Searching for Higgs decays to four bottom quarks at LHCb

Directeur du travail: Prof. Aurelio BayExpert externe: Victor Coco

Julien Rouvinet

1

Overview

Theoretical overview of

LHCb experiment

Study of the signature

Statistical analysis

Conclusion

h0 ! aa ! bb̄bb̄

h0 ! aa ! bb̄bb̄

2

Motivation for this work

This master thesis is inspired by a recent paper from David E. Kaplan and Matthew McEvoy*.

Why at LHCb? Because of the (muon) trigger and the excellent tracking and vertexing in the forward region

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* Searching for Higgs decays to four bottom quarks at LHCb, arXiv:hep-ph/09091521 v1, 8 September 2009

Higgs to four b’s decay

Models like NMSSM, Higgs composite and little Higgs contain a light neutral pseudo scalar particle

If its mass is about half the Higgs mass

➡Dominant Higgs decay: h0 → aa

To mimic the different models we introduce the coupling: 1

2!a

2H

†H

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Higgs to four b’s decay

The mass of the particle a is given by:

where is the Higgs vacuum expectation value

The decay rate is then:

It dominates the Higgs (ex. SM 115 GeV) decay if the coupling is:

!(h ! aa) "1

32!

"2v2

mh

!

1 #

4m2a

m2

h

m2

a=

1

2!v

2

v ! 246 [GeV]

! > 0.255

LHCb Experiment

Context: LHC and LHCb experiment

Acceptance:

L0 trigger: among other things, a muon with:

15 < ! < 300 mrad

Pt > 1.1 GeV

VELO: Vertex Locator → Displaced verticesM1-M5: Muons detectors

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QCD Backgrounds

Irreducible

contribution: Probability of proper b recognition is about 70% and the probability to confuse a b with a c is about 30% *

Also a contribution because t decays mostly in b. The production cross section of this background is small (3.78 [pb] from Alpgen)

bb̄bb̄

bb̄cc̄

bb̄tt̄

7* AURELIO BAY and CEDRIC POTTERAT, A b jet "seed" finder, LHCb-INT-2009-023 V. 1, 10 2009

Signal events generation

Pythia 6.325 for production, hadronization and decay of the events

Energy in the center of mass frame:

We set and to one

10‘000 signal events produced through gluons fusion

!

s = 14 TeV

!h0 pbarn ! 5.9 · 105

!bb 2.45 · 108 pbarn ! 1.5 · 1012

!ZW 4.33 · 106 pbarn ! 2.7 · 1010

!tt 2.75 · 103 pbarn ! 1.7 · 107

q q

V!

h0

V!

q q

VV

g q

h0

g q

b/t

g q

qh0

g q

gg

q q

h0

q q

b/t

q

q

V

h0

V!

q

h0

q

q q

!h0

!Ldt " 6.3 · 103 pbarn!1

h0 ! "̃0"̃0 ! 6

Nevts =

"LLHCb · !h0 · (h0 ! "̃0"̃0) · ("̃0 ! bqiqj)

2dt

(h0 ! "̃0"̃0) " 0.7 # 0.9 ("̃0 ! bqiqj) " 0.7 # 0.8

Nevnts " 200"000 #340"000

Z0

(Z0 ! "̃0"̃0) Z0

BR(h0 ! aa) BR(a ! bb̄)

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Signal events generation (2)

Masses: mh=115 [GeV/c2], ma=20 [GeV/c2]

We have also used mh=125 [GeV/c2], ma=35 [GeV/c2] for comparison

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NEW NEW

Background events generation

Generator: Alpgen v2.13

Energy in the center of mass:

700’000 and 500’000 have been generated

!

s = 14 TeV

bb̄bb̄ bb̄cc̄

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b quarks analysis (1)

b quarks hadronize into jets reconstructed with the cone algorithm:

Particles falling inside a cone of aperture ΔR in (η, ϕ) space are gathered.

where and

!Rij =!

(!i ! !j)2 + ("i ! "j)2 ! = !ln(tan("/2))

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b quarks analysis (2)

Only events with four b’s in the acceptance are considered

These four b’s are all signal ones or of one background type (simplest case)

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Kinematics: b quarks

To reconstruct jets, a radius Rcone= 0.6 is chosen

Two jets overlap if :

!R(b1, b2) < 2 · Rcone = 1.2

• Signal• bbbb• bbcc

➡ some jets are

overlapping

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Jets reconstruction

Cone algorithm: b quark as seed

We combine the jets onto pairs to get an a candidate

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Jets combinations

Three possibilities of jets combinations

Criteria: we minimize ΔR

For mh= 115 [GeV/c2] and ma= 20 [GeV/c2]: 99% of correct combinations

Only 75 % for mh= 125 and ma= 35 [GeV/c2]

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Jets combinations

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Jets combinations

Anti-correlation more pronounced for the signal

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a

a are well separated in ∆R

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Cuts on Pt

Cuts on Pt:

Pt(j1) + Pt(j2) < 60 GeV c!1

Pt(j3) + Pt(j4) > 20 GeV c!1

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• Signal• bbbb

Cut on ma

Mass cuts: 13 < ma < 25 [GeV/c2]

• Signal• bbbb• bbcc

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ma and cuts

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•Signal• bbbb• bbcc

Higgs’ reconstruction

Combination of the two a candidates

Smaller mass because of the loss of neutrini along with particles produced out of the acceptance, or lying outside the cone used for jets reconstructions

• Signal• bbbb• bbcc

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ProcessCross

sectionσ [pb]

% in

accept.

Total number of events after one

LHCb year

Prob. of 4b recognition

εb

Number passing the

cuts×εb

Higgs production 94 4.5 8.46 .103 (0.7)4 1.13 . 103

bbbb background 19661.5 2.2 8.65 . 105 (0.7)4 3.35 . 104

bbcc background 52053.3 2.2 2.34 . 106 (0.7)2 . (0.3)2 2.2 . 104

bbtt background 3.78 0.55 41.8 (0.7)4 6.2 . 10-2

Number of events per year:mh=115 [GeV], ma=20 [GeV]

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Number of events per year:mh=125 [GeV], ma=35 [GeV]

ProcessCross

sectionσ [pb]

% in

accept.

Total number of events after one

LHCb year

Prob. of 4b recognition

εb

Number passing the

cuts×εb

Higgs production 20 3.72 1.49 .103 (0.7)4 1.83 . 102

bbbb background 19661.5 2.2 8.65 . 105 (0.7)4 2.31 . 104

bbcc background 52053.3 2.2 2.34 . 106 (0.7)2 . (0.3)2 1.18 . 104

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Muons trigger only

Taking advantage of the muon trigger

BR of B-hadrons to muons is about 11% (verified in our simulation)

Events saved if a muon is produced with Pt>1.1 [GeV/c]

44% of signal events and 35% of background events

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Toy Monte Carlo

Testing the sensibility of our analysis to the variation of the number of signal events

Simulate 100 «LHCb experiments» with N signal events (+statistical variations) but with a fixed average background (+ statistical variations)

mh is fixed, the shape of the background is supposed known

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Toy Monte Carlo:mh=115 [GeV], ma=20 [GeV]

0

2000

4000

6000

8000

10000

12000

14000

16000

0 40 80 120 160 200Mass GeV/c2

N/2

0 G

eV/c

2

0

1

2

3

4

5

6

7

8

-800-600-400-200 0 200 400 600 800

MeanRMS

-15.03 171.6

Number of Events27

Toy Monte Carlo:mh=115 [GeV], ma=20 [GeV]

0255075

100125150175200

0 500 1000 1500 2000 2500 3000N of Events

N

02468

10121416

0 500 1000 1500 2000 2500 3000N of Events

N/

N

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With NMSSM: about 5σ for one

LHCb year

Toy Monte Carlo:mh=125 [GeV], ma=35 [GeV]

050

100150200250300

0 250 500 750 1000 1250 1500 1750 2000N of Events

N

01234567

0 250 500 750 1000 1250 1500 1750 2000N of Events

N/

N

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20 [pb] Higgs production: less than 1σ for one

LHCb year

Conclusion

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There seems to be a possibility worth to investigate to detect Higgs bosons at LHCb

The significance strongly depends on masses: a scan of different Higgs and a masses has to be made

Next step: Full simulation and incorporation of more complicated backgrounds

Back up slide: Smearing

Smoothing of the curves

Safe-side because we loose characteristics that could have been useful for discrimination

We assume the gain of one order of magnitude in statistics

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