Download Physics of the Large Hadron Collider Lecture 1: Fundamentals of the

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project

Document related concepts

Gauge fixing wikipedia , lookup

Topological quantum field theory wikipedia , lookup

Canonical quantization wikipedia , lookup

Quantum electrodynamics wikipedia , lookup

Relational approach to quantum physics wikipedia , lookup

Quantum field theory wikipedia , lookup

Kaluza–Klein theory wikipedia , lookup

Relativistic quantum mechanics wikipedia , lookup

T-symmetry wikipedia , lookup

Nuclear structure wikipedia , lookup

Quark wikipedia , lookup

Peter Kalmus wikipedia , lookup

Instanton wikipedia , lookup

An Exceptionally Simple Theory of Everything wikipedia , lookup

Renormalization wikipedia , lookup

Weakly-interacting massive particles wikipedia , lookup

Strangeness production wikipedia , lookup

Yang–Mills theory wikipedia , lookup

Light-front quantization applications wikipedia , lookup

Theory of everything wikipedia , lookup

History of quantum field theory wikipedia , lookup

Lepton wikipedia , lookup

Scalar field theory wikipedia , lookup

Renormalization group wikipedia , lookup

Higgs boson wikipedia , lookup

Elementary particle wikipedia , lookup

Supersymmetry wikipedia , lookup

Mathematical formulation of the Standard Model wikipedia , lookup

Quantum chromodynamics wikipedia , lookup

ALICE experiment wikipedia , lookup

Grand Unified Theory wikipedia , lookup

Higgs mechanism wikipedia , lookup

ATLAS experiment wikipedia , lookup

Search for the Higgs boson wikipedia , lookup

Technicolor (physics) wikipedia , lookup

Compact Muon Solenoid wikipedia , lookup

Minimal Supersymmetric Standard Model wikipedia , lookup

Standard Model wikipedia , lookup

Future Circular Collider wikipedia , lookup

Large Hadron Collider wikipedia , lookup

Transcript
Physics of the Large Hadron Collider
Lecture 1: Fundamentals of the LHC
Johan Alwall, SLAC
Michelson lectures at Case Western Reserve
April 13-16, 2009
Outline
●
Introduction: What is the LHC?
●
Fundamentals of QCD
●
Fundamentals of Electroweak Physics
●
The Standard Model and the Higgs
Johan Alwall - Fundamentals of the LHC
2
What is the LHC
Johan Alwall - Fundamentals of the LHC
3
What is the LHC
Johan Alwall - Fundamentals of the LHC
4
What is the LHC
●
14 TeV proton-proton collider
●
27 km (17 miles) in circumference
●
●
Luminosity: Initial: 1033 /cm2s (10 fb-1/year)
Nominal: 1034 /cm2s (100 fb-1/year)
4 detectors:
–
ATLAS, CMS: general-purpose detectors
–
ALICE: Specialized for heavy-ion collisions
–
LHC-B: One-arm large- detector for bottom-quark
physics
Johan Alwall - Fundamentals of the LHC
5
What is the LHC
The CMS (Compact Muon Solenoid) detector
cms.cern.ch
Johan Alwall - Fundamentals of the LHC
6
What is the LHC
The ATLAS detector
atlas.ch
Johan Alwall - Fundamentals of the LHC
7
What is the LHC
Real ATLAS events (from startup run)
Images from atlas.ch
“Splash” event, when the LHC beam
was steered into a magnet upstream
Cosmic ray muon event
from the detector
Johan Alwall - Fundamentals of the LHC
8
What is the LHC
Simulated ATLAS events
Images from atlas.ch
Supersymmetry event with jets
and muons
Higgs boson event with H→ZZ
recoiling agains jet
Johan Alwall - Fundamentals of the LHC
9
Fundamentals of QCD
●
●
●
The theory of strong interactions; describes
interactions between quarks and gluons
Represented by a gauge theory with the gauge
group SU(3)C (for color)
Non-abelian field theory
→ gluon carries color charge (self-interacting)
Johan Alwall - Fundamentals of the LHC
10
Fundamentals of QCD
Running of QCD coupling due to quantum
loops leads to “asymptotic freedom”:
Johan Alwall - Fundamentals of the LHC
11
Fundamentals of QCD
Can be understood in terms of screening and
anti-screening by vacuum bubbles:
Johan Alwall - Fundamentals of the LHC
12
Fundamentals of QCD
●
●
At low energy (~1 GeV) QCD is confining
–
No free color charges
–
Quarks/gluons always confined in hadrons
–
Different degrees of freedom at large and small
energies (quarks, gluons vs. hadrons, pions)
–
No analytic proof, but strong evidence from lattice
simulations that SU(3) is confining Millennium $1M Prize!
Coupling constant s 10 x stronger than
electromagnetic coupling EM (at Z mass)
Johan Alwall - Fundamentals of the LHC
13
Parton distribution functions
●
●
Probability for finding a quark or gluon in a
hadron
Function of x (momentum fraction of parton)
and 2 (momentum transfer in process)
Johan Alwall - Fundamentals of the LHC
14
Parton distribution functions
●
●
●
●
Probability for finding a quark or gluon in a
hadron
Function of x (momentum fraction of parton)
and 2 (momentum transfer in process)
Increases as power-law for small x and
logarithmically for large Q2
Behaviour governed by DGLAP equation
(Dokshitzer-Gribov-Lipatov-Altarelli-Parisi)
Johan Alwall - Fundamentals of the LHC
15
Parton distribution functions
●
●
●
●
Probability for finding a quark or gluon in a
hadron
Function of x (momentum fraction of parton)
and 2 (momentum transfer in process)
Increases as power-law for small x and
logarithmically for large Q2
Behaviour governed by DGLAP equation
(Dokshitzer-Gribov-Lipatov-Altarelli-Parisi)
Johan Alwall - Fundamentals of the LHC
16
Splitting functions and DGLAP
QCD bremsstrahlung – logarithmic divergences
as energy fraction z → 0 and virtuality k2 → 0
–
Leading contribution from the soft and collinear regions of
phase space
–
Subsequent emissions factorize (no interference in
soft/collinear regions) → Allows log resummation to all orders
–
Integration over multiple emissions gives evolution from
interaction scale 2 to hadronic scale ~ 1 GeV
Johan Alwall - Fundamentals of the LHC
17
Parton showering
●
●
●
Can describe QCD radiation in the soft and
collinear limit as subsequent independent
emissions, called “parton shower”.
Every hard interaction always associated with
extra QCD radiation
Initial high-energy
parton gives showerlike “jet”
Johan Alwall - Fundamentals of the LHC
18
Parton showering
At end of shower, need hadronization ansatz to
pass from partons to color neutral hadrons
–
Phenomenological models, based on general
behaviour of QCD, e.g. the Lund string model or
Herwig cluster model
Johan Alwall - Fundamentals of the LHC
19
Simulation of LHC collision
Hadronization,
hadron decay
Hard interaction
Parton
showers
Underlying
event / multiple
interactions
Johan Alwall - Fundamentals of the LHC
20
Missing transverse energy
●
●
●
●
Fundamental concept for hadron colliders
(will be used many times in these lectures)
Momentum fraction x1, x2 of incoming partons
→ event boosted in the lab frame
If invisible particle (e.g. neutrino) produced,
cannot determine boost along beam direction
Only the missing transverse momentum can be
measured:
where the sum is over all visible particles i
Johan Alwall - Fundamentals of the LHC
21
Fundamentals of Elecroweak Physics
●
Enrico Fermi explained beta decay of nuclei as
4-fermion interactions n → p e- e with mass
scale Mweak~ 90 GeV in denominator
(corresponding to intermediate vector boson!)
Johan Alwall - Fundamentals of the LHC
22
Fundamentals of Elecroweak Physics
●
●
●
●
Enrico Fermi explained beta decay of nuclei as
4-fermion interactions n → p e- e with mass
scale Mweak~ 90 GeV in denominator
(corresponding to intermediate vector boson!)
Looks like SU(2) gauge theory with massive
gauge vector bosons
Non-renormalizable, gives scattering probability
> 1 at high energies (unitarity violation)
Solution: Massless gauge bosons getting mass
from spontaneous symmetry breaking
Johan Alwall - Fundamentals of the LHC
23
The Higgs mechanism
●
●
●
Introduce scalar Higgs field which couples to the
massless gauge bosons and fermions
Let this field get a non-zero vacuum expectation
value due to Higgs potential
Rewrite
Johan Alwall - Fundamentals of the LHC
24
The Higgs mechanism
●
●
●
Gives mass to vector bosons and fermions in
proportion to their couplings to the Higgs
–
Gauge fields (gauge couplings):
–
Matter fields (Yukawa couplings):
Three of the four Higgs degrees of freedom
become longitudinal components of
and
mix to give Z and γ
Johan Alwall - Fundamentals of the LHC
25
The Standard Model
Johan Alwall - Fundamentals of the LHC
26
The Standard Model
●
QCD SU(3) x Weak SU(2) x Hypercharge U(1)
–
●
●
●
Electromagnetic A linear combination of W3 and B
Weak SU(2) dynamically broken
Three generations of weak doublet fermion
fields (left-handed doublets, right-handed singlets)
Top quark Yukawa coupling close to 1, all other
Yukawas small – no explanation for this
hierarchy within the model
Johan Alwall - Fundamentals of the LHC
27
The Standard Model
Scientific American
Johan Alwall - Fundamentals of the LHC
28
The Higgs boson mass
Higgs mass (and top and W masses before they
were seen) determined by precision measurements
–
Particle masses affect precision observables
through quantum loop contributions
–
Examples of observables used:
●
●
●
●
●
●
Mass and decay widths of the W and Z bosons
Branching ratios of Z boson to leptons, hadrons, bottom
Weak mixing angle sin2W
Forward-backward asymmetries at LEP
Top mass
EM, weak and QCD couplings EM, Weak and s
Johan Alwall - Fundamentals of the LHC
29
Indirect mass determinations
SM Higgs mass upper limit: 163 GeV
Excluded by LEP
Excluded by Tevatron
(March 2009)
Direct measurements
Indirect determination
Johan Alwall - Fundamentals of the LHC
30
Higgs boson decays
Johan Alwall - Fundamentals of the LHC
31
Higgs hunting at the Tevatron
CDF and D0 combined search, March 2009
Johan Alwall - Fundamentals of the LHC
32
Higgs hunting at the Tevatron
CDF individual search channels, March 2009
Johan Alwall - Fundamentals of the LHC
33
The Higgs boson mass, cont.
Higgs mass
only unknown
parameter of the Standard Model
–
95% CL upper limit from precision measurements
at 163 GeV
–
Must be <800 GeV to conserve unitarity of weak
boson scattering (i.e. do its job)
–
Like all masses in a quantum field theory, mass is
given by
where mH2 is given by quantum loop corrections
Johan Alwall - Fundamentals of the LHC
34
Naturalness and the Higgs
●
●
●
●
't Hooft: A theory is “natural” if the size of
corrections not too much larger than bare mass
“Fine-tuning”: The precision by which the bare
mass term must cancel the corrections
Corrections for elementary scalar are quadratic
in new-physics cutoff (for fermions and gauge
bosons, corrections are logarithmic)
Main loop corrections from strongest coupled
particles: top, W, Z and Higgs self-couplings
Johan Alwall - Fundamentals of the LHC
35
Higgs mass corrections
New
physics
cutoff
1% fine-tuning
10% fine-tuning
The “Veltman throat”
where loop corrections
cancel(excluded by
indirect SM bounds
at >95% CL)
Johan Alwall - Fundamentals of the LHC
36
The hierarchy problem
●
●
●
If there is no new physics besides the Standard
Model and gravity, the cutoff scale is
~MPl~1018 GeV, giving a finetuning of 10-34
To reduce finetuning to an “acceptable” 10%
level, there must be new physics at around 1
TeV, i.e. within reach for the LHC!
More about ideas for new physics that solves
the hierarchy problem, in the next lecture!
Johan Alwall - Fundamentals of the LHC
37
Summary and plan
Today I have talked about:
–
What is the LHC?
–
Fundamentals of QCD
●
●
●
–
Fundamentals of Electroweak Physics
●
–
Parton density functions
Parton showering, hadronization
Elements of simulations of LHC collisions
The Higgs mechanism
The Standard Model
●
●
Properties of the Higgs boson
The Hierarchy problem
Johan Alwall - Fundamentals of the LHC
38
Summary and plan
Next lecture: New Physics at the LHC
–
Problems with the Standard Model
–
Classes of solutions to the hierarchy problem
●
●
●
–
Supersymmetry
Extra dimentions
Little Higgs models
Other New Physics ideas
rd
3 lecture: Simulation at the LHC
Johan Alwall - Fundamentals of the LHC
39
Recommended reading
●
LHC:
–
●
QCD:
–
●
http://public.web.cern.ch/public/en/LHC/LHC-en.html
QCD and Collider Physics, Ellis, Sterling, Webber,
Cambridge 1996
Electroweak physics, Standard Model:
–
An introduction to Quantum Field Theory,
Peskin, Schröder, Westview 1995
–
Gauge Theory of Elementary Particle Physics,
Cheng, Li, Oxford 1988
Johan Alwall - Fundamentals of the LHC
40
Recommended reading
●
The Higgs boson:
–
●
Precision measurements of the Standard Model:
–
●
“Higgs Boson Theory and Phenomenology”,
Carena, Haber, Prog.Part.Nucl.Phys.50:63-152,2003
The LEP Electroweak Working Group,
http://lepewwg.web.cern.ch/LEPEWWG/
Higgs searches at the Tevatron:
–
The CDF and D0 Higgs pages:
http://www-cdf.fnal.gov/physics/new/hdg/hdg.html
http://wwwd0.fnal.gov/Run2Physics/WWW/results/higgs.htm
Johan Alwall - Fundamentals of the LHC
41