Searcharxiv⌕ Search

arXiv subjects

F. Krüger

Publications and source records attributed to F. Krüger.

35 records · Page 2Linked to original sources
arXiv2007

Magnetic fluctuations in n-type high-$T_c$ superconductors reveal breakdown of fermiology

By combining experimental measurements of the quasiparticle and dynamical magnetic properties of optimally electron-doped Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$ with theoretical calculations we demonstrate that the conventional fermiology approach cannot possibly account for the magnetic fluctuations in these materials. In particular, we perform tunneling experiments on the very same sample for which a dynamical magnetic resonance has been reported recently and use photoemission data by others on a similar sample to characterize the fermionic quasiparticle excitations in great detail. We subsequently use this information to calculate the magnetic response within the conventional fermiology framework as applied in a large body of work for the hole-doped superconductors to find a profound disagreement between the theoretical expectations and the measurements: this approach predicts a step-like feature rather than a sharp resonance peak, it underestimates the intensity of the resonance by an order of magnitude, it suggests an unreasonable temperature dependence of the resonance, and most severely, it predicts that most of the spectral weight resides in incommensurate wings which are a key feature of the hole-doped cuprates but have never been observed in the electron-doped counterparts. Our findings strongly suggest that the magnetic fluctuations reflect the quantum-mechanical competition between antiferromagnetic and superconducting orders.

cond-mat.supr-con↗
arXiv2004

More Model-Independent Analysis of b->s Processes

We study model-independently the implications of non-standard scalar and pseudoscalar interactions for the decays b ->s gamma, b -> s g, b -> s l^+l^- (l=e,mu) and B_s -> mu^+ mu^-. We find sizeable renormalization effects from scalar and pseudoscalar four-quark operators in the radiative decays and at O(alpha_s) in hadronic b decays. Constraints on the Wilson coefficients of an extended operator basis are worked out. Further, the ratios R_H = BR(B -> H mu^+ mu^-)/BR(B -> H e^+ e^-), for H=K^(*), X_s, and their correlations with B_s -> mu^+ mu^- decay are investigated. We show that the Standard Model prediction for these ratios defined with the same cut on the dilepton mass for electron and muon modes, R_H= 1 + O(m^2_mu/m^2_b), has a much smaller theoretical uncertainty (<1%) than the one for the individual branching fractions. The present experimental limit R_K < 1.2 puts constraints on scalar and pseudoscalar couplings, which are similar to the ones from current data on BR(B_s -> mu^+ mu^-). We find that new physics corrections to R_{K*} and R_{X_s} can reach 13% and 10%, respectively.

hep-ph↗
arXiv2002

Enhancement of Br(B_d -> mu^+ mu^-)/Br(B_s -> mu^+ mu^-) in the MSSM with Modified Minimal Flavour Violation and Large tan(beta)

We extend our previous analysis of the decays B_{d,s} -> mu^+ mu^- in the Minimal Supersymmetric Standard Model (MSSM) to include gluino and neutralino contributions. We provide analytic formulae, valid at large values of tan(beta), for the scalar and pseudoscalar Wilson coefficients arising from neutral Higgs boson exchange diagrams with gluinos and neutralinos. Together with the remaining contributions (W^{+/-}, H^{+/-}, charginos), and assuming the Cabibbo-Kobayashi-Maskawa (CKM) matrix to be the only source of flavour violation, we assess their implications for the branching fractions Br(B_{d,s} -> mu^+ mu^-). Of particular interest is the quantity R = Br(B_d -> mu^+ mu^-)/Br(B_s -> mu^+ mu^-), since (i) the theoretical errors cancel to a large extent, and (ii) it offers a theoretically clean way of extracting the ratio |V_{td}/V_{ts}| in the Standard Model (SM), which predicts R_SM ~ |V_{td}/V_{ts}|^2 ~ O(10^{-2}). Exploring three different scenarios of modified minimal flavour violation, we find that part of the MSSM parameter space can accommodate large Br(B_{d,s} -> mu^+ mu^-) branching fractions, while being consistent with various experimental constraints. More importantly, we show that the ratio R can be as large as O(1), while the individual branching fractions may be amenable to detection by ongoing experiments. We conclude that within the MSSM with large tan(beta) the decay rates of B_{d} -> mu^+ mu^- and B_{s} -> mu^+ mu^- can be of comparable size even in the case where flavour violation is due solely to the CKM matrix.

hep-ph↗
arXiv2002

Analysis of Neutral Higgs-Boson Contributions to the Decays B_s -> l^+l^- and B -> K l^+l^-

We report on a calculation of Higgs-boson contributions to the decays B_s -> l^+l^- and B -> K l^+l^- (l=e, mu) which are governed by the effective Hamiltonian describing b -> s l^+ l^-. Compact formulae for the Wilson coefficients are provided in the context of the type-II two-Higgs-doublet model (2HDM) and supersymmetry (SUSY) with minimal flavour violation, focusing on the case of large tan(beta). We derive, in a model-independent way, constraints on Higgs-boson-mediated interactions, using present experimental results on rare B decays including b -> s gamma, B_s -> mu^+ mu^-, and B -> K^(*) mu^+ mu^-. In particular, we assess the impact of possible scalar and pseudoscalar interactions transcending the standard model (SM) on the branching ratio of B_s -> mu^+ mu^- and the forward-backward (FB) asymmetry of mu^- in B -> K mu^+ mu^- decay. We find that the average FB asymmetry, which is unobservably small within the SM, and therefore a potentially valuable tool to search for new physics, is predicted to be no greater than 4% for a nominal branching ratio of about 6x10^{-7}. Moreover, striking effects on the decay spectrum of B -> K mu^+ mu^- are already ruled out by experimental data on the B_s -> mu^+ mu^- branching fraction. In addition, we study the constraints on the parameter space of the 2HDM and SUSY with minimal flavour violation. While the type-II 2HDM does not give any sizable contributions to the above decay modes, we find that SUSY contributions obeying the constraint on b -> s gamma can affect significantly the branching ratio of B_s -> mu^+ mu^-. We also comment on previous calculations contained in the literature.

hep-ph↗
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.

hep-ph↗
arXiv2001

Spontaneous CP Violation in the Next-to-Minimal Supersymmetric Standard Model

We re-examine spontaneous $\cp$ violation (SCPV) at the tree level in the context of the next-to-minimal supersymmetric standard model (NMSSM) with two Higgs doublets and a gauge singlet field. We analyse the most general Higgs potential without a discrete $Z_3$ symmetry, and derive an upper bound on the mass of the lightest neutral Higgs boson. We estimate $\epsi_K$ by applying the mass insertion approximation, finding that in order to account for the for the observed $\cp$ violation in the neutral kaon sector a non-trivial flavour structure in the soft-breaking $A$ terms is required and that the upper bound on the lightest Higgs-boson mass becomes stronger. We also discuss the implications of electric dipole moments of the electron and the neutron in SUSY models with SCPV.

hep-ph↗
arXiv2001

Spontaneous CP Violation in the Next-to-Minimal Supersymmetric Standard Model Revisited

We re-examine spontaneous CP violation at the tree level in the context of the next-to-minimal supersymmetric standard model (NMSSM) with two Higgs doublets and a gauge singlet field. We analyse the most general Higgs potential without a discrete Z_3 symmetry, and derive an upper bound on the mass of the lightest neutral Higgs boson consistent with present experimental data. We investigate, in particular, its dependence on the admixture and CP-violating phase of the gauge singlet field, as well as on tan(beta). To assess the viability of the spontaneous CP violation scenario, we estimate epsilon_K by applying the mass insertion approximation. We find that a non-trivial flavour structure in the soft-breaking A terms is required to account for the observed CP violation in the neutral kaon sector. Furthermore, combining the minimisation conditions for spontaneous CP violation with the constraints coming from K0-K0bar mixing, we find that the upper bound on the lightest Higgs-boson mass becomes stronger. We also point out that the electric dipole moments of electron and neutron are a serious challenge for SUSY models with spontaneous CP violation.

hep-ph↗
arXiv2000

Angular Distribution and CP Asymmetries in the Decays B->K^-pi^+e^-e^+ and B->pi^-pi^+e^-e^+

The short-distance Hamiltonian describing b->s(d)e^-e^+ in the standard model is used to obtain the decay spectrum of \bar{B}->K^-pi^+e^-e^+ and \bar{B}->pi^-pi^+e^-e^+, assuming the Kpi and pipi systems to be the decay products of K^* and rho respectively. Specific features calculated are (i) angular distribution of K^- (or pi^-) in the K^-pi^+ (or pi^-pi^+) centre-of-mass (c.m.) frame; (ii) angular distribution of e^- in the e^-e^+ c.m. frame; and (iii) the correlation between the meson and lepton planes. We also derive CP-violating observables obtained by combining the above decays with the conjugate processes B->K^+pi^-e^-e^+ and B->pi^-pi^+e^-e^+.

hep-ph↗
arXiv2000

Looking for Novel CP-Violating Effects in B->K^*l^+l^-

The CP-violating asymmetries in the exclusive decay B->K^*l^+l^- (l=e,mu,tau) are predicted to be exceedingly small in the standard model (SM), thereby offering an opportunity to assess various new-physics scenarios. We derive quantitative predictions for various integrated observables in B->K^*mu^+mu^- decay in the presence of physics beyond the SM with additional CP phases and an extended operator basis. In particular, a model-independent analysis of CP asymmetries that require the presence of unitarity phases, in addition to CP violation, is performed. We find that in the low dimuon invariant mass region 2m_mu s gamma rate and the upper limit on BR(B^0->K^{*0}mu^+mu^-), CP-violating effects of a few per cent are estimated, even in the presence of new physics with CP phases of O(1). By contrast, in the high dimuon invariant mass region M_{psi'}<M_{mu^+mu^-}<(M_B-M_{K^*}) significant CP-violating effects are possible. Given a branching ratio of 1.8 x 10^{-6}, the CP asymmetries can be quite substantial (\sim 20% or more), and thus may serve as a means of discovering physics transcending the SM.

hep-ph↗
arXiv2000

Flavour-Conserving CP Phases in Supersymmetry and Implications for Exclusive B Decays

We study rare exclusive B decays based on the quark-level transition b->s(d)l^+l^-, where l=e or mu, in the context of supersymmetric theories with minimal flavour violation. We present analytic expressions for various mixing matrices in the presence of new CP-violating phases, and examine their impact on observables involving B and \bar{B} decays. An estimate is obtained for CP-violating asymmetries in B->K^(*)l^+l^- and B->rho(pi)l^+l^- decays for the dilepton invariant mass region 1.2 GeV < M_{l^+l^-}< M_{J/psi}. As a typical result, we find a CP-violating partial width asymmetry of about -6% (-5%) in the case of B->pi (B->rho) in effective supersymmetry with phases of O(1), taking into account the measurement of the inclusive b->s gamma branching fraction. On the other hand, CP asymmetries of less than 1% are predicted in the case of B->K^(*). We argue that it is not sufficient to have additional CP phases of O(1) to observe large CP-violating effects in exclusive b->s(d)l^+l^- decays.

hep-ph↗
arXiv1999

A Real CKM Matrix and Physics Beyond the Standard Model

We study the possible existence of a real Cabibbo-Kobayashi-Maskawa (CKM) matrix, with CP violation originating from physics beyond the standard model (SM). We show that present experimental data allow for a real CKM matrix provided that new physics also contributes to Delta m_{B_d} by at least 20% of the SM contribution (for rho > 0), besides generating CP violation in the kaon sector. The naturalness of a real CKM matrix is studied within the framework of general multi-Higgs-doublet models with spontaneous CP violation. As an example, we discuss a specific two-Higgs-doublet model and its implications for CP asymmetries in non-leptonic neutral B-meson decays.

hep-ph↗
arXiv1997

CP Violation in Selected B Decays

We summarize the results of two papers in which we have studied CP violation in inclusive and exclusive decays b -> d e^+e^-. Two CP-violating effects are calculated: the partial rate asymmetry between b and \bar{b} decay, and the asymmetry between e^- and e^+ spectra for an untagged B/\bar{B} mixture. These asymmetries, combined with the branching ratio, can potentially determine the parameters (ρ, η) of the unitarity triangle. We also summarize a paper by Browder et al. on a possible CP-violating asymmetry in the inclusive reaction B -> K^-(K^{*-})X.

hep-ph↗
arXiv1997

CP Violation in the Exclusive Decays B -> pi e^+e^- and B -> rho e^+e^-

As a sequel to the calculation of the CP-violating asymmetry in the decay rates of b -> d l^+l^- and \bar{b} -> \bar{d} l^+l^-, we address in this paper the asymmetry in exclusive channels \bar{B} -> pi e^+e^- and \bar{B} -> rho e^+e^-, using form factors from two different models. In the invariant mass region 1GeV < \sqrt{s} < M_{J/psi}, the partial width asymmetry in the channel \bar{B} -> pi is -6% (-2%), and that in the channel \bar{B} -> rho, for one choice of form factors, is -5% (-2%), assuming CKM parameters eta = 0.34, rho = 0.3 (-0.3). We also calculate the forward-backward asymmetry A_{FB} of the e^- in the e^+e^- centre-of-mass system, and find average values (\bar{B} -> pi) \equiv 0, (\bar{B} -> rho)= -17 %, to be compared with the inclusive result (b -> d)= -9%. There is a CP-violating difference between A_{FB} and the corresponding asymmetry in the antiparticle channel \bar{A}_{FB}. Formulae are given that are applicable to any FCNC channel \bar{B} -> P_q (V_q) l^+l^-, q=s, d, with m_l \neq 0, including lepton spin effects. An approximate procedure is used to incorporate the rho, omega, and J/psi resonances.

hep-ph↗
arXiv1996

CP Violation in the Decay B \to X_d e^+ e^-

The decay $b\to d e^+e^-$ has an amplitude containing comparable contributions proportional to $V_{tb}V_{td}^{*}$, $V_{cb}V_{cd}^{*}$ and $V_{ub}V_{ud}^{*}$. These pieces involve different unitarity phases produced by $c\bar{c}$ and $u\bar{u}$ loops. The simultaneous presence of different CKM phases and different dynamical phases leads to a calculable asymmetry in the partial widths of $b\to d e^+e^-$ and $\bar{b}\to \bar{d} e^+e^-$. Using the effective Hamiltonian of the standard model, we calculate this asymmetry as a function of the $e^+e^-$ invariant mass. The effects of $ρ$, $ω$ and $J/ψ$ resonances are taken into account in the vacuum polarization of the $u\bar{u}$ and $c\bar{c}$ currents. As a typical result, an asymmetry of $- 5% (- 2%)$ is predicted in the nonresonant domain $1 GeV < m_{e^+e^-} < m_{J/ψ}$, assuming $η= 0.34$ and $ρ= 0.3 (-0.3)$. The branching ratio in this domain is $1.2\times 10^{-7} (3.3\times 10^{-7})$. Results are also obtained in the region of the $J/ψ$ resonance, where an asymmetry of $3\times 10^{-3}$ is expected, subject to certain theoretical uncertainties in the $b\to d J/ψ$ amplitude.

hep-ph↗
arXiv1996

Lepton Polarization in the Decays $B\to X_sμ^+μ^-$ and $B\to X_sτ^+τ^-$

The effective Hamiltonian for the decay $b\to s l^+ l^-$ predicts a characteristic polarization for the final state lepton, which can serve as an important test of the underlying theory. The lepton polarization has, in addition to a longitudinal component $P_L$, two orthogonal components $P_T$ and $P_N$, lying in and perpendicular to the decay plane which are proportional to $m_l/m_b$, and therefore significant for the $τ^+τ^-$ channel. The normal polarization component $P_N$ is a $T$-odd effect connected with the nonhermiticity of the effective Hamiltonian, arising mainly from $c\bar{c}$ intermediate states. We calculate all three polarization components for the decay $B\to X_sτ^+τ^-$ as a function of the lepton pair mass, and find average values $ $ = -0.37, $ $ = -0.63, $ $ = 0.03. By comparison, the $μ^-$ polarization is $ $ = -0.77, $ = \approx 0$.

hep-ph↗
arXiv1995

Angular Distribution and Polarization of Photons in the Inclusive Decay $Λ_b\to X_s\,γ$

We study the angular distribution of photons produced in the inclusive decay of a polarized $Λ_b$, $Λ_b\to X_s\,γ$, using the technique of Heavy Quark Effective Theory. Finite non-perturbative corrections are obtained relative to the free quark decay $b\to sγ$. These corrections affect significantly the intensity and polarization of photons emitted at small angles relative to the $Λ_b$ spin direction.

hep-ph↗