pp interactions can be cathegorized, according to their final state, as elastic, diffractive inelastic and inelastic. If the mass of the final state is small (Mx < 15 GeV) the event escapes undetected. Both CMS and ATLAS measured the total inelastic cross section for Mx greater than 15.7GeV with good precision and then extrapolated it to the full mass range using Monte Carlo models. The results

of the total inelastic pp cross section at √s = 7TeV and the extrapolated value are here presented for both experiments. The ATLAS measurement of the inelastic cross section as a function of the rapidity gap, used to tune the MC simulations in the low mass region, is also presented

Valentinetti, S.

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DOI 10.1393/ncc/i2014-11755-9Colloquia: LC13IL NUOVO CIMENTO Vol. 37 C, N. 2 Marzo-Aprile 2014Measurement of pp inelastic cross section at√s = 7TeV at LHCS. ValentinettiINFN, Sezione di Bologna and Universita` di Bologna - Bologna, Italyricevuto il 4 Febbraio 2014Summary. — pp interactions can be cathegorized, according to their final state,as elastic, diffractive inelastic and inelastic. If the mass of the final state is small(Mx < 15GeV) the event escapes undetected. Both CMS and ATLAS measured thetotal inelastic cross section for Mx greater than 15.7GeV with good precision andthen extrapolated it to the full mass range using Monte Carlo models. The resultsof the total inelastic pp cross section at√s = 7TeV and the extrapolated valueare here presented for both experiments. The ATLAS measurement of the inelasticcross section as a function of the rapidity gap, used to tune the MC simulations inthe low mass region, is also presented.PACS 13.75.Cs – Nucleon-nucleon interactions (including antinucleons, deuterons,etc.).PACS 25.40.Ep – Inelastic proton scattering.1. – IntroductionAt LHC energies the largest contribution to the total pp cross section, composed bythe elastic and inelastic part, is due to inelastic processes, that can be separated in non-diffractive (ND), single diffractive (SD), double diffractive (DD) and central diffractive(CD) components. The diffractive processes are characterized by a final state in whichthe produced particles are separated in rapidity η(1) by a gap due to the exchange of agluon ladder. The size of the gap is related to the mass of the dissociative system Mx(i.e. in case of single diffraction Δη ∝ ln ξ, where ξ = M2x/s with s the centre of massenergy). Since both CMS [1] and ATLAS [2] were designed to be highly sensitive tohigh-pT physics (high final state mass) and did not cover the whole rapidity range, onlythe measurement of the inelastic cross section in the mass range given by the detectoracceptance Mx > 15.7GeV was performed. The measurement was then extrapolated to(1) η = − ln[tan(θ \ 2)], where θ is the polar angle in a right-handed coordinate system withthe origin at the interaction point, the z-axis coincides with the beam line and the x-axis pointsto the centre of the LHC ring.c© Societa` Italiana di Fisica 193194 S. VALENTINETTIthe full mass range by using Monte Carlo (MC) simulations. The measured inelastic ppcross section served as an input to validate the phenomenological models and providedbasic information needed for tuning Monte Carlo (MC) generators.2. – Measurement of pp inelastic cross section at√s = 7TeV with CMSCMS measured the total inelastic cross section using two different methods [3]. Inthe first method, called Event Counting, events triggered by at least one of the HadronForward Calorimeters (HF) (2.9 < |η| < 5.2) were counted. In the second method, calledPile-up Counting, the number of pile-up events triggered by the inner tracker (|η| < 2.4)in a given bunch crossing was counted as a function of bunch luminosity. For theseanalyses seven runs taken in 2010 at√s = 7TeV at different values of average numberof collisions per bunch crossing (from 0.007 to 0.1) were used.Event counting method . In this method [4] the number of events that depositedat least 5GeV of energy in any of the two HF calorimeters were counted. The HFacceptance 2.9 < |η| < 5.2 corresponded to the kinematic range ξ < 5 · 10−6. In thisrange the inelastic cross section was evaluated as(1) σppinel(ξ < 5 · 10−6) =Ninel(1− fξ<5·10−6)Fpile−upξ · Lint ,Ninel was the number of selected events, background subtracted. Fpile-up was the pile-upcorrection factor: since the acceptance of the calorimeters was outside the acceptanceof the tracker, events with more than one interaction in the same bunch crossing cuoldnot be discarted and a pile-up correction was necessary. Fpile-up was evaluated fromdata using the ratio of the number of detected events to the number of triggers countedrequiring the coincidence of the two HF calorimeters. The detection efficiency ξ andthe contamination factors fξ<5·10−6 (fraction of selected events with low mass ≈ 0.02)were determined using three MC generators (PHYTIA6, PYTHIA8 and PHOJET) withan estimated uncertainties at 1% level [4]. The expected distributions of the kinematicvariable ξ for all events, events that deposit 4GeV and 5GeV of energy in the HF,respectively, are shown as a function of the kinematic variable ξ in fig. 1. The cut onξ > 5 · 10−6 for EHF > 5GeV guaranteed that more than 98% of total events weredetected. The final inelastic cross section value, obtained averaging the cross sectionsmeasured in runs at different pile-up conditions and shown in fig. 2, was(2) σppinel(ξ > 5 · 10−6) = 60.2± 0.2(stat)± 1.1(syst)± 2.4(lumi)mb.Pile-up counting method . This analysis [5] was based on the assumption that thenumber of inelastic pp interactions (n) in a given bunch crossing was a function of thetotal inelastic cross section σinel via the Poisson probability distribution function P (n):(3) P (n) =(L · σinel)nn!· e−L·σinelwhere L was the bunch luminosity. The number of reconstructed vertices that fulfilledquality requirements were counted at each bunch crossing. To obtain the true numberof vertices a bin-by-bin correction was applied to the visible number of vertices usingMEASUREMENT OF pp INELASTIC CROSS SECTION AT√s = 7TeV AT LHC 195Fig. 1. – Simulated distributions of the kinematic variable ξ for all events (white), events thatdeposit EHF > 4GeV (blue), studied for systematic effects, and EHF > 5GeV (green) for thefull mass range. The cut on ξ > 5 · 10−6 (dashed vertical line) guarantees that more than 98%of total events are detected.MC simulation. This accounted for vertex reconstruction efficiency, vertex merging andfakes. After the correction, the distributions of the fraction of pile-up events as a functionof luminosity for n = 0 . . . 8 pile-up events were fitted with a Poisson distribution. Thefinal value of the inelastic cross section was evaluated by averaging the nine values ofvisible cross section, as shown in fig. 3. The analysis was performed by using three setsof events, requiring respectively at least 1, 2 or 3 tracks for each vertex. The results forthe different samples wereσppvis(≥ 1 track) = 58.7± 2.0(syst)± 2.4(lumi)mb,(4)σppvis(≥ 2 track) = 57.2± 2.0(syst)± 2.4(lumi)mb,σppvis(≥ 3 track) = 55.4± 2.0(syst)± 2.4(lumi)mb.Fig. 2. – Inelastic pp cross section at 7TeV for ξ > 5 · 10−6 obtained from data sets collected atvarious pile-up conditions. The error bars show the statistical uncertainties combined with thesystematic ones. The final result of the cross section is obtained averaging the seven values at5GeV.196 S. VALENTINETTIFig. 3. – Visible inelastic pp cross section as a function of the number of pile-up vertices, inevents with at least one track with pT > 200 MeV and |η| < 2.4 for each vertex. The line is theresult of averaging the nine individual values of cross section.Extrapolation to the total inelastic cross section. Six different MC generators wereused to extrapolate the visible inelastic cross section to the total inelastic cross section,in the full mass range. All models showed similar trend for cross section at the measuredhadron level but there were substantial differences in the expectations for the total in-elastic pp cross section, as visible in fig. 4. The final extrapolated values of the inelasticcross section for the two analyses were respectively:σppinel(7TeV) = 64.5± 0.2(stat)± 1.1(syst)± 2.4(lumi)± 1.5(extr)mb,(5)σppinel(7TeV) = 68.0± 2.0(syst)± 2.4(lumi)± 4.0(extr)mb.The largest uncertainty came from extrapolation and was greater for the pile-up methodfor which the detector acceptance was smaller.Fig. 4. – Inelastic cross section extrapolated to the full mass range (first column) and measuredwith the event counting method (red filled circle, second column) and with the pile-up countingmethod (red squares, last three columns for three data sets). Other LHC experimental resultsare also presented. Data are compared to different MC generators with uncertainty of 1 mb (notshown).MEASUREMENT OF pp INELASTIC CROSS SECTION AT√s = 7TeV AT LHC 1973. – Measurement of pp inelastic cross section at√s = 7TeV with ATLASTo measure the inelastic cross section ATLAS used events triggered by the MinimumBias Trigger Scintillator detector (MBTS), composed by two sets of scintillator counterssituated on the inner face of the endcap calorimeters in the forward region of ATLAS.MBTS, the Inner Detector (ID) and the calorimeters covered together the pseudorapidityrange of |η| < 4.9, corresponding to ξ > 5 · 10−6 and Mx > 15.7GeV. The detectoracceptance didn’t permit the detection of events at low mass. To extrapolate to the fullmass range, data were corrected using MC simulations, that had to be validated by datathemselves.Total inelastic cross section. The data considered in this analysis [6] were collectedduring a single fill in 2010 at a centre of mass energy of 7TeV, corresponding to anintegrated luminosity of 20.3 ± 0.7μb−1, and with an average number of collisions perbunch crossing of 0.01, triggered by MBTS. Two samples were used: the inclusive, inwhich MBTS activity was required on any of the two detector sides, and the single side,in which activity was required on one side only. The inelastic cross section in the detectoracceptance was evaluated as(6) σppinel(ξ > 5 · 10−6) =N −Nbkgtrigg · Lint ×1− fξ<5·10−6sel,where N and Nbkg were the number of selected and background events respectively, triggwas the trigger efficiency (about 99.98% from MC simulations), Lint was the integratedluminosity, fξ<5·10−6 was the contamination as defined for CMS and sel was the offlineevent selection efficiency (100% for ξ > 5 · 10−6). To reduce the uncertainties due tothe dependence on MC generators, the fraction of diffractive events (i.e. the relativediffraction dissociation cross section), defined as(7) fD =σSD + σDD + σCDσinel,was constrained by using the measurement of the ratio of single side to inclusive eventsRSS = NSS/Nincl as shown in fig. 5. The fraction of dissociative events was estimatedto be 26.9+2.5−1.0%.The final value of the inelastic cross section inside the ATLAS acceptance was(8) σppinel(ξ > 5 · 10−6) = 60.3± 0.05(stat)± 0.5(syst)± 2.1(lumi)mband the one extrapolated to the full kinematic range was(9) σppinel(7TeV) = 69.1± 2.4(exp)± 6.9(extr)mb.These results are shown in fig. 6, where the present results are compared to the mea-surements obtained with previous experiments [8] and with different theoretical predic-tions [9-11].198 S. VALENTINETTIFig. 5. – Measured value of RSS presented as the grey horizontal line (systematic uncertainties).Predictions of several models are shown as a function of assumed relative diffractive contribution(fD) indicated by the markers. The value of fD that best match the value of RSS is 26.9+2.5−1.0%when the Donnachie-Landshoff model (DL, blue filled circle) is used.Rapidity gap cross section. To extrapolate the inelastic cross section to the low massrange where ATLAS didn’t have rapidity acceptance, different MC models were used.ATLAS performed a measurement of the differential inelastic cross section as a functionof the rapidity gap [12] used to validate the MC models of diffractive processes. For thisanalysis data collected in a single run in 2010 at√s = 7TeV with a negligible pile-upcontamination and at an integrated luminosity of 7μb−1 were used. The events werestudied as a function of the largest forward rapidity gap defined as follow: the calorime-ters and ID acceptance was divided into unit-Δη rings starting from the edge of thedetector (η = ±4.9); the observable forward rapidity gap ΔηF was defined as the largestof the forward rapidity regions, from the edge of the detector to the nearest track orcalorimeter cluster, in which no ring presented activity. The differential inelastic crosssection as a function of ΔηF is shown in fig. 7 for particles with pT > 200MeV. The datawere compared with PYTHIA8 predictions showing separately the contributions of thedifferent non diffractive and diffractive processes. At small values of ΔηF , the cross sec-Fig. 6. – Inelastic cross section versus the centre of mass energy. The ATLAS measurement forξ > 5 · 10−6 is shown as red filled circle and compared with the predictions of different models.The extrapolation to the full kinematic range is presented as blue filled triangle. Data fromdifferent experiments are shown as filled circles for pp and unfilled circles for pp.MEASUREMENT OF pp INELASTIC CROSS SECTION AT√s = 7TeV AT LHC 199Fig. 7. – Inelastic cross section as function of the forward rapidity gap for particles with pT >200MeV. The data are compared with PYTHIA8 predictions showing separately the differentdiffractive components.tion was dominated by the non diffractive component, that was exponentially suppressedas ΔηF increases. For intermediate values of ΔηF (from two to five), the differential crosssection showed a plateau. At high ΔηF (> 5) the cross section presented a rise due tothe diffractive components only. No MC model predicted the data distribution perfectly:the comparison of different MCs over a large rapidity region allowed ATLAS to tune therelative fractions of the different diffractive components. The differential cross sectionintegrated on ξ over ATLAS mass range is presented in fig. 8: the data were comparedto the predictions for ξ > 5 · 10−6. ATLAS results were shown together with TOTEMones, covery the low mass region (ξ < 10−6) [13].4. – ConclusionsCMS and ATLAS measurements of the pp inelastic cross section at√s = 7TeV inthe detector acceptance range that corresponded to a final-state mass of Mx > 15.7GeVwere presented. Each measurement was extrapolated to the full LHC mass range andFig. 8. – Differential inelastic cross section integrated on ξ over the mass range of the detectoracceptance as a function of ξ. Data are compared to ATLAS results for the total inelastic crosssection in the same mass range and to TOTEM measurement for low mass range.200 S. VALENTINETTIcompared to different MC generators. ATLAS measurement of the differential crosssection as a function of the rapidity gap was also shown and used to validate the MCmodels used for the extrapolation to the full mass range. Even if the general trend ofdata was well described by the MCs, no model was currently able to reproduce the datain the whole η and ξ range for√s = 7TeV.REFERENCES[1] CMS Collaboration, JINST, 3 (2008) S08004.[2] ATLAS Collaboration, JINST, 3 (2008) S08003.[3] CMS Collaboration, Phys. Lett. B, 722 (2013) 5.[4] CMS Collaboration, CMS-PAS-QCD-11-002 cdsweb.cern.ch/record/1433413.[5] CMS Collaboration, CMS-PAS-FWD-11-001 cdsweb.cern.ch/record/1373466.[6] ATLAS Collaboration, Nat. Comm., 2 (2011) 463.[7] Donnachie A. and Landshoff P. V., Nucl. Phys. B, 244 (1984) 322.[8] Abbot B. et al. [D0 Coll], Phys. Lett. B, 440 (1998) 189; Abe F. et al. [CDF Coll],Phys. Rev. Lett., 81 (1998) 5278, Adloff C. et al. [H1 Coll], Eur. Phys. J. C, 24 (2002)517; Derrik M. et al. [ZEUS Coll], Phys. Lett. B, 369 (1996) 55.[9] Achilli M. et al., Phys Rev D., 84 (2011) 094009.[10] Block M. M. and Halzen F., Phys. Rev. D, 72 (2005) 036006.[11] Shuler G. A. and Sjostrand T., Phys. Rev. D, 49 (1994) 2257.[12] ATLAS Collaboration, Eur. Phys. J. C, 72 (2012) 1926.[13] TOTEM Collaboration, EPL, 96 (2011) 21002.

Measurement of pp inelastic cross section at √s = 7TeV at LHC

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Measurement of pp inelastic cross section at √s = 7TeV at LHC

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