Searcharxiv⌕ Search

arXiv subjects

U. Nierste

Publications and source records attributed to U. Nierste.

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.

hep-ex↗
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.

hep-ph↗
arXiv2012

New Physics in B-Bbar mixing in the light of recent LHCb data

We perform model-independent statistical analyses of three scenarios accommodating New Physics (NP) in Delta F=2 flavour-changing neutral current amplitudes. In a scenario in which NP in B_d-B_d-bar and B_s-B_s-bar is uncorrelated, we find the parameter point representing the Standard-Model disfavoured by 2.4 standard deviations. However, recent LHCb data on B_s neutral-meson mixing forbid a good accommodation of the D0 data on the semileptonic CP asymmetry A_SL. We introduce a fourth scenario with NP in both M_12^d,s and Gamma_12^d,s, which can accommodate all data. We discuss the viability of this possibility and emphasise the importance of separate measurements of the CP asymmetries in semileptonic B_d and B_s decays. All results have been obtained with the CKMfitter analysis package, featuring the frequentist statistical approach and using Rfit to handle theoretical uncertainties.

hep-ph↗
arXiv2011

Anatomy of New Physics in B-Bbar mixing

We analyse three different New Physics scenarios for Delta F=2 flavour-changing neutral currents in the quark sector in the light of recent data on neutral-meson mixing. We parametrise generic New Physics contributions to B_q-Bbar_q mixing (q=d,s), in terms of one complex quantity Delta_q, while three parameters Delta_K^tt, Delta_K^ct and Delta_K^cc are needed to describe K-Kbar mixing. In Scenario I, we consider uncorrelated New Physics contributions in the B_d, B_s, and K sectors. In this scenario, it is only possible to constrain the parameters Delta_d and Delta_s whereas there are no non-trivial constraints on the kaon parameters. In Scenario II, we study the case of Minimal Flavour Violation (MFV) and small bottom Yukawa coupling and Scenario III is the generic MFV case with large bottom Yukawa couplings. Our quantitative analyses consist of global CKM fits within the Rfit frequentist statistical approach, determining the Standard Model parameters and the new physics parameters of the studied scenarios simultaneously. We find that the recent measurements indicating discrepancies with the Standard Model are well accomodated in Scenarios I and III with new mixing phases, with a slight preference for Scenario I that permits different new CP phases in the B_d and B_s systems. Within our statistical framework, we find evidence of New Physics in both B_d and B_s systems. The Standard-Model hypothesis Delta_d=Delta_s=1 is disfavoured with p-values of 3.6 sigma and 3.3 sigma in Scenarios I and III, respectively. We also present an exhaustive list of numerical predictions in each scenario. In particular, we predict the CP phase in B_s -> J psi phi and the difference between the B_s and B_d semileptonic asymmetries, which will be both measured by the LHCb experiment.

hep-ph↗
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.

hep-ph↗
arXiv2003

CP asymmetry in flavour-specific B decays beyond leading logarithms

We compute next-to-leading order QCD corrections to the CP asymmetry a_{fs} = Im(Gamma_{12}/M_{12}) in flavour-specific B_{d,s} decays such as B_d->X l nu or B_s->D^-_s pi^+. The corrections reduce the uncertainties associated with the choice of the renormalization scheme for the quark masses significantly. In the Standard Model we predict a^d_{fs} = -(5.0 +- 1.1)*10^{-4}. As a by-product we also obtain the width difference in the B_d system at next-to-leading order in QCD.

hep-ph↗
arXiv2002

Lifetimes of heavy hadrons beyond leading logarithms

The lifetime splitting between the B^+ and B_d^0 mesons has recently been calculated in the next-to-leading order of QCD. These corrections are necessary for a reliable theoretical prediction, in particular for the meaningful use of hadronic matrix elements computed with lattice QCD. Using results from quenched lattice QCD we find tau(B^+)/tau(B^0_d)=1.053 +/- 0.016 +/- 0.017, where the uncertainties from unquenching and 1/m_b corrections are not included. The lifetime difference of heavy baryons Xi_b^0 and Ξ_b^- is also discussed.

hep-ph↗
arXiv2002

Trilepton Events and B_s -> mu^+ mu^- : No-lose for mSUGRA at the Tevatron?

We study the Tevatron search potential for minimal supergravity (mSUGRA) and find two observables which reveal complementary information on the mSUGRA parameter space: the ``gold-plated'' decays of charginos/neutralinos to trilepton final states and the rare decay B_s-> mu^+ mu^-. For the universal gaugino mass, M_{1/2} below 250 GeV and for the universal scalar fermion mass, M_0 outside of 200-370 GeV we face a ``no-lose'' situation for the Tevatron: If tan(beta) < 30, the Tevatron has a chance to see the trilepton events but not B_s -> mu^+ mu^- during Run IIa, whereas for larger tan(beta) the trilepton events become invisible (at least with an integrated luminosity of 2 fb^{-1}) while B_s -> mu^+ mu^- is enhanced to an observable level. For this study we perform an updated analysis of the trilepton signature which includes the full set of the decay matrix elements and spin correlations. This leads to a new, more promising search reach for the Tevatron.

hep-ph↗
arXiv2002

The B^+ B_d^0 Lifetime Difference Beyond Leading Logarithms

We compute perturbative QCD corrections to the lifetime splitting between the charged and neutral $B$ meson in the framework of the heavy quark expansion. These next-to-leading logarithmic corrections are necessary for a meaningful use of hadronic matrix elements of local operators from lattice gauge theory. We find the uncertainties associated with the choices of renormalization scale and scheme significantly reduced compared to the leading-order result. We include the full dependence on the charm-quark mass $m_c$ without any approximations. Using hadronic matrix elements estimated in the literature with lattice QCD we obtain $τ(B^+)/τ(B^0_d)=1.053\pm 0.027\pm 0.017$, where the effects of unquenching and $1/m_b$ corrections are not yet included. The lifetime difference of heavy baryons $Ξ^0_b$ and $Ξ^-_b$ is also briefly discussed.

hep-ph↗
arXiv2002

B Physics at the Tevatron: Run II and Beyond

This report provides a comprehensive overview of the prospects for B physics at the Tevatron. The work was carried out during a series of workshops starting in September 1999. There were four working groups: 1) CP Violation, 2) Rare and Semileptonic Decays, 3) Mixing and Lifetimes, 4) Production, Fragmentation and Spectroscopy. The report also includes introductory chapters on theoretical and experimental tools emphasizing aspects of B physics specific to hadron colliders, as well as overviews of the CDF, D0, and BTeV detectors, and a Summary.

hep-ph↗
arXiv2001

Correlation of B_s -> mu^+ mu^- and (g-2)_mu in Minimal Supergravity

We analyse the rare decay mode B_s -> mu^+ mu^- in the minimal supergravity scenario (mSUGRA). We find a strong correlation with the muon anomalous magnetic moment (g-2)_mu. An interpretation of the recently measured excess in (g-2)_mu in terms of mSUGRA corrections implies a substantial supersymmetric enhancement of the branching ratio Br(B_s -> mu^+ mu^-): if (g-2)_mu exceeds the Standard Model prediction by 4*10^{-9}, Br(B_s -> mu^+ mu^-) is larger by a factor of 10-100 than in the Standard Model and within reach of Run-II of the Tevatron. Thus an experimental search for B_s -> mu^+ mu^- is a stringent test of the mSUGRA GUT scale boundary conditions. If the decay B_s -> mu^+ mu^- is observed at Run-II of the Tevatron, then we predict the mass of the lightest supersymmetric Higgs boson to be less than 120 GeV. The decay B_s -> mu^+ mu^- can also significantly probe the favoured parameter range in SO(10) SUSY GUT models.

hep-ph↗
arXiv2000

b -> s γand supersymmetry with large tanβ

We discuss the constraints on the supersymmetric parameter space from the decay mode b -> s γfor large values of tanβ. We improve the theoretical prediction for the decay rate by summing very large radiative corrections to all orders in perturbation theory. This extends the validity of the perturbative calculation to the large tanβregime. This resummation of terms of order alpha_s^n tan^{n+1}βuses a recently proposed effective lagrangian for the Yukawa interaction of bottom quarks. Moreover, we identify an additional source of tanβ-enhanced terms, which are of order alpha_s tanβand involve the charged Higgs boson, and analyze their behaviour in higher orders of perturbation theory. After correcting the current expressions for this rare decay branching ratio at next-to-leading order, we obtain that, contrary to recent claims, the presently measured branching ratio of b -> s γconstrains the supersymmetric parameter space in a relevant way, even if tanβis large.

hep-ph↗
arXiv2000

Effective lagrangian for the tbH^+ interaction in the MSSM and charged Higgs phenomenology

In the framework of a 2HDM effective lagrangian for the MSSM, we analyse important phenomenological aspects associated with quantum soft SUSY-breaking effects that modify the relation between the bottom mass and the bottom Yukawa coupling. We derive a resummation of the dominant supersymmetric corrections for large values of \tb to all orders in perturbation theory. With the help of the operator product expansion we also perform the resummation of the leading and next-to-leading logarithms of the standard QCD corrections. We use these resummation procedures to compute the radiative corrections to the \tbH, \Htb decay rates. In the large \tb regime, we derive simple formulae embodying all the dominant contributions to these decay rates and we compute the corresponding branching ratios. We show, as an example, the effect of these new results on determining the region of the \mH--\tb plane excluded by the Tevatron searches for a supersymmetric charged Higgs boson in top quark decays, as a function of the MSSM parameter space.

hep-ph↗
arXiv1999

Theoretical status of epsilon'/epsilon

I give a detailed introduction into the theoretical formalism for epsilon'/epsilon, which measures direct CP-violation in K -> pi pi decays. The current status of hadronic matrix elements and the strange quark mass is discussed. Several possible explanations of the unexpectedly high experimental results for epsilon'/epsilon are pointed out: A small strange quark mass, an enhancement of the hadronic parameter B_6^(1/2) from the sigma resonance, an underestimate of isospin breaking and possible new physics contributions in the sdZ-vertex and the sd-gluon-vertex.

hep-ph↗
arXiv1999

Next-to-Leading Order QCD Corrections to the Lifetime Difference of $B_s$ Mesons

We compute the QCD corrections to the decay rate difference in the $B_s-\bar B_s$ system, $ΔΓ_{B_s}$, in the next-to-leading logarithmic approximation using the heavy quark expansion approach. Going beyond leading order in QCD is essential to obtain a proper matching of the Wilson coefficients to the matrix elements of local operators from lattice gauge theory. The lifetime difference is reduced considerably at next-to-leading order. We find $(ΔΓ/Γ)_{B_s}=(f_{B_s}/210 MeV)^2 [0.006 B(m_b)+0.150 B_S(m_b)-0.063]$ in terms of the bag parameters $B, B_S$ in the NDR scheme. As a further application of our analysis we also derive the next-to-leading order result for the mixing-induced CP asymmetry in inclusive $b\to u\bar ud$ decays, which measures $\sin 2α$.

hep-ph↗
arXiv1999

A short look at epsilon'/epsilon

We analyze the theoretical implications of the new KTeV measurement of direct CP-violation in K->pi pi decays. The result is found consistent with the Standard Model for low values of the strange quark mass m_s. If the hadronic parameters B_6^{(1/2)} and B_8^{(3/2)} satisfy 2 B_6^{(1/2)} - B_8^{(3/2)} < 2, as suggested by lattice and 1/N_c calculations, we find an upper bound of 110 MeV for m_s (2 GeV)$. We parametrize potential new physics contributions to ε'/εand illustrate their correlation with upper bounds on m_s. Finally we discuss a non-perturbative mechanism, which is not contained in the existing calculations of B_6^{(1/2)}. This mechanism enhances B_6^{(1/2)} and thereby leads to a better understanding of the Delta I=1/2 rule and the high measured value of Re (ε^\prime/ε)

hep-ph↗