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Nuclear and Particle Physics 3 Nuclear and Particle Physics 3 = Particle Physics 2= Particle Physics 2
2
TopicsTopics
Recap of SM particles and forces Symmetry How to do a particle physics
experiment Fermilab, TeVatron collider DØ experiment
detector discovery of top quark
experimental tests of the SM Beyond the SM The Future: CERN, LHC, CMS Summary
3
u u u d d d e
b b b
c c c s s s
g g g g g g g g
Z W+/-
e
Quarks Leptons+2/3 -1/3 -1 0
I
II
III
Boso
ns
Ferm
ions
Particles of Standard ModelParticles of Standard Model
t t t
4
“every-day” matter
Proton
d u
d
e
Neutron
e
u d
u
Electron Electron Neutrino
Photon
5
Forces (interactions)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
Causes radioactivity
Gravitational interaction 10-40
Binds matter on large scales
6
What holds the world together?What holds the world together?
interaction
participants
relative strength
field quantum(boson)
strong
quarks
1
ggluon
electro-magnetic
charged particles
10-2
photon
weak
all particles
10-10
W± Z0
gravity
all particles
10-40
Ggraviton
7
Beta decay
d u
d
Neutron
Proton
u d
u
e
Electron e
Anti-electron Neutrino
W
Mean lifetime of a free neutron ~ 10.3 minutes Mean lifetime of a free proton > 1031 years!QuestionQuestion: Why doesn’t the neutron in the deuteron decay? Hint: deuteron mass = 1875.6 MeV/c2
8
SymmetrySymmetry
An object is called symmetric if one can do something with/to it (“perform a transformation”) without having changed it when one is finished with the procedure.
“symmetry” in physics: invariance under a transformation laws of nature are invariant under
certain transformations electromagnetic theory has
“gauge symmetry” which is related to charge conservation
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The Noether-TheoremThe Noether-Theorem
Emmi Noether (1882-1935)
Symmetry Conservation Law Symmetry Conservation Law
Laws of physics are independent of...Laws of physics are independent of...
origin of time axis energy conservation
origin of spatial axes momentum conservation
orientation of spatial axes angular mom. conservation
10
““Gauge Transformations”Gauge Transformations”
Hermann Weyl (1920): noted scale invariance of electromagnetism tried to unify general relativity and
electromagnetism conjectured that “Eichinvarianz”) (scale
invariance) may be also a local gauge theory of general relativity – did not work out
later realized that requiring the Schrödinger equation to be invariant under a local gauge (phase) transformation leads naturally to electromagnetic field and gives the form of the interaction of a charged quantum particle
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Global and local gauge Global and local gauge transformationstransformations
global transformation: same transformation carried out at all space-time
points (“everywhere simultaneously”) local transformation:
different transformations at different space-time points
globally invariant theories in general not invariant under local transformations
in quantum field theory, can restore invariance under local gauge (phase) transformation by introducing new force fields that interact with the original particles of the theory in a way specified by the invariance requirement
i.e. in this sense the dynamics of the theory is governed by the symmetry properties
can view these force fields as existing in order to permit certain local invariances to be true
electroweak interaction theory as well as QCD follow from gauge invariance which is a generalization of the gauge invariance of Maxwell’s equations
12
Particle physics experimentsParticle 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 remember de Broglie wavelength of a particle
= h/p in particle physics, relativistic effects
important; mass-energy equivalence plays an important role; in collisions, kinetic energy converted into mass energy;
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How to do a particle physics How to do a particle physics experimentexperiment
Outline of experiment: get particles (e.g. protons, antiprotons,…) accelerate them throw them against each other observe and record what happens analyze and interpret the data
ingredients needed: particle source accelerator and aiming device detector trigger (decide what to record) recording device sufficiently many people to:
o design, build, test, operate accelerator o design, build, test, calibrate, operate, and understand
detectoro analyze data
lots of money to pay for all of this
p p
14
Testing the Standard Model: Testing the Standard Model: Requirements for experimentsRequirements for experiments
Use highest available energy higher cross sections of rare processes can probe smaller distances
Be sensitive to rare processes Deviations from SM are rare (would have been seen
otherwise) need high beam intensities (“luminosity”) and detectors that can cope with this.
Detectors: Precise momentum measurement of charged particles Energy and direction measurement of neutral particles
and “jets” (from quarks and gluons) Identify leptons (e, μ, τ) all of this in whole solid angle -- detector should be
“hermetic” (be able to measure “missing energy” as signature of production of weakly interacting particle(s)
development of big “general-purpose” detectors Need to be able to recognize “new” phenomena among
huge background of “standard” processes need to understand background very well
15
How to get high energy qq How to get high energy qq collisionscollisions
Need Ecm to be large enough to o allow high momentum transfer (probe small distances)o produce heavy objects (top quarks, Higgs boson)o e.g. top quark production: e+e- tt, qq tt, gg tt, …
Shoot particle beam on a target (“fixed target”): o Ecm = 2Emc2 ~ 20 GeV for E = 100 GeV, m = 1 GeV/c2
Collide two particle beams (“collider”) :
o Ecm = 2E ~ 200 GeV for E = 100 GeV
__
__________
-- __ __ __
16
DetectorsDetectors use characteristic effects from interaction of particle with matter to detect, identify and/or measure properties of particle;
“transducer” to translate direct effect into observable/recordable (e.g. electrical) signal
example: our eye is a photon detector; “seeing” is performing a photon scattering
experiment:o light source provides photonso photons hit object of our interest -- absorbed, some
reemitted (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 neededo transmitted to brain for processing and interpretation
17
Bend angle momentumMuon
Experimental signature of a quark or gluon
Jet
Hadronic layers
Magnetized volumeTracking system
EM layersfine sampling
CalorimeterInduces shower
in dense material
Innermost tracking layers
use siliconMuon detector
Interactionpoint Absorber
material
“Missing transverse energy”
Signature of a non-interacting particle
Electron
Typical particle physics detector system
18
Fermi National Accelerator Fermi National Accelerator LaboratoryLaboratory
(near Batavia, Illinois)(near Batavia, Illinois)
Main Injector
Tevatron
DØCDF
Chicago
_ p source
Booster
19
FermilabFermilab
Fermi National Accelerator Laboratory (http://www.fnal.gov/) Founded 1972 One of the top laboratories for high energy
physics Near Batavia, Illinois (45 mi West of Chicago) presently world’s highest energy accelerator:
Tevatron = proton synchrotron, Emax=900GeV
Operated as collider: proton – antiproton collisions at Ecm = 1.9 TeV
Physics Program Collider experiments CDF, DØ, CMS neutrino physics: Minos, Mini-Boone Astrophysics: Auger Observatory, Sloan Sky
Survey ………….
20
FermiNational AcceleratorLaboratory
21
The The TeVTeVatron Collideratron 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 GeV980 GeV of energy,
so the total energy in the center of massis 1960 GeV = 1.96 TeV1960 GeV = 1.96 TeV
About 2,500,000 2,500,000 collisions per second
22
peak luminosity 1032 cm-1s-1 (5X1032 cm-1s-1 ) energy in c.m.s. 1.9 TeV, bunch crossing
time 396 ns expect integrated luminosity 6fb-1
Turn-on March 1, 2001 First collisions April 3, 2001
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Collisions at the TevatronCollisions at the Tevatron
we are interested in what’s inside the proton and antiproton “soft collisions” – peripheral
collisions – do not really probe internal structure
“hard collisions”: constituents interact
p p collisions qq(g) interactionsUnderlyingEvent
u
u
d
gq
q u
u
d
Hard Scatter
_ _
24
~700 scientists
and engineers
82 institutions
20 countries
140 Ph.D.
Dissertations
150 papers
published
The DØ CollaborationThe DØ Collaboration
25
The old DØ detector (before 1996)The old DØ detector (before 1996)
CalorimeterUranium-liquid Argon60,000 channels
Muon System1.9T magnetized Fe,Prop. drift tubes40,000 channels
Central Tracking
Drift chambers, TRD
26
DØ DØ CalorimeterCalorimeter
Uranium-Liquid Argon sampling calorimeter Linear, hermetic About 55000 cells
27
DDØ Ø Control Room 1993Control Room 1993
28
the upgraded DØ detectorthe upgraded DØ detector
29
DDØ Ø Detector in hall January 2001Detector in hall January 2001
30
DDØ Ø Detector in hall January 2001Detector in hall January 2001
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The Discovery of Top QuarkThe Discovery of Top Quark
1977 – 1992 Many null
results
1992 – 1993 A few
interesting events show up
1994, DØmt > 131
GeV/c2
1994, CDFFirst evidencemt ~ 170
GeV/c2
1995 – CDF, DØDiscovery!
32
e u c
e d s
e u c
e d s
PP
t2 3/
t 2 3/
W b1 3/
W
b 1 3/
Creating Top Anti-Top Quark pairsCreating Top Anti-Top Quark pairs
33
Artist’s impression of a top eventArtist’s impression of a top event
34
What do we actually “see”What do we actually “see”
Jet-1
Jet-2
Muon
Electron
Missing energy
jetsett _
jetsett _
35
““event display” of a event display” of a DØ top eventDØ top event
jetse tt -
36
Standard model vs experimentStandard model vs experiment
Theory works embarrassingly well! Has been tested by many hundreds of
precision measurements over last decade – 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
In electroweak theory, three parameters can be used to calculate
any electroweak quantity (use the three known best!)
lots more than three have been measuredo theory is over-constrained
37
Luminosity and cross Luminosity and cross sectionsection
Luminosity is a measure of the beam intensity (particles per area per second) ( L~1031/cm2/s )
“integrated luminosity” is a measure of the amount of data collected (e.g. ~100 pb-1)
cross section is measure of effective interaction area, proportional to the probability that a given process will occur.
o 1 barn = 10-24 cm2
o 1 pb = 10-12 b = 10-36 cm2 = 10- 40 m2
interaction rate: LdtnLdtdn /
38
Experimental tests of standard Experimental tests of standard modelmodel
we are interested in the “hard scattering processes”, i.e. central head-on collisions, since high momentum transfer probes small distances
most of the “hard scatters” are due to strong interactions between partons (quarks, gluons) leading to two ore more “jets” in the final state
such reactions represent a source of “background” to the search for other (rarer) processes that we want to study need to understand this background
in addition, measuring the cross section for these events is a test of QCD
39
Jet productionJet production
measured jet production cross section vs pt of the jet varies by 9 orders of magnitude over measured range;
good agreement with prediction
from QCD
40
Photon productionPhoton production photon production probes directly the quark
content of the proton:
41
Photon production vs theory predictionPhoton production vs theory prediction
good agreement over 6 orders of magnitude
42
W, Z productionW, Z production
43
W propertiesW properties
W width: upper points:
measured lowest two points
(“LEP1/SLD..”: calculated within SM from all other measured parameters
44
The SM works very well Why aren’t we satisfied ?The SM works very well Why aren’t we satisfied ?
SM, developed in the 1970’s, has been thoroughly tested in many experiments -- describes data extremely well
Why are we not happy with it? Lots of parameters
o Masses of charged leptons, neutrinos, quarks, W, Z, ….. not predicted by theory
o Where do they come from ? -- would like theory which predicts all of these parameters from first principles
EW. symmetry breaking mechanism via Higgs Boson is “put in by hand”
Higgs mass calculation within SM is not stable – “quadratic divergences”
Haven’t seen evidence for Higgs Boson yet SM becomes inconsistent at very high energies SM does not include gravity
45
What about gravity? Why three generations?
with different masses so many elementary particles
Why is the top quark so heavy? Why are W and Z bosons heavy and
’s light? No dark matter candidate
Open questionsOpen questions
46
… … would like “theory of would like “theory of everything”everything”
Looking for the “Theory of Everything” (TOE) that contains SM as approximation while solving SM
shortcomings many extensions proposed and considered:
o GUTs, technicolor, SUSY, … superstring theory,… Theorists need guidance from experiment to nail down
choice Frantically looking for deviations from SM unification
Electricity
Magnetism
Weak force
Strong force
Gravity string theory...
electromagnetism
electroweak force
GUTs
47
What do we need to do?What do we need to do? further refine tests of SM, hope to see deviations
from SM: higher energy increased precision and higher sensitivity
o More precision will constrain SM more tightly (e.g. SM relation between masses of top, W, Higgs)
new measurements that were not possible before Something new must show up at scale of 1 TeV
look for “new phenomena” (not predicted within SM): Supersymmetry Technicolor, compositeness,
new vector bosons, extra dimensions …. Higgs boson:
Mass expected below 200 GeV If seen, is it “SM Higgs”?
(mass, decay modes,… as predicted by SM?)
48
The Higgs ParticleThe Higgs Particle electroweak symmetry breaking
accomplished by Higgs field -- particles acquire mass by interaction with Higgs Field
Particle manifestation of Higgs Field (its “quantum”) is the Higgs Boson (spin 0)
Search for Higgs particle is one of the main motivations for research programs at present and future colliders: Fermilab (DØ and CDF) -- ongoing Large Hadron Collider at CERN
(from 2009) Next Linear Collider (planned)
49
Searches for the Higgs Searches for the Higgs BosonBoson
Searches for Higgs have been going on for a decade at LEP (CERN) and the Fermilab TeVatron collider direct searches for Higgs
production at LEP exclude mH < 114 GeV.
precision measurements of parameters of the W and Z bosons, combined with Fermilab’s top quark mass measurements
mH <~ 159 GeV. CDF and DØ exclude Higgs
mass range 159 to 170 GeV at 95% C.L.
159 @95% . .Hm GeV C L
GeVmH362787
)(114 directGeVmH
50
MMtoptop, M, MWW, M, MHiggsHiggs
relation between Mtop, MW, MHiggs measurement of Mtop, MW constrains MHiggs
Constraint on the mass of the Higgs boson: MH = 80+36
-27 GeV(within SM)
Light Higgs preferred by the SM with latest top and W mass
51
MMtoptop, , MMWW, ,
MMHiggsHiggs
Mtop (GeV)
52
Higgs – exclusion limitsHiggs – exclusion limits
53
Supersymmetry Supersymmetry
Symmetry between bosons and fermions such that all the presently observed particles have new, more massive super-partners (SUSY is a broken symmetry) Fermions have partners with spin 0 called “sfermions” :
squarks, selectrons, sneutrinos, … bosons have partners with spin ½ called “-inos”: photinos,
charginos, neutralinos Theoretically attractive:
additional particles cancel divergences in mH
SUSY closely approximates the standard model at low energies
allows unification of forces at much higher energies
provides a path to the incorporation of gravity and string theory:
Local Supersymmetry = Supergravity lightest stable particle cosmic
dark matter candidate masses depend on unknown
parameters, but expected to be 100 GeV - 1 TeV
54
Unification of forcesUnification of forces Unification of forces
Gauge couplings of em, weak and strong interactions vary
with log E within SM, they do not
become equal with SUSY corrections,
they coincide at E 1016 GeV (“GUT scale”
strong
weak
EM
cou
plin
g
log(energy)
55
Or…Extra Dimensions!?Or…Extra Dimensions!? Superstring theory:
can accommodate four forces (including gravity) need 11 dimensional world (time, 3 space + 7
“compacted dimensions” (very small -- less than 1mm in size))
Only gravity can communicate with/to other dimensions, its “strength” is diluted in ours (its influence is spread among all 10 spatial dimensions)
Experiments are underway searching for signals of these
dimensions.
The “other” dimensions
“Our World”
q
graviton
56
Status of searchesStatus of searches
Searches for SUSY, extra dimensions, other exotic particles predicted by extensions of standard model have been done during last decade at most HEP laboratories -- no evidence yet
Hope is to find Higgs, SUSY,… at the LHC
57
CERN CERN (Conseil Europ(Conseil Europééen pour la Recherche Nuclen pour la Recherche Nuclééaire)aire)
European Laboratory
for Particle Physics,
near Geneva,
Switzerland
(about 9km West of
Geneva, between
Meyrin,
and St.Genis,
straddling the Swiss-
French border)http://www.cern.ch
58
59
LHC ExperimentsLHC Experiments
Atlas
CMS
60
Large Hadron Collider (LHC)
Proton beams travel around the 27 km ring
in opposite directions, separate beam pipes.
In ultrahigh vacuum, 10-10 Torr. time for a single orbit, 89.92 μs.
beams controlled by superconducting electromagnets
o 1232 dipole magnets, 15 m each, bends beam.
o 392 quadrupole magnets, 5-7 m, focus beams.
o 8 inner triplet magnets are used to 'squeeze' the particles closer for collisions. Similar to firing needles 10 km apart with enough precision to meet in the middle.
61
LHC DetectorsLHC Detectors
ATLAS
CMS
62
CMS CollaborationCMS Collaboration
Slovak Republic
CERN
France
Italy
UK
Switzerland
USA
Austria
Finland
Greece
Hungary
Belgium
Poland
Portugal
Spain
Pakistan
Georgia
Armenia
UkraineUzbekistan
CyprusCroatia
China, PR
TurkeyBelarus
Estonia
India
Germany
Korea
Russia
Bulgaria
China (Taiwan)
IranSerbia
63
The CMS DetectorThe CMS Detector
HF
HOHB
HE
64
Transverse slice through CMS detectorClick on a particle type to visualise that particle in CMS
Press “escape” to exit
65
CMS Assembly HallCMS Assembly Hall
66
Megatile Scanner & DAQ at CERNMegatile Scanner & DAQ at CERNBuilt by Florida State HEP3Built by Florida State HEP3
67
Laser calibration and monitoring for Laser calibration and monitoring for HCALHCAL
68
CMS Physics outlookCMS Physics outlook
LHC started end 2009, but running at half design energy and low luminosity
Global fit of electroweak data to theory suggests mass of Higgs 100 to 200 GeV
LEP results of mass of Higgs > 114 GeV; latest CDF and DØ results indicate MHiggs < 159GeV
Standard model breaks down at E ~ 1 TeV New physics is expected at TeV scale. Discoveries should revolutionize
particle physics & provide window to physics at unification.
69
Accelerators Accelerators discoveries discoveries
1981 - 1987: CERN pp colliderE = 540 GeV W± (80 GeV),
Z0 (91 GeV) 1989 - 2011: Fermilab Tevatron
pp colliderE=1.8 (now 1.96) TeV
top quark (175 GeV) 2009: CERN LHC pp collider
E=7 TeV 14 TeV (in 2011?) discover SUSY and Higgs?
????: Linear e+e- ColliderE=1-2 TeV study Higgs in detail
_
_
70
1897 – ELECTRON J.J. Thomson + others 1909 – PROTON E. Rutherford 1932 – NEUTRON J. Chadwick 1964 – QUARK M. Gell-Mann, G. Zweig 1974 – CHARM S. Ting (BNL) , B. Richter
(SLAC) 1977 – BOTTOM L. Lederman (Fermilab) 1979 – GLUON1979 – GLUON JADE, Mark J, TASSO (DESY)JADE, Mark J, TASSO (DESY) 1983 – W, Z BOSONS1983 – W, Z BOSONS UA1, UA2 (CERN) (C. Rubbia)UA1, UA2 (CERN) (C. Rubbia) 1995 – TOP1995 – TOP DDØØ & CDF (Fermilab) & CDF (Fermilab)
2012 – HIGGS BOSON ? 2013 – SUPERSYMMETRY ? 2015 – GRAVITON ? 2020 – EXTRA DIMENSIONS ?
Summary -- outlookSummary -- outlook
71
What do HEP experimenters do?What do HEP experimenters do? Conceptual design of experiments and detectors
Simulation of physics reactions Simulation of detector response to background and
signal Design and construction of detectors and electronics
In close collaboration with engineers and technicians
Find compromise between desired performance and technical and financial constraints
Install, test, debug, calibrate detectors Study their performance Develop software tools for reconstruction and analysis Operate the detectors -- data taking
Shifts to ensure good data quality Maintenance, repair
Do physics analysis – discover things (sometimes), measure, test theory
Write and publish papers, present results at conferences and meetings