1. RHIC Spin Physics M. Grosse Perdekamp (University of Illinois and RBRC) Workshop on Hadron Structure and Hadron Spectroscopy August 1-3, 2005 Charles University, Prague

2. RHIC Spin Overview2 August 2 nd Overview o Scientific motivation Proton spin structure ∆G, ∆q/∆q, δq from QCD analysis of polarized proton-proton collisions o Novel experimental tool Polarized proton-proton collisions at high energies The experiments New from run 2005 o Initial results and outlook Helicity distributions Transverse spin physics W-physics and upgrades

3. RHIC Spin Overview3 August 2 nd Structure of the Basic Building Blocks of Matter vs Fundamental Forces in Nature fundamental bound state of matter interaction between the constituents of the bound state measurements with new tools + better precision theory predictions increasingly quantitative

4. RHIC Spin Overview4 August 2 nd Structure of the Basic Building Blocks of Matter vs Fundamental Forces in Nature hydrogen atom Quantum Electro Dynamics Doppler free saturation spectroscopy Lamb shift Example: Lamb Shift (1947)  QED pioneers QFT! Tomonaga, Feynman, Schwinger Lamb

5. RHIC Spin Overview5 August 2 nd Structure of the Basic Building Blocks of Matter vs Fundamental Forces in Nature Nucleon Quantum Chromo Dynamics DIS quark structure Friedman, Kendall, Taylor Gell Mann Nakano, Nishijima Nucleon structure: DIS/QCD

6. RHIC Spin Overview6 August 2 nd Structure of the Basic Building Blocks of Matter vs Fundamental Forces in Nature Nucleon Quantum Chromo Dynamics DIS with polarized beams and targets quark helicity dis. Bjorken sum rule

7. RHIC Spin Overview7 August 2 nd Proton Spin Structure in DIS Parton Distribution functions (PDF): Helicity average distribution quarks q(x) : well known gluons G(x) : moderately well known Helicity difference distribution quarks ∆q(x) : moderately well known gluons ∆G(x) : unknown Helicity flip (transversity) distribution quarks δq(x) : unknown Field started with polarized source and targets about 1975 Yale/SLAC collaboration 1975 – 2005 SLAC: E80, E130, E142, E143, E154, E155 CERN: EMC, SMC, COMPASS DESY: HERMES JLab : Halls A,B,C Future progress on ∆G(x), the flavor separation of ∆q(x) and δq(x) in DIS appears best possible at future polarized high luminosity e-p collider

8. RHIC Spin Overview August 2 nd AGS LINAC BOOSTER Polarized Source Spin Rotators Partial Snake Siberian Snakes 200 MeV Polarimeter AGS Internal Polarimeter Rf Dipole RHIC pC Polarimeters Absolute Polarimeter (H jet) P HENIX P HOBOS B RAHMS & PP2PP S TAR Siberian Snakes Run 04 P b ~45%, 55 bunches Run 05 AGS pC Polarimeter Helical Partial Snake Strong Snake Spin Flipper = 45% strong AGS snake installed *Complete* A novel experimental method A novel experimental method: Probing Proton Spin Structure in High Energy Polarized Proton Collisions at RHIC

9. RHIC Spin Overview9 August 2 nd Gluon polarization Flavor separation of quark polarizations Transverse spin struc- ture of the Nucleon available channels Inclusive jets, hadrons, photons and heavy flavor production Single lepton asymmetries A L (e,μ) in W-production (1) A TT with Collins- and Interfe- rence-FFs (2) A N + back-to-back correlations (3) A TT and A T In Drell Yan goals determine first moment of the spin dependent gluon Distribution. flavor separation of quark and anti-quark spin distributions measurements of Collins and Sivers distributions Proton Spin Structure in Polarized p-p Collisions in PHENIX

10. RHIC Spin Overview10 August 2 nd  G from a global NLO pQCD Analysis with projected future Direct Photon Data at 200 GeV from RHIC AAC Preliminary M. Hirai, H.Kobayashi, M. Miyama et al. Does NLO pQCD provide a reliable framework for the interpretation of polarized proton data in terms of polarized parton distribution functions?

11. RHIC Spin Overview11 August 2 nd I) Can one extract G(x,Q 2 ) from pp? II) NLO pQCD vs RHIC data Is pQCD applicable at RHIC?

12. RHIC Spin Overview12 August 2 nd Global QCD Analysis for G(x,Q 2 ) and q(x,Q 2 ): J. Pumplin et.al JEHP 0207:012 (2002) 10 -4 10 -3 10 -2 10 -1 0.5 x gluon down up-quarks anti-down Quark and Gluon Distributions error on G(x,Q 2 ) error for u(x,Q 2 ) +/- 10% +/- 5% error for d(x,Q 2 ) 10 -4 10 -3 10 -2 10 -1 0.5 x CTEQ6: use DGLAP Q 2 -evolution of quark and gluon distributions to extract q(x,Q 2 ) and G(x,Q 2 ) from global fit to data sets at different scales Q 2. H1 + Zeus F 2 CDF + D0 Jets CTEQ5M1 CTEQ6M

13. RHIC Spin Overview13 August 2 nd G(x,Q 2 ) and q(x,Q 2 ) + pQCD beautifully agree Tevatron + HERA! J. Pumplin et.al JEHP 0207:012 (2002) D0 Jet Cross Section ZEUS F 2

14. RHIC Spin Overview14 August 2 nd and at RHIC ? q(x,Q 2 ), G(x,Q 2 ) and D(z,Q 2 ) + pQCD are nicely consistent with experiment! o Good agreement between NLO pQCD calculations and experiment  can use a NLO pQCD analysis to extract spin dependent quark and gluon distributions from RHIC data! PHENIX π 0 cross section a |η|<0.35 Phys.Rev.Lett.91:241803,2003 STAR π 0 cross section a 3.4<η<4.0 Phys.Rev.Lett.92:171801,2004 gluon fragmentation !?

15. RHIC Spin Overview15 August 2 nd NLO-pQCD calculation –Private communication with W.Vogelsang –CTEQ6M PDF. –direct photon + fragmentation photon –Set Renormalization scale and factorization scale p T /2,p T,2p T Theory calculation show good agreement with the experimental cross section. Direct Photons: NLO pQCD vs RHIC data

16. Experimental Method a) Experiments b) Polarized proton run 2005

17. RHIC Spin Overview17 August 2 nd RHIC  four complementary experiments Two experimental tools needed for experiments at polarized colliders o Local polarimeter to verify local spin orientation o Relative luminosity monitoring  bunch sorted luminosity scalers!

18. RHIC Spin Overview18 August 2 nd Left Right Top Bottom * BBC West BBC East Interaction Vertex 3.3<|  |< 5.0 Local Polarimetry in STAR and PHENIX  PHENIX: Exploit A N in forward neutron production in ZDC (zero degree ha- dronic calorimeters) STAR: P L /P > 0.99 blue & yellow rotator off A(Φ) rotator on

19. RHIC Spin Overview19 August 2 nd Relative Luminosity ZDC/BBC Use BBC Counts to measure R high statistics (  ~ 51%) low background Use ZDC as a cross-check different systematics and kinematics achieved high precision in Run03/04 Error from relative luminosity R  R =  (L++/L+-) < 2.5 x 10 -4 “bunch sorted” relative luminosity scaler boards to record up to 24 different monitors for each bunch crossing.

20. RHIC Spin Overview20 August 2 nd STAR spin physics program: ∆G, ∆q/∆q, δq Large acceptance TPC and EMC -1<η<2

21. STAR EMC upgra- des for RHIC Spin e.g., STAR barrel (nearly completed) and endcap (completed 2005) em calorimeters permit triggering/reconstruction for jets, ,  0, … 2005 STAR online jet monitoring (TPC+BEMC)  0 reconstr’n from 2  ’s in EEMC+SMD p T (GeV/c) # Jets/Evt  di-Jet Event (Courtesy Steve Vigdor)

22. RHIC Spin Overview22 August 2 nd 2 central arms: electrons, photons, hadrons –charmonium J/ ,  ’  e  e  –vector meson   e  e  –high p T       –direct photons –open charm –hadron physics 2 muon arms: –“onium” J/ ,  ’,      –vector meson      –open charm PHENIX spin physics program: ∆G, ∆q/∆q, δq Excellent trigger and DAQ capabilities: multiple trigger signature important for spin physics can be taken in parallel with high bandwidth! See talks by D. Fields (overview), H. Torii (central arms) and Ming Liu (muon arms).

23. RHIC Spin Overview23 August 2 nd 100% transverse spin! Two spectrometer arms with good particle ID to high momenta BRAHMS: A N for charged π,K, p

24. RHIC Spin Overview24 August 2 nd RHIC Detector Status and Upgrades o All instrumentation is in place for the planned measurements on gluon polarization and transverse spin! o W-physics (flavor separation of quark and anti-quark polarizations) requires upgrades in PHENIX (muon trigger, funded by NSF) and STAR (forward tracking, grant proposal to DOE in preparation) which are planned to be complete by 2009 and 2010 respectively. o In PHENIX a central silicon tracking upgrade and a forward tungsten silicon calorimeter upgrade will significantly will enhance capabilities for jet and photon-jet physics. o A RHIC luminosity upgrade (RHIC II) for heavy ions with electron cooling will gain a factor 3-5 in luminosity from 2012.

25. RHIC Spin Overview25 August 2 nd Polarized Proton Run 2005 projected maximum Projected minimum

26. RHIC Spin Overview26 August 2 nd Did we meet our goals? (I) Sampled Luminosity PHENIX Goal 5.5 pb -1 P beam =45% delivered luminosity 3.8 pb -1 (sampled)  85x10 9 p-p collisions +0.16pb -1 transverse

27. RHIC Spin Overview27 August 2 nd (II) Luminosity weighted Polarization goal 45% luminosity weighted average polarizations

28. RHIC Spin Overview28 August 2 nd (III) Figure of Merit P b 2 P y 2 ∫Ldt PHENIX Goal 226 nb -1 recorded 206 nb -1

29. RHIC Spin Overview29 August 2 nd Transverse Polarization at p beam = 205 GeV Polarization ~ 30% Analyzing power of local polarimeter roughly the same at twice the energy!  Local Polarimeter can be used at higher beam energy needed for the W-physics program Raw Yellow Beam  =P beam A N @ PHENIX

30. Early Results and Outlook (I) Transverse Spin

31. RHIC Spin Overview31 August 2 nd E704: AN The observed asymmetries could result from Transversity x Spin-dep.fragmentation (Collins-Heppelmann effect), or Sivers effect or Higher-twist effects Sterman and Qiu Initial State Twist 3 Koike Final State Twist or a combination of the above left right Fermilab E-704 reported large asymmetries A N in pion productions E704 P T =0.5~2 GeV/c   0 – E704, PLB261 (1991) 201. π +/- - E704, PLB264 (1991) 462.

32. RHIC Spin Overview32 August 2 nd Run 5  reduce errors by a factor 3-5. STAR: A N for backward angles from 2003 data PHENIX A N (π 0 ) and A N (π 0 ) at |η|<0.35 C. Aidala, DIS 2004, to be published STAR A N (π 0 ) at 3.4<η<4.0 Phys.Rev.Lett.92:171801,2004 and (hep-ex/0502040)

33. RHIC Spin Overview33 August 2 nd BRAHMS: A N for charged pions x F x 100 p T vs X F x F x 100 A N for pions:  N = -0.08 +- 0.005 +- [0.02] in 0.17 < x F < 0.32 in 0.17 < x F < 0.32  N = +0.05 +- 0.005 +- [0.015] o From the ongoing 2005 run the statistical errors will be reduced by a factor 6-7 o New absolute RHIC polarimeter will reduce the systematic error by a factor 4  What can be learned from confronting model calculations with precision data on A N ?

34. RHIC Spin Overview34 August 2 nd A N for Protons from BRAHMS BRAHMS preliminary  Proton A N is compatible with 0 A N will be available for pions, kaons and protons from run 2005  What do we learn from comparing A N for different fragmentation functions?

35. RHIC Spin Overview35 August 2 nd Run 2006 High Precision Measurement of A N, Sivers from back-to-back Correlations STAR Phys. Rev. Lett. 92:171801, 2004 Boer and Vogelsang (hep-ph/0312320): azimuthal back to back correlation between hadrons in opposite hemisphere jets: Separation of intrinsic transverse quark spin (transversity) from trans- verse momentum effects (Sivers)? Clean channel for Sivers effect! STAR, PHENIX and BRAHMS STAR and PHENIX (I) (II)

36. RHIC Spin Overview36 August 2 nd Sensitivity Check for Back-to-Back Sivers Asymmetry (based on 2003 pp sample) Full di-jet Sivers Reduced for polarization and, “di-hadron smearing” Run03 p+p gamma-charged, 0.35/pb Given 0.35/pb of data, we should be able to get 1% statistical significance in A N using gamma-charged measurements of jet dphi Expected raw A N could be 3.5% Could also be as low as 0.5%, or as large as 10% x-dependence of Sivers? Effects from P=0.5, jet angle not aligned with transverse polarization, and fragmentation reduces raw A N to ~1.0% Have not evaluated systematic errors yet (underlying event…)

37. RHIC Spin Overview37 August 2 nd J.C. Collins, Nucl. Phys. B396, 161(1993) R. Jaffe, X.Jin, J. Tang Phys. Rev. D57 (1999)5920 J. Collins, S. Heppelmann, G. Ladinsky, Nucl.Phys. B420 (1994)565 Statistical sensitivity for A T with 32pb -1 STAR and PHENIX Transverse Spin Physics Channels at RHIC for “High” Luminosity STAR and PHENIX W. Vogelsang and M. Stratmann, RBRC Wrkshp on Transversity (2000)

38. RHIC Spin Overview38 August 2 nd Transverse spin program in PHENIX (and STAR) is luminosity limited A N yes, very good A N (back-to-back) good (Sivers signature!) A T (Collins FF) fair A T (Interference FF) limited A TT (Jets) not studied experimentally A T (Drell Yan) --- A TT ( Drell Yan) --- Direct photons --- (A N,A T (CFF, IFF)) RHIC by 2009 at 200 GeV ∫Ldt ~275pb -1 delivered ∫Ldt ~100pb -1 accepted (eg. PHENIX: vertex cut, trigger efficiencies, duty factor)  ∫Ldt ~25 pb -1 transverse RHIC II luminosity sufficient?  Dedicated Drell Yan experiment for transverse spin physics ?

39. RHIC Spin Overview39 August 2 nd Dedicated Experiment for Drell Yan ? Idea: Large acceptance calorimeter experiment at PHOBOS interaction point (10 o’clock) Physics: A TT (DY)  transversity A UT (DY )  Sivers A TT (Jets)  transversity, correlations unpolarized DY? Exp. Param.: Re-use existing EMC + HCAL elements, pad-chamber layer for charged particle tagging, BBC, ZDC (exists), utilize PHENIX FEE, DAQ and trigger for dead time free read out  -3<η<3, sample all verticies. Low cost? Advantages: - 100% transverse spin - take advantage of luminosity upgrades, large acceptance, wide vertex cuts and dead time free read out for maximum ∫Ldt

40. RHIC Spin Overview40 August 2 nd Expected effective Luminosity in 10 weeks 100% transverse running  125pb -1 delivered in 10 weeks x 0.6 (vertex, trigger efficiency, DAQ dead time, up-time x 2.5 (e-cooling) x 4 (mini quads) x 8 (acceptance) = 6000 pb -1

41. RHIC Spin Overview41 August 2 nd Projections for pp at 200 GeV and 320pb -1 O. Martin, M. Stratmann, A. Schaefer, W. Voglesang, Phys.Rev.D60:117502,1999 A TT for Drell Yan with PHENIX Result includes acceptance for PHENIX muon arms or alternatively for muon arms + central arms

42. RHIC Spin Overview42 August 2 nd Projections for pp at 200 GeV and 8000 pb -1 A TT for Drell Yan with PHENIX projections for 10 weeks of running, 5-10% higher polarization, with RHIC II luminosities and large acceptance Drell Yan 8fb -1, large acceptance detector for Drell Yan This measurement appears to be also possible at 500 GeV

43. Early Results and Outlook (II) Longitudinal Spin

44. RHIC Spin Overview44 August 2 nd First Exploratory Results on ∆G Preliminary PHENIX Data appears to prefer the GRSV- standard curve, consistent with the existing constraints from DIS QCD analysis. B. Jaeger et al. Phys. Rev. D67, 054005 (2003) Statistical resolution comparable to existing information - QCD analysis of g1(x,Q 2 ) - from DIS experiments. PHENIX A LL for Inclusive π 0 production at √s=200 GeV, 2003 + 2004 data with about ∫Ldt~0.3pb-1 and ~0.3 Run 5 (April –June 2005): absolute polarimeter gives ∆P b /P b ~7% ∫Ldt ~ 3pb -1, ~0.45  reduces statistical error by factor 7

45. RHIC Spin Overview45 August 2 nd Expectations from Run 2005 GRSV standard ∆G(x)  Gluon distribution from NLO pQCD fit to DIS data on A 1, Gluck Reya, Stratmann, Vogelsang Phys. Rev. D63:094005, 2001 A LL in inclusive π 0 production (PHENIX) A LL ( π 0 ) ∫Ldt=3pb -1 P=0.4 Vogelsang hep-ph/0405069 Input: ∆G(x)=G(x) GRSV: standard ∆G(x) ∆G(x)=0 ∆G(x)= -G(x) A LL in inclusive jet production (STAR) Jager, Stratmann, Vogelsang hep-ph/0404057 A LL (Jets) ∫Ldt=3pb -1 P=0.4 Input: ∆G(x)=G(x) GRSV: standard ∆G(x) ∆G(x)= -G(x) ∆G(x)=0 p T [GeV/c] run 05: ∫Ldt ~ 3.0 pb -1 P =0.45

46. RHIC Spin Overview46 August 2 nd  Final results on ∆G will come from combined NLO analysis of RHIC and DIS  RHIC measurements will span broad range in x with good precision. Multiple channels with independent theo. and exp. uncertainties.  Uncertainty through extrapolation to small x  s=200 GeV incl.  0 prod’n  s=500 GeV incl. jet prod’n ∆G Measurements by 2012 see Spin report to DOE http://spin.riken.bnl.gov/rsc/

47. RHIC Spin Overview47 August 2 nd Physics vs Luminosity and Polarization at RHIC 2005 2006 2007 2008 2009 …. 2012 (RHIC II) 10 pb -1 ……………………………………  275pb -1 …….. 950pb -1 √s= ……………………….. 200 GeV …………………......... 500 GeV| P= 0.5 0.6 0.7 …………………………………… Inclusive hadrons + Jets ~ 25% Transverse Physics Charm Physics direct photons bottom physics W-physics A LL (hadrons, Jets)A LL (charm) A LL (γ) AL(W)AL(W) L= 1x10 31 cm -2 s -1 6x10 31 cm -2 s -1 1.6x10 32 cm -2 s -1 see Spin report to DOE http://spin.riken.bnl.gov/rsc/ @ 200GeV @ 500GeV

48. Outlook (III) W physics (IV) Upgrades

49. RHIC Spin Overview49 August 2 nd W Z W Production in Polarized pp Collisions Single Spin Asymmetry in the naive Quark Parton Model Experimental Requirements:  tracking at high p T  event selection for muons difficult due to hadron decays and beam backgrounds. Parity violation of the weak interaction in combination with control over the proton spin orientation gives access to the flavor spin structure in the proton!

50. RHIC Spin Overview50 August 2 nd Quark Polarizations through Inclusive Single Longitudinal Spin Lepton Aymmstries A L (lepton)  Access to quark polarizations through measurements of inclusive longitudinal measurements of inclusive longitudinal single spin asymmetry? single spin asymmetry? –Yes! Complete theoretical treatment from first principles by Nadolsky and Yuan at NLO pQCD (Nucl. Phys.B 666(2003) 31). p T   GeV]  Machine and detector requirements: –∫Ldt=800pb-1, P=0.7 at √s=500 GeV –Upgrades: o Muon trigger in PHENIX o Forward tracking in STAR

51. RHIC Spin Overview51 August 2 nd Extraction of quark polarizations at LO  Machine and detector requirements: –∫Ldt=800pb-1, P=0.7 at √s=500 GeV –Upgrades: Muon trigger 2009 to 2012 running at √s=500 GeV is projected to yield ∫Ldt ~950pb -1

52. RHIC Spin Overview52 August 2 nd PHENIX Forward Upgrade: W-physics and jet+photon Muon Trigger Upgrade  RPC + FPGA based trigger logic Nose cone calorimeter  W-Si calorimeter Trigger RPC Locations RPC2 June 2005: $1.99M NSF MRI grant approved + $280k institutional support UC Riverside, Abilene Christian, Iowa State, U. Illinois, U. Colorado, Columbia, Georgia Tech., RBRC, Kyoto, Peking U.,

53. RHIC Spin Overview53 August 2 nd BNL, UC-Riverside, JINR-Dubna, Ewha, Moscow State U, Moscow Eng. Physics Institute, Charles Univ., Czech Tech Univ., Czech Inst of Physics, Yonsei The Nosecone Calorimeter Upgrade tungsten silicon calorimeter 1)two EMC compartments 1.5x1.5cm 2 pads, separated by π 0 /γ identifier with 0.5x0.5mm 2 pads and 2.5mm thick tungsten plates: L rad =10 2)one hadronic compartment with same pad size but the tungsten is 16mm thick.

54. RHIC Spin Overview54 August 2 nd prompt photon central arms GS95  G at low x using the NCC  Detection of both hadron jet and final state photon is possible with the NCC and new central arm tracking detectors. – Allows the determination of x G of the gluon on an event-by-event basis NCC  Significantly extends the range of x G for the prompt-g measurement down to ~0.001 at √s =200 GeV – Channel with highest analyzing power for gluon polarization in polarized p+p. – Sensitivity to shape of polarized gluon distribution over a large x range (important input to extrapolation of ∆G to low x)  ∆G with NCC at low-x through jet-g, p 0, e-m, open charm.

55. RHIC Spin Overview55 August 2 nd  Accelerator instrumentation and the experiments at RHIC are ready for physics with polarized protons complete.  A first long pp collision physics run just has been completed  We expect significant constraints for spin dependent gluon distribution in the proton.  For the years 2006 to 2008 we expect significant polarized proton running at √s=200GeV. About 25% of the running time will be spend with transverse polarization. By 2009 we expect (a) constraints on ∆G, for 0.04<x<0.4, through multiple channels with independent theoretical and experimental uncertainties and (b) quantitative results on the origin of transverse spin asymmetries.  Starting in 2009 spin running will be at √s=500GeV and taking advantage of various detector upgrades focusing on low x. Gluon polarization measurements will be extended to low x and for the first time parity violating W-production will be studied inn polarized proton- proton collisions. Summary see Spin report to DOE http://spin.riken.bnl.gov/rsc/

56. RHIC Spin Overview56 August 2 nd Example: High Energy Proton Polarimeters for p=20-250 GeV/c High Energy Polarimeter Requirement for RHIC Spin  Absolute RHIC polarimeter  Fast relative RHIC and AGS polarimeters for monitoring and tuning  Local Polarimeters to confirm spin orientation at collision point RHIC polarimetery relies on newly observed spin asymmetries: o Sizeable elastic proton-Carbon spin asymmetries at high energies  J. Tojo et al. Phys. Rev. Lett. 89:052302, 2002 o Very forward neutron asymmetries  A. Bazilevsky et al. AIP Conf. Proc. 675: 584-588, 2003 o Spin asymmetries in forward multiplicity production as seen by beam-beam counters in STAR  J. Kiryluk, AIP Conf. Proc. 675, 424 (2003)

57. RHIC Spin Overview57 August 2 nd Carbon CNI Polarimeter in the AGS: Polarization during Acceleration raw asymmetry = A N  P B 12+ 36- 36+ G  = 1.91 E beam intrinsic: G  = imperfection: G  = n each point = 50 MeV step 48- red line: simulation of polarization losses assuming constant A N

58. RHIC Spin Overview58 August 2 nd Run 04+05: The Polarized Jet Target for RHIC Polarized Hydrogen Gas Jet Target thickness of > 10 12 p/cm 2 polarization > 92.4% (+/-2)%! no depolarization from beam wake fields Silicon recoil spectrometer to measure The left-right asymmetry A N in pp elastic scattering in the CNI region to  A N < 10 -3 accuracy. Transfer this to the beam polarization Calibrate the p-Carbon polarimeters 2004 analysis P b = 0.39+/-0.03 Courtesy Sandro Bravar, and Yousef Makdisi

59. RHIC Spin Overview59 August 2 nd Jet Profile and TOF vs Energy Hor. pos. of Jet 10000 cts. = 2.5 mm Number of elastic pp events FWHM ~ 6 mm as designed recoil protons unambiguously identified ! 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 T Kin [MeV]