Chapter 3

Cosmic rays and thedevelopment of the physicsof elementary particles andfundamental interactions

By 1785 Coulomb found that electroscopes can discharge spontaneously, and notbecause of defective insulation. The brithish Crookes observed in 1879 that thespeed of discharge decreased when the pressure of the air inside the electroscopeitself was reduced. The discharge was then likely due to the ionization of theatmosphere. But what was the cause of atmospheric ionization?

The explanation of this phenomenon came in the beginning of the 20th cen-tury and paved the way to one of mankind’s revolutionary scientific discoveries:cosmic rays. We know today that cosmic rays are particles of extraterrestrialorigin which can reach high energy (much larger than we shall be ever able toproduce). They were the only source of high energy beams till the 1940’s. WorldWar II and the Cold War provided later new resources, both technical and polit-ical, for the study of elementary particles; technical resources included advancesin microwave electronics and the design of human-made particle accelerators,which allowed physicists to produce high energy particles in a controlled environ-ment. By about 1955 elementary particle physics would have been dominatedby accelerator physics, at least until the beginning of the 1990s when possibleexplorations with the energies one can produce on Earth started showing signsof saturation, so that nowadays cosmic rays got again the edge.

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Figure 3.1: The electroscope is a device for detecting electric charge. A typicalelectroscope (this configuration was invented at the end of the XVIII century)consists of a vertical metal rod from the end of which two gold leaves hang. Adisk or ball terminal is attached to the top of the rod, where the charge to betested is applied. To protect the leaves from drafts of air they are enclosed in aglass bottle. The gold leaves repel, and thus diverge, when the rod is charged.

3.1 The puzzle of atmospheric ionisation andthe discovery of cosmic rays

Spontaneous radioactivity (i.e., emission of particles from nuclei as a result ofnuclear instability) was discovered in 1896 by Becquerel. A few years later,Marie and Pierre Curie discovered that the elements Polonium and Radium(today called Radon) suffered transmutations generating radioactivity: suchtransmutation processes were then called “radioactive decays”. In the presenceof a radioactive material, a charged electroscope promptly discharges: it wasconcluded that some elements emit charged particles, which induce the forma-tion of ions in the air, causing the discharge of electroscopes. The discharge ratewas then used to gauge the level of radioactivity.

Following the discovery of radioactivity, a new era of research in dischargephysics was then opened, this period being strongly influenced by the discoveriesof the electron and of positive ions. During the first decade of the 20th centuryresults on the study of ionisation phenomena came from several researchers inEurope and in the New World.

Around 1900, C.T.R. Wilson in Britain and Elster and Geitel in Germanyimproved the technique for the careful insulation of electroscopes in a closedvessel, thus improving the sensitivity of the electroscope itself (Figure 3.2). Asa result, they could make quantitative measurements of the rate of spontaneousdischarge. They concluded that such a discharge was due to ionising agents

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Figure 3.2: Left: The two friends Julius Elster and Hans Geiter, gymnasiumteachers in Wolfenbuttel, around 1900. Right: an electroscope developed byElster and Geitel in the same period (private collection R. Fricke).

coming from outside the vessel. The obvious questions concerned the natureof such radiation, and whether it was of terrestrial or extra-terrestrial origin.The simplest hypothesis was that its origin was related to radioactive materi-als, hence terrestrial origin was a commonplace assumption. An experimentalconfirmation, however, seemed hard to achieve. Wilson tentatively made thevisionary suggestion that the origin of such ionisation could be an extremelypenetrating extra-terrestrial radiation; however, his investigations in tunnelswith solid rock overhead showed no reduction in ionisation and could thereforenot support an extra-terrestrial origin. An extra-terrestrial origin, though nowand then discussed, was dropped for many years.

The situation in 1909 was that measurements on the spontaneous dischargewere proving the hypothesis that even in insulated environments backgroundradiation did exist, and such radiation could penetrate metal shields. It was thusdifficult to explain it in terms of α (He nuclei) and β (electron) radiation; thus itwas assumed to be γ, i.e., made of photons, which is the most penetrating amongthe three kinds of radiation known at that time. Three possible sources for thepenetrating radiation were considered: an extra-terrestrial radiation possiblyfrom the Sun, radioactivity from the crust of the Earth, and radioactivity in theatmosphere. It was generally assumed that large part of the radiation came fromradioactive material in the crust. Calculations were made of how the radiationshould decrease with height.

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Figure 3.3: Left: Scheme of the Wulf electroscope (drawn by Wulf himself).The 17 cm diameter cylinder with depth 13 cm was made of Zinc. To the rightis the microscope that measures the distance between the two silicon glass wiresilluminated using the mirror to the left. According to Wulf, the sensitivity ofthe instrument, as measured by the decrease of the inter-wire distance, was 1volt. Right: an electroscope used by Wulf (private collection R. Fricke).

3.1.1 Experiments underwater and in height

Father Theodor Wulf, a German scientist and a Jesuit priest serving in theNetherlands and then in Rome, thought of checking the variation of radioactivitywith height to test its origin. In 1909, using an improved electroscope easier tothansport than previous instruments (Figure 3.3) in which the two leaves hadbeen replaced by metalised silicon glass wires, he measured the rate of ionisationat the top of the Eiffel Tower in Paris (300 m above ground). Supporting thehypothesis of the terrestrial origin of most of the radiation, he expected to findat the top less ionisation than on the ground. The rate of ionisation showed,however, too small a decrease to confirm the hypothesis. He concluded that, incomparison with the values on the ground, the intensity of radiation “decreasesat nearly 300 m [altitude] was not even to half of its ground value”; while withthe assumption that radiation emerges from the ground there would remainat the top of the tower “just a few percent of the ground radiation”. Wulf’sobservations were of great value, because he could take data at different hours ofthe day and for many days at the same place. For a long time, Wulf’s data wereconsidered as the most reliable source of information on the altitude effect in thepenetrating radiation. However he concluded that the most likely explanationof his puzzling result was still emission from the soil.

The conclusion that radioactivity was mostly coming from the Earth’s crustwas questioned by the Italian physicist Domenico Pacini, who developed an

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Figure 3.4: Left: Pacini making a measurement in 1910 (courtesy of the Pacinifamily). Right: the instruments used by Pacini for the measurement of ioniza-tion.

experimental technique for underwater measurements. He found a significantdecrease in the discharge rate when the electroscope was placed three metersunderwater, first in the sea of the gulf of Genova, and then in the Lake ofBracciano (Figure 3.4). He wrote: “Observations carried out on the sea duringthe year 1910 led me to conclude that a significant proportion of the pervasiveradiation that is found in air had an origin that was independent of directaction of the active substances in the upper layers of the Earth’s surface. ... [Toprove this conclusion] the apparatus ... was enclosed in a copper box so that itcould immerse in depth. [...] Observations were performed with the instrumentat the surface, and with the instrument immersed in water, at a depth of 3metres.” Pacini measured seven times during three hours the discharge of theelectroscope, measuring a ionization of 11.0 ions per cubic centimeter per secondon surface; with the apparatus at a 3 m depth in the 7 m deep sea, he measured8.9 ions per cubic centimeter per second. The difference of 2.1 ions per cubiccentimeter per second (about 20% of the total radiation) should be, in his view,attributed to an extraterrestrial radiation.

The need for balloon experiments became evident to clarify Wulf’s observa-tions on the effect of altitude (at that time and since 1885, balloon experimentswere anyway widely used for studies of the atmospheric electricity). The firsthigh-altitude balloon flight with the purpose of studying the properties of pene-trating radiation was arranged in Switzerland in December 1909 with a balloonfrom the Swiss aeroclub. Albert Gockel, professor at the University of Fri-bourg, ascended to 4500 m above sea level (a.s.l.); he made measurements upto 3000 m, and found that ionization did not decrease with height as it wouldbe expected from the hypothesis of a terrestrial origin. Gockel concluded “thata non-negligible part of the penetrating radiation is independent of the directaction of the radioactive substances in the uppermost layers of the Earth”.

In spite of Pacini’s conclusions, and of Wulf’s and Gockel’s puzzling resultson the dependence of radioactivity on altitude, physicists were however reluctant

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Figure 3.5: Left: Hess during the balloon flight in August 1912. Right: one ofthe electrometers used by Hess during his flight. This instrument is a versionof a commercial model of a Wulff electroscope especially modified by its man-ufacturer, Gunther & Tegetmeyer, to operate under reduced pressure at highaltitudes (Smithsonian Nationsl Air and Science Museum, Washington, DC).

to give up the hypothesis of a terrestrial origin. The situation was cleared upthanks to a long series of balloon flights by the Austrian physicist Victor Hess,who established the extra-terrestrial origin of at least part of the radiationcausing the observed ionisation.

Hess started his experiments by studying Wulf’s results; he carefully checkedthe data on gamma-ray absorption coefficients (due to the large use of radioac-tive sources he will lose a thumb), and after a careful planning he finalized hisstudies with balloon observations. The first ascensions took place in August1911. From April 1912 to August 1912 he had the opportunity to fly seventimes with three instruments (since for a given energy electrons have a shorterrange than heavier particles, one of the three instruments had a thin wall toestimate the effect of beta radiation). In the final flight, on August 7, 1912,he reached 5200 m (Figure 3.5). The results clearly showed that the ionisation,after passing through a minimum, increased considerably with height (Fig. 3.6).“(i) Immediately above ground the total radiation decreases a little. (ii) At al-titudes of 1000 to 2000 m there occurs again a noticeable growth of penetratingradiation. (iii) The increase reaches, at altitudes of 3000 to 4000 m, already50% of the total radiation observed on the ground. (iv) At 4000 to 5200 m theradiation is stronger [more than 100%] than on the ground”.

Hess concluded that the increase of the ionisation with height must be dueto radiation coming from above, and he thought that this radiation was ofextra-terrestrial origin. He also excluded the Sun as the direct source of thishypothetical penetrating radiation due to there being no day-night variation.

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Figure 3.6: Variation of ionisation with altitude. Left panel: Final ascent byHess (1912), carrying two ion chambers. Right panel: Ascents by Kolhorster(1913, 1914).

The results by Hess were later confirmed by Kolhorster in a number of flightsup to 9200 m: he found an increase of the ionisation up to ten times that at sealevel. The measured attenuation length of the radiation was about 1 km in airat NTP; this value caused great surprise as it was eight times smaller than theabsorption coefficient of air for gamma rays as known at the time.

Hess coined the name Hohenstrahlung after the 1912 flights. There wereseveral other names used to indicate the extraterrestrial radiation before “cosmicrays”, suggested later by Millikan and inspired by the “kosmische Srahlung”term used by Gockel in 1909, was generally accepted: Ultrastrahlung, kosmischeStrahlung, Ultra-X-Strahlung.

The idea of cosmic rays, despite the striking experimental proofs, was how-ever not immediately accepted (the Nobel prize for the discovery of cosmic rayswill be assigned to Hess only in 1936).

During the war in 1914 - 1918 and the following years thereafter very fewinvestigations of the penetrating radiation were performed. In 1926, however,Millikan and Cameron carried out absorption measurements of the radiation atvarious depths in lakes at high altitudes. Based upon the absorption coefficientsand altitude dependence of the radiation, they concluded that the radiation washigh energy gamma rays and that “these rays shoot through space equally inall directions” calling them “cosmic rays”.

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3.1.2 The nature of cosmic rays

Cosmic radiation was generally believed to be gamma radiation because of itspenetrating power (the penetrating power of relativistic charged particles wasnot known at the time). Millikan had put forward the hypothesis that gammarays were produced when protons and electrons form helium nuclei in the inter-stellar space.

A key experiment, which would decide the nature of cosmic rays (and inparticular if they were charged or neutral), was the measurement of the de-pendence of cosmic ray intensity on geomagnetic latitude. During two voyagesbetween Java and Genova in 1927 and 1928, the Dutch physicist Clay foundthat ionisation increased with latitude; this proved that cosmic rays interactedwith the geomagnetic field and, thus, they were mostly charged.

With the introduction of the Geiger-Muller counter tube1 in 1928, a new erabegan and soon confirmation came that the cosmic radiation is indeed corpuscu-lar. In 1933, three independent experiments by Alvarez & Compton, Johnson,Rossi, discovered that close to the Equator more cosmic rays were coming fromWest than from East: this is due to the interaction with the magnetic field ofthe Earth, and it demonstrated that cosmic rays are mostly positively chargedparticles. However, it would take until 1941 before it was established in anexperiment by Schein that cosmic rays were mostly protons.

3.2 Cosmic rays and the beginning of particlephysics

Thanks to the development of cosmic ray physics, scientists then knew thatastrophysical sources were providing very-high energy bullets entering the at-mosphere. It was then obvious to investigate the nature of such bullets, andto use them as probes to investigate matter in detail, along the lines of theexperiment made by Marsden and Geiger in 1909 (the Rutherford experiment,described in Chapter 2). Particle physics, the science of the fundamental con-stituents of the Universe, started with cosmic rays. Many of the fundamentaldiscoveries were made using cosmic rays.

In parallel, the theoretical understanding of the Universe was progressingquickly: at the end of the 1920s, scientists tried to put together relativity andquantum mechanics, and the discoveries following these attempts changed com-pletely our view of nature. A new window was going to be opened: antimatter.

1The Geiger-Muller counter is a cylinder filled with a gas, with a charged metal wire inside.When a charged particle enters the detector, it ionizes the gas, and the ions and the electronscan be collected by the wire and by the walls. The electrical signal of the wire can be amplifiedand read by means of an amperometer. The tension V of the wire is large (a few thousandvolt), in such a way that the gas is completely ionized; the signal is then a short pulse ofheight independent of the energy of the particle. Geiger-Muller tubes can be also appropriatefor detecting gamma radiation, since a photoelectron can generate an avalanche.

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3.2.1 Relativistic quantum mechanics and antimatter

Schrodinger’s equation

i~∂Ψ

∂t= − ~2

2m~∇2Ψ + VΨ

can be seen as the translation into the wave language of the hamiltonian equationof classical mechanics

H =p2

2m+ V ,

where the hamiltonian is represented by the operator

H = i~∂

∂t

and momentum by~p = −i~~∇ .

Since Schrodinger’s equation contains derivatives of different order with re-spect to space and time, it cannot be relativistically covariant, and thus, itcannot be the “final” equation. How can it be extended to be consistent withLorentz invariance?

Klein-Gordon equation

In the case of a free particle (V = 0), the simplest way to extend Schrodinger’sequation to take into account relativity is to write the hamiltonian equation

H2 = p2c2 +m2c4

=⇒ −~2 ∂2Ψ

∂t2= −~2c2~∇2Ψ +m2c4Ψ ,

or, in natural units, (− ∂2

∂t2+ ~∇2

)Ψ = m2Ψ .

This equation is known as Klein-Gordon equation, but it was first consideredas a quantum wave equation by Schrodinger and it was found in his notebooksfrom late 1925. Erwin Schrodinger had also prepared a manuscript applying itto the hydrogen atom; however he could not solve some fundamental problemsrelated to the form of the equation (which is not linear in energy, so that statesare not easy to combine), and thus he went back to the equation today knownby his name. In addition, the solutions of the Klein-Gordon equation do notallow for statistical interpretation of |Ψ|2 as a probability density - its integralwould not remain constant [?].

The Klein-Gordon equation displays one more interesting feature. Solutionsof the associated eigenvalue equation(

−m2 + ~∇2)ψ = E2

pψ

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have both positive and negative eigenvalues for energy. For every plane wavesolution of the form

Ψ(~r, t) = Nei(~p·~r−Ept)

with momentum ~p and positive energy

Ep =√p2 +m2 ≥ m

there is a solutionΨ∗(~r, t) = N∗ei(−~p·~r+Ept)

with momentum −~p and negative energy

E = −Ep = −√p2 +m2 ≤ −m.

Note that one cannot simply drop the solutions with negative energy as“unphysical”: the full set of eigenstates is needed, because, if one starts forma given wave function, this could evolve with time in a wavefunction that, ingeneral, has projections on all eigenstates (including those one would like to getrid of). We remind the reader that these are solutions of an equation describinga free particle.

Dirac equation

Dirac in 1928 searched for an alternative relativistic equation starting from thegeneric form describing the evolution of wave function, in the familiar form:

i~∂Ψ

∂t= HΨ

with a Hamiltonian operator linear in ~p, t (Lorentz invariance requires thatif the Hamiltonian has first derivatives with respect to time also the spatialderivatives should be of first order):

H = c~α · ~p+ βm .

This must be compatible with the Klein-Gordon equation, and thus

α2i = 1 ; β2 = 1

αiβ + βαi = 0

αiαj + αjαi = 0 .

Therefore, parameters ~α and β cannot be numbers. However, it works if they arematrices (and if these matrices are hermitian we guarantee that the hamiltonianwill be hermitian). The lowest order is 4×4 (a demonstration as well as a setof matrices which solve the problem can be found, for example, in [?]; however,

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this is not relevant for what follows). The wavefunctions Ψ must thus be 4-component vectors:

Ψ(~r, t) =

Ψ1(~r, t)Ψ2(~r, t)Ψ3(~r, t)Ψ4(~r, t)

.

We arrive at an interpretation of the Dirac equation, as a four-dimensionalmatrix equation in which the solutions are four-component wavefunctions calledspinors. Plane wave solutions are

Ψ(~r, t) = u(~p)ei(~p·~r−Et)

where u(~p) is also a 4-component spinor satisfying the eigenvalue equation

(c~α · ~p+ βm)u(~p) = Eu(~p) .

This equation has four solutions: two with positive energy E = +Ep and twowith negative energy E = −Ep. We will discuss later the interpretation of thenegative energy solutions.

Dirac’s equation was a success. First, it accounted “for free” for the existenceof two spin states (we remind that spin had to be inserted by hand in theSchrodinger equation of nonrelativistic quantum mechanics). In addition, sincespin is embedded in the equation, the Dirac’s equation

• allows computing correctly the energy splitting of atomic levels with thesame quantum numbers due to the spin-orbit interaction in atoms (fineand hyperfine splitting);

• explains the magnetic moment of point-like fermions.

The predictions on the values of the above quantities were incredibly preciseand still resist to experimental tests.

Hole theory and the positron

The problem of the negative energy states is that they must be occupied: ifthey are not, transitions from positive to negative energy states could occur,and matter would be unstable. Dirac postulated that the negative energy statesare completely filled under normal conditions. In the case of electrons (whichare fermions, and thus are bound to respect the Pauli exclusion principle) theDirac picture of the vacuum is a “sea” of negative energy states, each with twoelectrons (one with spin up and the other with spin down), while the positiveenergy states are mostly unoccupied (Figure 3.7). This state is indistinguishablefrom the usual vacuum.

If an electron is added to the vacuum, it is confined to the positive energyregion since all the negative energy states are occupied. If a negative energyelectron is removed from the vacuum, however, a new phenomenon happens:removing such an electron with E < 0, momentum -~p, spin -~S and charge

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Figure 3.7: Dirac picture of the vacuum. In normal conditions, the sea ofnegative energy states is totally occupied with two electrons in each level.

−e leaves a “hole” indistinguishable from a particle with positive energy E >0, momentum ~p, spin ~S and charge +e. This is similar to the formation ofholes in semiconductors. The two cases are equivalent descriptions of the samephenomena. Dirac thus predicted the existence of a new fermion with mass equalto the mass as the electron, but opposite charge. This particle, later called thepositron, is a kind of antiparticle for the electron, and is the prototype of a newfamily of particles: antimatter.

3.2.2 The discovery of antimatter

During his doctoral thesis (supervised by Millikan), Anderson was studying thetracks of cosmic rays passing through a cloud chamber2 in a magnetic field.

2The cloud chamber (see next chapter), invented by C.T.R. Wilson at the beginning of theXX century, was an instrument for reconstructing the trajectories of charged particles. Theinstrument is a container with a glass window, filled with air and saturated water vapour; thevolume could be suddenly expanded, bringing the vapour to a supersaturated (metastable)state. A charged cosmic ray crossing the chamber produces ions, which act as seeds forthe generation of droplets along the trajectory. One can record the trajectory by takinga photographic picture. If the chamber is immersed in a magnetic field B, momentum andcharge can be measured by the curvature. The working principle of bubble chambers is similarto that of the cloud chamber, but here the fluid is a liquid. Along the tracks’ trajectories,a trail of gas bubbles condensates around the ions. Bubble and cloud chambers provide acomplete information: the measurement of the bubble density and the range, i.e., the total

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In 1933 he discovered antimatter in the form of a positive particle of massconsistent with the electron mass, later called the positron (Figure 3.8). Theprediction by the Dirac’s equation was confirmed; this was a dramatic successfor cosmic-ray physics. Anderson shared with Hess the Nobel prize for physicsin 1936; they were nominated by Compton, with the following motivation.

“The time has now arrived, it seems to me, when we can saythat the so-called cosmic rays definitely have their origin at suchremote distances from the Earth that they may properly be calledcosmic, and that the use of the rays has by now led to results ofsuch importance that they may be considered a discovery of the firstmagnitude. [...] It is, I believe, correct to say that Hess was the firstto establish the increase of the ionization observed in electroscopeswith increasing altitude; and he was certainly the first to ascribewith confidence this increased ionization to radiation coming fromoutside the Earth”.

Why so late a recognition to the discovery of cosmic rays? Compton writes:

“Before it was appropriate to award the Nobel Prize for the dis-covery of these rays, it was necessary to await more positive evi-dence regarding their unique characteristics and importance in var-ious fields of physics”.

3.2.3 Cosmic rays and the progress of particle physics

After Anderson’s fundamental discovery of the antimatter, new experimentalresults in the physics of elementary particles in cosmic rays were guided andaccompanied by the improvement of tools for detection, and in particular by theimproved design of the cloud chambers and by the introduction of the Geiger-Muller tube. According to Giuseppe Occhialini, one of the pioneers of theexploration of fundamental physics with cosmic rays, the Geiger-Muller counterwas like the Colt in the Far West: a cheap instrument usable by everyone onone’s way through a hard frontier.

At the end of the 1920s, Bothe and Kolhorster introduced the coincidencetechnique to study cosmic rays with the Geiger counter. A coincidence circuitactivates the acquisition of data only when signals from predefined detectorsare received within a given time window. Coincidence circuits are widely usedin particle physics experiments and in other areas of science and technology.Walther Bothe shared the Nobel Prize for Physics in 1954 for his discoveryof the method of coincidence and the discoveries subsequently made thanks toit. Coupling a cloud chamber with a system of Geiger counters and using thecoincidence technique, it was possible to take photographs only when a cosmicray traversed a cloud chamber (we call today such a system a “trigger”). This

track length before the particle eventually stops, provide an estimate for the energy and themass; the angles of scattering provides an estimate for the momentum.

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Figure 3.8: A picture by Anderson showing the passage of a cosmic anti-electron,or positron, through a cloud chamber immersed in a magnetic field. One canunderstand that the particle comes from the bottom in the picture by the factthat, after passing through the sheet of material in the medium (and thereforelosing energy), the radius of curvature decreases. The positive charge is inferredfrom the direction of rotation in the magnetic field. The mass density is mea-sured by the bubble density (a proton would lose energy faster). Since mostcosmic rays come from the top, the probability of backscattering of a positronin an interaction is low: the first evidence for antimatter comes thus from anunconventional event.

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increased the chances of getting a significant photograph and thus the efficiencyof cloud chambers.

Soon afterward the discovery of the positron by Anderson, a new importantobservation was made in 1933: the conversion of photons into pairs of electronsand positrons. Dirac’s theory not only predicts the existence of anti-electrons,but it also provides that electron-positron pairs can be created from a singlephoton with energy large enough; the phenomenon was actually observes incosmic rays by Blackett (Nobel Prize for Physics in 1948) and Occhialini, whofurther improved in Cambridge the coincidence technique. The electron-positronpair production is a confirmation of the mass-energy equivalence. This is asimple and direct confirmation of what is predicted by the theory of relativity. Italso demonstrates the behaviour of light, confirming the quantum concept whichwas originally expressed as ”wave-particle duality”: the photon can behave asa particle.

In 1934 the Italian Bruno Rossi reported an observation of near-simultaneousdischarges of two Geiger counters widely separated in a horizontal plane duringa test of his equipment. In his report on the experiment, Rossi wrote “...itseems that once in a while the recording equipment is struck by very exten-sive showers of particles, which causes coincidences between the counters, evenplaced at large distances from one another.” In 1937 Pierre Auger, unawareof Rossi’s earlier report, detected the same phenomenon and investigated it insome detail. He concluded that extensive particle showers are generated byhigh-energy primary cosmic-ray particles that interact with air nuclei high inthe atmosphere, initiating a cascade of secondary interactions that ultimatelyyield a shower of electrons, photons, and muons that reach ground level. Thiswas the explanation of the spontaneous discharge of electroscope due to cosmicrays.

3.2.4 The µ lepton and the π mesons

In 1935 the Japanese physicist Yukawa, twenty-eight years old at that time,formulated his innovative theory explaining the “strong” interaction ultimatelykeeping together matter3. Strong interaction keeps together protons and neu-trons4 in the atomic nuclei. This theory has been sketched in the previous

3This kind of interaction was first conjectured and named by Isaac Newton at the end ofXVII century: “There are therefore Agents in Nature able to make the Particles of Bodies sticktogether by very strong Attractions. And it is the Business of experimental Philosophy to findthem out. Now the smallest Particles of Matter may cohere by the strongest Attractions, andcompose bigger Particles of weaker Virtue; and many of these may cohere and compose biggerParticles whose Virtue is still weaker, and so on for divers Successions, until the Progressionend in the biggest Particles on which the Operations in Chemistry, and the Colours of naturalBodies depend.” (I. Newton, Opticks).

4Although the neutron hypothesis was quite old – Rutherford conjectured the existenceof the neutron in 1920 in order to reconcile the disparity he had found between the atomicnumber of an atom and its atomic mass, and he modeled it as an electron orbiting a proton –its discovery happened only in 1932. James Chadwick performed a series of experiments at theUniversity of Cambridge, finding a radiation consisting of uncharged particles of approximatelythe mass of the proton; these uncharged particles were called neutrons. As we saw in the

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chapter, and requires a “mediator” particle of intermediate mass between theelectron and the proton, thus called meson – the word “meson” meaning “middleone”.

To account for the strong force, Yukawa predicted that the meson must havea mass of about one-tenth of GeV, a mass that would explain the rapid weaken-ing of the strong interaction with distance. The scientists studying cosmic raysstarted to discover new types of particles of intermediate masses. Anderson,who after the Nobel prize had become a professor, and his student Nedder-meyer, observed in 1937 a new particle, present in both positive and negativecharge, more penetrating than any other particle known at the time. The newparticle was heavier than the electron but lighter than the proton, and theysuggested for it the name “mesotron”. The mesotron mass, measured fromionization, was between 200 and 240 times the electron mass; this was fittinginto Yukawa’s prediction for the meson. Most researchers were convinced thatthese particles were the Yukawa’s carrier of the strong nuclear force, and thatthey were created when primary cosmic rays collide with nuclei in the upperatmosphere, as well as electrons emit photons when colliding with a nucleus.

The lifetime of the mesotron was measured studying its flow at various alti-tudes, in particular by Rossi in Colorado (Rossi had been forced to emigrate tothe United States to escape racial persecution); they claimed that the averagelifetime for a muon was about two microseconds (about a hundred times largerthan predicted by Yukawa for the particle that transmits the strong interaction,but not too far). The attenuation of the mesotron intensity with altitude alsoprovided a check of relativistic time dilation, confirming Einstein’s theory ofrelativity (in the absence of such a dilation, the mesotron would travel only ap-proximately 600 meters in average, and would not get through the atmosphereto the surface of the Earth). Rossi found also that at the end of its life themesotron decays into an electron and other neutral particles (neutrinos) thatdid not leave tracks in bubble chamber – the positive mesotron decays into apositive electron plus neutrinos.

Beyond the initial excitement, however, the picture did not work. In par-ticular, the Yukawa particle is the carrier of strong interactions, and thereforeit can not be highly penetrant - the nuclei of the atmosphere would absorb itquickly. Many theorists tried to find complicated explanations to save the the-ory. The correct explanation was however the simplest one: the mesotron wasnot the Yukawa particle.

A better analysis showed that there were likely two particles of similar mass.One of them (with mass of about 140 MeV), corresponding to the particlepredicted by Yukawa, was since then called pion (or π meson); it was created inthe interactions of cosmic protons with the atmosphere, and then interacted withthe nuclei of the atmosphere, or decayed. Among its decay products there wasthe mesotron, since then called the muon (or µ lepton), which was insensitiveto the strong force.

previous chapter, the same name had been given by Pauli to the (different) neutral particlehe had conjectured in 1930; Fermi proposed then to change the name of the Pauli particle to“neutrino”.

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Figure 3.9: Pion and muon: the decay chain π → µ→ e. The pion travels frombottom to top on the left, the muon horizontally, and the electron from bottomto top on the right. The missing momentum is carried by neutrinos. From C.F. Powell, P.H. Fowler & D. H. Perkins, The Study of Elementary Particles bythe Photographic Method (Pergamon Press 1959).

In 1947, Powell, Occhialini and Lattes, exposing nuclear emulsions (a kind ofvery sensitive photographic plates, with space resolutions of a few µm; we shalldiscuss them better in the next chapter) to cosmic rays on Mount Chacaltayain Bolivia, finally proved the existence of charged pions, positive and negative,while observing their decay into muons and allowing a precise determination ofthe masses. For this discovery Cecil Powell, the group leader, was awarded theNobel Prize in 1950.

Many photographs of nuclear emulsions, especially in experiments on bal-loons, clearly showed traces of pions decaying into muons (their masses werereported to be about 106 MeV), decaying in turn into electrons. In the de-cay chain π → µ → e (Figure 3.9) some energy is clearly missing, and can beattributed to neutrinos.

At this point the distinction between pions and muons was clear. The muonlooks like a “heavier brother” of the electron. After the discovery of the pion, themuon had no theoretical reason to exist (the physicist Isidor Rabi was attributedin the 40’s the famous quote: “Who ordered it?”). However, a new family wasinitiated: the family of leptons – including for the moment the electron and themuon, and their antiparticles.

The neutral pion

Before it was even known that mesotrons were not the Yukawa particle, the the-ory of mesons had great development. In 1938 a theory of charge symmetry wasformulated, conjecturing the fact that the forces between protons and neutrons,between neutrons and protons and between neutrons and protons are similar.This implies the existence of positive, negative and also neutral mesons.

The neutral pion was more difficult to detect than the charged one, due tothe fact that neutral particles do not leave tracks in cloud chambers and nuclearemulsions – and also to the fact, discovered only later, that it leaves only ap-proximately 10−16 s before decaying mosly into two photons. However, between1947 and 1950, the neutral pion was identified in cosmic rays by analyzing its

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decay products in showers of particles. So, after fifteen years of research, thetheory of Yukawa had finally complete confirmation.

3.2.5 Strange particles

In 1947, after the thorny problem of the meson had been solved, particle physicsseemed to be a complete science. Fourteen particles were known to physicists(some of them at the time were only postulated, and were going to be foundexperimentally later): the proton, the neutron (proton and neutron togetherbelong to the family of baryons, the Greek etymology of the word referringto the concept of “heavyness”) and the electron, and their antiparticles; theneutrino that was postulated because of an apparent violation of the principleof energy conservation; three pions; two muons; and the photon.

Apart from the muon, a particle that appeared unnecessary, all the othersseemed to have a role in nature: the electron and the nucleons constitute theatom, the photon carries the electromagnetic force, and the pion the strong force;neutrinos are needed for energy conservation. But once more in the history ofscience, when everything seemed understood, a new revolution was just aroundthe corner.

Since 1944 strange topologies of cosmic particles began to be photographedfrom time to time in cloud chambers. In 1947 G.D. Rochester and the C.C.Butler from the University of Manchester observed clearly in the photographsof bubble chamber a couple of tracks from a single point in the shape of letter“V”; the two tracks were deflected in opposite directions by an external magneticfield.

The analysis showed that the parent neutral particle had a mass of abouthalf a GeV (intermediate between the mass of a proton and that of a pion), anddisintegrated into a pair of oppositely charged pions. A broken track in a secondphotograph showed the decay of a charged particle, about the same mass, intoa charged pion and at least one neutral particle (figure 3.10).

These particles, which were produced only in high energy interactions, wereobserved only every few hundred photographs. They are known today as Kmesons (or kaons); kaons can be positive, negative or neutral.

A new family of particles had been discovered. The behaviour of theseparticles was somehow strange: the cross section for their production couldbe understood in terms of strong interactions; however, their decay time wasinconsistent with strong interaction, being too slow. These new particles werecalled “strange mesons”.

Later analyses indicated the presence of particles heavier than protons andneutrons. They decayed with a “V” topology into final states including protons,and they were called strange baryons, or hyperons (Λ, Σ, ...).

Strange particles appear to be always produced in pairs, indicating the pres-ence of a new quantum number.

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Figure 3.10: The first images of the decay of particles known today as K-mesonsor kaons - the first examples of “strange” particles. The image on the left showsthe decay of a neutral kaon. Being neutral it leaves no trace, but when it decaysinto two lighter charged particles (just below the central bar to the right), onecan see a “V”. The picture on the right shows the decay of a charged kaon intoa muon and a neutrino. The kaon reaches the top right corner of the chamberand the decay occurs where the track seems to bend sharply to the left (fromG. Rochester and C. Butler, 1947).

The τ-θ puzzle

In the beginning, the discovery of strange mesons was made complicated by theso-called τ -θ puzzle. A strange meson was disintegrating into two pions, andwas called the θ meson; another particle called the τ meson was disintegratinginto three pions. Both particles disintegrated via the weak force, and the twoparticles turned out to be indistinguishable other than their mode of decay.Their masses were identical, within the experimental uncertainties. Were theyin fact the same particle? It was concluded that they were (we are talking aboutthe K meson); this opened a problem related to the so-called parity conservationlaw, and we will discuss it better in the next chapter.

3.2.6 Mountain-top laboratories

The discovery of mesons, which had put the physical world in turmoil afterWorld War II, can be considered as the origin of the “modern” physics of ele-mentary particles.

The following years showed a rapid development of the research groups deal-ing with cosmic rays, along with a progress of experimental techniques of de-tection, exploiting the complementarity of cloud and bubble chambers, nuclearemulsions, and electronic coincidence circuits. The low cost of emulsions al-lowed the spread of nuclear experiments and the establishment of international

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collaborations.It became clear that it was appropriate to equip laboratories on top of the

mountains to study cosmic rays. Physicists from all around the world wereinvolved in a scientific challenge of enormous magnitude, taking place in smalllaboratories on the tops of the Alps, the Andes, the Rocky Mountains, theCaucasus.

Particle physicists used cosmic rays as the primary tool for their researchuntil the advent of particle accelerators in the 50’s, so that the pioneering resultsin this field are due to cosmic rays. For the first thirty years cosmic rays allowedto gain information on the physics of elementary particles. With the advent ofparticle accelerators, in the years since 1950, most physicists step from huntingto farming.

3.3 Particle hunters become farmers

The 1953, the Cosmic Ray Conference at Bagneres de Bigorre in the FrenchPyrenes was a turning point for the high energy physics. The technology of ar-tificial accelerators was progressing, and many cosmic ray physicists were movingto this new frontier. CERN, the European Laboratory for Particle Physics, wasgoing to be founded soon.

Also from the sociological point of view, important changes were in progress,and large international collaborations were formed. Only 10 years before, arti-cles for which the preparation of the experiment and the data analysis had beenperformed by many scientists were signed only by the group leader. But therecent G-stack experiment, an international collaboration in which cosmic rayinteractions were recorded in a series of balloon flights by means of a giant stackof nuclear emulsions, had introduced a new policy: all scientist contributing tothe result were authors of the publications5.

In the 1953 Cosmic Ray Conference contributions coming from acceleratorphysics were not accepted: the two methods of investigation of the nature of ele-mentary particles were kept separated. However, the French physicist Laprince-Ringuet, who was going to found CERN in 1954 together with scientists of thelevel of Bohr, Heisenberg, Powell, Auger, Edoardo Amaldi, and others, said inhis concluding remarks:

“If we want to draw certain lessons from this congress, let’s pointout first that in the future we must use the particle accelerators.Let’s point out for example the possibility that they will allow themeasurement of certain fundamental curves (scattering, ionization,range) which will permit us to differentiate effects such as the exis-tence of π mesons among the secondaries of K mesons. [...]

5At that time the number of signatures in one of the G-stack papers, 35, seemed enormous;in the XXI century things have further evolved, and the two articles announcing the discoveryof the Higgs particle by the ATLAS and CMS collaborations have 2931 and 2899 signaturesrespectively.

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Figure 3.11: The so-called “maximum accelerator” by Fermi (original drawingby Enrico Fermi reproduced from his 1954 speech at the annual meeting of theAmerican Physical Society). Courtesy of Fermi National Laboratory, Batavia,Illinois.

I would like to finish with some words on a subject that is dearto my heart and is equally so to all the ‘cosmicians’, in particularthe ‘old timers’. [...] We have to face the grave question: what isthe future of cosmic rays? Should we continue to struggle for a fewnew results or would it be better to turn to the machines? Onecan with no doubt say that that the future of cosmic radiation inthe domain of nuclear physics depends on the machines [...]. Butprobably this point of view should be tempered by the fact that wehave the uniqueness of some phenomena, quite rare it is true, forwhich the energies are much larger [...].”

It should be stressed the fact that, despite the great advances of the technol-ogy of accelerators, the highest energies will always be reached by cosmic rays.The founding fathers of CERN, in their Constitution (Convention for the Es-tablishment of a European Organization for Nuclear Research, 1953) explicitlystated that cosmic rays are one of the research items of the Laboratory.

A calculation made by Fermi about the maximum reasonably (and evenunreasonably) achievable energy by terrestrial accelerators is interesting in thisregards. In his speech “What can we learn from high energy accelerators”)held at the American Physical Society in 1954 Fermi had considered a protonaccelerator with a ring as large as the maximum circumference of the Earthas the maximum possible accelerator. Assuming a magnetic field of 2 tesla,it is possible to obtain a maximum energy of 5000 TeV: this is the energy ofcosmic rays just under the “knee”, the typical energy of galactic accelerators.Fermi estimated with great optimism, extrapolating the rate of progress of theaccelerator technology in the 1950s, that this accelerator could be constructed

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in 1994 and cost approximately 170 billion dollars (the cost of LHC is some 100times larger, and its energy is 1000 times smaller).

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