Recently M.Ambrosio et al have submitted a Letter of Intent to the SPSC, in which they propose to expose a neutrino detector (much smaller but of a type similar to Monolith) to the neutrinos that accompany the M2 muon beam in the CERN North Area. In fact a significant neutrino flux (almost 2 10^9 nu per SPS cycle) is produced during the muon running of COMPASS at full intensity, out of which some 5 10^8 would traverse a detector with a section of 2 meters diameter. These neutrinos pass well below the COMPASS experiment, located in experimental hall EHN2. The neutrino detector will be housed in a new experimental hall, with a floor level, depending on the site chosen, several meters underground. On their way towards the detector, the neutrinos would traverse one or several RF cavities, recuperated from LEP, which may induce neutrino oscillations. Another part of the proposal addresses the question of speed of flight of the neutrinos by measuring the time difference between the neutrino interaction and the time of the muon from the same pion decay. This note compares and proposes suitable locations for the different detector components

Gatignon, L

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EUROPEAN ORGANIZATION FOR NUCLEAR PHYSICSEuropean Laboratory for Particle PhysicsCERN – SL DIVISIONCERN-SL-2001-054  EAWHERE TO INSTALL THE I224 DETECTORL.GatignonAbstract:Recently M.Ambrosio et al have submitted a Letter of Intent to the SPSC, in which they propose toexpose a neutrino detector (much smaller but of a type similar to Monolith) to the neutrinos thataccompany the M2 muon beam in the CERN North Area. In fact a significant neutrino flux (almost2 109  per SPS cycle) is produced during the muon running of COMPASS at full intensity, out ofwhich some 5 108 would traverse a detector with a section of 2 meters diameter. These neutrinospass well below the COMPASS experiment, located in experimental hall EHN2. The neutrinodetector will be housed in a new experimental hall, with a floor level, depending on the site chosen,several meters underground. On their way towards the detector, the neutrinos would traverse one orseveral RF cavities, recuperated from LEP, which may induce neutrino oscillations. Another part ofthe proposal addresses the question of speed of flight of the neutrinos by measuring the timedifference between the neutrino interaction and the time of the muon from the same pion decay.This note compares and proposes suitable locations for the different detector components.Geneva, Switzerland8 October, 2001- 2 -TABLE OF CONTENTSIntroduction 31. Optimum place for neutrino detector 52. The RF cavity 83. The muon tagging detector 9Conclusions 11Acknowledgements 11- 3 -IntroductionRecently M.Ambrosio et al have submitted a Letter of Intent1 to the SPSC, in which they propose toexpose a neutrino detector (much smaller but of a type similar to Monolith) to the neutrinos thataccompany the M2 muon beam in the CERN North Area. In fact a significant neutrino flux (almost2 109  per SPS cycle) is produced during the muon running of COMPASS at full intensity, out ofwhich some 5 108 would traverse a detector with a section of 2 meters diameter. These neutrinospass well below the COMPASS experiment, located in experimental hall EHN2. The neutrinodetector will be housed in a new experimental hall, with a floor level, depending on the site chosen,several meters underground. On their way towards the detector, the neutrinos would traverse one orseveral RF cavities, recuperated from LEP, which may induce neutrino oscillations. The schematiclayout is shown in Figure 1.Figure 1:   Schematic layout of the M2 beam with COMPASS and the I224 detector.In the figure, the originally proposed location for the new experiment is indicated (‘I224 L.O.I.’),just downstream of EHN2. At this point the maximum neutrino flux can be recuperated within agiven detector section. However, simulations2 show that there is a significant muon background,almost 2 106 muons per SPS cycle when COMPASS runs at its nominal intensity of 2 108 muonsper SPS cycle. Also the detector would be quite deep underground. In fact the neutrinos follow astraight line with an upward slope of 9.69 mrad, which is a straight prolongation of the pion decaysection in the muon beam. Therefore the new experimental hall floor would be less deepunderground if the detector would be located further downstream. In Figure 2 we show the verticalseparation between the muon beam leaving the COMPASS experiment and the neutrino axis.                                                1 M.Ambrosio et al., A neutrino experiment at the CERN North Area, CERN/SPSC 2001-019, SPSC/I2242 Ch.Iselin, HALO, a computer program to calculate muon halo, CERN 74-17, 29 August 1974.- 4 -Figure 2:  The vertical separation between the M2 line and the neutrino axis versus Z positionThe green vertical line indicates the fence which limits the beam area, i.e. only downstream ofthat fence one has access with beam on3.In this note we look for an optimal location of the I224 apparatus as a compromise between variousparameters:1. Neutrino flux,2. Muon background,3. Level below ground of new experimental hall,4. Possibility to construct the hall and install the apparatus during COMPASS runs,5. Cost. However, no detailed cost estimate will be presented in this document.Separate chapters address the locations for the main neutrino detector, the RF cavity and the muontagging detector, respectively.                                                3 Please note that in general this applies only above ground level.- 5 -1. Optimum place for neutrino detector.In Figure 3 we show the neutrino flux into the detector as a function of its longitudinal distancefrom the COMPASS target. The detector section is defined as a circle with 1 meter radius.Figure 3 :  Neutrino flux into the I224 detector versus its distance from the COMPASS target.The COMPASS  target position is defined as 0 meters, the upstream end of the proposed location(in L.O.I I224) at the end of EHN2 is at 70 meters and the first location downstream of the beamarea fence is at about 250 meters. At that point the decrease of neutrino flux w.r.t. the L.O.I.position is 17%.In the L.O.I. the authors mention the significant muon background of about 2 106 muons per SPScycle. Their typical momentum is about 50 GeV/c, according to the simulation, with a maximumclose to 90 GeV/c. They come from the pion decay tube and have not yet ranged out. The authorspropose to install an iron shield of 25 meters long. This would be difficult to install and be quitecostly (at least 1 Meuro). An alternative solution would be to use the ground shield which is alreadypresent downstream of EHN2. Using the HALO program we calculated the muon flux as a functionof longitudinal position. The result is shown in Figure 4. There is no background beyond 450 m.The flux at the L.O.I. position is almost exclusively muons originating from the pion decay tube(the muons ‘miss’ the vertical bends B4+B5). As their remaining energy is low, they shouldessentially range out before the location of the fence. However, with increasing distance, thevertical separation between the muons from the COMPASS beam and the neutrino line decreasesand some beam muons scatter into the I224 detectors. A minimum is observed in the region around- 6 -the fence, at a level below 105 muons per SPS cycle. This without the need of an iron shield. Inreality the situation may be further optimized by deflecting the muons sideways after theCOMPASS target.Figure 4 : The muon background into the I224 detector as a function of its distance fromthe COMPASS target. The blue bars indicates muons from the decay volume,the red bars punch-through from muons traversing the COMPASS experiment.In Figure 5 we show the approximate height of the neutrino beam with respect to the ground level.It is clear that the cost of digging decreases the further downstream the hall is situated. However,there would be some additional cost in terms of road construction. A major advantage of installingthe hall just behind the fence of the beam zone is that, with certain constraints, access to the pit ispossible with beam on4. This location corresponds to the position 1375 to 1405 meters in Beatchbeam length and is indicated as ‘New position ?’ in the figure. It is also the location referred to inthe Addendum to the Letter of Intent5. The constraints are listed in a memo by TIS/RP6. In Figure 6the momentum spectrum of the neutrinos hitting the detector in this new position is compared withthe spectrum at the L.O.I. position.                                                4 These constraints imply that the access to the underground cavern must be via a shaft that is displaced laterally by asubstantial amount. Also the current in the last vertical bend must be limited by an interlock.5 M.Ambrosio et al., Addendum to the Letter of Intent I224, CERN/SPSC 2001-024, SPSC/I224 Add.1, 31 July 2001.6 S.Roesler et al., Memorandum, RP Considerations for the site selection of Experiment I224, copy available on the webat http://cern.ch/gatignon/i224rp.ps.- 7 -Figure 5: The level of the neutrino beam w.r.t. the EHN2 experimental hall.However, the construction of the hall is still not possible with beam on, therefore it would becumbersome and more costly7.These problems are avoided if the hall is located 600 meters downstream of the end of EHN2, i.e.just behind the CERN perimeter8. In that location no muons will reach the detector, even at thehighest energies available at the SPS. Therefore no constraints are imposed on access orconstruction at any time7. Also the hall is less deep: the muons pass only 6 meters below groundlevel. However, there is again a price to pay in neutrino flux: about a factor 2 less than at the L.O.I.location.There is a location, located some 1100 meters downstream of the end of EHN2, where the neutrinosreach the surface. At this location, still CERN property, a surface building could be installed. Thislocation is also quite close to an existing road. However, at that point the neutrino is reduced to 1/3of the flux at the L.O.I. location.                                                7 Or the COMPASS schedule would have to be adjusted8 The land behind the fence is still CERN property over quite some length. Therefore it is not excluded to construct abuilding in this area, though some negotiation may be involved.- 8 -        Figure 6:   The neutrino momentum spectrum at the suggested location (behind                            the fence) compared with the one at the L.O.I. position2. The RF cavityThe RF cavity should be located at a position sufficiently downstream of Bends 4 and 5 to havesufficient separation between the neutrino line and the M2 vacuum tube (at least 80 cm), but not sofar away that the neutrino flux has disappeared into the floor of the tunnel. This position is justbelow the passage of the river Lion. After informal discussions with some members of the SL/CTgroup, it seems that the best location is at about 770 meters from the T6 target, more or less at thepresent location of MIB 1+2, which would have to be displaced slightly.- 9 -3. The muon tagging detectorFor part of the physics program outlined in the Letter of Intent it is important to compare the time ofthe neutrino event with the time of the muon from the decay of the same pion. The HALO programallows to check where goes the muon partner to a neutrino that traverses the neutrino detector.   Figure 7: The neutrino momentum spectra at the neutrino detector, tagged anduntagged, at the L.O.I. position, compared with the tagged spectrum at the newproposed location as well as with the spectrum of all muons producedIn Figure 7 we show the momentum of the neutrinos ‘tagged’ with a ‘good’ muon at the COMPASSexperiment and compare this with the spectra of all neutrinos produced and of those traversing theneutrino detector. Only about 5% of the muons traversing the detector at the L.O.I. position areaccompanied by a ‘good’ muon in the COMPASS apparatus. For the 2nd location, i.e. behind thefence, this fraction is only about 2.7% of the neutrinos traversing the detector and only 1% of allneutrinos produced. The fraction tagged will be even lower for a detector located behind the CERNfence.For ‘good’ muons (i.e. those who never left the nominal aperture of the beam line), the neutrinomomentum pQ and the muon momentum pP are related to the parent pion momentum pS by therelationpQ = pS – pP – E/c- 10 -where E is the energy loss of the muon in the Beryllium absorber (amounting to about 2.8 GeV).For halo muons (which have traversed material) this relation reduces topQ < pS – pP – E/cThis explains why the tagged neutrinos are concentrated at low momenta. As the neutrinostraversing the detector are rather forward ones, they tend to originate from the highest momentumpion decays (well above the average of 177 GeV/c) and therefore their typical momenta are of theorder of 20 GeV/c as expected (177 + pS – 162.8 GeV/c).However, this disadvantage can probably be recuperated if a ‘tagging hodoscope’ would beinstalled in a suitable location, e.g. just downstream of SCRAPER 2, i.e. 719 meters from the centerof the T6 primary target.In Figure 8 we show the X and Y profiles of the muons correlated with a neutrino in the Monolithprototype at the new proposed location (1381 m, i.e. 180 meters from the L.O.I. position).Figure 8:  The profiles in the plane just behind SCRAPER 2 of muons from the same        pion decay as a neutrino in the Monolith prototype in the new location.A hodoscope covering 1 meter in X and 1 meter in Y would thus allow to tag some 99% of themuons accompanying the useful neutrinos. However, it should be pointed out that this detectorshould be a rather performant one. In fact the total rates are rather high at that location:‘Good’ muons: about 4 .108 per spill with RMS (X x Y) = 2.5 x 2.0 cm‘Halo’ muons: about 109 per spill with RMS (X x Y) = 70 x 50 cm- 11 -Therefore at least the central strips of a hodoscope should be rather finely segmented. Please notethat the centre of the flux is some 20 cm above the nominal beam axis in the vertical plane! Onepossible solution may be a Scintillating Fiber tracker (e.g. 1mm fibers), which also offers good timeresolution: it seems that resolutions of the order of 300 to 500 psec can be achieved.ConclusionsThe best compromise in terms of neutrino flux and minimum background seems to be the locationjust behind the CERN fence, i.e. some 200 meters behind EHN2. However, practical considerationsand cost lead to a preference for a more downstream location, behind the CERN fence. The fact thatconstruction and installation work is no longer restricted to winter periods may make it morerealistic to do the job in a limited time and budget. The neutrino flux just after the CERN fence, 600meters behind EHN2, is about half the one at the location suggested in the Letter of Intent. This fluxis reduced to about 1/3 of the L.O.I. values at the first location, 1100 metres downstream of EHN2,where a substantially cheaper hall at ground level can be considered. The land still belongs toCERN. The latter location is probably the most realistic as an overall compromise between physicsperformance, time schedules and cost.AcknowledgementsMany discussions with several authors of the Letter of Intent are acknowledged, in particular withA.Pullia, M.Paganoni, N.Redaelli and D.Cundy. The input from the TIS/RP group was very helpful,in particular discussions with D.Forkel-Wirth, G.Stevenson and S.Roesler.

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