- Email: [email protected]
Measurement of inclusive production of neutral hadrons from Z decays with L3 detector at LEP
IUCLEARPHYSIC.{ PROCEEDINGS SUPPLEMENTS EI.SKVIHR
Nuclear Physics B (Proc. Suppl.) 39B,C (1995) 153-156
Measurement of inclusive production of neutral hadrons from Z decays with L3 detector at L E P F. Becattini, a aUniversit~ di Firenze and INFN Sezione di Firenze, Largo E. Fermi 2, 50125, Firenze, Italy A measurement of inclusive rate and spectra of 7r°j/,K°,A, w and O' performed with L3 detector at LEP is described. Data are compared with perturbative QCD calculations.
1. I N T R O D U C T I O N Analytical calculations performed in the Modified Leading Log Approximation (MLLA) framework of perturbative QCD give a prediction for the spectra of hadrons produced after the parton shower evolution, once the Local Parton Hadron Duality (LPHD) [1,2] hypothesis is assumed. The spectrum function, including coherence effects of gluon radiation, known as l i m i t i n g s p e c t r u m , can be expressed for any hadron h as 1 dCrh _ N ( V f ~ ) " f ( v ~ , A ~ 1 l ; ( p ) , ah d ~ v
(1)
where {p = l o g ( v ~ / 2 p ) and v ~ is the total energy. The function f is specified in [1]. The only free parameters in (1) are N(v/-~) and A~/] which are to be determined with a fit to the data. N is an overall normalization factor which embodies all fragmentation effects, according to LPHD, while A~/] is a QCD scale parameter not directly related to AMS. In this report I will describe a measurement of inclusive rate and spectra of neutral hadrons ~r°,r/,K°,A and preliminary results for ca,r/' as well, performed with L3 detector at the e+e - collider LEP at x/~ ~ 91 GeV. Data are confronted with (1) and with Monte Carlo generators JETSET 7.3 [3] and HERWIG 5.4 [4] predictions. 2. L3 D E T E C T O R The L3 detector [5] consists of several specialized subdetectors arranged in a cylindrical geometry around the beam axis. These subdetectors are 0920-5632/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSDI 0920-5632(95)00062-3
a silicon vertex detector (SMD), a central tracking chamber, a high resolution electromagnetic calorimeter made of bismuth germanium oxide (BGO), a uranium and brass hadronic calorimeter and an accurate muon chamber system. They are installed within a 12 m diameter conventional magnet providing an uniform field of 0.5 T along the beam direction.
3. D A T A S A M P L E A N D E V E N T S E L E C TION The overall data sample used in this analysis consisted of approximately 929,000 Z --* q~ events collected by L3 during 1991 and 1992 runs, corresponding to an integrated luminosity of ~ 35 pb -1. 7r° and 7/are identified by means of their decays into photons pairs. Photons are detected as isolated energy clusters in BGO, selected requiring no tracks pointing to them and a cluster shape consistent with an electromagnetic shower. K ° and A are reconstructed using their decays into two charged particles, 7r+Tr- and 7rp respectively. Due to their long lifetimes (~_ 10 - l ° see.) the signature of these neutral hadrons is a clear secondary vertex within the tracking chamber region. ca is reconstructed in two different channels, ~r°7 and 7r+Tr-~r° respectively, while rf is detected by means of its decay into ~r+lr-r/. For all channels involving charged particles well measured tracks with momentum greater than _~ 100 MeV/c are required.
E Becattini/Nuclear Physics B (Proc. Suppl.) 39B, C (1995) 153-156
154
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,,,I,,,,I
....
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I,~
0.05 0.1 0.15 0.2
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Figure 1. xp spectra compared with JETSET. Bars include statistical and systematic errors.
Figure 2. xp spectra compared with HERWIG. Bars include statistical and systematic errors.
4. P A R T I C L E S S P E C T R A A N D M U L T I PLICITY MEASUREMENT
pected validity range 1 < 6 < l°g(v~/2A~/l)" A considerable feature of the ~v spectrum is the presence of a clear maximum which is due to suppression of small-x gluon emission and whose position ~ depends just on A~/I. Results from the fit are shown in table 2. The dependence of ~p on energy can be determined from the same function (1) once a value of A~fl has been fixed. This is shown in figure 6 together with measurements at lower energy. It is seen that the evolution of this parameter is succesfully described by MLLA predictions.
In order to measure particles spectra we chose the variables xp = 2p/v'~ and ~p = log(1/xp), well-suited for perturbative QCD analysis. We then divided our sample into different Xp, (p bins and for each bin, signals have been extracted from data using invariant mass distributions. Final spectra have been obtained after deconvolution from detector effects and division by efficiencies, both estimated with Monte Carlo. The xp spectra (fig. (1-4)) generally agree with JETSET and flERWIG predictions, with the noticeable exception of r/ whose spectrum is predicted to be too soft by both generators. Moreover, there is a clear discrepancy between data and HERWIG, concerning A spectrum. This effect has been investigated and found to be caused by the exceedingly high production of -- hadrons. Total multiplicities per hadronic Z decay have been determined by extrapolating those measured in the accessible xp-range using Monte Carlo generators spectrum. The (p spectra have been fitted to (1) in its ex-
5. C O N C L U S I O N S The study of inclusive hadrons production at the Z peak using L3 detector and data from lower energy experiments clearly indicates that perturbative QCD correctly describes both shape and energy evolution of spectra. A general pretty good agreement is also found with predictions from JETSET and HERWIG parton shower and hadronisation programs.
155
E Becattini/Nuclear Physics B (Proc. Suppl.) 39B, C (1995) 153-156
Table 1 Measured multiplicities. The measured value using J E T S E T and HERWIG for the efficiency calculation are reported separately. The average of the two is listed in the last column as the final result. The first error is statistical and the second systematic, w and r/ values are preliminary, estimated with J E T S E T and errors are only statistical. Hadron 7r° 7/ h "° A +/~ w ~l'
JETSET Meas. 9.77 0.95 1.02 0.37
HERWIG Pred. 9.63 1.21 1.08 0.37 1.33 0.67
Multiplicity
Meas. 8.60 0.88 1.01 0.37
Pred. 9.81 1.31 1.09 0.48
9.18 0.91 1.02 0.37 1.20 0.25
± ± ± ± ± ±
0.03 0.02 0.01 0.01 0.10 0.04
± ± ± ±
0.73 0.11 0.07 0.04
Table 2 Measured parameters of function (1). w values are preliminary and errors are only statistical. Hadron ~r° K° A +/~ w
0.492 0.123 0.102 0.041 0.140
± ± ± ± ±
N 0.051 0.016 0.005 0.004 0.011
A~]! (GeV) 0.147 ± 0.030 1.485 ± 0.234 0.832 ± 0.061 0.917 ± 0.178 1.105 ± 0.154
3.96 2.52 2.89 2.83 2.70
± ± 4± ±
~; 0.13 0.10 0.05 0.13 0.10
6. A C K N O W L E D G E M E N T S I would like to thank all members of the L3 collaboration and particularly Y.J. Pei and Y.F. Wang.
• DATA (c~-*'x°'f) • DATA (co-.z+x'x °) J:::
REFERENCES 1. Y. Dokshitzer, S. Troyan, Leningrad Preprint LNPI 92 (1984); Y. Azimov et al., Z. Phys. C 27 (1985) 65; C 31 (1986); V. Khoze et al., Lurid Preprint LU T P 90-12. 2. D. Amati, G. Veneziano, Phys. Lett. B 83 (1979) 87. 3. T. SjSstrand, Comp. Phys. Comm. 39 (1986) 347. 4. G. Marches±n± and B. Webber, Nucl. Phys. B 310 (1988) 461; G. Marches±n± et al., Comp. Phys. Comm. 67 (1992) 465. 5. L3 Collaboration, B. Adeva et al., Nucl. Instr. Meth. A 289 (1990) 35.
e-,-q
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0
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~
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0.08
0.12
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0,16
I ....
0.2
I
....
0.24
I.,I
0.28
Xp Figure 3. w xp-spectrum compared with J E T SET. Errors are only statistical.
] 56
E Becattini/Nuclear Physics B (Proc. Suppl.) 39B, C (1995) 153-156
5
• DATA ~QCD A~.~147 MeV
4
'~
,~O
2
0
0
2
4
/./ii ~ ' ' ~ j 1 2 3
6
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0,4
w K"
A
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p 0,6
0.8
1
1.2
1.4
1.6
P o r U c l e M o s s (GeV)
Figure 4. The inclusive (p spectra in comparison with fitted function (1) for 7r°, ~7, K° and A. The actual fitted region is drawn as a solid line. Error bars include statistical and systematic contributions.
4
• L3 [3 JADE
/
o ARGUS & Crystal Ball • TASSO
. /
~
.
"K°,,," ~ " K /
QCD (MLLA)
10
4s (GeV)
10
2
Figure 5. The energy dependence of ~p for 7r°, r/ and K °. The lines represent the QCD MLLA predictions extrapolated from our fitted values. D a t a from TASSO are K +.
Figure 6. The position of the m a x i m a ~ measured at LEP for different particles versus particle ]~1 ass.
Nuclear Physics B (Proc. Suppl.) 39B,C (1995) 153-156
Measurement of inclusive production of neutral hadrons from Z decays with L3 detector at L E P F. Becattini, a aUniversit~ di Firenze and INFN Sezione di Firenze, Largo E. Fermi 2, 50125, Firenze, Italy A measurement of inclusive rate and spectra of 7r°j/,K°,A, w and O' performed with L3 detector at LEP is described. Data are compared with perturbative QCD calculations.
1. I N T R O D U C T I O N Analytical calculations performed in the Modified Leading Log Approximation (MLLA) framework of perturbative QCD give a prediction for the spectra of hadrons produced after the parton shower evolution, once the Local Parton Hadron Duality (LPHD) [1,2] hypothesis is assumed. The spectrum function, including coherence effects of gluon radiation, known as l i m i t i n g s p e c t r u m , can be expressed for any hadron h as 1 dCrh _ N ( V f ~ ) " f ( v ~ , A ~ 1 l ; ( p ) , ah d ~ v
(1)
where {p = l o g ( v ~ / 2 p ) and v ~ is the total energy. The function f is specified in [1]. The only free parameters in (1) are N(v/-~) and A~/] which are to be determined with a fit to the data. N is an overall normalization factor which embodies all fragmentation effects, according to LPHD, while A~/] is a QCD scale parameter not directly related to AMS. In this report I will describe a measurement of inclusive rate and spectra of neutral hadrons ~r°,r/,K°,A and preliminary results for ca,r/' as well, performed with L3 detector at the e+e - collider LEP at x/~ ~ 91 GeV. Data are confronted with (1) and with Monte Carlo generators JETSET 7.3 [3] and HERWIG 5.4 [4] predictions. 2. L3 D E T E C T O R The L3 detector [5] consists of several specialized subdetectors arranged in a cylindrical geometry around the beam axis. These subdetectors are 0920-5632/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSDI 0920-5632(95)00062-3
a silicon vertex detector (SMD), a central tracking chamber, a high resolution electromagnetic calorimeter made of bismuth germanium oxide (BGO), a uranium and brass hadronic calorimeter and an accurate muon chamber system. They are installed within a 12 m diameter conventional magnet providing an uniform field of 0.5 T along the beam direction.
3. D A T A S A M P L E A N D E V E N T S E L E C TION The overall data sample used in this analysis consisted of approximately 929,000 Z --* q~ events collected by L3 during 1991 and 1992 runs, corresponding to an integrated luminosity of ~ 35 pb -1. 7r° and 7/are identified by means of their decays into photons pairs. Photons are detected as isolated energy clusters in BGO, selected requiring no tracks pointing to them and a cluster shape consistent with an electromagnetic shower. K ° and A are reconstructed using their decays into two charged particles, 7r+Tr- and 7rp respectively. Due to their long lifetimes (~_ 10 - l ° see.) the signature of these neutral hadrons is a clear secondary vertex within the tracking chamber region. ca is reconstructed in two different channels, ~r°7 and 7r+Tr-~r° respectively, while rf is detected by means of its decay into ~r+lr-r/. For all channels involving charged particles well measured tracks with momentum greater than _~ 100 MeV/c are required.
E Becattini/Nuclear Physics B (Proc. Suppl.) 39B, C (1995) 153-156
154
: e DATA]1;;O
10 2
11
11 . DAT: O
10
1C
10 2
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1 10
10 till
0
0.05
0.1
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0.1
0.15
,,
,,
0.2
0.05
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0
I
0.05 0.1 0.15 0.2 Xp
,,,I,,,,I
....
I ....
I,~
0.05 0.1 0.15 0.2
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Figure 1. xp spectra compared with JETSET. Bars include statistical and systematic errors.
Figure 2. xp spectra compared with HERWIG. Bars include statistical and systematic errors.
4. P A R T I C L E S S P E C T R A A N D M U L T I PLICITY MEASUREMENT
pected validity range 1 < 6 < l°g(v~/2A~/l)" A considerable feature of the ~v spectrum is the presence of a clear maximum which is due to suppression of small-x gluon emission and whose position ~ depends just on A~/I. Results from the fit are shown in table 2. The dependence of ~p on energy can be determined from the same function (1) once a value of A~fl has been fixed. This is shown in figure 6 together with measurements at lower energy. It is seen that the evolution of this parameter is succesfully described by MLLA predictions.
In order to measure particles spectra we chose the variables xp = 2p/v'~ and ~p = log(1/xp), well-suited for perturbative QCD analysis. We then divided our sample into different Xp, (p bins and for each bin, signals have been extracted from data using invariant mass distributions. Final spectra have been obtained after deconvolution from detector effects and division by efficiencies, both estimated with Monte Carlo. The xp spectra (fig. (1-4)) generally agree with JETSET and flERWIG predictions, with the noticeable exception of r/ whose spectrum is predicted to be too soft by both generators. Moreover, there is a clear discrepancy between data and HERWIG, concerning A spectrum. This effect has been investigated and found to be caused by the exceedingly high production of -- hadrons. Total multiplicities per hadronic Z decay have been determined by extrapolating those measured in the accessible xp-range using Monte Carlo generators spectrum. The (p spectra have been fitted to (1) in its ex-
5. C O N C L U S I O N S The study of inclusive hadrons production at the Z peak using L3 detector and data from lower energy experiments clearly indicates that perturbative QCD correctly describes both shape and energy evolution of spectra. A general pretty good agreement is also found with predictions from JETSET and HERWIG parton shower and hadronisation programs.
155
E Becattini/Nuclear Physics B (Proc. Suppl.) 39B, C (1995) 153-156
Table 1 Measured multiplicities. The measured value using J E T S E T and HERWIG for the efficiency calculation are reported separately. The average of the two is listed in the last column as the final result. The first error is statistical and the second systematic, w and r/ values are preliminary, estimated with J E T S E T and errors are only statistical. Hadron 7r° 7/ h "° A +/~ w ~l'
JETSET Meas. 9.77 0.95 1.02 0.37
HERWIG Pred. 9.63 1.21 1.08 0.37 1.33 0.67
Multiplicity
Meas. 8.60 0.88 1.01 0.37
Pred. 9.81 1.31 1.09 0.48
9.18 0.91 1.02 0.37 1.20 0.25
± ± ± ± ± ±
0.03 0.02 0.01 0.01 0.10 0.04
± ± ± ±
0.73 0.11 0.07 0.04
Table 2 Measured parameters of function (1). w values are preliminary and errors are only statistical. Hadron ~r° K° A +/~ w
0.492 0.123 0.102 0.041 0.140
± ± ± ± ±
N 0.051 0.016 0.005 0.004 0.011
A~]! (GeV) 0.147 ± 0.030 1.485 ± 0.234 0.832 ± 0.061 0.917 ± 0.178 1.105 ± 0.154
3.96 2.52 2.89 2.83 2.70
± ± 4± ±
~; 0.13 0.10 0.05 0.13 0.10
6. A C K N O W L E D G E M E N T S I would like to thank all members of the L3 collaboration and particularly Y.J. Pei and Y.F. Wang.
• DATA (c~-*'x°'f) • DATA (co-.z+x'x °) J:::
REFERENCES 1. Y. Dokshitzer, S. Troyan, Leningrad Preprint LNPI 92 (1984); Y. Azimov et al., Z. Phys. C 27 (1985) 65; C 31 (1986); V. Khoze et al., Lurid Preprint LU T P 90-12. 2. D. Amati, G. Veneziano, Phys. Lett. B 83 (1979) 87. 3. T. SjSstrand, Comp. Phys. Comm. 39 (1986) 347. 4. G. Marches±n± and B. Webber, Nucl. Phys. B 310 (1988) 461; G. Marches±n± et al., Comp. Phys. Comm. 67 (1992) 465. 5. L3 Collaboration, B. Adeva et al., Nucl. Instr. Meth. A 289 (1990) 35.
e-,-q
1I 10 "1
H
0
IJll
0.04
~
'
0.08
0.12
'1
' ' "
0,16
I ....
0.2
I
....
0.24
I.,I
0.28
Xp Figure 3. w xp-spectrum compared with J E T SET. Errors are only statistical.
] 56
E Becattini/Nuclear Physics B (Proc. Suppl.) 39B, C (1995) 153-156
5
• DATA ~QCD A~.~147 MeV
4
'~
,~O
2
0
0
2
4
/./ii ~ ' ' ~ j 1 2 3
6
K°
~ = 8 ~ MeV
0.4
3.5 4 3
0.2
A i" ~m=917 MeV
2.5
0.15
B o.3
2
"0 .e 0.2 ,r-
• L3 [] OPAL o DELPHI
4.5 4
0.2
0.5 ~. "0
~
'l z.e=1485MeV
0.4
0.1
0.1
+ 0
I
r
2
4
1.5
0
0.05 0
I
I
I
I
1
2
3
4
1
n°
K° ~
n
K 0.2
0,4
w K"
A
2-
p 0,6
0.8
1
1.2
1.4
1.6
P o r U c l e M o s s (GeV)
Figure 4. The inclusive (p spectra in comparison with fitted function (1) for 7r°, ~7, K° and A. The actual fitted region is drawn as a solid line. Error bars include statistical and systematic contributions.
4
• L3 [3 JADE
/
o ARGUS & Crystal Ball • TASSO
. /
~
.
"K°,,," ~ " K /
QCD (MLLA)
10
4s (GeV)
10
2
Figure 5. The energy dependence of ~p for 7r°, r/ and K °. The lines represent the QCD MLLA predictions extrapolated from our fitted values. D a t a from TASSO are K +.
Figure 6. The position of the m a x i m a ~ measured at LEP for different particles versus particle ]~1 ass.