The LHCspin project A polarised gas target at the Large Hadron Collider

. The goal of LHCspin is to develop, in the next few years, innovative solutions and cutting-edge technologies to access spin physics in polarised fixed-target collisions at high energy, exploring the unique kinematic regime o ff ered by LHC and exploiting new final states by means of the LHCb detector.


Introduction
The LHC delivers proton and lead beams with an energy of 7 TeV and 2.76 TeV per nucleon, respectively, with world's highest intensity.Fixed-target proton-gas collisions occur at a centre-of-mass energy per nucleon of up to 115 GeV in the case of a hydrogen target.The large centre-of-mass boost offers an unprecedented opportunity to investigate partons carrying a large fraction of the target nucleon momentum, i.e. large Bjorken−x values.
The LHCb detector [1] is a general-purpose forward spectrometer specialised in detecting hadrons containing c and b quarks, and the only LHC detector able to collect data in both collider and fixed-target mode.It is fully instrumented in the 2 < η < 5 region with a vertex locator (VELO), a tracking system, two Cherenkov detectors, electromagnetic and hadronic calorimeters and a muon detector.
The fixed-target physics program of LHCb is active since the installation of the SMOG device [2], enabling the injection of noble gases in the beam pipe section crossing the VELO detector at a pressure of O(10 −7 ) mbar.
With the SMOG2 upgrade [3], an openable gas storage cell, shown in Fig. 1 (left), has been installed in 2020 in front of the VELO.The cell boosts the target areal density by a factor of 8 to 35 depending on the injected gas species, and creates a localised beam-gas collision region which is well detached from the proton-proton vertices.As shown in Fig. 1 (right), full tracking efficiency is expected in the beam-gas region, despite its upstream position with respect to the VELO.Due to these developments, fixed-target data will be collected in the upcoming Run 3 with a novel reconstruction software allowing for the simultaneous datataking of beam-gas and beam-beam collisions.SMOG2 will offer a rich physics program  stand-alone pHe, (in red) overlapped pp and pHe and (in orange) pp and pAr events simulated considering the Run3 pp conditions (⌫ ⇠ 7.6, L ' 2 • 10 33 cm 2 s 1 ) and one fixed per-bunch beam-gas collision.Similar e ciencies and fake rates between beam-beam and beam-gas collisions and no pp performance loss when injecting the gas are observed.A steep evolution with z of the resolution in the SMOG2 cell is found instead, as a consequence of the larger uncertainty when extrapolating low-aperture VELO tracks upstream of the nominal LHCb interaction point.for the Run 3 and, at the same time, will also allow to investigate the dynamics of the novel beam-target system, setting the basis for future developments.
The LHCspin project [5,6] aims at extending the fixed-target physics program in Run 4 and Run 5 with the installation of a polarised gas target, bringing spin physics at LHC for the first time.The experimental setup of LHCspin is discussed in Sec. 2, while a selection of physics opportunities is presented in Sec. 3.

Experimental setup
The LHCspin experimental setup is in R&D phase and calls for the development of a new generation polarised target.The starting point is the setup of the HERMES experiment at DESY [7] and comprises three main components: an Atomic Beam Source (ABS), a Target Chamber (TC) and a diagnostic system.The ABS consists of a dissociator with a cooled nozzle, a Stern-Gerlach apparatus to focus the wanted hyperfine states, and adiabatic RFtransitions for setting and switching the target polarisation between states of opposite sign.The ABS injects a beam of polarised hydrogen or deuterium into the TC, which is located in the LHC primary vacuum.The TC hosts a T-shaped openable storage cell, sharing the SMOG2 design, and a dipole holding magnet (B = 300 mT), as shown in Fig. 2. The fitted amplitudes are compatible with the parameters used i showing no bias.Within the available statistics, corresponding to the plots, there is no sensitivity to fit for a second harmonic with the chosen the first harmonic amplitudes are summarised in Fig. 8 together with from the method described in Sec. 2. As expected, the amplitudes ar value and a mild, increasing trend is observed as xF (x) gets smaller (la scheme, Sivers amplitudes with around 10% error are expected to be of data-taking at LHCspin.The diagnostic system continuously analyses gas drawn from the TC and prises a target gas analyser to detect the molecular and thus the degree of dissociation, and a Breit-Rabi polarimeter to the relative population of the injected hyperfine states.An instantaneous luminosity of O(10 32 ) cm −2 s −1 is foreseen for p-H collisions during Run 4.

Physics case
The physics case of LHCspin covers three main areas: exploration of the wide physics potential offered by unpolarised gas targets, investigation of the nucleon spin and heavy-ion collisions.

Unpolarised gas targets
By means of the SMOG2 gas feed system, LHCspin will allow the injection of several species of unpolarised gases: H 2 , D 2 , He, N 2 , O 2 , Ne, Ar, Kr and Xe with negligible impact on the LHC beam lifetime.This gives an excellent opportunity to investigate parton distribution functions (PDFs) in both nucleons and nuclei in the large-x and intermediate Q 2 regime, which is especially affected by lack of experimental data and impact several fields from basic QCD tests to astrophysics.For example, the large acceptance and high reconstruction efficiency of LHCb on heavy flavour states enables the study of gluon PDFs, which are a fundamental input for theoretical predictions [8].Searches for an intrinsic charm component in the proton [9] and antiproton production in p-He collisions [10] are two other high-profile example measurements that have already been pioneered at LHCb.With the large amount of data to be collected, nuclear PDFs can also be investigated in greater detail, helping to shed light on the intriguing anti-shadowing effect [11], which is expected to be dominant in the x range covered with LHCspin.

Spin physics
Beside standard collinear PDFs, LHCspin will offer the opportunity to probe polarised quark and gluon distributions by means of proton collisions on polarised hydrogen and deuterium.For example, measurements of transverse-momentum dependent PDFs (TMDs) provide a map of parton densities in 3-dimensional momentum space.Light quark TMDs, especially in the high-x regime, can be accessed by measuring transverse single spin asymmetries (TSSAs) in Drell-Yan processes, while gluon densities, such as the gluon Sivers function, can be probed via heavy-flavour production.Such asymmetry can be large (Fig. 3, left) and well in reach with just 1 month of LHCspin data, as shown in simulated J/ψ → µ + µ − events (Fig. 3, right).
It is also attractive to go beyond a 3-dimensional description by building observables which are sensitive to Wigner distributions [13] and to measure the elusive transversity PDF, whose knowledge is currently limited to valence quarks at the leading order [14], as well as its integral, the tensor charge, which is of direct interest in constraining physics beyond the Standard Model [15].

Heavy-ion collisions
Thermal heavy-flavour production is negligible at the typical temperature of few hundreds MeV of the system created in ultra-relativistic heavy-ion collisions.Quarkonia states (cc, bb) are instead produced on shorter timescales, and their energy change while traversing the medium represents a powerful way to investigate Quark-Gluon Plasma (QGP) properties.
LHCb capabilities allow to both cover the aforementioned charmonia and bottomonia studies and to extend them to bottom baryons as well as exotic probes.QGP phase diagram exploration at LHCspin can be performed with a rapidity scan, complementing RHIC's Notice that here negative rapidities correspond to the forward region for the polarized proton.

IV. CONCLUSIONS
In this paper we have extended, and somehow completed, a detailed analysis of SSAs for J/ production in pp collisions within a phenomenological TMD scheme.This study started in a previous paper, where, employing the Color-Singlet Model for quarkonium formation, we compared the Generalized Parton Model and the Color-Gauge-Invariant GPM.It has been then continued quite recently in a second work, adopting the NRQCD framework within the GPM.Here we have eventually considered its extension within the CGI-GPM.The main interest of this analysis is to see whether and to what extent one can extract information on the poorly known gluon Sivers function, focusing only on this specific process.
We have considered all relevant subprocesses in NRQCD, both for the 2 ! 1 and the 2 ! 2 channels, including e↵ects of initial and final state interactions, in the one-gluon-exchange approximation.This leads to the introduction of new color factors, diagram by diagram, and the computation of modified hard scattering amplitudes.In such a way one can move the process dependence, coming from ISIs and FSIs, into the hard parts, factorizing the corresponding TMDs.One, well-known, outcome of this approach is the appearance of two independent gluon Sivers functions, referred to as the d-type and the f -type distributions.
We have then calculated the maximized contributions to A N , separately for the gluon and the quark Sivers e↵ects, adopting the kinematics of the PHENIX experiment, for which data are available.The main findings are that the quark as well as the d-type gluon Sivers functions, even if maximized, give almost negligible contributions to the SSA, leaving at work, as in the CSM, only the f -type GSF.On the other hand, within NRQCD this contribution is also generally quite small and could be relatively sizeable only at forward rapidities and P T around 2-3 GeV, at least for the two LDME sets considered.
Therefore, while within the GPM, the GSF could be easily constrained by PHENIX SSA data for J/ production alone, the situation in the CGI-GPM is quite di↵erent.Indeed, if one adopts the CSM, the f -type GSF (the only one active) gives still a potentially sizeable contribution; on the contrary, in full NRQCD it could be hardly constrained, and definitely not in the backward region.
We have also presented some maximized estimates of A N , for the kinematics reachable at LHC in a fixed target mode, showing similar features as those discussed for PHENIX setup.
More data, with higher statistics, could certainly help in shedding light on the role of the gluon Sivers function, as well as on its process dependence.

Marco Santimaria
/16 SQM2022 14 • Gluon Sivers function can be probed with quarkonia and open heavy-flavour production • broad range at a scale with lot of unique probes: ...
• predictions on with LHCspin kinematics A N J/Ψ → μ + μ − • An example analysis with ~1 month of LHCspin data: • Full LHCb simulation + polarisation emulation • Measuring with bins on • has a negligible impact in this example Notice that here negative rapidities correspond to the forward region for the polarized proton.

IV. CONCLUSIONS
In this paper we have extended, and somehow completed, a detailed analysis of SSAs for J/ production in pp collisions within a phenomenological TMD scheme.This study started in a previous paper, where, employing the Color-Singlet Model for quarkonium formation, we compared the Generalized Parton Model and the Color-Gauge-Invariant GPM.It has been then continued quite recently in a second work, adopting the NRQCD framework within the GPM.Here we have eventually considered its extension within the CGI-GPM.The main interest of this analysis is to see whether and to what extent one can extract information on the poorly known gluon Sivers function, focusing only on this specific process.
We have considered all relevant subprocesses in NRQCD, both for the 2 ! 1 and the 2 ! 2 channels, including e↵ects of initial and final state interactions, in the one-gluon-exchange approximation.This leads to the introduction of new color factors, diagram by diagram, and the computation of modified hard scattering amplitudes.In such a way one can move the process dependence, coming from ISIs and FSIs, into the hard parts, factorizing the corresponding TMDs.One, well-known, outcome of this approach is the appearance of two independent gluon Sivers functions, referred to as the d-type and the f -type distributions.
We have then calculated the maximized contributions to A N , separately for the gluon and the quark Sivers e↵ects, adopting the kinematics of the PHENIX experiment, for which data are available.The main findings are that the quark as well as the d-type gluon Sivers functions, even if maximized, give almost negligible contributions to the SSA, leaving at work, as in the CSM, only the f -type GSF.On the other hand, within NRQCD this contribution is also generally quite small and could be relatively sizeable only at forward rapidities and P T around 2-3 GeV, at least for the two LDME sets considered.
Therefore, while within the GPM, the GSF could be easily constrained by PHENIX SSA data for J/ production alone, the situation in the CGI-GPM is quite di↵erent.Indeed, if one adopts the CSM, the f -type GSF (the only one active) gives still a potentially sizeable contribution; on the contrary, in full NRQCD it could be hardly constrained, and definitely not in the backward region.
We have also presented some maximized estimates of A N , for the kinematics reachable at LHC in a fixed target mode, showing similar features as those discussed for PHENIX setup.
More data, with higher statistics, could certainly help in shedding light on the role of the gluon Sivers function, as well as on its process dependence.

Introduction
LHCSpin aims at installing a polarized gas target in front of the LHCb spectrometer [1], bringing, for the first time, polarized physics to the LHC.The project will benefit from the experience achieved with the installation of an unpolarized gas target at LHCb during the LHC Long Shutdown 2 [2,3].LHCb will then become the first experiment simultaneously running in collider and fixed-target mode with polarized targets, opening a whole new range of explorations to its exceptional spectrometer.
Among the main advantages of a polarized gas target are the high polarization achievable (>80%), the absence of unpolarized materials in the target (no dilution), the possiblity to flip the nuclear spin state very rapidly (order of minutes) such to efficiently reduce systematic effects and a negligible impact on the beam lifetime.
LHCSpin will offer a unique opportunity to probe polarized quark and gluon parton distributions in nucleons and nuclei, especially at high x and intermediate Q2, where experimental data are still largely missing.Beside standard collinear parton distribution functions (PDFs), LHCSpin will make it possible to study multidimensional polarized parton distributions that depend also on parton transverse momentum (transverse-momentumdependent PDFs, or TMDs).
The study of the multidimensional partonic structure of the nucleon, particularly including polarization effects, can test our knowledge of QCD at an unprecedented level of sophistication, both in the perturbative and nonperturbative regime.At the same time, an accurate knowledge of hadron structure is necessary for precision measurements of Standard Model (SM) observables and discovery of physics beyond the SM.
Due to the intricate nature of the strong interaction, it is indispensable to perform the widest possible suite of experimental measurements.In the time range covered by the next update of the ESPP, it will be ideal to have two new projects complementing each other: a new facility for polarized electron-proton collisions and a new facility for polarized proton-proton collisions.LHCSpin [4] stands out at the moment as the most promising candidate for the second type of project, going beyond the kinematic coverage and the accuracy of the existent experiments, especially on the heavy-quark sector.
The document comprises two main parts, describing the physics case and the hardware implementation, respectively.[12].simulated J/ψ → µ + µ − azimuthal asymmetries with a fit curve superimposed made of two terms.beam-energy scan, while flow measurements will benefit from the excellent particle identification performance of LHCb on charged and neutral light hadrons.An interesting topic joining heavy-ion collisions and spin physics is the dynamics of small systems which can be probed via ellipticity measurements in lead-ion collisions on polarised deuterons [16].

Figure 6 :
Figure6: Primary vertex reconstruction e ciency (top), resolution (middle) and fake rate (bottom) as a function of the z coordinate for minimum-bias (in blue) stand-alone pp, (in green) stand-alone pHe, (in red) overlapped pp and pHe and (in orange) pp and pAr events simulated considering the Run3 pp conditions (⌫ ⇠ 7.6, L ' 2 • 10 33 cm 2 s 1 ) and one fixed per-bunch beam-gas collision.Similar e ciencies and fake rates between beam-beam and beam-gas collisions and no pp performance loss when injecting the gas are observed.A steep evolution with z of the resolution in the SMOG2 cell is found instead, as a consequence of the larger uncertainty when extrapolating low-aperture VELO tracks upstream of the nominal LHCb interaction point.

Figure 1 .
Figure 1.Left: picture of the SMOG2 storage cell after installation.Right: simulated vertex reconstruction efficiency for simultaneous beam-gas and beam-beam collisions [4].

Figure 4 :Figure 5 :
Figure 4: Kinematic coverage in the x Q 2

•
Starting from the well established HERMES setup @ DESY ... to create the next generation of fixed-target polarisation techniques!• Cylindrical target cell with and • LHCb simulations show broader kinematic acceptance & higher efficiency at the same position of the SMOG2 cell, i.e. close to the VELO L = 20 cm D = 1 cm VELO vessel

Figure 2 .
Figure 2. Two views of the TC with the magnet coils (orange) and the iron return yoke (blue) enclosing the storage cell.VELO vessel and detector box are shown in green and grey, respectively.

FIG. 7 :
FIG. 7: Maximized values for AN for the process pp " !J/ + X at p s = 115 GeV and PT = 3 GeV as a function of xF (left panel) and at y = 2 as a function of PT (right panel), obtained adopting the CGI-GPM and GPM approaches, within the CS model and NRQCD (BK11 set).Notice that here negative rapidities correspond to the forward region for the polarized proton.

FIG. 7 :
FIG. 7: Maximized values for AN for the process pp " !J/ + X at p s = 115 GeV and PT = 3 GeV as a function of xF (left panel) and at y = 2 as a function of PT (right panel), obtained adopting the CGI-GPM and GPM approaches, within the CS model and NRQCD (BK11 set).Notice that here negative rapidities correspond to the forward region for the polarized proton.