Searches at ZEUS

Stefano Antonelli1 1CNAF, viale Berti Pichat 6/2, 40127 Bologna, Italy

DOI: http://dx.doi.org/10.3204/DESY-PROC-2012-02/294

The results of recent searches for scalar and vector leptoquarks and anomalous single top production in e± collisions at HERA are presented. The searches make use of the full ZEUS data set corresponding to 0.5 fb−1. No evidence for leptoquarks signals is found and limits are set on the Yukawa coupling, λ, as a function of the leptoquark mass for different leptoquark types within the Buchmüller-Rückl-Wyler model. No evidence for top production is found and upper limits on the single top cross section via flavour changing neutral current and on the anomalous coupling κtuγ are set.

1 Search for leptoquarks

Many extensions of the (SM) predict the existence of particles, such as lepto- quarks (LQs), carrying both and baryon numbers [1]. At HERA, LQs could be resonantly produced in the s-channel or exchanged in the u-channel between the initial state lepton of en- ergy 27.6 GeV and a quark coming from the of energy up to 920 GeV with subsequent decays into and quark or neutrino and quark, Fig. 1. These decays have a topology similar to deep-inelastic scattering (DIS) neutral current (NC) or charged current (CC). Analysis searches for deviations from the SM in the lepton-jet invariant mass spec- trum at different lepton scattering angle (θ∗) in the lepton-jet scattering frame to reduce DIS background. Two plots of mass spectra measured for NC-like and CC-like events for 106 pb−1 data set are showed in Fig. 2.

Since no evidence for any leptoquark sig- Figure 1: Leptoquark diagrams: s-channel LQ nal is found, limits are derived on the Yukawa production (left) and u-channel LQ exchange coupling λ as a function of the mass for dif- (right). ferent leptoquark states as described by the Buchmüller-Rückl-Wyler model (BRW) [1]. Limits are evaluated including also data recorded with the ZEUS detector in 1994-2000, as published in [2], for a total of 0.5 fb−1. Table 3 shows mass limit for the 14 BRW LQs at L L λ = 0.3. Figures 4 show the limits on the S0 and S1/2 compared to the limits from CMS [3], D0 [4] and L3 [5]. Assuming λ = 0.3, the mass limits range from 291 to 629 GeV.

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λ assuming states vector 7 + scalar 7 for Limits 3: Figure 10 10 •2 •1 1 ...... 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2

Nobs/Npred Events 10 10 10 0.5 1.5 10 0 2 4 6 8 0 2 4 6 8 0 320 300 280 260 240 220 200 180 160 140 120 100 •1 1 2 3 1 √ ZEUS (prel.)e SM (cos ZEUS (prel.)e SM withsyst.uncertainty(exp.+theory) s 318 = S θ *<0.4) 0 L ZEUS ZEUS (prel.)e L3 indirectlimit D0 pairprod. CMS pairprod. • • p (106pb p (106pb M M e.Tedmnn rcs o Msnl o rdcin hre cur- charged production, top single SM for process dominant The GeV. ZEUS LQ LQ QTp (F=2) Type LQ •1 •1 QTp (F=0) Type LQ , P , P GV ES(prel.) ZEUS (GeV) (prel.) ZEUS (GeV) e e =•0.27) =•0.27), cos ± p (498pb M LQ θ *<0.4 •1 (TeV) ) e • p → M ejs e • (GeV) X 435 504 V S 0 0 L L 326 293 V S 0 0 R R 291 343 V S ˜ ˜ Events 0 N /N

0 obs R pred 10 R 10 0.5 1.5 10 0 2 4 6 8 0 2 4 6 8 0 320 300 280 260 240 220 200 180 160 140 120 100 •1 1 2 1 466 629 V S 1 S 1 L L λ e 10 10 0 − L e SM (cos SM withsyst.uncertainty(exp.+theory) ZEUS (prel.)e ZEUS (prel.)e •2 •1 1 + S V p 292 322 lf)and (left) em 1 1 L p ...... 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 L / / θ 2 *<0.4) ape(os n h CSM NC the and (dots) sample 2 ape(os n h CSM CC the and (dots) sample + + p (104pb p (104pb S V coupling 324 300 1 1 R R / / 2 2 ZEUS •1 •1 , P , P M S V e e 409 293 ˜ ˜ =0.32), cos =0.32) M 1 1 L S L / / ejs 2 2 νjs S 1 L 1/2 L ZEUS / θ λ 2 *<0.4 L3 indirectlimit D0 pairprod. CMS pairprod. ZEUS (prel.)e nthe in rgt rmZEUS, from (right) nthe in = e + p √ M → ν 4

js ν ± πα (GeV) p (498pb X e − e cosθ M + p LQ 0 = I 2012 DIS p •1 (TeV) ) → ∗ → . 3 cosθ < e . − νX 0 X . 4 ∗ rent process ep → νtX, has a cross section of less than 1 fb [6, 7] so no sizeable production is ex- pected and any excess can be attributed to new physics. In several extensions of the SM [8], single top production can happen via a flavour changing neutral current (FCNC) process mediated by an effective coupling which allows a u−t tran- sition via a neutral vector boson γ (tuγ) or Z (tuZ), see Fig. 5 Due to the large Z mass the process is more sensitive to a coupling tqγ. Furthermore, the production of single is most sensitive to the tuγ cou- pling because large values of x, the fraction of Figure 5: Anomalous single-top production via the proton momentum carried by the struck flavour changing neutral current transitions at quark, are needed to produce a top quark and, HERA with subsequent decays t → bW + and + + at large x, u-quark parton distribution func- W → νe(νµ)e (µ+). tion (PDF) of the proton is dominant. The search has been performed in the electron and muon channels. Figure 6 shows the preselection plots in the muon (left) and electron (right) channels.

ZEUS ZEUS 2 2 10 2 10 ZEUS 0.37 fb•1 ZEUS 0.37 fb•1 10 2 SM 10 SM Events W Events Events W Events 10 10 10 10

1 1 1 1

•1 •1 •1 •1 10 10 20 30 40 50 60 70 80 10 0 0.5 1 1.5 2 2.5 3 10 0 10 20 30 40 50 60 70 80 10 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 µ µ p (GeV) θ (rad) pel (GeV) θel T T (rad)

2 2 2 10 10 102 10 Events Events Events Events

10 10 10 10

1 1 1 1

•1 •1 •1 •1 10 0 0.5 1 1.5 2 2.5 3 3.5 10 0 10 20 30 40 50 60 70 80 10 0 0.5 1 1.5 2 2.5 3 3.5 4 10 0 10 20 30 40 50 60 70 80 φacop miss φacop miss (rad) PT (GeV) PT (GeV)

102 2 2 10 102 10 Events Events Events Events 10 10 10 10

1 1 1 1

•1 •1 •1 •1 10 0 10 20 30 40 50 60 70 80 90 100 10 0 10 20 30 40 50 60 70 80 10 0 10 20 30 40 50 60 70 80 90 100 10 0 10 20 30 40 50 60 70 80 M (GeV) had M (GeV) had T PT (GeV) T PT (GeV)

Figure 6: Preselection plots in the muon (left) and electron (right) channels.

Since no excess of events above the SM expectations is observed, a further selection is made to improve the limit on FCNC cross section under the assumption√ of no signals. The 95% C.L. limit on the cross section is found to be: σ < 0.24 pb at s = 318 GeV. The limit on the cross section is converted into a limit on the coupling κγ : κγ < 0.18 (95% C.L.). This result has been combined with a previous ZEUS result [9] giving the following results: σ < 0.13 pb and κγ < 0.13 (95 % C.L.) [10]. Constraints on the anomalous top branching ratios (Br) t → uγ and t → uZ were also evaluated assuming a non-zero vZ . Figure 7 shows the ZEUS boundary in the (Bruγ , BruZ) plane compared to limits from [11], ALEPH [14], CDF [12], D0 [13]. For low values of vZ , resulting in branching ratios of t → uZ of less than 4%, this paper provides the current best limits.

DIS 2012 3 ZEUS 1 uZ ZEUS 0.5 fb•1 Br H1 t → uγ excluded by ZEUS CDF t → u(c)γ,u(c)Z D0 t → u(c)Z ALEPH t → u(c)γ,u(c)Z

10•1

10•2 10•3 10•2 10•1 Bruγ

Figure 7: ZEUS boundary in the (Bruγ , BruZ) plane. Also shown are boundaries of H1 [11], CDF [12], D0 [13] and ALEPH [14]. The shaded area is excluded. The dark shaded region denotes the area uniquely excluded by ZEUS.

References

[1] W. Buchmüller, R. Rückl, D. Wyler, “Leptoquarks in lepton-quark collisions”, Phys. Lett. B191 (1987) 442. Erratum in Phys. Lett B448, (1999) 320. [2] ZEUS Coll., S. Chekanov et al. , “Search for resonant decays to lepton+jet at DESY HERA and limits on leptoquarks”, Phys. Rev. D68 (2003) 052004. [3] Chatrchyan, Serguei and others , “Search for first generation scalar leptoquarks in the eν jj channel in pp collisions at sqrt(s)=7 TeV”, Phys. Lett. B703 (2003) 246. [4] Abazov, V.M. and others , “Search for pair production of first-generation leptoquarks in p pbar collisions at sqrt(s)=1.96 TeV”, Phys. Lett. B681 (2009) 224. [5] Acciarri, M. and others , “Search for manifestations of new physics in fermion pair production at LEP”, Phys. Lett. B489 (2000) 81. [6] T. Stelzer, D. Sullivan and S. Willenbrock, Phys. Rev. D56 (1997) 5919. [7] T. Moretti and K. Odagiri, Phys. Rev. D57 (1998) 3040. [8] T.Han, J.L.Hewett, Phys. Rev. D60 (1999) 074015. [9] ZEUS Coll., S. Chekanov et al., Phys. Lett. B559 (2003) 153 [10] ZEUS Coll., H. Abramowicz et al., Phys. Lett. B708 (2012) 27 [11] H1 Coll., F.D. Aaron et al., Phys. Lett. B678 (2009) 450 [12] CDF Coll., F. Abe et al., Phys. Rev. Lett. 80 (1998) 2525 [13] D0 Coll., V.M. Abazov et al., Phys. Lett. B701 (2011) 313 [14] Aleph Coll., A. Heister et al., Phys. Lett. B543 (2002) 173

4 DIS 2012