LFV in Tau Decays: Results and Prospects at the LHC Kristof De Bruyn On behalf of the ATLAS, CMS & LHCb Collaborations Tau 216 14th International Workshop on Tau Lepton Physics Beijing September 22th, 216 Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 1 / 25
Charged Lepton Flavour Violation Neutrino Oscillation Neutrino Masses Lepton Flavour Violation W τ ντ ν µ X γ µ + µ µ Loop suppressed B (m ν/m W ) 4 O(1 4 ) Ideal playground to search for Beyond the SM physics Many BSM models with TeV scale new physics predict rates within reach of current experiments. Is there a counterpart in the charged lepton sector? Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 2 / 25
LHC Experiments Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 3 / 25
Overview This Talk 1 τ µ µ + µ 2 Z τ ± µ 3 D e ± µ Other LHC Results H τ ± µ See preceding ATLAS & CMS Talks H τ ± e See preceding ATLAS & CMS Talks Z e ± µ ATLAS, PRD 9 (214) 721, arxiv:148.5774 τ pµ µ LHCb, PLB 724 (213), arxiv:134.4518 B (s) e ± µ LHCb, PRL 111 (213) 14181, arxiv:137.4889 Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 4 / 25
B(τ µ µ + µ ) @ ATLAS & LHCb Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 5 / 25
Neutrino-less τ Decay Strongly suppressed in the Standard Model W τ ν τ ν µ X γ µ + µ µ Can be enhanced by new physics at tree level (Z,... ) or loops (SUSY,... ) Current best limit: B(τ µ µ + µ ) < 2.1 1 8 @ 9% C.L. Belle, PLB 687 (21) 139, arxiv:11.3221 Experimental Signature No missing energy in τ decay Three muons with invariant mass m 3µ m τ Displaced vertex Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 6 / 25
ATLAS: Strategy ATLAS, EPJC 76 (216) 232, arxiv:161.3567 Analysis based on 2.3 fb 1 of data, collected at 8 TeV in 212 Experimental Setup Uses W τ ν τ decays (highest efficiency for trigger & reconstruction) B(τ µ µ + µ ) = N sig ɛ sig σ W τ ν τ L N sig = signal yield, ɛ sig = selection efficiency, σ = cross section, L = luminosity Relies on the W l ν cross-section measurement Estimate about (2.41 ±.8) 1 8 τ s produced ATLAS, PRD 85 (212) 724, arxiv:119.5141 Contamination from other sources (Z τ + τ, heavy flavour) is less than 3% Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 7 / 25
ATLAS: Strategy Selection 1 Cut-based loose selection ATLAS, EPJC 76 (216) 232, arxiv:161.3567 2 Train Boosted Decision Tree and apply loose cut x on BDT output x 3 Cut-based tight selection 4 Apply tight cut x 1 on BDT output, optimising for the expected B limit Fit Strategy Blinded analysis: ignore signal region: m 3µ [1713, 1841] MeV/c 2 1 Estimate background yield from mass sidebands using tight + x > x 2 Fit BDT output in region x > x 3 Extrapolate background yield for tight + x > x 1 Events / 3 MeV 25 2 15 Data (tight+x>x selection) Data (tight+x>x selection) 1 Fit to the SB data Fit uncertainty Signal (tight+x>x selection) ATLAS -1 s=8 TeV, 2.3 fb Sidebands (SB) Signal region Events /.5 25 2 15 SB data (tight+x>x Fit to the SB data Fit uncertainty Signal (tight+x>x selection) selection) ATLAS -1 s=8 TeV, 2.3 fb 1 1 5 5 15 16 17 18 19 2 21.8.6.4.2.2.4.6.8 1 m 3µ [MeV] BDT score Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 8 / 25
ATLAS: Results ATLAS, EPJC 76 (216) 232, arxiv:161.3567 Unblinding No events found in signal region Expected background N bkg =.193 ±.37 (stat) ±.131 (syst) (dominant uncertainty = extrapolation procedure) Efficiency ɛ sig =.231 ±.5 (Jet) ±.9 (MC) ±.25 (trig) ±.3 (reco) Limit: Outlook B(τ µ µ + µ ) < 3.76 1 7 Not competitive with results from B-factories or LHCb Demonstrates ATLAS potential for LFV searches @ 9% C.L. Helped improve muon trigger and reconstruction of low-p T muons (4 18 GeV/c) Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 9 / 25
LHCb: Strategy Analysis based on 3 fb 1 of data (full Run 1) LHCb, JHEP 2 (215) 121, arxiv:149.8548 Experimental Setup τ originate from b and c-hadron decays Normalisation mode: D s φ( µ + µ )π B(τ µ µ + µ ) = B(D s φ( µ + µ )π ) B(D s τ ν τ ) f Ds τ ɛ cal ɛ sig N sig N cal = α N sig N sig = signal yield, N cal = yield of calibration mode, ɛ = selection efficiency f Ds τ =.8 ±.3 is the fraction of τ that originates from D s mesons Input: b b, c c cross-sections LHCb, NPB 871 (213), arxiv:132.2864 Input: inclusive b D s, c D s, b τ and c τ branching fractions Normalisation factor: α(7 TeV) = (7.2 ±.98) 1 9 α(8 TeV) = (3.37 ±.5) 1 9 Different trigger: p T (µ) > 1.48 GeV/c (211) vs p T (µ) > 1.76 GeV/c (212) Corresponds to about 8.5 to 9 1 1 τ s produced Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 1 / 25
LHCb: Strategy Selection Cut-based loose selection Three likelihoods to distinguish signal from background 1 MVA exploiting the geometrical properties M 3body 2 Neural Network for muon particle identification M PID 3 Invariant mass m 3µ of three muons LHCb, JHEP 2 (215) 121, arxiv:149.8548 Fraction of candidates per bin.9.8 LHCb.7 Simulated τ µ µ + µ.6 Calibrated τ µ µ + µ.5 Data sidebands.4.3.2.1.2.4.6.8 1 M 3body response Fraction of candidates per bin 1.8 LHCb Simulated τ µ µ + µ.6 Calibrated τ µ µ + µ Data sidebands.4.2.2.4.6.8 1 M PID response Fit Strategy Fit invariant mass in each bin of M 3body M PID Blinded analysis: ignore signal region: (m τ ± 3) MeV/c 2 Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 11 / 25
LHCb: Results c 2 ) Candidates / (8.75 MeV/ 7 6 5 4 3 2 1 LHCb 16 17 18 19 m(µ µ + µ ) [MeV/c 2 ] CL s LHCb, JHEP 2 (215) 121, arxiv:149.8548 dashed: expected solid: observed 1.9.8.7.6.5.4.3.2.1 All bins consistent with background-only hypothesis Limit: Outlook B(τ µ µ + µ ) < 4.6 1 8 Still factor 2.5 worse than Belle Run 2: Might overtake Belle...... and be overtaken again by Belle II LHCb 2 4 6 8 1-8 B(τ µ µ + µ ) [ 1 ] @ 9% C.L. Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 12 / 25
Z τ ± µ @ ATLAS Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 13 / 25
ATLAS: Strategy ATLAS, arxiv:164.773 Analysis based on 2.3 fb 1 of data, collected at 8 TeV in 212 Experimental Setup Reconstructed in τ π π + π ν τ mode Background decomposition W + jets (using dedicated control region) Z τ + τ (modelled using Z µ + µ ) Combinatorial bkg (taken from same-sign data) Adopt data-driven techniques to control them Analysis strategy similar to H µ ± τ had ATLAS, JHEP 1511 (215) 211, arxiv:158.3372 But larger contribution from W + jets More detailed study of this component Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 14 / 25
ATLAS: Strategy Fit Strategy: Combined analysis of signal region + control regions Two signal search windows: SR1 & SR2 Control region for the modelling of W + jets (WCR) background Control region for t t events (normalisation) Regions defined in terms of the transverse mass miss l,et m 2pT l E miss (1 cos φ) with φ angle between l and the direction of E miss T [GeV] miss τ had, E T m T T ATLAS, 14 12 1 8 6 4 2 SR2 s = 8 TeV T -1 L dt = 2.3 fb 1 1.4 Fraction of Events ATLAS, arxiv:164.773 2 4 6 8 1 12 14 miss e, E m T T [GeV] Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 15 / 25 WCR SR1 1.2 1.8.6.4.2-3
ATLAS: Results ATLAS, arxiv:164.773 Signal Region 1 Signal Region 2 Events / 5 GeV 3.5 3 2.5 2 1.5 3 1 ATLAS SR1 µτ events had -1 s = 8 TeV L dt = 2.3 fb Data Z Z τµ ττ -3 BR=1 + jets (OS-SS) W + jets (OS-SS) Same Sign Others Syst. Unc. Events / 5 GeV 3 1 5 ATLAS SR2 µτ events had 4-1 s = 8 TeV L dt = 2.3 fb 3 2 Data Z Z τµ ττ Same Sign -3 BR=1 + jets (OS-SS) W + jets (OS-SS) Others Syst. Unc. Data / BKG 1.5 1.4 1.2 1.8.6 1 15 mµτ MMC [GeV] Data / BKG 1 1.4 1.2 1.8.6 Small deficit compared to background expectation Limit: B(Z τ ± µ ) < 1.69 1 5 1 15 1 15 mµτ MMC @ 95% C.L. mµτ MMC [GeV] [GeV] Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 16 / 25
D e ± µ @ LHCb Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 17 / 25
Forbidden Decay Strongly suppressed in the Standard Model D ū c d, s, b W W ν e ν µ X e + µ Enhanced in many beyond the SM theories: SUSY with R-parity violation: O(1 6 ) Leptoquarks: 4 1 8 Multiple Higgs doublets: 7 1 1 Previous best limit: B(D e ± µ ) < 2.6 1 7 @ 9% C.L. Belle, PRD 81 (21) 9112, arxiv:13.2345 Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 18 / 25
LHCb: Strategy Analysis based on 3 fb 1 of data (full Run 1) Experimental Setup Candidates selected from D + D π + decays Normalisation mode: D K π + LHCb, PLB 754 (216) 167, arxiv:1512.322 B(D e ± µ ) = B(D K π + ) ɛ cal N sig ɛ sig N cal N sig = signal yield, N cal = yield of calibration mode, ɛ = selection efficiency Efficiencies: ɛ sig = (4.4 ±.3) 1 4 ɛ cal = (2.5 ±.1) 1 6 Difference: keep only 1% of triggered D K π + events Arbitrary units.9.8.7.6.5.4.3 LHCb simulation D e + µ - D π + π - misid Backgrounds Main background: misid-ed D π + π Probability [ππ eµ] = (1.8 ±.4) 1 8.2 B(D π + π ) = (1.421 ±.25) 1 3.1. Electron momentum corrected for 182 184 186 188 19 m(eµ) [MeV/c 2 ] bremsstrahlung losses ( Tails) Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 19 / 25
LHCb: Strategy Fit Strategy Cut-based + BDT selection divide data in 3 bins LHCb, PLB 754 (216) 167, arxiv:1512.322 Simultaneous fit to m D and m = m D + m D Legend: D e ± µ D π + π Dashed: Combinatorial Bkg Most Bkg-like Candidates / [1.7 MeV/c 2 ] 8 LHCb 6 4 2 182 183 184 185 186 187 188 189 19 191 m(eµ) [MeV/c 2 ] Candidates / [.27 MeV/c 2 ] 1 LHCb 8 6 4 2 14 142 144 146 148 15 152 154 m(eµπ) - m(eµ) [MeV/c 2 ] Intermediate Most Sig-like Candidates / [1.7 MeV/c 2 ] Candidates / [1.7 MeV/c 2 ] 2 15 1 5 1 8 6 4 2 LHCb 182 183 184 185 186 187 188 189 19 191 m(eµ) [MeV/c 2 ] LHCb 182 183 184 185 186 187 188 189 19 191 m(eµ) [MeV/c 2 ] m D Candidates / [.27 MeV/c 2 ] Candidates / [.27 MeV/c 2 ] 25 2 15 1 5 1 8 6 4 2 LHCb 14 142 144 146 148 15 152 154 m(eµπ) - m(eµ) [MeV/c 2 ] LHCb 14 142 144 146 148 15 152 154 m(eµπ) - m(eµ) [MeV/c 2 ] m = m D + m D Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 2 / 25
LHCb: Results LHCb, PLB 754 (216) 167, arxiv:1512.322 Simultaneous fit to the 3 BDT bins gives: N sig = 7 ± 15 Limit: CL S Factor 2 improvement over Belle 1..9.8 LHCb.7.6.5.4.3.2.1. 2 4 B(D e ± µ ) < 1.3 1 8 Observed CLs Expected CLs - Median Expected CLs ± 1 σ Expected CLs ± 2 σ B(D 6 1-8 e ± µ ) ± @ 9% C.L. Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 21 / 25
Prospects for LHC Run 2 and Beyond Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 22 / 25
LHC Run 2 and Beyond GPD LHCb: 3fb 1 5fb 1 Upgrade 5fb 1?? (3fb 1 ) Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 23 / 25
Prospects for LHC Run 2 and Beyond Ambitious Plans: Sensitivities strongly depend on trigger settings These depend on pile-up, available bandwidth,... Difficult to predict no official numbers! ATLAS: Added insertable B layer in 214, for improved tracking Improved trigger and reconstruction for low-p T muons Aim for result on B(τ µ µ + µ ) with Run 2 data that is competitive with LHCb and/or Belle II CMS: Will replace the inner tracker during EYETS 216/17 (extra layer) Beneficial for B-physics programme Plans to study τ µ µ + µ with the future upgrade LHCb: Aim for a limit on B(τ µ µ + µ ) of O(1 9 ) O(1 1 ) with Run 2 data, may be competitive with Belle II Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 24 / 25
Conclusion ATLAS limit on the τ µ µ + µ branching ratio B(τ µ µ + µ ) < 3.76 1 7 @ 9% C.L. LHCb limit on the τ µ µ + µ branching ratio B(τ µ µ + µ ) < 4.6 1 8 @ 9% C.L. ATLAS limit on the Z τ ± µ branching ratio B(Z τ ± µ ) < 1.69 1 5 @ 95% C.L. LHCb limit on the D e ± µ branching ratio B(D e ± µ ) < 1.3 1 8 @ 9% C.L. Decays involving τ s play an important role in searches for charged lepton flavour violation Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau 216 25 / 25