# Presentation: 398637

## Jason Nielsen
(University of California, Santa Cruz)

for the CDF and DØ Collaborations

- VERTEX 2004

**Jason Nielsen**
- (University of California, Santa Cruz)
- for the CDF and DØ Collaborations

**Hadron Collider Physics 2007**

## What lies beyond the Standard Model?

- Most BSM Higgs results interpreted in MSSM framework with CP-conserving Higgs sector

- Consider adding a second Higgs doublet (2HDM)

- _Minimal Supersymmetric Model_: framework for gauge unification with stable electroweak energy scale
-  5 physical Higgs bosons h, H, A, H+, H-
-  parameterized fully at tree level by tan , mA
-  radiative corrections depend on value of stop mixing

- In large tan  regime, enhanced bb and  couplings give rise to large Higgs boson production rates at hadron colliders

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

**Notes:**
8 degrees of freedom yield 5 physical Higgs bosons
MSSM is an example of what is sometimes called the “Model 2 2HDM”, with Higgs doublets coupling separately to (up-type quarks and neutrinos) or to (down-type quarks and leptons)
At tree-level the lightest Higgs must be less the mZ; rad corrections can push this to 135 GeV
Stop mixing parameters modify this upper bound on mH
Tan beta is the ratio of the vevs of the two Higgs doublets: not clear what is the “allowable” range for that ratio, but a high tan beta is good for hadron collider searches.
Bottom quark loop means that production rate scales as tan2beta!

## Experimental results before Run 2

- LEP Higgs Working Group combined channels of 4 experiments
-  neutral Higgs bosons are produced via e+e- hA or hZ

- (Point with mh=100, mA=100 not excluded at 95% C.L.)

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

**Notes:**
What we know about the MSSM Higgs bosons comes from the combined LEP2 experiments, where neutral Higgs bosons are pair-produced or produced via Higgsstrahlung
In the least-excluded benchmark (maximal mixing), low values of tanbeta are excluded, but there is at least one region which is not excluded (100,100), noted by Gunion et al.
Results from H+ direct search assume charged Higgs decays to some combination of tau-nu and cs
There are more restrictive limits, based on the projection of the MSSM parameters into a tanbeta vs mH+ space
Those exclude all charged Higgs masses above 140 GeV, and all tan-beta between 0.9 and 1.5

## Charged Higgs searches in top decays

- Current limits on H+ mass allow top quark decay to bH+ for certain values of tan 

- Charged Higgs decays to  or cs modify final state ratios expected from simple Wb decays
-  taus vs e/ ; dilepton vs. lepton + jets

*CPSuperH*

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

**Notes:**
Charged Higgs limits from LEP around 100 GeV open a new channel for top quark decay
Typical decays from a unit charge boson: tau nu, cs
Precise branching ratios depend on tan beta and mH (shown by CPSuperH results)
Simple W decays are balanced between lepton flavors, but Higgs decay to tau nu
Throws the ratio out of balance, and we measure how much out of balance it could be

## CDF search for tH+b

- Search for lbj in 335 pb-1 sets limits on Br(tH+b)*Br(H+)
-  excludes values less than 0.5 for 80 < mH+ < 125 GeV/c2

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

## Fermiophobic Higgs Decaying to 3 

- If hf VV is very weak, then LEP2 limits from e+e-hf Z fail

- Alternative production mechanism is unique to hadron collider

- Viable at Tevatron as long as mH+ is not too large!

- Background rates in 3 final state are very low
-  fakes from Z or W estimated using measured fake rates
-  real tri-photon production extrapolated from di-photon sample

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

**Notes:**
Extrapolation assumes the Pythia ratio of 
tri-photon to di-photon is correct

## Fermiophobic Higgs search results

- Final 25 GeV cut on photonic vector HT cut rejects background

| Process | Events |
| --- | --- |
| 3 direct | 0.9  0.2 |
| 3 fake | 0.3  0.05 |
| Observed | 0  |

- LEP2 limits of 108 GeV/c2 assumed SM coupling hfVV

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

**Notes:**
Final selection is made on a VECTOR sum HT (not scalar), summed over the 3 “photons” only
Idea is that the 3 photons are imbalanced in true Higgs events
Background is balanced in pT.
Final background estimate is 1.2 events, and 0 are observed (there were 5 before the cut)

## MSSM Higgs bosons decaying to bb

- BR(Hbb) 90% but overwhelmed by non-resonant production

- Associated Higgs production has smaller QCD background

- Expect 3 b-tagged jets, look for peak in dijet mass spectrum
-  trigger on high-Q2 events with b-tagging at Level 3
-  trick is estimating bbb(b) background entirely from data

*(Dawson et al., hep-ph/0603112)*

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

**Notes:**
Because the bb couplings are enhanced, the bb channel would be a first place to look
Overwhelmed by bb production 
Sometimes called the “four-flavor number scheme” and the “five-flavor scheme”
(4FNS uses QCD corrections to qq and gg, while 5FNS uses a b-quark PDF)
There are corresponding s-channel diagrams for both
Analysis really is only possible if trigger has a separate b-jet channel: both Dzero and CDF have this
B-tagging eff is 50%, mistag rate 0.33%

## Estimating triple-tag background

- Starting from large double-tagged sample (270K events)
-  estimate “tag rate” to tag a third jet as a function of jet ET
-  measure for jets outside of dijet mass signal region
-  apply tag rate to all events in the dijet mass distribution
-  renormalize prediction to data outside of signal region

**How to check such a data-driven method?**

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

**Notes:**
It is evidently the tag rate dependence which can change the dijet mass shape
The 2.7 factor is because Dzero used only the Wbb generation
Need at least 2.0 to include Wcc as well

## Cross-check triple-tag background estimate

**TRIPLE-TAGGED SAMPLE**

- Ratios of jet cross sections calculated with ALPGEN MC

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

## Results from DØ

- 880 pb-1

**TRIPLE-TAGGED SAMPLE**

**(m****A****=120, tan********=60)**

- After normalizing background to data outside signal region:

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

**Notes:**
Note the size of a Higgs signal for tan beta = 60
Difficult to interpret results for other Higgs hypotheses from this plot, since normalization
changes for each Higgs mass.

## b(b) production cross section exclusion

- 880 pb-1

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

**Notes:**
Exclusion at 95% CL
Calculated using the CLs method which was developed for Higgs searches at LEP

## Exclusion in MSSM parameter space

- Uses tree-level scaling of cross section by tan2

- 880 pb-1

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

**Notes:**
Do not bother with no mixing, maximal mixing.  This is a simple scaling by tan beta, as 
Expected at tree level in the MSSM.
Challenging analysis which in the end excludes rates that only correspond to very high 
values of tan beta.
Luckily there is another hope...

## MSSM Higgs bosons decaying to 

- Avoid like-flavor  pair decays which have large Z bkgd

- _CDF two-cone algorithm reconstruction_

- Count 1 or 3 tracks inside inner cone
- Axis defined by a seed track pT>6 GeV/c

- Isolation annulus used for jet veto

- Visible mass

- is handle against Z

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

## Reconstructed mass spectra at CDF

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

**Notes:**
Reminder that “etau” means tau decay to electron + tau decay to hadrons

## Limit on total  production rate

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

## Exclusion in MSSM parameter space

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

**Notes:**
Exclusion projected into the tan beta mA plane for positive mu.
See the LEP2 exclusions again for low values of tan beta in both maximal and no mixing
Complementary Tevatron exclusion for very high values of tan beta.
Observed limit is somewhat worse than expected because of fluctuation at high ditau mass.

## Results from DØ

- Region of
- CDF excess

- Neural network tau ID operates on tau “minijets”

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

**Notes:**
In fact, there is a deficit in these two bins where the signal contribution is greatest.

## Results from DØ

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

## Summary

- Tevatron BSM Higgs searches cover basic CP-conserving MSSM benchmarks
- Exclusions limited to high values of tan 
- No discoveries reported yet!
- Neutral Higgs boson searches show worse observed limits than expected for Higgs masses around 160 GeV/c2
- Given expected resolution, likely to be fluctuations
- New results soon from 2.0 fb-1 Tevatron dataset on tape

- Hadron Collider Physics 2007,  Elba

- J. Nielsen

## Backup

- Hadron Collider Physics 2007,  Elba

- J. Nielsen