$\textit{N}$-naturalness is a novel solution to the electroweak hierarchy problem which posits $\textit{N}$ copies of the Standard Model with varying Higgs mass-squared parameters. Reheating proceeds through a “reheaton” particle that deposits most of its energy density into the Standard Model and small but potentially measurable fractions into the other copies. Typically the sector with the lightest negative Higgs mass-squared is identified as the Standard Model. We demonstrate that $\textit{N}$-naturalness admits a broader class of realizations in which the Standard Model is identified with a heavier sector, rather than being restricted to the lightest. This is made possible by resonant mixing between the reheaton and the Higgs, which generically causes one sector to be preferentially reheated and to acquire the largest share of the energy density, singling it out as the Standard Model. We demonstrate that this scenario is consistent with current cosmological bounds on new relativistic degrees of freedom and overclosure constraints from heavy stable relics, while future cosmic microwave background and high redshift surveys will probe significant portions of the remaining parameter space. Furthermore, we highlight the possibility of a novel stochastic gravitational wave spectrum from the many cosmological first order QCD phase transitions occurring across the other sectors.

United States

Brian Batell

Akshay Ghalsasi

Wenjie Huang

Matthew Low

$\textit{i}$-incidental $\textit{N}$-naturalness

The Forward Physics Facility (FPF) is a proposed extension of the HL-LHC program designed to exploit the unique scientific opportunities offered by the intense flux of high energy neutrinos, and possibly new particles, in the far-forward direction. Located in a well-shielded cavern 627 m downstream of one of the LHC interaction points, the facility will support a broad and ambitious physics program that significantly expands the discovery potential of the HL-LHC. Equipped with four complementary detectors—FLArE, FASER <italic>&#957;</italic>2, FASER2, and FORMOSA&#8212;the FPF will enable breakthrough measurements that will advance our understanding of neutrino physics, quantum chromodynamics, and astroparticle physics, and will search for dark matter and other new particles. With this Letter of Intent, we propose the construction of the FPF cavern and the construction, integration, and installation of its experiments. We summarize the physics case, the facility design, the layout and components of the detectors, as well as the envisioned collaboration structure, cost estimate, and implementation timeline.

Luis A. Anchordoqui

United Kingdom

John Kenneth Anders

Switzerland

Japan

Akitaka Ariga

Tomoko Ariga

Canada

David Asner

Jeremy Atkinson

Alan J. Barr

Larry Bartoszek

Hans Peter Beck

Germany

Florian U. Bernlochner

India

Bipul Bhuyan

Jianming Bian

Aleksey Bolotnikov

Silas Bosco

Jamie Boyd

Nick Callaghan

Gabriella Carini

Michael Carrigan

Kohei Chinone

Matthew Citron

Isabella Coronado

Peter Denton

Albert De Roeck

Milind V. Diwan

Sergey Dmitrievsky

Romania

Radu Dobre

Monica D’Onofrio

Jonathan L. Feng

Max Fieg

Elena Firu

Brazil

Netherlands

Reinaldo Francener

Haruhi Fujimori

Frank Golf

Yury Gornushkin

Kranti Gunthoti

Claire Gwenlan

Carl Gwilliam

Andrew Haas

Elie Hammou

Daiki Hayakawa

Christopher S. Hill

Dariush Imani

Tomohiro Inada

Sune Jakobsen

South Korea

Yu Seon Jeong

Kevin J. Kelly

Samantha Kelly

Luke Kennedy

Felix Kling

Umut Kose

Peter Krack

China

Jinmian Li

Yichen Li

Steven Linden

Ming Liu

Kristin Lohwasser

Adam Lowe

Belgium

Steven Lowette

Toni Mäkelä

Roshan Mammen Abraham

Christopher Mauger

Konstantinos Mavrokoridis

Josh Mcfayden

Hiroaki Menjo

Connor Miraval

Keiko Moriyama

Toshiyuki Nakano

Ken Ohashi

Toranosuke Okumura

Hidetoshi Otono

Vittorio Paolone

Saba Parsa

Junle Pei

Michaela Queitsch-Maitland

Mary Hall Reno

Sergio Rescia

Filippo Resnati

Adam Roberts

Juan Rojo

Hiroki Rokujo

France

Olivier Salin

Jack Sander

Sai Neha Santpur

Osamu Sato

Italy

Paola Scampoli

Ryan Schmitz

Matthias Schott

Anna Sfyrla

Dennis Soldin

Albert Sotnikov

Anna Stasto

George Stavrakis

Jacob Steenis

David Stuart

Juan Salvador Tafoya Vargas

Yosuke Takubo

Simon Thor

Poland

Sebastian Trojanowski

Yu Dai Tsai

The forward physics facility: Physics opportunities and conceptual design

<p>Heavy right-handed neutrinos are highly motivated due to their connection with the origin of neutrino masses via the seesaw mechanism. If the right-handed neutrino Majorana mass is at or below the weak scale, direct experimental discovery of these states is possible in laboratory experiments. However, there is no <i>a priori</i> basis to expect right-handed neutrinos to be so light since the Majorana mass is a technically natural parameter and could comfortably reside at any scale, including at scales far above the weak scale. Here we explore the possibility that the right-handed neutrino Majorana mass originates from electroweak symmetry breaking. Working within an effective theory with two Higgs doublets, a nonzero lepton number is assigned to the bilinear operator built from the two Higgs fields, which is then coupled to the right-handed neutrino mass operator. In tandem with the neutrino Yukawa coupling, following electroweak symmetry breaking a seesaw mechanism operates, generating the light Standard Model neutrino masses along with right-handed neutrinos with masses below the electroweak scale. This scenario leads to novel phenomenology in the Higgs sector, which may be probed at the LHC and at future colliders. There are also interesting prospects for neutrinoless double beta decay and lepton flavor violation. We also explore some theoretical aspects of the scenario, including the technical naturalness of the effective field theory and ultraviolet completions of the right-handed neutrino Majorana mass.</p>

Amit Bhoonah

Right-handed neutrino masses from the electroweak scale

We describe a novel variation of the mirror twin Higgs model in which the color gauge group in both sectors is extended to SU(4) and spontaneously broken to SU(3) exclusively in the visible sector. Through this process, the mirror $\textit{Z}$ symmetry is spontaneously broken, allowing for a phenomenologically viable electroweak vacuum alignment. This structure produces interesting collider signatures, including heavy vectors and fermions with fractional electric charges. The twin sector, with unbroken SU(4) , produces interesting cosmological characteristics, such as the possibility to reduce ∆$\textit{N}$ and stable spin-0 baryons. The enlarged top quark sector required by the extended color gauge symmetry preserves naturalness, with even less tuning than the original twin Higgs in many circumstances.

Thomas Cochran

Logan Page

Christopher Verhaaren

Extended color twin Higgs

We study dark gauge-mediated supersymmetry breaking (dark GMSB) in a theory with a new unbroken U(1) local symmetry and massless dark photon. Messenger fields charged under both Standard Model and dark gauge symmetries produce new soft supersymmetry-breaking terms due to gauge kinetic mixing between U(1) hypercharge and U(1) . We show that large kinetic mixing induces significant distortions to the superpartner spectra relative to conventional GMSB. Notably, shifts in the Higgs soft masses impact the conditions for electroweak symmetry breaking, lowering the $\textit{μ}$ parameter and yielding a relatively light Higgsino that may be accessible at the LHC. Furthermore, for very simple messenger representations, a very light bino-dark photino mixed state is present in the spectrum, which may be probed through exotic Higgs boson decays at future Higgs factories. We also examine the cosmological and phenomenological consequences of the messengers, the lightest of which is absolutely stable and carries fractional electric charge.

Yechan Kim

Hye-Sung Lee

Jiheon Lee

Dark gauge-mediated supersymmetry breaking with a massless dark photon

The recent direct detection of neutrinos at the LHC has opened a new window on high-energy particle physics and highlighted the potential of forward physics for groundbreaking discoveries. In the last year, the physics case for forward physics has continued to grow, and there has been extensive work on defining the Forward Physics Facility and its experiments to realize this physics potential in a timely and cost-effective manner. Following a 2-page Executive Summary, we first present the status of the FPF, beginning with the FPF’s unique potential to shed light on dark matter, new particles, neutrino physics, QCD, and astroparticle physics. We then summarize the current designs for the Facility and its experiments, FASER2, FASER $$\nu $$ 2, FORMOSA, and FLArE.

Jyotismita Adhikary

Luis Anchordoqui

Alan Barr

Albert Roeck

Milind Diwan

Jonathan Feng

Christopher Hill

Yu Jeong

Kostas Mavrokoridis

Josh McFayden

Matteo Vicenzi

Wenjie Wu

Scientific program for the Forward Physics Facility

We explore the feasibility of using TeV-energy muons to probe lepton-flavor-violating (LFV) processes mediated by an axionlike particle (ALP) <math><mi>a</mi></math> with mass <math><mi>O</mi><mo>(</mo><mn>10</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi><mo>)</mo></math>. We focus on <math><mi>μ</mi><mi>τ</mi></math> LFV interactions and assume that the ALP is coupled to a dark state <math><mi>χ</mi></math>, which can be either less or more massive than <math><mi>a</mi></math>. Such a setup is demonstrated to be consistent with <math><mi>χ</mi></math> being a candidate for dark matter, in the experimentally relevant regime of parameters. We consider the currently operating NA64-<math><mi>μ</mi></math> experiment and proposed <math><mrow><mi>FASER</mi><mi>ν</mi><mn>2</mn></mrow></math> detector as both the target and the detector for the process <math><mi>μ</mi><mi>A</mi><mo>→</mo><mi>τ</mi><mi>A</mi><mi>a</mi></math>, where <math><mi>A</mi></math> is the target nucleus. We also show that a possible future active muon fixed-target experiment operating at a 3 TeV muon collider or in its preparatory phase can provide an impressive reach for the LFV process considered, with future <math><mrow><mi>FASER</mi><mi>ν</mi><mn>2</mn></mrow></math> data providing a pilot study towards that goal. The implications of the muon anomalous magnetic moment <math><mo>(</mo><mi>g</mi><mo>−</mo><mn>2</mn><msub><mo>)</mo><mi>μ</mi></msub></math> measurements for the underlying model, in case of a positive signal, are also examined, and a sample UV completion is outlined.

Hooman Davoudiasl

Roman Marcarelli

Ethan T. Neil

Lepton-flavor-violating ALP signals with TeV-scale muon beams

Why does dark matter (DM) live longer than the age of the Universe? Here we study a novel sub-eV scalar DM candidate whose stability is due to the Pauli exclusion of its fermionic decay products. We analyze the stability of the DM condensate against decays, scatterings (i.e., evaporation), and parametric resonance, delineating the viable parameter regions in which DM is cosmologically stable. In a minimal scenario in which the scalar DM decays to a pair of new exotic fermions, we find that scattering can populate an interacting thermal dark sector component to energies far above the DM mass. This self-interacting dark radiation may potentially alleviate the Hubble tensions. Furthermore, our scenario can be probed through precise measurements of the halo-mass function or the masses of dwarf spheroidal galaxies since scattering prevents the DM from becoming too dense. On the other hand, if the lightest neutrino stabilizes the DM, the cosmic neutrino background (<math><mrow><mi>C</mi><mi>ν</mi><mi>B</mi></mrow></math>) can be significantly altered from the Lambda cold dark matter prediction and thus be probed in the future by <math><mrow><mi>C</mi><mi>ν</mi><mi>B</mi></mrow></math> detection experiments.

Wen Yin

Cosmic stability of dark matter from Pauli blocking

We study the prospects for observing exotic decays of the Standard Model Higgs boson <math><mi>h</mi></math> into light beyond the Standard Model scalars <math><mi>a</mi></math> with mass <math><msub><mi>m</mi><mi>a</mi></msub><mo>≲</mo><msub><mi>m</mi><mi>h</mi></msub><mo>/</mo><mn>2</mn></math> in the single Higgs and Higgs pair production channels at the high luminosity run of the Large Hadron Collider (HL-LHC). Discovery prospects for single Higgs production in the gluon-gluon fusion and vector boson fusion modes with the Higgs boson decaying via the exotic mode <math><mi>h</mi><mo>→</mo><mi>a</mi><mi>a</mi><mo>→</mo><mn>4</mn><mi>τ</mi></math> are analyzed at the HL-LHC. The projected sensitivity for exotic Higgs decays in the nonresonant Higgs pair production channel <math><mi>p</mi><mi>p</mi><mo>→</mo><mi>h</mi><mi>h</mi><mo>→</mo><mo>(</mo><mi>h</mi><mo>→</mo><mi>b</mi><mover><mi>b</mi><mo>¯</mo></mover><mo>)</mo><mo>(</mo><mi>h</mi><mo>→</mo><mi>a</mi><mi>a</mi><mo>→</mo><mn>4</mn><mi>τ</mi><mo>)</mo><mo>→</mo><mn>2</mn><mi>b</mi><mn>4</mn><mi>τ</mi></math> at the HL-LHC and a future <math><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>100</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></math> hadron collider (FCC-hh) are also estimated. Furthermore, we study HL-LHC’s potential reach for the Higgs-strahlung process in the <math><mn>2</mn><mi>b</mi><mn>4</mn><mi>τ</mi></math> channel, taking into account the contamination from nonresonant Higgs pair production. Finally, the potential reach for resonant Higgs pair production in the <math><mn>2</mn><mi>b</mi><mn>4</mn><mi>τ</mi></math> channel at the HL-LHC is also explored for several choices of <math><mo>{</mo><msub><mi>m</mi><mi>H</mi></msub><mo>,</mo><msub><mi>m</mi><mi>a</mi></msub><mo>}</mo></math>. Our studies suggest that significant improvements over existing bounds are achievable in several production channels, motivating new dedicated searches for <math><mi>h</mi><mo>→</mo><mi>a</mi><mi>a</mi><mo>→</mo><mn>4</mn><mi>τ</mi></math> at the HL-LHC and future colliders.

Amit Adhikary

CERN

Shankha Banerjee

Rahool Kumar Barman

Biplob Bhattacherjee

Camellia Bose

Zhuoni Qian

Michael Spannowsky

Prospects for exotic <math><mi>h</mi><mo>→</mo><mn>4</mn><mi>τ</mi></math> decays in single and di-Higgs boson production at the LHC and future hadron colliders

We study the prospects for probing the Nnaturalness solution to the electroweak hierarchy problem with future gravitational wave observatories. Nnaturalness, in its simplest incarnation, predicts N copies of the Standard Model with varying Higgs mass parameters. We show that in certain parameter regions the scalar reheaton transfers a substantial energy density to the sector with the smallest positive Higgs squared mass while remaining consistent with bounds on additional effective relativistic species. In this sector, all six quarks are much lighter than the corresponding QCD confinement scale, allowing for the possibility of a first-order chiral symmetry-breaking phase transition and an associated stochastic gravitational wave signal. We consider several scenarios characterizing the strongly-coupled phase transition dynamics and estimate the gravitational wave spectrum for each. Pulsar timing arrays (SKA), spaced-based interferometers (BBO, Ultimate-DECIGO, μAres, asteroid ranging), and astrometric measurements (THEIA) all have the potential to explore new regions of Nnaturalness parameter space, complementing probes from next generation cosmic microwave background radiation experiments.

Mudit Rai

Gravitational Waves from Nnaturalness