IL ~TUOVO CIME~TO VOT.. XXII I , X. 1 1 o Gennaio 196~

The Role of Hyperons in Extensive Air Showers and in Other High-Energy Phenomena.

B. PETERS

Institute ]or Theoretical Physics - University o] Copenhagen

(ricevuto il 28 Agosto 1961)

S u m m a r y . - - Experimental results obtained at the Geneva proton synchrotron support the hypothesis that in high energy nuclear colli~ sions the particle which carries away most of the energy is often a hyperon. In proton collisions at 25 GeV this occurs in about 20% of the cases. The fraction of hyperons appears to increase further with increasing proton energy and probably reaches a value close to 50% for energies above ~ 1000 GeV. I t is shown that in the atmosphere most pions, ~-mesons, and y-rays in the energy interval h'om a few times l011 eV to about 101~ eV are due to hyperon decay rather than to direct pion production and that a large fraction of the primary energy appearing in the electron-photon component is t ransmitted through this process. The presence of hyperons in the nuclear cascade in the core of air showers gives rise to a large number of characteristic phenomena, both in the a~- mosphere and underground, some of which may be related to phenomena recently reported in the literature. Since hyperon decays resul~ in a large excess of negative over positive mesons, charged eterminations on ~-mesons underground may give information on the relative yield of nucleons, nuclear isobars, and of strangeness one and strangeness two hyperons at high energies. I t is further shown that the simultaneous arrivM of two or more ~-mesons a~ great depth below ground signals the arrival and interaction of a complex primary nucleus at the top of the atmosphere.

1 . - I n t r o d u c t i o n .

The d e v e l o p m e n t a n d t he s t r ue tu rM fea tures of air showers d e p e n d c r i t -

ica l ly on the proper t ies of s t rong ly i n t e r a c t i n g p~rt icles i n the ene rgy r a n g e

above a few t imes 1011 eV. The l~rgest avMl~ble accelera tors ~re now app roach -

ing th is ene rgy region so t h a t some of the ' more r ecen t l ~ b o r a t o r y resu l t s are:

T H E R O L E OF H Y P E R O N S IN E ] I . T E N S I V E A I R S R O W E R S E T C . 89

applicable to the in te rpre ta t ion of cosmic ray phenomena in air showers wi thout major extrapolat ions.

I n Section 2 it is shown tha t exper imenta l results obta ined with the 25 GeV pro ton beam at CEI~I~ prove t h a t the part icle which carries the grea tes t a m o u n t of energy away f rom a nuclear collision is in an appreciable f rac t ion of cases

(10% to 30%) a hyperon ra the r t han a nucleon.

I t is also shown tha t informat ion obta ined f rom the s t udy of high energy

cosmic ray jets suggests s t rongly t ha t the f ract ion of these hyperons increases fur ther with energy; the emerging high energy baryons appear to dis tr ibute

themselves be tween nucleons and hyperons roughly according to their rela- t ive stat ist ical weights.

In Section 3 this result is applied to high energy interact ions in air where, in general, the emerging hyperon has a chance to decay before undergoing a nuclear collision. The decay pion receives a f ract ion of the hype ron energy which depends only on the angle of emission and averages 16 °/o for A-

and 20% for E-hyperons. The energy going to this single de layed pion re- presents, therefore, a ma jo r f ract ion of the energy given to the pion com- ponent as a whole.

When the energy of the emerging ba ryon exceeds a few t imes 1011eV

(corresponding to an energy for the incident nucleon which is about 20 °/o

higher), the decay pion carries more energy than any direct ly produced pion. This effect becomes more m arked as the collision energy increases, because the l abora to ry energy of direct ly produced pions emi t ted in the forward di- rection, increases essentially in propor t ion to the square-root of the energy of the incident particle, whereas t h a t of the decay pion increases linearly.

The energy b a n d in which conditions for observing delayed decay pions are mos t favourable extends f rom a few t imes 1011 eV to a value which is de termined b y the probabi l i ty of the hyperon in terac t ing before decay. The upper bounda ry of the region lies, therefore, near

and

2.3.1013 eV at sea level

3.3-10 la eV at moun ta in a l t i tude (2 000 meters)

1.9-101~ eV at an al t i tude of 18.5 k m (*).

These upper l imits refer to E+-hyperons; the energy l imits for E - and for AO-hyperons are lower by factors 2 and 3, respectively. Iqo such upper energy

l imit exists for the fas t decays such as E ° - ~ A ° ÷ y or Y * - - > Y ~ - ~ and for

other exci ted ba ryon states.

(') This is the altitude eolTesponding to one collision mean free pa th for protons, i .e. the altitude where the first hyperons of the nucleon cascade make their appearance.

~ 0 B. PETERS

Observable phenomena re la ted to the decay of high energy hyperons in to neut ra l pious are discussed in Section 4, while observable phenomena re la ted

to the decay of hyperons into charged pions and to the ~-mesons resul t ing

f rom such decays are discussed in Section 5. I t seems probable t ha t the pencil

beams in air shower cores repor ted b y VEI~I~OV et al. (~), and the observa t ion

o f bundles of ~-mesons below ground (HIGAS~I et aK (~)) are re la ted to the f requent change in strangeness n u m b e r which accompanies the interact ions

,of high energy baryons.

2. - The production of high energy hyperons in nuclear collisions.

Recent work with the p ro ton synchro t ron a t Geneva has established t h a t the average mul t ip l ic i ty of charged shower particles in collisions of 25 GeV

protons with l ight t a rge t nuclei is

A t the same p ro ton energy the ra t io K+/~ + in the secondary beam has been m e a s u r e d b y v. DARDEL et al. over a m o m e n t u m r a n g e f rom 2 to 18 GeV/e. An average value of abou t 25 % seems to app ly over the entire m o m e n t u m range and over the whole angular in terva l where meson product ion is signi-

f icant (Proc. Rochester High Energy Particle Con]erence 1960, p. 801). How- ever , p re l iminary results obta ined a t Brookhaven (8) indicate a value which m a y be lower b y as much as ~ fac tor two. (I t is as ye t not possible to assign prob- ab le errors to these values. I n this paper we are in teres ted pI~marfly in the .quali tat ive aspects of the result ing phenomena and shall ignore such errors

th roughout ) . Assuming charge independence, the two observat ions indicate t h a t on the

~verage 0.4 to 0.8 K-mesons with strangeness + 1 are produced per collision. The rat io K - / ~ - is found to be considerably lower t h a n the rat io K+/~ +

~nd ~verages abou t 6 %. This indicates t h a t K-mesons of negat ive strangeness

a re less a b u n d a n t th~n those of posi t ive strangeness b y a fac tor lying be tween

two and four. I t follows tha t 50% to 75% of K-mesons of s t rangeness + 1 ~re produced in association with hyperons ; one gets, therefore, on the average

0.2 to 0.6 hyperons per collision a t 25 GeV in a luminium and beryl l ium targets .

(1) S. lq. VER•OV, G. V. KI~LII~OV, Z. S. STRUGALSKI and G. B. KRISTIANSEN: J E T P (engl. ed.), 37, 848 (9960).

(2) S. HIGASI-II, T. 0SHIO, m. SHIBATA, K. WAKANABE DAXfl ~r. WATASE: -~TUOVO Cimento, 5, 597 (1957).

(z) ~;ote added in proo]. - W. F. BAKER, R. L. COOL, E. W. J~KI~S, T. F. KYOIA, S. J. LIND]~NBAUB~[, W. t . LOVE, D. Lt~ERS, J. A. NIEDERER, S. 0ZAKI, A. L. READ, J . J. RUSS]~L and L. C. L. YUAN: Phys. Rev. I~ett., 7, 101 (1961).

T H E R O L E OF t t Y P E I ~ O N S I N E X T E N S I V ~ A I R SHOWI~,t~S E T C . 91

I n such collisions the t a rge t nucleon or the nucleon serving as projecti le can be t r ans formed into a hyperon with equal probabi l i ty . Since it is known t h a t only pa r t of the incident energy is g iven to pions in these collisions, i t

follows tha t the bu lk of the energy is carried a w a y b y a hyperon in 10 °/o to 3 0 % of all cases. I n the remaining cases it is ca r r ied away b y a nucleon which

m a y be in the ground s ta te or in an exci ted s ta te . In cosmic r ay jets the f rac t ion of shower par t ic les which are not pions is

approx ima te ly independent of energy and has near ly the same value as t ha t ~ound at 25 GeV. However , the mul t ip l ic i ty of shower particles increases slowly with energy; a t 10 ~ GeV the n u m b e r of charged shower particles is somewhat less t han four t imes as high as a t 25 GeV i.e. n~ ~ 15.

Whe the r the rat io K + / K - remains as high as i t is a t lower energies is not known. Since even at 25 GeV the collision ene rgy a l ready exceeds grea t ly

the threshold for the product ion of K-meson pairs , i t is perhaps reasonable to

expec t t ha t the ra t io K + / K - ~ 2 to 4 represen t s some kind of sa tura t ion value. The large excess of slow posi t ive over slow negat ive K-mesons observed in emulsion stacks exposed in the equator ia l s t ra tosphere , while not const i tu t ing a proof, is a t least consistent with this view.

I f one assumes K + / K - ~ 2 to 4 also in t h e je t region i t follows tha t the average nmnbe r of hyperons per collision m o v i n g forward in the center of mass sys tem and carrying, therefore, a large f rac t ion of the incident energy lies be tween 0.4 and 1.0.

While this increase f rom abou t 10~o to 30 ~ a t 25 G e ¥ to perhaps

50~o or 70°~ a t ~--10 ~ GeV cannot be considered well established, an appre-

ciable increase is to be expected if one assumes t h a t a t sufficiently high

energy not only the charge s ta tes b u t also the different s ta tes of strangeness of the colliding baryons are d is t r ibuted according to their s tat is t ical weight.

3. - High energy pions in the atmosphere.

Pions produced in the centre of mass sy s t em of a high energy nucleon- nucleon collision m u s t be expected to have in mos t cases lower energies t han a pion emi t t ed b y a hyperon or b y a nucleon isobar which moves with a

veloci ty comparable to the l abo ra to ry ve loc i ty of the incident nucleon.

Express ing all energies in t e rms of the res t mass of the part icles indicated as subscript, one finds for direct ly produced p ions a l abora to ry energy

r " = J 2 - + - 1-- cos a ) ,

p where y~, ~ are the energy and angle of emission in the centre of mass sys tem of the collision. Since i t i t known tha t even in v e r y high energy jets pion

9 2 B. PETERS

energies in the C-system rarely exceed ~ 1 GeV, the spectrum of directly pro- duced pions is expected to fall steeply beyond laboratory energies of order

On the other hand, the laboratory energy of the delayed decay pions is known to be uniformly distributed throughout the energy interval

and

@ < y . < 3 F z .

Assuming an average loss of 20 % to directly instantaneously produced pions, the energies of the emerging hyperons are related t o those of the incident protons by

M. 2 6

The laboratory energy of the delayed pion lies, therefore, between the limits

0.05 < (%)A < 0.20e, ,

0.03 < (s=)~ < 0.30e.,

(~)A/e~ = 12.5 %,

(~)z /~o = 16.5 %.

I t is clear that on the average the decay pion receives a very appreciable fraction of the primary energy. Since its energy increases as F v while tha t of the directly produced mesons emitted into the forwards hemisphere increases only as fast as ~ / ~ , the energy of the decay meson will be higher than tha t of the other mesons when / 'p exceeds a few hundred GeV; it dominates comple-

tely at collision energies above 103 GeV. This result has a direct bearing on existing measurements of the so-called

inelasticity parameter in the nucleon cascade. The absorption of the n u c l e a r component in the atmosphere and the rate at which the incident primary cosmic radiation feeds energy into the photon-electron component at great altitude, both suggest t h a t the average energy going into meson production is fairly constant from below 103 to above 105 GeV and lies close to ~-- 30 %. Since these observations are carried out in a rarefied gas they include the energy of the delayed pion. This pion alone receives on the average

(~)" ~ 14% 8p

T H E R O L E OF H Y P E R O N S I N E X T E N S I V E A I R S H O W E R S E T C . 93

of the p r ima ry energy, leaving abou t 16 % to be shared among an average of perhaps 15 charged and 7 neut ra l d i rect ly produced pious and K-mesons. Even if one assumes t h a t hyperon product ion shows no increase be tween 25 GeV and the cosmic r ay energy region, the fair ly f requent p roduc t ion of delayed pious wi th an energy a t least one order of magn i tude greater t han t h a t of

direct ly produced mesons m u s t be expected to produce observable effects in the a tmosphere .

The fract ion of energy which a ve ry energetic nucleon loses to pion pro-

duction has been measured not only in the a tmosphere bu t also in nuclear

emulsions, i .e. , in condensed mat te r . Here one measures only the energy of the directly produced mesons (*). I t seems, therefore, reasonable t ha t the

measured values of energy lost to mesons (between 10° o and 20%) is lower t han the 30 % value obta ined f rom observat ions in the a tmosphere . Emuls ion results suggest a value for the inelast ici ty in nucleon-nucleon collisions which is not constant bu t decreases slightly with energy; they are, therefore, perhaps

in somewhat be t te r agreement with current theories of mul t ip le meson pro- duction.

Taking into account the large energy- independent contr ibut ion to the in- e last ic i ty made b y the delayed pion tends to improve , not only the agree-

men t between the two types of measurements , bu t also between the experi- men ta l results and theoret ical predictions.

The conclusions abou t the pion produc t ion der ived here follow directly f rom measurements a t the C E R N pro ton synchro t ron and the known proper- ties of hyperons. They contain essentially no fu r the r assumptions. Cascade hyperons and hyperons in exci ted states have been omi t t ed in this discussion because they p lay perhaps a minor role even a t high energies, bu t including t h e m obviously does not al ter the conclusions appreciably.

4. - High energy neutral pious in the atmosphere (**).

The large n u m b e r of hyperons appear ing in the nuclear cascade which one is compelled to pos tu la te on the basis of resul ts obta ined a t machine energy

(*) Actually, the situation is somewhat more complicated. As mentioned earlier, a fraction of the decay energy of some hyperons (and the decay energy of nuclear isobars, if they also play a role) will be given off without apparent time delay. This fraction will be observed in nuclear emulsions.

(*') Note added in p~'oo]. - In this and the following section we discuss some meas- urable effects which result from the production of high energy hyperons in the atmo- sphere. Some of these effects, namely those which do not depend on the compara-

94 ~. PETERS

implies the presence in the a tmosphere of ~ group of pious in equilibrium with nucleons, f rom which they differ in energy not by the factor of order fifty to

one hundred characterist ic of the directly produced pions, but by a factor

which is more nearly five. A t sea level and on moderately high mountains the

energy of this pion c o m p o n e n t should lie between 10 n eV and a few t imes

1012 eV. We shall first discuss the observabil i ty of events in which the hyperon

decays into ~ neutrM pion.

4"1. y - R a y s i n equi l ibriu m wi th nucleons i n the atmosphere. - The n u m b e r

of y-rays in the air (and, therefore, of locMty produced, photon-induced electron

jets) in the energy intervM between 100 and 5 000 GeV should occur with a

f requency which is roughly four to five times smMler than t h e number of

nucleons of energy 700 to 35000 GeV colliding in the air within a distance

of the order of one rad ia t ion length; the factor ½ to ¼ represents the p roduc t

of the probabilities tha t a fas t hyperon is produced and tha t it decays via

a 7: ° mode. This effect, whose presence can be deduced directly from the large hype roa

yield in the nuclear cascade, has some similarity with a phenomenon which has been discovered with t h e help of a diffusion chamber in the core of air

showers by V E ~ o v et al. (~). Bundles of 4 to ~5 parallel tracks within a

circle of ~ 4 cm diameter were found to appear frequently in the core of Mr

showers of size ~ ~ 3-104 particles. I f the bundles represent fully developed

electron cascades ini t iated b y v-rays entering from the outside, the energy of the y-rays is est imated to lie between 10 ~ and 10 ~-° eV. The initiating ~-ray

must then be produced w i th in 150 g/cm 2 above the apparatus. The authors.

point out tha t not more t h a n one strongly interacting particle of energy

>~ 10 ~2 eV is present in such showers with sufficient frequency to account for

the creation of a corresponding ::°-meson and conclude tha t such an expla- nat ion would indicate (( a m a r k e d role of nuqlear interactions in which the

main par t of the energy is concent ra ted in one ~°-meson ~. They also examine the possibil i ty tha t these narrow pencil beams are not

electrons bu t either =- or ~-mesons and find tha t there is no sat isfactory ex-

planat ion within the f r a m e w o r k of present knowledge of the product ion and

decay of elementary particles. I t seems plausible, therefore, to a t t r ibute the

tively long lifetime of hyperons nor on the negative charge excess among the decay pions can equally well be produced by nucleons in excited states. So far nothing is known about the frequency of occurrence of nuclear isobars at high energy. It is pos- sible that they also play an important role in the nuclear cascade but we shall not consider them here. We are grateful to Dr. V. I. ZA~s~I'I~ for pointing out their possible importance.

T H E R O L E OF t I Y P ~ R O N S I N :EXTE]NSIV3~ A I R S H O W E R S :ETC. 95

pencil beams to the neutral decay mode of high energy hyperons in the nuclear

cascade (*).

4"2. The energy spectrum o] T-rays in the atmosphere. - Pract ical ly all ener- getic y-rays in the atmosphere should be due to the decay of neutral pious.

I f ~ neutral pion of ~-10 a GeV originates in a pionizatiou process, the energy

of the incident nucleon must have been of order 10 s GeV; if, on the other hand,

it is due to the decay of a high energy hyperon, the energy of the nucleon ini-

t iat ing the reaction must have been of the order of 5.10 ~ GeV. Since nucleons

of 5.10 3 GeV are more ~bundant than those of 10 ~ GeV by a l~rge factor

(of order one hundred), it follows tha t practically ~lI y-r~ys of ]O a GeV or more

found in the atmosphere are produced via the hyperon process.

I t follows further tha t if the energy of the hyperon reaches the upper limits

discussed in Section 1, i.e. an energy so high tha t interact ion becomes more

probable than decay, the 7-ray spectrum should show a dec]inc. This occurs. at a hyperon energy of order

Fy ~-- __h° log P

where v~., ~y are the mean life and interaction mean free path of the hyperon

and ho~ p ( ~ ~y) are the atmospheric scale height and the pressure at the po in t

of observation~ respectively.

For neutral pions from Z+-decays this energy corresponds to y- ray energies

of about 4-103 G e¥ at sea level and mounta in alt i tude and to 2-10 ~ GeV at

a pressure of 200 g/cm% The limit for ~0 from A°-decay is three to four times lower.

Evidence for a rapid increase in the slope of the y - ray spectrum at about these energies has been reported by several investigators. I t is impor tan t to

note tha t the spectrum of T-rays produced locally by the nuclear active com-

ponent in condensed material should not (and apparent ly does not) show such a change of slope.

5. - High energy charged mesons from hyperon decay.

5"1. Charged pious in equglibrium with the nucleon component. - As shown

in the previous section, pious o~ euergy above ~ few times 10 ~- GeV originating

(*) The term <( m~clear cascade ~) is not meant to convey the concept of multipli- cation charac~erist, ic of the phol;on-elec~ron cascac]~. ~flt'he production of s~co~dary nucleons seems to play only a minor role. In the main, api)arently, a baryon proceeds through the atmosphere in a series of (( pioni~ing )) collisions (a term introduced by G. Cocco~*) interspersed by decays whenever it suffers a change in its strangeness number.

96 ~. ~ETEnS

in the decay of hyperons in the atmosphere are much more numerous than pions of the same energy produced in the pionization process. Therefore, wi thout detailed knowledge of the multiplici ty and energy distr ibution in ordinary pion production, it is possible to calculate the fraction of nucleons, hyperons and pions among the strongly interact ing particles of energy above a few hundred GeV. Apar t f rom the density distribution of the atmosphere and the known decay constants and branching ratios of hyperon decays, the variables in such a calculation are the interact ion cross-section of high energy baryons and pions in air and the fract ion of various types of hyperons and nuclear isobars among the baryons emi t ted into the forward hemisphere in the center of mass system. In terac t ion cross-sections measured at CERbT (~), indi- cate tha t asymptot ic , i.e. energy-independent, values are reached already at N 20 GeV. An exper imental determinat ion of the fract ion of hyperons among the baryons which carry away the bulk of energy from the collision seems possible, because of the large excess of negative over positive mesons among the decay products of hyperons (*). The nucleon component at a given alti- tude will show a positive excess due to some pr imary protons which have escaped in teract ion in the air overhead and have leaked through the atmos- phere to the point of observation. Charged pions produced in pionization processes will show also a small positive excess, representing effects of the excess posit ive charge among the pr imary nucleons incident on the atmos- phere. On the other hand, pions from hyperon decay yield a rat io ~-/~+ = - (1 4 /3 )= 4.7 if one assumes tha t all hyperons of strangeness S = - - I are produced with equal probabi l i ty and = - / = + ~ (28/3) = 9.3 if cascade particles are included with comparable statistical weight.

Therefore, al though it is difficult to distinguish experimental ly between the various types of high energy strongly interact ing particles, informat ion on the composition can be obtained by measuring not only the charged to neut ra l ra t io as has been done in the past, bu t the actual cha~ge, i.e. the number of positive, negative and neutral particles in the strongly interact ing component a t various energies. Quali tat ively one would expect mainly nucleons and negat ive pions (from hyperon decay) above a few hundred GeV and most ly hyperons and nucleons at the still higher energies at which hyperons are likely

to in terac t before decay.

5"2. High energy p.-mesons produced via hyperon decay. - Because of the compara t ive ly long mean life of the pion, 7:-~ decay at high energy is possible

(*) The negative excess of pious arises from ghe fact that ~he hyperons are asso- ciated in production with K-mesons of strangeness S= + 1 and are associated in decay with nucleons neither of which possess negative charge states.

(~) Proceedings of the 1960 International Conference on High Energy Physics at Rochester, page 799.

THE ROL]~ OF H Y P E R O N $ IN E X T E N S I V E AIR S H O W E R S ETC. 97

only in a very rarefied atmosphere, i.e. at great altitude. This is true both for ordinary ~-mesons and for those produced in the Y - - ~ - + ~ channel. Both types, therefore, become very rare as the energy increases. For reasons given

in the preceding section, the ~-mesons above a few hundred GeV must arise

through baryon decay, predominant ly via the hyperon channel. Such ix-mesons

receive on the average

1,~ m~ o/ ~ 10.5o/ - - / 0 . ' 0 'lT"t~

of the incident pr imary nucleon energy. (The largest energy which can be

transferred to a ~-meson by this process is slightly more than 35 °/o. )

I t follows tha t ~-mesons observed at a depth greater than ~ 1 5 0 0 meters

water equivalent should show a negative excess. I t seems possible to measure

the ratio ix+/(~+÷ ~-) at great depth by stopping ~-mesons in a large absorber

of high atomic number and recording the delayed pulses of their decay posi-

trons.

5"3. p.-Meso~s undergrou~td and the composition o] the primary cosmic ra-

diation. - Since ~-mesons at great depth below ground are produced mainly as a result of baryon decay, the nuclear interactions responsible for their pro-

duction occur mainly above an altitude of order

P0 hc ~ho log~- -- c~=),= + CT~/'B ~ 18.5 ÷ 8.3 "10 3y~ kilometers .

For ix-mesons of energy ~ 3 0 0 GeV (y~ ~z 2 700) this corresponds to an alti-

tude above 40 kin. I t is clear, therefor% tha t a pr imary nucleon incident on

the atmosphere cannot produce more than one ~-meson of this energy. I t

follows tha t ~-mesons at a depth of 1500 m w.e. or more should arrive singly if all cosmic ray primaries were protons. Two or more ~-mesons arriving at

the same time indicate simultaneous production of more than one energetic, un-

stable baryon at great altitude, and therefore the collision of a complex pr imary with a nucleus of the air. Thus, above a certain energy each pr imary nucleon

has a certain probabil i ty (which is a function of the atomic number of the

incident nucleus) of being represented by one ~-meson at great depth below

ground. Once this probabil i ty function is known, the composition of the pr imary

radiation can be determined by measuring the number of simultaneously

incident ~-mesons below ground.

At a depth of ~ 1 5 0 0 m w.e. these simultaneously arriving 9-mesons are

spread over an area which can be calculated easily, because the main contri-

bution to their spread comes from the transverse momen tum which the pion

7 - I I N u o v o C i m e ~ d o .

9 8 B. PETERS

receives in the hyperon decay. The mean transverse momen tum is given by

Yg

The various ~-mesons will fall, therefore~ inside a circle whose radius ~ r )> is of order

• / / IS

7r h~ y= - - I

Insert ing the value for the height of interaction~ h , derived above ~md making

use of the fact tha t

one finds

rho logpo/2~ 18.5 r h~. - - ~ . . . . k m

r - - r o r - - r o

where ro = (:~/4)c7: ~/'~ ,o 1 has the value r: / ' ~ - -

and ~.3 meters for A - > 7 : - + ~ decays ,

8.2 meters for E -+ ~ -+ ~ decays.

To this spread must be added a smM1 spread due to the scattering of the

it-mesons in the overlying earth. Bundles of pmesons which fall inside a target area of radius 2to have

energies of the order of

%T~y~ ~ ho logp/2~.

This corresponds to a pion energy of s. ~ 390 GeV and s~ ~ 290 GeV. The

corresponding pr imary particle must have an energy of the order of 2 500 GeV/nu-

cleon and the collision of the pr imary must occur above the altitude

ho ~ 2h0 logp/2~ = 37 k m .

The number of primaries with this energy and with atomic number Z~> 6 which

collide above ~ 3 7 km is estimated as ~pproximately equal to

2 per day, m ~ and s teradian.

Bundles of 9-m6sons ought to occur with comparuble frequency and should

therefore be detectable below ground.

T H E R O L E OF H Y P E R O N S I N E X T E N S I V E A I R S H O W E R S E T C . 9 .q

Whethe r the ~-meson bundles repor ted by HIOASI~I et al. (3) are due to this process of s imul taneous product ion of several hyperons by complex primaries at gre~t a l t i tude cannot be decided wi thout more extensive experi-

men ta l data.

5"4. Speculations on baryons and antibaryons. - While it seems plausible tha t in a high energy nucleon-nucleon interact ion the colliding particles m a y change electric charge and strangeness or m a y be raised to an excited state, it seems almost self-evident t ha t they will re ta in their ba ryon number : the particle which c~rries away mos t of the energy f rom the collision will there-

fore be a baryon, like the incident particle, even if one or more baryon-ant i - ba ryon pairs are created in the interaction. Such a belief e~nnot, however, be proven and represents ~n ext rapola t ion f rom experience gained at much

lower collision energies. Should the assumpt ion be invalid, it would give rise to some observable effects.

F r o m the rat io of in teract ion to absorpt ion mean free pa th of s t rongly in teract ing particles in the a tmosphere , one knows tha t at least up to col-

lision energies of ~ 1 0 ~ GeV there exists such a part icle which carries away most of the energy. One also has some (although not decivise) exper imenta l evidence to the effect that the ba ryon n u m b e r of the particle which carries the bulk of the energy cannot change f rom B = ] to B = 0; otherwise the energy of the nucleon cascade could a t some stage be t ransferred to a =0

meson, which would make it very difficult for the cascade to pene t ra te down to sea level.

I t is, however, conceivable tha t in the course of the cascade the role of principal carrier of energy devolves at t imes onto an ant ibaryon. When the incident baryon produces a large num ber of baryons and ant ibaryons in a collision, i t is not easy to see how a distinction between the incident and a secondary ba ryon could be mMntained and, therefore, it is not obvious t h a t upon emerging with mos t of its energy intact , its ba ryon label will have re- nlained securely a t tached. As far as existing exper imenta l evidence is con- cerned i t is only necessary to postula te tha t the absolute value of the ba ryon number remains unaffected in m o m e n t u m transfers which are large com- pared to a ba ryon mass.

Admi t t ed ly there is no reason for assuming tha t the particle which carries the principal energy and propagates the nuclear cascade can change f rom

baryon to an an t iba ryon and vice versa ; bu t there is also no conlpelling reason

to rule out such a phenomenon at ve ry high energies. I f the process occurred, then ant i -hyperons as well as hyperons mus t contr ibute to high energy pion (and ~-meson) production.

Most of the phenomena which we have discussed would not be affected. However , the fract ion of negat ively charged particles in the nuclear ~ctive

100 B. ~ETER8

c o m p o n e n t at m o u n t a i n a l t i t ude would increase a n d the nega t i ve excess a m o n g

u n d e r g r o u n d ~z-mesons would be reduced.

The a u t h o r is g rea t ly i n d e b t e d to his colleagues a t the T a t a I n s t i t u t e of

F u n d a m e n t a l Research, i n pa r t i cu l a r to Prof. M. G. K. 3/[EI~ON, Dr. ¥ASI~

PAL, a n d Dr. B. V. SHaEEKA~TA~ for m a n y v a l u a b l e a n d s t i m u l a t i n g dis-

cussions. H e w~nts to express his g r a t i t u d e also to Dr. t t . J. BHA~HA for

the hosp i t a l i t y e x t e n d e d to h im du r ing his r ecen t s t ay in B o m b a y .

R I A S S U N T O (*)

I risultat, i sperimerttali ottenuti col protonsincrotone di Ginevra daano valido sostegno all'ipotesi che nelle eo]lisioni nucleari di alta energia la particella che porta via con se la maggior parte dell'energia 5 spesso un iperone. Ne]le eo]lisioni di protoni di 25 GeV questo avviene in circa il 25°,0 dei easi. La proporzione di iperoni sembra crescm'e ancora aumentando il valore de]l'energia dei protoni e probabilmente raggiunge un valore prossimo al 70% per energie superiori a ~ 1000 GeV. Si mostra che nel- Fatmosfera la maggior parte dei pioni, mesoni ~ e raggi T nell ' intervallo di energia a par- tire da un ordine di 1011 eV fino a circa 10 la eV sono dovuti a decadimento di iperoni piut- ¢osto che ad una produzione diretta di pioni e che aria grande frazione dell'energia primaria che compare nella componente elettrone-fotolte vierm trasmessa Cramite.questo processo. La presenza di iperoni nelle cascate nucleari nel eentro deg]i sciami dell'aria dg origine a,d un grande numm'o di fenomeni c~/ratteristici, sia nell 'atmosfera ehe nel sottosuolo, alcuni dei quali possono essere rapportati a fenomeni recentemente deseritti negli arti- eoli su questi argomenti. Poich~ i decadimenti degli iperoni danno come risu]tato 'un grande eccesso di mesoni negativi rispetto a quelli positivi, la determinazione della carica dei mesoni ~z presenti nel sottosuolo pub dare informazioni sulla produzione relativa di isobari nueleari e eli iperoni di stranezza uno e di stranezza due ad alte energie. Si mostra altresi che l 'arrivo simultaneo di due o pig mesoni a grande profondit~ sotto terra segnala l 'arrivo e l ' interazione di un nucleo primario eomplesso al sommo dell'atmosfera.

(*) Tra&tz io~e a ct~ra della Redaz ione .