In Babar experiment they basically observe $\Upsilon(4s)$ resonance which decays to $B^0$ and $\bar{B}^0$ which eventually then decays to Kaon, pions, muons, etc. So, my question is: Is this a two jet event ? Or is it right to talk of jets in this case or just leave it saying that it was a two meson event?

  • $\begingroup$ $\Upsilon$ looks way to much like $\gamma$... Fun fact, if you google $\gamma(4S)$, the first result is the PDG page for $\Upsilon(4S)$. So even Google is confused! $\endgroup$ Jan 19, 2018 at 12:40

1 Answer 1


A jet is created when a quark or gluon is produced with a very high energy and subsequently hadronizes, creating a shower of particles.

The $\rm\Upsilon(4S)$ resonance has a mass of only $10.57~\rm GeV$, which is just barely enough to decay into $\rm B^0 \bar B^0$ in the first place. As such, the mesons are produced almost at rest (in the $\rm\Upsilon(4S)$ rest frame), no hadronization occurs, and you would not call this a two jet event.

You mainly get jets with more massive particles decaying, since there has to be enough energy available to produce multiple hadrons. Decay modes that produce jets will have either free quarks or free gluons listed in the final state. Since colored is confined, this always implies the formation of a jet. For instance, the top quark decays like $\rm t\rightarrow bW$, which will be observed as a jet and whatever the $\rm W$ decays into.

  • $\begingroup$ Also, if you are aware of, do these boosted B meson mostly decay after passing through the first tracking detector so that their decay can be observed or do they decay near the beam pipe and only their decay products(hadrons) are observed by the detector? $\endgroup$
    – kbg
    Jan 19, 2018 at 12:40
  • 2
    $\begingroup$ @kbg I don't know that much about BaBar, but $B^0$ mesons at that energy only travel around a millimeter on average before decaying, so I'd be surprised if they got tracking on them before the decayed. They presumably can reconstruct the vertex of the decay products and figure out pretty much exactly where it decayed, though. $\endgroup$ Jan 19, 2018 at 12:49
  • $\begingroup$ @Chris Do you know if all jets need a hadron to be defined? Are there non-hadronic jets ? $\endgroup$
    – partizanos
    Dec 24, 2020 at 3:02
  • $\begingroup$ @partizanos From an experimental point of view, a jet is just a whole bunch of particles concentrated in one place. Hadrons tend to make jets, and other particles do not, but in principle you could have a "jet" with no hadrons. $\endgroup$ Dec 25, 2020 at 8:34

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