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First operation of the FAMU experiment at the RIKEN-RAL high intensity muon beam facility

T0 review · 2 major / 4 minor · reviewed 2026-08-04 · deepseek-v4-flash

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Pith's one-line read The FAMU experiment has taken its final setup into operation at a pulsed muon facility, scanning the predicted muonic-hydrogen hyperfine resonance with 29 laser wavelengths and collecting 49 days of data; the first physics result is deferre

desk verdict Honest commissioning report: the integrated FAMU apparatus ran and took 49 days of data, but no physics result yet; the main fragility is the unverified in-situ laser-cavity alignment. read the letter →

arxiv 2509.10350 v1 pith:B5C7C52P submitted 2025-09-12 physics.atom-ph physics.ins-det

FAMU Collaboration , A. Adamczak , D. Bakalov , G. Baldazzi , M. Baruzzo , R. Benocci , R. Bertoni , M. Bonesini
show 35 more authors
S. Capra D. Cirrincione M. Clemenza L. Colace M. Danailov P. Danev A. De Bari C. De Vecchi D. Di Ferdinando E. Fasci R. Gaigher L. Gianfrani A. D. Hillier K. Ishida J. S. Lord A. Menegolli E. Mocchiutti S. Monzani L. Moretti G. Morgante C. Pizzolotto A. Pullia M. Pullia R. Ramponi H. E. Roman M. Rossella R. Rossini A. Sbrizzi M. Stoilov J. J. Suarez-Vargas G. Toci L. Tortora E. Vallazza K. Yokoyama A. Vacchi
This is my paper · ORCID
classification physics.atom-phphysics.ins-det PACS 36.10.-k32.30.-r42.62.Fi
keywords muonichydrogenhyperfinesplittingZemachradiuslaserspectroscopymuontransfermulti-passopticalcavityX-raydetectorsprotonstructure
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper reports that the FAMU experiment, after a long development phase, has run its final apparatus at a high-intensity pulsed muon beam. All key components—cryogenic hydrogen/oxygen target, multi-pass optical cavity, mid-infrared laser, and a ring of 34 X-ray detectors—were operated and characterized, and the laser was stepped across 29 wavelengths from 6788.400 to 6789.050 nm, covering the region where current theory places the ground-state hyperfine transition of muonic hydrogen. The purpose of this exercise is a first measurement of the hyperfine splitting, which would yield the proton's Zemach radius and constrain models of proton structure. The paper does not yet claim a resonance: it states that the laser/no-laser separation in the data is under investigation, with a first result to follow after a blinded analysis.

What carries the argument

The observable is not the excitation itself but its aftermath: a laser-excited muonic hydrogen atom in the triplet state gains about 0.12 eV of kinetic energy through subsequent collisions, and because the rate of muon transfer from hydrogen to oxygen rises sharply with collision energy, the time distribution of muonic-oxygen X-rays shifts relative to the no-laser case. The multi-pass optical cavity, with mirror reflectivity of about 99.89%, multiplies the transition probability by roughly 1/(1-R), making the weak magnetic-dipole excitation detectable. A beam hodoscope measures the muon flux for normalization, and the laser wavelength and energy are recorded shot-by-shot to build the resonan

What would settle it

Compare the delayed muonic-oxygen X-ray time spectra from laser-on and laser-off spills, batch by batch, across 6788.400–6789.050 nm. If no batch shows a statistically significant perturbation in the transfer-time distribution, the laser is not exciting muonic hydrogen in the target, contradicting the operational claim that the cavity illuminates the gas.

Watch

Extended reading notes

Core claim

On its own terms, the paper claims that the fully assembled apparatus—a cryogenic gas target with a multi-pass optical cavity, a tunable mid-infrared laser, and a mosaic of X-ray detectors—performed as designed during its first beam exposure, and that the resulting wavelength scan spans the region where theory places the 1S-hyperfine transition of muonic hydrogen. The measurement itself is not yet claimed: the separation of laser-on and laser-off data is under investigation, and a first value of the hyperfine splitting is promised to follow. What is established here is technical readiness and a dataset large enough (about 8.5 million triggers over four periods) for the planned resonance sear

Load-bearing premise

That the optical path of the infrared laser inside the cold, pressurized gas target is identical to the path through an external reference cavity, so the laser genuinely illuminates the hydrogen-oxygen gas.

Editorial extensions

If this is right

  • A resonance will appear as the wavelength at which the delayed muonic-oxygen X-ray time distribution deviates most from the no-laser distribution.
  • Once located, the resonance wavelength gives the 1S hyperfine splitting, from which the proton's Zemach radius can be extracted using the Fermi-energy relation and known QED corrections.
  • The reported gain stability and wavelength stabilization (a few percent and about 0.1 nm over hours) make the 2023-2024 dataset usable for the blinded analysis now being prepared.
  • The 2024 detector upgrade increases X-ray yield per trigger by about 25%, so future beam time can accumulate statistics faster than the 2023 runs.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Because the final physics result is explicitly deferred, the paper's present claim should be read as technical readiness; the decisive test is the upcoming blinded analysis, not the scan coverage itself.
  • The twin-cavity alignment assumption—that a reference cavity outside the target reproduces the optical path inside the target at 80 K and 7 bar—is unverified in situ; a direct measurement of infrared power leaving the target cavity during beam time would settle whether the laser actually illuminates the gas.
  • The unidentified ~110 keV peak seen in both hydrogen and hydrogen-oxygen delayed spectra could interfere with the oxygen X-ray selection if its origin is not resolved before unblinding.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 4 minor

Summary. This paper reports the first operation of the FAMU experiment in its final configuration at Port 1 of the RIKEN-RAL muon beam facility. It describes the detection principle (laser excitation of the muonic-hydrogen ground-state hyperfine transition, followed by muon transfer to oxygen and detection of characteristic X-rays), the full setup (beam monitor, cryogenic gas target with a multipass optical cavity, 6.8 µm laser, LaBr3:Ce detectors, and DAQ), and results from the July 2023 commissioning and four 2023–2024 physics runs. The paper explicitly defers the physics result: the laser/no-laser separation is still under investigation, and no resonance signal is claimed.

Significance. If the operational claims hold, this is an important technical milestone: it demonstrates a working apparatus for a difficult measurement of the muonic-hydrogen 1S hyperfine splitting, a quantity sensitive to the proton Zemach radius. The paper provides valuable characterisation data: target temperature/pressure control, detector gain stability and energy resolution, beam-momentum optimisation, laser wavelength/energy stability, and a 29-wavelength scan covering the predicted resonance region. The honest separation of technical readiness from physics results and the planned blinded analysis are strengths.

major comments (2)
  1. [Sec. 3.2 and Sec. 4.1] The in-target multipass-cavity alignment is inferred exclusively from the external 'twin' cavity. The text in Sec. 3.2 states the procedure 'guarantees'/'warrants' that the optical path in the target cavity is the same, but no quantitative in-situ verification is reported. Sec. 4.1 says the stepwise refilling is used 'to verify the optical cavity alignment,' but no measurement (e.g., transmitted or reflected laser power from the target cavity, or an alignment-sensitive signal) is shown. Because the physics scan requires the 6.8 µm beam to actually multipass through the cold, 7-bar gas, this is load-bearing for the claim of successful full-apparatus operation. Please either provide such evidence or explicitly state this as a residual limitation.
  2. [Sec. 5] The conclusion states that the laser 'enable[s] the excitation of a large number of muonic hydrogen atoms' and that the runs confirmed 'flawless operation of the experimental setup.' However, Sec. 4.2 reports that the laser/no-laser separation is 'currently under investigation' and no resonance signal is presented. The data so far demonstrate stable operation of the laser system and detectors, but not end-to-end laser-gas coupling. Please temper the conclusion to match the demonstrated scope.
minor comments (4)
  1. [Sec. 3.2 (Fig. 8 caption)] The caption states the red and 6.8 µm beams are 'well overlapped,' but no quantitative overlap measurement is described; please specify the procedure or tolerance.
  2. [Sec. 4.2 (Fig. 19)] The theory labels ('Ruth '24', 'Hagelstein '23', etc.) are not matched to references in the bibliography; please add citations.
  3. [Table 1] Caption: 'Average detectors performances' should read 'Average detector performances.'
  4. [Sec. 3.2] 'with warranty that' should be 'with the guarantee that' or 'ensuring that'; also 'merging' should be 'emerging' in the alignment description.

Circularity Check

0 steps flagged · score 2.0 of 10

No significant circularity: the paper reports new operational data; its physics claim is explicitly deferred, and its self-citations to prior FAMU measurements are independent support rather than constructional inputs.

full rationale

The paper's central claim is that the final FAMU apparatus was installed at RIKEN-RAL Port 1 and operated for 49 days, with all subdetectors characterized, the laser scanned over 29 wavelengths (6788.400-6789.050 nm), and data collected (Sec. 4.1, 4.2, Tab. 2). This is new empirical material, not derived from the cited inputs. The 1S-hfs result is explicitly deferred: Sec. 4.2 states laser/no-laser separation is 'currently under investigation' and a first result 'will then follow', so no measured resonance is being claimed. The detection principle uses the energy-dependent muon-transfer rate λpO(E) from Ref. [16] (Stoilov et al., Phys. Rev. A 107, 032823), an externally published measurement by collaboration members; this is real independent evidence, not a self-citation chain. Similarly, the 1.5% O2, 7 bar, 80 K target choice cites Ref. [12] (Pizzolotto et al., EPJ A 56, 185), again prior FAMU work that is externally validated. Beam-monitor calibration uses Eq. 6 with experimentally determined Qµ and a simulated geometric correction; the linearity plot (Fig. 6) checks it against an independent X-ray count, not against the quantity being predicted. The only genuinely weak point is the external 'twin' cavity alignment (Sec. 3.2): in-target 6.8 µm multipassing is inferred from a 632 nm alignment of a reference cavity, and no in-situ quantitative verification is reported. That is a correctness/verification limitation, not a circular reduction: the claim that wavelengths were scanned and data collected does not require the resonance to be present, and the paper does not use the twin-cavity assumption to define its observable. Therefore no step reduces to its own inputs by definition.

Assumptions & free parameters 3 free parameters · 4 assumptions · 0 invented entities

The paper is a commissioning report and introduces no new physics entities. Its central claims rely on previously measured muon transfer rates, theoretical hyperfine predictions, and the twin-cavity alignment assumption; the only parameters fitted in this paper are the beam monitor calibration constant and the choice of beam momentum.

free parameters (3)
  • flux calibration constant k = not stated numerically; derived from measured Q_mu and simulated W_j
    Eq. 6 converts total deposited charge Q_tot in the beam monitor to muon flux, using an experimentally determined single-muon charge Q_mu and simulated fiber-hit fractions; this calibration underpins the beam flux and data normalization claims.
  • beam momentum working point = 55 MeV/c
    Selected from commissioning data by maximizing the number of muon stops in the gas (Fig. 18); central to the stopping efficiency of the target.
  • target composition and pressure = 1.5% O2 in H2 at 7 bar
    Declared the optimal gas configuration for the 80-91 K target from earlier FAMU studies [12, 24]; these are chosen operational parameters, not derived in this paper.
assumptions (4)
  • domain assumption Energy dependence of the muon transfer rate lambda_pO(E)
    The detection method requires that the transfer rate from muonic hydrogen to oxygen changes sharply with collision energy (nearly an order of magnitude) at thermal energies, as measured by the same collaboration [16]. If this dependence were different, the observable would not respond to laser excitation.
  • domain assumption Theoretical predictions of the 1S-hfs wavelength define the scan range
    The scanned interval 6788.400-6789.050 nm is chosen from the latest theoretical predictions (Fig. 19); if the true resonance lies outside this interval, the 2023-2024 data would miss it.
  • domain assumption Twin-cavity alignment replicates the in-situ target cavity
    Sec. 3.2 assumes the external reference cavity has the same optical path as the target cavity at 80 K and 7 bar, ensuring the laser beam is actually multi-passed inside the gas target.
  • standard math Simulated fiber hit fractions W_j in flux calibration
    Eq. 6 relies on a Monte Carlo correction factor (W2 + W1/eta) for muons traversing one or two fibers; if the simulation is wrong, the absolute flux scale is biased.

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Cite this review

Pith. "Pith review of First operation of the FAMU experiment at the RIKEN-RAL high intensity muon beam facility." pith.science (2026). https://pith.science/paper/B5C7C52P

@misc{pith2026250910350,
  author       = {Pith},
  title        = {Pith review of: First operation of the FAMU experiment at the RIKEN-RAL high intensity muon beam facility},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/B5C7C52P}},
  note         = {Machine review of arXiv:2509.10350}
}
abstract

The FAMU experiment, supported and funded by the Italian Institute of Nuclear Physics (INFN) and by the Science and Technology Facilities Council (STFC), aims to perform the first measurement of the ground-state hyperfine splitting (1S-hfs) of muonic hydrogen ($\mu H$). This quantity is highly sensitive to the proton's Zemach radius $R_Z$. An experimental determination of $R_Z$ provides significant constraints on the parametrization of the proton form factors as well as on theoretical models describing the proton's electromagnetic structure. Following years of technological and methodological development, the FAMU experiment began operations in 2023 at Port 1 of the RIKEN-RAL muon beam line at the ISIS Neutron and Muon Source facility (Didcot, UK). In this paper, we first describe the unique detection technique employed by FAMU to determine the 1S-hfs of muonic hydrogen, followed by a detailed presentation of the final experimental layout. Finally, we report the first outcome from the 2023 commissioning run and from the initial physics runs performed in 2023 and 2024.

Figures

Figures reproduced from arXiv: 2509.10350 by the authors.

Figure 1
Figure 1. Scheme of the FAMU method to excite the 1S-hfs transition in muonic hydrogen through a MIR laser beam, with the subsequent transfer of the muon to molecular Oxygen and the detection of the typical µO X-rays as a signature of the transition. The proposal of the FAMU experiment was presented in 2013 at the Program Advisory Committee (PAC) of the RIKEN laboratory, co-owner at that time of the RIKEN-RAL pulsed muon beam… view at source ↗
Figure 2
Figure 2. Behaviour of the FAMU observable, the excess of delayed µO X-rays. Left: toy simulation of a hypothetical ∼10% effect on the FAMU observable (yellow coloured area). Right: illustration of a hypothetical resonance of the FAMU observable as a function of the laser wavelength, where the mean value is the resulting estimation of ∆E1S−hf s (the value chosen here as ∆E1S−hf s is the latest theoretical prediction, not a FA… view at source ↗
Figure 3
Figure 3. Left: photograph of the fully mounted FAMU experiment at RIKEN-RAL Port 1, taken in September 2023 after the commissioning but before the first beam time. Right: zoom on the FAMU target and detectors [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figures from the paper (19 more)
Figure 4
Figure 4. Figure 4: Scheme of the path followed by muons directed to the FAMU target in the RIKEN-RAL muon facility. The positions of the bending (RB) and quadrupole (RQ) magnets involved in the beam delivery to FAMU are labelled. 6 [PITH_FULL_IMAGE:figures/full_fig_p006_4.png]
Figure 5
Figure 5. Figure 5: Internal view of the FAMU beam monitor: the 32+32, 1 mm width scintillating fibers with TiO2 EMA paint coating can be seen in the middle. This detector, initially designed for beam focusing and centering, is now capable of measuring the muon beam flux. This has been ma…
Figure 6
Figure 6. Figure 6: Left: beam monitor flux linearity, tested by varying the flux through a dedicate de-tuning of bending magnet RB1, and comparing the charge deposited in the scintillating fibres (Qtot) with the number of X-rays measured by the detectors, which is proportional to the num…
Figure 7
Figure 7. Figure 7: Side section view of the target cryostat (CAD drawing), where the main components are marked. The three detector-holding rings are attached to the target cryostat around the target chamber (green volume), as shown in [PITH_FULL_IMAGE:figures/full_fig_p009_7.png]
Figure 8
Figure 8. Figure 8: Laser Injection Systems: sketch of the alignment procedure: a red laser beam, well overlapped with the 6.8 µm radiation, is directed towards the two identical Multi-pass Optical Cavity, namely the reference cavity and the target cavity. The injection is assured by alig…
Figure 9
Figure 9. Figure 9: Simplified sketch of the FAMU laser system representing the pump and signal formation and the coupling of the beams inside the non-linear (NL) crystal. DC1 and DC2 are dichroic mirror used respectively to superimpose the beams and to isolate the 6.78 µm. 10 [PITH_FULL…
Figure 10
Figure 10. Figure 10: Position of the three detector rings in the FAMU setup. The HPGe detector is omitted for improved illustration clarity. SiPM CAEN digi V1742 CAEN digi V1730 CAEN digi V1724 1’’ LaBr3 crystal 1’’ LaBr3 crystal HPGe crystal Hodo fibre Preamp. PMT SiPM NI amp. ORTEC amp …
Figure 11
Figure 11. Figure 11: Scheme of the DAQ electronics for all types of detectors (beam monitor fibres, scintillating crystals with different readout and HPGe detector) and close-up view of the main VME crate hosting the digitisers. 12 [PITH_FULL_IMAGE:figures/full_fig_p012_11.png]
Figure 12
Figure 12. Figure 12: The diagram illustrates the working principle of the laser trigger. Since the Cherenkov trigger arrives after the muons, it cannot be used to activate the laser. Instead, the solution is to use the kicker trigger: the PC is triggered by a signal that arrives 3.5 µs be…
Figure 13
Figure 13. Figure 13: Measured target pressure (left) and target temperature (right) as function of time during target filling and cooling. The procedure is illustrated in [PITH_FULL_IMAGE:figures/full_fig_p014_13.png]
Figure 14
Figure 14. Figure 14: Example of prompt X-rays from muon atoms of the material composing the FAMU target. The 511 keV prompt peak of e+e − annihilation is also highlighted. As it can be seen in [PITH_FULL_IMAGE:figures/full_fig_p015_14.png]
Figure 15
Figure 15. Figure 15: Gain variation as a function of the time of LaBr3:Ce detectors with PMT (LaBr, top-left) and SiPM (MIB, other panels) readout. The time on the x-axis is expressed in terms of the batch identification number (Batch N) used to uniquely identify each data sample within t…
Figure 16
Figure 16. Figure 16: FWHM energy resolution at 141 keV as a function of the detector name measured in all 34 detectors used in the September 2024 beam time identified by the Batch 3. 16 [PITH_FULL_IMAGE:figures/full_fig_p016_16.png]
Figure 17
Figure 17. Figure 17: Calibrated energy spectra (zoom in the region 40÷ 230 keV) obtained by adding up the pulses in all detectors of the same type in the sample with Batch 3. The left plot is obtained using 6 LaBr detectors, while the right plot results from the combination of 28 MIB dete…
Figure 18
Figure 18. Figure 18: Variation of the delayed µO X-rays as a function of beam momentum. By selecting pµ = 55 MeV/c, the number of muons stopped in the gas is maximised. 17 [PITH_FULL_IMAGE:figures/full_fig_p017_18.png]
Figure 19
Figure 19. Figure 19: Left: comparison among the latest theoretical predictions for the 1S-hfs and the spectral range covered in the 2023 and 2024 beam times. Right: histogram of the 29 wavelengths measured in 2023 and 2024 by the FAMU experiment. to be ∼ 80 pm wide. It is important to not…
Figure 20
Figure 20. Figure 20: (left) demonstrates the wavelength stability: each point of the plot represents the average of the wavelength over one hour of data taking [PITH_FULL_IMAGE:figures/full_fig_p018_20.png]
Figure 21
Figure 21. Figure 21: Number of X-rays per muon trigger collected in the Oxygen signal region by all LaBr3:Ce detectors (LaBr-MIB), as a function of the wavelength in two different data taking periods. oxygen contribution (right panel). Events under the Kα and Kβ/Kγ oxygen peaks correspond…
Figure 22
Figure 22. Figure 22: Subtraction of the normalised H2 delayed background, measured in September 2024, from the delayed spectrum in Batch 5, for all LaBr3:Ce detectors (LaBr-MIB). The plot on the left shows the two spectra, the plot on the right the net subtraction which is the net oxygen …

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

  1. Hadronic vacuum polarization in hydrogen-like atoms and ions amid the interplay of recoil and finite-size effects

    physics.atom-ph 2026-07 accept novelty 6.0 of 10

    The hVP contribution to the HFS in muonic hydrogen is 2.153(11) µeV, deviating from previous evaluations by ~10x the anticipated experimental precision, due to corrected recoil and finite-size interplay.

Reference graph

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