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ATLAS gives new insight into the internal structure of the proton

During the Lepton Photon Conference this week, the ATLAS collaboration presented a new paper that describes how partons interact within the proton


ATLAS new small wheel C lowering
The lowering of the New Small Wheel inside the ATLAS detector during second long shutdown (LS2). ATLAS’ new understanding of PDFs will be used in the search for new physics processes when the LHC restarts later this year. (Image: CERN)

While the Large Hadron Collider (LHC) is well known for smashing protons together, it is actually the quarks and gluons inside the protons – collectively known as partons – that are really interacting. Thus, in order to predict the rate of a process occurring in the LHC – such as the production of a Higgs boson or an as-yet-unknown particle – physicists have to understand how partons behave within the proton. This behaviour is described in parton distribution functions (PDFs), which describe what fraction of a proton’s momentum is taken by its constituent quarks and gluons.

Knowledge of PDFs has traditionally come from lepton–proton colliders, such as HERA at DESY. These machines use point-like particles, such as electrons, to directly probe the partons within the proton. Their research has revealed that, in addition to the well-known up and down quarks that are inside a proton, there is also a sea of other quark–antiquark pairs in the proton. This sea is theoretically made of all types of quarks, bound together by gluons. Now, studies of the LHC’s proton–proton collisions are providing a detailed look at PDFs, in particular the proton’s gluon and quark-type composition.

The ATLAS collaboration has just released a new paper combining LHC and HERA data to determine PDFs. The result uses ATLAS data from several different Standard Model processes, including the production of W and Z bosons, pairs of top quarks and hadronic jets (collimated sprays of particles). The strange quark’s contribution to PDFs was expected to be lower than that of lighter quarks. The new paper confirms a previous ATLAS result, which found that the strange quark is not substantially suppressed at small proton momentum fractions and extends this result to show how suppression kicks in at higher momentum fractions.

Several experiments and theoretical groups around the world are working to understand PDFs, as variance in these results could impact high-energy searches for physics beyond the Standard Model.

Achieving high-accuracy PDFs is needed if physicists are to find evidence for new-physics processes – which is where the ATLAS analysis contributes most powerfully.  The ATLAS collaboration is able to assess the correlations of the systematic uncertainties between their datasets and account for them – an ability put to great effect in their new PDF result. Such knowledge was not previously available outside ATLAS, making this result a new “vademecum” for global PDF groups.

Read the full article on the ATLAS website.

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