Cross section and asymmetry measurement of very forward … · 2015. 12. 18. · nent of the proton...
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Preliminary results of Aπ0
√s = 510 GeV at
mid-rapidity through a PHENIX experiment
I. Yoon∗1,∗2 for the PHENIX collaboration
One of the important functions of the relativisticheavy ion collider (RHIC) longitudinally polarized pro-ton program is to constrain the gluon-spin compo-nent of the proton (∆G) by measuring the doublehelicity asymmetry (ALL) of π
0 and jet production.Based on the results of deep inelastic scattering ex-periments, the quark-spin component of proton is only0.330 ± 0.011(Theo.) ± 0.025(Exp.) ± 0.028(Evol.).1)
The remaining spin might be carried by gluons or or-bital momentum. However, ∆G has not been mea-sured in detail as yet.2)
The measurement of ALL for π0 production (Aπ
s = 200 GeV has been successfully published andhas contributed to constraining ∆G.3) To explore thelower Bjorken x region, where dominant uncertaintyremains, longitudinally polarized proton-proton colli-sions in 2013 (Run13) were successfully carried outwith increased energy, i.e.
s = 510 GeV. Becauseof the increased energy, the measurement at
s = 510GeV can reach a lower x range, 0.01 < x, while theprevious measurement could reach only 0.02 < x. Themeasurement at
s = 510 GeV is ongoing and prelim-inary results have been released.
LL can be defined in terms of differences in cross-sections as
++ − dσπ0
++ + dσπ0+−
where σ++(+−) stands for π0 cross-section with the
same(opposite) helicity proton collisions. Because σπ0
can be described by the parton distribution function,the partonic reaction cross-section and fragmentationfunction and most of π0s are from quark-gluon orgluon-gluon scattering at mid-rapidity region, gluonhelicity distribution (∆g) is accessible by measuring
LL. This description is verified by comparing theπ0 cross-section between theoretical and experimentaldata.
Equation 1 can be rewritten in terms of experimentalobservables as
N++ +RN+−, R =
where PB(Y ) is the polarization of RHIC’s “Blue (Yel-low)” beam, N++(+−) is the yield of the π
0 candidatefrom the same (opposite) helicity collisions, and R isthe relative luminosity of the same and opposite helic-ity collisions.
∗1 RIKEN Nishina Center∗2 Seoul National University
As the collision rate increases in Run13, the effect ofmultiple collisions on one beam crossing becomes siz-able. The effect of multiple collisions is fully taken intoaccount for the relative luminosity measurement. Lu-minosity miscount by the finite resolution of the vertexwidth of luminosity detector is also considered. Theuncertainty of the relative luminosity is a dominantsystematic uncertainty of the measurement.
To reduce the combinatorial background in di-photon invariant distribution, several cuts are applied.Hits by a charged track, hadron, and previous cross-ings are rejected. To evaluate ALL for the remainingbackground, ALL of the background is also measuredand is subtracted to obtain physical asymmetry.Fig. 1 shows preliminary results of Aπ
LL measure-ment at
s = 510 GeV. The result covers 2 GeV/c< PT < 20 GeV/c (0.008 < xT < 0.08, where xT =2PT /
s). The DSSV14 theory curve is shown andit agrees with experimental data within uncertainty.The measurement at
s = 200 GeV is also shown forcomparison. Larger asymmetry is observed at highercollision energy.
Fig. 1. Preliminary result of Aπ0
510 (red line) and comparison with Aπ0
s = 200
GeV3) (blue line). DSSV14 theory curves2) are shown.
The grey and yellow bands indicate systematic uncer-
tainty due to the uncertainty of relative luminosity.
Global-scale uncertainty due to systematic uncertainty
of polarization is not shown. Open boxes denote point-
to-point systematic uncertainty.
Further analysis is ongoing to obtain the final result.
References1) A. Airapetian et al.: Phys. Rev. D 75, 012007 (2007).2) D. de Floran, R. Sassot, M. Stratmann and W. Vogel-
sang: Phys. Rev. Lett. 113. 012001 (2014).3) A. Adare et al.: Phys. Rev. D 90, 012007 (2014).
Cross section and asymmetry measurement of very forward neutralparticle production at RHIC
Y. Goto∗1 for the RHICf Collaboration
Although air shower observations at the surface ofthe earth have been carried out to understand the ori-gin of the ultrahigh energy cosmic rays, these obser-vations have uncertainties in the interpretation of theobserved data from the present phenomenological nu-clear collision models. The LHCf experiment has beenperformed at LHC for understanding cosmic ray gener-ation from the collider experiment data1–3). Precisionmeasurements of the very forward particle productionin the collider experiments improve the understandingof particle production processes in the nuclear colli-sions, and largely affect the interpretation of the ob-served data and the origin of the cosmic rays.
A large 10% single transverse-spin asymmetry(SSA) in neutron production from transversely polar-ized proton collisions was found at RHIC in 20024).This provides an important clue to study elementaryprocesses in air shower generation because the largeSSA indicates that there are dominant contributionsfrom processes that strongly interfere with each other.Because the SSA measurement of neutron productionprovides interference measurement with one pion ex-change with a spin flip, it is sensitive to a mesonexchange without spin flip5). Although the SSA ofvery forward neutrons at collision energies of 62 GeV,200 GeV, and 500 GeV was measured at RHIC, thetransverse-momentum resolution of the data was lim-ited.
We will perform a new collider experiment at RHIC,the so called RHICf experiment, which uses a LHCf de-tector (which is called the RHICf detector) with a highresolution and wide coverage of transverse-momentummeasurements in order to improve the studies per-formed at RHIC and LHC. RHIC is a dedicated ma-chine for QCD physics and it allows flexible operationto achieve our physics goals. In the RHICf experi-ment, we will have an improved transverse-momentumresolution using the RHICf detector. The LHCf ex-periment has measured very forward neutral particlecross section at collision energies of 0.9 TeV and 7 TeV.We will obtain precision measurements of very forwardneutral particles (neutron, photon, neutral pion) at acollision energy of 510 GeV at RHIC. These data ofvarious collision energies will provide an understand-ing of air shower generation and the limitations in theorigin of the ultrahigh energy cosmic rays.
The RHICf detector will be located in front of theZero-Degree Calorimeter (ZDC) in the north side, 18m from the PHENIX collision point. In this area, allcharged particles are swept out by the last dipole mag-
∗1 RIKEN Nishina Center
Fig. 1. Schematic view of the RHICf detector installation
and the structure of the calorimeter.
net for colliding beams at RHIC, and only neutral par-ticles are detected. The RHICf detector consists oftwo sampling EM (electromagnetic) calorimeters with44 interaction-length tungsten, 16 layers of scintilla-tors, and 8 layers of silicon strip detectors; one with25 mm × 25 mm and another with 32 mm × 32 mmcross sectional area. The ZDC detector is a samplinghadron calorimeter composed of Cu-W alloy absorberswith PMMA (Polymethyl methacrylate)-based opticalfibers. The detection, identification, and energy mea-surement of photons and neutrons are performed usingEM and hadron calorimeters. A schematic view of theRHICf detector installation and the structure of thecalorimeter are shown in Fig. 1.
We have proposed to perform a dedicated run for1 week in 2016 with 510 GeV polarized proton colli-sions6), and we are preparing for it. In the dedicatedRHICf run, we will use the normal accelerator con-dition of the RHIC, except the beta function valueat the PHENIX collision point, β∗ = 10 m, in orderto obtain parallel beam collisions. We need 12 h toachieve sufficient luminosity for cross section measure-ments of photons, neutrons, and neutral pions includ-ing the transverse-momentum scan. The SSA can bemeasured with the same data set with higher statisticsthan the measurements in the past.
References1) O. Adriani et al., Phys. Lett. B 715, 298 (2012).2) O. Adriani et al., Phys. Rev. D 86, 092001 (2012).3) O. Adriani et al., Phys. Lett. B 703, 128 (2011).4) Y. Fukao, M. Togawa et al., Phys. Lett. B 650, 325
(2007).5) B. Z. Kopeliovich, I. K. Potashnikova, I. Schmidt and
J. Soffer, Phys. Rev. D 84, 114012 (2011).6) Y. Itow et al. [RHICf Collaboration], arXiv:1409.4860
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Ⅱ-4. Hadron Physics RIKEN Accel. Prog. Rep. 48 (2015)RIKEN Accel. Prog. Rep. 48 (2015) Ⅱ-4. Hadron Physics
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