Optimization of the Neutrino Factory, revisited

Abstract

We perform the baseline and energy optimization of the Neutrino Factory
including the latest simulation results on the magnetized iron detector (MIND).
We also consider the impact of tau decays, generated by nu_mu to nu_tau or nu_e
to nu_tau appearance, on the mass hierarchy, CP violation, and theta_{13}
discovery reaches, which we find to be negligible for the considered detector.
For the baseline-energy optimization for small theta_{13}, we qualitatively
recover the results with earlier simulations of the MIND detector. We find
optimal baselines of about 2500 km to 5000 km for the CP violation measurement,
where now values of E_mu as low as about 12 GeV may be possible. However, for
large theta_{13}, we demonstrate that the lower threshold and the backgrounds
reconstructed at lower energies allow in fact for muon energies as low as 5 GeV
at considerably shorter baselines, such as FNAL-Homestake. This implies that
with the latest MIND analysis, low- and high-energy versions of the Neutrino
Factory are just two different versions of the same experiment optimized for
different parts of the parameter space. Apart from a green-field study of the
updated detector performance, we discuss specific implementations for the
two-baseline Neutrino Factory, where the considered detector sites are taken to
be currently discussed underground laboratories. We find that reasonable setups
can be found for the Neutrino Factory source in Asia, Europe, and North
America, and that a triangular-shaped storage ring is possible in all cases
based on geometrical arguments only.

Full text

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EURONU-WP6-10-28
IDS-NF-019
VT-IPNAS 10-19
IFIC/10-50
Optimization of the Neutrino Factory, revisited
January 19, 2011
Sanjib Kumar Agarwallaa, Patrick Huberb,
Jian Tangc, Walter Winterd
aInstituto de F´ ısica Corpuscular, CSIC-Universitat de Val` encia,
Apartado de Correos 22085, E-46071 Valencia, Spain
a,bDepartment of Physics, Virginia Tech, Blacksburg, VA24061, USA
c,dInstitut f¨ ur Theoretische Physik und Astrophysik, Universit¨ at W¨ urzburg,
D-97074 W¨ urzburg, Germany
Abstract
We perform the baseline and energy optimization of the Neutrino Factory including the
latest simulation results on the magnetized iron detector (MIND). We also consider the
impact of τ decays, generated by νµ→ ντ or νe→ ντ appearance, on the mass hierarchy,
CP violation, and θ13discovery reaches, which we find to be negligible for the considered
detector. For the baseline-energy optimization for small sin22θ13, we qualitatively recover
the results with earlier simulations of the MIND detector. We find optimal baselines of
about 2500km to 5000km for the CP violation measurement, where now values of Eµas
low as about 12 GeV may be possible. However, for large sin22θ13, we demonstrate that the
lower threshold and the backgrounds reconstructed at lower energies allow in fact for muon
energies as low as 5 GeV at considerably shorter baselines, such as FNAL-Homestake. This
implies that with the latest MIND analysis, low- and high-energy versions of the Neutrino
Factory are just two different versions of the same experiment optimized for different parts
of the parameter space. Apart from a green-field study of the updated detector performance,
we discuss specific implementations for the two-baseline Neutrino Factory, where the con-
sidered detector sites are taken to be currently discussed underground laboratories. We find
that reasonable setups can be found for the Neutrino Factory source in Asia, Europe, and
North America, and that a triangular-shaped storage ring is possible in all cases based on
geometrical arguments only.
aEmail: Sanjib.Agarwalla@ific.uv.es
bEmail: pahuber@vt.edu
cEmail: jtang@physik.uni-wuerzburg.de
dEmail: winter@physik.uni-wuerzburg.de
arXiv:1012.1872v2 [hep-ph] 18 Jan 2011

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1Introduction
Neutrino oscillation experiments have provided compelling evidence that the active neutri-
nos are massive particles [1], pointing towards physics beyond the Standard Model. In a
three-generation scenario, there are two characteristic mass squared splittings (∆m2
and three mixing angles (θ12,θ13,θ23) as well as a CP violating phase δCPaffecting neutrino
oscillations. A global fit of solar, atmospheric, accelerator and reactor neutrino oscilla-
tion experiments, yields the following parameter ranges for the oscillation parameters at
the 1σ level [2]: ∆m2
θ12= (34.4 ± 1.0)◦, θ23= (42.8+4.7
framework, there are still unknowns: the mass hierarchy (MH)–∆m2
or ∆m2
in the lepton sector. The experiment class, which may ultimately address these questions,
is the Neutrino Factory [4,5].
31,∆m2
21)
21= (7.59 ± 0.20) × 10−5eV2, |∆m2
−2.9)◦and θ13= (5.6+3.0
31| = (2.46 ± 0.12) × 10−3eV2,
−2.7)◦. Even, within the three flavor
31> 0 (normal ordering)
31< 0 (inverted ordering); the value of θ131, and whether there is CP violation (CPV)
In a Neutrino Factory, neutrinos are produced from muon decays in straight sections of a
muon storage ring. The feasibility and a possible design of a Neutrino Factory have been
subject of several, extensive international studies, such as in Refs. [6–8]. The International
Neutrino Factory and Superbeam Scoping Study [8–10] has laid the foundations for the
currently ongoing Design Study for the Neutrino Factory (IDS-NF) [5]. The goal of the IDS-
NF is to present a conceptual design report, a schedule, a cost estimate, and a risk assessment
for a Neutrino Factory facility by 2013. The IDS-NF defines a first-version baseline setup of
a high energy neutrino factory with Eµ= 25GeV and two baselines L1? 3000 − 5000km
and L2 ? 7500km (the “magic” baseline [11]) served by two racetrack-shaped storage
rings, with a muon energy of 25 GeV (for optimization questions, see, e.g., Refs. [12–19]).
This setup has been demonstrated to have excellent sin22θ13 reaches for addressing the
open questions in the three flavor scenario [17], to be robust against many potential new
physics effects [19,20] or systematical errors [21], and to be useful for degeneracy resolution
independently of the finally achieved luminosity [11]; for the physics case for the very long
baseline, see also Ref. [18].
The appearance signal in a neutrino factory consists of so called wrong-sign muons (e.g.,
from ¯ νe→ ¯ νµ) and therefore a detector which is capable of measuring the charge sign of
muons is required in order to distinguish this signal from the right-sign (e.g., from νµ→ νµ)
muon background. The most straightforward solution towards a high fidelity muon charge
measurement is a magnetized iron detector (MIND). With a MIND, the achievable levels
of muon charge identification allow for CP violation measurements in the muon neutrino
appearance channels [13,14].
The optimal MIND detector has backgrounds (such as from neutral currents or charge mis-
identification) at the level of about 10−3to 10−4, and the potential to measure the muon
charges at relatively low energies down to a few GeV. The importance of the precise location
of the detection threshold was discussed in detail in Refs. [17,22]. As the design of the
Neutrino Factory matures, more refined detector simulations have become available [23,24],
especially in comparison to the IDS-NF baseline 1.0 [5]. Compared to the older analyzes,
1Note, that recent hints for for θ13> 0 [3] are inconclusive and need to await more experimental data.
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these new simulations provide the detector response in terms of migration matrices mapping
the incident to the reconstructed neutrino energy for all individual signal and background
channels. An optimization of the cuts has lead to a lower threshold and higher signal
efficiencies than in previous versions, while the background level has been maintained in the
most recent analysis [24]. In addition, separate detector response functions for neutrinos
and anti-neutrinos are available, and it turns out that the ¯ νµdetection efficiency is better
than the νµdetection efficiency, which partially compensates for the different cross sections.2
The MIND detector has been studied in Ref. [23,24] as generic neutrino factory detector
and a specific detector of similar type is proposed for the India-based Neutrino Observatory
(INO) to measure atmospheric neutrinos [25]. The detector at INO may serve as a Neutrino
Factory far detector at a later stage.
Most recently, the background from τ decays was discussed for disappearance [26] and
appearance [27] channels. These taus arise from charged current interaction of ντwhich are
due to oscillation, e.g., for µ+stored:
App.: νe→ ντ→ τ−17%
Disapp.: ¯ νµ→ ¯ ντ→ τ+17%
The reason for these muons to contribute to the background is that the MIND cannot
resolve the second vertex from the τ decay, in contrast to OPERA-like emulsion cloud
chamber (ECC) [28]. In principle, the muons from τ decays carry information which may
be used for the standard oscillation [29,30] or new physics [31] measurements.
→ µ−(background) versus νe→ νµ→ µ−(signal)
→ µ+(background) versus ¯ νµ→ ¯ νµ→ µ+(signal)
(1)
(2)
An alternative version of the Neutrino Factory with respect to the IDS-NF baseline has
been proposed in Refs. [32–37]. The key difference is to replace the MIND with a magne-
tized totally active scintillator detector (TASD). The TASD, being fully active, has a lower
threshold and better energy resolution. The better detector performance and an optimiza-
tion of the front-end increasing the intensity have allowed a version of the Neutrino Factory
with Eµ∼ 5 GeV and a baseline possibly as short as L ? 1300km, corresponding to FNAL-
Homestake. This version is usually called “low energy Neutrino Factory” (LENF) and it
is found that the LENF has especially good performances for large sin22θ13. In addition,
the performance of a TASD allows to exploit the “platinum channel” (νµ→ νe), however it
turns out that it is of little practical value [36].
The recent simulation results for the MIND have made the performance margin between
TASD and MIND considerably smaller and therefore, we will show that the distinction
between the low- and high-energy Neutrino Factory is somewhat artificial and merely cor-
responds to two extreme corners of a common parameter space.
The phenomenological discussion of the Neutrino Factory has so far been performed mostly
in an abstract baseline-energy space. While the energy is a continuous variable, it is not
obvious that all baselines can be realized from any accelerator site. Therefore, we will present
a comparison of physics performances for a judicious choice of accelerator and detector
locations. It seems unlikely that a machine of the size and complexity of the accelerator
part of a Neutrino Factory would be built on a green-field site and therefore, we assume
2The difference in neutrino and anti-neutrino response is due to the different y-distributions [24].
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that it will be co-located with an existing, large accelerator facility. To be specific we
consider: CERN, the Fermi National Accelerator Laboratory (FNAL), the Japan Proton
Accelerator Research Complex (J-PARC), and the Rutherford Appleton Laboratory (RAL)
as potential accelerator sites [10]. For the choice of potential detector sites the issue is less
clear-cut, since, at the current stage, there is little information on the required amount
of rock overburden for a MIND (or TASD) to perform satisfactorily. Therefore, we make
the conservative choice and assume that a Neutrino Factory far detector requires a similar
amount of rock overburden as other neutrino experiments do. Under this assumption, a
natural choice of candidate detector sites is given by candidate detector sites for other
neutrino experiments. Fortunately, lists of candidate sites for general neutrino experiments
have been compiled for the US in response to the National Science Foundation (NSF) call for
proposals for a Deep Underground Science and Engineering Laboratory (DUSEL) [38] and
for Europe in the context of the Large Apparatus studying Grand Unification and Neutrino
Astrophysics (LAGUNA) [39] study. In North America, we consider eight locations: Soudan,
WIPP, Homestake, SNOLAB, Henderson, Icicle Creek, San Jacinto, and Kimballton. In
Europe, under LAGUNA, there are seven possible candidate sites: Pyh¨ asalmi in Finland,
Slanic in Romania, Boulby in UK, Canfranc in Spain, Fr´ ejus in France, SUNLAB in Poland,
and Umbria in Italy. Along with these seven sites, we also consider Gran Sasso National
Laboratory (LNGS) in Italy and Gran Canaria in Spain. We will complement these lists
of detector sites by the Asian facilities: the Kamioka mine in Japan, the proposed Chinese
underground laboratory at CJPL, YangYang in Korea, as well as INO in India.
This paper is organized as follows: We describe our methods and implementation in Sec. 2.
After that, we update the simultaneous optimization of baseline and muon energy in Sec. 3 in
a green-field scenario. In Sec. 4, we discuss the selection of specific sites, the site geometry,
and the possibility to use a triangular-shaped storage ring. Furthermore, in Sec. 5, we
quantify site-specific performance of the Neutrino Factory. Finally, we summarize and draw
our conclusions in Sec. 6. Details for the assumptions for the individual accelerator and
detector sites can be found in Appendix A. The sensitivity curves for all possible considered
site combinations are given Appendix B. The individual data files for the curves are available
for download at Ref. [40].
2 Simulation method and performance
In this section we describe our simulation method and we show the difference to the IDS-NF
1.0 in terms of event rates. We also compare the performance resulting from the different
detector simulations, and we compare the performance between one and two baselines.
2.1Simulation method
For the simulation of the Neutrino Factory, we use the GLoBES software [41, 42]. The
description of the experiment is based on Refs. [17,22], where we use the parameters from
the IDS-NF baseline setup 1.0 (IDS-NF 1.0) described in Ref. [5] (note number IDS-NF-
002). The detector description of this setup is based on Ref. [9], which has been updated
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in Refs. [23,24]. In this section, we compare these three detector descriptions, whereas we
use only the most recent version, Ref. [24], in the following sections. IDS-NF 1.0 uses two
magnetized iron calorimeters (fiducial mass 50 kt) at L = 4000 km and L = 7500 km. There
are two racetrack-shaped storage rings pointing towards these detectors, with a luminosity
of 2.5 × 1020useful muon decays per polarity, decay straight, and year, i.e., 1021useful
muon decay per year. We assume a running time of 10 years, i.e., 1022useful muon decay in
total. The parent muon energy is assumed to be Eµ= 25GeV. The considered oscillation
channels are:
νµappearance: νe→ νµfor µ+stored,
¯ νµappearance: ¯ νe→ ¯ νµfor µ−stored,
νµdisappearance: νµ→ νµfor µ−stored,
¯ νµdisappearance: ¯ νµ→ ¯ νµfor µ+stored.
(3)
(4)
(5)
(6)
Since the luminosity changes if one or two storage rings are required, i.e., one or two
baselines are operated, and the efficiency of a triangular-shaped ring, which will discuss
later, is different, it is convenient to re-parameterize luminosity in terms of a scale factor
(SF) [37]: SF=1 corresponds to the above mentioned parameters 2.5 × 1020useful muon
decays per polarity, decay straight, and year. If only one baseline is needed, then all muons
can be injected in the same storage ring, and SF=2. If, on the other hand, a storage ring
with a different geometry (such as a triangle) is used to point towards the two baselines
simultaneously, all muons will be injected into this ring, but the straight length towards each
detector will be smaller than in the racetrack case, i.e., 0<SF<2 in general. The scale factor
is then convenient to parameterize the obtained luminosity relative to the IDS-NF baseline
setup: SF>1: higher luminosity, SF<1: lower luminosity. Note that, in principle, the SF
can, for lower Eµ, also be increased by a re-optimization of the front-end and generally will
increase for lower energies due to the reduced decay losses during acceleration. For example,
a SF=2.8 for a low energy 4 GeV Neutrino Factory has been obtained in Ref. [35] compared
to SF=2.0. We will not consider this type of effect, since it depends on the accelerator
complex in a non-trivial fashion.
For the updated detector simulations, we use the migration matrices mapping the incident to
the reconstructed neutrino energies for all individual signal and background channels, which
can be directly implemented into GLoBES. Note that charge mis-identification, (electron)
flavor mis-identification and neutral current backgrounds are included. For the binning,
we then follow Ref. [23,24], where the migration matrices for the appearance channels are
given. For the disappearance channels, we use the same matrices.3In addition, we increase
the number of sampling points for high energies to avoid aliasing. This implementation will
be used throughout the remainder of this paper, unless indicated otherwise. It is denoted
by the label “new-NF”. Note that we also include signal (2.5%) and background (20%)
normalization errors, uncorrelated among all oscillation channels.
For the ντcontamination, we use the migration matrix from Ref. [27] for both the νe→ ντ
3That is somewhat on the conservative side, since we require charge identification and better results may
be obtained with an event sample without charge identification [17].
4