Please print the last picture of work sheet and analysis of a bubble chamber picture ,use rule,protractor to measure the picture you print.

and strictly follow the worksheet calculate with data you measured and write a lab report with showing all your measurement and calculation,discussion, and a perfect conclusion.

1

PHY 4822L (Advanced Laboratory):

Analysis of a bubble chamber picture

Introduction

In this experiment you will study a reaction between “elementary particles” by analyzing their

tracks in a bubble chamber. Such particles are everywhere around us [1,2]. Apart from the standard

matter particles proton, neutron and electron, hundreds of other particles have been found [3,4],

produced in cosmic ray interactions in the atmosphere or by accelerators. Hundreds of charged

particles traverse our bodies per second, and some will damage our DNA, one of the reasons for the

necessity of a sophisticated DNA repair mechanism in the cell.

2

Figure 1: Photograph of the interaction between a high-energy π–meson from the Berkeley

Bevatron accelerator and a proton in a liquid hydrogen bubble chamber, which produces two neutral

short-lived particles Λ0 and K0 which decay into charged particles a bit further.

Figure 2: illustration of the interaction, and identification of bubble trails and variables to be

measured in the photograph in Figures 3 and 4.

The data for this experiment is in the form of a bubble chamber photograph which shows bubble

tracks made by elementary particles as they traverse liquid hydrogen. In the experiment under

study, a beam of low-energy negative pions (π- beam) hits a hydrogen target in a bubble chamber.

A bubble chamber [5] is essentially a container with a liquid kept just below its boiling point (T=20

K for hydrogen). A piston allows expanding the inside volume, thus lowering the pressure inside

the bubblechamber. When the beam particles enter the detector a piston slightly decompresses the

liquid so it becomes “super-critical” and starts boiling, and bubbles form, first at the ionization

trails left by the charged particles traversing the liquid.

The reaction shown in Figure 1 shows the production of a pair of neutral particles (that do not leave

a ionized trail in their wake), which after a short while decay into pairs of charged particles:

π – + p → Λo + Ko,

3

where the neutral particles Λo and Ko decay as follows:

Λo → p + π-, Ko → π+ + π-.

In this experiment, we assume the masses of the proton (mp = 938.3 MeV/c2) and the pions (mπ+ =

mπ- = 139.4 MeV/c2) to be known precisely, and we will determine the masses of the Λ0 and the K0,

also in these mass energy units.

Momentum measurement

In order to “reconstruct” the interaction completely, one uses the conservation laws of (relativistic)

momentum and energy, plus the knowledge of the initial pion beam parameters (mass and

momentum). In order to measure momenta of the produced charged particles, the bubble chamber is

located inside a magnet that bends the charged particles in helical paths. The 1.5 T magnetic field is

directed up out of the photograph. The momentum p of each particle is directly proportional to the

radius of curvature R, which in turn can be calculated from a measurement of the “chord length” L

and sagitta s as:

r = [L2/(8s)] + [s/2] ,

Note that the above is strictly true only if all momenta are perfectly in the plane of the photograph;

in actual experiments stereo photographs of the interaction are taken so that a reconstruction in all

three dimensions can be done. The interaction in this photograph was specially selected for its

planarity.

In the reproduced photograph the actual radius of curvature R of the track in the bubble chamber is

multiplied by the magnification factor g, r = gR. For the reproduction in Figure 3, g = height of

photograph (in mm) divided by 173 mm.

The momentum p of the particles is proportional to their radius of curvature R in the chamber. To

derive this relationship for relativistic particles we begin with Newton’s law in the form:

F = dp/dt = e v×B (Lorentz force).

Here the momentum (p) is the relativistic momentum m v γ, where the relativistic γ-factor is defined

in the usual way

γ = [√(1- v 2/c2)]-1.

Thus, because the speed v is constant:

F = dp/dt = d(mvγ)/dt = mγ dv/dt = mγ (v2/R)(-r) = e v B (-r) ,

where r is the unit vector in the radial direction. Division by v on both sides of the last equality

finally yields:

mγv /R = p /R = e B ,

identical to the non-relativistic result! In “particle physics units” we find:

p c (in eV) = c R B , (1)

4

thus p (in MeV/c) = 2.998•108 R B •10-6 = 300 R (in m) B (in T)

Measurement of angles

Draw straight lines from the point of primary interaction to the points where the Λ0 and the K0

decay. Extend the lines beyond the decay vertices. Draw tangents to the four decay product tracks at

the two vertices. (Take care drawing these tangents, as doing it carelessly is a source of large

errors.) Use a protractor to measure the angles of the decay product tracks relative to the parent

directions (use Fig. 3 or 4 for measurements and Fig. 2 for definitions).

Note: You can achieve much better precision if you use graphics software to make the

measurements, rather than ruler and protractor on paper. Examples of suitable programs are

GIMP or GoogleSketchUp, both of which can be obtained for free.

Analysis

The laws of relativistic kinematics relevant to this calculation are written below. We use the

subscripts zero, plus, and minus to refer to the charges of the decaying particles and the decay

products.

p+sinθ+ = p-sinθ-

(2)

p0 = p+cosθ+ + p-cosθ-

(3)

E0 = E+ + E-,

where E+ = √(p+2c2 + m+2c4) , and E- = √(p-2c2 + m-2c4)

m0c2 = √(E02 – p02c2)

Note that there is a redundancy here. That is, if p+, p-, θ+, and θ- are all known, equation (2) is not

needed to find m0. In our two-dimensional case we have two equations (2 and 3), and only one

unknown quantity m0, and the system is over-determined. This is fortunate, because sometimes (as

here) one of the four measured quantities will have a large experimental error. When this is the case,

it is usually advantageous to use only three of the variables and to use equation (2) to calculate the

fourth. Alternatively, one may use the over-determination to “fit” m0, which allows to determine it

more precisely.

A. K0 decay

1. Measure three of the quantities r+, r-, θ+, and θ-. Omit the one which you believe would

introduce the largest experimental error if used to determine mK. Estimate the uncertainty of

your measurements.

2. Use the magnification factor g to calculate the actual radii R and equation (1) to calculate the

momenta (in MeV/c) of one or both pions.

3. Use the equations above to determine the rest mass (in MeV/c2) of the Ko.

4. Estimate the error in your result from the errors in the measured quantities.

5.

5

Β. Λo decay:

1. The proton track is too straight to be well measured in curvature. Note that θ+ is small and

difficult to measure, and the value of mΛ is quite sensitive to this measurement. Measure θ+, r-

and θ-. Estimate the uncertainty on your measurements.

2. Calculate mΛ and its error the same way as for the Ko.

3. Estimate the error in your result from the errors in the measured quantities.

4. Finally, compare your values with the accepted mass values (the world average) [3], and

discuss.

C. Lifetimes:

Measure the distance traveled by both neutral particles and calculate their speed from their

momenta, and hence determine the lifetimes, both in the laboratory, and in their own rest-frames.

Compare the latter with the accepted values [3]. Estimate the probability of finding a lifetime value

equal or larger than the one you found.

References:

[1] G.D. Coughlan and J.E. Dodd: “The ideas of particle physics”, Cambridge Univ. Press,

Cambridge 1991

[2] “The Particle Adventure”, http://particleadventure.org/

[3] Review of Particle Physics, by the Particle Data Group, Physics Letters B 592, 1-1109

(2004) (latest edition available on WWW: http://pdg.lbl.gov )

[4] Kenneth Krane: Modern Physics, 2nd ed.; John Wiley & Sons, New York 1996

[5] see, e.g. K. Kleinknecht: “Detectors for Particle Radiation”, Cambridge University Press,

Cambridge 1986;

R. Fernow: “Introduction to Experimental Particle Physics”, Cambridge University Press,

Cambridge 1986;

W. Leo: Techniques for Nuclear and Particle Physics Experiments :

A How-To Approach; Springer Verlag, New York 1994 (2nd ed.)

Note: Experiment adapted from PHY 251 lab at SUNY at Stony Brook (Michael Rijssenbeek)

6

Figure 3 : Photograph of the interaction between a high-energy π–meson from the Berkeley

Bevatron accelerator and a proton in a liquid hydrogen bubble chamber. The interaction produces

two neutral particles Λ0 and K0, which are short-lived and decay into charged particles a bit further.

The photo covers an area (H•W) of 173 mm • 138 mm of the bubble chamber. In this enlargement,

the magnification factor g = (height (in mm) of the photograph )/173 mm.

7

Fig. 4: Negative of picture shown in Fig. 3

Measurement

of

the

angle

θ

For

better

understanding

I

am

showing

you

a

different

particle

track

diagram

bellow.

Where

at

point

C

particle

𝜋! 𝑎𝑛𝑑 Σ!

are

created

and

the

Σ!

decays

into

𝜋∓ 𝑎𝑛𝑑 K!

particles

The

angle

θ

between

the

π−

and

Σ−

momentum

vectors

can

be

determined

by

drawing

tangents

to

the

π−

and

Σ−

tracks

at

the

point

of

the

Σ−

decay.

We

can

then

measure

the

angle

between

the

tangents

using

a

protractor.

Alternative

method

which

does

not

require

a

protractor

is

also

possible.

Let

AC

and

BC

be

the

tangents

to

the

π−

and

Σ−

tracks

respectively.

Drop

a

perpendicular

(AB)

and

measure

the

distances

AB

and

BC.

The

ratio

AB/BC

gives

the

tangent

of

the

angle180◦−θ.

It

should

be

noted

that

only

some

of

the

time

will

the

angle

θ

exceed

90◦

as

shown

here.

Determining

the

uncertainty

of

Measurements

In

part

B,

It

is

asked

to

estimate

the

uncertainty

of

your

measurements

of

𝜃

and

r.

Uncertainty

of

measurement

is

the

doubt

that

exists

about

the

result

of

any

measurement.

You

might

think

that

well-‐made

rulers,

clocks

and

thermometers

should

be

trustworthy,

and

give

the

right

answers.

But

for

every

measurement

-‐

even

the

most

careful

-‐

there

is

always

a

margin

of

doubt.

It

is

important

not

to

confuse

the

terms

‘error’

and

‘uncertainty’.

Error

is

the

difference

between

the

measured

value

and

the

‘true

value’

of

the

thing

being

measured.

Uncertainty

is

a

quantification

of

the

doubt

about

the

measurement

result

Since

there

is

always

a

margin

of

doubt

about

any

measurement,

we

need

to

ask

‘How

big

is

the

margin?’

and

‘How

bad

is

the

doubt?’

Thus,

two

numbers

are

really

needed

in

order

to

quantify

an

uncertainty.

One

is

the

width

of

the

margin,

or

interval.

The

other

is

a

confidence

level,

and

states

how

sure

we

are

that

the

‘true

value’

is

within

that

margin.

You

can

increase

the

amount

of

information

you

get

from

your

measurements

by

taking

a

number

of

readings

and

carrying

out

some

basic

statistical

calculations.

The

two

most

important

statistical

calculations

are

to

find

the

average

or

arithmetic

mean,

and

the

standard

deviation

for

a

set

of

numbers.

The

‘true’

value

for

the

standard

deviation

can

only

be

found

from

a

very

large

(infinite)

set

of

readings.

From

a

moderate

number

of

values,

only

an

estimate

of

the

standard

deviation

can

be

found.

The

symbol

s

is

usually

used

for

the

estimated

standard

deviation.

Suppose

you

have

a

set

of

n

readings.

Start

by

finding

the

average:

For

the

set

of

readings

x={16,

19,

18,

16,

17,

19,

20,

15,

17

and

13},

the

average

is

𝑥 = !!

!

=

17.

Next

find

(𝑥! − 𝑥)!

Then

𝑠 =

(!!!!)!

!

!!!

!!!

= 2.21

To

calculating

standard

uncertainty

u

when

a

set

of

several

repeated

readings

has

been

taken,

use

𝑢 =

𝑠

𝑛

(The

standard

uncertainty

of

the

mean

has

historically

also

been

called

the

standard

deviation

of

the

mean,

or

the

standard

error

of

the

mean.)

Lifetime

calculation

In

part

C

you

are

asked

to

determine

the

life

time

of

the

neutral

particles

from

their

momentums.

The

Σ−

lifetime

can

be

approximately

determined

using

the

measured

values

of

the

Σ−

track

lengths.

The

average

momentum

of

the

Σ−

particle

can

be

found

from

its

initial

and

final

values:

𝑝! =

1

2

(𝑝!! + 𝑝!!)

Where

𝑝!! 𝑎𝑛𝑑 𝑝!!

are

initial

and

final

momentums

of

Σ

particle.

And

can

be

found

using

the

measured

track

length

𝑙!.

The

length

of

time

that

the

Σ−lives(the

time

between

its

creation

and

decay)

is

𝑡 =

𝑙!

𝑣

𝑐𝑜𝑜𝑟𝑑𝑖𝑛𝑎𝑡𝑒 𝑠𝑦𝑠𝑡𝑒𝑚 𝑎𝑡 𝑟𝑒𝑠𝑡 𝑖𝑛 𝑡ℎ𝑒 𝑙𝑎𝑏

𝑡 =

𝑙!

𝑣

1 −

𝑣!

𝑐!

𝑐𝑜𝑜𝑟𝑑𝑖𝑛𝑎𝑡𝑒 𝑠𝑦𝑠𝑡𝑒𝑚 𝑚𝑜𝑣𝑖𝑛𝑔 𝑤𝑖𝑡ℎ 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒

𝑡 =

𝑚!𝑙!

𝑝!𝑐