Light (Anti-)Nuclei Production in the STAR experiment at RHIC

1. Light (Anti-)Nuclei Production in the STAR experiment at RHIC Jianhang Zhou Bonner Lab, Rice University

2. Collision Hadrons QGP Hadronization Hadrons Light Nuclei Chemical Freeze-out Thermal Freeze-out Introduction • Building blocks of the world: quarks and leptons • Quark confinement: No free quarks • Quark deconfinement: Quark Gluon Plasma (QGP) • Ultra relativistic heavy Ion collision experiments • Light nuclei study provides a probe for understanding the final freeze-out • The relation to cosmology: early universe from Big Bang is similar to the heavy ion collision experiments Jianhang Zhou, PhD thesis defense

3. Outline • Experiment facilities: RHIC, STAR, TPC, TOF • Transverse momentum spectra and related techniques Particle identification Coalescence model Transverse momentum spectra in Cu+Cu 200 GeV collisions • Elliptic flow and related techniques Event plane method, event plane shift, resolution Elliptic flow results in Cu+Cu 200 GeV • Blast Wave model fit to Au+Au 200 GeV • Search for anti-alpha particles • Summary Jianhang Zhou, PhD thesis defense

4. Experiment Facilities Relativistic Heavy Ion Collider (RHIC)　（2.4 miles circ.) at Brookhaven National Lab (BNL) Solenoidal Tracker at RHIC (STAR) 6 o’clock position at RHIC Jianhang Zhou, PhD thesis defense

5. Time Projection Chamber (TPC) PID method: Ionization Energy Loss dE/dx PID method: measure TOF=T(stop)-T(start), along with p from TPC, => calculate Mass Time of Flight (TOF) Structure: pVPD and TOF tray Particle Identification (PID) in STAR Jianhang Zhou, PhD thesis defense

6. TPCPID: N-sigma Distribution With tight track cuts, z-distribution of helium is background free. No need for background subtract. Proton and deuteron N-sigma distributions are fit by a Gaussian function plus a background. Jianhang Zhou, PhD thesis defense

7. The coalescence parameter (A is atomic number) For A=2, 3 : Baryon density related to yields: Coalescence Model The relation of the light nuclei invariant yield and the proton yield Jianhang Zhou, PhD thesis defense

8. Centrality and Number of participants Npart is the equivalent number of participant nucleons revolved in the collisons. RefMult is the number of primary tracks from the collision vertex. Data set TPC Track quality cuts: nHitsdEdx>15 nHitsFit>25 |Zvtx|<30, DCA<1 |pseudo-rapidity|<0.9 Data set: STAR Run-V Cu+Cu 200 GeV Trigger: Minimum Bias About 37 million events Jianhang Zhou, PhD thesis defense

9. DCA of d DCA of dbar Distance of Closest Approach (DCA) • DCA distribution of d indicates contamination by background • dbar is not contaminated by background • dbar is used in the analysis of coalescence parameters Jianhang Zhou, PhD thesis defense

10. Transverse momentum (pT) spectra pT spectra of pbar pT spectra of dbar • Tracking efficiency is the ratio of TPC reconstructed tracks to all the tracks. • Cu+Cu pbar and dbar spectra are calculated with tracking efficiency obtained from Au+Au for similar reference multiplicity. Jianhang Zhou, PhD thesis defense

11. BA vs pT/A • B2 and sqrt(B3) are close to each other in the same system • BA from Cu+Cu is larger than Au+Au, consistent with smaller coalescence volumes • BA increases slightly with increasing pT/A, consistent with decreasing coalescence volumes Jianhang Zhou, PhD thesis defense

12. 1/B2 vs <Npart> • <Npart> is the number of participant nucleons • 1/B2 is found to be linear with <Npart>, in all pT ranges • Consistent with that <Npart> is proportional to the coalescence volume Jianhang Zhou, PhD thesis defense

13. 1/BA comparison to Au+Au • B2 and sqrt(B3) in similar pT/A range • All Cu+Cu and Au+Au results shows 1/BA is proportional to <Npart> Jianhang Zhou, PhD thesis defense

14. Comparison to pion HBT volume The extracted B2 and sqrt(B3) are smaller for larger Npart, which is consistent with larger volume for more central collisions. The B2 & sqrt(B3) are consistent with HBT volumes HBT volume is calculated from the HBT correlation lengths along the longitudinal and transverse directions. Cu+Cu results at pT/A= 0.45GeV/c are consistent with pT=0.5 GeV/c pion HBT volume. Jianhang Zhou, PhD thesis defense

15. Baryon density dbar/pbar ratio as a measure of antibaryon phase space density v.s. beam energy. Data points from e+e- and γp collisions are also shown. Jianhang Zhou, PhD thesis defense

16. He3 and He3bar production Jianhang Zhou, PhD thesis defense

17. The azimuthal dependence of yield: Determine the event plane angle: Elliptic Flow (v2) The weight factors wi are chosen to be the transverse momentum pT. Jianhang Zhou, PhD thesis defense

18. (n=1,2,3,4…… ) Event plane angle shift Formula used for shift correction Jianhang Zhou, PhD thesis defense

19. Event Plane from FTPC Jianhang Zhou, PhD thesis defense

20. dbar yield versus azimuthal angle Jianhang Zhou, PhD thesis defense

21. pbar and dbar v2 for different background Use different background estimation to fit the N-sigma plots: Gaussian, Exponential, or no background Jianhang Zhou, PhD thesis defense

22. pbar and dbar v2 compared to Au+Au Pbar and Lambda v2: slightly higher in Cu+Cu than in Au+Au, for pT<1GeV/c. Mass dependence: larger v2 for smaller mass, in both Au+Au and Cu+Cu Negative dbar v2 was first observed in Au+Au Negative dbar v2 is observed again, in Cu+Cu Systematic errors are smaller than statistical errors Jianhang Zhou, PhD thesis defense

23. Blast-Wave model parameters y The 8 parameters of the blast-wave model are: T, rho0, rho2, Ry, Rx, s, , Δ The freeze-out distribution is infinite in z-direction, and elliptical in transverse(x-y) plane. The transverse shape is controlled by Rx, Ry. Ry r x Rx The parameter s corresponds to a surface diffuseness of the emission source. s =0 corresponds to a hard edge source. The flow rapidity is given by: Rho(r,φs) = r~ (rho0+rho2*cos(2φb)) where r~=sqrt((rcos(φs))2/Rx2+(rsin(φs))2/Ry2) The freeze-out is supposed to occur with a given distribution in longitudinal proper time =sqrt(t2-z2). We assume a Gaussian distribution peaked at 0 and with a width Δ Jianhang Zhou, PhD thesis defense

24. spectra fit(pi,K,p) : V2 fit (pi,K,p) : Fit all spectra and v2 for pi, K, p: (MinBias triggered) Total 2/ndf = 711.422/154 T (MeV) = 124.2 +-1.9 rho0= 0.88 +- 0.01 rho2 = 0.061 +-0.002 Rx/Ry= 0.89 +- 0.003 s = 0 +- 0 (fixed)  (fm/c) = 9.2 +- 0 (fixed)  (fm/c) = 0.03 +- 0 (fixed) Blast-Wave Fitting Jianhang Zhou, PhD thesis defense

25. Spectra and Blast-Wave Prediction Spectra of d (dbar) and He3 (He3bar) v.s. pT, for both central and MinBias. The corresponding BW fitting results are shown by solid and dashed lines. The green bands show proton data/BW ratio, as a comparison. BW describes proton very well, but overpredicts radial flow of d and He3. Jianhang Zhou, PhD thesis defense

26. (a) MB v2 vs. pT for He3+He3bar, d+dbar and dbar, and BW fitting. (b) d+dbar and He3+He3bar v2/A  v.s. pT/A. pbar and the Λ+Λbar v2 are also shown as comparison. BW fit 2/ndf = d+dbar : 3.1/2 He3+He3bar:4.3/2 (c) Low pT dbar and pbar v2/A v.s. Npart, and BW predictions. pT range for dbar: upper: 0.2<pT<0.7 GeV/c; lower: 0.7<pT<1.0 GeV/c. pT range for pbar: upper: pT<0.24 GeV/c; lower: 0.4<pT<0.48 GeV/c. V2 and Blast-Wave prediction Heavier nucleus deviates more from the scaling. Negative v2 is not correctly predicted by BW. Jianhang Zhou, PhD thesis defense

27. Search for anti-alpha • Anti-alpha has never been found. • Using TPC, 2 candidates are found in STAR Run-VII Au+Au collisions. Jianhang Zhou, PhD thesis defense

28. Track validity check • The candidate tracks are checked to be valid. • Confirmation needs further investigation or more candidates. • Upgraded TPC and TOF will provide enough statistics in the future Jianhang Zhou, PhD thesis defense

29. Summary • The pT spectra of pbar, dbar and He3bar in STAR Run-V Cu+Cu are studied and the coalescence parameters B2, B3 are calculated. B2 and sqrt(B3) are demonstrated to be comparable with each other and linear with <Npart> for different centralities. • B2 and sqrt(B3) from both Cu+Cu and Au+Au are compared. In similar pT range, they are consistent with each other and proportional with <Npart>. It is consistent with that the final freeze-out volume is proportional to <Npart>. • B2 and sqrt(B3) are also compared to pion HBT volumes. They are consistent with each other. • He3bar/He3 ratio in Cu+Cu are compared to pbar/p ratio. The comparison is consistent with coalescence model. • The elliptic flow (V2) of pbar and dbar in Cu+Cu are studied and compared to Au+Au. The comparison is consistent with mass dependence. A negative dbar v2 is observed. • Blast Wave model is used to fit pi/K/p pT spectra and v2. The fit results are used to predict light nuclei (d,He3) spectra and v2. The comparison shows consistence between data and BW predictions. • Search for anti-alpha in STAR Run-VII results in two candidates. Tracking information is checked. Further confirmation is needed. Future hope in finding more candidates depends on the upgraded TPC and the large area TOF system. Jianhang Zhou, PhD thesis defense

30. THE END THANK YOU! Jianhang Zhou, PhD thesis defense

31. Comparison to Big Bang Nucleosynthesis baryon density The ratio of the baryon density (D/H) in the universe (from BBN) to the baryon density from collider experiment is 3.60.4%. The ratio of the observed baryon in the universe to the total matter in the universe is about 4%. Possible explanation of the relation between these 2 numbers is unknown. Jianhang Zhou, PhD thesis defense