Search Results (10)

The search for New Physics (NP) beyond the Standard Model (SM) has been a central focus of particle physics, including in the context of B-meson decays involving $b\to s\ell \ell$ transitions. These transitions, mediated by flavour-changing neutral currents, are highly sensitive to small NP effects due to their suppression in the SM. While direct searches at colliders have not yet led to NP discoveries, indirect probes through semi-leptonic decays have revealed anomalies in observables such as the branching fraction $\mathcal{B}(B\to K\mu \mu )$ and the angular observable $P_5^{\prime }(B\to K^{\ast }\mu \mu )$. In order to assess the observed tensions, it is essential to ensure an accurate SM prediction. In this review, we examine the theoretical basis of the $B\to K^{(\ast )}\ell \ell$ decays, addressing in particular key uncertainties arising from local and non-local form factors. We also discuss the impact of QED corrections to the Wilson coefficients, as well as the effect of CKM matrix elements on the predictions and the tension with the experimental measurements. We discuss the most recent results, highlighting ongoing efforts to refine predictions and to constrain potential signs of NP in these critical decay processes. Full article

Tests of lepton flavour universality in rare decays of b hadrons mediated by flavour-changing neutral-current transitions constitute sensitive probes for physics beyond the standard model. In recent years, such tests have become increasingly precise and have attracted significant theoretical and experimental attention. In this article, we review the status of searches for lepton flavour universality violations in these processes and discuss prospects for future measurements at various facilities. Full article

The flavour anomalies are a set of experimental deviations from the Standard Model (SM) predictions in several observables involving decays of bottom quarks. In particular, tensions between theory and experiment in measurements involving a bottom quark decaying into a strange quark and a pair of muons have motivated much theoretical work to explore possible new physics explanations. This review summarises the tumultuous evolution of these tensions, focusing on the most recent experimental results and their implications for physics beyond the SM. We also discuss the prospects for future measurements and tests of the flavour anomalies at the LHC and other facilities. Full article

Flavour changing neutral current (FCNC) processes are described by loop diagrams in the Standard Model (SM), while in 331 models, based on the gauge group $\mathrm{SU}{\left(3\right)}_C\times \mathrm{SU}{\left(3\right)}_L\times \mathrm{U}{\left(1\right)}_X$, they are dominated by tree-level exchanges of a new heavy neutral gauge boson $Z^{\prime }$. By exploiting this feature, observables related to FCNC decays of K, $B_d$ and $B_s$ mesons can be considered in several variants of 331 models. The variants are distinguished by the value of a parameter $\beta$ that plays a key role in this framework. Imposing constraints on the $\Delta F=2$ observables, we select possible ranges for the mass of the $Z^{\prime }$ boson in correspondence to the values $\beta =\pm k/\sqrt{3}$, with $k=1,2$. The results are used to determine the impact of 331 models on $b\to s$ processes and on the correlations among them, in the light of new experimental data recently released. Full article

The Large Hadron Collider (LHC) is at the frontier of collider physics today, probing new physics at unprecedented energy scales. Many theories of physics beyond the Standard Model seek to elucidate the underlying mechanism of electroweak symmetry breaking. Given their large Yukawa couplings to the Higgs boson, third generations quarks of the Standard Model, and especially the top quark, play a key role in such theories. Therefore, new particles predicted by these theories often couple preferentially to top and bottom quarks. The favoured coupling to third generation can also be used to explain recently observed flavour physics anomalies in the LHCb, Babar or Belle experiments. This article will review recent searches for new physics performed by the ATLAS and CMS experiments at the LHC, in final states containing top and bottom quarks. In particular, searches for vector-like quarks, leptoquarks, and heavy scalar and gauge bosons will be discussed. Full article

This is a pedagogical introduction to the physics of confinement on $R^3\times S^1$, using $SU\left(2\right)$ Yang–Mills with massive or massless adjoint fermions as the prime example; we also add fundamental flavours to conclude. The small-$S^1$ limit is remarkable, allowing for controlled semiclassical determination of the nonperturbative physics in these, mostly non-supersymmetric, theories. We begin by reviewing the Polyakov confinement mechanism on $R^3$. Moving on to $R^3\times S^1$, we show how introducing adjoint fermions stabilizes center symmetry, leading to abelianization and semiclassical calculability. We explain how monopole–instantons and twisted monopole–instantons arise. We describe the role of various novel topological excitations in extending Polyakov’s confinement to the locally four-dimensional case, discuss the nature of the confining string, and the $\theta$-angle dependence. We study the global symmetry realization and, when available, present evidence for the absence of phase transitions as a function of the $S^1$ size. As our aim is not to cover all work on the subject, but to prepare the interested reader for its study, we also include brief descriptions of topics not covered in detail: the necessity for analytic continuation of path integrals, the study of more general theories, and the ’t Hooft anomalies involving higher-form symmetries. Full article

Motivated by the SU(2)${}_{\mathrm{CMB}}$ modification of the cosmological model $\Lambda$CDM, we consider isolated fuzzy-dark-matter lumps, made of ultralight axion particles whose masses arise due to distinct SU(2) Yang–Mills scales and the Planck mass $M_P$. In contrast to SU(2)${}_{\mathrm{CMB}}$, these Yang–Mills theories are in confining phases (zero temperature) throughout most of the Universe’s history and associate with the three lepton flavours of the Standard Model of particle physics. As the Universe expands, axionic fuzzy dark matter comprises a three-component fluid which undergoes certain depercolation transitions when dark energy (a global axion condensate) is converted into dark matter. We extract the lightest axion mass $m_{a,e}=0.675\times {10}^{-23}$ eV from well motivated model fits to observed rotation curves in low-surface-brightness galaxies (SPARC catalogue). Since the virial mass of an isolated lump solely depends on $M_P$ and the associated Yang–Mills scale the properties of an e-lump predict those of $\mu$- and $\tau$-lumps. As a result, a typical e-lump virial mass ∼$6.3\times {10}^{10}{M}_{\odot }$ suggests that massive compact objects in galactic centers such as Sagittarius A* in the Milky Way are (merged) $\mu$- and $\tau$-lumps. In addition, $\tau$-lumps may constitute globular clusters. SU(2)${}_{\mathrm{CMB}}$ is always thermalised, and its axion condensate never has depercolated. If the axial anomaly indeed would link leptons with dark matter and the CMB with dark energy then this would demystify the dark Universe through a firmly established feature of particle physics. Full article

Flavour anomalies have attracted a lot of attention over recent years as they provide unique hints for possible New Physics. Here, we consider a supersymmetric (SUSY) extension of the Standard Model (SM) with an additional anomaly-free gauge $U\left(1\right)$ group. The key feature of our model is the particular choice of non-universal charges to the gauge boson $Z^{\prime }$, which not only allows a relaxation of the flavour discrepancies but, contrary to previous studies, can reproduce the SM mixing matrices both in the quark and lepton sectors. We pay special attention to the latter and explicitly enumerate all parameters relevant for our calculation in the low-energy effective theory. We find regions in the parameter space that satisfy experimental constraints on meson mixing and LHC $Z^{\prime }$ searches and can alleviate the flavour anomalies. In addition, we also discuss the predictions for lepton-flavour violating decays $B^{+}\to K^{+}\mu \tau$ and $B^{+}\to K^{+}e\tau$. Full article

Symmetries in the Physical Laws of Nature lead to observable effects. Beyond the regularities and conserved magnitudes, the last few decades in particle physics have seen the identification of symmetries, and their well-defined breaking, as the guiding principle for the elementary constituents of matter and their interactions. Flavour SU(3) symmetry of hadrons led to the Quark Model and the antisymmetric requirement under exchange of identical fermions led to the colour degree of freedom. Colour became the generating charge for flavour-independent strong interactions of quarks and gluons in the exact colour SU(3) local gauge symmetry. Parity Violation in weak interactions led us to consider the chiral fields of fermions as the objects with definite transformation properties under the weak isospin SU(2) gauge group of the Unifying Electro-Weak SU(2) × U(1) symmetry, which predicted novel weak neutral current interactions. CP-Violation led to three families of quarks opening the field of Flavour Physics. Time-reversal violation has recently been observed with entangled neutral mesons, compatible with CPT-invariance. The cancellation of gauge anomalies, which would invalidate the gauge symmetry of the quantum field theory, led to Quark–Lepton Symmetry. Neutrinos were postulated in order to save the conservation laws of energy and angular momentum in nuclear beta decay. After the ups and downs of their mass, neutrino oscillations were discovered in 1998, opening a new era about their origin of mass, mixing, discrete symmetries and the possibility of global lepton-number violation through Majorana mass terms and Leptogenesis as the source of the matter–antimatter asymmetry in the universe. The experimental discovery of quarks and leptons and the mediators of their interactions, with physical observables in spectacular agreement with this Standard Theory, is the triumph of Symmetries. The gauge symmetry is exact only when the particles are massless. One needs a subtle breaking of the symmetry, providing the origin of mass without affecting the excellent description of the interactions. This is the Brout–Englert–Higgs Mechanism, which produces the Higgs Boson as a remnant, discovered at CERN in 2012. Open present problems are addressed with by searching the New Physics Beyond-the-Standard-Model. Full article

Anomalies recently observed in semileptonic $b\to c{\ell }^{-}{\overline{\nu }}_{\ell }$ and $b\to s{\ell }^{+}{\ell }^{-}$ transitions point to violation of Lepton Flavour Universality. Strategies for new analyses of different modes are required, in particular for the modes induced by the $b\to u$ transition. We describe the purely leptonic B decay, the $\overline{B}\to \pi {\ell }^{-}{\overline{\nu }}_{\ell }$ channel and the B semileptonic modes to $\rho (770)$ and $a_1(1260)$ in extensions of the Standard Model involving Lepton Flavour Universality violating $b\to u$ operators. In particular, we review the observables in the four-dimensional angular $\overline{B}\to \rho (\pi \pi ){\ell }^{-}{\overline{\nu }}_{\ell }$ and $\overline{B}\to a_1(\rho \pi ){\ell }^{-}{\overline{\nu }}_{\ell }$ distributions, suitable to pin down deviations from the Standard Model. We discuss the complementarity among the various modes for New Physics searches. Full article