Physics Letters B 757 (2016) 211–215
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Reconstructing semi-invisible events in resonant tau pair production
from Higgs
Partha Konar, Abhaya Kumar Swain
∗
Theoretical Physics Group, Physical Research Laboratory, Ahmedabad 380009, India
a r t i c l e i n f o a b s t r a c t
Article history:
Received
5 February 2016
Received
in revised form 11 March 2016
Accepted
24 March 2016
Available
online 31 March 2016
Editor:
G.F. Giudice
Keywords:
Higgs
Tau
lepton
Event
reconstruction
Hadron
Collider
We study the possibility of utilizing the constrained mass variable, M
2Cons
, in reconstructing the semi-
invisible
events originated from a resonant production at the LHC. While this proposal is effective for any
similar antler type production mechanism, here we demonstrate with a potentially interesting scenario,
when the Higgs boson decays into a pair of the third generation τ leptons. Buoyed with a relatively large
Yukawa coupling, the LHC has already started exploring this pair production to investigate the proper-
ties
of Higgs in the leptonic sector. Dominant signatures through hadronic decay of tau, associated with
invisible neutrinos compound the difficulty in the reconstruction of such events. Exploiting the already
existing Higgs mass bound, this new method provides a unique event reconstruction, together with a
significant enhancement in terms of efficiency over the existing methods.
© 2016 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 Large Hadron Collider (LHC), still lacking with its objec-
tive
in confirming any clear indication of new physics beyond the
Standard Model (SM), has nevertheless successfully discovered the
SM like Higgs boson at 125 GeV [1,2] and also made tremendous
progress in probing different properties of this newly discovered
scalar [3,4]. Owing to the relatively large Yukawa coupling, look-
ing
for the events where the Higgs decaying into third generation
τ ’s is the natural first step in exploring the interactions with the
leptonic modes. Full event reconstruction for such event topology
is especially more important, since the fermions from third gener-
ation
family held the key to the electro-weak symmetry breaking,
and moreover, can shed light on different aspects of the resonant
state such as, coupling structure, spin, CP properties. This, in turn,
can be exploited to constrain the effects coming from any possible
new physics.
The
CMS collaboration recently studied [5] the tau pair pro-
duction
from the Higgs boson, at center-of-mass energy of 7 and
8 TeV, corresponding to the integrated luminosity of 4.9 and
19.7fb
−1
.To explore these τ leptons, both hadronic and leptonic
decay modes are considered, resulting into six different final states
from the pair. This analysis reported an excess of events over the
background only hypothesis, with a local significance of 3.2 stan-
*
Corresponding author.
E-mail
addresses: konar@prl.res.in (P. Konar), abhaya@prl.res.in (A.K. Swain).
dard deviation corresponding to the Higgs boson mass at 125 GeV.
The study of τ pair final state at the LHC is rather onerous, mak-
ing
the significance smaller compared to other decay modes of the
Higgs boson. Difficulty lies in reconstructing the hadronic or lep-
tonic
decay modes of the tau lepton, especially in presence of the
invisible neutrinos at the final state.
There
are several techniques introduced for the study of h →
ττ process and we give an outline as follows.
• Collinear approximation [6] assumes that all the decay products
from the tau lepton are collinear. As a result, each neutrino,
among these decay products, takes some fraction of the tau
momenta. This unknown fraction can be determined by using
the measured momenta of the visible particles and missing
transverse momentum. This approximation is effective when
the Higgs is produced in association with hard jet(s), boosting
the tau pair system. Thus, a significant portion of events, pro-
ducing
the τ ’s back-to-back in transverse direction, remains
outside the preview of this method. Therefore, the overall sta-
tistical
significance from such study gets reduced.
• Missing mass calculator [7] replaces the collinear approximation
by constructing a probability function utilizing the angular in-
formation
in the event, to parameterize this under-constrained
system. Two remaining unsolved degrees of freedom are thus
fixed whereas, rest are solved using the four constraints with
τ mass-shell relations, and the missing transverse momenta.
http://dx.doi.org/10.1016/j.physletb.2016.03.070
0370-2693/
© 2016 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
.