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Physics at the FermilabTevatron Collider

Darien WoodNortheastern University

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Outline

• Introduction: collider experiments• The Tevatron complex (review)• Examples of physics studies at the Tevatron

– jet production – testing substructure– Search for extra space-time dimensions– Direct search for the Higgs boson– Precision measurements: W mass & top mass– Sleuth – is there anything new?

• What’s next

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Motivating Questions

• Do the laws of physics that we understand continue to work at the smallest scales that we can probe?

• Are the known “fundamental particles” (quarks, leptons, vector bosons)” truly fundamental, or are they made of something else?

• Is there a higher mass or energy scale at which new types of particles or interactions can be seen?

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Today’s highest energy beams: FermilabDØ

TeVatron

MainInjector

2 km

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Tevatron Collider

Beam:980 GeVprotons

“Target”:980 GeVantiprotons

Collision energy = 1.96 TeV

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Why use antiprotons?

Efficient annihilation of particle and antiparticle

Also, protons & antiprotons automatically travel in opposite orbits in the accelerator

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Collision statistics

• Each collision is a random event; many different kinds of new particles could be produced

• Theory does not predict what will happen on a given event, but it does predict the probability for certain things happening (like top quark production).

• Probability of producing a top and and anti-top is around 1 event in 1010

• Some other processes are even more rare

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Why Colliding Beams?

980 GeV protonHits proton at rest:

GeVs 30≈

980 GeV protonHits 980 GeV antiproton:

GeVs 1960≈

(see http://www-ed.fnal.gov/projects/exhibits/searching/)

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Major particle colliders (past, present & future)Lepton Colliders

• ~1974-1985, SPEAR– SLAC (Stanford, CA)– e+e-, ~3 GeV

• 1979-present, CESR– Cornell (Ithaca, NY)– e+e-, ~10 GeV

• ~1980-1990 PETRA, – DESY (Hamburg)– e+e-, ~35 GeV

• 1989-1998, SLC– SLAC (Stanford, CA)– e+e-, ~90 GeV

• 1989-2000, LEP– CERN (Geneva)– e+e-, ~200 GeV

• ~2020, NLC/TESLA– (CA, IL, Hamburg?)– e+e+, 500 GeV

Hadron Colliders• 1981-1990, SppbarS,

– CERN (Geneva)– p-pbar, ~630 GeV

• 1987-present, Tevatron– Fermilab (Batavia, IL) – p-pbar, ~2 TeV

• 2006?, LHC– CERN (Geneva)– 14 TeV pp

Mixed Colliders• 1992-present, HERA

– DESY (Hamburg) – ep, 30+280 GeV

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Experiment example #1: quark scattering

• Repeating Rutherfoord’s experiment, essentially

• Detect energy & angle of outgoing quarks• Note: quarks could be replaced with gluons – very hard to

distinguish• At what distance scale is physics tested?

quark in antiquark in

quark out

antiquark out

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A closer look

(Too) simple minded calculation:

m10GeV900

fmGeV2.1 18−≈⋅==pchcλ

But this is a swindle, because typically, only a small fraction of the proton energy goes into the hard collision:

Quarks are not free, so whatemerges is a collimated jet of hadrons along theoriginal quarkdirection

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An event observed in the detector:

Colors correspond to energydeposited in a “cell” of the calorimeter

(2-dimensional slice)

Point of collision

Charged tracks

Note that energy is concentratedin two narrow cones, or jets. Two-jet production is the most common hard scatter process

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Spectrum of jet transverse energy

ET (GeV)

1/(∆

η∆E

T)∫∫

d2 σ/(d

ETdη

)dE

Tdη

(fb

/GeV

)

DØ Data |ηjet| < 0.5

JETRAD

CTEQ3M, µ = 0.5 ET max

1

10

10 2

10 3

10 4

10 5

10 6

10 7

50 100 150 200 250 300 350 400 450 500

pin

pout ET

Hard collision:

soft collision:

ET

DØ Run 1

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Angular distribution of two jet events

0.5

0.75

1

1.25

1.5

200 400 600 800M (GeV/c2)

Cro

ss s

ectio

n ra

tio

JETRAD: CTEQ3M, µ = 0.5ET max, sep=1.3

Λ+ =1.5 TeVΛ+ =2.0 TeVΛ+ =2.5 TeVΛ+ =3.0 TeV

DØ Data

ΛΛΛΛ is the compositenessmass scale

This is whereyou would expect to see evidence of quark substrucure

The ratio of (forward+backward)/(central) is plotted

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Example 2: Looking for Extra Space-time Dimensions

0 250 500 750 10000

0.250.5

0.75110

-1

1

10

10 2

Data

Mass, GeV |cos(θ* )|

Eve

nts a)

0 250 500 750 10000

0.250.5

0.75110

-1

1

10

10 2

Total background

Mass, GeV |cos(θ* )|

Eve

nts b)

0 250 500 750 10000

0.250.5

0.75110

-1

1

10

10 2

SM+LED signal, η = 1 TeV-4

Mass, GeV |cos(θ* )|

Eve

nts c)

0

1

2

3

4

5

6

7

0 0.25 0.5 0.75 1|cos(θ*)|, M > 300 GeV

Eve

nts

|cos(θ*)|, M > 300 GeV|cos(θ*)|, M > 300 GeV|cos(θ*)|, M > 300 GeV

d)

RR<3x10-4m (n=2)R<2x10-15m (n=7)

qq

γ

γ

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Example 3: Direct Search for Higgs Boson

• The most important missing piece of the “Standard Model”• Responsible for giving mass to all particles with mass• Mass of Higgs itself is unknown, >115 GeV• Likes to be produced with W’s and Z’s, the carriers of the

weak interaction:

qbar’

q

W

W

H

µ

b

bbar

ν

What would you see in the detector for such an event? How could you be fooled?

Predicted frequency: ~1 in 2x1012

collisions

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Example 4: precision measurements

• With enough measurement precision and statistics, it is possible to “see” objects smaller than the wavelength:

• Similarly, the properties of lower mass particles can be distorted by the effects of “virtual” higher mass particles

Wavefront distorted by interference effects

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“Self-interference” of W boson

WW

W WH

t

b

GeV175≈tm

GeV80≈Wm GeV5≈bm

GeV?≈Hm

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W Boson

• Discovered at CERN in 1982• Now produced at both the

Tevatron and LEP• At DØ, its mass is measured

precisely using the decay mode

• Approximately 60,000 W events used in the mass measurement.

• Fit transverse mass (formed using quantities perpendicular to the beam direction)

• mass measured to less that 100 MeV (about 0.1%)

νeW →

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The top quark

• Discovered at the Tevatron in 1995

• Produced mostly in pairs

• almost every possible decay mode is used in its discovery and mass measurement

• about 90 events used in mass measurement.

• Mass measured to better than 5% by DØ

Xttpp +→

0

1

2

3

4

5

6

7

8

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Fit top quark mass (GeV/c2)

data

fit

bkgd

31 Events(5 tagged)

0

10

0

2

4

6

∆ ln

L

80 120 160 200 240 280

Fit mass100 260

LBNN

150 200True mass

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W mass precision

80.2

80.3

80.4

80.5

80.6

130 150 170 190 210

mH [GeV]114 300 1000

mt [GeV]

mW

[G

eV]

Preliminary

68% CL

∆α

LEP1, SLD Data

LEP2, pp− Data

Mass shift from virtual Higgs effects (?)

Note supressed zero

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DD Question: Is it possible to Question: Is it possible to perform a perform a datadata--driven driven search for new phenomena?search for new phenomena?

SleuthSleuth

A quasi-model-independent new physics search strategy

1) Define final states1) Define final states

2) Define variables2) Define variables

∑ WZTpγ

∑ lTp

∑ jTp

Tp/

3) Define regions3) Define regions4) Define "interestingness"4) Define "interestingness"

∑ =

−N

i

ib

ibe

0 !

5) Run 5) Run hypothetical hypothetical similar similar experimentsexperiments

6) Can Sleuth find something 6) Can Sleuth find something interesting?interesting?

topbkg

σ

7) Does Sleuth 7) Does Sleuth find anything find anything interesting in interesting in Run I data?Run I data?

(yes!)(yes!)

No.No. A systematic A systematic search of many search of many final states reveals final states reveals no evidence of new no evidence of new highhigh ppTT physics.physics.

8) Apply to Run II8) Apply to Run II

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Main points

• Creating collisions with lots of kinetic energy (in the center of mass system) makes it possible to create many different new particles

• Creation of massive particles requires more energy• The average result of collisions can be predicted by theory

(if the theory is correct) but each individual collision has a random outcome, so– Many different processes can be studied with the same experiment

(with different analysis procedures)– Detecting rare processes requires the accumulation and examinatio

of trillions of collisions.