Measurements of Cross Sections and Charged Pion Spectra in Proton-Carbon Interactions at 31 GeV/c
As neutrino long baseline experiments enter a new domain of precision, the careful study of systematic errors due to poor knowledge of production cross sections for pions and kaons require more dedicated measurements for precise neutrino flux predictions. The cosmic ray experiments require dedicated hadron production measurements to tune simulation models used to describe air shower profiles. Among other goals, the NA61/SHINE (SPS Heavy Ion and Neutrino Experiment) experiment at the CERN SPS aims at precision measurements (5% and below) for both neutrino and cosmic ray experiments: those will improve the prediction of the neutrino flux for the T2K experiment at J-PARC and the prediction of muon production in the propagation of air showers for the Auger and KASCADE experiments. NA61/SHINE took data during a pilot run in 2007 and in 2009 and 2010 with different carbon targets. The NA61/SHINE set-up and spectra for positive and negative pions obtained with the 2007 thin (4% interaction length) carbon target data are presented .
ETH, Institute for Particle Physics,
Keywords: p+C interaction, inelastic cross section, inclusive pion spectra
1 Physics motivation
The NA61/SHINE (SPS Heavy Ion and Neutrino Experiment) experiment at the CERN SPS pursues a rich physics program in various fields . First, precise hadron production measurements are performed for improving calculations of the neutrino flux in the T2K neutrino oscillation experiment , as well as for more reliable simulations of cosmic-ray air showers in the Pierre Auger and KASCADE experiments . Second, p+p, p+Pb and nucleus+nucleus collisions will be studied extensively at SPS energies. This article presents first NA61/SHINE results on charged pion spectra in p+C interactions at 31 GeV/c which are needed for an accurate neutrino flux prediction in the T2K experiment. The results are based on the data collected during the first NA61/SHINE run in 2007.
T2K is a long baseline neutrino experiment in Japan, which uses a high intensity neutrino beam produced at J-PARCaaaJapan Proton Accelerator Research Complex organized jointly by JAEA and KEK in Tokai, Japan. It aims to precisely measure the appearance and disappearance . In order to generate the neutrino beam a high intensity 30 GeV (kinetic energy) proton beam impinging on a 90 cm long graphite target is used, where and K mesons decaying into (anti)neutrinos are produced. The neutrino fluxes and spectra are then measured both at the near detector complex, 280 m from the target, and by the Super-Kamiokande (SK) detector located 295 km away from the neutrino source and 2.5 degrees off-axis. Neutrino oscillations are probed by comparing the neutrino flux measured at SK to the predicted one. In order to predict the flux at SK one uses the near detector measurements and extrapolates them to SK with the help of Monte Carlo simulations. Up to now, these Monte Carlo predictions are based on hadron production models only. For more precise predictions, which would allow the reduction of systematic uncertainties to the level needed for the T2K physics goals, measurements of pion and kaon production off carbon targets are essential . The purpose of the NA61/SHINE measurements for T2K is to provide this information at exactly the proton extraction energy of the J-PARC Main Ring synchrotron, namely 30 GeV kinetic energy (approximately 31 GeV/c momentum). Presently, the T2K neutrino beam-line is set up to focus positively charged hadrons, in such a way that it produces a beam. Spectra of positively charged pions presented in this paper constitute directly an essential ingredient in the neutrino flux calculation.
2 The NA61/SHINE detector
The NA61/SHINE experiment is a large acceptance hadron spectrometer in the North Area H2 beam-line of the CERN SPS. The schematic layout is shown in Fig. 1 together with the overall dimensions.
The main components of the current detector were constructed and used by the NA49 collaboration . A set of scintillation and Cherenkov counters as well as beam position detectors (BPDs) upstream of the spectrometer provide timing reference, identification and position measurements of the incoming beam particles. The main tracking devices of the spectrometer are large volume Time Projection Chambers (TPCs). Two of them, the vertex TPCs (VTPC-1 and VTPC-2 in Fig. 1), are located in a free gap of 100 cm between the upper and lower coils of the two superconducting dipole magnets. Their maximum combined bending power is 9 Tm. In order to optimize the acceptance of the detector at 31 GeV/c beam momentum, the magnetic field used during the 2007 data taking period was set to a bending power of 1.14 Tm. Two large TPCs (MTPC-L and MTPC-R) are positioned downstream of the magnets symmetrically to the beam line. The TPCs are filled with Ar:CO gas mixtures in proportions 90:10 for VTPCs and 95:5 for MTPCs. The particle identification capability of the TPCs based on measurements of the specific energy loss, , is augmented by time-of-flight measurements using Time-of-Flight (ToF) detectors. The ToF-L and ToF-R arrays of scintillator pixels have a time resolution of better than 90 ps . Before the 2007 run the experiment was upgraded with a new forward time-of-flight detector (ToF-F) in order to extend the acceptance. The ToF-F consists of 64 scintillator bars with photomultiplier (PMT) readout at both ends resulting in a time resolution of about 115 ps. The target under study is installed 80 cm in front of the VTPC-1. The results presented here were obtained with an isotropic graphite target of dimensions 2.5(W)2.5(H)2(L) cm and with a density of g/cm. The target thickness along the beam is equivalent to about 4% of a nuclear interaction length ().
3 Analysis techniques
This section presents the procedures used for data analysis to extract pion cross sections. Crucial for this analysis is the identification of the produced pions. Depending on the momentum interval, different approaches have been adopted, which lead also to different track selection criteria. The calibrated distributions as a function of particle momentum for positively and negatively charged particles are presented in Fig. 2.
The task is facilitated for the negatively charged pions, by the observation that more than 90% of primary negatively charged particles produced in p+C interactions at this energy are , and thus the analysis of spectra can also be carried out without additional particle identification.
In the low momentum region (less than about 1 GeV/c), it is sufficient to distinguish pions from electrons/positrons, kaons and protons by means of particle identification via measurements of specific energy loss () in the TPCs. A reliable identification of mesons was not possible at momenta above 1 GeV/c where the Bethe-Bloch (BB) curves for pions, kaons and protons cross each other. On the other hand, for mesons, where the contribution of and antiprotons is almost negligible, the analysis could be extended in momentum up to 3 GeV/c allowing consistency checks with the other analysis methods in the region of overlap.
High purity particle identification can be performed by combining the and information. Moreover, in the momentum range 1-4 GeV/c, where bands for different particle species overlap, particle identification is in general only possible using the method, see Figs. 3. The ToF-F detector was designed to cover the necessary acceptance in momentum and polar angle required by the T2K experiment, although limited to particle momenta above about 0.8 GeV/c.
Indeed, three analysis methods were applied to obtain pion spectra:
analysis of mesons via measurements of negatively charged particles ( analysis)
analysis of and mesons identified via measurements in the TPCs ( analysis at low momentum) and
analysis of and mesons identified via time-of-flight and measurements in the ToF-F and TPCs, respectively ( analysis) .
Each analysis yields fully corrected pion spectra with independently calculated statistical and systematic errors. The spectra were compared in overlapping phase-space domains to check their consistency. Complementary domains were combined to reach maximum acceptance.
The agreement between the spectra obtained by different methods is, in general, better than 10%. Note, that data points in the same (, ) bin from different analysis methods are statistically correlated as they result from the analysis of the same data set. In order to obtain the final spectra consisting of statistically uncorrelated points the measurement with the smallest total error was selected.
The analysis was based on a sample of 521 k events selected from the total sample of 667 k of registered and reconstructed events. This criterion essentially removes a contamination by interactions upstream of the target. For the event sample with the target removed, the selection reduces the number of events from 46 k to 17 k.
This section presents results on inelastic and production cross sections as well as on differential spectra of and mesons in p+C interactions at 31 GeV/c.
4.1 Inelastic and production cross sections
The total inelastic cross section is measured to be
The production cross section was calculated from the inelastic cross section by subtracting the quasi-elastic contribution . The result is :
The production cross section is compared to previous measurements in Fig. 4.
4.2 Spectra of and mesons
The and spectra presented in this section refer to pions produced in strong and electromagnetic processes in p+C interactions at 31 GeV/c.
The spectra are presented as a function of particle momentum in ten intervals of the polar angle. Both quantities are calculated in the laboratory system. The chosen binning takes into account the available statistics of the 2007 data sample, detector acceptance and particle production kinematics.
The final spectra are plotted in Figs. 5 and 6. For the purpose of a comparison of the data with model predictions the spectra were normalized to the mean multiplicity in all production interactions by dividing by . This avoids uncertainties due to the different treatment of quasi-elastic interactions in models as well as problems due to the absence of predictions for inclusive cross sections.
This work presents inelastic and production cross sections as well as positively and negatively charged pion spectra in p+C interactions at 31 GeV/c on the 2007 thin (4% interaction length) carbon target. These data are essential for precise predictions of the neutrino flux for the T2K long baseline neutrino oscillation experiment in Japan. Furthermore, they provide important input to improve hadron production models needed for the interpretation of air showers initiated by ultra high energy cosmic particles.
A much larger data set with both the thin and the T2K replica carbon targets was recorded in 2009 and 2010 and is presently being analysed. This will lead to results of higher precision for pions and extend the measurements to other hadron species such as charged kaons, protons, and . Analysis of the data collected with the T2K replica target during the 2007 run is in progress . The new data will allow a further significant reduction of the uncertainties in the prediction of the neutrino flux in the T2K experiment.
This work was supported by the following funding agencies:
the Hungarian Scientific Research Fund (grants OTKA 68506 and 79840),
the Polish Ministry of Science and Higher Education (grants 667/N-CERN/2010/0,
N N202 1267 36, N N202 287838 (PBP 2878/B/H03/2010/38), DWM/57/T2K/2007),
the Federal Agency of Education of the Ministry of Education and Science
of the Russian Federation (grant RNP 220.127.116.11.1547),
the Russian Academy of Sciences and
the Russian Foundation for Basic Research (grants 08-02-00018 and 09-02-00664),
the Ministry of Education, Culture, Sports, Science and Technology,
Japan, Grant-in-Aid for Scientific Research (grants 18071005, 19034011,
19740162, 20740160 and 20039012), the Toshiko Yuasa Lab.
(France-Japan Particle Physics Laboratory),
the Institut National de Physique Nucléaire et Physique des Particules
the German Research Foundation (grant GA 1480/2-1),
the Swiss National Science Foundation
(Investigator-Driven projects and SINERGIA) and the Swiss State Secretariat
for Education and Research (FORCE grants).
The authors also wish to acknowledge the support provided by the collaborating institutions, in particular, the ETH Zurich (Research Grant TH-01 07-3), the University of Bern and the University of Geneva.
Finally, it is a pleasure to thank the European Organization for Nuclear Research for a strong support and hospitality and, in particular, the operating crews of the CERN SPS accelerator and beam lines who made the measurements possible.
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