The Large Hadron Collider is mostly underground, as shown by the circle on this aerial view
of the site on the French-Swiss border.
The Future of Physics
Scientific American Magazine, February 2008
Edited by Andy Ross
The terascale is the realm of physics that comes into view when
two elementary particles smash together with a combined energy of around a
trillion electron volts, or one TeV. The machine that will take us to the
terascale — the ring-shaped Large Hadron Collider (LHC) at CERN — is now nearing
completion.
To ascend through the energy scales from electron volts to the terascale is to
travel from the domains of chemistry and solid-state electronics (electron
volts) to nuclear reactions (millions of electron volts) to the territory that
particle physicists have been investigating for the past half a century
(billions of electron volts).
What lies in wait for us at the terascale? No one knows.
LHC cooldown status as of February 23, 2008
The Large Hadron Collider
Edited by Andy Ross
The Large Hadron Collider (LHC), a short drive from Geneva, will
peer into the physics of the shortest distances and the highest energies ever
probed. For a decade or more, particle physicists have been eagerly awaiting a
chance to explore the terascale domain. Significant new physics is expected to
occur at these energies, such as the elusive Higgs particle (believed to be
responsible for imbuing other particles with mass) and the particle that
constitutes the dark matter that makes up most of the material in the universe.
The mammoth machine, after a nine-year construction period, is scheduled to
begin producing its beams of particles later this year. The commissioning
process is planned to proceed from one beam to two beams to colliding beams,
from lower energies to the terascale, from weaker test intensities to stronger
ones suitable for producing data at useful rates. Each step will produce
challenges for the more than 5,000 scientists, engineers and students
collaborating on the effort. The particle physics community is eagerly awaiting
the first results from the LHC.
To break into the new territory that is the terascale, the LHC's basic
parameters outdo those of previous colliders in almost every respect. It starts
by producing proton beams of far higher energies than ever before. Its nearly
7,000 magnets, chilled by liquid helium to less than two kelvins to make them
superconducting, will steer and focus two beams of protons traveling within a
millionth of a percent of the speed of light. Each proton will have about 7 TeV
of energy — 7,000 times a proton rest mass. When it is fully loaded and at
maximum energy, all the circulating particles will carry energy roughly equal to
the kinetic energy of about 900 cars traveling at 100 kilometers per hour.
The protons will travel in nearly 3,000 bunches, spaced all around the
27-kilometer circumference of the collider. Each bunch of up to 100 billion
protons will be the size of a needle, just a few centimeters long and squeezed
down to 16 microns in diameter (about the same as the thinnest of human hairs)
at the collision points. At four locations around the ring, these needles will
pass through one another, producing more than 600 million particle collisions
every second. The collisions, or events, will occur between the quarks and
gluons making up the protons. The most cataclysmic of the smashups will release
about 2 TeV.
Four giant detectors — the largest would roughly half-fill the Notre Dame
cathedral in Paris, and the heaviest contains more iron than the Eiffel Tower —
will track and measure the thousands of particles spewed out by each collision
occurring at their centers. Despite the detectors' vast size, some elements of
them must be positioned with a precision of 50 microns.
The nearly 100 million channels of data streaming from each of the two largest
detectors would fill 100,000 CDs every second. So instead of attempting to
record it all, the experiments will have what are called trigger and
data-acquisition systems, which act like vast spam filters, immediately
discarding almost all the information and sending the data from only the most
promising-looking 100 events each second to the LHC's central computing system
at CERN for archiving and later analysis.
A farm of a few thousand computers at CERN will turn the filtered raw data into
more compact data sets organized for physicists to comb through. Their analyses
will take place on a grid network comprising tens of thousands of PCs at
institutes around the world, all connected to a hub of a dozen major centers on
three continents that are in turn linked to CERN by dedicated optical cables.
In the coming months, all eyes will be on the accelerator. The final connections
between adjacent magnets in the ring were made in November 2007, and the eight
sectors are now being cooled almost to the cryogenic temperature required for
operation. After the operation of the sectors has been tested, first
individually and then together as an integrated system, a beam of protons will
be injected into one of the two beam pipes that carry them around the machine’s
27 kilometers.
The series of smaller accelerators that supply the beam to the main LHC ring has
already been checked out. The first injection of the beam will be a critical
step, and the LHC scientists will start with a low-intensity beam to reduce the
risk of damaging LHC hardware. Only when they have carefully assessed how that
pilot beam responds inside the LHC and have made fine corrections to the
steering magnetic fields will they proceed to higher intensities. For the first
running at the design energy of 7 TeV, only a single bunch of protons will
circulate in each direction.
As the full commissioning of the accelerator proceeds in this measured
step-by-step fashion, problems are sure to arise. The big unknown is how long
the engineers and scientists will take to overcome each challenge. If a sector
has to be brought back to room temperature for repairs, it will add months. The
four experiments — ATLAS, ALICE, CMS and LHCb — also have a lengthy process of
completion ahead of them, and they must be closed up before the beam
commissioning begins.
When everything is working together at the design luminosity, as many as 20
events will occur with each crossing of the needlelike bunches of protons. As
little as 25 nanoseconds pass between one crossing and the next. Product
particles sprayed out from the collisions of one crossing will still be moving
through the outer layers of a detector when the next crossing is already taking
place. Individual elements in each of the detector layers respond as a particle
of the right kind passes through it. The millions of channels of data streaming
away from the detector produce about a megabyte of data from each event, a
petabyte every two seconds.
The trigger system that will reduce this flood of data to manageable proportions
has multiple levels. The first level will receive and analyze data from only a
subset of all the detector’s components, from which it can pick out promising
events. This level-one triggering will be conducted by hundreds of dedicated
computer boards. They will select 100,000 bunches of data per second for more
careful analysis by the next stage, the higher-level trigger.
The higher-level trigger will receive data from all of the detector’s millions
of channels. Its software will run on a farm of computers, and with an average
of 10 microseconds elapsing between each bunch approved by the level-one
trigger, it will have enough time to reconstruct each event. It will project
tracks back to common points of origin and thereby form a coherent set of data
for the particles produced by each event.
The higher-level trigger passes about 100 events per second to the hub of the
LHC's global network of computing resources — the LHC Computing Grid. A grid
system combines the processing power of a network of computing centers and makes
it available to users who may log in to the grid from their home institutes.
The LHC grid is organized into tiers. Tier 0 is at CERN itself and consists in
large part of thousands of commercially bought servers, both PC-style boxes and,
more recently, blade systems looking like black pizza boxes, stacked in row
after row of shelves. Computers are still being purchased and added to the
system. The data passed to Tier 0 by the LHC data-acquisition systems will be
archived on magnetic tape.
Tier 0 will distribute the data to the 12 Tier 1 centers, which are located at
CERN itself and at 11 other major institutes around the world. Thus, the
unprocessed data will exist in two copies, one at CERN and one divided up around
the world. Each of the Tier 1 centers will also host a complete set of the data
in a compact form structured for physicists to carry out many of their analyses.
The full LHC Computing Grid also has Tier 2 centers, which are smaller computing
centers at universities and research institutes. Computers at these centers will
supply distributed processing power to the entire grid for the data analyses.
The LHC has experienced some setbacks along the way. Last March a magnet of the
kind used to focus the proton beams just ahead of a collision point (called a
quadrupole magnet) suffered a serious failure during a test of its ability to
stand up to the forces that could occur if its coils lost their
superconductivity (a mishap called quenching). Part of the supports of the
magnet had collapsed under the pressure of the test, producing a loud bang like
an explosion.
These magnets come in groups of three, to squeeze the beam first from side to
side, then in the vertical direction, and finally again side to side, a sequence
that brings the beam to a sharp focus. The LHC uses 24 of them, one triplet on
each side of the four interaction points. At first the LHC scientists did not
know if all 24 would need to be removed from the machine and brought
above-ground for modification, a time-consuming procedure that could have added
weeks to the schedule. The problem was a design flaw. CERN and Fermilab
researchers worked feverishly, identifying the problem and coming up with a
strategy to fix the undamaged magnets in the accelerator tunnel.
In June, CERN director general Robert Aymar announced that he had to postpone
the scheduled start-up of the accelerator from November 2007 to spring 2008. The
beam energy is to be ramped up faster to try to stay on schedule for doing
physics by July.
The LHC Remote Operations Center
One of the LHC cryogenic superconducting dipole magnets
A view along the LHC main tunnel
The LHC ATLAS detector under construction, with an engineer in the
foreground
The Coming Revolutions in Physics
Edited by Andy Ross
When physicists are asked to give a short answer to the
question of why we are building the Large Hadron Collider (LHC), we usually
reply "Higgs" — the Higgs particle, the last remaining undiscovered piece of
our current theory of matter. But the full story is more interesting. The
new collider provides the greatest leap in capability of any instrument in the
history of particle physics.
In this new world, we expect to learn what distinguishes two of the forces of
nature — electromagnetism and the weak interactions — with broad implications
for our conception of the everyday world. We will gain a new understanding of
simple and profound questions: Why are there atoms? Why chemistry? What makes
stable structures possible?
The search for the Higgs particle is only the first step. Beyond it lie
phenomena that may clarify why gravity is so much weaker than the other forces
of nature and that could reveal what the unknown dark matter that fills the
universe is. Even deeper lies the prospect of insights into the different forms
of matter, the unity of outwardly distinct particle categories and the nature of
spacetime. The LHC will help us refine these questions.
The Standard Model of particle physics can explain much about the known world.
The main elements of the Standard Model fell into place during the 1970s and
1980s. Yet even as the Standard Model has gained ever more experimental support,
a growing list of phenomena lies outside its purview, and new theoretical ideas
have expanded our conception of what a richer and more comprehensive worldview
might look like. Taken together, the continuing progress in experiment and
theory point to a very lively decade ahead.
Our current conception of matter comprises two main particle categories, quarks
and leptons, together with three of the four known fundamental forces,
electromagnetism and the strong and weak interactions. Gravity is, for the
moment, left to the side. Quarks, which make up protons and neutrons, generate
and feel all three forces. Leptons, the best known of which is the electron, are
immune to the strong force. What distinguishes these two categories is a
property akin to electric charge, called color. Quarks have color, and leptons
do not.
The guiding principle of the Standard Model is that its equations are
symmetrical. The equations remain unchanged when you change the perspective from
which they are defined, even when the perspective shifts by different amounts at
different points in space and time. The symmetry of the equations places very
tight constraints on them. These symmetries beget forces that are carried by
special particles called bosons.
In the Standard Model, the symmetry of the equations dictates the interactions
among particles that the theory describes. For instance, the strong nuclear
force follows from the requirement that the equations describing quarks must be
the same no matter how one chooses to define quark colors. The strong force is
carried by eight particles known as gluons. The other two forces,
electromagnetism and the weak nuclear force, are the electroweak forces and are
based on a different symmetry. The electroweak forces are carried by a quartet
of particles: the photon, Z boson, W+ boson and W– boson.
The theory of the electroweak forces was formulated by Sheldon Glashow, Steven
Weinberg and Abdus Salam. The weak force, which is involved in radioactive beta
decay, does not act on all the quarks and leptons. Each of these particles comes
in left-handed and right-handed varieties, and the beta-decay force acts only on
the left-handed ones — a striking fact still unexplained 50 years after its
discovery.
In the initial stages of its construction, the theory had two essential
shortcomings. First, it foresaw four long-range force particles—referred to as
gauge bosons—whereas nature has but one: the photon. The other three have a
short range, less than about 10–17 meter. According to Heisenberg’s uncertainty
principle, this limited range implies that the force particles must have a mass
approaching 100 GeV. The second shortcoming is that the family symmetry does not
permit masses for the quarks and leptons, yet these particles do have mass.
The way out here is to recognize that a symmetry of the laws of nature can be
broken. The needed theoretical apparatus was worked out in the 1960s by Peter
Higgs, Robert Brout, François Englert and others. The inspiration came from
superconductivity, in which certain materials carry electric current with zero
resistance at low temperatures. Although the laws of electromagnetism themselves
are symmetrical, the behavior of electromagnetism within the superconducting
material is not. A photon gains mass within a superconductor, thereby limiting
the intrusion of magnetic fields into the material.
This phenomenon is a prototype for the electroweak theory. If space is filled
with a type of superconductor that affects the weak interaction rather than
electromagnetism, it gives mass to the W and Z bosons and limits the range of
the weak interactions. This superconductor consists of particles called Higgs
bosons. The quarks and leptons also acquire their mass through their
interactions with the Higgs boson. By obtaining mass in this way, these
particles remain consistent with the symmetry requirements of the weak force.
The paradigm of quark and lepton constituents interacting by means of gauge
bosons completely revised our conception of matter and pointed to the
possibility that the strong, weak and electromagnetic interactions meld into one
when the particles are given very high energies. The electroweak theory shows
how the quarks and leptons might acquire masses but does not predict what those
masses should be. The electroweak theory is similarly indefinite in regard to
the mass of the Higgs boson itself. Many of the outstanding problems of particle
physics and cosmology are linked to the question of exactly how the electroweak
symmetry is broken.
Encouraged by a string of promising observations, theorists began to take the
Standard Model seriously enough to begin to probe its limits. Toward the end of
1976 Benjamin W. Lee, Harry B. Thacker, and I devised a thought experiment to
investigate how the electroweak forces would behave at very high energies. We
imagined collisions among pairs of W, Z and Higgs bosons. At the time of our
work, not one of these particles had been observed.
We noticed a subtle interplay among the forces generated by these particles.
Extended to very high energies, our calculations made sense only if the mass of
the Higgs boson were not too large — the equivalent of less than 1 TeV. If the
Higgs is lighter than 1 TeV, weak interactions remain feeble and the theory
works reliably at all energies. If the Higgs is heavier than 1 TeV, the weak
interactions strengthen near that energy scale and all manner of exotic particle
processes ensue. This mass threshold means that something new is to be found
when the LHC turns the thought experiment into a real one.
Experiments may already have observed the influence of the Higgs. The
uncertainty principle implies that particles such as the Higgs can exist for
moments too fleeting to be observed directly but long enough to leave a subtle
mark on particle processes. The Large Electron Positron collider at CERN, the
previous inhabitant of the tunnel now used by the LHC, detected the work of such
an unseen hand. Comparison of precise measurements with theory strongly hints
that the Higgs exists and has a mass less than about 192 GeV.
For the Higgs to weigh less than 1 TeV, as required, poses an interesting
riddle. Quantities such as mass are modified by quantum effects. Just as the
Higgs can exert a behind-the-scenes influence on other particles, other
particles can do the same to the Higgs. Those particles come in a range of
energies, and their net effect depends on where precisely the Standard Model
gives way to a deeper theory. If the model holds all the way to 1015 GeV, where
the strong and electroweak interactions appear to unify, particles with truly
titanic energies act on the Higgs and give it a comparably high mass. So why
does it appear to have a mass of no more than 1 TeV?
This tension is known as the hierarchy problem. One resolution would be a
precarious balance of additions and subtractions of the contending contributions
of different particles. Physicists are suspicious of immensely precise
cancellations that are not mandated by deeper principles. It seems likelier that
both the Higgs boson and other new phenomena will be found with the LHC.
Theorists have explored many ways to resolve the hierarchy problem.
Supersymmetry supposes that every particle has an as yet unseen superpartner
that differs in spin. If nature were exactly supersymmetric, the masses of
particles and superpartners would be identical, and their influences on the
Higgs would cancel each other out exactly. But in that case, physicists would
have seen the superpartners by now. So if supersymmetry exists, it must be a
broken symmetry. The net influence on the Higgs could still be acceptably small
if superpartner masses were less than about 1 TeV, within reach of the LHC.
Another option, called technicolor, supposes that the Higgs boson is not truly a
fundamental particle but is built out of as yet unobserved constituents. If so,
the Higgs is not fundamental. Collisions at energies around 1 TeV would allow us
to look within it and thus reveal its composite nature. Like supersymmetry,
technicolor implies that the LHC will set free a veritable menagerie of exotic
particles.
One more piece of evidence points to new phenomena on the TeV scale. The dark
matter that makes up the bulk of the material content of the universe appears to
be a novel type of particle. If this particle interacts with the strength of the
weak force, then the big bang would have produced it in the requisite numbers as
long as its mass lies between approximately 100 GeV and 1 TeV.
Opening the TeV scale to exploration means entering a new world of experimental
physics. Making a thorough exploration of this world is the top priority for
accelerator experiments. The answers will not only be satisfying for particle
physics, they will deepen our understanding of the everyday world.
The Guardian CERN page
June/July 2008
"The Large Hadron Collider at CERN will smash particles together to recreate the moments after the big bang. Some theories of spacetime suggest the particle collisions might create mini black holes. If that happened, I have proposed that these black holes would radiate particles and disappear. If we saw this at the LHC, it would open up a new area of physics, and I might even win a Nobel prize. But I'm not holding my breath." — Stephen Hawking
"Cathedrals were designed to celebrate the glory of God as manifested through the human spirit in words, music and art. The LHC has been engineered to celebrate and proclaim the glory of the natural world, and of our remarkable ability to comprehend it, as manifested through experimental science." — Lawrence Krauss
Plus articles by:
Brian Cox
A.C. Grayling
Michio Kaku
Martin Rees
and others
Peter Higgs, father of the 'God particle'
The Higgs boson is the particle that is thought to give everything else in the
universe mass. Its theistic nickname was coined by Leon Lederman, but Higgs
himself is no fan of the label. "I find it embarrassing because, though I'm not
a believer myself, I think it is the kind of misuse of terminology which I think
might offend some people."
Genesis machine poised to end quest for 'God particle'
"I sincerely hope I'm not the only one who's at least slightly worried about this mad scientist Peter Higgs and his 'Genesis machine'" — a reader
A Bluffer's Guide
Interactive: A graphic guide to the LHC, how it will work and the physics that
lies behind it all
Da
LHC is Superduper Fly — rap video
Smashing Idea
By William Booth
Washington Post, September 11, 2008
It is the biggest machine ever built. Everyone says it looks like
a movie set for a corny James Bond villain. They are correct. The machine is
attended by brainiacs wearing hard hats and running around on catwalks. They are
looking for the answer to the question: Where does everything in the universe
come from? Price tag: $8 billion plus.
The world's largest particle accelerator is buried deep in the earth beneath
herds of placid dairy cows grazing on the Swiss-French border. The thing has
been under construction for years, like the pyramids. Its centerpiece is a
circular 17-mile tunnel that contains a pipe swaddled in supermagnets
refrigerated to crazy-low temperatures, colder than deep space.
The idea is to set two beams of protons traveling in opposite directions around
the tunnel, redlining at the speed of light, generating wicked energy that will
mimic the cataclysmic conditions at the beginning of time, then smashing into
each other in a furious re-creation of the Big Bang — this time recorded by
giant digital cameras.
Wednesday, they fired this sucker up ...
AR This is so exciting. I
can hardly wait for the results — what will the Higgs look like?
