DØ Results on Diphoton Direct Production and Photon + b and c J et Production

In this note we present measurements of the direct photon pai r production cross sections using 8.5 fb−1 of data collected with the DØ detector at the Fermilab Tevatr on pp̄ collider at √ s = 1.96 TeV. The results are shown as di fferential distributions with respect to the photon pair mass , pair transverse momentum, azimuthal angle, and polar scattering angle in the CollinsSoper frame. We also present measurements of the differential cross section dσ/dpγT for the inclusive production of a photon in association with abor c-quark jet. The results are based on 8.7 fb −1 of data, and the measured cross sections are compared with ne xt-to-l ading order perturbative QCD calculations using di fferent sets of parton distribution functions as well as to pre dictions based on the kT-factorization QCD approach, and those from v arious Monte Carlo event generators.


Introduction
Direct photon production in hadron collisions provides a precise probe of perturbative Quantum Chromodynamics (pQCD), particularly as a test of soft gluon resummation.Diphoton production can proceed through Born-level and box diagrams in pQCD, as well as single or double parton fragmentation.In addition, high mass diphoton pairs form a major irreducible background to searches for Higgs boson production via the H → γγ decay channel.Therefore, the measurement of diphoton production cross sections at the Tevatron collider is important for both comparisons to QCD as well as for search for the Higgs or new phenomena producing diphoton resonances.In this note we present single differential cross-section for diphoton production, in bins of the diphoton invariant mass M γγ , total transverse momentum of the pair p γγ T , and polar scattering angle | cos θ * | in the frame with no net transverse momentum (computed as cos θ * = tanh (η 1 − η 2 )/2).We also compare diphoton production for the cases of small and large opening angles between the two photons.
Photon plus jet events can also be used as a probe of the partonic structure of the protons and antiprotons.Measurement of the production of isolated photons in association with jets initiated by b-or c-type quarks can provide a test of the Parton distribution Functions (PDFs) of these quark species in the colliding hadrons.
The data presented were taken in p p collisions at a center-of-mass energy of √ s = 1.96TeV, collected with the DØ detector, a detailed description of which can be found in Ref. [1].We select events with photons with calorimeter and track isolations and also satisfying shower shape cuts based on the calorirmeter and CPS.Please see a. e-mail: sawyer@phys.latech.edudetails in [2] and [3].Jets are defined by the Run II midpoint cone jet algorithm [4] with a cone radius of R cone = (∆y) 2 + (∆φ) 2 = 0.7 in rapidity y and azimuthal angle φ.

Direct Diphoton Production
The data used in this analysis were collected using a combination of triggers requiring at least two clusters of energy in the EM calorimeter with loose shower shape requirements and varying p T thresholds between 15 GeV and 25 GeV, and correspond to an integrated luminosity of 8.5 ± 0.5fb −1 .The two photons had to have p T > 18 and 17 GeV and be with |η| < 0.9 and a separation of ∆R(γγ) > 0.4.An artificial neural network was applied to further reject jet backgrounds; see [3] for additional details, as well as estimates of backgrounds, uncertainties, and details of the theoretical comparisons.
Cross sections were calculated as dσ dX = N γγ ǫ×A×L×∆ X where X represents the kinematical variable being studies (M γγ , p γγ T , ∆φ γγ , | cos θ * |) and ∆ X is the bin width in this variable.The event selection efficiency ǫ, acceptance A, and integrated L, along with the background estimation on the number of diphoton events N γγ contribute a total systemic uncertainty of around +15/-12%, relatively flat over most of the M γγ range.
Single differential cross section are shown in Double differential cross section are formed in two bins bins of the diphoton azimuthal opening angle ∆φ γγ : ∆φ γγ < π 2 and ∆φ γγ > π 2 .Figures 3 and 4 show the the cross sections for M γγ , p γγ T , and | cos θ * | in the two ∆φ γγ bins.

Photon + b-Jet Cross Section
The production of an isolated photon in association with a b-quark provides important tests of Quantum Chromodyamics, and can yield information about the b-quark and gluon PDFs.For low p T photons, production is dominated by the Compton-like scattering of a gluon (gb → γb), while for photons with p T ≥ 70 GeV quark-antiquark annihilation q q → γg → γb b dominates.Photons may also be produced in quark or gluon fragmentation, but this is suppressed by requiring the photon to be isolated.estimates of uncertainties, and details of the comparison to theoretical predictions.
In figure 6 we show the measured cross section for γ + b-jet production as a function of p γ T .along with the-oretical predictions.The uncertainties on the data points include statistical and systematic contributions added in quadrature.
In the figure 7 the ratio of data to the NLO pQCD predictions are shown.The uncertainties on the data include both statistical (inner error bar) and full uncertainties (entire error bar).

Photon + c-jet Cross Section
In a similar fashion to the γ + b analysis, we can also extract the fraction of c-quark jets from the liklihood template fits to the M S V distribution.This allows us to measure the cross section of γ + c-jets production as a function of p γ T .Please see [14] for complete analysis details. Figure 8 shows the resulting cross section measurement for γ + c-jet events, while figure 9 shows the ratio of the measured cross sections of γ + c-jet events to γ + b-jet events as a function of p γ T .The uncertainties on the data points include statistical and systematic contributions added in quadrature.In both plots, comparisons are made to pQCD and kT-factorization calculations as well as Monte Carlo generator predictions. .The measurements are compared to the NLO QCD calculations [11] using CTEQ6.6MPDFs (solid line).The predictions from SHERPA, PYTHIA and a "kT factorization" approach [12] are shown by the dash-dotted, dotted and dashed lines, respectively.LHCP 2013 14006-p.5

Fig- ures 1 and 2 .Figure 1 .Figure 2 .
Figure 1.(left) The diphoton cross section as a function of the invariant mass M γγ of the photon pair.(right) The diphoton cross section as a function of the transverse momentum p γγ T of the photon pair.Error bars include systematic uncertainties for both plots.

Figure 7 .Figure 6 .
Figure 7.The ratio of γ + b production differential cross sections between data and NLO QCD predictions with uncertainties for the rapidity regions |y| < 1.0 (a) and 1.5 < |y| < 2.5 (b).Also shown are the uncertainties on the theoretical QCD scales and the CTEQ6.6MPDFs.The ratio of NLO predictions with CTEQ6.6M to those with MSTW2008 and ABKM09NLO[13] are also shown.

Figure 8 .Figure 9 .
Figure 8.The γ + c-jet differential production cross sections as a function of p T .The horizontal error bars show the p T bins.The measurements are compared to the NLO QCD calculations using CTEQ6.6MPDFs (solid line).The predictions from SHERPA, PYTHIA, and "kT factorization" approach are shown by the dash-dotted, dotted and dashed lines, respectively.