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L. Massacrier

Publications and source records attributed to L. Massacrier.

At least 19 recordsLinked to original sources

Measurements of quarkonia production

Quarkonium production in high-energy hadronic collisions is a useful tool to investigate fundamental aspects of Quantum Chromodynamics, from the proton and nucleus structure to deconfinement and the properties of the Quark Gluon Plasma (QGP). In these proceedings, emphasis is made on few recent quarkonium results from the RHIC and LHC colliders in proton-proton (pp), proton-nucleus (p-A) and nucleus-nucleus (A-A) collisions. In addition, results for some key observables are compiled to discuss the state-of-the-art in quarkonium production, with a focus on quarkonium hadroproduction.

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Coherent J/$\psi$ photoproduction and polarization in peripheral Pb-Pb collisions with ALICE

Photonuclear reactions are induced by the strong electromagnetic field generated by ultrarelativistic heavy ions. These processes have been extensively studied in ultraperipheral collisions. In recent years, the observation of coherent J/$\psi$ photoproduction has been claimed in nucleus-nucleus (A-A) collisions with nuclear overlap, based on the measurement of an excess in the very low transverse momentum ($p_{\rm T}$) J/$\psi$ yield. Such quarkonium measurements can help constraining the nuclear gluon distribution at low Bjorken-$x$ and can shed light on the theory behind photon induced reactions in A-A collisions with nuclear overlap. In order to confirm the photoproduction origin of the very low-$p_{\rm T}$ J/$\psi$ yield excess, polarization measurement is a golden observable. It is indeed expected that the produced quarkonium would keep the polarization of the incoming photon due to $s$-channel helicity conservation. ALICE can measure inclusive and exclusive quarkonium production down to zero transverse momentum, at forward rapidity (2.5 $< y <$ 4) and midrapidity ($|y| <$ 0.9). In this contribution, we will report on the new preliminary measurement of the $y$-differential cross section and the first polarization analysis at LHC of coherently photoproduced J/$\psi$ in peripheral Pb-Pb collisions. Both measurements are conducted at forward rapidity in the dimuon decay channel.

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Heavy-flavour studies with a high-luminosity fixed-target experiment at the LHC

Extraction of the multi-TeV proton and lead LHC beams with a bent crystal or by using an internal gas target allows one to perform the most energetic fixed-target experiment ever. pp, pd and pA collisions at $\sqrt{s}$ = 115 GeV and Pbp and PbA collisions at $\sqrt{s_{\rm{NN}}}$ = 72 GeV can be studied with high precision and modern detection techniques over a broad rapidity range. Using the LHCb or the ALICE detector in a fixed-target mode offers unprecedented possibilities to access heavy-flavour production in a new energy domain, half way between the SPS and the nominal RHIC energy. In this contribution, a review of projection studies for quarkonium and open charm and beauty production with both detector set-ups used with various nuclear targets and the LHC lead beams is presented.

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A Fixed-Target Programme at the LHC: Physics Case and Projected Performances for Heavy-Ion, Hadron, Spin and Astroparticle Studies

We review the context, the motivations and the expected performances of a comprehensive and ambitious fixed-target program using the multi-TeV proton and ion LHC beams. We also provide a detailed account of the different possible technical implementations ranging from an internal wire target to a full dedicated beam line extracted with a bent crystal. The possibilities offered by the use of the ALICE and LHCb detectors in the fixed-target mode are also reviewed.

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Spin Physics with a fixed-target experiment at the LHC

The multi-TeV proton and ion beams of the LHC would allow for the most energetic fixed-target experiment ever. In particular, $pp$, $p$d and $p$A collisions could be performed at $\sqrt{s_{NN}}$ = 115~GeV, as well as Pb$p$ and PbA collisions at $\sqrt{s_{NN}}$ = 72~GeV, in a parasitic way by making use of the already existing LHCb and ALICE detectors in fixed-target mode. This would offer the possibility to carry out a ground-breaking physics program, to study the nucleon and nuclear structure at high $x$, the spin content of the nucleon and the phases of the nuclear matter from a new rapidity viewpoint. In this talk I focus on the spin physics axis of the full program developed so far by the AFTER@LHC study group.

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High luminosity fixed-target experiment at the LHC

By extracting the beam with a bent crystal or by using an internal gas target, the multi-TeV proton and lead LHC beams allow one to perform the most energetic fixed-target experiments ever and to study $pp$, $p$d and $p$A collisions at $\sqrt{s_{NN}}=115$ GeV and Pb$p$ and PbA collisions at $\sqrt{s_{NN}}=72$ GeV with high precision and modern detection techniques. Such studies would address open questions in the domain of the nucleon and nucleus partonic structure at high-$x$, quark-gluon plasma and, by using longitudinally or transversally polarised targets, spin physics. In this paper, we will review the technical solutions to obtain a high-luminosity fixed-target experiment at the LHC and will discuss their possible implementations with the ALICE and LHCb detectors.

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Coherent J/$ψ$ photoproduction in Pb-Pb collisions with nuclear overlap studied with ALICE at the LHC

The photoproduction of heavy vector mesons in the electromagnetic interactions of ultra-relativistic nuclei is sensitive to the gluon distribution in the nucleus and thus to cold nuclear matter effects like shadowing or parton saturation. Besides the well known observations of vector meson production in ultra-peripheral collisions, first observations of an excess over the expected hadronic J/$ψ$ production at very low transverse momentum ($p_T < $~0.3 GeV/$c$) in peripheral and semi-central nucleus-nucleus collisions both at LHC and RHIC energies were interpreted as the first sign of coherent J/$ψ$ photoproduction occurring in Pb-Pb collisions with nuclear overlap. The ALICE Collaboration published the J/$ψ$ coherent photoproduction cross sections in peripheral and semi-central Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76~TeV and forward rapidity ($2.5<y<4.0$). Using the LHC Run-2 data, ALICE presents preliminary results in peripheral Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV at mid-rapidity ($|y|<0.9$) and forward rapidity. Thanks to the very good tracking resolution of the central barrel, the extraction of the $p_T$-differential cross section was also possible, strengthening the photoproduction origin of the observed J/$ψ$ excess. The quantitative understanding of this low-$p_T$ excess poses significant theoretical challenges since the J/$ψ$ photoproduction depends on the collision dynamics as well as on the photon-flux and the photonuclear cross section. In this proceeding, we present the latest ALICE measurements on J/$ψ$ photoproduction cross section in peripheral Pb-Pb collisions, with emphasis on the new forward measurement in the dimuon decay channel at $\sqrt{s_{\rm NN}}$ = 5.02~TeV. These results will be discussed and compared to several model calculations of J/$ψ$ photoproduction in Pb-Pb collisions with nuclear overlap.

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Probing the high-x content of the nuclei in the fixed-target mode at the LHC

Using the LHCb and ALICE detectors in the fixed-target mode at the LHC offers unprecedented possibilities to study the quark, gluon and heavy-quark content of the proton and nuclei in the poorly known region of the high-momentum fractions. We review our projections for studies of Drell-Yan, charm, beauty and quarkonium production with both detector set-ups used with various nuclear targets and the LHC proton beams. Based on this, we show the expected improvement in the determination of the quark, charm and gluon proton and nuclear PDFs as well as discuss the implication for a better understanding of the cold-nuclear-matter effects in hard-probe production in proton-nucleus collisions.

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Single-Transverse-Spin Asymmetries in Exclusive Photo-production of J/psi in Ultra-Peripheral Collisions in the Fixed-Target Mode at the LHC and in the Collider Mode at RHIC

We investigate the potentialities offered by the study of J/psi exclusive photo-production in ultra-peripheral collisions at a fixed-target experiment using the proton and lead LHC beams (generically denoted as AFTER@LHC) on hydrogen targets and at RHIC in the collider mode. We compare the expected counting rates in both set-ups. Studying Single-Transverse-Spin Asymmetries (A_N) in such a process provides a direct path to the proton Generalised Parton Distribution (GPD) E_g(x,xi,t). We evaluate the expected precision on A_N for realistic conditions with the LHCb detector in pH(pol) and PbH(pol) collisions. We also discuss prospects with polarised deuterium and helium targets in the case of AFTER@LHC.

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A fixed-target programme at the LHC for heavy-ion, hadron, spin and astroparticle physics: AFTER@LHC

Thanks to its multi-TeV LHC proton and lead beams, the LHC complex allows one to perform the most energetic fixed-target experiments ever and to study with high precision pp, pd and pA collisions at sqrt(s_NN) = 115 GeV and Pbp and PbA collisions at sqrt(s_NN) = 72 GeV. We present a selection of feasibility studies for the production of quarkonia, open heavy-flavor mesons as well as light-flavor hadrons in pA and PbA collisions using the LHCb and ALICE detectors in a fixed-target mode.

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Physics perspectives with AFTER@LHC (A Fixed Target ExpeRiment at LHC)

AFTER@LHC is an ambitious fixed-target project in order to address open questions in the domain of proton and neutron spins, Quark Gluon Plasma and high-$x$ physics, at the highest energy ever reached in the fixed-target mode. Indeed, thanks to the highly energetic 7 TeV proton and 2.76 A.TeV lead LHC beams, center-of-mass energies as large as $\sqrt{s_{NN}}$ = 115 GeV in pp/pA and $\sqrt{s_{NN}}$ = 72 GeV in AA can be reached, corresponding to an uncharted energy domain between SPS and RHIC. We report two main ways of performing fixed-target collisions at the LHC, both allowing for the usage of one of the existing LHC experiments. In these proceedings, after discussing the projected luminosities considered for one year of data taking at the LHC, we will present a selection of projections for light and heavy-flavour production.

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Quarkonium-photoproduction prospects at a fixed-target experiment at the LHC (AFTER@LHC)

We report on the potentialities offered by a fixed-target experiment at the LHC using the proton and ion LHC beams (AFTER@LHC project) regarding the study of J/psi exclusive-photoproduction in pA and AA collisions. The foreseen usage of polarised targets (hydrogen, deuteron, helium) gives access to measurements of the Single-Transverse-Spin Asymmetries of this exclusive process, therefore allowing one to access the helicity-flip Generalised Parton Distribution (GPD) Eg. We detail the expected yields of photoproduced J/psi in proton-hydrogen and lead-hydrogen collisions and discuss the statistical uncertainties on the asymmetry measurement for one year of data taking at the LHC.

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Heavy-ion Physics at a Fixed-Target Experiment Using the LHC Proton and Lead Beams (AFTER@LHC): Feasibility Studies for Quarkonium and Drell-Yan Production

We outline the case for heavy-ion-physics studies using the multi-TeV lead LHC beams in the fixed-target mode. After a brief contextual reminder, we detail the possible contributions of AFTER@LHC to heavy-ion physics with a specific emphasis on quarkonia. We then present performance simulations for a selection of observables. These show that $Υ(nS)$, $J/ψ$ and $ψ(2S)$ production in heavy-ion collisions can be studied in new energy and rapidity domains with the LHCb and ALICE detectors. We also discuss the relevance to analyse the Drell-Yan pair production in asymmetric nucleus-nucleus collisions to study the factorisation of the nuclear modification of partonic densities and of further quarkonia to restore their status of golden probes of the quark-gluon plasma formation.

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First LHCb results from pA and Pb-Pb collisions

In 2015, the LHCb collaboration endorsed the proposal to pursue an ambitious heavy-ion physics program. In 2013, LHCb has demonstrated its capabilities to operate successfully in p-Pb and Pb-p collisions, leading already to several important publications in the field. The measurements of the nuclear modification factor and forward-backward production of prompt and displaced J/$ψ$, $ψ$(2S) and $Υ$(1S) states, as well as the production of prompt $D^{0}$ mesons, have allowed to extend the knowledge of Cold Nuclear Matter effects on open heavy flavours and quarkonium production. The measurement of Z-boson production, important to constrain nuclear PDFs, and the measurement of two-particle angular correlations, probing collective effects in the dense environment of high energy collisions, have also been performed. Furthermore, LHCb is the only experiment at the LHC that can be operated in fixed-target mode, owing to the injection of a small amount of gas inside the LHCb collision area. There have been several p-gas and Pb-gas data taking periods during Run 1 and beginning of Run 2. This fixed target programme is conducted at a center-of-mass energy of $\mathcal{O}$(100 GeV), and has great potential to bridge the gap of knowledge between SPS and RHIC in the domain of Quark Gluon Plasma physics. Finally, LHCb successfully participated to its first Pb-Pb data taking at the end of 2015. First studies show that up to semi-central Pb-Pb collisions can be successfully analysed to provide unique measurements in the forward region. In this proceeding, we will present a selection of LHCb results in pA collisions. We will also give some prospects for fixed target and Pb-Pb studies.

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Physics case for a polarised target for AFTER@LHC

We review a number of ideas put forward in favour of the use of a polarised target along with the proposed idea of a fixed-target experiment using the LHC beams -- AFTER@LHC. A number of recent studies have shown that single transverse-spin asymmetries (STSAs) are large enough to be precisely measured in the region accessible with AFTER@LHC, in particular as regards the Drell-Yan process as well as single-pion, isolated-photon and jet production. AFTER@LHC with a polarised target would also be the ideal experimental set-up to measure the gluon Sivers effect via a number of original quarkonium STSA studies. We discuss first figures-of-merit based on simulations for AFTER@LHC with a polarised target.

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