Exsperiment – Hands on CERN

1. Exsperiment – Hands on CERN Maiken Pedersen, Farid Ould-Saada, Eirik Gramstad University of Oslo

2. Du skal: • 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?

3. Exercise 1

4. Exercise 1 – Identify particles originating from a collision • 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

5. µ- Z µ+ e- Z e+ Z boson – neutral mediator of the weak force

6. W ν W ν W boson – electrically charged nediator of the weak

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

8. Simplified picture – what really happens? • 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

9. Z u u

10. Vi ønsker å studere de energirike kollisjonene • Fjerner derfor det vi ikke er interessert i

15. Initiation to Hypatia • Together, try to identify: • Z  e+e- • W  m+m- • Jet events – these are not interesting for us just now, and are referred to as background

16. Let´s go! On CD, go to folder “Exercise 1” • For Windows machines: click on Hypatia icon • For Linux machines: open terminal and type java –jar hypatia.jar A catalogue with collision events will open automatically Identify collisions and write result on the form given to you Did you manage to find a Higgs?

17. Exercise 2

18. Calculate the mass of the Z boson • 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

19. Formlulae and other help material • Energy and momentum are conserved • Energy of a particle at rest • Energy of a moving particle

20. Theoretical formula of the Z boson mass, or invariant mass • 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

21. Challenges with the invariant mass The goal is to find the right decay particles whose measured quantities will enter the calculation

22. 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!

23. When we redo the calculation, but this time only with only with highly energetic particles … After hundreds of measurements

24. Demonstration of a Z event and introduction to the mass calculation and plotting

25. Ready? Go! Open folder exercise 2 on the CD Open the calculation form you find here Open the file assigned to your group • 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 • Up to you to decide whether it corresponds to a decay to e+e- or m+m- in which case you consider the particles for the invariant mass calculation 4) Once done, we proceed with combing all your results

26. Combination and discussion of results

27. Why does the distribution expand around the most robabøe value 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

28. Why do we spend all that time to measure invariant masses? • 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?