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Single pion production in antineutrino induced charged current interactions
Volume 81B, number 3,4
PHYSICS LETTERS
26 February 1979
SINGLE PION PRODUCTION IN ANTINEUTRINO INDUCED CHARGED CURRENT INTERACTIONS T. BOLOGNE~E, J.P. ENGEL, J . L GUYONNET and J.L. RIESTER Centre de Recherches NuclEaires et Universit~ Louis Pasteur, Strasbourg, France
Received 15 December 1978
Results are presented on the exclusive charged current antineutrino production of one pion using the data of the Gargamelle propane experiment at CERN PS. The isospin structure of the charged weak current is studied as well as the energy dependence of the total cross section for 7r- antineutrino production, which is compared with the prediction of the Adler model.
1. Introduction. Single pion production by antineutrinos provides important information about the structure of the weak hadronic current. Results on neutral current antineutrino interactions, from the same experiment, have been published previously [1]. Here, for the first time, an analysis of the following three charged current reactions:
~+ p ~#+rr-p,
(1)
+ p ~/a+lr0n,
(2)
+ n -~ ~+rr- n,
(3)
is presented. For this experiment, the big heavy liquid bubble chamber Gargamelle ( " 8 m 3 of visible and ~3 m 3 of fiducial volume) was filled with propane and a small admixture of heavy freon CF3Br and was exposed to the CERN PS antineutrino beam (peaked at EV ~ 1.5 GeV). See ref. [1] for more experimental details. The results presented here are obtained using 80% of the total sample, i.e. ~3 X 105 pictures which have been scanned twice. The mean scanning efficiency is greater than 99% for each of the three channels analysed. 2. Selection criteria. Candidates for reactions (1), (2) and (3) are required to satisfy the following selection criteria: (i) the primary vertex has to be in the fiducial vol-
ume without any possible upstream sources; (ii) the #+ is defined as a leaving or decaying positive track; (iii) the lr- is any negative track ( p - background from neutrino events is negligible in these channels
[2]); (iv) the ~r0 is identified by 1 or 2 materialized 7's, pointing to the primary vertex with a probability greater than 1%; (v) the proton is a stopping track or any positive interacting track not identified as 7r+; (vi) candidates for reaction (1) must have only one proton which has a momentum greater than 240 MeV/c (low momentum protons came mainly from nuclear evaporation); (vii) candidates for reactions (2) and (3) have no proton. The total number of events selected, for reactions (1), (2) and (3), respectively, is: 282, 179,266. 3. Kinematical reconstruction, background and losses. A 0 constraint kinematical calculation has been
performed for events/~+Tr-n and ~+lr-p (neglecting the proton in the second case) in order to handle the two samples in the same way. The kinematical reconstruction of the #+~r0n events is not significant since only 36% of these events have 2 7's recognized. To test the reliability of the 0C calculation, the /a+lr-p events were also fitted with the free proton 393
Volume 81B, number 3,4
PHYSICS LETTERS
target hypothesis. 81 events satisfy this hypothesis. Their kinematical quantities 3C- and 0C-fitted are in appreciable agreement [2]. To reduce possible background from interactions o f incoming hadrons, events were rejected if the total longitudinal momentum ptot of the final state partiX cles along the beam direction is less than 0.6 GeV/c. For reactions (2) and (3), ptot is calculated using only X the muon and the pion: the percentage of genuine events lost because of this cut is found to be at most 2% [21. The following sources of background and losses have been considered: (j) background from ~ charged current interactions with two pions produced, one of which escapes detection (n ÷ misclassified as proton, n 0,+- absorbed, 3"s not detected); (jj) loss ofTr 0 with both 7's undetected; (jjj) background from two pion neutral current events with n + misidentified as/a +; (jv) nuclear effects: channel mixing, absorption and additional pion or proton production. Both (]) and (jj) are corrected applying the pion identification probabilities given in table 1. The 27r NC background (jjj) is negligible (<2%) [2]. The nuclear effects (jv) are evaluated using a Monte Carlo technique [3]. In table 2 the number of events selected and corrected for background and losses is shown. The errors also take into account the systematical error induced
Table 1 Pion identification probabilities. n+
n-
n°
2 no
P3'
0.85+-0.10
1
0.84+-0.05 0.48+-0.05 0.60+0.04
26 February 1979
by nuclear corrections [3].
4. Isospin analysis. The three amplitudes for charged current one pion production can be parametrized in terms of two complex isovector amplitudes S 1/2 and S 3/2 p r o d u c i n g / = 1/2 a n d / = 3/2 final states, respectively: S(bt+Tr-p) = ~($3/2 + 2S1/2), S(/.t+rr0n) = ~x/~-($3/2 _ S1/2),
S(/.t+rr-n) = $3/2.
Since our experimental data satisfy the inequality: o(/l%r-n)/[a0z+Tr0n) + o(/l%r-p)] = 0.83 < 3, resulting from the assumption that only the isovector exchange makes a contribution, there is no need to introduce an additional isotensor exchange amplitude. The total cross sections are deduced from the corrected number of events (table 2) taking into account the percentage of neutrons and protons in the liquid (45% and 55%, respectively). They are interpreted in terms of the ratio R ~ :
R ~= IS1/2[/IS3/21 = 1.14 + 0.23, and the phase 4)~ : Cv = arccos (Re ($3/2"S1/2)/1S3/211S1/21)= (94 -+ 13) °. This result is given in the total kinematical range of the experiment and it is consistent with the presence of a resonant I = 3/2 amplitude with a large I = 1/2 aN non-resonant contribution. The calculation of R v is performed also in the more interesting region of the A3/2, 3/2(1232) production, defined by the invariant mass cut: Wn- N ~< 1.4 GeV/c 2. In order to do so, we have to keep the phase fixed, since the 7r0n invariant mass cannot be determined within the present experimental conditions. A value of 90 ° was chosen, compatible with previous
Table 2 Background loss corrections for the total sample.
394
Topology
No. of observed events with Pxt°t > 0.6 GeV/c
No. corrected for 2n background
No. corrected for n° loss
No. corrected for nuclear effects
u+n-p ta+rr0n tt%r-n
263 +- 16 165 +-13 241 -+ 15
246 +- 17 152 +- 13 227 +- 17
246 +- 17 181 +- 19 227 +-17
309 +- 80 257 +-64 385 -+ 86
Volume 81B, number 3,4
PHYSICS LETTERS
Table 3 Background and loss corrections for the reduced sample, 0Cfitted, with n--nucleon invariant mass smaller than 1.4 GeV/c 2. Topology
No. of 0Cfitted events with Wn- N ~<1.4 GeV/c 2
No. corrected for 27r background
159 ± 13 178 ± 13
149 ± 12 168 ± 13
#+u-p ~+n-n
No. corrected for nuclear effects 190 • 50 291 ± 64
[ 9 ( o O r - p ) - -~oOr--n)]/o(rr-n)] 1/2.
=
Along with background and loss corrections, table 3 gives the number of 0C-fitted events with ~< 1.4 GeV/c 2. The total corrected sample gives: ,1 The result does not change sensibly for the values of q~ ranging between the experimental values found in the neup trino exoeriments: q~,NL = (89.2 ± 8.7) ° [6] ; CGGM +12 o = 75_16 ) [4].
t50 "_I
I
%
,~ I
!
!
Phase
-__
Breit-Wigner
a i I I I
tOO
I
_ _
/ I
":
R~ = 0.98 -+ 0.20. This indicates that, even in the A3/2, 3/2 region, there is a large I = 1/2 rr- N contribution. This has been already observed in neutrino pion production: R lV(GGM) = 0.71 + 0.17 (in propane [ 4 ] ) , RI(ANL ) v = 0.57 -+ 0.06 (in deuterium [5]).
neutrino results * With this hypothesis, the ratio R ~ = IS 1/21/I S 3/21 can be given in terms of the two cross sections o(zr-n) and o(Tr - p ) , as follows: R~
26 February 1979
Further information on the isospin structure o f the charged weak current can be extracted from the analysis of the invariant mass distribution of the l r - n system which is a pure I = 3/2 isospin state. Fig. l a shows WTr-n distribution: the A3/2, 3/2 (1232) signal is clearly visible above the phase space, but the uncertainty of the shape o f the phase space as well as nuclear effects do not allow a good fit to the Breit-Wigner A3/2, 3/2 function. In the W~r-p distribution of fig. l b , the A3/2, 3/2 production is less abundant. A3/2, 3/2 production by neutrinos in similar conditions has been recently published [6]. 5. Cross sections. T h e energy dependence of the total cross section has been obtained, for reaction + n -+/J+rr-n and for exclusive g - production, correcting the p + r r - n and # + r r - p 0C energy distributions, in
"u..150I'
Space
---
~I.. ,I,
i'I'
function o':
400[
I
> ID
t,4
o 50 O ~.
Phase Space Brelt-Wigner A,/, , ~ I ,
I I
I I
50
O
Z
z
/ O
0 iO0
i
I 1 I
vi 4,a
II) > tD
t
!
1.50 2.00 2.50 .-n m a s s ( C e V / c ' ) (a)
I
1.00
I
1.50 2.00 2.50 .-p mass (GeV/ci) (b)
Fig. 1. Invariant ~r-n (a) and ~r-p (b) mass distributions. The solid line is the three-body phase space distribution. The dotted line is a Breit-Wigner function of the A3/2,3/2 (1232) resonance. 395
Volume 81B, number 3,4
PHYSICS LETTERS
26 February 1979
~ror ( , - n )
O'TOT ( / t - N )
4 'o
3
x
"3>
ta 2
M ~= 1.5 GeV'?'/~
b
M~ =771 c-~v~,,,"
"' b
I
2
3
4 (a)
5
6
2 M A = t 5 GeV2/c~
'o,.+ 3
2 MA : .71 Gek,"~c~'
2
EgGeV 1
2
3
i 4
i
i
5
6
(a) ~E
W,lt- n ~< 1.4 GeV/c~"
I
I
•
7 8 E - GeV V
Wft- N ~ t.4GeV/¢ z
'9
,,,,~-
M~ =771 GeV~',¢~'
I
t
2
3
4 (b)
5
M~,~5 GeVZE'
lad
MZ= .7t GeV2/c4"
I 6 EgGe V
Fig. 2. Energy dependence of the total cross section for the reaction b- + n --*#+~r-n with and without the invariant mass cut WTr-n ~< 1.4 GeV/c2. The solid curves are the Adler model predictions [7]. the total and reduced (W+r-N ~< 1.4 GeV/c 2) samples. In these calculations the following effects can be neglected [2]: (i) pion absorption in 27r charged current events (<2%), (ii) lr0 charge exchange (~6%), (iii) 2rr neutral current background (<2%). Furthermore, corrections for background and losses are assumed to be energy independent. The experimental points shown in figs. 2 and 3 are compared with the theoretical predictions of the Adler model [7]. A good agreement is found in the region: W+r-N ~< 1.4 GeV/c 2 with a value of the axial mass squared M 2 ranging between 0.71 and 1.5 (GeV/c2) 2. We are grateful to our colleagues of the Gargamelle antineutrino propane collaboration for their assistance in the data reduction and for helpful suggestions. We
396
'0__
i 4
• 2
3
4
5 (b)
6
i 7 8 E-- GeV V
Fig. 3. Energy dependence of the total cross section for the n - exclusive production with and without the invariant mass cut Wn-N ~<1.4 GeV/c2 (N = neutron or proton). The solid curves are the Adler model predictions [7].
are also indebted to the scanning and measuring teams for their work in this experiment.
References [1] O. Erriques et al., Phys. Lett. 73B (1978) 350. [2] T. Bolognese, th6se de 3~me cycle, Strasbourg, CRN-HE/ 78-22 (1978). [3] C. Longuemare, th~se d'Etat, L.A.L. 78/40rsay (1978); M. Pohl, Aachen, preprint, to be published. [4] M. Pohl, PITHA-N.R. 105 (1978). [5] Levman,Topical Conf. on Neutrino physics at accelerators (Oxford, 3-7 July 1978). [6] W. Lerche et al., Phys. Lett. 78B (1978) 510; R.T. Ross et al., Proc. Purdue Conf. (1978); P. Schmid et al., Proc. Purdue Conf. (1978). [7] S. Adler et al., Ann. Phys. (NY) 50 (1968) 189.
PHYSICS LETTERS
26 February 1979
SINGLE PION PRODUCTION IN ANTINEUTRINO INDUCED CHARGED CURRENT INTERACTIONS T. BOLOGNE~E, J.P. ENGEL, J . L GUYONNET and J.L. RIESTER Centre de Recherches NuclEaires et Universit~ Louis Pasteur, Strasbourg, France
Received 15 December 1978
Results are presented on the exclusive charged current antineutrino production of one pion using the data of the Gargamelle propane experiment at CERN PS. The isospin structure of the charged weak current is studied as well as the energy dependence of the total cross section for 7r- antineutrino production, which is compared with the prediction of the Adler model.
1. Introduction. Single pion production by antineutrinos provides important information about the structure of the weak hadronic current. Results on neutral current antineutrino interactions, from the same experiment, have been published previously [1]. Here, for the first time, an analysis of the following three charged current reactions:
~+ p ~#+rr-p,
(1)
+ p ~/a+lr0n,
(2)
+ n -~ ~+rr- n,
(3)
is presented. For this experiment, the big heavy liquid bubble chamber Gargamelle ( " 8 m 3 of visible and ~3 m 3 of fiducial volume) was filled with propane and a small admixture of heavy freon CF3Br and was exposed to the CERN PS antineutrino beam (peaked at EV ~ 1.5 GeV). See ref. [1] for more experimental details. The results presented here are obtained using 80% of the total sample, i.e. ~3 X 105 pictures which have been scanned twice. The mean scanning efficiency is greater than 99% for each of the three channels analysed. 2. Selection criteria. Candidates for reactions (1), (2) and (3) are required to satisfy the following selection criteria: (i) the primary vertex has to be in the fiducial vol-
ume without any possible upstream sources; (ii) the #+ is defined as a leaving or decaying positive track; (iii) the lr- is any negative track ( p - background from neutrino events is negligible in these channels
[2]); (iv) the ~r0 is identified by 1 or 2 materialized 7's, pointing to the primary vertex with a probability greater than 1%; (v) the proton is a stopping track or any positive interacting track not identified as 7r+; (vi) candidates for reaction (1) must have only one proton which has a momentum greater than 240 MeV/c (low momentum protons came mainly from nuclear evaporation); (vii) candidates for reactions (2) and (3) have no proton. The total number of events selected, for reactions (1), (2) and (3), respectively, is: 282, 179,266. 3. Kinematical reconstruction, background and losses. A 0 constraint kinematical calculation has been
performed for events/~+Tr-n and ~+lr-p (neglecting the proton in the second case) in order to handle the two samples in the same way. The kinematical reconstruction of the #+~r0n events is not significant since only 36% of these events have 2 7's recognized. To test the reliability of the 0C calculation, the /a+lr-p events were also fitted with the free proton 393
Volume 81B, number 3,4
PHYSICS LETTERS
target hypothesis. 81 events satisfy this hypothesis. Their kinematical quantities 3C- and 0C-fitted are in appreciable agreement [2]. To reduce possible background from interactions o f incoming hadrons, events were rejected if the total longitudinal momentum ptot of the final state partiX cles along the beam direction is less than 0.6 GeV/c. For reactions (2) and (3), ptot is calculated using only X the muon and the pion: the percentage of genuine events lost because of this cut is found to be at most 2% [21. The following sources of background and losses have been considered: (j) background from ~ charged current interactions with two pions produced, one of which escapes detection (n ÷ misclassified as proton, n 0,+- absorbed, 3"s not detected); (jj) loss ofTr 0 with both 7's undetected; (jjj) background from two pion neutral current events with n + misidentified as/a +; (jv) nuclear effects: channel mixing, absorption and additional pion or proton production. Both (]) and (jj) are corrected applying the pion identification probabilities given in table 1. The 27r NC background (jjj) is negligible (<2%) [2]. The nuclear effects (jv) are evaluated using a Monte Carlo technique [3]. In table 2 the number of events selected and corrected for background and losses is shown. The errors also take into account the systematical error induced
Table 1 Pion identification probabilities. n+
n-
n°
2 no
P3'
0.85+-0.10
1
0.84+-0.05 0.48+-0.05 0.60+0.04
26 February 1979
by nuclear corrections [3].
4. Isospin analysis. The three amplitudes for charged current one pion production can be parametrized in terms of two complex isovector amplitudes S 1/2 and S 3/2 p r o d u c i n g / = 1/2 a n d / = 3/2 final states, respectively: S(bt+Tr-p) = ~($3/2 + 2S1/2), S(/.t+rr0n) = ~x/~-($3/2 _ S1/2),
S(/.t+rr-n) = $3/2.
Since our experimental data satisfy the inequality: o(/l%r-n)/[a0z+Tr0n) + o(/l%r-p)] = 0.83 < 3, resulting from the assumption that only the isovector exchange makes a contribution, there is no need to introduce an additional isotensor exchange amplitude. The total cross sections are deduced from the corrected number of events (table 2) taking into account the percentage of neutrons and protons in the liquid (45% and 55%, respectively). They are interpreted in terms of the ratio R ~ :
R ~= IS1/2[/IS3/21 = 1.14 + 0.23, and the phase 4)~ : Cv = arccos (Re ($3/2"S1/2)/1S3/211S1/21)= (94 -+ 13) °. This result is given in the total kinematical range of the experiment and it is consistent with the presence of a resonant I = 3/2 amplitude with a large I = 1/2 aN non-resonant contribution. The calculation of R v is performed also in the more interesting region of the A3/2, 3/2(1232) production, defined by the invariant mass cut: Wn- N ~< 1.4 GeV/c 2. In order to do so, we have to keep the phase fixed, since the 7r0n invariant mass cannot be determined within the present experimental conditions. A value of 90 ° was chosen, compatible with previous
Table 2 Background loss corrections for the total sample.
394
Topology
No. of observed events with Pxt°t > 0.6 GeV/c
No. corrected for 2n background
No. corrected for n° loss
No. corrected for nuclear effects
u+n-p ta+rr0n tt%r-n
263 +- 16 165 +-13 241 -+ 15
246 +- 17 152 +- 13 227 +- 17
246 +- 17 181 +- 19 227 +-17
309 +- 80 257 +-64 385 -+ 86
Volume 81B, number 3,4
PHYSICS LETTERS
Table 3 Background and loss corrections for the reduced sample, 0Cfitted, with n--nucleon invariant mass smaller than 1.4 GeV/c 2. Topology
No. of 0Cfitted events with Wn- N ~<1.4 GeV/c 2
No. corrected for 27r background
159 ± 13 178 ± 13
149 ± 12 168 ± 13
#+u-p ~+n-n
No. corrected for nuclear effects 190 • 50 291 ± 64
[ 9 ( o O r - p ) - -~oOr--n)]/o(rr-n)] 1/2.
=
Along with background and loss corrections, table 3 gives the number of 0C-fitted events with ~< 1.4 GeV/c 2. The total corrected sample gives: ,1 The result does not change sensibly for the values of q~ ranging between the experimental values found in the neup trino exoeriments: q~,NL = (89.2 ± 8.7) ° [6] ; CGGM +12 o = 75_16 ) [4].
t50 "_I
I
%
,~ I
!
!
Phase
-__
Breit-Wigner
a i I I I
tOO
I
_ _
/ I
":
R~ = 0.98 -+ 0.20. This indicates that, even in the A3/2, 3/2 region, there is a large I = 1/2 rr- N contribution. This has been already observed in neutrino pion production: R lV(GGM) = 0.71 + 0.17 (in propane [ 4 ] ) , RI(ANL ) v = 0.57 -+ 0.06 (in deuterium [5]).
neutrino results * With this hypothesis, the ratio R ~ = IS 1/21/I S 3/21 can be given in terms of the two cross sections o(zr-n) and o(Tr - p ) , as follows: R~
26 February 1979
Further information on the isospin structure o f the charged weak current can be extracted from the analysis of the invariant mass distribution of the l r - n system which is a pure I = 3/2 isospin state. Fig. l a shows WTr-n distribution: the A3/2, 3/2 (1232) signal is clearly visible above the phase space, but the uncertainty of the shape o f the phase space as well as nuclear effects do not allow a good fit to the Breit-Wigner A3/2, 3/2 function. In the W~r-p distribution of fig. l b , the A3/2, 3/2 production is less abundant. A3/2, 3/2 production by neutrinos in similar conditions has been recently published [6]. 5. Cross sections. T h e energy dependence of the total cross section has been obtained, for reaction + n -+/J+rr-n and for exclusive g - production, correcting the p + r r - n and # + r r - p 0C energy distributions, in
"u..150I'
Space
---
~I.. ,I,
i'I'
function o':
400[
I
> ID
t,4
o 50 O ~.
Phase Space Brelt-Wigner A,/, , ~ I ,
I I
I I
50
O
Z
z
/ O
0 iO0
i
I 1 I
vi 4,a
II) > tD
t
!
1.50 2.00 2.50 .-n m a s s ( C e V / c ' ) (a)
I
1.00
I
1.50 2.00 2.50 .-p mass (GeV/ci) (b)
Fig. 1. Invariant ~r-n (a) and ~r-p (b) mass distributions. The solid line is the three-body phase space distribution. The dotted line is a Breit-Wigner function of the A3/2,3/2 (1232) resonance. 395
Volume 81B, number 3,4
PHYSICS LETTERS
26 February 1979
~ror ( , - n )
O'TOT ( / t - N )
4 'o
3
x
"3>
ta 2
M ~= 1.5 GeV'?'/~
b
M~ =771 c-~v~,,,"
"' b
I
2
3
4 (a)
5
6
2 M A = t 5 GeV2/c~
'o,.+ 3
2 MA : .71 Gek,"~c~'
2
EgGeV 1
2
3
i 4
i
i
5
6
(a) ~E
W,lt- n ~< 1.4 GeV/c~"
I
I
•
7 8 E - GeV V
Wft- N ~ t.4GeV/¢ z
'9
,,,,~-
M~ =771 GeV~',¢~'
I
t
2
3
4 (b)
5
M~,~5 GeVZE'
lad
MZ= .7t GeV2/c4"
I 6 EgGe V
Fig. 2. Energy dependence of the total cross section for the reaction b- + n --*#+~r-n with and without the invariant mass cut WTr-n ~< 1.4 GeV/c2. The solid curves are the Adler model predictions [7]. the total and reduced (W+r-N ~< 1.4 GeV/c 2) samples. In these calculations the following effects can be neglected [2]: (i) pion absorption in 27r charged current events (<2%), (ii) lr0 charge exchange (~6%), (iii) 2rr neutral current background (<2%). Furthermore, corrections for background and losses are assumed to be energy independent. The experimental points shown in figs. 2 and 3 are compared with the theoretical predictions of the Adler model [7]. A good agreement is found in the region: W+r-N ~< 1.4 GeV/c 2 with a value of the axial mass squared M 2 ranging between 0.71 and 1.5 (GeV/c2) 2. We are grateful to our colleagues of the Gargamelle antineutrino propane collaboration for their assistance in the data reduction and for helpful suggestions. We
396
'0__
i 4
• 2
3
4
5 (b)
6
i 7 8 E-- GeV V
Fig. 3. Energy dependence of the total cross section for the n - exclusive production with and without the invariant mass cut Wn-N ~<1.4 GeV/c2 (N = neutron or proton). The solid curves are the Adler model predictions [7].
are also indebted to the scanning and measuring teams for their work in this experiment.
References [1] O. Erriques et al., Phys. Lett. 73B (1978) 350. [2] T. Bolognese, th6se de 3~me cycle, Strasbourg, CRN-HE/ 78-22 (1978). [3] C. Longuemare, th~se d'Etat, L.A.L. 78/40rsay (1978); M. Pohl, Aachen, preprint, to be published. [4] M. Pohl, PITHA-N.R. 105 (1978). [5] Levman,Topical Conf. on Neutrino physics at accelerators (Oxford, 3-7 July 1978). [6] W. Lerche et al., Phys. Lett. 78B (1978) 510; R.T. Ross et al., Proc. Purdue Conf. (1978); P. Schmid et al., Proc. Purdue Conf. (1978). [7] S. Adler et al., Ann. Phys. (NY) 50 (1968) 189.