nuSTORM: NEUTRINOS FROM STORED MUONS ∗

Alan D. Bross, Fermi National Accelerator Laboratory, Batavia, IL 60510, USAfor the nuSTORM Collaboration

AbstractNeutrino beams produced from the decay of muons in a

racetrack-like decay ring provide a powerful way to studyshort-baseline neutrino oscillation and neutrino interactionphysics. In this paper, I describe the facility, nuSTORM,and show how the unique neutrino beam at the facility willenable experiments of unprecedented precision to be car-ried out. I will present sensitivity plots that indicated thatthis approach can provide 10 σ confirmation or rejectionof the LSND/MinBooNE results and can be used to per-form neutrino interaction measurements of unprecedentedprecision. The unique ν beam available at the nuSTORMfacility has the potential to be transformational in our ap-proach to ν interaction physics, offering a “ν light source”to physicists from a number of disciplines.

INTRODUCTIONThe nuSTORM facility has been designed to deliver

beams of ↪ ↩ν e and ↪ ↩ν µ from the decay of a stored µ± beamwith a central momentum of 3.8 GeV/c and a momentumacceptance of 10% [1]. The facility is unique in that it will:

• Allow searches for sterile neutrinos of exquisite sen-sitivity to be carried out; and

• Serve future long- and short-baseline neutrino-oscillation programs by providing definitive measure-ments of ↪ ↩ν eN and ↪ ↩ν µN scattering cross sectionswith percent-level precision;

• Constitute the crucial first step in the development ofmuon accelerators as a powerful new technique forparticle physics.

The nuSTORM facility represents the simplest implemen-tation of the Neutrino Factory concept [2]. In our case, 120GeV/c protons are used to produce pions off a conventionalsolid target. The pions are collected with a magnetic hornand quadrupole magnets and are then transported to, andinjected into, a storage ring. The pions that decay in thefirst straight of the ring can yield muons that are capturedin the ring. The circulating muons then subsequently de-cay into electrons and neutrinos. We are using a storagering design that is optimized for 3.8 GeV/c muon centralmomentum. This momentum was selected to maximize thephysics reach for both ν oscillation and the cross sectionphysics. See Figure 1 for a schematic of the facility.

Muon decay yields a neutrino beam of precisely knownflavor content and energy. In addition, if the circulating

∗Work supported by DOE under contract DE-AC02-07CH11359

Figure 1: Schematic of the facility.

muon flux in the ring is measured accurately (with beam-current transformers, for example), then the neutrino beamflux is also accurately known. Near and far detectors areplaced along the line of one of the straight sections ofthe racetrack decay ring. Purpose-specific near detectorscan be located in the near hall and will measure neutrino-nucleon cross sections and can provide the first precisionmeasurements of νe and ν̄e cross sections. A far detec-tor at ' 2000 m would study neutrino oscillation physicsand would be capable of performing searches in both ap-pearance and disappearance channels. The experiment willtake advantage of the “golden channel” of oscillation ap-pearance νe → νµ (µ+ stored in the ring), where the re-sulting final state has a muon of the wrong-sign from inter-actions of the ν̄µ in the beam. This detector would need tobe magnetized for the wrong-sign muon appearance chan-nel, as is the case for the current baseline Neutrino Factorydetector [3]. A number of possibilities for the far detectorexist. However, a magnetized iron detector similar to thatused in MINOS is seen to be the most straight forward andcost effective approach.

NUSTORM FACILITY OVERVIEWThe components of nuSTORM are anticipated to consist

of six (6) functional areas consisting of the Primary Beam-line, Target Station, Transport Line, Muon Decay Ring,Near and Far Detector Halls [4].

These areas will be located south of the existing Main In-jector accelerator and west of Kautz Road on the Fermilabsite. In general terms, a proton beam will be extracted fromthe Main Injector at the existing MI-40 absorber, directedeast towards a new below grade target station, pion trans-port line and muon decay ring. The neutrino beam will bedirected towards a Near Detector Hall located 20 m East ofthe muon decay ring and towards the Far Detector locatedapproximately 1900 m away in the existing D0 AssemblyBuilding (DAB). Figure 2 shows the nuSTORM facilitycomponents as they will be sited near the Fermilab MainInjector. The nuSTORM facility will follow, wherever pos-sible (primary proton beam line, target, horn, etc.), NuMI[5] designs. Our plan is to extract one ”booster batch” at

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Figure 2: nuSTORM facility components.

120 GeV from the Main Injector (' 8× 1012 protons) andplace this beam on a carbon target. Forward pions are fo-cused by a horn into a capture and transport channel. Pionsare then “stochastically” injected into the Decay Ring (see[6]). Pion decays within the first straight of the Decay Ringcan yield a muon that is stored in the ring. Muon decaywithin the straight sections will produce ν beams of knownflux and flavor via: µ+ → e+ + ν̄µ + νe or µ− → e− + νµ+ ν̄e. For the implementation which is described here, wechose a 3.8 GeV/c storage ring to obtain the desired spec-trum of ' 2 GeV neutrinos. This means that we capturepions at a momentum of approximately 5 GeV/c.

FACILITY DETAILSAs mentioned above, the primary proton beam line and

target station (and its components, i.e., target and horn)for nuSTORM follow the NuMI designs. From the down-stream end of the horn, however, the nuSTORM beamsystem is longer similar to a conventional neutrino beam.From the downstream end of the horn, we continue thepion transport with several radiation-hard (MgO insulated)quadrupoles. Although conventional from a magnetic fieldpoint of view, the first two to four quads need special andcareful treatment in their design in order to maximize theirlifetime in this high-radiation environment. The pion beamis brought out of the Target Station and transported to theinjection point of the Decay Ring, which we have calledthe “Beam Combination Section” or BCS. Figure 3 showsthe pion transport line and the beginning of the DecayRing FODO straight section. The Decay Ring straight-section FODO cells were designed to have betatron func-

Figure 3: The G4beamline drawing from horn downstreamto the FODO cells. Red: quadrupole, Blue: dipole, White:drift.

tions βx, βy (the Twiss parameters) optimized for beam ac-ceptance and neutrino beam production (small divergencerelative to the muon opening angle (1/γ) from π → µ de-cay. Large betatron functions increase the beam size lead-ing to aperture losses, while smaller betatron functions in-crease the divergence of the muon beam. Balancing thesecriteria, we have chosen FODO cells with βmax ∼ 30.2 m,and βmin ∼ 23.3 m for the 3.8 GeV/c muons, which for the5.0 GeV/c pions, implies ∼ 38.5 m and ∼ 31.6 m for thepion’s βmax and βmin, respectively.

A large dispersion, Dx, is required at the injection point,in order to achieve π and µ beam separation. The BCSreadily reaches this goal. A schematic drawing of the in-jection scenario is shown in Figure 4. The pure sector

Figure 4: The schematic drawing of the injection scenario.

dipole for muons in the BCS has an exit angle for pionsthat is non-perpendicular to the edge, and the pure defo-cusing quadrupole in the BCS for muons is a combined-function dipole for the pions, with both entrance and exitangles non-perpendicular to the edges. The BCS will befollowed by a short matching section to the decay FODOcells (see Figure 3). The performance of the injection sce-nario can be gauged by determining the number of muonsat the end of the decay straight using a G4beamline sim-ulation. In this simulation, we were able to obtain 0.012muons per POT (see Figure 5). These muons have a widemomentum range (beyond that which the ring can accept,3.8 GeV/c± 10%) and thus will only be partly accepted bythe ring. The green region in Figure 5 shows the 3.8±10%GeV/c acceptance of the ring, and the red region shows

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the high momentum muons which will be extracted by themirror BCS at the end of the injection straight (see below).Within the acceptance of the decay ring, we obtain approx-imately 0.008 muons per POT.

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Figure 5: The muon momentum distribution at the end ofdecay straight.

Decay ringWe propose a compact racetrack ring design (480 m in

circumference) based on large aperture, separate functionmagnets (dipoles and quadrupoles). The ring is config-ured with FODO cells combined with DBA (Double BendAchromat) optics. The ring layout, including pion injec-tion/extraction points, is illustrated in Figure 6 and the cur-rent ring design parameters are given in Table 1. With the185 m length for the injection straight, ∼ 48% of the pionsdecay before reaching the arc. Since the arcs are set for thecentral muon momentum of 3.8 GeV/c, the pions remainingat the end of the straight will not be transported by the arc,making it necessary to guide the remaining pion beam intoan appropriate absorber. Another BCS, which is just a mir-ror reflection of the injection BCS, is placed at the end ofthe decay straight. It extracts the residual pions and muonswhich are in the 5±0.5 GeV/c momentum range. Theseextracted muons will enter the absorber along with pions inthis same momentum band and can be used to produce anintense low-energy muon beam.

Figure 6: Racetrack ring layout. Pions are injected into thering at the Beam Combination Section (BCS). Similarly,extraction of pions and muons at the end of the productionstraight is done using a mirror image of the BCS.

PERFORMANCEWith an exposure of 1021 120 GeV protons on target, we

obtain approximately 1.9 ×1018 useful µ decays. The ap-pearance of νµ, via the channel νe → νµ, gives nuSTORMbroad sensitivity to sterile neutrinos and directly tests theLSND/MiniBooNE anomaly [7]. A contour plot showingthe sensitivity of the νµ appearance experiment to sterile

Table 1: Decay Ring SpecificationsParameter Specification Unit

Central momentum Pµ 3.8 GeV/cMomentum acceptance ± 10%Circumference 480 mStraight length 185 mArc length 50 mArc cell DBARing Tunes (νx, νy) 9.72, 7.87Number of dipoles 16Number of quadrupoles 128Number of sextupoles 12

neutrinos, utilizing a boosted-decision-tree (BDT) multi-variate analysis, appears in Figure 7 for the exposure givenabove.

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Figure 7: 5 and 10 σ contours for the BDT analysis. The99% confidence level contours from a global fit to all ex-periments [8] are also shown.

REFERENCES[1] D. Adey et al.,, nuSTORM Proposal, arXiv:1308.6822.

[2] S. Geer, “Neutrino beams from muon storage rings: Charac-teristics and physics potential”, Phys.Rev., D57, 6989-6997,(1998).

[3] S. Choubey et al.,“Interim Design Report for the In-ternational Design Study for a Neutrino Factory”,arXiv:1112.2853 (2011).

[4] T. Lackowski et al., “nuSTORM Project Definition Report”,arXiv:1309.1389.

[5] A. G. Abramov et al.,“Beam optics and target conceptual de-signs for the NuMI project,” Nucl. Instrum. Meth. A 485, 209(2002).

[6] A. Liu et al., “nuSTORM Pion Beamline Design Update,”TUPBA18, these proceedings.

[7] K. N. Abazajian et al.,“Light Sterile Neutrinos: A White Pa-per,” arXiv:1204.5379 [hep-ph].

[8] J. Kopp et al., “Sterile Neutrino Oscillations: The Global Pic-ture,” JHEP 1305, 050 (2013).

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