Download HSB_Mclass_Notes_v1

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Hi everyone, my name is Hardeep and I am going to talk to you today about the Large Hadron Collider and
ATLAS.
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Here’s an outline of what I will talk about:
I will start by looking at the Large Hadron Collider, then talking about how we actually get collisions inside it. I
will then talk about the ATLAS experiment itself, some of the things we want it to achieve, how the detector is
split up into its various components and then a bit about selecting interesting events.
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The Large Hadron Collider, or the LHC as it is more conveniently known is, at 27km long in circumference, the
world’s largest and highest energy particle accelerator. The red line shows you the size of the LHC, you wouldn't
get to see it on Google Maps as it is roughly 100m underground under the Swiss and French borders on the
outskirts of Geneva.
The LHC is designed to collide two counter rotating beams of protons or heavy ions. The beams move around
the LHC ring inside a continuous vacuum chamber guided by powerful magnets. The magnets are
superconducting and are cooled by a huge cryogenics system to temperatures lower than those in space.
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There are four major detectors constructed at the LHC, located underground in large caverns excavated at
points along the ring.
ATLAS and CMS are general purpose detectors used to look for signs of new physics. The other detectors have
more specialised purposes: ALICE will study a form of matter called a quark–gluon plasma that existed shortly
after the Big Bang. LHCb will try to investigate why the Universe we can see is made of matter rather than antimatter when equal amounts of the two were created in the Big Bang.
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Focussing on proton-protons collisions, an important question to ask is how do you get the protons in the first
place? You need to start off with a bottle of hydrogen gas like the one shown, take the hydrogen molecules
inside and strip off the electrons to leave the protons. However these are not at the energy we need for
collisions.
You start the protons in a linear accelerator and then inject them through a series of circular accelerators
(smaller to larger) to build up the speeds. By the time they get to collisions in the LHC, they have an energy of 7
TeV which corresponds to speeds really close to the speed of light. When you think that it takes 7 seconds for
light to get from the Sun to Earth, this speed of the colliding protons is around 6 mph less than that. This means
they circulate around the ring over 11000 times a second.
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When the LHC is at full power, it will be capable of colliding 7 TeV protons with 7 TeV protons (1 TeV is about
the energy of a flying mosquito) making a collision with a centre of mass energy of 14 TeV. This is enough
energy to produce particles with masses up to roughly 10 times more massive than any particles currently
known – assuming that they exist.
At the LHC the best way of doing this is to collect the protons into bunches with 1.15×1011 protons in each one.
The LHC is designed to have 2808 bunches in the ring (this is enough that we need a small crossing angle to
avoid collisions in the wrong places). We squeeze the beam to its smallest size possible at the collision point
(16μm × 16μm × few cm).
Even so… protons are really small objects so although we squeeze our 100,000 million protons down to 16
microns we get ‘only’ around 20 collisions per crossing. Fortunately the protons are moving so fast that we get
40 million bunch crossings per second and close to 1 billion collisions per second.
Most protons miss each other and carry on around the ring time after time and the beams are kept circulating
for hours.
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So I would like to show a short video to sum this up.
This shows the protons going through the various stages of the LHC accelerator chain (LINAC2, PS Booster, PS,
then getting to SPS, splitting into two directions, going around the LHC rings many times). The circulation in the
LHC actually takes around 20 minutes. They then collide in the ATLAS detector.
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At 44m long and 25m in diameter (about the size of a five storey building), ATLAS is the largest volume detector
ever constructed for particle physics. Just to get the sense of the scale you can see the little people hanging out
around it. It also weighs 7000 tonnes which is the same as the Eiffel Towel.
The detector is designed to have a cylindrical geometry as the particles will radiate out evenly from the
interaction point at the centre of the detector.
The ATLAS experiment is a collaboration of approximately 3000 scientists from 173 institutions in 37 different
countries. It is big international effort in which the UK is heavily involved.
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ATLAS is intended to investigate many different types of physics that might become detectable in the energetic
collisions of the LHC. The purpose of this is to further our knowledge on how the universe is constructed and
works:
Some of these are confirmations or improved measurements of the Standard Model, while many others are
searches for new physical theories. Among the questions ATLAS will focus on are why particles have mass, what
the unknown 96% of the Universe is made of, and why gravity is weaker than the other forces by creating
microscopic black holes.
This is possible as not all collisions produce the same effects, and the types of collisions that we are trying to
study are extremely rare. Therefore we need huge numbers of ordinary collisions to see just a few of the
interesting ones.
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The reason that detectors are divided into many components is that each component tests for a special set of
particle properties. These components are stacked so that all particles will go through the different layers
sequentially.
Charged particles, like electrons and protons, are detected both in the tracking chamber and the
electromagnetic calorimeter.
Neutral particles, like neutrons and photons, are not detectable in the tracking chamber; they are only evident
when they interact with the detector. Photons are detected by the electromagnetic calorimeter, while neutrons
are evidenced by the energy they deposit in the hadron calorimeter.
Neutrinos are not shown on this chart because they rarely interact with matter, and can only be detected by
missing matter and energy. Muons can have their momentum measured in the muon spectrometer.
Now we are going to have a look at the various components in a little more detail from the centre going
outwards.
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The Inner Detector is the tracking chamber for the ATLAS detector. The outer radius of the Inner Detector is
1.15 m, and the total length 7 m. In the barrel region the high-precision detectors are arranged in concentric
cylinders around the beam axis, while the end-cap detectors are mounted on disks perpendicular to the beam
axis.
The pixel detectors are closest to the interaction point and need to provide the highest granularity and
precision. Further out from there are the semiconductor tracker and the transition radiation tracker. A track
from a charged particle like the red line shown here will make a hit wherever it passes a tracking component.
Up to 40 hits along the way to reconstruct the particle’s trajectory really well.
This is all contained in the Central Solenoid Magnet, which provides a nominal field of 2 T …
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The magnetic field, nearly 100,000 times stronger than the Earth's, is produced by a large electrical current
inside a hollow cylindrical coil. This field, parallel to the beam axis, deflects each charged particle coming from
the collision point.
As the magnetic field is really large it needs its own cryogenics system called a cryostat to keep it all at 4.5K.
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The calorimeter measures the energies of charged and neutral particles. It consists of metal plates (absorbers)
and sensing elements. Interactions in the absorbers transform the incident energy into a "shower" of particles
that are detected by the sensing elements.
In the inner sections of the calorimeter, the sensing element is liquid argon. The showers in the argon liberate
electrons that are collected and recorded. The number of electrons is proportional to the amount of energy
seen. This needs to be able to have a really high precision.
Say something about accordion geometry
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The hadronic calorimeter surrounds the electromagnetic calorimeter. It absorbs and measures the energies of
hadrons, including protons and neutrons, pions and kaons (electrons and photons have been stopped before
reaching it). The ATLAS hadronic calorimeters consist of steel absorbers separated by tiles of scintillating plastic.
Interactions of high energy hadrons in the plates transform the incident energy into a "hadronic shower" of
many low energy protons and neutrons, and other hadrons. This shower, when traversing the scintillating tiles,
causes them to emit light in an amount proportional to the incident energy.
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The ATLAS Barrel Toroid consists of eight superconducting coils, each in the shape of a round-cornered
rectangle, 5m wide, 25m long and weighing 100 tonnes, all aligned to millimetre precision.
Magnetic field around 4T and in a doughnut shape perpendicular to beam line (toroidal field)
You also need endcap toroids to maintain the shape of the fields the further out in the detector you go.
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Muons interact with matter almost entirely through their electric charge. However, because they are about 200
times more massive than electrons, they are much less affected by the electric forces of the atomic nuclei that
they encounter. They therefore do not produce the same kind of electromagnetic shower.
Any particles emerging from the calorimeter that are still energetic and whose paths point approximately to the
collision point are identified as muons. Although their momenta are measured in the Inner Detector, more
precise measurements are desired. This is done with a stronger field over a longer distance.
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If all the data from ATLAS would be recorded, this would fill 100,000 CDs per second. This would create a stack
of CDs 450 feet high every second, which would reach to the moon and back twice each year. The data rate is
also equivalent to 50 billion telephone calls at the same time. ATLAS actually only records a fraction of the data
(those that may show signs of new physics) and that rate is equivalent to 27 CDs per minute.
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Finished construction around Aug 2008
First collisions seen in Sep 2008
Commissioning to improve detector performance ongoing
2009 – taken data at 0.9 TeV & 2.36 TeV (world record)
Successfully taking data at 7 TeV until 2011
After work in 2012 will see collisions at 14 TeV
Big discoveries soon!