Dihadron Fragmentation Functions Research Articles

Asymmetries involving dihadron fragmentation functions: From DIS toe+e−annihilation

Using a model calculation of dihadron fragmentation functions, we fit the spin asymmetry recently extracted by HERMES for the semi-inclusive pion pair production in deep-inelastic scattering on a transversely polarized proton target. By evolving the obtained dihadron fragmentation functions, we make predictions for the correlation of the angular distributions of two pion pairs produced in electron-positron annihilations at BELLE kinematics. Our study shows that the combination of two-hadron inclusive deep-inelastic scattering and electron-positron annihilation measurements can provide a valid alternative to Collins effect for the extraction of the quark transversity distribution in the nucleon.

System size dependence of di-hadron correlations and fragmentation functions in Cu+Cu and Au+Au collisions at GeV

We present a study of di-hadron jet-like correlations in Cu+Cu and Au+Au collisions in an attempt to investigate the path length dependence of energy loss. The near- and away-side jet yields as a function of number of participants are investigated. The results are found to be equal within errors for the two systems at similar Npart. The di-hadron fragmentation functions are found to be unmodified on the near side, whereas on the away side they are suppressed but have the same shape in the heavy ion collisions as in d+Au collisions.

STAR results on medium properties and response of the medium to energetic partons

We report new STAR results on the consequences of highly energetic partons propagating through the medium formed in heavy-ion collisions using correlations as an experimental probe. The recent results providing insights about color factor effects and path length dependence of parton energy loss, system size dependence of di-hadron fragmentation functions, conical emission and ridge formation in heavy-ion collisions are presented.

Evidence for a transverse single-spin asymmetry in leptoproduction of π+π−pairs

A single-spin asymmetry was measured in the azimuthal distribution of pi(+)pi(-) pairs produced in semi-inclusive deep-inelastic scattering on a transversely polarized hydrogen target. For the first time, evidence is found for a correlation between the transverse target polarization and the azimuthal orientation of the plane containing the two pions. The corresponding single-spin asymmetry is expected to be related to the product of the little-known quark transversity distribution function and an unknown naive-T-odd chiral-odd dihadron fragmentation function.

Modified Dihadron Fragmentation Functions in Hot and Nuclear Matter

Medium modification of dihadron fragmentation functions due to gluon bremsstrahlung induced by multiple partonic scattering is studied in both deep-inelastic scattering (DIS) off large nuclei and high-energy heavy-ion collisions within the same framework of twist expansion. The modification for dihadrons is found to closely follow that for single hadrons, leading to a weak nuclear suppression of their ratios in DIS experiments. A mild enhancement of the near-side correlation of two high transverse momentum hadrons with increasing centrality is found in heavy-ion collisions due to trigger bias and the rise in parton energy loss with centrality. Successful comparisons between theory and experiment for multihadron observables in both confining and deconfined media offer comprehensive evidence for partonic energy loss as the mechanism of jet modification in dense matter.

Evolution equations for extended dihadron fragmentation functions

We consider dihadron fragmentation functions, describing the fragmentation of a parton in two unpolarized hadrons, and in particular extended dihadron fragmentation functions, explicitly dependent on the invariant mass, Mh, of the hadron pair. We first rederive the known results on Mh-integrated functions using jet calculus techniques, and then we present the evolution equations for extended dihadron fragmentation functions. Our results are relevant for the analysis of experimental measurements of two-particle-inclusive processes at different energies.

Modeling dihadron fragmentation functions

We present a model for dihadron fragmentation functions, describing the fragmentation of a quark into two unpolarized hadrons. We tune the parameters of our model to the output of the PYTHIA event generator for two-hadron semi-inclusive production in deep-inelastic scattering at HERMES. Once the parameters of the model are fixed, we make predictions for other unknown fragmentation functions and for a single-spin asymmetry in the azimuthal distribution of ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ pairs in semi-inclusive deep-inelastic scattering on a transversely polarized target at HERMES and COMPASS. Such asymmetry could be used to measure the quark transversity distribution function.

High- p T Hadron–Hadron Correlations in Vacuum and in Matter

Two-particle correlations within a single jet produced in deeply inelastic scattering (DIS) off a large nucleus as well as in heavy-ion collisions are studied. This is performed within the framework of the medium modi fied dihadron fragmentation functions. The modification occurs due to gluon bremsstrahlung induced by multiple scattering. The modified fragmentation functions for dihadrons are found to follow closely that of single hadrons leading to a weak nuclear suppression of their ratios as measured by HERMES in DIS experiments. Meanwhile, a moderate medium enhancement of the nearside correlation of two high-pT hadrons is found in central heavy-ion collisions, partially due to trigger bias caused by the competition between parton energy loss and the initial Cronin effect.

Evolution of the parton dihadron fragmentation functions

Quark and gluon parton dihadron fragmentation functions and their evolution are studied in the process of ${e}^{+}{e}^{\ensuremath{-}}$ annihilation. We provide definitions of such dihadron fragmentation functions in terms of parton matrix elements and derive the momentum sum rules and their connection to single hadron fragmentation functions. We parametrize results from the Lund Monte Carlo model JETSET as the initial conditions for the parton dihadron fragmentation functions at the scale ${Q}_{0}^{2}=2\text{ }\text{ }{\mathrm{GeV}}^{2}$. The evolution equations for the quark and gluon fragmentation functions are solved numerically and the results at different higher scales ${Q}^{2}$ agree well with JETSET results. The importance of the input from the single fragmentation functions is pointed out.

Dihadron fragmentation: in vacuum and in matter

Two particle correlations within a single jet produced in deeply inelastic scattering (DIS) off a large nucleus as well as in heavy-ion collisions are explored. This is performed within the framework of the medium modified dihadron fragmentation functions. The modification occurs due to gluon bremsstrahlung induced by multiple scattering. The modified fragmentation functions for dihadrons are found to follow closely that of single hadrons leading to a weak nuclear suppression of their ratios as measured by HERMES in DIS experiments. Meanwhile, a moderate medium enhancement of the near-side correlation of two high $p_T$ hadrons is found in central heavy-ion collisions, partially due to trigger bias caused by the competition between parton energy loss and the initial Cronin effect.

High pT hadron–hadron correlations

We propose the formulation of a dihadron fragmentation function in terms of parton matrix elements. Under the collinear factorization approximation and facilitated by the cut-vertex technique, the two hadron inclusive cross section at leading order (LO) in e+e− annihilation is shown to factorize into a short distance parton cross section and the long-distance dihadron fragmentation function. We also derive the DGLAP evolution equation of this function at leading log. The evolution equation for the non-singlet and singlet quark fragmentation function and the gluon fragmentation function are solved numerically with the initial condition taken from event generators. Modifications to the dihadron fragmentation function from higher twist corrections in DIS off nuclei are computed. Results are presented for the cases of physical interest.

Dihadron interference fragmentation functions in proton-proton collisions

We study the production of hadron pairs in proton-proton collisions, selecting pairs with large total transverse momentum with respect to the beam, and small relative transverse momentum, i.e., belonging to a single jet with large transverse momentum. We describe the process in terms of dihadron fragmentation functions. We consider the production of one pair in polarized collisions (with one transversely polarized proton) and the production of two pairs in unpolarized collisions. In the first case, we discuss how to observe the quark transversity distribution in connection with a specific class of dihadron fragmentation functions, named interference fragmentation functions. In the second case, we suggest how to determine the latter and also how to observe linearly polarized gluons.

Dihadron fragmentation function and its evolution

Dihadron fragmentation functions and their evolution are studied in the process of ${e}^{+}{e}^{\ensuremath{-}}$ annihilation. Under the collinear factorization approximation and facilitated by the cut-vertex technique, the two hadron inclusive cross section at leading order is shown to factorize into a short distance parton cross section and a long distance dihadron fragmentation function. We provide the definition of such a dihadron fragmentation function in terms of parton matrix elements and derive its Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution equation at leading log. The evolution equation for the nonsinglet quark fragmentation function is solved numerically with a simple ansatz for the initial condition and results are presented for cases of physical interest.

Dihadron fragmentation functions and high pT hadron–hadron correlations

We propose the formulation of a dihadron fragmentation function in terms of parton matrix elements. Under the collinear factorization approximation and facilitated by the cut-vertex technique, the two-hadron inclusive cross section at leading order in e+e− annihilation is shown to factorize into a short-distance parton cross section and the long-distance dihadron fragmentation function. We also derive the DGLAP evolution equation of this function at leading log. The evolution equation for the non-singlet quark fragmentation function is solved numerically with a simple ansatz for the initial condition and results are presented for cases of physical interest.