The Dawn of the LHC ERA

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The Dawn of the LHC ERA

• A Confrontation with Fundamental Questions

Michael Dine Quarknet, UCSC, 2008
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Aerial view of LHC
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Size of LHC
In a magnetic field B, a particle of charge q and
momentum momentum p travels in a circle of radius
R given by
At the LHC, the desired beam energy is 7 TeV and
the state of the art dipole magnets have a field
of 8 Tesla. Plugging in and converting units
gives a radius of 3 km and a circumference of 18
km. Addition of quadrupoles, RF cavities, etc.,
increases the circumference of LHC to 27 km.
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Magnet Pictures
2 in 1 superconducting dipole magnet
being installed in the CERN tunnel
LHC dipoles waiting to be installed.
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ATLAS Detector
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Tracker Pictures
Tracker
Inserting silicon detector into tracker
Inserting solenoid into calorimeter
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Calorimeter Installation
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Muon Toroids
Muon superconducting toroids.
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Endcap muon sector
Endcap Muon Sectors
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SCALE OF THE PROJECT

• The stored energy in the beams is equivalent
  roughly to the kinetic energy of an aircraft
  carrier at 10 knots (stored in magnets about 16
  times larger)
• There will be about a billion collisions per
  second in each detector.
• The detectors will record and store only
  approx. 100 collisions per second.
• The total amount of data to be stored will be 15
  petabytes (15 million gigabytes) a year.
• It would take a stack of CDs 20Km tall per year
  this much data.

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Today A Theorists View of the LHC

• Why is this machine, perhaps the largest
  scientific instrument ever built, interesting?
• What do we expect to learn? What questions might
  we hope to answer?

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The Standard Model
By 1980, the Standard Model of particle physics
offered a nearly complete picture of the
elementary particles and their interactions. Quar
ks and leptons, interacting through exchange of
gauge particles (photon, W, Zo, gluons).
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• quantum field theory, describing interactions
  between
• pointlike spin-1/2 particles (quarks and
  leptons)
• via exchange of spin-1 vector bosons (photon, W
  and Z, gluon)
• fundamental particles (fermions)
• 2 (particle pair)
• 3 (generations)
• 2 (anti-particles)

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By 1995, the strong and weak interactions were
understood at the sort of precision level of QED
in 1960. the Standard Model was triumphant no
interesting discrepancies. All questions in our
list answered (except general relativity)!
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time year

• Last missing particle in SM
• (EW symmetry breaking mass)
• Light SM Higgs preferred

MH 126 73 -48 GeV lt 280 GeV
(95 CL)
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Puzzles of the Standard Model

• The Standard Model possesses many parameters.
  Some are extremely peculiar e.g. me/mt 3 x
  10-6.
• The electric charges of the quarks and leptons
  are exact rational multiples of one another (e.g.
  QeQp). Why?
• General relativity cannot be combined sensibly
  with the Standard Model, without some significant
  modification.
• The Standard Model cannot account for most of the
  energy density of the universe. About 20 dark
  matter about 75 dark energy only 5 baryons.
• The Standard Model cannot explain why there are
  baryons at all (baryogenesis).

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The Hierarchy Problem (or the failure of
dimensional analysis)

• But, apart from our failure to discover it up
  to now, the Higgs field presents a deeper puzzle.
  It may be too heavy to see without an LHC but
  the real puzzle is that it is so light. Problem
  is one of dimensional analysis. We know there
  are large energy scales in nature. Biggest is
  the Planck mass, Mp GN1/2 1019 GeV
• Why isnt MH C Mp, where C 1?

g
H
e pi
e pi
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... doubled particle spectrum ... ?
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Solves hierarchy problem

• Now dimensional analysis requires greater care.
  It turns out that because of the symmetry,
• MH C Ms
• New physics at TeV (LHC!) scales
• Explains dark matter
• Gives prediction of strong interaction strength

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o
without SUSY

• ... BUT some of our puzzles
• solved ...
• Successful unification of
• forces
• Lightest susy particle stable, and
• produced in abundance to be dark matter
• Readily explains baryon asymmetry

with SUSY
Interaction energy in GeV
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l
c
l
c
l
c
q
q
l
l
c
g
q
l
q
Production and decay of superparticles at the
LHC. Here, jets, Leptons, missing energy.
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• I am a fan of the supersymmetry hypothesis I'm
  not alone. About 12,000 papers in the SPIRES
  data base (also a good fraction of your faculty).
  If true, quite exciting a new symmetry of
  physics, closely tied to the very nature of space
  and time. Dramatic experimental signatures. A
  whole new phenomenology, new questions. But
  neither the limited evidence nor these sorts of
  arguments make it true there is good
  experimental as well as theoretical reason for
  skepticism.
• This is not the only explanation offered for the
  hierarchy, and all predict dramatic phenomena in
  this energy range.
• Large extra dimensions
• Warped extra dimensions
• Technicolor
• Its just that way (anthropic?)

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Hypothetical answers to our fundamental questions

• Too many parameters
• Hierarchy
• Charge quantization
• Quantum general relativity
• Dark Matter
• Dark energy
• Baryogenesis

Other proposals have some success with each of
the starred items perhaps fair to see that
supersymmetry does best.
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STRING THEORY

• String theory, an extension of the ideas of grand
  unification, has pretensions to attack the
  remaining problems on this list
• A consistent theory of quantum gravity
• Incorporates gauge interactions, quarks and
  leptons, and other features of the Standard
  Model.
• Parameters of the model can be calculated, in
  principle.
• Low energy supersymmetry emerges naturally all
  of this proliferation, which seemed artificial,
  almost automatic.

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Has string theory delivered?

• String theory is hard. We dont have a
  well-understood set of principles. Some
  problems of quantum gravity are resolved, but
  many of the challenges remain.
• String theory seems able to describe a vast
  number of possible universes, only a small
  fraction of which are like ours.
• Until recently, no progress on one of the most
  difficult challenges to particle physics the
  dark energy.

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Dark Energy/Cosmological Constant

• About 3/4 of energy of universe. Satisfies
• p -r
• an energy density of the vacuum.
• Dimensional analysis L M4.

Mp4? MW4? (1076,108) Measured 10-47!
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Progress and Controversy

• Many states of string theory now known with
  properties close to those of the Standard Model.
  Possibly 10500 or more!
• Among these, a uniform distribution of L. So
  many consistent with observation.
• Banks, Weinberg in such a circumstance, only
  form galaxies in those states with L close to
  observation. Perhaps universe, in its history,
  samples all? (This argument actually predicted
  the observed value of the dark energy).

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Can string theorists make other predictions?

• Supersymmetry at LHC, or not?
• If yes, spectrum of superpartners?
• If no, alternatives (just a Higgs, large extra
  dimensions, warping?)
• Cosmology?

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We are at the dawn of a very exciting era. We
may resolve some of our fundamental questions.
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Popular Treatments of String Theory
Rhapsodic about string theory
Denounces string theory
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Should the public care?

• Green too focused on the mathematics of string
  theory, too little on what we actually see,
  observe in nature. Given string theorys limited
  successes, seems to me this should be end of
  the book material.
• Smolin some valid criticisms, but promoting his
  own agenda no more interest in physical
  phenomena than Green.

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Dine rant

• See handout. Not yet prepared to put on my
  website.