Studying the High X Frontier With a Fixed-Target Experiment at the Lhc Page: 4 of 6
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Studying the high x frontier with AFTER@LHC
that AFTER should be designed as a multi-purpose detector, capable of studying quarkonium and
isolated photon production. While the use of quarkonia as gluon probe (see e.g. [16]) relies on
a better understanding of their production mechanisms [17], these would be better constrained
thanks to the large quarkonium yields and precise measurements of their correlations, along with
the forthcoming LHC results.
In addition, it will be particularly interesting to investigate the gluon content of the neutron
and to see whether there is any deviation from that of the proton. A pioneering measurement was
done by E866 [18] at Fermilab, using T. A unique opportunity to get significant improvements can
be offered by AFTER, on two different sides: the precision and also the extension to lower x and
Q2 values by using the J/iy.
Another question concerns the gluon momentum tomography in the nucleon. Is there any
Sivers effect for the gluon? Is there any correlation, pinned down by the Boer-Mulders effect,
between the gluon kT and the gluon linear polarisation?
A first attempt to measure a SSA arising from the gluon Sivers effect was recently carried
out by the PHENIX collaboration [19] by looking at J/yi production in ppr collisions. Improve-
ments are needed. On top of larger luminosities (by orders of magnitude), AFTER could offer to
extend the measurement in various experimental probes sensitive to gluons, such as other quarko-
nium species (T, Xc, ...), B and D meson [20] production, prompt photon, photon-jet [21] or
double photon production [22]. Concerning the Boer-Mulders effect, linearly polarised gluons in-
side unpolarised protons [23] can be accessed with the study fo low-PT scalar and pseudoscalar
quarkonium3.
AFTER could also greatly contribute to the knowledge of the gluon nPDF in pA collisions,
in good complementarity with LHeC [1] (focusing at low x) and EIC future facilities [25] (at
intermediate x). On top of the x dependence, a more precise A dependence is needed. It is easier
in a fixed-target setup than in the collider mode thanks to the target versatility. Such results can
provide a much better evaluation of the nuclear matter effects on quarkonium and heavy-flavour
production, which is a key milestone towards any precision study of the deconfinement at RHIC
energies.
4. Heavy quark distribution at large x
The presence of an intrinsic charm component in the nucleon is still the subject of intense
debates in the particle and hadron physics community. At large x, whereas compelling indication
of intrinsic strangeness has been recently claimed to be found [26], a state-of-the-art global fit [27]
has shown that various realisations of intrinsic charm PDFs are allowed by the data. Similarly, the
existence of intrinsic bottom is surely worth some investigations (see e.g. [28].)
To advance the debate, dedicated measurements are absolutely needed; not only one. Several
probes are accessible at AFTER [2, 10], such as backward open charm and beauty production,
double quarkonium production, quarkonium plus open heavy flavour production, as well as the
production of prompt photon in association with a heavy quark [29, 30]. All these measurements
could also be completemented by a new measurement of the charm structure function in DIS.
3See [24] for a discussion of the Boer-Mulders effect in heavy flavour production in pp collisions.4
A. Rakotozafindrabe
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Rakotozafindrabe, A.; Anselmino, M.; Arnaldi, R.; Scomparin, E.; Brodsky, S.J.; Chambert, V. et al. Studying the High X Frontier With a Fixed-Target Experiment at the Lhc, article, November 1, 2013; United States. (https://digital.library.unt.edu/ark:/67531/metadc869975/m1/4/: accessed April 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.