[Landau ITP Seminars] Friday 29.11.2019

[Serge Krashakov]

Уважаемые сотрудники ИТФ,

На заседании Ученого совета ИТФ в пятницу 29 ноября будут заслушаны доклады:

1) Baerbel Rethfeld (Technische Universitaet Kaiserslautern, Germany)
*Relaxation dynamics of nonequilibrium electrons in laser-excited solids*

When an ultrashort laser pulse of visible light is absorbed by a solid, 
mainly the electrons in the material are excited. In metals, free 
electrons in the conduction band can directly absorb photons. In 
semiconductors and dielectrics, on the other hand, a band gap has to be 
overcome first, as almost no free electrons are present at room 
temperature in the unexcited material. Due to this excitation, the 
electronic system, or the so-called electron-hole plasma, is in a 
nonequilibrium state. A sequence of different relaxation processes 
transfers the material into a new equilibrium. Depending on the 
interaction associated with the particular relaxation process, it occurs 
on a characteristic timescale. On the basis of complete Boltzmann-type 
collision integrals, we calculate the transient distribution functions 
of electrons and phonons in different materials. We consider 
electron-electron interaction, different ionization processes, as well 
as electron-phonon coupling. By that we trace the relaxation cascade of 
nonequilibrium electrons after ultrafast heating. Distinct material 
properties enter through the density of states of the electrons in the 
conduction band. We study in particular noble metals, dielectrics and 
ferromagnets. In noble metals and ferromagnets, d-electrons play an 
important role, whereas in dielectrics two separated bands govern the 
dynamics and the ionization state may differ from. We show, that the 
electron distributions deviate from Fermi distributions for timescales 
up to a few picoseconds. While the initial thermalization within one 
band has an intrinsic timescale of typically only a few tens of 
femtoseconds, nonequilibrium occupations of the different bands as well 
as continous electron-phonon coupling can drive the conduction electrons 
out of equilibrium for much longer times [1, 2]. We present in detail 
the mutual influence of different interaction and relaxation processes.
[1] N. Brouwer and B. Rethfeld, Phys. Rev. B 95, 245139 (2017). [2] S.T. 
Weber and B. Rethfeld, Phys. Rev. B 99, 174314 (2019).


2) Б.Г. Захаров
*Color randomization of fast two-parton states in quark-gluon plasma in 
heavy ion collisions*

We study the color randomization of two-parton states produced after 
splitting of a primary fast parton in the quark-gluon plasma. We find 
that the color randomization of the two-parton states in the quark-gluon 
plasma produced in heavy ion collisions at RHIC and LHC energies is 
rather slow. At jet energies E= 100 and 500 GeV, for typical jet path 
length in the plasma in central Pb+Pb collisions, the SU(3)-multiplet 
averaged color Casimir of the two-parton states differs considerably 
from its value for the fully color randomized state. We evaluate the 
energy dependence for generation of the nearly collinear gluon-gluon 
pairs in the decuplet color state and quark-gluon pairs in the 
anti-sextet color states, that can lead to an anomalous baryon jet 
fragmentation, which are forbidden in vacuum for nucleon-nucleon 
collisions. Our results show that the baryon production via the color 
anomalous two-parton states can be important in the enhancement of the 
baryon/meson ratio observed in heavy ion collisions at RHIC and LHC.


3) Б.Г. Захаров
*Radiative parton energy loss and baryon stopping in heavy ion 
collisions* (короткий доклад)

We study the radiative energy loss contribution to proton stopping in 
heavy ion collisions. The radiative parton energy loss is calculated 
within the light-cone path integral approach to induced gluon emission. 
We have found that the radiative correction can fill in partly the 
midrapidity dip in the net proton rapidity distribution in AA collisions 
at center of mass energy \sqrt{s} about 10 GeV. This energy region is of 
great interest in connection with the beam energy scan program at RHIC 
(Brookhaven) and future experiments at collider NICA (Dubna) motivated 
by searching for the QCD critical point. We show that the net proton 
fluctuations at midrapidity, that have been proposed to be a good probe 
of the QCD critical point, may be dominated by the initial fluctuations 
of the proton flow, which, to a good accuracy, should be binomial, even 
in the presence of the critical point.


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