Dipole portal and neutrinophilic scalars at DUNE revisited The importance of the high-energy neutrino tail Maksym Ovchynnikov1 2Thomas Schwetz1yand Jing-Yu Zhu1 3z

2025-05-03 0 0 1.56MB 22 页 10玖币
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Dipole portal and neutrinophilic scalars at DUNE revisited:
The importance of the high-energy neutrino tail
Maksym Ovchynnikov,1, 2, Thomas Schwetz,1, and Jing-Yu Zhu1, 3,
1Institut f¨ur Astroteilchen Physik, Karlsruher Institut f¨ur Technologie (KIT),
Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
2Instituut-Lorentz for Theoretical Physics, Universiteit Leiden,
Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
3School of Physics and Astronomy and Tsung-Dao Lee Institute,
Shanghai Jiao Tong University, 800 Dongchuan Rd, Shanghai 200240, China
We estimate the sensitivity of the DUNE experiment to new physics particles interacting with
neutrinos, considering the dipole portal to heavy neutral leptons and a neutrinophilic scalar with
lepton-number 2 as examples. We demonstrate that neutrinos from the high-energy tail of the
DUNE flux, with energies Eν&510 GeV, may significantly improve the sensitivity to these
models, allowing to search for particles as heavy as '10 GeV. We also study the impact of the
so-called tau-optimized neutrino beam configuration, which slightly improves sensitivity to the new
physics models considered here. For both models, we consider new production channels (such as
deep-inelastic scattering) and provide a detailed comparison of different signatures in the detector.
CONTENTS
I. Introduction 1
II. High-energy neutrinos at DUNE 2
A. The DUNE experiment 2
B. Neutrino flux at DUNE 2
C. High-energy neutrinos and production of new
physics particles 4
III. Neutrino dipole portal 4
A. Phenomenology at DUNE 5
B. Discussion of results 6
IV. Neutrinophilic scalar portal 8
V. Conclusions 10
A. Flux of muon and electron neutrinos at DUNE 13
B. Dipole portal 14
1. Production 14
a. Decays of mesons 14
b. Neutrino up-scattering – inside 15
c. Neutrino up-scattering – outside 17
d. Comparison of different production
channels 18
2. Number of events at DUNE 19
3. The shape of the sensitivity curves 20
C. Neutrinophilic scalar portal 21
1. Production 21
a. Quasi-elastic production 21
b. Deep inelastic scattering production 22
maksym.ovchynnikov@kit.edu
schwetz@kit.edu
jing-yu.zhu@kit.edu
2. Number of events and discussion of
sensitivity 22
I. INTRODUCTION
In a broad class of Standard Model (SM) extensions,
an interaction of SM neutrinos with new particles in
the few GeV mass range is introduced. Such models
may be probed at neutrino experiments, where a high-
intensity neutrino beam is produced, see e.g. [1]. Ex-
amples of currently running and future neutrino beam
experiments are T2K [2,3], MiniBooNE [4], Micro-
BooNE [5] and DUNE [6]. Other promising facilities to
look for such new particles are LHC-based experiments
such as SND@LHC [7], FASERν[8], or beam-dump ex-
periments like SHADOWS [9] and SHiP [10].
The goal of this paper is to demonstrate the poten-
tial of the DUNE experiment to search for interactions
of new physics particles with neutrinos. We do this by
revising the sensitivity of DUNE to two example models
– a neutrinophilic scalar [11,12] and the dipole portal to
heavy neutral leptons (HNLs) [13,14]. We improve on
previous studies [11,12,15,16] in several aspects. First,
we include the high-energy neutrino tail in the calcula-
tion of the production rate, showing its importance to
extend the reach in the mass of new physics particles,
and we study the effect of the so-called tau-optimized
beam configuration. In particular, we demonstrate that,
depending on the model, DUNE may have sensitivity to
new physics particles with masses up to O(10 GeV) a
few times larger than what it was obtained in the previ-
ous literature. Furthermore, for both models, we consider
new production channels for the new particles. For the
dipole portal, we discuss additional detection signatures;
the ratio of the different signal types is a specific pre-
diction of the dipole portal, allowing us to identify this
model in case of detection.
arXiv:2210.13141v2 [hep-ph] 26 Mar 2023
2
Detector Stransverse Ldet Lto det nnucl
ND 2 ×6 m24 m 574 m 8.4×1029 m3
FD 12 ×14 m258.2 m 1285 km 8.4×1029 m3
Table I: Parameters of Near Detector and individual Far
Detector module at DUNE. The numbers are taken from [17,
18], where Stransverse,Ldet,Lto det, and nnucl stand for the
fiducial cross-sectional area of the detector, the fiducial length
of the detector, the distance to the proton collision point, and
the nucleon number density, respectively.
The paper is organized as follows. In Sec. II, we dis-
cuss the flux of neutrinos at DUNE and stress the impor-
tance of its high-energy tail for the production of heavy
particles. In Sec. III, we consider the dipole portal to
HNLs, reestimate the DUNE sensitivity by considering
new production channels such as deep inelastic scatter-
ing and new search signatures. In Sec. IV, we discuss
the neutrinophilic scalar portal, revising the sensitivity
in a similar way. Finally, we conclude in Sec. V. Tech-
nical details, expressions for matrix elements and cross
sections, as well as further discussion can be found in
several appendices.
II. HIGH-ENERGY NEUTRINOS AT DUNE
A. The DUNE experiment
DUNE [6] is the next-generation long-baseline neu-
trino oscillation experiment, primarily aiming at pre-
cisely measuring neutrino oscillation parameters, such as
the unknown CP violation phase in the lepton sector. On
the other hand, it is also a powerful tool to search for a
variety of new physics beyond the SM. By using a 120
GeV proton beam onto a graphite target with a beam
power of 1.2 MW, this experiment can provide 1.1 ×1021
PoT/year, generating a large flux of light mesons such as
π±, K±,0. A fraction of these mesons then would decay
inside a 194 meter long decay pipe, producing neutrinos.
As a result, DUNE provides the world’s most intense
neutrino beam with a wide range of neutrino energies
peaking at about 2.5 GeV. The interactions of neutri-
nos are supposed to be studied in two detectors – the
near detector (ND) and the far detector (FD). The for-
mer is designed to include a 67.2 ton Liquid Argon Time
Projection Chamber (LArTPC), a magnetized gaseous
argon time projection chamber, and a large, magnetized
monitor, which will be located 574 m downstream of the
neutrino target following the decay pipe. The FD will be
equipped with four 10-kton LArTPC modules at a dis-
tance of 1285 km from the target; one can refer to Fig 1.4
in Ref. [17] for the layout of the modules. The useful pa-
rameters of ND and FD are summarized in Table I.
In order to maximize the flux of neutri-
nos/antineutrinos in the direction of the DUNE
detectors, the charged particles need to be collimated
with respect to the beam axis by a magnetic horn
νe, ND, CP-opt.
νμ, ND, CP-opt.
ντ+ντ, ND,×105
ντ, FD,×105, CP-opt.
νe, ND, τ-opt.
νμ, ND, τ-opt.
ντ, FD,×105,τ-opt.
0.5 1 5 10 50 100
10-12
10-10
10-8
10-6
10-4
Eν
[
GeV
]
Φν[GeV-1PoT-1m-2]
Figure 1: Neutrino fluxes as defined in Eq. (1) of νe,µ,τ at
the DUNE ND and the flux of ντat the DUNE FD (due to
νµντoscillations), assuming neutrino operating mode of
the focusing horns. ντfluxes are magnified by a factor 105
for better visibility. The results for two beam configurations
are shown: the CP-optimized (solid), and the tau-optimized
(dashed). Note that the ντflux at the ND does not depend
on the horn configuration, since it originates from decays of
promptly decaying Dsmesons and τleptons.
system. In dependence on the operating mode (the one
for neutrino or antineutrino), correspondingly positively
or negatively charged particles would be collimated. The
flux in antineutrino mode is very similar to the one in
neutrino mode (see e.g., Fig. 5.4 in [6]), and we expect
similar sensitivities to the new physics models discussed
below for neutrino and antineutrino beam modes. To be
specific, in this work, we will only consider the neutrino
mode.
In addition, there are two horn configurations consid-
ered by the DUNE collaboration: the CP-optimized con-
figuration, which maximizes the flux at Eντ<5 GeV im-
portant to study CP violations in neutrino oscillations,
and the tau-optimized configuration, for which the flux
of τneutrinos at the FD with Eν<5 GeV is some-
what lower, but the higher-energy flux between 5 and
10 GeV gets significantly increased, which would result
in an order of magnitude higher number of neutrino deep-
inelastic scattering (DIS) events [6].
B. Neutrino flux at DUNE
We define the neutrino flux at a given detector as
Φν1
NPoT ·Stransverse
dNν
dEν
,(1)
where NPoT is the number of protons on target, Stransverse
is the transverse area of the detector (Table I), dNν/dEν
is the differential distribution of neutrinos traveling in the
direction of the detector. The fluxes at ND and FD for
various neutrino flavors, assuming the neutrino operating
mode, are shown in Fig. 1. Below, we briefly discuss their
main characteristics.
3
The electron and muon neutrinos at DUNE are pro-
duced mainly by decays of light long-lived mesons such
as π±, K±, K0
L, and muons. The low-energy part of the
spectra of νµ(Eνµ.6 GeV) and νe(Eνe.10 GeV)
originates from decays
π+νµ+µ+, µ+νe+ ¯νµ+e+(2)
correspondingly. The high-energy tail comes from decays
of kaons K+, K0
L. The relative suppression νeµcomes
from the fact that muons are long-lived, τµπ/K 102,
and only a small fraction of them, '102, decays
inside the decay pipe before being scattered/absorbed
in the material after the decay pipe. To obtain the
fluxes of these neutrinos, we use the publicly available
results of the detailed GEANT4 [1921] based simula-
tion (G4LBNF) of the LBNF beamline developed by the
DUNE collaboration [6,22]. Technical details are given
in Appendix A.
When considering the DUNE sensitivity to new physics
interacting with νe,µ, we will only consider the ND. The
reason is that the fluxes of νe/µ at the FD are much
smaller than at the ND due to the much smaller angular
coverage of the FD. To argue this point, let us compare
the products of the flux times the detector volume Vdet,
Φ×Vdet, at the ND and FD. Using that VFD/VND 800
if assuming four FD module (Table I), and neglecting the
oscillations for the moment, one has
ΦFD
νe/µ VFD
ΦND
νe/µ VND 800 ×Lto ND
Lto FD 2
= 1.6·104,(3)
where (Lto ND/Lto FD)22·107is the solid angle sup-
pression (see Table I). Thus, the FD is less relevant for
searching for hypothetical BSM particles interacting with
these flavors.
Let us now discuss τneutrinos. At the DUNE ND,
their main production channels are the decays
D+
sτ++ντ, τ+¯ντ+X(4)
and their charge conjugated channels, where Xdenotes
lepton or hadron final states. Since Dsand τdecay
promptly, their distribution (and hence the flux of τneu-
trinos and antineutrinos at ND) is not affected by the
horn configuration. However, the flux of these Ds-
originated ντs or ¯ντs at the FD would be too small com-
pared to the flux of ντoriginated from the oscillations
νµντ, and therefore the former can be safely ignored.
Only the latter (ντfrom oscillations) would be present
at the FD when we calculate the sensitivities of neutrino
mode to new physics.
The DUNE simulations did not include Dsmesons and
τleptons. To generate the τneutrino flux, we have used
the spectrum of Dsmesons from [23], then simulated the
decay chain (4) (see the Appendix A), and selected the
τneutrinos that point to the ND.
The relative flux suppression ντµat the ND para-
metrically behaves as
ΦND
ντ+¯ντ
ΦND
νµ'PppDs·Br(Dsτ)
Pppπ+
pEν'3·108×pEν.
(5)
Here, PppDs'4·106,Pppπ+6.3 are multiplic-
ities (the number of decayed mesons per PoT), taken
from [24], Br(Dsτ)0.055 [25], and pEνis a fac-
tor depending on the neutrino energy, varying from O(1)
at Eν.3 GeV to '103for Eν'50 GeV. The reason
of this behavior is that τleptons (and hence τneutrinos)
have much larger mean energy than pions and kaons (and
hence νe/µ) decaying inside the decay pipe.
Apart from the production in decays of mesons and τ
leptons, ντmay also be produced via oscillations νµ
ντ. This channel is not relevant for the ND, since the
typical neutrino oscillation length is much larger than the
distance from the target to ND. However, it becomes the
main contribution to the ντflux at the FD (see Sec. II).
To obtain the oscillated flux at the FD, we have first
extracted the flux of νµat the FD, and then convoluted
it with the oscillation probability
Pνµντ0.943 ×sin2m2L
4Eν(6)
assuming ∆m2= 2.523 ×103eV2and L1300 km
for all neutrinos. The resulting flux is
ΦFD
ντ= ΦFD
νµ×Posc 'ΦND
νµ×Lto ND
Lto FD 2
×Pνµντ,(7)
where Pνµντis O(1) at the FD. Since the oscillation
probabilities νµντand ¯νµ¯ντare the same up to
small CP-violating effects, independently on the oscillat-
ing neutrino energy, the property of the relative suppres-
sion of the antineutrino to neutrino fluxes in the neutrino
mode translates to the fluxes of τneutrinos and antineu-
trinos at the FD.
Therefore, the ντfluxes at the ND and FD have a
different origin. As a result, the conclusion that the FD
is not relevant for the DUNE sensitivities to new physics
becomes invalid in the case of interactions with τflavor.
To illustrate this, let us again compare the products of
the flux times the detector volume. For the CP-optimized
horn configuration, using Eqs. (5), (7), one has
ΦFD
ντ×VFD
ΦND
ντ×VND '1·103Posc(Eν)
pEν
.(8)
Considering the FD and Eν.5 GeV, both Posc and
pEνcan be of O(1), leading to the ratio in Eq. (8) being
very large. Hence, the FD may provide better sensitiv-
ities to new physics coupling to ντ[15]. However, since
Posc(Eν)E2
νat energies Eν&10 GeV, together with
large pEνthey cause the suppression of this ratio at large
energies. As a result, with the increase of the neutrino
4
Detector Horn conf. NνeNνµNντ
ND CP-optimized 1.3·1041.2·1027.5·1010
ND tau-optimized 1.0·1041.4·1027.5·1010
FD CP-optimized 1.8·1098.9·1091.5·108
FD tau-optimized 1.05 ·1091.4·1081.1·108
Table II: Numbers of neutrinos per PoT within the angular
acceptance of the DUNE detectors (ND or FD), assuming the
parameters of the detectors given by Table I. Neutrino oscil-
lations are included. Two horn configurations are assumed:
CP-optimized and tau-optimized. For FD, we report numbers
corresponding to one module.
energy, the ratio (8), being 1 at Eν=O(1 GeV),
quickly drops and becomes O(1) at Eν'15 GeV.
The situation is somewhat different for the tau-
optimized horn configuration, for which tau neutrinos
at the FD are more energetic on average (see Fig. 1),
although qualitatively the conclusions do not change.
Therefore, both the ND and FD may be important for
searching for new particles coupling to ντ, depending on
their mass.
The total numbers of neutrinos traveling in the direc-
tion of the DUNE detectors (with the parameters given
in Table I) per PoT are given in Table II.
C. High-energy neutrinos and production of new
physics particles
In Refs. [12,15], which have studied the sensitivity
of DUNE to new physics particles produced in scatter-
ings of electron and muon neutrinos, artificial cuts on
the neutrino spectrum have been imposed: Eν<6 GeV
and Eν<10 GeV correspondingly. This is reasonable
if one studies the production of particles Ywith mass
mYEν,max. Indeed, first, the production of such par-
ticles does not require large energies. Second, only a tiny
fraction of νe, νµhave energies Eν>510 GeV, see
Fig. 1.
However, as we show below, these cuts can significantly
underestimate the maximal mass of new physics particles
that may be searched for at DUNE. Namely, the sensitiv-
ity of DUNE estimated in [12,15] rapidly drops at masses
mY= 2.53 GeV, which is directly related to the cuts.
To understand this, let us look closer at the relevant Y
production processes (here without specifying the model
details). Correspondingly, they are1
ν+pY+p, ν +pY+µ+n, (9)
1Apart from the scatterings off nucleons, [15] considered scatter-
ings off electrons and nuclei, but the former requires much larger
neutrino energies to produce a particle with the given mass, while
the latter is suppressed due to nuclear form factors for large
masses mr1
nuclear.
ν+pY+p
ν+pY+n+μ,pT,Y>0.5 GeV
0.01 0.05 0.10 0.50 1 5 10
0.1
0.5
1
5
10
50
100
mY[GeV]
Eν,min [GeV]
Figure 2: Dependence of the minimal neutrino energies
E(1),(2)
ν,min , Eq. (11), required to produce a particle Yin the
scattering processes in Eq. (9), on the Ymass. The short-
and long-dashed black lines denote correspondingly the max-
imal energies of νe,µ and ντobtained in the simulation (see
Fig. 1).
where n, p are nucleons, and in the second process
a threshold for the invisible transverse momentum of
pT,Y > pT,min = 0.5 GeV is required to suppress back-
grounds (we will discuss this in more detail in Sec. IV).
The minimal energy Eν,min of the neutrino required to
produce the particle Yin these processes is, correspond-
ingly,
E(1)
ν,min =2mpmY+m2
Y
2mp
,(10)
E(2)
ν,min '
hmn+mµ+qm2
Y+p2
T,min +p2
T,min
2(mn+mµ)i2
m2
p
2mp
.
(11)
The behavior of E(1),(2)
ν,min as a function of mYis shown
in Fig. 2. We see from the figure that at DUNE it actu-
ally may be possible to produce much heavier particles
– up to mY'8 GeV (the production from νe/µ), or to
mY'10 GeV (from ντ) if neutrinos with energies up
to 100 GeV are taken into account, offering a potential
trade-off to the reduced flux at high energies.
In the next two sections, we will study how the DUNE
sensitivity to the mentioned models extends in detail.
We will consider both ND/FD CP-optimized and tau-
optimized horn configurations.
III. NEUTRINO DIPOLE PORTAL
The effective Lagrangian of the neutrino dipole portal
below the electro-weak (EW) scale is [13]
Ldipole =dα¯
Nσµν PLναFµν + h.c.,(12)
5
where ναis the SM neutrino of flavor α=e, µ, τ,σµν =
i
2[γµ, γν], PL= (1 γ5)/2, Fµν =µAννAµis the
electromagnetic field strength tensor, and Nis a Heavy
Neutral Lepton (HNL).
Note that this Lagrangian is not gauge-invariant and
therefore valid only at energies below the EW scale,
above which we need to consider UV-completions of the
operator in Eq. (12). Here we remain agnostic about
the UV origin of this new interaction and study its phe-
nomenological implications at energies below the EW
scale.
Motivated by the unsolved MiniBooNE [26],
ANITA [27,28], and muon g-2 anomalies [29,30],
the dipole portal provides another way to test the
existence of HNLs and has attracted a lot of attention
recently [15,16,3148]. Bounds on dαcome from various
laboratory, astrophysical and cosmological observations.
Laboratory constraints come from neutrino oscillation
experiments, dark matter detectors, and the observa-
tion of high-energy neutrinos in neutrino telescope by
studying coherent elastic neutrino-nucleus scattering,
elastic neutrino-electron scattering, deep inelastic inter-
actions, etc. Astrophysical constraints on dαarise from
supernova bursts, Big Bang Nucleosynthesis, or Cosmic
Microwave Background. We refer to Refs. [13,14,46]
for a compilation of various constraints.
A. Phenomenology at DUNE
The DUNE sensitivity to HNLs with a dipole por-
tal interaction has been studied previously in Refs. [15,
16]. The HNL production mechanism studied there was
mainly quasi-elastic (QE) neutrino up-scattering [13],
να+TN+T, (13)
where T=e, n/p, Ar or atomic nuclei in the crust along
the trajectory of the neutrino beam.
Here, we will include also the deep-inelastic (DIS) con-
tribution, να+p/n N+X, where Xis an arbitrary
hadronic state, see Fig. 3(a). Furthermore, in addition to
the neutrino up-scattering, we consider the production of
HNLs by decays of short-lived and long-lived mesons [13],
π±,0, K±,0, η, ρ0, see Fig. 3(b), (c) for two example dia-
grams. A detailed discussion of the relevant cross sections
and production rates is given in Appendix B 1. The HNL
production may occur either inside the detector (neutrino
up-scattering) or outside it (both neutrino up-scattering
and meson decays). In the latter case, HNLs need to
reach the detector in order to decay inside it and hence
be detected. As discussed in detail in Appendix B1d, in
most cases the production from meson decays plays only
a sub-leading role and the main production channel is ei-
ther inside or outside up-scattering. The only exception
is HNL production via dτat the ND.
The main HNL decay channels are
Nγ+να, N l++l+να,(14)
with l=e, µ. The dominant channel is the mono-photon
channel, with the decay width being
ΓNναγ=|dα|2m3
N
4π(15)
for Dirac neutrinos.
Above the di-electron and di-muon mass threshold,
the leptonic channel becomes available. This channel
has been considered in ref. [35] in the context of test-
ing the dipole portal at FASER and in ref. [40] in the
context of the T2K near detector. These processes are
sub-dominant. However, they have the advantage that it
is possible to reconstruct the decay vertex since we have
two charged particles. In the limit mN2ml, the decay
width behaves as
ΓNναl+lαEM|dα|2m3
N
12π2log m2
N
m2
l3(16)
The branching ratios of the leptonic decay modes are
shown in Fig. 4. At large masses mNml, the
suppression of the leptonic decay width with a factor
αEM/3π'103compared to the photon channel (15)
gets partially compensated by the logarithm.
The two decay modes lead to different experimental
signatures and imply different requirements for back-
ground rejection. Furthermore, if both decay channels
can be observed, their ratio is a specific prediction of the
model, serving as a smoking-gun signature.
The combination of the different production and decay
processes leads to different signatures of the dipole portal
at DUNE:
1. Monophoton – an event consisting of a single iso-
lated photon appearing inside the detector. This
type of events occur when the HNL is produced
outside of the detector (via e.g. decays of mesons
or by neutrino up-scatterings), then enters the de-
tector, and decays through Nν+γ.
2. Double-bang – an event inside the detector con-
sisting of two vertices: the one with recoil matter
particles (electrons, nucleons, nuclei) and the one
with a displaced monophoton or a pair of charged
leptons [16,32]. This type of signature appears if
a HNL is produced inside the detector via neutrino
up-scattering, and then travels a distance larger
than the DUNE spatial resolution, which is of order
lDUNE '1 cm [17].2
2Whether the recoil particle (and hence the HNL production
point) would be detected depends on the recoil energy of the
target particles. If it is below the DUNE energy detection thresh-
old, it will be not visible. In this case, instead of the double-bang
event, one would see a monophoton from the decaying HNL. In
our current estimates, we assume ideal recoil energy reconstruc-
tion efficiency.
摘要:

DipoleportalandneutrinophilicscalarsatDUNErevisited:Theimportanceofthehigh-energyneutrinotailMaksymOvchynnikov,1,2,ThomasSchwetz,1,yandJing-YuZhu1,3,z1InstitutfurAstroteilchenPhysik,KarlsruherInstitutfurTechnologie(KIT),Hermann-von-Helmholtz-Platz1,76344Eggenstein-Leopoldshafen,Germany2Instituut-...

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