The D and CDF experiments have each collected more than 8 fb..1 in Run II of Fermilab\u27s Tevatron, and have many recent search results which use up to 5.2 fb..1. Here I summarize the results of a variety of searches for physics beyond the Standard Model with an emphasis on searches for very exotic phenomena. I will present the status of model-inspired searches for several signatures of supersymmetry, as well as several other searches for several \hidden- valley  inspired models, all of which contain photons and jets in the nal state

Brau, B

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Tevatron Searches for New Physics with Photons and Jets

University of Massachusetts AmherstScholarWorks@UMass AmherstPhysics Department Faculty Publication Series Physics2011Tevatron Searches for New Physics with Photonsand JetsB BrauUniversity of Massachusetts - Amherst, bbrau@physics.umass.eduFollow this and additional works at: https://scholarworks.umass.edu/physics_faculty_pubsThis Article is brought to you for free and open access by the Physics at ScholarWorks@UMass Amherst. It has been accepted for inclusion in PhysicsDepartment Faculty Publication Series by an authorized administrator of ScholarWorks@UMass Amherst. For more information, please contactscholarworks@library.umass.edu.Recommended CitationBrau, B, "Tevatron Searches for New Physics with Photons and Jets" (2011). Physics Department Faculty Publication Series. 1099.Retrieved from https://scholarworks.umass.edu/physics_faculty_pubs/1099Tevatron Searches for New Physics with Photons and JetsBenjamin. P. BrauDepartment of Physics,1126 Lederle Graduate Research Tower,University of Massachusetts,Amherst, MA 0100, USAThe DØ and CDF experiments have each collected more than 8 fb−1 in Run II of Fermilab’sTevatron, and have many recent search results which use up to 5.2 fb−1. Here I summarizethe results of a variety of searches for physics beyond the Standard Model with an emphasison searches for very exotic phenomena. I will present the status of model-inspired searchesfor several signatures of supersymmetry, as well as several other searches for several “hidden-valley” inspired models, all of which contain photons and jets in the final state.1 IntroductionDespite its enormous success at explaining the interactions of the known particles, we are certainthat the Standard Model (SM) is incomplete. We now have overwhelming evidence that theuniverse contains mostly dark matter, yet the SM provides no explanation for it. Among themodels on the market, Supersymmetry (SUSY) is favored by many, in part because it naturallyhas a dark matter candidate in the lightest supersymmetric particle (LSP). Recently though,a class of models has been proposed that can generally be described as “hidden-valley” (HV)models. These are characterized by the general feature that they propose relatively light particlesone might expect to have already discovered, but haven’t been because some mechanism preventsthem from interacting with the SM particles, except gravitationally. These models therefore alsocan naturally provide a solution to the dark matter puzzle. Most collider experiments sensitiveto new higher energy scales have searched for SUSY in a variety of final states, and the Tevatronis no exception. Hidden-valley models are relatively recent, and searches for them at collidersare just emerging. I will discuss several searches for SUSY and HV models at the CDF and DØexperiments at the Tevatron with 2.6-5.2 fb−1.2 Searches for Sbottom with b jets and E/TIn the MSSM, if tanβ is large, there is a large mass splitting in the third generation. Thisleads to relatively low masses for the lightest scalar bottom quark (sbottom) state. In an R-parity conserving scenario, sbottoms are expected to be produced at the Tevatron in pairs viagluon-gluon fusion or qqbar annihilation. The sbottom is then kinematically required to decayinto bottom and the lightest neutralino (assumed to be the LSP), leading to final states whichinclude two b-jets and substantial missing transverse energy (MET) from the undetected LSP.Both Tevatron experiments have searched for bbottom in events with jets, E/T , and b-tags.arXiv:1007.2244v2  [hep-ex]  28 Jan 2011Figure 1: The ET of the leading jet (left) and limits in the mχ01-mb˜ neutrino and a b quark plane (right).The CDF collaboration has searched in 2.65 fb−1 of Run II data1, and measures the dominantbackground in this search from light-flavor jets misidentified as b-jets with control samples indata. Events are required to have a good primary vertex, and track activity consistent withenergy measured in the calorimeter. Events with exactly two well-measured jets with ET > 25GeV, not pointing along either jet direction, and no isolated high-pT tracks are selected for finalanalysis. The final selection criteria are optimized for two signal regions in ∆M = Mb˜1 −Mχ˜01 ,the mass difference between the sbottom and the neutralino. The background predictions forthe two regions are 133.8± 25.2 events and 47.6± 8.3 events for the high and low ∆M regionsrespectively. In the data, 139 events are observed in the high ∆M region and 38 in the low ∆Mregion. In the absence of any excess, limits are set in the neutralino-sbottom mass plane.DØ has performed an updated search2, which uses 5.2 fb−1 of Run II data. This searchis also for pair production of scalar bottom quarks, and is also interpreted in the context ofscalar third-generation leptoquarks. As in the CDF analysis, scalar bottom quarks are assumedto decay to a neutralino and a b quark. Events are selected by requiring two or three jetswith pT > 20 GeV/c, and no identified isolated electrons or muons. The E/T is required to begreater than 40 GeV, and not aligned with the direction of any jet. A neural network b-taggingalgorithm is employed to identify heavy-flavor jets, two of the jets are required to be consistentwith originating from a b quark. A final selection on the kinematics of the event, including thepT of the leading jet, E/T , and the scalar sum of the jets ET and E/T , HT , is optimized for thesmallest expected cross section limit for each signal mass considered.In the absence of a significant excess, this search has set 95% C.L. lower limits on theirproduction in the ( mb˜ , mχ01) mass plane, such as mb˜ > 247 GeV/c2 for a massless neutralino,and mχ01> 110 GeV/c2 for 160 < mb˜ < 200 GeV/c2. The ET of the leading jet and limits in themχ01−mb˜ plane are shown in Figure 1. Limits are also placed on third generation leptoquarks,which are assumed to decay to a tau neutrino and a b quark. The 95% C.L. lower limit on themass of a charge-1/3 third-generation scalar leptoquark is 247 GeV/c2.3 Search for Scalar top with c jets and E/TCDF has also searched for direct stop quark production, where the stop decays to a charmquark and a neutralino. In this search, the neutralino is assumed to be the LSP, and R-parityconservation is assumed. Therefore, the stop signature is two c jets and large missing transverseenergy from the LSP escaping detection.NN Output0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Events051015202530354045CDF DataHF MultijetsLight-flavor jetsTop-quarkElectroweak bosons)=70!")=125, m(t~Signal m()=70!")=135, m(t~Signal m()=70!")=115, m(t~Signal m(EventsCDF Run II Preliminary -1 L dt=2.6 fb#]2 Stop Mass [GeV/c60 80 100 120 140 160 180]2 Neutralino Mass [GeV/c406080100120]2 Neutralino Mass [GeV/c Observed Limit (95% CL))!1±Expected Limit (]2 Neutralino Mass [GeV/cc + m1o"# = mt~m1o"# + mb + mW = mt~mo = 56$LEP o = 0$LEP -1CDF 295 pb -1DØ 995 pbCDF Run II Preliminary -1 L dt=2.6 fb%Figure 2: Output discriminant from the heavy-flavor multi-jet neural network (left) and the 95% C.L. limits inthe mχ01−mt˜ mass plane (right).Events are selected by requiring at least two well-measured jets with ET > 25GeV andE/T > 50 GeV. A neural network is used to reject heavy flavor multi-jet events, and a secondtwo-dimensional neural network is employed to select charm jets. The background contributionfrom heavy-flavor production and light quark jets misidentified as heavy flavor are estimatedfrom the data. This search is optimized for a stop mass of 125 GeV/c2, and a neutralino mass of70 GeV/c2. The predicted background is 132.0± 24.4 events, and 115 events are observed. Fora neutralino mass of 70 GeV/c2, and mt˜ ranging from 115 to 125 GeV/c2, the expected signalranges from 82.4 to 90.2 events. No excess is observed and limits are set in the mχ01and mt˜plane, shown in Figure 2.4 Search for Gauginos in GMSBAfter the observation in Run 1 by CDF of a two electron, two photons, and E/T candidate event,a class of supersymmetry models with gauge-mediated SUSY breaking became of particulartheoretical interest. These models solve the “naturalness problem” and provide a low-mass darkmatter candidate. In this search, a scenario where the lightest neutralino, χ01, decays almostexclusively to a photon and a weakly interacting, stable gravitino is considered. The gravitinoescapes the detector without depositing any energy, and therefore can generate significant E/T .At the Tevatron, production of gaugino pairs would then result in two photons, E/T , and possiblyother products of the cascade decay.CDF has performed a search 3 for this signature with 2.6 fb−1 of Run II data. This search isoptimized for neutralino lifetimes of order a few ns or less, and utilizes a timing system allowingthe photon arrival time to be measured. A detailed model of the E/T resolution is employed toreject backgrounds from instrumental and non-collision sources, enhancing the sensitivity forlarge χ01 masses. This model enables a per-event figure of merit to be computed, allowing eventswhere the E/T is more likely to be the result of a mis-measured jet or other poorly reconstructedobject to be rejected, while retaining with high efficiency events with well-measured real E/T .Events are selected by requiring two photons with ET > 20 GeV. The E/T significance ofevents is required to be greater than 3, and the total sum ET of the observed jets and E/T inthe event is required to be more than 200 GeV. Finally, the two photons are required to notbe back-to-back by requiring the angle between them to be less than pi − 0.35 radians. AfterFigure 3: The E/T significance, showing the separation power for mismeasured QCD and electroweak sources ofE/T (left), and the limits in the mχ01and χ01 lifetime plane (right).all selection, the dominant background originates from electroweak production of a Z0 with twophotons, where the Z0 decays to neutrinos and results in significant E/T . The total predictedbackground in this search is 1.4± 0.4 events, and zero are observed. Limits are set at 95% C.L.in the mχ01and χ01 lifetime plane, shown in Figure 3. For a short-lived neutralino with lifetimeless than a nanosecond, the limit is 150 GeV/c2.5 Search for Hidden Valley Dark PhotonsRecently, a subset of HV models with SUSY have emerged which may simultaneously explainanomalies of the observed spectra of high-energy cosmic ray positrons and electrons, and providea solution to the origin of Dark Matter. They propose a scenario where dark matter canannihilate to pairs of so-called dark photons, which then would potentially themselves decay topairs of e+e− or µ+µ− through their mixing with the SM photon. At the Tevatron, gauginoproduction could lead to cascade decays of charginos and neutralinos which could include decaysto both SM photons and these dark photons, whose favored mass range is on the order of 1 GeV.The signature, then, will be a SM photon, two opposite charge leptons spatially close to eachother in the detector, and E/T from the escaping LSP. DØ has searched for this signature with4.1fb−1 of Run II data4.Events are selected by requiring a single photon and two leptons. The proximity of theleptons to each other requires a careful treatment of the isolation requirement placed on theleptons. The main background is QCD with photon conversions producing pairs of leptons.After selection, the dilepton invariant mass is used as a signal discriminant, where the signalwould appear as an excess at the dark photon mass. No significant excess is observed, andexclusion limits are set on the lightest chargino mass as a function of the dark photon mass,shown in Figure 4.6 Search for Hidden Valley Long-lived ParticlesAnother class of HV models predicts a new, confining gauge group, weakly coupled to the SM.Stable configurations of HV hadrons, “v-hadrons” in this model, can be long-lived. In onebenchmark model, the SM Higgs boson mixes with the HV Higgs boson. In this scenario, theSM Higgs could decay, through the mixing with the HV Higgs, to pairs of v-hadrons. Thev-hadrons then preferentially couple to the heavier SM quarks due to a helicity suppressionFigure 4: Dark Photon mass for the muon and electron channels with hypothetical signal shown in solid line(left),and limits in the dark photon mass - chargino mass plane (right).Figure 5: The smallest secondary vertex mass in the event (left), and limits on the SM Higgs boson productioncross section, for a v-hadron mass of 15 GeV/c2 (right).mechanism. The resulting signature is highly-displaced secondary vertices with a large numberof tracks from the b-quark decays. This has been searched for by DØ with 3.6 fb−1 5.Events are selected by searching for at least two separated four-track secondary vertices,with an impact parameter in the transverse plane, Lxy > 1.6 cm. The secondary vertex massand collinearity are used to select the signal region. No excess is observed in data. Assumingthe Higgs always decays to dark-sector v-hadrons, which themselves always decay to bb¯ pairs,limits are set on SM Higgs boson production, shown in Figure 5.7 ConclusionsThe CDF and DØ experiments are searching many models and signatures for evidence of newphysics. None is observed yet, but both experiments have now recorded more that 8 fb−1, andshould have results with this doubling of the data soon. Much of the Run II data is yet to beanalyzed, and additional data continues to increase sensitivity to new physics signals.8 AcknowledgmentsI would like to thank the conference organizers for the opportunity to attend the conference. Ialso thank my colleagues at DØ and my colleagues at CDF for help in preparation of this talk.References1. [arXiv:1005.3600v1 [hep-ex]].2. [arXiv:1005.2222v2 [hep-ex]].3. T. Aaltonen et al. [CDF Collaboration], “Search for Supersymmetry with Gauge-MediatedBreaking in Diphoton Events with Missing Transverse Energy at CDF II,” Phys. Rev. Lett.104, 011801 (2010) [arXiv:0910.3606 [hep-ex]].4. V. M. Abazov et al. [D0 Collaboration], “Search for dark photons from supersymmetrichidden valleys,” Phys. Rev. Lett. 103, 081802 (2009) [arXiv:0905.1478 [hep-ex]].5. V. M. Abazov et al. [D0 Collaboration], “Search for Resonant Pair Production of long-lived particles decaying to bb¯ in pp¯ collisions at√s = 1.96-TeV,” Phys. Rev. Lett. 103,071801 (2009) [arXiv:0906.1787 [hep-ex]].

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Tevatron Searches for New Physics with Photons and Jets

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