exsperiment – hands on cern
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Exsperiment – Hands on CERN. Maiken Pedersen, Farid Ould-Saada, Eirik Gramstad University of Oslo. 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 - PowerPoint PPT PresentationTRANSCRIPT
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Maiken Pedersen, Farid Ould-Saada, Eirik GramstadUniversity of Oslo
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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?
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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
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e+
e-
Z
µ-
µ+
Z
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W
ν νW
eW e W
e
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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
_
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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
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uu u uZ
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Vi ønsker å studere de energirike kollisjonene
Fjerner derfor det vi ikke er interessert i
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Together, try to identify:
Z e+e-
W +-
Jet events – these are not interesting for us just now, and are referred to as background
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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?
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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
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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
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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
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The goal is to find the right decay particles whose measured quantities will enter the calculation
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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!
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When we redo the calculation, but this time only with only with highly energetic particles …
After hundreds of measurements
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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
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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
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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?