Transcript Physics Searches and Results from the LHC - Indico

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PHYSICS SEARCHES
AND RESULTS FROM
THE LHC
Steven Kaneti
18 July 2012
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Outline of Talk
My goal is to explain (within time)
How to go from:
Outline of Theory
• Overview of Particle Physics
• The History Behind the Standard Model
• Motivation for the Large Hadron
To Experimental Basics
Collider
• Introduction to the LHC and the
Detector Experiments
• Description of the ATLAS detector
• The good stuff (a.k.a. recent
results):
• SM Higgs boson (-like) discovery
• Other searches for new physics
• Time for Questions Afterward
To Results and Interpretation
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Particle Physics
• Many of you probably have quite a
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good understanding of chemistry
In chemistry, you deal with matter at
the atomic and/or molecular level (1010m)
In particle physics, we deal with
matter on a much, much smaller
scale (10-15 – 10-18 m)
You cannot use visible light to probe
inside a proton
To probe inside matter, i.e. a proton
you must collide it with some particle
to split it apart
To achieve energies capable of
smashing a proton, you need to
accelerate your probe (i.e. proton,
antiproton, electron, or neutrino) to
very high energies, i.e. very close to
the speed of light
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A typical molecule is on the order of 10-10 m
To probe inside a hadron, you must break it apart
with a very high energy probe particle.
The higher the energy of the collision, the higher
the ability to probe the constituents
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The Road to the Standard Model
• By 1975, there were 6 known
leptons (𝑒,𝜇,𝜏,𝜈𝑒 ,𝜈𝜇 ,𝜈𝜏 ) but only 4
known quarks (u,d,c,s)
• This ruined the idea of a
symmetry between quarks and
lepton
• This idea was maintained by S.
Glashow, amongst others
• In 1977 bottom quark discovered
as a bound state of 𝑏𝑏, called an
upsilon
• This made the case stronger for
existence of 6th quark, the top
• Also, the existence of neutral
Weak boson (what we call the 𝑍)
was predicted
• This is part of the Electroweak
Unification
Winners of the 1979 Nobel Prize in Physics:
Sheldon Glashow, Steven Weinberg, Abdus Salam
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Discovery of 𝑊 +/𝑊 − Bosons
• 𝑊 boson mass
predicted by GWS
theory to be around
82 ± 2 GeV/c2
• 𝑊 boson was
discovered in
proton-antiproton
collisions by the
UA1 experiment at
CERN in 1983.
• Mass of the 𝑊
measured to be
80.38 ± 0.17 GeV/c2
• The process to
produce a 𝑊 boson
is:
𝑝+𝑝 →𝑊+𝑋
Mechanism for W production at
proton-antiproton colliders
X can be a number of final states that allow for baryon-number, charge, energy and momentum conservation
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Discovery of 𝑍
• The 𝑍 Boson was also discovered in
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1983 by at the CERN SPPS by the UA1
experiment in proton-antiproton
collisiions
𝑍 Boson mass was predicted to be
about 92 ± 2 GeV/c2
Measured 𝑍 Boson Mass: 91.2 ± 0.03
GeV/c2
𝑍 Bosons are neutral particles, so they
have to decay to particle-antiparticle
pairs
• Quark-antiquark pairs 𝑏𝑏
• Electron-positron pairs 𝑒 + 𝑒 −
• Muon-pairs
• Tau-pairs
• Neutrino-antineutrino pairs (𝜈𝜈)
Extremely important in proving the
GWS model of electroweak theory
GWS theory forms the basis for the
electroweak part of the standard model
(the other parts are Quantum
Chromodynamics, QCD, and Quantum
Electrodynamics, QED)
Feynman diagram of Z boson vertices; on the left is Z decay to
electron positron pair, on right decay to muon-antimuon pair.
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Discovery of Top Quark
Unlike the LHC, the Tevatron accelerator
ring is above ground
That’s CDF
That’s D0
• To restore the symmetry
between leptons and
quarks, a 6th quark was
needed.
• It was predicted to be
extremely massive
• Discovery of top quark
was announced in March
of 1995 at the Tevatron in
Fermilab, Chicago,
Illinois
• Observation by both CDF
and D0 experiments
• The process at the
Tevatron is: 𝑝 + 𝑝 → 𝑡 + 𝑡
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The top Quark
• The top quark is the most
massive subatomic
particle discovered to date
• Before the LHC began
running, only the tevatron
could produce top quarks
• Mass of top 𝑚𝑡 = 173.2
GeV/c2
• Because it’s lifetime is so
short, it cannot form
bound states of 𝑡𝑡 the way
other quarks can such as
𝑏𝑏
• Top quark physics is a
subject of much study at
the LHC
Feynman Diagrams showing how top quarks
Are produced
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The Standard Model (almost)
• So there you have it: we
have 6 types of quarks,
grouped into 3 “generations”
• We have 3 different types of
leptons, each with its own
neutrino*
• We have the force carriers
• The photon 𝛾 mediates the
electromagnetic force
• The gluon** 𝑔 mediates the
“strong” (nuclear) force
• The 𝑊 and the 𝑍 particles
mediate the weak force
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*Don’t forget, that all the charged particles have
corresponding antiparticles with opposite charge
Whether or not neutrinos are (Majorana) or not (Dirac)
their own antiparticles is still subject to investigation
**Also, there are 8 gluons
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But there’s Something Missing
• The Standard Model is a type of theory called a
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gauge field theory.
This requires ALL gauge bosons to be massless.
But we know that’s not true: the 𝑊 and 𝑍 have
large mass (about half the mass of a lead atom!!!)
• 𝑍 almost one hundred times the mass of proton!
This requires a mechanism whereby the 𝑊 and 𝑍
acquire mass
• This is the Higgs Mechanism
Higgs Mechanism predicts existence of a neutral,
spin zero (scalar) that represents the particle
associated with the Higgs field
• This is just the Standard Model Higgs;
• Some theories require multiple Higgs particles
However, the mass of the particle is not given by
the theory
• This is the main problem and has caused
physicists to search for it for over 35 years.
Peter Higgs
*The mechanism and field arte named
after Prof. Higgs, but it should be noted that many
other theoreticians contributed
to such a theory around the same time
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The Standard Model (In Full)
• The Standard Model combines
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knowledge obtained over the past
60+ years
Essentially, all matter is made up
of spin-½ quarks and spin-½
leptons.
Forces are mediated by the spin 1
gauge bosons (gamma, Z, W, g)
Particles acquire mass in the SM
by interacting with the Higgs field*
It describes essentially all known
particle interactions
• You can derive the interactions from
the “Lagrangian**” density of the
Standard Model
• However, the Standard Model is
not believed to be a truly
fundamental theory of particle
interactions, but valid at the
current (TeV) energy scale
• What we would like to know is what
*This statement is a bit oversimplified
**If you continue your studies with physics at
university level, you will learn Lagrangian mechanics
And field theory, and you will know exactly what this means .
lies beyond the Standard Model at
even higher energies than we can
currently achieve
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Are We There Yet?
Motivation for the Large Hadron Collider
• If we are to believe that the Standard
Model does describe nature at a
fundamental level, then we should be
able to observe predictions made by
the model
• This includes, but is not limited to, the
Higgs Boson.
• The origin of the matter/antimatter
asymmetry
• Understand the source of CP violation
• Further tests of quantum chromodynamics
(the theory of strong interaction)
• Observables not predicted by the
standard Model
• Supersymmetric particles
• Understand Dark Matter (possibly find dark
matter candidates)
• More “Exotic” Theories
• Technicolor
• Large Extra Dimensions
Black holes produced at a collider are expected to rotate and distort the spacetime around it
Photo courtesy of CERN courier
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Why a Proton-Proton Collider*?
• For instance, would we have a
Lots of gluons!
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*This is a slight detour, but hopefully this
will explain the various types of colliders,
and why the LHC is a proton-proton
collider
better chance of producing certain
processes if we collided electrons
with positrons?
• Charged particles radiate (give off
energy) when accelerated
• Electrons and positrons (which
have a mass roughly 1/1800 of
the proton) would lose much
energy
• Heavy charged particles tend to
radiate less when accelerated
• Why didn’t they build a protonantiproton collider like at Fermilab?
• Anti-protons are very difficult to
produce in large quantities
• High number of collisions
required every second at the
LHC
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The LHC Detector Experiments at the
LHC
• The LHC consists of the
accelerator and the
detector experiments
• ATLAS (general purpose
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detector)
CMS (general purpose
detector)
LHCb (designed for studying
b-quark physics)
ALICE (designed for studying
heavy ion collisions)
TOTEM
LHCf
MoEDAL
Cutaway schematic view the ATLAS detector
• What does it mean when
someone says a detector is
designed for a certain kind
of physics?
• I’ll talk about that more in a
little while
Cutaway schematic view the CMS detector
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More on the LHC: The Accelerator
• The LHC accelerator complex is massive!
• It is a 27 km ring that crosses into France from Switzerland
• ATLAS is the only detector in Switzerland proper
• The tunnel used for the experiment is the old tunnel from the Large
Electron-Positron Collider (a.k.a. LEP)
• The tunnel itself is about 3.8 m (around 12 ft) wide
We’re here right now
This is the runway of Geneva Cointrin airport
The protons are in the
ring as we speak. They circulate this ring
over 11000 times per second
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Another Aerial View for Perspective
Large Hadron Collider
Lake Geneva
27 km circumference
CMS
LHCb
ALICE
ATLAS
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The Accelerator (continued)
• LHC is not a perfect circle: it’s made of
eight arcs (each 2.45 km long)and eight
“insertions” (each 545 m long)
• The full LHC ring consists of 22 km of
curved section and 5 km of straight
sections
• The straight sections are used for injection,
beam cleaning, target experiments, beam
dumps, and, of course the four main
experiments
• Protons are made in an ion source and
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accelerated to 50 MeV in a linear
accelerator
They are then accelerated to 14 GeV in
proton synchrotron (PS) booster ring
After the PS, they are accelerated to 420
GeV in the SPS
They are then injected into the main ring
and achieve a final energy of (*currently)
4TeV per beam
Protons are accelerated in “bunches”;
each bunch has 1.15×1011 protons
• Why do we need so many?
r=4243 m
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The Star of the Show: The Magnets
• There are 1232 dipole magnets of 15 m length
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used to bend the beams
There are 392 quadrupole magnets each 5 to7
m long used to focus the beams
The two parallel beams are brought into a
crossing angle at the collision point using a
number of bending dipoles
Just prior to collision, another type of magnet
is used to squeeze the beam to roughly 16 𝜇m
× 16 𝜇m
LHC magnet coils are 14 m in length and inner
diameter 56 mm copper-niobium-titanium
cables
Magnets are cooled using pressurized
superfluid Helium
Magnets operate at a temperature of roughly
1.9 K (just above absolute zero), making the
LHC one of the coldest places in the universe
The magnets produce a nominal field of 8.33
Tesla
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This is tremendous; it’s about 100,000 times the magnetic
field of the earth
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Overview of ATLAS*
• A particle physics detector like ATLAS
consists of many different sub-systems
• All detectors operate on the same principles:
• The presence of a charged particle is detected
through it’s interaction with material
• Particles with zero electric charge such as neutrinos
will escape the detector
• We have to use the relativistic forms though since
these particles are traveling so fast
• We can use conservation laws like energy and
momentum to reconstruct particles that are not
directly detected
• Use a layered approach:
• Innner Detector provides tracking for charged
particles
• Calorimeters go after the inner detector to measure
energy of particles
• Spectrometer measures momentum of muons that
make it to the outer part of the detector
• The detectors used employ various technologies
• Silicon pixels/Silicon Strips
• Xenon gas Wire chamber for tracking
• Liquid argon for calorimetry
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*ATLAS is an acronym for A Toroidal LHC Apparatus
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The ATLAS Collaboration
• Modern Particle physics
experiments tend to be
quite big nowadays.
• ATLAS is one of the
biggest collaborations in
the history of science.
• ATLAS is an international
collaboration of people
from 3000 physicists from
37 countries
• This is three times the size of
my former high school
• These include, physicists,
engineers, technicians,
post-doctoral researchers,
doctoral (Ph.D.) students,
and undergraduate
students
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Tracking in ATLAS
• The Inner Detector in ATLAS provides
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tracking of particles.
The tracking allows for momentum
measurements based on the curvature
of a particle’s trajectory as it moves in
a magnetic field
The inner detector in ATLAS is made
of a mixture of two different silicon
technologies (pixel and strips) and
wire chambers, whereas in CMS, the
entire tracker is made from layers and
layers of silicon (pixels and strips)
Nearly all particle physics
experiments have some form of
tracking detector, and it’s usually made
of silicon that is placed in a strong
magnetic field to bend the trajectories
of charged particles
Magnetic field in ATLAS inner detector
is 2 Tesla.
• Provided by a superconducting
solenoid that weighs 5.5 tons!
• We will see the importance of tracking
in just a little bit.
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Silicon Detectors, The Basics
• Implant metalized electrodes in
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highly pure silicon
Apply (usually reverse) bias
voltage
High energy charged particles will
cause inonization in the bulk
material
Electrons will drift to positive
potential under the applied voltage
Holes will drift in the opposite
direction
The charge is collected by the
metalized electrodes
Amplifiers increase
The signal is then digitized, i.e. a
hit or no hit, and which strip
registered a hit and that
information is stored
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Back to The Inner Detector
• The ATLAS Inner Detector is itself
made of three systems
• a silicon Pixel detector: Pix
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Closest layer is 5 cm from interaction point
Over 1450 barrel modules
80 Million pixels cover 1.7 m2
Provides up to 3 space points
• a silicon strip detector: SCT
• Eight layers provide 4 space points
• SCT has 2112 barrel modules and
• Over 6 million readout channels for the SCT
• and a transition radiation tracker: TRT
• Made of straw tubes 4mm in diameter and
144 cm long
• Uses a mixture of Xe/CO2/O2
• Each End-cap consists of 18 wheels
• Provides up to 36 additional points for
reconstructing tracks
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The Components of the Inner Detector
PIXEL
SCT
TRT
The ATLAS SCT
Barrel being
inserted into the
Inner Detector
Structure
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The Electromagnetic Calorimeter
• The calorimeter is designed to measure
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The accordion geometry is not for musical purposes
It is to reduce time for readout of the signal
the energy of the particles emerging from
the interaction point
Electromagnetic calorimeter is good for
measuring energies of electrons,
positrons and photons
As a particle is stopped in material, it will
“shower” or cause a cascade of particles
Electromagnetic calorimeter consists of
lead-plate absorbers with polyimidecopper electrodes
The active material is Liquid Argon kept
at -185oC by a cryostat
The space filled with the Liquid argon is
under a very high electric potential (2000
V)
The signal is collected at the copper
electrodes, where it is amlified, shaped,
and digitized
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The Muon System
• Muons go virtually
undeflected through the inner
detector, the electromagnetic
calorimeter, and the hadronic
calorimeter
• This is due to their high mass
and the fact that they do not
shower in the calorimeters the
way hadrons do
• An air-core toroid provides
the magnetic field to bend the
trajectories of muons for
momentum measurement
schematic of the ATLAS Muon
system
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The ATLAS Big Wheel
• The end-cap disks of the muon system is
referred to as “big wheels”
• The big wheel is 25 meters in diameter
Hey! Who let that guy down in the pit?
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Why do we want to detect Muons? Why
do we want to Detect Photons?
• Muons are among the decay products of
• Certain Neutral particles such as
many different hadrons
For instance very high energy Muons
provide a very clean signature with
minimal background.
Also, certain bosons can decay into
muons
A virtual photon can decay to two muons
As we have seen before, a 𝑍 particle
can decay to two muons (𝑍 → 𝜇𝜇)
A Higgs particle can decay to a pair of 𝑍
bosons or a pair of 𝑊 bosons
So if we can find the muons from the
decay of both 𝑍’s (there will be four of
them), we can go back and use energy
and momentum conservation* to build
up the Higgs candidate 𝑝𝐻 = 𝑝𝑍1 +𝑝𝑍2
pions can decay to pairs of
photons
• Photons provide very good ways
to test the Standard Model
• Other, heavier particles can also
decay to pairs of photons
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• A diphoton decay is one of the best
channels to detect a Higgs boson
• Certain particles predicted in
theories with extra dimensions
predict particles that will decay to
photon pairs
• If you can find a Z going to two
photons, you have just discovered
new physics not accounted for in
the Standard Model
*Since the particles are moving close to the speed of light, we have to use the relativistic form
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Let’s Do Some Analysis Already!
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An Example Analysis: Let’s go looking for
the Higgs
• The Higgs particle is a neutral,
spin-0 boson (called a “scalar”;
spin 1 boson is a “vector boson”)
• It is believed to be responsible for
giving mass to other particles
(leptons, quarks, massive
bosons)
• How do we look for the Higgs?
• The Standard Model Higgs boson
has been hypothesized to decay into
various particles
• Subatomic particles may have many
decay modes, i.e. Z->qq, or Z->e+eor Z->mu+mu• The “preference” for the particle to
decay into a certain channel (i.e. two
photons, or two tau leptons) is called
it’s branching ratio
• The branching ratio for the Higgs in
different channels is function of it’s
mass
Looking at the branching ratio and the fact that LEP
has excluded masses below 114 GeV, which channel
do you think would be a good channel to search for
the Higgs?
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Relation Between the Top Quark, the 𝑊
Boson and The Higgs?
• Did scientists have an idea
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where to look?
Yes they did (well, kind of)
There’s a relationship between
the 𝑊 mass, the mass of the top
quark and the Higgs which helps
guide where to look
If you look at the left-most blue
band intersects the ellipses, it
says the most probably place to
find the higgs is between 115
and 127 GeV/c2
This does not mean that
scientists only look in that region
• They conduct Higgs searches at
masses all the way up to around
700 GeV/c2
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Doing Analysis
• Physicists have to analyze many, many (millions) collisions to
be able to study a certain process
• Because particle production is a quantum process, it behaves
probabilistically
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• The more collisions you have, the better chance you have of finding
the process you want
• So, how can we be sure that a pair of high energy muons
came from two 𝑍 bosons and not something else?
• How do we know those two 𝑍 bosons came from a Higgs and not
something else
• Physicists apply “cuts” to the data
• Cuts are a set of requirements on the particles that are detected
• We only look at particles (say electrons or muons) that have a certain
momentum
• This cuts down the possible sources of the final state particles
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The ATLAS Higgs Analysis
• ATLAS searches various
channels for the Higgs (as
mentioned in the previous slide),
but the most promising channels
are the diphoton and 4 muon or
4 electron channels
• Results began to start showing
excesses in the diphoton
channel around 125 GeV/c2last
summer, but there was just not
enough
• By December, it seemed that the
excesses seen in the summer
were not going away, thus they
were not just statistical
fluctuations in the data
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The ATLAS Higgs Analysis (cont’d)
• The 2012 analyses were
done using lots more data
taken over this year
• Combined data from 2011 at 7
TeV and 2012 at 8 TeV
• The excess in the two
photon channel shows a
clear peak around 126
GeV/c2
• CMS also saw clear
excesses around the same
mass
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Accumulating More Data
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Reaching the 5-Sigma Mark
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A Candidate 𝐻 → 𝑍𝑍 → 4𝜇 Event
As recently as
June 10!
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A Candidate 𝐻 → 𝑍𝑍 → 4𝑒 Event
What took you
all so long to
find me?
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So, What Do We Have Here?
• On July 4, 2012 a press conference was held by
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CERN
Both the ATLAS and CMS collaborations showed
their analysis of the 2012 data for the search of the
Standard Model Higgs in several channels
”We observe in our data clear signs of a new
particle, at the level of 5 sigma, in the mass region
around 126 GeV. The outstanding performance of
the LHC and ATLAS and the huge efforts of many Sarah was there at the big
announcement.
people have brought us to this exciting stage. A
little more time is needed to finalize these results, How cool is that?
and more data and more study will be needed to
determine the new particle’s properties.” – Fabiola
Gianotti
Both general-purpose detectors, ATLAS and CMS,
have shown sufficient evidence for a neutral scalar
(spin-0) particle with mass around 125-126 GeV/c2
We have a particle that is consistent with the
Standard Model Higgs boson
But, as the Director General said we still don’t
know what kind of Higgs it is:
“We can only call it a Higgs Boson – not the Higgs
Boson”- Director General Rolf Heuer
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So, is that it?
• To put it briefly: No
• If that is all there is left to discover,
These are all very technical problems with the SM
so if you have questions, please ask
then there will be lots of unhappy
physicists
• We are not done because there are still
many more questions to answer
• The Standard Model cannot be a
complete theory for several reasons
• (this is a sample from a very long list):
The masses of the particles in the SM
have to put in “by hand” so there’s no
prediction of what the masses will be
• Heirarchy problem (ask your local
theorist)
• Theres one “little” detail that I left out:
The Standard Model does not
account for gravitational interactions
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Ideas Beyond the Standard Model:
Supersymmetry
• Supersymmetry is a theory that dates
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back to the 1970’s (not really that
new)
It proposes a “symmetry” between
bosons and fermions, whereby all
fermions (spin ½ particles) have a
bosonic (spin 0 or spin 1) partner and
all the bosons would have a spin ½
partner)
This solves some of the problems of
the standard model, although the
theory does have some peculiar
quirks to it
Currently, no overwhelming evidence
for the existence of supersymmetric
particles (we probably would have
seen them if they were the same
mass as SM particles)
This means that Supersymmetry is a
“badly broken” symmetry since the
particle masses are so different
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The Minimally Supersymmetric Standard
Model
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ATLAS SUSY Searches: stop right there
• Earlier, I showed how top quarks
from the Standard Model can be
produced in 𝑝𝑝 and 𝑝𝑝 collisions
• Supersymmetric partners to the
top (called stop) are predicted to
be produced in pairs in 𝑝𝑝
collisions
• The process is 𝑞𝑞 → 𝑡𝑡 or 𝑔𝑔 → 𝑡 𝑡
• The stop quark will decay into a top
quark and the lightest
supersymmetric particle (LSP, 𝜒 0)
• LSP goes undetected, so there will
be missing energy
• Let’s take a look at the missing
energy distribution and how it
compares to the Standard Model
processes
• Do we see any significant excesses?
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Ideas Beyond the Standard Model:
Gravity at the TeraScale
• If you have noticed, one of the issues with the Standard Model
is that it ignores the gravitational force
• This is alright for describing particle interactions that we see because
gravity is so weak at the distance/energy scale we are probing
• Gravity is expected to become comparable in strength to the other
forces at very high energies
• To explain the weakness of the gravitational force, some models of the
universe predict extra dimensions in addition to the 3 space dimensions
(x,y,z) and 1 time dimension*
• These theories predict the particles of the Standard Model are confined
to a four-dimensional space (a “brane”) while gravity propagates in the
extra dimensions that are large (compared to the Planck Scale (10-35 m)
• One of these Models is the famous “R-S model” named after Lisa
Randall and Raman Sundrum
• In the minimal R-S model, a fifth dimension is assumed in which the
fifth dimension is compactified with length 𝜋𝑟𝑐 , where 𝑟𝑐 is the radius
• Gravitons would be confined to live (propagate) in the fifth dimension
*This is nothing new, original ideas of extra dimensions were first put forth in the 1920’s by mathematicians Theodor Kaluza and Oskar Klein
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Graviton Searches in ATLAS
• Graviton production would proceed
much like production of other heavy
particles at the LHC where a
resonance would be produced
• One of the more promising channels is
the diphoton channel
• Processes are 𝑞 𝑞 → 𝐺 → 𝛾𝛾 and 𝑔𝑔 →
𝐺 → 𝛾𝛾
• Some other promising channels are
dijets, 𝑍𝑍, and 𝑊𝑊
• Gravitons are searched for in the mass
range of starting around 450 GeV and
up
• Current limits are showing that there
are no excesses all the way out to
2TeV
• This means that if we find a graviton, it
will be quite massive (several times
larger than the most massive particles
currently measured).
The resonance would appear as a bump in the diphoton mass spectrum
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Black Hole Production at Colliders
• Black holes can be produced at the LHC if
the “Planck scale”* is on the order of several
TeV
• The gravitational force would be of sufficient
strength compared to the other forces
• These black holes are different from classical
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(astrophysical) black holes
Once produced, they would disintegrate via
Hawking radiation
After the Hawking evaporation phase, these
quantum black holes transform into highentropy “string states” which continue to
decay
The way you search for quantum black holes
is very similar to searches for gravitons: can
look for multiple jets and high energy
electrons or muons
It seems that the scale at which black holes
can be produced is over several (4 to 5) TeV!
Does gravity become strong at LHC
energies? Is is silly to think this way?
**The Planck scale is a fancy word used for the energy range
at which all forces start to unify
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Summary
• The LHC is an incredible machine that represents over 20 years of planning and
collaboration by thousands of people from many countries
• Hopefully, today you have seen:
• The Standard Model and some predictions even grander theories
• How the LHC operates and what the detectors consist of
• How scientists use the data from the experiments to look for signs of new physics
• The LHC and it’s experiments are all operating incredibly well and taking lots of
data
• It appears that we have discovered a Higgs-like particle
• We still have to confirm if it is the standard Model Higgs
• We will be continuing to take data until the end of this year for p-p collisions (look to have
between 20 and 30 inverse femtobarns of data (this is a lot more than 2010 and 2011 combined)
• The more data we have, the better the chance we have of discovering physics Beyond the
Standard Model
•
•
•
•
Supersymmetry
Dark Matter
Matter – Antimatter Asymmetry
Extra dimensions/Black Holes
• Our job is not done; we still have a lot more to discover an understand
• Are we just beginning the phase of discovery?
• Some people say the best things we can discover are the things nobody has predicted: “Who
ordered that?”
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So…
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Stay Tuned for More Interesting Results and
Discoveries from the LHC.
Thank you for Listening….
The End.
Any Questions?
50
LHC Accelerator
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LHC Collisions
• The probability of one particular proton in
one bunch colliding with a different proton
in the other oncoming bunch depends on
the proton size (~10-15 m) and the width of
the bunch (~16×10-6 m)
• The probability is extremely small 4 × 10-21
• The number of interactions can be
increased with more protons in the bunch
• On average there are about 50
interactions per crossing
• Most of the interactions do not produce
particles with high angles with respect to the
beam axis (which is what we actually want)
• There are only about 20 or so “effective”
interactions per event
• With 11245 crosses per second we get
• 11245 crosses/sec × 2808 = 31.6 million
crosses/sercond
• 31.6 × 106 crosses/sec × 20 collisions/cross =
600 million collisions/sec!
Candidate 𝑍 → 𝜇+ + 𝜇− event with
25 reconstructed vertices
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The ATLAS TRT
• TRT is the outer part of the
•
•
•
•
•
ATLAS Inner Detector
It is essentially a wh
Based on the use of straw
detectors which operate at
high rates
Xenon gas is used to detect
radiation photons coming from
electrons
Maximum straw length is 144
cm in the barrel, which
contains 50 000 straws
End-caps contain 320 000
radial straws
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Wire Chamber Detectors
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Particles Bending in Magnetic Field
• A charged particle moving in a constant magnetic field
•
•
•
•
will experience a force called the Lorentz Force
This force is proportional to the charge and the
velocity
Magnitude of force is given by 𝐹 = 𝑞𝑣𝐵 sin 𝜃
The force acts perpendicularly (cannot do work on the
particle, only change direction)
Use the “right-hand rule” to find direction of force on
positively-charged particle
•
•
•
Extend thumb in direction of velocity of
Extend Fingers in direction of magnetic field
Palm faces the direction of the force
• For negatively charged particles, use left hand version
of rule
• Very useful for tracking particles in our detectors
•
An illustration of the
right hand rule for the
Lorentz force
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How Much Does the LHC Cost?
• This total cost is shared by CERN’s 20 “member states”, with
additional contributions from “observer states”
• The total cost for the collider is about GBP 2.1 billion
• The total cost for the detectors is GBP 57 million
• The UK contributes over GBP 500 million; the third largest amount behind the
French and the Swiss
• The UK pays about GBP 95 million per year as an annual
subscription to contribute to research at CERN
• It’s direct contribution to the LHC is around GBP 34 million
• The annual cost for the LHC experiments is about GBP 3.5 billion
(USD 5.5 billion)
• The amount of electricity used by the LHC is immense: about 120
megawatts
• This is about as much as all homes in the entire Swiss Canton of Geneva
• The annual cost of the electricity is around GBP 15 million
• The computing resources cost over GBP 180 million
• When you add everything up, the total cost of finding the Higgs boson
is around GBP 8.5 billion (USD 13.25 billion)
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The Hadronic Calorimeter
• The Tile Calorimeter is
placed outside the liquid
Argon calorimeter
• Uses steel absorbers and
scintillating tiles as active
material
• Separated into barrel and
end-cap regions