CERN SPS Research Articles

Investigating pi ^{-} production in central ^{7}text {Be}+^{9}text {Be} and ^{40}text {Ar}+ ^{45}text {Sc} collisions at CERN SPS energies with PYTHIA8/Angantyr

We study pion production in proton–proton (pp) and in central ^{7}Be + ^{9}Be and ^{40}text {Ar}+ ^{45}text {Sc} collisions at CERN super proton synchrotron (SPS) energies within the Angantyr model, whose pp dynamics are modeled by PYTHIA 8.303 event generator. We have tuned the parameters of the default multi-partonic interaction (MPI) mechanism in PYTHIA 8.303 to describe simultaneously the measured rapidity and transverse momentum distributions of pions in inelastic pp-collisions in the beam momentum range from 20 to 158 GeV/c. We explore the influence of the standard Lund fragmentation and thermal string models, on the pi ^{-} observables, mainly from light(er) ion beams at CERN SPS energies. The role of primary hadron rescattering (HadSc) is also discussed. We find that the PYTHIA 8.303/Angantyr model employing both pp-tuned and thermal-string model with HadSc is able to reproduce the observed pi ^{-} yields in the light(er) ion beams at the whole CERN SPS energies.

Highlights from the NA61/SHINE strong-interactions programme

NA61/SHINE is a multipurpose fixed-target facility at the CERN SPS. The main goals of the NA61/SHINE strong-interactions programme are to discover the critical point of strongly interacting matter as well as to study the properties of the onset of deconfinement. In order to reach these goals, a study of hadron production properties is performed in nucleus-nucleus, proton-proton and proton-nucleus interactions as a function of collision energy and size of the colliding nuclei. In this contribution, the NA61/SHINE results from a strong interaction measurement programme are presented. In particular, the latest results from different reactions p+p, Be+Be, Ar+Sc, and Pb+Pb on hadron spectra, as well as intermittency, higher-order moments of multiplicity fluctuations and spectator induced electromagnetic effects are discussed.

The Gluon Exchange Model in Proton–Nucleus Collisions

We apply our recently formulated Gluon Exchange Model (GEM) to baryon production in proton-nucleus reactions involving N>1 proton-nucleon collisions. We propose a description scheme for the process of soft color octet (gluon) exchange, based on the assumption that probabilities to form an effective diquark are equal for all allowed pairs of quarks. The latter effective diquark can form either from two valence, one valence and one sea, or from two sea quarks. Consequently we calculate the probabilities for different color configurations involving diquarks of valence-valence, valence-sea and sea-sea type. These probabilities appear to depend on the number of exchanged gluons, which results in increasing baryon stopping as a function of the number of proton-nucleon collisions in the nucleus. As such, the nuclear stopping power appears to be governed by the emergence of new color configurations as a function of N rather than by the energy loss of the original valence diquark. The advantage of our approach lies in its high predictive power which makes it verifiable by the new, precise data on proton and neutron production from the CERN SPS. The latter verification, and a set of predictions for the N-dependence of the baryon stopping process, are included in the letter.

System size and energy dependence of proton rapidity spectra from NA61/SHINE at the CERN SPS

NA61/SHINE is an experiment at the CERN Super Proton Synchrotron. The main goals of the experiment are the search for the critical point of strongly interacting matter and the study of the properties of the onset of deconfinement. To reach these goals, the two-dimensional scan in beam momentum (13A − 150A GeV/c) and system size (p+p, Be+Be, Ar+Sc, Xe+La, Pb+Pb) was performed. In the final stage of the collision, the spectra of protons are only weakly affected by the effects of resonance decays and rescattering due to their large mass. Thus, proton rapidity distribution is particularly sensitive to the onset of deconfinement. This article presents experimental results on proton production in the collision energy range, which is most relevant to the onset of deconfinement. The procedure of measuring the proton rapidity spectra by NA61/SHINE is described, as well as Collaboration’s recent results from reactions of p+p, Be+Be and Ar+Sc. Presented experimental results are confronted with existing data and models.

Search for the critical point of strongly interacting matter (Intermittency analysis by NA61/SHINE at CERN SPS)

The existence and location of the QCD critical point is an object of both experimental and theoretical studies. The comprehensive data collected by NA61/SHINE during a two-dimensional scan in beam momentum (13A-150A GeV/c) and system size (p+p, p+Pb , Be+Be, Ar+Sc, Xe+La, Pb+Pb) allows for a systematic search for the critical point – a search for a non-monotonic dependence of various correlation and fluctuation observables on collision energy and size of colliding nuclei. In particular, fluctuations of particle number in transverse momentum space are studied. They are quantified by measuring the scaled factorial moments of multiplicity distribution.

K^{0}_{S} meson production in inelastic p+p interactions at 158\xa0text{ GeV }/c beam momentum measured by NA61/SHINE at the CERN SPS

The production of K^{0}_{S} mesons in inelastic p+p collisions at beam momentum 158 text{ GeV }/c (sqrt{s_{NN}}=17.3 text{ GeV }) was measured with the NA61/SHINE spectrometer at the CERN Super Proton Synchrotron. Double-differential distributions were obtained in transverse momentum and rapidity. The mean multiplicity of K^{0}_{S} was determined to be 0.162 pm 0.001 (stat.) pm 0.011 (sys.). The results on K^{0}_{S} production are compared with model predictions (EPOS 1.99, SMASH 2.0, PHSD and UrQMD 3.4 models) as well as with published world data.

Prospects for the NA60+ experiment at the CERN SPS

A new heavy-ion experiment on fixed target, NA60+, has been proposed at the CERN SPS for data taking in the next years. Its main goals will be focused on precision studies of thermal dimuons, heavy quark and strangeness production in Pb-Pb collisions at center-of-mass energies ranging from 5 to 17 GeV, which will provide a unique opportunity to investigate the region of the QCD phase diagram at high baryochemical potential (μB ~ 200 – 400 MeV). The key points of the NA60+ very broad and ambitious physics program will be described.

News from the strong interactions program of NA61/SHINE

NA61/SHINE (SPS Heavy Ion and Neutrino Experiment) is a fixedtarget experiment operating at the CERN SPS accelerator. The main goal of the strong interactions program of NA61/SHINE is to study the properties of the phase transition between confined matter and quark-gluon plasma by performing a two-dimensional scan in beam momentum and size of collided nuclei. Within this program, collisions of different systems (p+p, p+Pb, Be+Be, Ar+Sc, Xe+La, Pb+Pb) over a wide range of beam momenta (13A-150(8)A GeV/c) have been recorded. This contribution discusses the latest results of hadron production in p+p, Be+Be, Ar+Sc and Pb+Pb reactions measured by the NA61/SHINE. In particular, the results include charged kaons and pions spectra and higher-order moments of multiplicity and net charge distributions. The presented data are compared with the predictions of different theoretical models as well as the results from other experiments. Finally, the motivation and plans for future NA61/SHINE measurements are discussed.

Strangeness production in the NA61/SHINE experiment at the CERN SPS energy range

NA61/SHINE is a multi-purpose fixed-target experiment located at the H2 beamline of the CERN North Area. One of the main goals of the experiment is to study the phase transition and search for the critical point of the strongly interacting matter. Strangeness production is a long-known valuable probe for understanding particle production in high-energy physics due to the absence of strange valence quarks in the initial collision state. This talk will present the results of strangeness production in p+p, Be+Be, and Ar+Sc collisions in the SPS energy range (√sNN = 5.1 − 17.3 GeV) measured by NA61/SHINE. The talk will emphasise the importance of the results for discussion of the onset of deconfinement and onset of fireball. The obtained results will be compared to available world data and selected theoretical models.

Search for pseudoscalar bosons decaying into e+e− pairs in the NA64 experiment at the CERN SPS

We report the results of a search for a light pseudoscalar particle $a$ that couples to electrons and decays to $e^+e^-$ performed using the high-energy CERN SPS H4 electron beam. If such pseudoscalar with a mass $\simeq 17$ MeV exists, it could explain the ATOMKI anomaly. We used the NA64 data samples collected in the "visible mode" configuration with total statistics corresponding to $8.4\times 10^{10}$ electrons on target (EOT) in 2017 and 2018. In order to increase sensitivity to small coupling parameter $\epsilon$ we used also the data collected in 2016-2018 in the "invisible mode" configuration of NA64 with a total statistics corresponding to $2.84\times 10^{11}$ EOT. A thorough analysis of both these data samples in the sense of background and efficiency estimations was already performed and reported in our previous papers devoted to the search for light vector particles and axion-like particles (ALP). In this work we recalculate the signal yields, which are different due to different cross section and life time of a pseudoscalar particle $a$, and perform a new statistical analysis. As a result, the region of the two dimensional parameter space $m_a - \epsilon$ in the mass range from 1 to 17.1 MeV is excluded. At the mass of the ATOMKI anomaly the values of $\epsilon$ in the range $2.1 \times 10^{-4} < \epsilon < 3.2 \times 10^{-4}$ are excluded.

Selection of radiation tolerant commercial greases for high-radiation areas at CERN: Methodology and applications

At the European Laboratory for Particle Physics (CERN), commercial greases and oils are extensively used to lubricate moving devices operating in high-radiation areas, absorbing doses up to the MGy range. Due to their sensitivity to radiation, lubricants can become fluid or solid in operation, potentially causing component failures and compromising the operation of the accelerator complex. CERN is therefore developing methodologies to select radiation tolerant commercial lubricants for critical Beam Intercepting Devices (BIDs) such as high-power targets, dumps and collimators. A careful selection of greases is fundamental to reduce the risk of failure, to optimize equipment lifetime and to minimize maintenance and unwanted radiation exposure of personnel in high radiation areas. Three different parameters are considered to select commercial lubricants: the expected total dose absorbed in operation, the maintenance feasibility and the failure impact in terms of accelerator downtime and of personnel involvement in replacement procedures. Based on these criteria, an example of relevant application is presented: the lubrication of the new internal beam dump of the Super Proton Synchrotron (SPS). Experimental radiation damage data allowed the identification of two radiation tolerant commercial greases to lubricate the dump support jack assembly.

Study of charge carrier transport properties and lifetimes in HR GaAs:Cr with Timepix3

The response of a Timepix3 (256 × 256 pixels, pixel pitch 55 μm) detector with a 500 μm thick HR GaAs:Cr sensor was studied in proton beams of 125 MeV at the Danish Centre for Particle Therapy in Aarhus, Denmark and in a 120 GeV/c pion beam at the Super-Proton Synchrotron (SPS) at CERN. The sensor was biased at different voltages and irradiated at different angles. The readout chip was configured to operate in electron and hole collection modes. Measurements at grazing angles allowed to see elongated tracks with well-defined impact and exit points, so that charge carrier production depths could be determined in each pixel. We extracted the charge collection efficiencies and the charge carrier drift times as a function of the distance to the pixel plane. It was found that measured proton tracks are shorter in hole collection than in the case of electron collection, which is explained by the shorter lifetime of holes. At an angle of 60 degrees with respect to the sensor normal, the average track length in hole collection was ∼700 μm and 950 μm in electron collection mode. To understand the experimental findings, models describing the properties of HR GaAs:Cr were implemented into the Allpix2 simulation framework. We added previously presented experimental results describing the dependence of the electron drift velocity on the electric field and validated the response by comparing measurement and simulation for various X- and gamma-ray sources in the energy range of 10–60 keV. By comparison of the experimental and the simulated results, the mobility μ h and the lifetime of holes τ h were estimated as μ h = (320 ± 10) cm2/V/s and τ h = (4.5 ± 0.5) ns.

Multifold bunch splitting in a high-intensity medium-energy proton synchrotron

Bunch splitting is an RF manipulation method of changing the bunch structure, bunch numbers and bunch intensity in a high-intensity synchrotron that serve as the injector for a particle collider. An efficient way to realize bunch splitting is to use the combination of different harmonic RF systems, such as the two-fold bunch splitting of a bunch with a combination of fundamental harmonic and higher harmonic RF systems. The two-fold bunch splitting and three-fold bunch splitting methods have been experimentally verified and successfully applied in the CERN PS. In this paper, a generalized multifold bunch splitting method is given. The five-fold bunch splitting method using specially designed multi-harmonic RF systems was studied and tentatively applied to the medium-stage synchrotron (MSS), the third accelerator of the injector chain of the Super Proton-Proton Collider (SPPC), to mitigate the pileup effects and collective instabilities of a single bunch in the SPPC. The results show that the five-fold bunch splitting is feasible and both the bunch population distribution and longitudinal emittance growth after the splitting are acceptable, e.g., a few percent in the population deviation and less than 10% in the total emittance growth. Intensity-related effects and beam feedback issues during the splitting process have also been addressed.

A novel non-adiabatic approach to transition crossing in a circular hadron accelerator

Crossing the transition energy is always a delicate process, representing a potential source of strong perturbations of the dynamics of charged particle beams in a hadron circular accelerator. Since the first generation of multi-GeV rings, intense studies have been devoted to understanding the possible harmful mechanisms involved in transition crossing and to devise mitigation measures. Nowadays, several circular particle accelerators are successfully operating across the transition energy and this process is well mastered. In a completely different context, stable resonances of the traverse phase space have been proposed as new means of manipulating charged particle beams. While the original aim of such a proposal was multi-turn extraction from the CERN Proton Synchrotron to the Super Proton Synchrotron, many more applications have been proposed and studied in detail. In this paper, the two topics, i.e. transition crossing and stable resonances, have been brought together with the goal of providing a novel and non-adiabatic approach to perform a clean transition crossing. The idea presented here is that by judiciously using sextupoles and octupoles it is possible to generate stable islands of the horizontal phase space. These islands represent a second closed orbit whose properties can be selected independently of those of the standard, i.e. central, closed orbit. This provides a means of performing a non-adiabatic change of the transition energy experienced by the charged particles by displacing the beam between the two closed orbits.

Study of Kinetic Freeze-Out Parameters as a Function of Rapidity in pp Collisions at CERN SPS Energies.

We used the blast wave model with the Boltzmann–Gibbs statistics and analyzed the experimental data measured by the NA61/SHINE Collaboration in inelastic (INEL) proton–proton collisions at different rapidity slices at different center-of-mass energies. The particles used in this study were , , , , and . We extracted the kinetic freeze-out temperature, transverse flow velocity, and kinetic freeze-out volume from the transverse momentum spectra of the particles. We observed that the kinetic freeze-out temperature is rapidity and energy dependent, while the transverse flow velocity does not depend on them. Furthermore, we observed that the kinetic freeze-out volume is energy dependent, but it remains constant with changing the rapidity. We also observed that all three parameters are mass dependent. In addition, with the increase of mass, the kinetic freeze-out temperature increases, and the transverse flow velocity, as well as kinetic freeze-out volume decrease.

Probing hidden sectors with a muon beam: Total and differential cross sections for vector boson production in muon bremsstrahlung

Vector bosons, such as Dark Photon $A'$ or $Z'$, can couple to muons and be produced in the bremsstrahlung reaction $\mu^- + N \rightarrow \mu^- + N + A'(Z')$. Their possible subsequent invisible decay can be detected in fixed target experiments through missing energy/momentum signature. In such experiments, not only is the energy transfer to $A'(Z')$ important, but also the recoil muon angle $\psi_{\mu'}$. In this paper, we derive the total and the double differential cross-sections involved in this process using the phase space Weisz\"acker-Williams and improved Weisz\"acker-Williams approximations, as well as using exact-tree-level calculations. As an example, we compare the derived cross-sections and resulting signal yields in the NA64$\mu$ experiment that uses a 160 GeV muon beam at the CERN Super Proton Synchrotron accelerator. We also discuss its impact on the NA64$\mu$ expected sensitivity to explore the $(g-2)_\mu$ anomaly favoured region with a $Z'$ boson considering $10^{12}$ muons accumulated on target.

Measurements of {Xi left( 1530right) ^{0}} and {overline{Xi }left( 1530right) ^{0}} production in proton\u2013proton interactions at sqrt{s_{NN}}\xa0=\xa017.3\xa0text{ GeV } in the NA61/SHINE\xa0experiment

Double-differential yields of {Xi left( 1530right) ^{0}} and {overline{Xi }left( 1530right) ^{0}} resonances produced in p+p interactions were measured at a laboratory beam momentum of 158 text{ GeV }!/!c. This measurement is the first of its kind in p+p interactions below LHC energies. It was performed at the CERN SPS by the NA61/SHINE collaboration. Double-differential distributions in rapidity and transverse momentum were obtained from a sample of 26times 10^6 inelastic events. The spectra are extrapolated to full phase space resulting in mean multiplicity of {Xi left( 1530right) ^{0}} (6.73 pm 0.25pm 0.67)times 10^{-4} and {overline{Xi }left( 1530right) ^{0}} (2.71 pm 0.18pm 0.18)times 10^{-4}. The rapidity and transverse momentum spectra and mean multiplicities were compared to predictions of string-hadronic and statistical model calculations.

The CaloCube calorimeter for high-energy cosmic-ray measurements in space: performance of a large-scale prototype

The direct observation of high-energy cosmic rays, up to the PeV energy region, will increasingly rely on highly performing calorimeters, and the physics performance will be primarily determined by their geometrical acceptance and energy resolution. Thus, it is extremely important to optimize their geometrical design, granularity and absorption depth, with respect to the total mass of the apparatus, which is amongst the most important constraints for a space mission. CaloCube is an homogeneous calorimeter whose basic geometry is cubic and isotropic, obtained by filling the cubic volume with small cubic scintillating crystals. In this way it is possible to detect particles arriving from every direction in space, thus maximizing the acceptance. This design summarizes a three-year R&D activity, aiming to both optimize and study the full-scale performance of the calorimeter, in the perspective of a cosmic-ray space mission, and investigate a viable technical design by means of the construction of several sizable prototypes. A large scale prototype, made of a mesh of 5 × 5 × 18 CsI(Tl) crystals, has been constructed and tested on high-energy particle beams at CERN SPS accelerator. In this paper we describe the CaloCube design and present the results relative to the response of the large scale prototype to electrons.

New CERN SPS Beam Dump Imaging System

New CERN SPS Beam Dump Imaging System

Conceptual design of an off-momentum collimation system in the CERN Super Proton Synchrotron for High-Luminosity Large Hadron Collider proton beams

The CERN accelerator complex is undergoing an upgrade in order to meet the requirements of the High-Luminosity Large Hadron Collider (HL-LHC). One of the key needs is to increase the intensity of beams injected into the LHC from the current $1.15\ifmmode\times\else\texttimes\fi{}{10}^{11}$ to $2.3\ifmmode\times\else\texttimes\fi{}{10}^{11}\text{ }\text{ }{\mathrm{p}}^{+}/\mathrm{bunch}$. This requires a beam intensity at injection in the Super Proton Synchrotron (SPS), the last injector before the LHC, as high as $2.6\ifmmode\times\else\texttimes\fi{}{10}^{11}\text{ }\text{ }{\mathrm{p}}^{+}/\mathrm{bunch}$, given that a budget for losses of about 10% is included in the design. However, previous experience with high intensity beams suggests that the total amount of losses in the SPS can be even higher, causing an increased activation, faster aging, and consequent early failure of machine equipment, in addition to an increase in the requirements for preinjectors in order to still meet the HL-LHC target in spite of higher losses. In this paper, we propose a collimation system to be used with HL-LHC proton beams in the SPS in order to intercept and safely dispose of beam losses in ad hoc locations and therefore provide the machine with protection against activation and equipment aging. The design is based on a two-stage concept, where the primary stage intercepts particles that otherwise would be lost, and the secondary stage employing an absorber where lost particles are finally disposed of. Numerical simulations prove a cleaning efficiency of the proposed system of at least 80%, substantially reducing the spreading of particle losses around the machine. Furthermore, the performance of the collimation system has a low sensitivity to common machine errors, and the design is cost efficient, with minimal hardware changes.