⚛️ high-energy experiments

Evidence of the $e^+ e^-\to f_1(1285)$ reaction with the CMD-3 detector at VEPP-2000

Using the CMD-3 detector at the VEPP-2000 collider, researchers analyzed 109.1 pb $^{-1}$ of data to report 4.5 $\sigma$ evidence for the production of the C-even $f_1(1285)$ resonance in the $e^+ e^- \to \eta\pi^0\pi^0$ reaction via two-photon interaction, measuring a production cross section of $0.074 \pm 0.021 \pm 0.010$ nb.

Original authors: R. R. Akhmetshin, A. N. Amirkhanov, A. V. Anisenkov, V. M. Aulchenko, N. S. Bashtovoy, E. V. Bedarev, D. E. Berkaev, A. E. Bondar, A. V. Bragin, D. E. Chistyakov, V. S. Denisov, E. A. Eminov, D. A. Ep

Published 2026-08-05

📖 4 min read🧠 Deep dive

CC0 1.0

Original authors: R. R. Akhmetshin, A. N. Amirkhanov, A. V. Anisenkov, V. M. Aulchenko, N. S. Bashtovoy, E. V. Bedarev, D. E. Berkaev, A. E. Bondar, A. V. Bragin, D. E. Chistyakov, V. S. Denisov, E. A. Eminov, D. A. Epifanov, L. B. Epshteyn, A. L. Erofeev, G. V. Fedotovich, L. B. Fomin, A. O. Gorkovenko, A. A. Grebenuk, S. S. Gribanov, D. N. Grigoriev, F. V. Ignatov, V. L. Ivanov, A. S. Kasaev, S. V. Karpov, V. F. Kazanin, I. A. Koop, A. A. Korobov, A. N. Kozyrev, P. P. Krokovny, A. S. Kuzmin, T. A. Kuznetsov, I. B. Logashenko, P. A. Lukin, K. Yu. Mikhailov, I. V. Obraztsov, V. G. Petrochenko, N. A. Petrov, A. S. Popov, S. A. Rastigeev, Yu. A. Rogovsky, A. A. Ruban, A. E. Ryzhenenkov, A. V. Semenov, V. E. Shebalin, D. N. Shemyakin, B. A. Shwartz, D. B. Shwartz, E. P. Solodov, M. V. Timoshenko, V. M. Titov, A. A. Talyshev, S. S. Tolmachev, A. I. Vorobiov, D. S. Zhadan, Yu. V. Yudin

Original paper dedicated to the public domain under CC0 1.0 (http://creativecommons.org/publicdomain/zero/1.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, bustling dance floor where tiny particles called electrons and positrons (the electron's antimatter twin) are constantly colliding. Usually, when these two meet, they annihilate each other in a flash of energy, creating new particles. Most of the time, this happens through a "single-lane" highway where they exchange one photon (a particle of light) to create new matter. But there's a rare, secret backdoor: sometimes, they can interact by exchanging two photons at once. This "two-photon" path is incredibly quiet and hard to hear over the noise of the main highway, but it's the only way to create a special type of particle called a "C-even" resonance. Think of these particles as dancers who can only enter the room if two people push the door open simultaneously. Scientists are eager to find them because understanding how these particles are born helps us solve a massive cosmic mystery: why the muon (a heavier cousin of the electron) wobbles slightly differently than our current theories predict. If we can measure how often these rare particles appear, we get a better clue about the invisible forces shaping our universe.

Now, let's zoom in on a specific experiment where a team of scientists, using a giant detector called CMD-3 at the VEPP-2000 collider in Russia, decided to hunt for one of these elusive dancers: the f1(1285). This particle is like a heavy, short-lived balloon that pops almost instantly into three other pieces: an eta particle and two neutral pions. The team didn't just guess; they set up a precise trap. They smashed electrons and positrons together at a specific energy level, aiming right at the "resonance peak" where the f1(1285) is most likely to appear. They collected data from a massive energy scan (like sweeping a radio dial from 1.20 to 1.36 GeV) and then focused a dedicated spotlight on the exact energy of 1282 MeV, gathering a huge amount of data equivalent to 51.7 inverse picobarns of luminosity.

The results were exciting, though not a slam-dunk victory. After sifting through millions of collision events and filtering out the "noise" (background events that look similar but aren't the target), the team found a small but distinct bump in their data. At the specific energy of 1282 MeV, they spotted 26.5 ± 7.4 events that matched the signature of the f1(1285) decaying into the expected particles. This isn't just a fluke; the statistical significance of this finding is 4.5 sigma. In the world of particle physics, this is a very strong hint—like hearing a clear melody in a noisy room—but it falls just short of the "5 sigma" gold standard usually required to claim a definitive discovery. To be sure they weren't just seeing ghosts, they checked their "control sample," which was data collected at nearby energies where the f1(1285) shouldn't exist. In that control group, they found no indication of a signal, confirming that the bump they saw at 1282 MeV was real and not just a glitch in the machine.

So, what does this mean? The team interprets this as the first solid evidence (though not yet a confirmed discovery) of the f1(1285) being produced directly in an electron-positron collision via that rare two-photon interaction. They calculated the "production cross section" (a measure of how likely this event is to happen) to be 0.074 ± 0.021 ± 0.010 nb. From this, they derived the probability of the f1(1285) turning into an electron-positron pair, finding a branching fraction of (8.3 ± 2.3 ± 1.1) × 10⁻⁹. This number is incredibly small, which makes sense given how rare the process is. The results align well with previous, less certain measurements from the SND experiment and match theoretical predictions. While the scientists are cautious and call it "evidence" rather than a final proof, they have successfully caught a glimpse of this C-even resonance in the act of being created, adding a valuable piece to the puzzle of how the subatomic world works.

Technical Summary: Evidence of the $e^+e^- \to f_1(1285)$ Reaction with the CMD-3 Detector

Problem and Motivation
The production of a single $C$ -even resonance in $e^+e^-$ collisions is strictly forbidden via the dominant single-photon annihilation mechanism. Such production can only occur via two-photon annihilation, a process suppressed by a factor of $\alpha^2$ (the square of the fine structure constant). While previous experiments have searched for single $C$ -even resonances using this "inverse" reaction, they have historically only established upper limits. Notably, the BESIII Collaboration recently observed the $J^{PC}=1^{++}$ charmonium state $\chi_{c1}(1P)$ via an interference pattern, allowing for the extraction of its branching fraction to $e^+e^-$ .

The axial-vector resonance $f_1(1285)$ presents a specific theoretical interest because it does not decay into two real photons but can be produced via two virtual photons in $e^+e^-$ annihilation. Understanding this process is crucial for theoretical models, particularly regarding the light-by-light contribution to the anomalous magnetic moment of the muon ( $g-2$ ). Previous measurements by the SND collaboration indicated two candidate events for $e^+e^- \to f_1(1285) \to \pi^0\pi^0\eta$ , yielding a branching fraction of $B(f_1(1285) \to e^+e^-) = (5.1^{+3.7}_{-2.7}) \times 10^{-9}$ , though with significant uncertainties. This paper presents a new, more precise search for this reaction.

Methodology
The study utilizes data collected by the CMD-3 detector at the VEPP-2000 $e^+e^-$ collider. The dataset comprises:

Resonance Peak Data: A dedicated run at the center-of-mass energy $E_{c.m.} = 1282$ MeV (corresponding to the $f_1(1285)$ mass), with an integrated luminosity of $51.7 \text{ pb}^{-1}$ .
Scan Data: A scan over the energy range $E_{c.m.} = 1.20\text{--}1.36$ GeV with a 20 MeV step, providing a total integrated luminosity of $57.4 \text{ pb}^{-1}$ (totaling $109.1 \text{ pb}^{-1}$ for the study). This scan serves as a control sample to verify background levels.

Detector and Simulation:
The CMD-3 detector features a cylindrical drift chamber inside a 1.3 T superconducting solenoid, a barrel liquid-xenon (LXe) calorimeter, a barrel CsI crystal calorimeter, and an end-cap BGO calorimeter.

Signal Channel: The analysis targets the decay chain $e^+e^- \to f_1(1285) \to \pi^0\pi^0\eta$ , where $\eta \to \gamma\gamma$ . This channel is selected because it is forbidden for single-photon production.
Background Suppression: The primary background is $e^+e^- \to \omega\pi^0 \to \pi^0\pi^0\gamma$ , where soft photons mimic the signal. Events are reconstructed with six or more photons. A 6C kinematic fit is applied for the $e^+e^- \to \pi^0\pi^0\gamma\gamma$ hypothesis, constraining total energy/momentum and two photon pairs to the $\pi^0$ mass.
Selection Criteria: Events are retained if $\chi^2_{6\gamma} < 60$ and $\chi^2_{5\gamma} > 50$ (to suppress the 5-photon $\omega\pi^0$ background). Further cuts on the $\pi^0\gamma$ invariant mass ( $m(\pi^0\gamma) < 740$ MeV/ $c^2$ ) and a 2D selection around the $a_0(980)$ and $\eta$ masses are applied to maximize the signal-to-background ratio.
Monte Carlo (MC): Simulations use matrix elements corresponding to the dominant $f_1 \to a_0(980)\pi^0$ decay (73%) and a sub-dominant $f_1 \to f_0(500)\eta$ contribution. Detector response is modeled using GEANT4.

Key Results

Signal Observation: In the peak energy data ( $E_{c.m.} = 1282$ MeV), the analysis identified $26.5 \pm 7.4$ signal events in the $e^+e^- \to \eta\pi^0\pi^0$ channel.
Significance: The signal significance is calculated as 4.5 $\sigma$ (based on the likelihood ratio of the fit with and without the signal component).
Control Sample: Analysis of the scan data ( $E_{c.m.} = 1.20\text{--}1.36$ GeV) revealed no significant signal, with a fit yielding $2 \pm 2$ events, confirming that the background is flat and non-peaking.
Cross Section: The peak cross section for the reaction is determined to be:
$\sigma(e^+e^- \to f_1) = 0.074 \pm 0.021 \text{ (stat)} \pm 0.010 \text{ (syst)} \text{ nb}$
Branching Fraction: The corresponding branching fraction for the $f_1(1285) \to e^+e^-$ decay is calculated as:
$B(f_1(1285) \to e^+e^-) = (8.3 \pm 2.3 \text{ (stat)} \pm 1.1 \text{ (syst)}) \times 10^{-9}$

Significance and Claims
The authors interpret the 4.5 $\sigma$ observation as evidence for the direct production of a $C$ -even resonance in $e^+e^-$ collisions via the two-photon interaction. The measured cross section and branching fraction are consistent with previous results from the SND experiment and theoretical estimates.

The paper modestly frames this result as the "first evidence" of this specific process, noting that while the statistical significance is strong (4.5 $\sigma$ ), systematic uncertainties—primarily from background subtraction and selection criteria variations—prevent a claim of definitive "observation" at the 5 $\sigma$ discovery level. The result provides a significantly better estimate of the $f_1(1285) \to e^+e^-$ decay rate compared to direct decay searches, contributing to the understanding of light-by-light scattering contributions to the muon $g-2$ .

Technical Summary: Evidence of the e+e−→f1(1285)e^+e^- \to f_1(1285)e+e−→f1​(1285) Reaction with the CMD-3 Detector

More like this

Technical Summary: Evidence of the $e^+e^- \to f_1(1285)$ Reaction with the CMD-3 Detector