Search - Particle Physics and Particle Astrophysics

hep.shef.ac.uk

Search - Particle Physics and Particle Astrophysics

Along The Path to Understanding:

Searches for New Phenomena at

the Tevatron

Todd Adams

Florida State University

Sheffield University

May, 2007


Outline

• Introduction

• Extra gauge bosons

• W′, Z′

• Extra dimensions

• Randall-Sundrum gravitons

• Supersymmetry

• trileptons

• Long-lived particles

• More SUSY

• squarks/gluinos

• Summary

May, 2007 Searches at the Tevatron - T. Adams, FSU 2


• Standard Model

• 16 observed particles

• one unobserved (Higgs)

• 4 forces

• more than 30 years

• incomplete

• Many unanswered questions

• what is the meaning of flavor

• why is the weak force 10 32 times stronger than

gravity

• are there several forces or one force

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• Many theories (Beyond the Standard Model)

• supersymmetry

• technicolor

• leptoquarks

• extra dimensions

• string theory


Graviton (G)

our universe

p

Recoil

• Experimental data needed to determine which

might be correct

May, 2007 Searches at the Tevatron - T. Adams, FSU 5


The path to new

understanding is

through discovery

Many possibilities = many paths = many searches

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Fermilab’s Tevatron

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Run 1

• 1992-1996

• 0.1 fb -1

Run 2a

• 2001-2006

• 1 fb -1

Run 2b

• 2006-

• 3-7 fb -1 • 2 multipurpose experiments

May, 2007 Searches at the Tevatron - T. Adams, FSU 8


Multipurpose Detectors

tracking volume

EM calorimeter

hadron calorimeter

muon system

• Leptons at colliders

• electrons

• muons

• taus

• Other objects

• photons

• jets

• many hadrons

• missing E T

• Σp T = 0

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calorimeter

silicon tracker

fiber tracker

muon system

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Detector

central outer tracker

silicon tracker

end plug

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How to Perform a Search

1) Have an idea

• where to look, what to look for

2) Determine standard model processes

• make sure that simulation matches data

3) Determine data selection which optimizes sensitivity

4) Compare data to simulation

• estimate backgrounds

• estimate likelihood of

seeing signal

5) Determine what physics is

observed or excluded

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Extra Gauge Bosons

W′

Z′


W′ Search

Search for additional charged

gauge boson

• W′eν

• can also decay to muons or taus

• Events w/ electron and missing

transverse energy

• electron E T >30 GeV

• MET>30 GeV

• M T >150 GeV

May, 2007 Searches at the Tevatron - T. Adams, FSU 15


W′ Search

Data = 630 events

Bkgd = 623 ± 18 +83 -75 events

Data matches expectation from

standard processes

May, 2007 Searches at the Tevatron - T. Adams, FSU 16


W′ Search

• Limits

• lack of observation

mean a limit can be

set on the crosssection

times

branching fraction

• this is the

experimental result

• Models can be used to

interpret it

M W’ > 965 GeV @ 95% CL

May, 2007 Searches at the Tevatron - T. Adams, FSU 17


Z′ Search

• Some models predict heavier

versions of Z boson

• called Z′

• Z′ee

• can also decay to muons or taus

Search for events w/ two

energetic electrons

• electron E T >25 GeV

events displays

from CDF

May, 2007 Searches at the Tevatron - T. Adams, FSU 18


Two Electron Mass Search

Search for narrow high

mass resonances

• 150


Z′ Search Results

• Limits

• set limit on the

cross-section times

branching fraction

• Models can be used to

interpret it

• Limits on Z′

Z′ SM >923 GeV

Z′ I >729 GeV Z′ ψ >822 GeV Z′ χ >822 GeV Z′ η >891 GeV

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Extra Dimensions

Randall-Sundrum Gravitons

Graviton (G)

our universe

p

Recoil


Randall-Sundrum

Gravitons

• Use extra dimensions to address strength of gravity

• gravity works in all dimensions

• all other forces confined to “normal” 4d space

• Resonant production of pairs of electrons, muons or

photons at Tevatron

• observable in mass or

angular distributions

• recent searches in

dielectron mass

dielectron and diphoton

states

• no excess observed

May, 2007 Searches at the Tevatron - T. Adams, FSU 22


RS Gravitons

CDF

• M G >889 GeV

for k/M pl = 0.1

D0

• M G >865 GeV

for k/M pl = 0.1

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Supersymmetry

Charginos and neutralinos

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Supersymmetry (SUSY)

• Model which attempts to solve:

• explanation of the Higgs mechanism

• unification of the electroweak and strong forces

• the gravitational force strength issue

• Proposes

• each particle has a partner

• different spin: bosons ⇔ fermions

• no superpartners have been discovered dark matter

candidate

• R-parity – new quantum number

• normal = +1 SUSY = -1

• if conserved the lightest SUSY particle (LSP) is stable

May, 2007 Searches at the Tevatron - T. Adams, FSU 25


Charginos and

and Neutralinos in Trileptons

pp

→ ~ χ ±

1

~

χ

0

2

R-parity conserved

Trileptons:

~ ~ χ

±

1

pp

χ → lν

→ ~ χ ±

0

1

1

~

χ

0

2

χ~ 0

→ ll

~ χ

2

0

1

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Charginos and Neutralinos Trileptons

Trileptons:

Advantages

• small backgrounds

Disadvantages

~ ~ χ

± 0

χ~ 0

→ ll

0

1

1

2

χ → lν

2

χ ~

• 3 rd lepton is low p T

• small cross-section x

branching ratio

Techniques

• all 3 leptons

• 2 leptons + track

• same-sign leptons

combine techniques for best results

May, 2007 Searches at the Tevatron - T. Adams, FSU 27


3lep

Lumi

(pb -1 )

Bkgd

ee+l

CEM

ee+l

plug

eμ+l

μμ+l

high p T

μe+l

CEM

μe+l

plug

ee +

track

μμ+l

low p T

1034 954 1034 745 745 680 1013 976

0.44 ±

0.08

14 Combined Results

0.34 ±

0.10

0.28 ±

0.09

0.64 ±

0.18

0.42 ±

0.08

0.36 ±

0.07

0.97 ±

0.28

Data 0 0 0 1 0 0 3 1

0.42 ±

0.12

LS lep ee LS ee si LS e si e si LS e si μ LS eμ LS μμ LS

Lumi

(pb -1 )

Bkgd

993 993 993 971 971 1087

0.10 ±

0.10

0.50 ±

0.30

1.30 ±

0.30

1.70 ±

020

2.30 ±

0.50

Data 1 2 1 4 4 1

0.90 ±

0.10

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SUSY Interpretation

mSUGRA (inspired)

tanβ=3, A 0 =0, μ>0, m 0 =60

A. mSUGRA

no limit yet

B. MSSM

same as mSUGRA

without slepton mixing

M(χ 1± ) > 130 GeV

C. MSSM

set lepton BR to

same as W/Z

no limit yet

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• New channels

• μμl and eμl

4 Combined Results

Lumi

(pb -1 )

Bkgd

Data

eel 1000 0.76 ± 0.67 0

μμl 1100 0.32 ± 1.34 2

μel 1100 0.94 ± 0.40 0

LS μμ 1000 1.1 ± 0.4 1

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SUSY Limits from Trileptons

• Use 3 SUSY models

• mSUGRA inspired

• m(χ 1± ) ≈ m(χ 20 ) ≈ 2m( χ 10 )

• no slepton mixing

• large m 0

• W/Z decays dominate

• no sensitivity

• 3l-max

• m(slepton) slightly larger

than m( χ 20 )

• M(χ 1±

) > 141 GeV

• heavy squarks

• relax scalar mass unification

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Long-lived Particles

neutral, long-lived particles

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In 1937:

History Lesson

In 2007:

electron

proton

neutron

photon

positron

hypothesis of neutrino

hypothesis of Higgs boson

very nice model of particle physics

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“Who ordered that”

- I.I. Rabi (1937)

Discovery of the muon

Anderson and

Neddermeyer

1937

Street and Stevenson

new discovery ⇒ new understanding

other examples: strange particles, J/ψ, tau lepton

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Search for Neutral, Long-lived Particles

Search for production of

two neutral particles

• Look for decay away

from production point

• two muons p T >10 GeV

• Why

• NuTeV result

• Theory

Simulated

event

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NuTeV

Neutrinos at the Tevatron

neutrino deep-inelastic scattering

sin 2 θ w

structure functions

charm production

NuTeV

• 10 12 protons per minute (10 7 Watts)

• ~15x10 9 neutrinos per minute (in five

2ms pulses)

• 700 ton detector located 1.4 km from

neutrino production

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eam

Two muon tracks

NuTeV

Iron-scintillator

Calorimeter

Decay region with drift chambers

Muon Spectrometer

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Search for 2.2-15 GeV

object decaying

to leptons:

3 μμ events found

Expected background:

0.07 ± 0.01 events

No explanation to date

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Search for Neutral, Long-lived Particles

• NuTeV result unexplained

• could N 0 be produced at the

Tevatron

• Theory ideas

• RPV SUSY

• χ 1

0

pair production

• “Hidden valley” theory

Search for muon pairs that come

from point not near beamline

20 cm

5 cm

y

r

V

x

May, 2007 Searches at the Tevatron - T. Adams, FSU 39


Limits on NLLP Production

0 events observed

0.75 ± 1.1 ± 1.1 expected

Phys. Rev. Lett. 97

161802 (2006)

DØ sets limits on pair production

cross-section vs. lifetime

Excludes some interpretations of

NuTeV result

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More SUSY

squarks and gluinos

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Squarks and Gluinos

• Supersymmetry

• quarks ⇔ squarks

• gluons ⇔ gluinos

• often produced in pairs

~~ ~~ ~~

ppqq ppqg ppgg

• decays to quarks, gluons and

undetected neutralinos

~ ~ ~ ~

qqχ 0

1

gqqχ 0

1

• observed as jets with MET

• look for events with multiple,

high energy jets

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Three Combined Analyses

• Optimize in three search channels

• account for overlaps

“2-jets” “3-jets” “4-jets”

Dominate Mode ppqq ppqg ppgg

~~ ~~ ~~

# of expected events 7.47 ± 1.06 ± 1.17 6.10 ± 0.37 ± 1.21 33.35 ± 0.81 ± 5.24

Data 5 6 34

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The Standard Model Revisited

• Good News: The Standard Model is rock-solid

• one missing piece – Higgs boson

• no significant excesses in searches for new particles

• good agreement is standard processes

• W/Z production, top quark, B physics at Tevatron

• B physics at SLAC/KEK

• neutrino oscillations confirmed

• biggest discrepancy - g-2 results

• Bad News: No evidence of new physics

• something is needed to answer our outstanding

questions

May, 2007 Searches at the Tevatron - T. Adams, FSU 45


Summary

• The Tevatron has an exciting program of

searches for new phenomena using leptons

• I’ve shown some of the more recent ones

• extra dimensions, SUSY, extra particles

• Significant discovery potential remains

• Also, excellent preparation for LHC searches

May, 2007 Searches at the Tevatron - T. Adams, FSU 46

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