Collider Searches for Dark Matter

Transcription

1 Collider Searches for Dark Matter AMELIA BRENNAN COEPP-CAASTRO WORKSHOP 1 ST MARCH 2013

2 Introduction Enough introductions to dark matter (see yesterday) Even though we don t know if DM interacts with SM, we like to hope that it does Why colliders? Search methods What does a signal look like? Choosing your model Effective operators Some results Mono- Mono-jet Mono-W/Z Gravitation lensing in the galaxy cluster Abell 2218

3 Methods of investigation Direct detection DM-nucleon scattering Uncertainties: local DM density, backgrounds, calibration See E. Barberio s talk Indirect detection DM annihilation or decay Uncertainties: profile of DM halo in Milky Way, cosmic rays, understanding of astrophysical objects Collider searches looking for DM production Can only identify a DM candidate, can t prove it is DM

4 Why collider searches? Can see regions of parameter space not covered by other searches eg, for low mass WIMPs, the energy transfer in WIMP/nucleon scattering is ~prop. to m WIMP, can be too small for direct detection experiments Confirmation of potential signal Large interaction for direct detection large WIMP/nucleon cross section large production rate predicted at colliders

5 Collider searches a common technique Smash protons together at high enough energies that DM particles are produced These particles can t be detected, so appear as missing energy in the detector (similar to neutrinos) What s going on here? To detect and measure this missing energy, we need something to balance it against Mono-photon Mono-jet Mono-W Mono-Z Note: In practice we measure the missing energy transverse to the beam line. Generally defined as the magnitude of the vector sum of transverse momentum of all reconstructed particles

6 Model-dependent...? For meaningful results, need something to compare with our data need to decide how general our DM model will be Many models make very precise predictions about the DM candidate Supersymmetry: the LSP (often a neutralino) Extra dimensions: the lightest KK particle... and many more Compare predictions with data Fiddle with your model until the data naturally arises from the model

7 ... or model-independent? However, we may prefer to make our model as general as possible Any larger encompassing framework ignored Assume a generic form for DM: Scalar (real or complex) Fermion (Dirac or Majorana) Consider (non-renormalizable) contact interactions Assume intermediate state not accessible at collider energies Treat as an effective interaction Ignore what is happening here

8 Effective operators Can couple to quarks or leptons What we can constrain -2 ( 1 ) (q 2 q) Cut-off scale where effective theory is no longer valid Can be replaced by leptons Complex scalars Dirac fermions Real scalars

9 Effective operators scalar interaction vector interaction axial-vector interaction tensor interaction Can consider several at once but more common to treat one at a time place limit on

10 General method signal generation Not comprehensive, plenty of other generators out there! GEANT4 Simulations interactions between particles and detector matter Taken from: Computer Codes for Collider Studies, CoEPP wiki. digitization reconstruction Reconstructs the information in the same way as it does with data

11 General method - selection Make selections on generated data (Monte Carlo) and real data that maximise signal and minimise background eg, if signal includes a single Z, select events with a muon pair of opposite charge with invariant mass close to the Z mass Compare numbers of events for data and Monte Carlo If data matches with MC background, put limits on your model If they don t, check and check and check again! If they still don t, get excited!

12 Mono- search at ATLAS Search for Dark Matter Candidates and Large Extra Dimensions in events with a photon and missing transverse momentum in pp collision data at s = 7 TeV with the ATLAS detector (ATLAS Collaboration, arxiv: , 29/09/2012) Effective theory method coupling suppressed by effective cut-off mass scale ~ M/(g g q ) Mass of mediator Couplings to WIMP and SM WIMP is a Dirac fermion Test: - scalar (D1) - vector (D5) - axial-vector (D8) - tensor (D9) s = 7 TeV, 4.6fb -1 Selections: E T miss > 150 GeV Photon required, with p T > 150 GeV and < 2.37, tight identification, isolated Only one jet with p T > 30 GeV allowed No electrons or muons Main backgrounds: Irreducible: Z () + (dominates) W/Z + with unidentified lepton W/Z + jets with miscontructed electron or jet

13 Mono- search at ATLAS Operator m χ = 1 GeV m χ = 1.3 TeV scalar D1 31 GeV 5 GeV vector D5 585 GeV 156 GeV axial-vector D8 585 GeV 100 GeV tensor D9 794 GeV 188 GeV Exclusions at 90% CL Taken from: ATLAS Collaboration, arxiv:

14 Mono- search at ATLAS From axial-vector (D8) and tensor (D9) operators From scalar (D1) and vector (D5) operators Remember: only valid if assumption of effective theory holds true! Taken from: ATLAS Collaboration, arxiv:

15 Mono- search at LEP Can test coupling to leptons at a lepton collider LEP Shines Light on Dark Matter (Fox et al, arxiv: , 01/03/2011) WIMP is a Dirac fermion Test: - scalar (D1) (s-channel and t-channel) - vector (D5) - axial-vector (D8) Can be Fierz-transformed into combination of other operators

16 Mono- search at LEP Assuming DM couples to quarks with the same strength as leptons Assuming DM only couples to leptons Taken from: Fox et al, arxiv:

17 Mono-jet search at CMS Search for dark matter and large extra dimensions in monojet events in pp collisions at s = 7 TeV (CMS Collaboration, arxiv: , 25/06/2012) WIMP is a Dirac fermion Test: - vector (D5) - axial-vector (D8) jet s = 7 TeV, 5.0fb -1 Selections: E T miss > 200 GeV Jet with p T > 110 GeV and < 2.4 No more than two jets allowed ((j 1 j 2 ) < 2.5) No isolated electrons or muons Main backgrounds: Irreducible: Z () + jets (dominates) W+ jets with unidentifed/unisolated lepton tt, Z(ll) + jets, single t, QCD multijets

18 Mono-jet search at CMS Lower limits on, upper limits on at 90%CL axial-vector (D8) vector (D5) Taken from: CMS Collaboration, arxiv:

19 Mono-jet search at CMS Taken from: CMS Collaboration, arxiv: Best limit for m χ < 3.5 GeV (region not yet explored by direct detection experiments) Best limit for m χ in GeV range* *ATLAS may have better result by now

20 Mono-W search Don t have to search within a collider experiment, can appropriate their results Searches with Mono-Leptons (Y. Bai and T. Tait, arxiv: , 21/08/2012) WIMP is a Dirac fermion Can include isospin violation with ξ 1 Test: - vector (D5) - axial-vector (D8) Use results from CMS W search for single energetic lepton and missing transverse momentum 5fb -1, s = 7 TeV Consider these together due to interference effects

21 Mono-W search Lower limits for vector operator (D5) Lower limits for axialvector operator (D8) Best limits for unequal coupling to u- and d-type quarks. If signal is discovered, mono-lepton channel will help to check these couplings, including the relative sign between them. Taken from: Y. Bai and T. Tait, arxiv:

22 Mono-W search Spin-dependent, protons Spin-dependent, neutrons Taken from: Y. Bai and T. Tait, arxiv: Spin-independent, protons

23 Mono-Z search Collider searches for dark matter in events with a Z boson and missing energy (Carpenter et al, arxiv: , 22/12/2012) So far have assumed radiation of photon/jet/w only from the initial state Can also include interaction directly between WIMP and pairs of electroweak bosons WIMP is a Dirac fermion Test: - scalar (D1) - vector (D5) - axial-vector (D8) - tensor (D9) Use results from ATLAS measurement of ZZ ll events 4.6 fb -1, s = 7 TeV Suppressed rates due to branching ratio of Z to leptons, but systematic uncertainties scale better than for mono-/jet Taken from: Carpenter et al, arxiv:

24 Mono-Z search Taken from: Carpenter et al, arxiv:

25 Searching for the mono-z at Melbourne Bremmstrahlung of a Z boson was originally proposed by a group including Dr. Nicole Bell and (almost Dr.) Ahmad Galea from Melbourne University: Searching for Dark Matter at the LHC with a Mono-Z (Bell et al, arxiv: , 3/09/2012) Importantly, they do not use the effective operator technique, but a more explicit model for a Majorana fermion: scalar field Maj. fermoin

26 Searching for the mono-z at Melbourne Study to determine detection prospects Muon pair with invariant mass within 60 GeV window of Z mass E miss t > 150 GeV At least one muon with p T > 50 GeV (expect boosted Z) R = ( ) < 1 between muons m χ = 10 GeV We are currently in initial stages of performing this search with ATLAS data m χ = 30 GeV Taken from: Bell et al, arxiv:

27 Conclusion Colliders can be used to put limits on different dark matter models ATLAS and CMS can place the most stringent limits on spindependent WIMP-nucleon scattering cross sections for all WIMP masses, and on spin-independent cross sections for WIMP masses < ~10 GeV. Results valid assuming that effective operator method is valid

28 Backup

29 Production cross sections