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Investigation of like-sign dimuon production in neutrino and antineutrino reactions
Volume 86B, number 1
PHYSICS LETTERS
10 September 1979
INVESTIGATION OF LIKE-SIGN DIMUON PRODUCTION IN NEUTRINO AND ANTINEUTRINO REACTIONS J.G.H. de GROOT, T. HANSL, M. HOLDER, J. KNOBLOCH, J. MAY, P. PALAZZI, A. PARA, F. RANJARD, A. SAVOY-NAVARRO, D. SCHLATTER, J. STEINBERGER, W. von RUDEN and H. WAHL CERN, Geneva, Switzerland
F. EISELE, K. KLEINKNECHT, H. LIERL and H.J. WILLUTZKI lnstitut fiir Physik 1 der Universitiit, Dortmund, Germany
F. DYDAK, C. GEWENIGER, V. HEPP, K. TITTEL and J. WOTSCHACK Institut far Hochenergiephysik 1 der UniversitiitHeidelberg, Germany
P. BLOCH, B. DEVAUX, S. LOUCATOS, J £ . MERLO, B. PEYAUD, J. RANDER and R. TURLAY D.Ph.P.E., CEN-Saclay, France
F.L. NAVARRIA Istituto di Fisica dell'UniversitY, Bologna, Italy
Received 1 June 1979
290 events of the type vFe ~ ta-ta-X and 53 events from the reaction u--Fe--} Ia+z+Xwith Ev > 30 GeV and muon momenta p~ > 6.5 GeV/c have been observed in the CDHS detector. After subtracting the background from charged-current processes with one n or K meson of the hadronic shower decaying into t~-F (or t~+v),we obtain for neutrinos a rate of prompt like-sign dimuon production of (3.4 ± 1.8) × 10 -s relative to the rate of charged-current events with the same cuts, or (4.1 ± 2.2)% relative to the prompt ta-ju+ rate, and for antineutrinos the corresponding relative rates (4.3 ± 2.3) × 10 -s and (4.2 ± 2.3)%. A possible explanation for the events is charm pair production at a level of 10 -3 relative to all charged-current reactions.
The study of prompt like-sign dimuon production in neutrino reactions is a long-standing experimental problem [ 1 - 3 ] . While opposite-sign dimuon production exists at a rate of 10 - 2 relative to the charged-current rate and can be explained [4] in detail by single charm production and decay, the observed rate of occurrence of neutrino interactions with a pair of muons o f the same charge is smaller by an order of magnitude and is therefore comparable to the rate of background events due to a charged-current event associated with a nonprompt muon (/a- in the case o f a neutrino interaction) t Supported by the Bundesministerium f'~ Forschung und
Technologic, Bonn.
coming from the muonic decay of a pion or kaon from the hadron shower of the charged-current event. A prompt signal, however, could be indicative, amongst other processes, o f associated charm production by neutrinos, vFe ~ ~t-c~X, with the subsequent decay c --} s l a v . Evidence for the corresponding process in hadronic interactions has been reported recently [5]. We report here on an experiment done iri the CERN 350 GeV wideband horn-focussed neutrino beam and the 330 GeV antineutrino beam. The detector [6] consists of 19 modules of a magnetized iron calorimeter interspaced by 19 drift chambers. The total exposure corresponds to 1.1 × 1018(1.5 X 1018) protons on target for the (anti)neutrino run. We require both muons 103
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to traverse at least five adjacent drift chambers; 557 /2 /2 events and zero/2+/2+ events have been found in the neutrino exposure and 111/2+/2+ events in the antineutrino exposure. We then apply cuts in the fiducial volume, in the visible energy Evis> 30 GeV and in both muon momenta Pu > 6.5 GeV/c. 290 u ~/2 /2 and 53 F -+/2+/2+ events survive these cuts. In analogy with our trimuon analysis [7], we define as non-leading muon/22 the muon which has the smaller transverse momentum relative to the hadron shower axis W as given by the neutrino momentum Pv and the momentum of the "leading" muon P l , W = Pv - P l " The data then show a striking asymmetry between the two muons: the leading muon/21 is as energetic as the one in single-muon charged-current reactions, with (p 1)v = 31.9 GeV/c for the neutrino beam and (pl)b- = 31.8 GeV/c for the antineutrino beam, while the second muon is less energetic with (p2)v = 10.6 GeV/c and (p2)7 = 10.4 GeV/c, barely above the detection threshold. From the measured muon energies, E 1 and E2, and the hadron shower energy E h we calculate the visible energy Evi s = E 1 + E 2 + E h of the event and the scaling variable y = (E n + E2)/Evi s. Using the measured angle 01 o f the leading muon relative to the neutrino direction, we obtain x = 2E1Evi s sin2(O1/2)/((E 2 + Eh)Mp). In fig. 1 we show the distributions of/2 /2 and /2+/2+ events in these two scaling variables. The shape o f the x-distributions resembles the one for normal charged-current events, while the y-distributions are shifted towards high y. If the second muon comes from the hadron vertex, such a shift can be understood as being due to the momentum cut on the second muon. In order to clarify the origin o f the second muon, we calculate two further variables: the azimuthal angle A~b between the two muons in the plane perpendicular to the incident neutrino direction, and the transverse momentum p~ of the second muon relative to the hadron shower axis W. In fig. 2 we show the corresponding distributions of both/2 /2 and/2+/2+ events. For the A~ distribution, we require both muons to have a transverse momentum relative to the incident neutrino direction larger than 0.2 GeV/c. In these distributions, the events are concentrated around 180 °, with average values of (129 -+ 3) ° for neutrino induced events and (134 + 6) ° for antineutrino events, consistent with the second mu0n originating from the hadronic vertex. 104
10 September 1979
~
80 Q z t,u
60 ~o
b ~ --~.
20
~t+ i t +
10
20 02
0t~
06
0.8 x
' &' 0.6
0.8
x d
v . . . ~ p - i.t -
60 z
~.o
//// '-,-'
20
0.2
0.t~
016
018 y
0.2
0.4
0.6
0.8
Fig. 1. Distributions in the scaling variables. (a) x distribution of neutrino events v ~ #-#-. Co) x distribution of antineutrino events ~'--*#+#+. (c) y distribution of neutrino events v --*#-#-. (d) y distribution of antineutrino events ~ ~ t~+#+. A similar conclusion can be drawn from the p~ distributions. These transverse momenta have an average value o f 0.57 GeV/c and are below 2 GeV/c except for one/2 /2 event and two/2+/2+ events. From these distributions we therefore conclude that the bull o f the events is consistent with being produced in a charged-current reaction in connection with a second muon coming from the hadron shower. In view of the low observed rate of these events, a fair fraction of them has to come from lr and K mesons in the shower decaying into/.w. The main question is now what fraction o f the observed events comes from this background process, and whether there is a prompt signal remaining after subtraction o f this background. We have performed two detailed Monte Carlo calculations simulating charged-current neutrino events according to our measured scaling-variable distributions. In one program the Ir and K meson fragmentation functions and multiplicities were generated according to the data from vNe interactions [ 8 - 1 0 ] . The transverse momentum distribution of the hadrons was the one measured in vNe interactions, exp (--6roT) , with m T = (p2T + m2) 1/2. The absorption length o f pions in iron was taken from measurements in our test calorimeter [11], ~ = 18.6 -+ 1.0 cm. The average density o f the apparatus in the fiducial volume is p = 5.18 g/cm 3 . If the pion or kaon does not decay, a second and third
Volume 86B, number 1
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hadronic cascade is initiated. In the other program, longitudinal phase space is generated for all shower particles according to ref. [12], and the shower composition is done according to ref. [13]. The remainder of the calculation is the same as above. The results for a muon momentum cut off of 6.5 GeV/c agree within 5%. These calculations give for neutrino events the probability Pv of obtaining a/1- from rr or K decay within the detector with Pu > 6.5 GeV/c and E v > 30 GeV per inelastic charged-current neutrino interaction (W > 2 GeV, Q2 > 1 GeV2/c2). We obtainP v = (1.05 -+0.16) × 10 -4 and the corresponding number for antineutrino events, P~- = (0.70 +0.14) × 10 -4. The uncertainties in the absorption length (-+6%), the K/lr ratio (20%) and the hadron multiplicity (+0.5) contribute about equally to the error in the calculation. In order to check the Monte Carlo calculation, we have done an experiment in which lr- beams of 50, 75 and 100 GeV/c momentum were incident on our calorimeter [11], and the # leakage was measured. The measured rates and the ones calculated by this Monte Carlo program agree within 15%. For the (1.97 + 0.10) × 106 charged-current neutrino
10 September 1979
events and the (4.4 ---0.3) X 105 antineutrino events, the background calculation yields 207 -+33 like-sign dimuon events for the neutrino exposure and 31 ---7 for the antineutrino exposure. Another background source for the neutrino # g production is the feedthrough of trimuon events tt /1 /a+ where the/a + is not detected. The ratio W1/W2 of two probabilities, W1 of having two/a- with momenta above 6.5 GeV/c and a/1 + below the detection threshold of 4.5 GeV]c, and W2 of observing a trimuon event ju /a ta+ with all muons above 4.5 GeV/c was calculated [14] for the two trimuon production mechanisms [7], the electromagnetic and the hadronic one, and found to be WI/W2 = 0.30 + 0.03. Together with our observed [7] trimuon rate, this leads to a background of 13 + 3 events. The analogous consideration for antineutrino events leads to a background of 3 + 1 events. The least important source of background is due to spatial overlay of two charged-current events. From the lateral vertex distance of overlay events, we estimate this contribution to be 3 + 2 events for the neutrino case and negligible for the antineutrino case. In table 1 the event numbers and rates are summarized. After background subtraction 67 + 37 events re-
b
0
80 - o
V
20
/"
60 -~--/1 • t'~
j
J
10
40
20 60 °
120°
60°
A¢
o
'F
120°
A¢
,7 o
~
lO
,,>, 20 1.0
p~.
2.0 GeV/c
3.0
1.0 PT
2.0 GeVlc 3.0
Fig. 2. Distribution in A~b, the angle between the two muons in the projection onto a p h n e perpendicular to the neutrino direction: (a) neutrino events u ~ ~ - ~ - ; Co) antineutrino events ~ u+/~+. Distribution i n p ~ , the transverse momentum of the non-leadin8 rouen relative to the shower axis: (c) neutrino events v --, ~ - ~ - ; (d) antineutrino events ~"~ ~+~+. Broken line: =/K decay; full line: 7r/K decay and c~- production.
105
Volume 86B, number I
10 September 1979
PHYSICS LETTERS
Fable 1 Event numbers and rates of like-sign dimuons (LSD).
~t~ cut Dbserved events Frimuonbackgr. Dverlay backgr. •[K decay backgr.
6.5 GeV/c 290 13 3 207
Prompt signal (LSD)pr/(U+U-)pr (LSD)pr/I#
-+ 17 ± 3 ± 2 -+ 33
67 ~ 37 4.1 :~ 2.2% (3.4 ~- 1.8) X 10 -s
I0 GeV/c 91 7 64
53 3
± 7 ± 1
16 1
±4 ±1
± 10
31
± 7
10
±2
20 -+ 13 2.0 ± 1.3%
~: ROJ+la+/#+) = (4.3
10 - 5 , X 10 - 2 ,
+ 2.3) X 1 0 - 5 ,
R(la+la+/#+la-) = (4.2
+ 2.3) X 10 - 2 .
This excess o f events persists at the same level if we use a muon m o m e n t u m cut o f 10 GeV/c instead o f 6.5 GeV[c. For neutrino energies above 100 GeV, the result is R(la-la-/la-t.t +) = (4.2 + 2.4) X 10 - 2 for a momentum cut o f 6.5 GeV/c. There is therefore evidence here for p r o m p t like-sign dimuon production for neutrino and antineutrino interactions, both at a level o f about 4% relative to opposite-sign dimuon production. Previously Benvenuti et al. [3] have reported a signal for the case o f neutrinos. For a minimum muon momentum cut of 5 GeV/c the rate relative to opposite-sign dimuons was given as 6 ---5%. F o r the cut Pu > 10 GeV/c the signal was more significant, 12 + 5% relative to opposite sign dimuons. The mechanism for the production of a p r o m p t likesign dimuon signal is constrained b y the observed distributions in figs. 1 and 2. As noted above, the nonleading muon is very slow and is associated with the direction o f the hadron shower axis, and the mechanism is a charged-current reaction with a second muon associated to the hadron shower. Models involving heavy 106
10 GeV/c
± 9 ± 2
main for the neutrino exposure and 19 -+ 10 events for the antineutrino exposure. This excess o f observed events over background corresponds to the following rates relative to charged-current and opposite-sign dimuon events, averaged over the neutrino energy spectrum above 30 GeV:
~': R(t~-Ia-/la-) = (3.4 + 1.8) X RO-g-/U-U +) = (4.1 + 2.2)
6.5 GeV/c
19 ± 10 4.2 ± 2.3% (4.3 ± 2.3) × 10 -s
5 -+4 2.2 ± 1.7%
lepton cascades, which require an energgtic second muon with a rather isotropic distribution in A¢, cannot explain the bulk of these excess events. The associated production o f charmed particle pairs, however, is a possible production mechanism. We have performed Monte Carlo calculations on c~ production along the lines o f the model o f ref. [15], using slow rescaling [16], a transverse m o m e n t u m distribution of c~ according to exp ( - 6 ( p 2 + m2~-)1/2), and leptonic D ~ K*pu decay. We then compare two calculations to the data in figs. 1 and 2: one where we assume all data to be due to ~r/K decay and another one where we add n/K decay and cE production in the proportions given b y table 1. Both assumptions fit the data reasonably well. F o r the/~ /a y-distribution, the fit improves from X2 = 14.8/9 DF to X2 = 6.5/9 DF if the c~ production is added to the curve from lr/K decay, however, the kinematics for lr/K or charm origin o f the extra muon are rather similar and the observed distributions cannot help appreciably in distinguishing between n/K background and charm pair origin. If the observed signal is of c~ origin, it corresponds to the production o f cE pairs in both neutrino and antineutrino charged current collisions in (2 + 1) × 10 - 3 o f the interactions at these energies. This rate is much larger than the one calculated from gluon bremsstrahlung [15]. We would like to acknowledge helpful discussions with J. Smith and thank our technical collaborators for their assistance.
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PHYSICS LETTERS
References [1] [2] [3] [4]
A. Benvenuti et al., Phys. Rev. Lett. 35 (1975) 1199. M. Holder et al., Phys. Lett. 70B (1977) 396. A. Benvenuti et al., Phys. Rev. Lett. 41 (1978) 725. C.B. Barish et al., Phys. Rev. Lett. 36 (1976) 939; M. Holder et al., Phys. Lett. 69B (1977) 377; A. Benvenuti et al., Phys. Rev. Lett. 41 (1978) 1204. [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; B.C. Barish et al., Contrib. No. 1011 to XIXth Intern. Conf. on High energy physics (Tokyo, 1978). [6] M. Holder et al., Nucl. Instrum. Methods 148 (1978) 235. [7] T. Hansl et al., Phys. Lett. 77B (1978) 114; Nucl. Phys. B142 (1978) 381.
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[8 ] R.B. Palmer, Pro c. XIIIth Rencontre de Moriond (Les Arcs, March 1978), ed. Fronti6res, Vol. II, p. 361. [9] Seattle-Berkeley CoUab., T.H. Bumett et al., preprint VTL-PUB-50/51. [ 10] BEBC-ABCLOS CoUab., H. Emans, Thesis Bonn-IR-78-10 (1978). [11] M. Holder et al., Nucl. Instrum. Methods 151 (1978) 69. [12] S. Jadach, Comput. Phys. Commun. 9 0975) 297. [13] J. Bell et al., Fermilab-Pub-78/57-Exp.7420.045. [14] J. Smith, private communication. [15] B.L. Young, T.F. Walsh and T.C. Yang, Phys. Lett. 74B (1978) 111; H. Goldberg, Phys. Rev. Lett. 39 (1977) 1598; G.L. Kane, J. Smith and J.A.M. Vermaseren, Phys. Rev. D19 (1979) 1978. [16] R.B. Barnett, Phys. Rev. Lett. 36 (1976) 1163.
107
PHYSICS LETTERS
10 September 1979
INVESTIGATION OF LIKE-SIGN DIMUON PRODUCTION IN NEUTRINO AND ANTINEUTRINO REACTIONS J.G.H. de GROOT, T. HANSL, M. HOLDER, J. KNOBLOCH, J. MAY, P. PALAZZI, A. PARA, F. RANJARD, A. SAVOY-NAVARRO, D. SCHLATTER, J. STEINBERGER, W. von RUDEN and H. WAHL CERN, Geneva, Switzerland
F. EISELE, K. KLEINKNECHT, H. LIERL and H.J. WILLUTZKI lnstitut fiir Physik 1 der Universitiit, Dortmund, Germany
F. DYDAK, C. GEWENIGER, V. HEPP, K. TITTEL and J. WOTSCHACK Institut far Hochenergiephysik 1 der UniversitiitHeidelberg, Germany
P. BLOCH, B. DEVAUX, S. LOUCATOS, J £ . MERLO, B. PEYAUD, J. RANDER and R. TURLAY D.Ph.P.E., CEN-Saclay, France
F.L. NAVARRIA Istituto di Fisica dell'UniversitY, Bologna, Italy
Received 1 June 1979
290 events of the type vFe ~ ta-ta-X and 53 events from the reaction u--Fe--} Ia+z+Xwith Ev > 30 GeV and muon momenta p~ > 6.5 GeV/c have been observed in the CDHS detector. After subtracting the background from charged-current processes with one n or K meson of the hadronic shower decaying into t~-F (or t~+v),we obtain for neutrinos a rate of prompt like-sign dimuon production of (3.4 ± 1.8) × 10 -s relative to the rate of charged-current events with the same cuts, or (4.1 ± 2.2)% relative to the prompt ta-ju+ rate, and for antineutrinos the corresponding relative rates (4.3 ± 2.3) × 10 -s and (4.2 ± 2.3)%. A possible explanation for the events is charm pair production at a level of 10 -3 relative to all charged-current reactions.
The study of prompt like-sign dimuon production in neutrino reactions is a long-standing experimental problem [ 1 - 3 ] . While opposite-sign dimuon production exists at a rate of 10 - 2 relative to the charged-current rate and can be explained [4] in detail by single charm production and decay, the observed rate of occurrence of neutrino interactions with a pair of muons o f the same charge is smaller by an order of magnitude and is therefore comparable to the rate of background events due to a charged-current event associated with a nonprompt muon (/a- in the case o f a neutrino interaction) t Supported by the Bundesministerium f'~ Forschung und
Technologic, Bonn.
coming from the muonic decay of a pion or kaon from the hadron shower of the charged-current event. A prompt signal, however, could be indicative, amongst other processes, o f associated charm production by neutrinos, vFe ~ ~t-c~X, with the subsequent decay c --} s l a v . Evidence for the corresponding process in hadronic interactions has been reported recently [5]. We report here on an experiment done iri the CERN 350 GeV wideband horn-focussed neutrino beam and the 330 GeV antineutrino beam. The detector [6] consists of 19 modules of a magnetized iron calorimeter interspaced by 19 drift chambers. The total exposure corresponds to 1.1 × 1018(1.5 X 1018) protons on target for the (anti)neutrino run. We require both muons 103
Volume 86B, number 1
PHYSICS LETTERS
to traverse at least five adjacent drift chambers; 557 /2 /2 events and zero/2+/2+ events have been found in the neutrino exposure and 111/2+/2+ events in the antineutrino exposure. We then apply cuts in the fiducial volume, in the visible energy Evis> 30 GeV and in both muon momenta Pu > 6.5 GeV/c. 290 u ~/2 /2 and 53 F -+/2+/2+ events survive these cuts. In analogy with our trimuon analysis [7], we define as non-leading muon/22 the muon which has the smaller transverse momentum relative to the hadron shower axis W as given by the neutrino momentum Pv and the momentum of the "leading" muon P l , W = Pv - P l " The data then show a striking asymmetry between the two muons: the leading muon/21 is as energetic as the one in single-muon charged-current reactions, with (p 1)v = 31.9 GeV/c for the neutrino beam and (pl)b- = 31.8 GeV/c for the antineutrino beam, while the second muon is less energetic with (p2)v = 10.6 GeV/c and (p2)7 = 10.4 GeV/c, barely above the detection threshold. From the measured muon energies, E 1 and E2, and the hadron shower energy E h we calculate the visible energy Evi s = E 1 + E 2 + E h of the event and the scaling variable y = (E n + E2)/Evi s. Using the measured angle 01 o f the leading muon relative to the neutrino direction, we obtain x = 2E1Evi s sin2(O1/2)/((E 2 + Eh)Mp). In fig. 1 we show the distributions of/2 /2 and /2+/2+ events in these two scaling variables. The shape o f the x-distributions resembles the one for normal charged-current events, while the y-distributions are shifted towards high y. If the second muon comes from the hadron vertex, such a shift can be understood as being due to the momentum cut on the second muon. In order to clarify the origin o f the second muon, we calculate two further variables: the azimuthal angle A~b between the two muons in the plane perpendicular to the incident neutrino direction, and the transverse momentum p~ of the second muon relative to the hadron shower axis W. In fig. 2 we show the corresponding distributions of both/2 /2 and/2+/2+ events. For the A~ distribution, we require both muons to have a transverse momentum relative to the incident neutrino direction larger than 0.2 GeV/c. In these distributions, the events are concentrated around 180 °, with average values of (129 -+ 3) ° for neutrino induced events and (134 + 6) ° for antineutrino events, consistent with the second mu0n originating from the hadronic vertex. 104
10 September 1979
~
80 Q z t,u
60 ~o
b ~ --~.
20
~t+ i t +
10
20 02
0t~
06
0.8 x
' &' 0.6
0.8
x d
v . . . ~ p - i.t -
60 z
~.o
//// '-,-'
20
0.2
0.t~
016
018 y
0.2
0.4
0.6
0.8
Fig. 1. Distributions in the scaling variables. (a) x distribution of neutrino events v ~ #-#-. Co) x distribution of antineutrino events ~'--*#+#+. (c) y distribution of neutrino events v --*#-#-. (d) y distribution of antineutrino events ~ ~ t~+#+. A similar conclusion can be drawn from the p~ distributions. These transverse momenta have an average value o f 0.57 GeV/c and are below 2 GeV/c except for one/2 /2 event and two/2+/2+ events. From these distributions we therefore conclude that the bull o f the events is consistent with being produced in a charged-current reaction in connection with a second muon coming from the hadron shower. In view of the low observed rate of these events, a fair fraction of them has to come from lr and K mesons in the shower decaying into/.w. The main question is now what fraction o f the observed events comes from this background process, and whether there is a prompt signal remaining after subtraction o f this background. We have performed two detailed Monte Carlo calculations simulating charged-current neutrino events according to our measured scaling-variable distributions. In one program the Ir and K meson fragmentation functions and multiplicities were generated according to the data from vNe interactions [ 8 - 1 0 ] . The transverse momentum distribution of the hadrons was the one measured in vNe interactions, exp (--6roT) , with m T = (p2T + m2) 1/2. The absorption length o f pions in iron was taken from measurements in our test calorimeter [11], ~ = 18.6 -+ 1.0 cm. The average density o f the apparatus in the fiducial volume is p = 5.18 g/cm 3 . If the pion or kaon does not decay, a second and third
Volume 86B, number 1
PHYSICS LETTERS
hadronic cascade is initiated. In the other program, longitudinal phase space is generated for all shower particles according to ref. [12], and the shower composition is done according to ref. [13]. The remainder of the calculation is the same as above. The results for a muon momentum cut off of 6.5 GeV/c agree within 5%. These calculations give for neutrino events the probability Pv of obtaining a/1- from rr or K decay within the detector with Pu > 6.5 GeV/c and E v > 30 GeV per inelastic charged-current neutrino interaction (W > 2 GeV, Q2 > 1 GeV2/c2). We obtainP v = (1.05 -+0.16) × 10 -4 and the corresponding number for antineutrino events, P~- = (0.70 +0.14) × 10 -4. The uncertainties in the absorption length (-+6%), the K/lr ratio (20%) and the hadron multiplicity (+0.5) contribute about equally to the error in the calculation. In order to check the Monte Carlo calculation, we have done an experiment in which lr- beams of 50, 75 and 100 GeV/c momentum were incident on our calorimeter [11], and the # leakage was measured. The measured rates and the ones calculated by this Monte Carlo program agree within 15%. For the (1.97 + 0.10) × 106 charged-current neutrino
10 September 1979
events and the (4.4 ---0.3) X 105 antineutrino events, the background calculation yields 207 -+33 like-sign dimuon events for the neutrino exposure and 31 ---7 for the antineutrino exposure. Another background source for the neutrino # g production is the feedthrough of trimuon events tt /1 /a+ where the/a + is not detected. The ratio W1/W2 of two probabilities, W1 of having two/a- with momenta above 6.5 GeV/c and a/1 + below the detection threshold of 4.5 GeV]c, and W2 of observing a trimuon event ju /a ta+ with all muons above 4.5 GeV/c was calculated [14] for the two trimuon production mechanisms [7], the electromagnetic and the hadronic one, and found to be WI/W2 = 0.30 + 0.03. Together with our observed [7] trimuon rate, this leads to a background of 13 + 3 events. The analogous consideration for antineutrino events leads to a background of 3 + 1 events. The least important source of background is due to spatial overlay of two charged-current events. From the lateral vertex distance of overlay events, we estimate this contribution to be 3 + 2 events for the neutrino case and negligible for the antineutrino case. In table 1 the event numbers and rates are summarized. After background subtraction 67 + 37 events re-
b
0
80 - o
V
20
/"
60 -~--/1 • t'~
j
J
10
40
20 60 °
120°
60°
A¢
o
'F
120°
A¢
,7 o
~
lO
,,>, 20 1.0
p~.
2.0 GeV/c
3.0
1.0 PT
2.0 GeVlc 3.0
Fig. 2. Distribution in A~b, the angle between the two muons in the projection onto a p h n e perpendicular to the neutrino direction: (a) neutrino events u ~ ~ - ~ - ; Co) antineutrino events ~ u+/~+. Distribution i n p ~ , the transverse momentum of the non-leadin8 rouen relative to the shower axis: (c) neutrino events v --, ~ - ~ - ; (d) antineutrino events ~"~ ~+~+. Broken line: =/K decay; full line: 7r/K decay and c~- production.
105
Volume 86B, number I
10 September 1979
PHYSICS LETTERS
Fable 1 Event numbers and rates of like-sign dimuons (LSD).
~t~ cut Dbserved events Frimuonbackgr. Dverlay backgr. •[K decay backgr.
6.5 GeV/c 290 13 3 207
Prompt signal (LSD)pr/(U+U-)pr (LSD)pr/I#
-+ 17 ± 3 ± 2 -+ 33
67 ~ 37 4.1 :~ 2.2% (3.4 ~- 1.8) X 10 -s
I0 GeV/c 91 7 64
53 3
± 7 ± 1
16 1
±4 ±1
± 10
31
± 7
10
±2
20 -+ 13 2.0 ± 1.3%
~: ROJ+la+/#+) = (4.3
10 - 5 , X 10 - 2 ,
+ 2.3) X 1 0 - 5 ,
R(la+la+/#+la-) = (4.2
+ 2.3) X 10 - 2 .
This excess o f events persists at the same level if we use a muon m o m e n t u m cut o f 10 GeV/c instead o f 6.5 GeV[c. For neutrino energies above 100 GeV, the result is R(la-la-/la-t.t +) = (4.2 + 2.4) X 10 - 2 for a momentum cut o f 6.5 GeV/c. There is therefore evidence here for p r o m p t like-sign dimuon production for neutrino and antineutrino interactions, both at a level o f about 4% relative to opposite-sign dimuon production. Previously Benvenuti et al. [3] have reported a signal for the case o f neutrinos. For a minimum muon momentum cut of 5 GeV/c the rate relative to opposite-sign dimuons was given as 6 ---5%. F o r the cut Pu > 10 GeV/c the signal was more significant, 12 + 5% relative to opposite sign dimuons. The mechanism for the production of a p r o m p t likesign dimuon signal is constrained b y the observed distributions in figs. 1 and 2. As noted above, the nonleading muon is very slow and is associated with the direction o f the hadron shower axis, and the mechanism is a charged-current reaction with a second muon associated to the hadron shower. Models involving heavy 106
10 GeV/c
± 9 ± 2
main for the neutrino exposure and 19 -+ 10 events for the antineutrino exposure. This excess o f observed events over background corresponds to the following rates relative to charged-current and opposite-sign dimuon events, averaged over the neutrino energy spectrum above 30 GeV:
~': R(t~-Ia-/la-) = (3.4 + 1.8) X RO-g-/U-U +) = (4.1 + 2.2)
6.5 GeV/c
19 ± 10 4.2 ± 2.3% (4.3 ± 2.3) × 10 -s
5 -+4 2.2 ± 1.7%
lepton cascades, which require an energgtic second muon with a rather isotropic distribution in A¢, cannot explain the bulk of these excess events. The associated production o f charmed particle pairs, however, is a possible production mechanism. We have performed Monte Carlo calculations on c~ production along the lines o f the model o f ref. [15], using slow rescaling [16], a transverse m o m e n t u m distribution of c~ according to exp ( - 6 ( p 2 + m2~-)1/2), and leptonic D ~ K*pu decay. We then compare two calculations to the data in figs. 1 and 2: one where we assume all data to be due to ~r/K decay and another one where we add n/K decay and cE production in the proportions given b y table 1. Both assumptions fit the data reasonably well. F o r the/~ /a y-distribution, the fit improves from X2 = 14.8/9 DF to X2 = 6.5/9 DF if the c~ production is added to the curve from lr/K decay, however, the kinematics for lr/K or charm origin o f the extra muon are rather similar and the observed distributions cannot help appreciably in distinguishing between n/K background and charm pair origin. If the observed signal is of c~ origin, it corresponds to the production o f cE pairs in both neutrino and antineutrino charged current collisions in (2 + 1) × 10 - 3 o f the interactions at these energies. This rate is much larger than the one calculated from gluon bremsstrahlung [15]. We would like to acknowledge helpful discussions with J. Smith and thank our technical collaborators for their assistance.
Volume 86B, number 1
PHYSICS LETTERS
References [1] [2] [3] [4]
A. Benvenuti et al., Phys. Rev. Lett. 35 (1975) 1199. M. Holder et al., Phys. Lett. 70B (1977) 396. A. Benvenuti et al., Phys. Rev. Lett. 41 (1978) 725. C.B. Barish et al., Phys. Rev. Lett. 36 (1976) 939; M. Holder et al., Phys. Lett. 69B (1977) 377; A. Benvenuti et al., Phys. Rev. Lett. 41 (1978) 1204. [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; B.C. Barish et al., Contrib. No. 1011 to XIXth Intern. Conf. on High energy physics (Tokyo, 1978). [6] M. Holder et al., Nucl. Instrum. Methods 148 (1978) 235. [7] T. Hansl et al., Phys. Lett. 77B (1978) 114; Nucl. Phys. B142 (1978) 381.
10 September 1979
[8 ] R.B. Palmer, Pro c. XIIIth Rencontre de Moriond (Les Arcs, March 1978), ed. Fronti6res, Vol. II, p. 361. [9] Seattle-Berkeley CoUab., T.H. Bumett et al., preprint VTL-PUB-50/51. [ 10] BEBC-ABCLOS CoUab., H. Emans, Thesis Bonn-IR-78-10 (1978). [11] M. Holder et al., Nucl. Instrum. Methods 151 (1978) 69. [12] S. Jadach, Comput. Phys. Commun. 9 0975) 297. [13] J. Bell et al., Fermilab-Pub-78/57-Exp.7420.045. [14] J. Smith, private communication. [15] B.L. Young, T.F. Walsh and T.C. Yang, Phys. Lett. 74B (1978) 111; H. Goldberg, Phys. Rev. Lett. 39 (1977) 1598; G.L. Kane, J. Smith and J.A.M. Vermaseren, Phys. Rev. D19 (1979) 1978. [16] R.B. Barnett, Phys. Rev. Lett. 36 (1976) 1163.
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