Double diffractive cross-section measurement in the forward region at LHC

The first double diffractive cross-section measurement in the very forward region has been carried out by the TOTEM experiment at the LHC with center-of-mass energy of sqrt(s)=7 TeV. By utilizing the very forward TOTEM tracking detectors T1 and T2, which extend up to |eta|=6.5, a clean sample of double diffractive pp events was extracted. From these events, we measured the cross-section sigma_DD =(116 +- 25) mub for events where both diffractive systems have 4.7<|eta|_min<6.5 .

(Dated: May 11, 2014) The first double diffractive cross-section measurement in the very forward region has been carried out by the TOTEM experiment at the LHC with center-of-mass energy of √ s = 7 TeV.By utilizing the very forward TOTEM tracking detectors T1 and T2, which extend up to |η|=6.5, a clean sample of double diffractive pp events was extracted.From these events, we measured the cross-section σDD = (116 ± 25) µb for events where both diffractive systems have 4.7<|η|min<6.5.
Diffractive scattering represents a unique tool for investigating the dynamics of strong interactions and proton structure.These events are dominated by soft processes which cannot be calculated with perturbative QCD.Various model calculations predict diffractive cross-sections that are markedly different at the LHC energies [1][2][3].Double diffraction (DD) is the process in which two colliding hadrons dissociate into clusters of particles, and the interaction is mediated by an object with the quantum numbers of the vacuum.Experimentally, DD events are typically associated with a rapidity gap that is large compared to random multiplicity fluctuations.Rapidity gaps are exponentially suppressed in non-diffractive (ND) events [4], however when a detector is not able to detect particles with the transverse momentum (p T ) of a few hundred MeV, the identification of double diffractive events by means of rapidity gaps becomes very chal-lenging.The excellent p T acceptance of the TOTEM 50 detectors makes the experiment favorable for the mea-51 surement.Previous measurements of DD cross-section 52 are described in [5,6].In this novel measurement, the double diffractive crosssection was determined in the forward region.The method is as model-independent as possible.The DD events were selected by vetoing T1 tracks and requiring tracks in T2, hence selecting events that have two diffractive systems with 4.7<|η| min <6.5, where η min is the minimum pseudorapidy of all primary particles produced in the diffractive system.Although these events are only about 3% of the total σ DD , they provide a pure selection of DD events and the measurement is an important step towards determining if there is a rich resonance structure in the low mass region [9].To probe further, the η min range was divided into two sub-regions on each side, providing four subcategories for the measurement.
The analysis is structured in three steps.In the first step, the raw rate of double diffractive events is estimated: the selected sample is corrected for trigger efficiency, pile-up and T1 multiplicity, and the amount of background is determined.In the second step, the visible cross-section is calculated by correcting the raw rate for acceptance and efficiency to detect particles.In the last step, the visible cross-section is corrected so that both diffractive systems have 4.7<|η| min <6.5. in [11].The pile-up correction was calculated using the 134 formula:

This measurement uses data collected in
where j is the signature type, p pu =(1.5±0.4)% is the 136 pile-up correction factor for inelastic events [11], and 164 The number of ND events in the ND dominated control 165 sample, 2T2+2T1, has been determined as: where N 2T 2+2T 1 DD and N 2T 2+2T 1 SD were taken from MC for 167 the first iteration.Pythia was used as the default gen-3 ple, was calculated from MC as The number of ND events within the signal sample was estimated as where C j is the normalization factor deduced from the relative mismatch between the data and the total Pythia prediction in the signal sample: The SD background estimation starts from the calculation of the number of SD events in the SD dominated control sample, 1T2+0T1, by subtracting the number of other kind of events from the number of data events: was calculated with the ND estimation method and N 1T 2+0T 1 DD was taken from Pythia for the first iteration.To scale the number of SD events to the signal region, the ratio R j SD was calculated from data.The SD dominated data events that were used in the calculation of the ratio have exactly one leading proton seen by the RPs, in addition to the sample selections based on T2 and T1 tracks.By using the ratio the expected number of background SD events was calculated as The first estimate of σ DD was calculated with the ND, SD and CD background estimates described above.The background estimations were repeated with redefined the numbers of DD events were scaled with the ratio of σ measured DD /σ MC DD , and the numbers of SD and ND events were calculated using their estimation methods.
Next, the three steps were repeated until N 2T 2+0T 1  II.
The reliability of the background estimates was examined in the validation samples.In these samples, the total estimated number of events is consistent with the number of data events within the uncertainty of the estimate,   The visible DD cross-section was calculated using the where E is the experimental correction and the integrated  spect to QGSJET-II-03 [14] and Phojet was taken as the

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uncertainty.An additional correction was introduced for

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The TOTEM experiment[7]  is a dedicated experiment 54 to study diffraction, total cross-section and elastic scat-55 tering at the LHC.It has three subdetectors placed sym-56 metrically on both sides of the interaction point: Roman 57 Pot detectors to identify leading protons and T1 and T2 58 telescopes to detect charged particles in the forward re-59 gion.The most important detectors for this measure-60 ment are the T2 and T1 telescopes.T2 consists of Gas 61 Electron Multipliers that detect charged particles with 62 p T >40 MeV/c at pseudo-rapidities of 5.3<|η|<6.5[8].63 The T1 telescope consists of Cathode Strip Chambers 64 that measure charged particles with p T >100 MeV/c at 2 October 2011 at √ s=7 TeV during a low pile-up run with a special β * =90 m optics.The data were collected with the T2 minimum bias trigger.The trigger condition was that 3 out of 10 superpads in the same r − φ sector fired.A superpad consists of 3 radial and 5 azimuthal neighbouring pads, and it is sufficient that one out of 15 pads registered a signal for a superpad to be fired.After the offline reconstruction [10], the DD events were selected by requiring tracks in both T2 arms and no tracks in either of the T1 arms (2T2+0T1).T2 tracks with a χ 2 -fit probability smaller than 2% and tracks falling in the overlap region of two T2 quarters, i.e. tracks with 80 • <φ<100 • or 260 • <φ<280 • , were removed.The tracks in the overlap region were removed because simulation does not model well their response.In the paper, this full selection for visible cross-section is named I track .The four subcategories for the visible cross-section measurement were defined by the T2 track with minimum |η| on each side, |η + track | min and |η − track | min .The subcategory D11 track includes the events with 5.3<|η ± track | min <5.9,D22 track the events with 5.9<|η ± track | min <6.5, D12 track the events with 5.3<|η + track | min <5.9 and 5.9<|η − track | min <6.5, and D21 track the events with 5.9<|η + track | min <6.5 and 5.3<|η − track | min <5.9.Two additional samples were extracted for background estimation.A control sample for single diffractive (SD) events has at least one track in either of the T2 arms and no tracks in the opposite side T2 arm nor in T1 (1T2+0T1).A control sample for ND events has tracks in all arms of T2 and T1 detectors (2T2+2T1).Four additional exclusive data samples were defined for testing the background model validity: tracks in both arms of T2 123 and exactly in one arm of T1 (2T2+1T1), tracks in either 124 of T2 arms and in both T1 arms (1T2+2T1), tracks in 125 T2 and T1 in one side of the interaction point (1T2+1T1 126 same side) and tracks in T2 and T1 in the opposite side 127 of the interaction point (1T2+1T1 opposite side).Each 128 sample corresponds to one signature type j. 129 The number of selected data events was corrected for 130 trigger efficiency and pile-up.The trigger efficiency cor-131 rection c t was calculated from zero-bias triggered sample 132 in the bins of number of tracks.It is described in detail 133

137p
j is the correction for signature type changes due to 138 pile-up.The correction p j was determined by creating 139 a MC study of pile-up.A pool of signature types was 140 created by weighting each type with their probability 141 in the data.Then a pair was randomly selected, and 142 their signatures were combined.After repeating the se-143 lection and combination, the correction was calculated 144 as p j =N j combined /N j original .N j combined is the number of 145 selected combinations that have the combined signature 146 of j.The uncertainty in p j was determined by taking the 147 event type weights from Pythia 8 [12] and recalculating 148 p j .The corrected number of data events were calculated 149 with the formula N j = c t c j pu N j raw .150 The simulated T1 track multiplicity distribution pre-151 dicts a lower number of zero-track events than what was 152 observed in the data.The number of T1 tracks in the 153 simulation was corrected to match with the data by ran-154 domly selecting 10% (2%) of one-(two-)track events and 155 changing them to zero-track events.156 Three kinds of background were considered for the 157 analysis: ND, SD and central diffraction (CD).ND and 158 SD background estimation methods were developed to 159 minimize the model dependence, and the values of esti-160 mates were calculated iteratively.Since the CD back-161 ground is significantly smaller than the ND and SD ones, 162 its estimate (N CD ) was taken from simulation, using the 163 acceptance and σ CD =1.3 mb from Phojet [13].
final numbers of estimates in the I track control samples are shown in Table I, and the estimated numbers of background events in the signal sample are shown in Table

FIG. 1 .
FIG.1.Validation of background estimates for the full selection I track .Each plot shows the corrected number of events in data (black squares) and the combined estimate with background uncertainties.The combined estimate is the sum of ND estimate (cyan), CD estimate (green), SD estimate (blue) and DD estimate (red).The shaded area represents the total uncertainty of the background estimate. 212

215luminosity
L=(40.1±1.6)µb −1 .The experimental cor-216 rection includes the acceptance, the tracking and recon-217 struction efficiencies of T2 and T1 detectors, the fraction 218 of events with only neutral particles within detector ac-219 ceptance, and bin migration.The correction was esti-220 mated using Pythia, and the largest difference with re-4

TABLE I .
Estimated numbers of ND, SD, CD and DD events in the ND and SD background control samples.The numbers correspond to the full selection I track .

TABLE II .
Expected number of background events and observed number of data events passing the signal event selection 2T2+0T1.

TABLE III .
Summary of statistical and systematic uncertainties (µb).

TABLE IV .
Double diffractive cross-section measurements (µb) in the forward region.Both visible and true ηmin corrected cross-sections are given.The latter is compared to Pythia and Phojet predictions.Pythia estimate for total σDD=8.1 mb and Phojet estimate σDD=3.9mb.