Investigating W bosons at the LHC
Abstract : Comparing current theory to forthcoming data from the (soon to be) world’s highest energy collider will help us to understand more fully the basic
interactions between particles. This will be studied in the context of W bosons in a region that has not previously been observable.
The Large Hadron Collider (LHC) and LHCb
The LHC is a high energy particle physics colider near Geneva, on the border
between France and Switzerland. It is a circular accelerator with a circumference
of 27km which is designed to collide two proton beams. The actual accelerator is
100m underground and there are 4 main detectors at different points - one of
which is LHCb (the LHC beauty experiment - so called because of its aim to look
mainly at beauty quarks).
LHCb is a one arm forward spectrometer, which means that it looks at particles
which travel close to the beam but only in one direction. There are many different
parts which have to work together in order to be able to reconstruct the collisions
and the subsequent interactions and the first part (some would argue one of the
most important) was built at Liverpool. This is the Vertex Locator (or VeLo). It is
able to track to a very high resolution where the initial proton-proton collision took
place.
Fig 1. Table showing quark and lepton families
(1)
The Particle Zoo
Particle physics is the way we have of understanding the formation
of the universe and the way that matter behaves as we see it today.
The laws which govern the tiniest particles are the same laws that
define how stars and galaxies form and why. For this reason,
understanding the smallest things inputs a lot to how science and
technology develop.
There are many different particles that we know of today. The most
well-known are the constituents of the atom - the proton, neutron
and electron. However, there are plenty more, and the proton and
neutron are even made up from smaller particles themselves
(called quarks). A proton consists of two up-quarks and a down,
and the neutron has two downs and an up. There are six main
types of quark which all have an ‘opposite’ partner - an anti-quark.
An anti-particle always has exactly the same properties as it’s
particle partner except that its charge is of opposite sign.
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Fig 2. Aerial view of CERN, showing main detectors (2)
Principles of an accelerator
A circular accelerator, like the LHC,
aims to collide two beams of particles
travelling in opposite directions. In the
case of a circular accelerator, each
beam is created and then, using
magnetic fields and pulsed electrical
fields, the beams are made to
accelerate around the ring before being
brought together at an interaction point
. A detector is placed around the
interaction point and these are
designed specifically to look at a
particular research issue. When the
particles in the beam collide they are at
a very high energy (at the LHC, 7 TeV
each way) and this could allow new
particles and interactions to be
observed.
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The electron too has an anti-particle - the positron. In the same
family, the leptonic family, there are also muons and tauons. After
this there are neutrinos, photons, W and Z bosons and then many
more particles which all belong to different families and have a
whole range of properties.
Looking at Ws
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Although LHCb is mainly set up to look at beauty-quarks (or
bottom), it is also ideal for looking at W bosons in a very
important region of their production. A W boson is a particle
made up from an up quark and an anti-down quark or a
down quark and an anti-up quark. In the LHC, because the
particles being collided are protons, there is always an
abundance of up and down quarks. However, to make a W
you also need an anti-quark. These come from a ‘sea’ of
particles inside the protons which pop in and out of existence
very rapidly. We don’t understand this particle sea very well
and, as I’ll explain, looking at the W boson allows us to
understand a bit more about it.
Fig 3. Diagram of the LHCb layout (3)
Improving the Theory
In any kind of science theorists take data from experiments and apply their
knowledge to it to explain what’s going on. Their theories are then tested by further
experiments, and if it looks like they work then we adopt the best ‘model’ and use
that. In particle physics, it works exactly the same. A lot of data has been taken by
general purpose experiments so theorists have been able to come up with very
accurate theories for processes which manifest themselves here. In the region
where LHCb operates, these theories have been extrapolated so we have a fair
idea what’s happening but it hasn’t ever really been verified. The data we take will
enable us to see how well the theories work so that they can then be verified,
modified or totally changed!
Most general purpose detectors are built like a cylinder
around the beam and they mainly see particles which come
off transverse to the beam direction. LHCb looks at particles
which are travelling almost parallel to the beam line
In W production, the second quark has to come from the ‘sea’. The way that sea
quarks are produced involves another particle called a gluon. The W is only
produced in the forward direction if the sea quark has very little energy compared
to the valence quark (directly from the proton). We don’t know much about how
gluons behave at low energy and by observing the W production mechanisms and
measuring various parameters in this region, we can test the knowledge that we
have and hopefully build upon it.
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Fig 4. Diagram showing regions
observable in LHCb (4)
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Fig 5. Predictions by MSTW theory group (5)
References
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Figs 6 and 7. Theory predictions with errors for two different sets of assumptions
(5),(6)
http://keyhole.web.cern.ch/keyhole/theory/quarks_leptons.jpg
http://startswithabang.com/wp-content/uploads/2008/05/lhc-sim.jpg
http://public.web.cern.ch/public/Objects/LHC/LHCbSetup.jpg
http://indico.cern.ch/conferenceDisplay.py?confId=9588 - Jonathan Anderson
http://durpdg.dur.ac.uk/hepdata/mrs.html
http://www.phys.psu.edu/~cteq/ - CTEQ theory group
http://projects.hepforge.org/lhapdf/ - PDF theory group overview
Stephanie Donleavy, Dept of Physics
http://hep.ph.liv.ac.uk/~donleavy