Searches for New Physics with ZEUS Ioannis Gialas

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

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 EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

HERA p HERA DESY e Protons: 820 920 Ge. V Electrons or Positrons 27. 5 Ge. V EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

The detectors at HERA H 1 p Interaction point e+ Liquid Argon calorimeter Electrons Hadrons EESFYE, April 14, 2006 I. Gialas 44000 cells Searches for New Physics with ZEUS

The detectors at HERA ZEUS Electrons Hadrons 6000 cells 2 PMT/cell EESFYE, April 14, 2006 U Sci calorimeter I. Gialas Searches for New Physics with ZEUS

The detectors at HERA ZEUS e’ θe Proton remnant e p γh Current jet EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

HERA kinematics e or ν (k’) e (k) (x. P) p(P) q = Q 2 γ* , Ζ 0, W± } Proton remnant W • Q 2 : 4 momentum transfer squared • x : Fraction of proton momentum carried by the struck quark • y : Inelasticity parameter • W : Mass of the photon proton system EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

What does Q 2 show? d~0. 6 fm Rproton~ 1 fm d~0. 2 fm d~0. 02 fm EESFYE, April 14, 2006 Adopted from A. Caldwell I. Gialas Searches for New Physics with ZEUS

Single Vector Boson Production at HERA Main source of isolated leptons (SM cross section ~ 1. 1 pb @ 318 Ge. V) Background • Badly reconstucted NC and CC DIS events • Dilepton production EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

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: üSupersymmetry üMulti Higgs doublet models üExotic quarks (GUT’s, string theories, etc) EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

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 the process would proceed through anomalous couplings • Final state electron disappears down the beampipe in 65% of the events EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

Single top signature Backgrounds In the leptonic decay channel of the W • Isolated high energy lepton In leptonic channels • Significant missing transverse momentum • 2γ processes • Jet from the b quark decay In the hadronic channel • NC DIS • Single W production Hadronic channel • Three jets with • QCD • Mdijet ~MW • Mtotal ~ Mtop EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

Event Selection Data sets e+ p and e p 1998 – 2005 (HERA I & HERA II) A useful variable. Ecms = 318 Ge. V φacop pe. T >10 Ge. V θe >1. 5 φacop >0. 3 for PXT> 4 Ge. V Q 2 elec >5000 Ge. V 2 or PT>25 Ge. V Ε Pz < 50 Ge. V perpendicular to beam EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

98 – 99 e p control plots SM MC W production MC (SM) EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

04 – 05 Control Plots SM MC W production MC (SM) EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

Candidate event (4) EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

Candidate event (5) EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

Golden W production event from 1997 DESY note 99 054 EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

Isolated e candidates New results : To be shown in DIS 06 in Tsukuba EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

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 H 1 is not confirmed. EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

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 EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

Terminology (1) – R parity • R parity RP=( 1)3 B+L+2 S +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) EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS e+ d

Terminology (2) – some parameters Stop Branching ratios and Neutralino, chargino and gluino masses controlled by: • μ : mixes Higgs superfields • M 1, M 2, M 3 SUSY breaking parameters Related to U(1), SU(2), SU(3) gauginos • tanβ • EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

Decay modes to look for e+ + one jet (e J) e+ ν + many jets (ν MJ) f b d b f’ W+ νe e+ e+ + many jets (e MJ) EESFYE, April 14, 2006 d I. Gialas Searches for New Physics with ZEUS d

Selection for e+ many jet channel b e+ d • • Data sample 99 00 e+ p 65 pb 1 Ecms=318 Ge. V • • Q 2 DA>3000 Ge. V 2 Pt_antiparallel/Pt_parallel > 0. 05 (Pt of hadronic system) 0. 4<y. DA<0. 7 (100 280 Ge. V) • Data: 115 events EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

Invariant mass distributions • Good agreement between data and MC • No evidence of resonance • MLX>100 Ge. V EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

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% EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

Systematic checks (2) Systematic uncertainties on signal cross section • Uncertainty on the d quark pdf, from different CTEQ 6 parametrizations • Signal efficiency uncertainty EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

Limits on Minimal Supersymmetric Model Limits are calculated for different values of SUSY parameters 100 Ge. V< M 2 < 300 Ge. V (step 30 Ge. V) 300 Ge. V < μ < 300 Ge. V (step 20 Ge. V) Not considered scenario where: • neutralino not LSP • Mχ0 < 30 Ge. V (excl. by LEP) Yellow region: excluded at 95% C. L Red region: excluded in part of the SUSY parameters More restrictive limit up to masses ~240 Ge. V compared to low energy experiments (APV) EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

STOP m. SUGRA • Stop, chargino and neutralino masses and BR are determined by the value of m 0, m 1/2, tanb , sign(μ ). • Limit based on a Bayesian approach. • Red region: excluded at 95% C. L. The exclusion limit in the plane (m 0, m 1/2) 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 Ge. V (already excluded by LEP) Limits can rule out a stop with mass up to 270 Ge. V EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS

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’ 131 as a function of stop mass. • Direct limits on the stop mass was derived within the m. SUGRA model. For λ’ 131 =0. 3, Mstop < 270 Ge. V are excluded for most of the parameter space. • More restrictive limit up to masses ~240 Ge. V compared to low energy experiments (APV) EESFYE, April 14, 2006 I. Gialas Searches for New Physics with ZEUS