Table Of ContentUnderlying Event Studies for LHC Energies
Gergely Gábor Barnaföldi , András G. Agócs ,† and Péter Lévai
∗ ∗ ∗
KFKIResearchInstituteforParticleandNuclearPhysicsoftheHAS,
∗
29-33Konkoly-ThegeM.Str.H-1121Budapest,Hungary
1 †EötvösUniversity,
1 1/APázmányPéterSétány,H-1117Budapest,Hungary
0
2
Abstract.
n
UnderlyingeventwasoriginallydefinedbytheCDFcollaborationdecadesago.Hereweimprove
a
J theoriginaldefinitiontoextendouranalysisforeventswithmultiple-jets.Weintroduceadefinition
forsurroundingrings/beltsandbasedonthisdefinitionthejet-andsurrounding-belt-excludedareas
1
2 willprovideagoodunderlyingeventdefinition.We inverstigateourdefinitionviathemultiplicity
inthedefinedgeometry.Inparallel,meantransversemomentaoftheseareasalsostudiedinproton-
] protoncollisionsat√s=7TeVLHCenergy.
h
p Keywords: underlyingevent,jetphysics,LHC,proton-protoncollisions
- PACS: 13.85.-t,24.85.+p,25.75.-q,13.87.-a,24.10.Lx
p
e
h
[ INTRODUCTION
1
v Underlying event (UE) was originally defined by the CDF Collaboration [1] and used
5
to investigate properties of the remaining of the events, after jets were identified and
5
1 removed from there. The CDF definition of the underlying event is a simple tool in
4 order to work, however detailed structure or information on off-jet particles cannot be
.
1 obtained. On the other hand the definition is not capable to analyze more than 2-jet
0
structures.Thismotivateus todevelopanewdefinitionfortheunderlyingevent.
1
1 To enhance the informationcontent to be extracted from underlying events,we mod-
: ifiedtheaboveCDF’sdefinitionintroducingmultiplesurroundingbelts(SB)aroundthe
v
i identified jets [2, 3]. This new definition is immediately leads a more detailed analysis
X
of the underlying event, even in case of multiple jets. On the other hand, as a specific
r
a case of our new method, one can get the originally extracted physical observables cor-
respondingto theanalysisbased ontheCDF-definition.
In this short contribution we present the basic properties of the two ways of defining
underlying event. We recall the original CDF-based and our new definition of the
underlying events, which will be compared. We used two physical quantities for our
comparison: (i) the average hadron multiplicity within the defined areas and (ii) the
mean transverse momenta versus multiplicity in the given regions. Quantities were
investigatedforbothdefinitioninparallel.
Our analysis is based on jet production and identification in proton-proton collisions
at7TeV.WeusedtheLHC10e14jet-jetsamplegeneratedbyPYTHIA6.2[4]framework
withcone-based UA1[5]jet finder.
GENERALIZED DEFINITION OF THE UNDERLYING EVENT
Anydefinitionofunderlyingeventshouldstronglydependonajet-identificationmethod
applied in the analysis. There are various state-of-the-art development on this direc-
tion [7, 8], which are very promising. On the other hand there are still a problematics
of these definitions – the strong process dependence. E.g. changing from proton-proton
tonucleus-nucleuscollisionsneed tore-tunethepropertiesofthealgorithmsinorderto
findand separate jetsand thebaseline/backgroundofeach event[9].
The CDF-based underlying event definition corresponds to the jet identification in
caseofaone-(ortwo-)jetevents.Nearsidejetseasilydefinethetowardandtheopposite
awayregionsoftheeventgeometry.OuroriginalconceptwastoimprovetheCDF-based
definitionon atwo-foldedway:
• to develope an underlying event definition which is capable to handle multijet
events.
• to investigate the surrounding areas around identified jets, even without major
changes ofthejet-findigparameters in acaseofnucleus-nucleuscollision.
Theserequirementsareledustothedefinitionofsurroundingbeltsonthebasisofthe
event-backgroundsuchas’underlyingevent’,whichcompletelysatisfyourrequirements
above.
Inourmethod,weareusingjet-findingalgorithmsalso.Wedefinejets,thanbasedon
the physical properties of the concentrical surrounding belts and the remaining particle
multiplicities, a better background or baseline can be provided. On the other hand the
analysisofthesurroundingareaaroundtheidentifiedjets,canevengivefeedbackonthe
goodnessofthejetfinding parameters.
On Fig. 1 the visual comparison of the two definitions can be seen. Leftside of
the figure is for the CDF-based definition, the rightsidedisplays the SB-based one.
The two definitions can be summarized in a following ways, using the azimuth, F and
(pseudo)rapidity,(h or)yplane:
CDF-baseddefinition of the underlying event is based on the subtraction of two areas
of the whole measured acceptance: one around the identified near jet (toward
region) and another to the opposite (away) direction. Both regions are DF D h -
×
slices of the measured acceptance around the near jet and to the opposite, with the
fullD h range andDF = 60o inazimuth.
±
SB-based definition usesallidentifiedjetsoftheeventtosubtractthemfromtheback-
ground. Each jet can have an approximate dial-like area, around which concentric
bands (or rings) can be defined. If a jet cone angle, R= pDF 2+D h 2 is given, a
first ’SB ’ and a second ’SB ’ surrounding belt can be defined for any jet with the
1 2
thicknesses of d R and d R , respectively. Generally, d R =0.1 with respect
SB1 SB2 SBi
to the R 0.5 1 values. It is easy to see our underlying event definition is no
≈ −
longerjet-numberdependent.
Furthermore, increasing d R values, similar (but not the same) area can be cov-
SBi
ered as in the original CDF-based definition. In this way the two model can be
comparabletoo.
FIGURE1. Theschematicviewoftheunderlyingevent(UE)definedbytheCDF(leftpanel)andthe
surroundingbelts(SB,rightpanel).Detailsareinthetext.(Coloronline.)
Now we investigatethe basic properties of the areas and parallel the physical quanti-
tiesfortheselectedregions.
COMPARISON OF UE DEFINITIONS
Here we compare the details of the CDF- and surrounding belt (SB) based underlying
eventdefinitions.ForourtestweusedPYTHIA6-simulated[4]proton-protoncollisions
(Perugia-0 tune [6]), namely LHC10e14 jet-jet at 7 TeV center-of-mass energy with
150,000 events. This sample contains jets identified by UA1 method [5]. We restricted
ouranalysisto thesettingsof p =10GeV/cand p =20GeV/c.
THardMin THardMax
Primarily we investigated the multiplicities of various geometrical regions of the
generated events based on the full sample. After applying UA1 jet finding algorithm to
identifyjets,wecomparedtheselectedareas usingbothCDF- andSB-based definitions
of the underlying events. On left panel of Fig. 2 we plotted the multiplicities, N of the
i
CDF-selectedareas versusthetotaleventmultiplicity.HereN refers forfollowings:the
i
multiplicitiesoftheidentified’leading/nearjet’(bluesquares),thejet-excluded’toward’
area (greendots), the ’away’ side area to the opposite direction (purpledots), and the
CDF-defined underlying is event, ’transverse’ (pinkdots). The rightsideof the Fig. 2
standsfortheSB-basedunderlyingeventdefinitionwithmoreareas:themultiplicitiesof
theidentifiedleadingjetbluesquares),theawaysidejet (bluedots),multiplicityforthe
surrounding belts, SB , SB , SB , and SB are openredsquares,open
lead,1 lead,2 away,1 away,2
purpletriangles,openredcircles,openpurplediamondsrespectively. Finally orange
Ni120 leading jet Ni120 leading jet
away jet
100 taowwaayr dresg wio/on j(eCtDF) 100 SSSBBBlleeaadd,,12
SBaway,1
80 transverse 80 UEa2way,2
60 60
40 40
20 20
0 0
0 20 40 60 80 100 120 0 20 40 60 80 100 120
N N
tot tot
FIGURE 2. The multiplicity, N for the selected areas as the multiplicity of the total event, N .
i tot
Underlying event regions are defined on the leftpanelfor the CDF-based and on the rightpanelfor
thesurroundingbeltbaseddefinitions.Moredetailsareinthetext.(Coloronline.)
crossesdenote multiplicityfor the newly defined underlying eventUE outside all jets.
2
(Note,all colorin accordance withtheareas ofFig 1above.)
Fig. 2 shows multiplicity in almost all regions: N increases almost linearly with the
i
totalmultiplicity,intheN <120regionoftheeventforbothcases.IncaseoftheCDF-
tot
based definition, the away region gives the biggest contribution, and the jet belongs to
the smallest one. The transverse (underlying event) area lies between the two extremal
contribution. Moreover, it is interesting to see, after excluding the jet from the toward
region, the remaining area has almost the same multiplicity as the underlying event.
This shows the goodness of the jet finding algorithms and the "safety" of the CDF-
based underlying event definition (e.g. 1/3 of the whole acceptance far from any jet-
contaminatedareas).
ThemultiplicityrelationsoftheSB-baseddefinitiondiffersfromtheCDF-based.The
near jet has the same contribution, away side jet and the SBs have small fraction from
i
theN –duetothesmallareas.Ontheotherhand,thenewlydefinedunderlyingevent,
tot
UE dominatestheeventmultiplicitysinceit hasalmostthewholeacceptance.
2
In general themultiplicityfraction of thedefined areas are almostproportional to the
geometricalsurface,onlythejet-contentpartviolatesthisdependence,asFig.2displays.
Thus,theSB-basedUE haslargermultiplicitycomparingtotheCDF-basedone,which
2
mightgivesbetterstatisticsforan underlyingeventanalysis.
Secondly, the mean transverse momentum p of the selected areas is investigated
T
h i
includingespeciallytheunderlyingevent.Weplottedthe p vs.themultiplicityofthe
T
h i
totalevent,N andthe p vs.multiplicityoftheCDF-basedandSB-basedunderlying
tot T
h i
events.
On Fig. 3 we display the p vs. the multiplicity of the total event, N for both
T tot
h i
CDF-based (leftpanel) and SB-based (rightpanel) underlying event. We use the same
colorand mark encodingfortheselected regionsoftheeventas on Fig.2 above.
We found the mean-p distributions of the regions are similar in proton-proton col-
T
lisions. The identified leading jet has the highest values p 8 9 GeV/c,
T leading jet
h i ∼ −
which are decreasing to p 5 6 GeV/c as going to larger N , for both
T leading jet tot
h i ∼ −
[GeV/c]><piT 167890 lttateoorwaawwnadaasyirrnv ddrgeess rgj sewieot/on j(eCtDF) [GeV/c]><piT 167890 SSSSUlaewBBBBEaallaa2deewwyaaiaandd yyj,,g12,,e12 tjet
5 5
4 4
3 3
2 2
1 1
0 0
0 20 40 60 80 100 120 0 20 40 60 80 100 120
N N
tot tot
FIGURE3. Theaveragetransversemomentaversusthe totalmultiplicityoftheevents,N . Regions
tot
definedon theleftpanelarefor the CDF-basedand ontherightpanelare forthe SB-based definitions.
Moredetailsareinthetext.(Coloronline.)
definition’s cases. The mean-p for both underlying event cases are the same with the
T
constantvalue p 0.5GeV/c. Forthe CDF-based definitionthisis similarto the
T UEi
h i ∼
jet-excluded toward area also, and for the SB-based definition surrounding belts, SB
i
have also similar, but a slightly higher p 1.0 GeV/c. Differences between the
T SBi
h i ∼
left and the right panels are originating from the handling of the near and away side
jet. CDF-based definition contains the jet to the near (leading) direction, and fully or
partiallytotheoppositeawayregion: p 2 3GeV/cand p 1.5 2
T towards T away
h i ∼ − h i ∼ −
GeV/c. SB-based case since away side jets were also identified, p 2 3
T away jet
h i ∼ −
GeV/c.Wecanstategenerally,bothunderlying-eventdefinitiongivethesameresultand
they are differ only in the separation (or inclusion) of leading or away side jet to the
givenareas.
[GeV/c]><piT 167890 lttateoorwaawwnadaasyirrnv ddrgeess rgj sewieot/on j(eCtDF) [GeV/c]><piT 167890 SSSSUlaewBBBBEaallaa2deewwyaaiaandd yyj,,g12,,e12 tjet
5 5
4 4
3 3
2 2
1 1
0 0
0 20 40 60 80 100 120 0 20 40 60 80 100 120
N N
UE/CDF UE2
FIGURE4. Theaveragetransversemomentaversusthemultiplicityoftheunderlyingevens.Regions
are defined on theleftpanelfor the CDF UE and on therightpanelfor the surroundingbelt based UE
definitions.Moredetailsareinthetext.(Coloronline.)
FinallyonFig. 4wecomparedmean-p valuesvs.totheself-definition-givenunder-
T
lyingevents:CDF-based UE,N onleftpanelandSB-based N onrightpanel.
UE/CDF UE2
(Colors and marks are the same as above figures.) Here, the comparison shows slight
difference between the panels. A stronger decrease in the highest p -content regions
T
h i
present compared to Fig. 3. Furthermore, changing from N to N and N the
tot UE/CDF UE2
separation of the curves are more clear in both cases, especially at the largest p val-
T
h i
ues. In parallel themean-p valuesfortheunderlyingeventare almostthesamefor the
T
averagemultiplicityeventsand slightlyhigherfortherare ones.
CONCLUSIONS
Westudiedournew underlyingeventdefinitionin√s=7 TeVproton-protoncollisions
with 150,000 events. We investigatedand compared the multiplicitiesand the mean-p
T
vs.multiplicitiesfortheCDF-based and ourSB-based definition.
Wefoundthemultiplicityfractionofthedefinedregionsarealmostproportionaltothe
geometricalsurface,onlythejet-contentpartdiffers,duetotheseparation(orinclusion)
of the leading or away side jets. The SB-based underlying event, UE found to have
2
largermultiplicitycomparingtotheCDF-based,N one,whichmightgivesbetter
UE/CDF
statisticsfortheunderlyingeventanalysis.
The mean-p vs. N analysis led us to compare both definition on the same level.
T tot
We got the same dependence of the underlying event for both, CDF- and SB-based
cases. On theotherhand,theabovementionedjet and near/away-area handlingleads to
differences.
Finally, we compared our definitions by the mean-p vs. the self-defined underlying
T
eventmultiplicities,namelyN andN .Ourresultshaveshownbothdefinition
UE/CDF UE2
isreliable,and–duetothegeneralizeddefinitionofthesurroundingbeltbasedunderly-
ingevent–multiplejetsanddetailedanalysisofthesurroundingareascanbeperformed
inthefuture.
ACKNOWLEDGMENTS
ThisworkwassupportedbyHungarianOTKANK77816,PD73596andEötvösUniver-
sity. Authors (GGB & PL) are appreciate the local support by the UNAM, Mexico and
GGBthanksfortheJánosBolyaiResearch Scholarship oftheHAS.
REFERENCES
1. A.A.Affolderetal.[CDFCollaboration],Phys.Rev.D65,092002(2002).
2. P.LévaiandA.G.Agócs,PoSEPS-HEP2009,472(2009).
3. A. G. Agócs,G. G. Barnaföldi,andP. Lévai,arXiv:1011.5363[hep-ph],acceptedforthe Proc. of
HotQuarks2010Workshop.
4. T.Sjostrand,S.Mrenna,andP.Z.Skands,JHEP0605,026(2006).
5. G.Arnisonetal.[UA1Collaboration],CERN-EP/83-118,Phys.Lett.B132,214(1983).
6. P.Z.Skands,MCNET-10-08,CERN-PH-TH-2010-113,Phys.Rev.D82,074018(2010).
7. G.P.Salam,Eur.Phys.J.C67,637-686(2010).
8. M.Cacciari,G.P.Salam,S.Sapeta,JHEP1004,065(2010).
9. S.Salur,Nucl.Phys.A830,139(2009).
