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the harp experiment@xcite was designed to perform a systematic and precise study of hadron production for beam momenta between 1.5 and @xmath3 and target nuclei ranging from hydrogen to lead .
the detector was located at cern , in the ps beam.the daq recorded 420 million events during the years 2001 and 2002 .
the physics program of harp includes : a ) the measurement of pion yields for a variety of energies and targets relevant for he design of the proton driver of a future neutrino factory@xcite ; b ) the measurement of pion yields on low @xmath4 targets as well as on cryogenic oxygen and helium targets , useful to improve the precision of atmospheric neutrino flux calculations@xcite ; and c ) the measurement of pion and kaon yields , relevant for the calculation of the neutrino fluxes of experiments such as miniboone@xcite and k2k@xcite .
harp ( fig .
[ fig : harp ] ) is a large acceptance spectrometer , with two distinct regions . in the forward part of the apparatus ( up to polar angles of about 250 @xmath2 ) ,
the main tracking devices are a set of large drift chambers .
magnetic analysis is provided by a 0.4 @xmath5 dipole magnet and particle identification relies in the combination of a threshold cherenkov detector , a time - of - flight wall and an electromagnetic calorimeter . in the rest of the solid angle
the main tracking device is a tpc , which is complemented by a set of rpc detectors for time - of - flight measurements .
the target is located inside the tpc .
in addition , sophisticated beam instrumentation ( including three timing detectors and threshold cherenkov detectors ) provides identification of the incoming particle and allows the interaction time at the target to be measured .
given the immediate interest of the miniboone and k2k experiments in a measurement of the production cross sections for pions and kaons at the energies and targets relevant for their beam setups , the harp collaboration has given priority to the analysis of those particular data sets .
in particular , we present in this article an initial result relevant for the k2k experiment .
specifically , we measure pion yields from proton - nucleus collisions , for a thin al target ( @xmath6 ) and a beam energy of 12.9 @xmath7 .
the k2k neutrino beam is produced from the decay of hadrons ( primarily pions ) emanating from a @xmath8 al target , at the kek ps proton driver energies of 12.9 gev .
further details about the motivation of this analysis can be found in ref .
tracking of forward - going particles is done by a set of large drift chambers(ndc ) placed upstream and downstream of the dipole magnet .
the chambers were recuperated from the nomad experiment and their properties have been described elsewhere@xcite .
each ndc module is made of four chambers , and each chamber of three planes of wires with tilted angles @xmath9 , @xmath10 and @xmath11 .
the single - wire efficiency is of the order of @xmath12 , and the spatial resolution approximately @xmath13 .
the performance of the ndc is inferior to the one measured in nomad , where the hit efficiency was close to @xmath14 and the resolution approached @xmath15 .
this is mainly due to the fact that , for harp , the ndc used a gas mixture and voltage settings different from the ones in nomad , a non - flamable gas mixture . ] .    due to the low hit efficiency ,
the algorithm used in nomad , based in building up space points ( with an efficiency of about @xmath16 ) is not viable ( for harp we would obtain @xmath17 ) . instead , the reconstruction builds @xmath18 and @xmath19 track segments in each ndc module ( 12 hits maximum ) , which are fitted to a straight line model via a kalman filter fit@xcite .
next , the algorithm attempts all possible combinations ( which include at least a @xmath19 segment ) to connect tracking objects in the modules downstream of the dipole magnet .
combinations such as @xmath19 + @xmath18 or even @xmath19 + hits  not  associated are valid ways to build longer , @xmath19 segments .    to measure the momentum it is necessary to connect a ( @xmath19 ) segment downstream of the dipole with at least one space point upstream of the dipole . since one can impose the constraint that all tracks emanate from the event vertex in the case of thick target
, a vertex algorithm will provide the event vertex to a comparable precision.]this point is always known and therefore the necessary and sufficient condition for a particle emanating from the target to have its momentum measured is that a @xmath19 segment can be measured by the combination of downstream ndc chambers .
a measurement in the first ndc module is not strictly necessary to analyse the track momentum . ideally , one would like , of course , to connect a @xmath19 segment downstream with a @xmath19 segment in the ndc1 _ and _ the vertex point . however , the tracking efficiency of the upstream module is sizeable lower than the efficiency of the downstream modules .
this is due to , a ) a higher hit density in ndc1 and b ) the lack of redundance due to the fact that there is only a single module upstream of the dipole .
a too restrictive condition to accept a track such as matching a @xmath19 segment upstream of the dipole with a @xmath19 segment downstream results not only in lower efficiency , but also in a systematic error , arising from the uncertainties in the hit density expected in ndc1 which would be hard to estimate ( hit density depends on factors such as track multiplicity and opening angle , which are model - dependent ) . instead , by building a track _
always _ when a @xmath19 segment is present downstream of the dipole , one is practically independent of the tracking efficiency in ndc1 .    in order to quantify these considerations ,
we have considered three different types of tracks , depending on the matching between the downstream and upstream modules .
type i is a @xmath19-@xmath19 matching .
type ii matches a @xmath19 ( downstream ) with a @xmath18 ( upstream ) .
type iii are those where one has been unable to find either a @xmath19 or a @xmath18 segment in ndc1 and thus the track is built using the downstream modules and the vertex constraint .
particle identification ( pid ) in the forward region of the spectrometer combines the information provided by beam detectors and three systems located downstream the dipole magnet .
namely , a threshold cherenkov detector ( ckov ) , a time  of  flight wall ( tof ) , and an electromagnetic calorimeter ( electron identifier , eid ) .
these subsystems have been described in ref .
its combined information results in good pid over the whole range of relevant momenta , as well as redundancy due to overlaps .
pion / proton separation is provided by tof up to @xmath20 , and by the ckov above @xmath21 .
electron / pion separation is covered by the ckov below @xmath21 and by the eid above @xmath22 .
finally the kaon contamination can be estimated with the ckov above @xmath21 and with the ( tof ) below this energy .
about one sixth ( 1 million events ) of the `` k2k thin target '' data has been analysed .
the unnormalised pion production differential cross section can be computed as follows :    @xmath23 \label{eq : cross}\ ] ]    where the sum runs over track types ( i = @xmath19-@xmath19 , ii= @xmath19-@xmath18 , iii=@xmath19-vertex ) and the indices @xmath24 and @xmath25 correspond to true and reconstructed @xmath26 bins respectively .
@xmath27 is the geometrical acceptance , @xmath28 is the tracking efficiency , @xmath29 is the migration matrix from reconstructed bin @xmath25 to true bin @xmath24 , @xmath30 is the pion identification efficiency , @xmath31 is the pion purity and @xmath32 is the observed pion yield .
the pion purity is defined as @xmath33 , where @xmath34 is the number of true observed pions in the bin @xmath25 , and @xmath35 is the number of particles misidentified as pions in the same bin @xmath25 .
that means that @xmath31 is just the probability of correct identification of a pion . the pion identification efficiency , @xmath30 is the fraction of times that a tracked pion is identified as such .
[ fig : down_eff_p_th ] shows the spectrometer acceptance . for clarity of ilustration ,
the momentum acceptance has been computed in the region of good angular acceptance , ( @xmath36 , @xmath37 ) and conversely , the geometrical acceptance has been computed in the region of good momentum acceptance ( @xmath38 ) . as it can be seen
the acceptance is good for pion momenta above 1 @xmath0 , and still acceptable above @xmath39 . as illustrated in ref .
@xcite this permits the measurement of k2k neutrino fluxes up to @xmath40 , which amply covers the energy region where the atmospheric oscillation affects the neutrino spectrum maximally .
the angular acceptance is also very well matched to k2k physics requirement .    the tracking efficiency is shown in fig .
[ fig : track_eff_mc1_normalized_p_th ] as a function of @xmath41 and @xmath42 .
the contributions of track types i , ii and iii are normalized individually , so that the total efficiency is the sum of the three . notice that the loss in efficiency at low polar angles for tracks of type i ( due to saturation effects in ndc1 ) is largely compensated by tracks of type ii and iii , so that the total tracking efficiency is rather flat .
finally , the yield must be corrected by the pion efficiency and purity , which is computed from the data themselves , using samples of pions , protons electrons and kaons of various energies tagged with the help of the beam detectors . the pion correction factor defined as the ratio of purity over efficiency
is shown in fig .
[ fig : pion_corr_factor ] , for the three track types and for 1.5 , 3 and 5 gev beam particles .
preliminary studies show an almost diagonal migration matrix , where migration is mainly due to finite momentum and angular resolutions , but not to systematic effects ( momentum and angular biases , etc ) .
figure  [ fig : cross ] shows the pion yields as a function of momentum after succesive corrections ( e.g. , acceptance , tracking efficiency and pion correction factor ) .
no attempt to deconvolute the data ( e.g , correct for migrations ) have been done yet , and no absolute normalization is computed .
we have presented a preliminary analysis of one of harp data samples , corresponding to a thin al target , at beam energies of @xmath43 .
this analysis is relevant for the k2k experiment and a first step towards an upcoming measurement of the fully corrected production cross section .
99 the harp collaboration , cern - spsc/2003 - 027 , spsc - p-325 for a recent review of the neutrino factory see , for example j.j .
gomez - cadenas and d.a .
harris , ann.rev.nucl.part.sci.52:253-302,2002e see for example g.d .
barr et al . , phys.rev.d70:023006,2004 .
initial results on particle yields obtained by the harp experiment are presented . 
 the measurements correspond to proton  nucleus collisions at beam energies of 12.9 @xmath0 and for a thin al target of @xmath1 interacion legth . 
 the angular range considered is between 10 and 250 @xmath2 . 
 this results are the first step in the upcoming measurement of the forward production cross - section for the same target and beam energy , relevant for the calculation of the far  to  near ratio of the k2k experiment .