Post on 02-Jan-2016
description
Large p/ Ratio without Jet Correlations at RHIC and
LHC
Rudolph C. HwaUniversity of Oregon
November 14-20, 2006
Shanghai, China
The Omega challenge
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• In regions where fragmentation of jets is dominant
Dp = D R(p / ) = 1
• In regions where recombination is dominant
R(p /) ≈1
• In regions where is suppressed, compared to
q q
R(p / ) ? 1
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Subjects of this talk
Where R(p / ) ≈or ? 1
1. At midrapidity in central collisions
2. Strange hadron production
3. Forward production at RHIC: hard to find qbar at large x
4. pT<20 GeV/c at LHC: 2-jet recombination
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At a STAR collaboration meeting at BNL in Feb 2006, I showed a slide concluding the discussion on Omega production --
However, it is more urgent to answer the Omega challenge presented by the STAR data at this meeting.
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A prediction that can be checked now!
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Since shower partons make insignificant contribution to production for pT<8 GeV/c, no jets are involved.
Select events with or in the 3<pT<6 region, and treat them as trigger particles.
Thermal partons are uncorrelated, so all associated particles are in the
background.
Predict: no associated particles giving rise to peaks in , near-side or away-side.
The details behind this prediction will be presented by C.B.Yang in a parallel talk in 3.1.
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STARRuan (Tuesday, plenary) Barranikova (Wed, plena.) Bielcikova (Sunday, 3.1)At face value the data falsify the prediction and discredits RM.
Phantom jet
I now explain why the prediction was wrong and how the data above can be understood.
Recombination still works, but we need a new idea.
Yang’s talk tomorrow is still right.
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The core issue is the (seemingly) contradictory phenomena:
(1) means that there is no contribution from hard scattering, which is power-law behaved; hence, there is no jet.
The resolution is to recognize that it is a phantom jet.
(2) means that there is jet structure.
(1) spectrum is exponential up to 6 GeV/c.
(2) triggered events have associated particles.
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3<pt,trigger<4 GeV
pt,assoc.>2 GeVAu+Au 0-10%
preliminaryCalderon showed on Tuesday
preliminaryJet+Ridge ()Jet ()Jet)
yie
ld,
)
Npart
But p/ ratio depends on centrality.
A lot of action is going on in the ridge!
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J. Putschke, QM-1.3Jet+Ridge on near side
J
R≥1
Unidentified charged hadron
Jet+ridgeJet+ridge Jet onlyJet only
J/R~10-15%J/R~10-15%
trigger even lower!
J. Bielcikova (HP06) at lower pt(assoc)
J
R< 0.1
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So the ridge is important.
Radial expansion does not broaden the ridge under the peak in
The ridge has been interpreted as the recombination of enhanced thermal partons due to the energy loss to the medium by the passage of hard parton.
Longitudinal expansion results in broad ridge
Chiu & Hwa, PRC 72, 034903 (2005)
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Pedestal (ridge) in
P1,2 = dp2pmin(1,2)
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∫dN(T'T'−TT)
dp2|trig
2 < p2 < 4 GeV/c, P1 = 0.04
P1
T'(q) =Cqe−q/T '
T ’ adjusted to fit pedestal T = 15 MeV/c
That is for unidentified charged hadron trigger.Now, for triggered events, there is not enough statistics to separate Jet from Ridge.
I expect J/R<<1 for pT(assoc) as low as 1.5 GeV/c.
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Thus we have a ridge without any significant peak on top. The ridge would not be there without a hard scattering, but it is not a usual jet, because it contain no shower partons, only thermal partons.
Phantom Jet
One can see the usual peak when pT(assoc) is increased, and the ridge height will decrease.
When pT(trig) is low, and the trigger is , it is not in the jet, since s quark is suppressed in the shower partons.
The s quarks in the ridge form the .
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Resolution of the puzzleThe ridge contains thermalized partons:
u, d, s Hence, sss recombine to form the trigger .Other partons can form the
associated particles.(1) The pT distribution of is exponential.
(2) There are associated particles.
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The looks like a peak, but it is all ridge.Our earlier prediction that there is
no jet is still right, if ‘jet’ is meant to be the usual jet.
But we were wrong to conclude that there would be no associated particles, because a phantom jet is associated with the and it is the ridge that sits above the background.
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Since is among the particles in the ridge and is formed by TTT recombination, everything calculated previously remains valid, as to be reported by C.B. Yang.
See talks by J. Bielcikova, S. Blyth, and C.B. Yang in session 3.1 tomorrow.
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Predictions for triggered events:
• The ridge should be found in .
• The ridge has abundant u, d, s. So the associated particles
should have the characteristic feature of recombination, i.e., large p/ and /K ratios, ~O(1).
• Since the ridge arises out of enhanced thermal partons, the associated particles should have exponential pT distribution.
End of excursion to
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Forward Production
Au + Au→ h+ X
BRAHMS has data at √s=62.4 GeV
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Forward Production
Au + Au→ h+ X
BRAHMS has data at √s=62.4 GeV
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Only thermal partons contribute at large pT distribution is exponential
At large hard scattering with large pT is suppressed at kinematical boundary. Since there are no hard partons to generate the usual jets or phantom jets, there is no jet structure, not even ridges.
There should be no partners associated with triggers at pT≈2.5 Gev/c.
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Hwa & Yang, nucl-th/0605037
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Hwa & Yang, nucl-th/0605037 (to be revised)
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Two-jet recombination at LHC
New feature at LHC: density of hard partons is high.
High pT jets may be so dense that neighboring jet cones may overlap.
If so, then the shower partons in two nearby jets may recombine.
2 hard partons
1 shower parton from each
p
Hwa & Yang, PRL 97, 042301 (2006)
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Proton-to-pion ratio at LHC -- probability of overlap of 2 jet cones
single jet
(pT)~pT-
7
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The particle detected has some associated partners.
There should be no observable jet structure distinguishable from the
background.
10 < pT < 20 GeV/c
But they are part of the background of an ocean of hadrons from other jets.
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Summary
For both (a) forward production at RHIC
and (b) 10<pT<20 GeV/c at LHC,
we expect large p/ ratio and no associated particles above background.
The puzzle is resolved by recognizing the existence of ridge (without the usual jet) that constitutes the observed associated particles, while keeping the exponential pT dist of .
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Back-up slides
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Forward production of hadrons
PHOBOS, nucl-ex/0509034
Without knowing pT, it is not possible to determine xF Back et al, PRL 91, 052303
(2003)
' = η − ybeam
But now BRAHMS has pT distribution
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Forward ProductionAu + Au→ h+ X BRAHMS has data
at √s=62.4 GeVWhat is significant about it?
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BRAHMS, nucl-ex/0602018
AuAu collisions
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TT
TS
TTT
xF = 0.9
xF = 0.8 TFR
xF = 1.0
?
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Theoretically, can hadrons be produced at xF > 1?It seems to violate momentum conservation, pL > √s/2.
In pB collision the partons that recombine must satisfy
xii∑ <1
p
B
But in AB collision the partons can come from different nucleons
BA
xii∑ >1
(TFR)
In the recombination model the produced p and can have smooth distributions across the xF = 1 boundary.
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proton
pionHwa & Yang, PRC 73,044913 (2006)
: momentum degradation factor
• not constrained by data
• no regeneration of soft partons
• pT distribution not studied
• p/ to be determined
TRFFR
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Many issues to consider about forward production
2. Momentum degradation of partons in traversing nuclear medium. (baryon stopping in pA collisions) 3. Regeneration of soft partons and gluon conversion.4. More than one forward nucleons can contribute to the formation of a hadron at large x. (x>1 is possible)5. Transverse momentum distribution near the kinematical boundary. (suppression of hard scattering)6. Large p/ ratio.
7. Correlation at large .
1. Trans-fragmentation region (TRF) xF>1
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Forward production at low pT is not a process that can be studied in pQCD.
A model is needed to treat momentum degradation: valon model.
y 'G 'ν (y') = dyG(y)δ(y'yy'
1
∫ − ν )
1
NA
y’y
v
G 'ν (y') = G'νν=0
∞
∑ (y')ν ν
ν!e−ν
⎛
⎝⎜⎞
⎠⎟Poissonian average over ν
Fνq(x) = dy'G'ν∫ (y')K
xy'
⎛⎝⎜
⎞⎠⎟
x
Quark distribution after degradation
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Regeneration of soft partons and gluon conversion
Sum of parton momentum fractions:
%KNS (2)+ 2[2 %Lq(2) + %Ls(2)] + %Lg(2) =1
Gluon conversion
%KNS (2)+ 2[2 %L 'q(2) + %L 's(2)] =1
Momentum lost after ν collisions:
%KNS (2)+ 2[2 %L"q(2) + %L"s(2)] =1+ (1−
ν )
Valence quark
u,d sea quarks
momentum increase for sea quarks
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Contribution from different forward nucleons
H pAB =x
dNp
dx=
dx1x1∫
dx2x2
dx3x3
F(x1,x2 ,x3)Rp(x1,x2 ,x3,x)
xi <1 x1 + x2 + x3 =x>1
Fuud (x1, x2 , x3)=Fνu(x1)Fν
u(x2 )Fνd(x3)
HAB =x
dN
dx=
dx1x1∫
dx2x2
F(x1,x2 )R (x1,x2 ,x)
Fqq (x1, x2 )=Fνq(x1)Fν
q(x2 )
q is suppressed in forward region, so production is also suppressed.
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Proton Pion
Hwa & Yang, nucl-th/0605037
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Transverse momentum distribution
x
pT
dNhdxdpT
=Hh(x, )Vh(pT )
Hard-scattered partons near kinematical boundary is suppressed.
Thermal partons
T pT( ) =pT
dNth
dpT
=CpTe−pT /T
Cq =Cq in central region: chemical equilibrium
In forward region:
Cq ∝ Fνq(x, ), Cq ∝ Fν
q(x, )
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dNhpTdpT
=Hh(1)(pT ) + (pT )Hh
(2)(pT )
overlap probability
dplpll
∏⎛
⎝⎜⎞
⎠⎟∫ Fii '(k,k'; p1, p2 ,[ p3 ])Rh(p1, p2 ,[ p3 ]; pT )
Hh(2)(pT )=ξ2 dkdk'kfi (k)k' fi '∫
i,i '∑ (k')
Sij p1
k⎛⎝⎜
⎞⎠⎟Si '
j ' p2k'
⎛⎝⎜
⎞⎠⎟
pion p1 + p2 =pT
Given pT , k and k’ can be smaller, thus enhancing
fi(k)fi’(k’).Effect is even more pronounced for proton formation.
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(pT ) : pT−7
Limiting distribution for 1-jet fragmentation
(pT ) → 0
Does not approach limiting dist. for 1-jet
Fragmentation of a parton to a proton has very low probability, but recombination of shower partons from two jets increases the yield.