Search for Three-Jet Resonances in pp Collisions at ffiffiffi s p ¼ 7 TeV S. Chatrchyan et al.* (CMS Collaboration) (Received 16 July 2011; published 29 August 2011) A search for three-jet hadronic resonance production in pp collisions at a center-of-mass energy of 7 TeV has been conducted by the CMS Collaboration at the LHC, using a data sample corresponding to an integrated luminosity of 35 pb�1. Events with high jet multiplicity and a large scalar sum of jet transverse momenta are analyzed using a signature-based approach. The number of expected standard model background events is found to be in good agreement with the observed events. Limits on the cross section times branching ratio are set in a model of gluino pair production with an R-parity-violating decay to three quarks, and the data rule out such particles within the mass range of 200 to 280 GeV=c2. DOI: 10.1103/PhysRevLett.107.101801 PACS numbers: 13.85.Rm, 12.60.Jv, 13.87.Ce Searches for new physics in multijet final states, although experimentally challenging at hadron colliders, are sensitive to many extensions of the standard model (SM). For example, variations of technicolor models, re- sulting in heavy colored fermions that transform as octets under SUð3Þc, have been proposed in a variety of forms [1–4]. Other models incorporate R-parity-violating (RPV) decays of supersymmetric gluinos (~g) to three-quark final states, where the gluino represents a colored adjoint Majorana fermion [5–7]. In all cases, these high-mass resonances can be pair-produced, yielding a six-jet final state pp ! ~g ~gþX, where ~g ! 3 jets. Recent results from the Tevatron provide limits on gluino RPV decays for masses below 144 GeV=c2 [8]. This Letter presents the first results of a dedicated search for three-jet hadronic resonances in multijet events in pp collisions. The results are based on a data sample of proton-proton collisions at ffiffiffi s p ¼ 7 TeV, corresponding to an integrated luminosity of 35:1� 1:4 pb�1 [9], col- lected with the Compact Muon Solenoid (CMS) detector [10] at the large hadron collider (LHC) in the running period from March through November 2010. Events with at least six jets, each with high transverse momentum (pT), are selected and investigated for evidence of three-jet resonances consistent with strongly coupled supersymmet- ric particle decays. The event selection criteria are opti- mized in the context of the gluino model mentioned above. However, the generic features of the selection criteria provide a robust signature-based method that can be ap- plied to many extensions of the SM. The CMS detector is a multipurpose apparatus, de- scribed in detail in Ref. [10]. Here, we briefly describe the subdetectors most relevant to this analysis. The high- resolution silicon pixel and strip tracker provides charged tracking coverage for j�j< 2:4, where � ¼ � ln½tanð�=2Þ� is the pseudorapidity and � is the polar angle measured with respect to the counterclockwise pro- ton beam direction. Immersed in the 3.8 T magnetic field of the superconducting solenoid, the tracker provides trans- verse momentum resolution of approximately 1.5% for charged particles with pT � 100 GeV=c. Energy deposits of the jets are measured using electromagnetic (ECAL) and hadronic (HCAL) calorimeters. The ECAL has a barrel part and two endcaps, is composed of finely segmented crystals, and has an energy resolution of better than 0.5% for unconverted photons with transverse energies above 100 GeV. The ECAL barrel covers the pseudorapidity range j�j< 1:4 with a granularity of ����� ¼ 0:0174� 0:0174, where � is the azimuthal angle, and the endcaps cover 1:4< j�j< 3:0 with a granularity that decreases to 0:05� 0:05 for j�j � 3:0. A preshower de- tector consisting of two planes of silicon sensors inter- leaved with a total of three radiation lengths of lead is located in front of the ECAL endcaps. The HCAL extends up to j�j � 5:0 and its central and endcap regions consist of brass or scintillator sampling calorimeters that cover j�j< 3:0 with a granularity of ��� �� ¼ 0:087� 0:087 for central rapidities. The energy of charged pions and other quasistable hadrons is measured with the calo- rimeters (ECAL and HCAL combined) with a resolution of �E=E � 100%= ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E½GeV�p � 5%. Events are recorded using a two-tier trigger system. Objects satisfying the requirements at the first level (L1) are passed to the high level trigger (HLT) where the total recorded rate is limited to about �350 Hz. Triggers based on the sum of all transverse energy from jets (HT), reconstructed with only calorimeter information, are used to select recorded events. For the L1 trigger, theHT thresh- old is 50 GeV. The corresponding threshold for the HLT varies between 100 and 150 GeV, depending on the run period. *Full author list given at the end of the article. Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distri- bution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. PRL 107, 101801 (2011) P HY S I CA L R EV I EW LE T T E R S week ending 2 SEPTEMBER 2011 0031-9007=11=107(10)=101801(15) 101801-1 � 2011 CERN, for the CMS Collaboration http://dx.doi.org/10.1103/PhysRevLett.107.101801 http://creativecommons.org/licenses/by/3.0/ The CMS particle-flow algorithm [11] uses calorimeter information and combines it with reconstructed tracks to identify individual particles such as photons, leptons, and both neutral and charged hadrons within the jets. The energy of photons is directly obtained from the calibrated ECAL measurement. The energy of electrons is deter- mined from a combination of the track momentum at the main interaction vertex, the corresponding ECAL cluster energy, and the energy sum of all bremsstrahlung photons attached to the track. The energy of muons is obtained from the corresponding track momentum. The energy of charged hadrons is determined from a combination of the track momentum and the corresponding ECAL and HCAL en- ergy, corrected for zero-suppression effects, and calibrated for the nonlinear response of the calorimeters. Finally, the energy of neutral hadrons is obtained from the correspond- ing calibrated ECAL and HCAL energy. The particle-flow objects serve as input for jet reconstruction, performed using the anti-kT algorithm [12] with a distance parameter of 0.5 in �-� space. The jet energy resolution amounts typically to 15% at 10 GeV, 8% at 100 GeV, and 4% at 1 TeV, to be compared to about 40%, 12%, and 5% obtained when the calorimeters alone are used for jet clustering. Jet energy scale corrections [13] derived from Monte Carlo (MC) simulation are applied to account for the nonlinear and nonuniform response of the calorimeters. The jet momenta are first corrected to account for the presence of additional proton-proton interactions. Next, an exclusive sample of azimuthally back-to-back jets is used to derive a relative correction of up to 8% to remove the pseudorapidity dependence of the jet momentum re- sponse. Finally, the absolute scale of the jet momentum response is set by applying a factor determined from an exclusive sample of a well-measured photon azimuthally back-to-back with a single hadronic jet. Additionally in data, a small residual correction factor of about 1% is included to correct for differences in jet response between data and simulation. The combined corrections are on the order of 5%–10%, and their corresponding uncertainties range from 3% to 5%, depending on the measured jet’s pseudorapidity and energy. Jet quality criteria [14] are applied to remove misidentified jets arising primarily from calorimeter noise. For both data and simulated signal events, more than 99.8% of all selected jets satisfy these criteria. Pair-produced gluinos are used to model the signal. Gluino production and decay are simulated using the PYTHIA [15] MC program (v6.420), where each gluino decays to three jets through the �uds quark RPV coupling. This coupling is set such that the branching ratio B of the gluino to three light jets is 100%. The mass of the gluino is varied between 200 and 500 GeV=c2 in 50 GeV=c2 steps. The leading-order cross section from PYTHIA is 325 pb for a gluino mass of 200 GeV=c2, falling to � 1 pb for a gluino mass of 500 GeV=c2. For the generation of this signal all superpartners except the gluino are taken to be decoupled [7], the natural width of the gluino resonance is taken to be much smaller than the resolution of the detec- tor, and no intermediate particles are produced in the gluino decay. The next-to-leading-order (NLO) correction factors (K factors), with values ranging from 1.7 to 2.2, are calculated using the PROSPINO [16] program and are ap- plied to the leading-order cross sections, with uncertainties on the theoretical cross section that range from 15.5 to 17.1%. Simulation of the CMS detector is performed using GEANT4 [17]. Events recorded with the HT trigger are required offline to have a good reconstructed primary event vertex [18]. Pair-produced three-jet resonances naturally yield events with high jet multiplicity and large transverse energy. Thus we require events to contain at least six jets, and that the total scalar sum of the pT of those jets is larger than 425 GeV=c. The latter requirement also ensures that the trigger is fully efficient for these events. Jets are required to have pT > 45 GeV=c and j�j< 3:0, which also minimizes the effects from multiple proton-proton interactions. To reconstruct the gluinos, the six highest-pT jets are combined into all possible unique triplet combinations, resulting in 20 combinations of jet triplets. For signal events, each of the pair-produced gluinos corresponds to one of these 20 jet triplets, even in the case where all six jets come solely from the decay of these particles, leaving the 18 uncorrelated jet triplets as combinatorial back- ground. Thus, the overall background arises not only from SM events, described by quantum chromodynamics (QCD), but also from spurious jet triplet combinations in signal events themselves. We impose additional require- ments on each triplet to increase the signal sensitivity, while retaining as many signal triplets as possible. The invariant mass of background triplets is found to scale with the respective scalar sum of jet pT , while for signal triplets the mass is constant. To reduce background, we therefore require each jet triplet to satisfy the following relation: Mjjj < X3 i¼1 jpTji � �; (1) where Mjjj is the triplet invariant mass, P 3 i¼1 jpTji is the scalar sum of jet pT in the triplet (triplet scalar pT), and � is an offset adjusted to optimize signal sensitivity. Figure 1 shows the simulated triplet invariant mass versus the triplet scalar pT for a gluino mass of 250 GeV=c2, and the insert displays the invariant mass distribution before and after the requirement. For each event, all 20 triplet combinations are included. The value of � is determined by maximizing the ratio of the number of signal triplets to the sum of the number of signal plus background triplets in a 1 standard deviation (�) window around the center of the gluino mass peak. A common value of� ¼ 130 GeV=c2 is taken for all gluino masses considered, which gives an efficiency in PRL 107, 101801 (2011) P HY S I CA L R EV I EW LE T T E R S week ending 2 SEPTEMBER 2011 101801-2 signal events for triplets of 1 to 5%, and a background triplet selection efficiency of less than 0.05%. Even after the final selection, background remains from both QCD multijet events and uncorrelated triplets in gluino signal events. The latter only contribute minimally, and the shape of their distribution is found to be consistent with that of the dominant background, from QCD multijet events. These QCD multijet events arise from hard two- particle interactions combined with initial- and final-state radiation in the form of gluon jets. Although the cross section falls with increasing jet multiplicity (Njet), the underlying kinematic distributions are essentially the same among these events. Thus, we use a rescaled mass distribution of triplets in events with Njet ¼ 4, where the signal contributions are minimal, to estimate the shape of the background. Specifically, we select events in data with Njet ¼ 4 that satisfy all other selection criteria, form jet triplets, and require each to pass Eq. (1). TheMjjj values of these triplets are multiplied by the ratio of the average triplet scalar pT in data for events with Njet � 6 to the events with Njet ¼ 4, to account for expected minor kine- matic differences between the two samples. The resulting Mjjj distribution is then fit to an exponential function of the form: eP0þP1Mjjj , where P0 and P1 are free parameters. The slope P1 of the exponential function in the Njet � 6 sample is constrained to be equal to that found for the scaled Njet ¼ 4 fit within its uncertainties. This is verified in QCD simulation, and as a cross-check in data, we apply this procedure to predict the shape of the Mjjj distribution for an Njet ¼ 5 sample, where the QCD multijet back- ground is also expected to dominate, and find good agree- ment. To verify that the choice of the background model does not bias the derived limit, the exponential function is tested on an Njet � 6 sample, defined by the standard selection criteria without the requirement of Eq. (1) im- posed. The parameterization is found to be in agreement with the data in the fitted region, with the slope of the fit consistent with those of the Njet ¼ 4 and Njet ¼ 5 samples. To estimate the number of signal events expected after all selection criteria are applied, the sum of a Gaussian function that represents the signal and the exponential function that models the background is fitted to the simu- lated Mjjj distribution for each gluino mass. The fit is performed in the range 170