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 students at Nevis

May, 2005

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

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

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

Statistical Interpretations

Statistical techniques are used to

make measurements

look for any anomalies

rule out specific alternate theories

So far, we see excellent agreement with the standard model... within the errors we are able to measure, and predict, the events we see

bb MC 8840 events
->3763 events pred.

DØ Run II
~50pb-1

Z boson decaying to bottom quarks

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 > 14.4 ps-1 95%CL (world comb.)

Only at hadron colliders

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)

Difficult measurement! It’s doable, but will take time!

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

W (width)

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

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!

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

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

Top cross-section and mass : lepton +jets channel

ZH (nu nu b b) Search

WH (e 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, and 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!

D0 experiment, studying wines in France!

Andrew Haas

2003-11-03T14:52:15

Andy Haas