LFV in Tau Decays: Results and Prospects at the LHC. Tau th International Workshop on Tau Lepton Physics Beijing September 22th, 2016

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1 LFV in Tau Decays: Results and Prospects at the LHC Kristof De Bruyn On behalf of the ATLAS, CMS & LHCb Collaborations Tau th 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 / 25

2 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 / 25

3 LHC Experiments Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau / 25

4 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: τ pµ µ LHCb, PLB 724 (213), arxiv: B (s) e ± µ LHCb, PRL 111 (213) 14181, arxiv: Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau / 25

5 B(τ µ µ + µ ATLAS & LHCb Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau / 25

6 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 ) < % C.L. Belle, PLB 687 (21) 139, arxiv: 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 / 25

7 ATLAS: Strategy ATLAS, EPJC 76 (216) 232, arxiv: 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: 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 / 25

8 ATLAS: Strategy Selection 1 Cut-based loose selection ATLAS, EPJC 76 (216) 232, arxiv: 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 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 / 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 m 3µ [MeV] BDT score Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau / 25

9 ATLAS: Results ATLAS, EPJC 76 (216) 232, arxiv: 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 ) < Not competitive with results from B-factories or LHCb Demonstrates ATLAS potential for LFV 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 / 25

10 LHCb: Strategy Analysis based on 3 fb 1 of data (full Run 1) LHCb, JHEP 2 (215) 121, arxiv: 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: 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 τ s produced Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau / 25

11 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: Fraction of candidates per bin.9.8 LHCb.7 Simulated τ µ µ + µ.6 Calibrated τ µ µ + µ.5 Data sidebands M 3body response Fraction of candidates per bin 1.8 LHCb Simulated τ µ µ + µ.6 Calibrated τ µ µ + µ Data sidebands 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 / 25

12 LHCb: Results c 2 ) Candidates / (8.75 MeV/ LHCb m(µ µ + µ ) [MeV/c 2 ] CL s LHCb, JHEP 2 (215) 121, arxiv: dashed: expected solid: observed All bins consistent with background-only hypothesis Limit: Outlook B(τ µ µ + µ ) < Still factor 2.5 worse than Belle Run 2: Might overtake Belle and be overtaken again by Belle II LHCb B(τ µ µ + µ ) [ 1 9% C.L. Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau / 25

13 Z τ ± ATLAS Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau / 25

14 ATLAS: Strategy ATLAS, arxiv: 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: 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 / 25

15 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, SR2 s = 8 TeV T -1 L dt = 2.3 fb Fraction of Events ATLAS, arxiv: miss e, E m T T [GeV] Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau / 25 WCR SR

16 ATLAS: Results ATLAS, arxiv: Signal Region 1 Signal Region 2 Events / 5 GeV 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 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 mµτ MMC [GeV] Data / BKG Small deficit compared to background expectation Limit: B(Z τ ± µ ) < mµτ 95% C.L. mµτ MMC [GeV] [GeV] Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau / 25

17 D e ± LHCb Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau / 25

18 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: Multiple Higgs doublets: Previous best limit: B(D e ± µ ) < % C.L. Belle, PRD 81 (21) 9112, arxiv: Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau / 25

19 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: 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 LHCb simulation D e + µ - D π + π - misid Backgrounds Main background: misid-ed D π + π Probability [ππ eµ] = (1.8 ±.4) B(D π + π ) = (1.421 ±.25) Electron momentum corrected for m(eµ) [MeV/c 2 ] bremsstrahlung losses ( Tails) Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau / 25

20 LHCb: Strategy Fit Strategy Cut-based + BDT selection divide data in 3 bins LHCb, PLB 754 (216) 167, arxiv: 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 m(eµ) [MeV/c 2 ] Candidates / [.27 MeV/c 2 ] 1 LHCb m(eµπ) - m(eµ) [MeV/c 2 ] Intermediate Most Sig-like Candidates / [1.7 MeV/c 2 ] Candidates / [1.7 MeV/c 2 ] LHCb m(eµ) [MeV/c 2 ] LHCb m(eµ) [MeV/c 2 ] m D Candidates / [.27 MeV/c 2 ] Candidates / [.27 MeV/c 2 ] LHCb m(eµπ) - m(eµ) [MeV/c 2 ] LHCb 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 / 25

21 LHCb: Results LHCb, PLB 754 (216) 167, arxiv: Simultaneous fit to the 3 BDT bins gives: N sig = 7 ± 15 Limit: CL S Factor 2 improvement over Belle LHCb B(D e ± µ ) < Observed CLs Expected CLs - Median Expected CLs ± 1 σ Expected CLs ± 2 σ B(D e ± µ ) 9% C.L. Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau / 25

22 Prospects for LHC Run 2 and Beyond Kristof De Bruyn (CPPM) LFV in Tau Decays: Results and Prospects at the LHC Tau / 25

23 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 / 25

24 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 / 25

25 Conclusion ATLAS limit on the τ µ µ + µ branching ratio B(τ µ µ + µ ) < % C.L. LHCb limit on the τ µ µ + µ branching ratio B(τ µ µ + µ ) < % C.L. ATLAS limit on the Z τ ± µ branching ratio B(Z τ ± µ ) < % C.L. LHCb limit on the D e ± µ branching ratio B(D e ± µ ) < % 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 / 25