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Blast from the past: Constraints on the dark sector from the BEBC WA66 beam dump experiment

by Giacomo Marocco, Subir Sarkar

This Submission thread is now published as SciPost Phys. 10, 043 (2021)

Submission summary

As Contributors: Giacomo Marocco · Subir Sarkar
Preprint link: scipost_202011_00018v2
Date accepted: 2021-02-10
Date submitted: 2021-02-05 17:33
Submitted by: Marocco, Giacomo
Submitted to: SciPost Physics
Academic field: Physics
  • High-Energy Physics - Phenomenology
Approach: Phenomenological


We derive limits on millicharged dark states, as well as particles with electric or magnetic dipole moments, from the number of observed forward electron scattering events at the Big European Bubble Chamber in the 1982 CERN-WA-066 beam dump experiment. The dark states are produced by the 400~GeV proton beam primarily through the decays of mesons produced in the beam dump, and the lack of excess events places bounds extending up to GeV masses. These improve on bounds from all other experiments, in particular CHARM~II.

Published as SciPost Phys. 10, 043 (2021)

Author comments upon resubmission

"The authors make a point about the difference between charged and neutral pion decay distributions, owed to their markedly different lifetime and subsequent differing interactions with the target. However, it did not become clear to me to what extent this is actually taken into account? MadDump is used, but it appears its primary purpose is to infer the angular and energy differential chi distributions given the initial meson spectra? But what is assumed for the latter?

We model the initial meson spectra as proton-proton collisions scaled according to the nucleon number of the target nuclei; concretely, the scaling goes as A^{2/3}. We are aware of the subtleties hinted at by the Reviewer but in actual fact made a crude approximation since it makes little difference to our final result. Indeed (as was pointed out to us by W. Venus, private communication), there are two corrections to our results which are of *opposite* sign. That we ignore the contribution from the nuclear cascade makes our bounds conservative, whereas that we also ignore “reinteractions in the nucleus” (i.e. slowing down in nuclear matter of the quarks and gluons off which the DM particles are created) makes them less so. However these effects are at most 20-30% in the rates and probably cancel out - so the net effect would not be discernible on our final limit which goes as the 4th root of the rate.

"Finally, plots of the chi-distribution in angle and energy would be helpful."

We include a plot of 1/\sigma d\sigma/dE versus E for DM particles entering BEBC, coming from all channels for 3 nominal values of DM mass (3, 40 and 330 MeV). Also a plot of 1/\sigma d\sigma/d\theta against \theta for all the DM particles being produced in the beam dump, again for 3 DM masses (3, 40 and 330 MeV).

The curves are jagged because of limited statistics in running the simulations. We hope the overall trends reassures the Reviewer. Would they like us to keep these in a new version of the paper? (In that case we will probably need to run more simulations to make the figures look prettier.)

List of changes

Added plots of differential distribution of dark states in angle and energy. Added comment on this at end of page 4.

Reports on this Submission

Anonymous Report 1 on 2021-2-7 (Invited Report)


The authors have addressed my raised points, and the paper is ready to be published. I would like them to keep the additional figures. (Personally, I don't mind that they are a little jagged.)

  • validity: high
  • significance: high
  • originality: good
  • clarity: top
  • formatting: perfect
  • grammar: perfect

Author:  Subir Sarkar  on 2021-02-11  [id 1226]

(in reply to Report 1 on 2021-02-07)

We thank the Referee for accepting our responses to their queries and approving publication - the additional figures requested are included in the revised version.

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