Trimuon events observed in high-energy antineutrino interactions

Volume 85B, number 1

PHYSICS LETTERS

30 July 1979

TRIMUON EVENTS OBSERVED IN HIGH-ENERGY ANTINEUTRINO INTERACTIONS J.G.H. de GROOT, T. HANSL, M. HOLDER, J. KNOBLOCH, J. MAY, H.P. PAAR, P. PALAZZI, A. PARA, F. RANJARD, D. SCHLATTER, J. STEINBERGER, H. SUTER, W. von RI]DEN, H. WAHL, S. WHITAKER and E.G.H. WILLIAMS CERN, Geneva, Switzerland F. EISELE, K. KLEINKNECHT, H. LIERL, G. SPAHN and H.J. WILLUTZKI lnstitut fftr Physik I der Universitat, Dortmund, Germany W. DORTH, F. DYDAK, C. GEWENIGER, V. HEPP, K. TITTEL and J. WOTSCHACK lnstitut fur Hochenergiephysik I der Universitat, Heidelberg, Germany P. BLOCH, B. DEVAUX, S. LOUCATOS, J. MAILLARD, J.P. MERLO, B. PEYAUD, J. RANDER, A. SAVOY-NAVARRO and R. TURLAY D.Ph.P.E., CEN-Saclay, France and F.L. NAVARRIA Istituto di Fisica dell'Universitft, Bologna, Italy Received 30 April 1979 Eight #+#+#- events have been observed in the CDHS detector during the 330 GeV and 350 GeV antineutrino wideband beam exposures at CERN. The corresponding average trimuon rate relative to the single-muon rate is (1.8 -+ 0.6) × 10-s for visible energy ~ 30 GeV and muon momenta ~ 4.5 GeV/c. Some characteristics of these antineutrlno trimuon events are compared with t~-#-# ÷ events produced by neutrinos.

The origin of neutrino-induced trimuon events has been recently clarified [ 1 - 3 ] . The bulk of the events has been attributed to charged current interactions which are accompanied by a muon pair of either hadronic or internal bremsstrahlung origin. Possible new processes such as heavy lepton production and decay, or heavy quark cascade are not observed to contribute more than 10% of the measured rate. The comparison of antineutrino and neutrino production rates and characteristics might be expected to further clarify the question of the origin of tri1 Supported by Bundesministerium fur Forschung und Technologic.

muons particularly with respect to new production processes. However, up to now only one neutrino event, and this with unusual charges (+ - - ) , has been reported from a narrow-band beam exposure at CERN [4]. In this letter we report on the observation of additional eight trimuon events obtained with a wideband antineutrino beam at the CERN SPS. The events were observed with the CDHS detector, previously described [5]. The antineutrmo beam was produced with incident proton energies of 330 GeV and 350 GeV. The total intensities on target were respectively 1.25 × 1018 and 0.28 X 1018 protons. The horn and reflector currents were optimized such that the peak antineutrino flux was around 20 GeV. At high energies 131

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such a beam has a large contamination o f neutrinos. representative neutrino and ant]neutrino event spectra for a subsample of charged current events are shown in fig. 1 ; clearly the neutrino contamination becomes predominant above 100 GeV. The trimuon candidates were selected from the total sample of events by software, demanding three or more hits in at least two of the three planes o f four or more adjacent drift cha/nbers. This requirement has the consequence that each muon must have a minimum energy o f 4.5 GeV. The trimuon candidates were then scanned and reconstructed with an interactive program. Only events which occur in the first fourteen modules and within a radius of 1.6 m with respect to the beam axis have been retained. The total visible energy for each event was required to be at least 30 GeV. Our sample which survived these cuts then consists of 8/a+/a+/a- events and l 0 / a - / a - / a + events. In presenting the characteristics o f the observed

~WBB 1oo(:

CC

~+

CC

~/~

>

Q

100

C 0

k~ L..~ t-- I

I..1 i L- 3 10 i

k~

0

50

100

150

200

250

300

Evis (GeV)

Fig. 1. Antineutrino and neutrino charged current spectra obtained by the study of a subsample of charged current events in the wide-band beam antineutrmo exposure at 330 GeV. 132

30 July 1979

t n m u o n events It is useful to briefly review the variables used m our analysis; a detailed discussion ]s given in ref. [11 ]. In the present case the definitions refer to the/a+p+/a - events. The "leading m u o n " / a l is defined as that positive muon which minimizes the sum o f the transverse momenta o f the other two muons relative to the W boson axis. The/a2 IS the second positive muon and the/a3 is the negative muon. Their energies are respectively EUl , E u2, ~#3" The visible energy bwl s for each event ]s the sum Eul + Eu2 + E•3 + Eh, where E h is the hadromc energy measured m the calorimeter. The melastlc]ty is calculated a s y = Ehad/ Ev]s, where Eha d = E h +Eu2 + E u 3 . For each event there are three possible dimuon combinations/aflat for which we call the invariant masses rnt/and the azimuthal angle m the plane perpendicular to the neutrino direction ~b~/..Furthermore, the angle ~bl.23 is the azimuthal angle between the momentum o f the leading muon and the sum o f the momenta o f the two others as measured in a plane perpendicular to the neutrino direction. We attribute the bulk of the/a-/a-/a+ events to the neutrino contamination m the beam: their rate relative to the observed single/a- rate ]s (4.1 + 1.3) X l 0 - 5 which is in good agreement with the rate measured in ref. [1 ]. However, the neutrino contamination Is insufficient to account for the/a+/a+/a- events. The largest neutrino-reduced background for/a+/a+/a- events are dimuon events with an additional 7r, K decay, as shown in table 1. We expect 0.3/a+/a+/a- events m our sample from this source. Other sources of background such as charged current events with two successive rr or K decays or spatial overlay o f a dlmuon event with a charged current are found to be negligible. Possible direct production by neutrinos of/a+/a+/a- events can be estimated from the four/a+/a+/a- events observed with our sample of 76/a-/a-/a+ events described in ref. [1 ]. The/a+V+/a- events were in agreement with calculated antineutrmo background; we could assume, however, that they represent an exotic neutrino process, then we should expect that ~0.5/a+/a+/a- events m our present sample could be o f such a neutrino origin. Thus we have finally eight/a+/a÷/a- events which we believe are to be attributed to antineutrino interactions. After the background has been subtracted, the average rate o f the ant]neutrino trimuons (+ + - ) compared to the charged current/a÷ rate is (1.8 + 0.6) × 10 - 5 . These charged current events have an average

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PHYSICS LETTERS

Table 1 Calculated backgrounds from lr and K decays. Observed events

Expected background events dimuon+t~-

8++10 - + -

~dimuon+t~ ÷

vdlmuon+t~-

0.15 0.2

0.3 0.2

visible energy o f 57 GeV. As seen in n e u t r i n o t r l m u o n p r o d u c t i o n [2], the observed t r i m u o n rate is very energy d e p e n d e n t . Th~s is chiefly due to the effect o f the mini m u m m o m e n t u m r e q u i r e m e n t o f 4.5 G e V / c for each m u o n . Even with low + + - event numbers it is interesting to give the rate for Evl s > 100 GeV: we find three events above I 0 0 G e V , gwing a 3/a/1/a rate o f (0.9 -+ 0.5) × 10 - 4 . The 3/~/1/~ rate for different samples are summarized in table 2 for Evi s > 30 GeV and Evi s > 100 GeV. It is interesting to n o t e that if we take the definition o f leading m u o n given earlier and relax the charge constraint, all o f the eight ~u+/a+/a- events have a leading m u o n with a positive charge. (This same classification for the ~ - / a - / l + events yields nine o f the ten as a negatwe leading m u o n in good agreement w i t h an assignm e n t o f t h e s e / 1 - / ~ - / ~ + events to n e u t r i n o background.) F u r t h e r m o r e , the agreement b e t w e e n charge and transverse m o m e n t u m selection o f the leading m u o n favors antineutrino t r i m u o n p r o d u c t i o n mechanisms similar to that for neutrinos; heavy l e p t o n cascades for example do not exhibit such a charge agreement (e.g. L ÷, L 0 masses o f 7.0 and 3.5 GeV w o u l d give ~ 3 0 % negative leading m u o n s w i t h our selection m e t h o d ) . A c o m p a r i s o n with refs. [1] and [2] shows that the antineutrino 3/a/1/~ ratios are comparable to the corresponding ratios for neutrinos. With so few events it is Table 2 Relative rates of trimuon productions. 3t~/l~ rate

V÷V÷V-//a÷ this exper/~-~-~÷/~- lment ~t-~-/J÷/~t- [1,2] ta-Ja-~÷//a- [31 a) a) All E v.

vdlmuon+~+

-

Table 3 Average values for u+tz+v- and u-~t-v + events. This experiment ++--

Evis (GeV) Eha d (geV) P1 (GeV/c) P2 (GeV/c) Pa (GeV/c) m12 (GeV) mla(GeV) m23 (GeV) m123 (GeV) 01.2 (deg) ~1.3 (deg) ~1.23 (deg) PTS2 (GeV/c) PTS3 (GeV/c)

PS23(GeV/c)

Evis > 30 GeV

Evls > 100 GeV

(1.8 (4.1 (3.0 (6

(0.9 (1.1 (1.1 (1.2

± 0.6) X 10 -s ± 1.3) X 10 -s ± 0.4) X 10 -s ± 2) X 10 -s

difficult to compare distributions m detail. We report in table 3 the average values o f the more interesting variables o f the data where we include the n e u t r i n o - + events. The general behaviour o f antineutrinoand neutrino-induced t r i m u o n events are quite similar. One m a y notice, h o w e v e r , t w o differences: the average x = 0.15 for a n t i n e u t r m o events c o m p a r e d with 0.21 for n e u t r i n o evetns, and, respectively, the average q)1.23 = 81 o _+20 ° c o m p a r e d w i t h 138 ° -+ 7 ° (note that the observed difference in the effective mass m# is correlated with the ¢#). Concerning the distributions o f ten p - p - g + events,

± 0.5) X 10 --4 ± 0.4) X 10 -4 ± 0.25) X 10-4 ± 0.5) X 10-4

x y

Ref. [ 1 ] -- --+

90 ± 10 110 41 ± 10 44 25 ± 7 35 9.5 ± 1 14.5 12 ± 2 16 1.35± 0.15 1.9 1.5 ± 0.24 2.6 1.0 ± 0.15 0.83 2.2 ± 0.28 3.4 82 ± 20 90 63 ± 20 98 81 ± 20 96 0.81 ± 0.16 0.7 1.05 ± 0.19 0.91 1.552 0.25 1.4 0.15 ± 0.03 0.21 0.75 ± 0.06 0.69

----+

± 10 95 ± 8 37 ± 7 29 ± 3 13 ± 2.5 15 ± 0.33 2.3 ± 0.46 2.4 ± 0.17 0.63 ± 0.6 3.5 ± 20 136 ± 20 128 ± 20 138 ± 0.14 0.53 ± 0.21 0.59 ± 0.4 1.1 ± 0.07 0.21 ± 0.05 0.69

Number of events

8

10

76

Average Evls for CC events (GeV)

57

78

70

±5 ±3 ±3 ± 1.4 ± 1.5 ±0.14 ±0.18 ± 0.04 ±0.2 ±6 ±7 ±7 ± 0.06 ± 0.06 ±0.2 ± 0.02 ± 0.02

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Table 4 Calculated contributions of different processes for trlmuon events. Expected 3#/1# rate ++--

--

vbeam

vm~beam

vbeama)

0.8 × 10 -s 2.4 × 10 -s

0.8 × 10-s 2.2 × 10 -s

electromagnetic 0.5 × 10-s hadronlc 0.4 × 10-s

- - +

--

--+

a) v beam for runs refs. [1 ] and [2]. we should not expect sigmficant differences between them and the neutrino trimuon events in ref. [1 ]. This is indeed the case except for a lower value of <¢>, which may be due to a statistical fluctuation. The most probable explanation o f differences for the antlneutrino trimuons, assuming that these tnmuon events are also produced by normal charged currents with muon pair production from hadromc or electromagnetic ongin, is that the different average incident energy and kinematical conditions 0.e. v and ~y-dlstributions) change the relative contribution o f the hadromc and electromagnetic processes. This effect can be seen in table 4 where the predicted values ~1 for these two processes for the three different beam configurations are given. (Identical cuts Evi s > 30 GeV and Pu > 4.5 GeV/e, as required for our data have been used in these calculations.) According to these calculations, the expected contributions o f electromagnetic and hadronic p-pair production are approximately equal for antineutrino-induced trimuon events. Thus the average azimuthal angle is expected .1 Values calculated by J. Smith (Theory Division, CERN), to be published.

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[6] to be (q~.23) = 110 °, which is smaller than the corresponding value for neutrino-Induced trimuons (<~.23) = 138 ° [ 1 ] ) a n d which may be reflected In our lower value for the + + - events. Furthermore, the for the antineutrino-mduced trtmuons is calculated to be (x) = 0.14 [6]. Such a shift m (x) can be understood from the fact that these events are at h i g h y , (y) = 0.75, and therefore one expects that the antineutrino is scattered predominantly from sea antiquarks while the neutrmo scatters on valence quarks. An ad&tlonal shift o f x is expected from those electromagnetic processes where the/a pair lS ra&ated from the outgoing leading muons/21. Here the observed scahng variable,

x = 4EvlsEul sln2(O/2)/2MEha d , does not include the energy of the radiated pair which should be added to Eul. Again the lower (x) for the + + - events in our sample is not inconsistent with this wew. In conclusion, the observed/l+/~+# - events are consistent with an antlneutrmo origin. Furthermore, the data seem to be sufficiently explained by the same processes used to describe neutrino t n m u o n production. This agreement, however, should be viewed withm the limited statistics o f our event sample.

References [1] T. Hansl et al., Nucl. Phys. B142 (1978) 381. [2] T. Hansl et al., Phys. Lett. 77B (1978) 114. [3] A.K. Mann, Talk presented at the XIXth Intern. Conf. on High energy physics (Tokyo, Japan, August 23-30, 1978). [4] M. Holder et at., Phys. Lett. 70B (1977) 393. [5] M. Holder et al., Nucl. lnstrum. Methods 148 (1978) 235. [6] J. Smith, Phys. Lett. 85B (1979) 124.