Fireworks on the 4th of JulyR. Michael Barnett

Citation: American Journal of Physics 81, 85 (2013); doi: 10.1119/1.4773295 View online: http://dx.doi.org/10.1119/1.4773295 View Table of Contents: http://scitation.aip.org/content/aapt/journal/ajp/81/2?ver=pdfcov Published by the American Association of Physics Teachers Articles you may be interested in Mass, Speed, Direction: John Buridan's 14th-Century Concept of Momentum Phys. Teach. 51, 411 (2013); 10.1119/1.4820853 Fireworks on the 4th of July Phys. Teach. 51, 75 (2013); 10.1119/1.4775521 A HIGGS MEMO Am. J. Phys. 81, 5 (2013); 10.1119/1.4766452 Higgs: The Invention & Discovery of the “God” Particle: Steven Weinberg, Jim Baggott Phys. Teach. 50, 575 (2012); 10.1119/1.4767509 Physics To Go online newsletter passes 100th edition Phys. Teach. 48, 559 (2010); 10.1119/1.3502521

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GUEST COMMENT

Fireworks on the 4th of July

The February editions of The Physics Teacher and Ameri-can Journal of Physics include a poster by the ContemporaryPhysics Education Project with the title: “The Higgs Boson– Born on the 4th of July,” covering the Higgs boson newsreported on July 4, 2012. For an extensive description of thediscovery of the Higgs-like particle, see Don Lincoln’s arti-cle in The Physics Teacher.1

After half a century of waiting, the drama was intense.

Physicists slept overnight outside the auditorium to get seats

for the seminar at the CERN lab in Geneva, Switzerland. Ten

thousand miles away on the other side of the planet, at the

world’s most prestigious international particle physics confer-

ence, hundreds of physicists from every corner of the globe

lined up to hear the seminar streamed live from Geneva (see

Fig. 1). And in universities from North America to Asia physi-

cists and students gathered to watch the streaming talks.

In 1964, six theoretical physicists hypothesized a new field

(like an electromagnetic field) that would permeate all of

space and solve a critical problem for our understanding of

the universe. Independently, other physicists were construct-

ing a theory of the fundamental particles, eventually called

the “Standard Model,” that would prove to be phenomenally

accurate. These otherwise unrelated efforts turned out to be

intimately interconnected. The Standard Model needed a

mechanism to give fundamental particles mass. The field

theory devised by Peter Higgs, Robert Brout, Francois Eng-

lert, Gerald Guralnik, Carl Hagen, and Thomas Kibble did

just that.

Peter Higgs realized that, in analogy with other quantum

fields, there would have to be a particle associated with this

new field. It would have intrinsic spin of zero and therefore

be a boson, a particle with integer spin (unlike fermions,

which have half-integer spin: 1/2, 3/2, etc.), and indeed it

soon became known as the Higgs boson. The only drawback

was that no one had seen it.

Unfortunately, the theory that predicted its existence

didn’t specify the mass of the Higgs boson. Everyone hoped

it would be fairly light so that existing accelerators could dis-

cover it. But as the years went by it became clear that the

Higgs boson would be extremely massive, and most likely

beyond the reach of all machines built prior to the Large

Hadron Collider (LHC).

By the time the LHC started collecting data in 2010,

experiments at other accelerators had shown that the mass of

a Higgs boson had to be greater than about 115 GeV. The

LHC experiments planned to search for evidence anywhere

in the mass range 115–600 GeV or even up to 1,000 GeV.

On July 4, the leaders of the ATLAS2 and CMS3 experi-

ments were presenting their latest results on the search for

the Higgs boson. Rumors were flying that they were going to

report more than search results, but what was it? Indeed,

when the talks were presented, both experiments reported

that they had evidence for a “Higgs-like” boson with a mass

around 125 GeV. There was definitely a particle there, and if

it wasn’t the Higgs it was a very good mimic. The evidence

was far from weak; they were five sigma results, meaning

less than one chance in a million of the data being only a sta-

tistical fluctuation.

The data were convincing but not perfect, and there were

significant shortcomings. For one thing, the limited statistics

collected by July 4 couldn’t establish if the rate at which this

Higgs candidate decays to various collections of less massive

particles (the “branching ratios”) are those predicted by the

Standard Model.4

How does one know when one sees a collision event if it

is a candidate for a Higgs boson? There are unique character-

istics that make these events stand out.

Higgs bosons decay into other particles almost instantly

after they are produced, so we only see the products of the

decay. The most common decays (among those we are capa-

ble of seeing) are those to:

• a b-quark and its antiquark (b�b),• a tau lepton and its antiparticle (sþs�),• two photons (cc),• two W bosons (WþW�),• two Z bosons (Z0Z0).

A technicality: For a 125-GeV Higgs boson, the decay to

two Z bosons is not possible because Z bosons have a mass

of 91 GeV so the pair has a mass of 182 GeV, which is more

than 125 GeV. However, what we do observe is the decay to

a Z boson and a virtual Z boson (Z�) whose effective mass is

much less.

This Z Z� decay mode is quite easy for the ATLAS and

CMS experiments to detect because the Z boson sometimes

decays into an electron/antielectron pair or a muon/antimuon

pair. So in the collision of two protons, one of the manyFig. 1. Physicists applaud the Higgs boson news at the International Confer-

ence on High Energy Physics in Melbourne (July 4, 2012).

85 Am. J. Phys. 81 (2), February 2013 http://aapt.org/ajp VC 2013 American Association of Physics Teachers 85

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particles produced on rare occasions is a Higgs boson H,

which occasionally decays as H ! Z þ Z�. Each of these Zbosons then (occasionally) decays as either Z ! e� þ eþ

or Z ! l� þ lþ. The end result is that we will sometimes

see (in addition to some unrelated particles) four muons, or

four electrons, or two muons and two electrons.

Figure 2 shows a neon sign of an actual event recorded by

ATLAS in which four muons (shown in neon) were pro-

duced. The other particles are shown in the background.

While decays of this kind had been observed for the new

particle by July 4, the rates at which they occur were still

uncertain. It was not even known if the newly discovered

particle has the right quantum numbers—that is, whether it

has the spin and parity required of a Higgs boson.

In other words, the July 4 particle looks like a duck, but

we need to make sure it swims like a duck and quacks like a

duck. This work is continuing. There is a major conference

in March 2013, which comes after the conclusion of this

year’s proton-proton collision run at the LHC. Physicists

look forward with great anticipation to seeing what the

experiments report about branching ratios, spin, and parity,

the properties essential to confirm that this really is the Higgs

boson.

Most physicists believe it is; it is difficult to create a

theory with a massive particle having significantly different

properties. However, the confirmation—or not—will eventu-

ally come from the data.

The discovery of the Higgs boson is an enormous clue

about the mechanism for giving mass to fundamental par-

ticles, as conceived by Higgs, Brout, Englert, Guralnik,

Hagen, and Kibble. What is this mechanism? It is a mathe-

matical theory for which an overly simplified cartoon (see

Fig. 3) can be used to demonstrate its essential nature.

Fundamental particles get their masses from the Higgs

mechanism. However, most of the ordinary mass of the

Fig. 2. A neon sign showing an actual event recorded by ATLAS, which

might reflect the decay of a Higgs boson into four muons, shown in neon.

The other particles are shown in the background.

Fig. 3. A cartoon helps to understand the Higgs mechanism. (a) Imagine that a room full of physicists chattering quietly is like space filled with the Higgs field

(top left). A well-known scientist walks in (top center), creating a disturbance as he moves across the room and attracting a cluster of admirers with each step

(top right). This cluster of admirers increases his resistance to movement; in other words, he acquires mass, just like a particle moving through the Higgs field.

(b) On the other hand, if a rumor crosses the room (bottom left), it creates the same kind of clustering, but this time among the scientists themselves (bottom

right). In this analogy, these clusters are the Higgs particles. # 1996 CERN. We thank CERN for the use of these images and text; the concept was inspired by

Professor David J. Miller of University College London.

86 Am. J. Phys., Vol. 81, No. 2, February 2013 86

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universe does not come from this mechanism. Ordinary mass

comes mostly from the masses of protons and neutrons,

which are not fundamental particles, but particles made of

quarks.

Only a small part of the masses of neutrons and protons

comes from the mass of their constituents, the quarks. Most

of the mass comes from the kinetic energy of the quarks, via

E ¼ mc2. So the Higgs is not the source of most of the mass

of protons and neutrons.

The universe would be dramatically different if fundamen-

tal particles had no mass. There might be particles somewhat

similar to protons and neutrons, but there would be no atoms.

There would be no significant brightness. It would be a dark

universe with no people…and no fun. Whatever else it

teaches us, discovering the Higgs boson assures us of fun.

Certainly watching these grand experiments is amazing fun.

R. Michael BarnettLawrence Berkeley National Laboratory

ACKNOWLEDGMENTS

The author greatly appreciates valuable input from PaulPreuss (LBNL), Howard Haber (UC Santa Cruz), and mem-bers of the Contemporary Physics Education Project.

1Don Lincoln, “The Higgs Boson: Is the End in Sight?” Phys. Teach. 50,

332–337 (2012).2The ATLAS experiment, <www/atlas.ch/news/2012/atlas-and-the-higgs.

html>.3The CMS experiment, <cms.web.cern.ch/news/observation-new-particle-

mass-125-gev>.4Additional information on the Standard Model can be found online at The

Particle Adventure, <ParticleAdventure.org>.

87 Am. J. Phys., Vol. 81, No. 2, February 2013 87

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