Pentaquark searches in ep interactions at HERA

1. IFAE – Incontri di Fisica delle Alte Energie Catania – March 30th – April 2nd 2005 Pentaquark searches inep interactions at HERA Alessandro Polini - INFN Bologna • Outline: • Introduction • Strange Pentaquarks: • + (ZEUS) • -- (ZEUS) • Charm Pentaquarks: • ΘC (H1, ZEUS) • Conclusions DESY Hamburg (D) A. Polini

2. What are Pentaquarks? • QCD: only colourless states can exist as free particles (confinement) • conventional wisdom: bound multiquark states (hadrons): qq (mesons) qqq (baryons) • other colourless combinations not forbidden: qqqq (tetraquarks) qqqqq (pentaquarks) ... • can they be bound tightly enough to have narrow width? A. Polini

3. udssd Strange Pentaquarks • some QCD models available: e.g. Diakonov, Petrov, Polyakov 1997: uudds = q+ m~1530 MeV, G < 15 MeV • Θ+ K+n is a manifestly exotic state (uudds) (B=+1, S=+1). • Θ K0Sp is not manifestly exotic. Can be interpreted as a Σ* (uudds). A. Polini

4. Evidence for Pentaquarks (Fixed Target) q+ observed by several fixed target experiments: • q+ n K+, LEPS, CLAS, SAPHIR m~1540±5 MeV, width ~ resolution • q p K0s, DIANA, ITEP, HERMES, SVD, COSY-TOF m~1527±3 MeV, width ~ resolution Evidence for --(ddssu), 0(udssu) by NA49 (pp s =17 GeV) Fixed target experiments: Pentaquarks produced from valence quarks A. Polini

5. Pentaquarks in High Energy Collisions? • Look in high CMS energy experiments, e.g. deep inelastic ep scattering (DIS) at HERA Pentaquark can be produced in fragmentation • Measurement of „established“, q+ (I=0, uudds) • Investigation of isospin triplet (q0, q++) ? • what about antipentaquarks uudds = q- ? • what about ddssu = -- ? • Heavy flavour Pentaquarks (c,...) uuddc/uuddc = qc A. Polini

6. Photon structure: Proton structure: Test pQCD approximation for gluon radiations: DGLAP, BFKL,CCFM … HERA e±p scattering Factorisation: Photon structure  Matrix element  Proton structure  Fragmentation √s = 300-318 GeV (Energy C.M.) W = m(g*p) (Hadronic Energy) Q2 = -q2 (Photon Virtuality) x = Bjorken x (energy carried by the struc parton in the proton) y = electron inelasticity s em 27.6 GeV ME Q2~0 (<1 GeV2) Photoproduction - Real Photon-Proton Scattering Q2>1 GeV2 Deep Inelastic Scattering - Scattered e±in main detector - *probing the proton structure - At High Q2 ~ M(Z0) weak contributions Z0,W± becoming relevant 820/920 GeV A. Polini

7. Search of Θ+ at HERA • K0Sp spectrum has many reported “Σ bumps” (observed by experiments at low significance). 1480 MeV (near threshold), 1560 MeV, 1580 MeV,… • A “window” exists 15001550 MeV where no states are reported. • A narrow state at ~1530 MeV in KSp would be consistent with the exotic Θ+ A. Polini

8. Search of Θ+ K0Sp in ZEUS • L=121 pb-1 (the full HERA I DIS sample) • Reconstruct K0sp(p) invariant mass • K0s Selection: PT(K0s) >0.3 GeV; |(K0s)|<1.5 ;  870x103 candidates • (anti) proton from dE/dx in the central tracking chamber • P(p)<1.5 GeV; dE/dx> 1.15 mips ;  60 % Purity • Systematics Checks: selection cuts, particle misidentification, reflections etc. K0s p k k p + - A. Polini

9. K0sp (+c.c.) Invariant Mass Spectrum • K0sp (+c.c.) invariant mass in bins of Q2 • No signal at ~1530 MeV for Q2>1 GeV2 • Bump at ~1520 MeV for Q2>10 GeV2 • seen also for Q2>1 GeV2 and W<125 GeV • Other bump at ~1465 MeV at high Q2 • Montecarlo normalized in the high mass region does not reproduce well the shape of the data. Unestablished PDG  bumps  (1480)  (1560) …? A. Polini

10. Θ+ Signal 1560 Σ bumps: 1480 1660 Clean signal in Σ bump-free region: 2 Gaussians fit + 3-par background M=1521.5±1.5(stat) -1.7(syst)MeV Stat. Signif. ~ 4.6  Breit-Wigner fit:  = 8±4 MeV 1580 1620 +2.8 1670 Monte Carlo normalized here “Σ bumps” not in Monte Carlo Signal seen in both charges (pK0s fit (96±34)~ 3) - pK0santipentaquark? (Results stable for Q2 > 15-30 GeV2) A. Polini

11. Θ+ Cross Section Determine cross-section using Σ+ MC (forced to be Θ+) in bins of Q2min + cross-section (Q2min=20 GeV) vis= 125±27(stat)-28(syst) pb +36 A. Polini

12. Cross Section and Production Ratio vs Q2 • +/((1116)p) production ratio R vs Q2min ~ 4% • R sets a lower limit on +/ production since + nK+decay also expected • R not compatible with other (non ep) experiments (HERA-B, ALEPH…) … phase space dependence? 4% A. Polini

13. q++ K+p (+c.c.) spectrum Is q+ member of a I=1 triplet? (q0, q+, q++) Search for q++ K+p (+c.c.) Select p,K± using dE/dx Known L0(1520) resonance seen in p K- (+c.c.) M = 1519.5 ± 1.0 MeV G = 15.6 ± 1.0 MeV consistent with PDG. No resonance in K+p (+c.c.) No signal for q++(1530) A. Polini

14. Search for NA49 signal (X--) in ZEUS ZEUS: • Analysis similar to NA49 using DIS data • Same 121 pb-1 DIS sample as for the Θ+ search. • Reconstruct Λ using dE/dx to ID protons A. Polini

15. Ξπ Mass Spectrum • No Ξ3/2 signal around 1860 MeV • Known non–exotic state Ξ0(1530) well visible. • The ratio R=N(Ξ3/2 )/N(Ξ0 (1530) ) around 1860 MeV is <0.29 at 95% C.L. • Not necessarily incompatible with NA49. Fragmentation region probed. A. Polini

16. Search for charmed PQ, qc D*p, in H1 D0ps • 96-00 Data, 75 pb-1, DIS: 1<Q2<100 GeV2 • particle identification via dE/dx • Normalised likelihood based on measured dE/dx & expectations • for , K, p and resolution: L()+L(K)+L(p) = 1 kp 3400 events with D* in DIS D* final selection (mass difference 3 tracks – 2 tracks) Proton identification via dE/dx qc Invariant Mass A. Polini

17. qc D*p significance estimate Background + signal hypothesis fit: • Background only hypothesis: Nb=51.7±2.7 • background fluctuation probability: 4x10-8 (Poisson)= 5.4  (Gauss) • Independent confirmation: Signal observed also in Photoproduction A. Polini

18. Search for ΘC  D*p at ZEUS • ZEUS has made a search in 126 pb-1 of DIS and photoproduction data. • Both Kππ and Kππππ decay modes of D* were used. (H1 uses Kππ only). • Altogether, 62600 D* are found. 13500 in DIS alone. • If 1% of D* came from ΘC expect >600 ΘC. A. Polini

19. Search of ΘC at ZEUS MC simulation of ΘC at 1% of observed D*s Proton identified via dE/dx Procedure refined—using 2 probability of a proton hypothesis rather than a simple cut as in Θ+ “1%” signal is ruled out for the whole sample at 9σ For DIS only ruled out at 5σ A. Polini

20. ΘC ZEUS vs H1 comparison ZEUS with “H1 Selection” Kππ channel only; -1.5 < η(D*) < 1.0 DIS: 0.05 > y > 0.7; PT(D*)>1.5 GeV Photoproduction: 0.2 > y > 0.8; PT(D*)>2.0 GeV ~3400 D* ~5900 D* ~4900 D* ~11500 D* A. Polini

21. ΘC Summary • H1 collaboration has reported an observation of a narrow state ΘC  D*p at 3099 MeV in DIS. • The signal corresponds to ~1% of the observed number of D* (51±11 ΘC observed) • H1 also reports a compatible state observed in photoproduction (Q2≈0). • No resonance structure observed from >60x103 D*. • “1%” of D* signal is ruled out at 9σ for all and 5σ for DIS only. • The upper limit of observed D*s originating from ΘC is 0.23% (95% C.L.) After acceptance correction: (assuming PT(ΘC) and η(ΘC) same as those of D*): the upper limit of D*s, in the measured kinematic region, originating from ΘC is 0.16% (at 95% C.L.) The results from ZEUS are not compatible with the ones by H1. A. Polini

22. Conclusions and Outlook Pentaquarks situation still unclear: • Θ KSp: A narrow state observed at 1520 MeV at Q2>20 GeV2 (ZEUS). Maybe the Θ+ pentaquark reported by fixed target experiments. • Ξ3/2 Ξπ: No state observed by ZEUS. Result may still be compatible with NA49: fragmentation region probed. • ΘC  D*p: 5.4 signal in H1. No state observed by ZEUS. A production at 1% of the D* rate is ruled out by ZEUS. Results are not compatible. Great expectations from HERA II data • Improved detectors (Tracking, Silicon, Trigger) • Higher Luminosity. HERA II running till mid 2007 A. Polini

23. Backup Slides A. Polini

24. Comparison of +Experiments A. Polini

25. Published Null Experiments A. Polini

26. A. Polini