Femtoscopy in heavy ion collisions: Wherefore, Whence,

1
Femtoscopy in heavy ion collisionsWherefore,
Whence, Whither?

• Mike Lisa
• Ohio State University

• Wherefore (why?)
• motivation (basic) formalism

• Whence (from where?)
• systematics over 2 decades

• Whither (to where?)
• or wither...?

MAL, Pratt, Soltz, Wiedemann Ann Rev Nucl Part
Sci 55 (2005)
http//www-rnc.lbl.gov/TBS
2
Spacetime - The characteristic of H.I.C.

• Non-trivial space-time - the hallmark of R.H.I.C.
• Initial state dominates further dynamics
• Intermediate state impt element in exciting
  signals
• Final state
• Geometric structural scale is THE defining
  feature of QGP

• Temporal scale sensitive to deconfinement
  transition (?)

3
Extraction of length scales
maximum-likelihood fit to
usually used (even for non-id)
4
Reminder

• Two-particle interferometry p-space separation ?
  space-time separation

source sp(x) homogeneity region
Sinyukov(95) ? connections with whole source
always model-dependent
Pratt-Bertsch (out-side-long) decomposition
designed to help disentangle space time
5
Disintegration timescale - INITIAL expectation
3D 1-fluid Hydrodynamics
Rischke Gyulassy, NPA 608, 479 (1996)
with transition
?
?

• Long-standing favorite signature of QGP
• increase in ?, ROUT/RSIDE due to deconfinement ?
  confinement transition
• hoped-for turn on as QGP threshold is reached

6
Two decades of systematics
Lisa, Pratt, Soltz, Wiedemann
R 5 fm

• Pion HBT _at_ Bevalac largely confirming nuclear
  dimensions
• Since 90s increasingly detailed understanding
  and study w/ high stats

7
Two decades of systematics
Lisa, Pratt, Soltz, Wiedemann

• Pion HBT _at_ Bevalac largely confirming nuclear
  dimensions
• Since 90s increasingly detailed understanding
  and study w/ high stats

8
Repeating most basic sanity check at relativistic
energies...
Forget homogeneity regions or fancy stuff. Do
femtoscopic length scales increase when they
should?
p-p correlations big bump ? small source

• Also
• SPS NA44(99),NA49(00)
• RHIC STAR(05)

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• Generalize A1/3 ?Npart1/3
• not bad _at_ RHIC!
• connection w/ init. size?

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• Generalize A1/3 ?Npart1/3
• not bad _at_ RHIC!
• connection w/ init. size?
• Heavy and light data from AGS, SPS, RHIC
• ?s-ordering in geometrical Rlong, Rside
• Mult K(?s)Npart
• source of residual ?s dep?
• ...Yes! common scaling
• final state drives radii, not init. geometry
• (breaks down ?s lt 5 GeV)

NB not constant density
LPSW nucl-ex/0505014
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We are not alone...
Entropy determines everything at bulk level
(soft sector) ? c.f. Helens talk
NB scaling violated ?s lt 4 GeV (as with
femtoscopy)
12
Refinement chemical effects

• different behaviour below/above AGS
• violates universal scaling
• baryon ? meson dominance
• neglect time/dynamics gross F.O. geometry
  appears determined by
• chemistry
• universal mean free path 1 fm (!?)

13
Messages from systematics

• AB, b, Npart systematics
• sanity check on overall size dependence ?
• final state multiplicity/chemistry determines
  rough geometry...

...and that geometry is 2x initial size
collective/flow-like expansion? ? probe
anisotropically!
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Strongly-interacting 6Li released from an
asymmetric trap OHara, et al, Science 298 2179
(2002)
What can we learn?
transverse FO shape collective velocity ?
evolution time estimate check independent of
RL(pT)
Teaney, Lauret, Shuryak nucl-th/0110037
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• observe the source from all angles with respect
  to RP
• expect oscillations in HBT radii

16

• observe the source from all angles with respect
  to RP
• expect oscillations in HBT radii (including new
  cross-terms)

R2out-sidelt0 when ?pair135º
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Measured final source shape
R2out-sidelt0 when ?pair135º
ever see that symmetry at ycm ?
model-dependent, but see Retiere MAL PRC70
044907 2004
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Measured final source shape
Expected evolution
?
model-dependent, but see Retiere MAL PRC70
044907 2004
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Evolution of size and shape - the rule of two
1/2 shape reduction
x2 size increase
Initial size/shape estimated by Glauber
calculation Final config according to Retiere
MAL PRC70 044907 2004
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Anisotropic sanity check

• non-trivial excitation function
• does it make sense? Is it related to bulk
  dynamics?
• YES

(in a toy model yes - fordiscusison if asked)
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Messages from systematics

• AB, b, Npart systematics
• sanity check on overall size dependence ?
• final state multiplicity/chemistry determines
  rough geometry...
• ?, b, ?s systematics
• sanity check on shape evolution ?
• geometric evolution consistent with collective
  bulk dynamics (flow)

look for dynamic signatures substructure

• more than T versus mass
• more than spectral shapes
• more than v2
• ...
• flow is defined by space-momentum correlations
• only femtoscopy can probe substructure
• of dynamic bulk behaviour

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Geometric substructure?
random (non-)system all observers measure
the whole source
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Flow-generated substructure
random (non-)system all observers measure
the whole source

• Specific predictions ofbulk global collective
  flow
• space-momentum (x-p) correlations
• faster (high pT) particles come from
• smaller source
• closer to the edge

24
Strong flow confirmed by all expts...
25
Flow-dominated Blast-wave model PRC70 044907
(2004)
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• Decreasing R(pT)
• usually attributed to collective flow
• flow integral to our understanding of R.H.I.C.
  taken for granted
• femtoscopy the only way to confirm x-p
  correlations impt check

• Non-flow possibilities
• cooling, thermally (not collectively) expanding
  source
• combo of x-t and t-p correlations

early times small, hot source
late times large, cool source
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• Decreasing R(pT)
• usually attributed to collective flow
• flow integral to our understanding of R.H.I.C.
  taken for granted
• femtoscopy the only way to confirm x-p
  correlations impt check

• Non-flow possibilities
• cooling, thermally (not collectively) expanding
  source
• combo of x-t and t-p correlations

28

• Decreasing R(pT)
• usually attributed to collective flow
• flow integral to our understanding of R.H.I.C.
  taken for granted
• femtoscopy the only way to confirm x-p
  correlations impt check

• Non-flow possibilities
• cooling, thermally (not collectively) expanding
  source
• combo of x-t and t-p correlations
• hot core surrounded by cool shell
• important ingredient of Buda-Lund hydro
  picturee.g. Csörgo LörstadPRC54 1390 (1996)

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Each scenario generates x-p correlations

• Decreasing R(pT)
• usually attributed to collective flow
• flow integral to our understanding of R.H.I.C.
  taken for granted
• femtoscopy the only way to confirm x-p
  correlations impt check

but
?x2?-p correlation yes ?x?-p correlation yes

• Non-flow possibilities
• cooling, thermally (not collectively) expanding
  source
• combo of x-t and t-p correlations
• hot core surrounded by cool shell
• important ingredient of Buda-Lund hydro
  picturee.g. Csörgo LörstadPRC54 1390 (1996)

?x2?-p correlation yes ?x?-p correlation no
t
?x2?-p correlation yes ?x?-p correlation no
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Flow-dominated Blast-wave model PRC70 044907
(2004)
?
K
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RHIC femtoscopy with wide variety of particle
species
R(vSNN, b, Npart, A, B, mT, y, ?, PID1, PID2)
? prelim or final result available
32
Trends, soft sector, and RHI history
A. Wetzler (2005)

• unlike hard and intermediate sectors, soft
  sector has decades of systematics

Then p-dependent potentials, surface effects,
hard-cores, absoption...
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model comparisons

• unlike hard and intermediate sectors, soft
  sector has decades of systematics
• single-point agreement a bit unsatisfying...
• femtoscopic model comparisons mostly HBT radii
  in b0 collisions
• RHIC models only expected to work at highest
  energies...

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Comparison to perfect hydro calculations

• model sensitivity (not just R5)
• Rout too big (too long emission time?)
• Rlong too big (too long evolution time?)
• Rside too small (?)

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Comparison to Boltzmann/cascade models
better! (better than perfect...?)
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Whats the difference?
freeeze-out........ EoS............viscosity
hydro
cascade
should be the same IF source is Gaussian
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Non-Gaussian Effects and Model Comparisons -
Hydro/BW
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Wherefore Whence
R(vSNN, b, Npart, A, B, mT, y, ?, PID1, PID2)

• size shape timescale estimate
• space-momentum substructure of a flowing
  systemdynamics and evolution of
• radial flow (size and shift)
• elliptic flow (shape and timescale)
• directed flow (shape and tilt) did not
  show due to time
• hadronic particle potentials / phase shifts
• Femtoscopy
• well-calibrated, well-established tool
• the one most directly connected to space-time -
  THE defining feature of R.H.I.C.
• huge set of systematics of consistent
  measurements
• sensitive to physics in models
• with care the femtoscope is a precision tool to
  apply to new systemsLHC, pp, etc

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Whither?

• non-id correlations for b?0 Retiere Lisa
  PRCPRC70 044907 (2004)
• invert the problem low-energy phaseshifts from
  exotic pairs (e.g. ?-?) talk of H. Gos P.
  Chaloupta QM05
• space-time substructure in pp ?? - direct
  comparison talk of Z. ChajeckiChajecki et
  al, WW05
• imaging - beyond sizes and shifts Brown,
  Danielewicz, Pratt, others
• first-order azimuthally-sensitive femtoscopy -
  spacetime aspects of v1Lisa et al, PLB489 287
  PLB496 1
• relative jets, spin.... (?)
• continue/expand rich program _at_ ALICE/LHC

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THEEND
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Strong flow confirmed by all expts...
Central (10) AuAu (PbPb) collisions at y0
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A simple estimate ?0 from ?init and ?final

• BW ? ?X, ?Y _at_ F.O. (?X gt ?Y)
• hydro flow velocity grows t
• From RL(mT) ?0 9 fm/c
• consistent picture
• Longer or shorter evolution times
• inconsistent
• toy estimate ?0 ?0(BW) 9 fm/c
• too short to account for expansion??
• Need a real model comparison? asHBT workable
  evolutionary clock constraint for models

MAL ISMD03
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1D projections a limited view
STAR PRC71 044906 (2005)

• Usually, quality of data and fit shown in 1D
  projections
• Narrow integration best
• limited view of data
• see talks of Adam, Scott, Sandra
• tomorrow a better way

out
Gaussian fit (remember not Gaussian CF)
side
long
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The perennial non-Gaussianness

• Source has never been fully Gaussian. c.f. J.
  Sullivan _at_ SPS
• periodically re-discovered, with little change
  information condensation needed to observe
  systematic data trends
• non-Gaussianness _at_ RHIC reported in first and
  subsequent HBT measurements
• imaging is probably best solution (but even
  then...)

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The perennial non-Gaussianness

• CF is mostly Gaussian
• STAR tried Edgeworth
• functional expansion
• (Csorgo 2000)
• among few quantitative estimates
• of non-Gaussian shape

STAR PRC71 044906 (2005)

• 20 effect in Rlong! systematic error...?
• appears fit captures dominant length scale

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Another implication of strong flow mT scaling
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• transverse shape
• non-trivial excitation function
• increased flowtime ? rounder FO geometry _at_ RHIC
• insufficient flowxtime to become in-plane

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AuAu ?sNN 2.3 GeV b?5 fm
E895, PLB496 1 (2000)
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AGS

• transverse shape
• non-trivial excitation function
• increased flowtime ? rounder FO geometry _at_ RHIC
• insufficient flowxtime to become in-plane
• Spatial orientation
• another handle on flow time
• HUGE tilts _at_ AGS!!
• RHIC?
• QGP-induced orientation?

STAR soon
?
?
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v1 predictions (QGP invoked)
x-p transverse-longitudinal coupling may be
affected in early (v1) stage
L.P. Csernai, D. Rohrich Phys. Lett. B 458
(1999) 454
J. Brachmann et al., Phys. Rev. C. 61 024909
(2000)
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AGS

• transverse shape
• non-trivial excitation function
• increased flowtime ? rounder FO geometry _at_ RHIC
• insufficient flowxtime to become in-plane
• Spatial orientation
• another handle on flow time
• HUGE tilts _at_ AGS!!
• RHIC?
• QGP-induced orientation?
• requires true 3D dynamical model (explicitly
  non-B.I.)

STAR soon ?
?
?