Collision Pairs Research Articles

Jet quenching and effects of non-Gaussian transverse-momentum broadening on dijet observables

We study, at a qualitative level, production of jet pairs in ultrarelativistic nuclear collisions within a framework combining High Energy Factorisation (HEF) and in-medium propagation of jet particles that takes into account stochastic transverse forces as well as medium-induced radiation. We find that the resulting di-jet observables feature the behaviour deviating from that of jet-pairs which undergo transverse-momentum broadening following the Gaussian distribution.

Evaluation of the generalized bernoulli trial-transient adaptive subcell (GBT-TAS) collision scheme in treating rarefied gas flows

Collision process, the most complicated evolution term in the Boltzmann or Kac stochastic equations, is treated statistically in the direct simulation Monte Carlo (DSMC) method. The treatment of collision pair selection is of crucial importance in DSMC. A subsequent of several collision algorithms called Bernoulli-trials family schemes have been put forward, which were mathematically based on the Kac stochastic equation. The purpose of this paper is to report an extensive evaluation of the most recent variant of the Bernoulli Trials schemes, i.e., Generalized Bernoulli-trial (GBT) in treating standard problems such as cavity flow and hypersonic flow around a cylinder. The key feature of the GBT collision algorithm is its adaption to any number of particle pairs (Nsel) and its lower computational costs compared to standard schemes. The results show that the GBT scheme, compared to the standard “No Time Counter (NTC)”, “Nearest neighbor (NN)” and “Simplified Bernoulli Trials (SBT)” schemes, successfully captures the collision frequency in the cavity flow, shock wave and wake structure in the cylinder flow and held the same level of accuracy as the other standard collision schemes using a wide range of desired Nsel. A comparison of two variants of the GBT scheme (Scheme 1 and Scheme 2) indicates that Scheme-1 performs the simulation with a lower computational cost. The results prove that using a variable particle per subcell (PPSC) control approach significantly improves the collision probability accuracy and performance of the GBT scheme, especially in regions with high requirements for accurate collision process calculation.

Comment on "Creation of Electron-Positron Pairs in Photon-Photon Collisions Driven by 10-PW Laser Pulses".

Comment on "Creation of Electron-Positron Pairs in Photon-Photon Collisions Driven by 10-PW Laser Pulses".

Inelastic Processes of Electron Interaction with Chalcogens in the Gaseous Phase (a Review)

Complex research of elementary pair collision processes occurring when low-energy (0–70 eV) electrons pass through chalcogen (S, Se, Te) vapor has been carried out in the evaporation temperature intervals of those elements (T = 320÷700 K for sulfur, 420÷490 K for selenium, and 400÷600 K for tellurium). The vapor compositions of indicated elements are studied using the mass spectroscopy method. The radiation spectra are analyzed in the wavelength interval from 200 to 600 nm with the help of optical spectroscopy. Using highly monoenergetic electron beams, the total (integral) formation cross-sections for positive and negative S, Se, and Te ions are measured. It is found that, under the experimental conditions, the main components of chalcogen vapor are molecules containing 2 to 8 atoms. At the energies of bombarding electrons below 10 eV, the emission spectra mainly consist of bands of diatomic molecules, and, at higher energies (E > 15 eV), there appear separate atomic and ionic lines. At E = 50 eV, the lines of singly charged ions are the most intense ones. It is shown that the most effective reaction channel is the interaction of electrons with diatomic molecules of indicated elements, whereas other processes are mainly associated with the decay of polyatomic molecules. The excitation and ionization thresholds for interaction products are found by analyzing the energy dependences of process characteristics. Specific features are also observed in the energy dependences of the excitation and ionization functions. Doubly charged ions of diatomic sulfur molecules, as well as selenium and tellurium atoms, are revealed for the first time. The appearance of triply charged ions of diatomic sulfur molecules is also detected. The main contribution to the total (integral) effective ionization cross-section of both positive and negative ions is proved to be made by the interaction processes of electrons with diatomic molecules S2, Se2, and Te2. Besides the experimental research, a detailed theoretical study is carried out. Calculations with a theoretical analysis of their results are performed for the structural characteristics of homoatomic sulfur, Sn, selenium, Sen, and tellurium, Ten, molecules with n = 2÷8; namely, interatomic distances, ionization potentials, electron affinity energies, and dissociation energies. The energy characteristics are applied to calculate the appearance energies for singly and doubly charged ionic fragments of those molecules at the dissociative ionization. The obtained results are carefully compared with the available experimental and theoretical data.

Multiple-scattering production of lepton pairs in ultraperipheral heavy-ion collisions

Ultra-relativistic peripheral collisions of heavy ions at LHC can produce copies of numbers of lepton pairs via the two-photon process. Since the energy of the heavy ions is so high, multi-pair production cross-sections of the light leptons especially electrons are quite large so that it is possible to measure them experimentally. To calculate the multi-pair production probabilities, first, we should have an impact parameter dependence cross-section. We have obtained a well-behaved impact parameter dependence cross-section and by using the Monte-Carlo methods, we have calculated multi-pair production cross-sections of electrons and muons in Pb-Pb heavy-ion collisions at LHC energies. We have also used some experimental restrictions in our calculation to compare our findings with the experimental results.

Measurement of the central exclusive production of charged particle pairs in proton-proton collisions at sqrt{s} = 200 GeV with the STAR detector at RHIC

We report on the measurement of the Central Exclusive Production of charged particle pairs h+h− (h = π, K, p) with the STAR detector at RHIC in proton-proton collisions at sqrt{s} = 200 GeV. The charged particle pairs produced in the reaction pp → p′ + h+h− + p′ are reconstructed from the tracks in the central detector and identified using the specific energy loss and the time of flight method, while the forward-scattered protons are measured in the Roman Pot system. Exclusivity of the event is guaranteed by requiring the transverse momentum balance of all four final-state particles. Differential cross sections are measured as functions of observables related to the central hadronic final state and to the forward-scattered protons. They are measured in a fiducial region corresponding to the acceptance of the STAR detector and determined by the central particles’ transverse momenta and pseudorapidities as well as by the forward-scattered protons’ momenta. This fiducial region roughly corresponds to the square of the four-momentum transfers at the proton vertices in the range 0.04 GeV2< −t1, −t2< 0.2 GeV2, invariant masses of the charged particle pairs up to a few GeV and pseudorapidities of the centrally-produced hadrons in the range |η| < 0.7. The measured cross sections are compared to phenomenological predictions based on the Double Pomeron Exchange (DPE) model. Structures observed in the mass spectra of π+π− and K+K− pairs are consistent with the DPE model, while angular distributions of pions suggest a dominant spin-0 contribution to π+π− production. For π+π− production, the fiducial cross section is extrapolated to the Lorentz-invariant region, which allows decomposition of the invariant mass spectrum into continuum and resonant contributions. The extrapolated cross section is well described by the continuum production and at least three resonances, the f0(980), f2(1270) and f0(1500), with a possible small contribution from the f0(1370). Fits to the extrapolated differential cross section as a function of t1 and t2 enable extraction of the exponential slope parameters in several bins of the invariant mass of π+π− pairs. These parameters are sensitive to the size of the interaction region.

Kink-antikink collision in a Lorentz-violating ϕ4 model

In this work, kink-antikink collision in a two-dimensional Lorentz-violating ϕ4 model is considered. It is shown that the Lorentz-violating term in the proposed model does not affect the structure of the linear perturbation spectrum of the standard ϕ4 model, and thus there exists only one vibrational mode. The Lorentz-violating term impacts, however, the frequency and spatial wave function of the vibrational mode. As a consequence, the well-known results on ϕ4 kink-antikink collision will also change. Collisions of kink-antikink pairs with different values of initial velocities and Lorentz-violating parameters are simulated using the Fourier spectral method. Our results indicate that models with larger Lorentz-violating parameters would have smaller critical velocities vc and smaller widths of bounce windows. Interesting fractal structures existing in the curves of maximal energy densities of the scalar field are also found.

Cs-Ar optical amplifier with a saturation intensity of 10 kW-cm-2 and single-pass extraction efficiency of 28% at 852.2 nm.

Optical amplification by the stimulated emission of Cs(6p2P3/2)-Ar atomic pairs, observed in pump-probe experiments over a ∼290 GHz-wide spectral region lying to the red of the Cs D2 line (852.1 nm), has been realized by photoexciting thermalized, ground state Cs-Ar atoms in the 834-849 nm wavelength interval. When the gain medium is pumped at the peak of the CsAr B2Σ1/2+←X2Σ1/2+ transition at 836.7 nm, maximum gain occurs between 852.2 nm and 852.3 nm and >28% of the energy stored in the upper laser level is extracted with 8 ns (FWHM) probe pulses in a single pass. From the measured rate of saturation of the extracted pulse energy with increasing probe intensity, the product of γ0L and Esat, the saturation pulse energy, is measured directly to be 400 ± 20 µJ and the lower limit for the saturation intensity (Isat) of this amplifier is estimated to be 10 kW-cm-2 at 852.2 nm. Circularly polarizing the optical pump beam increases the optical-to-optical conversion efficiency by 20%, and the storage lifetime of the upper laser level is observed from temporally-resolved gain spectra to be 5 ± 1 ns. Pump excitation spectra also reveal a significant contribution from Ar-Cs-Ar (3-body) photoassociation and suprathermal Ar atoms generated by the dissociation of the CsAr B2Σ1/2+ complex. Multipass-amplifier geometries with broad-bandwidth probe signals are expected to yield upper state energy extraction efficiencies above 50%. This alkali-rare gas amplifier demonstrates the efficiencies available with the storage of energy in, and optical extraction from, excited atomic collision pairs.

Search for heavy Higgs bosons decaying to a top quark pair in proton-proton collisions at sqrt{s} = 13 TeV

A search is presented for additional scalar (H) or pseudoscalar (A) Higgs bosons decaying to a top quark pair in proton-proton collisions at a center-of-mass energy of 13 TeV. The data set analyzed corresponds to an integrated luminosity of 35.9 fb−1 collected by the CMS experiment at the LHC. Final states with one or two charged leptons are considered. The invariant mass of the reconstructed top quark pair system and variables that are sensitive to the spin of the particles decaying into the top quark pair are used to search for signatures of the H or A bosons. The interference with the standard model top quark pair background is taken into account. A moderate signal-like deviation compatible with an A boson with a mass of 400 GeV is observed with a global significance of 1.9 standard deviations. New stringent constraints are reported on the strength of the coupling of the hypothetical bosons to the top quark, with the mass of the bosons ranging from 400 to 750 GeV and their total relative width from 0.5 to 25%. The results of the search are also interpreted in a minimal supersymmetric standard model scenario. Values of mA from 400 to 700 GeV are probed, and a region with values of tan β below 1.0 to 1.5, depending on mA, is excluded at 95% confidence level.

Hadron blind Cherenkov counters

A Hadron Blind Detector, or HBD, is a novel form of Cherenkov counter that is designed to provide high efficiency for detecting highly relativistic electrons while being essentially blind to most hadrons. In this paper, we present some of the most prominent realizations of the HBD concept in real experiments. We describe the first implementation of an HBD that was made in the CERES experiment at CERN using a spectrometer based on a doublet of hadron blind RICH detectors for the measurement of low-mass electron pairs in pA and AA collisions at the SPS. We next present a detailed account of a more extensive realization of the HBD that was made in the PHENIX experiment for the measurement of low-mass electron pairs in central heavy-ion collisions at RHIC, followed by a description of a very similar detector that is currently under construction at J-PARC for the measurement of vector mesons through their e+e− decay in pA collisions. We conclude with a brief discussion of possible evolutions of the HBD concept as well as possible developments and uses of HBDs in experiments at the future Electron Ion Collider.

Evidence for electroweak production of two jets in association with a Zγ pair in pp collisions at [formula omitted] with the ATLAS detector

Evidence for electroweak production of two jets in association with a Zγ pair in s=13 TeV proton–proton collisions at the Large Hadron Collider is presented. The analysis uses data collected by the ATLAS detector in 2015 and 2016 that corresponds to an integrated luminosity of 36.1fb−1. Events that contain a Z boson candidate decaying leptonically into either e+e− or μ+μ−, a photon, and two jets are selected. The electroweak component is measured with observed and expected significances of 4.1 standard deviations. The fiducial cross-section for electroweak production is measured to be σZγjj−EW=7.8±2.0fb, in good agreement with the Standard Model prediction.

Flow of Hagedorn singularities and phase transitions in large N gauge theories

We investigate the singularity structure of the $(-1)^F$ graded partition function in QCD with $n_f \geq 1$ massive adjoint fermions in the large-$N$ limit. Here, $F$ is fermion number and $N$ is the number of colors. The large $N$ partition function is made reliably calculable by taking space to be a small three-sphere $S^3$. Singularites in the graded partition function are related to phase transitions and to Hagedorn behavior in the $(-1)^F$-graded density of states. We study the flow of the singularities in the complex "inverse temperature" $\beta$ plane as a function of the quark mass. This analysis is a generalization of the Lee-Yang-Fisher-type analysis for a theory which is always in the thermodynamic limit thanks to the large $N$ limit. We identify two distinct mechanisms for the appearance of physical Hagedorn singularities and center-symmetry changing phase transitions at real positive $\beta$, inflow of singularities from the $\beta=0$ point, and collisions of complex conjugate pairs of singularities.

Production of Heavy Particle Pairs via Photon–Photon Processes at the LHC

We discuss production of $W^+ W^-$ pairs and $t \bar t$ quark-antiquark pairs in proton-proton collisions induced by two-photon fusion including transverse momenta of incoming photons. The unintegrated inelastic fluxes (related to proton dissociation) of photons are calculated based on modern parametrizations of deep inelastic structure functions in a broad range of $x$ and $Q^2$. We focus on processes with single and double proton dissociation. Highly excited remnant systems hadronise producing particles that can be vetoed in the calorimeter. We calculate associated gap survival factors. The gap survival factors depend on the process, mass of the remnant system and collision energy. The rapidity gap survival factor due to remnant fragmentation for double dissociative (DD) collisions is smaller than that for single dissociative (SD) process. We observe approximate factorisation: $S_{R,DD} \approx S_{R,SD}^2$ when imposing rapidity veto. For the $W^+W^-$ final state, the remnant fragmentation leads to a taming of the cross section when the rapidity gap requirement is imposed. Also for $t \bar t$ quark-antiquark pairs such a condition reverses the hierarchy observed for the case when such condition is taken into account. Our results imply that for the production of such heavy objects as $t$ quark and $\bar t$ antiquark the virtuality of the photons attached to the dissociative system are very large ($Q^2 <$ 10$^{4}$ GeV$^2$). A similar effect is observed for the $W^+ W^-$ system.

Combination of searches for Higgs boson pairs in pp collisions at [formula omitted] with the ATLAS detector

This letter presents a combination of searches for Higgs boson pair production using up to 36.1fb−1 of proton–proton collision data at a centre-of-mass energy s=13 TeV recorded with the ATLAS detector at the LHC. The combination is performed using six analyses searching for Higgs boson pairs decaying into the bb¯bb¯, bb¯W+W−, bb¯τ+τ−, W+W−W+W−, bb¯γγ and W+W−γγ final states. Results are presented for non-resonant and resonant Higgs boson pair production modes. No statistically significant excess in data above the Standard Model predictions is found. The combined observed (expected) limit at 95% confidence level on the non-resonant Higgs boson pair production cross-section is 6.9 (10) times the predicted Standard Model cross-section. Limits are also set on the ratio (κλ) of the Higgs boson self-coupling to its Standard Model value. This ratio is constrained at 95% confidence level in observation (expectation) to −5.0<κλ<12.0 (−5.8<κλ<12.0). In addition, limits are set on the production of narrow scalar resonances and spin-2 Kaluza–Klein Randall–Sundrum gravitons. Exclusion regions are also provided in the parameter space of the habemus Minimal Supersymmetric Standard Model and the Electroweak Singlet Model.

Sampling the mu nu SSM for displaced decays of the tau left sneutrino LSP at the LHC

Within the framework of the mu nu mathrm{SSM}, a displaced dilepton signal is expected at the LHC from the decay of a tau left sneutrino as the lightest supersymmetric particle (LSP) with a mass in the range 45–100 GeV. We compare the predictions of this scenario with the ATLAS search for long-lived particles using displaced lepton pairs in pp collisions, considering an optimization of the trigger requirements by means of a high level trigger that exploits tracker information. The analysis is carried out in the general case of three families of right-handed neutrino superfields, where all the neutrinos get contributions to their masses at tree level. To analyze the parameter space, we sample the mu nu SSM for a tau left sneutrino LSP with proper decay length ctau > 0.1 , hbox {mm} using a likelihood data-driven method, and paying special attention to reproduce the current experimental data on neutrino and Higgs physics, as well as flavor observables. The sneutrino is special in the mu nu mathrm{SSM} since its couplings have to be chosen so that the neutrino oscillation data are reproduced. We find that important regions of the parameter space can be probed at the LHC run 3.

High resolution technique for studying the energy exchange near the shock wave front

The structure and properties of the shock wave front propagating in binary gas was investigated by multi-channel emission spectroscopy and electrostatic probe located in a flow core. The experimental conditions allowed to consider the energy exchange processes in the front like interaction of isolated cold heavy molecules Mo (CO)6 with shock wave front of light noble gas (He, Ar). Measurements were carried out in a weak incident shock waves (Mach number M = (2.5 ÷ 3.6) in the high vacuum shock tube at equilibrium temperature of 850–1400 K and pressure of 0.1–1.1 bar, Mo(CO)6 concentration 5.0 × 1013−3.0 × 1014 cm−3. An influence of heavy molecules on thermodynamics was negligible and free paths of “heavy” molecules between collisions “heavy-heavy” were longer then 1 mm. Nearby shock wave front the conductivity and the electron precursor zones caused by ionization and charge separation were detected and measured. The dependence of probe current on the energy of pair collisions was found. The length of charge separation in SW front was measured and free electrons concentration was estimated.

Searching for Odderon in Exclusive Reactions

We discuss the possibility to use the pp → ppфф process in identifying the odderon exchange. So far, there is no unambiguous experimental evidence for the odderon, the charge conjugation C = −1 counterpart of the C = +1 pomeron. Last year, the results of the TOTEM collaboration suggest that the odderon exchange can be responsible for a disagreement of theoretical calculations and the TOTEM data for the elastic proton-proton scattering. Here, we present recent studies for the central exclusive production (CEP) of фф pairs in proton-proton collisions. We consider the pomeron-pomeron fusion to фф (PP → фф) through the continuum processes, due to the ^t- and ^u-channel reggeized ф-meson, photon, and odderon exchanges, as well as through the s-channel resonance process (PP → f2(2340) → фф). This f2 state is a candidate for a tensor glueball. The amplitudes for the processes are formulated within the tensor-pomeron and vector-odderon approach. Some model parameters are determined from the comparison to the WA102 experimental data. The odderon exchange is not excluded by the WA102 data for high фф invariant masses. The measurement of large Mфф or Ydiff events at the LHC would therefore suggest the presence of the odderon exchange. The process is advantageous, as here the odderon does not couple to protons.

Central Exclusive Production at LHCb

The LHCb collaboration has measured central exclusive production of J/ф, ф(2S), and ϒ mesons as well as J/фJ/ф, J/фф(2S), ф(2S)ф(2S), and xcxc meson pairs in proton-proton collisions. The analyses of ϒ and charmonium pairs are performed at the centre-of-mass energies of 7 TeV and 8 TeV, and those of J/ф and ф(2S) are done at 7 TeV and 13 TeV. The analysis at 13 TeV involves the use of new shower counters. These allow a reduction in the background by vetoing events with activity in an extended region in rapidity. The measurements of central exclusive production at LHCb are sensitive to gluon distributions for Bjorken-x values down to 2×10−6 (at 13 TeV). An overview of the LHCb results is presented and compared to existing measurements of other experiments and theoretical calculations.

Electromagnetic Interactions of Nuclei at the FCC-hh Collider

At the Future Circular Collider (FCC-hh) at CERN in addition to proton-proton collisions it is planned to study interactions of 208Pb nuclei with the collision energy $$\sqrt {{{s}_{{NN}}}} = 39.4$$ TeV, which is 7–8 times higher than that achieved at the LHC. The electromagnetic dissociation (EMD) of $$^{{208}}$$ Pb nuclei, along with the electron capture resulting from the multiple production of e $$^{ + }$$ e $$^{ - }$$ pairs in ultraperipheral collisions at the LHC, lead to a significant luminosity decay, and secondary ions impose a well localized thermal load on superconducting magnets. As expected, such effects are reduced in collisions of light and medium nuclei. In the present work the EMD cross sections for collisions of $$^{{40}}$$ Ar, $$^{{40}}$$ Ca, $$^{{63}}$$ Cu, $$^{{78}}$$ Kr, $$^{{84}}$$ Kr, $$^{{115}}$$ In, $$^{{129}}$$ Xe, $$^{{208}}$$ Pb and $$^{{238}}$$ U at the LHC and the planned FCC-hh were calculated in order to assess the operation conditions of the colliders. The results were compared with results of the RELDIS model, and the uncertainties of calculations of the EMD cross sections associated with using of various approximations of the total nuclear photoabsorption cross sections were estimated.

Extending the constraint for axion-like particles as resonances at the LHC and laser beam experiments

We study the discovery potential of axion-like particles (ALP), pseudo-scalars weakly coupled to Standard Model fields, at the Large Hadron Collider (LHC). Our focus is on ALPs coupled to the electromagnetic field, which would induce anomalous scattering of light-by-light. This can be directly probed in central exclusive production of photon pairs in ultra-peripheral collisions at the LHC in proton and heavy-ion collisions. We consider non-standard collision modes of the LHC, such as argon-argon collisions at sNN=7 TeV and proton-lead collisions at sNN=8.16 TeV, to access regions in the parameter space complementary to the ones previously considered for lead-lead and proton-proton collisions. In addition, we show that, using laser beam interactions, we can constrain ALPs as resonant deviations in the refractive index induced by anomalous light-by-light scattering effects. If we combine the aforementioned approaches, ALPs can be probed in a wide range of masses from the eV scale up to the TeV scale.