Download The Royal Society of Edinburgh The Large Hadron Collider – What It

The Royal Society of Edinburgh
The Large Hadron Collider –
What It Might Tell Us About the Universe
Professor Peter Clarke FRSE
Professor of Physics, University of Edinburgh
Tuesday 2 June 2015
Greenwood Conference Centre, Dreghorn, Irvine
Report by Kate Kennedy
There are many things we do not know about the Universe, including what the nature is of
the dark matter of which most of it is made; why it seems to be accelerating apart; why there
is any matter left in it at all anyway; and, until recently, what is the origin of mass. The last
question was answered when the Large Hadron Collider (LHC) finished its three-year
running period in 2013 with the discovery of the Higgs boson. Since then, the LHC has
undergone a major upgrade and is due to start its second running period in 2015. This talk
explained the importance of the Higgs boson discovery and looked at what the LHC will
hope to find in the next three years. The talk also covered some of the wider outstanding
unsolved questions, such as dark matter and dark energy and the way we try to understand
these phenomena.
There are many outstanding questions about the Universe. Indeed, we don’t know what 95%
of it is made of, though experts believe this comprises around 25% dark matter and 70%
dark energy. As an example, stars and galaxies move very differently to how we would
expect if they were comprised of only conventional matter that we can see – leading to the
need for the existence of dark matter. Everything is made of atoms of varying numbers,
comprising protons, neutrons and electrons. Until recently, however, we did not understand
the origin of mass and this is what the search for the Higgs boson, and its subsequent
discovery in 2012, sought to explain. Furthermore, it is not really understood why there is
actually any matter still in existence in the Universe; since, originally, there would have been
equal quantities of matter and anti-matter created which should then have collided and
annihilated each other, leaving nothing but photons.
CERN, the European Centre for Nuclear Research, is located on the French/Swiss border
close to Geneva and has become the most important site in the world for particle physics.
After starting with a small circular accelerator, the size has increased to the point where the
current tunnel is now 27km in circumference and is located at a depth of 100 metres. Around
the circuit, there are four main access points where the detectors are located. Within the
tunnels, there are two beams of protons orbiting separately in opposite directions. The
protons are guided around the circle using over 1200 superconducting magnets that produce
a very large vertical magnetic field. Superconducting magnets use liquid helium cooling to
operate at minus 271 degrees Centigrade, and carry a huge electrical current using
superconducting wire, which has no electrical resistance when very cold. At points around
the tunnel, the beams are brought together to collide. This results in thousands of particles
being created. At those points, large, continuously-operating ‘experiments’ detect which
particles are produced, take measurements such as their momentum, and track where they
go. Essentially, the information from the detectors can be used to reconstruct and explain
what has happened in the collision process. At CERN, there are four major detectors that
run continuously 24 hours every day for many months. They create enormous amounts of
data, of the order of 20 petabytes a year (one petabyte equals one million gigabytes). The
analysis requires around a million programs to be run each day, which requires the use of
400,000 computers all around the world.
Professor Clarke explained that the LHC makes new particles by accelerating matter and
anti-matter protons and then colliding them. The resulting annihilation effectively produces
a fireball of pure energy, from which new particles can be created. Following the principle of
E = Mc2, the heavier the mass of the particle, the more energy is required. Consequently, to
create and detect heavier particles such as the ‘Higgs boson’, which weighs over a hundred
times more than a single proton, larger accelerators are required to produce both high
energy and a very high collision rate. The LHC is essentially a “factory for producing stuff
you otherwise could not create”.
Notwithstanding that, there is so much we don’t know and understand about the Universe;
what we do know is understood in significant detail. Indeed, 99% of the matter we know
about comprises protons, neutrons and electrons. These are essentially formed of ‘quarks’,
of which there are two main types – ‘up quarks’ and ‘down quarks’. Additionally, there are
‘neutrinos’, of which there are thousands travelling through us all the time. Until recently,
however, we did not know what gave mass to the electrons or to the quarks. A solution to
this problem was proposed by theoreticians including Higgs, Brout and Englert, and is
universally known as the ‘Higgs Mechanism’. Essentially, it proposes that what was believed
to be empty space is not actually empty space, but is filled with the ‘Higgs field’, which
attaches itself to particles and, in doing so, gives them mass. The field itself cannot be seen;
however, even in the absence of particles passing through, there is still apparent activity
because the field can have its own particles, Higgs bosons, which were finally detected by
the LHC in 2012.
Another principal question being considered by the research at the LHC is why there is any
matter and anti-matter left. The approach being taken is to examine how matter behaves
differently to anti-matter, as there must have been such an imbalance in the early Universe.
The LHCb experiment is specifically designed for this purpose and measures these
differences precisely to characterise and search for new sources of this phenomenon.
Other research being conducted at the LHC is the search for ‘dark matter’. Dark matter
candidates are associated with a theory of ‘super symmetry’, which proposes that for all the
particles we know about, there is a set of partner particles that differ by one half unit of ‘spin’.
It is one of these partner particles which could account for some dark matter. Since particles
decay and, in doing so, transmute into other particles, the objective is to measure the entire
decay chain and determine if there is energy which missed the detectors and was
unaccounted for. This could be the signature of dark matter; however, none has ever been
found as yet. Professor Clarke believes, if we are lucky, this could be the next big discovery.
The Vote of Thanks was offered by Dr Giles Hammond.
Opinions expressed here do not necessarily represent the views of the RSE, nor of its Fellows
The Royal Society of Edinburgh, Scotland’s National Academy, is Scottish Charity No. SC000470