Particle Physics Masterclass

Particle Physics Masterclass 2009

Timetable

Welcome to the Physics Department at Royal Holloway. The Particle Physics Masterclass is one of many being held at universities throughout the country in 2009. They are intended to give sixth-formers and their teachers an opportunity of learning more about the subject through talks, activities and by talking to active researchers in the field.

The detailed timetable for our course is below. There are three activity sessions during the day and the order in which you do them depends on the group to which you are assigned. Demonstrators will take you to the locations for these activities.

9:30 Arrival, refreshments in T1189:45 Welcome (Bourne LT1)9:50 The structure of matter

talk by RHUL lecturer in Bourne LT110:35 Introduction to the activity sessions10:45 Activity session11:45 Activity session12:45 Lunch (see over page)13:15 Tea/coffee available in T11813:45 ATLAS and the LHC

talk by RHUL lecturer in Bourne LT114:30 Activity session15:30 Refreshments in T11816:00 End

Group A Group B Group C10:45 Lancaster Particle Physics

PackageParticle Physics detectors Measurements using

simulated events from the LHC

11:45 Measurements using simulated events from the LHC

Lancaster Particle Physics Package

Particle Physics detectors

14:30 Particle Physics detectors Measurements using simulated events from the LHC

Lancaster Particle Physics Package

An important feature of the day is that you will be in close contact with physicists carrying out research in particle physics. Make sure that you take the opportunity to discuss any questions you have about the subject with them.

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Particle Physics Masterclass

LUNCH

If you have brought your own sandwiches you may eat them outside on the lawn and enjoy our beautiful campus in spring bloom or, in the case of wet weather, T118 will be open.

Alternatively, you can purchase sandwiches at the College Shop (see map* ref 6).

Or if you prefer a hot/cold meal, salad, etc. you can buy that in the Crossland Suite which edges Founders West courtyard (map ref 1C, near the information point I ).

There is also Café Jules for drinks and snacks just opposite the department in the

International Building (map ref 15).

After lunch at about 1:15 there will be refreshments available in T118.

*campus map inside back cover

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Particle Physics Masterclass

Activity 1The Lancaster Particle Physics PackageIntroduction

The Lancaster Particle Physics package is designed for sixth form students and illustrates some of the techniques used by particle physicists to determine properties of the particles they are studying. It was originally produced in 1996 and a second edition was released in web-based form in 2005. In the exercises set out here you will learn about:

non-relativistic collisions; what happens when particles and antiparticles annihilate; motion of particles in a magnetic field; measuring the mass and lifetime of particles;

To get started open a web browser, go to the site http://lppp.lancs.ac.uk and read the welcome page.

Exercise 1 – non-relativistic collisions

Click on the pool tab (note the four screens called Introduction, Collisions, Masses and Calculation.)

Read this page and study the animation at the bottom.

Click on the links to learn about Conservation of Kinetic Energy and Conservation of momentum.

Click on Collisions or next at the top of the screen and work through the next page. You may wish to study a number of different collisions by clicking the Fire button a number of times.

Move on to the Masses page, use the fire button to produce collisions and for each one answer the question about the relative masses of the two balls.

Summarize your observations about and below

1. When m1 = m2 the sum of and is always _______________

2. When m1 > m2 the sum of and is always _______________

3. When m1 < m2 the sum of and is always _______________Go to the final page by clicking on calculations and work your way through this page to learn how particle physicists use particle scattering to determine the masses of different particles.

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Particle Physics Masterclass Exercise 2 - particle-antiparticle annihilation

Click on the annihilation tab and work through the next section to learn how new particles can be produced when an electron and its antiparticle, the positron, annihilate to produce new particles provided sufficient energy is available.

It is suggested that you omit the section Stationary Target and move straight to Colliding Beams to learn about the threshold energy for the production of a +- pair. Carry out the exercise on this page.

Read the Conclusion section noting particularly that determining the threshold energy for the production of a particle is one of the ways in which particle physicists can determine the mass of the particle.

Exercise 3 - motion in a magnetic field

Click on the magnetic field tab, and read and understand the Introduction page. Go to the page Experiment and make sure that you read and understand the section that you reach by clicking on the link Making Radius Measurements.

Carry out the experiment and learn how to use this method to determine the mass of particles and hence identify them. On the options tab set the magnetic field to 1T and the incident beam energy to 1 GeV. Complete the table below. Note that it is very difficult to identify the green and pink pairs.

Colour Particle/antiparticleGreyYellowGreenPink

Exercise 4 - particle mean lifetime

By now you are an expert in using this package but nevertheless this last exercise should still be quite a challenge. Click on the lifetime tab and read the Kaon Decay page and then proceed to the Kaon Mass/Lifetime page. See how well you can manage it to learn how particle physicists measure both the mass and the mean lifetime of a particle that decays, in this case within a small fraction of a microsecond. After you have measured three events yourself the programme will do some automatically for you (options tab). Select (say) 30 and click the Fire button. Observe how the mass plot and the lifetime plot approach Gaussian and exponential distributions respectively. Record your mean mass and lifetime and compare them with the accepted values of 0.498MeV/c2 and 0.895 x 10-10s.

Mean mass: ± MeV/c2 Mean lifetime: ± x 10-10s

Activity 2

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Particle Physics Masterclass

Measurements using simulated events from LHCIntroductionIn this exercise, you will be making different measurements using simulated events from LHC. The Large Hadron Collider (LHC) was completed recently and uses a 27km tunnel at CERN in Swizterland. The LHC collides protons with protons at a center of mass of 14TeV. It will be the collider operating at the highest energy once it starts this Autumn.

Exercise 1 – Identifying different types of ATLAS simulated eventsIn pairs of 2, you will look at 20 ATLAS simulated events and try to associate each of them with an even type. There will be an introduction by your demonstrator as to how to do this and you have a summary sheet next to your computer. Bring up the Atlantis Canvas and Atlantis GUI from the Start bar at the bottom of your screen. In the Atlantis GUI under File select: Read Events Locally. Pick the group of events written on your strip of paper. Each event can be of the following type:

1) W e2) W 3) Z ee4) Z 5) Background6) Hll, l=e,

Remember that there is only 1 Higgs event for your entire group so it might not be among the 20 events you are looking at. Tabulate your results in the following table and when you are

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Particle Physics Masterclass done bring them to the demonstrators, who will collect all the results from your group. This kind of collective work is done all the time in particle physics!

You can use the following table to help you out (type is the number: 1-6):

Event number type Event number type1 112 123 134 145 156 167 178 189 19

10 20 Number of events from each type (to give to your demonstrator as soon as you are done):

1 2 3 4 5 6

DiscussionAre the numbers of type 1 and 2 roughly the same? What about 3 and 4? Do you have more W events or more Z events? Why is that?What is the uncertainty on each number?Discuss your results (and the reasons for them) with your demonstrators! Feel free to ask them what is a W and Z particle and what it is that they hope to find with the ATLAS detector!

Extra exercisesIf you are done before the rest of the group you can proceed and do the other group of events to hunt for the Higgs event!

Exercise 2 – Measuring the mass of the Higgs particle

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This exercise makes use of the Lancaster Particle Physics package: go to their web site: http://lppp.lancs.ac.uk and click on the Higgs tab. In that section there are 4 sub-sections: LHC, Higgs, Detector and Measurement. Read through the first 3 sub-sections, some of it contains information that you already know may be.

The measurement using LHC eventsUnder the Measurement sub-section, you will find an exercise which lets you identify the elusive Higgs boson, a quest that is central to many researchers around the world (including at RHUL)!

In groups of 3, discuss and try out different configurations of Magnetic Field and Energy Cutoff by going through the simulation for 3 or 4 events and then selecting some numbers of autoevents. You want a significance close to 5 (you click Fit when the simulation is done with the autoevents). When you’re done, give your best results to the demonstrators so they can tally them. Use the following table to record your results:

Try Magnetic Field

Energy Cutoff

Number of

autoevents

Number of events

used

Efficiency (used/auto)

Mass Significance

123

DiscussionAll through this activity feel free to have discussions with your colleagues and demonstrators about what you are learning. Examples of topics could be:

1) Why is a particle detector so big?

2) What are the main differences between the LEP and LHC accelerators? What sort of consequences do these have on the event pictures you have seen?

3) Why is finding the Higgs boson particle such an important goal of the LHC? What other ways are there to identify the Higgs boson other than the 2 photons final state?

4) Have you heard of a future accelerator that would collide electrons and positrons just like LEP did? What fundamental difference is there between LEP and this future accelerator?

5) Ask your demonstrators to talk about their own particle physics project!

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Activity 3Cosmic rays and their detection

Cosmic rays that reach the earth’s surface are mainly muons (a particle which behaves like an electron but is 200 times heavier). Cosmic rays were first discovered in 1912 by Victor Hess who flew his balloon to an altitude of 5000m and discovered that the higher he went the amount of ionising radiation increased. This observation led him to conclude that the radiation was coming from outer space and not from the earth.

High energy protons, accelerated by supernovae remnants, hit the upper atmosphere and produce showers of particles. These showers contain particles called pions, which are short lived and decay to muons. The muons produced travel at speeds close to the speed of light and are therefore able to reach the earth’s surface even though their mean lifetime is 2x10-6 s. Advances in accelerator design have enabled particle physicists to access energies close to that of the big bang, but which are still 100,000,000 times less than the highest energy cosmic rays. The origin of such high energy cosmic rays is still a mystery.

Scintillation counters

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Figure 1. Hess and his balloon

Figure 2. Schematic of an extensive air shower

Particle Physics Masterclass

Scintillation counters, in various forms, are widely used in particle physics. A scintillation counter consists of two main components, a scintillator and a light measuring device (usually a photomultiplier tube). A scintillator is a material that emits light after being excited by ionising particles (e.g. electrons, photons, etc.). However, if a scintillator is to be useful for detecting particles it needs to be transparent to its own scintillation light. There are two types of scintillator: organic (e.g. plastic) and inorganic (e.g. crystalline Sodium Iodide). The mechanism by which the two types emit light is different. Organic scintillators in general respond quicker than inorganic scintillators.

A photomultiplier tube can be used to measure the amount of light produced by the scintillator. The photomultiplier works by converting a photon into an electron, which is then accelerated by a high voltage, and collides with a piece of shaped metal called a dynode. More electrons are ejected from the dynode in the collision and these are accelerated towards the next dynode (see figure 3). Thus the number of electrons produced at the end of the dynode chain is enough to give a measurable signal.

The surface of the scintillator is usually of a different geometry to that of the photomultiplier. Therefore, it is necessary to couple the two using a light guide. The scintillation light is guided by total internal reflection with as little loss as possible.

In particle physics detectors, scintillation counters can be used to measure the energies as well as detect the presence, with accurate timing, of the various particles produced in a collision.

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Figure 4. A light guide

Figure 3. Schematic of a photomultiplier tube

Particle Physics Masterclass

Measurement of the mean muon lifetime

The equipment used to measure the muon lifetime consists of 30x20x20 cm3 plastic scintillator, two photomultiplier tubes with light guides, timing electronics and a computer. An incoming muon deposits energy within the scintillator generating a signal that starts the timing electronics. It then waits 25.4 s for a stop signal before resetting itself. Most muons will pass through the scintillator. However, if the muon is brought to rest it will decay producing a second signal due to the electron produced by the decay. This stops the timing electronics. The time between the start and stop signals is then sent to the computer, which makes a histogram of the results.

The experiment has been running for the past couple of days to accumulate enough data to make a measurement of the mean lifetime. Take around 5 measurements of number of counts (N) and time (t) from the histogram on the screen and plot ln(N) against time.

The number of entries in the histogram can be described by , where is the mean muon lifetime. Taking the log of both sides yields . Therefore, by fitting a straight line to the measurements you have taken, you can estimate the value of the mean lifetime. How can you estimate the uncertainty in your value? Why does the time that the muon lived before entering the experiment not matter for your measurement? The rest mass of a muon is 0.1057 GeV/c2. Estimate how far on average a muon will travel before decay if its energy is (a) 1 GeV, (b) 10 GeV and (c) 100 GeV.

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teN −∝consttN +−= l

Particle Physics Masterclass

The cloud chamber

The cloud chamber, invented by C.T.R. Wilson in 1911, has played an important role in many particle physics discoveries. Wilson was awarded the Nobel Prize in Physics in 1927 for this invention. When an electrically charged particle such as a proton, electron or muon passes through the chamber it creates a visible track of droplets. The device you will see during the Masterclass is a diffusion cloud chamber, shown schematically below.

The diffusion cloud chamber consists of a chamber base and the observation chamber. The chamber base contains a cooling element which cools the black metal plate (45cmx45cm) at the bottom of the observation chamber to about -30°C. On the top you can see a grid of fine heating wires which helps to keep the hood free from condensation. Iso-propyl alcohol is fed by a tube and is allowed to drip into the gutter which runs around the whole circumference. You will also see that there is a heating wire in the gutter.

The alcohol evaporates and diffuses from the upper, warmer area to the cold chamber bottom. There the alcohol is condensed into tiny droplets and flows back into the reservoir hidden in the base. You can easily see the liquid alcohol on the edges of the metal plate. Right above the thin liquid layer at the bottom, a zone of super-saturated alcohol vapour is formed (about 1 cm thick), due to the temperature gradient. When a charged particle passes through this ‘active layer’, it collides with the air molecules, liberating electrons and leaving behind a trail of ionisation. The ions trigger the condensation of the alcohol and a visible trail of alcohol droplets is formed. Photons create only indirectly a trail when, e.g. they eject an electron from atom (Compton scattering), which produces a trail of ionisation.

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Particle Physics Masterclass

The will see a number of different types of tracks. Some are cased by alpha particles emitted by radioactive impurities in the base of the chamber. Because alpha particles are heavy and do not move very fast, they deposit almost all of their energy in a very localised region and appear as small ‘mushroom clouds’, short and dense. Some of the tracks are low energy electrons, knocked out of their atoms by cosmic rays. Because electrons are relatively light, they are buffeted around by collisions with air nuclei and therefore leave jagged tracks. The straight tracks are higher energy cosmic rays. These are mostly muons, which come from cosmic ray showers initiated by a high energy proton when it collides with the earth's atmosphere. Other tracks are electrons and positrons from electromagnetic showers created when cosmic rays strike the roof of the building.

The cloud chamber can also be used to see the tracks left by electrons emitted in beta decay. A very weak strontium-90 (90Sr) can be mounted in left side of the chamber and the electrons from individual decays can be seen. By placing a magnet under the chamber, the electrons will travel in a curved path. Those with high energy travel almost straight, and those with lower energy are more curved. If you watch the chamber for a minute or so you should be able to convince yourself that not all electrons come out with the same energy. This observation is what led Pauli to propose that an additional particle is emitted in beta decay, which is now called the neutrino (). This particle is in fact classified now as an electron-type antineutrino and what we observe in the cloud chamber are the electrons from 90Sr -> 90Y + e-

+ e.

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Useful Websites for Schools

Of interest to schools ....http://www.pp.rhul.ac.uk/ScoolsGoodies about particle physics from our site, including a new "slide show" introduction to particle physics, the new version of our summary sheets and posters and much more.

You can find some of the following links on: http://www.physics.ox.ac.uk/documents/WorkExp/WebGuide/default.htmlList produced by Gerald Myatt (University of Oxford) email: g.myatt1@physics.ox.ac.uk

Particle Physics UKhttp://www.particlephysics.ac.ukParticlePhysics.ac.uk was launched November 2005 and replaces the old PPUK website. It is intended for use by the general public interested in particle physics, as well as journalists, and members of the research community.

LHC and the UKhttp://www.lhc.ac.ukEverything about the LHC for students, teachers, the media and physicists.

Interactions.Orghttp://www.interactions.org/It provides links to current particle physics news from the world's press, photos and graphics and links to education and outreach programs.

Big Bang Sciencehttp://hepwww.rl.ac.uk/pub/bigbang/part1.htmlPPARC’s booklet introducing particle physics, especially at CERN and its Large Electron Positron collider (LEP).

The Particle Adventurehttp://durpdg.dur.ac.uk/lbl/particleadventure/Learn about basic particle physics in an interactive "Particle Adventure", a web-site from the Contemporary Physics Education Project (CPEP), which is mirrored in Durham. See http://durpdg.dur.ac.uk/lbl/particleadventure/other/education/ for classroom activities and http://www.cpepweb.org/ for more information about the CPEP.

SLAC Virtual Visitor Centerhttp://www2.slac.stanford.edu/vvc/Default.htmAn excellent site from the Stanford Linear Accelerator Center in California, home to the 2-mile long linear electron accelerator. There is a lot of good information here.

All about neutrinoshttp://wwwlapp.in2p3.fr/neutrinos/aneut.html An informative guide to the history of neutrinos and the various puzzles surrounding them.

High-energy physics made painlesshttp://www-ed.fnal.gov/painless/htmls/index.htmlArticles from Ferminews - the newsletter of Fermilab - which aim to explain ideas in particle physics in everyday language.

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Particle Physics Masterclass

"Hands on" Particle Physics

Events in DELPHIhttp://www.physics.ox.ac.uk/Documents/ParticleDemos/DelphiIntro/index.htmlAn introduction to interpreting events in one of the four LEP detectors at CERN. If you can run Java you can also try rotating and zooming events at http://hepwww.rl.ac.uk/WIRED/.

Hands on CERNhttp://hands-on-cern.physto.se/For those with Java, the chance to rotate and zoom real events from the DELPHI detector. Click on the event display, read the instructions and select the detector components. Useful in conjunction with the above two sites.

Identifying events at LEPhttp://hepwww.ph.man.ac.uk/~wyatt/events/A self-guided tutorial aimed at sixth-formers which explains how to understand event pictures from the OPAL detector at LEP, together with a five-part challenge (with the answers!).

Making top quark data accessible ...http://www-ed.fnal.gov/samplers/hsphys/activities/top_quark_intro.htmlUse conservation of momentum to calculate the mass of the top quark, complete with pages for students and pages for teachers.

Lancaster Particle Physicshttp://lppp.lancs.ac.ukInformation about a computer package designed to help A-level students who have chosen one of the Particle Physics options currently available.

Seeing particleshttp://www.ep.ph.bham.ac.uk/user/watkins/seeweb/BubbleChamber.htmExercises aimed at schools, based on the interpretation of photographs of particle tracks in bubble chambers.

Beginners guide to high energy physicshttp://www-physics.mps.ohiostate.edu/~cleo/hep/root.htmlSome detail about how experiments and detectors work, based on the CLEO detector at the Cornell Laboratory, New York, but not for absolute beginners! Includes some do-it-yourself analysis but at a fairly sophisticated level.

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Major Particle Physics Labs and Detectors

CERN - Europe's main centre for particle physicshttp://www.cern.ch/Public/Home of particle physics in Europe and the invention of the World Wide Web. The site includes general information about particle physics, links to experiments, and some information for teachers at http://microcosm.web.cern.ch/Microcosm/teachers/home.

Welcome to the DELPHI experimenthttp://www.cern.ch/Delphi/Welcome.htmlMore detailed information on the DELPHI detector, with some good event pictures for the more advanced student (under "About DELPHI - DELPHI transparencies").

The OPAL detectorhttp://www.cern.ch/Opal/tour/detector.htmlMore detailed information about OPAL - for the seriously interested. There is also good tutorial on typical events in OPAL, with pictures (in both GIF and Postscript formats) - again for the keener enthusiast - at http://www.cern.ch/Opal/events/opalpics.html.

The ATLAS detectorhttp://atlas.chThe biggest of the detectors being built for CERN's next accelerator, the Large Hadron Collider (LHC). The Education pages are on a server in the US, but the Public pages are at CERN.

The CMS detectorhttp://cms.cern.chThe smaller "general purpose detector" being built for LHC. ("C" is for compact!).

The LHCb detector http://lhcb-public.web.cern.ch/lhcb-public/The experiment at the LHC that may help us understand why we aren’t made from antimatter.

DESY laboratory, Hamburghttp://www.desy.de/html/home/fastnavigator.htmlThe home of HERA, the world’s only electron-proton collider. The links on this page take you mainly to sites in English, but the navigation bar at the top at present takes you to German pages.

The H1 experiment at HERAhttp://www-h1.desy.de/An introduction to one of the major experiments on the HERA collider at DESY, including a "tour" and event displays.

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The ZEUS experiment at HERAhttp://www-zeus.desy.de/The second of the two major experiments investigating electron-proton collisions at DESY. There are some interesting attempts to explain the physics behind the published papers at http://www-zeus.desy.de/zeus_papers/paper_summary/

BaBar and the Missing Antimatterhttp://hepwww.rl.ac.uk/BaBarpub/An introduction to the BaBar experiment and its search for subtle differences between matter and antimatter. A teaching package on special relativity and its application to events in BaBar can be found at http://www.hep.manchester.ac.uk/babarph/babarteach/intro.html.

Physics at Fermilabhttp://www.fnal.gov/pub/inquiring/physics/index.htmlAn introduction to aspects of particle physics from Fermilab in the US.

Gran Sasso http://www.lngs.infn.it/Particle physics underground, without accelerators, at an international laboratory in Italy, under the Gran Sasso Massif.

Neutrino experimentshttp://hepunx.rl.ac.uk/neutrino-industry/ A starting point for anyone who wants to know about the wide variety of experiments with neutrinos

Other interesting sites

Astronomy Picture of the Dayhttp://www.star.ucl.ac.uk/~apod/apod/astropix.htmlA different picture each day, with a brief explanation.

The Electronic Nobel Museumhttp://www.nobel.se/All about all the Nobel prizes, from 1901 to the present day, including videos of the Nobel lectures for the most recent awards.

The Internet Pilot to Physicshttp://physicsweb.org/TIPTOP/Includes the opportunity for some interactive "experiments" which illustrate a range of physics from Ohm’s law, to chaotic pendulums, to controlling a nuclear reactor!

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