Abramowicz, H., Abusleme, A., Afanaciev, K., Alipour Tehrani, N.,
Balázs, C., Benhammou, Y., Benoit, M., Bilki, B., Blaising, J. J.,
Boland, M. J., Boronat, M., Borysov, O., Božović-Jelisavčić, I.,
Buckland, M., Bugiel, S., Burrows, P. N., Charles, T. K., Daniluk, W.,
Dannheim, D., ... Zgura, I. S. (2017). Higgs physics at the CLIC
electron–positron linear collider.
European Physical Journal C:
Particles and Fields
,
77
(7), [475]. https://doi.org/10.1140/epjc/s10052-
017-4968-5
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Eur. Phys. J. C (2017) 77:475
DOI 10.1140/epjc/s10052-017-4968-5
Regular Article - Experimental Physics
Higgs physics at the CLIC electron–positron linear collider
H. Abramowicz
1
,A.Abusleme
2
, K. Afanaciev
3
, N. Alipour Tehrani
4
, C. Balázs
5
, Y. Benhammou
1
, M. Benoit
6
,
B. Bilki
7
, J.-J. Blaising
8
, M. J. Boland
9
, M. Boronat
10
, O. Borysov
1
, I. Božovi´c-Jelisavˇci´c
11
, M. Buckland
12
,
S. Bugiel
13
,P.N.Burrows
14
, T. K. Charles
5
, W. Daniluk
15
, D. Dannheim
4
, R. Dasgupta
13
, M. Demarteau
7
,
M. A. Díaz Gutierrez
2
, G. Eigen
16
, K. Elsener
4
, U. Felzmann
9
, M. Firlej
13
,E.Firu
17
,T.Fiutowski
13
,J.Fuster
10
,
M. Gabriel
18
, F. Gaede
4,19
, I. García
10
, V. Ghenescu
17
, J. Goldstein
20
, S. Green
21
,C.Grefe
4,b,d
, M. Hauschild
4
,
C. Hawkes
23
, D. Hynds
4
, M. Idzik
13
,G.Kaˇcarevi´c
11
, J. Kalinowski
24
, S. Kananov
1
,W.Klempt
4
, M. Kopec
13
,
M. Krawczyk
24
, B. Krupa
15
, M. Kucharczyk
15
, S. Kulis
4
, T. Laštoviˇcka
25
,T.Lesiak
15
,A.Levy
1
,I.Levy
1
,
L. Linssen
4
, S. Luki´c
11,b
, A. A. Maier
4
, V. Makarenko
3
, J. S. Marshall
21
,V.J.Martin
22
,K.Mei
21
,
G. Milutinovi´c-Dumbelovi´c
11
,J.Moro´n
13
, A. Moszczy´nski
15
, D. Moya
26
, R. M. Münker
4,e
, A. Münnich
4,f
,
A. T. Neagu
17
, N. Nikiforou
4
, K. Nikolopoulos
23
, A. Nürnberg
4
, M. Pandurovi´c
11
, B. Pawlik
15
, E. Perez Codina
4
,
I. Peric
27
, M. Petric
4
, F. Pitters
4,g
,S.G.Poss
4
, T. Preda
17
, D. Protopopescu
28
, R. Rassool
9
, S. Redford
4,b,h
,
J. Repond
7
, A. Robson
28
, P. Roloff
4,a,b
,E.Ros
10
, O. Rosenblat
1
, A. Ruiz-Jimeno
26
, A. Sailer
4
, D. Schlatter
4
,
D. Schulte
4
, N. Shumeiko
3,c
, E. Sicking
4
,F.Simon
18,b
, R. Simoniello
4
, P. Sopicki
15
, S. Stapnes
4
,R.Ström
4
,
J. Strube
4,i
,K.P.
´
Swientek
13
, M. Szalay
18
,M.Tesaˇr
18
, M. A. Thomson
21,b
, J. Trenado
29
, U. I. Uggerhøj
30
,
N. van der Kolk
18
, E. van der Kraaij
16
, M. Vicente Barreto Pinto
6
, I. Vila
26
, M. Vogel Gonzalez
2,j
,M.Vos
10
,
J. Vossebeld
12
,M.Watson
23
, N. Watson
23
, M. A. Weber
4
, H. Weerts
7
, J. D. Wells
31
, L. Weuste
18
,A.Winter
23
,
T. Wojto ´n
15
,L.Xia
7
,B.Xu
21
,A.F.
˙
Zarnecki
24
, L. Zawiejski
15
, I.-S. Zgura
17
1
Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel
2
Pontificia Universidad Católica de Chile, Santiago, Chile
3
National Scientific and Educational Centre of Particle and High Energy Physics, Belarusian State University, Minsk, Belarus
4
CERN, Geneva, Switzerland
5
Monash University, Melbourne, Australia
6
Département de Physique Nucléaire et Corpusculaire (DPNC), Université de Genève, Geneva, Switzerland
7
Argonne National Laboratory, Argonne, IL, USA
8
Laboratoire d’Annecy-le-Vieux de Physique des Particules, Annecy-le-Vieux, France
9
University of Melbourne, Melbourne, Australia
10
IFIC, CSIC-University of Valencia, Valencia, Spain
11
Vin ˇca Institute of Nuclear Sciences, University of Belgrade, Belgrade, Serbia
12
University of Liverpool, Liverpool, UK
13
Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Crakow, Poland
14
Oxford University, Oxford, UK
15
The Henryk Niewodnicza´nski Institute of Nuclear Physics Polish Academy of Sciences, Crakow, Poland
16
Department of Physics and Technology, University of Bergen, Bergen, Norway
17
Institute of Space Science, Bucharest, Romania
18
Max-Planck-Institut für Physik, Munich, Germany
19
DESY, Hamburg, Germany
20
University of Bristol, Bristol, UK
21
Cavendish Laboratory, University of Cambridge, Cambridge, UK
22
University of Edinburgh, Edinburgh, UK
23
School of Physics and Astronomy, University of Birmingham, Birmingham, UK
24
Faculty of Physics, University of Warsaw, Warsaw, Poland
25
Institute of Physics of the Academy of Sciences of the Czech Republic, Prague, Czech Republic
26
IFCA, CSIC-University of Cantabria, Santander, Spain
27
Karlsruher Institut für Technologie (KIT), Institut für Prozessdatenverarbeitung und Elektronik (IPE), Karlsruhe, Germany
28
University of Glasgow, Glasgow, UK
29
University of Barcelona, Barcelona, Spain
30
Aarhus University, Aarhus, Denmark
31
Physics Department, University of Michigan, Ann Arbor, MI, USA
123
475 Page 2 of 41 Eur. Phys. J. C (2017) 77:475
Received: 29 August 2016 / Accepted: 2 June 2017
© The Author(s) 2017. This article is an open access publication
Abstract The Compact Linear Collider (CLIC) is an option
for a future e
+
e
−
collider operating at centre-of-mass ener-
gies up to 3 TeV, providing sensitivity to a wide range of
new physics phenomena and precision physics measure-
ments at the energy frontier. This paper is the first com-
prehensive presentation of the Higgs physics reach of CLIC
operating at three energy stages:
√
s = 350 GeV, 1.4 and
3 TeV. The initial stage of operation allows the study of
Higgs boson production in Higgsstrahlung (e
+
e
−
→ ZH)
and WW-fusion (e
+
e
−
→ Hν
e
¯ν
e
), resulting in precise mea-
surements of the production cross sections, the Higgs total
decay width Γ
H
, and model-independent determinations of
the Higgs couplings. Operation at
√
s > 1 TeV provides
high-statistics samples of Higgs bosons produced through
WW-fusion, enabling tight constraints on the Higgs boson
couplings. Studies of the rarer processes e
+
e
−
→ t
¯
tH and
e
+
e
−
→ HHν
e
¯ν
e
allow measurements of the top Yukawa
coupling and the Higgs boson self-coupling. This paper
presents detailed studies of the precision achievable with
Higgs measurements at CLIC and describes the interpreta-
tion of these measurements in a global fit.
Contents
1 Introduction ......................
2 Experimental environment at CLIC ..........
3 Overview of Higgs production at CLIC .......
4 Event generation, detector simulation and reconstruction
5 Higgs production at
√
s = 350 GeV .........
6 WW-fusion at
√
s > 1TeV ..............
7 ZZ-fusion .......................
8 Top Yukawa coupling .................
9 Double Higgs production ...............
10 Higgs mass ......................
11 Systematic uncertainties ...............
12 Combined fits .....................
13 Summary and conclusions ..............
References .........................
a
e-mail:
philipp.roloff@cern.ch
b
Corresponding Editors: e-mail:
clicdp-higgs-paper-editors@cern.ch
c
Deceased
d
Now at University of Bonn, Bonn, Germany
e
Also at University of Bonn, Bonn, Germany
f
Now at European XFEL GmbH, Hamburg, Germany
g
Also at Vienna University of Technology, Vienna, Austria
h
Now at Paul Scherrer Institute, Villigen, Switzerland
i
Now at Pacific Northwest National Laboratory, Richland,Washington,
USA
j
Now at University of Wuppertal, Wuppertal, Germany
1 Introduction
The discovery of a Higgs boson [
1,2] at the Large Hadron
Collider (LHC) provided confirmation of the electroweak
symmetry breaking mechanism [
3–8] of the Standard Model
(SM). However, it is not yet known if the observed Higgs
boson is the fundamental scalar of the SM or is either a more
complex object or part of an extended Higgs sector. Precise
studies of the properties of the Higgs boson at the LHC and
future colliders are essential to understand its true nature.
The Compact Linear Collider (CLIC) is a mature option
for a future multi-TeV high-luminosity linear e
+
e
−
collider
that is currently under development at CERN. It is based on a
novel two-beam acceleration technique providing accelerat-
ing gradients of 100 MV/m. Recent implementation studies
for CLIC have converged towards a staged approach. In this
scheme, CLIC provides high-luminosity e
+
e
−
collisions at
centre-of-mass energies from a few 100 GeV up to 3 TeV.
The ability of CLIC to collide e
+
e
−
up to multi-TeV energy
scales is unique. For the current study, the nominal centre-
of-mass energy of the first energy stage is
√
s = 350 GeV.
At this centre-of-mass energy, the Higgsstrahlung and WW-
fusion processes have significant cross sections, providing
access to precise measurement of the absolute values of
the Higgs boson couplings to both fermions and bosons.
Another advantage of operating CLIC at
√
s ≈ 350 GeV is
that it enables a programme of precision top quark physics,
including a scan of the t
¯
t cross section close to the pro-
duction threshold. In practice, the centre-of-mass energy of
the second stage of CLIC operation will be motivated by
both the machine design and results from the LHC. In this
paper, it is assumed that the second CLIC energy stage has
√
s = 1.4 TeV and that the ultimate CLIC centre-of-mass
energy is 3 TeV. In addition to direct and indirect searches
for Beyond the Standard Model (BSM) phenomena, these
higher energy stages of operation provide a rich potential for
Higgs physics beyond that accessible at lower energies, such
as the direct measurement of the top Yukawa coupling and a
direct probe of the Higgs potential through the measurement
of the Higgs self-coupling. Furthermore, rare Higgs boson
decays become accessible due to the higher integrated lumi-
nosities at higher energies and the increasing cross section
for Higgs production in WW-fusion. The proposed staged
approach spans around twenty years of running.
The following sections describe the experimental condi-
tions at CLIC, an overview of Higgs production at CLIC,
and the Monte Carlo samples, detector simulation, and event
reconstruction used for the subsequent studies. Thereafter,
Higgs production at
√
s = 350 GeV, Higgs production in
WW-fusion at
√
s > 1 TeV, Higgs production in ZZ-fusion,
the measurement of the top Yukawa coupling, double Higgs
production, and measurements of the Higgs boson mass are
presented. The paper concludes with a discussion of the mea-
123
Eur. Phys. J. C (2017) 77:475 Page 3 of 41 475
surement precisions on the Higgs couplings obtained in a
combined fit to the expected CLIC results, and the system-
atic uncertainties associated with the measurements.
The detailed study of the CLIC potential for Higgs physics
presented here supersedes earlier preliminary estimates [
9].
The work is carried out by the CLIC Detector and Physics
(CLICdp) collaboration.
2 Experimental environment at CLIC
The experimental environment at CLIC is characterised by
challenging conditions imposed by the CLIC accelerator
technology, by detector concepts optimised for the pre-
cise reconstruction of complex final states in the multi-TeV
energy range, and by the operation in several energy stages
to maximise the physics potential.
2.1 Accelerator and beam conditions
The CLIC accelerator design is based on a two-beam accel-
eration scheme. It uses a high-intensity drive beam to effi-
ciently generate radio frequency (RF) power at 12 GHz. The
RF power is used to accelerate the main particle beam that
runs parallel to the drive beam. CLIC uses normal-conducting
accelerator structures, operated at room temperature. These
structures permit high accelerating gradients, while the short
pulse duration discussed below limits ohmic losses to tol-
erable levels. The initial drive beams and the main elec-
tron/positron beams are generated in the central complex
and are then injected at the ends of the two linac arms. The
feasibility of the CLIC accelerator has been demonstrated
through prototyping, simulations and large-scale tests, as
described in the Conceptual Design Report [
10]. In particular,
the two-beam acceleration at gradients exceeding 100 MV/m
has been demonstrated in the CLIC test facility, CTF3. High
luminosities are achievable by very small beam emittances,
which are generated in the injector complex and maintained
during transport to the interaction point.
CLIC will be operated with a bunch train repetition rate of
50 Hz. Each bunch train consists of 312 individual bunches,
with 0.5ns between bunch crossings at the interaction point.
The average number of hard e
+
e
−
interactions in a sin-
gle bunch train crossing is much less than one. However,
for CLIC operation at
√
s > 1 TeV, the highly-focussed
intense beams lead to significant beamstrahlung (radiation
of photons from electrons/positrons in the electric field of
the other beam). Beamstrahlung results in high rates of inco-
herent electron–positron pairs and low-Q
2
t-channel multi-
peripheral
γγ
→ hadron events, where Q
2
is the negative
of the four-momentum squared of the virtual space-like pho-
ton. In addition, the energy loss through beamstrahlung gen-
erates a long lower-energy tail to the luminosity spectrum
Fig. 1 The luminosity spectrum for CLIC operating at
√
s = 3TeV,
where
√
s
′
is the effective centre-of-mass energy after beamstrahlung
and initial state radiation [
11]
that extends well below the nominal centre-of-mass energy,
as shown in Fig.
1. Both the CLIC detector design and the
event reconstruction techniques employed are optimised to
mitigate the influence of these backgrounds, which are most
severe at the higher CLIC energies; this is discussed further
in Sect.
4.2.
The baseline machine design allows for up to ±80% longi-
tudinal electron spin-polarisation by using GaAs-type cath-
odes [
10]; and provisions have been made to allow positron
polarisation as an upgrade option. Most studies presented
in this paper are performed for zero beam polarisation and
are subsequently scaled to account for the increased cross
sections with left-handed polarisation for the electron beam.
2.2 Detectors at CLIC
The detector concepts used for the CLIC physics stud-
ies, described here and elsewhere [
11], are based on the
SiD [
12,13] and ILD [13,14] detector concepts for the Inter-
national Linear Collider (ILC). They were initially adapted
for the CLIC 3 TeV operation, which constitutes the most
challenging environment for the detectors in view of the
high beam-induced background levels. For most sub-detector
systems, the 3 TeV detector design is suitable at all energy
stages, the only exception being the inner tracking detec-
tors and the vertex detector, where the lower backgrounds
at
√
s = 350 GeV enable detectors to be deployed with a
smaller inner radius.
The key performance parameters of the CLIC detector
concepts with respect to the Higgs programme are:
– excellent track-momentum resolution of σ
p
T
/ p
2
T
2 ·
10
−5
GeV
−1
, required for a precise reconstruction of lep-
tonic Z decays in ZH events;
– precise impact parameter resolution, defined by a
5 µm and b 15 µmGeVinσ
2
d
0
= a
2
+b
2
/( p
2
sin
3
θ)to
123
475 Page 4 of 41 Eur. Phys. J. C (2017) 77:475
Fe Yoke
3.3 m
(a)
Fe Yoke
2.6 m
(b)
Fig. 2 Longitudinal cross section of the top right quadrant of the CLIC_ILD (a) and CLIC_SiD (b) detector concepts
provide accurate vertex reconstruction, enabling flavour
tagging with clean b-, c- and light-quark jet separation;
– jet-energy resolution σ
E
/E 3.5% for light-quark jet
energies in the range 100 GeV to 1 TeV, required for the
reconstruction of hadronic Z decays in ZH events and
the separation of Z → q ¯q and H → q¯q based on the
reconstructed di-jet invariant mass;
– detector coverage for electrons extending to very low
angles with respect to the beam axis, to maximise back-
ground rejection for WW-fusion events.
The main design driver for the CLIC (and ILC) detec-
tor concepts is the required jet-energy resolution. As a result,
the CLIC detector concepts [
11], CLIC_SiD and CLIC_ILD,
are based on fine-grained electromagnetic and hadronic calo-
rimeters (ECAL and HCAL), optimised for particle-flow
reconstruction techniques. In the particle-flow approach, the
aim is to reconstruct the individual final-state particles within
a jet using information from the tracking detectors com-
bined with that from the highly granular calorimeters [
15–
18]. In addition, particle-flow event reconstruction provides
a powerful tool for the rejection of beam-induced back-
grounds [
11]. The CLIC detector concepts employ strong
central solenoid magnets, located outside the HCAL, pro-
viding an axial magnetic field of 5 T in CLIC_SiD and 4 T in
CLIC_ILD. The CLIC_SiD concept employs central silicon-
strip tracking detectors, whereas CLIC_ILD assumes a large
central gaseous Time Projection Chamber. In both con-
cepts, the central tracking system is augmented with silicon-
based inner tracking detectors. The two detector concepts
are shown schematically in Fig.
2 and are described in detail
in [11].
2.3 Assumed staged running scenario
The studies presented in this paper are based on a scenario
in which CLIC runs at three energy stages. The first stage is
at
√
s = 350 GeV, around the top-pair production threshold.
The second stage is at
√
s = 1.4 TeV; this energy is chosen
because it can be reached with a single CLIC drive-beam
complex. The third stage is at
√
s = 3 TeV; the ultimate
energy of CLIC. At each stage, four to five years of running
with a fully commissioned accelerator is foreseen, provid-
ing integrated luminosities of 500 fb
−1
, 1.5 and 2 ab
−1
at
350 GeV, 1.4 and 3 TeV, respectively.
1
Cross sections and
integrated luminosities for the three stages are summarised
in Table
1.
3 Overview of Higgs production at CLIC
A high-energy e
+
e
−
collider such as CLIC provides an exper-
imental environment that allows the study of Higgs boson
properties with high precision. The evolution of the leading-
order e
+
e
−
Higgs production cross sections with centre-of-
mass energy, as computed using the Whizard 1.95 [
20]
program, is shown in Fig.
3 for a Higgs boson mass of
126 GeV [
21].
The Feynman diagrams for the three highest cross section
Higgs production processes at CLIC are shown in Fig.
4.At
√
s ≈ 350 GeV, the Higgsstrahlung process (e
+
e
−
→ ZH)
1
As a result of this paper and other studies, a slightly different staging
scenario for CLIC, with a first stage at
√
s = 380 GeV to include precise
measurements of top quark properties as a probe for BSM physics, and
the next stage at 1.5 TeV, has recently been adopted and will be used for
future studies [
19].
123