Download Discovery of the Higgs Particle

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UNIVERSITY OF
WATERLOO
Department of Physics & Astronomy
University of Waterloo
Waterloo, Ontario, Canada
N2L 3G1
Spring 2013
Number 146
Discovery of the Higgs Particle
Cover:
The Higgs particle is discovered!
Contents
Evidence for the Higgs Particle – a Scientific
Detective Story………………………………………...3
A 50 year search for the origin of mass culminates in the
discovery of the Higgs boson. Find out here how it was
done.
From the Editor
In this issue of PHYS13 we celebrate the
discovery of the Higgs boson. This particle, anticipated
to exist for nearly 50 years, was finally discovered at the
Large Hadron Collider at CERN last year. In this issue
we describe what this discovery is, why it is important,
and (in the Young Physicists' Corner) what other
alternatives have been considered.
This issue marks my last as the editor of
PHYS13. I have enjoyed working with the various
concepts and ideas of physics and of thinking how to
present them to the PHYS13 audience. This particular
issue was co-edited with Richard Epp. Richard recently
rejoined the Physics & Astronomy Dept at Waterloo
after having spent several years working on scientific
outreach at the Perimeter Institute.
I am pleased to say that Richard Epp will be
taking on this position with the next issue. I am
delighted to pass the mantle on to him.
Robert Mann
The Large Hadron Collider………………………….8
Here is a look at the machine that finally found the
Higgs particle.
Person of Interest……………………………………12
Meet Richard Epp, UW’s newest addition to the Physics
& Astronomy Department
Phys13news is published four times a year by the
Department of Physics and Astronomy at the University
of Waterloo. Our policy is to publish anything relevant
to high school and first-year university physics, or of
interest to high school physics teachers and their senior
students. Letters, ideas and articles of general interest
with respect to physics are welcomed by the editor. You
can reach the editor by paper mail, fax or email.
Paper:
Phys13news
Department of Physics and Astronomy
University of Waterloo
Waterloo, ON N2L 3G1
Fax:
E-mail:
519-746-8115
[email protected]
Editor:
Robert Mann
Young Physicists Corner……………………………13
What other alternatives exist for the origin of mass?
Find out here what might have been true.
Editorial Board:
N. Afshordi, D. Hawthorn, M. Mariantoni,
C. O’Donovan
Puzzles, Problems and Solutions
Publisher: Judy McDonnell
The SIN Bin………………………………………….17
Printing:
UW Graphics
Anagrammatic Physics……………………………...19
Phys13news / Spring 2013
Page 2
Evidence for the Higgs Particle
― a Scientific Detective Story
Natalia Toro
Perimeter Institute
On July 4, 2013, a curious online broadcast from
the European Center for Nuclear Research (CERN)
drew half a million live viewers, many of them waking
up before dawn to watch a broadcast at 3am in Eastern
North America. On-site at CERN, people had to camp
outside the door overnight to get seats. While this
turnout pales in comparison with the audiences for
international sporting events, it was remarkably high
given that the main event was a pair of two rather
technical scientific talks.
periodic table (see Figure 1). Some of the particles in
this periodic table are the building blocks of familiar
objects: the electron, and the “up” and “down” quarks
that bind into protons and neutrons. The Standard Model
also has six other types of charged matter, but you won’t
find them inside stable atoms. These less familiar
particles, such as the muon and top quark, are unstable:
they decay into the electron and up and down quarks on
timescales of a microsecond or shorter (sometimes much
shorter: the top quark lifetime is about 10-24 seconds, a
time so short it has never been measured directly!).
Though exotic, these particles are mathematically very
similar to the electron and up and down quarks. For
example, atoms with muons taking the place of electrons
have been made in the lab, and they behave much like
electronic atoms scaled down to a smaller size.
What garnered so much attention for these talks
was, in part, the world-wide nature and superhuman
scale of the teams doing the analysis. The two scientists
who spoke represented the ATLAS and CMS
collaborations, each a group of over 3000 scientists from
35 countries, who over the past decades designed and
built particle detectors the size of a small office building
at the Large Hadron Collider (LHC), and the software
and computing tools required to analyze the data these
detectors would produce. The analysis techniques
themselves were years in the making.
But an even greater reason for the large
audience (by the standards of scientific talks) was the
sense, even among people who barely understood the
analysis, that this result would turn a page in scientific
history that has lasted over forty years. The two talks
presented evidence for a new kind of particle, whose
properties were strikingly like those expected for the socalled “Higgs boson”. Conjectured in the 1960s, the
Higgs boson differed in important ways from any
elementary particle ever seen before. The Higgs field
associated with this new particle was thought to fill all
of space, and to be responsible for the masses of certain
elementary particles―a bold claim, but a solid one, as
something like a Higgs field is needed to make a
mathematically consistent theory of other known
particles.
The Higgs and the Standard Model
To understand why the Higgs field is such a vital
part of our Universe, we must delve briefly into the socalled “Standard Model” of matter and forces that it
completes.
The Standard Model is a theoretical framework
that describes all known types of matter and their
interactions (forces). The Standard Model is often
summarized as a list of particles, like a 20th-century
Phys13news / Spring 2013
th
Fig. 1: The 20 -century periodic table of elementary
particles, which represents all known types of matter and
their interactions. Credit Fermi National Accelerator
Laboratory.
But the Standard Model is much more than a
catalog of building blocks―it also encodes the laws of
these matter particles’ interactions. Four kinds of interparticle
force
have
been
seen:
gravity,
electromagnetism, and the strong and weak nuclear
forces (there should also be a Higgs-force, but we’ll get
to that a bit later). Each interaction is associated with a
new kind of particle. This is not as weird as it sounds at
first. To see how it works, imagine watching one electric
charge “A”, as another charge “B” far away is wiggled
back and forth. When charge B wiggles towards charge
A, the electric force on A goes up. When B wiggles
away, the force goes down. But Maxwell’s equations for
electricity respect special relativity: the up-and-down of
the force can’t happen instantly, but travels at the speed
of light. In fact, the little wiggle is a pulse of light. In a
Page 3
quantum version of electromagnetic theory, there is a
smallest wave or quantum of light, called the photon.
Likewise, the laws for all the other forces predict wave
solutions. The quanta of weak-force waves (called the
W or Z particles) and of strong-force waves (called
gluons) have both been detected, while gravity-waves
have not yet been seen. Unlike the other force-particles,
the W and Z particles are massive, which leads to
exponential weakening of the weak force at large
distances (hence the name “weak,” although at short
distances, the strength of this interaction is actually quite
respectable).
What makes some particles turn into “stuff” in
the macroscopic world, while others carry forces? One
important ingredient (though not the only one) is a
property called spin―a kind of angular momentum
carried by a single particle. All electrons (and protons,
quarks, and muons) carry the same amount of spin: ½
times Planck’s constant, which has units of length times
momentum, or angular momentum. All photons and
gluons carry 1 times Planck’s constant, all gravitons
carry 2 times…and Higgs particles carry no angular
momentum at all. The pattern is clear: all the particles
with integer spin (called bosons) carry forces, the ones
with half-integer spin (called fermions) make up matter.
Though the full story is complicated, many important
properties follow from two basic facts: (1) no two
fermions can occupy the same state, which causes even
slow-moving fermions to exert pressure on each other
that bosons lack (known as Pauli’s exclusion principle),
and (2) multi-particle interactions can produce single
bosons―the waves associated with a time-varying
force, but can only produce fermions in pairs
(ultimately, this is because total angular momentum,
counting both particles’ intrinsic spin and the “orbital
angular momentum” from their relative motion, is
conserved, and the latter is always an integer times
Planck’s constant).
The Standard Model has been quite successful:
the force laws can be used to compute the lifetime and
decay patterns of each particle, to precisely predict how
a muon spins in a magnetic field, and much more. But it
is mathematically inconsistent. The problem is a very
subtle one: the W and Z particles themselves feel a weak
force, which grows too fast at short distances. To keep
the theory consistent, there must be some new force that
precisely cancels this rapid growth. In the simplest
consistent version of the theory, this force is associated
with a single Higgs particle, though more general
versions of this setup are also possible.
recognized its relevance to the problem of W and Z
bosons. An important feature of this model is that the
value of the Higgs field in any region of space changes
the effective masses of the W and Z, as well as all the
basic matter particles of the Standard Model, as they
propagate through that region. In our Universe, the
equilibrium value of the Higgs field is at some point,
where the W, Z, and basic matter particles all have nonzero masses. But for some special value of the Higgs
field, the W and Z particles would have zero mass (as
would all the Standard Model matter particles, but not
the Higgs particle itself). The existence of this
“symmetric point” is key to the theory’s selfconsistency.
According to this picture, the Higgs field plays a
truly essential role in our world: the separation of the
actual value of the Higgs field from its symmetric point
determines both the mass of the electron (which in turn
dictates the size of atoms) and the mass of the W
particle (which determines the rate for radioactive beta
decays). A weaker Higgs field would make atoms
smaller and their nuclei less stable. Though it is
sometimes said that the Higgs particle is “the origin of
mass”, this isn’t quite true: the Higgs field is responsible
for less than one percent of your mass or mine (which
comes mostly from energy stored in the strong nuclear
force within protons and neutrons). But even this subpercent correction to the proton and neutron masses is
significant: if it were much smaller, the proton would be
heavier than the neutron, making all charged nuclei
unstable to decay into clumps of neutrons, meaning
that if the Higgs field were just a little bit different, we
might not have chemistry at all. Like Atlas holding up
the world in Greek myths, the Higgs field has a big job
to do!
Young physicists like me learned about the
Standard Model from textbooks that include a Higgs
field from the very beginning, so it’s easy for us to take
the existence of the Higgs particle for granted―our
forerunners predicted it long ago! But the arguments
from which the Higgs was predicted are very subtle;
they rely on thought experiments well beyond any
conditions observed in a lab, and require great
confidence in our mathematical understanding of subatomic physics. That these arguments were correct―and
could be formulated precisely enough to find evidence
for the Higgs particle, is a true triumph for theoretical
physics!
Knowing What to Look For
The basic structure that Higgs (and
simultaneously Brout, Englert, Guralnik, Hagen, and
Kibble) envisioned in 1964 is quite general, and has
already found important applications in the physics of
superconductors. Soon after, Weinberg and Salam
Phys13news / Spring 2013
The hunt for Higgs particles is almost as old as
the Standard Model itself. Part of the challenge is that
the theory couldn’t predict how heavy the Higgs particle
should be―only that it must be lighter than about ten
Page 4
times the mass of the W and Z bosons, or 1000 times the
mass of a proton. (Physicists usually measure masses in
units of Giga-electron-volts or GeV. This unit relates the
energy, E=mc2, needed to produce a new particle of
mass m, to the kinetic energy of an electron accelerated
through an electric potential of a billion volts. In this
manner of speaking, the mass of a proton is about one
GeV.) But any mass lighter than this could have
worked. Moreover, there could be one Higgs particle, or
more than one with different interaction strengths, if two
different types of field together controlled the masses of
the W, Z, and matter.
But it is often useful to consider the simplest
possibility first, at least to track one’s progress, as long
as it remains consistent with the data. The simplest
case―the “standard” in “Standard Model”―is that only
one Higgs field determines the masses of the W and Z.
Then the strength of the Higgs field’s interactions with
each of the particles, their “Higgs-charges”, could be
reverse-engineered from the particles’ masses. The
heaviest particles―the top quark, W, and Z, would have
the largest Higgs-charge. The Higgs-charge of the
bottom quark (the next-heaviest) should be forty times
weaker than that of the top, and that of the much lighter
electron about 30,000 times weaker. The massless
photon and gluon must have exactly zero Higgs-charge,
but would have other kinds of interaction with a Higgs
particle (loosely analogous to the way an oscillating
magnet can produce electromagnetic waves, even
though it is electrically neutral). These could be
precisely calculated in the Standard Model, and turn out
to be crucial to its discovery.
The hunt for the Higgs particle intensified in the
1990s, with the Large Electron-Positron Collider (LEP)
at CERN colliding electrons and anti-electrons (also
called positrons) at record-breaking energies of 209
GeV. Although proton colliders like the LHC can reach
40 times higher energies, electron-positron collisions
had distinct advantages in looking for a new particle.
When an electron and positron interact, their entire
energy can go into the creation of one or two heavy
particles. In contrast, when two protons collide at the
LHC, most of their kinetic energy goes into breaking up
the protons themselves into constituent quarks and
gluons, which then recombine into many tens of lowerenergy particles (many physicists jokingly call the LHC
a “trash-can-on-trash-can” collider). Only a small
fraction of their energy gets concentrated into a region
small enough to produce a Higgs particle.
By the year 2000, experimentalists working at
LEP could exclude the minimal “Standard Model Higgs
boson” for masses less than 114 GeV. At this point, it
was shut down and dismantled to make room for the
LHC, which would run in the very same tunnel
underneath Switzerland and France. The Tevatron at
Phys13news / Spring 2013
Fermilab in the US (a proton-antiproton collider) made
inroads to search for the Higgs at masses around 150–
170 GeV, using a different technique. But it would be
more than a decade before any experiment would move
the LEP lower Higgs-mass limit of 114 GeV any higher,
or be sensitive to a Higgs at the theoretical upper limit
around 1000.
How to Catch a Higgs
The first LHC collisions in 2010 brought good
news: with higher energy than any previous collider,
they could finally search the whole mass range for a
Standard Model Higgs with a few years’ data. But
would they find a Standard Model Higgs, or would they
instead find the first hints that Nature is more
complicated than our expectations? Most particle
physicists were confident in the essential idea of the
“Higgs mechanism”―that some field(s) that fills all
space is responsible for the masses of the known
elementary particles, and that there must be a new
particle(s) associated with this field―but there were
many credible (some would say, expected) ways that a
real-world Higgs particle(s) could behave differently
than the Standard Model Higgs particle, and perhaps be
more difficult to find.
Using the calculated Higgs-charges of each
particle, physicists could predict (for each possible
Higgs mass) how often the Standard Model Higgs boson
would be produced in different kinds of particle
collisions, and the relative frequencies of different
decays. This information was critical to finding the
Higgs. In fact, one of the key requirements used in
designing the LHC experiments (what detector materials
to use and how deep each layer should be) was
maximizing their ability to find the remnants of a
decaying Higgs particle. And in the fast-paced
environment of the LHC, you have to know what you’re
looking for before you look for it.
When it is running, the LHC collides “bunches”
of protons 20 million times per second. Each collision
produces a barrage of particles (remember the trashcans) that must be traced through four major
components of the detectors in order to reconstruct the
“story” of the collision. And each component is
complicated. The ATLAS experiment, for example,
reads out the responses of over 100 million detector
elements after each collision: a high-resolution digital
“image” of the event. This is simply too much data to
save (the equivalent of over a million blu-ray disks
every hour!). And so each experimental team has
devised an elaborate―and fast―set of automatic
conditions to determine which events should be saved.
Many of these “trigger” conditions were also designed
specifically with the Higgs decays in mind.
Page 5
This is What a Higgs Looks Like
One of the most distinctive things a Higgs
particle can do is to decay to two photons, each carrying
an energy equal to half of the Higgs particle’s mass (in
the Higgs particle’s center-of-mass frame). Another
distinctive pattern is for the Higgs to decay into two Z’s,
which in turn each decay into electron and anti-electron
or muon and anti-muon pairs, leading to four “leptons”
(a general term for electrons, muons, and their antiparticles) traveling through the detector. See Figure 2.
(Note that even if the Higgs is too light to decay into
two real Z particles, as we now know it to be, it can still
decay into the four leptons through a quantum effect
called a “virtual” Z.)
Fig. 2: A reconstruction of what a four-lepton collision event “looks” like in the ATLAS detector. This event is one entry in
the histograms in Figure 3. The tracks traveling all the way out of the detector are a muon and anti-muon; the ones that
stop in the blue region are an electron and anti-electron. But are these tracks the footprints of a Higgs particle? We can
never know for sure―it might just as well be a “background” event.
Both are rare processes: only about one in a thousand
Higgs particles is expected to have each of these specific
decay patterns. But they are useful anyway for two
reasons. First, the production of four leptons or two
photons from other “background” processes, i.e.,
collisions that do not involve a Higgs particle, is also
very rare. Second, the energies and directions of the
photons and leptons can be measured very accurately.
These can be combined into an “invariant mass”:
Phys13news / Spring 2013
assuming the hypothesis that the two photons (or four
leptons) came from the decay of one particle, then that
“parent” particle’s energy must have been equal the sum
of the photon energies, using energy conservation.
Similarly, the parent particle’s momentum-vector must
be the sum of the two photon momentum-vectors.
Putting these together, one can compute the mass of the
(still hypothetical) parent particle from its energy and
momentum.
Page 6
The pairs that came from “background” processes will
be spread out at random over a wide range of calculated
“invariant mass”. But when the pairs really came from a
Higgs particle, the invariant mass will always equal the
actual Higgs particle’s mass (up to measurement errors).
By making a histogram of the signal and background
events, you can eventually start to see a “bump” of real
Higgs decays over a plateau of background events. See
Figure 3. The rarity of two-photon and four-lepton
processes make for a small bump, but the background
plateau is also small, and the precise measurement of the
particles’ energies and directions make the bump
narrower and easier to spot.
Fig. 3: Histograms of the “invariant mass” for Higgs-like events with two photons (left) or four leptons (right). A sharp bump,
like the one seen here at 125 GeV in both histograms, is the sure sign of a particle decaying. The figure on the right also
shows a bump at 90 GeV, from Z bosons decaying into four leptons. (The figures shown here have more data than the
ones from July 2012, so the bumps show up a bit more clearly now than they did at the time of discovery.)
When we picture discovery, most people imagine
a flash of inspiration like the proverbial apple that hit
Isaac Newton on the head. But most scientific
discoveries happen slowly, and the Higgs particle
discovery was no exception. Even after the decades of
theoretical study and of design and construction of the
LHC and experiments, it took years of effort to reach the
level of precision shown in the figures above, as well as
years’ worth of data to obtain the evidence for a Higgs
boson. Most of the LHC events―even the ones that
were saved by the “trigger” requirement―are irrelevant
to the Higgs search, albeit interesting for other purposes.
The clean, simple figures we see now make it easy to
forget how much meticulous work was required to
construct them. A small part of this drama played out in
real-time as the ATLAS and CMS experiments
presented intermediate Higgs search results at
conferences. By December 2011, both experiments
reported some tentative evidence for a Higgs particle,
but it was not yet clear-cut. The CMS collaboration
found bumps at two different masses, each one
consistent with either a Higgs particle or with an
unlucky fluctuation (a few extra background events that
just happened to line up at the same mass). Building the
confidence to claim an “observation” of the Higgs
particle―the standard criterion is a signal five times
larger than a typical one-standard-deviation fluctuation
of the background―took work, time, and more work.
Phys13news / Spring 2013
Where next?
Presently, the LHC is shut down for a massive
upgrade that will nearly double the energy carried by its
colliding protons, and further increase the rate of
collisions. The discovery of a Higgs particle marks the
end of one chapter in fundamental physics, but also the
beginning of a new one. The “Standard Model” with one
Higgs particle is not the only possibility. As soon as the
Higgs particle was discovered, it was immediately
turned around into a new tool for measuring Nature: do
the different decay processes happen at the rates
predicted for the simplest kind of Higgs particle? Or
does it show some hints that its behavior is more
complicated?
This kind of measurement is tricky: just like you
can’t know for sure whether a coin is fair by tossing it
four times, there is an inevitable uncertainty in
measuring the average Higgs production rate from just a
handful of events. The uncertainty can be reduced only
by taking more data, and continuing to optimize the
analysis. For example, early data suggested that the
observed Higgs particle decayed to two photons about
50% more often than expected; this result triggered a lot
of theoretical work because it’s hard to accommodate in
consistent theories, but has since settled back near the
expected value. The hunt is also on for different Higgs
Page 7
decay modes (like decays into two bottom quarks) that
have not yet been seen, to further test whether this
particle’s behavior matches theoretical predictions. For
now, the Higgs particle looks uncannily similar to the
predictions of the simplest models, but there is a lot
more to see.
While testing specific theories, we are also on the
look-out for surprises, like new particles that might not
be produced except in Higgs decays. Having learned
what the Higgs particle looks like, one can also look for
new ways of producing it, and see if it leads to further
surprises.
One of the weirdest things about the Higgs
particle is that it has no spin angular momentum. Not
only is this unique among the known particles, it is
confusing. The conservation of angular momentum
significantly restricts the interactions of other particles;
this seeming detail helps to explain why some fermions
can consistently be much lighter than others. But this
protection fails for the Higgs, which has no angular
momentum; if it interacts with any heavy particles, then
quantum effects of these interactions would typically
drag its mass up, too. And yet, we think there are
particles heavier than the Higgs. So if angular
momentum doesn’t protect it, what does? Many
theories have been proposed―some, which did away
with an elementary Higgs particle altogether, have
already been ruled out by the discovery of a Higgs
particle. Others, like supersymmetry, predict new
particles that should be accessible at the LHC. Nothing
has been found yet, but we are not done looking.
The Large Hadron Collider
Ross Diener
McMaster University
Depending on who you ask, the Large Hadron
Collider, or LHC, can be a lot of things. To many
people, it is something they learned about from the plots
of popular fiction like Angels and Demons and The Big
Bang Theory. To crackpots, it is an alleged doomsday
device that might create black holes or other exotic stuff
and destroy the Earth. Ask the folks at Guinness and
they will tell you that the LHC is the world’s most
powerful particle accelerator, and a scientist will agree,
making sure to add that it is also the biggest and most
expensive science experiment ever. Someone might
even tell you that the LHC is the subject of a rap song,
the “LHC Rap,” a surprisingly informative and accurate
ode to the LHC that can be found on YouTube [1].
If you ask a particle physicist, though, you will
probably get an answer so full of jargon that it might as
well be in a different language. After all, even the name
of the LHC contains a strange word: hadron. So it might
seem daunting to try to learn more about the LHC.
Luckily, there is a particle physics cheat sheet, and it has
been included in Figure 1. This will start you on your
road to particle physics fluency.
The effort to mount the LHC project was
monumental, but it is paying off. The LHC has already
confirmed a remarkable forty-year-old prediction of the
Higgs particle.
Measurements of the Higgs will
improve with more data, teaching us more about where
the masses of the elementary particles come from and
why Nature works the way it does. The LHC has a lot
left to teach us about the puzzles of the Universe.
Further Reading
An introduction to the Higgs field and Higgs
particle for a general audience:
http://profmattstrassler.com/articles-and-posts/the-higgsparticle/the-higgs-faq-2-0/
Animations of the accumulating events in the fourlepton and two-photon channels:
https://twiki.cern.ch/twiki/pub/AtlasPublic/HiggsPublic
Results//4l-FixedScale-NoMuProf2.gif
and
https://twiki.cern.ch/twiki/pub/AtlasPublic/HiggsPublic
Results//Hgg-FixedScale-Short2.gif
Phys13news / Spring 2013
Fig. 1: The particle physics cheat sheet. The quarks and
leptons are in the blue bubbles, and the bosons are in
the purple bubble.
As far as physicists know, the particles in Figure
1 are all elementary particles, in that they are not made
up of smaller particles. The purple bubble contains the
bosons, and each of these is associated with a force that
can act on the stuff in the blue bubbles. The blue
bubbles contain the quarks and leptons that make up
matter, and are classified separately, mostly because the
quarks interact with the strong force and the leptons
don’t. The more detailed properties and interactions of
these fundamental particles are all described by what’s
called the Standard Model of particle physics, and many
physical phenomena can be explained by this model
Page 8
with astounding accuracy. There are notable exceptions,
though, like the nature of dark matter, and augmenting
this cheat sheet with a dark matter particle by
uncovering it at the LHC would delight most particle
physicists.
reach the threshold energy required to produce a new
particle whose mass is, say, 3 TeV/c2. This particle
would have never been observed before because there
simply wasn’t enough energy to do so in previous
experiments.
You might recognize one or two elementary
particles, like the photon, which makes up light, or the
electron that lives in atoms and flows through wires as
current. The other particles are probably less familiar,
and with good reason. The neutrinos and the W and Z
bosons interact very weakly with familiar matter. For
example, neutrinos from the Sun usually pass right
through the entire Earth without “hitting” (interacting
with) anything. The muon and tau are heavy versions of
the electron that decay so quickly that they are not
normally observed on human timescales. The quarks and
gluons, on the other hand, are “confined,” usually inside
protons and neutrons, so we don’t see them directly.
Another reason physicists are excited about the
LHC is that more garbage (elementary particles) will be
produced there than in any other particle physics
experiment in history. As any yard sale aficionado will
tell you, it sometimes takes a lot of searching through
rubbish before you find something good, and the LHC is
no exception. The new particles being searched for
might only appear once in every few billion proton
collisions. So to stand a chance of seeing the new
particle amidst all the rubbish, and to really be sure it is
there, takes a lot of collisions. Luckily, the LHC is a
monumental machine that is up to the task.
The Standard Model tells us that at the low
energies of everyday life, the gluons responsible for the
strong force bind the quarks together into composite
particles called hadrons. The hadrons familiar to most
people are the proton and the neutron, but there are
many, many others. Inside the hadrons, the quarks and
gluons interact and move around like marbles in a
pouch, or, as my PhD supervisor says, like garbage in a
garbage can. At the LHC, these “garbage can” hadrons,
usually protons, are smashed together, or collided, and
garbage flies everywhere. Then it is up to
experimentalists to look through the garbage and find
interesting physics there, except the garbage is actually
elementary particles.
Picking through garbage doesn’t sound like a
very fun job, so why are physicists so excited about the
LHC? One reason is that garbage will be flung at
unprecedentedly high energies. As of this writing, the
LHC has accelerated protons to energies of 4 trillion
electron volts, or TeV, each. (An electron volt is the
amount of kinetic energy an electron gains when it is
accelerated through a potential difference of one volt.)
When the protons are collided head on, this gives a total
energy transfer of 8 TeV, with plans to eventually
increase this number to 14 TeV. To put this in
perspective, the next highest energy particle accelerator,
the Tevatron in Illinois, reached a maximum collision
energy of 2 TeV, so the LHC will eventually reach
energies almost ten times as large.
Enormous amounts of energy are exciting,
because it takes increasingly large energies to create
heavier particles, à la E = mc2. This equation means, for
instance, that if a collision brings a particle with kinetic
energy E to rest, that kinetic energy can become a new
particle with mass m = E/c2―a kind of 21st century
alchemy. So the LHC might be the first machine to
Phys13news / Spring 2013
It really is hard to imagine the magnitude of the
LHC. As you might know, experiments can be a lot of
work. Maybe one of the dozen resistors in your circuit
decided to burn out, and the whole circuit had to be
rebuilt. Or perhaps you’ve spent hours retaking all your
measurements because a thermometer was poorly
calibrated. Even if you have experienced such woes,
they are tiny compared to the LHC. It is impossible to
envision building a machine that, on an average day,
accelerates bunches of hundreds of billions of protons to
nearly the speed of light, collides them once every 45
nanoseconds, and then detects the products of these
collisions and sends the data to physicists around the
world for analysis [2]. It is very impressive. One means
of understanding the grandeur of the LHC is to discuss
its actual size. The apparatus resides in a circular tunnel
28 km in circumference and 100 m underground, that
straddles the border of Switzerland and France, near
Geneva. The tunnel was originally built for a different
particle collider with an equally imaginative name, the
Large Electron-Positron (LEP) Collider, which ran in
the 1990s and discovered the Z boson. In order to
upgrade the facility for protons, the tunnel was fitted
with a series of over 1200 superconducting
magnets―electromagnets made from coiled wire with
zero resistance―that are meant to deflect the otherwise
straight paths of protons and other hadrons into a circle.
Since the wires only superconduct at low temperatures,
liquid helium is used to keep them at a chilly 1.9 Kelvin
(1.9 Celsius degrees above absolute zero). That’s right,
somewhere on Earth there’s a 28 km tunnel where the
temperature is always kept at about 270 degrees below
the freezing point of water. That’s colder than the
coldest places in deep space.
For more perspective, consider this: when
constructing the LHC, each individual magnet was
lowered, one at a time, through the only entrance to the
tunnel that was big enough to fit them. Then that magnet
Page 9
was delivered by trolley to its place within the
underground ring. That’s an average distance of 14 km,
or halfway around the tunnel. In order to not damage the
magnets, they had to be moved at the slow rate of 2
km/h. So just bringing the magnets to their places in the
tunnel took around 10,000 hours of labour.
All that effort was worth it, though. Once
assembled, the 1200 magnets can bend the path of
hadrons around the ring even when they are moving at
extraordinary energies, at which point they are smashed
together in one of the various building-sized detectors
that are placed along the ring. For perspective, a
drawing of the ATLAS detector, with humans drawn to
scale, is in Figure 2. These detectors capture and analyze
the final products of a collision and the different
detectors’ capabilities are suited to their scientific goals.
The four main detectors are nicely summarized by the
chorus of the aforementioned LHC rap, which states:
LHCb sees where the antimatter’s gone
ALICE looks at collisions of lead ions
CMS and ATLAS are two of a kind
They’re looking for whatever new particles they can find.
The first two detectors on the list are smaller
experiments. LHCb is short for Large Hadron Collider
beauty experiment, and it represents a collaboration
ofaround 600 physicists [3]. This experiment
investigates the properties of hadrons that are made out
of bottom quarks (“b” stands for “bottom” or “beauty,”
depending on how fanciful you feel), and others as well,
which are copiously produced in proton collisions at the
LHC. ALICE, or A Large Ion Collider Experiment, is a
collaboration of 1200 scientists [4]. This experiment
differs from the others in that it predominantly looks at
the products of lead ions collisions instead of protons.
The nucleus of a lead ion contains dozens of neutrons
and protons, making them like garbage trucks in
comparison to the garbage cans that are protons. Their
collisions result in vast numbers of energetic particles
being produced. This is just a glimpse of the physics
behind these two experiments, and the interested reader
can consult the references for more information.
This brings us to the general purpose detectors:
CMS (Compact Muon Solenoid) and ATLAS (A
Toroidal LHC Apparatus) [5]. These larger
collaborations, with around 3000 scientist each, are the
newsmakers responsible for the discovery of the Higgs
boson, and it is a safe bet that if the LHC discovers
anything revolutionary, it will be done with these
detectors.
Both CMS and ATLAS are actively looking for
signs of new physics. After a disastrous first attempt in
September 2008, the LHC was successfully turned on in
November of 2009. Shortly afterwards, it broke the
world record as the world’s most powerful particle
accelerator, and ran exceptionally well until a planned
shutdown in early 2013. An upgraded LHC will
recommence operations in 2015. In the short three years
that CMS and ATLAS have had to take data, they have
already resulted in over 200 published scientific articles
each. So far, two new particles have been discovered at
the LHC: the Higgs [6], which you can read about in
more detail in an accompanying article, and χ b (3P) [7],
another hadron to add to the long list.
Fig. 2: The ATLAS Detector (Image courtesy of Argonne National Laboratory)
Phys13news / Spring 2013
Page 10
This is a natural point to discuss the various
revolutionary ideas the LHC might confirm. Are there
extra dimensions? How about new fundamental forces?
What is dark matter made of? This is what an LHC
article usually discusses. But the results from the LHC
have so far been surprisingly standard. That is, most
tests of exotic models are giving null results. For
example, dark matter is assumed to be very stable,
which is a good assumption since it is believed to have
been hanging around our universe for quite some time.
This means that if a dark matter particle is created in a
collision, it is very unlikely to decay into other particles
before it flies through the detector. It is also supposed to
interact very weakly with normal matter, or we would
have seen it already (it wouldn’t be “dark”). So it is
expected to fly right through the detector without being
detected (either directly, or through the particles it
decays into). But it would carry energy away with it.
This kind of thing can be searched for at the LHC, and it
is called missing energy. The problem is that, so far,
missing energy searches are consistent with the Standard
Model. There hasn’t been any evidence in favour of dark
matter, or the numerous other exotic physics scenarios
that predict missing energy. Nor has there been evidence
in other types of searches. So, rather than enumerating
the various unconfirmed exotic physics scenarios that
can be tested with the LHC, which I urge the reader to
explore in the next article, I am going to argue that the
LHC will be a success even if it doesn’t discover
anything exotic.
The LHC has already advanced technology. For
example, there will probably be a global energy crisis in
the next century, but the LHC could mitigate that
problem by yielding new insights into energy-saving
superconductor technology. After all, the machine itself
is a big ring of superconducting magnets, and future
upgrades to the machine are driving research in this
field. The LHC is also a marvel of modern computing.
Analyzing the LHC data from trillions of particle
collisions is an immense task, as is distributing it to
thousands interested physicists around the world. It is no
wonder that the Worldwide LHC Computing Grid is the
world’s largest, and is a prototype for future large scale
grid computing.
The LHC will also find a use once it is done
smashing particles. Chalk River Labs is a nuclear
research laboratory located in Ontario. It was originally
opened in the 1940s to study nuclear physics. Nowadays
it produces around half of the world’s medical isotopes.
Cyclotrons originally used to study particle physics are
now proton beams being used to treat cancer. Old
particle smashers are finding use in archaeology and
forensics as well. Somebody will find a job for the
Large Hadron Collider one it has retired from its career
as a particle physicist, so building it is doubly useful.
Phys13news / Spring 2013
The LHC is also relatively inexpensive; it is
estimated to have cost only ten billion dollars. That
might seem like a lot of money, but it isn’t when you
realize that five billion dollars is the approximate cost of
an aircraft carrier, and the recent cost of bailing out one
insurance company is the same amount it cost a previous
generation to put a man on the moon. So, one could say
that governments have lots of cash that they spend on
arguably less important things. The LHC is but a minor
expense, so who cares if it uncovers something.
Lastly, the LHC will produce thousands of highly
trained individuals worldwide. Many of these
individuals will make a career of physics, but many
won’t. There are physicists working in banks and on
Wall Street, building software, developing products for
industry, and, of course, teaching. The skills that they
developed to work on the LHC will be put to use
elsewhere. These people didn’t learn these skills to
become bankers. They wanted to learn physics because
it is exciting. In fact, I am one of those people. I have
been put to work on LHC-related research, and I have
learned a great deal, because particle physics is exciting.
If I leave physics, I will take my skills and tackle the
economic crisis, or the impending energy crisis I
mentioned. The amount of human currency that the
LHC produces is priceless. Thanks to the LHC, I’ve
been trained to better understand and communicate
physics, and you can’t argue with that; you just read this
whole article.
References:
[1] The LHC Rap, and various others:
http://www.katemcalpine.com/scirap html
[2] The public website for the LHC:
http://home.web.cern.ch/about/accelerators/largehadron-collider
[3] The public website for LHCb:
http://lhcb-public.web.cern.ch/lhcb-public/
[4] The public website for ALICE:
http://aliceinfo.cern.ch/Public/Welcome html
[5] The official websites of ATLAS and CMS:
http://atlas.ch/, http://cms.web.cern.ch/.
[6] ATLAS Collaboration, "Observation of a new
particle in the search for the Standard Model Higgs
boson with the ATLAS detector at the LHC" Phys.
Lett. B 716, 1 (2012) arXiv:1207.7214; CMS
Collaboration, "Observation of a new boson at a
mass of 125 GeV with the CMS experiment at the
LHC" Phys. Lett. B 716, 30 (2012) arXiv:1207.7235
[7] ATLAS Collaboration, "Observation of a new chi b
state in radiative transitions to Upsilon(1S) and
Upsilon(2S) at ATLAS" Phys. Rev. Lett. 108,
152001 (2012) arXiv:1112.5154
Page 11
Person of Interest
Richard Epp
With great enthusiasm, the
department of Physics and Astronomy
at the University of Waterloo is
welcoming back one of our best
teachers, Richard Epp. Richard joins
us from Perimeter Institute for
Theoretical Physics, where he founded
its educational outreach program, and was the driving
force behind it for several years. There he developed
programs for high school students (the International
Summer School for Young Physicists, or ISSYP), and
their teachers (Einstein Plus), and is the mind behind the
popular Alice & Bob in Wonderland science animations.
“I think the most fun thing I did at Perimeter was
work on those Alice & Bob animations. They’re a great
way to get students engaged in the process of
science―to question their understanding of reality.
Teachers too. I always smile at how teachers react to the
“How does a flashlight work?” episode. None of them
foresee that coming! Or the “What keeps us stuck to the
Earth?” episode: The fact that when an apple falls it is
not the apple accelerating down, but the ground
accelerating up, is a total mind warp. We really do live
in an Alice in Wonderland world, where things are not
always what they seem. They’re a powerful hook for
kids, and a great class-discussion starter. Working with
Dave Fish and other amazing high school physics
teachers, we expanded the three Alice & Bob modern
physics episodes into a full-fledged teacher resource
called Revolutions in Science. I’m really proud of that
work…”
In recognition of his long service to educational
outreach at Perimeter Institute, Richard was recently
awarded the Queen Elizabeth II Diamond Jubilee Medal.
Over the years he has ping-ponged between ‘outreach
guy’ at Perimeter Institute, and lecturer at the University
of Waterloo Department of Physics and Astronomy,
before finally accepting a Continuing Lecturer position
at the University of Waterloo.
Richard’s fascination with science and
technology started early. He grew up tinkering in his
father’s basement workshop―a treasure trove of tools
and raw materials in electric motors, electronics, high
voltage, wood and metal working supplies, fluids and
pumps, and junk of all kinds. He built everything from
rockets, mile-high kites, and exploding things (we won’t
get into that chapter of his life), to robots, a voice
synthesizer, and even a functioning self-sustained life
support system (OK, just for a fish, but still…).
“As a kid I learned how to program a
microprocessor in machine language, and hook it up to
Phys13news / Spring 2013
motors, lights, and other things. I fell in love with “the
machine”. Its logic, and what you could make it
do―anything you could imagine. I programmed the
thing to calculate the square root of a number using just
Boolean algebra and shifting of bits, and even to talk…”
As he was finishing his Masters degree in
electrical & computer engineering at the University of
Manitoba, working on satellite antenna design, he
realized that it was the physics of electromagnetism
itself that was more interesting than the applications.
“I was working on waveguides and corrugated
horn antennas. It’s fascinating how electric charge and
current on metal surfaces generate electric and magnetic
fields, and how the mechanisms of electric and magnetic
induction work together to move energy through
vacuum at the speed of light. Like yin and yang. I fell in
love with the inner ‘gears and wheels’ of how nature
works…”
He wandered to the other side of campus and
earned a PhD degree in theoretical physics.
“One day I spent an afternoon with a physics
professor who showed me, in his beautiful handwriting
on the blackboard, how something called ‘Lagrangian
mechanics’ reduces everything I learned in engineering
to a simple ‘stationary action’ principle. How cool is
that? I was hooked, and haven’t looked back since…”
It is moments like this that inspire Richard as a teacher
and prompted him to return to the University.
“I like being with students—I wanted to be back
interacting with them on a more regular basis. I’ve
always loved teaching, and the continual learning of
new physics that goes with it. I’m also really excited
about this new opportunity to become a better teacher. I
just finished a Teaching Excellence Academy where I
learned a whole lot about the different ways that
students learn, and new ideas about more effective
teaching strategies, and how to properly articulate
learning outcomes and design courses better aligned to
those outcomes. Teaching is a beautiful art, that’s highly
rewarding.”
In his previous three-year stint as a lecturer in the
department, Richard received stellar teaching
evaluations from his students, and designed a course in
general relativity for second-year students that was very
popular. Needless to say, we are glad to have him back.
“I think my most important task as a teacher is to
inspire students, and help them discover and nurture
their own passions. With the first-year course I just
developed―PHYS 175: Introduction to the Universe,
this was easy. I got to talk to the students about black
holes, cosmic inflation, the mysteries of dark matter and
Page 12
dark energy, etc. (This is my day job―I can hardly
believe how lucky I am!) I also told the students “I care
less about what you know, and more about what you can
figure out”. After all, and most importantly, we’re
training students to think like physicists.”
Richard’s current research involves gaining a
deeper, more geometrical understanding of the nature of
energy and momentum, and the mechanisms by which
extended objects move and interact through the warping
of spacetime.
“I thought electromagnetism was amazing…then
I found out about general relativity. The thing about
general relativity is that energy and warped spacetime
go hand-in-hand. For example, I recently figured out
that, when you throw a rock in empty space, and it
moves in a straight line at constant speed, there’s
actually a lot of ‘gears and wheels’ going on. As the
rock moves, the space in front of it needs to warp, and
this takes energy (like compressing a spring), energy it
gets from the ‘unwarping’ of the space behind it (like
releasing the spring). Energy in the warping of space
flows around and through the rock, from behind to in
front, and this is what we call ‘momentum’. It’s so
simple, and yet so beautiful…”
Richard plans to use these ideas developed in his
research to enhance his teaching. “I hope to find ways
to use such deeper insights into motion to enrich my
PHYS 121 course this fall [first-year physics for physics
majors]. There’s a lot of potentially mind-numbing
Newtonian mechanics in that course; I hope I can
capture their imaginations with hints of fascinating
things they might be learning down the road―Einstein’s
ideas, I mean. And quantum. And thermo. And E & M.
And…”
But his number one priority is his young
daughter. “I love capturing her imagination the most.
One day I introduced her to building ‘airships’ with
helium balloons, and now she’s off and running, sending
her My Little Ponies on great adventures to the Moon
and planets. She already has theories about how the Sun
works, and her own weird cosmology. Kids are
amazing!”
Young Physicists Corner
The Tangled Substructure of
Quarks and Leptons
Laura Henderson
University of Waterloo
The original rishon (”primary” in Hebrew) model
is a scheme proposed in 1979 by Haim Harari and
independently by Michael A. Shupe that hypothesized
quarks and leptons are not fundamental particles, but
rather are different combinations of two smaller
particles, the “T” (“Tohu”/“third”) and “V” (“vaVohu”/“void”) rishon.[1][6] This model was primarily
motivated by the large number of quarks (five flavours
in three colours each) and leptons (six flavours) that
were known to exist at the time, along with their antiparticle partners. As Harari said, “It is simply unlikely
that more than twenty building blocks of matter are
fundamental.”[1] Additionally these particles were
arranged in three generations of quarks and leptons with
particles having the same observational properties, other
than mass and stability being divided among each one.
He asserted that these two facts “hinted at a common
sub-structure.”[1] Today these patterns have been
extended in the standard model to include an additional
three flavours of quarks giving us 24 fundamental
particles (if colour is included) and three generations.
Harrari also hoped that a smaller number of
fundamental particles would allow for an easier
extension to a unified supergravity theory. This hope is
still alive today as work by Sundance O. BilsonThompson, Fotini Markopoulou and Lee Smolin in 2006
showed that the V and T rishons of the original model
could be extended to topological braids which in turn
could provide some insight into the definition of
particles in loop quantum gravity.[4][5]
As an alternative to the Higgs mechanism, the
Rishon model – even though experimentally disfavoured
-- provides some interesting insights into other possible
physics at the shortest distance scales we can probe.
Links:
http://www.perimeterinstitute.ca/outreach/students/progr
ams/international-summer-school-young-physicistsissyp
http://www.perimeterinstitute.ca/outreach/teachers/progr
ams-and-opportunities/einsteinplus-0
http://www.perimeterinstitute.ca/outreach/students/game
s-and-videos/alice-bob-wonderland
http://www.perimeterinstitute.ca/outreach/teachers/class
-kits/revolutions-science
Phys13news / Spring 2013
Fig. 1: The fermions of the standard model [2]
Page 13
and allows the individual helons to become distinct. It
is also worth noting that three is the smallest number
from which non-trivial braided structure can be formed
that cannot be smoothly deformed into simpler structure
(i.e. untwisted).
Figure 2 just shows the first generation of the
composite fermions; however, higher generations can
easily be defined to have a more complicated pattern
such as more crossings. For example, as seen in figure
3, the electron neutrino has two crossing while the muon
neutrino has three.
Interactions between particles can be seen as well
as the division and joining of particles which allow
tweedles to be exchanged between helons. As a result, it
is very easy to construct the vertices of the weak
interactions provided each of the three helons in the
interacting triplets are accounted for as seen in figure 4.
Since the W± bosons are trivially braided, they do not
remove, “complexity,” from braid structure of the
composite fermion, meaning charge is transferred but
generation is not changed under weak interactions.
Muons can only transform into muon neutrinos (see
figure 5) and likewise for tau leptons. This is exactly
what is expected for leptons, but the same behaviour
holds for quarks as well. There is currently no
mechanism to allow non-zero Cabibbo angles in the
current model.
Fig. 5: Tweedle exchange in a weak interaction. Details
about the H + helons (upper left) and the H 0 helon (lower
left) are provided. [4]
Fig. 2: Pictorial diagrams of the braided triplets forming
the first generation. The (3) on the quarks represents
that there are three permutations. [4] For example, as
seen in Fig. 3, the electron neutrino has two crossing
while the muon neutrino has three.
In colour interactions, the triplets do not split
apart during the interaction, but instead stay stacked on
top of each other forming “super-strands” where the
charge on these strands is equal to the charges of each of
its component strands. These stacks lead to the last
assumption about helon mechanics: when two or more
braids are stacked, they must combine to produce the
same total charge on all three super-strands.
Fig. 3: The braided triplet of the electron neutrino vs the
muon neutrino. [5]
As stated previously, the gauge bosons are
upbraided triplets and as a result, must have an integer
charge. The construction of the W+ and W- and bosons
are self-explanatory, but the Z boson and the photon are
less so. While they both have zero total charge, the Z
boson still has explicit left-handed and right-handed
twists and the photon does not. This indicates that they
can be smoothly deformed into each other and accounts
for the Weinberg mixing angle between them. It can
also be shown that conservation of braid number leads
to conservation of lepton number and baryon number.
Phys13news / Spring 2013
Fig. 6: A muon transforming into a muon neutrino.[4]
The two gluons that are superpositions of colour
and anti-colour can be seen as triplets with some strands
untwisted strands (like the photon) and some counter
twisted strands (like the Z boson). The remaining six
consist of the six permutations of one H - , one H 0 and
one H + helon.
Page 16
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