Quark Couplings Research Articles

Constraining the minimal flavor violating leptoquark explanation of the RD(*) anomaly

There has been persistent disagreement between the Standard Model (SM) prediction and experimental measurements of $R_{D^{(*)}}=\mathcal{B}(\bar B \rightarrow D^{(*)} \tau \bar\nu_\tau)/\mathcal{B}(\bar B \rightarrow D^{(*)} l \bar\nu_l)$ $(l=e,\mu)$. This anomaly may be addressed by introducing interactions beyond the Standard Model involving new states, such as leptoquarks. Since the processes involved are quark flavor changing, any new states would need to couple to at least two different generations of quarks, requiring a non-trivial flavor structure in the quark sector while avoiding stringent constraints from flavor-changing neutral current processes. In this work, we look at scalar leptoquarks as a possible solution for the $R_{D^{(*)}}$ anomaly under the assumption of $\it{minimal~flavor~violation}$ (MFV). We investigate all possible representations for the leptoquarks under the SM quark flavor symmetry group, consistent with asymptotic freedom. We consider constraints on their parameter space from self-consistency of the MFV scenario, perturbativity, the FCNC decay $b\to s\bar\nu\nu$ and precision electroweak observables. We find that none of the scalar leptoquarks can explain the $R_{D^{(*)}}$ anomaly while simultaneously avoiding all constraints within this scenario. Thus scalar leptoquarks with MFV-generated quark couplings do not work as a solution to the $R_{D^{(*)}}$ anomaly.

Search for single production of vector-like quarks decaying to a top quark and a mathrm {W} boson in proton\u2013proton collisions at sqrt{s} = 13 ,text {TeV}

A search is presented for the single production of vector-like quarks in proton–proton collisions at sqrt{s}=13,text {TeV} . The data, corresponding to an integrated luminosity of 35.9,text {fb}^{-1}, were recorded with the CMS experiment at the LHC. The analysis focuses on the vector-like quark decay into a top quark and a mathrm {W} boson, with one muon or electron in the final state. The mass of the vector-like quark candidate is reconstructed from hadronic jets, the lepton, and the missing transverse momentum. Methods for the identification of mathrm {b} quarks and of highly Lorentz boosted hadronically decaying top quarks and mathrm {W} bosons are exploited in this search. No significant deviation from the standard model background expectation is observed. Exclusion limits at 95% confidence level are set on the product of the production cross section and branching fraction as a function of the vector-like quark mass, which range from 0.3 to 0.03,text {pb} for vector-like quark masses of 700 to 2000,text {GeV}. Mass exclusion limits up to 1660,text {GeV} are obtained, depending on the vector-like quark type, coupling, and decay width. These represent the most stringent exclusion limits for the single production of vector-like quarks in this channel.

Pion constituent quark couplings strong form factors: A dynamical approach

Form factors for pions interactions with constituent quarks are investigated as the leading effective couplings obtained from a one loop background field method applied to a global color model. Two pion field definitions are considered and the resulting eleven form factors are expressed in terms of components of the quark and gluon propagators that compose only two momentum dependent functions. A momentum dependent Goldberger Treiman relation is also obtained as one of the ratios between the form factors. The resulting form factors with pion momenta up to 1.5 GeV are exhibited for different quark effective masses and two different non-perturbative gluon propagators and they present similar behavior to fittings of experimental data from nucleons form factors. The corresponding pseudoscalar averaged quadratic radii (a.q.r.) and correction to the axial a.q.r. are presented as functions of the sea quark effective mass, being equal respectively to the scalar and vector ones at the present level of calculation.

Yang–Mills theory in the SO(2,3)⋆ model of noncommutative gravity

According to the standard cosmological model, thermodynamic conditions of the early Universe were such that nuclear matter existed in the state of quark–gluon plasma, rather than hadrons. On the other hand, it is generally believed that quantum gravity effects become ever more stronger as we approach the Big Bang, in particular, we expect that the phenomenon of space–time noncommutativity will be significant. Thus we are led to consider the properties of quarks and gluons in noncommutative space–time. For this, we employ the [Formula: see text] model of noncommutative gravity. As a first step towards the full theoretical treatment of the effects of noncommutativity on quark–gluon plasma, our main goal in this paper is to consistently incorporate Yang–Mills gauge fields in the [Formula: see text] framework and investigate their coupling to gravity that arises due to space–time noncommutativity. We construct an action that is invariant under deformed [Formula: see text] gauge transformations and expand it perturbatively in orders of the canonical deformation parameter [Formula: see text] via Seiberg–Witten map. In particular, we analyze the flat-space–time limit and demonstrate that residual noncommutativity induces various new couplings of quarks and gluons.

The direct coupling of light quarks to heavy di-quarks

In the limit mQ>mQvrel>mQvrel2≫ΛQCD hadronic states with two heavy quarks Q should be describable by a version of HQET where the heavy quark is replaced by a di-quark degree of freedom. In this limit the di-quark is a small (compared with 1/ΛQCD) color anti-triplet, bound primarily by a color Coulomb potential. The excited Coulombic states and color six states are much heavier than the color anti-triplet ground state. The low lying spectrum of hadrons containing two heavy quarks is then determined by the coupling of the light quarks and gluons with momentum of order ΛQCD to this ground state di-quark. In this short paper we calculate the coefficient of leading local operator (Sv†Sv)(q¯γμvμq) that couples this color anti-triplet di-quark field Sv (with four-velocity v) directly to the light quarks q in the low energy effective theory. It is O(1/(αs(mQvrel)mQ2)). While our work is mostly of pedagogical value we make an estimate of the contribution of this operator to the masses of Ξbbq baryon and TQQq¯q¯ tetraquark using the non-relativistic constituent quark model.

Neutron star matter with Δ isobars in a relativistic quark model

The effect of strange interactions in neutron star matter and the role of the strange meson-hyperon couplings are studied in a relativistic quark model where the confining interaction for quarks inside a baryon is represented by a phenomenological average potential in an equally mixed scalar-vector harmonic form. The hadron-hadron interaction in nuclear matter is then realized by introducing additional quark couplings to $\sigma$, $\omega$, $\rho$, $\sigma^*$ and $\phi$ mesons through mean-field approximations. The meson-baryon couplings are fixed through the SU(6) spin-flavor symmetry and the SU(3) flavor symmetry to determine the hadronic equation of state (EoS). We find that the SU(3) coupling set gives the potential depth between $\Lambda$s around $-5$ MeV and favours a stiffer EoS.The radius for the canonical neutron star lies within a range of $12.7$ to $13.1$ km.

Global and optimal probes for the top-quark effective field theory at future lepton colliders

We study the sensitivity to physics beyond the standard model of precise top-quark pair production measurements at future lepton colliders. A global effective-field-theory approach is employed, including all ten dimension-six operators of the Warsaw basis which involve a top-quark and give rise to tree-level amplitudes that interfere with standard-model {e}^{+}{e}^{-}to toverline{t}to b{W}^{+}overline{b}{W}^{-} ones in the limit of vanishing b-quark mass. Four-fermion and CP-violating contributions are taken into account. Circular-collider-, ILC- and CLIC-like benchmark run scenarios are examined. We compare the constraining power of various observables to a set of statistically optimal ones which maximally exploit the information contained in the fully differential b{W}^{+}overline{b}{W}^{-} distribution. The enhanced sensitivity gained on the linear contributions of dimension-six operators leads to bounds that are insensitive to quadratic ones. Even with statistically optimal observables, two centre-of-mass energies are required for constraining simultaneously two- and four-fermion operators. The impact of the centre-of-mass energy lever arm is discussed, that of beam polarization as well. A realistic estimate of the precision that can be achieved in ILC- and CLIC-like operating scenarios yields individual limits on the electroweak couplings of the top quark that are one to three orders of magnitude better than constraints set with Tevatron and LHC run I data, and three to two hundred times better than the most optimistic projections made for the high-luminosity phase of the LHC. Clean global constraints can moreover be obtained at lepton colliders, robustly covering the multidimensional effective-field-theory space with minimal model dependence.

Determination of electroweak parameters in polarised deep-inelastic scattering at HERA

The parameters of the electroweak theory are determined in a combined electroweak and QCD analysis using all deep-inelastic e^+p and e^-p neutral current and charged current scattering cross sections published by the H1 Collaboration, including data with longitudinally polarised lepton beams. Various fits to Standard Model parameters in the on-shell scheme are performed. The mass of the W boson is determined as m_W=80.520pm 0.115~mathrm {GeV} . The axial-vector and vector couplings of the light quarks to the Z boson are also determined. Both results improve the precision of previous H1 determinations based on HERA-I data by about a factor of two. Possible scale dependence of the weak coupling parameters in both neutral and charged current interactions beyond the Standard Model is also studied. All results are found to be consistent with the Standard Model expectations.

Search for new physics in dijet angular distributions using proton\u2013proton collisions at sqrt{s}=13hbox {TeV} and constraints on dark matter and other models

A search is presented for physics beyond the standard model, based on measurements of dijet angular distributions in proton–proton collisions at sqrt{s}=13hbox {TeV}. The data collected with the CMS detector at the LHC correspond to an integrated luminosity of 35.9,text {fb}^{-1}. The observed distributions, corrected to particle level, are found to be in agreement with predictions from perturbative quantum chromodynamics that include electroweak corrections. Constraints are placed on models containing quark contact interactions, extra spatial dimensions, quantum black holes, or dark matter, using the detector-level distributions. In a benchmark model where only left-handed quarks participate, contact interactions are excluded at the 95% confidence level up to a scale of 12.8 or 17.5TeV, for destructive or constructive interference, respectively. The most stringent lower limits to date are set on the ultraviolet cutoff in the Arkani–Hamed–Dimopoulos–Dvali model of extra dimensions. In the Giudice–Rattazzi–Wells convention, the cutoff scale is excluded up to 10.1TeV. The production of quantum black holes is excluded for masses below 5.9 and 8.2TeV, depending on the model. For the first time, lower limits between 2.0 and 4.6TeVare set on the mass of a dark matter mediator for (axial-)vector mediators, for the universal quark coupling g_{mathrm {mathrm {q}}} =1.0.

Reducing the quadratic divergence in the Higgs boson mass squared without top partners

We examine a model with multiple scalar fields to see whether it is possible to reduce the fine- tuning of the SM Higgs mass without introducing low scale top partners. Our approach may be regarded as a generalization of the condition proposed by Veltman, who attempted to predict the Higgs mass using the criterion that the various low energy contributions to the quadratic divergence of the Higgs mass cancel. Although the Veltman condition predicts the wrong Higgs mass in the Standard Model, it can still be adapted to extended Higgs sectors. Furthermore, theories with additional Higgs bosons can lead to suppressed Yukawa couplings of the top quark to the 125 GeV Higgs, making the associated one-loop divergence smaller. Here, we review possible extensions of the Standard Model where the Veltman condition could be realized, and study in detail one minimal model with two extra scalar fields. For this model and for a cutoff of 5 TeV, we show that the overall fine-tuning can be considerably lowered without introducing low-scale Landau poles, albeit the Higgs sector will be strongly coupled at the cutoff. Models where the top Yukawa coupling is reduced, in particular, will be within the reach of the upcoming LHC searches.

LHC signals for singlet neutrinos from a natural warped seesaw mechanism. I

Recently, it was shown in arXiv:1512.06742 that a straightforward implementation of the type I seesaw mechanism in a warped extra dimensional framework is in reality a {\em natural} realization of "inverse" seesaw, i.e., the Standard Model (SM) neutrino mass is dominantly generated by exchange of pseudo-Dirac {\em TeV}-mass SM singlet neutrinos. By the AdS/CFT correspondence, this scenario is {\em dual} to these singlet particles being composites of some new strong dynamics, along with the SM Higgs boson, with the rest of the SM particles being mostly elementary. We study signals from production of these heavy neutrinos at the Large Hadron Collider (LHC). We focus on the scenario where the strong sector has a global $SU(2)_{\rm L} \times SU(2)_{\rm R} \times U(1)_{\rm X}$ symmetry; such a left-right (LR) structure being motivated by consistency with the electroweak (EW) precision tests. The singlet neutrinos are charged under $SU(2)_{\rm R} \times U(1)_{\rm X}$ symmetry, thus can be produced from $W^{ \pm }_R$ exchange, as in four-dimensional (4D) LR symmetric models. However, the direct coupling of light quarks to $W^{ \pm }_R$ is negligible, due to $W^{ \pm }_R$ also being composite; nonetheless, a sizable coupling can be induced by mixings among the various types of $W^{ \pm }$ bosons. Furthermore, $W^{ \pm }_R$ decays dominantly into the singlet and {\em composite} partner of charged lepton. This heavy charged lepton, in turn, decays into SM lepton, {\em plus} $Z$/Higgs, thus the latter can be used for extra identification of the signal. For a benchmark scenario with $W^{ \pm }_R$ of mass 2 TeV and singlet neutrino of mass 750 GeV, we find that, in both the di-lepton + di-jet + Higgs and tri-lepton + Higgs channels, significant evidence can be seen at the LHC14 for an integrated luminosity of 300/fb and that even discovery is possible with slightly more luminosity.

Light vector and axial mesons effective couplings to constituent quarks

Low energy effective couplings of baryons' constituent quarks to light vector and axial mesons are derived by considering quark polarization for a dressed one gluon exchange quark interaction. The quark field is splitted into two components, one for background constituent quarks and another one for quark-antiquark states, light mesons and the scalar chiral condensate. By considering a large quark effective mass derivative expansion, several effective coupling constants are resolved as functions of the original model parameters and of components of the quark and gluon propagators. Besides the leading single vector meson-constituent quark gauge-type effective coupling, several two-vector and axial mesons-constituent quark couplings are also obtained in the next leading order. Among these, vector and axial mesons mixings induced by constituent quark currents are found.Approximated and exact ratios between the effective coupling constants in the limit of very large quark effective mass and numerical estimates are exhibited. Numerical results of the corresponding form factors and of the (strong) vector meson quadratic radius are also presented.

Mass scale of vectorlike matter and superpartners from IR fixed point predictions of gauge and top Yukawa couplings

We use the IR fixed point predictions for gauge couplings and the top Yukawa coupling in the minimal supersymmetric model (MSSM) extended with vectorlike families to infer the scale of vectorlike matter and superpartners. We quote results for several extensions of the MSSM and present results in detail for the MSSM extended with one complete vectorlike family. We find that for a unified gauge coupling ${\ensuremath{\alpha}}_{G}>0.3$ vectorlike matter or superpartners are expected within 1.7 TeV (2.5 TeV) based on all three gauge couplings being simultaneously within 1.5% (5%) from observed values. This range extends to about 4 TeV for ${\ensuremath{\alpha}}_{G}>0.2$. We also find that in the scenario with two additional large Yukawa couplings of vectorlike quarks the IR fixed point value of the top Yukawa coupling independently points to a multi-TeV range for vectorlike matter and superpartners. Assuming a universal value for all large Yukawa couplings at the grand unified theory scale, the measured top quark mass can be obtained from the IR fixed point for $\mathrm{tan}\ensuremath{\beta}\ensuremath{\simeq}4$. The range expands to any $\mathrm{tan}\ensuremath{\beta}>3$ for significant departures from the universality assumption. Considering that the Higgs boson mass also points to a multi-TeV range for superpartners in the MSSM, adding a complete vectorlike family at the same scale provides a compelling scenario where the values of gauge couplings and the top quark mass are understood as a consequence of the particle content of the model.

Low energy constituent quark and pion effective couplings in a weak external magnetic field

An effective model with pions and constituent quarks in the presence of a weak external background electromagnetic field is derived by starting from a dressed one gluon exchange quark-quark interaction.By applying the auxiliary field and background field methods, the structureless pion limit is considered to extract effective pion and constituent quark couplings in the presence of a weak magnetic field. The leading terms of a large quark and gluon masses expansion are obtained by resolving effective coupling constants which turn out to depend on a weak magnetic field. Two pion field definitions are considered for that. Several relations between the effective coupling constants and parameters can be derived exactly or in the limit of very large quark mass at zero and weak constant magnetic field. Among these ratios, the Gell Mann Oakes Renner and the quark level Goldberger Treiman relations are obtained. In addition to that, in the pion sector, the leading terms of Chiral Perturbation Theory coupled to the electromagnetic field is recovered. Some numerical estimates are provided for the effective coupling constants and parameters.

Search for single production of vector-like top partner decaying to Wb at egamma collision

In a simplified model including an SU(2) singlet T quark with charge 2/3, we investigate the single vector-like T production at the high energy egamma collision. We study the observability of the vector-like T focusing on the Trightarrow Wb decay channel with Wrightarrow lbar{nu } at sqrt{s}=2.0 TeV. In this analysis, only two free parameters are involved, namely the T quark coupling strength for single production g^{*} and the mass m_{T}. We scan the parameter space and find that the correlation region of g^{*}in [0.24, 0.5] and m_{T}in [800, 1360] GeV can be excluded with integrated luminosity L=100,hbox {fb}^{-1} and the correlation region of g^{*}in [0.13, 0.5] and m_{T}in [800, 1620] GeV can be excluded with integrated luminosity L=1000 ,hbox {fb}^{-1} at 2sigma level.

Chiral symmetry restoration and quark deconfinement beyond mean field in a magnetized PNJL model

We study chiral symmetry restoration and quark deconfinement beyond mean field approximation in a magnetized PNJL model. The feedback from mesons to quarks modifies the quark coupling constant and Polyakov potential. As a result, the separate critical temperatures for the two phase transitions at mean field level coincide and the magnetic catalysis becomes inverse magnetic catalysis, when the meson contribution is included.

Search for Flavour Changing Neutral Current Couplings of Higgs-up Sector Quarks at Future Circular Collider (FCC-eh)

Search for Flavour Changing Neutral Current Couplings of Higgs-up Sector Quarks at Future Circular Collider (FCC-eh)

Measurement of the D-meson Nuclear Modification Factor and Elliptic Flow in Pb–Pb Collisions at sNN = 5.02 TeV with ALICE at the LHC

Heavy-flavour hadrons are effective probes to study the Quark-Gluon Plasma (QGP) formed in ultra-relativistic heavy-ion collisions. The ALICE Collaboration measured the D-mesons (D0, D[Formula: see text], D*[Formula: see text] and D[Formula: see text]) production in Pb–Pb collisions at [Formula: see text] = 5.02 TeV. The in-medium energy loss can be studied by means of the nuclear modification factor ([Formula: see text]). The comparison between the D[Formula: see text] and the non-strange D-meson [Formula: see text] can help to study the hadronisation mechanism of the charm quark in the QGP. In semi-central collisions the measurement of the D-meson elliptic flow, [Formula: see text], at low [Formula: see text] allows to investigate the participation of the heavy quarks in the collective expansion of the system while at high [Formula: see text] it constrains the path-length dependence of the energy loss. Furthermore the Event-Shape Engineering (ESE) technique is used to measure D-meson elliptic flow in order to study the coupling of the charm quarks to the light quarks of the underlying medium.

Measurement of the D-meson nuclear modification factor and elliptic flow in Pb–Pb collisions at √SNN = 5.02 TeV with ALICE at the LHC

Heavy-flavour hadrons are recognised as a powerful probe for the characterisation of the deconfined medium created in heavy-ion collisions, the Quark-Gluon Plasma (QGP). The ALICE Collaboration measured the production of D0, D+, D*+ and [see formula in PDF] mesons in Pb–Pb collisions at [see formula in PDF] = 5.02 TeV. The measurement of the nuclear modification factor (RAA) provides a strong evidence of the in-medium parton energy loss. The comparison between the [see formula in PDF] and the non-strange D-meson RAA can help to study the hadronisation mechanism of the charm quark in the QGP. In mid-central collisions, the measurement of the D-meson elliptic flow v2 at low transverse momentum (pT) gives insight into the participation of the charm quark into the collective motion of the system, while at high pT it constrains the path-length dependence of the energy loss. The [see formula in PDF] v2, measured for the first time at the LHC, is found to be compatible to that of non-strange D mesons and positive with a significance of about 2.6 σ. The coupling of the charm quark to the light quarks in the underlying medium is further investigated for the first time with the application of the Event-Shape Engineering (ESE) technique to D-meson elliptic flow.

Probing light-quark Yukawa couplings via hadronic event shapes at lepton colliders

We propose a novel idea for probing the Higgs boson couplings through the measurement of hadronic event shape distributions in the decay of the Higgs boson at lepton colliders. The method provides a unique test of the Higgs boson couplings and of QCD effects in the decay of the Higgs boson. It can be used to probe the Yukawa couplings of the light quarks and to further test the mechanism of electroweak symmetry breaking. From a case study for the proposed Circular Electron-Positron Collider, assuming a hypothesis of SM-like theory, light-quark couplings with a strength greater than 9% of the bottom-quark Yukawa coupling in the standard model can be excluded.