Possible Higgs searches at LHCb

A preliminary study investigating the feasibility of finding the SM Higgs at LHCb

Jonathan Anderson (UCD)

Outline

What are the possible Higgs discovery channels at LHCb?What about the gold-plated channel?Gold-plated channel and its backgroundsCross-sections and branching ratios for this channelThe effects of the small LHCb angular acceptanceKinematic cuts used and their effectsOverall conclusions

Higgs decay modes

In the range 130 Gev < MH < 300 Gev, H WW and H ZZ dominate. While for MH < 130 Gev H bb dominates.

The QCD backgrounds swamp the hadronic final states so we must look for final states containing leptons or photons.

A possible exception at LHCb might be H bb.

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The Gold Plated channel

H ZZ 4L is known as the ‘gold plated’ Higgs decay channel.

It divides into two regions, 130Gev < MH < 2MZ

Contains one real Z and one off mass shell Z Branching ratio is smaller Background elimination more difficult

MH > 2MZ Contains Two real Z’s Higher branching ratio easier to eliminate background

H ZZ 4μ was investigated

Backgrounds for gold-plated channel

Reducible backgrounds gg (Z/)bb 4 gg, qq tt WbWb

4 gg, qq bb 4

Irreducible backgrounds gg, qq (Z/)(Z/)

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Cross-sections and branching ratios

Process x BR (fb)

H 4 (MH=130 GeV) 0.903

H 4 (MH=150 GeV) 1.459

H 4 (MH=180 GeV) 0.7627

H 4 (MH=200 GeV) 2.756

H 4 (MH=250 GeV) 2.2163

H 4 (MH=300 GeV) 1.7643

(Z/)(Z/ ) 4 20.618 *

(Z/)(Z/ ) 2µ2 41.236 *

(Z/)bb (Z 2 µ) 5.63 x 103

bb 4m > 200 Gev) 1.303 x 104

tt WbWb (W ) 5.637 x 103

All processes generated in pythia(One year of running equates to 2 fb-1)* Includes factor (1.3) to account for gg (Z/*)(Z*/ *)

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LHCb angular acceptance

The acceptance for LHCb is 0.015 0.245

LHCb acceptanceProcess Percentage within

acceptance

H 4 (MH=130 Gev) 2.042

H 4 (MH=150 Gev) 1.672

H 4 (MH=180 Gev) 1.284

H 4 (MH=200 Gev) 1.08

H 4 (MH=250 Gev) 0.53

H 4 (MH=300 Gev) 0.351

(Z/)(Z/ ) 4 2.91

(Z/)bb (Z 2 µ) 4.678

bb 4m > 200 Gev) 19.39

tt WbWb (W ) 1.72

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Muon pair wise invariant masses

Higgs (200 Gev) ZZ

Zbb tt_ _

Muon four wise invariant masses

Zbb tt

Higgs (200 Gev) ZZ

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Kinematic cuts for MH > 2MZ

MH (Gev) 200 250 300

|M12 – MZ| <

and|M34 – MZ| <

5

5

10

10

15

15

|M1234 – MH| < 4.5 9 13.5

All muon impact parameters <

30μm 30μm 30μm

Effectiveness of high mass cuts

Process 200 Gev 250 Gev 300 Gev Events

H 4 (MH=200 GeV) 73.87 ± 2.6%

200k

H 4 (MH=250 GeV) 71.21 ±2.9%

200k

H 4 (MH=300 GeV) 70.37 ±2.9%

200k

(Z/)(Z/ ) 4 6.136 ± 0.001%

6.55 ±0.001%

5.14 ±0.001%

100k

(Z/)bb (Z 2 µ) 0 ±0.00017%

0 ±0.00017%

0 ±0.00017%

600k

bb 4m > 200 Gev)

0 ±0.0001%

0 ±0.0001%

0 ±0.0001%

1M

tt WbWb (W ) 0 ±0.00017%

0 ±0.00017%

0 ±0.00017%

600k

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Overall signal against background

For 2 fb-1 of data (equivalent to one year)

MH (Gev) Signal Background Significance

200 0.044 ± 0.00114

0.0734 ± 0.00002

1.148

250 0.0168 ± 0.00048

0.0784 ± 0.0002

0.424

300 0.0088 ± 0.00026

0.0614 ± 0.0002

0.251

Gold-plated channel does not have any discovery potential at LHCbHowever it seems as though H bb is more promisingWork to be done Investigate H bb more fully, starting with

LHCb thesis…

Conclusions and further work

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