arXiv subjects

A. J. Buras

Publications and source records attributed to A. J. Buras.

At least 19 recordsLinked to original sources

arXiv2019

The Belle II Physics Book

We present the physics program of the Belle II experiment, located on the intensity frontier SuperKEKB $e^+e^-$ collider. Belle II collected its first collisions in 2018, and is expected to operate for the next decade. It is anticipated to collect 50/ab of collision data over its lifetime. This book is the outcome of a joint effort of Belle II collaborators and theorists through the Belle II theory interface platform (B2TiP), an effort that commenced in 2014. The aim of B2TiP was to elucidate the potential impacts of the Belle II program, which includes a wide scope of physics topics: B physics, charm, tau, quarkonium, electroweak precision measurements and dark sector searches. It is composed of nine working groups (WGs), which are coordinated by teams of theorist and experimentalists conveners: Semileptonic and leptonic B decays, Radiative and Electroweak penguins, phi_1 and phi_2 (time-dependent CP violation) measurements, phi_3 measurements, Charmless hadronic B decay, Charm, Quarkonium(like), tau and low-multiplicity processes, new physics and global fit analyses. This book highlights "golden- and silver-channels", i.e. those that would have the highest potential impact in the field. Theorists scrutinised the role of those measurements and estimated the respective theoretical uncertainties, achievable now as well as prospects for the future. Experimentalists investigated the expected improvements with the large dataset expected from Belle II, taking into account improved performance from the upgraded detector.

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arXiv2019

Opportunities in Flavour Physics at the HL-LHC and HE-LHC

Motivated by the success of the flavour physics programme carried out over the last decade at the Large Hadron Collider (LHC), we characterize in detail the physics potential of its High-Luminosity and High-Energy upgrades in this domain of physics. We document the extraordinary breadth of the HL/HE-LHC programme enabled by a putative Upgrade II of the dedicated flavour physics experiment LHCb and the evolution of the established flavour physics role of the ATLAS and CMS general purpose experiments. We connect the dedicated flavour physics programme to studies of the top quark, Higgs boson, and direct high-$p_T$ searches for new particles and force carriers. We discuss the complementarity of their discovery potential for physics beyond the Standard Model, affirming the necessity to fully exploit the LHC's flavour physics potential throughout its upgrade eras.

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arXiv2013

Implications of LHCb measurements and future prospects

During 2011 the LHCb experiment at CERN collected 1.0 fb-1 of sqrt{s} = 7 TeV pp collisions. Due to the large heavy quark production cross-sections, these data provide unprecedented samples of heavy flavoured hadrons. The first results from LHCb have made a significant impact on the flavour physics landscape and have definitively proved the concept of a dedicated experiment in the forward region at a hadron collider. This document discusses the implications of these first measurements on classes of extensions to the Standard Model, bearing in mind the interplay with the results of searches for on-shell production of new particles at ATLAS and CMS. The physics potential of an upgrade to the LHCb detector, which would allow an order of magnitude more data to be collected, is emphasised.

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arXiv2012

Proceedings of the 2nd Workshop on Flavor Symmetries and Consequences in Accelerators and Cosmology (FLASY12)

These are the proceedings of the 2nd Workshop on Flavor Symmetries and Consequences in Accelerators and Cosmology, held 30 June 2012 - 4 July 2012, Dortmund, Germany.

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arXiv2012

The Rare Decay K^+ -> pi^+ nu nubar at the Next-to-Next-to-Leading Order in QCD

We calculate the charm quark contribution to the rare decay K+ -> pi+ nu anti-nu in the next-to-next-to-leading order of QCD. This new contribution reduces the theoretical uncertainty in the relevant parameter Pc from +/- 10.1% down to +/- 2.4%, corresponding to scale uncertainties of +/- 1.3%, +/- 1.0%, +/- 0.006 and +/- 1.2 degrees in BR(K+ -> pi+ nu anti-nu) and in |V_td|, sin(2 beta) and gamma extracted from the K -> pi nu anti-nu system. The error in Pc = 0.37 +/- 0.04 is now fully dominated by the current uncertainty of +/- 3.8% in the charm quark mass mc. We find BR(K+ -> pi+ nu anti-nu) = (8.0 +/- 1.1) * 10^-11, where the quoted error stems almost entirely from the present uncertainties in mc and the Cabibbo-Kobayashi-Maskawa elements.

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arXiv2010

Flavor Physics in the Quark Sector

One of the major challenges of particle physics has been to gain an in-depth understanding of the role of quark flavor and measurements and theoretical interpretations of their results have advanced tremendously: apart from masses and quantum numbers of flavor particles, there now exist detailed measurements of the characteristics of their interactions allowing stringent tests of Standard Model predictions. Among the most interesting phenomena of flavor physics is the violation of the CP symmetry that has been subtle and difficult to explore. Till early 1990s observations of CP violation were confined to neutral $K$ mesons, but since then a large number of CP-violating processes have been studied in detail in neutral $B$ mesons. In parallel, measurements of the couplings of the heavy quarks and the dynamics for their decays in large samples of $K, D$, and $B$ mesons have been greatly improved in accuracy and the results are being used as probes in the search for deviations from the Standard Model. In the near future, there will be a transition from the current to a new generation of experiments, thus a review of the status of quark flavor physics is timely. This report summarizes the results of the current generation of experiments that is about to be completed and it confronts these results with the theoretical understanding of the field.

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arXiv2010

Anatomy and Phenomenology of FCNC and CPV Effects in SUSY Theories

We perform an extensive study of FCNC and CP Violation within Supersymmetric (SUSY) theories with particular emphasis put on processes governed by b->s transitions and of their correlations with processes governed by b->d transitions, s->d transitions, $D^0-\bar D^0$ oscillations, lepton flavour violating decays, electric dipole moments and (g-2)_mu. We first perform a comprehensive model-independent analysis of Delta F=2 observables and we emphasize the usefulness of the R_b-gamma plane in exhibiting transparently various tensions in the present UT analyses. Secondly, we consider a number of SUSY models: the general MSSM, a flavour blind MSSM, the MSSM with Minimal Flavour Violation as well as SUSY flavour models based on abelian and non-abelian flavour symmetries that show representative flavour structures in the soft SUSY breaking terms. We show how the characteristic patterns of correlations among the considered flavour observables allow to distinguish between these different SUSY scenarios. Of particular importance are the correlations between the CP asymmetry S_psi phi and B_s->mu^+μ^-, between the anomalies in S_phi K_S and S_psi phi, between S_phi K_S and d_e, between S_psi phi and (g-2)_mu and also those involving lepton flavour violating decays. In our analysis, the presence of right-handed currents and of the double Higgs penguin contributions to B_s mixing plays a very important role. We propose a "DNA-Flavour Test" of NP models including Supersymmetry, the Littlest Higgs model with T-parity and the Randall-Sundrum model with custodial protection, with the aim of showing a tool to distinguish between these NP scenarios, once additional data on flavour changing processes become available.

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arXiv2009

Low Energy Probes of CP Violation in a Flavor Blind MSSM

We analyze the low energy implications of a flavor blind supersymmetric scenario (where the CKM matrix is the only source of flavor violation) in the presence of new CP violating but flavor conserving phases in the soft sector. We find that the best probes of this rather restricted scenario are i) the electric dipole moments (EDMs) of the electron (d_e) and the neutron (d_n) and ii) flavor changing and CP violating processes in B systems, like the CP asymmetries in b->s gamma and B->phi(eta^')K_S, i.e. A_CP(b->s gamma) and S_phi(eta^')K_S, respectively. The non-standard values for S_phi(eta^')K_S, measured at the B factories, can find a natural explanation within our scenario and this would unambiguously imply i) positive and often large (non-standard) values for A_CP(b->s gamma) and ii) a lower bound for the electron and neutron EDMs at the level of d_e,n > 10^-28 e cm. Moreover, we predict positive New Physics (NP) contributions to epsilon_K which could be welcomed in view of the recently lowered Standard Model value for epsilon_K. Interestingly, an explanation for the non-standard values for S_phi(eta^')K_S can also naturally lead to an explanation for the anomaly of the muon anomalous magnetic moment. Finally, we outline the role and the interplay of the direct NP searches at the LHC with the indirect searches performed by low energy flavor physics observables.

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arXiv2008

Flavour physics of leptons and dipole moments

This chapter of the report of the ``Flavour in the era of the LHC'' Workshop discusses the theoretical, phenomenological and experimental issues related to flavour phenomena in the charged lepton sector and in flavour-conserving CP-violating processes. We review the current experimental limits and the main theoretical models for the flavour structure of fundamental particles. We analyze the phenomenological consequences of the available data, setting constraints on explicit models beyond the Standard Model, presenting benchmarks for the discovery potential of forthcoming measurements both at the LHC and at low energy, and exploring options for possible future experiments.

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arXiv2008

B, D and K decays

With the advent of the LHC, we will be able to probe New Physics (NP) up to energy scales almost one order of magnitude larger than it has been possible with present accelerator facilities. While direct detection of new particles will be the main avenue to establish the presence of NP at the LHC, indirect searches will provide precious complementary information, since most probably it will not be possible to measure the full spectrum of new particles and their couplings through direct production. In particular, precision measurements and computations in the realm of flavour physics are expected to play a key role in constraining the unknown parameters of the Lagrangian of any NP model emerging from direct searches at the LHC. The aim of Working Group 2 was twofold: on one hand, to provide a coherent, up-to-date picture of the status of flavour physics before the start of the LHC; on the other hand, to initiate activities on the path towards integrating information on NP from high-pT and flavour data.

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arXiv2006

Particle-Antiparticle Mixing, epsilon_K, Delta Gamma_q, A_SL^q, A_CP(B_d -> psi K_S), A_CP(B_s -> psi phi) and B -> X_{s,d} gamma in the Littlest Higgs Model with T-Parity

We calculate a number of observables related to particle-antiparticle mixing in the Littlest Higgs model with T-parity (LHT). The resulting effective Hamiltonian for Delta F=2 transitions agrees with the one of Hubisz et al., but our phenomenological analysis goes far beyond the one of these authors. In particular, we point out that the presence of mirror fermions with new flavour and CP-violating interactions allows to remove the possible Standard Model (SM) discrepancy between the CP asymmetry S_{psi K_S} and large values of |V_ub| and to obtain for the mass difference Delta M_s < (Delta M_s)_SM as suggested by the recent result by the CDF collaboration. We also identify a scenario in which simultaneously significant enhancements of the CP asymmetries S_{phi psi} and A_SL^q relative to the SM are possible, while satisfying all existing constraints, in particular from the B -> X_s gamma decay and A_CP(B -> X_s gamma) that are presented in the LHT model here for the first time. In another scenario the second, non-SM, value for the angle gamma=-(109+-6) from tree level decays, although unlikely, can be made consistent with all existing data with the help of mirror fermions. We present a number of correlations between the observables in question and study the implications of our results for the mass spectrum and the weak mixing matrix of mirror fermions. In the most interesting scenarios, the latter one turns out to have a hierarchical structure that differs significantly from the CKM one.

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arXiv2005

The Discovery Potential of a Super B Factory

The Proceedings of the 2003 SLAC Workshops on flavor physics with a high luminosity asymmetric e+e- collider. The sensitivity of flavor physics to physics beyond the Standard Model is addressed in detail, in the context of the improvement of experimental measurements and theoretical calculations.

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arXiv2003

The CKM Matrix and the Unitarity Triangle

This report contains the results of the Workshop on the CKM Unitarity Triangle, held at CERN on 13-16 February 2002 to study the determination of the CKM matrix from the available data of K, D, and B physics. This is a coherent document with chapters covering the determination of CKM elements from tree level decays and K and B meson mixing and the global fits of the unitarity triangle parameters. The impact of future measurements is also discussed.

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arXiv2003

$ΔM_{d,s}$, $B^0_{d,s}\toμ^+μ^-$ and $B\to X_sγ$ in Supersymmetry at Large $\tanβ$

We present an effective Lagrangian formalism for the calculation of flavour changing neutral and charged scalar currents in weak decays including $SU(2)\times U(1)$ symmetry breaking effects and the effects of the electroweak couplings $g_1$ and $g_2$. We apply this formalism to the MSSM with large $\tanβ$ with the CKM matrix as the only source of flavour violation, heavy supersymmetric particles and light Higgs bosons. We give analytic formulae for the neutral and charged Higgs boson couplings to quarks including large $\tanβ$ resummed corrections in the $SU(2) \times U(1)$ limit and demonstrate that these formulae can only be used for a semi-quantitative analysis. In particular they overestimate the effects of large $\tanβ$ resummed corrections. We give also improved analytic formulae that reproduce the numerical results of the full approach within $5-10%$. We present for the first time the predictions for the branching ratios $B^0_{s,d}\to μ^+μ^-$ and the $B^0_{d,s}-\bar B^0_{d,s}$ mass differences $ΔM_{d,s}$ that include simultaneously the resummed large $\tanβ$ corrections, $SU(2)\times U(1)$ breaking effects and the effects of the electroweak couplings. We perform an anatomy of the correlation between the increase of the rates of the decays $B^0_{s,d}\toμ^+μ^-$ and the suppression of $ΔM_s$, that for large $\tanβ$ are caused by the enhanced flavour changing neutral Higgs couplings to down quarks. We take into account the constraint from $B\to X_s γ$ clarifying some points in the calculation of the large $\tanβ$ enhanced corrections to this decay.

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arXiv2002

Correlation between Delta M_s and B^0_{s,d} --> mu^+ mu^- in Supersymmetry at Large tan beta

Considering the MSSM with the CKM matrix as the only source of flavour violation and heavy supersymmetric particles at large $\tanβ$, we analyze the correlation between {\it the increase} of the rates of the decays $B^0_{s,d}\to μ^+μ^-$ and {\it the suppression} of $ΔM_s$, that are caused by the enhanced flavour changing neutral Higgs couplings to down-type quarks. We give analytic formulae for the neutral and charged Higgs couplings to quarks including large $\tanβ$ resummed corrections in the $SU(2)\times U(1)$ limit and comment briefly on the accuracy of this approximation. For $0.8\le (ΔM_s)^{\rm exp}/(ΔM_s)^{\rm SM}\le 0.95$ we find $6\cdot 10^{-7}\ge BR(B^0_s\ra μ^+μ^-)^{\rm max} \ge 4\cdot 10^{-8}$ and $1.4\cdot 10^{-8}\ge BR(B^0_d\ra μ^+μ^-)^{\rm max}\ge 1\cdot 10^{-9}$. For $(ΔM_s)^{\rm exp} \ge (ΔM_s)^{\rm SM}$ substantial enhancements of $B^0_{s,d}\raμ^+μ^-$ relative to the expectations based on the Standard Model are excluded. With $(ΔM_s)^{\rm exp}>15.0/$ps a conservative analysis of $(ΔM_s)^{\rm SM}$ gives $BR(B^0_s\ra μ^+μ^-)\simlt1.2\cdot10^{-6}$ and $BR(B^0_d\ra μ^+μ^-)\simlt3\cdot10^{-8}$. However, we point out that in the less likely scenario in which the squark mixing is so large that the neutral Higgs contributions dominate $ΔM_s$, the rates for $B^0_{s,d}\to μ^+μ^-$ increase with increasing $ΔM_s$ and the bounds in question are weaker. Violation of all these correlations and bounds would indicate new sources of flavour violation.

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arXiv2002

QCD Corrections to B -> X_{d,s} nu nubar, B_{d,s} -> l^+l^-, K -> pi nu nubar and K_L -> mu^+ mu^- in the MSSM

We compute for the first time QCD corrections to the rare decays B -> X_{d,s} nu nubar, B_{d,s} -> l^+ l^-, K -> pi nu nubar and K_L -> mu^+ mu^-, where l=e or mu, in the context of a supersymmetric extension of the Standard Model (SM) with minimal flavour violation and new operators, in addition to those present in the SM. Assuming that the gluino is heavy, we consider an effective theory which consists of charged and neutral Higgs particles, charginos and squarks. We evaluate the QCD corrections to box and Z^0-penguin diagrams with top-quark, charged Higgs boson, chargino and squark exchanges, as well as to neutral Higgs boson penguin diagrams. We provide a compendium of analytic formulae for the Wilson coefficients, which are valid for arbitrary values of tan(beta) (the ratio of the vacuum expectation values of the two Higgs fields) except for the case of the neutral Higgs-boson contributions. These contributions have been obtained at large tan(beta), which may compensate for the inevitable suppression by the masses of the light leptons in decays based on the b -> s (d) l^+ l^- transition. We investigate the dependence of the various branching ratios on the renormalization scale mu, which is the main theoretical uncertainty in the short-distance calculation. We find that the mu dependence of the branching ratios is considerably reduced once the QCD corrections are taken into account. The contributions of new operators are found to be dominant at large tan(beta) in B_{d,s} -> mu^+ mu^- while they are subleading in B -> X_{d,s} nu nubar and completely negligible in kaon decays.

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arXiv2000

epsilon'/epsilon and Rare K and B Decays in the MSSM

We analyze the CP violating ratio epsilon'/epsilon and rare K and B decays in the MSSM with minimal flavour and CP violation, including NLO QCD corrections and imposing constraints on the supersymmetric parameters coming from epsilon, B_{d,s}^0-\bar B_{d,s}^0 mixings, B to X_s gamma, Delta rho in the electroweak precision studies and from the lower bound on the neutral Higgs mass. We provide a compendium of phenomenologically relevant formulae in the MSSM. Denoting by T(Q) the MSSM prediction for a given quantity normalized to the Standard Model result we find the ranges: 0.53 < T(epsilon'/epsilon) < 1.07, 0.65 < T(K^+ to pi^+ nu nubar) < 1.02, 0.41 < T(K_L to pi^0 nu nubar) < 1.03, 0.48 < T(K_L to pi^0 e^+ e^-) < 1.10, 0.73 < T(B to X_s nu nubar) < 1.34 and 0.68 < T(B_s to mu^+ mu^-) < 1.53. We point out that these ranges will be considerably reduced when the lower bounds on the neutral Higgs mass and tan beta improve. Some contour plots illustrate the dependences of the quantities above on the relevant supersymmetric parameters. As a byproduct of this work we update our previous analysis of epsilon'/epsilon in the SM and find in NDR epsilon'/epsilon = (9.2^{+6.8}_{-4.0}), a value 15 % higher than in our 1999 analysis.

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arXiv2000

Universal Unitarity Triangle and Physics Beyond the Standard Model

We make the simple observation that there exists a universal unitarity triangle for all models, like the SM, the Two Higgs Doublet Models I and II and the MSSM with minimal flavour violation, that do not have any new operators beyond those present in the SM and in which all flavour changing transitions are governed by the CKM matrix with no new phases beyond the CKM phase. This universal triangle can be determined in the near future from the ratio (Delta M)_d/(Delta M)_s and sin(2 beta) measured first through the CP asymmetry in B_d^0 to psi K_S and later in K to pi nu nubar decays. Also suitable ratios of the branching ratios for B to X_{d,s} nu nubar and B_{d,s} to mu^+ mu^- and the angle gamma measured by means of CP asymmetries in B decays can be used for this determination. Comparison of this universal triangle with the non-universal triangles extracted in each model using epsilon, (Delta M)_d and various branching ratios for rare decays will allow to find out in a transparent manner which of these models, if any, is singled out by experiment. A virtue of the universal triangle is that it allows to separate the determination of the CKM parameters from the determination of new parameters present in the extensions of the SM considered here.

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