1. Supersymmetry measurements with ATLAS Tommaso Lari (CERN/INFN Milano) On behalf of the ATLAS Collaboration After we have discovered New Physics, can we understand what it is?

2. Tommaso Lari2 January 5th-9th, 2009 Overview Supersymmetry. What we might know from inclusive searches. Measurements possible with very first data (~1 fb -1 ) Some of the possibilities with high luminosity Beyond masses: spin measurements Conclusions

3. Tommaso Lari3 January 5th-9th, 2009 Wanted (live or dead): SUSY Add to each SM boson (fermion) a fermionic (bosonic) partner. Partners should not be too heavy (< 1 TeV) to solve the hierarchy problem MSSM: ~100 free parameters (all possible SUSY breaking terms in the EW scale effective lagrangian) Constrained models have few parameters, with assumptions mSUGRA parameters

4. Tommaso Lari4 January 5th-9th, 2009 Typical LHC scenario Abundant production of strongly interacting scalar quarks and gluinos They decay to some SU(2)xU(1) gaugino and jets Decay chain ends with stable, invisible LSP Signatures: Missing energy+jets+something Examples of something: nothing, 1,2,3 leptons (e,m), , , Z, h Corresponding searches sensitive to a large number of SUSY models/parameters, but also to other new physics with similar signatures

5. Tommaso Lari5 January 5th-9th, 2009 What might we know from inclusive analyses? ATLAS 10 fb -1 ATLAS 10 fb -1 First step: establish excess over Standard Model expectations, make sure it is from new physics The Atlas Collaboration, Observation of events with large transverse missing energy and high p T jets in pp collisions at  s=1x TeV Points to production of strongly interacting particles with undetectable particles in final state. It might be SUSY or something else.

6. Tommaso Lari6 January 5th-9th, 2009 Information to establish SUSY? Each SM particle has a superpartner Their spin differ by ½ The couplings are the same SUSY mass relation holds Information desired Production cross sections Masses of new particles Angular distribution of decays Branching ratios Observables Inclusive observables are for example cross sections, rates of specific search channels, average pt of photons, etc. Exclusive analysis (this talk) isolate specific decay chains. Most of the work so far aims at measuring the masses of new particles. Spin measurements from angular distributions also possible in some cases.

7. Tommaso Lari7 January 5th-9th, 2009 Some comments on models SUSY models have typically long decay chains with several particles in the final state The SUSY combinatorial background is usually much larger (and less known!) than the Standard Model background For a realistic study of the feasiblity of a measurement technique, simulation of the decay chain of interest is not enough. All the SUSY production cross section for a specific point in a model parameter space is needed The results I show have been obtained with mSUGRA benchmarks The techniques should be applicable whenever the relevant decay chain is open But the precision of the measurements IS model dependent

8. Tommaso Lari8 January 5th-9th, 2009 mSUGRA benchmarks Benchmarks have been chosen requiring that neutralino relic density matches DM constraints SUn = mSUgra benchmark n (no reference to simmetry groups!)

9. Tommaso Lari9 January 5th-9th, 2009 Benchmarks details For this talk I will show results for SU3: in bulk region, squark and gluino masses 600-700 GeV SU4: just beyond Tevatron limits, squark and gluino masses ~400 GeV

10. Tommaso Lari10 January 5th-9th, 2009 Some references Many results shown are from the recently published The ATLAS Collaboration, Measurement from Supersymmetric events, in Expected Performance of the ATLAS experiment, CERN-OPEN-2008-020, pages1611-1636. Summarizes three years of studies by the collaboration, focus is on initial data (~1 fb -1, moderately understood detector), all results are with full simulation I will present also some earlier published work to show what else may be done with more (~300 fb -1 ) integrated luminosity B.K. Gjelsten et al., A detailed analysis of the measurement of SUSY masses with the ATLAS detector at LHC, ATL-PHYS-2004-007 M. Biglietti et al., Study of the second Lightest neutralino spin measurement with The ATLAS detector at LHC, ATL-PHYS-PUB-2007-004 G. Polesello and D.R.Tovey, JHEP 05 (2004) 071. U. De Sanctis et al., Eur. Phys. J. C52, 743.

11. Tommaso Lari11 January 5th-9th, 2009 The edge method With two undetected particles with unknown mass in the final state it is not possible to reconstruct mass peaks The typical approach is to look for minima (thresholds) and maxima (edges) of visible invariant mass products 2 two-body decays: the invariant mass of p,q (massless SM particles) has a maximum at and a triangular shape if the spin of particle b is zero. 3 successive two-body decays Four invariant mass combinations of the three visible particles: (12), (13), (23), (123) For the first three minimum is zero: only one constraint. The last has both non-trivial minimum and maximum: five constraints in total on four unknown masses. If sufficiently long decay chains can be isolated and enough endpoints measured, then the masses of the individual particles can be obtained

12. Tommaso Lari12 January 5th-9th, 2009 The two-lepton edge Experimentally very clean Lepton 4-momentum measured with good resolution and very small energy scale uncertainty (ultimate ~0.1%) Lepton flavour unambiguos The combinatorial background cancels in the flavour subtracted distribution: ATLAS Physics TDR M ll (GeV) The relevant decay chain is open in a large fraction of SUSY parameter space.

13. Tommaso Lari13 January 5th-9th, 2009 Dilepton edge SU3 (bulk point), two body decays Fitting function: triangle smeared with a gaussian SU4 (low-mass point near Tevatron limits), three body decay. Fitting function: theoretical three-body decay shape with gaussian smearing In reality more luminosity is needed to discriminate two-body and three-body decays from the shape of the distribution. With 1 fb -1 both fitting functions give reasonable  2.

14. Tommaso Lari14 January 5th-9th, 2009 Lepton+jets combinations Lepton+jets combinations give further mass relations The two jets with highest p T are likely from squark decay – but which one belongs to the right decay chain?

15. Tommaso Lari15 January 5th-9th, 2009 Lepton+jets combinations llq edge llq threshold lq max edge lq min edge For this particular benchmark (bulk point SU3) all constraints measurable with 1 fb -1 !

16. Tommaso Lari16 January 5th-9th, 2009 Mass and parameter fits From these edges it is possible to derive the masses of particles in the decay and place limits on parameters of constrained models. Large statistical errors with 1 fb -1. Mass differences better measured than absolute masses. Sparticle Expected precision (100 fb -1 ) q L  3%  0 2  6% l R  9%  0 1  12% ~ ~ ~ ~ ATLAS SPS1a, fast simulation, 100 fb -1 SU3, full simulation, 1 fb -1

17. Tommaso Lari17 January 5th-9th, 2009 Tau lepton edges Taus experimentally more difficult than electrons and muons  Can only identify hadronically decaying taus, with smaller efficiency and larger jet fake rate than for first two generations  Neutrino energy not measured – no sharp edge! However they carry unique information  Information on the mass of the scalar tau in the decay chain  Tau BRs are enhanced over first two generations at large tan , and it may be that   2 →  is the only two-body decay open.  The polarization of taus also carries interesting information (different in various SUSY breaking models). Feasiblity of polarization measurements still under investigation. ~ ~

18. Tommaso Lari18 January 5th-9th, 2009 Measurement of  edge The inflection point of the  invariant mass fit function is in a linear relation with the endpoint Systematics from the (unknown) tau polarization Measurement of both endpoint and polarization is under investigation SU3, full sim., 1 fb -1

19. Tommaso Lari19 January 5th-9th, 2009 An hadronic-only signature If A is pair produced and A → B LSP, the endpoint of is the mass of A (if true m(LSP) is used). Applicable to mSUGRA q R as BR(q R → q  0 1 ~ 1) Analysis requires two hard jets and large missing energy SU3, full sim., 1 fb -1 Sharp endpoint is visible A linear fit gives while true q R mass is 611 GeV ~ ~ ~ ~

20. Tommaso Lari20 January 5th-9th, 2009 A 3 rd generation example Using the low-mass SU4 point with large BRs in 3 rd generation squarks Study decay chain Fully reconstruct hadronic top, and subtract jjb combinatorial background with jet pairs in W sidebands SU4, full sim., 200 pb -1 For this very low mass point, the tb edge is in principle visible with very low statistics In practice, need good undertanding of detector (b-tagging, jet reconstruction) before attaching this channel

21. Tommaso Lari21 January 5th-9th, 2009 High luminosity possibilities With 1 fb -1, many measurements may already be possible for favourable SUSY scenarios The high luminosity potential studied in the past in fast simulation, for example for SPS1a point in B.K.Gjelsten et al., ATL-PHYS-2004-007 With 300 fb -1 many measurements are limited by JES sistematics Scalar lepton, gluino, scalar bottom masses also measured Parameter Expected precision (300 fb -1 ) m 0  2% m 1/2  0.6% tan(  )  9% A 0  16% Parameter constraints (assuming mSUGRA)

22. Tommaso Lari22 January 5th-9th, 2009 Dark Matter connection The unseen LSP particle is a natural DM candidate. Within a given model, we can determine the parameter space compatible with measurements and compute the corresponding the relic density Exercise done in JHEP 05 (2004) 071 using SPS1a 300 fb -1 simulated measurements, and within mSUGRA.   h 2 = 0.1921  0.0053 log 10 (   p /pb) = -8.17  0.04

23. Tommaso Lari23 January 5th-9th, 2009 Focus Point study Interesting information possible from few measurements In Focus Point region relic density ok because gaugino mass parameters (M 1, M 2,  ) are of the same order giving a large Higgsino component to  0 1 For SU2 benchmark, two lepton edges observable. Using only this info, a fit of gaugino mass parameters, assuming unification relation M 1 = 0.5 M 2 (but not mSUGRA) tells that indeed  ~ M 1 ATLAS 300 fb -1    →    l + l -    →    l + l - M 1 (GeV)  (GeV ) tan  Eur. Phys. J. C52, 743

24. Tommaso Lari24 January 5th-9th, 2009 Test of spin hypothesis Important to measure spin of new particles; it’s a fundamental check to ensure that what we have discovered is SUSY! The charge asymmetry is diluted because: 1.Usually not possible to discriminate neat and far leptons: we sum ql far and ql near distributions 2. The charge coniugate decay gives the opposite asymmetry. Cancellation not exact at a pp collider however.

25. Tommaso Lari25 January 5th-9th, 2009 Spin measurement Cuts on EtMiss and jet pt to reject SM 2 opposite sign electrons or muons; combinatorial background subtracted using For SU3 point, 10 fb -1 already enough to exclude charge symmetry SU3 point: 19.3 pb x 3.8% Ratio squarks/antisquarks ~3 ATLAS-PHYS-PUB-2007-004 ATLAS

26. Tommaso Lari26 January 5th-9th, 2009 Conclusions If SUSY discovery, long path to understand the nature of the involved signal In favourable scenarios (gluino or squark mass of the order of 600 GeV) ATLAS has the potential to isolate specific decay chains and measure several kinematic endpoints already with an integrated luminosity of the order of 1 fb -1 (assuming well understood detector). The reconstruction of a (large part of) the SUSY mass spectrum and a clue on the underlying physics model (including whether it is really SUSY) will require exploiting the full high luminosity potential of the LHC

27. Tommaso Lari27 January 5th-9th, 2009 Backup slides

28. Tommaso Lari28 January 5th-9th, 2009 Gluino and sbottom mass peaks Once the mass of    is known, it is possible to get the    four- momentum using p(     = ( 1-m(     m(ll) ) p ll valid for lepton pairs with invariant mass close to the edge. The    can be combined with b jets to get the gluino and sbottom masses in the decay chain g → bb → bb    ~ ~ ~ ~ ATLAS SPS1a 100 fb -1 SPS1a, fast sim., 300 fb -1