Photon-rejection Power of the Light Dark Matter eXperiment in an 8 GeV Beam

The Light Dark Matter eXperiment (LDMX) is an electron-beam fixed-target experiment designed to achieve comprehensive model independent sensitivity to dark matter particles in the sub-GeV mass region. An upgrade to the LCLS-II accelerator will increase the beam energy available to LDMX from 4 to 8 GeV. Using detailed GEANT4-based simulations, we investigate the effect of the increased beam energy on the capabilities to separate signal and background, and demonstrate that the veto methodology developed for 4 GeV successfully rejects photon-induced backgrounds for at least $2\times10^{14}$ electrons on target at 8 GeV.Comment: 28 pages, 20 figures; corrected author lis

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements

Measurement of the ratio R=BR(t->Wb)/BR(t->Wq) at CDF

In the Stardard Model of elementary particles, the top quark completes the third quarks generation. It was directly observed in 1995 during Tevatron Run I at $\sqrt{s}=1.8$ TeV by both CDF and D0 experiments \cite{top1,top2,top3}. It is the most massive elementary known particle up until now, with a mass of 172.7 $\pm$ 1.1(stat + syst) GeV/c$^{2}$ \cite{masstop}, about 35 times larger than the mass oft the next heavy quark and very close to the scale of the electroweak symmetry breaking. Produced in Tevatron in proton-antiproton collisions via strong interactions, top quark decays trough weak interaction to a $W$ boson and a down-type quark $q$ ($d$,$s$,$b$) before forming hadrons, giving the possibility to study the properties of a \textit{bare} quark. In the Standard Model the decay rate is proportional to $\left|V_{tq}\right|^{2}$, the Cabibbo-Kobayashi-Maskawa (CKM) matrix element. Since the assumption of three generation of quarks and the unitarity of the CKM matrix lead to $\left |V_{tb}\right|=0.99915^{+0.00003}_{-0.00005}$ \cite{PDG}, it can be assumed that top quark decays exclusively to $Wb$. On the other hand, if more than three generation of quarks are allowed, the constraint on $\left|V_{tb}\right|$ is removed and lower values are possible, affecting top cross section measurements, B mixing and CP violation. A direct measurement of $\left|V_{tb}\right|$ matrix element can be obtained measuring the single top production cross section, but a value can be extracted from the top quark decay rate in the $t \bar t$ channel. It is possible to define $R$ as the ratio of the branching fractions: \begin{equation} R =\frac{\mathscr{B}(t\rightarrow Wb)}{\mathscr{B}(t\rightarrow Wq)} =\frac{\left|V_{tb}\right|^2}{\left|V_{tb}\right|^2+\left|V_{ts}\right|^2+\left|V_{td}\right|^2} \end{equation} expected to be $0.99830^{+0.00006}_{-0.00009}$ if the same constraints are assumed. In this analysis we measured directly the the ratio of the branching fractions R using a data sample corresponding to 7.5 fb$^{-1}$ collected at the CDF detector at $\sqrt{s}=$1.96 TeV. The analysis is performed in the lepton plus jets (l+jets) channel, where one $W$ boson, coming from $t\bar t \rightarrow W^{+}qW^{-}\bar q$, decays hadronically while the second decays in a charged lepton and a neutrino. CDF performed several measurements of $R$ both during Run I and Run II, combinating the l+jets channel with the dilepton channel, where both of $W$ bosons produced by top pairs decay leptonically. The last measurement found a central value of $R=1.12^{+0.21}_{-0.19}$(stat)$^{+0.17}_{0.13}(syst)$ using an integrated luminosity of 162 pb$^{-1}$, extracting $R>0.61$ at 95\% CL. The D\O\ collaboration has measured recently $R$, using 5.4 fb$^{-1}$, with a simultaneous fit on the top pair production cross section, in the l+jets and dilepton channels. Their result is $R=0.90\pm0.04$(stat+syst) and $R>0.79$ at 95\% CL. Since the uncertainty on the central value measured by CDF was dominated by the statistical error, we decided to perform a new measurement adding the new datasets. My analysis is based on the determination of the number of b-jets in $t \bar t$ events using the l+jets sample with more than three jets in the final state. We consider events in which the charged leptons are either electrons or muons. Identification of jets coming from b-quark fragmentation (b-jet \textit{tagging}) is performed by the \textit{SecVtx} algorithm, based on the reconstruction of displaced secondary vertices. We divided our sample in subsets according to the type of lepton, number of jets in the final states and events with zero, one or two tags. The comparison between the total prediction, given by the sum of the expected $t\bar t$ events and background estimate, and the observed data in each subsample is made using a Likelihood function. Our measured value for R is that one which maximizes the Likelihood, i.e. gives the best match between the observed events and prediction. Our final measurement of R is obtained recursively performing a simultaneous fit also to top pair production cross section. We obtain $R=0.92 \pm 0.07$ (stat+syst) and $\sigma_{p\bar p \rightarrow t \bar t} = 7.4 \pm 0.6$ pb. Assuming the unitarity of the CKM matrix and three generation of quarks we obtained $\left|V_{tb}\right| = 0.95 \pm 0.04$, in agreement with the Standard Model prediction

Advanced alignment of the ATLAS tracking system

In order to reconstruct the trajectories of charged particles, the ATLAS experiment exploits a tracking system built using different technologies, silicon planar modules or microstrips (PIX and SCT detectors) and gaseous drift tubes (TRT), all embedded in a 2T solenoidal magnetic field. Misalignments of the active detector elements and deformations of the structures (which can lead to \textit{Weak Modes}) deteriorate resolution of the track reconstruction and lead to systematic biases on the measured track parameters. The applied alignment procedures exploit various advanced techniques in order to minimise track-hit residuals and remove detector deformations. For the LHC Run II, the Pixel Detector has been refurbished and upgraded with the installation of a new pixel layer, the Insertable B-layer (IBL)

Search for scalar top quarks decaying into scalar tau leptons with ATLAS at sqrt{s} =8 TeV

This thesis presents a search for Supersymmetry carried out in a particular scenario arising from the Gauge Mediated Supersymmetry breaking mechanism that assumes a massless gravitino as lightest supersymmetric particle, a scalar tau lepton as next-to-lightest supersymmetric particle and the top squark as the lightest among the quark superpartners. The analysis is performed using the data collected by ATLAS at a centre-of-mass energy √s = 8 TeV during 2012 data taking, for a total of 20.3 fb−1 of integrated luminosity of proton-proton collisions. Scalar top quark candidates are searched for in events with either two light leptons, one hadronically decaying tau and one light lepton or two hadronically decaying taus in the final state. No significant excess over the Standard Model expectation is found and the results are interpreted as 95% confidence lower limits not top squark and scalar tau masses. Depending on the scalar tau mass, lower limits between 490 and 650 GeV are placed on the top squark mass within the model considered. This thesis presents also the results of the track-based alignment of the ATLAS Inner Detector during 2015 data taking campaign and the characterisation of the mechanical deformation of the Insertable B-Layer as function of the operating temperature

Operational Experience and Performance with the ATLAS Pixel detector with emphasis on radiation damage

The tracking performance of the ATLAS detector relies critically on its 4-layer Pixel Detector, that has undergone significant hardware and software upgrades to meet the challenges imposed by the higher collision energy, pileup and luminosity that are being delivered by the Large Hadron Collider, with record breaking instantaneous luminosities of 1.3 x 10^34 cm-2 s-1 recently surpassed. The key status and performance metrics of the ATLAS Pixel Detector are summarised, and the operational experience and requirements to ensure optimum data quality and data taking efficiency are described, with special emphasis to radiation damage experience

Searches for direct pair production of third generation squarks with the ATLAS detector

Naturalness arguments for weak-scale supersymmetry favour supersymmetric partners of the third generation quarks with masses not too far from those of their Standard Model counterparts. Top or bottom squarks with masses of a few hundred GeV can also give rise to large direct pair production rates at the LHC. The talk presents recent ATLAS results from searches for direct stop and sbottom pair production, using 20/fb of 8 TeV pp collision data, and prospects for 13 TeV Run-2 data are also included

Single Event Upsets in the ATLAS IBL Frontend ASICs

During operation at instantaneous luminosities of up to 1.5 /s/cm^2 the frontend chips of the ATLAS innermost pixel layer (IBL) experienced single event upsets affecting its global registers as well as the settings for the individual pixels, causing, amongst other things loss of occupancy, noisy pixels, and silent pixels. A quantitative analysis of the single event upsets as well as the operational issues and mitigation techniques will be presented

Alignment of the ATLAS Inner Detector Upgraded for the LHC Run II

ATLAS is a multipurpose experiment at the LHC proton-proton collider. Its physics goals require high resolution, unbiased measurement of all charged particle kinematic parameters. These critically depend on the layout and performance of the tracking system, notably quality of its offline alignment. ATLAS is equipped with a tracking system built using different technologies, silicon planar sensors (pixel and micro-strip) and gaseous drift- tubes, all embedded in a 2T solenoidal magnetic field. For the LHC Run II, the system has been upgraded with the installation of a new pixel layer, the Insertable B-layer (IBL). Offline track alignment of the ATLAS tracking system has to deal with about 700,000 degrees of freedom (DoF) defining its geometrical parameters. The task requires using very large data sets and represents a considerable numerical challenge in terms of both CPU time and precision. The adopted strategy uses a hierarchical approach to alignment, combining local and global least squares techniques. An outline of the track based alignment approach and its implementation within the ATLAS software will be presented. Special attention will be paid to integration to the alignment framework of the IBL, which plays the key role in precise reconstruction of the collider luminous region, interaction vertices and identification of long-lived heavy flavor states. Techniques allowing to pinpoint and eliminate tracking systematics due to alignment as well as strategies to deal with time-dependent variations will be briefly covered. The first results from Cosmic Ray commissioning runs and status from proton-proton collision in LHC Run II will be discussed