Flavor Asymmetry Research Articles

Flavor asymmetry from the nonperturbative nucleon sea

Flavor asymmetry from the nonperturbative nucleon sea

Measurement of J/ψ and [formula omitted] production in p + p and p + d interactions at 120 GeV

We report the p+p and p+d differential cross sections measured in the SeaQuest experiment for J/ψ and ψ(2S) production at ▪ beam energy covering the forward x-Feynman (xF) range of 0.5<xF<0.9. The measured cross sections are in good agreement with theoretical calculations based on the nonrelativistic QCD (NRQCD) using the long-distance matrix elements deduced from a recent global analysis of proton- and pion-induced charmonium production data. The σψ(2S)/σJ/ψ cross section ratios are found to increase as xF increases, indicating that the qq¯ annihilation process has larger contributions in the ψ(2S) production than the J/ψ production. The σpd/2σpp cross section ratios are observed to be significantly different for the Drell-Yan process and J/ψ production, reflecting their different production mechanisms. We find that the σpd/2σpp ratios for J/ψ production at the forward xF region are sensitive to the d¯/u¯ flavor asymmetry of the proton sea, analogous to the Drell-Yan process. The transverse momentum (pT) distributions for J/ψ and ψ(2S) production are also presented and compared with data collected at higher center-of-mass energies.

Kaon superfluidity in the early Universe

Previously, it was found that pion superfluidity could be realized in the quantum chromodynamics (QCD) epoch of the early Universe, when lepton flavor asymmetry |le+lμ| is large enough to generate a charge chemical potential |μQ| larger than vacuum pion mass. By following the same logic, kaon superfluidity might also be possible when |le+lμ| is so large that |μQ| becomes larger than vacuum kaon mass. Such a possibility is checked by adopting Ginzburg-Landau approximation within the three-flavor Polyakov–Nambu–Jona-Lasinio model. Consider the case with full chemical balance, though kaon superfluidity could be stable compared to the chiral phases with only σ condensations, it would get killed by the more favored homogeneous pion superfluidity. If we introduce mismatch between s and d quarks, kaon superfluidity would require so large s quark density that such a state is impossible in the early Universe. Published by the American Physical Society 2024

QCD angular momentum in N → Δ transitions

N→Δ transitions offer new possibilities for exploring the isovector component of the QCD quark angular momentum (AM) operator causing the Ju−d flavor asymmetry in the nucleon. We extend the concept of QCD AM to transitions between baryon states, using light-front densities of the energy-momentum tensor in transversely localized states. We calculate the N→Δ transition AM in the 1/Nc expansion, connect it with the Ju−d flavor asymmetry in the nucleon, and estimate the values using lattice QCD results. In the same setup we connect the transition AM to the transition GPDs sampled in hard exclusive electroproduction processes with N→Δ transitions, enabling experimental study of the transition AM.

CJ15 global PDF analysis with new electroweak data from the STAR and SeaQuest experiments

We present updates to a recent CTEQ-Jefferson Lab (CJ) global analysis of parton distribution functions with a new set of electroweak data that provide unique access to quark flavor separation in the proton. In particular, recent WW and ZZ boson measurements from the STAR experiment at RHIC put additional constraints on light quarks and antiquarks near the valence regime. The new measurement of the Drell-Yan lepton pair production ratio in p+pp+p and p+dp+d collisions by the SeaQuest experiment at Fermilab extends the large-xx coverage of the previous E866 experiment and sheds new light on the light antiquarks distribution. In this report, the impact of these new data sets on parton distribution functions will be presented with emphasis given to the flavor asymmetry of the light antiquark sea at large values of the parton momentum xx.

Hadronic structure on the light front. V. Diquarks, nucleons, and multiquark Fock components

This work is a continuation in our series of papers, that addresses quark models of hadronic structure on the light front, motivated by the QCD vacuum structure and lattice results. In this paper we focus on the importance of diquark correlations, which we describe by a quasi-local four-fermion effective 't Hooft interaction induced by instantons. The same interaction is also shown to generate extra quark-antiquark pair of the "sea". Its higher order iteration can be included via "pion-mediation": both taken together yield a quantitative description of the observed flavor asymmetry of antiquarks sea. Finally we discuss the final step needed to bridge the gap between hadronic spectroscopy and parton observables, by forward DGLAP evolution towards the chiral upper scale of $\sim 1\, {\rm GeV}^2$.

Quark orbital angular momentum of ground-state octet baryons

Here we study the quark orbital angular momentum of the ground octet baryons employing an extended chiral constituent quark model, within which the baryon wave functions are taken to be superposition of the traditional $qqq$ and the $qqqq\overline{q}$ higher Fock components. Coupling between the two configurations is estimated using the $^{3}P_{0}$ quark-antiquark creation mechanism, and the corresponding coupling strength is determined by fitting the sea flavor asymmetry of the nucleon. The obtained numerical results show that the quark angular momentum of the nucleon, $\mathrm{\ensuremath{\Sigma}}, \mathrm{\ensuremath{\Lambda}}$, and $\mathrm{\ensuremath{\Xi}}$ hyperons, are in the range 0.10--0.30. In addition, the quark angular momentum of all the hyperons are a little bit smaller than that of the nucleon. And the octet baryons spin fractions taken by the intrinsic quark orbital angular momentum could be up to $60%$ in present model.

Generalized parton distributions of sea quarks in the proton from nonlocal chiral effective theory

We calculate the spin-averaged generalized parton distributions (GPDs) of sea quarks in the proton at zero skewness from nonlocal covariant chiral effective theory, including one-loop contributions from intermediate states with pseudoscalar mesons and octet and decuplet baryons. A relativistic regulator is generated from the nonlocal Lagrangian where a gauge link is introduced to guarantee local gauge invariance, with additional diagrams from the expansion of the gauge link ensuring conservation of electric charge and strangeness. Flavor asymmetries for sea quarks at zero and finite momentum transfer, as well as strange form factors, are obtained from the calculated GPDs, and results compared with phenomenological extractions and lattice QCD.

Cosmic QCD transition for large lepton flavor asymmetries

We study the impact of large lepton flavor asymmetries on the cosmic QCD transition. Scenarios of unequal lepton flavor asymmetries are observationally almost unconstrained and therefore open up a whole new parameter space for the cosmic QCD transition. We find that for large asymmetries, the formation of a Bose-Einstein condensate of pions can occur and identify the corresponding parameter space. In the vicinity of the QCD transition scale, we express the pressure in terms of a Taylor expansion with respect to the complete set of chemical potentials. The Taylor coefficients rely on input from lattice QCD calculations from the literature. The domain of applicability of this method is discussed.

Baryon Asymmetry of the Universe from Lepton Flavor Violation.

Charged-lepton flavor violation (CLFV) is a smoking-gun signature of physics beyond the standard model. The discovery of CLFV in upcoming experiments would indicate that CLFV processes must have been efficient in the early Universe at relatively low temperatures. In this Letter, we point out that such efficient CLFV interactions open up new ways of creating the baryon asymmetry of the Universe. First, we quote the two-loop corrections from charged-lepton Yukawa interactions to the chemical transport in the standard model plasma, which imply that nonzero lepton flavor asymmetries summing up to B-L=0 are enough to generate the baryon asymmetry. Then, we describe two scenarios of what we call leptoflavorgenesis, where efficient CLFV processes are responsible for the generation of primordial lepton flavor asymmetries that are subsequently converted to a baryon asymmetry by weak sphaleron processes. Here, the conversion factor from lepton flavor asymmetry to baryon asymmetry is suppressed by charged-lepton Yukawa couplings squared, which provides a natural explanation for the smallness of the observed baryon-to-photon ratio.

Cosmology Meets Functional QCD: First-Order Cosmic QCD Transition Induced by Large Lepton Asymmetries.

The lepton flavor asymmetries of the Universe are observationally almost unconstrained before the onset of neutrino oscillations. We calculate the cosmic trajectory during the cosmic QCD epoch in the presence of large lepton flavor asymmetries. By including QCD thermodynamic quantities derived from functional QCD methods in our calculation, our work reveals for the first time the possibility of a first-order cosmic QCD transition. We specify the required values of the lepton flavor asymmetries for which a first-order transition occurs for a number of different benchmark scenarios.

Instanton-induced effects in interquark forces, light-front wave functions and formfactors

Exclusive processes are traditionally described by perturbative hard blocks and “distribution amplitudes" (DAs), matrix elements of operators of various chiral structure and twist. One paper (with I.Zahed) calculate instanton contribution to hard blocks, which is found comparable to perturbative one in few-GeV2 Q2 region of interest. Another paper aims at comprehensive wave functions of mesons, baryons and pentaquarks. The last ones are also included as 5-quark component of the baryons. The calculation, using ’t Hooft operator, gives x-dependence and magnitude of the antiquark PDF. It explains long standing issue of strong flavor asymmetry of antiquark sea. The third paper (also with I.Zahed) is semi-review on the instanton-sphaleron processes in QCD and electroweak theories, with emphasis on their possible experimental observation via double diffractive events at LHC and RHIC. Insert your english abstract here.

On the evaluation of the Gottfried sum and d̄−ū asymmetry with the use of the TMM method

In this paper, we obtain the integrated flavor asymmetry of the sea quarks in the proton, [Formula: see text], with the help of the truncated moments approach elaborated in our previous papers. We use the difference between the light sea-quark distributions [Formula: see text] extracted from Drell–Yan (DY) NuSea/E866 measurements of the cross-section ratio [Formula: see text] and from the recent global analysis of deep inelastic scattering (DIS) [Formula: see text] data incorporating the reanalyzed neutron structure function. In our analysis, we also include the most recent DY data from the Fermilab SeaQuest/E906 experiment.

Reentrant pion superfluidity and cosmic trajectories within a PNJL model

In this work, we self-consistently explore the possibility of charged pion superfluidity and cosmic trajectories in early Universe under the framework of Polyakov-Nambu--Jona-Lasinio model. By taking the badly constrained lepton flavor asymmetries $l_{\rm e}$ and $l_\mu$ as free parameters, the upper boundaries of pion superfluidity phase are consistently found to be around the pseudocritical temperature at zero chemical potentials. So the results greatly support the choice of $T=0.16~{\rm GeV}$ as the upper boundary of pion superfluidity in the previous lattice QCD study. Take $l_{\rm e}+l_\mu=-0.2$ as an example, we demonstrate the features of pion condensation and the associated cosmic trajectories with the evolution of early Universe. While the trajectory of electric chemical potential reacts strongly at both the lower and upper boundaries of reentrant pion superfluidity, the trajectories of other chemical potentials only respond strongly at the upper boundary.

Axial charges of the proton within an extended chiral constituent quark model

We have performed a study of the isovector, octet and singlet axial charges of the proton in an extended chiral constituent quark model, where all the possible $uudq\bar{q}$~($q=u,d,s$) five-quark Fock components in the proton wave function are taken into account. The $^3P_0$ quark-antiquark creation mechanism is assumed to account for the transition coupling between three- and five-quark components in proton, and the corresponding transition coupling strength is fixed by fitting the intrinsic sea flavor asymmetry $\bar{d}-\bar{u}$ data for proton. Accordingly, with all the parameters fixed by empirical values, the probabilities of the intrinsic five-quark Fock components in proton wave function should be $\sim30 - 50\%$, which lead to the numerical results for quark spin $\Delta u$, $\Delta d$ and $\Delta s$, as well the axial charges of proton consistent with the experimental data and predictions by other theoretical approaches.

Lepton flavor asymmetries and the mass spectrum of primordial black holes

We study the influence of lepton flavour asymmetries on the formation and the mass spectrum of primordial black holes. We estimate the detectability of their mergers with LIGO/Virgo and show that the currently published gravitational wave events may actually be described by a primordial black hole spectrum from non-zero asymmetries. We suggest to use gravitational-wave astronomy as a novel tool to probe how lepton flavour asymmetric the Universe has been before the onset of neutrino oscillations.

Flavor-dependent radiative corrections in coherent elastic neutrino-nucleus scattering

We calculate coherent elastic neutrino-nucleus scattering cross sections on spin-0 nuclei (e.g. 40Ar and 28Si) at energies below 100 MeV within the Standard Model and account for all effects of permille size. We provide a complete error budget including uncertainties at nuclear, nucleon, hadronic, and quark levels separately as well as perturbative error. Our calculation starts from the four-fermion effective field theory to explicitly separate heavy-particle mediated corrections (which are absorbed by Wilson coefficients) from light-particle contributions. Electrons and muons running in loops introduce a non- trivial dependence on the momentum transfer due to their relatively light masses. These same loops, and those mediated by tau leptons, break the flavor universality because of mass-dependent electromagnetic radiative corrections. Nuclear physics uncertainties significantly cancel in flavor asymmetries resulting in subpercent relative errors. We find that for low neutrino energies, the cross section can be predicted with a relative precision that is competitive with neutrino-electron scattering. We highlight potentially useful applications of such a precise cross section prediction ranging from precision tests of the Standard Model, to searches for new physics and to the monitoring of nuclear reactors.

Pion Condensation in the Early Universe at Nonvanishing Lepton Flavor Asymmetry and Its Gravitational Wave Signatures.

We investigate the possible formation of a Bose-Einstein condensed phase of pions in the early Universe at nonvanishing values of lepton flavor asymmetries. A hadron resonance gas model with pion interactions, based on first-principle lattice QCD simulations at nonzero isospin density, is used to evaluate cosmic trajectories at various values of electron, muon, and tau lepton asymmetries that satisfy the available constraints on the total lepton asymmetry. The cosmic trajectory can pass through the pion condensed phase if the combined electron and muon asymmetry is sufficiently large: |l_{e}+l_{μ}|≳0.1, with little sensitivity to the difference l_{e}-l_{μ} between the individual flavor asymmetries. Future constraints on the values of the individual lepton flavor asymmetries will thus be able to either confirm or rule out the condensation of pions during the cosmic QCD epoch. We demonstrate that the pion condensed phase leaves an imprint both on the spectrum of primordial gravitational waves and on the mass distribution of primordial black holes at the QCD scale, e.g., the black hole binary of recent LIGO event GW190521 can be formed in that phase.

Measurement of Longitudinal Single-Spin Asymmetry for W± Production in Polarized Proton+Proton Collisions at STAR

The contribution from the sea quark polarization to the nucleon spin is an important piece for the complete understanding of the nucleon spin structure. The production of W ± bosons in longitudinally polarized p+p collisions at the RHIC collider at Brookhaven National Laboratory provides a unique probe of the sea quark polarization, through the parity-violating single-spin asymmetry, AL . At the STAR experiment, the W bosons that decay through the W → ev channel at mid-rapidity (|η <1.3) can be effectively determined with the Electromagnetic Calorimeters and Time Projection Chamber. The STAR measurements of AL for W boson from datasets taken in 2011 and 2012 at =510 GeV have been included in the global analysis of polarized parton distribution functions, and provided significant constraints on the helicity distribution functions of and quarks. The final AL results from 2013 STAR data sample are reported, which is about three times larger than the total integrated luminosity of previous years. The combined results of AL for 2011-2013 data are also given. A flavor asymmetry of light sea quark helicity distribution, , is confirmed from a re-weighting of global analysis NNPDFpol1.1 after including the new AL results. In addition, results on the double-spin asymmetries ALL for W ±, and AL for Z/γ* production are also reported.

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.