Directed flow in AuAu collisions at 62 Ge

Directed flow in Au+Au collisions at 62 Ge. V Gang Wang for STAR Collaboration q. Introduction Directed Flow Wiggle Previous result 3 Methods ZDC upgrade ZDC-SMD method q. Physics results 10% - 70% Centrality dependence Pt dependence Integrated v 1 Limiting fragmentation q Summary Gang Wang, KSU

Flow Among different kinds of flow, such as radial flow, directed flow, elliptic flow and higher harmonics, directed flow is the first discovered type of flow (20 years ago). Radial Flow (isotropic) Flow Directed Flow Anisotropic Flow Elliptic Flow Higher Harmonics 2021/10/21 Gang Wang, KSU 2

Directed Flow • Directed flow: the first harmonic (v 1) in the Fourier expansion of the particle azimuthal anisotropy with respect to the reaction plane • It’s likely influenced by the compression in the initial stages of the collision, and makes a probe for the early stage of the collision. v 1 changes with the collision energy. <px> or v 1 rapidity J. -Y. Ollitrault, Nucl. Phys. A 638, 195 c (1998) 2021/10/21 Gang Wang, KSU 3

Wiggle Models predicted that anti-flow/3 rd flow component, with QGP v 1(y) flat or a wiggle structure. J. Brachmann et al. PRC 61, 024909 (2000) 2021/10/21 L. P. Csernai, D. Rohrich PRL 458 (1999) 454 Gang Wang, KSU 4

Wiggle Models without QGP also predicted a wiggle structure in peripheral or midperipheral events. M. Bleicher and H. Stöcker, PLB 526, 309 (2002) D M Q R Baryon stopping and positive space-momentum correlation D M u. RQ R. Snellings, H. Sorge, S. Voloshin, F. Wang, N. Xu, PRL 84, 2803 (2000) 2021/10/21 Gang Wang, KSU 5

Previous result STAR Collaboration, PRL 92, 062301 (2004) 2021/10/21 Gang Wang, KSU 6

Experimental complications At RHIC, most detectors cover the central rapidity region where the directed signal is small and the analysis procedures are easily confused by non-flow effects (azimuthal correlations not related to the reaction plane orientation). We try to reduce sensitivity to non-flow effects: v 1{3} • the three-particle cumulant method • by Aihong Tang v 1{EP 1, EP 2} • the event plane method with mixed harmonics • by Markus Oldenburg, Ilya Selyuzhenkov v 1{ZDC-SMD} • the standard method with the 1 st-order event plane reconstructed from neutral fragments of the incident beams • the event plane is determined from the sideward deflection of spectator neutrons (“bounce-off”) measured in ZDC-SMD • by Gang Wang 2021/10/21 Gang Wang, KSU 7

STAR ZDC ØEach of the RHIC experiments has a pair of Zero Degree Calorimeter detectors for beam monitoring, triggering, and locating interaction vertices. ØZDCs detect neutrons emitted along beam directions and measure their total energy (multiplicity). ØBaseline ZDCs have no transverse segmentation, which motivates upgrade. 2021/10/21 Gang Wang, KSU 8

STAR ZDC-SMD is an 8 channel by 7 channel hodoscope that sits directly on the face of the 2 nd ZDC module 2021/10/21 Gang Wang, KSU 9

Raw event plane from ZDC-SMD The raw event plane distributions are not flat. We can use the inverse of the bin content as psi weight in the analysis. R STA 2021/10/21 ary n i m i Prel Gang Wang, KSU 10

Flatten event plane distribution E 877 Collaboration, Phys. Rev. C 56, 3254 (1997) R STA 2021/10/21 ary n i m i Prel Gang Wang, KSU 11

Comparison between 2 analysis The analysis with psi weight gives the same result as the one with flat psi. 50% -- 60% ary R STA 2021/10/21 min i l e r P Gang Wang, KSU 12

Systematic study: 4 terms In analysis: To systematically study the method with ZDC-SMD, we can use the sub event plane from only east or west ZDC-SMD, instead of the full event plane, and even break down the correlation into X and Y direction. For example: Definitions of 4 terms: 2021/10/21 Gang Wang, KSU 13

Systematic study : Corrections East and west correction: When east and west ZDC-SMD have different RP resolutions, we have to use a 3 rd 1 st-order event plane ( ) as an independent reference to correct this effect. E 877 Collaboration, Phys. Rev. C 55, 1420 (1997) They are inverse of each other where denotes another 1 st-order event plane, e. g. from FTPC 2021/10/21 Gang Wang, KSU 14

Systematic study : Corrections X - Y correction: (from Sergei V. and Ilya S. ) When detectors like TPC and FTPC don’t have the same sensitivity in X and Y direction, we have to replace the “ 2” in the numerator with for X direction or for Y direction. For example: 2021/10/21 Gang Wang, KSU 15

Systematic study : Comparison After corrections applied, the 4 separate terms get much closer to each other. 50% -- 60% ary R STA 2021/10/21 Gang Wang, KSU min i l e r P 16

Systematic study : Comparison The average of the 4 separate terms almost coincides with the result of the full-event-plane analysis. 50% -- 60% ary R STA 2021/10/21 min i l e r P Gang Wang, KSU 17

Charged hadrons: Eta dependence ary STA 2021/10/21 Gang Wang, KSU R min i l e r P 18

Charged hadrons : Eta dependence nary. NA 49, QM 99 i m i l e R Pr Centrality by Centrality A ST 2021/10/21 Gang Wang, KSU 19

Charged hadrons: Pt dependence The magnitude of v 1 is observed to increase with particle transverse momentum and then appears to saturate. nary i m i l e R Pr STA 2021/10/21 Gang Wang, KSU 20

Charged hadrons: Integrated v 1 • v 1 is integrated over 0. 15 Ge. V < pt < 2 Ge. V. • The magnitude of v 1 decreases with centrality. R STA 2021/10/21 • v 1 in the more forward pseudorapidity region varies more strongly with centrality than in the region closer to midrapidity. nary i m i l Pre Gang Wang, KSU 21

Limiting Fragmentation It has been observed that particle emission(both spectra and flow) as a function of rapidity in the vicinity of beam rapidity appears unchanged over a wide range of beam energies, a pattern known as limiting fragmentation. R STA nary i m i l Pre The hypothesis holds here. 2021/10/21 Gang Wang, KSU 22

Summary ØWe present the first measurements of charged particle directed flow in Au+Au collisions at 62 Ge. V. ØThe analysis has been performed using three different methods and the results agree very well with each other. ØSTAR ZDC-SMD now allows to determine the reaction plane from the bounceoff of fragmentation neutrons, the first measurement of this type at RHIC. ØCharged particles in the pseudorapidity region covered by the STAR TPC and FTPCs (up to eta = 4. 0) flow in the opposite direction to the fragmentation nucleons. ØOver the pseudorapidity range studied, the v 1 for charged particles is found to vary monotonically with pseudorapidity at all centralities. ØThere’s no evidence so far for pion wiggle. We are working on v 1 of identified particles. ØOur data provide further support for the limiting fragmentation picture. 2021/10/21 Gang Wang, KSU 23

• Backup Slides 2021/10/21 Gang Wang, KSU 24

Event Plane Resolution for ZDC-SMD R STA 2021/10/21 ary n i m i Prel Gang Wang, KSU 25

EOS data: pion flow E 895 data 2021/10/21 Gang Wang, KSU 26