Transverse Parton Distribution Research Articles

Moments of nucleon unpolarized, polarized, and transversity parton distribution functions from lattice QCD at the physical point

Moments of nucleon unpolarized, polarized, and transversity parton distribution functions from lattice QCD at the physical point

Observation of triple J/ψ meson production in proton-proton collisions at √ s = 13 TeV

The first observation of the three J/ψ meson production in proton-proton collisions at a center-of-mass energy of 13 TeV in final states with three µ +µ − pairs is reported. The analysis is based on a data sample recorded by the CMS experiment at the CERN LHC corresponding to an integrated luminosity of 133 fb−1 . The pp → J/ψ J/ψ J/ψ X process is observed with a significance in excess of five standard deviations. The fiducial cross section for this process is found to be σ(pp → J/ψ J/ψ J/ψ X) = 272+141 −104(stat)±17(syst) fb. The result is compared to theoretical expectations for the production of three J/ψ mesons in single (SPS), double- (DPS), and triple- (TPS) parton scattering processes. Under the most economical assumption of factorization of multiple hard scattering probabilities in terms of SPS cross sections, the measured final state is found to be dominated by DPS and TPS contributions. A value of the associated DPS effective cross section parameter of σeff,DPS = 2.7 +1.4 −1.0 (exp)+1.5 −1.0 (theo) mb, related to the transverse distribution of partons in the proton, is derived.

Transversity parton distribution function of the nucleon using the pseudodistribution approach

We present a determination of the non-singlet transversity parton distribution function (PDF) of the nucleon, normalized with respect to the tensor charge at $\mu^2=2$ GeV$^2$ from lattice quantum chromodynamics. We apply the pseudo-distribution approach, using a gauge ensemble with a lattice spacing of 0.094 fm and the light quark mass tuned to a pion mass of 358 MeV. We extract the transversity PDF from the analysis of the short-distance behavior of the Ioffe-time pseudo-distribution using the leading-twist next-to-leading order (NLO) matching coefficients calculated for transversity. We reconstruct the $x$-dependence of the transversity PDF through an expansion in a basis of Jacobi polynomials in order to reduce the PDF ansatz dependence. Within the limitations imposed by a heavier-than-physical pion mass and a fixed lattice spacing, we present a comparison of our estimate for the valence transversity PDF with the recent global fit results based on single transverse spin asymmetry. We find the intrinsic nucleon sea to be isospin symmetric with respect to transversity.

StringSpinner - adding spin to the PYTHIA string fragmentation

StringSpinner is a plugin allowing for the introduction of spin effects in the hadronization part of the PYTHIA 8 event generator. In this first version the implementation is limited to handling pesudoscalar mesons in events without parton showers. The spin effects are generated by propagating the quark polarization along the fragmentation chain according to the rules of the quantum mechanical string+P03 model. By using parametrizations for the transversity parton distribution functions, StringSpinner allows to simulate the polarized semi inclusive deep inelastic scattering process on protons and neutrons. The instructions to use StringSpinner and to modify the parametrizations of the transversity distribution functions are presented, as well as an example of the calculation of the Collins asymmetries as obtained from simulations of the transversely polarized semi inclusive deep inelastic scattering process. Program summaryProgram Title: StringSpinnerCPC Library link to program files:https://doi.org/10.17632/37jrpmyz3t.1Developer's repository link:https://gitlab.com/albikerbizi/stringspinner.gitLicensing provisions: GNU GPL v2 or laterProgramming language: C++, FortranNature of problem: To analyse and interpret the experimental data on the study of the nucleon spin structure, as well as to make predictions for future experiments simulations are needed.Solution method: Introduce spin effects in the hadronization part of a complete Monte Carlo event generator, largely used in the community of high energy particle physicists.

Flavor decomposition of the nucleon unpolarized, helicity, and transversity parton distribution functions from lattice QCD simulations

We present results on the quark unpolarized, helicity and transversity parton distributions functions of the nucleon. We use the quasi-parton distribution approach within the lattice QCD framework and perform the computation using an ensemble of twisted mass fermions with the strange and charm quark masses tuned to approximately their physical values and light quark masses giving pion mass of 260 MeV. We use hierarchical probing to evaluate the disconnected quark loops. We discuss identification of ground state dominance, the Fourier transform procedure and convergence with the momentum boost. We find non-zero results for the disconnected isoscalar and strange quark distributions. The determination of the quark parton distribution and in particular the strange quark contributions that are poorly known provide valuable input to the structure of the nucleon.

Factorization and transverse phase-space parton distributions

We first revisit impact-parameter dependent collisions of ultra-relativistic particles in quantum field theory. Two conditions sufficient for defining an impact-parameter dependent cross section are given, which could be violated in proton-proton collisions. By imposing these conditions, a general formula for the impact-parameter dependent cross section is derived. Then, using soft-collinear effective theory, we derive a factorization formula for the impact-parameter dependent cross section for inclusive hard processes with only colorless final-state products in hadron and nuclear collisions. It entails defining thickness beam functions, which are Fourier transforms of transverse phase-space parton distribution functions. By modelling non-perturbative modes in thickness beam functions of large nuclei in heavy-ion collisions, the factorization formula confirms the cross section in the Glauber model for hard processes. Besides, the factorization formula is verified up to one loop in perturbative QCD for the inclusive Drell-Yan process in quark-antiquark collisions at a finite impact parameter.

A constrained fit of the valence transversity distributions from dihadron production

We present a constrained analysis of the valence transversity Parton Distribution Functions from dihadron production in semi-inclusive DIS. While usual extractions of the transversity distributions rely explicitly on the fulfillment of the Soffer bound, the present analysis releases that restriction to implement further constraints through the method of Lagrange multipliers. The results are quantitatively comparable to previous analyses in the kinematical range of data; the qualitative impact translates into an increased flexibility in the functional form.

Transverse parton distribution and fragmentation functions at NNLO: the gluon case

We calculate in this paper the perturbative gluon transverse momentum dependent parton distribution functions (TMDPDFs) and fragmentation functions (TMDFFs) using the exponential regulator for rapidity divergences. We obtain results for both unpolarized and linearly polarized distributions through next-to-next-to leading order in strong coupling constant, and through mathcal{O} (ϵ2) in dimensional regulator. We find a nontrivial momentum conservation sum rule for the linearly polarized component for both TMDPDFs and TMDFFs in the mathcal{N} = 1 super-Yang-Mills theory. The TMDFFs are used to calculate the two-loop gluon jet function for the energy-energy correlator in Higgs gluonic decay in the back-to-back limit.

Studying minijets and MPI with rapidity correlations

We propose and carry a detailed study of an observable sensitive to different mechanisms of minijet production. The observables measure how the transverse momenta of hadrons produced in association with various trigger objects are balanced as a function of rapidity. It is shown that the observables are sensitive to the model parameters relevant for the minijet production mechanisms: low-p_mathrm{T} cutoff regulating jet cross-section, transverse distribution of partons in protons and parton distribution functions. We perform our test at different charge-particle multiplicities and collision energies. The Monte Carlo models, which describe many features of the LHC data, are found to predict quite different results demonstrating high discriminating power of the proposed observables. We also review mechanisms and components of Herwig, Pythia, and Sherpa Monte Carlo models relevant to the minijet production.

A note on Pretzelosity TMD parton distribution

We show that the transverse-momentum-dependent parton distribution, called as Pretzelosity function, is zero at any order in perturbation theory of QCD for a single massless quark state. This implies that Pretzelosity function is not factorized with the collinear transversity parton distribution at twist-2, when the struck quark has a large transverse momentum. Pretzelosity function is in fact related to collinear parton distributions defined with twist-4 operators. In reality, Pretzelosity function of a hadron as a bound state of quarks and gluons is not zero. Through an explicit calculation of Pretzelosity function of a quark combined with a gluon nonzero result is found.

Transversity parton distribution functions from lattice QCD

We present the first direct calculation of the transversity parton distribution function within the nucleon from lattice QCD. The calculation is performed using simulations with the light quark mass fixed to its physical value and at one value of the lattice spacing. Novel elements of the calculations are non-perturbative renormalization and extraction of a formula for the matching to light-cone PDFs. Final results are presented in the $\overline{\rm MS}$ scheme at a scale of $\sqrt{2}$ GeV.

Semi-inclusive production of two back-to-back hadron pairs in e+e− annihilation revisited

The cross section for back-to-back hadron pair production in $e^+e^-$ annihilation provides access to the dihadron fragmentation functions (DiFF) needed to extract nucleon parton distribution functions from the semi-inclusive deep inelastic scattering (SIDIS) experiments with two detected final state hadrons. Particular attention is given to the so-called interference DiFF (IFF), which makes it possible to extract the transversity parton distribution of the nucleon in the collinear framework. However, previously unnoticed discrepancies were recently highlighted between the definitions of the IFFs appearing in the collinear kinematics when reconstructed from DiFFs entering the unintegrated fully differential cross sections of SIDIS and $e^+e^-$ annihilation processes. In this work, to clarify this problem we rederive the fully differential cross section for $e^+e^-$ annihilation at the leading-twist approximation. We find a mistake in the definition of the kinematics in the original expression that systematically affects a subset of terms and that leads to two significant consequences. First, the discrepancy between the IFF definitions in the cross sections for SIDIS and $e^+e^-$ annihilation is resolved. Second, the previously derived azimuthal asymmetry for accessing the helicity dependent DiFF $G_1^\perp$ in $e^+e^-$ annihilation vanishes, which explains the nonobservation of this asymmetry in the recent experimental searches by the ${\tt BELLE}$ Collaboration. We discuss the recently proposed alternative option to extract $G_1^\perp$.

Constraints on the double-parton scattering cross section from same-sign W boson pair production in proton-proton collisions at sqrt{s}=8 TeV

A first search for same-sign WW production via double-parton scattering is performed based on proton-proton collision data at a center-of-mass energy of 8 TeV using dimuon and electron-muon final states. The search is based on the analysis of data corresponding to an integrated luminosity of 19.7 fb−1. No significant excess of events is observed above the expected single-parton scattering yields. A 95% confidence level upper limit of 0.32 pb is set on the inclusive cross section for same-sign WW production via the double-parton scattering process. This upper limit is used to place a 95% confidence level lower limit of 12.2 mb on the effective double-parton cross section parameter, closely related to the transverse distribution of partons in the proton. This limit on the effective cross section is consistent with previous measurements as well as with Monte Carlo event generator predictions.

Collinear parton distributions and the structure of the nucleon sea in a light-front meson-cloud model

The unpolarized, helicity and transversity parton distribution functions of the nucleon are studied within a convolution model where the bare nucleon is dressed by its virtual meson cloud. Using light-front time-ordered perturbation theory, the Fock states of the physical nucleon are expanded in a series involving a bare nucleon and two-particle, meson-baryon, states. The bare baryons and mesons are described with light-front wave functions (LFWFs) for the corresponding valence-parton components. Using a representation in terms of overlap of LFWFs, the role of the non-perturbative antiquark degrees of freedom and the valence quark contribution at the input scale of the model is discussed for the leading-twist collinear parton distributions. After introducing perturbative QCD effects through evolution to experimental scales, the results are compared with available data and phenomenological extractions. Predictions for the nucleon tensor charge are also presented, finding a very good agreement with recent phenomenological extractions.

Nucleon parton distributions in a light-front quark model

Continuing our analysis of parton distributions in the nucleon, we extend our light-front quark model in order to obtain both the helicity-independent and the helicity-dependent parton distributions, analytically matching the results of global fits at the initial scale mu sim 1 GeV; they also contain the correct Dokshitzer–Gribov–Lipatov–Altarelli–Parisi evolution. We also calculate the transverse parton, Wigner and Husimi distributions from a unified point of view, using our light-front wave functions and expressing them in terms of the parton distributions q_v(x) and delta q_v(x). Our results are very relevant for the current and future program of the COMPASS experiment at SPS (CERN).

Exclusive ω meson muoproduction on transversely polarised protons

Exclusive production of ω mesons was studied at the COMPASS experiment by scattering 160 GeV/c muons off transversely polarised protons. Five single-spin and three double-spin azimuthal asymmetries were measured in the range of photon virtuality 1 (GeV/c)2<Q2<10 (GeV/c)2, Bjorken scaling variable 0.003<xBj<0.3 and transverse momentum squared of the ω meson 0.05 (GeV/c)2<pT2<0.5 (GeV/c)2. The measured asymmetries are sensitive to the nucleon helicity-flip Generalised Parton Distributions (GPD) E that are related to the orbital angular momentum of quarks, the chiral-odd GPDs HT that are related to the transversity Parton Distribution Functions, and the sign of the πω transition form factor. The results are compared to recent calculations of a GPD-based model.

Nucleon helicity and transversity parton distributions from lattice QCD

We present the first lattice-QCD calculation of the isovector polarized parton distribution functions (both helicity and transversity) using the large-momentum effective field theory (LaMET) approach for direct Bjorken-x dependence. We first review the detailed steps of the procedure in the unpolarized case, then generalize to the helicity and transversity cases. We also derive a new mass-correction formulation for all three cases. We then compare the effects of each finite-momentum correction using lattice data calculated at Mπ≈310 MeV. Finally, we discuss the implications of these results for the poorly known antiquark structure and predict the sea-flavor asymmetry in the transversely polarized nucleon.

Exploring universality of transversity in proton-proton collisions

We consider the azimuthal correlations of charged hadron pairs with large total transverse momentum and small relative momentum, produced in proton-proton collisions with one transversely polarized proton. One of these correlations directly probes the chiral-odd transversity parton distribution in connection with a chiral-odd interference fragmentation function. We present predictions for this observable based on previous extractions of transversity (from charged pion pair production in semi-inclusive deep-inelastic scattering) and of the interference fragmentation function (from the production of back-to-back charged pion pairs in electron-positron annihilations). All analyses are performed in the framework of collinear factorization. We compare our predictions to the recent data on proton-proton collisions released by the STAR collaboration at RHIC, and we find them reasonably compatible. This comparison confirms for the first time the predicted role of transversity in proton-proton collisions and it allows to test its universality.

Studies of the 3D Structure of the Nucleon at JLab

Studies of the 3D structure of the nucleon encoded in transverse momentum dependent distribution and fragmentation functions of partons and generalized parton distributions are among the key objectives of the JLab 12 GeV upgrade and the electron ion collider. Main challenges in extracting 3D partonic distributions from precision measurements of hard scattering processes include clear understanding of leading twist QCD fundamentals, higher twist effects, and also correlations of hadron production in target and current fragmentation regions. In this contribution we discuss some ongoing studies and future measurements of spin-orbit correlations at Jefferson Lab.

Beyond-Standard-Model Tensor Interaction and Hadron Phenomenology.

We evaluate the impact of recent developments in hadron phenomenology on extracting possible fundamental tensor interactions beyond the standard model. We show that a novel class of observables, including the chiral-odd generalized parton distributions, and the transversity parton distribution function can contribute to the constraints on this quantity. Experimental extractions of the tensor hadronic matrix elements, if sufficiently precise, will provide a, so far, absent testing ground for lattice QCD calculations.