Precision Flavour Physics Research Articles

Flavour physics as a window to new physics searches

Flavour physics is one of the essential elements of the Standard Model (SM). Experimental studies of weak decays of B and D mesons at B factories have, together with the discovery of the Higgs boson at LHC, provided the final confirmation of the validity of the SM. The present generation of precision flavour physics experiments is looking for departures from the SM. We discuss studies of anomalies in b hadron decays, and studies of rare decays, which are a very promising method for searching for new physics. This article is part of the theme issue ‘The particle-gravity frontier’.

Phenomenology of flavorful composite vector bosons in light of B anomalies

We analyze the flavor structure of composite vector bosons arising in a model of vectorlike technicolor — often called hypercolor (HC) — with eight flavors that form a one-family content of HC fermions. Dynamics of the composite vector bosons, referred to as HC ρ in this paper, are formulated together with HC pions by the hidden local symmetry (HLS), in a way analogous to QCD vector mesons. Then coupling properties to the standard model (SM) fermions, which respect the HLS gauge symmetry, are described in a way that couplings of the HC ρs to the left-handed SM quarks and leptons are given by a well-defined setup as taking the flavor mixing structures into account. Under the present scenario, we discuss significant bounds on the model from electroweak precision tests, flavor physics, and collider physics. We also try to address B anomalies in processes such as B → K(∗)μ+μ− and Bto {D}^{left(ast right)}tau;overline{nu} , recently reported by LHCb, Belle, (ATLAS, and CMS in part). Then we find that the present model can account for the anomaly in B → K(∗)μ+μ− consistently with the other constraints while it predicts no significant deviations in Bto {D}^{left(ast right)}tau;overline{nu} ν from the SM, which can be examined in the future Belle II experiment. The former is archived with the form C9 = −C10 of the Wilson coefficients for effective operators of b → sμ+μ−, which has been favored by the recent experimental data. We also investigate current and future experimental limits at the Large Hadron Collider (LHC) and see that possible collider signals come from dijet and ditau, or dimuon resonant searches for the present scenario with TeV mass range. To conclude, the present b → sμ+μ− anomaly is likely to imply discovery of new vector bosons in the ditau or dimuon channel in the context of the HC ρ model. Our model can be considered as a UV completion of conventional U(1)′ models.

Performance of the LHCb RICH detectors during the LHC Run II

The LHCb RICH system provides hadron identification over a wide momentum range (2–100GeV/c). This detector system is key to LHCb's precision flavour physics programme, which has unique sensitivity to physics beyond the standard model. This paper reports on the performance of the LHCb RICH in Run II, following significant changes in the detector and operating conditions. The changes include the refurbishment of significant number of photon detectors, assembled using new vacuum technologies, and the removal of the aerogel radiator. The start of Run II of the LHC saw the beam energy increase to 6.5TeV per beam and a new trigger strategy for LHCb with full online detector calibration. The RICH information has also been made available for all trigger streams in the High Level Trigger for the first time.

Finite-volume QED corrections to decay amplitudes in lattice QCD

We demonstrate that the leading and next-to-leading finite-volume effects in the evaluation of leptonic decay widths of pseudoscalar mesons at $O(\ensuremath{\alpha})$ are universal; i.e. they are independent of the structure of the meson. This is analogous to a similar result for the spectrum but with some fundamental differences, most notably the presence of infrared divergences in decay amplitudes. The leading nonuniversal, structure-dependent terms are of $O(1/{L}^{2})$ [compared to the $O(1/{L}^{3})$ leading nonuniversal corrections in the spectrum]. We calculate the universal finite-volume effects, which requires an extension of previously developed techniques to include a dependence on an external three-momentum (in our case, the momentum of the final-state lepton). The result can be included in the strategy proposed in Ref. [N. Carrasco et al.,Phys. Rev. D 91, 074506 (2015).] for using lattice simulations to compute the decay widths at $O(\ensuremath{\alpha})$, with the remaining finite-volume effects starting at order $O(1/{L}^{2})$. The methods developed in this paper can be generalized to other decay processes, most notably to semileptonic decays, and hence open the possibility of a new era in precision flavor physics.

The Belle II imaging Time-of-Propagation (iTOP) detector

High precision flavor physics measurements are an essential complement to the direct searches for new physics at the LHC ATLAS and CMS experiments. Such measurements will be performed using the upgraded Belle II detector that will take data at the SuperKEKB accelerator. With 40x the luminosity of KEKB, the detector systems must operate efficiently at much higher rates than the original Belle detector. A central element of the upgrade is the barrel particle identification system. Belle II has built and installed an imaging-Time-of-Propagation (iTOP) detector. The iTOP uses quartz optics as Cherenkov radiators. The photons are transported down the quartz bars via total internal reflection with a spherical mirror at the forward end to reflect photons to the backward end where they are imaged onto an array of segmented Micro-Channel Plate Photo-Multiplier Tubes (MCP-PMTs). The system is read out using giga-samples per second waveform sampling Application-Specific Integrated Circuits (ASICs). The combined timing and spatial distribution of the photons for each event are used to determine particle species. This paper provides an overview of the iTOP system.

Constraining top-Higgs couplings at high and low energy

The study of the couplings of the Higgs boson and of the top quark plays a preeminent role at the LHC, and could unveil the first signs of new physics. I will discuss the interplay of direct and indirect probes of certain classes of top and Higgs couplings. Including constraints from collider observables, precision electroweak tests, flavor physics, and electric dipole moments (EDMs), I will show that indirect probes are competitive, if not dominant, for both the CP-even and CP-odd top and Higgs couplings we considered. I will discuss the role of theoretical uncertainties, associated with hadronic and nuclear matrix elements, and indicate targets to further improve the constraining power of EDM experiments.

Dark matter interpretations of ATLAS searches for the electroweak production of supersymmetric particles in s = 8 $$ \sqrt{s}=8 $$ TeV proton-proton collisions

A selection of searches by the ATLAS experiment at the LHC for the electroweak production of SUSY particles are used to study their impact on the constraints on dark matter candidates. The searches use $20\,{\rm fb}^{-1}$ of proton-proton collision data at $\sqrt{s}=8$ TeV. A likelihood-driven scan of a five-dimensional effective model focusing on the gaugino--higgsino and Higgs sector of the phenomenological minimal supersymmetric Standard Model is performed. This scan uses data from direct dark matter detection experiments, the relic dark matter density and precision flavour physics results. Further constraints from the ATLAS Higgs mass measurement and SUSY searches at LEP are also applied. A subset of models selected from this scan are used to assess the impact of the selected ATLAS searches in this five-dimensional parameter space. These ATLAS searches substantially impact those models for which the mass $m(\tilde{\chi}^0_1)$ of the lightest neutralino is less than 65 GeV, excluding 86% of such models. The searches have limited impact on models with larger $m(\tilde{\chi}^0_1)$ due to either heavy electroweakinos or compressed mass spectra where the mass splittings between the produced particles and the lightest supersymmetric particle is small.

Belle II Status and Plans

To deepen our understanding of nature it is essential to complement the direct searches for new physics at the LHC by high precision flavor physics measurements. The next-generation B factory experiment Belle II at the upgraded KEKB accelerator, SuperKEKB, is designed to perform such measurements. The status of the collider and the Belle II detector is presented in this article.

Physics Opportunities at the Next Generation of Precision Flavor Physics Experiments

With the next-generation experiments, flavor physics is fully entering the era of precision measurements. Its focus is shifting from testing the Standard Model to finding and characterizing New Physics contributions. We review the opportunities offered by future flavor experiments and discuss the expected sensitivities of the most important measurements. We also present some examples of measurable deviations from the Standard Model in the flavor sector generated in a selection of New Physics models, which demonstrate the potentially major contribution of precision flavor physics to the effort of going beyond the Standard Model.

Future of B physics at CDF and DØ

In this contribution to the panel discussion on “The Future of Hadron B Experiments” held at the 8 th International Conference on B Physics at Hadron Machines (Beauty 2002) at Santiago de Compostela, Spain, June 17–21, 2002, we explore the physics potential for B physics at CDF and DØ in five years and beyond. After a brief introduction to precision flavour physics, we concentrate our discussion on the future of CP violation by evaluating the prospects for measuring the CKM angles β, γ and α at the Tevatron Collider experiments CDF and DØ by the end of Run II.

Impact of B physics on model building and vice versa: An example

We motivate that the start-up of the B factories has opened a new precision flavour physics era, with an important effect on model building. Using as an example a left-right model with spontaneous CP violation, we will show how the inclusion of the new experimental data on B physics observables, together with the old observables coming from kaon physics, has significantly widened our capacity to strongly constrain the parameter space up to the point to exclude models. On the contrary, using certain hypotheses, mainly concerning isospin, we discuss how theory may help us to ‘test’ the data on charged, neutral and mixed B → πK decays once experimental errors will be reduced.

Precision flavour physics and supersymmetry

We review the salient features of a comparative study of the profile of the CKM unitarity triangle, and the resulting CP-violating phases α, β and γ in B decays, in the standard model and in several variants of the minimal supersymmetric standard model (MSSM), reported recently by us. These theories are characterized by a single phase in the quark flavour mixing matrix and give rise to well-defined contributions in the flavour-changing-neutral-current transitions in K and B decays. We analyse the supersymmetric contributions to the mass differences in the B d 0– B d 0 and B s 0– B s 0 systems, ΔM d and ΔM s , respectively, and to the CP-violating quantity | ε| in K decays. Our analysis shows that the predicted ranges of β in the standard model and in MSSM models are very similar. However, precise measurements at B-factories and hadron machines may be able to distinguish these theories in terms of the other two CP-violating phases α and γ.