AWAKE - creating a path for high energy particle physics

Scientific concepts, discoveries, and nature phenomena mentioned

Particle-physics accelerator status (2021–2022 context)

• LHC shutdown and upgrades: accelerator performance and detector systems were being improved before resuming data taking (targeted around March 2022).
• Experiment upgrades:
  • LHCb upgrade of a Ring Imaging Cherenkov (RICH) detector, including mirror installation and photodetectors.
  • Purpose: identify charged particles using Cherenkov light patterns.

Key detector physics: Cherenkov radiation & particle ID (LHCb)

• Cherenkov effect: charged particles traveling faster than light in a medium emit a cone of radiation.
• RICH technique: detect the cone using mirrors to form a ring image.
• Particle discrimination: use the ring pattern to distinguish pions vs kaons vs protons.
• Scientific role: crucial for LHCb’s data-taking when the LHC restarts.

Higgs-boson coupling studies and limits (ATLAS)

• Higgs coupling to different fermion generations:
  • Known behavior: Higgs coupling scales with mass; larger mass → larger coupling.
  • Previously constrained mainly to the 3rd generation; the talk emphasizes sensitivity opening to the 2nd generation.
• Higgs coupling to charm quarks (ATLAS):
  • Reported as about 26× larger than the Standard Model expectation (as a current limit/constraint).
• Goal: use more data to refine measurements and test Standard Model consistency.

Searches for new particles (ATLAS)

• “Xeno”: described as a supersymmetric particle related to the Higgs (context suggests higgs-sector SUSY searches).
  • Earlier work explored one region; new exclusions cover larger phase space, leaving reduced viable parameter space.
• A nominal “chi” particle:
  • Mentioned a search for a decay involving four heavy quarks (final state with 4 “b-like/top-like” heavy quarks, as described).
  • Sensitivity emphasized for high masses (~2000–3000 GeV).

Neutrino physics: oscillation anomaly and the sterile-neutrino question

• MiniBooNE (since 2007):
  • Reported an excess of electron-like events compared to Standard Model expectations.
  • One interpretation: evidence for an additional neutrino flavor, a sterile neutrino.
• MicroBooNE:
  • A liquid argon time projection chamber (LArTPC) detector.
  • Better at distinguishing photons vs electrons than MiniBooNE’s earlier techniques.
  • New result (stated as published at the end of October in the talk):
    • No evidence of the previously observed excess.
    • The excess is now explained well by Monte Carlo / background expectations.
• Implication stated: from this channel, there is currently no evidence supporting the sterile-neutrino-related interpretation.

Novel accelerator technology: Wakefield / plasma acceleration (AWAKE)

• AWAKE (Advanced Wakefield Experiment, at CERN) aims to create a new kind of particle accelerator using plasmas.
• Core motivation: probe smaller distance scales (higher energies) to continue exploring particle structure.
• Conceptual relation highlighted:
  • Smaller measurable distances → higher particle momentum/energy.
• Why today’s colliders have limits:
  • Circular accelerators:
    • limited by synchrotron radiation (radiative energy loss) for light particles,
    • limited by magnet strength for protons (maximum bending field).
  • Linear accelerators:
    • limited by required length given achievable RF accelerating gradients.

Plasma wakefield acceleration mechanism

• Plasma: ionized gas with free electrons and ions.
• Wakefield concept:
  • A “driver” excites a plasma oscillation, forming a strong electric-field structure (the wake / bubble).
  • A “witness” electron bunch injected later rides the wake to gain energy.
• Driver examples discussed:
  • Historically: laser pulse driver (the talk cites 1979 as an early concept),
  • In AWAKE: a proton bunch driver from CERN.

Self-modulation of long proton bunches (crucial AWAKE challenge)

• Problem:
  • Proton bunch length (~10 cm) is much longer than the wake/bubble scale (~1 mm).
  • Without correction, the long bunch would disrupt the plasma’s needed microstructure.
• Solution:
  • Self-modulation instability converts the long proton bunch into a train of micro-bunches.
  • Micro-bunch spacing matches the plasma wavelength, enabling coherent driving of the wake.
• Analogy used: pushing a swing at the right rhythm amplifies the motion.

AWAKE experiment details and results (as described)

• Location/setup:
  • AWAKE beamline at CERN; the beam travels through a long tunnel to the experiment region (about ~1 km mentioned).
• Plasma source:
  • Rubidium vapor, heated and laser-ionized to create the plasma.
  • Engineering complexity: many temperature sensors (80 stated).
• Electron acceleration performance (early runs):
  • First running period: 2016–2018.
  • Initial beam turn-on around December 12, 2016 (described as days before shutdown).
  • Reported results:
    • observation of proton micro-bunch modulation,
    • demonstrated electron acceleration to several GV in a few meters,
    • mention of sub-picosecond time resolution for proton bunch evolution.

Future goals / Run 2 and physics applications of high-energy electrons

• Planned separation of functions:
  • optimize modulation (driver → wake formation) separately from acceleration (electron trapping/energy gain).
• Run 2:
  • began in July (as stated),
  • goal: accelerate electron bunches suitable to begin particle-physics experiments.
• Example physics directions listed:
  • Dark matter searches via “dark photons” (fixed-target style with weakly interacting photons),
  • high-energy fixed-target experiments and exploration of new energy regimes,
  • possible electron–proton / electron–ion colliders at higher energies than conventional proposals.

Plasma acceleration for broader (non-high-energy) applications

The talk notes plasma-accelerator techniques could also support:

• compact ion beam generation by using a laser to excite plasma,
• potential cancer therapy applications via localized energy deposition with ion beams.

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Researchers or sources featured (mentioned explicitly)

People

• Professor Alan Caldwell (speaker; professor, AWAKE head at CERN)
• Christie Duffy (UKRI Future Leadership Fellow; MicroBooNE physics coordinator, Oxford)
• Giulio Villani (Dennis Wilkinson Prize; first-year performance awardee)
• Martin Tatt (Donald Perkins Prize; first-year performance awardee)
• Peter Griffin Hicks (John Adams Prize; first-year performance awardee)
• Nathan Zurich (European Physical Society young experimental physicist awardee)
• Jake Flower (poster prize mentioned)
• Sonia Zamani (poster prize mentioned)
• Chacilia Toshiri (poster/prize mention)
• Luigi Marquez (2020 ATLAS thesis award)
• Danielle (introduced the speaker; name unclear in subtitles)

Experiments / institutions / collaborations

• Oxford Particle Physics Christmas Lecture (event context)
• LHC; CERN
• ATLAS
• LHCb
• ZEUS (spokesperson noted in Caldwell’s bio)
• HERA accelerator complex (context for ZEUS)
• MiniBooNE
• MicroBooNE
• iPERk / Hyper-K (mentioned as “iper k” in Japan)
• DUNE (mentioned in the U.S.)
• SPS (used for the proton driver in AWAKE description)
• AWAKE (Advanced Wakefield Experiment)
• Isis (mentioned in a student bio; produces neutrons and muons)
• IOP and IOP meetings (award/poster contexts)

Scientific “sources” cited by context

• Rutherford (early scattering interpretation)
• Geiger and Marsden (early scattering experiments)
• McAllister and Hofstadter (proton size resolution via scattering; credited in the talk)
• Feynman lectures (educational source referenced)
• Enrico Fermi (“ultimate particle accelerator” idea)
• 1979 (historical reference for laser-driven plasma wakefield concept)
• Monte Carlo simulations (used in MicroBooNE result explanation; “Monte Carlo expectations”)