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Josh McFayden

This is my personal webpage. Find out more below about me and my work as an experimental particle physicist at the Large Hadron Collider.

About Me

Particle Physics at CERN

I am an experimental particle physicist. Ultimately, my job is to work out why the world around us, and the universe, is the way it is. I’m currently a Royal Society University Research Fellow at the University of Sussex. The experiments I work on are the ATLAS Experiment and the FASER Experiment, both based at the Large Hadron Collider, LHC, at CERN (European Laboratory for Particle Physics) in Geneva, Switzerland.

Why particle physics?

When I was younger I always got into trouble taking things apart because I wanted to understand how they work. My research at the LHC is kind of similar - we are essentially trying to reverse-engineer the building blocks of the universe. We try to understand the universe’s smallest components, fundamental particles. What properties do they have? How do they interact with each other? How does all that give rise to the world around us? Does what we see agree with our best models of how these particles should look?! This is done by trying to recreate conditions just after the Big Bang in the laboratory.

The most exciting thing about my job is that when you’re making increasingly accurate measurements or searching for new particles, every so often you reach a point in your research where you are the first person on the planet to have ever looked at this particular thing. That’s what fundamental research is all about and that’s why I love it!

Short bio

I started my PhD on ATLAS during the extremely exciting period of first collisions delivered by the LHC. Since then I have made major contributions across ATLAS in several areas: physics analysis, Monte Carlo (MC) modelling, trigger and computing. This includes significant analysis roles leading to publications from Higgs, Top, Standard Model, B-physics, Exotics and SUSY groups with world-leading precision (measurements) or limits on the existence of new physics (searches). To further broaden my experimental particle physicist expertise to include hands-on detector development in January 2019 I joined the FASER collaboration where I am currently jointly responsible for the construction and commissioning of the scintillator and calorimeter sub-detectors.

Main research interest

My primary research interest is Higgs boson and Top quark measurements on ATLAS. The discovery of the Higgs boson in 2012 is one of the major scientific achievements of this millenium. Measuring the Higgs in more detail is now a critical priority of LHC research. I have led a team of physicists in measuring Higgs boson production in association with a top quark pair (tt̄H).

Below you can find a few details about my main areas of work, my publications, my main skills, contact details and more. Enjoy!

Outreach and Public Engagement

I enjoy communicating the work we do in particle physics and science in general with wider audiences. I also think engaging the general public in the important and broader benefit of our work is really important. Here are a few examples of my work.

Higgs Anniversary Lecture

Public lecture at University of Sussex for the 10th anniversary of the Higgs boson discovery. Recording

CERN Video

CERN Video on a measurement I led, looking for new physics. YouTube

Royal Society Science Book Prize

The Science Book Prize is one of the Royal Societies most high profile engagement activities. I was on the judging panel in 2022. Royal Society Page

FASER Experiment Twitter

I’m responsible for the Twitter account of the FASER Experiment, reaching an audience of tens of thousands of people. @FASERexperiment. I’m also active on my personal Twitter account @JoshMcFayden

Latest News

HSF Monte Carlo Tuning Workshop

HSF Monte Carlo Tuning Workshop I was one of the organisers of an online-only workshop organised through the HEP Software Foundation (HSF) on Monte Carlo (MC) Event Generator Tuning.

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ttW discrepancy remains!

Tension in ttW remains after new predictions! At the recent LHC Top Working Group meeting the latest ttW measurements were presented, along with a new state-of-the-art NNLO calculation.

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My Research

Here is an overview of my main research activities.

Most of my time is spent working on the ATLAS Experiment, where my main activities are researching the Higgs boson, measuring top quark production and improving Monte Carlo event generators.

I spend about 20% of my time working on the FASER Experiment, searching for new weakly-interacting long-lived particles. My main contributions are on the scintillator and calorimeter detectors.

I am also contributing to the longer term future of the LHC experiment programs by coordinating improvement of the next generation of event generators through the HEP Software Foundation.

My Publications

I am an author of several hundred ATLAS publications (all members of the collaboration sign all papers), two FASER publications and several publications from external collaborations.

Below are the highlights of the papers I have made major contributions to, arranged into categories. Links to the publication webpage and other information can be found on each item.

First Direct Observation of Collider Neutrinos with FASER at the LHC

First Direct Observation of Collider Neutrinos with FASER at the LHC

Phys. Rev. Lett. 131, 031801

First Direct Observation of Collider Neutrinos with FASER at the LHC.

Journal arXiv CERN press Sussex news

First Results from the Search for Dark Photons with the FASER Detector at the LHC

First Results from the Search for Dark Photons with the FASER Detector at the LHC

CERN-FASER-CONF-2023-001

New sensitivity to Dark Photons.

Public note CERN press

ttW inclusive and differential cross-section measurement

ttW inclusive and differential cross-section measurement

ATLAS-CONF-2023-019

First differential measurement of ttW production.

Documents Briefing

The Forward Physics Facility at the High-Luminosity LHC

The Forward Physics Facility at the High-Luminosity LHC

J. Phys. G: Nucl. Part. Phys. 50 030501

Physics, infrastructure, detector, simulation studies and on future directions for the FPF’s physics potential.

arXiv Journal

Forward Physics Facility Site and Physics Potential

Forward Physics Facility Site and Physics Potential

Physics Reports, Volume 968, 2022, Pages 1-50

First differential measurement of ttW production.

arXiv Journal

Test of the universality of τ and μ lepton couplings in W boson decays

Test of the universality of τ and μ lepton couplings in W boson decays

Nature Physics 17, 813–818 (2021)

Lepton Flavour Universality measurement in tt̄ events.

Journal Documents CERN video CERN Courier ATLAS Briefing CERN Seminar LBNL press CERN EP Newsletter

Challenges in Monte Carlo event generator software for HL-LHC

Challenges in Monte Carlo event generator software for HL-LHC

Comput Softw Big Sci 5, 12 (2021)

Editor and HSF Gen WG Convener.

Journal arXiv

tt̄H in the multileptons channel at 13 TeV

tt̄H in the multileptons channel at 13 TeV

ATLAS-CONF-2019-045

Latest tt̄H measurement in the multi-leptons channel and associated analysis of tt̄W background.

Documents More info

Measurement of Z+b(b) at 13 TeV

Measurement of Z+b(b) at 13 TeV

JHEP 07 (2020) 44

Measurements of the production cross-section for a Z boson in association with b-jets in proton-proton collisions at √s=13 TeV with the ATLAS detector.

Journal Documents arXiv

Early-Career Researchers views on the 2020 European Strategy Update

Early-Career Researchers views on the 2020 European Strategy Update

arXiv:2002.02837

Report of ECFA Early-Career Researchers Debate on 2020 European Strategy Update for Particle Physics

arXiv

Simplified Template Cross Section for tt̄H

Simplified Template Cross Section for tt̄H

arXiv:2003.01700

Les Houches 2019: Physics at TeV Colliders Standard Model Working Group Report.

arXiv

tt̄H in the multileptons channel at 8 TeV

tt̄H in the multileptons channel at 8 TeV

PLB 749(2015)519-541

Search for the associated production of the Higgs boson with a top quark pair in multilepton final states with the ATLAS detector.

Journal Documents arXiv

Detecting and studying high energy collider neutrinos with FASER

Detecting and studying high energy collider neutrinos with FASER

EPJC (2020)80:61

Physics prospects for FASERv.

Journal arXiv

tt̄Z and tt̄W production at 13 TeV

tt̄Z and tt̄W production at 13 TeV

ATLAS-CONF-2016-003

Measurement of the tt̄Z and tt̄W production cross sections in multilepton final states using 3.2 fb-1 of pp collisions at 13 TeV at the LHC

Documents

A Roadmap for HEP Software and Computing R&D for the 2020s

A Roadmap for HEP Software and Computing R&D for the 2020s

Comput Softw Big Sci 3, 7 (2019)

Input to MC generators section.

Journal arXiv

FASERv Technical Proposal

FASERv Technical Proposal

arXiv:2001.03073

FASERv is a proposed small and inexpensive emulsion detector designed to detect collider neutrinos for the first time and study their properties

arXiv

Standard Model Physics at the HL-LHC and HE-LHC

Standard Model Physics at the HL-LHC and HE-LHC

arXiv:1902.04070

The physics reach of the HL-LHC in the realm of strong and electroweak interactions and top quark physics.

arXiv

tt̄Z and tt̄W production at 8 TeV

tt̄Z and tt̄W production at 8 TeV

JHEP 11(2015)172

Measurement of the tt̄Z and tt̄W production cross sections in pp collisions at √s = 8 TeV with the ATLAS detector.

Journal DocumentsarXiv

tt̄Z and tt̄W production at 8 TeV

tt̄Z and tt̄W production at 8 TeV

ATLAS-CONF-2014-038

Evidence for the associated production of a vector boson (W, Z) and top quark pair in the dilepton and trilepton channels in pp collision data at √s=8 TeV collected by the ATLAS detector at the LHC

Documents

First ATLAS tt̄Z search

First ATLAS tt̄Z search

ATLAS-CONF-2012-126

Search for tt̄+Z production in the three lepton final state with 4.7 fb−1 of √s=7 TeV pp collision data.

Documents

Measurement of di-B-hadron pair production at 8 TeV

Measurement of di-B-hadron pair production at 8 TeV

JHEP 11 (2017) 062

Measurement of b-hadron pair production with the ATLAS detector in proton-proton collisions at √s=8 TeV.

Journal Documents arXiv

Measurement of Z+jets at 13 TeV

Measurement of Z+jets at 13 TeV

EPJC 77(2017)361

Measurements of the Production Cross Section of a Z boson in Association with Jets in pp collisions at √s=13 TeV with the ATLAS Detector.

Journal Documents arXiv

Is there room for improvement in the description of the gluon splitting to heavy quarks?

Is there room for improvement in the description of the gluon splitting to heavy quarks?

arXiv:1803.07977

Les Houches 2017: Physics at TeV Colliders Standard Model Working Group Report

arXiv

Search for heavy VH resonances at 13 TeV

Search for heavy VH resonances at 13 TeV

JHEP 03(2018)174

Search for heavy resonances decaying into a W or Z boson and a Higgs boson in final states with leptons and b-jets in 36 fb−1 of √s=13 TeV pp collisions with the ATLAS detector.

Journal Documents arXiv

Search for heavy VH resonances at 13 TeV (partial dataset)

Search for heavy VH resonances at 13 TeV (partial dataset)

PLB 765(2017)32-52

Search for new resonances decaying to a W or Z boson and a Higgs boson in the llbb, lνbb, and ννb channels with pp collisions at √s = 13 TeV with the ATLAS detector.

Journal Documents arXiv

All-hadronic stop pair production at 8 TeV

All-hadronic stop pair production at 8 TeV

JHEP09(2014)015

Search for direct pair production of the top squark in all-hadronic final states in proton-proton collisions at √s=8 TeV with the ATLAS detector.

Journal Documents arXiv

All-hadronic stop pair production at 7 TeV

All-hadronic stop pair production at 7 TeV

PRL 109,211802(2012)

Search for a Supersymmetric Partner to the Top Quark in Final States with Jets and Missing Transverse Momentum at √s=7 TeV.

Journal Documents arXiv

Direct sbottom pair production at 7 TeV

Direct sbottom pair production at 7 TeV

PRL 108,181802(2012)

Search for Scalar Bottom Quark Pair Production with the ATLAS Detector in pp Collisions at √s=7 TeV.

Journal Documents arXiv

Short CV

Here is a quick overview of my responsibilities, employments, education and awards.

ATLAS Top WG convener

Oct 2024 – present

Sussex News

LHC MC WG convener

Jan 2024 – present

ATLAS Sussex Group Deputy Team Leader

Nov 2022 – present

FASER2 Contact Person

Oct 2022 – present

LHC Higgs Cross Section Working Group ttH/tH Subgroup Convener

Jan 2021 – present

HEP Software Foundation Generators WG Convener

Nov 2018 – Dec 2022

ATLAS Physics Modelling Group Convener

Oct 2016 – Sep 2018

Organising Committee ECFA Early Career Researchers

Oct 2019 – Dec 2020

ATLAS Monte Carlo Production Coordinator

Oct 2014 – Jan 2016

Senior Research Fellow

Oct 2023 – present

University of Sussex

Royal Society University Research Fellow

Nov 2020 – present

University of Sussex

Project Scientist

Apr 2020 – Oct 2020

Lawrence Berkeley National Laboratory

COFUND CERN Fellow

Apr 2017 – Mar 2020

CERN

Post-doctoral Research Associate

Sep 2013 – Mar 2017

University College London

PhD

Oct 2009 - Sep 2013

University of Sheffield

Thesis

Masters of Physics with Mathematics

Sep 2005 - Jul 2009

University of Sheffield

My Student Story

Institute of Physics HEPP Group Prize

April 2021

Sussex News

Springer Thesis Prize

July 2014

Springer Thesis Prize

After obtaining my Masters degree and PhD from the University of Sheffield in the UK, I have gone on to study at University College London (UCL), the European Organization for Nuclear Research (CERN), Lawrence Berkeley National Laboratory (LBNL) and am now based at the University of Sussex.

I have been entrusted with significant ATLAS-wide leadership responsibilities and have been made Analysis Coordinator (selected to oversee all members of an analysis team) for several analyses across different physics groups: Higgs, Top , SUSY and B-physics.

I have recently held two major responsibilities outside ATLAS as HEP Software Foundation (HSF) Generators WG convener and Organising Committee member of Early Career Researchers selected by ECFA to give feedback on the European Strategy Update.

My Skills

After more than 10 years working on cutting edge scientific experiments I have accumulated a wide range of skills and competencies. A snapshot of the areas of expertise I have developed are shown below.

Big Data Analysis

Extensive experience in huge scale data consolidation, categorisation, analysis and statistical interpretation.

Machine Learning

BDT, MVA, Neural Networks, Tensor Flow, Keras.

Statistical Interpretations

Statistical interpretation of large datasets.

Version Control

Management of complicated software structures, including unit testing and containerisation using git CI/CD.

Containers

Analysis preservation and custom simulation images using Docker.

3D Modelling & Printing

FDM and SLA.

Distributed Computing

Large scale grid computing and High Performance Computing.

Databases

Database skimming with SQL.

Metadata

Designed and implemented metadata schema for categorisation, bookkeeping of over 100,000 datasets.

GPUs

Now learning to optimise MC event generators for GPUs!

Web Design

I am proficient in html, css, javascript and php.

Contact

Contact Details

The easiest way to get in contact with me is via email. I am also quite active on Twitter (see right). I’ve also left here some other places to find more information about me and my activities in case they are useful:

Public Talks

Unravelling LEP-era Lepton Flavour Universality discrepancy with ATLAS

LBNL Research Progress Meeting - 21st July 2020

Abstract: A remarkable feature of the Standard Model (SM) is that each lepton flavour (electron, muon, tau) is equally likely to interact with a W boson. This is known as Lepton Flavour Universality. In a recent ATLAS measurement, a novel technique using events with top-quark pairs has been exploited to test the ratio of the probabilities for tau leptons and muons to be produced in W boson decays, R(τ/μ). In the SM, R(τ/μ) is expected to be unity, but a longstanding tension with this prediction has existed since the LEP era, where, from a combination of experiments, R(τ/μ) was measured to be 2.7σ higher than the SM expectation. If the LEP result were confirmed to O(1%) precision it would correspond to an unambiguous discovery of beyond the SM physics. This latest ATLAS measurement obtains this level a precision - join the RPM to discover whether or not it agrees with LEP!

Top: ATLAS results from precision to rarity

CERN LHC Seminar- 9th June 2020

Abstract: The abundant production of top quark events at the LHC allows testing the standard model in unprecedented ways. The relative probability for W bosons to decay to a muon or a tau-lepton is measured, selecting events in which a top quark pair decays to two opposite-sign leptons plus jets through W bosons. The measurement confirms the universality of tau and muon couplings to the W boson with the highest precision, to date. In a second analysis, charged leptons are again used to select di- and multi-lepton final states with high jet multiplicity. The kinematic, multiplicity and flavour properties are input to a multivariate discriminant to recognise and establish evidence for the extremely rare production of four top quarks in an event.

FASER status report and the FASERnu proposal

Physics Beyond Colliders Working Group Meeting - 6th November 2019

Abstract:

Prospects for Higgs & precision SM physics at HL-LHC with the ATLAS detector

EPS-HEP 2019 - 11th July 2019

Abstract: The Large Hadron Collider (LHC) has been successfully delivering proton-proton collision data at the unprecedented center of mass energy of 13 TeV. An upgrade is planned to increase the instantaneous luminosity delivered by LHC in what is called HL-LHC, aiming to deliver a total of about 3000/fb of data to the ATLAS detector at a center of mass energy of 14 TeV. To cope with the expected data-taking conditions ATLAS is planning major upgrades of the detector.

In this contribution we present an overview of the precision physics measurement of the Standard Model, with particular focus on the electro-weak and Higgs sectors. Prospects for precision determination of Standard Model fundamental parameters, as the weak mixing angle, the Higgs couplings, di-Higgs observation, vector boson scattering processes, as well as measurement for rare Higgs decays are presented.

Such studies formed the basis of the ATLAS Collaboration input to the recent HL/HE-LHC Yellow-Report. An executive summary of this report was then submitted as input to the European Strategy process.

Measurements of V+jets with the ATLAS detector

QCD@LHC 2017 - 28th August 2017

Abstract: The production of jets in association with vector bosons is an important process to study QCD in a multi-scale environment. The ATLAS collaboration has performed measurements of vector boson+jets cross sections, differential in several kinematic variables, in proton-proton collision data taken at center-of-mass energies of 8TeV and 13TeV. The measurements are compared to state-of-the art theory predictions and can be used to constrain the proton structure.

We have also studied the jet production rates at different resolution scales. In particular, we present a measurement of the splitting scales in the kt jet-clustering algorithm for final states containing a Z-boson candidate at a centre-of-mass energy of 8 TeV. The data are corrected for detector effects and are compared to state-of-the-art Monte Carlo predictions.

Tackling gluon splitting to bb

CERN Collider Cross-talk - 15th June 2017

Abstract: Many LHC measurements and searches exploit final states that include b-tagged jets. In several such analyses the modelling of heavy flavour production can be a vital component of the background estimation and a significant source of systematic uncertainty. In many cases, e.g. VH(→bb) and ttH(→bb), the dominant modelling uncertainty comes from the theory prediction of g→bb. We make the most precise measurement of g→bb at the LHC yet, with a new technique that examines b-hadron pair production in the B→J/ψ(→μ+μ−)+X and B→μ+X decay mode. The measurement exploits the excellent angular and momentum resolution for muon reconstruction of the ATLAS detector using 11.4 fb−1 of p-p collisions recorded at √s = 8 TeV. The total cross section as well as several kinematic variables sensitive to b-hadron production modes are measured. These measurements are compared to MC predictions from widely used event generators, providing new constraints on heavy flavour production and the possibility to reduce the associated systematic uncertainties.

Tackling gluon splitting to bbbar with a measurement of b-hadron pair production in the J/ψ+μ final state at ATLAS

University of Sussex - 11th May 2017

Abstract: Many LHC measurements and searches exploit final states that include b-tagged jets. In several such analyses the modelling of heavy flavour production can be a vital component of the background estimation and a significant source of systematic uncertainty. In many cases, e.g. VH(→bb) and ttH(→bb), the dominant modelling uncertainty comes from the theory prediction of g→bb. We make the most precise measurement of g→bb at the LHC yet, with a new technique that examines b-hadron pair production in the B→J/ψ(→μ+μ−)+X and B→μ+X decay mode. The measurement exploits the excellent angular and momentum resolution for muon reconstruction of the ATLAS detector using 11.4 fb−1 of p-p collisions recorded at √s = 8 TeV. The total cross section as well as several kinematic variables sensitive to b-hadron production modes are measured. These measurements are compared to MC predictions from widely used event generators, providing new constraints on heavy flavour production and the possibility to reduce the associated systematic uncertainties.

Search for ttH and tH production (not including H→bb) at the LHC

Top 2015 - 17th September 2015