Where Particle Physics is now

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Where Particle Physics is now

• The standard model
• Why it works
• How we check it
• Successes, Electroweak QCD
• Gaps in the model, masses,
• neutrino oscillations,
• matter excess and CP violation
• How beauty quarks help

• The central question
• Electroweak Symmetry Breaking
• The Higgs mechanism
• and the Higgs field
• The Higgs boson
• What if no Higgs boson?
• Even more baffling mysteries.
• Map of our knowledge and ignorance.

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Beautiful standard model (SM) containing
Particles spin 1/2 fermions
u (10MeV)
t (175GeV)
c (1.5GeV)
charge 2/3
Quarks
d (10MeV)
b (5GeV)
s (300MeV)
charge -1/3
e (0.51MeV)
? (1.777GeV)
? (106MeV)
charge 1
Leptons
?e (lt3eV)
?? (lt18.2MeV)
?? (lt0.19MeV)
charge 0
Why all these different masses?
We dont know!
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The standard model also contains
Force carriers spin 1 (vector) bosons
? m?0
The photon responsible for Electromagnetism. Inte
racts with charge NOT WITH ITSELF.
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Why does the standard model work at all?
The three forces (except QCD at low
energies) are weak enough for perturbation
theory so can use Feynmann
graphs simple recipe for a calculation.
Corrections to simplest tree graphs are
a few . Higher orders smaller still,
but calculable.
Tree graphs for LEP processes
Compton scattering (and S.R.)
? decay in nuclei or in sun
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Much of the checking of the standard model has
come from CERN
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Cutaway view of a particle physics experiment
OPAL at the LEP ee- collider (1989 2000)
Beam particles stay in vacuum pipe through
centre. Interaction products come out
into wrap-around layered detectors. 1st layer,
Trackers (thin Si chips or gas wires) pick up
ionisation from fast charged particles
leptons, mesons, protons etc. Curvature in
magnetic field gives sign and momentum. 2nd
layer. Calorimeters make charged and neutral
particles interact in heavy material (Pb glass
or iron plates). Output pulses are proportional
to deposited energy. 3rd layer. Muon chambers
look for penetrating tracks. Only neutrinos
escape.
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A clear event in OPAL
Beams eye view - blue muon tracks are almost
straight because high momentum. - small
energies in calorimeters (yellow is Pb glass of
ecal, electro- magnetic, pink is iron plates
of hcal, hadronic). - hits (red arrows) in
outer muon chambers
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An OPAL event with an extra particle
Side view - electrons leave blue tracks, then
energy (yellow) in ecal. - extra photon seen only
in forward calorimeter (green). - these
radiative events come at exactly the
calculated rate.
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Two quark jets in OPAL. Perturbative and
non-perturbative QCD effects.
Beams eye view - two bundles of blue charged
meson tracks, bent by field. - energy in ecal
(yellow) and hcal (pink), in line with
jets. BUT WHY DONT WE SEE THE QUARKS?
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QCDs subtleties confirmed by jets at LEP
meson
As quarks fly apart, gluon field stretches,
accumulating energy until it can snap and form a
new quark antiquark pair. Goes on until all
energy used up, leaving jets of mesons.
meson
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Counting the clear event rate gave us the number
of neutrinos
Just what the Big Bang nuclear abundances
require!
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Evidence for neutrino oscillations two
confirmed results.
1. Too few neutrinos from sun. ?e disappear (to
?? and/or ?? say SNO, SuperK)
2. Atmospheric neutrinos. From cosmic ray
interactions in the upper atmosphere e.g.
SuperKamiokande results
Numbers of e-like events are as expected. Numbers
of ?-like fall off at large zenith angle.
Large zenith angle means the neutrinos were
produced on the other side of the earth so they
had a long flight path. ?? ??x (possibly to ??,
we want to prove that)
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Tests of neutrino oscillations
Accelerators can produce neutrino beams of known
energy and lepton flavour. MINOS, for example
Source of ?? at Fermilab. Detector 730 km
away in Minnesota. Should see a clear
reduction in events with muons if favoured
parameters from Superkamiokande are true. Results
in 2005.
CERN to Gran Sasso OPERA experiment, similar
baseline, should see ?? interactions produce
decaying ? leptons in emulsion. Results in 2007?
Next generation of neutrino experiments may
use muon storage rings - after 2010.
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Heavy quarks give extra information
An OPAL event with 2 beauty particles decaying
It looks like an ordinary quark- antiquark event
in the normal view, but in close-up
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Close-up of beauty event
The same tracks as in the previous picture,
but extrapolated back from the high precision Si
tracker, close in to the beam. Most of them do
not point back to the ee- interaction but come
from the decay- points of two mesons
containing beauty quarks. Decay lifetimes about
10-10s.
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Extra information from Beauty 1.
We compared the rate of beauty production at LEP
(and SLC) with the rate for light quarks. Beauty
is slightly enhanced. Predicted the top quark
mass from the loop correction to Z0 ? b antib,
even before Fermilab discovered the top quark
directly.
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Extra information from Beauty 2.
There is 1 matter particle in the universe for
every 109 photons. WHY? The John Wayne answer
wont do
Thats the way it is kid!
In equilibrium after the big bang we had an
alphabet soup of particles and photons.
Thermodynamics says that everything that could
exist would be in equilibrium with the soup, so
all particles - including quarks should be
there, if their mass was below twice the average
photon energy in the soup
ALL IN ROUGHLY EQUAL NUMBERS So where did
all the antiquarks go?
They met quarks and annihilated - except for 1 in
109 of them. There was a slight excess of
eligible quarks, which therefore survived and
form todays matter.
We are not sure what caused the excess, but we
have some beautiful theorems from Sakharov-
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Sakharovs theorems on matter excess
1. Sometime very early in the Big Bang there was
a phase transition. Things went out of
equilibrium. Maybe the temperature was dropping
through the mass region (E mc2 kT) of some
particle must have happened many times.
2. At this temperature one of the important
interactions violated CP symmetry. (Whats that?
Space reflection of the
wavefunction exchange of particles
antiparticles) We have CP violation in K0 meson
decays in the standard model well described, but
not enough to do the job for Sakharov. Where
else might we look at it?
In beauty meson decays. Experiments now running
at Stanford and KEK (Japan). Early results say
YES - there is CP violation but still
consistent with the low rate in in K0 decays.
3. Protons should decay. Weve looked. Present
lifetime gt 1031 years c.f. age of universe
1010 years.
LOTS MORE TO DO HERE IN NEXT 20 YEARS
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Gaps in the standard model
Questions wed like answered 1. Why all the
fermion masses? 2. What is happening with the
neutrinos? 3. Why the matter excess. Was
Sakharov right about CP and pdk ?
The central question 4. Why are
mz and mw so big, when m? 0 ?
If all the vector bosons were massless we could
have used an Electroweak field theory like
quantum electrodynamics
good to 1 part in 109, because it
can be renormalised. Higgs ( precursors and
collaborators) showed that Electroweak
theory with massive vector bosons could also be
renormalised IF
Something else amazing happened in the very early
universe Electroweak Symmetry was Broken (EWSB)
and a Higgs field appeared.
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How the Higgs field may have appeared
The very early universe was hot the most stable
state had zero Higgs field. All three
vector bosons were massless.
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The Higgs mechanism 1
Imagine the vacuum in the form of a cocktail
party of political workers, uniformly spread
across the room.
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The Higgs mechanism 2
A beloved ex prime-minister enters and is
immediately surrounded by well-wishers.
The cluster of admirers gives her extra
mass, i.e. more inertia just as an
electron acquires extra mass from the lattice in
a semiconductor or the W and Z from the
Higgs field in vacuum.
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The Higgs boson 1
A scandalous rumour is launched into the party.
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The Higgs boson 2
The partygoers clump to transmit the rumour, just
as they clumped around the ex-leaderine.
Similar dilaton effects occur in solids.
The clump can travel like a particle. In the
vacuum such a clump in the Higgs field is a
Higgs boson. It has spin0.
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The smoking gun? If there is a Higgs boson - it
all may be true!
Where do we look for it? EWSB doesnt give mh
BUT perturbation theory might. Lots of good
EW measurements mZ, mW, mt,?em, Gfermi
etc.plus-
At Fermilab
At LEP many processes
or total hadron rate
Combine all EW measurements into one fit-
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Where we think the Higgs boson is
Blue band is best fit of data from LEP,
Fermilab etc. to all available SM
measurements. Lowest chi2 gives most likely
value for mass, With 95 confidence that chi2 gt
2.7, corresponds to mh lt 196 GeV. Yellow area
ruled out by LEPs direct search. Still plenty
of room to find SM Higgs.
chi2/ndf
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Where can it be found? (we looked at LEP and
maybe saw a hint of a signal)
1. Perhaps at the Fermilab Tevatron 2 TeV proton
antiproton collider, running now but will need 5
years to get enough statistics.
2. Certainly (if it is there) at the CERN Large
Hadron Collider (LHC). Being built in the LEP
tunnel. Runs for Physics in 2007.
3. And (if it is there) at a Linear ee-
Collider. Worldwide consensus that a 500 GeV to 1
TeV Linear Collider is needed to complement the
LHC. Ready by 2012?
We will need all 3 to sort out what is going on.
Is it a Standard model Higgs? Do we have
Supersymmetry?
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A LEP Higgs candidate event
Could have been
The two blue jets with multicoloured hits in
the calorimeters
The two red tracks, with yellow hits in the
calorimeters
Or it could just have been
We did not have quite enough energy to be sure
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How a light Higgs boson would stand out at the
Linear Collider
The Linear Collider would pin down all of the
vital properties of a light Higgs boson. The LHC
would discover it but only measure a few of its
parameters.
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If we do not find the Higgs boson there, things
are even more interesting. Whoever heard of
physics effects that behaved AS IF there was an
elementary particle to cause them, but it was not
there when we looked closely?
Superconductivity physicists did! When the
resistance disappears in a piece of cold Lead or
Niobium the current is carried by particles
called Cooper pairs. They behave like
elementary particles but they are actually
clustering effects of the the lattice which
induce pairs of electrons to stick
together. Maybe something like that makes the
Higgs field? Is it WW- pairs?
Or is it top-quark pairs?
We dont know
but the LHC and Linear Collider
could tell us.
Both are needed to tie down alternative EWSB
models
without a simple Higgs.
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Todays most baffling mysteries
1. What caused Cosmic Inflation? Needed by
Cosmologists for a number of reasons. E.g. to
explain near isotropy of Cosmic microwave
background radiation.
Dumping of energy from the Higgs field as it
rolled down to the minimum is exactly the sort of
effect needed to generate inflation.
2. What is the Dark Energy? Recent
measurements of distant supernovae suggest that
the expansion rate of the universe is
accelerating very slightly like a very feeble
continuation of inflation. 60 of mass of
universe?
Could it be that the minimum of the
Higgs potential is not exactly zero? No one
knows how to get 10-60 of the original E0.
3. Do the forces all come together at some very
energetic scale?
Maybe in two stages Electroweak and QCD at GUT
scale, gravity at Planck scale?
4. But how do we do Quantum gravity?
SUSY may help!
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Map of our knowledge and ignorance.
Before massless bosons B0, (W, W0, W-),
Spontaneous appearance of Higgs field
ElectroWeak Symmetry Broken in the early
universe. HOW?
Planck scale?
Unified forces break up at GUT scale?
Cosmic inflation?
Higgs boson?
MIXING
Experiments and ideas needed. Come and join in!
Dark energy? Accelerating expansion?
?, Z0, (W, W- )
QCD
Fermion masses and couplings
Massive bosons short range weak intn
Massless photon long range E B
CP violation? Matter excess? Neutrino oscillation?
Gravity
Nuclei, atoms, ions, molecules solids, plasmas,
liquids, gases galaxies, stars,
planets, chemistry, biology, electronics, acupunct
ure,skiing.