Ulrich Nierste

Charm quark contribution to K+ ---> pi+ nu anti-nu at next-to-next-to-leading order

The authors calculate the complete next-to-next-to-leading order QCD corrections to the charm contribution of the rare decay K{sup +} {yields} {pi}{sup +}{nu}{bar {nu}}. They encounter several new features, which were absent in lower orders. They discuss them in detail and present the results for the two-loop matching conditions of the Wilson coefficients, the three-loop anomalous dimensions, and the two-loop matrix elements of the relevant operators that enter the next-to-next-to-leading order renormalization group analysis of the Z-penguin and the electroweak box contribution. The inclusion of the next-to-next-to-leading order QCD corrections leads to a significant reduction of the theoretical uncertainty from {+-} 9.8% down to {+-} 2.4% in the relevant parameter P{sub c}(X), implying the leftover scale uncertainties in {Beta}(K{sup +} {yields} {pi}{sup +}{nu}{bar {nu}}) and in the determination of |V{sub td}|, sin 2{beta}, and {gamma} from the K {yields} {pi}{nu}{bar {nu}} system to be {+-} 1.3%, {+-} 1.0%, {+-} 0.006, and {+-} 1.2{sup o}, respectively. For the charm quark {ovr MS} mass m{sub c}(m{sub c}) = (1.30 {+-} 0.05) GeV and |V{sub us}| = 0.2248 the next-to-leading order value P{sub c}(X) = 0.37 {+-} 0.06 is modified to P{sub c}(X) = 0.38 {+-} 0.04 at the next-to-next-to-leading order level with the latter error fully dominated by the uncertainty in m{sub c}(m{sub c}). They present tables for P{sub c}(X) as a function of m{sub c}(m{sub c}) and {alpha}{sub s}(M{sub z}) and a very accurate analytic formula that summarizes these two dependences as well as the dominant theoretical uncertainties. Adding the recently calculated long-distance contributions they find {Beta}(K{sup +} {yields} {pi}{sup +}{nu}{bar {nu}}) = (8.0 {+-} 1.1) x 10{sup -11} with the present uncertainties in m{sub c}(m{sub c}) and the Cabibbo-Kobayashi-Maskawa elements being the dominant individual sources in the quoted error. They also emphasize that improved calculations of the long-distance contributions to K{sup +} {yields} {pi}{sup +}{nu}{bar {nu}} and of the isospin breaking corrections in the evaluation of the weak current matrix elements from K{sup +} {yields} {pi}{sup 0}e{sup +}{nu} would be valuable in order to increase the potential of the two golden K {yields} {pi}{nu}{bar {nu}} decays in the search for new physics.

Effective Lagrangian for the t̄bH+ interaction in the MSSM and charged Higgs phenomenology

Abstract 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 tan β 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 t→b H + , H + →t b decay rates. In the large tan β 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 M H + – tan β plane excluded by the Tevatron searches for a supersymmetric charged Higgs boson in top quark decays, as a function of the MSSM parameter space.

Anatomy of new physics in B-B 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 Bq - Bq-bar 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 Bd, Bs, 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, where Delta=Delta_d=Delta_s=Delta_K^{tt}. We show that Delta must then be real, so that no new CP phases can be accomodated, and express the remaining parameters Delta_K^{cc} and Delta_K^{ct} in terms of Delta in this scenario. Scenario III is the generic MFV case with large bottom Yukawa couplings. In this case, the Kaon sector is uncorrelated to the B_d and B_s sectors. As in the second scenario one has Delta_d=Delta_s=Delta, however, now with a complex parameter Delta. 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 Ithat permits different new CP phases in the Bd and Bs systems. Within our statistical framework, we find that the Standard-Model hypothesis Delta_d=Delta_s=1 is disfavoured with p-values of 3.4 sigma and 3.1 sigma in Scenarios I and III respectively.

b→sγ and supersymmetry with large tanβ

Abstract We discuss the constraints on the supersymmetric parameter space from the decay mode b → sγ for large values of tan β . 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 tan β regime. This resummation of terms of order α s n tan n +1 β is based on a recently proposed effective Lagrangian for the Yukawa interaction of bottom quarks. Moreover, we identify an additional source of tan β -enhanced terms, which are of order α s tan β and involve the charged Higgs boson, and analyse 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 measured branching ratio of b → sγ constrains the supersymmetric parameter space in a relevant way, even if tan β is large.

Correlation of B(s) --> mu(+)mu(-) and (g - 2)(mu) in minimal supergravity.

We analyze the decay mode B(s)-->mu(+)mu(-) in minimal supergravity (mSUGRA). We find that the recently measured excess in (g-2)(mu), if interpreted within mSUGRA, is correlated with a substantial enhancement of the branching ratio Beta(B(s)-->mu(+)mu(-)): if (g-2)(mu) exceeds the standard model prediction by 4 x 10(-9), Beta(B(s)-->mu(+)mu(-)) is larger by a factor of 10-100 and within reach of Run-II of the Tevatron. Thus the search for B(s)-->mu(+)mu(-) is a stringent test of the GUT scale relations of mSUGRA. An observation of B(s)-->mu(+)mu(-) at the Tevatron implies a mass of the lightest SUSY Higgs boson below 120 GeV. B(s)-->mu(+)mu(-) can also significantly probe SO(10) SUSY GUT models.

Next-to-leading order QCD corrections to the lifetime difference of Bs mesons

Abstract We compute the QCD corrections to the decay rate difference in the B s – B s system, Δ Γ 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 ( Δ Γ/Γ) B s =(f B s /210 MeV ) 2 [0.006B(m b )+0.150B 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→u u d decays, which measures sin2 α .

Correlation of Bs --> µ+µ and (g 2)µ in Minimal Supergravity

Athanasios Dedes, Herbert K. Dreiner and Ulrich Nierste 1 Physikalisches Institut, Universität Bonn, Nußallee 12, D-53115 Bonn, Germany 2 CERN, TH Division, CH-1211 Geneva 23, Switzerland We analyse the rare decay mode Bs → μ+μ− in the minimal supergravity scenario (mSUGRA). We find a strong correlation with the muon anomalous magnetic moment (g − 2)μ. An interpretation of the recently measured excess in (g − 2)μ in terms of mSUGRA corrections implies a substantial supersymmetric enhancement of the branching ratio B(Bs → μ+μ−): if (g−2)μ exceeds the Standard Model prediction by 3 ·10−9, B(Bs → μ+μ−) is larger by a factor of 100 than in the Standard Model and well within reach of Run-II of the Tevatron. Thus an experimental search for Bs → μ+μ− is a stringent test of the mSUGRA GUT scale boundary conditions. If the decay Bs → μ+μ− is observed at Run-II of the Tevatron, then we predict the mass of lightest supersymmetric Higgs boson to be less than 118 GeV. The decay Bs → μ+μ− can also significantly probe the favoured parameter range in SO(10) SUSY GUT models, possibly excluding them.

Impact of a Higgs boson at a mass of 126 GeV on the standard model with three and four fermion generations.

We perform a comprehensive statistical analysis of the standard model (SM) with three and four generations using the latest Higgs search results from LHC and Tevatron, the electroweak precision observables measured at LEP and SLD, and the latest determinations of M(W), m(t), and α(s). For the three-generation case we analyze the tensions in the electroweak fit by removing individual observables from the fit and comparing their predicted values with the measured ones. In particular, we discuss the impact of the Higgs search results on the deviations of the electroweak precision observables from their best-fit values. Our indirect prediction of the top mass is m(t) =175.7(-2.2)(+3.0) GeV at 68.3% C.L., which is in good agreement with the direct measurement. We also plot the preferred area in the M(W)-m(t) plane. The best-fit Higgs boson mass is 126.0 GeV. For the case of the SM with a perturbative sequential fourth fermion generation (SM4) we discuss the deviations of the Higgs signal strengths from their best-fit values. The H → γγ signal strength now disagrees with its best-fit SM4 value at more than 4σ. We perform a likelihood-ratio test to compare the SM and SM4 and show that the SM4 is excluded at 5.3σ. Without the Tevatron data on H → bb the significance drops to 4.8σ.

Status of the two-Higgs-doublet model of type II

A bstractWe determine the allowed parameter space of the CP-conserving two-Higgs-doublet model (2HDM) of type II with a softly broken Z2 symmetry. Our analysis includes theoretical constraints from vacuum stability and perturbativity as well as experimental constraints from signal strengths of the 126 GeV Higgs boson, the non-observation of additional Higgs resonances and electroweak precision and flavour observables. If the 126 GeV resonance is interpreted as the light CP-even Higgs boson of the 2HDM our analysis shows that scenarios where the couplings of this boson deviate substantially from those of the SM Higgs boson are disfavoured at one standard deviation and completely excluded for small values of tan β. We also discuss bounds on the masses of the heavy 2HDM Higgs bosons and their implications for the possible decay modes of these particles. We find that the region in which both non-standard neutral Higgs bosons are simultaneously lighter than 300 GeV is excluded at two standard deviations.

b ---> s gamma and supersymmetry with large tan Beta

Abstract We discuss the constraints on the supersymmetric parameter space from the decay mode b → sγ for large values of tan β . 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 tan β regime. This resummation of terms of order α s n tan n +1 β is based on a recently proposed effective Lagrangian for the Yukawa interaction of bottom quarks. Moreover, we identify an additional source of tan β -enhanced terms, which are of order α s tan β and involve the charged Higgs boson, and analyse 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 measured branching ratio of b → sγ constrains the supersymmetric parameter space in a relevant way, even if tan β is large.