Studying Hard Interactions in Nuclei with d-Au Collisions at RHIC

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Studying Hard Interactions in Nuclei with d-Au
Collisions at RHIC

• Prof. Brian A Cole
• Columbia University

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Why p-A (d-A) Collisions ?

• Probe Initial-State Effects at RHIC
• Shadowing of Nuclear PDFs
• Parton saturation
• Cronin effect
• p? broadening of hard processes
• Its becoming clear that these are all due to or
  reflect the same underlying physics
• Unique feature of RHIC measurements
• Ability to constrain centrality i.e. impact
  parameter range of d-A collisions.

peripheral
central
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A-A Hard Scattering Rates

• Parton flux density ? thickness
• For point-like interactions
• dNhard / dA ? product of nuclear Ts
• Integrate over transverse area
• Then
• Nbinary (also known as Ncoll) is fiction
• no successive nucleon-nucleon scattering !
• Just a convenience (pure number not fm-2)

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d-Au Centrality

• soft scatters of n/p
• Parameterize event multiplicity over some ?? at
  large ? vs ?n, ?p.
• Cut data according to fraction of total ?dA.
• For each, determine TdAu
• e.g for PHENIX (in )
• 0-20, 20-40, 40-60, 60-88
• Define

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High-p? Single Particles _at_ ?0

• PHENIX (e.g.) single particle spectra
• RdA vs centrality
• RdA increases as ?TAU ? increases.
• Difference between ? and all hadrons ?
• Partly syst. errors
• Partly real.
• Can now measure p? range of Cronin enhancement.
• And its dependence on nuclear thickness.

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? Dependence of High-p? Spectra

• RdA decreases with increasing ? at all p?.
• For low p? (how low is low) not surprising
• Well known that forward soft production in p-A
  collisions does not increase ? ?TA?.
• But, change between ?0, 1 even at high p?.
• Effect of (anti) shadowing ?

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BRAHMS A closer look

• Rcp shows suppression increases with TAu
• Clearer than RdA (pp data?)
• Suppression smooth in ?
• But see h/h- difference !
• Reflects Z1 of d ??
• Rcp with (hh-)/2 still shows suppression.

Rcp
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Model Comparisons (I)

• Vitev(nucl-th/0302002)
• pQCD w/ shadowing
• Include self-consistent p? broadening, dE/dx
• Both elastic radiative
• ? correct enhancement at mid-rapidity
• But EKS anti-shadowing overestimates RdA
• Predict RdA gt1 at y3
• dE/dx small effect.
• But significant dE/dx effects at y-3.

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Model Comparisons (II)
A. Accardi Quark Matter 2004 (also
nucl-th/0402101)

• Describe hard scattering in nuclear rest frame.
• Cronin effect from multiple semi-hard
  scattering
• Include unitarity corrections
• Fit to p-p Fermilab p-A
• Reproduces y0 ?0 Rcp
• But not y3
• Even if opacity increased x3
• BRAHMS data changed
• But p? dependence wrong

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STAR Di-jet kT

• Assume

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PHENIX Di-jet KT

• No jet reconstruction in PHENIX (yet)
• But can measure KT via two-hadron ??
  correlations.
• Additional broadening from fragmentation.
• But can be measured in single jet.
• Then
• Study vs p?h1
• KT Same in p-p, d-Au ?!
• Sensitivity ??
• More work needed.

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Summary Conclusions

• Strong nuclear modification of hard scattering in
  d-Au collisions _at_ GeV.
• Cronin enhancement for 2 lt p? lt 8 GeV/c.
• ? dependence of RdA / Rcp
• Nuclear effects increase with TAu.
• Suppression at forward ?
• Clearest in BRAHMS RdAu
• The difference between RdA, Rcp worrisome
• pQCD shadowing Cronin dE/dx cannot easily
  reproduce suppression at y3.
• Rapid variation with ? at mid-rapidity also.
• Need quantitative CGC predictions vs ?.
• Alternative measure of p? broadening via jets
  under way in STAR PHENIX.

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d (?) - A Collisions

• d-A required by aperture of IR bend magnets
• Properties of deuteron
• A2, Z1
• RRMS
• Long exp. tail to WF
• Nuclear thickness
• d-Au Collisions
• Plot Tp vs Tn
• Naively, nucleon-nucleon collisions T ? 42
  fm2
• ?dAu