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Like-sign dimuon events produced in narrow-band neutrino and antineutrino beams
Volume 70B, number 3
PHYSICS LETTERS
10 October 1977
LIKE-SIGN DIMUON EVENTS PRODUCED IN NARROW-BAND NEUTRINO AND ANTINEUTRINO
BEAMS
M. HOLDER, J. KNOBLOCH, J. MAY, H.P. PAAR, P. PALAZZI, D. SCHLATTER, J. STEINBERGER, H. SUTER, H. WAHL and E.G.H. WILLIAMS CERN, Geneva, Switzerland F. EISELE, C. GEWENIGER, K. KLEINKNECHT, G. SPAHN and H-J, WILLUTZKI Institut fffr Physik +, Universifft't, Dortmund. Germany W. DORTH, F. DYDAK, V. HEPP, K. TITTEL and J. WOTSCHACK lnstitut ffir Hochenergiephysik t, Universit(#, Heidelberg, Germany P. BLOCH, B. DEVAUX, M. GRIMM, J. MAILLARD, B. PEYAUD, J. RANDER, A. SAVOY-NAVARRO and R. TURLAY D.Ph.P.E., CEN-Saclay, France F.L. NAVARRIA Istituto di Fisica dell'Universitd, Bologna, Italy
Forty seven events of the type v + Fe ~ ~- + #- + X and nine events of the type ~ + Fe ~ ~+ + ~+ + X have been observed, and zero events with like-sign muons of the opposite sign. Most of the observed events can be attributed to the background of normal charged current events, in which one of the pions or kaons of the hadron shower decays, and so produces a second muon. The remaining events correspond to rates of the order of (3 +-2) X 10-4 of those for charged current events. They may very well be due to the hadronic production of a charm-anticharm pair, with subsequent decay of one member of the pair. The observed rates correspond to charm-anticharm pair production in 0.5-1% of the hadron showers. No evidence can be found for a lepton-cascade origin of the observed events.
Several dimuon events of the type in which both muons have the same charge have recently been reported in v and ~ reactions at high energy [1]. However, the relevant background due to 7r ~ / 2 and K ~ / 2 decay has perhaps not been discussed adequately, so that the direct origin of these events cannot be considered established. The possible existence of such events and their eventual properties are however of interest in connection with questions concerning the origin of opposite sign dimuon, and of trimuon events. In particular, some models constructed to explain certain features of some of the observed trimuon events [2], on the basis of postulated cascading heavy leptons,
predict a substantial number of like-sign dimuon events with the property that both muons are energetic and unassociated with the hadron shower [3]. Some like-sign dimuon production may also be expected, although with u n k n o w n cross section, due to the production and subsequent decay of charm-anticharm pairs in the hadron showers of charged current events:
* Supported by Bundesministerium fiir Forschung und Technologic.
For these reactions, one of the muons would, in general, have a low energy and would be associated with the
396
v + Fe ~ / ~ - + C + C + X [--~/.t- + ... and ~+ Fe-+/a+ + C + C + X [+/2+ + ....
Volume 70B, number 3
PHYSICS LETTERS
10 October 1977 I
7
I v
15
6
ul
10
tJa
5 140 ~20
v
I
0
100
02
04
x
I
t
06
08
F 10
80 Q 60 40 10
30
25
20 15 10 Events / 5 GeV
5
50
100 150 E I GeV
200
k%-k~v
250 6
Fig. 1. M o m e n t u m distributions for the two muons, and their correlations. The calculated background due to ~r ~ # and K ---, ~ decay is cross hatched.
I hadron shower. In either case, the charges would be negative for production by neutrinos, and positive for production by antineutrinos. We present here results for like-sign muon production in the same 200 GeV narrow-band neutrino and antineutrino exposure for which the results for opposite-sign dimuons were reported recently [4]. F o r t y seven - - events and zero + + events were observed in the neutrino exposure, and nine + + events and zero - - events in the antineutrino exposure. In comparison, 257 and 58 opposite-sign events were observed for neutrino and antineutrino exposures respectively. The uncorrected rates are therefore approximately 15% o f the opposite-sign rates. Averaged over the neutrino energy interval 30 < E v < 200 GeV, they are roughly 1200 times smaller than the charged-current event rates. In fig. 1 the m o m e n t u m distributions and correlations o f the two muons are shown. Here muon 1 refers to the more energetic, and muon 2 to the less energetic o f the two muons. In fig. 2 the x- and y-distributions are shown, where x and y are the inclusive reaction variables:
2E~.~(E,1 + E,2
+Ehad)sin2(0,/2)
X=
Mp'(Eha d + E , 2 )
02
04
06 Y
o18 !,0
Fig. 2. Distributions in x and iny for the two muons. The calculated background due to 7r~ # and K --, p decay is shown dashed in the y-distribution. and Y = (Ehad +Eu2)/(Eu 1 +Eu 2 +Ehad), where Eul , Eu2 and Eha d are the energies of the first muon, the second muon and the hadron shower respectively, and 0 u is the laboratory angle o f the first muon with respect to the incident neutrino. In fig. 3 we show the distribution in the angle made by the projection o f the two muons onto the plane perpendicular to the incident neutrino. In all o f these figures we show the neutrino and antineutrino data together, not because they should be similar, but because there are so few antineutrino events. In fig. 1 the first muon is distributed in energy very much like the muon in normal charged-current events, with an average energy o f 52 GeV. However, the second muon has a strikingly small energy; the observed average energy o f 7.0 GeV is only 2.5 GeV above the detection threshold o f ~4.5 GeV required in the present analysis. 397
Volume 70B, number 3
PHYSICS LETTERS
v
1(3
I
0
20
~060
801001'201401 1;0180 A~
Fig. 3. Distribution of the angle between the two muons, as seen in the projection onto the plane perpendicular to the incident neutrino. The x-distribution of fig. 2 is similar to that for normal charged-current events, with (x) = 0.28 in both cases. However the y-distribution is shifted to large y , with (y) = 0.57, compared to ~0.44 for (y) for neutrino, and ~0.30 for antineutrino charged-current reactions. In fig. 3 we see that the second muon is correlated in general with the hadron shower, at 180 ° in azimuth to the first muon. Because of the low observed rates of like-sign dimuons, the background due to pion and kaon decay becomes important. We have calculated this background on the basis of the known mean free path of pions and kaons in iron, and the properties of neutrinoproduced showers and their subsequent development [5]. The main qualitative features of this background are in agreement with the observed properties of the like-sign dimuon events: 1. The second (decay) muon energy is very low. This is expected because the energy of the shower is divided among several particles, and because the Lorentz time dilation favours the decay of low-momentum particles. 2. The x-distribution is that of normal charged-current events, because this background is due to normal charged-current reactions. 3. The expected second muon flux is roughly proportional to the hadron shower energy. The y-distribution is therefore shifted to large y. 4. The second muon direction is associated with the hadron shower since the muon is produced in the decay of a shower particle. 398
10 October 1977
The calculated y-distribution and muon spectrum agree quite well with the data, but the total calculated background rates are lower than the observed rates. In the case of neutrinos, there are 30 calculated background events compared to 47 observed events, and in the case of antineutrinos, there are five calculated compared to nine observed events. We estimate the probable error in the calculation to be approximately 25%, so that it is quite possible that very few, if any of the observed dimuons have a direct origin. If the excess of observed events over background is considered significant then the like-sign dimuon rate which remains after this background subtraction is of the order of (3 +- 2) × 10 - 4 of that of the charged-current rate, and of the order of (0.05 +- 0.03) of that of the opposite-sign dimuon rate, both for neutrinos and antineutrinos. In any case, some like-sign dimuons must be expected from the decay of one of the charmed particles in the hadronic production of charmed-anticharmed pairs, which must occur sometimes. The observed properties of the dimuon events do not contradict this origin. The observed rates correspond to a charmed-anticharmed pair production of the order of 0.5-1.0% in neutrino interactions at energies of the order of 100 GeV, if an efficiency of 0.3 for muon detection and a branching ratio of 0.15 for the/~-decay of the charmed particle are assumed [4]. From this process we would then also expect ~1 trimuon in the same data sample, and this is not in disagreement with our data [6]. On the other hand, models which explain certain features of trimuon events observed at FNAL [2] on the basis of cascading heavy leptons, predict like-sign dimuons with energetic second muons, correlated in direction with the first muon [3]. There are no such events in the data presented here, so that we may put an upper limit for their frequency at ~5 × 10 -5 of the charged current rate.
References
[1] A. Benvenuti et al., Phys. Rev. Lett. 35 (1975) 1199. [2] B.C. Barish et al., Phys. Rev. Lett. 38 (1977) 577; A. Benvenuti et al., Phys. Rev. Lett. 38 (1977) 1110. [3] V. Barger, T. Gottschalk, D.V. Nanopoulos, J. Abad and R.J.N. Phillips, Univ. of Wisconsin preprint COO-595. [4] M. Holder et al., Phys. Lett. 69B (1977) 377. [5] A.L. Grant, Nucl. Inst. Meth. 131 (1975) 167. [6] M. Holder et al., Phys. Lett. 70B (1977)
PHYSICS LETTERS
10 October 1977
LIKE-SIGN DIMUON EVENTS PRODUCED IN NARROW-BAND NEUTRINO AND ANTINEUTRINO
BEAMS
M. HOLDER, J. KNOBLOCH, J. MAY, H.P. PAAR, P. PALAZZI, D. SCHLATTER, J. STEINBERGER, H. SUTER, H. WAHL and E.G.H. WILLIAMS CERN, Geneva, Switzerland F. EISELE, C. GEWENIGER, K. KLEINKNECHT, G. SPAHN and H-J, WILLUTZKI Institut fffr Physik +, Universifft't, Dortmund. Germany W. DORTH, F. DYDAK, V. HEPP, K. TITTEL and J. WOTSCHACK lnstitut ffir Hochenergiephysik t, Universit(#, Heidelberg, Germany P. BLOCH, B. DEVAUX, M. GRIMM, J. MAILLARD, B. PEYAUD, J. RANDER, A. SAVOY-NAVARRO and R. TURLAY D.Ph.P.E., CEN-Saclay, France F.L. NAVARRIA Istituto di Fisica dell'Universitd, Bologna, Italy
Forty seven events of the type v + Fe ~ ~- + #- + X and nine events of the type ~ + Fe ~ ~+ + ~+ + X have been observed, and zero events with like-sign muons of the opposite sign. Most of the observed events can be attributed to the background of normal charged current events, in which one of the pions or kaons of the hadron shower decays, and so produces a second muon. The remaining events correspond to rates of the order of (3 +-2) X 10-4 of those for charged current events. They may very well be due to the hadronic production of a charm-anticharm pair, with subsequent decay of one member of the pair. The observed rates correspond to charm-anticharm pair production in 0.5-1% of the hadron showers. No evidence can be found for a lepton-cascade origin of the observed events.
Several dimuon events of the type in which both muons have the same charge have recently been reported in v and ~ reactions at high energy [1]. However, the relevant background due to 7r ~ / 2 and K ~ / 2 decay has perhaps not been discussed adequately, so that the direct origin of these events cannot be considered established. The possible existence of such events and their eventual properties are however of interest in connection with questions concerning the origin of opposite sign dimuon, and of trimuon events. In particular, some models constructed to explain certain features of some of the observed trimuon events [2], on the basis of postulated cascading heavy leptons,
predict a substantial number of like-sign dimuon events with the property that both muons are energetic and unassociated with the hadron shower [3]. Some like-sign dimuon production may also be expected, although with u n k n o w n cross section, due to the production and subsequent decay of charm-anticharm pairs in the hadron showers of charged current events:
* Supported by Bundesministerium fiir Forschung und Technologic.
For these reactions, one of the muons would, in general, have a low energy and would be associated with the
396
v + Fe ~ / ~ - + C + C + X [--~/.t- + ... and ~+ Fe-+/a+ + C + C + X [+/2+ + ....
Volume 70B, number 3
PHYSICS LETTERS
10 October 1977 I
7
I v
15
6
ul
10
tJa
5 140 ~20
v
I
0
100
02
04
x
I
t
06
08
F 10
80 Q 60 40 10
30
25
20 15 10 Events / 5 GeV
5
50
100 150 E I GeV
200
k%-k~v
250 6
Fig. 1. M o m e n t u m distributions for the two muons, and their correlations. The calculated background due to ~r ~ # and K ---, ~ decay is cross hatched.
I hadron shower. In either case, the charges would be negative for production by neutrinos, and positive for production by antineutrinos. We present here results for like-sign muon production in the same 200 GeV narrow-band neutrino and antineutrino exposure for which the results for opposite-sign dimuons were reported recently [4]. F o r t y seven - - events and zero + + events were observed in the neutrino exposure, and nine + + events and zero - - events in the antineutrino exposure. In comparison, 257 and 58 opposite-sign events were observed for neutrino and antineutrino exposures respectively. The uncorrected rates are therefore approximately 15% o f the opposite-sign rates. Averaged over the neutrino energy interval 30 < E v < 200 GeV, they are roughly 1200 times smaller than the charged-current event rates. In fig. 1 the m o m e n t u m distributions and correlations o f the two muons are shown. Here muon 1 refers to the more energetic, and muon 2 to the less energetic o f the two muons. In fig. 2 the x- and y-distributions are shown, where x and y are the inclusive reaction variables:
2E~.~(E,1 + E,2
+Ehad)sin2(0,/2)
X=
Mp'(Eha d + E , 2 )
02
04
06 Y
o18 !,0
Fig. 2. Distributions in x and iny for the two muons. The calculated background due to 7r~ # and K --, p decay is shown dashed in the y-distribution. and Y = (Ehad +Eu2)/(Eu 1 +Eu 2 +Ehad), where Eul , Eu2 and Eha d are the energies of the first muon, the second muon and the hadron shower respectively, and 0 u is the laboratory angle o f the first muon with respect to the incident neutrino. In fig. 3 we show the distribution in the angle made by the projection o f the two muons onto the plane perpendicular to the incident neutrino. In all o f these figures we show the neutrino and antineutrino data together, not because they should be similar, but because there are so few antineutrino events. In fig. 1 the first muon is distributed in energy very much like the muon in normal charged-current events, with an average energy o f 52 GeV. However, the second muon has a strikingly small energy; the observed average energy o f 7.0 GeV is only 2.5 GeV above the detection threshold o f ~4.5 GeV required in the present analysis. 397
Volume 70B, number 3
PHYSICS LETTERS
v
1(3
I
0
20
~060
801001'201401 1;0180 A~
Fig. 3. Distribution of the angle between the two muons, as seen in the projection onto the plane perpendicular to the incident neutrino. The x-distribution of fig. 2 is similar to that for normal charged-current events, with (x) = 0.28 in both cases. However the y-distribution is shifted to large y , with (y) = 0.57, compared to ~0.44 for (y) for neutrino, and ~0.30 for antineutrino charged-current reactions. In fig. 3 we see that the second muon is correlated in general with the hadron shower, at 180 ° in azimuth to the first muon. Because of the low observed rates of like-sign dimuons, the background due to pion and kaon decay becomes important. We have calculated this background on the basis of the known mean free path of pions and kaons in iron, and the properties of neutrinoproduced showers and their subsequent development [5]. The main qualitative features of this background are in agreement with the observed properties of the like-sign dimuon events: 1. The second (decay) muon energy is very low. This is expected because the energy of the shower is divided among several particles, and because the Lorentz time dilation favours the decay of low-momentum particles. 2. The x-distribution is that of normal charged-current events, because this background is due to normal charged-current reactions. 3. The expected second muon flux is roughly proportional to the hadron shower energy. The y-distribution is therefore shifted to large y. 4. The second muon direction is associated with the hadron shower since the muon is produced in the decay of a shower particle. 398
10 October 1977
The calculated y-distribution and muon spectrum agree quite well with the data, but the total calculated background rates are lower than the observed rates. In the case of neutrinos, there are 30 calculated background events compared to 47 observed events, and in the case of antineutrinos, there are five calculated compared to nine observed events. We estimate the probable error in the calculation to be approximately 25%, so that it is quite possible that very few, if any of the observed dimuons have a direct origin. If the excess of observed events over background is considered significant then the like-sign dimuon rate which remains after this background subtraction is of the order of (3 +- 2) × 10 - 4 of that of the charged-current rate, and of the order of (0.05 +- 0.03) of that of the opposite-sign dimuon rate, both for neutrinos and antineutrinos. In any case, some like-sign dimuons must be expected from the decay of one of the charmed particles in the hadronic production of charmed-anticharmed pairs, which must occur sometimes. The observed properties of the dimuon events do not contradict this origin. The observed rates correspond to a charmed-anticharmed pair production of the order of 0.5-1.0% in neutrino interactions at energies of the order of 100 GeV, if an efficiency of 0.3 for muon detection and a branching ratio of 0.15 for the/~-decay of the charmed particle are assumed [4]. From this process we would then also expect ~1 trimuon in the same data sample, and this is not in disagreement with our data [6]. On the other hand, models which explain certain features of trimuon events observed at FNAL [2] on the basis of cascading heavy leptons, predict like-sign dimuons with energetic second muons, correlated in direction with the first muon [3]. There are no such events in the data presented here, so that we may put an upper limit for their frequency at ~5 × 10 -5 of the charged current rate.
References
[1] A. Benvenuti et al., Phys. Rev. Lett. 35 (1975) 1199. [2] B.C. Barish et al., Phys. Rev. Lett. 38 (1977) 577; A. Benvenuti et al., Phys. Rev. Lett. 38 (1977) 1110. [3] V. Barger, T. Gottschalk, D.V. Nanopoulos, J. Abad and R.J.N. Phillips, Univ. of Wisconsin preprint COO-595. [4] M. Holder et al., Phys. Lett. 69B (1977) 377. [5] A.L. Grant, Nucl. Inst. Meth. 131 (1975) 167. [6] M. Holder et al., Phys. Lett. 70B (1977)