Flavour Physics Measurements Research Articles

Heavy flavor machine learning algorithms for fast data processing in sPHENIX

The sPHENIX experiment at RHIC utilizes the first new heavy ion detector since the switch on of the LHC experiments. It’s optimized for precision jet and heavy flavor physics measurements, and recorded its first collisions in spring 2023. sPHENIX uses a tracking system with streaming readout and barrel calorimetry to reconstruct the collision topology. Event plane detectors, minimum bias detectors and zero-degree calorimeters are used to characterize the event. The streaming readout detectors are capable of recording 10% of the minimum bias rate, in addition to triggered events, in p+p collisions which will enable unabated, precision b-hadron and heavy flavor jet measurements at RHIC. An AI-assisted hardware trigger demonstrator is under development to sample the remaining 90% of minimum-bias p+p collisions with an aim for further deployment at the EIC.

Chasing the two-Higgs doublet model in the di-Higgs boson production

We investigate the di-Higgs production at the Large Hadron Collider in the two-Higgs doublet model (2HDM). In particular, we study the production of an extra neutral Higgs boson $\phi$ in association with the Standard Model (SM) Higgs boson $h$ in the Higgs alignment limit. We analyze two scenarios where the additional Higgs $\phi$ is CP-even or -odd state with a large top-Yukawa interaction. The leading contribution of this production comes from the top-quark loop-induced gluon-fusion channel $gg \to h\phi$. The measurement of the $h\phi$ production can probe the quartic couplings in the Higgs potential as well as the top-Yukawa couplings. Imposing both theoretical constraints (from the perturbative unitarity and the vacuum stability bounds) and experimental bounds (from the SM Higgs and flavor physics measurements) on the 2HDM parameter space, we calculate the production cross-section of $gg \to h\phi$. Furthermore, we scrutinize these processes in the parameter spaces where the CMS di-tau and di-photon excesses around 100$\,$GeV, and/or the muon $g-2$ anomaly can be accommodated.

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.

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.

Prospects for light charged scalars in a three-Higgs-doublet model with Z3 symmetry

The stringent constraints from the direct searches for exotic scalars at the LHC as well as indirect bounds from flavor physics measurements have imposed severe restrictions on the parameter space of new physics models featuring extended Higgs sectors. In the Type-II 2HDM, this implies a lower bound on the charged Higgs masses of $\cal O$(600 GeV). In this work we analyze the phenomenology of a Z3HDM in the alignment limit focusing on the impact of flavor physics constraints on its parameter space. We show that the couplings of the two charged Higgs bosons in this model feature an additional suppression factor compared to Type-II 2HDM. This gives rise to a significant relaxation of the flavor physics constraints in this model, allowing the charged Higgs masses to be as low as $\cal O$(200 GeV). We also consider the constraints coming from precision electroweak observables and the observed diphoton decay rate of the 125 GeV Higgs boson at the LHC. The bounds coming from the direct searches of nonstandard Higgs bosons at the LHC, particularly those from resonance searches in the ditau channel, prove to be very effective in constraining this scenario further.

A comparative study of the S_1 and U_1 leptoquark effects in the light quark regime

We study the phenomenology of two leptoquarks, the Standard Model SU(2) singlets S_1 and U_1, with regard to the latest experimental data from the low-energy flavor physics measurements, LHC, and the IceCube neutrino experiment. We consider a scenario when scalar (vector) leptoquark S_1 (U_1) couples exclusively to the down quark and the neutrinos (charged leptons) of all flavors, where the leptoquark in question couples to the SM lepton doublets. The couplings of S_1 (U_1) to the up-type quarks and the charged leptons (neutrinos) are in turn uniquely determined via SU(2) symmetry. We find that the most important constraints on the leptoquark parameter space originate from flavor physics measurements, followed by the LHC search limits that take over the flavor physics ones in the large LQ mass regime. We furthermore show that S_1 (U_1) marginally improves (spoils) the fit of the current IceCube data with respect to the SM case within the region of parameter space that is otherwise consistent with various low-energy flavor physics measurements and the latest LHC input. Our study offers an up-to-date analysis for these two leptoquarks in view of the latest experimental data.

Reconstructing $$K^0_S$$ and $$\varLambda $$ in the CEPC baseline detector

The proposed Circular Electron Positron Collider (CEPC) is expected to produce up to 1 trillion Z bosons in 2 years and has a great potential on the flavor physics. The $$K^0_S$$ and $$\varLambda $$ are important physics signatures for many flavor physics measurements and are abundantly produced in the CEPC Z pole program. Using a full simulation tool, we study the CEPC baseline detector performance of the $$K^0_S/\varLambda $$ reconstruction with fully charged final states. For the $$K^0_S/\varLambda $$, the baseline detector reaches an inclusive efficiency/purity of $$40.6/92.4\%$$ and $$27.3/86.4\%$$, and a mass resolution of 1.20/0.40 (0.24/0.036%) MeV, respectively. We also quantify the potential improvements, by improving the low transverse momentum track reconstruction and implementing particle identification capabilities in ideal conditions. In addition, we discuss the potential application on the detector material and alignment studies.

Common explanation to the RK(*) , RD(*) , and ε′/ε anomalies in a 3HDM with right-handed neutrinos and connections to neutrino physics

Scalar theories can account for the current ${R}_{{D}^{(*)}}$ measurements through a vector operator ${\overline{c}}_{L}{\ensuremath{\gamma}}_{\ensuremath{\mu}}{b}_{L}{\overline{\ensuremath{\tau}}}_{L}{\ensuremath{\gamma}}^{\ensuremath{\mu}}{\ensuremath{\nu}}_{L}$ induced at the loop level. Once the vector contribution is considered on top of a subdominant tree-level scalar component, the predicted value of ${R}_{{D}^{(*)}}$ falls within the $1\ensuremath{\sigma}$ region indicated by the experiments. We explicitly demonstrate this claim in the framework of a three Higgs doublet model extended with GeV-scale right-handed neutrinos by matching the anomalous signal for perturbative values of the involved couplings and respecting the bounds from complementary flavor physics measurements. Remarkably, we furthermore show that the proposed framework can be employed to also simultaneously explain the present ${R}_{{K}^{(*)}}$ measurement, as well as the deviation in ${\ensuremath{\epsilon}}^{\ensuremath{'}}/\ensuremath{\epsilon}$ currently being debated in the literature. These results are obtained by considering the contribution of relatively light right-handed neutrinos which are fundamental in mediating the processes behind the anomalous signals. In this way, our findings reveal a new possible connection that links the flavor anomalies to the phenomenology of extended Higgs sector and neutrino physics.

ReDecay: a novel approach to speed up the simulation at LHCb

With the steady increase in the precision of flavour physics measurements collected during LHC Run 2, the LHCb experiment requires simulated data samples of larger and larger sizes to study the detector response in detail. The simulation of the detector response is the main contribution to the time needed to simulate full events. This time scales linearly with the particle multiplicity. Of the dozens of particles present in the simulation only the few participating in the signal decay under study are of interest, while all remaining particles mainly affect the resolutions and efficiencies of the detector. This paper presents a novel development for the LHCb simulation software which re-uses the rest of the event from previously simulated events. This approach achieves an order of magnitude increase in speed and the same quality compared to the nominal simulation.

Heavy Higgs searches: flavour matters

We point out that the stringent lower bounds on the masses of additional electrically neutral and charged Higgs bosons crucially depend on the flavour structure of their Yukawa interactions. We show that these bounds can easily be evaded by the introduction of flavour-changing neutral currents in the Higgs sector. As an illustration, we study the phenomenology of a two Higgs doublet model with a Yukawa texture singling out the third family of quarks and leptons. We combine constraints from low-energy flavour physics measurements, LHC measurements of the 125 GeV Higgs boson rates, and LHC searches for new heavy Higgs bosons. We propose novel LHC searches that could be performed in the coming years to unravel the existence of these new Higgs bosons.

The Belle II imaging Time-of-Propagation (iTOP) detector

High precision flavor physics measurements are an essential complement to the direct searches for new physics at the LHC ATLAS and CMS experiments. Such measurements will be performed using the upgraded Belle II detector that will take data at the SuperKEKB accelerator. With 40x the luminosity of KEKB, the detector systems must operate efficiently at much higher rates than the original Belle detector. A central element of the upgrade is the barrel particle identification system. Belle II has built and installed an imaging-Time-of-Propagation (iTOP) detector. The iTOP uses quartz optics as Cherenkov radiators. The photons are transported down the quartz bars via total internal reflection with a spherical mirror at the forward end to reflect photons to the backward end where they are imaged onto an array of segmented Micro-Channel Plate Photo-Multiplier Tubes (MCP-PMTs). The system is read out using giga-samples per second waveform sampling Application-Specific Integrated Circuits (ASICs). The combined timing and spatial distribution of the photons for each event are used to determine particle species. This paper provides an overview of the iTOP system.

Flavour physics and flavour symmetries after the first LHC phase

Based on flavour symmetries only, there are two ways to give rise to an effective description of flavour physics in the quark sector close to the CKM picture: one is based on $U(3)_q\times U(3)_u\times U(3)_d$ (or equivalent) and the other on $U(2)_q\times U(2)_u\times U(2)_d$ (or equivalent). In this context we analyze the current status of flavour physics measurements and we compare their impact, in the specific case of supersymmetry, with the direct searches of new particles at the LHC, present or foreseen.

Standard Model and Flavor Physics Measurements with the ATLAS Detector

Standard Model and Flavor Physics Measurements with the ATLAS Detector

Belle II Status and Plans

To deepen our understanding of nature it is essential to complement the direct searches for new physics at the LHC by high precision flavor physics measurements. The next-generation B factory experiment Belle II at the upgraded KEKB accelerator, SuperKEKB, is designed to perform such measurements. The status of the collider and the Belle II detector is presented in this article.

The LHCb trigger and its performance in 2011

This paper presents the design of the LHCb trigger and its performance on data taken at the LHC in 2011. A principal goal of LHCb is to perform flavour physics measurements, and the trigger is designed to distinguish charm and beauty decays from the light quark background. Using a combination of lepton identification and measurements of the particles' transverse momenta the trigger selects particles originating from charm and beauty hadrons, which typically fly a finite distance before decaying. The trigger reduces the roughly 11 MHz of bunch-bunch crossings that contain at least one inelastic pp interaction to 3 kHz. This reduction takes place in two stages; the first stage is implemented in hardware and the second stage is a software application that runs on a large computer farm. A data-driven method is used to evaluate the performance of the trigger on several charm and beauty decay modes.

The LHCb Trigger: Present and Future

Abstract LHCb is a single arm spectrometer covering the pseudo-rapidity range between 1.9 and 4.9, and has been optimised to perform flavour physics measurements at the LHC. The present two stage trigger system is able to select charm and beauty decay products with high efficiency due the highly inclusive approach of triggering on partially reconstructed decays and the use of a novel multivariate classifier at the second stage. The trigger can select both leptonic and purely hadronic decays. The performance of the trigger is determined from the data itself without having to rely on Monte-Carlo simulation and is presented. LHCb has recently submitted their upgrade LOI, which mainly aims at profiting from much larger luminosities by moving towards a single fully software based trigger. The upgrade strategy and expected performance are presented.

SuperB, the super flavor factory

Heavy flavor physics measurements, in particular B and τ physics results from the B Factories, currently provide strong constraints on models of physics beyond the Standard Model. SuperB, a next generation asymmetric collider with 50 to 100 times the luminosity of existing colliders, can, in a dialog with LHC and ILC, provide unique insights into New Physics phenomena seen at those machines.

Early flavor physics at the LHC

We discuss here the physics potential of the three LHC experiments ATLAS, CMS and LHCb for early flavor physics measurements.