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Dimuon and vector-meson production in p-W and S-W interactions at 200 GeVc nucleon
NUCLEAR PHYSICS A
Nuclear Physics A566 (1994) 95c-102~ North-Holland, Amsterdam
Dimuon and vector-meson 200 GeV/c/nucleon
production
M. A. Mazzoni a for the HELIOS/3
in p-W and S-W interactions
at
collaboration
“INFN Rome, Italy The focus of this experiment is on dimuons at low MT but the mass range up to the is also covered. Dimuons are measured over a wide rapidity interval, ranging from nearly central to very forward rapidities. We present the experimental results in different kinematic regions and as a function of charged multiplicity. The observed dimuon spectra are also compared to conventional sources of lepton pairs production.
J/$
1. EXPERIMENTAL
SET-UP
The experimental configuration is shown in fig.1. The sulphur or proton beam was defined by scintillator and quartz counters. A Cherenkov counter was used for the proton run. The six scintillators of a petal hodoscope provided the interaction trigger. Two silicon counters provided fast on-line information on charged multiplicity and detailed off-line multiplicity spectra. The first counter consisted of 384 pads in 12 sectors covering the pseudo-rapidity region of 1.6 5 7 5 3.7; the second one, with 288 pads in 24 sectors, covered the region of 2.6 5 q 5 5.5. Th e forward part of this counter (3.7 < q 5 5.5, N the spectrometer acceptance) was used in the trigger. Three multiplicity thresholds were used. An absorber optimized for low transverse mass measurements was placed 25 cm after a 12% X; rod target. The absorber was an alumina cone 180 cm long (6 Xi) followed by 100 cm of iron (more than 5 X;). An 8 mrad conic hole followed by a 60 cm long tungsten rod starting at 146 cm from the target on the beam axis insured that ion fragments interacted far from the target. The large acceptance muon spectrometer consisted of a dipole magnet with a vertical 4.1 Tm field and 7 multiwire proportional chambers for a total of 32 X,Y,U,V planes. A scintillator hodoscope Hl was placed at the exit of the magnet, while an 80 cm iron hadron filter was sandwiched between two trigger hodoscopes H2 and H3. H2 and H3 provided the first level dimuon trigger. The dimuon trigger was completed by requiring the reconstruction of at least two X-Z track projections in chambers PC3, PC5 and PC6. For most of the sulphur runs, triggers were eliminated when more than six slabs of Hl fired. 2. DATA
REDUCTION
AND
ANALYSIS
During the 1990 data taking with the 200 GeV/ c sulphur beam and the tungsten This number was reduced to 3.2 x 10’ target we collected 9 x lo6 dimuon triggers. events by requiring sulphur beam conditions, compatibility of the two silicon ring detector 0315-9414/94/$01.00
0 1994 - Elsevier Science B.V. All rights reserved.
96~
MA Mazzoni et al. I Dimuon and vector-meson production
Figure 1. Experimental
apparatus
information and exactly two reconstructed by vertex and acceptance cuts decreased analysis out of which 71.4% were /J+ p -, resulted in 45000 dimuons for the proton - PP’
tracks in the spectrometer. The target selection the number to 2.3 x lo6 dimuons used in the 11.6% p+p+ and 17% /.-CL-. Similar selection beam run with 96% p+p-, 1% /J+P+ and 3%
2.1. Beam selection and interaction clean-up The information from the various beam counters were used off-line to reject events from ions other than sulphur as well as pile-up events. The fraction of events rejected varies from 9 to 20% depending on the beam intensity. Information from the 2 silicon counters were used to reduce the contamination due to interactions occurring outside the target. We eliminated the events (21 1%) that showed a high multiph~ity in the second counter and a very small multip~city in the first, that were probably due to interactions in the first ring counter. Events from upstream interactions were rejected by a cut on the pattern correlation between the two counters. The cut was defined by looking at data taken with and without the target and then verified with a simple geometrical Monte Carlo program. This procedure removed 21 3% of the events. 2.2. momentum reconstruction The momentum reconstructed in the spectrometer is corrected for the average energy loss in the dump as estimated by Monte Carlo [4]. The reconstructed angles for each muon are estimators of the true angles, but are greatly influenced by the multiple scattering in the dump which changes the true angle and gives an apparent shift of the track. Another independent estimator for each angle can be determined [l, 8] by extrapolating the reconstructed track to a Z-plane at which the two effects are uncorrelated; the angles
M.A. Mazzoni et al. I Dimuon and vector-meson production
Sulphur Figure 2. Measured dimuon mass spectra and combinatorial
9lc
Protons background for S and p
are then determined from the target position and the intersection plane. The two independent estimators are averaged.
of the track with this
2.3. Combinatorial Background Two uncorrelated muons from rr meson or K meson decay can be measured in the same event and taken as a dimuon. The combinatorial background can be estimated from the like-sign p pairs as
where the coefficient R depends on the mean number of p+ and p- accepted by the spectrometer. We determined R by a full Monte Carlo simulation [6], using the VENUS 3.11 generator [7] to produce events and to decay short living particles, and Geant 3.14 to simulate the apparatus. The values obtained for R are 1.09 f 0.02 for the sulphur runs and 1.57 f 0.10 for the proton runs. To obtain the correct shape of the combinatorial background and reduce the statistical errors we used p pairs from different like-sign events, while the overall normalization is given by real like-sign events. The importance of the combinatorial background is illustrated in fig.2 which shows the measured dimuon mass spectrum and the combinatorial background. 2.4. Absorber Background A problem present in low mass dimuon measurements is the background produced by secondary hadrons interacting in the absorber. The reconstruction algorithm imposes the dimuon vertex to coincide with the target and therefore shifts the effective masses of dimuons produced in the absorber towards lower values. A big part of this background is rejected by an appropriately chosen cut. A point of maximum approach to the beam axis Z, was calculated for each muon track together with an error estimate uz,. We kept only muons for which Z,,/az,, < 3 and 2, < 250 cm
M.A. Maztoni et al. t D~~Ko~ and ve&i~r-pesos ~rod~t~on
!?ac
16 16
Protons
Sulphur
Figure 3. S and p dimuon mass spectra before and after subtraction of dump background
We estimate spectral shape and absolute rate of the remaining background (fig.3) with a Monte Carlo tuned to fit the results of special runs with rr beams of 25, 50 and 100 GeV/c impinging on the dump face and 200 GeV/c protons on the dump W rod. 2.5. Acceptance and Resolution The acceptance and resolution of the apparatus were extensively studied by Monte Carlo methods [5]. The resolution was checked at the 9 and at J/lc, mass, where the values are 82 and 90 MeV/c2 respectively; the mass scale was also verified at these masses. In order to remove events with very low acceptance, we restricted ourselves to the kinematic region
MT 2 4(7 - 23)
and
MT > \1(2M,)‘+
(~)2
where Pm;,, = 7.5GeV is given by the energy loss in the absorber and the strength of the magnetic field. 2.6. Multiplicity Analysis The multiplicity of charged particle measured in the pseudorapidity region 3.5 < 7 < 5.2 was used to group our data according to their centrality. The most important factors that limited our resolution were the high occupancy and the contamination due to secondary processes. The fluctuations due to the second effect play a very important role at low multip~cities. To correct for these effects we used a Monte Carlo simulation with VENUS 3.11 as generator and GEANT 3.14 for a full detector simulation. We divided the data in classes of overall occupancy as measured by the total number of firing pads in the detector. For each class we tabulated the relationship between the number of firing pads in a
M.A. Mazzoni et al. I Dimuon and vector-meson production
99c
given rapidity region, and the mean number of charged particles produced in the primary collision in the same region. Th ese look-up tables were then used to correct the raw multiplicity. The achieved resolution, defined as the standard deviation of the ratio (Mult,,,. - MuItt,,)/MuZt,,,) ranged from about 50% for peripheral collisions down to less than 15% for very central collisions. We grouped our data into 4 multiplicity classes, with enough statistics for the subsequent analysis. For each class we estimated the impact parameter and the average number of projectile participants (VENUS 3.11, FRITIOF[lO]). 2.7. Normalization to charged particle production Together with the dimuon triggers we acquired also events with no muon requirement and the same multiplicity thresholds. These events were used to normalize the dimuon yield to the number of charged particles in each multiplicity class. The (a) ratios were calculated at the trigger level, so that the multiplicity trigger acceptances canceled:
where a = multiplicity component of the trigger, p = multiplicity class S,,& = dimuon and nomuon triggers before downscaling (moo) = average charged multiplicity BP, 13, = dimuon and nomuon effective beams N,, N, = dimuon and nomuon triggers taken N,p, Nap = dimuon and nomuon triggers taken per multiplicity class. The various triggers contributing to each multiplicity class were then combined. 3. VECTOR
MESONS
The effective mass spectra between 0.3 and 2 GeV/c’ F(m)=
S[O.GB,(m)+
were fitted by the function
G,(m)+ &G.(m)]+ C(m)
where Gi are gaussians describing the w and @ meson peaks, the BP is a Breit-Wigner
Wm) (m2
7r-1m21?,_>,,(m)
- m$
+ m2r~_,1T(m)
convoluted with a gaussian resolution. The mean values of the resonances were fixed at their nominal masses and the gaussian variances take into account the experimental resolution.The constant 0.6 is the ratio c(p)/ u ( w ) measured in p-p interactions [3]. The function C(m)
= Pse pm= + pbeP6”
represents the continuum line shape. For both proton and sulphur runs three pi and three Y bins were considered. results show an enhancement of the ratio +/(p + w) from proton to sulphur data.
The The
I ooc
M.A. Mazzoni et al. i Dimuon and vector-meson production
Table 1 Transverse momentum pT F: 0.35 0.35 < pi 5 0.6 PT > 0.6
dependence of @/(p + w) S-W 0.424 i 0.083 0.320 f 0.056 0.398 f 0.034
P-W 0.142 f 0.016 0.167 i 0.018 0.165 f 0.018
Table 2 Rapidity dependence of @/(p + w) s-w
Y I 3.9 3.9 < Y 5 4.4 Y > 4.4
P-W 0.366 f 0.024 0.066 f 0.007 0.037 f 0.008
0.491 f 0.049 0.229 f 0.037 0.224 f 0.024
ratio is independent of pi within the errors (Table 1) and drops when going from nearly central to forward rapidity (Table 2). The multiplicity dependence of + and p + w is presented in Tab. 3 and Fig.4, where the
Table 3 Multiplicity
dependence of vector mesons lO’(p + w)/charged
protons 20 < Ntmw I: 95 95 < N+ I 130 130 < Nfory, 5 160 160 > N,,w
8.52 8.26 5.61 4.90 5.51
f f f f f
0.20 1.40 0.33 0.56 0.87
lO‘%/charged 1.38 2.19 1.94 2.04 2.65
f 0.15 It: 0.56 j, 0.20 f 0.18 f 0.26
@lb 0.157 f 0.294 f 0.334 f 0.407 f 0.470 f
+- WI 0.010 0.061 0.075 0.031 0.055
horizontal error bars reflect the uncertainty of the Monte Carlo calculations relating the measured charged multiplicity to the number of projectile participants. The ratio @/‘(p + w) clearly increases with centrality in S-W. Going from p-W to S-W (p + w),‘charged d ecreases while @/charged increases slightly. 4. INTERMEDIATE
MASS
REGION
Three sources are expected to contribute to the signal observed in the mass region 1 5 MeM 2 3GeV/c2: the tails of the resonances (p, w, a), the Dreh-Yan process and the muonic decay of charmed particles. To compare these sources with the measured dimuon spectra an extrapolation from p-p data to p-W and S-W is necessary as well as a simulation of detector response ([ll]). In this study the selection according to the multiplicity was replaced by a selection according to the impact parameter. The fitted number of p mesons was used for the absolute normalization.
M.A. Mazzoni et al. / Dimuon and vector-meson production
No. Proj. Participants Figure 4. Vector mesons production
as function of the number of projectile
The p cross section was extrapolated a,sw(kin.reg.)
participants
from p-p to S-W (or p-W) using
=
where cry is an experimental value and the ratios appearing in the formula are those predicted by QGSM ([12]). The Drell-Yan cross section corresponding to a given multiplicity class was defined in terms of impact parameter b &s
where 7’s~ is the thickness function for S-W collisions and oc is the nucleon-nucleon cross section (corrected for isospin). Drell-Yan events were generated using PYTHIA ([13]) with the Duke Owens structure functions, intrinsic parton kI adjusted to get (py) close to 0.7 - O.BGeV/c, K = 2.5 f 0.5. Charm events were generated using PYTHIA tuned to reproduce experimental differential distribution of B/a. The cross section per nucleon was taken from ((14j). We assume a linear A dependence. The intermediate muon continuum observed in central p-W (fig.5 can be explained by the contributions from Drell-Yan, resonances and charm particle decays. The data show no indication of any dimuon source not being considered. The intermediate muon continuum observed in central S-W (fig.5) is ~5 times larger than what is expected from the extrapolation of the known sources. The difference cannot be explained by errors and uncertainties of the extrapolation and normalization procedure. The observed excess can be an indication that either there is a dimuon source not taken into account or some of the known processes are different in the dense nuclear matter.
#up moss
[
G&/c2 I
Figure 5. Dimuon mass spectra with the expected these sources is shown as one-sigma band
pp. mass l SeVjc’
sources superimposed.
3
The sum of
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
G. London, NELIOS note 319. S. Dagan, HELIOS-3 note 30. T. Akesson et al., First measurement of branching ratio w” -+ ,u+P-, to be published M. Sarris, HELIOS-3 note 26.
S. Dagan, HELIOS-3 note 33; J. Bystricky and G. London, HELIOS-3 note 34. M. A. Mazzoni, HELIOS-3 note 50. K. Werner, Phys. Rev. Lett. 62 (1989) 2460. J. G. Branson et al., Phis. Rev. Lett. 38 (1977) 1334. T. Akesson et al., Nucl. Phys. B333 (1990) 48. B. Andersson, G. Gustafsson and B. Nilsson-Almqvist, Nucl. Phys. B281 (1987) 289. I. K&k, HELIOS-3 note 58. N. S. Smehn, K. K. Gudima and V. D. Toneev, NATO AS1 series B: Physics 2166 (1989) 473 and preprint GSI 89-52. 13. T. Sjijstrand and M. Bengtsson, Comp. Phys. Comm. 43 (1975) p191; T. Sjijstrand, PYTHIA 5.6 and JETSET 7.3 Physics and manual, CERN TH-6488/92. 14. 0. Erriquez et al., Physica Scripta 33 (1986) 202; S. Barlag, 2. Phys. C39 (1988) 451
Nuclear Physics A566 (1994) 95c-102~ North-Holland, Amsterdam
Dimuon and vector-meson 200 GeV/c/nucleon
production
M. A. Mazzoni a for the HELIOS/3
in p-W and S-W interactions
at
collaboration
“INFN Rome, Italy The focus of this experiment is on dimuons at low MT but the mass range up to the is also covered. Dimuons are measured over a wide rapidity interval, ranging from nearly central to very forward rapidities. We present the experimental results in different kinematic regions and as a function of charged multiplicity. The observed dimuon spectra are also compared to conventional sources of lepton pairs production.
J/$
1. EXPERIMENTAL
SET-UP
The experimental configuration is shown in fig.1. The sulphur or proton beam was defined by scintillator and quartz counters. A Cherenkov counter was used for the proton run. The six scintillators of a petal hodoscope provided the interaction trigger. Two silicon counters provided fast on-line information on charged multiplicity and detailed off-line multiplicity spectra. The first counter consisted of 384 pads in 12 sectors covering the pseudo-rapidity region of 1.6 5 7 5 3.7; the second one, with 288 pads in 24 sectors, covered the region of 2.6 5 q 5 5.5. Th e forward part of this counter (3.7 < q 5 5.5, N the spectrometer acceptance) was used in the trigger. Three multiplicity thresholds were used. An absorber optimized for low transverse mass measurements was placed 25 cm after a 12% X; rod target. The absorber was an alumina cone 180 cm long (6 Xi) followed by 100 cm of iron (more than 5 X;). An 8 mrad conic hole followed by a 60 cm long tungsten rod starting at 146 cm from the target on the beam axis insured that ion fragments interacted far from the target. The large acceptance muon spectrometer consisted of a dipole magnet with a vertical 4.1 Tm field and 7 multiwire proportional chambers for a total of 32 X,Y,U,V planes. A scintillator hodoscope Hl was placed at the exit of the magnet, while an 80 cm iron hadron filter was sandwiched between two trigger hodoscopes H2 and H3. H2 and H3 provided the first level dimuon trigger. The dimuon trigger was completed by requiring the reconstruction of at least two X-Z track projections in chambers PC3, PC5 and PC6. For most of the sulphur runs, triggers were eliminated when more than six slabs of Hl fired. 2. DATA
REDUCTION
AND
ANALYSIS
During the 1990 data taking with the 200 GeV/ c sulphur beam and the tungsten This number was reduced to 3.2 x 10’ target we collected 9 x lo6 dimuon triggers. events by requiring sulphur beam conditions, compatibility of the two silicon ring detector 0315-9414/94/$01.00
0 1994 - Elsevier Science B.V. All rights reserved.
96~
MA Mazzoni et al. I Dimuon and vector-meson production
Figure 1. Experimental
apparatus
information and exactly two reconstructed by vertex and acceptance cuts decreased analysis out of which 71.4% were /J+ p -, resulted in 45000 dimuons for the proton - PP’
tracks in the spectrometer. The target selection the number to 2.3 x lo6 dimuons used in the 11.6% p+p+ and 17% /.-CL-. Similar selection beam run with 96% p+p-, 1% /J+P+ and 3%
2.1. Beam selection and interaction clean-up The information from the various beam counters were used off-line to reject events from ions other than sulphur as well as pile-up events. The fraction of events rejected varies from 9 to 20% depending on the beam intensity. Information from the 2 silicon counters were used to reduce the contamination due to interactions occurring outside the target. We eliminated the events (21 1%) that showed a high multiph~ity in the second counter and a very small multip~city in the first, that were probably due to interactions in the first ring counter. Events from upstream interactions were rejected by a cut on the pattern correlation between the two counters. The cut was defined by looking at data taken with and without the target and then verified with a simple geometrical Monte Carlo program. This procedure removed 21 3% of the events. 2.2. momentum reconstruction The momentum reconstructed in the spectrometer is corrected for the average energy loss in the dump as estimated by Monte Carlo [4]. The reconstructed angles for each muon are estimators of the true angles, but are greatly influenced by the multiple scattering in the dump which changes the true angle and gives an apparent shift of the track. Another independent estimator for each angle can be determined [l, 8] by extrapolating the reconstructed track to a Z-plane at which the two effects are uncorrelated; the angles
M.A. Mazzoni et al. I Dimuon and vector-meson production
Sulphur Figure 2. Measured dimuon mass spectra and combinatorial
9lc
Protons background for S and p
are then determined from the target position and the intersection plane. The two independent estimators are averaged.
of the track with this
2.3. Combinatorial Background Two uncorrelated muons from rr meson or K meson decay can be measured in the same event and taken as a dimuon. The combinatorial background can be estimated from the like-sign p pairs as
where the coefficient R depends on the mean number of p+ and p- accepted by the spectrometer. We determined R by a full Monte Carlo simulation [6], using the VENUS 3.11 generator [7] to produce events and to decay short living particles, and Geant 3.14 to simulate the apparatus. The values obtained for R are 1.09 f 0.02 for the sulphur runs and 1.57 f 0.10 for the proton runs. To obtain the correct shape of the combinatorial background and reduce the statistical errors we used p pairs from different like-sign events, while the overall normalization is given by real like-sign events. The importance of the combinatorial background is illustrated in fig.2 which shows the measured dimuon mass spectrum and the combinatorial background. 2.4. Absorber Background A problem present in low mass dimuon measurements is the background produced by secondary hadrons interacting in the absorber. The reconstruction algorithm imposes the dimuon vertex to coincide with the target and therefore shifts the effective masses of dimuons produced in the absorber towards lower values. A big part of this background is rejected by an appropriately chosen cut. A point of maximum approach to the beam axis Z, was calculated for each muon track together with an error estimate uz,. We kept only muons for which Z,,/az,, < 3 and 2, < 250 cm
M.A. Maztoni et al. t D~~Ko~ and ve&i~r-pesos ~rod~t~on
!?ac
16 16
Protons
Sulphur
Figure 3. S and p dimuon mass spectra before and after subtraction of dump background
We estimate spectral shape and absolute rate of the remaining background (fig.3) with a Monte Carlo tuned to fit the results of special runs with rr beams of 25, 50 and 100 GeV/c impinging on the dump face and 200 GeV/c protons on the dump W rod. 2.5. Acceptance and Resolution The acceptance and resolution of the apparatus were extensively studied by Monte Carlo methods [5]. The resolution was checked at the 9 and at J/lc, mass, where the values are 82 and 90 MeV/c2 respectively; the mass scale was also verified at these masses. In order to remove events with very low acceptance, we restricted ourselves to the kinematic region
MT 2 4(7 - 23)
and
MT > \1(2M,)‘+
(~)2
where Pm;,, = 7.5GeV is given by the energy loss in the absorber and the strength of the magnetic field. 2.6. Multiplicity Analysis The multiplicity of charged particle measured in the pseudorapidity region 3.5 < 7 < 5.2 was used to group our data according to their centrality. The most important factors that limited our resolution were the high occupancy and the contamination due to secondary processes. The fluctuations due to the second effect play a very important role at low multip~cities. To correct for these effects we used a Monte Carlo simulation with VENUS 3.11 as generator and GEANT 3.14 for a full detector simulation. We divided the data in classes of overall occupancy as measured by the total number of firing pads in the detector. For each class we tabulated the relationship between the number of firing pads in a
M.A. Mazzoni et al. I Dimuon and vector-meson production
99c
given rapidity region, and the mean number of charged particles produced in the primary collision in the same region. Th ese look-up tables were then used to correct the raw multiplicity. The achieved resolution, defined as the standard deviation of the ratio (Mult,,,. - MuItt,,)/MuZt,,,) ranged from about 50% for peripheral collisions down to less than 15% for very central collisions. We grouped our data into 4 multiplicity classes, with enough statistics for the subsequent analysis. For each class we estimated the impact parameter and the average number of projectile participants (VENUS 3.11, FRITIOF[lO]). 2.7. Normalization to charged particle production Together with the dimuon triggers we acquired also events with no muon requirement and the same multiplicity thresholds. These events were used to normalize the dimuon yield to the number of charged particles in each multiplicity class. The (a) ratios were calculated at the trigger level, so that the multiplicity trigger acceptances canceled:
where a = multiplicity component of the trigger, p = multiplicity class S,,& = dimuon and nomuon triggers before downscaling (moo) = average charged multiplicity BP, 13, = dimuon and nomuon effective beams N,, N, = dimuon and nomuon triggers taken N,p, Nap = dimuon and nomuon triggers taken per multiplicity class. The various triggers contributing to each multiplicity class were then combined. 3. VECTOR
MESONS
The effective mass spectra between 0.3 and 2 GeV/c’ F(m)=
S[O.GB,(m)+
were fitted by the function
G,(m)+ &G.(m)]+ C(m)
where Gi are gaussians describing the w and @ meson peaks, the BP is a Breit-Wigner
Wm) (m2
7r-1m21?,_>,,(m)
- m$
+ m2r~_,1T(m)
convoluted with a gaussian resolution. The mean values of the resonances were fixed at their nominal masses and the gaussian variances take into account the experimental resolution.The constant 0.6 is the ratio c(p)/ u ( w ) measured in p-p interactions [3]. The function C(m)
= Pse pm= + pbeP6”
represents the continuum line shape. For both proton and sulphur runs three pi and three Y bins were considered. results show an enhancement of the ratio +/(p + w) from proton to sulphur data.
The The
I ooc
M.A. Mazzoni et al. i Dimuon and vector-meson production
Table 1 Transverse momentum pT F: 0.35 0.35 < pi 5 0.6 PT > 0.6
dependence of @/(p + w) S-W 0.424 i 0.083 0.320 f 0.056 0.398 f 0.034
P-W 0.142 f 0.016 0.167 i 0.018 0.165 f 0.018
Table 2 Rapidity dependence of @/(p + w) s-w
Y I 3.9 3.9 < Y 5 4.4 Y > 4.4
P-W 0.366 f 0.024 0.066 f 0.007 0.037 f 0.008
0.491 f 0.049 0.229 f 0.037 0.224 f 0.024
ratio is independent of pi within the errors (Table 1) and drops when going from nearly central to forward rapidity (Table 2). The multiplicity dependence of + and p + w is presented in Tab. 3 and Fig.4, where the
Table 3 Multiplicity
dependence of vector mesons lO’(p + w)/charged
protons 20 < Ntmw I: 95 95 < N+ I 130 130 < Nfory, 5 160 160 > N,,w
8.52 8.26 5.61 4.90 5.51
f f f f f
0.20 1.40 0.33 0.56 0.87
lO‘%/charged 1.38 2.19 1.94 2.04 2.65
f 0.15 It: 0.56 j, 0.20 f 0.18 f 0.26
@lb 0.157 f 0.294 f 0.334 f 0.407 f 0.470 f
+- WI 0.010 0.061 0.075 0.031 0.055
horizontal error bars reflect the uncertainty of the Monte Carlo calculations relating the measured charged multiplicity to the number of projectile participants. The ratio @/‘(p + w) clearly increases with centrality in S-W. Going from p-W to S-W (p + w),‘charged d ecreases while @/charged increases slightly. 4. INTERMEDIATE
MASS
REGION
Three sources are expected to contribute to the signal observed in the mass region 1 5 MeM 2 3GeV/c2: the tails of the resonances (p, w, a), the Dreh-Yan process and the muonic decay of charmed particles. To compare these sources with the measured dimuon spectra an extrapolation from p-p data to p-W and S-W is necessary as well as a simulation of detector response ([ll]). In this study the selection according to the multiplicity was replaced by a selection according to the impact parameter. The fitted number of p mesons was used for the absolute normalization.
M.A. Mazzoni et al. / Dimuon and vector-meson production
No. Proj. Participants Figure 4. Vector mesons production
as function of the number of projectile
The p cross section was extrapolated a,sw(kin.reg.)
participants
from p-p to S-W (or p-W) using
=
where cry is an experimental value and the ratios appearing in the formula are those predicted by QGSM ([12]). The Drell-Yan cross section corresponding to a given multiplicity class was defined in terms of impact parameter b &s
where 7’s~ is the thickness function for S-W collisions and oc is the nucleon-nucleon cross section (corrected for isospin). Drell-Yan events were generated using PYTHIA ([13]) with the Duke Owens structure functions, intrinsic parton kI adjusted to get (py) close to 0.7 - O.BGeV/c, K = 2.5 f 0.5. Charm events were generated using PYTHIA tuned to reproduce experimental differential distribution of B/a. The cross section per nucleon was taken from ((14j). We assume a linear A dependence. The intermediate muon continuum observed in central p-W (fig.5 can be explained by the contributions from Drell-Yan, resonances and charm particle decays. The data show no indication of any dimuon source not being considered. The intermediate muon continuum observed in central S-W (fig.5) is ~5 times larger than what is expected from the extrapolation of the known sources. The difference cannot be explained by errors and uncertainties of the extrapolation and normalization procedure. The observed excess can be an indication that either there is a dimuon source not taken into account or some of the known processes are different in the dense nuclear matter.
#up moss
[
G&/c2 I
Figure 5. Dimuon mass spectra with the expected these sources is shown as one-sigma band
pp. mass l SeVjc’
sources superimposed.
3
The sum of
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
G. London, NELIOS note 319. S. Dagan, HELIOS-3 note 30. T. Akesson et al., First measurement of branching ratio w” -+ ,u+P-, to be published M. Sarris, HELIOS-3 note 26.
S. Dagan, HELIOS-3 note 33; J. Bystricky and G. London, HELIOS-3 note 34. M. A. Mazzoni, HELIOS-3 note 50. K. Werner, Phys. Rev. Lett. 62 (1989) 2460. J. G. Branson et al., Phis. Rev. Lett. 38 (1977) 1334. T. Akesson et al., Nucl. Phys. B333 (1990) 48. B. Andersson, G. Gustafsson and B. Nilsson-Almqvist, Nucl. Phys. B281 (1987) 289. I. K&k, HELIOS-3 note 58. N. S. Smehn, K. K. Gudima and V. D. Toneev, NATO AS1 series B: Physics 2166 (1989) 473 and preprint GSI 89-52. 13. T. Sjijstrand and M. Bengtsson, Comp. Phys. Comm. 43 (1975) p191; T. Sjijstrand, PYTHIA 5.6 and JETSET 7.3 Physics and manual, CERN TH-6488/92. 14. 0. Erriquez et al., Physica Scripta 33 (1986) 202; S. Barlag, 2. Phys. C39 (1988) 451