Transcript E 2

Identifying particles & their
properties in detectors
An inquiry based learning scenario – for able A Level students
who have recently studied the particle physics content
-an extension & enhancement
or
use as a practice for synoptic questions at the end of the
course
Time estimate/suggestion: ½ hr intro in one lesson +
homework + 1 ½ hrs next lesson
Phase 1:Question
Eliciting Activities
A brief introduction to be
done in class
• Stimulate curiosity
• Define questions from current
knowledge
What particles have we been
learning about?
and
Hadrons
• Baryons
- made up of 3 quarks
eg protons & neutrons
•
Mesons
- made up of 2 quarks
eg pions & kaons
Leptons
……Which also have
their antiparticles
but no sub structure
Force carriers – Bosons
• We have a theory that
the forces that particles
experience arise from
exchange of particles
called bosons
- g photons for em forces
- g gluons for the strong force
between quarks
- W & Z for the weak force
which explains things like b
decay & nuclear reactions in
stars…
And we know…
•
•
•
•
Energies
Masses
Charges
Momentum
& conservation rules (charge, baryon
number, lepton number, strangeness…)
for particle interactions….
What evidence do we have for
this?
• Physicists have
designed and carried
out experiments
• Collected and
analysed data
• Using particle
detectors
A brief history……
• As an a particle passes
through a cloud chamber, it
collides with the the gas
particles inside, ionising
atoms.
• The supercooled,
supersaturated alcohol
vapour condenses around
the ions formed leaving a
vapour trail to show up the
particle path.
Spark chambers were widely used
in the 1970s…
• Spark-chamber detectors consist
of metal plates placed in a sealed
box filled with a gas such as
helium, neon or a mixture of the
two. As a charged particle travels
through the detector, it will ionise
atoms of the gas between the
plates. At the same time,
detectors above and below the
chamber activate a high voltage
to the plates to create an electric
field, producing sparks on the
particle’s exact trajectory.
Bubble chambers were more useful
research tools…..
• The bubble chamber, invented
by Donald Glaser in 1952,
consists of a tank of unstable
(superheated) liquid – for
example, hydrogen or a mixture
of neon and hydrogen at a
temperature of about 30K. This
liquid is very sensitive to the
passage of charged particles,
which initiate boiling as a result
of the energy they deposit by
ionising the atoms as they force
their way through the liquid.
• The liquid is prepared and
held under a pressure of
about 5 atmospheres
(1atm=105 Pa).
• Just before the beam arrives
from the accelerator, the
pressure is reduced to about
2 atmospheres making the
liquid superheated.
• As charged beam particles
pass through the liquid they
deposit energy by ionising
atoms and this causes the
liquid to boil along their
paths.
• Some beam particles may also
collide with an atomic nucleus ( a
proton in a hydrogen atom) – this is
what we want to study - and the
charged particle products of such
interactions also ionise the liquid
causing trails of bubbles to form.
• The bubbles formed are allowed to
grow for a few ms, and when they
have reached a diameter of about 1
mm, a flash photograph is taken
(on several views so as to enable
the interactions to be reconstructed
in 3-dimensions).
• The pressure is then increased
again to clear the bubbles and
await the arrival of the next burst of
beam particles.
Modern detectors are more complex
and rely on electronics & computer
technology….
• But work with similar underlying principles
What are the principles used?
• Ionisation of a medium to show the paths
of charged particles
• Magnetic fields to exert forces on charged
particles and so bend their paths – to
identify charge and enable momentum to
be calculated
• Absorbing materials to stop particles and
so enable energy to be calculated
Phase 2.1: Define questions
from current knowledge
• Which particles can we detect – are there any
we can’t ?
• How do we find their
- trajectory
- charge
- momentum
- energy ?
• What characteristics do we look for in the
particle tracks to identify which particle it is?
Phase 2.2: Plan & conduct
simple investigations
A homework suggestion
GOALS:
• To learn more about detectors and the
characteristics of particle paths in them
• To make some observations and
measurements
PREPARATION
Explore the physics of bubble chambers at:
http://teachers.web.cern.ch/teachers/archiv/HST2005/bu
bble_chambers/BCwebsite/index.htm
&
Explore the physics of the ATLAS detector at:
http://atlas.ch/
- Click on “multimedia” and then “how atlas works” and
“animated clips”
- Click on “e-tours” and look at these too.
• Start the Minerva software from the portal
toolbox – click on “toolbox” at the top of
the page, scroll down to Minerva 2D
analysis tool, and click the “start Minerva”
red box.
• Read through the introduction, using the
forward and backward arrows, and work
through the 5 tutorial examples.
• Check your score…and try more examples
if you have time!
Phase 2.3: Propose preliminary
explanations
Teacher coordinated activity in following lesson
STUDENTS FEED BACK SOME POSSIBLE
EXPLANATIONS TO THE QUESTIONS
POSED PREVIOUSLY:
Which particles can we detect – are
there any we can’t ?
• Most particles can be
detected by various
sections of a modern
detector
• Neutrinos have no
charge and very little
mass and rarely interact
with matter – we detect
their presence only by
noting “missing” energy
& momentum in
collisions
Typical detector parts
What characteristics do we look for in the
particle tracks to identify which particle it is?
• Small charged particles,
like electrons & positrons,
leave tracks in the
tracking chamber (where
magnetic fields are also
applied to enable
momentum
measurement) and
deposit all of their energy
in the em calorimeter,
where it can be
measured.
• Neutral particles, like a
photon, can deposit
energy in the em
calorimeter, but leave no
track in the tracking
chamber
…………….
•
•
•
Charged particles, consisting of
quarks, like protons, leave tracks in
the tracking chamber (where a
magnetic field is also applied to
enable momentum measurement)
and deposit their energy in the
hadronic calorimeter, where it can be
measured.
Neutral particles, consisting of
quarks, like neutrons, also deposit
energy in the hadronic calorimeter,
but leave no track in the tracking
chamber
Muons pass through all the detector
layers, leaving tracks, and depositing
small amounts of energy in all
calorimeters. In the muon
spectrometer, a large magnetic field
is applied which enables momentum
measurement.
Interactions of particles with the detectors Summary
e+
n leaves no track at all
How do we find the particle
- trajectory
• By the ionisation it causes in the matter through which it
passes.
• Tracking devices reveal the paths of electrically charged
particles through the trails they leave behind. There are
similar every-day effects: high-flying airplanes seem
invisible, but in certain conditions you can see the trails
they make. In a similar way, when particles pass through
suitable substances the interaction of the passing
particle with the atoms of the substance itself can be
revealed.
• Most modern tracking devices do not make the tracks of
particles directly visible. Instead, they produce tiny
electrical signals that can be recorded as computer data.
A computer program then reconstructs the patterns of
tracks recorded by the detector, and displays them on a
screen.
How do we find the particle
- charge?
• The charge on a particle can be determined by
the curvature of its path in a magnetic field
eg electron path in a
bubble chamber (Electrons
spiral because they are much
lighter than all other charged
particles and lose energy quickly
by another process called
bremsstrahlung )
A positron, with opposite
charge, would spiral in
the opposite direction
Motion of charged particle in
magnetic fields
• The direction of the force on the particle is
determined by Fleming’s Left hand Rule:
The current direction is the
direction
in which a POSITIVE charge is
travelling.
For a negative charge, this
direction is reversed, which
reverses the force direction
This force provides a centripetal
force from which we can deduce
particle momentum
• F = Bqv
• F = mv2 / r
➱ mv2 / r = Bqv
and momentum
P = mv = Bqr
Hence a particle’s
momentum can be
calculated from the
radius of curvature
of its path
– this
happens in the tracking
chambers of all
detectors
How do we find the particle
- energy?
• A calorimeter measures the energy lost by a
particle that goes through it. It is usually
designed to entirely stop or ‘absorb’ all of the
particles coming from a collision, forcing them to
deposit all of their energy within the detector.
• Calorimeters typically consist of layers of
‘passive’ or ‘absorbing’ high–density material
(lead for instance) interleaved with layers of
‘active’ medium such as liquid argon.
.
• Electromagnetic calorimeters measure the energy of light particles –
electrons and photons – as they interact with the electrically charged
particles inside matter.
e-
High energy
e-
e-
g
e+
The high energy e- interacts with the absorbing
material, producing a shower of low energy e-, e+,
g, until it stops. The shower of low energy particles
passes into the active material, ionising atoms.
The free e- so created are attracted towards
copper electrodes, where the charge is measured.
From this, the original energy of the high energy
e- can be calculated
•Hadronic calorimeters sample the energy of hadrons (particles containing
quarks, such as protons and neutrons) as they interact with atomic nuclei
High energy
p
p
p
The high energy p interacts with an atomic
nucleus in the absorbing plates, leading to a
shower of particles. These particles enter a
scintillating material, causing it to radiate light.
Long fibres carry the light to devices where
the intensity is measured and converted to an
electric current, from which the energy of the
incoming proton is measured.
Phase 3.2: Gather evidence
from observation
• Identifying Z decays to electron + positron or to muon + antimuon & estimating Z mass
e- + e+
Z
ee+
Z
mm+
or
Z
m- + m+
Group follow on activity
in class – teacher intro
to background theory,
followed by class
activity
Working in groups
•
In the Minerva 2D Analysis Tool page, click on the “masterclass
resources” box, and scroll down to computer set up. Choose a
suitable version (depending on class size) and download the sets of
events – click save, then right click on saved file and extract all (from
the zip file)
• Start the Minerva software from the portal toolbox – click on
“toolbox” at the top of the page, scroll down to Minerva 2D analysis
tool, and click the “start Minerva” red box.
• To display the events of a given group, go to File (upper left corner
of the right panel), then click on Read Events Locally, select the
Minerva file from where you have saved it, select the events folder
and then the group you want to display, and click Open.
TEACHERS MAY WANT TO LIAISE WITH THE IT TECHNICIAN IN
SCHOOL TO MAKE THIS EVENTS FOLDER ACCESSIBLE IN A
SPECIFIED FILE ON THE SCHOOL NETWORK
…….
• Print off the Atlantis Instructions, Summary sheet
and Overview sheet in the paperwork section on
this page.
• Each group takes a sample of 20 events from
the Minerva web site and identifies the events
within this set that possibly show a Z boson
decaying to either e- & e+ or m- & m+
• For each such event, calculate the invariant
mass of the Z particle
What is invariant mass??
• The invariant mass or rest mass, is a
characteristic of the total energy and momentum
of an object or a system of objects that is the
same in all frames of reference
• When the system as a whole is at rest, the
invariant mass is equal to the total energy of the
system divided by c2, which is equal to the mass
of the system as measured on a scale.
In general…..using SI units…
2
E
=
2
2
pc
+
2
4
mc
where m is the invariant mass or rest mass.
If a Z boson decays into e- + e- ,
then energy and momentum must be conserved:
∑ Ee , Ee = EZ and ∑ pe , pe = pZ
remembering p is a vector quantity!
Then mZ can be calculated: m2 = E2 - p2c2
c4
Units
Particle physicists work with less familiar
units that simplify the equation:
2
E
E is measured
in GeV
=
2
p
+
P is measured
In GeV/c
often just called GeV in
the software
2
m
m is measured
in Gev/c2
1 eV = energy gained by e- when accelerated through a PD
of 1V
= 1.6 x 10-19 J
1 GeV = 109 eV
Using these units…
2
m
m comes out in
in Gev/c2
=
when
2
E
-
E is measured
in GeV
&
2
p
p is
measured
in GeV/c
Process to calculate Z invariant
mass:
In this exercise we are going to make
some approximations – our results won’t
be exact but we will have learnt a process
and how to think along the same lines as a
particle physicist!!
2
E
=
2
p
+
2
m
• For electrons and muons,
m << p
So we can approximate that
E = p
• Z particles have a big mass, so we
can’t use this approximation for Z
bosons!
Once you have identified a
Z
e- + e+ event…
• Click on the hand symbol near the top of
the GUI box of the software, then click on
one of the electron/positron tracks
• Note the momentum components along
the 3 axes, px, py and pz
• Assume Ee = p e by our approximation,
= (px2 + py2 + pz2)1/2
• Repeat the process for the other
electron/positron track
• Calculate the invariant mass of the original Z
boson in each case:
mZ =
[ (Ee + Ee)2 - (px e + px e)2 - (py e + py e)2 - (py e + py
2 ]1/2
)
e
An excel spread sheet could be designed to do
this
Repeat this procedure for events showing Z
decaying to muon + anti-muon.
Phase 4: Collating and
discussing results
• Groups come back together and tabulate
values of mass calculated for the possible
Z boson
• A histogram of frequency against mass is
plotted
• Discussion of whether Z mass is positively
identified and to what accuracy
Phase 5: Discussion of
measurement technique
• The Higgs boson and the concept of the Higgs field was
postulated by Peter Higgs in the 1960s to try to explain
why particles have such diverse masses.
• Its maximum mass and modes of decay have been
mathematically predicted but it is the only particle in the
current standard model that has never been observed
experimentally
• Physicists will use very similar techniques to the one
used in this project to look for signature Higgs events
and determine the Higgs mass
• And if the Higgs is not found…..a new theory has to
emerge!