D 0 Mixing and CP Violation at the BABAR Experiment

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1 D 0 Mixing and CP Violation at the BABAR Experiment Giulia Casarosa INFN & Università di Pisa Società Italiana di Fisica XCV Congresso Nazionale Bari, 28 settembre - 3 ottobre Giulia Casarosa D 0 Mixing and CP Violation at the BABAR Experiment 1 Ottobre /14

2 Outline 1 Introduction to Mixing and CP Violation in the Charm Sector 2 Peculiarity of D Mixing & New Physics (NP) 3 Analysis on D Mixing at the BABAR experiment D 0 K + π (Wrong Sign) D 0 K + K, K π + (lifetime ratio) 4 New analysis in BABAR: D 0 K 0 Sh + h, h = K, π peculiarity of the method status and expectations 5 Conclusions Giulia Casarosa D 0 Mixing and CP Violation at the BABAR Experiment 1 Ottobre /14

3 Part I Mixing in the Charm Sector Giulia Casarosa D 0 Mixing and CP Violation at the BABAR Experiment 1 Ottobre /14

4 D 0 and D 0 are created as flavour eigenstate by the strong interaction, they mix via the weak interaction. Their evolution is obtained by solving: i ( ) D 0 (t) t D 0 = ( M i ( ) (t) 2 Γ) D 0 (t) D 0 (t) diagonalized by the mass eigenstates: D 1,2 = p D 0 ± q D 0 where q p = M 12 iγ 12 /2 M 12 iγ 12/2 and q 2 + p 2 = 1 (assuming CPT invariance) Mixing Parameters: strong phase weak phase x D = m1 m2 Γ y D = Γ1 Γ2 2Γ CP Violation (CPV) can occour in 3 ways: λ f = q Ā f p A f = λ f e i(δ f +φ) in decay: A f Ā f ( A f = D 0 H f, Ā f = D 0 H f ) in mixing: q/p 1 in the interference between decay and mixing: arg ( λ f ) 0 Giulia Casarosa D 0 Mixing and CP Violation at the BABAR Experiment 1 Ottobre /14

5 Peculiarity of D Mixing & NP 1 Important measurement for the Standard Model: complete the picture of neutral meson mixing in the SM; virtual down type quarks involved in mixing loop (only in D system). 2 Standard Model predictions: important uncertainties on SM prediction due to long distance diagrams 3 Important contribution in the search for New Physics: D exhibits the smallest mixing and CPV NP detection is facilitated x D >> y D or CP violation would be a sign of NP; NP models must account for this measure important constraints on NP models; (2006) A. Petrov, Int.J.Mod.Phys.A21:5686 u c W d, s, b d, s, b W D 0 KK D 0 ππ theory model index Giulia Casarosa D 0 Mixing and CP Violation at the BABAR Experiment 1 Ottobre /14 c u

6 Part II D Mixing at the BABAR Experiment Giulia Casarosa D 0 Mixing and CP Violation at the BABAR Experiment 1 Ottobre /14

7 PRL 98, (2007) where: arxiv:hep-ex/ R D = (DCS decay rate) / (CF decay rate) x = x D cos δ Kπ + y D sin δ Kπ D 0 K + π wrong sign decay time distribution discriminate DCS and MIX-CF by the time dependance of the WS decay of a meson which was a D 0 at t = 0: T WS (t) e Γt`R D + R D y Γt + x 2 +y 2 (Γt) 2 2 (in the limit of CP conservation and x D, y D << 1) y = x D sin δ Kπ + y D cos δ Kπ δ Kπ = strong phase between the DCS and the CF decay (not measurable at B-Factories) Evidence for D 0 mixing (3.9σ) & No evidence for CPV R D = (0.303 ± ± 0.01)% x 2 = ( ± 0.03 ± 0.021)% y = (0.97 ± 0.44 ± 0.31)% N sig = 4k L int = 384 fb 1 Giulia Casarosa D 0 Mixing and CP Violation at the BABAR Experiment 1 Ottobre /14

8 arxiv:hep-ex/ submitted to PRD D 0 K + K, K π + a lifetime difference measurement One manifestation of D mixing is differing D 0 decay time distributions for decays to different CP eigenstates: y CP = <τ Kπ> <τ KK > 1 (in the limit of x D, y D << 1 and no CPV in mixing and interference) < τ Kπ > is the D 0 mean lifetime of decays to a state of indefinite CP < τ KK > is the D 0 mean lifetime of decays to a CP-even final state untagged sample: N Kπ = 2.7M N KK = 264k L int = 384 fb 1 note that the D flavour may be tagged or not Kπ KK Evidence for D 0 mixing (3.3σ) y CP = (1.12 ± 0.26 ± 0.22)% combining the tagged y CP and the untagged y CP : Evidence for D 0 mixing (4.1σ): y CP = (1.16 ± 0.22 ± 0.18)% tagged y CP ref: PRD 78, (2008) Giulia Casarosa D 0 Mixing and CP Violation at the BABAR Experiment 1 Ottobre /14

9 ongoing analysis D 0 K 0 Sh + h, h = K, π a Time Dependent Analysis on the Dalitz Plot unique method to measure directly the mixing parameters, x D & y D The Dalitz Plot (DP): (m 2 K 0 S π, m2 K 0 S π+ ) many resonances contribute and interfere: A f (m 2, m2 + ) = P r areiφr A r(m 2, m2 + ) + a NRr iφ NR ; m 2 + sensitivity over the DP a Dalitz model is assumed and all the amplitudes and phases are extracted in the fit; m 2 in the limit of no direct CP violation we can write the PDF as a function of only one amplitude, let s say A f, and extract all the parameters from data NO strong phase rotation of the mixing parameters!! Γ(D 0 f, t) A f 2 e Γt h 1+ λ f 2 2 cosh(y D Γt)+ 1 λ f 2 2 i cos(x D Γt) Reλ f sinh(y D Γt) Imλ f sin(x D Γt) λ f = q Ā f p A f the sensitivity to mixing is amplified by the fact that λ f varies on the DP: λ f = λ f (m 2, m2 + ) there can be big interferences and relative phases. Giulia Casarosa D 0 Mixing and CP Violation at the BABAR Experiment 1 Ottobre /14

10 ongoing analysis L int = 468 fb 1 D 0 K 0 Sh + h, h = K, π The Method The decay chain is fully reconstructed: D + D 0 π s + (or c.c.), D 0 (D 0 ) K 0 S h+ h, K 0 S π+ π D flavour is tagged by the charge of the (slow) pion emitted by the D ; a beam spot constraint to the D decay reconstruction is applied. Selection of the events e + e c c events have high D 0 momentum in the CM frame momentum cut to reject D 0 from B B events (p D 0 > 2.5 GeV/c); quality cuts in order to have well measured tracks and a well measured D 0 lifetime (e.g. p t(tracks) > 100 MeV/c, σ t < 1ps) dedicated cuts on K 0 S flight length and collinearity angle in order to remove bkg events with the same final state as signal (D 0 KKππ or D 0 4π). Discrimination between signal and background events use the distribution of the variables m D 0 and m in order to discriminate signal events from bkg categories events: m= m D m D 0, m D 0 = the reconstructed mass of the D 0 ; very good resolution on m due to the small Q-value of the D reaction. Giulia Casarosa D 0 Mixing and CP Violation at the BABAR Experiment 1 Ottobre /14

11 bkg categories: Cat2 = true D 0 & misrec π s Cat3 = misrec D 0 & true π s Cat4 = misrec D 0 & misrec π s D 0 K 0 Sπ + π Fit to MonteCarlo events m D 0 m D 0 proper time the purity of the selection is greater than 98% proper time fit to full MC: τ = (412.4 ± 0.8) ps compatible with the generated lifetime we found no bias on lifetime fitting a full MC sample implemented with mixing Giulia Casarosa D 0 Mixing and CP Violation at the BABAR Experiment 1 Ottobre /14

12 D 0 K 0 Sh + h, h = K, π Status and Expectations blind fit and evaluation of systematics is done: statistical errors: σ stat x = 0.23%, σ stat y = 0.20% most important sources of systematic error: source σx syst /σx stat σy syst /σy stat Dalitz Model 49% 41% Fit Bias 32% 32% Final Fit Cuts 24% 26% Mistag 16% 16% Efficiency Map 16% 9% world single measurement best sensistivity on x D and y D expected BLIND FIT RESULT N.B.: the central point is moved by an unknown quantity PRELIMINARY Giulia Casarosa D 0 Mixing and CP Violation at the BABAR Experiment 1 Ottobre /14

13 Part III Conclusions Giulia Casarosa D 0 Mixing and CP Violation at the BABAR Experiment 1 Ottobre /14

14 Conclusions Measuring x D and y D is important for SM Physics and for New Physics models which must be able to account for the result of this measure; BABAR had evidence of mixing in D K + π wrong sign decay time analysis and D KK, Kπ lifetime difference analysis but no evidence for CP violation; The Time Dependent Analysis on the Dalitz Plot D 0 K 0 Sh + h (h = K, π) allows a direct measurement of the mixing parameters; this analysis is actually ongoing in BABAR. Giulia Casarosa D 0 Mixing and CP Violation at the BABAR Experiment 1 Ottobre /14