The 750GeV diphoton excess: who introduces it?

0.62 MB·

by  Hao Zhang

The 750GeV diphoton excess: who introduces it?
Hao Zhang1
1Department of Physics, University of California, Santa Barbara, California 93106, USA
Recently, both ATLAS and CMS collaborations report an excess at 750GeV in the diphoton
invariantmassspectrumat13TeVLHC.Ifitisanewscalarproducedvialoopinducedgluon-gluon
fusionprocess,itisimportanttoknowwhatistheparticleintheloop. Inthiswork,weinvestigate
the possibility of determine the fraction of the contribution from the standard model top-quark in
the loop.
I. INTRODUCTION particles. Iftheoperatorisfromtop-quarkloop,itiswell
known that the amplitude square could be written as2
6 repAonrteedxcbeyssbionthdiAphToLtAonS iannvdariCanMtSmcaoslslabsopreacttirounms aist |M(gg →φ)|2 = αs2c2t√GFMφ4(cid:12)(cid:12)(cid:12)m2t (cid:20)2+(cid:32)1− 4m2t(cid:33)
1 13TeV LHC [1, 2]. Although more data is needed to 96 2π2 (cid:12)Mφ2 Mφ2
0
n 2 mrmeaosokreneatdnhecafiennaaitteh7u5cn0oGdnrceelVudsapionandp,ewritsidiatsphppirseoa4br5aoGbnellyVin.aeInthoianetfxepowflawainenetkehwse, ×f(cid:32)Mmφ2t2(cid:33)(cid:21)(cid:12)(cid:12)(cid:12)(cid:12)2, (2)
a excess [3–149]. A new scalar which is produced via the
J where
gluon-gluonfusion(ggF)channelattheLHCisoneofthe
7  (cid:16) (cid:17)
] mweosdtispcoupsusltahrecpanodssiidbaitlietyofotfhdisisteixncgeussis.hIinngatfhoermqqe¯rawndorbk¯b, f(x)≡ (cid:104) 2a(cid:16)rcsin√2 2√1x , (cid:17)(cid:105)2 x> 14, (3)
h initial state production channel from the ggF [114]. We −1 log 1+√1−4x −iπ , x< 1.
p showedthatwecanknowwhethertheggFprocessisthe 2 1− 1−4x 4
p- dominant production mode in the near future. If ggF is Here we introduce ct to describe the contribution to cφ
e the dominant production mode, it is important to know from the top-quark loop. We have
h where is this loop induced effective operator from. Is
[ there a significant contribution from exotic colored par- cφ =ct+cNP, (4)

ticle in the loop?
1 where c is the contribution to c from the exotic col-
v In this work, we try to answer this question. If the NP φ
ored particles. Our aim is investigating the size of c
5 excess is confirmed by data in the future, we suggest the NP
and c . The pp → φ → tt¯is one of the possible channel.
5 experimentalistslookforthett¯γγsignalattheLHCRun- t
3 II,whichcanbeusedtomeasurethetop-quarkcontribu- However, there are some disadvantages of this channel.
1 First, the result of this channel depends on
tion in the loop. The reasons are explained in detail in
0
. Sec. II.InSec. III,westudytheLHCphenomenologyof Br(φ→γγ)
1 this signal. A simple simulation for a 100TeV pp collider Br(φ→tt¯) . (5)
0
is also shown there. Our conclusions are summarized in
6
Sec. IV. This ratio is highly model dependent. Even when we
1
: fix the production mechanism, it still depend on the de-
v
tails in the φ decay. Such a dependence will make the
i
X II. THE SIGNAL conclusion weaker. Second, it has been well known that
the “peak” in the tt¯invariant mass spectrum from this
r
a Welimitourdiscussionona750GeVscalarresonance process is suffered by the interference effect with the
φ produced at the LHC via effective operator1 SM top-pair production [150, 151]. This interference ef-
fect will smear the “peak” and make the discovery of
α c
s φφGa Ga,µν. (1) the signal very difficult at the LHC, especially for the φ
12πv µν
which is heavier than 700GeV. These reasons make the
Such a loop-induced effective operator could be gener- pp → φ → tt¯not be a good channel to investigate the
ated through a SM top-quark loop or some colored NP contribution of the top-quark in the φ production. The
1 If the particles in the loop is from new physics (NP) at cutoff 2 ThereisanassumptionthattheLorentzstructureoftheinter-
scale Λ, from the point of the effective field theory view, the action between φ and the SM top-quark is φt¯t. The discussion
interactionshouldbeexpandedinaccordingtotheorderof1/Λ for a pseudo-scalar with φt¯γ5t interaction and a generic scalar
butnot1/vwherevisthescaleoftheelectroweakspontaneously withφt¯eiθγ5tissimilarly. Peoplecandiscussnon-renormalizable
symmetry breaking (ESSB). Here we absorb the cutoff scale Λ interactions between φ and the SM top-quark. However, the
intoc forformallysimplicity. existenceofthemmeansNPeffectiveintheproduction.
φ
2
pp → tt¯φ → tt¯tt¯channel is helpful to solve the second V104
problem [152]. But it still highly depends on the details Ge gg scenario, s=13 TeV, 3.2fb-1
obfactkhgeroduencadys,oaftthleearsetso∼n1afnbce(.∼4Bfebc)autt¯stet¯ocfrotshsesleacrtgieonSMis s/40103 ATLAS data
needed to exclude (discover) a 750GeV scalar which de- nt Background-only fit
cays to tt¯ with 95% (∼ 5σ) confidence level (C.L.) at ve102 Best-fit signal strength
E
14TeV LHC with 3000fb−1 integrated luminosity. The
advantage of this channel is that it is c -independent 10
NP
andthuscanbeusedtomeasuretheabsolutevalueofc .
t
Toavoidthesedisadvantages,wenoticethatiftheSM
1
top-quark contribute to the ggF production, there must
be the pp → tt¯φ → tt¯γγ process. The Br(φ→γγ) de-
10- 1
pendence is cancelled when we take the ratio 200 400 600 800 1000 1200 1400 1600
σ(pp→tt¯φ→tt¯γγ) ∝(cid:12)(cid:12)(cid:12) ct (cid:12)(cid:12)(cid:12)2. (6) mg g [GeV]
σ(pp→φ→γγ) (cid:12)c +c (cid:12)
t NP
FIG.1. Thebest-fitresultoftheLHCRun-IIdiphotonexcess
This ratio only depends on c /c , which tells us
that if c = 0, the tt¯γγ sNigPnaltevent number is with the ggF production mode [114].
NP
uniquely determined by the diphoton signal strength,
and is a perfect observable to measure the size of the
We calculate the 750GeV SM Higgs-like scalar ggF cross
contribution from the top-quark loop. Then the rel-
section at 13TeV LHC to next-to-leading order (NLO)
ative tt¯γγ signal strength µ, which is the ratio be-
QCD level using MCFM7.0 [160] with CT10 PDF [161].
tween the σ(pp→tt¯φ→tt¯γγ) and the cross section
Therenormalizationandfactorizationscalesarebothset
σ(pp→tt¯φ→tt¯γγ) from the rescaling of the inclu-
top to be 375GeV. The inclusive cross section is 565fb. The
sive diphoton signal strength with c = 0 assumption,
NP top-pair associated production cross section of the SM
is just
Higgs-like scalar is calculated using MCFM7.0 to lead-
(cid:12)(cid:12)(cid:12)1+ cNP(cid:12)(cid:12)(cid:12)−2. (7) isnhgowonrdetor (hLaOve) waiKth-fMacStTorW∼20108[L16O3]P. DTFhe[1r6e2n],orwmhaiclihzais-
(cid:12) c (cid:12)
t
tionandfactorizationscalesarebothsettobe548.2GeV.
The total cross section is 2.4fb at 13TeV LHC, 3.25fb at
III. PHENOMENOLOGY 14TeV LHC, and 1.00pb at 100TeV pp collider. Thus we
have
To predict the tt¯γγ signal strength, we first fit the σ(pp→tt¯φ→tt¯γγ)=σ(pp→tt¯φ)Br(φ→γγ)
diphotonexcess. Inthiswork, wetakethedatafromthe
σ(pp→tt¯φ)
ATLAS collaboration as example. We generate parton = σ(pp→φ+X)
level events using MadGraph5 [153] with CT14llo par- σ(pp→φ+X)
ton distribution function (PDF) [154]. For ggF process, ×Br(φ→γγ)
pp → φ+nj events are generated to n=1. The MLM =52.3×10−3fb (9)
matching scheme is used to avoid the double counting in
the parton showering. All parton level events are show- at 13TeV LHC,
ered using PYTHIA6.4 with Tune Z2 parameter assign-
σ(pp→tt¯φ→tt¯γγ)=70.6×10−3fb (10)
ment [155, 156]. We use DELPHES3 to mimic the de-
tector effects [157, 158]. The b-tagging efficiency (and
at 14TeV LHC, and
the charm and light jets mis-tagging rates) is tuned to
beconsistentwiththeresultshowninRef. [159]. People σ(pp→tt¯φ→tt¯γγ)=21.8fb (11)
could find more details of this fitting in [114]. We re-
show the result in FIG. 1. With this result, the unfolded at 100TeV pp collider.
signal cross section3 In this work, we check both the dileptonic and semi-
leptonic decay modes of the top-quark pair in the tt¯γγ
σ(pp→φ+X)Br(φ→γγ)=12.3fb. (8)
events. We add some preselection cuts on the recon-
structed objects as follows:
3 ThisresultdependsonthecutacceptancefromtheMonteCarlo • Photon: The transverse energy of the leading
(MC)simulation,whichwillnotbeexactly. Forexample,ifthe
(subleading) photon should be larger than 40 (30)
photon identification rate from DELPHES is not perfectly the
same to the real case, it will be part of the systematic error of GeV. The pseudo-rapidity of the photons should
the unfolding. However, most of these errors will be partially satisfy
cancelledwhenwetaketheratiobetweentheinclusivecrosssec-
tionssincetheyalsoappearinthett¯φMCsimulation. |ηγ|<1.37, or 1.52<|ηγ|<2.37. (12)
3
We add the isolation cut for the photon. The ratio • Same-flavor dilepton events: Events are re-
between the summation of the transverse momen- quired to have either exactly two opposite-sign
tumofthetracksina∆R=0.4coneregionaround muons or two opposite-sign electrons. To suppress
thereconstructedphotonandthetransverseenergy the backgrounds from the Z+jets and heavy flavor
of the photon should be smaller than 0.022 (tight decay, the invariant mass of the dilepton system
selection). m is required to be
(cid:96)(cid:96)
• Electron: Electron in the pseudo-rapidity region m(cid:96)(cid:96) >60GeV, |m(cid:96)(cid:96)−mZ|>10GeV. (19)
|ηe|<1.37, or 1.52<|ηe|<2.47 (13) The missing transverse energy E/T of the signal
events must be larger than 30GeV.
is reconstructed if its transverse momentum is
• Semi-leptonic events: Events are required to
larger than 25GeV. The ratio between the summa-
have one and only one charged lepton and at least
tion of the transverse momentum of the tracks in
four jets. TheE/ and the transverse mass m of
a ∆R = 0.2 cone region around the reconstructed T T
the missing transverse energy and the charged lep-
electron and the transverse momentum of the elec-
ton is required to be
tron should be smaller than 0.1. Electrons which
are within ∆R < 0.4 of any reconstructed jet are E/ >40GeV, or m >50GeV (20)
T T
removed from the event.
for electron events and
• Muon: Muon should satisfy
E/ +m >60GeV (21)
T T
|ηµ|<2.5, pµ >25GeV. (14)
T for muon events.
Theratiobetweenthesummationofthetransverse
• eµ events: Events are required to have a pair of
momentumofthetracksina∆R=0.2coneregion
opposite-sign electron and muon. No more cut is
around the reconstructed electron and the trans-
added.
verse momentum of the electron should be smaller
All of the results of 100TeV pp collider are get with the
than 0.1. Muons which are within ∆R < 0.4 of
simple assumption that the parameters of the detector
any reconstructed jet are removed from the event
and the cuts are the same to the LHC.
to reduce the background from muons from heavy
The irreducible SM background is the pp→tt¯γγ pro-
flavor decays .
cess. There are some reducible SM backgrounds such as
• Jet: Jetsarereconstructedusinganti-k algorithm
T pp→tt¯γj,
with radius parameter R=0.4. They are accepted
if pp→tt¯jj,
pp→V +jets.
|ηj|<2.5, pj >25GeV. (15)
T However, the non-tt¯+X backgrounds would be highly
In additional, b-jets are required to be in suppressed by the cuts [164, 165]. And the tt¯jj,tt¯γj
backgrounds will be suppressed by the mis-identification
|ηb|<2.4. (16)
rate of a (or two) jet(s) to photon. In this prelimi-
nary analysis, we will only consider the irreducible SM
The signal events are required to have at least one background pp→tt¯γγ and neglect the irreducible back-
charged lepton, two isolated hard photons and at least
grounds. Although the signal cross section is not large,
one b-tagged jet. Then they are separated into same-
due to the extremely energetic diphoton cut, the back-
flavor dilepton events, eµ events and semi-leptonic
groundeventsnumberisexpectedtobequitesmall. The
events.
results are shown in TABLE I and FIG. 2. To discover
Some additional cuts are added for the three different
theNPintheproductionprocess,weneedtoexcludethe
signal events sample. The cuts are generally a combi-
c =0hypothesiswhichmeansµ=1. Weseparatethe
NP
nation of the SM top-pair cuts and high invariant mass
invariant mass region into fifteen bins and check the ex-
diphoton cuts [1, 164, 165]. First of all, the invariant
clusionsignificantofthesignalwithstrengthµ[166,167]
mass of the leading and subleading photons m must
γγ (cid:115)
satisfy (cid:20)L(µ{s}+{b}|{b})(cid:21)
CL ≡ −2log , (22)
b L({b}|{b})
|m −750GeV|<150GeV. (17)
γγ
ThetransverseenergyEγ1 (Eγ2)oftheleading(sublead- where s and b are events numbers of the signal and the
T T background, respectively. If the cross sections of the sig-
ing) photon must satisfy
nal and background are σ and σ respectively and the
s b
Eγ1 (cid:18)Eγ2 (cid:19) luminosity is L, we have
T >0.4 T >0.3 . (18)
mγγ mγγ s=σsL, b=σbL. (23)
4
b] b] b] 1
bin [f10-3 s=13 TeV LHC bin [f10-3 s=14 TeV LHC bin [f10-1 s=100 TeV pp-collider
/ / /
s s s
10-4 10-4 10-2
10-5 7SS5MM0 bbGkkeggVdd:: dmeip++hjjeeotttsso ncch hsaaignnnnnaeelll 10-5 7SS5MM0 bbGkkeggVdd:: dmeip++hjjeeotttsso ncch hsaaignnnnnaeelll 10-3 7SS5MM0 bbGkkeggVdd:: dmeip++hjjeeotttsso ncch hsaaignnnnnaeelll
SM bkgd: em channel SM bkgd: em channel SM bkgd: em channel
SM bkgd: mm channel SM bkgd: mm channel SM bkgd: mm channel
SM bkgd: ee channel SM bkgd: ee channel 10-4 SM bkgd: ee channel
10-6600 650 700 750 800 850 900 10-6600 650 700 750 800 850 900 600 650 700 750 800 850 900
mgg [GeV] mgg [GeV] mgg [GeV]
FIG.2. Thediphotoninvariantmassdistributionofbothsignalandbackgroundeventsaftercuts. Thesignalstrengthµisset
to be 1.
TABLEI.Thesignalandbackgroundscrosssectionsafterthecutsat13TeVLHC,14TeVLHCand100TeVppcollider. The
unit in this table is ab.
Channel e+jets channel µ+jets channel eµ channel ee channel µµ channel
13TeV background cross section (ab) 0.190 0.206 0.0298 0.0110 0.0117
13TeV signal cross section (ab) 0.823 0.873 0.104 0.0374 0.0403
14TeV background cross section (ab) 0.241 0.263 0.0363 0.0131 0.0152
14TeV signal cross section (ab) 1.11 1.16 0.137 0.0486 0.0523
100TeV background cross section (ab) 24.5 26.0 2.35 0.956 1.03
100TeV signal cross section (ab) 197 194 19.0 7.65 7.06
The likelihood function is defined by L102
C
CL 13TeV
L({x}|{n})≡(cid:89)xniiexp(−xi). (24) CLbs 13TeV
i Γ(ni+1) 30 CLb 14TeV
20 CLs 14TeV
People can also get the significant of confirming the top- CLb 100TeV
quark loop contribution (excluding ct = 0 hypothesis) 10 CLs 100TeV
which can be defined as
(cid:115)
(cid:20) L({b}|µ{s}+{b}) (cid:21) 3
CL ≡ −2log . (25)
s L(µ{s}+{b}|µ{s}+{b}) 2
1
From FIG. 3, we find that with the full data from the 1 10 102 103
Integrated Luminosity [fb-1]
high-luminosity (HL) LHC, a 3σ C.L. (nearly 5σ C.L.)
exclusion of the c = 0 (c = 0) hypothesis can be
NP t
reached. At 100TeV pp collider, the 3σ C.L. exclusion
FIG.3. Thesignificantofdiscoveryandexclusionofthetop-
of the c =0 hypothesis will be reached with 13.8fb−1
NP quark-loop only scenario at the LHC and the 100TeV pp col-
integrated luminosity. To measure µ precisely, a 100TeV lider versus the integrated luminosity.
pp collider is necessary. We define the uncertainty δµ of
the signal strength by
IV. CONCLUSION
(cid:115)
(cid:20) (cid:21)
L((µ+δµ){s}+{b}|µ{s}+{b})
−2log =1.
L(µ{s}+{b}|µ{s}+{b}) Recently, an excess at 750GeV in the diphoton invari-
(26) ant mass distribution is reported by ATLAS and CMS
At100TeVppcollider, thesignalstrengthµcanbemea- collaboration with the LHC Run-II data. Many works
sured with about 20% relative uncertainty with 100fb−1 appears online to explain this excess. If it is confirmed
integratedluminosity,about3%relativeuncertaintywith bythefuturedata,itwillbethefirstparticlebeyondthe
3000fb−1 integratedluminosity,andlessthan1%relative SM discovered at high energy colliders. And the particle
uncertainty with 3ab−1 integrated luminosity (see FIG. physics SM must be extended. It will be very important
4). to understand the production and the decay properties
5
60 ticles might be discovered in the searching of heavy col-
%] ored particles. However, it depends on the mass and the
y [ 100TeV pp collider decay modes of the exotic colored particle. Comparing
aint 40 100fb-1 with directly searching for the exotic colored particle,
ert 3000fb-1 our method can give a definitely answer of the role of
nc 20 30000fb-1 the SM top-quark and whether there is exotic contribu-
u
e tions in the production of this 750GeV resonance, with
ativ 0 either a positive or a null result. We show that with the
el HL-LHC data, 95% C.L. exclusion of the top-quark-only
R
hypothesiscanbereached. Itwillbeastronghintofthe
- 20
existenceoftheroleofnewparticlesintheproductionof
the 750GeV resonance.
- 40
0 0.5 1 1.5 2 Inadditional, thecontributionfromtheSMtop-quark
m
in the production of the 750GeV resonance can be pre-
cisely measured with the 100TeV pp collider in future
[168, 169]. With 3ab−1 integrated luminosity, the rela-
FIG.4. Therelativeuncertainty(δµ/µ)ofthesignalstrength tive uncertainty could be smaller than 1%. This result,
at 100TeV pp collider. We only consider statistical uncer- with the result from searching pp→tt¯φ→tt¯tt¯, can help
tainty.
usunderstandthepropertiesofthenewresonance, espe-
cially its production.
of the new particle. A ggF produced exotic scalar is one
of the most popular explanation of this excess. In this
work, we suggest a method to investigate the role of the V. ACKNOWLEDGMENTS
SM top-quark in the loop-induced effective operator.
If we are lucky, and there are new particles which con- The work of H. Zhang is supported by the U.S. DOE
tribute to the loop-induced effective operator, these par- under Contract No. DE-SC0011702.
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