exsperiment – hands on cern

Maiken Pedersen, Farid Ould-Saada, Eirik GramstadUniversity of Oslo

1. Get to know Hypatia event display: a visualisation program for particle collisions• Identify particles in a

detector• Identify particles

originating from a• Compete to find as

many correct answers as possible? *

2. Work with real data from LHC• Identify decays of

the Z boson• Find the mass of the

Z boson

*: Kan vi svare på dette? Hva med for ekte data?

We will identify Z boson W boson Higgs boson

Remember that these decay immediately

Why?

To identify these, remember how they decay and how this looks in a detector

e+

e-

Z

µ-

µ+

Z

W

ν νW

eW e W

e

Higgs boson is neutral and, if heavy enough, can decay into two Z particles, which can further decay in a lepton pair

H

Z

Z

e

_e

_

We said that 2 protons collide and produce particles such as W, Z, H, which further decay to pairs of leptons• You have heard that it is the quarks in the proton

that make these particles in so-called hard collisions

• The “rest” of the proton is responsible for additional activity in form of jets of hadrons

• W, Z and H can also lead to other more complicated combinations of particles

uu u uZ

Vi ønsker å studere de energirike kollisjonene

Fjerner derfor det vi ikke er interessert i

Together, try to identify:

Z e+e-

W +-

Jet events – these are not interesting for us just now, and are referred to as background

1) On CD, go to folder “Exercise 1”1) For Windows machines: click on Hypatia

icon2) For Linux machines: open terminal and

typejava –jar hypatia.jar

2) A catalogue with collision events will open automatically

3) Identify collisions and write result on the form given to you

4) Did you manage to find a Higgs?

You are among the first students to analyse, outside CERN, real data fro the ATLAS detector!

Usually, these data are only available for ATLAS researchers, but we have the permission to share with you a sample in form of collision event display

From these picture you will be able to infer the Z boson mass

You will calculate the mass from the decay products of the Z with the help of: Conservation laws Mass - Energy relation Simple geometry

You will then collect the result from the mass calculation for each collision in a histogram

Energy of a particle at rest

1 2, , Energifør etter etterE E E E E E

1 2

, bevegelsesmengdefør etter

etter

p p pp p p

Energy of a moving particle

2 , masse lysfartenE mc m c

2 2 2( ) ( )E mc pc

Energy and momentum are conserved

After some gymnastics and application of conservation laws we arrive to Z mass formula

The following quantities enter this formula

Mass Energy Momentum

of the 2 decay products

2 2 2 2 2 21 2 1 2 1 2( ) ( ) 2 2zm c m c m c E E p p c

The goal is to find the right decay particles whose measured quantities will enter the calculation

We cambine pairs of particles and calculate the invariant mass

Each result goes into a histogram

Each time we calculate a given mass, say 100 for example, we add it to the histogram

After hundreds of measurements

BUT: Here is no indication that a particle of certain mass has decayed into the 2 particles we combined …

We must clean up!

When we redo the calculation, but this time only with only with highly energetic particles …

After hundreds of measurements

1) Open folder exercise 2 on the CD2) Open the calculation form you find

here3) Open the file assigned to your group

1) Here you find the picture of real collision events where a Z boson has probably been produced and decayed into a pair of electrons or muons

2) Up to you to decide whether it corresponds to a decay to e+e- or +- in which case you consider the particles for the invariant mass calculation

4) Once done, we proceed with combing all your results

The wrold-average value of the Z boson mass is measured to be 91.1876 +- 0.0021 GeV• What does this mean?

Why is the distribution so wide? • Particle´s natural width• Measurement uncertainty• Uncertainty relation

Key to new discoveries goes through tools and methods we already know they work

Wish to understand the weak force Don't we understand it already? At very high energies, we think that all forces are

unified into one original force One of our goals is to look for some indication of

such a unification

If a new “weak” force reveals itself at high energies we are exploring, a new heavy Z boson could be the carrier of the new force

The invariant mass of the decay products would feature a similar distribution as for the Z but at muhc higher values

Z´ – have you found it?