Flavor Physics Research Articles

Flavour dynamics and violations of the CP symmetry

An overview of flavour physics and CP-violating phenomena is presented. The Standard Model quark-mixing mechanism is discussed in detail and its many successful experimental tests are summarized. Flavour-changing transitions put very stringent constraints on new-physics scenarios beyond the Standard Model framework. Special attention is given to the empirical evidences of CP violation and their important role in our understanding of flavour dynamics.The current status of the so-called flavour anomalies is also reviewed.

New perspective in searching for axionlike particles from flavor physics

We propose new perspective in searching for axion-like particles (ALPs) from quark and lepton flavor physics: measurements of the time-dependent CP asymmetry in $B^0 \to K_S^0 \pi^0 \gamma$ and the branching ratio of $B_s \to e^\pm \mu^\mp$ decay possess, along with the anomalous magnetic moment of muon. In the mass range of sub-GeV, accessible by the flavorful ALPs search, the experimental sensitivity for these flavor observables reaches the maximum at around the pion mass scale (called the {\it sweetest} spots), where a couple of loopholes (unexplored regions) for the ALP parameter space have heretofore been present, because of an unavoidable contamination with pion background events. The proposed complementary probes can precisely determine the ALP coupling to photon at these {\it sweetest} spots/loopholes, and will significantly help cover whole parameter spaces in the ALP search including the present loopholes in the future.

New flavor physics in di- and trilepton events from single-top production at the LHC and beyond

The associated production of a single-top with opposite-sign same-flavor (OSSF) di-leptons, $pp \to t \ell^+ \ell^-$ and $ pp \to t \ell^+ \ell^- + j$ ($j=$light jet), can lead to striking tri-lepton $pp \to \ell^\prime \ell^+ \ell^- + X$ and di-lepton $pp \to \ell^+ \ell^- + j_b + X$ ($j_b=b$-jet) events at the LHC, after the top decays. Although these rather generic multi-lepton signals are flavor-blind, they can be generated by new 4-Fermi flavor changing (FC) $u_i t \ell \ell$ scalar, vector and tensor interactions ($u_i \in u,c$), which we study in this paper; we match the FC $u_i t \ell \ell$ 4-Fermi terms to the SMEFT operators and also to different types of FC underlying heavy physics. The main backgrounds to these di- and tri-lepton signals arise from $t \bar t$, $Z$+jets and $VV$ ($V=W,Z$) production, but they can be essentially eliminated with a sufficiently high invariant mass selection on the OSSF di-leptons, $m_{\ell^+ \ell^-}^{\tt min}(OSSF) > 1$ TeV; the use of $b$-tagging as an additional selection in the di-lepton final state case also proves very useful. We find, for example, that the expected 95\% CL bounds on the scale of a tensor(vector) $u t \mu \mu$ interaction, with the current $\sim 140$ fb$^{-1}$ of LHC data, are $\Lambda < 5(3.2) $ TeV or $\Lambda < 4.1(2.7)$ TeV, if analyzed via the di-muon $\mu^+ \mu^- + j_b$ signal or the $e \mu^+ \mu^-$ tri-lepton one, respectively. The expected reach at the HL-LHC with 3000 fb$^{-1}$ of data is $\Lambda < 7.1(4.7)$ TeV and $\Lambda < 2.4(1.5)$ TeV for the corresponding $u t \mu \mu$ and $c t \mu \mu$ operators. We also study the potential sensitivity at future 27 TeV and 100 TeV high-energy LHC successors and also discuss the possible implications of this class of FC 4-Fermi effective interactions on lepton non-universality tests at the LHC.

In pursuit of new physics with and decays at the high-precision Frontier

The decays and play a key role for the determination of the – (q = d, s) mixing phases ϕ d and ϕ s , respectively. The theoretical precision of the extraction of these quantities is limited by doubly Cabibbo-suppressed penguin topologies, which can be included through control channels by means of the SU(3) flavour symmetry of strong interactions. Using the currently available data and a new simultaneous analysis, we discuss the state-of-the-art picture of these effects and include them in the extracted ϕ q values. We have a critical look at the standard model predictions of these phases and explore the room left for new physics. Considering future scenarios for the high-precision era of flavour physics, we illustrate that we may obtain signals for physics beyond the standard model with a significance well above five standard deviations. We also determine effective colour-suppression factors of , and decays, which serve as benchmarks for QCD calculations of the underlying decay dynamics, and present a new method using information from semileptonic and decays.

Common origin of radiative neutrino mass, dark matter and leptogenesis in scotogenic Georgi-Machacek model

We explore the phenomenology of the Georgi-Machacek model extended with two Higgs doublets and vector fermion doublets invariant under SU(2)L×U(1)Y×Z4×Z2. The Z4 symmetry is broken spontaneously while the imposed Z2 symmetry forbids triplet fields to generate any vacuum expectation value and leading to an inert dark sector providing a viable candidate for dark matter and generate neutrino mass radiatively. Another interesting feature of the model is leptogenesis arising from decay of vector-like fermions. A detailed study of the model is pursued in search for available parameter space consistent with the theoretical and experimental observations for dark matter, neutrino physics, flavor physics, matter-antimatter asymmetry in the Universe.

Searching for heavy Higgs in supersymmetric final states at the LHC

In this work, we analyse and demonstrate possible strategies to explore extended Higgs sector of the Minimal Supersymmetric Standard Model (MSSM). In particular we concentrate on heavy Higgs decays to electroweakinos. We analyse the Higgs to electroweakino decays in the allowed MSSM parameter space after taking into account 13 TeV LHC searches for supersymmetric particles and phenomenological constraints such as flavour physics, Higgs measurements and dark matter constraints. We explore some novel aspects of these Higgs decays. The final states resulting from Higgs to electroweakino decays will have backgrounds arising from the Standard Model as well as direct electroweakino production at the LHC. We demonstrate explicit kinematical differences between Higgs to electroweakino decays and associated backgrounds. Furthermore, we demonstrate for a few specific example points, optimised analysis search strategies at the high luminosity LHC (HL-LHC) run. Finally, we comment on possible search strategies for heavy Higgs decays to exotic final states, where the lightest chargino is long lived and leads to a disappearing track at the LHC.

Emergent 2HDM in the Low-Skiba-Smith little Higgs model: Musings from flavor and electroweak physics

The low energy effective theory ($\sim$ TeV) of the little-Higgs model with $SU(6)/Sp(6)$, as proposed by Low, Skiba and Smith (LSS), exhibits a two-Higgs doublet model (2HDM) structure. The symmetry dictates interesting Yukawa patterns, translating to non-trivial fermion couplings with both of the Higgs doublets. The couplings of the scalars with the fermions can induce flavor changing neutral currents (FCNC), which get constraints from flavor physics observables such as BR$(B\rightarrow X_s\gamma)$, $B_s - \bar{B}_s$ mixing etc. The precision measurement of $Z b \bar{b}$ vertex, the top and Higgs mass along with other Higgs coupling measurements at the Large Hadron Collider (LHC) also enforce severe restrictions on the LSS model. Direct LHC search results of beyond the Standard Model (BSM) particles also impose bounds on the masses. We probe the LSS model in view of the above constraints through a random scan in the multi-dimensional parameter space. We observe, on contrary to the general 2HDM scenario, the emergent 2HDM from the LSS model is less constrained from the flavor data and the $Z b \bar{b}$ measurement but is severely constrained form the electroweak (EW) searches at the LHC. From the flavor data and $Z b \bar{b}$, we find that the charged Higgs mass is relaxed with $\tan\beta$ being restricted to $0.5-5$, whereas the charged Higgs mass is pushed to larger than 1 TeV along with $\tan\beta$ being further restricted to $< 3$ when the LHC bounds are incorporated.

– Modern ARtificial Theoretical phYsicist A C++ framework automating theoretical calculations Beyond the Standard Model

Studies Beyond the Standard Model (BSM) will become more and more important in the near future with the rapidly increasing amount of data from different experiments around the world. The full study of BSM models is in general an extremely time-consuming task involving long and difficult calculations. It is in practice not possible to do exhaustive predictions in these models by hand. Here we present MARTY, a new C++ framework that fully automates calculations from the Lagrangian to physical quantities such as amplitudes or cross-sections. It can fully simplify, automatically and symbolically, physical quantities in a very large variety of models and compute Wilson coefficients in effective theories. This will considerably facilitate BSM studies in flavour physics. Contrary to the existing public codes in this field MARTY aims at providing a unique, free, open-source, powerful and user-friendly tool for high-energy physicists studying predictive BSM models, in effective or full theories up to the one-loop level, which does not rely on any external package. With a few lines of code one can gather final expressions that may be evaluated numerically for statistical analysis. Program summaryProgram Title: MARTYCPC Library link to program files:https://doi.org/10.17632/y7n82jcj8d.1Developer’s repository link: https://marty.in2p3.frLicensing provisions: GPLv3Programming language: C++Supplementary material: GNU-GSL [1], LoopTools [2]Nature of problem: Beyond the Standard Model phenomenology often requires one-loop theoretical calculations. These calculations must be done analytically because a numerical evaluation of such quantities is not possible, the number of terms being too large for any computer to handle. The required simplifications are very long and error prone, which justifies the effort to automate them. A dedicated computer algebra system is therefore needed. Some packages written with Mathematica [3] – a private and closed software for symbolic manipulations – can perform such calculations. There are usually multiple packages to interface, the user needs to pay for Mathematica and the calculation of Wilson coefficients for BSM and at the one-loop level is complicated to automate.Solution method: MARTY is a code written for BSM phenomenology that comes with its own computer algebra system, CSL, and automates the calculation of theoretical quantities at the one-loop level, in a very large variety of BSM models. This provides a unique C++ code, free and open-source, that can be used to do model building and calculate any kind of amplitudes, squared amplitudes or Wilson coefficients in BSM scenarios starting from a given Lagrangian. From the developers point of view, having such a code, independent of any other framework using only the C++ standard library (C++17 standard), is a great opportunity to develop it even further implementing new simplification procedures, different types of calculation and more.

Current bounds and future prospects of light neutralino dark matter in the NMSSM

Unlike its minimal counterpart, the Next to Minimal supersymmetric Standard Model (NMSSM) allows the possibility that the lightest neutralino could have a mass as small as $\sim 1 {\rm GeV}$ while still providing a significant component of relic dark matter (DM). Such a neutralino can provide an invisible decay mode to the Higgs as well. Further, the observed SM-like Higgs boson ($H_{125}$) could also have an invisible branching fraction as high as $\sim 19\%$. Led by these facts, we first delineate the region of parameter space of the NMSSM with a light neutralino ($M_{{\tilde{\chi}}_{1}^{0}} < 62.5 {\rm GeV}$) that yields a thermal neutralino relic density smaller than the measured relic density of cold dark matter, and is also compatible with constraints from collider searches, searches for dark matter, and from flavor physics. We then examine the prospects for probing the NMSSM with a light neutralino via direct DM detection searches, via invisible Higgs boson width experiments at future $e^+e^-$ colliders, via searches for a light singlet Higgs boson in $2b2\mu$, $2b2\tau$ and $2\mu2\tau$ channels and via pair production of winos or doublet higgsinos at the high luminosity LHC and its proposed energy upgrade. For this last-mentioned electroweakino search, we perform a detailed analysis to map out the projected reach in the $3l+{\rm E{\!\!\!/}_T}$ channel, assuming that chargino decays to $W {\tilde{\chi}}_{1}^{0}$ and the neutralino(s) decay to $Z$ or $H_{125}$ + ${\tilde{\chi}}_{1}^{0}$. We find that the HL-LHC can discover SUSY in just part of the parameter space in each of these channels, which together can probe almost the entire parameter space. The HE-LHC probes essentially the entire region with higgsinos (winos) lighter than 1 TeV (2 TeV) independently of how the neutralinos decay, and leads to significantly larger signal rates.

A GPU based multidimensional amplitude analysis to search for tetraquark candidates

The demand for computational resources is steadily increasing in experimental high energy physics as the current collider experiments continue to accumulate huge amounts of data and physicists indulge in more complex and ambitious analysis strategies. This is especially true in the fields of hadron spectroscopy and flavour physics where the analyses often depend on complex multidimensional unbinned maximum-likelihood fits, with several dozens of free parameters, with an aim to study the internal structure of hadrons. Graphics processing units (GPUs) represent one of the most sophisticated and versatile parallel computing architectures that are becoming popular toolkits for high energy physicists to meet their computational demands. GooFit is an upcoming open-source tool interfacing ROOT/RooFit to the CUDA platform on NVIDIA GPUs that acts as a bridge between the MINUIT minimization algorithm and a parallel processor, allowing probability density functions to be estimated on multiple cores simultaneously. In this article, a full-fledged amplitude analysis framework developed using GooFit is tested for its speed and reliability. The four-dimensional fitter framework, one of the firsts of its kind to be built on GooFit, is geared towards the search for exotic tetraquark states in the B^0 rightarrow J/psi K pi decays and can also be seamlessly adapted for other similar analyses. The GooFit fitter, running on GPUs, shows a remarkable improvement in the computing speed compared to a ROOT/RooFit implementation of the same analysis running on multi-core CPU clusters. Furthermore, it shows sensitivity to components with small contributions to the overall fit. It has the potential to be a powerful tool for sensitive and computationally intensive physics analyses.

A View of Flavour Physics in 2021

Based on a view of current flavour physics and motivated by the hierarchy problem and by the pattern of quark masses and mixings, I describe a picture of flavour physics that should give rise in a not too distant future to observable deviations from the SM in Higgs compositeness and/or in B-decays with violations of Lepton Flavour Universality, as hinted by current data, or perhaps even in supersymmetry, depending on the specific realisation.

Single-top final states as a probe of top-flavoured dark matter models at the LHC

Models incorporating flavoured dark matter provide an elegant solution to the dark matter problem, evading the tight LHC and direct direction constraints on simple WIMP models. In Dark Minimal Flavour Violation, a simple framework of flavoured dark matter with new sources of flavour violation, the constraints from thermal freeze-out, direct detection experiments, and flavour physics create well-defined benchmark scenarios for these models. We study the LHC phenomenology of four such scenarios, focusing on final states where a single top quark is produced accompanied by no jets, one jet from the fragmentation of light quarks or a b-tagged jet. For each of these signatures we develop a realistic LHC analysis, and we show that the proposed analyses would increase the parameter space coverage for the four benchmarks, compared to existing flavour-conserving LHC analyses. Finally we show the projected discovery potential of the considered signatures for the full LHC statistics at 14 TeV, and for the High Luminosity LHC.

Isospin Breaking in Lattice QCD Computations of Decay Amplitudes

The remarkable recent progress in the precision of Lattice QCD computations for a number of physical quantities relevant for flavour physics has motivated the introduction of isospin-breaking effects, including in particular electromagnetic corrections, to the computations. The isospin breaking corrections are necessary to fully exploit this improved precision for the determination of the fundamental parameters of the Standard Model, including the CKM matrix elements, and to look for deviations from experimental measurements which might signal the presence of new physics. Together with colleagues from Rome, we have developed and implemented a framework for including isospin-breaking corrections in leptonic decays $P\to\ell\bar\nu_\ell(\gamma)$, where $P$ is a pseudoscalar meson and $\ell$ a charged lepton, and the theoretical framework and numerical results are reviewed below. The status of our studies to extend this framework to semileptonic decays $P_1\to P_2\ell\bar\nu_\ell(\gamma)$, where $P_{1,2}$ are pseudoscalar mesons, is also presented.

CP asymmetries in the rare top decays t→cγ and t→cg

The rare radiative flavour changing top decays $t\to c\gamma$ and $t\to cg$ (and the even rarer $t\to u\gamma$ and $t\to ug$) have been processes of interest for decades as they offer a key probe for studying top quark properties. However an explicit analytical study of the branching ratios and $CP$ asymmetries resulting from these loop level processes has thus far evaded attention. In this work, we provide the formulation for the $CP$ asymmetry resulting from the total kinetic contribution of the loop integrals and their imaginary parts, as well as an updated numerical computation of the predicted Standard Model (SM) branching fractions. These rare processes are suppressed in the SM by the Glashow-Iliopoulos-Maiani (GIM) mechanism. However, the results presented here can easily be exported for use in minimal extensions of the SM including vector-like quarks or in Two-Higgs-Doublet models where radiative fermionic decay processes can be enhanced relative to the SM by several orders of magnitude. Such processes provide an experimentally clean signature for new fundamental physics and can potentially be tested by current collider experiments. These topical beyond the SM theories are an elegant means to provide improved global fits to the latest results emerging from flavour physics, CKM and precision electroweak measurements.

The ATLAS Level-1 topological processor: Experience and upgrade plans

During Run 2 (2015–2018) the Large Hadron Collider has provided, at the World’s highest energy frontier, proton–proton collisions to the ATLAS experiment with high instantaneous luminosity (up to [Formula: see text]), placing stringent operational and physics requirements on the ATLAS trigger system in order to reduce the 40 MHz collision rate to a manageable event storage rate of 1 kHz, while not rejecting interesting collisions. The Level-1 trigger is the first rate-reducing step in the ATLAS trigger system with an output rate of up to 100 kHz and decision latency of less than 2.5 [Formula: see text]s. In Run 2, an important role was played by the Level-1 Topological Processor (L1Topo). This innovative system consists of two blades designed in AdvancedTCA form factor, mounting four individual state-of-the-art processors, and providing high input bandwidth and low latency data processing. Up to 128 topological trigger algorithms can be implemented to select interesting events by applying kinematic and angular requirements on electromagnetic clusters, hadronic jets, muons and total energy reconstructed in the ATLAS apparatus. This resulted in a significantly improved background rejection and enhanced acceptance of physics signal events, despite the increasing luminosity. The L1Topo system has become more and more important for physics analyses making use of low energy objects, commonly present in the Heavy Flavor or Higgs physics events, for example. An overview of the L1Topo architecture, simulation and performance results during Run 2 is presented alongside with upgrade plans for the L1Topo system to be installed for the future Run 3 data taking period.

Radial distribution of charm quarks in jets in high-energy heavy-ion collisions

Heavy flavor physics in high-energy heavy-ion collisions is a promising and active area to study the mass dependence of the “jet quenching” effects both at the RHIC and the LHC. In this talk, we present the first theoretical study on the D0 meson radial distributions relative to the jet axis both in p+p and Pb+Pb collisions at sNN=5.02TeV, where a nice agreement of our results with experimental data is observed. The in-medium parton propagations are described by a Monte Carlo transport model which uses the next-to-leading order (NLO) plus parton shower (PS) event generator SHERPA as input and includes elastic (collisional) and inelastic (radiative) in-medium interaction of heavy flavor jet. We find that, at low D0 meson pT, the radial distribution significantly shifts to larger radius indicating a strong diffusion effect, and the diffusion effects decrease quickly with pT, which is consistent with the recent CMS measurements. We demonstrate that the angular deviation of charm quarks is sensitive to Ds but not qˆ, which may provide new constrains on the collisional and radiative heavy quark energy loss.

Heavy flavor physics with the sPHENIX MAPS vertex tracker upgrade

The sPHENIX detector at the Relativistic Heavy Ion Collider will measure a suite of unique jet and Upsilon observables with unprecedented statistics and kinematic reach. A MAPS-based vertex detector upgrade to sPHENIX, the MVTX, will provide a precise determination of the impact parameter of tracks relative to the collision vertex in high multiplicity heavy ion collisions. The MVTX utilizes the latest generation of MAPS technology to provide precision tracking with high tracking efficiency over a broad momentum range in the high luminosity RHIC environment. These new capabilities will enable precision measurements of open heavy flavor observables, covering an unexplored kinematic regime at RHIC. The physics program, its potential impact, and recent detector development of the MVTX will be discussed.

Inelastic fermion dark matter origin of XENON1T excess with muon (g − 2) and light neutrino mass

Motivated by the recently reported excess in electron recoil events by the XENON1T collaboration, we propose an inelastic fermion dark matter (DM) scenario within the framework of a gauged Lμ−Lτ extension of the standard model which can also accommodate tiny neutrino masses as well as anomalous muon magnetic moment (g−2)μ. A Dirac fermion DM, naturally stabilised due to its chosen gauge charge, is split into two pseudo-Dirac mass eigenstates due to Majorana mass term induced by singlet scalar which also takes part in generating right handed neutrino masses responsible for type I seesaw origin of light neutrino masses. The inelastic down scattering of heavier DM component can give rise to the XENON1T excess for keV scale mass splitting with lighter DM component. We fit our model with XENON1T data and also find the final parameter space by using bounds from (g−2)μ, DM relic, lifetime of heavier DM, inelastic DM-electron scattering rate, neutrino trident production rate as well as other flavour physics, astrophysical and cosmological observations. A tiny parameter space consistent with all these bounds and requirements will face further scrutiny in near future experiments operating at different frontiers.

Anomalies in B -meson decays and the muon g−2 from dark loops

We explore a class of models which can provide a common origin for the recently observed evidence for lepton flavor universality violation in $b \to s l^+ l^- $ decays, the dark matter (DM) problem and the long-standing muon $(g-2)$ anomaly. In particular, both anomalies in the $B$ meson decays and the muon $(g-2)$ can be explained by the additional one-loop diagrams with DM candidates. We first classify several simple models according to the new fields' quantum numbers. We then focus on a specific promising model and perform a detailed study of both DM and flavor physics. A random scan over the relevant parameter space reveals that there is indeed a large parameter space which can explain the three new physics phenomena simultaneously, while satisfying all other flavor and DM constraints. Finally, we discuss some of the possible new physics signatures at the Large Hadron Collider.

A connection between flavour anomaly, neutrino mass, and axion

We propose a minimal model in which the flavour anomaly in the b → sμ+μ− transition is connected to the breaking of Peccei-Quinn (PQ) symmetry. The flavour anomaly is explained from new physics contribution by introducing one generation of heavy quark and heavy lepton which are vector-like under the standard model (SM) gauge group but charged under a local U(1)X group. They mix with the SM quarks and leptons, inducing flavour-changing Z′ couplings, which generates the b → sμ+μ− anomaly at tree level. On the other hand the new fermions are chiral under the global Peccei-Quinn(PQ) symmetry. The pseudo-Goldstone boson coming from the spontaneous breaking of the PQ symmetry becomes an axion, solving the strong CP problem and providing a cold dark matter candidate. The same symmetry prevents the right-handed neutrino from having a Majorana mass term. But the introduction of a neutrino-specific Higgs doublet allows neutrino to have Dirac mass term without fine-tuning problem. The model shows an interplay between axion, neutrino, dark matter, and flavour physics.