Volume 49B, number 5 PHYSICS LETTERS 13 May 1974

L O W E R L I M I T F O R H E A V Y M U O N MASS O B T A I N E D F R O M T H E D A T A

O F T H E C E R N N E U T R I N O E X P E R I M E N T

v

A.E. ASRATYAN, S.S. GERSHTEIN, V.S. KAFTANOV, M.A. KUBANTZEV, V.V. LAPIN, V.N. FOLOMESHKIN and D.N. KHOVANSKY

Institute for High Energy Physics, Serpukhov, USSR

Received 17 April 1974

A lower limit for the heavy muon (heavy lepton/~'- which can have the same lepton number as t~- and v~) mass M., > 1.8 GeV at 90% confidence level is obtained with the help of the bubble chamber "Gargamelle" data in the C~RN neutrino experiment. This limit should not be confused with the known limit for an M + (heavy lepton M ÷ which can have the same lepton number as ~- and v~).

The data of the recent neutrino experiments at CERN [1, 2] and Batavia [3] (in particular, the observation of neutral currents) allow to make further steps in search for heavy leptons. The proper- ties of heavy leptons have been discussed many times~. Till recently the lower limit for mass of heavy lepton which might be produced in neutrino interactions in the reaction

v n -~ la ' -p (1)

wasM ~ 1 GeV/c 2 [7] (CERN data of 1963-64) ~. A similar value for the lower limit was obtained from the e+e - colliding beam data [8] specifically for lepton ~ who possesses its own neutrino v x.

If heavy leptons do exist, the reaction of the type - ~ f - - r +

v ( ~ ) + Y /a ( / a ) +hadrons (2)

with further decays

U '-+ -~ v~, ( ~ ) + hadrons, (3)

/.t '± ~ v~ ('flu) + e ± + ve (re)' (4)

u - , + u ± + ( v ) . (5)

We have calculated the dependence of the number of production events upon its mass under conditions of the CERN neutrino experiment. The total cross

sections for heavy muon production in reactions (2) were calculated with the parton model in accordance with [6]. They should be considered as a lower limit, as this model, valid within the limits of large q2 and v, understate the cross sections in the near-threshold region, where the contribution of two-body processes are of importance. The estimations show that account of elastic channel (1) only increases the number of the events for the CERN neutrino spectrum by (70-100)% (see the curves in fig. 1). Relative probabilities for decays (3) and (4) were taken to be equal to 60% and 20% correspondingly in the mass region 1.5-2 GeV. Experimental selection criteria introduced in [ 1,2] (Ehadrons > 1 GeV for decay mode (3) and Evia~ole > 1 GeV for mode (4))# were adopted in the calcula- tions, and the scanning efficiency was taken to be equal to 100%.

From the comparison with the experimental data it follows that if one tries to treat neutral current effect completely as due to/a' production, its mass should be equal t o M = 1.35 + 0.05 GeV. However, this result is known to be incompatible with the number of the electron events.

In estimating heavy muon mass from the electron (positron) events two procedures were used. In the first we assume that excess over unity of the

* References are to be looked for in [4 - 6]. * All the limits are at 90% confidence level.

# Unlike the usual neutr ino reactions (with charged as well as with neutral currents) the influence o f these l imitat ions is restricted: less than 10% for hadronic decay mode and negligible for the electron mode.

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Volume 49B, number 5 PHYSICS LETTERS 13 May 1974

m'

m'

P'" 7 "~ +l~drens

NC ew.ts \ \ \ ~ , \ f without bac~grounct \ \ \ \ ~

\ \

\

MIi-[.GeV/&]

Fig. 1. Search for heavy muon in "Gargamelle" by hadronic decay mode. The broken line has been calculated with the parton model. The solid line takes into account the elastic channel contribution.

experimental ratio [2]

o (v e N -+ e + hadrons) = 1.26 + 0.23 (6)

o (v u N-+/a + hadrons)

is determined by the heavy muon production effect. In the second one we used the data on absence of

positron production, accompanied by protons only, in the antineutrino beam [2]. In such analysis the calculations possess less theoretical uncertainties as only an elastic reaction

p-" u'* n (7) is taken into account. Both procedures gave identical results M > 1.8 GeV with 90% confidence level (see fig. 2).

From the comparison of the calculations and experimental data one may conclude that there is no possibility to explain the whole observed neutral

current effect as owing to heavy muon production. The limit M > 1.8 GeV permits a 30% contr ibut ion

to the neutral current events owing to the possible

production of heavy muons.

v) z

2

0,+ N -,-I d" +X \\ ~ P " " e" + roe+ ~)~

\ \ ~ zsoooo picturts

\ ' , , \ ~f

f¢ 0 4 2

M~' [GeV/dJ

Fig. 2. Search for heavy muon in "Gargamelle" by the electron decay mode. For neutrino the broken line has been calculated with the patton model. Curve 1 takes into account the elastic channel contribution. For antineutrino curve 2 has been calculated for an elastic channel only (see the text).

-

I " ;- '~ ?~.v) Fig. 3. Relative probabilities of heavy muon decay along the different hadronic channels. It was assumed that the con- tribution Jf the axial part of the weak interaction was equal to the vector contribution.

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Volume 49B, number 5 PHYSICS LETTERS 13 May 1974

We based our analysis on the results of the CERN experiment only as the first Batavia measurements [3] are of very preliminary character. Carrying out of such experiments in Batavia are of particular interest. If heavy muon mass - 2 GeV, a sharp increase of muonless events contribution should take place. It should be noted that in the region of large masses search for heavy leptons with the electron decay mode may present some difficulties as from the recent ex- periments with colliding e+e- beams [lo] it follows that relative probability of heavy muon decay into hadrons may considerably increase (see fig. 3). In this case one can reveal heavy muon production in- vestigating, for example, the dependence of the number of muonless events on neutrino energy, and the energy distribution of the secondary hadrons.

References

[l] F.J. Hasert, S. Kale and W. Krenz, et al., Phys. Lett. B46 (1973) 138.

[2] T. Eichten, H. Deden and F.J. Hasert, et al., Phys. Lett. B46 (1973) 281.

[3] A. Benvenuti, D.C. Cheng and D. Cline, et al., Phys. Rev. Lett. 32 (1973).

[4] M. Perl, preprint SLAC-PUB-1062, 1972. [5] S.S. Gerstein, L.G. Lansberg, and V.N. Folomeshkin,

preprint IHEP 72-l 15, 1972. [6] J.D. Bjorken and C.H. Llewellyn Smith, Phys. Rev. D7

(1973) 887. [ 71 S.S. Gerstein and V.N. Folomeshkin, Yadernaja Fiz., 8

(1968) 768. [ 81 M. Bernardini, D. Bollini and P.L. Brumini, et al.,

Nuovo Cim., 17A (1973) 383. [9] Y.S. Tsai, Phys. Rev. D4 (1971) 2821.

[_I01 G. Cosme, B. Jean-Marie and S. Jullian, et al., Phys. Lett. B40 (1972) 68; M. Grili, E. Larocci and P. Spillantini, et al., Nuovo Cim., Al3 (1973) 593; A. Litke, G. Hanson and A. Hoffman, et al., Phys. Rev. Lett. 30 (1973) 1189.

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