Observation of the Z$\to\psi\ell^+\ell^-$ decay in pp collisions at $\sqrt{s}=$ 13 TeV

• Published in: Physical review letters Vol. 121; no. 14
• Main Authors: Sirunyan, Albert M., Tumasyan, Armen, Adam, Wolfgang, Ambrogi, Federico, Asilar, Ece, Bergauer, Thomas, Brandstetter, Johannes, Brondolin, Erica, Dragicevic, Marko, Erö, Janos, Escalante del Valle, Alberto, Flechl, Martin, Fruehwirth, Rudolf, Ghete, Vasile Mihai, Hrubec, Josef, Jeitler, Manfred, Krammer, Natascha, Krätschmer, Ilse, Liko, Dietrich, Madlener, Thomas, Mikulec, Ivan, Rad, Navid, Rohringer, Herbert, Schieck, Jochen, Schöfbeck, Robert, Spanring, Markus, Spitzbart, Daniel, Taurok, Anton, Waltenberger, Wolfgang, Wittmann, Johannes, Wulz, Claudia-Elisabeth, Zarucki, Mateusz, Chekhovsky, Vladimir, Mossolov, Vladimir, Suarez Gonzalez, Juan, de Wolf, Eddi A., Di Croce, Davide, Janssen, Xavier, Lauwers, Jasper, Pieters, Maxim, van de Klundert, Merijn, van Haevermaet, Hans, van Mechelen, Pierre, van Remortel, Nick, Abu Zeid, Shimaa, Blekman, Freya, d'Hondt, Jorgen, de Bruyn, Isabelle, de Clercq, Jarne, Deroover, Kevin, Flouris, Giannis, Lontkovskyi, Denys, Lowette, Steven, Marchesini, Ivan, Moortgat, Seth, Moreels, Lieselotte, Python, Quentin, Skovpen, Kirill, Tavernier, Stefaan, van Doninck, Walter, van Mulders, Petra, van Parijs, Isis, Beghin, Diego, Bilin, Bugra, Brun, Hugues, Clerbaux, Barbara, de Lentdecker, Gilles, Delannoy, Hugo, Dorney, Brian, Fasanella, Giuseppe, Favart, Laurent, Goldouzian, Reza, Grebenyuk, Anastasia, Kalsi, Amandeep Kaur, Lenzi, Thomas, Luetic, Jelena, Postiau, Nicolas, Starling, Elizabeth, Thomas, Laurent, Vander Velde, Catherine, Vanlaer, Pascal, Vannerom, David, Wang, Qun, Cornelis, Tom, Dobur, Didar, Fagot, Alexis, Gul, Muhammad, Khvastunov, Illia, Poyraz, Deniz, Roskas, Christos, Trocino, Daniele, Tytgat, Michael, Verbeke, Willem, Vermassen, Basile, Vit, Martina, Zaganidis, Nicolas, Bakhshiansohi, Hamed, Bondu, Olivier, Brochet, Sébastien, Bruno, Giacomo
• Format: Journal Article
• Language: English
• Published: American Physical Society 2018
• Subjects: High Energy Physics - Experiment; Physics; muon: pair production; experimental results; J/psi > > muon+ muon; Z0 > > 2muon+ 2muon; J/psi: leptonic decay; CMS; S044.92; branching ratio: ratio: measured; dimuon: mass spectrum; CERN LHC Coll; electron: pair production; S044.91; Elementary Particles and Fields; Z0: rare decay; phase space; Z0 > > J/psi lepton+ lepton; p p: scattering; p p: colliding beams; 13000 GeV-cms; J/psi: associated production
• ISSN: 0031-9007; 1079-7114
• DOI: 10.1103/PhysRevLett.121.141801

This Letter presents the observation of the rare Z boson decay Z→ψℓ+ℓ-. Here, ψ represents contributions from direct J/ψ and ψ(2S)→J/ψX, ℓ+ℓ- is a pair of electrons or muons, and the J/ψ meson is detected via its decay to μ+μ-. The sample of proton-proton collision data, collected by the CMS experiment at the LHC at a center-of-mass energy of 13 TeV, corresponds to an integrated luminosity of 35.9  fb-1. The signal is observed with a significance in excess of 5 standard deviations. After subtraction of the ψ(2S)→J/ψX contribution, the ratio of the branching fraction of the exclusive decay Z→J/ψℓ+ℓ- to the decay Z→μ+μ-μ+μ- within a fiducial phase space is measured to be B(Z→J/ψℓ+ℓ-)/B(Z→μ+μ-μ+μ-)=0.67±0.18(stat)±0.05(syst).