Nuclear Modification Factor of Neutral Pions in the Forward and Backward Regions in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>p</mml:mi><mml:mtext>−</mml:mtext><mml:mi>Pb</mml:mi></mml:mrow></mml:math> Collisions
R. Aaij, A. S. W. Abdelmotteleb, C. Abellán Beteta, F. Abudinén, T. Ackernley, B. Adeva, M. Adinolfi, H. Afsharnia, C. Agapopoulou, C. Aidala, S. Aiola, Z. Ajaltouni, S. Akar, K. Carvalho Akiba, J. Albrecht, F. Alessio, M. Alexander, A. Alfonso Albero, Zakariya Aliouche, P. Álvarez Cartelle, S. Amato, J. L. Amey, Y. Amhis, L. An, L. Anderlini, M. Andersson, A. Andreianov, M. Andreotti, D. Ao, F. Archilli, A. Artamonov, M. Artuso, Kenenbek Arzymatov, E. Aslanides, M. Atzeni, B. Audurier, S. Bachmann, Marie Bachmayer, J. J. Back, A. Bailly-reyre, P. Baladrón Rodríguez, V. Balagura, W. Baldini, J. Baptista de Souza Leite, M. Barbetti, R. J. Barlow, S. Barsuk, W. Barter, M. Bartolini, F. Baryshnikov, J. M. Basels, G. Bassi, B. Batsukh, A. Battig, A. Bay, A. Beck, M. Becker, F. Bedeschi, I. Bediaga, A. Beiter, V. Belavin, S. Belin, V. Bellée, K. Belous, I. Belov, I. Belyaev, G. Bencivenni, E. Ben-Haim, 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, N. V. Biesuz, S. Bifani, P. Billoir, A. Biolchini, M. Birch, F. C. R. Bishop, A. Bitadze, A. Bizzeti, M. Bjørn, Michele Piero Blago, T. Blake, F. Blanc, S. Blusk, D. Bobulska, J. A. Boelhauve, O. Boente García, T. Boettcher, A. S. Boldyrev, N. Bondar
Abstract
The nuclear modification factor of neutral pions is measured in proton-lead collisions collected at a center-of-mass energy per nucleon of 8.16 TeV with the LHCb detector. The ${\ensuremath{\pi}}^{0}$ production cross section is measured differentially in transverse momentum (${p}_{T}$) for $1.5<{p}_{T}<10.0\text{ }\text{ }\mathrm{GeV}$ and in center-of-mass pseudorapidity (${\ensuremath{\eta}}_{\mathrm{c}.\mathrm{m}.}$) regions $2.5<{\ensuremath{\eta}}_{\mathrm{c}.\mathrm{m}.}<3.5$ (forward) and $\ensuremath{-}4.0<{\ensuremath{\eta}}_{\mathrm{c}.\mathrm{m}.}<\ensuremath{-}3.0$ (backward) defined relative to the proton beam direction. The forward measurement shows a sizable suppression of ${\ensuremath{\pi}}^{0}$ production, while the backward measurement shows the first evidence of ${\ensuremath{\pi}}^{0}$ enhancement in proton-lead collisions at the LHC. Together, these measurements provide precise constraints on models of nuclear structure and particle production in high-energy nuclear collisions.