Supersymmetry searches at ATLAS and CMS

In this thesis, my work in the Compact Muon Solenoid (CMS) experiment on the search for the neutral Minimal Supersymmetric Standard Model (MSSM) Higgs decaying into two muons is presented. The search is performed on the full data collected during the years 2011 and 2012 by CMS in proton-proton collisions at CERN Large Hadron Collider (LHC). The MSSM is explored within the most conservative benchmark scenario, m_h^{max}, and within its modified versions, m_h^{mod +} and m_h^{mod -}. The search is sensitive to MSSM Higgs boson production in association with a b\bar{b} quark pair and to the gluon-gluon fusion process. In the m_h^{max} scenario, the results exclude values of tanB larger than 15 in the m_A range 115-200 GeV, and values of tanB greater than 30 in the m_A range up to 300 GeV. There are no significant differences in the results obtained within the three different scenarios considered. Comparisons with other neutral MSSM Higgs searches are shown.

The Compact Muon Solenoid (CMS) experiment at CERN's Large Hadron Collider (LHC) is scheduled to come on-line in 2007. Fermilab will act as the CMS Tier-1 centre for the US and make experiment data available to more than 400 researchers in the US participating in the CMS experiment. The US CMS Users Facility group, based at Fermilab, has initiated a project to develop a model for optimizing movement of CMS experiment data between CERN and the various tiers of US CMS data centres and to design a WAN emulation facility which will enable controlled testing of unmodified or modified CMS applications and TCP implementations locally under conditions that emulate WAN connectivity. The WAN emulator facility is configurable for latency, jitter, and packet loss. The initial implementation is based on the NISTnet software product. In this paper we will describe the status of this project to date, the results of validation and comparison of performance measurements obtained in emulated and real environment for different applications including multistreams GridFTP. We also will introduce future short term and intermediate term plans, as well as outstanding problems and issues.

The Compact Muon Solenoid (CMS) experiment at the CERN Large Hadron Collider (LHC) uses Resistive Plate Chambers (RPCs) as a trigger-dedicated muon detector. The RPC subdetector presently consists of 912 chambers, providing a ≈3 ns time resolution for the pseudorapidity region |η| <; 1.6. Up to 18 on-detector Front-End Boards (FEBs) per RPC are responsible for signal processing. Each FEB is configured and monitored through its Inter-Integrated Circuit (I <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> C) interface to ensure correct and safe detector operation. The resulting ~1.2k I <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> C lines, serving 3 to 6 FEBs apiece, are accessible through Control Boards (CBs) in token rings. An online control and monitoring system has recently been developed to automatically configure and continuously monitor FEB parameters. A fine-grained software model of hardware and detector components, organized in a dual hierarchical tree structure, facilitates command and data flow. A novel web-based user-interface allows control of and navigation through the system for efficient problem tracing. This paper covers the front-end control and monitoring system for the RPC subdetector; the online software components that allow for flexible configuration and comprehensive monitoring are described.

The Compact Muon Solenoid (CMS) is one of the general purpose experiments at the CERN Large Hadron Collider (LHC). CMS computing relies on different grid infrastructures to provide computational and storage resources. The major grid middleware stacks used for CMS computing are gLite, Open Science Grid (OSG) and ARC (Advanced Resource Connector). Helsinki Institute of Physics (HIP) hosts one of the Tier-2 centers for CMS computing. CMS Tier-2 centers operate software systems for data transfers (PhEDEx), Monte Carlo production (ProdAgent) and data analysis (CRAB). In order to provide the Tier-2 services for CMS, HIP uses tools and components from both ARC and gLite grid middleware stacks. Interoperation between grid systems is a challenging problem and HIP uses two different solutions to provide the needed services. The first solution is based on gLite-ARC grid level interoperability. This allows to use ARC resources in CMS without modifying the CMS application software. The second solution is based on developing specific ARC plugins in CMS software.

The Higgs boson is the only elementary particle predicted by the Standard Model (SM) that has not yet been observed experimentally. If it exists, it explains the spontaneous electroweak symmetry breaking and the origin of mass for gauge bosons and fermions. We test the validity of the SM by performing a search for the associated production of a Higgs boson and a W boson in the channel where the Higgs boson decays to a bottom-antibottom quark pair and the W boson decays to a charged lepton and a neutrino (the WH channel). We study a dataset of proton-antiproton collisions at a centre-of-mass energy √s = 1.96 TeV provided by the Tevatron accelerator, corresponding to an integrated luminosity of 5.7 fb-1, and recorded using the Collider Detector at Fermilab (CDF).We select events consistent with the signature of exactly one charged lepton (electron or muon), missing transverse energy due to the undetected neutrino (MET) and two collimated streams of particles (jets), at least one of which is required to be identified as originating from a bottom quark. We improve the discrimination of Higgs signal from backgrounds through the use of an artificial neural network. Using a Bayesian statistical inference approach, we set for each hypothetical Higgs boson mass in the range 100-150 GeV/c2 with 5 GeV/c2 increments a 95% credibility level (CL) upper limit on the ratio between the Higgs production cross section times branching fraction and the SM prediction. Our main original contributions are the addition of a novel charged lepton reconstruction algorithm with looser requirements (ISOTRK) with respect the electron or muon tight criteria (TIGHT), as well as the introduction of a novel trigger-combination method that allows to maximize the event yield while avoiding trigger correlations and that is used for the ISOTRK category. The ISOTRK candidate is a high-transverse-momentum good-quality track isolated from other activity in the tracking system and not required to match a calorimeter cluster, as for a tight electron candidate, or an energy deposit in the muon detector, as for a tight muon candidate. The ISOTRK category recovers real charged leptons that otherwise would be lost in the non-instrumented regions of the detector. This allows the reconstruction of more W boson candidates, which in turn increases the number of reconstructed WH signal candidate events, and therefore improves the sensitivity of the WH search. For the TIGHT charged lepton categories, we employ charged-lepton-dedicated triggers to improve the rate of WH signal acceptance during data taking. Since there is no ISOTRK-dedicated trigger at CDF, for the ISOTRK charged lepton category we employ three MET-plus-jets-based triggers. For each trigger we first identify the jet selection where the trigger efficiency is flat with respect to jet information (transverse energy and direction of motion in the transverse plane for the two jets in the event) and then we parametrize the trigger efficiency as a function of trigger MET. On an event-by-event basis, for each trigger we compute a trigger efficiency as a function of trigger parametrization, trigger MET, jet information, trigger prescale and information about whether the trigger is defined or not. For the ISOTRK category we combine the three triggers using a novel method, which allows the combination of any number of triggers in order to maximize the event yield while avoiding trigger correlations. On an event-by-event basis, only the trigger with the largest efficiency is used. By avoiding a logical 'OR' between triggers, the loss in the yield of events accepted by the trigger combination is compensated by a smaller and easier-to-compute corresponding systematic uncertainty. The addition of the ISOTRK charged lepton category to the TIGHT category produces an increase of 33% in the WH signal yield and a decrease of 15.5% to 19.0% in the median expected 95% CL cross-section upper limits across the entire studied Higgs mass interval. The improvement in analysis sensitivity is smaller than the improvement in signal yield because the ISOTRK category has a smaller signal over background ratio than the TIGHT category, due to the looser ISOTRK reconstruction criteria. The observed (median expected) 95% CL SM Higgs upper limits on cross section times branching ratio vary between 2.39 x SM (2.73 x SM) for a Higgs mass of 100 GeV/c2 to 31.1 x SM (31.2 x SM) for a Higgs mass of 150 GeV/c2, while the value for a 115 GeV/c2 Higgs boson is that of 5.08 x SM (3.79 x SM). The novel trigger combination method is already in use by several CDF analyses. It is applicable to any analysis that uses triggers based on MET and jets, such as supersymmetry searches at the ATLAS and CMS experiments at the Large Hadron Collider. In its most general form, the method can be used by any analysis that combines any number of different triggers.

Recent results on studies of hard scattering processes and jet quenching in nuclear collisions from the ATLAS and CMS experiments at the Large Hadron Collider (LHC) are reviewed. They are based on high statistics samples of PbPb collisions at √ sNN = 2.76 TeV collected at the LHC in 2011, and pp collisions at √ s = 2.76 TeV and pPb collisions at √ sNN = 5.02 TeV collected in 2013. New results for the jet nuclear modification factor, RAA, confirm a strong suppression of jet production. Modifications of jet fragmentation properties in PbPb collisions, are observed. In pPb collisions jets are not strongly modified, but some non-scaling behavior of jet rates in collisions with different centralities is observed by ATLAS. New measurements for the electroweak boson production in pPb collisions allow a more precise testing of parton distributions in lead nuclei with the conclusion that the EPS09 description of nuclear modifications of parton distribution functions is preferred.

Sources of machine-induced background in the ATLAS and CMS detectors at the CERN Large Hadron Collider

This thesis presents two research works carried out within the Compact Muon Solenoid (CMS) experiment at the CERN Large Hadron Collider (LHC): the system validation of the scintillator section of the High Granularity Calorimeter (HGCAL), and a search for a heavy resonance coupling to top quarks and produced in association with top quark pairs in proton–proton collisions at center-of-mass energies of 13 and 13.6 TeV. For the High-Luminosity LHC (HL-LHC) phase, the endcap calorimeters of CMS will be replaced by the HGCAL, a high-precision sampling calorimeter designed to cope with the extreme radiation and pileup conditions expected at the HL-LHC. In regions where radiation levels remain moderate, the hadronic section employs scintillator tiles coupled to silicon photomultipliers (SiPM-on-tile) as active materials. Each HGCAL tilemodule, the basic detector unit of this region, accommodates up to 144 SiPM-on-tile channels read out by dedicated front-end electronics. The performance of several tilemodules was evaluated during test-beam campaigns at the DESY-II facility using 3 GeV electron beams. The measurements serve to validate the response and uniformity of the detector components and to confirm that they meet the design specifications for HL-LHC operation. A search for a new heavy resonance that couples exclusively to top quarks and is produced in association with a top quark pair is presented. The analysis uses proton–proton collision data collected with the CMS detector at center-of-mass energies of 13 TeV (2016–2018) and 13.6 TeV (2022). The search targets the final state with two oppositely charged leptons from the W boson decays of the nonresonant top quarks, and two high-energy jets reconstructing the hadronically decaying resonant top quarks. In this scenario, the two top quarks from the resonance decay are expected to be highly Lorentz-boosted. Their hadronic decays are reconstructed using jets clustered with a variable-radius algorithm and identified with a dedicated top quark tagger based on a boosted decision tree (BDT). Events are selected in a final state containing opposite-sign leptons and b-tagged jets, and the resonance mass is reconstructed from pairs of tagged top jets. The analysis probes resonance masses between 500 GeV and 4 TeV and searches for local excesses in the reconstructed mass spectrum. The results are interpreted in the context of models with vector-like, scalar and pseudoscalar resonances produced in association with top quarks, representing the first such interpretation performed with CMS data.

The highlight of our last budget period, June 1, 2010, to May 31, 2013, was the discovery of the Higgs boson by the ATLAS and CMS experiments at the CERN Large Hadron Collider (LHC), announced on July 4, 2012, and for which François Englert and Peter Higgs were awarded the 2013 Nobel Prize in Physics on October 8, 2013. The Higgs boson was postulated in 1964 to explain how elementary particles obtain mass and was the missing piece of the Standard Model. However, the Standard Model does not describe everything that we know. There are many unanswered questions, such as how can the Higgs boson have the mass that we have observed, are there more Higgs bosons, why is there more matter than antimatter, and what is the invisible dark matter, which constitutes about 85% of the matter in the universe. Our group played a significant role in the discovery of the Higgs boson and in subsequent analyses. We also carried out searches for new physics, in ways that could help elucidate some of the remaining questions. Our role in the CMS detector focused on the Tracker, a silicon strip outer tracker and pixel inner tracker.

The Compact Muon Solenoid (CMS) detector installed at the CERN Large Hadron Collider (LHC) has an extensive muon system which provides information simultaneously for identification, track reconstruction and triggering of muons. As a consequence of the extreme particle rate and high integrated charge, the essentiality to upgrade the LHC has given rise to the High Luminosity phase of the LHC (HL-LHC) project so that the CMS muon system will be upgraded with superior technological challenges. The CMS GEM collaboration offers a solution to equip the high-eta region of the muon system for Phase 2 (after the year 2017) with large-area triple-layer Gas Electron Multiplier (GEM) detectors, since GEMs have the ability to provide robust and redundant tracking and triggering functions with an excellent spatial resolution of order 100 micron and a high particle rate capability, with a close to 100% detection efficiency. In this contribution, the present status of the triple-GEM project will be reviewed, and the significant achievements from the start of the R&amp;D in 2009 will be emphasized.

These proceedings summarise the recent results on jet cross-section and jet substructure measurements from the ATLAS and CMS experiments at the CERN Large Hadron Collider (LHC). Jets, originating from quarks and gluons, are key probes of Quantum Chromodynamics (QCD) at the highest available energies. Inclusive and multi-differential jet cross-section measurements provide stringent tests of perturbative QCD at next-to-leading and next-to-next-to-leading order and offer important constraints on parton distribution functions, in particular on the gluon density at high momentum fractions. Substructure observables, such as the Lund Jet Plane, event shape variables, and jet track functions, probe the detailed dynamics of parton branching, fragmentation, and hadronization, enabling precision comparisons between data and theoretical predictions beyond the traditional DGLAP paradigm. These measurements also provide benchmarks for the tuning of Monte Carlo event generators and are increasingly relevant for the optimization of machine-learning–based jet tagging algorithms. Furthermore, the ATLAS and CMS Collaborations have extracted the strong coupling constant $\alpha_S$ from jet observables, extending the measurement range up to the multi-TeV regime. The observed running of $\alpha_S$ is consistent with the world average, providing one of the most precise high-energy tests of asymptotic freedom. Together, these results deepen our understanding of QCD, improve the modelling of jets in simulations, and set the stage for further precision studies with the upcoming High-Luminosity LHC program.

The Electromagnetic Calorimeter (ECAL) is one of the particle detectors of the Compact Muon Solenoid (CMS) experiment at the CERN Large Hadron Collider (LHC). For more than ten years, the CMS ECAL Detector Control System (DCS) and the CMS ECAL Safety Systems (ESS) have supported the experiment operation, contributing to its high availability and safety. The evolution of both systems to fulfil new requirements and constraints, in addition to optimizations towards improving usage and processes automation, led to several changes to their original design. This paper presents the current software/hardware architecture of both CMS ECAL control and safety systems and reviews the major changes applied to both systems during the past years. Furthermore, in view of the CMS Phase-II upgrade of this sub-detector, the corresponding plans for the control and safety systems are also discussed.

The CMS experiment has produced a large number of new measurements with data collected during Run 1 by the CERN Large Hadron Collider (LHC). In this report a few results in Higgs and top physics will be mentioned. After a shutdown in 2013 and 2014, the LHC restarted proton-proton collisions at the unprecedented center-of-mass energy of 13 TeV in June 2015. The data collected until August 2015 have yielded interesting events and allowed the extraction of the first results of searches at the high-energy end of mass spectra, where the effect of the higher collision energy is largest.

The event reconstruction at the Compact Muon Solenoid (CMS) experiment at the CERN Large Hadron Collider (LHC) is predominantly based on the Particle Flow algorithm. This algorithm for a global event description uses the information from all subdetector systems, unlike the previous, traditional approaches that were focused on the localized information in each subdetector. These traditional methods use the raw information (tracks, hits), while the Particle Flow algorithm completely reconstructs the event by identifying and reconstructing the comprehensive list of final-state particles (photons, electrons, muons, charged and neutral hadrons), resulting in superior reconstruction of jets, missing transverse energy, tau leptons, electrons and muons. This approach also allows for efficient identification and mitigation of the pileup effect. The concept and performance of the Particle Flow algorithm, together with the prospects for its development in the context of the upgraded CMS detector, are presented in this overview.

The Compact Muon Solenoid (CMS) experiment at the CERN Large Hadron Collider (LHC) is undergoing an upgrade program to prepare for the upcoming High-Luminosity era of the LHC. The design of a new timing layer used to measure minimum ionizing particles (MIPs) with a time resolution of (~) 30 ps) and coverage up to pseudo-rapidity |η| = 3.0 is presented. The MIP timing detector (MTD) will be composed of a central barrel region instrumented with LYSO:Ce crystals, and two end-caps instrumented with radiation tolerant low gain avalanche detectors (LGADs). The precision timing information from the MTD will be used to reduce the effects of high pileup on object reconstruction.