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Detection of atmospheric muons using ALICE detectors
Nuclear Physics B (Proc. Suppl.) 212–213 (2011) 293–298 www.elsevier.com/locate/npbps
Detection of atmospheric muons using ALICE detectors B. Alessandro a , A. Fern´ andez T´ellez b , M. Rodr´ıguez Cahuantzi b , M.A. Subieta V´ asquez c , V. Canoa b on behalf of the ALICE Collaboration Roman a
Istituto Nazionale di Fisica Nucleare, sezione di Torino, ITALY
b
Benem´erita Universidad Aut´ onoma de Puebla, MEXICO
c
Dipartimento di Fisica Universit` a di Torino and INFN sezione di Torino, ITALY
A large number of atmospheric muon events were recorded during 2009 for the calibration, alignment and commissioning of most of the ALICE (A Large Ion Collider Experiment at the CERN LHC) detectors. Specific triggers, not used during the LHC collisions, were implemented to take these data. Some triggers select atmospheric muons, with zenith angle between 0o and 60o , crossing the central barrel of ALICE and reconstructed with the TPC (Time Projection Chamber). The muon multiplicity of the event, and for each muon the momentum, the sign, the direction and the spatial coordinates are measured. We present a first analysis of these events with correlations between some observables. Another trigger selects horizontal muons with zenith angle between 65o and 85o . These muons are detected and reconstructed with the Forward Muon Spectrometer. The angular distribution and some characteristics of these rare events are discussed.
1. INTRODUCTION The ALICE experiment has been designed to study heavy ion collisions at the collider LHC at CERN [1]. Actually it is taking data with p-p √ collisions at s = 7 TeV and in√November 2010 the first Pb-Pb interactions at s = 2.75 TeV will be detected. The location of the experiment, underground with 30 meters of overburden, is suitable to detect atmospheric muons belonging to extensive air showers, making it possible to study topics connected with cosmic ray physics. At this depth only the atmospheric muons with an energy larger than 15 GeV can reach the apparatus. In this paper we discuss the performances of ALICE in the detection of these muons. In particular we analyse the main observables measured with the central detectors for muons arriving with zenith angle between 0o and 60o , and the capabilities in detecting and analysing the horizontal muons (65o − 85o ), with the Forward Muon Spectrometer.
0920-5632/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2011.03.039
2. TRIGGER AND TRACKING WITH CENTRAL DETECTORS The ALICE setup is shown in Fig. 1, the detectors or devices involved in the cosmic ray physics are marked with a red line. Some of them are used for trigger purpose others for tracking muons. The first trigger is given by the subdetector ACORDE. It consists of an array of 60 scintillator modules located on the three top octants of the L3 magnet and the trigger is given by the coincidence of two of them. Other two triggers are given by the Time-Of-Flight (TOF) a MRPC large area detector with cylindrical surface surrounding the main tracking detectors of ALICE. One trigger called ’TOF single’ is given by the coincidence of two pads in opposite side and select mainly single muons, while the ’TOF cosmic’ is given by the coincidence of four pads located everywhere with the purpose of detecting higher muon multiplicity. The last trigger is given by the coincidence of two hits in the opposite half of the external layer of the Silicon Pixel Detector (SPD) and selects mainly single muon crossing the Inner Tracking System (ITS).
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B. Alessandro et al. / Nuclear Physics B (Proc. Suppl.) 212–213 (2011) 293–298
Figure 2. An atmospheric muon crossing the TPC is reconstructed with two tracks (up and down). Figure 1. ALICE setup, the red lines mark the detectors or devices involved in cosmic ray physics.
The atmospheric muons crossing the experiment are tracked by the Time Projection Chamber (TPC) [2]. Since the TPC is optimized to track particles created in p-p or Pb-Pb collisions, one muon crossing the TPC is usually reconstructed as two different tracks, one developing in the upper part of the detector (up) and the other in the lower part (down), as we can see in Fig. 2. An algorithm has been written to match these two tracks to obtain one entire long track and count the correct number of muons crossing the TPC. The algorithm has been tested for low and high multiplicity (up to 200 muons) and compared with a visual scan, using the standard ALICE event display.
an high energy muon interacting, usually with the iron yoke of the magnet, and producing a shower of particles as shown in the right part of Fig. 3. These two samples of events are divided with some proper cuts after the matching of the tracks. In this paper we will analyse only the first category.
3. ANALYSIS OF THE DATA WITH CENTRAL DETECTORS A small data sample taken in Summer 2009, with magnetic field of 0.5 Tesla, have been analysed; corresponding to 2.5 effective days of data taken. Two main categories of events were detected and reconstructed by the TPC : atmospheric muon and interaction events. An example of the first type is shown in the left part of Fig. 3 in which we see that all the muons of the bundle are parallel, a property used in the matching algorithm. The second type are due to
Figure 3. On the left a typical atmospheric muon events with all the muons parallel, on the right the shower of particles created by a muon interaction in the iron yoke.
After applying the muon track reconstruction method (matching algorithm and selection cuts) described above, we obtain the uncorrected Muon Multiplicity Distribution for each one of the four
B. Alessandro et al. / Nuclear Physics B (Proc. Suppl.) 212–213 (2011) 293–298
triggers used and shown in Fig. 4. As we can see in this figure, most of the events are single muon given by “TOF single” trigger, while the highest multiplicity are obtained with ACORDE and “TOF cosmic” triggers. No events with very high multiplicity have been seen in this short period of data taking. This multiplicity distribution is one of the powerful tools to extract information on primary cosmic ray composition used in underground experiment. In such studies, this plot has to be compared with the multiplicity distribution generated from primary proton or iron nuclei, using cosmic shower simulation programs.
295
the heavy component. From simulation studies we have seen than more than 70% of the muons reaching ALICE have a momentum less than 100 GeV/c, so the minimum requirement is to measure the momentum with a good resolution at least up to this value. Since the TPC is optimized for lower momentum measurement, of the order of 10 GeV/c and less, a detailed study of the performances of the TPC at high momentum has been developed. As a first approach we try to get the resolution using only the real data, taking advantage of the reconstruction of the muon in two tracks. Using the measurements of the momentum for the upper track (Pu ) and its error (σu ) and for the lower track (Pd and σd ), we have defined two variables to evaluate the momentum P : P u + Pd 2 σd2 σu2 = Pu + Pd 2 2 2 σu + σd σu + σd2
Pmean =
(1)
Pweight
(2)
Pmean is the standard mean value between up and down momentum while Pweight weights the two momenta with their errors. The correlation between these two momenta (Fig. 5) reflects the fact that there is no asymmetry top-bottom in the TPC. In the following we choose Pweight as a measurement of the momentum for muons with two tracks. In order to estimate the resolution (R) of the momentum at different energies using the experimental data we define : Figure 4. Uncorrected Muon Multiplicity Distribution for different triggers.
R=
Taking advantage of the present magnetic field (0.5 Tesla intensity) provided by the ALICE solenoid magnet, we can measure the muon momenta, a quantity usually not measurable in standard underground cosmic ray apparatus. A correlation between multiplicity of the event and average momentum of the muon bundle could be a new approach for composition studies and give interesting results in disentangle the light from
where P tu and P td are the transverse momentum of up and down tracks. The resolution is defined as a function of 1/P t because the measured quantity in the TPC is the curvature of the track that is proportional to 1/P t and its probability density function is Gaussian. The resolution is given as the Gaussian width of the R variable for different interval of the momentum (Pweight ) and is shown in Fig. 6 for some experimental runs. The error is stable and does
1/P tu − 1/P td (1/P tu + 1/P td )/2
(3)
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B. Alessandro et al. / Nuclear Physics B (Proc. Suppl.) 212–213 (2011) 293–298
Figure 5. Correlation between Pmean and Pweight .
not depend on the run, its value is around 50% at Pweight = 100 GeV/c. In order to improve this resolution, we have to use the total length of the muon track (up and down), and fit this long track to measure the momentum. We have seen with simulations that this procedure is equivalent to using the covariant matrix of the track up and down and calculate the covariant momentum P cov. After the introduction in the ALICE off-line code (AliRoot) of this new variable, we did some simulations to calculate its resolution. An improvement of a factor 2 in all the range of the momentum studied has been found, so the estimated resolution at P cov = 100 GeV/c is around 25-30%. We think that this resolution should be improved in the future with a better calibration of the TPC and a new tracking program optimized for atmospheric muons. In Fig. 7 we shown the uncorrected Muon Momentum Distribution for vertical events (0o − 20o zenith angle). The next step will be the measurement of the Muon Momentum Distribution corrected for all the effects up to 200 GeV/c and of the ratio mu+/mu-. A correlation between muon mul-
Figure 6. Resolution in the measurement of the momentum of the muon for various runs.
tiplicity and muon momentum should be an interesting new approach to address composition analysis starting from the data. The direction of the muons has also been obtained and shown in Fig. 8 in which is plotted the correlation between the azimuth and zenith angle. We can see some characteristics of the environment surrounding the apparatus. The two green zones correspond to the muons passing through the two shafts. In these directions they loose less energy and the number arriving in ALICE is bigger. The two white zone (lack of events) are the directions in which muons cross the end and empty part of the cylinder of the TOF, and no trigger signal can be sent. This correlation allows to apply immediately some cuts to have a clean sample of events depending on the type of the analysis required. For example, events with zenith angle in the range 0o −20o (vertical muons) described before are not affected by shaft effects.
B. Alessandro et al. / Nuclear Physics B (Proc. Suppl.) 212–213 (2011) 293–298
Figure 7. Uncorrected Muon Momentum Distribution for vertical muons.
297
Figure 8. Correlation between the azimuth and the zenith angle of the muons.
4. DETECTIONS OF HORIZONTAL MUONS WITH THE FORWARD MUON SPECTROMETER The Forward Muon Spectrometer is composed by 5 RPC tracking chambers, a dipole magnet and two trigger chambers located after an iron absorber (see Fig. 1). It was designed to detect forward muons created in the LHC collisions. We have made use of this detector to track and analyse horizontal atmospheric muons. The aim is to evaluate its capabilities in measuring with a good precision the angular and momentum distributions and the ratio mu+/mu-. Ten days of data has been taken during Summer 2009 for a total of more than 8000 events. More than 5000 events have the right direction from central detector to muon spectrometer, like collision events, and we consider them a good sample to be analysed.
Figure 9. muons.
The zenith angle distribution of these muons is given in Fig. 9, as we can see most of them are in the range between 65o and 85o . A central and
narrower interval of zenith angle between 70o and 80o has been chosen to get the uncorrected Muon
Angular distribution of horizontal
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B. Alessandro et al. / Nuclear Physics B (Proc. Suppl.) 212–213 (2011) 293–298
the measurement of the ratio mu+/mu- in interval of momentum has to be done at surface level. From simulation programs we have calculated the energy loss of muons crossing the rock at different zenith angle and we have corrected with these values the previous muon distribution to show the uncorrected Muon Momentum Distribution at surface level (Fig. 11). The next step of our work will be the measurement of the ratio mu+/mu- with a precise evaluation of the systematic errors both for vertical muons with the TPC and for horizontal muons with the Muon Spectrometer. REFERENCES Figure 10. Uncorrected Muon Momentum Distribution of horizontal muons at the level of ALICE experiment.
Figure 11. Uncorrected Muon Momentum Distribution of horizontal muons at surface level.
Momentum Distribution at the level of ALICE and shown in Fig. 10. Since a muon crossing the rock with zenith angle of 70o loses about 50 GeV of the energy while a muon with an angle of 80o loses about 80 GeV,
1. ALICE Collaboration et al 2004 J. Phys. G: Nucl. Part. Phys. 30 1517. 2. ALICE TPC Collaboration, J. Alme et al., ”The ALICE TPC, a large 3-dimensional tracking device with fast readout for ultrahigh multiplicity events.”, Physics. InsDet/10011950 (2010).
Detection of atmospheric muons using ALICE detectors B. Alessandro a , A. Fern´ andez T´ellez b , M. Rodr´ıguez Cahuantzi b , M.A. Subieta V´ asquez c , V. Canoa b on behalf of the ALICE Collaboration Roman a
Istituto Nazionale di Fisica Nucleare, sezione di Torino, ITALY
b
Benem´erita Universidad Aut´ onoma de Puebla, MEXICO
c
Dipartimento di Fisica Universit` a di Torino and INFN sezione di Torino, ITALY
A large number of atmospheric muon events were recorded during 2009 for the calibration, alignment and commissioning of most of the ALICE (A Large Ion Collider Experiment at the CERN LHC) detectors. Specific triggers, not used during the LHC collisions, were implemented to take these data. Some triggers select atmospheric muons, with zenith angle between 0o and 60o , crossing the central barrel of ALICE and reconstructed with the TPC (Time Projection Chamber). The muon multiplicity of the event, and for each muon the momentum, the sign, the direction and the spatial coordinates are measured. We present a first analysis of these events with correlations between some observables. Another trigger selects horizontal muons with zenith angle between 65o and 85o . These muons are detected and reconstructed with the Forward Muon Spectrometer. The angular distribution and some characteristics of these rare events are discussed.
1. INTRODUCTION The ALICE experiment has been designed to study heavy ion collisions at the collider LHC at CERN [1]. Actually it is taking data with p-p √ collisions at s = 7 TeV and in√November 2010 the first Pb-Pb interactions at s = 2.75 TeV will be detected. The location of the experiment, underground with 30 meters of overburden, is suitable to detect atmospheric muons belonging to extensive air showers, making it possible to study topics connected with cosmic ray physics. At this depth only the atmospheric muons with an energy larger than 15 GeV can reach the apparatus. In this paper we discuss the performances of ALICE in the detection of these muons. In particular we analyse the main observables measured with the central detectors for muons arriving with zenith angle between 0o and 60o , and the capabilities in detecting and analysing the horizontal muons (65o − 85o ), with the Forward Muon Spectrometer.
0920-5632/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2011.03.039
2. TRIGGER AND TRACKING WITH CENTRAL DETECTORS The ALICE setup is shown in Fig. 1, the detectors or devices involved in the cosmic ray physics are marked with a red line. Some of them are used for trigger purpose others for tracking muons. The first trigger is given by the subdetector ACORDE. It consists of an array of 60 scintillator modules located on the three top octants of the L3 magnet and the trigger is given by the coincidence of two of them. Other two triggers are given by the Time-Of-Flight (TOF) a MRPC large area detector with cylindrical surface surrounding the main tracking detectors of ALICE. One trigger called ’TOF single’ is given by the coincidence of two pads in opposite side and select mainly single muons, while the ’TOF cosmic’ is given by the coincidence of four pads located everywhere with the purpose of detecting higher muon multiplicity. The last trigger is given by the coincidence of two hits in the opposite half of the external layer of the Silicon Pixel Detector (SPD) and selects mainly single muon crossing the Inner Tracking System (ITS).
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B. Alessandro et al. / Nuclear Physics B (Proc. Suppl.) 212–213 (2011) 293–298
Figure 2. An atmospheric muon crossing the TPC is reconstructed with two tracks (up and down). Figure 1. ALICE setup, the red lines mark the detectors or devices involved in cosmic ray physics.
The atmospheric muons crossing the experiment are tracked by the Time Projection Chamber (TPC) [2]. Since the TPC is optimized to track particles created in p-p or Pb-Pb collisions, one muon crossing the TPC is usually reconstructed as two different tracks, one developing in the upper part of the detector (up) and the other in the lower part (down), as we can see in Fig. 2. An algorithm has been written to match these two tracks to obtain one entire long track and count the correct number of muons crossing the TPC. The algorithm has been tested for low and high multiplicity (up to 200 muons) and compared with a visual scan, using the standard ALICE event display.
an high energy muon interacting, usually with the iron yoke of the magnet, and producing a shower of particles as shown in the right part of Fig. 3. These two samples of events are divided with some proper cuts after the matching of the tracks. In this paper we will analyse only the first category.
3. ANALYSIS OF THE DATA WITH CENTRAL DETECTORS A small data sample taken in Summer 2009, with magnetic field of 0.5 Tesla, have been analysed; corresponding to 2.5 effective days of data taken. Two main categories of events were detected and reconstructed by the TPC : atmospheric muon and interaction events. An example of the first type is shown in the left part of Fig. 3 in which we see that all the muons of the bundle are parallel, a property used in the matching algorithm. The second type are due to
Figure 3. On the left a typical atmospheric muon events with all the muons parallel, on the right the shower of particles created by a muon interaction in the iron yoke.
After applying the muon track reconstruction method (matching algorithm and selection cuts) described above, we obtain the uncorrected Muon Multiplicity Distribution for each one of the four
B. Alessandro et al. / Nuclear Physics B (Proc. Suppl.) 212–213 (2011) 293–298
triggers used and shown in Fig. 4. As we can see in this figure, most of the events are single muon given by “TOF single” trigger, while the highest multiplicity are obtained with ACORDE and “TOF cosmic” triggers. No events with very high multiplicity have been seen in this short period of data taking. This multiplicity distribution is one of the powerful tools to extract information on primary cosmic ray composition used in underground experiment. In such studies, this plot has to be compared with the multiplicity distribution generated from primary proton or iron nuclei, using cosmic shower simulation programs.
295
the heavy component. From simulation studies we have seen than more than 70% of the muons reaching ALICE have a momentum less than 100 GeV/c, so the minimum requirement is to measure the momentum with a good resolution at least up to this value. Since the TPC is optimized for lower momentum measurement, of the order of 10 GeV/c and less, a detailed study of the performances of the TPC at high momentum has been developed. As a first approach we try to get the resolution using only the real data, taking advantage of the reconstruction of the muon in two tracks. Using the measurements of the momentum for the upper track (Pu ) and its error (σu ) and for the lower track (Pd and σd ), we have defined two variables to evaluate the momentum P : P u + Pd 2 σd2 σu2 = Pu + Pd 2 2 2 σu + σd σu + σd2
Pmean =
(1)
Pweight
(2)
Pmean is the standard mean value between up and down momentum while Pweight weights the two momenta with their errors. The correlation between these two momenta (Fig. 5) reflects the fact that there is no asymmetry top-bottom in the TPC. In the following we choose Pweight as a measurement of the momentum for muons with two tracks. In order to estimate the resolution (R) of the momentum at different energies using the experimental data we define : Figure 4. Uncorrected Muon Multiplicity Distribution for different triggers.
R=
Taking advantage of the present magnetic field (0.5 Tesla intensity) provided by the ALICE solenoid magnet, we can measure the muon momenta, a quantity usually not measurable in standard underground cosmic ray apparatus. A correlation between multiplicity of the event and average momentum of the muon bundle could be a new approach for composition studies and give interesting results in disentangle the light from
where P tu and P td are the transverse momentum of up and down tracks. The resolution is defined as a function of 1/P t because the measured quantity in the TPC is the curvature of the track that is proportional to 1/P t and its probability density function is Gaussian. The resolution is given as the Gaussian width of the R variable for different interval of the momentum (Pweight ) and is shown in Fig. 6 for some experimental runs. The error is stable and does
1/P tu − 1/P td (1/P tu + 1/P td )/2
(3)
296
B. Alessandro et al. / Nuclear Physics B (Proc. Suppl.) 212–213 (2011) 293–298
Figure 5. Correlation between Pmean and Pweight .
not depend on the run, its value is around 50% at Pweight = 100 GeV/c. In order to improve this resolution, we have to use the total length of the muon track (up and down), and fit this long track to measure the momentum. We have seen with simulations that this procedure is equivalent to using the covariant matrix of the track up and down and calculate the covariant momentum P cov. After the introduction in the ALICE off-line code (AliRoot) of this new variable, we did some simulations to calculate its resolution. An improvement of a factor 2 in all the range of the momentum studied has been found, so the estimated resolution at P cov = 100 GeV/c is around 25-30%. We think that this resolution should be improved in the future with a better calibration of the TPC and a new tracking program optimized for atmospheric muons. In Fig. 7 we shown the uncorrected Muon Momentum Distribution for vertical events (0o − 20o zenith angle). The next step will be the measurement of the Muon Momentum Distribution corrected for all the effects up to 200 GeV/c and of the ratio mu+/mu-. A correlation between muon mul-
Figure 6. Resolution in the measurement of the momentum of the muon for various runs.
tiplicity and muon momentum should be an interesting new approach to address composition analysis starting from the data. The direction of the muons has also been obtained and shown in Fig. 8 in which is plotted the correlation between the azimuth and zenith angle. We can see some characteristics of the environment surrounding the apparatus. The two green zones correspond to the muons passing through the two shafts. In these directions they loose less energy and the number arriving in ALICE is bigger. The two white zone (lack of events) are the directions in which muons cross the end and empty part of the cylinder of the TOF, and no trigger signal can be sent. This correlation allows to apply immediately some cuts to have a clean sample of events depending on the type of the analysis required. For example, events with zenith angle in the range 0o −20o (vertical muons) described before are not affected by shaft effects.
B. Alessandro et al. / Nuclear Physics B (Proc. Suppl.) 212–213 (2011) 293–298
Figure 7. Uncorrected Muon Momentum Distribution for vertical muons.
297
Figure 8. Correlation between the azimuth and the zenith angle of the muons.
4. DETECTIONS OF HORIZONTAL MUONS WITH THE FORWARD MUON SPECTROMETER The Forward Muon Spectrometer is composed by 5 RPC tracking chambers, a dipole magnet and two trigger chambers located after an iron absorber (see Fig. 1). It was designed to detect forward muons created in the LHC collisions. We have made use of this detector to track and analyse horizontal atmospheric muons. The aim is to evaluate its capabilities in measuring with a good precision the angular and momentum distributions and the ratio mu+/mu-. Ten days of data has been taken during Summer 2009 for a total of more than 8000 events. More than 5000 events have the right direction from central detector to muon spectrometer, like collision events, and we consider them a good sample to be analysed.
Figure 9. muons.
The zenith angle distribution of these muons is given in Fig. 9, as we can see most of them are in the range between 65o and 85o . A central and
narrower interval of zenith angle between 70o and 80o has been chosen to get the uncorrected Muon
Angular distribution of horizontal
298
B. Alessandro et al. / Nuclear Physics B (Proc. Suppl.) 212–213 (2011) 293–298
the measurement of the ratio mu+/mu- in interval of momentum has to be done at surface level. From simulation programs we have calculated the energy loss of muons crossing the rock at different zenith angle and we have corrected with these values the previous muon distribution to show the uncorrected Muon Momentum Distribution at surface level (Fig. 11). The next step of our work will be the measurement of the ratio mu+/mu- with a precise evaluation of the systematic errors both for vertical muons with the TPC and for horizontal muons with the Muon Spectrometer. REFERENCES Figure 10. Uncorrected Muon Momentum Distribution of horizontal muons at the level of ALICE experiment.
Figure 11. Uncorrected Muon Momentum Distribution of horizontal muons at surface level.
Momentum Distribution at the level of ALICE and shown in Fig. 10. Since a muon crossing the rock with zenith angle of 70o loses about 50 GeV of the energy while a muon with an angle of 80o loses about 80 GeV,
1. ALICE Collaboration et al 2004 J. Phys. G: Nucl. Part. Phys. 30 1517. 2. ALICE TPC Collaboration, J. Alme et al., ”The ALICE TPC, a large 3-dimensional tracking device with fast readout for ultrahigh multiplicity events.”, Physics. InsDet/10011950 (2010).