4th July: H(125) Birthday
Sinéad Farrington and Bill Murray
The Higgs Boson as a toddler
•  Three years ago today the discovery of the 125 GeV
Higgs boson was announced
•
Simultaneously at CERN and at the major “International
Conference of High Energy Physics” in Melbourne, Australia
•
So today is a special day for particle physicists as well as for
Warwick!
Sinead Farrington, University of Warwick
2
Higgs Boson a little older…
•  As a theoretical concept, the Higgs boson passed its
own 50th birthday last year
•  1964 saw Brout, Englert and, independently, Higgs
formulate a relativistic description of spontaneous
symmetry breaking in gauge theories
•
predicted the existence of a boson and provided a mechanism
for massive mediators of the weak force (the W and Z bosons)
•  Thus began an almost
half-century-long experimental search
Sinead Farrington, University of Warwick
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Where to look for the Higgs?
•  But the theory did not predict the Higgs Boson’s mass
•  Very different composition of PRODUCTION and DECAY mechanisms
depending on mass
PRODUCTION
DECAY
Sinead Farrington, University of Warwick
Are (were) there any clues?
…SPS, LEP, Tevatron, LHC
Sinead Farrington, University of Warwick
5
Are (were) there any clues?
•  Most likely Higgs mass: 95+30-24 GeV (from indirect evidence)
•  Mass>115 GeV (direct searches)
•  Remember E=mc2, need high energy to produce a Higgs boson
of these masses and a lot of data as Higgs production is RARE
Sinead Farrington, University of Warwick
Particle Collisions
Two
at very which
high energy
Newparticles
particlescollide
are produced
we detect and study
?
Sinead Farrington, University of Warwick
7
A Particle Detector
“Onion shell” structure enables reconstruction of particles
Use this capability to reconstruct particle interactions of
special interest
Sinead Farrington, University of Warwick
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The ATLAS experiment
Sinead Farrington, University of Warwick
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The ATLAS experiment
• ~3000
collaborators
worldwide
• Warwick
became
a collaborating
institute in 2012
Sinead Farrington, University of Warwick
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Recording the data
•  Collision rate is ~40MHz, but can only record ~1kHz
•
Trigger system selects interesting events
•  Thereafter, distributed computing is key
•
Use tens of thousands of computers around the world (the
GRID)
Sinead Farrington, University of Warwick
11
What does a Higgs decay look like?
•  Decay to two photons
•
Experimental signature
•
•
2 deposits in electromagnetic
calorimeter and absence of matching
charged particle trajectories
Experimental challenges
•
•
Calibration
Background source: photons produced in other ways
Sinead Farrington, University of Warwick
Sinead Farrington, University of Warwick
Higgsγγ Results
1) Plot the invariant
mass
•
M=E2-|p|2
Sinead Farrington, University of Warwick
Higgsγγ Results
2) Evaluate
probability for the
“signal” to be a
statistical fluke
Most significant indication of signal is at 126.5 GeV:
Gold standard of observation 5 σ
• 7.4 σ
(Corresponds to 1 in 2 million chance)
Sinead Farrington, University of Warwick
Hè ττ
e+/µ+
H
νντ
e/µ
+
h- ντ
h
τ+
τ-
h+
-/µe
(K/π)
νντ
h- +νe/µ
h τ
h- (K/π)
Sinead Farrington, University of Warwick
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H ττ results
•  Observation at 4.5σ
•  Evidence that the Higgs
boson decays to fermions
and so can give them
mass
Sinead Farrington, University of Warwick
What have we discovered?
Compatible with Standard Model
(but still large uncertainties)!
Sinead Farrington, University of Warwick
Future of Higgs Physics
•  LHC Run 2 has begun
•
Higher energy, higher collision rates
•  Key properties of this new boson (spin/CP) will take
some time to ascertain
•
•
This was always anticipated
In fact we are fortuitous in nature’s choice for the Higgs mass –
all decay modes are accessible at this point
•  Key to characterising this particle are
•
•
Production and decay rates (to greater precision)
Intrinsic quantum numbers
•  Switch from search mode to precision physics
Sinead Farrington, University of Warwick
Higgs at 3
years
Warwick Physics
4th July 2015
There are therefore Agents in Nature able to make the Particles of Bodies stick together by very strong Attractions. And it is the business of experimental Philosophy to find them out.
W.Murray
Glasses by
Siobhan
Murray
What does the Higgs mean?
1
The Standard Model
?
W.Murray
2
The problem
One problem was, the maths said all the particles
should weigh nothing
That is not true
But it is hard to change the equations, add mass
a mass term violates 'gauge symmetry'
Peter Higgs, in 1964, had an idea.....
W.Murray
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Higgs' idea of mass
Think of a
fish tank:
W.Murray
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Empty the tank
The fish will
call this an
empty tank
but it still has
water in it
W.Murray
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Higgs theory
Maybe the same applies to us?
What if empty space is actually full of stuff
But stuff which acts in a special way
It slows you down moving through it
And we feel that drag as mass
If we could get outside then we would be massless
But we cannot.
How do we know if its true?
If we kick the 'water' (or Higgs field) we should make it ripple
The ripple is called the Higgs boson
But how do we kick the Higgs field?
Build the LHC!
Volunteer
W.Murray
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Higgs production
Gluon fusion
Associated
VBF
The most important 3 processes
Gluon fusion dominates rate
– Top loop
Vector boson fusion/associated
– Extra 'tags' to identify process
– Comparing rates is informative
W.Murray
gluon fusion
VBF
WH
ZH
ttH
Higgs Production
fractions
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Higgs decay modes
H→ZZ
– ZZ→llll: Golden mode
– ZZ→llνν: Good High mass
– ZZ→llbb: Also high-mass
H→WW
– WW→lνlν: First sensitive
– WW→lνqq: highest rate
H→γγ
– Rare, best for low mass
H→ττ
– Uses VBF, low mass
H→bb
– ttH, WH, ZH common but hard
W.Murray
bb
ττ
cc
gg
γγ
WW
ZZ
8
What do we know?
Experimentally study
production*decay
There are six with
significance over 2
i.e. error below 50%
Total of 11 with error
below 100%
They will improve
with more data
We are learning a lot
about this new
particle!
W.Murray
9
Interpreting the decays
Lets compare Higgs decays to ZZ and γγ
The γγ is about 2 per mille
The ZZ is 2.6%, ten times bigger
Once you remember we only see 6%2 of the ZZ
But this is very strange
bb
ττ
cc
gg
γγ
WW
ZZ
The Z weighs 91 GeV normally
The Higgs wegths 125 GeV
So when a Higgs decays to ZZ:
one has mass ~ 91 GeV,
But the other has maybe 30 – far less than a Z should
QM allows this, but only rarely
Once you allow for that you find the Higgs interaction
with the Z is 107 times stronger than with the photon
W.Murray
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Strong Higgs-Z coupling
Why does the Z interact strongly with this new particle?
Z is the force particle for the weak nuclear force
As the photon is for electromagnetism.
A photon interacts with charged particles
And a Z interacts proportional to the 'Weak charge'
So if the Z interacts strongly with the Higgs, it must carry
weak charge.
So the decay on the right happens
fast
But hold on!
The Z is neutral (no weak charge)
Just like the photon has no EM charge
So where does the Higgs charge go?
W.Murray
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The Higgs field is the answer
The answer is the diagram I gave you
is wrong
The Higgs does not decay to 2 Zs
Its emits two Zs (we saw them!)
But the Higgs itself slips off into the sea of
Higgs field surrounding us.
We have now measured the
production of a Higgs from colliding
two Ws or Zs
And its decay into them
Both of these rely on the field to
provide or absorb thhe Higgs boson
The field IS all around us
W.Murray
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Implications
This discovery is a triumph of human ingenuity
It confirms that the the universe obeys mathematical laws
That is not obvious
For the first time in the history of physics we have a
consistent description of the fundamental constituents of
matter and ther interactions that can be extrapolated up
to very high energy
The equations of the SM accurately describe the matter
we are made from
e.g. the magnetic field of an electron
But no explanation of:
Dark matter
The matter matter asymmetry
We do not understand what the Universe is made of
W.Murray
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What about the Higgs field?
Central to the Higgs mechanism is the field filling space
It filled the Universe about 10-10 s after the Big Bang
Unlike light, you turn it off and it is still there
More like water filling the sea
The density of this field is eaily ruled out by cosmology
It is 120 orders of magnitude larger than dark energy – and the
opposite sign
But remember: we don't have a quantum theory of gravity
I explained the best evidence we have
So do we really expect you to believe its there?
We should measure the self-coupling of the Higgs
This is what generates the field.
We might learn something from studying events with two
Higgs bosons
But that will be a very hard road to follow...decades
W.Murray
14
What does mH teach us?
The SM mH=94+29-24GeV from SM
Agrees with 125.09±0.21GeV we measured pretty well
This assumed no unknown particles
The SM is doomed
The Universe appears to be unstable
At Ultra-high energies the Higgs field will implode
Wait long enough (> Universe lifetime) and this will happen
The SM is incredibly fine tuned
Quantum corrections add 1016GeV to the Higgs mass
But we measure 125GeV
Did it really start as -1016? It would need to be 'just right'
This suggests something is missing with a mass near
the Higgs boson mass
Could it be dark matter?
W.Murray
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What comes next?
We have found a Higgs boson
This confirms a 'Higgs Field' filling space
Unlike light, you turn it off and it is still there
More like water filling the sea
But it is much denser than lead...
1964
This is something radically new
It is not like matter, not like a force
Extending Newtons vision
It is a Higgs field, something completely new.
Now we need to understand it
LHC started up again this year
At 13TeV and higher collision rate
To learn more about the the Higgs boson
And maybe we will find something else too!
W.Murray
2012
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