First look at time-dependent CP violation using early Belle II data

TL;DR

This paper reports early results from Belle II, demonstrating detector performance and initial sensitivity estimates for measuring CP violation parameters and exploring New Physics in B meson decays using initial data from the upgraded collider.

Contribution

First assessment of Belle II's detector performance and sensitivity to CP violation parameters using early data from the SuperKEKB collider.

Findings

Achieved initial reconstruction efficiency for key decay modes.

Estimated Belle II sensitivity to CKM angles and New Physics contributions.

Demonstrated feasibility of time-dependent CP violation measurements in early data.

Abstract

Time dependent CP-violation phenomena are a powerful tool to precisely measure fundamental parameters of the Standard Model and search for New Physics. The Belle II experiment at the SuperKEKB energy-asymmetric $e^+e^-$ collider is a substantial upgrade of the B factory facility at the Japanese KEK laboratory. The design luminosity of the machine is $8\times{10}^{35}~cm^{-2}s^{-1}$ and the Belle II experiment aims to record $50~ab^{-1}$ of data, a factor of 50 more than its predecessor. From February to July 2018, the machine has completed a commissioning run, achieved a peak luminosity of $5.5\times10^{33}~cm^{-2}s^{-1}$ , and Belle II has recorded a data sample of about $0.5~fb^{-1}$ . Main operation of SuperKEKB has started in March 2019. This early data set is used to establish the performance of the detector in terms of reconstruction efficiency of final states of interest for the measurement of time dependent CP violation, such as $J/\psi K_0$ , $\eta' K_0$, and $\phi K_0$. A first assessment of the B flavor tagging capabilities of the experiment will be given, along with estimates of the Belle II sensitivity to the CKM angles $\phi_1/\beta$ and $\phi_2/\alpha$ and to potential New Physics contributions in penguin amplitudes dominated decays and in $b\to s\gamma$ transitions. In this paper we will present estimates of the sensitivity to $\phi_1$ in the golden channels $b\to c\bar{c} s$ and in the penguin-dominated modes $B_0\to \eta' K_0,\quad\phi K_0,\quad K_0\pi_0(\gamma)$. A study for the time-dependent analysis of $B_0\to\pi_0\pi_0$, relevant for the measurement of $\phi_2$, and feasible only in the clean environment of an $e^+e^-$ collider, will also be given.

Full text

Main operation of SuperKEKB has started in March 2019. This early data set is used to establish the performance of the detector in terms of reconstruction efficiency of final states of interest for the measurement of time dependent CP violation, such as $\mathup{{{J\mspace{-2.0mu}/\mspace{-2.0mu}\psi}}}\mathup{{{K}}^{\scriptstyle{0}}}$ , $\mathup{{{\eta}}^{\scriptstyle{\prime}}}\mathup{{{K}}^{\scriptstyle{0}}}$, and $\phi\mathup{{{K}}^{\scriptstyle{0}}}$. A first assessment of the B flavor tagging capabilities of the experiment will be given, along with estimates of the Belle II sensitivity to the CKM angles $\phi_{1}/\beta$ and $\phi_{2}/\alpha$ and to potential New Physics contributions in penguin amplitudes dominated decays and in $b\to s\gamma$ transitions.

In this talk we will present estimates of the sensitivity to $\phi_{1}$ in the golden channels $\mathup{{{b}}}\to\mathup{{{c}}}\mathup{{\overline{{c}}}}\mathup{{{s}}}$ and in the penguin-dominated modes $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{\eta}}^{\scriptstyle{\prime}}}\mathup{{{K}}^{\scriptstyle{0}}},\quad\mathup{{{\phi}}}\mathup{{{K}}^{\scriptstyle{0}}},\quad\mathup{{{K}}^{\scriptstyle{0}}}\mathup{{{\pi}}^{\scriptstyle{0}}}(\gamma)$. A study for the time-dependent analysis of $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{\pi}}^{\scriptstyle{0}}}\mathup{{{\pi}}^{\scriptstyle{0}}}$, relevant for the measurement of $\phi_{2}$, and feasible only in the clean environment of an $\mathup{{{e}}^{\scriptstyle{+}}}\mathup{{{e}}^{\scriptstyle{-}}}$ collider, will also be given.

I Introduction

The Belle II experiment Abe et al. (2010) is currently taking data at SuperKEKB Ohnishi et al. (2013) $\mathup{{{e}}^{\scriptstyle{+}}}\mathup{{{e}}^{\scriptstyle{-}}}$ collider at KEK, in Tsukuba, Japan. Its main goal is to search for physics beyond the Standard Model (SM) in $\mathup{{{B}}}$, $\mathup{{{D}}}$, and $\mathup{{{\tau}}}$ decays, performing precise measurements of quantities for which the SM provide reliable calculation or looking for exotic particles and decays. The SuperKEKB asymmetric collider works at a center of mass energy corresponding (or close to) the ${\mathup{{{\Upsilon}}}}{\mathup{{\left({4S}\right)}}}$ resonance. The target integrated luminosity is $50~{}ab^{-1}$ in about five year of operation, with a ultimate instantaneous luminosity of $8\times 10^{35}~{}cm^{-2}s^{-1}$, 40 times larger than that of KEKB, its predecessor.

Belle II has collected an integrated luminosity of about 500 pb*-1* in year 2018 (Phase 2) with an incomplete detector, where only a small fraction of the silicon vertex sub-detector was installed. Starting from march 2019, Belle II is collecting more data at ${\mathup{{{\Upsilon}}}}{\mathup{{\left({4S}\right)}}}$ center of mass energy, with all sub-detectors fully installed and operational.

One of the major goal of the Belle II physics program Kou et al. (2018) is to improve the measurement of the CP violating parameter of Cabibbo-Kobayashi-Maskawa (CKM) matrix, via the time-dependent CP violation (TDCPV) technique in the $\mathup{{{B}}}$ sector.

In this contribution, we will present the expected sensitivity on CKM Unitary Triangle (UT) angles $\phi_{1}/\beta$, both in tree- and penguin-mediated transition, and $\phi_{2}/\alpha$. Additionally, searches for New Physics using TDCPV techniques in the $\mathup{{{K}}^{\scriptstyle{0}}}\mathup{{{\pi}}^{\scriptstyle{0}}}(\gamma)$ decay will be described.

II Belle II detector

The Belle II detector is major upgrade of the previous Belle one for almost all subsystem. Particularly relevant for the TDCPV analysis is the new vertex detector, and the particle identifier (PID) ones.

The vertex detector comprises two layers of silicon pixel (PXD), the closest placed at $r=14~{}mm$ from the beam line, and four layers of silicon strips (SVD). In spite of the reduced boost of the $\mathup{{{B}}}$ particle at SuperKEKB with respect to KEKB ($\beta\gamma=0.28$ vs $0.45$), an improvement of secondary vertex precision, and so of $\Delta{t}$ of about 30% is expected. The tracking system is completed by a central drift chamber (CDC), installed inside a solenoid providing a 1.5 T magnetic field. The CDC is new, larger than the one used at Belle, with smaller cell size and longer lever arm.

Two new PID detectors have been installed to provide separation between charged hadrons (mostly $\mathup{{{\pi}}}$ and $\mathup{{{K}}}$). In the barrel region, the Imaging Time Of Propagation (iTOP) detector is used, and the Aerogel Cherenkov (ARICH) covers the forward region. Both are based on the Cherenkov effect. The iTOP uses precise determination of time and position of arrival of Cherenkov photons to an array of Micro Channel Plate Photo Multiplier Tube. The photons are produced by charged particle crossing 2 cm of finely polished quartz. The ARICH uses an aerogel radiator with focusing capability, and collect Cherenkov photons via Hybrid Avalanche Photo Detector. The CDC provide additional information on PID measuring the specific ionization energy deposit. The overall performances are expected to provide an efficiency for identification of $\mathup{{{\pi}}}$ ($\mathup{{{K}}}$) greater than $90\%$ with a mis-identification rate lower than $10\%$ up to $4~{}\,\text{GeV}/c$ for particle momentum.

The improved PID, together with better algorithm based on multivariate selectors, is expected to increase the flavour tagging effective efficiency to $\sim 37\%$, to be compared to $\sim 30\%$ obtained at Belle.

III Measurement of $\phi_{1}/\beta$

The most precise determination of $\sin 2\beta$ is obtained from TDCPV analysis of tree mediated $\mathup{{{b}}}\to\mathup{{{c}}}$ processes, dominated by the decay $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{J\mspace{-2.0mu}/\mspace{-2.0mu}\psi}}}\mathup{{{K}}^{\scriptstyle{0}}}$. An improved measurement might indicate New Physics in case of incompatibility with other measurement of the UT parameters. The measurement of $\sin 2\beta$ in $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{c}}}\mathup{{\overline{{c}}}}\mathup{{{K}}^{\scriptstyle{0}}}$ decays will be soon dominated by systematics uncertainties, thanks to the large dataset expected at Belle II. Dominant irreducible systematics come from alignment of vertex detector, $\Delta{t}$ resolution, and tag-side interference.

Independent determination of time dependent $CP$ asymmetry $S$ will come from penguin dominated $\mathup{{{b}}}\to\mathup{{{s}}}\mathit{{q}{}_{\mspace{-3.0mu}\scriptstyle{}}^{\mspace{-1.0mu}\scriptstyle{}}}\mathit{{\mkern 4.0mu\overline{\mkern-4.0mu{q}}}{}_{\mspace{-2.0mu}\scriptstyle{}}^{\mspace{0.0mu}\scriptstyle{}}}$ ($\mathit{{q}{}_{\mspace{-3.0mu}\scriptstyle{}}^{\mspace{-1.0mu}\scriptstyle{}}}=\mathup{{{s}}},\mathup{{{d}}},\mathup{{{u}}}$). These decays are suppressed with respect to $\mathup{{{b}}}\to\mathup{{{c}}}$, due to the presence of loop amplitudes, but New Physics can introduce new phases in the loop, and shift the value of $S$ as measured in the tree-dominated diagram. A significant difference would be a clear indication of New Physics, as shown in Fig. 1.

The expected sentitivity for several final states have been studied: the most interesting are $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{\eta}}^{\scriptstyle{\prime}}}\mathup{{{K}}^{\scriptstyle{0}}}$ and $\mathup{{{\phi}}}\mathup{{{K}}^{\scriptstyle{0}}}$. The former has many different final states, from the $\mathup{{{\eta}}^{\scriptstyle{\prime}}}$ decay, all including neutrals. The latter have final states with $\mathup{{{K}}}$ and $\mathup{{{\pi}}}$, and the PID performances will be critical. These two channels are also the cleaner from the thoerotical point of view, so the comparison of $S$ to that of tree-dominated diagram is more sensitive.

Given the suppressed amplitudes, the precision of $S$ in most of the decay channels is expected to be dominated by statistical uncertainties. A notable exception is the decay $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{\eta}}^{\scriptstyle{\prime}}}\mathup{{{K}}^{\scriptstyle{0}}}$, where the systematics uncertainties are expected to be similar to the statistical one around 10-20\mbox{,\text{ab}^{\text{$-$1}}}.

To probe for New Physics, it is also interesting to look for TDCPV when none is expected in the SM. An example is the decay channel $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{K}}{}_{\scriptstyle{S}}^{\scriptstyle{0}}}\mathup{{{\pi}}^{\scriptstyle{0}}}\mathup{{{\gamma}}}$, which includes many neutrals in the final states, making very hard, if possible at all, for LHCb. The decay $\mathup{{{b}}}\to{\mathup{{{s}}}\gamma_{R}}$ is helicity suppressed with respect to the analogous with $\gamma_{L}$, but New Physics can enhance the $b\to{s\gamma_{R}}$ decay rate. The interference between $\mathup{{{B}}^{\scriptstyle{0}}}\to{f_{CP}}\gamma_{R}$ and $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{\overline{{B}}}^{\scriptstyle{0}}}\to{f_{CP}}\gamma_{R}$ happens only for helicity suppressed decay, and a measurement of time dependent asymmetry $S$ in this channel larger than SM prediction (few % Ball et al. (2007)) would be an evidence of New Physics.

A summary of expected precision on time dependent ($S$) and direct ($A$) CP violation parameters is presented in Table 1.

IV Measurement of $\phi_{2}/\alpha$

The second UT angle $\phi_{2}/\alpha$ can be measured using the decay $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{\pi}}}\mathup{{{\pi}}}$ as well as $\mathup{{{\rho}}}\mathup{{{\rho}}}$, using an isospin analysis Gronau and London (1990), due to the fact that there are two amplitudes of comparable size but different weak phase. The isospin techniques requires to determine the magnitude and phases of $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{\pi}}^{\scriptstyle{+}}}\mathup{{{\pi}}^{\scriptstyle{-}}}$ as well as $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{\pi}}^{\scriptstyle{0}}}\mathup{{{\pi}}^{\scriptstyle{0}}}$. Previous B-factories measurements include both for $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{\pi}}}\mathup{{{\pi}}}$, but the phase of $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{\pi}}^{\scriptstyle{0}}}\mathup{{{\pi}}^{\scriptstyle{0}}}$ has not been measured, resulting in a eightfold ambiguity in the determination of $\phi_{2}$. By measuring the phase, namely the time dependent asymmetry of $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{\pi}}^{\scriptstyle{0}}}\mathup{{{\pi}}^{\scriptstyle{0}}}$, the ambiguity can be reduced by a factor of two or four. Given the neutral final state, this decay channel is very difficult to study at LHCb.

At Belle II, the high integrated luminosity allows to study TDCPV for $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{\pi}}^{\scriptstyle{0}}}\mathup{{{\pi}}^{\scriptstyle{0}}}$ using the Dalitz decays $\mathup{{{\pi}}^{\scriptstyle{0}}}\to\mathup{{{\gamma}}}\mathup{{{e}}^{\scriptstyle{+}}}\mathup{{{e}}^{\scriptstyle{-}}}$ and the $\mathup{{{\gamma}}}\to\mathup{{{e}}^{\scriptstyle{+}}}\mathup{{{e}}^{\scriptstyle{-}}}$ conversion on beam pipe and first layers of vertex detector. The $\mathup{{{B}}^{\scriptstyle{0}}}$ decay vertex can be reconstructed from the $\mathup{{{B}}^{\scriptstyle{0}}}$ flight direction and that of $\mathup{{{e}}^{\scriptstyle{+}}}\mathup{{{e}}^{\scriptstyle{-}}}$, with a vertex resolution only 50% worse than that obtained with charged final state, such as $J/\psi K^{0}$. With the full integrated luminosity of 50\,\text{ab}^{\text{-1}}, Belle II expects to reconstruct $\sim 270$ $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{\pi}}^{\scriptstyle{0}}}\mathup{{{\pi}}^{\scriptstyle{0}}}$ signal events with Dalitz decay, and $\sim 50$ with a $\mathup{{{\gamma}}}$ conversion. The foreseen precision on $S$ would be $\sim 0.28$, and the impact on reducing the ambiguity on $\phi_{2}$ is shown in Fig. 2 for four different possible values of $S_{\mathup{{{\pi}}^{\scriptstyle{0}}}\mathup{{{\pi}}^{\scriptstyle{0}}}}$.

The $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{\pi}}}\mathup{{{\pi}}}$ will provide a resolution on $\phi_{2}$ around $2^{\circ}$: combining with the similar isospin analysis of $\mathup{{{\rho}}}\mathup{{{\rho}}}$ final state, the ultimate precision will be $0.6^{\circ}$, using the full statistics.

V Conclusion and outlook

The Belle II experiment is taking data at SuperKEKB in Tsukuba, Japan. In the physics program of the experiment, the study of time dependent CP violation will play a major role. The ultimate precision on $\phi_{1}$ and $\phi_{2}$ is expected to go below $\sim 1^{\circ}$, with interesting prospect in term of New Physics searches.

Looking at B-factory (Belle and BaBar) past history, with an integrated luminosity around 10\,\text{fb}^{\text{-1}}, Belle II could re-discover TDCPV in the $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{J\mspace{-2.0mu}/\mspace{-2.0mu}\psi}}}\mathup{{{K}}{}_{\scriptstyle{S}}^{\scriptstyle{0}}}$ final state, as well the decay into $\mathup{{{\eta}}^{\scriptstyle{\prime}}}\mathup{{{K}}{}_{\scriptstyle{S}}^{\scriptstyle{0}}}$. Larger dataset, around 40\,\text{fb}^{\text{-1}} would be required for TDCPV in the latter channel as well as in the $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{\pi}}^{\scriptstyle{+}}}\mathup{{{\pi}}^{\scriptstyle{-}}}$ one. Even larger \sim 100\mbox{,\text{fb}^{\text{$-$1}}} were needed for TDCPV in ${\mathup{{{\phi}}}\mathup{{{K}}{}_{\scriptstyle{S}}^{\scriptstyle{0}}}}$. For channel $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{K}}{}_{\scriptstyle{S}}^{\scriptstyle{0}}}\mathup{{{\pi}}^{\scriptstyle{0}}}\mathup{{{\gamma}}}$, Belle II can provide interesting improvement with few \,\text{ab}^{\text{-1}}, while the full dataset of 50\,\text{ab}^{\text{-1}} will be needed for the challenging $\mathup{{{B}}^{\scriptstyle{0}}}\to\mathup{{{\pi}}^{\scriptstyle{0}}}\mathup{{{\pi}}^{\scriptstyle{0}}}$.

More information about the Belle II physics program can be found on Kou et al. (2018).