UMN-TH-420122 FTPI-MINN-2225 Enhancing Searches for Heavy QCD Axions via Dimuon Final States Raymond T. Co1 2Soubhik Kumar3 4yand Zhen Liu1z

2025-05-06 0 0 1.44MB 27 页 10玖币

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UMN-TH-4201/22, FTPI-MINN-22/25

Enhancing Searches for Heavy QCD Axions via Dimuon Final States

Raymond T. Co,1, 2, ∗Soubhik Kumar,3, 4, †and Zhen Liu1, ‡

1School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA

2William I. Fine Theoretical Physics Institute,

University of Minnesota, Minneapolis, MN 55455, USA

3Berkeley Center for Theoretical Physics, Department of Physics,

University of California, Berkeley, CA 94720, USA

4Theoretical Physics Group, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

Heavy QCD axions are well-motivated extensions of the QCD axion that address the qual-

ity problem while still solving the strong CP problem. Owing to the gluon coupling, critical

for solving the strong CP problem, these axions can be produced in signiﬁcant numbers in

beam dump and collider environments for axion decay constants as large as PeV, relevant for

addressing the axion quality problem. In addition, if these axions have leptonic couplings,

they can give rise to long-lived decay into lepton pairs, in particular, dominantly into muons

above the dimuon threshold and below the GeV scale in a broad class of axion models.

Considering existing constraints, primarily from rare meson decays, we demonstrate that

current and future neutrino facilities and long-lived particle searches have the potential to

probe signiﬁcant parts of the heavy QCD axion parameter space via dimuon ﬁnal states.

∗rco@umn.edu;0000-0002-8395-7056

†soubhik@berkeley.edu;0000-0001-6924-3375

‡zliuphys@umn.edu;0000-0002-3143-1976

arXiv:2210.02462v1 [hep-ph] 5 Oct 2022

CONTENTS

I. Introduction 2

II. Heavy QCD Axion EFT 5

A. Axion Quality Problem and Heavy QCD Axions 5

B. Lagrangian and Low Energy Eﬀective Theory 7

C. Axion Decay 7

III. Beam Dump Considerations 8

A. Production 9

B. Detection 10

C. Enhanced Eﬀective Detector Length 11

D. Results 13

1. Future Projections 14

2. Existing Constraints 15

IV. Examples of UV Completion 17

V. Conclusion and Outlook 19

Acknowledgements 20

References 20

I. INTRODUCTION

The quantum chromodynamics (QCD) axion was proposed to address the strong CP problem [1],

which concerns the fact that the CP violation in the strong interactions is experimentally constrained to

be less than O(10−10) via the non-observation of neutron electric dipole moment [2–5] as opposed to the

theoretical expectation of O(1) [1]. In the Peccei-Quinn (PQ) mechanism [6,7], the QCD axion [8,9]

is coupled to the gluons, and upon conﬁnement the QCD dynamics generates a QCD axion potential

with a CP-conserving minimum and a mass ma'5.7 meV ×(109GeV/fa) [10,11] with fabeing the

axion decay constant. As the QCD axion relaxes to this minimum, the strong CP problem is solved

dynamically.

While this is an elegant mechanism to address the strong CP problem, in the minimal realization it

suﬀers from the axion quality problem [12–16]. To illustrate this, we can model the axion a1as a (pseudo)

Nambu-Goldstone boson residing in the PQ ﬁeld Φ ∼faeia/fa, which is charged under a global and

1From now on, we will use the phrase ‘axion’ to denote both the standard QCD axion and its heavier variants. The phrase

‘axion-like particles’ (ALPs), on the other hand, will be reserved for pseudoscalars not addressing the strong CP problem,

as often done in the current literature.

anomalous U(1)PQ symmetry. Since gravitational eﬀects are expected to break global symmetries [17–

23], including U(1)PQ, we expect Planck scale suppressed terms such as L ⊃ Φn/Mn−4

Pl to arise. Written

in terms of the axion, we then have L ⊃ fn

acos(na/fa+ϕn)/Mn−4

Pl where ϕnis the complex phase of

the coeﬃcient of this term. For n > 4, these contributions can potentially drive the axion away from the

CP-conserving minimum due to the random nature of the phases ϕn, and consequently spoil the solution

to the strong CP problem. The situation is exacerbated in minimal scenarios with large values of fa,

as is the case with the conventional QCD axion. This scenario concerns ma.O(10) meV, which is

very sensitive to the above corrections. Even with the lowest decay constant faallowed by astrophysical

bounds [24–33], fa'108GeV, all Planck-suppressed operators up to dimension-8 have to be severely

constrained to avoid shifting the potential minimum by O(10−10). It is then vital to understand why the

PQ symmetry is of such a high quality.

This problem is signiﬁcantly relaxed if the gravitational corrections are suppressed and/or if the CP-

conserving potential is strengthened so that the potential is more stable against CP-violating corrections.

The former is achieved by fa108GeV, which then requires ma&100 MeV to avoid various astro-

physical bounds. The latter is the case if the axion mass is much larger than that dictated by the strong

dynamics—as in the so-called heavy QCD axion models. The axion mass can be enhanced in ways that

still preserve the CP symmetry [34–50]. (See also [51–53] for early work on raising the axion mass.)

In this regime inspired by the axion quality problem where ma&100 MeV and fa108GeV, the

heavy QCD axions are more strongly interacting with the Standard Model and can be produced and

searched for in the collider and beam dump experiments. In this regard, various approaches have been

pursued at beam dump, ﬂavor, and collider experiments [54–76], both for heavy QCD axions and more

generally ALPs. Furthermore, heavy QCD axions or ALPs can also play important roles in astrophysics

and cosmology, such as explaining the dark matter and baryon abundance [77–84].

In addition to the deﬁning gluon coupling, the axion may couple universally to all the other Standard

Model (SM) gauge bosons as predicted by grand uniﬁcation and also to the SM fermions in a broad class

of theories, including the DFSZ models [85,86], or to new heavy quarks as in the KSVZ models [87,88].

The coupling to fermions implies that the axion may dominantly decay into a pair of fermions when kine-

matically allowed, opening the possibility of unique experimental signatures. Speciﬁcally, in this work,

we propose a search for heavy QCD axions, with masses above the dimuon threshold and below the GeV

scale, at various neutrino and beam dump experiments. For these masses, axions may dominantly decay

into a pair of muons, as we will describe in detail below. As examples, we focus on neutrino experiments

utilizing the liquid argon time projection chamber (LArTPC) [89] technology, such as the Short-Baseline

Near Detector (SBND) [90], ICARUS [90] and Deep Underground Neutrino Experiment (DUNE) [91].

The dimuon ﬁnal state can be particularly useful from the background mitigation perspective, especially

after applying an invariant mass cut. While the dielectron ﬁnal state can also be interesting, for the

benchmark models that we consider, the branching ratio to dielectrons is subdominant compared to that

into diphotons. Above the GeV range, axions would predominantly decay hadronically and constitute

a diﬀerent class of signatures explored in [92] in the context of DUNE. Therefore we focus on the mass

range between the dimuon threshold and O(GeV). Through the gluon coupling, the axions are produced

via its mixing with SM mesons produced when the beams hit the target or the absorber. The axions can

then propagate to and decay within the LArTPC of the experiments, where the muons will leave two

distinct minimally ionizing tracks. We also perform similar analyses with long-lived particle searches in

the context of SHiP [93] and FASER 2 [94–96].

Recently, such a search has been performed for the ArgoNeuT detector [97] using data collected in

2009-2010 in the Neutrinos at the Main Injector (NuMI) beamline [98] at Fermilab, and an important

constraint in the axion parameter space is obtained in the mass range mabetween 0.2-0.9 GeV for an

axion decay constant faaround 10-100 TeV [99]. The dimuon signatures have also been exploited in

Refs. [100,101] for LHCb and Ref. [102] for CHARM, respectively, where axions are assumed to be

produced from the coupling with the top quark instead. Similarly, utilizing LArTPC but assuming the

absence of the axion-fermion coupling, Ref. [92] analyzes the sensitivity of the DUNE detector with the

gluon coupling, and Ref. [60] shows prospects for a DUNE-like detector without the gluon coupling.

We illustrate the various experimental setups in Fig. 1. Three aspects characterize this. Firstly, the

axion production can be dominated by either the target or the absorber located further downstream.

While the absorber would receive less ﬂux compared to the target, due to the proximity of the absorber

to the detector, it can dominate the experimental sensitivity. This was found to be the case for the

ArgoNeuT search in Ref. [97].

Secondly, while each detector is typically on the same axis as its associated beam line, sometimes

a detector can be more sensitive if axions produced from a separate, simultaneously operating oﬀ-axis

beam reach it [103,104]. As an example, while the ICARUS detector is nominally associated and on

the same axis with the 8 GeV Booster Neutrino Beam (BNB), due to its large volume it can receive

a large ﬂux of axions produced as the 120 GeV protons at the NuMI beam hit the NuMI target, even

if the NuMI beam axis does not pass through ICARUS directly. This increased sensitivity to NuMI

beam compared to BNB has to do with the fact that the 120 GeV NuMI beam produces a larger ﬂux of

axions and also the fact that ICARUS is not too oﬀ-axis to lose that ﬂux. We will make a quantitative

comparison between the results with the two beams in Sec. III.

Thirdly, since our search strategy involves a dimuon ﬁnal state and the produced muons from axion

decay are often very energetic, they can penetrate the earth/material before the detector. Thus to

consider such events, we include axion decays both inside and outside of the detector. This increase in

eﬀective decay volume can have an important eﬀect on the experimental sensitivity, as we will illustrate in

the context of DUNE near detector in Sec. III. To give another example in this context, in the ArgoNeuT

search [97], we considered an extra decay length of 63 cm before the detector front panel.

This work is organized as follows. In Sec. II we review the axion quality problem and motivate how

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Beam

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Absorber

Target

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Beam Axis

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µ+

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µ

µ+

µ

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On-axis

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O↵-axis

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Decay inside

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Decay outside

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Decay

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Location

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Detector

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Orientation

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Production

Location

FIG. 1. In this schematic diagram, the three columns show possible production locations, beam directions with

respect to the detector, and decay locations. The axions can be produced from the beam at the target or at the

absorber. The detector, shown by the gray cylinder, may be on- or oﬀ-axis from the beam. Lastly, the axion may

decay inside or before entering the detector.

heavy QCD axions can improve it. Then we describe the couplings of the heavy QCD axion to the SM

using an eﬀective ﬁeld theory (EFT) framework. After describing the various decay modes of the axion,

in Sec. III we study the details of axion production in various neutrino and beam dump experiments.

With these results at hand, we derive the projected reach that SBND, ICARUS, DUNE, FASER 2, and

SHiP may be able to achieve, and then summarize the existing constraints on the axion parameter space

coming mostly from rare meson decays. In Sec. IV we describe examples of UV completions that can

give rise to the axion EFT under consideration. We conclude in Sec. V.

II. HEAVY QCD AXION EFT

We start our general analysis with an EFT approach, and we will present examples of UV realization

in Sec. IV. After reviewing the role the heavy axions play in the context of the quality problem, we

describe the EFT and summarize the decay modes of the axion.

A. Axion Quality Problem and Heavy QCD Axions

As alluded to in the introduction, the axion quality problem is that U(1)PQ breaking contributions

could generally arise from gravitational corrections. Such corrections can give rise to new CP non-

conserving minima, and as the axion dynamically relaxes to such a minimum, the strong CP problem

reappears. To be quantitative, we ﬁrst consider the case of the standard QCD axion which couples to

QCD as

L ⊃ αs

8πa

+¯

θGa

µν ˜

Ga,µν .(1)

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UMN-TH-4201/22,FTPI-MINN-22/25EnhancingSearchesforHeavyQCDAxionsviaDimuonFinalStatesRaymondT.Co,1,2,SoubhikKumar,3,4,yandZhenLiu1,z1SchoolofPhysicsandAstronomy,UniversityofMinnesota,Minneapolis,MN55455,USA2WilliamI.FineTheoreticalPhysicsInstitute,UniversityofMinnesota,Minneapolis,MN55455,USA3Berkel...

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UMN-TH-420122 FTPI-MINN-2225 Enhancing Searches for Heavy QCD Axions via Dimuon Final States Raymond T. Co1 2Soubhik Kumar3 4yand Zhen Liu1z.pdf

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UMN-TH-420122 FTPI-MINN-2225 Enhancing Searches for Heavy QCD Axions via Dimuon Final States Raymond T. Co1 2Soubhik Kumar3 4yand Zhen Liu1z

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