Transcript hdwsmp2011 - FSU High Energy Physics

Overview of Particle Physics
-- the path to the Standard Model
1
Topics
 “Prehistory” (A.A.)
o “prehistory” -- 19th century
o electron, radioactivity, nucleus
 historical flashback over development of the
field
o cosmic rays
o spectroscopy era, particle zoo
o collider era
 standard model of particle physics
2
Atoms, Nucleus
 electron: first hint that atom not indivisible
 natural radioactivity  understanding of
composition of atom, nucleus
 atom = nucleus surrounded by electrons (Geiger,
Marsden, Rutherford, 1906 -1911)
 hydrogen nucleus = proton, is component of all
nuclei (1920)
 neutron (Bothe, Becker, Joliot-Curie, Chadwick,
1930 – 1932)
Neutrino predicted by Pauli 1930 to solve puzzle in
some nuclear decays (“beta decays”)
3
Status of satisfaction
Atoms are made of electrons and nucleus
Nuclei are made of protons and neutrons
 everything looks simple and tidy
 But….
4
Cosmic rays
 Charged particles coming
from
the sky
 discovered by Victor Hess (1912)
 Observed on mountains and
from balloons
 intensity increases with height
above Earth
o must come from “outer space”
 Lots come from the Sun
 Mostly low energy protons and
nuclei
 Very high energy cosmic rays are
from outside the Solar System,
but probably from within the galaxy
5
6
Cosmic rays  new “elementary” particles
 new detectors (cloud chambers, emulsions)
exposed to cosmic rays  discovery of many new
particles
positron (anti-electron) : predicted by Dirac
(1928), discovered by Anderson 1932
 muon (μ): 1937 Nedermeyer, Anderson, Street,
Stevenson
 pion (π) predicted by Yukawa (1935), observed
1947 (Lattes, Occhialini, Powell)
 strange particles (K, Λ, Σ,…..
7
Cloud chamber
 Container filled with gas (e.g. air), plus vapor close to
its dew point (saturated)
 Passage of charged particle  ionization;
 Ions form seeds for condensation
 condensation along path of particle
 path of particle becomes visible as chain of droplets
8
Positron discovery
 Positron (anti-electron)
 predicted by Dirac
(1928) -- needed for
relativistic quantum
mechanics
 existence of
antiparticles doubled
the number of known
particles!!
 Cloud chamber with

lead sheet in middle
 Particle loses energy
going through lead
 Track going downward
(has lower energy
after lead
– more curved)
 Positron made by “pair
production”
9
Anderson and his cloud chamber
10
Muon discovery
 Nedermeyer, Anderson, Street, Stevenson 1936-1937,
using cloud chambers
 Found mass to be between that of electron and proton --called “mesotron”
 Later other particles with intermediate mass found – called
“mesons”
 Mesotron renamed “muon” μ -- very different from mesons
(no strong interaction, spin ½) -- was like an electron, but
heavier
 I. Rabi: “who ordered that?”
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“Strange particles”
 Kaon: discovered 1947; first called “V” particles, later
“strange” because they did not decay as expected
K0 production and decay
in a bubble chamber
12
Towards the “Particle Zoo”
Accelerators, beam lines
 increase energy of particles and steer
them where you want them  become
independent of cosmic rays
 can make new particles and use them as
projectiles  can do planned
experiments with probes of your choice
 new detectors: bubble chamber, wire
chamber, ….
13
Bubble chamber
 Operating principle:
 Vessel, filled with (e.g.) liquid hydrogen at a
temperature above the normal boiling point but held
under a pressure of about 10 atmospheres by a large
piston to prevent boiling.
 After passage of particles, move piston to reduce
pressure  boiling point lowered  boiling starts
along particle tracks  bubbles develop.
 Let bubbles grow (about 3 milliseconds), then tracks
are photographed (flash); provide stereo views of
tracks by use of several cameras .
 Then move piston back  recompress the liquid 
collapse bubbles before boiling all over.
 Invented by Glaser in 1952 (when he was drinking beer)
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--
_
 p p 
_
p n K0 K- + - 0
_
 n + p  3 pions
 0  ,   e+ e K0  + -
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Particle Zoo
 1940’s to 1960’s :
Plethora of new particles
discovered (mainly in cosmic rays):
o μ-, π±, π0, K, Λ0, Σ+ , Σ0 , Ξ,….
Accelerators, better detectors:
o Yet more “stable particles, Plus many
(100’s) particles decaying very fast
(“resonances”) (decay after 10-21 sec)
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The Particle Zoo!
e,
±
0
 , ,
±
0
0
K , K S, K L,
0
+
 , p, n,  ,
0
 , , , …
ρ, ω, φ, f, Δ, N, ……
±
e,
±
,
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Too many particles, chaos
 question:
 Can nature be so messy?
 are all these particles really
intrinsically different?
 or can we recognize patterns or
symmetries in their nature (charge,
mass, flavor) or the way they behave
(decays)?
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Symmetry to the rescue – Periodic table of particles
 1962: “eightfold way”, “flavor SU(3)” symmetry (Gell-Mann,
Ne’eman)
 allows classification of particles into “multiplets”
 Mass formula relating masses of particles in same
multiplet
 Symmetry can be explained by quark hypothesis
 quark model (Gell-Mann, Zweig 1964) :
three different kinds of quarks (u, d, s)
 Explains grouping of all strongly interacting particles
 Allows prediction of new particle Ω- , with all of its
properties (mass, spin, expected decay modes,..)
 subsequent observation of Ω- with expected properties
at BNL (1964)
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ΩBNL
1964
http://www.bnl.gov/bnlweb/history/Omega-minus.asp
 eight-fold way  quark model – particles
made up of three different “quarks” – u, d, s
 p = uud, n = udd,… Ω- = sss
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Quark discovery
Deep inelastic scattering (1968, Kendall, Taylor)
Shoot electrons of high energy on protons
proton
u
electron
d
u
 sometimes electron can penetrate deep into
proton: “deep” => probe inside structure (like
Rutherford scattering probed inside of nucleus)
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22
“Standard Model”
 refinement of these ideas, more quarks, “color”,
gauge field theory  Standard Model
 matter particles – quarks and leptons
 force carriers – “gauge bosons”
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Contemporary
Physics
Education
Project24
“every-day” matter
Proton
Neutron
d
u
u
u
d
Photon
d

Electron
e
Electron Neutrino
e
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No isolated quarks
_
q’
q
_
q’
q
q
q
q
q
_
_
q’
_
q’ q
q’
_
_
q’ q
_
q’ q
two separating
quarks
q’
_
q’
_
q’
q
q
q
q
_
_
q’ q
q’
_
_
q’ q
_
q’ q
q’
_
q’
jets
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Forces (interactions)
 Strong interaction
1
 Binds protons and neutrons to form
nuclei
 Electromagnetic interaction 10-2
 Binds electrons and nuclei to form
atoms
 Binds atoms to form molecules etc.
 Weak interaction  10-10
 changes “flavors” (e.g.  decay)
 important in stars’ energy “production”
 Gravitational interaction
10-39
 Binds matter on large scales
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Particles of Standard Model
Leptons
-1/3
-1
0
u
u
u
d
d
d
e
e
c
c
c
s
s
s


t
t
t
b
b
b
t
t
g
g
g
g
g
g
g
g
I
II
III

Z
W±
Bosons Fermions
+2/3
Quarks
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Electromagnetic interaction
Proton
exchange of photons
q1
Photon
q1q2
F k 2
r
Electron
q2
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The Strong Force
d
u
g
u
Strong force caused by
the exchange of gluons
d
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Weak interaction
 Beta decay
Neutron
u
d
Mean lifetime of a free
neutron ~ 10.3 minutes
Proton
d
u
d
Mean lifetime of a free
proton > 1031 years!
u
WElectron
e
Exchange of W±
e
Anti-electron Neutrino
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Testing the Standard Model
 The Standard Model works very well, but we want
to want to make sure
want to probe small structures, create massive
particles
 need more powerful accelerators – big colliders
 more sophisticated and big detectors
 many people necessary => big collaborations
 resources concentrated in large laboratories,
effort international in scope
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The Colliders
 Tevatron collider
Colliding bunches of protons and anti protons;
bunches meet each other every
_ detectors (DØ
396 ns in the center of two
and CDF) (steered apart at other places)
particle has ~ 980 GeV of energy,
 soEach
the total energy in the center of mass
is 1960 GeV = 1.96 TeV
 About 10M collisions per second
LHC
Colliding bunches of protons; bunches
 meet
each other every 25ns in the
center of 4 detectors
particle has ~ 3.5TeV of energy,
 Each
so the total energy in the center of mass
is 7 (=> 14) TeV

Susan Blessing
About 400M collisions per second
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Fermi
National
Accelerator
Laboratory
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 peak luminosity 4X1032 cm-1s-1 )
 energy in c.m.s. 1.9 TeV, bunch crossing time 396 ns
 total integrated luminosity 10fb-1
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Modern particle physics detectors
 today’s particle physics detectors:
 combine many detection techniques
 “Russian doll” like structure -- many layers
surrounding interaction region
 “general purpose detectors” – detect, identify
and measure as many different kinds of particles
as possible, (nearly) complete coverage of
interaction region (“hermetic”)
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Identifying particles
37
DØ Detector
Central Tracking
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Artist’s impression of a top event
39
Double b-tagged +jets event
B decay
Primary vertex
b jet
MTC
Primary vertex
IP
B decay
b jet
40
Standard Model
 A theoretical model of
interactions of elementary
particles, based on quantum field
theory
 “Matter particles”
 Quarks: up, down,
charm,strange, top, bottom
 Leptons: electron, muon, tau,
neutrinos
 “Force particles”
 Gauge Bosons
o  (electromagnetic force)
o W, Z (weak, electromagnetic)
o g gluons (strong force)
 Higgs boson (not yet observed)
 Mass
But note: gravity not part of SM
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Summary
 we’ve come a long way ……
 technical breakthroughs in accelerators and detectors
allowed new discoveries
 New theoretical ideas provided new understanding
 Standard Model (theory of particle interactions) works
embarrassingly well!
 Has been tested by many hundreds of precision
measurements over last four decades – very few
measurements differ by more than 1 or 2 standard
deviations
 Even some amount of frustration – always hope to see
experimental result in disagreement with theory
 But there are some open questions …………………
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The End
43
44
Testing the Standard Model
 want to probe small structures, create massive
particles
 need more powerful accelerators – big colliders
 more sophisticated and big detectors
 many people necessary => big collaborations
 resources concentrated in large laboratories,
effort international in scope
45
The TeVatron Collider
 Tevatron collider
 Colliding bunches of protons and anti-protons;
bunches meet each other every 396 ns in the
center of two detectors (DØ and CDF)
(steered apart at other places)
 Each particle has ~ 980 GeV of energy,
so the total energy in the center of mass
is 1960 GeV = 1.96 TeV
 About 2,500,000 collisions per second
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Fermilab aerial view
47
Fermilab TeVatron tunnel
48
the new DØ detector
49
DØ Detector in hall January 2001
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The Discovery of Top Quark
1977 – 1992
Many null results
1992 – 1993
A few interesting
events show up
1994, CDF
First evidence
mt ~ 170 GeV/c2
1995 – CDF, DØ
Discovery!
1994, DØ
mt > 131 GeV/c2
51
Creating Top Anti-Top Quark pairs
b
P
t
+
t
b
e  t uc
+
-1/ 3
+2 / 3
W
+
+
+
 e   t d s
P
-2 / 3
+1/ 3
e  t uc
-
W
-
-
-
 e   t d s
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-
What do we actually “see”
_
t t e+ jets
Muon
Jet-1
Jet-2
Missing energy
Electron
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“event display” of a DØ
top event
t t  e  + jets
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Ωb (http://www.fnal.gov/pub/presspass/images/DZero-Omega-discovery.html
 2008 DØ experiment
at Fermilab:
 discover brother
of Ω- , the Ωb
 Ω- = sss,
Ωb = ssb,
 theory predicts
properties, decay
modes, ..
 confirmed by
experiment
55
Summary
 we’ve come a long way ……
 technical breakthroughs in accelerators and detectors
allowed new discoveries and new understanding
 Standard Model (theory of particle interactions) works
embarrassingly well!
 Has been tested by many hundreds of precision
measurements over last three decades – very few
measurements differ by more than 1 or 2 standard
deviations
 Even some amount of frustration – always hope to see
experimental result in disagreement with theory
 But there are some open questions …………………
56
Higgs search – status 2011

combination of over 40 analyses by 100 physicists in two experiments
57
Higgs decay
Branching Fraction
 Decay modes depend on mass
 A low-mass Higgs will decay
primarily to bb
 high-mass Higgs will decay
primarily to W+W A low-mass Higgs (below 130 GeV)
is favored by supersymmetric
models
 There were “hints” of Higgs
production at >≈115 GeV, just
before LEP was turned off.
Higgs Mass (GeV/c2)
58
Future of DØ
The Tevatron stopped running at the end of September
with 10 fb-1 of data recorded per experiment.
So there’s plenty of data analysis for the next few years.
59
Higgs Decay Modes
Decay modes depend on Higgs mass
A low-mass Higgs will
_ decay
primarily to bb
Branching Fraction
A high-mass Higgs will decay
primarily to W+W-
A low-mass Higgs (below 130 GeV/c2)
is favored by supersymmetric models.
And, those “hints” of Higgs
production at 115 GeV/c2,
just before LEP was turned off.
Higgs Mass (GeV/c2)
60
Summary
 many different types of accelerators have been developed
for nuclear and particle physics research
 different techniques suitable for different particles and
energy regimes
 most accelerators in large research laboratories use
several of these techniques in a chain of accelerators
 active research going on to develop new accelerating
techniques for future applications
 many types of accelerators have found applications in
fields other than nuclear and particle physics (e.g. medicine,
ion implantation for electronics chips, condensed matter
research, biology,….)
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Summary
 Particle detection is based on interaction of
particles with material in the detector; detectors
usually have some “amplification” mechanism to render
result of this interaction observable
 Many detection techniques have been developed over
the last century
 breakthrough in detection techniques often led to
breakthrough discoveries
 many of the detectors and/or techniques that were
originally developed for basic research in nuclear or
particle physics are now used in other fields; they
often have led to advances in medical diagnosis (e.g.
MRI, PET,….)
62
63
A Century of Particle Physics
J.J Thomson
Top quark
1995
Electron – 1897
64
Sizes and
distance scales
 visible light:
wavelength
≈5∙10-7m
 virus 10-7m
 molecule 10-9m
 atom 10-10m
 nucleus 10-14m
 nucleon 10-15m
 quark <10-18m
65
The Building Blocks of a Dew Drop
 dew drop: 1021 molecules
of water.
 Each molecule = one
oxygen atom and two
hydrogen atoms (H2O).
 Atom: nucleus
surrounded by electrons.
 Electrons bound to the
nucleus by photons
 nucleus of a hydrogen
atom = single proton.
 Proton: three quarks,
held together by gluons
just as photons hold the
electron to the nucleus
in the atom
66
Very early era (19th century)
 chemistry, electromagnetism
 discharge tubes, “canal rays”, “cathode rays”
 photoelectric effect (Hertz, 1887)
 radioactivity (Becquerel, 1895)
 X-rays (Röntgen, 1895)

67
68
What holds the world together?
interaction
strong
electromagnetic
weak
gravity
participants
quarks
charged
particles
all
particles
all
particles
1
10-2
10-10
10-39
g
gluon

relative strength
field quantum
(boson)
photon
W±
Z0
G
graviton
69
The CMS Detector
HF
HE
HB
HO
70
DØ detector
Muon System
1.9T magnetized Fe,
Prop. drift tubes
40,000 channels
Central Tracking
Calorimeter
Uranium-liquid Argon
60,000 channels
71
Transverse slice through CMS detector
72
Seeing = photon scattering experiment
 our eye is a photon detector; (photons = light “quanta” = packets
of light -- see “photoelectric effect”)
 “seeing” is performing a photon scattering experiment:
o light source provides photons
o photons “interact” with object of our interest -- some absorbed,
some scattered, reflected
o some of scattered/reflected photons make it into eye; focused onto
retina;
o photons detected by sensors in retina (photoreceptors -- rods and
cones)
o transduced into electrical signal (nerve pulse)
o amplified when needed
o transmitted to brain for processing and interpretation
73
HOW TO SEE SMALL THINGS
 “seeing an object”
= detecting light that has been reflected off the
object's surface
 “visible light”= those electromagnetic waves that our eyes can
detect
 “wavelength” of e.m. wave (distance between two successive
crests) determines “color” of light
 if size of object is much smaller than wavelength, then wave is
hardly influenced by object
 wavelength of visible light: between 410-7 m (violet) and 7 10-7
m (red);
 diameter of atoms: 10-10 m  can’t see them with “ordinary”
(visible) light
 generalize meaning of seeing:
 seeing is to detect effect due to the presence of an object
 quantum theory  “particle waves”, with wavelength 1/(m v)
 use accelerated (charged) particles as probe, can “tune”
wavelength by choosing mass m and changing velocity v
 this method is used in electron microscope, as well as in
“scattering experiments” in nuclear and particle physics
74
Particle physics experiments
 Particle physics experiments:
 collide particles to
o produce new particles
o reveal their internal structure and laws of their
interactions by observing regularities, measuring
cross sections,...
 colliding particles need to have high energy
o to make objects of large mass
o to resolve structure at small distances
 to study structure of small objects:
o need probe with short wavelength: use particles with
high momentum to get short wavelength
o Particles behave as if they had a wavelength  = h/p
 mass-energy equivalence (E = mc2) plays an
important role; in collisions, kinetic energy
converted into mass energy;
75
ACCELERATORS
 are devices to increase the energy of charged particles;
 magnetic fields to shape (focus and bend) the trajectory of the
particles;
 electric fields for acceleration.
 types of accelerators:
 electrostatic (DC) accelerators
o Cockcroft-Walton accelerator (protons up to 2 MeV)
o Van de Graaff accelerator (protons up to 10 MeV)
o Tandem Van de Graaff accelerator (protons up to 20 MeV)
 resonance accelerators
o cyclotron (protons up to 25 MeV)
o linear accelerators
 electron linac: 100 MeV to 50 GeV
 proton linac: up to 70 MeV
 synchronous accelerators
o synchrocyclotron (protons up to 750 MeV)
o proton synchrotron (protons up to TeV)
o electron synchrotron (electrons from 50 MeV to 90 GeV)
 storage ring accelerators (colliders)
76
Van de Graaff accelerator
 use powersupply to deposit charges on belt; pick charges
off at other end of belt and deposit on “terminal”
 now rubber belt replaced by “pellet” chain – “pelletron”
77
Cyclotron
 two hollow metal chambers (“dees”)
with “gap” between them
 dees connected to AC voltage
source - one dee positive when
other negative  electric field in
gap between dees, but no electric
field inside the dees;
 source of protons in center,
everything in vacuum chamber;
 whole apparatus in magnetic field
perpendicular to plane of dees;
 frequency of AC voltage such that
particles always accelerated when
reaching the gap between the dees;
 in magnetic field, particles are
deflected: p = qBR
p = momentum, q = charge,
B = magnetic field strength,
R = radius of curvature
 radius of path increases as
momentum of proton increases
time for passage always the same
as long as momentum proportional
to velocity;
this is not true when velocity
becomes too big (relativistic
effects)
78
Synchrotron
 synchrotron
 Magnetic field B (to keep particles on circle) synchronized
with electric field (for acceleration);
magnetic field increases during acceleration,
radius of orbit fixed.
 synchrotron is most common accelerator used in particle
physics
 first synchrotrons:
 Cosmotron (Brookhaven), 3.3 GeV, 1953
 Bevatron (Berkeley): 6.2 GeV, 1954
 PS (CERN) 26 -> 28 GeV, 1959
 AGS (Brookhaven) 30 -> 33 GeV, 1960
79
80
81
Particle physics experiments
 Particle physics experiments:
 collide particles to
o produce new particles
o reveal their internal structure and laws of their
interactions by observing regularities, measuring
cross sections,...
 colliding particles need to have high energy
o to make objects of large mass
o to resolve structure at small distances
 to study structure of small objects:
o need probe with short wavelength: use particles with
high momentum to get short wavelength
o Particles behave as if they had a wavelength  = h/p
 mass-energy equivalence (E = mc2) plays an
important role; in collisions, kinetic energy
converted into mass energy;
82
Standard Model
 A theoretical model of
interactions of elementary
particles, based on quantum field
theory
 Symmetry:
 SU(3) x SU(2) x U(1)
 “Matter particles”
 Quarks: up, down,
charm,strange, top, bottom
 Leptons: electron, muon, tau,
neutrinos
 “Force particles”
 Gauge Bosons
o  (electromagnetic force)
o W, Z (weak, electromagnetic)
o g gluons (strong force)
 Higgs boson
 spontaneous symmetry
breaking of SU(2)
 mass
83
Detector Cartoon
Measure transverse
momentum/energy
**
*
Tracking chamber
Toroidal magnet
**
*
Tracking chamber
Calorimeter
(dense material)
Solenoidal magnet
*
*
*
*
Electron
*
*
**
Muon
** *
** *
**** **
Jet
Tracking chamber
Neutrino
(missing energy)
84
Detector Cartoon
**
*
Measure transverse
momentum/energy
Tracking chamber
Toroidal magnet
**
*
Tracking chamber
Calorimeter
(dense material)
Solenoidal magnet
*
*
*
*
*
*
*
*
Electron
Muon
**
**
**
**
Jet
*
*
*
*
Tracking chamber
Neutrino
(missing energy)
Susan Blessing
85