Searches for New Physics with ZEUS

1. Searches for New Physics with ZEUS Ioannis Gialas University of Aegean April 14, 2006 EESFYE, Ioannina I. Gialas Searches for New Physics with ZEUS

2. Some searches for new Physics at HERA Single top production • Physics motivation • Event selection • Some candidate events • Results Stop production • Physics motivation • Event selection • Systematic checks • Limits I. Gialas Searches for New Physics with ZEUS

3. p HERA e DESY HERA Protons: 820 - 920 GeV Electrons or Positrons 27.5 GeV I. Gialas Searches for New Physics with ZEUS

4. p Interaction point Liquid Argon calorimeter Electrons 44000 cells Hadrons The detectors at HERA - H1 e+ I. Gialas Searches for New Physics with ZEUS

5. Electrons Hadrons 6000 cells 2 PMT/cell U-Sci calorimeter The detectors at HERA - ZEUS I. Gialas Searches for New Physics with ZEUS

6. p Proton remnant e’ θe e γh Current jet The detectors at HERA - ZEUS I. Gialas Searches for New Physics with ZEUS

7. e or ν (k’) e (k) q = -Q2 γ* , Ζ0, W± (xP) } Proton remnant p(P) W HERA kinematics • Q2: 4-momentum transfer squared • x : Fraction of proton momentum carried by the struck quark • y : Inelasticity parameter • W : Mass of the photon-proton system I. Gialas Searches for New Physics with ZEUS

8. What does Q2 show? d~0.6 fm Rproton~ 1 fm d~0.2 fm d~0.02 fm I. Gialas Searches for New Physics with ZEUS Adopted from A. Caldwell

9. Single Vector Boson Production at HERA • Main source of isolated leptons (SM cross section ~ 1.1 pb @ 318 GeV) • Background • Badly reconstucted NC and CC DIS events • Dilepton production I. Gialas Searches for New Physics with ZEUS

10. Why single leptons? • Many processes indicating new physics would demonstrate themselves with isolated lepton in the final state. • Supersymmetric processes (eg. ) • R-parity violating supersymmetric processes (eg. Stop production) • Top quark production at a rate higher than predicted by SM. Such enhancement could be achieved through flavor changing neutral current process (FCNC) which could be attributed to: • Supersymmetry • Multi-Higgs-doublet models • Exotic quarks (GUT’s, string theories, etc) I. Gialas Searches for New Physics with ZEUS

11. Single top production • Flavor Changing Neutral Current events has a small cross section (<1 fb) in the SM • If events at HERA are attributed to single top production then the process would proceed through anomalous couplings • Final state electron disappears down the beampipe in 65% of the events I. Gialas Searches for New Physics with ZEUS

12. Single top signature • Backgrounds • In leptonic channels • 2γ processes • NC DIS • Single W production • Hadronic channel • QCD • In the leptonic decay channel of the W • Isolated high energy lepton • Significant missing transverse momentum • Jet from the b-quark decay • In the hadronic channel • Three jets with • Mdijet ~MW • Mtotal~ Mtop I. Gialas Searches for New Physics with ZEUS

13. Data sets e+-p and e--p 1998 – 2005 (HERA-I & HERA-II) Ecms = 318 GeV peT >10 GeV θe >1.5 φacop >0.3 for PXT> 4 GeV Q2elec >5000 GeV2 or PT>25 GeV Ε-Pz < 50 GeV A useful variable. φacop perpendicular to beam Event Selection I. Gialas Searches for New Physics with ZEUS

14. SM MC W production MC (SM) 98 – 99 e- - p control plots I. Gialas Searches for New Physics with ZEUS

15. 04 – 05 Control Plots SM MC W production MC (SM) I. Gialas Searches for New Physics with ZEUS

16. Candidate event (4) I. Gialas Searches for New Physics with ZEUS

17. Candidate event (5) I. Gialas Searches for New Physics with ZEUS

18. Golden W production event from 1997 DESY note 99-054 I. Gialas Searches for New Physics with ZEUS

19. Isolated e-candidates New results : To be shown in DIS06 in Tsukuba I. Gialas Searches for New Physics with ZEUS

20. Summary isolated leptons • Production of events with an isolated high energy lepton and high missing PT can indicate production mechanisms beyond the SM. • A search for this type of events was undertaken using a total of 106 pb-1. • The measured event rate is consistent with SM predictions • The excess in this type of events observed by H1 is not confirmed. I. Gialas Searches for New Physics with ZEUS

21. Supersymmetric searches • HERA is limited in the search for supersymmetric particles due to the restriction of pair-production of SUSY particles. • However, for mechanisms allowing single supersymmetric particle production it could still further the experimental limits I. Gialas Searches for New Physics with ZEUS

22. e+ d Terminology (1) – R-parity • R-parity RP=(-1)3B+L+2S +1 for particles -1 for sparticles R-parity conservation Implies pair production of supersymmetric particles Stable LSP (cold dark matter candidate) R-parity violation Resonant supersymmetric particle production Supersymmetric particles can decay back to SM particles R-parity violating terms λ are dimensionaless free parameters For stop production in ep colliders (lightest Sparticle) I. Gialas Searches for New Physics with ZEUS

23. Terminology (2) – some parameters Stop Branching ratios and Neutralino, chargino and gluino masses controlled by: • μ: mixes Higgs superfields • M1,M2,M3 SUSY breaking parameters Related to U(1), SU(2), SU(3) gauginos • tanβ I. Gialas Searches for New Physics with ZEUS

24. e+ + one jet (e-J) e+ ν + many jets (ν-MJ) f b f ’ b d W+ νe e+ e+ + many jets (e-MJ) d d Decay modes to look for I. Gialas Searches for New Physics with ZEUS

25. b e+ d Selection for e+-many jet channel • Data sample 99-00 e+-p 65 pb-1 • Ecms=318 GeV • Q2DA>3000 GeV2 • Pt_antiparallel/Pt_parallel > 0.05 (Pt of hadronic system) • 0.4<yDA<0.7 (100-280 GeV) Data: 115 events I. Gialas Searches for New Physics with ZEUS

26. Invariant mass distributions • Good agreement between data and MC • No evidence of resonance • MLX>100 GeV I. Gialas Searches for New Physics with ZEUS

27. Systematic checks (1) Systematic uncertainties on NC-SM as a function of reconstructed mass • SM pdf uncertainties • CAL energy scale • Different Monte Carlo generators • Integrated luminosity ±2% I. Gialas Searches for New Physics with ZEUS

28. Systematic checks (2) Systematic uncertainties on signal cross section • Uncertainty on the d-quark pdf, from different CTEQ6 parametrizations • Signal efficiency uncertainty I. Gialas Searches for New Physics with ZEUS

29. Limits on Minimal Supersymmetric Model • Limits are calculated for different • values of SUSY parameters • 100 GeV< M2 < 300 GeV (step 30 GeV) • ­300 GeV < μ < 300 GeV (step 20 GeV) • Not considered scenario where: • neutralino not LSP • Mχ0< 30 GeV (excl. by LEP) • Yellow region: excluded at 95% C.L • Red region: excluded in part of the • SUSY parameters • More restrictive limit up tomasses • ~240 GeV compared tolow energy experiments (APV) I. Gialas Searches for New Physics with ZEUS

30. STOP - mSUGRA • Stop, chargino and neutralino masses and BR are determined by the value of m0, m1/2, tanb , sign(μ ). • Limit based on a Bayesian approach. • Red region: excluded at 95% C.L. The exclusion limit in the plane (m0, m1/2) was defined as the region of the plane for which R < 0.05, where R is the ratio between the signal and the SM likelihoods. • Blue region: No solutions. • Yellow region: Mχ0 < 30 GeV (alreadyexcluded by LEP) • Limits can rule out a stop with • mass up to 270 GeV I. Gialas Searches for New Physics with ZEUS

31. Stop search summary • No evidence for resonances in the decay channels with jets and one high-Pt lepton was found. • Results were interpreted in terms of R-parity violating MSSM setting constraints on the Yukawa coupling λ’131as a function of stop mass. • Direct limits on the stop mass was derived within the mSUGRA model. For λ’131=0.3, Mstop < 270 GeV are excluded for most of the parameter space. • More restrictive limit up tomasses ~240 GeV compared tolow energy experiments (APV) I. Gialas Searches for New Physics with ZEUS