# Presentation: 170580

## Don Lincoln
Fermilab

- Don Lincoln
- Fermilab

- When a Proton Meets an Antiproton
- Don Lincoln

## 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.

## Periodic Table

*All* atoms are made
- of protons, neutrons
- and electrons

- Helium

- Neon

- Proton

- Neutron

- Electron

- Gluons hold quarks together
- Photons hold atoms together

## All particles have ‘anti-particles’, which have similar properties, but opposite electrical charge

Particles
u,c,t    +2/3
d,s,b   -1/3
e,,   -1

Anti-particles
u,c,t    -2/3
d,s,b   +1/3
e,,   +1

- Particles
- u,c,t    +2/3
- d,s,b   -1/3
- e,,   -1
- Anti-particles
- u,c,t    -2/3
- d,s,b   +1/3
- e,,   +1

- Anti-Matter

## Slide 5

## Now 
(15 billion years)

**Now **
**(15 billion years)**

**Stars form**
**(1 billion years)**

**Atoms form **
**(300,000 years)**

**Nuclei form **
**(180 seconds)**

**??? (Before that)**

**Nucleons form    **
**(10****-10**** seconds)**

**Quarks differentiate             (10****-34**** seconds?)**

## Slide 7

## Slide 8

##   The Main Injector upgrade was completed in 1999.

**Increasing ‘Violence’ **
**of Collision **

**Expected**
**Number**
**of**
**Events**

**Run II**

**Run I**

**Increased reach **
**for discovery physics**
**at highest masses**

**Huge statistics **
**for precision physics**
**at low mass scales**

**Formerly rare processes**
**become high statistics**
**processes**

- 1

- 10

- 100

- 1000

****  The Main Injector upgrade was completed in 1999.
- 		 ****  The new accelerator increases the number of
-                       possible collisions per second by 10-20.
- 			     ****  DØ and CDF have undertaken massive
-                                        upgrades to utilize the increased
-                                              collision rate.
**	****	****	****	****                  ******  Run II began March 2001

## 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.

## Typical Collider Detector: Run II

- 30’

- 30’

- 50’

- Weighs 5000 tons
- Can inspect 3,000,000 collisions/second
- Will record 50 collisions/second
- Records approximately 10,000,000 bytes/second
- Will record 1015  (1,000,000,000,000,000) bytes in the next run (1 PetaByte).

## Remarkable Photos

- This collision is the most violent
- ever recorded.  It required that
- particles hit within 10-19 m or
- 1/10,000 the size of a proton

- In this collision, a top and
- anti-top quark were created,
- helping establish their existence

## Coolest

- Coolest
- Detector
-      at
- Fermilab

- Collider
- Detector
-      at
- Fermilab

## Slide 14

## Why Colliders?

-  Energy Conservation

-  Momentum Conservation

## Special Relativity

- Clever (lazy?) Theorists
- c = 1

- Note:

- Low momentum
- means large
- mass

## Mmax = 42 GeV

- Why Colliders?

- 900 GeV           0 GeV

- 900 GeV          900 GeV

- Mmax = 42 GeV

- Mmax = 1800 GeV

- Note: GeV is an energy unit

## Proton

- u

- u

- d

- u

- u

- d

- AntiProton

-   Proton-AntiProton
- Scattering

## Collision Facts

- Very messy collisions
- Hundreds of objects after collision
- Need to simplify the measurement

***Experimentalist's View***

## Feynman Diagrams

- Make particles

- Scatter particles

- Note:  In subsequent
- slides, only representative
- diagrams are shown, not
- all.

*Concentrate on scattered *
*particles, ignore spectators.*

## Stuff to Study

- Quantum ChromoDynamics

- Violence of Collision
- Frequency of Collision
- Exotic Particles?
- Number of Output Objects

- quark

- antiquark

- gluon

- quark

- antiquark

- quark

- quark

- gluon

- quark

- gluon

- quark

- quark

- gluon

- quark

- gluon

- quark

- gluon

## We’re in luck!

- Quarks can’t exist, except when they are confined

- Miracle

- As quarks leave a collision, they change into a ‘shotgun blast’ of particles called a *‘jet’*

- 

## 107:1 !

**Increasing ‘Violence’ **
**of Collision **

**Expected**
**Number**
**of**
**Events**

**Data**

- 1

- 10

- 100

- 1000

- Hiding things?

- Most Likely:Least Likely
- 107:1 !

## ~20% over   7 

- Theory agrees within
- ~20% over   7
- orders of magnitude!

- CDF reports a
- slight excess of
- events at high p
- as compared to DØ
- Difference is not
- understood, but within
- global errors

## Stuff to Study

- ElectroWeak

- Frequency of W & Z Boson Creation
- Mass of W & Z Boson
- Frequency of Decay Modes

- quark

- antiquark

- W boson

- quark

- antiquark

- quark

- antiquark

- W boson

- electron

- neutrino

- quark

- antiquark

- Z boson

- electron

- antielectron

## Measuring Mass: Quicky Review of Special Relativity

- 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

- {

- {

- {

- Not Lorentz Invariant

- Lorentz Invariant

- A

- 1

- 2

## Neutrinos can penetrate

_*light-years*_ of lead

- How

- do you detect

- them?

- Transverse momentum
- is conserved

- Need a ‘fully surrounding’
- (4) detector

## Sometimes Z bosons

- are created in collisions with
- other objects

- quark

- antiquark

- gluon

- antiquark

- quark

- Z

- e

- e

## The Needle in the Haystack: Run I

- There are 2,000,000,000,000,000                    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

## Top Facts

- 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***

## 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

- 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

- Top Facts

## Combining Viewpoints

## The Challenge

- End on view top + antitop

- Algorithm

- Algorithm
- +
- Reality

- 

- 

- 

- 

- 

- 

- Jets in
- “God Mode”

- Jets in
- “Don Mode”

- Don’t know who goes with what

- Know
-    (1)  W   +                              MW2 = (E + E)2 - (p +p)2
-    (2)  Mt = Manti-t
-    (3)  t  W + b                                 anti-t  W + b

- jet
- 
- 

- jet
- jet
- jet

- Note:
- combinations

- 4 quarks, 1 lepton, 1 neutrino

*Guess!!!!*

## 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???**

## In 1964, Peter Higgs postulated a physics 

- 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*.

- Undiscovered!

## “LEP observes significant Higgs

- “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.
- What we see is consistent with being
- a Higgs Particle.  But it could end up
- being nothing.
- It’s Fermilab’s turn.

## 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

- 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

- Higgs Hunting at the Tevatron

## Is a Fermilab Higgs Search Credible?

- LEP incorrect
-  Rule out with 95% certainty by ~2003
- LEP correct
-  Similar quality evidence ~2004-2005
- “Discovery” quality evidence ~2007
- Higgs exists but is heavier than                             LEP suggests
- Depends on how heavy
- DØ and CDF have a good shot on seeing ‘maybe’ and possibly ‘absolutely’ quality evidence

**m****H**** probability ****density, J. Ellis ****(hep-ph/0011086)**

## Is a Fermilab Higgs Search Credible?:      Good News/Bad News

- 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.

## 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?

## Slide 41