arXiv subjects

Peter Stangl

Publications and source records attributed to Peter Stangl.

31 records · Page 2Linked to original sources

arXiv2020

The Dark Side of 4321

The evidence of Dark Matter (DM) is one of the strongest observational arguments in favour of physics beyond the Standard Model. Despite expectations, a similar evidence has been lacking so far in collider searches, with the possible exception of $B$-physics discrepancies, a coherent set of persistent deviations in a homogeneous dataset consisting of $b \to c$ and $b \to s$ semi-leptonic transitions. We explore the question whether DM and the $B$ discrepancies may have a common origin. We do so in the context of the so-called 4321 gauge model, a UV-complete and calculable setup that yields a $U_1$ leptoquark, the by far most successful single mediator able to explain the $B$ anomalies, along with other new gauge bosons, including a $Z^\prime$. Adding to this setup a 'minimal' DM fermionic multiplet, consisting of a ${\bf 4}$ under the 4321's $SU(4)$, we find the resulting model in natural agreement with the relic-density observation and with the most severe direct-detection bounds, in the sense that the parameter space selected by $B$ physics is also the one favoured by DM phenomenology. The DM candidate is a particle with a mass in the WIMP range, freeze-out dynamics includes a co-annihilator (the 'rest' of the ${\bf 4}$ multiplet), and the most important gauge mediator in the DM sector is the $Z^\prime$.

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arXiv2019

Computing Tools for the SMEFT

The increasing interest in the phenomenology of the Standard Model Effective Field Theory (SMEFT), has led to the development of a wide spectrum of public codes which implement automatically different aspects of the SMEFT for phenomenological applications. In order to discuss the present and future of such efforts, the "SMEFT-Tools 2019" Workshop was held at the IPPP Durham on the 12th-14th June 2019. Here we collect and summarize the contents of this workshop.

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arXiv2019

B-decay discrepancies after Moriond 2019

Following the updated measurement of the lepton flavour universality (LFU) ratio R_K in B -> Kll decays by LHCb, as well as a number of further measurements, e.g. R_K* by Belle and B_s -> mu mu by ATLAS, we analyse the global status of new physics in b -> s transitions in the weak effective theory at the b-quark scale, in the Standard Model effective theory at the electroweak scale, and in simplified models of new physics. We find that the data continues to strongly prefer a solution with new physics in semi-leptonic Wilson coefficients. A purely muonic contribution to the combination C_9 = -C_10, well suited to UV-complete interpretations, is now favoured with respect to a muonic contribution to C_9 only. An even better fit is obtained by allowing an additional LFU shift in C_9. Such a shift can be renormalization-group induced from four-fermion operators above the electroweak scale, in particular from semi-tauonic operators, able to account for the potential discrepancies in b -> c transitions. This scenario is naturally realized in the simplified U_1 leptoquark model. We also analyse simplified models where a LFU effect in b -> sll is induced radiatively from four-quark operators and show that such a setup is on the brink of exclusion by LHC di-jet resonance searches.

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arXiv2019

Flavour anomalies and (fundamental) partial compositeness

Several measurements of B-meson decay observables show deviations from Standard Model (SM) predictions, some of them hinting at violation of lepton flavour universality (LFU). I discusses how the anomalies in rare B decays can be explained by partial compositeness. Partial compositeness is a key ingredient of models with a composite Higgs boson and generically leads to violation of LFU. After presenting a simple model with partial compositeness that is able to explain the anomalies in rare B decays, the flavour phenomenology of a minimal UV completion of a composite Higgs model with partial compositeness is discussed: the minimal fundamental partial compositeness (MFPC) model. A virtue of the MFPC model is its capability of serving both as a solution to the naturalness problem of the SM and as an explanation of the flavour anomalies in rare B-meson decays. In view of recent new measurements, the results on which this proceedings contribution is based are updated.

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arXiv2019

A Global Likelihood for Precision Constraints and Flavour Anomalies

We present a global likelihood function in the space of dimension-six Wilson coefficients in the Standard Model Effective Field Theory (SMEFT). The likelihood includes contributions from flavour-changing neutral current B decays, lepton flavour universality tests in charged- and neutral-current B and K decays, meson-antimeson mixing observables in the K, B, and D systems, direct CP violation in K -> ππ, charged lepton flavour violating B, tau, and muon decays, electroweak precision tests on the Z and W poles, the anomalous magnetic moments of the electron, muon, and tau, and several other precision observables, 265 in total. The Wilson coefficients can be specified at any scale, with the one-loop running above and below the electroweak scale automatically taken care of. The implementation of the likelihood function is based on the open source tools flavio and wilson as well as the open Wilson coefficient exchange format (WCxf) and can be installed as a Python package. It can serve as a basis either for model-independent fits or for testing dynamical models, in particular models built to address the anomalies in B physics. We discuss a number of example applications, reproducing results from the EFT and model building literature.

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arXiv2018

Direct Constraints, Flavor Physics, and Flavor Anomalies in Composite Higgs Models

Composite Higgs models (CHMs) offer an elegant solution to the naturalness problem of the Standard Model (SM). Their direct effects at particle colliders like the Large Hadron Collider (LHC) are thus of central interest. While no direct effects have been observed so far, there are recent indirect hints for new physics (NP) coming from measurements of rare B meson decays. This thesis studies direct collider constraints on CHMs as well as the question if these models can explain the hints for NP in rare B decays. The first part of this thesis gives a self-contained introduction to all main concepts of CHMs used in the remainder of the work. These concepts are then applied in several phenomenological analyses. In the context of global numerical analyses of two explicit CHMs, the direct constraints on vector, fermion and scalar resonances are studied in detail. The prospects of various decay channels for observing or excluding still viable parameter points of the considered models are discussed. Model independent analyses of the hints for NP in rare B decays are performed in the context of the $B \to K^* μ^+ μ^-$ anomaly as well as the hints for violation of lepton flavor universality (LFU) found in measurements of the observables $R_K$ and $R_{K^*}$. A simple CHM is presented that can explain the anomalies in rare B decays by partially composite left-handed muons. The flavor physics of a much more ambitious model, which is based on a UV completion of effective CHMs called fundamental partial compositeness (FPC), is investigated in detail. Taking into account all relevant constraints from electroweak scale physics and low-energy flavor observables, it is shown that this model can explain the anomalies found in rare B decays.

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arXiv2018

Interpreting Hints for Lepton Flavor Universality Violation

We interpret the recent hints for lepton flavor universality violation in rare $B$ meson decays. Based on a model-independent effective Hamiltonian approach, we determine regions of new physics parameter space that give a good description of the experimental data on $R_K$ and $R_{K^*}$, which is in tension with Standard Model predictions. We suggest further measurements that can help narrowing down viable new physics explanations. We stress that the measured values of $R_K$ and $R_{K^*}$ are fully compatible with new physics explanations of other anomalies in rare $B$ meson decays based on the $b \to s μμ$ transition. If the hints for lepton flavor universality violation are first signs of new physics, perturbative unitarity implies new phenomena below a scale of $\sim 100$ TeV.

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arXiv2017

Flavor Physics and Flavor Anomalies in Minimal Fundamental Partial Compositeness

Partial compositeness is a key ingredient of models where the electroweak symmetry is broken by a composite Higgs state. Recently, a UV completion of partial compositeness was proposed, featuring a new strongly coupled gauge interaction as well as new fundamental fermions and scalars. We work out the full flavor structure of the minimal realization of this idea and investigate in detail the consequences for flavor physics. While CP violation in kaon mixing represents a significant constraint on the model, we find many viable parameter points passing all precision tests. We also demonstrate that the recently observed hints for a violation of lepton flavor universality in $B\to K^{(*)}\ell\ell$ decays can be accommodated by the model, while the anomalies in $B\to D^{(*)}τν$ cannot be explained while satisfying LEP constraints on $Z$ couplings.

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arXiv2017

Status of the $B\to K^*μ^+μ^-$ anomaly after Moriond 2017

Motivated by recent results by the ATLAS and CMS collaborations on the angular distribution of the $B \to K^* μ^+μ^-$ decay, we perform a state-of-the-art analysis of rare $B$ meson decays based on the $b \to s μμ$ transition. Using standard estimates of hadronic uncertainties, we confirm the presence of a sizable discrepancy between data and SM predictions. We do not find evidence for a $q^2$ or helicity dependence of the discrepancy. The data can be consistently described by new physics in the form of a four-fermion contact interaction $(\bar s γ_αP_L b)(\bar μγ^αμ)$. Assuming that the new physics affects decays with muons but not with electrons, we make predictions for a variety of theoretically clean observables sensitive to violation of lepton flavour universality.

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arXiv2017

Electroweak symmetry breaking and collider signatures in the next-to-minimal composite Higgs model

We conduct a detailed numerical analysis of the composite pseudo-Nambu-Goldstone Higgs model based on the next-to-minimal coset $\text{SO}(6)/\text{SO}(5)\cong\text{SU}(4)/\text{Sp}(4)$, featuring an additional SM singlet scalar in the spectrum, which we allow to mix with the Higgs boson. We identify regions in parameter space compatible with all current experimental constraints, including radiative electroweak symmetry breaking, flavour physics, and direct searches at colliders. We find the additional scalar, with a mass predicted to be below a TeV, to be virtually unconstrained by current LHC data, but potentially in reach of run 2 searches. Promising indirect searches include rare semi-leptonic $B$ decays, CP violation in $B_s$ mixing, and the electric dipole moment of the neutron.

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arXiv2015

Direct and indirect signals of natural composite Higgs models

We present a comprehensive numerical analysis of a four-dimensional model with the Higgs as a composite pseudo-Nambu-Goldstone boson that features a calculable Higgs potential and protective custodial and flavour symmetries to reduce electroweak fine-tuning. We employ a novel numerical technique that allows us for the first time to study constraints from radiative electroweak symmetry breaking, Higgs physics, electroweak precision tests, flavour physics, and direct LHC bounds on fermion and vector boson resonances in a single framework. We consider four different flavour symmetries in the composite sector, one of which we show to not be viable anymore in view of strong precision constraints. In the other cases, all constraints can be passed with a sub-percent electroweak fine-tuning. The models can explain the excesses recently observed in $WW$, $WZ$, $Wh$ and $\ell^+\ell^-$ resonance searches by ATLAS and CMS and the anomalies in angular observables and branching ratios of rare semi-leptonic $B$ decays observed by LHCb. Solving the $B$ physics anomalies predicts the presence of a dijet or $t\bar t$ resonance around 1 TeV just below the sensitivity of LHC run 1. We discuss the prospects to probe the models at run 2 of the LHC. As a side product, we identify several gaps in the searches for vector-like quarks at hadron colliders, that could be closed by reanalyzing existing LHC data.

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arXiv2015

Lepton flavour universality violation from composite muons

We describe a possibility to explain the $2.6σ$ deviation from lepton flavour universality observed by the LHCb collaboration in $B^+\to K^+\ell^+\ell^-$ decays in the context of minimal composite Higgs models. We find that a sizable degree of compositeness of partially composite muons can lead to a good agreement with the experimental data. Our construction predicts a new physics contribution to $B_s$-$\bar{B}_s$ mixing. Additionally, it accounts for the deficit in the invisible $Z$ width measured at LEP.

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arXiv2015

Violation of lepton flavour universality in composite Higgs models

We investigate whether the $2.6σ$ deviation from lepton flavour universality in $B^+\to K^+\ell^+\ell^-$ decays recently observed at the LHCb experiment can be explained in minimal composite Higgs models. We show that a visible departure from universality is indeed possible if left-handed muons have a sizable degree of compositeness. Constraints from $Z$-pole observables are avoided by a custodial protection of the muon coupling. The deficit in the invisible $Z$ width at LEP is explained in the same region of parameters.

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