Central Diffraction in Proton-Proton Collisions at $\sqrt{s}=7$\,TeV with ALICE at LHC

0.25 MB·English

by  Felix Reidt

Central Diffraction in Proton-Proton Collisions
√
at s = 7 TeV with ALICE at LHC
Felix Reidt
for the ALICE Collaboration
PhysikalischesInstitut,ImNeuenheimerFeld226,69120Heidelberg,Germany
3
1
0 Abstract. A double-gap topology is used for filtering central-diffractive events from a proton-
√
2 proton minimum-bias data sample at a centre-of-mass energy s=7TeV. This topology is

n defined by particle activity in the ALICE central barrel and absence of particle activity
a outside. The fraction of events satisfying the double-gap requirement RDG is found to be
J 7.63±0.02(stat.)±0.87(syst.)×10−4.Thebackgroundofthisdouble-gapfractionisestimatedby

5 studying the contributions of non-diffractive, single- and double-diffractive dissociation processes
1 asmodelledbyMonteCarloeventgenerators,andisfoundtobeabout10%.

Keywords: Double-GapFraction,CentralDiffraction,ALICE,LHC
]
x PACS: 12.40.Nn,12.38.Aw
e
-
p
e INTRODUCTION
h
[
The ALICE experiment [1] at the Large Hadron Collider (LHC) consists of the cen-
1
tral barrel in the pseudorapidity range of −0.9 < η < 0.9 and a muon spectrometer at
v
7 −2.5<η <−4.0. Additional detectors for triggering and event classification are avail-
0 able in the range −3.7 < η < 5.1. Double-gap events can be identified by requiring
5
activity in the central barrel and by the absence of activity in the more forward regions.
3
. At LHC energies, about 25mb, i.e. one third of the inelastic hadronic cross section, are
1
0 of diffractive origin as e.g. measured by ALICE in a previous study [2]. These diffrac-
3 tiveeventsaremediatedbyaPomeron[3]exchange.Central-diffractiveeventsshowthe
1
characteristic double-gap topology due to their double-Pomeron exchange. The central-
:
v
diffractivecrosssectionispredictedtobeabout800µb[4].
i
X
r
a ALICE DETECTORS
ThisanalysisfocusesonthecentralbarrelofALICE.TheinnermostdetectoristheInner
Tracking System (ITS) which consists of six layers. The first two layers are Silicon
Pixel Detectors (SPD) covering −2.0<η <2.0 and −1.4<η <1.4. The ITS is used
for vertex reconstruction and, together with the Time-Projection Chamber (TPC), for
tracking. In the forward region, the VZERO scintillator hodoscope for trigger purposes
is located at −3.7 < η < −1.7 and 2.8 < η < 5.1. In addition multiplicity-density
measurements can be performed by the Forward-Multiplicity Detector (FMD) situated
at−3.4<η <−1.7and1.7<η <5.0.Itisasiliconstripdetectorbuiltformultiplicity
density measurements. The low material budget and low magnetic field of 0.5T lead
to a low single particle p -threshold of 17MeV/c for 50% detection efficiency. Further
T
detailsontheALICEdetectorsystemcanbefoundin [1].
DOUBLE-GAP TOPOLOGY
Central diffraction results from the two incoming protons interacting via double-
Pomeron fusion. The centrally produced system is accompanied by large rapidity
gaps. In central diffractive processes, the protons can either stay intact or break up.
The double-gap topology can be exploited as filter for central diffraction. However, at
topology level, Pomeron, Reggeon or photon exchanges cannot be distinguished. The
contribution from Reggeon exchanges is assumed to be comparable to lower energies,
while the Pomeron contribution is expected to increase with energy. The ALICE pseu-
dorapiditycoverageandthecorrespondinggapdefinitionsareshowninFig.1.Thegaps
are defined by the absence of activity induced by the passage of particles in the trigger
signals of VZERO, FMD and SPD in the pseudorapidity ranges of −3.7 < η < −0.9
and 0.9 < η < 5.1. The central activity is determined using SPD in the range of
−0.9<η <0.9. This double-gap configuration is a trade-off of large gaps suppressing
backgroundandareasonablecoverageforthecentrallyproducedsignal.
VZERO-C VZERO-A
-3.7 to -1.7 2.8 to 5.1
FMD-C SPD outer layer FMD-A
-3.4 to -1.7 -1.5 to 1.5 1.7 to 5.0
SPD inner layer
-2 to 2
-4 -3 -2 -1 0 1 2 3 4 5 6 η
central
gap C gap A
activity
-3.7<η<-0.9 0.9<η<5.1
-0.9<η<0.9
FIGURE1. ALICEη coveragebyindividualdetectorsusedinthisstudyanddouble-gaptopology.
ANALYSIS
√
The double-gap analysis is based on minimum-bias data at s = 7TeV taken during
the 2010 run. This data set was acquired at low luminosity and low event pileup.
The minimum-bias trigger is the logical-OR of the two VZERO and the SPD trigger
signal, called MB . A total of 300 million MB triggers are used for this analysis.
or or
Luminositydeterminationwithinthisdatasampleispossiblebyusingtheminimum-bias
cross section σ available from van-der-Meer scans and the MB trigger count.
MBand and
The MB trigger is derived from the coincidence of the two VZERO subdetectors.
and
Furthermore, a monitoring of the gap-fraction for all involved detectors is done on a
run-by-runbasis.Runsdeviatingbymorethan3σ fromthedata-takingperiodmeanare
rejected. In addition to the run selection, an event selection is applied. Furthermore, a
vertex within ±4cm of the interaction point in direction of the beam axis is required to
ensure a continuous gap coverage. Moreover, pileup is rejected by identifying multiple
verticesreconstructedfromtheSPDinformation.Thenumberofdouble-gapeventsN
DG
andthenumberofMB eventsN areanalysedonarun-by-runbasis.
AND MBand
First data-driven systematic-error estimates are available. The event selection can
influencetheratioofN toN ,duetothevertexrequirement.Forexample,double-
DG AND
diffractivedissociativeeventsdonotnecessarilycontainavertex.Thiseffectisestimated
tobeabout6%.Furthermore,thecentralactivitycanbemissed,leadingtoabiastolower
cross section of approximately 5%. On the contrary, missing a particle in the gap shifts
thecrosssectionabout5%towardshighervalues.Theuncorrelatederrorcontributionis
estimatedtobe1.4%fromthespreadofthedata-takingperiodmeans.
DOUBLE-GAP FRACTION
Thedouble-gapfractionR isameasureforthepotentialamountofcentral-diffractive
DG
eventswithintheminimum-biasdata.IntheALICEproton-protondataitisfoundtobe
N
R ( 2.8 ,±0.9, 4.2 )= DG =(7.63±0.02(stat.)±0.95(syst.))·10−4 . (1)
DG
(cid:124)(cid:123)(cid:122)(cid:125) (cid:124)(cid:123)(cid:122)(cid:125) (cid:124)(cid:123)(cid:122)(cid:125) NMBand
∆η1 ηact. ∆η2
In Fig. 2, the fraction R is shown as a function of the run number. It shows a very
DG
uniformbehaviourandnoapparentdependenceontherunconditions.
)
ap not corrected for detector pp, s= 7 TeV
G 0.001 acceptance and efficiency
e
bl
u
o0.0008
D
D
P
S0.0006
-
D
M
single run (stat. error) period mean (stat. error)
F
0-0.0004 period mean (sys. error) combined mean (stat. error)
V
( combined mean (sys. error)
G
RD0.0002 Gap Activity Gap
06/09/2012 Δη=2.8 Δη=4.2
η
0 -0.9 0.9 ×103
116 118 120 122 124 126 128 130 132
systematic error estimated from the spread of the Run Number
period means, doesn't include detector effects
√
FIGURE2. Double-gapfractioninproton-protoncollisionsat s=7TeV.
COMPARISON TO MONTE CARLO GENERATORS
The data simulated using PHOJET and PYTHIA6 were used in order to estimate the
amountofnon-diffractive,singleanddoublediffractive-dissociativeinteractionshaving
adouble-gaptopology.BothPHOJETandPYTHIA6areusedwithatuneforsingleand
doublediffractivedissociationbutwithoutcentraldiffraction[2]inordertogeneratethe
datasetforcomparisons.However,furthertuningisneededsincegeneratormultiplicity
distributions are too low by up to 30% and 40% in the central and forward region,
respectively. A first estimate of the bias caused by this deviation assumes the gaps to
be uncorrelated and the multiplicity to be distributed according to a negative binomial
distribution. This estimate leads to a reduction of the double-gap fraction seen in MC
dataasshowninTable1.Bothgeneratorsareunabletodescribethedouble-gapfraction
TABLE 1. Double-gap fraction in Monte Carlo datasets, before
andafterfirsttuning
R (PYTHIA6) R (PHOJET)
DG DG
nottunedformultiplicity 3.4·10−4 0.5·10−4
firsttuning 1.0·10−4 0.2·10−4
7.63·10−4observedinALICEdata.Thesefindingscanbeinterpretedashintsforcentral
diffraction, since the simulated background amounts only to about 10%. However, the
ALICE acceptance and efficiency for central-diffractive events need to be studied using
MonteCarlodata.
SUMMARY AND OUTLOOK
ALICE is well suited to measure central diffraction due to the low p -threshold
T
and the low luminosity data set. The double-gap fraction derived from data of
7.63±0.02(stat.)±0.87(syst.)·10−4 is in excess by about a factor 10 over the
expected background simulated by PYTHIA6 and PHOJET. The evaluation of sys-
tematic uncertainties will be refined in the future by tuning the MC simulations of the
backgroundsandthegapsignalwillbeturnedintoacrosssectionmeasurementmaking
the result more universal. Further studies will determine the properties of this signal by
studyingthegap-sizedependenceofthedouble-gapfractionandspecificstates.
ACKNOWLEDGMENTS
This work is supported by the German Federal Ministry of Education and Research
underpromotionalreference06HD197DandbyWP8ofthehadronphysicsprogramme
ofthe7th and8th EUprogrammeperiod.
REFERENCES
1. K.Aamodtetal.(ALICECollaboration),JINST 3,S08002(2008).
2. B.Abelevetal.(ALICECollaboration)(2012),arXiv:1208.4968.
3. S. Donnachie, G. Dosch, P. Landshoff, and O. Nachtmann, Pomeron Physics and QCD, Cambridge
UniversityPress,2005.
4. R.CiesielskiandK.Goulianos(2012),arxiv:1205.1446.