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تسجيل مستخدم جديدLHC Forward Physics
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The goal of this report is to give a comprehensive overview of the rich field of forward physics, with a special attention to the topics that can be studied at the LHC. The report starts presenting a selection of the Monte Carlo simulation tools currently available, chapter 2, then enters the rich phenomenology of QCD at low, chapter 3, and high, chapter 4, momentum transfer, while the unique scattering conditions of central exclusive production are analyzed in chapter 5. The last two experimental topics, Cosmic Ray and Heavy Ion physics are presented in the chapter 6 and 7 respectively. Chapter 8 is dedicated to the BFKL dynamics, multiparton interactions, and saturation. The report ends with an overview of the forward detectors at LHC. Each chapter is correlated with a comprehensive bibliography, attempting to provide to the interested reader with a wide opportunity for further studies.
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The following effects in the nearly forward (soft) region of the LHC are proposed to be investigated: 1) At small |t| the fine structure of the cone (Pomeron) shouldbe scrutinized: a) a break of the cone near $tapprox - 0.1 ~ GeV$^2, due to the two-p
ion threshold, and required by t-channel unitarity, is expected, and b) possible small-period oscillations between $t=0$ and the dip region. 2) In measuring the elastic $pp$ scattering and total $pp$ cross section at the LHC, the experimentalists are urged to treat the total cross section $sigma_t,$ the ratio $rho$, the forward slope $B$ and the luminosity ${cal L}$ as free arameters, and to publish model-independent results on ${dN/{dt}}.$ 3) Of extreme interest are the details of the expected diffraction minimum in the differential cross section. Its position, expected in the interval $0.4<-t<1$ GeV$^2$ at the level of about $10^{-2} {rm mb} cdot$ GeV$^{-2}div 10^{-1} {rm mb}cdot$ GeV$^{-2}$, cannot be predicted unambiguously, and its depth, i.e. the ratio of $dsigma/dt$ at the minimum to that at the subsequent maximum (about $-t=5 $GeV$^2$, which is about 5 is of great importance. 4) The expected slow-down with increasing $|t|$ of the shrinkage of the second cone (beyond the dip-bump), together with the transition from an exponential to a power decrease in $-t$, will be indicative of the transition from soft to hard physics. Explicit models are proposed to help in quantifying this transition. 5) In a number of papers a limiting behavior, or saturation of the black disc limit (BDL) was predicted. This controversial phenomenon shows that the BDL may not be the ultimate limit.
A rapidity gap program with great potential can be realized at the Large Hadron Collider, LHC, by adding a few simple forward shower counters (FSCs) along the beam line on both sides of the main central detectors, such as CMS. Measurements of single
diffractive cross sections down to the lowest masses can be made with an efficient level-1 trigger. Exceptionally, the detectors also make feasible the study of Central Diffractive Excitation, and in particular the reaction g + g to g + g, in the color singlet channel, effectively using the LHC as a gluon-gluon collider.
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 inte
nse 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).
Recent data from LHC13 by the TOTEM Collaboration on $sigma_{tot}$ and $rho$ have indicated disagreement with all the Pomeron model predictions by the COMPETE Collaboration (2002). On the other hand, as recently demonstrated by Martynov and Nicolescu
(MN), the new $sigma_{tot}$ datum and the unexpected decrease in the $rho$ value are well described by the maximal Odderon dominance at the highest energies. Here, we discuss the applicability of Pomeron dominance through fits to the textit{most complete set} of forward data from $pp$ and $bar{p}p$ scattering. We consider an analytic parametrization for $sigma_{tot}(s)$ consisting of non-degenerated Regge trajectories for even and odd amplitudes (as in the MN analysis) and two Pomeron components associated with double and triple poles in the complex angular momentum plane. The $rho$ parameter is analytically determined by means of dispersion relations. We carry out fits to $pp$ and $bar{p}p$ data on $sigma_{tot}$ and $rho$ in the interval 5 GeV - 13 TeV (as in the MN analysis). Two novel aspects of our analysis are: (1) the dataset comprises all the accelerator data below 7 TeV and we consider textit{three independent ensembles} by adding: either only the TOTEM data (as in the MN analysis), or only the ATLAS data, or both sets; (2) in the data reductions to each ensemble, uncertainty regions are evaluated through error propagation from the fit parameters, with 90 % CL. We argument that, within the uncertainties, this analytic model corresponding to soft Pomeron dominance, does not seem to be excluded by the textit{complete} set of experimental data presently available.
The TOTEM experiment with its detectors in the forward region of CMS and the Roman Pots along the beam line will determine the total pp cross-section via the optical theorem by measuring both the elastic cross-section and the total inelastic rate. TO
TEM will have dedicated runs with special high-beta* beam optics and a reduced number of proton bunches resulting in a low effective luminosity between 1.6 x 10^{28} cm^{-2} s^{-1} and 2.4 x 10^{29} cm^{-2} s^{-1}. In these special conditions also an absolute luminosity measurement will be made, allowing the calibration of the CMS luminosity monitors needed at higher luminosities. The acceptance of more than 90 % of all leading protons in the Roman Pot system, together with CMSs central and TOTEMs forward detectors extending to a maximum rapidity of 6.5, makes the combined CMS+TOTEM experiment a unique instrument for exploring diffractive processes. Scenarios for running at higher luminosities necessary for hard diffractive phenomena with low cross-sections are under study.
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