Observation of “elastic” hyperon production by antineutrinos

Volume

40B, number

5

PHYSICS

OBSERVATION

OF “ELASTIC”

7 August

LETTERS

HYPERON

PRODUCTION

1972

BY ANTINEUTRINOS

T. EICHTEN, H. FAISSNER, S. KABE, W. KRENZ, J. Von KROGH,, J. MORFIN and K. SCHULTZE III Physlkahsches Instltut der technischen

Hochschule,

Aachen,

Germany

J. LEMONNE, J. SACTON, W. Van DONINCK and P. VILAINT Insfitu te for High Energies, II L B , V. II B. Brussels, Be&urn D.C. CUNDY, D. HAIDT*, P. MUSSET, U. NGUYEN-KHAC, S. NATALI, J.B.M. PATTISON, D.H. PERKINSTP, C.A. RAMM, W. VENUS and H.W. WACHSMUTH CERN, Geneva, Swztzerland

V. BRISSON, B. DEGRANGE,

M. HAGUENAUER, and P. PETIAU

Ecole Polytechmque,

E. BELLOTTI,

F. JACQUET, L. KLUBERG

Pans, France

S. BONETTI, M. BRINI-PENZO, C. CONTA, E. FIORINI, C. FRANZINETTI** and A. PULLIA

Istltuto dl Ftslca dell’UmverWh,

Mdano and IN F N. Mlano, Italy

B. AUBERT, L.M. CHOUNET, P. HEUSSE, L. JAUNEAU, A. LAGARRIGUE, A.M. LUTZ, C. PASCAUD and J.P. VIALLE Laboratolre de l’Ac&l~rateur

Lmiazre, Orsay, France

F. BULLOCK, M.J. ESTEN, T.W. JONES, J. MCKENZIE, A.G. MICHETTE***, G. MYATTtt ,R.H. SCHAFER and R.G. WORTHINGTON Umverszty College, University of London, England Received The productton estimate

of A and X0 hyperons

the productton

for the process

cross-sectton

by antineutrmos

on protons.

5 June 1972 has been observed

The result 1s compared

These events have been used to

wtth the predrctions

of the Cabibbo

theory

Vp --t ~.I’A.

The observation of the leptomc decays of hyperons leads one to expect the following strangeness changing reactions by antmeutrmos: (i)V+p+p+ +A, (ii)iitp+p+ +X0, (iii) V + n + p+ t C- . t Chercheur Agree’i l’I.I.S.N., Belgrque. * On leave from Aachen, Germany. tt And at the Umverstty of Oxford, England. ** Presently at the University of Turm, Italy. *** Grant from the Science Research CounciL

The cross-sections for these reactions have been predicted using the Cabibbo theory [l] , and their values, integrated over the CERN antmeutrino spectrum, are grven in table 1 as a function of M, the mass parameter in the dipole formula for the axial and vector form-factors, F = ( 1+q2/kf2)-2. Typically, one expects cross-sections in the region of 10w40 cm2/proton. In this paper we report on the first observation of hyperon production, using as detector the new large heavy liquid bubble chamber Gargamelle [2] , filled with freon (CF,Br, density 1.5 g/cm3, radiation

Volume

40B. number

PHYSICS

5

Table 1 Theorettcal hyperon productron cross-sectrons m units 1@40cm2/proton or neutron averaged over the CERN neutrmo spectrum for energres above 1 GeV. The mass eter M rs that m the dipole form factor formula F = = (1 +4*/M*)-*. VdUes of other parameters OCablbbo f= 0 45, d= 0.78 M(GeV/c*)

oA/proton

0.3 0.4 06 08 10 1.2 1.4

0.44 0.66 1.3 22 3.1 42 5.2

o&neutron 0.19 0 29 0 63 1.1 17 23 30

of antrparam= 0.24,

oZO(proton -___ 0.09 0.15 0 31 0.55 08 1.15 1.6

length 11 cm). This chamber has drmensrons (4.8 m length X 1.85 m diameter), wnh vissrble volume 7 m3, and a magnetic field of 20 kG. The large visible volume gives very hrgh efficiencies for the detection of y-rays (> 90%) and the rdentrflcatron of muon secondaries. So far 250 000 antmeutrino pictures have been taken with the chamber exposed to the CERN antmeutrmo beam, using 26 GeV protons. The ii flux and spectrum were momtored continuously throughout the runs by means of muon counters [3] placed at various depths and radial positions m the shreldmg. The followmg events have been observed: (a) (b) (c) (d)

10 events with I_~+A(A+ n-p), 2 events with /J+Z~(C~ + rA), 8 events with I.~+A+ IT’S,p’s etc., 8 events with /J+ + Y + K + etc. (associated production).

In thus analysis we consider only the charged decay mode A + rr- t p, since rt is possible to confuse the y’s from the mode A -+ no + n with those from no’s produced directly at the primary vertex. In additron, we have observed two possible cases of smgle Cproduction, but because of the drffcultres mvolved in their detection and identrfrcatron we shall not consider them further. Most events of category (c) are presumably due to the quasi-elastic process of Y* resonance productron, wrth ot without pion absorption u-r the nucleus. How-

LETTERS

I August

1972

ever, 3 of the 8 events have only a very low enery proton or neutron (< 30 MeV) whilst the 5 others contain prons. We consider that the 3 events should be included m events of type (a). The characteristics of the event types (a) and (b) are shown m table 2. One source of background m the elastic candidates 1s due to associated production of A K” and where the K” 1s not detected. Of the 8 associated production events seen, only 1 consists of the combination A + K”, without prons and could have been confused wrth elastic hyperon production rf the K” had not been detected. Allowmg for neutral and long lived K” decays we estimate that the background from associated production is 1.5 f 1.5 events. Another possible background can arise because most of the A’s are of low momentum and the track lengths of the proton and pron are normally very short. These could be simulated by neutron stars havmg two proton tracks. Such a neutron background would have an apparent “decay length” of - 60 cm, whereas the true A’s have a decay length of a few centrmetres. Furthermore neutron stars would be expected to give a broad mvarrant mass drstrrbutron. Fig. 1 shows a scatter plot of the A momentum of the

.

5 Frg. 1 Momentum

versus decay length dates.

LA cm,.

lb

for A’s of cci\ candi-

PHYSICS LETTERS

Volume 40B, number 5

7 August 1972

Table 2 Observed single hyperon events

Type

E~(=‘%s,ble)

L(m)

(GeV) 0.9 1.2 1.6 1.8 1.9 2.0 2.1 2.2 3.0 3.3 2.0 1.3 24 1.3 1.4

0.06 0.08 0 05 0.33 0.01 0.01 0.30 0.23 0.64 0.47 0.15 0 16 001 0.24 0.04

0.54 0 95 1.33 1.50 159 180 1.66 1.97 2.60 3.00 1.77 1.19 2.07 0.72 1.14

0.56 0 32 0 44 0 64 0.59 0.27 0.87 0.39 0.72 0.54 0 27 0 25 1 .oo 1 05 0.53

1 120 1.167 1.107 1 110 1 090 1 113 1 120 1.115 1.109 1.110 1.122 1.110 1 117 1.228 1 170

0.8 7.6 1.0 3.3 1.9 0.6 0.8 14 41 1.4 0.7 1.6 65 _ _

Remarks

neutrun star < 30 MeV proton < 30 MeV proton < 30 MeV

type (a) events against the distance of the decay point from the origin. The decay length drstributron 1s normal, and we conclude from the scatter plot that the neutron star background 1s very small, and 1s < 1 event out of the observed 13 events. From the drscussron of backgrounds we conclude that we have definitely observed examples of single A production by antmeutrinos, the number of events bemg 11.5 * 1.5 after background subtractions. In converting from the number of events observed to the true A production rate in freon, various correction factors must be applied. These are as follows: 1) The measured scanning efficiency yields a correction factor of 1.1:::. 2) Loss of A’s of very short decay time grves a factor of 1.2. 3) Correctron for decay of A’s through the neutral model A + rr” + n gives a factor of 1.5. When these are taken into account, the corrected number of I_~+Aevents becomes 23Ti2. From the calculated antineutrmo flux through the visible volume of the chamber, we thus obtain an observed crosssection for p+A production in feron, averaged over the spectrum, of

The reactions (r), (11)and (hi) refer to those on srngle nucleons, and comparison of theore trc al crosssections with data from events in complex nuclei involves consideration of vanous nuclear effects. The important ones come from the process of nuclear absorption and the production of Zhyperons transforming to A’s in their passage through nuclear matter. Monte-Carlo calculations [4] using the experimentally determmed hyperon-nucleon cross-sections [ 51 indicate that for hyperon production the A absorption IS - 15% and the Z-A conversion -20%. As shown m table 1, the total Z production rate IS - 0.75 of the A productron rate, depending only weakly on the form-factor. Takmg these two factors mto account, the elementary cross-section on free protons is calculated to be 1 .O f 0.5 of the observed rate per proton m freon, where the quoted error represents a generous estimate of the uncertainty m the above factors. Thus, from the observed 13 elastic A-production events m freon by antineutrtnos we deduce a crosssection on free protons of

+Oe6 X OA(freon) = l e3-0.3

Comparmg this figure with the values m table 1, we conclude that, within the errors the experimental result 1s compatible with the theoretical prediction

10e40 cm2/proton

where the error mcludes a 15% uncertainty flux.

m the 3

cA@) = 1.3::;

x 10P4’ cm2/proton.

Volume

40B, number

5

PHYSICS

using the “standard” electro-magnetic form-factors (M = MA = Mv = 0.84 GeV). The relative numbers of Co and A-hyperons observed is also in agreement with theoretical predictions, which involve the AZ = i rule. This paper presents the first experimental results from Gargamelle. We would therefore hke to acknowledge the Comrmssariat a 1’Energie Atomique which constructed the chamber and the team who brought it into successful and reliable operation. In particular we should like to pay tribute to the original project leader, J. Lutz, who died m October 1970. It is a pleasure to express out thanks to Prof. A. Rousset and the other members of the CERN TC-L group who have carried the technical responsibility

LETTERS

7 August

1972

for the experiment. We also thank the CERN PS operational staff, and the scanning and programmmg personnel in the various laboratories.

(11 N. Cabrbbo, Phys. Rev. Lett 10 (1963) 531, N. Cabrbbo and F. Chtlton, Phys. Rev 137 (1965) 1628. [2] International Colloquium on Bubble chambers (Herdelberg) CERN 67-26, 1967. [ 31 D. Bloess, 3.B M. Pattrson, G Plass, D. Rusch, W. Venus and H.W. Wachsmuth, Nuclear Instr. Methods 91 (1971) 605. [4] M. Goosens, to be published. [S] Data Group Compilation UCRL-2000 YN