Hadron Production cross-sections

Transcription

1 1 Hadron Production cross-sections E. Radicioni a a Istituto nazionale di Fisica Nucleare - Bari - Italy Hadron production measurements for neutrino experiments is a well established field since the 7s. In recent years, interest in such studies was revived and new generation of low-to-medium energy (from 1 to 1 GeV) hadron production experiments were built or proposed. Such experiments all share a basic design, consisting in the presence of open-geometry spectrometers, capable of - as close as possible - full angular coverage and full particle identification. New results are now provided by Harp; the future will most likely bring more measurements thanks to MIPP and NA Introduction and motivations It is claimed that Neutrino Oscillation searches have reached the era of precision measurements; this precision is not backed by an equal level of understanding of Hadron Production on neutrino beam targets, which is a fundamental component in understanding neutrino beams and their extrapolation to neutrino detector sites. Precise and comprehensive studies of particle yields from the interaction of protons and charged pions with nuclei, at incident particle momenta from few GeV/c to 1 GeV/c, are relevant in several areas of neutrino physics, and in recent years, interest in such studies was revived for several reasons. The design of alternative neutrino beams would profit a lot from a more detailed knowledge of the hadron production cross-sections, optimizing the figure of merit (and the cost) of the so-called neutrino factory (the proton driver energy ranges from a few GeV to 24 GeV, depending on design choices). [1,2] The study of atmospheric neutrinos provides strong evidence for neutrino oscillations[3,4]. In this field most of the uncertainty in the calculation of atmospheric neutrino fluxes comes from the limited understanding of hadron interactions. Different Monte-Carlo simulations, provide estimates which can differ by as much as 3% [5 9]. The relevant energy range for primary particles is, in this case, from a few GeV to 1 GeV, while target material should be close to the constituents of the atmosphere, namely N 2 and O 2. In either cases, detailed knowledge of the hadron cross sections at the relevant energies is now considered a must. 2. Historical overview Since the 7 s, in order to make a reliable prediction of the neutrino flux, the community of experimenters has always been committed to measure the hadron production cross-sections. This should not be surprising, because being able to describe the neutrino beam composition relies on the detailed knowledge of the primary hadron interaction cross sections. The first generation of hadron production experiments was mostly based on measurements of particle yields along instrumented beam lines, making a smart use of existing facilities. Typically they were single arm spectrometers with a small phase space coverage in the forward region (< 15 mrad), and characterized by low statistic and/or limited number of data points. The overall scale error, arising from the uncertainties in the spectrometer acceptances and in the absolute calibration of the primary proton beam intensity, was estimated to be 15%. The experiments usually quoted as Allaby [1] and Eichten [11] used a low energy (2

2 2 GeV/c) of primary proton momentum at the CERN PS. At higher energy the NA2 single-arm spectrometer [12], performed a secondary particle energy scan in the range 6-3 GeV/c at the H2 beam line of the CERN SPS. Finally, in more recent times (1996) the SPY [13] experiment at CERN made use of the NA56 instrumented beam line. SPY was dedicated to study the WANF neutrino beam at CERN, but also the new CNGS beam [14] should benefit from this measurements. 3. Present measurements and future plans Even modern Hadron Production measurements are affected by several (for neutrino physics) limitations. The most obvious ones are Sparse measurements, with low statistics Limited acceptance makes extrapolation difficult. On the other hand, modeling and extrapolation of soft processes are not obvious in Monte-Carlo simulations, which should then be calibrated to data. Recent Heavy-Ions experiments have some p-a hadron production data. Unfortunately, since the p-a data sample is only used for comparison with A-A data, and it is obtained with the same trigger conditions (i.e. centrality triggers) as A-A. Neutrino physics usually needs data with unbiased triggers. The present experiments (HARP [15] and MIPP [16] ) are designed to combine a large, full phase space acceptance with low systematic errors and high statistic. These experiments share a modern design, mostly derived from Heavy-Ions experiments, that includes open geometry spectrometers, (almost) full solid angle acceptance and particle identification capabilities. In addition, since an overall precision of a few percent is required to reach the final aim, this new generation of experiments are designed to collect events at high rate and with small trigger biases. 4. HARP The HARP experiment at the CERN PS (approved in 1999) is a special case, since it is the first experiment built on-purpose for Neutrino Physics, designed to match all the requirements previously listed, and hopefully marks the beginning of Hadron Production measurements at the same level of precision reached by modern Neutrino Oscillation experiments. HARP is performing extensive measurements of hadron production cross sections and secondary particle yields in the energy range 2-15 GeV over the full solid angle, using a large set of cryogenic and solid (thin and thick) targets. To measureme the momenta and indentify the particle types at large angle, the experiment includes a Time Projection Chamber (TPC) positioned in a solenoidal magnet, for tracking and de/dx measurements, and a system of Resistive Plate Chambers (RPC) to help in discrimination of particle type. The forward spectrometer covers polar angles up to 25 mrad, and it is made by a dipole magnet and large planar drift chambers for particle tracking. The particle identification is performed with a combination of Time- Of-Flight (TOF), Cerenkov, and calorimeter information. Redundancy in particle identification has been sought, with a view to cross-calibrating efficiencies and obtain a few percent overall accuracy in the cross section measurements by minimizing the systematic errors. By making use of a fast readout (event rate 2.5KHz), very demanding (and unprecedented ) for a TPC, the HARP detector was able to collect few millions of events per setting (a setting is a combination of target type and material, beam energy and polarity) insuring small statistical errors. About 3 TB of data corresponding to 42 millions of events (and more that 1 different settings) have successfully been stored in 2 years of data taking. HARP has a large physics program that covers the measurement of pion yields for a variety of energies and targets relevant for the Neutrino Factory Design, including special cryogenic useful for atmospheric neutrino flux calculations, as well the measurements of hadron production cross sections using the replica targets of the Mini-

3 3 BooNE [17] and K2K [18] experiments. HARP has completed the detector calibration up to readiness for data analysis and several data settings are being actively analyzed. At the moment the outstanding priorities are: The pion production using the K2K and MiniBoone targets (proton beam of respectively 12.9 and 8.9 GeV/c) given the immediate interest for the neutrino community the pion yields from 3,5,8 GeV/c protons on Tantalum targets, fitting the Neutrino Factory and protons driver conditions Additional results are expected in the future Pion production using the K2K & MiniBoone targets The K2K experiment is most sensitive to uncertainties in the predicted neutrino spectrum in the energy range between.5 and 1 GeV. The distortion of the spectrum measured by the far detector is predicted to be maximal in this range according to the neutrino oscillation parameters measured in atmospheric neutrino experiments [19]. The transmission properties of the beam line in the K2K experiment are such that the relevant angular region for pions at production is below 25 mrad, while the relevant momentum region starts at 1 GeV/c and is essentially exhausted at 5 GeV/c, matching very well the HARP forward spectrometer acceptance. The measurement of pion production for the K2K experiment using a 5%λ Al target and incident protons of 12.9 GeV/c momentum was recently completed [2]. The data, after correction for measurement acceptances and resolutions, were fitted with a Sanford-Wang empirical parametrization to make comparison easier with similar measurements, and to use HARP data in the K2K beam Monte-Carlo. The absolute normalization was performed using minimum-bias pre-scaled beam triggers. The results for positively charged pions within a momentum range from.75 GeV/c to 6.5 GeV/c and within an angular range from 3 mrad to 21 mrad are shown in Fig 1. The overall error on total cross-section is 4.7%. The main use of this measurement is to predict dσ π / dp (mb / (GeV/c)) mrad p (GeV/c) dσ π / dω (mb / sr) GeV/c θ (mrad) Figure 1. π+ production cross-section as function of the pion momentum (left) and angle (right) for p-al collisions at 12.9GeV/c. The points show the HARP measurements, the dotted curve the best-fit Sanford-Wang parametrization the far-near ratio R, for the muon neutrino disappearance search. In the Fig 2 the far-near ratio is plotted as a function of the neutrino energy, comparing the HARP prediction to the original pion production cross-section assumed in K2K. The reduction on the flux uncertainties is well visible. A similar analysis making use 5%λ Berillium target and incident protons of 8.9 GeV/c momentum was performed to predict momentum and the angular distribution of the neutrino beam of the MiniBoone experiment. Also in this case, the relevant phase space region for pions is below 21 mrad with momenta between.75 and 5 GeV/c, well fitting the HARP forward spectrometer acceptance. Several comparisons were done with pion production data available in the literature from aluminum and beryllium targets with proton momenta between 1 and 15 GeV/c. The agreement is ranging from good to 3% lower depending on the experiment Pion production in proton-tantalum interactions One of the main motivations of the HARP experiment is to measure pion yields for a quantitative design of a proton driver of a future neutrino factory. At the moment the CERN scenario make provision for a 3 GeV/c proton linac with an high-z target [21], but also higher energies are being considered.

4 ) Proton beam on Ta 5 % λ at 3 GeV/c (1 Φ far /Φ near E ν (GeV) pt (GeV/c) Figure 2. Far-to-near muon neutrino fluxes ratio as a function of neutrino energy Eν in the K2K experiments : filled circle with errors bars are the HARP predictions and the empty squares with error boxes show the K2K model results. Figure 3. Transverse momentum distribution of the forward going π+ (full circle/red) and π- (empty circle/blue) produced by 3 GeV protons impinging a Tantalum target in the HARP detector 5. MIPP & NA49 For momenta below 1 GeV/c the prediction of different hadron production generators exhibit discrepancies both in the total yields and in the relative abundance of negative and positive particles. These discrepancies become larger at lower incoming proton energy (3-5 GeV). For this reason the analysis of data obtained using a sample of 3 GeV/c protons impinging a 5% interaction lenght Tantalum target is considered urgent and, therefore, were analysed first. The TPC was mainly used as tracking and particle identification detector, and the beam instrumentation was used to select the type of the incoming beam particles. The secondary pions of both signs in the θ range between 45 and 12 and between 1 MeV/c and 6 MeV/c in transverse momentum were studied, and an estimation of the differential raw pion yields and its efficiency correction was obtained. This analysis is still in progress. The absolute normalization and a detailed error estimation in also under way. Fig 3 shows the transverse momentum distribution of pions of both polarities (45 Θ 9 ): this is the most interesting sample for the design of the proton driver of a future neutrino factory. The MIPP experiment (FNAL-E97), approved in 21, after an engineering run started the data taking in 25. It make use of secondary beams of protons, anti-protons, π ±, K ± from the 12 GeV main injector at Fermilab, and it measures particle production cross sections on various nuclei in the energy range from 5 to 85 GeV/c. This experiment is complementary to HARP and shares with it the same design with an open geometry spectrometer. To measure momenta it makes use of the E91 TPC [22] and applies a combination of de/dx, TOF, Cerenkov and Ring- Imaging Cerenkov (RICH) technologies to identify the nature of the particles. The main motivations of the experiment are the non-perturbative QCD effects, and the the pion production crosssection. The latter measurement will include settings with the NUMI [23] target and a liquid Nitrogen target for atmospheric neutrino flux evaluation. At the moment 8 millions of triggers were collected successfully. An upgrade of the TPC electronics is being considered to cope with the need for high statistic. In few years from now the T2K experiment, at the new Hadron Facility in Tokay (Japan) [24],

5 5 will use an off-axis neutrino beam obtained from 5 GeV/c protons, with the ambitious task to measure θ 13 ) (or to put a stringent limit to it). Similarly to K2K, in T2K the systematic error in the far/near ratio will become relevant, making the physics results dependent from the Monte- Carlo predictions. To mitigate this problem a hadron production measurement will be required. The re-use of an excellent existing facility, like the NA49 experiment at CERN [25], is being seriously considered. This Heavy-Ions experiment is located on the H2 fixed target station on the CERN SPS, using secondary beams of π,k and protons in the momentum range from 4 to 35 GeV/c. The tracking is done using a set of large TPCs and the particle identification is provided both by the TPCs and a TOF system. NA49 has already made measurements for atmospheric neutrino studies using a 158 GeV/c proton beam impinging a Carbon targets. Now a new collaboration is forming to restart the data taking in 27. The physics program will include hadron production on T2K targets, and on light targets for atmospheric neutrinos. Similar to MIPP, an upgrade or re-configuration of the read out electronics is required to allow a high event rate and, therefore, a large enough un-biassed event sample. 6. Conclusions Hadron production for neutrino experiments is a well established field since the 7s. Every generation of neutrino experiments was accompanied by hadron production measurements. In the future, the search for small-sized effects (like θ 13 ) and the claimed high precision era in the neutrino experiments will require an equivalent precision in the knowledge of the relevant hadronic cross-sections. In this sense it is not surprising that in recent years we have seen a boost in these activities, with increasing quality, complexity (and cost) of hadron production experiments. New data are now provided by HARP, and the future looks promising thanks to MIPP and NA49. REFERENCES 1. B. Autin,Prospective study of muon storage rings at CERN, CERN 99-2 (1999). 2. A.Blondel et al. ECFA/CERN Studies of a European Neutrino Factory Complex CERN ECFA Y. Fukuda (SuperKamiokande Collaboration), B433 (1998) 9; 81 (1998) T. Kajita, B77 (1999) T.K.Gaisser et al.,phys.rev.d54 (1996) M.Honda,Nucl.Phys.B77 (1999) G.Battistoni et al., ph/9748 (1999). 8. V. Agrawal (Bartol group), D53 (1996) E.V. Bugaev and V.A. Naumov (BN group),b232 (1989) J.V.Allaby et al.,cern report 7-12 (197). 11. T.Eichten et al.,nucl.phys.b44,(1972) H.W.Atherton et al.,cern report 8-7 (198). 13. G.Ambrosini et al.,eur.phys.j. C1 (1999) K. Elsener, CNGS,CERN-SL-2-18-EA. 15. M.G.Catanesi et al., CERN-SPSC (1999). 16. Y. Fisyak et al.,fermilab-p E. Church et al.,[miniboone Collaboration], FERMILAB-P Y. Oyama, K2K (KEK to Kamioka) neutrino oscillation experiment at KEK-PS,(hep-ex/98314) 19. Y.Ashie et al.,phys.rev.d (25) 2. M.G.Catanesi et al., Measurement of the production cross-section of positive pions in p - Al collisions at 12.9 GeV/c,accepted by Nuclear Physics B. 21. M. Apollonio et al., Oscillation Physics with a Neutrino Factory. CERN TH I. Chemakin et al. [BNL-E-91 Collaboration] (NUCL-EX 9929), UCRL-JC Berkeley, CA : Lawrence Berkeley Nat. Lab., 22 Feb p. 23. E. Ables et al. [MINOS Collaboration], FERMILAB-P Y. Itow et al., The JHF-Kamioka neutrino project hep-ex/ A. Panagiotou et al., CERN-SPSLC P264.