Ênergy frontieR: the There and back Again or Breese quinN University M ississippi of

Ênergy frontieR:Ênergy frontieR:thethe

There and back There and back AgainAgain

oror

Breese quinNBreese quinNUniversity University MMississippiississippiofof

Quarknet Workshop 2008B. Quinn University of Mississippi

2

TheThe Standard modeL Standard modeL

EM

Strong

Weak}

up/down1968 SLACstrange 1964 BNLcharm 1974 SLAC/BNLbottom 1977 Fermilabtop 1995 Fermilab

D0/CDFphoton 1905 Planck/Einsteingluon 1979 DESYW/Z 1983 CERN

electron 1897 Thomsone-neutrino 1956 Reactor muon 1937 Cosmic Raysmu-neutrino 1962 BNLtau 1976 SLACtau-neutrino 2000 Fermilab

ud d

d

u

u

electronneutron

proton..electron neutrino


3

The The Standard modeLStandard modeL

This model has successfully described or predicted almost everything we’ve seen in almost 100 years of particle physics.

However it does have some weaknesses

Too many free parameters

Predictive powers wane as we move past the Electroweak (EW) regime towards the TeV scale and beyond

EM

Strong

Weak}


4

TheThe Energy FrontieR Energy FrontieRIt takes higher and higher energy to probe smaller and smaller distance scalesHigher energy = Earlier time

We’re looking back in time to the early universe

Accelerators have now gotten us to about 10-12

seconds, or equivalently, close to the TeV energy scale

This is the energy frontier!


5

What might lie beyond the frontier?Where does mass come from?

Is the SM just a low-energy effective theory masking higher more fundamental symmetries?

Where does gravity fit in?

TheThe Energy FrontieR Energy FrontieR

What might lie beyond the frontier?


6

how Do we get how Do we get TherE?TherE?


7

the the Fermilab TevatroNFermilab TevatroNThe world’s most powerful particle accelerator

…until LHC at CERN, 14 TeV, 2008

Main Injector(new)

Tevatron

DØCDF

Chicago

p source

Booster

Run I1.8 TeV

1992-1996

Run II1.96 TeV2001-?

ppLinac


8

AA Generic detectoR Generic detectoRHow do we “see” the events?

Observe, identify, and measure the decay particles

Charged particle trajectories,vertices

Charged particle momentum

e, energy

p, n, ,etc. energy

identification

Infer missing energy ()<


9

AA Specific detectoR: Specific detectoR: DDØØ

Silicon Microstrip Tracker900 silicon devices, 1M channelsCentral Fiber Tracker

16 layers, 80k channels, Calorimeter, 50k ChannelsLiquid argon calorimeterwith uranium absorber

Muon Scintillators

Muon Chambers

Borg DØBorg DØ

separated at birth?separated at birth?


10

thethe D DØ collectivEØ collectivE

A collaboration of >650 physicists from more than 80 institutions in 19 countries

CollaboratioNCollaboratioN


11

24/7 Data collectioN24/7 Data collectioN

Proton-antiprotons collide at 7MHz or seven million times per second

Tiered electronics pick successively more interesting events

Level 1 2 kHz

Level 2 1 kHz

About 100 crates of electronics readout the detectors and send data to a Level 3 farm of 100+ CPUs that reconstruct the data

Level 3: 50 events or 12.5 Mbytes of data to tape per second

Per year: 500 million events


12

A A Common EvenTCommon EvenTA sample Ze+e- event

Collect many such events and measure the Z mass from the distribution of calculated masses for each event

Very important calibration sample


13

Something Something a littlea little

more more interestinG?interestinG?The top quark - it’s HUGE!


14

Why Study the Top?Why Study the Top?The Standard Model (SM) predicts all top properties given mt

In the SM, the top and W mass constrain the mass of the Higgs Boson via EW radiative corrections

In 1964, Peter Higgs postulated a physics mechanism which gives all particles their mass.

This mechanism is a field which permeates the universe and is mediated by a particle called the Higgs boson.

22 / WtW mmm

W Wt

W WH

)/ln( WHW mmm


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Why Study the Top?Why Study the Top?The Standard Model (SM) predicts all top properties given mt

In the SM, the top and W mass constrain the mass of the Higgs Boson via EW radiative corrections Since the top is so heavy, it should have particularly strong coupling to the Higgs

Likely to play a significant role in EW Symmetry Breaking (mW, mZ)

Top decay time (τ ~ 410-25 s) < hadronization time

Opportunity to study bare quarks

In general, quarks do not exist as free particles. qq pairs are pulled from the vacuum to produce stable hadrons with 2 quarks (mesons) or 3 quarks (baryons)

This process is called hadronization

Since the top quark decays before that happens, we can study quark properties such as spin polarization here and no where else


16

Run II, the Top Quark & Run II, the Top Quark & DDØØ

Most all that we knew about the top quark before Run II came from the DØ and CDF Run I data

1992-1996: ~100 events / experimentTop mass and production cross section at s = 1.8 TeV

mt = 174.3 ± 5.1 GeV (DØ and CDF combined)

σtt = 5.7 ± 1.6 pb (DØ); σtt = 6.5+1.7-1.4 pb (CDF)

At this point in Run II we have 40 times more data, with a few thousand top quark events. Should yield:

mt ±3 GeV, along with mW ±40 MeVConstrain the Higgs mass: mH 40%mH

At s = 1.96 TeV, σtt = 10%σtt Standard Model: σtt = 8.8 pbLarger than SM: new physics (resonance production, anomalous couplings)Smaller than SM: non-SM decays, e.g. tHb


17

Production: Cross SectionsProduction: Cross Sections

Top events are very rareOne collision out of every 3109 produced a tt pair

To study processes with small cross sections, one needs

High luminosity to produce enough interesting eventsMethods for distinguishing those events from more copious backgrounds


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Top Physics at DTop Physics at DØØ: : DecaysDecays

BR(tWb) ~ 100%Significant deviation would indicate new physics

Top decays before hadronization

Can study polarization of a free quark

21%

15%

15%1.3%2.6%1.3%

44%

τ+X+jetse+jetse+ee++all hadronic

Jets: showers of particles produced from quarks

bbbbb

qqqql-l-W-

t

bbbbb

qql+qql+W+

t

lepton+jets{

dilepton {all jets


19

Decay: What Does Decay: What Does a Top Event Look Like?a Top Event Look Like?

BR(tWb) ~ 100%W qq or W lSo top events should have either

6 jets from quarks4 jets, 1 charged lepton, 1 4 jets, 1 charged lepton, 1 neutrinoneutrino2 jets, 2 charged leptons, 2 neutrinos

2 jets must come from b quarks

These jets can be distinguished from light quark jets

Powerful background rejection tool


20

b-taggingb-tagging

Lifetime1.5 ps c 0.5 mm

Displaced secondary vertex

Requires precise tracking near the primary collision point: silicon trackers, new for DØ in Run II!

Flight Length ~ few mm

Collision

Impact Parameter

Decay Vertex

B Decay Products


21

pt 48 GeVMEt 55 GeVW pt 51 GeVJetEt 154 GeVApla 0.160Ht 517 GeV

Run II: Run II: +jets +jets CandidateCandidate

2 jets are tagged as b -jets2 jets are tagged as b -jets

passes topological selectionpasses topological selection

ttttjjjjjjjjcandidatecandidate


22

Run II: Run II: +jets +jets CandidateCandidate

2 jets tagged with displaced secondary vertices

Track in the calorimeterand muon tracker


23


What we’ve measured at this point: top mass


24


What we’ve measured at this point: cross section


25

is thereis there Something Something a bita bit more exotiC?more exotiC?

At the frontier will we find…At the frontier will we find…The origin of mass?The origin of mass? The higgß The higgß

ParticlEParticlE

Higher symmetry?Higher symmetry? SupersymmetrY SupersymmetrY - SUSY- SUSY

A place for gravity?A place for gravity? ExtrA ExtrA

dimensionßdimensionß


26

the the HiggßHiggß

Peter Higgs showed that Electroweak unification implied the existence of a molasses-like field (the Higgs field), which gives particles their mass.

Predicts at least one Higgs particle – not yet discovered.The strength of a particle’s interaction with the Higgs particle determines its mass.

e

t

Higgs Field

HH


27

Past Higgß searches and Past Higgß searches and current limitS current limitS

Over the last decade or so, experiments at LEP or the European e+e– collider have been searching for the Higgs.

Direct searches for Higgs production, similar to our Z mass measurement exclude mH < 114 GeV.

Precision measurements of electroweak parameters combined with DØ’s new* Run I top quark mass measurement, favor mH = 117 GeV with an upper limit of mH = 251 GeV.

* Nature 10 June 2004


28

Search for Search for hwhw productioN productioN

One very striking and distinctive signature

Look for an electron =

track + EM calorimeter energy

neutrino = missing transverse energy

two b quarks = two jets each with a

secondary vertex from the long lived quarks.

p

pW*

qq’

H

We

b

b


29

a Higgß candidatEa Higgß candidatETwo events found, consistent with Standard Model Wbb production

12.5 pb upper cross section limit for ppWH where Hbb mH=115 GeV.

By the end of Run II and combining all channels, we should have sensitivity to ~130 GeV.


30

SM is “ugly” – it requires too much fine tuning of too many free parameters. May only be an effective theory valid to some energy scale beyond which higher symmetries may manifest as new particles and forces.Supersymmetry, or SUSY is the leading candidate.

Every particle and force carrier has a massive super-partner (squark, gluino, etc.)

Theoretically attractive:SUSY closely approximates SM at low energiesAllows unification of forces at much higher energiesProvides a path to incorporate gravity and string theory: Local Supersymmetry = SupergravityLightest stable particle cosmic dark matter candidate

Masses expected to be 100 GeV - 1 TeV

SupersymmetrYSupersymmetrY

??


31

the the Golden tri-leptoN Golden tri-leptoN SUSY SUSY

SignaturÊSignaturÊIn one popular model the charged and neutral partners of the gauge and Higgs bosons, the charginos and neutralinos, are produced in pairsDecay into fermions and the Lightest Supersymmetric Particle (LSP), a candidate for dark matter.

The signature is particularly striking:Three leptons = track + EM calorimeter energy or tracks + muon tracks (could be eee, ee, e, , ee , etc… ).neutrino= missing transverse energy


32

Tri-lepton search ResultßTri-lepton search Resultß

In four tri-lepton channels three events total found.

Consistent with Standard Model expectation of 2.9 events.

Here is a like-sign muon candidate


33

Extra dimensionSExtra dimensionS

Gravity (in particular, its weakness) does not fit into the SM. It can be accommodated in a higher dimensional world (t + 3D + “n” extra spatial dim.).

Only gravity can propagate in the extra dimensions which are hidden to us because we are confined to the 4-D surface, or brane.

Gravity would therefore be as strong as the other forces, but since it spreads its influence in the other dimensions, it appears weakened or diluted from our viewpoint.

4-D Universe4-D Universe

extra extra dimensionsdimensions

q

graviton

graviton

q


34

Extra dimensionSExtra dimensionS

Gravitons could be produced in collisions, leave our 3-D world, go into the other dimensions… …come back and decay into two photons or electrons that appear to come from nowhere.

An unexpectedly high rate of such pairs at high energies would signal the presence of extra dimensions.

p

pG

qq’

e

e


35

High mass candidate High mass candidate eventßeventß

ee pair pair


36

Extra dimension Extra dimension limitSlimitS

Di-electrogmagnetic objects are collected and the mass calculated (just as in our Z plot a few minutes ago).

The observed mass spectrum is compared to a linear combination of

SM signals

Instrumental backgrounds

Extra Dimension Signals

No evidence is found for hidden dimensions, @ 95%CL

n = 2, 170 m

n = 3, 1.5 nm

n = 4, 5.7 pm

n = 5, 0.2 pm

n = 6, 21 fm

n = 7, 4.2 fm

Note the long mass tail


37

We’ve just begun We’ve just begun toto explorEexplorE

All of these results represent at most a quarter of our data

Documents

Ênergy frontieR: the There and back Again or Breese quinN University M ississippi of