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Measurement of the Triple-Differential Dijet Cross Section in Proton-Proton Collisions at √S = 8 Tev and Constraints on Parton Distribution Functions

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Measurement of the Triple-Differential Dijet Cross Section in Proton-Proton Collisions at √S = 8 Tev and Constraints on Parton Distribution Functions EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN) CERN-EP-2017-061 2017/11/13 CMS-SMP-16-011 Measurement of the triple-differential dijet cross section in proton-proton collisions at ps = 8 TeV and constraints on parton distribution functions The CMS Collaboration∗ Abstract A measurement is presented of the triple-differential dijet cross section at a centre- 1 of-mass energy of 8 TeV using 19.7 fb− of data collected with the CMS detector in proton-proton collisions at the LHC. The cross section is measured as a function of the average transverse momentum, half the rapidity separation, and the boost of the two leading jets in the event. The cross section is corrected for detector effects and com- pared to calculations in perturbative quantum chromodynamics at next-to-leading or- der accuracy, complemented with electroweak and nonperturbative corrections. New constraints on parton distribution functions are obtained and the inferred value of the strong coupling constant is a (M ) = 0.1199 0.0015 (exp) +0.0031 (theo), where M S Z ± 0.0020 Z is the mass of the Z boson. − Published in the European Physical Journal C as doi:10.1140/epjc/s10052-017-5286-7. arXiv:1705.02628v2 [hep-ex] 9 Nov 2017 c 2017 CERN for the benefit of the CMS Collaboration. CC-BY-4.0 license ∗See Appendix A for the list of collaboration members 1 1 Introduction The pairwise production of hadronic jets is one of the fundamental processes studied at hadron colliders. Dijet events with large transverse momenta can be described by parton-parton scat- tering in the context of quantum chromodynamics (QCD). Measurements of dijet cross sections can be used to thoroughly test predictions of perturbative QCD (pQCD) at high energies and to constrain parton distribution functions (PDFs). Previous measurements of dijet cross sections in proton-(anti)proton collisions have been performed as a function of dijet mass at the SppS,¯ ISR, and Tevatron colliders [1–6]. At the CERN LHC, dijet measurements as a function of dijet mass are reported in Refs. [7–11]. Also, dijet events have been studied triple-differentially in transverse energy and pseudorapidities h1 and h2 of the two leading jets [12, 13]. In this paper, a measurement of the triple-differential dijet cross section is presented as a func- tion of the average transverse momentum pT,avg = (pT,1 + pT,2)/2 of the two leading jets, half of their rapidity separation y = y y /2, and the boost of the dijet system y = y + y /2. ∗ j 1 − 2j b j 1 2j The dijet event topologies are illustrated in Fig. 1. | 2 y 3 − 1 y | 1 2 = ∗ y 2 1 0 0 1 2 3 y = 1 y + y b 2| 1 2| Figure 1: Illustration of the dijet event topologies in the y∗ and yb kinematic plane. The dijet system can be classified as a same-side or opposite-side jet event according to the boost yb of the two leading jets, thereby providing insight into the parton kinematics. The relation between the dijet rapidities and the parton momentum fractions x1,2 of the incom- pT y1 y2 ing protons at leading order (LO) is given by x1,2 = ps (e± + e± ), where pT = pT,1 = pT,2. For large values of yb, the momentum fractions carried by the incoming partons must corre- spond to one large and one small value, while for small yb the momentum fractions must be approximately equal. In addition, for high transverse momenta of the jets, x values are probed above 0.1, where the proton PDFs are less precisely known. The decomposition of the dijet cross section into the contributing partonic subprocesses is shown in Fig. 2 at next-to-leading order (NLO) accuracy, obtained using the NLOJET++ pro- gram version 4.1.3 [14, 15]. At small yb and large pT,avg a significant portion of the cross section 2 3 Event reconstruction and selection corresponds to quark-quark (and small amounts of antiquark-antiquark) scattering with vary- ing shares of equal- or unequal-type quarks. In contrast, for large yb more than 80% of the cross section corresponds to partonic subprocesses with at least one gluon participating in the interaction. As a consequence, new information about the PDFs can be derived from the mea- surement of the triple-differential dijet cross section. The data were collected with the CMS detector at ps = 8 TeV and correspond to an integrated 1 luminosity of 19.7 fb− . The measured cross section is corrected for detector effects and is com- pared to NLO calculations in pQCD, complemented with electroweak (EW) and nonperturba- tive (NP) corrections. Furthermore, constraints on the PDFs are studied and the strong coupling constant aS(MZ) is inferred. 2 The CMS detector The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal diam- eter, providing a magnetic field of 3.8 T. Within the solenoid volume are a silicon pixel and strip tracker, a lead tungstate crystal electromagnetic calorimeter (ECAL), and a brass and scin- tillator hadron calorimeter (HCAL), each composed of a barrel and two endcap sections. The silicon tracker measures charged particles within the pseudorapidity range h < 2.5. It consists j j of 1440 silicon pixel and 15 148 silicon strip detector modules. The ECAL consists of 75 848 lead tungstate crystals, which provide coverage in pseudorapidity h < 1.48 in a barrel region and j j 1.48 < h < 3.0 in two endcap regions. In the region h < 1.74, the HCAL cells have widths j j j j of 0.087 in pseudorapidity and 0.087 in azimuth (f). In the h-f plane, and for h < 1.48, the j j HCAL cells map on to 5 5 arrays of ECAL crystals to form calorimeter towers projecting ra- × dially outwards from close to the nominal interaction point. For h > 1.74, the coverage of the j j towers increases progressively to a maximum of 0.174 in Dh and Df. Within each tower, the energy deposits in ECAL and HCAL cells are summed to define the calorimeter tower energies, subsequently used to provide the energies and directions of hadronic jets. The forward hadron (HF) calorimeter extends the pseudorapidity coverage provided by the barrel and endcap de- tectors and uses steel as an absorber and quartz fibers as the sensitive material. The two halves of the HF are located 11.2 m from the interaction region, one on each end, and together they provide coverage in the range 3.0 < h < 5.2. Muons are measured in gas-ionisation detectors j j embedded in the steel flux-return yoke outside the solenoid. A more detailed description of the CMS detector, together with a definition of the coordinate system used and the relevant kinematic variables, can be found in Ref. [16]. 3 Event reconstruction and selection Dijet events are collected using five single-jet high-level triggers [17, 18], which require at least one jet with pT larger than 80, 140, 200, 260, and 320 GeV, respectively. At trigger level the jets are reconstructed with a simplified version of the particle-flow (PF) event reconstruction described in the following paragraph. All but the highest threshold trigger were prescaled in the 2012 LHC run. The triggers are employed in mutually exclusive regions of the pT,avg spectrum, cf. Table 1, in which their efficiency exceeds 99%. The PF event algorithm reconstructs and identifies particle candidates with an optimised com- bination of information from the various elements of the CMS detector [19]. The energy of photons is directly obtained from the ECAL measurement, corrected for zero-suppression ef- fects. The energy of electrons is determined from a combination of the electron momentum at 3 gg jets qiqi jets qiqi jets gg jets qiqi jets qiqi jets gq jets (xg < xq) qiqj jets qiqj jets gq jets (xg < xq) qiqj jets qiqj jets gq jets (xg > xq) gq jets (xg > xq) 8 TeV 8 TeV 1.0 1.0 0.8 0.8 0.6 0.6 Subprocess fraction 0.4 Subprocess fraction 0.4 0.2 0.2 0 yb < 1 0 yb < 1 0 y * < 1 1 y * < 2 0.0 0.0 200 300 500 1000 200 300 500 1000 pT, avg [GeV] pT, avg [GeV] gg jets qiqi jets qiqi jets gg jets qiqi jets qiqi jets gq jets (xg < xq) qiqj jets qiqj jets gq jets (xg < xq) qiqj jets qiqj jets gq jets (xg > xq) gq jets (xg > xq) 8 TeV 8 TeV 1.0 1.0 0.8 0.8 0.6 0.6 Subprocess fraction 0.4 Subprocess fraction 0.4 0.2 0.2 0 yb < 1 1 yb < 2 2 y * < 3 0 y * < 1 0.0 0.0 200 300 500 200 300 500 1000 pT, avg [GeV] pT, avg [GeV] gg jets qiqi jets qiqi jets gg jets qiqi jets qiqi jets gq jets (xg < xq) qiqj jets qiqj jets gq jets (xg < xq) qiqj jets qiqj jets gq jets (xg > xq) gq jets (xg > xq) 8 TeV 8 TeV 1.0 1.0 0.8 0.8 0.6 0.6 Subprocess fraction 0.4 Subprocess fraction 0.4 0.2 0.2 1 yb < 2 2 yb < 3 1 y * < 2 0 y * < 1 0.0 0.0 200 300 500 200 300 pT, avg [GeV] pT, avg [GeV] Figure 2: Relative contributions of all subprocesses to the total cross section at NLO as a func- tion of pT,avg in the various y∗ and yb bins.
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