Physics Letters B 728 (2014) 602–606
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Physics Letters B
www.elsevier.com/locate/physletb
Searching for intrinsic charm in the proton at the LHC
V.A. Bednyakov
a
, M.A. Demichev
a
, G.I. Lykasov
a
, T. Stavreva
b,∗
, M. Stockton
c
a
Joint Institute for Nuclear Research, Dubna 141980, Moscow region, Russia
b
Laboratoire de Physique Subatomique et de Cosmologie, UJF, CNRS/IN2P3, INPG, 53 avenue des Martyrs, 38026 Grenoble, France
c
Department of Physics, McGill University, Montreal, QC, Canada
article info abstract
Article history:
Received 21 May 2013
Received in revised form 9 December 2013
Accepted 9 December 2013
Available online 16 December 2013
Editor: D.F. Geesaman
Despite rather long-term theoretical and experimental study, the hypothesis of the non-zero intrinsic (or
valence-like) heavy quark component of the proton distribution functions has not yet been confirmed or
rejected. The LHC with pp-collisions at
√
s = 7–14 TeV will obviously supply extra unique information
concerning the above-mentioned component of the proton. To use the LHC potential, first of all, one
should select the parton-level (sub)processes (and final-state signatures) that are the most sensitive to the
intrinsic heavy quark contributions. To this end inclusive production of c
(b)-jets accompanied by photons
is considered. On the basis of performed theoretical study it is demonstrated that investigation of the
intrinsic heavy quark contributions looks very promising at the LHC in processes like pp
→γ +c(b) + X .
© 2013 The Authors. Published by Elsevier B.V.
1. Introduction
The Large Hadron Collider (LHC) opens up new and unique
kinematical regions with high accuracy for the investigation of the
structure of the proton, in particular for the study of the parton
distribution functions (PDFs). It is well known that the precise
knowledge of the PDFs is very essential for the verification of the
Standard Model and the search for New Physics.
By definition, the PDF f
a
(x, μ) is a function of the proton mo-
mentum fraction x carried by parton a (quark q or gluon g)atthe
momentum transfer scale
μ. For small values of μ, corresponding
to long distance scales less than 1
/μ
0
, the PDF currently can-
not be calculated from the first principles of QCD [1].At
μ > μ
0
the f
a
(x, μ) can be obtained by means of solving the perturbative
QCD evolution equations (DGLAP) [2].At
μ < μ
0
some progress in
calculation of the PDFs has been achieved within the lattice meth-
ods [1]. The unknown (input for the evolution) functions f
a
(x, μ
0
)
usually can be found empirically from some “QCD global analysis”
[3,4] of a large variety of data typically at
μ > μ
0
.
In general, almost all pp processes at LHC energies, includ-
ing Higgs boson production, are sensitive to the charm f
c
(x, μ)
*
Corresponding author.
E-mail addresses: Vadim.Bednyakov@cern.ch (V.A. Bednyakov),
Mikhail.Demichev@cern.ch (M.A. Demichev), gennady.lykasov@cern.ch
(G.I. Lykasov), stavreva@lpsc.in2p3.fr (T. Stavreva), mark.stockton@cern.ch
(M. Stockton).
or bottom f
b
(x, μ) PDFs. Nevertheless, within the global analysis
the charm content of the proton at
μ ∼ μ
c
and the bottom at
μ ∼ μ
b
are both assumed to be negligible. Here μ
c
and μ
b
are
typical energy scales relevant to the c- and b-quark QCD excitation
in the proton. These heavy quark components arise in the pro-
ton only perturbatively with increases in the Q
2
-scale through the
gluon splitting in the DGLAP Q
2
evolution [2]. Direct measure-
ment of open charm and open bottom production in deep inelastic
processes (DIS) confirms the perturbative origin of heavy quark fla-
vors [5]. However, modern descriptions of these experimental data
are not sensitive enough to the above-mentioned perturbative sea
heavy quark distributions at relatively large x values (x
> 0.1).
Analyzing hadroproduction of so-called leading hadrons Brod-
sky et al. [6,7] (about thirty years ago) has assumed co-existing
of extrinsic and intrinsic contributions to the quark–gluon structure
of the proton. The extrinsic (or ordinary) quarks and gluons are
generated on a short time scale associated with large-transverse-
momentum processes. Their distribution functions satisfy the stan-
dard QCD evolution equations. The intrinsic quarks and gluons exist
over a time scale which is independent of any probe momen-
tum transfer. They can be associated with a bound-state (zero-
momentum transfer regime) hadron dynamics and one believes
they have a nonperturbative origin.
It was shown in [7] that
the existence of intrinsic heavy quark
pairs c
¯
c, and b
¯
b within the proton state can be due to the virtue
of gluon-exchange and vacuum-polarization graphs. On this basis,
within the MIT bag model [8], the probability to find a five-quark
component
|uudc
¯
c bound within the nucleon bag is non-zero and
can be about 1–2%.
0370-2693 © 2013 The Authors. Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.physletb.2013.12.031
Open access under CC BY license.
Open access under CC BY license.