B !! D˝ ˝ at LHCb Miriam Gandelman on behalf of the LHCb … · 2015. 8. 26. · B!!D˝ ˝ at...
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B → → → D * τν * τν * τν at LHCb Miriam Gandelman on behalf of the LHCb Collaboration UFRJ, Rio de Janeiro, Brazil Lepton Photon 2015 - 15-22 August Ljubljana, Slovenia Motivation Lepton universality requires equal couplings between the gauge bosons and the three lepton families. No deviation of such behaviour has yet been observed although some hints of lepton non-universality eects in B + → K + e + e - and B + → K + μ + μ - decays have been seen [1]. A large class of models that extend the Standard Model propose additional interactions involving enhanced couplings to the third family that would violate this principle. Semileptonic decays of b hadrons to τ leptons provide a sensitive probe for these eects. The presence of additional charged Higgs bosons can have significant eect on the rate of ¯ B 0 → D *+ τ - ¯ ν [2]. BaBar and Belle have recently reported updated measurements of the ratios [3, 4]: R(D * ) ≡B ( ¯ B 0 → D *+ τ - ¯ ν )/B ( ¯ B 0 → D *+ μ - ¯ ν ) and R(D ) ≡B ( ¯ B 0 → D + τ - ¯ ν )/B ( ¯ B 0 → D + μ - ¯ ν ) which show deviations from the Standard Model predictions as shown in the figure below. Within the Standard Model, these ratios dier from unity mainly because of phase-space eects due to the dierence in charged lepton masses. In this work we present the first measurement of R(D * ) in hadron collisions at the LHC with data collected by LHCb in 2011 and 2012. The LHCb Detector The LHCb detector [5] is a single-arm forward spectrometer covering the pseudo-rapidity range 2 < η < 5, designed for the study of particles containing b or c quarks. The detector includes a high-precision tracking system consisting of a silicon- strip vertex detector surrounding the pp interaction region, a large-area silicon-strip detector located upstream of a dipole magnet with a bending power of about 4 Tm , and three stations of silicon-strip detectors and straw drift tubes placed downstream of the magnet. The tracking system provides a measurement of momentum, p , of charged particles with a relative uncertainty that varies from 0.5% at low momentum to 1.0% at 200 GeV/c. The minimum distance of a track to a primary vertex (PV), the impact parameter, is measured with a resolution of (15+29/p T )μm, where p T is the component of the momentum transverse to the beam, in GeV/c. Dierent types of charged hadrons are distinguished using information from two ring-imaging Cherenkov detectors. Photons, electrons and hadrons are identified by a calorimeter system consisting of scintillating- pad and preshower detectors, an electromagnetic calorimeter and a hadronic calorimeter. Muons are identified by a system composed of alternating layers of iron and multi-wire proportional chambers. Trigger selection I no hardware trigger requirement of μ p T to preserve distinct kinematic distributions for the signal and normalization channels. I one high p T particle in the event independent of the D *+ μ. I software trigger: accept D 0 → K - π + with candidates satisfying p T > 2 GeV/c. I K - π + : p > 5 GeV/c and at least one with p T > 1.5 GeV/c. I reconstructed mass consistent with the known D 0 mass. Oine Selection 1 well identified K - and π from D 0 decay. 2 D 0 mass within 3 σ of measured value. 3 D 0 π + must form a vertex. 4 Δm = m(D 0 π + ) - m(D 0 ) within 2.5 σ of the resolution. 5 μ well identified with 3<p<100 GeV/c forming a vertex with the D 0 and well separated from the primary vertex. 6 D *+ μ should have invariant mass < 5280 MeV/c 2 and their momentum vector must point to the primary vertex. 7 MVA trained to select a sample coming from a B meson. Data Sample The data analysed in this work correspond to the integrated luminosity of 1.0 fb -1 and 2.0 fb -1 collected at proton-proton center-of-mass energies of 7 TeV (2011) and 8 TeV (2012) respectively. The signal channel ¯ B 0 → D *+ τ - ¯ ν with τ - → μ - ¯ ν μ ν τ and the normalization channel ¯ B 0 → D *+ μ - ¯ ν produce identical visible states and are reconstructed by a common procedure. References [1] LHCb collaboration, R. Aaij et al., Test of lepton universality using B → K + ‘ + ‘ - decays, Phys. Rev. Lett.113(2014) 151601, arXiv:1406.6482. [2] M. Tanaka, Charged Higgs eects on exclusive semitauonic B decays, Z. Phys. C67 (1995) 321, arXiv:hep-ph/9411405. [3] Belle collaboration, A. Matyja et al., Observation of B 0 → D *- τ + ν τ decay at Belle, Phys. Rev. Lett. 99 (2007) 191807, arXiv:0706.4429. [4] BaBar collaboration, B. Aubert et al., Observation of the semileptonic decays B → D * τ - ¯ ν τ and evidence for B → Dτ - ¯ ν τ , Phys. Rev. Lett. 100 (2008) 021801, arXiv:0709.1698. [5] LHCb collaboration, A. A. Alves Jr. et al., The LHCb Detector at the LHC, JINST 3 (2008) S08005. Separation of the signal from the normalization channel I Three variables calculated in the B rest frame: I m 2 miss =(p μ B - p μ D - p μ μ ) 2 I q 2 =(p μ B - p μ D ) 2 I the muon energy E μ This variables are plotted for simulated events in the figures below. In one case, the true values are shown and in the other, the reconstructed variables obatained using the definition of the B rest frame. To calculate the B rest frame, the B momentum vector in the laboratory frame is determined from the unit vector connecting the B decay point and the primary vertex. The z (beam axis) component is approximated by (p B ) z = m B m reco (p reco ) z , where m reco and p reco are the mass and momentum of the reconstructed final state particles. Results (LHCb-PAPER-2015-025, arXiv:1506.08614) The binned m 2 miss , q 2 and E μ distributions in data are fitted with a maximum likelihood method with three dimensional templates representing the signal, the normalization and the background sources. ) 4 /c 2 (GeV miss 2 m -2 0 2 4 6 8 10 Pulls -2 2 -2 0 2 4 6 8 10 5000 10000 15000 20000 LHCb 4 /c 2 < 2.85 GeV 2 0.40 < q − ) 4 /c 2 Candidates / (0.3 GeV ) 4 /c 2 (GeV miss 2 m -2 0 2 4 6 8 10 Pulls -2 2 -2 0 2 4 6 8 10 10000 20000 30000 LHCb 4 /c 2 < 6.10 GeV 2 2.85 < q ) 4 /c 2 Candidates / (0.3 GeV ) 4 /c 2 (GeV miss 2 m -2 0 2 4 6 8 10 Pulls -2 2 -2 0 2 4 6 8 10 5000 10000 15000 20000 25000 LHCb 4 /c 2 < 9.35 GeV 2 6.10 < q ) 4 /c 2 Candidates / (0.3 GeV ) 4 /c 2 (GeV miss 2 m -2 0 2 4 6 8 10 Pulls -2 2 -2 0 2 4 6 8 10 1000 2000 3000 4000 LHCb 4 /c 2 < 12.60 GeV 2 9.35 < q ) 4 /c 2 Candidates / (0.3 GeV * (MeV) μ E 500 1000 1500 2000 2500 Pulls -2 2 500 1000 1500 2000 2500 1000 2000 3000 4000 LHCb 4 /c 2 < 2.85 GeV 2 0.40 < q − Candidates / (75 MeV) * (MeV) μ E 500 1000 1500 2000 2500 Pulls -2 2 500 1000 1500 2000 2500 2000 4000 6000 8000 10000 12000 LHCb 4 /c 2 < 6.10 GeV 2 2.85 < q Candidates / (75 MeV) * (MeV) μ E 500 1000 1500 2000 2500 Pulls -2 2 500 1000 1500 2000 2500 5000 10000 15000 LHCb 4 /c 2 < 9.35 GeV 2 6.10 < q Candidates / (75 MeV) * (MeV) μ E 500 1000 1500 2000 2500 Pulls -2 2 500 1000 1500 2000 2500 1000 2000 3000 4000 LHCb 4 /c 2 < 12.60 GeV 2 9.35 < q Candidates / (75 MeV) Data ν τ D* → B X')X ν l → ( c D*H → B ν D**l → B ν μ D* → B Combinatorial μ Misidentified Conclusions The ratio of branching fractions R(D * ) is measured to be 0.336 ± 0.027 (stat) ± 0.030 (syst). This is in good agreement with previous measurements and is 2.1 standard deviations greater than the SM expectation of 0.252 ± 0.003. This is the first measurement of any decay of a b hadron into a final state with tau leptons at a hadron collider. M. Gandelman [email protected] Instituto de Física Universidade Federal do Rio de Janeiro www.if.ufrj.br http://lhcb.cern.ch/
Transcript of B !! D˝ ˝ at LHCb Miriam Gandelman on behalf of the LHCb … · 2015. 8. 26. · B!!D˝ ˝ at...
B →→→ D∗τν∗τν∗τν at LHCbMiriam Gandelman on behalf of the LHCb Collaboration
UFRJ, Rio de Janeiro, Brazil
Lepton Photon 2015 - 15-22 August Ljubljana, Slovenia
Motivation
Lepton universality requires equal couplingsbetween the gauge bosons and the three leptonfamilies. No deviation of such behaviour has yetbeen observed although some hints of leptonnon-universality e�ects in B+ → K+e+e− andB+ → K+µ+µ− decays have been seen [1]. A largeclass of models that extend the Standard Modelpropose additional interactions involving enhancedcouplings to the third family that would violate thisprinciple. Semileptonic decays of b hadrons to τleptons provide a sensitive probe for these e�ects.The presence of additional charged Higgs bosonscan have significant e�ect on the rate ofB̄0→ D∗+τ−ν̄ [2].
BaBar and Belle have recently reported updatedmeasurements of the ratios [3, 4]:R(D∗) ≡ B(B̄0→ D∗+τ−ν̄)/B(B̄0→ D∗+µ−ν̄)and R(D) ≡ B(B̄0→ D+τ−ν̄)/B(B̄0→ D+µ−ν̄)which show deviations from the Standard Modelpredictions as shown in the figure below.
Within the Standard Model, these ratios di�erfrom unity mainly because of phase-space e�ectsdue to the di�erence in charged lepton masses. Inthis work we present the first measurement ofR(D∗) in hadron collisions at the LHC with datacollected by LHCb in 2011 and 2012.
The LHCb Detector
The LHCb detector [5] is a single-arm forward spectrometer covering the pseudo-rapidity range 2 < η <5, designed for the study of particles containing b or c quarks. The detector includes a high-precisiontracking system consisting of a silicon- strip vertex detector surrounding the pp interaction region, alarge-area silicon-strip detector located upstream of a dipole magnet with a bending power of about 4Tm , and three stations of silicon-strip detectors and straw drift tubes placed downstream of themagnet. The tracking system provides a measurement of momentum, p , of charged particles with arelative uncertainty that varies from 0.5% at low momentum to 1.0% at 200 GeV/c. The minimumdistance of a track to a primary vertex (PV), the impact parameter, is measured with a resolution of(15+29/pT)µm, where pT is the component of the momentum transverse to the beam, in GeV/c. Di�erenttypes of charged hadrons are distinguished using information from two ring-imaging Cherenkovdetectors. Photons, electrons and hadrons are identified by a calorimeter system consisting ofscintillating- pad and preshower detectors, an electromagnetic calorimeter and a hadronic calorimeter.Muons are identified by a system composed of alternating layers of iron and multi-wire proportionalchambers.
Trigger selection
I no hardware trigger requirement of µ pT topreserve distinct kinematic distributionsfor the signal and normalization channels.
I one high pT particle in the eventindependent of the D∗+µ.
I software trigger: accept D0→ K−π+ withcandidates satisfying pT > 2 GeV/c.
I K−π+: p > 5 GeV/c and at least one withpT > 1.5 GeV/c.
I reconstructed mass consistent with theknown D0 mass.
O�ine Selection
1 well identified K− and π from D0 decay.2 D0 mass within 3 σ of measured value.3 D0 π+ must form a vertex.4 ∆m = m(D0π+)−m(D0) within 2.5 σ ofthe resolution.
5 µ well identified with 3<p<100 GeV/cforming a vertex with the D0 and wellseparated from the primary vertex.
6 D∗+µ should have invariantmass < 5280 MeV/c2 and their momentumvector must point to the primary vertex.
7 MVA trained to select a sample comingfrom a B meson.
Data Sample
The data analysed in this work correspond to the integrated luminosity of 1.0 fb−1 and 2.0fb−1 collected at proton-proton center-of-mass energies of 7 TeV (2011) and 8 TeV (2012)respectively. The signal channel B̄0→ D∗+τ−ν̄ with τ−→ µ−ν̄µντ and the normalizationchannel B̄0→ D∗+µ−ν̄ produce identical visible states and are reconstructed by a commonprocedure.
References
[1] LHCb collaboration, R. Aaij et al., Test of lepton universality using B → K+`+`− decays, Phys. Rev. Lett.113(2014) 151601, arXiv:1406.6482.
[2] M. Tanaka, Charged Higgs e�ects on exclusive semitauonic B decays, Z. Phys. C67 (1995) 321, arXiv:hep-ph/9411405.
[3] Belle collaboration, A. Matyja et al., Observation of B0 → D∗−τ+ντ decay at Belle, Phys. Rev. Lett. 99 (2007) 191807, arXiv:0706.4429.
[4] BaBar collaboration, B. Aubert et al., Observation of the semileptonic decays B → D∗τ−ν̄τ and evidence for B → Dτ−ν̄τ , Phys. Rev. Lett. 100 (2008) 021801, arXiv:0709.1698.
[5] LHCb collaboration, A. A. Alves Jr. et al., The LHCb Detector at the LHC, JINST 3 (2008) S08005.
Separation of the signal from the normalization channel
I Three variables calculated in the B rest frame:I m2
miss = (pµB − pµD − p
µµ)2
I q2 = (pµB − pµD)2
I the muon energy EµThis variables are plotted for simulated events in thefigures below. In one case, the true values are shownand in the other, the reconstructed variables obatainedusing the definition of the B rest frame.
To calculate the B rest frame, the B momentum vector inthe laboratory frame is determined from the unit vectorconnecting the B decay point and the primary vertex.The z (beam axis) component is approximated by(pB)z =
mBmreco
(preco)z, where mreco and preco are themass and momentum of the reconstructed final stateparticles.
Results (LHCb-PAPER-2015-025, arXiv:1506.08614)
The binned m2miss, q
2 and Eµ distributions in data are fitted with a maximum likelihood method with threedimensional templates representing the signal, the normalization and the background sources.
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Dataντ D*→B
X')Xν l→(c D*H→B ν D**l→B νµ D*→B
CombinatorialµMisidentified
Figure 1: Distributions of m2miss (left) and E⇤
µ (right) of the four q2 bins of the signal data with fitprojections overlaid. Below each panel di↵erences between the data and fit are shown, normalizedby the Poisson uncertainty in the data. The bands give the 1� template uncertainties.
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Conclusions
The ratio of branching fractions R(D∗) is measured to be 0.336 ± 0.027 (stat) ± 0.030 (syst). This is in goodagreement with previous measurements and is 2.1 standard deviations greater than the SM expectation of 0.252 ±0.003. This is the first measurement of any decay of a b hadron into a final state with tauleptons at a hadron collider.
M. [email protected] de FísicaUniversidade Federal do Rio de Janeirowww.if.ufrj.br http://lhcb.cern.ch/
