The final piece of an all-new detector has completed the first leg of its
journey towards unlocking some of the most enduring mysteries of the
universe.
The 41-million-pixel Vertex Locator (VELO) was assembled at the University
of Liverpool. It was assembled from components made at different institutes,
before it traveled to its home at the Large Hadron Collider beauty (LHCb)
experiment at CERN.
Once installed in time for data-taking, it will attempt to answer the
following questions:
- Why is the universe made of matter, not antimatter?
- Why does it exist at all?
- What else is out there?
A fine balance at the dawn of space and time
In the moments immediately after the Big Bang, the universe was caught in a
fine balance between matter and antimatter.
From what we understand about the laws of nature, these forms of matter
should have annihilated each other and left behind a universe filled only
with light. Yet, against all odds, matter somehow gained the advantage and
something was left to form the universe we know today.
Our best understanding of the physics of the Big Bang tells us that matter
and antimatter were created in equal quantities. When they made contact in
the (far smaller and far denser) early universe, all of their combined mass
should have been violently transformed into pure energy. Why, and how,
matter survived the encounter is one of the most profound mysteries in
modern science.
The current theory is that, although matter and antimatter were created as
almost perfect mirror images, there must have been some tiny misbalance, or
blemish. This meant that some were not perfect reflections. This difference,
however tiny, might have been enough to give matter the edge.
Through the looking glass
Scientists have already found a small crack in the mirror, called
charge-parity (CP) violation. This means that, in some cases, the symmetry
of the matter and antimatter reflection becomes broken.
This results in a particle that is not the perfect opposite of its twin, and
this "broken symmetry" may mean that one particle could have an advantage
over the other.
When this symmetry is broken, an antimatter particle may decay at a
different rate to its matter counterpart. If enough of these violations
occurred after the Big Bang, it might explain why matter survived.
By behaving differently to their antimatter equivalents, it is possible that
matter particles with broken symmetry took just a little bit longer to
decay. If this caused matter to stick around just a little bit longer, it
could explain how it was the last one standing.
The deep unknown
Why matter survived is not the only mystery in the universe. There is
another issue puzzling scientists: what might dark matter be?
Dark matter is an elusive, invisible type of matter that supplies the
gravitational glue to keep stars moving around galaxies. Because we do not
yet know what dark matter is, it could be that there are other, new
particles and forces in the universe that we have not yet seen.
Discovering anything new could reveal a radically different picture of
nature to the one we have. New particles like these could announce
themselves by subtly changing the way the particles we can see behave,
leaving small but detectable traces in our data.
The beauty and charm of VELO
The new VELO detector, which will replace the old VELO detector, will be
used to investigate the subtle differences between matter and antimatter
versions of particles that contain subatomic particles. These are known as
beauty quarks and charm quarks.
These exotic quark-containing particles, also known as B and D mesons, are
produced during collisions within the Large Hadron Collider (LHC). They are
difficult to study because mesons are very unstable and decay out of
existence within a fraction of a fraction of a second.
When they decay, however, they actually transform into something else.
Scientists believe that, by studying these different decays and their
properties, VELO data will help LHCb to reveal the fundamental forces and
symmetries of nature.
Incredibly precise measurements
The new VELO detector will sit as close as possible to where the particles
collide within the LHCb experiment. These particles decay in less than a
millionth of a millionth of a second and travel only a few millimeters.
Therefore, this close proximity will give the device the best possible
chance of measuring their properties.
VELO's sensitivity and proximity to the LHC's beams will allow it to take
incredibly precise measurements of the particles as they decay.
By comparing these readings to predictions made by the Standard Model (the
guiding theory of particle physics) scientists can look for deviations that
might hint at new particles in nature. They can also look for CP violations
or other reasons why matter and antimatter behave differently.
These deviations could revolutionize our understanding of why the universe
is what it is.
Building on the legacy of the old
The VELO may be brand new and cutting-edge but it will be building on the
legacy of the previous VELO detector. The VELO has a state-of-the-art pixel
detector made up of grids of tiny squares of silicon that gives
high-resolution even in the challenging radiation environment near the LHC
beams.
Its predecessor, with its lines of stacked silicon detectors, helped the
LHCb make discoveries, including:
- New states of matter.
- Incredibly rare beauty quark decays.
- Differences between matter and antimatter charm quarks.
- The first intriguing indication of as yet unexplained behavior in beauty quark decay.
Glimpses of particle behavior
UK VELO project leader Professor Themis Bowcock, from the University of
Liverpool, said: "The data captured by the old VELO detector has given us
really tantalizing glimpses of particle behavior. To make progress, we need
to turn this into a really thorough, forensic investigation and this is
where the new VELO detector comes in. It gives us the precise set of eyes we
need to observe particles at the level of detail we need. Quite simply, the
VELO makes our whole physics program possible on LHCb."
Unprecedented detail
New VELO will be able to capture these decays in unprecedented detail.
Couple this with upgraded software and super-fast readout electronics that
will allow beauty and charm quarks to be pinpointed in real-time. Scientists
will have a device that allows them to track and analyze decays that were
previously too difficult to reconstruct.
What also makes the new VELO detector unique is that scientists can lift it
out of the way as they prepare the particle beams for collisions. Then, they
can move it mechanically into place when LHCb is ready to collect data.
This allows scientists to capture clear information from the first particles
that radiate from the collisions without unnecessary wear and tear from the
beam.