Tommaso Lari (CERN/INFN Milano) On behalf of the ATLAS Collaboration Supersymmetry measurements with ATLAS After we have discovered New Physics, can we understand what it is?
Download ReportTranscript Tommaso Lari (CERN/INFN Milano) On behalf of the ATLAS Collaboration Supersymmetry measurements with ATLAS After we have discovered New Physics, can we understand what it is?
Tommaso Lari (CERN/INFN Milano) On behalf of the ATLAS Collaboration Supersymmetry measurements with ATLAS After we have discovered New Physics, can we understand what it is? 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 January 5th-9th, 2009 Tommaso Lari 2 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 mSUGRA parameters MSSM: ~100 free parameters (all possible SUSY breaking terms in the EW scale effective lagrangian) Constrained models have few parameters, with assumptions January 5th-9th, 2009 Tommaso Lari 3 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), t, g, Z, h Corresponding searches sensitive to a large number of SUSY models/parameters, but also to other new physics with similar signatures January 5th-9th, 2009 Tommaso Lari 4 What might we know from inclusive analyses? First step: establish excess over Standard Model expectations, make ATLAS sure it is from new physics 10 fb-1 The Atlas Collaboration, Observation of events with large transverse missing energy and high pT 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. ATLAS January 5th-9th, 2009 Tommaso Lari 10 fb-1 5 Information to establish SUSY? Information desired Observables Each SM particle has a superpartner Their spin differ by ½ The couplings are the same SUSY mass relation holds Production cross sections Masses of new particles Angular distribution of decays Branching ratios 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. January 5th-9th, 2009 Tommaso Lari 6 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 January 5th-9th, 2009 Tommaso Lari 7 mSUGRA benchmarks Benchmarks have been chosen requiring that neutralino relic density matches DM constraints SUn = mSUgra benchmark n (no reference to simmetry groups!) January 5th-9th, 2009 Tommaso Lari 8 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 January 5th-9th, 2009 Tommaso Lari 9 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. January 5th-9th, 2009 Tommaso Lari 10 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 January 5th-9th, 2009 Tommaso Lari 11 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 The relevant decay chain is open in a large fraction of SUSY parameter space. Mll (GeV) January 5th-9th, 2009 Tommaso Lari 12 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 c2. January 5th-9th, 2009 Tommaso Lari 13 Lepton+jets combinations January 5th-9th, 2009 Lepton+jets combinations give further mass relations The two jets with highest pT are likely from squark decay – but which one belongs to the right decay chain? Tommaso Lari 14 Lepton+jets combinations llq edge lqmax edge llq threshold lqmin edge For this particular benchmark (bulk point SU3) all constraints measurable with 1 fb-1 ! January 5th-9th, 2009 Tommaso Lari 15 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. SPS1a, fast simulation, 100 fb-1 SU3, full simulation, 1 fb-1 ATLAS Sparticle Expected precision (100 fb-1) ~ qL 3% ~c02 6% ~ lR 9% ~c01 12% January 5th-9th, 2009 Tommaso Lari 16 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 tanb, and it may be ~0 → tt ~ is the only two-body decay open. that c 2 The polarization of taus also carries interesting information (different in various SUSY breaking models). Feasiblity of polarization measurements still under investigation. January 5th-9th, 2009 Tommaso Lari 17 Measurement of tt edge SU3, full sim., 1 fb-1 January 5th-9th, 2009 The inflection point of the tt 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 Tommaso Lari 18 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). ~ ~ ~ 0 ~ 1) Applicable to mSUGRA qR as BR(qR → q c 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 January 5th-9th, 2009 Tommaso Lari 19 A 3rd generation example Using the low-mass SU4 point with large BRs in 3rd 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 January 5th-9th, 2009 Tommaso Lari 20 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 constraints (assuming mSUGRA) Parameter m0 m1/2 tan(b) A0 January 5th-9th, 2009 Tommaso Lari Expected precision (300 fb-1) 2% 0.6% 9% 16% 21 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. Wch2 = 0.1921 0.0053 log10(scp/pb) = -8.170.04 January 5th-9th, 2009 Tommaso Lari 22 Focus Point study Interesting information possible from few measurements In Focus Point region relic density ok because gaugino mass parameters (M1, M2, m) are of the same order giving a large Higgsino component to c01 For SU2 benchmark, two lepton edges observable. Using only this info, a fit of gaugino mass parameters, assuming unification relation M1 = 0.5 M2 (but not mSUGRA) tells that indeed m ~ M1 m (GeV) c0→c0l+lc0→c0l+l- tanb Eur. Phys. J. C52, 743 ATLAS 300 fb-1 January 5th-9th, 2009 Tommaso Lari M1 (GeV) 23 M1 (GeV) 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 qlfar and qlnear distributions 2. The charge coniugate decay gives the opposite asymmetry. Cancellation not exact at a pp collider however. January 5th-9th, 2009 Tommaso Lari 24 Spin measurement SU3 point: 19.3 pb x 3.8% Ratio squarks/antisquarks ~3 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 ATLAS ATLAS ATLAS-PHYS-PUB-2007-004 January 5th-9th, 2009 Tommaso Lari 25 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 January 5th-9th, 2009 Tommaso Lari 26 Backup slides January 5th-9th, 2009 Tommaso Lari 27 Gluino and sbottom mass peaks Once the mass of c0is known, it is possible to get the c0fourmomentum using p(c0)= ( 1-m(c0)/m(ll) ) pll valid for lepton pairs with invariant mass close to the edge. The c0can be combined with b jets to get the gluino and sbottom ~ → bbc0 masses~ in the decay chain g~ → bb SPS1a, fast sim., 300 fb-1 SPS1a, fast sim.,ATLAS 300 fb-1 SPS1a 100 fb-1 ~ January 5th-9th, 2009 Tommaso Lari 28