TALK PDF file

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TALK PDF file

Searching for Higgs decays to

four bottom quarks at LHCb

Julien Rouvinet

Directeur du travail: Prof. Aurelio Bay

Expert externe: Victor Coco

1


Overview

Theoretical overview of

LHCb experiment

Study of the h0 → aa → b signature

¯bb¯b Statistical analysis

Conclusion

2

h0 → aa → b ¯ bb ¯ b


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

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

3


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 λa2 H † H

4


Higgs to four b’s decay

The mass of the particle a is given by:

where v 246 [GeV] is the Higgs vacuum

expectation value

The decay rate is then:

m 2 a = 1

2 λv2

Γ(h → aa) 1

32π

λ 2 v 2

mh

It dominates the Higgs (ex. SM 115 GeV) decay

if the coupling is:

λ>0.25

5


1 − 4m2 a

m 2 h


LHCb Experiment

Context: LHC and LHCb experiment

Acceptance:

15


Irreducible

QCD Backgrounds

b ¯ bb ¯ b

contribution: Probability of proper b

recognition is about 70% and the probability to

confuse a b with a c is about 30% *

b¯bc¯c Also a b contribution because t decays mostly

in b. The production cross section of this

background is small (3.78 [pb] from Alpgen)

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

7


h

Signal events generation

Pythia 6.325 for production, hadronization and

decay of the events

0 pbarn → 5.9 · 105

bb 2.45 · 10 8 pbarn → 1.5 · 10 12

ZW 4.33 · 10 6 pbarn → 2.7 · 10 10

Energy in the center of mass frame:

tt 2.75 · 10 3 pbarn → 1.7 · 10 7

q

h 0

q

√ s = 14 TeV

We set and to one

10‘000 signal events produced through gluons

fusion

g q

h 0

g q

BR(h0 → aa) BR(a → b ¯ b)

g q

q

g q

8

h 0


NEW

Signal events generation (2)

Masses: mh=115 [GeV/c 2 ], ma=20 [GeV/c 2 ]

We have also used mh=125 [GeV/c 2 ], ma=35

[GeV/c 2 ] for comparison

9

NEW


Background events generation

Generator: Alpgen v2.13

Energy in the center of mass:

√ s = 14 TeV

700’000 b and 500’000 have been

generated

¯bb¯b b¯bc¯c 10


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 ∆Rij =

and

(ηi − ηj) 2 +(φi − φj) 2 η = −ln(tan(θ/2))

11


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)

12


To reconstruct jets, a

radius Rcone= 0.6 is chosen

Two jets overlap if :

∆R(b1,b2) < 2 · Rcone =1.2

➡ some jets

are

overlapping

Kinematics: b quarks

13

• Signal

• bbbb

• bbcc


Jets reconstruction

Cone algorithm: b quark as seed

We combine the jets onto pairs to get an a

candidate

14


Jets combinations

Three possibilities of jets combinations

Criteria: we minimize ΔR

For mh= 115 [GeV/c 2 ] and ma= 20 [GeV/c 2 ]: 99% of

correct combinations

Only 75 % for mh= 125 and ma= 35 [GeV/c 2 ]

15


Jets combinations

16


Anti-correlation more

pronounced for the

signal

Jets combinations

17


a are well separated

in ∆R

a

18


Cuts on Pt:

Pt(j1)+Pt(j2) < 60 GeV c −1

Pt(j3)+Pt(j4) > 20 GeV c −1

Cuts on Pt

19

• Signal

• bbbb


Cut on ma

• Signal

• bbbb

• bbcc

Mass cuts: 13 < ma < 25 [GeV/c 2 ]

20


ma and cuts

•Signal

• bbbb

• bbcc

21


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

22

• Signal

• bbbb

• bbcc


Process

Number of events per year:

mh=115 [GeV], ma=20 [GeV]

Cross

section

σ [pb]

%

in

accept.

Total number of

events after one

LHCb year

Prob. of 4b

recognition

εb

Number

passing the

cuts

Higgs production 94 4.5 8.46 . 10 3 (0.7) 4 1.13 . 10 3

bbbb background 19661.5 2.2 8.65 . 10 5 (0.7) 4 3.35 . 10 4

bbcc background 52053.3 2.2 2.34 . 10 6 (0.7) 2 . (0.3) 2 2.2 . 10 4

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

23

×εb


Process

Number of events per year:

mh=125 [GeV], ma=35 [GeV]

Cross

section

σ [pb]

%

in

accept.

Total number of

events after one

LHCb year

Prob. of 4b

recognition

εb

Number

passing the

cuts

Higgs production 20 3.72 1.49 . 10 3 (0.7) 4 1.83 . 10 2

bbbb background 19661.5 2.2 8.65 . 10 5 (0.7) 4 2.31 . 10 4

bbcc background 52053.3 2.2 2.34 . 10 6 (0.7) 2 . (0.3) 2 1.18 . 10 4

24

×εb


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

25


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

26


N/20 GeV/c 2

16000

14000

12000

10000

8000

6000

4000

2000

0

Toy Monte Carlo:

mh=115 [GeV], ma=20 [GeV]

0 40 80 120 160 200

Mass GeV/c 2

27

8

7

6

5

4

3

2

1

Mean

RMS

-15.03

171.6

0

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

Number of Events


N

N/ N

200

175

150

125

100

75

50

25

0

Toy Monte Carlo:

mh=115 [GeV], ma=20 [GeV]

0 500 1000 1500 2000 2500 3000

N of Events

16

14

12

10

8

6

4

2

0

0 500 1000 1500 2000 2500 3000

N of Events

28

With NMSSM:

about 5σ for one

LHCb year


N

N/ N

300

250

200

150

100

50

0

Toy Monte Carlo:

mh=125 [GeV], ma=35 [GeV]

0 250 500 750 1000 1250 1500 1750 2000

N of Events

7

6

5

4

3

2

1

0

0 250 500 750 1000 1250 1500 1750 2000

N of Events

29

20 [pb] Higgs

production: less

than 1σ for one

LHCb year


Conclusion

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

30


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

31

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