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ATLAS and the vision for particle physics
John Womersley
Director of Science Strategy
STFC
John Womersley
What is the universe made of?
•
A very old question, and one that has been approached in many ways
•
The only reliable way to answer this question is by observation and direct
enquiry of nature, through experiments
– The ‘scientific method’ is one of the greatest human inventions
John Womersley
John Womersley
The structure of matter
•
Centuries of experimentation and subsequent theoretical synthesis have
led to an understanding of
– Molecules, made of atoms – electrons orbiting nuclei
– Chemistry – the interactions of these electrons
– Nuclear physics – nuclei made of protons and neutrons
– Quarks – the components of protons and neutrons
•
Culminates in what we call the “Standard Model”
– A theory of matter and forces
– A quantum field theory
describing
point-like matter particles
quarks and leptons
which interact by
exchanging
force carrying particles
photons, W± and Z, gluons
Lightest particles stable
make up everyday matter
John Womersley
So we understand what matter is made of, then?
Yes – but there are two big problems.
First: a problem with what’s in the Standard Model
John Womersley
The revolution is coming
John Womersley
The revolution is coming
•
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The standard model makes precise and accurate predictions
It provides an understanding of what protons, neutrons, atoms, stars,
you and me are made of
But (like capitalism!) it contains the
seeds of its own destruction
•
Its spectacular success in describing phenomena
at energy scales below 1 TeV is based on
– At least one unobserved ingredient
• the Higgs Boson
– Whose mass is unstable in quantum mechanics
• requires additional new forces or particles to fix
– And in any case has an energy density 1060 times too great to exist in
the universe we live in
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The way forward is through experiments at particle accelerators
John Womersley
Why accelerators?
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Today’s universe is cold and empty: only the stable relics and leftovers of
the big bang remain. The unstable particles have decayed away with time,
and the symmetries that shaped the early universe have been broken as it
has cooled.
We use particle accelerators to pump sufficient energy into a point in
space to re-create the short-lived particles and uncover the forces and
symmetries that existed in the earliest universe.
•
Accelerators, which were invented to study the structure of matter, are
also tools to study the structure of space-time, the fabric of the universe
itself
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With current accelerators we are exploring the forces that governed the
universe when it was about one trillionth of a second (one picosecond) old
We know the energy scale that is required to reach the region where the
“revolution” happens: ~ 1 TeV in the parton-parton centre of mass
 we can design accelerators to do this
John Womersley
John Womersley
The Large Hadron Collider
The Large Hadron Collider is the first accelerator that will
clearly reach the required energies
It will – we believe – transform particle physics
John Womersley
Experimental challenges
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Doing physics at a supercollider is not trivial:
– The luminosity required is set by small cross sections for new physics
• multiple pp collisions per bunch crossing
– Each hadron is a broadband projectile of quarks and gluons
– Triggering, pattern recognition…
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Fortunately nature has given us some help
– W and Z – calibrate leptons
– And at LHC – the top quark – calibrate jets, b-tags and MET
John Womersley
How to catch a Top quark
Neutrino
Muon
W b
t
Wt
b
John Womersley
Second big problem:
what’s not in the Standard Model
John Womersley
Meanwhile, back in the universe …
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•
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What shapes the cosmos?
– Old answer: the mass it
contains, through gravity
But we now know
– There is much more mass than
we’d expect from the stars we
see, or from the amount of
helium formed in the early
universe
• Dark matter
– The velocity of distant galaxies
shows there is some kind of
energy driving the expansion of
the universe, as well as mass
slowing it down
• Dark Energy
We do not know what 96% of the
universe is made of!
Quarks
and
leptons
4%
John Womersley
Don’t let the
bright lights fool
you
The stars are only
a few percent of
what’s out there
The galaxies and
the entire
universe itself
have been shaped
by
invisible dark
matter…
… and dark matter
is not any of the
standard model
particles we are
familiar with
John Womersley
Particle
Physics
Experiments
Accelerators
Underground
Astronomy
Experiments
Telescopes
Satellites
Quantum
Field
Theory
(Standard
Model)
Standard
Cosmology
Model
10–18 m
1026 m
A Quantum Universe
John Womersley
Particle
Physics
Experiments
Accelerators
Underground
Astronomy
Experiments
Telescopes
Satellites
Quantum
Field
Theory
(Standard
Model)
Standard
Cosmology
Model
Consistent understanding?
10–18 m
1026 m
A Quantum Universe
John Womersley
Particle
Physics
Experiments
Accelerators
Underground
WIMPs
Astronomy
Experiments
Telescopes
Satellites
Quantum
Field
Theory
(Standard
Model)
Standard
Cosmology
Model
Dark Matter
Consistent understanding?
?
Goals for particle physics
- can we detect cosmic dark matter on earth?
- is it consistent with cosmological observations?
John Womersley
•
Dark Matter  low rate, small energy
deposits
– Very sensitive detectors
– Well shielded
– Underground to avoid cosmic
rays
1100 m
The Boulby Underground facility is
opened, 2003
John Womersley
Boulby Underground facility
ZEPLIN II liquid xenon detector in
shield and associated gas system
Interactions in the xenon
UK Dark Matter program
– Designed and constructed a series
of experiments
– Currently commissioning the
ZEPLIN II detector over half a mile
underground
• Uses Liquid Xenon to measure
scintillation light and
ionisation from dark matter
John Womersley
Intriguingly, dark matter points to the same place
where the standard model starts to break down …
Standard
Model
Higgs Boson
etc.
Dark matter
particles
mass and
interactions
John Womersley
Particle
Physics
Experiments
Accelerators
Underground
Quantum
Field
Theory
(Standard
Model)
WIMPs
Astronomy
Experiments
Telescopes
Satellites
Standard
Cosmology
Model
Supersymmetry
Dark Matter
Consistent understanding?
?
Goals for particle physics
- can we discover supersymmetry at colliders? Something else?
- is it consistent with cosmic dark matter?
John Womersley
What is this “Supersymmetry?”
•
•
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A proposed enlargement of the standard model
– We know all the particles have corresponding antiparticles
– If supersymmetry is correct, they would also have new, but much
more massive relatives called superpartners
Theoretically this is very nice
– eliminates mathematical problems in standard model
– allows unification of forces at much higher energies
– provides a path to the incorporation of gravity and string theory
These nice properties come at a cost: lots of new particles!
– multiple Higgs bosons
– squarks and gluinos, sleptons, charginos and neutralinos
– their masses depend on unknown parameters
– None of these particles has yet been seen – but they are expected to
be within reach of current accelerators
Lightest supersymmetric particle has all the
right properties for cosmic dark matter
John Womersley
How would we make a discovery?
•
•
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Standard model predicts how
Highest missing ET event
many events expected as a
function of missing ET
Supersymmetry models modify
this prediction: more events expected
Challenge: how do you calibrate the SM + detector expectation for MET?
Are there ways to select two event samples, one where a signal is
expected and one where it is not expected, as a benchmark?
John Womersley
Time to revisit the Higgs Boson
•
•
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Photons of light and W and Z particles interact with the same strength
– “Electroweak unification”
Yet while the universe (and this room) is filled with photons, the W and Z
are massive and mediate a weak force inside atomic nuclei
Where does their mass come from?
Massive W, Z
Massless fields
mix
Higgs field
Massless 
Higgs boson
The “Higgs Mechanism”
•
This Higgs field has never been seen. Is this picture correct?
– A question to be answered experimentally
– One clear prediction: there is a neutral particle which is a quantum
excitation of the Higgs field
• The “Higgs boson”
John Womersley
Particle
Physics
Experiments
Accelerators
Underground
Astronomy
Experiments
Telescopes
Satellites
Quantum
Field
Theory
(Standard
Model)
Standard
Cosmology
Model
Higgs Field
Dark Energy
Consistent understanding?
NO! > 1060
Goals for particle physics
- what can we learn about the Higgs field?
- is it as simple as we think?
John Womersley
Does there have to be a Higgs?
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No one has seen this particle – so why do we think it exists?
– The W and Z have mass
– Precision measurements of Top quark and W properties
– Ultimate test: “WW scattering”
q
W
X
W
q
•
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probability becomes > 1 as energy
increases – unless there is a Higgs
This is a real experiment – can’t have a
nonsense answer
The Higgs doesn’t have to be a single elementary particle.
But something has to play its role
John Womersley
Higgs searches
•
If the standard model is correct, we
can predict the particles into which
the Higgs boson will decay
– e.g. if its mass is relatively low, it
will decay to two photons
– Then see the Higgs as an excess
of events with two photons that
correspond to a particular mass
… except in real
life the Higgs
signal will not be
coloured red!
PbWO4 crystals
High resolution photon detector
for CMS
(UK responsibility)
VPT light detector
John Womersley
Particle
Physics
Experiments
Accelerators
Underground
Astronomy
Experiments
Telescopes
Satellites
Quantum
Field
Theory
(Standard
Model)
Standard
Cosmology
Model
Small CP violation
in quark decays
Matter dominates
Consistent understanding?
Not really
Goals for particle physics
- search for new sources of CP violation using quark mixing
- search for CP violation in the neutrino sector
John Womersley
Indirect searches for new physics
•
•
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Measure the rate of the rare decay
of Bs and Bd  
In the Standard Model,
cancellations lead to a very small
decay probability
– 3  10-9 and 10-10
New particles (e.g. SUSY)
contribute additional ways for this
to happen, increase probability
– up to 10-6
Mass of muon pairs
Current best limits
(Tevatron)
– Probability
(Bs  )
< 5.8  10-8
– Probability
(Bd  )
< 1.8  10-8
John Womersley
Particle
Physics
Experiments
Accelerators
Underground
Astronomy
Experiments
Telescopes
Satellites
Quantum
Field
Theory
(Standard
Model)
Standard
Cosmology
Model
Small CP violation
in quark decays
Matter dominates
Consistent understanding?
Not really
Goals for particle physics
- search for new sources of CP violation using quark mixing
- search for CP violation in the neutrino sector
John Womersley
The Neutrino Programme
MINOS
• Operations
and
analysis
Technology demonstration
T2K
• Secure a strong role in detector and
accelerator development and in
physics analysis
• Build up UK neutrino community
Learn more about
neutrino mixing angles
(govern CP violation)
MICE
• Demonstrate
muon cooling
Explore CP violation
Neutrino Factory
• Build community, international
scoping study  design study
• RAL is one credible site
John Womersley
T2K
•
•
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Primary aim
– Measure final mixing angle θ13
(µ → e)
– CP violation if large
Tokai to Super-Kamiokande
– 50 GeV PS, 0.75MW (Phase I)
– Existing SK Detector, off-axis
• ‘narrow band’ ~ 1 GeV 
beam
Accelerator construction underway
– Beam in April 2009(!)
J-PARC
neutrino
beamline
•
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UK participation
– RAL, QMUL, IC, Sheffield,
Liverpool, Lancaster, Warwick
Focus on
– Beamline and beam dump
• Accelerator expertise
– Near (280m) detector
• ECAL funding TBD
John Womersley
Make no little plans…
Harwell Science and
Innovation Campus
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ISIS
ISIS 1MW
upgrade
ESS-class 5MW
spallation source
Neutrino factory
Future multi-TeV
muon collider
John Womersley
Scientific Conclusions
•
We have a theory – the standard model – which makes precise and
accurate predictions but which we know is incomplete
– theoretically – points to the Higgs boson (or something else)
– experimentally – dark matter and dark energy
•
By connecting experiments at particle accelerators and in underground
labs with astronomical observations we can understand far more about
the universe than from either approach alone
– What is the cosmic dark matter?
Is it leftovers of Supersymmetry?
– Is the universe filled with energy?
How does this relate to the Higgs field?
– Why is the universe made of matter?
Is this related to neutrinos?
John Womersley
In the past year, particle physics has stumbled into crisis
(at least in the UK and US)
The field’s internal planning and priority-setting processes have
worked – but they have somehow become disconnected from an ability
to convince external stakeholders to support these priorities (notably
the ILC)
The potential for excitement and transformative physics at the LHC is
the best hope to re-frame the debate and start to remedy this situation
Such opportunities come rarely - let’s not waste this one!
John Womersley
Questions, comments…
John Womersley
Particle
Physics
Experiments
Accelerators
Underground
Astronomy
Experiments
Telescopes
Satellites
Quantum
Field
Theory
(Standard
Model)
Standard
Cosmology
Model
Supersymmetry
Extra Dimensions
Quantum Gravity
Inflation
Consistent understanding?
Superstrings!
Goals for particle physics
– can we see evidence of extra dimensions?
John Womersley
Calorimeter
(energy measurement)
Superconducting Magnets
Tracker
protons
protons
Muon detectors
(penetrating particles)
physicists
John Womersley
Muon
Electron
every
25 ns …
Jet
(experimental signature
of a quark or gluon)
John Womersley
Missing transverse energy
(experimental signature
of a non-interacting particle)
John Womersley
What? Extra dimensions?
•
•
String theories predict that there are actually 10 or 11 dimensions of
space-time
The “extra” dimensions may be too small to be detectable at energies less
than ~ 1019 GeV
– To a tightrope walker, the tightrope is one-dimensional: he can only
move forward or backward
– But to an ant, the rope has an extra dimension: the ant can travel
around the rope as well
John Womersley
Detecting extra dimensions
•
If there are particles than can travel around the extra dimension(s), we’d
interpret this motion as being additional mass
– If the dimension is small, the motion would be quantized
– would look like a series of new, more massive relatives of a known
particle
• “Kaluza-Klein modes”
•
But what if none of the known particles can enter the extra dimension
except for gravity?
– We (the things we are made of) may be trapped on a (3+1)-dimensional
“brane” – the surface of a 10 or 11 dimensional universe
– This could explain why gravity seems so weak
– Extra dimensions could be large – even infinite
– The energies required to “see” them could be much lower
• within reach of current accelerators?
John Womersley
We are searching
•
Look for a “Kaluza-Klein”
excitation of the graviton
– Assumed to decay to two
electrons or photons
Putative signal
data
High-mass electron pair event
mass = 475 GeV, cos * = 0.01
•
Look for enhancement to the
production of pairs of high
energy photons or electrons
•
See no deviation from 3+1
dimensions
– We can set limits on the
size and properties of extra
dimensions
John Womersley