arXiv subjects

Ulrich Nierste

Publications and source records attributed to Ulrich Nierste.

100 records · Page 6Linked to original sources

arXiv2000

B_{s,d} -> l+ l- in a Two-Higgs-Doublet Model

We compute the branching fractions of B_{s,d} -> l+ l- in the type-II two-Higgs-doublet model with large tan(beta). We find that the parameters of the neutral Higgs sector of the two-Higgs-doublet model cancel in the result, so that the branching fractions depend only on the charged Higgs mass and tan(beta). For large values of tan(beta) and a charged Higgs mass above the bound from b -> s gamma, we find that the branching fractions can be enhanced by up to an order of magnitude or suppressed by up to a factor of two compared to the Standard Model result. We point out that previous calculations in the literature are gauge-dependent due to the omission of an important diagram, which gives the dominant contribution in the 't Hooft-Feynman gauge. We have analyzed in detail the region of the (M_{H^+}, tan(beta)) plane to be probed by searches for B_s -> mu+ mu- in Run II of the Tevatron. Since the branching fraction increases like tan^4(beta), this decay mode is complementary to b -> s gamma and efficiently probes the large tan(beta) region. For tan(beta)=60, an integrated luminosity of 20 fb^{-1} in an extended Run II will probe charged Higgs masses up to 260 GeV, if the background to B_s -> mu+ mu- is small. For the same value of tan(beta), the LHC may be able to explore charged Higgs masses up to 1 TeV using this decay.

hep-ph↗

arXiv1997

Probing penguin coefficients with the lifetime ratio tau(B_s)/tau(B_d)

We calculate penguin contributions to the lifetime splitting between the B_s and the B_d meson. In the Standard Model the penguin effects are found to be opposite in sign, but of similar magnitude as the contributions of the current-current operators, despite of the smallness of the penguin coefficients. We predict tau(B_s)/tau(B_d) -1 = ( -1.2 +/- 10.0 ) * 10^(-3) * (f_{B_s}/190 MeV)^2 where the error stems from hadronic uncertainties. Since penguin coefficients are sensitive to new physics and poorly tested experimentally, we analyze the possibility to extract them from a future precision measurement of tau(B_s)/tau(B_d). Anticipating progress in the determination of the hadronic parameters epsilon_1, epsilon_2 and f_{B_s}/f_{B_d} we find that the coefficient C_4 can be extracted with an uncertainty of order |Delta C_4|= 0.1 from the double ratio (tau(B_s)-tau(B_d))/(tau(B^+)-tau(B_d)), if |epsilon_1-epsilon_2| is not too small.

hep-ph↗

arXiv1997

Penguin Diagrams, Charmless B-Decays and the ``Missing Charm Puzzle''

We calculate the contributions of penguin diagrams with internal u or c quarks to various inclusive charmless B-decay rates. Further we analyze the influence of the chromomagnetic dipole operator Q_8 on these rates. We find that the rates corresponding to B-bar --> X_{u u-bar s}, B-bar --> X_{d d-bar s}, B-bar --> X_{s s-bar s}, B-bar --> X_{s s-bar d} and B --> X_{d d-bar d} are dominated by the new penguin contributions. The contributions of Q_8 sizably diminish these rates. Despite of an increase of the total charmless decay rate by 36 % the new contributions are not large enough to explain the charm deficit observed by ARGUS and CLEO. We predict n_c=1.33 +/- 0.06 for the average number of charmed particles per B-decay in the Standard Model. Then the hypothesis of an enhancement of the chromomagnetic dipole coefficient C_8 by new physics contributions is analyzed. We perform a model independent fit of C_8 to the experimental data. If the CKM structure of the new physics contribution is the same as in the Standard Model, |C_8(M_W)| must be enhanced by a factor of 9 to 16 in order to explain the observed charm deficit.

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arXiv1996

Phenomenology of epsilon_K in the top era

Todays key information on the shape of the unitarity triangle is obtained from the well-measured quantity epsilon_K characterizing the CP-violation in |Delta S|=2 transitions. The phenomenological analysis requires the input of four key quantities: The magnitudes of the CKM elements V_cb and V_ub, the top quark mass and the non-perturbative parameter B_K. In the recent years all of them have been determined with increasing precision. In order to keep up with this progress the |Delta S|=2-hamiltonian had to be obtained in the next-to-leading order (NLO) of renormalization group improved perturbation theory. I present the NLO results for the QCD coefficients eta_1 and eta_3, which have been calculated by Stefan Herrlich and myself, and briefly sketch some aspects of the calculation. Then I give an update of the Unitarity Triangle using the summer 1996 data for the input parameters. The results for the improved Wolfenstein parameters rho-bar and eta-bar and the CKM phase delta are -0.21 < rho-bar < 0.22, 0.27 < eta-bar < 0.43 and 57 degrees < delta < 122 degrees. The range for the quantity sin(2 beta) entering CP asymmetries in B-decays is found as 0.46 < sin(2 beta) < 0.79. The given ranges correspond to one standard deviation in the input parameters. Finally I briefly discuss the K_L-K_S mass difference.

hep-ph↗

arXiv1996

The Complete |Delta S|=2 Hamiltonian in the Next-To-Leading Order

We present the complete next-to-leading order short-distance QCD corrections to the effective \dstwo -hamiltonian in the Standard Model. The calculation of the coefficient $η_3$ is described in great detail. It involves the two-loop mixing of bilocal structures composed of two \dsone\ operators into \dstwo\ operators. The next-to-leading order corrections enhance $η_3$ by 27\% to $η_3=0.47 \errorpm{+0.03}{-0.04}$ thereby affecting the phenomenology of $ε_K$ sizeably. $η_3$ depends on the physical input parameters $m_t$, $m_c$ and $\laMSb$ only weakly. The quoted error stems from renormalization scale dependences, which have reduced compared to the old leading log result. The known calculation of $η_1$ and $η_2$ is repeated in order to compare the structure of the three QCD coefficients. We further discuss some field theoretical aspects of the calculation such as the renormalization group equation for Green's functions with two operator insertions and the renormalization scheme dependence caused by the presence of evanescent operators.

hep-ph↗

arXiv1995

Higgs Sector Renormalization Group in the MS-bar and OMS Scheme

We discuss different aspects of the Higgs self-interaction in the MS-bar and the on-mass-shell (OMS) scheme. The running coupling λ(μ) is investigated in great detail. The three-loop coefficient of the β-function in the OMS scheme is derived, and the three-loop running coupling is calculated. The breakdown of perturbation theory for large Higgs masses M_H is analyzed in three physical observables for which two-loop results are known. Requiring the dependence on the renormalization scale to diminish order-by-order in λ, we find that perturbation theory breaks down for M_H=O(700 GeV) in Higgs decays. Similarly, M_H must be smaller than O(400 GeV) for perturbatively calculated cross sections to be trustworthy up to cm energies of O(2 TeV). If the Higgs sector shall be perturbative up to the GUT scale, the Higgs must be lighter than O(150 GeV). For the two-loop observables examined, the apparent convergence of the perturbation series is better in the OMS scheme than in the MS-bar scheme.

hep-ph↗

arXiv1995

Phenomenology of $\varepsilon_K$ Beyond Leading Logarithms

I present the QCD short distance coefficient $η_3$ of the $ΔS=2$ hamiltonian in the \emph{next-to-leading order} (NLO) of renormalization group improved perturbation theory. Since now all QCD-factors $η_1$, $η_2$ and $η_3$ are known with NLO accuracy, a much higher precision in the analysis of $\varepsilon_K$ can be achieved. The CKM phase $δ$, $|V_{td}|$ and the mass difference $Δm_{B_s}$ in the $\text{B}_s^0-\overline{\text{B}}_s^0$-system are predicted from the measured values of $\varepsilon_K$ and $Δm_{B_d}$. Finally I briefly look at the \kkmd. This work has been done in collaboration with Stefan Herrlich.

hep-ph↗

arXiv1995

Indirect CP-Violation in the Neutral Kaon System Beyond Leading Logarithms and Related Topics

I describe in detail the calculation of the coefficients eta_3 and eta_1 of the low-energy Delta S=2 -hamiltonian describing K^0-K^0-bar-mixing in the next-to-leading order of renormalization group improved (RG) perturbation theory. First a general introduction into the application of the operator expansion and of RG methods to weak processes is presented. This includes the RG formalism for Green's functions with two operator insertions. Second I discuss the elimination of unphysical operators with emphasis on evanescent operators. Then the two-loop calculations needed for eta_3 and eta_1 are shown and finally the implications on the phenomenology of epsilon_K and the K_L-K_S mass difference are discussed. The results of this thesis have been published separately, but they are explained in more detail here.

hep-ph↗

arXiv1995

Evanescent Operators, Scheme Dependences and Double Insertions

The anomalous dimension matrix of dimensionally regularized four-quark operators is known to be affected by evanescent operators, which vanish in $D=4$ dimensions. Their definition, however, is not unique, as one can always redefine them by adding a term proportional to $(D-4)$ times a physical operator. In the present paper we compare different definitions used in the literature and find that they correspond to different renormalization schemes in the physical operator basis. The scheme transformation formulae for the Wilson coefficients and the anomalous dimension matrix are derived in the next-to-leading order. We further investigate the proper treatment of evanescent operators in processes appearing at second order in the effective four-fermion interaction such as particle-antiparticle mixing, rare hadron decays or inclusive decays.

hep-ph↗

arXiv1995

Indirect CP-Violation in the Neutral Kaon System Beyond Leading Logarithms

We have calculated the short distance QCD coefficient η_3 of the effective |ΔS|=2-hamiltonian in the next-to-leading order. Since now all coefficients η_1, η_2 and η_3 are known beyond the leading log approximation, one can achieve a much higher precision in the theoretical analysis of ε_K. The measured value for ε_K$ yields a lower bound on each of |V_{cb}|, |V_{ub}/V_{cb}|, the top quark mass $m_t$ and the non- perturbative parameter B_K as a function of the remaining three quantities. We discuss the implications on the CKM phase δ, |V_{td}| and the key quantity for all CP-violating processes, Im λ_t = Im [V^*_{ts} V_{td}]. These quantities and the improved Wolfenstein parameters \barρand \barηare tabulated and the shape of the unitarity triangle is discussed. We compare the range for |V_{td}| with the one obtained from the analysis of B^0--\bar{B^0} mixing. For 0.037 \leq |V_{cb}| \leq 0.043, 0.06 \leq |V_{ub}/V_{cb}| \leq 0.10 and 0.65 \leq B_K \leq 0.85 we find from a combined analysis of ε_K and the B^0--\bar{B^0} mixing paramater x_d: 49^{\circ} \leq δ\leq 146^{\circ}, 7.4 \cdot 10^{-3} \leq |V_{td}| \leq 12.4 \cdot 10^{-3}, 0.85 \cdot 10^{-4} \leq \imag λ_t \leq 1.60 \cdot 10^{-4}, -0.36 \leq \barρ\leq 0.28 and 0.21 \leq \barη\leq 0.44. We predict the mass difference of the $B_s^0$ system to lie in the range 6.5 ps^{-1} \leq Δm_{B_s} \leq 28 ps^{-1}. Finally we have a 1995 look at the K_L--K_S mass difference.

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