Strange particle production by antineutrinos

Volume 70B, number 3 PHYSICS LETTERS 10 October 1977

S T R A N G E P A R T I C L E P R O D U C T I O N BY A N T 1 N E U T R I N O S

O. ERRIQUES, M.T. FOGLI MUCIACCIA, S. NATALI, S. NUZZO Istituto di Fisica dell'Universita, Bari and LN.F.N. Bari, Italy

A. HALSTEINSLID, K. MYKLEBOST, A. ROGNEBAKKE, O. SKJEGGESTAD Institute of Physics, University o f Bergen, Norway

S. BONETTI, D. CAVALLI, A, PULLIA, M. ROLL1ER Istituto di Fisica dell' Universita, Milan and LN.F.N. Milan, Italy

G. BONNEAUD, B. ESCOUBES, J.L. GUYONNET, D. HUSS 1, M. PATY, C. RACCA, J.L. RIESTER, M. SCHAEFFER

Centre de Recherches Nucleaires et Universitd Louis Pasteur, Strasbourg, France

D. ALLASIA, V. BISI, C. FRANZINETTI, D. GAMBA, A. MARZARI-CHIESA, L. RICCATI, A. ROMERO, R. SACCO

[stituto di Fisica dell' Universita, Torino and LN.F.N. Torino, ltaly

F.W. BULLOCK, M.J. ESTEN, T.W. JONES, A.G. MICHETTE 2 University College London, London, UK

Received 20 June 1977

Cross sections are presented for antineutrino production of A, E ° and K ° in strangeness changing reactions. Associated production reactions (AS = 0) have been observed in the charged and the neutral current channels. For the elastic reaction Vp --, #÷A, estimates have been made of the axial transition form factor.

The strange particle events have been observed in 3 runs (~200 000 photos) of Gargamelle in the CERN PS wideband antineutrino beam, representing a total integrated flux of about 5 X 1015 antineutrinos. The chamber was filled with propane and a small admix- ture of freon (CF3Br) giving a radiation length of 64 cm for the first two runs and 47 cm for the third r u n .

A study has been made of antineutrino interactions producing strange particles in the channels (a) to (f) listed in table 1. The channels (a) to (e) represent charged current (CC) interactions and channel (f) represents a neutral current (NC) interaction.

Selection and identification o f events. Most o f the photos have been scanned twice for antineutrino reactions with one or more associated V 0 candidate de-

1 Also at Universit6 du Haut-Rhin, Mulhouse, France. 2 Now at Rutherford Laboratory, Chilton.

caying into two charged particles. Within this sample a further search has been made for additional V 0 decaying into neutral modes, and for charged strange particles. More than 50% of the V 0 candidates have a stopping proton and 2 events have arr + decaying at rest, thus excluding a priori the K 0 and A hypothesis respectively. For 3 events 8-rays on the leaving posi- tive track of the V 0 exclude the A hypothesis. For V 0 candidates with a flight path, (distance from the production vertex to the decay vertex), in the interval 0 .4 -50 cm we attempt a 3 constraint (3C) kinematical fit for one or both of the decay modes A ~ p + n - and K 0 ~ 7r + + ~r-, leaving the momentum of the V 0 as the only unknown. After the fit no event is am- biguous between the A and the K 0 hypothesis.

All candidates for the elastic channel ~p ~ g+A have been further subjected to a 3C fit at the production vertex with the antineutrino energy E u as the only unknown and assuming the F to interact with a

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free proton. Of the 24 elastic candidates, including 4 events with an observable proton of momentum <270 MeV/c at the production vertex, 9 events gave a 3C fit with an acceptable chi-square value (X 2 < 8). We consider these 9 events to be free from any background, e.g. from associated production with non- observed K 0. Of the 3 observed 2; 0 candidates one event gives a similar 3C production fit for F-'p ~/a+S 0.

Corrections for background, loss and misidentifi- cation of events. The requirement that the V 0 should give a 3C fit at the decay vertex (known line of flight) is expected to reduce the background of two-prong neutron stars to a negligible amount. We expect, on the other hand, that the requirement of a 3C fit may lead to a loss of V 0 which make a nuclear scatter before decay. From the known cross sections for A in our sample should scatter before decay. In fact one scatter with a visible recoil has been observed and is included in our sample.

The average geometric detection probabilities for A (88%) and K 0 (93%) have been calculated from the probability for each observed V 0 to decay between 0.4 cm and 50 cm from the production vertex (unless the V 0 could have left the visible volume before going 50 cm). For the ~.0's it is necessary to include also the 7 detection probability (60%) which has been determined from the large sample of lr 0 production events available in the experiment. In addition to correction for these geometric detection efficiencies and a scanning efficiency of 97%, each observed A is multi- plied by a factor 1.56 and each observed K 0 by a factor 2.91 to account for the neutral decay modes and non-observation of K0L .

The A, y0 and K 0 may be trapped inside the nucleus in which they are produced. Monte Carlo estimates show that (4 +- 2)% of the elastically produced A and S 0 hyperons should be trapped in carbon. How- ever, some A hyperons are produced by 2; conversion, and some S 0 appear due to S - conversion. Using the known cross sections for hyperon-nucleon scattering [1], the AI = ½ rule, and the Y~-/A production ratio from the Cabbibo theory, we estimate the net effect of trapping and conversion to be (0 + 4)% for A and E0. No correction has been made for K 0 trapping.

Some A hyperons, particularly of low momentum, give a decay proton or It- of too short range to be measured or detected. This effect has been estimated by Monte Carlo calculations for the elastic channel

Fp ~/I+A. Using a lower momentum cut for the proton of 230 MeV/c and for the 7r- of 50 MeV/c, we estimate a loss of about 8% of the A's in the elastic "channel. Consequently we have applied a further correction factor of (8 + 2)% to the A and 2; 0 sample.

There are sources of.background in the particular channels due to wrong identification of events. The erroneous identification of ~0 from A's produced in association with n o with one visible 3' appears to be a serious background in our E0 events. From the effective mass spectrum of generated A3, events, where all A's have been combined with all 3"'s from the channel u-'N ~/a+Tr0X, we find that (11 +- 3)% of the random A3' combinations could have been classified as ~;0. This gives an estimated background of 0.9 A even: among our 3 observed 2; 0 candidates. On the other hand we estimate from a geometric detection probability of 60% for ~"s that the background of 2;0 events in the elastic A sample is 1.2 events. The inelastic A sample has not been corrected for Z0 background since no inelastic ~0 candidate has been observed.

Due to the small detection probability for K 0, we expect background in the I ASI = 1 channels from associated production events (AS = 0). From the 2 observed CC, AS = 0 events with both A and K 0 decaying via the charged mode, we expect the background in the A sample to be 4 events, of which 2 would be in the elastic channel. Similar background in the K 0, [ASI = 1 sample is expected to be 1 event. In fact one event is observed with K 0 -+ lr+~r - and 2 3"s pointing to a common point in space about 20 cm downstream from the production vertex. This event could thus be interpreted as a K0A event with the A decaying into lr 0 and neutron; the event is included in table 1.

From the observation of one neutral current event where both the A and the K 0 decay by the charged mode, we expect 2 events where the K 0 is either K O or K 0 ~ Ir01r 0. One NC event has been observed with A ~ pTr- and a Dalitz pair and 33"s from the primary vertex. Since the effective mass of (e+e-33,) is found to be 543 +- 54 MeV, we interpret this event as a NC event with a A and a K 0 ~ 7r°rr 0 decaying close to the production vertex; the event is included in table 1.

Results. The observed and corrected number of events in each channel are given in table 1, together with the estimated cross sections * 1. The errors on the

,1 See footnote on next page.

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Table 1 Observed and corrected number of events and cross sections, for different product ion channels. (N-nucleon, X = anything)

Channel Observed Corrected Cross sections events events (X 104o cm2/nucleon)

(a) Fp --, .u+A 24 41.0 -+ 8.6 1.40 -+ 0.41

(b) __., p,+Eo 3 7.3_+ 4.1 0.25 -+ 0.11

(c) u-N~ u+A + X 18 <33.3 _+ 8.1 <0.64 -+ 0.19

(d) ~ u+K ° + X 5 <16.1 -+ 7.3 <0.31 -+ 0.16

(e) --, u+AK° + X (CC) 3 11.9-+8.4 0.23_+0.17

(f) --, FAK ° + X (NC) 2 6.0 _+ 6.0 0.12 _+ 0.12

cross sections include 20% uncertainty in the F flux estimate. Since K + are difficult to detect; the inelastic A channel and K 0 channel may contain background from associated production with K +, and the cross sections for these channels should be regarded as an upper limit.

The cross sections for the elastic channel Fp ~/2+A for three energy intervals are compared in fig. 1 a to predictions of the Cabibbo theory [2]. The curves are calculated*2 for a value of the Cabibbo angle sin 0 c = 0.23, for a ratio o f d to f coupling o f d/(d + f) = 0.658 [3], and for the form factor parametrized as FA, v = (1 + q2[M2,v)-2. Averaged over the b- energy interval 0.5 < E v < 10 GeV we have a(-ffp ~ / I+A) = (1.40 + 0.41) 10 -40 cm 2. This value, with its one standard deviation errors, is displayed in fig. lb together with the theoretical predictions of the Cabibbo theory as a function o f M A. For M V = 840 MeV/c 2, as found in electron scattering, we find that the magni- tude of the observed cross section is too low to allow any substantial axial coupling. Thus the preferred value o f M A is small, i.e. M A < 100 MeV/c 2. For M A --- M V = 840 MeV/c 2 the theoretical cross section is 1.7 standard deviations larger than the empirical value. Note, however, that the determination o f M A from the observed cross section depends crucially on the knowledge of the antineutrino flux. For the case with equal vector and axial vector form factors masses, we obtain a best fit f o r M A = M v = ( 6 9 0 + 110)90 MeV/c2"

,1 For the CC and NC associated product ion events correc- t ions are made to the 2 and 1 events respectively, observed to decay into the charged mode.

,2 For the interference te rm in ref. 12] a plus sign has been used. All terms with m u have been included.

o ;c c )

V3 u3 o , , t j

0 ~ 2 V3 <, Ld

a)

~ ~ - - -- Mv= MA= 1.5

i i

i / /

z ~ MA= 1.5-I

2 4 6 8 10 12 1/, E~ ( GeV )

(9

0 "~4- /Mv=MA 2 z (05 < E~< 10GeV) 7

~ 3 - / , / ' ~ My = 8/,0

. . . . . . . . . . . - ; i . . . . . . . . ]

u I EXP. -- / ' f ~l ........... -I . . . . . . . . . . . .

200 4 0 600 8 0 1000 1200 M A (MeV/c z )

Fig. 1. (a) The points show measured cross sections for the elastic reaction Fp ~ ~+A for three energy intervals 0.5 < E u

< 1.5 GeV, 1.5 < Ev < 3.0 GeV, E v > 3.0 GeV. The curves represent predictions f rom the Cabibbo theory for various values o f the axial and vector form factor masses, M V and MA, in uni ts o f G e V / c 2 . (b) The exper imental cross section (EXP) with its one s tandard deviation limits are displayed for 0.5 < E v < 10 GeV. The curves represent the cross sections expected from the Cabibbo theory as a funct ion o f the axial form factor mass M A.

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. . . . THEORY ( Mf f My= 0.84 GeV/c z)

8 _ ~ EXPERIMENT L~ 3C-PRODUCTION FIT

U.l I I I I - .;

_ 4 ( G e V I c ) 2

Fig. 2. The full drawn histogram shows the measured q2 distribution where the cross-hatched parts represent the 9 events that make a 3C production fit. The dashed histogram illu- strates the distribution expected from the Cabibbo theory for M A = M V = 0.84 C, eV/c 2 using the known antineutrino flux spectrum.

An independent and flux independent estimate of the axial form factor can be obtained from the q2 dis- t r ibution of the A's. Two Maximum Likelihood estimates have been performed: For fixed value of the vector form factor mass, M V = 840 MeV/c 2 we obtain: M A = (930 + 250) MeV/c 2. For M A = M v we obtain M2A M v = (720 -+ 100) MeV/c 2. The experimental q values are displayed in fig. 2, where the shaded portions of the histogram represent the 9 events with a 3C product ion fit, and the dashed histogram shows

the theoretical expectat ions for M A = M V = 840 MeV/c 2. Eichten et al. [4] have studied the same transition form factor in freon, but with smaller statistics.

Their experimental cross section was: o(b'p ~ /a+A) = (1 3+0", 9-) X 10 - 4 0 cm2/proton.

• - 0 7 In t~le same film as the strange particle events we

have also studied the reaction v-p ~ bt+n. Averaged over our ~ s p e c t m m we find the following ratios between strangeness changing and strangeness conserv-

ing elastic reactions:

R (e l a s t i c ) - o(~-p ~ # + A ) _ (3,9 +- 1 •1)%. o(~p ~ u + n)

Theoretical values for R(elastic) can be predicted by the Cabibbo theory. In the limit of exact SU(3) the masses in the axial form factors for ~p -~ ~+A and ~p ~ ~+n can be put equal. For M2V = 0.71 (GeV/c2) 2 the ratio can for E~ ~ co to a good approximation be expressed as [5]

0.9 + 0 . 5 M 2 R(elastic) = ~ tg20c

1 . 7 + 1 . 6 M 2

from which we obtain the theoretical upper and lower limit * 3, for M A = 0 and M A -~ oo respectively, as 2.6% < R(elastic) < 4.5%, which is in good agreement with the experimental value.

The ratio of 2; 0 to A elastic cross sections found from table 1 is 0.18 --- 0.11. From the Cabibbo theory

we expect, for MA(Z ) = MA(A ) = M V = 840 MeV, a ratio of 0•30.

4:3 We have verified that the used asymptotic expression for R(elastic) is a very good approximation even at our antineutrino energies. It may be noted that o(~p --* ~+A) is less sensitive to M A than MV, and opposite for o(~p --, tz+n).

References

[1] O. Benary et al. (Particle Data Group) UCRL-20000YN (1970).

[21 N. Cabibbo and F. Chilton, Phys. Rev. B137 (1965) 1628. [3] M.M. Nagels et al., Nucl. Phys. B109 (1976) 1. [4] T. Eichten et al., Phys. Lett. 40B (1972) 593. [5] C. Jarlskog, private communication. In a forthcoming

final report, containing the full statistics of the experiment, a more detailed discussion of the theoretical as well as the experimental questions will be presented•

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Strange particle production by antineutrinos