High pT physics in the forward direction

1
High pT physics in the forward direction

• Michael Murray
• University of Kansas

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How far forward is forward?
At 200GeV the longitudinal scaling region is
about 3.1 units wide. Therefore one might say
ygt2.3 is forward at RHIC. Extrapolating to LHC
this implies ygt3.6 is forward.
?-ybeam
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What pT is high pT?
Spectra have an exponential shape below pT lt1GeV
then a power law for pT gt2 GeV
XT 2pT/vs spectra deviate from a power law for
pT below 1-2GeV
Phenix found that the index of the power law
depends on centrality
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Are definitions of high pT and forward rapidity
independent?

• For the sake of this talk I will assume that they
  are and take pT gt 1.5GeV and ygt 2.3
• These could be tested at different rapidities or
  x values by comparing spectra from NA49 and
  BRHAMS at 20, 63 and 200GeV.

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Particle ID at high momentum
BRAHMS RICH
Phenix muons 1.2ltylt2.4
STAR Lead-glass 3.4lt?lt4.0
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Can we understand forward pp physics?

• We would like to be able to explain spectra in
  terms of parton distribution functions plus a
  fragmenation scheme.

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pp? ?0 at y3-4 consistent with NLO
STAR nucl-ex/0602011
KKP frag. func. has higher g-gt? than Kretzer
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pp??, k, p _at_y3 vS200GeV
?
Brahms Preliminary
Geometrical acceptance corrections applied as
well as absorption and decay in flight. Trigger
bias (20) is also corrected. Normalization to
total inelastic cross-section 41 mb
K
BRAHMS Preliminary
p
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NLO roughly consistent with ?-/?, k-/k _at_ y3
At high pT the ?-/? ratio is consistent with
dominance of valence quarks
pp at 200GeV
The pions are not corrected for weak decays
(small).
pT (GeV/c)
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Forward p/? is large
y3.3
y3.0
p/p contradicts strong g -gt p or p fragmentation
BRAHMS Preliminary
p/?
ee- p/?
p/?-
Red proton/? Blue p/ ?-
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NLO pQCD can explain pp spectra at y3
?-
k-
pbar
p
BRAHMS Preliminary
pT (GeV)
Calc by Vogelsang
KKP FF does a better job compared to Kretzer
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Summary of forward pp results

• .
• Using KKP fragmentation functions NLO pQCD can
  describe ??-, k-. This implies a dominance of g q
  and g g processes at y3 as was found for ??0 at
  y0.
• This dosent work for protons at y3. The large
  p/?, at high pT is interesting.

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For AA life is more complicated

• What is the initial state of the nucleus?
• How important is energy loss at forward angles?
• What about radial, elliptic and directed flow?
• Does recombination of quarks dominate high pT

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Probing the initial state of the nucleus
Increasing y or vs
Increasing pT
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Suppression in dAu at Forward Rapidities
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Energy Loss in hot medium
X. N. Wang et al.
The number of interactions (each emitting a
gluon) depends on the density of the medium.
Gluon density of the formed medium (rapidity
dependent)
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At vs 200GeV, RAA is independent of ?
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But dN/dy falls by factor of 2
In the parton-hadron duality approach, each
parton produces one hadron.
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AuAu vs CuCu
?3.2
h 3.2
AuAu
CuCu
Now when we explore the forward CuCu system it
behaves similarly to the AuAu.
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RAA for ?, p independent of y
p
?
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RAA at y3 increases with mass
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RAA vs. r L dL, for 200GeV AuAu y0
PHENIX changes L by varying centrality and
reaction plane.
?
I
I
I
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What controls RAA?
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Brahms see a rough scaling with energy density
ET(dN/dy)/?R2
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At 63GeV AuAu Rcp drops near kinementic limit
Brahms Preliminary
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AuAu 200 GeV v2 vs ?
CHARGED HADRONS
Underlying spectra seem to be affecting the v2
dependence on h.
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Recombination versus Fragmentation
dN dq
log
hadron momentum
suppressed by power-law
p
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Pbar/?- falls with rapidity
The shapes of the pT dependence have been
reproduced by calculations involving
recombination.
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p/ ?- increases with centrality
In recombination model probability for N quarks
to combine increases as densityN.
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Recombination may explain this
Hwa et al
P ?
pT
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P/? versus system at 200GeV
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Pbar/? for depends only on Npart
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p/ ?- versus Npart
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Recombination
Dramatic difference between y0 and 3. There are
2.5 more protons than pions at 2.5 GeV/c
Forward production of protons is favored in
recombination because of scarcity of anti-quarks
R. Hwa et al. nucl_th/0605037
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Pushing the kinematic limit
If these particles have pT gt 350 MeV they are
beyond the kinematic limit. This would be strong
evidence for recombination.
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Beyond the kinematic limit at 62GeV
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CMS TOTEM forward detectors

• HF,CASTOR,ZDC TOTEM Quasi-full acceptance at
  LHC

HF
CMS IP
ZDC
RPs_at_220m
RPs_at_150m
T1/T2, Castor
5.32 ? 6.86
Acceptance (neutral) ?lt(5.cm) /140.m lt400 µrad
? gt 8.5 , pT lt O(2. GeV/c)
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Alice, LHCb

• Good capabilities for heavy-Q,
• QQbar at low-x

2.5lt ?lt 4
2 lt ? lt 5
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Summary

• Forward measurements suggest an interplay of
  energy loss and initial state effects
• It is possible that recombination will be able to
  explain data
• One needs to carefully understand flow
• LHC offers many possibilities in 2008

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Backup slides
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Particle Nuclear Modification Factors
200 GeV
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Comparing two systems at low energy near
mid-rapidity
These results are consistent with weak effects of
a dense medium and a return of strong SPS Cronin
type enhancements
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p/p ratio integrated over 1.5ltpTlt2.5 GeV/c
at forward rapidity increase with centrality
smaller dependence seen at midrapidity
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pion
kaon
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