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Ulrich Haisch

Publications and source records attributed to Ulrich Haisch.

At least 91 records · Page 5Linked to original sources
arXiv2013

Constraints on CP-violating Higgs couplings to the third generation

Discovering CP-violating effects in the Higgs sector would constitute an indisputable sign of physics beyond the Standard Model. We derive constraints on the CP-violating Higgs-boson couplings to top and bottom quarks as well as to tau leptons from low-energy bounds on electric dipole moments, resumming large logarithms when necessary. The present and future projections of the sensitivities and comparisons with the LHC constraints are provided. Non-trivial constraints are possible in the future, even if the Higgs boson only couples to the third-generation fermions.

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arXiv2013

New Physics in Gamma_12^s: (sbar b) (taubar tau) Operators

Measurements performed at the Tevatron of both the like-sign dimuon charge asymmetry in B_d,s-meson samples and the mixing-induced CP asymmetry in B_s -> J/psi phi depart from their standard model (SM) predictions. This could be an indication for new CP phases in Delta B = 2 transitions, preferentially in B_s-Bbar_s mixing. The experimental situation, however, remained inconclusive, as it favored values of the element Gamma_12^s of the decay matrix in the B_s-meson system that are notably different from the SM expectation, suggesting the presence of new physics in the Delta B = 1 sector as well. The very recent LHCb measurements of B_s -> J/psi phi and B_s -> J/psi f_0, which do not find any evidence for a new-physics phase in the element M_12^s of the mass matrix, point into this direction as well. In this article, we explore the potential size of non-standard effects in Gamma_12^s stemming from dimension-six operators with flavor content (sbar b) (taubar tau). We show that since the existing constraints imposed by tree- and loop-level mediated B_d,s-meson decays are quite loose, absorptive new physics of this type can in part explain the current data. Our model-independent conclusions are finally contrasted with explicit analyses of the new-physics effects in B_s-Bbar_s mixing that can arise from leptoquarks or Z' bosons.

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arXiv2013

(No) New physics in B_s mixing and decay

The status of possible new-physics signals in B_s,d-meson mixing and decay is reviewed. In particular, it is emphasized that the recent LHCb results, that find no evidence for a non-standard phase in B_s-barB_s mixing, make a consistent explanation of the D0 data on the like-sign dimuon charge asymmetry notoriously difficult. In order to clarify the inconclusive experimental situation, independent measurements of the semileptonic asymmetries are needed.

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arXiv2013

Loop corrections to Delta N_eff in large volume models

In large volume models reheating is driven by the decays of the volume modulus to the visible sector, while the decays to its axion partners result in dark radiation. In this article we discuss the impact of loop corrections on the only model-independent visible decay channel: the decay into Higgs pairs via a Giudice-Masiero term. Including such radiative effects leads to a more precise determination of the relative fraction of dark radiation, since by contrast all loop corrections to the volume axion decay mode are Planck suppressed. Assuming an MSSM spectrum and that the Giudice-Masiero coupling is fixed at the string scale by a shift symmetry in the Higgs sector, we arrive at a prediction for the effective number of neutrinos. The result turns out to be too large to be consistent with data, highly disfavouring the minimal model.

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arXiv2013

On minimal Z' explanations of the B->K*mu+mu- anomaly

Recently LHCb has announced a discrepancy of 3.7 sigma in one of the theoretically clean observables accessible through studies of angular correlations in B->K*mu+mu-. We point out that in the most minimal Z' setup that can address this anomaly there is a model-independent triple-correlation between new physics (NP) in B->K*mu+mu-, B_s-Bbar_s mixing, and non-unitarity of the quark-mixing matrix. This triple-correlation can be cast into a simple analytic formula that relates the NP contribution DeltaC9 to the Wilson coefficient of the semileptonic vector operator to a shift in the mass difference DeltaM_{B_s} and a violation of |Vud|^2+ |Vus|^2+ |Vub|^2 = 1. In contrast to the individual observables the found relation depends only logarithmically on the Z' mass. We show that that our findings allow for useful future tests of the pattern of NP suggested by the B->K*mu+mu- anomaly.

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arXiv2013

The impact of heavy-quark loops on LHC dark matter searches

If only tree-level processes are included in the analysis, LHC monojet searches give weak constraints on the dark matter-proton scattering cross section arising from the exchange of a new heavy scalar or pseudoscalar mediator with Yukawa-like couplings to quarks. In this letter we calculate the constraints on these interactions from the CMS 5.0/fb and ATLAS 4.7/fb searches for jets with missing energy including the effects of heavy-quark loops. We find that the inclusion of such contributions leads to a dramatic increase in the predicted cross section and therefore a significant improvement of the bounds from LHC searches.

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arXiv2013

On the importance of loop-induced spin-independent interactions for dark matter direct detection

The latest results from LHC searches for jets in association with missing transverse energy place strong bounds on the scattering cross section of dark matter. For the case of spin-dependent or momentum suppressed interactions these limits seem to be superior to the bounds from direct detection experiments. In this article, we show that loop contributions can significantly alter this conclusion and boost direct detection bounds, whenever they induce spin-independent interactions. This effect is most striking for tensor and pseudotensor interactions, which induce magnetic and electric dipole moments at loop level. For axialvector and anapole interactions a relevant contribution to direct detection signals arises from loop-induced Yukawa-like couplings between dark matter and quarks. We furthermore compare the resulting bounds to additional constraints on these effective operators arising from indirect searches and relic density requirements.

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arXiv2012

Loop-induced dark matter direct detection signals from gamma-ray lines

Improved limits as well as tentative claims for dark matter annihilation into gamma-ray lines have been presented recently. We study the direct detection cross section induced from dark matter annihilation into two photons in a model-independent fashion, assuming no additional couplings between dark matter and nuclei. We find a striking non-standard recoil spectrum due to different destructively interfering contributions to the dark matter nucleus scattering cross section. While in the case of s-wave annihilation the current sensitivity of direct detection experiments is insufficient to compete with indirect detection searches, for p-wave annihilation the constraints from direct searches are comparable. This will allow to test dark matter scenarios with p-wave annihilation that predict a large di-photon annihilation cross section in the next generation of experiments.

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arXiv2012

MSSM: Cornered and Correlated

Inspired by the latest results of ATLAS and CMS on the search for the standard model (SM) Higgs scalar, we discuss in this article the correlations between Higgs-boson properties, low-energy observables, such as B -> X_sgamma, B_s -> mu^+mu^-, and (g-2)_mu, and the dark matter (DM) relic density. We focus on the corners of the MSSM parameter space where the pp -> h -> gammagamma signal is enhanced due to the presence of a light stau state. In this region tan(beta), M_A, A_t, and mu take large values, and we find striking correlations between many of the considered observables. In particular, the B -> X_sgamma branching fraction is enhanced, while the B_s -> mu^+mu^- rate tends to be below the SM expectation. In contrast, the Higgs-boson couplings show good overall agreement with the preliminary experimental determinations, the DM abundance is consistent with observation, and the discrepancy in (g-2)_mu is reduced. The predicted deviations and found correlations could be tested in the near future and hence may become very valuable as guidelines and consistency checks.

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arXiv2012

Charm-Quark Contribution to K_L -> mu^+ mu^- at Next-to-Next-to-Leading

We calculate the charm-quark contribution to the decay K_L -> mu^+ mu^- in next-to-next-to-leading order of QCD. This new contribution reduces the theoretical uncertainty in the relevant parameter P_c from +/-22% down to +/-7%, corresponding to scale uncertainties of +/-3% and +/-6% in the short-distance part of the branching ratio and the determination of the Wolfenstein parameter rhobar from K_L -> mu^+ mu^-. The error in P_c = 0.115 +/- 0.018 is now in equal shares due to the combined scale uncertainties and the current uncertainty in the charm-quark mass. We find BR (K_L -> mu^+ mu^-)_SD = (0.79 +/- 0.12) x 10^-9, with the present uncertainty in the Cabibbo-Kobayashi-Maskawa matrix element V_td being the dominant individual source in the quoted error.

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arXiv2012

Higgs Production in a Warped Extra Dimension

Measurements of the Higgs-boson production cross section at the LHC are an important tool for studying electroweak symmetry breaking at the quantum level, since the main production mechanism gg-->h is loop-suppressed in the Standard Model (SM). Higgs production in extra-dimensional extensions of the SM is sensitive to the Kaluza-Klein (KK) excitations of the quarks, which can be exchanged as virtual particles in the loop. In the context of the minimal Randall-Sundrum (RS) model with bulk fields and a brane-localized Higgs sector, we derive closed analytical expressions for the gluon-gluon fusion process, finding that the effect of the infinite tower of virtual KK states can be described in terms of a simple function of the fundamental (5D) Yukawa matrices. Given a specific RS model, this will allow one to easily constrain the parameter space, once a Higgs signal has been established. We explain that discrepancies between existing calculations of Higgs production in RS models are related to the non-commutativity of two limits: taking the number of KK states to infinity and removing the regulator on the Higgs-boson profile, which is required in an intermediate step to make the relevant overlap integrals well defined. Even though the one-loop gg-->h amplitude is finite in RS scenarios with a brane-localized Higgs sector, it is important to introduce a consistent ultraviolet regulator in order to obtain the correct result.

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arXiv2011

Bounds on Warped Extra Dimensions from a Standard Model-like Higgs Boson

We point out that the discovery of a light Higgs boson in the γγ, ZZ and WW decay channels at the LHC, with cross sections not far from the predictions of the Standard Model, would have important implications for the parameters of warped extra-dimension models. Due to loop effects of Kaluza-Klein particles, these models predict a significant reduction of the Higgs production cross section via gluon-gluon fusion, combined with an enhancement of the ratio Br(h->γγ)/Br(h->ZZ). LHC measurements of these decays will probe Kaluza-Klein masses up to the 10 TeV range, exceeding by far the reach for direct production.

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arXiv2011

Complete NNLO QCD Analysis of B -> X_s l^+ l^- and Higher Order Electroweak Effects

We complete the next-to-next-to-leading order QCD calculation of the branching ratio for B -> X_s l^+ l^- including recent results for the three-loop anomalous dimension matrix and two-loop matrix elements. These new contributions modify the branching ratio in the low-q^2 region, BR_ll, by about +1% and -4%, respectively. We furthermore discuss the appropriate normalization of the electromagnetic coupling alpha and calculate the dominant higher order electroweak effects, showing that, due to accidental cancellations, they change BR_ll by only -1.5% if alpha(mu) is normalized at mu = O(m_b), while they shift it by about -8.5% if one uses a high scale normalization mu = O(M_W). The position of the zero of the forward-backward asymmetry, q_0^2, is changed by around +2%. After introducing a few additional improvements in order to reduce the theoretical error, we perform a comprehensive study of the uncertainty. We obtain BR_ll(1 GeV^2 <= q^2 <= 6 GeV^2) = (1.57 +- 0.16) x 10^-6 and q_0^2 = (3.76 +- 0.33) GeV^2 and note that the part of the uncertainty due to the b-quark mass can be easily reduced.

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arXiv2011

Massive Color-Octet Bosons: Bounds on Effects in Top-Quark Pair Production

A critical survey of the existing direct and indirect constraints on massive spin-one color octets is presented. Since such new degrees of freedom appear in any extension of the color gauge group to a product of at least two SU(3) factors, we keep our discussion as independent as possible from the underlying theory. In the framework of scenarios that involve flavor non-universal couplings, we show that excessive flavor-changing neutral currents can be avoided by a suitable alignment in flavor space. Constraints from electroweak precision observables and direct production at hadron colliders still leave space for sizable new-physics effects in top-quark pair production, in particular a large forward-backward asymmetry. In this context, we derive a model-independent upper bound on the asymmetry that applies whenever top-antitop production receives the dominant corrections from s-channel exchange of a new single color-octet resonance.

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arXiv2010

Chromomagnetic Dipole-Operator Corrections in Bbar -> X_sgamma at O(beta_0 alpha_s^2)

We calculate the fermionic corrections to the photon-energy spectrum of Bbar -> X_sgamma which arise from the self-interference of the chromomagnetic dipole operator Q_8 at next-to-next-to-leading order by applying naive non-abelianization. The resulting O(beta_0 alpha_s^2) correction to the Bbar -> X_sgamma branching ratio amounts to a relative shift of +0.12% (+0.27%) for a photon-energy cut of 1.6 GeV (1.0 GeV). We also comment on the potential size of resummation and non-perturbative effects related to Q_8.

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arXiv2010

Top-Quark Forward-Backward Asymmetry in Randall-Sundrum Models Beyond the Leading Order

We calculate the ttbar forward-backward asymmetry, A_FB^t, in Randall-Sundrum (RS) models taking into account the dominant next-to-leading order (NLO) corrections in QCD. At Born level we include the exchange of Kaluza-Klein (KK) gluons and photons, the Z boson and its KK excitations, as well as the Higgs boson, whereas beyond the leading order (LO) we consider the interference of tree-level KK-gluon exchange with one-loop QCD box diagrams and the corresponding bremsstrahlungs corrections. We find that the strong suppression of LO effects, that arises due to the elementary nature and the mostly vector-like couplings of light quarks, is lifted at NLO after paying the price of an additional factor of alpha_s/(4 pi). In spite of this enhancement, the resulting RS corrections in A_FB^t remain marginal, leaving the predicted asymmetry SM-like. As our arguments are solely based on the smallness of the axial-vector couplings of light quarks to the strong sector, our findings are model-independent and apply to many scenarios of new physics that address the flavor problem via geometrical sequestering.

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arXiv2010

The Custodial Randall-Sundrum Model: From Precision Tests to Higgs Physics

We reexamine the Randall-Sundrum (RS) model with enlarged gauge symmetry SU(2)_L x SU(2)_R x U(1)_X x P_LR in the presence of a brane-localized Higgs sector. In contrast to the existing literature, we perform the Kaluza-Klein (KK) decomposition within the mass basis, which avoids the truncation of the KK towers. Expanding the low-energy spectrum as well as the gauge couplings in powers of the Higgs vacuum expectation value, we obtain analytic formulas which allow for a deep understanding of the model-specific protection mechanisms of the T parameter and the left-handed Z-boson couplings. In particular, in the latter case we explain which contributions escape protection and identify them with the irreducible sources of P_LR symmetry breaking. We furthermore show explicitly that no protection mechanism is present in the charged-current sector confirming existing model-independent findings. The main focus of the phenomenological part of our work is a detailed discussion of Higgs-boson couplings and their impact on physics at the CERN Large Hadron Collider. For the first time, a complete one-loop calculation of all relevant Higgs-boson production and decay channels is presented, incorporating the effects stemming from the extended electroweak gauge-boson and fermion sectors.

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