More “hands-on” Particle Physics; Learning with ATLAS at CERN Lynne Long, School of Physics & Astronomy, University of Birmingham, Edgbaston, B15 2TT

Is Particle Physics at A level boring? Big media interest in the start up of the Large Hadron Collider (LHC) in Geneva has switched on a lot of school students into asking questions……….. but particle physics seems, all too often, to become a tedious affair of learning names and rules in A Level classrooms! Talk for IOP coordinator meeting Sept 11th 2010

A major part of the Learning with ATLAS project has involved research groups adapting particle track analysis software to enable teachers to incorporate some “hands on” investigation into their teaching of particle physics, and so enhance this part of the curriculum for their students. Talk for IOP coordinator meeting Sept 11th portal.eu

Using MINERVA software – free download This is an opportunity for students to be led through the investigative processes that “real” researchers use, and gain a real insight into how we can investigate things so small and so fast or short lived we can’t hope to see them! The process is not as complex as it first appears, uses maths that A Level students can cope with, and leads to real results – initial trials with students have led to largely positive feedback, especially in using the software tools - “I felt like a real researcher!” Talk for IOP coordinator meeting Sept 11th 2010

Particle Detectors – How do they detect particles? Students will have met the idea of detecting particles when looking at  and maybe  tracks in school cloud chambers – the nature/direction/shape of the track being an indication of what sort of particle may have passed through. Talk for IOP coordinator meeting Sept 11th 2010

They may have looked at spark chamber tracks; or bubble chamber pictures and used Flemming’s Left Hand Rule to identify the charge on a particle from the way it turns or spirals in a magnetic field – the radius of curvature will depend on the particle’s momentum. They may have considered the particles they are “seeing” as decaying from other, much shorter lived, particles formed from the collision energy in the accelerator Talk for IOP coordinator meeting Sept 11th 2010

Modern particle detectors Modern particle detectors have a series of layers and collect far more data, far more quickly, using fast electronics/computers, but work on largely the same physics principles. Talk for IOP coordinator meeting Sept 11th It is their sheer size that is often the first thing to capture the imagination. The ATLAS detector is 44 metres long and 22 metres in diameter and is placed at one of the collision points around a 27 km accelerating ring, about 100 metres underground at CERN in Geneva.

Talk for IOP coordinator meeting Sept 11th 2010 It has large magnets to curve the particle tracks and an “onion-like” structure – various layers to track and measure the particles’ energy and momentum as they pass through.

Recognising Particle Tracks…. Small 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 em calorimeter, where it can be measured. Neutral particles, like a photon, can deposit energy in the em calorimeter, but leave no track in the tracking chamber Talk for IOP coordinator meeting Sept 11th 2010

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 n 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 small amounts of energy in all calorimeters. In the muon spectrometer, a large magnetic field is applied which enables momentum measurement. Talk for IOP coordinator meeting Sept 11th 2010

Now students and teachers have access to software that will enable them to measure particle properties in same way that researchers at CERN do.

Getting started Log onto or do a google search for “Learn with ATLAS at CERN” to find the portal site. To access the toolbox, you will need to register with a user name and a password – seems a bind but is very straightforward. Your students can access the site from home too, register and investigate for themselves, complete homework assignments etc. Talk for IOP coordinator meeting Sept 11th 2010

Click on the” Tool-Box” tag at the top of the page, scroll down to the MINERVA 2D Analysis Tool and click the Masterclass resources box. Click again on Masterclass resources and scroll down to “ computer set up”…and read instructions! The download and extracting files took only a few minutes on my computer – even the biggest file - and it worked on the home computer too! Talk for IOP coordinator meeting Sept 11th 2010

If you are going to use these packages in school, it is worth involving the IT technician in putting the software on the school intranet well in advance of your lesson – school systems often won’t allow downloads to be stored and saved. Your students will have less trouble doing this on their home computers! Clicking on the ATLANTIS jar file opens the software pages. Talk for IOP coordinator meeting Sept 11th 2010

Useful paperwork At this point, it is worth going through the introductory ppt slides in the preparation for the exercise section – they give a brief introduction to identifying which particles make which tracks. Next, scroll to the paperwork section on the web page, just after the computer set up section. Save and print off the results page, Atlantis Instructions and summary pages. These are useful crib sheets to keep by the computer for identifying which particles are indicated by which tracks etc. Talk for IOP coordinator meeting Sept 11th 2010

Doing the exercise & practising using the software Try dividing your class into groups and going through the exercise described in the exercise section. Experience shows that with a bit of help and encouragement, most students enjoy playing with the software and trying to identify which of the 5 specified events Talk for IOP coordinator meeting Sept 11th 2010

2 electrons with high pT Z  ee (>10GeV )

Talk for IOP coordinator meeting Sept 11th muons with high pT Z  (>10GeV )

Talk for IOP coordinator meeting Sept 11th electron with high pT Large missing E T W  e (>10GeV )

Talk for IOP coordinator meeting Sept 11th muon with high pT Large missing E T W  (>10GeV )

Talk for IOP coordinator meeting Sept 11th 2010 Mainly jets – particle bunch Background Only occasionally an electron or muon

Finding out if W and Z decay equally often to electrons and muons adds a bit of an “investigation” feel Note that there is a cheat sheet available for teachers by ing Peter Watkins Monika Wielers Talk for IOP coordinator meeting Sept 11th 2010

Interesting extensions Try clicking on the CERN repository” Tab at the top of the main page of the Portal website. Then, in the “Explore Cern” box, click “search for learning missions” and search for “Identifying Particles and their Properties in Detectors” – an example that I uploaded a few months ago! Clicking on the title when it comes up will open the mission/scenario information. Clicking on the “Educational Material file” opens up a powerpoint presentation which may have some useful slides, pictures and ideas that collate general information on the history of particle detectors and the main physics principles on which they work, as well as suggesting a few preparatory activities that students could be asked to do. Talk for IOP coordinator meeting Sept 11th 2010

Example Slide F = Bqv F = mv 2 / r ➱ mv 2 / r = Bqv and momentum P = mv = Bqr Hence a particle’s momentum can be calculated from the radius of curvature of its path – this happens in the tracking chambers of all detectors Talk for IOP coordinator meeting Sept 11th 2010

Later is an exercise that students can work through to calculate the invariant mass of a Z 0 particle decaying to electron + positron or  + +  -. The Z 0 decays in around s, so doesn’t travel very far! The only way to recognise its existence is to study the paths and the properties of the particles into which it decays. Talk for IOP coordinator meeting Sept 11th 2010

What is invariant mass?? The invariant mass or rest mass, is a characteristic of the total energy and momentum of an object or a system of objects that is the same in all frames of reference When the system as a whole is at rest, the invariant mass is equal to the total energy of the system divided by c 2, which is equal to the mass of the system as measured on a scale. Talk for IOP coordinator meeting Sept 11th 2010

In general….. using SI units… E 2 = p 2 c 2 + m 2 c 4 where m is the invariant mass or rest mass. If a Z 0 boson decays into e - + e , then energy and momentum must be conserved: ∑ E e, E e = E Z and ∑ p e, p e = p Z remembering p is a vector quantity! Then m Z can be calculated: m 2 = E 2 - p 2 c 2 c 4 Talk for IOP coordinator meeting Sept 11th 2010

Units Particle physicists work with less familiar units that simplify the equation: E 2 = p 2 + m 2 E is measured in GeV P is measured In GeV/c often just called GeV in the software m is measured in Gev/c 2 1 eV = energy gained by e - when accelerated through a PD of 1V = 1.6 x J 1 GeV = 10 9 eV Talk for IOP coordinator meeting Sept 11th 2010

Using these units… m 2 = E 2 - p 2 E is measured in GeV & m comes out in in Gev/c 2 when p is measured in GeV/c Talk for IOP coordinator meeting Sept 11th 2010

E 2 = p 2 + m 2 For electrons and muons, m << p So we can approximate that E = p Or we can put in the known masses of the electron or positron,  + or  ‑ ( in the correct units, m e = 0.5 MeV/c 2 and m   = 107 MeV/c 2 ) and calculate the energy, E, from the momentum and the mass. Talk for IOP coordinator meeting Sept 11th 2010

Once students have identified a Z0 e ‑ + e+ or Z0  - +  + event, they can Talk for IOP coordinator meeting Sept 11th 2010 Click on the hand – “pick” - symbol near the top of the GUI box of the software, then click on one of the electron/positron tracks (blue track in inner detector) Note the momentum components along the 3 axes, p x, p y and p z (watch the signs!) P x = GeV P y = GeV P z = GeV P x = GeV P y = GeV P z = GeV P x = GeV P y = GeV P z = GeV

The momentum components must be added up separately as momentum is a VECTOR quantity: P xZ = (p x e + p x p ) p yZ = (p y e + p y p ) p zZ = (p z e + p z p ) and p Z = [(p x e + p x p ) 2 + (p y e + p y p ) 2 + (p z e + p z p ) 2 ] 1/2 using Pythagoras’ Theorum to add vector quantities in 3 dimensions Talk for IOP coordinator meeting Sept 11th 2010

Assuming E e = p e by our approximation, then the energy of the electron is given by E e = (p xe 2 + p ye 2 + p ze 2 ) 1/2 or, more accurately, E e = [ m e 2 + (p xe 2 + p ye 2 + p ze 2 )] 1/2 The process is then repeated for the positron track E p = (p xp 2 + p yp 2 + p zp 2 ) 1/2 or, more accurately, E p = [ m p 2 + (p xp 2 + p yp 2 + p zp 2 )] 1/2 Talk for IOP coordinator meeting Sept 11th 2010

So now m Z = E z 2 - p z 2 = [(E e + E p ) 2 - [(p x e + p x p ) 2 - (p y e + p y p ) 2 - (p z e + p z p ) 2 ] 1/2 Initially, students trialling this idea were taken aback by so many equations, but soon understood where they were coming from, and designed their own spreadsheets to work out answers that didn’t rely on pushing calculator buttons in the right order! Talk for IOP coordinator meeting Sept 11th 2010

Collating and discussing results Dividing the students into groups and asking them to identify Z events (either decay to e + /e - or to  + /  - ) in a specified, downloaded group, and then measure the Z 0 mass, using the calculations above, is a good way of collating a good amount of data in a short time, as well as promoting group collaboration – “like real researchers”! NB there are not that many Z events in each group of 20 – maybe 1 or 2! Talk for IOP coordinator meeting Sept 11th 2010

Discussion of measurement technique The Higgs boson and the concept of the Higgs field was postulated by Peter Higgs in the 1960s to try to explain why particles have such diverse masses. Its maximum mass and modes of decay have been mathematically predicted but it is the only particle in the current standard model that has never been observed experimentally Physicists will use very similar techniques to the one used in this project to look for signature Higgs events and determine the Higgs mass And if the Higgs is not found…..a new theory has to emerge! Talk for IOP coordinator meeting Sept 11th 2010

Portal Development The idea of the portal - portal.eu – and the Learning with CERN project is to build a central repository of interesting resources for teachers, by teachers. We hope you will find time to explore the site and other tools in the toolbox contributed by colleagues in Europe, upload your own favourite videos/pictures and other educational material, and write your own learning missions/scenarios that can be used by others too. portal.eu We are always interested in any feedback from teachers or students – please Peter Watkins or Lynne Long with any comments or suggestions. Talk for IOP coordinator meeting Sept 11th 2010