The excess in high-Q^2 events at HERA is introduced and possible explanations
within the framework of R-parity violating supersymmetry are discussed.Comment: LaTeX, 5 pages, 2 figures. Talk presented at the International
  Workshop on Physics "Beyond the Standard Model: from Theory to Experiment",
  Valencia, Spain (October 13 - 17, 1997), to appear in the Proceeding

Raychaudhuri, Sreerup

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arXiv

The excess in high-Q/sup 2/ events at HERA is introduced and possible explanations within the framework of R-parity violating supersymmetry are discussed. (13 refs).The excess in high-Q^2 events at HERA is introduced and possible explanations within the framework of R-parity violating supersymmetry are discussed

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R-Parity Violation and the HERA Events.Sreerup RaychaudhuriTheory Division, CERN, CH 1211 Geneva 23, Switzerland.The excess in high-Q2 events at HERA is introduced and possible ex-planations within the framework of R-parity violating supersymmetryare discussed.Early this year, the H1 and ZEUS Collaborations at the HERAfacility at DESY, Hamburg, created a sensation by announcing 1that they had an excess in back-scattered positrons (in e+p colli-sions) over the deep inelastic scattering (DIS) predictions of theStandard Model (SM). Though statistics were low, it appeared tobe a hint of new physics beyond the SM, and a series of theoreti-cal papers investigating the eect followed. Subsequently, the twocollaborations presented 2 the results of later runs, in which theyagain had an excess, though at a lower level. More data have beencollected in the subsequent run of HERA, which are still beinganalysed. This is the current situation. Thus, the case for newphysics at HERA | though weaker than it was initially thoughtto be | still remains the best experimental hint of physics beyondthe SM.The scattering of positrons (beam energy E0e = 27:5 GeV) fromprotons (beam energy E0p = 820 GeV) at HERA can lead to neutralcurrent (NC) nal states with a positron (e+) and a jet (J) describ-able by any two of the following observables: Ee; e; EJ ; J whereEe; EJ are the energies and e; J are the scattering angles respec-tively (with respect to the proton beam direction). The observedexcess can be clearly seen in Fig.1(a), which shows the measuredcross-section as a function of a cut on the minimum Q2 of theevents. As errors are large, the excess is at the 2 level, whichmeans that they could well be due to statistical fluctuations. It is1also clear that the H1 Collaboration sees a larger deviation fromthe SM than the ZEUS Collaboration.0.0010.010.11101000 10000 20000 30000 40000Q2min (GeV2)σ(Q2>Q2 min(pb)SM200 GeV220 GeV(b)(a)Figure 1: (a) Comparison of SM Prediction with data (from Ref.2). The solidline shows the DIS prediction. (b) Eects of a resonance of mass 200 GeV and220 GeV (solid lines) for 01i1pB:R:(e+J) = 0:04.It is, of course, possible to dismiss the excess as an artefact.Nevertheless, it is interesting to seek theoretical explanations of theeect, in case it should turn out to be a genuine one. Three kinds ofsolution have, in fact, been tried. These are (a) attempts to modifythe proton structure functions 3, (b) explanations in terms of con-tact interactions arising out of new physics at a high scale 4, and(c) the postulate that a new particle resonance, of mass around200 GeV, is causing the eect. The rst two options have beenshown not to work well 3, and it is, thus, to the third possibilitythat we must appeal. A resonance produced at HERA in a e+q ore+g collision must have the quantum numbers of a leptoquark ora leptogluon respectively. As solutions with vector leptoquarks orleptogluons do not t the kinematic proles of the observed eventswell, a better option is to postulate a scalar leptoquark resonance.Such particles are predicted in various extensions of the SM, of2which one is the R-parity-violating version of the minimal super-symmetric extension of the SM, where the squarks can behave asleptoquarks.The Lagrangian describing this last interaction is given by:L = 0ijkh‘iPRdkeuLj − (i$ j)i + H:c: (1)where PR =12(1 + γ5) and euL is the scalar superpartner of theleft-chiral u-quark. The i; j; k are generation indices. If we wish toobtain reasonably large cross-sections, the only relevant couplingsare 0121, 0131, 132 corresponding to e+d ! ecL, e+d ! etL, ande+s ! etL respectively 5. (For the last option, it is a sea s-quarkwhich contributes: the flux being small, one requires a large 0132 toget the observed excess 6.) The calculation is simple and Fig.1(b)shows the results as a function of the minimum Q2. It is obviousthat the data are compatible with resonant masses in the 200{220 GeV range, with H1 data preferring the lower and ZEUS datapreferring the upper mass value. There will also be some variationwith the coupling strength, which is not shown in the gure. Onemust hope that more data will lead to a better idea of the mass.An intensive search has also been carried out at the Tevatron fora pair of leptoquarks (or squarks), which can be produced throughthe same interaction as the above. The signal would be a dielectronand one or two jets. These searches have proved negative 7, leadingto the bound meq > 240 Gev for branching ratio B:R:(e+J) = 1,206 GeV for B:R:(e+J) = 0:5. Thus, if the HERA events are to beexplained by a leptoquark/squark resonance of mass around 200GeV, this particle must have a B:R:(e+J) of 0.4 or less. This ispossible for a charge-23leptoquark if it mixes with another lepto-quark, but such models look a little contrived. On the other hand, asquark can easily have R-parity conserving decay modes to gauginostates. In a sense, therefore, the negative results of the leptoquarksearches at the Tevatron make supersymmetry the most attractiveexplanation of the HERA excess. Another interesting possibility 83{ that of seeing like-sign dileptons from gluino pair-production inthe event of a ecL resonance around 200 GeV { has been investi-gated by the CDF Collaboration with negative results 9; this leadsto fresh bounds on the gluino mass if the HERA events have agenuine supersymmetric solution.The question which immediately arises as a result of the abovereflections is: are the other decay channels observable ? The mostlikely other decay channel, to e + J , must lead, at HERA, to acorresponding excess in the charged current (CC) DIS data. Infact, the H1 Collaboration has found a tentative CC excess; theZEUS Collaboration has practically none 2. As the eJ decay modewill have a branching ratio of 0.6 or more, one must postulate lowdetection eciencies, in which case a mixed leptoquark solutionruns into diculties. In supersymmetry, however, it is possibleto have scenarios with low eciencies. One of these 10 assumes alight sneutrino, of mass 60 { 80 GeV, in addition to a ecL squark(with a 0121 coupling). Unfortunately, this scenario also predictsan excess of four-jet events at LEP, which has not been found(despite early excitement over the ALEPH Collaboration’s results).Another suggestion 11 that a resonant etL may directly decay intoa ebL (which goes to ed through 0131) leads to problems with thenumber of jets and is rather dicult to t in with -parameterconstraints.Two possible solutions 12 which do not have these drawbacksrequire a et1 resonance, where met1 − mebL < 35 GeV and met1 >me+1 > mebL , and a mixed stop state is required to have the smallsplitting in stop-sbottom masses. It is worth mentioning that thereare theoretical drawbacks associated with a light ecL which do notapply 13 in the case of a et. One could now have (a) a neutralino01 lying between the +1 and theebL in mass, which leads to thedecay chain et1 ! be+1 ! bf f 0 e01 ! bf f 0bebL ! bf f 0bd. For theassumed mass-spectrum most of the jets resulting from the cascadedecays are soft and would pass undetected. Single hard jets would4result from the last decay and the observed level of excess CCevents can be explained. Alternatively (b) the chargino producedin the rst decay can undergo the Cabibbo-Kobayashi-Maskawa-suppressed decay e+1 ! cebL where ebL ! d as before. Once again,one can have single jet events, though one requires rather largevalues of the soft supersymmetry-breaking parameter  to obtainthe observed CC excess. Fortunately, such values do not contradictcurrent low-energy bounds on supersymmetric models.To conclude, a great deal of excitement has been generatedby the HERA excess for which it seems that R-parity violatingsupersymmetry is the best explanation. With the present level ofstatistics it is hard to make a more denite statement. Furtherresults from HERA are, therefore, eagerly awaited.1. M. Adlo et al (H1 Collaboration), Z. Phys. C74, 191 (1997); J. Bre-itweig et al (ZEUS Collaboration), Z. Phys. C74, 207 (1997).2. B. Straub, (for H1 and ZEUS Collaborations), Talk presented at LP97,Hamburg (July 1997).3. G. Altarelli, Talk presented at LP97, Hamburg (July 1997), hep-ph/9710434.4. V. Barger et al, Phys. Lett., B404, 147 (1997).5. D. Choudhury and S. Raychaudhuri, Phys.Lett. B401, 54 (1997);G. Altarelli et al, Nucl. Phys. B506, 3 (1997); H. Dreiner andP. Morawitz, Nucl. Phys. B503, 55 (1997); J. Kalinowski et al, Z. Phys.C74, 595 (1997); T. Kon and T. Kobayashi, Phys. Lett. B409, 265(1997).6. J. Ellis et al, Phys. Lett. B408, 252 (1997).7. F. Abe et al (CDF Collaboration), Fermilab preprint FERMILAB-PUB-97/280-E (August 1997); B. Klima (D0 Collaboration), Talk presentedat EPS97, Jerusalem (August 1997).8. D. Choudhury and S. Raychaudhuri, Phys. Rev., D56, 1778 (1997).9. M. Chertok and T. Kamon (CDF Collaboration), private communica-tion.10. G. Altarelli et al, Nucl. Phys. B506, 29 (1997).11. T. Kon et al, hep-ph/9707355.12. M. Carena et al, Phys. Lett. B (to appear), hep-ph/9707458.13. A. Joshipura et al, in these Proceedings and hep-ph/9712219.50.0010.010.11101000 10000 20000 30000 40000Q2min (GeV2)σ(Q2>Q2 min(pb)SM200 GeV220 GeV(b)

R-Parity Violation and the HERA Events

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