Azimuthal correlations of forward dijets in d Au collisions

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Azimuthal correlations of forward dijets in dAu
collisions

• Cyrille Marquet
• Columbia University
• RIKEN BNL Research Center

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Outline and Motivation
- after the first d-Au run at RHIC, there was a
lot of new results on single inclusive particle
production at forward rapidities
d Au ? h X
the spectrum and
the modification factor were studied
y increases
the suppressed production (RdA lt 1) was predicted
in the Color Glass Condensate picture of the
high-energy nucleus
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Measurements at RHIC with d-Au
trustable theory difficult when the two
particles are separated by a large interval in
rapidity
a first attempt
Kharzeev, Levin and McLerran (2005)
qualitative features OK, quantitative results
NO
it is a simpler problem when the two particles
have similar rapidities
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Gluon saturation and the CGC
x parton longitudinal momentum fraction
kT parton transverse momentum
the distribution of partons as a function of x
and kT
QCD linear evolutions
DGLAP evolution to larger kT (and a dilute
hadron) BFKL evolution to smaller x (and a dense
hadron)
dilute/dense separation characterized by the
saturation scale Qs(x)
weakly-coupled regime
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The CGC wavefunction
- obeys the JIMWLK equation
a functional equation for the small-x evolution of
Jalilian-Marian, Iancu, McLerran, Weigert,
Leonidov, Kovner
- is used to compute physical observables
the x evolution of the cross-sections is encoded
in the x evolution of
functions of Wilson lines, describing the
scattering of partons off the CGC
we will use this formalism to compute forward
particle production
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Single inclusive particle production
final state transverse momentum k and rapidity y
and
the partons probed in the process have
high energies and forward rapidities are needed
to probe small values of x
involves
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RdA and forward pion spectrum
RdA
first comparisons to data
Kharzeev, Kovchegov and Tuchin (2004) Kharzeev,
Levin and Nardi (2005)
more recent work from qualitative to
quantitative agreement
Dumitru, Hayashigaki and Jalilian-Marian (2006)
pT - spectrum
shows the importance of both evolutions xA (CGC)
and xd (DGLAP)
shows the dominance of the valence quarks of the
deuteron
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Two particles at forward rapidities
final state
moderate values of xd, typically 0.5 dominant
partonic process
k1, k2 gtgt ?QCD ? collinear factorization of
the quark density
y1 y2 3 both h1 and h2 in forward
hemisphere
dAu ? h1h2X
feasible in d-Au collisions at RHIC
A
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2- 4- and 6-point functions
the scattering off the CGC is expressed through
the following correlators of Wilson lines
need more than the 2-point function no kT
factorization
same conclusions in heavy-quark
production and two-gluon production
Blaizot, Gélis and Venugopalan (2004)
Jalilian-Marian and Kovchegov (2004)
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MV model BK evolution
- then I evolve the correlators to the value
of needed using BK evolution
- finally I can compute the cross-section
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RHIC vs LHC
the CGC picture would be even better tested
value of xA at LHC ltlt RHIC
but at RHIC, one has xh 0.5, and the dominant
partonic process is
RHIC
LHC
the value of xd at the LHC is slightly smaller
than at RHIC
including the gluon-initiated processes
and in the calculation will be needed
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Conclusions and Outlook

• I obtained CGC predictions (MV model BK
  evolution) for the two-particle spectrum, when
  both particles are produced at forward rapidities
• ? need to include hadron fragmentation for final
  results
• ? need comparison with standard NLOQCD
  calculations
• - the predictions for the fully differential
  spectrum are not directly comparable with the
  data (unless fully differential spectra can be
  measured)
• ? need integrations over the experimental y and
  pT ranges

future measurements
- at RHIC with the new dAu run data
- at the LHC with pPb collisions ?