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Search for New Particles in Two-Jet Final States in 7 Tev Proton-Proton Collisions with the ATLAS Detector at the LHC

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Search for New Particles in Two-Jet Final States in 7 Tev Proton-Proton Collisions with the ATLAS Detector at the LHC Search for New Particles in Two-Jet Final States in 7 TeV Proton-Proton Collisions with the ATLAS Detector at the LHC The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation ATLAS Collaboration et al. "Search for New Particles in Two-Jet Final States in 7 TeV Proton-Proton Collisions with the ATLAS Detector at the LHC." Physical Review Letters, 105, 161801 (2010) © 2010 CERN, for the ATLAS Collaboration. As Published http://dx.doi.org/10.1103/PhysRevLett.105.161801 Publisher American Physical Society Version Final published version Citable link http://hdl.handle.net/1721.1/60850 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. week ending PRL 105, 161801 (2010) PHYSICAL REVIEW LETTERS 15 OCTOBER 2010 Search for New Particles in Two-Jet Final States in 7 TeV Proton-Proton Collisions with the ATLAS Detector at the LHC G. Aad et al.* (ATLAS Collaboration) (Received 13 August 2010; published 11 October 2010) A search for new heavy particles manifested as resonances in two-jet final states is presented. The data were produced in 7 TeV proton-proton collisions by the LHC and correspond to an integrated luminosity of 315 nbÀ1 collected by the ATLAS detector. No resonances were observed. Upper limits were set on the product of cross section and signal acceptance for excited-quark (qÃ) production as a function of qà mass. à These exclude at the 95% C.L. the q mass interval 0:30 <mqà < 1:26 TeV, extending the reach of previous experiments. DOI: 10.1103/PhysRevLett.105.161801 PACS numbers: 13.85.Rm, 12.60.Rc, 13.87.Ce, 14.80.Àj Two-jet (dijet) events in high-energy proton-proton (pp) The analysis technique consisted of a model- collisions are usually described in the standard model (SM) independent search for a dijet mass resonance on top of a by applying QCD to the scattering of beam-constituent smooth and rapidly falling spectrum and relied on the quarks and gluons. Several extensions beyond the SM measured mjj distribution to estimate the background level predict new heavy particles, accessible at LHC energies, to this new possible signal. In the absence of an observed that decay into two energetic partons. Such new states may new physics signal, upper limits were determined on prod- include an excited composite quark qÃ, exemplifying quark ucts of cross section () and signal acceptance (A) for substructure [1–3], an axigluon predicted by chiral color several qà test masses for a standard set of model models [4,5], a flavor-universal color-octet coloron [6,7], parameters. or a color-octet techni- meson predicted by models of The ATLAS detector [18] is a multipurpose particle extended technicolor and top-color-assisted technicolor physics apparatus with a forward-backward symmetric [8–11]. cylindrical geometry and near 4 coverage in solid angle Particularly sensitive to such new objects is the [19]. The overall layout of the detector is dominated by its jj four superconducting magnet systems, which comprise a pdijetffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi invariant mass observable, defined as m 2 2 thin solenoid surrounding inner tracking detectors and ðEj1 þ Ej2 Þ ðp~ j1 þ p~ j2 Þ , where E and p~ are the jet energy and momentum, respectively. Several experiments three large toroids with an eightfold azimuthal symmetry. have examined mjj distributions in search of new reso- The calorimeters, which are surrounded by an extensive nances [12–17]; recently, 1:13 fbÀ1 of pp collision data at muon system, are of particular importance to this analysis. j j 3 2 the Fermilab Tevatron collider have excluded the existence In the pseudorapidity region < : , high-granularity à liquid-argon (LAr) electromagnetic sampling calorimeters of excited quarks q with mass 260 <m à < 870 GeV q are used. An iron-scintillator tile calorimeter provides [16]. This Letter reports the first search by the ATLAS hadronic coverage in the range jj < 1:7. The end-cap experiment [18] at the LHC for such massive particles in pffiffiffi and forward regions, spanning 1:5 < jj < 4:9, are instru- pp collisions at a center-of-mass energy of s ¼ 7 TeV, mented with LAr calorimetry for both electromagnetic and based on a data sample corresponding to an integrated 1 hadronic measurements. luminosity of 315 nbÀ . The analysis presented here fo- The data sample was collected during stable periods of cused on a search for excited quarks because of the acces- 7TeVpp collisions using a trigger configuration requiring sible predicted cross section [2,3] for such particles and the the lowest-level hardware-based calorimeter jet trigger to benchmark nature of the model that allows limits on the satisfy a nominal transverse energy threshold of 15 GeV acceptance times cross section to be set for resonant states [20]. This trigger had an efficiency greater than 99% for with intrinsic widths narrower than the experimental events with at least one jet with transverse energy higher resolution. than 80 GeV. Jets were reconstructed by using the anti-kT jet cluster- ing algorithm [21] with a radius parameter R ¼ 0:6. The *Full author list given at the end of the article. inputs to this algorithm were clusters of calorimeter cells seeded by cells with energy significantly above the mea- Published by The American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distri- sured noise. Jet four-vectors were constructed by perform- bution of this work must maintain attribution to the author(s) and ing a four-vector sum over these cell clusters, treating each the published article’s title, journal citation, and DOI. as an ðE; p~Þ four-vector with zero mass. These were cor- 0031-9007=10=105(16)=161801(19) 161801-1 Ó 2010 CERN, for the ATLAS Collaboration week ending PRL 105, 161801 (2010) PHYSICAL REVIEW LETTERS 15 OCTOBER 2010 rected for the effects of calorimeter noncompensation and 104 inhomogeneities by using transverse-momentum (p ) and Data T Events ATLAS -dependent calibration factors based on Monte Carlo Fit 103 q*(500) s = 7 TeV (MC) corrections and validated with extensive test-beam q*(800) -1 jj ∫ L dt = 315 nb and collision-data studies [20,22]. The m observable was q*(1200) computed without unfolding jets to hadrons or partons. 102 In order to suppress cosmic-ray and beam-related back- grounds, events were required to contain at least one 10 primary collision vertex, defined by at least five recon- structed charged-particle tracks, each with a position, when 1 extrapolated to the beam line, of jzj < 10 cm. Events with at least two jets were retained if the highest pT jet (the j1 80 GeV B 2 500 1000 1500 ‘‘leading’’ jet) satisfied pT > and the next-to- j2 30 GeV leading jet satisfied pT > ; this ensured that the 0 data sample had high and unbiased trigger and jet recon- -2 (D - B) / struction efficiencies. Those events containing a poorly 500 1000 1500 15 GeV jj measured jet with pT > were vetoed to prevent Reconstructed m [GeV] cases where a jet was incorrectly identified as one of the two leading jets [23]; this affected the event selection by FIG. 1 (color online). The data (D) dijet mass distribution less than 0.5%. The two leading jets were required to (filled points) fitted by using a binned background (B) distribu- satisfy several quality criteria [23] and to lie outside de- tion described by Eq. (1) (histogram). The predicted qà signals tector regions where the jet energy was not yet measured in [2,3] for excited-quark masses of 500, 800, and 1200 GeV are an optimal way, such as the interval 1:3 < jjetj < 1:8. overlaid, and the bin-by-bin significance of the data-background Finally, both jets were required to be in the pseudorapidity difference is shown. region jjetj < 2:5, and their pseudorapidity difference was required to satisfy jj1 À j2 j < 1:3. These cuts, which acceptance (A), which included reconstruction and trigger suppress high-mass SM multijet background, were deter- efficiencies near 100%, was found to range from 31% for mined by performing an optimization of the potential mqà ¼ 300 GeV to 48% for mqà ¼ 1:7 TeV [29]. The à à signal from q decays (using a q mass of 1 TeV) compared choice of dijet mass binning was motivated by the dijet with the SM background. There were 132 433 candidates mass resolution of the signal. The predicted experimental jj 11% 300 GeV that satisfied these requirements. width ranged from mjj =m at mqà ¼ to jj The final event sample was selected by requiring the jj =m 7% at m à ¼ 1:7 TeV and was dominated by jj 200 GeV m q dijet invariant mass to satisfy m > in order to the detector energy resolution. jj eliminate any potential kinematic bias in the m distribu- The background shape was determined by fitting the tions from the selection requirements on the jet candidates. observed spectrum with the function [16] There were 37 805 events in this sample, which formed the jj p2 p3þp4 lnx m distribution shown in Fig. 1. fðxÞ¼p1ð1 À xÞ x ; (1) MC signal events were generated by using the excited- à pffiffiffi quark (qg ! q ) production model [2,3]. The excited where x mjj= s, such that fð1Þ¼0 and fð0Þ!þ1, à q 1=2 p4 lnx quark was assumed to have spin and quarklike and pf1;2;3;4g are free parameters.
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