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Thorsten Feldmann

Publications and source records attributed to Thorsten Feldmann.

At least 55 records · Page 3Linked to original sources
arXiv2010

Lepton Flavour Violation in the Presence of a Fourth Generation of Quarks and Leptons

We calculate the rates for the charged lepton flavour violating (LFV) decays l_i -> l_j gamma, tau -> l pi, tau -> l eta('), mu^- -> e^-e^+e^-, the six three-body leptonic decays tau^- -> l_i^- l_j^+ l_k^- and the rate for mu-e conversion in nuclei in the Standard Model (SM3) extended by a fourth generation of quarks and leptons (SM4), assuming that neutrinos are Dirac particles. We also calculate branching ratios for K_L -> mu e, K_L -> pi^0 mu e, B_{d,s} -> mu e, B_{d,s} -> tau e and B_{d,s} -> tau mu. We find that the pattern of the LFV branching ratios in the SM4 differs significantly from the one encountered in the MSSM, allowing to distinguish these two models with the help of LFV processes in a transparent manner. Also differences with respect to the Littlest Higgs model with T-parity are found. Most importantly the branching ratios for l_i -> l_j gamma, tau -> l pi, tau -> l eta('), mu^- -> e^-e^+e^-, tau^- -> e^-e^+e^-, tau^- -> mu^- mu^+ mu^-, tau^- -> e^- mu^+ mu^- and tau^- -> mu^- e^+ e^- can all still be as large as the present experimental upper bounds but not necessarily simultaneously. Also the rate for mu-e conversion in nuclei can reach the corresponding upper bound.

hep-ph↗
arXiv2010

The Impact of a 4th Generation on Mixing and CP Violation in the Charm System

We study D0-D0 mixing in the presence of a fourth generation of quarks. In particular, we calculate the size of the allowed CP violation which is found at the observable level well beyond anything possible with CKM dynamics. We calculate the semileptonic asymmetry a_SL and the mixing induced CP asymmetry eta_fS_f which are correlated with each other. We also investigate the correlation of eta_fS_f with a number of prominent observables in other mesonic systems like epsilon'/epsilon, Br(K_L -> pi0 nu nu), Br(K+ -> pi+ nu nu), Br(B_s ->mu+ mu-), Br(B_d -> mu+ mu-) and finally S_psi phi in the B_s system. We identify a clear pattern of flavour and CP violation predicted by the SM4 model: While simultaneous large 4G effects in the K and D systems are possible, accompanying large NP effects in the B_d system are disfavoured. However this behaviour is not as pronounced as found for the LHT and RSc models. In contrast to this, sizeable CP violating effects in the B_s system are possible unless extreme effects in eta_fS_f are found, and Br(B_s ->mu+ mu-) can be strongly enhanced regardless of the situation in the D system. We find that, on the other hand, S_psi phi > 0.2 combined with the measured epsilon'/epsilon significantly diminishes 4G effects within the D system.

hep-ph↗
arXiv2008

Is there a non-Standard-Model contribution in non-leptonic b -> s decays?

Precision measurements of branching fractions and CP asymmetries in non-leptonic b -> s decays reveal certain "puzzles" when compared with Standard Model expectations based on a global fit of the CKM triangle and general theoretical expectations. Without reference to a particular model, we investigate to what extent the (small) discrepancies observed in (B -> J/psi K), (B -> phi K) and (B -> K pi) may constrain new physics in (b -> s q qbar) operators. In particular, we compare on a quantitative level the relative impact of different quark flavours q=c,s,u,d.

hep-ph↗
arXiv2008

Soft-Collinear Effective Theory: Recent Results and Applications

Soft-collinear effective theory (SCET) has become a standard tool to study the factorization of short- and long-distance effects in processes involving low-energetic (soft) particles and high-energetic/low-virtuality (collinear) modes. In this contribution I give a brief overview on recent results for inclusive and exclusive B decays and on applications in collider physics.

hep-ph↗
arXiv2008

Light-cone sum rules in soft-collinear effective theory

We derive light-cone sum rules (LCSRs) for exclusive B-meson decays into light energetic hadrons from correlation functions within soft-collinear effective theory (SCET). In these sum rules the short-distance scale refers to ``hard-collinear'' interactions with virtualities of order (Lambda_{QCD}*m_b). Hard scales (related to virtualities of order m_b^2) are integrated out and enter via external coefficient functions in the sum rule. Soft dynamics is encoded in light-cone distribution amplitudes for the B-meson, which describe both the factorizable and non-factorizable contributions to exclusive B-meson decay amplitudes. As an example, we provide a detailed study of the SCET sum rule for the B -> pi transition form factor at large recoil, including radiative corrections from hard-collinear loop diagrams at first order in the strong coupling constant. We find remarkable conceptual and numerical differences with the heavy-quark limit of the conventional LCSR approach in QCD.

hep-ph↗
arXiv2008

Modelling light-cone distribution amplitudes from non-relativistic bound states

We calculate light-cone distribution amplitudes for non-relativistic bound states, including radiative corrections from relativistic gluon exchange to first order in the strong coupling constant. We distinguish between bound states of quarks with equal (or similar) mass, m_1 ~ m_2, and between bound states where the quark masses are hierarchical, m_1 >> m_2. For both cases we calculate the distribution amplitudes at the non-relativistic scale and discuss the renormalization-group evolution for the leading-twist and 2-particle distributions. Our results apply to hard exclusive reactions with non-relativistic bound states in the QCD factorization approach like, for instance, (B_c -> eta_c l nu) or (e^+ e^- -> J/psi eta_c). They also serve as a toy model for light-cone distribution amplitudes of light mesons or heavy B and D mesons, for which certain model-independent properties can be derived. In particular, we calculate the anomalous dimension for the B meson distribution amplitude phi_B^-(w) in the Wandzura-Wilczek approximation and derive the according solution of the evolution equation at leading logarithmic accuracy.

hep-ph↗
arXiv2008

Large Top Mass and Non-Linear Representation of Flavour Symmetry

We consider an effective theory (ET) approach to flavour-violating processes beyond the Standard Model (SM), where the breaking of flavour symmetry is described by spurion fields whose low-energy vacuum expectation values are identified with the SM Yukawa couplings. Insisting on canonical mass dimensions for the spurion fields, the large top-quark Yukawa coupling also implies a large expectation value for the associated spurion, which breaks part of the flavour symmetry already at the UV scale Lambda of the ET. Below that scale, flavour symmetry in the ET is represented in a non-linear way by introducing Goldstone modes for the partly broken flavour symmetry and spurion fields transforming under the residual symmetry. As a result, the dominance of certain flavour structures in rare quark decays can be understood in terms of the 1/Lambda expansion in the ET. We also discuss the generalization to 2-Higgs-doublet models with large tan(beta).

hep-ph↗
arXiv2006

Recent Developments in Soft-Collinear Effective Theory

Soft-collinear effective theory provides a systematic theoretical framework to describe the factorization of short- and long-distance QCD dynamics in hard-scattering processes that contain both, soft and energetic particles/jets. I present a short guide to recent theoretical achievements and to phenomenological applications in heavy B-meson decays.

hep-ph↗
arXiv2005

The B --> pi form factor from light-cone sum rules in soft-collinear effective theory

Recently, we have derived light-cone sum rules for exclusive B-meson decays into light energetic hadrons from correlation functions within soft-collinear effective theory. In these sum rules the short-distance scale refers to ``hard-collinear'' interactions with virtualities of order Λ_{QCD} m_b. Hard scales (related to virtualities of order m_b^2) are integrated out and enter via external coefficient functions in the sum rule. Soft dynamics is encoded in light-cone distribution amplitudes for the B-meson, which describe both the factorizable and non-factorizable contributions to exclusive B-meson decay amplitudes. Factorization of the correlation function has been verified to one-loop accuracy. Thus, a systematic separation of hard, hard-collinear, and soft dynamics in the heavy-quark limit is possible.

hep-ph↗
arXiv2004

Non-factorizable Contributions to $B \to ππ$ Decays

We investigate to what extent the experimental information on $B \to ππ$ branching fractions and CP asymmetries can be used to better understand the QCD dynamics in these decays. For this purpose we decompose the independent isospin amplitudes into factorizable and non-factorizable contributions. The former can be estimated within the framework of QCD factorization for exclusive $B$ decays. The latter vanish in the heavy-quark limit, $m_b \to \infty$, and are treated as unknown hadronic parameters. We discuss at some length in which way the non-factorizable contributions are treated in different theoretical and phenomenological frameworks. We point out the potential differences between the phenomenological treatment of power-corrections in the ``BBNS approach'', and the appearance of power -suppressed operators in soft-collinear effective theory (SCET). On that basis we define a handful of different (but generic) scenarios where the non-factorizable part of isospin amplitudes is parametrized in terms of three or four unknowns, which can be constrained by data. We also give some short discussion on the implications of our analysis for $B \to πK$ decays. In particular, since non-factorizable QCD effects in $B \to ππ$ may be large, we cannot exclude sizeable non-factorizable effects, which violate $SU(3)_F$ flavour symmetry, or even isospin symmetry (via long-distance QED effects). This may help to explain certain puzzles in connection with isospin-violating observables in $B \to πK$ decays.

hep-ph↗
arXiv2003

Short- and long-distance QCD effects in B-meson decays

Various exclusive and inclusive decays of B mesons are studied, at present, with dedicated experiments at ``B factories''. In order to compete with the experimental accuracy, we need a reliable theoretical framework to compute strong interaction effects in a hadronic environment. I discuss how the separation of (perturbatively calculable) short-distance QCD effects from (non-perturbative) long-distance phenomena helps to obtain precise theoretical predictions.

nucl-th↗
arXiv2002

Forward-backward and isospin asymmetry for B -> K* l+ l- decay in the standard model and in supersymmetry

We discuss two dedicated observables in exclusive B -> K* l+ l- decay that can be used to study effects of physics beyond the standard model, namely the forward-backward asymmetry in the lepton spectrum and the isospin-asymmetry between decays of charged and neutral B mesons. We consider the region of large recoil-energy (i.e. small invariant mass of the lepton pair), and employ the QCD factorization approach to exclusive B meson decays. Sub-leading effects in the heavy quark mass expansion have been taken into account for the calculation of the isospin-asymmetry. We give predictions for decay asymmetries in the standard model, and its supersymmetric extension with minimal flavor violation, using parameter values allowed by current experimental constraints on B->X_s gamma decay.

hep-ph↗
arXiv2002

Heavy-to-light decays at large recoil: Systematic treatment of short- and long-distance QCD effects

Heavy quark decays into energetic, collinear quarks and gluons are discussed within an effective theory that accomplishes the factorization of soft and hard strong interaction effects. We derive the relevant effective Lagrangian, and perform the matching of the heavy quark current, including power corrections of order 1/m, where m is the heavy quark mass. We apply our framework to heavy-to-light form factors.

hep-ph↗
arXiv2001

Beyond "naive" factorization in exclusive radiative B-meson decays

We apply the QCD factorization approach to exclusive, radiative B meson decays in the region of small invariant photon mass. We calculate factorizable and non-factorizable corrections to leading order in the heavy quark mass expansion and next-to-leading order in the strong coupling constant. Phenomenological consequences for the B -> K* gamma decay rate and the B -> K* ell+ ell- forward-backward asymmetry are discussed.

hep-ph↗
arXiv1999

Quark structure of pseudoscalar mesons

I review to which extent the properties of pseudoscalar mesons can be understood in terms of the underlying quark (and eventually gluon) structure. Special emphasis is put on the progress in our understanding of eta-eta' mixing. Process-independent mixing parameters are defined, and relations between different bases and conventions are studied. Both, the low-energy description in the framework of Chiral Perturbation Theory and the high-energy application in terms of light-cone wave functions for partonic Fock states, are considered. A thorough discussion of theoretical and phenomenological consequences of the mixing approach will be given. Finally, I will discuss mixing with other states pi^0, eta(c), ...).

hep-ph↗
arXiv1998

Mixing and decay constants of pseudoscalar mesons: Octet-singlet vs. quark flavor basis

Although eta-eta' mixing is qualitatively well understood as a consequence of the U(1)_A anomaly in QCD together with a broken SU(3) flavor symmetry, until recently the values of decay and mixing parameters of the eta and eta' were only approximately known, e.g. values for the octet-singlet mixing angle between -20 degrees and -10 degrees could be found in the literature. New experimental data, especially for the reactions gamma gamma* -> eta, eta' and B -> eta' K, together with new theoretical results from higher order corrections in chiral perturbation theory stimulated a phenomenological re-analysis of this subject, which led to a coherent qualitative and quantitative picture of eta-eta' mixing and even of eta-eta'-eta_c mixing.

hep-ph↗
arXiv1998

Interpolation Formulas for the Eta-Gamma and Eta'-Gamma Transition Form Factors

The new CLEO and LEP data on the eta-gamma and eta'-gamma transition form factors have renewed the interest in simple interpolation formulas, valid at any value of momentum transfer. We are going to show that recent theoretical and phenomenological results on eta-eta' mixing lead to two-pole forms, where each pole term resembles the Brodsky/Lepage interpolation formula for the pi-gamma case and depends on the mixing and decay parameters in a simple fashion. The parameters, entering the occasionally used one-pole formulas, on the other hand, cannot be interpreted theoretically in a simple way.

hep-ph↗