Tau Lepton Physics at Belle II Petar Rados (DESY) on behalf of the Belle II collaboration International Workshop on e + e - Collisions from Phi to Psi BINP, Novosibirsk, 1 March 2019
Outline 1) Overview of SuperKEKB and the Belle II experiment 2) First 𝛖 lepton physics results using early data 3) Prospects for 𝛖 lepton physics at Belle II 4) Summary and outlook Tau Lepton Physics at Belle II, 1/3/2019 P. Rados 1
SuperKEKB Accelerator • New facility to search for physics beyond the SM by studying B , D meson and τ lepton decays • Energy asymmetric electron-positron collider (7-4 GeV) • Higher beam currents compared to KEKB, and can achieve 50 nm vertical beam spot size at IP: SuperKEKB KEKB • Unprecedented design luminosity of 8.0 × 10 35 cm -2 s -1 @KEK • First beams and commissioning in 2016, Belle II Tsukuba, Japan detector rolled in 2017 Tau Lepton Physics at Belle II, 1/3/2019 P. Rados 2
Belle II Detector ~7.5m • Major upgrade of the Belle detector: Pixel Detector { Tracking Silicon Vertex Detector e - (7 GeV) Central Drift Chamber ~7m { TOP Counter Particle ID e + (4 GeV) Aerogel RICH Counter Electromagnetic Calorimeter K L / Muon Detector Tau Lepton Physics at Belle II, 1/3/2019 P. Rados 3
Belle II as 𝛖 -factory, and schedule • Belle II is not only a B- factory, but a next-generation τ lepton factory - σ ( e + e - →ϒ (4s)) = 1.05 nb, σ ( e + e - → τ + τ - ) = 0.92 nb • Over its lifetime Belle II aims to record 50 ab -1 of e + e - We are here collision data (x50 that of Belle) - 4.6 × 10 10 τ -pair events unique environment to study τ lepton physics with - high precision! Phase III Phase II (starting later this month) (2018, 3 months) • Data taking in Phase II was performed with all subsystems, except full vertex detector • VXD now installed and ready for Phase III Tau Lepton Physics at Belle II, 1/3/2019 P. Rados 4
First collisions • First collisions recorded by Belle II on 26 th April 2018 • During Phase II (April-July) about 500 pb -1 of data was recorded • Good performance of the subsystems. Clear mass peaks observed from both tracks and photons. • τ leptons also observed… π 0 →ɣɣ K S → π + π - Tau Lepton Physics at Belle II, 1/3/2019 P. Rados 5
̂ Tau leptons in early Belle II data 3 π ± + n π 0 • Targeting e + e - →τ + τ - with 3-by-1 prong decay: 𝛖 tag → ± 𝛏 𝛏̅ 𝛖 𝛖 signal → 3 π ± 𝛏 𝛖 + n π 0 ℓ ℓ ν τ • Events required to fire CDC track trigger: 291 pb -1 of usable data • Event topology and kinematic selections tailored to suppress qq ̅ and ee ɣ backgrounds, driven by: p ⋅ T ∑ - thrust value = , large for the signal since both 𝛖 leptons are boosted (back-to-back) | p h | signal h τ − - total visible energy , bellow √ s for the signal due to the three undetected neutrinos e − e + 1400 1800 Events Events Belle II Simulation Belle II Simulation 1600 1200 τ + tag 1400 τ τ τ τ 1000 1200 u u +d d +s s u u +d d +s s c c c c 800 1000 ee ee γ γ other other 800 600 600 400 400 ν τ ¯ 200 200 0 0 ν ℓ 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 0 2 4 6 8 10 12 ℓ + Visible energy in CMS [GeV] Thrust value Tau Lepton Physics at Belle II, 1/3/2019 P. Rados 6
Tau leptons in early Belle II data • After trigger + offline selections, we have agreement between the data and MC inclusive decay : 𝛖 signal → 3 π ± 𝛏 𝛖 + n π 0 exclusive decay : 𝛖 signal → 3 π ± 𝛏 𝛖 900 Events 1000 Events Belle II 2018 (Preliminary) Belle II 2018 (Preliminary) Data Data 800 MC total (stat ⊕ sys) MC total (stat ⊕ sys) 800 ∫ 700 ∫ -1 -1 L dt = 291 pb L dt = 291 pb ± ± 3 3 τ → π ν τ → π ν sig sig 600 other other τ → τ → 600 sig sig 500 q q q q 400 other other 400 300 200 200 100 0 0 Data / MC Data / MC 1.5 1.5 1 1 0.5 0.5 0 0.5 1 1.5 2 2.5 3 3.5 0 0.5 1 1.5 2 2.5 3 3.5 M [GeV] M [GeV] 3 ± 3 ± π π • Clear evidence for e + e - →τ + τ - in the Phase II data, and a demonstration of the capacity for missing energy analyses with Belle II Tau Lepton Physics at Belle II, 1/3/2019 P. Rados 7
e + e - → τ + τ - event candidate π π π µ hits in KLM Tau Lepton Physics at Belle II, 1/3/2019 P. Rados 8
Tau lepton mass measurement • First m τ measurement at Belle II was performed with a pseudomass Events 600 Belle II 2018 (Preliminary) technique developed by the ARGUS collaboration: Data MC total (stat ⊕ sys) 500 ∫ -1 L dt = 291 pb ± 3 τ → π ν sig M 2 3 π + 2( E beam − E 3 π )( E 3 π − P 3 π ), M min = 400 other τ → sig q q 300 other • M min distribution in the 𝛖 signal → 3 π ± 𝛏 𝛖 decay channel is fitted to an 200 empircal edge pdf (where m* is an estimator for m τ ): 100 F ( M min ; a , b , c , d , m *) = ( aM min + b ) × tan − 1 [( m * − M min )/ c ] + dM min + 1 0 Data / MC 1.5 1 70 ) 2 0.5 Events / ( 5.0 MeV/c Belle II 60 2018 (Preliminary) 0 0.5 1 1.5 2 2.5 3 3.5 50 M [GeV] min Data 40 ∫ -1 L dt = 291 pb • A fit to the pdf in the 1.7-185 GeV region yields 30 an m τ measurement of: 20 Belle II (2018) 10 2 m = (1776.4 ± 4.8) MeV/c τ m τ = 1776.4 ± 4.8 ( stat ) MeV 0 2 Pull 0 which is in good agreement with previous 2 − 1.7 1.72 1.74 1.76 1.78 1.8 1.82 1.84 measurements. 2 M (GeV/c ) min Tau Lepton Physics at Belle II, 1/3/2019 P. Rados 9
Prospects for tau lepton physics • The Phase III of data taking is expected to start later this month • The huge amount of data to be delivered (50 ab -1 , 2019 - 2027) will enable a broad program of τ lepton physics: arXiv:1808.10567 ‣ Searches for Lepton Flavour Violation (LFV) ‣ CP violation ‣ Second class currents ‣ and much more… (Michel parameters, precision m τ , electric dipole moment, …) • A brief overview of the program is provided here. More info can be found in The Belle II Physics Book . Tau Lepton Physics at Belle II, 1/3/2019 P. Rados 10
Searches for charged LFV • LFV has been established for the neutrinos, but what about their charged partners (e, µ and τ )? Standard Model New Physics • In the SM, charged LFV decays via neutrino oscillation are highly suppressed and immeasurably small: δ m 2 Br ( ℓ 1 → ℓ 2 γ ) SM ∝ ( 2 W ) ν ∼ 10 − 54 - 10 − 49 m 2 • Observation of charged LFV would be a clear signature for New Physics! - Br enhanced in many NP models (10 -10 -10 -7 ) - SUSY, extended Higgs sector, seesaw, leptoquarks, non- universal Z’, and many more µ→ e : stringent bounds exist from MEG - 𝛖 →µ /e : weaker bounds (Belle, BaBar and CLEO) - • As heaviest lepton, NP can have preferential τ LFV couplings Tau Lepton Physics at Belle II, 1/3/2019 P. Rados 11
Prospects for 𝛖 LFV decays • Due to their large mass, τ leptons provide a wide variety of LFV (and LNV) decay modes to study: τ → ℓγ - radiative: } ‣ “golden channels” for discovery: τ → µ ɣ , τ → µµµ τ → ℓℓℓ - leptonic: ‣ complementary: semileptonic modes allow us to test LFV couplings b/w quarks and leptons, and better discriminate b/w NP models τ → ℓ h ( h ) - semileptonic: • So far, searches for τ LFV decays mostly occurred at last-gen B factories • Upper limits had approached the regime sensitive to NP (10 -10 -10 -7 ) arXiv:1808.10567 Extrapolating from Belle results (50 ab -1 ): Belle II will push the current bounds forward by at least one order of magnitude! Tau Lepton Physics at Belle II, 1/3/2019 P. Rados 12
CP violation in 𝛖→ K s π ± ν 𝛖 + n π 0 𝛖 → K s π ± 𝛏 𝛖 Phys.Rev D85 (2012) 031102 • Due to CP violation in the kaon sector, τ→ K s π ± ν τ decays in the SM have a nonzero decay-rate asymmetry: BaBar (2012) A τ = Γ ( τ + → π + K 0 ν τ ) − Γ ( τ − → π − K 0 s ¯ s ν τ ) Γ ( τ + → π + K 0 ν τ ) + Γ ( τ − → π − K 0 s ¯ s ν τ ) ‣ SM prediction: (3.6 ± 0.1) ⨉ 10 -3 ‣ BaBar measurement: (-3.6 ± 2.3 ± 1.1) ⨉ 10 -3 (2.8 σ ) • An improved A τ measurement is a priority at Belle II 𝛖 → K s π ± 𝛏 𝛖 + π 0 𝛖 → K s π ± 𝛏 𝛖 + 2 π 0 BaBar (2012) BaBar (2012) Tau Lepton Physics at Belle II, 1/3/2019 P. Rados 13
CP violation in 𝛖→ K s π ± ν 𝛖 • CP violation could also arise from a charged scalar boson exchange. It would be detected as a difference in the decay angular distributions: PRL 107 (2011) 131801 Belle (2011) • With 50 ab -1 of data, Belle II is expected to provide a x70 more precise measurement: | A CP | < (0.5-3.8) ⨉ 10 -4 (assuming central value A CP = 0) Tau Lepton Physics at Belle II, 1/3/2019 P. Rados 14
Recommend
More recommend
