Andreas Weiler

The flavor of the composite pseudo-goldstone Higgs

We study the flavor structure of 5D warped models that provide a dual description of a composite pseudo-Goldstone Higgs. We first carefully re-examine the flavor constraints on the mass scale of new physics in the standard Randall-Sundrum-type scenarios, and find that the KK gluon mass should generically be heavier than about 21 TeV. We then compare the flavor structure of the composite Higgs models to those in the RS model. We find new contributions to flavor violation, which while still are suppressed by the RS-GIM mechanism, will enhance the amplitudes of flavor violations. In particular, there is a kinetic mixing term among the SM fields which (although parametrically not enhanced) will make the flavor bounds even more stringent than in RS. This together with the fact that in the pseudo-Goldstone scenario Yukawa couplings are set by a gauge coupling implies the KK gluon mass to be at least about 33 TeV. For both the RS and the composite Higgs models the flavor bounds could be stronger or weaker depending on the assumption on the value of the gluon boundary kinetic term. These strong bounds seem to imply that the fully anarchic approach to flavor in warped extra dimensions is implausible, and there have to be at least some partial flavor symmetries appearing that eliminate part of the sources for flavor violation. We also present complete expressions for the radiatively generated Higgs potential of various 5D implementations of the composite Higgs model, and comment on the 1–5 percent level tuning needed in the top sector to achieve a phenomenologically acceptable vacuum state.

ΔF = 2 observables and fine-tuning in a warped extra dimension with custodial protection

We present a complete study of ΔS = 2 and ΔB = 2 processes in a warped extra dimensional model with a custodial protection of ZbLL, including eK, ΔMK, ΔMs, ΔMd, ASLq, ΔΓq, ACP(Bd→ψKS) and ACP(Bs→ψ). These processes are affected by tree level contributions from Kaluza-Klein gluons, the heavy KK photon, new heavy electroweak gauge bosons ZH and Z, and in principle by tree level Z contributions. We confirm recent findings that the fully anarchic approach where all the hierarchies in quark masses and weak mixing angles are geometrically explained seems implausible and we confirm that the KK mass scale MKK generically has to be at least ~ 20 TeV to satisfy the eK constraint. We point out, however, that there exist regions in parameter space with only modest fine-tuning in the 5D Yukawa couplings which satisfy all existing ΔF = 2 and electroweak precision constraints for scales MKK 3 TeV in reach of the LHC. Simultaneously we find that ACP(Bs → ψ) and AsSL can be much larger than in the SM as indicated by recent results from CDF and DO data. We point out that for Bd,s physics ΔF = 2 observables the complex (ZH,Z) can compete with KK gluons, while the tree level Z and KK photon contributions are very small. In particular we point out that the ZdiLjL couplings are protected by the custodial symmetry. As a by-product we show the relation of the RS flavour model to the Froggatt-Nielsen mechanism and we provide analytic formulae for the effective flavour mixing matrices in terms of the fundamental 5D parameters.

A GIM mechanism from extra dimensions

We explore how to protect extra dimensional models from large avor changing neutral currents by using bulk and brane avor symmetries. We show that a GIM mechanism can be built in to warped space models such as Randall-Sundrum or composite Higgs models if avor mixing is introduced via UV brane kinetic mixings for right handed quarks. We give a realistic implementation both for a model with minimal avor violation and one with next-to-minimal avor violation. The latter does not suer from a CP problem. We consider some of the existing experimental constraints on these models implied by precision electroweak tests.

Simple flavor protection for the Randall-Sundrum model

We present a simple variation of warped flavor models where the hierarchies of fermion masses and mixings are still explained but dangerous flavor violating effects in the Kaon sector are greatly reduced. The key new ingredients are two horizontal U(1) symmetries. These symmetries forbid flavor violation in the down quark sector (with the exception of small IR brane localized kinetic mixing terms for the left-handed quarks) while allowing for flavor violation in the up quark sector. The leading flavor constraints come from D − D̄ mixing, and are safely satisfied for the KK mass scale of order 3 TeV. Our analysis of the flavor constraints also includes the constraints due to the (usually ignored) localized kinetic mixing terms. We also comment on the effects of the additional U(1) gauge bosons. The Randall-Sundrum (RS) model [1] provides an interesting framework for new physics beyond the Standard Model (SM). Not only does it address the electroweak hierarchy problem because the Higgs sector localized on the IR brane has effectively a TeV cut-off scale, but it also offers an explanation of the SM fermion mass hierarchies via localized profiles of fermions in the extra dimension [2, 3]. As SM fermions get their masses from Yukawa interactions with the Higgs on the IR brane, one can localize the light fermions close to the UV brane to make their effective Yukawa couplings hierarchically small. The simplest “anarchic approach” assumes that the Yukawa couplings in the 5D theory are all O(1) in natural units. Then the localization of the SM fermions leads to the effective 4D mass matrices which naturally incorporate hierarchies both in the mass eigenvalues and the CKM mixing angles [4]. Theories of flavor quite generally predict new sources of flavor violation. In the RS model, flavor changing neutral currents (FCNC’s) arise already at the tree level, due to flavor violating couplings of the Kaluza-Klein (KK) modes to the quark mass eigenstates. These same KK modes play the important role of stabilizing the electroweak scale, and naturalness implies that the lightest KK mode should not be heavier than ∼ 3 TeV. At first sight this spells disaster for the model, since tight experimental bounds on FCNCs would normally imply a much higher KK scale, MKK > 10 –10 TeV. However, the RS model has a built-in flavor protection usually referred to as RS-GIM [3,5]: the localization mechanism responsible for the mass hierarchies also suppresses FCNC’s. RS-GIM is powerful enough to suppress below the experimental sensitivity almost all effective ∆F = 2 four-fermion operators generated by the exchange of KK modes. The exception is the left-right (LR) operator O4 = (s̄LdR)(s̄RdL) whose imaginary part is tightly bounded by measurements of CP violation in the kaon sector. It turns out that the coefficient of this operator comes out roughly a factor of 100 too large for a ∼ 3 TeV KK gluon [6], if one assumes no tuning among the input parameters. Thus, some form of flavor symmetry is required if we insist that the RS model addresses both the electroweak naturalness problem and the flavor hierarchies. One possible approach is to require a full-fledged 5D GIM mechanism, where all tree-level FCNC’s are completely absent. This is indeed possible [7] using U(3) flavor symmetries in the bulk, and assuming that the only source of flavor violation are fermion kinetic terms localized on the UV brane. The price is, however, that the hierarchy of fermion masses and mixings is left completely unexplained. One would prefer a solution that incorporates just enough flavor symmetries to eliminate the dangerous sources of flavor violation, while still allowing for the generation of flavor hierarchies. An attempt in this direction is the “5D MFV” proposal of [8], which postulates that only two spurions are responsible for breaking the flavor symmetry, generating both the bulk mass matrices and the brane Yukawa matrices. This assumption does not suppress FCNCs by itself and an additional alignment of bulk masses and brane Yukawa terms has to be imposed [8, 9]. Recently, an economical model based on a U(3) symmetry acting in the bulk and broken on the IR brane was proposed in [10]. This symmetry, while allowing for down quark mixing via Yukawa couplings on the IR brane, enforces a degeneracy of the bulk masses in the down right quark sector which ensures universal couplings to vector KK modes. Flavor symmetries have also proved successful in warped models of the lepton sector, where they can explain the observed neutrino mixing

Light Nondegenerate Squarks at the LHC

Experimental bounds on squarks of the first two generations assume their masses to be eightfold degenerate and consequently constrain them to be heavier than ∼1.4u2009u2009TeV when the gluino is lighter than 2.5xa0TeV. The assumption of squark-mass universality is neither a direct consequence of minimal flavor violation (MFV), which allows for splittings within squark generations, nor a prediction of supersymmetric alignment models, which allow for splittings between generations. We reinterpret a recent CMS multijet plus missing energy search allowing for deviations from U(2) universality and find significantly weakened squark bounds: A 400xa0GeV second-generation squark singlet is allowed, even with exclusive decays to a massless neutralino, and, in an MFV scenario, the down-type squark singlets can be as light as 600xa0GeV, provided the up-type singlets are pushed up to 1.8xa0TeV, for a 1.5xa0TeV gluino and decoupled doublet squarks.

Constraining the dipole moments of the top quark

DESY, Notkestrasse 85, D-22603 Hamburg, Germany(Dated: July 18, 2011)We investigate the direct and indirect bounds on dipole operators involving the top quark. Acareful analysis shows that the experimental upper limit on the neutron electric dipole momentstrongly constrains the chromo-electric dipole of the top. We improve previous bounds by twoorders of magnitude. This has signi cant implications for new physics models and it also meansthat CP violation in top pair production mediated by dipole operators will not be accessible at theLHC. The CP conserving chromo-magnetic dipole moments are constrained by recent measurementsof the tx16tspectrum by the ATLAS collaboration. We also update the indirect constraints on electricand magnetic dipole moments from radiative b! stransitions, nding that they can be considerablylarger than their colored counterparts.Introduction

Particle-Antiparticle Mixing, epsilon(K), Delta Gamma(q), A**q(SL), A(CP) (B(d) ---> psi K(S)), A(CP) (B(s) ---> psi phi) and B ---> X(s,d gamma) in the Littlest Higgs Model with T-Parity

We calculate a number of observables related to particle-antiparticle mixing in the Littlest Higgs model with T-parity (LHT). The resulting effective Hamiltonian for Delta F=2 transitions agrees with the one of Hubisz et al., but our phenomenological analysis goes far beyond the one of these authors. In particular, we point out that the presence of mirror fermions with new flavour and CP-violating interactions allows to remove the possible Standard Model (SM) discrepancy between the CP asymmetry S_{psi K_S} and large values of |V_ub| and to obtain for the mass difference Delta M_s X_s gamma decay and A_CP(B -> X_s gamma) that are presented in the LHT model here for the first time. In another scenario the second, non-SM, value for the angle gamma=-(109+-6) from tree level decays, although unlikely, can be made consistent with all existing data with the help of mirror fermions. We present a number of correlations between the observables in question and study the implications of our results for the mass spectrum and the weak mixing matrix of mirror fermions. In the most interesting scenarios, the latter one turns out to have a hierarchical structure that differs significantly from the CKM one.

Polarized tops from stop decays at the LHC

In supersymmetric models, scalar top quarks, or stops, generically have parity-violating couplings to top quarks. As a result, tops produced in stop decays should be polarized. In this paper, we will argue that this effect may be observable at the LHC with realistic integrated luminosities, provided that one of the stops is copiously produced and can decay to a top and a neutralino. We define the ``effective stop mixing angle, which determines the degree of top polarization, and discuss the prospects for a measurement of this angle at the LHC. If some information about the neutralino mixing matrix is available, this measurement can be used to constrain the mixing angle in the stop sector, one of the most important ingredients in assessing the naturalness of electroweak symmetry breaking in the MSSM.

Bound on minimal universal extra dimensions from B ¯ → X s γ

We reexamine the constraints on universal extra dimensional models arising from the inclusive radiative Bbar -> X_s gamma decay. We take into account the leading order contributions due to the exchange of Kaluza-Klein modes as well as the available next-to-next-to-leading order corrections to the Bbar -> X_s gamma branching ratio in the standard model. For the case of one large flat universal extra dimension, we obtain a lower bound on the inverse compactification radius 1/R > 600 GeV at 95% confidence level that is independent of the Higgs mass.

Strong signatures of right-handed compositeness

A bstractRight-handed light quarks could be significantly composite, yet compatible with experimental searches at the LHC and precision tests on Standard Model couplings. In these scenarios, that are motivated by flavor physics, one expects large cross sections for the production of new resonances coupled to light quarks. We study experimental strong signatures of right-handed compositeness at the LHC, and constrain the parameter space of these models with recent results by ATLAS and CMS. We show that the LHC sensitivity could be significantly improved if dedicated searches were performed, in particular in multijet signals.