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Physics Prospects at the Belle II Experiment Available online at www.sciencedirect.com Nuclear and Particle Physics Proceedings 263–264 (2015) 15–23 www.elsevier.com/locate/nppp Physics prospects at the Belle II experiment Phillip Urquijoa aSchool of Physics, The University of Melbourne, Parkville 3010, Australia. Abstract A review of the flavour physics program for the Belle II experiment is presented, including projections for precision on key observables. The Belle II experiment is located at the second generation asymmetric e+e− collider SuperKEKB. It will be used to search for new phenomena at the flavour frontier. Keywords: Flavour, B-physics, Charm-physics, electron-positron collider, CKM. 1. Introduction interactions, excluding gravity, it does not provide an- swers to many fundamental questions. The SM does The physics goals of Belle II, as a next generation not explain why there should be only three generations flavour factory, are to search for new physics (NP) in of elementary fermions and why there is an observed the flavour sector at the precision frontier, and to fur- hierarchy in the fermion masses. The origin of mass ther reveal the nature of QCD in describing matter. The of fundamental particles is explained within the SM by SuperKEKB facility is designed to collide electrons and spontaneous electroweak symmetry breaking, resulting positrons at centre-of-mass energies in the regions of the in the Higgs boson. However, the Higgs boson does Υ resonances. Most of the data will be collected at the not account for neutrino masses. It is also not yet Υ (4S ) resonance, which is just above threshold for B- clear whether there is a only single SM Higgs boson meson pair production where no fragmentation particles or whether there may be a more elaborate Higgs sec- are produced. The accelerator is designed with asym- tor with other Higgs-like particle as in supersymmetry metric beam energies to provide a boost to the centre- or other NP models. At the cosmological scale, there of-mass system and thereby allow for time-dependent is the unresolved problem with the matter-antimatter charge-parity (CP) symmetry violation measurements. asymmetry in the universe. While the violation of CP The boost is slightly less than that at KEKB, which symmetry (CPV) is a necessary condition for the evolu- is advantageous for analyses with neutrinos in the fi- tion of a matter-dominated universe, the observed CPV nal state that require good detector hermeticity. Su- within the quark sector that originates from the complex × 35 −2 −1 perKEKB has a design luminosity of 8 10 cm s , phase of the Cabibbo-Kobayashi-Maskawa (CKM) ma- about 40 times larger that of KEKB. This luminosity trix is many orders of magnitude too small to explain × 10 τ will produce a total of 5 10 b, c and pairs over a the dominance of matter in the universe. Hence, there period of 8 years. The first data taking run for physics must exist undiscovered sources of the CP asymmetry. analyses is anticipated to begin in 2017. Furthermore, the elements of the CKM matrix exhibit The Standard Model (SM) is, at the current level of a roughly diagonal hierarchy, even though the SM does experimental precision and at the energies reached so not require this. This may indicate the presence of a new far, is the best tested theory. Despite its tremendous suc- mechanism, such as a flavour symmetry, that exists un- cess in describing the fundamental particles and their broken at a higher energy scale. Considering the open questions that in the SM remain unanswered, it is fair to conclude that the present theory is an extremely suc- Email address: [email protected] (Phillip Urquijo) http://dx.doi.org/10.1016/j.nuclphysbps.2015.04.004 2405-6014/© 2015 Elsevier B.V. All rights reserved. 16 P. Urquijo / Nuclear and Particle Physics Proceedings 263–264 (2015) 15–23 cessful but phenomenological description of subatomic • Are there quark FCNCs beyond the SM? It is of processes at the energy scales up to O(1 TeV). Many great interest to measure b → sνν¯ transitions such New Physics (NP) scenarios have been proposed to ex- as B → K(∗)νν¯, part of a class of decays with large plain these shortcoming of the SM, where new particles missing energy. It is also important to improve FC- and new processes arise. NCs measurements of b → d, b → s and c → u Experiments in high energy physics are designed to transitions. address the above questions through searches of NP us- • ing complementary approaches. At the energy frontier, Are there sources of LFV beyond the SM? Neutrino the LHC experiments are able to discover new particles experiments have found large mixing between the ν ν produced in proton-proton collisions at a centre-of-mass μ and τ, raising the question: are there flavour changing processes such as τ → μγ visible at the energy of up to 14 TeV. Sensitivity to the direct produc- −8 tion of a specific new particle depends on the cross sec- 10 level? LFV in charged lepton decay is also tion and on the size of the data sample. At the intensity a key prediction in many neutrino mass generation frontier, signatures of new particles or processes can be mechanisms. observed through measurements of suppressed flavour • Are there new operators with quarks enhanced by physics reactions or from deviations from SM predic- NP? It is crucial to measure forward-backward tions. An observed discrepancy can be interpreted in asymmetries as a function of the q2 of the dilep- terms of NP models. This is the approach of Belle II. 2 + − ton, AFB(q ), in inclusive b → s decays and The sensitivity of Belle II to NP depends on the in charged weak interactions. Another example is strength of the flavour violating couplings of the NP. measuring the rates and asymmetries in all B → / ff The mass reach for new particle process e ects can Kπ modes to a precision that we can determine O / 2 be as high as (100 TeV c ) if the couplings are en- whether or not there are enhanced electroweak hanced compared to the SM [1]. In the past, measure- penguins. ments of processes quantum corrections have given ac- cess to high mass scale physics before accelerators were • Does nature have multiple Higgs bosons? Many available to directly probe these scales. Belle II and extensions to the SM, such as two-Higgs-doublet SuperKEKB will exploit our strengths at the intensity models, predict charged Higgs bosons in addition frontier by moving beyond a simple observation of a to a neutral SM-like Higgs. The charged Higgs will NP effect to its detailed characterisation through over- be searched for in flavour transitions to τ leptons, ∗ constraining measurements in several related flavour including B → τν and B → D( )τν. physics reactions. • Does NP enhance CPV via D0 − D¯ 0 mixing to an 1.1. Flavour physics questions to be addressed by Belle observable level? The SM predicts negligible CPV II in this case. Hence CPV in the D system is a sign Further study of the quark sector, is necessary to re- of NP. veal NP at high mass scales beyond the direct reach It is worth noting that not only will Belle II measure of the LHC that may manifest in flavour observables. the current array of CKM observables with unprece- There are several important questions that can only dented precision, it will also allow measurements of a be addressed by further studies of flavour physics, de- large number of new observables and new modes rele- scribed in turn below. vant to NP in the quark sector. • Are there new CP violating phases? Answers will require new measurements of time-dependent 1.2. Advantages of SuperKEKB and Belle II CPVinb → s decays such as B → φK0, ηK0.If There are many experimental reasons to choose Su- there is NP in b → d transitions, precise measure- perKEKB and Belle II to address these puzzles in ments of CKM parameters in mixing and in tree flavour physics. processes will be required. • Running on the Υ(4S ) resonance produces a very • Are there right-handed currents from NP? Ap- clean sample of B0B¯ 0 pairs in a quantum correlated proaches include measurements of time-dependent 1−− state. The low background environment allows → ∗0 → 0 π0 γ CPVinB K ( KS ) , triple-product CPV for reconstruction of final states containing pho- → π0 ρ± η η 0 asymmetries in B VV decays, and semileptonic tons from decays of , , , etc.. Neutral KL decays B → Vν, V = D∗,ρ. mesons are also efficiently reconstructed. P. Urquijo / Nuclear and Particle Physics Proceedings 263–264 (2015) 15–23 17 0(±) (∗) • Detection of the decay products of one B allows form measurements in B and Bs meson decays, the flavour of the other B to be tagged. charm physics, τ lepton physics, spectroscopy, and elec- troweak measurements. A large number of planned • Due to low track multiplicities and detector occu- measurements will over-constrain the SM as well as its pancy, the B, D and τ reconstruction efficiency is extensions and will shed light on the nature of NP. high and the trigger bias is low. This reduces cor- rection and systematic uncertainties in many types of measurements, e.g. Dalitz plot analyses. 2.1. CKM matrix metrology • With asymmetric beam energies the Lorentz boost Belle II can improve on all the measurements of the of the e+e− system is large enough so that B or D Unitarity Triangle (UT) angles, α, β, γ to a precision ◦ ◦ ◦ mesons travel an appreciable distance before de- of about 1 ,0.3 and 1.5 , respectively.
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