Thomas Flacke

Singlets in composite Higgs models in light of the LHC 750 GeV diphoton excess

Models of compositeness can successfully address the origin of the Higgs boson, as a pseudo-Nambu-Goldstone boson (pNGB) of a spontaneously broken global symmetry, and flavor physics via the partial compositeness mechanism. If the dynamics is generated by a confining gauge group with fermionic matter content, there exists only a finite set of models that have the correct properties to account for the Higgs and top partners at the same time. In this paper, we explore the theory space of this class of models; remarkably, all of them contain—beyond the pNGB Higgs—a pNGB singlet, a, which couples to Standard Model gauge bosons via Wess-Zumino-Witten interactions, thus providing naturally a resonance in the diboson at the LHC. With the assumption that the recently reported diphoton excess at 750 GeV at the LHC arises from the a resonance, we propose a generic approach on how to delineate the best candidate for composite Higgs models with top partners. We find that constraints from other diboson searches severely reduce the theory space of the models under consideration. For the models which can explain the diphoton excess, we make precise and testable predictions for the width and other diboson resonance searches.

Phenomenology of relaxion-Higgs mixing

A bstractWe show that the relaxion generically stops its rolling at a point that breaks CP leading to relaxion-Higgs mixing. This opens the door to a variety of observational probes since the possible relaxion mass spans a broad range from sub-eV to the GeV scale. We derive constraints from current experiments (fifth force, astrophysical and cosmological probes, beam dump, flavour, LEP and LHC) and present projections from future experiments such as NA62, SHiP and PIXIE. We find that a large region of the parameter space is already under the experimental scrutiny. All the experimental constraints we derive are equally applicable for general Higgs portal models. In addition, we show that simple multiaxion (clockwork) UV completions suffer from a mild fine tuning problem, which increases with the number of sites. These results favour a cut-off scale lower than the existing theoretical bounds.

Composite scalars at the LHC: the Higgs, the Sextet and the Octet

A bstractWe present a phenomenological theory of scalar particles that transform as a sextet and an octet of QCD interactions. These particles may arise as light bound states of a fundamental dynamics giving rise to a composite Higgs boson and partial compositeness for the top. As a concrete example, we discuss an explicit UV completion based on the SU(4)/Sp(4) coset, where QCD colour is carried by additional fundamental fermions charged under the confining gauge group. Top partners, as well as potentially even lighter coloured scalars, arise as bound states of the coloured fermions. We study production and detection at LHC Run I and II of the octet and sextet, setting lower limits on masses and couplings to Standard Model particles using existing 8 TeV analyses. We finally explore prospects for the ongoing 13 TeV Run II: we focus on final states with two same sign leptons, that have the potential to discriminate the sextet.

Search strategies for TeV scale fermionic top partners with charge 2/3

A bstractSearches for fermionic top partners at the TeV scale will bring forward a new final state kinematic regime and event topologies, where Run I search strategies will inevitably fail. We propose concrete search strategies for singly produced charge 2/3 fermionic top partners (T′) adequate for LHC Run II. Our analysis spans over all of the T′ decay modes (i.e. tZ, th and Wb) where we present detailed discussion of the search performances, signal efficiencies and backgrounds rates. Our LHC Run II search proposals utilize signatures with large missing energy and leptons, as well as jet substructure observables for tagging of boosted heavy SM states, customized b-tagging tactics and forward jet tagging. We analyze the prospects for discovery and exclusion of T′ models within the framework of partially composite quarks at the LHC Run II. Our results show that the LHC Run II has good prospects for observing T′ models which predict single production cross section of σT′ ∼ 70 − 140 (30 − 65) fb for MT′ = 1 (1.5) TeV respectively with 100 fb−1 of integrated luminosity, depending on the branching ratios of the T′. Similarly, we find that cross sections of σT′ ∼ 27 − 60 (13 − 24) fb for MT′ = 1 (1.5) TeV respectively can be excluded with the same amount of data. Our results are minimally model dependent and can be applied to most T′ models where ΓT′ ≪ MT′ .

Di-boson signatures as standard candles for partial compositeness

A bstractComposite Higgs Models are often constructed including fermionic top partners with a mass around the TeV scale, with the top partners playing the role of stabilizing the Higgs potential and enforcing partial compositeness for the top quark. A class of models of this kind can be formulated in terms of fermionic strongly coupled gauge theories. A common feature they all share is the presence of specific additional scalar resonances, namely two neutral singlets and a colored octet, described by a simple effective Lagrangian. We study the phenomenology of these scalars, both in a model independent and model dependent way, including the bounds from all the available searches in the relevant channels with di-boson and di-top final states. We develop a generic framework which can be used to constrain any model containing pseudo-scalar singlets or octets. Using it, we find that such signatures provide strong bounds on the compositeness scale complementary to the traditional EWPT and Higgs couplings deviations. In many cases a relatively light scalar can be on the verge of discovery as a first sign of new physics.

Constraints on split universal extra dimensions from electroweak precision tests

We present strongly improved electroweak precision constraints on the split-UED model. We find that the dominating effect arises from contributions to the muon decay rate by the exchange of even-numbered W-boson Kaluza-Klein modes at tree-level, which so far have not been discussed in the context of UED models. The constraints on the split-UED parameter space are translated into bounds on the mass difference of the first Kaluza-Klein mode of fermions and the lightest Kaluza-Klein mode, which will be tested is the LHC.

Constraints on universal extra dimensions from W ′ searches

Thomas Flacke, 2 Arjun Menon, and Zack Sullivan Institut für Theoretische Physik und Astrophysik, Universität Würzburg, D-97074 Würzburg, Germany Department of Physics, Korea Advanced Institute of Science and Technology, 335 Gwahak-ro, Yuseong-gu, Daejeon 305-701, Korea Department of Physics and Astronomy, University of Oregon, Eugene, Oregon 97405, USA Department of Physics, Illinois Institute of Technology, Chicago, Illinois 60616-3793, USA (Dated: July 16, 2012)

Improved bounds on universal extra dimensions and consequences for Kaluza-Klein dark matter

We study constraints on models with a flat universal extra dimension in which all standard model fields propagate in the bulk. A significantly improved constraint on the compactification scale is obtained from the extended set of electroweak precision observables accurately measured at LEP1 and LEP2. We find a lower bound of M{sub c}{identical_to}R{sup -1}>700(800) GeV at the 99% (95%) confidence level. We also discuss the implications of this constraint on the prospects for the direct and indirect detection of Kaluza-Klein dark matter in this model.

Aspects of axion phenomenology in a slice of AdS5

Motivated by multi-throat considerations, we study the phenomenological implications of a bulk axion in a slice of AdS5 with a large extra dimension: k ~ 10−2 eV, kR(1). In particular, we compare axion physics with a warped geometry to axions in flat compactifications. As in flat compactification scenarios, we find that the mass of the axion can become independent from the underlying Peccei-Quinn scale. Surprisingly, we find that in warped extra dimensions the axions invisibility, cosmological viability, and basic phenomenology remain essentially unaltered in comparison to axions in flat compactifications.

Probing vector-like quark models with Higgs-boson pair production

A bstractWe investigate Higgs-boson pair production at the LHC when the final state system arises from decays of vector-like quarks coupling to the Higgs boson and the Standard Model quarks. Our phenomenological study includes next-to-leading-order QCD corrections, which are important to guarantee accurate predictions, and focuses on a detailed analysis of a di-Higgs signal in the four b-jet channel. Whereas existing Run II CMS and ATLAS analyses are not specifically designed for probing non-resonant, vector-like-quark induced, di-Higgs production, we show that they nevertheless offer some potential for these modes. We then investigate the possibility of distinguishing between the various di-Higgs production mechanisms by exploiting the kinematic properties of the signal.