Physics at the High-Energy Frontier:
...an introduction to the physics at D0

Dr. Andy Haas

Post-doc Researcher

Columbia / Nevis

D0 / ATLAS

Talk to Summer REU's

May 30, 2006

The Standard Model

The long quest to understand:
“What is this stuff?”
“What is it made of?”
“What holds it together?”
“What makes it move?”

We've come a long way... there are:

6 quarks

6 leptons

3 forces

and gravity

Atoms

Nuclei

Quarks

Quarks, Leptons, and Forces

Big Questions

Why 3 generations of particles?

Why is there matter everywhere, and no anti-matter?

Where do the particles get their mass?

What is all this Dark Matter?

Big Answers ?

Higgs Bosons?

Grand Unification?

Supersymmetry?

Strings?

What we study

Measure the properties and interactions of the particles we know about (with greater accuracy)

quarks and the strong interaction (QCD)

W and Z bosons

bottom quarks and hadrons

top quarks

Search for new particles we think should exist

Higgs bosons

Supersymmetric particles

Search for any new physics (particles and/or interactions) beyond the standard model

Fermilab Tevatron Accelerator – Run II

1 km radius p-pbar super-syncrotron

World's most powerful: 1.96 TeV

36 bunches of p and pbar

396 ns crossing period

25 cm long interaction region

One store every day or so...

DØ

Tevatron Performance

Run II began 5 years ago (2001)

Instantaneous luminosity has steadily increased

More than 1 fb-1 delivered so far

Should collect 4-8 fb-1 in the next few years

The DØ Detector for Run II

Tracking Systems

Calorimeters

Showers are induced by sheets of depleted Ur

Cells measure the ionization in liquid Ar gaps

Cells are arranged in a projective geometry

Highly granular (.1x.1)(-)

Jets are almost completely contained: >20 hadronic interaction lengths

2.3mm

Muon System

Used in this analysis for:

b-tagging studies...

jet energy corrections

A big toroidal Iron magnet:
~2000 Ton and 1.9 Tesla

Only muons with |p|>3 GeV penetrate the magnet

Very low background thanks to heavy shielding

3 drift-tube layers and scintillating panels determine the position and timing of muons

Coverage out to |eta|<2.0

A-layers and scintillators create the best muon trigger ever built

Trigger System

Level 1

Calorimeter EM and TOT ET (.2x.2)(-)

Muon stubs

Central high-pT axial tracks

Level 2

Clustered calorimeter EM and TOT ET

Missing transverse calorimeter ET

Matched muon segments

Displaced high-pT axial tracks

Level 3 trigger algorithms:

Jets (no split/merge)

Central stereo tracks

Muons matched to central stereo tracks

Primary vertex

Jet lifetime b-tags

Level 1
2.5 MHz

Level 2

1.6 kHz

Level 3

1 kHz

Tape

50 Hz

Track Reconstruction

Reconstruct detector energy into clusters

Pattern recognition combines clusters into tracks

Efficiency for pT>1GeV is about 95% in the central region, 85% for the forward

Path resolution is ~20 microns

An axial view of a 3-jet event in data,

with reconstructed clusters and tracks

IP res. (m)

Primary Vertex Reconstruction

Common locations of tracks along the beam-line are clustered into “primary vertices”

There can be more than one interaction in a crossing

Very unlikely to have more than one interesting event in a single crossing

About 0.4 interactions per crossing on average for this data set - depends on instantaneous luminosity

The interesting vertex is selected based on the momenta of the vertices' tracks

This “hard-scatter” primary vertex is used to calculate jet ET

Jet Reconstruction

The ET in (0.1x0.1)(-) towers is summed, and seeds are found

Energy is clustered in cones of ∆R=sqrt(∆2+∆2)=0.5 around the seeds

The midpoints (in -) between stable cones are also used as seeds

The unique cones with
ET>8 GeV are merged or split, depending on whether they share more or less than 50% of a jet's ET

Jets' ET are calibrated, using the jet energy scale (JES)

accounts for out-of-cone showering and the underlying event, on average

EM

Hadronic

MET

b-Jet Identification

B

IP

Identify jets containing bottom hadrons

main weapon again the huge QCD background

b-hadrons travel ~3 mm before decaying

Their mass leads to an opening angle between tracks --- large IP significances

Additional large IP significance tracks may arise from daughter decays (charms, tau)

High-IP-significance tracks are used to find the secondary vertex(es)

Jets with a secondary vertex in ∆R<0.5 are “tagged” as b-jets

Backgrounds consist of:

mis-tags: poorly reconstructed tracks which randomly happen to form a vertex

long-lived strange / charm decays
or photon to e+e- conversions

“gluon splitting” into nearly collinear charm or bottom pairs

3 mm

Evidence for displaced vertex

Lifetime b-tagging

- Tracks with large IPs
- Secondary vertices

Run II preliminary!

neg tag

pos tag

DØ Run II preliminary

B

B

Secondary Vertices in Jets

Simulation

To compare theory to data, the events from theory must be simulated

Advanced methods used to simulate

particle creation (Monte Carlo)

particle interactions with matter and the detector elements
(GEANT)

electronic responses of the detector

Ongoing analyses

QCD

Inclusive jet cross section, dijet mass and angular distributions, diffraction, …

Heavy flavor

Resonance reconstructions, masses, lifetimes, branching fractions, rare decays, Bs mixing, …

Electroweak

W/Z cross sections, dibosons and anomalous couplings, charge and rapidity asymmetry, …

Top Quark

Top quark pair production cross section measurements, top quark mass and decay properties, search for single top quark production, …

Higgs

Searches within the standard model, searches in supersymmetic models, and other searches

New phenomena searches

Higgs bosons, supersymmetry, leptoquarks, large extra dimensions, Z’ ,…

QCD

In some ways, the simplest measurement at the Tevatron, but in practice, difficult.

Tests our basic understanding of the proton, and QCD

Measure the inclusive jet cross-section

vs. pT of the jet

3 bins of |eta|

Measure the di-jet invariant mass spectrum

Also compared to theory

Experimental errors are large still

Can also measure the di-jet angular correlation... how back-to-back are the two leading jets?

Tests NLO in perturbation theory

b - Physics

(u,c,t) quarks interact with (d,s,b) quarks with these probabilities:

(u,c,t)V(d,s,b) ...
where V is the CKM matrix

In the standard model, there are just four parameters...

... lamba, rho, A ... and eta... which is complex... CP violation!

We want to know, is this really true? Is the standard model right?

Or are there other sources of CP violation?

There are also hundreds of other properties of b-hadrons to measure!

Masses, lifetimes, branching ratios... which test QCD... very interesting
for “heavy” quarks

Bs mixing

∆ms/∆md







Bd fully mixes in about 4.1 lifetimes

∆md = 0.5020.006 ps-1 (world comb.)

Measured with great precision by Belle & BaBar

Bs fully mixes in <0.15 lifetimes!!

∆ms ~= 18 ps-1 (world comb.)

To measure Bs mixing, need:

Tag initial state flavor (what was produced, a B or a Bbar?)

Tag final state flavor (what decayed was a B or a Bbar?)

Yield: as much decays as possible (flavor tagging is imperfect)

Proper decay length: Lxy and =pT/mB (mix prob vs decay time)

D0 made the first measurement of this... this year!

Z (→ee)
also have mu, tau, and soon... b!

W (width)

Next step... measure the W mass... very very important parameter!

A Needle in a Haystack

2.5 Million collisions each second

~1000 top quarks made each year

A few Higgs bosons each year (perhaps!)

Impossible to record every event... 2 TB/s

Just keep the interesting events... trigger !

top quarks in action

In the SM: BR(tWb)~100%, classify topologies according to W decays from ttbar:

dilepton: 2 high pT leptons, 2 b-jets, large Etmis

Small BR, but cleaner signal and small systematics. No b-tagging

lepton+jets: 1 high pT lepton, 4 jets (2 b’s), large Etmis

Larger yield, larger bkgUse event topology, b-tagging (and SLT)

all jets: 6 jets (2 b’s)

Swamped by bkg, very challenging, but impossible at LHC! Use NN

Top decays

jet

jet

jet

b

b



p

p

E T

jet

b

b





p

p

E T

Top-quark Physics

~90% of (tt)

~10% of (tt)

~30% of (t)

~70% of (t)

Pair production through strong interaction

(tt) ~ 7.5 pb at 1.96TeV
Mt = 174.35.1 GeV

Measure cross-section more accurately

Measure the mass more accurately

Does it always decay the way the SM
predicts it should?

Does it have any weird interactions???

Single top production via EW interaction

(t) ~ 2.86 pb at 1.96TeV

Very difficult to observe because of

large backgrounds

Would be a nice measurement:

measure Vtb directly!

Top cross-section and mass : lepton +jets channel

ZH (nu nu b b) Search

More to the Story?

Hierarchy problem:

Why is mh << mpl ?

Hierarchy stability problem:

QFT predicts radiative corrections

How does mh stay << mpl ?

Can the gauge couplings unify?

Needed for Grand Unified Theories (GUT)

Ruled out in the SM !

Supersymmetry to the Rescue?

Boson and fermion loop integral contributions differ by a factor of (-1)

A fermion for every boson:
scalar field masses are stabilized

Supersymmetry is slightly broken

Supersymmetry demands an even number of Higgs doublets

A light higgs is predicted
(mh<mZ at tree-level)

With 2 Higgs doublets, couplings unify at 1016 GeV

This is the MSSM

Higgses in Supersymmetry

Higgs fields come in pairs
(Hu and Hd)

5 Higgs bosons : h, H, A, H+, H-

tan = Hu / Hd

Cross-sections for bbh grow
like tan2!

H

h

A

total

Higgs Mass (GeV)

tanB = 30

tan

A

Highly

degenerate
region

3 b-tagged jets

Set limits on bh→bbb

Search for Higgses in Supersymmetry
(bh→bbb)

squarks/gluinos
(jets + MET)

sbottoms : (jets + btagging + MET)

charginos / neutralinos (3 leptons + MET)

Searching for Supersymmetry...

Other New Phenomena... have an open mind!

Search for L.E.D.

Search for Z' boson

Many other searches!

Large Extra Dimensions

Heavy Z' gauge bosons

R-parity violating Supersymmetry

Gauge-mediated Supersymmetry breaking: di-photon excesses

Lepto-quarks

Technicolor

Massive stable particles

...

Summary

There's a huge amount of physics to study

It takes a lot of people

We have a lot of data on tape, more is coming in every day

D0 and CDF are the only places currently capable of making these measurements and searches for new phyiscs

We study the known particles and interactions

We look for new particles and interactions we expect to exist

We look for surprises!

Andrew Haas

2003-11-03T14:52:15