NEWS IN FOCUS
IN FOCUS NEWS
LHC
2.0
CMS
Higher energy
he worlds most powerful particle collider is poised to
roar once again into action after a two-year hiatus. At
the end of March, the Large Hadron Collider (LHC)
at CERN, Europes particle-physics lab near Geneva, Switzerland, will start smashing particles together at a faster rate and
with higher energies than ever before. Were standing on the
threshold of a completely new view of the Universe, says Tara
Shears, a particle physicist at the University of Liverpool, UK.
The first run began in earnest in November 2009 and ended
in February 2013. The LHC collided particles mainly protons
but also heavier particles such as lead ions at high enough
energies to discover the Higgs boson in 2012, which garnered
those who predicted the subatomic particle a Nobel prize.
In the next run, set to last three years, energies will rise to an
eventual 14 teraelectronvolts (TeV; see Hardware rebooted).
One hope is that higher energies will produce evidence for
supersymmetry, an elegant theory that could extend the standard model of particle physics (see Desperately seeking SUSY).
They could also shake out particles of dark matter, the invisible
substance that is thought to make up 85% of the matter in the
Universe (see Decays decoded).
More collisions will enable more-precise study of the Higgs
nature (see The Higgs factory) and will provide clarity on
anomalies hinted at in run 1 (see Known unknowns).
In the first run we had a very strong theoretical steer to look
for the Higgs boson, says Shears. This time we dont have any
signposts that are quite so clear.
A new view of the Universe
ALICE
LHCb
ATLAS
The LHC is a 27-kilometre ring that
circulates beams of protons accelerated
to near the speed of light in opposite
directions. At four points, the two beams
collide, creating showers of particles
that are analysed by four detectors:
CMS, ATLAS, ALICE and LHCb.
20052013
Hardware rebooted
Beams are composed
of bunches of billions
of protons, which
travel at close to the
speed of light.
RUN 1: 20
10,000 new electrical
connectors fitted
between magnets will
divert current if there
is a fault.
Renovated cryogenics keep
magnets cold enough to maintain
a superconducting state, in which
they have no resistance and so
generate high current.
Higher energy
Beam pipe
Vacuum
092013
Beam
Collision energy will increase
from the 8 TeV of run 1 to
13 TeV and probably up to
14 TeV by the end of run 2.
The machine was initially
supposed to run at this
energy before it was damaged
by a short circuit in 2008.
RUN 2: 2015
2018
Proton bunches
spaced at
50-nanosecond (ns)
intervals
If the LHC makes supersymmetric particles, their lifetimes
will be fleeting. But physicists can deduce their presence
from the more-stable decay products. In at least one case,
such SUSY clues could also be evidence for dark matter.
Top quark
Up
quark
Quark
Standard-model
particles
Gluon
Hypothetical
SUSY particles
Gluon
collision
Some theories
suggest that the stop
would be the lightest
SUSY squark,
making it the easiest
to detect because it
would show up in
lower-energy collisions
than the others.
Stop
squark
Gluino
Gluino
Sbottom
squark
Squark
LHC experiments
discovered the Higgs
boson but they did not
produce enough of the
particles to examine
their properties in
much depth.
B quarks
Photons
Upgrades will increase CERNs
annual electricity bill by 20%
to 60 million (US$65 million).
Not to scale
At each of the four interaction
points, the number of
protonproton collisions will
increase from 600 million to
more than 1 billion per second,
thanks in part to a collision
area that has reduced from
75 to 48 micrometres across.
W bosons
Deviations in the decay
frequencies could hint
at behaviour that does
not fit with the standard
model. There may even
prove to be more than
one type of Higgs.
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The Higgs is detected
through the particles
it decays into, each of
which is expected to
be produced with a
particular frequency.
Quarks
Missing
energy
Known unknowns
More collisions will help to resolve some ongoing mysteries.
One of these concerns an anomaly in the way a transient
particle called a B+ meson decays.
The B+ meson can decay
in two ways that should
be equally rare.
Z bosons
48 m
More collisions
Missing
energy
Physicists will look for these quarks, and see whether
their total energy and momentum adds up to that of
the two gluons that sparked the collision. Just the right
amount of missing energy would suggest the presence
of neutralinos and, by a process of deduction, the
other supersymmetric particles in the decay chain.
The Higgs factory
Higgs
boson
Proton bunches in run 2
are smaller, closer together
and have higher energies
than those in run 1.
Neutralino
The neutralino would
have almost no
interaction with normal
matter meaning that
it would slip through
the LHCs detectors
making it a candidate
constituent of dark
matter.
More collisions
25 ns
Magnets will shrink
the diameter of the
proton beam.
Proton
(made of
quarks and
gluons)
Bottom
quark
Gluon
The gluino is superpartner of the gluon, which
carries the strong force that binds the quarks in
protons. So both squarks and gluinos should
show up more often in protonproton collisions
than should other supersymmetric particles.
Magnet
The inside of the beam pipe
has been coated with a
protective material to make
the vacuum more secure.
Decays decoded
Higher energies mean that the LHC can produce heavier particles (because of E = mc2)
and perhaps some of those predicted by the theory of supersymmetry, or SUSY. An extension
to the standard model of particle physics, SUSY postulates a giant 'superpartner' for each
known particle, and would offer explanations for mysteries such as the nature of dark matter.
B Y E L I Z A B E T H G I B N E Y / I L L U S T R AT I O N B Y N I K S P E N C E R
Superconducting magnets will
operate at higher currents to
provide the force needed to
steer the more energetic
beams in a circle.
Upgrades to the LHC will allow it to fire
proton beams at higher rates and
energies than it did in its first run.
Desperately seeking SUSY
Made up of different
combinations of
quarks
Tau leptons
Kaon
Muons
Negatively charged,
much heavier than
electrons
In run 1, the LHCb detector
saw the electron decay
pathway occurring 25% more
often, which could suggest
the influence of particles
beyond the standard model.
But further examples in run 2
are needed to confirm that
this is not a statistical fluke.
Kaon
B+ meson
Electrons
Negatively charged,
more stable than
muons
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