LHCb

Abstract:

<jats:p>A novel measurement technique of strong-phase differences between the decay amplitudes of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msup><a:mi>D</a:mi><a:mn>0</a:mn></a:msup></a:math> and <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:msup><c:mrow><c:mover accent="true"><c:mrow><c:mi>D</c:mi></c:mrow><c:mrow><c:mo accent="true" stretchy="true">¯</c:mo></c:mrow></c:mover></c:mrow><c:mrow><c:mn>0</c:mn></c:mrow></c:msup></c:mrow></c:math> mesons is introduced which exploits quantum-correlated <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mrow><h:mi>D</h:mi><h:mrow><h:mover accent="true"><h:mrow><h:mi>D</h:mi></h:mrow><h:mrow><h:mo stretchy="true">¯</h:mo></h:mrow></h:mover></h:mrow></h:mrow></h:math> pairs produced by <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:msup><l:mi>e</l:mi><l:mo>+</l:mo></l:msup><l:msup><l:mi>e</l:mi><l:mo>−</l:mo></l:msup></l:math> collisions at energies above the <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"><n:mi>ψ</n:mi><n:mo stretchy="false">(</n:mo><n:mn>3770</n:mn><n:mo stretchy="false">)</n:mo></n:math> production threshold, where <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline"><r:mrow><r:mi>D</r:mi><r:mover accent="true"><r:mrow><r:mi>D</r:mi></r:mrow><r:mrow><r:mo stretchy="true">¯</r:mo></r:mrow></r:mover></r:mrow></r:math> pairs are produced in both even and odd eigenstates of the charge-conjugation symmetry. Employing this technique, the first determination of a <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline"><v:mrow><v:msup><v:mrow><v:mi>D</v:mi></v:mrow><v:mrow><v:mn>0</v:mn></v:mrow></v:msup><v:mi>–</v:mi><v:msup><v:mrow><v:mover accent="true"><v:mrow><v:mi>D</v:mi></v:mrow><v:mrow><v:mo accent="true" stretchy="true">¯</v:mo></v:mrow></v:mover></v:mrow><v:mrow><v:mn>0</v:mn></v:mrow></v:msup></v:mrow></v:math> relative strong phase is reported with such data samples. The strong-phase difference between <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"><ab:mrow><ab:msup><ab:mrow><ab:mi>D</ab:mi></ab:mrow><ab:mrow><ab:mn>0</ab:mn></ab:mrow></ab:msup><ab:mo stretchy="false">→</ab:mo><ab:msup><ab:mrow><ab:mi>K</ab:mi></ab:mrow><ab:mrow><ab:mo>−</ab:mo></ab:mrow></ab:msup><ab:msup><ab:mrow><ab:mi>π</ab:mi></ab:mrow><ab:mrow><ab:mo>+</ab:mo></ab:mrow></ab:msup></ab:mrow></ab:math> and <db:math xmlns:db="http://www.w3.org/1998/Math/MathML" display="inline"><db:mrow><db:msup><db:mrow><db:mover accent="true"><db:mrow><db:mi>D</db:mi></db:mrow><db:mrow><db:mo accent="true" stretchy="true">¯</db:mo></db:mrow></db:mover></db:mrow><db:mrow><db:mn>0</db:mn></db:mrow></db:msup><db:mo stretchy="false">→</db:mo><db:msup><db:mrow><db:mi>K</db:mi></db:mrow><db:mrow><db:mo>−</db:mo></db:mrow></db:msup><db:msup><db:mrow><db:mi>π</db:mi></db:mrow><db:mrow><db:mo>+</db:mo></db:mrow></db:msup></db:mrow></db:math> decays, <jb:math xmlns:jb="http://www.w3.org/1998/Math/MathML" display="inline"><jb:msubsup><jb:mi>δ</jb:mi><jb:mrow><jb:mi>K</jb:mi><jb:mi>π</jb:mi></jb:mrow><jb:mi>D</jb:mi></jb:msubsup></jb:math>, is measured to be <lb:math xmlns:lb="http://www.w3.org/1998/Math/MathML" display="inline"><lb:msubsup><lb:mi>δ</lb:mi><lb:mrow><lb:mi>K</lb:mi><lb:mi>π</lb:mi></lb:mrow><lb:mi>D</lb:mi></lb:msubsup><lb:mo>=</lb:mo><lb:msup><lb:mrow><lb:mo stretchy="false">(</lb:mo><lb:msubsup><lb:mn>192.8</lb:mn><lb:mrow><lb:mo>−</lb:mo><lb:mn>12.4</lb:mn><lb:mo>−</lb:mo><lb:mn>2.4</lb:mn></lb:mrow><lb:mrow><lb:mo>+</lb:mo><lb:mn>11.0</lb:mn><lb:mo>+</lb:mo><lb:mn>1.9</lb:mn></lb:mrow></lb:msubsup><lb:mo stretchy="false">)</lb:mo></lb:mrow><lb:mo>∘</lb:mo></lb:msup></lb:math>, using a dataset corresponding to an integrated luminosity of <pb:math xmlns:pb="http://www.w3.org/1998/Math/MathML" display="inline"><pb:mn>7.13</pb:mn><pb:mtext> </pb:mtext><pb:mtext> </pb:mtext><pb:msup><pb:mi>fb</pb:mi><pb:mrow><pb:mo>−</pb:mo><pb:mn>1</pb:mn></pb:mrow></pb:msup></pb:math> collected at center-of-mass energies between 4.13–4.23 GeV by the BESIII experiment.</jats:p>

Abstract:

The first dedicated $Z$ -boson mass measurement at the LHC with $Z\to {\mu }^{+}{\mu }^{-}$ decays is reported. The dataset uses proton-proton collisions at a center-of-mass energy of 13 TeV, recorded in 2016 by the LHCb experiment, and corresponds to an integrated luminosity of $1.7{\mathrm{fb}}^{-1}$ . A template fit to the ${\mu }^{+}{\mu }^{-}$ mass distribution yields the following result for the $Z$ -boson mass: $m_Z=91,185.7\pm 8.3\pm 3.9\mathrm{MeV}$ , where the first uncertainty is statistical and the second systematic. This result is consistent with previous measurements and predictions from global electroweak fits.

Abstract:

The ${\mathrm{\Xi }}_c^{+}{\pi }^{-}{\pi }^{+}$ spectrum is investigated using proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$ , collected by the LHCb experiment during 2016–2018. Four states are observed with high significance, and their masses and widths are measured to be $m[{\mathrm{\Xi }}_c(2815{)}^{+}]=2816.65\pm 0.03\pm 0.03\pm 0.23\mathrm{MeV}$ , $\mathrm{\Gamma }[{\mathrm{\Xi }}_c(2815{)}^{+}]=2.07\pm 0.08\pm 0.12\mathrm{MeV}$ , $m[{\mathrm{\Xi }}_c(2923{)}^{+}]=2922.8\pm 0.3\pm 0.5\pm 0.2\mathrm{MeV}$ , $\mathrm{\Gamma }[{\mathrm{\Xi }}_c(2923{)}^{+}]=5.3\pm 0.9\pm 1.4\mathrm{MeV}$ , $m[{\mathrm{\Xi }}_c(2970{)}^{+}]=2968.6\pm 0.5\pm 0.5\pm 0.2\mathrm{MeV}$ , $\mathrm{\Gamma }[{\mathrm{\Xi }}_c(2970{)}^{+}]=31.7\pm 1.7\pm 1.9\mathrm{MeV}$ , $[{\mathrm{\Xi }}_c(3080{)}^{+}]=3076.8\pm 0.7\pm 1.3\pm 0.2\mathrm{MeV}$ , $\mathrm{\Gamma }[{\mathrm{\Xi }}_c(3080{)}^{+}]=6.8\pm 2.3\pm 0.9\mathrm{MeV}$ , where the uncertainties are statistical, systematic, and due to the limited precision on the ${\mathrm{\Xi }}_c^{+}$ mass, respectively. The ${\mathrm{\Xi }}_c(2923{)}^{+}$ baryon is observed for the first time, and is consistent with being the isospin partner of the previously observed ${\mathrm{\Xi }}_c(2923{)}^0$ state. Most of the measured parameters are more precise than existing world averages.

Abstract:

A search for the doubly-charmed-baryon decay Ξcc++→Ξc0π+π+ is performed using proton-proton collision data collected by the LHCb experiment at a centre-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 5.4 fb−1. A significant structure consistent with the Ξcc++ baryon is observed in the Ξc0π+π+ invariant-mass spectrum. Using the Ξcc++→Λc+K−π+π+ decay as the normalisation channel, the branching fraction ratio, BΞcc++→Ξc0π+π+BΞcc++→Λc+K−π+π+, is measured to be 1.37 ± 0.18 (stat) ± 0.09 (syst) ± 0.35 (ext). This measurement provides critical input for testing QCD factorisation methods in the weak decays of doubly-heavy baryons, particularly in quantifying nonperturbative effects such as final-state interactions and resonance contributions to the hadronisation process.

Abstract:

The elliptic and triangular flow of charged particles are measured using two-particle angular correlations in pPb collisions in the pseudorapidity range 2.0 < |η| < 4.8. The data sample was collected by the LHCb experiment in 2016 at a centre-of-mass energy per nucleon pair of sNN=8.16 TeV, containing in total approximately 1.5 billion collision events. Non-flow contributions are obtained in low-multiplicity collisions and subtracted to extract the flow harmonics. The results are presented as a function of event multiplicity and hadron transverse momentum. Comparisons with a full (3+1)D dynamic model indicate that it overestimates the measured elliptic flow. A comparison between the forward and backward regions reveals no significant differences in flow parameters, suggesting that final-state effects may dominate over initial-state effects in the origin of flow in small systems.