A comprehensive collection of results on longitudinal double-spin asymmetries is presented for charged pions and kaons produced in semi-inclusive deep-inelastic scattering of electrons and positrons on the proton and deuteron, based on the full HERMES data set. The dependence of the asymmetries on hadron transverse momentum and azimuthal angle extends the sensitivity to the flavor structure of...
The production of $W$ bosons in longitudinally polarized proton-proton collisions at the Relativistic Heavy Ion Collider (RHIC) provides an ideal tool to study the spin-flavor structure of the proton, through the measurement of the parity-violating single-spin asymmetry, $A_L$. STAR has measured $A_L$ for $W$ boson production from datasets taken in 2011 and 2012 which provided significant...
The contribution of the gluon helicity to the spin of the proton is being studied through the use of the unique capability of the Relativistic Heavy Ion Collider (RHIC) to collide polarized protons at $\sqrt{s}=200\,$GeV and $\sqrt{s}=510\,$GeV. The kinematic coverage of the Solenoidal Tracker At RHIC (STAR) allows access to gluons through quark-gluon and gluon-gluon scattering processes which...
A measurement of beam-helicity asymmetries for single-hadron production in deep-inelastic scattering is presented. Data from the scattering of 27.6 GeV electrons and positrons off gaseous hydrogen and deuterium targets were collected by the HERMES experiment. The asymmetries for charged pions and kaons as well as for protons and anti-protons are presented binned either separately or...
The CLAS12 detector at Jefferson Laboratory (JLab) started data taking with a polarized 10.6 GeV electron beam, interacting with an unpolarized liquid hydrogen target in February 2018. One of the first quantities which could be extracted from the new data is the moment $A_{LU}^{sin(\phi)}$ corresponding to the polarized electron beam spin asymmetry in semi-inclusive deep inelastic scattering....
In 2016 and 2017, together with DVCS data, the COMPASS Collaboration has collected a considerable amount of Deep Inelastic Scattering events scattering a 160 GeV/c muon beam off a liquid hydrogen target. A first analysis of a small
subsample of these data has allowed to extract preliminary results on the amplitudes of three azimuthal modulations, $A^{\cos \phi_h}_{UU}$, $A^{\cos...
The multiplicities of charged hadrons produced in the deep inelastic scattering (DIS) of muons off a proton target as a function of the hadron transverse momentum $P_{hT}$ are being measured by COMPASS using the data collected in 2016 and 2017 with a $160~\mathrm{GeV}/c$ muon beam and a liquid hydrogen target. Preliminary results of this analysis using a fraction of the full collected sample...
With the upgrade of JLab 12 GeV, a new era for the exploration of partonic degress of freedom in hadronic physics has started. In particular SIDIS is a key reaction to understand partonic structure of hadrons and the physics of hadronizaion provided a solid understanding and consistency of QCD factorization theorems to describe the data. In this talk I will review a recent progress to...
The extraction of unpolarized transverse momentum dependent functions (TMDs) has received a lot of attention in recent years. A succesful extraction of these funtions require careful consideration of the interplay between perturbative and non-perturbative components in the definitions of the TMDs. In this talk, I will discuss how our knowledge of the unpolarized TMDs may be improved in light...
Describing qT-dependent SIDIS distributions requires consideration of two different regimes which involve two different factorization schemes. In both cases, the necessary ingredients include parton distribution and fragmentation functions (PDFs and FF), which are traditionally extracted in statistical analyses with collinear observables. In this talk I will present examples of how the errors...
I present the latest extraction of unpolarized quark transverse momentum dependent parton distribution functions (TMDPDFs) and the non-perturbative part of TMD evolution kernel from the global analysis of Drell-Yan and $Z$-boson production data. The analysis is performed at the next-to-next-to-leading order (NNLO) in perturbative QCD, using the $\zeta$-prescription. The estimation of the...
Recently COMPASS has reported unexpected results concerning K− over K+ multiplicity ratio for kaons produced in DIS with a large fraction of the virtual-photon energy, PLB 786 (2018) 390. The obtained ratio is significantly below the lower limit given by NLO pQCD.
In order to provide more information about the observed phenomenon we will show for the first time results concerning pbar over p...
We present preliminary COMPASS results on pion and kaon multiplicities produced in semi-inclusive deep inelastic scattering of 160 GeV muons off a pure proton (LH$_2$) target. The results constitute a large data set of more than 600 points for pions and 600 for kaons, covering a large $x$, $Q^2$ and $z$ domain in a fine binning. The results from the sum of the z-integrated multiplicities...
The Belle experiment at the asymmetric e+e- collider KEKB provides a large data set not only for the exploration of flavor physics but also for precision QCD studies. The clean initial state is particularly well suited to investigate the process of high-energetic partons fragmenting into final state hadrons. Various results related to unpolarized and polarized fragmentation functions have been...
The description of the polarized quark hadronization process is one of the most challenging problems in strong interactions. The various single hadron and dihadron fragmentation functions, that quantify this process, are determined by analyzing the inclusive production of hadrons in electron-positron annihilation process. These, in turn, are used to extract the transverse momentum dependent...
We connect at the operator level the inclusive gauge-invariant quark propagator and the quark to single hadron fragmentation correlator in QCD. Exploiting a new spectral decomposition for the quark propagator, we then derive a complete set of momentum sum rules for the quark fragmentation functions into a single unpolarized hadron up to twist-three. Known results are recovered, and new sum...
Dihadron production in Semi-Inclusive Deep Inelastic Scattering (SIDIS) represents a novel probe for
nucleon structure, as well as for spin-orbit correlations in hadronization. Measurements of the beam
spin asymmetry from dihadron production provide access to the twist-3 PDF $e(x)$, which can be related to
the transverse polarization dependence of the transverse force experienced by a struck...
We present our work on the inclusion of spin effects in the hadronization part of the PYTHIA 8 event generator. These effects are restricted to the emission of pseudoscalar mesons and are obtained from the propagation of the quark polarization along the fragmentation chain according to the rules of the ${}^3P_0$ model.
The interface between PYTHIA 8 and the ${}^3P_0$ model and the results on...
In recent years information on the transversity distribution $h_1$ has been obtained
combining the Collins asymmetry results from semi-inclusive deep inelastic scattering
(SIDIS) data on transversely polarized nucleon targets and the information on the
fragmentation function of a transversely polarized quark from the asymmetries measured
in $e^+e^-$ annihilation into hadrons. An alternative...
The tensor charge, namely the first Mellin moment of the chiral-odd transversity parton distribution, is not connected to operators entering the Standard Model Lagrangian at tree level. It could represent a low-energy footprint of new physics happening at much higher scales, hence it is important to know it with very high precision. I will present the latest results obtained by extracting the...
Transverse single-spin asymmetries (TSSAs) of proton-proton collisions have a history of revealing the richness of QCD. Large TSSAs were originally discovered in fixed target experiments in the mid 1970s. However they have been found to persist in collisions up to $ \sqrt{s} = 510 $ GeV and transverse momenta up to about $7$ GeV/c, well into the perturbative regime of QCD, and yet their...
The PHENIX experiment at RHIC studied the $A$-dependence of transverse,
single-spin asymmetries in production of charged hadrons in collisons
of polarized protons with aluminum and gold nuclei, as well as with
protons. The measurements covered forward rapidities, between 1.4 and
2.4, over the transverse-momentum range of 1.8 to 7 GeV$/c$. We report
on the observation of a significant asymmetry...
The exploration of the transverse spin structure of the nucleon by measuring spin (in)dependent azimuthal asymmetries in semi-inclusive DIS and in Drell-Yan processes is one of the main objectives of the COMPASS experiment at CERN (SPS, M2 beamline). During the first phase of the experiment (2002-2011) a series of SIDIS measurements were performed, using a longitudinally polarized muon beam...
We study the differential cross section in the unpolarized $\pi^- N$ Drell-Yan process, using transverse momentum dependent factorization up to next-to-logarithmic order of QCD and extract the nonperturbative Sudakov form factor for the pion in the evolution formalism of the unpolarized TMD distribution function, by fitting the experimental data collected by the E615 Collaboration at Fermilab....
COMPASS is a fixed-target experiment in operation at the CERN North Area (SPS, M2 beam-line) since 2002. An important part of its broad physics programme is dedicated to the exploration of the transverse spin-structure of the nucleon studying target spin-(in)dependent azimuthal asymmetries arising in the Semi-Inclusive DIS (SIDIS) and Drell-Yan (DY) cross-sections. Recently, COMPASS has also...
Measuring lepton angular distributions of Drell-Yan process provides a powerful tool to explore the reaction mechanisms and related parton distributions. For example, the Lam-Tung relation has been proposed as a benchmark of the pQCD effect in Drell-Yan process. Nevertheless, the violation of Lam-Tung relation was observed in the measurements of fixed-target experiments. Precision data of...
We discuss production of Drell-Yan lepton pairs at hadron colliders
in the framework of the Parton Reggeization Approach, which includes
off-shell initial state effects in a gauge-invariant way. Other
possible prescriptions to restore gauge-invariance of
hard-scattering coefficient with off-shell initial-state partons are
also investigated and significant differences for the...
The dilepton angular distribution in vector particle decays can be described through a set of five SO(3) rotational-invariant observables. These observables are derived as invariants of the spacial part of the hadronic tensor (density matrix) expressed in terms of angular coefficients. The restrictions on the invariants following from the positivity of the hadronic tensor are obtained....
In this talk I will present our work on an improved implementation of the Collins Soper Sterman formalism for combining transverse-momentum-dependent (TMD) factorization and collinear factorization in semi-inclusive DIS (SIDIS). I will focus on our recent extension to the case of polarized observables; in particular the Sivers contribution to the transversely polarized target cross section. We...
The Sivers function is benchmark for spin physics. The systematic calculation of this observable in QCD has been object of many studies. We discuss here a methodology for calculation which is allows to rederive systematically the NLO calculation and that can be used for all other twist-3 TMDs.
We explore the complete cross-section for the production of unpolarized hadrons in semi-inclusive deep-inelastic scattering up to power-suppressed $\mathcal{O}(1/Q^2)$ terms in the Wandzura–Wilczek-type approximation, which consists in systematically assuming that $\bar{q}gq$-correlators are much smaller than $\bar{q}q$-correlators. Under the applicability of Wandzura–Wilczek-type...
Within the so-called color gauge invariant generalized parton model (CGI-GPM), which includes initial (ISI) and final state (FSI) interactions in a transverse momentum dependent formalism, we present a phenomenological analysis of available data on single spin asymmetries for pion and $D$-meson production in $pp$ collisions at RHIC. This allows us, for the first time, to put preliminary...
We study the production of prompt-photons at RHIC in the context of a generalised parton model framework, with a view to obtain information on the gluon Sivers function (GSF). At RHIC energy (√s = 200 GeV), the Compton process, gq → γq contributes significantly to the production of direct-photons at midrapidity and dominates it in the negative (backward) rapdity region. We find that for direct...
The presently published Boer-Mulders (BM) function, for a given quark flavour, was extracted from data on semi-inclusive deep inelastic scattering (SIDIS) using the simplifying assumption that it is proportional to the Sivers function for that flavour. We argued that such an assumption is theoretically unacceptable and replaced it with an analogous, theoretically acceptable, relation for the...
Flavor, spin and partonic transverse momenta are important characteristics for parton distribution functions (PDFs), allowing a proliferation of possibilities. This proliferation can provide novel information into the non-perturbative structure of nucleons as well as new probes for high energy processes. Wilson lines are an important ingredient in the operator definitions of transverse...
One of the most striking phenomena arising from the extension of collinear factorization (inspired in Bjorken's parton model) to Transverse Momentum Dependent (TMD) factorization is the appearance of azimuthal modulation induced by the polarization of partons with nonzero transverse momentum even inside unpolarized hadrons. Linearly polarized gluons inside unpolarized hadrons is one of these...
We report on our recent study on the extraction of the gluon Transverse Mo-
mentum Dependent distributions (TMDs) inside unpolarised protons, using vector
quarkonium pair production at the LHC. In this work, we show how J/ψ pair pro-
duction is an ideal process to pin down the gluon TMDs and generate large azimuthal
asymmetries ; we also present a first fit of the average kT parameter for a...
The J/psi, a bound state of charm and anti-charm quark with spin 1 state, decays into spin ½ lepton pairs with a large branching ratio. Its production in $p$+$p$ collisions shed light on inner workings of charmonium production that is dominated by gluon-gluon interaction at RHIC energy. Hadronization of charmonium in unpolarized $p$+$p$ collisions is also accessible in robust nonrelativistic...
Studying light-cone (standard) parton distribution functions (PDFs) through Euclidean correlators in lattice QCD is currently a very active field of research. In particular, the quasi-distributions (quasi-PDFs) suggested by Ji have attracted a lot of attention. Quasi-PDFs converge to their respective standard distributions if the hadron momentum goes to infinity. We explore the...
We present a first calculation of the heavy flavour contribution to the longitudinally polarized DIS structure function $g_1^Q$, differential in the transverse momentum or the rapidity of the observed heavy antiquark $\overline Q$. The results are obtained at next-to-leading order accuracy with a newly developed parton-level Monte Carlo generator that also allows one to study observables...
The basis light front quantization (BLFQ) approach has been developed for solving many-body bound state problems in quantum field theories. We investigate several aspects of the nucleon properties such as electromagnetic form factors, generalized parton distributions (GPDs) etc. using the framework of BLFQ. We consider the light front wavefunctions obtained by diagonalizing the effective...
We study the single longitudinal-spin asymmetry of dihadron production in semi-inclusive deep inelastic scattering process in which the transverse momentum of the final-state hadron pairs is integrated out. In Particular, we investigate origins of the $\sin \phi_R$ azimuthal asymmetry for which we take into account the coupling of the twist-3 distributions $h_L$ and the dihadron framgentation...
We investigate the transverse single-spin asymmetry with a $\sin\phi_{S_h}$ modulation for the transversely polarized proton and lambda production in semi-inclusive inelastic scattering process, where $\phi_{S_h}$ is the azimuthal angle of the transverse spin of the final hadron. Theoretically, the spin asymmetry can be interpreted by the convolution of the twist-3 transverse momentum...
One of the challenges of hadronic physics is to fully understand the structure of the proton. In particular, there is nowadays a great interest in the decomposition of its total angular momentum into orbital angular momentum (OAM) and intrinsic
spin, as well as identifying contributions from valence quarks, sea quarks and gluons.
The most common decompositions of OAM are the Jaffe-Manohar...
I will present two complementary analyses of nearly all existing Deeply Virtual Compton Scattering (DVCS) data allowing to explore the proton structure within the framework of Generalised Parton Distributions (GPDs). The first analysis is based on a classic approach, where parameterisations of DVCS amplitudes are constructed in a model-dependent way to fulfil the basic properties of GPDs. The...
Part of the COMPASS-II program is dedicated to the investigation of Generalized Parton Distributions (GPDs) via lepton-induced hard exclusive reactions, in particular Deeply Virtual Compton Scattering (DVCS) and Deeply Virtual Meson Production. Data have been taken in 2016 -17 using a high intensity muon beam of 160 GeV and a 2.5 m-long liquid hydrogen target surrounded by a barrel-shaped...
Generalized Parton Distributions (GPDs) provide the opportunity to obtain a 3-dimensional description of a nucleon in terms of its constituent partons. Moreover, GPDs are related, via QCD-based sum rules, to total angular momentum, mass and pressure distributions inside the nucleon. GPDs are experimentally accessible via the deeply virtual Compton scattering (DVCS), i.e. the absorption of a...
The form factors of the energy-momentum tensor can be accessed
via studies of GPDs in hard exclusive reactions. In this talk
we present recent results on the EMT densities in the bag model.
The simplicity and lucidity of this quark model allow us to
investigate many general concepts which have recently attracted
interest, including shear, pressure, radial and tangential forces.
We also present...
The electromagnetic probe has proven to be a very efficient way to access the three-dimensional
structure of the nucleon, particularly thanks to the exclusive Compton processes. We explore the hard photoproduction of a large invariant mass diphoton in the kinematical regime where the diphoton is nearly forward and its invariant mass is the hard scale enabling to factorize the scattering...
The transverse momentum parton distribution $f_{1T}^\perp$, known as Sivers function, is odd under the naive time-reversal transformation and gives rise to a transverse single spin asymmetry (tSSA) in semi-inclusive DIS (SIDIS). An intuitive picture to understand the origin of this asymmetry is to relate the average Sivers transverse momentum to the impact parameter parton distribution E,...
