1. Tales From the Front A Physicist Reports from the Trenches
Don Lincoln
Fermilab
Don Lincoln, Fermilab

2. What’s the Point?
High Energy Particle Physics is a study of the smallest pieces of matter.
It investigates (among other things) the nature of the universe immediately after the Big Bang.
It also explores physics at temperatures not common for the past 15 billion years (or so).
It’s a lot of fun.
Don Lincoln, Fermilab

3. Periodic Table Helium Neon All atoms are made of protons, neutrons
and electrons u u u d d d
Electron
Neutron
Proton
Gluons hold quarks together
Photons hold atoms together
Don Lincoln, Fermilab

4. Don Lincoln, Fermilab

5. Stars form (1 billion years)
Now (15 billion years)
Stars form (1 billion years)
Atoms form (300,000 years)
Nuclei form (180 seconds)
Protons and neutrons form (10-10 seconds)
Quarks differentiate (10-34 seconds?)
Fermilab
4×10-12 seconds
LHC
10-13 Seconds
??? (Before that)
Don Lincoln, Fermilab

6. Don Lincoln, Fermilab

7.  The Main Injector upgrade was completed in 1999.
 The new accelerator increases the number of
possible collisions per second by
 The major detectors have undertaken
massive upgrades to utilize the
increased collision rate.
 Run II began March 2001
Expected
Number of
Events
Huge statistics
for precision physics
at low mass scales
1000
Formerly rare processes
become high statistics
processes
100 10
Increased reach
for discovery physics
at highest masses
Run II 1
Run I
Increasing ‘Violence’
of Collision
Don Lincoln, Fermilab

8. Don Lincoln, Fermilab

9. How Do You Detect Collisions?
Use one of two large multi-purpose particle detectors at Fermilab (DØ and CDF).
They’re designed to record collisions of protons colliding with antiprotons at nearly the speed of light.
They’re basically cameras.
They let us look back in time.
Don Lincoln, Fermilab

10. DØ Detector: Run II Weighs 5000 tons
Can inspect 3,000,000 collisions/second
Will record 50 collisions/second
Records approximately 10,000,000 bytes/second
Will record (1,000,000,000,000,000) bytes in the next run (1 PetaByte).
30’
30’
50’
Don Lincoln, Fermilab

11. Highlights from Run
Limits set on the maximum size of quarks (it’s gotta be smaller than 1/1000 the size of a proton)
Supported evidence that Standard Model works rather well (didn’t see anything too weird)
Studied quark scattering, b quarks, W bosons
Top quark discovery 1995
Don Lincoln, Fermilab

12. The Needle in the Haystack: Run I
There are 2,000,000,000,000, possible collisions per second.
There are 300,000 actual collisions per second, each of them scanned.
We write 4 per second to tape.
For each top quark making collision, there are 10,000,000,000 other types of collisions.
Even though we are very picky about the collisions we record, we have 65,000,000 on tape.
Only 500 are top quark events.
We’ve identified 50 top quark events and expect 50 more which look like top, but aren’t.
Run II ×10
Don Lincoln, Fermilab

13. Top Facts Theorist’s View Discovery announced March 1995
Produced in pairs
Decays very rapidly ~10-24 seconds
You can’t see top quarks!!!
Six objects after collision
Theorist’s View
Don Lincoln, Fermilab

14. Top Facts In each event, a top and anti-top quark is created.
~100% of the time, a top quark decays into a bottom quark and a W boson.
A W boson can decay into two quarks or into a charged lepton and a neutrino.
So, an event in which top quarks are produced should have:
6 quarks
4 quarks, a charged lepton and a neutrino
2 quarks, 2 charged leptons and 2 neutrinos
Don Lincoln, Fermilab

15. Top Facts 2 quarks 2 leptons 3 to 1 2 neutrinos 1,000,000 to 1 Tau
stuff
(hard)
The types of
collisions one gets
in top-creating
collisions are not
unique to top.
In fact, there are many other
ways that one can make top-like
collisions.
You have to figure out how to pick the
ones you want.
6 quarks
4 quarks
1 lepton
1 neutrino
20 to 1
Top Facts
Don Lincoln, Fermilab

16. Experimentalist's View
Top Facts
Very messy collisions
Hundreds of objects after collision
Need to simplify the measurement
Experimentalist's View
Don Lincoln, Fermilab

17.  Quarks can’t exist, except when they are confined We’re in luck!
Miracle q
As quarks leave a collision, they change into a ‘shotgun blast’ of particles called a ‘jet’  q
We’re in luck!
Don Lincoln, Fermilab

18. Where Did the Energy Go?
Don Lincoln, Fermilab

19. Combining Viewpoints
Don Lincoln, Fermilab

20. Measuring Mass: Quicky Review of Special Relativity
For a particle (A), energy, momentum and mass are related:
Let this particle (A) decay into two particles (1) & (2)
High School Physics  Energy and momentum are conserved.
Lorentz Invariant 1 { A { {
Not Lorentz Invariant 2
Lorentz Invariant
Don Lincoln, Fermilab

21. The Challenge 4 quarks, 1 lepton, 1 neutrino End on view top + antitop
Jets in
“God Mode”
Jets in
“Don Mode”  
Algorithm +
Reality 
Algorithm   
Guess!!!!
Don’t know who goes with what
Know
(1) W   +  MW2 = (Em + En)2 - (pm +pn)2
(2) Mt = Manti-t
(3) t  W + b anti-t  W + b
Note:
combinations
jet m n
jet
Don Lincoln, Fermilab

22. Top Quark Run I: The Summary
The top quark was discovered in 1995
Mass known to 3% (the most accurately known quark mass)
The mass of one top quark is 175 times as heavy as a proton (which contains three quarks)
Why???
Don Lincoln, Fermilab

23. In 1964, Peter Higgs postulated a physics
mechanism which gives all particles their mass.
This mechanism is a field which permeates the
universe.
If this postulate is correct, then one of the signatures
is a particle (called the Higgs Particle). Fermilab’s
Leon Lederman co-authored a book on the subject
called The God Particle.
top
bottom
Don Lincoln, Fermilab
Undiscovered!

24. “LEP observes significant Higgs candidates for a mass of 115 GeV
with a statistical significance of
2.7 and compatible with the
expected rate and distribution of
search channels.”
Chris Tully, Fermilab Colloquium
13-Dec-2000
Non-Expert Translation:
Maybe we see something, maybe we
don’t. The likelihood of error is ~1%.
What we see is consistent with being
a Higgs Particle. But it could end up
being nothing.
It’s Fermilab’s turn.
Don Lincoln, Fermilab

25. Higgs Hunting at the Tevatron
If you know the Higgs mass, then the production cross section and decays are all calculable within the Standard Model
inclusive Higgs cross section is quite high:
~ 1pb = 1000 events/year
but the dominant decay H  bb is swamped by background
thus the best bet appears to be associated production of H plus a W or Z
leptonic decays of W/Z help give
the needed background rejection
~ 0.2 pb = 200 events/year
Don Lincoln, Fermilab

26. Is a Fermilab Higgs Search Credible?
mH probability density, J. Ellis (hep-ph/ )
LEP incorrect
Rule out with 95% certainty by ~2003
LEP correct
Similar quality evidence ~
“Discovery” quality evidence ~2007
Higgs exists but is heavier than LEP suggests
Depends on how heavy
DØ has a good shot on seeing ‘maybe’ and possibly ‘absolutely’ quality evidence
Don Lincoln, Fermilab

27. Is a Fermilab Higgs Search Credible?: Good News/Bad News
×10 more data than Run I
Bad News
×1/10 less likely to be created than top quark
So it’s a wash...similar problem to Run I top search
Except...
Events which look like Higgs but aren’t are much more numerous.
An irony...top quarks are a big piece of the ‘noise’ obscuring Higgs searches.
Don Lincoln, Fermilab

28. Data-Model Comparison
Don Lincoln, Fermilab

29. Data-Model Comparison
Don Lincoln, Fermilab

30. Run II: What are we going to find?
I don’t know!
Improve top quark mass and measure decay modes.
Do Run I more accurately
Supersymmetry, Higgs, Technicolor, particles smaller than quarks, something unexpected?
Don Lincoln, Fermilab