Physics Letters B 777 (2018) 294–297
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
ISR corrections to associated HZ production at future Higgs factories
Mario Greco
a,b
, Guido Montagna
c,d
, Oreste Nicrosini
d
, Fulvio Piccinini
d,∗
, Gabriele Volpi
c
a
Dipartimento di Matematica e Fisica, Università di Roma 3, via della Vasca Navale 84, 00146 Roma, Italy
b
INFN, Sezione di Roma 3, via della Vasca Navale 84, 00146 Roma, Italy
c
Dipartimento di Fisica, Università di Pavia, via A. Bassi 6, 27100 Pavia, Italy
d
INFN, Sezione di Pavia, via A. Bassi 6, 27100 Pavia, Italy
a r t i c l e i n f o a b s t r a c t
Article history:
Received
10 November 2017
Accepted
22 December 2017
Available
online 27 December 2017
Editor:
L. Rolandi
Keywords:
Higgs
boson
Electron–positron
colliders
QED
corrections
We evaluate the QED corrections due to initial state radiation (ISR) to associated Higgs boson production
in electron–positron (e
+
e
−
) annihilation at typical energies of interest for the measurement of the Higgs
properties at future e
+
e
−
colliders, such as CEPC and FCC–ee. We apply the QED Structure Function
approach to the four-fermion production process e
+
e
−
→ μ
+
μ
−
b
¯
b, including both signal and background
contributions. We emphasize the relevance of the ISR corrections particularly near threshold and show
that finite third order collinear contributions are mandatory to meet the expected experimental accuracy.
We analyze in turn the rôle played by a full four-fermion calculation and beam energy spread in precision
calculations for Higgs physics at future e
+
e
−
colliders.
© 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
The discovery of a new scalar particle at the LHC in 2012 by the
ATLAS [1] and CMS collaborations [2] has opened a new chapter
in particle physics, and immediately has triggered the question of
the real nature of this boson. The determination of the spin-parity
quantum numbers and the couplings to other Standard Model (SM)
particles strongly suggest it to be the Higgs boson, i.e. the elemen-
tary
scalar particle responsible for the mechanism of electroweak
symmetry breaking. However, from the available data it can not be
concluded yet that we have found the SM Higgs boson and not, for
instance, one of the scalars postulated within possible extensions
of the SM.
Therefore,
various lepton collider Higgs factories [3–10] are un-
der
consideration to study in detail the properties of the new
particle to great accuracies, because of the much more favorable
experimental environment than that at hadron colliders. Amongst
the many candidates of Higgs factories, one can distinguish two
main categories.
The
first one exploits the possibility of s-channel Higgs reso-
nant
production, which is especially important, due to the narrow
width of the Higgs boson, of about 4 MeV, as predicted by the
SM. In particular, a muon collider Higgs factory [3,6] could produce
the Higgs boson in the s-channel and perform an energy scan to
*
Corresponding author.
E-mail
address: fulvio.piccinini@pv.infn.it (F. Piccinini).
map out the Higgs resonance line shape at tens of MeV level [11].
This approach would provide the most direct measurement of the
Higgs boson total width, the Yukawa coupling to muons and other
fermions, and would also enable to simply investigate the existence
of other scalar bosons predicted by natural extensions of the SM.
This case has been studied in detail in Refs. [12–14], with a partic-
ular
emphasis on the rôle of the Initial State Radiation (ISR) effects
– which are quite important [15,16] due to the s-channel resonant
production of the very narrow Higgs boson – and the evaluation of
the background processes.
The
second category includes the possibility of ultra-high lumi-
nosity
electron–positron (e
+
e
−
) colliders, such as the CEPC, FCC–
ee,
the International Linear Collider and CLIC, which have been
proposed [4,5,7–10] with the precise aim of observing the Higgs
signal mainly through the reaction e
+
e
−
→ HZ at different center
of mass (c.m.) energies, but also measuring possibly the Yukawa
coupling to electrons. In these cases, the ISR effects could be quite
sizable because of the smallness of the electron mass, in particular
in the vicinity of the threshold of HZ production. A study at LEP
time of those effects, as well as of the background processes, was
given in Ref. [17].
In
this paper, we focus on the process of associated HZ produc-
tion
in e
+
e
−
annihilation, with the aim of providing a comprehen-
sive
and accurate theoretical approach to precision measurements
of the Higgs boson parameters at future Higgs factories. To this
end, we consider for definitiveness the cleanest production chan-
nel
given by the four-fermion process e
+
e
−
→ μ
+
μ
−
b
¯
b, including
both signal and background contributions, and induced in the sig-
https://doi.org/10.1016/j.physletb.2017.12.056
0370-2693/
© 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.