Document 7141640

Download Report

Transcript Document 7141640

Public Lecture at the International Workshop on Particle Physics and
Cosmology, University of Oklahoma, Norman 2009
Einstein spoke of the “incomprehensible comprehensibility” of
nature. Consciously or not, this viewpoint drives much of what
we do in science, especially in astronomy, astrophysics and
particle physics. When we see surprising or interesting features
in nature, we believe we should be able, over time, to
understand them. This view has historical support.
LHC/Fermi-GLAST – two instruments to extend our
Aerial view of LHC
Muon Toroids
Muon superconducting
Toroids in the ATLAS
Detector at the LHC
What are we hoping to learn with these instruments?
Convergence of particle physics, astrophysics
and cosmology:
1. What are the basic laws of nature: an
ingredient in any study of the universe
(compare nuclear physics, stars)?
2. What is the composition of the universe?
3. How did the universe get to be as it is?
Particle physicists, in the past few decades,
have determined completely the laws of
nature which govern phenomena on scales as
small as 10-17 cm. Embodied in the Standard
Model, which describes the strong nuclear
force, the weak nuclear force, and
electromagnetism (light, electricity,
This model has been subjected to stringent
PDG Wall Chart
Previous generation of instruments:
Stanford Linear Accelerator
Quarks were discovered at SLAC
Later, precision studies of quarks, leptons, W, Z,
gluons at CERN, SLAC, Fermilab
1. CERN (Geneva, site of LHC): LEP collided
electrons, positrons. Precision studies of the
weak interactions. [In same tunnel as LHC]
2. SLAC: SLAC Linear collider, new
technology, beams smaller than human hair
collided with enormous energies. Similar
3. Fermilab: collide protons, antiprotons at
very high energies. Precision studies of the
strong interactions.
The TEVATRON at Fermilab
60 km
p source
p _
~ 1.5 fb-1
Main Injector
& Recycler
~ 1.2 fb-1
s =1.8 - 1.96 TeV, t = 396 ns
Run I 1987 (92)-95 Lint ~ 125 pb-1
Run II 2001-09
4-9 fb-1
from the
- ArnulfHighlightsQuadt – 2.11.2005,
CDF & DØ data taking ε ~90%
By 1995, the strong and weak interactions were understood and
tested with high precision. Closely parallel to the triumph of
Quantum Electrodynamics, associated with Feynman, Schwinger,
Tomanaga, Lamb. No interesting discrepancies.
Puzzles with this picture:
1. Many ``fundamental constants” – masses of
quarks and leptons, strength of the
interactions (17 in all). Shouldn’t it be
possible to understand these?
2. Einstein’s General Theory of Relativity is not
compatible with this structure, but we know
that this describes gravitation in the universe
very well.
3. Related to (2), we don’t understand why
gravity is so “weak”.
4. Some of the constants in (1) are very
surprising. E.g. there is one called µ, which is
just a pure number, but µ < 10-9
Possible solutions (much more about these shortly):
1. For the puzzle of the weakness of gravity, a
hypothetical new symmetry of nature, called
supersymmetry. Turns out to also explain some of the
constants: the strength of the strong interactions
related to the strength of the electromagnetic and weak
2. For the puzzle of quantum gravity, string theory.
3. For the question of µ, a hypothetical particle called the
axion (subject of searches at Livermore)
4. For the puzzle of the many constants, string theory
Meanwhile, over the same period, astronomers
and astrophysicists established:
1. The big bang really happened. The universe (at
least what we can hope to see of it) is 15 billion
years old; its history is well understood from three
minutes until the present. We have some evidence
of phenomena at much earlier times (10-25 sec after
the big bang).
2. The universe consists of about 5% baryons
(protons and neutrons), 25% dark matter, 70%
dark energy.
Detailed study of the CMBR:
From satellites and earth based (balloon)
experiments. Most recently the WMAP
Detailed information about the universe:
1. What is the dark matter?
2. What is the dark energy?
3. Why is there matter at all?
4. What happened at the very early
stages of the big bang (something
called inflation, but what is it?)
5. What came before?
None of these questions can be answered within our
present knowledge of the laws of nature!
All of our cosmic questions are tied to the questions
from particle physics:
Supersymmetry ! Dark Matter
Supersymmetry ! Baryons
Axions ! Dark Matter
String theory ! Possible explanation of inflation
String theory ! Possible explanation of dark energy
String theory ! May explain what came before
Magnet Pictures
2 in 1 superconducting
dipole magnet being
installed in the CERN tunnel
LHC dipoles waiting to be installed.
Detecting Particle Collisions
When high energy particles collide, they produce many more particles.
gg  H  Z0 Z0         
Simulation of an event
in ATLAS detector.
White lines are the four
muons. The other track
are due to particles
from quarks in the
ATLAS Detector
Tracker Pictures
Inserting silicon detector into tracke
Inserting solenoid into calorimeter
Calorimeter Installation
Muon Toroids
Muon superconducting
Endcap Muon Sectors
Endcap muon sector
 The stored energy in the beams is equivalent roughly
to the kinetic energy of an aircraft carrier at 10 knots
(stored in magnets about 16 times larger)
 There will be about a billion collisions per second in
each detector.
 The detectors will record and stores “only” around 100
collisions per second.
 The total amount of data to be stored will be 15
petabytes (15 million gigabytes) a year.
It would take a stack of CDs 20Km tall per year to store
this much data.
Collide two protons each with energy 7TeV.
(1TeV is roughly the kinetic energy of a flying mosquito. This
energy is squeezed into a region 10-12 of a mosquito.)
The total energy in the
beam is comparable to an
aircraft carrier moving
at about 10 knots.
LHC Accident: Fall 2008
Electrical failure at a magnet junction: damage to
several magnets, large release of helium; design
flaws exposed, currently being assessed. Delay of a
few to many months possible, situation should be
clearer this week.
Information on the machine status is
available on the web
LHC Commissioning - home.htm
Update from the DG (edited)
Subject: LHC Performance Workshop, Chamonix 2009 - Message from the
Director-General - Message du Directeur général
Date: Fri, 6 Feb 2009 19:17:41 +0100
From: Rolf Heuer <[email protected]>
To: cern-personnel <[email protected]>
Many issues were tackled in Chamonix this week, and important recommendations made. Under a
proposal submitted to CERN management, we will have physics data in late 2009, and there is a strong
recommendation to run the LHC through the winter and on to autumn 2010 until we have substantial
quantities of data for the experiments. With this change to the schedule, our goal for the LHC's first
running period is an integrated luminosity of more than 200 pb-1 operating at 5 TeV per beam, sufficient
for the first new physics measurements to be made. This, I believe, is the best possible scenario for the
LHC and for particle physics.
Since the incident, enormous progress has been made in developing techniques to detect any
small anomaly. These will be used in order to get a complete picture of the resistance in the
splices of all magnets installed in the machine. This will allow improved early warning of
any additional suspicious splices during operation. The early warning systems will be in
place and fully tested before restarting the LHC.
What Might the LHC Discover?
The short answer: we don’ t know!
But there are plenty of speculations, motivated by the
questions on our lists. We can’t review them all, and
it is likely that none of our guesses are right. But, as
a prototype, we’ll consider the most popular one:
What is supersymmetry?
Symmetry between
Fermions ↔ Bosons
(force carrier)
... doubled particle spectrum ... ☹
Why supersymmetry (maybe?)
Higgs field: very heavy, mass > 116 GeV (more than 100
times mp). Can’t be too much more.
Real question: why so light?
Dimensional analysis: mH ¼ Mp = 1018 GeV.
In quantum field theory, there really are contributions to
the Higgs mass which are this large unless either
1. The Higgs particle is a composite, with a size a ¼
2. Nature is supersymmetric
Why Supersymmetry Solves this
“Hierarchy Problem”
Lorentz: Model for electron as a blob of charge of size r.
Ecoul = e2/r
Einstein: Energy = mass £ c2; me = {e2/r c2}
But we know r < 10-17 cm
me > 10 ¼ 10 mp!
Dirac theory of electron fixes this (Weisskopf) –
roughly speaking the positrons cancel off the big
contribution of the Coulomb field.
In supersymmetry, the extra particles cancel the big
contributions to the Higgs particles if their masses
are not too different than mH.
If supersymmetry is there, LHC will find it!
(Fermilab has looked and will continue)
Discovery of Supersymmetry is Likely to
Answer Several Questions in Our Lists
1. Explain why gravity is weak (mH ¿ Mp)
2. Supersymmetry -- (almost) for free – explains the value of
the strong coupling in terms of the couplings of weak
interactions and electromagnetism.
3. Supersymmetric theories – for free – almost always possess
a candidate for the dark matter, a WIMP (weakly
interacting massive particle).
4. Supersymmetry can readily explain the excess of matter over
If supersymmetry accounts for the dark
matter, we ought to be able to find it
1. Search in mines for (rare) collisions of dark
matter particles with ordinary
2. Dark matter particles might annihilate
frequently near the galactic center – see
energetic particles in Fermi/GLAST.
If dark matter particles are from
supersymmetry, they will sometimes meet
and annihilate in areas where they are most
dense; the products of these annihilations can
be seen by GLAST, other instruments.
Already some tantalizing evidence (esp. from
an Italian satellite, PAMELA) for such
Being greedy, physicists speculate about the other
questions on the list. The structure with the potential to
address all of them: Sting Theory
A contentious subject.
•What has it explained?
•When will it be tested?
String Theory
•For reasons that are still not understood, assuming
that the fundamental entities are strings rather than
point particles automatically gives a sensible
quantum theory of gravity (General Relativity).
•At the same time, these theories automatically give
structures which look remarkably like the Standard
As so often, the issues are exaggerated and
misrepresented by the antagonists.
But trust me; I speak with authority (I hang out with
string theorists and I went to high school with Smolin)
•String theory has taught us that quantum mechanics and
gravity can get along – something not widely believed
before (e.g. Hawking). Smolin is wrong when he says he
has an alternative which accomplishes this, but this is
not really so important.
•What theorists have studied – string theory and related
objects – are definitely unrealistic models. They have
the right to believe that more realistic theories exist and
to speculate on their properties, but at the moment they
are groping. Only some inklings of the underlying
Could the LHC discover string theory?
Maybe. String theory may predict
supersymmetry, the spectrum (masses) of the new
particles. It might predict (a real long shot, but
terribly exciting if true) extra dimensions of space
which could be observed, black holes…
So now we wait and see. Theorists,
experimentalists, working hard to be
ready to interpret the data as it starts to
come in, hopefully within less than a
Extra Slides
In a magnetic field B, a particle of charge q and momentum p
travels in a circle of radius R given by
At the LHC, the desired beam energy 7 TeV and the
state of the art dipole magnets have a field of 8 Tesla.
Plugging in and converting units gives a radius of 3 km
and a circumference of 18 km.