# Presentation: 845065

## Search for Scalar Top and Scalar Bottom Quarks at the Tevatron

- Shaohua Fu
- Fermilab
*For the D**Ø and CDF collaborations*
- PANIC 05, Oct. 24-28
- Santa Fe, NM

## Tevatron at Fermilab – Run II

- Run II goal: 4 ~ 9 fb-1 in 2009
- Analyses presented here use up to 340 pb-1

- Shaohua Fu, Fermilab

- PANIC05, Oct. 24-28

## DØ and CDF Detectors

- Both general purpose detectors well understood and highly efficient
- Precise tracking with silicon vertex detector
- Excellent calorimeters and muon chambers coverage

- Shaohua Fu, Fermilab

- PANIC05, Oct. 24-28

## Scalar Top and Scalar Bottom

- Supersymmetry
- SUSY overcomes some of the theoretical problems in the SM by introducing new degrees of freedom
- Every SM particle has supersymmetric partners:
- quarks/leptons  0-spin squarks/sleptons, gauge bosons  1⁄2-spin gauginos
- R-parity: *R**p* = (-1)3B+L+2S = +1 (SM) ; = -1 (SUSY)
- If R-parity is conserved (RPC), the LSP is stable  a good dark matter candidate
- Small violation of R-parity is not excluded (RPV)  additional couplings and the LSP decays into SM particles
- Stop and Sbottom (in generic MSSM model)
- Large top mass  substantial mixing between SUSY partners of the L- and R-handed top quark; thus the lightest stop could be the lightest squark
- Sbottom is also expected to be light at large tan
- Stop/sbottom can be pair produced through gluon fusion and quark-antiquark annihilation at the Tevatron, and stop can decay in various modes

- Shaohua Fu, Fermilab

- PANIC05, Oct. 24-28

**Notes:**
In the frame of generic MSSM model (or mSUGRA), so neutralino is LSP.
A quantum number called R-parity.
Within the Minimal Supersymmetric extensionof the Standard Model (MSSM), the large top quark mass can induce a large mixing between the superpartners of the left and right helicity states of the top quark and consequently can lead to a substantial mass splitting between the two scalar top mass eigenstates. This mixing depends in part on the top Yukawa coupling constant (Higgs-to-top coupling) which is largely due to the heavy top quark mass. Thus it is likely that the lighter scalar top may be the lightest squark, and it can be even lighter than the top quark.
If tan is large, then there can be a large mass splitting in the sbottom sector, yielding low mass for one of the sbottom which could be in the reach of the Tevatron energy.

## Search Channels

- Scalar top
- R-parity conservation decay modes:
- Decay channel                   (on-shell chargino) is difficult to explore at the Tevatron due to the high chargino mass limit of LEP2
- Decay channel                   has been extensively explored, but it might not be the dominant decay for stop masses accessible at the Tevatron
- 3-body decays via virtual chargino:
- Decay channel                       is almost closed for most of the stop mass within the Tevatron reach, due to slepton mass limit of LEP2
-  Decay                         dominates if the sneutrino mass is much greater than the *W* mass, but it has very limited potential for the Tevatron
- We explored decay channel                      which dominates if the sneutrino mass is of the same order as the *W*  mass. If        is the LSP:
- R-parity violation decay modes:

- Shaohua Fu, Fermilab

- PANIC05, Oct. 24-28

**Notes:**
Stop  top + neutralino is also kinematically forbidden in accessible parameter region.
RPV: BR(stopb+tau)~100% if stop is the LSP. If the LSP is the lightest neutralino with the NLSP being the stop, stopc+neutralino is possible. However, large ’333 etc. can still give the dominant BR for this channel.

## Search Channels

- Scalar bottom
- Dominant decay mode:                        (assuming       is the LSP)
- Direct production of sbottom pair, decaying into
- Sbottom quarks from gluino decays

- Shaohua Fu, Fermilab

- PANIC05, Oct. 24-28

## Search for Stop:		

- Search in channel
- Integrated luminosity (L) = 339 pb-1
- Main background
*Z*/*   / 
- QCD multi-jet
- (1S)  
*WW*
- Top pair
- Selection
- pT(1)>8 GeV, pT(2)>6 GeV
- 2-dim. cut on MET, (1,MET) plane (MET>20-50 GeV)
- At least 1 jet with ET(j)>15 GeV
*b*-tag based on the impact parameter of the tracks in the jet
- m[75,120] GeV for MET<50 GeV
- After cuts
- Top pair dominates background (2.3 out of 2.9 events)
- 1 event left in data (signal ~3 events)

- Use spectra of HT = jets |ET| to discriminate between top and stop-signal

- After pre-selection

| HT bin (GeV) | Background | Data |
| --- | --- | --- |
| [0, 40] | 0.11  0.03 | 0 |
| [40, 80] | 0.89  0.43 | 0 |
| [80, 120] | 0.75  0.13 | 0 |
| [120, 160] | 0.56  0.07 | 1 |
| > 160 | 0.57  0.08 | 0 |

- Shaohua Fu, Fermilab

- PANIC05, Oct. 24-28

**Notes:**
Used trigger DIMUON.
Stop pair production cross section ~pb.
Final state: two muons, two b quark jets, and missing transverse energy.
QCD background estimation: from the data by considering the ratio of opposite- to like-sign data events surviving muon isolation cuts.
It’s shown in the plot that the ttbar background is dominating at slightly higher values of HT than the two signal points. Instead of cutting on HT, which would harm the efficiency of some points, we define HT bins with a width of 40 GeV. For each signal point, the numbers (of expected signal, background events and data events) of these several bins are then combined into a final cross section upper limit. This approach has the advantage of using the full HT distribution for each signal point while optimally combining the information obtained from each HT bin.
Signal efficiency ~ a few percent. Signal ~ 3 events.
Systematic uncertainties: jet energy scale 4-22%, b-tagging 0-10%, muon id 3-7%, luminosity 6.5%, trigger efficiency 3%, ttbar cross section.

## Search for Stop:		

- Obtain exclusion limits assuming
- 	and equal *BR* in three lepton flavors
- Improvement w.r.t. Run I in low m(stop, sneutrino) region due to low pT() requirement
- Preferred channel: e (+jets) – 2*BR*, much lower *Z*/Drell-Yan background – expect significant improvement by new analysis in e channel

- 95% CL excluded region. Also shown is DØ Run I result with 108 pb-1 luminosity in the e final state.

- Shaohua Fu, Fermilab

- PANIC05, Oct. 24-28

**Notes:**
Limit calculation: modified frequentist approach (CLS) – likelihood ratio method.
Not much improvement compared to Run I because of: a final state having half the signal branching ratio and fifteen time more non-QCD background.

## Search for Stop:		

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- Search for Stop:

- Assume       is the stable LSP, and
- L = 163 pb-1
- Background
- QCD multi-jet
*W*/*Z*+jets, top, di-boson
- Selection
- ET(j1)>35 GeV, ET(j2)>25 GeV
- MET > 55 GeV
- No isolated lepton (e, ), veto 
- Heavy-flavor-tagging on one charm jet (based on track impact parameter)
- Result
- Upper cross section limits, but no additional exclusion in mass plane

| Heavy-flavor-tag | No | Yes |
| --- | --- | --- |
| Background | 105  12 | 8.3  2.3 |
| Data | 119 | 11 |

- CDF Run II projections based on integrated luminosity = 1, 2, 4 fb-1

- With heavy-flavor-tag

**Notes:**
Used trigger MET45.
The signature is a pair of acolinear heavy flavor jets in the transverse plane, large missing ET, and no isolated high pT leptons in the final state.
Top background includes single top and top pair production.
QCD background estimation: extrapolate independent inclusive jet sample from low missing ET to high missing ET region.
Signal efficiency ~ up to 2%. Signal ~ a few events (2-15).
Systematic uncertainties: jet energy scale 6-33%, gluon radiation ISR/FSR 10%, choice of PDF 2%, statistical 5%, luminosity 6%, heavy-flavor-tagging 10%, trigger efficiency 2%.
No additional exclusion in mass plane because of: higher MET trigger threshold, larger systematic uncertainties in jet energy scale and heavy flavor tagging, as compared to Run I.
Limit calculation: Bayesian approach, with a flat prior for the signal cross section and Gaussian priors for acceptance and background uncertainties.

## Search for Stop (RPV):		

- Search channel
- L = 332 pb-1
- Background
- QCD
*W*/*Z* + jets
- Top, di-boson
- Selection
- 1 lepton (e, ) pT(*l*) > 10 GeV (from ***lep*)
- 1 ***had* pT(**) > 15 GeV
- At least 2 jets ET(j) > 20 GeV
- Veto *Z*, veto *W* (mT(*l*, MET) < 35 GeV)
- Final cut ST=pT(*l*)+pT(**)+MET>110 GeV
- Result
- Njet = 0, 1 are control regions, Njet  2 is signal region

| (Signal region) | e +  |  +  | Sum |
| --- | --- | --- | --- |
| Background | 1.27 0.29 | 0.99 0.35 | 2.26 0.46 |
| Data | 1 | 1 | 2 |

- Loose cut
- ST > 80 GeV

- Final cut
- ST > 110 GeV

- Shaohua Fu, Fermilab

- PANIC05, Oct. 24-28

**Notes:**
’333 (lambda) is lepton-number violating term.
Used Lepton plus Track triggers.
The final state is either an electron or a muon from the tau leptonic decay as well as hadronically decaying tau lepton, and two or more jets.
A “blind” analysis technique is employed, where the data in the signal region is “blinded” until we fully optimize the event selection criteria for signal events and estimate the signal event acceptance and the background events in both the control regions and the signal region. At that point we freeze the criteria and examine the data in the signal region for possible signal. Control regions: MET>35 GeV, and/or njet=0,1.
The QCD background is estimated using the lepton isolation distribution using a data sample of non-isolated leptons.
W+jet background is negligible in the signal region – estimated from the contral regions.
Signal efficiency ~ up to 2%. Signal ~ a few events.
Systematic uncertainties: PDF, ISR/FSR, jet energy scale, MET simulation, ID efficiency and isolation efficiency for e mu tau, total is 11% to 7% (for stop mass 100 to 170 GeV).

## Search for Stop (RPV):		

- Obtain limits assuming
- Stop mass limit
- mstop > 155 GeV (nominal)
- mstop > 151 GeV (conservative)
- Since the stop pair production process is very similar to the pair production of the third generation scalar leptoquark (*LQ*3)
- 	and their NLO cross sections are very close to each other, the same mass limit is applicable to *LQ*3

- CDF Run I: mstop > 122 GeV (L = 106 pb-1)

- Shaohua Fu, Fermilab

- PANIC05, Oct. 24-28

**Notes:**
CDF Run I luminosity = 105-106 pb-1
We set a 95% confidence level lower limit on the stop mass at 155 GeV with the next-to-leading order calculation of the cross section. If we include theoretical uncertainties in the cross section calculation due to the renormalization scales and Parton Distribution Functions, a conservative limit of 151 GeV is obtained.
Projected 95% CL upper limit of cross section is shown as a grey band (corresponds to a 68% probability range of possible limit if data were to follow SM background expectation).
The NLO cross section for LQ3 becomes identical to that for stop pair in the limit of heavy gluino, and they are very close to each other for the existing limits on gluino mass.

## Search for Sbottom (Direct Production)

- Search channel
- L = 310 pb-1
- Background
*W*/*Z* + jets
- Top, di-boson
- QCD vanishes at large MET
- Selection
- 2 jets acoplanar (3rd jet allowed), MET
- Veto events with isolated e, , track ()
- Single *b*-tag
- Optimized cuts depending on sbottom mass: ET(j1) > 40-70 GeV, ET(j2) > 15-40 GeV, MET > 60-100 GeV

- After *b*-tag, optimized cuts low set

| Cuts | Before b-tag | Opt. low | Opt. med. | Opt. high |
| --- | --- | --- | --- | --- |
| Background | 1335  48 | 38.6  2.8 | 19.6  1.7 | 4.40  0.44 |
| Data | 1433 | 36 | 15 | 2 |
| Signal for
 | 68.8  2.3
(140, 80) | 35.0  1.2
(140, 80) | 21.6  0.7
(160, 75) | 6.10  0.17
(205, 60) |

- Shaohua Fu, Fermilab

- PANIC05, Oct. 24-28

**Notes:**
At the Tevatron, scalar bottom quarks are produced in the same way and with the same cross section as scalar top quarks.
Used trigger JETMET.
Final state: two acoplanar b jets and missing transverse energy.
QCD background estimation: we examined our selected data sample without the MET cut and attribute the exponential rise at low MET to QCD background. A fit of QCD to MET<60 GeV is extrapolated to MET>60 GeV, provides an estimate of 80 QCD events. These events can be assumed not to be enriched in heavy flavors, we expect that after b-tagging only a few QCD events will survive. For a MET>60 GeV cut, the QCD contribution becomes negligible.
For higher masses of sbottom the mean MET and jet ET become substantially larger than expectations from the SM backgrounds, this provides a new lever for improving the sensitivity to signal (S/B).
Signal efficiency ~ a few percent.
Systematic uncertainties: luminosity 6.5%, SM background cross section 15%, signal NLO cross section 11-15%, MC statistics 5%, jet energy scale 22% for SM background and 16% for signal, jet ID 3%, b-tagging 7%, trigger efficiency 5%, lepton isolation efficiency 5%.

## Search for Sbottom (Direct Production)

- Obtain exclusion limits assuming
- Significant improvement compared to the previous measurements
- Future improvement: double *b*-tag, better Jet Energy Scale and optimization of ET(j), MET cuts

- 95% CL exclusion contour

- Shaohua Fu, Fermilab

- PANIC05, Oct. 24-28

**Notes:**
Limit calculation: modified frequentist approach (CLS) – likelihood ratio method.

## Search for Sbottom (from Gluino Decay)

- Channel
- L = 156 pb-1
- Background
- QCD multi-jet,
*W*/*Z*+jets, top, di-boson
- Selection
- At least 3 jets ET(j) > 15 GeV
- MET > 80 GeV
- Angular cuts between jets and MET
- Veto leptons (e, , )
- One or more jets *b*-tagged

| # b-tag | Nb-tag = 1 | Nb-tag  2 |
| --- | --- | --- |
| Background | 16.4  3.7 | 2.6  0.7 |
| Data | 21 | 4 |

- 95% CL exclusion, mass limit set up to:

- Shaohua Fu, Fermilab

- PANIC05, Oct. 24-28

**Notes:**
Assume sbottom is lighter than gluino so that the decay is kinematically allowed. Cross section of gluino production is larger than direct sbottom production.
Used trigger MET35.
Final state: four b jets and missing ET.
Main background is QCD multijet before b-tagging and then bbar after b-tagging, where MET arises due to mis-measurements of the jet energy. In addition, QCD heavy-flavor jets could generate MET if the heavy quark decays semi-leptonically. MET due to mis-measurements of the jet energy is aligned with the direction of jet in the transverse plane – apply angular cut to veto.
QCD background estimation: the contribution of light-flavor jets which are misidentified as b-jets is estimated using jet data – negative impact parameter “negative tags” corrected by a factor obtained from MC. The contribution from real b-jet is estimated using MC.
Signal efficiency a few % to 10%. Signal ~ tens of events: 20-60 events for single tag, 10-60 events for double tag.
Systematic uncertainties: calorimeter energy scale 16-25% for background and 10% for signal, bbar background normalization 15%, tagging efficiency 7% for single tag and 14% for double tag, W/Z+jets cross section 12%, luminosity 6%, trigger efficiency 2.5%, ISR/FSR increase for small mass difference between gluino and sbottom ~ 5-8%.
Limit calculation: Bayesian likelihood method.
D0 also performed search for squarks and gluinos in jets + MET final state. And leptoquark searches in jets + MET final state.

## Conclusions

- DØ and CDF have searched for scalar top and scalar bottom quarks in various channels, using integrated luminosity of up to 340 pb-1
- Assuming one dominating decay channel and interpreting exclusion in terms of masses of stop/sbottom and decay products
- In some channels, substantial improvement over Run I results and extension of LEP excluded regions.
- Especially in RPV stop search and in sbottom searches
- We are now analyzing the 1fb-1 data, hoping for a discovery; otherwise there will be substantial improvement of the limits

- Shaohua Fu, Fermilab

- PANIC05, Oct. 24-28

## Backup

- Shaohua Fu, Fermilab

- PANIC05, Oct. 24-28

## Backup

- DØ *b*-tagging – Jet lifetime probability (JLIP)
- Use the signed impact parameter significance of tracks associated to a jet (R<0.5 cone matching) to identify jets with long lived particles (mostly *b*-jets)
- DØ: stop  *b* + ** + sneutrino

- After pre-selection

- Shaohua Fu, Fermilab

- PANIC05, Oct. 24-28

**Notes:**
Prospino NLO stop/sbottom pair production cross section as a function of mass at the Tevatron Run II. The two outer thinner lines correspond to a change in both the renormalization and factorization scales by a factor of 2 (upper one) and a factor of 0.5 (lower one) respectively.

## Backup

- CDF: stop  *c* + neutralino

- CDF RPV: stop  *b* + **

- Before heavy-flavor-tag

- No mass limit

- Njet = 0 (control region)

- Shaohua Fu, Fermilab

- PANIC05, Oct. 24-28

## Backup

- DØ: sbottom direct production

- CDF: sbottom from gluino decay

- Before *b*-tagging

- Before *b*-tagging

- Control region (1 lepton) for cross check

- Shaohua Fu, Fermilab

- PANIC05, Oct. 24-28