Mikael Chala

Tricking Landau–Yang: How to obtain the diphoton excess from a vector resonance ☆

Abstract We show that contrary to naive expectations the recently observed diphoton excess can be explained by a vector resonance, which decays to a photon and a light scalar s , followed by a decay of the scalar into two photons: Z ′ → γ s → 3 γ . As the two photons from the scalar decay are highly boosted, the experimental signature is an apparent diphoton final state. In fact all the necessary ingredients are naturally present in Z ′ models: Additional fermions with electroweak quantum numbers are required in order to render the theory anomaly free and naturally induce the required effective couplings, while the hidden Higgs which gives mass to the Z ′ can be very light. In particular no new coloured states are required in this framework. We also show that in such a setup the width of the resonance can be rather large, while all couplings remain perturbative.

Constraining Dark Sectors with Monojets and Dijets

A bstractWe consider dark sector particles (DSPs) that obtain sizeable interactions with Standard Model fermions from a new mediator. While these particles can avoid observation in direct detection experiments, they are strongly constrained by LHC measurements. We demonstrate that there is an important complementarity between searches for DSP production and searches for the mediator itself, in particular bounds on (broad) dijet resonances. This observation is crucial not only in the case where the DSP is all of the dark matter but whenever — precisely due to its sizeable interactions with the visible sector — the DSP annihilates away so efficiently that it only forms a dark matter subcomponent. To highlight the different roles of DSP direct detection and LHC monojet and dijet searches, as well as perturbativity constraints, we first analyse the exemplary case of an axial-vector mediator and then generalise our results. We find important implications for the interpretation of LHC dark matter searches in terms of simplified models.

h→γγ excess and dark matter from composite Higgs models

A bstractComposite Higgs Models are very appealing candidates for a natural realization of electroweak symmetry breaking. Non minimal models could explain the recent Higgs data from ATLAS, CMS and Tevatron experiments, including the excess in the amount of diphoton events, as well as provide a natural dark matter candidate. In this article, we study a Composite Higgs model based on the coset SO(7)/G2. In addition to the Higgs doublet, one SU(2)L singlet of electric charge one, κ±, as well as one singlet η of the whole Standard Model group arise as pseudo-Goldstone bosons. κ± and η can be responsible of the diphoton excess and dark matter respectively.

Single vectorlike quark production at the LHC

Abstract A gluon resonance G of mass below 1 TeV could be the origin of the t t ¯ forward–backward asymmetry observed at the Tevatron provided that new decay modes G → q Q ¯ , with q a standard quark and Q its massive excitation, make G broad enough. We consider all the different cases, with q the top, the bottom or a light quark and dominant decay modes Q → W q ′ or Q → Z q . We show that current experimental searches are unable to probe the model, but that minimal departures from these analyses can explore a large region of its parameter space for the current LHC luminosity. This includes the challenging case with the new quarks decaying mostly into light quark flavors. In some channels not only the heavy quark but also the massive gluon can be reconstructed, which would stablish the origin of the t t ¯ asymmetry. Similar analyses can be applied to more general models with new massive gluons and vectorlike quarks.

Renormalization group constraints on new top interactions from electroweak precision data

A bstractAnomalous interactions involving the top quark contribute to some of the most difficult observables to directly access experimentally. They can give however a sizeable correction to very precisely measured observables at the loop level. Using a model-independent effective Lagrangian approach, we present the leading indirect constraints on dimension-six effective operators involving the top quark from electroweak precision data. They represent the most stringent constraints on these interactions, some of which may be directly testable in future colliders.

Observable effects of general new scalar particles

A bstractWe classify all possible new scalar particles that can have renormalizable linear couplings to Standard Model fields and therefore be singly produced at colliders. We show that this classification exhausts the list of heavy scalar particles that contribute at the tree level to the Standard Model effective Lagrangian to dimension six. We compute this effective Lagrangian for a general scenario with an arbitrary number of new scalar particles and obtain flavor-preserving constraints on their couplings and masses. This completes the tree-level matching of the coefficients of dimension five and six operators in the effective Lagrangian to arbitrary extensions of the Standard Model.

Unified explanation for dark matter and electroweak baryogenesis with direct detection and gravitational wave signatures

A minimal extension of the Standard Model that provides both a dark matter candidate and a strong first-order electroweak phase transition (EWPT) consists of two additional Lorentz and gauge singlets. In this paper we work out a composite Higgs version of this scenario, based on the coset SO(7)/SO(6). We show that by embedding the elementary fermions in appropriate representations of SO(7), all dominant interactions are described by only three free effective parameters. Within the model dependencies of the embedding, the theory predicts one of the singlets to be stable and responsible for the observed dark matter abundance. At the same time, the second singlet introduces new CP-violation phases and triggers a strong first-order EWPT, making electroweak baryogenesis feasible. It turns out that this scenario does not conflict with current observations and it is promising for solving the dark matter and baryon asymmetry puzzles. The tight predictions of the model will be accessible at the forthcoming dark matter direct detection and gravitational wave experiments.

Global constraints on lepton-quark contact interactions

The Large Hadron Collider can do precision physics at a level that is competitive with electroweak precision constraints when probing physics beyond the Standard Model. We present a simple yet general parameterization of the effect of an arbitrary number of lepton-quark contact interactions on any di-lepton observable at hadron colliders. This parameterization can be easily adopted by the experimental collaborations to put bounds on arbitrary combinations of lepton-quark contact interactions. We compute the corresponding bounds from current di-lepton resonance searches at the LHC and find that they are competitive with and often complementary to indirect constraints from electroweak precision data. We combine all current constraints in a global analysis to obtain the most stringent bounds on lepton-quark contact interactions. We also show that the high-energy phase of the LHC has a unique potential in terms of discovery and discrimination power among different types of lepton-quark contact interactions.

LHC bounds on Lepton Number Violation mediated by doubly and singly-charged scalars

A bstractThe only possible doubly-charged scalar decays into two Standard Model particles are into pairs of same-sign charged leptons, H±± → l±l±, l = e, μ, τ, or gauge bosons, H±± → W±W±; being necessary the observation of both to assert the violation of lepton number. However, present ATLAS and CMS limits on doubly-charged scalar production are obtained under specific assumptions on its branching fractions into dileptons only. Although they can be extended to include decays into dibosons and lepton number violating processes. Moreover, the production rates also depend on the type of electroweak multiplet H±± belongs to. We classify the possible alternatives and provide the Feynman rules and codes for generating the corresponding signals for pair and associated doubly-charged scalar production, including the leading contribution from the s-channel exchange of electroweak gauge bosons as well as the vector-boson fusion corrections. Then, using the same analysis criteria as the LHC collaborations we estimate the limits on the H±± mass as a function of the electroweak multiplet it belongs to, and obtain the bounds on the lepton number violating processes pp → H±±H∓∓ → ℓ±ℓ±W∓W∓ and pp → H±±H∓ → ℓ±ℓ±W∓Z, ℓ = e, μ, implied by the ATLAS and CMS doubly-charged scalar searches.

New Higgs Production Mechanism in Composite Higgs Models

A bstractComposite Higgs models are only now starting to be probed at the Large Hadron Collider by Higgs searches. We point out that new resonances, abundant in these models, can mediate new production mechanisms for the composite Higgs. The new channels involve the exchange of a massive color octet and single production of new fermion resonances with subsequent decays into the Higgs and a Standard Model quark. The sizable cross section and very distinctive kinematics allow for a very clean extraction of the signal over the background with high statistical significance. Heavy gluon masses up to 2.8 TeV can be probed with data collected during 2012 and up to 5 TeV after the energy upgrade to