HARP and NA6 (SHINE) hadron production experiments Boris A. Popov [for the HARP and NA6 Collaborations] LPNHE, 4 place Jussieu, 755, Paris & DLNP, JINR, Joliot-Curie 6, 498, Dubna Abstract. The hadroproduction experiments HARP and NA6 (SHINE) as well as their implications for neutrino physics are discussed. Recent HARP measurements have already been used for precise predictions of neutrino beams in K2K and MiniBooNE/SciBooNE experiments and are also being used to improve the atmospheric neutrino flux predictions and to help in the optimization of neutrino factory and super-beam designs. First preliminary data from NA6 are of significant importance for a precise prediction of a new neutrino beam at J-PARC to be used for the first stage of the T2K experiment. Both HARP and NA6 provide a large amount of input for validation and tuning of hadroproduction models in Monte-Carlo generators. Keywords: hadron production, neutrino beams PACS: 3.75.-n, 3.75.Cs, 3.85.-t, 3.85.Ni, 25.4.-h,25.8.Hp THE HARP EXPERIMENT The HARP experiment [, 2] at the CERN PS was designed to make measurements of hadron yields from a large range of nuclear targets and for incident particle momenta from.5 GeV/c to 5 GeV/c. The main motivations are the measurement of pion yields for a quantitative design of the proton driver of a future neutrino factory [3], a substantial improvement in the calculation of the atmospheric neutrino flux [4] and the measurement of particle yields as input for the flux calculation of accelerator neutrino experiments, such as K2K [5, 6], MiniBooNE [7] and SciBooNE [8]. In addition to these specific aims, the data provided by HARP are valuable for validating hadron production models used in simulation programs. To provide a large angular and momentum coverage of the produced charged particles the HARP experiment makes use of a large-acceptance spectrometer consisting of a forward and large-angle detection system. A detailed description of the experimental apparatus can be found in Ref. [2]. The forward spectrometer based on large area drift chambers [9] and a dipole magnet complemented by a set of detectors for particle identification (PID): a time-offlight wall [] (TOFW), a large Cherenkov detector (CHE) and an electromagnetic calorimeter covers polar angles up to 25 mrad which is well matched to the angular range of interest for the measurement of hadron production to calculate the properties of conventional neutrino beams. The large-angle spectrometer based on a Time Projection Chamber (TPC) located inside a solenoidal magnet has a large acceptance in the momentum and angular range for the pions relevant to the production of the muons in a neutrino factory. It covers the large majority of the pions accepted in the focusing system of a typical design. The neutrino beam of a neutrino factory originates from the decay of muons which are in turn the decay products of pions. A large amount of data collected by the HARP experiment with thin (5% of nuclear interaction length, λ I ) and cryogenic targets have already been analyzed and published [, 3, 4, 5, 6, 7, 8, 9, 2, 2, 22]. Those results cover all the physics subjects discussed above. Results obtained with the HARP forward spectrometer A detailed description of established experimental techniques for the data analysis in the HARP forward spectrometer can be found in Refs. [, 2, 3, 4]. Only a brief summary is given here.
The absolutely normalized double-differential cross-section for a process like p+target π + +X can be expressed in bins of pion kinematic variables in the laboratory frame, (p π,θ π ), as where: d 2 σ π+ d pdω (p π,θ π ) = A N A ρt p Ω Npot Nπ+ (p π,θ π ), () d2 σ π+ d pdω is the cross-section in cm2 /(GeV/c)/srad for each (p π,θ π ) bin covered in the analysis. A N A ρ is the reciprocal of the number density of target nuclei. t is the thickness of the target along the beam direction. p and Ω are the bin sizes in momentum and solid angle ( p = p max p min ; Ω = 2π(cos(θ min ) cos(θ max ))). Npot is the number of protons on target after event selection cuts. N π+ (p π,θ π ) is the yield of positive pions in bins of true momentum and angle in the laboratory frame. Eq. () can be generalized to give the inclusive cross-section for a particle of type α d 2 σ α d pdω (p,θ) = A N A ρt p Ω Npot M pθα p θ α N α (p,θ ), (2) where reconstructed quantities are marked with a prime and M pθα p θ α is the inverse of a matrix which fully describes the migrations between bins of true and reconstructed quantities, namely: lab frame momentum, p, lab frame angle, θ, and particle type, α. There is a background associated with beam protons interacting in materials other than the nuclear target (parts of the detector, air, etc.). These events are subtracted by using data collected without the nuclear target in place after normalization to the same number of protons on target. This procedure is referred to as the empty target subtraction. The event selection is performed in the following way: a good event is required to have a single, well reconstructed and identified beam particle impinging on the nuclear target. A downstream trigger in the forward trigger plane (FTP) is also required to record the event, necessitating an additional set of unbiased, pre-scaled triggers for absolute normalization of the cross-section. These pre-scaled triggers (e.g. /64) are subject to exactly the same selection criteria for a good beam particle as the event triggers allowing the efficiencies of the selection to cancel, thus adding no additional systematic uncertainty to the absolute normalization of the result. Secondary track selection criteria have been optimized to ensure the quality of the momentum reconstruction as well as a clean time-of-flight measurement while maintaining high reconstruction and particle identification efficiencies [2, 3]. The first HARP physics publication [] reported measurements of the π + production cross-section from a thin 5% λ I aluminum target at 2.9 GeV/c proton momentum. This corresponds to the energies of the KEK PS and the target material used by the K2K experiment. The results obtained in Ref. [] were subsequently applied to the final neutrino oscillation analysis of K2K [6], allowing a significant reduction of the dominant systematic error associated with the calculation of the so-called far-to-near ratio from 5.% to 2.9% (see [] and [6] for a detailed discussion) and thus an increased K2K sensitivity to the oscillation signal. The next HARP goal was to contribute to the understanding of the MiniBooNE and SciBooNE neutrino fluxes. They are both produced by the Booster Neutrino Beam at Fermilab which originates from protons accelerated to 8.9 GeV/c by the booster before being collided against a beryllium target. A fundamental input for the calculation of the resulting ν µ flux is the measurement of the π + cross-sections from a thin 5% λ I beryllium target at exactly 8.9 GeV/c proton momentum. The double-differential inelastic cross-section for the production of positive pions from collisions of 8.9 GeV/c protons with beryllium have been measured in the kinematic range from.75 GeV/c p π 6.5 GeV/c and.3 rad θ π.2 rad, subdivided into 3 momentum and 6 angular bins [3]. A typical total uncertainty of 9.8% on the double-differential cross-section values and a 4.9% uncertainty on the total integrated cross-section are obtained. These HARP results have been used for neutrino flux predictions [24] in the MiniBooNE [25] and SciBooNE experiments [8]. Sanford and Wang [26] have developed an empirical parametrization for describing the production cross-sections of mesons in proton-nucleus interactions. This parametrization has the functional form: d 2 σ(p+a π + + X) d pdω (p,θ) = exp[c c 3 p c 4 p c 5 beam c 6 θ(p c 7 p beam cos c 8 θ)]p c 2 ( p p beam ), (3)
FIGURE. Measurements of the double-differential production cross-sections of π + (open circles) and π (closed circles) from 2 GeV/c protons on carbon (left), N 2 (middle) and O 2 (right) target as a function of pion momentum, p, in bins of pion angle, θ, in the laboratory frame. Different panels show different angular bins. The error bars shown include statistical errors and all (diagonal) systematic errors. The curves show the Sanford-Wang parametrization of Eq. (3) with parameters given in Ref. [4] (solid line for π + and dashed line for π ). where X denotes any system of other particles in the final state, p beam is the proton beam momentum in GeV/c, p and θ are the π + momentum and angle in units of GeV/c and radians, respectively, d 2 σ/(d pdω) is expressed in units of mb/(gev/c sr), dω 2π d(cosθ), and the parameters c,...,c 8 are obtained from fits to meson production data. HARP data have been fitted using this empirical parametrization. This is a useful tool to compare and combine different data sets. In [22] a global Sanford-Wang parametrization is provided for forward production of charged pions as an approximation of all the studied datasets. It can serve as a tool for quick yields estimates. The next HARP analysis was devoted to the measurement of the double-differential production cross-section of π ± in the collision of 2 GeV/c protons (and pions) with a thin 5% λ I carbon target [4]. The results are shown in Fig.. These measurements are important for a precise calculation of the atmospheric neutrino flux and for a prediction of the development of extended air showers. Simulations predict that collisions of protons with a carbon target are very similar to proton interactions with air. This hypothesis could be directly tested with the HARP data. Measurements with cryogenic O 2 and N 2 targets [5] (also shown in Fig. ) confirm that p-c data can be indeed used to predict pion production in p-o 2 and p-n 2 interactions. It is important to emphasize that recently a systematic campaign of measurements has been performed by HARP: all thin target data taken with pion beams are analyzed and published now [6]. Fig. 2 shows the dependence on the atomic number A of the pion yields in π -A and π + -A interactions averaged over two forward angular regions (.5 rad θ <.5 rad and.5 rad θ <.25 rad) and four momentum regions (.5 GeV/c p <.5 GeV/c,.5 GeV/c p < 2.5 GeV/c, 2.5 GeV/c p < 3.5 GeV/c and 3.5 GeV/c p < 4.5 GeV/c), for the four different incoming beam momenta (from 3 GeV/c to 2 GeV/c). Similar results have recently been obtained for incoming protons [22]. The advantage of these measurements is that they are performed with the same detector, thus related systematic uncertainties are minimized. Results obtained with the HARP large-angle spectrometer First results on the measurements of the double-differential cross-section for the production of charged pions in p-a collisions emitted at large angles from the incoming beam direction have been obtained for the full set of available nuclear targets using an initial part of the accelerator spill to avoid distortions in the TPC [7, 8, 9]. The pions were produced by proton beams in a momentum range from 3 GeV/c to 2 GeV/c hitting a target with a
FIGURE 2. The dependence on the atomic number A of the charged pion production yields in π Be, π C, π Al, π Cu, π Sn, π Ta, π Pb (two left plots) and in π + Be, π + C, π + Al, π + Cu, π + Sn, π + Ta, π + Pb (two right plots) interactions averaged over two forward angular regions (.5 rad θ <.5 rad and.5 rad θ <.25 rad) and four momentum regions (.5 GeV/c p <.5 GeV/c,.5 GeV/c p < 2.5 GeV/c, 2.5 GeV/c p < 3.5 GeV/c and 3.5 GeV/c p < 4.5 GeV/c), for the four different incoming beam momenta (from 3 GeV/c to 2 GeV/c). FIGURE 3. (Top plots): The dependence on the beam momentum of the π + (left) and π (right) production yields in p Be, p C, p Al, p Cu, p Sn, p Ta, p Pb interactions averaged over the forward angular region (.35 rad θ <.55 rad) and momentum region MeV/c p < 7 MeV/c. The results are given in arbitrary units, with a consistent scale between the left and right panel. Data points for different target nuclei and equal momenta are slightly shifted horizontally with respect to each other to increase the visibility. (Bottom plots): The dependence on the atomic number A of the pion production yields in p Be, p Al, p C, p Cu, p Sn, p Ta, p Pb interactions averaged over the forward angular region (.35 rad θ <.55 rad) and momentum region MeV/c p < 7 MeV/c. The results are given in arbitrary units, with a consistent scale between the left and right panel.
x y 3 m z φ x VERTEX MAGNETS MTPC L ToF L VTX VTX 2 BEAM TARGET PSD BPD /2/3 He BEAM PIPE VTPC GTPC VTPC 2 ToF F : Main detector upgrades TPC READOUT MTPC R ToF R FIGURE 4. The layout of the NA6 (SHINE) setup (top view, not to scale) with the basic upgrades indicated in red. thickness of 5% λ I. The angular and momentum range covered by the experiment ( MeV/c p < 8 MeV/c and.35 rad θ < 2.5 rad) is of particular importance for the design of a neutrino factory. Track recognition, momentum determination and particle identification were all performed based on the measurements made with the TPC (see [7] for more details). Recently, after the development of a special algorithm to correct for the TPC dynamic distortions [27] and validation of the TPC performance with benchmarks based on real data [28], full spill data have been analyzed and published for incoming protons [2]. Fig. 3 illustrates the dependences of the measured charged pion yields on the beam momentum and on the atomic number A in p Be, p Al, p C, p Cu, p Sn, p Ta, p Pb interactions averaged over the forward angular region (.35 rad θ <.55 rad) and momentum region MeV/c p < 7 MeV/c. Similar results are now obtained for incoming pions [2]. An additional analysis is currently going on to compare the pion yields measured with the short (5% λ I ) and long (% λ I ) nuclear targets [29]. All HARP measurements are being compared with Monte-Carlo model predictions available within GEANT4 [3] and MARS [3], as well as more recently with the GiBUU model [32]. Some models provide a reasonable description of HARP data in some regions [4, 5, 6, 7, 8, 9, 2, 2], while there is no model which would describe all aspects of the data. THE NA6 (SHINE) EXPERIMENT The physics program of the NA6 or SHINE (where SHINE SPS Heavy Ion and Neutrino Experiment) experiment at the CERN SPS [33] consists of three main subjects. In the first stage of data taking (27-29) measurements of hadron production in hadron-nucleus interactions needed for neutrino (T2K [34]) and cosmic-ray (Pierre Auger [35] and KASCADE [36]) experiments will be performed. At later stages of the NA6 experiment hadron production in proton-proton and proton-nucleus interactions needed as reference data for a better understanding of nucleusnucleus reactions will be studied, energy dependence of hadron production properties will be measured in p+p and p+pb interactions as well as in nucleus-nucleus collisions, with the aim to identify the properties of the onset of deconfinement and find evidence for the critical point of strongly interacting matter. The NA6 (SHINE) apparatus is a large acceptance hadron spectrometer at the CERN SPS for the study of the hadronic final states produced in interactions of various beam particles (π, p, C, S and In) with a variety of fixed targets at the SPS energies. The layout of the NA6 (SHINE) setup is shown in Fig. 4. The main components of the current detector were constructed and used by the NA49 experiment [37]. The main tracking devices are four large volume TPCs, which are capable of detecting up to 7% of all charged particles created in the reactions studied. Two of them, the vertex TPCs (VTPC- and VTPC-2), are located in the magnetic field of two super-conducting dipole
magnets (maximum bending power of 9 Tm) and two others (MTPC-L and MTPC-R) are positioned downstream of the magnets symmetrically with respect to the beam line. One additional small TPC, the so-called gap TPC (GTPC), is installed on the beam axis between the vertex TPCs. The setup is supplemented by time of flight detector arrays two of which (ToF-L/R) were inherited from NA49 and can provide a time measurement resolution of σ to f 6 ps. The particle identification in NA6 is based on the differential energy loss de/dx measured in the TPCs combined with the mass squared measurements based on the ToF information. ] 4 /c 2 [GeV m 2.5.5 8 6 ] 4 /c 2 [GeV m 2.5.5 5 4 3 ] 4 /c 2 [GeV m 2.5.5 3 25 2 5 4 2 -.5 2 -.5 -.5 5 -.6.8.2.4.6.8 2 de/dx [MIP] -.6.8.2.4.6.8 2 de/dx [MIP] -.6.8.2.4.6.8 2 de/dx [MIP] FIGURE 5. Examples of combined ToF and de/dx PID performance for positively charged particles in the momentum range 2-3 GeV/c (left), 3-4 GeV/c (middle) and 4-5 GeV/c (right). Four clear accumulations corresponding to positrons, pions, kaons and protons are observed. Status and plans The NA6 (SHINE) experiment was approved at CERN in June 27. The first pilot run was performed during October 27. For the 27 run a new forward time of flight detector (ToF-F) was constructed in order to extend the acceptance of the NA6 (SHINE) setup for pion and kaon identification as required for the T2K measurements. Two carbon (isotropic graphite) targets were used during the 27 run: ) a 2 cm-long target (about 4% λ I ) with density ρ =.84 g/cm 3, the so-called thin target; 2) a 9 cm long cylinder of 2.6 cm diameter (about.9 λ I ), the so-called T2K replica target with density ρ =.83 g/cm 3. In total about.5 6 events were registered for the NA6 physics programme. This includes about 67 k events with the thin target, 23 k events with the T2K replica target and 8 k events without target (empty target events). The main goals of the 27 NA6 (SHINE) run were reached [38, 39], namely: the NA6 (SHINE) apparatus, including the new ToF-F system, was run successfully and detector prototypes were installed and tested. pilot physics data on interactions of 3 GeV/c protons on the thin and T2K replica carbon targets were registered. the NA6 reconstruction and simulation software has been set up and used successfully to process the events from the pilot run. calibrations of all detector components have been performed successfully and preliminary uncorrected identified particle spectra have been obtained. high quality of track reconstruction and particle identification similar to NA49 has been achieved, see Fig. 5. the data as well as new detailed simulations confirm that the NA6 detector acceptance and particle identification capabilities cover the phase space required by the T2K experiment. The new ToF-F system extends significantly the NA49 PID acceptance in the domain relevant for the T2K measurements. a preliminary result of σ inel = 254.7 ±. (stat) ± 6.6 (syst) mb was obtained for the total inelastic p-c cross section at 3 GeV/c. This value is in good agreement with previous measurements. The main motivation and goal of the analysis of the 27 data is to obtain the inclusive spectra of pions and kaons produced in p-c interactions at 3 GeV/c and emitted from the T2K target. This will allow the absolute determination of the neutrino flux of the T2K long baseline neutrino oscillation experiment at J-PARC [34]. NA6 measurements of hadroproduction with % accuracy will allow the determination of the far-to-near ratio at the level of 2-3% precision. This in turn will constrain enough the systematic errors to allow considerable improvement in the T2K physics achievements.
Θ [mrad] 5 45 4 35 Entries 3493 7 6 5 Θ [mrad] 5 45 4 35.2. 3 25 2 4 3 3 25 2.8.6 5 2 5.4 5 5.2 2 4 6 8 2 4 p [GeV/c] 2 4 6 8 2 4 p [GeV/c] FIGURE 6. Left: Uncorrected distribution in (p,θ) plane for negatively charged particles at the primary vertex as measured with the thin target. Right: corrected spectrum of negatively charged pions produced in p-c interactions at 3 GeV/c. The phase space relevant for T2K is fully covered. The analysis of the data taken during the 27 run is well under way. Several analysis techniques are used in parallel to cross-check the results. First preliminary fully corrected spectra for the negatively charged pions have been obtained, see Fig. 6, and the work continues to make full use of detector PID capabilities, to minimize systematic biases and to estimate systematic errors. In 28 the TPC read-out and DAQ upgrades have been performed for the NA6 apparatus. During the commissioning phase the design value for the event rate of about 7 Hz has been reached. Unfortunately, no physics data could be taken in 28 due to the LHC incident. During the forthcoming 29 physics run the NA6 collaboration plans to collect enough data for p-c interactions at 3 GeV/c to cover fully the T2K needs. Data for the other NA6 physics goals will also be collected. CONCLUSIONS The HARP experiment has already made important contributions to the cross-section measurements relevant for neutrino experiments: results with Al target have been used for the final K2K publication; measurements with Be target have been used for the neutrino flux predictions in MiniBooNE/SciBooNE and for the first MiniBooNE oscillation paper; results on charged pion production on C, N 2 and O 2 targets relevant for atmospheric neutrino flux predictions are published; measurements for the neutrino factory studies using the full data set with all targets are also published; cross-section measurements for forward production of charged pions with incident pions on targets from Be to Pb have been recently published; more production cross-section measurements are now finalized and are being prepared for publication (forward production with incident protons on the full set of targets, large-angle production with pion beams, productions with long targets, etc.). HARP measurements are currently being used to validate and tune Monte- Carlo hadron production models. The NA6 (SHINE) experiment at the CERN SPS is making significant progress towards hadroproduction measurements in p-c interactions at 3 GeV/c relevant for precise predictions of a new neutrino beam at J-PARC to be used for the first stage of the T2K experiment. First results from NA6 will soon be released. ACKNOWLEDGMENTS It is a pleasure to thank the organizers of the NUINT 9 conference at Sitjes for the invitation to participate in this very interesting conference and for the possibility to present these results on behalf of the HARP and NA6 Collaborations. This work was supported in part by the Russian Foundation for Basic Research (grant 8-2-8).
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