Electron neutrino tagging through tertiary lepton detection
aa r X i v : . [ h e p - e x ] J u l Electron neutrino tagging through tertiary leptondetection
L. Ludovici a and F. Terranova ba I.N.F.N., Sezione di Roma Sapienza, Rome, Italy b I.N.F.N., Laboratori Nazionali di Frascati, Frascati (Rome), Italy
Abstract
We discuss an experimental technique aimed at tagging electron neutrinos inmulti-GeV artificial sources on an event-by-event basis. It exploits in a novelmanner calorimetric and tracking technologies developed in the framework ofthe LHC experiments and of rare kaon decay searches. The setup is suited forslow-extraction, moderate power beams and it is based on an instrumented decaytunnel equipped with tagging units that intercept secondary and tertiary leptonsfrom the bulk of undecayed π + and protons. We show that the taggers are ableto reduce the ν e contamination originating from K e decays by about one orderof magnitude. Only a limited suppression ( ∼ ν e producedby the decay-in-flight of muons; for low beam powers, similar performance asfor K e can be reached supplementing the tagging system with an instrumentedbeam dump. PACS: 14.60.Pq, 29.40.Vj, 95.55.Vj
Introduction
The identification of the initial flavour of neutrinos produced by artificial sourcesthrough the detection of the associated lepton (“neutrino tagging”) is a possibilitythat has been envisaged many decades ago [1, 2]. Its realization, however, must over-come major experimental challenges and, in spite of numerous proposals [3, 4, 5, 6, 7],a facility operating with ν e or ν µ neutrinos tagged on an event-by-event basis is stillto come. In a tagged neutrino facility, a precise knowledge of the neutrino flavour atthe source can be achieved identifying the associated lepton in coincidence with theoccurrence of a neutrino interaction at the far detector. As it will be shown in thefollowing, the exploitation of this correlation requires time resolutions below 1 ns bothfor the taggers and for the neutrino detectors. In the past, two approaches have beenpursued. The former - dating back to 1969 [1] - is targeted to the identification of ν µ from kaon decay: it takes advantage of the large difference in Q-value between π and K decays to isolate leptons from kaons without intercepting muons from π or the un-decayed parent mesons. The second approach focuses on the identification of positronsin order to either select a pure ν e beam for physics measurements [6] or to veto the ν e contamination in ν µ beams from π decay-in-flight (“anti-tagging” [7]). The techniquethat we discuss in this paper combines both approaches in a novel manner; moreover,it takes advantage of the outstanding progresses in high-rate radiation-hard detectorsthat have been achieved for the calorimetry in the forward region of LHC experimentsand for the study of very rare kaon decays. As in [7], the tagging setup discussed here-after is especially suited to suppress the ν e contamination in ν µ beams from the decayin flight of multi-GeV pions, i.e. to reduce the intrinsic contamination of beam-related ν e events at experiments seeking for ν µ → ν e transitions. It can also be employed toreduce the systematic error in the knowledge of the flavor contamination at source or toselect a pure ν e subsample for physics studies. The physics reach of such experimentsranges from the study of anomalous short baseline oscillations [8, 9, 10, 11] and lowenergy cross sections [12], which employs low power proton beams on solid targets, upto ambitious “Superbeam” facilities [13, 14, 15, 16] utilizing MW-class proton driversto address the study of subdominant ν µ → ν e transitions at the atmospheric scale. Allthese experiments are limited by systematic errors [17, 18] mainly arising from the finitepurity of the neutrino source: as a consequence, in the last decade the developmentof novel facilities designated to overcome the purity constraints of π -based beams hasbeen at focus of intense R&D efforts [19]. In this paper, the tagging principle and theconceptual design of a facility aimed at a substantial reduction of the ν e contaminationis discussed in Sec. 2. Simulation and performance for background rejection in a setup“scraping” the secondary beam until the hadron dump is studied in Sec. 3. Finally, thespecial case of an additional instrumented beam dump at the end of the decay tunnelis discussed in Sec. 4. 2 Tagging of electron neutrinos
Artificial sources of ν µ at energies larger than ∼
100 MeV can be produced by thetwo-body decay in flight of pions π + → µ + ν µ , which in turn are created from protonsimpinging on thick targets [20]. The source also contains ν µ originating from two-body and three-body decays of charged and neutral kaons. However, it is intrinsicallypolluted by ν e originating from three-body decays of K + ( K e : K + → π ν e e + ), from thedecay-in-flight of secondary muons along decay tunnel (DIF: π + → µ + ν µ → e + ν e ν µ ν µ )and from the decays of neutral kaons. Tuning of the pion momentum selected bymagnetic lenses after the primary target, of the transfer line up to the decay tunneland, finally, of the length and radius of the tunnel itself help in reducing the ratio ν e / ν µ below 1%, although a ν e contaminations in the 1-0.1% range is unavoidable in anyrealistic configuration. In a very broad neutrino energy range, i.e. from sub-GeV [21]up to tens of GeV [22], the ν e contamination is dominated by π + DIF and by K e , withthe addition of a minor contribution from semileptonic decay of K L ( K L → π − e + ν e ) andcharged kaons decaying before the bending magnets. All these decays produce positronsin the final state, whose spectral distribution follows from three-body kinematics andfrom the specific Q-value of the reaction. In the forward region along the decay tunnel,positrons are swamped by the bulk of undecayed hadrons, by muons resulting fromthe two-body decay of π + and by secondary protons within the acceptance of thefocusing system. Fig. 1 shows the polar angle distribution θ of positrons, muons andelectrons with respect to the axis of the decay tunnel for a specific beam configuration(“benchmark beamline” - see Sec. 3) resulting from the decay in flight of π + and K + with 8.5 GeV mean energy. At the entrance of the decay tunnel, mesons and protonsare assumed to be uniformly distributed in a 10 ×
10 cm area with a polar angle smallerthan 3 mrad (black solid curve in Fig. 1-left). On top of the beam divergence, positronsfrom K e (Fig. 1-right) show a large intrinsic divergence due to three-body kinematics,with a mean θ value of 88 mrad. The mean polar angle of muon-neutrinos producedby the source is of the order of 27 mrad (blue dot-dashed line in Fig. 1-left), i.e. ithas a value intermediate between the π + and positron beam divergence. Similarly, the θ distribution of positrons from DIF has a mean value of 28 mrad (black solid line ofFig. 1-right).On the other hand, the polar angle of the accompany muons in π + → µ + ν µ alongthe decay tunnel is extremely small (red dashed in Fig. 1-left) and, in fact, comparablewith the beam divergence of the parent π + . This muon focusing effect is due to 2-bodykinematics and to the fact that the rest mass of the muon is comparable with the pionrest mass. The emission angle θ of the muon is, therefore,tan θ = sin θ ∗ γ (1 /β ∗ + cos θ ∗ ) (1) γ being the Lorentz boost of the parent pion ( β ≃
1) in the laboratory frame, while θ ∗ and β ∗ = ( m π − m µ ) / ( m π + m µ ) ≃ .
26 are the emission angle and the muon velocity3 (rad) θ a . u . θ Figure 1: Left: θ angle distribution between the propagation direction of pions (blacksolid line), muons (red dashed) and ν µ (blue dot-dashed) and the axis of the decaytunnel. Right: θ distribution for positrons resulting from DIF (black solid line) and K e decays (red dashed line). 4n the pion rest frame, respectively. On the contrary, β ∗ = 1 for neutrinos; hence, theemission angle in the laboratory frame at large θ ∗ (cos θ ∗ ≃
0) is much wider thanfor µ + . The above considerations point toward a tagging setup designed to performa destructive (calorimetric) measurement of the positrons and intercepting secondaryparticles emerging from the primary pion beam up to the end of the decay tunnel.The setup is shown schematically in Fig. 2. It consists of a set of cylindrical e.m.calorimeters with a geometry and readout similar to the ATLAS forward calorimeter(FCAL [24]) but with much shorter length (10 X ). The FCAL is a liquid Argoncalorimeter designed to operate in a high radiation density environment. It ensuresradiation hardness up to 0.5 GRad/y, fast response to cope with the 25 ns beamcrossing of the LHC and reduced sensitivity to event pile-up thanks to a very smalldrift length (0.27 mm). Such small gap allows for full particle drift in 61 ns and,therefore, relieves the detector of the problem of ion build-up [23]. Unlike ATLAS, themodules considered here are built with inner radii of variable size, so that all primarymesons and most secondary muons reach the beam dump without intercepting thecalorimeters. Differently from early proposals [3, 7] there is no material installed alongthe trajectory of the undecayed pions up to the end of the decay tunnel; therefore,irrespective to the tagging performance, the flux of ν µ at the far detector remainsunchanged after the installation of the modules. In front of each module, a high speedtracker of granularity comparable to the FCAL ( ∼ ∼ π or hard bremsstrahlung. In the occurrence of a neutrino interaction atthe far location, a time coincidence with a charged particle in the tracker is sought for,once accounting for the propagation delay due to the source to the detector distance. Inaddition, an electromagnetic deposit beyond a given threshold is required in the FCALarea adjacent to the tracker hits within a time window comparable with the drift timeof the calorimeter. The presence of such energy deposit indicates the simultaneousproduction of a neutrino with a positron, tagging the event at the far location as a “ ν e at source”. The finite efficiency and geometrical acceptance of the modules (e.g. dueto positron produced along the pion flight direction) limits the capability to veto ν e (Sec. 3.1). Fake vetoes due to accidental overlaps of muons with photons, e.m. showerleakage along the modules and µ → e misidentification induces additional dead-time,causing a drop in statistics (Sec. 3.2). As discussed in Secs. 3 and 4, in order to achievea ν e suppression rate of about one order of magnitude and a sizable detector livetime,the tracker must be able to operate with rates up to ∼
200 kHz/cm ( ∼
20 MHz/cm in the proximity of the beam dump) and a time resolution better than 1 ns. Suchrequirements point toward the use of fast semiconductor detectors : they share withcollider physics applications the constraints coming from radiation hardness [25] andwith rare kaon decay physics applications the need for small material budget and sub-ns Other options based on scintillators or gaseous detectors can be envisaged, especially in the areasfar from the secondary meson beam, where the requirements on the particle rate can be relaxed below10 kHz/cm (see Sec. 3). µm ) and the constraints on the time resolution can be relaxedby about one order of magnitude (1 ns versus 100 ps); still, fast trackers for neutrinotagging applications represent a technological challenge due to the large area neededfor full coverage of the calorimeters in the region where particle rates are high ( ∼
10 m for rates higher than than 10 kHz/cm ). The requirement of a time resolution betterthan 1 ns poses strict constraints to the neutrino detector, as well, thus narrowingthe choice of technologies that can be employed. Cherenkov detectors, in particular,offer the advantage of extremely fast response at the expense of reduced light yieldwith respect to scintillators or gaseous detector. In recent years, water Cherenkovneutrino detectors have achieved resolutions below 1 ns for masses up to 50 kton [27],although fast triggering from liquid and solid scintillators, from UV light in liquefiednoble gases and from gaseous detectors still represent viable alternatives. Finally, itis worth noticing that an absolute time calibration between the instrumented decaytunnel and the neutrino detector must match the above-mentioned resolutions. Forshort baseline experiments, it can easily be achieved with atomic clocks, while thetechnique currently exploited by long baseline experiments and based on the GlobalPositioning System [28] does not fulfill this constraint (∆ t ≃
10 ns), so that a directresynchronization of the clocks would be periodically needed.
The tagging concept outlined in Sec. 2 has been tested against a specific beam con-figuration in order to quantify the performance of the ν e suppression. As alreadymentioned, the two most relevant contributions to the ν e beam contamination are dueto K e and DIF. The beam setup considered hereafter (“benchmark beamline”) is sim-ilar to the one studied in [7] and it is suited for short baseline ν e appearance searches,along the line of [8, 9, 10, 11]. In fact, the beamline of [7] has been considered as anupgrade of the I216 Proposal [29] at the CERN PS; here, it is used as a benchmark andthe number of pions produced per extraction, together with the proton mean powerare considered as free parameters. The neutrino beam in this benchmark configurationis produced by a slow extraction of protons (1 s) from a 19.2 GeV booster. In oursimulation [33] protons are dumped in a cylindrical beryllium target (110 cm lengthand 3 mm diameter) producing secondary particles, which are momentum-selected andtransported up to the decay tunnel by a magnetic focusing system. The focusing sys-tem necessarily relies on quadrupole-dipole magnets [20] due to the long extractionspill and has not been simulated in details (halo muons and off-momentum particlesare, therefore, neglected). Similarly to [7], we assume the focusing system to havean angular acceptance of 80 µSr and a momentum bite of ±
20% around a nominalmomentum of 8.5 GeV. All selected secondary particles are focused at the entrance6igure 2: Schematics of the tagging setup (lateral view, not in scale). The blue&yellow(solid) boxes indicate the position and size of the tagging modules (tracker andcalorimeter) along the 80 m decay tunnel. The lines labeled (1) and (2) show theaverage θ angles for positrons from K e and DIF, respectively. The orange (gridded)box represents the instrumented dump (Sec. 4).7f the decay tunnel, uniformly distributed in a 10 ×
10 cm window. Here, in order tosimulate the beam divergence, the secondary particles are randomly distributed up toan angle of 3 mrad with respect to the axis of the decay tunnel. We cross-checkedour simulation with a Sanford-Wang [30] parametrization of pion production data [31]and with π + and K + data taken with 19.2 GeV proton on beryllium [32]. The decaytunnel is 80 m long with a 2.5 m radius (Fig. 2). We neglect here the production of K L and K − at the target, whose ν e contribution at the far detector depends on the detailsof the focusing system. With respect to traditional neutrino beams, this contributionis further suppressed due to the bending between the primary target and the decaytunnel; it is, thus, negligible with respect to the K e and DIF contributions even aftertagging suppression. Assuming a primary beam intensity of 2 × protons-on-target(pot) per extraction [7], we evaluate from simulation a particle density at the entranceof the decay tunnel of about R ≡
100 MHz/cm during the 1 s-long spill. It corre-sponds to N π = 10 π + per extraction over the whole surface, with a mean K + /π + of4.1%. Any increase of the neutrino flux achieved by an increase of the time the boosteris dedicated to the neutrino beamline (“duty cycle”) does not affect the tagging per-formance . However, a power increase obtained raising the number of mesons R perextraction, e.g. increasing the proton beam intensity and energy or the acceptance ofthe focusing system, increases the particle rate at the taggers and challenge the taggingperformance. For the benchmark beamline in our simulation, Fig. 3 shows the spectraof all π + and K + produced and crossing a circle of 1.4 m radius, located 2 m down-stream the target and the spectra of π + and K + accepted by the focusing and bendingsystem. The neutrino flux in a far detector located 800 m from the decay tunnel is alsoshown in Fig. 3. The neutrino beam is a narrow-band beam of 1 . × − ν/ pot / m at a far detector. It gives ∼ . × events every 10 pot in a 1 kton detector. Themean ν µ energy is 3.5 GeV and the ν e contamination is 0.1%.Along the decay tunnel, four tagging stations have been simulated. They are located z =20, 40, 60 and 80 m far from the entrance of the tunnel ( z = 0). The stations havea cylindrical geometry with 2.5 m outer radius and variable inner radii of 12, 17, 21and 26 cm, corresponding to an angular opening of 6, 4.25, 3.5 and 3.25 mrad. Variableangular openings are employed since the source at z = 0 is non-pointlike: in this casemost of undecayed primary π + and K + reach the beam dump at z = 80 m throughthe central holes of the modules and only secondaries are scraped by the tagging units.Each module has a material budget of 10 X of lead , which has been simulated inGEANT4 [34]. The calorimeter response to energy deposit has not been simulatedin details: for e.m. deposits (positrons, electrons, photons) the reconstructed energyis drawn from the deposited energy and smeared according to the FCAL measuredresolution [35]. In the present case, the energy resolution is dominated by the samplingterm, which is assumed to be 30%/ √ E with E expressed in GeV. Since the tagger For the benchmark beamline ( E p = 19 . × pot every7 s corresponds to a mean beam power of 9 kW and a duty cycle of 1/7=14%. In fact, in the special case of the Atlas FCAL, copper has been employed as passive material. omentum (GeV/c)10 -8 -7 -6 -5 -4 -3 -2 -1 Figure 3: Muon neutrino flux (hatched histogram) in a detector located 800m fromthe target. Spectra of π + (black solid line) and K + (red dotted) 2 m downstream thetarget. Spectra of π + (black dashed) and K + (red dot-dashed) selected by the focusingand bending system. 9 nergy (GeV)0 0.5 1 1.5 2 2.5 3 3.5 a . u . -4 -3 -2 -1 µ and π Energy (GeV)0 1 2 3 4 5 a . u . and DIF from K + e Figure 4: Left: energy at the tagger for undecayed π + (black solid line) and µ + (reddashed). Right: energy at the tagger from e + originating by K e (red dashed) and DIF(black solid) giving a ν e at the far detector (“ ν e at source”).thickness is just 33% of the Pb interaction length, pions deposit only a small fractionof their energy in the tagger. Such deposit is mip-like in about 74% of the case and itexhibits strong energy fluctuations in the occurrence of hard hadronic interactions. Itis shown in Fig. 4-left (black line) and superimposed with the energy deposit of muonsfrom π + decay (red dashed line).Fig. 4-right shows the reconstructed energy of positrons from K e and DIF, whena ν e reaches the far detector. The energy has been smeared according to the FCALresolution and accounting for lateral leakage. For each event we require a ν e withenergy larger than 0.5 GeV hitting the far detector within its geometrical acceptance.We assumed a source-to-detector distance of 800 m [7]. The detector surface in theplane perpendicular to the neutrino beam is 10 ×
10 m . Lateral leakage is marginal( ∼ R c ) around theimpact point is collected. R c has been optimized empirically: larger collecting radiiare detrimental for π /e separation and for the effect of event pile-up while radii muchsmaller than 2 cm reduce the visible energy of the positrons, which gets closer to amip-like deposit.Finally, a “ ν e at source” is defined as a charged particle triggered by the trackerwith an energy deposit in the calorimeter greater than 300 MeV, in coincidence witha ν e interactions at the far detector. Again, only neutrinos with energy larger than0.5 GeV are considered. The inefficiency of the tracker has been neglected togetherwith subdominant effects as the albedo resulting from backscattered electrons or thedifferent energy response for the hadronic and e.m. component in the core ( R c < K e ± . ± .
3% 89 . ± .
3% 91 . ± . ± . ± .
7% 76 . ± .
6% 80 . ± . ν e from K e (first line) and DIF (second line) at source vetoedby the tagging system. The first two columns indicate the tagging efficiency assumingonly the scraping of the secondary beam as described in the text. NS (no-smearing)shows the efficiencies obtained considering an ideal e.m. calorimeter (i.e. a negligiblesampling term). The last two columns show the improvement gained introducing aninstrumented beam-dump and applying a tighter cut of 1 GeV on the reconstructedenergy inside the dump region (R <
26 cm at the last tagging module). Errors are dueto finite MC statistics.of the π + shower. Results in terms of K e and DIF suppression are listed in Tab. 3.1.The numbers in bold indicate the veto efficiency for events giving a ν e at the far detectorcoming from K e and DIF, respectively. The tagging system, therefore, is powerful invetoing the K e contamination, while the performance are poorer for DIF ν e . This isdue both to the smaller angular spread of DIF e + and to the different lifetime of kaonsand muons. Since the γcτ of the kaon (64 m) is comparable with the length of thedecay tunnel, ν e from K e decay are produced earlier than ν e from DIF, generatingpositrons that are intercepted by the tagging modules. On the other hand, due to thelonger pion and muon lifetime ( γ π cτ π ≃
478 m) most DIF occur in the proximity of thebeam dump and positrons impinge upon the dump at a radius smaller than the innerradius of the last tagging unit. In Tab. 3.1, the improvement in vetoing the DIF isshown in the third column, assuming that the last tagging unit has no inner hole, i.e.instrumenting the beam dump (see Sec. 4), and applying a tighter cut of 1 GeV in thisarea. As anticipated, the improvement in the K e rejection is marginal (+3%) whilethe rejection of DIF increases substantially (+16%). The second and fourth columnsare computed without energy smearing (“NS”) and show the impact of the samplingterm of the e.m. calorimeter on the tagging performance. Finally, it is worth noticingthat, beyond tagging, the instrumented decay tunnel allows for a precise measurementof the secondary beam and, therefore, it contributes to reduce systematic errors on the ν flux and composition. The particle rate at the tagging units is clearly dominated by the 2-body decay of the π + with µ + impinging on the calorimeters along the decay tunnel. In the beam dump(see Sec. 4), it mainly depends on the rate of undecayed π + , plus a small correctiondue to K + and secondary protons that reaches the end of the beamline. Fig. 5 showsthe average particle rate during the 1 s extraction as a function of the tagger radius.As anticipated, the rate at the taggers is dominated by muons, with peak values of11 adius (cm)10 15 20 25 30 35 40 ) r a t e ( H z / c m Figure 5: Rate at the first (20 m distance - empty dots), second (40 m, triangles), third(60 m, full dots) and fourth (80 m, squares) tagging module. The error bars (visibleonly at large radii) show the errors due to finite MC statistics.about 200 kHz/cm . As a consequence, the peak rate of pile-up is P R = 2 × cm − s − S ∆ T cal (2) S being the collection surface πR c ≃
12 cm and ∆ T cal is the integration time of thedetector. For the above-mentioned FCAL, it corresponds to 61 ns if operated in full-drift mode, but it drops below 25 ns in the standard LHC readout configuration, whichexploits the fast rise of the signal. Since piling-up particles are mainly constituted bymuons, PR up to ∼ P R ≃ .
06, so thatbeam powers up to a few hundreds of kW would be sustainable. In particular, a pile-upof 3 is reached by a beam power of 330 kW, assuming a duty cycle similar to the oneof the benchmark beamline (14%).Accidental coincidences are due to events classified as “ ν e at source” by the taggingsystem in the same time window ( ∼ ν e at source in coincidence with ν µ or ν µ → ν e interactions in the detector. The probability of having a fake coincidencebetween an event at the far detector and a “ ν e at source” from the tagging system isgiven by the rate of positrons impinging on the trackers times the squared sum of thetime resolution of the tracker and the neutrino detector (∆ t ). The rate of accidentalsis, therefore, (cid:2) N π f πe + ǫ DIF + N K f Ke + ǫ K e (cid:3) · ∆ t ≃ . × · ∆ t ; (3)12 DIF and ǫ K e are the tagging efficiency for DIF and K e positrons , N π ≃ s − the rate of pions at z = 0 and N K ≃ . × − · N π the kaons at z = 0. Thefraction of π + ( K + ) giving a positron in the decay tunnel from DIF ( K e ) is labeled f πe + ( f Ke + ) and, in the benchmark beamline is ≃ . f Ke + = BR ( K e )(1 − e − γ K cτ K /L ), L being the tunnel length. Eq. 3 sets the scale of the timeresolution needed at the tracker and neutrino detector. For ∆ t = 1 ns, the fraction of ν µ → ν e interactions at the far detector wrongly tagged as “ ν e at source” is 1.3%, i.e.the effective livetime of the detector is about 99%. It sets more stringent limit thanpile-up to the scalability of this technique up to the Superbeams: already an orderof magnitude increase of N π (100 kW for a 14% duty cycle) would bring the livetimebelow 70%. To cope with Superbeam powers, time resolutions of the order of a fewhundreds of ps would be needed both at the tracker and at the neutrino detector.Finally, it is worth noting that Eq. 3 demonstrates quantitatively what stated inSec. 2, i.e. the need of a high speed tracker in front of the calorimeter units with timeresolution of ∼ ν e at source” due to finite detector resolution ( µ → e or π → e misidentification). For the bulk of muons in the 2-body decay of π + , the misidenti-fication probability is 5 . × − , resulting into a probability of a fake veto of 0.3%.However, two-body hadronic decays of K + can give a relatively large rate of false tag-ging. This is due to the fact that pions are produced at large angles in association withphotons from π decay. Since the π → e misidentification probability is large ( ∼ K + → π + π (1.4%) is comparable with the rate dueto genuine ν e sources. Clearly, the poor π/e rejection capability in the tagger is dueto the fact that the longitudinal development of the hadronic versus e.m. shower isnot exploited for π/e separation (see Sec. 4). Other sources of background are listedin Tab 3.2 and do not exceed 1%. As shown in Sec. 3, the performance of the tagging system are excellent for the sup-pression of the ν e from K e while a significant fraction of the positrons produced bythe decay-in-flight of muons impinges upon the hadron dump at the end of the decaytunnel with a radius smaller than the inner radius of the last tagging station. Fig. 6shows the particle rate at the last tagging module at radii smaller than R min = 26 cm.For the benchmark beamline, the rates do not exceed 20 MHz/cm , so that it wouldbe conceivable to instrument also the dump with a fast tracker followed by a FCALunit. Three issues, however, have to be addressed, which in principle limit the tagging Unlike Tab.3.1, these efficiencies (63% and 83% respectively) are computed for all positrons,irrespective of the energy and direction of the outcoming ν e . π + → µ + ν µ µ → e misid. 0.3% µ + → e + ¯ ν µ ν µ DIF genuine e + K + → µ + ν µ µ → e misid. < K + → π + π π → e misid. 1.4% K + → π + π + π − π → e misid. 0.7% K + → π e + ν e e + K + → π µ + ν µ µ → e misid. < K + → π + π π π → e misid. < ǫ π and ǫ p of their initial energy. The value of ǫ π,p has been computed by simulation andit turns out to be ǫ π = 5 . ± .
5% for pions and ǫ p = 4 . ± .
5% for protons in 10 X ofPb. In the benchmark beamline the proton yield at the dump is large and comparablewith the one of the undecayed pions: for each extraction 1 . × protons are dis-tributed at the entrance of the decay tunnel in the 10 ×
10 cm surface. On the otherhand, if the beamline is operated in antineutrino mode ( π − selection after the primarytarget), the corresponding antiproton contribution is below the one of K − . Neglectingthe small contribution of undecayed K + , the yearly integrated dose is, therefore N π [(1 − f ) ǫ π E π + f E mip ] + N p ǫ p E p M ≃ . × − Gy / spill (4)which corresponds to 25 kGy for a module weight of M = 115 kg (10 X of Pb)and 10 pot. In Eq. 4 f represents the fraction of decayed pions (15%), E π and E p the mean pion and proton energy and E mip the energy released by a muon in 10 X .The integrated dose is, therefore, much below the safety operation limit of FCAL(5 MGy/y [24]).Once more, the issue that poses the main impediment to installing an instrumenteddump at higher beam powers is the rate of false tags from random coincidences. Here,the number of accidentals at the far detector per neutrino interaction scales as:[ N π (1 − f ) ǫ π → e + N p ǫ p → e ] · ∆ t ≃ · ǫ π → e (5)with ∆ t ≃ ǫ π → e ( ǫ p → e ) representing the fraction of pions (protons) identifiedas electrons. In this case, even at low power (benchmark beamline), the π/e misiden-tification rate must be kept well below the value measured at the tagger (see Sec. 3.2).This is quite an ambitious task, since the energy of the pions is much larger than thepositron spectrum in DIF events and, in order to reduce pile-up, only a small area14round the impact point can be used to collect the deposited energy. In this case,better performance can be obtained aligning two FCAL modules of different thick-ness: the first one (FCAL1) is a standard 10 X tagger module, which is followedby a second hadronic module FCAL2 of about three interaction lengths (thickness:51 cm). This configuration is quite similar to the one employed in the ATLAS forwardregion [24, 36]. In this case, a more powerful π/e and p/e separation can be achievedselecting events in the E E E E R c > ǫ π → e has been computed from the distribution of E E <
300 MeV) in FCAL2. Here, the misidenti-fication probability ǫ π → e drops to 3%. This value is still too high for the benchmarkbeamline, especially when neutrinos are produced and the proton contamination hasto be accounted for (see Eq. 5).Finally, the tagging performance of the dump is challenged by event pile-up, whichis in fact entangled with the evaluation of ǫ π → e . For the benchmark beamline, evenneglecting the proton contribution, the pion density at the dump is N π (1 − f ) /πR min ≃ . So, for an effective integration window ∆ T cal of 25 ns, the pile-up ratePR is N π (1 − f ) πR min · πR c · ∆ T cal ; (6)it corresponds to 1.2 event for R c = 2 cm. Once accounting for pile-up, the π → e misidentification probability grows up to ∼
10% at constant detection efficiency forDIF positrons. As a consequence, even if further suppression of the pion backgroundmight be achieved exploiting the transverse shower profile or a better longitudinal seg-mentation, the instrumented beam dump can be fruitfully employed only for moderatebeam powers ( ∼ π + per extraction) or, equivalently, for time resolutions below1 ns. At larger beam powers, instead of exploiting the instrumentation of the dump,it is more convenient to reduce the angular spread at the entrance of the decay tunnel(3 mrad in the present case) and, therefore, increase the geometrical acceptance of thescraping taggers at lower R min . In the last decade, outstanding progresses have been achieved in the development offast, radiation hard detectors for tracking and particle identification. Such efforts havebeen motivated by challenging requests from modern experiments at colliders - firstlythe LHC - and from experiments in the field of rare kaon decays. In this paper, wediscussed an application of these technologies aimed at tagging ν e neutrinos in beams15 adius (cm)0 5 10 15 20 25 ) r a t e ( H z / c m Figure 6: Charged particle rate (squares) and muon rate (dots) at the last module for
R <
26 cm (instrumented beam dump). E ( G e V ) E ( G e V ) a . u . Figure 7: Energy deposition in the first 10 X ( E
1) and in the subsequent 3 λ I ( E R c = 2 cm centered at the impact point of undecayed pions in thebenchmark beamline. 16riginating from the decay-in-flight of charged pions and based on scraping of secondaryand tertiary leptons along the decay tunnel. For a specific beam configuration ofmoderate power ( ∼
10 kW for a 14% duty cycle), we have shown that the taggingsystem can achieve a suppression of 86% of the ν e background from K e decays andof about 60% of the ν e from the decay-in-flight (DIF) of µ + . This setup can also beemployed for beams of larger power (up to ∼
100 kW at the same duty cycle) withoutsignificant loss in performance. At beamlines of a few kW power, the tagging efficienciesfor DIF ν e can be further improved (+16%) by an additional instrumented beam dumplocated at the end of the decay tunnel; at larger powers the use of the instrumenteddump is limited by the rate of accidentals due to π → e and p → e misidentification. Acknowledgments
We wish to express our gratitude to U. Dore, P. Loverre, M. Mezzetto and F. Rongafor many interesting discussions and a very careful reading of the manuscript. We aregrateful to A. Blondel and R. Steerenberg for useful information on the CERN-PS. Aspecial thank to R. Felici, whose suggestions speeded up the completion of this work.
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