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While the Standard Model is very succesful in describing all the measurements of particle interactions currently available, it cannot be the whole story. It contains too many arbitrary parameters and the Higgs mechanism of electroweak symmetry breaking requires unnatural fine-tuning. Appealing theoretical frameworks which address these shortcomings have been suggested; examples include supersymmetry and new flavor symmetries. The spectrum of new states occuring in these models should be visible in the Tevatron Collider energy range. It is also necessary to search for completely unexpected new phenomena.
Searches for phenomena beyond the Standard Model have been an important part of D0's physics menu to date. Their importance will continue to increase as greater accumulated luminosity stretches the reach into unexplored territory. The upgraded detector will maintain the unique D0 capabilities that have aided such searches in the past --- the hermeticity of the calorimeter system, the excellent missing ET resolution, and the large eta coverage for lepton identification --- and allow us to exploit the expanded capabilities of the detector to extend further into that territory.
Some of the searches we have already undertaken are summarized in the table below. The top quark is included in the table as a cross section comparison. The production cross sections are rough estimates of the range accessible at the Tevatron's CM energy for Run Ia's luminosity. The table shows current D0 mass limits from Run Ia, for completed searches, and the estimated discovery reach for 1 fb-1, for some of the searches. (The discovery reach is the mass which could be observed with a 10-event signal.)
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| | |
Possible signals | Production cross | Current Mass limit | Discovery reach
| sections | |
|(over accessible | (model dependent) | 1 fb-1
| mass range) | |
-------------------|------------------|--------------------|-----------------
t tbar | 6.4 +- 2.2 pb | 199 +- 30 GeV | ---
| | (measured) |
Right-handed W | 0.5-1000 pb | 540 GeV |
W' | 0.5-0.0001sigma_W| 600 GeV | ~ 1 TeV
Z' | 0.1-0.001 sigma_Z| 480 GeV | ~ 1 TeV
scalar 1st gen. | | |
leptoquarks | 1.0-100 pb | 130 GeV | ~ 240 GeV
scalar 2nd gen. | | |
leptoquarks | 1.0-100 pb | 90 GeV | ~ 240 GeV
squark/gluino pairs| 5-1000 pb | 140-200 GeV | ~ 200-320 GeV
gaugino pairs | 0.5-10 pb | < 47 GeV | ~ 90 GeV
stop | 0.1-100 pb | in progress |
b' | 10-1000 pb | in progress |
q* | 0.1-100 pb | in progress | 700 GeV
___________________|__________________|____________________|_________________
The table does not tell the complete story, however --- while some exotic objects appear to be ruled out to quite large masses, this sometimes masks assumptions about branching ratios and/or coupling constants. There is still territory to be covered which requires triggering and analysis cuts to remain at fairly low ET for the primary objects --- jets, leptons, and missing ET --- and this should be an important consideration for the upgraded detector. Four examples of searches which will be advanced with the upgraded DO detector are the Z' search, the gaugino search, the stop search, and the squark-gluino search, described below.
The current mass limit on a heavy Z' from D0's search is 480 GeV, assuming the coupling of the Z' to leptons is identical to the Standard Model Z. The main determinant of the acceptance for this search is the eta acceptance for lepton triggering and identification. The current D0 triggers for electrons reach out to eta < 2.5. The discovery reach for Z' and W' in using electrons is shown here as a function of integrated luminosity. For the upgraded detector, these searches will benefit from an approximately doubled acceptance through the use of muon as well as electron final states over all eta < 3.
Our current searches for supersymmetric gaugino pairs seek to probe the mass region from the LEP limit of approximately 47 GeV upward. While the production processes proceed through the electroweak interaction, the low mass still gives large enough cross sections to be explored with current data. The cleanest signature for Wino1 - Zino2 pair production is the 3 lepton + missing ET topology. (The missing ET comes from an escaping lightest supersymmetric particle, or LSP.) The highest ET lepton in such events has a mean ET of only 24 GeV for a Wino1 of mass 50 GeV, and the third lepton has a mean ET of only 9 GeV. The upgraded detector is well-matched to this physics as it will operate with efficient (and unprescaled) electron and muon triggers for lepton transverse momenta down to pT ~ 10 GeV/c, and multilepton triggers may include muons as low as pT ~ 1.5 GeV/c. The gaugino search also requires full pseudorapidity coverage for lepton identification and measurement, as illustrated in the table below, which shows acceptance figures for a Wino{1} with a mass of 50~GeV.
_______________________________________________ | | eta range | eee | e mu mu | (Leading e) | (pT(e) > 5 GeV) ------------|----------------|----------------- 0.0 -- 1.0 | 58.3% | 57.1% 1.4 -- 2.5 | 21.0% | 22.6% 2.5 -- 3.4 | 2.2% | 3.0% | Any electron | Any electron |(pT(e) > 5 GeV) | (pT(e) > 5 GeV) 1.1 -- 1.4 | 27.2% | 13.2% 1.4 -- 4.0 | 47.1% | 26.3% ____________|________________|_________________
While the leading lepton is predominantly central, the acceptance for the three-lepton state is a factor of ~ 3 larger for D0's full pseudorapidity coverage (|eta|<3) than if lepton identification were restricted to |eta|< 1.
Within current SUSY theories, the large mass of the top quark carries the extremely interesting implication that its supersymmetric partner, the stop, will be split into two mass eigenstates, one of which will be significantly less massive than the top quark itself. We have already carried out a search for this low mass stop in the region where m(stop) < m(W) + m(b), and will soon publish a limit contour in m(stop) vs. m(LSP). This is another search for which a relatively low mass region will remain interesting through Run II. The offline analysis for this search imposed a cut on missing ET of 40 GeV, and a lower cut would have been desirable. We aim to maintain a trigger on pure missing ET, or missing ET + 1 or 2 jets, with a threshold of < 50 GeV and no prescale. While missing ET trigger rates require some further study in the high luminosity regime, there appear to be no major problems. The reduced bunch spacing will mean that the number of interactions per crossing will be similar to that in Run Ib, and the faster calorimeter electronics will keep the pileup from out-of-time events in the calorimeter manageable. The upgraded inter-cryostat detector is also important for maintaining missing ET precision.
The search for the leptonic decay modes of the stop, in the mass region heavier than the W but lighter than the top quark, will be very like the search for the top from 90 GeV to 200 GeV.
The search for squark-gluino pair production, like the stop search, uses both leptonic and hadronic decay modes and relies heavily on the measurement of missing ET. Our published limit [18] of 212 GeV for m(squark)=m(gluino) is from the decay signature of multiple jets plus missing ET, and the leptonic analysis is under way. In conjunction with the searches for stop and gauginos, a combined analysis of these three SUSY signatures promises to make a significant contribution by either discovering SUSY or ruling out that portion of its (nearly infinite) parameter space most attractive to theorists.
It is important to stress that for the regime where the SUSY partners are in the range of 50-250 GeV, the Tevatron is the best place to make a SUSY discovery. The LHC detectors will have difficulty triggering at the low lepton ET's and missing ET required to cover this range, in the presence of a large number of overlapping minimum bias events.
Many other searches for phenomena beyond the Standard Model have been and are being performed with the Run I data sets. Among these other searches are those for leptoquarks, b' quarks, excited quarks, W' bosons, right-handed W's, heavy neutral leptons, charged Higgs, neutral Higgs, and massive stable particles. In addition, non-Standard Model behavior is being sought in distributions from known particles as well. Anomalous coupling analyses of W and Z data have produced limits, as will studies of QCD jet distributions. All of these searches will continue to be interesting in the upgrade era. Except for the QCD jet distribution studies, the acceptance, and therefore reach, of these searches depends on the ability to trigger on and identify leptons and missing ET. Since limits on several interesting new particles will remain below 150 GeV into Run II, we have aimed to preserve our capacity to trigger as low as possible in ET and missing ET, and to make the eta coverage for triggering and tracking as broad as possible.