Authors: Gianluigi Arduini, CERN, Kristiane Bernhard-Novotny, CERN, Joerg Jaeckel, University of Heidelberg, Gunar Schnell, UPV/EHU & Ikerbasque Bilbao, and Claude Vallée, CPPM-Marseille

The Physics Beyond Colliders (PBC) Study was launched in 2016 to explore the opportunities offered by CERN’s unique accelerator and experimental area complex and expertise to address some of the outstanding questions in particle physics through experiments complementary to the high-energy frontier. Together with the Large Hadron Collider (LHC) experiments, the PBC proposals form a synergistic partnership, which fosters an ecosystem beyond collider-based research and diversifies CERN’s science programme at the precision and intensity frontiers.

The fifth PBC annual workshop was held from 25 to 27 March at CERN to explore new ideas and avenues aiming to answer open questions of the Standard Model and beyond, and to provide updates of ongoing projects.

The Super Proton Synchrotron (SPS) North Area (NA) is one of the major fixed-target experimental facilities available at CERN and it is at the very heart of many present and proposed explorations for Beyond the Standard Model (BSM) physics. The NA includes an underground cavern (ECN3) for experiments requiring high-energy/high-intensity proton beams. Several proposals have been made for experiments to operate in ECN3 in the next decade and beyond. All of them require higher intensity proton beams than currently available. One of these proposals studied within PBC, SHiP (Search for Hidden Particles), aiming for a comprehensive investigation of the Hidden Sector in the GeV mass range at a dedicated Beam Dump Facility (BDF) [1], has been recently approved. Together with the activities of NA64, an experiment leading the searches for light dark particles with a versatile setup suited for electron [2], positron [3], muon [4] and hadron beams [5], this will significantly strengthen CERN’s focus towards dark-sector searches.

The FASER [6] and SND [7] experiments, now taking data at the LHC and originated in the first phase of the PBC initiative, contribute to both New Physics searches and to the study of very high-energy neutrinos. The proposed Forward Physics Facility (FPF), located in the line of sight of the interaction point 1 of the High Luminosity LHC (HL-LHC) 620 m away from it, could increase sensitivity to BSM physics by a factor of about 10,000 over FASER and it could allow for the detection of  thousands of neutrinos at TeV-energies per day with the potential of contributing to the measurement of parton-distribution functions with improved precision, benefitting the HL-LHC physics reach. The experiment consists of a series of sub-detectors of relatively small size. The FPF detectors’ layout definition and the corresponding integration studies have made significant progress as one of the main PBC-supported studies in view of the publication of a document describing the facility’s technical infrastructure by mid-2024.

proANUBIS [8], CODEX-beta [9] and MATHUSLA [10] are also actively being studied and would be located at large angles to the collision line of sight at the ATLAS, LHCb and CMS experiments.

Remaining in the realm of the Standard Model, a new NA60+[11] experiment with lead ions and NA61/SHINE[12] with light ions aim to uncover the onset of the Quantum Chromo Dynamics (QCD) phase transition at energy scales only accessible at the SPS, holding promise to decode the phases of nuclear matter in the non-perturbative regime of QCD. Understanding QCD means further to unravel the emergent properties of baryons and mesons. The AMBER [13] experiment plans to determine the charge radii of kaons and pions and to perform meson spectroscopy, in particular with kaons, within a wide range of experimental activities proposed beyond the next accelerator long Lshutdown (LS3). A substantial study has been carried out to enhance the number of identifiable kaons in the hadron beam delivered to AMBER. This could be achieved by improving the vacuum conditions and by the implementation of a dedicated optics in the beamline to the experiment.

To complement results obtained at AMBER’s predecessors COMPASS, HERA, and other experiments using a polarized beam and/or target, the LHCSpin collaboration  presented their proposal [14] to open a new frontier and to introduce spin physics at the LHC with a gaseous polarised target following the successful commissioning of the SMOG2 unpolarised-gas cell [15]. This would result in a new probe for studying collective phenomena at the LHC. Moreover, this would provide access to the multi-dimensional nucleon structure in a kinematic domain of hitherto limited exploration and make use of new probes, for instance by using charm mesons.

The TWOCRYST collaboration aims to demonstrate the feasibility and the performance of a possible fixed-target experiment in the LHC to measure electric and magnetic dipole moments (EDMs and MDMs) of charmed baryons [16], offering a complementary platform for the study of Charge-Parity (CP) violation in the Standard Model. These baryons would be generated by the collision of the protons of the secondary beam halo channelled by a crystal onto a target. MDM and EDM would be determined by measuring the baryon spin precession in the strong electric field of a crystal installed immediately downstream of the target.

The conceptual design of a beamline to produce a tagged neutrino beam to improve the precision of neutrino cross-section measurements has been developed combining the ENUBET [17] and NuTag [18] proposals. This design would significantly increase the amount of tagged neutrinos generated within a given geometric acceptance and energy band.

The Gamma Factory (GF) collaboration, which aims to demonstrate the principle of the Gamma Factory in the SPS, reported the progress achieved at IJCLab (France) in the development of the laser system required for this facility. The GF scheme is based on resonant excitation of ultra-relativistic partially stripped ions (that could be made available at the SPS and LHC) with a laser beam tuned to the atomic transition frequencies, followed by the process of spontaneous emission of photons. The resonant excitation of atomic levels of highly ionised atoms (ions) is possible due to the large energies of the ions generating a Doppler frequency boost of the counter-propagating laser beam photons by a factor of up to 2g, where g is the relativistic factor. Spontaneously-emitted photons produced in the direction of the ion beam, when seen in the laboratory frame, have their energy boosted by a further factor of 2g. As a consequence, the process of absorption and emission results in a frequency boost of the incoming photon of up to 4g ². In the GF scheme, the SPS (LHC) atomic beams play the role of photon “frequency converters” of eV-photons into keV (MeV) X-rays (γ-rays). These intense and quasi-monochromatic beams could be used in a variety of atomic, nuclear and particle physics experiments [19] and they could potentially find application to energy production or nuclear-waste transmutation as well as the generation of intense positron and muon beams for future accelerator facilities.

High quality factor superconducting radio-frequency cavities, similar to those used for the acceleration of charged particles in accelerators, can also be used to detect axions (hypothetical particles that might be able to explain both the strong CP violation problem and account for dark matter) and even gravitational waves, and they can also be of interest for developing multi-qubit systems. The design and fabrication of a superconducting cavity for the heterodyne detection of axion-like particles over a wide range of masses [20] is the subject of a joint project between PBC and the CERN Quantum Technology Initiative. Atom Interferometry is another subject of common interest between the two CERN initiatives and PBC has demonstrated the technical feasibility of installing an atom interferometer with a baseline of 100 m in one of the LHC access shafts [21].

The charged-particle EDM collaboration presented the status of their approach to build a prototype ring that would validate the main concepts of a ring required to perform the first direct measurement of a proton EDM [22] and evaluate the sensitivity reach of such measurement.

The proposed injectors of the Future Circular electron-positron Collider (FCC-ee) [23] will significantly expand the variety of the offer of the CERN accelerator complex in terms of beam types and parameters, potentially opening up the possibility of new experiments. New ideas have been also presented, ranging from the measurement of molecular EDMs at the ISOLDE (Isotope Separator On Line DEvice) Radioactive Ion Beam Facility, over the prospects for antiproton physics at the Antiproton Decelerator (AD) and the Extra Low ENergy Antiproton (ELENA) ring, to the measurement of the gravitational effect of the LHC beam.

With these highlights in stock, many fruitful discussions, the annual workshop concluded as a resounding success. The PBC community thanked Claude Vallée (CPPM, Marseille), who retired as PBC co-coordinator and co-founder of the PBC initiative, after almost a decade of integral work, and welcomed Gunar Schnell (UPV/EHU & Ikerbasque, Bilbao) who will take on this role.

A small part of the community who contributes with lively discussions and innovative proposals and projects to the success of PBC.
Credit: K. Bernhard-Novotny (CERN)

[1] SHiP Collaboration, BDF/SHiP at the ECN3 high-intensity beam facility, CERN-SPSC-2022-032 ; SPSC-I-258

[2] Yu. M. Adreev et al. , Search for Light Dark Matter with NA64 at CERN,     Phys.Rev.Lett. 131 (2023) 16, 161801

[3] Yu. M. Adreev et al. , Probing light dark matter with positron beams at NA64,     Phys.Rev.D 109 (2024) 3, L031103

[4] Yu. M. Adreev et al. , Exploration of the Muon g−2 and Light Dark Matter explanations in NA64 with the CERN SPS high energy muon beam, arxiv:2401.01708 ; accepted by PRL

[5] S. Gninenko et al., Test of vector portal with dark fermions in the charge-exchange reactions in the NA64 experiment at CERN SPS, arxiv:2312.01703

[6] H. Abreu et al., First Direct Observation of Collider Neutrinos with FASER at the LHC, Phys.Rev.Lett. 131 (2023) 3, 031801

[7] R Albanese et al., Observation of Collider Muon Neutrinos with the SND@LHC Experiment, Phys.Rev.Lett. 131 (2023) 3, 031802

[8] A Shah et al., Searches for long-lived particles with the ANUBIS experiment, PoS EPS-HEP2023 (2024) 051 / A Shah et al., Installation of proANUBIS – a proof-of-concept demonstrator for the ANUBIS experiment, PoS LHCP2023 (2024) 168

[9] C Aielli et al., The Road Ahead for CODEX-b, arXiv:203.07316

[10] C Alpigani et al., An Update to the Letter of Intent for MATHUSLA: Search for Long-Lived Particles at the HL-LHC, arXiv:2009.01693

[11] NA60+ Collaboration, Letter of Intent: the NA60+ experiment, CERN-SPSC-2022-036; SPSC-I-259, Geneva, 2022, https://cds.cern.ch/record/2845241

[12] NA61/SHINE Collaboration, Addendum to the NA61/SHINE Proposal: A Low-Energy Beamline at the SPS H2, CERN-SPSC-2021-028 / SPSC-P-330-ADD-12, Geneva 2021, https://cds.cern.ch/record/2783037/files/SPSC-P-330-ADD-12.pdf

[13] C Quintas et al., The New AMBER Experiment at the CERN SPS, Few Body Syst. 63 (2022) 4, 72

[14] P. Di Nezza et al., The LHCspin Project, Acta Phys.Polon.Supp. 16 (2023) 7, 7-A4

[15] C. Boscolo Meneguolo, et al., Study of beam-gas interactions at the LHC for the Physics Beyond Colliders fixed-target study, JACoW proceedings (2019)

[16] S. Aiola et al., Progress towards the first measurement of charm baryon dipole

moments, Phys. Rev. D 103, 072003 (2021).

[17] F Acerbi et al., Design and performance of the ENUBET monitored neutrino beam, Eur.Phys.J.C 83 (2023) 10, 964

[18] A Baratto-Roldan et al., NuTag: proof-of-concept study for a long-baseline neutrino beam, arXiv:2401.17068

[19] D. Budker, M. Gorchtein, M. W. Krasny, A. Pálffy, A. Surzhykov (editors), Physics Opportunities with the Gamma Factory, Annalen der Physik, Volume 534, Issue 3 (2022)

[20] A Berlin et al., Heterodyne Broadband Detection of Axion Dark Matter, Phys. Rev. D 104, L111701

[21] G. Arduini et al., A Long-Baseline Atom Interferometer at CERN: Conceptual Feasibility Study, arXiv:2304.00614", CERN-PBC-REPORT-2023-002, Geneva, 2023, https://cds.cern.ch/record/2851946

[22] F. Abusaif, et al., Storage ring to search for electric dipole moments of charged particles: Feasibility study, CERN Yellow Reports: Monographs, CERN-2021-003, Geneva, 2021, https://cds.cern.ch/record/2654645, doi=10.23731/CYRM-2021-003

[23] M. Benedikt et al. (editors), Future Circular Collider Study. Volume 2: The Lepton Collider (FCC-ee) Conceptual Design Report, CERN-ACC-2018-0057, Geneva, December 2018. Published in Eur. Phys. J. ST.

Author: Kees van der Beek

Sara Bolognesi: Laureate of the Summer 2021 EPS Emmy Noether Distinction

Kees van der Beek, chair of the EPS Equal Opportunities Committee, spoke to Sara Bolognesi of CEA-IRFU in Saclay, France, laureate of the Summer 2021 EPS Emmy Noether Distinction on her work, her interactions with other communities, research funding, reconciling work and family life, and mentoring of young physicists.

Kees van der Beek (KvdB): My very warmest congratulations with the Summer 2021 Emmy Noether Distinction for your contributions to, and, indeed, leading role in the CMS and T2K experiments! Can you explain what your current scientific interests are, why your experiments are important, and what the stakes are?

Sara Bolognesi (SB): My present scientific interest is in neutrino oscillations. Neutrinos are very interesting particles, but very difficult to study! This is because they are hard to produce, and once you produced them, they are hard to detect, because of their extremely weak interaction with matter. Therefore, very large amounts of neutrinos must be produced for any given experiment, and huge detectors are needed to obtain the necessary sensibility to pronounce oneself on physical effects related to them. However, building such huge instruments is well worth it, since neutrino physics is one of the most promising avenues to push our understanding of fundamental physics beyond our present interpretation, the Standard Model. The T2K (Tokai to Kamioka) experiment seeks to quantify neutrino oscillations (evolution of one neutrino type into another) through measurement of the so-called mixing parameters. This can, given sufficient sensitivity, unveil the symmetries in the neutrino mass ordering and flavour mixing, and most importantly, a possible violation of charge-parity (CP). This would be a crucial discovery, while CP-violation has been measured in quark sector, this would be a new fundamental source of CP-violation and the first in the lepton sector. We have, so far, made significant steps towards a measurement of possible violation of CP symmetry in neutrino physics, but experiments have to be made more sensitive – which is my aim and that of my team. Remarkably, since the collisions of neutrinos with the detector material involve their complex, many-body interaction with the multiplicity of particles composing the target nuclei, reaching the required accuracy requires an adequate comprehension of the nuclear physics involved. This is true for both the accurate characterisation of the emitted neutrino flux, as for the understanding of the scattering cross-sections in the remote detector. What I love about my work is the fact that it therefore involves many different communities – every day, I learn something new!

KvdB: Is the search for new physics the reason why you made a spectacular move from Higgs physics in the framework of the CMS collaboration to neutrino physics, and this, right after the discovery of the Higgs, when results were ready for the reaping? How did you decide this shift?

SB: Indeed, after the discovery of the Higgs, the entire team was extremely excited. However, in spite of the Higgs having been discovered, there are many questions to which the standard model cannot provide answers. In particular, it cannot possibly be valid to arbitrary high-energy scales, so there must be something beyond. An illuminating overview presented by Hiroshi Murayama from Berkeley at a Higgs workshop in 2013 made it very clear to me that neutrinos are an extremely promising window to such very high-energy scales. In particular, the standard model cannot explain why neutrinos have mass, nor why they oscillate the way they do. Both these phenomena determine the numerical values of a great many parameters, so understanding them would be a particularly important step into our further comprehension of nature, and, in particular, the existence of as-yet hidden symmetries. Practically, I was greatly helped by the job opportunity formulated by CEA-IRFU, that did not only propose a permanent position, but did not require previous experience in the field of neutrino physics – indeed, they were very open to candidates form other fields. This allowed me to settle and establish myself both as a scientist and in my personal life. As a particle physicist, the learning curve in neutrino physics was steep, but I feel I was truly helped both in my institute and by the welcoming attitude of the neutrino community.

KvdB:What are the most satisfying – and more difficult parts of your work?

SB: I love the interaction between many communities and between experimentalists and theorists that characterizes neutrino physics. The most difficult part of my position is securing the necessary financial resources – we are not trained for that as physicists! Here again, I see the need to go out and obtain funding as an opportunity to learn, even if this part of the job takes up more and more of our time. We, as physicists, should accept the manner the world we live in functions. We must, before publicizing our work in physics and asking for funding, stop and really ask ourselves whether what we project to do is truly worth of funding. To have to reflect on this and then explain to non-experts why society should fund physics is an important and necessary part of our job. For me, frustration arises when decisions are made based on political priorities rather than scientific arguments. While we need a realistic compromise due to the boundary conditions posed by the world we live in, our primary goal should always be driven by physics arguments.

More fundamentally, there are better ways in which a funding process could work. Notably, the very nature of fundamental physics research requires, at the least, medium-term funding based on a vision and multi-year strategy submitted by the team, lab, institute, or collaboration submitting the request, and not the calls for short-term, individualistic projects that we see all too often today. At the same time, I’m very worried by the inertia that comes with increasing size of the collaborations and cost of the experiments. This not only slows their development but also makes it very difficult to react and adapt the overall strategy to physics evidence when new results are obtained.

I, obviously, do not hold the perfect recipe but our compass should always point to the long-term objective of advancing physics, no matter how difficult this could be from a political or funding point of view.

KvdB: You are obviously very passionate about physics, and that since a very young age. Where did you get this passion, and how did you choose physics?

SB: (laughs) You will be surprised to know that at the outset, I first started on a literary, and not on a scientific path in my secondary school studies!  It was my professor of philosophy in secondary school who suggested that we read simple texts on modern physics to open our mind. These were simple texts that addressed issues such as particle-wave duality, the nature of light, matter, and their interactions, that had a very large impact on me. I realised that this touched on something so fundamental for the understanding of our world that I could not accept to ignore it: I wanted to learn more about it! My subsequent enrolment in the physics programme at the university of Torino has lead to two life-changing experiences. The first was my participation in the CMS-Torino group as of my third year of studies, a group with several women in leadership positions.  All had a rich social and family life, as well as being highly successful physicists, which allowed me to project myself in my own possible future. The second was my work at CERN, in a truly multicultural environment. This was, to me, as much as a scientific experience, a truly human experience that made me decide that this is what I wanted for the rest of my life. In the neutrino community, which involves close collaboration between physicists from Europe, Japan, and the Americas, I find this multicultural, tolerant, and very human ambiance once again.

KvdB: Did you ever have problems reconciling your work and your family?

SB: There have been some difficult moments, but, honestly, I am working in an environment and for an employer that is extremely respectful of the balance between work and one’s private life, to the point where the balance we can achieve here is envied by our foreign collaborators. For instance, when my partner and I adopted our children, my professional environment was extremely respectful of our choice and very helpful when I returned to the laboratory. I cannot help but think that this is related to the fact that the head of my laboratory, the head of the IRFU Institute, and the head of our CEA Direction are all women. A difficult moment was the advent of the COVID-19 pandemic and the first lockdown - even if I realise that the situation was much harder for so many others. Where I had, over two years, established a good work-family life balance, this was now, all of a sudden, overturned. Here I was working from home, with three children by my side, and required to school them! The real problem here is not, in my opinion, one of gender, but that of attaining equilibrium between family life and professional life in general, whatever the family’s composition. I am very fortunate in that my husband fully participates in family tasks, including during the COVID-19 period; having a family that supports me in my professional challenges is very important for me.

KvdB: You have had many role models in Torino. Do you consider yourself to be a role model now?

SB: I hope I am! All the more so since, in my group today, there are nearly as many women as men. We do discuss gender issues as well as family issues, especially with younger women. I tell them that their life choice is, of course, theirs. However, they should never make this choice based on fear. Being afraid that one cannot be a woman and a physicist at the same time, of “not being able to”, must never be a criterion for choosing work over one’s private life or vice versa. Taking responsibility for one’s choice however comes with effort, the effort to make it work, and the effort to find one’s correct personal balance. The message I wish to convey is: if you want a career in physics, go for it, if you love physics, you will manage!

Kees van der Beek (KvdB): You are in a position of ever increasing responsibilities. Do you have ideas on how an academic, scientific environment can help empower women active in its midst?

Sara Bolognesi (SB): That’s a tough question! There are no easy solutions to this. Nevertheless, I think two things can help. The first, and most effective in my opinion, is tutoring, through examples. When one meets a young woman in doubt about her career choice, having a role model with whom she can interact or a tutor that serves as an example and build her self-confidence can really help. At T2K we also have a Diversity group that reaches out to young women in this sense. The second, and more general point is that we all, women and men, should make an effort to make our professional environment less aggressive. Even though academic discussion can be passionate, we should always be careful to respect the other, and not try to, for example, undermine the other’s self-confidence. Speak out, discuss, argue, with passion and conviction, but do so as if you were speaking to a close family member, your daughter or son, with respect and understanding. Science is an environment for discussion, where no one holds the absolute truth.

Sara Bolognesi acting on the valves of the gas system of the near detector (ND280) of T2K - image credit: Sara Bolognesi

The High Energy and Particle Physics Division of the EPS is happy to announce the 2021 EPS HEPP prizes.

The 2021 EPS High Energy and Particle Physics prize is awarded to Torbjörn Sjöstrand and Bryan Webber for the conception, development and realisation of parton shower Monte Carlo simulations, yielding an accurate description of particle collisions in terms of quantum chromodynamics and electroweak interactions, and thereby enabling the experimental validation of the Standard Model, particle discoveries and searches for new physics.

The 2021 Giuseppe and Vanna Cocconi Prize is awarded to the Borexino Collaboration for their ground-breaking observation of solar neutrinos from the pp chain and CNO cycle that provided unique and comprehensive tests of the Sun as a nuclear fusion engine.

The 2021 Gribov Medal is awarded to Bernhard Mistlberger for his groundbreaking contributions to multi-loop computations in QCD and to high-precision predictions of Higgs and vector boson production at hadron colliders.

The 2021 Young Experimental Physicist Prize of the High Energy and Particle Physics Division of the EPS is awarded to Nathan Jurik for his outstanding contributions to the LHCb experiment, including the discovery of pentaquarks, and the measurements of CP violation and mixing in the B and D meson systems; and to Ben Nachman for exceptional contributions to the study of QCD jets as a probe of QCD dynamics and as a tool for new physics searches, his innovative application of machine learning for characterising jets, and the development of novel strategies on jet reconstruction and calibration at the ATLAS experiment.

The 2021 Outreach Prize of the High Energy and Particle Physics Division of the EPS is awarded to Uta Bilow and Kenneth Cecire for the long-term coordination and major expansion of the International Particle Physics Master Classes to include a range of modern methods and exercises, and connecting scientists from all the major LHC and Fermilab experiments to school pupils across the world; and to Sascha Mehlhase for the design and creation of the ATLAS detector and other interlocking-brick models, creating an international outreach program that reaches to an unusually young audience.

All prizes will be awarded in a ceremony on July 26, 2021 during the virtual EPS-HEP 2021 conference: https://www.eps-hep2021.eu/