arXiv subjects

Lars Hofer

Publications and source records attributed to Lars Hofer.

32 records · Page 2Linked to original sources

arXiv2014

COLLIER -- A fortran-library for one-loop integrals

We introduce the fortran-library COLLIER for the numerical evaluation of one-loop scalar and tensor integrals in perturbative relativistic quantum field theories. Important features are the implementation of dedicated methods to achieve numerical stability for 3- and 4-point tensor integrals, the support of complex masses for internal particles, and the possibility to choose between dimensional and mass regularization for infrared singularities. COLLIER supports one-loop N-point functions up to currently N=6 and has been tested in various NLO QCD and EW calculations.

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arXiv2013

EW and QCD One-Loop Amplitudes with RECOLA

We present the computer code RECOLA for the computation of EW and QCD amplitudes in the Standard Model at next-to-leading order. One-loop amplitudes are represented as linear combinations of tensor integrals whose coefficients are calculated by means of recursive relations similar to Dyson-Schwinger equations. A novel treatment of colour enables us to recursively construct the colour structure of the amplitude efficiently. RECOLA is linked with the library COLLIER for the computation of the tensor integrals.

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arXiv2013

Electroweak corrections to Z + 2 jets production at the LHC

We present results for the electroweak radiative corrections to the production of a leptonically decaying Z boson in association with two jets at the LHC. Tree-level and one-loop amplitudes have been obtained with the computer code Recola for the recursive generation of tree-level and one-loop amplitudes in the Standard Model. The one-loop integrals have been calculated with the tensor-integral library Collier. The electroweak corrections turn out to be small for inclusive cross sections but can become large in tails of distributions.

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arXiv2013

Complete resummation of chirally-enhanced loop-effects in the MSSM with non-minimal sources of flavor-violation

In this article we present the complete resummation of the leading chirally-enhanced corrections stemming from gluino-squark, chargino-sfermion and neutralino-sfermion loops in the MSSM with non-minimal sources of flavor-violation. We compute the finite renormalization of fermion masses and the CKM matrix induced by chirality-flipping self-energies. In the decoupling limit Msusy>>v, which is an excellent approximation to the full theory, we give analytic results for the effective gaugino(higgsino)-fermion-sfermion and the Higgs-fermion-fermion vertices. Using these vertices as effective Feynman rules, all leading chirally-enhanced corrections can consistently be included into perturbative calculations of Feynman amplitudes. We also give a generalized parametrization for the bare CKM matrix which extends the classic Wolfenstein parametrization to the case of complex parameters lambda and A.

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arXiv2013

Status of the B -> pi K puzzle and its relation to B_s -> phi pi and B_s -> phi rho decays

Some discrepancies between theory and experiment in the B -> pi K decays suggest the possibility of a new physics contribution with the structure of an electroweak penguin amplitude. If such a scenario is realised in nature, the branching ratios of the decays B_s -> phi pi and B_s -> phi rho can be enhanced by about one order of magnitude. We review and update the current status of the B -> pi K puzzle and its implications for the decays B_s -> phi pi and B_s -> phi rho.

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arXiv2013

Recursive generation of one-loop amplitudes in the Standard Model

We introduce the computer code Recola for the recursive generation of tree-level and one-loop amplitudes in the Standard Model. Tree-level amplitudes are constructed using off-shell currents instead of Feynman diagrams as basic building blocks. One-loop amplitudes are represented as linear combinations of tensor integrals whose coefficients are calculated similarly to the tree-level amplitudes by recursive construction of loop off-shell currents. We introduce a novel algorithm for the treatment of colour, assigning a colour structure to each off-shell current which enables us to recursively construct the colour structure of the amplitude efficiently. Recola is interfaced with a tensor-integral library and provides complete one-loop Standard Model amplitudes including rational terms and counterterms. As a first application we consider Z+2jets production at the LHC and calculate with Recola the next-to-leading-order electroweak corrections to the dominant partonic channels.

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arXiv2011

Phenomenological consequences of radiative flavor violation in the MSSM

In this article we investigate the consequences of radiative flavor violation (RFV) in the Minimal Supersymmetric Standard Model (MSSM). In this framework the small off-diagonal elements of the Cabibbo-Kobayashi-Maskawa (CKM) matrix and the small quark masses of the first two generations are generated from the trilinear supersymmetry-breaking terms. The impact of RFV on flavor-physics observables is studied in detail. We focus on the limiting cases in which the CKM matrix is either generated in the down-sector, i.e. by the soft SUSY-breaking mass insertions delta^dLR_i3 (i=1,2), or in the up-quark sector, i.e. by the mass insertions delta^uLR_i3. In the first case we find an enhancement of b->s gamma, which constrains the allowed range of sparticle masses (Fig. 3). In addition, neutral Higgs penguins significantly contribute to B_s,d->mu^+mu^- and, if also delta^dLR_32 is different from zero, these Higgs effects are capable of explaining the observed CP phase in the B_s system. If, on the other hand, the CKM generation takes place in the up-sector, |epsilon_K| receives additional positive contributions enforcing large squark and gluino masses (see Fig. 8). In this case also the rare decay K->pi nu nu receives sizable contributions. In conclusion we find that for SUSY masses around 1 TeV RFV is an interesting alternative to Minimal Flavor Violation (MFV).

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arXiv2011

The MSSM with a softly broken U(2)^3 flavor symmetry

In this article we review the phenomenological consequences of radiative flavor-violation (RFV) in the MSSM. In the model under consideration the U(3)^3 flavor symmetry of the gauge sector is broken in a first step to U(2)^3 by the top and bottom Yukawa couplings of the superpotential (and possibly also by the bilinear SUSY-breaking terms). In a second step the remaining U(2)^3 flavor symmetry is softly broken by the trilinear A-terms in order to obtain the measured quark masses and the CKM matrix of the Standard Model (SM) at low energies. The phenomenological implications of this model depend on the actual choice of the SUSY breaking A-terms. If the CKM matrix is generated in the down sector (by A^d), Bs->mu^+mu^- receives non-decoupling contributions from Higgs penguins which become important already for moderate values of tan(beta). Also the Bs mixing amplitude can be significantly modified compared to the SM prediction including a potential induction of a new CP-violating phase (which is not possible in the MSSM with MFV).

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arXiv2011

Probing new physics in electroweak penguins through B_d and B_s decays

An enhanced electroweak penguin amplitude due to the presence of unknown new physics can explain the discrepancies found between theory and experiment in the B -> pi K decays, in particular in A_CP(B^- -> pi^0 K^-) - A_CP(\bar{B}^0 -> pi^+ K^-), but the current precision of the theoretical and experimental results does not allow to draw a firm conclusion. We argue that the \bar{B}_s -> phi rho^0 and \bar{B}_s -> phi pi^0 decays offer an additional tool to investigate this possibility. These purely isospin-violating decays are dominated by electroweak penguins and we show that in presence of a new physics contribution their branching ratio can be enhanced by about an order of magnitude, without violating any constraints from other hadronic B decays. This makes them very interesting modes for LHCb and future B factories. In arXiv:1011.6319 we have performed both a model-independent analysis and a study within realistic New Physics models such as a modified-Z^0-penguin scenario, a model with an additional Z' boson and the MSSM. In this article we summarise the most important results of our study.

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arXiv2011

B_s -> phi rho^0 and B_s -> phi pi^0 as a handle on isospin-violating New Physics

The 2.5 sigma discrepancy between theory and experiment observed in the difference A_CP(B^- --> pi^0 K^-)-A_CP(Bbar^0 --> pi^+ K^-) can be explained by a new electroweak penguin amplitude. Motivated by this result, we analyse the purely isospin-violating decays B_s --> phi rho^0 and B_s --> phi pi^0, which are dominated by electroweak penguins, and show that in presence of a new electroweak penguin amplitude their branching ratio can be enhanced by up to an order of magnitude, without violating any constraints from other hadronic B decays. This makes them very interesting modes for LHCb and future B factories. We perform both a model-independent analysis and a study within realistic New Physics models such as a modified-Z^0-penguin scenario, a model with an additional Z' boson and the MSSM. In the latter cases the new amplitude can be correlated with other flavour phenomena, such as semileptonic B decays and B_s-Bbar_s mixing, which impose stringent constraints on the enhancement of the two B_s decays. In particular we find that, contrary to claims in the literature, electroweak penguins in the MSSM can reduce the discrepancy in the B --> pi K modes only marginally. As byproducts we update the SM predictions to Br(Bbar_s --> phi pi^0)=1.6^{+1.1}_{-0.3}*10^{-7} and Br(Bbar_s --> phi rho^0)=4.4^{+2.7}_{-0.7}*10^{-7} and perform a state-of-the-art analysis of B --> pi K amplitudes in QCD factorisation.

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arXiv2010

Electroweak penguins in isospin-violating B_s decays

The 2.5 sigma discrepancy between theory and experiment observed in the difference Delta ACP = ACP(B- --> pi0 K-) - ACP(Bb0 --> pi+ K-) can be explained by a new electroweak (EW) penguin amplitude. Motivated by this result, we have analyzed the purely isospin-violating decays B_s --> phi pi0 and B_s phi rho0, which are dominated by EW penguins. Our results extend the analysis in arXiv:0910.2809 and have recently been published in arXiv:1011.6319. Here we give a brief overview of the outcome.

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arXiv2010

Resummation of tan-beta-enhanced supersymmetric loop corrections beyond the decoupling limit

We study the Minimal Supersymmetric Standard Model with Minimal Flavour Violation for the case of a large parameter tanbeta and arbitrary values of the supersymmetric mass parameters. We derive several resummation formulae for tanbeta-enhanced loop corrections, which were previously only known in the limit of supersymmetric masses far above the electroweak scale. Studying first the renormalisation-scheme dependence of the resummation formula for the bottom Yukawa coupling, we clarify the use of the sbottom mixing angle in the supersymmetric loop factor Delta_b. As a new feature, we find tan-beta-enhanced loop-induced flavour-changing neutral current (FCNC) couplings of gluinos and neutralinos which in turn give rise to new effects in the renormalisation of the Cabibbo-Kobayashi-Maskawa matrix and in FCNC processes of B mesons. For the chromomagnetic Wilson coefficient C_8, these gluino-squark loops can be of the same size as the known chargino-squark contribution. We discuss the phenomenological consequences for the mixing-induced CP asymmetry in B_d -> phi K_S. We further quote formulae for B_s -> mu^+ mu^- and B_s-B_s-bar mixing valid beyond the decoupling limit and find a new contribution affecting the phase of the B_s-B_s-bar mixing amplitude. Our resummed tan-beta-enhanced effects are cast into Feynman rules permitting an easy implementation in automatic calculations.

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arXiv2009

The MSSM with large tan(beta) beyond the decoupling limit

For large values of tan(beta) interesting effects arise in the MSSM due to the enhancement of down-quark self-energies. These effects are well-studied within the decoupling limit, i.e. in the limit of supersymmetric masses far above the electroweak scale. In this article I discuss those issues which emerge from a treatment of these effects that goes beyond the decoupling limit.

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arXiv2009

Gluino-mediated FCNCs in the MSSM with large tan(beta)

We present results of our study of tan(beta)-enhanced loop corrections in the Minimal Supersymmetric Standard Model (MSSM) with Minimal Flavour Violation (MFV). We demonstrate that these corrections induce flavour changing neutral current (FCNC) gluino couplings which have a large impact on the Wilson coefficient C_8 of the chromomagnetic operator. To illustrate the phenomenological consequences of this gluino-squark contribution to C_8, we briefly discuss its effect on the mixing-induced CP asymmetry in the decay B_d -> phi K_S.

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