Rare Tau Decays Yifan Jin Belle collaboration 8th June 2020 [email protected] Outline • Tau at e+e− colliders • Tau LFV searches • Recent results of rare tau decays − − + − τ → π ντ ℓ ℓ (ℓ = e or µ) − − τ → ℓ ν$ℓ ντ γ (ℓ = e or µ) • Summary 1 τ and e+e− colliders • τ lepton discovered back in 1975 has now entered the era of precision. In SM, it only decays via the charged weak interaction. τ decays to leptons: very clear signature → precise measurement of SM parameters/ contributions from BSM. Hadronic τ decays: useful tools to study low energy QCD. Due to its short lifetime (~90 um), the precision of τ results are still way behind that of e/µ. But thanks to its larger mass, contributions of BSM would be enhanced w.r.t. e/µ. • e+e− collider is the best environment for precision: Low background, high trigger efficiency, good π0 and γ reconstruction. Initial beam condition → missing energy and missing mass. CLEO, ARGUS, SLD, L3, OPAL, DELPHI, ALEPH, CMD3, BES3, BaBar, Belle, etc… https://meetings.triumf.ca/indico/event/48/session/11/contribution/68 2 BaBar Detector BaBar and Belle � = �" #$ = 10.58 GeV Common features in design. Products of τ+τ− are two back to back narrow jets. 3 Flavor-factories B-factory, Charm-factory, and Tau-factory. �(bb/): 1.05 nb �(cc/): 1.30 nb �(ττ): 0.92 nb • 9×108 τ+τ− pair events, collected by Belle, still the largest statistics of τ. • LHC experiments also obtain huge statistics of τ from W and heavy flavor decays. 4 e+ e− → Numerous processes occur at Υ 4� , but distinguishable. A thorough study has been done for τ pair skim. KKMC → generation of τ+τ−. TAUOLA → τ lepton decays. 5 Tau physics program @ colliders SM precise measurements: − − − − • branching fraction of hadronic decays τ → Ksπ ντ , τ → ϕK ντ … − − 0 − • spectrum of hadronic currents τ → π π ντ , τ → hhhντ … nd − − • Search for 2 class current τ → π ηντ • Measurement of |Vus| Br of decays with strange, spectral moments τ property measurements: • mass • lifetime • EDM • Michel parameters τ → 3πντ BSM searches: • τ LFV • τ CPV 0 − − τ → 3ℓ , τ → ℓγ , τ → ℓhh , τ → ℓV … τ → Ksπ ντ 6 τ Lepton Flavor Violation 7 τ LFV • In neutrino sector, LFV is already seen. So, what about charged leptons? " %&! ' • Highly suppressed in SM, ∝ ( " ) . &# Not reachable in current experiments. τ SUPER-RARE • Extensions of SM predict enhancement up to the sensitivity of current experiments. • Waiting for surprise from the heaviest lepton τ. τ → ℓℓℓ, τ → ℓγ, τ → ℓhh, τ → ℓV0, τ → ℓS0, τ → ℓP0, τ → ∧ h … 8 τ→µ�, τ→µµµ Reference τ→µ� τ→µµµ SM + � mixing EPJ C8 (1999) 513 10-45 - -9 -10 SM + heavy Maj �R PRD 66 (2002) 034008 10 10 Non-universal Z’ PLB 547 (2002) 252 10-9 10-8 SUSY SO(10) PRD 68 (2003) 033012 10-8 10-10 τ mSUGRE + seesaw PRD 66 (2002) 115013 10-7 10-9 SUSY higgs PLB 566 (2003) 217 10-10 10-7 BaBar: B(τ→µγ) < 4.4x10-8 PRL 104, 021802 (2010) Belle: B(τ→µγ) < 4.5x10-8 Phys. Lett. B 666, 16 (2008) Belle: B(τ→3µ) < 2.1x10-8 Phys. Lett. B 687, 139 (2010) BaBar: B(τ→3µ) < 3.3x10-8 PRD 81, 111101 (R) (2010) LHCb: B(τ→3µ) < 4.6x10-8 JHEP 02, 121 (2015) ATLAS:B(τ→3µ) < 3.8x10-7 Eur. Phys. J. C 76, 232 (2016) CMS: B(τ→3µ) < 8.9x10-8 Recent CMS results of a search for τ->3µ decays by Guenakh Mitselmakher 9 τ LFV • A variety of channels have been searched. • Tremendous contributions from several experiments. • We can be more ambitious with Belle II. • ∎ ∎ ∎ CMS • ATLAS HFLAV summary plot for τ LFV decays, overlaid with Belle II extrapolation to 50 ab-1 assuming zero background 10 Search for rare decay − − + − τ → π ντ ℓ ℓ 11 − − + − τ → π ντ ℓ ℓ P. Roig, PRD 88, 033007(2013) Using Resonance Chiral Theory The first three are QED contributions of a γ emitted off the τ, π and vertex. The solid square (triangle) represents the vector (axial-vector) current contribution. Central value of Br Error band of Br (Theoretical prediction) (Theoretical prediction) + − -5 -5 -5 τ → π ντ e e 1.710 x 10 [1.4x10 , 2.8x10 ] + − -6 -7 -5 τ → π ντ µ µ 1.938 x 10 [3x10 , 1x10 ] Belle tries to access these modes for the first time. 12 Physics Motivations 1, Major BKG to the LFV and LNV searches: τ → ℓℓℓ, ℓππ, πℓℓ, whose upper limits on branching fraction have been reduced to the order of 10-8. (π-µ mis- id ~%, π-e mis-id ~‰) PRD. 85, 011301 (2012) Br < 1.3x10-5 MiniBooNE PRL. 102, 101802 (2009) PRL. 121, 221801 (2018) 2, Probe the New Physics contribution: Heavy long-lived sterile neutrinos. ± ∓ ∗ 3, ℱ(±)∓*∗ (the π W γ vertex in d and e). + − + − Same vertex in cross channel π → ℓ νl e e : Br(π → e νe e e ) = (3.2 ± 0.5) x -9 + − 10 , lepton universality can be checked. Br(π → µ νµ e e ) is not observed yet, thus can be inferred. Phys. Lett. B 222 533 (1989) 13 − − + − τ → π ντ e e • Blind analysis of counting events. • Difficulty: contamination from π0, − − + − calibrated by τ → π e e γ ντ. − − . Signal mode: Main BKG(80%): τ → π π ντ (Belle pion form factor) PRD 78, 072006 (2008) π e Dalitz decay of π. �-conversion ντ e Br=1.174±0.035% e e π e e π � τ ντ ντ � � e+ e− π0 0 τ + τ τ π e − + − ν=τ e e e τ τ 1-prong decay ν=τ ν=τ 1-prong decay 1-prong decay 14 2 − − + − 1000 DATA τ → π ντ e e ρ-(→π-e+e-γ)ν ρ-(→π-γγ)ν 800 other-ττ non-ττ 5-lepton Invariant Mass of 3 prongs is used 600 MC signal to define the sideband/signal box. Events/0.025 GeV/c L = 562 fb-1 Observed MC BKG MC signal 400 DATA/MC χ2 / ndf 37.95 / 33 events events events Prob 0.2539 p0 0.9841 ± 0.0138 10243 10083±504 202.5±8.1 200 In the sideband, data agree with the 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 known BKG within 1�. M(πee) [GeV/c2] 2 400 DATA 350 ρ-(→π-e+e-γ)ν ρ-(→π-γγ)ν 300 other-ττ Observed events MC BKG events 250 non-ττ 5-lepton 1365 954 ± 45 Events/0.025 GeV/c Signal Region 200 MC signal 150 There is a clear excess of data (closed circles) over the expected background 100 L = 562 fb-1 (colorful histogram). (7.0 � , first 50 observation) 0 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 M(πee) [GeV/c2] 15 − − + − Br(τ → π ντ e e ) Contents Syst. error MC size 3.7% Meanwhile, it is found that the cut ee → ττ 0.3% on invariant mass of three prongs Trigger 1.2% has strong model dependence. 0 For two extreme cases: π veto 1.9% -5 Br_A = (1.46 ± 0.13 ± 0.21) x 10 Br’s of BKG 4.4% -5 Br_V = (3.01 ± 0.27 ± 0.43) x 10 Luminosity 4.7% And a partial Br (M>1.05 GeV/c2) Tracking 4.7% is determined: -6 PID 11.1% (5.09 ± 0.53 ± 0.85 ± 0.11) x 10 stat. syst. model. Total: 14.4% PRD. 100, 071101(R) (2019) 16 − − + − τ → π ντ µ µ − − + − • Main background is from τ → π π π ντ, where two π’s are misidentified as µ’s. − − + − • Difficulty: PID (π fake rate), calibrated by τ → π π π ντ. − − + − Signal mode: Main BKG (82%): τ → π π π ντ π → µ misID µ π π π π π π π ντ µ ντ ντ 0 τ τ τ π e+ e− e+ e− e+ e− τ τ τ ν=τ ν=τ ν=τ 1-prong decay 1-prong decay 1-prong decay 17 250 DATA − − + − π-π+π-ν τ → π ντ µ µ - + - 0 200 π π π π ν other-ττ Events/0.4 cm non-ττ Transverse distance of µ+µ− vertex is 150 5-lepton MC signal used to define the sideband/signal box. 100 -1 Observed MC BKG MC signal events L = 562 fb events events DATA/MC χ2 / ndf 14.04 / 17 50 Prob 0.6644 505 477±23 11.4±0.4 p0 0.9912 ± 0.0662 In the sideband, data agree with the 0 0.2 1.2 2.2 3.2 4.2 5.2 6.2 7.2 8.2 9.2 10.2 known BKG within 1�. Rxy [cm] 250 DATA Syst. of NBKG Observed MC BKG - + - events events π π π ν MC size 1.7% π-π+π-π0ν 2578 2244 ± 109 200 other-ττ Luminosity 1.4% non-ττ ee → ττ 0.3% Events/0.005 cm 150 5-lepton The excess (signal yield) MC signal Tracking 1.4% is 334 ± 51 ± 109 with a Trigger 0.3% 100 -1 significance of 2.8σ. L = 562 fb PID 3.7% Br < 1.14 x 10-5 @90% C.L. Br’s of BKG 1.0% 50 π-µ mis-id 1.5% PRD.