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I remember reading that the background contains $q \bar{q}$ giving 4 leptons while studying the 4 lepton decay channel of Higgs boson. Since LHC is a $pp$ collider, I was wondering the source of anti-quark $\bar{q}$.

Or is it correct to say that both $q$ and $\bar{q}$ are produced simultaneously during collision?

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  • $\begingroup$ Z/$\gamma$ -> q-qbar, W$\pm$ -> q-q'bar, etc $\endgroup$
    – JamieBondi
    Commented Mar 12, 2017 at 4:35
  • $\begingroup$ The proton itself, apart from the quark content uud, has a lot of virtual q-qbar pairs and gluons propagating in the proton. $\endgroup$
    – JamieBondi
    Commented Mar 12, 2017 at 4:43
  • $\begingroup$ @JamieBondi So can I say that there are virtual mesons present in a proton? $\endgroup$
    – kg__
    Commented Mar 12, 2017 at 4:50
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    $\begingroup$ Virtual quarks wouldn't form mesons/baryons. They are very energetic (in QFT jargon, they are off-shell). The coupling strength of QCD is small at high energies, so virtual quarks won't bind into bound states. $\endgroup$
    – JamieBondi
    Commented Mar 12, 2017 at 5:00
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    $\begingroup$ It is indeed correct to say that both $q$ and $\overline{q}$ are produced during collisions. They must be produced in equal numbers to conserve baryon number. $\endgroup$
    – dukwon
    Commented Mar 12, 2017 at 10:16

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It is simple to understand if you try to write down feynman diagrams for proton proton giving a Higgs :

higgs

gluons give pairs of quark antiquark. The spectator protons will also give jets or even more gluons might be radiated because the energies are high.

The Higgs has high couplings(proportional to mass) with the top-antitop loop , and so this is dominant for two gamma decays .

Higgs to quark antiquark pairs is only limited by the mass of the Higgs. Searches are on at LHC to find the Higgs to b bbar channel.

For the general case, the gluon can generate all quark antiquark pairs that conserve energy and momentum for the interaction.

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