Discovery potential of top- partners in a realistic composite Higgs model with early LHC data

Discovery potential of top-

partners in a realistic composite Higgs model with early LHC data

Günther Dissertori, Elisabetta Furlan, Filip Moortgat, Pascal Nef

Kick-off Meeting Of The LHCPhenoNet Initial Training Network Valencia, February 1st 2011

JHEP09(2010)019

composite Higgs model

‣ try to explain why the Higgs boson is light

‣ the Higgs boson is a composite state of some new, strongly interacting sector

‣ this new sector has a global SO(5) symmetry that is broken to SO(4) at the scale f

‣ get 4 real Goldstone bosons with the correct quantum numbers to be identified with the Higgs doublet

‣ the Higgs is the (pseudo) Goldstone boson associated to this symmetry breaking

‣ its mass is naturally light, as it is generated at loop level and not sensitive to radiative corrections above the compositeness scale Λ ~ 2 π f

‣ we choose f = 500 GeV in order not to have too large fine-tuning

‣ similar idea as for pions (π⁺, π^-, π⁰) in QCD!!

composite Higgs model

‣ we could also have other composite states of the new sector, for example new quarks!

‣ mixing of the top-quark with such new quarks could explain the large top-mass

‣ we consider the case of multiplets of new quarks that transform under the fundamental representation of SO(5)

‣ one multiplet contains three quarks of charge +2/3, one of charge -1/3, and one of charge +5/3

‣ the new quarks can help in restoring the agreement of the model with electroweak precision tests ...

‣ couplings of Higgs to gauge bosons are reduced w.r.t. the SM (shift of S and T parameters)

‣ the new quarks contribute to electroweak precision observables

‣ scan parameter space for points that are consistent with EWPT and CDF mass limits

‣ consider Peskin-Takeuchi S & T parameters and coupling

‣ the case of one multiplet of new quarks is very constrained

‣ the mass spectrum and possible decay chains are quite fixed

Z Z

t₁

composite Higgs model

Z

¯ t₁

t1

¯b

b

Peskin-Takeuchi T

Z → b

¯ b

Z → b

¯ b

‣ for two multiplets of new quarks, the constraints are much weaker

‣ much richer collider phenomenology

‣ focus on distinctive signatures of the two multiplet model

‣ either two charge 5/3 quarks below 500 GeV (XX signature)

‣ or t1, t2, x1, and b1 with Δm < 60 GeV below 500 GeV (4 of SO(4))

‣ upper mass bound of 500 GeV for early detection

‣ we define 30 benchmark points that show these signatures

g

g

g t1

t¯1

Z t

l⁺ l⁻

b ν

l⁺

W⁻

q^"

q¯

¯b W⁺

composite Higgs model

find large region of parameter space to be compatible with current electroweak precision data!

detector simulation &

selection of jets and leptons

‣ implement the model consistently in the MadGraph event generator

‣ produce 10⁵ events for pair-production of the new quarks for all 30 benchmark points

‣ fast detector simulation with Delphes

‣ using the CMS detector specifications

‣ searching for an excess over the SM expectation in multi-lepton final states

‣ select electrons and muons with pt > 20 GeV and |η| < 2.4

‣ relative isolation

‣ select cleaned jets with pt > 50 GeV and |η|< 3.0 (anti-kt with cone radius 0.5)

P_t

p_t,lept < 0.05 where P_t =

tracks�

i�=µ,e

p_t,i in cone ∆R < 0.3

final state selection

‣ lepton configuration vs. jet multiplicity for 200 pb^-1

‣ signal: benchmark point with cross section 10.78 pb^-1 (LO)

‣ total SM background

‣ SF = same flavour, OF = opposite flavour,

‣ SS = same sign, OS = opposite sign (at least one lepton has a different sign) lepton configuration

10-2

10-1

1 10

2l SS 2l OS SF 2l OS OF 3l SS 3l OS 4l 5l 0j

1j 2j 3j 4j 5j 6j 7j 8j 9j

jet multiplicity

10 102

103

104

105

2l SS 2l OS SF 2l OS OF 3l SS 3l OS 4l 5l 0j

1j 2j 3j 4j 5j 6j 7j 8j 9j

jet multiplicity

lepton configuration

lower limit at 95% C.L. upper limit at 95% C.L.

# of events # of events

‣ number of signal events divided by the total number of background events from SM

0 0.5 1 1.5 2 2.5 3 3.5 4

2l SS 2l OS SF 2l OS OF 3l SS 3l OS 4l 5l 0j

1j 2j 3j 4j 5j 6j 7j 8j 9j

lepton configuration

jet multiplicity

S/B

final state selection &

inclusive discovery potential

most promising final states

SS dilepton with 3 or 4 j OS trilepton with 2 or 3 j

(GeV) pT

0 50 100 150 200 250 300 350 400 450 500

events / 15 GeV

0 2 4 6 8 10 12

(GeV) pT

0 50 100 150 200 250 300 350 400 450 500

events / 15 GeV

0 2 4 6 8 10

12 hardest jet for

!

pT

lBP 18 BP 10

SM background

‣ robust, cut-based analysis for two benchmark points (BP) with a large total cross section

‣ both BPs show a 4 of SO(4) signature

‣ one BP has in addition two charge 5/3 quarks below 500 GeV

‣ both have a large cross section of 5.52 pb (4) and 10.78 pb(XX)

‣ selection cuts:

‣ preselection: same-sign di-lepton

‣ cut 1: at least two jets with pt > 50 GeV

‣ cut 2: hardest jet pt > 90 GeV

‣ cut 3: ht > 300 GeV

after preselection

two promising

benchmark points

(GeV) hT

0 200 400 600 800 1000 1200

events / 30 GeV

0 10 20 30 40 50 60 70

(GeV) hT

0 200 400 600 800 1000 1200

events / 30 GeV

0 10 20 30 40 50 60 70

L=200 pb-1

!

for hT

lBP 18 BP 10

SM background

‣ for 200 pb^-1 and our selection cuts

‣ 62 and 41 signal events

‣ 6.9 background events

‣ 5σ achieved with and pb^-1

‣ LHC experiments have sensitivity already now

‣ ATLAS & CMS have collected about

~35 pb^-1 in the 2010 runs each

‣ first results will be public soon...

24⁺¹⁶₋₁₂

46⁺²⁵₋₂₂ only after requiring

same-sign di-leptons

two promising

benchmark points

x 1 mass reconstruction

‣ how to reconstruct the mass of a charge 5/3 quark? x1 → t W⁺

‣ traditional method

(e.g. CMS PAS EXO-08-008)

‣ identify the decay using the same-sign di-leptons from the decay of the one quark

‣ reconstruct the mass of the other, hadronically decaying quark

x

x ¯

x¯1

x1

g

g

g

W⁺

W⁻

q^"

q¯

q^"

q¯

¯b ν

l⁺

ν

l⁺

b t

¯ t

W⁺

W⁻

x 1 mass reconstruction

‣ how to reconstruct the mass of a charge 5/3 quark? x1 → t W⁺

‣ traditional method

(e.g. CMS PAS EXO-08-008)

‣ identify the decay using the same-sign di-leptons from the decay of the one quark

‣ reconstruct the mass of the other, hadronically decaying quark

x

x ¯

x¯1

x1

g

g

g

W⁺

W⁻

q^"

q¯

q^"

q¯

¯b ν

l⁺

ν

l⁺

b t

¯ t

W⁺

W⁻

‣ our idea: use the invariant mass distribution of the SS di-leptons to reconstruct the x1

mass in the decay

x

→ tW

→ bW

W

→ bl

l

ν

ν

x 1 mass reconstruction with 200 pb -1

‣ selecting signal events with our selection cuts

‣ kinematic configuration of x1 decay is identical to

‣ Kraml & Raklev:

analytic expression for Mlc in gluino decay

‣ fit tail of SS di-lepton inv mass

distribution to reconstruct x1 mass!

‣ 200 pb^-1 is sufficient to get x1 mass within ~ 30 GeV

˜

g → t ¯ t ˜

, ˜ t

→ c χ ˜

(GeV) Mll

0 100 200 300 400 500 600

events / 28 GeV

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

L=200 pb-1

0

for Mll

signal + SM SM

m_fit = 368.6 ± 33.4 GeV m_x1 = 365 GeV

applying the method to the benchmark points

‣ for both discussed signatures of the model, the excess of SS di-lepton events is produced by the contributions of various new quarks

‣ t1 and x1 are the lightest new quarks: x1 gives the largest contribution!

‣ Q: is 200 pb^-1 (~50 events) enough to reveal the presence of new quarks in addition to x1?

applying the method to the benchmark points

‣ for both discussed signatures of the model, the excess of SS di-lepton events is produced by the contributions of various new quarks

‣ t1 and x1 are the lightest new quarks: x1 gives the largest contribution!

‣ Q: is 200 pb^-1 (~50 events) enough to reveal the presence of new quarks in addition to x1?

idea:

‣ if only x1 is present, a fit of the tail of Mll leads to a fairly accurate estimate of the x1

mass.

‣ with a given mass hypothesis, we can calculate the pair production cross section for the heavy top-partner

‣ knowing mass and cross section, we can use MC to compare the observation with the expected distribution due to x1 with the fitted mass.

‣ if various quarks are present, the above leads to a discrepancy between the single x1

hypothesis and the observation.

applying the method to the benchmark points

‣ SS di-lepton invariant mass of signal + SM (red)

‣ signal produced by t1, x1, t2, b1, t3, x2, t4

‣ 62 signal and 7 SM events for 200 pb^-1

‣ mx1= 365 GeV, fit gives mfit =395 ± 25 GeV

‣ blue distributions due to pair-produced x1 with fitted mass ± error on mass.

‣ expect 11 ± 3 events from x1 only with fitted mass

events / 28 GeV

0 2 4 6 8 10 12

L=200 pb-1

0

for Mll

BP10 + SM

only + SM expectation for x1

‣ large discrepancy of 51 unexplained signal events!

‣ evidence for contribution of additional quarks

conclusions

‣ composite Higgs models are attractive solutions to the hierarchy problem

‣ large top mass explained by mixing of top with heavy top-partners

‣ SS di-lepton and tri-lepton final states were found to be promising for a potential discovery

‣ outlined new method to reconstruct the mass of a charge 5/3 top-partner via its leptonic decay

‣ does not rely on jets

‣ the method can be used to judge if the signal is compatible with the presence of one charge 5/3 quark only

Backup

analogy to pions

‣ the three pions have the same problem as the Higgs:

they are light scalars

‣ explanation:

‣ the pions are composite states of up- and down-quarks

‣ the quark Lagrangian has a global SU(2)L x SU(2)R symmetry that is (spontaneously) broken to SU(2)V

‣ get three light, real Goldstone bosons, the pions!

‣ composite Higgs: adopt solution from pions

the quark multiplets

‣ we consider vector-like multiplets of top partners Ψ that transform under the fundamental representation of SO(5)

‣ Q: SU(2)L doublet with the same quantum numbers as qL = (tL , bL)

‣ X: SU(2)L doublet with hypercharge 7/6

‣ its upper component is a charge 5/3 quark

‣ T: singlet with hypercharge 2/3

‣ same quantum numbers as tR

Ψ = (Q, X, T ) ⇒ (5) = (2, 2) ⊕ (1, 1)

electroweak precision tests

reduction of

W-h couplings UV physics

S

‣ # of jets after preselection cut

Njets

0 1 2 3 4 5 6 7 8 9 10

events

10-2

10-1

1 10 102

Njets

0 1 2 3 4 5 6 7 8 9 10

events

10-2

10-1

1 10 102

L=200 pb-1

!

number of jets for lBP 18 BP 10

SM background

we require

# jets ≥ 2

two promising

benchmark points

x 1 mass reco using the endpoint

‣ the x1 mass can be extracted from the endpoint of the SS di-lepton invariant mass distribution via:

(GeV) Mll

0 100 200 300 400 500 600

events / 28 GeV

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

L=200 pb-1

0

for Mll

signal + SM SM

m_fit = 368.6 ± 33.4 GeV m_x1 = 365 GeV

m_x1 =

���M_l^max

1l₂

�2

+ m²_t� ��

M_l^max

1l₂

�2

+ m²_W� M_l^max_1l2

M

1l₂

= 308 GeV