Science1 publisher2 min readPublished
A drop in J/psi production at sub-proton scales strains conventional nuclear shadowing
ALICE turned near-miss lead collisions into a photon beam and scanned momentum transfer down to a resolution of 0.2 femtometers, about a quarter of a proton. The drop in J/psi production it found there was not expected.
The Scientist · Science desk
What happened
- The ALICE collaboration published in Physical Review Letters the first multidimensional measurement of incoherent J/psi photonuclear production, tracking interaction energy and momentum transfer together.
- The data come from LHC Run 2, where lead nuclei pass close to one another without colliding and their electromagnetic fields act as beams of high-energy photons that can produce a J/psi in the other nucleus.
- Varying the momentum transfer moved the effective focus to resolutions of 0.6, 0.3 and 0.2 femtometers, the finest corresponding to structures about one-quarter the size of a proton.
- At the smallest scale the team saw a drop in J/psi production that conventional nuclear shadowing struggles to explain, and attributes it to gluons beginning to behave collectively.
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Why it matters
- capability Sensitivity to local gluon density turns lead-beam data the LHC already recorded into a probe of structure smaller than a proton, so this class of measurement does not wait on a new accelerator.
- precedent A data set indexed by two variables sets a harder bar for model builders than a single point: a calculation now has to fit a surface in energy and momentum transfer, and can fail in more than one place.
- constraint Until the suppression's size and uncertainty are on the table, a reader of the announcement cannot say how badly shadowing does, only that its authors think it does badly.
Momentum transfer is the setting that changes the resolution. "By varying the momentum transfer, our experiment effectively changes the focus of our microscope," said Daniel Tapia Takaki, the University of Kansas physicist who helped lead the analysis with colleagues at the Czech Technical University in Prague [5][3]. Three settings, with a factor of three in scale between the coarsest and the finest [14].
Most measurements of this kind average the gluon distribution over an entire nucleus [8]. An average cannot separate a nucleus that is uniformly short of gluons from one with dense patches sitting next to thin ones. Incoherent J/psi production responds to local changes in density instead [8]. That is what brings structures smaller than a proton into range.
Take the finest setting at face value. If 0.2 femtometers is a quarter of a proton, the proton in this comparison is about 0.8 femtometers across [13]. Tapia Takaki offered a second picture: a nucleus enlarged to the size of a football stadium, with the best resolution fine enough to distinguish features only a few yards wide on the field [11].
"At these extraordinary scales, we observe evidence that the gluons begin to behave collectively," Tapia Takaki said [10]. The University of Kansas release presents the measurement as a way to separate two competing explanations of small-scale gluon behaviour, but it does not quantify the drop, and shadowing is the only one of the two explanations it identifies by name [9][16][17].
Scoring one calculation against another takes the measured suppression at each setting and its uncertainty. The scan covers one production channel in one species of nucleus at three resolutions [5]. A drop that conventional shadowing cannot fit is evidence against those calculations before it is a measurement of whatever replaces them.
"Understanding how gluons behave inside nuclei is therefore essential to understanding how matter itself acquires its mass and structure," Tapia Takaki said [12].
What to watch
- The Physical Review Letters figures: the measured suppression at each resolution setting, with uncertainties, is what lets anyone rank shadowing calculations against collective-gluon ones.
- Whether calculations based on collective gluon behaviour reproduce the full dependence on both energy and momentum transfer, not just the one suppressed setting.
- Whether ALICE pushes the scan below 0.2 femtometers, or repeats it on a lighter nucleus, with later lead-beam data.