Unraveling The Physics Behind Modified Higgs Couplings -- LHC vs. a Higgs Factory

0.42 MB·English

by  Steven B. Giddings

Unraveling The Physics Behind Modified Higgs Couplings –
LHC vs. a Higgs Factory
Steven B. Giddings,1 Tao Liu,1 Ian Low,2,3,4 and Eric Mintun1
1Department of Physics, University of California, Santa Barbara, CA 93106, USA
2High Energy Physics Division, Argonne National Laboratory, Argonne, IL 60439, USA
3Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA
4Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA 93106, USA
Strongly modified hγγ and hgg couplings indicate new electroweak and color mediators, respec-
tively, with a light mass and a significant coupling to the Higgs boson. We point out the Higgs
boson could have a significant decay width into the mediators and propose uncovering the hidden
new physics through such exotic decays, which can probe the Higgs coupling with the mediators
directly. Focusing on the electroweak mediators, we study a simplified model using as an example
final states with tau leptons and neutrinos. Because one of the mediators is off-shell and its decay
3 products are extremely soft, it is challenging to make a discovery at the Large Hadron Collider. A
1 HiggsfactorysuchastheInternationalLinearCollider,however,couldserveasadiscoverymachine
0 for the EW mediators even in an early stage.
2

n
a Introduction–OnJuly4th,2012CERNannouncedthe hancement of the hγγ coupling indicates new EW states
J observation of a Higgs-like boson at the Large Hadron thati)arelight,ontheorderofafewhundredsGeV,and

0 Collider (LHC) with a mass at around 126 GeV [1, 2]. ii) couple to the Higgs boson significantly. Therefore, if
1 Preliminary results based on decay branching ratios in- the enhancement persists in the future, a top priority

dicate a genuine Higgs boson, not an imposter [3], while will be to devise strategies to search for these light “EW
]
h signal strengths in all observed channels are also con- mediators” and to probe their couplings with the Higgs
p sistent with those expected from a Standard Model boson. (There are ways to hide these new light states
- (SM)Higgsboson,exceptinthediphotonchannelwhere from direct search and precision EW constraints by, for
p
e early data suggest an enhancement over the SM rate of example, assigning a new ”parity” to the new particles
h O(50%) [4, 5]. [9].) AsmokinggunsignalofEWmediatorsisamodified
[ Theenhancedeventrateinthediphotonchannelcould rate in Higgs decays into Zγ final states [13], which cor-

1 arise from modifying any of the following quantities: relates with deviations from the SM width in the dipho-
v 1) the production cross-section, which at leading order ton channel [7–10]. Another indirect probe lies in elec-
4 comes from the gluon fusion process, 2) the total width, troweak production of the mediators at the LHC, with
2 which is dominated by the partial width of Higgs to b¯b search for their decays into SM particles [8, 10, 14]. The
3
[6, 7], and 3) the partial width of Higgs to diphoton [7– former however can not identify the mediators directly
2
. 10]. 1) and 2) alter the signal rate in all channels, while while the latter does not involve couplings between the
1
3)onlyaffectsthediphotonchannel. Currentexperimen- Higgs boson and the mediators.
0
3 tal fits favor a SM Higgs-gluon-gluon (hgg) coupling and In this letter we propose searching for the new physics
1 an enhanced Higgs-to-diphoton (hγγ) coupling [5, 11], behind the modified hγγ coupling via exotic Higgs de-
: although the statistics is limited and uncertainty quite cays. Ifthemediatorsdecaytolightparticles,anon-shell
v
i large. Higgscandecaytothemediators,atleastoneofwhichis
X
The hγγ and hgg couplings are of special importance. off-shell,muchlikeHiggsdecaystooff-shellW/Z bosons.
r On the experimental side, these couplings enter into the The partial width of this exotic Higgs decay depends on
a
gg →h→γγ channel,whichisthemaindiscoverychan- the available modes and phase space for the subsequent
nel of the Higgs boson at the LHC. On the theoretical mediator decays, which can only be computed in a spe-
side, both couplings are induced only at the loop-level cific model. However, it could be significant, especially
and serve as indirect probes of any new particles with a intheparameterregiongivingrisetoastronglymodified
significantcouplingtotheHiggs[7–10,12]. Inparticular, hγγ couplingwherethemediatorsarelightandcoupleto
newelectroweak(colored)particlescouplingtotheHiggs theHiggssignificantly. Asimilarstrategycanbeapplied
would necessarily modify the hγγ (hgg) couplings. More for studying the hgg coupling, if a strong modification is
importantly, if electroweak (EW) symmetry breaking is indicated by the LHC measurements in the near future.
natural, new particles with significant couplings to the To illustrate this strategy, we will work in a simplified
Higgs must exist to soften the quadratic divergences in model with a EW scalar mediator, φ, assuming for ex-
the Higgs mass. As a result, there are intricate connec- ample that it mainly decays into tau and tau neutrino.
tionsbetweenmodificationsinthehγγandhggcouplings One implementation of this is the MSSM with a gauged-
and the naturalness of TeV scale physics [9, 12]. U(1) extension [10], where the diphoton width can be
PQ
It was shown in Ref. [7–10] that a possible strong en- enhanced either by EW vector-like fermions which are
2
requiredfortheU(1) anomalycancellationorbytheir
PQ
(cid:45)0.5 0.2
superpartners. ThesechargedmediatorscandecaytoSM 1.5
particles and (or) their superpartiners and hence avoid 1.2
3.2 0.8(cid:42)(cid:42)
overproduction in the early Universe. We will see that,
(cid:45)1.0
given a Higgs mass ∼ 126 GeV, one of the mediators
is very off-shell and it is difficult to search for such de-
caysattheLHC,althoughwithsomeoptimisticassump- cΦ (cid:45)1.5
tions it might be feasible. We then turn to a Higgs fac- 2.1
1.8
torysuchastheInternationalLinearCollider(ILC)with
√
s = 250 GeV and show that the discovery potential is (cid:45)2.0
fairly promising. Therefore the Higgs factory can be not
only a precision machine, but also a discovery machine 0.05
(cid:45)2.5
for the truth behind the strongly modified hγγ or hgg
80 90 100 110 120
couplings.
m (cid:72)GeV(cid:76)
Φ
ExoticDecayWidth–Thepartialdecaywidthofh→
FIG. 1: Contours for Γ /ΓSM (red, solid) and
φφdependsonthreephysicalparameters: themassofthe Γ /ΓSM (black, dashedh)→.φφ h→ττ¯
h→γγ h→γγ
scalarmediator,m ,itstotalwidth,Γ ,anditscoupling
φ φ
with the Higgs, c , which is defined as in c vhφφ† with
φ φ
v ≈246 GeV being the Higgs vacuum expectation value.
The contours of Γ /ΓSM and Γ /ΓSM are
Asacomparison,thechangeinthehγγcouplingdepends h→φφ h→ττ¯ h→γγ h→γγ
showninFig.1. Thesecontoursindicateastrongpositive
on two physical parameters: m and c [9].
φ φ
correlation between Γ and Γ . We see the par-
Extendingthecalculationsin[15],itiseasytofindthe h→φφ h→γγ
tialwidthofh→φφcouldbesizableintheparameterre-
partial decay width of h→φφ
gion where the diphoton width is enhanced significantly.
Γ =(cid:90) u1dm2 (cid:90) u2dm2 dΓh→φφ . (1) For the benchmark (blue star) with mh = 126 GeV,
h→φφ + − dm2dm2 mφ =92 GeV and cφ =1, a Higgs-to-diphoton enhance-
0 0 + −
ment of 60% leads to a partial decay width Γ ∼
h→φφ
where m± is the invariant mass of φ±, u1 = m2h, u2 = 0.5×ΓShM→ττ¯. Recall BRSM(h → ττ¯) ≈ 6.4% for a 126
(m −m )2, and GeVSMHiggs,sosuchanexoticdecaymodepotentially
h +
can lead to significant implications for colliders. A cau-
dΓ c2v2 (cid:113) tionary remark is that the exotic partial width becomes
h→φφ = φ m2 −(m +m )2
dm2dm2 16πm3 h + − very small if both mediators in the decay are only al-
+ − h
(cid:113) lowed to be off-shell (i.e., if mφ > mh), in which case
× m2h−(m+−m−)2P+P− . (2) our method may not be applicable. But, such a scenario
usuallyrequiresalargerfine-tuningtoescapetheoretical
P are the propagators of φ±: and experimental constraints, given similar modification
±
strength for the hγγ coupling.
m Γ 1
P = ± ± . (3)
± π (m2 −m2)2+m2Γ2
± φ ± ± LHC Study – We first consider the discovery potential
at the LHC. Since each mediator decays into τ plus ν ,
Note here m is the invariant mass of the possibly off- τ
± the signature is 2τ+/p , which is very similar to the SM
shellφ,andΓ isthetotalwidthofφatthecorrespond- T
± h→ττ decay. In this work we only consider the vector-
ingm ,whichismodel-independent. Forillustration,we
± boson fusion (VBF) production of the Higgs, which has
consider a simplified model, assuming φ → τ +ν pre-
τ the best signal-to-background ratio in the SM h → ττ¯
dominantly. Thiswassuggestedin[10]. Thenthepartial
search [16]. In particular, we use the dileptonic channel
width Γ is
± 2τl+/pT to illustrate the LHC sensitivity (with l=e,µ).
c2 m It is well-known that the VBF selection cuts have con-
Γ = φνττ ± , (4) taminations from the gluon fusion channel, which we in-
± 16π
clude in our simulations.
where cφνττ is the φνττ coupling in the mass eigenbasis. √In the simulation, the events are generated for the
Inthisanalysis,weassumecφνττ =0.6,comparablewith s = 14 TeV LHC with MadGraph5 [17] and are show-
theEWcoupling. Formφ ∼100GeV,theon-shelldecay ered with Pythia6 [18]. Here the contributions from un-
width is then derlying events and pile-up are also turned on, with the
pile-up cross section taken to be 0.25 mb. Jet clustering
Γφ ∼0.7 GeV . (5) is done in FastJet [19] using the anti-kt algorithm with a
3
Cut 1 TwojetswithpT >20GeVeach,mjj >650GeV, Hardest Lepton Pt
|∆η|>3.5,andη η <0. TotaljetH >80GeV,
1 2 T Signal
andnoadditionaljetswithpT >30GeVbetween 0.18
forward jets. Higgs to di-tau
0.16
Cut 2 Twoopposite-signleptons,harderwith10GeV< Z to di-tau
p <20GeV,softerwith10GeV<p <15GeV 0.14
T T Higgs to WW
|η|<2.3 for electrons; |η|<2.1 for muons.
0.12
Cut 3 Invariant lepton mass mll < 20 GeV, /p > Di-boson
T 0.1
40 GeV . TTbar
0.08
0.06
TABLE I: Cuts for the LHC analysis.
0.04
0.02
conesizeofR=0.5. LeptonisolationrequiresthenetpT 0 10 20 30 40 50 60 70 80 90 100
of particles in a cone of size R=0.3 to be less than 10% GeV
of the lepton’s p . The cross sections of the processes
T Second Hardest Lepton Pt
with no Higgs involved are obtained from Pythia, scaled
0.14 Signal
by the appropriate k-factor [20–23], while the cross sec-
Higgs to di-tau
tions of the other ones are simply scaled from the SM 0.12
predictions by comparing with the SM h → ττ¯ width. Z to di-tau
The selection cuts are summarized in Table I. 0.1 Higgs to WW
Fig.2showsthenormalizedp distributionsofthetwo Di-boson
T 0.08
final-state leptons for the signal and the backgrounds. TTbar
Weseethatthatthesecondhardestleptonforthesignal 0.06
has an extremely soft p distribution peaked at below 5
T
0.04
GeV, because the off-shell φ carries very little invariant
mass. The feature exemplifies the challenge of making
0.02
this discovery at the LHC, as the standard dilepton se-
lection cut in Table I eliminates most of the signal. One 0
5 10 15 20 25 30
mightconsiderforgoingthesoftleptonandmakingasin- GeV
gle lepton selection. However, in this case the signal is
completelyoverwhelmedbytheW+j background,which FIG. 2: Normalized distributions of lepton pT.
issuppressedbydileptonselectionandthusnotincluded
inoursimulation. Intheend,thisanalysissharessimilar 14 TeV Signal h→τlτl h→WW Z →τlτ¯l tt¯ Di-boson
σ (pb) 0.06 0.11 0.27 0.72 8.0 0.17
background with the SM h → τ τ search, which mainly
l l
Cut 1 1539 3041 6393 24757 9377 4421
includes tt¯, di-boson and (Z → ττ¯)+2j [16]. (On the
Cut 2 33 66 74 327 11 13
other hand, the (Z →l+l−)+2j background is removed
Cut 3 16 2 16 40 2 4
by the lower cut on /p and not included.) In addition, √
T S/ B ∼2σ
the h → τ τ decay itself is a background for the exotic
l l
decay search. In addition to the VBF and dilepton se-
lections, in Table I we further require a maximum value TABLE II: Cut flows in the LHC analysls. Events produced
in m . For signal events, m tends to be small since are for 100 fb−1 of data. Gluon fusion contamination is in-
ll ll
cluded in the VBF selection. Production cross sections are
one of the leptons is soft. An even stronger minimum
after preselection cuts which are different for different pro-
cut on the missing energy does not help much after the
cesses.
VBF cut, since both the signal and background events
left tend to have a relatively large missing energy.
The cut flows for the signal and the backgrounds are a beam polarization (Pe−,Pe+)=(0.8,−0.6) for the ILC
summarized in Table II, where a luminosity of 100 fb−1 and focus on the process (l± =e±,µ±)
is assumed. We see that the search sensitivity is not too
promising,unlessnewtechniquesforidentifyingverysoft e+e− →Z(h→φφ→ττ¯ντν¯τ)→l+l−l+l−+/pT . (6)
leptons are developed. This decay topology provides an extremely clean labora-
tory, with the main background being Z +(Z/h → ττ¯).
ILC Study–NextwedemonstratethataHiggsfactory Tri-boson production, ZWW, with all of them decaying
such as the ILC could serve as a discovery machine for leptonically, is subdominant and also included.
the Higgs decay to light EW mediators, even during its The selection cuts are summarized in Table III. Given
√
earlyrunwith s=250GeV.Tobeginwith,weassume that the fourth charged lepton, the one from the off-
4
√
Cut 1 Threeleptonsl (i=1,2,3),with|cosθ |<0.99, s=250 GeV signal Zττ¯ ZWW
E > 3 GeV aind E < 20 GeV. Fourlith-lepton Xsection (fb) 0.93 27.81 0.02
li l3
(with |cosθ |<0.99 and E >10 GeV) veto. Events 10000 10000 10000
l4 l4
Cut 2 m =91.2±5 GeV, |cosθ |<0.8 Cut 1 2420 1854 1404
l1l2 l1l2
Cut 3 /p >70 GeV Cut 2 1272 575 329
Cut 4 12T5 GeV <mrec < 150 GeV Cut 3 821 93 258
h
Cut 4 820 3 255
√
S/ B ∼5.2σ
TABLEIII:CutsfortheILCanalysis. θ andθ arepolar
li l1l2
angles of the leptons and the reconstructed Z boson (l1+l2) √
w.r.t. the beam, respectively. mrec is the Higgs recoil mass. TABLEIV:CutflowsintheILCanalysis. S/ Biscalculated
h
for 40 fb−1 of data.
Higgs Recoil Mass mrhec signal √
Z+ditau made with about 40 fb−1 of data at s=250 GeV.
0.5 ZWW
Conclusion – We have shown that light EW mediators
0.4 contributing to a strongly enhanced Higgs-to-diphoton
width could show up in exotic Higgs decays. We then
0.3 proposedusingsuchdecaystouncoverthemediatorsand
exploretheircouplingstotheHiggs. Ingeneraloneofthe
0.2 mediatorsintheexoticHiggsdecayisfaroff-shellandits
decay products are very soft, which makes it difficult to
0.1 search for at the LHC, unless new techniques for identi-
fyingsofttracksareintroduced. Ontheotherhand,such
0 discoveries can be made with a relatively small amount
0 50 100 150 200 250 √
GeV of data at the ILC with s=250 GeV. Obviously a de-
tailed comparison between the discovery reaches at the
FIG. 3: Normalized distributions of the Higgs recoil mass. LHC and the ILC, as well as generalizations to other
types of mediators and decay final states, are warranted,
and will be reported elsewhere [25].
shell mediator decay, is extremely soft, we use a three-
lepton selection and introduce a hard fourth-lepton veto Acknowledgments
to enhance the signal. In addition, due to a relatively
√
small s, the angular distribution of the Z boson in We acknowledge discussions with Claudio Campag-
e+e− → Zh is flat in cosθ [24], while the Z bosons in nari,TomDanielson,JoeLykken,VyacheslavKrutelyov,
the Zττ¯ events are more forward because most of the Jim Olsen, Yanjun Tu and El Carlos Wagner. S.G. and
Zττ¯ events are from the di-Z production, which pro- T.L. are supported in part by DOE under grant DE-
ceeds via t-channel processes. So in Table III we require FG02-91ER40618, and S.G. is supported in part by a
|cosθ | < 0.8 to suppress the Zττ¯ background. Fur- Simons Foundation Fellowship, 229624, to Steven Gid-
l1l2
thersuppressionisachievedbydemanding/p >50GeV. dings. I.L. is supported in part by DOE under Contract
T
In Fig. 3 we show the normalized distribution of the No. DE-AC02-06CH11357 (ANL) and No. DE-FG02-
(cid:113) √ 91ER40684 (Northwestern), and by the Simons Founda-
Higgs recoil mass, mrec ≡ s−2 sE +m2 , for
h l1l2 l1l2 tion under award No. 230683. Work at KITP is sup-
both signal and backgrounds, where the peak at the
ported by the National Science Foundation under Grant
m = 126 GeV for the signal is difficult to miss. This
h No. PHY11-25915.
figuredemonstratestheadvantageofknowingthecenter-
of-mass energy in a lepton collider such as the ILC:
[1] G. Aad et al. , Phys. Lett. B 716, 1 (2012).
the Higgs mass can be reconstructed precisely even with
[2] S. Chatrchyan et al. , Phys. Lett. B 716, 30 (2012).
missing particles in the final state. Our last cut in Ta-
[3] I. Low, J. Lykken and G. Shaughnessy, Phys. Rev. D
ble III utilizes mrhec to cut away the diboson background 86, 093012 (2012); J. Ellis and T. You, JHEP 1209,
Z+(Z →ττ¯),whichispeakedatmZ inFig.3. Itisalso 123 (2012); J. R. Espinosa et al., JHEP 1212, 045
interestingtoseethatbothZττ¯andZWW backgrounds (2012);M.MontullandF.Riva,JHEP1211,018(2012);
receivecontributionsfromZ+(h→WW/ττ¯)processes. D. Carmi et al. , JHEP 1210, 196 (2012); T. Plehn and
M. Rauch, Europhys. Lett. 100, 11002 (2012).
The cut flows for the signal and the backgrounds are
[4] G. Aad et al. , ATLAS-CONF-2012-162.
summarized in Table IV. For the benchmark that we are
[5] S. Chatrchyan et al. , CMS-PAS-HIG-12-045.
considering, the signal cross s√ection is about half of the [6] M. S. Carena et al. , Eur. Phys. J. C 26, 601 (2003);
SM h→ττ¯. We see that a S/ B ≈5σ discovery can be M. Carena et al., Phys. Rev. D 84, 095010 (2011).
5
[7] M. Carena et al., JHEP 1203, 014 (2012). 713, 68 (2012); G. Aad et al. [ATLAS Collaboration],
[8] M. Carena et al., JHEP 1207, 175 (2012). JHEP 1209, 070 (2012).
[9] M. Carena, I. Low and C. E. M. Wagner, JHEP 1208, [17] J. Alwall et al., JHEP 1106, 128 (2011).
060 (2012) [18] T. Sjostrand et al., JHEP 0605, 026 (2006).
[10] H.An,T.LiuandL.-T.Wang,Phys.Rev.D86,075030 [19] M. Cacciari et al., Eur. Phys. J. C 72, 1896 (2012).
(2012). [20] R. Hamberg et al., Nucl. Phys. B 359, 343 (1991)
[11] G. Aad et al. , ATLAS-CONF-2012-127. [Erratum-ibid. B 644, 403 (2002)].
[12] I. Low, R. Rattazzi and A. Vichi, JHEP 1004, 126 [21] A. Broggio et al., JHEP 1205, 151 (2012).
(2010). [22] J.M.CampbellandR.K.Ellis,Phys.Rev.D60,113006
[13] J. S. Gainer et al., Phys. Rev. D 86, 033010 (2012). (1999).
[14] N. Arkani-Hamed et al., arXiv:1207.4482 [hep-ph]. [23] S. Dittmaier et al. , arXiv:1101.0593 [hep-ph].
[15] A. Grau et al., Phys. Lett. B 251, 293 (1990); I. Low, [24] A. Djouadi, Phys. Rept. 457, 1 (2008).
J. Lykken, JHEP 1010, 053 (2010). [25] S.B.Giddings,T.Liu,I.LowandE.Mintun,inprogress.
[16] S.Chatrchyanet al.[CMSCollaboration],Phys.Lett.B