Quark Couplings Research Articles

Exploring SMEFT couplings using the forward–backward asymmetry in neutral current Drell–Yan production at the LHC

Neutral current Drell–Yan (DY) lepton-pair production is considered to study Z-boson quark couplings. Using the open-source fit platform xFitter, we investigate the impact of high-statistics measurements of the neutral current DY (NCDY) forward–backward asymmetry AFB near the weak boson mass scale in the present and forthcoming stages of the Large Hadron Collider (LHC). Besides recovering earlier results on the AFB sensitivity to parton distribution functions, we analyze the precision determination of Z-boson couplings to left-handed and right-handed u-quarks and d-quarks, and explore Beyond-Standard-Model contributions using the Standard Model Effective Field Theory (SMEFT) framework. We perform a sensitivity study and comment on the role of the AFB asymmetry for the electroweak SMEFT fit and precision Z-boson physics at the LHC and high-luminosity HL-LHC.

Heavy-to-light form factors to three loops

We compute three-loop corrections of O(αs3) to form factors with one massive and one massless quark coupling to an external vector, axialvector, scalar, pseudoscalar, or tensor current. We obtain analytic results for the color-planar contributions, for the contributions of light-quark loops, and the contributions with two heavy-quark loops. For the computation of the remaining master integrals we use the “expand and match” approach which leads to semianalytic results for the form factors. We implement our results in a and a ortran code which allows for fast and precise numerical evaluations in the physically relevant phase space. The form factors are used to compute the hard matching coefficients in Soft-Collinear Effective Theory for all currents. The tensor coefficients at lightlike momentum transfer are used to extract the hard function in B¯→Xsγ to three loops. Published by the American Physical Society 2024

Light axion-like particles and the pseudoscalar mesonic decays M⃗M′νν¯

The axion-like particle (ALP) is one kind of the best-motivated new particles. We consider its production from the pseudoscalar mesonic decays M→M′a , with M being a pseudoscalar meson B or K. The upper limits on the flavor-conserving ALP–quark coupling parameter g u are obtained by assuming the ALP to be an invisible particle. We find that the most severe constraint on g u comes from the decay K+→π+νν¯ for 0.05 GeV ≤ M a ≤ 0.35 GeV, while the decays B+,0→K+,0νν¯ and B+,0→π+,0νν¯ can also generate significant constraints.

Spectrum of S - and P -wave ccq¯q¯′ (q¯,q¯′=u¯,d¯,s¯) systems in a chiral SU(3) quark model

Inspired by the resonance Tcc+(3875) recently observed by the LHCb Collaboration, we systematically explore the S- and P-wave ccq¯q¯′ (q¯,q¯′=u¯,d¯,s¯) systems in a chiral SU(3) quark model. The Hamiltonian contains the kinetic energy, the one-gluon-exchange (OGE) potential, the confinement potential, and the one-boson-exchange (OBE) potential stemming from the coupling of quark and chiral fields. The Schrödinger equation is solved by use of the variational method with the spacial trial wave functions chosen as Gaussian functions. It is found that the lowest state has a mass 3879 MeV, isospin and spin-parity IJP=01+, and quark constituent ccu¯d¯, in agreement with the experimentally observed Tcc+(3875). This state is approximately at the calculated DD* threshold and has a root-mean-square radius of about 0.48 fm. These demonstrate that the Tcc+(3875) can be accommodated as a stable and compact tetraquark sate in the chiral SU(3) quark model. All the other S- and P-wave ccq¯q¯′ (q¯,q¯′=u¯,d¯,s¯) states lie about 100 to a few hundreds MeV higher than the corresponding meson-meson thresholds and thus are not suggested to be candidates of stable and compact tetraquark states due to their fall-apart decays to two mesons. Published by the American Physical Society 2024

Aiming for tops of ALPs with a muon collider

Future muon colliders with center-of-mass energy of O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(1 − 10) TeV can provide a clean high-energy environment with advantages in searches for TeV-scale axion-like particles (ALPs), pseudo-Nambu-Goldstone bosons associated with spontaneously broken global symmetries, which are widely predicted in physics beyond the Standard Model (SM). We exploit ALP couplings to SM fermions, and guided by unitarity constraints, build a search strategy focusing on the ALP decay to top quark pairs at muon colliders. It is found that a large parameter space of TeV-scale ALPs with TeV-scale decay constants can be probed by utilizing the ALP-top quark coupling.

Quantum corrections and the minimal Yukawa sector of SU(5)

It is well known that the SU(5) grand unified theory, with the standard model quarks and leptons unified in 5¯ and 10 and the electroweak Higgs doublet residing in five-dimensional representations, leads to the relation Yd=YeT between the Yukawa couplings of the down-type quarks and the charged leptons. We show that this degeneracy can be lifted in a phenomenologically viable way when quantum corrections to the tree-level matching conditions are taken into account in the presence of one or more copies of gauge singlet fermions. The one-loop threshold corrections arising from heavy leptoquark scalar and vector bosons, already present in the minimal model, and heavy singlet fermions can lead to realistic Yukawa couplings, provided their masses differ by at least 2 orders of magnitude. The latter can also lead to a realistic light neutrino mass spectrum through the type I seesaw mechanism if the color partner of the Higgs stays close to the Planck scale. Most importantly, our findings demonstrate the viability of the simplest Yukawa sector when quantum corrections are considered and sizable threshold effects are present. Published by the American Physical Society 2024

Single-Top Quark Physics at the LHC: From Precision Measurements to Rare Processes and Top Quark Properties

Since the initial measurements of single-top quark production at the Tevatron in 2009, tremendous progress has been made at the LHC. While LHC Run 1 marked the beginning of a precision era for the single-top quark measurements in some of the main production mechanisms, LHC Run 2 witnessed the emergence and exploration of new processes associating top quark production with a neutral boson. In this paper, we review the measurements of the three main production mechanisms (t-channel, s-channel, and tW production), and of the associated production with a photon, a Z boson, or a Higgs boson. Differential cross-sections are measured for several of these processes and compared with theoretical predictions. The top quark properties that can be measured in single-top quark processes are scrutinized, such as Wtb couplings and top quark couplings with neutral bosons, and the polarizations of both the W boson and top quark. The effective field theory framework is emerging as a standard for interpreting property measurements. Perspectives for LHC Run 3 and the HL-LHC are discussed in the conclusions.

SMEFT is falsifiable through multi-Higgs measurements (even in the absence of new light particles)

By embedding the Standard Model effective field theory (SMEFT) in the more general Higgs effective field theory (HEFT), we expose correlations among the coefficients of the latter that, if found to be violated in future data, would lead to the experimental falsification of the SMEFT framework. These are derived from the necessary symmetric point of HEFT and analyticity of the SMEFT Lagrangian that allows the construction of the SMEFT expansion, as laid out by other groups, and properties at that point of the Higgs-flare function coupling Goldstone and Higgs bosons, of the Higgs potential V(h) and of the Higgs-top quark coupling function .

Measurements of inclusive and differential cross sections for the Higgs boson production and decay to four-leptons in proton-proton collisions at sqrt{s} = 13 TeV

Measurements of the inclusive and differential fiducial cross sections for the Higgs boson production in the H → ZZ → 4ℓ (ℓ = e, μ) decay channel are presented. The results are obtained from the analysis of proton-proton collision data recorded by the CMS experiment at the CERN LHC at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb−1. The measured inclusive fiducial cross section is 2.73 ± 0.26 fb, in agreement with the standard model expectation of 2.86 ± 0.1 fb. Differential cross sections are measured as a function of several kinematic observables sensitive to the Higgs boson production and decay to four leptons. A set of double-differential measurements is also performed, yielding a comprehensive characterization of the four leptons final state. Constraints on the Higgs boson trilinear coupling and on the bottom and charm quark coupling modifiers are derived from its transverse momentum distribution. All results are consistent with theoretical predictions from the standard model.

Simple Scenario of Photons Emission from Anti Charm–Gluon Interaction using QCD Theory

In this work, the chromodynamics (QCD) theory was used to investigate the photon rate which was produced in interactions of anti-charm with gluon. The simple quantum scenario theory was implemented to give the rate equation that describes the collision of quark and gluon at a chemical potential .The photon emission rate was evaluated from the anti-charm-gluon interaction of the cg → dγ collision at the temperature of the system in the range of 180 - 360 MeV with different critical temperatures (e.g. 116.575, 139.891, 157.377 and 174.863 MeV) with photons energy GeV under the assumption that fugacity of quark and gluon are =0.08 and =0.02 respectively . The photon rate increases with decreasing the coupling of quark and gluon according to a decrease in the temperature of the system from 360 MeV to 180 MeV. The photon emission spectrum was calculated and discussed using a photon energy of 1GeV to 10GeV with different critical temperatures. In terms of QCD theory, the quantitative accomplishment was made for a unique six-flavor number nf = 4 + 2 of photon emission. The photon rate reaches minimum with photon energy E=10 GeV, it reflects the less coupling for the cg → dγ interaction.

Properties of quark-matter cores in massive hybrid stars

Using the constraints from astrophysical observations and heavy-ion experiments, we investigate the equation of state (EOS) of hybrid star matter and the properties of quark-matter cores in hybrid stars. The quark matter interactions in hybrid stars are described based on 3-flavor Nambu-Jona-Lasinio model with various vector and vector-isovector coupling constants. In this work, we find that the hybrid star matter EOS is more sensitive to the strength of the vector interaction, and the EOS becomes stiffer with increasing vector strength $R_V$. The vector-isovector interaction characterized by the coupling constant $R_{IV}$ make main contribution to the hadron-quark mixed phase. Meanwhile, we note that a step change of both the sound velocity and the polytropic index $\gamma$ occurs in the hadron-quark phase transition, and it is restored with the decrease of nucleon and lepton degrees of freedom in the high density quark phase. Although the coupling constants increase the hybrid star maximum mass up to $2.08M_{\odot}$, they also decrease the mass and radius of the quark core and the mixed core. With different quark coupling constants, we also find that the maximum mass and radius of the quark matter core in a stable hybrid star can reach $0.80M_{\odot}$ and 6.95 km, which are close to half of the maximum mass and radius of the complete star. However, properties of quark matter have no effect on the $M = 1.4M_{\odot}$ hybrid star as a result of no quark matter inner core, which can also be confirmed by the criterion of the polytropic index, and thus our results also indicate that the quark interactions have no effect on the tidal deformability $\Lambda_{1.4}$ of hybrid stars.

Erratum: Pion constituent quark couplings strong form factors: A dynamical approach [Phys. Rev. D 99 , 014001 (2019)

Received 24 March 2023DOI:https://doi.org/10.1103/PhysRevD.107.079902Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasForm factorsParticle interactionsPhysical SystemsQuark modelTechniquesFunctional analytical methodsParticles & Fields

Global fit of modified quark couplings to EW gauge bosons and vector-like quarks in light of the Cabibbo angle anomaly

There are two tensions related to the Cabibbo angle of the CKM matrix. First, the determinations of Vus from Kμ2, Kℓ3, and τ decays disagree at the 3σ level. Second, using the average of these results in combination with β decays (including super-allowed β decays and neutron decay), a deficit in first-row CKM unitarity with a significance of again about 3σ is found. These discrepancies, known as the Cabibbo Angle anomaly, can in principle be solved by modifications of W boson couplings to quarks. However, due to SU(2)L invariance, Z couplings to quarks are also modified and flavour changing neutral currents can occur. In order to consistently assess the agreement of a new physics hypothesis with data, we perform a combined analysis for all dimension-six Standard Model Effective Field Theory operators that generate modified W couplings to first and second generation quarks. We then study models with vector-like quarks, which are prime candidates for a corresponding UV completion as they can affect W -quark couplings at tree level, and we perform a global fit including flavour observables (in particular loop effects in ∆F = 2 processes). We find that the best fit can be obtained for the SU(2)L doublet vector-like quark Q as it can generate right-handed W-u-d and W-u-s couplings as preferred by data.

Constraints on the doublet left-right symmetric model from Higgs data

We study the constraints on the doublet left-right symmetric model (DLRSM) arising due to the Higgs data. The SU(2)L symmetry of this model is broken by three vacuum expectation values, κ1, κ2 and vL. Most studies of this model assume that the ratios r = κ2/κ1 and w = vL/κ1 are very small. In this work, we study the constraints imposed on r and w by the Higgs data from LHC. We consider the most general scalar potential and calculate the masses of the CP-even scalars and the couplings of the lightest of these scalars to itself, to W and Z gauge bosons and to the third generation quarks. We find that there is no lower bound on either r or w. Equating the mass of the lightest CP-even scalar to 125 GeV leads to an upper limit w < 6.7. The requirement that the Yukawa coupling of the quarks to the Higgs bidoublet of the model should be perturbative yields the upper bounds r < 0.8 and w < 3.5. The Yukawa coupling of the bottom quark to the lightest CP-even scalar strongly disfavours value of r, w < 0.1 and shows a marked preference for values of w ∼ mathcal{O} (1).

Resolving the flavor structure in the MFV-SMEFT

We constrain the flavor structure of Wilson coefficients in the Standard Model Effective Field Theory (SMEFT) from data. In the SMEFT, new physics effects in couplings of up-type and down-type quarks are related through the Cabibbo-Kobayashi-Maskawa mixing matrix. We exploit this relation to pin down potential new sources of flavor symmetry breaking in a global analysis of high- and low-energy data from the LHC, LEP, and b factory experiments. We demonstrate the power of such an analysis by performing a combined fit of effective four-quark and two-quark couplings contributing to a large set of flavor, top-quark, electroweak, and dijet observables. All four sectors are needed to fully resolve the flavor structure of left-chiral four-quark couplings without leaving blind directions in the parameter space. Although we work in the framework of minimal flavor violation, our strategy applies as well to other flavor patterns, like U(2) flavor symmetry or leptoquark scenarios.

PHENIX Probing QCD Matter Through Heavy Flavor and Quarkonium at RHIC

Recent results from the PHENIX experiment on heavy flavors and quarkonia production in p + Al, p + Au, d + Au, and Au + Au collision systems at √ SNN = 200 GeV are summarized. The results are carried out by the measurements of the nuclear modification factors and elliptic flow. The nuclear modification factors measurements give insight into the energy loss of heavy quarks in the quark-gluon plasma medium along their path lengths. The elliptic flow measurements are a good tool to investigate the coupling of heavy quarks with the medium. The measurements are presented as a function of centrality, rapidity, and transverse momentum. The interpretations of the results in light of our current theoretical models, and comparison to LHCb and ALICE measurements are presented.

Phenomenological implications of the new Littlest Higgs model with T-parity

We investigate the parameter space of the new Littlest Higgs model with T-parity (NLHT) recently introduced to cure some pathologies of the original LHT. The model requires extra fermion content and additional pseudo-Goldstone bosons. While the heavy top quark sector is similar, there are both T-odd and T-even heavy quarks and leptons with masses proportional to just two sets of Yukawa matrices in flavor space, one more than in the LHT. The new scalars are a singlet and real triplet, T-odd, with masses controlled by gauge and Yukawa couplings, independent of the spontaneous symmetry breaking scale f, and hence potentially light. Imposing that no mass exceeds the cutoff scale, applying current lower bounds on vector-like quarks and assuming a simplified model with mass degenerate heavy fermions compatible with the heavy photon as dark matter constituent, we find that f gets constrained within the interval between 2 and 3 TeV, the common Yukawa coupling of heavy leptons gets fixed and the Yukawa coupling of heavy quarks becomes greatly correlated to the top quark Yukawa couplings. The particle spectrum is then bounded from below and above, with the (lightest) heavy photon at about 0.5 TeV, not far from the heavy leptons, the new scalars below 1 TeV, the usual complex scalar triplet close to the heavy weak bosons at about 1.5 to 2.5 TeV, and the heavy quarks and top quark partners between 2 and 5 TeV. The new scalars decay predominantly to a standard and a T-odd lepton and have a width comparable to that of the Higgs.

Probing nuclear observables via primordial nucleosynthesis

We study the dependence of primordial nuclear abundances on fundamental nuclear observables such as binding energies, scattering lengths, neutron lifetime, etc. by varying these quantities. The numerical computations were performed with four publicly available codes, thus facilitating an investigation of the model-dependent (systematic) uncertainties on these dependences. Indeed deviations of the order of a few percent are found. Moreover, accounting for the temperature dependence of the sensitivity of the rates to some relevant parameters leads to a reduction of the sensitivity of the final primordial abundances, which in some cases is appreciable. These effects are considered to be relevant for studies of the dependence of the nuclear abundances on fundamental parameters such as quark masses or couplings underlying the nuclear parameters studied here.

Associated production of heavy Higgs bosons with a mathrm{b}overline{mathrm{b}} pair in the nonholomorphic MSSM and LHC searches

In the NonHolomorphic Supersymmetric Standard Model (NHSSM), the Yukawa couplings of the bottom quark (yb) and the tau lepton (yτ) might receive substantial supersymmetric (SUSY) radiative corrections which have prominent dependencies on the NHSSM-specific trilinear soft parameters, {A}_b^{prime } and {A}_{tau}^{prime } , respectively, in addition to their well-known dependence on tan β as is already present in the Minimal SUSY Standard Model (MSSM). We study to what extent these could affect the production cross sections of the heavy Higgs bosons (H and A) in association with a pair of b-quarks and their decay branching ratios, in particular, to a tau overline{tau} pair and compare them with those obtained in the MSSM. Requiring compliance with the recently observed upper bounds on the product of their total cross section and the branching ratio to tau overline{tau} at the 13 TeV run of the Large Hadron Collider (LHC), with data worth 139 fb−1, results in an altered exclusion region in the customary mA− tan β plane in the framework of the NHSSM when compared to what is derived by the LHC experiments within an MSSM setup. Such alterations are estimated to be pronounced only for large tan β (≥ 40) and for large, negative {A}_b^{prime } when one finds a reinforced exclusion of the mA − tan β plane with an excluded mA value larger by ≈ 200 GeV, compared to the MSSM case, at tan β = 60. On the other hand, the maximum relaxation in mA, for a similarly large but positive {A}_b^{prime } , barely exceeds ≈ 100 GeV as a result of complementary variations in production cross sections and decay branching fractions of the heavy, neutral Higgs bosons.

Onset of Color Transparency in Holographic Light-Front QCD

The color transparency (CT) of a hadron, propagating with reduced absorption in a nucleus, is a fundamental property of QCD (quantum chromodynamics) reflecting its internal structure and effective size when it is produced at high transverse momentum, Q. CT has been confirmed in many experiments, such as semi-exclusive hard electroproduction, eA→e′πX, for mesons produced at Q2&gt;3GeV2. However, a recent JLab (Jefferson Laboratory) measurement for a proton electroproduced in carbon eC→e′pX, where X stands for the inclusive sum of all produced final states, fails to observe CT at Q2 up to 14.2 GeV2. In this paper, the onset of CT is determined by comparing the Q2-dependence of the hadronic cross sections for the initial formation of a small color-singlet configuration using the generalized parton distributions from holographic light-front QCD. A critical dependence on the hadron’s twist, τ, the number of hadron constituents, is found for the onset of CT, with no significant effects from the nuclear medium. This effect can explain the absence of proton CT in the present kinematic range of the JLab experiment. The proton is predicted to have a “two-stage” color transparency with the onset of CT differing for the spin-conserving (twist-3, τ=3) Dirac form factor with a higher onset in Q2 for the spin-flip Pauli (twist-4) form factor. In contrast, the neutron is predicted to have a “one-stage” color transparency with the onset at higher Q2 because of the dominance of its Pauli form factor. The model also predicts a strong dependence at low energies on the flavor of the quark current coupling to the hadron.