How to Identify Different New Neutrino Oscillation Physics Scenarios at DUNE_2

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How to Identify Diﬀerent New Neutrino Oscillation

Physics Scenarios at DUNE

Peter B. Denton,1Alessio Giarnetti,2and Davide Meloni2

1High Energy Theory Group, Physics Department, Brookhaven National Laboratory, Upton, NY

11973, USA

2Dipartimento di Matematica e Fisica, Universit`a di Roma Tre Via della Vasca Navale 84, 00146

Rome, Italy

E-mail: pdenton@bnl.gov,alessio.giarnetti@uniroma3.it,

davide.meloni@uniroma3.it

Abstract: Next generation neutrino oscillation experiments are expected to measure the

remaining oscillation parameters with very good precision. They will have unprecedented

capabilities to search for new physics that modify oscillations. DUNE, with its broad band

beam, good particle identiﬁcation, and relatively high energies will provide an excellent en-

vironment to search for new physics. If deviations from the standard three-ﬂavor oscillation

picture are seen however, it is crucial to know which new physics scenario is found so that it

can be veriﬁed elsewhere and theoretically understood. We investigate several benchmark

new physics scenarios by looking at existing long-baseline accelerator neutrino data from

NOvA and T2K and determine at what sensitivity DUNE can diﬀerentiate among them.

We consider sterile neutrinos and both vector and scalar non-standard neutrino interac-

tions, all with new complex phases, the latter of which could conceivably provide absolute

neutrino mass scale information. We ﬁnd that, in many interesting cases, DUNE will have

good model discrimination. We also perform a new ﬁt to NOvA and T2K data with scalar

NSI.

10000-0002-5209-872X

20000-0001-8487-8045

30000-0001-7680-6957

arXiv:2210.00109v2 [hep-ph] 24 Feb 2023

Contents

1 Introduction 2

2 New Physics Scenarios 2

2.1 Vector Non-Standard Neutrino Interaction 3

2.2 Scalar Non-Standard Neutrino Interaction 4

2.3 Sterile Neutrino 6

3 Benchmark Scenarios 7

3.1 Vector NSI Motivated by NOvA and T2K 7

3.2 Scalar NSI Motivated by NOvA and T2K 7

3.3 Sterile Neutrino Motivated by NOvA and T2K 7

4 DUNE Analysis Details 9

5 Single Scenario Results 10

5.1 Vector NSI 10

5.2 Scalar NSI 12

5.3 Sterile neutrinos 15

6 Diﬀerentiating the Models 16

6.1 Method 16

6.2 Discussion 16

6.2.1 True Vector NSI 16

6.2.2 True Scalar NSI 17

6.2.3 True Sterile Neutrino 17

7 Conclusions 19

A Approximate Expressions for Neutrino Oscillation Probabilities 19

A.1 Vector NSI 20

A.2 Scalar NSI 20

A.3 Sterile neutrinos 21

B NSI Fits To NOvA, T2K, and Reactor Data 22

B.1 Vector NSI 22

B.2 Scalar NSI 22

– 1 –

1 Introduction

Neutrino oscillation physics is on the advent of reaching the precision era. Current long-

baseline accelerator experiments NOvA [1] and T2K [2] are making the ﬁrst appearance

measurements in accelerator neutrinos and the next-generation accelerator experiments HK

[3] and DUNE [4] will measure the appearance probabilities precisely providing a wealth

of information about the three remaining oscillation unknowns: the octant of θ23, the

atmospheric mass ordering, and the value of the complex phase δ.

These powerful experiments will provide the strongest tests yet of the standard three-

ﬂavor oscillation hypothesis. In the event there is new physics, however, it is important to

check if that new physics can be robustly identiﬁed compared to alternatives. To address

this question, we use several benchmark new physics points, motivated by the slight tension

in the NOvA and T2K data [5], see also [6–11] and ﬁrst test the level at which DUNE can

identify them, and then the level at which they can be diﬀerentiated. As scalar NSI has

not yet been tested with T2K and NOvA data, we derive the ﬁrst constraints on scalar

NSI with existing long-baseline data.

Testing these benchmarks against no new physics and against each other will pro-

vide a fairly comprehensive overview of the capability of DUNE to correctly identify the

new physics scenarios and the parameters within the scenario that are preferred, although

partial degeneracies among diﬀerent new physics scenarios for precise values of the param-

eters may weakened this sensitivity in some cases. HK will also have sensitivity to many

of these scenarios and their combination will be particularly powerful for disentangling

things. Nonetheless, as we will show in section 6, DUNE alone will provide very good

model discrimination capabilities.

2 New Physics Scenarios

There are numerous new physics scenarios aﬀecting neutrino oscillations [12,13] including

neutrino decay [14–34], Lorentz invariance violation and CPT violation [35–50], background

dark matter ﬁelds [51–64], neutrino decoherence or wave-packet separation [65–73], unitary

violation [74–76], non-standard neutrino interactions (NSI) [5,77–83], and sterile neutrinos

[81,84–87]. We focus on the last two: NSI and sterile neutrinos. NSI provides a general

framework for quantifying modiﬁcations to the neutrino oscillation probability due to a

new interaction with matter particles. We further split NSIs into vector NSIs, which

act like the regular matter eﬀect [77] but possibly in a diﬀerent basis or with a diﬀerent

dependence on the matter particles, and scalar NSI [88], which acts like a new mass term

for neutrinos sourced by matter particles. We focus on NSI with oﬀ-diagonal couplings

as these are the parameters for which appearance measurements are particularly crucial

to constrain. Diagonal NSI are best constrained with solar and atmospheric neutrino

oscillations and neutrino scattering. Sterile neutrinos are well motivated extensions to the

standard three-ﬂavor neutrino oscillation picture due to theoretical arguments based on

the fact that neutrinos have mass, as well as due to a host of confusing anomalies [89–95]

which could be explained by new light sterile neutrinos in the m4∼1 eV range. Sterile

– 2 –

neutrino searches are also generally related to those involving unitary violation but are

a bit more comprehensive in their ability to directly probe the new mass scale. In the

following subsections we review the formalism of each of these three cases as they apply

for long-baseline neutrino oscillations.

2.1 Vector Non-Standard Neutrino Interaction

Since the introduction of the matter eﬀects in neutrino oscillations, the possibility that

neutrinos can undergo NSIs with matter has been widely studied. Focusing only on neutral

current vector NSI, which dominates over the axial-vector current assuming comparable

coupling strengths, we can describe vector NSI using an eﬀective theory approach. The

Lagrangian now includes the following terms:

Leff

vector NSI =−2√2GFX

f,α,β

εf

αβ(¯ναγρνβ)( ¯

fγρf),(2.1)

where GFis the Fermi constant, εf

αβ is the parameter which describes the strength of the

NSI, fis a ﬁrst generation SM charged-fermion (e,u, or d) and αand βdenote the neutrino

ﬂavors e,µor τ. The εparameter can be related to the parameters in a simpliﬁed model or

even a UV complete scenarios. Since these details do not aﬀect oscillations within a single

experiment, we focus only on the εeﬀective parameter. Notice that in this subsection we

will only consider interactions mediated by vector particles.

The presence of such interactions modiﬁes the neutrino oscillation Hamiltonian to

H=1

2E



UM2U†+a





1 + εee εeµ εeτ

ε∗

eµ εµµ εµτ

ε∗

eτ ε∗

µτ εττ









,(2.2)

where Uis the PMNS matrix [96,97], M2= diag(0,∆m2

21,∆m2

31), a= 2√2GFNeE, and

Neis the electron number density. Due to the hermiticity of the Hamiltonian matrix, the

diagonal NSI couplings εαα must be real, while the non-diagonal ones are in general complex

and can be written as εαβ =|εαβ |eiφαβ . Since we can subtract a matrix proportional to

the identity without changing the oscillation probabilities, only two of the diagonal NSI

parameters are independent. The Hamiltonian level NSI parameters relevant for neutrino

oscillations, those without superscripts εαβ, are related to the Lagrangian level NSI terms

via

εαβ =X

f∈{e,u,d}

εf

αβ ,(2.3)

where Nfis the number density of fermion f. When looking at NSI in the Sun and the

Earth simultaneously one must consider the Lagrangian level NSI to accurately translate

between them. Here we are only considering experiments in the Earth’s crust so we can

safely work with the Hamiltonian level parameters. Many studies of vector NSI exist in

the literature, see e.g. [5,6,78,98,99].

– 3 –

Several analyses of oscillation data1have been considered under various assumptions.

A recent global analysis of oscillation data in the context of NSIs has estimated the con-

straints on the NSI parameters in the context of both LMA (Large Mixing Angle solution

of the solar neutrino problem) and LMA-Dark results are shown, with the diﬀerence mainly

aﬀecting εee. The LMA-Dark solution [100,105,115–121] is the solution with εee ' −2

and the opposite sign2on ∆m2

31, ∆m2

21, and δ. For a recent discussion of LMA-Dark in the

context of the latest reactor constraints see [105]. We note that while the allowed values

in the global analysis [123] they ﬁnd might seem to disfavor some of the values preferred

in recent analyses long-baseline data [5,6] used in this paper (see table 6), it is easy to see

that the constraints on real NSI and NSI with a large complex component can be quite

diﬀerent.

It might appear that charged lepton ﬂavor violating probes would always be stronger

than those from oscillations, but numerous UV complete models with large εαβ &0.1 exist

in the literature where oscillations provide the strongest probes [79,124–131]. All of these

models can be recast into the language of NSI which is exactly what makes NSI such an

attractive BSM scenario to investigate.

In order to gain a good understanding of the impact of vector NSI on oscillation

experimental data, we derive approximate expressions for the vector NSI contribution to

neutrino oscillations in matter in appendix Aby performing a perturbative expansion in

various parameters known to be small.

2.2 Scalar Non-Standard Neutrino Interaction

In addition to a vector mediator, one can consider diﬀerent Lorentz structure for the

underlying theory behind a new neutrino interaction. Scalar NSI has been investigated in

the context of some neutrino oscillation experiments as well as early universe constraints

[88,113,132–137]. All previous studies, to our knowledge, focused on the diagonal scalar

NSI parameters; instead, we focus here on the oﬀ-diagonal parameters. Early universe

constraints and ﬁfth-force probes may be stronger than terrestrial probes in many cases,

although not necessarily all, depending primarily on the mediator mass [113]. Given the

highly disparate environments between the early universe and terrestrial oscillations for

which an UV complete model may behave diﬀerently, in addition to some hints for a

new interaction in early universe data [138,139], we consider this scenario in DUNE data

nonetheless. That said, we do caution the reader to be aware of important non-oscillation

constraints on scalar NSI.

The eﬀective Lagrangian for scalar NSI is:

Leff

scalar NSI =yfyαβ

(¯νανβ)( ¯

ff),(2.4)

1Scattering data is also sensitive to NSI [100–105], although these data sets have a non-trivial dependence

on the mediator mass, while oscillation data is essentially [106–114] independent of it.

2We take the deﬁnition of the three mass eigenstates as |Ue1|>|Ue2|>|Ue3|. Thus θ12 <45◦by

deﬁnition and the sign of ∆m2

21 has been measured experimentally with solar neutrinos. Some deﬁne the

mass eigenstates by m1< m2,|Ue1|>|Ue3|, and |Ue2|>|Ue3|. In this case ∆m2

21 >0 by deﬁnition and

the octant of θ12 is to be determined experimentally. See [121,122].

– 4 –

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摘要：

HowtoIdentifyDierentNewNeutrinoOscillationPhysicsScenariosatDUNEPeterB.Denton,1AlessioGiarnetti,2andDavideMeloni21HighEnergyTheoryGroup,PhysicsDepartment,BrookhavenNationalLaboratory,Upton,NY11973,USA2DipartimentodiMatematicaeFisica,UniversitadiRomaTreViadellaVascaNavale84,00146Rome,ItalyE-mail:pd...

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