UU Collisions at RHIC Columbia Experimental HeavyIon Research
U+U Collisions at RHIC Columbia Experimental Heavy-Ion Research Group Journal Club 27 Feb 2007 David L. Winter 27 Feb 2007 D. Winter: U+U Collisions at RHIC
27 Feb 2007 D. Winter: U+U Collisions at RHIC 2
Outline • Introduction to the 238 U 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 27 Feb 2007 D. Winter: U+U Collisions at RHIC 3
238 U 27 Feb 2007 D. Winter: U+U Collisions at RHIC 4
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 Isotope Atomic Mass (ma/u) Natural Abundance (atom %) Nuclear Spin (I) Magnetic Moment (m/m. N) Q (barn) 233 U 234. 04 0. 0055 0 0 235 U 235. 04 0. 7200 7/2 -0. 35 238 U 238. 05 99. 2745 0 0 197 Au 196. 96 100 3/2 0. 14 0. 547 63 Cu 62. 92 69. 17 3/2 2. 22 -0. 22 27 Feb 2007 D. Winter: U+U Collisions at RHIC 4. 936 5
Electric Quadrupole Moments Q(U)>0 • Non-zero quadrupole moment indicates that the charge distribution is not spherically symmetric • Q 0 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 27 Feb 2007 D. Winter: U+U Collisions at RHIC 6
Accelerating Ions at RHIC 27 Feb 2007 D. Winter: U+U Collisions at RHIC 7
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-to-RHIC Transfer Line 27 Feb 2007 D. Winter: U+U Collisions at RHIC 8
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 k. V 27 Feb 2007 D. Winter: U+U Collisions at RHIC 9
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 27 Feb 2007 D. Winter: U+U Collisions at RHIC 10
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 Me. V/A with +77 charge • • AGS accelerates (Au) bunches to ~9 Ge. V/A At the AGS exit, ions are fully stripped Transported to RHIC via the AGS-to-RHIC (At. R) line In ~ 2 min, RHIC can acclerate ions to top energy 27 Feb 2007 D. Winter: U+U Collisions at RHIC 11
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 O 2 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 A 488 (2003) 245 -263 27 Feb 2007 D. Winter: U+U Collisions at RHIC 12
Future Capabilities of RHIC Scaled results from ½ length prototype exceed RHIC needs 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, 3 He 27 Feb 2007 D. Winter: U+U Collisions at RHIC 13
Location of EBIS W. Fischer, PANIC 05 27 Feb 2007 D. Winter: U+U Collisions at RHIC 14
U+U: Anisotropy and Jet Quenching 27 Feb 2007 D. Winter: U+U Collisions at RHIC 15
1 a. U+U: Anisotropic Flow • The final momentum anisotropy v 2 is driven by the initial spatial eccentricity ex • Systematic studies of v 2 at midrapidity in Au+Au and Pb+Pb of different centralities show: – v 2/ ex scales with (initial entropy density of overlap region) • 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 27 Feb 2007 D. Winter: U+U Collisions at RHIC 16
1 b. 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-ofplane directions • Again, full-overlap U+U to the rescue 27 Feb 2007 D. Winter: U+U Collisions at RHIC 17
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 27 Feb 2007 a = 0. 75, from fit to Au+Au ks tuned to central Au+Au also D. Winter: U+U Collisions at RHIC 18
Initial Energy and Entropy Density vs. Npart Conversion of entropy density to energy density assumes ideal quarkgluon gass EOS Larger energy density in central U+U yields larger lever arm to probe approach to ideal hydro 27 Feb 2007 D. Winter: U+U Collisions at RHIC 19
Multiplicity and Eccentricity Probabilities Model fluctuations with probability density for n = d. Nch/dy Initial eccentricity in overlap region Integrate over F <n>(F) computed from transverse integral over s(r. T; F) 27 Feb 2007 Eccentricity probability distribution for cuts shown to the left • Full-overlap collisions vary from 0 -0. 25 D. Winter: U+U Collisions at RHIC 20
Aside: Multiplicity Fluctuations nucl-ex/0409015 Total multiplicity Multiplicity of 4 highest centrality bins Analogous centrality-selected (b=0) multiplicity distribution 27 Feb 2007 D. Winter: U+U Collisions at RHIC 21
Estimating Radiative Energy Loss Look familiar? • Compare energy loss of inward-moving partons • t 0: parton density constant • t: includes dilution due to longitudinal expansion • Difference in e-loss between in- and outemission is 2 x Au+Au – Better discriminating power 27 Feb 2007 D. Winter: U+U Collisions at RHIC 22
U+U: Multiplicity and Source Deformation 27 Feb 2007 D. Winter: U+U Collisions at RHIC 23
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 d. Nch/dy Tuning a and ks (initial entropy density of overlap region) Integrate over F to obtain multiplicity probability distribution. 27 Feb 2007 D. Winter: U+U Collisions at RHIC 24
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 27 Feb 2007 D. Winter: U+U Collisions at RHIC 25
Cutting on number of spectators tight loose ~0 -5% • Number of spectator nucleons: Nspec = 2 x 238 - Npart • Selecting low-spectator events biases sample towards – b ~ 0 and F 1, 2 ~ 0 – Symmetry axes of nuclei approximately parallel • Result: single-peaked mult dist whose center shifts left as spectator cut loosens 27 Feb 2007 D. Winter: U+U Collisions at RHIC 26
Effect on eccentricity distribution • For sufficiently tight spectator cuts, expect events corresponding to left edge of mult dists to have larger contribution from side-onside 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 27 Feb 2007 D. Winter: U+U Collisions at RHIC 27
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 27 Feb 2007 D. Winter: U+U Collisions at RHIC 28
References • “Tandem Injected Relativistic Heavy Ion Facility at Brookhaven, Present and Future” P. Thieberger et al. , NIM A 268 (1988) 513 -521 • “The RHIC Design Review” H. Hahn et al. , NIM A 499 (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) 27 Feb 2007 D. Winter: U+U Collisions at RHIC 29
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