Download The Standard Model and Beyond

The Standard Model
and Beyond
Maxim Perelstein, LEPP/Cornell U.
NYSS APS/AAPT Conference, April 19, 2008
The basic question of particle
physics: What is the world made
of? What is the smallest
indivisible building block of
matter? Is there such a thing?
In the 20th century, we made
tremendous progress in
observing smaller and smaller
objects
Today’s accelerators allow us to
study matter on length scales as
short as 10^(-18) m
The world’s largest particle accelerator/collider: the
Tevatron (located at Fermilab in suburban Chicago)
4 miles long, accelerates protons and
antiprotons to 99.9999% of speed of
light and collides them head-on, 2
million collisions/sec.
The CDF
detector
The
control
room
Particle Collider is a Giant
Microscope!
• Optics: diffraction limit, ∆min ≈ λ
• Quantum mechanics: particles waves,
λ ≈ h̄/p
• Higher energies
•
•
•
shorter distances:
Nucleus: ∆ ∼ 10−13 cm
Colliders today: E ∼ 100 GeV
proton mass Mp c2 ∼ 1 GeV
∆ ∼ 10−16 cm
Colliders in near future: E ∼ 1000 GeV ∼ 1 TeV
∆ ∼ 10−17 cm
Particle Colliders Can Create
New Particles!
•
All naturally occuring matter consists of particles of just a
few types: protons, neutrons, electrons, photons, neutrinos
•
Most other known particles are highly unstable (lifetimes
<< 1 sec)
do not occur naturally
•
In Special Relativity, energy and momentum are conserved,
but mass is not: energy-mass transfer is possible! E = mc2
•
So, a collision of 2 protons moving relativistically can result
in production of particles that are much heavier than the
protons, “made out of” their kinetic energy
•
This is how most elementary particles are discovered!
Another basic question: how did the Universe
begin?
High-energy particle collisions, today seen only in
accelerators, were quite common in the hightemperature, high-density universe within the first
second after the Big Bang!
All our knowledge about subatomic physics is summarized in the Standard Model
- the most successful Physics theory ever!
[from: particleadventure.org]
16 different elementary particles
have been observed in collider
experiments: 12 “matter
particles” and 4 “force particles”
Matter particles are further
divided into leptons and quarks
There are 6 leptons and 6 quarks:
3 “generations”, 2 leptons and 2
quarks in each
Particles in each row (e.g. u, c and
t quarks) are identical except for
their masses: t is heavier than c,
which is heavier than u
(The Periodic Table - just like Chemistry, but
much simpler and way cooler!)
Some Basic Properties of Matter Particles:
•
Each matter particle has an antiparticle, with exactly the
same mass but opposite electric charge
•
Quarks do not exist as free objects, but only as constituents
of “baryons” (a bound state of 3 quarks) and “mesons” (a
bound state of a quark and an antiquark)
•
•
¯ π + = ud,
¯ ...
Examples: p = uud, n = udd, p̄ = ūūd,
•
Most particles are unstable (decay into other particles, with
lifetimes <<1 sec)
•
Exceptions: electron, 3 neutrinos, 2 baryons: proton (uud)
and neutron (udd), and their antiparticles, are STABLE
100’s of baryons and mesons have been observed, all can be
understood as bound states of the known quarks
FOUR FUNDAMENTAL FORCES:
Gravitational force:
motion of planets, rockets, apples, ...
Electromagnetic force:
electricity, radiowaves, light, ...
Weak force:
origin of radioactivity
Strong force:
binds protons and neutrons in the nucleus
Gravity is very weak – a small magnet can balance
the gravitational effect of the entire Earth, BUT:
Only one type of gravitational charge (always attractive),
forces add up – very relevant over long distances (while
E&M charges cancel)
Modern Picture of Forces: forces between “matter
particles” are due to exchange of “force particles”
For example: Electromagnetic force
between electrons is due to a photon
exchange
“Quantum Electrodynamics” - combines Maxwell’s theory
of electromagnetism, Special Relativity and Quantum
Mechanics
Electric
and
magnetic
forces are described by
emission and absorption of
photon – a particle of zero
mass, the force carrier of
electromagnetism
Feynman
“Feynman Diagram”
[Nobel prize 1965, with
Schwinger, Tomonaga]
Weak Interactions
Weak force is described by a
theory just like electrodynamics,
but instead of photon, mediating
particles are the W and Z bosons
●
Weak force is short-range, with
range about 10−15 cm:
Vweak
●
e−r/r0
∝
r
This implies that the W and Z
bosons are massive:
h̄
M∼
∼ 100 GeV
cr0
●
Discovery of W and Z: CERN, 1983
[Nobel prize 1984: Rubbia, van der Meer]
Feynman diagram for
the neutron beta-decay
EM-Weak Unification
• EM and Weak forces
2 -
2
d!/dQ (pb/GeV )
HERA I high Q e p
become equally strong
at short distances of
order 10−15 cm
H1 e-p NC
ZEUS e-p NC 98-99
SM e-p NC (CTEQ5D)
2
10
1
10
10
10
10
10
10
10
-1
-2
• Same theory describes
-3
-4
both forces in a unified
framework
H1 e-p CC
ZEUS e-p CC 98-99
SM e-p CC (CTEQ5D)
-5
-6
-7
y < 0.9
10
3
10
4
2
2
Q (GeV )
[blue=EM, red=weak]
[Nobel prize 1979: Glashow,
Salam, Weinberg]
Strong Interactions
Strong force is also described by a theory very similar to
electrodynamics, the force particle is the gluon
Due to peculiar nature of the gluon, the strong force grows
with distance between charges: V ∝ r
Only quarks experience the strong force, leptons are immune to
it (neutral). This explains why quarks are confined and leptons
are not!
[Nobel prize 2004: Gross,
Politzer, Wilczek]
At short distances, the strong force gets weaker - the closer
together you bring the quarks, the more freedom they feel!
(”asymptotic freedom”)
Gravitational Interactions
Gravitational force is supposedly described by a theory very
similar to electrodynamics, the force particle is the graviton
Just like photon is a quantum of electromagnetic wave,
graviton would be a quantum of gravitational wave
Gravitaional waves are predicted by General Relativity, and
their indirect effects have been seen, but NO direct
observation so far!
LIGO gravitational wave detectors in Hanford,
Washington and Livingston, Louisiana
This concludes our brief tour of matter particles...
and their interactions/force particles!
Predictive Power of the
Standard Model
•
The Standard Model is not just a list of particles and a
classification - it is a theory that makes detailed, precise
quantitative predictions!
•
Consider a head-on collision of a 100 GeV electron and a
100 GeV antielectron (”positron”). Possible outcomes:
e+ e− , µ+ µ− , τ + τ − , pp̄, W + W − , e+ e+ e− e− , . . .
•
Quantum mechanics: there is no way to know for sure
which outcome will occur in a given collision, but the SM
predicts probabilities (”cross sections”) of each outcome,
plus details like directions of the produced particles, etc.
•
Works spectacularly well! (some predictions experimentally
verified to 0.1% accuracy)
Anything Left to Discover?
•
The Standard Model has been the accepted theory of
particle physics since ~1974
•
No statistically significant deviations from the Standard
Model predictions have been experimentally observed to
date, in spite of years of effort
•
Most aspects of the model have already been
experimentally verified
•
~10 Nobel prizes have already been awarded for theoretical
and experimental work related to the Standard Model
•
So, is particle physics over?
NO!
Truffle 1: The Higgs Boson
•
The Higgs boson: a hugely important aspect of the Standard
Model still awaits experimental verification!
•
SM postulates that the Universe is filled with a uniform
“Higgs field” (somewhat similar to electric/magnetic fields)
•
A massless particle moving in a Higgs field is equivalent to a
massive particle
Higgs field “gives mass to all particles”
•
Different particles have different masses due to different
strength of their interaction with the Higgs field (”charge”)
Where Is The Higgs Boson?
•
Relativity + Quantum Mechanics guarantee that any field
must have a particle associated with it: e.g. electric/magnetic
field
photon, gravitational field
graviton, etc.
•
•
Higgs field
•
Maybe this aspect of the SM is wrong! (Alternative theories
of mass generation exist, although none is as appealing...)
•
Maybe the Higgs is just too heavy - theory only predicts a
range for its mass, with upper bound well above the current
sensitivity...
•
Need a more powerful accelerator/collider to test this!
Higgs particle (or Higgs boson)
Experiments at the Tevatron and other colliders have
hunted for the Higgs for years and have not found it!
Large Hadron Collider (LHC) at CERN (Geneva,
Switzerland) will turn on in Summer 2008!
The LHC: 7 TeV protons (7 times more powerful
than the Tevatron!), 17 miles long, few GEuros
If Higgs exists, the LHC will find it!
LHC cooldown status (this morning)
[from http://lhc.web.cern.ch/lhc/]
Truffle 2: Beyond the Higgs
•
It is highly unlikely that the Higgs boson will be the only
new particle discovered at the LHC
•
It’s obvious: in a generic quantum field theory, any scalar
field mass term receives quadratically divergent radiative
corrections, and severe fine-tuning is required to maintain a
hierarchy between the scalar mass and the cutoff of the
theory (Ken Wilson, 1970s)
•
In other words, if the Higgs is there, something must
“stabilize” it!
Supersymmetry (a.k.a.
SUSY)
•
Supersymmetry is the
idea that there is a
“superpartner” for each
elementary particle:
selectron, smuon, stau,
squarks, photino, wino,
higgsino, etc.
•
This would stabilize the
Higgs (trust me!)
•
Many superpartners
could be discovered at
the LHC!
However, Higgs+SUSY is not the only possible
mechanism to generate masses...
Alternatives include: technicolor, extended technicolor,
walking technicolor, composite Higgs, large extra
dimensions, warped extra dimensions, universal extra
dimensions, gauge-Higgs unification, Little Higgs models,
Higgsless models, twin Higgs, ...
Good news: all these theories will be tested (and most of
them ruled out) by the LHC experiments!
What is the Universe REALLY
Made Of?
•
Particle physicist’s answer:
stable particles - protons,
neutrons, electrons, neutrinos
•
(Why not antiprotons,
positrons, etc.? another puzzle
- maybe next time?)
•
But astronomical observations
indicate that the known
particles make only about 4%
of the stuff in the Universe!!!
Truffle 4: Dark Matter
Rotation velocities of satellites of galaxies imply that
galaxies have large halos of non-luminous matter
What Is Dark Matter Made Of?
•
The simplest explanation would be that dark matter
consists of ordinary gas (protons, neutrons, electrons) that,
for some reason, did not collapse into stars and remained
cold
•
However, this simple hypothesis is ruled out by
measurements of the number of baryons
(protons+neutrons) in the universe - there is not enough
baryons to account for the observed dark matter
•
The most sensitive measurement is provided by observing
inhomogeneities of cosmic microwave background - a
snapshot of the Universe at a tender age of 300000 years
(vs 14 Gyr now!)
•
Conclusion: there MUST be a new stable particle (not in the
Standard Model!) that makes up dark matter
•
Dark matter has been observed only indirectly, through
its gravitational effects
•
Why hasn’t the dark matter particle been seen in
colliders? It’s either too heavy, or interacts too weakly
with ordinary matter...
•
The most attractive theoretical idea is a WIMP - Weakly
Interacting Massive Particle
•
Many extensions of the Standard Model (e.g.
supersymmetry) contain WIMPs
•
WIMPs could be produced directly at the LHC and
studied in the lab - direct connection between collider
physics and cosmology!
Conclusions
•
The Standard Model of particle physics is a simple theory
describing a very wide range of observations
•
SM allows for precise quantitative predictions, many of
which have been verified with better than 1% accuracy
•
Remaining piece of the SM - the Higgs boson - is about to
be tested at the Large Hadron Collider at CERN
•
Strong theoretical reasons to expect physics beyond the SM
to show up in the same experiments, for the first time
•
Dark Matter cannot be explained in the SM, possibly will be
directly produced at the LHC