The
introduction of ‘quarks’ as fundamental constituents of matter marked a
revolutionary step forward in our understanding of the physical world. As
revolutionary as the introduction of atoms, about five decades earlier.
Quarks
and electrons and the forces between them: that is all that is needed to build
up the material world.
As an
example let us look at the proton, the simplest of all atomic nuclei, the
nucleus of the hydrogen atom. It consists of three quarks ‘swimming’ in the
force field that binds them together. As in taxonomy we label the quarks with
(trivial) names for ease of description. Two of the three quarks in the proton
are ‘u’ quarks and one is a ‘d’ quark. So a proton can be labeled ‘uud’. Quarks
have a property that initially was impeding their acceptance as real, physical
objects: their electrical charge is not equal to an integer multiple of the
electron charge, the fundamental charge e.
The u quark has positive charge 2/3 |e|
and the d quark negative charge -1/3 |e|. A neutron is then ‘udd’. A third
quark was required to ‘build’ the particles that were known at the time of the
construction of the quark model: the s quark, with the same charge as the d
quark. Using all three quark species for making combinations of three quarks it
is possible to make ten combinations that form the so called baryon decuplet –
baryon is the generic name for ‘proton like particles’, consisting of three
quarks.
A
spectacular confirmation of this quark picture was the discovery of the Ω-
(Omega-minus) particle (in 1964): the ‘sss’ state that completes the baryon
decuplet.
‘Baryon
spectroscopy’ has revealed the existence of many hundreds of three quark states,
including ‘excitations’ of the ground states. Although the underlying quark
picture is very simple, the reality built on it, is very complex. What purpose
this complexity serves is unknown. Perhaps this is what Steven Weinberg meant when
he wrote: The more the universe seems comprehensible, the more it seems
pointless.
The story does not end here. Three more quark
species were discovered, the ‘c’, ‘b’ and ‘t’ quarks: charm, beauty and truth,
or, somewhat more down to earth: charm, bottom and top. There are six quarks
species, or, as the more commonly used nomenclature says: six quark flavors. (Why
six? Nobody knows, but a lot can be said about it. I will not do that here.) So
we can now build baryons by combinations of three quarks where each quark has
one of six flavors. The ‘decuplet’ referred to above, thereby turns into a ’56-plet’.
(Please verify!).
The CMS collaboration at the Large Hadron Collider
published this week the first ever detection of an excited state of the three
quark combination ‘usb’. This particle is called ‘xi-star sub b’. It belongs to
the ’56-plet’ I suspect. It is the good old ‘decuplet’ xi-star (‘uss’) with one
‘s’ quark replaced by a ‘b’ quark. For CMS this result is a by-product: the
real aim is to find the Higgs boson and other ‘new physics’. But the
measurement of this new baryon is an achievement that would have been deemed
impossible a few years ago – it is possible thanks to superbly functioning
state of the art technology of both the CMS setup and the LHC accelerator.
Frontiers of technology were pushed back to achieve this result and new
knowledge was added in a wonderful contribution to scientific culture. The
article is freely available on arXiv.org prior to publication in an ‘official’
journal. Enjoy!
There was more news this week. The Dutch scientist
Fouchier and his group have created a mutation of the virus (H5N1) causing bird
flu that can be transferred to humans. This finding has been publicized by the
scientists months ago, but they were not allowed to publish, because of the
possible abuse of their results by terrorists. Good sense has finally prevailed
and the arguments in favor of publication put forward by the authors have won
(increased knowledge of the influenza virus, the subsequent development of
vaccines, etc.) The publication is yet to appear and will, I understand, be
published in Science. The main author, Fouchier, is relieved that finally the
work can be published, and rightly so. He is quoted to have said: ‘together
with the editors of Science we will now write the text in such a way that it is
understandable for a broad audience, also in view of all the commotion that has
surrounded our work’. Bravo! But... Science is not accessible to the general
public. It is not Open Access, but published behind a pay-wall. I urge Fouchier
and his co-workers to publish the article in a place that is freely accessible
for everybody interested, as soon as it is written. I am almost sure that
Science will not allow this...
Yet more news this week. Nature published an
article co-authored by a leading Dutch scientist, Van den Broeke, entitled: ‘Antarctic
ice sheet loss driven by basal melting of ice shelves’. A dramatic thinning is
observed of the floating Antarctic ice sheet by warming up ‘from below’ by streams
of warmed up seawater. If I understood correctly. The result was highlighted on
Dutch national television and possible dramatic consequences for sea level rise
were explicitly mentioned. This article has to appear in the public domain. The
work has been publicly funded and it is not reasonable that the public should
pay 32 dollars to Nature to be allowed to familiarize itself with these
important findings!
All rich scientific results referred to above
should be freely available for all of us – general public, politicians and
scientists alike!
Jos Engelen
April 29, 2012
