138 Proceedings of the LHCP2015 Conference, St. Petersburg, Russia, August 31 - September 5, 2015 Editors: V.T. Kim and D.E. Sosnov Search for new physics in dijet resonant signatures and recent results from Run 2 with the CMS experiment GIULIA D’IMPERIO Universit`aLa Sapienza and INFN Roma On behalf of the CMS Collaboration Abstract. A search for narrow resonances in proton-proton collisions at a center-of-mass energy of √s = 13 TeV is presented. The dijet invariant mass distribution of the two leading jets is measured with the CMS detector using early data from Run 2 of the Large Hadron Collider. The dataset presented here was collected in July 2015 and corresponds to an integrated luminosity of 42 1 pb− . The highest observed dijet mass is 5.4 TeV. The spectrum is well described by a smooth parameterization and no evidence for new particle production is observed. Upper limits at a 95% confidence level are set on the cross section of narrow resonances with masses above 1.3 TeV. When interpreted in the context of specific models the limits exclude: string resonances with masses below 5.1 TeV; scalar diquarks below 2.7 TeV; axigluons and colorons below 2.7 TeV; excited quarks below 2.7 TeV; and color octet scalars below 2.3 TeV. Introduction Deep inelastic proton-proton (pp) collisions often produce two or more energetic jets when the constituent partons are scattered with large transverse momenta (pT ). The invariant mass of the two jets with the largest pT in the event (the dijet) has a spectrum that is predicted by quantum chromodynamics (QCD) to fall steeply and smoothly with increasing dijet mass (m jj) [1]. Many extensions of the standard model predict the existence of resonances at the TeV scale that couple to partons (quarks and/or gluons), and therefore accessible at the proton-proton collisions of the Large Hadron Collider (LHC). The object of this search are narrow resonances, with small natural width compared to the experimental resolution, decaying to a pair of partons. The two partons will result in the detector as a pair of back-to-back hadronic jets in the transverse plane. If they come from the decay of a resonance, a bump corresponding to the mass of the resonance over the smoothly falling dijet mass distribution of the QCD processes will appear. The search is extremely powerful and promising at the beginning of LHC run 2, since the new center-of-mass energy of √s = 13 TeV is almost twice with respect to the past. The increase in energy corresponds to much larger cross sections, especially at high mass, as can be seen in Fig. 1 left, where the ratio of the parton luminosity factor between 13 and 8 TeV is shown as a function of the resonance mass 1 MX. The results presented here make use of a 42 pb− dataset collected by the CMS experiment [2] in the first three weeks of data taking in July 2015. These data already exceed the sensitivity of the 2012 dijet search for resonance masses greater than about 5 TeV, as it is shown in Fig. 1 right. Figure 1 right [3] and represents an estimate of the system mass that can be probed in BSM searches at one collider setup (“collider 2”, e.g. LHC 8 TeV with 20 fb−1) given an established system mass reach of some other 1 collider setup (“collider 1”, e.g. LHC 13 TeV with 0.04 fb− .). The mass where the grey diagonal line crosses the green (different green lines for different final states), indicates≈ the point where the two setups have the same sensitivity. Where the green lines go below the grey, the 13 TeV datasets start to be more statistically powerful than the 8 TeV full dataset. This happens around 5 TeV if we consider the average of all partonic channels. G. D’Imperio, Search for new physics in dijet resonant signatures and recent results from Run 2 with ... 139 Proceedings of the LHCP2015 Conference, St. Petersburg, Russia, August 31 - September 5, 2015 WJS2013 100 7 http://cern.ch/collide Editors: V.T. Kim and D.E. Sosnov q q ratios of LHC parton luminosities: 13 TeV / 8 TeV -1 Σ q q- 6 q ig i i g g r-reach by G.P. Salam by G.P. r-reach gg_ 5 Σqq qg 4 10 and A. Weiler and A. or 8.00 TeV, 20.00 fb or 8.00 TeV, 3 luminosity ratio 2 1 MSTW2008NLO f system[TeV] mass 1 100 1000 0 M (GeV) 0 1 2 3 4 5 6 7 X system mass [TeV] for 13.00 TeV, 0.04 fb-1 FIGURE 1. Left: the parton luminosity ratio between √s = 13 TeV and 8 TeV as a function of the resonance mass MX for qq, qg and gg final states. The ratio increases quickly at high dijet mass. Right: estimate of the system mass that can be probed in 1 BSM searches at LHC run 1 setup (on y-axis √s = 8 TeV with 20 fb− ) given the system mass reach of run 2 setup (on x-axis 1 √s = 13 TeV with 0.04 fb− ). The different green lines indicates the different final states (qq, qg, gg). ≈ Jet reconstruction 2 2 The dijet analysis uses Particle Flow [4] jets with anti-kT algorithm [5] and width parameter ∆R = ∆η +∆φ = 0.4 (PF ak4 jets). This definition satisfies the requirements of infrared and collinear safety, and the jet energy is corrected using MC and data-based techniques in order to take into account the pile-up extra energy, the non-uniformity of the response across the detector and the residual difference in the absolute scale of the energy between data and MC. CMS has developed jet quality criteria (“Jet ID”) for PF jets which are found to retain the vast majority of real jets in the simulation while rejecting most fake jets arising from calorimeter and/or readout electronics noise. In addition to the Jet ID, all the PF ak4 jest are required to have a minimum pT > 30 GeV and to be in the tracker coverage region η < 2.5. For the leading jet the pT cut is pT > 60 GeV. | | The dijet analysis choice, as in the past, is to recluster in a larger cone the corrected PF ak4 jets that pass the selection described above, and use wide jets to reconstruct the invariant mass of the dijet system. This allows to contain better the energy of the hadrons in presence of final state radiation (FSR), and thus improves the dijet mass resolution with the resonance peak resulting both closer to the nominal mass and narrower. In the phase of the analysis preparation the optimization of the cone width has been studied in order to minimize the expected upper limits on the cross sections and the value ∆R = 1.1 is found to be optimal. Event selection and trigger studies The most relevant selection criteria are: Wide i The dijet mass calculated using wide jets m jj > 1.2TeV. During run 1 this cut was set to 890 GeV. ii The angular separation between the two wide jets ∆ηWide < 1.3. During run 1 this cut was the same. Opti- | jj | mization studies on 13 TeV Monte Carlo indicate that this value is still optimal. The reason of (i) is that the trigger turn-on curve is complete around 1.2 TeV (see below in this section). The requirement (ii) is a cut on the ∆ηWide between the jets. This quantity is related to the emission angle of the final | jj | partons with respect to the beam line in the center-of-mass reference frame (the scattering angle θ∗): ∆η cos θ∗ = tanh( ) (1) 2 and the cut ∆ηWide < 1.3 corresponds to cos θ < 0.57. This criterion is introduced to improve the signal over | jj | ∗ background ratio, excluding the region close to cos θ∗ = 1 where most of the QCD processes concentrates. The analysis, with this choice, remains inclusive with respect to different new physics hypotheses. 140 LHCP2015 Conference, St. Petersburg, Russia, August 31 - September 5, 2015 The event with the highest dijet mass passing the full selection is shown in Fig. 2 left. The dijet mass of this clean dijet event is 5.4 TeV, greater then the highest mass event observed in 2012 (of 5.2 TeV), confirming that this analysis exceeds the run 1 sensitivity above 5 TeV. The PFHT800 is the main unprescaled≈ trigger that is used for this analysis. The trigger selection is based on the scalar sum of the transverse momenta of all the jets in the event (HT ) with a threshold around 800 GeV and carries a large part of the fully hadronic physics at CMS. The PFHT475 is a prescaled path, based on the same HT selection of the main unprescaled one, with a lower threshold (around 450 GeV). It used as reference to study the relative trigger efficiency, since its turn-on region is far enough from the one of PFHT800, and it has a relatively small prescale ( (100)), that allows to collect a sufficient statistics to study the turn on region of the main trigger. The turn on curve O Wide as a function of m jj is shown in Fig. 2 right. 2015, 13 TeV 1.2 CMS Preliminary 1 0.8 PF HT > 800 GeV Relative Efficiency 0.6 0.4 0.2 0 500 1000 1500 2000 Dijet Mass [GeV] FIGURE 2. Left: display of the highest dijet mass event in the ρ-φ view of the CMS detector.