Search for prompt neutrinos in 70 GeV pN collisions

Volume 79B, number 4, 5 PHYSICS LETTERS 4 December 1978

SEARCH FOR PROMPT NEUTRINOS IN 70 GeV pN COLLISIONS

A.E. AS RATYAN a, V.Sh. EPSTEIN a, R.M. FAKHRUTDINOV b, V.N. GORYACHEV b, E.A. GR1GORIEV a, N.V. KALGANOV a, V.S. KAFTANOV a, V.D. KHOVANSKY a, S.A. KNYAZEV h, V.I. KOCHETKOV b, Yu.G. KORNELYUK a, M.A. KUBANTSEV a, A.V. KULIKOV b, V.I. KURBAKOV b, I.P. MAKSIMOV a, A.I. MUKHIN b, V.F. PERELYGIN b V.G. PLATONOV b, R.A. RZAEV, A.N. ROSANOV a, M.S. RYABININ a, M.M. SAVITSKY a, A.V. SAMOILOV b, B.M. SEREZHIN a, V.V. SHAMANOV a, V.G. SHEVCHENKO a, V.A. SMOTRYAEV a, Yu.M. SVIRIDOV b, I.S. TROSTIN a, A.A. VOLKOV b, A.S. VOVENKO b, V.A. YARBA b, A.A. ZAYTZEV a, S.A. ZELDOVICH a, S.A. ZHEMANOV a, V.P. ZHIGUNOV b, Yu.A. ZUDIN b a Institute o f Theoretical and Experimental Physics, Moscow, USSR b Institute .for High Energy Physics, Serpukhov, USSR

Received 3 July 1978

Results of a search for prompt neutr inos in a beam-dump exper iment with 70 GeV protons are reported. The observed rate is Upr/n = (2.7 -+ 2.2) X 10 -s for mean neutr ino energy (Ep) ,m 5 GeV. An est imation on charmed meson product ion as

a possible source of prompt neutrinos is given. Production rate of the axion relative to rr ° mesons is found to be less than 6 X 10 -9 (95%C.L.).

Prompt leptons in hadron collision are usually associated with production of short-lived particles. A number of experimental results exist on charged lepton production [ 1 ] but very frew are known on prompt neutrinos [21. The advantages of a search for leptonic decays of new particles by neutrino detection were discussed in detail by Pontecorvo [3].

Preliminary results of the present beam-dump experiment were reported at the Neutrino-77 conference [4]. When the final analysis of the data was completed,

results of three CERN beam-dump experiments at 400 GeV had been published [5].

The present experiment [4] has been performed with a 70 GeV proton beam dumped onto an iron target inserted between a vacuum decay tube and an iron muon shielding (fig. 1). Three targets made up of steel plates 1 cm thick were used with average density P/PFe = +1, 1/2, 1/3.

All measurements were done with a fast ejected 70 GeV proton beam with average intensity 3 X 1012p/p.

M U O N S H I E L D N E U T R I N O D E T E C T O R

3 8 0 rn , 6 0 rn ~ _ 2 0 rn

~ " ~ BEAM - DU M P ~/~///////'///////~ - 'I T ~ f l I I I I / I I I I I I I I I I I I A

A C C E L E R A T O R

Fig. 1. The layout of the beam-dump experiment.

l 497


T A R G ET - C A L O R I M E T E R M u O N - S P E C T R O M E TER

~'-2.

11 identical modules (each 4chamberslcounter) 6 ma

i 2 B 4__5 6 7___8 9 I0 11

KHHI SC Lou~nters

6 m \ Spark chambers 12 cm AI

nets 36 cm Fe each

!i'l A 4 m

'I i / AI 2.Bcm Spark chambers

Fig. 2. Spark chamber neutr ino detector.

/

Ma( nets

Proton intensity was measured by current transformers with 1-2% accuracy. Total proton flux on targets was 7.4 X 1017 p. Intensity and profile of the muon flux in the shielding were permanently monitored.

The spark chamber neutrino detector placed 80 m downstream the target, consisted of a production part and muon spectrometer with magnetized iron (fig. 2). The production part contained 44 optical spark chambers 5 m 2 each, alternated with 28 mm AL filters (total weight 20 tons) and 11 layers of 4 m 2 planes of scintillation counters placed after every four chambers.

Spark chambers were triggered by triple coincidence of any three adjacent layers of counters (Ci, Ci+I, Ci+2) , in coincidence with the proton beam. Due to 3/~s spill time of the proton beam the detector was practically insensitive to cosmic rays.

Calibrations with pions and electrons showed that electromagnetic showers with energy deposition above 1 GeV were easily separated from hadrons. Muons with E u ~> 1 GeV were reliably identified in the mag- netic spectrometer.

We have found 261 neutral-induced events in the production part with vertices uniformly distributed throughout the fiducial volume of 10 tons. This number corresponds to the expected number of 220 ± 44 neutrino events predicted from the production experiment [6] and muon fluxes in the shielding.

Two groups of events (called uu-sample and u e- sample) were selected for further analysis:

(i) vu-sample of 84 events with a track passing through 72 cm of iron in the magnet (E u ~> 1 GeV);

(ii) Ue-sample of 22 events which satisfied the fol-

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lowing criteria: presence of e.m. shower with energy deposition

~> 1 GeV starting directly from the vertex; no track of penetration length/> 72 cm in iron

recorded in the magnet. Note that the uu-sample is rather clean but the %-

sample contains background of n0's produced by neutral currents or charged currents where muons missed the spectrometer.

Extrapolation to an infinite-density target is a straightforward way to extract the signal of prompt neutrinos. Corresponding event rates for three target densities are presented in table 1. The u u- and %- samples must be corrected for detection efficiency to obtain the total numbers of uuN and ueN charged current interactions. Calculated correction factors are 1.1 and 2.4 for the u e- and uu_ samples, respectively.

The extrapolation procedure (see fig. 3) gives the following numbers of prompt neutrino events: 69 -+ 56u u and 15+I 0 for v e (normalized to the total proton flux 7.4 × 1017). Errors quoted are purely statistical.

Another approach to obtain the prompt neutrino signal is to subtract the background from the observed numbers of events. Search for v e-events has an appa- rent advantage, as pion and kaon decays produce predominantly muonic neutrinos. Therefore further analysis will be made only for the re-sample.

The two main sources of background are: (i) re-events produced by conventional sources

(K, A, Z - decays); N i = 9.0 + 2.0, this number is based on K-production data at 70 GeV [6] ;

(ii) vu-events without identified muon and with a shower from rr 0 attached to the vertex.


Table 1

Event distributions Total (Eu> a) (GeV)

PFe/P 1.0 2.0 3.0 -- Proton flux 4.97 × 1017 1.90 × 1017 0.54 X 1017 7.4 X 10 Iv -

Total number of events 130 100 31 261 - l#-events 43 34 7 84 6 +- 1 1e-candidates 13 6 2 21 5 +- 1

a) The energy measurement is based on calibrations in pion and electron beams.

To estimate this background we have selected events similar to those of the Ue-sample but with a gap ( 1 - 5 spark chambers) between the vertex and the e.m. shower (further called 7r 0 events). We have found 10 such events. This corresponds to the expected 7r 0 background of 6 -+ 3 events in our Pe-sample. This background can also be estimated in a different way, i.e. from the number of events with an identified muon and a shower attached directly to the primary

30

20,

IO

"6

E 400 z

300

I I I

200

100

[

Ve samp le I i

a

40

I

b

I i

PFe / Ptarg

Fig. 3. Extrapolation procedure to infinite density of the beam- dump target. All points are normalized to the total proton flux 7.4 X 1017. Corrections for detection efficiencies are made,

vertex. Since 6 such events were observed, the estimated rr 0 background in the Ue-sample is 12 -+ 5 events. Both methods of 7r 0 background evaluation are statistically independent and as an average one obtains Nii = 8-+2.5.

Subtracting the background from both sources, we get Npr = 6 -+ 6 as prompt Ue-signal.

Ambiguities associated with conversion length deter- ruination for events with large hadron mult ipl ici ty lead to systematic uncertainties comparable with statistical errors. A quite different approach is to analyse distri-

5

7

6

5

4

3

2

"8 0 8 b

7 10 events z

6

0 O. 2 0.4 0.6 0.8 1.0 y = EvL~ - Esh

Evis

Fig. 4. Yvis distributions for two classes of events: (a) candidates for %-interactions (re-sample); (b) events with visible distance from tile vertex to the point of gamma conversion (rr ° sample).

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Volume 79B, number 4, 5 PItYSICS LETTERS 4 December 1978

butions of events in the variable

Yvis = ( E v i s - Esh)/Evis"

Evi s is the visible deposited energy, Esh is the energy of an electromagnetic shower.

The distributions are essentially different for v e- and n0-samples (see fig. 4). The cutoff Yvis < 0.4 re- duces the Ue-sample to 14 events with a small background from neutral pions. The calculated background in the same Yvis range from K, A, 2 is 6 + 2 events. Therefore the prompt Ue-signal is 8 .+ 5 events, which should be multiplied by 1.4 + 0.4 to obtain the total signal equal to 1 1 + 8. All the estimates on the prompt neutrino yield are in agreement with each other and Npr = 1 1 -+ 9 was taken as a final number.

Using OvN = 0.74 X 10 38Evcm2, R = ~N/OvN = 0.42 [7] and a mean neutrino energy (Eve) = 5 GeV, we obtain that the prompt neutrino flux into the solid angle of our detector (4.5 X 10 . 4 st) is

(Pe + ~e)pr = (2.7 -+2.2) X 10 -6 ,

per incident proton + 1 This result can also be presented as the ratio of

neutrino to pion production rates in p ro ton-nuc le i collisions (u e + Fe)/(n + + n ). Pion yields within the solid angle of the detector were calculated from the production experiment at the same energy [6], The ratio is

(Ue+ ~e)/(n++ rr ) = (2.7 -+ 2.2) X 10 -5 .

The estimate of the D[) production cross section in p ro ton-nuc leon interactions obtained on the basis of the charmed nreson production model [8] is

o . B =(1.9 + 1.5) X 10-30cm2 ,

assuming that DE) are produced - A 2/3 and

o . B = (0.5 -+0.4) X 10-30cm2 ,

,1 We assume that the prompt neutrino and antineutrino spectra are identical.

for linear A dependence, B is the branching ratio to electron decay modes.

The existence of a light pseudoscalar meson (axion) has recently been suggested by Weinberg and Wilczek [9,10]. Its interactions in our detector would have led to muonless events looking like neutron stars.

The comparison of charged current (N cc = 84 X 2.4 = 202 u~-interactions, N cc = 9 re-interactions ) and neutral current (N Nc = N cc X 0.25 = 53) neutrino- induced events with a total number of 261 observed events leaves us with - 3 -+ 23 events which can be at- tributed to axion interactions. Under the assumption that pion and axion production cross sections in pN- interactions are in the same ratio as their interaction cross sections, the limit can be obtained: (axion yield)/ (n O yield) < 6 X 10 -9 (95% C.L.), or the ratio of the observed rates to the theoretically predicted [91 ones

of Oprod X Oin t is less than 3 X 10 - 2 .

The authors are indebted to the accelerator staff for the i r cooperation in running the experiment and to the scanning personnel of both laboratories for their efforts.

References

[1] L.M. Lederman, Phys. Rep. 26 (1976) 149; B.A. Dolgoshein, Proc. Intern. Conf. Neutrino-77, Vol. 1, p. 281.

[2] H. Fraissner et al., Phys. Lett. 60B (1976) 401. [3] B. Pontecorvo, JETP 69 (1975) 452. [4] A.E. Asratyan et al., Proc. Intern. Conf. Neutrino-77,

Vol. 1, p. 299. [5] P. Alibran et al., Phys. Lett. 74B (1978) 134;

T. Hansl et al., Phys. Lett. 74B (1978) 139; P.C. Bosetti et al., Phys. Lett. 74B (1978) 143.

[6] C.W. Akerlof et al., preprint IHEP 77-86 (1977). [7] A.E. Asratyan et al., Proc. Intern. Conf. Neutrino-77,

Vol. 2, p. 52. [81 M. Bourquin and J.M. Galliard, Nucl. Phys. Bl14 (1976)

334. [9] S. Weinberg, Phys. Rev. Lett. 40 (1978) 223.

[10] F. Wilczek, Phys. Rev. Lett. 40 (1978) 279.

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Search for prompt neutrinos in 70 GeV pN collisions