The ATLAS experiment is now taking data from 13 TeV proton-proton collisions. The increased collision energy and rate in these Run 2 collisions will allow physicists to carry out stronger tests of many theoretical conjectures, including several theories that predict more massive versions of force-carrying particles like the W and Z bosons. Some models suggest that these massive copies, called W’ and Z’ bosons, would exist because matter particles are linked by even more symmetries than the three kinds of fundamental symmetry we already recognise. Alternatively, these massive bosons might represent the familiar W and Z bosons, moving in extra dimensions of space.

Recent ATLAS results in the search for new massive particles, like the W’ and Z’, have provided another reason to eagerly anticipate the Run 2 dataset. While the search did not turn up any evidence of new particles, it has pointed to an interesting mass region to explore as Run 2 data streams in.

ATLAS,physics briefing,updates
Figure 1: Display of one of the events selected as a candidate W’ event decaying to WZ. The signals from the Z candidate jet are marked in red, while the signals from the W candidate jet are marked in green. The W’ candidate has a reconstructed mass of 2.26 TeV. (Image: CERN)

With additional data from Run 2, ATLAS physicists should soon be able to probe the existence of these di-boson resonances at even higher masses, and determine whether or not the Run 1 result near 2 TeV was just a coincidence.

The analysis, performed using 8 TeV LHC collision data collected in 2012, is one of several ATLAS searches for heavy particles decaying into a pair of weak gauge bosons, or “di-boson resonances”. Searches for di-boson resonances are made complicated by the fact that the weak gauge bosons themselves decay, so experiments have to identify the granddaughters of the hypothetical new particle.

In this search, the ATLAS team looked for events where both of the weak gauge bosons decayed to quark-antiquark pairs, and each of the quark-antiquark pairs were reconstructed as single jets in the ATLAS calorimeter (see Figure 1). As this is the most likely way for a pair of weak bosons to decay, the search maximized the chance of spotting the decay of a rare new particle.

The analysis team succeeded in rejecting over 99.95% of background events by applying novel filtering techniques that rely on the subtle differences between the particle jets produced by massive boson decays and those produced by single quarks or gluons. In the reduced sample, new di-boson resonances would have been clearly noticeable even if they had been produced at a rate hundreds of times smaller than that of Higgs bosons decaying into W boson pairs. No new resonances were observed, but a few more candidates were collected than were expected with a mass near 2 TeV (see Figure 2). With additional data from Run 2, ATLAS physicists should soon be able to probe the existence of these di-boson resonances at even higher masses, and determine whether or not the Run 1 result near 2 TeV was just a coincidence.

ATLAS,physics briefing,updates
Figure 2: Measured distribution of the di-jet mass after selecting W’ candidates in the WZ final state. The data (black points) is compared to the expected background (blue line). The signals expected for a W’ boson with masses of 1.5, 2.0, or 2.5 TeV from an extended gauge model are displayed for illustration purposes. (Image: CERN)

Links:

  1. Search for high-mass diboson resonances with boson-tagged jets in proton-proton collisions at 8 TeV with the ATLAS detector (arXiv: 1506.00962): http://arxiv.org/abs/1506.00962