PoS(CKM2016)142 http://pos.sissa.it/ of data, which 1 − system. We focus 0 D - 0 D decays. − π + π 0 S K → 0 violation in the D CP , and 0 π − π + K → resonance with the goal of collecting 50 ab 0 ) D S , 4 − ( π ϒ + K → 0 D † ∗ collisions at or near the − [email protected] e + -violation measurements at Belle II Speaker. On behalf of the Belle II Collaboration is a large increase over that recordeda by large the Belle sample and of BaBar experiments.Belle charm II This data for will decays. measuring provide time-dependent mixingon In and measurements this of report we present the expected sensitivity of The Belle II experiment is undere construction at the KEK laboratory in Japan. Belle II will study ∗ † Copyright owned by the author(s) under the terms of the Creative Commons c 9th International Workshop on the CKM28 Unitarity November Triangle - 3 December 2016 Tata Institute for Fundamental Research (TIFR), Mumbai, India Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0).

A. J. Schwartz Prospects for time-dependent mixing and Physics Department, University of Cincinnati, Cincinnati,E-mail: Ohio 45221 CP CP CP PoS(CKM2016)142 , , ] ]. ) 3 − → 5 Γ π 0 2 + ( 0 [ flavor D / K 0 . Some 6= 1 identifies x D ∆Γ → - system has 0 ± 0 . = 0 π 2 D D A. J. Schwartz y D m violation in the - 0 experiment such µ and D -violating parame- − CP meson decays, and Γ 50 e is determined by re- / + CP D × D e m in “wrong-sign” mixing was obtained in ∆ 0 φ or even whether and = , and x D x y - B 0 > D y , and | and p violation in the x / q | decay; the charge of the CP , − y π , 0 x violation depend on measuring decay times . The pixel layers will lie only 14 mm and D decays. 1 reconstruction), high flavor-tagging efficiency − → CP π + + ∗− ρ 1 π − − − D (right) plots the residuals of decay time for a large π π π are the complex coefficients relating the → or + + + decays. The RMS of this distribution is 140 fs, which 2 , and 0 p 0 + K K K D − η π , 0 K → → → and η + ]. In all cases the flavor of the neutral D 0 0 0 6 q K [ D D D → ] experiments, with subsequent observations made by LHCb [ − 2 + → ∗ π 0 D + D π , where , 0 S φ + K π 0 → ) = D 0 p violation in decays, and also for / D ] and BaBar [ → q 1 − violation by Belle II could indicate new physics. are the differences in masses and decay widths between the two mass eigenstates ( + CP CP CP K ∗ + D , and CP ∆Γ (left) shows the resolution on the impact parameter of tracks with respect to the IP 0 K flavor. All results given are preliminary. reconstruction (and thus π 2 0 0 ]. However, the current precision on the mixing parameters → − and π 4 and Arg D 0 π | m D + p or ∆ is the mean decay width, is insufficient to determine whether / and K 0 q Figure There are several advantages of performing these measurements at an Measurements of time-dependent mixing and To study the sensitivity of Belle II to parameters The Belle experiment at the KEK laboratory in Japan is now being upgraded to the Belle II decays. Within this broad physics program, measuring mixing and γ | Γ D → τ 0 eigenstates with the mass eigenstates.D We focus on three benchmark decay modes: parameters of the vertex detector are listed in Table 22 mm from the interaction point (IP), and the smallest pixelas size will a be function 55 of trackare momentum, corresponding as results obtained from fromapproximately BaBar. Monte half Carlo that The (MC) of figure simulation.sample BaBar. indicates of Also that MC Figure shown the Belleis almost II half resolution the will decayprompt time be resolution of BaBar (270 fs). Similar results have been obtained for in neutral charm system is an2007 important by goal. the Belle The [ firstand evidence CDF [ for To establish this, higher statistics measurementsnot are needed. yet been observed. Theobservation of predicted rate within the Standard Model (SM) is very small, and an quiring that it originate from a 2. Decay-time resolution precisely. Due to ansuperior to improved that vertex of Belle detector, andwill the BaBar. use Whereas decay Belle four time used layers a resolution of four-layer of silicon-strip silicon detector, Belle strips Belle II II plus should two be layers of silicon pixels, as shown in Fig. 3. Mixing and Time-dependent mixing and CP violation at Belle II 1. Introduction experiment. The goalobtained. of Belle The II experiment is is to designed collect to search a for data new set physics corresponding in to 50 times what Belle where as Belle II rather thanlent at a machine: lower backgrounds, higher trigger efficiency, excel- ters the and with low dilution, and numerous control samplespresent with Belle which II to prospects study systematics. for measuring In this the report mixing we parameters PoS(CKM2016)142 ) of 1 3.1 (3.1) (3.2) − , ) ) 2 2 ). A. J. Schwartz ) ) ) ) ) ) 2 z z z z t t | ( ( ( ( ) Γ Γ , and 50 ab ( ( 1 − 2 2 − 0 0 π 512 512 512 768 y y , , , , + ) ) ) ) 4 4 + + K φ φ φ φ 2 2 ( ( ( ( 0 0 decays. The decay times → x x , 20 ab 0 ∓ 1 2 2

768 768 768 768 π − ) is the ratio of amplitudes D p p q q ( D ±

R K ( A decays and a separate sample of ) ) ) / z z z ) + ) + → ) 0 ) D ( ( ( t t z + R D D ( Γ Γ 5050 3072000 4608000 π )( )( 240 240 240 − × × is the strong phase difference between , , , φ φ ) ) 160 K m) or strips ) ) ) δ , φ φ φ µ ) 60 85 sin sin ( ( ( → , , 0 0 φ 0 x x ( 75 75 75 55 70 D , , , , and ( ( − + ( δ φ φ 2 A | ( cos cos cos 2 y 0 0 | y y ) ]. The parameter ( ( + 7 + D D δ π R R − sin p q K

x (mm) ( p p q q → − ), there are a total of 438600 decays [

0 1 = − D + + + amplitudes. This strong phase difference can be measured at BESIII 0 ( π time distributions are simultaneously fitted. Backgrounds are not yet D D y + Layers and segmentation of the Belle II vertex detector. − 0 R R , A π / δ K D ( ( − ) t t − Γ Γ K → sin π − − y and e e Type Distance from IP Size or pitch Number of pixels → + stripsstripsstripsstrips 38 80 104 135 50 50 50 50 pixelspixels 14 22 CP Table 1: 0 0 + K D D D δ ) = ) = → 0 0 0 5 6 2 1 3 4 cos D D and D ( ( x ( Layer + Left: layers of the Belle II vertex detector, which consists of two layers of silicon pixels and four -tagged = dt π A dt dN dN | ) with a Gaussian resolution function; this results in products of exponential functions and 0 − decays, we perform a “toy” MC study as follows. We generate an ensemble of 1000 MC x CP K 3.2 − After generation, the decay times are smeared by the expected decay time resolution of 140 fs, decays. The number of events in each sample corresponds to 5 ab π → 0 0 + data. For the full dataset (50are ab obtained by sampling from the following probability density functions (PDFs): and the resulting and ( included in this study; aerrors first increase look by at only backgrounds a indicates small that amount. when The they PDFs are used included, for the the fit fitted are the convolution of Eqs. ( K experiments, with each experiment consistingD of a sample of using where Time-dependent mixing and CP violation at Belle II D squared Figure 1: layers of silicon strips. Right: a. cutaway view of the vertex detector. PoS(CKM2016)142 , | ]. p 8 ππ / K q | δ 0039, . 0 cos = y amplitudes y 0 + A. J. Schwartz π + ππ K π δ − K ]. The projections of sin 0258 and 9 . x , and the fitted parame- → 0 049%. This precision is ◦ . 0 − 0 = D = x = 1 00 y − y and δ 0 . In practice, to extract values for 4 π and + 50 ab is fixed to 10 π decays: uncertainty on “rotated” mixing ]. . The final Belle II sensitivity for , for three values of integrated luminosity. ππ − of Belle II data. The generated decay 2 1 − φ ππ K violation and backgrounds are neglected K 10 1 − π δ K 057% and − . δ + → 0 and K sin CP 0 | y p = 20 ab D → / x 0 + q | δ 3 D . The input values are 1 ππ 3 − K δ ]. Possible 5 ab 0.370.26 0.230.20 0.1716 0.15 0.09 0.10 9.2 0.05 5.7 decays using an MC simulation based on EVTGEN [ 9 cos 0 x π = − (%) π 00 | ) x + p ◦ (%) (%) ( / K . This precision is a significant improvement over that which Belle 0 0 0 0 0 q ◦ y | x π -violating parameters π π → δ δ δφ Parameter δ 0 − − − is 6 CP decays. The RMS of this distribution is 140 fs. π D π π decays corresponding to 50 ab φ + − + + . For this study the strong phase 0 K K ρ K K π + M − K → → → , and on π 0 0 0 = 0 y → + D 0 D D . The results are shown in Fig. K ππ y D and m is the strong phase difference between , → 0 + x 0 and Left: Belle II track impact parameter with respect to the IP as a function of track momentum, as ππ π Preliminary results of a toy MC study of D 0 K x 1%, and that for δ D . 0 The mixing parameters are We study mixing in → + < ∗ times are smeared by a resolutionplot of 140 is fs, the and isobar the model decay model used used by to BaBar generate [ and fit the Dalitz ters are For this study we225000 generate an ensemble of 10 experiments, with each experiment consisting of in this phase of the study. More details are given in Ref. [ a typical fit (one experiment in the ensemble) are shown in Fig. 4. Mixing in Table 2: Time-dependent mixing and CP violation at Belle II is parameters evaluated at and the RMS of the distributionsapproximately of residuals one are order of magnitude better than that achieved by BaBar [ Figure 2: Error functions. The preliminary results are listed in Table obtained from MC simulation.D Right: Belle II residuals of decay vertex position, from MC simulation of and BaBar achieved. where PoS(CKM2016)142 . t ≡ ]. − (5.2) (5.1) 10 m ) ) xt xt and ( ( 2 A. J. Schwartz ) cos cos + π ! ! P 2 2 , | | f f +   ) ) S A A | 0 K | , and the decay time xt xt 2 P 2 ( (

− (

p q m p q sin sin

≡ ,

) requires dividing the data y +   − + − ∗ ∗ 2 f f m 2 m | | f f A and A f f x A A | | A A

p p q q   ) + ) + yt yt ( ( 2Im 2Im ; this can in principle be measured at BESIII violation is by fitting the time-dependent Dalitz − − ππ − − − ) ) cosh cosh K 4 π π π δ (in addition to yt yt CP ! ! ( ( + + events for one typical experiment in the ensemble [ + 2 2 φ | | 0 π π π f f 049%. 0 0 0 π . S S S sinh sinh 0 A − K K A K | and | π   2 = | 2 ∗ ∗

+ f f

→ → → y p p q K p 0 q 0 δ 0 / A

A

q f D D f D | → + decays. 0 + A A 2 0 2 | D | p p f q q π f ]. Middle and right: projections of the left-most plot. The RMS values of +   A A | | π 057% and 10 . − 0 decays. One calculates the observables 2Re 2Re K ( ( ( decays and fitting the two subsamples simultaneously. = t t − + + x 0 → Γ Γ violation in π 2 2 δ − − D of data [ + e e CP 1 π CP D CP CP 0 S − and K ) = ) = 0 and performs an unbinned maximum likelihood fit to 0 0 -violating parameters D → 2 D D Projections of the fit to ) 0 ( ( Left: preliminary results of fitting an ensemble of 10 experiments, with each experiment corre- CP − -tagged D π requires knowledge of the strong phase dt dt P dN dN y CP + Another method for measuring mixing and The time-dependent PDFs are S 0 K and Figure 4: P To fit for sample into plot for ( using 5. Mixing and Figure 3: sponding to 50 ab these distributions are Time-dependent mixing and CP violation at Belle II x PoS(CKM2016)142 , ], and 11 -waves. , 101802 ) depend S , 211802 ], the decay 110 5.2 98 ππ A. J. Schwartz 11 decays [ , 211803 (2007). and and − 98 π π 5.1 + K π of data [ 0 S 1 K ) are scaled by the square − ) → ) 0 , Phys. Rev. Lett. stat φ , Phys. Rev. Lett. ◦ ( σ D -waves, were obtained from a σ S . These errors are conservative, ) that do not decrease with more , Phys. Rev. Lett.

mixing ) decays. These amplitudes must 2 irred mixing using the CDF-II detector f p q 0 irred σ 0

σ oscillations D → σ - mixing 0 ). ) + 0 0 0 2 1 D - D )( D − -anti-D 0 ( - 2 ) 0 0 y decays using 976 fb − f ( − σ 50 ab π → 5 / + 0 π from a Belle analysis of D ) (10 0 S ) 2 Belle φ x − K ( L Observation of D ( 0.190.06 0.150.11 0.06 0.1550.11 0.04 10.7 0.054 0.05 0.069 4.5 0.073 3.8 4.1 σ → Evidence for D · (10 Evidence for D 0 ) , and Observation of D | D p 2 syst / tot q σ along with the precision estimated for Belle II. This precision is | σ syst irred stat , 3 + y σ σ σ , x 2 stat σ ( Belle Belle Belle Belle II q , 231802 (2013). = (CDF Collaboration), 111 (Belle Collaboration), (Belle Collaboration), (LHCb Collaboration), et al. ) is the decay amplitude for Belle II f et al. et al. σ ] are listed in Table et al. A ) that should decrease with luminosity such as errors due to background modeling as without being “rotated” by an unknown strong phase difference. The results obtained ( 11 y f syst Precision obtained for σ A and (2007). Phys. Rev. Lett. R. Aaij T. Aaltonen M. Staric B. Aubert (2013). The authors thank the CKM 2016 organizers for a well-run workshop and excellent hospitality. An important advantage of this measurement is that the fitted PDFs (Eqs. x [3] [4] [1] [2] as the simple scaling used doesvertex detector not as account compared for to the that improved of decay Belle time (see resolution Section of the Belle II be modeled, and themeasurement. parameters For of a Belle the analysis model of must also be fitted or else taken from a previous Acknowledgements This research is supported by the U.S. Department of Energy. References Table 3: The magnitudes and phases ofseparate the time-independent isobars, fit to and the parameters data. of the the precision expected for Belle II as obtained by scaling the Belle errors (see text). obtained as follows. The statistical errors of the Belle measurement ( Time-dependent mixing and CP violation at Belle II where model used consisted of 14 intermediate resonances modeled by isobars, plus on by Belle [ root of the ratio of luminosities.errors The ( systematic errors are divided intodetermined two from categories: control “reducible” samples; anddata “irreducible” such errors as ( uncertainty in decaycalculated time as resolution due to detector misalignment. The total error is PoS(CKM2016)142 , 023001 41 -lepton A. J. Schwartz τ strong phase difference in , Chin. Phys. C + π − K , 211801 (2009). → mixing from a time-dependent 0 103 mixing and search for indirect CP D , Nucl. Instrum. Meth. Phys. Res., Sect. A - 0 0 mixing sensitivity estimation at Belle II in D 0 - , 091103 (2014). 0 D - 89 0 D Averages of b-hadron, c-hadron, and 6 , Phys. Rev. Lett. Measurement of the D Measurement of D , Phys. Rev. D decays , 227 (2014). Measurement of D 0 via time-dependent amplitude analysis π 0 − 734 π π decays − , arXiv:1612.07233 (2016). + − π K π + + K → π 0 0 S → K 0 → (BESIII Collaboration), , Phys. Lett. B 0 0 (BaBar Collaboration), (Heavy Flavor Averaging Group), D (Belle Collaboration), 0 The EvtGen particle decay simulation package et al. D et al. et al. → et al. ) , 152 (2001). 3770 ( properties as of summer 2016 462 (2017). T. Peng D. J. Lange, B. Aubert amplitude analysis of D L.-K. Li, Y.-Q. Chen, W.-B. Yan, and Z.-P. Zhang, violation using D Throughout this paper, charge-conjugate modes are implicitly includedM. unless Ablikim stated otherwise. Y. Amhis ψ wrong-sign decays D [8] [9] [6] [7] [5] [11] [10] Time-dependent mixing and CP violation at Belle II