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An upper limit to the cross section for the reaction νμe−→νμe− at SPS energies
Volume 84B, number 3
PHYSICS LETTERS
2 July 1979
AN UPPER LIMIT TO THE CROSS SECTION FOR THE REACTION Pue
~ v ue
AT SPS ENERGIES
D. BERTRAND c, 2, G. BERTRAND-COREMANS e, R. BLAES d, 3, F.W. BULLOCK e, M. DEWlT c, a, B. ESCOUBES d, E. FETT b, A. HAATUFT b, A. HALSTEINSLID b, T.W. JONES e, J.G. MORFIN a, K. MYKLEBOST b, J.M. OLSEN b, p. PETITJEAN d, M. POHL a, F. RAHIMI d, P.N. R A T O F F e,4, j . SACTON c, S. DE UNAMUNO d, P. VILAIN c, 2, H. WEERTS a, L.C. WELCH a, s a 111.Physikalisches Institut, R WTH Aachen, Germany b Dept. of Physics, University of Bergen, Bergen, Norway c Inter-University Institute for High Energies, ULB-VUB, Brussels, Belgium d CBLL, Centre de Recherches Nucleaires, Strasbourg, France e University College London, England Received 22 March 1979
Candidates for the purely leptonic process ff~e- ~ flue- have been searched for in the bubble chamber Gargamelle exposed to the CERN-SPS antineutrino wide-band beam. No single e-, of energy greater than 1 GeV, was found in a total of 230000 pictures, corresponding to 7400 charged current events. This leads to an upper limit for the observed cross section of Oobs < 1.6 × 10 -42 (Eff/GeV) cm 2 (90% C.L.). Interpretation of this value in terms of the standard Weinberg-Salam model yields an upper limit to the mixing parameter sin20w < 0.39 at 90% C.L.
Study o f the process vu+e-~Yu+e-
,
(1)
provides one o f the most stringent experimental tests of gauge theories unifying electromagnetic and weak interactions. In contrast to neutral current reactions on nucleon targets, it is a completely calculable process. This enables a direct comparison between theoretical predictions and experiment, without involving models o f hadron structure. Experimental results are" available at antineutrino energies o f a few GeV [1,2] and recently also at higher energies [3,4]. The experiment was performed with the heavy liquid bubble chamber Gargamelle, exposed to the CERN-SPS antineutrino wide-band beam. With 330 GeV incident protons the antineutrino spectrum peaks 1 Aspirant FNRS. 2 Chercheur qualifi~ FNRS. 3 Also at University de Haute Alsace, France. 4 Supported by Science Research Council Studentship. s Now at University of Indiana. 354
at 20 GeV and extends beyond 150 GeV (fig. 1). The chamber was filled with a 90 mol% C3H8/10 mol% CF3Br mixture, with a radiation length o f 61 cm. This ensures a good detection efficiency for electrons, because particles emitted in the forward direction have a mean potential path o f 2.3 m. The total sample o f 230000 pictures was scanned twice for isolated electrons, positrons and gammas occurring in a fiducial volume o f 5.6 m 3. An event was defined as isolated if it had no visible upstream source. The energy o f the particle and its angle with respect to the beam were constrained by: E > 1 GeV; 0 < 3 ° to reduce the background. Electrons and positrons had to show at least two different electromagnetic signatures which could be any combination o f spiralisation, bremsstrahlung, delta rays, trident production and, in addition for positrons, annihilation. Furthermore it was required that no positron (electron) originated within the first 7 cm of projected track length * 1 + t This corresponds to about 20 cm in space.
Volume 84B, number 3
PHYSICS LETTERS
2 July 1979
scattering, asymmetric gamma-rays or Compton electrons and high energy gammas misclassified as electrons, have to be considered. The first process, the reaction (v e + n ~ e - + p) would only be accepted if the proton were not detected. The background expected can be estimated from those events with a visible proton in the final state and the ratio [5]
10-
lO-1
%
'<
v~. 104-
Ve
5
25
45
65
85
105
125
,
145
f
165
E (GeV)
Fig. 1. The relative spectrum of the four components of the CERN-SPS ~ beam.
of the electron (positron) candidate. In the complete sample 0 electrons, 2 positrons and 2 gammas were found satisfying the above criteria. To determine the signal loss introduced by the selection criteria, a sample o f 520 gammas with energy greater than 1 GeV was analysed. For electrons of energy greater than 1 GeV, (74 -+ 7)% had at least two signatures, in agreement with the results of a Monte Carlo simulation. This simulation also indicates that the identification efficiency increases gradually with energy so that this value should be taken as a lower limit in the expected energy range relevant to reaction 1 with the SPS b-spectrum. Electrons can be misclassified as gammas if they have shower development within the first seven centimeters o f projected track length. From the above mentioned gamma sample this loss has been found to be
(5 + 1)%. Because no electrons were found satisfying the kinematical criteria, the scan-efficiency was taken to be 0.89 -+ 0.10, as for isolated gammas with energy greater than 0.5 GeV and no angle cut. Even though no candidate and thus no possible background event was observed, it has to be shown that this fact is not in disagreement with background expectation. Three sources of background, elastic r e - n e u t r o n
No. o f It-(0 < 3 °) + 0p Ru = - = 0.06 -+ 0 . 0 3 . No. o f # - + lp However since no ( e - + 1p) events and only 1 ( e + mp, m > 1) event was found, this background is negligible. The second source of background, asymrrtetric gammas and Compton electrons, was evaluated both theoretically and experimentally. Above 1 GeV the ratio of single electrons from this source to the total number of gammas is less than 1% theoretically. From the measured gamma sample, this ratio was found to be (0.4 -+ 0.4)%. With two isolated gammas found, less than 0.02 events are expected from this background. The last possible background comes from gammas, where the positron and the electron have not separated after 7 cm of projected track length. The percentage of gammas where this occurs has also been determined from the measured sample and found to be (0.3 -+ 0.3)%. The total amount of background from all possible sources is thus compatible with zero events found. To calculate the upper limit for the cross section, the measured muon flux in the beam shielding was used to determine the antineutrino flux. Using OCt = (0.29 -+ 0.02) X 10-38(E~/GeV) cm 2, 7400 + 800 charged current events are predicted in the fidu zial volume. This prediction has been checked by mea3uring a sample o f charged current events in a restricted fiducial volume. The number o f events found (138 -+ 12) agrees with the flux prediction (160 -+ 16). With no electrons found, the upper limit to the observed cross section is then e-
ao~ s < 1.6 X 10 -42 (E~/GeV) cm 2
(90% C.L.).
In the context of the Salam-Weinberg model, the cross section can be expressed as a function of sin20w . Defining a likelihood function depending on sin20w, the detection efficiency and folding in the antineutrino spectrum to account for the 1 GeV cut, the upper limit 355
Volume 84B, number 3
PHYSICS LETTERS
culation and E. Heijne, who supplied the muon data. We have also benefitted greatly from discussions with H. Wachsmuth and J.B.M. Pattison.
on sin20w is sin20w < 0.39
(90% C.L.).
This would correspond to a fully corrected cross section o f o ( ~ ' u e - ) < 2.7 X 10-42(Eff/GeV) cm 2
(90% C.L.).
We wish to thank the SPS and Gargamelle operating staff and the scanning teams in all laboratories, who made this experiment possible. We also gratefully acknowledge the assistance o f E. Fiorini in the flux cal-
356
2 July 1979
References [1] J. Blietschau et al., Nucl. Phys. B114 (1976) 189. [2] H. Faissner et al., Phys. Rev. Lett. 41 (1978) 213. [3] P. Petiau, Proe. Topical Conf. on Neutrino physics at accelerators (Oxford, 1978)p. 275. [4] R.P. Roe, Proc. Topical Conf. on Neutrino physics at accelerators (Oxford, 1978) p. 285. [5] P. Alibran et al., Phys. Lett. 74B (1978) 422.
PHYSICS LETTERS
2 July 1979
AN UPPER LIMIT TO THE CROSS SECTION FOR THE REACTION Pue
~ v ue
AT SPS ENERGIES
D. BERTRAND c, 2, G. BERTRAND-COREMANS e, R. BLAES d, 3, F.W. BULLOCK e, M. DEWlT c, a, B. ESCOUBES d, E. FETT b, A. HAATUFT b, A. HALSTEINSLID b, T.W. JONES e, J.G. MORFIN a, K. MYKLEBOST b, J.M. OLSEN b, p. PETITJEAN d, M. POHL a, F. RAHIMI d, P.N. R A T O F F e,4, j . SACTON c, S. DE UNAMUNO d, P. VILAIN c, 2, H. WEERTS a, L.C. WELCH a, s a 111.Physikalisches Institut, R WTH Aachen, Germany b Dept. of Physics, University of Bergen, Bergen, Norway c Inter-University Institute for High Energies, ULB-VUB, Brussels, Belgium d CBLL, Centre de Recherches Nucleaires, Strasbourg, France e University College London, England Received 22 March 1979
Candidates for the purely leptonic process ff~e- ~ flue- have been searched for in the bubble chamber Gargamelle exposed to the CERN-SPS antineutrino wide-band beam. No single e-, of energy greater than 1 GeV, was found in a total of 230000 pictures, corresponding to 7400 charged current events. This leads to an upper limit for the observed cross section of Oobs < 1.6 × 10 -42 (Eff/GeV) cm 2 (90% C.L.). Interpretation of this value in terms of the standard Weinberg-Salam model yields an upper limit to the mixing parameter sin20w < 0.39 at 90% C.L.
Study o f the process vu+e-~Yu+e-
,
(1)
provides one o f the most stringent experimental tests of gauge theories unifying electromagnetic and weak interactions. In contrast to neutral current reactions on nucleon targets, it is a completely calculable process. This enables a direct comparison between theoretical predictions and experiment, without involving models o f hadron structure. Experimental results are" available at antineutrino energies o f a few GeV [1,2] and recently also at higher energies [3,4]. The experiment was performed with the heavy liquid bubble chamber Gargamelle, exposed to the CERN-SPS antineutrino wide-band beam. With 330 GeV incident protons the antineutrino spectrum peaks 1 Aspirant FNRS. 2 Chercheur qualifi~ FNRS. 3 Also at University de Haute Alsace, France. 4 Supported by Science Research Council Studentship. s Now at University of Indiana. 354
at 20 GeV and extends beyond 150 GeV (fig. 1). The chamber was filled with a 90 mol% C3H8/10 mol% CF3Br mixture, with a radiation length o f 61 cm. This ensures a good detection efficiency for electrons, because particles emitted in the forward direction have a mean potential path o f 2.3 m. The total sample o f 230000 pictures was scanned twice for isolated electrons, positrons and gammas occurring in a fiducial volume o f 5.6 m 3. An event was defined as isolated if it had no visible upstream source. The energy o f the particle and its angle with respect to the beam were constrained by: E > 1 GeV; 0 < 3 ° to reduce the background. Electrons and positrons had to show at least two different electromagnetic signatures which could be any combination o f spiralisation, bremsstrahlung, delta rays, trident production and, in addition for positrons, annihilation. Furthermore it was required that no positron (electron) originated within the first 7 cm of projected track length * 1 + t This corresponds to about 20 cm in space.
Volume 84B, number 3
PHYSICS LETTERS
2 July 1979
scattering, asymmetric gamma-rays or Compton electrons and high energy gammas misclassified as electrons, have to be considered. The first process, the reaction (v e + n ~ e - + p) would only be accepted if the proton were not detected. The background expected can be estimated from those events with a visible proton in the final state and the ratio [5]
10-
lO-1
%
'<
v~. 104-
Ve
5
25
45
65
85
105
125
,
145
f
165
E (GeV)
Fig. 1. The relative spectrum of the four components of the CERN-SPS ~ beam.
of the electron (positron) candidate. In the complete sample 0 electrons, 2 positrons and 2 gammas were found satisfying the above criteria. To determine the signal loss introduced by the selection criteria, a sample o f 520 gammas with energy greater than 1 GeV was analysed. For electrons of energy greater than 1 GeV, (74 -+ 7)% had at least two signatures, in agreement with the results of a Monte Carlo simulation. This simulation also indicates that the identification efficiency increases gradually with energy so that this value should be taken as a lower limit in the expected energy range relevant to reaction 1 with the SPS b-spectrum. Electrons can be misclassified as gammas if they have shower development within the first seven centimeters o f projected track length. From the above mentioned gamma sample this loss has been found to be
(5 + 1)%. Because no electrons were found satisfying the kinematical criteria, the scan-efficiency was taken to be 0.89 -+ 0.10, as for isolated gammas with energy greater than 0.5 GeV and no angle cut. Even though no candidate and thus no possible background event was observed, it has to be shown that this fact is not in disagreement with background expectation. Three sources of background, elastic r e - n e u t r o n
No. o f It-(0 < 3 °) + 0p Ru = - = 0.06 -+ 0 . 0 3 . No. o f # - + lp However since no ( e - + 1p) events and only 1 ( e + mp, m > 1) event was found, this background is negligible. The second source of background, asymrrtetric gammas and Compton electrons, was evaluated both theoretically and experimentally. Above 1 GeV the ratio of single electrons from this source to the total number of gammas is less than 1% theoretically. From the measured gamma sample, this ratio was found to be (0.4 -+ 0.4)%. With two isolated gammas found, less than 0.02 events are expected from this background. The last possible background comes from gammas, where the positron and the electron have not separated after 7 cm of projected track length. The percentage of gammas where this occurs has also been determined from the measured sample and found to be (0.3 -+ 0.3)%. The total amount of background from all possible sources is thus compatible with zero events found. To calculate the upper limit for the cross section, the measured muon flux in the beam shielding was used to determine the antineutrino flux. Using OCt = (0.29 -+ 0.02) X 10-38(E~/GeV) cm 2, 7400 + 800 charged current events are predicted in the fidu zial volume. This prediction has been checked by mea3uring a sample o f charged current events in a restricted fiducial volume. The number o f events found (138 -+ 12) agrees with the flux prediction (160 -+ 16). With no electrons found, the upper limit to the observed cross section is then e-
ao~ s < 1.6 X 10 -42 (E~/GeV) cm 2
(90% C.L.).
In the context of the Salam-Weinberg model, the cross section can be expressed as a function of sin20w . Defining a likelihood function depending on sin20w, the detection efficiency and folding in the antineutrino spectrum to account for the 1 GeV cut, the upper limit 355
Volume 84B, number 3
PHYSICS LETTERS
culation and E. Heijne, who supplied the muon data. We have also benefitted greatly from discussions with H. Wachsmuth and J.B.M. Pattison.
on sin20w is sin20w < 0.39
(90% C.L.).
This would correspond to a fully corrected cross section o f o ( ~ ' u e - ) < 2.7 X 10-42(Eff/GeV) cm 2
(90% C.L.).
We wish to thank the SPS and Gargamelle operating staff and the scanning teams in all laboratories, who made this experiment possible. We also gratefully acknowledge the assistance o f E. Fiorini in the flux cal-
356
2 July 1979
References [1] J. Blietschau et al., Nucl. Phys. B114 (1976) 189. [2] H. Faissner et al., Phys. Rev. Lett. 41 (1978) 213. [3] P. Petiau, Proe. Topical Conf. on Neutrino physics at accelerators (Oxford, 1978)p. 275. [4] R.P. Roe, Proc. Topical Conf. on Neutrino physics at accelerators (Oxford, 1978) p. 285. [5] P. Alibran et al., Phys. Lett. 74B (1978) 422.