35 research outputs found

### Accessing quark helicity in $e^+e^-$ and SIDIS via dihadron correlations

The correlation between the longitudinal polarization of a fragmenting quark
and the transverse momenta of the produced hadrons was predicted over two
decades ago. Nevertheless, experimental searches in the electron-positron
annihilation process, both through the so-called jet handedness measurements by
the {\tt SLD} Collaboration and more recently via the measurements of the
azimuthal asymmetry containing the helicity-dependent dihadron fragmentation
function (DiFF) by the $\textrm{BELLE}$ Collaboration, did not yield a signal.
We will first discuss our recent explanation of the zero result at
$\textrm{BELLE}$, and the two new methods for accessing the helicity-dependent
DiFFs both in the electron-positron annihilation experiments, and in the
semi-inclusive deep inelastic scattering (SIDIS) experiments with a
longitudinally polarized target. We will also for the first time describe yet
another, new method for accessing the helicity-dependent DiFFs in SIDIS using
polarized beam asymmetry. Finally, we will present a new Monte Carlo
calculation of the specific Fourier moments of the helicity-dependent DiFF
entering in to the new asymmetries, performed within the extended quark-jet
model, and compare the results to those for the interference DiFF.Comment: 5 pages, 1 figure. To appear in PoS DIS201

### Studies of Azimuthal Modulations in Two Hadron Fragmentation of a Transversely Polarised Quark

We study the azimuthal modulations of dihadron fragmentation functions
(DiFFs) of a transversely polarised quark using an NJL-jet based model that
incorporates the Collins effect for single hadron emission. The DiFFs are
extracted as Monte Carlo (MC) averages of corresponding multiplicities using
their probabilistic interpretation. To simplify the model and highlight the
possible mechanisms that create this modulation, we choose the elementary
Collins function to be proportional to the elementary unpolarised fragmentation
and a constant probability ($P_{SF}$) for the quark to flip its spin after a
single hadron emission. Moreover, as a leading order calculation, only one of
the produced hadrons in the decay chain of the quark is produced with
elementary Collins modulation. We calculate the dependence of the polarised
DiFFs on various angles such as the azimuthal angle of the single hadron and
the angle of the two hadron production plane $\varphi_R$ for several values of
$P_{SF}$. We observe that the polarised DiFFs for oppositely charged pion pairs
exhibit a $\sin(\varphi_R)$ modulation. This effect is induced purely via the
elementary Collins effect and persists even when the quark completely
depolarises after a single hadron emission ($P_{SF}=0.5$). Moreover, similar
sine modulations are present in the distribution of pion pairs with respect to
the azimuthal angle of their total transverse momentum, $\varphi_T$.Comment: 10 pages, 12 figures - small updates and added references, to comply
with the version to be published in PL

### 1/Nc Expansion in QCD: Double-Line Counting Rules and the Undeservingly Discarded U(1) Ghost

The 1/Nc expansion is one of the very few methods we have for generating a
systematic expansion of QCD at the energy scale relevant to hadron structure.
The present formulation of this theory relies on the double-line notation for
calculating the leading order of a diagram in the 1/Nc expansion, where the
local SU(Nc) gauge symmetry is substituted by a U(Nc) symmetry and the
associated U(1) ghost field is ignored. In the current work we demonstrate the
insufficiency of this formulation for describing certain non-planar diagrams.
We derive a more complete set of Feynman rules that include the U(1) ghost
field and provide a useful tool for calculating both color factors and 1/Nc
orders of all color-singlet diagrams

### Collins Fragmentation Function within NJL-jet Model

The NJL-jet model is extended to accommodate hadronization of a transversely
polarized quark in order to explore the Collins effect within a multihadron
emission framework. This is accomplished by calculating the polarized quark
spin flip probabilities after a pseudoscalar hadron emission and the elementary
Collins functions. The model is used to calculate the number densities of the
hadrons produced in the polarized quark's decay chain. The full Collins
fragmentation function is extracted from the sine modulation of the polarized
number densities with respect to the polar angle between the initial quark's
spin and hadron's transverse momentum. Two cases are studied here. First, a toy
model for elementary Collins function is used to study the features of the
transversely polarized quark-jet model. Second, a full model calculation of
transverse momentum dependent pion and kaon Collins functions is presented. The
remarkable feature of our model is that the 1/2 moments of the favored Collins
fragmentation functions are positive and peak at large values of z but decrease
and oscillate at small values of z. The 1/2 moments of the unfavored Collins
functions have comparable magnitude and opposite sign to the favored functions,
vanish at large z and peak at small values of z. This feature is observed for
both the toy and full models and can be attributed to the quark-jet picture of
hadronization. Moreover, the transverse momentum dependencies of the model
Collins functions differ significantly from the Gaussian form widely used in
the empirical parametrizations. Finally, a naive interpretation of the
Schafer-Teryaev sum rule is proven not to hold in our model, where the
transverse momentum conservation is explicitly enforced. This is attributed to
the sizable average transverse momentum of the remnant quark that needs to be
accounted for to satisfy the transverse momentum sum-rule.Comment: 15 pages, 22 figures. v2 - minor changes/additions to conform to the
journal published versio

### Monte-Carlo Approach to Calculating the Fragmentation Functions in NJL-Jet Model

Recent studies of the fragmentation functions using the Nambu--Jona-Lasinio
(NJL) - Jet model have been successful in describing the quark fragmentation
functions to pions and kaons. The NJL-Jet model employs the integral equation
approach to solve for the fragmentation functions in quark-cascade description
of the hadron emission process, where one assumes that the initial quark has
infinite momentum and emits an infinite number of hadrons. Here we introduce a
Monte Carlo (MC) simulation method to solve for the fragmentation functions,,
that allows us to relax the above mentioned approximations. We demonstrate that
the results of MC simulations closely reproduce the solutions of the integral
equations in the limit where a large number of hadrons are emitted in the quark
cascade. The MC approach provides a strong foundation for the further
development of the NJL-Jet model that might include many more hadronic emission
channels with decays of the possible produced resonances, as well as inclusion
of the transverse momentum dependence (TMD), all of which are of considerable
importance to the experimental studies of the transverse structure of hadrons.Comment: 5 pages, 3 figures, Proceedings of "TROPICAL QCD II Workshop

### Kaon fragmentation function from NJL-jet model

The NJL-jet model provides a sound framework for calculating the
fragmentation func- tions in an effective chiral quark theory, where the
momentum and isospin sum rules are satisfied without the introduction of ad hoc
parameters [1]. Earlier studies of the pion fragmentation func- tions using the
Nambu-Jona-Lasinio (NJL) model within this framework showed good qualitative
agreement with the empirical parameterizations. Here we extend the NJL-jet
model by including the strange quark. The corrections to the pion fragmentation
function and corresponding kaon fragmen- tation functions are calculated using
the elementary quark to quark-meson fragmentation functions from NJL. The
results for the kaon fragmentation function exhibit a qualitative agreement
with the empirical parameterizations, while the unfavored strange quark
fragmentation to pions is shown to be of the same order of magnitude as the
unfavored light quark's. The results of these studies are expected to provide
important guidance for the analysis of a large variety of semi-inclusive data.Comment: 6 pages, 9 figures, Proceedings of "Achievements and New Directions
in Subatomic Physics: Workshop in Honour of Tony Thomas's 60th Birthday

### Sivers Effect in Dihadron Electroproduction

The Sivers effect in polarized SIDIS can be measured in two hadron production
as sine modulations involving the azimuthal angles $\varphi_T$ and $\varphi_R$
of both the total and the relative transverse momenta of the hadron pair,
complementary to the conventional single hadron studies. In this talk we
briefly present the results obtained in our recent work [1] and [2]. We also
present the leading order parton model expression for the two hadron SIDIS
cross section for different choices of the relative transverse momentum that
dismiss the seeming contradiction of our results with previous work. Finally,
we show the numerical predictions for the corresponding single spin asymmetries
in the kinematics of COMPASS experiment obtained using the modified version of
the $\tt{LEPTO}$ Monte Carlo event generator that includes the Sivers effect.Comment: Proceedings of Transversity 2014; 6 pages, 7 figure