Benedikt Kloss for the BESIII Collaboration Institute of...
Transcript of Benedikt Kloss for the BESIII Collaboration Institute of...
Benedikt Kloss for the BESIII Collaboration
Institute of Nuclear Physics – Mainz University
Twelfth Conference on the Intersections of Particle and Nuclear Physics May 2015, Vail, Colorado
Benedikt Kloss - Universität Mainz 2
A new U(1) gauge boson γ‘ („dark photon“) might be the connection between the standard model and a dark sector:
dark sector kinetic mixing standard model
Benedikt Kloss - Universität Mainz 3
Existing exclusion limits on the dark photon (γ’) mass and mixing parameter (ε):
Dark photon coupling strength to SM matter:
Mixing parameter:
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ε 2 =α 'α
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α '
GOAL: make a contribution at
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BEPCII Collider:
• located in Beijing, China • symmetric e+e- collider • 2 GeV < ECMS < 4.6 GeV • data taken at : 2.9 fb-1
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s = 3.77 GeV
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Main Drift Chamber
CsI Crystal Calorimeter
1T superconducting solenoid magnet
Time-Of-Flight System
Muon Chamber
Benedikt Kloss - University of Mainz (Graphic produced by Matthias Ulrich, Gießen)
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• photon emitted in the initial state • CMS energy lowered by the energy of the emitted photon ⇒ measurements at different energies possible
fixed Ecms = 3.77 GeV
new energy
E [GeV]
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EMC EMC
tagged: photon hits EMC
untagged: photon leaves the detector
Two different analysis types: • tagged: photon is detected in the Electromagnetic Calorimeter • untagged: photon leaves the detector (most probable case)
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Two different analysis types: • tagged: photon is detected in the Electromagnetic Calorimeter • untagged: photon leaves the detector (most probable case)
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TAGGED&
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boarders&of&EMC&
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Idea: Search for the ISR processes
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e+e− →γ ISR ʹ′ γ →γ ISRµ+µ−
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e+e− →γ ISR ʹ′ γ →γ ISRe+e−
and
Use untagged ISR events and 2.9 fb-1 data, taken at 3.77 GeV.
at
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Idea: Search for the ISR processes
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e+e− →γ ISR ʹ′ γ →γ ISRµ+µ−
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e+e− →γ ISR ʹ′ γ →γ ISRe+e−
and
Irreducible background:
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e+e− →γ ISRγ*→γ ISRµ+µ−
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e+e− →γ ISRγ*→γ ISRe+e−
QED process, same signature in
detector!
Dark photon signal would appear as peak on the QED background. ⇒ Peak search!
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distance to interaction point Rxy < 1.0 cm Rz < 10.0 cm
acceptance of charged tracks 0.4 rad < θ< π – 0.4 rad
to supress background PID to select µ or e
# charged tracks = 2
total charge = 0
# photons = 0 (untagged analysis)
missing photon angle < 0.1 rad or > π – 0.1 rad
1C kinematic fit χ21C < 20
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e+e− →µ+µ−γ ISREvent selection: and
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e+e− →e+e−γ ISR
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MC simulated with PHOKHARA Eur.Phys.J. C24, 71-82 (2002) Phys. Rev. D77, 114005 (2008)
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MCγ-µ+µ MCγ-π+π
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MCγ-µ+µ MCγ-π+π
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QED process - irreducible background
Data-MC efficiency corrections applied on MC.
See talk on Wednesday:
“Measurement of the π+π- cross section at BESIII”
(PPHI, 16:40)
PRELIMINARY
J/ψ
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data
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MC simulated with BABAYAGA 3.5
QED process - irreducible background
Nucl. Phys. B758, 227-253 (2006)
PRELIMINARY MC not corrected
for data-MC differences.
J/ψ
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/ ndf 2χ 196 / 145 0a 1.288e+02± 1.638e+05 1a 9.571e+01± -3.663e+05 2a 4.362e+01± 3.155e+05 3a 1.83e+01± -1.16e+05 4a 5.177e+00± 1.687e+04
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Fit of the continouus mass spectrum in data with a polynomial and look for
a peak in data:
fit to data
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p(x) = a0 + a1x + a2x2 + a3x
3 + a4x4
PRELIMINARY
To get rid of the MC prediction:
Spare the region around J/ψ.
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/ ndf 2χ 196 / 145 0a 1.288e+02± 1.638e+05 1a 9.571e+01± -3.663e+05 2a 4.362e+01± 3.155e+05 3a 1.83e+01± -1.16e+05 4a 5.177e+00± 1.687e+04
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Fit of the continouus mass spectrum in data with a polynomial and look for
a peak in data:
No peaking structure found. ⇒ Set exclusion limit.
[GeV]*γm1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4
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significance significance
fit to data
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p(x) = a0 + a1x + a2x2 + a3x
3 + a4x4
PRELIMINARY
PRELIMINARY PRELIMINARY
To get rid of the MC prediction:
µ e
Spare the region around J/ψ.
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event selection
compare with MC prediction
fit the invariant mass distribution in data
search for a peaking structure
calculate the exclusion limit between 1.5 GeV/c2 and 3.4 GeV/c2
check whether huge background is left
none is found
get rid of any data-MC comparison in the final result
below 1.5 GeV/c2: pion background is dominant
above 3.4 GeV/c2: too close to production threshold
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90% confidence level (CL) calculated with the algorithm by Rolke et al. (TRolke)
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data - fit CL = 90%
combined statistics, including systematics
Nucl.Instrum.Meth., A551, 493-503 (2005)
PRELIMINARY
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J. D. Bjorken, R. Essig, P. Schuster, and N. Toro, Phys. Rev., D80, 075018 (2009)
€
dσ(e+e− →γ 'γ ISR → l+l−γ ISR )dσ(e+e− →γ * γ ISR → l+l−γ ISR )
=3π
2N fl+ l− ⋅
ε 2
α⋅mγ '
δm
We want to calculate it in bins of the mixing parameter ε:
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mass resolution, determined with MC
fine structure constant
number of decay modes of dark photon containing phase space
(contains R ratio, see next slide)
dark photon mass
mixing parameter determined exclusion limit
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dσ(e+e− →γ 'γ ISR → l+l−γ ISR )dσ(e+e− →γ * γ ISR → l+l−γ ISR )
=3π
2N fl+ l− ⋅
ε 2
α⋅mγ '
δm
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taken from PDG 2014
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R =σ(e+e− →hadrons)σ(e+e− →µ+µ−)
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Γtot = Γ(γ '→e+e−) +Γ(γ '→µ+µ−)⋅ (1+ R( s))€
N fl+ l− =
ΓtotΓ(γ '→ l+l−)
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Γ(γ '→ l+l−) =αε 2
3mγ '2 mγ '
2 − 4ml2 (mγ '
2 + 2ml2) Phys. Rev. D88, 015032 (2013)
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uncertainty of the R ratio
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Completely dominated by the uncertainty of the R ratio (everywhere above 5%)
taken from PDG 2014
Systematic uncertainty is estimated and implemented bin-by-bin (possible with TRolke algorithm1)
1 see https://root.cern.ch/root/html/tutorials/math/Rolke.C.html
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• Goal of the analysis is to search a dark photon signal using and events
• no evidence has been found between 1.5 and 3.4 GeV/c2
• an exclusion limit with 90% confidence has been calculated with the TRolke algorithm in bins of the mass and mixing parameter of the dark photon
• values down to ε < 710-3 can be excluded
• best exclusion limit in this mass range
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µ+µ−γ ISR
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e+e−γ ISR
Thank you for your attention!
PRELIMINARY
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e+e− →µ+µ−γ ISR
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e+e− →e+e−γ ISR
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DLL(µ,e) = 2⋅ log p(µ)p(e)
⎛
⎝ ⎜
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⎠ ⎟ > 0
,e)µDLL(-10 -8 -6 -4 -2 0 2 4 6 8 10
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