Download Kscenario - Elementary Particle Physics Group

Identifying strange particles &
determining their properties in
the ATLAS experiment
People
Particles in ATLAS
In a particle collision in
ATLAS, a large number
of particles are
produced. Which are
they, and how are they
constructed?
Many are constructed of quarks
and antiquarks
Quarks make up the Hadrons
• Baryons
- made up of 3 quarks
eg protons & neutrons
•
Mesons
- made up of 2 quarks
eg pions & kaons
There are also leptons
……Which also have
their antiparticles but
no sub structure –
they are elementary
Force carriers
• The forces that particles
experience arise from
exchange of force
carriers
- g photons for electromagnetic
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
What evidence do we have for
this?
• Physicists have
designed and carried
out experiments with:
- Cosmic rays
– Particles accelerated in
particle laboratories
• Using more and more
sophisticated particle
detectors
• The bubble chamber has
been a very useful detector
to visualise particle collisions
and particle decays.
• A charged particle passing
through the (superheated)
liquid causes the liquid to boil
along their paths.
• A magnetic field causes the
particles to bend.
Classical bubble chamber image
The observation of a short-lived
neutral kaon in a bubble chamber
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Modern detectors are very complex
and rely on advanced electronics &
computer technology
½% of the ATLAS members
Detecting particles
• Which particles can we detect – are there any
we can’t ?
• How do we find their
- charge
- momentum
- energy ?
• What characteristics do we look for to identify
which particle it is?
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
Detector homework
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 the ATLAS detector at:
http://atlas.ch/
- Click on “multimedia” and then “how atlas works” and
“animated clips”
- Particularly Episode2:
http://www.atlas.ch/multimedia/htmlnc/feature_episode2.html
- Construction of ATLAS in 3 minutes:
http://www.atlas.ch/multimedia/htmlnc/built_in_three_minutes.html
- Click on “e-tours” and look at these too.
- Study “Physics with ATLAS” report on the Learning
with ATLAS portal.
Download the Minerva software at
http://atlas-minerva.web.cern.ch/atlasminerva
Read through the introduction and, using the
Minerva help and instructions pdf ,
work through the 5 tutorial examples.
Strange Particle event files
• There are four event files on the Minerva
event location for the Strange Particle
explorations, each containing 20 event
files with 25 events in each.
• Choose one of the files
– Lambda 0.9 TeV (real collision data)
– Lambda MC 0.9 TeV (simulated event data)
– V0 0.9 TeV (real collision data)
– V0 MC 0.9 TeV (simulated event data)
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Student feed back
Which particles can we detect?
Which characteristics do we look for?
How are the particles detected?
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?
• 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 electromagnetic
calorimeter, where it can
be measured.
• Neutral particles, like a
photon, can deposit
energy in the
electromagnetic
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
very 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
The particle trajectory and charge
• Tracking devices reveal the paths of electrically charged
particles through the trails they leave behind. When
particles pass through the detector material, they ionise
the atoms of the material. The ionised atoms give rise to
a feeble electric current.
• Most modern tracking devices 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.
• The charge on a particle is determined by the curvature
of its path in a magnetic field
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
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 ‘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
‘absorbing’ high–density material (lead or steel)
interleaved with layers of ‘active’ medium such
as a scintillator.
.
• Electromagnetic calorimeters measure the energy of 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 a large number of
low energy e-, e+, g. The numerous low energy
particles pass into the active material, ionising
atoms. The created e- 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
The high energy p interacts with an atomic
High energy
p
p
p
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.
Gather evidence from
observation
A K0 particle produced in a proton-proton collision,
and decaying in the Inner Detector of ATLAS
K0 particle features
• Features to determine
– The mass
– The lifetime
– Its decay
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•
Working in groups
On the Minerva website http://atlas-minerva.web.cern.ch/atlas%2Dminerva/
click on masterclass resources 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)
The Strange Particle event files - the K0 and L0 event files - are
presently being uploaded, and MINERVA is being modified for this
scenario.
• Locate the file atlantis.jar inside the AtlantisJavaMinerva folder.
Double click this file and MINERVA will begin, as long as you have a
recent version of Java installed, version 1.5 or later. If you need
Java installing please go to www.java.com and download the
software from the website. The default events are events which are
shown in the introductory slides. 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.
…….
• Print off the Instructions for Atlantis, Summary
sheet and Overview sheet in the paperwork
section on this page.
• Each group takes a sample of 25 events from
the Minerva web site and identifies the events
within this set that possibly show the decay of a
K0 particle
• For each such event, calculate the invariant
mass of the K0 particle and determine its lifetime
Exploration hints
• Preparation of the event display (Data tab, InDet
tab and Cuts tab might already be set at start-up)
– “Data” tab
– Click “Data” tab and under “Status”, then “InDet” and
tick “Track Collection”, “Space Point” and
“RecVertex”.
– “InDet” tab
– Make the collision point (primary vertex) and the
decay points (secondary vertices) well visible by
clicking on “InDet” tab, then “RecVertex” and set the
“Symbol size” to 7. Tick “Force symb” to see vertex
also on top display.
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Exploration hints cont.
• “Cuts” tab
– Set pT to > 0.5 GeV (pT is the momentum in the direction at right
angle to the colliding beams). Tick the box, and highlight the
number. Enter the new number and do “Return”. This removes
quite a few tracks and makes it easier to explore the event.
– Set z0 < 20 cm (z0 is the z coordinate of the primary vertex and
should be rather close to 0) and d0Loose to < 4 cm (d0 is the
distance the track misses the primary vertex)
– The d0 cut can be used during the exploration of the event.
Normally require d0> 0.5 mm. This focuses the interest on the
particles from secondary vertices. Note that this cut can
sometimes also remove a track from the secondary vertex. By
removing the cut, all the tracks from the primary vertex are also
seen.
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Exploration hints cont.
• Exploration
– Use both display projections to inspect primary and secondary
vertices close to the collision point, typically within 40 cm from
the collision point. Use the zoom facility to focus on that part of
the detector. Use the information from both views as some tracks
and vertices are easier to observe in one of the views.
Sometimes the vertices are somewhat displaced from the real
vertex. The best is to complement the display with your own
pattern recognition capacity
• During the exploration of the event, it is useful to switch
from including and excluding the d0 cut to see tracks that
come from the primary and the secondary vertex
respectively.
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Special relativity
• High energies, several GeV per particle
• High speed, close to c, speed of light
• Need to use Special relativity
– Albert Einstein 1905
– Important contributions from Hendrik Lorentz
and Henri Poincaré
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What is invariant mass??
• The invariant mass, is a characteristic of
the total energy and momentum of a
system of particles.
• It is the same in all frames of reference –
it is invariant.
• The invariant mass is the mass of the
decaying particle.
In general…..using SI units…
E2 = p2c2 + m2c4
where m is the invariant mass or just mass.
Energy and momentum must be conserved when
the K0 particle decays into a p+ and a p- .
Then :
E = Ep+ + Ep- and p = pp+ + ppremembering that p is a vector quantity!
Then mK 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 charged particle accelerated
through a voltage of 1V
1 eV = 1.6 x 10-19 J
1 GeV = 109 eV
1 TeV = 1012 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
Once you have identified a
K0
p+ + p- event…
• Click on “pick” at the top of the GUI box of the
software, then click on the two pion tracks one
after the other
• The three components of the momentum will be
displayed.
• Calculate the invariant mass of the original K0
particle in each case:
mK = [ (Ep+ + Ep-)2 - (px p+ + px p-)2 - (py p+ + py p-)2
- (pz p+ + pz p-)2 ]1/2
An excel spread sheet could be designed to do this
Estimating the K0 mass
• Explore the K0 events, and determine the
mass from the momenta of the two pions.
• Repeat it for each K0 particle
• Make a histogram of the measured values,
and determine the average mass of the
K0.
• Estimate the uncertainty.
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The lifetime
• Most particles are unstable.
• How long they live depends both on their lifetime
and on their speed relative to the observer, that
is us.
• The lifetime we observe is the particle lifetime at
rest multiplied with the gamma factor (also called
the Lorentz factor)
• The gamma factor, g = 1/(1-v2/c2)1/2
• The gamma (or Lorentz) factor shows up
”everywhere” in special relativity.
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Estimating the lifetime
• Explore the K0 events, and determine the
decay distance, the distance from the
collision point to the decay point.
• Determine the speed of the K0 particle
• It is often rather close to c, the speed of light.
• Determine the lifetime of each K0 particle,
divided by the gamma factor
• Make a histogram of the measured values,
and determine the lifetime of the K0.
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The exponential decrease
of the measured lifetime
• The measured lifetime of a particle follows an
exponential curve given by:
– N(t) = No e-t/t
– Which describes the number of particles found at times t for a
lifetime of t. t is the measured lifetime divided with gamma, the
Lorentz factor.
– Plotting the number of measured particles with a lifetime t on a
logarithmic plot makes it rather easy to determine the particle
lifetime.
– However, a complication is that the detection efficiency varies as
a function of time, and that has to be corrected for.
– Without corrections, an average value of the measured lifetimes
give an approximate value (order of magnitude) of the lifetime.
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Collating and discussing
results
• Groups come back together and tabulate values
of mass and lifetimes calculated for the K0
particles
• A histogram of frequency against mass is plotted
• A histogram of frequency against lifetime is
plotted
• Discussion of whether the K0 is positively
identified and to what accuracy
Discussion of measurement
technique
• The K0 particle decay can be “seen” in the
detector
• The decay of very shortlived particles can not be
seen in the detector
• Can the same technique still be used?
• Which complications could there be to use the
technique for “invisible”, very shortlived
particles?
Discussion of the results
• What is your best estimate of the mass of the K0
particle?
• What could the uncertainty be due to?
• What is your best estimate of the lifetime of the
K0 particle?
• Discuss ways to determine the lifetime more
correctly and more precisely.
• What could the uncertainty be due to?
• How far would the K0 particle typically move if
the gamma factor is 1?
46
K0 particle and antiparticle
• The K0 particle and the K0 antiparticle are
different particles
– The K0 is composed of an s quark and a u
quark
– The K0 (the anti K0) is composed of an s
quark and an u quark
– The K0 and the K0 are different particles as
they are composed of different quarks
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Lifetime reconstruction
A straightforward exponential does not give the correct lifetime.
It is roughly a factor of two too high.
Weighted events are used, which take into account acceptance
and inefficiencies.
Discuss how inefficiencies can arise.
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