Fri·Apr 29, 2022Friday·April 29, 2022

1. 01*

   Two-particle azimuthal correlations in $\gamma$p interactions using pPb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 8.16 TeV

   CMS Collaboration

   The first measurements of the Fourier coefficients ($V_{n\Delta }$) of the azimuthal distributions of charged hadrons emitted from photon-proton ($\gamma$p) interactions at the LHC are presented. The data are extracted from 68.8 nb${}^{-1}$ of ultra-peripheral proton-lead (pPb) collisions at $\sqrt{s_{\mathrm{NN}}}$ = 8.16 TeV using the CMS detector. The high energy lead ions produce a flux of photons that can interact with the oncoming proton. This $\gamma$p system provides a set of unique initial conditions with multiplicity lower than in photon-lead collisions but comparable to recent electron-positron and electron-proton data. The $V_{n\Delta }$ coefficients are presented in ranges of event multiplicity and transverse momentum ($p_{\mathrm{T}}$) and are compared to corresponding hadronic minimum bias pPb results. For a given multiplicity range, the mean $p_{\mathrm{T}}$ of charged particles is smaller in $\gamma$p than in pPb collisions. For both the $\gamma$p and pPb samples, $V_{1\Delta }$ is negative, $V_{2\Delta }$ is positive, and $V_{3\Delta }$ consistent with 0. For each multiplicity and $p_{\mathrm{T}}$ range, $V_{2\Delta }$ is larger for $\gamma$p events. The $\gamma$p data are consistent with model predictions that have no collective effects.

   nucl-exhep-exPLB(2023)·40 citations
2. 02*

   Measurement of the nuclear modification factor of $b$-jets in 5.02 TeV Pb+Pb collisions with the ATLAS detector

   ATLAS Collaboration

   This paper presents a measurement of $b$-jet production in Pb+Pb and $pp$ collisions at $\sqrt{s_{{}_{\text{NN}}}}=5.02$ TeV with the ATLAS detector at the LHC. The measurement uses 260 ${\text{pb}}^{-1}$ of $pp$ collisions collected in 2017 and 1.4 ${\text{nb}}^{-1}$ of Pb+Pb collisions collected in 2018. In both collision systems, jets are reconstructed via the anti-$k_t$ algorithm. The $b$-jets are identified from a sample of jets containing muons from the semileptonic decay of $b$-quarks using template fits of the muon momentum relative to the jet axis. In $pp$ collisions, $b$-jets are reconstructed for radius parameters $R=0.2$ and $R=0.4$, and only $R=0.2$ jets are used in Pb+Pb collisions. For comparison, inclusive $R=0.2$ jets are also measured using 1.7 ${\text{nb}}^{-1}$ of Pb+Pb collisions collected in 2018 and the same $pp$ collision data as the $b$-jet measurement. The nuclear modification factor, $R_{\text{AA}}$, is calculated for both $b$-jets and inclusive jets with $R=0.2$ over the transverse momentum range of 80--290 GeV. The nuclear modification factor for $b$-jets decreases from peripheral to central collisions. The ratio of the $b$-jet $R_{\text{AA}}$ to inclusive jet $R_{\text{AA}}$ is also presented and suggests that the $R_{\text{AA}}$ for $b$-jets is larger than that for inclusive jets in central Pb+Pb collisions. The measurements are compared with theoretical calculations and suggest a role for mass and colour-charge effects in partonic energy loss in heavy-ion collisions.

   nucl-exhep-exEPJC(2023)·49 citations
3. 03*

   Theoretical analysis of the extraction of neutron skin thickness from coherent {\pi}0 photoproduction off nuclei

   F. Colomer🇧🇪 · P. Capel🇧🇪 · M. Ferretti🇩🇪 · J. Piekarewicz🇺🇸 · C. Sfienti🇩🇪 · M. Thiel🇩🇪 · V. Tsaran🇩🇪 · M. Vanderhaeghen🇩🇪

   Background: Coherent {\pi}0 photoproduction on heavy nuclei has been suggested as a reliable tool to infer neutron skin thicknesses. To this aim, various experiments have been performed, especially on 208Pb. Purpose: We analyze the sensitivity of that reaction process to the nucleonic density, and especially to the neutron skin thickness, for 12C, 40Ca and 208Pb, for which reliable data exist, and on 116,124Sn, for which measurements have been performed in Mainz. We study also the role played by the {\pi}0-nucleus final-state interaction. Method: A model of the reaction is developed at the impulse approximation considering either plane waves or distorted waves to describe the {\pi}0-nucleus scattering in the outgoing channel. Results: Our calculations are in good agreement with existing data, especially for 208Pb. The sensitivity of the theoretical cross sections to the choice of the nucleonic density is small, and below the experimental resolution. Conclusions: Coherent {\pi}0 photoproduction is mostly an isoscalar observable that bares no practical sensitivity to the neutron skin thickness. To infer that structure observable it should be coupled to other reaction measurements, such as electron scattering, or by comparing experiments performed on isotopes of the same chemical element.

   ↳ nucl-thnucl-exPRC(2022)·11 citations
4. 04*

   Observation of the $\gamma \gamma \to \tau \tau$ process in Pb+Pb collisions and constraints on the $\tau$-lepton anomalous magnetic moment with the ATLAS detector

   ATLAS Collaboration

   This Letter reports the observation of $\tau$-lepton pair production in ultraperipheral lead-lead collisions, $\text{Pb+Pb}\to \text{Pb}(\gamma \gamma \to \tau \tau )\text{Pb}$, and constraints on the $\tau$-lepton anomalous magnetic moment, $a_{\tau }$. The dataset corresponds to an integrated luminosity of 1.44 nb${}^{-1}$ of LHC Pb+Pb collisions at $\sqrt{s_{{}_{\text{NN}}}}=5.02$ TeV recorded by the ATLAS experiment in 2018. Selected events contain one muon from a $\tau$-lepton decay, an electron or charged-particle track(s) from the other $\tau$-lepton decay, little additional central-detector activity, and no forward neutrons. The $\gamma \gamma \to \tau \tau$ process is observed in Pb+Pb collisions with a significance exceeding 5 standard deviations, and a signal strength of ${\mu }_{\tau \tau }=1.03_{-0.05}^{+0.06}$ assuming the Standard Model value for $a_{\tau }$. To measure $a_{\tau }$, a template fit to the muon transverse-momentum distribution from $\tau$-lepton candidates is performed, using a dimuon ($\gamma \gamma \to \mu \mu$) control sample to constrain systematic uncertainties. The observed 95% confidence-level interval for $a_{\tau }$ is $-0.057<a_{\tau }<0.024$.

   ↳ hep-exnucl-exPRL(2023)·148 citations
5. 05*

   First concurrent extraction of the leading-order scalar and spin proton polarizabilities

   E. Mornacchi (1)🇩🇪 · S. Rodini (2)🇩🇪 · B. Pasquini (3 and 4)🇮🇹 · P. Pedroni (4) ((1) Institut für Kernphysik, Johannes Gutenberg-Universität Mainz, Mainz, Germany, (2) Institut für Theoretische Physik, Universität Regensburg, Regensburg, Germany, (3) Dipartimento di Fisica, Università degli Studi di Pavia, Pavia, Italy, (4) INFN Sezione di Pavia, Pavia, Italy)🇮🇹

   We performed the first simultaneous extraction of the six leading-order proton polarizabilities. We reached this milestone thanks to both new high-quality experimental data and an innovative bootstrap-base fitting method. These new results provide a self-consistent and fundamental benchmark for all future theoretical and experimental polarizability estimates.

   ↳ hep-phnucl-exnucl-thPRL(2022)·8 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.