle level for a chosen model Provide single lepton efficiencies (εfid) as a function of p and η in that fiducial volume A % upper-limit on the cross section in the new model is then defined as; fid = N R = fid Ldt vis fid Events / GeV Simulation s=8 ev Reconstructed Predicted Uncorrected Vector-like B quark model

12 In the Standard Model (SM) neutrinos are massless From neutrino oscillation experiments: ype-iii SeeSaw: Introduction CONRADICION At least two neutrinos of all three generations are massive he origin and smallness of neutrino masses? SeeSaw models are proposed as an effective framework in order to provide answers to these puzzles ype-iii SeeSaw Mechanism Introduces at least two fermionic triplets that generate neutrino masses he lightest fermionic triplet; N + and N -, N - Approximately degenerate masses N and N ± decay into a W, Z or H and accompanying charged or neutral lepton

13 ype III SeeSaw: Event Selection At least four leptons (e or μ) in the event Z candidate: leptons with opposite sign and same flavour, invariant mass within ± GeV of the Z mass Bachelor lepton: hird lepton candidate, closest in φ to the reconstructed Z Fourth lepton: Highest p e or μ candidate remaining in the event 3

14 ype III SeeSaw: Backgrounds MC based estimates ZZ (*) - dominant background Veto events with a second Z boson with m`+` m Z Remains a small and irreducible contribution from ZZ * Z+jets (Z+bb/cc) - small contribution Validated with data in the control region ttv, VVV - small contribution < GeV Events / GeV ALAS Preliminary s = 8 ev, L dt = 5.8 fb Data Bkg (stat) WW ZZ W VVV t tv Single op Z+jets W+jets ZZ Control Region Events / GeV 3 ALAS Preliminary 4 Data Bkg (stat) WW ZZ W VVV s = 8 ev, L dt = 5.8 fb t tv Single op Z+jets W+jets Z+jets Control Region m Z(ll)l m Z(ll)l 4

15 ype III SeeSaw: Results Events / GeV ALAS Preliminary s = 8 ev, L dt = 5.8 fb Data Bkg (sys stat) WW ZZ W VVV t tv Single op Z+jets W+jets N [ GeV] N [3 GeV] W l) BF(N ) ± 3 Observed % C.L. upper limit Expected % C.L. upper limit Expected limit ± Expected limit ± ± ± ± (pp N N ),BF(N Z l ) BF(N W l)= ± ± ± (pp N N ) BF(N Z l ) BF(N W l) Z l ± BF(N ) ± N (pp N Ldt = 5.8 fb, s = 8 ev V =.55 e V V =.63 µ = m Z(ll)l m N m N distribution p at 4 GeV =. Exclusion limits on m N No significant deviation from the SM is observed % upper limits are placed on σb by using the CLs method B(N ±! Z`±)B(N! W `±) Limits depend on For B =, mn up to 35 GeV is excluded (same as expected) For nominal mass dependent B, mn up to 45 GeV is excluded (43 GeV expected) 5

16 For nominal mass dependent B, m N > 45 GeV 6 CONCLUSIONS Presented three recent ALAS analyses with multi-lepton final states No significant excesses observed wo lepton final states - Excited e/μ search: Analysis is performed with 3 fb of s = 8 ev data An upper limit is set at % CL on the excited lepton mass m` For = Λ, m` >. ev hree lepton final states - Model independent search: Analysis is performed with.3 fb of s = 8 ev data Provided model independent limits on σvis Limits vary from.9 fb to 34 fb Single lepton efficiencies are provided in terms of p and η to derive model-dependent limits Four lepton final states - ype-iii SeeSaw model heavy fermions search: Analysis is performed with 5.8 fb of s = 8 ev data Limits are set on the mass of the N states at % CL For B =, m N > 35 GeV

17 REFERENCES Search for excited electrons and muons in s = 8 ev proton-proton collisions with the ALAS detector New J. Phys Search for New Phenomena in Events with hree Charged Leptons at s = 8 ev with the ALAS detector ALAS-CONF-3-7 Search for ype III SeeSaw Model Heavy Fermions in Events with Four Charged Leptons using 5.8 fb of s = 8 ev data with the ALAS Detector ALAS-CONF-3-9 7

18 BACK UP 8

19 Excited e/μ: Event/Object Selection γγ trigger pleading,subleading > {4 GeV, 3 GeV} ee η <.47, crack region excluded Electron medium ID Isolation: Eel (ΔR =.) < 7 GeV Single muon trigger μμ µ,µ p > 5 GeV Muon hit quality requirements Muon Spectrometer three station hits Impact parameter requirements µ p p Isolation: (ΔR =.3) / < 5% Opposite sign pair p > 3 GeV η <.47, crack region excluded ight photon definition Isolation: Eiso (ΔR =.4) < GeV or Eiso /p < % ΔR (l, γ) >.7 9 γ

20 hree Charged Leptons: Systematic Uncertainties Sample systematics from most representative signal regions

21 hree Charged Leptons: Other Distributions Events / GeV 4 Data 3 3 e/µ off-z s= 8 ev Ldt =.3 fb VV(V) Reducible tt+v(v) Syst. Unc. Events / GeV e/µ + τ off-z s= 8 ev Ldt =.3 fb Data Reducible VV(V) tt+v(v) Syst. Unc. Data/Bkg E Data/Bkg E E Events / GeV e/µ on-z s= 8 ev Ldt =.3 fb Data VV(V) Reducible tt+v(v) Syst. Unc. Events / GeV e/µ + τ on-z s= 8 ev Ldt =.3 fb Data Reducible VV(V) tt+v(v) Syst. Unc. Data/Bkg Data/Bkg

22 hree Charged Leptons: All Limits I s=8 ev Ldt =.3 fb Observed Expected Exp ± σ Exp ± σ s=8 ev Ldt =.3 fb jets H GeV Observed Expected Exp ± σ Exp ± σ 3 e/µ : on-z e/µ + τ : on-z 3 e/µ : on-z e/µ + τ : on-z Min p lep Inclusive E Inclusive E E H jets GeV 3 e/µ : off-z e/µ + τ : off-z 3 e/µ : off-z e/µ + τ : off-z Inclusive 5 5 Inclusive 5 5 Inclusive Inclusive Min. p lep Min. p lep E E s=8 ev Ldt =.3 fb jets H GeV Observed Expected Exp ± σ Exp ± σ s=8 ev Ldt =.3 fb Observed Expected Exp ± σ Exp ± σ 3 e/µ : on-z e/µ + τ : on-z 3 e/µ : on-z e/µ + τ : on-z Inclusive E Inclusive E E H jets GeV Inclusive 6 5 Inclusive e/µ : off-z e/µ + τ : off-z 3 e/µ : off-z e/µ + τ : off-z Inclusive E Inclusive E Inclusive 6 5 Inclusive 6 5

23 hree Charged Leptons: All Limits II s=8 ev Ldt =.3 fb W GeV m Observed Expected Exp ± σ Exp ± σ m W GeV 3 e/µ : on-z e/µ + τ : on-z Inclusive 6 Inclusive 6 s=8 ev Ldt =.3 fb E GeV Observed Expected Exp ± σ Exp ± σ s=8 ev Ldt =.3 fb Observed Expected Exp ± σ Exp ± σ 3 e/µ : on-z e/µ + τ : on-z 3 e/µ : on-z e/µ + τ : on-z E GeV Inclusive 6 Inclusive b-tags Inclusive N btag b-tags 3 e/µ : off-z e/µ + τ : off-z 3 e/µ : off-z e/µ + τ : off-z Inclusive 6 Inclusive 6 3 Inclusive b-tags Inclusive b-tags

24 ype III SeeSaw: Systematic Uncertanties Summary of the systematic uncertainties on the normalisation of the signal (mn = GeV) and background contributions, given in percentages he luminosity uncertainty is 3.6% 4