IPM

Matti Järvinen came to the Institute in October 2014. He had earned his PhD
in 2007 in Helsinki and had done postdocs at the University of southern Danmark
and at the University of Crete. During the first few months since his arrival, he
finished two projects related to holographic QCD. In collaboration with researchers
from the University of Helsinki, the University of Crete, and the University of Ice-
land, he analyzed [12] corrections to the equation of state for holographic QCD at
small temperatures. In earlier work it had been noticed that the pressure obtained
in the holographic model (V-QCD) was trivial at low temperatures. It did not in-
clude contributions from hadron states which should appear at leading order in the
Veneziano limit where the model was defined. In the new article, these corrections to
the pressure were estimated simply by using known expressions for the free hadron
gas, containing the pions and massive states distributed according to the Hagedorn
spectrum. By matching with the high temperature pressure obtained directly by us-
ing the holographic model, it was demonstrated that a second or a third order chiral transition could be obtained.
The other recent work [13] discusses the dependence of holographic QCD on the
quark mass. A phase diagram as a function of the quark mass and the ratio of the
number of flavours and colours x = Nf /Nc was sketched. The dependence on the
quark mass of observables such as the bound state masses, the chiral condensate,
the S-parameter, and the critical temperatures was studied. Many of the results
were argued to be universal, i.e., independent of the details of the holographic model,
and compared to explicit computations in the V-QCD models. The effect of adding
four-fermion operators in QCD was also discussed.
In the period from February 2015 to October 2016 M. Järvinen has finished two
projects. First, the study of a generalization of the Bjorken flow in heavy-ion collisions
which was done in collaboration with G. Policastro (ENS) and U. Gürsoy (Univer-
sity of Utrecht) and started in late 2014 was finished in late 2015 and published in
JHEP [14]. They used a non-conformal holographic background to demonstrate that
the cooling of the plasma is slowed down at late times due to the breaking of the con-
formal symmetry. Second, a study QCD at finite θ-angle and the physics of the axial
anomaly, using a holographic model with full backreaction of the quarks to the gluon
dynamics was just finished in September 2016 [15].
This extensive study went beyond the level of effective free theories, but also recovered the results of effective field theories in regimes where they can be used. It was done in collaboration with D. Arean (MPI Munich), I. Iatrakis (University of Utrecht), and E. Kiritsis (University of Crete and APC, Paris).
Two more projects of M. Järvinen will be finished soon. He has continued collabo-
ration with N. Jokela (University of Helsinki) and M. Lippert (Long Island University)
on applying top-down holography to condensed matter physics. They will show that
the DC and optical conductivities of the striped ground state (i.e., spontaneaously
broken translation symmetry) in a D3-D7 brane configuration which was constructed
in their earlier work involve interesting novel features.
He is also studying backreacted holographic QCD under strong magnetic fields
with U. Gürsoy, I. Iatrakis, and G. Nijs from the University of Utrecht. In partic-
ular they will demonstrate that holographic models are able to reproduce in detail
the decrease of the chiral condensate with increasing magnetic field near the critical
temperature of QCD, coined “inverse magnetic catalysis”, which has been recently
observed on the lattice.
M. Järvinen has several additional ongoing projects. As a continuation of [14],
quasi normal modes of the decaying plasma are being studied. Near a critical point
of the system, certain two-point correlators of the energy-momentum tensor can be
solved exactly, showing that hydrodynamics breaks down near the critical point. His
project in collaboration with C. Bachas (ENS) on the Kondo effect has extended to
a more generic study of holographic Wilson lines and related supersymmetric em-
beddings of D-branes which realize flows on the N = 4 super Yang-Mills with a
defect.
Recently M. Järvinen has also started projects on the dynamics of holographic
QCD in the conformal window with I. Iatrakis and E. Kiritsis, as well as a project with
T. Alho (University of Iceland) on the overall comparison of back-reacted holographic
QCD to lattice and experimental data.