Measurement of the inelastic proton-lead cross section at √sNN=5.02 TeV

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The inelastic hadronic cross section in proton-lead collisions at a centre-of-mass energy per nucleon pair of 5.02 TeV is measured with the CMS detector at the LHC. The data sample, corresponding to an integrated luminosity of $L=12.6\pm0.4~{\rm nb}^{-1}$, has been collected with an unbiased trigger for inclusive particle production. The cross section is obtained from the measured number of proton-lead collisions with hadronic activity produced in the pseudo rapidity η ranges between 3 and 5 and/or −5 and −3, corrected for photon-induced contributions, experimental acceptance, and other instrumental effects. The inelastic cross section is measured to be $\sigma_{\rm inel}({\rm pPb})=2061\pm3({\rm stat})\pm34({\rm lumi})\pm72({\rm lumi})~{\rm mb}$. Various Monte Carlo generators, commonly used in heavy ion and cosmic ray physics, are found to reproduce the data within uncertainties. The value of $\sigma_{\rm inel}({\rm pPb})$ is compatible with that expected from the proton-proton cross section at 5.02 TeV scaled up within a simple Glauber approach to account for multiple scatterings in the lead nucleus, indicating that further net nuclear corrections are small.


Figure Caption
click on it to get it click on it to get it Fig.1: Distribution of the energy deposited in the HF calorimeter ($E_{\rm HF}$) for the single-arm (left) and double-arm (right) event selections, for a data sample corresponding to $1.31~{\rm nb}^{-1}$ recorded with an unbiased trigger. The contribution from noise is obtained from a random trigger normalised to the same number of triggers as that in the collision data. The average number of $\gamma{\rm p}$ processes simulated with STARLIGHT+DPMJET and STARLIGHT+ PYTHIA is treated as background and stacked on top. Four hadronic interaction models (EPOS, DPMJET, HIJING, and QGSJETII) are overlaid and normalised to the number of data events with $E_{\rm HF}>10~{\rm GeV}$, where the contribution from the background is small. The vertical line represents the threshold energy of 8 GeV (4 GeV) for the single-arm (double-arm) selection used in this analysis.
click on it to get it Fig.2: Acceptance versus purity of the two event selections, as derived from the EPOS and QGSJETII generators. The symbols indicate different values of the $E_{\rm HF}$ thresholds. The chosen thresholds are marked with squares.
click on it to get it Fig.3: Inelastic hadronic cross sections for pPb collisions as a function of the centre-of-mass energy. The measurement described here (circle, with error bars obtained from the quadratic sum of all uncertainties) is compared to lower energy data (squares and triangles) [2, 39, 40] and to different model predictions (curves).

Preliminary PAS (Physics Analysis Summary)


A measurement of the inelastic proton-lead (pPb) cross section at a nucleon-nucleon centre-of-mass energy of 5.02 TeV is presented using the CMS detector at the LHC. Inelastic collisions are tagged using the forward calorimeters at pseudorapidities $3<|\eta|<5$. Two different event selections are used: (i) a coincidence of both sides of the detector, and (ii) a single-sided event selection. These two selections have different sensitivity to contributions from photon induced ($\gamma$p) collisions and hadronic diffractive interactions. The value of the hadronic inelastic cross section is measured within the CMS acceptance and extrapolated to its total value. The photon-induced contribution is subtracted. The final result is $\sigma_{\rm inel}=2.06\pm0.08~$b. The uncertainty is dominated by the luminosity determination. This measurement of the inelastic cross section is consistent with the expectation from the Glauber approach.


Preliminary Figures

Figure Caption
click on it to get it click on it to get it Fig.1: Distribution of EHF. The noise rate (random trigger with empty bunch crossings) is matched to the zero bias trigger rate. The average of $\gamma$p simulated with STARLIGHT+DPMJET and STARLIGHT+PYTHIA is added to the background as well. Three hadronic interaction models (EPOS-LHC, HIJING, and QGSJETII-04) are also shown. These models are normalised to the number of zero bias events above 10 GeV, where the contribution from the background is low. The vertical lines represent the threshold energies used in this analysis.
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Fig.2: Top panel: Efficiency for different selection thresholds for EHF for three hadronic interaction models are shown. Bottom panel: Visible cross section for noise (randomly triggered, empty bunch crossings) events and photo-nuclear events passing the selection. The vertical lines represent the threshold energies used in this analysis.
click on it to get it Fig.3: Illustration of a single-diffractive event where the proton disintegrates and three particles (dashed lines) are within the acceptance of the HF calorimeter. The particle with the highest energy (dotted line) determines EHF.
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Fig4: Distribution of EHF for three MC generators: EPOS-LHC (top), QGSJETII-04 (middle) and HIJING (bottom). The single-arm selection is on the left side and the double-arm selection on the right side. Five stacked categories of inelastic collisions are shown: single-diffractive where the lead ion (SD1) or the proton (SD2) dissociates, double-diffractive (DD), central-diffractive (CD), and non-diffractive (ND) events are shown for each.
click on it to get it click on it to get it Fig5: Examples of ultra-peripheral proton-nucleus interactions. Left panel: A quasi-real photon from the field of the lead ion fluctuates into a neutral $\rho$ meson (circle) which interacts with the incoming proton via a colour-singlet state (pomeron exchange) and decays into $\pi^+$ and $\pi^-$ mesons. Right panel: A quasi-real photon $\gamma^*$ interacts with a parton leading to the break-up of the proton.
click on it to get it click on it to get it Fig.6: Left panel: Inelastic cross section values derived from Eq. (17) for one example run. One value for each 23.31~s period is added with weight 1/variance determined by the Poisson distribution. The averages of the two event selections are indicated by markers on the x-axis. Right panel: Measured inelastic cross sections for the two selection criteria as a function of analysed runs (the horizontal lines show the weighted mean of the values for each event selection criterion).
click on it to get it Fig.7: The measured proton-lead cross section for visible, visible hadronic, and hadronic inelastic contribution. The systematic uncertainty on $\sigma$inel is shown as defined in Table 3. For $\sigma$vis,had the extrapolation and modelling uncertainties do not enter. In addition, the uncertainty on electromagnetic events does not enter for $\sigma$vis. Model predictions from the hadronic interaction models EPOS-LHC and QGSJETII-04 are given for the visible hadronic and hadronic inelastic cross section.
click on it to get it Fig.8: Measured proton-lead hadronic inelastic cross section compared to model predictions.
click on it to get it Fig.9: Hadronic inelastic proton-lead cross section as a function of centre-of-mass energy. The data are from this work (CMS) and [2830]. The lines show predictions by the hadronic interaction models EPOS-LHC, QGSJETII-04 and a Glauber calculation based on the pp cross section from the COMPETE fit that has been extended by the TOTEM measurement at a centre-of-mass energy of 7 TeV.

-- ColinBaus - 09 Dec 2013

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Topic revision: r12 - 2015-09-25 - ColinBaus
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