Transcript ATLAS and CMS

What Questions Remain in Particle
Physics?
 Why are there three types of quarks and leptons of each
charge?
 Why do the particles haves the masses they do?
 Are there more types of particles and forces to be
discovered at yet higher-energy accelerators?
 Are the quarks and leptons really fundamental, or do they,
too, have substructure?
 How can the gravitational interactions be included in the
standard model?
Richard E. Hughes
Atlas and CMS; p.1
The Higgs
 The “Rule Book” that particle physicist use to describe the universe
includes the observed 6 quarks and 6 leptons
 This rule book also has 3 forces, each of which has associated particles
 1) Electromagnetic: for electricty ad magnetism (photons)
 2) Strong force: (hold the nucleus together (gluons)
 3) Weak force: beta decay, powers the sun (W and Z)
 Gravity (4th force) is not included (more on this later)
 But there is a 5th force, which explains why particles – any particle has
mass. This force has a field associated with it, just like the gravitional
force has the gravitational field associated with it.
 The Standard Model proposes that there is another field not yet
observed, a field that is almost indistinguishable from empty space. We
call this the Higgs field. We think that all of space is filled with this
field, and that by interacting with this field, particles acquire their
masses. Particles that interact strongly with the Higgs field are heavy,
while those that interact weakly are light.
 The Higgs field has at least one new particle associated with it, the
Higgs particle (or Higgs boson).
Richard E. Hughes
Atlas and CMS; p.2
The Higgs Mechanism
 See http://www.hep.ucl.ac.uk/~djm/higgsa.html
 To understand the Higgs mechanism, imagine that a room full of
physicists chattering quietly is like space filled with the Higgs field
Richard E. Hughes
Atlas and CMS; p.3
 a well-known scientist walks in, creating a disturbance as he moves
across the room and attracting a cluster of admirers with each step
Richard E. Hughes
Atlas and CMS; p.4
 this increases his resistance to movement, in other words, he acquires
mass, just like a particle moving through the Higgs field
Richard E. Hughes
Atlas and CMS; p.5
 if a rumor crosses the room
Richard E. Hughes
Atlas and CMS; p.6
 it creates the same kind of clustering, but this time among the
scientists themselves.
 In this analogy, these clusters are the Higgs particles.
Richard E. Hughes
Atlas and CMS; p.7
Grand Unified Theories
 Physicists hope that a Grand Unified Theory will unify the strong, weak,
and electromagnetic interactions. There have been several proposed
Unified Theories, but we need data to pick which, if any, of these
theories describes nature.
 If a Grand Unification of all the interactions is possible, then all the
interactions we observe are all different aspects of the same, unified
interaction.
Richard E. Hughes
Atlas and CMS; p.8
Super Sysmmetry!
 Many physicists have developed theories of supersymmetry, particularly
in the context of Grand Unified Theories. The supersymmetric theories
postulate that every particle we observe has a massive "shadow"
particle partner. For example, for every quark there may be a so-called
"squark" tagging along.
Richard E. Hughes
Atlas and CMS; p.9
String Theory
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Modern physics has good theories for quantum mechanics, relativity, and
gravity. But these theories do not quite work with each other. There are
problems caused by our living in three spatial dimensions. If we lived in more
than three dimensions, these problems would naturally resolve themselves.
String Theory, one of the recent proposals of modern physics, suggests that in
a world with three ordinary dimensions and some additional very "small"
dimensions, particles are strings and membranes. Yes, membranes in extra
dimensions are weird and hard to visualize. And what are "small dimensions?"
Richard E. Hughes
Atlas and CMS; p.10
Extra Dimensions
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Extra Dimensions String theory and other new proposals require more than three space
dimensions. These extra dimensions could be very small, which is why we don't see them.
How can there be extra, smaller dimensions?
Think about an acrobat and a flea on a tight rope. The acrobat can move forward and
backward along the rope. But the flea can move forward and backward as well as side to
side. If the flea keeps walking to one side, it goes around the rope and winds up where it
started. So the acrobat has one dimension, and the flea has two dimensions, but one of
these dimensions is a small closed loop.
So the acrobat cannot detect any more than the one dimension of the rope, just as we can
only see the world in three dimensions, even though it might well have many more. This is
impossible to visualize, precisely because we can only visualize things in three dimensions!
Richard E. Hughes
Atlas and CMS; p.11
The Theory of Everything
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The long range goal of physics is to unify all the forces, so that gravity would be
combined with the future version of the Grand Unified Theory. Then the
gravitational interaction would be thought of as quantized, like the other forces,
so that the gravitational force is transmitted by particles called gravitons.
This poses a formidable problem. Einstein showed us that the gravitational force
arises due to curvature in the fabric of spacetime. Thus, the task is to quantize
spacetime to produce the desired gravitons. Achieving this type of quantum
field theory is quite a challenge both conceptually and mathematically.
Richard E. Hughes
Atlas and CMS; p.12
How do we find Higgs and SUSY?
 We already have a proton-antiproton collider at Fermilab
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There are two detectors studying these collisions: CDF and D0
They *might* see hints of this new physics
BUT THEY HAVE NOT SEEN ANYTHING LIKE THESE PARTICLES!
YET!
 What are the difficulties?
 Not enough energy
 Not enough “luminosity”
 What to do: Build something even bigger
 More energy in the Collider
 Better Detectors
Richard E. Hughes
Atlas and CMS; p.13
The Large Hadron Collider
 LHC = Large Hadron Collider; located at CERN
 It is large: 27km in circumference
 It collides “hadrons” in this case: protons are collided with other protons
Richard E. Hughes
Atlas and CMS; p.14
What is CERN?
 The major European International Accelerator Laboratory located near
Geneva, Switzerland. (originally called Centre European pour Rechearche
Nucleaire).
 The World Wide Web originated in this laboratory in 1989 when its
staff proposed a multimedia, hyperlinked system of documents. This
laboratory has sometimes been referred to as the "home of the Web."
The NCSA staff developed the first graphical browser, Mosaic, for the
World Wide Web. Mosaic was released (free) for public use in 1990.
Richard E. Hughes
Atlas and CMS; p.15
Another View
 The LHC is 27 km in circumference, about 100m underground
 To curve the protons in the circle will require ~1300 superconducting
magnets
 Each about 14m long, generating an 8T field
Richard E. Hughes
Atlas and CMS; p.16
LHC Detectors
General-purpose
Higgs
SUSY
??
B-physics
CP Violation
Richard E. Hughes
Heavy Ions
Quark-gluon plasma
General-purpose
Higgs
SUSY
??
Atlas and CMS; p.17
Fermilab Vs LHC
Parameter
Fermilab
LHC
Type of Collider
Protonantiproton
Proton-proton
C.M. Energy
2 TeV
14 TeV
Size of ring
6.3km circum
27km circum
Collisions every
396nsec
25nsec
Main Detectors
CDF & D0
CMS & Atlas
Luminosity
2 x 1032cm-2sec-1
100 x 1032cm-2sec-1
Richard E. Hughes
Atlas and CMS; p.18
The LHC Tunnel
 This is the underground tunnel of the Large Hadron Collider (LHC)
accelerator ring, where the proton beams are steered in a circle by
magnets.
Richard E. Hughes
Atlas and CMS; p.19
How Fast are the Protons moving?
 Accelerators raise the energy and therefore speed of
particles. However, the particle speed gets to be near that
of light before the particle has traveled very far. After
that, the energy can continue to increase quickly, but the
particle speed very slowly gets closer and closer to the
speed of light.
Richard E. Hughes
Energy of Proton
Fraction of
Speed of Light
1eV
0.00005
1GeV (109eV)
0.875
1 TeV (1012eV)
0.99999956
7 TeV
0.999999991
Atlas and CMS; p.20
What is a TeV?
Energies are often expressed in units of "electron-volts". An electron-volt (eV) is
the energy acquired by a electron (or any particle with the same charge) when it
is accelerated by a potential difference of 1 volt.
 Typical energies involved in atomic processes (processes such as chemical
reactions or the emission of light) are of order a few eV. That is why batteries
typically produce about 1 volt, and have to be connected in series to get much
larger potentials.
 Energies in nuclear processes (like nuclear fission or radioactive decay) are
typically of order one million electron-volts (1 MeV).
 The highest energy accelerator now operating (at Fermilab) accelerates protons
to 1 million million electron volts (1 trillion electron volts, 1 TeV =1012 eV).
 The Large Hadron Collider (LHC) at CERN will accelerate each of two counterrotating beams of protons to 7 TeV per proton.
Richard E. Hughes
Atlas and CMS; p.21
A collision at the LHC in action
Richard E. Hughes
Atlas and CMS; p.22
The two Giants!
ATLAS A Toroidal LHC ApparatuS
CMS Compact Muon Solenoid
µ
µ
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Atlas and CMS; p.23
Richard E. Hughes
Atlas and CMS; p.24
ATLAS Detector
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Transverse slice through CMS detector
Click on a particle type to visualise that particle in CMS
Press “escape” to exit
Richard E. Hughes
Atlas and CMS; p.26
An Event at CMS
Richard E. Hughes
Atlas and CMS; p.27
An Event at ATLAS
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A simulated collision event viewed along
the beampipe. The event is one in which a
mini-blackhole was produced and decayed
immediately. The black area in the center
with many particle tracks represents the
inner detector (pixel detector,
semiconductor tracker, and transition
radiation tracker), which has been
enormously magnified relative to the rest
of the detector (in this view) . The colors
of the thin tracks have no significance.
The thick yellow lines are the two
electrons in this event. The green area is
the electromagnetic calorimeter, while
the red area is the hadronic calorimeter.
The green and red histograms show the
energy deposits by particles in the
electromagnetic and hadronic
calorimeters. A muon was added by hand
to the event to show how it would look in
the detector; it is a thick blue line in the
inner detector and orange in the (blue)
muon chambers.
http://pdg.lbl.gov/atlas/atlas_photos/ful
ldetector/fulldetector/atlas_event_cros
s.html
Richard E. Hughes
Atlas and CMS; p.28
The Detectors are LARGE!
This is just one small piece of the CMS detector, called the barrel yoke:
Richard E. Hughes
Atlas and CMS; p.29
The CMS Cavern
Richard E. Hughes
Atlas and CMS; p.30
Some Fun Facts about CMS
 The total mass of CMS is approximately 12500 tonnes - double that
of ATLAS (even though ATLAS is ~8x the volume of CMS)
 The CMS tracker comprises ~250 square metres of silicon detectors
- about the area of a 25m-long swimming pool
 The silicon Pixel detector comprises (in its basic form) more than
23 million detector elements in an area of just over 0.5 square
metres
 The lead tungstate crystals forming the ECAL are 98% metal (by
mass) but are completely transparent
 The 80000 crystals in the ECAL have a total mass equivalent to
that of ~24 adult African elephants - and are supported by 0.4mm
thick structures made from carbon-fibre (in the endcaps) and glass
fibre (in the barrel) to a precision of a fraction of a millimetre
 The brass used for the endcap HCAL comes from recuperated
artillery shells from Russian warships
Richard E. Hughes
Atlas and CMS; p.31
Some Fun Facts about CMS
 The CMS magnet will be the largest solenoid ever built
 The maximum magnetic field supplied by the solenoid is 4 Tesla approximately 100000 times the strength of the magnetic field
of the earth
 The amount of iron used as the magnet return yoke is roughly
equivalent to that used to build the Eiffel Tower in Paris
 The energy stored in the CMS magnet when running at 4 Tesla
could be used to melt 18 tonnes of solid gold
 During one second of CMS running, a data volume equivalent to
10,000 Encyclopaedia Britannica is recorded
 The data rate handled by the CMS event builder (~500 Gbit/s) is
equivalent to the amount of data currently exchanged by the
world's Telecom networks
 The total number of processors in the CMS event filter equals the
number of workstations at CERN today (~4000)
Richard E. Hughes
Atlas and CMS; p.32
What happens when the LHC Starts?
 The expected start date for LHC (and thus CMS and Atlas)
is around 2008
• If H0 found at the expected Standard Model mass, it will
validate the GWS Electroweak Theory and complete the model.
• Measurements of the Higgs couplings and comparison with
particle masses will verify mass-generating mechanism.
• A lighter than 130 GeV/c2 mass Higgs boson could support a
theory beyond the Standard Model, known as Supersymmetry.
• If a Higgs boson with a mass < 1 TeV is not found, it would
indicate that the Electroweak symmetry must be broken by a
means other than the Higgs mechanism.
Richard E. Hughes
Atlas and CMS; p.33
Puzzle
View along beam
line of the inner
tracking, with a H
4m event
superimposed. The m
are very high energy,
so leave straight
tracks originating
from the centre and
travelling to the
outside
Find 4 straight tracks.
Richard E. Hughes
Atlas and CMS; p.34
Puzzle solution
Make a “cut” on the
Transverse momentum
Of the tracks: pT>2 GeV
Richard E. Hughes
Atlas and CMS; p.35
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Since the two proton beams are traveling
in separate beam pipes passing through
oppositely directed magnetic fields, how
do they ever collide? At certain locations
around the ring, called "collision points",
there are no magnetic fields, and the
protons are moving in straight lines. At
those places, the two beams can be
brought together into a single vacuum
enclosure and allowed to collide head on.
The protons come in roughly cylindrical
bunches a few centimeters long and a few
millionths of a meter in radius. The
distance from one bunch to the next is
7.5 m. Since it takes light 25 billionths of
a second (25 nanoseconds or 25 ns) to
travel 7.5 m, and the protons are
practically moving at the speed of light,
head-on meetings between bunches at
every collision point occur every 25 ns, or
40 million times per second.
Richard E. Hughes
Atlas and CMS; p.36
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How Many Collisions?
If two bunches of protons meet head on, the
number of collisions between protons of one
beam and protons from the other might be ten,
one, or even zero. How often are there actually
collisions? For a fixed bunch size, this depends
on how many protons there are in each bunch,
and how large each proton is.
Actually a proton can be roughly thought of as
being about 10-15 meter in radius. If you had
bunches 10-6 meters in radius, and only, say, 10
protons in each bunch, the chance of even one
proton-proton collision when two bunches met
would be extremely small.
On the other hand, if each bunch had a billionbillion (1018) protons so that its entire cross
section were just filled with protons, every
proton from one bunch would collide with one
from the other bunch, and you would have a
billion-billion collisions per bunch crossing.
The LHC situation is in between these two
extremes, a few collisions (up to 20) per bunch
crossing, which requires about a billion protons
in each bunch
Richard E. Hughes
Atlas and CMS; p.37