Flavor Physics Research Articles

Requirement analysis for dE/dx measurement and PID performance at the CEPC baseline detector

The Circular Electron-Positron Collider (CEPC) can be operated not only as a Higgs factory but also as a Z-boson factory, offering great opportunities for flavor physics studies where Particle Identification (PID) is critical. The baseline detector of the CEPC could record TOF and dE/dx information that can be used to distinguish particles of different species. We quantify the physics requirements and detector performance using physics benchmark analyzes with full simulation. We conclude that at the benchmark TOF performance of 50ps, the dE/dx resolution should be better than 3% for incident particles in the barrel region with a relevant momentum larger than 2GeV/c. This performance leads to an efficiency/purity for K± identification of 97%/96%, for D0→π+K− reconstruction of 68.19%/89.05%, and for ϕ→K+K− reconstruction of 82.26%/77.70%, providing solid support for relevant CEPC flavor physics measurements.

A new KS → πeν branching fraction measurement from KLOE-2

The KLOE-2 Collaboration continues the KLOE long-standing tradition of flavour physics precision measurements in the kaon sector with a new KS → πeν branching fraction measurement with 1.63 fb−1 KLOE data acquired at the DAΦNE Frascati ϕ-factory. The strategy to achieve this new result has been presented together with the combination with our previous BR(KS → πeν) measurement, based on an independent data sample, which allows the total precision to be improved by almost a factor of two, and a new derivation of f +(0)|Vus |.

The Upgrade of LHCb VELO

LHCb physics achievements to date include some of the world’s most precise flavour physics measurements, including the CKM phase γ and the discovery of CP violation in the charm sector. These achievements have been possible thanks to the enormous data samples collected and the performance of the subdetectors, with the efficiency and precision of the silicon vertex detector (VELO) being a major contributor to this success. The experiment has been upgraded to run at higher luminosity, requiring 40 MHz readout. The VELO Upgrade modules are composed of hybrid pixel detectors and electronics circuits glued onto a cooling substrate, made of thin silicon plates with embedded micro-channels, allowing for the circulation of evaporative CO 2 . The detectors are located in vacuum, separated from the beam by a thin aluminium box. The upgraded VELO is composed of 52 modules placed along the beam axis divided into two retractable halves. The design, production, installation and commissioning of the VELO Upgrade system is presented together with test results. • LHCb experiment relies on the VELO detector for accurate vertex reconstruction. • The VELO Upgrade is the brand new silicon pixel tracker of LHCb. • Detector comprises 52 double sided modules with 4 pixel sensors each. • Micro-channels circulating evaporative CO 2 allows for efficient heat dissipation. • Modules Electrical and thermal performance validated in operation conditions.

Cosmological imprints of Dirac neutrinos in a keV-vacuum 2HDM* *Supported by the National Natural Science Foundation of China (12135006, 12075097, 12047527, 11775092), as well as by the Fundamental Research Funds for the Central Universities (CCNU20TS007, CCNU19TD012, CCNU22LJ004)

The Dirac neutrino masses could be simply generated by a neutrinophilic scalar doublet with a vacuum being dramatically different from the electroweak one. While the case with an eV-scale vacuum has been widely explored previously, we exploit in this work the desert where the scalar vacuum is of scale. In this regime, there would be rare hope to probe the keV-vacuum neutrinophilic scalar model via the lepton-flavor-violating processes, which makes it distinguishable from the widely considered eV-scale vacuum. Although such a keV-vacuum scenario is inert in the low-energy flavor physics, we show that the baryogenesis realized via the lightest Dirac neutrino can be a natural candidate in explaining the baryon asymmetry of the Universe. Furthermore, the Dirac neutrinos with a keV-vacuum scalar can generate a shift of the effective neutrino number within the range , which can be probed by the future Simons Observatory experiments. In particular, the model with a minimal value can already be falsified by the future CMB Stage-IV and Large Scale Structure surveys, providing consequently striking exploratory avenues in the cosmological regime for such a keV-vacuum scenario.

Probing the h cbar{c} coupling at a Future Circular Collider in the electron-hadron mode

We study the production of a neutral Higgs boson at a Future Circular Collider in the electron-hadron mode (FCC-eh) through the leading process e^- p rightarrow nu _e h q assuming the decay channel h rightarrow c bar{c} , where h is the Standard Model (SM)-like state discovered at the Large Hadron Collider (LHC). This process is studied in the context of a 2-Higgs Doublet Model Type III (2HDM-III) embedding a four-zero texture in the Yukawa matrices and a general Higgs potential, where both Higgs doublets are coupled with up- and down-type fermions. Flavour Changing Neutral Currents (FCNCs) are well controlled by this approach through the adoption of a suitable texture once flavour physics constraints are taken in account. Considering the parameter space where the signal is enhanced and in agreement with both experimental data and theoretical conditions, we analyse the aforementioned signal by taking into account the most important SM backgrounds, separating c-jets from light-flavour and gluon ones as well as b-jets by means of efficient flavour tagging. We find that the h c bar{c} coupling strength can be accessed with good significance after a luminosity of 1 ab^{-1} for a 50 TeV proton beam and a 60 GeV electron one, the latter with a 80% (longitudinal) polarisation.

Interesting Directions in Flavor Physics

Interesting Directions in Flavor Physics

Simulation study of particle identification using cluster counting technique for the BESIII drift chamber

The particle identification of charged hadrons, especially for the separation of K and π, is crucial for the flavour physics study. Ionization measurement with the cluster counting technique, which has a much less fluctuation than traditional dE/dx measurement, is expected to provide better particle identification for the BESIII experiment. Simulation studies, including a Garfield++ based waveform analysis and a performance study of K/π identification in the BESIII offline software system have been performed. The results show that K/π separation power and PID efficiency would be improved appreciably in the momentum range above 1.2 GeV/c using cluster counting technique even with a conservative resolution assumption.

A silicon vertex detector with timing for the Upgrade II of LHCb

The LHCb Upgrade II aims at maximizing the potential for heavy flavour physics at high luminosity-LHC. The Vertex Locator (VELO) plays an important role in this task due to its excellent tracking and vertexing abilities. During Run 5 and 6, the detector will undergo through a substantial increase in data rate, pile-up and radiation damage. To achieve the same vertex reconstruction efficiency as the current Upgrade I detector, the introduction of timing and its possible implementations are discussed. Additional requirements for new sensor and ASIC technologies, introduced to cope with increased radiation damage, data rate and timing, are also presented. Finally, possible new detector layouts, adapted to expected conditions, are discussed and their impact on the RF-foil and vacuum tank geometries are treated.

MUonE, muon g−2 and electroweak precision constraints within 2HDMs

Two Higgs doublet models are attractive scenarios for physics beyond the Standard Model. In particular, lepton-specific manifestations remain contenders to explain the observed discrepancy between the anomalous magnetic moment of the muon $a_\mu$ predicted within the Standard Model and recent observations at Fermilab and BNL. Dominant uncertainties that affect $a_\mu$ have motivated the MUonE experiment to access the hadronic vacuum polarisation contribution that impacts $a_\mu$ via elastic muon-electron scattering. In this work, we contrast the high precision that is achievable within the MUonE context with constraints from flavour physics, precision electroweak constraints and LHC searches as well as their extrapolations for a range of two Higgs doublet models with a softly broken $\mathbb{Z}_2$ symmetry. We find that the sensitivity of MUonE does not extend beyond the parameter regions that are already excluded by other constraints. MUonE will therefore provide a detailed measurement of the hadronic vacuum polarisation contribution which then transparently informs $a_\mu$ interpretations in 2HDMs without modifications of correlations from beyond the Standard Model interactions. In passing we extend earlier results of LHC and flavour projections to lepton-specific 2HDM (Types X and Y) scenarios, and comment on the possibility of modifying the value of the W-boson mass; we briefly discuss the implications for a strong first-order electroweak phase transition for these models.

Heavy Flavor Physics at the EIC with the ECCE detector

The proposed Electron-Ion Collider (EIC) will operate high-energy high-luminosity electron+proton and electron+nucleus collisions to solve several unresolved fundamental questions. Due to their large masses (mc,b > ΛQCD), heavy quarks and their hadron products are ideal probes to study the nucleon/nuclear parton distribution functions in the high Bjorken-x (xBJ > 0.1) region and explore the hadronization process within the unconstrained kinematic region. Recently, the Electron-Ion Collider Comprehensive Chromodynamics Experiment (ECCE) consortium detector conceptual design has been selected as the reference design for the EIC project detector. The precise momentum and spatial resolutions provided by the ECCE tracking detector enable a series of open heavy flavor and quarkonia measurements. The physics projections of these proposed heavy flavor measurements in simulation studies using the ECCE detector design will be presented.

Role of non-Gaussian quantum fluctuations in neutrino entanglement

The flavor evolution of neutrinos in environments with large neutrino number densities is an open problem at the nexus of astrophysics and neutrino flavor physics. Among the many unanswered questions pertaining to this problem, it remains to be determined whether neutrino-neutrino coherent scattering can give rise to nontrivial quantum entanglement among neutrinos, and whether this can affect the flavor evolution in a meaningful way. To gain further insight into this question, here we study a simple system of two interacting neutrino beams and obtain the exact phase space explored by this system using the Husimi quasiprobability distribution. We observe that the entanglement induced by the coupling leads to strong delocalization in phase-space with largely non-Gaussian quantum fluctuations. The link between the neutrino entanglement and quantum fluctuations is illustrated using the one- and two-neutrino entropy. In addition, we propose an approximate phase-space method to describe the interacting neutrinos problem, where the exact evolution is replaced by a set of independent mean-field evolutions with a statistical sampling of the initial conditions. The phase-space approach provides a simple and accurate method to describe the gross features of the neutrino entanglement problem. Applications are shown using time-independent and time-dependent Hamiltonians in the nonadiabatic regime.

Inclusive weak-annihilation decays and lifetimes of beauty-charmed baryons

Imbalanced beauty-charmed baryons $ \Xi_{bc}^{+,0} $ are of great significance to the development of heavy flavor physics. In this work, we study the inclusive weak-annihilation decays of $\Xi_{bc}^{+,0}$ and their contributions to the $\Xi_{bc}^{+,0}$ lifetimes. For the calculation of the inclusive $\Xi_{bc}^{+,0}\to X_{cs}$ decay width where $X_{cs}$ stands for the sum of the final states with charm number +1 and strange number -1, we work in the heavy diquark effective theory which provides us with a convenient technical tool to construct the operator product expansion. The $\Xi_{bc}^{+,0}$ is considered to be a superposition of two states with one containing a spin-0 $bc$ diquark and the other one containing a spin-1 $bc$ diquark. It is found that both the $\Xi_{bc}^{+,0}$ lifetimes and the $\Xi_{bc}^{+,0}\to X_{cs}$ branching ratios are very sensitive to the $bc$ spin in $\Xi_{bc}^{+,0}$. The $\Xi_{bc}^{+,0}\to X_{cd}$ results are also presented. As $\Xi_{bc}^+$ has a longer lifetime than $\Xi_{bc}^{0}$ and bigger branching ratios of similar decay channels, the exclusive decays $\Xi_{bc}^+\to D^{(*)+}\Lambda$, $\Xi_{bc}^+\to \Lambda_c^+\bar{K}^{(*)0}$, $\Xi_{bc}^+\to D^{(*)+}K^-p$, and $\Xi_{bc}^+\to D^{0}\bar{K}^{(*)0}p$ are more promising for experimental searches of $\Xi_{bc}^{+,0}$ at the LHC comparing with exclusive decay $\Xi_{b c} ^{0} \to D^0pK^-$.

The flavourful present and future of 2HDMs at the collider energy frontier

We study the intersection of flavour and collider physics for Two-Higgs-Doublet models of Type I and II. Drawing from the flavour precision-LHC exotics search complementarity, we also provide a projection of the future sensitivity that can be achieved in light of currently available analyses. On the one hand, we find that the parameter space of the 2HDM can be explored significantly further with more data from the LHC with some complementarity with flavour physics. On the other hand, flavour physics results alongside their projections remain powerful tools to constrain the model space in regions where direct sensitivity to new states via exotics searches is lost. Our results further high-light the recently observed flavour physics anomalies as important drivers of new physics searches in the future; we also touch on implications for a strong first order electroweak phase transition.

Unified origin of dark matter self interactions and low scale leptogenesis

We propose a novel and minimal framework where a light scalar field can give rise to dark matter (DM) self-interactions, while enhancing the CP symmetry required for successful baryon asymmetry of the Universe via leptogenesis route. For demonstration purpose we choose to work in a scotogenic seesaw scenario where the lightest among the right handed neutrinos (RHN), introduced for generating light neutrino masses radiatively, play the role of DM while the heavier two can play non-trivial roles in generating DM relic as well as lepton asymmetry. While dark matter self-interactions mediated by an additional singlet scalar can alleviate the small scale issues of cold dark matter paradigm, the same scalar can give rise to new one-loop decay processes of heavy RHN into standard model leptons providing an enhanced contribution to CP asymmetry, even with sub-TeV scale RHN mass. The thermally under-abundant relic of DM due to large annihilation rates into its light mediator receives a late non-thermal contribution from a heavier RHN. With only five new particles involved in the scotogenic seesaw, each having non-trivial roles in generating DM relic and baryon asymmetry, the model can explain non-zero neutrino mass while being verifiable at different experiments related to DM direct detection, flavour physics and colliders. The mechanism we demonstrated here by using a scotogenic seesaw scenario is also applicable to other models.

The charged Higgs from the Bottom-Up: probing flavor at the LHC

We systematically study model-independent constraints on the three generic charged Higgs couplings to b-quarks and up-type quarks. While existing LHC searches have focussed on the tb coupling, we emphasize that the LHC plays a crucial role in probing also ub and cb couplings, since constraints from flavor physics are weak. In particular we propose various new searches that can significantly extend the present reach on the parameter space by: i) looking for light charged Higgses that decay into ub-quarks, ii) probing charged Higgs couplings to light and top quarks using multi-b-jet signatures, iii) looking for single b-quarks in low-mass dijet searches, iv) searching for charge asymmetries induced by charged Higgs production via ub couplings.

Flavour anomalies and dark matter assisted unification in SO(10) GUT

With the recent experimental hint of new physics from flavor physics anomalies, combined with the evidence from neutrino masses and dark matter, we consider a minimal extension of SM with a scalar leptoquark and a fermion triplet. The scalar leptoquark with couplings to leptons and quarks can explain lepton flavor non-universality observables RK, {R}_{K^{left(ast right)}} , {R}_{D^{left(ast right)}} and RJ/ψ. Neutral component of fermion triplet provides current abundance of dark matter in the Universe. The interesting feature of the proposal is that the minimal addition of these phenomenologically rich particles (scalar leptoquark and fermion triplet) assist in realizing the unification of the gauge couplings associated with the strong and electroweak forces of standard model when embedded in the non-supersymmetric SO(10) grand unified theory. We discuss on unification mass scale and the corresponding proton decay constraints while taking into account the GUT threshold corrections.

Sensitivity and constraints to the 2HDM soft-breaking Z2 parameter m12

In this letter we study the specific sensitivity and constraints to the soft breaking Z2 parameter m12 of the Two Higgs Doublet Model (2HDM). m12 is considered here as an input parameter of the model together with the masses of the Higgs bosons, mh, mH, mA, mH±, the ratio of the two Higgs vacuum expectation values, tan⁡β, and cos⁡(β−α), with α and β the mixing angles of the CP-even and CP-odd Higgs sector, respectively. We explore in particular the sensitivity to m12 in the decays of the lightest Higgs boson h to two photons, assuming: 1) that h is the state observed at the LHC at ∼125GeV, and 2) the h couplings to the SM particles are SM-like, by going to the alignment limit, cos⁡(β−α)=0. The aim of this work is to demonstrate possible constraints on m12 from the present measurement of the di-photon signal strength, μγγ. These constraints are relevant in the region of the 2HDM parameter space that is allowed by all the other constraints, specifically, theoretical constraints as well as experimental constraints from LHC Higgs rate measurements, Higgs boson searches, flavor physics and precision observables. The exploration is performed in the four different Yukawa types of 2HDM, where the allowed region by all the other constraints depends on the model type. We also analyze the case that the Higgs-boson mass spectrum accommodates a possible new world average of mW including the recent CDF measurement.

Model with two scalar leptoquarks: R2 and S3

We discuss a model that can accommodate the $B$-physics anomalies, based on combining two scalar leptoquarks, ${R}_{2}$ and ${S}_{3}$, of mass $\mathcal{O}(1\text{ }\text{ }\mathrm{TeV})$, and that we proposed in our previous paper. We update the analysis of its parameter space and show that a model remains viable and consistent with a number of low energy and high energy flavor physics constraints. Since the model predicts a nonzero new physics phase, we discuss the possibility to test its contribution to the neutron electric dipole moment and to the angular distributions of the exclusive $b\ensuremath{\rightarrow}c\ensuremath{\tau}\overline{\ensuremath{\nu}}$ decays. We find that the model can provide a significant enhancement to $\mathcal{B}(B\ensuremath{\rightarrow}{K}^{(*)}\ensuremath{\nu}\ensuremath{\nu})$ and provides both the upper and lower bounds to $\mathcal{B}(B\ensuremath{\rightarrow}{K}^{(*)}\ensuremath{\mu}\ensuremath{\tau})$.

Phenomenology of a flavored multiscalar Branco-Grimus-Lavoura-like model with three generations of massive neutrinos

In this paper, we present several possible anomaly free implementations of the Branco-Grimus-Lavoura (BGL) model with two Higgs doublets and one singlet scalar. The model also includes three generations of massive neutrinos that get their mass via a type-I seesaw mechanism. A particular anomaly free realization, which we dub $\ensuremath{\nu}\mathrm{BGL}$-1 scenario, is subjected to an extensive phenomenological analysis, from the perspective of flavor physics and collider phenomenology.

Searches for long-lived particles at the future FCC-ee

The electron-positron stage of the Future Circular Collider, FCC-ee, is a frontier factory for Higgs, top, electroweak, and flavour physics. It is designed to operate in a 100 km circular tunnel built at CERN, and will serve as the first step towards ≥100 TeV proton-proton collisions. In addition to an essential and unique Higgs program, it offers powerful opportunities to discover direct or indirect evidence of physics beyond the Standard Model. Direct searches for long-lived particles at FCC-ee could be particularly fertile in the high-luminosityZrun, where 5 × 1012Zbosons are anticipated to be produced for the configuration with two interaction points. The high statistics of Higgs bosons,Wbosons and top quarks in very clean experimental conditions could offer additional opportunities at other collision energies. Three physics cases producing long-lived signatures at FCC-ee are highlighted and studied in this paper: heavy neutral leptons (HNLs), axion-like particles (ALPs), and exotic decays of the Higgs boson. These searches motivate out-of-the-box optimization of experimental conditions and analysis techniques, which could lead to improvements in other physics searches.