Abstract
A dedicated sample of Large Hadron Collider proton-proton collision data at centre-of-mass energy sqrt{s} = 8 TeV is used to study inclusive single diffractive dissociation, pp → X p. The intact final-state proton is reconstructed in the ATLAS ALFA forward spectrometer, while charged particles from the dissociated system X are measured in the central detector components. The fiducial range of the measurement is −4.0 < log10ξ < −1.6 and 0.016 < |t| < 0.43 GeV2, where ξ is the proton fractional energy loss and t is the squared four-momentum transfer. The total cross section integrated across the fiducial range is 1.59 ± 0.13 mb. Cross sections are also measured differentially as functions of ξ, t, and ∆η, a variable that characterises the rapidity gap separating the proton and the system X . The data are consistent with an exponential t dependence, dσ/dt ∝ eBt with slope parameter B = 7.65 ± 0.34 GeV−2. Interpreted in the framework of triple Regge phenomenology, the ξ dependence leads to a pomeron intercept of α(0) = 1.07 ± 0.09.
Highlights
Experimental conditionsATLAS is a multipurpose apparatus covering almost the entire solid angle around its LHC collision point [17].1 This measurement makes use of the sensitivity of the inner tracking detector (ID) and the minimum-bias trigger scintillators (MBTS) to the components of the dissociating system X
The Herwig7 prediction is broadly in line with the shape of the ∆η distribution, but exhibits an even larger excess in normalisation. This may be partly due to the operational definition of the SD process that is adopted in the default SD model normalisation, which is derived from a rapidity gap measurement that contains a DD admixture [33]
Differential cross sections are measured as a function of the fractional proton energy loss ξ, the squared four-momentum transfer t, and the size ∆η of the pseudorapidity interval on the same side of the interaction point as the intact proton extending from η = ±2.5 to the closest charged particle with smaller |η| and pT > 200 MeV
Summary
ATLAS is a multipurpose apparatus covering almost the entire solid angle around its LHC collision point [17].1 This measurement makes use of the sensitivity of the inner tracking detector (ID) and the minimum-bias trigger scintillators (MBTS) to the components of the dissociating system X. The main detectors consist of 20 layers of 64 fibres each, arranged in 10 overlapping pairs in two perpendicular (u, v) orientations at 45◦ to the (x, y) coordinates, read out by an array of multi-anode photomultiplier tubes These main detectors are supplemented by scintillating tiles, which provide trigger signals. The data were taken in a high-β∗ quadrupole configuration, which provides beams of almost collinear protons at zero crossing angle This allows the sensitive components of ALFA to be placed as close as possible to the beam, enabling detection of protons at very small deflection angles. A special beam optics configuration [20] was in place for the data used here, incorporating ‘parallel-to-point’ focusing in the vertical plane, such that the y coordinate of the proton impact point in a Roman pot detector at fixed z depends only on the scattering angle and the energy loss.
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