Generation Quarks Research Articles

Investigating the Sterile Neutrino Parameters with QLC in 3 + 1 Scenario

In the scenario with four generation quarks and leptons and using a 3+1 neutrino model having one sterile and the three standard active neutrinos with a 4×4 unitary transformation matrix, UPMNS4, we perform a model-based analysis using the latest global data and determine bounds on the sterile neutrino parameters i.e., the neutrino mixing angles. Motivated by our previous results, where, in a quark-lepton complementarity (QLC) model we predicted the values of θ13PMNS=9−2+2° and θ23PMNS=40.60−0.3+0.1°. In the QLC model the nontrivial correlation between CKM4 and PMNS4 mixing matrix is given by the correlation matrix Vc4. Monte Carlo simulations are performed to estimate the texture of Vc4 followed by the calculation of PMNS4 using the equation, UPMNS4=UCKM4·ψ4−1·Vc4, where ψ4 is a diagonal phase matrix. The sterile neutrino mixing angles, θ14PMNS, θ24PMNS and θ34PMNS are assumed to be freely varying between 0−π/4 and obtained results which are consistent with the data available from various experiments, like NovA, MINOS, SuperK, Ice Cube-DeepCore. In further investigation, we analytically obtain approximately similar ranges for various neutrino mixing parameters Uμ42 and Uτ42.

Dark matter and naturalness

The Standard Model of particle physics is governed by Poincaré symmetry, while all other symmetries, exact or approximate, are essentially dictated by theoretical consistency with the particle spectrum. On the other hand, many models of dark matter exist that rely upon the addition of new added global symmetries in order to stabilize the dark matter particle and/or achieve the correct abundance. In this work we begin a systematic exploration into truly natural models of dark matter, organized by only relativity and quantum mechanics, without the appeal to any additional global symmetries, no fine-tuning, and no small parameters. We begin by reviewing how singlet dark sectors based on spin 0 or spin frac{1}{2} should readily decay, while pure strongly coupled spin 1 models have an overabun-dance problem. This inevitably leads us to construct chiral models with spin frac{1}{2} particles charged under confining spin 1 particles. This leads to stable dark matter candidates that are analogs of baryons, with a confinement scale that can be naturally \U0001d4aa(100)TeV. This leads to the right freeze-out abundance by annihilating into massless unconfined dark fermions. The minimal model involves a dark copy of SU(3) × SU(2) with 1 generation of chiral dark quarks and leptons. The presence of massless dark leptons can potentially give rise to a somewhat large value of ∆Neff during BBN. In order to not upset BBN one may either appeal to a large number of heavy degrees of freedom beyond the Standard Model, or to assume the dark sector has a lower reheat temperature than the visible sector, which is also natural in this framework. This reasoning provides a robust set of dark matter models that are entirely natural. Some are concrete realizations of the nightmare scenario in which dark matter may be very difficult to detect, which may impact future search techniques.

Bounds on CP-violating Higgs-gluon interactions: the case of vanishing light-quark Yukawa couplings

We investigate CP-violating interactions involving the Higgs boson and gluons within an effective field theory approach, focusing on the specific class of new-physics scenarios where the Yukawa couplings of light quarks are zero or strongly suppressed compared to the standard-model expectations. We compute the contributions of the most relevant higher-dimensional operators of Weinberg type to the electric dipole moment of the neutron (nEDM), which are induced by Feynman diagrams that involve an effective CP-violating Higgs-gluon coupling and top-quark loops. The resulting nEDM sensitivities and prospects are discussed and compared to the existing and expected LHC bounds. We find that future nEDM searches can set non-trivial constraints on CP-violating Higgs-gluon interactions even if the Higgs only couples to the third generation of quarks.

Probing Higgs couplings to light quarks via Higgs pair production

We consider the potential of the Higgs boson pair production process to probe the light quark Yukawa couplings. We show within an effective theory description that the prospects of constraining enhanced first generation light quark Yukawa couplings in Higgs pair production are similar to other methods and channels, due to a coupling of two Higgs bosons to two fermions. Higgs pair production can hence also probe if the Higgs sector couples non-linearly to the light quark generations. For the second generation, we show that by employing charm tagging for the Higgs boson pair decaying to c overline{c} γγ, we can obtain similarly good prospects for measuring the charm Yukawa coupling as in other direct probes.

Bounding the charm Yukawa coupling

The study of the properties of the observed Higgs boson is one of the main research activities in High Energy Physics. Although the couplings of the Higgs to the weak gauge bosons and third generation quark and leptons have been studied in detail, little is known about the Higgs couplings to first and second generation fermions. In this article, we study the charm quark Higgs coupling in the so-called $\kappa$ framework. We emphasize the existence of specific correlations between the Higgs couplings that can render the measured LHC Higgs production rates close to the SM values in the presence of large deviations of the charm coupling from its SM value, $\kappa_c = 1$. Based on this knowledge, we update the indirect bounds on $\kappa_c$ through a fit to the precision Higgs measurements at the LHC. We also examine the limits on $\kappa_c$ arising from the radiative decay $H \to J/\psi + \gamma$, the charm quark-associated Higgs production, charm quark decays of the Higgs field and charge asymmetry in $W^{\pm} + H$ production. Estimates for the future LHC sensitivity on $\kappa_c$ at the high luminosity run are provided.

Composite Higgs models with a hidden sector

We discuss the phenomenology of composite Higgs models that naturally produce a Standard Model-like Higgs boson with a mass of $126$ GeV. The effective theory below the compositeness scale is weakly coupled in these models, and the goldstone sector acts as a portal between the third generation of quarks and a hidden gauge sector. The addition of hidden-sector fermions gives rise to a calculable effective scalar potential with a naturally light scalar resonance. The generic prediction of these theories is the existence of additional pseudo-Nambu Goldstone bosons with electroweak-scale cross sections and masses. In this paper we analyze the collider signatures for some simple concrete realizations of this framework. We find that despite the existence of additional weakly and strongly coupled particles that are kinematically within reach of current experiments, the generic signatures are difficult to resolve at the LHC, and could remain well-hidden in the absence of an $e^+ e^-$ Higgs factory such as the CEPC or a surface detector such as MATHUSLA.

Exotic decays of top partners: Mind the search gap

Many standard model extensions, including composite Goldstone Higgs models, predict vector-like fermionic top-partners at the TeV scale. The intensive search programmes by ATLAS and CMS focus on decays into a 3rd generation quark and an electroweak boson (W,Z,h). However, underlying models of partial compositeness contain additional states that give rise to exotic top partner decays. We consider a well-motivated scenario in which a charge-2/3 top-partner decays into a pseudo-scalar, T→ta, with a→gg or bb¯ dominating below the tt¯ threshold. We show that the constraints on the top partner mass from QCD pair production are substantially weakened, still allowing a top partner mass as light as 400 GeV.

LHC constraints on a mediator coupled to heavy quarks

We apply LHC data to constrain a simplified extension of the Standard Model containing a new spin−1 mediator R, which does not couple to first generation quarks, and a spinor dark matter particle χ. We recast ATLAS and CMS searches for final states containing one or more jet(s) + E̸T, with or without b tags, as well as searches for di–jet resonances with b or t tagging. We find that LHC constraints on the axial vector couplings of the mediator are always stronger than the unitarity bound, which scales like mR/mt. If R has a sizable invisible branching ratio, the strongest LHC bound on both vector couplings and axial vector coupling comes from a di–jet + E̸T search with or without double b tag. These bounds are quite strong for mR<1 TeV, even though we have switched off all couplings to valence quarks. Searches for a di–jet resonance with double b tag lead to comparable bounds with the previous results even if R→χχ¯ decays are allowed; these are the only sensitive LHC searches if the invisible branching ratio of R is very small or zero.

Aligned and Spontaneous Flavor Violation.

We present a systematic spurion setup called aligned flavor violation (AFV) that allows for new physics couplings to quarks that are aligned with standard model (SM) Yukawa couplings, but do not necessarily share their hierarchies nor are family universal. Additionally, we show that there is an important subset of AFV called spontaneous flavor violation (SFV), which naturally arises from UV completions where the quark family number and CP groups are spontaneously broken. Flavor-changing neutral currents are strongly suppressed in SFV extensions of the SM. We study SFV from an effective field theory perspective and demonstrate that SFV new physics with significant and preferential couplings to first or second generation quarks may be close to the TeV scale.

Dark mesons at the LHC

A new, strongly-coupled “dark” sector could be accessible to LHC searches now. These dark sectors consist of composites formed from constituents that are charged under the electroweak group and interact with the Higgs, but are neutral under Standard Model color. In these scenarios, the most promising target is the dark meson sector, consisting of dark vector-mesons as well as dark pions. In this paper we study dark meson production and decay at the LHC in theories that preserve a global SU(2) dark flavor symmetry. Dark pions — like the pions of QCD — can be pair-produced through resonant dark vector meson production, pp → ρD → πDπD, and decay in one of two distinct ways: “gaugephobic”, when {pi}_Dto f{overline{f}}^{prime } generally dominates; or “gaugephilic”, when πD → W + h, Z + h dominates once kinematically open. Unlike QCD, the decay πD0 → γγ is virtually absent due to the dark flavor symmetry. We recast a vast set of existing LHC searches to determine the current constraints on (and future opportunities for) dark meson production and decay. When {m}_{rho_D} is slightly heavier than 2{m}_{pi_D} and ρD±,0 kinetically mixes with the weak gauge bosons, the 8 TeV same-sign lepton search strategy sets the best bound, {m}_{pi_D} > 500 GeV. Yet, when only the ρD0 kinetically mixes with hypercharge, we find the strongest LHC bound is {m}_{pi_D} > 130 GeV, that is only slightly better than what LEP II achieved two decades ago. We find the relative insensitivity of LHC searches, especially at 13 TeV, can be blamed mainly on their penchant for high mass objects or large missing energy. Dedicated searches would undoubtedly yield substantially improved sensitivity. We provide a GitHub page to speed the implementation of these searches in future LHC analyses. Our findings for dark meson production and decay provide a strong motivation for model-independent searches of the form pp → A → B + C → SM SM + SM SM where the theoretical prejudice is for SM to be a 3rd generation quark or lepton, W, Z, or h.

QCD evolution of superfast quarks

Recent high-precision measurements of nuclear deep inelastic scattering at high x and moderate 6 < Q$^2$ < 9GeV$^2$ give a rare opportunity to reach the quark distributions in the {\it superfast} region, in which the momentum fraction of the nucleon carried by its constituent quark is larger than the total fraction of the nucleon at rest, x>1. We derive the leading-order QCD evolution equation for such quarks with the goal of relating the moderate-Q$^2$ data to the two earlier measurements of superfast quark distributions at large 60 < Q$^2$ < 200~GeV$^2$. Since the high-Q$^2$ measurements gave strongly contradictory estimates of the nuclear effects that generate superfast quarks, relating them to the high-precision, moderate-Q$^2$ data through QCD evolution allows us to clarify this longstanding issue. Our calculations indicate that the moderate-Q$^2$ data at $x\lesssim 1.05$ are in better agreement with the high-Q$^2$ data measured in (anti)neutrino-nuclear reactions which require substantial high-momentum nuclear effects in the generation of superfast quarks. Our prediction for the high-Q$^2$ and x>1.1 region is somewhat in the middle of the neutrino-nuclear and muon-nuclear scattering data.

Looking for B→Xsℓ+ℓ− in a nonminimal universal extra dimensional model

Non-vanishing boundary localised terms significantly modify the mass spectrum and various interactions among the Kaluza-Klein excited states of 5-Dimensional Universal Extra Dimensional scenario. In this scenario we compute the contributions of Kaluza-Klein excitations of gauge bosons and third generation quarks for the decay process $B\rightarrow X_s\ell^+\ell^-$ incorporating next-to-leading order QCD corrections. We estimate branching ratio as well as Forward Backward asymmetry associated with this decay process. Considering the constraints from some other $b \to s$ observables and electroweak precision data we show that significant amount of parameter space of this scenario has been able to explain the observed experimental data for this decay process. From our analysis we put lower limit on the size of the extra dimension by comparing our theoretical prediction for branching ratio with the corresponding experimental data. Depending on the values of free parameters of the present scenario, lower limit on the inverse of the radius of compactification ($R^{-1}$) can be as high as $\geq 760$ GeV. {Even this value could slightly be higher if we project the upcoming measurement by Belle II experiment.} Unfortunately, the Forward Backward asymmetry of this decay process would not provide any significant limit on $R^{-1}$ in the present model.

Nonstandard signatures of vectorlike quarks in a leptophobic 221 model

We consider vector-like quarks in a leptophobic 221 model characterized by the gauge group $SU(2)_L\times SU(2)_2 \times U(1)_X$, where the $SU(2)_2$ is leptophobic in nature. We discuss about the pattern of mixing between Standard Model quarks and vector-like quarks and how we prevent tree level flavour-changing interactions in the model. The model also predicts tauphilic scalars decaying mostly to tau leptons. We consider a typical signal of the model in the form of pair production of top-type vector-like quarks which decays to the tauphilic scalars and a third generation quark. We analyze the resulting final state signal for the 13 TeV LHC, containing $\geq 3j(1b) \, + \geq 2\tau \, + \geq 1l$ and discuss the discovery prospects of such vector-like quarks with non-standard decay modes.

Three fermion generations with two unbroken gauge symmetries from the complex sedenions

We show that three generations of leptons and quarks with unbroken Standard Model gauge symmetry SU(3)_ctimes U(1)_{em} can be described using the algebra of complexified sedenions {mathbb {C}}otimes {mathbb {S}}. A primitive idempotent is constructed by selecting a special direction, and the action of this projector on the basis of {mathbb {C}}otimes {mathbb {S}} can be used to uniquely split the algebra into three complex octonion subalgebras {mathbb {C}}otimes {mathbb {O}}. These subalgebras all share a common quaternionic subalgebra. The left adjoint actions of the 8 {mathbb {C}}-dimensional {mathbb {C}}otimes {mathbb {O}} subalgebras on themselves generates three copies of the Clifford algebra {mathbb {C}}ell (6). It was previously shown that the minimal left ideals of {mathbb {C}}ell (6) describe a single generation of fermions with unbroken SU(3)_ctimes U(1)_{em} gauge symmetry. Extending this construction from {mathbb {C}}otimes {mathbb {O}} to {mathbb {C}}otimes {mathbb {S}} naturally leads to a description of exactly three generations.

Current status of MSSM Higgs sector with LHC 13 TeV data

ATLAS and CMS collaborations have reported the results on the Higgs search analyzing $\sim 36$ fb$^{-1}$ data from Run-II of LHC at 13 TeV. In this work, we study the Higgs sector of the phenomenological Minimal Supersymmetric Standard Model, in light of the recent Higgs data, by studying separately the impact of Run-I and Run-II data. One of the major impacts of the new data on the parameter space comes from the direct searches of neutral CP-even and CP-odd heavy Higgses ($H$ and $A$, respectively) in the $H/A \to \tau^{+} \tau^{-}$ channel which disfavours high $\tan\beta$ regions more efficiently than Run-I data. Secondly, we show that the latest result of the rare radiative decay of $B$ meson imposes a slightly stronger constraint on low $\tan \beta$ and low $M_A$ region of the parameter space, as compared to its previous measurement. Further, we find that in a global fit Run-II light Higgs signal strength data is almost comparable in strength with the corresponding Run-I data. Finally, we discuss scenarios with the Heavy Higgs boson decaying into electroweakinos and third generation squarks and sleptons.

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.

New physics from high energy tops

Precision measurements of high energy top quarks at the LHC constitute a powerful probe of new physics. We study the effect of four fermion operators involving two tops and two light quarks on the high energy tail of the toverline{t} invariant mass distribution. We use existing measurements at a center of mass energy of 13 TeV, and state of the art calculations of the Standard Model contribution, to derive bounds on the coefficients of these operators. We estimate the projected reach of the LHC at higher luminosities and discuss the validity of these limits within the Effective Field Theory description. We find that current measurements constrain the mass scale of these operators to be larger than about 1–2 TeV, while we project that future LHC data will be sensitive to mass scales of about 3–4 TeV. We apply our bounds to constrain composite Higgs models with partial compositeness and models with approximate flavor symmetries. We find our limits to be most relevant to flavor non-universal models with a moderately large coupling of the heavy new physics states to third generation quarks.

A minimal U(1)′ extension of MSSM in light of the B decay anomaly

Motivated by the RK and RK⁎ anomalies from B decays, we extend the minimal supersymmetric model with a non-universal anomaly-free U(1)′ gauge symmetry, coupling non-universally to the lepton sector as well as the quark sector. In particular, only the third generation quarks are charged under this U(1)′, which can easily evade the dilepton bound from the LHC searches. An extra singlet is introduced to break this U(1)′ symmetry allowing for the μ-term to be generated dynamically. The relevant constraints of Bs−B¯s mixing, D0−D¯0 mixing and the LHC dilepton searches are considered. We find that in the allowed parameter space this U(1)′ gauge interaction can accommodate the RK and RK⁎ anomalies and weaken considerably the Z′ mass limits while remaining perturbative up to the Planck scale.

S3 flavored left-right symmetric model of quarks

We construct a model based on the electroweak gauge group $\rm SU(2)_L \times SU(2)_R \times U(1)_{B-L}$ augmented by an $S_3$ symmetry. We assign nontrivial $S_3$ transformation properties to the quarks and consequently we need two scalar bidoublets. Despite the extra bidoublet we have only six Yukawa couplings thanks to the discrete symmetry. Diagonalization of the quark mass matrices shows that at the leading order only the first two generations mix, resulting in a block diagonal CKM matrix, and the first generation quarks are massless. Inclusion of subleading terms produce an acceptable CKM matrix up to corrections of order $\lambda^4$. As for the first generation quark masses, we obtain satisfactory value of $m_u/m_d$. The masses themselves, though being in the same ballpark, turn out to be somewhat smaller than the accepted range.

A flavorful factoring of the strong CP problem

Motivated by the intimate connection between the strong CP problem and the flavor structure of the Standard Model, we present a flavor model that revives and extends the classic mu = 0 solution to the strong CP problem. QCD is embedded into a SU(3)1 × SU(3)2 × SU(3)3 gauge group, with each generation of quarks charged under the respective SU(3). The non-zero value of the up-quark Yukawa coupling (along with the strange quark and bottom-quark Yukawas) is generated by contributions from small instantons at a new scale M ≫ ΛQCD. The Higgsing of SU(3)3 → SU(3)c allows dimension-5 operators that generate the Standard Model flavor structure and can be completed in a simple renormalizable theory. The smallness of the third generation mixing angles can naturally emerge in this picture, and is connected to the smallness of threshold corrections to overline{theta} . Remarkably, overline{theta} is essentially fixed by the measured quark masses and mixings, and is estimated to be close to the current experimental bound and well within reach of the next generation of neutron and proton EDM experiments.