Search for Dark Matter and Large Extra Dimensions in the photon + MET final state in pp Collisions at sqrt ( s ) = 13 TeV

A search is conducted for dark matter pair-production and for graviton production predicted by the ADD large extra dimensions model in a final state with a photon and missing transverse energy in pp collisions at sqrt(s) = 13 TeV. Data taken by the CMS experiment at the CERN LHC in 2016 corresponding to an integrated luminosity of 12.9 fb-1 is analyzed. We find no deviation from the Standard Model prediction for this final state, and achieve an extension of the current limits on parameter space. Presented at DAE-HEP Symposium XXII DAE-BRNS High Energy Physics Symposium Search for Dark Matter and Large Extra Dimensions in the photon + MET final state in pp Collisions at √ s = 13 TeV Ashim Roy On behalf of the CMS Collaboration Saha Institute of Nuclear Physics, HBNI, Kolkata-64, India ashim.roy@saha.ac.in, Abstract. A search is conducted for dark matter pair-production and for graviton production predicted by the ADD large extra dimensions model in a final state with a photon and missing transverse energy in pp collisions at √ s = 13 TeV. Data taken by the CMS experiment at the CERN LHC in 2016 corresponding to an integrated luminosity of 12.9 fb−1 is analyzed. We find no deviation from the Standard Model prediction for this final state, and achieve an extension of the current limits on parameter space. A search is conducted for dark matter pair-production and for graviton production predicted by the ADD large extra dimensions model in a final state with a photon and missing transverse energy in pp collisions at √ s = 13 TeV. Data taken by the CMS experiment at the CERN LHC in 2016 corresponding to an integrated luminosity of 12.9 fb−1 is analyzed. We find no deviation from the Standard Model prediction for this final state, and achieve an extension of the current limits on parameter space.


Introduction
A search in pp collisions at the Large Hadron Collider (LHC) for a final state consisting of a photon (γ) of large transverse momentum (p T ) and missing transverse momentum (E miss T ) is used to investigate two extensions to the standard model (SM). We search for dark matter (DM), which is considered to be the dominant non-baryonic contribution to the matter density of the universe [1], its detection and identification in ground-based and spaceborne experiments remain elusive. At LHC, production of DM particles may be inferred from pp collisions with large E miss T , if the DM particles indirectly interact with the standard model (SM) quarks and gluons via forces of the electroweak scale.
The second SM extension is the large extra dimension model [2], which is motivated by the hierarchy problem, namely the large gap between the electroweak (M EW ) and Planck (M Pl ) scales. This model postulates that there exist n compactified extra dimensions, in which gravitons can propagate freely, and that the true scale of the gravitational interaction in this 4 + n dimension spacetime (M D ) is of the same order as M EW . The compactification length scale R of the additional dimensions can be large from a particle physics perspective, admitting a near-continuous mass spectrum of the Kaluza-Klein graviton states. Production of such gravitons at the LHC will therefore also manifest itself as events with broadly distributed in E miss T . Events with large missing transverse momentum exist only if there are visible objects recoiling against the invisible particles. Among the many possibilities, a recoiling photon ( γ ) has the advantage of being identifiable with high efficiency and purity. The results of the search are interpreted in terms of the DM simplified models as proposed by the CMS-ATLAS Dark matter forum group [3] and the ADD graviton production.

Event Selection and Background Estimation
The analyzed data sample corresponds to an integrated luminosity of 12.9 fb −1 is collected by CMS detector [4]. Events are required to have E miss T > 170 GeV and at least one photon with p γ T > 175 GeV in the central region (|η| < 1.44) of the detector and vetoed with well-identified electrons and muons. Events are rejected if the minimum azimuthal opening angle (∆φ) between E miss T and up to four leading jets is less than 0.5 radian. This requirement significantly suppresses the background where jet energy mismeasurement gives rise to large E miss T . Only jets with p T > 30 GeV and |η| < 5 are considered. The candidate photon and E miss T must also be separated by more than 2 radians. The major background from the Z(→ νν) + γ and W (→ lν) + γ processes is estimated using simulated events with NNLO QCD and NLO EWK corrections and cross-checked with control data dominated by well-reconstructed Z(→ l + l − ) + γ and W (→ lν) + γ events. Background from jets or electrons mis-identified as photons is estimated by measuring the mis-identification rates in control samples in data. Finally, non-collision backgrounds due to beam halo and ECAL spikes are estimated by fits to distributions of the azimuthal angle (φ) and the electromagnetic cluster seed time.

Results and Interpretation
A total of 400 events are observed in data, which is in agreement with the total expected background of 414.6 ± 38 events. Because no excess with respect to the SM prediction is observed, limits are set on the considered DM production models and ADD extra dimension scenarios. For each signal model, a 95% confidence level (CL) cross section upper bound is obtained utilizing the asymptotic CL s prescription [5][6][7]

Limits on simplified dark matter models
The DM simplified models are designed to facilitate the comparison and translation of various DM search results by limiting the degrees of freedom of the DM production interaction. In the models considered in this analysis, Dirac DM particles couple to a vector or axial-vector mediator, which in turn has couplings to the SM quarks. Model points are identified by a set of four parameters: the DM mass m DM , the mediator mass M med , the universal mediator coupling to quarks g q and the mediator coupling to DM g DM . In this analysis, we fix the values of g q and g DM to 0.25 and 1, respectively, and scan the M med -m DM plane. Figure 1 shows the cross section upper limits with respect to the corresponding