Identifying strange particles & determining their

properties in the ATLAS experiment

People

Particles in ATLAS

In a particle collision in ATLAS, a large number of particles are produced. Which are they, and how are they constructed?

Many are constructed of quarks and antiquarks

Quarks make up the Hadrons

• Baryons

- made up of 3 quarks

eg protons & neutrons

• Mesons

- made up of 2 quarks

eg pions & kaons

There are also leptons

……Which also have their antiparticles but no sub structure – they are elementary

Force carriers

• The forces that particles experience arise from exchange of force carriers

- photons for electromagnetic forces

- g gluons for the strong force between quarks

- W & Z for the weak force which explains things like decay & nuclear reactions in stars

What evidence do we have for this?

• Physicists have designed and carried out experiments with:- Cosmic rays– Particle accelerated in

particle laboratories

• Using more and more sophisticated particle detectors

• The bubble chamber has been a very useful detector to visualise particle collisions and particle decays.

• A charged particle passing through the (superheated) liquid causes the liquid to boil along their paths.

• A magnetic field causes the particles to bend.

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Classical bubble chamber image

The observation of a short-livedneutral kaon in a bubble chamber

Modern detectors are very complex and rely on advanced electronics &

computer technology

½% of the ATLAS members

Detecting particles• Which particles can we detect – are there any

we can’t ?• How do we find their - charge - momentum - energy ?

• What characteristics do we look for to identify which particle it is?

What are the principles used?

• Ionisation of a medium to show the paths of charged particles

• Magnetic fields to exert forces on charged particles and so bend their paths – to identify charge and enable momentum to be calculated

• Absorbing materials to stop particles and so enable energy to be calculated

Detector homework

GOALS:

• To learn more about detectors and the

characteristics of particle paths in them

• To make some observations and measurements

PREPARATION

Explore the physics of the ATLAS detector at:http://atlas.ch/- Click on “multimedia” and then “how atlas works” and

“animated clips”- Particularly Episode2: http://www.atlas.ch/multimedia/html-nc/feature_episode2.html- Construction of ATLAS in 3 minutes:

http://www.atlas.ch/multimedia/html-nc/built_in_three_minutes.html

- Click on “e-tours” and look at these too.- Study “Physics with ATLAS” report on the Learning with

ATLAS portal.

Download the Minerva software at

http://atlas-minerva.web.cern.ch/atlas-minerva

Read through the introduction and, using the Minerva help and instructions pdf ,

work through the 5 tutorial examples.

Student feed back

Which particles can we detect?

Which characteristics do we look for?

How are the particles detected?

Which particles can we detect – are there any we can’t ?

• Most particles can be detected by various sections of a modern detector

• Neutrinos have no charge and very little mass and rarely interact with matter – we detect their presence only by noting “missing” energy & momentum in collisions

Typical detector parts

What characteristics do we look for in the particle tracks to identify which particle it is?

• Charged particles, like electrons & positrons, leave tracks in the tracking chamber (where magnetic fields are also applied to enable momentum measurement) and deposit all of their energy in the electromagnetic calorimeter, where it can be measured.

• Neutral particles, like a photon, can deposit energy in the electromagnetic calorimeter, but leave no track in the tracking chamber

…………….• Charged particles, consisting of

quarks, like protons, leave tracks in the tracking chamber (where a magnetic field is also applied to enable momentum measurement) and deposit their energy in the hadronic calorimeter, where it can be measured.

• Neutral particles, consisting of quarks, like neutrons, also deposit energy in the hadronic calorimeter, but leave no track in the tracking chamber

• Muons pass through all the detector layers, leaving tracks, and depositing very small amounts of energy in all calorimeters. In the muon spectrometer, a large magnetic field is applied which enables momentum measurement.

Interactions of particles with the detectors - Summary

e+

leaves no track at all

The particle trajectory and charge

• Tracking devices reveal the paths of electrically charged particles through the trails they leave behind. When particles pass through the detector material, they ionise the atoms of the material. The ionised atoms give rise to a feeble electric current.

• Most modern tracking devices produce tiny electrical signals that can be recorded as computer data. A computer program then reconstructs the patterns of tracks recorded by the detector, and displays them on a screen.

• The charge on a particle is determined by the curvature of its path in a magnetic field

Motion of charged particle in magnetic fields

• The direction of the force on the particle is determined by Fleming’s Left hand Rule:

The current direction is the direction in which a POSITIVE charge is travelling.

For a negative charge, this direction is reversed, which reverses the force direction

This force provides a centripetal force from which we can deduce

particle momentum• F = Bqv• F = mv2 / r➱ mv2 / r = Bqvand momentum P = mv = Bqr

Hence a particle’s momentum can be calculated from the radius of curvature of its

How do we find the particle - energy?

• A calorimeter measures the energy lost by a particle that goes through it. It is usually designed to entirely ‘absorb’ all of the particles coming from a collision, forcing them to deposit all of their energy within the detector.

• Calorimeters typically consist of layers of ‘absorbing’ high–density material (lead or steel) interleaved with layers of ‘active’ medium such as a scintillator.

.

• Electromagnetic calorimeters measure the energy of electrons and photons as they interact with the electrically charged particles

inside matter.

• Hadronic calorimeters sample the energy of hadrons (particles containing quarks, such as protons and neutrons) as they interact with atomic nuclei

High energy e-

e-

e-

e+

pHigh energy p

p

The high energy e- interacts with the absorbingmaterial, producing a shower of a large number of low energy e-, e+, . The numerous low energy particles passes into the active material, ionising atoms. The created e- are attracted towards copper electrodes, where the charge is measured. From this, the original energy of the high energy e- can be calculated

The high energy p interacts with an atomic nucleus in the absorbing plates, leading to a shower of particles. These particles enter a scintillating material, causing it to radiate light. Long fibres carry the light to devices where the intensity is measured and converted to an electric current, from which the energy of the incoming proton is measured.

Gather evidence from observation

A K0 particle produced in a proton-proton collision,and decaying in the Inner Detector of ATLAS

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K0 particle features

• Features to determine– The mass– The lifetime– Its decay

Working in groups• On the Minerva website http://atlas-minerva.web.cern.ch/atlas%2Dminerva/ click on masterclass resources and scroll down to computer set up. Choose

a suitable version (depending on class size) and download the sets of events – click save, then right click on saved file and extract all (from the zip file)

The K0 event files are not yet uploaded. As the ATLAS data taking just started, and the K0 data are very new and interesting to the particle physics community, the ATLAS management has not yet approved their use outside the ATLAS collaboration. However, this approval is expected rather soon.

• Locate the file atlantis.jar inside the AtlantisJavaMinerva folder. Double click this file and MINERVA will begin, as long as you have a recent version of Java installed, version 1.5 or later. If you need Java installing please go to www.java.com and download the software from the website. The default events are events which are shown in the introductory slides. To display the events of a given group, go to File (upper left corner of the right panel), then click on Read Events Locally, select the minerva file from where you have saved it, select the events folder and then the group you want to display, and click Open.

…….

• Print off the Instructions for Atlantis, Summary sheet and Overview sheet in the paperwork section on this page.

• Each group takes a sample of 20 events from the Minerva web site and identifies the events within this set that possibly show the decay of a K0 particle

• For each such event, calculate the invariant mass of the K0 particle and determine its lifetime

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Special relativity

• High energies, several GeV per particle

• High speed, close to c, speed of light

• Need to use Special relativity– Albert Einstein 1905– Important contributions from Hendrik Lorentz

and Henri Poincaré

What is invariant mass??

• The invariant mass, is a characteristic of the total energy and momentum of a system of objects.

• It is the same in all frames of reference – it is invariant.

• The invariant mass is the mass of the decaying particle.

In general…..using SI units…

E2 = p2c2 + m2c4

where m is the invariant mass or just mass.

Energy and momentum must be conserved when the K0 particle decays into a + and a - .

Then :

E = E + Eand p = p + p

remembering that p is a vector quantity!

Then mK can be calculated: m2 = E2 - p2c2

c4

Units Particle physicists work with less familiar

units that simplify the equation:

E2 = p2 + m2

E is measured in GeV P is measured in GeV/c(often just called GeV in the software)

m is measured in Gev/c2

1 eV = energy gained by charged particle accelerated through a voltage of 1V 1 eV = 1.6 x 10-19 J1 GeV = 109 eV 1 TeV = 1012 eV

Using these units…

m2 = E2 - p2

E is measured in GeV &

m comes out in in Gev/c2 when

p is measured in GeV/c

E2 = p2 + m2

• For the high energy pions, the momentum is large compared to the mass

m << pThe mass term can often be disregarded,

and we can approximate that

E = p for each of the pions

Once you have identified a K0 + + -event…

• Click on “pick” at the top of the GUI box of the software, then click on the two pion tracks one after the other

• The three components of the momentum will be displayed.

• Calculate the invariant mass of the original K0 particle in each case:

mK = [ (E+ + Epx + + px )2 - py + + py )2

pz + + pz )2 ]1/2

An excel spread sheet could be designed to do this

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Estimating the K0 mass

• Explore the K0 events, and determine the mass from the momenta of the two pions.

• Repeat it for each K0 particle

• Make a histogram of the measured values, and determine the average mass of the K0.

• Estimate the uncertainty.

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The lifetime

• Most particles are unstable.• How long they live depends also on their speed

relative to the observer, that is us.• The lifetime we observe is the particle lifetime at

rest multiplied with the gamma factor (also called the Lorentz factor)

• The gamma factor, = 1/(1-v2/c2)1/2

• The gamma (or Lorentz) factor shows up ”everywhere” in special relativity.

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Estimating the lifetime

• Explore the K0 events, and determine the decay distance, the distance from the collision point to the decay point.

• Determine the speed of the K0 particle• It is often rather close to c, the speed of light.

• Determine the lifetime of each K0 particle, divided by the gamma factor

• Make a histogram of the measured values, and determine the lifetime of the K0.

Collating and discussing results

• Groups come back together and tabulate values of mass and lifetimes calculated for the K0 particles

• A histogram of frequency against mass is plotted• A histogram of frequency against mass is plotted• Discussion of whether the K0 is positively identified

and to what accuracy

Discussion of measurement technique

• The K0 particle decay can be “seen” in the detector

• The decay of very shortlived particles can not be seen in the detector

• Can the same technique still be used?• Which complications could there be to use the

technique for “invisible”, very shortlived particles?

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Discussion of the results

• What is your best estimate of the mass of the K0 particle?

• What could the uncertainty be due to?• What is your best estimate of the lifetime of the K0

particle?• Discuss ways to determine the lifetime more

correctly and more precisely.• What could the uncertainty be due to?• How far would the K0 particle typically move if the

gamma factor is 1?

K0 particle and antiparticle

• The K0 particle and the K0 antiparticle are different particles– The K0 is composed of an s quark and a u

quark– The K0 (the anti K0) is composed of an s quark

and an u quark– The K0 and the K0 are different particles as

they are composed of different quarks

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Lifetime reconstructionA straightforward exponential does not give the correct lifetime.

It is roughly a factor of two too high.

Weighted events are used, which take into account acceptance

and inefficiencies.

Discuss how inefficiencies can arise.