Read The Higgs Boson: Searching for the God Particle Online
Authors: Scientific American Editors
The Higgs Boson: Searching for the God Particle (31 page)
A third, highly provocative idea is that the hierarchy
problem will go away on closer examination,
because space has additional dimensions
beyond the three that we move around in.
Extra dimensions might modify how the forces
vary in strength with energy and eventually
meld together. Then the melding—and the onset
of new physics—might not happen at 10
12
TeV but at a much lower energy related to the
size of the extra dimensions, perhaps only a few
TeV. If so, the LHC could offer a peek into those
extra dimensions.
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. Whatever resolves the hierarchy
problem will probably suggest a candidate
for the dark matter particle.
Revolutions on the Horizon
Opening the TeV scale to exploration means
entering a new world of experimental physics.
Making a thorough exploration of this world—
where we will come to terms with electroweak
symmetry breaking, the hierarchy problem and
dark matter—is the top priority for accelerator
experiments. The goals are well motivated and
matched by our experimental tools, with the
LHC succeeding the current workhorse, Fermilab’s
Tevatron collider. The answers will not
only be satisfying for particle physics, they will
deepen our understanding of the everyday
world.
But these expectations, high as they are, are
still not the end of the story. The LHC could
well find clues to the full unification of forces or
indications that the particle masses follow a rational
pattern. Any proposed interpretation
of new particles will have consequences for rare
decays of the particles we already know. It is
very likely that lifting the electroweak veil will
bring these problems into clearer relief, change
the way we think about them and inspire future
experimental thrusts.
Cecil Powell won the 1950 Nobel Prize in
Physics for discovering particles called pions—
proposed in 1935 by physicist Hideki Yukawa
to account for nuclear forces—by exposing
highly sensitive photographic emulsions to cosmic
rays on a high mountain. He later reminisced:
“When [the emulsions] were recovered
and developed in Bristol, it was immediately apparent
that a whole new world had been revealed.
. . . It was as if, suddenly, we had broken
into a walled orchard, where protected trees
had flourished and all kinds of exotic fruits had
ripened in great profusion.” That is just how I
imagine our first look at the TeV scale.
Sidebar: Hidden Symmetry That Shapes Our World
If there were no Higgs mechanism, what a different world it would
be! Elementary particles of matter such as quarks and electrons
would have no mass. Yet that does not mean the universe would contain
no mass. An underappreciated insight from the Standard Model is
that particles such as the proton and neutron represent matter of a
novel kind. The mass of a proton, in contrast to macroscopic matter, is
only a few percent of its constituent masses. (In fact, quarks account
for not more than 2 percent of the proton’s mass.) Most of the mass
arises through the original form of Albert Einstein’s famous equation,
m = E/c
2
, from the energy stored up in confining the quarks in a tiny
volume. In identifying the energy of quark confinement as the origin
of proton and neutron mass, we explain nearly all the visible
mass of the universe, because luminous matter is
made mostly of protons and neutrons in stars.
Quark masses do account for an important
detail of the real world: that the neutron is slightly
more massive than the proton. One might expect
the proton to be the more massive one, because
its electric charge contributes to its intrinsic energy—a source of self-energy the neutron lacks. But
quark masses tip the balance the other way. In the
no-Higgs zone, the proton would outweigh the neutron.
Radioactive beta decay would be turned on its
head. In our world, a neutron sprung from a nucleus
decays into a proton, electron and antineutrino
in about 15 minutes, on average. If quark masses were to vanish, a
free proton would decay into a neutron, positron and neutrino. Consequently,
hydrogen atoms could not exist. The lightest “nucleus”
would be one neutron rather than one proton.
In the Standard Model, the Higgs mechanism differentiates electromagnetism
from the weak force. In the absence of the Higgs, the strong
force among quarks and gluons would induce the distinction. As the
strong interaction confined the colored quarks into colorless objects like
the proton, it would also act to distinguish the weak and electromagnetic
interactions, giving small masses to the W and Z bosons while leaving
the photon massless. This manifestation of the strong force would
not give any appreciable mass to the electron or the quarks.
If it, rather than the Higgs, operated, beta decay would
operate millions of times faster than in our world.
Some light nuclei would be produced in the early
no-Higgs universe and survive, but they would
not form atoms we would recognize. An atom’s
radius is inversely proportional to the electron’s
mass, so if the electron has zero mass, atoms—
less than a nanometer across in our world—would
be infinitely big. Even if other effects gave electrons
a tiny mass, atoms would be macroscopic. A
world without compact atoms would be a world
without chemistry and without stable composite
structures like our solids and liquids.
Credit: Ian Worpole
-Originally published: Scientific American 298(2) 46-53 (February 2008)
Fermilab Provides More Constraints on the Elusive Higgs Boson
by John Matson
The Higgs particle, the last piece of the Standard Model of particle physics
menagerie that has yet to be observed, is running out of places to hide—if, that is, it
exists at all. Fermi National Accelerator Laboratory in Batavia, Ill., today narrowed
the range of mass where the Higgs might be found.
The Higgs boson, named for British physicist Peter Higgs, is believed to give other
elementary particles, such as the heavy W and Z bosons, their mass, so finding it or
proving it does not exist would have major implications in ground-up
interpretations of how the world works.
"This is a very interesting time in particle physics, because we have this Standard
Model, which explains everything we've observed and everything we know about for
the last 30 years with no significant deviations. And, yet, we know that the Standard
Model can't be the whole story of nature," says John S. Conway, a physicist at the
University of California, Davis, and a member of the Collider Detector at Fermilab
(CDF) collaboration, one of two teams involved in the new mass-range results.
Many of the lingering questions in physics could be answered or at least clarified
when the model's missing piece is located. "Whatever we discover," Conway adds,
"it's going to be astounding."
Previous collider experiments had placed a lower bound of 114 giga-electron volts
(GeV), a measure that can be used for particle mass, on the Higgs, and theoretical
calculations require it to be less than 185 GeV. The new Fermilab results, from its
Tevatron collider, rule out a Higgs mass between 160 and 170 GeV. (All of these
constraints are at the 95 percent confidence level, according to Fermilab.)
Collider experiments such as those at the Tevatron smash particles together at
extremely high energies and observe what is produced, including some exotic but
short-lived particles. "We look for the signature of things we know are there and
things we think might be there, like the Higgs," says physicist Craig Blocker of
Brandeis University in Waltham, Mass., also a member of the CDF team. "If the Higgs had a mass in this fairly narrow range" of 160 to
170 GeV, he says, "we should have seen it, we had a good chance to see it."
Conway says the extension of the excluded Higgs masses at Fermilab is "a really exciting development." All the same, he thinks the
Higgs, if it is to be found, will be first seen at the more powerful Large Hadron Collider (LHC) near Geneva, Switzerland, which is slated
to come back online later this year after an aborted start-up last September. (Both Conway and Blocker are also working on physics
projects at the LHC.) "It is a bit of a race" to find the Higgs, Conway says, "but if I had to bet money, I would bet on the LHC."
There are some scenarios, however, in which Fermilab—enjoying its continuing status as particle physics top dog while the LHC is
sidelined—might win that race. If the Higgs happened to have a mass around 150 GeV, which Conway believes is unlikely—evidence
points to a lighter particle in the neighborhood of 120 GeV, he says—the Tevatron could find it relatively soon. Alternately, with more
time to gather data, the Tevatron could close in even tighter on the Higgs by inching its lower mass bound upward.
But what if the entire mass window were exhausted—if experiments showed that the Higgs, or something like it, didn't exist at all? "That
would basically mean we have a very deep and fundamental lack of understanding of what is going on in the Standard Model," Blocker
says. "If there's not something like the Higgs or something similar giving masses to the W and Z, we have no clue as to how that's
happening."
-Originally published: Scientific American online, March 13, 2009.
A Higgs Setback: Did Steven Hawking Just Win the Most Outrageous Bet in Physics History?
by Amir Aczel
CERN
A few years ago,
celebrated British
physicist Stephen
Hawking was widely
reported in the press
to have placed a
provocative public bet
that the LHC (along
with all particle
accelerators that
preceded it) would
never find the Higgs
boson, the so-called “God particle” believed responsible for having
imbued massive particles with their mass when the universe was
very young.
His pronouncement caused a stir in the global physics community,
and the Scottish physicist Peter Higgs, whose name had gotten attached to the hypothetical particle (Higgs had done some work in the
1960s, as had several other physicists, paving the way for the theoretical existence of the mass-imparting boson) took the challenge
personally, complained about Hawking, and later lamented that to answer Hawking’s challenge would have been “like criticizing the late
Princess Diana.”
In fact, informal polls of physicists over the last decade have shown that an overwhelming majority believed that the existence of the
Higgs was a foregone conclusion and that all that was needed was simply to run the LHC long enough: the Higgs would eventually show
up. Hawking—known for controversial and contrarian pronouncements—was seen as simply throwing around his weight.
But the Higgs boson never appeared. Running continually at an unprecedented energy level of seven trillion electron volts since March
31, 2010, the LHC has been amassing petabytes of data that are being analyzed by a grid of interlinked computers worldwide in search
of the missing boson. And yesterday, August 22, at the Biennial International Symposium on Lepton-Photon Interactions at the Tata
Institute of Fundamental Research in Mumbai, India, the bombshell was dropped: CERN scientists declared that over the entire range
of energy the Collider had explored—from 145 to 466 billion electron volts—the Higgs boson is excluded as a possibility with a 95%
probability.
The search for the Higgs is a statistical hunt that involves looking at the particles that emanate from the high-energy collisions of
protons inside the LHC, measuring their energies and directions of flight, as well as other parameters, and trying to assess whether it is
likely that some of these particles result from the decay of a Higgs boson created by the collision. These assessments carry a probability
measure, such as 95%, 99%, or—as traditionally required in particle physics for a “definitive” conclusion about the existence of a new
particle: 99.99997% (this is the infamous “five-sigma” requirement).
To be sure, the new, negative results presented in Mumbai yesterday are of a different nature. They state that, with a 95% probability,
the Higgs does not exist within the range of energies the LHC has so far explored, between 145 and 466 billion electron volts. The
probability of nonexistence is not overwhelming—there is still a 5% chance that the Higgs is hiding somewhere within this energy range.
And, more importantly, the lower energy range from 114 to just under 145 billion electron volts, a region of energy that Fermilab has
determined, through earlier experiments, may harbor the Higgs, has not been ruled out. But the Higgs is quickly running out of places
to hide. Lower energy levels have been accessible to smaller accelerators, such as the Tevatron at Fermilab and the LEP—the LHC’s
predecessor at CERN—and neither collider had found it. Perhaps the Higgs does not exist at all.
So while CERN will continue its search for the Higgs at least until the end of this year, if no positive results about the Higgs should come
out, Stephen Hawking—betting against the entire world of physics, as it were—would be able to cash in on his wager. And in that case,
Congress may feel that even though its 1993 decision to cancel the American alternative to CERN—the Superconducting Super
Collider—was generally met with chagrin by the American physics community, it may have been the right move one after all: to spend
billions of taxpayer dollars in search of a particle that likely does not exist would have been wasteful.
But if the Higgs doesn’t exist, where does mass in the universe come from? Theories that go beyond the “standard model” of particle
physics (of which the Higgs is the keystone—the one missing piece needed to explain how the universe we know came to be) may be
necessary. Steven Weinberg, who in his landmark 1967 paper on the unification of the electromagnetic and the weak interactions had
made key use of the Higgs for “breaking the symmetry” and separating the electromagnetic from the weak forces, has since gone beyond
the standard model in his research. Weinberg has proposed a theory called Technicolor, within which the primeval symmetry of our
universe can be broken through a different mechanism than the action of the elusive Higgs. But to prove the validity of the Technicolor
theory may require an energy level that would dwarf that available to the LHC—at an equally astronomical cost.
-Originally published: Scientific American online August 23, 2011