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The away-side ridge in two-dimensional correlation data from STAR. Lanny Ray University of Texas at Austin STAR Collaboration. Our Philosophy and Challenge. The philosophy behind the analysis presented here is based on an old and simple idea: - PowerPoint PPT Presentation
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The away-side ridge in two-dimensional correlation data from STAR
Lanny RayUniversity of Texas at Austin
STAR Collaboration
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Outline: Definitions p-p and QCD Centrality evolution Same-side structures Away-side ridge Implications
The philosophy behind the analysis presented here is based on an old and simple idea:• measure p-p and try to understand those data in terms of QCD, PDFs, FFs.• then move to p+Au (d+Au) and understand those data (initial state, sequential scattering effects)• then move to Au-Au and see if the data require anything else• interpret data with new physics mechanisms, beyond that in p-p, p-A only when the data demand it.
This is not a new idea, e.g. in Sept. 1992 at a STAR collaboration meetingMiklos Gyulassy outlined this very program.
I hope that this workshop will live up to its title of critically assessing the data and itsInterpretation.
Our Philosophy and Challenge
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measures number of correlated pairs per final state particle
Event 1
Event 2
ρsibling(p1,p2)
ρreference(p1,p2)
Correlation measure
1),(),(
baba
dddN
ref
sibch
ref
normalized ratio of 2D binned histograms;
acceptance cancellation;two-track ineff. corrections
square-rootof ref(a,b);
(for space)
Fill 2D histograms (a,b), e.g. (1,2), (1,2), (1-2,1-2), (pt1,pt2), etc.
ρ(p1,p2) = 2 particle density
Motivated by p-p superposition null hypothesis
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SS - same side
AS - away side
LS – like sign US – unlike sign
Two-component soft + semi-hard structure
away-side bumpat 1 GeV/c is
independent ofcharge pair
same-side bump at 1 GeV/c for US;soft structure varies
√
ref
√
ref
0.151.0
10.0
p t (GeV/c)
√
ref
√
ref
Transverse momentum correlations in p-p
ref
200 GeV p-p minbiastt yp
transverse rapidity
}/)ln{( 0mpmy ttt
STAR Preliminary
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yt1
yt2
refρΔρ
p-p transverse correlations
ηΔ
φΔ
p-p axial correlations
refρΔρ
semi-hard component
ηΔ
φΔ
refρΔρ
soft component
ηΔ
φΔ
refρΔρ
Longitudinal Fragmentation: 1D Gaussian on ηΔ
HBT peak at origin, LS pairs only
This looks like a jet – but is it?2D Gaussian + away-side ridge
STAR Preliminary21
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Angular correlations for p-p
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“Jets” in UA1Energy clusters were measured by UA1 down to 5 GeV/c in p-pbar collisions at sqrt{s} = 200 – 900 GeV; accounted for with pQCD; back-to-back angular correlations and pt structures are “jet-like.”
C. Albajar (UA1) Nucl. Phys. B 309, 405 (1988)
pQCD calc.
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Relation of UA1 jets to p-p correlationsSTAR observes angular correlated charged hadron pairs with pt ~ 1 – 1.5 GeV/ccorresponding to typical parton pt of order >(3/2)(2)(1 – 1.5) = 3-4.5 GeV/c,well within reach of the UA1 data and pQCD descriptions.
X.-N.Wang and M. Gyulassy implemented the UA1 observations into their Monte Carlo code HIJING (PYTHIA) (Phys. Rev. D 44, 3501 (1991)) using standard PDFs, pQCD parton-parton cross sections, and standard fragmentation functions. The low-pt parton energy follows a power-law until a cut-off at Q0 ~ 2 GeV. The majority of produced jets have Q0 ~ few GeV and only produce a few hadronic fragments.
In Au-Au we cannot hope to identify these low Q0 jets directly. There is alsono identifiable trigger particle at lower pt. Thus we use both angular and (pt1,pt2)correlations for all pairs as suggested by Xin-Nian Wang a long time ago (Phys. Rev. D 46, R1900 (1992)). These soft, untriggered jets are known in the literature as minijets.
How would minijets appear in our 2D () angular correlations?
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proton
proton
NN cmparton-parton cm
Jet-A
Jet-B
0
Num
ber
of p
airs
A-AB-B
A-BB-A
Num
ber
of p
airs
0
A-AB-BA-B B-A
sum overmany events
small angle peak
large azimuth peakΦ1 - Φ
2
η1 - η2
(n)ρΔρ(n)
ref
p-p 200 GeVSTAR preliminary
2121 ,
Example: di-minijets
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p + p at 200 GeVMinijets in HIJING & PYTHIA
We conjecture that the bump at 1 GeV/c in the (yt,yt) and the above angular correlations are generated by the fragmentation of semi-hard scattered partons.
These minijets are simply jets with no low pt cut-off.
soft pt pairsremoved
STAR Preliminary
HijingJets on
HijingJets off
additionalsharp peak:
HBT, conversionelectrons
ref 4
ref
ref
~20% agreement
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Au-Au centrality evolution of the 2Dcorrelations
(Expectation is that minijets will be thermalized.)
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(yt1,yt2) correlations for same-side pairs
Like Sign
Unlike Sign
STAR Preliminary
Au-Au 200 GeV
HBT
peripheral
peripheral
central
central
pp
pp
minijets persist;pt dissipation
From M. Daugherity’s Ph.D Thesis (2008)
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(yt1,yt2) correlations for away-side pairsAu-Au 200 GeV
Like Sign
Unlike Sign
pp
pp
STAR Preliminary
peripheral
peripheral
central
central
minijets persist;pt dissipation
minijets
From M. Daugherity’s Ph.D Thesis (2008)
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84-93%
28-38%
74-84%
18-28%
64-74% 55-64% 46-55%
9-18% 5-9% 0-5%
proton-proton
note: 38-46% not shown
We observe the evolution of several correlation structures including the same-side low pt ridge
refρΔρ
ηΔ
φΔ
ηΔ
φΔ
refρΔρ
Analyzed 1.2M minbias 200 GeV Au+Au events; included all tracks with pt > 0.15 GeV/c, |η| < 1, full φ
STAR Preliminary
200 GeV Au-Au data
From M. Daugherity’s Ph.D Thesis (2008)
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Proton-Proton fit function
= +
STAR Preliminary
longitudinal fragmentation1D gaussian
HBT, e+e-2D exponential
refρΔρ
refρΔρ
refρΔρ
ηΔφΔ ηΔ
φΔ ηΔφΔ
Au-Au fit function Use proton-proton fit function plus cos(2φΔ) quadrupole term (~ elliptic flow).
Note: from this point on we’ll include entire momentum range instead of using soft/hard cuts ηΔ
φΔ
dipole
quadrupolecos(2φΔ)
Fit function
Same-side “Minijet” Peak, 2D gaussian
Away-side -cos(φ)
“soft” “hard”
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Same-side correlation structure
< /2
The “soft” ridge
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)1(1
)(
xAA ppAA
Deviations from binary scaling represent new
physics unique to heavy ion collisions
Binary scaling: Kharzeev and Nardi model
200 GeV62 GeVrefρ
Δρ
constant widths
STAR Preliminary STAR Preliminary STAR PreliminaryPeak Amplitude Peak η Width Peak φ Width
binpppartpp
bin
chrg
binAA xNnNxn
NNNA
2/)1( Amplitude
Same-side 2D Gaussian & binary scaling - AuAuStatistical and fitting errors are~5-10%
Systematic error is 9% of correlation amplitude
2/part
bin
NN
peripheral central
HIJING 1.382 default parameters, 200 GeV, quench off
172D Gaussian amplitude, -width, volume scale with particle density in Au-Au
Peripheral bins are compressed.
Peak Amplitude
Npart Npart
Npart
Peak η WidthSTAR PreliminarySTAR Preliminary
200 GeV62 GeV
Depends on formation time (assumed 1 fm/c), difficult to compare energies.
εBJ εBJ
Peak Amplitude Peak η WidthBjorken Energy Density
STAR PreliminarySTAR Preliminary200 GeV62 GeV
Transverse Particle Density
Peak amplitude width aspect ratio volume
Sd
dNch /23~
S = overlap area(Monte Carlo
Glauber) STAR Preliminary
Does the transition point scale?
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Does the transition from narrow to broad ηΔ occur quickly or slowly?
data - fit (except same-side peak)
Shape changes little from peripheral to the transition
The transition in same-side ridge occurs quickly
STAR Preliminary83-94% 55-65%
Large change within ~10%
centrality
46-55%
Smaller change from transition to most central
low-pt manifestation of the “ridge”
0-5%
ηΔ width
Transition – close-up (Au-Au 200 GeV)
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Expected behavior:
Comparison with data:
Implications: superposition model
Minijet shape unchanged, amplitude follows binary scaling.
Minijet peak on (yt,yt) unchanged except for amplitude.
Superposition model approximates data to the transition point,implying an approximately transparent system,
but radically fails at higher density, more central collisions.
STAR Preliminary
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Minijets & Quadrupole
Below the transition the Au-Au system appears transparent, i.e. no collisional pressure build-up, no flow,
at least up to the transition point.If a few secondary collisions (LDL) produce v2 in peripheral collisions, why aren’t the minijets affected (same pt range)?
What mechanism(s) produces a smooth v2?
2/ amplitude quadrupole 22vNch
expected v2?
STAR Preliminary
STAR Preliminary
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Away-side correlation structure
> /2
The away-side ridge
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The dipole matches the centrality dependence of the same-side Gaussian and shows the same transition point. It’s origin is pt conservation: global + jets
Away-side ridge (dipole) – local pt conservation
calculated global pt conservation
Low-x
parto
n
KT ~ 1 GeV/c
p z
Q ~ 2 GeV minijets, nucleon KT , acoplanarity
Low-x parton
200 GeV62 GeV
φΔ
0
0
φΔ-3 - 3
KT broadening
)cos( 0
events1,2,3…
sumevents
away-side
STAR Preliminary
Hijing – jets on(no soft pt)
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pt correlations remain (yt,yt) dissipates but amplitude remains at minijet yt
same-side 2D Gaussian remains However, same-side yield decreases unless enough hadrons from surface are correlated with minijet. Some jets will lose away-side partner, reducing –cos() away-side pt escapes, but is dispersed among many more pairs
Implications: opaque core + hadronic coronaExpected behavior:
Comparison with data:
Ratio of away-side ridgeto same-side Gaussian
is ~constant from peripheralto most-central;
data are inconsistent with this scenario
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Mach Cone ?Au+Au 200 GeV 0-5% most-central
Shown are Data – Quadrupole whichisolates the away-side dipole structure
STA
R P
relim
inar
y
No indication of a double away-side ridge – no Mach cone
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Please stop using ZYAM!ZYAM method assumes non-overlapping peaks above background – questionable
Example: fake “data” generatedby same-side Gaussian + dipole
v2{2} obtainedfrom due to SS Gauss.
)2cos(
ZYAM proceduresubtracts v2 and adjusts offset.
ZYAMdeducedcorrelation
wrong amplitude!spurious shape!
nonphysical
Always plot the raw correlation data v2 is being over-subtracted; v2{2} or [v2{2} + v2{4}]/2 includes SS peak contribution, i.e. non-flow, which is larger at higher pt where this analysis is usually done.
Plots from Tom Trainor
ZYAM
input correlation
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Summary and Conclusions Correlation structures in minbias p-p collisions can be understood with pQCD, PDFs, and standard FFs.
Minijets (jets with no low-pt cut-off) are an essential component of RHIC collisions and are manifest in (yt,yt) and 2D angular correlations.
Up to the transition minijets escape from the entire collision volume (binary scaling) with little broadening as if from a transparent medium.
The transition, observed in the same-side peak, is also observed in the away-side ridge.
The transition is not manifest in v2 (quadrupole) implying that v2 is not affected by post-impact rescattering.
The minimum-bias away-side correlations do not show evidence of a double-hump structure reported in leading particle-associated analyses.