Particles

Question 1

What are quarks?

Answer 1

1. They are fundamental particles that make up particles such as protons and neutrons
2. They exert the strong nuclear force on one another
3. They are the building blocks for hadrons and anti hadrons

Question 2

What is strangeness?

Answer 2

• Strangeness is only conserved some of the time
  1. Strangeness, like baryon number, is a quantum number and it can only rake a certain set of values
  2. Strange particles, such as kaons, are created via the strong interaction but decay via the weak interaction
  3. Strangeness is conserved in the strong interaction, but not in the weak interaction
  4. That means strange particles are always produced in pairs (e.g. K+ and K-). One has a strangeness of +=1, and the other has a strangeness of -1, so overall the strangeness of 0 is conserved

Question 3

What are baryons made from?

Answer 3

• Evidence for quarks came from hitting protons with high energy electrons
• The way the electrons scattered showed that there were three concentrations of charge (quarks) inside the proton
• Baryons are made from three quarks

Question 4

What are mesons?

Answer 4

1. Pions are just made from a combination of up, down, anti-up and anti-down quarks
2. Kaons have strangeness so you need to put in s qaurks as well (remember, the s quark has a strangeness of S= -1)
3. Pi- meson is just the antiparticle of the pi+ meson
4. The K- meson is the antiparticle of the K+ meson and the antiparticle of a pi^0 meson is itself

Question 5

What does the weak interaction do to the quark type?

Answer 5

• The weak interaction is something that changes the quark type
  1. In Beta- decay a neutron is change into a proton: in other words udd change into uud
  2. In means turning a d quark into a u quark, only the weak interaction can do this
  3. Some unstable isotopes like carbon-11 decay by beta+ emission
  4. In this case a proton change to a neuron, so a u quark change to a d quark

Question 6

What four properties are conserved in particle interactions? Describe them

Answer 6

1. Charge is always conserved: In any particle interaction, the total charge after the interaction must be equal to the total change before the interaction
2. Baryon number is always conserved: just like with charge, in any particle interaction, the baryon number after the interaction must be equal to the baryon number before the interaction
3. Strangeness is conserved in strong interactions: the only way to charge the type of quark is with the weak interaction, so in strong interactions there has to be the same number of strange quarks at the beginning as at the end. In weak interactions, strangeness can change by -1,0+1. The interaction K- + p -> n + pi^0 is fine for change and baryon number but not for strangeness, so it won’t happen. The negative kaon has an s quark in it
4. Conservation of the lepton number: the different types of lepton number have to be conserved separately

Question 7

What are atoms made up of?

Answer 7

• Inside every atom there is a nucleus continuing protons and neutrons and protons and neutrons are both know as nucleons
• Orbiting this core are the electrons and this is the nuclear model of the atom

Question 8

What are the relative charges of protons, neutrons and electrons?

Answer 8

Proton: +1
Neutron: 0
Electron: -1

Question 9

What are the relative masses of protons, neutrons and electrons?

Answer 9

Proton: 1
Neutron: 1
Electron: 0.0005

Question 10

What is the proton number?

Answer 10

• The proton number is the number of protons in the nucleus
  1. The proton number is sometimes called the atomic number, and has the symbol Z and Z is just the number of protons in the nucleus
  2. Its the proton number that defines an element, no two elements will have the same number of protons
  3. In a neutral atom, the number of electrons equals the number of protons (a particle with a different number of electrons to protons is called an ion)
  4. The element’s reactions and chemical behaviour depend in the number of electrons, so the proton number tells you a lot about its chemical properties

Question 11

What is the nucleon number?

Answer 11

• The nucleon number is the total number of protons and neutrons
  1. The nucleon number os also called the mass number, and has the symbol A
  2. It tells you how many protons and neutrons are in the nucleus
  3. Since each proton or neutron has a relative mass of (approximately) 1 and the electrons weigh vitally nothing, the number of nucleons is the same as the atoms relative mass

Question 12

What are isotopes?

Answer 12

-Isotopes have the same proton number, but different nucleon numbers
1. Atoms with the same number of protons but different numbers of neutrons are called isotopes
2. Changing the number of neutrons doesn’t change the chemical properties
3. The number of neutrons affects the stability of the nucleus though
4. Unstable nuclei may be radioactive and decay over time into different nuclei that are more stable
E.G. Hydrogen has three natural isotopes: hydrogen, deuterium and tritium:
-Hydrogen has 1 proton and 0 neutrons
-Deuterium has 1 proton and 1 neutron
-Tritium: has 1 proton and 2 neutrons

Question 13

How can radioactive isotopes be used to tell you how old stuff is?

Answer 13

1. All living things contain the same percentage of radioactive carbon-14 taken in from the atmosphere
2. After they die, the amount of carbon-14 inside them decreases over time as to decays to stable elements
3. Scientists can calculate the approximate age of archaeological finds made from dead organic matter (e.g. wood, bone) by using the isotopic data (amount of each isotope present) to find the percentage of radioactive carbon-14 that’s left in the object

Question 14

What is the specific charge of a particle equal to?

Answer 14

• The specific charge of a particle is equal to its charge over mass
  1. The specific charge of a particle is the root of its charge to its mass given in columns per kilogram (C Kg-1)
  2. To calculate the specific charge you just divide the charge in C by the mass in kg
  3. You could be asked to find the specific charge of any particle, from a fundamental particle like an electron, to the nucleus of an atom or ion

Question 15

What are photons?

Answer 15

• Photons are packets of electromagnetic radiation
  1. Visible light is just one type of electromagnetic radiation
  2. The electromagnetic spectrum is a continuous spectrum of all the possible frequencies of electromagnetic radiation
  3. The frequency, f and wavelength lamda are linked by f= c/lamda where c = 3x10^8 ms-1 is the seeds of light in a vacuum *sometimes called the speed of light in vacuo)
  4. Electromagnetic radiation exists as photons of energy and there energy of a photon depends on the frequency of the radiation
• E=hf=hc/lamda (h is the Planck constant equal to 6.63 x 10-34 Js)

Question 16

How can you create matter and antimatter?

Answer 16

• You can create matter and antimatter from energy
  1. You’ve probably hear about the equivalence of energy and mass and it all comes from Einstein’s Special Theory of Relativity
  2. Energy can turn into mass and mass can turn into energy if you know how
  3. The rest energy of a particle is just the ‘energy equivalent’ of the particle;s mass, measure in Mev
  4. You a work it all using the formula E=mc^2

Question 17

What happens when energy is converted into mass?

Answer 17

1. When energy is converted into mass you get equal amount of matter and antimatter
2. Fire two protons at each other an high speed and you’ll need up with a lot of energy at the point of impact
3. This energy might be converted into more particles
4. If an extra proton is formed then there will always be an antiproton to go with it
5. Its called pair production

Question 18

What is each particle-antiparticle pair produced from?

Answer 18

• Each particle-antiparticle pair is produced from a single photon
  1. Energy that gets converted into matter and antimatter is in the from of a photon
  2. Pair production only happens if one photon has enough energy to produce that much mass; only gamma ray photon have enough energy
  3. It also tends to happen near a nucleus, which helps to conserve momentum
  4. You usually get electron-positron pairs produced (rather than any other pair), because they have a relatively low mass

Question 19

What is the minimum energy for a photon to undergo pair production?

Answer 19

• The minimum energy for a photon to undergo pair production is the total rest energy of the particles produced
• The particle and antiparticle each have a rest energy of Esubscript0 so Emin=hfmin=2Esubscript0

Question 20

What is the opposite of pair production?

Answer 20

• The opposite of pair-production is annihilation
  1. When a particle meets its anti-particle the result is annihilation
  2. All the mass of the particle and antiparticle gets converted back to energy
  3. Antiparticles can usually inly exist for a fraction of a second before this happens, so you don’t get them in ordinary matter
  4. An annihilation is between a particle-antiparticle pair, which both have a rest energy Esubscript0
• Both photons need to have a minimum energy, Emin, which when added together equals at least 2E0 for energy to be conserved in this interaction
• So: 2Emin = 2E0 and Emin=hfmin = E0
• The electron and positron annihilate and they mass is converted into eh energy of a pair of gamma ray photons to conserve momentum

Question 21

What are forces caused by?

Answer 21

• Forces are caused by particle exchange
• You can’t have instantaneous action at a distance (accord to Einstein, anyway), so when two particles interact something must happen to let one particle know that the other one’s there and that’s the idea behind exchange particles
  1. Repulsion: each time the ball is thrown or caught the propel get pushed apart and it happens because the ball carries momentum
• Particle exchange also explains attraction, but you need a bit more imagination
  2. Attraction: each time a boomerang is thrown or caught the people get pushed together

Question 22

What are these exchange particles called?

Answer 22

• These exchange particles are called gauge bosons
• The repulsion between two protons is caused by the exchange of virtual photons, which are the gauge boson fo the electromagnetic force
• Gauge bosons are the virtual particle and they only exist for a very short time

Question 23

What are the four fundamental forces? What are their gauge bosons and particles affected?

Answer 23

1. Electromagnetic, virtual photon (symbol gamma) and charged particles only are affected
2. Weak, W+, W-, all types of particles are affected
3. Strong, pions (pi+, pi-, pi0) and hadrons are the only particles affected (acts between quarks)

Question 24

What happens the larger the mass of the gauge bosons?

Answer 24

• The larger the mass of the gauge boson, the shorter the range of forces
  1. The W bosons have a mass of about 100 times that of a proton, which gives the weak force a very short range. Creating a virtual W particle uses so much energy that it can only exist for a very short time and it can’t travel far
  2. On the other hand, the photon has zero mass, which gives you a force with infinite range

Question 25

How can you use diagrams to show whats going in and whats coming out?

Answer 25

• Particle interaction can be hard to get your head around, and a neat way of solving probes is by drawing simple diagrams of particle interactions rather than doing calculations
  1. Gauge bosons are represented by wiggly lines
  2. Other particles are represented by straight lines

Question 26

What are the rules for drawing particle interaction diagrams?

Answer 26

1. Particles are represented by straight lines with arrow heads drawn on them
2. Exchange articles are resented by wavy lines (A W- particle going to the left has the same effect as a W+ particle going to the right)
3. Incoming particles start at the bottom of the diagram and move upwards
4. The baryons and leptons can’t cross from one side to the other
5. Make sure the charges on both sides are balanced. The W bosons carry charge from one side of the diagram to the other

Question 27

What are hadrons?

Answer 27

• Hadrons are particles that feel the strong nuclear force (e.g. Protons and neutrons)
  1. The nucleus of an atom is made up of protons and neutrons
  2. Since the protons are positively caged they need a strong force to hold them together and this called the strong nuclear force or the strong interaction
  3. Not all particles can feel the strong nuclear force, the ones that CAN are called hadrons (leptons are an example of particles that can’t)
  4. Hadrons aren’t fundamental particles, and they are made up of smaller particles called quarks
  5. there are two types of hadrons: baryons (and anti-baryons) and mesons. They are classified according to the number of quarks that make them up

Question 28

What type of particle are protons and neutrons?

Answer 28

• Protons and neutrons are baryons
  1. it is helpful to think of protons and neutrons as two versions of the same particle-the nucleon, they just have different electric charges
  2. Protons and neutrons are both baryons
  3. There are other baryons that you don’t get in normal matter, like sigmas and they are shorts lived and you don’t need to know about them

Question 29

Why is a proton a special baryon?

Answer 29

1. The proton is the only stable baryon (they do not decay)
2. All baryons, except the proton are unstable
3. This means that they delay to become other particles
4. The particles a baryon ends up as depends on what it started as, but it always includes a proton
   - All baryons expect protons decay to a proton

Question 30

Are there anti baryons?

Answer 30

1. The antiparticles of protons and neutrons (antiprotons and antineutrons) are anti baryons
2. BUT antiparticles are annihilated when they meet hr e corresponding particle, which means that you don’t find antibaryons in ordinary matter

Question 31

Whta is the number of baryons in an interaction called?

Answer 31

• The number of baryons in an interaction is called the baryon number
• The total baryon number in any particle interaction never changes
  1. The baryon number is the number of baryons (a bit like nucleon number but including unusual baryons like sigma too)
  2. The proton and the neutrons each have a baryon number B=+1
  3. Antibaryons have a baryon number B=-1
  4. Other particles (i.e. things that aren’t baryons) are given a baryon number B=0
  5. Baryon number is a quantum number that must be conserved in any interaction and that means it can only take on a certain set of values
  6. When an interaction happens, the baryon number on either side of the interaction has to be the same and you can use this fact to predict whether an interaction will happen and if the numbers don’t match, it can’t happen

Question 32

What are neutrons in terms of baryons? What is formed when neutron decays?

Answer 32

• Neutrons are baryons that decay into protons
  1. Beta decay involved a neutron changing into a proton and this happens when there are more neutrons than protons in a nucleus or when a neutron is by itself, outside of a nucleus
  2. Beta decay is caused by the weak interaction
  3. When a neutron decays, it forms a proton, an electron and an antineutrino
• Electrons and antineutrinos are not baryons, they are leptons and so they have a baryon number B=0
• Neutrons and protons are baryons, so have have a baryon number B=1
• This means that the batons number of both sides are equal (to 1), so the interaction can happen

Question 33

What are mesons?

Answer 33

• The second type of hadron you need to know about is the meson
• The mesons you need to know about are pions and kaons
  1. All mesons are unstable and have a baryon number B=0 (because they are not baryons)
  2. Pions na kaons were discovered in cosmic rays, cosmic ray showers are a source of both particles and you can observe the tracks of these particles with a cloud chamber
  3. Mesons interact with baryons via the strong force

Question 34

What are pions?

Answer 34

1. Pions (pi-mesons) are the lightest mesons
2. You get three versions with different electric charges pi+, pi0,pi- and you get loads of pions in high energy particle collisions like those studied at the CERN particle accelerator

Question 35

What are kaons?

Answer 35

1. Kaons (K-mesons) are heavier and more unstable than pions
2. You get different ones like K+ and K0
3. Kaons have a very short lifetime and decay into pions

Question 36

What are leptons?

Answer 36

• Leptons (e.g. electrons and neutrinos) do not feel the strong nuclear force
  1. Leptons are fundamental particles and they don’t feel the strong nuclear force
  2. They only really interact with other particle;s via the weak interaction (along with a bit of gravitational force and the electromagnetic force as well if they’re charged)
  3. Electrons (e-) are stable and very family but there are also other leptons, such as the muon, that are just like heavy electrons
  4. Muons are unstable and decay eventually into ordinary electrons 5
  5. The electron and muon leptons each come with their own neutrino

Question 37

What are neutrinos?

Answer 37

1. Neutrinos have zero, or almost zero mass, and zero electric charge, so they don’t do much
2. Neutrinos only tea part in weak interactions
3. In fact, a neutrino can pass right through the Earth without anything happening to it

Question 38

How do you count the number of leptons?

Answer 38

• You have to count the types of lepton separately
  1. Just like the baryon number the lepton number is just the number of leptons
  2. Each lepton is given a lepton number of +1, but the electron and muon types of lepton have to be counted separately
  3. You get different types of lepton number, Lsubscripte and Lsubscriptmu
  4. All the leptons and lepton0neutrinos have they own antiparticle too
  5. They have the opposite charge and lepton number to their matching particles, for example the antiunion has charge +1, Le=0 and Lmu=-1

Question 39

Why is there no such thing as a free quark?

Answer 39

1. If you blasted a proton with enough energy you could not operate out the quarks
2. Your energy just gets changed into more quarks and antiquarks, it’s pair production again and you just make mesons
3. It is not possible to get a quark by itself, this is called quark confinement

Question 40

Why are we still searching for particles? How do we do it?

Answer 40

• As time goes on, our knowledge and understanding of particle physics changes
  1. New theories are created to try and explain observations from experiments and sometimes physicists hypothesise a new particle and the properties they expect to have e.g. the neutrino was hypothesised due to observations of beta decay
  2. Experiments to try and find the existence of this new particle are then carried out and results from different experiments are combined to try to confirm the new particle. if it exists the theory is more likely to be correct and the scientific community start to accept it-it’s validated
  3. It is not quite that simple though and experiments in particle physics often need particles travelling at incredibly high speeds (close to the speed of light) and this can only be achieved using particle accelerators, and these huge pieces of equipment are very expensive to build and run. This means that large groups of scientist and engineers form all over the world have to collaborate to be able to fund these experiments

Question 41

What is an example of searching for particles?

Answer 41

1. Paul Dirac predicted the existence of antimatter in 1928. His theory was validated with the observation of the positron and, over the years more and more observations of antiparticles and nowadays, its accept that antimatter exists, but there are still questions. For example, there should have been equal amounts of matter and antimatter created when the universe was formed, but almost everything we observe is made of matter
2. Scientist are trying to figure out what happened to all the antimatter by studying the differences in behaviours of matter and antimatter particles using the Larger Hadron Collider (LHC) at CERN. The LHC is a 17 mile log particle accelerator costing around £3 billion to build and £15 million per year to run. Some 10,000 scientists from 100 countries are involved
   - ATLAS, just one of many experiments the LHC at CERN is used for, involves around 3000 scientists from 38 different countries

Question 42

What does the the strong nuclear force do?

Answer 42

• The strong nuclear force binds nucleons together
  1. There are several different forces acting on the nucleons in a nucleus
  2. The two you already know about are electrostatic forces from the protons’ electric charges, and gravitational forces due to the masses of the particles
  3. If you do the calculations you find the repulsion from the electrostatic force is much much bigger than the gravitational attraction and if these were the only forces acting in the nucleus, the nucleons would fly apart
  4. So there must be another attractive force that holds the nucleus together, called the strong nuclear force

Question 43

What is the strong nuclear force?

Answer 43

1. The strong iteration is an extremely short range force (10-14 to 10-15 fm (the scale of the nucleus) and it acts between quarks)
2. The exchange particle is the gluon
3. It has a very high magnitude
4. acts between quarks so it is the force than holds nucleons together and it is also the force than holds the nuclei together
5. Experiments also show that the strong nuclear force works equally between all nucleons. This means that the size of the force is the same whether its proton-proton, neutron-neutron or proton-neutron
6. At very small separations, the strong nuclear force mart be repulsive or it would crush the nucleus to a point

Question 44

What does the size of the strong nuclear force vary with?

Answer 44

• The size of the strong nuclear force varies with nucleon separation
• The strong nuclear force can be plotted on a graphite show how it changes with the distance of separation between nucleons
• If the electrostatic force is also plotted you can see the relationship between these two forces
  1. The strong nuclear force is repulsive for very small separations of nucleons
  2. As nucleon operation increases past about 0.5 fm, the strong nuclear force become attractive, it reaches a maximum attractive value and then falls rapidly towards zero after about 3fm
  3. The electrostatic repulsive force extends over a much larger range (indefinitely actually)

Question 45

When does alpha emission happen?

Answer 45

• Alpha emission happens in very big nuclei
  1. Alpha emission only happens in very big nuclei, like uranium and radium
  2. The nuclei of these atoms are just too massive for the strong nuclear force to keep them stable
  3. When an alpha particle is emitted: the proton number decreases by two, and the nucleon number decreases by four
• Alpha particles have very short range-only a few cm in air. This can be seen by observing the tracks left by alpha particles in a cloud chamber. You could also use a Geiger counter (a device that measures the amount of ionising radiation). Bring it up close to the alpha source, the move it way slowly and observe how the count rate drops

Question 46

When does beta emission happen?

Answer 46

• Beta minus emissions happens in neutron-rich nuclei
  1. Beta-minus (usually just called beta) decay is the emission of an electron from the nucleus along with an antineutrino
  2. Beta decay happens in isotopes that are unstable due to being ‘neutron rich’ (i.e. they have too many more neutrons than protons in their nucleus)
  3. When a nucleus ejects beta particle, one of the neutrons in the nucleus is changed into proton
• The proton number increases by one and the nucleon number stays the same
• In beta decay, you can get a tiny neutral particle called an antineutrino relates. This antineutrino carries away some energy and momentum

Question 47

How were neutrinos first hypothesised?

Answer 47

• Neutrinos were first hypothesised our to observations of beta decay
  1. Scientists originally thought that the only particle emitted from the nucleus during beta decay was an electron
  2. However, observations showed that the very of the particles after the beta decay was less that it was before, which didn’t fit with the principle of conservation of energy
  3. In 1930 Wolfgang Pauli suggested another particle was being emitted too, and it carried away the missing energy. This particle has to be neutral (or charge wouldn’t be conserved in beta decay) and had to have zero or almost zero mass (as it had never been detected)
  4. Other discovers led to Pauli’s theory becoming accepted and the particle was named the neutrino (we now know this particle was an antineutrino)
  5. The neutrino was eventually observed 25 years later, providing evidence for Pauli’s hypothesis

Question 48

Why is gravity usually ignored?

Answer 48

1. Particle physicists never bother about gravity because it’s so incredibly feeble compared with other types of interaction
2. Gravity only really matter when you’ve got bug masses like stars and planets
3. In the strong nuclear force, pions are described as being exchanged between nucleons and you might also see it describe as gluons being exchanged between quarks

Question 49

What are gluons?

Answer 49

1. Most of the mass of a proton is due to the energy of the interactions between the quarks and the gluons that hold the quarks together via the strong nuclear force
2. These gluons come into and go out of existence as they exchange between quarks
3. The energy required to do this is included in the mass of the proton or the neutron
4. The up and down quark are much less massive than the other quarks

Question 50

What is the strange quark?

Answer 50

The strange quark is observed in particles in high altitude due to tot he interaction of high energy cosmic rays within the upper atmosphere

Question 51

What is deep inelastic scattering?

Answer 51

• It involves firing electrons at protons at very high energies
• Elastic scattering fore compile involved two particle such as two protons colliding with each other and rebounding off each other with the same kinetic energy and the word ‘elastic’ in this case means that no kinetic energy is lost
• Inelastic scattering involse the conversion of kinetic energy into other forms of energy and in this case the electrons penetrate into the proton ad interact with the quarks (via the exchange photon), kinetic energy is converted into mass as the proton shatters, producing a shower of other particles

Question 52

What are exchange particles?

Answer 52

Exchange particles are particles involved with the interaction of particles via the four fundamental forces of nature on the quant scale. Exchange particles are only created, emitted and absorbed and destroyed between the interacting particles

Question 53

What is the electromagnetic force?

Answer 53

1. On the quantum scale, two protons can interact with each other by exchanging virtual (exist for minute amounts of time) photons via the electromagnetic interaction
2. The photons are exchanged in both directions and exit for only minute amounts of time
3. The photons acts over ‘infinite distances’, however the strength of the forced decrease with an inverse square relationship of distance 1/r^2
4. This means that if the distance between the particles is doubles the force is reduced by a factor of four

Question 54

Why are the photons called virtual photons?

Answer 54

They are created, interact and decay all within the time of interaction as so only exist during the time of interaction

Question 55

What is the weak force?

Answer 55

• The Weak force is known to be responsible for Beta minus and Beta plus radioactive decay, electron capture and electron-proton collisions
• Three exchange particles: W+, W- and Z0
• About. a million times weaker than the strong nuclear force
• Acts at about 10-18 m (about 0.1% of the diameter of a proton)

Question 56

What is gravity?

Answer 56

• Fundamental force and it acts over indicate distances and it acts between masses
• Weakest of four fundamental forces
• Exchange particle is graviton

Question 57

What happens as two electrons scatter off each other?

Answer 57

1. Two electrons meet, exchange photons and scatter away from each other
2. The photon symbol gamma indicated that this is an example of electromagnetic interaction

Question 58

What happens in B+ decay?

Answer 58

1. A proton decays in to a neutron nd a W+ exchange article which subsequently decays into a positron and an electron neutrino
2. Example of weak interaction

Question 59

What happens in B- decay?

Answer 59

1. A neutron decaying into a apron and a W- exchange particle, which subsequently decays into an electron and an electron antineutrino
2. Example of weak interaction

Question 60

What is muon decay?

Answer 60

Muons are unstable and decay via the weak interaction into three electron and one of which is a electron(or positron) and the other two are neutrinos