Photon polarisation in b->s gamma using B->K*e+e- at LHCb

0.19 MB·English

by  Michelle Nicol

Photon polarisation in b → sγ using B → K∗e+e− at LHCb
MichelleNicol(onbehalfoftheLHCbcollaboration)1,a
Laboratoiredel’Acce´le´rateurLine´aire,Orsay,France
Abstract. Theb → sγtransitionproceedsthroughflavourchangingneutralcurrents,andthusisparticularly
sensitive to the effects of new physics. An overview of the method to measure the photon polarisation at the
LHCbexperimentviaanangularanalysisofB→K∗e+e−atlowq2ispresented.ThestatusoftheB→K∗µ+µ−
√
2 analysiswith309pb−1ofppcollisionsat s=7TeVatLHCbisalsogiven.
1
0
2
1 Introduction

n
a
J Althoughthebranchingratioofb→ sγhasbeenmeasured

4 to be consistent with Standard Model (SM) predictions,
2 new physics could still be present and detectable through
the analysis of details of the decay process. In particular,

] thephotonfromthebispredominantlylefthandedinthe
x
SM,whereasadditionalrighthandedcurrentscanarisein
e
- certain new physics models, such as the Left-Right sym-
p
metricmodels,orinsomesupersymmetricmodels[1].Ac-
e
cesstothepolarisationinformationisavailableviaanan-
h
gularanalysisofB→ K∗e+e−.
[
Hadronicformfactorsrendertheoreticalpredictionover
1 the whole q2 (the dilepton invariant mass squared) range
v difficult.However,ithasbeenshownthattheseuncertain-
0
tiesarecontrollableatlowq2,wherethephotontermdom-
0
0 inates, and certain asymmetries providing information on
5 thephotonpolarisationcanbeformed.[2]
.
1
0
2 2 B → K∗µ+µ− status at LHCb
1 Fig. 1. A and F as a function of q2, as measured at LHCb
FB L
: withB→K∗µ+µ−[3].TheSMpredictionsaregivenbythecyan
v √
i With 309 pb−1 of pp collisions at s=7 TeV, collected (light)band,andthispredictionintegratedintheq2 binsisindi-
X in three months during the first half of 2011, the forward catedbythepurple(dark)regions.
r backwardasymmetryofthedileptonsystem,A hasbeen
a measured [3] using B → K∗µ+µ− events, (aFsBis shown
inFig.2),alongwith F ,theK∗ longitudinalpolarisation
L
(Fig.2);aninputrequiredforthephotonpolarisationmea-
surement.Theseobservableshavebeenmeasuredasbeing
in good agreement with SM predictions, [4], implying a
SM like Wilson Coefficient C , but still allowing for the
7
existenceofC(cid:48) (righthandedcurrents).Asstressedabove,
7
themeasurementismostsensitiveatlowq2.Itwouldthere-
forebepreferabletoperformtheanalysisusingelectrons.
However,experimentallyitismorechallengingtoobserve
electronsthanmuons,primarilyduetothefactthatmuons
provideaverycleansignaturetotriggeron.With309pb−1
ofLHCbdata, B → K∗µ+µ− intheq2 range0-2GeVhas
beenobserved,asisshown,alongwithotherq2 ranges,in
Fig. 2. The mass distributions of B → K∗µ+µ− in six q2 bins.
Fig.2.Withtherestofthe2011data,onecanexpecttosee
Thesolidlineshowsafitwithadouble-Gaussiansignalcompo-
aB→ K∗e+e−signal.
nent(thin-greenline)andanexponentialbackgroundcomponent
(dashed-redline).
a e-mail:[email protected]
EPJWebofConferences
When expressed in terms of the helicity amplitudes, for
smallrealvaluesof AR,oneobtainsA(2) ≈−2Aright.
AL T Aleft
4 B → K∗e+e− Monte Carlo studies
Although work is ongoing on the analysis of the B →
K∗e+e−data,andyieldpredictionsfromMonteCarlo(MC)
havebeenvalidatedusingthecontrolchannelB→K∗J/Ψ
withJ/Ψ →(e+e−),thereisnotyet,atthetimeofthiscon-
ference,enoughdatatoperformtheanalysisortestthefit-
tingprocedure.ToyMCstudieshavethereforebeencarried
Fig. 3. Definition of the angles φ, θK and θL in the decay B → out for this purpose [6]. 190k signal events were gener-
K∗e+e−. atedusingEvtGen,andseparatedintofilescontaining250
events:thepredictedyieldsfromMCstudieswith2fb−1at
3 Analysis formalism acentreofmassenergyof14TeV,excludingeffectsfrom
LHCb’s high level trigger. By performing the fit on each
B → K∗e+e− canbeuniquelydescribedbyfourvariables: file,itisshownthatwith200-250signaleventsandasig-
q2andthreeangularvariables,θ ,θ andφ,(thedefinitions naltobackgroundratiooftheorderof1,aprecisionof0.2
L K
of which can be seen in Fig. 3). Following the formalism isattainableonA2,equivalenttoanaccuracyonthefrac-
T
as described in [5], the differential decay distribution can tion of wrongly polarised photons of 0.1. An example of
bewrittenintermsofthesevariablesas: oneofthefitsforonetoyMCstudycanbeseeninFig.4.
Theanalysisalsodemonstratesthatthemeasurementsare
dΓ
= not sensitive to the knowledge of the angular acceptance,
dq2dcosΘdcosΘ dφ
l K andhenceshallnotbesystematicslimited.
9
[I (cosΘ )+I (cosΘ )cos2Θ+
32π 1 K 2 K l
I (cosΘ )sin2Θ cos2φ+I (cosΘ )sin2Θ cosφ+
3 K l 4 K l
I (cosΘ )sinΘ cosφ+I (cosΘ )cosΘ+
5 K l 6 K l
I (cosΘ )sinΘ sinφ+I (cosΘ )sin2Θ sinφ+
7 K l 8 K l
I (cosΘ )sin2Θ sin2φ] (1)
9 K l
WhenmeasuringthisrateatLHCb,the3Dangularaccep-
tance,ε(cosΘ,cosΘ ,φ)mustalsobetakenintoaccount.
l K
It is assumed to be factorisable as the products of ε , the
1
acceptanceasafunctionofφ,andε ,theacceptanceasa
D
functionofcosΘ andcosΘ .Furthermore,assumingthat
K L
ε isanevenfunction,Equation1canbesimplifiedbyper-
1
formingtheφtransformationthatifφ>0,thenφ=φ+π.A
similartransformationcanbeperformedforcosΘ .Equa-
L
tion1canthenbewrittenas: Fig.4.ExampleofthefitofcosΘ ,cosΘ andφforonetoyMC
L K
studycontaining250signaleventsandnobackgroundevents.
dΓ
=
dq2dcosΘdcosΘ dφ
l K
9
[I (cosΘ )+I (cosΘ )cos2Θ+
32π 1 K 2 K l
I (cosΘ )sin2Θ cos2φ+I (cosΘ )sin2Θ sin2φ] References
3 K l 9 K l
×ε (cosΘ,cosΘ ) (2)
D l K 1. for example E. Lunghi and J. Matias, J. High Enerfy
In order to minimize theoretical uncertainties, it is desir- Physics,04,(2007)058
able to measure ratios of the amplitudes. Neglecting the 2. Y.GrossmanandD.Pirjol,J.HighEnerfyPhysics,06,
lepton mass, the remaining I terms in equation 2 can be (2009)029
writtenintermsofthreesuchparameters,F ,A(2),A : 3. LHCB-CONF-2011-039
L T Im 4. C. Bobeth, G. Hiller and D. van Dyk, J. High Enerfy
|A |2 Physics,07,(2010)098
F = 0
L |A0|2+|A⊥|2+(cid:12)(cid:12)(cid:12)A(cid:107)(cid:12)(cid:12)(cid:12)2 5.0C94F0.0K9ruger and J. Matias, Physics Rev, D71, (2005)
(cid:12) (cid:12)
A(2) = |A⊥|2−(cid:12)(cid:12)A(cid:107)(cid:12)(cid:12)2 (3) 6. J.LeFranc¸ois,M.H.Schune,LHCb-2009-008
T |A⊥|2+(cid:12)(cid:12)(cid:12)A(cid:107)(cid:12)(cid:12)(cid:12)2
(cid:61)(A∗ A )−(cid:61)(A∗ A )
A = ⊥L ⊥L ⊥R ⊥R
Im |A0|2+|A⊥|2+(cid:12)(cid:12)(cid:12)A(cid:107)(cid:12)(cid:12)(cid:12)2