7th th In Inter ernational l Workshop on on Multi ltiple Part rtonic In Interactions at t th the LH LHC . Preview fr from RHIC Run 15 p-p and p-Au Forw rward Neutral Pio ion Pro roduction fr from Transversely Polarized Pro rotons Steve Heppelmann * Penn State University ( STAR) STAR * Supported by NSF 1
Transverse Single Spin Asymmetries (TSSA) A N A N Scattering Process Factorizes into 3 parts 1) Parton distribution : Select a quark from the incident proton and a parton from the target proton 2) Hard scattering: Scatter the quark from a parton in the target proton does not depend on transverse spin. 3) Universal Jet Fragmentation : Color neutralize the scattered quark , pulling partons from one of the protons Possible sources of non-zero A N : 1) “Sivers Effect” with Transverse Spin Dependent initial parton momentum components.. 2) “Collins Effect” with Transverse Spin Dependent Fragmentation. 2
RHIC Collisions at STAR, between Polarized Protons and Pol Polariz ized Pr Proto tons ns or Nucl clei • The hard parton cross sections do depend on the longitudinal spins of colliding partons. • The hard parton cross sections do not depend on the transverse components of parton spin for two reasons. 1. Dependence of scattering amplitude on transverse spin implies helicity flip amplitudes. 2. Dependence of the cross section on transverse spin implies interference between amplitudes of different phases. Leading twist amplitudes do not provide the required STAR phases changes. • Dependence of hard cross sections on transverse spin does not come from the hard parton cross section but is expected to involve initial and final state or “higher twist” effects. 3
* Parton Momentum Direction Proton Momentum Direction * Transverse Momentum Dependent Parton Distributions (TMD) Spin Dependent Fragmentation • Parton Angular Momentum • Wilson Line for phase change Sivers Collins / 0 q / P P P P P 1993 1990 parton T T T T T Higher P P T A Twist T A N N 2 P 2 P 1991 T T 1 A N 2 P T
p p X • 0 – E704, PLB261 (1991) 201. • +/- - E704, PLB264 (1991) 462 . s=20 GeV, p T =0.5-2.0 GeV/c: 0 , ( Large X up quark scattering ) F ( Large X downquark scattering ) F Fermi Lab Fixed Target Energies Strong historical evidence that forward pion production transverse polarized pion production reflects the i nteractions of large momentum “u” and “d” quarks correlated with the transverse spin of the proton. 5
The FMS is illuminated by forward scattering Cone = 35mR Nphotons=2 From the RHIC blue beam 0 A Z<.8 N and backward scattering from M<0.4 GeV the yellow beam. No significant Esoft < 0.5 backward asymmetry is seen. STAR Run 12 Preliminary s =200 GeV STAR 6
RHIC Run 12 2012 STAR FMS @ s=200 GeV Selection : N 2( in cone ) STAR Run 12 Preliminary photons E 6 GeV & E 6 GeV 1 2 E E 2 1 Z 0.7 E E 2 1 STAR Run 12 Preliminary 2 M 0.4 GeV c 1,2 E 0.5 GeV soft Cone : 35 mR 35 mR =2.8 =3.1 2 Mass GeV c 2 Mass GeV c =3.4 =3.7 STAR STAR Run 12 Preliminary STAR Run 12 Preliminary 2 2 Mass GeV c Mass GeV c Examples of Run 12 Mass Distributions: 7 35<E<55GeV, four pseudo-rapidity ( ) regions.
A N vs. Energy, averaged over pseudo-rapidity . Compare 3 selection criteria based on presence of 2 nd photon energy (>6 GeV) outside the cone ( 35mR cone) STAR Run 12 Preliminary s =200 GeV For 0 ‘s with X F <0.45: Events with “opposite side” photons or “no” photons have similar A N . Same side photons lead to much reduced A N . For 0 ‘s with X F >0.45: Observation of additional Photons reduce A N . STAR 8
STAR FMS Run Run 11 (2 (2011) ) 5 500 GeV tr transverse pola larized pp. t Energy for π 0 s and jet-like multiple photon events. A N vs. s. EM-Je Jet π 0 -Jets – 2 photon-EM-Jets with M γγ <0.3 Z γγ <0.8 EM-Jets – with no. photons >2 STAR 9
Newest STAR FMS Data Transversely ( s= polarized p-p and p-Au ( s=200 GeV) Run 15 (2 (2015) Event Selection (inclusive: 0 + X) 1) Collect photons within 35 mR cones. 2) 0 mass |M-.135|< 0.12 GeV 3) P T (transverse momentum) and E (energy) Bins 4) For photon pair, Z<.7 (Z=|E photon1 -E photon2 )/(E photon1 +E photon2 )|) 2) Beam Beam Counter (BBCE) cuts (gold or away side proton breakup cut) 3) Require P T above trigger threshold. STAR 10
What we need to learn from new p-Au RHIC Run 15 (2015) 1. Correlating TSSA A N with other observables like - R pA - Fragmentation universality. - Collision centrality. 2. Do the surprising aspects of A N seen in pp persist in pA scattering or are they “Filtered” away. - Surprising transverse momentum dependence of A N . - Surprising increase in A N with more exclusive production. 11
TSSA A N : Dependence on p-Au Gold Breakup Multiplicity (perhaps related to centrality) East (Au direction) multiplicity and summed photo-multiplier signals in Beam-Beam Counter (BBC) Initial state Consider the dependence on the distribution of summed photo-tube light from 16 small cells of BBC Final state To BBC East 0 to FMS West STAR Red circles on both the FMS and the BBC scintillator tiles shown the loation of pseudo-rapidy = 3.3. 12
The p-Au Asymmetry depends upon BBC The TSSA A N is obtained from selected 0 events in p-Au collisions. charged particle distribution from gold breakup This Example with 0 within (0.55<X F <0.65) and (2.55GeV <p T [GeV/c]<3.05) in the East BBC (and to lesser extent similar away side proton breakup in pp collisions) N N Raw A in 10 Cos( ) bins For now, that will be included as a systematic N N uncertainty in the measured A N and is the Raw A ( ) P P Cos 0 1 dominant systematic uncertainty. This P dependence will be fully characterized in the 1 A N Beam Polarization future. 13 STAR
STAR RHIC Run 15: (2015) s=200 GeV A N pp A N pp A N pp A N pAu A N pAu A N pAu 0 p p X 0 p Au X TSSA A N STAR Run 15 STAR Run 15 Inclusive 0 event selection STAR Run 15 s=200 GeV s=200 GeV s=200 GeV described above Preliminary Preliminary Preliminary Error bars represent statistical errors only. Luminosity pAu=204.6 nb -1 Luminosity pp=34.8 pb -1 Average Polarization: A N pp A N pp A N pp pp 55.6 . 2 % A N pAu A N pAu A N pAu pAu 60.4 2% Shaded bands represent systematic uncertainty, dominated by dependence of A N on observed East BBC energy STAR Run 15 (gold or proton breakup charge STAR Run 15 STAR Run 15 s=200 GeV s=200 GeV s=200 GeV multiplicity) Preliminary Preliminary Preliminary STAR 14 14
Run 15 2015 pp s=200 GeV Data FMS 0 + 1 EM Cluster ( Cluster Energy>3 GeV) Example showing suppression of 0 A N for 2 nd EM Cluster Distribution in (pseudo-rapidity) vs. (azimuthal angle) jet – like events. This shows 2 photon cluster FMS events , with a 0 (0.25<X F <0.35) Relative to 0 Direction - 0 Second E&M photon cluster (E>3 GeV), STAR Run 15 Preliminary outside the primary 35 mR 0 cone. 2 nd EM Photon Cone Cluster Distribution - 0 < 100 mRad STAR Run 15 Preliminary 0 P T (GeV/c) STAR 15
Distribution of Event with 2 EM Energy Cone Clusters Cone radius=35mR Event Distribution for Two FMS Event Distribution for Two FMS Clusters in 2015 p-Au. Clusters in 2015 p-p. First cluster contains 0 0.25 X 0.35 F 0 ( ) 3.55 GeV c p 4.05 GeV c T 0 ( ) for 2 nd cluster momentum Direction relative to 0 direction (pseudo-rapidity) vs. (azimuthal angle) 16 STAR Run 15 Preliminary STAR Run 15 Preliminary
Comparison of 0 A N for second Cone of energy “Near” or “Far” from 0 STAR Run 15 p-Au x F = 0.3. s=200 GeV STAR Run 15 p-p x F = 0.3. s=200 GeV Dependence of 0 A N on the location of second Dependence of 0 A N on the location of second forward EM particle in FMS. forward EM particle in FMS. 2 nd EM Cone Cluster 2 nd EM Cone Cluster (E>3 GeV) (Angle >100 mR from 0 ) (E>3 GeV) (Angle >100 mR from 0 ) 2 nd EM Cone Cluster 2 nd EM Cone Cluster (E>3 GeV) (Angle <100 mR from 0 ) (E>3 GeV) (Angle <100 mR from 0 ) STAR Run 15 pp Preliminary STAR Run 15 pAu Preliminary Dominant errors are statistical Dominant errors are statistical 0 P T ( GeV/c ) 0 P T ( GeV/c ) STAR
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