# A method of finding the critical point in finite density QCD

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A method of finding the critical point in finite density QCD
Shinji Ejiri (Brookhaven National Laboratory) Existence of the critical point in finite density lattice QCD Physical Review D77 (2008) [arXiv: ] Canonical partition function and finite density phase transition in lattice QCD Physical Review D 78 (2008) [arXiv: ] Tools for finite density QCD, Bielefeld, November 19-21, 2008

QCD thermodynamics at m0
Interesting properties of QCD Measurable in heavy-ion collisions Critical point at finite density Location of the critical point ? Distribution function of plaquette value Distribution function of quark number density Simulations: Bielefeld-Swansea Collab., PRD71,054508(2005). 2-flavor p4-improved staggered quarks with mp 770MeV 163x4 lattice ln det M: Taylor expansion up to O(m6) quark-gluon plasma phase hadron phase RHIC SPS AGS color super conductor? nuclear matter mq T

Effective potential of plaquette
Physical Review D77 (2008) [arXiv: ]

Effective potential of plaquette Veff(P) Plaquette distribution function (histogram)
SU(3) Pure gauge theory QCDPAX, PRD46, 4657 (1992) First order phase transition Two phases coexists at Tc e.g. SU(3) Pure gauge theory Gauge action Partition function histogram histogram Effective potential

Problem of complex quark determinant at m0
Problem of Complex Determinant at m0 Boltzmann weight: complex at m0 Monte-Carlo method is not applicable. Configurations cannot be generated. (g5-conjugate)

Distribution function and Effective potential at m0 (S.E., Phys.Rev.D77, 014508(2008))
Distributions of plaquette P (1x1 Wilson loop for the standard action) (Weight factor at m=0) (Reweight factor) R(P,m): independent of b,  R(P,m) can be measured at any b. Effective potential: 1st order phase transition? m=0 crossover non-singular + = ?

Reweighting for b(T) and curvature of –lnW(P)
Change: b1(T) b2(T) Weight: Potential: + = Curvature of –lnW(P) does not change. Curvature of –lnW(P,b) : independent of b.

m-dependence of the effective potential
Critical point Crossover + = m= reweighting Curvature: Zero 1st order phase transition + = m= reweighting Curvature: Negative

Sign problem and phase fluctuations
Complex phase of detM Taylor expansion: odd terms of ln det M (Bielefeld-Swansea, PRD66, (2002)) |q| > p/2: Sign problem happens. changes its sign. Gaussian distribution Results for p4-improved staggered Taylor expansion up to O(m5) Dashed line: fit by a Gaussian function Well approximated q: NOT in the range of [-p, p] histogram of q

Effective potential at m0 (S.E., Phys.Rev.D77, 014508(2008))
Results of Nf=2 p4-staggared, mp/mr0.7 [data in PRD71,054508(2005)] detM: Taylor expansion up to O(m6) The peak position of W(P) moves left as b increases at m=0. at m=0 Solid lines: reweighting factor at finite m/T, R(P,m) Dashed lines: reweighting factor without complex phase factor.

Truncation error of Taylor expansion
Solid line: O(m4) Dashed line: O(m6) The effect from 5th and 6th order term is small for mq/T  2.5.

Curvature of the effective potential
Nf=2 p4-staggared, mp/mr0.7 at mq=0 + = ? Critical point: First order transition for mq/T  2.5 Existence of the critical point: suggested Quark mass dependence: large Study near the physical point is important.

Physical Review D 78 (2008) 074507[arXiv:0804.3227]
Canonical approach Physical Review D 78 (2008) [arXiv: ]

Canonical approach Canonical partition function
Effective potential as a function of the quark number N. At the minimum, First order phase transition: Two phases coexist.

First order phase transition line
In the thermodynamic limit, Mixed state First order transition Inverse Laplace transformation by Glasgow method Kratochvila, de Forcrand, PoS (LAT2005) 167 (2005) Nf=4 staggered fermions, lattice Nf=4: First order for all r.

Canonical partition function
Fugacity expansion (Laplace transformation) canonical partition function Inverse Laplace transformation Note: periodicity Derivative of lnZ Integral Arbitrary m0 Integral path, e.g. 1, imaginary m axis 2, Saddle point

Integral Saddle point Inverse Laplace transformation Saddle point approximation (valid for large V, 1/V expansion) Taylor expansion at the saddle point. At low density: The saddle point and the Taylor expansion coefficients can be estimated from data of Taylor expansion around m=0. Saddle point:

Canonical partition function in a saddle point approximation Chemical potential Saddle point: saddle point reweighting factor Similar to the reweighting method (sign problem & overlap problem)

Saddle point in complex m/T plane
Find a saddle point z0 numerically for each conf. Two problems Sign problem Overlap problem

Technical problem 1: Sign problem
Complex phase of detM Taylor expansion (Bielefeld-Swansea, PRD66, (2002)) |q| > p/2: Sign problem happens. changes its sign. Gaussian distribution Results for p4-improved staggered Taylor expansion up to O(m5) Dashed line: fit by a Gaussian function Well approximated q: NOT in the range [-p, p] histogram of q

Technical problem 2: Overlap problem Role of the weight factor exp(F+iq)
The weight factor has the same effect as when b (T) increased. m*/T approaches the free quark gas value in the high density limit for all temperature. free quark gas free quark gas free quark gas

Technical problem 2: Overlap problem
Density of state method W(P): plaquette distribution Same effect when b changes. for small P linear for small P linear for small P

Reweighting for b(T)=6g-2
(Data: Nf=2 p4-staggared, mp/mr0.7, m=0) Change: b1(T) b2(T) Distribution: Potential: + = Effective b (temperature) for r0 (r increases)  (b (T) increases)

Overlap problem, Multi-b reweighting Ferrenberg-Swendsen, PRL63,1195(1989)
When the density increases, the position of the importance sampling changes. Combine all data by multi-b reweighting Problem: Configurations do not cover all region of P. Calculate only when <P> is near the peaks of the distributions. Plaquette value by multi-beta reweighting peak position of the distribution ○ <P> at each b P

Chemical potential vs density
Approximations: Taylor expansion: ln det M Gaussian distribution: q Saddle point approximation Two states at the same mq/T First order transition at T/Tc < 0.83, mq/T >2.3 m*/T approaches the free quark gas value in the high density limit for all T. Solid line: multi-b reweighting Dashed line: spline interpolation Dot-dashed line: the free gas limit Nf=2 p4-staggered, lattice Number density

Summary Effective potentials as functions of the plaquette value and the quark number density are discussed. Approximation: Taylor expansion of ln det M: up to O(m6) Distribution function of q=Nf Im[ ln det M] : Gaussian type. Saddle point approximation (1/V expansion) Simulations: 2-flavor p4-improved staggered quarks with mp/mr  0.7 on 163x4 lattice Existence of the critical point: suggested. High r limit: m/T approaches the free gas value for all T. First order phase transition for T/Tc < 0.83, mq/T >2.3. Studies near physical quark mass: important. Location of the critical point: sensitive to quark mass

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