How low can you go: Physics of LowPt Phenomena

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How low can you go?Physics of Low-Pt Phenomena

• Craig Ogilvie, MIT
• 9 Dec 1998

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Last Week HBT

• Choose a source distribution that ignores
  momentum dependence e.g
• Construct several C(q) each for range of pair
  momentum k
• Fit correlation function with
• R is an effective source parameter
• HBT source parameter, correlation length
• refrain from usage of radius or source size
• R(k) probes geometry and dynamics that correlate
  particles

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Today

• low-pt physics
• large-scale phenonema
• survey of existing data
• AGS, CERN
• resonances, Coulomb
• experimental difficulties
• RHIC

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Basic Argument

• particles at low momenta have large wavelength gt
  sensitive to any large-scale phenomena
• candidates for large-scale phenomona
• QGP of this size may exist, but is there any
  coherence over this scale?
• collective modes of the plasma
• decay of disorientated vacuum

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AA at RHIC
wide range of soft/hard collisions occur
partons from hard scattering color-field
between quarks
what is this material ? not color singlets!!
proto-QGP?
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Time-Development
1) high-pt particles leave, hadronize 2)
remaining system cools, hadronizes to a range of
momenta, low-pt hadrons, especially if any
large-scale phenomena exists at late
stage of hadronization van Hove Emission from
cold QGP 3) do these hadrons scatter with
hotter hadrons (potential to lose info.) or
have hotter hadrons left system
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Other Sources of low-pt hadrons

• Decay of resonances
• e.g. D -gt pp , DQ 155 MeV,
• especially in baryon-rich AGS, SPS
• r-gtpp, DQ 492 MeV
• Bose-dynamics

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low-pt in pp

• no evidence for any strong, unusual feature at
  low pt

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low-pt from pA
Helios Z.Phys C 46, 361, (90) also Phys.Lett67B,
355 (77) also E910 pA, QM97

• extra pions at low pt above exponential
• also at higher-pt (pre-scattering Cronin-effect)

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E866, PRC57, R466 (98)

• large, p-, p difference attributed to Coulombgt
  size

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Systematics of p low-pt rise E866E917
AuAu QM97

• low-pt rise largest at lower-energies
• decay of resonances relatively more impt at lower
  energies ?

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Systematics versus rapidity

• largest at target and projectile rapidities
• consistent with resonance decay
• QGP-like explanations should peak at mid-rapidity

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Systematics vs rapidity
B.Jacak private comm SA (SPS) problems with
fitting ranges etc., but trend is
clear consistent with resonance decay QGP-like
explanations should peak at mid-rapidity.
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Hadronic Models

• Models reproduce low-pt rise
• decay of D-resonances late in the reaction
• 1AGeV B.A.Li, W. Bauer Phys. Lett B254, 335 (91)
• 10AGeV G.E.Brown et al, Phys.Lett.B253, 19 (91)
• possibility of Bose-enhancement
• 160AGeV HW Barz et al, Phys. Lett. B287, 40 (92),
  Welke et al, Phys. Rev C 45, 1403 (92)
• No summary paper yet
• data trends
• consistent model treatment

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Experimental Difficulties (I)

• tracking efficiency strongly varies
• PID contamination often results are quoted as h-
• p-, K-, e-

helios Zphys C
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Experimental Difficulties (II)

• Scattering from apparatus, producing tracks that
  should not point back to target e.g. magnet E877
  _at_AGS
• presented at major conf 1992-95, e.g. QM95
• large theoretical effort
• never published

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Experimental Difficulties (III)

• Secondary interactions in thick target
• paticles from first reaction collide with other
  nuclei in target gt extra particles concentrated
  at low-pt

NA35 Monte Carlo private comm. J.Simon-Gallo
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• Acceptances close comparisons will depend on
  tracking efficicency, resolution, PID dE/dx vs p,
  dE/dx vs E stopped

W.Busza LBNL Feb 97 (sorry its crooked)
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RHIC

• given history of results that have come and gone,
  extreme skepticism in HI field about low-pt
  results
• RHIC multiple experiments will be able to
  cross-check
• how does low-pt yield change with rapidity
• discriminate between resonance and potential QGP
• openness
• demonstration of backgrounds
• efficiency, ghosts vs. pt

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Summary

• low-pt sensitive to large-scale phenomena
• emission from cold QGP
• collective motion in plasma
• decay of disoriented vacuum
• any scattering after hadronization will remove
  info.
• low-pt in pA, AA, consistent decay of resonances
• difference p-, p Coulomb interaction gt size
• experimental difficulties
• RHIC opportunities across several experiments