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

Don Lincoln, Fermilab

Tales From the FrontA Physicist Reports from the Trenches

Don LincolnFermilab


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 madeof protons, neutronsand electrons

Helium Neon

u

du u

d d

Proton NeutronElectron

Gluons hold quarks togetherPhotons hold atoms together



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)

Fermilab4×10-12 seconds

LHC10-13 Seconds


Don Lincoln, FermilabIncreasing ‘Violence’ of Collision

ExpectedNumber

ofEvents

Run II

Run I

Increased reach for discovery physicsat highest masses

Huge statistics for precision physicsat low mass scales

Formerly rare processesbecome high statisticsprocesses

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



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


• 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


6 quarks

2 quarks2 leptons

2 neutrinos

Taustuff

(hard)

4 quarks1 lepton

1 neutrino

The types ofcollisions one gets

in top-creating collisions are not

unique to top.

In fact, there are many otherways 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


Top Facts• Very messy

collisions

• Hundreds of objects after collision

• Need to simplify the measurement

Don Lincoln, FermilabWe’re in luck!

Quarks can’t exist, except when they are confined

MiracleqAs quarks leave a collision, they change into a ‘shotgun blast’ of particles called a

‘jet’

q


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.

222

22242222 1

pEM

MpEc

cMcpE

Lorentz Invariant

221

221

2A

2121

)()(M :So

;

ppEE

pppEEE AA

{ {{Not Lorentz Invariant

Lorentz Invariant

A

1

2


The ChallengeEnd 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

121

3

1

4

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 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

Undiscovered!


“LEP observes significant Higgscandidates for a mass of 115 GeVwith a statistical significance of 2.7 and compatible with theexpected rate and distribution ofsearch channels.”

Chris Tully, Fermilab Colloquium13-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

mH 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?

Documents

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