arXiv subjects
Publications and source records attributed to Gudrun Hiller.
We explain why vector-like fermions are natural candidates to lift the Standard Model vacuum instability. Results are further discussed from the viewpoint of criticality. Several models allow for vector-like quarks and leptons in the TeV-range which can be searched for at the LHC.
We perform a model-independent analysis of $|Δb|=|Δd|=1$ processes to test the standard model and probe flavor patterns of new physics. Constraints on Wilson coefficients are obtained from global fits to $B^+ \to π^+ \,μ^+μ^-$, $B^0_s\to \bar{K}^{*0}\, μ^+μ^-$, $B^0\toμ^+μ^-$, and radiative $B \to X_d \,γ$ decays data. The fits are consistent with the standard model but leave sizable room for new physics. Besides higher-statistics measurements and more data in theory-friendly bins of the dilepton mass, further complementary observables such as angular distributions of $B_s^0 \to \bar{K}^{*0}\,\ell^+ \ell^-$, $B \to ρ\,\ell^+ \ell^-$ or the baryonic modes $Ξ_b \to Σ\,\ell^+ \ell^-$, $Ω_b^- \to Ξ^- \, \ell^+ \ell^-$ are necessary to resolve the significant degeneracy in the fit for the semileptonic four-fermion operators. Assuming minimal quark flavor violation, the $b \to s$ global fit implies tight constraints on the $b \to d$ couplings, and hence allows to test this paradigm with improved data. Another benefit from $|Δb|=|Δd|=1$ processes is to shed light on the $B$-anomalies in $|Δb|=|Δs|=1$ modes from a new angle. Specifically, studies of lepton flavor-specific and dineutrino modes are informative on the lepton flavor structure. Rare $b \to d \, \ell \ell, ν\bar ν$ decays can be studied at high luminosity flavor facilities LHCb, Belle II, and a future $Z$-factory.
We address the notorious metastability of the standard model (SM) and promote it to a model building task: What are the new ingredients required to stabilize the SM up to the Planck scale without encountering subplanckian Landau poles? Using the SM extended by vector-like fermions (VLFs), we chart out the corresponding landscape of Higgs stability. We find that the gauge portal mechanism, triggered by new SM charge carriers, opens up sizeable room for stability in a minimally invasive manner. We also find models with Higgs criticality, and Yukawa portals opening up at stronger coupling. Several models allow for VLFs in the TeV-range, which can be searched for at the LHC. For nontrivial flavor structure severe flavor-changing neutral current constraints arise which complement those from stability, and push lower fermion masses up to $\mathcal{O}(10^3\,\text{TeV})$.
We present a global fit of Drell-Yan, top-quark and beauty-physics data in the Standard Model Effective Field Theory framework using the Minimal Flavor Violation ansatz for the quark-flavor structure. The different energy scales are thereby connected by applying the renormalization group equation and matching onto the Weak Effective Theory in order to incorporate $b\to s$ FCNCs at low energies into the fit. We find that the combination of the different sectors greatly improves the bounds on the Wilson coefficients.
We analyze rare charm baryon decays within the standard model and beyond. We identify all null test observables in unpolarized $Λ_c \to p \ell^+ \ell^-$, $\ell=e, μ$ decays, and study the new physics sensitivities. We find that the longitudinal dilepton polarization fraction $F_L$ is sensitive to electromagnetic dipole couplings $C_{7}^{(\prime)}$, and to the right-handed 4-fermion vector coupling $C_{9}^\prime$. The forward-backward asymmetry, $A_{\rm FB}$, due to the GIM-suppression a standard model null test already, probes the left-handed axialvector 4-fermion coupling $C_{10}$; its CP-asymmetry, $A_{\rm FB}^{\rm CP}$, probes CP-violating phases in $C_{10}$. Physics beyond the standard model can induce branching ratios of dineutrino modes $Λ_c \to p ν\bar ν$ up to a few times $10^{-5}$, and one order of magnitude smaller if lepton flavor universality is assumed, while standard model rates are negligible. Charged lepton flavor violation allows for striking signals into $e^\pm μ^\mp$ final states, up to $10^{-6}$ branching ratios model-independently, and up to order $10^{-8}$ in leptoquark models. Related three-body baryon decays $Ξ_c \to Σ\ell \ell$, $Ξ_c \to Λ\ell \ell$ and $Ω_c \to Ξ\ell \ell$ offer similar opportunities to test the standard model with $|Δc|=|Δu|=1$ transitions.
We study weak radiative $|Δc|=|Δu|=1$ decays of the charmed anti-triplett ($Λ_c$, $Ξ_c^{+}$, $Ξ_c^{0}$) and sextet ($Σ_c^{++}$, $Σ_c^+$, $Σ_c^0$, $Ξ_c^{\prime +}$, $Ξ_c^{\prime 0}$, $Ω_c$) baryons in the standard model (SM) and beyond. We work out $SU(2)$ and $SU(3)_F$-symmetry relations. We propose to study self-analyzing decay chains such as $Ξ_c^+ \to Σ^+ (\to p π^0) γ$ and $Ξ_c^0 \to Λ(\to p π^-) γ$, which enable new physics sensitive polarization studies. SM contributions can be controlled by corresponding analysis of the Cabibbo-favored decays $Λ_c^+ \to Σ^+ (\to p π^0) γ$ and $Ξ_c^0 \to Ξ^0 (\to Λπ^0) γ$. Further tests of the SM are available with initially polarized baryons including $Λ_c \to p γ$ together with $Λ_c \to Σ^+ γ$ decays, or $Ω_c \to Ξ^0 γ$ together with $Ω_c \to (Λ,Σ^0) γ$. In addition, CP-violating new physics contributions to dipole operators can enhance CP-asymmetries up to few percent.
Rare $\vert Δc \vert=\vert Δu \vert=1$ processes uniquely probe flavor in the Standard Model and beyond from the up-type quark sector. Opportunities to search for BSM physics in charm arise from the severe GIM suppression which kills SM contributions to leptonic axial-vector contributions and suppresses CP violation. Semileptonic decays of charmed hadrons offer a variety of clean null test observables, featuring new physics effects which are even enhanced by resonance contributions. In particular, angular observables in three- and self-analyzing four-body baryon decays, such as $Λ_c\to p \ell^+\ell^-$ and $Ξ_c^+\toΣ^+(\to pπ^0)\ell^+\ell^-$, $Ξ_c^0\toΛ^0\,(\to p π^-)\ell^+\ell^-$ and $Ω_c^0\toΞ^0\,(\to Λ^{0} π^0)\ell^+\ell^-$ disentangle possible new physics effects in electromagnetic dipole couplings $C_7^{(\prime)}$ and (axial-)vector 4-fermion ones $C^{(\prime)}_{9\,(10)}$. There is sensitivity to BSM couplings as small as $\sim 0.01$.
$SU(2)_L$-invariance links charged dilepton and dineutrino couplings. Phenomenological implications are worked out for flavor changing neutral currents involving strange, charm, beauty and top quark transitions in a model-independent way. We put forward novel tests of lepton universality and lepton flavor conservation in $|Δc|= |Δu|=1$ and $|Δb|= |Δq|=1$, $q=d,s$, transitions suitable for the experiments Belle II and BES III. Single top production plus dileptons uniquely probes semileptonic four-fermion $|Δt|= |Δq'|=1$, $q'=u,c$, couplings and further study at the LHC is encouraged. Tests with single top production associated with dileptons and missing energy are suitable for study at future $e^+ e^-$ or muon colliders.
We study the full angular distribution of semileptonic rare charm baryon decays in which the secondary baryon undergoes weak decay with sizable polarization parameter, $Ξ_c^+\toΣ^+\,(\to pπ^0)\ell^+\ell^-$, $Ξ_c^0\toΛ^0\,(\to p π^-)\ell^+\ell^-$ and $Ω_c^0\toΞ^0\,(\to Λ^{0} π^0)\ell^+\ell^-$. Such self-analyzing decay chains allow for seven additional observables compared to three-body decays such as $Λ_c \to p \ell^+ \ell^-$, with different sensitivities to the $|Δc|=|Δu|=1$ weak couplings. Opportunities to test the standard model in $c \to u$ transitions with standard model null tests and other angular observables are worked out. We show that a joint model-independent analysis of the leptonic $A_{\text{FB}}^\ell$, hadronic $A_{\text{FB}}^{\text{H}}$, and combined forward-backward asymmetry $A_{\text{FB}}^{\ell\text{H}}$ together with the fraction of longitudinally produced leptons, $F_L$, is able to pin down the dipole couplings $C_{7},C^\prime_{7}$ and the semileptonic (axial-) vector ones $C_{10},C^\prime_{9},C^\prime_{10}$. $A_{\text{FB}}^{\text{H}}$ is also accessible with dineutrino $c \to u ν\bar ν$ modes and probes right-handed currents.
We search for weakly interacting fixed points in extensions of the minimally supersymmetric standard model (MSSM). Necessary conditions lead to three distinct classes of anomaly-free extensions involving either new quark singlets, new quark doublets, or a fourth generation. While interacting fixed points arise prolifically in asymptotically free theories, their existence is significantly constrained as soon as some of the non-abelian gauge sectors are infrared free. Performing a scan over $\sim 200$k different MSSM extensions using matter field multiplicities and the number of superpotential couplings as free parameters, we find mostly infrared conformal fixed points, and a small subset with ultraviolet ones. All settings predict low-scale supersymmetry-breaking and a violation of $R$-parity. Despite of residual interactions, the running of couplings out of asymptotically safe fixed points is logarithmic as in asymptotic freedom. Some fixed points can be matched to the Standard Model though the matching scale comes out too low. Prospects for higher matching scales and asymptotic safety beyond the MSSM are indicated.
$U(1)^\prime$ extensions of the standard model with generation-dependent couplings to quarks and leptons are investigated as an explanation of anomalies in rare $B$-decays, with an emphasis on stability and predictivity up to the Planck scale. To these ends, we introduce three generations of vector-like standard model singlet fermions, an enlarged, flavorful scalar sector, and, possibly, right-handed neutrinos, all suitably charged under the $U(1)^\prime$ gauge interaction. We identify several gauge-anomaly free benchmarks consistent with $B_s$-mixing constraints, with hints for electron-muon universality violation, and the global $b \to s$ fit. We further investigate the complete two-loop running of gauge, Yukawa and quartic couplings up to the Planck scale to constrain low-energy parameters and enhance the predictive power. A characteristic of models is that the $Z^\prime$ with TeV-ish mass predominantly decays to invisibles, i.e. new fermions or neutrinos. $Z^\prime$-production can be studied at a future muon collider. While benchmarks feature predominantly left-handed couplings $C_9^μ$ and $C_{10}^μ$, right-handed ones can be accommodated as well.
We present a systematic global analysis of dineutrino modes $b \to q \,ν\bar ν$, $q=d,s$, and charged dilepton $b \to q \,\ell^+ \ell^-$ transitions. We derive improved or even entirely new limits on dineutrino branching ratios including decays $B^0 \to (K^0 , X_s)\, ν\bar ν$, $B_s \to ϕ\,ν\bar ν$ and $B^0 \to (π^0, ρ^0)\, ν\bar ν$ from dineutrino modes which presently are best constrained: $B^+ \to (K^+,π^+, ρ^+) \,ν\bar ν$ and $B^0 \to K^{*0} \,ν\bar ν$. Using SMEFT we obtain new flavor constraints from the dineutrino modes, which are stronger than the corresponding ones from charged dilepton rare $b$-decay or Drell-Yan data, for $e τ$ and $ττ$ final states, as well as for $μτ$ ones in $b \to s$ processes. The method also allows to put novel constraints on semileptonic four-fermion operators with top quarks. Implications for ditau modes $b \to s \, τ^+ τ^-$ and $b \to d \, τ^+ τ^-$ are worked out. Furthermore, the interplay between dineutrinos and charged dileptons allows for concrete, novel tests of lepton universality in rare $B$-decays. Performing a global fit to $b \to s \,μ^+ μ^-, \,s γ$ transitions we find that lepton universality predicts the ratio of the $B^0 \to K^{*0} \,ν\bar ν$ to $B^0 \to K^0 \,ν\bar ν$ ($B^+ \to K^+ \,ν\bar ν$) branching fractions to be within 1.7 to 2.6 (1.6 to 2.4) at $1\,σ$, a region that includes the standard model, and that can be narrowed with improved charged dilepton data. There is sizable room outside this region where universality is broken and that can be probed with the Belle II experiment. Using results of a fit to $B^0 \to μ^+ μ^-$, $B^0_s\to \bar{K}^{\ast 0}\,μ^+μ^-$ and $ B^+ \to π^+\, μ^+ μ^-$ data we obtain an analogous relation for $|Δb|=|Δd|=1$ transitions.
$SU(2)_L$-invariance links charged dilepton $\bar q\,q^\prime\,\ell^+\,\ell^-$ and dineutrino $\bar q\, q^\prime\, \barν\,ν$ couplings. This connection can be established using the Standard Model Effective Field Theory framework, and allows to perform complementary experimental tests of lepton universality and charged lepton flavor conservation with flavor-summed dineutrino observables. We present its phenomenological implications for the branching ratios of rare charm decays $c\to u\,ν\,\barν$ and rare $B$ decays $b\to s\,\barν\,ν$ decays.
Following our earlier work we establish kinematic endpoint relations for baryon decays using the Wigner-Eckart theorem and apply them to $\frac{1}{2} \to \frac{1}{2}$ and $\frac{1}{2} \to \frac{3}{2}$ baryon transitions. We provide angular distributions at the kinematic endpoint which hold for the generic $d=6$ model-independent effective Hamiltonian. Moreover, we explicitly verify the endpoint relations using an explicit form factor parametrisation and clarify constraints on helicity based form factors to evidence endpoint relations. Our results provide guidance for phenomenological parameterisations, consistency checks for theory computations and experiment. Results are applicable to ongoing and future new physics searches at LHCb, BES III and Belle II with rare semileptonic-, dineutrino-and charged-modes, which include $Λ_b \to Λ^{(*)} \ell \ell, Λ_b \to Λ^{(*)} νν$, $Ω_b \to Ω\ell \ell$, $Λ_c \to p \ell \ell$, $Σ\to p \ell \ell$ and $Λ_b \to Λ_c^{(*)} \ell ν$
Evidence for electron-muon universality violation that has been revealed in $b\to s \ell\ell$ transitions in the observables $R_{K,K^*}$ by the LHCb Collaboration can be explained with spin-1 leptoquarks in $SU(2)_L$ singlet $V_1$ or triplet $V_3$ representations in the ${\cal{O}}(1-10)$ TeV range. We explore the sensitivity of the high luminosity LHC (HL-LHC) and future proton-proton colliders to $V_1$ and $V_3$ in the parameter space connected to $R_{K,K^*}$-data. We consider pair production and single production in association with muons in different flavor benchmarks. Reinterpreting a recent ATLAS search for scalar leptoquarks decaying to $b μ$ and $j μ$, we extract improved limits for the leptoquark masses: For gauge boson-type leptoquarks ($κ=1$) we obtain $M_{V_1}> 1.9$ TeV, $M_{V_1}> 1.9$ TeV, and $M_{V_1}> 1.7$ TeV for leptoquarks decaying predominantly according to hierarchical, flipped and democratic quark flavor structure, respectively. Future sensitivity projections based on extrapolations of existing ATLAS and CMS searches are worked out. We find that for $κ=1$ the mass reach for pair (single) production of $V_1$ can be up to 3 TeV (2.1 TeV) at the HL-LHC and up to 15 TeV (19.9 TeV) at the FCC-hh with $\sqrt{s}=100$ TeV and $20 \, \mbox{ab}^{-1}$. The mass limits and reach for the triplet $V_3$ are similar or higher, depending on flavor. While there is the exciting possibility that leptoquarks addressing the $R_{K,K^*}$-anomalies are observed at the LHC, to fully cover the parameter space $pp$-collisions beyond the LHC-energies are needed.
We perform global fits within Standard Model Effective Field Theory (SMEFT) combining top-quark pair production processes and decay with $b\rightarrow s$ flavor changing neutral current transitions and $Z \to b \bar b$ in three stages: using existing data from the LHC and $B$-factories, using projections for the HL-LHC and Belle II, and studying the additional new physics impact from a future lepton collider. The latter is ideally suited to directly probe $\ell^+\ell^-\rightarrow t\bar t$ transitions. We observe powerful synergies in combining both top and beauty observables as flat directions are removed and more operators can be probed. We find that a future lepton collider significantly enhances this interplay and qualitatively improves global SMEFT fits.
We present constraints on Wilson coefficients from a combined fit to data from top-quark and beauty-physics measurements within the Standard Model effective field theory. We consider present data on top-quark pair production and decay, $b\to s$ flavor changing neutral currents, and $Z\to b\bar b$ decay, as well as different future scenarios. These comprise projections for HL-LHC, Belle II, and a lepton collider using the example of CLIC. We investigate opportunities for detecting deviations from the Standard Model hinted at by present data on $b\to s$ transitions. In the fit, we find strong synergies between top-quark and beauty physics that allow to tighten constraints on various coefficients.
We compute 10 radiative three-body decays of charged charmed mesons $D^+ \to P^+ P^0 γ$ and $D_s \to P^+ P^0 γ$, $P=π, K$, in leading order QCDF, HH$χ$PT and the soft photon approximation. We work out decay distributions and asymmetries in the standard model and with new physics in the electromagnetic dipole operators. The forward-backward asymmetry is suitable to probe the QCD frameworks, in particular the $s$-channel dependent weak annihilation contributions in QCDF against the markedly different resonance structure in HH$χ$PT. These studies can be performed with Cabibbo-favored modes $D_s \to π^+ π^0 γ$, $D^+ \to π^+ \overline{K}^0 γ$ and $D_s \to K^+ \overline{K}^0 γ$ with ${\cal{O}}(10^{-4}-10^{-3})$-level branching ratio, which are standard model-like and induced by different hadronic dynamics. Understanding of the latter can therefore be improved in a data-driven way and sharpens the interpretation of standard model tests. Singly Cabibbo-suppressed modes such as $ D^+ \to π^+ π^0 γ$, $D_s \to π^+ K^0 γ$, $D_s \to K^+ π^0 γ$ with branching ratios within $\sim 10^{-5}-10^{-4}$ are sensitive to new physics that can be signalled in the forward-backward asymmetry and in the CP-asymmetry of the rate, ideally in the Dalitz region but also in single differential distributions. Results complement those with neutral $D^0 \to PP γ$ decays.