62 research outputs found
Anomalous fermion number nonconservation: Paradoxes in the level crossing picture
In theories with anomalous fermion number nonconservation, the level crossing
picture is considered a faithful representation of the fermionic quantum number
variation. It represents each created fermion by an energy level that crosses
the zero-energy line from below. If several fermions of various masses are
created, the level crossing picture contains several levels that cross the
zero-energy line and cross each other. However, we know from quantum mechanics
that the corresponding levels cannot cross if the different fermions are mixed
via some interaction potential. The simultaneous application of these two
requirements on the level behavior leads to paradoxes. For instance, a naive
interpretation of the resulting level crossing picture gives rise to charge
nonconservation. In this paper, we resolve this paradox by a precise
calculation of the transition probability, and discuss what are the
implications for the electroweak theory. In particular, the nonperturbative
transition probability is higher if top quarks are present in the initial
state.Comment: 26 pages, 6 figure
The contribution of NLO and LPM corrections to thermal dilepton emission in heavy ion collisions
Recently lots of efforts have been made to obtain the next to leading order
and Landau-Pomeranchuk-Migdal corrections to the thermal dilepton emission rate
in perturbative QCD. Here we apply these results to the plasma created in heavy
ion collisions and see wether these corrections improve the comparison between
theoretical calculations and experimental results for the invariant mass
dependence of the dilepton emission rate. In particular, we simulate the
quark-gluon plasma produced at RHIC and LHC using a 2+1-dimensional viscous
hydro model. We compare our results to STAR experiment and comment on the need
for a non-perturbative determination of the dilepton rate at low invariant
mass.Comment: 9 pages, 11 figure
A novel Bayesian approach to spectral function reconstruction
We present a novel approach to the inference of spectral functions from
Euclidean time correlator data that makes close contact with modern Bayesian
concepts. Our method differs significantly from the maximum entropy method
(MEM). A new set of axioms is postulated for the prior probability, leading to
an improved expression, which is devoid of the asymptotically flat directions
present in the Shanon-Jaynes entropy. Hyperparameters are integrated out
explicitly, liberating us from the Gaussian approximations underlying the
evidence approach of the MEM. We present a realistic test of our method in the
context of the non-perturbative extraction of the heavy quark potential. Based
on hard-thermal-loop correlator mock data, we establish firm requirements in
the number of data points and their accuracy for a successful extraction of the
potential from lattice QCD. An improved potential estimation from previously
investigated quenched lattice QCD correlators is provided.Comment: 4 pages, 4 figure
A gauge invariant Debye mass and the complex heavy-quark potential
Following the original idea of Debye, we define and extract a gauge-invariant
screening mass from the complex static in-medium heavy-quark potential
, recently obtained from lattice QCD. To this end we derive a
field theoretically motivated analytic formula that faithfully reproduces both
the screened real- as well as the imaginary part of the lattice potential with
a single temperature dependent fit parameter . Using values of the real
part of in a gluonic medium, we obtain Debye masses compatible
with predictions from HTL perturbation theory.Comment: 13 pages, 2 figure
Can the Standard Model CP violation near the W-bags explain the cosmological baryonic asymmetry?
In the scenario of cold electroweak baryogenesis, oscillations of the Higgs
field lead to metastable domains of unbroken phase where the Higgs field nearly
vanishes. Those domains have also been identified with the bags,
a non-topological soliton made of large number () of gauge quanta
and heavy (top and anti-top) quarks. As real-time numerical studies had shown,
sphalerons (topological transition events violating the baryon number) occur
only inside those bags. In this work we estimate the amount of CP violation in
this scenario coming from the Standard Model, via the Cabibbo-Kobayashi-Maskawa
(CKM) quark mixing matrix, resulting in top-minus-antitop difference of the
population in the bags. Since these tops/anti-tops are "recycled" by
sphalerons, this population difference leads directly to the baryonic asymmetry
of the Universe. We look at the effect appearing in the 4th order in weak
diagrams describing interference of different quark flavor contributions. We
found that there are multiple cancellations of diagrams and clearly
sign-definite effect appears only in the 6th order expansion over
flavor-dependent phases. We then estimate contributions to these diagrams in
which weak interaction occurs (i) inside, (ii) near and (iii) far from the
b-bags, optimizing the contributions in each of them. We conclude
that the second ("near") scenario is the dominant one, producing CP violation
of the order of , in our crude estimates. Together with the baryon
violation rate of about , previously demonstrated for this scenario,
it puts the resulting asymmetry close to what is needed to explain the observed
baryonic asymmetry in the Universe. Our answer also has a definite sign, which
apparently seems to be the correct one.Comment: 7 pages, 2 figure
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