Perspectives of Nuclear Physics in Europe - European Science ...

The longest-range parts of these forces arise from pion
exchange and so can be calculated from the same chiral
EFT as used for NN interactions. Here the smaller
scattering lengths and weaker pion-hyperon couplings
mean that Weinberg’s expansion scheme is appropriate.
However, there is a clear need for a larger and better
database, as current analyses are not able to pin down
the sizes of the singlet and triplet scattering lengths. Here
too, proton-proton collisions with strangeness in the final
state, and measurements of polarization observables
can offer another means for determining these fundamental
parameters. Experiments on processes such as
pp→pKΛ are now under way at COSY.
Hypernuclei, nuclear systems with one or more
bound hyperons, can provide an alternative route to
learning about the hyperon-nucleon interaction. There
has been considerable experimental progress in the
γ-spectroscopy of their low-lying excited states, at KEK,
JLab and Frascati. This has led, for example, to the first
determination of the spin-spin terms in the YN interaction.
Hypernuclei also offer a unique possibility to
study the non-mesonic weak decays ΛN→NN, due to the
Pauli principle hindering the pionic decay modes. Such
processes can provide the primary means of exploring
the strangeness-changing weak interactions between
baryons.
Work on EFTs has also led to the important insight
that few-body systems with short-range interactions
and a large scattering length a display universal features
that do not depend on the details of their structure or
their interactions at short distances. In the two-body
sector, these universal properties are familiar, arising
from the leading terms of the effective-range expansion.
They include, for instance, the relation between
the binding energy and depend the scattering. In the
three-body sector, universality leads to the Efimov effect
– the accumulation below threshold of bound states of
a three-body system when the two-body bound states
are located exactly at the dissociation threshold – and
a log-periodic dependence of scattering observables
on the energy and the scattering length. These features
have been observed in ultracold atomic systems but are
also expected to appear in few-nucleon systems, halo
nuclei, and even in hadron physics. Here, for example,
the X(3872) is a very loosely bound lying within
half an MeV of the threshold. Such a shallow state
should display universal features, which can be tested by
studying D 0 X(3872) scattering in final-state interactions
of B c decays. These universal properties are embodied
within the framework of EFTs, making these the best tool
for such calculations. Universality provides fascinating
links to other fields of physics, spanning energies from
meV to TeV, and such connections should be made even
stronger in the future.
Another area that has received much attention is the
so-called “medium modification” of hadron properties
in hot or dense matter. Changes to the vector mesons’
spectral functions (mass and width) have been proposed
for two decades as possible signals of modifications of
the QCD condensates in the nuclear medium. Reduced
vector-meson masses have been predicted for vector
mesons and searched for over the years in several
experiments, making use of different energy regimes
and collision systems, to cover a range of densities and
temperatures. As of today, experiments have been conducted,
albeit with rather low statistics, at GSI, KEK,
JLab, BNL, and CERN. None of the published results
gives a clear and unambiguous signal of non-trivial inmedium
effects. Moreover, since the experiments cover
different phase space regions, their results are not easily
compared. Nonetheless, they do point towards a sizeable
broadening of the vector states in matter.
Physics Perspectives
There are very bright prospects for studies of hadronic
interactions in both the short and long term. In particular,
the J-PARC and FAIR facilities should produce abundant
data on hadronic systems with strange and charm as well
as light quarks. On the theoretical side, LQCD and EFT
methods are now capable of dealing with the multi-scale
problems that necessarily arise in such systems. With
the new area of charm-quark nuclear physics, it will also
be able to learn about the interplay of and transition from
non-perturbative to perturbative QCD dynamics.
The upcoming huge body of data for J/ψ and ψ’
decays from the BES-III experiment at BEPCII will offer
multiple opportunities to further investigate fundamental
properties of QCD encoded in the decay and final-state
dynamics. For example, it has recently been shown that
the light quark mass difference m u -m d can be extracted
form a combined analysis of ψ’→h c π 0 and η’ c →χ c0 π 0
decays, the latter being measurable with PANDA at
FAIR. Other tests are related to chiral dynamics with
strange quarks and SU(3) flavour breaking. In this field there
is a close collaboration between theory and experiment
with the intended goal to maximize the physics output
from the upcoming premier hadron physics facility FAIR.
In Europe, there will be dedicated chiral perturbation
theory tests, tests of the discrete symmetries C,P,T and
the search for physics beyond the Standard Model in
dedicated η and η’ decay studies at MAMI, ELSA and
COSY.
Lattice QCD calculations are now starting to include
the so-called disconnected contributions, which will
allow for precise calculations of the isospin zero (1/2) ππ
(πK) scattering lengths employing Lüscher-type formulae.
Similarly, a systematic analysis of all meson-baryon and
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