Transcript Slajd 1
Rola spinu w elastycznym rozpraszaniu pp i pC w RHIC-u Andrzej Sandacz Seminarium Fizyki Wielkich Energii UW Warszawa, 4 marca 2005 Why interesting to study spin effects in elastic scattering? Scientific interest Constraints from general principles of Field Theory e.g. single spin-flip / nonflip amplitude ~ ln s as s → ∞ Non-perturbative region of QCD ( | t | < 0.05 GeV2 ) but spin-flip probes smaller distances ( ~ 0.2 fm ) in nucleon than non-flip interaction ( ~ 1 fm ) Details of static constituent quark structure of nucleon If spin-flip present, e.g. compact diquark in the nucleon or anomalous color-magnetic moment of quarks or isoscalar magnetic moment of the nucleon At high energy exchange of Pomeron dominant Pomeron coupling to nucleon spin? Practical interest - beam polarimetry at RHIC can be traced back to 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 observables: 3 cross sections 5 spin asymmetries double spin flip 2 s, t | M | spin non–flip ? 3 s, t | M | double spin flip 4 s, t | M | single spin flip 5 s, t | M | AN ANN 1 s, t | M | AN ( s, t ) d 4 2 Im 5* 1 2 3 4 dt s ANN ( s, t ) d 4 2 dt s 25 Re1*2 3*4 formalism well developed, however not much data ! only AN studied / measured to some extent M The Very Low t Region Ahadronic ACoulomb INTERFERENCE CNI = Coulomb – Nuclear Interference around t ~ 103 (GeV/c)2 scattering amplitudes modified to include also electromagnetic contribution ihad ihad iem ei hadronic interaction described in terms of Pomeron (Reggeon) exchange electromagnetic = |Ahadronic + ACoulomb|2 single photon exchange P unpolarized clearly visible in the cross section d/dt polarized “left – right” asymmetry AN + g charge magnetic moment AN & Coulomb Nuclear Interference the left – right scattering asymmetry AN arises from the interference of the spin non-flip amplitude with the spin flip amplitude (Schwinger) * had had * em AN C1em C 2 flip non flip flip non flip 1)p pphad in absence of hadronic spin – flip contributions AN is exactly calculable (Kopeliovich & Lapidus): AN (t) 8 Z y 3 / 2 totpA t 1 y AN 2 pA 2 8 Z m p tot 1 y hadronic spin- flip modifies the QED “predictions” interpreted in terms of Pomeron spin – flip and parametrized as 5had r5 t 1 Im 1had 3had mp 2 Some AN measurements in the CNI region pC Analyzing Power pp Analyzing Power E704@FNAL p = 200 GeV/c PRD48(93)3026 E950@BNL p = 21.7 GeV/c PRL89(02)052302 no hadronic spin-flip AN(%) with hadonic spin-flip no hadronic spin-flip -t r5pC Fshad / Im F0had Re r5 = 0.088 0.058 Im r5 = 0.161 0.226 highly anti-correlated RHIC pp accelerator complex RHIC pC “CNI” polarimeters absolute pH polarimeter BRAHMS & PP2PP PHOBOS RHIC Siberian Snakes PHENIX STAR Siberian Snakes Spin Rotators 5% Snake LINAC Pol. Proton Source BOOSTER AGS 200 MeV polarimeter 20% Snake AGS quasi-elastic polarimeter Rf Dipoles AGS pC “CNI” polarimeter Polarimetry : Impact on RHIC Spin Physics Single Spin Asymmetries Physics Asymmetries 1 N N AN PB N N 1 Nleft N right PB AN Nleft N right recoil Double Spin Asymmetries ALL 1 N N G 2 PB N N measurements measured spin asymmetries normalized by P to extract Physics Spin Observables RHIC Spin Program requires P / P ~ 0.05 normalization scale uncertainty polarimetric process with large and known A B beam beam N – pC elastic scattering in CNI region, AN ~ 1 – 2 % – fast measurements – requires absolute calibration polarized gas jet target The Road to Pbeam with the JET target Requires several independent measurements 0 JET target polarization Ptarget (Breit-Rabi polarimeter) 1 AN for elastic pp in CNI region: AN = - 1 / Ptarget eN’ 2 Pbeam = 1 / AN eN” 1 & 2 can be combined in a single measurement: Pbeam / Ptarget = - eN’ / eN” “self calibration” works for elastic scattering only 3 CALIBRATION: ANpC for pC CNI polarimeter in covered kinematical range: ANpC = 1 / Pbeam eN”’ (1 +) 2 + 3 measured simultaneously with several insertions of carbon target 4 BEAM POLARIZATION: Pbeam = 1 / ANpC eN”” to experiments at each step pick-up some measurement errors: AN Pbeam Pt arg et e e 6% expected A Pbeam Pt arg et precision e pp e pC N pC transfer calibration measurement target polarization cycle +/0/- ~ 500 / 50 / 500 sec polarization to be scaled down due to a ~3% H2 background: minus polarization 0.96 the JET thickness of 1 1012 atoms/cm2 record intensity 0.94 the JET ran with an average intensity of 11017 atoms / sec 0.98 pol. JET target polarization & performance plus polarization Ptarget ~ 0.924 ± 0.018 (current understanding) no depolarization from beam wake fields observed ! 2.5 h time 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 ANbeam (t ) ANtarget (t ) for elastic scattering only! Pbeam = Ptarget . eNbeam / eNtarget B HAVE “design” azimuthal coverage one Si layer only smaller energy range reduced bkg rejection power Recoil particle ID ; Correlation of TOF and Energy mp 1 tof Distance 2 TR TOF ns TOF< +/-8 ns width line Mp TR 0 prompt events TR MeV 1 calibration alpha source (241Am, 5.486 MeV) 2 GeV/c2 Forward scattering particle ID ; Correlation of Energy and position (ch) TOF vs TR Si detector of 16channels vertical strips ch# recoil angle #4 #16 TOF #1 #1 TR #16 #1 TOF < +/-8 ns #16 TR 2m p sin 2 R 241Am TR MeV analysis 2m p ch # 2 1 Ch# 16 “ONLINE” measured asymmetries & Results ONLINE statistical errors only no background corrections data divided into 3 p energy energy bins no dead layer corrections an example: 750 < EREC < 1750 keV no systematic studies no false asymmetries studies “Target”: no run selection average over beam polarization blue beam with alternating bunch polarizations: … “Beam”: average over target polarization Pbeam e beam Pt arg et e t arg et 1 run ~ 1 hour good uniformity from run to run (stable JET polarization) JET polarization reversed each ~ 5 min. Pbeam = 37 % 2 % Pbeam (pC CNI) = 38 % No major surprises ? (statistical errors only !) AN for pp pp @ 100 GeV this expt. E704@FNAL data (from this expt. only) fitted with CNI prediction [TOT = 38.5 mbarn, r = 0, = 0] no hadronic fitted with: spin-flip N f CNI N – “normalization factor” N = 0.98 0.03 c2 ~ 5 / 7 d.o.f. data in this t region being analyzed the errors shown are statistical only (see previous slide) no need of a hadronic spin – flip contribution to describe these data however, sensitivity on 5had in this t range low Setup for pC scattering – beam direction 6 1 5 2 4 3 Ultra thin Carbon ribbon Target (3.5g/cm2 ,10m) the RHIC polarimeters inside RHIC ring @IP12 Si strip detectors (ToF, EC) 30cm RHIC 2 rings recoil carbon ions detected with Silicon strip detectors 2 72 channels read out with WFD (increased acceptance by 2) very large statistics per measurement (~ 20 106 events) allows detailed analysis – bunch by bunch analysis – channel by channel (each channel is an “independent polarimeter”) – 45o detectors: sensitive to vertical and radial components of Pbeam unphysical asymmetries Event Selection & Performance TOF, ns Tkin= ½ MR(dist/ToF)2 Typical mass reconstruction MR ~ 11 GeV M ~ 1 GeV non-relativistic kinematics Carbon Prompts Alpha Alpha C* Prompts EC, keV Carbon MR, GeV - 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 ( M ~ 1 GeV) - very high rate: 105 ev / ch / sec Raw asymmetry (t) @ 100 GeV (RHIC) higher –t range Regular calibration measurements good agreement btw X90 vs. X45 X-90 X-45 False asymmetry ~0 X-average Radial asymmetry Cross asymmetry False asymmetry ~0 0.02 0.03 0.04 -t (GeV/c)2 0.01 0.02 Regular polarimeter runs measurements taken simultaneously with Jet -target very stable behavior of measured asymmetries -t (GeV/c)2 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 AN for pC pC @ 100 GeV r5pC Fshad / Im F0had 1 contour statistical errors only no hadronic spin-flip spread of r5 values from syst. uncertainties with hadronic spin-flip systematic uncertainty “forbidden” asymmetries best fit with hadronic spin-flip Kopeliovich – Truemann model PRD64 (01) 034004 hep-ph/0305085 The Setup of PP2PP p1 p2 1x , 1y 2x ,2y Principle of the Measurement • Elastically scattered protons have very small scattering angle θ*, hence beam transport magnets determine trajectory scattered protons • The optimal position for the detectors is where scattered protons are well separated from beam protons • Need Roman Pot to measure scattered protons close to the beam without breaking accelerator vacuum Beam transport equations relate measured position at the detector to scattering angle. xD Dx yD Dy = a11 a21 a31 a41 Lxeff a22 a32 a42 a13 a23 a33 a43 a14 a24 y Leff a44 x0 *x y0 *y x0,y0: Position at Interaction Point Θ*x Θ*y : Scattering Angle at IP xD, yD : Position at Detector ΘxD, ΘyD : Angle at Detector Elastic Event Identification An elastic event has two collinear protons, one on each side of IP p1 p2 1x , 1y 2x , 2y 1. It also has eight Si detector “hits”, four on each side. 2. Clean trigger: no hits in the other arm and in inelastic counters. 3. The vertex in (z0) can be reconstructed using ToF. Angle (hit) Correlations Before the Cuts Events with only eight hits Note: the background appears enhanced because of the “saturation” of the main band Experimental Determination of AN Use Square-Root-Formulae to calculate spin ( , ) and false asymmetries (, ) In this formulae luminosities, apparatus asymmetries and efficiencies cancel Where Pb Py ANN cos 2 ASS sin 2 can be neglected wrt 1 ( < 0.03 ) Systematic error ~ 15% (icluding 12.5% incertainty on beam polarisation) Brief summary on AN experimental results at CNI region √s [GeV] -t range [GeV2] typical ΔAN (E704, 1993) 19 0.002 – 0.05 0.01 pp (RHIC jet, 2004) 14 0.001 – 0.009 0.002 0.01 – 0.03 0.004 0.002 pp pp (PP2PP, 2004) 200 pC (E950, 2002) 6 0.009 – 0.045 pC (RHIC Cpol,2004) 14 0.007 – 0.05 < 0.001 Both pC results consistent; 5% single spin-flip contribution, significantly different from 0 Both low energy pp results consistent; do not require spin-flip pp result at collider energy indicate spin-flip Another spin puzzle ? A consistent theoretical description of the data Regge poles model, ( T.L. Trueman ) dominant contributions P, f, ω, a2, ρ exchanges I=0 I=1 Fit to unpolarized pp and ppbar data → g0P, g0f, g0ω, g0a, g0ρ( s, t ) pC polarised gs s ,t I = 0 exchanges only Assumption: pC data t t s go s ,t P goP s ,t f gof s ,t go s ,t m m → pp polarised jet data ‘degeneracy’: λP, λf, λω real, energy independent λP = 0.09 ± 0.014, λf = –0.30 ± 0.02, λω = 0.19 ± 0.10 all five exchanges contribute ( f, a2 ) → R+ ( C = +1 ) λP = 0.09 (fixed), and ( ω, ρ ) → R- ( C = -1 ) jet data → λR+ = –0.32, λR- = 1.09 compensation by ρ contribution (mostly) → almost no net spin-flip pp at collider contributions of all Reggeons but Pomeron decrease ~ 10 % spin-flip due to Pomeron exchange expected Summary and outlook New precise data on AN in pp and pC scattering at small t from RHIC Small but significantly different from 0 spin-flip contribution, even at √s = 200 GeV (Model dependent) estimate of Pomeron single spin-flip contribution 0.09 ± 0.02 Possible results from RHIC in future AN in increased energy range up to √s = 500 GeV double-spin asymmetry ANN sensitive to hypothetical Odderon (C = -1 partner of Pomeron) 2 moderate t (0.15 – 1.5 GeV ) dramatic spin dependence seen in pp scattering at low energies large t ( > 2 GeV2) d σ/ d t ~ t -8 3 vector exchanges (gluons/ color singlets) between pairs of quarks