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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN–SPSC/2001–031SPSC/M 67229 October, 2001Status reportof the HARP experiment1. IntroductionThe physics objective of HARP is a systematic and precise study of hadron production for beammomenta between 2 and 15 GeV/c, for target nuclei ranging from hydrogen to lead.The experiment was approved on February 17, 2000, after which the construction of the experi-mental apparatus started. The HARP spectrometer comprises 15 different hardware subsystems(see Table 1), some of which had to be constructed from scratch, while others could be recuper-ated from earlier experiments. It turned out, however, that the recuperation resulted in majorrefurbishment and modification work, and in nearly all cases required the use of new readoutelectronics. The construction of 13 out of the 15 subsystems proceeded on schedule, and theyTable 1: HARP subsystemshardware subsystem Mechanics Electronics1 Beam Cherenkov counters re-used new2 Beam scintillator counters re-used/new new3 Beam MWPCs re-used/new re-used/new4 TPC new new5 Solenoid magnet re-used6 Inner TPC trigger counter new new7 RPCs new new8 Forward trigger plane re-used/new new9 Driftchambers re-used re-used/new10 Dipole magnet re-used11 Cherenkov counter new new12 TOF wall new new13 Electron identifier re-used new14 Cosmic trigger wall re-used15 Beam muon identifier re-used newwere largely ready for commissioning and data taking in April 2001 when the PS started op-eration. However, the construction of the TPC and the Cherenkov counter lead to unexpectedproblems at the time of component integration. The (commercial) technology of the micro–flexcables which connect the densely packed chips of the 4000 preamplifiers of the TPC pads withthe driver channels, did not perform as well as had been seen in tests before contract adjudica-tion. In addition, the threshold Cherenkov counter, which is filled with 35 m3 of the expensiveC4F10 gas, had initially an unacceptable gas leak rate.After these problems were overcome, HARP started data taking in the second half of August2001, with all 15 subsystems in operation.Since then, HARP has been recording data around the clock, with no appreciable down-time.All subsystems work to satisfaction. Problems, still to be sorted out, concern a larger TPC datavolume per event, and a larger background from triggers on non-interacting beam particles thananticipated. Both problems are under intense study and are expected to be solved in due time.2. Hardware: status and performance22.1 Beam Cherenkov and beam TOF countersThe beam particles to be distinguished from each other are p, pi, K, µ and e, all between 2 and15 GeV/c. Above 5 GeV/c, pions and protons are discriminated by Cherenkov counters in theT9 beam line, one 5 m and the other 3 m long. At 5 GeV/c and below, the Cherenkov countersare used for electron tagging. At low momentum, the Cherenkov counters are complemented bytwo TOF scintillation counters with time resolution of ∼ 150 ps, 21.4 m apart.The particle separation capabilities at 3 GeV/c beam momentum by time-of flight between thetwo scintillation counters of the beam TOF system is shown in Fig. 1. Beam electrons are taggedby the beam Cherenkov counters. Pions are well separated from protons.Figure 1: Beam particle separation by time-of-flight at 3 GeV/c momentum (electrons taggedby the beam Cherenkov counters).2.2 Beam MWPCsThe position and direction of incoming beam particles are precisely measured by three 10 cm ×10 cm MWPCs with 1 mm wire spacing, located in the free space after the last quadrupole ofthe T9 beam line.Fig. 2 shows the extrapolated position of incoming 3 GeV/c beam particles at the position of theForward Trigger Plane, demonstrating that non-interacting beam particles pass well within a 6cm diameter hole, machined out in the trigger plane to prevent these particles from triggering.2.3 TPCAfter the problem of the micro–flex cables was solved, the TPC was commissioned withoutfurther problems, thus validating the design of the instrument. In particular, the HV field cagesheld voltage without sizeable leak current, the gas was free of electron-absorbing contaminations,3Figure 2: Reconstruction of the position of beam particles at the position of the Forward TriggerPlane. The circle denotes the hole in the Forward Trigger Plane.and the wire chambers for the pad readout worked flawlessly. No oscillations were observed inthe preamplifiers. The digitization electronics worked as designed.Out of the 4000 pads, 95% work as expected. The remaining pads are either absent, presum-ably due to problems with a few micro–flex cables which could not be recovered in the repaircampaign, or noisy.The presently only known problem with the TPC is too high a data volume per event, arisingfrom noisy pads. This problem can be eliminated once more stringent cuts have been appliedduring readout, to suppress empty channels. However, this can only be done after a detailedunderstanding of the pulseheights and pulseshapes will have been achieved.The TPC is routinely calibrated by shots of laser light which emit electrons into the gas volumeat the HV membrane, at precisely known locations. This accurately measures the drift velocityand distortions of the drift path from a straight line. To highlight the performance of the TPC,Fig. 3 shows the pulse amplitude as a function of time of such laser signals. This is in linewith the expectation from the 100 ns digitization intervals, folded with longitudinal diffusionin the gas and the preamplifier response function. Another calibration of the TPC stems fromthe reconstruction of cosmic-muon tracks which traverse the centre of the TPC. They permitthe reconstruction of two independent track segments which will give two momenta of oppositesign. The comparison of the absolute values of these two momenta gives a measurement of themomentum resolution.4Figure 3: Pulse amplitude vs time of laser signals.Yet another calibration is provided by the addition of radioactive 83Kr gas. Such data havebeen successfully taken already. The calibration through the measurement of the low-energyphoton energy spectrum of this krypton isotope is very sensitive on the equalization of the padresponse, which is currently under study.2.4 RPCsThe 46 RPC chambers which are to discriminate by time-of-flight between charged pions andelectrons, are working smoothly ever since their installation. From the calibration of somechambers in the T10 testbeam, a time resolution of ∼ 150 ps is expected, considerably betterthan the design figure of 300 ps. The study of their performance with secondary particlesoriginating from the target requires the prior reconstruction of tracks in the TPC.2.5 DriftchambersThe drift chambers, including their preamplifiers, have been recuperated from the NOMAD ex-periment. The NOMAD gas (40% argon + 60% ethane) permits good efficiency but is flammableand met next to unsurmountable difficulties for reasons of safety. It was therefore decided toemploy the non-flammable mixture 90% argon + 9% carbon dioxide + 1% methane.The disadvantage of this mixture is that the efficiency per wire plane is significantly below 100%.5The efficiencies have been measured from the trajectories of non-interacting beam particles. Thetypical efficiency per wire plane is ∼ 90%, see fig. 4. Notice that a few seemingly lower efficienciesstem from chambers located outside the beam particle trajectories and have no meaning for lackof statistics. The low efficiency per wire plane is compensated by the use of 12 planes perdriftchamber module which still permits full tracking efficiency once a suitable reconstructionalgorithm is employed.Plane number10 20 30 40 50 60Hit efficiency020406080100Hit Efficiency For Run# 9770Figure 4: Hit efficiency per wire plane of the NOMAD driftchambers, for non-interacting beamparticles.2.6 Cherenkov counterThe gas used in the Cherenkov counter is C4F10 (perfluorobutane), eight times heavier than air.After the repair the gas leak rate dropped to ∼ 4 litres per hour (from a 35 m3 volume), or 1 kgper day, which is acceptable. The Cherenkov counter works without problems. The calibrationof the light response is under way.2.7 TOF wallThe TOF wall has been newly constructed. Regular calibration with laser pulses shows excellentstability with time (better than 100 ps over one month). The calibration of the time response isunder way. Preliminary results for the time separation of 3 GeV/c beam protons and pions areshown in fig. 5. The measured time resolution represents the convolution of the time resolutionsof the TOF-B counter in the beam line, which serves as time reference, and the TOF wall. Theyindicate an intrinsic time resolution of the TOF wall of ∼ 150 ps, considerably better than thedesign figure of 300 ps.2.8 Electron identifierThe primary purpose of the electron identifier is to discriminate electrons from pions, since pionsmay occasionally disguise themselves in the Cherenkov counter as electrons when accompaniedby knock-on electrons. The electron identifier consists of one plane of 62 calorimeter modules 56 Tw6 ≤ Tw7  - TofB∆t (ns)0100200300400500600700-97 -96 -95 -94 -93 -92 -91 -90 -89 -88Figure 5: Time of flight for 3 GeV/c beam protons and pions.r.l. thick (‘em1’), followed by one plane of 80 calorimeter modules 11 r.l. thick (‘em2’).The calibration of the energy response of the electron identifier is under way. Figure 6 showsfirst results on the energy deposition in the two planes by 5 GeV/c hadrons and electrons.3. Software: status and performanceIn HARP, established modern software-engineering procedures were adopted:• User requirements were defined and then transformed into a set of software requirements;• the architectural Object-Oriented design was worked out in an iterative process, leadingto the identification of the software domains, the definition of their dependence structureand of all interfaces;• the detailed software design and code production in C++ language were implemented;• unit testing, system testing and release procedures were defined and implemented.Complementing the above approach, a number of accompanying decisions have been taken andimplemented:• OBJECTIVITY as database, including the storage of physics data;7Figure 6: Response of the electron identifier for 5 GeV/c electrons and hadrons.• a reduced version of GAUDI as common framework for all HARP software components;• GEANT4 for simulation and detector geometry representation;• DATE for data acquisition and monitoring;• ROOT as event display and analysis tool;• PVSS for detector control.Overall, the HARP experience with this approach to software is positive. The first softwarerelease coincided with the start of the technical run in October 2000. Both the simulation andanalysis skeletons were made available. Since then, new versions have been regularly producedwhich add in a coherent way more and more functionality. Today, data can be simulated down tothe digitization level. The development of algorithms for a high-performance reconstruction code8is intensely under way. Experience with real data is vital for the fine-tuning of the algorithmsand their parameters.The basic software for data acquisition and monitoring was already available at the time of thetechnical run. Since then, it has evolved with the coming into operation of more subsystems,and has considerably increased in functionality. The system works according to specificationsand is remarkably stable.On a whole, HARP has enjoyed very good collaboration with IT Division on matters of datahandling. Technical difficulties that have been initially experienced are now largely resolved.However, a problem with quick access to data persists. The data storage system of CERN,CASTOR, is still based on tape technology. Given that HARP has not been granted a dedicatedtape drive, contrary to other experiments, the data processing time is dominated by waiting fortapes to be staged, which is sometimes short but can be unacceptably long when rapid feedbackfrom data is needed.The time schedule of HARP required the availability of the CERN-recommended PVSS tool fordetector control when it was not yet ready for use in a real experiment. An interim solutionemploying LabView had to be adopted for detector control.The HARP collaboration is committed to using the winter shutdown efficiently for data analysis,in order to maximize feedback and prepare optimal conditions for the 2002 data taking.4. Data taking: status and plansTable 2 summarizes the targets which are planned to be measured. The emphasis will beon precise cross-section measurements with thin solid and liquid targets. The selected beammomenta are 2, 3, 5, 8, 10, 12 and 15 GeV/c, positive and negative. This programme whichrepresents the bulk of data taking, already involves some 150 settings.There will also be measurements with selected thick targets, with positive beams only. Inaddition, there will be ‘empty’ target runs and other runs with specially shaped targets, with aview to estimating background and systematic errors.Measurements with the goal of determining the neutrino yield of the K2K target (Al) at 12.9GeV/c, and of the MiniBooNE target (Be) at 8 GeV/c are also planned.Up to the PS shutdown starting on November 5, data will have been taken for some 60 days. Itis expected that by then the following data will be taken taken with at least 100 k good eventsper setting (the present data acquisition rate is ∼ 150 events per spill):• thin Be, Al, Ta and Pb targets at momenta 3, 5, 12 and 15 GeV/c, positive and negative;• thick Ta and Pb targets at +3 GeV/c;• some K2K target measurements at +12.9 GeV/c;• some MiniBooNE target measurements at +8 GeV/c;• ‘empty’ target runs, etc.The liquid targets and the associated cryogenics are nearly ready. A test installation is plannedto take place in November.9During the winter shutdown, several minor repairs (e.g. the broken micro–flex cables in theTPC) will be undertaken. However, no major detector upgrade or new construction is planned.HARP is approved for 140 days of running in 2002, which should permit the experiment toachieve its physics objectives.Table 2: Summary of HARP targetsMaterial Z A 0.02λ (cm) 1.0λ (cm) CommentBe 4 9.01 0.81 40.70Be 4 9.01 0.5 λ (MiniBooNE)Be 4 9.01 MiniBooNE target replicaC 6 12.01 0.76Al 13 26.98 0.79 39.44Al 13 26.98 0.5 λ (K2K)Al 13 26.98 K2K target replicaCu 29 63.55 0.30 15.00Cu 29 63.55 ‘button’ targetCu 29 63.55 ‘skew’ targetSn 50 118.71 0.45Ta 73 180.95 0.19 9.58Pb 82 207.2 0.34 17.05H2 1 1 6 cmD2 1 2 6 cmN2 7 14.01 6 cmO2 8 16.00 6 cm10

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