Transcript Review of high-pT physics at RHIC and evidences of

High-pT Physics at RHIC and
Evidences of Recombination
Rudolph C. Hwa
University of Oregon
International Symposium on Multiparticle Dynamics
Sonoma, CA, July 2004
1
In collaboration with
Chunbin Yang
Hua-zhong Normal University
Wuhan, China
2
Outline
Anomalies at high pT according to the
“standard model”
Alternative to the “standard model” at high pT
• Recombination in fragmentation
• Shower partons
• Inclusive distributions at all pT
• Dihadron correlations
All anomalies can be understood in terms of
parton recombination
3
Conventional approach to hadron production
at high pT
h
D(z)
A
q
A
Hard scattering near the surface because of
energy loss in medium --- jet quenching.
4
If hard parton fragments in vacuum,
then the fragmentation products should
be independent of the medium.
Particle ratio should
depend on the FF D(z)
only.
h
D(z)
q
The observed data reveal
several anomalies
according to that picture.
5
Anomaly #1
R
R
1
p/π
p/π
Not possible in
fragmentation model:
Dp / q  D /q
Dp / q
D / q
u
6
Anomaly #2
in pA or dA collisions
Cronin Effect
Cronin et al, Phys.Rev.D (1975)
h
q
p
A
dN

(pA  X)  A ,
dpT
 1
k broadening by multiple
T
scattering in the initial state.
Unchallenged for ~30 years.
If the medium effect is before fragmentation,
then  should be independent of h=  or p
 >
p

RHIC expt (2003)
7
RHIC data from dAu collisions at 200 GeV per NN pair
Ratio of central to peripheral collisions: RCP
dN h 1
central 
dpT N Coll
h
RCP ( pT ) 
dN h 1
 peripheral 
dpT NColl
PHENIX and STAR experiments found (2002)
p

RCP
( pT )  RCP
( pT )
Can’t be explained by fragmentation.
8
Anomaly # 2
p
RCP ( pT

)  RCP ( pT )
9
Anomaly #3
Jet structure
Hard parton  jet { (p1) + (p2) + (p3) + ···· }
trigger particle
associated particles
The distribution of the associated particles should
be independent of the medium if fragmentation
takes place in vacuum.
10
Fuqiang Wang (STAR) nucl-ex/0404010
pp
Anomaly #3
Jet structure for Au+Au collisions is
different from that for p+p collisions
11
Anomaly #4
Azimuthal anisotropy
Anomaly #5
Forward-backward asymmetry
at intermediate pT
Come back later when there is time.
Resolution of the anomalies
12
How can recombination solve the puzzles?
hadron momentum
Parton
distribution
(log
scale)
p
p1+p2
(recombine)
higher yield
p
q
(fragment)
heavy penalty
13
The black box of fragmentation
q
1
z
A QCD process from quark to
pion, not calculable in pQCD

Momentum fraction z < 1
D/q
Phenomenological
fragmentation
function
1
z
14
Let’s look inside the black box of fragmentation.
q
1
z

fragmentation
gluon radiation
quark pair creation
Although not calculable in pQCD (especially when Q2 gets low),
gluon radiation and quark-pair creation and subsequent
hadronization nevertheless take place to form pions and
other hadrons.
15
Description of fragmentation by recombination
hard parton
fragmentation
meson
shower
partons
recombination
dx1 dx 2
xD(x)  
Fq,q (x1 ,x 2 )R (x1, x 2 , x)
x1 x 2
known from data (e+e-, p, … )
can be
determined
known from
recombination model
16
Shower parton distributions
 x2 

F (x1 , x2 )  S (x1 )S 
1 x1 
(i )
qq '
d
K L
L K
j
Si  
L L
G G
q'
i
s
valence
Ls  u
Ls  d
Ks 

Gs  g
K  K NS  L
sea
Sud,d ,u,u(sea)  L
s
u
q
i
Ks  K NS  Ls
5 SPDs are determined from 5 FFs.
LL
KNS L
GG
L Ls
G Gs
R
RK





DSea
DV
DG
DKSea
DKG
17
Shower Parton Distributions
u -> u valence
g -> u
u -> d
u -> s
g -> s
Hwa & CB Yang, hep-ph/0312271
18
BKK fragmentation functions
19
Once the shower parton distributions are known,
they can be applied to heavy-ion collisions.
The recombination of thermal partons with shower
partons becomes conceptually unavoidable.
h
Conventional
approach
A
D(z)
q
A
20
Once the shower parton distributions are known,
they can be applied to heavy-ion collisions.
The recombination of thermal partons with shower
partons becomes conceptually unavoidable.
h
Now, a new
component
21

d
u
hard parton
(u quark)
22
Inclusive distribution of pions in any direction
p
dN
dp1 dp2
p

Fqq ( p1 , p2 )R (p1, p2 , p)
dp
p1 p2
p1 p2
 (p1  p2  p)
p
Pion
pT Distribution
dN
1
 3
pdp p
p
0 dp1Fqq (p1 , p  p1 )
23
Pion formation:
qq
distribution
Fqq  TT  TS  (SS)  (SS)
1
2
Proton formation:
Fuud  TTT
T
thermal
S
shower
uud distribution
 TTS  T(SS)1  (SSS)1  T(SS)2  ((SS)1S)2  (SSS)3
soft
component
soft semi-hard
components
usual
fragmentation
(by means of
recombination)
24
Thermal distribution
Contains hydrodynamical properties, not included in our model.
Fit low-pT data to determine C & T.
T( p1)  p1
dN qth
dp1
 Cp1 exp( p1 / T)
Shower distribution in AuAu collisions
S(p2 )   i  dkkfi (k)Sij (p2 /k)
fraction of hard
partons that get
out of medium to
produce shower
hard parton
momentum
distribution of
hard parton i in
AuAu collisions
calculable
SPD of parton j
in shower of hard
parton i
25
fi (k) 
hard
dN i
kdkdy
y0
density of hard partons
with pT = k
Input: parton distributions CTEQ5L
nuclear shadowing EKS98
hard scattering pQCD
Srivastava, Gale, Fries, PRC 67, 034903 (2003)
dN je t
C
K
2
d pT dy y 0
(1 pT / B) 
C, B,  are tabulated for i=u, d, s, u, d, g
K=2.5
26
2-shower partons in 1 jet:
(SS)1(p1 , p2 )     dkk fi (k)Si j (
i
p1 j' p2
)Si (
)
k
k  p1
R (p1 , p2 , p)
Di ( p / k)
2-shower partons in 2 jets:
(SS)2 ( p1 , p2 )   y    dkk fi (k )Sij (
i
p1
p
)     dk' k' fi' (k' )Si'j' ( 2 )
k
k'
i'
Negligible at RHIC, but can be important at LHC.
27
 production in AuAu central collision at 200 GeV
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
TS
Hwa & CB Yang, nucl-th/0401001, PRC(2004)
28
Proton production in AuAu collisions
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
TSS
TTS+TSS
29
Anomaly #1
Proton/pion ratio
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
resolved
30
Monte Carlo Calculation
in Coalescence model
Greco, Ko & Levai, PRL (2003)
31
Production of  and  in central Au+Au
Hwa & Yang,
nucl-th/0406072
32
Anomaly #2 d+Au collisions (to study the Cronin Effect)
peripheral
central
d
d
more T
 more TS
dN h 1
central 
dpT N Coll
h
RCP ( pT ) 
dN h 1
 peripheral 
dpT NColl
less T
 less TS
more TS

1
less TS
33
d+Au collisions
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Pions
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
No pT broadening by multiple scattering in the initial state.
Medium effect due to thermal-shower recombination.
Hwa & CB Yang, nucl-th/0403001, PRL(2004)
34
Proton
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Thermal-shower
recombination is
negligible.
nucl-th/0404066
35
Nuclear Modification Factor
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Anomaly
#2
p
RCP
 RCP
because 3q  p, 2q  
36
Jet Structure
Anomaly #3
Jet structure in Au+Au
different from that in p+p collisions
Since TS recombination is more important
in Au+Au than in p+p collisions,
we expect jets in Au+Au to be
different from those in p+p.
Consider dihadron correlation in the same jet
on the near side.
37
Trigger at 4 < pT < 6 GeV/c
p+p:
mainly SS
Au+Au:
mainly TS
fragmentation
Associated particle
q3
q4
 i  dkkfi(k)
p1 (trigger)
q1
q2
k
p2 (associated)
S(q1)T(q3 )R(q1,q3, p1)
trigger
S(q2 )T(q4 )R(q2,q4, p2 )
associated
38
There are other contributions as well.
trigger
associated
(TS)
+
[TS]
(TS)
+
[SS]
(SS)
+
[TS]
(SS)
+
[SS]
too small for Au+Au
but the only term for p+p
39
Associated particle distribution in p2
with trigger at p1
dN
dp2
p1
dN
i  dkkfi (k)
(k)
dp1 dp2

dN
i  dkkfi (k)
(k)
dp1
trigger at p1 integrated over 4 < p1 < 6 GeV/c
40
Central Au+Au collisions
+
(TS)[TS]
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
(TS)[SS]+(SS)[TS]
(SS)[SS] for p+p collisions much lower
41
Central Au+Au collisions
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Hwa & Yang, nucl-th/0407081
42
Centrality dependence of associated-particle dist.
Ratios
Distributions
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Anomaly #3
Au+Au central
d+Au
peripheral
QuickTime™
and a
p+p
TIFF (LZW) decompressor
are needed to see this picture.
d+Au central
d+Au peripheral
Jets in Au+Au and in p+p are very different.
43
Anomaly #4
Azimuthal anisotropy
v2: coeff. of 2nd harmonic of  distribution
PHENIX,
PRL 91 (2003)
v2(p) > v2()
at pT > 2.5 GeV/c
44
Molnar and Voloshin, PRL 91, 092301 (2003).
Parton coalescence implies that v2(pT)
scales with the number of constituents
STAR data
45
Anomaly #5 Forward-backward asymmetry at intermed. pT
in d+Au collisions
backward
forward
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
STAR preliminary data
46
Less soft partons in forward (d) direction
than in backward (Au) direction.
Less TS recombination in forward than
in backward direction.
It is natural for parton recombination to result
in forward-backward asymmetry
More interesting behavior found in
large pT and large pL region.
47
Summary
Traditional picture
0
2
4
6
8
soft
10
pT
10
pT
hard
pQCD + FF
More realistic picture
0
2
4
6
soft
semi-hard
(low)
(intermediate)
thermal-thermal
thermal-shower
8
hard
(high)
shower-shower
48
All anomalies at intermediate pT can be
understood in terms of recombination of
thermal and shower partons
Recombination is the hadronization process.
At pT > 9 GeV/c fragmentation dominates,
but it can still be expressed as
shower-shower recombination.
Thus we need not consider fragmentation
separately any more.
49
50
Backup slides
51
Rapidity dependence of RCP in d+Au collisions
BRAHMS
RCP < 1 at =3.2
nucl-ex/0403005
Central more suppressed
than peripheral collisions
Interpreted as possible signature of Color Glass Condensate.
52
BRAHMS
data
=0
RCP > 1
=3.2
RCP < 1
At forward rapidity, parton x is degraded (stopping)
more for central than for peripheral
x distributions at various b
accounted for by recombination
central < peripheral

RCP < 1 at large 
53
Forward RCP in d+Au collisions
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
54
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
55
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
56
Forward pT spectra in d+Au collisions
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
57
At very high pT and very high energy
like at LHC, then the density of hard
partons produced will be so high as to
make
jet-jet recombination
very important .
The effect should be dramatic.
58
near side
away side
59
PHENIX
60
Thermal partons
No prediction from our model (without hydro input)
Fit data on  spectrum for pT < 2GeV/c
C = 23.2 GeV-1 ,
T = 0.317 GeV
Shower partons

fraction of hard partons that can get out
of the dense medium to produce shower
Au+Au:  adjusted to fit the data at intermediate pT.
 = 0.07
d+Au:
 =1
61