Download CERN and Bubbel Chamber Detective

Bubble Chamber Detective
The story so far: When physicists looked at cosmic rays and the first particle accelerators they found
positrons, muons and pions. Particles were accelerated at higher and higher energies and for the next 30
years the bubble chambers allowed physicists to detect hundreds of different ‘elementary’ particles. The
following rules will allow you to analyze photographs of such events from a bubble chamber at CERN
in Switzerland, the home of the Large Hadron Collider.
 Centripetal acceleration a = v2/r. The force of a magnetic field on a charged particle is given by
F = qvB = m v2/r. Therefore the momentum of that charged particle is given by p = mv = qBr.
 Charge and momentum are always conserved. The particles have a charge of either +1e, -1e or 0.
 The bubble chamber is filled with liquid hydrogen - a source of protons and electrons.
 Moving charged particles leave a ‘bubble trail’. Neutral and stationary particles leave no trail.
Part 1: For each question, select the correct answer and write a brief justification.
1) The parallel lines are the paths of negative kaons. Which way are they traveling?
a)Up the page
b)Down the page
2) What is the direction of the magnetic field?
a)Into the page
b)Out of the page
c)Right
3) What are the charges of the particles that leave point A?
a)Two negative
b)Positive- top, negative-below
d)Left
c)Positive-bottom, negative below
4) What is the charge of the particle that produces the event at point A?
a)Positive
b)Negative
c)Neutral
5) Which particle coming from A has more momentum?
a)Positive
b)Negative
c)The momenta are equal
6) In which general direction was the neutral particle traveling before it decayed? How do you know?
7) How many particles leave point B?
a)Two
b)At least two
c)Three
8) What are the charges of the two charged particles leaving B?
d) At least three
a)The right is positive, the left is negative
b)The left is negative, the right is positive
9) Is charge conserved at B?
a)No, we started with one negative and end up with one negative and one positive.
b)Yes. The kaon must have interacted with a positive particle that was not moving.
10) What happened at C? This is not a gentle curve. There is a kink in the trail to the right. There has
been a change in momentum. This means that a negative particle
a)collided with a neutral particle.
b)collided with a positive particle
c)decayed into a positive and a neutral
d)decayed into a negative and a neutral
11) What happens at D? A neutral particle
a)collides with a neutral particle.
c)decays into a positive and a negative particle
b)collides with a positive particle
12) Which of the particles that travel between points D and E has the larger momentum?
13) The total momentum of the two particles from D is along the line joining D and E. Does the neutral
particle that decays at point D come from point C or B?
Part 2
1) In what direction are the beam particles traveling?
2) The lightest particles in these photos are the electrons. They will have the greatest acceleration and as
a result they will radiate energy fastest. What will their paths look like? Hint: r = mv/qB
3) The electron trails are a result of the beam particles colliding with electrons that are attached to
hydrogen nuclei. How many electron trails can you see? What is the direction of the magnetic field?
4) The beam particles are negative kaons. Why aren’t their trails curving to the right?
5) The heaviest and slowest particles are protons. They will have very dense bubble trails. Find two.
6) What happened at p?
7) What happened at q?
8) What happened at r?
9) What happened at s?
Bubble Chamber Detective, Part One: (Lower Energy)
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F
D
C
A
B
Bubble Chamber Detective, Part One: (Lower Energy)
Bubble Chamber Detective: CERN 1970
Assumed Preparation: The lesson assumes that the students are already familiar with protons, electrons
and neutrons. They are also familiar with the mathematics of circular motion, magnetic force on a
charged particle and conservations laws.
Subversive Physics: This lesson is a good application/extension of magnetic forces on charged
particles.
The Activity: Part 1 has small groups work through a structured inquiry worksheet dealing with the
concepts – not mathematics. With a small set of rules they analyze what has happened in the photograph.
Part 2 is similar except that the questions are less structured and the energy is higher. How a bubble
chamber works can be modeled with a bottle of soda water, an old black and white television and a
magnet.
Further Information:
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Internet: The Particle Adventure http://www.particleadventure.org/ is a friendly, non- threatening
introduction to particle physics for teachers and students. More bubble chamber exercises and other
particle physics resources can be found at http://roberta.tevlin.ca/
Books:
1) *** “Understanding the Universe: from Quarks to the Cosmos” by Don Lincoln, 2004. This book
does an excellent job of explaining how particle physicists know what it is that they know. Chapter 6 is
devoted to Accelerators and Detectors.
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2) **** "The Quantum World: Quantum Physics for Everyone" by Kenneth W. Ford, 2004. This
book is great. It combines the ideas of quantum mechanics with the ideas in particle physics in a very
clear, easy but thorough read.
3) *** "Quarks, Leptons and the Big Bang" by Jonathan Allday, 1999. The book does a very thorough
job of developing the Standard Model. It gets really mathematical in parts, but you can skip those parts
if you aren't interested and still appreciate the rest of the book.
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Review this lesson
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Bubble Chamber Detective
Assumed Preparation: This lesson assumes that the student is familiar with classical momentum, the
force of a constant magnetic field on a charge (F = qvB) and circular motion (a = v2/r). It does not
require that they know what a kaon is.
Subversive Physics: This can be fit into a course involving the movement of charges under
electromagnetic forces and wherever you might discuss cathode ray tubes or mass spectrometers. It
introduces students to the idea that there are many particles beyond the electron, proton and neutron and
that these can be made from pure energy.
The Activity: The lesson uses bubble chamber photographs from CERN and is a modification of one of
their resources. You can find much more on the CERN website and at http://roberta.tevlin.ca.
Part 1 Answers
1) The parallel lines are the paths of negative kaons. Which way are they traveling?
a)Up the page
b)Down the page
Decays from a point - like B - are more likely than two particles colliding and stopping. There are
seven parallel tracks at the bottom and only six at the top. The kaons must be moving up the page.
2) What is the direction of the magnetic field?
a)Into the page
b)Out of the page
c)Right
d)Left
The beam of negative particles is bending to the right, so the field must be either in or out of the
page. By the left hand rule (or reversed right hand rule) it is into the page
3) What are the charges of the particles that leave point A?
a)Two negative
b)Positive- top, negative-below
c)Positive-bottom, negative below
The bottom one curves right, just like the kaon, so it must also be negative. The top curves left and
must be positive. (It turns out that these are pions.)
4) What is the charge of the particle that produces the event at point A?
a)Positive
b)Negative
c)Neutral
Charge must be conserved, so it should be neutral. Neutral particles leave no trail.
5) Which particle coming from A has more momentum?
a)Positive
b)Negative
c)The momenta are equal
The magnetic force causes the curving and qvB = mv2/r. Therefore momentum, mv = qBr. All of
the particles all have charges of +/- one elementary charge. Furthermore, the field B is constant.
Therefore the momentum is directly proportional to the radius. The negative one is curving just a
little bit more, i.e. it has a smaller radius, so its momentum is smaller.
6) Which general direction was the neutral particle traveling before it decayed? How do you know?
In order to conserve momentum, it must have been moving up and to the left but not halfway
between the two tracks. It should be slightly steeper than that because the top particle has more
momentum. If you found the two radii, you could determine the precise path because mv = qBr.
7) How many particles leave point B?
a)Two
b)At least two
c)Three
d) At least three
There are two tracks, so there are at least two. To conserve momentum there should also be
another one to the left and there is further evidence of it at A. There may be other neutral
particles.
8) What are the charges of the two charged particles leaving B?
a)The right is positive, the left is negative
b)The left is negative, the right is positive
The one on the right is curving left so it must be positive. The one on the left is curving right, so
the other one must be negative.
9) Is charge conserved at B?
a)No, we started with one negative and end up with one negative and one positive.
b)Yes. The kaon must have interacted with a positive particle that was not moving.
Charge conservation is such a long-standing law, that it is more likely we are missing a piece of the
puzzle than that the law has been violated. The hydrogen bubble chamber is full of stationary
protons and what we see here is a collision between a kaon and a proton.
10) What happened at C? This is not a gentle curve. There is a kink in the trail to the right. There has
been a change in momentum. This means that a negative particle
a)collided with a neutral particle.
b)collided with a positive particle
c)decayed into a positive and a neutral
d)decayed into a negative and a neutral
The trail bends at C. There must be another particle to conserve momentum. It can’t be seen, so it
must be neutral. The liquid hydrogen is a source of many protons and electrons, but there are
almost no neutral particles to collide with. Furthermore, there is evidence of a neutral particle
‘downstream’ at D.
11) What happens at D? A neutral particle
a)collides with a neutral particle.
c)decays into a positive and a negative particle
b)collides with a positive particle
The left one is clearly negative. The other one curves very slightly to the left and therefore is
positive. The total charge after is zero and therefore the total charge before must be zero. Either a
neutral particle decays or collides with a neutral particle. The second is not very likely.
12) Which of the particles that travel between points D and E has the larger momentum?
Again, based on constant magnitude of q and the constant field B, momentum is inversely
proportional to the radius of curvature. The one on the right has more momentum.
13) The total momentum of the two particles from D is along the line joining D and E. Does the neutral
particle that decays at point D come from point C or B?
It’s a close call, but a line joining the two intersection points is the direction of C not B.
Part 2 : Answers
1) In what direction are the beam particles traveling?
Toward the top.
2) The lightest particles in these photos are the electrons. They will have the greatest acceleration and as
a result they will radiate energy fastest. What will their paths look like? Hint: r = mv/qB
The velocities are all close to c, so the momentum depends most on the mass. The electron
has the smallest mass and therefore will form the tightest arcs. As the electron slows down, it will
spiral inwards.
3) The electron trails are a result of the beam particles colliding with electrons that are attached to
hydrogen nuclei. How many electron trails can you see? What is the direction of the magnetic field?
There are seven largish electron trails and many smaller ones. If you look closely you can
see that they all spiral to the right, so the field must be into the page. If one spiraled the other way
it would be positrons
4) The beam particles are negative kaons. Why aren’t their trails curving to the right?
They must be, but it’s hard to see because they curve so little. The kaons are moving really
fast. If you lay a ruler alongside of the tracks and look closely, you can see that they curve slightly
to the right.
5) The heaviest and slowest particles are protons. They will have very dense bubble trails. Find two.
They need to be dense and curving to the left. They also end up stopping after a short
distance. There is one leaving r and another leaving p.
6) What happened at p?
We see the trails of two positive particles leaving p. There is only one incoming trail. To
conserve charge and incoming positive particle must have hit a stationary proton. It is not a kaon
but something that formed ‘downstream’. We should also suspect that the incoming particle is not
a kaon because its path is not parallel to the kaons. The outgoing particle on the right has already
been identified as a proton, so we have a proton before and after. If the other particle is the same
before and after, then this was just an elastic collision. We don’t have enough information to
determine if that is the case.
7) What happened at q?
At q we see a trail of an incoming kaon producing a positive particle on the left and another
charged particle on the right. To conserve charge, the kaon must have interacted with a stationary
proton. We can’t see a curve for the particle to the right but to conserve charge, it must be
negative. It looks like there is a third particle coming from q but it is just the continuation of a
kaon trail that is further into or out of the page.
8) What happened at r?
A negative particle (not a kaon) from q has produced a negative particle, a proton and
another particle that must be negative to conserve charge.
9) What happened at s?
The trail has a kink in it. To conserve momentum a charged particle must have decayed a
particle with the same charge and a neutral particle moving to the left. The charged particle is
curving to the right and must be negative.