©2004 richard e. hughes fermilab; p.1 studying the fundamental particles particle physicists see...

Fermilab; p.1©2004 Richard E. Hughes

Studying the Fundamental Particles Particle physicists see the world as made up of a small

number of fundamental particles: QUARKS: up, down, charm, strange, top, and bottom LEPTONS: electron, electron-neutrino, muon, muon-neutrino,

tau, tau-neutrino Force carrying particles: photon, W/Z boson, gluon, graviton Special “mass generating particle”: Higgs

Special Features Only the up and down quarks, and the electron, are in the

matter around us The masses of the particles vary wildly: the up, down and

electron are much less massive than a hydrogen atom, while the top quark is more massive than a gold atom!

The Higgs particle – which we think can help explain these masses of the particles – is predicted by our theory, but has not been observed (yet!)


The Fundamental Particles


Studying the Fundamental Particles

Some reasonable questions to ask: Are there any other “fundamental particles”? Why are the masses of these fundamental particles so different?

Why is the top quark so massive? Does the Higgs particle really exist?

To answer these questions we need: To be able to make at least some of these fundamental particles To be able to study them in great detail Since particles like the top quark are very massive, we will need

a lot of energy to do this (Remember, E=mc2)

One way to do this: Use “Particle Accelerators” and “colliders” to get the necessary

high energy to make interesting particles Use “Particle Detectors” to take “photographs” of these newly

created particles


Particle Accelerators Accelerators are machines used to speed up particles to very

high energies. This way, we achieve two things: We decrease the particle’s wavelength, so we can use it to probe

inside atoms, nuclei, even quarks. We increase its energy, and since E = mc2, we can use that energy to

create new, massive particles that we can study.

Tevatron Accelerator at FermilabTevatron Accelerator at Fermilab


FNAL: Fermi National Accelerator Laboratory

Fermilab is located in Batavia, Illinois (about an hour west of Chicago).

Fermilab is home to the Tevatron, the world’s highest-energy particle accelerator.

Fermilab is also a park, with 1,100 acres of prairie-restoration land!

Danger of working too hard at physics!


The Accelerator Complex

Linac

Cockcroft-Walton

Booster

Anti-protons

Tevatron


Practical Facts

FNAL -- about 2500 employees work there on the payroll.

On any given day, there are probably another 1500-2000 users on location

FNAL budget is about $320 million dollars per year

Power cost

Is this worth it? Currently, the US Gov’t spends less than 0.5% of its total GNP

on (ALL) knowledge based scientific research.


Beam Facts How many particles in the beam?

10^14 protons 10^14 antiprotons Grouped in 36 packets

Total collision energy of 1.8TeV Each packet has the energy of a

car! But the beam has the width of a

human hair! Protons and antiprotons are

circulated in opposite directions about a four-mile-long tunnel. The beams are focused and steered by over a thousand superconducting magnets

When running Fermilab uses 60MW of electricity (about what is used by a small city in the summertime)

How fast are they moving? Really fast! 0.999999c


Collisions are important events! After particles have been accelerated, they collide either with a target

(fixed target experiments) or with each other (colliding beam experiments).

These collisions are called events. New particles are created in such a collision. Most of them quickly decay,

but we can look at their decay products using detectors.

More energy in initial state to make new particles


Our detectors are HUGE!

ALEPHALEPH detector at CERN

CDFCDF detector at FNAL

A lot of HEP detectors are as big asa house -- several stories high!A lot of HEP detectors are as big asa house -- several stories high!


The CDFII Collaboration

700+ scientists

55+ institutions

11+ countriesStudentsPostdoc’s

ProfessorsResearch Scientists


A brief history of CDF

1985: First collisions with partial detector

1987: Core detector in place. Jet physics

1988/9: “Run 0” – we got 4x the expected data see lots of W/Z’s

1992-1995: “Run I” – add silicon detector. Discover the top quark

2001-?: Run II era begins with essentially a new detector, higher collision energy, and more data. We want to discover what hasn’t even been thought of.


CDF detector roll-in Feb 2001Detector weight: 5000 tons. Don’t drop on your toe!

“Channels”: Approximately 2 million.

Cost of detector: About $400 million (materials and construction only,no salaries).


Trying to find a top quark!

What happens when we collide a proton and an anti-proton? Some jargon: a collision is called an “Event” If they hit nearly head on, then the energy in this collision can

turn into new particles.

What kinds of particles are created? Many different kinds are possible, as long as the total mass is

less than what you would get from E=mc2! Aside from this, exactly which kinds of particles are created is

random, although some particles are more likely than others to be created.

New particles

p p

New particles



It is possible that a given proton-antiproton collision could make a pair of top quarks (actually one top and one antitop)

But this is very rare: only 1 in every 10 billion collisions! Luckily, we have ~2 million collisions/second

(Doing the math: pair of top quarks made every 1½ hours!)

Let’s imagine this happens. How do we know we have a top quark in this “event”?

t

p t p


Pattern Recognition It turns out that when top

quarks are created, they don’t live very long….only about 1 yoctosecond….

Each top quark decays into two other particles: a b quark (or anti-quark) and one of the force carrier particles: the W boson.

Both the W boson and the b quarks also decay The b’s decay into a spray of

particles called a “jet” The W’s decay in two ways:

Sometimes 2 “jets” Sometimes a lepton (electron,

muon, tau) and a neutrino

jetjet



jet

jet

t

p p

b (jet)

W+

W-

b (jet)

electron

t

So here is what we can look forEvents in which one W decayed to a lepton and neutrino, while the other W decayed to two jetsIncluding the two b quarks, we want events which have a lepton, a neutrino, and 4 “jets”


What does an event look like?



A Candidate top-antitop event

Jet 1

Jet 3

Jet 2

Jet 4

electron

neutrino


How do we identify top-antitop events?

Top pair events have an “M.O”: for example: an electron, a neutrino, and 4 “jets”

How hard is it to find them? BACKGROUND: Things that share the above “M.O.” but are not

top events There are about as many “BACKGROUND” events expected as

top events

How do we tell the difference? We use Advanced Analysis Techniques Examples:

Genetic algorithmsNeural Networks…..


Artificial Neural Networks


Constructing an Artificial Neural Network


What does the data look like?

Mostly top quarks up here, about 90 total events

Mostly background events down here, about 430 total events


A more enriched sample?

Remember that every top-antitop event has two b quarks

Background events tend to NOT have b quarks Is it possible to identify events in which there are 2 b

jets? YES! Use a device called a silicon vertex detector (SVX)

q, l-

q’,

t

p p

bW+

W-

b

q, l+

q’,

t


The SVX

About 1 million channels of info Extremely precise

mesaurements Precision of ~40 microns (width of

human hair)

Excellent b-quark “tagger”


Hey, that looks just like….top!

Require top “M.O”: an electron, a neutrino, and 4 “jets” But additionally require that at least 1 of the 4 jets be

identifed as a b-quark by the SVX What does the neural net say for these events?

They are almost ALL top quarks!


What Now? Now we have the world’s largest (only) collection of top quarks.

And we are continually adding to the collection. What can we learn about this quark?

Since the top quark is so massive, maybe it can tell us about mass itself. Theorist Chris Hill of Fermilab claims that an understanding of the

origin of mass would rank as "an achievement on a par with the greatest scientific strides in history, like Newton's establishing the universal law of gravitation or Einstein's connection of energy to mass and the speed of light."

©2004 richard e. hughes fermilab; p.1 studying the fundamental particles particle physicists see...

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