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First Study of the Nuclear Response to Fast Hadrons via Angular Correlations between Pions and Slow Protons in Electron-Nucleus Scattering

S. J. Paul, M. Arratia, H. Hakobyan, W. Brooks, A. Acar, P. Achenbach, J. S. Alvarado, W. R. Armstrong, N. A. Baltzell, L. Barion, M. Bashkanov, M. Battaglieri, F. Benmokhtar, A. Bianconi, A. S. Biselli, F. Bossù, S. Boiarinov, K. -T. Brinkmann, W. J. Briscoe, V. Burkert, T. Cao, D. S. Carman, P. Chatagnon, H. Chinchay, G. Ciullo, P. L. Cole, A. D'Angelo, N. Dashyan, R. De Vita, A. Deur, S. Diehl, C. Djalali, R. Dupre, H. Egiyan, A. El Alaoui, L. Elouadrhiri, P. Eugenio, M. Farooq, S. Fegan, A. Filippi, C. Fogler, G. Gavalian, G. P. Gilfoyle, R. W. Gothe, B. Gualtieri, M. Hattawy, F. Hauenstein, T. B. Hayward, M. Hoballah, M. Holtrop, Yu-Chun Hung, Y. Ilieva, D. G. Ireland, E. L. Isupov, D. Jenkins, H. S. Jo, D. Keller, M. Khandaker, A. Kim, V. Klimenko, I. Korover, A. Kripko, V. Kubarovsky, L. Lanza, S. Lee, P. Lenisa, X. Li, D. Marchand, V. Mascagna, B. McKinnon, T. Mineeva, V. Mokeev, E. F. Molina Cardenas, C. Munoz Camacho, P. Nadel-Turonski, T. Nagorna, K. Neupane, S. Niccolai, G. Niculescu, M. Osipenko, A. I. Ostrovidov, M. Ouillon, P. Pandey, M. Paolone, L. L. Pappalardo, R. Paremuzyan, E. Pasyuk, C. Paudel, W. Phelps, N. Pilleux, P. S. H. Vaishnavi, S. Polcher Rafael, L. Polizzi, J. W. Price, Y. Prok, A. Radic, T. Reed, J. Richards, M. Ripani, J. Ritman, G. Rosner, S. Schadmand, A. Schmidt, R. A. Schumacher, Y. Sharabian, S. Shrestha, E. Sidoretti, D. Sokhan, N. Sparveris, M. Spreafico, S. Stepanyan, I. I. Strakovsky, S. Strauch, M. Tenorio, F. Touchte Codjo, R. Tyson, M. Ungaro, S. Vallarino, C. Velasquez, L. Venturelli, H. Voskanyan, E. Voutier, Y. Wang, D. P. Watts, U. Weerasinghe, X. Wei, M. H. Wood, L. Xu, Z. Xu, M. Zurek

Abstract

We report on the first measurement of angular correlations between high-energy pions and slow protons in electron-nucleus ($eA$) scattering, providing a new probe of how a nucleus responds to a fast-moving quark. The experiment employed the CLAS detector with a 5-GeV electron beam incident on deuterium, carbon, iron, and lead targets. For heavier nuclei, the pion-proton correlation function is more spread-out in azimuth than for lighter ones, and this effect is more pronounced in the $πp$ channel than in earlier $ππ$ studies. The proton-to-pion yield ratio likewise rises with nuclear mass, although the increase appears to saturate for the heaviest targets. These trends are qualitatively reproduced by state-of-the-art $eA$ event generators, including BeAGLE, eHIJING, and GiBUU, indicating that current descriptions of target fragmentation rest on sound theoretical footing. At the same time, the precision of our data exposes model-dependent discrepancies, delineating a clear path for future improvements in the treatment of cold-nuclear matter effects in $eA$ scattering.

First Study of the Nuclear Response to Fast Hadrons via Angular Correlations between Pions and Slow Protons in Electron-Nucleus Scattering

Abstract

We report on the first measurement of angular correlations between high-energy pions and slow protons in electron-nucleus () scattering, providing a new probe of how a nucleus responds to a fast-moving quark. The experiment employed the CLAS detector with a 5-GeV electron beam incident on deuterium, carbon, iron, and lead targets. For heavier nuclei, the pion-proton correlation function is more spread-out in azimuth than for lighter ones, and this effect is more pronounced in the channel than in earlier studies. The proton-to-pion yield ratio likewise rises with nuclear mass, although the increase appears to saturate for the heaviest targets. These trends are qualitatively reproduced by state-of-the-art event generators, including BeAGLE, eHIJING, and GiBUU, indicating that current descriptions of target fragmentation rest on sound theoretical footing. At the same time, the precision of our data exposes model-dependent discrepancies, delineating a clear path for future improvements in the treatment of cold-nuclear matter effects in scattering.

Paper Structure

This paper contains 7 sections, 6 equations, 3 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Experimental setup
  3. Event selection and observables
  4. Uncertainties
  5. Results and discussion
  6. Summary and conclusions
  7. Acknowledgements

Figures (3)

  • Figure 1: Illustration of nuclear deep‑inelastic scattering featuring a leading charged pion and secondary protons in the final state.
  • Figure 2: Correlation functions for D, C, Fe, and Pb (from top to bottom, first four rows), with $\Delta\phi$ on the $x$ axis within each panel and different bins of $\Delta Y$ for each column. These are compared to calculations from the BeAGLE model Chang:2022hkt (curves). The horizontal caps in the error bars represent the systematic uncertainties, while the outer bars represent the total systematic and statistical uncertainty (added in quadrature). Bottom row: same for the ratios between the nuclear and deuterium correlation functions.
  • Figure 3: Top row: RMS width $\sigma$ as a function of $\sqrt[3]A$ (left-most panel), and as a function of $\Delta Y$ for each target (other panels). Bottom row: same for the azimuthal broadening, $b$. The horizontal caps in the error bars represent the systematic uncertainties, while the outer bars represent the total systematic and statistical uncertainty (added in quadrature). The results are compared to the predictions from the BeAGLEChang:2022hkt, eHIJING PhysRevD.110.034001, and GiBUUBuss:2011mx models, as well as analogous measurements of the dipion channel from Ref. PhysRevC.111.035201 (gray, open squares). The broadenings for the dipion channel were reported in Ref. PhysRevC.111.035201 with respect to deuterium, and have been re-calculated with respect to carbon in order to compare them to the results of this work.