Measurement of the Nuclear Modification Factor and Prompt Charged Particle Production in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>p</mml:mi></mml:math>-Pb and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>p</mml:mi><mml:mi>p</mml:mi></mml:math> Collisions at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msqrt><mml:mrow><mml:msub><mml:mrow><mml:mi>s</mml:mi></mml:mrow><mml:mrow><mml:mi>N</mml:mi><mml:mi>N</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:msqrt><mml:mo>=</mml:mo><mml:mn>5</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>TeV</mml:mi></mml:mrow></mml:math>
R. Aaij, C. Abellán Beteta, T. Ackernley, B. Adeva, M. Adinolfi, H. Afsharnia, C. Aidala, Simone Aiola, Z. Ajaltouni, S. Akar, J. Albrecht, F. Alessio, M. Alexander, A. Alfonso Albero, Z. Aliouche, G. Alkhazov, P. Álvarez Cartelle, S. Amato, J. L. Amey, Y. Amhis, L. An, L. Anderlini, A. Andreianov, M. Andreotti, F. Archilli, A. Artamonov, M. Artuso, K. Arzymatov, E. Aslanides, M. Atzeni, B. Audurier, Marie Bachmayer, M. Bachmayer, J. J. Back, P. Baladrón Rodríguez, V. Balagura, W. Baldini, J. Baptista de Souza Leite, R. J. Barlow, S. Barsuk, W. Barter, M. Bartolini, F. Baryshnikov, J. M. Basels, G. Bassi, B. Batsukh, A. Battig, A. Bay, M. Becker, F. Bedeschi, I. Bediaga, A. Beiter, V. Belavin, S. Belin, V. Bellée, K. Belous, I. Belov, I. Belyaev, G. Bencivenni, A. Berezhnoy, A. Berezhnoy, R. Bernet, D. Berninghoff, H. C. Bernstein, C. Bertella, A. Bertolin, C. Betancourt, F. Betti, Ia. Bezshyiko, S. Bhasin, J. Bhom, L. Bian, M. S. Bieker, S. Bifani, Pierre Billoir, M. Birch, F. C. R. Bishop, A. Bitadze, A. Bizzeti, M. Bjørn, M. P. Blago, T. Blake, F. Blanc, S. Blusk, D. Bobulska, J. A. Boelhauve, O. Boente García, T. Boettcher, A. Boldyrev, A. Bondar, N. Bondar, S. Borghi, M. Borisyak, M. Borsato, J. T. Borsuk, Sonia Amina Bouchiba, T. J. V. Bowcock, A. Boyer, C. Bozzi, M. J. Bradley
Abstract
The production of prompt charged particles in proton-lead collisions and in proton-proton collisions at the nucleon-nucleon center-of-mass energy $\sqrt{{s}_{NN}}=5\text{ }\text{ }\mathrm{TeV}$ is studied at LHCb as a function of pseudorapidity ($\ensuremath{\eta}$) and transverse momentum (${p}_{T}$) with respect to the proton beam direction. The nuclear modification factor for charged particles is determined as a function of $\ensuremath{\eta}$ between $\ensuremath{-}4.8<\ensuremath{\eta}<\ensuremath{-}2.5$ (backward region) and $2.0<\ensuremath{\eta}<4.8$ (forward region), and ${p}_{T}$ between $0.2<{p}_{T}<8.0\text{ }\text{ }\mathrm{GeV}/c$. The results show a suppression of charged particle production in proton-lead collisions relative to proton-proton collisions in the forward region and an enhancement in the backward region for ${p}_{T}$ larger than $1.5\text{ }\text{ }\mathrm{GeV}/c$. This measurement constrains nuclear PDFs and saturation models at previously unexplored values of the parton momentum fraction down to ${10}^{\ensuremath{-}6}$.