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First collider neutrinos detected

Electron neutrino charged-current interaction

Since their discovery 67 years ago, neutrinos from a range of sources – solar, atmospheric, reactor, geological, accelerator and astrophysical – have provided ever more powerful probes of nature. Although neutrinos are also produced abundantly in colliders, until now no neutrinos produced in such a way had been detected, their presence inferred instead via missing energy and momentum. 

A new LHC experiment called FASER, which entered operations at the start of Run 3 last year, has changed this picture with the first observation of collider neutrinos. Announcing the result on 19 March at the Rencontres de Moriond, and in a paper submitted to Physical Review Letters on 24 March, the FASER collaboration reconstructed 153 candidate muon neutrino and antineutrino interactions in its spectrometer with a significance of 16 standard deviations above the background-only hypothesis. Being consistent with the characteristics expected from neutrino interactions in terms of secondary-particle production and spatial distribution, the results imply the observation of both neutrinos and antineutrinos with an incident neutrino energy significantly above 200 GeV. In addition, an ongoing analysis of data from an emulsion/tungsten subdetector called FASERν revealed a first electron–neutrino interaction candidate (see image). 

“FASER has directly observed the interactions of neutrinos produced at a collider for the first time,” explains co-spokesperson Jamie Boyd of CERN. “This result shows the detector worked perfectly in 2022 and opens the door for many important future studies with high-energy neutrinos at the LHC.” 

The extreme luminosity of proton–proton collisions at the LHC produces a large neutrino flux in the forward direction, with energies leading to cross-sections high enough for neutrinos to be detected using a compact apparatus. FASER is one of two new forward experiments situated at either side of LHC Point 1 to detect neutrinos produced in proton–proton collisions in ATLAS. The other, SND@LHC, also reported its first results at Moriond. The team found eight muon–neutrino candidate events against an expected background of 0.2, with an evaluation of systematic uncertainties ongoing. 

Covering energies between a few hundred GeV and several TeV, FASER and SND@LHC narrow the gap between fixed-target and astrophysical neutrinos. One of the unexplored physics topics to which they will contribute is the study of high-energy neutrinos from astrophysical sources. Since the production mechanism and energy of neutrinos at the LHC is similar to that of very-high-energy neutrinos from cosmic-ray collisions with the atmosphere, FASER and SND@LHC can be used to precisely estimate this background. Another application is to measure and compare the production rate of all three types of neutrinos, providing an important test of the Standard Model.

Beyond neutrinos, the two experiments open new searches for feebly interacting particles and other new physics. In a separate analysis, FASER presented first results from a search for dark photons decaying to an electron-positron pair. No events were seen in an almost background-free analysis, yielding new constraints on dark photons with couplings of 10–5 to 10–4 and masses of between 10 and 100 MeV, in a region of parameter space motivated by dark matter. 

New insights into CP violation via penguin decays

LHCb figure 1

At the recent Moriond Electroweak conference, the LHCb collaboration presented a new, high-precision measurement of charge–parity (CP) violation using a large sample of B0s→ ϕϕ decays, where the ϕ mesons are reconstructed in the K+K final state. Proceeding via a loop transition (b → sss, such “penguin” decays are highly sensitive to possible contributions from unknown particles and therefore provide excellent probes for new sources of CP violation. To date, the only known source of CP violation, which is governed by the Cabibbo–Kobayashi–Maskawa matrix in the quark sector, is insufficient to account for the huge excess of matter over antimatter in the universe; extra sources of CP violation are required.

A B0s or B0s meson can change its flavour and oscillate into its antiparticle at a frequency Δms/2π, which has been precisely determined by the LHCb experiment. Thus a B0s meson can decay either directly to the ϕϕ state or via changing its flavour to the B0s state. The phase difference between the two interfering amplitudes changes sign under CP transformations, denoted ϕs for B0s or –ϕs for B0s decays. A time-dependent CP asymmetry can arise if the phase difference ϕs is nonzero. The asymmetry between the decay rates of initial B0s and B0s mesons to the ϕϕ state as a function of the decay time follows a sine wave with amplitude sin(ϕs) and frequency Δms/2π. In the Standard Model (SM) the phase difference is predicted to be consistent with zero, ϕSMs  = 0.00 ± 0.02 rad.

This is the most precise single measurement to date

The observed asymmetry as a function of the B0sϕϕ decay time and the projection of the best fit are shown in figure 1 for the Run 2 data sample. The measured asymmetry is diluted by the finite decay-time resolution and the nonzero flavour mis-identification rate of the initial B0s or B0s state, and averaged over two types of linear polarisation states of the ϕϕ system that have CP asymmetries with opposite signs. Taking these effects into account, LHCb measured the CP-violating phase using the full Run 2 data sample. The result, when combined with the Run 1 measurement, is ϕs = –0.074 ± 0.069 rad, which agrees with the SM prediction and improves significantly upon the previous LHCb measurement. In addition to the increased data sample size, the new analysis benefits from improvements in the algorithms for vertex reconstruction and determination of the initial flavour of the B0s or B0s mesons.

This is the most precise single measurement to date of time-dependent CP asymmetry in any b → s transition. With no evidence for CP violation, the result can be used to derive stringent constraints on the parameter space of physics beyond the SM. Looking to the future, the upgraded LHCb experiment and a planned future phase II upgrade will offer unique opportunities to further explore new-physics effects in b → s decays, which could potentially provide insights into the fundamental origin of the puzzling matter–antimatter asymmetry.

Beauty quark production versus particle multiplicity

ALICE figure 1

Measurements of the production of hadrons containing heavy quarks (i.e. charm or beauty) in proton–proton (pp) collisions provide an important test of the accuracy of perturbative quantum chromodynamics (pQCD) calculations. The production of heavy quarks occurs in initial hard scatterings of quarks and gluons, whereas the production of light quarks in the underlying event is dominated by soft processes. Thus, measuring heavy-quark hadron production as a function of the charged-particle multiplicity provides insights into the interplay between soft and hard mechanisms of particle production.

Measurements in high-multiplicity pp collisions have shown features that resemble those associated with the formation of quark–gluon plasma in heavy-ion collisions, such as the enhancement of the production of particles with strangeness content and the modification of the baryon-to-meson production ratio as a function of transverse momentum (pT). These effects can be explained by two different types of models: statistical hadronisation models, which evaluate the population of hadron states according to statistical weights governed by the masses of the hadrons and a universal temperature, or models that include hadronisation via coalescence (or recombination) of quarks and gluons which are close in phase space. Both predict an enhancement of the baryon-to-meson and strange-to-non-strange hadron ratios as a function of charged-particle multiplicity.

In the charm sector, the ALICE collaboration has recently observed a multiplicity dependence of the pT-differential Λc+/D0 ratio, smoothly evolving from pp to lead–lead collisions, while no dependence was observed for the Ds+-meson production yield compared to the one of the D0 meson. Measurements of these phenomena in the beauty sector are needed to shed further light on the hadronisation mechanism.

To investigate beauty-quark production as a function of multiplicity and to put it in relation with that of charm quarks, ALICE measured for the first time the fraction of D0 and D+ originating from beauty-hadron decays (denoted as non-prompt) as a function of transverse momentum and charged-particle multiplicity in pp collisions at 13 TeV, using the Run 2 dataset. The measurement exploits different decay-vertex topologies of prompt and non-prompt D mesons with machine-learning classification techniques. The fractions of non-prompt D mesons were observed to somewhat increase with pT from about 5 to 10%, as expected by pQCD calculations (figure 1). Similar fractions were measured in different charged-particle multiplicity intervals, suggesting either no or only mild multiplicity dependence. This suggests a similar production mechanism of charm and beauty quarks as a function of multiplicity.

The possible influence of the hadronisation mechanism was investigated by comparing the measured D-meson non-prompt fractions with predictions based on Monte Carlo generators such as PYTHIA 8. A good agreement was observed with different PYTHIA tunes, with and without the inclusion of the colour-reconnection mechanism beyond the leading colour approximation (CR-BLC), which was introduced to describe the production of charm baryons in pp collisions. Only the CR-BLC “Mode 3” tune that predicts an increase (decrease) of hadronisation in baryons for beauty (charm) quarks at high multiplicity is disfavoured by the current data.

The measurements of non-prompt D0 and D+ mesons represent an important test of production and hadronisation models in the charm and beauty sectors, and pave the way for future measurements of exclusive reconstructed beauty hadrons in pp collisions as a function of charged-particle multiplicity.

Searching for electroweak SUSY: a combined effort

CMS figure 1.

The CMS collaboration has been relentlessly searching for physics beyond the Standard Model (SM) since the start of the LHC. One of the most appealing new theories is supersymmetry or SUSY – a novel fermion-boson symmetry that gives rise to new particles, “naturally” leads to a Higgs boson almost as light as the W and Z bosons, and provides candidate particles for dark matter (DM).

By the end of LHC Run 2, in 2018, CMS had accumulated a high-quality data sample of proton–proton (pp) collisions at an energy of 13 TeV, corresponding to an integrated luminosity of 137 fb–1. With such a large data set, it was possible to search for the production of strongly interacting SUSY particles, i.e. the partners of gluons (gluinos) and quarks (squarks), as well as for SUSY partners of the W and Z bosons (electroweakinos: winos and binos), of the Higgs boson (higgsinos), and of the leptons (sleptons). The cross sections for the direct production of SUSY electroweak particles are several orders of magnitude lower than those for gluino and squark pair production. However, if the partners of gluons and quarks are heavier than a few TeV, it could be that the SUSY electro­weak sector is the only one accessible at the LHC. In the minimal SUSY extension of the SM, electroweakinos and higgsinos mix to form six mass eigenstates: two charged (charginos) and four neutral (neutralinos). The lightest neutralino is often considered to be the lightest SUSY particle (LSP) and a DM candidate.

CMS has recently reported results, based on the full Run 2 dataset, from searches for the electroweak production of sleptons, charginos and neutralinos. Decays of these particles to the LSP are expected to produce leptons, or Z, W and Higgs bosons. The Z and W bosons subsequently decay to leptons or quarks, while the Higgs boson primarily decays to b quarks. All final states have been explored with complementary channels to enhance the sensitivity to a wide range of electroweak SUSY mass hypotheses. These cover very compressed mass spectra, where the mass difference between the LSP and its parent particles is small (leading to low-momentum particles in the final state) as well as uncompressed scenarios that would instead produce highly boosted Z, W and Higgs bosons. None of the searches showed event counts that significantly deviate from the SM predictions.

CMS maximised the output of the Run 2 dataset, providing its legacy reference on electroweak SUSY searches

The next step was to statistically combine the results of mutually exclusive search channels to set the strongest possible constraints with the Run 2 dataset and interpret the results of searches in different final states under unique SUSY-model hypotheses. For the first time, fully leptonic, semi-leptonic and fully hadronic final states from six different CMS searches were combined to explore models that differ depending on whether the next-to-lightest supersymmetric partner (NLSP) is “wino-like” or “higgsino-like”, as shown in the left and right panels of figure 1, respectively. The former are now excluded up to NLSP masses of 875 GeV, extending the constraints obtained from individual searches by up to 100 GeV, while the latter are excluded up to NLSP masses of 810 GeV.

With this effort, CMS maximised the output of the Run 2 dataset, providing its legacy reference on electroweak SUSY searches. While the same data are still being used to search for new physics in yet uncovered corners of the accessible phase-space, CMS is planning to extend its reach in the upcoming years, profiting from the extension of the data set collected during LHC Run 3 at an unprecedented centre-of-mass energy of 13.6 TeV.

Euclid to link the largest and smallest scales

Euclid payload module

Untangling the evolution of the universe, in particular the nature of dark energy and dark matter, is a central challenge of modern physics. An ambitious new mission from the European Space Agency (ESA) called Euclid is preparing to investigate the expansion history of the universe and the growth of cosmic structures over the last 10 billion years, covering the entire period over which dark energy is thought to have played a significant role in the accelerating expansion. The 2 tonne, 4.5 m tall and 3.1 m diameter probe is undergoing final tests in Cannes, France, after which it will be shipped to Cape Canaveral in Florida and inserted into the faring of a SpaceX Falcon 9 rocket, with launch scheduled for July. 

Let there be light

Euclid, which was selected by ESA for implementation in 2012 with a budget of about €600 million, has four main objectives. The first is to investigate whether dark energy is real, or whether the apparent acceleration of the universe is caused by a breakdown of general relativity on the largest scales. Second, if dark energy is real, Euclid will investigate whether it is a constant energy spread across space or a new force of nature that evolves with the expansion of the universe. A third objective is to investigate the nature of dark matter, the mass of neutrinos and whether there exist other, so-far undetected fast-moving particle species, and a fourth is to investigate statistics and properties of the early universe that seeded large-scale structures. To meet these goals, the six-year Euclid mission will use a three-mirror system to direct light from up to a billion galaxies across more than a third of the sky towards a visual imager for photometry and a near-infrared spectrophotometer.

So far, the best constraints on the geometry and expansion history of the universe come from cosmic-microwave background (CMB) surveys. Yet these missions are not the best tracers of the curvature, neutrino masses and expansion history, nor for identifying possible exotic subcomponents of dark matter. For this, large surveys on galaxy clustering are required. Euclid will use three methods to achieve this. The first is redshift-space distortions, which combines how fast galaxies move away from us due to the expansion of the universe and how fast galaxies move towards a region of strong gravitational pull in our line-of-sight; measuring these deformations in galactic positions enables the growth rate of structures as well as gravity to be investigated. The second is baryonic acoustic oscillations (BAOs), which arose when the universe was a plasma made from baryons and photons and set a characteristic scale that is related to the sound horizon at recombination. After recombination, photons decoupled from visible matter while baryons were pulled in by gravity and started to form bigger structures, with the BAO scale imprinted in galaxy distributions. BAOs thus serve as a ruler to trace the expansion rate of the universe. The third method, weak gravitational lensing, occurs when light from a background source is bent around a massive foreground object such as a galaxy cluster, from which the distribution of dark matter can be inferred. 

As the breadth and precision of cosmological measurements increase, so do the links with particle physics. CERN and the Euclid Consortium (which consists of more than 2000 scientists from 300 institutes in 13 European countries, the US, Canada and Japan) signed a memorandum of understanding in 2016 after Euclid gained CERN recognised-experiment status in 2015. The collaboration was motivated by technical synergies for the mission’s Science Ground Segment (SGS), which will process about 850 Gbit of compressed data per day – the largest of any ESA mission to date. CERN is contributing with the provision of critical software tools and related support activities, explains CERN aerospace and environmental applications coordinator Enrico Chesta: “CernVM–FS, developed by the EP-SFT team to assist high-energy physics collaborations to deploy software on the distributed computing infrastructure used to run data-processing applications, has been integrated into Euclid SGS and will be used for software continuous deployment among the nine Euclid science data centres.” 

Competitive survey

Euclid’s main scientific objectives also align closely with CERN’s physics challenges. A 2019 CERN-TH/Euclid workshop identified overlapping areas of interest and options for scientific visitor programmes, with topics of potential interest including N-body CMB simulations, redshift space distortions with relativistic effects, model selection of modified gravity, and dark-energy and neutrino-mass estimation from cosmic voids. Over the coming years, Euclid will provide researchers with data against which they can test different cosmological models. “Galaxy surveys have been happening for decades and have grown in scale, but we didn’t hear much about it because the CMB was, until now, more accurate,” says theorist Marko Simonović of CERN. “With Euclid there will be a competitive survey that is big enough to be comparable to CMB data. It is exciting to see what Euclid, and other new missions such as DESI, will tell us about cosmology. And maybe we will even discover something new.”

Higgs Hunting 2023

The origin of Electroweak symmetry breaking is one of the central topics of research in fundamental physics. The discovery of a Higgs boson at CERN on July 4th, 2012, following a hunt that spanned several decades and multiple colliders, changed the landscape of these investigations and provided key evidence for the Brout-Englert-Higgs mechanism of mass generation through the spontaneous breaking of Electroweak symmetry.

Almost ten years later, the hunt goes on several fronts, in particular for:

  • New physics through precision studies of the properties of the Higgs boson : in particular its mass, spin and couplings to other Standard Model particles.
  • New production and decay modes, in particular in processes involving multiple Higgs bosons which provide key insight into the shape of the Higgs potential.
  • New Higgs-like states and signals for physics beyond the Standard Model.

The 13th workshop of the « Higgs Hunting » series organized on September 11-13th, 2023 will present an overview of these topics, focusing in particular on new developments in the LHC Run-2 analyses, detailed studies of Higgs boson properties and possible deviations from Standard Model predictions. Highlights will also include a first look at LHC Run-3 analyses, prospects from studies at future colliders, and recent theoretical developments.

The workshop will be held in person in Orsay and Paris to continue the Higgs Hunting tradition of lively discussions during and after the sessions. Remote participation will also be made possible for those unable to attend in person. No registration fees are asked for remote participation..

TAUP 2023

The International Conference on Topics in Astroparticle and Underground Physics is a biennial Conference started in 1989. The purpose of TAUP2023 is to bring together theorists and experimentalists working in Astroparticle Physics, to review and discuss the status and prospects of the field.

The main topics of the TAUP2023 conference are:

  • Cosmology and Particle Physics
  • Dark matter and its detection
  • Neutrino physics and astrophysics
  • Gravitational waves
  • High-Energy astrophysics and cosmic rays

Lattice 2023

40th International Symposium on Lattice Field Theory

The International Symposium on Lattice Field Theory is an annual conference that attracts scientists from around the world. Originally started as a place for physicists to discuss their recent developments in lattice gauge theory, nowadays the conference is the largest of its type and has grown to include areas like algorithms and machine architectures, code development, chiral symmetry, physics beyond the standard model, and strongly interacting phenomena in low-dimensions.

The scientific program of this conference will include plenary talks and parallel sessions on the following topics:

  • Algorithms and Artificial Intelligence
  • Hadronic and Nuclear Spectrum and Interactions
  • Particle Physics Beyond the Standard Model
  • QCD at Non-zero Density
  • QCD at Non-zero Temperature
  • Quantum Computing and Quantum Information
  • Quark and Lepton Flavor Physics
  • Software Development and Machines
  • Standard Model Parameters
  • Structure of Hadrons and Nuclei
  • Tests of Fundamental Symmetries
  • Theoretical Developments
  • Vacuum Structure and Confinement

CHARM 2023

The international CHARM 2023 conference will be held in Siegen, Germany from July 17 to July 21, 2023, hosted by the University of Siegen as an in-person event with in-person presentations only.

The purpose of the CHARM 2023 Workshop is to bring together theorists and experimentalists working in charm physics to discuss recent results in this area, including the impact on and from theory as well as projections for results to be expected from upcoming experimental facilities.

This year’s conference will cover the following topics:

  • Charm facilities – Status and future
  • Charmed meson and baryon spectroscopy
  • Exotic hadrons
  • Production of charm and charmonia
  • Hidden and open charm in media
  • Light hadronic spectroscopy from decays of charm and charmonia
  • Leptonic, semileptonic rare charm decays (including form factors, BSM models, LFV)
  • Rare charm decays to photons, neutrinos and invisibles (dark photons, axions)
  • Hadronic charm decays and CP-violation
  • D mixing
  • Tau lepton physics
  • Averages for HFLAV and PDG

SUSY 2023

The 30th International Conference on Supersymmetry and Unification of Fundamental Interactions

The Conferences on Supersymmetry and Unification of Fundamental Interactions (SUSY) are among the largest international events where particle physicists come together to discuss innovative ideas pertaining to fundamental interactions among elementary particles.

The aim of the SUSY conference is to review and discuss recent research related to supersymmetric theories and all other approaches to physics beyond the Standard Model in all aspects, including formal theory, phenomenology, astrophysics, experiment, etc.

The University of Southampton is responsible for organising the 30th International Conference on Supersymmetry and Unification of Fundamental Interactions (SUSY 2023).

Registration and Abstract submission will open on 20th April and close on 16th June for both the SUSY 2023 conference and the pre-SUSY school.

 

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