Heavy flavour results from ATLAS

0.17 MB·English

by  Paul Bell

Heavy flavour results from ATLAS
P.J.Bella
Universite´deGene`ve,1211Gene`ve4,Switzerland
Abstract. Aselectionofheavy-flavourphysicsresultsfromtheATLASexperimentispresented, basedon
datacollectedinproton-protoncollisionsattheLHCduring2010.Differentialcross-sectionsfortheproduction
2
ofheavyflavours,charmoniumandbottomoniumstatesandD-mesonsarepresentedandcomparedtovarious
1
theoreticalmodels.ResultsofB-hadronlifetimemeasurementsarealsoreported.
0
2
Keywords. HeavyFlavour
n
a
J
4 1 Introduction V] 1
2 Thegoalsoftheheavy-flavourphysicsprogrammeatAT- b / Ge ∫s L = d 7t =Te 1V.4 pb-1 EMlueoctnr odna tdaa 2ta0 120010
x] LduAcStioanrewtiothtienstththeeSotraentidcaarldmMooddeelsl(fSoMrh)eaanvdy-tfloasveoaurcrhpfroor- µ [pT10-1 FFOONNLLLL ((NNLLOO)+NLL)
-e new physics through rare decays or new sources of CP σ / d PPyotwhHiae ×g +0P.4y9t hniaorm.
p violation.Theseproceedingspresentanon-exhaustivese- d PowHeg+Herwig
e lectionofanalysescompletedduring2011basedon2010 10-2
h data,dividedintocross-sectionmeasurements(Section2)
[
andlifetimemeasurements(Section3).
1 Details of the ATLAS detector may be found in [1]. 10-3 |η|<2.0 excluding 1.37<|η|<1.52
v The sub-detectorsof greatest importance to the analyses
3 presentedhereare in the InnerDetector (ID)tracker and NLL ATLAS
3 Muon Spectrometer (MS) systems. In addition, in many FO1.5
9 σ
4 cases the data collectionhas reliedon specific B-physics σ/
triggerselections implementedin the HigherLevelTrig-
.
1 ger(HLT). 1
0
2
1
2 Cross-sectionmeasurements
: 0.5
v
i 2.1 Heavy-flavourcross-sectionmeasurements
X 8 10 12 14 16 18 20 22 24 26
p [GeV]
ar At low transverse momentum (pT . 30 GeV) the inclu- T
sivespectraofchargedelectronsormuonsaredominated Fig. 1. Electron and muon differential cross-sections from
by decays of heavy-flavour hadrons. Using 1.13 pb−1 of heavy-flavourproductionasafunctionofp for|η|<2.0exclud-
T
early2010data,forwhichnon-isolatedlow-p electrons ing1.37 < |η| < 1.52.Theratioofthemeasuredandpredicted
T
were recorded without any HLT selection bias, electron cross-sectionstotheFONLLcalculationisgiveninthebottom.
and muonsignals are extractedfrom the dominantback- ThePYTHIA(LO)cross-sectionsarenormalisedtothedata.
groundof QCD fakes, conversions(in the electron case)
and pion and kaon decays in flight (in the muon case).
2.2 Quarkoniumcross-sectionmeasurements
Thewell-understoodtheoreticalpredictionforthecontri-
butionfromW/Z/γ∗productionisthensubtractedtoleave
Despitebeingamongthemoststudiedoftheboundquark
theheavy-flavourcomponentofthesignal.
systems, there is still no clear understanding of the pro-
The results [2] for electrons and muons within 7 <
duction mechanisms for quarkonium states like the J/ψ
p < 26GeVand|η| < 2.0(excluding1.37 < |η| < 1.52)
T
andtheΥthatcanconsistentlyexplainboththeproduction
areshowninFig.1,comparedtothepredictionsfromthe
cross-section and spin alignment measurements in e+e−,
FONLLtheoreticalframework,inwhichtheheavyquark
hadron and heavy ion collisions. Data from the LHC al-
productioncross-sectioniscalculatedinpQCDbymatch-
lowtestsoftheoreticalmodelsofquarkoniumproduction
ingtheFixedOrderNLOtermswithNLLhigh-p resum-
T
inanewenergyregime.
mation. Good agreementwith the FONLL calculation is
TheinclusiveJ/ψandΥ(1S)productioncross-sections
observed,includingthesensitivitytotheNLLterms.
aremeasuredatATLASinthedi-muondecaychannelus-
a e-mail:[email protected] ing2.3pb−1and1.1pb−1of2010data,respectively[3,4].
EPJWebofConferences
The true numbersof J/ψ or Υ(1S)candidatesare recov-
V]102
ered from the observeddi-muon pairs by applying event e Inclusive cross-section
G ATLAS |y |<0.75
wficeiiegnhctystfoacutnofroslbdetihnegrdeestpeormnsieneodftahlemdoestteecntotirr,ewlyitfhrothmetehfe- y [nb/ 10 SCpMinS- alignm|yJeJ//ψψn|t< e1n.2velope
data.(MonteCarloisusedtoprovidetriggerefficienciesin d 1
T
p
finerbinningthanwouldbepossiblewiththedatastatis- d
tics,withscalefactorsbeingobtainedfromdata.)TheJ/ψ σ2/d10-1
andΥ(1S)yieldsarethendeterminedfromfitstothedi- µ+-)10-2
muonmassinbinsof pTandη. µ→ ATLAS
In the J/ψ analysis, a correction for the detector ac- ψ10-3 ∫s= 7 TeV
J/ L dt = 2.2 pb-1
ceptance (the probability for muons to fall into the fidu- Br(
cialvolumeofdetector)isalsoappliedinordertoobtain 7 8910 20 30 40 50
a total cross-section measurement. Since this correction pJ/ψ [GeV]
T
dependsontheunknownspinalignmentoftheJ/ψ,anen-
velopeofallpossiblepolarisationassumptionsistakenas V] Data 2010
asunreamddeintitonnoalacthceeoprteatniccealcuonrrceecrttiaoinntiys.aFpoprlitehde,aΥn(d1tSh)emfideua-- b/Ge 102 ATLAS NRQCD, Pythia8
p
cspiaalccerowshs-esreectthioenmisuroenpsoartreeddienteacrteesdtr,ifcrteeedorefgainoynaosfspuhmaspe- µ-) [ (CdiSreMct oNnlLy)O
tions about the spin alignment. The J/ψ and Υ(1S) in- µ+ 10
clusive cross-sections in an example rapidity (y) bin are →
showninFig.2.FortheΥ(1S)case,theresultsarecom- S)
paredtopredictionsfromPYTHIA8.135usingtheNRQCD 1
(
frameworkandtoNLOQCDcalculationsasimplemented ϒ( 1
in MCFM. Significant deviations from the data are seen BR |yϒ(1S)|<1.2
iftnoeecbdoo-ndthtorwipbrnuetdferiocatbmioounhtsig,ahthfeaorcutmogrhasoMsfstCwtaFotMesatwdtohheeiscThneovwatatirsnocenls.utidmeaatnedy × dy dpT10-1 |ηsµ=|7< T2e.V5,,∫ pLµTd>t=41 .1G3 epVb-1
σ/
OftheJ/ψinclusivecross-section,thenon-promptfrac- 2d 0 5 10 15 20 25 30
tion coming from B-hadrondecays can be identified ex- pϒ(1S) [GeV]
perimentallyduetotheassociateddisplacementoftheJ/ψ T
vertexinthetransverseplane,L ,duetothelonglifetime
xy Fig.2.(Top) Inclusive J/ψproduction cross-sectionasafunc-
of the parent B-hadron. A simultaneous unbinned maxi-
tionof p fortheexample J/ψrapiditybinof|y| < 0.75,com-
mumlikelihoodfittothe J/ψinvariantmassandpseudo- T
pared to the equivalent results from CMS. (Bottom) Inclusive
properdecaytime,τ = LxymPJ/DψG/pTJ/ψ, isusedtoextract Υ(1S) production cross-section asa function of pT for theex-
thisfractionfromthedata,allowingthepromptandnon- ampleΥ(1S)rapiditybin|y| < 1.2,forthegivenkinematicac-
promptcross-sectionstobeobtained.Theresultsinanex- ceptanceofthetwomuons.Theresultiscomparedtothecolour-
amplerapiditybinareshowninFig.3.Forthenon-prompt singletmodel(CSM)NLOpredictionforwhichtheshadedarea
component,good agreementis seen with the predictions showsthechangeinthepredictionwhenvaryingtherenormal-
ofFONLL.Forthepromptcomponent,thedataarecon- isation and factorisation scales (nominally m ) by a factor of
T
sistentwithNNLO∗calculations. two.TheCSMNLOcalculationaccountsonlyfordirectproduc-
tionofΥ(1S)mesonsandexcludesanyfeed-downfromexcited
states.TheNRQCDpredictionasimplementedinPYTHIA8is
alsoshownforaparticularchoiceofparameters.
2.3 Dmesoncross-sectionmeasurements
Using an integratedluminosityof1.1 nb−1, D∗±, D± and D± (yield 1546± 81) and D± (yield 304± 51) mesons,
S
D± charmed mesons with p > 3.5 GeV and |y| < 2.1 massesconsistentwithPDGworldaveragesarefound.
s T
are reconstructed using tracks measured in the ATLAS UsingMonteCarlotocorrectforthedetectorresponse,
ID[5].TakingtheexampleoftheD∗±,whichisidentified the D∗± and D± production cross-sections (in the kine-
in the decaychannel D∗± → D0π±s → (K−π+)π±s, where maticacceptancefortheD-mesonsof pT > 3.5GeVand
the π± is the slow pion in the D∗± decay frame, pairs of |y| < 2.1) are found, as shown in in Fig. 4 (Bottom) for
s
oppositely-charged tracks with p > 1.0 GeV are com- the D∗± example. Within the large theoretical uncertain-
T
binedto form D0 candidates,withkaonandpionmasses ties,NLOQCDpredictionsareseentobeconsistentwith
assumedinturnforeachtrackinordertocalculatethein- thedata,theuncertaintyonwhichisdominatedbythesta-
variantmass. Any additionaltrack, with p > 0.25 GeV tisticalerror.
T
andachargeoppositetothatofthekaontrack,isassigned
thepionmassandcombinedwiththeD0candidatetoform
a D∗± candidate.A clear signalisseen inFig. 4(Top)in 3 Lifetime measurements
thedistributionofthemassdifference∆M = M(Kππ )−
s
M(Kπ) at the nominal value of M(D∗±)− M(D0). From 3.1 B-hadronaveragelifetime
afittothe∆M distribution,a D∗± yieldof2310±130is
obtained,anditsmassfoundtobe145.41±0.03MeV,in PrecisemeasurementsofB-hadronlifetimesallowtestsof
agreementwiththePDGworldaverage.Similarlyforthe theoreticalpredictionsfrom the Heavy Quark Expansion
HadronColliderPhysicssymposium(HCP-2011)
n 1 V3000
on fractio 00..89 ACCTMDLFSA Ss s ==s17=. 9T76e T VTe,eVV,|,yJ|/y|ψyJ|/J<ψ/ψ1|<|.<200.7.65 er 0.5 Me2500 ATDrwLigraoAhtnta-Sgc -h2ca0hPra1grre0ge eclo icmmombininbaiantairoytniosns s = 7 TeV Lint = 1.1 nb-1
ψ productimpt J/ 00000.....34567 A∫TLSL dpAti Sn~- a2l.i3g npmb-e1nt envelope Combinations p112050000000 fit : N(D*±) = 2310 ± 130
Non-pro 00..21 500 σ∆m Mod (=∆ 1M4)5 .=4 10 .±4 60 .±0 30 .M03e VMeV
0
0 0.14 0.145 0.15 0.155 0.16 0.165 0.17
1 10 pJ/ψ [GeV] ∆M = M(Kππs) - M(Kπ) [GeV]
T
eV] Non-prompt cross-section GeV]GeV] ATLASPreliminary s = 7 TeV Lint = 1.1 nb-1
dy [nb/G 101 FSAOpTiLNnA-LaSLli gB|yn→Jm/ψ e|J<n/0ψt .Xe7n5velope ±±µµb/) [D*b/) [D*110022 DPPOOatWWa HH20EE1GG0--PHYETRHWIAIG
non-promptσ/dpT1100--21 σσ(/dpd(/dpdTT110011 MC@NLO
2d10-3
µ-) ATLAS
µ→ψ+ 10-4 ∫sL= d 7t =T e2V.2 pb-1 1100--11
Br(J/10-5 10 pJ/ψ [GeV] 55 1100 1155 2200 2255 3300ppTT((DD33**±±55)) [[GGeeVV44]]00
T
Fig. 4. (Top) The distribution of the mass difference, ∆M =
V] M(Kππ )−M(Kπ),for D∗± candidates.Thedashedlinesshow
promptσ2dy [nb/Ge/dpdT1110010--0-1321 PromSCNNApTpoLNitlOLLno cAO-u raCSro*l oi sEgClsonv|yo-aumsJlopr/eψe uoSc|n<rtrii tan0oS etg.ni7ionnle5nvgt elMelotopdeel tcstH2hheu.E1eecr.tvRNideToWisLnhsOteIsfrrGoiebbQrpauarnCDtneidsDd∗os±ennMscmthafCoolectwsr@huoelwntNahstrfiLieooatOnesngsrsate-ficoosfmhuufralanPDtrtcsOeg.t-deimWo(tnBechHsoeoooEmotfntrGobspemi-tTpniPc)rafYaootilDdrTouudnHinffcasIecteAadearren,ctwnPadotiimOintaththWplyieancHroesr|fyEood|tslGhits<doe--
µ-) POWHEG-PYTHIAcalculation.
µ→+ 10-4 AsT=L 7A STeV
ψBr(J/10-5 ∫L dt = 2.2 pb-1 Theinclusivelifetimemeasurementgivestheaverage
lifetime of the admixture of B-hadrons produced at the
0 10 20 30 40 50 60 70
pJ/ψ [GeV] LHCanddecayingtofinalstatesincludinga J/ψ.Asdis-
T
cussed above, J/ψ mesons produced from the decays of
B-hadronsarenon-prompt,havingadisplaceddecayver-
Fig. 3. (Top) J/ψ non-prompt to inclusive fraction as a func-
tex due to the B-hadronlifetime. The average B-lifetime
tion of the J/ψ p compared to equivalent results from CMS
T
andCDF.(Middle)Non-promptJ/ψproductioncross-sectionas isthusextractedfromthedatabyperforminganunbinned
a function of J/ψ p , compared to predictions from FONLL. maximumlikelihoodfitsimultaneouslytothe J/ψinvari-
T
(Bottom)Prompt J/ψproductioncross-sectionasafunctionof ant mass and the pseudo-proper decay time. In order to
J/ψ p ,comparedtopredictionsfromNLOandNNLO∗ calcu- extract the real lifetime of the B-hadrons, which are not
T
lations,andtheColourEvaporationModel.Allplotsareinthe fully reconstructed, a correction for the smearing intro-
exampleJ/ψrapiditybin|y|<0.75. ducedbytheuseofthepseudo-properdecaytimeinthefit
isrequired.Thiscorrection(“F-factor”)isobtainedfrom
Monte Carlo, with the J/ψ spectrumin B-hadrondecays
re-weightedtomatchBaBardata.Theinvariantmassand
pseudo-properdecay time projectionsof the fit, with the
framework,whichcanpredictlifetimeratiosfordifferent
respectivepulldistributions,areshowninFig.5.Theav-
B-hadron species with per cent level accuracy. With the
erageB-lifetimeismeasuredtobe:
goalofpreparingtheway forprecisionmeasurementsof
B-hadron lifetimes, an average lifetime measurement of hτ i=1.1489±0.016(stat.)±0.043(syst.)ps, (1)
B
inclusive B → J/ψX → µµX is made on the full 2010
dataset (35 pb−1) [6]. With orders of magnitude higher withthemainsystematicuncertaintyoriginatingfromthe
statisticsthanforfullyreconstructedexclusive B-mesons uncertainty (in 2010) on the radial alignment of the ID.
orB-baryoncandidates,theinclusivesampleenablesade- Thisresultisinagreementwiththemostrecentonefrom
tailedinvestigationofthedecaylengthresolutionandthe the CDF collaborationand the expectedaveragelifetime
impact of the residual misalignment of the tracking sys- computed using PDG lifetimes and production fractions
tem. fromdifferentB-hadronspecies.
EPJWebofConferences
ing certain selections then fully reconstructthe B0 or B0
nts / [ 0.003 Gev ] 567800000000 7CB aoTcmekVbg irdnoaeutdan dfit component ATLLiAntS = 3P5r eplbim-1inary cpbmarinaonsnpdseei,ddriadnmteeaocsx.radiySmeitrniumctmoeelteτihxke=etrlBaiLhc-hoxtyaotmdhdreBfio/yntiptieosTlgdBfe,,utclhmlaeyanrsrwbseeciatohunnsdtsehtdlreiufiicenntdtveiamadnr,eiutahninnes-t
e
Ev 400 eachchannel.
300 ForB0thelifetimeisfoundtobe
d
200
100 τ =1.51±0.04(stat.)±0.04(syst.)ps (2)
B0
0 d
2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5
Invariant Mass [Gev] andforB0
s
Pulls / [0.003 GeV ] ----02343211 whereaτmBa0si=ns1o.u4r1c±eo0f.0sy8s(tsetmata.)ti±cu0n.0c5er(tsayinstty.)cposm, esfro(3m)
2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 thedetectoralignment.Withalimitednumberof B0 can-
Invariant Mass [Gev] s
didates (463 ± 26) the current measurement ignores the
s ] 104 lifetimedifferenceofthemasseigenstates.However,within
3 p ATLAS Preliminary thecurrentprecisionthefittedvalueagreeswellwiththe
7
0.0 Lint = 35 pb-1 worldaverageoftheaveragelifetime(1.472±0.026ps).
s / [ 103 7C oTmeVb idnaetda fit
ent Signal component
v
E Background component
102 4 Summary
TheATLASheavy-flavourphysicsprogrammehasmade
10 manyimportantmeasurementsofproductioncross-sections
andmassesandlifetimesofvariousheavy-flavourstates.
-2 0 2 4 6 8 10
Pseudo-proper Time [ps] Inseveralcasesthesearealreadychallengingcurrentthe-
Pulls / [ 0.073 ps ] --20211 odwtireoaemnytiocafnaonlsrdtmrfrauoattirduoeerneldsoemacfteatatyhhsseeu.rLeTexHmhpeCeenraietbmsnielaeirnntgydtyatlsroeetaegprciecmhrhnfeeoi.sqrMmufeoosrtrhCepeoaPsvveeevrmsi,ottelhhaaee--
-2 0 2 4 6 8 10 surements has been particularly dependent on the excel-
Pseudo-proper Time [ps]
lent performanceof the ATLAS ID, MS and trigger sys-
Fig.5. Invariantmass(Top)andpseudo-properdecaytime(Bot- tems.
tom)projections(withtheirrespectivepulldistributions)ofthe
simultaneous fit to these distributions in the B-hadron average
lifetime measurement, showing the data and the signal, back- References
groundandcombinedfitresults.
1. TheATLASCollaboration,JINST,3,(2008),S08003
3.2 B0 and B0 lifetimes 2. TheATLASCollaboration,arXiv:1109:0525[hep-ex],
d s submittedtoPhys.Lett.B(2011)
3. The ATLAS Collaboration, Nucl. Phys. B850, 3,
The lifetimes of B0 and B0 mesons are determined from
d s (2011),387-444
their exclusive decay modes B0 → J/ψK∗0 and B0 →
d s 4. The ATLAS Collaboration, Phys. Lett. B705, 1-2,
J/ψφ,usingthe J/ψdecaytoadi-muonfinalstate,based
(2011),9-27
on an integrated luminosityof 40 pb−1 [7]. The study of
5. The ATLAS Collaboration, ATLAS-CONF-2011-017
the B0 → J/ψφ decay is of special interest as it allows
s (2011)
themeasurementofthe B0 mixingphasewhichcangen-
s 6. The ATLAS Collaboration, ATLAS-CONF-2011-145
erateCPviolationinthischannel.TheSMpredictionfor
(2011)
this CP violating phase is small meaning that any mea-
7. The ATLAS Collaboration, ATLAS-CONF-2011-092
suredexcesswouldbe a clear indicationofnew physics.
(2011)
Inadditionthelight(B )andheavy(B )masseigenstates
L H
have two distinct decay widths Γ and Γ which have
L H
beendeterminedattheTevatronusingatechniqueoftime-
dependent angular analyses that allow the simultaneous
extractionoftheCP-evenandCP-oddamplitudes.TheB0
d
channel provides a valuable testing ground for measure-
mentsoftheB0decayduetotheequivalenteventtopology
s
withadvantageofhigherstatistics.
The K∗0 or φ candidates are reconstructed by select-
ing all pairs of oppositely charged tracks not previously
identified as muons, fitted to a common vertex with the
twomuontracksfromtheJ/ψ.Quadrupletsoftrackspass-