Tales From the Front A Physicist Reports from the Trenches

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Tales From the FrontA Physicist Reports from the
Trenches
Don Lincoln Fermilab
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Whats 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).
• Its a lot of fun.

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Periodic Table
Helium
Neon

• All atoms are made
• of protons, neutrons
• and electrons

Electron
Neutron
Proton
Gluons hold quarks together Photons hold atoms
together
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• 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 410-12 seconds LHC 10-13 Seconds
??? (Before that)
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• ? 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

Expected Number of Events
Huge statistics for precision physics at low
mass scales
1000
Formerly rare processes become high
statistics processes
100
Increased reach for discovery physics at highest
masses
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Run II
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Run I
Increasing Violence of Collision
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How Do You Detect Collisions?

• Use one of two large multi-purpose particle
  detectors at Fermilab (DØ and CDF).
• Theyre designed to record collisions of protons
  colliding with antiprotons at nearly the speed of
  light.
• Theyre basically cameras.
• They let us look back in
  time.

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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 1015 (1,000,000,000,000,000) bytes
  in the next run (1 PetaByte).

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30
50
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Highlights from 1992-1996 Run

• Limits set on the maximum size of quarks (its
  gotta be smaller than 1/1000 the size of a
  proton)
• Supported evidence that Standard Model works
  rather well (didnt see anything too weird)
• Studied quark scattering, b quarks, W bosons
• Top quark discovery 1995

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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.
• Weve identified 50 top quark events and expect
  50 more which look like top, but arent.

Run II 10
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Top Facts

• Discovery announced March 1995
• Produced in pairs
• Decays very rapidly 10-24 seconds
• You cant see top quarks!!!
• Six objects after collision

Theorists View
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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

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2 quarks 2 leptons 2 neutrinos
3 to 1
1,000,000 to 1
Tau stuff (hard)
6 quarks
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.
4 quarks 1 lepton 1 neutrino
20 to 1
Top Facts
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Top Facts

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

Experimentalist's View
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Were in luck!
Quarks cant exist, except when they are confined
Miracle
As quarks leave a collision, they change into a
shotgun blast of particles called a jet
?
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Where Did the Energy Go?
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Combining Viewpoints
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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
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A
Not Lorentz Invariant
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Lorentz Invariant
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The Challenge
4 quarks, 1 lepton, 1 neutrino
End on view top antitop
Jets in God Mode
Jets in Don Mode
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Algorithm Reality
Algorithm
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?
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Guess!!!!
Dont 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 jet jet
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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???
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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). Fermilabs Leon
Lederman co-authored a book on the subject
called The God Particle.
Undiscovered!
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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 dont. The likelihood of error is
1. What we see is consistent with being a
Higgs Particle. But it could end up being
nothing. Its Fermilabs turn.
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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

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Is a Fermilab Higgs Search Credible?
mH probability density, J. Ellis
(hep-ph/0011086)

• 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

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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 its a wash...similar problem to Run I top
  search
• Except...
• Events which look like Higgs but arent are much
  more numerous.
• An irony...top quarks are a big piece of the
  noise obscuring Higgs searches.

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Data-Model Comparison
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Data-Model Comparison
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Run II What are we going to find?

• I dont know!

Improve top quark mass and measure decay
modes. Do Run I more accurately
Supersymmetry, Higgs, Technicolor, particles
smaller than quarks, something unexpected?