Higgs width from interferometry in the γγ channel at the LHC

Francesco Coradeschi∗(University of Firenze & INFN, section of Firenze)

Padova - June 4, 2015

∗ In collaboration with D. De Florian, L. Dixon, N. Fidanza, S. Hoece, H. Ita, Y. Liand J.Mazzitelli

Introduction

I Using interference effects in pp → γγ + X , we may boundHiggs width much better than in direct measurements (whichcan’t really do much, see e.g. [CMS] 1407.0558)

I Important test of SM (and constraint for NP)

I Similar idea used in WW /ZZ channels using far off-shellmeasurements [Caola,Melnikov] 1307.4935, [Campbell,Ellis,Williams] 1311.3589

I Can get relatively close to SM value of ∼ 4 MeV(Γ . 4ΓSM , [CMS] 1405.3455, [ATLAS] 1503.01060)

I Could be invalidated, see e.g.[Englert,Soreq,Spannowsky] 1405.0285,1410.5440, [Logan] 1412.7577

I γγ more direct: uses the shift of the resonance peak, onlydepends on near-resonance behaviour (but. . . trickier!)

Resonance-continuum interference

I It all boils down to a Breit-Wigner:

δσi1i2→H(→γγ) =

−2(s −m2H)

Re (Ai1i2→HAH→γγA∗cont)(s −m2

H)2 + m2HΓ2

−2mHΓIm (Ai1i2→HAH→γγA∗cont)

(s −m2H)2 + m2

HΓ2

I Real part of interference antisymmetric (around the peak)

I Imaginary part of interference symmetric

Mass Shift[Martin] 1208.1533

I Lineshape smeared by finite detector resolution (∼ 1 GeV;approx. with a Gaussian of width σMR)

Mass shift from real part

[Martin] arXiv:1208.1533, arXiv:1303.3342

[deFlorian et al.] arXiv:1303.1397

� Smear lineshape with Gaussian of width 1.7 GeV (∼ detector resolution)

124.95 125 125.05M

!! [GeV]

-100

-50

0

50

100

d"

/dM

!!

[fb

/Ge

V]

→

110 115 120 125 130 135 140M

!! [GeV]

-0.2

-0.1

0.0

0.1

0.2

d"

int/d

M!! [fb

/GeV

]

1.3 GeV1.5 GeV1.7 GeV2.0 GeV2.4 GeV

"MR

=

� Re-fitting to Gaussian of mass M + δM gives δM ∼ 100 MeV

Stefan Hoche Interferometry in γγ 4

Mass Shift[Martin] 1208.1533

I Symmetric part: same shape as the resonance, ∼ 1% effect onthe cross-section [Dixon,Siu] hep-ph/0302233

I Antisymmetric part: shifts the peak roughly proportionally toGaussian width

I Re-fitting the lineshape to a Gaussian centered at mH + δmH

⇒ δmH ∼ −100 MeV (in the inclusive case; details later)

I For comparison latest experimental fit gives [ATLAS-CMS] 1503.07589

mγγH = 125.07± 0.25 (stat.)± 0.14 (syst.) GeV

m4lH = 125.15± 0.37 (stat.)± 0.15 (syst.) GeV

Mass Shift[Martin] 1208.1533, [F.C.,De Florian,Dixon,Fidanza,Hoece,Ita,Li,Mazzitelli] 1504.05215

I Shift vs. experimental mass resolution σMR in γγ + 2j

1.0 1.5 2.0 2.5 3.0!35

!30

!25

!20

!15

!10

!5

0

"MR !GeV"

#mH!MeV

"

#$% jj #&2.8pT ,H &40 GeV , #$% jj #&2.8pT ,H &80 GeV , #$% jj #&2.8

#$% jj #&0#$% jj #&5

New Physics

I Use Effective Lagrangian parametrization[Giudice,Grojean,Pomarol,Rattazzi] hep-ph/0703164

L = −[αs

8πcgbgGa,µνG

µνa +

α

8πcγbγFµνF

µν] h

v

(+ similar contr. for W /Z ) with cγ = cg = 1 in the SM

⇒ σ 'c2gc

2γS

mHΓ

I We can keep the signal strength µ = σ/σSM = 1 and varyΓSM

⇒ δmH ∼ cgcγ '√

Γ/ΓSM

New Physics[Dixon,Li] 1305.3854

I Shift vs. Γ/ΓSM (at µ = 1) in inclusive γγ

0 5 10 15 20!400

!300

!200

!100

0

100

200

300

"H !"HSM

#M

H!MeV

Constructive Interf.

Destructive Interf. "SM#

New Physics[F.C.,De Florian,Dixon,Fidanza,Hoece,Ita,Li,Mazzitelli] 1504.05215

I Shift vs. Γ/ΓSM (at µ = 1) in γγ + 2j

0 5 10 15 20!350

!300

!250

!200

!150

!100

!50

0

"! "SM

#m

H$$"MeV

#

Inclusive case (@ NLO)[Dixon,Li] 1305.3854

I Representative diagrams

NLO (gg): +

+ +

LO (gg): H LO (qg):

Inclusive case (@ NLO)[Dixon,Li] 1305.3854

I Plots using a test σMR = 1.7 GeV

120 122 124 126 128 1300

1

2

3

4

M!! !GeV"

d"sig#dM!

!!fb#G

eV"

Higgs Signal # NLO $gg%Higgs Signal # LO $gg%

120 122 124 126 128 130

!0.10

!0.05

0.00

0.05

0.10

M"" !GeV"d#

int #dM"

"!fb#G

eV" Interference $ NLO $gg%Interference $ LO $qg%Interference $ LO $gg%

Results: inclusive case[Dixon,Li] 1305.3854

I Shift as a function of jet veto

10 20 30 40 50 60

!120

!100

!80

!60

!40

!20

0

pT ,veto ! GeV

"M

H!MeV

NLO "gg# # LO "qg#NLO "gg#LO "gg#

Results: inclusive case[Dixon,Li] 1305.3854

I K-factor larger for resonant part → relative size ofinterference smaller compared to LO estimate

10 20 30 40 50 60

!120

!100

!80

!60

!40

!20

0

pT ,veto ! GeV

"M

H!MeV

NLO "gg# # LO "qg#NLO "gg#LO "gg#

Control mass

Need something to measure the shift against. . .

I Ideally, the h→ ZZ channel: δmZZH � δmγγ

H [Kauer,Passarino] 1206.4803.However, expect bigger systematics (different final state)[CMS] 1407.0558, [ATLAS] 1406.3827

I May use inclusive h→ γγ itself

I h→ γγ + 2j : include VBF production [F.C.,De

Florian,Dixon,Fidanza,Hoece,Ita,Li,Mazzitelli] 1504.05215

I For comparison latest experimental fit gives [ATLAS-CMS] 1503.07589

mγγH = 125.07± 0.25 (stat.)± 0.14 (syst.) GeV

m4lH = 125.15± 0.37 (stat.)± 0.15 (syst.) GeV

Control mass

Inclusive h→ γγ as control mass [Dixon,Li] 1305.3854

I qg -gg channel cancellations ⇒ pT ,H dependence

I When pT ,H & 40 GeV, the shift drops almost to 0

I Experimental systematics largely cancel

0 20 40 60 80 100!120

!100

!80

!60

!40

!20

0

20

pT ,H ! GeV

"M

H!MeV

H#g!q $ O"%S3#H#g!q $ O"%S3#!O"%S2#H#g $ O"%S3#

Control mass

Need something to measure the shift against. . .

I Ideally, the h→ ZZ channel: δmZZH � δmγγ

H [Kauer,Passarino] 1206.4803.However, expect bigger systematics (different final state)[CMS] 1407.0558, [ATLAS] 1406.3827

I May use h→ γγ itself

I h→ γγ + 2j : include VBF production

I For comparison latest experimental fit gives [ATLAS-CMS] 1503.07589

mγγH = 125.07± 0.25 (stat.)± 0.14 (syst.) GeV

m4lH = 125.15± 0.37 (stat.)± 0.15 (syst.) GeV

γγ + 2j channel (@ LO)[F.C.,De Florian,Dixon,Fidanza,Hoece,Ita,Li,Mazzitelli] 1504.05215

I Some representative diagrams

γγ + 2j channel (@ LO)[F.C.,De Florian,Dixon,Fidanza,Hoece,Ita,Li,Mazzitelli] 1504.05215

I First useful channel to include VBF production

I Smaller rate, signal-to-background still good

I GF and VBF quite cleanly separated by kinematics

Results: γγ + 2j channel[F.C.,De Florian,Dixon,Fidanza,Hoece,Ita,Li,Mazzitelli] 1504.05215

I Shift (and signal) as a function of jet separation |∆jj |min

!20

!10

0

10"m

H!MeV

"

M jj#400GeV

VBFGFSum

0 1 2 3 4 5 6 7 80.001

0.01

0.1

1

10

#$% jj min

Signal!fb"

Results: γγ + 2j channel[F.C.,De Florian,Dixon,Fidanza,Hoece,Ita,Li,Mazzitelli] 1504.05215

I Shift has opposite sign in GF and VBF

I In general, effect smaller in magnitude with respect to theinclusive case, plus cancellation. . .

I Not good to measure shift, but good control case!

Results: γγ + 2j channel[F.C.,De Florian,Dixon,Fidanza,Hoece,Ita,Li,Mazzitelli] 1504.05215

I Shift (and signal) as a function of minimum Higgs pT

!20

!15

!10

!5

0

5

"mH!MeV

"M jj#400GeV

#$% jj ##2.8VBF

GF

Sum

0 20 40 60 80 100 120 140 1600.00.51.01.52.02.53.0

pT ,Hmin !GeV"

Signal!fb"

Conclusions

I In this phase, we need to test the SM in as many differentways as we can think of

I The Higgs width is an important theoretical quantity (→overall coupling strenght), but not directly measurable

I The mass shift from interference gives a competitive indirectmeasure (at least complementary) of Γ

I The VBF-enriched γγ + 2j channel also provides a candidatecontrol mass