Anatomy of the ATLAS diboson anomaly

We perform a general analysis of new physics interpretations of the recent ATLAS diboson excesses over standard model expectations in LHC Run I collisions. First, we estimate a likelihood function in terms of the truth signal in the $WW$, $WZ$, and $ZZ$ channels, finding that the maximum has zero events in the $WZ$ channel, though the likelihood is sufficiently flat to allow other scenarios. Second, we survey the possible effective field theories containing the standard model plus a new resonance that could explain the data, identifying two possibilities, viz. a vector that is either a left- or right-handed $SU(2)$ triplet. Finally, we compare these models with other experimental data and determine the parameter regions in which they provide a consistent explanation.

Figure 1

A simple picture of the $WW$ (red), $WZ$ (green), and $ZZ$ (blue) signal regions used in [1], in the $m_j$–$m_j$ plane of the two fat jets in an event. We also show our labelling of disjoint signal regions $A,B,C,D,E,F$.

Figure 2

$p$-values as a function of production cross section of the 2 TeV resonance $X$ times branching ratio for each decay channel. The $p$-value has been minimized over the other two signal regions for each line. The horizontal line shows the 95% upper bound. The efficiencies have been unfolded.

Figure 3

Joint constraints on the values of $\sigma \times \mathrm{Br}$ for different decay channels of a diboson resonance from the ATLAS fat jets analysis of the Run I LHC before efficiencies. The darkest region corresponds to 70% CL, whereas the next darkest region corresponds to 95% CLİn each panel, the best-fit point is denoted by a white dot.

Figure 4

Joint constraints on the values of $\sigma \times \mathrm{Br}$ for different decay channels of a diboson resonance from the ATLAS fat jets analysis of the Run I LHC, where one of $s_{WW}$, $s_{WZ}$ or $s_{ZZ}$ is set to zero (i.e. before efficiency corrections). We show the 70% and 95% preferred regions. In each case, the best-fit point is denoted by a white dot.

Figure 5

Invariant mass distribution near the 2 TeV resonance in each channel for $s_{WW}=1$, $s_{WZ}=223$, $s_{ZZ}=0$.

Figure 6

The branching ratios of the ${\rho }^{+}$ and ${\rho }^0$ of the left-handed triplet model, as a function of their coupling to quarks $g_q$ over their coupling to bosons $g_{\rho }$. Note the equal branching ratios to $Wh$ and $WZ$, and also to $Zh$ and $WW$, as predicted by [17]. The dijet branching ratios of the ${\rho }^{+}$ and ${\rho }^0$ overlap (black curves), as do their diboson branching ratios (blue and red curves).

Figure 7

Preferred regions in the plane of the $SU(2{)}_L$ triplet’s coupling to bosons, $g_{\rho }$, and quarks, $g_q$, as determined from the number of events observed in the six disjoint signal regions (see Table 1). We show the 95%, 99% preferred regions by the inner and outer pair of solid lines, respectively. Also shown are the 95% CL limits on a $W^{\prime }$ model from [31] (ATLAS W’) and [5] (CMS W’) which should be similar to the limits on the $SU(2{)}_L$ triplet considered here, the limit from the CMS search for resonances decaying to $Wh,Zh\to qqb\overline{b}$ [32] (“CMS Vh”), and the limit from the CMS dijet resonance search [33] (“CMS jj”). The line denoted “LEP I” depicts the approximate constraint from electroweak precision tests at LEPI. The region above each broken line is excluded.

Figure 8

Preferred regions in the plane of the $SU(2{)}_R$ triplet’s coupling to bosons, $g_{\rho }$, and quarks, $g_q$. We show the 95%, 99% preferred regions by the inner and outer pair of solid lines, respectively. Also shown are the 95% CL limits on a $W^{\prime }$ model from [31] (ATLAS W’) and [5] (CMS W’) which should be similar to the limits on the $SU(2{)}_R$ triplet considered here, the limit from the CMS search for resonances decaying to $Wh,Zh\to qqb\overline{b}$ [32] (“CMS Vh”), and the limit from the CMS dijet resonance search [33] (“CMS jj”). The region above each broken line is excluded.