Spin 2004 Trieste October 14, 2004 Alessandro Bravar Spin Dependence in Elastic Scattering in the CNI Region: p  p  pp & p  C  pC A. Bravar, I. Alekseev, G. Bunce, S. Dhawan, R. Gill, H. Huang, W. Haeberli, G. Igo, O. Jinnouchi, A. Khodinov, K. Kurita, Z. Li, Y. Makdisi, A. Nass, H. Okada, S. Rescia, N. Saito, H. Spinka, E. Stephenson, D. Svirida, D. Underwood, C. Whitten, T. Wise, J. Wood, A. Zelenski

Spin 2004 Alessandro Bravar RHIC beams + internal targets  fixed target mode  s ~ 14 GeV The Elastic Process: Kinematics recoil proton or Carbon (polarized) proton beam scattered proton polarized proton target or Carbon target essentially 1 free parameter: momentum transfer t = (p 3 – p 1 ) 2 = (p 4 – p 2 ) 2 <0 + center of mass energy s = (p 1 + p 2 ) 2 = (p 3 – p 4 ) 2 + azimuthal angle  if polarized !  elastic pp kinematics fully constrained by recoil proton only !

Spin 2004 Alessandro Bravar Helicity Amplitudes for spin ½ ½  ½ ½ Scattering process described in terms of Helicity Amplitudes  i All dynamics contained in the Scattering Matrix M (Spin) Cross Sections expressed in terms of spin non–flip double spin flip spin non–flip double spin flip single spin flip ?  M  identical spin ½ particles formalism well developed, however not much data ! only A N studied / measured to some extent observables: 3  -sections 5 spin asymmetries

Spin 2004 Alessandro Bravar The Very Low t Region around t ~  10  3 (GeV/c) 2 A hadronic  A Coulomb  INTERFERENCE CNI = Coulomb – Nuclear Interference scattering amplitudes modified to include also electromagnetic contribution hadronic interaction described in terms of Pomeron (Reggeon) exchange electromagnetic single photon exchange  = |A hadronic + A Coulomb | 2 unpolarized  clearly visible in the cross section d  /dtcharge polarized  “left – right” asymmetry A N magnetic moment + P 

Spin 2004 Alessandro Bravar the left – right scattering asymmetry A N arises from the interference of the spin non-flip amplitude with the spin flip amplitude (Schwinger) in absence of hadronic spin – flip contributions A N is exactly calculable (Kopeliovich & Lapidus): hadronic spin- flip modifies the QED “predictions” interpreted in terms of Pomeron spin – flip and parametrized as A N & Coulomb Nuclear Interference  1) p  pp had A N (t)

Spin 2004 Alessandro Bravar can be traced back to

Spin 2004 Alessandro Bravar Some A N measurements in the CNI region pp Analyzing Power no hadronic spin-flip -t A N (%) p = 200 GeV/c PRD48(93)3026 p = 21.7 GeV/c PRL89(02) with hadonic spin-flip no hadronic spin-flip pC Analyzing Power r 5 pC  F s had / Im F 0 had Re r 5 =  Im r 5 =   highly anti-correlated

Spin 2004 Alessandro Bravar RHIC pp accelerator complex BRAHMS & PP2PP STAR PHENIX AGS LINAC BOOSTER Pol. Proton Source Spin Rotators 20% Snake Siberian Snakes 200 MeV polarimeter AGS quasi-elastic polarimeter Rf Dipoles RHIC pC “CNI” polarimeters PHOBOS RHIC absolute pH polarimeter Siberian Snakes AGS pC “CNI” polarimeter 5% Snake

Spin 2004 Alessandro Bravar Polarimetry : Impact on RHIC Spin Physics  measured spin asymmetries normalized by P B to extract Physics Spin Observables  RHIC Spin Program requires  P beam / P beam ~ 0.05  normalization  scale uncertainty  polarimetric process with large  and known A N – pC elastic scattering in CNI region, A N ~ 1 – 2 % – fast measurements – requires absolute calibration  polarized gas jet target Physics Asymmetries Single Spin Asymmetries Double Spin Asymmetries measurements recoil

Spin 2004 Alessandro Bravar The Road to P beam with the JET target Requires several independent measurements 0 JET target polarization P target (Breit-Rabi polarimeter) 1 A N for elastic pp in CNI region: A N = - 1 / P target  N ’ 2 P beam = 1 / A N  N ” 1 & 2 can be combined in a single measurement: P beam / P target = -  N ’ /  N ” “ self calibration” works for elastic scattering only 3 CALIBRATION: A N pC for pC CNI polarimeter in covered kinematical range: A N pC = 1 / P beam  N ”’ (1 +) measured simultaneously with several insertions of carbon target 4 BEAM POLARIZATION: P beam = 1 / A N pC  N ”” to experiments at each step pick-up some measurement errors: transfer calibration measurement expected precision

Spin 2004 Alessandro Bravar p  p  p p

Spin 2004 Alessandro Bravar p  p  pp and p  p   pp with a Polarized Gas Jet Target RHIC polarized proton beams polarized gas JET target

Spin 2004 Alessandro Bravar The Atomic H Beam Source separation magnets (sextupoles) H 2 dissociator Breit-Rabi polarimeter focusing magnets (sextupoles) RF transitions holding field magnet recoil detectors record beam intensity 100% eff. RF transitions focusing high intensity B-R polarimeter OR P z + OR P z - H = p + + e -

Spin 2004 Alessandro Bravar the JET ran with an average intensity of 1  atoms / sec the JET thickness of 1  atoms/cm 2 record intensity target polarization cycle +/0/- ~ 500 / 50 / 500 sec polarization to be scaled down due to a ~3% H 2 background: P target ~ ± (current understanding) no depolarization from beam wake fields observed ! JET target polarization & performance pol. minus polarization plus polarization 2.5 h time

Spin 2004 Alessandro Bravar The Polarized Jet Target under development Dissociator stage Baffle location Sextupoles 1-4 Sextupoles 5-6 Profile measurement BRP vacuum vessel Electronics racks Turbo pump controllers Dissociator RF systems Vac. gauges monitors Target chamber & beam pipe adapters Recoil spectrometer silicon detectors

Spin 2004 Alessandro Bravar Recoil Si spectrometer 6 Si detectors covering the blue beam => MEASURE energy (res. < 50 keV) time of flight (res. < 2 ns) scattering angle (res. ~ 5 mrad) of recoil protons from pp  pp elastic scattering HAVE “design” azimuthal coverage one Si layer only  smaller energy range  reduced bkg rejection power B A N beam (t )  A N target (t ) for elastic scattering only! P beam =  P target.  N beam /  N target

Spin 2004 Alessandro Bravar Jet-Target Holding Magnetic Field (1.0)  Bdl ~ 0 displacement (cm) B z (Gauss) p = 30 MeV/c (|t|~10 -3 ) p = 100 MeV/c (|t|~10 -2 ) kGauss Helmholtz coils almost no effect on recoil proton trajectories: left – right hit profiles & left – right acceptances almost equal (also under reversal of holding field)

Spin 2004 Alessandro Bravar pp elastic data collected Hor. pos. of Jet cts. = 2.5 mm Number of elastic pp events FWHM ~ 6 mm as designed recoil protons unambiguously identified ! 100 GeV ~ 1.8  10 6 events for 1.5  10 –3 < -t < 1.0  10 –2 GeV 2 similar statistics for 1.0  10 –2 < -t < 3.0  10 –2 GeV 2 24 GeV ~ 300 k events CNI peak A N 1 < E REC < 2 MeV prompt events and beam-gas  source calibration recoil protons elastic pp  pp scattering background 118 cts. subtracted JET Profile: measured selecting pp elastic events ToF vs E REC correlation T kin = ½ M R (dist/ToF) 2  ToF <  8 ns

Spin 2004 Alessandro Bravar TDC vs ADC individual channels Energy - Position correlations T kin   2 (i.e. position 2 ) pp elastic events clearly identified ! fully absorbed protons punch through protons recoil energy punch through recoil protons position

Spin 2004 Alessandro Bravar not corrected for the magnetic field M X 2 [GeV 2 ] Mp2Mp2 inelastic threshold number of events (a. u.) FWHM ~ 0.1 GeV 2 M 2 X (GeV 2 ) proton M 2 X distribution 80 cm from target convoluted with spectrometer Resolution  M 2 X 2  M 2 X ~ 0.1 GeV 2 Missing Mass M X 100 GeV M2XM2XM2XM2X simulations

Spin 2004 Alessandro Bravar A N for p  p  100 GeV source of systematic errors: 1  P TARGET = 2 % (normalization error) 2 from backgrounds <  false asymmetries: small statistical errors only preliminary

Spin 2004 Alessandro Bravar this expt. A N for p  p  100 GeV data in this t region being analyzed preliminary data (from this expt. only) fitted with CNI prediction [  TOT = 38.5 mbarn,  = 0,  = 0] fitted with: N  f CNI N – “normalization factor” N = 0.98  0.03  2 ~ 5 / 7 d.o.f. the errors shown are statistical only (see previous slide) no need of a hadronic spin – flip contribution to describe these data however, sensitivity on  5 had in this t range low no hadronic spin-flip

Spin 2004 Alessandro Bravar ONLINE  statistical errors only no background corrections no dead layer corrections no systematic studies no false asymmetries studies no run selection “ONLINE” measured asymmetries & Results data divided into 3 p energy energy bins “Target”: average over beam polarization “Beam”: average over target polarization an example: 750 < E REC < 1750 keV  P beam  = 37 %  2 %  P beam (pC CNI)  = 38 % No major surprises ? (statistical errors only !) blue beam with alternating bunch polarizations:    … good uniformity from run to run (stable JET polarization) JET polarization reversed each ~ 5 min. 1 run ~ 1 hour work in progress !

Spin 2004 Alessandro Bravar p  C  p C

Spin 2004 Alessandro Bravar Setup for pC scattering – the RHIC polarimeters  recoil carbon ions detected with Silicon strip detectors  2   72 channels read out with WFD (increased acceptance by 2)  very large statistics per measurement (~ 20  10 6 events) allows detailed analysis – bunch by bunch analysis – channel by channel (each channel is an “independent polarimeter”) – 45 o detectors : sensitive to vertical and radial components of P beam  unphysical asymmetries Ultra thin Carbon ribbon Target (3.5  g/cm 2,10  m) beam direction RHIC  2 rings 2 Si strip detectors (ToF, E C ) 30cm inside RHIC

Spin 2004 Alessandro Bravar Event Selection & Performance - very clean data, background < 1 % within “banana” cut - good separation of recoil carbon from  (C*   X) and prompts may allow going to very high |t| values -  (Tof) <  10 ns (    ~ 1 GeV) - very high rate: 10 5 ev / ch / sec E C, keV TOF, ns Typical mass reconstruction Carbon Alpha C*  Prompts Alpha Carbon Prompts M R, GeV M R ~ 11 GeV   ~ 1 GeV T kin = ½ M R (dist/ToF) 2 non-relativistic kinematics

Spin 2004 Alessandro Bravar A N p  C  pC at 3.9, 6.5, 9.7 & 21.7 GeV only statistical errors are shown normalization errors: ~ 10 % (at 3.9) ~ 15 % (at 6.5) ~ 20 % (at 21.7) systematic errors: < 20 % - backgrounds - pileup - RF noise recoil Carbon energy (keV) p = 3.9 GeV p = 21.7 GeV p = 9.7 GeV preliminary data CNI peak ~ 4 %  P B  ~ 73 %  P B  ~ 65 %  P B  ~ 47 % p = 6.5 GeV A N (%) momentum transfer –t (GeV 2 /c 2 )  P B  ~ 60 % statistical errors only (AGS)

Spin 2004 Alessandro Bravar A N p  C  pC: Energy Dependence A N (%) Beam Energy (GeV) preliminary data t = GeV 2 t = GeV 2 t = GeV 2 t = GeV 2 statistical errors only only statistical errors are shown systematic errors as for previous slide E ? Asymptotic regime No energy dependence ?

Spin 2004 Alessandro Bravar Raw asymmetry 100 GeV (RHIC) X-90X-45 X-average Regular calibration measurements Cross asymmetry Radial asymmetry False asymmetry ~0 higher –t range False asymmetry ~0 good agreement btw X90 vs. X t (GeV/c) 2 Regular polarimeter runs measurements taken simultaneously with Jet -target very stable behavior of measured asymmetries Polarimeter dedicated runs (high -t) Signal attenuation (x1/2) to reach higher – t Normalized at overlap region to regular runs Zero crossing measured with large significance preliminary

Spin 2004 Alessandro Bravar pC Systematics: each detector channel covers same t range  72 independent measurements of A N width ~ stat. error single meas. channel by channel raw asymmetry sources of systematic uncertainties: 1  P BEAM = 7.8 % (normalization) [P BEAM =  0.030, stat. error] 2 energy scale ~ 50 keV for lowest |t| bin (from detector dead layer) NB these are “external” factors not “intrinsic” limitations Fit with sine function (phase fixed)

Spin 2004 Alessandro Bravar A N for p  C  100 GeV no hadronic spin-flip with hadronic spin-flip “forbidden” asymmetries systematic uncertainty best fit with hadronic spin-flip Kopeliovich – Truemann model PRD64 (01) hep-ph/ r 5 pC  F s had / Im F 0 had preliminary statistical errors only spread of r 5 values from syst. uncertainties 1  contour

Spin 2004 Alessandro Bravar Summary  measured A N pp for elastic pp  pp scattering at 100 GeV with very high accuracy (statistical and systematic) – |t| range: < |t| < (GeV/c) 2 – soon A N in |t| range of < |t| < (GeV/c) 2 – soon A NN in same |t| range (stat. err.  2.5 larger)  pp data well described by CNI – QED predictions (“S – LK”) no need for a hadronic spin-flip term  measured A N pC for elastic pC  pC scattering at 100 GeV (RHIC) – zero crossing around |t| ~ 0.03 (GeV/c) 2  pC data require substantial hadronic spin-flip !  measured A N pC for pC  pC scattering over 3.5 < E b < 24 GeV (AGS) – E b < 10 GeV/c: almost no t dependence & departure from “CNI” shape