Search for Quark Compositeness with the Dijet Centrality Ratio in $pp$ Collisions at $\sqrt{s}=7\mathrm{TeV}$

A search for quark compositeness in the form of quark contact interactions, based on hadronic jet pairs (dijets) produced in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$, is described. The data sample of the study corresponds to an integrated luminosity of $2.9{\mathrm{pb}}^{-1}$ collected with the CMS detector at the LHC. The dijet centrality ratio, which quantifies the angular distribution of the dijets, is measured as a function of the invariant mass of the dijet system and is found to agree with the predictions of the standard model. A statistical analysis of the data provides a lower limit on the energy scale of quark contact interactions. The sensitivity of the analysis is such that the expected limit is 2.9 TeV; because the observed value of the centrality ratio at high invariant mass is below the expectation, the observed limit is 4.0 TeV at the 95% confidence level.

Figure 1

(a) Event counts corrected for trigger prescales for inner (solid circles) and outer (open boxes) dijets and (b) the observed $R_{\eta }$ as functions of $m_{jj}$. We compare $R_{\eta }$ with predictions for QCD from pythia6 (dashed line), NLO calculations (dotted line), and NLO plus nonperturbative corrections (solid line) and its uncertainty (band).

Figure 2

The observed dijet centrality ratio as a function of $m_{jj}$ compared with the null (QCD) hypothesis (solid line), including the total systematic uncertainty (band), and to hypotheses of quark contact interactions with $\Lambda =3\mathrm{TeV}$ (dotted line) and 4 TeV (dashed line).

Figure 3

Summary of the limit for the contact interaction scale $\Lambda$. We show ${\mathcal{R}}_{\mathrm{LL}}$ versus $\Lambda$ for the data (solid line), the 95% ${\mathrm{CL}}_s$ (dashed line), and the SM expectation (dotted line) with $1\sigma$ (dark) and $2\sigma$ (light) bands.