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Status and perspectives of HARP
Nuclear Physics B (Proc. Suppl.) 138 (2005) 408–410 www.elsevierphysics.com
Status and perspectives of HARP Mario Campanellia∗ a
University of Geneva, 24 Q. E.Ansermet, Geneva, Switzerland
The HARP experiment has been taking data during the summer of 2001 and 2002 at CERN on the PS beam. Its primary goal has been to measure low-energy hadron cross-sections, and angular distributions of decay products. These measurements, presently quite incomplete, will be of primary use for the understanding of neutrino beams, both artificial and produced in cosmic rays. Detector performances and some first results will be presented, as well as a road map to get to the final measurements.
1. INTRODUCTION The present data on low-energy hadron production cross-sections for proton on fixed target are quite poor and incomplete. These data are extremely important for neutrino physics, in particular: • to understand neutrino production in K2K[1] and MiniBOONE[2] • to understand the rate of νe to νµ , therefore the background in future superbeams • hadron production for atmospheric neutrino fluxes • pion production in a neutrino factory
2. HARP DETECTOR AND DATA TAKING To fulfill HARP’s physics goals, we have to measure secondary particles at large and small angles, up to momenta of few GeV, and have a good pion/kaon separation, at least in the lowenergy part of the spectrum. This is achieved by surrounding the target by a large TPC, to measure and identify large-angle particles. For lowangle particles, a forward spectrometer is made of four drift chamber stations, with a dipole magnet between the first two stations and a threshold Cerenkov for particle ID between the second and third one. After the last spectrometer station, a system of calorimeter and TOF ensures particle identification in the forward region. A drawing of the HARP detector is shown in figure 1.
• to provide an input to hadronic MC (in particular Geant4) drift chambers
As an example, available data on pion production only cover the low-energy part of the spectrum, while the very important high-energy section has to be extrapolated, with large systematic uncertainties. Similarly, sub-GeV neutrinos in K2K and MiniBOONE (the region most affected by oscillations) are produced by decay of pions of about 2 GeV in the forward region (between 0 and 300 mrad), and angular-dependent data are not available. ∗ For
the HARP collaboration
0920-5632/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2004.11.095
time-of-flight scintillators beam-muon identifier
HARP PS 214
electron identifier
cosmics trigger wall
TPC + RPCs in solenoid magnet
threshold Cherenkov dipole magnet T9 beam
FTP + RPCs
Figure 1. The HARP detector
409
M. Campanelli / Nuclear Physics B (Proc. Suppl.) 138 (2005) 408–410
3. Performances of Harp 3.1. Beam detectors The T9 beam of the CERN PS is nominally a proton beam, with additional components of pions, kaons, electrons, and even deuterium and tritium, whose relative fraction depends on beam momentum. Since we need to discriminate the incoming beam particle on an event-by-event basis, we have used two beam Cerenkov and two beam TOF, at a relative distance of 21.4 meters, with a 170 ps resolution. The Cerenkov allows 100% efficiency for e/pi separation at the highest energies (12.9 GeV of the K2K run), while the TOF provides p/k/pi separation for the low energies (< 5 GeV). 3.2. The time-projection chamber The TPC is the most complex HARP subdetector; it completely surrounds the target and is used to identify tracks produced at θ > 100 mrads. Since these kinds of events are scarce in the backward region, a full detector calibration is only possible with lasers, x-ray sources and cosmic rays. During data-taking, we noticed some problems due to electronic cross-talk, field disomogeneities, and space-charge effects. Most of these effects are now well-understood and corrected, and the present momentum resolution is a factor 2 better than the uncorrected one (∆pt /pt = 0.75pt + 0.06). Additional improvement is expected from the correction of the crosstalk, due to electronic pick-up in the pad plane, that has been measured pad-by-pad after the end of data-taking. Then it was modeled by capacity coupling, and introduced in the Monte Carlo and filtered out in the reconstruction code. 3.3. The forward spectrometer The forward spectrometer is composed of four stations with a 1.5 T magnet between the first
two. Each station is made of four planes composed of three layers with respective rotation of 120◦ . These stations have been reused from the Nomad experiment[3]; however, due to stricter CERN safety regulations, non-flammable gas had to be used, reducing the efficiency for each chamber from above 95% to about 80%. The use of a new matching algorithm between the planes based on singlets, and not on triplets, as it was in Nomad, allows a recovery of efficiency for each station larger than 95%. After calibration and alignment, produced by an iterative procedure using cosmic rays, the spectrometer resolution is in good agreement with expectations and well reproduced by the Monte Carlo (figure 2).
δP (GeV)
The HARP collaboration is composed of 124 members from 24 institutes. The data-taking in 2001 and 2002 collected over 30 tbytes of data, corresponding to a few million events for targets ranging from hydrogen to lead and beam energies from 3 to 15 GeV. Overall, 420 million events were recorded.
0.9 0.8
A0 A1 A2
0.3443E-02 0.3560E-02 0.5931E-03
-0.1146E-01 0.2515E-01 0.7987E-02
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
2
4
6
8 P (GeV)
Figure 2. Resolution of the drift chambers
3.4. Particle identification Forward particle identification in HARP is achieved combining information from the Cerenkov detector located between the second and third chamber plane, the electron identifier (a lead-scintillating fiber calorimeter) and the timeof-fly system at the end of the apparatus. The Cerenkov has a volume of 31m3 , filled with C4 F10, with a refraction index n=1.0014, corresponding
410
M. Campanelli / Nuclear Physics B (Proc. Suppl.) 138 (2005) 408–410
The electron identifier, reused from Chorus, is 0 identiused for electron/pion separation, and π√ fication. The energy resolution of 23%/ E is in good agreement with that measured from Chorus, convoluted with the T9 beam specifications.
Figure 3. Cerenkov light yield as a function of momentum for pions
to a threshold for pions of 2.6 GeV. The TOF wall [4] has been continuously calibrated using a laser for stability checks, plus every 2-3 months with cosmic rays to measure the relative time offset of the photo multipliers. This resulted in a time resolution of 70 ps, resulting in a 3σ separation between pions and protons below 4.5 GeV, and between pions and kaons below 2.4 GeV.
160
55.31 P1 P2 P3 P4 P5 P6
140 120 100 80 60
/ 50 137.6 14.13 0.1819 31.56 15.98 0.1863
5.128 0.4192E-02 0.4231E-02 2.632 0.1149E-01 0.1102E-01
20
11
12
13
14
15
16
17
18
4. CONCLUSIONS The need to collect data on hadronic interactions in the neutrino physics community has lead a small collaboration to build and operate in a very short time the HARP detector. We took data in the summers of 2001 and 2002, using a large variety of targets and beam energies. Overall, the detector showed good behavior, although the TPC still needs further improvement for optimal performance. The most relevant region for oscillations of neutrinos from a νm u beam is the sub-GeV, that corresponds to pion momenta smaller than 1 GeV, and a production angle smaller than 200 mrad. HARP has showed to be in perfect shape to perform relevant measurements in this part of the phase space. REFERENCES
40
0
3.5. Sub-detector matching The different sub-detectors in the forward region (particle identification and drift chambers) have been aligned using mainly non-interacting beam particles collected with spurious triggers. Beam-halo particles, as well as low-angle scattering, have been used to match the drift chambers and the TPC, both before and after the bending magnet. Particles are tracked through the whole detector using a Kalman filter algorithm, and PID information from the different sub-detectors are combined.
19
Time of flight (ns)
Figure 4. π/p separation by TOF at a momentum of 3 GeV/c
1. M.H. Ahn et al., K2K Collaboration, Phys. Rev. Lett. 90 (2003) 041801. 2. I. Stancu et al., MiniBooNE collaboration, FERMILAB-TM-2207. 3. M. Anfreville et al., Nucl. Instr. and Methods A 481 (2002) 339. 4. G. Barichello et al., INFN-AE-02-01,HARPMEMO-02-001.
Status and perspectives of HARP Mario Campanellia∗ a
University of Geneva, 24 Q. E.Ansermet, Geneva, Switzerland
The HARP experiment has been taking data during the summer of 2001 and 2002 at CERN on the PS beam. Its primary goal has been to measure low-energy hadron cross-sections, and angular distributions of decay products. These measurements, presently quite incomplete, will be of primary use for the understanding of neutrino beams, both artificial and produced in cosmic rays. Detector performances and some first results will be presented, as well as a road map to get to the final measurements.
1. INTRODUCTION The present data on low-energy hadron production cross-sections for proton on fixed target are quite poor and incomplete. These data are extremely important for neutrino physics, in particular: • to understand neutrino production in K2K[1] and MiniBOONE[2] • to understand the rate of νe to νµ , therefore the background in future superbeams • hadron production for atmospheric neutrino fluxes • pion production in a neutrino factory
2. HARP DETECTOR AND DATA TAKING To fulfill HARP’s physics goals, we have to measure secondary particles at large and small angles, up to momenta of few GeV, and have a good pion/kaon separation, at least in the lowenergy part of the spectrum. This is achieved by surrounding the target by a large TPC, to measure and identify large-angle particles. For lowangle particles, a forward spectrometer is made of four drift chamber stations, with a dipole magnet between the first two stations and a threshold Cerenkov for particle ID between the second and third one. After the last spectrometer station, a system of calorimeter and TOF ensures particle identification in the forward region. A drawing of the HARP detector is shown in figure 1.
• to provide an input to hadronic MC (in particular Geant4) drift chambers
As an example, available data on pion production only cover the low-energy part of the spectrum, while the very important high-energy section has to be extrapolated, with large systematic uncertainties. Similarly, sub-GeV neutrinos in K2K and MiniBOONE (the region most affected by oscillations) are produced by decay of pions of about 2 GeV in the forward region (between 0 and 300 mrad), and angular-dependent data are not available. ∗ For
the HARP collaboration
0920-5632/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2004.11.095
time-of-flight scintillators beam-muon identifier
HARP PS 214
electron identifier
cosmics trigger wall
TPC + RPCs in solenoid magnet
threshold Cherenkov dipole magnet T9 beam
FTP + RPCs
Figure 1. The HARP detector
409
M. Campanelli / Nuclear Physics B (Proc. Suppl.) 138 (2005) 408–410
3. Performances of Harp 3.1. Beam detectors The T9 beam of the CERN PS is nominally a proton beam, with additional components of pions, kaons, electrons, and even deuterium and tritium, whose relative fraction depends on beam momentum. Since we need to discriminate the incoming beam particle on an event-by-event basis, we have used two beam Cerenkov and two beam TOF, at a relative distance of 21.4 meters, with a 170 ps resolution. The Cerenkov allows 100% efficiency for e/pi separation at the highest energies (12.9 GeV of the K2K run), while the TOF provides p/k/pi separation for the low energies (< 5 GeV). 3.2. The time-projection chamber The TPC is the most complex HARP subdetector; it completely surrounds the target and is used to identify tracks produced at θ > 100 mrads. Since these kinds of events are scarce in the backward region, a full detector calibration is only possible with lasers, x-ray sources and cosmic rays. During data-taking, we noticed some problems due to electronic cross-talk, field disomogeneities, and space-charge effects. Most of these effects are now well-understood and corrected, and the present momentum resolution is a factor 2 better than the uncorrected one (∆pt /pt = 0.75pt + 0.06). Additional improvement is expected from the correction of the crosstalk, due to electronic pick-up in the pad plane, that has been measured pad-by-pad after the end of data-taking. Then it was modeled by capacity coupling, and introduced in the Monte Carlo and filtered out in the reconstruction code. 3.3. The forward spectrometer The forward spectrometer is composed of four stations with a 1.5 T magnet between the first
two. Each station is made of four planes composed of three layers with respective rotation of 120◦ . These stations have been reused from the Nomad experiment[3]; however, due to stricter CERN safety regulations, non-flammable gas had to be used, reducing the efficiency for each chamber from above 95% to about 80%. The use of a new matching algorithm between the planes based on singlets, and not on triplets, as it was in Nomad, allows a recovery of efficiency for each station larger than 95%. After calibration and alignment, produced by an iterative procedure using cosmic rays, the spectrometer resolution is in good agreement with expectations and well reproduced by the Monte Carlo (figure 2).
δP (GeV)
The HARP collaboration is composed of 124 members from 24 institutes. The data-taking in 2001 and 2002 collected over 30 tbytes of data, corresponding to a few million events for targets ranging from hydrogen to lead and beam energies from 3 to 15 GeV. Overall, 420 million events were recorded.
0.9 0.8
A0 A1 A2
0.3443E-02 0.3560E-02 0.5931E-03
-0.1146E-01 0.2515E-01 0.7987E-02
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
2
4
6
8 P (GeV)
Figure 2. Resolution of the drift chambers
3.4. Particle identification Forward particle identification in HARP is achieved combining information from the Cerenkov detector located between the second and third chamber plane, the electron identifier (a lead-scintillating fiber calorimeter) and the timeof-fly system at the end of the apparatus. The Cerenkov has a volume of 31m3 , filled with C4 F10, with a refraction index n=1.0014, corresponding
410
M. Campanelli / Nuclear Physics B (Proc. Suppl.) 138 (2005) 408–410
The electron identifier, reused from Chorus, is 0 identiused for electron/pion separation, and π√ fication. The energy resolution of 23%/ E is in good agreement with that measured from Chorus, convoluted with the T9 beam specifications.
Figure 3. Cerenkov light yield as a function of momentum for pions
to a threshold for pions of 2.6 GeV. The TOF wall [4] has been continuously calibrated using a laser for stability checks, plus every 2-3 months with cosmic rays to measure the relative time offset of the photo multipliers. This resulted in a time resolution of 70 ps, resulting in a 3σ separation between pions and protons below 4.5 GeV, and between pions and kaons below 2.4 GeV.
160
55.31 P1 P2 P3 P4 P5 P6
140 120 100 80 60
/ 50 137.6 14.13 0.1819 31.56 15.98 0.1863
5.128 0.4192E-02 0.4231E-02 2.632 0.1149E-01 0.1102E-01
20
11
12
13
14
15
16
17
18
4. CONCLUSIONS The need to collect data on hadronic interactions in the neutrino physics community has lead a small collaboration to build and operate in a very short time the HARP detector. We took data in the summers of 2001 and 2002, using a large variety of targets and beam energies. Overall, the detector showed good behavior, although the TPC still needs further improvement for optimal performance. The most relevant region for oscillations of neutrinos from a νm u beam is the sub-GeV, that corresponds to pion momenta smaller than 1 GeV, and a production angle smaller than 200 mrad. HARP has showed to be in perfect shape to perform relevant measurements in this part of the phase space. REFERENCES
40
0
3.5. Sub-detector matching The different sub-detectors in the forward region (particle identification and drift chambers) have been aligned using mainly non-interacting beam particles collected with spurious triggers. Beam-halo particles, as well as low-angle scattering, have been used to match the drift chambers and the TPC, both before and after the bending magnet. Particles are tracked through the whole detector using a Kalman filter algorithm, and PID information from the different sub-detectors are combined.
19
Time of flight (ns)
Figure 4. π/p separation by TOF at a momentum of 3 GeV/c
1. M.H. Ahn et al., K2K Collaboration, Phys. Rev. Lett. 90 (2003) 041801. 2. I. Stancu et al., MiniBooNE collaboration, FERMILAB-TM-2207. 3. M. Anfreville et al., Nucl. Instr. and Methods A 481 (2002) 339. 4. G. Barichello et al., INFN-AE-02-01,HARPMEMO-02-001.