Large Hadron Collider Run Research Articles

Trigger systems of LHC experiments

Interaction rates at CERN's Large Hadron Collider (LHC) increase with the collider's luminosity and collision energy. All LHC experiments and in particular their trigger systems have to take this fact into account in order to ensure efficient physics data taking while at the same time keeping data rates at an acceptable level. This paper presents an overview of the most important trigger upgrades that have happened since the end of LHC Run 1 in 2012, describes work currently underway and presents an outlook on major improvements planned for the medium-term future.

Novel SM-like Higgs decay into displaced heavy neutrino pairs in U(1)\u2032 models

We examine the observability of heavy neutrino (νh) signatures of a U(1)′ enlarged Standard Model (SM) encompassing three heavy Majorana neutrinos alongside the known light neutrino states at the the Large Hadron Collider (LHC). We show that heavy neutrinos can be rather long-lived particles producing distinctive displaced vertices that can be accessed in the CERN LHC detectors. We concentrate here on the gluon fusion production mechanism gg → H1,2 → νhνh, where H1 is the discovered SM-like Higgs and H2 is a heavier state, yielding displaced leptons following νh decays into weak gauge bosons. Using data collected by the end of the LHC Run 2, these signatures would prove to be accessible with negligibly small background.

The LHC Run 2 ATLAS trigger system: design, performance and plans

In many high-energy physics experiments, online selection is vital to collect the most interesting collisions out of the large data volume. The ATLAS experiment at the Large Hadron Collider (LHC) utilizes a trigger system that consists of a hardware Level-1 (L1) and a software based high-level trigger (HLT), reducing the event rate from the LHC bunch-crossing rate of 40 MHz to an average recording rate of around 1000 Hz. In Run 2 of the LHC, started in spring 2015, the LHC is operating at a centre-of-mass energy of 13 TeV providing an instantaneous luminosity up to 1.4 ⋅ 1034 cm−2s−1 so far. The ATLAS trigger system has to cope with these challenges, while maintaining or improving upon the efficiency to select relevant physics processes. In this paper, the ATLAS trigger system for the LHC Run 2 is reviewed. Secondly, the impressive performance improvements in the HLT trigger algorithms used to identify leptons, hadrons and global event quantities like missing transverse energy is shown. Electron, muon and photon triggers covering transverse energies from a few GeV to several TeV are essential for signal selection in a wide variety of ATLAS physics analyses to study Standard Model (SM) processes and to search for new phenomena. Finally, further developments planned for the rest of the LHC Run 2 are discussed. These include two new hardware subsystems for topological selections at L1 and full-scan tracking at the input to the HLT.

Characterization of the first prototype of the ALICE SAMPA ASIC for LHC Run 3 and beyond

A Large Ion Collider Experiment (ALICE) at the Large Hadron Collider (LHC) is planing to upgrade its Time Projection Chamber (TPC) due to the expected higher Pb-Pb collision-rates in the next running period (Run 3) of the LHC starting in 2020. In the upgraded TPC, Gas Electron Multiplier (GEM) chambers and continuous readout system will replace Multi-Wire Proportional (MWP) chambers and conventional triggered readout. In the continuous readout, GEM signals will be processed using a 32 channel SAMPA ASIC. The first version of the SAMPA (MPW1) was delivered in 2014 and the production of final version is in progress. In this paper, the test results obtained for charge injection to the device using pulse generator as well as GEM detector prototype are reported.

Type II seesaw model and multilepton signatures at hadron colliders

We investigate multilepton signatures, arising from the decays of doubly charged and singly charged Higgs bosons in the Type-II Seesaw model. Depending on the vacuum expectation value of the triplet $v_{\Delta}$, the doubly and singly charged Higgs bosons can decay into a large variety of multi-lepton final states. We explore all possible decay modes corresponding to different regimes of $v_{\Delta}$, that generate distinguishing four and five leptonic signatures. We focus on the 13 TeV Large Hadron Collider (LHC) and further extend the study to a very high energy proton-proton collider (VLHC) with a center-of-mass energy of 100 TeV. We find that a doubly charged Higgs boson of masses around 375 GeV can be discovered at immediate LHC runs. A heavier mass of 630 GeV can instead be discovered at the high-luminosity run of the LHC or at the VLHC with 30 $\rm{fb}^{-1}$.

The CMS Level-1 Calorimeter Trigger for the LHC Run II

Results from the completed Phase 1 Upgrade of the Compact Muon Solenoid (CMS) Level-1 Calorimeter Trigger are presented. The upgrade was performed in two stages, with the first running in 2015 for proton and heavy ion collisions and the final stage for 2016 data taking. The Level-1 trigger has been fully commissioned and has been used by CMS to collect over 43 fb−1 of data since the start of the Run II of the Large Hadron Collider (LHC). The new trigger has been designed to improve the performance at high luminosity and large number of simultaneous inelastic collisions per crossing (pile-up). For this purpose it uses a novel design, the Time Multiplexed Trigger (TMT), which enables the data from an event to be processed by a single trigger processor at full granularity over several bunch crossings. The TMT design is a modular design based on the μTCA standard. The trigger processors are instrumented with Xilinx Virtex-7 690 FPGAs and 10 Gbps optical links. The TMT architecture is flexible and the number of trigger processors can be expanded according to the physics needs of CMS. Sophisticated and innovative algorithms are now the core of the first decision layer of the experiment. The system has been able to adapt to the outstanding performance of the LHC, which ran with an instantaneous luminosity well above design. The performance of the system for single physics objects are presented along with the optimizations foreseen to maintain the thresholds for the harsher conditions expected during the LHC Run II and Run III periods.

Operational experience with the CMS pixel detector in LHC Run II

The CMS pixel detector was repaired successfully, calibrated and commissioned for the second run of Large Hadron Collider during the first long shutdown between 2013 and 2015. The replaced pixel modules were calibrated separately and show the expected behavior of an un-irradiated detector. In 2015, the system performed very well with an even improved spatial resolution compared to 2012. During this time, the operational team faced various challenges including the loss of a sector in one half shell which was only partially recovered. In 2016, the detector is expected to withstand instantaneous luminosities beyond the design limits and will need a combined effort of both online and offline teams in order to provide the high quality data that is required to reach the physics goals of CMS. We present the operational experience gained during the second run of the LHC and show the latest performance results of the CMS pixel detector.

First results of CMS RPC performance at 13 TeV

The muon spectrometer of the CMS (Compact Muon Solenoid) experiment at the Large Hadron Collider (LHC) is equipped with a redundant system made of Resistive Plate Chambers (RPCs) and Drift Tube (DT) chambers in the barrel, RPC and Cathode Strip Chambers (CSCs) in the endcap region. In this paper, the first results of the performance of the RPC system during 2015 with the LHC running at 13 TeV is presented. The stability of the RPC performance, in terms of efficiency, cluster size and noise, is reported.

A brief review of measurements of electroweak bosons at the LHCb experiment in LHC Run 1

The LHCb experiment is one of four major experiments at the large hadron collider (LHC). Despite being designed for the study of beauty and charm particles, it has made important contributions in other areas, such as the production and decay of W and Z bosons. Such measurements can be used to study and constrain parton distribution functions (PDFs), as well as to test perturbative quantum chromodynamics (QCD) in hard scattering processes. The angular structure of Z boson decays to leptons can also be studied and used to measure the weak mixing angle. The phase–space probed by LHCb is particularly sensitive to this quantity, and the LHCb measurement using the dimuon final state is currently the most precise determination of [Formula: see text] at the LHC. LHCb measurements made using data collected during the first period of LHC operations (LHC Run 1) are discussed in this review. The paper also considers the potential impact of related future measurements.

Bounds on universal extra dimension from LHC run I and II data

We discuss the collider bounds on minimal Universal Extra Dimension (mUED) model from LHC Run-I and II data. The phenomenology of mUED is determined by only two parameters namely, the compactification scale ($R^{-1}$) of the extra dimension and cutoff scale ($\Lambda$) of the theory. The characteristic feature of mUED is the occurrence of nearly degenerate mass spectrum for the Kaluza-Klein (KK) particles and hence, soft leptons, soft jets at the collider experiments. The degree of degeneracy of KK-mass spectrum crucially depends on $\Lambda$. The strongest direct bound on $R^{-1}$ ($\sim $950 GeV for large $\Lambda$) arises from a search for a pair of soft dimuons at the Large Hadron Collider (LHC) experiment with 8 TeV center-of-mass energy and $20~{\rm fb}^{-1}$ integrated luminosity. However, for small $\Lambda$ and hence, small splitting within the first KK-level, the bounds from the dimuon channel is rather weak. On the other hand, the discovery of 126 GeV Higgs boson demands small $\Lambda$ to prevent the scalar potential form being unbounded from below. We discuss LHC monojet searches as a probe of low $\Lambda$ region of mUED parameter space. We also compute bounds on the mUED parameter space from 13 TeV multijets results.

The LHCb Silicon Tracker

The LHCb experiment is dedicated to the study of heavy flavour physics at the Large Hadron Collider (LHC). The primary goal of the experiment is to search for indirect evidence of new physics via measurements of CP violation and rare decays of beauty and charm hadrons. The LHCb detector has a large-area silicon micro-strip detector located upstream of a dipole magnet, and three tracking stations with silicon micro-strip detectors in the innermost region downstream of the magnet. These two sub-detectors form the LHCb Silicon Tracker (ST). This paper gives an overview of the performance and operation of the ST during LHC Run 1. Measurements of the observed radiation damage are shown and compared to the expectation from simulation.

Classifying the bosonic quartic couplings

The larger center-of-mass energy of the Large Hadron Collider Run 2 opens up the possibility of a more detailed study of the quartic vertices of the electroweak gauge bosons. Our goal in this work is to classify all operators possessing quartic interactions among the electroweak gauge bosons that do not exhibit triple gauge-boson vertices associated to them. We obtain all relevant operators in the non-linear and linear realizations of the $SU(2)_L \otimes U(1)_Y$ gauge symmetry.

The upgraded Pixel Detector of the ATLAS Experiment for Run 2 at the Large Hadron Collider

During Run 1 of the Large Hadron Collider (LHC), the ATLAS Pixel Detector has shown excellent performance. The ATLAS collaboration took advantage of the first long shutdown of the LHC during 2013 and 2014 and extracted the ATLAS Pixel Detector from the experiment, brought it to surface and maintained the services. This included the installation of new service quarter panels, the repair of cables, and the installation of the new Diamond Beam Monitor (DBM). Additionally, a completely new innermost pixel detector layer, the Insertable B-Layer (IBL), was constructed and installed in May 2014 between a new smaller beam pipe and the existing Pixel Detector. With a radius of 3.3cm the IBL is located extremely close to the interaction point. Therefore, a new readout chip and two new sensor technologies (planar and 3D) are used in the IBL. In order to achieve best possible physics performance the material budget was improved with respect to the existing Pixel Detector. This is realized using lightweight staves for mechanical support and a CO2 based cooling system.This paper describes the improvements achieved during the maintenance of the existing Pixel Detector as well as the performance of the IBL during the construction and commissioning phase. Additionally, first results obtained during the LHC Run 2 demonstrating the distinguished tracking performance of the new Four Layer ATLAS Pixel Detector are presented.

The ATLAS FTK system: how to improve the physics potential with a tracking trigger

After a very successful data taking run, the ATLAS experiment is being upgraded to cope with the higher luminosity and higher center of mass energy that the Large Hadron Collider (LHC) will provide in the next years. The Fast Tracker (FTK) trigger system, part of the ATLAS trigger upgrade program, is a highly parallel hardware device processor based on a mixture of advanced technologies. FTK provides global track reconstruction in the full inner silicon detector, with resolution comparable to the offline algorithms, in approximately 100 microseconds, allowing a fast and precise detection of the primary and secondary vertex information. The track and vertex information is then used by the high-level trigger algorithms, allowing highly improved trigger performance for the most difficult signatures, such as b-jets. The expected physics performance of FTK in the harsh environment at high pile-up and high luminosities at LHC Run2 are presented.

CP violating Two-Higgs-Doublet Model: constraints and LHC predictions

Two-Higgs-Doublet Models (2HDMs) are amongst the simplest extensions of the Standard Model. Such models allow for tree-level CP Violation (CPV) in the Higgs sector. We analyse a class of CPV 2HDM (of Type-I) in which only one of the two Higgs doublets couples to quarks and leptons, avoiding dangerous Flavour Changing Neutral Currents. We provide an up to date and comprehensive analysis of the constraints and Large Hadron Collider (LHC) predictions of such a model. Of immediate interest to the LHC Run 2 is the golden channel where all three neutral Higgs bosons are observed to decay into gauge boson pairs, $WW$ and $ZZ$, providing a smoking gun signature of the CPV 2HDM.

The ATLAS Data Acquisition System: from Run 1 to Run 2

The experience gained during the first period of very successful data taking of the ATLAS experiment (Run 1) has inspired a number of ideas for improvement of the Data Acquisition (DAQ) system that are being put in place during the so-called Long Shutdown 1 of the Large Hadron Collider (LHC), in 2013/14. We have updated the data-flow architecture, rewritten an important fraction of the software and replaced hardware, profiting from state of the art technologies. This paper summarizes the main changes that have been applied to the ATLAS DAQ system and highlights the expected performance and functional improvements that will be available for the LHC Run 2. Particular emphasis will be put on explaining the reasons for our architectural and technical choices, as well as on the simulation and testing approach used to validate this system.

A revisit to a compressed supersymmetric spectrum with 125 GeV Higgs

A compressed spectrum was initially proposed as an explanation for the elusiveness of low-energy supersymmetry (SUSY). Some characteristic signals at the Large Hadron Collider (LHC), such as mono-jet + \slashed{E}_T, had been propounded as its trademark signals. However, later investigations suggested that lower limits on the supersymmetric particle masses would be quite stringent in spite of compression. Also, most compressed SUSY scenarios studied so far are only partially compressed. In this backdrop, we make an exhaustive analysis of the compressed SUSY scenarios for the 13 TeV run of LHC, keeping the level of compression in the entire spectrum as high as possible. A broad class of benchmark spectra are thus considered, after ensuring consistency with the observed Higgs mass as well as the dark matter constraints. The rates of observable events in the high-energy run are obtained through detailed simulation, for both the multi-jet +\slashed{E}_T and mono-jet + \slashed{E}_T final states. Our conclusion is that the former is still more efficient to reveal a compressed SUSY spectrum first, while the latter can serve as a useful confirmatory channel.

ATLAS Inner Tracker Performance at the Beginning of the LHC Run 2

The ATLAS experiment performs studies of proton-proton collisions at the Large Hadron Collider (LHC) at CERN. The Inner Detector is a part of the ATLAS apparatus placed nearest the interaction point, designed to measure charged particles momenta and their trajectories, and to reconstruct vertices of decays of physics objects created in collisions. During the LHC technical stop 2013-2015, the Inner Detector underwent several upgrades and improvements, most notably an additional Pixel Detector layer was installed. This document describes the improvements done in the Inner Detector and its combined performance in the first year of data taking after the LHC restart in 2015.

LHCb Vertex Locator: Performance and radiation damage in LHC Run 1 and preparation for Run 2

LHCb is a dedicated experiment to study New Physics in the decays of heavy hadrons at the Large Hadron Collider (LHC) at CERN. Heavy hadrons are identified through their flight distance in the Vertex Locator (VELO). The VELO comprises 42 modules made of two n+-on-n 300μm thick half-disc silicon sensors with R- and Φ-measuring micro-strips. In order to allow retracting the detector, the VELO is installed as two movable halves containing 21 modules each. The detectors are operated in a secondary vacuum and are cooled by a bi-phase CO2 cooling system. During data taking in LHC Run 1 the LHCb VELO has operated with an extremely high efficiency and excellent performance. The track finding efficiency is typically greater than 98%. An impact parameter resolution of less than 35μm is achieved for particles with transverse momentum greater than 1GeV/c. An overview of all important performance parameters will be given. The VELO sensors have received a large and non-uniform radiation dose of up to 1.2×10141MeV neutron equivalent cm−2 during the first LHC run. Silicon type-inversion has been observed in regions close to the interaction point. The preparations for LHC Run 2 are well under way and the VELO has already recorded tracks from injection line tests. The current status and plans for new operational procedures addressing the non-uniform radiation damage are shortly discussed.

The ATLAS jet trigger performance in LHC Run I and Run II updates

The Large Hadron Collider (LHC) provides proton-proton collisions with a nominal rate of 40 MHz, and the ATLAS trigger performs the first event selections online during data- taking. The ATLAS jet trigger is an important element of the ATLAS trigger system, selecting collision events containing high transverse energy jets, to provide data samples for studies ranging from Standard Model physics to searches for new physics at the LHC. During LHC Run I, the first LHC operation period from 2010 to 2012, the ATLAS jet trigger system improved as experience developed with triggering in a high luminosity and high event pileup environment. For the next LHC operation period, Run II, the system is being updated for further improved performance and stability. In this contribution, performance and improvements of the ATLAS jet trigger in Run I are presented. Updates for Run II are also shown.