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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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Dark Matter:Indirect Detection & LHC searches
Nicole BellThe University of Melbourne, Australia
in collaboration with
Ahmad Galea, Amelia Brennan (Melbourne) Thomas Jacques, James Dent, Lawrence Krauss (Arizona)
Tom Weiler
(Vanderbilt)
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Nicole Bell, U. Melbourne The LHC, Particle Physics & the Cosmos, Auckland, 13 July 2012
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production (collider searches)
annihilation (indirect detection)
scattering(direct
detection)
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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annihilation (indirect detection)
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modes are
important!
production (collider searches)
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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Indirect detection ~
dark matter annihilation
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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Parameterize the annihilation cross section as:
<v> = a + bv2 + …
a -- from s-wave (L=0) annihilationb -- both s-wave and p-wave (L=1) contributions
The Lth partial wave contribution is suppressed as v2L
In galactic halos, v~ 10-3c, so only the s-wave contribution will be significant.
However, in many models, s-wave annihilation to a fermion pair is helicity suppressed by a factor of 2
DMf mm
Annihilation cross section
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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Majorana neutralinos annihilate to a fermion pair via:
• t- and u-channel exchange of sfermions helicity suppressed
• s-channel exchange of Zhelicity suppressed
• s-channel exchange of higgs suppressed by yukawa couplings
2 vevfm
Example: SUSY
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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Cao, Ma, Shaughnessy, PLB 2009.Dark matter = gauge-singlet Majorana fermion =
Example of suppressed annihilation
SUSY analogue
Annihilation of bino dark matter to fermions via exchange of sfermions
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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Emission of a photon can lift this suppression: Bergstrom, PLB 225, 372, 1989;Flores, Olive, Rudaz, PLB 232, 377, 1989;Bringmann, Bergstrom, Edsjo, 2008; Barger, Gao, Keung, Marfatia, 2009,…
Lifting the suppression (photons)
The photon carries away a unit of angular momentum no longer helicity suppressed.
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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Lifting the suppression (photons)
Final state radiation (FSR) “Virtual internal bremsstrahlung”
(VIB) Effect most pronounced for near-degenerate
and
(i.e. coincides with the co-annihilation region)
Bringmann, Bergstrom, Edsjo, 2008
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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Bergstrom, Bringmann, Edsjo, PRD 2008
Bringmann, Huang, Ibarra, Weniger, arXiv:1203.1312
Bremsstrahlung
signals Gamma rays Positrons
Supposed FERMI gamma ray line at ~130GeV fit by bremsstrahlung signal with m~150GeV, arXiv:1203.1312
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
11Lifting the suppression: electroweak (W,Z) bremsstrahlung
Radiating a W or Z boson can also lift the suppression Both VIB and FSR are present similar to
brem, except for W/Z mass effects.
distinct phenomenology: W and Z bosons decay to charged leptons, neutrinos, gammas, and hadrons hadron production even for “leptophillic” models
Bell, Dent, Jacques & Weiler, PRD 2010.
Bell, Dent, Galea, Jacques, Krauss & Weiler, PLB 2011, arXiv:1104.3823
Ciafaloni, Cirelli, Comelli, De Simone, Riotto & Urbano, JCAP 2011
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
12W brem
rate larger than photon brem
rate, except near threshold
Bell, Dent, Galea, Jacques, Krauss and Weiler, arXiv:1104.3823
Adding all the bremsstrahlung processes:
(in the limit where the W/Z mass is negligible)
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Ratio of eW and e+e-
cross sections
• Annihilation to e+e-, e+e-Z & eW dominates over e+e-• Enhancement by up to three orders of magnitude!
(v~10-3c, Galactic halo)
Bell, Dent, Galea, Jacques, Krauss and Weiler, arXiv:1104.3823
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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Annihilation spectra:
/ W/ Z brem
p
γ
ν
e
Bell, Dent, Jacques & Weiler, arXiv:1101.3357
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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Maximum allowed brem
cross sections
Bremsstrahlung can’t make significant contribution to e+ flux, without overproducing pbar
/pp
Bell, Dent, Jacques & Weiler, arXiv:1101.3357
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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Kachelriess, Serpico and Solberg PRD 2009.
Models with no helicity suppression
EW-brem still occurs, but is subdominant
W/Z decays ensures there is at least a minimal yield of hadrons, photons, charged leptons and neutrinos.
neutrinos-only
(+ W/Z brem)
electrons-only e+e- (+ W/Z brem)
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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Neutrinos from the Sun• Dark matter accumulates in the centre of Sun • Capture rate and (if in equilibrium) the annihilation rate
controlled by WIMP-nucleon scattering cross section• Neutrinos are the only annihilation products able to escape• Hard spectra (WW) more detectable than soft spectra (bb)• EW brem annihilation mode distinctive hard spectra
(D.Hooper)
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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Solar WIMP limits-
limits depend on the assumed annihilation spectrum
IceCube, arXiv:1112.1840
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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Neutrinos from the Sun –
EW brem
Bell, Brennan, Jacques, arXiv:1206.297
EW brem relative muon track rates - IceCube
EW brem
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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LHC dark matter searches with electroweak bremsstrahlung
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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The dominant DM production process at the LHC may be:
But this process is invisible to the detectors (DM stable, weakly interacting)
We need a visible particle in the final state, to recoil against some missing transverse energy,
e.g.
boson gauge qq
Dark matter visible as high pT state + missing ET
Dark matter at the LHC
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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gauge boson is a gluon or photon monojet or monophoton searchesFox, Harnik, Koop, Tsai, 1109.4398, 1103.0240Beltran, Hooper, Kolb, Krusberg, Tait, 1002.4137Frandsen, et al, 1204.3839CMS collaboration, 1204.0821, 1206.5663ATLAS collaboration, 1106.5327…
gluon radiation – high cross section & large backgrounds
monophoton– complementary to (but less constraining than) monojets
electroweak channels? – relatively low backgrounds
Dark matter at the LHC
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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Consider the Z decay to muons:
Signature: pair of high pT muons which reconstruct to a Z, recoiling against missing ET.
We consider bremsstrahlung of Z bosons
Z easily reconstructed invariant mass
Leptonic Z decays backgrounds are low purely EW channel competitive with hadronic jet searches
Z bremsstrahlung
Zqq
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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Signal: pair of muons + missing ET Zqq
Backgrounds
EW backgrounds:
QCD backgrounds:
ZZ background has large missing ET & is kinematically most similar to our signal
Z+jets is a large background, but can be removed with kinematic cuts
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similar model as before, but now we take the scalar to be
charged under SU(3), c~(3,2,1/3),
(i.e.
and
are analogous to the neutralino and squark. But, importantly, note we don’t have a gluon.)
Z bremsstrahlung
–
example model
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(Includes a cut on invariant mass of the muon pair, and an inclusive muon pT cut of 50 GeV)
Signal and Backgrounds –
before cuts
Bell, Dent, Galea, Jacques, Krauss & Weiler, in preparation
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signal/background –
as function of R cut
Z boosted muons co-linear signal has small R
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Cuts: - missing ET > 100 GeV- R < 1
Signal and Backgrounds –
after cuts
Bell, Dent, Galea, Jacques, Krauss & Weiler, in preparation
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Muon
pT
Missing ET
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Nicole Bell, CoEPP, The University of Melbourne The LHC, Particle Physics and the Cosmos, 13 July 2012
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Signal falls away for large dark matter mass
(note: couplings chosen to be consistent with relic density)
m=500 GeV
m=300 GeV
m=1000 GeV
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Comparison with monojet
searchesAt 7 TeV, 1fb-1, and same model parameterssimilar sensitivity
Monojet analysisFox et al.arXiv:1109.4398
Z brem analysisBell, Dent, Galea, Jacques, Krauss, Weiler, in preparation
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Electroweak bremsstrahlung - indirect detection Annihilation to a fermion pair is often helicity suppressed Internal bremsstrahlung of , W & Z removes the suppression;
Allows indirect detection of otherwise unobservable physics Interesting multimessenger signals. Antiproton limits important. Distinctive neutrino signal for solar WIMP searches
Conclusions
Electroweak bremsstrahlung - colliders Signal is a gauge boson plus missing transverse energy Large signal/background Complementary to monojet and monophoton searches
3-body final states are important for dark matter searches