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Search for light Higgs in Y(1S)→ gamma lepton-pairs

Nasra Sultana&

Tomasz Skwarnicki

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Motivation• Some NMSSM models (Dermisek, Gunion, McElrath: hep-

ph/0612031) predict existence of a new non-SM-like higgs boson a0 (pseudo-scalar) with ma < 2mb to avoid fine-tuning of parameters in electroweak symmetry breaking

• Such light higgs avoids the LEP limit mH > 100GeV based on e+e- →ZH(→bb) searches since its mass is below the threshold for decay to bb.

• In this scenario also SM-like higgs boson h (scalar) also avoids the LEP lower mass limit since Br(h→ bb) is much smaller than Br(h →a0a0)

• The perfect place to search for a0 is in radiative decays of Upsilon meson, Υ → a0.

• Such an a0 decays predominantly into heaviest pair of fermions available (Br(a0→ )~0.9 for ma>2m)

• We have studied the decay Υ → a0 followed by a0→ (or a0→ for ma<2m

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Previous results are very old

• ARGUS: Phys.Lett.B154:452,1985 Υ(2S) → Υ(1S), Υ(1S)→ )

• CUSB: Phys.Rev.D35:2883,1987 Υ(1S)→ X

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Method (a0→ )

• To study the decay Υ (1S)→ a0, a0→ we tag Υ(1S) via Υ(2S) → Υ(1S).

• By tagging the Υ(1S) we eliminate events with photon coming from initial state radiation in tau pair production (e+e-→ ), a serious background for the reactions e+e-→ Υ(1S)→ a0.

• The channel a0 → is selected by using 1-prongdecays, requiring missing energy (neutrinos!) and at least one leptonic decay: →or→e

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Cuts details• Numbers of charged tracks = 4• -0.015 < Recoil-Mass() – M(Y(1S)) < +0.015 GeV• Require at least one of the remaining 2 charged tracks to be an

electron or muon candidate:– eE/P-1 | < 0.15, DEDX:e– : depthmu >1, muqual=0, 0.15< E< 0.45 , DEDX:

• Select the highest energy photon in the good barrel part (E> 0.06 GeV) which does not make a mass within 3 with any other photon as a candidate for Υ(1S) →a. The 0 veto suppress →, →→ background

• Sum up energy of all other photon candidates: Eneutral

• Imbalance of total energy: E + Echarged + Eneutral – Ecm < -0.5 GeV • Mass of neutrals (except for the highest energy ) plus the 1-

prong not required to be a lepton < 2 GeV• cos(1-prong and )< 0.99 to suppress final state radiation.

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Recoil mass – Υ(1S) mass

Signal region

Side band

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9.4 M Υ(2S) decays

Photon Energy distribution in the rest frame of Υ(1S)

Scaled side bands (non Y(1S) background)

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Photon Energy distribution in the rest frame of Υ(1S) after side band subtraction

e+e- → Υ(1S), Υ(1S)→ ll MCs

scaled by PDG BRs

Sideband-subtracted data

Data above 200 MeV saturated by

e+e- → Υ(1S),Υ(1S)→

Within errors all data well described by Υ(1S)→ ll

We used Υ(1S)→ MC to optimize our data selection procedure.

Υ(1S)→ MC+ → MC

+ → ee MC

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Photon Energy distribution for various ma

e+e- → Υ(1S)Υ (1S)→ a0, a0→ signal MonteCarlo10,000 events for each

mass

BKH’s fix to MC energy resolution is on

Peaks are fitted with a Crystal Ball function

Signal MC:

ma = 9 GeV

ma = 8 GeV

ma = 7 GeV

ma = 6 GeV

ma = 5 GeV

ma = 4 GeV

ma = 8.5 GeV

ma = 9.15 GeV

ma = 9.30 GeV

ma = 9.35 GeV

ma = 9.41 GeV

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Efficiency obtained from fits to signal MC and interpolated for the regions in between.

Fits to MC data (previous slide)

Polynomial fit to interpolate to other photon energies (used in calculation of upper limits on signal BR)

Plotted efficiencies based on phase-space MC

Multiply them by 0.91 to account for 1+cos2θ

distribution for Υ→ a

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Energy resolution obtained from fits to signal MC (points) and interpolation to other energies (solid line).

0.7309(1

0.0060.0114 0. .046 0.03 00) 1E

EE

E

Obtained by BKH and Selina (CBX 02-22) from fits to single MC (before the MC resolution fix)

Factor from fits to our MC

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Photon spectrum with binning comparable to expected signal width

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Setting upper limits on signal yield• At each energy fit CB line shape with

width determined from MC on top of linear background in the ± ln(E) = 0.5 range around the peak

• Fix signal amplitude at values minimize with respect to the background parameters, then plot the fit likelihood as a function of the signal amplitude

• Determine 90% U.L. on the signal amplitude by integral of the likelihood function

Example for ln(E in MeV)=7.590%

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Systematic errors Contribution Value

Neglected angular correlations and

helicity correlations in decays

20 %

Track reconstruction (per event) 4 %Photon detection 3 %Number of Y(2S) decays 1.5%

Error on BR(Υ(2S)→Y(1S)) 3.2%Total systematic error 21 %

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Upper Limit on product branching ratio Br(Υ(1S)→ a0)*Br(a0→ ) as function of ma

Br(Υ(1S)→ a0)*Br(a0→ )

= Ns / ( * NΥ(2S) * Br(Υ(2S) → Υ(1S) )

Br(Υ(2S) → Υ(1S))=18.8 % PDG’06

Upper limits are loose at low photon energies (E<150 MeV) since our analysis was optimized for intermediate and high energies.

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CLEO III

We have improved ULs by about an order of magnitude or more.

We are constraining NMSSM models. Many models with 2m<ma<7.5 GeV (represented

by red points) ruled out by our results.

Switch to a0→ for ma<2m(blue points)

- see next!

From Dermisek, Gunion, McElrath: hep-ph/0612031NMSSM consistent with all previous results

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a0→

• Two identified muons:– depthmu >1, muqual=0, 0.15< E< 0.45 , DEDX:

– RICH: 2K12

2K22

0

• | E + Echarged – Ecm |< 0.25 GeV

e+e- → Υ(1S), Υ(1S)→ MC

scaled by PDG BRs

( includes tiny Υ(1S)→ contribution)

Data (sideband subtraction very small)

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a0→

maGev Efficiency ()%

0.3 5.6±0.2

0.5 5.4±0.2

1.0 5.6±0.2

2.0 5.4±0.2

3.0 6.1±0.2

Data Signal MCma=3 GeV 0.5 GeV

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• Using =5.4% Br(Υ(1S)→ a0)*Br(a0→ ) < 2.5 x 10-5 (90% C.L.) Eliminates most of NMSSM

models for ma<2m (blue points)

Concerns about ability of our MC to correctly predict tracking efficiency for

very small ma (no opening angle between tracks)

Do not intend to show any a0 → +- results in public at this point

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a0→

• Using =4.3% Br(Υ(1S)→ a0)*Br(a0→ ) < 3.2 x 10-5 (90% C.L.)

• Same concerns about MC as for

ma

GeV

Efficiency ()%

1.0 4.3±0.2

2.0 7.6±0.3

3.0 6.9±0.3

4.0 6.6±0.2

Data Signal MCma=4 GeV 2.0 GeV

• Two identified kaons:– RICH: 2

K20

– DEDX:

– depthmu <1

• | E + Echarged – Ecm | < 0.25 GeV

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Summary and plans• We have obtained meaningful limits on

Br(Υ(1S)→ a0)*Br(a→ ) and *Br(a0→ ) • Future work:

– Study effects of angular correlations in MC to reduce systematic error

– Try separate set of cuts to optimize for high a0 masses (E< 150 MeV) ?

– Study track reconstruction efficiency for low a0 masses in a0→ with e+e- →followed by conversion (→e+e-)

– David McKeen and Jon Rosner performed theoretical calculations which indicated that direct Y(1S) production (e+e- → Y(1S)) will be more effective than Y(2S) →Y(1S) in setting limits for low mass a0→. We will investigate this with CLEO data and MC.

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CLEO III

We have improved ULs by about an order of magnitude or more.

We are constraining NMSSM models. Many models with 2m<ma<7.5 GeV (represented

by red points) ruled out by our results.

Switch to a→ for ma<2m(blue points)

- see next!

From Dermisek, Gunion, McElrath: hep-ph/0612031NMSSM consistent with all previous results