Session U13: LHC Searches II

Search for Vector-like Quarks using a Combination of Decay Channels in pp Collisions at $\sqrt{s}$ = 8 TeV Collected with the ATLAS Detector

Vector-like quarks are predicted in several beyond the Standard Model theories. In some models, there is strong coupling to third generation quarks. The vector-like heavy quarks T and B can decay into several different channels involving third generation quarks. The ATLAS experiment has searched for vector-like quarks in several different decay channels using data collected in pp collisions at $\sqrt{s}$ = 8 TeV. We present search results by combining the results from the individual decay channel searches. The combination search improves the sensitivity to observing or excluding vector-like quarks.   

Search for New Physics in the Photon$+$MET Final State

With the recent discovery of the Higgs boson at the Large Hadron Collider, the next goals of the Compact Muon Solenoid (CMS) Experiment include characterizing this new particle and probing for new physics beyond the Standard Model (SM). We present a search for new physics which results in a final state consisting of a low pt photon, low missing transverse energy and low jet multiplicity, which can arise from models involving dark matter production or exotic decays of the Higgs in low scale supersymmetry breaking scenarios. This analysis is extremely challenging due to the lack of a fully reconstructed final state, the low energy spectrum of the final state objects, as well as the large SM backgrounds.   

Search for a heavy higgs bosons that decays to light higgs bosons in the minimally supersymmetric standard model using $\tau$ final states

A search for the extension of the higgs sector to the two higgs doublet model is presented. Decays of the heavy scalar (H) and pseudo-scalar (A) higgs bosons in their decays $H\rightarrow hh \rightarrow \tau\tau bb$ and $A\rightarrow Zh \rightarrow ll\tau\tau$ include the standard model-like higgs in the final state. Background estimations use a data driven approach. A binned maximum likelihood fit to the signal monte carlo using the di-tau secondary vertex fit algorithm improves discrimination as compared to previous analyses. This search is performed using a 19.5 fb$^{-1}$ data sample at $\sqrt{s}$ = 8 TeV collected by the CMS experiment at the LHC.   

Discovering Unexpected Signals through Background Ranking

The Matrix Element Method (MEM) has become an important tool in experimental particle physics, as it provides optimal sensitivity in using data to distinguish between models that could explain that data. However, it has generally required some concrete model to describe the potential signal. Motivated by the possibility of surprises at the Large Hadron Collider (LHC), we develop MEM-based methods for determining the presence of new physics without assumptions about the signal process responsible for the new physics. One such method, which we believe is especially robust with respect to systematic uncertainties, involves the ranking of events with respect to the value of background matrix element.   

Improved search for $ZH\rightarrow \ell^{+}\ell^{-}b\bar{b}$ using 9.7 fb$^{-1}$ of data collected by the D0 detector

We present an improved search for the standard model Higgs boson produced in association with a $Z$ boson, using 9.7 fb$^{-1}$ of $p\bar{p}$ collision data collected by D0 detector at $\sqrt{s} =1.96$ TeV. Events are selected with two electrons or two muons that are consistent with the decay of a $Z$ boson, and at least two reconstructed jets (including at least one $b$-tagged jet). Four dedicated random forests of decision trees (RFs) are trained in order to distinguish the signal with $t\bar{t}$, $Z$+Heavy Flavor jets, $Z$+Light Flavor jets and diboson background events respectively. The final discriminant is trained separately in five regions according to the output of the RFs. Upper limits on the $ZH$ production cross-section times branching ratio to two $b$-jets are set at 95$\%$ C.L. We also use the minimum walking technicolor model ($\rho_{TC} \rightarrow Z\pi_{TC} \rightarrow \ell^+\ell^-b\bar{b}$) to interpret the results we obtained.   

Search for Supersymmetry in Diphoton Events with Large Missing Transverse Momentum in 8 TeV pp collisions with the ATLAS Detector

We describe a search for physics processes beyond the standard model using events with two photons and large missing transverse energy. The search analyzes 20.3 $fb^{-1}$ of data taken with pp collisions at $\sqrt{s}$ = 8 TeV by the ATLAS experiment at the Large Hadron Collider. The search for new physics considers two different production modes, electroweak and strong, as well as lower and higher mass binos which yield different kinematic distributions. The rates and kinematic distributions are found to be consistent with Standard Model expectations, and limits are set on possible new physics scenarios with the final state of two photons and missing transverse energy.   

Search for weakly-produced supersymmetric particles in the multi-lepton and missing energy channel at the ATLAS experiment

With null results so far from all the searches for signatures from the high-rate strong production of supersymmetric particles, the search for rarer signatures from the electroweak production of gauginos could provide an accessible discovery mode at the 8 TeV LHC. This talk presents the latest results from the ATLAS experiment on pair production of charginos and neutralinos, which then decay to yield a multi-lepton final state with missing transverse energy. We interpret these results for a simplified model where the gauginos decay via sleptons, and for a simplified model where the gauginos decay via W and Z bosons. We also present the results of searches for these two decay chains in the context of phenomenological MSSM.