<p>In relativistic heavy-ion collisions, the formation of a deconfined and thermalized state of partons, known as quark-gluon plasma (QGP), leads to enhanced production of strange hadrons in contrast to proton-proton (<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>p</mi><mi>p</mi></mrow></math>) collisions, which are taken as baseline. This observation is known as strangeness enhancement in heavy-ion collisions and is considered one of the important signatures that can signify the formation of QGP. However, in addition to strangeness enhancement, recent measurements hint at observing several heavy-ion-like features in high multiplicity <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>p</mi><mi>p</mi></mrow></math> collisions at energies available at the CERN Large Hadron Collider. Alternatively, event shape observables, such as transverse spherocity, transverse sphericity, charged particle flattenicity, and relative transverse activity classifiers, can fundamentally separate hard interaction-dominated jetty events from soft isotropic events. These features of event shape observables can probe the observed heavy-ion-like features in <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>p</mi><mi>p</mi></mrow></math> collisions with significantly reduced selection bias and can bring all collision systems on equal footing. In this article, we present an extensive summary of the strange particle ratios to pions as a function of different event classifiers using the PYTHIA 8 model with color reconnection and rope hadronization mechanisms to understand the microscopic origin of strangeness enhancement in <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>p</mi><mi>p</mi></mrow></math> collisions and also prescribe the applicability of these event classifiers in the context of strangeness enhancement. Charged-particle flattenicity is found to be most suited for the study of strangeness enhancement, and it shows a quantitative enhancement similar to that seen for the analysis based on the number of multiparton interactions.</p>

India

Suraj Prasad

Bhagyarathi Sahoo

CERN

Sushanta Tripathy

Neelkamal Mallick

Raghunath Sahoo

Probing strangeness with event topology classifiers in <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>p</mi><mi>p</mi></mrow></math> collisions at energies available at the CERN Large Hadron Collider with the rope hadronization mechanism in PYTHIA

The ITk is a new silicon tracker for the ATLAS experiment designed to increase detector resolution, readout capacity, and radiation hardness, in preparation for the larger number of simultaneous proton–proton interactions at the High Luminosity LHC. This paper presents the first direct measurement of the material budget of an ATLAS ITk pixel module, performed at a testbeam at the CERN Proton Synchrotron via the multiple scattering of low energy positrons within the module volume. Using a four plane telescope of thin monolithic pixel detectors from the MALTA Collaboration, scattering datasets were recorded at a beam energy of $$1.2\,\text {GeV}$$ . Kink angle distributions were extracted from tracks derived with and without information from the ITk pixel module, and were fit to extract the RMS scattering angle, which was converted to a fractional radiation length $$x/X_0$$ . The average $$x/X_0$$ across the module was measured as $$[0.89 \pm 0.01 \text { (resolution)} \pm 0.01 \text { (subtraction)} \,\,\pm \,\, 0.08 \text {~(beam momentum band)}]\%$$ , which agrees within uncertainties with an estimate of $$0.88\%$$ derived from material component expectations.

United Kingdom

Simon Koch

Switzerland

Brian Moser

Antonín Lindner

United States

Valerio Dao

Ignacio Asensi

Daniela Bortoletto

Marianne Brekkum

Florian Dachs

Germany

Hans Joos

Norway

Milou Rijnbach

Abhishek Sharma

Ismet Siral

Carlos Solans

Yingjie Wei

Measuring the ATLAS ITk pixel detector material via multiple scattering of positrons at the CERN PS

In this article, we reexamine the formulation for extraction of Knudsen number ( ), the ratio of shear viscosity to entropy density ( $$\eta /s$$ ), within the incomplete thermalization scenario, using eccentricity scaling of elliptic flow ( $$v_{2}$$ ) of final state hadrons. Data on centrality dependence of charged hadron $$v_{2}$$ in Xe–Xe and Pb–Pb collisions, measured by ALICE and CMS collaborations at LHC are analyzed for this purpose. Results have been compared using two different models of collision namely Glauber Monte Carlo and TRENTO. The measured $$v_{2}$$ , even for $$\sqrt{s_{NN}}=5.02$$ TeV most central Pb–Pb collisions is found to be below the ideal hydrodynamic limit by at least $$15 \%$$ . Extracted $$\eta /s$$ of the medium exhibit heavy dependence on the employed initial conditions. Impact of the Pade approximants based alternative formulation of the functional relation between $$v_{2}$$ and are also investigated in detail. Our studies reconfirm the importance of clarifying the initial state configuration as well as the inbuilt ambiguities in employing the transport approach based on incomplete equilibrium in a strongly coupled regime at LHC.

Arun Yadav

Partha Bhaduri

Subhasis Chattopadhyay

Reexamination of eccentricity scaling of elliptic flow and incomplete thermalization scenario in heavy-ion collisions at energies available at CERN large hadron collider

The DsTau(NA65) experiment at CERN was proposed to measure an inclusive differential cross-section of $$D_s$$ production with decay to tau lepton and tau neutrino in – interactions. The DsTau detector is based on the nuclear emulsion technique, which provides excellent spatial resolution for detecting short-lived particles like charmed hadrons. This paper presents the first results of the analysis of the pilot-run (2018 run) data and reports the accuracy of the proton interaction vertex reconstruction. High precision in vertex reconstruction enables detailed measurement of proton interactions, even in environments with high track density. The measured data has been compared with several Monte Carlo event generators in terms of multiplicity and angular distribution of charged particles. The multiplicity distribution obtained in p–W interactions is tested for KNO-G scaling and is found to be nearly consistent. The interaction length of protons in tungsten is measured to be $$93.7 \pm 2.6~\text {mm}.$$ The results presented in this study can be used to validate event generators of – interactions.

Japan

S. Aoki

A. Ariga

T. Ariga

N. Charitonidis

S. Dmitrievsky

Romania

R. Dobre

Türkiye

N. Ergin

E. Firu

Y. Gornushkin

A. Guler

D. Hayakawa

F. Kling

K. Kodama

M. Komatsu

U. Kose

M. Miura

T. Nakano

A. Neagu

T. Okumura

C. Oz

H. Rokujo

O. Sato

S. Vasina

M. Yoshimoto

E. Yuksel

Study of proton–nucleus interactions in the DsTau/NA65 experiment at the CERN-SPS

We study the system size dependence of heavy quark (HQ) observables at a center-of-mass energy of <math><mrow><msqrt><msub><mi>s</mi><mrow><mi>N</mi><mi>N</mi></mrow></msub></msqrt><mo>=</mo><mn>5.02</mn></mrow></math> TeV to explore whether it can provide further constraints on the physical processes which are involved: energy loss of HQs in the quark gluon plasma (QGP), hadronization, and hadronic rescattering. We use the EPOS4HQ approach to study <math><mi>p</mi><mo>−</mo><mi>p</mi></math> and 0–10% central O-O, Ar-Ar, Kr-Kr, and Pb-Pb reactions and investigate in detail the momentum change of heavy quarks from creation until their detection as part of a hadron as well as the enhancement of heavy baryon production at low <math><msub><mi>p</mi><mi>T</mi></msub></math>. We investigate furthermore the origin and the system size dependence of the azimuthal correlations between the heavy quark <math><mi>Q</mi></math> and the heavy antiquark <math><mover><mi>Q</mi><mo>¯</mo></mover></math> and how one can bypass the problem that correlations are washed out due to the combinatorial background. We conclude that a systematic study of the system size dependence of the momentum loss allows separating the momentum loss due to the passage through the QGP from the momentum change due to hadronization, and that the <math><mrow><mi>Q</mi><mover><mi>Q</mi><mo>¯</mo></mover></mrow></math> correlations yield insight into the different pQCD processes in which the heavy quarks are created.

France

Jiaxing Zhao

Joerg Aichelin

Pol Bernard Gossiaux

Klaus Werner

System-size dependence of energy loss and correlations of heavy mesons at energies available at the CERN Large Hadron Collider

The estimation of the antibaryon to baryon ratio is considered to be a useful tool for studying baryon number transport in <math><mrow><mi>p</mi><mi>p</mi></mrow><mo>,</mo><mo> </mo><mrow><mi>p</mi><mi>A</mi></mrow></math>, and <math><mrow><mi>A</mi><mi>A</mi></mrow></math> collisions. For this paper, the Pythia event generator with various tunes is used to measure the <math><mrow><mover><mi>Λ</mi><mo>¯</mo></mover><mo>/</mo><mi>Λ</mi></mrow></math> ratio as a function of rapidity (<math><mi>y</mi></math>), transverse momentum (<math><msub><mi>p</mi><mi>T</mi></msub></math> ), and multiplicity within both ALICE and LHCb acceptances. The results obtained using various Monte Carlo data for <math><mrow><mi>p</mi><mi>p</mi></mrow></math> collisions at <math><mrow><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>0.9</mn></mrow></math> and 7 TeV are compared with the experimentally measured values of <math><mrow><mover><mi>Λ</mi><mo>¯</mo></mover><mo>/</mo><mi>Λ</mi></mrow></math> ratio of ALICE and LHCb experiments. Out of the various studied tunes of Pythia 8.3, the string junction model is found to be the most successful to describe the experimentally observed results. Evidence of a considerable amount of baryon number transportation from the beam fragmentation to the ALICE and LHCb acceptances could be recognized.

Banajit Barman

Nur Hussain

Buddhadeb Bhattacharjee

Baryon number transportation over full rapidity space in <math><mrow><mi>p</mi><mi>p</mi></mrow></math> collisions at energies available at the CERN Large Hadron Collider

A search for dark sectors is performed using the unique M2 beam line at the CERN Super Proton Synchrotron. New particles (<math><mi>X</mi></math>) could be produced in the bremsstrahlung-like reaction of high-energy 160 GeV positively charged muons impinging on an active target, <math><mi>μ</mi><mi>N</mi><mo>→</mo><mi>μ</mi><mi>N</mi><mi>X</mi></math>, followed by their decays, <math><mi>X</mi><mo>→</mo><mtext>invisible</mtext></math>. The experimental signature would be a scattered single muon from the target, with about less than half of its initial energy and no activity in the subdetectors located downstream from the interaction point. The full sample of the 2022 run is analyzed through the missing-energy/momentum channel, with total statistics of <math><mo>(</mo><mn>1.98</mn><mo>±</mo><mn>0.02</mn><mo>)</mo><mo>×</mo><msup><mn>10</mn><mn>10</mn></msup></math> muons on target. We demonstrate that various muonphilic scenarios involving different types of mediators, such as scalar or vector particles, can be probed simultaneously with such a technique. For the vector case, besides a <math><msub><mi>L</mi><mi>μ</mi></msub><mo>−</mo><msub><mi>L</mi><mi>τ</mi></msub></math> <math><msup><mi>Z</mi><mo>′</mo></msup></math> vector boson, we also consider an invisibly decaying dark photon (<math><msup><mi>A</mi><mo>′</mo></msup><mo>→</mo><mtext>invisible</mtext></math>). This search is complementary to NA64 running with electrons and positrons, thus opening the possibility to expand the exploration of the thermal light dark matter parameter space by combining the results obtained with the three beams.

Yu. M. Andreev

D. Banerjee

B. Banto Oberhauser

J. Bernhard

Italy

P. Bisio

P. Crivelli

E. Depero

A. V. Dermenev

S. V. Donskov

R. R. Dusaev

T. Enik

V. N. Frolov

Greece

A. Gardikiotis

S. V. Gertsenberger

S. Girod

Chile

S. N. Gninenko

M. Hösgen

R. Joosten

V. A. Kachanov

Y. Kambar

A. E. Karneyeu

E. A. Kasianova

G. Kekelidze

B. Ketzer

D. V. Kirpichnikov

M. M. Kirsanov

V. N. Kolosov

V. A. Kramarenko

L. V. Kravchuk

N. V. Krasnikov

S. V. Kuleshov

V. E. Lyubovitskij

V. Lysan

V. A. Matveev

R. Mena Fredes

R. G. Mena Yanssen

Spain

L. Molina Bueno

M. Mongillo

D. V. Peshekhonov

V. A. Polyakov

Canada

B. Radics

K. M. Salamatin

V. D. Samoylenko

D. A. Shchukin

O. Soto

H. Sieber

V. O. Tikhomirov

I. V. Tlisova

A. N. Toropin

M. Tuzi

B. M. Veit

P. V. Volkov

V. Yu. Volkov

I. V. Voronchikhin

J. Zamora-Saá

A. S. Zhevlakov

Shedding light on dark sectors with high-energy muons at the NA64 experiment at the CERN SPS

True muonium (<math><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>−</mo></msup></math>) is one of the heaviest and smallest electromagnetic bound states not containing hadrons, and has never been observed so far. In this work we show that the spin-1 TM state (ortho-TM) can be observed at a discovery level of significance in three months at the CERN SPS North-Area H4A beam line, using 43.7 GeV secondary positrons. In this way, by impinging the positrons on multiple thin low-Z targets, ortho-TM, which decays predominantly to <math><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></math>, can be produced from <math><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup><mo>→</mo><mi>TM</mi></math> interaction on resonance (<math><msqrt><mi>s</mi></msqrt><mo>∼</mo><mn>2</mn><msub><mi>m</mi><mi>μ</mi></msub></math>).

Ruben Gargiulo

Stefano Palmisano

Elisa Di Meco

Feasibility study of true muonium discovery with the CERN-SPS H4 positron beam

We investigate charged-hadron production in relativistic heavy-ion collisions of asymmetric systems within a nonequilibrium-statistical framework. Calculated centrality-dependent pseudorapidity distributions for p-Pb collisions at <math><mrow><msqrt><msub><mi>s</mi><mtext>NN</mtext></msub></msqrt><mo>=</mo><mn>5.02</mn></mrow></math> and 8.16 TeV are compared with data from the Large Hadron Collider. Our approach combines a relativistic diffusion model with formulations based on quantum chromodynamics while utilizing numerical solutions of a Fokker-Planck equation to account for the shift and broadening of the fragmentation sources for particle-production with respect to the stopping (net-baryon) rapidity distributions. To represent the centrality dependence of charged-hadron production in asymmetric systems over a broad region of pseudorapidities, the consideration and precise modeling of the fragmentation sources, along with the central gluon-gluon source, is found to be essential. Specifically, this results in an inversion of the particle-production amplitude from backward to forward dominance when transitioning from central to peripheral collisions, in agreement with recent ATLAS and ALICE p-Pb data at <math><mrow><msqrt><msub><mi>s</mi><mtext>NN</mtext></msub></msqrt><mo>=</mo><mn>5.02</mn><mspace width="0.16em"></mspace><mi>T</mi><mi>e</mi><mi>V</mi></mrow></math>.

Philipp Schulz

Georg Wolschin

Relativistic diffusion model for hadron production in <math><mi>p</mi></math>-Pb collisions at energies available at the CERN Large Hadron Collider

In this study, we demonstrate the potential of the NA64 experiment at CERN SPS to search for New Physics processes involving  $$e\rightarrow \mu $$  <math> <mrow> <mi>e</mi> <mo>→</mo> <mi>μ</mi> </mrow> </math>   transitions after the collision of 100 GeV electrons with target nuclei. A new Dark Sector leptonic portal in which a scalar boson  $$\varphi $$  <math> <mi>φ</mi> </math>   could be produced in the lepton-flavor-changing bremsstrahlung-like reaction,  $$eN\rightarrow \mu N\varphi $$  <math> <mrow> <mi>e</mi> <mi>N</mi> <mo>→</mo> <mi>μ</mi> <mi>N</mi> <mi>φ</mi> </mrow> </math>  , is used as benchmark process. In this work, we develop a realistic Monte Carlo simulation of the NA64 experimental setup implementing the differential and total production cross-section computed at exact tree-level and applying the Weiszäcker–Williams phase space approximation. Using this framework, we investigate the main background sources and calculate the expected sensitivity of the experiment. The results indicate that with minor setup optimization, NA64 can probe a large fraction of the available parameter space compatible with the muon  $$g-2$$  <math> <mrow> <mi>g</mi> <mo>-</mo> <mn>2</mn> </mrow> </math>   anomaly and the Dark Matter relic predictions in the context of a new Dark Sector leptonic portal with  $$10^{11}$$  <math> <msup> <mn>10</mn> <mn>11</mn> </msup> </math>   EOT. This result paves the way to the exploration of lepton-flavour-changing transitions in NA64.