Search for Low-mass Dark-Sector Gauge Boson with the BABAR Detector

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by  Romulus Godang

Search for Low-mass Dark-Sector Gauge Boson
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with the BABAR Detector
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] RomulusGodang∗†
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UniversityofSouthAlabama
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- 411UniversityBoulevard,North
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Mobile,AL36688
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h E-mail: [email protected]
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v Wereportsearchesforanewmuonicdarkforcemediatedbyagaugeboson(Z)couplingonlyto
3 thesecondandthirdleptonfamilies.TheexistenceoftheZ′bosonisprobedine+e−→m +m −Z′,
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7 Z′ →m +m − events, with an analysis based on the full data sample collected with the BABAR
1 detector at the PEP-II e+e− collider. No significant signal is observed. Limits on dark-sector
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. coupling constants are derived; these improve upon current bounds, and further constrain the
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0 allowedparameterspace.
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38thInternationalConferenceonHighEnergyPhysics
3-10August2016
Chicago,USA
∗
Speaker.
†BABARCollaboration
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommons
Attribution-NonCommercial-NoDerivatives4.0InternationalLicense(CCBY-NC-ND4.0). http://pos.sissa.it/
ShortTitleforheader RomulusGodang
1. Introduction
The microscopic nature of dark matter is currently unknown. Models of physics beyond
Standard Model predict the existence of a new non-Abelian gauge group Higgs with gauge bo-
son masses below 10 GeV [1]. The WIMP hypothesis suggested that dark matter is assumed to
consistofstableparticlewithlowmasses. Suchnewgaugebosonsaretypicallyinteractwithother
Standard Model elementary particles. Based on the Lm −Lt model [2] one of the most promising
candidatesbasedonthegaugingtheexistingapproximateglobalsymmetriesoftheStandardModel
(SM)isthegaugegroupassociated withthedifference betweenmuonandtau-lepton number. The
gauge Lm −Lt model portal to the Z′ has all features of being couples only to the leptons of the
second andthirdgeneration.
2. Data andEvent Selection
WeusedthedatacollectedbytheBABARdetectorwiththetotalluminosityof514fb−1. Most
of the data were taken at the ¡ (4S) resonance including about 28 fb−1 data at ¡ (3S) and 14 fb−1
dataat¡ (2S)and48fb−1dataattheoff-resonance. The¡ (4S)resonancedecaystoapairofB¯B[3].
Weused about 5% ofthe data set tovalidate and optimize the analysis method. Thedata are only
examined afterfinishfinalizing theanalysis method. Forthebackground study wegenerate signal
MonteCarlo(MC)samples.
Signal MCevents aregenerated using the MadGraph 5[4], whichcalculates matrix elements
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for the sample. The MC then were showered using the Pythia 6 [5] for a bout 30 different Z
mass hypotheses. The main background comes from the QED processes. We generate the direct
processes of e+e− →m +m −m +m − using the Diag36 [6], which includes the full set of the lowest
orderdiagrams. NotethatDiag36doesnotincludeinitialstateradiation(ISR)samples. Theevents
of the process of e+e− → e+e−(g ) is generated using the BHWIDE [7] and the MC events of
e+e− →m +m −(g ) and e+e− →t +t −(g ) are generated using the KK [8]. The off-resonance data
samples, e+e− →q¯q (q= u, d, s, c), are simulated using the EvtGen [9]. The detector acceptance
andreconstruction efficiencyaredeterminedusingMCsimulation basedonGEANT4[10]
3. Measurement ofZ′ →m +m −
Weselecteventscontainingexactlytwopairsofoppositelychargedtracks,consistentwiththe
topology of the process: e+e− → m +m −Z′ and Z′ → m +m − final state. The muons are identified
by particle identification algorithms for each track. Werequire the sum of energies of the electro-
magnetic clusters thatarenotassociated toanytrackmustbelessthan200MeV.Wefinallyreject
events that comes from the ¡ (3S) and ¡ (2S), where ¡ (2S,3S) → p +p −¡ (1S), ¡ (1S) → m +m −
decays if the dimuon combination is within 100 MeV of the ¡ (1S) where pions are misidentified
asmuons.
ThereduceddimuonmassiscalculatedusingthefollowingequationmR=qm2m +m −−4m2m in
logscaleisshowninFig.1. Themostdominantsamplesiscomingfromthedirectdecayofe+e−→
m +m −m +m − process. The contribution from the decay of ¡ (2S)→p +p −J/y , J/y →m +m − as
shown around 3 GeV. The signal efficiency at low masses is about 35% and it rises to about 50%
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ShortTitleforheader RomulusGodang
Figure 1: The distributionof the reduceddimuonmass, mR =qm2m +m −−4m2m in log scale togetherwith
theMonteCarlopredictionsofvariousprocesseswithnormalizedtothedataluminosity.
aroundhighermassofthereduceddimuonmass. WeexcludetheJ/y regionwhencalculating the
correctionfactorsbyfittingthesimulatedandreconstructedreduceddimuonmassesintherangeof
1<m <9GeV.Thesignalyieldisextractedbuaseriesofunbinnedlikelihoodfitstothereduced
R
dimuonmassspectrumwithintherangeof0.212<m <10GeVand0.212<m <9GeVforthe
R R
¡ (4S) resonance data and ¡ (2S) and ¡ (3S) resonances data, respectively. Weexclude a region of
±30 MeV around the nominal known J/y mass. We probe a total of 2219 mass hypothesis. The
cross section of e+e− → m +m −Z′, Z′ → m +m − is extracted as a function of Z′ mass as shown in
Fig. 2 (left). The gray band indicates the excluded region. We find the largest local significance
is 4.3s around Z′ mass of 8.2 GeV that is corresponding to the global significance of 1.6s and it
is consistent with the zero-hypothesis. We also derive 90% confidence level (CL)Bayesian upper
limitonthecrosssectionofe+e−→m +m −Z′,Z′→m +m − asshowninFig.2(right).
Figure2: (Left)Themeasurementof e+e−→m +m −Z′, Z′ →m +m − crosssectionwith its statistical sig-
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nificanceasafunctionoftheZ mass. Theexcludedregionisindicatedbythegrayband. (Right)Thelimit
onthecrosssections (e+e−→m +m −Z′,Z′→m +m −)asafunctionoftheZ′ mass. Theexcludedregionis
indicatedbythegrayband.
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ShortTitleforheader RomulusGodang
We consider all uncertainties to be uncorrelated except for the uncertainties of the luminos-
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ity and efficiency. We finally extract the corresponding 90% CL on the coupling parameter g by
assuming the equal magnitude vector couplings muons, taus and the corresponding neutrinos to-
gether withtheexisting limitsfromBorexinoandneutrino experiments asshowninFig.3. Weset
downto7×10−4 nearthedimuonthreshold.
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Figure3:Upperlimitonthenewgaugecouplingg asafunctionofthemassofZ togetherwiththeexisting
limitsfromBorexinoandneutrinoexperiments.
4. Conclusion
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Inconclusion,wehaveperformedthefirstdirectmeasurementofZ productionfromthedecay
of e+e− → m +m −Z′, Z′ → m +m − an e+e− collider at BABAR. No significant signal is observed
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forZ massesintherange of0.212 -10GeV.Wesetlimitsonthecoupling parameters g downto
7×10−4. Weset astrongest bounds formanyparameter space below 3GeV. Weexclude mostof
theremaining[parameterspacepreferredbythediscrepancy betweenthecalculatedandmeasured
anomalous magneticmomentofthemuonabovethedimuonthreshold [11].
The author would like to thank the organizers of the 38th International Conference on High
Energy Physics, Chicago, United States ofAmerica, the BABARCollaboration, the University of
SouthAlabama,andtheUniversityofMississippi.
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ShortTitleforheader RomulusGodang
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