Purely flavor-changing Z' bosons and where they might hide

A plethora of ultraviolet completions of the Standard Model have extra U(1) gauge symmetries. In general, the associated massive $Z^\prime$ gauge boson can mediate flavor-changing neutral current processes at tree level. We consider a situation where the $Z^\prime$ boson couples solely via flavor-changing interactions to quarks and leptons. In this scenario the model parameter space is, in general, quite well constrained by existing flavor bounds. However, we argue that cancellation effects shelter islands in parameter space from strong flavor constraints and that these can be probed by multipurpose collider experiments like ATLAS or CMS as well as LHCb in upcoming runs at the LHC. In still allowed regions of parameter space these scenarios may help to explain the current tension between theory and experiment of $(g-2)_\mu$ as well as a small anomaly in $\tau$ decays.Comment: 44 pages, 22 figures; v2: Corrected factor of -1, updated references, improved discussion of tau decays; v3: Minor corrections, further extended discussion of tau decay

Phenomenology of Extra Abelian Gauge Symmetries

In the search for new physics, models of extra U(1) symmetries provide a simple but versatile theoretical framework to attenuate shortcomings of the Standard Model (SM). In an effort to systematically survey the physical prospects of such theories, we study three different phenomenological aspects of extra U(1) symmetries. First, studying Flavor-Changing Neutral Currents (FCNC) mediated by an effective heavy Z' boson, we identify the most stringent constraints for all possible combinations of lepton and quark flavor violating couplings.In regions of parameter space where severe bounds from meson mixing are avoided, LHC resonance searches can provide the leading constraint. Such scenarios are able to explain the observed (g-2)_\mu and tau decay anomalies. Second, we consider the possibility that the hidden photon is the gauge boson of the anomaly-free groups U(1)_{B-L}, U(1)_{L_\mu-L_e}, U(1)_{L_e-L_\tau} or U(1)_{L_\mu-L_\tau}. Taking into account loop-induced kinetic mixing, we perform a comprehensive analysis of all hidden photon constraints in these scenarios. While generically stringent bounds arise due to neutrino couplings, for U(1)_{L_{\mu}-L_\tau} an explanation of the (g-2)_\mu anomaly is possible. Finally, extending the previously considered U(1)_{L_\mu-L_\tau} model by a vector-like fermion \chi, we find that the (g-2)_\mu anomaly and the dark matter relic abundance can be simultaneously explained

Potential of FX06 to prevent disease progression in hospitalized non-intubated COVID-19 patients — the randomized, EU-wide, placebo-controlled, phase II study design of IXION

Background: More than 2.7 million hospitalizations of COVID-19-infected patients have occurred in Europe alone since the outbreak of the coronavirus in 2020. Interventions against SARS-CoV-2 are still in high need to prevent admissions to ICUs worldwide. FX06, a naturally occurring peptide in humans and other mammals, has the potential to reduce capillary leak by improving endothelial dysfunction and thus preventing the deterioration of patients. With IXION, we want to investigate the potential of FX06 to prevent disease progression in hospitalized, non-intubated COVID-19 patients. Methods: IXION is an EU-wide, multicentre, placebo-controlled, double-blinded, parallel, randomized (2:1) phase II clinical study. Patient recruitment will start in September 2022 (to Q2/2023) in Germany, Italy, Lithuania, Spain, Romania, Portugal, and France. A total of 306 hospitalized patients (>= 18 years and < 75 years) with a positive SARS-CoV-2 PCR test and a COVID-19 severity of 4-6 according to the WHO scale will be enrolled. After randomization to FX06 or placebo, patients will be assessed until day 28 (and followed up until day 60). FX06 (2 x 200 mg per day) or placebo will be administered intravenously for 5 consecutive days. The primary endpoint is to demonstrate a difference in the proportion of patients with progressed/worsened disease state in patients receiving FX06 compared to patients receiving placebo. Secondary endpoints are lung function, oxygen saturation and breathing rate, systemic inflammation, survival, capillary refill time, duration of hospital stay, and drug accountability. Discussion: With IXION, the multidisciplinary consortium aims to deliver a new therapy in addition to standard care against SARS-CoV-2 for the clinical management of COVID-19 during mild and moderate stages. Potential limitations might refer to a lack of recruiting and drop-out due to various possible protocol violations. While we controlled for drop-outs in the same size estimation, recruitment problems may be subject to external problems difficult to control for

A Consistent Theory of Kinetic Mixing and the Higgs Low-Energy Theorem

Extensions of the Standard Model of particle physics with new Abelian gauge groups allow for kinetic mixing between the new gauge bosons and the hypercharge gauge boson, resulting in mixing with the photon. In many models the mixing with the hypercharge gauge boson captures only part of the kinetic mixing term with the photon, since the new gauge bosons can also mix with the neutral component of the $SU(2)_L$ gauge bosons. We take these contributions into account and present a consistent description of kinetic mixing for general Abelian gauge groups both in the electroweak symmetric and the broken phase. We identify an effective operator that captures the kinetic mixing with $SU(2)_L$ and demonstrate how renormalisable contributions arise if the charged fields only obtain their masses from electroweak symmetry breaking. For the first time, a low-energy theorem for the couplings of novel Abelian gauge bosons with the Standard Model Higgs boson is derived from the one-loop kinetic mixing amplitudes.Comment: 6 pages, 1 figur

Flavor structure of anomaly-free hidden photon models

Extensions of the Standard Model with an Abelian gauge group are constrained by gauge anomaly cancellation, so that only a limited number of possible charge assignments is allowed without the introduction of new chiral fermions. For flavor universal charges, couplings of the associated hidden photon to Standard Model fermions are flavor conserving at tree level. We show explicitly that even the flavor-specific charge assignments allowed by anomaly cancellation condition lead to flavor-conserving tree-level couplings of the hidden photon to quarks and charged leptons if the Cabibbo-Kobayashi-Maskawa or Pontecorvo-Maki-Nakagawa-Sakata matrix can be successfully reconstructed. Further, loop-induced flavor-changing couplings are strongly suppressed. As a consequence, the structure of the Majorana mass matrix is constrained and flavor-changing tree-level couplings of the hidden photon to neutrino mass eigenstates are identified as a means to distinguish the U(1)B−L gauge boson from any other anomaly-free extension of the Standard Model without new chiral fermions. We present a comprehensive analysis of constraints and projections for future searches for a U(1)B−L gauge boson, calculate the reach of resonance searches in B meson decays and comment on the implications for nonstandard neutrino interactions