83,407 research outputs found
Open-charm systems in cold nuclear matter
We study the spectral distributions of charmed meson with quantum
numbers in cold nuclear matter applying a self-consistent and covariant
many-body approach established previously for the nuclear dynamics of kaons. At
leading orders the computation requires as input the free-space two-body
scattering amplitudes only. Our results are based on the s-wave meson-nucleon
amplitudes obtained recently in terms of a coupled-channel approach. The
amplitudes are characterized by the presence of many resonances in part so far
not observed. This gives rise to an intriguing dynamics of charmed mesons in
nuclear matter. At nuclear saturation density we predict a pronounced two-mode
structure of the mesons with a main branch pushed up by about 32 MeV. The
lower branch reflects the coupling to two resonance-hole states that are almost
degenerate. For the we obtain a single mode pushed up by about 18 MeV
relative to the vacuum mode. Most spectacular are the results for the
meson. The presence of an exotic resonance-hole state gives rise to a rather
broad and strongly momentum dependent spectral distribution.Comment: 11 pages, 3 figure
Chiral symmetry and strangeness at SIS energies
In this talk we review the consequences of the chiral SU(3) symmetry for
strangeness propagation in nuclear matter. Objects of crucial importance are
the meson-baryon scattering amplitudes obtained within the chiral
coupled-channel effective field theory. Results for antikaon and
hyperon-resonance spectral functions in cold nuclear matter are presented and
discussed. The importance of the Sigma(1385) resonance for the subthreshold
antikaon production in heavy-ion reaction at SIS is pointed out. The in-medium
properties of the latter together with an antikaon spectral function based on
chiral SU(3) dynamics suggest a significant enhancement of the pi \Lambda ->
bar K N reaction in nuclear matter.Comment: 10 pages, 2 figures, invited talk at Erice 200
Effects of vinyl substitutions on resonance Raman spectra of (bacterio)chlorophylls
Soret resonance and Qy preresonance Raman spectra are reported and compared for a series of (bacterio)chlorophylls. Chlorophyll a, 2-acetylchlorophyll a, bacteriochlorophyll a and 2-vinylbacteriochlorophyll a were studied in the non-protic solvent tetrahydrofuran. These experiments were designed to identify Raman bands corresponding to the stretching mode(s) of the vinyl group at the C-2 position of ring I of chlorophyll a and 2-vinylbacteriochlorophyll a, and to ascertain whether additional bands corresponding to Ca Cm and/or Cb Cb vibrations could be observed in the 1615-1660 cm-1 region. Raman spectra of chlorophyll a and 2-vinylbacteriochlorophyll a exhibit a 1625 cm-1 band, which is absent from the Raman spectra of 2-acetylchlorophyll a and bacteriochlorophyll a. It is assigned to the vC2a C2b mode of the vinyl group. No other band can be definitively assigned to any mode predominantly arising from vinyl motions. The acetyl-containing molecules 2-acetylchlorophyll a and bacteriochlorophyll a give rise to a ca. 1070 cm-1 band, which appears to be related to the presence of the acetyl substituent. The 1615-1660 cm-1 region of the Raman spectra of all four derivatives did not contain any additional band which could be ascribed to modes involving the vCa Cm and/or Cb Cb coordinates
Baryon resonances from chiral coupled-channel dynamics
We discuss the formation of s- and d-wave baryon resonances as predicted by
the chiral SU(3) symmetry of QCD. Based on the leading order term of the chiral
Lagrangian a rich spectrum of molecules is generated, owing to final-state
interactions. The spectrum of s- and d-wave baryon states with zero and
non-zero charm is remarkably consistent with the empirical pattern. In
particular, the recently announced Sigma_c(2800) of the BELLE collaboration is
reproduced with realistic mass and width parameters. Similarly, the d-wave
states Lambda_c(2625) and Xi_c(2815) are explained naturally to be chiral
excitations of J^P=3/2^+ states. In the open-charm sector exotic multiplet
structures are predicted. These findings support a radical conjecture: meson
and baryon resonances that do not belong to the large-N_c ground state of QCD
should be viewed as hadronic molecular state
On the method of typical bounded differences
Concentration inequalities are fundamental tools in probabilistic
combinatorics and theoretical computer science for proving that random
functions are near their means. Of particular importance is the case where f(X)
is a function of independent random variables X=(X_1, ..., X_n). Here the well
known bounded differences inequality (also called McDiarmid's or
Hoeffding-Azuma inequality) establishes sharp concentration if the function f
does not depend too much on any of the variables. One attractive feature is
that it relies on a very simple Lipschitz condition (L): it suffices to show
that |f(X)-f(X')| \leq c_k whenever X,X' differ only in X_k. While this is easy
to check, the main disadvantage is that it considers worst-case changes c_k,
which often makes the resulting bounds too weak to be useful.
In this paper we prove a variant of the bounded differences inequality which
can be used to establish concentration of functions f(X) where (i) the typical
changes are small although (ii) the worst case changes might be very large. One
key aspect of this inequality is that it relies on a simple condition that (a)
is easy to check and (b) coincides with heuristic considerations why
concentration should hold. Indeed, given an event \Gamma that holds with very
high probability, we essentially relax the Lipschitz condition (L) to
situations where \Gamma occurs. The point is that the resulting typical changes
c_k are often much smaller than the worst case ones.
To illustrate its application we consider the reverse H-free process, where H
is 2-balanced. We prove that the final number of edges in this process is
concentrated, and also determine its likely value up to constant factors. This
answers a question of Bollob\'as and Erd\H{o}s.Comment: 25 page
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