1. Physics at the LHC with ATLAS
Ian Hinchliffe (LBNL)
1 April 2010
“Particle physics is the unbelievable in pursuit of the unimaginable. To pinpoint
the smallest fragments of the Universe you have to build the biggest machine
in the world. To recreate the first millionths of a second of creation you have to
focus energy on an awesome scale.”
The Guardian

2. The fundamental questions
• The mass problem
– Why are some particles heavier than others?
– How do they get mass?
• The matter anti-matter problem
– We are made of matter not anti-matter
– The early universe had both matter and anti-
matter: what happened?
• The Dark matter problem: what is it?
• The Dark energy problem
• LHC is expected to contribute to all these
(except the last one)

3. What are you made of?
• Flesh and blood
• Molecules
• Atoms
• Electrons protons and neutrons
• Electrons and quarks
– These are “points”
– No substructure
–
Helium atom (used to keep LHC cold)

4. The fundamental matter particles

5. What holds you together:
fundamental forces
• Gravity: mediated by graviton: only important if
• Others are neutralized: planets, you
• Elementary particle is very heavy (none seen)
• Strong: Mediated by gluon
• Holds protons together
• Short range (size of nucleus)
• No free quarks
• Electromagnetic: mediated by photon
• Long range (infinite)
• Holds atoms and molecules together
• Weak: mediated by W and Z bosons
• Short range (1% of strong range)

6. Forces

7. The Standard Model
• Should really be called a theory (like “theory of
relativity”)
• Developed over last 40 years
• Describes all interactions
– Except gravity
• Calculations and measurements agree to
0.00001%
• A few hints that it may be incomplete

8. The Standard Model incomplete
• Cannot explain some phenomena
– Neutrino masses (can be fixed in an ad-hoc way)
• Mechanism of mass generation not tested
– Could be Higgs boson (coming next)
– Or something more complicated
• Too many arbitrary parameters (18)
– Must be determined by experiment
• Forces not unified
• What about gravity?

9. What is a Higgs?
• Here's a video of that speech
• http://videoglossary.lbl.gov/2009/higgs-boson/
Peter Higgs visiting ATLAS

10. The ATLAS Experiment

11. Underground across the street from CERN

12. The scale of ATLAS
Half size of
Notre Dame
The size is determined by the precision needed for physics

13. Assembly underground

14. A Worldwide Collaboration
Approx 2900 people
700 Students
170 universities
Took 17 years to build
Will operate for 20 years

15. How does it work I
• Protons collide (currently 3.5+3.5 TeV)
– Really proton constituents collide
– Energy is shared out among them
– Each collision is different
• New particles are made
• How many do you get?
– How heavy are they? (LHC energy and sharing)
– How many protons do you have? (LHC luminosity)
– How strongly do they couple to proton constituents?
(details of the new particle itself)

16. How does it work II
• New particles will be unstable (usually)
– Decay into standard model particles
– Usually decay very fast: cannot see them before
decay
• Detector sees high energy particles coming out in all
directions (most importantly NOT along the proton
direction)
– Must measure these
– Quarks and gluons appear as jets of particles (mainly
pions)
– Don't loose anything (except neutrinos)
• Deduce what was made from these observations

17. How does it work III
• Different detector components measure different
particles:
– Bend charged particles in magnetic field
• Follow the track and measure the energy
– Absorb photons
– Absorb neutrons (takes more material)
– Muons penetrate
– If there are neutrinos, the sum of their energies can
be inferred (provided detector has no holes)
• Illustration on next slide
–

18. How does it work IV

19. Animation with actual event added

20. When will we see a Higgs?
• Its mass is unknown (greater than 100 times proton)
• Small production rate
– It has very small couplings to proton constituents
• The decays that are most distinct might be very rare
– Depends on the mass
– Common decays are difficult to distinguish from junk
• Needs at least 10000000000000000 proton proton
interactions
– we got 600000 on Tuesday
– Expect to get close by the end of next year
– Easier at 14 TeV

21. What might a Higgs event look like?
• H--> ZZ >>  ee
• Two muons
• Two electrons
• A jet
• Some junk

22. Will the LHC see Dark Matter?
• Astrophysical observations
– Motion of stars and galaxies
• More “stuff” than we can see
• Universe is mostly not made of “stuff
like us”
• Dark matter
– Clumps near galaxies
• Could be cloud of particles
– Must be very weakly interacting
– Very sensitive experiments
search for these
– Mass should be about 100 times
proton
• Could be produced directly at LHC

23. Extra Dimensions
• We have no real reason why there are 4 dimensions
• Could there be more
• Consider a tightrope walker
– Moves in two dimensions (time+rope)
• Insect on same rope
– Moves in three dimensions
• LHC
– Higher energy
– Shorter distances
– May see more >4 dim
–

24. The future
• The discovery era is beginning
– Lots of theoretical ideas
• Supersymmetric particles
• Extra dimensions
• ..
– Need to know which are physics and which are
sophistry
• Expect to operate LHC for 20 years
• Plans afoot to increase intensity by factor of ten
(2020?)
• Stay tuned...

25. Some more from Tuesday
ATLAS control
room at CERN

26. More events