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AT _ T H E _ F R O N T I E R _ O F _ P H Y S I C S
The Strange Flavor of
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AMONG THE PAYO F FS OF QCD CA LC U L AT I O N S
ARE FUNDA M E N TAL PARAMETERS CALLED CKM
M ATRIX ELEMENTS.
Who says physicists aren’t fun? When we want to talk about the fundamental
particles of matter, the rock-bottom, indivisibly smallest of the small things
deep within the nucleus of an atom, we can be grateful that the person who
thought them up read James Joyce and had a sense of humor.
In 1963, theoretical physicist Murray Gell-Mann needed a name for his idea for the building blocks of protons,
neutrons and related subatomic particles, till then thought to be indivisible. He settled on quark, a word that
combines a dog’s bark with a sea gull’s squawk and that in German means curd, the fundamental constituent of
cheese. Gell-Mann’s quarks, which originated as a mathematical glimmer in his mind, have since been substantiated
in many experiments, though no one — for reasons inherent in the nature of quarks — has yet tasted, touched, seen
or in any way directly observed one.
Gell-Mann won a 1969 Nobel Prize, and by the early 1970s, quarks were key ingredients in a powerful theory,
called quantum chromodynamics or QCD, that describes the strongest force in nature — the interactions that hold
together the nucleus of an atom. QCD is a major component of modern physics’ integrated theory of the fundamental
particles and their interactions, called The Standard Model.
“The Standard Model,” says Indiana University physicist Steven Gottlieb, “has been enormously successful,
having passed all the experimental tests to which it’s been put. But what we know about it is incomplete, because
it’s been difficult to extract many of the most interesting predictions of QCD.”
Enter supercomputing. Though frightfully challenging and demanding the most powerful systems that can be
built, computational simulations of QCD are crucial to expanding what’s known in high energy and nuclear physics.
Gottlieb began doing QCD computations in the early 1980s, and since the 1990s he’s been part of a nationwide
team of theoretical physicists who collaborate to gain maximum advantage from the available computing time. They
call themselves the MILC collaboration, an acronym, MImd Lattice Computation, that points both to scientific computing (multiple instruction, multiple data — mimd — the technology of massively parallel processing) and to the
computational approach — the “lattice” — that makes it feasible to solve QCD equations.
LeMieux, PSC’s terascale system, has been a major boost to their work. Beginning in early 2001 with the
TCSini prototype and since October, when grownup LeMieux came online, the MILC team has milked every
drop of computing available to it from this system. The result is their most realistic QCD calculations yet.
“We’ve made exceptional progress,” says Gottlieb, “that wouldn’t have been possible without this system.”
M AY THE FORCES BE WITH YOU: THE STA N DARD MODEL & CKM MATRIX ELEMENTS
The Standard Model is a pedestrian name for one of the more glorious achievements of science. It’s a tidy summing up
of much of the knowledge that has blossomed, if not exploded, from the 20th century revolutions of quantum theory and
relativity. It links the tiniest particles inside an atom with the big bang and what we know about evolution of the cosmos.
Four forces — that’s all you need to know. From the standpoint of modern physics, everything that happens is at
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root level a result of gravity, electromagnetism, the strong force and
the weak force. For The Standard Model, gravity is the stepchild that
doesn’t quite fit. The weakest of the four — weaker even than the weak
force — gravity affects us only when matter aggregates into huge
masses, such as planets and stars.
The Standard Model’s fame lies in bringing the other three forces
under one umbrella. It includes a unified theory of electromagnetism
and the weak force (electroweak theory) and QCD, the theory of the
strong force. In other words, if a dream of modern physics is (and it
is) a theory of everything (TOE), The Standard Model is close to
three-fourths of a TOE.
The Standard Model sums up what we’ve learned about fundamental particles and how they interact at the extremes, such as
when atoms are smashed in a collider and “decay” into other
particles. Most fundamental particles exist only for fleeting
instants as traces from these high-energy events, in which
scientists force them out from hiding.
elements. Named for three physicists,
Cabibbo, Kobayashi and Maskawa, CKM
matrix elements are of great interest to
contemporary physics. They are fundamental
parameters of The Standard Model, similar
to Planck’s constant at the core of quantum
theory. They define the intersection between
the strong and weak force in events that
involve both. Some of the CKM elements are
known. Others are more intractable. Pinning
them down could lead the way to deeper
levels of understanding in particle physics.
QUARKS, MESONS & WEAK DECAY
According to QCD, quarks come in six “flavors”
— up, down, strange, charm, bottom and top
(in ascending order by mass) — that mix in
FOR THE FIRST TIME IN QCD, LAT T I C E S
I N C LUDE DY N A M I CAL STRANGE QUARKS.
WEAK DECAY
Called a Feynman diagram, after Nobel physicist Richard Feynman who originated them, this
drawing by Steve Gottlieb starts on the left with a B meson (B+). The strong force confines an up
_
quark (u) and bottom anti-quark ( b) inside the B meson, where they interact until they annihilate.
“For a moment,” says Gottlieb, “there’s a W boson (W+), carrier of the weak force. Then that W
boson, because it’s a virtual particle, decays to an anti-electron, also called a positron (e+), and
an electron neutrino (νe).”
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As successful as The Standard Model has been, physicists know
it’s a plateau in the progress of science — a grand edifice that with
every piece of knowledge added reveals another question to be
resolved. QCD calculations are the only way “to confront experiment
with theory,” says PSC physicist and co-scientific director Ralph
Roskies — to compute quantities needed to answer some of the
questions. While we spend hundreds of millions annually on powerful
accelerator experiments, QCD calculations — which aren’t backed with
this level of resources — are an equal partner in the intellectual effort.
“We need simulations to determine a number of the basic
parameters of The Standard Model,” says Gottlieb, “for quantitative
understanding of physical phenomena controlled by the strong
interactions, and to do precision tests of the Standard Model.”
Among the payoffs ahead — awaiting accurate QCD calculations to
interact with experimental data — are quantities called CKM matrix
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different combinations to form many varied
species of the subatomic particle zoo. The
big particles of the nucleus, protons and
neutrons, are bundles of three quarks. Two
ups and a down make a proton, an up and two
downs a neutron.
From the mid-1960s forward, accelerator
experiments have set out to break quarks free
from the larger nuclear particles they inhabit.
No luck. The problem, as it’s been theorized
within QCD, is that when you add great
amounts of energy to a proton — trying to
smash it in an accelerator — rather than
freeing a quark, the added energy becomes
new matter. Out of thin air, another particle
appears. The particle that forms is a meson — a quark paired with
an anti-quark.
There are many mesons, and one of them in particular, the B
meson — B for the bottom anti-quark (paired with an up or down
flavored quark) — is the subject of a tantalizing question, one the
MILC group is aiming at with LeMieux. The question — “weak decay”
— is one of the places where QCD calculations are crucial to
determining CKM matrix elements.
The weak force — in some ways the most mysterious of the four
forces — is responsible for nuclear decay, of which the best known
example is radioactivity. Because of the weak interaction, the vast
majority of fundamental particles when left to themselves disintegrate to one of only a few apparently stable particles — prominently
including protons, electrons and neutrinos.
With a B meson, a “leptonic” weak decay happens when the heavy
anti-quark and light quark annihilate each other due to the weak
interaction. (Bottom, the next to heaviest quark, is 1,000 times
Members of the MILC group, colleagues and canine friends at a recent
Lattice QCD conference in Salt Lake City (left to right): Carleton DeTar,
heavier than up, the lightest.) What comes out of this
decay, roughly analogous to sex change, is no longer a
nuclear particle but a pair of non-nuclear particles. Called
leptons (Greek for light and swift), the most prevalent are
electrons and neutrinos.
Gottlieb and his colleagues and many others want to
accurately describe how the weak interaction contributes
to this leptonic decay. But the strong interaction gets in
the way. The quark and anti-quark stay inside the meson,
interacting through the strong force before they annihilate.
“If we don’t understand how they interact strongly,” says
Gottlieb, “we can’t get the detailed probability of how
they’ll decay.”
Major accelerator experiments now underway, one at
Stanford and another in Japan, are focused on producing
B mesons and analyzing their decay. At the same time,
with big help from LeMieux, the MILC group is zeroing in
on a quantity — called the B meson decay constant — that’s
needed, along with data from the experiments, to arrive at
the CKM matrix elements that define this decay.
Eric Gregory, Tom Burch, James Osborn, Roberta Toussaint, Doug
BUILDING A BETTER LAT T I C E
Toussaint, Steve Gottlieb, Phillip Toussaint.
THE MILC QCD COLLABORAT I O N
Claude Bernard
WA S H I N GTO N U N I V E R S I T Y
Tom DeGrand
U N I V E R S I T Y O F C O LO R A D O
Carleton DeTar, James Osborn
U N I V E R S I T Y O F U TA H
Steven Gottlieb
INDIANA UNIVERSITY
Urs M. Heller
F LO R I DA S TAT E U N I V E R S I T Y
James Hetrick
UN I V E R S I T Y
OF THE
PAC I F I C
Robert Sugar
UNIVERSITY
OF
CA L I FO R N I A, SA N TA B A R B A R A
Tom Burch, Eric Gregory, Doug Toussaint
UNIVERSITY OF ARIZONA
The prerequisite to calculating the B meson decay constant
is realistic QCD lattices, a scaffold-like framework that
divides the continuum of space-time into discrete regions
in which to compute physical quantities. “We’re generating
the most realistic set of lattices we’ve ever produced,” says
Gottlieb of the past year’s work with LeMieux. The major
part of the MILC effort has been to generate the best lattices
possible with available resources. With over a million singleprocessor hours of LeMieux (nearly 3,000 hours on 512
processors), their new lattices are more finely resolved than
ever (grid spacing is .09 femtometers, a million-billionth
of a meter).
Significantly, the LeMieux lattices also reflect a more
realistic representation of dynamical quarks — the sealike background of quark, anti-quark pairs that affect
QCD physics. Prior QCD lattices have included only up
and down dynamical quarks. The new lattices, for the
first time in QCD, include dynamical strange quarks.
Preliminary analysis indicates that these quarks influence
the value of the decay constant.
With the new lattices and more computations, the MILC
group is progressively closing in on the elusive pieces of
knowledge it seeks. As powerful an explanatory work as it
is, The Standard Model is replete with mystery. “Why are the
quark masses what they are?,” asks Gottlieb. “The top quark
is 175 times the mass of the proton. It’s amazing. Quarks and
leptons come in pairs of three generations. Why?”
The CKM matrix elements represent a key that may
unlock the next layer of mystery. “A more basic, deeper
theory,” says Gottlieb, “would have to make an accurate
prediction of the CKM matrix elements. Until we can
accurately extract these parameters, there will be uncertainty
when people propose more fundamental theories. We all
think there may be something beyond The Standard Model,
something we don’t yet understand.”
M O R E I N F O R M AT I O N : http://www.psc.edu/science/milc.html
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