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Adam Falkowski

Publications and source records attributed to Adam Falkowski.

At least 55 records · Page 3Linked to original sources
arXiv2016

Implications of the 750 GeV gamma-gamma Resonance as a Case Study for the International Linear Collider

If the gamma-gamma resonance at 750 GeV suggested by 2015 LHC data turns out to be a real effect, what are the implications for the physics case and upgrade path of the International Linear Collider? Whether or not the resonance is confirmed, this question provides an interesting case study testing the robustness of the ILC physics case. In this note, we address this question with two points: (1) Almost all models proposed for the new 750 GeV particle require additional new particles with electroweak couplings. The key elements of the 500 GeV ILC physics program---precision measurements of the Higgs boson, the top quark, and 4-fermion interactions---will powerfully discriminate among these models. This information will be important in conjunction with new LHC data, or alone, if the new particles accompanying the 750 GeV resonance are beyond the mass reach of the LHC. (2) Over a longer term, the energy upgrade of the ILC to 1 TeV already discussed in the ILC TDR will enable experiments in gamma-gamma and e+e- collisions to directly produce and study the 750 GeV particle from these unique initial states.

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arXiv2016

Phenomenology of a 750 GeV Singlet

We study the recently reported excess in the diphoton resonance search by ATLAS and CMS. We investigate the available parameter space in the combined run-1 and run-2 diphoton data and study its interpretation in terms of a singlet scalar field which possibly mixes with the Standard Model Higgs boson. We show that the mixing angle is already strongly constrained by high-mass Higgs searches in the diboson channel, and by Higgs coupling measurements. While a broad resonance is slightly favored, we argue that the signal is consistent with a narrow-width singlet which couples to colored and electromagnetically-charged vector-like fermions. Dijet signals are predicted and may be visible in upcoming analyses. Allowing for additional decay modes could explain a broader resonance, however, we show that monojet searches disfavor a large invisible width. Finally, we comment on the possible relation of this scenario to the naturalness problem.

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arXiv2016

Di-photon portal to warped gravity

The diphoton excess around 750 GeV observed by ATLAS and CMS can be interpreted as coming from a massive spin-2 excitation. We explore this possibility in the context of warped five-dimensional models with the Standard Model (SM) fields propagating in the bulk of the extra dimension. The 750 GeV resonance is identified with the first Kaluza-Klein (KK) excitation of the five-dimensional graviton that is parametrically lighter than KK resonances of SM fields. Our set-up makes it possible to realize non-universal couplings of the spin-2 resonance to matter, and thus to explain non-observation of the 750 GeV resonance in leptonic channels. Phenomenological predictions of the model depend on the localization of fields in the extra dimension. If, as required by naturalness arguments, the zero modes of the Higgs and top fields are localized near the IR brane, one expects large branching fractions to ttbar, hh, WW, and ZZ final states. Decays to a Z boson and a photon can also be observable when the KK graviton couplings to the SM gauge fields are non-universal.

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arXiv2016

On the exotic Higgs decays in effective field theory

We discuss exotic Higgs decays in an effective field theory where the Standard Model is extended by dimension-6 operators. We review and update the status of 2-body lepton- and quark-flavor violating decays involving the Higgs boson. We also comment on the possibility of observing 3-body flavor-violating Higgs decays in this context.

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arXiv2015

Global constraints on anomalous triple gauge couplings in effective field theory approach

We present a combined analysis of LHC Higgs data (signal strengths) together with LEP-2 WW production measurements. To characterize possible deviations from the Standard Model (SM) predictions, we employ the framework of an Effective Field Theory (EFT) where the SM is extended by higher-dimensional operators suppressed by the mass scale of new physics $Λ$. The analysis is performed consistently at the order $Λ^{-2}$ in the EFT expansion keeping all the relevant operators. While the two data sets suffer from flat directions, together they impose stringent model-independent constraints on the anomalous triple gauge couplings. As a side product, we provide the results of the combined fit in different EFT bases.

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arXiv2015

Model independent constraints on four-lepton operators

We obtain constraints on four-lepton interactions in the effective field theory with dimension-6 operators. To this end, we combine the experimental input from Z boson measurements in LEP-1, W boson mass and decays, muon and tau decays, lepton pair production in LEP-2, neutrino scattering on electrons, and parity violating electron scattering. The analysis does not rely on any assumptions about the flavor structure of the dimension-6 operators. Our main results are the confidence intervals for Wilson coefficients of 16 lepton-flavor conserving four-lepton operators, together with the full correlation matrix. Consequences for leptophilic models beyond the Standard Model are discussed.

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arXiv2015

Rosetta: an operator basis translator for Standard Model effective field theory

We introduce Rosetta, a program allowing for the translation between different bases of effective field theory operators. We present the main functions of the program and provide an example of usage. One of the Lagrangians which Rosetta can translate into has been implemented into FeynRules, which allows Rosetta to be interfaced into various high-energy physics programs such as Monte Carlo event generators. In addition to popular bases choices, such as the Warsaw and Strongly Interacting Light Higgs bases already implemented in the program, we also detail how to add new operator bases into the Rosetta package. In this way, phenomenological studies using an effective field theory framework can be straightforwardly performed.

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arXiv2015

Lepton Flavor Non-Universality in B-meson Decays from a U(2) Flavor Model

We address the recent anomalies in semi-leptonic $B$-meson decays using a model of fermion masses based on the $U(2)$ flavor symmetry. The new contributions to $b \to s \ell \ell$ transitions arise due to a tree-level exchange of a $Z^\prime$ vector boson gauging a $U(1)$ subgroup of the flavor symmetry. They are controlled by a single parameter and are approximately aligned to the Standard Model prediction, with constructive interference in the $e$-channel and destructive interference in the $μ$-channel. The current experimental data on semi-leptonic $B$-meson decays can be very well reproduced without violating existing constraints from flavor violation in the quark and lepton sectors. Our model will be tested by new measurements of $b \to s \ell \ell$ transitions and also by future electroweak precision tests, direct $Z^\prime$ searches, and $μ$-$e$ conversion in nuclei.

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arXiv2015

Electroweak constraints on flavorful effective theories

We derive model-independent constraints arising from the Z and W boson observables on dimension six operators in the effective theory beyond the Standard Model. In particular, we discuss the generic flavor structure for these operators as well as several flavor patterns motivated by simple new physics scenarios.

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arXiv2015

A second Higgs from the Higgs portal

In the Higgs portal framework, the Higgs field generally mixes with the Standard Model singlet leading to the existence of two states, one of which is identified with the 125 GeV scalar observed at the LHC. In this work, we analyse direct and indirect constraints on the second mass eigenstate and the corresponding mixing angle. The existence of the additional scalar can be beneficial as it can stabilise the otherwise-metastable electroweak vacuum. We find parameter regions where all of the bounds, including the stability constraints, are satisfied. We also study prospects for observing the decay of the heavier state into a pair of the 125 GeV Higgs-like scalars.

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arXiv2015

Model-independent precision constraints on dimension-6 operators

We discuss electroweak precision constraints on dimension-6 operators in the effective theory beyond the standard model. We identify the combinations of these operators that are constrained by the pole observables (the W and Z masses and on-shell decays) and by the W boson pair production. To this end, we define a set of effective couplings of W and Z bosons to fermions and to itself, which capture the effects of new physics corrections. This formalism clarifies which operators are constrained by which observable, independently of the adopted basis of operators. We obtain numerical constraints on the coefficients of dimension-6 operator in a form that can be easily adapted to any particular basis of operators, or any particular model with new heavy particles.

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arXiv2014

Displaced Vertices from X-ray Lines

We present a simple model of weak-scale thermal dark matter that gives rise to X-ray lines. Dark matter consists of two nearly degenerate states near the weak scale, which are populated thermally in the early universe via co-annihilation with slightly heavier states that are charged under the Standard Model. The X-ray line arises from the decay of the heavier dark matter component into the lighter one via a radiative dipole transition, at a rate that is slow compared to the age of the universe. The model predicts observable signatures at the LHC in the form of exotic events with missing energy and displaced leptons and jets. As an application, we show how this model can explain the recently observed 3.55 keV X-ray line.

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arXiv2014

New Observables for CP Violation in Higgs Decays

Current experimental data on the 125 GeV Higgs boson still allow room for large CP violation. The observables usually considered in this context are triple product asymmetries, which require an input of four visible particles after imposing momentum conservation. We point out a new class of CP violating observables in Higgs physics which require only three reconstructed momenta. They may arise if the process involves an interference of amplitudes with different intermediate particles, which provide distinct strong phases in the form of the Breit-Wigner widths, in addition to possible weak phases that arise from CP violating couplings of the Higgs in the Lagrangian. As an example, we propose a forward-backward asymmetry of the charged lepton in the three-body Higgs decay, $h\to \ell^-\ell^+ γ$, as a probe for CP-violating Higgs couplings to $Zγ$ and $γγ$ pairs. Other processes exhibiting this type of CP-violation are also discussed.

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arXiv2014

Exotic Higgs decays in the golden channel

The Higgs boson may have decay channels that are not predicted by the Standard Model. We discuss the prospects of probing exotic Higgs decays at the LHC using the 4-lepton final state. We study two specific scenarios, with new particles appearing in the intermediate state of the 4-lepton Higgs decay. In one, Higgs decays to a Z boson and a new massive gauge boson, the so-called hidden photon. In the other, Higgs decays to an electron or a muon and a new vector-like fermion. We argue that the upcoming LHC run will be able to explore a new parameter space of these models that is allowed by current precision constraints. Employing matrix element methods, we use the full information contained in the differential distribution of the 4-lepton final state to extract the signal of exotic decays. We find that, in some cases, the LHC can be sensitive to new physics even when the correction to the total 4-lepton Higgs rate is of the order of a percent. In particular, for the simplest realization of the hidden photon with the mass between 15 and 65 GeV, new parameter space can be explored in the LHC run-II.

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arXiv2014

From Tevatron's top and lepton-based asymmetries to the LHC

We define a lepton-based asymmetry in semi-leptonic ttbar production at the LHC. We show that the ratio of this lepton-based asymmetry and the ttbar charge asymmetry, measured as a function of the lepton transverse momentum or the ttbar invariant mass is a robust observable in the Standard Model. It is stable against higher order corrections and mis-modeling effects. We show that this ratio can also be a powerful discriminant among different new physics models and between them and the Standard Model. Finally, we show that a related ratio defined at the Tevatron is also robust as a function of the ttbar invariant mass.

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arXiv2013

Vector-like leptons: Higgs decays and collider phenomenology

We study the impact of heavy vector-like leptons on several observables in collider and low-energy physics. These states, present in many well-motivated extensions of the Standard Model, can induce lepton flavour violation and non-standard Higgs decays. We study these effects in an effective model inspired by the composite Higgs scenario. After deriving bounds on the mass and production cross-section of the vector-like states using recent LHC data on multilepton searches, we discuss the modification of the Higgs decays to dilepton, diphoton and $Zγ$ final states as well as low-energy observables like radiative lepton decays, the anomalous magnetic moment of the muon and the electric dipole moment of the electron. We find several interesting correlations. In particular, we show that branching fractions of lepton flavour-violating Higgs decays at an observable level are prohibited due to the strong bounds on the radiative lepton decays.

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