Outline

Searching for charm with ZEUS and HERA:

Tales of a classical physicist or physical classicist

Philip AllfreySomerville College

MCR/SCR Symposium, 25 January 2007 2 / 16

Outline

What are HERA and ZEUS?

What is a charm quark?

What have I measured?

MCR/SCR Symposium, 25 January 2007 3 / 16

Motivation

Particle physics investigates what the Universe is made up of and how it works at the most fundamental level

Lucretius, De Rerum Natura, Quote 1

•Four fundamental forces (EM, strong, weak, gravity)

•Which are mediated by the exchange of particles (, W, Z, g, g)

•Between matter particles (quarks, leptons)

Not a new discipline, dates back to the Greek and Roman atomic philosophers, Democritus, Leucippus, Epicurus and Lucretius

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What is a quark?

Baryons (e.g. protons) consist of three quarks, held together by the exchange of gluons

“Three quarks for Muster Mark”

Proton

Quark

-James Joyce, Finnegan’s Wake

The force between the quarks gets stronger with distance, so they are permanently confined inside the proton

Lucretius, Quote 2

MCR/SCR Symposium, 25 January 2007 5 / 16

What is a charm quark?

Lucretius, Quote 3

There are six flavours of quark,only up and down occur in everyday matter

up charm top

down strange bottom

Charm and top are heavier copies of up

Strange and bottom are heavier copies of down

(Baryons consist of three quarks, mesons of two)

Different combinations of quarks result in different baryons/mesons

Energy equal to the mass of the quark is required to produce it (E=mc2)

Quarks can be produced in interactions between subatomic particles

Accelerate particles, collide them and see what comes out

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What happens when particles collide?

2) Quarks move apart, creating more quarks because of the nature of the strong force

1) Initial particles interact and produce (eg) a quark-antiquark pair

3) Eventually end up with lots of baryons and mesons

D0

D+

K

c

Experimentally, have to reconstruct (1) by measuring (3)

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The HERA Accelerator

ZEUS

920 GeV Protons

27.5 GeV Electrons

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The HERA Tunnel

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The ZEUS Detector

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The ZEUS Detector

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What do we look for?

We want to find a given particle in a particular decay mode, e.g.

c→ 0 → (p)

Take all combinations of the right number of tracks, add their masses together, and see if they have the mass of the particle we are looking for

However, the lightest ones live long enough to leave tracks in the detector

c

0

p

Baryons and mesons decay within fractions of a second

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How do we improve our chances?

First, reduce the number of events we’re looking at

10 million collisions per second, most of them are uninteresting.

Use a real-time selection mechanism to accept/reject events before writing them to tape. Reduces rate to approx 5 per second

Eliminate tracks which don’t match the physical properties of the decay

E.g. excluding all tracks to the left of the dotted line significantly improves the signal

Second, reduce the number of tracks we’re looking at

Signal

Background

MCR/SCR Symposium, 25 January 2007 13 / 16

What did I do?

Normal selection cuts did not result in a signal

Should give zero for fake candidate, one for true particle

Instead of applying cuts to several variables individually, combine them into one ‘super-variable’

Using this to discriminate between true and false candidates worked!

Signal

Background

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What results did I get?

D0 D+

Dsc

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What can I do with this?

• Could also look at how cross-section varies with quantities like charm quark momentum

• Can feed into description of the proton structure

• Compare these to theoretical predictions

(NB this result is just for one decay at one energy, theory claim to describe all decays at all energies)

• Calculate the probability for the initial reaction to occur (‘cross-section’) from the number of events in the peak

• For c→ 0 decay can measure the asymmetry parameter, only about 4 previous measurements, so can affect the world average, could also investigate its dependence on electron polarisation

MCR/SCR Symposium, 25 January 2007 16 / 16

Summary

What are HERA and ZEUS?

What is a charm quark?

What have I measured?

One type of the fundamental particles which make up matter

An acclerator and detector with which quarks and other things can be studied

Decays of four different charmed hadrons

Lucretius, Quote 4

MCR/SCR Symposium, 25 January 2007 17 / 16

String Theory