Searches for Lepton flavour violating H / Z → τ l decays with the ATLAS detector at 8 TeV On behalf of ATLAS collaboration Hartger Weits (NIKHEF) April 11, 2016 1 / 28
Lepton Flavour Violation is an established fact ☞ 2001 at Sudbury Neutrino ☞ neutrino mixing can be incorporated Observatory by introducing PMNS matrix ν e ν 1 = V PMNS ν µ ν 2 ν τ ν 3 ☞ This makes LFV Z & H decays possible: l 1 l 1 W ν i W D1: D2: ν i ν j W l 2 l 2 l l ☞ nobel prize 2015: for the discovery of ☞ However, prediction ν SM of BF( Z → τ l ) ∼ 10 − 54 [1] neutrino oscillations, which shows that neutrinos have mass 2 / 28
Collider experiments well suited for production of leptons most sensitive Z → τ l searches stem from LEP Br( Z → τ e ) < 9 . 8 × 10 − 6 [2, 3] ◮ Br( Z → τµ ) < 1 . 2 × 10 − 5 , ◮ they had a cleaner environment, we have more statistics TD TE TS TK TV ST PA DELPHI Interactive Analysis 1 35 0 2 0 0 0 Act Beam: 45.6 GeV Run: 26154 DAS : 25-Aug-1991 ( 37) ( 35) ( 0) ( 4) ( 0) ( 0) ( 0) Evt: 2958 21:46:38 0 0 0 0 0 0 0 Deact Proc: 1-Oct-1991 Scan: 4-Dec-1992 ( 0) ( 1) ( 0) ( 3) ( 0) ( 0) ( 0) Y Z X H → τ l new measurement Br( H → τµ ) = 0 . 84 +0 . 39 ◮ CMS found 2 . 4 σ excess : − 0 . 37 % [4]. ◮ no excess in electron channel: Br( H → τ e ) < 0 . 7 % (preliminary results [5]) 3 / 28
Search for H / Z → e τ / µτ decays in the τ had channel Missing Mass Calculator [6] M MMC : invariant mass of the Z or H τ l quadratic equation p 2 z , ν + α p z , ν + β = 0 most likely solution L = P (∆ R ) × P ( � E T ) Events Events 4 10 Z → τ τ Z → τ τ 3-prong decays τ 1-prong decays τ 3 10 3 10 2 10 2 10 10 10 1 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.1 0.2 0.3 0.4 0.5 Δ R Δ R 4 / 28
Data–driven methods & Monte Carlo corrections ATLAS Simulation Preliminary, Z + jets ATLAS Simulation Preliminary, W + jets → τ τ -3 -3 10 10 × × 140 140 [GeV] Fraction of Events [GeV] Fraction of Events 3 2 120 120 1.8 Data–driven 2.5 1.6 miss miss 100 100 T T E E 1.4 2 , , had 80 had 80 1.2 T T Z → ττ : τ τ m 1.5 m 1 60 60 0.8 ☞ from Z → µµ 1 40 40 0.6 0.4 0.5 QCD multi-jets: 20 20 0.2 0 0 0 0 ☞ OS/SS symmetry 20 40 60 80 100 120 140 20 40 60 80 100 120 140 miss miss e , E e , E m [GeV] m [GeV] T T T T ∫ -1 ATLAS Simulation Preliminary, H 125 → e τ ATLAS Preliminary, s = 8 TeV L dt = 20.3 fb -3 × -3 10 10 × 140 140 [GeV] Fraction of Events [GeV] Fraction of Events Control Regions 2.5 1.4 120 120 1.2 2 miss miss 100 100 T T 1 t / t ¯ E E t , , had had 80 80 1.5 0.8 T T τ τ m m W +jets 60 60 0.6 1 SR2 WCR 40 40 Z / VV → ll 0.4 0.5 SR1 20 20 0.2 0 0 0 0 20 40 60 80 100 120 140 20 40 60 80 100 120 140 miss miss e , E e , E m T [GeV] m T [GeV] T T 5 / 28
Z → µτ had [7] × 3 × 3 10 10 Events / 5 GeV Events / 5 GeV 5 ATLAS Preliminary ATLAS Preliminary Data Data 3.5 → τ µ -3 → τ µ -3 µ τ Z BR=10 µ τ Z BR=10 SR1 events SR2 events had → τ τ had → τ τ Z + jets (OS-SS) Z + jets (OS-SS) ∫ 4 ∫ 3 -1 -1 = 8 TeV L dt = 20.3 fb = 8 TeV L dt = 20.3 fb s s W + jets (OS-SS) Same Sign Same Sign W + jets (OS-SS) 2.5 Others Others 3 Syst. Unc. Syst. Unc. 2 2 1.5 1 1 0.5 0 0 Data / BKG Data / BKG 1.4 100 150 1.4 100 150 1.2 1.2 1 1 0.8 0.8 0.6 0.6 100 150 100 150 MMC MMC [GeV] [GeV] m µ τ m µ τ ◮ Br( Z → τµ ) = − 1 . 6 +1 . 3 − 1 . 4 × 10 − 5 , best fit value ◮ Br( Z → τµ ) < 1 . 69(2 . 58) × 10 − 5 , observed (expected) 95 % C.L 6 / 28
H → µτ had [8] ◮ Br( H → τµ ) = 0 . 77 ± 0 . 66 %, best fit value ◮ Br( H → τµ ) < 1 . 85(1 . 24) %, observed (expected) 95 % C.L. 7 / 28
H → e τ had [7] ◮ Br( H → τ e ) = − 0 . 47 1 . 08 − 1 . 18 %, best fit value ◮ Br( H → τ e ) < 1 . 81(2 . 07) %, observed (expected) 95 % C.L. 8 / 28
Search for H → e τ / µτ decays in the τ lep channel ℎ ℓ ℓ ’ ℓ ’ The final discriminant used in this channel is the collinear mass m coll defined as: � � � 2 p ℓ 1 p ℓ 2 T + E miss m coll = (cosh∆ η − cos∆ φ ) . (1) T T This quantity is the invariant mass of two massless particles, τ and l 1 , computed with the approximation that the decay products of the τ lepton, l 2 and ν , are collinear to the τ , and that the E miss originates from the ν . T 9 / 28
H → e τ lep / µτ lep [7] Dilepton events are divided into two mutually exclusive samples: ◮ µ e sample : p µ T ≥ p e : H → µτ → µ e νν would be here T T > p µ ◮ e µ sample : p e T Events/10 GeV Events/10 GeV ATLAS Preliminary ATLAS Preliminary 3 ∫ 3 ∫ 10 10 -1 -1 s = 8 TeV L dt = 20.3 fb s = 8 TeV L dt = 20.3 fb Data e SR Data e SR µ µ noJets noJets Symm. background Symm. background 2 2 10 10 Tot. background Tot. background Post-fit uncertainty Post-fit uncertainty H → τ (BR=1%) µ 10 10 1 1 2 2 Data/Bkg Data/Bkg 1.5 1.5 1 1 0.5 0.5 0 0 0 50 100 150 200 250 300 350 400 450 0 50 100 150 200 250 300 350 400 450 m [GeV] m [GeV] coll coll ◮ Br( H → µτ ) < 1 . 79(1 . 73) %, Br( H → e τ ) < 1 . 36(1 . 48) % 10 / 28
Combined Results ATLAS Preliminary ATLAS Preliminary Expected 1 σ Expected 1 σ ± ± σ σ Expected ± 2 Expected ± 2 ∫ ∫ -1 -1 s = 8 TeV L dt = 20.3 fb s = 8 TeV L dt = 20.3 fb Observed Observed Excluded Excluded τ , SR1 τ , SR1 e µ had had τ τ e , SR2 µ , SR2 had had e τ , Comb τ , Comb µ had had τ τ e , SR µ , SR lep noJets lep noJets e τ , SR τ , SR µ lep withJets lep withJets τ τ e , Comb µ , Comb lep lep τ τ e , Comb , Comb µ 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 → τ → τ 95% CL upper limit on Br( H e ), % 95% CL upper limit on Br( H µ ), % ◮ Combined result: Br( H → µτ ) < 1 . 43(1 . 01) %, Br( H → e τ ) < 1 . 21(1 . 48) % 11 / 28
Complementary low energy decay: τ → 3 µ [9] 25 Events / 30 MeV ATLAS Data (tight+x>x selection) 0 Data (tight+x>x selection) -1 s =8 TeV, 20.3 fb 1 20 ¯ Fit to the SB data ℓ k Fit uncertainty Sidebands (SB) Signal (tight+x>x selection) Signal region 0 15 Z ℓ k 10 ℓ i ℓ j 5 ¯ 0 1500 1600 1700 1800 1900 2000 2100 m [MeV] 3 µ ◮ trained BDT, predict event count from sidebands invariant mass m 3 µ ◮ Br( τ → 3 µ ) < 3 . 76 × 10 − 7 (3 . 94 × 10 − 7 ) observed (expected) at 90% C.L. 12 / 28
Conclusion ◮ LHC offers a new opportunity to look for charged lepton flavour violating decays ◮ interesting from the standpoint of new physics models w.r.t. neutrino oscillations → unambiguous sign of new physics ◮ several searches 1 have been performed at ATLAS with different techniques template fit using M MMC H / Z → l τ had : τ l H → l τ lep : completely data–driven technique on symmetry argument τ → 3 µ : counting experiment after BDT selection Z → e µ : bump hunting ◮ no significant excess found ◮ determining more Higgs properties at ATLAS ◮ Z → τµ will be competitive with LEP after Run 2 and/or τ lep ◮ τ → 3 µ : expected to be competitive with Belle result with Run2 data and trigger improvement 1 Z → e µ is an older analysis,see backup, most sensitive limit 13 / 28
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References II CMS Collaboration. Search for lepton-flavour-violating decays of the Higgs boson to etau and emu at sqrt(s)=8 TeV. 2015. A. Elagin, P. Murat, A. Pranko, and A. Safonov. A New Mass Reconstruction Technique for Resonances Decaying to di-tau. Nucl.Instrum.Meth. , A654:481–489, 2011. Robert Clarke et al. Search for lepton flavour violating decays of the Higgs and Z bosons with the ATLAS detector. Technical Report ATL-COM-PHYS-2015-1362, CERN, Geneva, Nov 2015. 15 / 28
References III Georges Aad et al. Search for lepton-flavour-violating H → µτ decays of the Higgs boson with the ATLAS detector. JHEP , 11:211, 2015. Georges Aad et al. Probing lepton flavour violation via neutrinoless τ − → 3 µ decays with the ATLAS detector. 2016. Robert H. Bernstein and Peter S. Cooper. Charged Lepton Flavor Violation: An Experimenter’s Guide. Phys. Rept. , 532:27–64, 2013. Shikma Bressler, Avital Dery, and Aielet Efrati. Asymmetric lepton-flavor violating Higgs boson decays. Phys. Rev. , D90(1):015025, 2014. 16 / 28
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