U U Collisions at RHIC

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UU Collisions at RHIC

• Columbia Experimental Heavy-Ion Research Group
  Journal Club
• 27 Feb 2007

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(No Transcript)
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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
• UU Collisions
• Anisotropic Flow and Jet Quenching
• Multiplicity distribution and source deformation

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238U
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Fun facts about Uranium

• Z 92, A233, 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/mN) Q (barn)
233U 234.04 0.0055 0 0
235U 235.04 0.7200 7/2 -0.35 4.936
238U 238.05 99.2745 0 0
197Au 196.96 100 3/2 0.14 0.547
63Cu 62.92 69.17 3/2 2.22 -0.22
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Electric Quadrupole Moments
Q(U)gt0

• 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

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Accelerating Ions at RHIC
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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

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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

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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

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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

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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

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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, 3He?

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Location of EBIS
W. Fischer, PANIC05
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UU Anisotropy and Jet Quenching
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1a. UU Anisotropic Flow

• The final momentum anisotropy v2 is driven by the
  initial spatial eccentricity ex
• Systematic studies of v2 at midrapidity in AuAu
  and PbPb of different centralities show
• v2/ ex scales with
• Predictions from ideal hydro agree with data only
  in the highest RHIC energy at almost central
  AuAu collisions
• Need to increase beyond the 25
  fm-2 available in central AuAu
• UU to the rescue full-overlap collisions could
  achieve 40 fm-2

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1b. UU Jet Quenching

• Experiments show that in semi-peripheral AuAu
  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 AuAu lacks
  resolving power of the path length difference
  between in- and out-of-plane directions
• Again, full-overlap UU to the rescue

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Full-overlap (b0 and coplanar) UU 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 _at_ z0
Binary collision density
Wounded nucleon density

• 0.75, from fit to AuAu
• ks tuned to central AuAu also

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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 UU yields
larger lever arm to probe approach to ideal hydro
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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

ltngt(F) computed from transverse integral over
s(rTF)
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Aside Multiplicity Fluctuations
nucl-ex/0409015
Total multiplicity Multiplicity of 4 highest
centrality bins
Analogous centrality-selected (b0) multiplicity
distribution
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Estimating Radiative Energy Loss
Look familiar?

• 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 AuAu
• Better discriminating power

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UU Multiplicity and Source Deformation
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2. Multiplicity Distribution for Full-overlap UU

• Assuming we can select full-overlap (b0,
  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 probabilit
y distribution.
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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 UU
  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
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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 F1,2 0
• Symmetry axes of nuclei approximately parallel
• Result single-peaked mult dist whose center
  shifts left as spectator cut loosens

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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
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Summary

• The authors show that full-overlap UU collisions
  at RHIC can be used to
• Test the hydro behavior of elliptic flow to
  energy densities much higher than available to
  non-central AuAu
• 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 UU collision program at
  RHIC is feasible

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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 UU Collisions U. Heinz and A.
  Kuhlman, PRL 94, 132301 (2005)
• Multiplicity distribution and source deformation
  in full-overlap UU collisions A. Kuhlman and U.
  Heinz, PRC 72, 037901 (2005)