PHYS 3313 – Section 001 Lecture #22
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Transcript of PHYS 3313 – Section 001 Lecture #22
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Wednesday, Nov. 28, 2012
PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
1
PHYS 3313 – Section 001Lecture #22
Wednesday, Nov. 268, 2012Dr. Jaehoon Yu
• Particle Accelerators• Particle Physics Detectors• Hot topics in Particle Physics• What’s coming in the future?
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Wednesday, Nov. 28, 2012
PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
2
Announcements• Your presentations are in classes on Dec. 3 and Dec. 5
– All presentation ppt files must be sent to me by 8pm this Sunday, Dec. 2• Final exam is 11am – 1:30pm, Monday, Dec. 10
– You can prepare a one 8.5x11.5 sheet (front and back) of handwritten formulae and values of constants for the exam
– No formulae or values of constants will be provided!• Planetarium extra credit
– Tape one side of your ticket stubs on a sheet of paper with your name on it– Submit the sheet on Wednesday, Dec. 5
• Please be sure to fill out the feedback survey. • Colloquium this Wednesday at 4pm in SH101
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
3Wednesday, Nov. 28, 2012
• What are elementary particles?– Particles that make up all matters in the universe
• What are the requirements for elementary particles?– Cannot be broken into smaller pieces– Cannot have sizes
• The notion of “elementary particles” have changed from early 1900’s through present– In the past, people thought protons, neutrons, pions, kaons, ρ-
mesons, etc, as elementary particles• Why?
– Due to the increasing energies of accelerators that allowed us to probe smaller distance scales
• What is the energy needed to probe 0.1–fm?– From de Broglie Wavelength, we obtain
Introduction
P
c
c
2000 /MeV c197fm0.1fm
MeVc
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
4Wednesday, Nov. 28, 2012
• Classical forces:– Gravitational: every particle is subject to this force, including
massless ones• How do you know?
– Electromagnetic: only those with electrical charges– What are the ranges of these forces?
• Infinite!!– What does this tell you?
• Their force carriers are massless!!– What are the force carriers of these forces?
• Gravity: graviton (not seen but just a concept)• Electromagnetism: Photons
Forces and Their Relative Strengths
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
5Wednesday, Nov. 28, 2012
• What other forces?– Strong force
• Where did we learn this force? – From nuclear phenomena– The interactions are far stronger and extremely short ranged
– Weak force• How did we learn about this force?
– From nuclear beta decay– What are their ranges?
• Very short– What does this tell you?
• Their force carriers are massive!• Not really for strong forces
• All four forces can act at the same time!!!
Forces and Their Relative Strengths
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
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• The ranges of forces also affect interaction time– Typical time for Strong interaction ~10-24sec
• What is this time scale?• A time that takes light to traverse the size of a proton (~1 fm)
– Typical time for EM force ~10-20 – 10-16 sec– Typical time for Weak force ~10-13 – 10-6 sec
• In GeV ranges, the four forces (now three since EM and Weak forces are unified!) are different
• These are used to classify elementary particles
Interaction Time
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
7Wednesday, Nov. 28, 2012
• Before the quark concepts, all known elementary particles were grouped in four depending on the nature of their interactions
Elementary Particles
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
8Wednesday, Nov. 28, 2012
• How do these particles interact??– All particles, including photons and neutrinos, participate in
gravitational interactions– Photons can interact electromagnetically with any particles
with electric charge– All charged leptons participate in both EM and weak
interactions– Neutral leptons do not have EM couplings– All hadrons (Mesons and baryons) respond to the strong
force and appears to participate in all the interactions
Elementary Particle Interactions
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
9Wednesday, Nov. 28, 2012
• All particles can be classified as bosons or fermions– Bosons follow Bose-Einstein statistics
• Quantum mechanical wave function is symmetric under exchange of any pair of bosons
• xi: space-time coordinates and internal quantum numbers of particle i
– Fermions obey Fermi-Dirac statistics• Quantum mechanical wave function is anti-symmetric under
exchange of any pair of Fermions
• Pauli exclusion principle is built into the wave function– For xi=xj,
Elementary Particles: Bosons and Fermions
1 2 3, , ,... ...B i nx x x x x
1 2 3, , ,... ...F i nx x x x x
F
1 32 , ,... ., ..B i nx xx x x
1 32 , ,... ., ..F i nx xx x x
F
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
10Wednesday, Nov. 28, 2012
• Bosons– All have integer spin angular momentum– All mesons (consists of two quarks) are bosons
• Fermions– All have half integer spin angular momentum– All leptons and baryons (consist of three quarks) are fermions
• All particles have anti-particles– What are anti-particles?
• Particles that has same mass as particles but with opposite quantum numbers– What is the anti-particle of
• A π0?• A neutron?• A K0?• A Neutrino?
Bosons, Fermions, Particles and Antiparticles
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
11Wednesday, Nov. 28, 2012
• When can an interaction occur?– If it is kinematically allowed– If it does not violate any recognized conservation laws
• Eg. A reaction that violates charge conservation will not occur– In order to deduce conservation laws, a full theoretical
understanding of forces are necessary• Since we do not have full theory for all the forces
– Many of general conservation rules for particles are based on experiments
• One of the clearest conservation is the lepton number conservation– While photon and meson numbers are not conserved
Quantum Numbers
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
12Wednesday, Nov. 28, 2012
• Can the decay occur?– Kinematically??
• Yes, proton mass is a lot larger than the sum of the two masses– Electrical charge?
• Yes, it is conserved• But this decay does not occur (<10-40/sec)
– Why?• Must be a conservation law that prohibits this decay
– What could it be?• An additive and conserved quantum number, Baryon number (B)• All baryons have B=1• Anti-baryons? (B=-1)• Photons, leptons and mesons have B=0
• Since proton is the lightest baryon, it does not decay.
Baryon Numbers0p e π
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13Monday, Nov. 27, 2006 PHYS 3446, Fall 2006Jae Yu
The Standard Model of Particle Physics• Prior to 70’s, low mass hadrons (mesons and baryons) are
thought to be the fundamental constituents of matter, despite some new particles that seemed to have new flavors– Even lightest hadrons, protons and neutrons, show some indication
of substructure• Such as magnetic moment of the neutron
– Raised questions whether they really are fundamental particles• In 1964 Gell-Mann and Zweig suggested independently that
hadrons can be understood as composite of quark constituents– Recall that the quantum number assignments, such as strangeness,
were only theoretical tools rather than real particle properties
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14Monday, Nov. 27, 2006 PHYS 3446, Fall 2006Jae Yu
The Standard Model of Particle Physics• In late 60’s, Jerome Friedman, Henry Kendall and
Rich Taylor designed an experiment with electron beam scattering off of hadrons and deuterium at SLAC (Stanford Linear Accelerator Center) – Data could be easily understood if protons and neutrons
are composed of point-like objects with charges -1/3e and +2/3e.
– A point-like electrons scattering off of point-like quark partons inside the nucleons and hadrons• Corresponds to modern day Rutherford scattering• Higher energies of the incident electrons could break apart the
target particles, revealing the internal structure
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15Monday, Nov. 27, 2006 PHYS 3446, Fall 2006Jae Yu
The Standard Model of Particle Physics• Elastic scatterings at high energies can be described well with
the elastic form factors measured at low energies, why?– Since the interaction is elastic, particles behave as if they are point-
like objects without a substructure• Inelastic scatterings cannot be described well w/ elastic form
factors since the target is broken apart– Inelastic scatterings of electrons with large momentum transfer (q2)
provides opportunities to probe shorter distances, breaking apart nucleons
– The fact that the form factor for inelastic scattering at large q2 is independent of q2 shows that there are point-like object in a nucleon• Bjorken scaling
• Nucleons contain both quarks and glue particles (gluons) both described by individual characteristic momentum distributions (Parton Distribution Functions)
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16Monday, Nov. 27, 2006 PHYS 3446, Fall 2006Jae Yu
The Standard Model of Particle Physics• By early 70’s, it was clear that hadrons (baryons and mesons)
are not fundamental point-like objects• But leptons did not show any evidence of internal structure
– Even at high energies they still do not show any structure– Can be regarded as elementary particles
• The phenomenological understanding along with observation from electron scattering (Deep Inelastic Scattering, DIS) and the quark model
• Resulted in the Standard Model that can describe three of the four known forces along with quarks, leptons and gauge bosons as the fundamental particles
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17Monday, Nov. 27, 2006 PHYS 3446, Fall 2006Jae Yu
Quarks and Leptons• In SM, there are three families of leptons
– Increasing order of lepton masses– Convention used in strong isospin symmetry, higher member of
multiplet carries higher electrical charge• And three families of quark constituents
• All these fundamental particles are fermions w/ spin
e
e
ud
cs
tb
+2/3
-1/3
Q
0-1
Q
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Monday, Aug. 27, 2012 PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
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The Standard Model
Monday, Aug. 27, 2012 PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
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Discovered in 1995, ~175mp
• Total of 16 particles make up the matter in the universe! Simple and elegant!!!
• Tested to a precision of 1 part per million!
Make up most ordinary matters ~0.1mp
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20Monday, Nov. 27, 2006 PHYS 3446, Fall 2006Jae Yu
Quark Content of Mesons• Meson spins are measured to be integer.
– They must consist of an even number of quarks – They can be described as bound states of quarks
• Quark compositions of some mesons– Pions Strange mesons
π
π
0π
ud
ud
12uu dd
K
K 0K 0K
us
us
ds
ds
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21Monday, Nov. 27, 2006 PHYS 3446, Fall 2006Jae Yu
Quark Content of Baryons• Baryon spins are measured to be ½ integer.
– They must consist of an odd number of quarks – They can be described as bound states of three quarks based on the
studies of their properties• Quark compositions of some baryons
– Nucleons Strange baryons Other Baryons– s=1 s=2
• Since baryons have B=1, the quarks must have baryon number 1/3
p
n
uud
udd
0 0
udsuusudsdds
0
ussdss
uuu
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22Monday, Nov. 27, 2006 PHYS 3446, Fall 2006Jae Yu
Z and W Boson Decays• The weak vector bosons (discovered in early 1980’s) couple
quarks and leptons – Thus they decay to a pair of leptons or a pair of quarks
• Since they are heavy, they decay instantly to the following channels and their branching ratios– Z bosons: MZ=91GeV/c2 – – – – W bosons: MW=80GeV/c2
– –
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23Monday, Nov. 27, 2006 PHYS 3446, Fall 2006Jae Yu
Z and W Boson Search Strategy• The weak vector bosons have masses of 91 GeV/c2 for Z and 80
GeV/c2 for W• While the most abundant decay final state is qqbar (2 jets of
particles), the multi-jet final states are also the most abundant in collisions
– Background is too large to be able to carry out a meaningful search• The best channels are using leptonic decay channels of the bosons
– Especially the final states containing electrons and muons are the cleanest• So what do we look for as signature of the bosons?
– For Z-bosons: Two isolated electrons or muons with large transverse momenta (PT)
– For W bosons: One isolated electron or muon with a large transverse momentum along with a signature of high PT neutrino (Large missing ET).
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24Monday, Nov. 27, 2006 PHYS 3446, Fall 2006Jae Yu
What do we need for the experiment to search for vector bosons?
• We need to be able to identify isolated leptons– Good electron and muon identification– Charged particle tracking
• We need to be able to measure transverse momentum well– Good momentum and energy measurement
• We need to be able to measure missing transverse energy well– Good coverage of the energy measurement (hermeticity)
to measure transverse momentum imbalance well
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
25Wednesday, Nov. 28, 2012
• How can one obtain high energy particles?– Cosmic ray Sometimes we observe 1000TeV cosmic rays
• Low flux and cannot control energies too well
• Need to look into small distances to probe the fundamental constituents with full control of particle energies and fluxes– Particle accelerators
• Accelerators need not only to accelerate particles but also to– Track them– Maneuver them– Constrain their motions to the order of 1m or better
• Why?– Must correct particle paths and momenta to increase fluxes and control
momenta
Particle Accelerators
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
26Wednesday, Nov. 28, 2012
• Depending on what the main goals of physics are, one needs different kinds of accelerator experiments
• Fixed target experiments: Probe the nature of the nucleons Structure functions– Results also can be used for producing secondary particles for further
accelerations Tevatron anti-proton production• Colliders: Probes the interactions between fundamental
constituents– Hadron colliders: Wide kinematic ranges and high discovery potential
• Proton-anti-proton: TeVatron at Fermilab, at CERN• Proton-Proton: Large Hadron Collider at CERN (turned on early 2010)
– Lepton colliders: Very narrow kinematic reach, so it is used for precision measurements• Electron-positron: LEP at CERN, Petra at DESY, PEP at SLAC, Tristan at KEK,
ILC in the med-range future• Muon-anti-muon: Conceptual accelerator in the far future
– Lepton-hadron colliders: HERA at DESY
Particle Accelerators
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
27Wednesday, Nov. 28, 2012
• Cockcroft-Walton Accelerator– Pass ions through sets of aligned DC electrodes at successively
increasing fixed potentials– Consists of ion source (hydrogen gas) and a target with the electrodes
arranged in between– Acceleration Procedure
• Electrons are either added or striped off of an atom• Ions of charge q then get accelerated through series of electrodes, gaining kinetic
energy of T=qV through every set of electrodes
Electrostatic Accelerators: Cockcroft-Walton
• Limited to about 1MeV acceleration due to voltage breakdown and discharge beyond voltage of 1MV.
• Available commercially and also used as the first step high current injector (to ~1mA).
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
28Wednesday, Nov. 28, 2012
• Energies of particles through DC accelerators are proportional to the applied voltage
• Robert Van de Graaff developed a clever mechanism to increase HV– The charge on any conductor resides on its outermost
surface– If a conductor carrying additional charge touches another
conductor that surrounds it, all of its charges will transfer to the outer conductor increasing the charge on the outer conductor, thereby increasing voltage higher
Electrostatic Accelerators: Van de Graaff
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
29Wednesday, Nov. 28, 2012
• Sprayer adds positive charge to the conveyor belt at corona points
• Charge is carried on an insulating conveyor belt
• The charges get transferred to the dome via the collector
• The ions in the source then gets accelerated to about 12MeV
• Tandem Van de Graff can accelerate particles up to 25 MeV
• This acceleration normally occurs in high pressure gas that has very high breakdown voltage
Electrostatic Accelerators: Van de Graaff
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
30Wednesday, Nov. 28, 2012
• Fixed voltage machines have intrinsic limitations in their energy due to breakdown
• Machines using resonance principles can accelerate particles to even higher energies
• Cyclotron developed by E. Lawrence is the simplest and first of these
• The accelerator consists of– Two hallow D shaped metal chambers
connected to alternating HV source– The entire system is placed under
strong magnetic field
Resonance Accelerators: Cyclotron
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
31Wednesday, Nov. 28, 2012
• While the D’s are connected to HV sources, there is no electric field inside the chamber due to Faraday effect
• Strong electric field exists only in the gap between the D’s
• An ion source is placed in the gap• The path is circular due to the perpendicular
magnetic field• Ion does not feel any acceleration inside a
D but gets bent due to magnetic field• When the particle exits a D, the direction of
voltage can be changed and the ion gets accelerated before entering into the D on the other side
• If the frequency of the alternating voltage is just right, the charged particle gets accelerated continuously until it is extracted
Resonance Accelerators: Cyclotron
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
32Wednesday, Nov. 28, 2012
• For non-relativistic motion, the frequency appropriate for alternating voltage can be calculated from the fact that the magnetic force provides centripetal acceleration for a circular orbit
• In a constant angular speed, ω=v/r. The frequency of the motion is
• Thus, to continue accelerating the particle, the electric field should alternate in this frequency, cyclotron resonance frequency
• The maximum kinetic energy achievable for an cyclotron with radius R is
Resonance Accelerators: Cyclotron
2vmr
v qBr mc
2f
π
22 2 2
max max 21 12 2
qBRT mv m R
mc
vBqc
ω
2qBmcπ
12
q Bm cπ
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
33Wednesday, Nov. 28, 2012
• Accelerates particles along a linear path using resonance principle• A series of metal tubes are located in a vacuum vessel and connected
successively to alternating terminals of radio frequency oscillator• The directions of the electric fields changes before the particles exits the
given tube• The tube length needs to get longer as the particle gets accelerated to
keep up with the phase• These accelerators are used for accelerating light particles to very high
energies
Resonance Accelerators: Linear Accelerator
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
34Wednesday, Nov. 28, 2012
• For very energetic particles, the relativistic effect must be taken into account
• For relativistic energies, the equation of motion of a charge q under magnetic field B is
• For v ~ c, the resonance frequency becomes
• Thus for high energies, either B or should increase• Machines with constant B but variable are called synchro-
cyclotrons• Machines with variable B independent of the change of is
called synchrotrons
Synchroton Accelerators
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
35Wednesday, Nov. 28, 2012
• Electron synchrotrons, B varies while is held constant
• Proton synchrotrons, both B and varies• For v ~ c, the frequency of motion can be expressed
• For an electron
• For magnetic field strength of 2Tesla, one needs radius of 50m to accelerate an electron to 30GeV/c.
Synchroton Accelerators
/( )
0.3 )p GeV cpcR m
qB B Tesla
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
36Wednesday, Nov. 28, 2012
• Synchrotons use magnets arranged in a ring-like fashion.
• Multiple stages of accelerations are needed before reaching over GeV ranges of energies
• RF power stations are located through the ring to pump electric energies into the particles
Synchroton Accelerators
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Comparisons between Tevatron and LHC• Tevatron: A proton-anti proton collider at 2TeV
– Need to produce anti-protons using accelerated protons at 150GeV
– Takes time to store sufficient number of anti-protons• Need a storage accelerator for anti-protons
– Can use the same magnet and acceleration ring to circulate and accelerator particles
• LHC: A proton-proton collier at 14TeV design energy– Protons are easy to harvest– Takes virtually no time to between a fresh fill of particles into
the accelerator– Must use two separate magnet and acceleration rings
Wednesday, Nov. 28, 2012
PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
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Monday, Aug. 27, 2012 PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
Fermilab Tevatron and LHC at CERN• World’s Highest Energy proton-anti-proton collider
– 4km circumference– Ecm=1.96 TeV (=6.3x10-7J/p 13M Joules on the
area smaller than 10-4m2)– Equivalent to the kinetic energy of a 20t truck at the
speed 81mi/hr 130km/hr• ~100,000 times the energy density at the ground 0 of the
Hiroshima atom bomb– Was shut down at 2pm CDT, Sept. 30, 2011– Vibrant other programs running!!
Chicago
Tevatron p
p CDF DØ
• World’s Highest Energy p-p collider– 27km circumference, 100m underground– Design Ecm=14 TeV (=44x10-7J/p 362M
Joules on the area smaller than 10-4m2) Equivalent to the kinetic energy of a B727
(80tons) at the speed 193mi/hr 312km/hr ~3M times the energy density at the ground 0 of the
Hiroshima atom bomb• First 7TeV collisions on 3/30/10 The highest energy
humans ever achieved!!• First 8TeV collisions in 2012 on April 5, 2012
38
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Monday, Aug. 27, 2012 PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
39
LHC @ CERN Aerial View
Geneva Airport
ATLAS
CMS
France
Swizerland
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
40Wednesday, Nov. 28, 2012
• Subatomic particles cannot be seen by naked eyes but can be detected through their interactions within matter
• What do you think we need to know first to construct a detector?– What kind of particles do we want to detect?
• Charged particles and neutral particles– What do we want to measure?
• Their momenta• Trajectories• Energies• Origin of interaction (interaction vertex)• Etc
– To what precision do we want to measure?• Depending on the answers to the above questions we use
different detection techniques
Particle Detectors
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41Monday, Nov. 27, 2006 PHYS 3446, Fall 2006Jae Yu
Particle Detection Techniques
InteractionPoint
electron
photon
jet
muonneutrino -- or any non-interacting particle missing transverse momentum
B
Scintillating FiberSilicon Tracking
Charged Particle Tracks
Calorimeter (dense)
EM hadronic
Energy
Wire Chambers
Mag
net
Muon Tracks
We know x,y starting momenta is zero, butalong the z axis it is not, so many of our measurements are in the xy plane, or transverse
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
The ATLAS and CMS Detectors
42
• Fully multi-purpose detector with emphasis on lepton ID & precision E & P• Weighs 7000 tons and 10 story tall• Records 200 – 400 collisions/second• Records approximately 350 MB/second• Record over 2 PB per year 200*Printed material of the US Lib. of Congress
Monday, Aug. 27, 2012
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
43Wednesday, Nov. 28, 2012
• Two types of scintillators–Organic or plastic
• Tend to emit ultra-violate• Wavelength shifters are needed to reduce attenuation• Faster decay time (10-8s)• More appropriate for high flux environment
– Inorganic or crystalline (NaI or CsI)• Doped with activators that can be excited by electron-hole
pairs produced by charged particles in the crystal lattice• These dopants can then be de-excited through photon
emission• Decay time of order 10-6sec• Used in low energy detection
Scintillation Counters
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
44Wednesday, Nov. 28, 2012
• Scintillation detectors consist of a doped plastic that emits lights when a particle loses its energy via atomic excitation and transition back to lower energy states
• The light produced by scintillators are usually too weak to see– Photon signal needs amplification through photomultiplier
tubes• Gets the light from scintillator directly or through light guide
– Photocathode: Made of material in which valence electrons are loosely bound and are easy to cause photo-electric effect (2 – 12 cm diameter)
– Series of multiple dynodes that are made of material with relatively low work-function» Operating at an increasing potential difference (100 – 200 V) difference
between dynodes
Scintillation Detectors & Photo-multiplier Tube
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
45Wednesday, Nov. 28, 2012
Scintillation Detector Structure
Scintillation Counter
PMT
Readout Electronics
HV PS
Light Guide/WavelengthShifter
Scope
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
46Wednesday, Nov. 28, 2012
Some PMT’sSuper-Kamiokande detector
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
47Wednesday, Nov. 28, 2012
• Scintillator + PMT can provide time resolution of 0.1 ns. – What position resolution does this corresponds to?
• 3cm• Array of scintillation counters can be used to measure
the time of flight (TOF) of particles and obtain their velocities– What can this be used for?
• Can use this to distinguish particles with about the same momentum but with different mass
– How?• Measure
– the momentum (p) of a particle in the magnetic field– its time of flight (t) for reaching some scintillation counter at a distance L from
the point of origin of the particle– Determine the velocity of the particle and its mass
Time of Flight
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
48Wednesday, Nov. 28, 2012
• What is the Cerenkov radiation (covered in CH2)?– Emission of coherent radiation from the excitation of atoms and
molecules • When does this occur?
– If a charged particle enters a dielectric medium with a speed faster than light in the medium
– How is this possible?• Since the speed of light is c/n in a medium with index of refraction n, if the
particle’s β>1/n, its speed is larger than the speed of light• Cerenkov light has various frequencies but blue and ultraviolet
band are most interesting– Blue can be directly detected w/ standard PMTs– Ultraviolet can be converted to electrons using photosensitive
molecules mixed in with some gas in an ionization chamber
Cerenkov Detectors
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
49Wednesday, Nov. 28, 2012
• Threshold counters– Particles with the same momentum but with different mass will start emitting
Cerenkov light when the index of refraction is above a certain threshold– These counters have one type of gas but could vary the pressure in the chamber to
change the index of refraction to distinguish particles– Large proton decay experiments use Cerenkov detector to detect the final state
particles, such as p e+π0
• Differential counters– Measure the angle of emission for the given index of refraction since the emission
angle for lighter particles will be larger than heavier ones• Ring-imaging Cerenkov Counters (RICH)
– An energetic charged particle can produce multiple UV distributed about the direction of the particle
– The now stopped BaBar experiment at Stanford Linear Accelerator Center (SLAC) used RICH as the primary detector system
Cerenkov Detectors
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
50Wednesday, Nov. 28, 2012
Super Kamiokande A Differential Water Cerenkov Detector
• Kamioka zinc mine, Japan• 1000m underground• 40 m (d) x 40m(h) SS• 50,000 tons of ultra pure H2O• 11200(inner)+1800(outer) 50cm
PMT’s• Originally for proton decay
experiment• Accident in Nov. 2001, destroyed
7000 PMT’s• Dec. 2002 resumed data taking• This experiment was the first to
show the neutrinos oscillate
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
51Wednesday, Nov. 28, 2012
Super-K Event Displays
Stopping 3
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
52Wednesday, Nov. 28, 2012
• Semiconductors can produce large signal (electron-hole pairs) for relatively small energy deposit (~3eV)– Advantageous in measuring low energy at high resolution
• Silicon strip and pixel detectors are widely used for high precision position measurements– Due to large electron-hole pair production, thin layers (200 – 300 m)
of wafers sufficient for measurements– Output signal proportional to the ionization loss– Low bias voltages sufficient to operate– Can be deposit in thin stripes (20 – 50 m) on thin electrode– High position resolution achievable– Can be used to distinguish particles in multiple detector configurations
• So what is the catch?– Very expensive On the order of $30k/m2
Semiconductor Detectors
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
53Wednesday, Nov. 28, 2012
DØ Silicon Vertex Detector234
9 811
1
67
5
1012
1
6…...
12
34
Barrels F-Disks H-DisksChannels 387120 258048 147456Modules 432 144 96I nner R 2.7 cm 2.6 cm 9.5 cmOuter R 9.4 cm 10.5 cm 26 cm
One Si detector
BarrelDisk
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
54Wednesday, Nov. 28, 2012
• Magnetic measurement of momentum is not sufficient for physics, why?– The precision for angular measurements gets worse as
particles’ momenta increases– Increasing magnetic field or increasing precision of the
tracking device will help but will be expensive– Cannot measure neutral particle momenta
• How do we solve this problem?– Use a device that measures kinetic energies of particles
• Calorimeter– A device that absorbs full kinetic energy of a particle– Provides signal proportional to deposited energy
Calorimeters
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
55Wednesday, Nov. 28, 2012
• Large scale calorimeter were developed during 1960s– For energetic cosmic rays– For particles produced in accelerator experiments
• How do high energy EM (photons and electrons) and Hadronic particles deposit their energies?– Electrons: via bremsstrahlung– Photons: via electron-positron conversion, followed by
bremsstrahlung of electrons and positrons– These processes continue occurring in the secondary
particles causing an electromagnetic shower losing all of its energy
Calorimeters
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
56Wednesday, Nov. 28, 2012
Electron Interactions in material (showering)
Photon, γ
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
57Wednesday, Nov. 28, 2012
• Hadrons are massive thus their energy deposit via brem is small
• They lose their energies through multiple nuclear collisions• Incident hadron produces multiple pions and other secondary
hadrons in the first collision• The secondary hadrons then successively undergo nuclear
collisions• Mean free path for nuclear collisions is called nuclear
interaction lengths and is substantially larger than that of EM particles
• Hadronic shower processes are therefore more erratic than EM shower processes
Calorimeters
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
58Wednesday, Nov. 28, 2012
• High energy particles require large calorimeters to absorb all of their energies and measure them fully in the device (called total absorption calorimeters)
• Since the number of shower particles is proportional to the energy of the incident particles
• One can deduce the total energy of the particle by measuring only the fraction of their energy, as long as the fraction is known Called sampling calorimeters– Most the high energy experiments use
sampling calorimeters
Sampling Calorimeters
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
59Wednesday, Nov. 28, 2012
EM
Hadron
How particle showers look in detectors
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
60Wednesday, Nov. 28, 2012
Principles of Calorimeters
Total absorption calorimeter: See the entire shower energy
Sampling calorimeter: See only some fraction of shower energy
For EM
Absorber plates
visE fE
For HAD visE fE
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
61Wednesday, Nov. 28, 2012
Example Hadronic Shower (20GeV)
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
62Wednesday, Nov. 28, 2012
Conventional Neutrino Beam
• Use large number of protons on target to produce many secondary hadrons (π, K, D, etc)
• Let π and K decay in-flight for beam– π+ 99.99%, K 63.5%– Other flavors of neutrinos are harder to make
• Let the beam go through thick shield and dirt to filter out and remaining hadrons, except for – Dominated by
p
Good target
Good beam focusing
Long decay region
Sufficient dump
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
63Wednesday, Nov. 28, 2012
How can we select sign of neutrinos?• Neutrinos are electrically neutral• Need to select the charge of the secondary hadrons
from the proton interaction on target• NuTeV experiment at Fermilab used a string of magnets
called SSQT (Sign Selected Quadrupole Train)
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
64Wednesday, Nov. 28, 2012
• Calorimeter– 168 FE plates & 690tons– 84 Liquid Scintillator– 42 Drift chambers interspersed
A Typical Neutrino Detector: NuTeV
• Solid Iron Toroid• Measures Muon momentum• p/p~10%
Continuous test beam for in-situ calibration
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
65Wednesday, Nov. 28, 2012
Charged Current Events
Neutral Current Events
How Do Neutrino Events Look?
x-view
y-view
x-view
y-viewNothing is coming in!!!
Nothing is coming in!!!
Nothing is going out!!!
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PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu
66Wednesday, Nov. 28, 2012
Source of Cleaner Neutrino BeamMuon storage ring can generate 106 times higher flux and well understood, high purity neutrino beam significant reduction in statistical uncertainty But e and from muon decays are in the beam at all times Deadly for traditional heavy target detectors
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• Why is the mass range so large (0.1mp – 175 mp)?• How do matters acquire mass?
– Higgs mechanism but where is the Higgs?• Why is the matter in the universe made only of particles?• Neutrinos have mass!! What are the mixing parameters, CP
violations and mass ordering?• Why are there only three apparent forces?• Is the picture we present the real thing?
– What makes up the 96% of the universe?– How about extra-dimensions?
• Are there any other theories that describe the universe better?– Does the super-symmetry exist?
• Where is new physics?
April 24, 2012 Searchees for the Higgs and the Future Dr. Jaehoon Yu
What’s the current hot issues?
67
- Higgs mechanism, did we find the Higgs?
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Is the Higgs particle discovered? Dr. Jaehoon Yu
68July 6, 2012
What is the Higgs and What does it do?• When there is perfect symmetry, one cannot tell
directions!• Only when symmetry is broken, can one tell directions • Higgs field works to break the perfect symmetry and
give mass– This field exists right now amongst us so that we have
mass• Sometimes, this field spontaneously generates a
particle, the Higgs particle• So the Higgs particle is the evidence of the existence
of the Higgs field!
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Is the Higgs particle discovered? Dr. Jaehoon Yu
69July 6, 2012
How do we look for the Higgs?• Higgs particle is so heavy they decays into some
other particles very quickly• When one searches for a new particle, you look for
the easiest way to get at them• Of these the many signatures of the Higgs, some
states are much easier to find, if it were the Standard Model one– H γγ– H ZZ* 4e, 4, 2e2, 2e2 and 22 – H WW*2e2 and 22 – And many more complicated signatures
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Is the Higgs particle discovered? Dr. Jaehoon Yu
70July 6, 2012
How do we look for the Higgs?• Identify the Higgs candidate events
• Understand fakes (backgrounds)
• Look for a bump!!
e- (μ-)e+ (μ+)
e-
e+
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The ATLAS and CMS DetectorsSub System ATLAS CMS
Design
Magnet(s) Solenoid (within EM Calo) 2T3 Air-core Toroids
Solenoid 3.8TCalorimeters Inside
Inner TrackingPixels, Si-strips, TRTPID w/ TRT and dE/dx
Pixels and Si-stripsPID w/ dE/dx
EM CalorimeterLead-LAr Sampling
w/ fine longitudinal segmentationLead-Tungstate Crys. Homogeneous
w/o longitudinal segmentation
Hadronic Calorimeter Fe-Scint. & Cu-Larg (fwd) Brass-scint. & Tail Catcher
Muon Spectrometer SystemAcc. ATLAS 2.7 & CMS 2.4
Instrumented Air Core (std. alone) Instrumented Iron return yoke411
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Oct. 23, 2012 Recent LHC Higgs Results LCWS12, Jae Yu, U. Texas at Arlington
72
Amount of LHC Data
20100.05 fb-1
at 7 TeV
2012:16 ~17fb-1
at 8 TeV thus far~1fb-1//week
20115.6 fb-1
at 7 TeV
4th July announcement
Max inst. luminosity:~ 7.7 x1033 cm-2 s-1
Superb performance!!
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73
Experiment’s design value (expected to bereached at L=1034 !)
Z event from 2012 data with 25 reconstructed vertices
The BIG challenge in 2012: PILE-UP
Z
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“raw” mass spectrum
weighted: wi ~S/B in each category i
After all selections: 59059 events
peak above a large smooth background,relies upon excellent mass resolution
Data sample mH of max local significance significance obs. (exp. SM H) 2011 126 GeV 3.4 σ (1.6) 2012 127 GeV 3.2 σ (1.9) 2011+2012 126.5 GeV 4.5 σ(2.5) ATLAS2011+2012 125.5GeV 4.1 σ(2.8) CMS
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75
pT (e,e,μ,μ)= 18.7, 76, 19.6, 7.9 GeV, m (e+e-)= 87.9 GeV, m(μ+μ-) =19.6 GeV12 reconstructed vertices
2e2 candidate event w/ M2e2=123.9GeV
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76
Excluded at 95% CL:112-122, 131-559 GeV
All Channel Combined Exclusion
Only little sliver in 122 – 135 GeV and high mass left
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77
5.9σ
All Channel Combined Significance
ATLAS
CMS
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ATLAS and CMS Combined Higgs – end of 2011
Oct. 23, 2012 78Recent LHC Higgs Results LCWS12, Jae Yu, U. Texas at Arlington
Standard Model Higgs excluded in 110.0 <MH<117.5 GeV, 118.5 <MH< 122.5 GeV, and 129<MH<539 GeV & 127.5<MH<543GeV
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79
Evolution of the excess with time
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80
Evolution of the excess with time
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81
Evolution of the excess with time
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82
Evolution of the excess with time
07/12 CERN Prel.
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83
Evolution of the excess with time
07/12 CERN Prel.
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So have we seen the Higgs particle?• The statistical significance of the finding is over 5 standard
deviation– Level of significance: 99.99994%– We can be wrong once if we do the same experiment 1,740,000 times
• So did we find the Higgs particle?– We have discovered a new particle, the heaviest boson we’ve seen thus
far • Since this particle decays to two spin 1 particles, the possible spin states of this
new boson is either 0 or 2!– It has some properties consistent with the Standard Model Higgs
particle– We, however, do not have enough data to precisely measure all the
properties – mass, life time, the rate at which this particle decays to certain other particles, etc – to definitively determine
July 6, 2012 84Is the Higgs particle discovered? Dr. Jaehoon Yu
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So why is this discovery important?• This is the giant first in completing the Standard
Model• Will help understand the origin of mass and the
mechanism at which mass is acquired• Will help understand the origin and the structure of
the universe and the inter-relations of the forces• Will help us make our lives better• Generate excitements and interests on science and
train the next generation
July 6, 2012 85Is the Higgs particle discovered? Dr. Jaehoon Yu
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Long Term LHC Plans• 2012 run will end with ~25fb-1
– Combined with 2011 run (5.6fb-1), a total of 30fb-1
• 2013 – 2014: shutdown (LS1) to go to design energy (13 – 14TeV) at high inst. Luminosity
• 2015 – 2017: √s=13 – 14TeV, L~1034, ~100fb-1
• 2018: Shut-down (LS2)• 2019 – 2021: √s~=13 – 14TeV, L~2x1034, ~300fb-1
• 2022 – 2023: Shut-down (LS3)• 2023 – 2030(?): √s=13 – 14TeV, L~5x1034 (HL-LHC),
~3000fb-1
Oct. 23, 2012 Recent LHC Higgs Results LCWS12, Jae Yu, U. Texas at Arlington
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July 6, 2012
What next? Future Linear Collider• Now that we have found a new boson, precision measurement of the
particle’s properties becomes important• An electron-positron collider on a straight line for precision
measurements• 10~15 years from now (In Dec. 2011, Japanese PM announced that
they would bid for a LC in Japan)• Takes 10 years to build the detector
L~31km
Is the Higgs particle discovered? Dr. Jaehoon Yu
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Circumference ~6.6km~300 soccer fields
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Bi-product of High Energy Physics Research
July 6, 2012 88Is the Higgs particle discovered? Dr. Jaehoon Yu
Can you see what the object is?
WWW Came from HEP!!!
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July 6, 2012 Is the Higgs particle discovered? Dr. Jaehoon Yu
GEM Application Potential FAST X-RAY IMAGING
Using the lower GEM signal, the readout can be self-triggered with energy discrimination:
A. Bressan et al, Nucl. Instr. and Meth. A 425(1999)254F. Sauli, Nucl. Instr. and Meth.A 461(2001)47
9 keV absorption radiography of a small mammal (image size ~ 60 x 30 mm2)
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So what?• The LHC opened up a whole new kinematic regime
– The LHC performed extremely well in 2011 and 2012!• Accumulated 22fb-1 thus far, and still have a weeks to go – additional ~1fb-1 expected!
• Searches conducted with 4.8fb-1 at 7TeV and 5.8fb-1 at 8TeV of data• Observed a neutral boson couple to vector bosons and whose measured mass is
– At 5.9σ/5.0σ significance, corresponds to 1.7x10-9 bck fluctuation probability!– Compatible with production and decay of SM Higgs boson
• Excluded MH=112 – 122 and 131 – 559GeV (ATLAS) @95% CL• Linear collider and advanced detectors are being developed for future precision
measurements of Higgs and other newly discovered particles• Outcome and the bi-product of HEP research impacts our daily lives
– WWW came from HEP– GEM will make a large screen low dosage X-ray imaging possible
• Many technological advances happened through the last 100 years & coming 100 yrs• Continued sufficient investment to forefront scientific endeavors are absolutely
necessary for the future!Oct. 23, 2012 Recent LHC Higgs Results
LCWS12, Jae Yu, U. Texas at Arlington90