Big surprises come in small nuclei
Electron scattering measurements from nuclei have shown a clear difference in
how quarks are distributed in large nuclei compared to small nuclei, a
phenomenon called the EMC effect. Our understanding of the origins of the
effect is limited by the difficulty of modeling large, complex nuclei.
Therefore, even basic information about the effect has been elusive, such as
the underlying mechanism that causes the difference and whether it depends on
the mass or density of the nucleus.
Jefferson Lab experiment E03-103 made precise new measurements of the EMC
effect for four light nuclei. The large difference between 3He and 4He, the
lightest nuclei measured, rules out mass-dependent explanations. The large
difference between 3He and 9Be, which have similar densities but very
different masses, rules out density-dependent models. The low density of 9Be
is related to its unusual structure; most of the time it is in a configuration
with two 4He-like clusters and an additional neutron orbiting around each
other. The orbiting clusters yield a large radius and an anomalously low
average density, even though most nucleons are contained within the high local
densities of the clusters. This clearly demonstrates that the microscopic
structure of nuclei, usually neglected in high energy measurements, is of
critical importance. The results also suggest that the EMC effect may be
entirely generated within these small, high-density clusters, where densities
can briefly approach those in a neutron star.
***
lg12369
Blast Shocks and marbles falling upon sand
In the 1940s, G. I. Taylor, a famous physicist and hydrodynamicist,
managed to deduce the energy released by a nuclear bomb blast,
classified information then, from photographs of such explosions.
Since then, blasts have been studied in a variety of settings and are
believed to arise in the explosion of stars raising important
questions about the relevance of the analysis developed by Taylor. In
a recent paper to be published in Physical Review Letters, we show
that such a complex phenomenon can be produced in the lab by simply
letting a marble impact a sand layer. If the sand layer is made to
flow, the impact of the marble makes a hole in the layer. This hole
expands rapidly as if it were a small explosion. And the expansion of
the hole indeed follows the rules for blast shocks. With one major
difference, energy losses are present and can be accounted for in a
simple way. It may well be that such experiments help in the
understanding of phenomena as far away as a dying star and provide a
simple system to further study the blast problem.
***
LG12995
Taming the charmonium wilderness
Since its discovery in the early 70s until the beginning of the new century,
charmonium spectroscopy was considered a safe haven in the tumultuous
landscape of hadronic physics. It seemed to be fairly well understood and no
unusual or unexplained features seemed to exist. However, during the last
years experiments have proved how utterly wrong we were. No less than
eighteen new mesons have been recently reported in this once upon a time
quite and peaceful energy region. Their understanding as simple
quark-antiquark pairs has proved to be nearly impossible and therefore
several different attempts to dissect them have been performed. Their success
in the ambitious task of obtaining a full picture of the whole charmonium
spectra has been, at best, limited.
In our work we have attempted a different approach. Making use of standard
few-body techniques developed, for example, to study the deuteron, we have
examined the possible existence of meson-meson molecular states hidden within
the charmonium spectra. Hence, our idea was not to describe the properties of
a carefully chosen set of states, but to determine the possible existence of
molecular structures. We have carefully examined all possible quantum
numbers establishing boundaries for the existence of molecular states
that might be identified with some of the new reported mesons. This is
the first time such a global and ambitious study has been performed
and we hope it will encourage an experimental effort to confirm or rule out
the molecular character of some particular states.
***
LK11836
Refrigerated ions last longer!
We have measured the lifetime of negative helium ions stored in an ion trap
at very low temperature to avoid any influence on the result by the thermal
radiation from the surroundings. Even though no visible light is seen in the
thermal radiation of an object at room temperature, the infrared radiation
is sufficiently intense and energetic to destroy a weekly bound system like
negative helium rapidly enough to severely disturb a measurement of the
intrinsic lifetime (found to be 359 microsecs in the present experiment). By
building a very small electrostatic ion-beam trap and mounting it in a
vacuum chamber which can be cooled down to 10 Kelvin, we have for the first
time reached a situation where the thermal radiation can be neglected and
the lifetime determined directly without the need for any assumptions or
corrections.
