Download SET 2 Option J — Particle physics J1. This question is about

SET 2
Option J — Particle physics
J1.
This question is about fundamental interactions.
The diagram below shows two fundamental interaction vertices, one for the electromagnetic
and the other for the weak interactions.
Z
photon
charged
fermion
charged
fermion
Electromagnetic interaction
(a)
charged
fermion
charged
fermion
Weak interaction
Explain why in both vertices the incoming and the outgoing charged fermions above must
have the same electric charge.
[2]
.......................................................................
.......................................................................
(b)
The process e   e        occurs mostly via the electromagnetic interaction and
less frequently via the weak interaction.
Draw a Feynman diagram for the process e  e        according to
(i)
the electromagnetic interaction.
[1]
(ii)
the weak interaction.
[1]
(This question continues on the following page)
2209-6509
3640
(Question J1 continued)
(iii) Identify the virtual particles in each of the Feynman diagrams that you drew in (i)
and (ii).
Virtual particle in (i):
[1]
...............................................
Virtual particle in (ii): . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
(c)
State one reason why the process e  e        is more likely to involve the
electromagnetic interaction rather than the weak interaction.
[1]
.......................................................................
.......................................................................
(d)
Electrons are accelerated to a total energy E and then collide with stationary positrons.
Determine the minimum total energy E of the electron for the reaction e   e       
to take place.
Electron rest mass = 0.511 MeVc–2.
Muon rest mass = 105 MeVc–2.
[3]
.......................................................................
.......................................................................
.......................................................................
.......................................................................
Turn over
2209-6509
3740
J2.
This question is about particle dectors.
(a)
(i)
Describe the operating principle of the bubble chamber.
[2]
..................................................................
..................................................................
..................................................................
..................................................................
(ii)
Outline how two particle properties or characteristics are measured using a
bubble chamber.
[2]
..................................................................
..................................................................
..................................................................
..................................................................
..................................................................
(b)
The proportional wire spark chamber has now replaced the bubble chamber.
Outline two advantages of this detector compared to the bubble chamber.
.......................................................................
.......................................................................
.......................................................................
.......................................................................
2209-6509
3840
[2]
J3.
This question is about quarks.
The diagram below shows the eight spin 12 baryons made out of the three lightest quarks,
the up (u), the down (d) and the strange (s). In this plot baryons belonging to the same
horizontal line have the same strangeness (S) and those along the same slanted line have the
same charge (Q).
Q = –1
Q=0
Q=1
S=0
S = –1
S = –2
(a)
(i)
On the diagram above draw a circle around the point representing the neutron.
[1]
(ii)
State the quark content of the neutron.
[1]
..................................................................
(b)
The   baryon is unstable and decays according to the reaction     0   .
The quark content of the particles involved is    ssd ,  0  uds and   du .
State and explain whether the interaction involved in this decay is electromagnetic,
strong or weak.
[3]
.......................................................................
.......................................................................
.......................................................................
Turn over
2209-6509
3940
J4.
This question is about cosmology and strings.
(a)
It is assumed that in the first 10–2 s after the Big Bang quarks behaved as free particles
that could not bind into nucleons.
Suggest a reason for this.
[2]
.......................................................................
.......................................................................
.......................................................................
(b)
At a time of 10–2 s after the Big Bang the average thermal energy per particle in the
universe was approximately 50 MeV.
Estimate the temperature of the universe 10–2 s after the Big Bang.
[2]
.......................................................................
.......................................................................
(c)
In the very early universe it is thought that the total number of particles was only very
slightly larger than the number of antiparticles.
Explain why the matter in the present universe is made predominantly by particles and
not antiparticles.
[2]
.......................................................................
.......................................................................
.......................................................................
(d)
Outline why physicists were led to consider string theories.
[2]
.......................................................................
.......................................................................
(e)
Many string theories suggest that space is 10 dimensional rather than the usual 3+1
dimensions (3 for space and 1 for time).
Assuming that string theory is correct, explain why we are not aware of the extra
dimensions.
.......................................................................
.......................................................................
4040
[2]
Option J — Particle physics
J1.
(a)
(b)
electric charge must be conserved at each vertex;
and both the photon and the Z are neutral;
(i)
[2]
μ+
antimuon
e+
positron
[1]
photon
electron
e–
(ii)
muon
μ–μ+
e+
positron
antimuon
[1]
Z
electron
e–
muon
μ–
Award [0] if arrow directions are not consistent with labels.
(iii)
virtual particle in (i): photon;
virtual particle in (ii): Z;
(c)
need both correct to award the mark
[1]
because the strength of the electromagnetic interaction is not the same/is greater
than the strength of the weak interaction;
or
(d)
the weak interaction diagram contains a very heavy intermediate particle as
opposed to a massless particle in the electromagnetic interaction;
[1]
the energy needed to create the muons at rest is 2  105  210 MeV ;
210 2  2  0.511  E  2  0.5112 ;
giving E  43 GeV ;
[3]
4041
J2.
(a)
(i)
charged particles (moving through hydrogen near its boiling point) ionize
atoms;
visible bubbles appear along path of the particle;
[2]
the sign of the electric charge by seeing which way the particle bends in the
magnetic field;
the momentum by measuring the radius of the circular path (and knowing
that most particles have a charge equal to  e);
the rate of energy loss from the density of the bubbles;
[2 max]
no photographs need to be taken (whose analysis is extremely difficult and time
consuming);
there is no “dead time” as in a bubble chamber in between successive
photographs;
the information is digital and so can be processed easier;
[2 max]
(ii)
(b)
J3.
(a)
(b)
(i)
circle around the point with Q = S = 0;
[1]
(ii)
udd;
[1]
weak;
the reaction violates strangeness;
the other interactions conserve strangeness/only the weak interaction violates
strangeness;
or
weak;
the reaction changes quark flavour;
and only the weak does that;
[3]
4042
J4.
(a)
the average kinetic energy of the quarks was very high because the temperature
was very high;
the quarks moved with too much kinetic energy that prevented them from binding
/ the average kinetic energy was greater than the binding energy of the nucleons;
4043
[2]
(b)
4044
(2)
(c)
(d)
(e)
in the early universe/when the temperature was very high particles collided with
antiparticles to create photons and the photons materialized into particleantiparticle pairs;
As the temperature dropped, the second process became impossible and so the
universe was left with (the small number of excess particles);
[2]
there has been a long attempt to provide a quantum theory of gravitation;
but all theories based on point particles have failed;
[2]
the extra dimensions are supposed to be curved into a compact/closed/
finite/Calabi-Yau space;
whose typical linear size is unobservably small;
[2]
4045