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Belle II Project Proceedings of the 8th International Conference on Quarks and Nuclear Physics (QNP2018) Downloaded from journals.jps.jp by Deutsches Elek Synchrotron on 11/15/19 Proc. 8th Int. Conf. Quarks and Nuclear Physics (QNP2018) JPS Conf. Proc. 26, 011016 (2019) https://doi.org/10.7566/JPSCP.26.011016 Belle II Project Kenkichi Miyabayashi1 1Department of Physics, Nara Women’s University, Kita-Uoya-Nishi-machi, Nara 630-8506, Japan E-mail: [email protected] (Received February 25, 2019) Belle II experiment is a new generation experiment with a luminosity frontier e+e− colliding beam with the SuperKEKB accelerator. In order to identify the effective degree of freedom which is a key to understand hadrons from quarks and gluons, spectroscopy of heavy-flavored hadrons was carried out at the Belle and BaBar experiments, and lead to observations of exotic candidates such as so- called XYZ states. Prospect of hadron spectroscopy at the Belle II experiment as well as the recent status of the accelerator and the detector commissioning is reported. KEYWORDS: SuperKEKB, Belle II experiment, High luminosity e+e− colliding beam experiment, Heavy flavored hadron spectroscopy 1. Introduction In the last decades, running of αS and parton distribution in nucleons were established, and as a result, we have good knowledge at the region where perturbative QCD is applicable. We have still unrevealed aspects at the non-perturbative region. Two B-factories, the BaBar experiment at SLAC and the Belle experiment at KEK, achieved two order of magnitude larger statistics than the older e+e− colliding beam experiments such as the ARGUS, CLEO, PEP, PETRA, TRISTAN and LEP experiments. Exploiting the high statistics data, B-factory experiments have brought not only their primary mission physics such as CP violation measurements and rare B and τ decay studies but also rich variety of hadron physics results, especially for so-called XYZ states and charm baryon excited states. Now new generation of B-factory experiment, the Belle II experiment is starting with featuring the SuperKEKB collider. Expected contribution for QCD and recent status of detector and accelerator construction and commissioning are discussed. 2. Legacy of B-factories Belle experiment took data from 1999 until 2010 and accumulated the integrated luminosity of 1 ab−1 including 711 fb−1 containing 772 million BB, while BaBar experiment recorded the data corresponding to the integrated luminosity of 550 fb−1. In total, more than 1.5 ab−1 were gotten. These high statistics data contain a variety of particle production processes from e+e− collisions: (1) B meson decays, (2) e+e− ! qq¯ continuum production where q = u; d; s; c, (3) initial state radi- ation to produce neutral vector states, (4) double charmonium production, (5) two photon collisions, and (6) higher Υ states’ transitions down to lower states emitting photon or light hadrons. Since each production process has its favored quantum number to produce the hadronic states, a variety of production mechanism brings us possibility to study various states. At the same time, both detector systems of these two experiments were designed, constructed and operated to realize high momen- tum and energy resolution for both charged and neutral particles with excellent particle identification capability as well as superb spacial resolution of the decay vertex. Originally such fancy instrumenta- tions were made for studying the CP violation in B decays which required demanding measurements, ■■■1 011016-1 ©2019 The Author(s) This article is published by the Physical Society of Japan under the terms of the Creative Commons Attribution 4.0 License. Any further distribution of this work must maintain attribution to the author(s) and the title of the article, journal citation, and DOI. Proceedings of the 8th International Conference on Quarks and Nuclear Physics (QNP2018) Downloaded from journals.jps.jp by Deutsches Elek Synchrotron on 11/15/19 JPS Conf. Proc. 26, 011016 (2019) 011016-2 and at the same time those were quite powerful tools to explore hadron spectroscopy. The most striking discovery was X(3872) [1] which is an extraordinary narrow resonance despite its mass above the DD threshold and its decay into J/ψπ+π−. Nowadays, an agreeable interpretation is that it is an admixture of a DD∗ molecule and the charmonium having the same quantum number, JPC = 1++ [2] where J is spin, P and C are eigenvalues for parity and charge conjugation, respectively. The reason is that it does not conflict with any of the compilation of knowledge; (1) B(X(3872) ! D0D∗0)=B(X(3872) ! J/ψπ+π−) is about 10 [3], (2) D+D∗− component can explain J/ψπ+π− and J/ψπ+π−π0 decay modes coexist, (3) a pure molecule is too fragile to have prompt production in high energy in pp¯ or pp collisions at Tevatron/LHC, (4) still unseen χc1(2P) is due to the fact that another χc1(2P) dominant state would become broad because of its higher mass caused by the level splitting from mixing. Branching fractions of the radiative decays into J/ψγ and (2S )γ also support the admixture scenario [4]. Reaching such a plausible interpretation is a remarkable progress in this topic. Charged quarkonium-like states are a firm evidence of the hadrons which can not be explained + + as a usual qq¯ meson. Zc(4430) decaying to the (2S )π final state was discovered by Belle exper- iment in B ! (2S )π+K three-body decay. Thanks to the large b-hadron production cross section, + LHCb experiment confirmed the Zc(4430) production with one-order-of-magnitude higher yield of B meson decays, and revealed its resonant nature by performing an Argand diagram approach. + + Two charged bottomonium-like states of Zb(10610) and Zb(10650) were discovered in Belle data accumulated at the Υ(10860) resonance, corresponding to 121 fb−1 [5]. They appeared in the + + + + following five bottomonium + one charged pion modes; Υ(1S )π , Υ(2S )π , Υ(3S )π , hb(1P)π and + ∗ ∗ ∗ hb(2P)π . Since their masses are quite close to the BB and B B threshold, it is natural to interprete them as such molecular states. In fact, one B meson in an event was fully reconstructed and using + ∗ + ∗ ∗ partial reconstruction technique, Zb(10610) ! BB and Zb(10650) ! B B were found to be dominant decay modes [6] . The molecular picture also explains co-existence of the decays to Υ and hb. As a conventional heavy quarkonium, co-existence of the decays to these two states are unusual because the spin flip of heavy quarks is suppressed [7]. + − The Yc(4260) resonance was found in the J/ψπ π final state and the charged charmonium-like + object Zc(3900) was found to be contributing as an intermediate state [8]. Such phenomena can be interpreted as the charm counterpart with respect to the Zb states in Υ(10860) decays. Studying charm baryon excited states is important to see if the di-quark is functional as an ef- fective degree of freedom to form hadrons, because one constituent is c quark which is heavy, then the remained two light quarks can strongly correlate to form a di-quark. In order to investigate the structure of charm baryons, performing measurements for the decays of excited states to a charm baryon + light hadron(s) and a charm meson + a baryon bring us very important information to know where color string is cut when strong decay happens. This kind of information just has started to be + + − + + got in our hand. Ξc(3055) and Ξc(3080) were observed in both ΛcK π and ΛD modes while + − + Ξc(2980) was only seen in the ΛcK π final state [9]. Determination of quantum numbers for these excited states is also important, however currently available statistics is not adequate to perform it. For all the items described above, further studies, such as quantum number determination, search for unseen but predicted decay modes by a plausible theoretical interpretation and exploring partner states, require higher statistics data being hoped for in the successor project, the Belle II experiment. 3. Belle II experiment at SuperKEKB e+e− collider The SuperKEKB collider is designed to reach the 8 × 1035 cm−2s−1 luminosity with the storing e+ and e− beams at 4.0 GeV and 7.0 GeV, respectively. The design luminosity is 40 times as high as the one achieved by the KEKB collider. The essential means to get such a high luminosity is the β function squeezed at the interaction point by factor 20, and increased current by the factor of 2. In ■■■2 Proceedings of the 8th International Conference on Quarks and Nuclear Physics (QNP2018) Downloaded from journals.jps.jp by Deutsches Elek Synchrotron on 11/15/19 JPS Conf. Proc. 26, 011016 (2019) 011016-3 order to make the small β function possible, larger beam crossing angle of 83 mrad was chosen, to be compared with the one at KEKB of 22 mrad. The Belle II detector is a 4π magnetic spectrometer having high momentum and energy resolution with particle identification capability and excellent decay vertex resolution. The phase-1 commissioning was done from February until June in 2016 to carry out initial shake- down of the accelerator alone as well as baking of the inner surface of the beam pipes. The phase-2 commissioning was carried out from March until July in 2018 featuring the Belle II detector at its roll-in position with the Belle solenoid magnetic field and the final focus system comprised by the superconducting focusing magnets (QCS). The vertex detector system (VXD) was not its final form, as the devices covered a limited solid angle to monitor beam background.
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