arXiv subjects

Gudrun Hiller

Publications and source records attributed to Gudrun Hiller.

At least 19 recordsLinked to original sources

arXiv2026

Constraining the real singlet extension of the Standard Model: implications for vacuum stability

Amongst the simplest extensions of the Standard Model is the addition of a real singlet scalar field with profound phenomenological consequences. The singlet can catalyze a strong first-order electroweak phase transition, necessary for successful electroweak baryogenesis. Furthermore, the current prediction of a metastable Higgs vacuum can be lifted towards absolute stability. While current data mainly constrain the scalar mixing angle, future measurements of the Higgs self-coupling and direct searches for additional scalar states will probe much larger parts of the viable parameter space. We report on a combined study of theoretical and experimental constraints on the real singlet extension, highlighting here in particular its implications for vacuum stability.

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arXiv2026

Constraining the real scalar singlet extension of the SM

The real scalar singlet extension of the standard model provides a minimal framework in which the Higgs sector can realise a strong first-order electroweak phase transition and improve the stability of the electroweak vacuum. We combine the electroweak phase transition and high-scale vacuum stability with current and projected collider probes, including precision Higgs measurements, the Higgs trilinear coupling, EWPO and resonant searches for a heavy singlet-like scalar in $ZZ$ and di-Higgs final states. Focusing on a singlet heavier than the standard model Higgs, we find that there is parameter space compatible with a strong first-order electroweak phase transition for singlet-like scalar masses up to nearly 1 TeV. Deviations in the Higgs self-coupling can be larger than those in the Higgs--$Z$ coupling, making Higgs-potential measurements a key probe. We find that the HL-LHC will test a large fraction of the parameter space, while the FCC will provide ultimate discovery and model-discrimination capabilities.

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arXiv2026

Probing baryon number with missing energy

Quark portal interactions $qqqN$ with a light singlet fermion $N$ make baryon number testable through missing transverse energy (MET). We find that present LHC data constrain scales up to 10 TeV (MET plus jet), 8 TeV (MET plus top) and 11 TeV (MET plus bjet). With increasing mass, or larger portal couplings, $N$ becomes less long-lived, and gives clean displaced vertex signatures, which encourage dedicated searches. We also narrow down viable mesogenesis models with color triplet scalars to a mass range $\sim y \,\cdot 3 \, \text{TeV}$ (with charm) and $\sim y \, \cdot 5 \, \text{TeV}$ (charmless) couplings $y$ to $b$ and lighter quarks, a window that can be scrutinized by HL-LHC. Interactions also induce rare decays of type baryon (meson) to meson (baryon) plus invisible, which complement high-$p_T$ searches and can prove baryon number violation. We explore charm decays $\Lambda_c \to (\pi,K) + \mathrm{invisible}$. Their branching ratios are subject to sizable hadronic uncertainties and require high luminosity flavor facilities such as a Tera-Z facility (FCC-ee, CEPC). Branching ratios of top quarks into one or two $b$-jets plus $N$ can reach few$\times 10^{-6}$.

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arXiv2025

Probing invisible particles with charm

We point out opportunities to probe invisible particles, left- and right-handed neutrinos, axion-like particles (ALPs) and dark photons $(Z^\prime)$ with rare decays of charm hadrons. We employ and recast existing searches in $D \to (\pi, \omega) X$, $D^ 0 \to X$ and $\Lambda_c \to p X$, where $X$ denotes one of the above invisible final states including dineutrinos. The branching ratios are clean null tests of the standard model, yet, are essentially unconstrained for some parameters of light new physics, limited only by weak lifetime constraints at the level of $\mathcal{O}(10^{-1})$. On the other hand, if models are probed, branching ratios still reach up to $10^{-3}$ ($Z^\prime$) and $10^{-4}$ (ALPs). Chirality-preserving operators from heavy new physics in the dimension six standard model effective theory (SMEFT) imply tighter upper limits, up to few $\times 10^{-5}$. Constraints on chirality-flipping heavy new physics, such as lepton number violation from dimension seven SMEFT, or with light sterile neutrinos, are weaker, with branching ratios up to few$\times 10^{-4}$. Sensitivities to different couplings arise with $\Lambda_c \to p X $ and $D \to \pi \pi X$ decays, in particular in relation with the other modes. Processes can be studied at running and future experiments with high charm luminosities, BESIII, Belle II, a super-tau-charm factory (STCF) and $Z$-factories, such as the FCC-ee and the CEPC.

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arXiv2025

Beyond Universality: Probing Lepton Flavor in the SMEFT

We present a global analysis of lepton-flavor-specific operators in the Standard Model Effective Field Theory (SMEFT), combining data from collider and flavor physics experiments. We systematically explore various lepton-flavor scenarios, including flavor-specific, universal, and democratic patterns, while employing a minimal flavor violation (MFV) ansatz in the quark sector. We constrain a set of 17 dimension-six SMEFT operators using a Bayesian fitting approach. Our analysis yields stringent bounds on the Wilson coefficients, probing new physics scales up to $\mathcal{O}(1000)$\,TeV. The strongest constraints are obtained in lepton-flavor universal and lepton-flavor democratic scenarios, in particular for the electroweak dipole operators. We further provide posterior-based predictions for $B \to K^{(*)} \nu \bar \nu$ and $B \to (\pi, \rho) \nu \bar \nu$ decays, highlighting their role as probes of the MFV hypothesis in upcoming Belle II measurements. Our results demonstrate the complementarity of collider and flavor observables, the impact of flavor assumptions on global SMEFT fits, and establish robust limits on extensions of the Standard Model in the lepton sector.

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arXiv2025

New opportunities for rare charm from $Z\to c\bar{c}$ decays

We analyze the potential of rare charm decays as probes of new physics at a high-luminosity flavor facility operating at the $Z$ pole, such as the FCC-ee or CEPC. In particular, we identify clean null-test observables in $D^0 \to \pi^+ \pi^- \nu\bar{\nu}$ and in polarized $\Lambda_c^+ \to p \ell^+ \ell^-$ decays with $\ell=e, \mu$. Complementarity with the LHC and HL-LHC flavor programs arises from the characteristic features of a Tera-$Z$ environment: the capability to study missing-energy modes and charm production with significant polarization. We improve the theoretical description of $D^0 \to \pi^+ \pi^- \nu\bar{\nu}$ decays and work out the phenomenology of polarization-induced null-test observables in $\Lambda_c^+ \to p \ell^+ \ell^-$ decays. In regions of dilepton mass near the $\phi$ resonance, polarization asymmetries can reach $O(5 \%)$ for muons and $O(14 \%)$ for electrons times the $\Lambda_c^+$ polarization. We also point out synergies between the dineutrino and the dilepton modes using the SMEFT framework of heavy new physics. Using the IDEA detector concept at FCC-ee, we find in simulation studies that dineutrino branching fractions as low as $\sim 2 \times 10^{-7}$ can be probed, which reaches well into the parameter space of new physics, and also allows for discrimination of lepton flavor structures. Furthermore, the measurement of asymmetries in $\Lambda_c^+ \to p \mu^+ \mu^-$ at $O(1 \%)$ will be possible. Similar sensitivities are expected for dielectron final states, although robust predictions will require further dedicated studies.

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arXiv2025

BSM reach of rare charm decays, including the rising star $\Lambda_c\to p\mu^+\mu^-$

We perform the first global fit of rare charm transitions using recent data on $D^0\to \mu^+\mu^-$, $D^+\to\pi^+\mu^+\mu^-$, $\Lambda_c\to p\mu^+\mu^-$ and $D^0\to\pi^+\pi^-\mu^+\mu^-$ decays, including angular observables. We work out constraints on new physics in the framework of the weak effective field theory. While angular observables in $D^0\to\pi^+\pi^-\mu^+\mu^-$ decays provide sensitivities to different QCD models, future null tests in $\Lambda_c\to p \mu^+\mu^-$ look more promising to extract limits because of less hadronic uncertainties. This marks $\Lambda_c\to p \mu^+\mu^-$ as the rising star of rare charm decays.

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arXiv2025

Kaon Physics: A Cornerstone for Future Discoveries

The kaon physics programme, long heralded as a cutting-edge frontier by the European Strategy for Particle Physics, continues to stand at the intersection of discovery and innovation in high-energy physics (HEP). With its unparalleled capacity to explore new physics at the multi-TeV scale, kaon research is poised to unveil phenomena that could reshape our understanding of the Universe. This document highlights the compelling physics case, with emphasis on exciting new opportunities for advancing kaon physics not only in Europe but also on a global stage. As an important player in the future of HEP, the kaon programme promises to drive transformative breakthroughs, inviting exploration at the forefront of scientific discovery.

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arXiv2025

Teaming up MET plus jet with Drell-Yan in the SMEFT

The Standard Model Effective Field Theory (SMEFT) is a widely utilized framework for exploring new physics effects in a model-independent manner. In previous studies, Drell-Yan collider data has emerged as a promising signature due to its energy enhancement relative to Standard Model predictions. We present recent works, extending this approach by also considering the "missing energy + jet" signature, which can constrain related dineutrino couplings and similarly benefits from energy enhancement. The combination of these observables allows for constraining a broader selection of operators and helps resolve flat directions in a global analysis. Overall, the bounds probe the multi-TeV range, with the strongest reaching up to $10\; \text{TeV}$ for four-fermion interactions and $7 \;\text{TeV}$ for gluonic dipole interactions. Furthermore, we find that low energy flavor observables improve limits by up to a factor of three for dipole operators. We also estimate sensitivities to new physics at future hadron colliders including the $\sqrt{s} = 27 \;\text{TeV}$ HE-LHC and the $\sqrt{s} = 100 \; \text{TeV}$ FCC-hh.

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arXiv2025

Total Drell-Yan in the flavorful SMEFT

We perform a global analysis of Drell-Yan production of charged leptons and dineutrinos, the latter in missing energy plus jet events, in proton-proton collisions within the Standard Model Effective Field Theory (SMEFT). The combination allows for the removal of flat directions, sharper limits and to probe more couplings than the individual observables, which we show performing a fit to LHC-data. We also find that limits have only mild dependence on lepton flavor patterns; hierarchies in quark flavors are driven by the parton distribution functions. The strongest constraints are on couplings involving the first and second generation quarks, exceeding 10 TeV. Combining flavor and high-$p_T$ data, the limits on electroweak and gluon dipole operators can be improved, by up to a factor of three, highlighting once more that a more global approach increases sensitivities significantly. We also estimate the improvements in reach over existing data for the high-luminosity LHC (HL-LHC) by $\sim 1.5$, and future collider options such as the high-energy LHC (HE-LHC) by $\sim 3$ and FCC-hh by $\sim 8$. To maximize the new physics reach kinematic cuts and binning needs to be adjusted to each quark-flavor separately.

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arXiv2024

Effective field theory analysis of rare $|\Delta c|=|\Delta u|=1$ charm decays

We perform a global analysis of $|\Delta c| = |\Delta u| = 1$ transitions using recent data on $D^0 \to \mu^+\mu^-$, $D^+ \to \pi ^+\,\mu^+\mu^-$, $\Lambda_c \to p\,\mu^+\mu^-$, and $D^0 \to \pi^+\pi^-\,\mu^+\mu^-$ decays, and work out constraints on new physics Wilson coefficients $\mathcal{C}_{7,9,10}^{(\prime)}$. While results are consistent with the standard model, we find sizeable room for new physics that can be cleanly signaled with null test observables, not probed with searches in other sectors such as kaon and $b$-decays. The decay $D^0 \to \pi^+\pi^-\,\mu^+\mu^-$ requires better understanding of hadronic contributions to be competitive in the current fit. Progress can be achieved by precision study of the double differential decay rate in the dipion and dimuon masses, together with improved theory modelling and $D \to \pi \pi$ transition form factors. On the other hand, the 4-body decay is an important contributor to the future global analysis due to its angular distributions that probe complementary combinations of Wilson coefficients, and as a QCD laboratory. The decay $\Lambda_c \to p\,\ell^+\ell^-$ is the rising star due to the simplicity of a 3-body decay with available form factors from lattice QCD, sensitivity to both 4-fermion and electromagnetic dipole couplings and its null test forward-backward asymmetry.

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arXiv2024

Missing Energy plus Jet in the SMEFT

We study the production of dineutrinos in proton-proton collisions, with large missing transverse energy and an energetic jet as the experimental signature. Recasting a search from the ATLAS collaboration we work out constraints on semileptonic four-fermion operators, gluon and electroweak dipole operators and $Z$-penguins in the SMEFT. All but the $Z$-penguin operators experience energy-enhancement. Constraints on gluon dipole operators are the strongest, probing new physics up to 14 TeV, and improve over existing ones from collider studies. Limits on FCNC four-fermion operators are competitive with Drell-Yan production of dileptons, and improve on those for tau final states. For left-handed $|Δs|=|Δd|=1$ and right-handed $|Δc|=|Δu|=1$ transitions these are the best available limits, also considering rare kaon and charm decays. We estimate improvements for the $3000 \;\mathrm{fb}^{-1}$ High Luminosity Large Hadron Collider.

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arXiv2024

Vacuum Stability in the Standard Model and Beyond

We revisit the stability of the Standard Model vacuum, and investigate its quantum effective potential using the highest available orders in perturbation theory and the most accurate determination of input parameters to date. We observe that the stability of the electroweak vacuum centrally depends on the values of the top mass and the strong coupling constant. We estimate that reducing their uncertainties by a factor of two to three is sufficient to establish or refute SM vacuum stability at the $5\sigma$ level. We further investigate vacuum stability for a variety of singlet scalar field extensions with and without flavor using the Higgs portal mechanism. We identify the BSM parameter spaces for stability and find sizable room for new physics. We further study the phenomenology of Planck-safe models at colliders, and determine the impact on the Higgs trilinear, the Higgs-to-electroweak-boson, and the Higgs quartic couplings, some of which can be significant. The former two can be probed at the HL-LHC, the latter requires a future collider with sufficient energy and precision such as the FCC-hh.

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arXiv2023

Implications of an enhanced $B \to K ν\bar ν$ branching ratio

Rare decays mediated by $b \to s ν\bar ν$ transitions have been reported by the Belle II experiment. The branching ratio of the decay $B^+ \to K^+ ν\bar ν$ is found to be enhanced with respect to the standard model value. If taken at face value, the implications are profound: either lepton flavor universality is violated at the (multi)-TeV-scale, or light new physics is involved. This holds in general if $\mathcal{B}(B^+ \to K^+ ν\bar ν)$ exceeds $1.2 \cdot 10^{-5} \, (1.3 \cdot 10^{-5})$ at $1 σ$ ($2 σ$), which tightens with a decreasing upper limit on $\mathcal{B}(B \to K^*ν\bar ν)$, that is in reach of the Belle II experiment. In view of the strong constraints on electron-muon universality violation in $|Δb|=|Δs|=1$ processes, viable explanations are heavy, $(5-10)$-TeV tree-level new physics mediators that couple only to tau-flavors, or lepton flavor violating ones. In addition, couplings of similar size to both left- and right-handed quarks are generically required, implying non-minimal BSM sectors which are carefully balanced against flavor constraints. The decay $B_s^0 \to \text{invisibles}$ can shed light on whether new physics is light or heavy. In the former case, branching ratios can be as large as $10^{-5}$.

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arXiv2023

More Synergies from Beauty, Top, $Z$ and Drell-Yan Measurements in SMEFT

We perform a global analysis of Beauty, Top, $Z$ and Drell-Yan measurements in the framework of the Standard Model effective theory (SMEFT). We work within the minimal flavor violation (MFV) hypothesis, which relates different sectors and generations beyond the $SU(2)_L$-link between left-handed top and beauty quarks. We find that the constraints on the SMEFT Wilson coefficients from the combined analysis are stronger than the constraints from a fit to the individual sectors, highlighting synergies in the global approach. We also show that constraints within MFV are strengthened compared to single-generation fits. The strongest bounds are obtained for the semileptonic four-fermion triplet operator $C_{lq}^{(3)}$, probing scales as high as $18$ TeV, followed by the gluon dipole operator $C_{uG}$ with $7$ TeV, and other four-fermion and penguin operators in the multi-TeV range. Operators with left-handed quark bilinears receive order one contributions from higher orders in the MFV expansion induced by the top Yukawa coupling as a result of the FCNC $b \to s μμ$ anomalies combined with the other sectors. We predict the $68\%$ credible intervals of the dineutrino branching ratios within MFV as $5.3 \cdot 10^{-6} \leq {\cal{B}}(B^0 \to K^{* 0} ν\barν) \leq 12.8 \cdot 10^{-6}$ and $ 2.5 \cdot 10^{-6} \leq {\cal{B}}(B^+ \to K^+ ν\barν) \leq 5.9 \cdot 10^{-6}$, which include the respective Standard Model predictions, and are in reach of the Belle II experiment. We show how future measurements of the dineutrino branching ratios can provide insights into the structure of new physics in the global fit.

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arXiv2023

Old and new anomalies in charm

The recent LHCb determination of the direct CP asymmetries in the decays $D^0 \to K^+ K^-, π^+ π^-$ hints at a sizeable breaking of two approximate symmetries of the SM: CP and U-spin. We aim at explaining the data with BSM physics and use the framework of flavorful $Z^\prime$ models. Interestingly, experimental and theoretical constraints very much narrow down the shape of viable models: Viable, anomaly-free models are electron- and muon-phobic and feature a light $Z^\prime$ of 10-20 GeV coupling only to right-handed fermions. The $Z^\prime$ can be searched for in low mass dijets or at the LHC as well as dark photon searches. A light $Z^\prime$ of $\sim$ 3 GeV or $\sim$ 5-7 GeV can moreover resolve the longstanding discrepancy in the $J/ψ, ψ^\prime$ branching ratios with pion form factors from fits to $e^+ e^- \to π^+ π^-$ data, and simultaneously explain the charm CP asymmetries. Smoking gun signatures for this scenario are $Υ$ and charmonium decays into pions, taus or invisbles.

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arXiv2023

Two is better than one: The U-spin-CP anomaly in charm

The recent measurement of the CP-asymmetry in the decay $D \to K^+ K^-$ by LHCb, combined with $ΔA_{\text{CP}}$, evidences a sizable CP-asymmetry in $D \to π^+ π^-$ decays, which requires a dynamical enhancement of standard model higher-order contributions over tree-level ones by a factor of two. The data furthermore imply huge U-spin breaking, about 4-5 times larger than the nominal standard model one of $\lesssim 30 \%$ in charm. Enhanced breakdown of the two approximate symmetries points to models that violate U-spin and CP and disfavors flavor singlet contributions such as chromomagnetic dipole operators as explanations of the data. We analyze the reach of flavorful $Z^\prime$ models for charm CP-asymmetries. Models feature explicit U-spin and isospin breaking, allowing for correlations with $D \to π^0 π^0$ and $D^+ \to π^+ π^0$ decays with corresponding CP-asymmetries at a similar level and sign as $D \to π^+ π^-$, about $ {\cal{O}}(1-2) \cdot 10^{-3}$. Experimental and theoretical constraints narrow down the shape of viable models: anomaly-free models are leptophobic -- or at least electro- and muo-phobic -- with light $Z^\prime$ below ${\cal{O}}(20)$ GeV, and can be searched for in low mass dijets at the LHC, $Υ$ and charmonium decays, and dark photon signatures. A $Z^\prime$ around $\sim 3$ GeV or $\sim (5-7)$ GeV can relieve the tensions in the $J/ψ\to π^+ π^-$ and $ψ^\prime \to π^+ π^-$ branching ratios with pion form factors from fits to Babar and JLab data, and simultaneously explain the charm CP asymmetries. Models also feature sizable branching ratios into light right-handed neutrinos or vector-like dark fermions, which can be searched for in $e^+ e^- \to$~hadrons + invisibles at Belle II and BESIII. Due to the low new physics scale dark fermions may induce an early Landau pole which requires UV-completion near the TeV-scale.

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arXiv2023

Synergies of Drell-Yan, beauty, top, and Z observables in MFV-SMEFT

The Standard Model Effective Field Theory (SMEFT) is a powerful tool to search for new physics in a model-independent way. We explore the synergies arising from different types of observables in a combined, global SMEFT fit. Specifically, we investigate the combination of top-quark measurements, $b\to s$ flavor changing neutral current transitions, $Z\to b \bar b$ and $Z\to c \bar c$, as well as Drell-Yan data from the LHC. We also examine the impact of Minimal Flavor Violation (MFV) as a flavor pattern in the global fit. We find that the combination of high-p$_T$ with flavor physics observables provides powerful synergies that significantly improve the fit and enable more precise tests of various SMEFT operators. By incorporating different observables, we are able to remove flat directions in the parameter space and infer on the flavor structure based on the MFV parameterization. In particular, we find that MFV significantly strengthens the constraints in comparison to a flavor-specific approach. Furthermore, our analysis yields a prediction for the dineutrino branching ratios ${\cal{B}}(B \to K^{(*)} ν\bar ν)$ within MFV, which can be tested experimentally at Belle II.

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