Unpolarized Distribution Research Articles

Dynamic changes in mitochondrial localization in human neocortical basal radial glial cells during cell cycle.

Mitochondria play critical roles in neural stem/progenitor cell proliferation and fate decisions. The subcellular localization of mitochondria in neural stem/progenitor cells during mitosis potentially influences the distribution of mitochondria to the daughter cells and thus their fates. Therefore, understanding the spatial dynamics of mitochondria provides important knowledge about brain development. In this study, we analyzed the subcellular localization of mitochondria in the fetal human neocortex with a particular focus on the basal radial glial cells (bRGCs), a neural stem/progenitor cell subtype attributed to the evolutionary expansion of the human neocortex. During interphase, bRGCs exhibit a polarized localization of mitochondria that is localized at the base of the process or the proximal part of the process. Thereafter, mitochondria in bRGCs at metaphase show unpolarized distribution in which the mitochondria are randomly localized in the cytoplasm. During anaphase and telophase, mitochondria are still localized evenly, but mainly in the periphery of the cytoplasm. Mitochondria start to accumulate at the cleavage furrow during cytokinesis. These results suggest that the mitochondrial localization in bRGCs is tightly regulated during the cell cycle, which may ensure the proper distribution of mitochondria to the daughter cells and, thus in turn, influence their fates.

Polarised cross sections for vector boson production with Sherpa

Measurements of vector boson polarisation in vector boson production processes offer a powerful probe of the electroweak symmetry breaking mechanism, scrutinising the Standard Model and new physics scenarios alike. Since massive vector bosons can only be observed as intermediate particles, polarised cross section templates from simulation are necessary to extract their polarisation from measurable unpolarised distributions. In this work we present an extension of the Sherpa Monte-Carlo event generator allowing the simulation of polarised cross sections for vector boson production processes. Based on the narrow-width approximation, polarised cross sections of all possible polarisation combinations for an arbitrary number of intermediate vector bosons can be simulated in a single simulation run. In addition, it is possible to directly predict the interference between different intermediate polarisation states, and various differing polarisation definitions can be studied simultaneously. Besides the simulation of polarised cross sections at fixed LO and LO+PS accuracy as well as in multijet-merged calculations, we also present parton-shower-matched polarised cross sections with approximate NLO QCD corrections in the vector boson production processes. We demonstrate that the differences of this approximation to full NLO QCD predictions are small and it thus opens up the possibility for fully-simulated calculations at the hadron level including polarisation information and higher-order QCD effects for the first time.

Sivers function of sea quarks in the light-cone model

We calculate the Sivers function of u¯ and d¯ quarks using the overlap representation within the light-cone formalism. The light-cone wave functions of the proton are obtained in terms of the |q¯qB〉 Fock states motivated by the meson-baryon fluctuation model. We consider the final-state interaction at the level of one gluon exchange. In a simplified scenario, the Sivers function of u¯ and d¯ can be expressed as the convolution of the Sivers function of the pion inside the proton and the unpolarized distribution of q¯ inside the pion. The model parameters are fixed by fitting the unpolarized sea quark distributions to the known parameterizations. We present the numerical results for f1⊥u¯/p(x,kT2) and f1⊥d¯/p(x,kT2). The first transverse moment of the sea quark Sivers functions in our model are find to be negative and the magnitude is about 0.004 at most.

Kaon generalized parton distributions and light-front wave functions in the Nambu\u2013Jona-Lasinio model

Kaon generalized parton distributions (GPDs) and the leading Fock state light-front wave functions are investigated in the framework of Nambu–Jona-Lasinio model with proper time regularization. In addition, we compared the form factors, parton distribution functions, and generalized form factors obtained from them, respectively. The first Mellin moments of GPDs result in the form factors of local currents. The second Mellin moments of vector GPDs are related to gravitational form factors, the quark mass distribution theta _2 and the quark pressure distribution theta _1. When taking a Fourier transform of GPDs in impact parameter space, we can get the mean-squared impact parameter for the quarks of the kaon: langle {varvec{b}}_{bot }^2rangle _K^u=0.149 fm^2, langle {varvec{b}}_{bot }^2rangle _K^s=0.088 fm^2. This means that the kaon s quark is nearer to the center of transverse momentum than the u quark. We also give the light-cone energy radius for the quarks of the kaon from the mass distribution theta _2: r_{E,LC}^{u,K}=0.187 fm, r_{E,LC}^{s,K}=0.167 fm, and the light-cone charge radius from quark form factors of the kaon: r_{c,LC}^{u,K}=0.390 fm, r_{c,LC}^{s,K}=0.296 fm, which means that the s quark has a smaller extent than the u quark. The light-front transverse-spin distributions rho _u^1left( {varvec{b}}_{bot },{varvec{s}}_{perp }right) and rho _u^2left( {varvec{b}}_{bot },{varvec{s}}_{perp }right) show distortions, the average shift are langle b_{bot }^yrangle _1^u=0.116 fm and langle b_{bot }^yrangle _2^u=0.083 fm. On the kinematic domain associated with the valence-quark dominance, the unpolarized Wigner distribution from light-front wave functions is sharply peaked. It extends as the transverse position variable increases in magnitude and has a domain of negative support. Through the comparison of distributions from the two methods, we find that they give the same multi-dimensional mapping of the kaon in the Nambu–Jona-Lasinio model.

Extraction of the Sivers function from SIDIS, Drell-Yan, and W\xb1/Z boson production data with TMD evolution

We perform a global fit of the available polarized Semi-Inclusive Deep Inelastic Scattering (SIDIS), polarized pion-induced Drell-Yan (DY) and W±/Z boson production data at N3LO and NNLO accuracy of the Transverse Momentum Dependent (TMD) evolution, and extract the Sivers function for u, d, s and for sea quarks. The Qiu-Sterman function is determined in a model independent way via the operator product expansion from the extracted Sivers function. The analysis is supplemented by additional studies, such as the estimation of applicability region, the impact of the unpolarized distributions’ uncertainties, the universality of the Sivers functions, positivity constraints, the significance of the sign-change relation, and the comparison with the existing extractions.

QCD resummation on single hadron transverse momentum distribution with the thrust axis

We derive the transverse momentum dependent (TMD) factorization and resummation formula of the unpolarized transverse momentum distribution (jT) for the single hadron production with the thrust axis in an electron-positron collision. Two different kinematic regions are considered, including small transverse momentum limit jT « Q, and joint transverse momentum and threshold limit jT « Q(1 − zh) « Q, where Q and zh are the hard scattering energy and the observed hadron momentum fraction. Using effective theory methods, we resum logarithms ln(Q/jT) and ln(1 − zh) to all orders. In the end, we present the differential cross sections and Gaussian widths calculated for the inclusive charged pion production and find that our results are consistent with the measurements reported by the Belle collaboration.

Nucleon quasi-parton distributions in the large Nc limit

In this letter, we investigate the nucleon quasi-parton distribution functions in the chiral quark soliton model. We derive a set of sum-rules depending on the velocity of the nucleon and on the Dirac matrix defining the distribution functions. We present numerical results for the isosinglet unpolarized distribution, in which we find that the anti-quark distribution breaks the positivity condition at nucleon velocities of v≈0.99(PN≈7.0MN) and smaller. We found that, for the isosinglet unpolarized case, a large nucleon momentum is required for the quasi-parton distribution to get close enough to the usual parton distribution function.

Unified Description of Polarized and Unpolarized Quark Distributions in the Proton.

We propose a unified new approach to describe polarized and unpolarized quark distributions in the proton based on the gauge-gravity correspondence, light-front holography, and the generalized Veneziano model. We find that the spin-dependent quark distributions are uniquely determined in terms of the unpolarized distributions by chirality separation without the introduction of additional free parameters. The predictions are consistent with existing experimental data and agree with perturbative QCD constraints at large longitudinal momentum x. In particular, we predict the sign reversal of the polarized down-quark distribution in the proton at x=0.8±0.03, a key property of nucleon substructure which will be tested very soon in upcoming experiments.

Structure functions at small x from worldlines: Unpolarized distributions

The world-line representation of quantum field theory is a powerful framework for the computation of perturbative multi-leg Feynman amplitudes. In particular, in gauge theories, it provides an efficient way, via point particle Grassmann functional integrals, to compute spinor and color traces in these amplitudes. Further, semi-classical approximations to quantum mechanical world-line trajectories provide useful intuition in a wide range of dynamical problems. We develop here the world-line approach to compute deeply inelastic structure functions in the small x Regge limit of QCD. In particular, in a shockwave approximation valid in this limit, we show how one recovers the well-known dipole model for unpolarized structure functions. In a follow-up work, we will discuss the world-line computation of polarized structure functions at small x.

Accessing Gluon Parton Distributions in Large Momentum Effective Theory.

Gluon parton distribution functions (PDFs) in the proton can be calculated directly on Euclidean lattices using large momentum effective theory (LaMET). To realize this goal, one has to find renormalized gluon quasi-PDFs in which power divergences and operator mixing are thoroughly understood. For the unpolarized distribution, we identify four independent quasi-PDF correlators that can be multiplicatively renormalized on the lattice. Similarly, the helicity distribution can be derived from three independent multiplicatively renormalizable quasi-PDFs. We provide a LaMET factorization formula for these renormalized quasi-PDFs from which one can extract the gluon PDFs.

Boer-Mulders effect in the unpolarized pion induced Drell\u2013Yan process at COMPASS within TMD factorization

We investigate the theoretical framework of the cos 2phi azimuthal asymmetry contributed by the coupling of two Boer-Mulders functions in the dilepton production unpolarized pi p Drell–Yan process by applying the transverse momentum dependent factorization at leading order. We adopt the model calculation results of the unpolarized distribution function f_1 and Boer-Mulders function h_1^perp of pion meson from the light-cone wave functions. We take into account the transverse momentum dependent evolution effects for both the distribution functions of pion and proton by adopting the existed extraction of the nonperturbative Sudakov form factor for the pion and proton distribution functions. An approximate kernel is included to deal with the energy dependence of the Boer-Mulders function related twist-3 correlation function T_{q,F}^{(sigma )}(x,x) needed in the calculation. We numerically estimate the Boer-Mulders asymmetry nu _{BM} as the functions of x_p, x_pi , x_F and q_T considering the kinematics at COMPASS Collaboration.

Boer-Mulders function of the pion and the qT -weighted cos2φ asymmetry in the unpolarized π−p Drell-Yan process at COMPASS

We calculate the two leading-twist transverse momentum dependent distribution functions of the pion meson, the unpolarized distribution $f_{1\pi}(x,\bm{k}^2_T)$ and the Boer-Mulders function $h_{1\pi}^\perp (x,\bm{k}^2_T)$, using the pion wave functions derived from a light-cone approach. The evolution effect of the first $\bm k_T$-moment of the pion Boer-Mulders function is studied by employing an approximate evolution kernel. Using the model resulting distributions, we predict the transverse momentum weighted $\cos2\phi$ azimuthal asymmetry in the unpolarized $\pi^- p$ Drell-Yan process which can be measured at COMPASS in the near future.

Nucleon structure functions and longitudinal spin asymmetries in the chiral quark constituent model

We have analysed the phenomenological dependence of the spin independent ($F_1^{p,n}$ and $F_2^{p,n}$) and the spin dependent ($g_1^{p,n}$) structure functions of the nucleon on the the Bjorken scaling variable $x$ using the unpolarized distribution functions of the quarks $q(x)$ and the polarized distribution functions of the quarks $\Delta q(x)$ respectively. The chiral constituent quark model ($\chi$CQM), which is known to provide a satisfactory explanation of the proton spin crisis and related issues in the nonperturbative regime, has been used to compute explicitly the valence and sea quark flavor distribution functions of $p$ and $n$. In light of the improved precision of the world data, the $p$ and $n$ longitudinal spin asymmetries ($A_1^p(x)$ and $A_1^n(x)$) have been calculated. The implication of the presence of the sea quarks has been discussed for ratio of polarized to unpolarized quark distribution functions for up and down quarks in the $p$ and $n$ $\frac{\Delta u^p(x)}{u^p(x)}$, $\frac{\Delta d^p(x)}{d^p(x)}$, $\frac{\Delta u^n(x)}{u^n(x)}$, and $\frac{\Delta d^n(x)}{d^n(x)}$. The ratio of the $n$ and $p$ structure functions $R^{np}(x)=\frac{F_2^n(x)}{F_2^p(x)}$ has also been presented. The results have been compared with the recent available experimental observations. The results on the spin sum rule have also been included and compared with data and other recent approaches.

Pax8 controls thyroid follicular polarity through cadherin-16.

ABSTRACTOrganization of epithelial cells during follicular lumen formation is crucial for thyroid morphogenesis and function of the thyroid gland; however, the molecular mechanisms underlying this are poorly understood. To investigate this process, we established three-dimensional (3D) epithelial culture model systems using Fischer rat thyroid (FRT) cells or murine primary thyrocytes that developed polarized spherical structures with a central lumen, mimicking thyroid follicles. Using microarray-based differential expression analysis of FRT cells grown under 2D or 3D conditions, followed by RNA-mediated interference (RNAi) and morphogenetic analysis, we identified a key role for the thyroid transcription factor Pax8 and its target cadherin-16 (Cdh16) in the generation of polarized follicle-like structures. Silencing Pax8 expression inhibited the acquisition of apical–basal membrane polarity and impaired lumen formation. Both laminin and β1-integrin (Itgb1) expression was reduced, and cell cytoskeleton polarized distribution was altered. Silencing Cdh16 expression also led to the formation of defective structures characterized by very low laminin expression at the follicle–matrix interface, downregulation of Itgb1, and unpolarized distribution of cell cytoskeleton. Our results demonstrate that Pax8 controls apical–basal follicular polarization and follicle formation through Cdh16.

A check-up for the statistical Parton model

We compare the Parton distributions deduced in the framework of a quantum statistical approach for both the longitudinal and transverse degrees of freedom with the unpolarized distributions measured at HERA and with the polarized ones proposed in a previous paper, which have been shown to be in very good agreement also with the results of experiments performed after that proposal. The agreement with HERA data in correspondence to very similar values for the “temperature” and the “potentials” found in the previous work gives a robust confirm of the statistical model. The unpolarized distributions are compared also with the result of NNPDF. The free parameters are fixed mainly by data in the range (0.1, 0.5) for the x variable, where the valence Partons dominate, and in the small x region for the diffractive contribution. This feature makes the parametrization proposed here very attractive.

Quasi-parton distribution functions: A study in the diquark spectator model

A set of quasi-parton distribution functions (quasi-PDFs) have been recently proposed by Ji. Defined as the matrix elements of equal-time spatial correlations, they can be computed on the lattice and should reduce to the standard PDFs when the proton momentum Pz is very large. Since taking the Pz→∞ limit is not feasible in lattice simulations, it is essential to provide guidance for which values of Pz the quasi-PDFs are good approximations of standard PDFs. Within the framework of the spectator diquark model, we evaluate both the up and down quarks' quasi-PDFs and standard PDFs for all leading-twist distributions (unpolarized distribution f1, helicity distribution g1, and transversity distribution h1). We find that, for intermediate parton momentum fractions x, quasi-PDFs are good approximations to standard PDFs (within 20–30%) when Pz≳1.5–2 GeV. On the other hand, for large x∼1 much larger Pz>4 GeV is necessary to obtain a satisfactory agreement between the two sets. We further test the Soffer positivity bound, and find that it does not hold in general for quasi-PDFs.

Exploring the Flavor Dependence of Partonic Transverse Momentum

Using recently published HERMES data on multiplicities in semi-inclusive deep-inelastic scattering, we discuss the flavor dependence of unpolarized transverse-momentum dependent distribution and fragmentation functions. We find convincing indications that favored fragmentation into pions have smaller average transverse momentum than unfavored ones and fragmentation into kaons. We find weaker indications of flavor dependence in the transverse-momentum dependence of parton distributions.

Transverse Momentum Dependent Gluon Distributions at the LHC

Linearly polarized gluons inside an unpolarized proton contribute to the transverse momentum distributions of (pseudo)scalar particles produced in hadronic collisions, such as Higgs bosons and quarkonia with even charge conjugation (ηc, ηb, χc0, χb0). Moreover, they can produce azimuthal asymmetries in the associated production of a photon and a J/ψ or a Υ particle, in a kinematic configuration in which they are almost back to back. These observables, which can be measured in the running experiments at the LHC, could lead to a first extraction of both the polarized and the unpolarized gluon distributions and allow for a study of their process and energy scale dependences.

New Results in the Quantum Statistical Approach to Parton Distributions

We will describe the quantum statistical approach to parton distributions allowing to obtain simultaneously the unpolarized distributions and the helicity distributions. We will present some recent results, in particular related to the nucleon spin structure in QCD. Future measurements are challenging to check the validity of this novel physical framework.

Unbiased spin-dependent Parton Distribution Functions

We present the first unbiased determination of spin-dependent, or polarized, Parton Distribution Functions (PDFs) of the proton. A statistically sound representation of the corresponding uncertainties is achieved by means of the NNPDF methodology: this was formerly developed for unpolarized distributions and is now generalized to the polarized here for the first time. The features of the procedure, based on robust statistical tools (Monte Carlo sampling for error propagation, neural networks for PDF parametrization, genetic algorithm for their minimization, and possibly reweighting for including new data samples without refitting), are illustrated in detail. Different sets of polarized PDFs are obtained at next-to-leading order accuracy in perturbative quantum chromodynamics, based on both fixed-target inclusive deeply-inelastic scattering data and the most recent polarized collider data. A quantitative appraisal on the potential role of future measurements at an Electron-Ion Collider is also presented. We study the stability of our results upon the variation of several theoretical and methodological assumptions and we present a detailed investigation of the first moments of our polarized PDFs, compared to other recent analyses. We find that the uncertainty on the gluon distribution from available data was substantially underestimated in previous determinations; in particular, we emphasize that a large contribution to the gluon may arise from the unmeasured small-x region, against the common belief that this is actually rather small. We demonstrate that an Electron-Ion Collider would provide evidence for a possible large gluon contribution to the nucleon spin, though with a sizable residual uncertainty.