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DOE OSTI2023

Measurement of the non-prompt D-meson fraction as a function of multiplicity in proton-proton collisions at $\sqrt{s}$ = 13 TeV

The fractions of non-prompt (i.e. originating from beauty-hadron decays) D 0 and D + mesons with respect to the inclusive yield are measured as a function of the charged-particle multiplicity in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV with the ALICE detector at the LHC. The results are reported in intervals of transverse momentum (pT) and integrated in the range 1 < p T < 24 GeV/c. The fraction of non-prompt D 0 and D + mesons is found to increase slightly as a function of p T in all the measured multiplicity intervals, while no significant dependence on the charged-particle multiplicity is observed. In order to investigate the production and hadronisation mechanisms of charm and beauty quarks, the results are compared to PYTHIA 8 as well as EPOS 3 and EPOS 4 Monte Carlo simulations, and to calculations based on the colour glass condensate including three-pomeron fusion.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2024

Measurement of beauty production via non-prompt charm hadrons in p–Pb collisions at $\sqrt{s_{\textrm{NN}}}$ = 5.02 TeV

The production cross sections of D 0 , D + , and Λ$^{+}_{c}$ hadrons originating from beauty-hadron decays (i.e. non-prompt) were measured for the first time at midrapidity in proton–lead (p–Pb) collisions at the center-of-mass energy per nucleon pair of $\sqrt{s_{\textrm{NN}}}$ = 5.02 TeV. Nuclear modification factors (R pPb ) of non-prompt D 0 , D + , and Λ$^{+}_{c}$ are calculated as a function of the transverse momentum (p T ) to investigate the modification of the momentum spectra measured in p–Pb collisions with respect to those measured in proton–proton (pp) collisions at the same energy. The R pPb measurements are compatible with unity and with the measurements in the prompt charm sector, and do not show a significant p T dependence. The p T -integrated cross sections and p T -integrated R pPb of non-prompt D 0 and D + mesons are also computed by extrapolating the visible cross sections down to p T = 0. The non-prompt D-meson R pPb integrated over p T is compatible with unity and with model calculations implementing modification of the parton distribution functions of nucleons bound in nuclei with respect to free nucleons. The non-prompt Λ$^{+}_{c}$/D 0 and D + /D 0 production ratios are computed to investigate hadronisation mechanisms of beauty quarks into mesons and baryons. The measured ratios as a function of p T display a similar trend to that measured for charm hadrons in the same collision system.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2021

Measurement of b -quark fragmentation properties in jets using the decay $B^±$ → $J/ψK^±$ in pp collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

The fragmentation properties of jets containing b-hadrons are studied using charged B mesons in 139 fb -1 of pp collisions at $\sqrt{s}$ = 13 TeV, recorded with the ATLAS detector at the LHC during the period from 2015 to 2018. The B mesons are reconstructed using the decay of B ± into J/ψK ± , with the J/ψ decaying into a pair of muons. Jets are reconstructed using the anti-k t algorithm with radius parameter R = 0.4. The measurement determines the longitudinal and transverse momentum profiles of the reconstructed B hadrons with respect to the axes of the jets to which they are geometrically associated. These distributions are measured in intervals of the jet transverse momentum, ranging from 50 GeV to above 100 GeV. The results are corrected for detector effects and compared with several Monte Carlo predictions using different parton shower and hadronisation models. The results for the longitudinal and transverse profiles provide useful inputs to improve the description of heavy-flavour fragmentation in jets.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2022

Measurement of beauty production via non-prompt D 0 mesons in Pb-Pb collisions at $ \sqrt{{\textrm{s}}_{\textrm{NN}}} $ = 5.02 TeV

The production of non-prompt D 0 mesons from beauty-hadron decays was measured at midrapidity (|y| < 0.5) in Pb-Pb collisions at a nucleon-nucleon center-of-mass energy of $\sqrt{s_{NN}}$= 5.02 TeV with the ALICE experiment at the LHC. Their nuclear modification factor (R AA ), measured for the first time down to p T = 1 GeV/c in the 0–10% and 30–50% centrality classes, indicates a significant suppression, up to a factor of about three, for p T > 5 GeV/c in the 0–10% central Pb-Pb collisions. The data are described by models that include both collisional and radiative processes in the calculation of beauty-quark energy loss in the quark-gluon plasma, and quark recombination in addition to fragmentation as a hadronisation mechanism. The ratio of the non-prompt to prompt D 0 -meson R AA is larger than unity for p T > 4 GeV/c in the 0–10% central Pb-Pb collisions, as predicted by models in which beauty quarks lose less energy than charm quarks in the quark-gluon plasma because of their larger mass.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2023

Charm production and fragmentation fractions at midrapidity in pp collisions at $\sqrt{\textrm{s}}$ = 13 TeV

Measurements of the production cross sections of prompt $D^{0}$, $D^{+}$, $D^{*+}$, $D^{+}_{s}$, $Λ^{+}_{c}$, and $Ξ^{+}_{c}$ charm hadrons at midrapidity in proton-proton collisions at $\sqrt{\textrm{s}}$ = 13 TeV with the ALICE detector are presented. The D-meson cross sections as a function of transverse momentum (p T ) are provided with improved precision and granularity. The ratios of p T -diferential meson production cross sections based on this publication and on measurements at diferent rapidity and collision energy provide a constraint on gluon parton distribution functions at low values of Bjorken-x (10 —5 –10 —4 ). The measurements of $Λ^{+}_{c}$ ($Ξ^{+}_{c}$) baryon production extend the measured p T intervals down to p T = 0(3) GeV/c. These measurements are used to determine the charm-quark fragmentation fractions and the $c$$\overline{c}$ production cross section at midrapidity (|$y$| < 0.5) based on the sum of the cross sections of the weakly-decaying ground-state charm hadrons $D^{0}$, $D^{+}$, $D^{+}_{s}$, $Λ^{+}_{c}$, and $Ξ^{0}_{c}$ and, for the first time, $Ξ^{+}_{c}$, and of the strongly-decaying J/ψ mesons. The frst measurements of $Ξ^{+}_{c}$and $Σ^{0,++}_{c}$ fragmentation fractions at midrapidity are also reported. A signifcantly larger fraction of charm quarks hadronising to baryons is found compared to e + e — and ep collisions. The $c$$\overline{c}$ production cross section at midrapidity is found to be at the upper bound of state-of-the-art perturbative QCD calculations

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2024

Event generation with Sherpa 3

Sherpa is a general-purpose Monte Carlo event generator for the simulation of particle collisions in high-energy collider experiments. We summarise new developments, essential features, and ongoing improvements within the Sherpa 3 release series. Physics improvements include higher-order electroweak corrections, simulations of photoproduction and hard diffraction at NLO QCD, heavy-flavour matching in NLO multijet merging, spin-polarised cross section calculations, and a new model of colour reconnections. In addition, the modelling of hadronisation, the underlying event and QED effects in both production and decay has been improved, and the overall event generation efficiency has been enhanced.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2025

Multiplicity dependence of ${\Xi }_{\text{c}}^{+}$ and ${\Xi }_{\text{c}}^{0}$ production in pp collisions at $\sqrt{s}=13$ TeV

The first measurement at midrapidity (|y| < 0.5) of the production yield of the strange-charm baryons $Ξ$$^{+}_{c}$ and $Ξ$$^{0}_{c}$ as a function of transverse momentum (p T ) in different charged-particle multiplicity classes in proton-proton collisions at $\sqrt{s}$ = 13 TeV with the ALICE experiment at the LHC is reported. The $Ξ$$^{+}_{c}$ baryon is reconstructed via the $Ξ$$^{+}_{c}$ → $Ξ$ – π + π + decay channel in the range 4 < p T < 12 GeV/c, while the $Ξ$$^{0}_{c}$ baryon is reconstructed via both the $Ξ$$^{0}_{c}$ → $Ξ$ – π + and $Ξ$$^{0}_{c}$ → $Ξ$ – e + ν e decay channels in the range 2 < p T < 12 GeV/c. The baryon-to-meson ($Ξ$$^{0}_{c}$ + /D 0 ) and the baryon-to-baryon ($Ξ$$^{0}_{c}$ + /Λ$^{+}_{c}$) production yield ratios show no significant dependence on multiplicity. In addition, the observed yield ratios are not described by theoretical predictions that model charm-quark fragmentation based on measurements at e + e − and e − p colliders, indicating differences in the charm-baryon production mechanism in pp collisions. A comparison with different event generators and tunings, including different modelling of the hadronisation process, is also discussed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2021

Υ production and nuclear modification at forward rapidity in Pb–Pb collisions at $\sqrt{S_{NN}}$ = 5.02 TeV

The production of Υ mesons in Pb–Pb collisions at a centre-of-mass energy per nucleon pair $\sqrt{S_{NN}}$ = 5.02 TeV is measured with the muon spectrometer of the ALICE detector at the LHC. The yields as well as the nuclear modification factors are determined in the forward rapidity region 2.5 < y < 4.0, as a function of rapidity, transverse momentum and collision centrality. The results show that the production of the Υ(1S) meson is suppressed by a factor of about three with respect to the production in proton– proton collisions. For the first time, a significant signal for the Υ(2S) meson is observed at forward rapidity, indicating a suppression stronger by about a factor 2–3 with respect to the ground state. The measurements are compared with transport, hydrodynamic, comover and statistical hadronisation model calculations.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2023

Measurement of beauty-strange meson production in Pb–Pb collisions at s NN = 5.02 TeV via non-prompt D s + mesons

The production yields of non-prompt ${D}^{+}_{s}$ mesons, namely ${D}^{+}_{s}$ mesons from beauty-hadron decays, were measured for the first time as a function of the transverse momentum ($p_T$) at midrapidity (|$y$| < 0.5) in central and semi-central Pb–Pb collisions at a centre-of-mass energy per nucleon pair $\sqrt{s{NN}}$ = 5.02 TeV with the ALICE experiment at the LHC. The ${D}^{+}_{s}$ mesons and their charge conjugates were reconstructed from the hadronic decay channel ${D}^{+}_{s}$ → $φπ^+$, with $φ$ → K - K + , in the 4 < $p_T$ < 36 GeV/c and 2 < $p_T$ < 24 GeV/ $c$ intervals for the 0–10% and 30–50% centrality classes, respectively. The measured yields of non-prompt ${D}^{+}_{s}$ mesons are compared to those of prompt ${D}^{+}_{s}$ and non-prompt $D^0$ mesons by calculating the ratios of the production yields in Pb–Pb collisions and the nuclear modification factor $R_{AA}$. The ratio between the $R_{AA}$ of non-prompt ${D}^{+}_{s}$ prompt ${D}^{+}_{s}$ mesons, and that between the $R_{AA}$ of non-prompt ${D}^{+}_{s}$ and non-prompt $D^0$ mesons in central Pb–Pb collisions are found to be on average higher than unity in the 4 < $p_T$ < 12 GeV/ $c$ interval with a statistical significance of about 1.6σ and 1.7σ, respectively. The measured $R_{AA}$ ratios are compared with the predictions of theoretical models of heavy-quark transport in a hydrodynamically expanding QGP that incorporate hadronisation via quark recombination.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2023

First measurement of $Ω^0_c$ production in pp collisions at $\sqrt{s}=$13 TeV

The inclusive production of the charm–strange baryon $Ω^0_c$ is measured for the first time via its hadronic decay into Ω – π + at midrapidity(|y| < 0.5) in proton–proton (pp) collisions at the centre-of-mass energy $\sqrt{s}=$ 13 TeV with the ALICE detector at the LHC. The transverse momentum (p T ) differential cross section multiplied by the branching ratio is presented in the interval 2 < p T < 12 GeV/c. The p T dependence of the $Ω^0_c$-baryon production relative to the prompt D 0 -meson and to the prompt $Ξ^0_c$-baryon production is compared to various models that take different hadronisation mechanisms into consideration. In the measured p T interval, the ratio of the p T -integrated cross sections of $Ω^0_c$ and prompt $Λ^+_c$ baryons multiplied by the Ω – π + branching ratio is found to be larger by a factor of about 20 with a significance of about 4σ when compared to e + e – collisions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2023

First measurement of prompt and non-prompt D* + vector meson spin alignment in pp collisions at $\sqrt{s}$ = 13 TeV

This letter reports the first measurement of spin alignment, with respect to the helicity axis, for D* + vector mesons and their charge conjugates from charm-quark hadronisation (prompt) and from beauty meson decays (non-prompt) in hadron collisions. The measurements were performed at midrapidity (|y| < 0.8) as a function of transverse momentum (p T ) in proton–proton (pp) collisions collected by ALICE at the centre-of-mass energy $\sqrt{s}$ = 13 TeV. The diagonal spin density matrix element $ρ$00 of D* + mesons was measured from the angular distribution of the D* + → D 0 (→ K - π + )π + decay products, in the D* + rest frame, with respect to the D* + momentum direction in the pp centre of mass frame. The $ρ$00 value for prompt D* + mesons is consistent with 1/3, which implies no spin alignment. However, for non prompt D* + mesons an evidence of $ρ$00 larger than 1/3 is found. The measured value of the spin density element is $ρ$00 = 0.455±0.022(stat.)±0.035(syst.) in the 5 < p T < 20 GeV/c interval, which is consistent with a Pythia 8 Monte Carlo simulation coupled with the EvtGen package, which implements the helicity conservation in the decay of D* + meson from beauty mesons. In non-central heavy-ion collisions, the spin of the D* + mesons may be globally aligned with the direction of the initial angular momentum and magnetic field. Based on the results for pp collisions reported in this letter it is shown that alignment of non-prompt D* + mesons due to the helicity conservation coupled to the collective anisotropic expansion may mimic the signal of global spin alignment in heavy-ion collisions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2024

Measurement of 3 Λ H production in Pb–Pb collisions at √ s NN = 5.02 TeV

The first measurement of 3 Λ H and $^{3}_{\overline{Λ}}$$\overline{H}$ differential production with respect to transverse momentum and centrality in Pb–Pb collisions at √ s NN = 5.02 TeV is presented. The 3 Λ H has been reconstructed via its two-charged-body decay channel, i.e., 3 Λ H → 3 He → π - . A Blast-Wave model fit of the p T -differential spectra of all nuclear species measured by the ALICE collaboration suggests that the kinetic freeze-out surface is consistent with that of other nuclei. The ratio between the integrated yields of 3 Λ H and 3 He is compared to predictions from the statistical hadronisation model and the coalescence model, with the latter being favoured by the presented measurements.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2025

Search for emerging jets in pp collisions at $\sqrt{s} = 13.6$ TeV with the ATLAS experiment

A search for emerging jets is presented using 51.8 fb −1 of proton–proton collision data at $\sqrt{s} = 13.6$ TeV, collected by the ATLAS experiment during 2022 and 2023. The search explores a hypothetical dark sector featuring ‘dark quarks’ that are charged under a confining gauge group and couple to the standard model (SM) via a new mediator particle. These dark quarks undergo showering and hadronisation within the dark sector, forming long-lived dark mesons that decay back into SM particles. This results in jets that contain multiple displaced vertices known as emerging jets. The analysis targets events with pairs of emerging jets, produced either through a vector mediator, Z′, in the s-channel, or a scalar mediator, Φ, in the t-channel. No significant excess over the SM background is observed. Assuming a dark pion proper decay length between 5 mm and 50 mm, Z′ mediator masses between 600 GeV and 2550 GeV are excluded for quark and dark quark coupling values of 0.01 and 0.1, respectively. For a quark dark-quark coupling of 0.1, Φ mediator masses between 600 GeV and 1375 GeV are excluded. These results represent the first direct search targeting emerging jet pair production via a Z′ mediator, as well as the first study of emerging jet production mediated by a scalar particle exchanged in the t-channel.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2023

Light (anti)nuclei production in Pb-Pb collisions at s N N = 5.02 TeV

The measurement of the production of deuterons, tritons and 3 He and their antiparticles in Pb-Pb collisions at $\sqrt{s_{NN}}$= 5.02 TeV is presented in this article. The measurements are carried out at midrapidity (|y| < 0.5) as a function of collision centrality using the ALICE detector. The p T -integrated yields, the coalescence parameters and the ratios to protons and antiprotons are reported and compared with nucleosynthesis models. The comparison of these results in different collision systems at different center-of-mass collision energies reveals a suppression of nucleus production in small systems. In the Statistical Hadronisation Model framework, this can be explained by a small correlation volume where the baryon number is conserved, as already shown in previous fluctuation analyses. However, a different size of the correlation volume is required to describe the proton yields in the same data sets. The coalescence model can describe this suppression by the fact that the wave functions of the nuclei are large and the fireball size starts to become comparable and even much smaller than the actual nucleus at low multiplicities.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2019

Measurement of $K_S^0$ and $\Lambda^0$ production in $t \bar{t}$ dileptonic events in pp collisions at $\sqrt{s} =$ 7 TeV with the ATLAS detector

Measurements of $K_S^0$ and Λ 0 production in $t\bar{t}$ final states have been performed. They are based on a data sample with integrated luminosity of 4.6 fb -1 from proton–proton collisions at a centre-of-mass energy of 7 TeV, collected in 2011 with the ATLAS detector at the Large Hadron Collider. Neutral strange particles are separated into three classes, depending on whether they are contained in a jet, with or without a $b$-tag, or not associated with a selected jet. The aim is to look for differences in their main kinematic distributions. A comparison of data with several Monte Carlo simulations using different hadronisation and fragmentation schemes, colour reconnection models and different tunes for the underlying event has been made. The production of neutral strange particles in $t \bar{t}$ dileptonic events is found to be well described by current Monte Carlo models for $K_S^0$ and Λ 0 production within jets, but not for those produced outside jets.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2021

Measurement of the branching fraction of the ${{B} ^0} {\rightarrow }{{D} ^+_{s}} {{\pi } ^-} $ decay

A branching fraction measurement of the ${{B} ^0} {\rightarrow }{{D} ^+_{s}} {{\pi } ^-} $ decay is presented using proton–proton collision data collected with the LHCb experiment, corresponding to an integrated luminosity of $5.0\,\text {fb} ^{-1} $ . The branching fraction is found to be ${\mathcal {B}} ({{B} ^0} {\rightarrow }{{D} ^+_{s}} {{\pi } ^-} ) =(19.4 \pm $ $1.8\pm 1.3 \pm 1.2)\times 10^{-6}$ , where the first uncertainty is statistical, the second systematic and the third is due to the uncertainty on the ${{B} ^0} {\rightarrow }{{D} ^-} {{\pi } ^+} $ , ${{D} ^+_{s}} {\rightarrow }{{K} ^+} {{K} ^-} {{\pi } ^+} $ and ${{D} ^-} {\rightarrow }{{K} ^+} {{\pi } ^-} {{\pi } ^-} $ branching fractions. This is the most precise single measurement of this quantity to date. As this decay proceeds through a single amplitude involving a $b{\rightarrow }u$ charged-current transition, the result provides information on non-factorisable strong interaction effects and the magnitude of the Cabibbo–Kobayashi–Maskawa matrix element $V_{ub}$ . Additionally, the collision energy dependence of the hadronisation-fraction ratio $f_s/f_d$ is measured through ${{\overline{B}} {}^0_{s}} {\rightarrow }{{D} ^+_{s}} {{\pi } ^-} $ and ${{B} ^0} {\rightarrow }{{D} ^-} {{\pi } ^+} $ decays.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2020

(Anti-)deuteron production in pp collisions at $\sqrt{s}=13 \ \text {TeV}$

The study of (anti-)deuteron production in pp collisions has proven to be a powerful tool to investigate the formation mechanism of loosely bound states in high-energy hadronic collisions. In this paper the production of $\text {(anti-)deuterons}$ is studied as a function of the charged particle multiplicity in inelastic pp collisions at $\sqrt{s}=13$ TeV using the ALICE experiment. Thanks to the large number of accumulated minimum bias events, it has been possible to measure (anti-)deuteron production in pp collisions up to the same charged particle multiplicity ( ${\mathrm {d} N_{ch}/\mathrm {d} \eta } \sim 26$ ) as measured in p–Pb collisions at similar centre-of-mass energies. Within the uncertainties, the deuteron yield in pp collisions resembles the one in p–Pb interactions, suggesting a common formation mechanism behind the production of light nuclei in hadronic interactions. In this context the measurements are compared with the expectations of coalescence and statistical hadronisation models (SHM).

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2022

Production of light (anti)nuclei in pp collisions at $\sqrt{s} = 5.02$ TeV

The study of the production of nuclei and antinuclei in pp collisions has proven to be a powerful tool to investigate the formation mechanism of loosely bound states in high-energy hadronic collisions. In this paper, the production of protons, deuterons and 3He and their charge conjugates at midrapidity is studied as a function of the charged-particle multiplicity in inelastic pp collisions at √s=5.02 TeV using the ALICE detector. Within the uncertainties, the yields of nuclei in pp collisions at √s=5.02 TeV are compatible with those in pp collisions at different energies and to those in p–Pb collisions when compared at similar multiplicities. The measurements are compared with the expectations of coalescence and Statistical Hadronisation Models. The results suggest a common formation mechanism behind the production of light nuclei in hadronic interactions and confirm that they do not depend on the collision energy but on the number of produced particles.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗