Probing QCD with Photons and Jets at the ATLAS detector

Abstract. This contribution gives an overview of the recent measurements of the differential cross sections for final states involving photons and/or jets at the centre-of-mass energies of 8 and 13 TeV, published by the ATLAS Collaboration. The results are compared with several next-to-leading order calculations and with the latest predictions of various Monte Carlo generators. New measurements of transverse energy-energy correlations and their associated asymmetries in multi-jet events at 8 TeV are also presented. Both measurements are used to extract the strong coupling constant and test the renormalization group equation.


Introduction
The study of the production of jets and photons at the Large Hadron Collider (LHC) provides a powerful tool for understanding perturbative QCD (pQCD). The production of prompt photons in protonproton collisions, provides a testing ground for pQCD in a cleaner enviroment than in jet production, since the colorless photon originates directly from the hard interaction. The prompt photon analyses described in this contribution include both photons coming from the direct contribution and from the fragmentation process [1]. An isolation requirement is applied in order to reduce the background from photons produced during hadronisation. Moreover, the jet production measurements are used to contrain the gluon density in the proton and can be exploited to extract the strong coupling constant (α s ) in specific final state topologies. This paper presents the recent published measurements of the cross section of final states involving photons and/or jets at center-of-mass energies of 8 and 13 TeV with the data collected by ATLAS detector [2]. We also present a measurement of the transverse energy-energy correlations (TEEC) and their associate asymmetries (ATEEC) in multi-jet events at 8 TeV and a method to extract α s from them.  Figure 1. Ratio of the NLO pQCD predictions from Jetphox based on the MMHT2014 PDFs to the measured cross sections for isolated-photon production (solid lines) as a function of E γ T in four |η| bins [3]. The black error bars represent the experimental uncertainties, the dot-dashed lines represent the uncertainty due to the luminosity measurement and the shaded bands display the theoretical uncertainty. The ratio of the NLO pQCD predictions based on the CT14 (dashed lines) or NNPDF3.0 (dotted lines) PDF sets to the data are also included. description of the data, except for E γ T ≥ 500 GeV in the regions of |η γ | < 0.6 and 0.6 < |η γ | < 1.37. The photon energy is the dominant systematic uncertainty at high E γ T while, at low E γ T the uncertainty related to the background subtraction method dominates. The uncertainties are larger in the |η γ |forward regions. In Figure 1, the NLO pQCD predictions of Jetphox [6], using the MMHT2014 PDF set, are shown, providing an adequate description of the data within the theoretical uncertainties, which are larger than the experimental ones.

Photon + jet production
Measurements of the cross sections for the production of an isolated photon in association with up to three jets have been studied with the ATLAS detector at 8 TeV [7]. The photon is required to have E γ T > 130 GeV and |η γ | < 2.37. The jets are reconstructed using the anti-k t algorithm [8] with radius parameter 0.6. The differential cross sections for photon plus one jet are measured as functions of E γ T and p jet1 T with p jet1 T > 100 GeV. For the measurements of the cross sections as a function of m γ−jet1 and the scattering angle in the center-of-mass frame (| cos θ * |), an additional requirement is imposed: m γ−jet1 > 467 GeV. The NLO QCD predictions of Jetphox, corrected for hadronisation and underlying-event effects, give a good description of the measured cross section distributions in both shape and normalisation. In particular, the measured differential cross section as a function of | cos θ * | and its scale dependence is consistent with the dominance of processes in which a quark is being exchanged. Photon plus two-jets production is investigated by measuring differential cross sections as functions of E γ T and p a better description of p jet2 T and the angular distributions, respect to those of Pythia, which are based on 2 → 2 processes. The NLO QCD predictions of BlackHat [9] provide a good description of the measured differential cross sections, except for E γ T > 750 GeV, as shown in Figure 2. Photon plus three-jets production is characterised by measuring the differential cross sections as     Figure 2. Measured cross section for isolated-photon plus two jets production (dots) as a function of E γ T (left) and of the azimuthal separation between the photon and the sub-leading jet, ∆φ γ−jet2 , (right) [7]. The NLO QCD predictions of BlackHat based on the CT10 PDF set (solid lines) are also included for comparison. The bottom part of both figures shows the ratio of the NLO QCD prediction to the measured cross-section. The black error bars respresent the experimental uncertainties while the shaded bands are the theoretical uncertainties of the calculation.
functions of E γ T and p jet3 T and the azimuthal separation between the final state objects for p jet1 T > 100 GeV, p jet2 T > 65 GeV and p jet3 T > 50 GeV. Sherpa describes better than Pythia p jet3 T , while both of them give a good description of the angular distributions. The NLO QCD predictions of BlackHat provide a good description of the measured differential cross sections. The dynamics of the isolated-photon production in association with a jet have been also studied using the dataset collected with the ATLAS detector in 2015, with an integrated luminosity of 3.2 fb −1 [10]. Photons are required to have E γ T above 125 GeV. Jets are reconstructed using the anti-k t with radius parameter R = 0.4 and are required to have p jet T greater than 100 GeV and |y jet | < 2.37. Measurements of isolated-photon plus jet cross sections are presented as functions of the leading photon tranverse energy, the leading jet transverse momentum p jet−lead T , the azimuthal separation between the photon and the jet, the invariant mass of the photon-jet system and | cos θ * |. The differential cross sections as a function of m γ−jet and | cos θ * | are performed in an unbiased phase-space by requiring: |η γ + y jet−lead | < 2.37, | cos θ * | < 0.83 and m γ−jet > 450 GeV. The LO predictions of Pythia and Sherpa give a good description of the data, except for p jet−lead T in the case of Pythia. The fixed-order NLO QCD calculations of Jetphox, corrected for hadronisation and underlying event effects, and the multi-leg NLO QCD plus parton shower calculations of Sherpa describe the measured cross sections within the theoretical uncertainties, larger than the experimental ones, as shown in Figure 3. The comparison with these predictions that use different PDF sets, show that the description of the data achieved  does not depend on the used PDF set. The measured dσ/| cos θ * | is consistent with the dominance of processes in which a quark has been exchanged.

Photon pair production
Measurements of a photon pair production has been studied at a center-of-mass energy of 8 TeV [11]. The leading and sub-leading photons are required to have E γ(1) T > 40 GeV and E γ (2) T > 30 GeV respectevely and |η γ | < 2.37 both. Differential cross sections are mesured as functions of the main variables of the photon pair system, together with the trasverse component of the diphoton tri-momentum respect to the thrust axis (a T ). The fixed-order QCD calculations of Diphox [12] and Resbos [13] at NLO, 2γNNLO at NNLO [14] are compared to the data. These predictions are unable to reproduce the measured cross sections: Resbos and Diphox miss higher orders, while the NNLO calculations of 2γNNLO improve the description, but are still insufficient to describe the data. The predictions of a parton-level calculation of varying the jet multiplicity up to NLO 1 , matched to the parton shower algorithm in Sherpa 2.2.1 provide a good description of the data. Figure 4 shows the comparison between the measured differential cross section as a function of a T and the fiducial cross section compared to the different pQCD calculations.

Inclusive jet production at 8 TeV
The inclusive jet cross-sections have been measured using the ATLAS 8 TeV dataset [15]. Jets are reconstructed with the anti-k t algorithm with jet radius parameter values of R = 0.4 and R = 0.6, in the kinematic region of the jet transverse momentum from p T > 70 GeV and |y jet | < 3. The cross sections are measured double-differentially in the jet transverse momentum and rapidity, as shown in Figure 5 (left). A fair agreement has been found in the comparison between the measured cross sections and the fixed-order NLO QCD calculations for different PDF sets, shown in Figure 5

Inclusive and dijet production at 13 TeV
The inclusive jet and dijet cross sections have also been measured using the 3.2 fb −1 ATLAS dataset at 13 TeV [16]. The jets are reconstructed using the anti-k t with radius parameter R = 0.4. The cross sections were measured following the same method as for the 8 TeV analysis. The double-differentially cross sections for the 13 TeV measurement are shown in Figure 6 (left). NLO and NNLO pQCD calculations for the inclusive jet measurement, corrected for non-perturbative and electroweak effects, are compared to the measured cross sections. As for the 8 TeV measurement, a good agreement has been found when considering the jet cross sections in individual jet rapidity bins independently. No significant deviations between the inclusive jet cross sections and the fixed order NNLO QCD calculations, corrected for non-perturbative and electroweak effects, are observed when using the transverse momentum of each jet, p jet T , as QCD scale, as shown in Figure 6 (right).  [16]. (Right) Ratio of the inclusive jet cross-section predicted by NLO (red) and NNLO (black) QCD predictions to the cross-section in data as a function of the jet p T in each jet rapidity bin. The error bars indicate the total theory uncertainty. The grey band shows the total uncertainty in the measurement.
In Figure 7 (left), the double-differential dijet production cross sections are presented as a function of the dijet invariant mass from 300 GeV to 9 TeV and half absolute rapidity separation between the two leading jets (y * ), up to y * <3. A fair agreement has been found in the comparison between the

Transverse energy-energy correlations and α s extraction
Transverse energy-energy correlations and the relative asymmetries in multi-jet events have been measured using the 8 TeV ATLAS dataset [17]. The data were binned in six intervals of the scalar sum of the transverse momenta of the two leading jets, H T2 = p T1 + p T2 and compared to the NLO pQCD predictions, corrected for hadronisation and multi-parton interaction effects. The comparison shows that the data are compatible with the theoretical predictions, within the uncertainties. The results are then used to determine the strong coupling constant α s and its evolution with a chosed scale Q = H T2 /2, by means of a χ 2 fit to the theoretical predictions for both TEEC and ATEEC in each energy bin. The results of the χ 2 fit, together with other experiment determinations are shown in Figure 8

Summary
In this paper high-precision measurements involving photons and jets at 8 TeV are presented together with the first results at 13 TeV. All the mesurements are in good agreement with the pQCD predictions ICNFP 2017 within the theoretical uncertainties, expecially those coming from missing higher order terms in the calculations. For the future, the ATLAS collaboration plans to improve these studies with higher statistics in order to explore the new energy regime opened by the LHC at 13 TeV and to provide valuable physics inputs to PDF and α s fits.