Dijets confirm the Standard Model

31 May 2012

Dijet measurements provide an excellent tool not only to probe high transverse-momentum parton interactions to study QCD but also to look for signs of new phenomena beyond the Standard Model. Thanks to the outstanding performance of the LHC in 2011, the ATLAS experiment recorded nearly 30,000 events with dijet masses above 2 TeV and even observed dijet masses up to 4.6 TeV.


The collaboration has used the full 2011 data sample – corresponding to nearly 5 fb–1 of integrated luminosity – for a measurement of the dijet cross-section as a function of mass and rapidity difference. The data were first corrected for detector effects – paying particular care to the effect of possible multiple interactions per beam crossing – and the measured cross-sections were then compared with various predictions of QCD. While there are small deviations in some models at the higher end of the spectrum, overall the agreement with QCD is reasonably good.

QCD predicts that the cross-section falls steeply with dijet mass. New, as yet unobserved, particles would typically give rise to resonances or bumps on top of this smoothly falling spectrum. ATLAS observes no bumps, allowing limits to be set on a number of theories that predict such particles.

Angular distributions can also be used to search for deviations from the Standard Model. They are typically measured in bins of dijet mass, where the scattering angle is transformed into a variable known as χ (see figure). The Standard Model predicts that these distributions should be relatively flat, while many theories beyond the Standard Model predict a rise at low values of χ.

The measured distributions are found to be in agreement with QCD predictions, allowing limits to be set on various models for new physics. For one of these models, where quarks are no longer fundamental particles but are instead composite objects, this analysis sets a limit on the compositeness scale – the scale of the constituent binding energies – at 7.8 TeV.

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