U+U Collisions at RHIC

1. U+U Collisions at RHIC Columbia Experimental Heavy-Ion Research Group Journal Club 27 Feb 2007

2. D.Winter: U+U Collisions at RHIC

3. Outline • Introduction to the 238U nucleus • Fun facts • Definition of quadrupole moment • How do we accelerate ions at RHIC? • Overview • Tandem source/acceleration • Onward to RHIC • U+U Collisions • Anisotropic Flow and Jet Quenching • Multiplicity distribution and source deformation D.Winter: U+U Collisions at RHIC

4. 238U D.Winter: U+U Collisions at RHIC

5. Fun facts about Uranium • Z = 92, A=233, 235, 238 (three natural isotopes) • Not rare – more common than beryllium or tungsten • Solid at 298 K • Metallic grey in color D.Winter: U+U Collisions at RHIC

6. Electric Quadrupole Moments • Non-zero quadrupole moment indicates that the charge distribution is not spherically symmetric • Q0 is the classical form of the calculation • Represents the departure from spherical symmetry in the rest frame of the nucleus • Q is the quantum mechanical form • Takes into account the nuclear spin I and projection K in the z-direction Q(U)>0 D.Winter: U+U Collisions at RHIC

7. Accelerating Ions at RHIC D.Winter: U+U Collisions at RHIC

8. Overview of the Transport to RHIC • LINAC for source of protons • Two Tandem Van-der-Graff accelerators available • Allows asymmetric collisions, for example • Heavy-ion Transfer Line • AGS Booster • AGS • AGS-to-RHIC Transfer Line D.Winter: U+U Collisions at RHIC

9. Originating Source of Heavy Ions • Positive Cs ions strike sputter target • Ions emerging from target have picked up one electron • Ions accelerated thru extraction potential of approximately 25 kV D.Winter: U+U Collisions at RHIC

10. Accelerating Ions at the Tandem • Beam passing thru carbon foils strips off electrons • Multiple stages of acceleration/stripping used (2 or 3 depending on A of species) • Au Ions exit the tandem in +32 state D.Winter: U+U Collisions at RHIC

11. Tandem to RHIC • Heavy Ion Transfer Line transports ions (with no additional stripping or acceleration) to the Booster • Foil at the Booster exit strips all but two tightly bound K-shell electrons • Au ions exit the booster at 95 MeV/A with +77 charge • AGS accelerates (Au) bunches to ~9 GeV/A • At the AGS exit, ions are fully stripped • Transported to RHIC via the AGS-to-RHIC (AtR) line • In ~ 2 min, RHIC can acclerate ions to top energy D.Winter: U+U Collisions at RHIC

12. Current Capabilities of RHIC • RHIC can accelerate range of species from p to Au • Which ions specifically? Those which can be easily produced from a sputter source • Major issue: U does not form an abundant negative ion, making acceleration from sputter target a challenge • Using a sputter target drilled out in the middle to allow O2 into bleed in – result: UO- ions accelerated (Benjamin et al. 1999) • “Uranium is a viable species but must be considered as a future upgrade, since at present, an adequate source for Uranium does not exist at Brookhaven and further R & D will be needed to achieve this goal” • H. Hahn et al., NIM A488 (2003) 245-263 D.Winter: U+U Collisions at RHIC

13. Future Capabilities of RHIC EBIS: Electron Beam Ion Source • Replace 35-year-old tandem by 2009 • Advantages: • Simpler operation at lower cost • Simpler booster injection • New species available: U, 3He Scaled results from ½ length prototype exceed RHIC needs D.Winter: U+U Collisions at RHIC

14. Location of EBIS W. Fischer, PANIC05 D.Winter: U+U Collisions at RHIC

15. U+U: Anisotropy and Jet Quenching D.Winter: U+U Collisions at RHIC

16. (initial entropy density of overlap region) 1a. U+U: Anisotropic Flow • The final momentum anisotropy v2 is driven by the initial spatial eccentricity ex • Systematic studies of v2 at midrapidity in Au+Au and Pb+Pb of different centralities show: • v2/ ex scales with • Predictions from ideal hydro agree with data only in the highest RHIC energy at almost central Au+Au collisions • Need to increase beyond the ~ 25 fm-2 available in central Au+Au • U+U to the rescue: full-overlap collisions could achieve ~ 40 fm-2 D.Winter: U+U Collisions at RHIC

17. 1b. U+U: Jet Quenching • Experiments show that in semi-peripheral Au+Au collisions fast partons suffer more energy loss in the direction perpendicular to the RP compared to the in-plane direction • Small size of fireball in semi-periph Au+Au lacks resolving power of the path length difference between in- and out-of-plane directions • Again, full-overlap U+U to the rescue D.Winter: U+U Collisions at RHIC

18. Full-overlap (b=0 and coplanar) U+U Collisions Very important assumption: we can select these collisions with tight spectator cuts “Side-on-side” “Tip-on-tip” Or “Edge-on-edge” Initial entropy density in transverse plane @ z=0 Binary collision density Wounded nucleon density • = 0.75, from fit to Au+Au ks tuned to central Au+Au also D.Winter: U+U Collisions at RHIC

19. Initial Energy and Entropy Density vs. Npart Conversion of entropy density to energy density assumes ideal quark-gluon gass EOS Larger energy density in central U+U yields larger lever arm to probe approach to ideal hydro D.Winter: U+U Collisions at RHIC

20. Multiplicity and Eccentricity Probabilities Model fluctuations with probability density for n = dNch/dy Initial eccentricity in overlap region Integrate over F • Eccentricity probability distribution for • cuts shown to the left • Full-overlap collisions vary from 0-0.25 <n>(F) computed from transverse integral over s(rT;F) D.Winter: U+U Collisions at RHIC

21. Aside: Multiplicity Fluctuations nucl-ex/0409015 Total multiplicity Multiplicity of 4 highest centrality bins Analogous centrality-selected (b=0) multiplicity distribution D.Winter: U+U Collisions at RHIC

22. Estimating Radiative Energy Loss • Compare energy loss of inward-moving partons • t0: parton density constant • t: includes dilution due to longitudinal expansion • Difference in e-loss between in- and out- emission is 2x Au+Au • Better discriminating power Look familiar? D.Winter: U+U Collisions at RHIC

23. U+U: Multiplicity and Source Deformation D.Winter: U+U Collisions at RHIC

24. (initial entropy density of overlap region) 2. Multiplicity Distribution for Full-overlap U+U • Assuming we can select full-overlap (b=0, coplanar nuclei) collisions with ZDC signal, cutting on multiplicity we can select different spatial deformations of overlap zone Centrality dependence of dNch/dy Tuning a and ks Integrate over F to obtain multiplicity probability distribution. D.Winter: U+U Collisions at RHIC

25. Allowing for misalignment • Slightly misaligned tip-on-tip and fully aligned side-on-side collisions can have the same Npart (and ZDC signal) • Assessing the effect of imperfect overlap requires the inclusion of noncentral U+U collisions • In general, need to characterize collision with 5 variables • Impact parameter b • Euler angles of orientation of U: W = (F, b) Initial entropy density becomes: Region of full-overlap events D.Winter: U+U Collisions at RHIC

26. Cutting on number of spectators • Number of spectator nucleons: Nspec = 2 x 238 - Npart • Selecting low-spectator events biases sample towards • b ~ 0 and F1,2 ~ 0 • Symmetry axes of nuclei approximately parallel • Result: single-peaked mult dist whose center shifts left as spectator cut loosens tight loose ~0-5% D.Winter: U+U Collisions at RHIC

27. Effect on eccentricity distribution • For sufficiently tight spectator cuts, expect events corresponding to left edge of mult dists to have larger contribution from side-on-side collisions • Therefore, cutting on low spectators and low multiplicity should select strongly deformed overlap regions • Loosening the spectator cut broadens the eccentricity distributions • Allows contributions from non-zero impact parameter • Thus ex can exceed 0.25 Impact: have ability to select spatial deformation of collision zone D.Winter: U+U Collisions at RHIC

28. Summary • The authors show that full-overlap U+U collisions at RHIC can be used to: • Test the hydro behavior of elliptic flow to energy densities much higher than available to non-central Au+Au • Produce highly-deformed reaction zones to explore more detailed study of path-length dependence of energy loss by a fast parton as it passes thru the plasma • Full-overlap collisions can be selected by tight cuts on the number of spectators (i.e. ZDC signal) • Further cuts on the multiplicity of low-spectator events can discriminate between degrees of spatial deformation of the fireball • Via correlation with “side-on-side-ness” of collision • This approach is reasonably robust against trigger inefficiencies • Extracting physics from U+U collision program at RHIC is feasible D.Winter: U+U Collisions at RHIC

29. References • “Tandem Injected Relativistic Heavy Ion Facility at Brookhaven, Present and Future” P. Thieberger et al., NIM A268 (1988) 513-521 • “The RHIC Design Review” H. Hahn et al., NIM A499 (2003) 245-263 • “Anisotropic Flow and Jet Quenching in Ultrarelativistic U+U Collisions” U. Heinz and A. Kuhlman, PRL 94, 132301 (2005) • “Multiplicity distribution and source deformation in full-overlap U+U collisions” A. Kuhlman and U. Heinz, PRC 72, 037901 (2005) D.Winter: U+U Collisions at RHIC