Quark Polarization Research Articles

Searching for heavy charged Higgs bosons through top quark polarization

We study the production of a heavy charged Higgs boson at the Large Hadron Collider (LHC) in [Formula: see text] within a 2-Higgs Doublet Model (2HDM). The chiral structure of the [Formula: see text] coupling can trigger a particular spin state of the top quark produced in the decay of a charged Higgs boson and, therefore, is sensitive to the underlying mechanism of the Electroweak Symmetry Breaking (EWSB). Taking two benchmark models (2HDM type-I and 2HDM type-Y) as an example, we show that inclusive rates, differential distributions and forward–backward asymmetries of the top quark’s decay products can be used to search for heavy charged Higgs bosons and also as model discriminators.

What can we learn from the global spin alignment of ϕ mesons in heavy-ion collisions?

We propose that a significant positive deviation from 1/3 for the spin density matrix element $\rho_{00}$ of the $\phi$ meson may indicate the existence of a mean field of the $\phi$ meson generated in heavy-ion collisions. This explains why STAR preliminary data for the $\phi$ meson's $\rho_{00}$ are much larger than 1/3 while the data of $\Lambda$ and $\bar{\Lambda}$ polarization seem not to allow such a significant and positive deviation. The contribution may be from the polarization of the strange quark and antiquark through the $\phi$ field, an effective mode of the gluon field in strong interaction. We show that $\rho_{00}$ for the $\phi$ meson is a good analyzer for fields even if they may strongly fluctuate in space-time.

Measurement of differential cross sections and charge ratios for t-channel single top quark production in proton\u2013proton collisions at sqrt{s}=13,text {Te}text {V}

A measurement is presented of differential cross sections for t-channel single top quark and antiquark production in proton–proton collisions at a centre-of-mass energy of 13,text {Te}text {V} by the CMS experiment at the LHC. From a data set corresponding to an integrated luminosity of 35.9,text {fb}^{-1}, events containing one muon or electron and two or three jets are analysed. The cross section is measured as a function of the top quark transverse momentum (p_{mathrm{T}} ), rapidity, and polarisation angle, the charged lepton p_{mathrm{T}} and rapidity, and the p_{mathrm{T}} of the text {W}{}{} boson from the top quark decay. In addition, the charge ratio is measured differentially as a function of the top quark, charged lepton, and text {W}{}{} boson kinematic observables. The results are found to be in agreement with standard model predictions using various next-to-leading-order event generators and sets of parton distribution functions. Additionally, the spin asymmetry, sensitive to the top quark polarisation, is determined from the differential distribution of the polarisation angle at parton level to be 0.440 pm 0.070, in agreement with the standard model prediction.

Single spin asymmetry AULsin(ϕh−ϕR) in dihadron semi-inclusive DIS

In this paper, we study the single longitudinal spin asymmetry ${A}_{UL}^{\mathrm{sin}({\ensuremath{\phi}}_{h}\ensuremath{-}{\ensuremath{\phi}}_{R})}$ of dihadron production in semi-inclusive deep inelastic scattering (SIDIS) via helicity-dependent dihadron fragmentation function (DiFF), which describes the correlation of the longitudinal polarization of a fragmenting quark with the transverse momenta of the produced hadron pair. Recently, experimental searching for this azimuthal asymmetry in dihadron SIDIS by the COMPASS Collaboration yielded a very small signal. Here we calculate this unknown T-odd DiFF ${G}_{1}^{\ensuremath{\perp}}$ using a spectator model to access the asymmetry and clarify why the signal is very small. The transverse momentum dependent factorization method, in which the transverse momentum of the final state hadron pair leaves unintegrated, has been applied. We estimate the $\mathrm{sin}({\ensuremath{\phi}}_{h}\ensuremath{-}{\ensuremath{\phi}}_{R})$ asymmetry at the kinematics of COMPASS experiments and compare with the data. What's more, the predictions on the same asymmetry are also made at the electron ion collider.

Probing the C P nature of the top quark Yukawa at hadron colliders

We analyze the prospects of probing the C P -odd itilde{kappa}overline{t}gamma 5th interaction at the LHC and its projected upgrades, the high-luminosity and high-energy LHC, directly using associated on-shell Higgs boson and top quark or top quark pair production. To this end we first construct a C P -odd observable based on top quark polarization in Wb → th scattering with optimal linear sensitivity to tilde{kappa} . For the corresponding hadronic process pp → thj we present a method of extracting the phase-space dependent weight function that allows to retain close to optimal sensitivity to tilde{kappa} . We project future sensitivity to the signal in pp → t(→ ℓ\U0001d708b)h(→ b overline{b} )j. We also propose novel C P -odd observables for top quark pair production in association with the Higgs, pp → t overline{t} h, with semileptonically decaying tops and h → b overline{b} , that rely solely on measuring the momenta of leptons and b-jets from the decaying tops without having to distinguish the charge of the b-jets. Among the many possibilities we single out an observable that can potentially probe tilde{kappa} ∼ 0.1 at the high-energy LHC with 2\U0001d70e confidence.

Light quark vacuum polarization at the physical point and contribution to the muon g−2

We report on the computation of the connected light quark vacuum polarization with 2+1+1 flavors of HISQ fermions at the physical point and its contribution to the muon anomalous magnetic moment. Three ensembles, generated by the MILC collaboration, are used to take the continuum limit. The finite volume correction to this result is computed in the (Euclidean) time-momentum representation to NNLO in chiral perturbation theory. We find $a_\mu^{ll}({\rm HVP})=(659\pm 20\pm 5\pm 5\pm 4)\times 10^{-10}$, where the errors are statistical and estimates of residual uncertainties from taking the continuum limit, scale setting, and truncation of chiral perturbation theory at NNLO. We compare our results with recent ones in the literature.

Polarization of cascade hyperons and antihyperons

Polarizations of cascade hyperons (Ξ0 and Ξ−) and antihyperons are especially interesting because they are very sensitive to the mechanism of their production. Hyperon polarization for existing data is considered in framework of the model of chromomagnetic polarization of quarks (CPQ). Hyperon and antihyperon polarization predictions are presented for a wide range of c.m.s. energy and Feynman variable xF.

Recent results on cold-QCD from RHIC

Polarized proton-proton collisions at the Relativistic Heavy Ion Collider (RHIC) provide unique opportunities to study the spin structure of the nucleon. We will highlight recent results on the nucleon spin structure from the STAR and PHENIX experiments at RHIC: (1) A sizable gluon polarization in the proton is measured with longitudinal double spin asymmetries of jet and hadron production; (2) Longitudinal single spin asymmetries in W boson production improve constraints on the sea quark polarization. The new spin asymmetry results for W boson confirmed the SU(2) flavor asymmetry of the light sea quark polarization in the proton; (3) Transverse spin effects in hadronic systems offer new implications on parton distribution functions in the collinear and transverse momentum dependent frameworks. We will also discuss near term plans for the STAR forward detector upgrade and prospects for proton-proton and protonion collisions in the years beyond 2021 at STAR.

Rotation induced charged pion condensation in a strong magnetic field: A Nambu–Jona-Lasino model study

We investigate the possibility of charged pion condensation in the presence of parallel rotation and magnetic field within the Nambu--Jona-Lasinio model with quarks as the fundamental degrees of freedom. Previous study based on non-interacting Klein-Gordon theory for pions showed that the charged pions will undergo Bose-Einstein condensation under this circumstance [Y. Liu and I. Zahed, Phys. Rev. Lett. ${\bf 120}$, 032001 (2018)]. In this work, we take into account the internal quark structures of charged pions self-consistently through quark polarization loops in an interacting theory, i.e., the Nambu--Jona-Lasino model. The stability of the system is explored against the formation of a nonzero expectation value of the composite charged pion field $\bar{u}i\gamma_5d$. We find that two competing effects are induced by the rotation: the isospin enhancement which favors charged pion condensation and the spin breaking which disfavors the condensation. For a strong magnetic field ($\sqrt{eB}\sim1{\rm GeV}$) and system size of a few fermi, the isospin enhancement effect is stronger than the spin breaking one, and the charged pion condensation becomes energetically favored beyond a critical angular velocity of a few MeV.

Measurement of the top quark polarization and tt¯ spin correlations using dilepton final states in proton-proton collisions at s=13 TeV

Measurements of the top quark polarization and top quark pair ($\mathrm{t\bar{t}}$) spin correlations are presented using events containing two oppositely charged leptons (e$^+$e$^-$, e$^\pm\mu^\mp$, or $\mu^+\mu^-$) produced in proton-proton collisions at a center-of-mass energy of 13 TeV. The data were recorded by the CMS experiment at the LHC in 2016 and correspond to an integrated luminosity of 35.9 fb$^{-1}$. A set of parton-level normalized differential cross sections, sensitive to each of the independent coefficients of the spin-dependent parts of the $\mathrm{t\bar{t}}$ production density matrix, is measured for the first time at 13 TeV. The measured distributions and extracted coefficients are compared with standard model predictions from simulations at next-to-leading-order (NLO) accuracy in quantum chromodynamics (QCD), and from NLO QCD calculations including electroweak corrections. All measurements are found to be consistent with the expectations of the standard model. The normalized differential cross sections are used in fits to constrain the anomalous chromomagnetic and chromoelectric dipole moments of the top quark to $-$0.24 $<C_\text{tG}/\Lambda^{2}$ $<$ 0.07 TeV$^{-2}$ and $-$0.33 $< C^{I}_\text{tG}/\Lambda^{2}$ $<$ 0.20 TeV$^{-2}$, respectively, at 95% confidence level.

Quantum kinetic theory of spin polarization of massive quarks in perturbative QCD: Leading log

We present the quantum kinetic equation for spin polarization of massive quarks in leading log order of perturbative QCD, which describes time evolution of the spin density matrix in momentum space of a massive quark interacting with a background QCD plasma. We find that the time evolution operator of the spin density matrix, or the quantum kinetic collision terms, are universally of order $\alpha_s^2\log(1/\alpha_s)$ in terms of the QCD coupling constant $\alpha_s=g^2/(4\pi)$. Our quantum kinetic equation is valid for an arbitrary quark mass $m\gg m_D\sim gT$, where $m_D$ is the Debye mass, and can be used to study relaxation dynamics of spin polarization of massive quarks in perturbative QCD regime.

Boosted top quark polarization

In top quark production, the polarization of top quarks, decided by the chiral structure of couplings, is likely to be modified in the presence of any new physics contribution to the production. Hence the same is a good discriminator for those new physics models wherein the couplings have a chiral structure different than that in the Standard Model (SM). In this note we construct probes of the polarization of a top quark decaying hadronically, using easily accessible kinematic variables such as the energy fraction or angular correlations of the decay products. Tagging the boosted top quark using the usual jet sub structure technique we study robustness of these observables for a benchmark process, $W^{\prime} \to tb$. We demonstrate that the energy fraction of b-jet in the laboratory frame and a new angular variable, constructed by us in the top rest frame, are both very powerful tools to discriminate between the left and right polarized top quarks. Based on the polarization sensitive angular variables, we construct asymmetries which reflect the polarization. We study the efficiency of these variables for two new physics processes where which give rise to boosted top quarks: (i) decay of the top squark in the context of supersymmetry searches, and (ii) decays of the Kaluza-Klein(KK) graviton and KK gluon, in Randall Sundrum(RS) model. Remarkably, it is found that the asymmetry can vary over a wide range about +20\% to -20\%. The dependence of asymmetry on top quark couplings of the new particles present in these models beyond the SM (BSM) is also investigated in detail.

Fingerprinting the top quark FCNC via anomalous Ztq couplings at the LHeC

A study of the top quark \emph{Flavour Changing Neutral Current} (FCNC) through $Z$-boson has been performed in the proposed future $e^-p$ collider for the energy, $E_{e(p)} = 60~(7000)$~GeV. We considered an effective theory where the anomalous FCNC couplings are of vector and tensor nature. The effect of these couplings is probed in the single top production along with the scattered electron. The polar angle $\theta$ of the electrons coming out of the primary vertex in association with the top quark polarization asymmetries constructed from the angular distribution of the secondary lepton arising from the top decay, allow to distinguish the Lorentz structure of the coupling. From a multi-parameter analysis, we obtain a reach of ${\cal O} (10^{-2})$ in the case of $Ztu$ and $Ztc$ couplings at an integrated luminosity of 2~ab$^{-1}$ at 95\% C.L.

Polarization of top and chargino from stop decay in natural SUSY

Top squark (stop) plays a crucial role in the naturalness of supersymmetry (SUSY) models, which has been extensively searched for in the LHC experiments. Due to the parity-violating couplings, top quark and chargino thus produced from stop decay are expected to be polarized. We investigate the polarization of top quark and chargino, from which we find that the mass differences between neutralino and stop/chargino can affect the degree of polarization of top quark/chargino in the stop decay. We then examine the top polarization effects on kinematics of stop decay in natural SUSY (NSUSY) at the LHC and the results show that several observables have the discriminating power for different polarized top quark at the LHC, which may be useful for testing the nature of stop couplings in NSUSY.

Nucleon strangeness polarization from varLambda /bar{varLambda } hyperon production in polarized proton\u2013proton collision at RHIC

We calculate the inclusive production of a polarized varLambda or bar{varLambda } hyperon from the single longitudinally polarized proton and proton (pp) collision at RHIC. By comparing the data reported by the STAR Collaboration, we find that this process is sensitive to the polarization of strange and antistrange quarks of the proton in the experimental range. By introducing asymmetric coefficients with the minimization of chi ^2, we further identify the asymmetry of the polarized strange–antistrange quarks in the proton sea and find that the first moment is varDelta s approx -,0.025pm 0.019 for strange quark and varDelta bar{s} approx -,0.001pm 0.012 for antistrange quark, with central values agreeing with the light-cone meson–baryon fluctuation model prediction, the recent lattice QCD determination of strangeness polarization and results from a global QCD analysis given by the Jefferson Lab Angular Momentum (JAM) Collaboration.

Polarization of top quark in vector-like quark decay

Vector-like quarks (VLQs) are attractive extensions to the Standard Model. They mix with the SM quarks and can lead to rich phenomenology. Determination of VLQ's interaction structure with the SM is then an important issue, which can be inferred from the decay products of VLQs, such as top quark. We calculate the spin-analyzing powers for charged leptons from top quark in VLQ decay for various VLQ scenarios. We find that the top polarization effect will be helpful to distinguish different natures of the VLQ couplings with the SM.

Nucleon spin structure and measurements on sea quark polarization at RHIC

The nucleon spin structure in terms of its parton constituents is a fundamental question in QCD. The sea quark contribution to the nucleon spin is an important piece for a complete understanding of the nucleon spin structure. The production of W bosons in longitudinally polarized proton-proton collisions at RHIC provides a unique probe for the sea quark polarization, through the parity-violating single-spin asymmetry A L = ( σ + − σ − ) / ( σ + + σ − ) . At RHIC, W ’s can be detected via the leptonic channel W → e ν with the kinematics of the decay lepton alone. Longitudinally polarized proton-proton collision events at s =500 GeV were recorded at RHIC through the years 2009, 2011, 2012 and 2013. Both STAR and PHENIX experiments completed their measurements on longitudinal single spin asymmetries for W bosons as a function of decay lepton rapidity. These results provide new constraints on sea quark polarizations through the QCD global analyses, which clearly indicate the existence of a flavor asymmetry in the polarization of the light-quark sea, Δ u ¯ > Δ d ¯ , in the proton for parton momentum fraction range of 0.05 x d ¯ > u ¯ over a wide x range has been observed. On the other hand, the phenomenological studies showed that the spin transfer to Λ ¯ anti-hyperons provides a natural connection to helicity or transversity distributions of strange and anti-strange quarks in the proton. STAR experiment made the first and an improved measurement on the longitudinal spin transfer D LL to Λ and Λ ¯ hyperons in longitudinally polarized proton-proton collisions at 200 GeV. STAR also completed the first measurement on transverse spin transfer D LL to Λ and Λ ¯ hyperons in transversely polarized proton-proton collisions at 200 GeV. These data and future measurements with higher precision and larger momentum fraction in the forward region at RHIC will shed new lights on the polarized parton distributions for strange and anti-strange quark in the nucleon.

Top quark polarization as a probe of charged Higgs bosons

We study the production and decay of a heavy charged Higgs boson in the $bg\to tH^-$ and $H^-\to b\bar t$ processes at the Large Hadron Collider (LHC). We show that the chiral structure of the $H^-t\bar b$ vertex entering both stages is sensitive to the underlying Higgs mechanism of Electro-Weak Symmetry Breaking (EWSB) and we specifically demonstrate that one could distinguish between two popular realizations of a 2-Higgs Double Model (2HDM) embedding a new $H^\pm$ state, i.e., those with Type-I and -Y Yukawa couplings. The chiral structure of such a vertex, which is different in the two cases, in turn triggers a particular spin state of the top quark which is then transmitted to its decay products. Hence, both inclusive rates and exclusive observables can be used to extract the presence of such a charged Higgs boson state in LHC data.

Induced charges and quark polarization operator at A0 background

The presence of the A0 condensate in quark-gluon plasma leads to Z(3) symmetry breaking and Furry's theorem violation. Due to this phenomenon the color charges are induced and other even-number diagrams are allowed. We calculate the first diagram of this series – tadpole diagram with one gluon line and find induced color charges in the plasma. Next, quark polarization operator in presence is calculated and partially investigated.

Next-to-Next-to-Leading Order Calculation of Valence Quark Polarization in the Valon Model

We used the “valon model” to study the valence quarks polarization inside nucleon at Next-to-Next-to-leading order (NNLO) approximation. The results show a good agreement with available experimental data and also with those from global analysis presented in Taghavi-Shahri et al. (Phy. Rev. D 93, 114024, 2016).