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Comparison of ø production by π, K and p incident on protons at 10 GeV
Volume 68B, number 4
COMPARISON
PHYSICS LETTERS
OF # PRODUCTION ON PROTONS
20 June 1977
BY A, K AND p INCIDENT
AT 10 CeV
R. BALDI, T. BijHRINGER, P.A. DORSAZ, V. HUNGERBUHLER, M.W. KIENZLE-FOCACCI, M. MARTIN, A. MERMOUD, C. NEF and P. SIEGRIST University of Geneva, Switzerland
Received 2 March 1977 @production has been measured at 10 GeV/c in the reactions hp -+ h@p, with h = n-, K’, p. The G/W production ratio is compatible with the value expected from the w-6 mixing angle for n incident, thus requiring no violation of the Zweig rule, whereas the p incident data indicates a ratio bigger than expected.
The discovery of the narrow width vector mesons J//J’s has raised questions on the foundation and limits of the validity of the Zweig rule [l ,2] . In this respect it is interesting to compare experimentally the production of nearly pure sS states (as @,f ‘) in reactions containing no other particle with strange quarks, to reactions containing strange particles. Specifically, we present results on 4 production in the reactions hp+h@p,withh=n-,K’,pand@+K+K-. The data have been obtained by a counter.experiment at the CERN PS in an unseparated beam of 10 GeV/c momentum. The experiment was designed to study the reaction K’p + KOrr*p, measuring the recoil proton in the momentum transfer range 0.05 < Itl < 1 (GeV/c)2 and the direction of the forward particles. Since the system was not saturated by this channel we have also collected events with three forward particles produced by incident n and p. The experimental set-up consists of 1) An incident beam spectrometer which measures momentum and direction of the incident particles by scintillation hodoscopes and beam transport magnets. It identifies the particles by four threshold Cerenkov counters, two set to count rr and K, two only n. The contamination on 71and p by K is less than 10-4. 2) A recoil detector, consisting of 6 MWPC planes, time-of-flight and range counters, which measures the direction and momentum of the recoil proton. 3) A set of 8 MWPC planes to measure the direction of the forward emitted particles. Their momenta are not measured. A detailed description of the apparatus can be found in ref. [3].
Events corresponding
to the channel
hp+hK+K-pwithh=n-,K”,p are selected by the following requirements: A recoil proton and three forward tracks coming from the proton vertex must be reconstructed. For each reaction we choose a kinematical region of the recoil proton such that the total system acceptance does not vary by more than 30 per cent. In terms of the momentum transfer t and of the missing mass to the recoil proton M, these regions are 0.1 < I tj < 0.8 (G~V/C)~ and 1.4
1.9 GeV/c2 for n-,
1.6
< 2.0 GeV/c2 for K* ,
2.0
< 2.5 GeV/c2 for p.
The momentum conservation is used to calculate the three forward particle momenta. The channel is then selected with a cut of *lo MeV on the energy balance AE = Einc + mp - Erecoil -
ig3
Ei
using the mass hypotheses hKK to calculate the energies Eiof the three forward particles. Examples of energy balance histograms for rr, K and p are shown in fig. 1. The background level in the A,!? distribution is very high (-70% for n, 40% for K and 90% for p) but decreases by about 30% at the K+K- threshold. We think 381
Volume 68B, number 4
PHYSICS LETTERS
K+P
UK+
K+K-p
K+K-p
-LO
-
I
I
I
20
0
20
I
LO
AEIMEVI
Fig. 1. Energy balance AE = Emc + mp - Erecoil- X+1,3 Ei for the reactions hp -+ hK+K-p with h = II-, K*, p.
this background is mainly due to hp + hn+n-(nnO)p contamination; less than 1% of hp + hK+K-p events fulfill also the hypothesis hp + hn+n-p. In the case where h is n or p, there are three possible hKK mass hypotheses, all three are plotted in fig. 1. In the limits 1AEl < 10 MeV, about 30% of the events have more than one hKK assignment. The number of events in each hK+K- channel is estimated by fitting a second order polynomial to the AE spectrum in the interval 40 to -10 MeV, +lO to +40 MeV and counting the excess of events in the 382
20 June 1911
range -10 to +lO MeV. We give in table 1 the number of events together with the partial cross sections Ao(hKK) in the measured AM and At intervals for each channel. The cross section Au is calculated taking into account the system acceptance and efficiency and the events lost by the AE cut. We also parametrize the tdependence of the data with the exponential form do/dt = exp(u + b Itl). For each reaction we obtain a good fit; the parameter b is used to obtain the t-integrated cross section do (table 1). In order to estimate $ production cross sections, the background subtraction is done as function of the K+K- effective mass. The resulting spectra are shown in fig. 2. The 4 signal dominates the spectra of K* incident and is also present in p induced reactions. The ‘IIinduced spectrum shows a three standard deviations effect above background. Under the hypothesis that the $ is produced in each channel, we have fitted the data with a Breit-Wigner form (mass, width and amplitude free) plus a polynomial background taking into account the experimental mass resolution o = 5 MeV. The I$ mass and width are consistent between the different reactions (table 2) and agree well with the world average [4] . From the fitted number of events in the $-peak we calculate partial cross sections Au(@) and t-integrated cross sections da, taking into account the 4 + K+K- branching ratio of 0.466 [4] . For the t-integration we use the slope b of the full hKK channel whenever too few @events prevent from taking an independent measurement. (We notice in fact that KKK and Kc$ t-dependence are compatible within errors (table l).) The quoted errors in table 1 take into account the statistical errors and the uncertainty of the fit on the background line. For the systematic errors we estimate an error of 5 per cent on the corrections due to acceptance and inefficiency of the system, 12 per cent for the method of subtracting background and 25 per cent whenever we have to extrapole on total cross sections without measuring the t-dependence directly. To test the Zweig rule we calculate the G/o production ratio R = do (hp + @hp)/do(hp + ohp). For o production we use cross sections measured in the corresponding channels at the same energy [5-81 and we scale them to our mass intervals (table 1). In the SU(3) model the observed w and C#J are not pure singlet and octet members of the vector nonet. From the Cell-Mann-Okubo quadratic mass formula
Volume 68B, number 4
PHYSICS LETTERS
20 June 1977
Table 1 Cross sections and comparison to w production Reaction
Mass interval Ar%f(GeV/c2)
K-p+
1.6 - 2.0 1.6 - 2.0
K-K+K-p -+K-@P
Events
Ao* (crb)
Slope b** (GeV/c)-2
67 36
4.25 f 0.13 5.15 f 0.21
5.8 + 0.8 5.6 f 0.3
7.73 f 0.24 9.2 f 0.4
78700 f 800 383 f 86
9.0 + 0.1 0.13 ?r 0.03
6.0 f 0.3 -
16.7 f 0.2 0.24 f 0.05
37
14.5 + 1.1
0.77
50
0.020 f 0.005 (0.008)
2143+ 905 f
do*** (pb)
n-p -, s-K+K-p +n@p
1.4 - 1.9 1.4 - 1.9
K+p + K+K+K-p -+ K+@p
1.6 - 2.0 1.6 - 2.0
6899i 2504i
116 62
5.85 * 0.09 6.41 f 0.15
4.8 + 0.2 5.3 f 0.1
9.9 f 0.2 11.2 f 0.2
pp +pK+K-p -cP@P
2 2
7706 f 366 721 f 73
2.2 f 0.1 0.60 i 0.06
4.7 f 0.6 -
3.7 f 0.2 1.0 * 0.1
- 2.5 - 2.5
w production? (pb)
Ratio @/wtt
14.7 f
0.63
0.7
*16
*10
f 0.04 (0.23)
0.006 i 0.003 (0.002) f 0.06 (0.30)
* Partialcross section in the measured m and Ar (0.1-0.8 (GeV/c)?) interval. The systematic error is estimated to i15 per cent. **Parameter b of an exponential tit to the data do/dt = exp(a + blrl). ***t-integrated cross section. The systematic error is estimated to be k30 percent. t Cross sections from refs. [5-81 scaled to our mass interval Akf, and corrected for the branching ratio 0.899 (ref. [4]). tt In parentheses we indicate the systematic error.
d) PP-K+
a) K-p-K+K-K-p
I.1
Mass (K’K-I Gev/c’
K- pp
c) K’p-K’K-K+p
I.
1.05
Fig. 2. K+K- effective mass in 5 MeV bins, after background subtraction, for the channels a) K-p+ c) K+p+ K+K-K+p, d) pp -* K+K-pp.
I.1 MassCK’K-)Gev/c*
K+K-K-P,
b) n-P--t K+K-n-p, 383
Volume 68B, number 4
PHYSICS LETTERS
20 June 1911
Table 2 I#Jmass and width from a Breit-Wigner fit
x is mcldent o TT incident l p madent
Reaction
Mass (MeV)
Width (MeV)
P(x2)
K-p + K-Q p n-p-+ IT-@p K+p+ K+@p PP -+P@P
1020.4 1020 1020.0 1022
6 4 9 11
0.3 0.3 0.05 0.90
World average [4]
1019.7 f 0.3
R
f 0.3 *1 It 0.2 +1
+2 +2 *2 *6
4.1 2 0.2
This angle is greater than the ideal mixing angle by A8 = (3.5 f 0.1)’ so that the observed $Jis not a pure ST state. If we assume that the Zweig rule holds exactly no sS pair can be produced in reactions where no other s-quark is present. We can produce the $ only via its dd and uii components. The ratio R between r$ and w production, in reactions with no s-quark, is then expected to be R = tg2Ar3 [2] i.e. R = 0.0037 + 0.0003. The experimental ratios are compatible within the systematic errors with this expectation. For incident protons we find a value R = 0.020 + (0.005 statistical error, +0.008 systematic error) which corresponds to Ar3 = (8.0 + 1.7)“. For incident pions we find R = 0.006 + (0.003 statistical error, ?0.002 systematic error) which corresponds to A8 = (4.4 + 1.5)‘. We may notice, however, that r$ seems to be produced more abundantly by p incident, than by rr incident. A comparison of the value R found in different reactions [9-131 at various energies (fig. 3) indicates a similar behaviour: all n-induced data are consistent with the value R expected by mixing angle, whereas the p and p induced data are systematically higher. Such a trend, if confirmed, may indicate that the nonstrange content in the #Jis not the complete production mechanism, but that $Jis produced also from strange quarks in the incident particle, with protons containing more ss pairs than pions.
384
acknowledge
the hospitality
1
0.002
(using the experimental masses of the neutral vector mesons [4] ) one finds a mixing angle of (38.8 + 0.1)“.
We gratefully
This experiment
10-2
and tech-
0
5
10
15
20
Pint (GeV/cI
Fig. 3. Ratio between @and w production for reactions with no s valence quark present. A compilation as function of energy of different reactions.
nical assistance of CERN, as well as the loan of equipment by the EP Division. We thank the Fonds National Suisse de la Recherche Scientifique for support of the project, and the technicians of the University of Geneva for expert assistance.
References [l] G. Zweig, CERN-TH 412 (1964) unpublished. [2] HJ. Lipkin,Phys. Lett. B60 (1976) 371. [ 31 R. Baldi et al., Large. acceptance spectrometer for simple event topologies, paper submitted to the EPS Conference, Palermo, Italy (1975). [4] Particle Data Group, Rev. Mod. Phys. 48 (1976). [5] S.P. Almeida et al., Phys. Rev. 174 (1968) 1638. [6] N.M. Cason et al.,Phys. Rev. D l(l970) 851. [7] N.N. Biswas, Phys. Rev. 134 B (1964) 901. [8] ABCLV and BBCMS Collaboration CERN-EPPHYS 76[9] [lo] [ 111 [12] [13]
E*Cohen et al., Phys. Rev. Lett. 38 (1977) 269. P.L. Woodworth et al., Phys. L&t. B65 (1976) 89. G. Hentschel, thesis - MPI-PAE/Exp. El. 56 (1976). R.A. Donald et al.,Phys. Lett. B 61 (1976) 210. V. Blobel et al., Phys. Lett. B 59 (1975) 88.
COMPARISON
PHYSICS LETTERS
OF # PRODUCTION ON PROTONS
20 June 1977
BY A, K AND p INCIDENT
AT 10 CeV
R. BALDI, T. BijHRINGER, P.A. DORSAZ, V. HUNGERBUHLER, M.W. KIENZLE-FOCACCI, M. MARTIN, A. MERMOUD, C. NEF and P. SIEGRIST University of Geneva, Switzerland
Received 2 March 1977 @production has been measured at 10 GeV/c in the reactions hp -+ h@p, with h = n-, K’, p. The G/W production ratio is compatible with the value expected from the w-6 mixing angle for n incident, thus requiring no violation of the Zweig rule, whereas the p incident data indicates a ratio bigger than expected.
The discovery of the narrow width vector mesons J//J’s has raised questions on the foundation and limits of the validity of the Zweig rule [l ,2] . In this respect it is interesting to compare experimentally the production of nearly pure sS states (as @,f ‘) in reactions containing no other particle with strange quarks, to reactions containing strange particles. Specifically, we present results on 4 production in the reactions hp+h@p,withh=n-,K’,pand@+K+K-. The data have been obtained by a counter.experiment at the CERN PS in an unseparated beam of 10 GeV/c momentum. The experiment was designed to study the reaction K’p + KOrr*p, measuring the recoil proton in the momentum transfer range 0.05 < Itl < 1 (GeV/c)2 and the direction of the forward particles. Since the system was not saturated by this channel we have also collected events with three forward particles produced by incident n and p. The experimental set-up consists of 1) An incident beam spectrometer which measures momentum and direction of the incident particles by scintillation hodoscopes and beam transport magnets. It identifies the particles by four threshold Cerenkov counters, two set to count rr and K, two only n. The contamination on 71and p by K is less than 10-4. 2) A recoil detector, consisting of 6 MWPC planes, time-of-flight and range counters, which measures the direction and momentum of the recoil proton. 3) A set of 8 MWPC planes to measure the direction of the forward emitted particles. Their momenta are not measured. A detailed description of the apparatus can be found in ref. [3].
Events corresponding
to the channel
hp+hK+K-pwithh=n-,K”,p are selected by the following requirements: A recoil proton and three forward tracks coming from the proton vertex must be reconstructed. For each reaction we choose a kinematical region of the recoil proton such that the total system acceptance does not vary by more than 30 per cent. In terms of the momentum transfer t and of the missing mass to the recoil proton M, these regions are 0.1 < I tj < 0.8 (G~V/C)~ and 1.4
1.9 GeV/c2 for n-,
1.6
< 2.0 GeV/c2 for K* ,
2.0
< 2.5 GeV/c2 for p.
The momentum conservation is used to calculate the three forward particle momenta. The channel is then selected with a cut of *lo MeV on the energy balance AE = Einc + mp - Erecoil -
ig3
Ei
using the mass hypotheses hKK to calculate the energies Eiof the three forward particles. Examples of energy balance histograms for rr, K and p are shown in fig. 1. The background level in the A,!? distribution is very high (-70% for n, 40% for K and 90% for p) but decreases by about 30% at the K+K- threshold. We think 381
Volume 68B, number 4
PHYSICS LETTERS
K+P
UK+
K+K-p
K+K-p
-LO
-
I
I
I
20
0
20
I
LO
AEIMEVI
Fig. 1. Energy balance AE = Emc + mp - Erecoil- X+1,3 Ei for the reactions hp -+ hK+K-p with h = II-, K*, p.
this background is mainly due to hp + hn+n-(nnO)p contamination; less than 1% of hp + hK+K-p events fulfill also the hypothesis hp + hn+n-p. In the case where h is n or p, there are three possible hKK mass hypotheses, all three are plotted in fig. 1. In the limits 1AEl < 10 MeV, about 30% of the events have more than one hKK assignment. The number of events in each hK+K- channel is estimated by fitting a second order polynomial to the AE spectrum in the interval 40 to -10 MeV, +lO to +40 MeV and counting the excess of events in the 382
20 June 1911
range -10 to +lO MeV. We give in table 1 the number of events together with the partial cross sections Ao(hKK) in the measured AM and At intervals for each channel. The cross section Au is calculated taking into account the system acceptance and efficiency and the events lost by the AE cut. We also parametrize the tdependence of the data with the exponential form do/dt = exp(u + b Itl). For each reaction we obtain a good fit; the parameter b is used to obtain the t-integrated cross section do (table 1). In order to estimate $ production cross sections, the background subtraction is done as function of the K+K- effective mass. The resulting spectra are shown in fig. 2. The 4 signal dominates the spectra of K* incident and is also present in p induced reactions. The ‘IIinduced spectrum shows a three standard deviations effect above background. Under the hypothesis that the $ is produced in each channel, we have fitted the data with a Breit-Wigner form (mass, width and amplitude free) plus a polynomial background taking into account the experimental mass resolution o = 5 MeV. The I$ mass and width are consistent between the different reactions (table 2) and agree well with the world average [4] . From the fitted number of events in the $-peak we calculate partial cross sections Au(@) and t-integrated cross sections da, taking into account the 4 + K+K- branching ratio of 0.466 [4] . For the t-integration we use the slope b of the full hKK channel whenever too few @events prevent from taking an independent measurement. (We notice in fact that KKK and Kc$ t-dependence are compatible within errors (table l).) The quoted errors in table 1 take into account the statistical errors and the uncertainty of the fit on the background line. For the systematic errors we estimate an error of 5 per cent on the corrections due to acceptance and inefficiency of the system, 12 per cent for the method of subtracting background and 25 per cent whenever we have to extrapole on total cross sections without measuring the t-dependence directly. To test the Zweig rule we calculate the G/o production ratio R = do (hp + @hp)/do(hp + ohp). For o production we use cross sections measured in the corresponding channels at the same energy [5-81 and we scale them to our mass intervals (table 1). In the SU(3) model the observed w and C#J are not pure singlet and octet members of the vector nonet. From the Cell-Mann-Okubo quadratic mass formula
Volume 68B, number 4
PHYSICS LETTERS
20 June 1977
Table 1 Cross sections and comparison to w production Reaction
Mass interval Ar%f(GeV/c2)
K-p+
1.6 - 2.0 1.6 - 2.0
K-K+K-p -+K-@P
Events
Ao* (crb)
Slope b** (GeV/c)-2
67 36
4.25 f 0.13 5.15 f 0.21
5.8 + 0.8 5.6 f 0.3
7.73 f 0.24 9.2 f 0.4
78700 f 800 383 f 86
9.0 + 0.1 0.13 ?r 0.03
6.0 f 0.3 -
16.7 f 0.2 0.24 f 0.05
37
14.5 + 1.1
0.77
50
0.020 f 0.005 (0.008)
2143+ 905 f
do*** (pb)
n-p -, s-K+K-p +n@p
1.4 - 1.9 1.4 - 1.9
K+p + K+K+K-p -+ K+@p
1.6 - 2.0 1.6 - 2.0
6899i 2504i
116 62
5.85 * 0.09 6.41 f 0.15
4.8 + 0.2 5.3 f 0.1
9.9 f 0.2 11.2 f 0.2
pp +pK+K-p -cP@P
2 2
7706 f 366 721 f 73
2.2 f 0.1 0.60 i 0.06
4.7 f 0.6 -
3.7 f 0.2 1.0 * 0.1
- 2.5 - 2.5
w production? (pb)
Ratio @/wtt
14.7 f
0.63
0.7
*16
*10
f 0.04 (0.23)
0.006 i 0.003 (0.002) f 0.06 (0.30)
* Partialcross section in the measured m and Ar (0.1-0.8 (GeV/c)?) interval. The systematic error is estimated to i15 per cent. **Parameter b of an exponential tit to the data do/dt = exp(a + blrl). ***t-integrated cross section. The systematic error is estimated to be k30 percent. t Cross sections from refs. [5-81 scaled to our mass interval Akf, and corrected for the branching ratio 0.899 (ref. [4]). tt In parentheses we indicate the systematic error.
d) PP-K+
a) K-p-K+K-K-p
I.1
Mass (K’K-I Gev/c’
K- pp
c) K’p-K’K-K+p
I.
1.05
Fig. 2. K+K- effective mass in 5 MeV bins, after background subtraction, for the channels a) K-p+ c) K+p+ K+K-K+p, d) pp -* K+K-pp.
I.1 MassCK’K-)Gev/c*
K+K-K-P,
b) n-P--t K+K-n-p, 383
Volume 68B, number 4
PHYSICS LETTERS
20 June 1911
Table 2 I#Jmass and width from a Breit-Wigner fit
x is mcldent o TT incident l p madent
Reaction
Mass (MeV)
Width (MeV)
P(x2)
K-p + K-Q p n-p-+ IT-@p K+p+ K+@p PP -+P@P
1020.4 1020 1020.0 1022
6 4 9 11
0.3 0.3 0.05 0.90
World average [4]
1019.7 f 0.3
R
f 0.3 *1 It 0.2 +1
+2 +2 *2 *6
4.1 2 0.2
This angle is greater than the ideal mixing angle by A8 = (3.5 f 0.1)’ so that the observed $Jis not a pure ST state. If we assume that the Zweig rule holds exactly no sS pair can be produced in reactions where no other s-quark is present. We can produce the $ only via its dd and uii components. The ratio R between r$ and w production, in reactions with no s-quark, is then expected to be R = tg2Ar3 [2] i.e. R = 0.0037 + 0.0003. The experimental ratios are compatible within the systematic errors with this expectation. For incident protons we find a value R = 0.020 + (0.005 statistical error, +0.008 systematic error) which corresponds to Ar3 = (8.0 + 1.7)“. For incident pions we find R = 0.006 + (0.003 statistical error, ?0.002 systematic error) which corresponds to A8 = (4.4 + 1.5)‘. We may notice, however, that r$ seems to be produced more abundantly by p incident, than by rr incident. A comparison of the value R found in different reactions [9-131 at various energies (fig. 3) indicates a similar behaviour: all n-induced data are consistent with the value R expected by mixing angle, whereas the p and p induced data are systematically higher. Such a trend, if confirmed, may indicate that the nonstrange content in the #Jis not the complete production mechanism, but that $Jis produced also from strange quarks in the incident particle, with protons containing more ss pairs than pions.
384
acknowledge
the hospitality
1
0.002
(using the experimental masses of the neutral vector mesons [4] ) one finds a mixing angle of (38.8 + 0.1)“.
We gratefully
This experiment
10-2
and tech-
0
5
10
15
20
Pint (GeV/cI
Fig. 3. Ratio between @and w production for reactions with no s valence quark present. A compilation as function of energy of different reactions.
nical assistance of CERN, as well as the loan of equipment by the EP Division. We thank the Fonds National Suisse de la Recherche Scientifique for support of the project, and the technicians of the University of Geneva for expert assistance.
References [l] G. Zweig, CERN-TH 412 (1964) unpublished. [2] HJ. Lipkin,Phys. Lett. B60 (1976) 371. [ 31 R. Baldi et al., Large. acceptance spectrometer for simple event topologies, paper submitted to the EPS Conference, Palermo, Italy (1975). [4] Particle Data Group, Rev. Mod. Phys. 48 (1976). [5] S.P. Almeida et al., Phys. Rev. 174 (1968) 1638. [6] N.M. Cason et al.,Phys. Rev. D l(l970) 851. [7] N.N. Biswas, Phys. Rev. 134 B (1964) 901. [8] ABCLV and BBCMS Collaboration CERN-EPPHYS 76[9] [lo] [ 111 [12] [13]
E*Cohen et al., Phys. Rev. Lett. 38 (1977) 269. P.L. Woodworth et al., Phys. L&t. B65 (1976) 89. G. Hentschel, thesis - MPI-PAE/Exp. El. 56 (1976). R.A. Donald et al.,Phys. Lett. B 61 (1976) 210. V. Blobel et al., Phys. Lett. B 59 (1975) 88.