Measurements of polarization and spin correlation and observation of entanglement in top quark pairs using <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>lepton</mml:mi><mml:mo>+</mml:mo><mml:mtext>jets</mml:mtext></mml:mrow></mml:math> events from proton-proton collisions at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msqrt><mml:mi>s</mml:mi></mml:msqrt><mml:mo>=</mml:mo><mml:mn>13</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>TeV</mml:mi></mml:math>
Aram Hayrapetyan, A. Tumasyan, W. Adam, Janik Walter Andrejkovic, L. Benato, T. Bergauer, S. Chatterjee, K. Damanakis, M. Dragicevic, Priya Sajid Hussain, M. Jeitler, Natascha Krammer, A. Li, D. Liko, I. Mikulec, J. Schieck, R. Schöfbeck, D. Schwarz, M. Sonawane, W. Waltenberger, C.-E. Wulz, X. Janssen, T. Van Laer, P. Van Mechelen, Nordin Breugelmans, Jorgen D'Hondt, Soumya Dansana, A. De Moor, M. Delcourt, Felix Heyen, Steven Lowette, I. Makarenko, D. Müller, S. Tavernier, M. Tytgat, G. P. Van Onsem, S. Van Putte, D. Vannerom, B. Bilin, B. Clerbaux, Aloke Kumar Das, I. De Bruyn, G. De Lentdecker, Hugues Evard, L. Favart, P. Gianneios, J. Jaramillo, Mohammad A. Khalilzadeh, Fakhri Alam Khan, Kyeongpil Lee, A. Malara, S. Paredes, Muhammad Aamir Shahzad, Laurent Thomas, M. Vanden Bemden, C. Vander Velde, P. Vanlaer, M. De Coen, D. Dobur, G. Gökbulut, Yanwen Hong, J. Knolle, Luka Lambrecht, David Marckx, K. Mota Amarilo, K. Skovpen, N. Van Den Bossche, Jan van der Linden, Liam Wezenbeek, A. Benecke, A. Bethani, G. Bruno, C. Caputo, J. De Favereau De Jeneret, C. Delaere, I. S. Donertas, A. Giammanco, Ahmet Oguz Guzel, Sandhya Jain, Vincent Lemaître, J. Lidrych, Paola Mastrapasqua, T. T. Tran, S. Turkcapar, G. A. Alves, E. Coelho, G. Correia Silva, C. Hensel, T. Menezes De Oliveira, C. Mora Herrera, P. Rebello Teles, M. Soeiro, E. J. Tonelli Manganote, A. Vilela Pereira, W. L. Aldá Júnior, M. Barroso Ferreira Filho, H. Brandao Malbouisson, W. Carvalho, J. Chinellato, E. M. Da Costa
Abstract
Measurements of the polarization and spin correlation in top quark pairs (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>t</a:mi><a:mover accent="true"><a:mi>t</a:mi><a:mo stretchy="false">¯</a:mo></a:mover></a:math>) are presented using events with a single electron or muon and jets in the final state. The measurements are based on proton-proton collision data from the LHC at <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msqrt><e:mi>s</e:mi></e:msqrt><e:mo>=</e:mo><e:mn>13</e:mn><e:mtext> </e:mtext><e:mtext> </e:mtext><e:mi>TeV</e:mi></e:math> collected by the CMS experiment, corresponding to an integrated luminosity of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mn>138</g:mn><g:mtext> </g:mtext><g:mtext> </g:mtext><g:msup><g:mi>fb</g:mi><g:mrow><g:mo>−</g:mo><g:mn>1</g:mn></g:mrow></g:msup></g:math>. All coefficients of the polarization vectors and the spin correlation matrix are extracted simultaneously by performing a binned likelihood fit to the data. The measurement is performed inclusively and in bins of additional observables, such as the mass of the <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mi>t</i:mi><i:mover accent="true"><i:mi>t</i:mi><i:mo stretchy="false">¯</i:mo></i:mover></i:math> system and the top quark scattering angle in the <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mi>t</m:mi><m:mover accent="true"><m:mi>t</m:mi><m:mo stretchy="false">¯</m:mo></m:mover></m:math> rest frame. The measured polarization and spin correlation are in agreement with the standard model. From the measured spin correlation, conclusions on the <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:mi>t</q:mi><q:mover accent="true"><q:mi>t</q:mi><q:mo stretchy="false">¯</q:mo></q:mover></q:math> spin entanglement are drawn by applying the Peres-Horodecki criterion. The standard model predicts entangled spins for <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:mi>t</u:mi><u:mover accent="true"><u:mi>t</u:mi><u:mo stretchy="false">¯</u:mo></u:mover></u:math> states at the production threshold and at high masses of the <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"><y:mi>t</y:mi><y:mover accent="true"><y:mi>t</y:mi><y:mo stretchy="false">¯</y:mo></y:mover></y:math> system. Entanglement is observed for the first time in events at high <cb:math xmlns:cb="http://www.w3.org/1998/Math/MathML" display="inline"><cb:mi>t</cb:mi><cb:mover accent="true"><cb:mi>t</cb:mi><cb:mo stretchy="false">¯</cb:mo></cb:mover></cb:math> mass, where a large fraction of the <gb:math xmlns:gb="http://www.w3.org/1998/Math/MathML" display="inline"><gb:mi>t</gb:mi><gb:mover accent="true"><gb:mi>t</gb:mi><gb:mo stretchy="false">¯</gb:mo></gb:mover></gb:math> decays are spacelike separated, with an expected and observed significance of above 5 standard deviations. © 2024 CERN, for the CMS Collaboration 2024 CERN