a short introduction to hbt qinghui zhang
DESCRIPTION
A Short introduction to HBT Qinghui Zhang. Heavy Ion Collisions requires understanding particle distributions in coordinate and momentum space. GGLP Goldhaber, Goldhaber, Lee and Pais (1960) First application of “intensity interferometry” in particle physics HBT - PowerPoint PPT PresentationTRANSCRIPT
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A Short introduction to A Short introduction to HBTHBT
Qinghui ZhangQinghui Zhang
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Heavy Ion Collisions requires understanding Heavy Ion Collisions requires understanding particle distributions in coordinate particle distributions in coordinate and and
momentum spacemomentum space
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GGLPGGLP Goldhaber, Goldhaber, Lee and Pais Goldhaber, Goldhaber, Lee and Pais
(1960)(1960) First application of “intensity First application of “intensity
interferometry” in particle physicsinterferometry” in particle physics HBTHBT
Hanbury-Brown and Twiss (1950’s)Hanbury-Brown and Twiss (1950’s) Revolutionary development of Revolutionary development of
intensity interferometry in astronomyintensity interferometry in astronomy B-EB-E
Bose-Einstein (1920’s)Bose-Einstein (1920’s) Role of Bose statistics in fluctuationsRole of Bose statistics in fluctuations
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NeglectsNeglects Momentum dependence of sourceMomentum dependence of source Quantum mechanics Quantum mechanics up to up to x and yx and y Final State Interactions Final State Interactions afterafter x and y x and yC2(q) contains shape informationC2(q) contains shape information
y
X
1
2
Source
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qOUT R
Sphere vs hemisphere
That is, squared Fourier Transforms measure (only) relative separations of source coordinates
Very different sources (x) can give very similar distributions in relative separation
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)(
),(),(
)()(,|)(|1)(
0
0212121
422
rqtqxq
qqppEEppq
xdexqqqC iqx
Fourier Transform provides one time and three space extensions of source
BUT:
That is, q0 is not an independent quantity in the F.T.(e.g., note that )
Fourier Transform has support in only half of the q0-q plane
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xdexVqq
tVrqrqtVqxq
Vqq
xdexqqqC
tVrqiPAIR
PAIRPAIR
PAIR
iqx
PAIR 4)(
0
422
)();()(
)()(
)()(,|)(|1)(
So Fourier transform (+ two-particle kinematics) already provides conclusion:
22
22 )();(|)(|1)( tVrVqCqqC PAIRPAIR
Additional dynamic effects (expansion, thermal source, …) will also lead to systematic dependence of extracted “radii” on VPAIR (or mT or kT or …)
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There are other decompositions...There are other decompositions...
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)])((2
1exp[);(
)](2
1exp[~),(
22222222
2
2
2
2
2
2
2
2
PAIRzzyyxxPAIR
zyx
VqRqRqRqVq
t
R
z
R
y
R
xtr
Simplest possible (yet still very useful) case:Simplest possible (yet still very useful) case:
)])((2
1exp[);(
)](2
1exp[~),(
22222222
2
2
2
2
2
2
2
2
PAIRzzyyxxPAIR
zyx
VqRqRqRqVq
t
R
z
R
y
R
xtr
Choose frame so that x = “Out” (that is, parallel to VPAIR) y = “Side” z = “Long”:
y
z x
222222
2222222222
22222222
""
))((
ySIDEPAIRxOUTxy
PAIRxOUTOUTOUTLONGLONGSIDESIDE
PAIRzzyyxx
RRVRRRRSymmetry
VRRwithRqRqRq
VqRqRqRq
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Most experiments with charged Most experiments with charged tracking and particle identification have tracking and particle identification have HBT results:HBT results:
BNL AGS:BNL AGS: E859, E866E859, E866
E877E877
E895E895 CERN SPS:CERN SPS:
NA44NA44
NA49NA49
WA98WA98
RHICRHIC
PHENIX STARPHENIX STAR
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The Great Puzzle: Why so little variation with ECM ? Why are lifetimes () ~ 0 ??
( especially at RHIC )
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systematic errors in experimental data is systematic errors in experimental data is required:required: Definition of CDefinition of C22
Coulomb correctionsCoulomb corrections Role of Role of , dependence of R on same, dependence of R on same Contributions from resonancesContributions from resonances
(especially when comparing between (especially when comparing between different experiments)different experiments)
A better understanding of theoretical A better understanding of theoretical assumptions and uncertainties is assumptions and uncertainties is required:required: Resonance contributionsResonance contributions Lorentz effectsLorentz effects Modeling of expansion, velocity profilesModeling of expansion, velocity profiles
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Our favorite (charged) bosons never Our favorite (charged) bosons never have pure plane-wave stateshave pure plane-wave states Even our ideal-case Fourier transform Even our ideal-case Fourier transform
becomes a “Coulomb transform”becomes a “Coulomb transform” This particular case is easy to treat This particular case is easy to treat
analytically via “Gamow” or “Coulomb” analytically via “Gamow” or “Coulomb” corrections:corrections:
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Some indication from both AGS and SPS Some indication from both AGS and SPS that Rthat RSideSide(--) > R(--) > RSideSide(++)(++) E877 (AGS): RE877 (AGS): RSideSide(--) ~ (1.5 +/- 0.4)*R(--) ~ (1.5 +/- 0.4)*RSideSide(+(+
+)+) NA44 (SPS): NA44 (SPS):
RRTSideTSide(--) > R(--) > RTOutTOut(--)(--)
RRTSideTSide(++) < R(++) < RTOutTOut(++)(++)
- Ze
Ze-
Note external Coulomb
•Systematic in RSIDE
•Cancels in ROUT
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2)0(
1)0()()(
2
4
qC
qxdexq iqx
This is parameterized away via C2(q) = 1+| (q)|2 , ~0.1-0.5
“Understood” via assumed resonance contributions:
versus observed value of 1.1-1.5
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How do resonances affect How do resonances affect ? ? By creating a “halo” around the source By creating a “halo” around the source
“core”:“core”: Some culprits:Some culprits:
(( = 150 MeV = 150 MeV S ~ S ~ 1.3 fm )1.3 fm )
KKKK ( ( = 50 MeV = 50 MeV S ~ 4 S ~ 4 fm ) fm )
(( = 8.4 MeV = 8.4 MeV S ~ 20 S ~ 20 fm ) fm )
(( = 0.2 MeV = 0.2 MeV S ~ 200 S ~ 200 fm ) fm )
(and many more…)(and many more…)
Effect: CEffect: C2 2 develops structure near q=0 develops structure near q=0
to describe the various sources:to describe the various sources:
fCORE fCORE 2(1-fCORE )fCORE (1-fCORE )(1-fCORE )
0
1
2
0 50 100
q (MeV/c)
C2(
q)
Core
Halo
Combination
]GeV/c[%)1(~%)1(~ pp
p
Momentum resolution
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Use ~this in cascade codes
Using Wigner function:Using Wigner function: See for example:See for example:
Chao, Gao and ZhangChao, Gao and Zhang
Phys. Rev. C 49,3224 (1994)Phys. Rev. C 49,3224 (1994)
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Expect CExpect C22 to depend on q to depend on q andand K K
B.R. Schlei and N. Xu:B.R. Schlei and N. Xu:
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Final state phase space points xi are
weighted by 1 + cos[(pa-pb)(xa-xb)]
RQMD vs. Data for HBTRQMD vs. Data for HBT
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This approach isThis approach is PlausiblePlausible WRONGWRONG
Final state phase space points xi are
weighted by 1 + cos[(pa-pb)(xa-xb)]
Advantage: Uses precisely what codes produce
Disadvantage Can lead to non-physical oscillations
in C2 First noted empirically by Weiner et al
and Later matin et al. Theoretical analysis by Q.H. Zhang et
al. Phys. Lett. B 407, 33 (1997)
Quantitatively, is it significant for heavy ion collisions? Answer 1: No (Empirical) Answer 2: Not yet? (see next slide)
with
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Study these effects with a source that Study these effects with a source that explicitly parameterizes x-p explicitly parameterizes x-p correlations:correlations:
Examples for R=5fm, P=200 MeV/c
-10 -8 -6 -4 -2 0 2 4 6 8 10-500
-400
-300
-200
-100
0
100
200
300
400
500
R (fm)
0
1
2
0 100 200q (MeV/c)
C2(
q)
Naive Correct Usual (used in MC's)
S=0.33
-10 -8 -6 -4 -2 0 2 4 6 8 10-500
-400
-300
-200
-100
0
100
200
300
400
500
R (fm)
0
1
2
0 100 200q (MeV/c)
C2(
q)
Naive Correct Usual (used in MC's)
S=0.9
-10 -8 -6 -4 -2 0 2 4 6 8 10-500
-400
-300
-200
-100
0
100
200
300
400
500
R (fm)
0
1
2
0 100 200q (MeV/c)
C2(
q)
Naive Correct Usual (used in MC's)
S=0
x
K
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Q. Can one remove these pathologies Q. Can one remove these pathologies from ~classical predictions?from ~classical predictions?
A. Yes (Q.H. Zhang A. Yes (Q.H. Zhang et alet al.): .): Smear the phase space points (x,p) with Smear the phase space points (x,p) with
minimum uncertainty wave-packets:minimum uncertainty wave-packets:
with with ~ 1 fm ~ 1 fm Pictorially:Pictorially:
-10 -8 -6 -4 -2 0 2 4 6 8 10-500
-400
-300
-200
-100
0
100
200
300
400
500
R (fm)-10 -8 -6 -4 -2 0 2 4 6 8 10
-500
-400
-300
-200
-100
0
100
200
300
400
500
R (fm)
=
- 1 0 - 8 - 6 - 4 - 2 0 2 4 6 8 1 0
- 5 0 0
- 4 0 0
- 3 0 0
- 2 0 0
- 1 0 0
0
1 0 0
2 0 0
3 0 0
4 0 0
5 0 0
R ( f m )x This can be done analytically with
parameterization on previous slide:
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What are the Lorentz properties of CWhat are the Lorentz properties of C22(q)?(q)?
How to write our favorite practice Gaussian in an explicitly Lorentz invariant way?
Answered in F.B. Yano and S.E. Koonin, Phys. Lett. B78, 556 (1978).
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Can Lorentz effects “distort” information?Can Lorentz effects “distort” information? Apply Yano-Koonin-Podgoretzsky result to Apply Yano-Koonin-Podgoretzsky result to
another toy model:another toy model:
p1
p2
VPAI
RBeamaxis
uS
Apply this to ~1-d motion in “Out” direction:
Nota Bene: This last case allows ROUT<RSIDE
even for~ RSIDE
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The chief lesson from last 10-15 years The chief lesson from last 10-15 years of (theoretical) HBT work:of (theoretical) HBT work:
The “radii” are a (potentially) The “radii” are a (potentially) complicated mixture ofcomplicated mixture of Space-time distributionSpace-time distribution Spatial flow gradients Spatial flow gradients Temperature gradients Temperature gradients
More correct terminology:“radii” “lengths of homogeneity”
In the presence of source dynamics, “radii” depend on mean pair(energy, momentum, transverse mass, …)
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But : ROUT/RSIDE trend completely eliminates one “hydro” calculation
Soff, Bass, Dumitru, Phys. Rev. Lett. 86, 3981
(2001)
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Plotted versus KPlotted versus KTT, there is an amazing , there is an amazing
consistency between data from AGS, SPS, and consistency between data from AGS, SPS, and RHIC, spanning a factor of 100 in CM RHIC, spanning a factor of 100 in CM energy(!):energy(!):
SPSAGS
RHIC
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There is now an extensive There is now an extensive experimental and theoretical experimental and theoretical literature built on 20+ years of literature built on 20+ years of “HBT” studies“HBT” studies
We’re now ready to start doing We’re now ready to start doing things right:things right: ExperimentallyExperimentally
Measure resonance contributionsMeasure resonance contributions Measure like and unlike particle effectsMeasure like and unlike particle effects Understand systematic errorsUnderstand systematic errors
TheoreticallyTheoretically Cascade codes for RHICCascade codes for RHIC Right parametrizationRight parametrization Coulomb systematicsCoulomb systematics
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Multipion correlationsMultipion correlations
Each events has large number of Each events has large number of pions, therefore, multi-pion pions, therefore, multi-pion correlation will affect correlation will affect
Two-pion interferometryTwo-pion interferometry
(1): If the pion correlation formula (1): If the pion correlation formula is right?is right?
(2): If it is not right, which kind of (2): If it is not right, which kind of formula should we use!formula should we use!
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Seems right!Seems right!
Zhang, Phys. Rev. C58, 22 (1998)Zhang, Phys. Rev. C58, 22 (1998) Proved that for a class model, the Proved that for a class model, the
present pion correlation function present pion correlation function formula is right.formula is right.
However generally, the formula is However generally, the formula is not right!not right!
Zhang, Phys. Rev. C59,1646 (1999).Zhang, Phys. Rev. C59,1646 (1999).
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The formula is :The formula is :
C(p1,p2)=C(p1,p2)=
A(p1,p2)[1+R(p1,p2)]A(p1,p2)[1+R(p1,p2)]
A(p1,p2) is a function of p1,p2.A(p1,p2) is a function of p1,p2.
R(p1,p2) can be written as the same R(p1,p2) can be written as the same as above.as above.
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Three-pions correlationsThree-pions correlations
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Some of slides taken from Some of slides taken from
W. A. Zajc, R. Wilson. W. A. Zajc, R. Wilson.
Thank you!Thank you!