Dihadron Fragmentation Functions Research Articles

Transversity Distributions and Tensor Charges of the Nucleon: Extraction from Dihadron Production and Their Universal Nature.

We perform the first global quantum chromodynamics (QCD) analysis of dihadron production for a comprehensive set of data in electron-positron annihilation, semi-inclusive deep-inelastic scattering, and proton-proton collisions, from which we extract simultaneously the transversity distributions of the nucleon and π^{+}π^{-} dihadron fragmentation functions. We incorporate in our fits known theoretical constraints on transversity, namely, its small-x asymptotic behavior and the Soffer bound. We furthermore show that lattice-QCD results for the tensor charges can be successfully included in the analysis. This resolves the previously reported incompatibility between the tensor charges extracted from dihadron production data and lattice QCD. We also find agreement with results for the transversity and tensor charges obtained from measurements on single-hadron production. Overall, our work demonstrates for the first time the universal nature of all available information for the transversity distributions and the tensor charges of the nucleon.

Number Density Interpretation of Dihadron Fragmentation Functions.

We present a new quantum field-theoretic definition of fully unintegrated dihadron fragmentation functions (DiFFs) as well as a generalized version for n-hadron fragmentation functions. We demonstrate that this definition allows certain sum rules to be satisfied, making it consistent with a number density interpretation. Moreover, we show how our corresponding so-called extended DiFFs that enter existing phenomenological studies are number densities and also derive their evolution equations. Within this new framework, DiFFs extracted from experimental measurements will have a clear physical meaning.

The hadron pair forward-backward asymmetry in the electron positron annihilation process

The forward-backward asymmetry is an important measurable quantity which can enable independent determination of the neutral-current couplings of fermions. In this paper, we extend the definition of the asymmetry from the partonic level to the hadronic level by calculating this asymmetry of the hadron pair in the semi-inclusive electron position annihilation process. Semi-inclusive implies that a back-to-back jet is also measured in addition to the hadron pair. Due to the limitation of the factorization theorem, we calculate this process up to leading order twist-4 level by applying the collinear expansion formalism. After obtaining the differential cross section, we calculate the forward-backward asymmetries and show them in terms of the corresponding di-hadron fragmentation functions. Di-hadron fragmentation functions are introduced to describe the hadron pair productions in the fragmentation process. With available parameterization of the functions, we present a numerical estimation of the forward-backward asymmetry. On the flip side, measurements of the forward-backward asymmetry would give strict restrictions of parameterizations of the di-hadron fragmentation functions. We also present a numerical estimation of the twist-4 di-hadron fragmentation functions in order to illustrate their contributions.

Hadron pair production in semi-inclusive electron positron annihilation process at twist-4

Fragmentation functions are important quantities in describing the hadronization process in high energy reactions. They can induce various azimuthal modulations which can be measured to reveal correlations between transverse momenta and polarizations. Without introducing initial state uncertainties, electron positron annihilation process is known as an ideal process to investigate the fragmentation functions. In this paper, therefore, we calculate the hadron pair production in the semi-inclusive electron positron annihilation process e^+ + e^- rightarrow h_1+h_2 + {bar{q}} +X at twist-4 to study dihadron fragmentation functions. Here {bar{q}} denotes an antiquark that corresponds to a jet of hadrons in experiments. Together with single hadron fragmentation functions, dihadron fragmentation functions can provide additional ways to extract nucleon parton distribution functions from the semi-inclusive deeply inelastic scattering experiments with two detected final state hadrons. We calculate the differential cross section of the hadron pair production semi-inclusive electron positron annihilation process at twist-4 level. The calculation is carried out by using the collinear expansion method. We also calculate azimuthal asymmetries in terms of dihadron fragmentation functions. Contributions from four-quark correlator are also taken into account. Both the electromagnetic and weak interactions are considered in this paper.

Multidimensional partial wave analysis of SIDIS dihadron beam spin asymmetries at CLAS12

Dihadron beam spin asymmetries provide a wide range of insights into nucleon structure and hadronization. Recent measurements at CLAS12 provide the first empirical evidence of nonzero G_1^\perpG1⊥, the parton helicity-dependent dihadron fragmentation function (DiFF) encoding spin-momentum correlations in hadronization. These measurements also allow for a point-by-point extraction of the subleading-twist PDF e(x)e(x) in a collinear framework. We observe different behavior of the asymmetries in different invariant mass regions, motivating a fully multidimensional study. The DiFFs also expand in terms of partial waves, each corresponding to the interference of dihadrons of particular polarizations. Altogether a fully multidimensional partial wave analysis is needed, and this presentation will summarize the efforts and results obtained thus far.

A new method to extract the valence transversity distributions

A new method is suggested for the extraction of uu-quark and dd-quark transversity distributions, using single spin asymmetry (SSA) data in semi-inclusive deep inelastic scattering (SIDIS) processes, where they couple to the Collins or the di-hadron fragmentation functions. We discuss a recent suggestion to extract the transversity distribution using the concept of difference asymmetries and their ratios, which avoids the requirement of Collins function. We suggest new measurements, involving ratios of polarized cross-sections, that would directly probe the ratio h_1^{d_v}/h_1^{u_v}h1dv/h1uv. We also show some numerical estimates.

Unpolarized dihadron fragmentation functions in nonlocal chiral quark model

We have calculated the unpolarized dihadron fragmentation functions (uDiFFs) of pions and kaons using the nonlocal chiral-quark model (NLChQM) and evolved our results to the transferred momentum scale Q2=4GeV2 by the QCD evolution equations. These uDiFFs have also been computed in the Nambu–Jona-Lasinio-jet (NJL-jet) model for the sake of comparison. We find that there is substantial difference between the results of these two models. Furthermore, the DiFFs of u→π+π− and g→π+π− at Q2=109GeV2 in these two models are presented in comparison with the parametrizations fitted by the Monte Carlo event generator JETSET.

Extraction of the valence transversity distributions from SIDIS data

The transversity distribution for $u$ and $d$ quarks is usually extracted from data on spin asymmetries in Semi Inclusive Deep Inelastic Scattering (SIDIS): however, due to its chiral odd nature, it has to be coupled to another chiral odd function, typically the Collins or the di-hadron fragmentation function. A recent suggestion of considering SIDIS data involving ratios of spin asymmetries and avoiding a knowledge of the Collins function, is briefly discussed. New measurements, involving ratios of cross sections, are suggested. They would allow a direct extraction of the transversity ratio, $h_1^{d_V}/h_1^{u_V}$. Numerical estimates are given.

Single spin asymmetry AULsin(2ϕh−2ϕR) in dihadron semi-inclusive DIS

The single longitudinal spin asymmetry $A_{UL}^{\sin(2\phi_h-2\phi_R)}$ of dihadron production in semi-inclusive deep inelastic scattering (SIDIS) is examined through helicity-dependent dihadron fragmentation function (DiFF) $G_1^\perp$. The correlation of the longitudinal polarization of a fragmenting quark with the transverse momenta of the produced hadron pair is illustrated by this DiFF. The experimental investigation for this azimuthal asymmetry in dihadron SIDIS by the COMPASS Collaboration has lately yielded a very small signal. Here, the unknown T-odd dihadron fragmentation function $G_1^\perp$ utilizing a spectator model is computed. The model has been successfully used to describe the dihadron production in both the unpolarized and the single polarized processes 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 is left unintegrated, has been considered. The $\sin(2\phi_h-2\phi_R)$ asymmetry at the COMPASS kinematics is estimated and we compare it with the data. Besides, the predictions on the same asymmetry are also made at the HERMES and Electron Ion Collider.

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.

Double spin asymmetry in dihadron production in SIDIS off a longitudinally polarized nucleon target

In this paper we study the double longitudinal spin asymmetry of dihadron production in semi-inclusive deep inelastic scattering (SIDIS). We calculate a unknown twist-3 dihadron fragmentation function $\widetilde{D}^\sphericalangle$ within a spectator model which has been used successfully in describing the dihadron production in both the unpolarized and the single polarized processes. The collinear picture, in which the transverse momentum of the final state hadron pair is integrated out, has been considered. The $\cos\phi_R$ azimuthal asymmetry arises from the coupling $e_L H_1^\sphericalangle$ and the coupling $g_1 \widetilde{D}^\sphericalangle$ is studied. We estimate the $\cos\phi_R$ asymmetry at the kinematics of COMPASS and compare with the data. The prediction at the future Electron Ion Collider (EIC) has also been presented.

CosϕR asymmetry of dihadron production in double longitudinally polarized SIDIS

We present a study on the double longitudinal-spin asymmetry of dihadron production in semi-inclusive deep inelastic scattering, in which the total transverse momentum of the final-state hadron pairs is integrated out. In particular, we investigate the origin of the $\cos\phi_R$ azimuthal asymmetry for which we take into account the coupling of the helicity distribution $g_1$ and the twist-3 dihadron fragmentation function $\widetilde{D}^{\sphericalangle}$. We calculate the $s$-wave and $p$-wave interference term $\widetilde{D}^{\sphericalangle}_{ot}$ in a spectator model. We estimate the $\cos\phi_R$ asymmetry at the kinematics of COMPASS which is collecting data on dihadron production in polarized deep inelastic scattering. The prediction of the same asymmetry at JLab 12GeV and a future EIC are also presented. Our study indicates that measuring the $\cos\phi_R$ asymmetry in SIDIS may be a ideal way to probe the dihadron fragmentation function $\widetilde{D}^{\sphericalangle}$.

Single-spin asymmetry in dihadron production in SIDIS off longitudinally polarized nucleon targets

We study the single longitudinal-spin asymmetry of dihadron production in semi-inclusive deep inelastic scattering process. We consider the collinear picture in which the transverse momentum of the final-state hadron pair is integrated out, such that the $\sin \phi_R$ azimuthal asymmetry arises from the coupling $h_L\, H_{1}^{\sphericalangle}$ as well as the coupling $g_1 \,\widetilde{G}^{\sphericalangle}$. We calculate the unknown twist-3 dihadron fragmentation function $\widetilde{G}^{\sphericalangle}$ using a spectator model which is successful in describing the dihadron production in the unpolarized process. Using the spectator model results for the quark distributions and dihadron fragmentation functions, we estimate the $\sin \phi_R$ asymmetry of dihadron production in SIDIS at the kinematics of COMPASS and compare it with the COMPASS preliminary data. In addition, the prediction on the $\sin \phi_R$ asymmetry at the typical kinematics of the future Electron Ion Collider is also presented. In order to test the reliability of the spectator model estimate, we compare the model result for the distribution $h_L$ with the Wandzura-Wilczek approximation for that distribution, and compare $H_{1}^{\sphericalangle}$ with the existing parametrization. Although the asymmetry is dominated by the $h_L H_{1}^{\sphericalangle}$ term, we find that the contribution from the $g_1\, \widetilde{G}^{\sphericalangle}$ term should also be taken into account in certain kinematical region.

Semi-inclusive back-to-back production of a hadron pair and a single hadron in e+e\u2212 annihilation

Inclusive hadron production in e+e− annihilation has long been used to study both single hadron fragmentation functions (FF) and dihadron fragmentation functions (DiFF). In particular, the polarized DiFFs can be accessed in electron-positron annihilation by measuring azimuthal correlations between two back-to-back pairs of hadrons in the center of mass system, where the relevant structure functions can be expressed as convolutions of two (polarized) DiFFs. Here we explore the advantages of measuring the inclusive back-to-back production of a single hadron on one side against a hadron pair on the opposite side of the detector in two jet events. The leading twist cross section for this process contains convolutions of the corresponding single hadron FFs on one side and the DiFFs for the hadron pair on the other side, which furnishes several interesting new opportunities. A measurement of the unpolarized cross section with a number of different types of observed hadrons will help in untangling the quark flavor dependence of the unpolarized DiFFs, when the results are analyzed together with the inclusive measurements of dihadron pairs, such as those recently performed by the BELLE collaboration. Even more interesting, with a polarized hyperon on one side we can study the quark spin-dependent DiFFs of an unpolarized hadron pair on the other side. This, in turn, will allow us to test the universality of the spin-dependent DiFFs entering the cross sections of electron-positron annihilation and semi-inclusive deep inelastic scattering processes.

Accessing Quark Helicity through Dihadron Studies.

We present a new proposal to study the helicity-dependent dihadron fragmentation functions (DiFF), which describe the correlations of the longitudinal polarization of a fragmenting quark with the transverse momenta of the produced hadron pair. Recent experimental searches for this DiFF via azimuthal asymmetries in back-to-back hadron pair production in e^{+}e^{-} annihilation by the BELLE Collaboration did not yield a signal. Here we propose a new way to access this DiFF in e^{+}e^{-} annihilation, motivated by the recently recalculated cross section of this reaction, which explains why there was in fact no signal for the BELLE Collaboration to see. In this new approach the azimuthal asymmetry is weighted by the virtual photon's transverse momentum square multiplying sine and cosine functions of difference of azimuthal angles of relative and total momentum for each pair. The integration over the virtual photon's transverse momentum has the effect of separating the convolution between the helicity-dependent DiFFs in the quark and antiquark jets and results in a nonzero collinear expression containing Fourier moments of helicity-dependent DiFFs. A second new measurement is also proposed for two-hadron production in semi-inclusive deep inelastic scattering, where the asymmetry is weighted in a similar way for a single pair. This results in a collinear factorized form of the asymmetry, which includes the quark helicity parton distribution function and the same helicity-dependent DiFF, as in e^{+}e^{-} production and will allow us to check the universality of this DiFF.

First Extraction of Transversity from a Global Analysis of Electron-Proton and Proton-Proton Data.

We present the first extraction of the transversity distribution in the framework of collinear factorization based on the global analysis of pion-pair production in deep-inelastic scattering and in proton-proton collisions with a transversely polarized proton. The extraction relies on the knowledge of dihadron fragmentation functions, which are taken from the analysis of electron-positron annihilation data. For the first time, the transversity is extracted from a global analysis similar to what is usually done for the spin-averaged and helicity distributions. The knowledge of transversity is important for, among other things, detecting possible signals of new physics in high-precision low-energy experiments.

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 $G_1^\perp$.

Dihadron fragmentation functions in the quark-jet model: Transversely polarized quarks

Within the most recent extension of the quark-jet hadronization framework, we explore the transverse-polarization-dependent dihadron fragmentation functions (DiFFs) $H_1^\sphericalangle$ and $H_1^\perp$ of a quark into $\pi^+\pi^-$ pairs. Monte Carlo (MC) simulations are employed to model polarized quark hadronization and calculate the corresponding number densities. These, in turn, are used to extract the Fourier cosine moments of the DiFFs $H_1^\sphericalangle$ and $H_1^\perp$. A notable finding is that there are previously unnoticed apparent discrepancies between the definitions of the so-called interference DiFF (IFF) $H_1^\sphericalangle$, entering the cross sections for two-hadron semi-inclusive electroproduction, and those involved in the production of two pairs of hadrons from back-to-back jets in electron-positron annihilation. This manuscript completes the studies of all four leading twist DiFFs for unpolarized hadron pairs within the quark-jet framework, following our previous work on the helicity-dependent DiFF $G_1^\perp$.

The Tensor and the Scalar Charges of the Nucleon from Hadron Phenomenology

We discuss the impact of the determination of the nucleon tensor charge on searches for physics Beyond the Standard Model. We also comment on the future extraction of the subleading-twist PDF e(x) from Jefferson Lab soon-to-be-released Beam Spin Asymmetry data as well as from the expected data of CLAS12 and SoLID, as the latter is related to the scalar charge. These analyses are possible through the phenomenology of Dihadron Fragmentation Functions related processes, which we report on here as well.

Dihadron fragmentation functions in the quark-jet model: Longitudinally polarized quarks

We study the dihadron fragmentation functions (DiFF) of longitudinally polarized quarks into pion pairs. The quark-jet framework is used to model the sequential hadronization of a polarized quark into hadrons, where the polarization transfer to the remnant quark in each hadron emission step is calculated using the spin density matrix formalism. Using Monte Carlo (MC) simulations of the hadronization process, we find a nonvanishing helicity dependent DiFF, $G_1^\perp$ , which is also related to the longitudinal jet handedness. A method is developed for extracting the angular moments of this DiFF, which enter the expressions for the azimuthal asymmetries for an electron-positron annihilation process into two pairs of hadrons from back-to-back jets and the dihadron production in semi-inclusive deep inelastic scattering. Finally, we derive explicit integral expressions for DiFFs where only two hadrons are emitted by a quark and use them to validate our MC results for both unpolarized and helicity dependent DiFFs.