A Measurement of the J/Psi Spin Alignment in CDF Run II Data

We present a measurement of the spin alignment of J/&psi mesons having rapidity |y| < 0.6 and 5 &le p_T< 30 GeV/c using the J/&psi &rarr &mu⁺ &mu^- decay channel. The analysis is performed using data from pp collisions at 1.96 TeV with an integrated luminosity of 800 pb^-1 taken from June 2004 to February 2006 with the CDF II detector at the Fermilab Tevatron. Promptly-produced J/&psi mesons are isolated from those produced in heavy flavor decays by impact parameter selections on the two muon tracks. The analysis procedure is similar to that used in Run I, employing a template method to account for acceptance and trigger efficiency effects. We use experimentally-derived trigger efficiency funtions to produce the templates.

We measure the polarization &alpha_B=-0.066 ± 0.050 for the B-decay events.
We find negative (longitudinal) polarization in the higher pT region for the prompt J/&psi events, in contrast to expectations from NRQCD.

Recent theory prediction for the prompt J/&psi polarization [1]:
[1] V.A. Khoze, A.D. Martin, M.G. Ryskin, and W.J. Stirling, Eur. Phys. J. C 39, 163-171 (2005), hep-ph/0410020

• Figure 1: The invariant mass distribution of the final J/&psi candidate events
  (a) prompt J/&psi mesons (.eps) (.gif),
  (b) J/&psi mesons from B-decays (.eps) (.gif)


• Figure 2: The invariant mass distributions of the final sample to measure the prompt J/&psi spin alignment, separated into six different p_T ranges of J/&psi
  (a) 5 &le p_T < 6 GeV/c (.eps) (.gif),
  (b) 6 &le p_T < 7 GeV/c (.eps) (.gif),
  (c) 7 &le p_T < 9 GeV/c (.eps) (.gif),
  (d) 9 &le p_T < 12 GeV/c (.eps) (.gif),
  (e) 12 &le p_T < 17 GeV/c (.eps) (.gif),
  (f) 17 &le p_T < 30 GeV/c (.eps) (.gif)


• Figure 3: The invariant mass distributions for the J/&psi candidates from B-decays, separated into six different p_T ranges of J/&psi
  (a) 5 &le p_T < 6 GeV/c (.eps) (.gif),
  (b) 6 &le p_T < 7 GeV/c (.eps) (.gif),
  (c) 7 &le p_T < 9 GeV/c (.eps) (.gif),
  (d) 9 &le p_T < 12 GeV/c (.eps) (.gif),
  (e) 12 &le p_T < 17 GeV/c (.eps) (.gif),
  (f) 17 &le p_T < 30 GeV/c (.eps) (.gif)


• Figure 4: MC templates for J/&psi decays
  (a) 5 &le p_T < 6 GeV/c (.eps) (.gif),
  (b) 6 &le p_T < 7 GeV/c (.eps) (.gif),
  (c) 7 &le p_T < 9 GeV/c (.eps) (.gif),
  (d) 9 &le p_T < 12 GeV/c (.eps) (.gif),
  (e) 12 &le p_T < 17 GeV/c (.eps) (.gif),
  (f) 17 &le p_T < 30 GeV/c (.eps) (.gif)


• Figure 5: Pulls from the polarization fits with 160 toy Monte Carlo experiments (.eps) (.gif)


• Figure 6: The sideband-subtracted ct distributions of J/&psi candidate events
  (a) before applying the prompt selection requirement (.eps) (.gif)
  (b) after applying the prompt selection requirement (.eps) (.gif)


• Figure 7: Polarization fit results for the prompt J/&psi signal
  (a) 5 &le p_T < 6 GeV/c (.eps) (.gif),
  (b) 6 &le p_T < 7 GeV/c (.eps) (.gif),
  (c) 7 &le p_T < 9 GeV/c (.eps) (.gif),
  (d) 9 &le p_T < 12 GeV/c (.eps) (.gif),
  (e) 12 &le p_T < 17 GeV/c (.eps) (.gif),
  (f) 17 &le p_T < 30 GeV/c (.eps) (.gif)


• Figure 8: Prompt J/&psi polarization (.eps) (.gif)


• Figure 9: The sideband-subtracted ct distribution for the J/&psi candidates from B-decays (.eps) (.gif)


• Figure 10: Polarization of J/&psi mesons from B-decays (.eps) (.gif)


• Figure 11: Run II results reanalyzed in the same J/&psi p_T bins used in Run I and compared to Run I results (.eps) (.gif)