X-Git-Url: http://git.treefish.org/~alex/phys/proceedings_lattice13.git/blobdiff_plain/d9fffa925e5bd54a6484370c7f4365e52265fd13..81a169949997087e2e55fd6d80e5edce62725844:/proceed.tex

diff --git a/proceed.tex b/proceed.tex
index 2f3e030..12e7768 100644
--- a/proceed.tex
+++ b/proceed.tex
@@ -6,10 +6,9 @@
 \usepackage{dsfont}
 \usepackage{subfigure}
 
-\title{Solving the sign problem of scalar, two-flavored electrodynamics 
-for finite chemical potential and exploring its full phase-diagram}
+\title{Solving the sign problem of two flavored scalar electrodynamics at finite chemical potential}
 
-\ShortTitle{Solving the sign problem of scalar electrodynamics at final chemical potential}
+\ShortTitle{Solving the sign problem of two-falvored scalar electrodynamics at finite chemical potential}
 
 \author{Ydalia Delgado
 \\Institut f\"ur Physik, Karl-Franzens Universit\"at, Graz, Austria
@@ -28,11 +27,10 @@ for finite chemical potential and exploring its full phase-diagram}
 We explore two-flavored scalar electrodynamics on the lattice, which has a complex phase problem 
 at finite chemical potential. By rewriting the action in terms of dual variables 
 this complex phase problem can be solved exactly. The dual variables are links and plaquettes, subject to non-trivial 
-constraints, which have to be respected by the Monte Carlo algorithm. 
-Therefore, for the simulation we use a local update and the surface worm algorithm (SWA). 
+constraints, which have to be respected by the Monte Carlo algorithm. For the simulation we use a local update that always obeys the constraints and the surface worm algorithm (SWA). 
 The SWA is a generalization of the Prokof'ev Svistunov 
 worm algorithm concept to simulate the dual representation of abelian Gauge-Higgs models on a lattice. 
-We also assess the performance of the SWA and compare it with a local update in the dual representation. 
+We also assess the performance of the SWA and compare it with the local update algorithm in the dual representation. 
 Finally, we determine the full phase diagram of the model.
 }
 
@@ -55,17 +53,17 @@ has not been achieved yet.
 For some models or QCD in limiting cases, it is possible to deal with the complex phase 
 problem (e.g. \cite{solve-sign-problem}).  Among the different techniques, we use the dual representation,
 which has been shown to be a very powerful method that can solve the complex 
-phase problem without making any approximation of the partition sum, i.e. it is an exact method \cite{dual}.  
-In the following we present another example where the dual representation can be applied succesfully.  We consider a compact
-U(1) gauge field coupled with two complex scalar fields with opposite charge. We explore the full phase diagram
-as a function of the gauge coupling, the mass parameter and the chemical potential, which has not yet been studied in detail.
-At finite density we present some preliminary results.
+phase problem of different models \cite{dual} without making any approximation of the partition sum.  
+In the following we present another example where the dual representation can be applied successfully.  
+We consider a compact U(1) gauge field coupled with two complex scalar fields with opposite charge \cite{prl}. 
+We explore the full phase diagram as a function of the inverse gauge coupling and the mass parameter, 
+and present some preliminary results at finite $\mu$.
 
 After mapping the degrees of freedom of the system to its dual variables, the weight in the 
 partition sum is positive and real and usual Monte Carlo techniques can be applied.  However, 
 the dual variables, links and plaquettes for this model, are subject to non-trivial constraints.
 Therefore one has to choose a proper algorithm in order to sample the system efficiently.  In our case, we have
-used two different Monte Carlo algorithms:  A local update (LMA) \cite{z3} and an extension \cite{swa} of the
+used two different Monte Carlo algorithms:  A local update algorithm (LMA) \cite{z3} and an extension \cite{swa} of the
 Prokof'ev Svistunov worm algorithm \cite{worm}.   Here we present
 some technical comparison of both algorithms in addition to the physics of the model.
  
@@ -186,31 +184,33 @@ flux variables as the new degrees of freedom.
 \section{Monte Carlo simulation}
 \vspace*{-1mm}
 \noindent
-Because the dual variables are subject to non-trivial constraints, they cannot be updated randomly.
-The most straight forward way to update the system is to change complete allowed objects.  In order to
-increase the acceptance rate we use the smallest possible structures.  This algorithm is called local update
-(LMA) and was used in \cite{z3,swa,prl}.  Other possibility is to use an extension of the worm
+Because the dual variables are subject to non-trivial constraints, they cannot be modified randomly during the update.
+A straight forward way to sample the system is to change allowed surfaces.  
+Thus we choose the smallest possible structures in order to
+increase the acceptance rate.  This algorithm is called local update
+(LMA) and was used in \cite{z3,swa,prl}.  Another possibility is to use an extension of the worm
 algorithm \cite{worm}, the so called surface worm algorithm \cite{swa}.  For this model we use both algorithms and
 assess their performance.
 
-First, we start describing the LMA. It consists of the following updates:
+\subsection{Local update algorithm}
+Let us begin by describing the LMA. It consists of the following updates:
 \begin{itemize}
 \vspace*{-1mm}
-\item A sweep for each unconstrained variable $\overline{l}$ and $\overline{k}$ 
+\item A sweep for each unconstrained variable $\overline{l}$ and $\overline{j}$ 
 rising or lowering their occupation number by one unit.
 %
 \vspace*{-1mm}
 \item ``Plaquette update'': 
 It consists of increasing or decreasing a plaquette occupation number
 $p_{x,\nu\rho}$ and
-the link fluxes (either $l_{x,\sigma}$ or $k_{x,\sigma}$) at the edges of $p_{x,\nu\rho}$ by $\pm 1$ as 
+the link fluxes (either $l_{x,\sigma}$ or $j_{x,\sigma}$) at the edges of $p_{x,\nu\rho}$ by $\pm 1$ as 
 illustrated in Fig.~\ref{plaquette}. The change of $p_{x, \nu \rho}$ 
-by $\pm 1$ is indicated by the signs $+$ or $-$, while the flux variables $l$($k$) are denoted by the red(blue) lines
+by $\pm 1$ is indicated by the signs $+$ or $-$, while the flux variables $l$($j$) are denoted by the red(blue) lines
 and we use a dashed line to indicate a decrease by $-1$ and a full line for an increase by $+1$.
 %
 \vspace*{-1mm}
 \item ``Winding loop update'': 
-It consists of increasing or decreasing the occupation number of both link variables $l$ and $k$ by 
+It consists of increasing or decreasing the occupation number of both link variables $l$ and $j$ by 
 one unit along a winding loop in any of the 4 directions.  This update is very important because the winding loops
 in time direction are the only objects that couple to the chemical potential.
 %
@@ -234,7 +234,7 @@ probability computed from the local weight factors.
 \end{center}
 \vspace{-4mm}
 \caption{Plaquette update: A plaquette occupation number is changed by $+1$ or
-$-1$ and the links $l$ (red) or $k$ (blue) of the plaquette are changed simultaneously. The
+$-1$ and the links $l$ (red) or $j$ (blue) of the plaquette are changed simultaneously. The
 full line indicates an increase by +1 and a dashed line a decrease by $-1$. 
 The directions $1 \le \nu_1 < \nu_2 \le 4$
 indicate the plane of the plaquette.} \label{plaquette}
@@ -252,11 +252,10 @@ the directions $1 \leq \nu_1 < \nu_2 < \nu_3 \leq 4$.} \label{cube}
 \vspace*{-2mm}
 \end{figure}
 
+\subsection{Worm algorithm}
 \noindent
 Instead of the plaquette and cube updates we can use the worm algorithm.
-Here we will shortly describe the SWA (see \cite{swa} for a detailed description) 
-for the variable $l$ (red).
-The algorithm for the other type of link variable works in exactly the same way.
+Here we will shortly describe the SWA (see \cite{swa} for a detailed description).
 
 The SWA is constructed by breaking up the smallest update, i.e., the plaquette update 
 into smaller building blocks called ``segments'' 
@@ -266,25 +265,28 @@ In the SWA the constraints are temporarily violated at a link
 $L_V$, the head of the worm, and the two sites at its endpoints.
 The admissible configurations are produced using 3 steps:
 \begin{enumerate}
-\item The worm starts by changing the flux by $\pm 1$ at a randomly chosen link (step 1 in Fig.~\ref{worm}).
+\item The worm starts by changing either the $l$ or $j$  flux by $\pm 1$ at 
+a randomly chosen link (step 1 in Fig.~\ref{worm}, a worm for $l$ fluxes starts).
 \item The first link becomes the head of the worm $L_V$.
 The defect at $L_V$ is then propagated through the lattice by 
-attaching segments, which are chosen in such a way that the constraints are always 
+attaching segments of the same kind of flux as the first segment, 
+which are chosen in such a way that the constraints are always 
 obeyed (step 2 in Fig.~\ref{worm}).
 \item The defect is propagated through the lattice until the worm decides to
 end with the insertion of another unit of link flux at $L_V$ (step 3 in Fig.~\ref{worm}).
  
 \end{enumerate}
-A full sweep consists of $V_4$ worms using the SWA plus a sweep of the unconstraint 
-variables $\overline{l}$ and $\overline{k}$,
-and a sweep of winding loops (as explained in the LMA).
+A full sweep consists of $V_4$ worms with the $l$ fluxes and $V_4$ worms with the $j$ fluxes, 
+plus a sweep of the unconstrained 
+variables $\overline{l}$ and $\overline{j}$,
+and a sweep of winding loops (as explained for the LMA).
 
 \begin{figure}[h]
 \begin{center}
 \includegraphics[width=\textwidth,clip]{pics/segments}
 \end{center}
 \vspace{-4mm}
-\caption{Examples of positive (lhs.) and negative segments (rhs.) 
+\caption{Examples of segments for the links $l$ (lhs.) and $j$ (rhs.) 
 in the $\nu_1$-$\nu_2$-plane ($\nu_1 < \nu_2$).
 The plaquette occupation numbers are changed as indicated by the signs. 
 The full (dashed) links are changed by $+1$ ($-1$). The empty link shows
@@ -300,32 +302,62 @@ $L_V$ where the constraints are violated.} \label{segments}
 \vspace{-4mm}
 \caption{Illustration of the worm algorithm.  See text for an explanation.} \label{worm}
 \vspace{-2mm}
-\end{figure}
+\end{figure} 
 
 
-\section{Algorithm Assessment}
+\section{Results}
 \vspace{-1mm}
 \noindent
-For the assessment of both algorithms we used two different models, the U(1) gauge-Higgs model but couple
-only to one scalar field (see \cite{swa}) and the model presented in this proceedings.  In both cases we 
-analyzed the bulk observables (and their fluctuations): 
-$U_P$ which is the derivative wrt. $\beta$ and $|\phi|^2$ (derivative wrt. 
-$\kappa$).  First we checked the correctness of the SWA comparing the results for different 
+In this section we describe the numerical analysis.  We first show the assessment of both algorithms
+and then the physics of the model.  In both cases we use thermodynamical observables and their fluctuations.
+We study in particular three observables:  The first and second derivatives with respect to the inverse
+gauge coupling $\beta$, i.e., the plaquette expectation value and its susceptibility,
+
+\begin{equation}
+\langle U \rangle = \frac{1}{6 N_s^3 N_t}\frac{\partial}{\partial \beta} \ln\ Z\quad , \quad
+\chi_{U} = \frac{1}{6 N_s^3 N_t}\frac{\partial^2}{\partial \beta^2} \ln\ Z\ .
+\end{equation}
+
+\noindent We also consider the particle number density $n$ 
+and its susceptibility which are the derivatives 
+with respect to the chemical potential,
+
+\begin{equation}
+n  = \frac{1}{N_s^3 N_t}\frac{\partial}{\partial \mu} \ln\ Z\quad , \quad
+\chi_{n} = \frac{1}{N_s^3 N_t}\frac{\partial^2}{\partial \mu^2} \ln\ Z\ .
+\end{equation}
+
+\noindent Finally, we analyze the derivatives with respect to $M^2$,
+
+\begin{equation}
+\langle |\phi|^2 \rangle = \frac{1}{N_s^3 N_t}\frac{\partial}{\partial M^2} \ln\ Z\quad , \quad
+\chi_{|\phi|^2} = \frac{1}{N_s^3 N_t}\frac{\partial^2}{\partial (M^2)^2} \ln\ Z\ .
+\end{equation}
+
+\subsection{Algorithm assessment}
+\noindent
+For the comparison of both algorithms we considered the U(1) gauge-Higgs model coupled
+with two (as described above) and with only one scalar field \cite{swa}.  
+First we checked the correctness of the SWA comparing the results for different 
 lattices sizes and parameters.  Examples for the one flavor model are shown in \cite{swa}.
-Fig.~\ref{obs} shows two observables for the two flavor case.  
+Fig.~\ref{obs} shows two observables for the two flavor case.
+The figure on the top shows 
 $\langle |\phi|^2 \rangle$ (lhs.) and its susceptibility (rhs.) as a function of $\mu$ 
-for point ``f'' (see phase diagram) on a lattice of size $12^3 \times 60$.  
-We observe very good agreement among the different algorithms.
+at $\beta = 0.85$ and $M^2 = 5.325$ on a lattice of size $12^3 \times 60$.  This point is located
+in the Higgs phase and does not show any phase transition.  The plot on the bottom shows 
+the particle number $n$ (lhs.) and its susceptibility (rhs.) as a function of $\mu$
+for $\beta = 0.75$ and $M^2 = 5.73$ on a lattice of volume $12^3 \times 60$.  This plot shows
+the transition from the confining phase to the Higgs phase. 
+We observe very good agreement between both algorithms.
 
 \begin{figure}[h]
 \begin{center}
-\includegraphics[width=\textwidth,clip]{pics/f}
-\includegraphics[width=\textwidth,clip]{pics/f}
+\hbox{\includegraphics[width=\textwidth,clip]{pics/aphi}}
+\hbox{\hspace{4mm}\includegraphics[width=0.97\textwidth,clip]{pics/bn}}
 \end{center}
-\vspace{-2mm}
-\caption{Observables $\langle |\phi|^2 \rangle$ (lhs.) and $\chi_\phi$ (rhs.) 
-as a function of $\mu$ for point f on a $12^3 \times 60$ lattice size.
-We compare results from the SWA (circles) and the LMA (crosses).} \label{obs}
+\vspace{-6mm}
+\caption{Observables as a function of $\mu$ for different parameters on a $12^3 \times 60$ lattice.
+We compare results from the SWA (circles) and the LMA (triangles).} \label{obs}
 \vspace*{-2mm}
 \end{figure}
 
@@ -339,7 +371,7 @@ On the other hand, when the constrained links have a very low acceptance rate
 the worm algorithm has difficulties to efficiently sample the 
 system because it modifies the link occupation number in every move, while the LMA has a sweep with only
 closed surfaces. The plot on the rhs. of Fig.~\ref{auto} shows how $\overline{\tau}$ for
-$U_P$ is larger for the SWA than for the LMA.  But this can be overcome by offering
+$U$ is larger for the SWA than for the LMA.  But this can be overcome by offering
 a sweep of cube updates.
 
 \begin{figure}[t]
@@ -352,15 +384,12 @@ of parameters.  Left: parameters close to a first order phase transition.
 Right: low acceptance rate of the variable $l$.  Both simulations correspond
 to a $16^4$ lattice.  Data taken from \cite{swa}.} \label{auto}
 \vspace*{-2mm}
-\end{figure} 
-
+\end{figure}
 
-\section{Results}
-\vspace{-1mm}
-\noindent
+\subsection{Physics}
 One of the main results of these studies so far and already published in \cite{prl} is the full phase diagram of the considered model in the $\beta$-$M^2$ plane at $\mu=0$ and some selected chemical potential driven phase transitions of the measured observables. For the sake of completeness we here again want to show the obtained phase diagram, but as a proceedings-extra also present some plots which show the shifting of the phase-boundaries at $\mu \neq 0$ and measurements of the dual occupation numbers.
 
-\subsection{Phase-diagram at $\mu=0$}
+\subsubsection*{Phase-diagram at $\mu=0$}
 \noindent
 We studied the different transition lines in Fig.~\ref{phasediagram} using finite size analysis of the measured observables $\langle U \rangle$ and $\langle |\phi|^2 \rangle$ and the corresponding susceptibilities, finding that the phase boundary separating  Higgs- and
 confining phase is strong first order, the line separating confining- and Coulomb phase is  of weak
@@ -378,7 +407,7 @@ inflection points of $\chi_n$.}
 \label{phasediagram}
 \end{figure}
 
-\subsection{Phase-boundaries at $\mu \neq 0$}
+\subsubsection*{Phase-boundaries at $\mu \neq 0$}
 \noindent
 In Fig.~\ref{muphases} we plot the observables $\langle U \rangle$, $\langle |\phi|^2 \rangle$, $\langle n \rangle$ as function of $\beta$ and $M^2$ for four different values of the chemical potential $\mu=0,0.5,1,1.5$.
 
@@ -393,12 +422,13 @@ For all observables it can be seen that the phase-boundaries in general become m
 \label{muphases}
 \end{figure}
 
-\subsection{Dual occupation numbers}
+\subsubsection*{Dual occupation numbers}
 \noindent
 The dual reformulation of a problem makes it possible to look at the same physics from a different perspective by studying the dynamics of the dual degrees of freedom instead of the conventional ones. This being a feature we find especially exciting about rewriting to dual variables, we here want to present an example.
 
 \noindent
-In Fig.~\ref{occutrans_plaq} we plot the plaquette expectation value $\langle U \rangle$ and the corresponding susceptibility $\chi_U$ as function of the chemical potential, for two different volumes $12^3\times60$ and $16^3\times60$. We see that for the larger volume the transition is shifted slightly towards lower chemical potential, but the volume dependence seems to be reasonably small. The parameters $\beta$ and $M^2$ are fixed to $\beta=0.75$ and $M^2=5.73$. Increasing the chemical potential takes us from the confining- to the Higgs-phase where we cross the phase boundary at some critical value of $\mu$, which is $\mu\simeq2.65$ for the larger and $\mu\simeq2.7$ for the smaller lattice, telling us that the Higgs phase is tilted towards the confining phase in $\mu$-direction. Below the critical value of the chemical potential both $\langle U \rangle$ and $\chi_U$ are independent of $\mu$, which is typical for a Silverblaze type transition.
+In Fig.~\ref{occutrans_plaq} we plot the plaquette expectation value $\langle U \rangle$ and the corresponding susceptibility $\chi_U$ as function of the chemical potential, for two different volumes $12^3\times60$ and $16^3\times60$. We see that for the larger volume the transition is shifted slightly towards lower chemical potential, but the volume dependence seems to be reasonably small. The parameters $\beta$ and $M^2$ are fixed to $\beta=0.75$ and $M^2=5.73$. Increasing the chemical potential takes us from the confining- to the Higgs-phase where we cross the phase boundary at some critical value of $\mu$, which is $\mu\simeq2.65$ for the larger and $\mu\simeq2.7$ for the smaller lattice, telling us that the Higgs phase is tilted towards the confining phase in $\mu$-direction. Below the critical value of the chemical potential both 
+$\langle U \rangle$ and $\chi_U$ are independent of $\mu$, which is characteristic for a Silverblaze type transition \cite{cohen}.
 
 \noindent
 Then in Fig.~\ref{occutrans} we show the occupation numbers of all dual link variables $\bar{j}$, $\bar{l}$, $j$, $l$ and dual plaquette variables $p$ just below (top) and above (bottom) the critical chemical potential $\mu_c$. Here blue links/plaquettes depict positive occupation numbers, green links/plaquettes depict negative occupation numbers and links/plaquettes with $0$-occupation are spared out. It can be seen that below $\mu_c$ links and plaquettes are hardly occupied, while above $\mu_c$ they are highly occupied. In that sense the Silverblaze transition shown in Fig.~\ref{occutrans_plaq} can be understood as condensation phenomenon, which is a new perspective on the underlying physics we gained from the dual reformulation of the problem.
@@ -425,10 +455,10 @@ We thank Hans Gerd Evertz
 for numerous discussions that helped to shape this project and for 
 providing us with the software to compute the autocorrelation times. 
 This work was supported by the Austrian Science Fund, 
-FWF, DK {\it Hadrons in Vacuum, Nuclei, and Stars} (FWF DK W1203-N16)
-and by the Research Executive Agency (REA) of the European Union 
-under Grant Agreement number PITN-GA-2009-238353 (ITN STRONGnet).
- 
+FWF, DK {\it Hadrons in Vacuum, Nuclei, and Stars} (FWF DK W1203-N16). Y.~Delgado is supported by
+the Research Executive Agency (REA) of the European Union 
+under Grant Agreement number PITN-GA-2009-238353 (ITN STRONGnet), HP2 and TRR 55.
+
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 \bibitem{reviews}
   P.~Petreczky,
@@ -545,6 +575,13 @@ under Grant Agreement number PITN-GA-2009-238353 (ITN STRONGnet).
   arXiv:1306.1495 [hep-lat].
   %%CITATION = ARXIV:1306.1495;%%
   
+\bibitem{prl}
+  Y.~D.~Mercado, C.~Gattringer and A.~Schmidt,
+  %``Dual lattice simulation of the U(1) gauge-Higgs model at finite density - an exploratory proof-of-concept study,''
+  Phys.\ Rev.\ Lett.\  {\bf 111} (2013) 141601
+  [arXiv:1307.6120 [hep-lat]].
+  %%CITATION = ARXIV:1307.6120;%%
+  
 \bibitem{z3}
   C.~Gattringer and A.~Schmidt,
   %``Gauge and matter fields as surfaces and loops - an exploratory lattice study of the Z(3) Gauge-Higgs model,''
@@ -566,13 +603,6 @@ under Grant Agreement number PITN-GA-2009-238353 (ITN STRONGnet).
   %``Worm Algorithms for Classical Statistical Models,''
   Phys.\ Rev.\ Lett.\  {\bf 87} (2001) 160601.
   %%CITATION = PRLTA,87,160601;%%
-  
-\bibitem{prl}
-  Y.~D.~Mercado, C.~Gattringer and A.~Schmidt,
-  %``Dual lattice simulation of the U(1) gauge-Higgs model at finite density - an exploratory proof-of-concept study,''
-  Phys.\ Rev.\ Lett.\  {\bf 111} (2013) 141601
-  [arXiv:1307.6120 [hep-lat]].
-  %%CITATION = ARXIV:1307.6120;%%
 
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@@ -600,6 +630,13 @@ Nucl.\ Phys.\ B {\bf 318} (1989) 705.
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   arXiv:0711.0818 [cond-mat.str-el].
   %%CITATION = ARXIV:0711.0818;%% 
+  
+\bibitem{cohen}
+T.D.~Cohen,
+  %``Functional integrals for QCD at nonzero chemical potential and zero density,''
+  Phys.\ Rev.\ Lett.\  {\bf 91} (2003) 222001.
+  %[hep-ph/0307089].
+  %%CITATION = HEP-PH/0307089;%%
 
 \end{thebibliography}
 
