NEW RESULTS ON MIXING FROM LEP STEPHEN ARMSTRONG European Organization for Nuclear Research (CERN) EP Division Geneva, Switzerland Flavour Physics and CP Violation (FPCP) Philadelphia, Pennsylvania, USA May 16,
OUTLINE Mixing Phenomenology LEP and the LEP Experiments Experimental Strategy New/Improved Analyses from () Results and Interpretation Conclusion S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page 1
MIXING PHENOMENOLOGY _ B s - B s oscillation frequency proportional to mass difference m s b W u,c,t u,c,t _ W W _ s b s u,c,t b W + Measurement of m s (and m d ) m s m d s B s B s B B s s m m BBd permits extraction of CKM elements V ts F V td = b F BBd B B BBd Time-dependent asymmetry between "mixed" and "unmixed" decays P mix (t) = P (_ Bs (t) = Γ e Γ ) st unmix s [1 cos( m s t)] Assuming CP conservation and small lifetime differences + u,c,t s ξ contains theo. uncertainties roughly 6% S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page
Large Electron Positron (LEP) Collider LEP1:1989-1995 at E cm close to 91 GeV LEP: 1995- at E cm = 13-9 GeV OPAL L3 Large Electron-Positron storage ring (LEP) 7 km, 45 GeV < E < 14 GeV Super Proton Synchrotron (SPS) 7 km, E= GeV Proton Synchrotron (PS).6 km, E=3.5 GeV Electron-Positron Accumulator (EPA).1 km, E=6 MeV LEP Linear Injector system (LIL) E 1 = MeV, E =6 MeV DELPHI LEP1 Data used for Heavy Flavour Analyses: 1991-1995 4 Million Hadronic Z Decays per exp. LEP Accelerator and Experiments dismantled to make way for LHC S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page 3
A CANDIDATE EVENT DALI Run=1514 Evt=4177 e + e Ζ bb _ (if no mix) +X B x +X π K π K Gev EC 8 Gev HC.cm Y".3cm π K D s Kπ IP RO TPC X".cm # T>4 man.cut S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page 4
EXPERIMENTAL STRATEGY Select Candidates and Determine their Event Purity Tag Initial and Final States Measure Proper Time Fit m s Final State Same Side Candidate K f IP Initial State l + K Opposite Side S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page 5
Bs SELECTION AT LEP 4 selection categories of increasing sample size/decreasing purity 3 new/improved analyses Fully Exclusive, DELPHI Criteria Sample Size/Purity Fully reconstructed: D s ( * ) (π +, a 1 +, ρ + ) D K (π +, a 1 + ) 5-8 candidates 5-8% purity small sample size is compensated by excellent resolution Semi-Exclusive, DELPHI, OPAL D s ( * ) l + ν l D s ( * ) + hadrons 1-1 3 candidates 4-6% purity Semi-Inclusive, DELPHI, OPAL l + + X 1 4-1 5 candidates 1 - % purity Fully Inclusive DELPHI Inclusive secondary vertices 5x1 5 candidates 1% purity ("natural") S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page 6
NEW FULLY EXCLUSIVE Bs SELECTION Entries/4 MeV/c 15 1 5 15 1 5 3 1 Main Peak Satellite Region D s π + (π, γ) D s a 1 + (π, γ) Combined Data MC MC 4 4.4 4.8 5. 5.6 6 Mass (GeV/c ) D s ( * ) (π +, a 1 +, ρ + ) Topology D s π + (a 1 +, ρ + ) Event Purity 1 event classes based on decay Purity from helicity angle, m(d s ) Candidate Events 3 candidates in main peak 48 candidates in satellite region 11 candidates with purity > 8% S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page 7
IMPROVED SEMI-EXCLUSIVE Bs SELECTION D s ( * ) l + ν l Topology D s l + ν (ν) Event Purity m(d s or φ) resonant fraction NN based discriminant for signal vs b D s DX (D l) Entries/5 MeV/c 8 7 6 5 4 3 D s hadronic 1 97 candidates 36 candidates Data Fit Wrong sign 1 1.7 1.8 1.9.1. D s Reco. Mass (GeV/c ) Entries/ MeV/c S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page 8 1 8 6 4 D s φl + ν Data Fit Wrong sign 1 1. 1.4 1.6 φ Mass (GeV/c )
IMPROVED SEMI-INCLUSIVE Bs SELECTION NN-based b l discriminant: lepton p and p T E miss E ν Jet/Track kinematics lepton impact parameter 1 4 w.r.t. Charm vertex Data qq simulation udsc and fake l b c l b c l b l Inclusive Charm candidate recovered photon l + ν Same Side l + + X IP Opposite Side 1 5 1 4 NN-based Event b-tag using b-hadron content of Same and Opposite Sides Track Impact Parameters Secondary Vertices b Hadron Mass lepton p and p T Data qq simulation bb cc uds 1 3 Cut selection yields: 74, 6 candidates 1 3 Cut 1 1-1 -.8 -.6 -.4 -...4.6.8 1 NN (b l) Output -1 -.8 -.6 -.4 -...4.6.8 1 NN Event b-tag Output S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page 9
IMPROVED SEMI-INCLUSIVE Bs SELECTION Event Purity determined with NN-based discriminant Vertex charge and charge multiplicity: q l Σ w i κ (1) q i, Σw i κ (), q l Σw i p i κ (3) q i K from Fragmentation and B Decay: K ± (w.r.t. l charge), K estimators, m(k + K ) Entries/.4 5 15 1 5 qq simulation udsc b baryons B + Data B d -1 -.8 -.6 -.4 -...4.6.8 1 Neural Network Output S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page 1 b-hadron fraction 1.9.8.7.6.5.4.3..1 B + simulation Bs 17% sample has purity above % Bd b baryons -1 -.8 -.6 -.4 -...4.6.8 1 Neural Network Output
INITIAL AND FINAL STATE TAGGING Determine Particle/Antiparticle State of at Production (Decay) FINAL STATE TAGGING Fully Exclusive, DELPHI D s ( * ) (π +, a 1 +, ρ + ) D K (π +, a 1 + ) Charges of Decay Products Semi-Exclusive, DELPHI, OPAL D s ( * ) l + ν l : lepton charge D s ( * ) + hadrons : D s ( * ) charge Semi-Inclusive, DELPHI, OPAL l + + X : lepton charge (account for b c l mistag) Fully Inclusive DELPHI NN-based charge dipole method S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page 11
INITIAL STATE TAGGING Draw upon information from both Same and Opposite sides Same Side Same Side Information Primary Vertex charge Fragmentation Kaon K f IP Initial State l + K Opposite Side Information Hemisphere "Jet" charge Primary Vertex charge Secondary Vertex charge K ± and l ± charge(s) Opposite Side Combine all information into single Tagging discriminant: performance evaluated as Mistag Rate η S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page 1
NEW INITIAL STATE TAGGING Entries/. Opposite Side Information combined using NN: 3 5 15 Data qq simulation udsc bb b hadrons b hadrons opposite only Additional Same Side Information: Fragmentation Kaon (NN selected, charge signed) "Jet" charges (excluding decay products) cos θ( ), p( ), N tracks Entries/.4 5 Data qq simulation udsc bb b hadrons b hadrons same + opposite ηeff = 4% 1 _ b hadrons b hadrons 15 5.1..3.4.5.6.7.8.9 1 NN Output (Opposite Side) 1 5 _ b hadrons b hadrons -1 -.8 -.6 -.4 -...4.6.8 1 NN Output (Initial State Tag) S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page 13
PROPER TIME MEASUREMENT Determine Proper Time (i.e., meson lifetime in its rest frame) t = l m p σt = ( m p σl + ( t σp p ) ) Term diminished as osc. << τ Decay Length (l): Distance from Primary to Seconary Vertex Two Ingredients Momentum (p) Event Selection Dependent PV SV Primary Vertex independent of analysis Secondary Vertex dependent upon event selection (impact upon resolution) typical σ l = 5 µm Fully Enclusive: sum of decay products Inclusive: Jet momentum Correct for p ν in semileptonic (event energy-momentum cons.) S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page 14
OSCILLATION FIT Construct Signal Likelihood: N comp for every candidate i: Li = Σ f j i [ (1 η j i ) P j unmix (ti) + η j i P j mix (ti)] j j denotes signal and background components oscillating oscillating B d non-osc. b Hadrons udsc events f j i : prob. of candidate i from j (event-byevent purity) η j i : prob. of candidate i osc. if from j (event-by-event initial and final state tag) p.d.f. of decay proper time for unmixed/mixed candidates in component j with experimental effects (e.g., σ l, σ p ) Elements of the Likelihood are evaluated event-by-event S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page 15
THE AMPLITUDE METHOD Introduce an Amplitude A into probabilities: P mix (t) = Γ e Γ st unmix s [1 cos( m s t)] ± Γ s e Γ st [1 Acos(ωt)] Maximize Likelihood with respect to Amplitude A for a given test frequency ω Permits combination of different analyses/experiments results A = for ω << m s A = 1 for ω = m s ω excluded at 95% C.L. if A + 1.645σ A < 1 Analysis Sensitivity: expected limit at 95% C.L. ± S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page 16
RESULTS OF THE THREE NEW ANALYSES Amplitude 8 6 4 data±1σ 1.645σ data±1.645σ Fully Exclusive Semi-Exclusive (D s lepton) Semi-Inclusive (lepton) data±1.645σ (stat only) Amplitude data±1σ 95% CL limit 11.5 ps -1 1.645σ expected limit 14.3 ps -1 data±1.645σ data±1.645σ (stat only) -1 - - -1.5.5 5 7.5 1 1.5 15 17.5.5 5.5 5 7.5 1 1.5 15 17.5.5 5.5 5 7.5 1 1.5 15 17.5.5 5 ω (ps -1 ) Amplitude 8 6 4 data±1σ 95% CL limit 7. ps -1 1.645σ data±1.645σ -1 expected limit 7.5 ps data±1.645σ (stat only) 3.5 3.5 1.5 1.5 -.5 ω (ps -1 ) ω (ps -1 ) m s Observed (Expected) Lower Limits at 95% C.L. m s >.4 ps 1 (.3 ps 1 ) m s > 7. ps 1 (7.4 ps 1 ) m s > 11.4 ps 1 (14. ps 1 ) NEW! (was 7. ps 1 (6.6 ps 1 )) (was 9.5 ps 1 (9.8 ps 1 )) COMBINATION OF THREE ANALYSES m s > 1.9 ps 1 (15.7 ps 1 ) S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page 17
DELPHI AND OPAL RESULTS Amplitude 7 6 5 4 DELPHI D s ( * ) l + ν l Eur. Phys. J. C16 () 555 Amplitude 5 4 3 OPAL D s ( * ) l + ν l Eur. Phys. J. C19 (1) 41 3 1 1-1 - -1-3 - -4-3 5 1 15 m s (ps -1 ) m s > 7.4 ps 1 (8.1 ps 1 ) also Fully and Semi-Exclusive: m s > 4. ps 1 (3. ps 1 ) Semi-Inclusive: m s > 7.3 ps 1 (1.6 ps 1 ) Fully Inclusive: m s > 1.1 ps 1 (6.1 ps 1 ) -5 4 6 8 1 1 14 m s (ps -1 ) m s > 1. ps 1 (4.1 ps 1 ) also Semi-Inclusive: m s > 5.1 ps 1 (6.7 ps 1 ) S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page 18
WORLD COMBINATION Combine LEP (new, DELPHI, OPAL) with CDF and SLD Amplitude.5 1.5 1.5 -.5-1 World average (prel.) data ± 1 σ 95% CL limit 14.9 ps -1 1.645 σ sensitivity 19.3 ps -1 data ± 1.645 σ data ± 1.645 σ (stat only) l (91-95, no D s l, adjusted) D s l (91-95) B (91-) s CDF lφ/l (9-95) DELPHI +D s h (9-95) DELPHI D s l+φl (9-95 prel) DELPHI l (9-95, prel) DELPHI vtx (9-95, prel) OPAL l (91-95) OPAL D s l (91-95) SLD l+d (96-98, prel.) SLD dipole (96-98, prel.) SLD D s (96-98, prel.) amplitude (sensitivity).43 ±.69 ±.16 (13.7 ps -1 ) 3.83 ± 1.49 ±.3 ( 7.5 ps -1 ) -.47 ± 1.15 ±.47 (.4 ps -1 ) -.14 ±. ±.51 ( 5.1 ps -1 ).45 ± 3.58 ± 1.93 ( 3. ps -1 ) -.43 ± 1.51 ±.35 ( 8.7 ps -1 ) -.19 ± 1.18 ±.19 ( 9.9 ps -1 ) -.43 ± 3.67 ±.56 ( 6.1 ps -1 ) -1.5 ±.34 ± 1.91 ( 7. ps -1 ) -3.63 ± 3.5 +.4 -.4 ( 4. ps -1 ).67 ± 1.7 +.5 -.39 ( 6.3 ps -1 ).41 ±.99 +.1 -.3 ( 8.6 ps -1 ) 1.38 ± 1.75 +.1 -.45 ( 1.7 ps -1 ) -1.5.5 5 7.5 1 1.5 15 17.5.5 5 m s (ps -1 ) World average (prel.).39 ±.39 B Oscillations Working Group -6-4 - 4 amplitude at m s = 15. ps -1 (19.3 ps -1 ) m s > 14.9 ps 1 (19.3 ps 1 ) at 95% C.L. No measurement, but data are consistent with signal expectation around m s = 16 to 18 ps 1 S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page 19
CONCLUSIONS No observation of Bs mixing Bs mixing analyses continue to improve New/Improved results () Lower Limit of ms > 14.9 ps 1 far below sensitivity of 19.3 ps 1 Possibile hint of signal between 16-18 ps 1 Look forward to new results from CDF and D S.R. Armstrong FPCP Philadelphia, PA, USA May 16, Page