status of the qcd phase diagram from the...
TRANSCRIPT
Status of the QCD phase diagram from the lattice
Owe Philipsen
QGHXM St. Goar, 15.03.2011
Is there a critical end point in the QCD phase diagram?
Is it connected to a chiral phase transition?
Imaginary chemical potential: rich phase structure, benchmark for models!
The conference location...
The conference location...
...a critical point for Rhine navigation
The QCD phase diagram established by experiment:
B
Nuclear liquid gas transition with critical end point
QCD phase diagram: theorist’s view
T
µ
confined
QGP
Color superconductor
Tc!
~170 MeV
~1 GeV?
Expectation based on models: NJL, NJL+Polyakov loop, linear sigma models, random matrix models, ...
Until 2001: no finite density lattice calculations, sign problem!
earl
y un
iver
se
heavy ion collisio
ns
compact stars ?
B
QGP and colour SC at asymptotic T and densities by asymptotic freedom!
The Monte Carlo method, zero density
QCD partition fcn:
links=gauge fields
lattice spacing a<< hadron << L ! thermodynamic behaviour, large V !
Monte Carlo by importance sampling U
det M e!Sgauge
T =1
aNtNt !", a! 0Continuum limit:
Here:
Z =!
DU"
f
det M(µf , mf ;U) e!Sgauge(!;U)
Nt = 4! 10 a ! 0.1" 0.3 fm staggered fermions
Theory: how to calculate p.t., critical temperature
= crit. exponent
The order of the p.t., arbitrary quark masses
chiral p.t.restoration of global symmetry in flavour space
µ = 0
deconfinement p.t.: breaking of global symmetry
SU(2)L ! SU(2)R ! U(1)A
Z(3)
anomalous
chiral critical line
deconfinement critical line
How to identify the critical surface: Binder cumulant
B4(!̄!) !"("!̄!)4#
"("!̄!)2#2V !"
$%
!
"
#
1.604 3d Ising1 first $ order3 crossover
1.3
1.4
1.5
1.6
1.7
1.8
1.9
0.018 0.021 0.024 0.027 0.03 0.033 0.036
B4
am
L=8
L=12
L=16
Ising
B4(m, L) = 1.604 + bL1/!(m ! mc0), ! = 0.63µ = 0 :
university-logo
Intro Tc CEP Results Discussion Concl. Others Strategy
Observable: Binder cumulant
• Probability distribution of order parameter
- distinguishes crossover (Gaussian) vs 1rst order (2 peaks)
- 2nd order: scale-invariant distribution with known Ising exponents
- encoded in Binder cumulant
• Measure B4(!̄!) ! "("!̄!)4#"("!̄!)2#2
!!!"("!̄!)3#=0
=
"
#
$
3 crossover
1 first$order1.604 3d Ising
for V % #
0
0.5
1
1.5
2
2.5
3
3.5
4
-1 -0.5 0 0.5 1
First order
Crossover
V1V2 > V1
V3 > V2 > V1V=$$"
1.3
1.4
1.5
1.6
1.7
1.8
1.9
0.018 0.021 0.024 0.027 0.03 0.033 0.036
B4
am
L=8
L=12
L=16
Ising
• Finite volume, µ= 0: B4(am) = 1.604+ c(L)(am$amc
0)+ . . . , c(L) % L1/&
Ph. de Forcrand INT, Aug. 2008 Controlled crit. pt.
How to identify the order of the phase transition
x! xc
parameter along phase boundary, T = Tc(x)
Hard part: order of p.t., arbitrary quark masses
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 0.01 0.02 0.03 0.04
ams
amu,d
Nf=2+1
physical point
mstric - C mud
2/5
phys.point
00
N = 2
N = 3
N = 1
f
f
f
m s
sm
Gauge
m , mu
1st
2nd orderO(4) ?
2nd orderZ(2)
2nd orderZ(2)
crossover
1st
d
tric
!
!
Pure
deconf. p.t.
chir
al p
.t.
physical point: crossover in the continuum Aoki et al 06
chiral critical line on de Forcrand, O.P. 07
consistent with tri-critical point at
But: chiral O(4) vs. 1st still open Di Giacomo et al 05, Kogut, Sinclair 07 anomaly! Chandrasekharan, Mehta 07
Nt = 4, a ! 0.3 fm
mu,d = 0, mtrics ! 2.8T
Nf = 2UA(1)
chiral critical line
µ = 0
The nature of the phase transition at the physical point Fodor et al. 06
...in the staggered approximation...in the continuum...is a crossover!
The nature of the transition for phys. masses Aoki et al. 06
How to identify the critical surface: Binder cumulant
B4(!̄!) !"("!̄!)4#
"("!̄!)2#2V !"
$%
!
"
#
1.604 3d Ising1 first $ order3 crossover
1.3
1.4
1.5
1.6
1.7
1.8
1.9
0.018 0.021 0.024 0.027 0.03 0.033 0.036
B4
am
L=8
L=12
L=16
Ising
B4(m, L) = 1.604 + bL1/!(m ! mc0), ! = 0.63µ = 0 :
university-logo
Intro Tc CEP Results Discussion Concl. Others Strategy
Observable: Binder cumulant
• Probability distribution of order parameter
- distinguishes crossover (Gaussian) vs 1rst order (2 peaks)
- 2nd order: scale-invariant distribution with known Ising exponents
- encoded in Binder cumulant
• Measure B4(!̄!) ! "("!̄!)4#"("!̄!)2#2
!!!"("!̄!)3#=0
=
"
#
$
3 crossover
1 first$order1.604 3d Ising
for V % #
0
0.5
1
1.5
2
2.5
3
3.5
4
-1 -0.5 0 0.5 1
First order
Crossover
V1V2 > V1
V3 > V2 > V1V=$$"
1.3
1.4
1.5
1.6
1.7
1.8
1.9
0.018 0.021 0.024 0.027 0.03 0.033 0.036
B4
am
L=8
L=12
L=16
Ising
• Finite volume, µ= 0: B4(am) = 1.604+ c(L)(am$amc
0)+ . . . , c(L) % L1/&
Ph. de Forcrand INT, Aug. 2008 Controlled crit. pt.
How to identify the order of the phase transition
x! xc
parameter along phase boundary, T = Tc(x)
The ‘sign problem’ is a phase problem
importance sampling requirespositive weights
Dirac operator:
Lattice QCD at finite temperature and density
Difficult (impossible?): sign problem of lattice QCD
Z =
!
DU [detM(µ)]fe!Sg[U ]
positivity governed by !5-hermiticity: D/ (µ)† = !5D/ (!µ")!5
"det(M) complex for SU(3), µ #= 0
"real positive for SU(2), µ = iµi
N.B.: all expectation values are real, but MC importance sampling impossible
The following methods evade the sign problem, they don’t cure it!
N.B.: all expectation values real, imaginary parts cancel, but importance sampling config. by config. impossible!
D/ (µ)† = !5D/ (!µ!)!5
Z =
!DU [detM(µ)]fe!Sg[U ]
Lattice QCD at finite temperature and density
Difficult (impossible?): sign problem of lattice QCD
Z =
!
DU [detM(µ)]fe!Sg[U ]
positivity governed by !5-hermiticity: D/ (µ)† = !5D/ (!µ")!5
"det(M) complex for SU(3), µ #= 0
"real positive for SU(2), µ = iµi
N.B.: all expectation values are real, but MC importance sampling impossible
The following methods evade the sign problem, they don’t cure it!
µu = !µd
Same problem in many condensed matter systems!
Finite density: methods to evade the sign problem
Reweighting:
Optimal: use |det| in measure, reweight in phase
Taylor expansion:
Imaginary : no sign problem, fit by polynomial, then analytically continue
All require !
U
Sµ=0 finite µ
~exp(V) statistics needed,overlap problem
coefficients one by one, convergence?
requires convergencefor analytic continuation
µ/T < 1
!O"(µi) =N!
k=0
ck
" µi
!T
#2k, µi # $iµ
!O"(µ) = !O"(0) +!
k=1
ck
" µ
!T
#2k
Z =!
DU det M(0)det M(µ)det M(0)
e!Sg
use for MC calculate
µ = iµi
inte
gran
d
Comparing approaches: the critical line
university-logo
Intro Tc CEP Results Discussion Concl.
The good news: curvature of the pseudo-critical line
All with Nf = 4 staggered fermions, amq = 0.05,Nt = 4 (a! 0.3 fm)
PdF & Kratochvila
4.8
4.82
4.84
4.86
4.88
4.9
4.92
4.94
4.96
4.98
5
5.02
5.04
5.06
0 0.5 1 1.5 2
1.0
0.95
0.90
0.85
0.80
0.75
0.70
0 0.1 0.2 0.3 0.4 0.5!
T/T
c
µ/T
a µ
confined
QGP<sign> ~ 0.85(1)
<sign> ~ 0.45(5)
<sign> ~ 0.1(1)
D’Elia, Lombardo 163
Azcoiti et al., 83
Fodor, Katz, 63
Our reweighting, 63
deForcrand, Kratochvila, 63
imaginary µ
2 param. imag. µ
dble reweighting, LY zeros
Same, susceptibilities
canonical
Agreement for µ/T ! 1
Ph. de Forcrand INT, Aug. 2008 Controlled crit. pt.
de Forcrand, Kratochvila LAT 05
; same actions (unimproved staggered), same massNt = 4, Nf = 4
Test of methods: comparing Tc(µ) de Forcrand, Kratochvila 05
The calculable region of the phase diagram
T
µ
confined
QGP
Color superconductor
Tc!
need
Upper region: equation of state, screening masses, quark number susceptibilities etc.under control
µ/T <! 1 (µ = µB/3)
The (pseudo-) critical temperature
Tc(µ)Tc(0)
= 1! !(Nf , mq)! µ
T
"2+ . . .
de Forcrand, O.P. 03D’Elia, Lombardo 03
! ! Nf
Nc
Curvature rather small
Toublan 05
Tc(µ)Tc(0)
= 1! 0.059(2)(4)! µ
T
"2+ O
#! µ
T
"4$
Curvature of crit. line from Taylor expansion2+1 flavours, Nt=4, 8 improved staggered
Extrapolation to chiral limit assuming O(4),O(2) scaling of magn. EoS
Consistent with determinations at finite mass with imag. chem. pot.
Phase boundary in the chiral limit hotQCD 11
scaling form of magnetic EoS
=2!T
t0msh!(1!!)/!" dfG(z)
dz
z = h1/!"/t
Phase boundary for physical masses Endrödi et al. 11
Curvature of crit. line from Taylor expansion2+1 flavours, Nt=6,8,10 improved staggered
Observables
Continuum extrapolation:
!̄!r, "s
t =T ! Tc
Tc
Comparison with freeze-out curve
freeze-out
Much harder: is there a QCD critical point?
Much harder: is there a QCD critical point?
1
Much harder: is there a QCD critical point?
12
Fodor,Katz JHEP 04
abrupt change: physics or problem of the method? Splittorff 05; Han, Stephanov 08
Lee-Yang zero:
Critical point from reweighting
physical quark masses, unimproved staggered fermionsNt = 4, Nf = 2 + 1
Approach 1a: CEP from reweighting Fodor, Katz 04
Splittorf 05, Stephanov 08
Approach 1b: CEP from Taylor expansion
p
T 4=!!
n=0
c2n(T )" µ
T
#2n
Nearest singularity=radius of convergenceµE
TE= lim
n!"
!""""c2n
c2n+2
"""", limn!"
""""c0
c2n
""""
12n
0.75
0.8
0.85
0.9
0.95
1
1.05
1.1
1.15
1.2
0 1 2 3 4 5 6
µB / Tc(0)
T / Tc(0)
!2 !4
nf=2+1, m"=220 MeVnf=2, m"=770 MeV
CEP from [5]CEP from [6]
Bielefeld-Swansea-RBC
FK
Gavai, Gupta
Nf = 2
improved staggered Nt = 4
Predictivity ?
0
1
2
3
4
5
6
7
8
0.85 0.9 0.95 1 1.05 1.1
T/Tc(0)
! !2(p/T4)!4(p/T4)!2("B)!4("B)
Radius of convergence necessary condition, but can it proof the existence of a CEP?
Hadron resonance gas
C.Schmidt, hotQCD 09
Different definitions agree only for not n=1,2,3,... CEP may not be nearest singularity, control of systematics?
n!"
Approach 2: follow chiral critical line surface
mc(µ)mc(0)
= 1 +!
k=1
ck
" µ
!T
#2k
chir
al p
.t.
chir
al p
.t.
hard/easyde Forcrand, O.P. 08,09
Finite density: chiral critical line critical surface
phys.point
00
N = 2
N = 3
N = 1
f
f
f
m s
sm
Gauge
m , mu
1st
2nd orderO(4) ?
2nd orderZ(2)
2nd orderZ(2)
crossover
1st
d
tric
!
!
Pure
* QCD critical point
crossover 1rst0
!
Real world
X
Heavy quarks
mu,dms
!
QCD critical point DISAPPEARED
crossover 1rst0
!
Real world
X
Heavy quarks
mu,dms
!
mc(µ)
mc(0)= 1 +
!
k=1
ck
" µ
!T
#2k
T
µ
confined
QGP
Color superconductor
m > mc(0)
Tc
T
!
confined
QGP
Color superconductor
m > > mc(0)
Tc
!
transition strengthens transition weakens
Curvature of the chiral critical surface
de Forcrand, O.P. 08,09
On coarse lattice exotic scenario: no chiral critical point at small density
Weakening of p.t. with chemical potential also for:
-Heavy quarks de Forcrand, Kim, Takaishi 05
-Light quarks with finite isospin density Kogut, Sinclair 07
-Electroweak phase transition with finite lepton density Gynther 03
Towards the continuum: Nt = 6, a ! 0.2 fm
phys.point
00
N = 2
N = 3
N = 1
f
f
f
m s
sm
Gauge
m , mu
1st
2nd orderO(4) ?
2nd orderZ(2)
2nd orderZ(2)
crossover
1st
d
tric
!
!
Pure
Nt=4
mc!(Nt = 4)
mc!(Nt = 6)
! 1.77
Physical point deeper in crossover region as
Cut-off effects stronger than finite density effects!
Preliminary: curvature of chiral crit. surface remains negative de Forcrand, O.P. 10
No chiral critical point at small density, other crit. points possible
a! 0
Nf = 3 de Forcrand, Kim, O.P. 07Endrodi et al 07
Nt=6
Same statement with different methods
Endrödi et al., 11 findweakening of crossover
Study suitably defined width of crossover region
strengthening of transition
Chiral/deconf. and Z(3) transitions at imaginary
Nf=4: D’Elia, Di Renzo, Lombardo 07 Nf=2: D’Elia, Sanfilippo 09 Nf=3:
Strategy: fix , measure Im(L), order parameter at
determine order of Z(3) branch/end point as function of m
µi
T= !µi
T=
!
3, !
µ
de Forcrand, O.P. 10
0 0.5 1 1.5 2 2.5 3 3.5 4
T
µi
T /(!3 )
ordered
disordered
ordered
disordered
Results:
1
1.5
2
2.5
3
5.216 5.217 5.218 5.219
B4(Im
(L))
!
m=0.040
L=8L=12L=16
1
1.5
2
2.5
3
-0.6 -0.4 -0.2 0 0.2 0.4 0.6
B4(Im
(L))
(!-!c)L1/"
m=0.040
L=8L=12L=16
! = 0.33
B4 at intersection has large finite size corrections (well known), more stable!
B4(!, L) = B4(!c,!) + C1(! " !c)L1/! + C2(! " !c)2L2/! . . .
! = 0.33, 0.5, 0.63
for 1st order, tri-critical, 3d Ising scaling
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.01 0.1 1
!
quark mass
Second order
Tricritical
First order
On infinite volume, this becomes a step function, smoothness due to finite L
T
m
Phase diagram at fixedµi
T=
!
3, !
triple line
triple line
3d Ising
ordered phase
disordered phase
Critical lines at imaginary µ
phys.point
00
N = 2
N = 3
N = 1
f
f
f
m s
sm
Gauge
m , mu
1st
2nd orderO(4) ?
2nd orderZ(2)
2nd orderZ(2)
crossover
1st
d
tric
!
!
Pure
µ = i!T
3µ = 0
-Connection computable with standard Monte Carlo! -Here: heavy quarks in eff. theory
triple
tricritical1st ordertriple
1st order
2nd order 3d Ising
mu,d
ms
?
Critical surfaces
shape, sign of curvature determined by tric. scaling!
Similar chiral crit. surface: tric. line renders curvature negative!
de Forcrand, O.P. 10
(µ/T)2
ms!
mu,d!
0crossover
1.O.chiral 1.O.
deconf.
tric.
x phys. point
(µ/T)2
ms!
mu,d!
0crossover
1.O.chiral 1.O.
deconf.
tric.
x phys. point
-("/3)2-("/3)2-("/3)2-("/3)2
0.5
1
1.5
2
2.5
3
3.5
4
-1.5 -1 -0.5 0 0.5 1
mc
(µ/T)2
Potts: QCD, Nt=1, strong coupling series:Langelage, O.P. 09
mc
T(µ2) =
mtric
T+ K
!"!
3
#2+
" µ
T
#2$2/5
tri-critical scaling: exponent universal
6.5
7
7.5
8
8.5
9
9.5
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5
mc /
T
(µ/T)2
Imaginary µ Real µ
6.6+1.6((!/3)2+(µ/T)2)2/5
Conclusions
Reweighting, Taylor: indications for critical point, systematics ?
Chiral crit. surface, deconfinement crit. surface: Transitions weaken with chemical potential
For lattices a~0.3, 0.2 fm no chiral critical point for
Still possible: chiral critical point at large chemical potential non-chiral critical point(s)
µ/T <! 1
?