1. How does the LHC work? What is the 'Higgs boson' What might be next?
CERN: The Higgs boson and the
High Energy Frontier?
Bill Murray
Warwick University /
RAL, STFC
26th January 2017
How does the LHC work?
What is the 'Higgs boson'
What might be next?

3. Lots of questions... Mass (pre-LHC problem!)
Without Higgs mechanism particles are massless:
Gravity
there is no gravity in this model
Neutrino Mass
Neutrinos have mass – but how? We do not know
Dark matter
Most matter in the Universe is something unknown
Dark energy
An unknown force accelerates the Universe expansion
Matter-antimatter asymmetry
Where did the antimatter go after the big bang?
The naturalness problem
I will explain this later

4. Mass: Higgs idea
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 et al, in 1964, had
an idea.....

5. What is Higgs' mechanism?
Doublet of SU(2)L, Φ=(Φ1,Φ2)
Potential respects SU(2)L
But Vacuum does not!
Fermions:
Interact with Higgs field slows them down  generates mass
3 degrees of freedom
in Boson masses
4th becomes fundamental
scalar
Bosons:
SU(2)L interact, gain mass
U(1) and SU(3)c do not, massless
Go to 3rd year particle physics course!

6. Higgs' idea of mass
Think of a fish tank:

7. Empty the tank The fish will call this an empty tank
but it still has water in it

8. Higgs theory Maybe the same applies to us?
What if empty space is actually full of stuff
A field of weak charge everywhere
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!

9. A huge accelerator: CERN's LHC
CMS
Geneva
Airport
LHCb
Alice
ATLAS
CERN
© Photo CERN

10. LHC operation LHC accelerates protons to 6.5 TeV (6500 GeV)
Previous CERN proton accelerator got to 630 GeV
1eV is the energy an electron gets from a 1V battery
1 GeV is the energy required to make a proton (E=mc2)
Speed is c
They circle the 27km tunnel 11,000 times / second
The pass the accelerating cavities a million time in only 100 seconds
Increasing their energy is not really the problem
Making the bending them is the tough job
Consider the lateral acceleration…. 1017G
8.3Tesla magnets
12000 amps in a wire, coiled ~80 times
This is superconducting, or I2R losses would be huge

11. LHC beam Beam energy is dangerous! Enough to melt 2 tonnes of copper
Design
2012
2016
Bunches
2808
1380
2041
Protons / bunch
1011
Particle energy
7000GeV
4000GeV
6500GeV
Stored energy
350MJ
140MJ
280MJ
Beam energy is dangerous!
Enough to melt 2 tonnes of copper
Or burn a hole through the magnet system
Also 11GJ stored in the magnetic fields
~25tonnes of TNT
Caused damage & 1 year delay in 2008

12. Data Delivered “fb-1” measure amount of data
proton-proton collisions
5.5fb-1 in 2011, 22.8fb-1 in 7/8 TeV
4+39fb-1 in 13 TeV
i.e. more data in 2016
than the previous years
combined
No complete results yet

13. LHC collisions The beams have 6.5 TeV: 13 TeV in collision
But a proton is a bag of quarks
Only rarely do two high-energy quarks collide head on
Approximately two collisions in 1010 makes one
How do we recognize a Higgs if we make one?
Decays immediately to known particles
Need a detector that can measure and characterize the particles

14. ATLAS detector

15. What do we see in collisions?

16. Here is an event with 2 muons
Most particles are stopped at the green calorimeter
Muons get right to the outside
Two are seen here
They are heavy copies of electrons
They both come from the same collision
Are they connected?

18. How to find the Higgs? The Higgs boson breaks up almost instantly
There are lots of ways it can decay:
Two photons
Two Z bosons
WW, bb, ττ, ….
The problem is background
We smashed 2x1015 protons in LHC already
We need to sort through all that to find Higgs candidates
Clean, well measured decays are important
Even if rare
Decay to 2 photons and 2 Zs were used for the discovery

19. H→γ γ Not a common decay 2 per 1000 Higgs
But can measure photon energies well
Extract mass
Photon is neutral, so no track
But a cluster of energy in 'ECAL'
But photons are light – lots of light comes out of collisions

20. Two photon mass: Is there a bump?
2011/2012
2015/2016
Lots of events have 2 photons
So look for excess of ~500 Higgs bosons
Small peak at 125: seen in 2012 and 2016 (phew)

21. H→ZZ→llll

22. Four-lepton mass: Yup, bump!
Red/purple are known background sources
Background reasonably described
Note peak at 91 from Z to four leptons
Clear peak at 125 – both in 2012 and 2016 data

23. OK, so what? Well...lots Does it interact with matter, or only forces?
Now we see H→ττ, a fermion
Is it a Higgs
It looks like spin 0 – that’s pretty suggestive
Is it the Higgs
Got to check all its decays are right
Is it the only Higgs boson?
String theory, combining QM+Gravity, expects 5 or more.
Does it decay to Dark Matter?
This would mean an invisible decay..we are looking.
But I stick to two questions:
The Brout-Englert-Higgs field
Naturalness

24. What about the Higgs field?
The Higgs mechanism needs the field filling space
Unlike light, you turn it off and it is still there
~2 Higgs bosons / fm3
The density of the field is cosmologically ridiculous
It is 120 orders of magnitude larger than dark energy – and the opposite sign
Remember: we don't have a quantum theory of gravity
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 at once
But that will be a very hard road to follow...

25. How to see Higgs self-interaction
Need to produce 2 Higgs bosons
Only the LHC can hope to do this today
They are much rarer than one Higgs
The best Higgs modes: γγ and ZZ→llll have BR of and respectively
If we want HH→ γγγγ we can make one by 2035!!
Not enough to measure
So we need to try to use more abundant modes
e.g. HH→ γγbb has 300 predicted – but can they be seen?
This is one of the major reasons we are planning to run LHC to 2035.
Warwick working on replacement ATLAS tracker

26. A tale of two top loops
Z and Higgs bosons are neutral and interact with top quarks
They can split (Uncertainty) into a tt pair and reform again
This changes measured properties
Precise measurements of Z bosons at LEP (etc.) in 1990s predicted mt=173±12 GeV
C/f 173±1 GeV from measurement
Conclusion: Loops are real and work as we expect

27. A tale of two top loops
Higgs boson properties are also changed by top loop
But maths is different because H is spin 0, Z spin 1
Effect is to correct measured mass to be
M2H,meas = M2H,bare + p2max/c2
pmax is the highest momentum allowed before theory changes
e.g. scale of Gravity? ~ 1018 GeV
So the fact we see 125GeV says our theory stops around 125GeV!

28. Naturalness
This suggests something is missing with a mass near the Higgs boson mass
Supersymmetry would fix this with a top-partner the ‘stop’
And it has a dark matter candidate too...
But there are many other ideas
This makes us very enthusiastic about the 2016 LHC data
Energy raised from 8→13 TeV
Anything might be round the next corner….

29. What comes next? We have found a Higgs boson
This confirms a 'Higgs Field' filling space
Unlike light, you turn it off and it persists
But it is much denser than lead...
This is something radically new
It is not like matter, not like a force
Newton, 1704, described world as these
It is a Higgs field, something new.
Now we need to understand it
LHC just produced large amounts of high energy (13TeV) data
maybe we will find something else too!
1964
2012

30. The complete theory: We have maths that describes particles and forces
But we cannot discuss that here

31. Particle identification
Look at me! I'm a neutrino
Reconstruct long-lived particles in the detector
Basically only a few types: photons(γ), e-, μ-, p+, π+, n, ν
Can identify type by interaction pattern in the detector
Measure the momentum by bending in magnetic field
Electrons, muons and photons especially useful
There are none in a proton so their presence is suggestive
We can measure their energy very well

32. Evidence: H to ZZ The measured HZZ rate is about 10xHγγ
After allowing for Z→ll Br
So HZZ must be single vertex, not a loop
The Z interacts with weak charge
ZZH vertex shows the H must be weak charged
But Z is neutral (Charge and weak charge)
So in H→ZZ where does the charge go?

33. Evidence: H to ZZ The measured HZZ rate is about 10xHγγ
After allowing for Z→ll Br
So HZZ must be single vertex, not a loop
The Z interacts with weak charge
ZZH vertex shows the H must be weak charged
But Z is neutral (Charge and weak charge)
So in H→ZZ where does the charge go?
It is really a 4-point coupling
One leg 'grounded' in the vacuum
The ZZ decay needs vacuum help
With a (weak) charge!
The apparent 3 point couplings come
from expanded about v
There IS a field
∂ μ ϕ ∂ μ ϕ

34. LHC running

35. LHC magnets

36. The 4 big detectors LHC has 4 points where the beams cross ATLAS
Designed to see what happens at the highest energy
'General Purpose' – ready for anything
e.g. Higgs, Supersymmetry, dark matter, black holes
CMS
Similar to ATLAS. 2 experiments check each other
LHCb
Studies b quarks, for matter-antimatter differences
LHC energy & luminosity means lots of particles
ALICE
Studies the quark gluon plasma
A soup of free quarks when two lead nuclei collide
LHC switches to Pb+Pb for last few weeks of year

37. The Higgs self-interaction
The Brout-Englert-Higgs theory replaces a mass term (mφ2) by a two-term piece (-mφ2+φ4)
Φ is the field density
The field-energy, or action, has a minimum away from zero
It is this that means the Universe sits in a high Higgs density state