Nuclear Dependence of the Transverse-Single-Spin Asymmetry for Forward Neutron Production in Polarized p+A Collisions at √sNN = 200 GeV

This is an electronic reprint of the original article. This reprint may differ from the original in pagination and typographic detail. Author(s): Title: Year: Version: Please cite the original version: All material supplied via JYX is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. Nuclear Dependence of the Transverse-Single-Spin Asymmetry for Forward Neutron Production in Polarized p+A Collisions at √sNN = 200 GeV PHENIX Collaboration PHENIX Collaboration. (2018). Nuclear Dependence of the Transverse-Single-Spin Asymmetry for Forward Neutron Production in Polarized p+A Collisions at √sNN = 200 GeV. Physical Review Letters, 120(2), Article 022001. https://doi.org/10.1103/PhysRevLett.120.022001 2018 Nuclear Dependence of the Transverse-Single-Spin Asymmetry for Forward Neutron Production in Polarized p+ACollisions at ffiffiffiffiffiffiffiffi sNN p= 200 GeV C. Aidala,40 Y. Akiba,51,52,†M. Alfred,22 V. Andrieux,40 K. Aoki,30 N. Apadula,27 H. Asano,33,51 C. Ayuso,40 B. Azmoun,7 V. Babintsev,23 A. Bagoly,16 N. S. Bandara,39 K. N. Barish,8S. Bathe,5,52 A. Bazilevsky,7M. Beaumier,8R. Belmont,12 A. Berdnikov,54 Y. Berdnikov,54 D. S. Blau,32,43 M. Boer,35 J. S. Bok,45 M. L. Brooks,35 J. Bryslawskyj,5,8 V. Bumazhnov,23 C. Butler,20 S. Campbell,13 V. Canoa Roman,57 R. Cervantes,57 C. Y. Chi,13 M. Chiu,7I. J. Choi,24 J. B. Choi,10,* Z. Citron,62 M. Connors,20,52 N. Cronin,57 M. Csanád,16 T. Csörgő,17,63 T. W. Danley,46 M. S. Daugherity,1G. David,7,57 K. DeBlasio,44 K. Dehmelt,57 A. Denisov,23 A. Deshpande,52,57 E. J. Desmond,7A. Dion,57 D. Dixit,57 J. H. Do,64 A. Drees,57 K. A. Drees,6 M. Dumancic,62 J. M. Durham,35 A. Durum,23 T. Elder,20 A. Enokizono,51,53 H. En’yo,51 S. Esumi,60 B. Fadem,41 W. Fan,57 N. Feege,57 D. E. Fields,44 M. Finger,9M. Finger, Jr.,9S. L. Fokin,32 J. E. Frantz,46 A. Franz,7A. D. Frawley,19 Y. Fukuda,60 C. Gal,57 P. Gallus,14 P. Garg,3,57 H. Ge,57 F. Giordano,24 Y. Goto,51,52 N. Grau,2S. V. Greene,61 M. Grosse Perdekamp,24 T. Gunji,11 H. Guragain,20 T. Hachiya,51,52 J. S. Haggerty,7K. I. Hahn,18 H. Hamagaki,11 H. F. Hamilton,1S. Y. Han,18 J. Hanks,57 S. Hasegawa,28 T. O. S. Haseler,20 X. He,20 T. K. Hemmick,57 J. C. Hill,27 K. Hill,12 R. S. Hollis,8K. Homma,21 B. Hong,31 T. Hoshino,21 N. Hotvedt,27 J. Huang,7S. Huang,61 K. Imai,28 J. Imrek,15 M. Inaba,60 A. Iordanova,8 D. Isenhower,1Y. Ito,42 D. Ivanishchev,50 B. V. Jacak,57 M. Jezghani,20 Z. Ji,57 X. Jiang,35 B. M. Johnson,7,20 V. Jorjadze,57 D. Jouan,48 D. S. Jumper,24 J. H. Kang,64 D. Kapukchyan,8S. Karthas,57 D. Kawall,39 A. V. Kazantsev,32 V. Khachatryan,57 A. Khanzadeev,50 C. Kim,8,31 D. J. Kim,29 E.-J. Kim,10 M. Kim,55 M. H. Kim,31 D. Kincses,16 E. Kistenev,7J. Klatsky,19 P. Kline,57 T. Koblesky,12 D. Kotov,50,54 S. Kudo,60 K. Kurita,53 Y. Kwon,64 J. G. Lajoie,27 E. O. Lallow,41 A. Lebedev,27 S. Lee,64 M. J. Leitch,35 Y. H. Leung,57 N. A. Lewis,40 X. Li,35 S. H. Lim,35,64 L. D. Liu,49 M. X. Liu,35 V.-R. Loggins,24 S. Lökös,16,17 K. Lovasz,15 D. Lynch,7T. Majoros,15 Y. I. Makdisi,6M. Makek,65 M. Malaev,50 V. I. Manko,32 E. Mannel,7 H. Masuda,53 M. McCumber,35 P. L. McGaughey,35 D. McGlinchey,12,35 C. McKinney,24 M. Mendoza,8W. J. Metzger,17 A. C. Mignerey,38 D. E. Mihalik,57 A. Milov,62 D. K. Mishra,4J. T. Mitchell,7G. Mitsuka,52 S. Miyasaka,51,59 S. Mizuno,51,60 P. Montuenga,24 T. Moon,64 D. P. Morrison,7S. I. M. Morrow,61 T. Murakami,33,51 J. Murata,51,53 K. Nagai,59 K. Nagashima,21 T. Nagashima,53 J. L. Nagle,12 M. I. Nagy,16 I. Nakagawa,51,52 H. Nakagomi,51,60 K. Nakano,51,59 C. Nattrass,58 T. Niida,60 R. Nouicer,7,52 T. Novák,17,63 N. Novitzky,57 R. Novotny,14 A. S. Nyanin,32 E. O’Brien,7 C. A. Ogilvie,27 J. D. Orjuela Koop,12 J. D. Osborn,40 A. Oskarsson,36 G. J. Ottino,44 K. Ozawa,30,60 V. Pantuev,25 V. Papavassiliou,45 J. S. Park,55 S. Park,51,55,57 S. F. Pate,45 M. Patel,27 W. Peng,61 D. V. Perepelitsa,7,12 G. D. N. Perera,45 D. Yu. Peressounko,32 C. E. PerezLara,57 J. Perry,27 R. Petti,7M. Phipps,7,24 C. Pinkenburg,7R. P. Pisani,7A. Pun,46 M. L. Purschke,7P. V. Radzevich,54 K. F. Read,47,58 D. Reynolds,56 V. Riabov,43,50 Y. Riabov,50,54 D. Richford,5T. Rinn,27 S. D. Rolnick,8M. Rosati,27 Z. Rowan,5J. Runchey,27 A. S. Safonov,54 T. Sakaguchi,7H. Sako,28 V. Samsonov,43,50 M. Sarsour,20 K. Sato,60 S. Sato,28 B. Schaefer,61 B. K. Schmoll,58 K. Sedgwick,8R. Seidl,51,52 A. Sen,27,58 R. Seto,8 A. Sexton,38 D. Sharma,57 I. Shein,23 T.-A. Shibata,51,59 K. Shigaki,21 M. Shimomura,27,42 T. Shioya,60 P. Shukla,4 A. Sickles,24 C. L. Silva,35 D. Silvermyr,36 B. K. Singh,3C. P. Singh,3V. Singh,3M. J. Skoby,40 M. Slunečka,9 K. L. Smith,19 M. Snowball,35 R. A. Soltz,34 W. E. Sondheim,35 S. P. Sorensen,58 I. V. Sourikova,7P. W. Stankus,47 S. P. Stoll,7T. Sugitate,21 A. Sukhanov,7T. Sumita,51 J. Sun,57 S. Syed,20 J. Sziklai,63 A. Takeda,42 K. Tanida,28,52,55 M. J. Tannenbaum,7S. Tarafdar,61,62 A. Taranenko,43,56 G. Tarnai,15 R. Tieulent,20,37 A. Timilsina,27 T. Todoroki,60 M. Tomášek,14 C. L. Towell,1R. S. Towell,1I. Tserruya,62 Y. Ueda,21 B. Ujvari,15 H. W. van Hecke,35 S. Vazquez-Carson,12 J. Velkovska,61 M. Virius,14 V. Vrba,14,26 N. Vukman,65 X. R. Wang,45,52 Z. Wang,5Y. Watanabe,51,52 Y. S. Watanabe,11 C. P. Wong,20 C. L. Woody,7C. Xu,45 Q. Xu,61 L. Xue,20 S. Yalcin,57 Y. L. Yamaguchi,52,57 H. Yamamoto,60 A. Yanovich,23 P. Yin,12 J. H. Yoo,31 I. Yoon,55 H. Yu,45,49 I. E. Yushmanov,32 W. A. Zajc,13 A. Zelenski,6 S. Zharko,54 and L. Zou8 (PHENIX Collaboration) 1Abilene Christian University, Abilene, Texas 79699, USA 2Department of Physics, Augustana University, Sioux Falls, South Dakota 57197, USA 3Department of Physics, Banaras Hindu University, Varanasi 221005, India 4Bhabha Atomic Research Centre, Bombay 400 085, India 5Baruch College, City University of New York, New York, New York 10010, USA PHYSICAL REVIEW LETTERS 120, 022001 (2018) 0031-9007=18=120(2)=022001(9) 022001-1 Published by the American Physical Society 6Collider-Accelerator Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA 7Physics Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA 8University of California–Riverside, Riverside, California 92521, USA 9Charles University, Ovocný trh 5, Praha 1, 116 36, Prague, Czech Republic 10Chonbuk National University, Jeonju 561-756, Korea 11Center for Nuclear Study, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan 12University of Colorado, Boulder, Colorado 80309, USA 13Columbia University, New York, New York 10027, USA and Nevis Laboratories, Irvington, New York 10533, USA 14Czech Technical University, Zikova 4, 166 36 Prague 6, Czech Republic 15Debrecen University, H-4010 Debrecen, Egyetem t´er 1, Hungary 16ELTE, Eötvös Loránd University, H-1117 Budapest, Pázmány P. s. 1/A, Hungary 17Eszterházy Károly University, Károly Róbert Campus, H-3200 Gyöngyös, Mátrai út 36, Hungary 18Ewha Womans University, Seoul 120-750, Korea 19Florida State University, Tallahassee, Florida 32306, USA 20Georgia State University, Atlanta, Georgia 30303, USA 21Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8526, Japan 22Department of Physics and Astronomy, Howard University, Washington, DC 20059, USA 23IHEP Protvino, State Research Center of Russian Federation, Institute for High Energy Physics, Protvino 142281, Russia 24University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA 25Institute for Nuclear Research of the Russian Academy of Sciences, prospekt 60-letiya Oktyabrya 7a, Moscow 117312, Russia 26Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic 27Iowa State University, Ames, Iowa 50011, USA 28Advanced Science Research Center, Japan Atomic Energy Agency, 2-4 Shirakata Shirane, Tokai-mura, Naka-gun, Ibaraki-ken 319-1195, Japan 29Helsinki Institute of Physics and University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä, Finland 30KEK, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan 31Korea University, Seoul 136-701, Korea 32National Research Center “Kurchatov Institute,”Moscow 123098 Russia 33Kyoto University, Kyoto 606-8502, Japan 34Lawrence Livermore National Laboratory, Livermore, California 94550, USA 35Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA 36Department of Physics, Lund University, Box 118, SE-221 00 Lund, Sweden 37IPNL, CNRS/IN2P3, Univ Lyon, Universit Lyon 1, F-69622, Villeurbanne, France 38University of Maryland, College Park, Maryland 20742, USA 39Department of Physics, University of Massachusetts, Amherst, Massachusetts 01003-9337, USA 40Department of Physics, University of Michigan, Ann Arbor, Michigan 48109-1040, USA 41Muhlenberg College, Allentown, Pennsylvania 18104-5586, USA 42Nara Women’s University, Kita-uoya Nishi-machi Nara 630-8506, Japan 43National Research Nuclear University, MEPhI, Moscow Engineering Physics Institute, Moscow 115409, Russia 44University of New Mexico, Albuquerque, New Mexico 87131, USA 45New Mexico State University, Las Cruces, New Mexico 88003, USA 46Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701, USA 47Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 48IPN-Orsay, Universit´e Paris-Sud, CNRS/IN2P3, Universit´e Paris-Saclay, BP1, F-91406 Orsay, France 49Peking University, Beijing 100871, People’s Republic of China 50PNPI, Petersburg Nuclear Physics Institute, Gatchina, Leningrad region 188300, Russia 51RIKEN Nishina Center for Accelerator-Based Science, Wako, Saitama 351-0198, Japan 52RIKEN BNL Research Center, Brookhaven National Laboratory, Upton, New York 11973-5000, USA 53Physics Department, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima, Tokyo 171-8501, Japan 54Saint Petersburg State Polytechnic University, St. Petersburg 195251 Russia 55Department of Physics and Astronomy, Seoul National University, Seoul 151-742, Korea 56Chemistry Department, Stony Brook University, SUNY, Stony Brook, New York 11794-3400, USA 57Department of Physics and Astronomy, Stony Brook University, SUNY, Stony Brook, New York 11794-3800, USA 58University of Tennessee, Knoxville, Tennessee 37996, USA 59Department of Physics, Tokyo Institute of Technology, Oh-okayama, Meguro, Tokyo 152-8551, Japan 60Center for Integrated Research in Fundamental Science and Engineering, University of Tsukuba, Tsukuba, Ibaraki 305, Japan 61Vanderbilt University, Nashville, Tennessee 37235, USA 62Weizmann Institute, Rehovot 76100, Israel PHYSICAL REVIEW LETTERS 120, 022001 (2018) 022001-2 63Institute for Particle and Nuclear Physics, Wigner Research Centre for Physics, Hungarian Academy of Sciences (Wigner RCP, RMKI) H-1525 Budapest 114, P.O. Box 49, Budapest, Hungary 64Yonsei University, IPAP, Seoul 120-749, Korea 65Department of Physics, Faculty of Science, University of Zagreb, Bijenička c. 32 HR-10002 Zagreb, Croatia (Received 3 April 2017; revised manuscript received 26 September 2017; published 8 January 2018) During 2015, the Relativistic Heavy Ion Collider (RHIC) provided collisions of transversely polarized protons with Au and Al nuclei for the first time, enabling the exploration of transverse-single-spin asymmetries with heavy nuclei. Large single-spin asymmetries in very forward neutron production have been previously observed in transversely polarized pþpcollisions at RHIC, and the existing theoretical framework that was successful in describing the single-spin asymmetry in pþpcollisions predicts only a moderate atomic-mass-number (A) dependence. In contrast, the asymmetries observed at RHIC in pþA collisions showed a surprisingly strong Adependence in inclusive forward neutron production. The observed asymmetry in pþAl collisions is much smaller, while the asymmetry in pþAu collisions is a factor of 3 larger in absolute value and of opposite sign. The interplay of different neutron production mechanisms is discussed as a possible explanation of the observed Adependence. DOI: 10.1103/PhysRevLett.120.022001 Understanding forward particle production in highenergy hadron collisions is of great importance, because most of the energy goes in the forward direction, and therefore informs our understanding of overall particle production. This has particular importance in studies of ultrahigh-energy cosmic rays, where extraction of the cosmic ray distributions from air shower measurements depends on models of forward particle production in the interaction with nuclei in the air [1–3]. Mechanisms for forward particle production are not well understood, as perturbative quantum chromodynamics (pQCD) is not applicable at small momentum transfers and diffractive production mechanisms are not well modeled. To better understand production mechanisms, the measurement of the single-spin asymmetry AN, describing the azimuthal asymmetry of particle production relative to the spin direction of the transversely polarized beam or target, provides crucial tests and deeper insight beyond just cross-section measurements. The spin degree of freedom has served as a strong discriminator between theoretical models. For example, the origin of the large asymmetries discovered in forward meson production in pþpcollisions from ffiffiffi s p¼4.9–19.4GeV [4–11] and later confirmed at ffiffiffi s p¼62.4–500 GeV at the Relativistic Heavy Ion Collider (RHIC) [12–17] has been under intensive discussion for three decades and still remains an open question [18]. Despite substantial theoretical attempts to reproduce data in the pQCD regime using the conventional 2→2 parton scattering processes, the latest multiplicitydependent ANmeasurements from RHIC [19] indicate that a significant contribution to the asymmetry may be of a diffractive nature. Another important approach in forward particle production is to study the nuclear dependence in pþAcollisions. In the perturbative region, theoretical approaches based on color-glass-condensate models predicted that hadronic AN should decrease with increasing A[20–24], while some approaches based on pQCD factorization predicted that AN would stay approximately the same for all nuclear targets [25]. On the other hand, almost no theoretical or experimental studies are available in the nonperturbative region or diffractive scattering with polarized probes on nuclei, and interesting phenomena may be hidden in this unexplored region. In the case of forward neutron production in pþp collisions, production cross sections [26–28] were successfully explained in terms of one-pion exchange [29–33]. However, that model could not explain the sizable ANin very forward (near zero degree) neutron production, discovered at RHIC in pþpcollisions at ffiffiffi s p¼200 GeV [28]. To reproduce the experimental asymmetry, an interference between the spin-flip πexchange and a nonspin-flip a1-Reggeon exchange was necessary [33]. Kopeliovich, Potashnikova, and Schmidt considered nuclear absorption effects as a source for a possible A dependence of ANand found only a small effect [34]. In this Letter, we report the first measurements of ANfor very forward neutron production in collisions between polarized protons and nuclei (Al and Au) at ffiffiffiffiffiffiffiffi sNN p¼ 200 GeV recorded in 2015 with the PHENIX detector [35].Forpþpcollisions 18 RHIC stores were used and one store each for pþAl and pþAu measurements, with a typical store length of 8 h. The average beam polarization in pþp,pþAl, and pþAu data samples was 0.515 0.002,0.59 0.02, and 0.59 0.04, respectively, Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3. PHYSICAL REVIEW LETTERS 120, 022001 (2018) 022001-3 with an additional global uncertainty of 3% from the polarization normalization [36,37]. The experimental setup using a zero-degree calorimeter (ZDC) [38] and a position-sensitive shower-maximum detector (SMD) is similar to the one used for pþpdata [39]. The ZDC comprises three modules located in series at 18 m away from the collision point. The ZDC has an acceptance in the transverse plane of 10 ×10 cm2, with a total of 5.1 nuclear interaction lengths (or 149 radiation lengths), and an energy resolution of ∼25%–20% for 50–100 GeV neutrons. The SMD comprises x-y(horizontal-vertical) scintillator strip hodoscopes inserted between the first and second ZDC modules (approximately at the position of the maximum hadronic shower) and provides a position resolution of ∼1cm for 50–100 GeV neutrons. These detectors are located downstream of the RHIC DX beam splitting magnet, so that near beam-momentum charged particles from collisions are expected to be swept into the beam lines and out of the ZDC acceptance (see Fig. 1). To accommodate asymmetric pþAcollisions of beams with different rigidity, the DX magnets were moved horizontally [40]. In this special setup for the present measurement, the proton beam was angled off axis by ∼2mrad relative to the nominal beam direction at the collision point, with a crossing angle with the Au (Al) beam of 2.0 (1.1) mrad. Correspondingly, the ZDC was moved by 3.6 cm (2 mrad) to keep zero-degree neutrons at the ZDC center (see Fig. 1). The data were collected with triggers employing the ZDC and beam-beam counters (BBCs) [41]. Only the north ZDC detector, facing the incoming polarized proton beam, was used in this analysis. Two BBCs are located at 144 cm from the nominal collision point along the beam pipe and are designed to detect charged particles in the pseudorapidity range of ð3.0–3.9Þwith full azimuthal coverage. The ZDC inclusive trigger required the energy deposited in the ZDC to be greater than 15 GeV. The ZDC ⊗BBC-tag trigger in addition required at least one hit in each of the BBCs, and the ZDC ⊗BBC-veto trigger required no hits in both BBCs. The latter two sets represent mutually exclusive but not complete subsets of the ZDC inclusive triggered data. As described in detail in Ref. [39], event selection and neutron identification cuts include (i) a total ZDC energy cut of 40–120 GeV, (ii) at least two SMD strips fired (above threshold) in both xand ydirections and a nonzero (above threshold) energy in the second ZDC module (to reject photons), and (iii) an acceptance cut of 0.5<r<4.0cm for the reconstructed radial distance rfrom the determined beam center (to reduce the impact of the position resolution and edge effects in the asymmetry measurements). The raw asymmetry [ϵNðϕÞ] is calculated using the square-root formula [39] for each azimuthal angle (ϕ) bin. The polarization normalized Afit Nis then extracted from the fit to a sine function ϵNðϕÞ¼PAfit Nsinðϕ−ϕ0Þ;ð1Þ where Pis the proton beam polarization and ϕ0is the polarization direction in the transverse plane. Figure 2compares ϵNðϕÞ=P results for ZDC inclusive samples from pþp,pþAl, and pþAu collisions and shows the nuclear dependence of Afit N, including a sign change from negative in pþpcollisions to positive in pþAu collisions. The Afit Nwas measured separately in each PHENIX data-taking segment, typically 60 min long, and then the weighted average was calculated. The obtained Afit Nis then corrected for backgrounds and detector responses. The main background contribution comes from protons, generated by elastic, diffractive, and hard processes. Protons from elastic and diffractive reactions travel close to the beam line and are swept by the DX magnet to the right (toward negative xin Fig. 1). Only a small fraction of such protons scattered by large angles, larger than 4–5 mrad, fall in the ZDC acceptance. Because the cross section for these reactions falls sharply with the scattering angle, these protons contribute mainly on the right side of the ZDC. This contribution was evaluated from the particle position distribution as measured by the SMD and found to be 9% and 32% in the inclusive ZDC and ZDC ⊗BBC-veto FIG. 1. ZDC location and beam orbits of a proton (blue) beam and a heavy-ion (yellow) beam in the special stores used for this analysis; the zaxis shows the nominal beam direction, and the dashed line represents the zero-degree neutron trajectory. DX and D0 are the RHIC beam bending dipole magnets. (rad)φ P/ N ∈ -0.2 -0.1 0 0.1 0.2 n+X→+p ↑ p (rad)φ n+X→+Al ↑ p = 200 GeV NN s >0.5 F x <2.2 mradθ0.3< ZDC inclusive PHENIX (rad)φ -1 0 1 -1 0 1 -1 0 1 n+X→+Au ↑ p FIG. 2. Afit Nfit of ZDC inclusive samples. PHYSICAL REVIEW LETTERS 120, 022001 (2018) 022001-4 triggeredsamples,respectively,inpþpcollisions,<2% in both samples in pþAcollisions, and negligible in ZDC ⊗BBC-tag samples of both pþpand pþAcollisions. The significant suppression of elastic and diffractive proton background relative to the neutron signal in pþA collisions can be understood as due to the stronger magnetic fields in the DX magnets. Correspondingly, the minimum scattering angle for the elastic and diffractive proton backgrounds to reach the ZDC acceptance increases from 3.8 to 5 mrad, leading to a cross section reduction by an order of magnitude. The contribution of the charged hadron background from hard scattering processes, distributed nearly uniformly over the ZDC acceptance, was estimated using PYTHIA 6[42] with a GEANT 3[43] detector simulation. However, from previous studies where a charge veto counter was installed in front of the ZDC to measure the charged hadron background, it was found that the simulation underestimates the proton background by a factor of ∼2[39]. Therefore, the hard scattering background contribution from the simulation was scaled by a factor of 2 with an uncertainty equal to the size of the increase. In pþp collisions, this background fraction resulted in 63%, 31.5%, and 12 6% in ZDC, ZDC ⊗BBC-veto, and ZDC ⊗BBC-tag triggered samples, respectively. In pþA collisions, due to the increased neutron signal from electromagnetic (EM) processes (to be discussed later), the relative background contributions are expected to be smaller. Therefore, the measured asymmetries in pþA collisions were not corrected for background, but one-sided systematic uncertainties (in the direction of the asymmetry magnitude increase) equal to the upper 1σlimit of the background fractions taken from the pþpcase, i.e., 9%, 4.5%, and 18%, were conservatively assigned in ZDC, ZDC ⊗BBC-veto, and ZDC ⊗BBC-tag triggered samples, respectively. From the considerations above, only the pþpasymmetries were corrected for backgrounds according to AS N¼ANfit −reffAB N 1−reff ;ð2Þ where AS Nand AB Nstand for signal and background asymmetries, respectively, and reff is the “effective”background fraction in the reconstructed neutron sample. The parameter reff accounts for the dilution of the background effect in Afit Nin the case when the background contributes preferably on one side of the detector (as from elastic or diffractive protons). This effect, which was studied in the simulation, comes from a specific way the left and right sides of detector acceptance are combined in the squareroot formula for asymmetry calculation. The background asymmetry AB Nwas evaluated from the comparison of asymmetries with and without the charge veto cut from the 2008 data when the charge veto counter was available and then used in Eq. (2). The asymmetries AB Nwere found to be consistent with zero within statistical uncertainties for all triggers. After a background correction, AS Nresults for pþpfrom 2008 and 2015 data were found to be consistent within statistical uncertainties. Asymmetries from 2015 data were used in the final results. Besides charged hadrons, the other background sources are photons and K0mesons. From the PYTHIA 6simulation, their contribution after the analysis cuts was evaluated to be below 3% in all collision systems and triggers and was neglected in the asymmetry results. The measured asymmetries are affected by detector resolutions and other detector systematic effects (e.g., edge effects), as well as by the uncertainty in the shape of the neutron production cross section vs pTand xF, the size of the asymmetry, and the assumption for the shape of ANðpTÞ within the pTrange sampled in this analysis. These effects were studied in detail with a GEANT 3Monte Carlo simulation. The fully corrected transverse-single-spin asymmetry ANwas calculated as AN¼AS N=Cϕ, where the correction factor Cϕwas calculated in the simulation as the ratio of the measured asymmetry to the average input asymmetry over the neutron sample collected with experimental cuts used in the analysis. The biggest variation in Cϕcomes from the position resolution uncertainty and the assumption for ANðpTÞ. The position resolution in the simulation vs data was confirmed from the comparison of the shower shape and its fluctuations in SMD strips. The simulation was tuned to data by varying noise and thresholds in the SMD channels, as well as by introducing a cross talk effect, similar to Ref. [39]. An overall value of 3% was assigned to the Cϕuncertainty. For the shape of ANðpTÞ,it was modeled as ANðpTÞ¼const (as was assumed in Ref. [39]) and ANðpTÞ∝pT(which is supported by theory in the pTrange relevant here [33]). The difference of 3% was included in the Cϕuncertainty. The final correction factor applied to the measured asymmetries is Cϕ¼0.855 0.036. Note that the Cϕvalue here is higher than the one in our previous publication [39] mainly due to two reasons: First, a more realistic ANðpTÞ∝pT assumption was used in this analysis, and, second, the optimized SMD thresholds reduced the smearing effect. In addition to the beam polarization, background, and smearing correction (Cϕ) discussed above, the other sources of systematic uncertainties are the ZDC and SMD gain calibrations (including threshold variation) and location of the beam center on the ZDC plane. The latter is among the dominant uncertainties in this data, contributing 0.002–0.010 to the ANuncertainty. It was estimated by calculating the asymmetry for varying assumptions of the beam axis projection on the ZDC plane, 1cm in the horizontal and 0.5cm in the vertical direction from the ZDC center, which reflect the uncertainty in ZDC alignment relative to the beam axis. PHYSICAL REVIEW LETTERS 120, 022001 (2018) 022001-5 The analyzed data correspond to the neutron sampled pT in the range smaller than 0.25 GeV=c peaked at about 0.1GeV=c, which is defined mainly by detector acceptance and which is affected by detector resolutions. Because of the varying contribution of different processes to neutron production, the sampled pTdistribution may vary in different collision systems and in different triggered data. Figure 3shows the differences in the radial distributions, which is related to the neutron production cross section dσ=dpTby pT∝r[39]. From a comparison with the simulation assuming different slope parameters b, in the parameterization dσ=dpT∼e−b·pT, the data were found to be consistent with b¼4ðGeV=cÞ−1for all collision systems in ZDC ⊗BBC-tag triggered data and b¼4,6, and 8ðGeV=cÞ−1in pþp,pþAl, and pþAu collisions, respectively, in a ZDC ⊗BBC-veto triggered sample, with uncertainty σb¼1ðGeV=cÞ−1reflecting its sensitivity to SMD gain calibration and thresholds. These variations lead to a difference in the average pTsampled in different collision systems and triggers by as much as 10%. As can be also judged from Fig. 3, due to the small detector acceptance, the sampled pTdistribution shows a very modest dependence on the slope of the input pTdistribution, particularly at low pT(or r), which is most responsible for the dilution of the measured asymmetry. As a consequence, the variation of the correction factor Cϕdue to different slope parameters bdiscussed above was less than 1%. Figure 4and Table Isummarize the results for ANin forward neutron production in pþp,pþAl, and pþAu collisions, for ZDC inclusive, ZDC ⊗BBC-tag, and ZDC ⊗BBC-veto samples. In addition to the 3% scale uncertainty from polarization normalization, common to all points, the other part of the polarization uncertainty is correlated for different triggers in a particular collision system. The presented asymmetries in pþpcollisions are consistent with our previous publication [39], albeit with larger systematic uncertainties in these data due to a larger background (unlike this measurement, the charged veto counter was used in Ref. [39] to suppress the background) and larger variations due to the uncertainty of the beam position on the ZDC plane. From Fig. 4, the Adependence of ANfor inclusive neutrons is strong. Compared to the ANof pþpcollisions, the observed asymmetry in pþAl collisions is much smaller, while the asymmetry in pþAu collisions is a factor of 3 larger in absolute value and of opposite sign. This behavior is unexpected, because the theoretical framework using πand a1-Reggeon interference can predict only a moderate nuclear dependence, and there is no known mechanism to flip the sign of ANwithin this framework [34]. The asymmetries requiring BBC hits are remarkably different. Once BBC hits are required (ZDC ⊗BBC-tag), the drastic behavior of the inclusive ANvanishes and its sign stays negative, approaching AN¼0at large A.In contrast, the strong Adependence is amplified once no hits in the BBC are required (ZDC ⊗BBC-veto). While the BBCs cover a limited acceptance, the requirement (or veto) of hits in the BBC should place constraints on the activity near the detected neutron and thus the corresponding production mechanism. One possibility to explain the present results is a contribution from EM interactions, which have been demonstrated to be important for reactions with small momentum transfer, e.g., in ultraperipheral heavy ion collision at RHIC [44–47] and Large Hadron Collider [48–51], including forward neutron production in pþA collisions [52], and polarization observables in fixed target experiments [53,54]. Although it was ignored in the interpretation for the pþpdata [34], EM interactions become increasingly important for large atomic number (Z) (a) (b) FIG. 3. The rdistribution of the (a) ZDC ⊗BBC-tag sample and (b) ZDC ⊗BBC-veto sample for three collision systems. (atomic mass number)A 0 100 200 N A 0 0.2 0.4 uAlAp 3% scale uncertainty not shown =200 GeV NN s n+X at →+A ↑ p < 2.2 mradθ > 0.5, 0.3 < F x PHENIX ZDC inclusive BBC-tag⊗ ZDC BBC-veto⊗ ZDC FIG. 4. Forward neutron ANin pþAcollisions for A¼1(p), 27 (Al), and 197 (Au), for ZDC inclusive, ZDC ⊗BBC-tag, and ZDC ⊗BBC-veto triggered samples; color bars are systematic uncertainties, and statistical uncertainties are smaller than the marker size; the 3% scale uncertainty (not shown) is from the polarization normalization uncertainty. Data points are shifted horizontally for better visibility. PHYSICAL REVIEW LETTERS 120, 022001 (2018) 022001-6 nuclei, as the EM field of the nucleus is a rich source of virtual photons, increasing as Z2. Forward neutrons in the final state can be produced through nonresonant photo-πþ production and neutron decay channel from photonucleon excitation processes, such as the Δresonance [55]. According to a Monte Carlo study [52], the neutron and its associated πþproduced through this process are substantially boosted towards the proton beam direction, so that only a small fraction of pions would be detected by the BBC. Thus, a large fraction of EM processes are expected to be suppressed in the ZDC ⊗BBC-tag events while enhanced in the ZDC ⊗BBC-veto events. Here, it is noted that the importance of EM processes in pþAcollisions is also hinted at in the present data: The ratio between reconstructed neutrons in ZDC ⊗BBC-veto and ZDC ⊗BBC-tag samples increases from smaller than 0.5 in pþpto ∼1(∼5)inpþAl (pþAu) collisions. In addition, a faster drop of the neutron production cross section with pTin pþAcollisions in ZDC ⊗BBC-veto triggered data discussed in Fig. 3(b) is consistent with the increasing role of EM processes that have a softer pT distribution than hadronic processes. Similarly in the asymmetry measurements, contributions of different production mechanisms may be suppressed or enhanced by different event selection triggers. Hence, while the result for the ZDC ⊗BBC-tag sample may be explained by the conventional pion and a1-Reggeon interference mechanism [34], that for the ZDC ⊗BBCveto triggered sample could be explained by contributions from interference with EM amplitudes [55], which are expected to be enhanced in that data set. However, there could be other mechanisms, such as diffractive scattering, which is also expected to be enhanced by a ZDC ⊗BBCveto trigger. Therefore, further studies are needed to fully understand the present results. In summary, we observe an unexpectedly strong A dependence in ANof inclusive forward neutron production in polarized pþAcollisions at ffiffiffiffiffiffiffiffi sNN p¼200 GeV. Furthermore, a distinctly different behavior of ANwas observed in two oppositely trigger-enhanced data sets. These surprising behaviors could be explained by a contribution of EM interactions, which may be sizable for heavy nuclei. Further studies of the production mechanism including EM contributions and diffractive scattering would have an impact not only to hadron physics but also to cosmic-ray science, where measurements of high-energy cosmic rays depend on models of forward particle production in the interactions with nuclei in the air. Spin asymmetry measurements not only provide a unique discriminating power for the models of particle production but also will contribute to our understanding of the origin of the transverse spin asymmetries in hadronic collisions. We thank the staff of the Collider-Accelerator and Physics Departments at Brookhaven National Laboratory, especially the CA-D staff for providing beams with a special tune for these measurements, and the staff of the other PHENIX participating institutions for their vital contributions. We also thank Boris Kopeliovich and Michal Kˇ relina for providing us with theoretical calculations of the elastic proton cross sections and for useful discussions. We acknowledge support from the Office of Nuclear Physics in the Office of Science of the Department of Energy, the National Science Foundation, Abilene Christian University Research Council, Research Foundation of SUNY, and Dean of the College of Arts and Sciences, Vanderbilt University (USA), Ministry of Education, Culture, Sports, Science, and Technology and the Japan Society for the Promotion of Science (Japan), Conselho Nacional de Desenvolvimento Científico e Tecnológico and Fundação de Amparo `a Pesquisa do Estado de São Paulo (Brazil), Natural Science Foundation of China (People’s Republic of China), Croatian Science Foundation and Ministry of Science and Education (Croatia), Ministry of Education, Youth and Sports (Czech Republic), Centre National de la Recherche Scientifique, Commissariat `al’Énergie Atomique, and Institut National de Physique Nucl´eaire et de Physique des Particules (France), Bundesministerium für Bildung und Forschung, Deutscher Akademischer TABLE I. ANfor forward neutron production in pþp,pþAl, and pþAu collisions, for ZDC inclusive, ZDC ⊗BBC-tag, and ZDC ⊗BBC-veto samples. pþppþAl pþAu Inclusive BBC tag BBC veto Inclusive BBC tag BBC veto Inclusive BBC tag BBC veto AN−0.054 −0.064 −0.031 −0.013 −0.057 0.073 0.157 −0.015 0.234 Statistical uncertainty 0.001 0.002 0.004 0.002 0.003 0.003 0.002 0.005 0.002 Systematic uncertainty: Background 0.007 0.009 0.017 −0.001 −0.010 þ0.004 þ0.015 −0.003 þ0.012 Smearing 0.002 0.003 0.001 <0.001 0.002 0.003 0.007 <0.001 0.010 Beam position 0.009 0.006 0.010 0.004 0.004 0.006 0.002 0.004 0.008 Polarization <0.001 <0.001 <0.001 <0.001 0.002 0.003 0.011 0.001 0.017 Calibration 0.003 0.001 0.007 0.001 0.004 0.004 0.004 0.009 0.006 Total systematic 0.012 0.011 0.021 þ0.004 −0.004 þ0.007 −0.012 þ0.009 −0.008 þ0.020 −0.014 þ0.009 −0.010 þ0.025 −0.022 PHYSICAL REVIEW LETTERS 120, 022001 (2018) 022001-7 Austausch Dienst, and Alexander von Humboldt Stiftung (Germany), J. Bolyai Research Scholarship, EFOP, the New National Excellence Program (ÚNKP), NKFIH, and OTKA (Hungary), Department of Atomic Energy and Department of Science and Technology (India), Israel Science Foundation (Israel), Basic Science Research Program through NRF of the Ministry of Education (Korea), Physics Department, Lahore University of Management Sciences (Pakistan), Ministry of Education and Science, Russian Academy of Sciences, Federal Agency of Atomic Energy (Russia), VR and Wallenberg Foundation (Sweden), the U.S. Civilian Research and Development Foundation for the Independent States of the Former Soviet Union, the Hungarian American Enterprise Scholarship Fund, the U.S.–Hungarian Fulbright Foundation, and the U.S.–Israel Binational Science Foundation. *Deceased. †PHENIX spokesperson. [email protected] [1] D. d’Enterria, R. Engel, T. Pierog, S. Ostapchenko, and K. Werner, Constraints from the first LHC data on hadronic event generators for ultra-high energy cosmic-ray physics, Astropart. Phys. 35, 98 (2011). [2] K. H. Kampert and M. Unger, Measurements of the cosmic ray composition with air shower experiments, Astropart. Phys. 35, 660 (2012). [3] O. Adriani et al. (LHCf Collaboration), Measurements of longitudinal and transverse momentum distributions for neutral pions in the forward-rapidity region with the LHCf detector, Phys. Rev. D 94, 032007 (2016). [4] R. D. Klem, J. E. Bowers, H. W. Courant, H. Kagan, M. L. Marshak, E. A. Peterson, K. Ruddick, W. H. Dragoset, and J. B. Roberts, Measurement of Asymmetries of Inclusive Pion Production in Proton-Proton Interactions at 6 and 11.8GeV=c,Phys. Rev. Lett. 36, 929 (1976). [5] W. H. Dragoset, J. B. Roberts, J. E. Bowers, H. W. Courant, H. Kagan, M. L. Marshak, E. A. Peterson, K. Ruddick, and R. D. Klem, Asymmetries in inclusive proton-nucleon scattering at 11.75 GeV=c,Phys. Rev. D 18, 3939 (1978). [6] J. Antille, L. Dick, L. Madansky, D. Perret-Gallix, M. Werlen, A. Gonidec, K. Kuroda, and P. Kyberd, Spin dependence of the inclusive reaction pþpðpolarizedÞ→ π0þXat 24 GeV=c for high-pTπ0produced in the central region, Phys. Lett. 94B, 523 (1980). [7] S. Saroff et al., Single-Spin Asymmetry in Inclusive Reactions p↑þp→πþþX,π−þX, and pþXat 13.3 and 18.5GeV=c,Phys. Rev. Lett. 64, 995 (1990). [8] C. E. Allgower et al., Measurement of analyzing powers of πþand π−produced on a hydrogen and a carbon target with a22-GeV=c incident polarized proton beam, Phys. Rev. D 65, 092008 (2002). [9] D. L. Adams et al. (FNAL E704 Collaboration), Analyzing power in inclusive πþand π−production at high xðFÞwith a 200 GeV polarized-proton beam, Phys. Lett. B 264, 462 (1991). [10] D. L. Adams et al. (FNAL E581 and E704 Collaborations), First results for the two-spin parameter ALL in π0production by 200 GeV polarized protons and antiprotons, Phys. Lett. B 261, 197 (1991). [11] D. L. Adams et al. (FNAL E704 Collaboration), Measurement of single spin asymmetry in η-meson production in p↑pand ¯ p↑pinteractions in the beam fragmentation region at 200 GeV=c,Nucl. Phys. B510, 3 (1998). [12] I. Arsene et al. (BRAHMS Collaboration), Single-Transverse-Spin Asymmetries of Identified Charged Hadrons in Polarized pp Collisions at ffiffiffi s p¼62.4GeV, Phys. Rev. Lett. 101, 042001 (2008). [13] B. I. Abelev et al. (STAR Collaboration), Forward NeutralPion Transverse Single-Spin Asymmetries in pþpCollisions at ffiffiffi s p¼200 GeV, Phys. Rev. Lett. 101, 222001 (2008). [14] L. Adamczyk et al. (STAR Collaboration), Transverse single-spin asymmetry and cross section for π0and η mesons at large Feynman xin p↑þpcollisions at ffiffiffi s p¼200 GeV, Phys. Rev. D 86, 051101(R) (2012). [15] A. Adare et al. (PHENIX Collaboration), Measurement of transverse-single-spin asymmetries for midrapidity and forward-rapidity production of hadrons in polarized pþ pcollisions at ffiffiffi s p¼200 and 62.4 GeV, Phys. Rev. D 90, 012006 (2014). [16] S. Heppelmann et al. (STAR Collaboration), Large pT forward transverse single spin asymmetries of π0mesons at ffiffiffi s p¼200 and 500 GeV from STAR, Proc. Sci., DIS2013 (2013) 240. [17] A. Adare et al. (PHENIX Collaboration), Cross section and transverse single-spin asymmetry of ηmesons in p↑þp collisions at ffiffiffi s p¼200 GeV at forward rapidity, Phys. Rev. D90, 072008 (2014). [18] E. C. Aschenauer et al., The RHIC cold QCD plan for 2017 to 2023: A portal to the EIC, arXiv:1602.03922. [19] M. M. Mondal (STAR Collaboration), Measurement of the transverse single-spin asymmetries for π0and jet-like events at forward rapidities at STAR in pþp collisions at ffiffiffi s p¼500 GeV, Proc. Sci., DIS2014 (2014) 216. [20] D. Boer, A. Dumitru, and A. Hayashigaki, Single transverse-spin asymmetries in forward pion production at high energy: Incorporating small-xeffects in the target, Phys. Rev. D 74, 074018 (2006). [21] D. Boer and A. Dumitru, Polarized hyperons from pA scattering in the gluon saturation regime, Phys. Lett. B 556, 33 (2003). [22] D. Boer, A. Utermann, and E. Wessels, The saturation scale and its x-dependence from polarization studies, Phys. Lett. B671, 91 (2009). [23] Z.-B. Kang and F. Yuan, Single spin asymmetry scaling in the forward rapidity region at RHIC, Phys. Rev. D 84, 034019 (2011). [24] Y. V. Kovchegov and M. D. Sievert, New mechanism for generating a single transverse spin asymmetry, Phys. Rev. D 86, 034028 (2012). [25] J.-W. Qiu, in Proceedings of the RIKEN/RBRC Workshop: Forward Physics at RHIC, 2012, Upton, New York (Brookhaven National Laboratory, Upton, 2012), Vol. 111, p. 741. PHYSICAL REVIEW LETTERS 120, 022001 (2018) 022001-8