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Outline. Searching for charm with ZEUS and HERA: Tales of a classical physicist or physical classicist. Philip Allfrey Somerville College. Outline. What is a charm quark?. What are HERA and ZEUS?. What have I measured?. Motivation.

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slide1

Outline

Searching for charm with ZEUS and HERA:Tales of a classical physicist or physical classicist

Philip AllfreySomerville College

slide2

Outline

What is a charm quark?

What are HERA and ZEUS?

What have I measured?

MCR/SCR Symposium, 25 January 2007

slide3

Motivation

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

  • Four fundamental forces (EM, strong, weak, gravity)
  • Which are mediated by the exchange of particles (g, 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

 Lucretius, De Rerum Natura, Quote 1

MCR/SCR Symposium, 25 January 2007

slide4

What is a quark?

“Three quarks for Muster Mark”

-James Joyce, Finnegan’s Wake

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

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

 Lucretius, Quote 2

Proton

Quark

MCR/SCR Symposium, 25 January 2007

slide5

top

up

charm

bottom

down

strange

What is a charm quark?

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

Charm and top are heavier copies of up

Strange and bottom are heavier copies of down

Different combinations of quarks result in different baryons/mesons

(Baryons consist of three quarks, mesons of two)

 Lucretius, Quote 3

Quarks can be produced in interactions between subatomic particles

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

 Accelerate particles, collide them and see what comes out

MCR/SCR Symposium, 25 January 2007

slide6

What happens when particles collide?

D0

p

D+

K

Lc

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

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

3) Eventually end up with lots of baryons and mesons

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

MCR/SCR Symposium, 25 January 2007

slide7

The HERA Accelerator

920 GeV Protons

ZEUS

27.5 GeV Electrons

MCR/SCR Symposium, 25 January 2007

slide8

The HERA Tunnel

MCR/SCR Symposium, 25 January 2007

slide9

The ZEUS Detector

MCR/SCR Symposium, 25 January 2007

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The ZEUS Detector

MCR/SCR Symposium, 25 January 2007

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What do we look for?

Baryons and mesons decay within fractions of a second

p

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

p

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

Lc→L0p→ (pp)p

L0

 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

Lc

p

MCR/SCR Symposium, 25 January 2007

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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

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

Signal

Background

 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

MCR/SCR Symposium, 25 January 2007

slide13

What did I do?

Normal selection cuts did not result in a signal

Signal

Background

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

Should give zero for fake candidate, one for true particle

Using this to discriminate between true and false candidates worked!

MCR/SCR Symposium, 25 January 2007

slide14

What results did I get?

D+

D0

Lc

Ds

MCR/SCR Symposium, 25 January 2007

slide15

What can I do with this?

  • Calculate the probability for the initial reaction to occur (‘cross-section’) from the number of events in the peak
  • Could also look at how cross-section varies with quantities like charm quark momentum
  • 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)
  • Can feed into description of the proton structure
  • For Lc→L0pdecay 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

slide16

Summary

What is a charm quark?

One type of the fundamental particles which make up matter

What are HERA and ZEUS?

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

What have I measured?

Decays of four different charmed hadrons

 Lucretius, Quote 4

MCR/SCR Symposium, 25 January 2007

slide17

String Theory

MCR/SCR Symposium, 25 January 2007