Selected Topics on Diffraction with the STAR Detector at RHIC. 1. Physics with Roman Pots Tagged Forward Protons

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1 Selected Topics on Diffraction with the STAR Detector at RHIC Włodek Guryn for the STAR Collaboration 1. Physics with Roman Pots Tagged Forward Protons Central Exclusive Production Single Diffraction Dissociation Elastic scattering Diffraction in pau, pal Forward π 0 production with leading protons 2. Preliminary results in pp at s=200 GeV 3. Plans 1

2 Few Examples Elastic and Inelastic Processes p + p p + p elastic p + p p + X SDD p + p p + X + p diffractive X= particles, glueballs For each proton vertex one has t four-momentum transfer ξ = Δp/p M X invariant mass In terms of QCD, Pomeron exchange consists of the exchange of a color singlet combination of gluons. Hence, triggering on forward protons at high (RHIC) energies predominantly selects exchanges mediated by gluonic matter. 2

3 Central Production at High Energies M X Colliding protons interact via a colour singlet exchange and remain intact after the interaction. In the collider experiment those protons follow magnetic field of the accelerator and remain in the beam pipe. A system of mass M X is produced, whose decay products are present in the central detector region. Tagging on forward protons assures rapidity gap (modulo) soft rescattering processes, which fill the gap. Such effect is quantified by gap survival probability factor. 3

4 Central Exclusive Production in DPE In the Central Exclusive Production process there is a momentum balance between the central system M X and the outgoing protons. M X = ξ 1 ξ 2 s - invariant mass p M X p For each proton vertex one has t four-momentum transfer ξ = Δp/p The massive system could form resonances. We expect that because of the constraints provided by the double Pomeron interaction, glueballs, hybrids, and other states coupling preferentially to gluons, will be produced with much reduced backgrounds compared to standard hadronic production processes. 4

5 Glueball Spectrum Sparse spectrum! )% # "! New I=0 mesons starting with GeV 0 -+, GeV No J PC -exotic glueballs until * $ +, )$ ( ' & $ "" # %""! "" # ""! %!" # %!"!!" #!"! "! $ "! & "! $!!!!! &!! & - % +,./, at 4 GeV % The glueball spectrum from an anisotropic lattice study ) $ Colin Morningstar, Mike Peardon Phys. Rev. D60 (1999) !!!!!!!! "# $ 5

6 The Relativistic Heavy Ion Collider RHIC is a QCD Laboratory: Nucleus-Nucleus collisions (AuAu, CuCu, UU ); Asym. Nucl. (dau, pau, CuAu); Polarized proton-proton; erhic - Future 6

7 How to measure Implementa0on at STAR Need detectors to measure forward protons: t - four-momentum transfer squared and ξ = Δp/p, MX invariant mass Roman Pots of PP2PP and; Detector with good acceptance and particle ID to measure central system STAR!"#$%&''&(!$&.)*/+.0*/-&!"#$%&'&(!$& ))*)+),*)-& DX Q4 Q3 D0 1. Roman Pots (RP) detectors to measure forward protons 2. Staged implementation for wide kinematic coverage Phase I, low-t coverage run 2009 Phase II*, current, no special conditions required Run 15 and Run 17 Phase II with bigger acceptance, new detectors will be needed. 7

8 Implementa0on at STAR Need detectors to measure forward protons: t - four-momentum transfer squared and ξ = Δp/p, MX invariant mass Roman Pots of PP2PP and; Detector with good acceptance and particle ID to measure central system STAR 8

9 Phase I preliminary results 9

10 Layout of the setup at STAR in 2015 and beyond Phase II* In this configuration Diffraction Program is able to acquire large data samples without special conditions. New DX D0 chambers 10

11 The pp2pp Setup Roman Pot - vessel Detector package placed inside the Roman Pot Roman Pot Station PP2PP and in

12 Roman Pot Operation (part II) Out In Routine operation of Roman Pots at 8σ y of the beam 12

13 Elastic scattering data The ongoing analyses in pp Central Exclusive Production (CEP) Single Diffraction Dissociation (SDD) p + p p + p elastic p + p p + X + p diffractive X= particles, glueballs p + p p + X SDD 13

14 Elastic Scattering collinearity 14

15 Elastic Scattering de/dx in Si # Events ADC count 15

16 Elastic Scattering: Special four - hour data taking Collected 9 M elastic triggers at the end of a store. Three vernier scans were performed. Roman Pots were moved to 18 mm from the beam for the last 1.5 hrs of data taking => 6σ y of the beam. t min 0.02 (GeV/c) 2 p + p p + p elastic It was a very smooth data taking period. We expect a good total and elastic σ measurement from RHIC based on the above data set. 16

17 CEP Event Selection two mesons Exactly 2 opposite-sign tracks in TPC matched with hits in Time-of-Flight detector Consistency between z-component of vertex reconstructed in TPC and vertex reconstructed using proton timing in Roman Pots z vtx TPC z vtx RP < 3σ Protons (consistent with ξ = 0) not collinear (to remove elastic events as described above)!"!!"! p 1 + p2 Veto in large BBC tiles (2.1 < η < 3.3) to confirm rapidity gap; Particle ID determined by (de/dx de/dx π, K ) < 3σ T > 60MeV / c Momentum balance between central system M X and protons measured in the Roman Pots 17

18 Geometrical Acceptance of the STAR experiment at s = 200 GeV Majority of protons in exclusive π+π production have very low momentum loss ξ < 0.05 Acceptance in -t range [0.03, 0.3] (GeV/c) 2 - Fractional momentum loss of protons in p + p p + π + + π + p not acceptance-corrected, statistical errors only Four-momentum transferred squared in p + p p + π + + π - + p not acceptance-corrected, statistical errors only Counts / East Roman Pots West Roman Pots 2 /c 2 Counts / 0.01 GeV 500 East Roman Pots West Roman Pots ξ = (p - p) / p t [GeV /c ] 18

19 CEP π + π - Sample: Missing Momentum 19

20 CEP π + π - Sample: Missing Momentum Detection and momentum reconstruction of all final state particles provides the ability to ensure exclusivity of the system via momentum balance check 20

21 Invariant Mass Distribution M X (ππ) miss Invariant mass of π π, p T < 0.1 GeV/c, not acceptance-corrected, statistical errors only 2 Events / 0.05 GeV/c PRELIMINARY p + p p + π + π + p s = 200 GeV < -t < 0.3 GeV /c opposite-sign same-sign ~2.5% of our data sample from fast offline Inv. mass m π π [GeV/c ] Small Background after momentum balance cut! 1. broad structure extending from π+π threshold to approximately 1 GeV/c 2 ; 2. sharp drop at about 1 GeV/c 2 ; 3. resonance-like structure between GeV/c 2 ; ~70K events expected for M x (π + π - ) > 1 GeV/c 2 21

22 Compare with CDF Result on π + π - Central Production (M. Żurek at this Conference) miss Invariant mass of π π, p T < 0.1 GeV/c, not acceptance-corrected, statistical errors only 2 Events / 0.05 GeV/c PRELIMINARY p + p p + π + π + p s = 200 GeV < -t < 0.3 GeV /c opposite-sign same-sign Inv. mass m π π [GeV/c ] pp p + π + π + p Phys.Rev. D91 (2015) 9, pp gap π + π gap Note that STAR essential features are the same as at other colliders Similar spectrum found by AFS at ISR (pp) and by CDF ( pp, no pp tagging rapidity gap method) 22

23 Invariant Mass Distribution M X (ΚΚ) prominent peak around GeV/c 2 some enhancement at f2(1270)/f0(1370) region) In spectrum measured by WA102 (fixed target) there is significant contribution from f0(980) not seen by STAR (most probably an effect of limited acceptance at low masses (low K pt )) Expect 10 4 exclusive K+K events at full statistics allowing measurement of cross-section and Partial Waves Analysis. 23

24 Particle Spectra in SDD in pp at s=200 GeV 24

25 Particle Ratios in SDD and CD 25

26 Summary of particle ratios 1. Preliminary results on π+/π and K+/K ratios are about 1 in SD and CD and agree with STAR previous non-diffractive measurements. 2. SD preliminary results on ratio are greater than STAR non-diffractive measurements. pp pp 3. Preliminary results on ratio in SD may indicate that baryon number transfer is smaller in the outgoing proton direction. 4. Comparisons with different generators, e.g. PYTHIA8, HIJING, are also planned to understand the dynamics of baryon number transport. 26

27 Physics related to Roman Pots in Run 15 Tagging exclusive γ+p in p+p /A+p : Access to Generalized Parton Distribution. π 0 production in the forward direction using STAR FMS and Roman Pots. Ultra Peripheral Collisions (UPC) with tagged forward protons in pau and pal. 27

28 Tagging exclusive γ+p in p+p /A+p : Access to Generalized Parton Distribution p γ Get quasi-real photon from a proton (p) or nucleus (A) Ensure dominance of γ from one identified proton by selecting very small t1, while t2 of typical hadronic size γ* small-t1 large impact parameter b (UPC) Final state lepton pair Time-like Compton Scattering p time-like Compton scattering: detailed access to GPD if we have transverse target polarization Challenging to suppress all backgrounds p, A p γ Z 2 A 2 Final state lepton pair not from γ* but from J/ψ Similar measurements in Ultra Peripheral Au+Au collisions at RHIC Estimates for J/ψ compatible with RHIC upgraded L Transverse target spin asymmetry calculable with GPDs Elke Aschenauer: Work in collaboration with Jakub Wagner, Dieter Mueller, Markus Diehl 28

29 Summary 1. STAR experiment at RHIC has suitable conditions to study diffractive physics in polarized pp and pa. 2. We had a very successful data taking run in 2015 both pp and pa. 3. Routine operation of Roman Pots at 8σ y of the beam was achieved. 4. In 2015 STAR collected large sample of high quality diffractive events including CEP-dedicated data. 5. Preliminary mass distributions of exclusively produced pion and kaon pairs look very promising allowing PWA on full data sample. 6. Study of particle production in SDD and CP looks very encouraging. 7. We are looking forward to proton-proton data run in 2017 at s = 510 GeV will be collected (larger kinematic region) hence comparison of results from two energy regimes will be possible. 29

PrHEP JHW2002. Experiments on high energy reactions in the diffractive regime at LHC. 1. Introduction. Twenty-sixth Johns Hopkins Workshop

$PrHEP JHW2002. Experiments on high energy reactions in the diffractive regime at LHC. 1. Introduction. Twenty-sixth Johns Hopkins Workshop$ PROCEEDINGS Experiments on high energy reactions in the diffractive regime at LHC Helsinki Institute for Physics, c/o CERN, Route de Meyrin, CH-1211 Geneva 23, Switzerland E-mail: Stefan.Tapprogge@cern.ch