TiTle : Beam-Energy Dependence of the Directed Flow of Deuterons in Au+Au Collisions

PAs : Xionghong He, Hao Qiu, Nu Xu, Ning Yu  

Target Journal :  Physical Review C

Paper draft : drupal.star.bnl.gov/STAR/blog/hexh/paper-draft-deuteron-v1

Analysis note : drupal.star.bnl.gov/STAR/blog/hexh/analysis-note-deuteron-v1

We present a measurement of first order azimuthal anisotropy (v₁(y)) of deuterons from Au+Au collisions at √(s_NN)  = 7.7, 11.5,
14.5, 19.6, 27 and 39 GeV recorded with the STAR experiment at Relativistic Heavy Ion at Collider (RHIC). The energy dependence
of v₁(y) slope (dv₁/dy|_y=0) for deuterons, for intermediate-centrality(10-40%) of Au+Au collisions, is extracted and compared with
that of protons and Λ's. While v₁(y) slopes of protons and Λ's are mostly negative at √(s_NN) > 10 GeV, v₁(y) slopes of deuterons
are positive or consistent with zero. A strong enhancement of the v₁(y) slope of deuterons is seen at the lowest collision energy
(the largest baryon density) √(s_NN)  = 7.7 GeV. The AMPT calculation can qualitatively reproduce the general trend of the v₁(y) 
slope of deuterons, but predicts larger magnitudes than the measurements.

Motivation

 

• Net-baryons dv1/dy changes sign and shows a minimum near 14.5 GeV
• Mesons and all anti-baryons slope is negative at all energies
• Stronger directed flow observed for heavier nuclei at intermediate energies

                              How about light nuclei directed flow at BES?

Provide information to understand the collision evolution and light nuclei production mechanism.

Fig.1 (a)The dE/dx of charged tracks plotted against momentum in Au+Au collisions at √(s_NN) = 19.6 GeV. The curves
are Bichsel expectation values of the corresponding particles.  (b) Particle m² versus momentum from the TOF at
√(s_NN) = 19.6 GeV. The bands, from bottom to top, correspond to π⁺, K⁺, protons, and deuterons, respectively.

Fig.2 The z distribution for deuteron from TPC in various pT ranges in Au+Au collisions at √(s_NN) = 19.6 GeV.
The dashed line is Gaussian fit representing the distribution for deuteron. The dot-dashed curve is Gaussian fit denoting
contributions from π+, K+ and proton. The plotted uncertainties are statistical only.

Fig.3 The detection maps for (a) protons and (b) deuterons in rapidity-pT phase space in intermediate-centrality (10-40%)
Au+Au collisions at √(s_NN) = 19.6 GeV. The dashed-lines denote the regions used for calculation of v1 of protons and deuterons, respectively.

Fig.4 The 1st-order event plane (Ψ₁) resolution as a function of centrality of Au+Au collision at √(s_NN) = 7.7, 11.5, 14.5, 19.6, 27
and 39 GeV. The Ψ1 is reconstructed with the BBC detectors and its resolution is estimated by the correlation of sub-Ψ₁ from east BBC
and west BBC. Data presented later (10-40% centrality) is indicated by the dashed-line box.

Fig.5 Rapidity dependence of v1 for protons (circles) and deuterons (dots) in intermediate-centrality (10-40%) Au+Au collisions
at √(s_NN) = 7.7, 11.5, 14.5, 19.6, 27 and 39 GeV. The lines at the midrapidity (|y| < 0.6) are the fit with linear functions to
extract the slopes. The plotted uncertainties are statistical only.

Fig.6 Directed flow slope near the midrapidity (dv₁/dy) as a function of beam energy for intermediate-centrality (10-40%) Au+Au collisions.
Dots represent deuterons. Circles and squares are the published results in [20] for protons and Λ, respectively. The band denotes the AMPT
calculation for deuterons. Statistical uncertainties (bars) and systematic uncertainties (horizontal lines) are shown separately. For visibility,
the data points are staggered horizontally.

Summary
In summary, we present the mid-rapitidy directed flow v₁(y) of deuterons from Au+Au collisions at √(s_NN)  = 7.7-39 GeV Au+Au collisions. At 10-40% centrality, the v₁(y) slopes , dv1/dy, are all >= 0 and show a strong increase at lowest collision energy at √(s_NN)  = 7.7 GeV. At energy >= 10 GeV,
The positive deuteron v1(y) slopes are constrasted with the previously observed v1(y) slopde of protons from the same collision centralities. The simplest coalescence mechanism for deuteron production is therefore disfavored. 
production predicts a ratio of two between deuteron and proton v₁(y) slopes, which is disfavored by the data at √(s_NN)  > 10 GeV.
The AMPT transport model overestimated the values of deuteron v₁(y) slopes at most measured collision energies.

Supporinting materials
presentations in PWG
www.star.bnl.gov/protected/heavy/qiuh/other/hexh_Deuteronv1ForPWG_20180508.pdf
drupal.star.bnl.gov/STAR/system/files/hexh_Deuteronv1ForPWG_20190529.pdf

drupal.star.bnl.gov/STAR/system/files/Deuteronv1_Xionghong_20190703.pdf
drupal.star.bnl.gov/STAR/system/files/Xionghong_deuteronv1_20190731.pdf

drupal.star.bnl.gov/STAR/system/files/hexh_Deuteronv1_20190814.pdf
drupal.star.bnl.gov/STAR/system/files/hexh_Deuteronv1ForPWG_20190918_0.pdf