Quarks are
the elementary constituents from which Hadrons
(baryons and
mesons) are made. There are three main types
(or flavors) of quark known as Up,
Down and Strange.
Each type of quark has different properties of mass, charge, baryon number
and strangeness as shown in the table below.
























There are also three
other much more massive quarks: Charm, Top and Bottom (sometimes called
Truth and Beauty) but these are much rarer and the Top/Truth quark was
only discovered in 1997.
Quarks also have a
corresponding antiquark which has similar properties but opposite
and equal charge. Antiquarks have the same symbol as the quark but with
an underline i.e. an antidown has the symbol d.
Any particle containing
quarks is termed a Hadron, however
quarks do not ever exist on their own but bound into quarkantiquark
pairs or triplets by the strong
force. This is known as quark confinement.
In a triplet there
will be three quarks (q q q) or
three antiquarks (qqq
) but never a mixture, for example a proton consists of two up quarks and
a down quark, these are called Baryons. In a pair there will always
be one quark and one antiquark (q
q )but never two
quarks or two antiquarks, an example of this is the Pion, these are caled
Mesons.
.
The following table
shows the quark configuration and properties of some common Hadrons (These
are the ones relevant to the AQA ALevel syllabus 2001)




















































During a nuclear reaction properties such as charge (Q), baryon number (B) and Strangeness (S) are conserved.
In a reaction involving the strong force all three (Q,B,S) are conserved. But in reactions involving the weak force only charge and baryon number are conserved.
From Hadrons 1
2) K^{+} > p^{0} + m^{} + n_{m} cannot occur because conservation of charge means that there cannot be a +1 charge on one side and a 0 + (1) +0 = 1 charge on the other.
3) n > p + e^{} + n_{e}
Charges:
LHS has Q = 0 charge
RHS has Q = (+1) +
(1) + 0 = 0 charge
Therefore charge is
conserved
Baryon number:
LHS has B = 1
RHS has B = 1 + 0
+ 0 = 1
Therefore baryon number
is conserved
Lepton number:
LHS has L = 0
RHS has L = 0 + (+1)
+ (1) = 0
Therefore lepton number
is conserved
5) The particle tracks are due to a magnetic field perpendicular to the plane of the particle motion. In accordance with the left hand rule this causes the +ve particles (e^{+}) to curve anticlockwise and so generally be on the right of the bubble chamber. However ve particles (e^{}, W^{} and p^{ }) rotate clockwise and so tend to be on the right of the bubble chamber. A particle with no charge ( p, K^{0}, X^{0} and L^{0 }) will have a straight path and will leave no visible track. The more massive the particle ( p = BqR) the larger the initial radius of the curve, so low mass charged particles will form a tightly wound spiral.
Charge, lepton
number and baryon number are conserved for all the events, here are two
examples:
Charge: Before: 1 + 1 = 0 After: 1 + 1 + 0 = 0 Baryon number: Before: 1 + 0 = 1 After: 1 + 0 + 0 = 1 Lepton number: Before: 0 + 0 = 0 After: 0 + 0 + 0 = 0 Therefore Q, B and L are conserved. 
Charge: Before: 1 After: 1 + 0 = 1 Baryon number: Before: 1 After: 1 + 0 = 1 Lepton number: Before: 0 After: 0 + 0 = 0 
7) This is likely to be a strong interaction because Q,B,L and S are all conserved.
8) This is a
weak interaction as strangeness changes by +1 in each decay. Remember
strangeness only changes in weak interactions and is conserved in strong
and electromagnetic interactions.
10) Time for light to cross an atom:
Diameter of atom:
approx 10^{9} m
Speed of light: approx
3x10^{8} ms^{1}
Therfore time taken to cross atom = 10^{17} s
The lifespan of a L^{0} hyperon is about 10^{10}. From above it can be seen that light can cross an atom 10 million times in the lifetime of a L^{0} hyperon. So in some respects it lives quite a long time. Elementary particle resonances may be as short lived as 10^{23} seconds !
Distance = 0.5
x 3x10^{8} x 1.2x10^{10}
= 0.018 m
12) These energies
have been calculated using relativistic mechanics.
See Eg: Relativity
Problems 2 . Click here
to see the relativistic formula for kinetic energy.
Speed /c  0.1  0.5  0.9  0.95  0.99  0.995  
KE (J)  Proton  8x10^{13}  2x10^{11}  2x10^{10}  3x10^{10}  9x10^{10}  1x10^{9} 
Truth quark  1x10^{11}  5x10^{10}  5x10^{9}  8x10^{9}  2x10^{8}  3x10^{8} 
The truth quark on its own has a K.E. 23 times that of a proton at comparable speeds.