Evidence for the bottom baryon resonance state Lambda_b* with the CDF II detector

0.11 MB·English

by  Igor V. Gorelov

Evidence for the bottom baryon resonance state L 0
3 b∗
1
with the CDF II detector
0
2

n
a
J

5

]
x IgorV. Gorelov
∗
e
(onbehalfoftheCDFCollaboration)
-
p
UniversityofNewMexico,Albuquerque,USA
e
h E-mail: [email protected]
[

1
Usingdatafrom ppcollisionsat√s=1.96TeVrecordedbytheCDFIIdetectorattheFermilab
v
9 Tevatron,wepresentevidencefortheexcitedresonancestateL 0initsfullyreconstructeddecay
b∗
4 modetoL 0p p +whereL 0 L +p withL + pK p +.Theanalysisisbasedonadatasample
9 b − b → c − c → −
0 correspondingtoanintegratedluminosityof9.6fb 1collectedbyanonlineeventselectionbased
−
.
1 ontracksdisplacedfromthe ppinteractionpoint.Thelocalsignificanceoftheobservedsignalis
0 4.6s whilethesignificanceofthesignalforthesearchregionis3.5s . Themassoftheobserved
3
1 stateisfoundtobe5919.5 0.35(stat) 1.72(syst)MeV/c2.
± ±
:
v
i
X
r
a
36thInternationalConferenceonHighEnergyPhysics,
July4-11,2012
Melbourne,Australia
Speaker.
∗
©Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
L 0 withCDFII IgorV.Gorelov
b∗
Baryons with a heavy quark Q can be viewed as a useful laboratory for quantum chromody-
namics (QCD) in its confinement domain. An experimental measurement of a new heavy quark
baryon state adds another constraint in sampling the confinement QCD force with experimental
data.
The first result on bottom baryon resonances was obtained by CDF with the discovery of the
S-wave S b(∗) states in the L b0p ± decay modes [1]. Recently CDF has confirmed this observation
presenting the measurements of the masses and widths of the S (∗)± baryons [2]. In this report,
b
wepresent evidence for theP-wavebottom resonance L 0, predicted atamassscale tobe nextto
b∗
the established S b(∗) baryons. Wehave searched forcandidate L b∗0 baryons withthecomplete data
sampleof9.6fb 1. Ourresultprovidesanadditional contribution tothecurrentlysmallnumberof
−
heavyquarkbaryonobservations.
The models describing the heavy hadrons in the framework of heavy quark effective theories
(HQET)[3]treat aheavy baryon as asystem consisting of aheavy quark Qconsidered as astatic
color source with mass m L and of a light diquark qq with a gluon field [4]. Thence the
bottom bquark and the spQin≫less [QuCdD]diquark make the lowest-lying singlet ground state JP = 1+,
2
the experimentally well established L 0 baryon [5]. When the [ud] diquark acquires an orbital
b
excitation with L = 1 relative to the heavy quark b, the two excited states L 0 emerge with the
b∗
samequarkcontentasasingletL 0,withisospinI=0butwithtotalspinJP= 1− andJP= 3−[6].
b 2 2
Theseisoscalarstatesarethelowest-lyingP-wavestatesthatcandecaytothesingletL 0 viastrong
b
processes involving emissionofapairofsoftpions–giventheparityPisconserved andprovided
sufficient phase space is available. Both L 0 particles are classified as bottom baryon resonant
b∗
states.
Several recent theoretical predictions on masses of the excited heavy baryons L 0 are avail-
b∗
able [8, 9, 10]. Based on the predictions, the mass difference M(L 0) M(L 0) for the first,
b∗ − b
JP = 1− state, ispredicted tobeof 300 310MeV/c2. Themasssplitting between different JP
2 ∼ −
states, M(L b∗0,JP= 23−)−M(L b∗0,JP= 12−),isevaluatedtobeoforder10−17MeV/c2.
The component of the CDFII detector [11] most relevant to this analysis is the charged par-
ticle tracking system. The tracking system operates in a uniform axial magnetic field of 1.4T
generated byasuperconducting solenoidal magnet. Theinnertrackingsystemcomprises threesil-
icon detectors: layer 00 (L00), the silicon vertex detector (SVX II), and the intermediate silicon
layers(ISL)[12]. Alargeopencellcylindrical driftchamber, thecentraloutertracker(COT)[13],
completes the CDF detector tracking system. The silicon tracking system provides fine resolu-
tion on a transverse impact parameter d of s 35m m (with the 28m m beam spot size in-
0 d0 ≃ ≈
cluded). The combined track transverse momentum resolution of the whole tracking system is
s (p )/p 0.07%p [GeV/c] 1.
T T T −
≃
Thisanalysis relies onathree-level trigger used for theonline event selection to collect large
datasamplesofmultibodyhadronicdecaysofb-flavorstates. Werefertothisasthedisplacedtwo-
tracktrigger. ThetriggerrequirestwotracksintheCOTwith p >2.0GeV/cforeachtrack[14]. A
T
further requirement thattheimpact parameter d ofeach tracklieintherange 0.12 1mm makes
0
−
aneffectiveselectionoflong-livedb-flavorparticles[15]. Finally,thedistanceL inthetransverse
xy
plane between the beam axis and the intersection point of the twotracks projected onto their total
transverse momentumisrequiredtobegreaterthan200m m.
2
L 0 withCDFII IgorV.Gorelov
b∗
Usingthedatasetcollectedwiththedisplacedtwo-tracktrigger,wereconstructtheL 0 candi-
b∗
datestatesintheexclusivestrongdecayL 0 L 0p p +followedbytheweakdecaysL 0 L +p
b∗ → b s− s b → c b−
andL + pK p +[16]. TheanalysisoftheL 0massdistributionsisperformedusingtheQvalue,
c → − b∗
where Q=m(L b0p s−p s+)−m(L b0)−2mp , m(L b0) is the reconstructed L c+p b− mass and mp is the
known charged pion mass. The mass resolution of the L 0 signal and most of the systematic un-
b
certainties cancel inthemassdifference spectrum. Wesearch fornarrow structures inthe Qvalue
spectrum withintherangeof6 45MeV/c2 motivatedbythetheoretical estimates[8,9,10].
−
Theanalysis begins withreconstruction oftheL + pK p + decay by fitting three tracks to
c −
→
a common vertex. Standard quality requirements are applied to each track, and only tracks with
p > 400MeV/c are used. All tracks are refit using pion, kaon and proton mass hypotheses to
T
properly correct for the differences in multiple scattering and ionization energy loss. No particle
identification is used in this analysis. The invariant mass of the L + candidate is required to be
c
within 18MeV/c2oftheworld-averageL +mass[5]. ThemomentumvectoroftheL + candidate
c c
±
isthenextrapolated tointersect withafourth trackthat isassumed tobeapion, toform theL 0
b →
L +p candidate. The L 0 vertex is subjected to a three-dimensional kinematic fit with the L +
c b− b c
candidate mass constrained to its world-average value [5]. The probability of the constrained L 0
b
vertex fit must exceed 0.01% [2]. The proton from the L + candidate is required to have p >
c T
2.0GeV/c to contribute to the trigger decision. The momentum criterion for the p from the L 0
b− b
has been optimized by maximizing the score function S /(1+√B), where S is the number of
L 0 L 0
b b
L 0 signal events obtained from the fit of the L +p invariant mass experimental spectrum and B
b c b−
is the number of events in the sideband region, 50 90MeV/c2, of theL 0 Q value experimental
− b∗
spectrum. Thesidebandregionboundariesaremotivatedbythesignalpredictionsin[8,9,10]. The
sideband spectrum isparametrized by asecond order Chebyshev polynomial. Therequirement of
p (p )>1.0GeV/c corresponds to the maximum of the score function. The momentum criteria
T b−
both forproton andp candidates favor theseparticles tobethetwocontributing tothedisplaced
b−
two-tracktriggerdecision. TokeeptheslowpionsofL 0decayingwithinthekinematicacceptance
b∗
oftheCDFtrackreconstruction, theL 0 candidate musthave p (L 0)greater than 9.0GeV/c. This
b T b
corresponds to the maximum of the score function S /(1+√B), where S is the L 0 signal
MC MC b∗
reconstructedintheMCsimulationandBisthenumberofeventsinthepreviouslydefinedsideband
regionoftheL 0 Qvaluespectrum.
b∗
To suppress prompt backgrounds from the primary interactions, the decay vertex of the L 0
b
is required to be distinct from the primary vertex. To achieve this, cuts on the proper lifetime,
ct(L 0)>200m m, and its significance, ct(L 0)/s >6.0, are applied. The first requirement con-
b b ct
firms the trigger while the second one is set using MC simulation data to be fully efficient for
theL 0 signal. We define the proper lifetime as ct(L 0)=L m c/p , where m is the world-
b∗ b xy L 0 T L 0
b b
averagemassoftheL 0 [5]. Theprimaryvertexisdeterminedevent-by-event whencomputingthis
b
vertex displacement. We require the L + vertex to be associated with a L 0 decay by applying a
c b
cut on the proper lifetime ct(L +), where the corresponding quantity L (L +) is calculated with
c xy c
respect to the L 0 vertex. The requirement ct(L +) > 100m m reduces contributions from L +
b c − c
baryons directly produced in pp interactions and from the random combination of tracks faking
L + candidates (which may have negative ct(L +) values). To reduce combinatorial background
c c
and contributions from partially reconstructed decays, we require L 0 candidates to point to the
b
3
L 0 withCDFII IgorV.Gorelov
b∗
2
c
V/10000
e CDF Run II Preliminary
M
20 8000 L » 9.6 fb-1
er
p
s
ate 6000
d
di
n
a 4000
c
N(L 0) » 15418 cands.
b
2000
0
5 5.2 5.4 5.6 5.8 6 6.2 6.4
Mass(L + p -) [GeV/c2]
c b
Figure1: InvariantmassdistributionofL 0 L +p candidateswiththeprojectionofamassfitoverlaid.
b → c b−
Thebluelineshowsthebackgroundwhiletheredonecorrespondstothesignalplusbackground.
primaryvertexbyrequiringtheimpactparameterd (L 0)nottoexceed80m m. Bothlattercuts[2]
0 b
arefullyefficientfortheL 0 signal.
b∗
Figure1showsaprominentL 0 signal intheL +p invariant massdistribution, reconstructed
b c b−
with the criteria explained above. The fit model describing the invariant mass distribution com-
prisestheGaussianL 0 L +p signalontopofabackground shapedbyseveralcontributions [1,
b → c b−
2, 17]. A binned maximum-likelihood fit finds a signal of approximately 15400 candidates at the
expectedL 0 mass,withasignaltobackground ratioaround1:1.
b
To reconstruct theL 0 candidates, each L 0 candidate with an invariant mass within a region
b∗ b
of5.561 5.677GeV/c2 iscombinedwithapairofoppositely chargedtrackseachassignedtothe
−
pion hypothesis. The L 0 mass region corresponds to an area of 3s around the L 0 signal peak
b ± b
as determined by a fit to the spectrum of Fig. 1. To increase the efficiency for reconstructing L 0
b∗
decays nearthekinematic threshold, thequality criteria applied tosoftpion tracks areloosened in
comparison with those applied to tracks used for the L 0 candidates. The basic COT and SVX II
b
hit requirements are imposed on the p tracks, and only tracks with p >200MeV/c having hits
s± T
inbothtrackers andwithavalidtrackfitanderrormatrixareaccepted.
To reduce the background level, a kinematic fit is applied to the resulting combinations of
L 0p p + candidates to constrain them to originate from a common point. TheL 0 candidates are
b s− s b
not constrained to a nominalL 0 mass in this fit. Furthermore, since the bottom baryon resonance
b
originates and decays atthe primary vertex, the soft pion tracks are required to originate from the
primary vertex by requiring an impact parameter significance d (p )/s smaller than 3 [1, 2].
0 s± d0
Thisrequirement corresponds tothemaximalvalueofthescorefunction S /(1+√B).
MC
The experimental L 0 Q value distribution is shown in Fig. 2. A narrow structure at Q
b∗ ∼
21MeV/c2 isclearlyseen. Theprojection ofthecorresponding unbinned likelihood fitissuperim-
posed on the graph. The fit function includes a single narrow signal structure on top of a smooth
background. ThesignalisparametrizedbytwoGaussianswiththesamemeanvalueandwiththeir
4
L 0 withCDFII IgorV.Gorelov
b∗
2 24
c
V/ 22 CDF Run II Preliminary
e
M 20 L » 9.6 fb-1
5
1. 18
r
e 16
p
s 14
e
at 12
d
ndi 10
a
C 8
6
4
2
0
0.01 0.02 0.03 0.04 0.05 0.06 0.07
Q (L *fi0 L 0p -p +) [GeV/c2]
raw b b s s
Figure 2: The projection of the unbinned fit with a blue line showing background only. The binned Q
valuedistributionofL 0candidatesisshownfortherange0.006 0.075GeV/c2. Thesoftpiontrackshave
b∗ −
transversemomentumabove0.2GeV/c.
widthsandweightssetaccording toMonteCarlosimulation studies. Thebackground isdescribed
byasecond orderChebyshev polynomial. Theparameters ofinterest aretheposition ofthesignal
and its yield. The negative logarithm of the extended likelihood function is minimized over the
unbinned set of Q values observed for the candidates in our sample. The Q value spectrum is fit
overtherange6 75MeV/c2. Thefitfinds17.3+5.3 signalcandidatesatQ=20.96 0.35MeV/c2.
− 4.6 ±
−
The significance of the signal is determined using a log-likelihood ratio statistic [18, 19],
D= 2ln(L /L ). We define hypothesis H corresponding to the presence of a L 0 signal on
− 0 1 1 b∗
top of the background. The statistic D is used as a c 2 variable with two degrees of freedom to
derive p values for observing a deviation as large as is in our data or larger, assuming H is true.
0
Therefore our baseline signal fit has a local significance of 4.6s . The significance for a Q search
windowof6 45MeV/c2hasbeendeterminedbyrunningstatisticalpseudo-experiments inwhich
−
theH hypothesisisgeneratedbutfitwiththeH hypothesisandthecorrespondinglog-likelihood
0 1
ratio statistic is calculated for each trial. The fraction of the generated trials having D above the
value returned by the fits of the experimental data determines the significance. For this case the
significance hasbeenfoundtobe3.5s .
The systematic uncertainties on the mass considered in our analysis derive from the CDF
tracker momentum scale (the dominant contribution); the resolution model described by the sum
of two Gaussians; and the choice of a background model. The uncertainties on the measured
massdifferences duetothemomentumscaleofthelow-p p tracksareestimated fromthelarge
T s±
calibrationsampleofD + D0p +events. Thescalefactortobeappliedtothesoftpiontransverse
∗ s
→
momentumisfoundtocorrectthedifferencebetweentheexperimental QvalueinD + decaysand
∗
its world-average value [5]. The same factor applied for the soft pions in a full Monte-Carlo
simulation ofL 0 L 0p p + decays yields aQvalue change of 0.28MeV/c2. Wetake thefull
b∗ → b s− s −
5
L 0 withCDFII IgorV.Gorelov
b∗
Table1: Summaryofsystematicuncertainties.
Source Value, Comment
MeV/c2
Momentumscale 0.28 Propagatedfromhigh
±
statisticscalibration
D + sample;
∗
100%ofthefound
adjustmentvalue.
Signalmodel 0.11 MCunderestimates
±
theresolution; choiceof
themodel’sparameters
Background model 0.03 Consider3-rd,4-thpower
±
polynomials
Total: 0.30 Addedinquadrature
±
Table2: Summaryofthefinalresults. Thefirstuncertaintyisstatisticalandthesecondissystematic.
Value MeV/c2
Q 20.68 0.35(stat) 0.30(syst)
± ±
D M 299.82 0.35(stat) 0.30(syst)
± ±
M(L 0) 5919.5 0.35(stat) 1.72(syst)
b∗ ± ±
value of the change as the uncertainty and adjust by 0.28 0.28MeV/c2 the Q value found by
− ±
thefitoftheL 0 experimentalspectrum. Thesystematicuncertainties aresummarizedinTable1.
b∗
The analysis results are arranged in Table 2. From the measured L 0 Q value we extract
b∗
the absolute masses using the known value of the p mass and the CDF L 0 mass measurement,
± b
m(L 0)=5619.7 1.2(stat) 1.2(syst)MeV/c2,asobtained inanindependent sample[20]. The
b ± ±
L 0 massstatisticalandsystematicuncertainties contributetothesystematicuncertaintyontheL 0
b b∗
absolute mass. The result is closest to the calculation in [10]. Our result is consistent with the
stateL 0(5920) recently observed by the LHCbCollaboration [22]. Thelower production rate of
b∗
bottom hadrons at the Tevatron combined with the low efficiency for soft pion tracks make this
sampleinsensitive tothepresence oftheL 0(5912)stateobserved byLHCb.
b∗
In conclusion, we have conducted a search for the L 0 L 0p p + resonance state in its
b∗ → b −
Q value spectrum, and a narrow structure has been identified. The narrow structure has a local
significance of4.6s andisinterpreted asevidence foraL 0 signal. Thesignificance ofthesignal
b∗
for thesearch region of6 45MeV/c2 is3.5s . Ourresult confirms thestateL 0(5920) observed
− b∗
bytheLHCbCollaboration [22].
Acknowledgments
Theauthorisgrateful tohiscolleagues fromtheCDFB-PhysicsWorkingGroupforuseful
6
L 0 withCDFII IgorV.Gorelov
b∗
suggestions andcommentsmadeduringpreparation ofthistalk.
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7