The STAR collaboration reported precision measurements on the transverse
single spin asymmetries for the production of forward $\pi^0$ mesons from
polarized proton collisions at $\sqrt{s}=$ 200 GeV. To disentangle
contributions to measured forward asymmetries one has to look beyond inclusive
$\pi^0$ production to the production of forward jets or direct photons. In
2006, STAR with the Forward Pion Detector++ (FPD++) in place, collected 6.8
pb$^{-1}$ of forward data with an average polarization of 60%. FPD++ had
sufficient acceptance for "jet-like" objects, which are clustered responses of
an electromagnetic calorimeter primarily sensitive to incident photons,
electrons and positrons. For these objects, the angle of the outgoing leading
$\pi^0$ with respect to the fragmenting parton was reconstructed, thus enabling
us to disentangle the contributions to the forward $\pi^0$ asymmetries. The
simulated data set shows that on average there are approximately 2.5
fragmenting mesons per one "jet-like" object, making them reasonably "jetty".
Preliminary results provide no evidence of measured contributions to the
asymmetry from jet fragmentation, implying the Sivers distribution functions
play a substantial role in producing the large inclusive forward $\pi^0$
asymmetries. A similar effort was made in the mid-rapidity $(|\eta|<1)$ region
of the STAR detector, where 2.2 pb$^{-1}$ of data was collected. We present
progress made by making measurements of the azimuthal asymmetry of leading
charged pions in jets produced by transversely polarized proton collisions.Comment: 6 pages, 4 figures, proceedings of the Transversity 2011 worksho

Poljak, N.

English

arXiv

The STAR Collaboration reported precision measurements on the transverse-single-spin asymmetries for the production of forward π0 mesons from polarized proton collisions at

√s = 200 GeV. To disentangle contributions to measured

forward asymmetries one has to look beyond inclusive π0 production to the production of forward jets or direct photons. In 2006, STAR with the Forward Pion Detector++ (FPD++) in place, collected 6.8 pb−1 of forward data with an average polarization of 60%. FPD++ had sufficient acceptance for “jet-like” objects, which are clustered responses of an electromagnetic calorimeter primarily sensitive to incident photons, electrons and positrons. For these objects, the angle of the outgoing leading π0 with respect to the fragmenting parton was reconstructed, thus

enabling us to disentangle the contributions to the forward π0 asymmetries. The simulated data set shows that on average there are approximately 2.5 fragmenting mesons per one “jet-like” object, making them reasonably “jetty”. Preliminary results provide no evidence of measured contributions to the asymmetry from jet fragmentation, implying the Sivers distribution functions play a substantial role in producing the large inclusive forward π0 asymmetries. A similar effort was made in the 

mid-rapidity (|η| < 1) region of the STAR detector, where 2.2 pb−1 of data was collected. We present progress made by making measurements of the azimuthal asymmetry of leading charged pions in jets produced by transversely polarized proton collisions

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DOI 10.1393/ncc/i2012-11199-3Colloquia: Transversity 2011IL NUOVO CIMENTO Vol. 35 C, N. 2 Marzo-Aprile 2012STAR results and perspectives on transverse-spin asymmetriesN. Poljak for the STAR CollaborationUniversity of Zagreb - Zagreb, Croatiaricevuto il 5 Novembre 2011; approvato il 21 Dicembre 2011pubblicato online il 23 Marzo 2012Summary. — The STAR Collaboration reported precision measurements on thetransverse-single-spin asymmetries for the production of forward π0 mesons frompolarized proton collisions at√s = 200GeV. To disentangle contributions to mea-sured forward asymmetries one has to look beyond inclusive π0 production to theproduction of forward jets or direct photons. In 2006, STAR with the Forward PionDetector++ (FPD++) in place, collected 6.8 pb−1 of forward data with an aver-age polarization of 60%. FPD++ had sufficient acceptance for “jet-like” objects,which are clustered responses of an electromagnetic calorimeter primarily sensitiveto incident photons, electrons and positrons. For these objects, the angle of theoutgoing leading π0 with respect to the fragmenting parton was reconstructed, thusenabling us to disentangle the contributions to the forward π0 asymmetries. Thesimulated data set shows that on average there are approximately 2.5 fragment-ing mesons per one “jet-like” object, making them reasonably “jetty”. Preliminaryresults provide no evidence of measured contributions to the asymmetry from jetfragmentation, implying the Sivers distribution functions play a substantial role inproducing the large inclusive forward π0 asymmetries. A similar effort was madein the mid-rapidity (|η| < 1) region of the STAR detector, where 2.2 pb−1 of datawas collected. We present progress made by making measurements of the azimuthalasymmetry of leading charged pions in jets produced by transversely polarized pro-ton collisions.PACS 13.88.e+ – Polarization in interactions and scattering.PACS 13.85.Ni – Inclusive production with identified hadrons.PACS 14.20.Dh – Protons and neutrons.1. – IntroductionFor a particle produced in a collision of transversely polarized protons, the analyzingpower AN is equal to the difference of spin-up and spin-down cross sections divided bytheir sum. AN is just one example of a transverse-single-spin asymmetry, which is ex-pected to be near zero in a leading-twist collinear perturbative QCD description of parti-cle production [1]. In the high-rapidity region, the measured cross sections for productionc© Societa` Italiana di Fisica 193194 N. POLJAK for the STAR COLLABORATIONof neutral pions (π0) produced with large Feynman-x (2pL/√s) and moderate pT in ppcollisions are found to be in agreement with next-to-leading order pQCD calculations at√s = 200GeV and are included in global fits on fragmentation functions (FFs) [2, 3].“Jet-like” structures were found in two-particle correlations involving a forward pion, asexpected from pQCD. Precision measurements of the forward π0 asymmetry as a functionof xF and pT were reported, showing large AN at large xF [4]. The measured xF depen-dence is in qualitative agreement with the Sivers effect [5, 6] expectations. Theoreticalunderstanding of the forward π0 production data continues to evolve [7].Several classes of models try to explain the observed asymmetries. Simulations indi-cate that the kinematics of observed events come from processes that reside firmly in theTransverse Moment Distribution (TMD) regime. Hence, we will only discuss two possi-ble contributions to the observed asymmetries within the TMD framework. The Siverseffect manifests itself as an asymmetry in the forward jet or gamma production whilethe Collins effect [8] manifests itself as an asymmetry in the forward jet fragmentation.Both effects introduce a transverse scale kT to produce the observed asymmetries. TheCollins mechanism introduces kT in the FFs, making it sensitive to correlations of thehadron transverse momenta and parton transversity. To distinguish between the mech-anisms, one has to look beyond inclusive π events to direct gamma or “jet-like” objects.Since the Collins effect is a spin-dependent azimuthal modulation of hadrons around thethrust axis of an outgoing quark, integrating over the full azimuthal angle would cancelit, leaving only the Sivers effect responsible for any measured asymmetry.The forward (〈η〉 ≈ 3.3) electromagnetic detector present during the RHIC runningin the year 2006 was specifically designed to have sufficient acceptance for “jet-like”objects. A “jet-like” object is a clustered response of an electromagnetic calorimeterprimarily sensitive to incident photons, electrons and positrons. Theoretical predictionsfor the estimated π0 Collins contribution to the asymmetry for the observed process(pp → π0 + “jet-like” + X) show it to be negligible [9]. Due to isospin symmetry the π0FFs are half the sum of those for charged pions (which are similar in size and of oppositesign), making the π0 Collins FF very small. The data obtained were used to separatethe Sivers/Collins contributions to the asymmetries observed for the π0 events.A similar exploratory analysis was done at the mid-rapidity region (|η| < 1) of theSTAR detector, which is well suited for full jet reconstruction. Observation of the az-imuthal distribution of charged pions inside hadronic jets would in a similar way allowsone to find the Collins moment of any measured asymmetries.2. – Experimental setupThe forward STAR calorimeters prior to 2006 measured the inclusive π0 cross sectionas well as the single beam spin asymmetry for their inclusive production. In 2006 thedetectors placed on the West STAR platform were upgraded to a detector called theForward Pion Detector ++ (FPD++) (fig. 1). A detailed discussion of the detectorsetup and calibration can be found in [10].The mid-rapidity (0 < φ < 2π,−1 < η < 2) section of the STAR detector (fig. 1)consists of a large solid angle detector well-suited for full jet reconstruction [11]. TheTime-Projection Chamber (TPC) provides charged particle identification and momen-tum measurement [12]. The Barrel and Endcap Electromagnetic Calorimeters (BEMC,EEMC) detect neutral particles and serve for triggering [13, 14]. TPC and EMC ac-ceptances allow charged particle and jet detection over a pseudorapidity |η| < 1 in themid-rapidity analysis. The Beam-Beam Counter (3.3 < |η| < 5) measures polarizationdirection, luminosity and provides a minimum bias trigger.STAR RESULTS AND PERSPECTIVES ON TRANSVERSE-SPIN ASYMMETRIES 195Fig. 1. – Left panel: The schematic view of the front face of the FPD++ forward electro-magnetic calorimeter. Right panel: The schematic view of the mid-rapidity part of the STARdetector, showing the STAR coordinate system and four of the essential detector components.The FPD++ is located to the right of the EEMC, outside of the mid-rapidity part.To aid the understanding of the unpolarized results, full PYTHIA/GEANT [15] sim-ulations have been made with adequate statistics. In forward analysis, we used PYTHIA6.222, which predates tunings related to “underlying event” for midrapidity Tevatrondata, since these tunings impact forward production at RHIC energies. The newer ver-sions of PYTHIA, used for mid-rapidity analysis, reduce agreement between data andsimulation in the forward region. Energy deposition computed by GEANT for PYTHIAgenerated events was digitized and was run through the same algorithms as the data.3. – ResultsIn the analysis of the forward data, we formed “jet-like” clusters in an event byconsidering energy depositions > 0.4GeV in all FPD++ cells. A cluster consists of Ncells, where N is the maximal subset of cells found to be within a cone of radius 0.5 in η-φspace. Assuming that the energy deposition is from photons originating from the collisionvertex, the xF and pT for the cluster are given by the vector sum of momenta from eachcell. Clusters with at least 10 cells having cluster pT > 1.5GeV/c and xF > 0.23 arerequired in the analysis. A requirement that the “jet-like” cluster centroid is within thecalorimeter volume by at least two large cell widths is also imposed.The energy deposition profiles and the invariant mass distributions of the “jet-like”objects were found both in data and simulations. Throughout the xF range of the eventsamples the properties of the “jet-like” objects in the data and the simulations comparedwell. A subset of data containing both a reconstructed π0 and a “jet-like” object in asingle event was extracted. Events from this subset were characterized with the help ofPYTHIA, showing that on average the π0 carries ≈ 90% of the energy of the “jet-like”object (fig. 2). Further, the reconstructed “jet-like” thrust axis (reconstructed from theenergy deposition in the calorimeter) agrees well with the direction of either a hard-scattered parton or a radiated parton. On average, there are 2.5 fragmenting mesons perone “jet-like” object, making the objects reasonably jetty.196 N. POLJAK for the STAR COLLABORATIONFig. 2. – Left panel: The fraction of the event energy carried by the leading π0 in data andsimulations. Right panel: The component of the π0 momentum perpendicular to the “jet-like”object axis (kT ). In both panels the error bars are statistical.The component of the π0 momentum perpendicular to the “jet-like” object axis (kT )was found to be in the domain of TMD fragmentation (fig. 2). Systematic studies of the“jet-like” object model ensured that no special point in the model parameter space wasselected. By changing the model parameters by 10% both on data and simulations, wefound that the changes in the unpolarized results are small and smooth. The data andsimulations follow the same trends when changing a single parameter.To separate the Collins/Sivers contributions to the measured asymmetries, we definedthe angle γ as the azimuthal angle of the π0 with respect to the reaction plane (fig. 3).Fig. 3. – Left panel: Schematic view of the γ angle in the right detector module. Right panel:The dependence of the asymmetry on the γ angle. The lines on the points represent statisticalerrors and the gray area the systematic errors.STAR RESULTS AND PERSPECTIVES ON TRANSVERSE-SPIN ASYMMETRIES 197Fig. 4. – The dependence of the measured mid-rapidity Collins asymmetry on z and jT . Thelines on the points represent statistical errors and the gray area the systematic errors. Detailsof the Collins moment calculation (on the vertical axis) can be found in [16].γ is defined mirror symmetrically (clockwise for the left module, counterclockwise for theright module), so that for both detector modules γ ≈ 0 corresponds to π0 having largerpT than the “jet-like” object. For a given bin in γ defined this way, an event reconstructedin the left module from an up-polarized proton measures the same fragmentation as anevent reconstructed in the right module from a down-polarized proton.The unpolarized γ angle distribution was found both in data and simulations andshows reasonable agreement [10]. The peaking near γ = 0 in the shape of the distributionis a detector acceptance effect. In most cases both the π0 and the thrust axis of the“jet-like” object are reconstructed in the small cells. The χ2 for fitting a constant tothe spin-averaged γ distribution systematically decreases in an analysis that restrictsthe acceptance to have the thrust axis of the “jet-like” object increasingly centered ona calorimeter module. Consequently, the shape of the spin-averaged γ distribution isdue to finite detector acceptance. The pT dependence of the “jet-like” production crosssection strongly favors low pT events. Coupled with detector acceptance, this enhancesthe number of events with γ ≈ 0.The dependence of the asymmetry on the γ angle was found. The data was separatedinto 4 equally sized bins in cos(γ). For each of the bins, the asymmetry was calculatedby means of a cross-ratio formula:ANf(γ) =√N↑LN↓R −√N↓LN↑R√N↑LN↓R +√N↓LN↑R.(1)Here, N stands for the number of measured events in a given bin, for a given module (Lor R) and a given polarization direction (↑ or ↓). By forming a geometric mean in each γbin, the detector effects are minimized. On the plot of AN vs. cos(γ) (fig. 3) the Collinscontribution is proportional to the slope parameter, since then the fragmentation is γangle-dependent. If the slope is found to be consistent with zero, while the asymmetryis larger than zero, one has isolated the Sivers effect, since fragmentation is not involved,and one is left with just the initial state kT . On the figure, the lines on the points representstatistical errors and the black area the systematic errors. The xF > 0 and the xF < 0points have been slightly horizontally shifted to improve visibility. The asymmetry for198 N. POLJAK for the STAR COLLABORATIONnegative xF values is consistent with zero, while the positive xF values show a positiveasymmetry with a significance of 1σ, but no dependence on cos(γ). Hence, the Collinseffect is not found to be present in the production of forward neutral pions.To reconstruct jets from the mid-rapidity data, the Midpoint Cone Algorithm wasused. A TPC track or EMC tower with an energy of 0.5GeV or become the seed fora jet with cone radius of 0.7 in the η-φ space. Event triggers required a minimumADC sum threshold for one of 12 separate “jet patches”. A cut of pT > 10GeV isimposed to optimize access to quark-scattering events. Charged pions were identified inthe TPC. Cuts on the number of track points and the distance of closest approach totheir common vertex define an acceptable charge track. A cut of fixed width in nσ, theGaussian distribution of log(dE/dx) about a momentum-dependent centroid, is used toseparate charged pions from other charged particles. In the analysis, only charged pionsthat are leading particles are kept. Only jet triggers in the forward half of the detectorwith respect to either RHIC polarized beam are analyzed. The Collins moment of theasymmetry (fig. 4) is measured with respect to either kT (denoted by jT in this analysis)or z(= pπ/pjet). The measurements have large uncertainties and we expect significantimprovements in the near future. Further details of the analysis can be found in [16].4. – Conclusion and outlookThe FPD++ was used to analyze forward “jet-like” objects. The data show goodagreement with the simulated sample of events. The “jet-like” objects were characterizedand were shown to be “jetty”. For the event sample containing both a reconstructed π0and a “jet-like” object, the γ angle and kT were found. The kT was shown to be inthe domain of TMD fragmentation. Preliminary results from the correlation analysisof pions and “jet-like” objects provide no evidence for a Collins signature. Analysis ofmid-rapidity jets has large uncertainties, expected to be improved in the near future.For the forward rapidity data, we aim to open up the π0 acceptance from small cellsto large and small cells in both calorimeter modules, thereby increasing the statistics ofthe sample and further addressing the γ distribution shape. We aim to improve the mid-rapidity uncertainties with inclusion of additional simulation statistics and new analysismethods.REFERENCES[1] Kane G. L., Pumplin J. and Repko W., Phys. Rev. Lett., 41 (1978) 1689.[2] Adams J. et al., Phys. Rev. Lett., 97 (2006) 152302.[3] de Florian D., Sassot R. and Stratmann M., Phys. Rev. D, 75 (2007) 114010.[4] Abelev B. I. et al., Phys. Rev. Lett., 101 (2008) 222001.[5] Sivers D. W., Phys. Rev. D, 41 (1990) 83.[6] Sivers D. W., Phys. Rev. D, 43 (1991) 261.[7] Kang Z., Qiu J., Vogelsang W. and Yuan F., Phys. Rev. D, 83 (2011) 094001.[8] Collins J., Nucl. Phys. B, 396 (1993) 161.[9] D’Alesio U., Murgia C. and Pisano F., Phys. Rev. D, 83 (2011) 034021.[10] Poljak N., J. Phys. Conf. Ser., 295 (2011) 012102.[11] Ackermann K. H. et al., Nucl. Instrum. Methods A, 499 (2003) 624.[12] Anderson M. et al., Nucl. Instrum. Methods A, 499 (2003) 659.[13] Beddo M. et al., Nucl. Instrum. Methods A, 499 (2003) 725.[14] Allgower C. E. et al., Nucl. Instrum. Methods A, 499 (2003) 740.[15] Sjostrand T., Lonnblad L. and Mrenna S., arXiv:hep-ph/0108264v1 (2001) 1-425.[16] Fersch R., J. Phys. Conf. Ser., 295 (2011) 012048.

STAR results and perspectives on transverse spin asymmetries

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