University of Tsukuba | Center for Computational Sciences

Research in Particle Physics
2+1 flavor QCD at Physical Point on very large lattices (master-field simulations)
Hadrons are the constituents of atomic nuclei. Computing the mass
spectrum of hadrons from first principles of the quantum
chromodynamics (QCD), the fundamental theory of strong interaction
described by quarks and gluons, is a principal subject in particle
physics.
After quenched and succeeding 2 flavor QCD simulations by the CPPACS, those studies were extended to 2+1 flavor QCD byincorporating the dynamical strange quark, though the degenerate
up-down quark mass was much heavier than the physical one. On thePACS-CS and the T2K computers, we have succeeded in reaching thephysical point. This calculation is followed by a larger volume
simulation on the K computer.
Our current project is aiming to control and remove systematic errors
due to the previous simulations on a finite volume with a finite latticespacing. We are performing so called master-field simulations on verylarger (10fm)4 volume with several lattice spacings using theOakforest-PACS.

Fig1a:
Relative difference of the light hadron spectrum from theexperiment. Inputs are only the pion, kaon, and omega baryon
masses to determine the up-down and strange quark masses, andthe lattice cutoff, respectively. Our results show good agreementwith the experiment albeit errors are still not quite small forsome of the hadrons.
[K-.I. Ishikawa et al., https://arxiv.org/abs/1511.09222]

Fig. 1b:
A comparison of pseudoscalar decay constants, fπ and fK, on(10fm)4 and (5fm)4. We detect 0.66% and 0.26% finite volume
effect on fπ and fK, respectively. The effect is very small andnegligible to compare the corresponding experiments. Now, we
can control and remove the finite volume effect completely byusing the master-field simulations.
[K-.I. Ishikawa et al., Phys. Rev. D 99, 014504]

Exploring QCD phase diagram
Investigating the phase structure of QCD at non-zero temperature and density is very
important to understand properties of strongly interacting matters under extreme
conditions. It is known that the order of the phase transition depends on the massand the number of flavors of quarks and there should be so-called critical endlines,lines of second order phase transitions, in certain space of quark masses as shown in
Fig. 2a.
To determine the shape of the critical endline in the small quark mass region we arecarrying out lattice QCD simulations at finite temperature with 2+1 as well as 3
degenerate quark flavors on Cygnus and Oakforest-PACS. Fig. 2b shows our recent
estimation of the critical pion mass in 3 flavor QCD in the continuum limit including a
new calculation with the temporal lattice extent of 12, where the new result gives asmaller upper bound than that of our previous calculation.
Fig. 2a:Expected quark mass dependence of theorder of the QCD phase transition. Our goal
is to determine the shape of the criticalendline shown as a red curve in the lowerleft corner.

contact address: pr@ccs.tsukuba.ac.jp

Fig. 2b:
Our recent estimation of the critical pion mass,mπ,E, in 3 flavor QCD. The continuum extrapolation
including new data sets with the temporal extent
of 12 gives an upper bound mπ,E ≲ 110 MeV.
[Y. Kuramashi et al., Phys. Rev. D 101, 054509]

https://www.ccs.tsukuba.ac.jp/