LHC Predictions from a Tevatron Anomaly in the Top Quark Forward-Backward Asymmetry

Abstract

We examine the implications of the recent CDF measurement of the top-quark
forward-backward asymmetry, focusing on a scenario with a new color octet
vector boson at 1-3 TeV. We study several models, as well as a general
effective field theory, and determine the parameter space which provides the
best simultaneous fit to the CDF asymmetry, the Tevatron top pair production
cross section, and the exclusion regions from LHC dijet resonance and contact
interaction searches. Flavor constraints on these models are more subtle and
less severe than the literature indicates. We find a large region of allowed
parameter space at high axigluon mass and a smaller region at low mass; we
match the latter to an SU(3)xSU(3)/SU(3) coset model with a heavy vector-like
fermion. Our scenario produces discoverable effects at the LHC with only 1-2
inverse femtobarns of luminosity at 7-8 TeV. Lastly, we point out that a
Tevatron measurement of the b-quark forward-backward asymmetry would be very
helpful in characterizing the physics underlying the top-quark asymmetry.

Full text

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SLAC-PUB-14364
LHC Predictions from a Tevatron Anomaly in the Top Quark
Forward-Backward Asymmetry
Yang Bai, JoAnne L. Hewett, Jared Kaplan and Thomas G. Rizzo
SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA
Abstract
We examine the implications of the recent CDF measurement of the top-quark forward-backward
asymmetry, focusing on a scenario with a new color octet vector boson at 1-3 TeV. We study several
models, as well as a general effective field theory, and determine the parameter space which provides
the best simultaneous fit to the CDF asymmetry, the Tevatron top pair production cross section, and
the exclusion regions from LHC dijet resonance and contact interaction searches. Flavor constraints
on these models are more subtle and less severe than the literature indicates. We find a large region
of allowed parameter space at high axigluon mass and a smaller region at low mass; we match the
latter to an SU(3)1× SU(3)2/SU(3)ccoset model with a heavy vector-like fermion. Our scenario
produces discoverable effects at the LHC with only 1 − 2 fb−1of luminosity at√s = 7 − 8 TeV.
Lastly, we point out that a Tevatron measurement of the b-quark forward-backward asymmetry
would be very helpful in characterizing the physics underlying the top-quark asymmetry.
arXiv:1101.5203v1 [hep-ph] 27 Jan 2011

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1 Introduction
CDF recently announced a 3.4σ deviation from the Standard Model (SM) in a measurement of the
t¯t forward-backward asymmetry [1]. Their study updates previous analyses that observed deviations
of roughly 2σ [2, 3], but they also go further, demonstrating that the t¯t asymmetry is dominated by
events with large invariant mass Mt¯t. This rise in the asymmetry at high energy could be interpreted
as further evidence of a new physics signal characterized by energies at or above the weak scale. In
this work we will investigate explanations of the asymmetry based on new physics [4] [5][6, 7, 8, 9],
examine what can be done next at the Tevatron and the LHC to either rule out or confirm as well as
characterize models that explain the anomaly.
An important first question is whether we should consider models that are wholly responsible for
the asymmetry, or models where interference between new physics and QCD generates the asymmetry.
After unfolding their acceptance, CDF observed [1] a parton-level asymmetry At¯t
in t¯t events with Mt¯t> 450 GeV and in the t¯t center of mass frame (see Table 1), compared to the
SM prediction of At¯t
FB(SM) = 0.088 ± 0.013. This is a large effect. If new physics alone generates the
forward-backward asymmetry, a straightforward interpretation of these numbers suggests that new
physics must be responsible for roughly half of all t¯t production above 450 GeV, even in a scenario
FB= 0.475 ± 0.114
where the new physics yields a maximal forward-backward asymmetry! However, a new heavy particle
produced in the s-channel will have to contend with searches for resonances and contact interactions
at both the Tevatron and the LHC [9]. For this reason, we will focus on models where the new physics
interferes with QCD to produce the asymmetry, allowing for a smaller total contribution to production
rates from new physics.
In order to interfere with SM top production, new physics processes must produce t¯t pairs in a
state with the same quantum numbers as in QCD production. This means that the t¯t must form a
color octet, Lorentz vector state, so if there is a new particle in the s-channel coupling to tops, it must
be a heavy color octet vector boson1. Such a ‘heavy gluon partner’ G?can arise in a wide variety
of models [10], including top-color models [11][12], technicolor [13], warped extra dimensions [14][15],
universal extra dimensions [16] and chiral color [17][18][19]. As we will discuss below, G?particles are
constrained by flavor physics and collider searches, although we disagree with some exclusion claims
in the literature. For convenience we will present a phenomenological low-energy effective field theory
and some simple coset models, and we will ignore the issue of anomaly cancellation. Throughout we
will use the term ‘axigluon’ as a shorthand for ‘color octet vector boson’.
1In principle, colored particles with spin > 1 might also interfere off-shell, but roughly speaking, this is due to the
contribution of the vector particles they ‘eat’ to become massive.
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SelectionMt¯t< 450 GeVMt¯t> 450 GeV
Parton Level Data
−0.116 ± 0.146 ± 0.047
|∆y| < 1.0
0.026 ± 0.104 ± 0.056
0.475 ± 0.101 ± 0.049
|∆y| > 1.0
0.611 ± 0.210 ± 0.147
Selection
Parton Level Data
Table 1: Unfolded parton level data for At¯t
FBfrom the CDF study [1].
New physics produced in the t-channel [6] could also generate a substantial forward-backward
asymmetry. Naively, one might imagine that a t-channel contribution would not rise with energy, but
in fact t-channel physics produces a large energy dependence that might fit the CDF results nicely.
However, producing t¯t pairs via a new t-channel process generates non-trivial flavor issues; moreover,
new t-channel effects already have significant tension with Tevatron data as they give rise to a large
number of like-sign top pairs [6]. Since the LHC is a pp collider, a new t-channel state could be
discovered soon by searches for like-sign tops. We will not discuss these models any further here.
Future studies at both the Tevatron and the LHC could shed light on the nature of the t¯t asym-
metry. The Tevatron could provide a great deal of insight into the physics responsible for the t¯t
asymmetry by studying a potential b¯b asymmetry, using semi-leptonic b decays to tag the sign of
b-jets. If the heavy color octet G?has large enough couplings to generate a sizable asymmetry, it
should be visible in dijet resonance or contact interaction searches this year (with 1 − 2 fb−1of lu-
minosity) at the LHC. In fact, we will see below that a sizable portion of the possible parameter
space has already been excluded by the LHC! Measurements of the t¯t production cross section are
also sensitive to new physics, and a new G?could be discovered as a resonance decaying to t¯t. Fixing
the new physics contribution to At¯t
product of the G?coupling to light quarks and tops must essentially be held fixed to account for the
FB, we will see that dijet and t¯t searches are complementary, as the
asymmetry. Whatever the verdict, in all likelihood new physics explanations for the asymmetry will
be much better understood after the next year of operations at the LHC.
The rest of this paper is organized as follows. Section 2 introduces an effective field theory for a
heavy color octet vector boson and then discusses various collider bounds that constrain it. In Section 3
we introduce specific coset models for G?, and define the parameter space for each model that provides
a fit to the CDF asymmetry data at 90% and 95% C.L. We also show how the constraints from
other collider data (σt¯t, dijet resonance, and dijet contact interaction searches) impact this parameter
space. In Section 4 we explore the contributions in the flavor sector from our models and show that
meson mixing does not necessarily restrict the relevant parameter space. Section 5 delineates various
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signatures of the heavy color octet at the LHC. Lastly, Section 6 contains our conclusions.
2 An Axigluon?
Our goal is to examine if a new color octet vector boson G?can explain the discrepancy in the
recently measured t¯t forward-backward asymmetry. To this end, let us begin with a general low-
energy effective Lagrangian coupling G?to the SM quarks. To suppress dangerous FCNC effects it
will prove advantageous to assume universal vector and axial couplings to the first two generations,
gq
Vand gq
we have rescaled these couplings in terms of the strong coupling gs. The tree-level differential cross
section for q¯ q → t¯t will be [20, 7]
dˆ σq¯ q→t¯t
dcosθ∗
9ˆ s
+?1 − 4m2+ c2?gt2
− 4gq
(ˆ s − M2
where m2= m2
A, while the couplings to the top quark and bottom quark will be gt
Vand gt
A. For convenience
=α2
s
π√1 − 4m2
??1 + 4m2+ c2?
A(gq 2
?
1 −
2gq
Vgt
Vˆ s(M2
G?)2+ M2
G? − ˆ s)
(ˆ s − M2
ˆ s2
G?)2+ M2
G?Γ2
G
+
gt2
V(gq 2
V+ gq 2
G?)2+ M2
A)ˆ s2
G?Γ2
(ˆ s − M2
G
?
V+ gq 2
A)
(ˆ s − M2
− 2gq
G?Γ2
G
ˆ s2
Agt
Ac
?
ˆ s(M2
G?)2+ M2
G? − ˆ s)
G?Γ2
G
Vgt
V
(ˆ s − M2
G?)2+ M2
G?Γ2
G
??
(1)
t/ˆ s, c =√1 − 4m2cosθ∗, and θ∗is the angle between the top quark and the incoming
quark in the center of mass frame. The forward-backward asymmetry arises solely from the last line
in this equation, so to obtain a positive asymmetry we must have gq
We will be considering MG? ? 1 TeV, so at the Tevatron we have ˆ s < M2
support of the parton distribution functions. In this limit the interference term between the QCD
Agt
A< 0.
G? over most of the
and resonance contributions dominates over the pure resonance term, and we expect the following
qualitative features:
• the asymmetry is directly proportional to (−gq
Agt
A)
• relative to the pure QCD cross section, the asymmetric term grows ∝ ˆ s, so the asymmetry will
grow with the invariant mass squared M2
t¯t
• the total t¯t cross section at low energies will decrease with increasing gq
Additionally, we see that the sign and magnitude of gq
high energies, and that for larger gq
V, our qualitative discussion based on the parametric expansion
in ˆ s/M2
G? breaks down more quickly.
Vgt
V.
Vgt
Vdetermine the behavior of the asymmetry at
Vgt
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Axigluons face constraints from flavor-changing neutral currents (FCNCs), from t¯t resonance
searches, and from measurements of the total t¯t cross section. Furthermore, any axigluon that couples
to q¯ q is subject to constraints from dijet resonance and dijet contact interaction searches – not just at
the Tevatron but at the LHC! As we will see, the strongest constraints on our models will come from
the LHC, and the prospects of a discovery or a definitive exclusion this year are extremely good. This
is striking proof that we have entered the LHC era!
Before considering the details, note that at least qualitatively, once we fix gq
constraints will be orthogonal. Increasing |gq
afoul of constraints; one might avoid altering the t¯t cross section by increasing gq
the expense of decreasing the asymmetry itself. Now let us discuss the restrictions from the t¯t cross
Agt
A, the t¯t and dijet
Agt
A| increases the asymmetry, but eventually one runs
Vgt
V, but only at
section and the dijet searches; we will leave a discussion of flavor to Section 4.
2.1 The t¯t Cross Section
Theoretical predictions [21] of the t¯t cross section have been verified at the Tevatron [22]. An axigluon
will modify both the total t¯t cross section and its shape as a function of the invariant mass Mt¯t,
so Tevatron t¯t measurements restrict axigluon models. However, we cannot set a precise constraint
without a more detailed understanding of the experimental and theoretical details. The current t¯t
resonance searches at the Tevatron [23][24][25][26][27] have not set a limit on a possible resonance with
mass beyond ∼ 800 GeV due to a lack of statistics in their published analyses.
In our analysis below, we will display contours corresponding to an axigluon contribution of 20%
and 40% to the total QCD t¯t cross section for Mt¯t> 450 GeV. These contours should be viewed as a
suggestion of the Tevatron sensitivity, and not as hard bounds. One might instead consider using a
larger cut on the top invariant mass. This would result in a larger relative axigluon contribution to the
cross section, but would compete with the uncertainties on the QCD cross section which grow rapidly
at higher invariant mass. After some experimentation, we chose this Mt¯t> 450 GeV requirement as
a reasonable and conservative compromise between these two factors.
For illustrative purposes, we show the Mt¯tdistributions in the upper panel of Fig. 1. With the
exception of the magenta line, the other curves on the histogram represent a 1 TeV axigluon. For
the red(blue) lines, the coupling of the axigluon is of QCD strength and Γ/M = 0.1(0.2) has been
assumed. For the green(cyan) histogram, the coupling strengths of the axigluon have been doubled
and now Γ/M = 0.4(0.8) has been assumed. One can see that a 1 TeV axigluon with a narrow width
will appear as a t¯t resonance.
4