Thu·Aug 20, 2015Thursday·August 20, 2015

1. 01*

   Using spectator distributions to measure the initial geometry fluctuation

   Md Nasim🇺🇸

   A study of eccentricity (${\epsilon }_n$) fluctuations and its possible impact on final state momentum anisotropy ($v_n$) for symmetric collisions are presented in the framework of Glauber model. Effect of fluctuations of nucleon's position on the initial geometry has been studied using a new method, where the difference between oppositely moving spectators is taken as a measurement of eccentricity fluctuations. This study shows that higher harmonics ($n$ =3, 4 and 5) of eccentricity are less sensitive to fluctuations in transverse plane compared to the 2${}^{nd}$ harmonic. Position fluctuations in transverse plane will increase ${\epsilon }_2$ and hence possibly $v_2$ for the most central nucleus-nucleus collisions. For semi-central and peripheral collisions, the fluctuations have opposite effect, it deceases the eccentricity ${\epsilon }_2$. The fluctuation of initial geometry can be studied in collider experiments by studying the spectator distribution on the both sides of the beam.

   ↳ nucl-thhep-phnucl-exIJMPE(2015)·0 citations
2. 02*

   First observation of long-lived ${\pi }^{+}{\pi }^{-}$ atoms

   DIRAC Collaboration

   After observing and investigating the double-exotic ${\pi }^{+}{\pi }^{-}$ atom with the ground state lifetime $\tau$ of about $3\times 10^{-15}$~s, the upgraded DIRAC experiment at the CERN PS accelerator observes for the first time long-lived states of the same atom with lifetimes of about $10^{-11}$~s and more. The number of characteristic pion pairs resulting from the breakup (ionisation) of long-lived ${\pi }^{+}{\pi }^{-}$ atoms amounts to $436\pm 61$, corresponding to a signal-to-error ratio of better than 7 standard deviations. This observation opens a new possibility to measure energy differences between $p$ and $s$ atomic states and so to determine $\pi \pi$ scattering lengths.

   ↳ hep-exnucl-exphysics.ins-detPLB(2015)·19 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.