CERN LHC Collider Research Articles

The LiTE-DTU: A Data Conversion and Compression ASIC for the Readout of the CMS Electromagnetic Calorimeter

The High-Luminosity phase of operation of the CERN LHC collider (HL-LHC) will pose new challenges to the detectors and their readout electronics. In particular, the CMS barrel electromagnetic calorimeter will require a full redesign of the electronic readout chain in order to cope with the increase in luminosity and trigger rate. In this framework, a new Application Specific Integrated Circuit (ASIC) integrating A/D conversion, lossless data compression and high speed transmission has been developed and tested. The ASIC, named LiTE-DTU, is designed in a commercial CMOS 65 nm process and embeds two 12-bit, 160 MS/s Analog to Digital Converters (ADCs), a data selection and compression logic, and a 1.28 Gb/s output serial link. The high-speed 1.28 GHz clock is generated internally from the 160 MHz input by a clock multiplication Phase-Locked Loop (PLL). The circuit has been designed implementing radiation tolerant techniques in order to work in the harsh environment of the HL-LHC upgrade. The LiTE-DTU is currently in the pre-production phase. A sample of 600 chips has been tested and incorporated into front end boards for systems performance testing.

A comparison of direct and indirect determinations of the masses of the Higgs Boson and the top quark

The indirect estimation of the Higgs Boson mass from electroweak radiative corrections within the Standard Model is compared with the directly measured value obtained by the ATLAS and CMS Collaborations at the CERN LHC collider. Treating the direct measurement of mH as input, the Standard Model indirect estimation of the top-quark mass is also obtained and compared with its directly measured value. A model-independent analysis finds an indirect value of mH of ≃70 GeV, below the directly measured value of 125.7±0.4 GeV and an indirect value: mt=177.3±1.0 GeV, above the directly measured value: 173.21±0.87 GeV. A goodness-of-fit test to the Standard Model using all Z-pole observables and mW has a χ2 probability of ≃2%. The reason why probability values about a factor of ten larger than this, and indirect estimates of mH about 30 GeV higher, have been obtained in recent global fits to the same data is recalled.

Characterization of the Outer Barrel modules for the upgrade of the ALICE Inner Tracking System

ALICE is one of the four large detectors at the CERN LHC collider, designed to address the physics of strongly interacting matter, and in particular the properties of the Quark-Gluon Plasma using proton-proton, proton-nucleus, and nucleus-nucleus collisions. Despite the success already reached in achieving these physics goals, there are several measurements still to be finalized, like high precision measurements of rare probes (D mesons, Lambda baryons and B mesons decays) over a broad range of transverse momenta. In order to achieve these new physics goals, a wide upgrade plan was approved that combined with a significant increase of luminosity will enhance the ALICE physics capabilities enormously and will allow the achievement of these fundamental measurements. The Inner Tracking System (ITS) upgrade of the ALICE detector is one of the major improvements of the experimental set-up that will take place in 2019–2020 when the whole ITS sub-detector will be replaced with one realized using a innovative monolithic active pixel silicon sensor, called ALPIDE. The upgraded ITS will be realized using more than twenty-four thousand ALPIDE chips organized in seven different cylindrical layers, for a total surface of about ten square meters. The main features of the new ITS are a low material budget, high granularity and low power consumption. All these peculiar capabilities will allow for full reconstruction of rare heavy flavour decays and the achievement of the physics goals. In this paper after an overview of the whole ITS upgrade project, the construction procedure of the basic building block of the detector, namely the module, and its characterization in laboratory will be presented.

Insights from the ALICE quark-gluon coloured world at the LHC

Interactions among heavy ions at the CERN LHC collider recreate the state of matter as it was a few micro-seconds after the Big Bang: the Quark Gluon Coloured World (QGCW). At a centre-of-mass energy of 2.76 TeV the produced QGCW reaches unprecedented temperatures, volumes and durations allowing more refined studies of its properties. Data collected during LHC Run1 (from 2010 to 2013) already provided new and sometimes unexpected results, and pointed to intriguing similarities among high multiplicity events produced in different colliding systems, namely pp, p-Pb and Pb-Pb. A comprehensive review of these results is presented, showing how heavy-ion physics is a powerful tool to shed light on QCD in such extreme conditions when multiple phase transitions should occur to go from the QGCW to our present non-coloured world.

Diphoton decay for a 750 GeV scalar boson in a U(1)X model

In the context of a nonuniversal and anomaly free U(1)X extension of the standard model, we examine the decay of a 750 GeV scalar singlet state, ξχ, as a possible explanation of the observed diphoton excess announced by the ATLAS and CMS collaborations at CERN-LHC collider. The one-loop decay to photons is allowed through three heavy singlet quarks and one charged Higgs boson into the loop. We obtain, for different width approximations and for masses of the exotic singlet quarks in the region [900,3000] GeV, a production cross section σ(pp→ξχ→γγ) compatible with ATLAS and CMS collaborations data. We also include another scalar singlet, σ, as a dark matter candidate that may couple with the 750 GeV scalar at tree level with production cross sections in agreement with ATLAS and CMS.

Diphoton Higgs Decay in an U(1)′ model

We explore the parameter regions on the scalar coupling in a nonuniversal U(1)′ extension of the Standard Model free from anomalies with a complex scalar dark matter particle. Using recent data from the CERN-LHC collider, we study the signal strenght of the diphoton Higgs decay, which imposes very stringent bounds to the scalar couplings and other scalar parameters, including parameters associated to the dark matter.

Scalar coupling limits and diphoton Higgs decay from LHC in anU(1)′model with scalar dark matter

In the context of an nonuniversal $U(1)'$ extension of the standard model free from anomalies, we introduce a complex scalar singlet candidate to be dark matter. In addition, an extra scalar doublet and a heavy scalar singlet are required to provide masses to all fermions and to break spontaneously the symmetries. From unitarity and stability of the Higgs potential, we find the full set of bounds and order relations for the scalar coupling constants. Using recent data from the CERN-LHC collider, we study the signal strenght of the diphoton Higgs decay $R_{\gamma \gamma}$, which imposes very stringent bounds to the scalar couplings and other scalar parameters. We obtain constraints in different scenarios of the space of parameters, where decays into dark matter may or may not contribute according to the mass of the scalar dark matter candidate. By assuming that the lightest scalar boson of the model corresponds to the observed Higgs boson, we evaluate deviations from the SM of the trilineal Higgs self-coupling. The conditions from unitarity, stability and Higgs diphoton decay data allow trilineal deviations in the range $0 \leq \delta g \lesssim -72\%$.

Probing triple-W production and anomalous WWWW coupling at the CERN LHC and future O 100 $$ \mathcal{O}(100) $$ TeV proton-proton collider

Triple gauge boson production at the LHC can be used to test the robustness of the Standard Model and provide useful information for VBF di-boson scattering measurement. Especially, any derivations from SM prediction will indicate possible new physics. In this paper we present a detailed Monte Carlo study on measuring WWW production in pure leptonic and semileptonic decays, and probing anomalous quartic gauge WWWW couplings at the CERN LHC and future hadron collider, with parton shower and detector simulation effects taken into account. Apart from cut-based method, multivariate boosted decision tree method has been exploited for possible improvement. For the leptonic decay channel, our results show that at the sqrt{s}=8(14)[100] TeV pp collider with integrated luminosity of 20(100)[3000] fb-1, one can reach a significance of 0.4(1.2)[10]sigma to observe the SM WWW production. For the semileptonic decay channel, one can have 0.5(2)[14]sigma to observe the SM WWW production. We also give constraints on relevant Dim-8 anomalous WWWW coupling parameters.

The origin and status of the third neutrino

1. The three lepton families The elementary particles known today fall in two categories: twelve spin particles (fermions), the building blocks of matter, and twelve spin 1 particles (bosons), which are the mediators of all the forces but gravity. The last element in the Standard Model is a spin 0 particle, thought to be at the origin of the masses and called the “Higgs” after the name of one of the discoverers of the theory. Not yet detected, it is searched for at the new CERN LHC collider. The fermions come in three different groups, called families, of identical structure. The reason for that is unknown. Each family is made of a doublet of quarks, of charge +2/3 q and –1/3 q (q is the proton charge), and a doublet of leptons, one of charge –q and one neutral. The neutral leptons are collectively called neutrinos, but are three different particles, distinguished by an additive quantum number called “lepton flavour”. The electron (e) and the electron-neutrino ( e) have one unit of electronic flavour (–1 their antiparticles); the muon ( ) and the muon-neutrino ( μ) have one unit of muonic flavour and similarly for the tau ( ) and the tau-neutrino ( ). The charged leptons are distinguished by their masses (increasing with the family number) and lifetimes, neutrinos only by their lepton flavours. Neutrinos are produced in states of definite flavour, as e, μ or , in pairs with an antiparticle of the same and opposite flavour. Elementary interactions conserve flavours. So that, by definition, the electron-neutrino ( e) is the neutral lepton produced with a positron (e), the μ is the one produced with a + and the is the one produced with a . And electron-anti-neutrino is the neutral particle produced with an e, etc. Neutrinos cannot be detected directly. However, when one of them interacts with the matter producing a charged lepton, the latter can be detected. The identification of the charged lepton gives the flavour of the neutrino: if it is an electron (e) it was a e etc. Experiments show that neutrinos born with a flavour produce charged leptons of the same flavour, provided the ratio L/E between distance from production to interaction points and neutrino energy is not very large, namely if the oscillation phenomenon has no time to develop. Indeed, experiments in underground laboratories have shown that neutrinos do not behave as assumed in the Standard Model, they do change, “oscillate”, between one flavour and another. The evidence has gradually grown in the last four decades, by studying the es produced by the fusion reactions in the core of the Sun and the μs indirectly produced by the cosmic rays collisions in the atmosphere. Confirmations came by experiments with artificial neutrino sources: proton accelerators (producing mainly μ) and nuclear power reactors ( e ). There two types of experiments. In a disappearance experiment the flux of neutrinos of a certain flavour is known at production; if the flux is measured at a (large) distance and found to be less than expected, the oscillation to another flavour is inferred. In an appearance experiment a flavour not present at production is searched. Oscillations happen because neutrinos of definite flavour are not stationary states (mass eigenstates). The latter, 1, 2 and 3, do not change and have definite masses, m1, m2 and m3. The two basis are linked by an orthogonal transformation that can be expressed in terms of three rotations, through angles that we shall call 12, 23 and 13, and of phase factors. If neutrinos are Dirac particles, as assumed in the Standard Model, all but one of the phase factors can be absorbed, as in the case of quarks, in the wave functions of the states. However, neutrino and antineutrino might be two states of the same particle, namely ‘Majorana particles’. In this case two more phases, which we shall call M

Δr in the Two-Higgs-Doublet Model at full one loop level—and beyond

After the recent discovery of a Higgs-like boson particle at the CERN LHC-collider, it becomes more necessary than ever to prepare ourselves for identifying its standard or non-standard nature. The Electroweak parameter Delta r relating the values of the gauge boson masses [MW,MZ] and the Fermi constant [G_F] is the traditional observable encoding high precision information of the electroweak physics at the quantum level. In this work we present a complete quantitative study of Delta r in the framework of the general (unconstrained) Two-Higgs-Doublet Model (2HDM). First of all we report on a systematic analysis of Delta r at the full one loop level in the general 2HDM, which to our knowledge was missing in the literature. Thereby we extract a theoretical prediction for the mass of the W-boson in this model, taking MZ, \alpha_{em} and G_F as experimental inputs. We find typical corrections leading to mass shifts $\delta MW \sim 20-40 MeV$ which help to improve the agreement with the experimentally measured value, in a degree no less significant than in the MSSM case. In the second part of our study we extend our calculation beyond the mere one-loop order. We devise an effective Lagrangian approach that captures the dominant higher order quantum effects on delta rho (viz. that part of Delta r describing the breaking of the approximate SU(2) custodial symmetry) in the limit of large Higgs boson self-interactions. This limit constitutes a telltale property of the general 2HDM which is unmatched by e.g. the MSSM. Our conclusion is that the Electroweak precision program to be conducted at the LHC, and maybe at a future linear collider, can nicely complement the direct searches. Should these distinctive loop effects be eventually found they would signal a smoking gun hinting at non-standard Higgs physics.

Search for a heavy Standard Model Higgs boson in the channel [formula omitted] using the ATLAS detector

A search for a heavy Standard Model Higgs boson decaying via H→ZZ→ℓ+ℓ−qq¯, where ℓ=e,μ, is presented. The search is performed using a data set of pp collisions at s=7 TeV, corresponding to an integrated luminosity of 1.04 fb−1 collected in 2011 by the ATLAS detector at the CERN LHC collider. No significant excess of events above the estimated background is found. Upper limits at 95% confidence level on the production cross section (relative to that expected from the Standard Model) of a Higgs boson with a mass in the range between 200 and 600 GeV are derived. Within this mass range, there is at present insufficient sensitivity to exclude a Standard Model Higgs boson. For a Higgs boson with a mass of 360 GeV, where the sensitivity is maximal, the observed and expected cross section upper limits are factors of 1.7 and 2.7, respectively, larger than the Standard Model prediction.

The ALICE Zero Degree Calorimeter in pp physics

ALICE is the dedicated heavy-ion experiment at the CERN LHC collider. The experiment main aim is to study the transition of hadronic matter to the deconfined plasma of quarks and gluons (QGP) predicted by Lattice QCD calculations. Proton-proton and proton-nucleus data will be used as reference. The ALICE Zero Degree Calorimeter system is made of two sets of forward hadronic calorimeters placed on both sides relative to the interaction point. Each set consists of a neutron and a proton calorimeter. The ZDC has been designed to trigger on the collision centrality in heavy-ion interactions. In proton-proton collisions they measure the energy flow of the leading neutrons and protons. Thanks to the forward rapidity region covered, they can be used to select different event topologies: minimum bias, non diffractive and diffractive events. Results from the 2009 run at s = 900 GeV as well as from 2010 data taking at s = 7 TeV are presented, putting the emphasis on the detector performance.

Commissioning and operation of the ATLAS Pixel Detector at the CERN LHC collider

The physics program at the CERN LHC collider started in autumn 2009. Since then, LHC regularly delivers collisions of its two proton beams. This talk was devoted to the commissioning and early operation of the ATLAS Pixel Detector. The Pixel Detector is working nicely and all of the required performance like efficiency, resolution and low noise were met. The fraction of working modules is as high as 97.4%. The Pixel Detector fully participates in the reconstruction of charged particles trajectories, and is a key element in finding primary and secondary vertices and in tagging short-lived particles.

Heavy quark signals from radiative corrections to theZ′boson decay in 3-3-1 models

One-loop corrections to the ${Z}^{\ensuremath{'}}$ decay width are derived and analyzed in the framework of the general form of the 3-3-1 models. We identify two important sources of corrections: oblique corrections associated to the ${Z}^{\ensuremath{'}}$ propagator through vacuum polarizations induced by virtual particle-antiparticle pairs of new heavy quarks $J$, and vertex corrections to the ${Z}^{\ensuremath{'}}q\overline{q}$ vertex through virtual exchange of new ${K}^{{Q}_{1,2}}$ gauge bosons. Fixing a specific renormalization scheme, we obtain dominant oblique corrections that exhibit a quadratic dependence on the $J$ quark mass, which are absorbed into two oblique parameters: a global parameter ${\ensuremath{\rho}}_{f}^{\ensuremath{'}}$ which modify the ${Z}^{\ensuremath{'}}$ decay width, and a parameter ${\ensuremath{\kappa}}_{f}^{\ensuremath{'}}$ that define effective ${Z}^{\ensuremath{'}}$ couplings. Numerical results in an specific 3-3-1 model gives a strong contribution of the oblique corrections from about 1.3% in the $d(s)$ quark channel to 10.5% in the neutrino channel, for ${m}_{J}=2\text{ }\text{ }\mathrm{TeV}$. The vertex corrections contribute to the oblique corrections up to 1.4% for the same channel and ${m}_{J}$ value. For $pp$ collisions at the CERN LHC collider, we find that the corrections significantly modify the shape of the cross section distributions for ${e}^{+}{e}^{\ensuremath{-}}$ and $t\overline{t}$ final states, where the distributions including the radiative corrections increases up to 1.23 times the tree-level distribution for the dielectron events and to 1.07 for the top events when ${m}_{J}=3\text{ }\text{ }\mathrm{TeV}$.

Associated production of Higgs bosons and heavy quarks at the LHC: Predictions with thekT-factorization approach

In the framework of the ${k}_{T}$-factorization approach, we study the production of Higgs bosons associated with a heavy (beauty or top) quark pair at the CERN LHC collider conditions. Our consideration is based mainly on the off-shell gluon-gluon fusion subprocess ${g}^{*}{g}^{*}\ensuremath{\rightarrow}Q\overline{Q}H$. The corresponding matrix element squared has been calculated for the first time. We investigate the total and differential cross sections of $b\overline{b}H$ and $t\overline{t}H$ production taking into account also the non-negligible contribution from the $q\overline{q}\ensuremath{\rightarrow}Q\overline{Q}H$ mechanism. In the numerical calculations we use the unintegrated gluon distributions obtained from the Ciafaloni-Catani-Fiorani-Marchesini evolution equation. Our results are compared with the leading predictions of the collinear factorization of QCD.

Ultraperipheral collisions with the ATLAS detector

The CERN LHC collider will operate with lead ions at s of 5.5 TeV/n. The ATLAS detector, designed to study high- p T physics in pp mode of the LHC, has potential to study ultraperipheral collisions of heavy ions in a wide spectrum of observables. The physics program includes study of production of electron and muon pairs; spectroscopy of single mesons; measurement of meson pairs and search for new states.

Heavy-ion physics with the ATLAS detector

The CERN LHC collider will operate with lead ions at √s of 5.5 TeV/nucleon. The ATLAS detector, designed to study high-pT physics in the pp mode of the LHC, has the potential to study ultrarelativistic heavy-ion collisions in a full range of observables characterized extremely dense matter and the formation of a quark-gluon plasma. The ATLAS physics program includes global event measurements (particle multiplicities, transverse momentum), suppression of heavy-quarkonia production, jet quenching, and a study of ultraperipheral collisions.

LHC studies relevant to PeV - EeV cosmic ray physics

At the CERN LHC collider, where p-p collision energies will be equivalent to 100 PeV cosmic ray interactions, the ambiguity of interaction models needed for the interpretation of V.H.E. cosmic ray data is recognized, and relevant inclusive data at high pseudorapidities from p-p and nucleus-nucles interactions will be obtained. In this context, the parameters and plans of the CMS, CASTOR, and TOTEM detectors will be noted.

Signals from R-parity violating top quark decays at LHC

We evaluate the potential of the CERN LHC collider to observe rare decays of the top quark in channels involving R-parity violating ( ) interactions. We stress the importance of calculating top quark production and decay simultaneously as a true 2→4 process. The process of t pair production followed by decay of one of the top quarks is analyzed with fast detector simulation. We show that intermediate supersymmetric particles can be observed as resonances even if they are heavier than the top quark due to the significant off-shell top quark mass effects. The approach where the top quark is produced on-mass-shell and then decays into 2- or 3-body final state would in general lead to incorrect kinematical distributions and rates. The rates of the 2→4 process with top quark production and 3-body decay depend on the total width of the heavy intermediate sfermion which could, therefore, be measured indirectly. We find that the LHC collider offers a unique potential to study rare top quark decays in the framework of supersymmetry with broken R-parity for branching fractions of top decays as low as 10−6.

Feasibility study for the detection of D0 → K−π+ decays in Pb–Pb collisions at LHC with ALICE

The ALICE experiment at the CERN LHC collider is devoted to the study of heavy-ion collisions at a centre of mass energy of 5.5 TeV per nucleon. The study of open charm production in nucleus–nucleus collisions at the LHC allows investigation of the mechanisms of heavy quark production and energy loss in the hot and dense medium formed in the early stage of the collision. In addition, the open charm production cross section is the natural normalization for the study of J/ψ production. The reconstruction of exclusive hadronic decays of charm particles is the only way to obtain a direct measurement of their transverse momentum distribution. We present the results of a feasibility study for the detection of D0 → K−π+ decays in Pb–Pb collisions with ALICE.