- Email: [email protected]
First results from the ALICE experiment
Nuclear Physics B (Proc. Suppl.) 207–208 (2010) 329–332 www.elsevier.com/locate/npbps
First results from the ALICE experiment C. Cheshkov Institut de Physique Nucl´eaire de Lyon, CNRS-IN2P3, Bˆatiment Paul Dirac, 4 Rue Enrico Fermi, 69622 Villeurbanne Cedex, France
Abstract We present the results from the first series of minimum-bias measurements with the ALICE detector at the CERN LHC. Charged-particle pseudorapidity density, multiplicity distribution and transverse momentum spectra were measured using data collected in 2009 and 2010 at three different centre-of-mass energies of 0.9, 2.36 and 7 TeV. The results are compared to previous proton-antiproton data and to model predictions. Besides the valuable physics outcome the first ALICE results show also the exceptional operation of both the LHC machine and the ALICE experiment. 1. Introduction We present the first results on the primary chargedparticle1 pseudorapidity density√and the multiplicity distribution in pp collisions at s = 0.9, 2.36 and 7 TeV [1–3], as well as the primary charged-particle transverse momentum spectra at 0.9 TeV [4], obtained with the ALICE detector [5] at the CERN LHC. Our results are compared with other experimental data and with the predictions from various models. The measured global collision properties are useful to study QCD in nonperturbative regime, to constrain models and event generators, and to gain understanding for further measurements of hard and rare processes. It is worth to note that the 0.9 and 2.36 TeV data samples taken during the commissioning of the LHC provide a unique possibility to compare the LHC results with the corresponding data from p p¯ collisions. 2. Event class definition In order to compare our results with those of other experiments, the analysis presented here was performed for different event classes. For the 0.9 and 2.36 TeV data, the first event class (INEL) corresponds to all inelastic interactions including single-diffractive (SD), double-diffractive (DD) and non-diffractive (ND) processes. The second one, non-single-diffractive (NSD), Email address: [email protected] (C. Cheshkov) charged particles are defined as charged particles produced in the collision and their decay products excluding weak decays from strange particles 1 Primary
0920-5632/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2010.10.080
includes only double-diffractive and non-diffractive processes. The generated Monte-Carlo data uses SD/INEL and DD/INEL fractions that match the previously measured cross-sections at these energies. In order to address the uncertainty in the kinematics of the diffractive events, two Monte-Carlo generators have been used, namely Pythia [6] and Phojet [7]. Due to lack of experimental data for diffractive cross-sections, the 7 TeV data was treated in a different way. The inelastic event class was defined as all the processes that give at least one charged particle in the central region of |η| < 1 (INEL>0). This definition allows to reduce the model dependencies and compare the results with the other LHC experiments. 3. Detectors, trigger and data-sets The ALICE experiment has a complex layout consisting of various detector subsystems [5]. Our analysis is based on a subset of the ALICE detectors. The multiplicity is measured mainly by the Silicon Pixel Detector (SPD), which represent the two innermost layers of the Inner Tracking System (ITS). It surrounds the central beryllium beam pipe with two cylindrical layers, at radii of 3.9 cm and 7.6 cm, and covers the pseudorapidity ranges |η| < 2 and |η| < 1.4 for the inner and the outer layer, respectively. The SPD has about 9.8 million pixels of size 50 × 425 μm2 distributed among 1200 readout chips. The second detector used is the ALICE V0. It consists of two arrays of plastic scintillators located on each side of the interaction region at z = 3.3 m and z = −1.7 m, with acceptance of 2.8 < η < 5.1
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C. Cheshkov / Nuclear Physics B (Proc. Suppl.) 207–208 (2010) 329–332
and −3.7 < η < −1.7, respectively. The central detector subsystems are placed inside a solenoidal magnet which provides a longitudinal field of 0.5 T. The transverse momentum measurement involved also one of the main ALICE detectors - the Time Projection Chamber (TPC) [8]. The TPC is a huge cylindrical drift detector with a central high-voltage membrane and two readout planes at the end-caps. The active volume is limited by 85 < r < 247 cm and −250 < z < 250 cm, in the radial and longitudinal direction, respectively. The central membrane at z = 0 divides the 90 m3 drift volume into two halves. The readout end-caps are equipped with 72 multi-wire proportional chambers. The transverse momentum resolution of the TPC is σ(pT )/pT = 0.012 ⊕ 0.007pT , where pT is in GeV/c. Table 1 summarizes the trigger algorithms and datasets used for the multiplicity measurement at all three energies. The relatively low interaction rate allowed to
use a loose minimum-bias trigger based on the following trigger signals. The BPTX is a signal coming from the so called beam pick-up counters, it requires presence of the two proton beams. The SPD trigger is generated by the pixel detector if at least one of the pixel chips is fired. The V0OR signal is a logical OR of signals from all the V0 scintillators. In order to reject beam-induced background events, an offline trigger selection based on SPD and V0 data was applied as well. The selection requires a limited number of unassociated hits in the two layers of SPD. Moreover, it uses the timing information from the V0 detector in order to reject events with hits in V0 which have an arrival time before the bunchcrossing. The NSD event sample was analyzed by using V0AND offline trigger requiring the presence of hits in both sides of the V0 detector. This approach allowed to reduce model-dependent corrections.
one hit in the inner and one in the outer layer of SPD, pointing to the primary vertex. The primary vertex is reconstructed by correlating the hits on the two layers of the SPD. Only tracklets pointing to the primary vertex are selected for the analysis. The number of primary charged particles is estimated by counting number of reconstructed and selected tracklets. The obtained result is then corrected for: − geometrical acceptance and detector efficiency; − contamination from weak decays, gamma conversions and secondary interactions; − undetected particles below 50 MeV/c transversemomentum cut-off; − combinatorial background from accidental association of hits in the two SPD layers, estimated from data by counting pairs of hits with large difference between their azimuthal angles; − vertex reconstruction efficiency and bias; − trigger bias using control triggers and Monte-Carlo simulation of the trigger logic. The analysis of multiplicity distribution implied further selection of events with |zvertex | < 5.5 cm where the geometrical acceptance does not depend on the track pseudorapidity. The multiplicity distribution was then extracted by an unfolding of the measured one by means of the detector-response function, which takes into account the acceptance and the detector efficiency. The corrections for the trigger bias and the vertex reconstruction efficiency were applied after the unfolding. The transverse momentum spectrum of charged particles was obtained using trigger, event selection and Monte-Carlo generation similar to the multiplicity measurements. However the analysis was based on tracks reconstructed with the ITS and the TPC in the pseudorapidity range of |η| < 0.8. Due to residual relative misalignment between the ITS and the TPC, the tracks were treated in a special way. The transverse momentum was measured by the TPC, while the impact parameter to the primary vertex was given by the ITS. This allowed for an unbiased primary track selection. The primary vertex was reconstructed by correlating the reconstructed tracks. In case there were no reconstructed tracks in the ITS and the TPC, the vertex from the SPD tracklets was used. And in case of a single track, the point of the closest approach to the beam axis was taken.
4. Data analysis
5. Results and discussion
The measurement of charged-particle multiplicities is based mainly on the so called tracklets, short track segments which are formed from hits on the two layers of the SPD. Tracklets are reconstructed as pairs of hits,
Fig.1 and Fig.2 show the charged-particle density as a function of pseudorapidity obtained for INEL and NSD √ collisions at s = 0.9, 2.36 and 7 TeV, respectively. Our results are in good agreement with p p¯ data at 0.9 TeV
Table 1: The summary of the LHC centre-of-mass energy, solenoid magnetic field, trigger logic and corresponding data-sets used in the analysis. Energy,TeV
Mag.field,T
Trigger
0.9 0.9 2.36 7
0 0.5 0.5 0.5
BPTX&SPD BPTX&(V0OR|SPD) BPTX&SPD BPTX&(V0OR|SPD)
Number of collisions 284 150000 40000 380000
C. Cheshkov / Nuclear Physics B (Proc. Suppl.) 207–208 (2010) 329–332
331
Figure 3: A comparison of the measured multiplicity distributions for INEL collisions at 0.9 TeV (left) and 2.36 TeV (right) to models for the pseudorapidity range of |η| < 1. The error bars represent statistical uncertainties while shaded area systematic ones. The ratios between the measured values and model calculations are shown in the lower part of the figures. The shaded areas represent the combined uncertainties.
Figure 1: The measured charged-particle pseudorapidity density for INEL and NSD collisions as a function of the pseudorapidity at 0.9 TeV (left) and 2.36 TeV (right). The results are compared with the data from other experiments at the same energies (top) and with various Monte-Carlo generators (bottom).
Figure 4: The measured multiplicity distributions for INEL>0 event sample Figure 2: Left: the measured charged-particle pseudorapidity density for √
INEL>0 collisions as a function of the pseudorapidity at s = 7 TeV. The result is compared with various Monte-Carlo generators. Right: the charged-particle pseudorapidity density in the central pseudorapidity region of |η| < 0.5 for INEL and NSD collisions, and in |η| < 1 for INEL>0 collisions, as a function of the centre-of-mass energy. The lines indicate a fit with a power-law dependence on the energy.
from the UA5 experiment [9] and with pp NSD data at 0.9 and 2.36 TeV from the CMS experiment [10]. The comparison with model predictions shows best consistency with the Pythia tune ATLAS-CSC [11]. Phoject [7] is systematically below for 2.36 and especially 7 TeV data. The other two Pythia tunes, D6T [12] and Perugia-0 [13], are significantly lower at all three energies. The corresponding multiplicity distributions of charged particles are presented in Fig.3 and Fig.4. The comparison with the models is quite similar to the case of charged-particle densities. The distributions were fitted satisfactorily with a Negative-Binomial Distribution (NBD) (see Fig.4,left). The normalized differential primary charged-particle yield for INEL collisions at 0.9 TeV is given in Fig.5,top left. While the transverse momentum spectra are fitted quite satisfactorily by the modified Hagedorn function [14], the high pT tail of the spectrum is described much better by a power-law fit. The comparison with mod-
for the pseudorapidity range of |η| < 1. The error bars represent statistical uncertainties while shaded area systematic ones. Left: the results at three energies are shown together with the NBD fits (lines). Right: the 7 TeV result is compared to the model predictions. The ratio between the measured values and model calculations are shown in the lower part. The shaded areas represent the combined uncertainties.
els gives a picture opposite to the one of the chargedparticle multiplicities (see Fig.5,top right). This time the Perugia-0 tune provides the best agreement on the spectral shape. The D6T tune is similar to Perugia0, but underestimates the data at high pT . Both Phojet and ATLAS-CSC tune fail to describe the data. A comparison with data from other experiments has been done using the NSD event sample. Fig.5,bottom, gives a comparison with the recent results from the ATLAS [15] and the CMS [16] experiments and from p p¯ data from the UA1 collaboration [17], measured in different pseudorapidity regions. The overall agreement is good though at higher pT our data are above these two measurements. This disagreement may be related to the different pseudorapidity acceptances of these experiments. For completeness, in Fig.6 we provide a comparison of average transverse momentum in INEL collisions as a function of event multiplicity. The measurement was done by splitting the data into bins of number of detected particles and performing a correction using the
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C. Cheshkov / Nuclear Physics B (Proc. Suppl.) 207–208 (2010) 329–332
Figure 6: A comparison of the average transverse momentum of charged particles in INEL collisions at 0.9 TeV as a function of the charged-particle multiplicity (nch ) with model predictions, for 0.5 < pT < 4 GeV/c (left) and 0.15 < pT < 4 GeV/c (right). The ratios between the Monte-Carlo and the data are shown in the lower parts of the figures. The shaded areas illustrate the combined uncertainties.
Acknowledgements The author is grateful to the organizers of the QCD’10 Conference for the kind invitation to present the first results from the ALICE experiment.
Figure 5: Top left: the normalized differential primary charged-particle yield for INEL collisions at 0.9 TeV, averaged in the pseudorapidity range of |η| < 0.8. The line shows a fit with the modified Hagedorn function. Top right: the yield is compared to the Pythia and the Phojet generators. Bottom left: the yield is compared to the results from ATLAS and CMS at the same energy. Bottom right: the normalized invariant primary charged-particle yield is compared to the results from the UA1 experiment in p p¯ at the same energy. The yield is computed assuming all particles are pions. The shaded areas on all figures illustrate combined uncertainties.
detector-response function. 6. Conclusions A set of first minimum-bias measurements has been performed with the ALICE experiment at three LHC beam energies. The average observed multiplicity increases much faster with the energy than it is predicted by the existing models. The obtained precise results on the multiplicity and transverse momentum distributions are essential for tuning and constraining the Monte-Carlo generators which presently do not describe the data satisfactorily. The success of these complex measurements demonstrates the excellent operation of the LHC machine and the ALICE experiment from the beginning of the physics run in 2009.
References [1] K. Aamodt et al. [ALICE Collaboration], Eur.Phys.J. C65, 111-125 (2010). [2] K. Aamodt et al. [ALICE Collaboration], Eur.Phys.J. C68, 89108 (2010). [3] K. Aamodt et al. [ALICE Collaboration], Eur.Phys.J. C68, 345-354 (2010). [4] K. Aamodt et al. [ALICE Collaboration], arXiv:1007.0719 [hep-ex] (2010). [5] K. Aamodt et al. [ALICE Collaboration], J.Instrum. 3, S08002 (2008). [6] T. Sj¨ostrand, S. Mrenna, P. Skands, J.High Energy Phys. 0605 (2006) 026. [7] R. Engel, J. Ranft, S. Roesler, Phys.Rev. D52, 1459 (1995). [8] J. Alme et al. [ALICE Collaboration], accepted for publication in Nucl.Instrum.Meth.A, arXiv:1001.1950 [physics.ins-det]. [9] G.J. Alner et al. [UA5 Collaboration], Z.Phys. C33, 1 (1986). [10] V. Khachatryan et al. [CMS Collaboration], JHEP 2010, 02 041 (2010). [11] A. Moraes [ATLAS Collaboration], ATLAS Note ATL-COMPHYS-2009-119 (2009). [12] M.G. Albrow et al., arXiv:hep-ph/0610012 (2006). [13] P.Z. Skands, arXiv:0905.3418 [hep-ph] (2009). [14] R. Hagedorn, Riv.Nuovo Cim. 6 1 (1983). [15] G. Aad et al. [ATLAS Collaboration], submitted to Phys.Lett. B, arXiv:1003.3124 [hep-ex] (2010). [16] V. Khachatryan et al. [CMS Collaboration], JHEP 02 02 041 (2010). [17] C. Albajar et al. [UA1 Collaboration], Nucl.Phys. B335, 261 (1990).
First results from the ALICE experiment C. Cheshkov Institut de Physique Nucl´eaire de Lyon, CNRS-IN2P3, Bˆatiment Paul Dirac, 4 Rue Enrico Fermi, 69622 Villeurbanne Cedex, France
Abstract We present the results from the first series of minimum-bias measurements with the ALICE detector at the CERN LHC. Charged-particle pseudorapidity density, multiplicity distribution and transverse momentum spectra were measured using data collected in 2009 and 2010 at three different centre-of-mass energies of 0.9, 2.36 and 7 TeV. The results are compared to previous proton-antiproton data and to model predictions. Besides the valuable physics outcome the first ALICE results show also the exceptional operation of both the LHC machine and the ALICE experiment. 1. Introduction We present the first results on the primary chargedparticle1 pseudorapidity density√and the multiplicity distribution in pp collisions at s = 0.9, 2.36 and 7 TeV [1–3], as well as the primary charged-particle transverse momentum spectra at 0.9 TeV [4], obtained with the ALICE detector [5] at the CERN LHC. Our results are compared with other experimental data and with the predictions from various models. The measured global collision properties are useful to study QCD in nonperturbative regime, to constrain models and event generators, and to gain understanding for further measurements of hard and rare processes. It is worth to note that the 0.9 and 2.36 TeV data samples taken during the commissioning of the LHC provide a unique possibility to compare the LHC results with the corresponding data from p p¯ collisions. 2. Event class definition In order to compare our results with those of other experiments, the analysis presented here was performed for different event classes. For the 0.9 and 2.36 TeV data, the first event class (INEL) corresponds to all inelastic interactions including single-diffractive (SD), double-diffractive (DD) and non-diffractive (ND) processes. The second one, non-single-diffractive (NSD), Email address: [email protected] (C. Cheshkov) charged particles are defined as charged particles produced in the collision and their decay products excluding weak decays from strange particles 1 Primary
0920-5632/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2010.10.080
includes only double-diffractive and non-diffractive processes. The generated Monte-Carlo data uses SD/INEL and DD/INEL fractions that match the previously measured cross-sections at these energies. In order to address the uncertainty in the kinematics of the diffractive events, two Monte-Carlo generators have been used, namely Pythia [6] and Phojet [7]. Due to lack of experimental data for diffractive cross-sections, the 7 TeV data was treated in a different way. The inelastic event class was defined as all the processes that give at least one charged particle in the central region of |η| < 1 (INEL>0). This definition allows to reduce the model dependencies and compare the results with the other LHC experiments. 3. Detectors, trigger and data-sets The ALICE experiment has a complex layout consisting of various detector subsystems [5]. Our analysis is based on a subset of the ALICE detectors. The multiplicity is measured mainly by the Silicon Pixel Detector (SPD), which represent the two innermost layers of the Inner Tracking System (ITS). It surrounds the central beryllium beam pipe with two cylindrical layers, at radii of 3.9 cm and 7.6 cm, and covers the pseudorapidity ranges |η| < 2 and |η| < 1.4 for the inner and the outer layer, respectively. The SPD has about 9.8 million pixels of size 50 × 425 μm2 distributed among 1200 readout chips. The second detector used is the ALICE V0. It consists of two arrays of plastic scintillators located on each side of the interaction region at z = 3.3 m and z = −1.7 m, with acceptance of 2.8 < η < 5.1
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C. Cheshkov / Nuclear Physics B (Proc. Suppl.) 207–208 (2010) 329–332
and −3.7 < η < −1.7, respectively. The central detector subsystems are placed inside a solenoidal magnet which provides a longitudinal field of 0.5 T. The transverse momentum measurement involved also one of the main ALICE detectors - the Time Projection Chamber (TPC) [8]. The TPC is a huge cylindrical drift detector with a central high-voltage membrane and two readout planes at the end-caps. The active volume is limited by 85 < r < 247 cm and −250 < z < 250 cm, in the radial and longitudinal direction, respectively. The central membrane at z = 0 divides the 90 m3 drift volume into two halves. The readout end-caps are equipped with 72 multi-wire proportional chambers. The transverse momentum resolution of the TPC is σ(pT )/pT = 0.012 ⊕ 0.007pT , where pT is in GeV/c. Table 1 summarizes the trigger algorithms and datasets used for the multiplicity measurement at all three energies. The relatively low interaction rate allowed to
use a loose minimum-bias trigger based on the following trigger signals. The BPTX is a signal coming from the so called beam pick-up counters, it requires presence of the two proton beams. The SPD trigger is generated by the pixel detector if at least one of the pixel chips is fired. The V0OR signal is a logical OR of signals from all the V0 scintillators. In order to reject beam-induced background events, an offline trigger selection based on SPD and V0 data was applied as well. The selection requires a limited number of unassociated hits in the two layers of SPD. Moreover, it uses the timing information from the V0 detector in order to reject events with hits in V0 which have an arrival time before the bunchcrossing. The NSD event sample was analyzed by using V0AND offline trigger requiring the presence of hits in both sides of the V0 detector. This approach allowed to reduce model-dependent corrections.
one hit in the inner and one in the outer layer of SPD, pointing to the primary vertex. The primary vertex is reconstructed by correlating the hits on the two layers of the SPD. Only tracklets pointing to the primary vertex are selected for the analysis. The number of primary charged particles is estimated by counting number of reconstructed and selected tracklets. The obtained result is then corrected for: − geometrical acceptance and detector efficiency; − contamination from weak decays, gamma conversions and secondary interactions; − undetected particles below 50 MeV/c transversemomentum cut-off; − combinatorial background from accidental association of hits in the two SPD layers, estimated from data by counting pairs of hits with large difference between their azimuthal angles; − vertex reconstruction efficiency and bias; − trigger bias using control triggers and Monte-Carlo simulation of the trigger logic. The analysis of multiplicity distribution implied further selection of events with |zvertex | < 5.5 cm where the geometrical acceptance does not depend on the track pseudorapidity. The multiplicity distribution was then extracted by an unfolding of the measured one by means of the detector-response function, which takes into account the acceptance and the detector efficiency. The corrections for the trigger bias and the vertex reconstruction efficiency were applied after the unfolding. The transverse momentum spectrum of charged particles was obtained using trigger, event selection and Monte-Carlo generation similar to the multiplicity measurements. However the analysis was based on tracks reconstructed with the ITS and the TPC in the pseudorapidity range of |η| < 0.8. Due to residual relative misalignment between the ITS and the TPC, the tracks were treated in a special way. The transverse momentum was measured by the TPC, while the impact parameter to the primary vertex was given by the ITS. This allowed for an unbiased primary track selection. The primary vertex was reconstructed by correlating the reconstructed tracks. In case there were no reconstructed tracks in the ITS and the TPC, the vertex from the SPD tracklets was used. And in case of a single track, the point of the closest approach to the beam axis was taken.
4. Data analysis
5. Results and discussion
The measurement of charged-particle multiplicities is based mainly on the so called tracklets, short track segments which are formed from hits on the two layers of the SPD. Tracklets are reconstructed as pairs of hits,
Fig.1 and Fig.2 show the charged-particle density as a function of pseudorapidity obtained for INEL and NSD √ collisions at s = 0.9, 2.36 and 7 TeV, respectively. Our results are in good agreement with p p¯ data at 0.9 TeV
Table 1: The summary of the LHC centre-of-mass energy, solenoid magnetic field, trigger logic and corresponding data-sets used in the analysis. Energy,TeV
Mag.field,T
Trigger
0.9 0.9 2.36 7
0 0.5 0.5 0.5
BPTX&SPD BPTX&(V0OR|SPD) BPTX&SPD BPTX&(V0OR|SPD)
Number of collisions 284 150000 40000 380000
C. Cheshkov / Nuclear Physics B (Proc. Suppl.) 207–208 (2010) 329–332
331
Figure 3: A comparison of the measured multiplicity distributions for INEL collisions at 0.9 TeV (left) and 2.36 TeV (right) to models for the pseudorapidity range of |η| < 1. The error bars represent statistical uncertainties while shaded area systematic ones. The ratios between the measured values and model calculations are shown in the lower part of the figures. The shaded areas represent the combined uncertainties.
Figure 1: The measured charged-particle pseudorapidity density for INEL and NSD collisions as a function of the pseudorapidity at 0.9 TeV (left) and 2.36 TeV (right). The results are compared with the data from other experiments at the same energies (top) and with various Monte-Carlo generators (bottom).
Figure 4: The measured multiplicity distributions for INEL>0 event sample Figure 2: Left: the measured charged-particle pseudorapidity density for √
INEL>0 collisions as a function of the pseudorapidity at s = 7 TeV. The result is compared with various Monte-Carlo generators. Right: the charged-particle pseudorapidity density in the central pseudorapidity region of |η| < 0.5 for INEL and NSD collisions, and in |η| < 1 for INEL>0 collisions, as a function of the centre-of-mass energy. The lines indicate a fit with a power-law dependence on the energy.
from the UA5 experiment [9] and with pp NSD data at 0.9 and 2.36 TeV from the CMS experiment [10]. The comparison with model predictions shows best consistency with the Pythia tune ATLAS-CSC [11]. Phoject [7] is systematically below for 2.36 and especially 7 TeV data. The other two Pythia tunes, D6T [12] and Perugia-0 [13], are significantly lower at all three energies. The corresponding multiplicity distributions of charged particles are presented in Fig.3 and Fig.4. The comparison with the models is quite similar to the case of charged-particle densities. The distributions were fitted satisfactorily with a Negative-Binomial Distribution (NBD) (see Fig.4,left). The normalized differential primary charged-particle yield for INEL collisions at 0.9 TeV is given in Fig.5,top left. While the transverse momentum spectra are fitted quite satisfactorily by the modified Hagedorn function [14], the high pT tail of the spectrum is described much better by a power-law fit. The comparison with mod-
for the pseudorapidity range of |η| < 1. The error bars represent statistical uncertainties while shaded area systematic ones. Left: the results at three energies are shown together with the NBD fits (lines). Right: the 7 TeV result is compared to the model predictions. The ratio between the measured values and model calculations are shown in the lower part. The shaded areas represent the combined uncertainties.
els gives a picture opposite to the one of the chargedparticle multiplicities (see Fig.5,top right). This time the Perugia-0 tune provides the best agreement on the spectral shape. The D6T tune is similar to Perugia0, but underestimates the data at high pT . Both Phojet and ATLAS-CSC tune fail to describe the data. A comparison with data from other experiments has been done using the NSD event sample. Fig.5,bottom, gives a comparison with the recent results from the ATLAS [15] and the CMS [16] experiments and from p p¯ data from the UA1 collaboration [17], measured in different pseudorapidity regions. The overall agreement is good though at higher pT our data are above these two measurements. This disagreement may be related to the different pseudorapidity acceptances of these experiments. For completeness, in Fig.6 we provide a comparison of average transverse momentum in INEL collisions as a function of event multiplicity. The measurement was done by splitting the data into bins of number of detected particles and performing a correction using the
332
C. Cheshkov / Nuclear Physics B (Proc. Suppl.) 207–208 (2010) 329–332
Figure 6: A comparison of the average transverse momentum of charged particles in INEL collisions at 0.9 TeV as a function of the charged-particle multiplicity (nch ) with model predictions, for 0.5 < pT < 4 GeV/c (left) and 0.15 < pT < 4 GeV/c (right). The ratios between the Monte-Carlo and the data are shown in the lower parts of the figures. The shaded areas illustrate the combined uncertainties.
Acknowledgements The author is grateful to the organizers of the QCD’10 Conference for the kind invitation to present the first results from the ALICE experiment.
Figure 5: Top left: the normalized differential primary charged-particle yield for INEL collisions at 0.9 TeV, averaged in the pseudorapidity range of |η| < 0.8. The line shows a fit with the modified Hagedorn function. Top right: the yield is compared to the Pythia and the Phojet generators. Bottom left: the yield is compared to the results from ATLAS and CMS at the same energy. Bottom right: the normalized invariant primary charged-particle yield is compared to the results from the UA1 experiment in p p¯ at the same energy. The yield is computed assuming all particles are pions. The shaded areas on all figures illustrate combined uncertainties.
detector-response function. 6. Conclusions A set of first minimum-bias measurements has been performed with the ALICE experiment at three LHC beam energies. The average observed multiplicity increases much faster with the energy than it is predicted by the existing models. The obtained precise results on the multiplicity and transverse momentum distributions are essential for tuning and constraining the Monte-Carlo generators which presently do not describe the data satisfactorily. The success of these complex measurements demonstrates the excellent operation of the LHC machine and the ALICE experiment from the beginning of the physics run in 2009.
References [1] K. Aamodt et al. [ALICE Collaboration], Eur.Phys.J. C65, 111-125 (2010). [2] K. Aamodt et al. [ALICE Collaboration], Eur.Phys.J. C68, 89108 (2010). [3] K. Aamodt et al. [ALICE Collaboration], Eur.Phys.J. C68, 345-354 (2010). [4] K. Aamodt et al. [ALICE Collaboration], arXiv:1007.0719 [hep-ex] (2010). [5] K. Aamodt et al. [ALICE Collaboration], J.Instrum. 3, S08002 (2008). [6] T. Sj¨ostrand, S. Mrenna, P. Skands, J.High Energy Phys. 0605 (2006) 026. [7] R. Engel, J. Ranft, S. Roesler, Phys.Rev. D52, 1459 (1995). [8] J. Alme et al. [ALICE Collaboration], accepted for publication in Nucl.Instrum.Meth.A, arXiv:1001.1950 [physics.ins-det]. [9] G.J. Alner et al. [UA5 Collaboration], Z.Phys. C33, 1 (1986). [10] V. Khachatryan et al. [CMS Collaboration], JHEP 2010, 02 041 (2010). [11] A. Moraes [ATLAS Collaboration], ATLAS Note ATL-COMPHYS-2009-119 (2009). [12] M.G. Albrow et al., arXiv:hep-ph/0610012 (2006). [13] P.Z. Skands, arXiv:0905.3418 [hep-ph] (2009). [14] R. Hagedorn, Riv.Nuovo Cim. 6 1 (1983). [15] G. Aad et al. [ATLAS Collaboration], submitted to Phys.Lett. B, arXiv:1003.3124 [hep-ex] (2010). [16] V. Khachatryan et al. [CMS Collaboration], JHEP 02 02 041 (2010). [17] C. Albajar et al. [UA1 Collaboration], Nucl.Phys. B335, 261 (1990).