Physics Letters B 729 (2014) 108–111
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Physics Letters B
www.elsevier.com/locate/physletb
Simulating full QCD at nonzero density using the complex Langevin
equation
Dénes Sexty
Institut für Theoretische Physik, Universität Heidelberg, Germany
article info abstract
Article history:
Received 14 October 2013
Received in revised form 7 January 2014
Accepted 8 January 2014
Available online 10 January 2014
Editor: J.-P. Blaizot
The complex Langevin method is extended to full QCD at non-zero chemical potential. The use of gauge
cooling stabilizes the simulations at small enough lattice spacings. At large fermion mass the results are
compared to the HQCD approach, in which the spatial hoppings of fermionic variables are neglected,
and good agreement is found. The method allows simulations also at high densities, all the way up to
saturation.
© 2014 The Author. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/3.0/). Funded by SCOAP
3
.
The determination of the phase diagram of finite density QCD
is one of the great problems of theoretical physics today. One is
interested in averages defined with the Euclidean path integral
f [U ]
=
1
Z
DUe
−S
g
[U ]
det M(μ, U ) f [U ], (1)
where S
g
[U ] is the Yang–Mills action of the gauge fields and
M
(μ, U ) is the Dirac-matrix of the quark fields. Naive lattice simu-
lations at
μ = 0 using importance sampling are made unfeasible by
the fact that the determinant of the fermion matrix is a complex
number in general. Various methods have been invented to cir-
cumvent the problem, but these are of limited use [1],mostlybe-
ing applicable for
μ/T 1. An exception is the complex Langevin
method [2], which is not limited to small chemical potential. It
has been demonstrated that this method allows for the solution of
thesignprobleminvarioussystems[3–7], but in some cases also
non-physical results are delivered [8–12]. In this Letter I demon-
strate that the algorithm can be extended to full QCD with light
quark masses on lattices with sufficiently small lattice spacings.
The complex Langevin method is based on setting up a com-
ple
x Langevin equation (CLE) in an enlarged manifold, which is the
complexification of the original field space [2]. The original the-
ory is recovered by taking expectation values of the analytically
continued observables. For SU
(N) gauge theories this complexifi-
cation is SL
(N, C). This method can also be applied to other cases
where the action becomes complex, e.g. the case of real time
evolution, where the complexity of the action is much ‘larger’, us-
ing the Minkowskian formulation of the path integral [13–15],or
Yang–Mills theory with
Θ-term [16]. In this work I am concerned
E-mail address: d.sexty@thphys.uni-heidelberg.de.
with finite density physics, where the complexity of the action is
present at non-zero chemical potential. The analytic understanding
of the breakdowns and successes of the complex Langevin method
has improved in the last few years [17–21], one can gain an in-
sight whether the results are trustworthy using requirements such
as the fast decay of the distributions.
Recently an important breakthrough in this field was the de-
v
elopment of a ‘gauge cooling’ algorithm for the CLE method [7],
where the gauge symmetry of the system is used to ensure a well
localized distribution in the complexified field space, and thus con-
vergence to the correct results.
In this work the CLE method is applied to the lattice discretiza-
tion
of full QCD, i.e. for the action
S
eff
[U ]=S
g
[U ]−
N
F
4
ln det M
(μ, U ) (2)
where S
g
[U ] is the Wilson plaquette action for the SU(3) link
variables, and M
(μ, U ) is the unimproved staggered fermion de-
terminant for N
F
fermion flavors
M(μ, U )
xy
=mδ
xy
+
ν
η
ν
(x)
2a
e
δ
ν4
μ
U
ν
(x)δ
x+a
ν
,y
−e
−δ
ν4
μ
U
−1
ν
(x −a
ν
)δ
x−a
ν
,y
,
(3)
where x and y indices represent spacetime coordinates, and η
μ
(x)
are the staggered sign functions. Periodic (antiperiodic) boundary
conditions are used in space (time) directions. The fermion matrix
fulfills the symmetry condition:
x
M(μ, U )
xy
y
= M
†
−
μ
∗
, U
yx
(4)
with the “staggered γ
5
matrix”,
x
= (−1)
x
1
+x
2
+x
3
+x
4
. This symme-
try leads to det M
(−μ
∗
, U ) = (det M(μ, U ))
∗
. This means that the
determinant becomes complex for Re
μ = 0, making a simulation
http://dx.doi.org/10.1016/j.physletb.2014.01.019
0370-2693/
© 2014 The Author. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/). Funded by
SCOAP
3
.