Collision Channel Research Articles

Measurement of the inclusive and fiducial tbar{t} production cross-sections in the lepton+jets channel in pp collisions at sqrt{s} = 8~hbox {TeV} with the ATLAS detector

The inclusive and fiducial tbar{t} production cross-sections are measured in the lepton+jets channel using 20.2~hbox {fb}^{-1} of proton–proton collision data at a centre-of-mass energy of 8 TeV recorded with the ATLAS detector at the LHC. Major systematic uncertainties due to the modelling of the jet energy scale and b-tagging efficiency are constrained by separating selected events into three disjoint regions. In order to reduce systematic uncertainties in the most important background, the W text {+,jets} process is modelled using Z+ jets events in a data-driven approach. The inclusive tbar{t} cross-section is measured with a precision of 5.7% to be sigma _{text {inc}}(tbar{t}) = 248.3 pm 0.7 , ({mathrm {stat.}}) pm 13.4 , ({mathrm {syst.}}) pm 4.7 , ({mathrm {lumi.}})~text {pb}, assuming a top-quark mass of 172.5 GeV. The result is in agreement with the Standard Model prediction. The cross-section is also measured in a phase space close to that of the selected data. The fiducial cross-section is sigma _{text {fid}}(tbar{t}) = 48.8 pm 0.1 , ({mathrm {stat.}}) pm 2.0 , ({mathrm {syst.}}) pm 0.9 , ({mathrm {lumi.}})~text {pb} with a precision of 4.5%.

Double Resummation for Higgs Production.

We present the first double-resummed prediction of the inclusive cross section for the main Higgs production channel in proton-proton collisions, namely, gluon fusion. Our calculation incorporates to all orders in perturbation theory two distinct towers of logarithmic corrections which are enhanced, respectively, at threshold, i.e., large x, and in the high-energy limit, i.e., small x. Large-x logarithms are resummed to next-to-next-to-next-to-leading logarithmic accuracy, while small-x ones to leading logarithmic accuracy. The double-resummed cross section is furthermore matched to the state-of-the-art fixed-order prediction at next-to-next-to-next-to-leading accuracy. We find that double resummation corrects the Higgs production rate by 2% at the currently explored center-of-mass energy of 13TeV and its impact reaches 10% at future circular colliders at 100TeV.

A VCSEL-based backbone extended-reach optical fibre network: Supporting up to 10 Gbps flexible access networks for Africa

Abstract In this paper, using vertical cavity surface-emitting lasers (VCSELs) and Raman amplification, a low power consuming, energy-efficient and cheap technique to transmit high speed data signals over optical fibre is experimentally demonstrated. Several channels are joined through dense wavelength division multiplexing (DWDM) technique and transmitted over a single fibre link. With a 24.7 dBm forward Raman pump, a 8 dB flat gain is distributed over a spectral width of 5.2 nm (650 GHz). Moreover, different wavelength paths are realized by tuning the emission of the VCSEL from 1546.8 nm to 1552 nm while adjusting its bias currents. In realizing extended-reach, two DWDM/flexible channels spaced at 50 GHz and with each transmitting at 10 Gbps have been transmitted for 76.8 km while incurring a 3.2 dB power penalty as measured at a bit-error-rate (BER) threshold of 10−9. In a typical network, the wavelength adjustment of the VCSEL avoids incidences of denial of service whenever there is either fibre-cuts, increased network traffic or channel collisions. Finally, by combining Raman amplification and chromatic dispersion mitigation using negative dispersion shifted fibres, this work presents robust VCSEL-based technique that is tailored to provide 10 Gbps per channel for backbone optical fibre supporting long-haul access networks in Africa.

Finite-Length Analysis of Irregular Repetition Slotted ALOHA in the Waterfall Region

A finite-length analysis is introduced for irregular repetition slotted ALOHA (IRSA) that enables to accurately estimate its performance in the moderate-to-high packet loss probability regime, i.e., in the so-called waterfall region. The analysis is tailored to the collision channel model, which enables mapping the description of the successive interference cancellation process onto the iterative erasure decoding of low-density parity-check codes. The analysis provides accurate estimates of the packet loss probability of IRSA in the waterfall region as demonstrated by Monte Carlo simulations.

Cooling of trapped ions by resonant charge exchange

The two most widely used ion cooling methods are laser cooling and sympathetic cooling by elastic collisions (ECs). Here we demonstrate another method of cooling ions that is based on resonant charge exchange (RCE) between the trapped ion and the ultracold parent atom. Specifically, trapped Cs$^{+}$ ions are cooled by collisions with co-trapped, ultracold Cs atoms and, separately, by collisions with co-trapped, ultracold Rb atoms. We observe that the cooling of Cs$^{+}$ ions by Cs atoms is more efficient than cooling of Cs$^{+}$ ions by Rb atoms. This signals the presence of a cooling mechanism apart from the elastic ion-atom collision channel for the Cs-Cs$^{+}$ case, which is cooling by RCE. The efficiency of cooling by RCE is experimentally determined and the per-collision cooling is found to be two orders of magnitude higher than cooling by EC. The result provides the experimental basis for future studies on charge transport by electron hopping in atom-ion hybrid systems.

CRT Sequences With Applications to Collision Channels Allowing Successive Interference Cancellation

Protocol sequences are periodic zero-one sequences for the scheduling of packet transmissions in a time-slotted channel. A special class of protocol sequences, called shift-invariant sequences, plays a key role in achieving the information-theoretic capacity of the collision channel without feedback. This class of shift-invariant protocol sequences has the property that the pairwise Hamming crosscorrelation functions are invariant to relative delay offsets. However, the common period of shift-invariant sequences grows exponentially as a function of the number of supported users. In this paper, we consider a family of protocol sequences, whose period increases roughly as a quadratic function of the number of the users, and show that it is close to shift-invariant by establishing a bound on the pairwise Hamming crosscorrelation. The construction is based on the Chinese remainder theorem (CRT), and hence the constructed sequences are called CRT sequences. Applications to collision channel allowing successive interference cancellation at the receiver are discussed.

Contention Resolution Mechanisms for Multi-Channel Cognitive Radio Ad Hoc Networks

In this article, the channel hopping (CH) method for the rendezvous process of secondary users in multi-channel CRAHNs is overviewed. With the evolution of the Internet of Things, design criteria of CH sequences targeting these applications are discussed. While channel collision among CH sequences is inherent in realistic CRAHNs, most prior works focused on sequence designs without addressing the channel collision problem. Hence, the concept of contention resolution in CRAHNs is introduced in this article, and a "multi-round" contention resolution mechanism is studied. To quantify the performance, the RS rate of the mechanism is presented and validated with computer simulation. Finally, the rendezvous time savings, which, in turn, can enhance data throughput, of the multi-round mechanism over a single-round one are investigated.

Measurement of normalized differential mathrm{t}overline{mathrm{t}} cross sections in the dilepton channel from pp collisions at sqrt{s}=13 TeV

Normalized differential cross sections for top quark pair production are measured in the dilepton (e+e−, μ+μ−, and μ∓e±) decay channels in proton-proton collisions at a center-of-mass energy of 13 TeV. The measurements are performed with data corresponding to an integrated luminosity of 2.1 fb−1 using the CMS detector at the LHC. The cross sections are measured differentially as a function of the kinematic properties of the leptons, jets from bottom quark hadronization, top quarks, and top quark pairs at the particle and parton levels. The results are compared to several Monte Carlo generators that implement calculations up to next-to-leading order in perturbative quantum chromodynamics interfaced with parton showering, and also to fixed-order theoretical calculations of top quark pair production up to next-to-next-to-leading order.

A MAC Protocol Based on RS-Polar Coding for Airborne Tactical Networks

For the features of high speed moving, sparse distribution, diversity of data service, etc., and high reliability and low latency of QoS demand when transmitting time-sensitive information in airborne tactical networks, we present a random access multi-channel medium access control protocol based on RS-Polar coding technology. In the protocol, by adopting error correcting coding technology of RS-Polar cascaded, the multi-packet reception technology, it can ensure the real-time and reliability of packet transmission without the use of channel reservation and time-slot allocation, and improve the network throughput effectively. By establishing the channel collision model and the multi packet reception model in the bursts propagation stage, the mathematical expressions of packet success probability, packet end-to-end delay and network throughput are derived. The simulation results show that packet end-to-end delay of the protocol is less than 2ms, and when the load is less than 2000 packet/s, the packet success probability is higher than 99%, so the protocol can meet the performance requirements of the airborne tactical network effectively.

ZZ→ℓ+ℓ−ℓ′+ℓ′− cross-section measurements and search for anomalous triple gauge couplings in 13 TeV pp collisions with the ATLAS detector

Measurements of $ZZ$ production in the $\ell^{+}\ell^{-}\ell^{\prime +}\ell^{\prime -}$ channel in proton-proton collisions at 13 TeV center-of-mass energy at the Large Hadron Collider are presented. The data correspond to 36.1 $\mathrm{fb}^{-1}$ of collisions collected by the ATLAS experiment in 2015 and 2016. Here $\ell$ and $\ell'$ stand for electrons or muons. Integrated and differential $ZZ \to \ell^{+}\ell^{-}\ell^{\prime +}\ell^{\prime -}$ cross sections with $Z \to \ell^+\ell^-$ candidate masses in the range of 66 GeV to 116 GeV are measured in a fiducial phase space corresponding to the detector acceptance and corrected for detector effects. The differential cross sections are presented in bins of twenty observables, including several that describe the jet activity. The integrated cross section is also extrapolated to a total phase space and to all Standard-Model decays of $Z$ bosons with mass between 66 GeV and 116 GeV, resulting in a value of $17.3 \pm 0.9$ [$\pm 0.6$ (stat.) $\pm 0.5$ (syst.) $\pm 0.6$ (lumi.)] pb. The measurements are found to be in good agreement with the Standard-Model predictions. A search for neutral triple gauge couplings is performed using the transverse momentum distribution of the leading $Z$-boson candidate. No evidence for such couplings is found and exclusion limits are set on their parameters.

Interplay of break-up and transfer processes in reactions involving weakly-bound systems 11Work done in collaboration with Antonio Moro and Kouichi Hagino

In this note we illustrate some applications of a simple model which has been devised to clarify the reaction mechanism and the interplay of different reaction channels (elastic, inelastic, transfer, break-up) in heavy-ion collisions. The model involves two potential wells moving in one dimension and few active particles; in spite of its simplicity, it is supposed to maintain the main features, the properties and the physics of the full three-dimensional case. Special attention is given to the role of continuum states in reactions involving weakly-bound systems, and different approximation schemes (as first-order or coupled-channels) as well as different continuum discretization procedures are tested. In the case of two active particles the reaction mechanism associated with two-particle transfer and the effect of pairing intearction are investigated.

Wireless network-level partial relay cooperation: A stable throughput analysis

In this work, we study the benefit of partial relay cooperation. We consider a two-node system consisting of one source and one relay node transmitting information to a common destination. The source and the relay have external traffic and in addition, the relay is equipped with a flow controller to regulate the incoming traffic from the source node. The cooperation is performed at the network level. A collision channel with erasures is considered. We provide an exact characterization of the stability region of the system and we also prove that the system with partial cooperation is always better or at least equal to the system without the flow controller.

Shedding Light on Hexaquarks

Several new findings in the four, five and six quark systems reheat the interest in the field of multiquark states (beyond the trivial [Formula: see text] and [Formula: see text]). A lot of progress has recently been made in the 6q sector, on both the theoretical and experimental side. A resonance like structure observed in double-pionic fusion to the deuteron, at M = 2.38 GeV with [Formula: see text] = 70 MeV and [Formula: see text] has been consistently observed in a wealth of reaction channels, supporting the existence of a resonant dibaryon state - the [Formula: see text]. These studies include measurement of all the principle strong decay channels in pn collisions in the quasifree mode by the WASA-at-COSY and HADES collaborations. The internal structure of the [Formula: see text] is largely unknown. It can contain various ”hidden color” 6q configurations, [Formula: see text] molecular states with angular momentum L = 0,2,4,6 as well as meson-assisted dressed dibaryon structures. The large set of experimental data obtained to date gives some constraints on the internal structure of the [Formula: see text] dibaryon, but does not settle the issue. The [Formula: see text] is the only multiquark state which can be produced copiously at current facilities, offering unique access to information beyond its basic quantum numbers, particularly its physical size and internal structure.

Blue straggler populations beyond the Milky Way

Although the formation of blue straggler stars (BSSs) is routinely attributed to stellar interactions in binary systems, the relative importance of the direct collision and slow(er) stellar coalescence formation channels is still poorly understood. We selected a sample of 24 Magellanic Cloud star clusters for which multi-passband Hubble Space Telescope images are available to address this outstanding question. We compiled a BSS database, containing both traditional and evolved BSSs. We found a robust correlation between the number of BSSs in a cluster’s core and its core mass, NBSS,core ∝ M0.51±0.07 core, which supports the notion that BSS formation is linked to a population’s binary fraction. At low stellar collision rates, the mass-normalised number of BSSs does not appear to depend on the collision rate, which implies that the coalescence-driven BSS formation channel dominates. Comparison with simulations suggests that stellar collisions contribute less than 20% to the total number of BSSs formed.

Probing resonant energy transfer in collisions of ammonia with Rydberg helium atoms by microwave spectroscopy.

We present the results of experiments demonstrating the spectroscopic detection of Förster resonance energy transfer from NH3 in the X1A1 ground electronic state to helium atoms in 1sns 3S1 Rydberg levels, where n = 37 and n = 40. For these values of n, the 1sns 3S1 → 1snp 3PJ transitions in helium lie close to resonance with the ground-state inversion transitions in NH3 and can be tuned through resonance using electric fields of less than 10 V/cm. In the experiments, energy transfer was detected by direct state-selective electric field ionization of the 3S1 and 3PJ Rydberg levels and by monitoring the population of the 3DJ levels following pulsed microwave transfer from the 3PJ levels. Detection by microwave spectroscopic methods represents a highly state selective, low-background approach to probing the collisional energy transfer process and the environment in which the atom-molecule interactions occur. The experimentally observed electric-field dependence of the resonant energy transfer process, probed both by direct electric field ionization and by microwave transfer, agrees well with the results of calculations performed using a simple theoretical model of the energy transfer process. For measurements performed in zero electric field with atoms prepared in the 1s40s 3S1 level, the transition from a regime in which a single energy transfer channel can be isolated for detection to one in which multiple collision channels begin to play a role has been identified as the NH3 density was increased.

Vehicular Radio Access to Unlicensed Spectrum

Connection to/among vehicles has been bound to one of the dominant wireless use cases in the upcoming decade. To support the booming applications, scarce bandwidth allocated for vehicle connection has become a performance bottleneck and rigidifies further evolution of future technologies. This concern consequently drives the utilization of unlicensed spectrum with ample bandwidth, on which communications nevertheless should comply with regulatory requirements such as listen-before-talk, which results in unique challenges. For next generation vehicular radio access to unlicensed spectrum, in this article, we develop dual designs to inherit the distinct engineering spirits of 3GPP LTE and IEEE 802.11ax. The proposed LTE-based vehicular radio access should integrate 3GPP vehicle-to-everything connection, licensed assisted access, and device-todevice proximity services to address the particular issues of sidelink shared channel collision and resource uncertainty on uplink, downlink, and sidelink. The proposed IEEE 802.11ax-based vehicular direct radio access emphasizes providing a qualityof-service-guaranteed sidelink access scheme to enable direct vehicle data exchange. The proposed designs pave a foundation to deploy vehicle-to-everything in unlicensed spectrum.

New CRT sequence sets for a collision channel without feedback

Protocol sequences are binary and periodic sequences used for deterministic multiple access in a collision channel without feedback. In this paper, we focus on user-irrepressible (UI) protocol sequences that can guarantee a positive individual throughput per sequence period with probability one for a slot-synchronous channel, regardless of the delay offsets among the users. As the sequence period has a fundamental impact on the worst-case channel access delay, a common objective of designing UI sequences is to make the sequence period as short as possible. Consider a communication channel that is shared by $M$ active users, and assume that each protocol sequence has a constant Hamming weight $w$. To attain a better delay performance than previously known UI sequences, this paper presents a CRTm construction of UI sequences with $w=M+1$, which is a variation of the previously known CRT construction. For all non-prime $M\geq 8$, our construction produces the shortest known sequence period and the shortest known worst-case delay of UI sequences. Numerical results show that the new construction enjoys a better average delay performance than the optimal random access scheme and other constructions with the same sequence period, in a variety of traffic conditions. In addition, we derive an asymptotic lower bound on the minimum sequence period for $w=M+1$ if the sequence structure satisfies some technical conditions, called equi-difference, and prove the tightness of this lower bound by using the CRTm construction.

Measurements of top-quark pair differential cross-sections in the lepton+jets channel in pp collisions at sqrt{s}=13 TeV using the ATLAS detector

Measurements of differential cross-sections of top-quark pair production in fiducial phase-spaces are presented as a function of top-quark and toverline{t} system kinematic observables in proton-proton collisions at a centre-of-mass energy of sqrt{s}=13 TeV. The data set corresponds to an integrated luminosity of 3.2 fb−1, recorded in 2015 with the ATLAS detector at the CERN Large Hadron Collider. Events with exactly one electron or muon and at least two jets in the final state are used for the measurement. Two separate selections are applied that each focus on different top-quark momentum regions, referred to as resolved and boosted topologies of the toverline{t} final state. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations by means of calculated χ2 and p-values.

Inelastic processes in collisions of lithium positive ions with hydrogen anions and atoms

Inelastic processes in the low-energy collisions Li3+ + H−, Li2+ + H, Li2+ + H- and Li+ + H are investigated for all collisional channels with the excited ionic lithium states Li2+ (nl) and Li+ (1s nl) up to and including the corresponding ion-pair states for the temperature range 1000–20 000 K. For all possible processes in the Li3+ + H- and Li2+ + H collisions inelastic cross sections and rate coefficients are calculated for the transitions between the ion-pair channel Li3+ + H- and the 35 below lying contributing Li2+ (nl) + H channels. It is found that the highest values of cross sections and rate coefficients are obtained for the recombination processes and their inverse, the ion-pair formation processes, involving the Li2+ (3l), Li2+ (4l), and Li2+ (5l) states. For the processes in the Li2+ + H- and Li+ + H collisions, cross sections and rate coefficients are calculated for all transitions between 34 Li+ (1s nl) + H channels lying below Li2+ + H- plus this ion-pair channel. In this case the highest rate coefficients correspond to the recombination processes with the Li+(1s3l1,3L) and Li+(1s4l1,3L) final states, as well as their inverse processes of ion-pair production. Rate coefficient values for these most efficient processes are rather high, of the order of 10−8 cm3/s. This leads to total recombination rate coefficients in Li3+ + H- and Li2+ + H- collisions with values larger than 10−7 cm3/s.Graphical abstract

Measurement of inclusive and differential cross sections in the H \u2192 ZZ* \u2192 4\u2113 decay channel in pp collisions at sqrt{s}=13 TeV with the ATLAS detector

Inclusive and differential fiducial cross sections of Higgs boson production in proton-proton collisions are measured in the H → ZZ* → 4ℓ decay channel. The proton-proton collision data were produced at the Large Hadron Collider at a centre-of-mass energy of 13 TeV and recorded by the ATLAS detector in 2015 and 2016, corresponding to an integrated luminosity of 36.1 fb−1. The inclusive fiducial cross section in the H → ZZ* → 4ℓ decay channel is measured to be 3.62 ± 0.50(stat)− 0.20+ 0.25 (sys) fb, in agreement with the Standard Model prediction of 2.91 ± 0.13 fb. The cross section is also extrapolated to the total phase space including all Standard Model Higgs boson decays. Several differential fiducial cross sections are measured for observables sensitive to the Higgs boson production and decay, including kinematic distributions of jets produced in association with the Higgs boson. Good agreement is found between data and Standard Model predictions. The results are used to put constraints on anomalous Higgs boson interactions with Standard Model particles, using the pseudo-observable extension to the kappa-framework.