Transcript The Higgs Boson - Particle Physics Group

The Higgs Boson
What it is and how to find it
Roger Barlow
Manchester University
Particle Physics: the Goal
To deduce the laws of physics
using the minimum number
of arbitrary assumptions
"What really interests me is
whether God had any
choice in the creation of the
world." --Albert Einstein
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Elementary Particles:
(1) The electron
eKnown for 100 years
• Very common
• Very light: mass of 9.109 10-31 kg
• Very small (pointlike?)
• Described by Quantum Mechanics. Wave
function (r,t), a solution of the
Schrödinger Equation –(ħ2/2m)2  =E 
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(2) The photon
Argument:Wave function  has an arbitrary phase
Constant change of phase:  ei does not change physics
It would be ‘nice’ if variable change of phase:  ei(r)  did not change
physics…but  terms mess up Schrödinger Equation
Modify S.E. new term –(ħ2/2m) (-ieA)2  =E 
And if   ei then A A+(1/e)  (Gauge Transformation)
A(r) describes another particle: Gauge Boson. Spin 1, interacts with
electron, has zero mass (no A2 term)… the photon
Hence electromagnetism,Maxwell’s Equations,Etc Everything
predicted except the actual value of e
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(3) The positron
Relativity: Schrödinger Equation replaced by Dirac
Equation
-iħa.(-ieA)+m=E 
 is not just one complex function but 4.
Extra components describe spin (up/down) and
particle/antiparticle
Antiparticle has opposite charge
Many more processes possible
e-
QuantumElectroDynamics
QED
e+
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(4) The quark
quark - like an electron (has charge,
spin ½, has antiparticle)
But also has an extra (triple) quantum
number. Called ‘colour’ – red (1,0,0),
green (0,1,0), blue (0,0,1)
Needed because of the Pauli Exclusion
Principle in particles such as the D++ ,
made of 3 otherwise identical quarks.
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(5) The gluon
Argue: the choice of red-green-blue axes arbitrary. Physics
should not change if we switch around
Or even if we rotate the axes in r-g-b space. Rotation
matrix R…
Even if R varies with position+time… extra  R terms in
equations.
Need extra function in equation with appropriate gauge
transformation
New massless particle –
Another Gauge Boson
the gluon
Similar to QED but more complicated due to matrix
structure:QuantumChromoDynamics - QCD.
Arbitrary constant is much larger than e. Strong force.
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Pause for breath
Understand Electromagnetism and the
Strong (nuclear) force, apart from a few
arbitrary(?) constants. And technical
details of calculations
That’s everything except gravity and beta
decay. Not a ‘Theory of Everything’ but a
‘Theory of quite a lot’
Can’t do gravity…. But should manage beta
decay
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Beta decay as it ought to be…
np e- 
du e- 
Quarks in protons/neutrons/nuclei are in two
‘flavours’: u and d. (Different charges and
masses)
u and d are two states of the same fundamental entity
- the quark
e and  are two states of the same fundamental
entity – the lepton
(Weak) isospin up or down.
Run gauge theory argument again for up-down…
predicts Gauge Bosons W+, W0, We
W-

d
u
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Slight(?) problem
Gauge Bosons have got to be massless.* Or the
Gauge Invariance of the equations breaks down.
• Photons
• Gluons 
• The W bosons 
They exist alright – but have masses ~80 GeV.
Theory stuck here for some time
* Mass: The minimum energy needed to create a
particle
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The Higgs Field
Suppose there is a field called H(r,t)
that interacts with the electron,
quark, W etc
OK, why not
Suppose that the lowest-energy
stats is not H(r,t)=0 but H(r,t)=V
Seriously weird
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Masses that are not masses
1. As a W propagates through space and time, it
interacts with this nonzero Higgs field…
2. Which gives it an energy….
3. Even if it has no kinetic or potential energy…
4. Which means it has, to all intents and
purposes, a mass. Without breaking gauge
invariance
Happens to quarks and leptons too
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The Standard Model
• Quarks and Leptons (x3
‘generations’)
• Gauge Symmetries for the
Weak, Strong and EM force
• Higgs mechanism giving
masses to the W bosons
• Also mixing/unifying Weak and
EM forces
• Also explains weak decays
between generations (with a
few more parameters)
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Is the Standard Model true?
Yes!
Predicts W/Z mass ratio
Predicts cross sections and
branching ratios in many
many particle decays
Accounts for parity violation
Accounts for CP violation in
K and B sectors
No experimental results in
disagreement
No!
Does not predict quark and
lepton masses
Or coupling constants…
28 free parameters
altogether
Or why there are 3
generations
Or why there is parity
violation
Higgs is an ad-hoc addition
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Testing Higgs: from field to particle
Higgsness
H?
• Quantum excitations of the H field are H particles
(Same as any particle, though usually about 0)
• The Higgs coupling of any particle is proportional
to its mass.
(actually the other way round…)
H is best made by massive particles
H will decay to the heaviest allowed particles
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Is the Higgs true?
• Probably not – it’s a very arbitrary kludge
• Many alternative theories have been
proposed that are more
elegant/beautiful/natural
• All have very similar effects until you get to
high (TeV) energies
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First Attempt: LEP
q
Collide electrons and positrons at
energies of 200 GeV
e+
Z
Z*
H
e-
q
b
b
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Saw some events, but..
Consistent with
background
MH>114 GeV
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Second Attempt: the LHC
Proton proton
collisions at 14
TeV
Start operation
next year
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Experiments: ATLAS and CMS
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Common
features
Tracking
• Magnetic Field
• Measure charged particle
tracks with drift chambers or
Silicon
• Curvature gives momentum
Calorimetry
• Material so Neutral particles
interact
• Measure total energy by
scintillator etc
Muon detection
• Muons get through the
calorimeter
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Looking for signals
Decay depends on
MH
Plots shows signal
if MH fairly large
Smaller values
more difficult
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Handling the data
• Collision rate 40 MHz
• Several events/collision
• Each event gives
massive amount of data
• Massive data stream.
>10 TB/y
Handled by Grid of
• Tiny number of
computers
all
over
interesting events
Europe - and the world
10,000+ CPUs
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Third Attempt: the ILC
Electron positron
collisions at 1 TeV
Still at the design
stage
Straight (not circular)
Chicago? Japan??
38 km? $6Bn?
Slide 24/26
Start 2015+?
Why?
• LHC is a proton-proton
collider
• Protons are made of
quarks
• LHC is actually a quarkquark collider
• Quarks share proton
energy in a random way
500 GeV
500 GeV
1 TeV
Precision measurements
junk
77TeV
TeV
?
7 TeV
junk
Exploration
• A 14 TeV protonproton collision gives
a whole spectrum of
energies for quarkquark collisions
• And the unused
energy appears as
Slide 25/26
background particles
The Future
•
•
•
•
•
LHC will start next year
First serious data 2008+
Interesting results 2-3 years? after that
Should find Higgs - probably not quite as expected
Other new particles/new effects predicted by speculative
models (SUSY? GUTs?)
• Exploration will be followed by precision measurements
at the ILC
• Build Beyond the Standard Model theory with fewer
arbitrary parameters
• Understand the universe we live in a little bit better
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