Partonic Structure Of Nucleon Research Articles

Azimuthal Asymmetries for eA/eN Semi-Inclusive DIS and Its Nuclear Dependence

We applied collinear expansion to the semi-inclusive deeply inelastic lepton-nucleon (nucleus) scattering process [Formula: see text] with both polarized beam and polarized target up to twist-3, and unpolarized process up to twist-4. The differential cross section and azimuthal asymmetries are expressed in terms of gauge invariant twist-3 and twist-4 TMD parton distribution/correlation functions. Measurements of such azimuthal asymmetries provide methods to study different spin and transverse momentum aspects of the partonic structure of nucleon. We further study the nuclear dependence of azimuthal asymmetries and adopt Gaussian ansatz for TMD distribution/correlation functions to estimat the semi-quantitive behaviour of the nuclear dependence. We predict the A-dependence of azimuthal asymmetries which can be tested in the planned EIC’s.

On the origin of negative binomial multiplicity distributions in proton-nucleus collisions

We investigate multiplicity distributions in ultrarelativistic proton-nucleus collisions in a string model approach. We try to disentangle the different sources of multiplicity fluctuations: nuclear structure, the parton structure of nucleons and the fragmentation of colour strings. We show that the multiplicity distribution can be written as a continuous superposition ∫ dN W(N) P(N;n) of poissonian distributions P with variable mean values N, the weight factor W(N) representing nuclear and nucleonic structure, whereas the poisson distribution P reflects fluctuations due to string fragmentation. By using the string model VENUS, we show that W(N) can be well approximated by a gamma distribution G(k, N;N) , leading to a negative binomial distribution N(k, N;n) for the multiplicity distribution, with the same parameters as for G ! This has interesting consequences: for small 〈 n〈 (small rapidity window) the fragmentation fluctuations dominate; for large 〈 n〉 they are negligible!

Negative binomial multiplicity distributions for proton-nucleus collisions from a string model

Multiplicity distributions of charged particles produced in collisions of ultrarelativistic protons with nuclei are studied in a string model. Recently measured multiplicity distributions in proton-nucleus reactions display a negative binomial shape. The string model VENUS reproduces the data well. We trace back within the model the origin of the negative binomial form of the multiplicity distributions. We find that the final multiplicity distribution can be expressed as a continuous superposition of Poisson distributions with different mean multiplicities ∫dNW(N)P(N, n). The Poisson distribution represents the string fragmentation, whereas the weight factorW(N) reflects the string mass distribution. This quantity has contributions due to both the nuclear structure and the parton structure of nucleons. We show thatW(N) is very well approximated by a gamma-distribution . This leads to a negative binomial distribution\(\mathcal{N}(k,\bar N;n)\) with the same parameters as for . It is shown that the fluctuations in the multiplicity are dominated by the string fragmentation fluctuations for small rapidity intervals and by string mass and number fluctuations for large rapidity windows.