PhD Research

Research Overview

I am a part of the STAR collaboration at Brookhaven National Lab (BNL). STAR is the Solenoidal Tracker At RHIC, and RHIC is the Relativistic Heavy Ion Collider. RHIC is the only collider in the world capable of colliding high-energy polarized proton beams. The beams travel in opposite directions around RHIC’s 3.86 km two-lane racetracks. The STAR detector sits at the six o'clock position with respect to RHIC as indicated in the figure below.

The aim of my PhD is to study the Collins effect in STAR’s 2015 200 GeV p+Au dataset, where the proton is transversely polarized. The Collins effect [1, 2] describes the relationship between the quark transversity and the Collins fragmentation function. This effect manifests itself as a final-state transverse single-spin asymmetry (TSSA), where hadrons produced from the fragmenting transversely polarized quark are azimuthally distributed about the axis of the parent jet. See the diagram below for more details about the Collins effect. STAR has measured this effect in transversely polarized p+p collisions at center-of-mass energies of 200 and 500 GeV, and a non-zero Collins effect measurement has been observed for identified pions in jets [3, 4]. 

However, the universality of the Collins fragmentation function is subject to debate [2, 5, 6, 7, 8, 9], and studying the Collins effect in p+Au will provide a test for the universality of the Collins fragmentation function. For the Collins asymmetry to arise, an overall phase from the scattering amplitudes is needed to have a non-vanishing TSSA. The overall phase is expected to arise from the fragmentation and hadronization processes of the outgoing transversely polarized quark [2, 7]. Comparing p+p to p+Au Collins measurements can, experimentally, identify potential contributions to the overall phase from outside the fragmentation and hadronization processes of the outgoing transversely polarized quark [5, 6], because the Au nucleus offers a large color field for possible initial- or final-state color connections. If color entanglement due to color connections with the Au nucleus is observed in p+Au, then the factorization and universality of the Collins fragmentation function become questionable.

References

[1] J. C. Collins, “Fragmentation of transversely polarized quarks probed in transverse momentum distributions,” Nucl. Phys. B,    vol. 396, pp. 161–182, 1993, arXiv:hep-ph/9208213.

[2] F. Yuan, “Azimuthal asymmetric distribution of hadrons inside a jet at hadron collider,” Phys. Rev. Lett., vol. 100, p. 032003, 2008, arXiv:0709.3272 [hep-ph].

[3] M. Abdallah et al., “Azimuthal transverse single-spin asymmetries of inclusive jets and identified hadrons within jets from polarized pp collisions at √s= 200 GeV,” Phys. Rev. D, vol. 106, no. 7, p. 072010, 2022, arXiv:2205.11800v2 [hep-ex].

[4] L. Adamczyk et al., “Azimuthal transverse single-spin asymmetries of inclusive jets and charged pions within jets from polarized-proton collisions at √s= 500 GeV,” Phys. Rev. D, vol. 97, no. 3, p. 032004, 2018, arXiv:1708.07080 [hep-ex].

[5] J. Collins and J.-W. Qiu, “ kT factorization is violated in production of high-transverse-momentum particles in hadron-hadron collisions,” Phys. Rev. D, vol. 75, p. 114014, 2007, arXiv:0705.2141 [hep-ph].

[6] T. C. Rogers and P. J. Mulders, “No Generalized TMD-Factorization in Hadro-Production of High Transverse Momentum Hadrons,” Phys. Rev. D, vol. 81, p. 094006, 2010, arXiv:1001.2977 [hep-ph].

[7] F. Yuan, “Collins Asymmetry at Hadron Colliders,” Phys. Rev. D, vol. 77, p. 074019, 2008, arXiv:0801.3441 [hep-ph].

[8] Z.-B. Kang, A. Prokudin, F. Ringer, and F. Yuan, “Collins azimuthal asymmetries of hadron production inside jets,” Phys. Lett. B, vol. 774, pp. 635–642, 2017, arXiv:1707.00913 [hep-ph].

[9] Z.-B. Kang, X. Liu, F. Ringer, and H. Xing, “The transverse momentum distribution of hadrons within jets,” JHEP, vol. 11, p. 068, 2017, arXiv:1705.08443 [hep-ph].

Selected Presentations