The Quantum Universe

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The Quantum Universe

• John Womersley
• Director of Particle Physics
• Rutherford Appleton Laboratory, UK

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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

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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 particlesquarks and leptons which
  interact byexchanging force carrying
  particlesphotons, W and Z, gluons

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So we understand what matter is made of,
then?Yes but there are two big problems.
First a problem with whats in the Standard
Model
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a revolução está vindo!
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the revolution is coming

• The standard model makes precise and accurate
  predictions
• It provides an understanding of what protons,
  neutrons, atoms, stars, you and me are made of

• 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
• The way forward is through experiments at
  particle accelerators

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Why accelerators?

• Todays 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.
  
• But every kind of particle that ever existed is
  still there, in the quantum fluctuations of empty
  space. Empty space still knows about all the
  equations, all the symmetries that governed the
  formation of the universe.

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• 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
• With current accelerators we are exploring the
  forces that governed the universe when it was
  about one trillionth of a second (one picosecond)
  old

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Fermilab
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• Fermilabs Tevatron collider started operations
  in 1988
• Run I 1992-95
• Run II 2001-09 50 more data, increased
  energy

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Detectors
CDF
D-Zero

• Surround the collision points with arrays of
  instrumentation to intercept the particles
  produced
• large (thousands of tons)
• complex (many subsystems, 106 107 channels of
  electronics)
• designed and built by collaborations of
  university and laboratory physicists

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Tracker
Superconducting Magnet
protons
antiprotons
3 LayerMuon System
Electronics
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Proton-antiproton collisions

• Proton beams can be accelerated to very high
  energies (good)
• But the energy is shared among many constituents
  quarks and gluons (bad)
• To select high-energy collisions look for
  outgoing particles produced with high momentum
  perpendicular to the beamline (transverse
  momentum)
• Do this 2½ million times a second, as the
  collisions happen
• triggering

Transverse momentum
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Computer programs reconstruct the particle
trajectories and energies in each collision (each
event)
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Displaced vertex tagging

• The ability to identify b-type quarks is very
  important
• signatures for the Higgs boson and many other
  interesting things
• b quark forms a B-meson, travels 1mm before
  decaying

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Silicon sensor
Wire bonds
Silicon sensor
HDI (flex circuit readout)
SVX2e readout chips
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Measuring ladder position after insertion
Zeiss coordinate measuring machine at Fermilabs
Silicon Detector Facility
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What do physicists actually do?

• Design and build hardware
• Detectors, electronics
• Write software
• Operate the detector
• Interpret data
• Present, refine, discuss our results among
  ourselves
• Publish papers

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The work of many peopleExample The DØ
detector was built and is operated by an
international collaboration of 670 physicists
from 80 universities and laboratories in 19
nationsgt 50 non-USA 120 graduate students
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Second big problem whats not in the
Standard Model
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Meanwhile, back in the universe

• What shapes the cosmos?
• Old answer the mass it contains, through gravity
• But we now know
• There is much more mass than wed 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!

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Dont let the bright lights fool you The stars
are only a few percent of whats 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
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A Quantum Universe
Particle Physics Experiments Accelerators Undergro
und
Astronomy Experiments Telescopes Satellites
1018 m
1026 m
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A Quantum Universe
Particle Physics Experiments Accelerators Undergro
und
Astronomy Experiments Telescopes Satellites
Quantum Field Theory (Standard Model)
Standard Cosmology Model
Consistent understanding?
1018 m
1026 m
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WIMPs
Particle Physics Experiments Accelerators Undergro
und
Astronomy Experiments Telescopes Satellites
Quantum Field Theory (Standard Model)
Standard Cosmology Model
Dark Matter
Consistent understanding?
?
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• Dark Matter ? low rate, small energy deposits
• Very sensitive detectors
• Well shielded
• Underground to avoid cosmic rays

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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

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• Intriguingly, dark matter points to the same
  place where the standard model starts to break
  down

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WIMPs
Particle Physics Experiments Accelerators Undergro
und
Astronomy Experiments Telescopes Satellites
Quantum Field Theory (Standard Model)
Standard Cosmology Model
Dark Matter
Supersymmetry
Consistent understanding?
?
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What is this Supersymmetry?

• 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

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How would we make a discovery?

• Standard model predicts how many events expected
  as a function of missing ET
• Supersymmetry models modify this prediction
  more events expected
• We found one very high missing-ET event in the
  first year of data
• Will we find more?

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Indirect searches for new particles

• 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
Carry out a blind analysis

• Current best limits
• Observe no events
• Probability (Bs ? ????) lt 2 ? 10
• Probability (Bd ? ????) lt 5 ? 10-8
• Will keep getting better

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Time to revisit the Higgs Boson

• 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?
• 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

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Particle Physics Experiments Accelerators Undergro
und
Astronomy Experiments Telescopes Satellites
Quantum Field Theory (Standard Model)
Standard Cosmology Model
Dark Energy
Higgs Field
Consistent understanding?
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Does there have to be a Higgs?

• 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
• probability becomes gt 1 as energy
  increases unless there is a Higgs
• This is a real experiment cant have a
  nonsense answer
• The Higgs doesnt have to be a single elementary
  particle.
• But something has to play its role

q
W
X
W
q
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Higgs searches

• Current searches at the Tevatron are 20-100 times
  less sensitive than will be needed to find a
  Higgs
• We will get a factor 20 more data, but by itself
  that wont be enough
• Experiments are improving their techniques
• should be able to say something interesting,
  provided the mass of the Higgs is low enough

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The Top Quark

• The top quark offers an indirect window on to the
  Higgs
• Because it is the most massive particle known, it
  interacts most strongly with the Higgs
• Precise measurements of the top mass can tell us
  about the Higgs mass
• Measurements of the way the top quark is produced
  and how it decays may hint at new phenomena
  associated with the Higgs
• The top is in principle just a very heavy quark
  so we can calculate its behavior in detail
• Look for any surprises, anomalies

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How to catch a Top quark
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Top production

• If the top is just a very heavy quark, we can
  precisely calculate its expected production rate
  (cross section) in proton-antiproton collisions

PLB 626, 35 (2005)
L230 pb-1
?1 secondary vertex tag
Expected Top Signal
Looks very much as expected
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Top mass

• Because its mass is so large, the top quark
  should decay very rapidly (yoctoseconds) into a W
  boson and a b quark the W decays even more
  rapidly into either two quarks or a lepton
  neutrino
• The top mass can be reconstructed from the energy
  of the b and of the W decay products
• It can be measured quite precisely at the 2-3
  level
• In the Standard Model, the top mass W mass and
  Higgs are all related
• Hence we can check if it is allconsistent yes,
  so far
• And get an indirect measurementof the Higgs mass
• Points to a rather light Higgs

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How does top decay?

• Does it really decay always to a W plus a
  b-quark?
• Can test by using the silicon detector to
  identify b quarks
• Distinguish b from?b by charge of particles seen
• All consistent with SM
• i.e. 100 top ? Wb
• Does it decay to a W through the normal weak
  interaction?
• Can test by measuring the angular distribution of
  the W decay
• All consistent with standard weak decays

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Particle Physics Experiments Accelerators Undergro
und
Astronomy Experiments Telescopes Satellites
Quantum Field Theory (Standard Model)
Standard Cosmology Model
Quantum GravityInflation
SupersymmetryExtra Dimensions
Consistent understanding?
Superstrings!
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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

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Detecting extra dimensions

• If there are particles than can travel around the
  extra dimension(s), wed 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 (31)-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?

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We are searching

• Look for a Kaluza-Klein excitation of the
  graviton
• Assumed to decay to two electrons or photons

Putative signal
data

• Look for enhancement to the production of pairs
  of high energy photons or electrons
• See no deviation from 31 dimensions
• We can set limits on the size and properties of
  extra dimensions

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Where do we go from here?
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The Large Hadron Collider
14 TeV proton-proton collider at CERN
Magnets being installed
Over half the dipole magnets completed
First beam in 2007
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The ATLAS and CMS detectors
CMS mid-2005
PbWO4 crystals
Final Barrel assembly at CERNSeptember 2005
ATLAS mid 2005
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The International Linear Collider

• Discoveries at the Tevatron or LHC will leave us
  more questions than answers
• Have we really discovered the Higgs
• Right quantum numbers?
• Does it couple to mass?
• Have we really discovered supersymmetry?
• Superpartners have same properties as their
  partners?
• Have we really discovered dark matter?
• Does it have the right properties?
• An electron-positron linear collider is the way
  to answer these questions

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• The ILC is a high priority for the US Department
  of Energy, provided it is affordable and
  scientifically justified
• Seen as a fully international project
• Northern Illinois (near Fermilab) is a candidate
  site
• Just to show the scale

Fermilab site
US study version 47 km long
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International Linear Collider

• 500 GeV, upgradeable to 1 TeV
• Accelerator technology chosen
• Global design group established

Professor Barry BarishCaltech

• Timeline
• 12/05
• Baseline configuration (done)
• 12/06
• Reference design report with cost estimate
• 2008
• Technical design report
• 2010
• Construction decision?

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Conclusions

• We have 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?
• What is the structure of space and time? Are
  there extra dimensions?

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The quantum universe is a wonderful
placePerhaps the most wonderful aspect is that
it is possible for us to understand it
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Questions, comments
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Particle Physics Experiments Accelerators Undergro
und
Astronomy Experiments Telescopes Satellites
Quantum Field Theory (Standard Model)
Standard Cosmology Model
Matter dominates
Small CP violationin quark decays
Consistent understanding?
Not really