# Presentation: 128110

## Tales From the FrontA Physicist Reports from the Trenches

- Tales From the FrontA Physicist Reports from the Trenches

- Don Lincoln
- Fermilab

## 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

## Slide 4

## 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?)

- ??? (Before that)

- Fermilab
- 4×10-12 seconds
- LHC
- 10-13 Seconds

## Slide 6

##   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.
- 			     ****  The major detectors have undertaken
-                                           massive upgrades to utilize the
-                                              increased collision rate.
**	****	****	****	****                  ******  Run II began March 2001

## Slide 8

## 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.

## DØ 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).

## Highlights from 1992-1996 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

## 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

## Top Facts

- 6 quarks

- 2 quarks
- 2 leptons
- 2 neutrinos

- Tau
- stuff
- (hard)

- 4 quarks
- 1 lepton
- 1 neutrino

- 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.

- 1,000,000 to 1

- 20 to 1

- 3 to 1

## Top Facts

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

***Experimentalist's View***

## 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’*

- 

## Where Did the Energy Go?

## Combining Viewpoints

## 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

## 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.  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.

## 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

## 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 has 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

- Good 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.

## Data-Model Comparison

## Data-Model Comparison

## 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?