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.

United States

Luis A. Anchordoqui

United Kingdom

John Kenneth Anders

Switzerland

Japan

Akitaka Ariga

Tomoko Ariga

Canada

David Asner

Jeremy Atkinson

Alan J. Barr

Larry Bartoszek

Brian Batell

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

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

<p>With no concrete evidence for nongravitational interactions of dark matter to date, it is natural to wonder whether dark matter couples predominantly to the Standard Model (SM)’s neutrinos. Neutrino interactions (and the possible existence of additional neutrinophilic mediators) are substantially less understood than those of other SM particles, yet this picture will change dramatically in the coming decades with new neutrino sources. One potential new source arises with the construction of a high-energy muon collider (MuCol); due to muons’ instability, a MuCol is a source of high-energy collimated neutrinos. Importantly, since the physics of muon decays (into neutrinos) is very well-understood, this leads to a neutrino flux with systematic uncertainties far smaller than fluxes from conventional high-energy (proton-sourced) neutrino beams. In this work, we study the capabilities of a potential neutrino detector, “<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>MuCol</mi><mi>ν</mi></mrow></math>”, placed <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mo>∼</mo><mn>100</mn><mtext> </mtext><mtext> </mtext><mi mathvariant="normal">m</mi></math> downstream of the MuCol interaction point. The <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>MuCol</mi><mi>ν</mi></mrow></math> detector would be especially capable of searching for a neutrinophilic mediator <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>ϕ</mi></math> through the mononeutrino scattering process <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>ν</mi><mi>μ</mi></msub><mi>N</mi><mo stretchy="false">→</mo><msup><mi>μ</mi><mo>+</mo></msup><mi>ϕ</mi><mi>X</mi></math>, exceeding searches from other terrestrial approaches for <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>m</mi><mi>ϕ</mi></msub></math> in the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mo>∼</mo><mi>few</mi></mrow></math> MeV- ten GeV range. Even with a 10 kg-yr exposure, <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>MuCol</mi><mi>ν</mi></mrow></math> is capable of searching for well-motivated classes of thermal freeze-out and freeze-in neutrino-portal dark matter.</p>

Neutrino-portal dark matter detection prospects at a future muon collider

Proton-proton collisions at energy-frontier facilities produce an intense flux of high-energy light particles, including neutrinos, in the forward direction. At the LHC, these particles are currently being studied with the far-forward experiments FASER/FASER$\textit{ν}$ and SND@LHC, while new dedicated experiments have been proposed in the context of a Forward Physics Facility (FPF) operating at the HL-LHC. Here we present a first quantitative exploration of the reach for neutrino, QCD, and BSM physics of far-forward experiments integrated within the proposed Future Circular Collider (FCC) project as part of its proton-proton collision program (FCC-hh) at $$ \sqrt{s} $$ ≃ 100 TeV. We find that 10 electron/muon neutrinos and 10 tau neutrinos could be detected, an increase of several orders of magnitude compared to (HL-)LHC yields. We study the impact of neutrino DIS measurements at the FPF@FCC to constrain the unpolarised and spin partonic structure of the nucleon and assess their sensitivity to nuclear dynamics down to $\textit{x}$ ∼ 10 with neutrinos produced in proton-lead collisions. We demonstrate that the FPF@FCC could measure the neutrino charge radius for $\textit{ν}$ and $\textit{ν}$ and reach down to five times the SM value for $\textit{ν}$ . We fingerprint the BSM sensitivity of the FPF@FCC for a variety of models, including dark Higgs bosons, relaxion-type scenarios, quirks, and millicharged particles, finding that these experiments would be able to discover LLPs with masses as large as 50 GeV and couplings as small as 10 , and quirks with masses up to 10 TeV. Our study highlights the remarkable opportunities made possible by integrating far-forward experiments into the FCC project, and it provides new motivation for the FPF at the HL-LHC as an essential precedent to optimize the forward physics experiments that will enable the FCC to achieve its full physics potential.

Tanjona Rabemananjara

FPF@FCC: neutrino, QCD, and BSM physics opportunities with far-forward experiments at a 100 TeV Proton Collider

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

Neutrino trident production of dilepton pairs is well recognized as a sensitive probe of both electroweak physics and physics beyond the Standard Model. Although a rare process, it could be significantly boosted by such new physics, and it also allows the electroweak theory to be tested in a new regime. We demonstrate that the forward neutrino physics program at the Large Hadron Collider offers a promising opportunity to measure for the first time, dimuon neutrino tridents with a statistical significance exceeding <math><mn>5</mn><mi>σ</mi></math>, improving on the previous claims at the <math><mo>∼</mo><mn>3</mn><mi>σ</mi></math> level by the CHARM-II and CCFR collaborations while accounting for additional backgrounds later identified by the NuTeV collaboration. We present predictions for various proposed experiments and outline a specific experimental strategy to identify the signal and mitigate backgrounds, based on “reverse tracking” dimuon pairs in the <math><mrow><mi>FASER</mi><mi>ν</mi><mn>2</mn></mrow></math> detector. We also discuss prospects for constraining beyond Standard Model contributions to neutrino trident rates at high energies.

Wolfgang Altmannshofer

Subir Sarkar

Keping Xie

Bei Zhou

Discovering neutrino tridents at the Large Hadron Collider

Despite being mostly secluded, dark sector particles may feebly interact with photons via a small mass-dimension 4 millicharge, a mass-dimension 5 magnetic and electric dipole moment, or a mass-dimension 6 anapole moment and charge radius. If sufficiently light, the LHC may produce an intense and collimated beam of these particles in the far forward direction. We study the prospects of searching for such dark sector particles with electromagnetic form factors via their electron scattering signature in the Forward Liquid Argon Experiment (FLArE) at the Forward Physics Facility (FPF). We find that FLArE can provide new probes of sub-GeV dark particles with dipole moments and strong sensitivities for millicharged particles in the 100 MeV to 100 GeV region. This complements other search strategies using scintillation signatures or dark matter direct detection and allows for probing strongly interacting dark matter motivated by the EDGES anomaly. Along with the FORMOSA detector, this leads to a very versatile and leading experimental program in the search for millicharged particles in the FPF.

Jui-Lin Kuo

Yu-Dai Tsai

FLArE up dark sectors with EM form factors at the LHC forward physics facility

Simplified models of light new physics provide a convenient benchmark for experimental searches for new physics signatures, including dark matter (DM). However, additional detection modes can arise in less simplified and more realistic scenarios where new degrees of freedom are invoked. In this study, we introduce a nonminimal model based on a popular dark photon portal to DM where the mediator mass is obtained by interactions with the dark Higgs boson which acts as a long-lived particle. We further add to this scenario a new heavy DM species secluded from the Standard Model. In this model, which involves light and heavy particles in the dark sector, we find some new interesting phenomenological features that lead to complementary probes in intensity frontier searches for light long-lived particles, indirect detection searches for dark matter, and cosmic microwave background surveys. We also find possible nonlocal effects in the DM indirect detection searches that could significantly affect the usual detection strategies.

Krzysztof Jodłowski

Leszek Roszkowski

Indirect detection of long-lived particles in a rich dark sector with a dark vector portal

The FASER experiment at the LHC is designed to search for light, weakly-interacting particles produced in proton-proton collisions at the ATLAS interaction point that travel in the far-forward direction. The first results from a search for dark photons decaying to an electron-positron pair, using a dataset corresponding to an integrated luminosity of 27.0 <math><msup><mrow><mtext>fb</mtext></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></math> collected at centre-of-mass energy <math><msqrt><mrow><mi>s</mi></mrow></msqrt><mo>=</mo><mn>13.6</mn></math> TeV in 2022 in LHC Run 3, are presented. No events are seen in an almost background-free analysis, yielding world-leading constraints on dark photons with couplings <math><mi>ϵ</mi><mo>∼</mo><mn>2</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>5</mn></mrow></msup><mo>−</mo><mn>1</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>4</mn></mrow></msup></math> and masses <math><mo>∼</mo><mn>17</mn><mtext> MeV</mtext><mo>−</mo><mn>70</mn><mtext> MeV</mtext></math>. The analysis is also used to probe the parameter space of a massive gauge boson from a U(1)<math><msub><mrow></mrow><mrow><mi>B</mi><mo>−</mo><mi>L</mi></mrow></msub></math> model, with couplings <math><msub><mrow><mi>g</mi></mrow><mrow><mi>B</mi><mo>−</mo><mi>L</mi></mrow></msub><mo>∼</mo><mn>5</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>6</mn></mrow></msup><mo>−</mo><mn>2</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>5</mn></mrow></msup></math> and masses <math><mo>∼</mo><mn>15</mn><mtext> MeV</mtext><mo>−</mo><mn>40</mn><mtext> MeV</mtext></math> excluded for the first time.