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Recently the TOTEM experiment at the LHC has released measurements at $sqrt{s} = 13$ TeV of the proton-proton total cross section, $sigma_{tot}$, and the ratio of the real to imaginary parts of the forward elastic amplitude, $rho$. Since then an intense debate on the $C$-parity asymptotic nature of the scattering amplitude was initiated. We examine the proton-proton and the antiproton-proton forward data above 10 GeV in the context of an eikonal QCD-based model, where nonperturbative effects are readily included via a QCD effective charge. We show that, despite an overall satisfactory description of the forward data is obtained by a model in which the scattering amplitude is dominated by only crossing-even elastic terms, there is evidence that the introduction of a crossing-odd term may improve the agreement with the measurements of $rho$ at $sqrt{s} = 13$ TeV. In the Regge language the dominant even(odd)-under-crossing object is the so called Pomeron (Odderon).
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We present a systematic study of the normalized symmetric cumulants, NSC(n,m), at the eccentricity level in proton-proton interactions at $sqrt s!=! 13$ TeV within a wounded hot spot approach. We focus our attention on the influence of spatial correlations between the proton constituents, in our case gluonic hot spots, on this observable. We notice that the presence of short-range repulsive correlations between the hot spots systematically decreases the values of NSC(2,3) and NSC(2,4) in mid-to-ultra central collisions while increases them in peripheral interactions. In the case of NSC(2,3) we find that, as suggested by data, an anti-correlation of $varepsilon_2$ and $varepsilon_3$ in ultra-central collisions, i.e. NSC(2,3)$<0$, is possible within the correlated scenario while it never occurs without correlations. We attribute this fact to the decisive role of correlations on enlarging the probability of interaction topologies that reduce the value of NSC(2,3) and, eventually, make it negative. Further, we explore the dependence of our conclusions on the values of the hot spot radius and the repulsive core distance. Our results add evidence to the idea that considering spatial correlations between the subnucleonic degrees of freedom of the proton may have a strong impact on the initial state properties of proton-proton interactions [1].
We compute the leading order (LO) $qgto q gamma$ and next-to-leading order (NLO) $ggto q{bar q} gamma$ contributions to inclusive photon production in proton-proton (p+p) collisions at the LHC. These channels provide the dominant contribution at LO and NLO for photon transverse momenta $k_{gammaperp}$ corresponding to momentum fractions of $xleq 0.01$ in the colliding protons. Our computations, performed in the dilute-dense framework of the Color Glass Condensate effective field theory (CGC EFT), show that the NLO contribution dominates at small-$x$ because it is sensitive to $k_perp$-dependent unintegrated gluon distributions in both of the protons. We predict a maximal $10%$ modification of the cross section at low $k_{gammaperp}$ as a direct consequence of the violation of $k_perp$-factorization. The coherence effects responsible for this modification are enhanced in nuclei and can be identified from inclusive photon measurements in proton-nucleus collisions. We provide numerical results for the isolated inclusive photon cross section for $k_{gammaperp}leq 20$ GeV in p+p collisions that can be tested in the future at the LHC.
We analyse the transverse momentum ($p_{rm T}$)-spectra as a function of charged-particle multiplicity at midrapidity ($|y| < 0.5$) for various identified particles such as $pi^{pm}$, $K^{pm}$, $K_S^0$, $p+overline{p}$, $phi$, $K^{*0} + overline {K^{*0}}$, and $Lambda$ + $bar{Lambda}$ in proton-proton collisions at $sqrt{s}$ = 7 TeV using Boltzmann-Gibbs Blast Wave (BGBW) model and thermodynamically consistent Tsallis distribution function. We obtain the multiplicity dependent kinetic freeze-out temperature ($T_{rm kin}$) and radial flow ($beta$) of various particles after fitting the $p_{rm T}$-distribution with BGBW model. Here, $T_{rm kin}$ exhibits mild dependence on multiplicity class while $beta$ shows almost independent behaviour. The information regarding Tsallis temperature and the non-extensivity parameter ($q$) are drawn by fitting the $p_{rm T}$-spectra with Tsallis distribution function. The extracted parameters of these particles are studied as a function of charged particle multiplicity density ($dN_{ch}/deta$). In addition to this, we also study these parameters as a function of particle mass to observe any possible mass ordering. All the identified hadrons show a mass ordering in temperature, non-extensive parameter and also a strong dependence on multiplicity classes, except the lighter particles. It is observed that as the particle multiplicity increases, the $q$-parameter approaches to Boltzmann-Gibbs value, hence a conclusion can be drawn that system tends to thermal equilibrium. The observations are consistent with a differential freeze-out scenario of the produced particles.
Proton-nucleus (p+A) collisions have long been recognized as a crucial component of the physics programme with nuclear beams at high energies, in particular for their reference role to interpret and understand nucleus-nucleus data as well as for their potential to elucidate the partonic structure of matter at low parton fractional momenta (small-x). Here, we summarize the main motivations that make a proton-nucleus run a decisive ingredient for a successful heavy-ion programme at the Large Hadron Collider (LHC) and we present unique scientific opportunities arising from these collisions. We also review the status of ongoing discussions about operation plans for the p+A mode at the LHC.
Recently, the CMS Collaboration has published identified particle transverse momentum spectra in high multiplicity events at LHC energies $sqrt s $ = 0.9-13 TeV. In the present work the transverse momentum spectra have been analyzed in the framework of the color fields inside the clusters of overlapping strings, which are produced in high energy hadronic collisions. The non-Abelian nature is reflected in the coherence sum of the color fields which as a consequence gives rise to an enhancement of the transverse momentum and a suppression of the multiplicities relative to the non overlapping strings. The initial temperature and shear viscosity to entropy density ratio $eta/s$ are obtained. For the higher multiplicity events at $sqrt s $ =7 and 13 TeV the initial temperature is above the universal hadronization temperature and is consistent with the creation of de-confined matter. In these small systems it can be argued that the thermalization is a consequence of the quantum tunneling through the event horizon introduced by the confining color fields, in analogy to the Hawking-Unruh effect. The small shear viscosity to entropy density ratio $eta/s$ near the critical temperature suggests that the matter is a strongly coupled Quark Gluon Plasma.
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