Study of supersymmetric tau final states with Atlas at LHC: discovery prospects and endpoint determination

Transcription

1 Study of supersymmetric tau final states with Atlas at LHC: discovery prospects and endpoint determination University of Bonn Outlook: supersymmetry: overview and signal LHC and ATLAS invariant mass distribution background and selection cuts endpoint measurement Maria Laach 7 1

2 Motivation for Supersymmetry Standard Model: fermions matter bosons force mediation generation of mass via Higgs mechanism Shortcomings: if R-Parity is conserved: stable LSP quarks u s b leptons d e νe c µ νµ t τ ντ + anti γ (em), g (strong), Z, W± (weak) with supersymmetric extension: no explanation for dark matter and dark energy unification of the coupling constants hierarchy problem / finetuning bosonic and fermionic contributions cancel energy dependence changes with particle content of the theory Maria Laach 7

3 Supersymmetry - Overview Supersymmetric Extension of the Standard Model: symmetry of fermions and bosons: every particle has a supersymmetric partner with spin ±1/ known particles don't built superpartners particle number doubles SUSY particles not observed so far heavy SUSY particles symmetry must be broken Minimal Supersymmetric Standard Model (MSSM): soft susy breaking terms lead to 15 add. free parameters R-parity conservation: R = -1 SUSY-particles particle content with supersymmetry R= 1 3 B L s +1 SM pair production of SUSY particles lightest SUSY particle (LSP) must be stable msugra: SUSY breaking via gravity only 5 additional free parameters left: m: scalar mass at GUT scale q quarks squarks q l l leptons sleptons neutrinos sneutrinos photon photino ±, ±, Z W, Z-Bosons W Z wino, zino W g g gluons gluinos + ± higgs-bosons h, H, A, H higgsinos H 1, H, H 1, H 1,,3,4 ± 1, m1/: fermion mass at GUT scale tanβ: ratio of Higgs vacuum expectation values A: coupling constant Higgs-Sfermion-Sfermion sgnµ: sign of higgsino mixing parameter µ Maria Laach 7 3

4 Signal Signal Channel: χ -> τ±τ χ1 two typical ATLAS points in the msugra parameter space: SU1: coannihilation-region SU3: bulk-region SU1 SU3 m 7 GeV 1 GeV m½ 35 GeV 3 GeV A GeV GeV 6 + 9GeV 3GeV Tanβ Sgnμ m(τ 1 χ1) l+-=e,µ,τ BR(χ-> e+e- χ1 ) BR(χ-> µ+µ- χ1 ).5 BR(χ-> τ+τ- χ1 ) (SU1).1 BR(χ-> τ+τ- χ1 ) (SU3) -> factor 4 to 1 more taus than electrons/muons from χ-decays goal: SUSY-masses can be measured via combinations of invariant masses in the decay chain here: mττ Maria Laach 7 4

5 The Atlas-Detector at the LHC Large Hadron Collider: 7 km circumference (built in LEP-tunnel) proton-proton-collisions at 14 TeV luminosity: cm-s-1 bunch crossing every 5 ns 111 particles per bunch A Toroidal LHC Apparatus: length: 46m diameter: m weight: 7 t study done with fast simulation (ATLFAST) of the detector: four-vectors of particles smeared with gaussian Maria Laach 7 5

6 Invariant Mass Distribution: Exspectation kinematic endpoint at max m = m m 1 m 1 m 1 m 1 max m 76GeV SU1 98GeV SU3 = events /1GeV/1fb-1 visible decay products LSP not detectable no mass peak endpoint for ττ washed out due to neutrinos (not detectable) events /1GeV/1fb-1 all decay products of τs generator level, level χ -decays ATLFAST: fast detector simulation SU3 Mττ[GeV] detector level, level all ττ (OS), fakes not included yet tail: combinatorial background! only hadronically decaying taus are considered: τ± >π±ν+nπ (1 prong) Mττ[GeV] Atlfast data samples: (Herwig) ± ± ± τ >π π π ν+nπ (3 prong) 11..4: 1.4M ev. 73 fb-1 SU3, SU3 6k ev. 77 fb-1 SU : 1.5 M ev. 79 fb-1 SU3 Maria Laach 7 6

7 pt-dependence of the rejection 11..4: Atlfast B fake tau jets parameterized in pt for three ranges of pseudorapidity: η <.7,.7< η <1.5 and 1.5< η <.5 The ATLFAST Validation Task Force: Performance of the ATLAS fast simulation ATLFAST ATL-COM-PHYS-7-1 Tau-ID: parameterizations based on full simulation = ln tan tested different efficiencies: 3 %: poor statistics 7 %: too many fakes 5% 1..6: TauRec/Tau1p3p performance of calorimeter based TauRec and track based Tau1p3p algorithm parametrized via tables of efficiency and rejection values for different pt and default mean efficiencies (used here): TauRec: 5 % Tau1p3p: 35 % (1p) / 8 % (3p) Maria Laach 7 whole study done in Athena currently confirming results in Athena

8 Background Took subsample of official production QCD-Jets: done by SUSY WG, gen. with Alpgen: : Z, tt, W, multijets, bb 1..64: Z, tt, Wbb, sliced multijets Z + (1-5)Jets: Z -> νν Z -> ll: ττ, µµ, ee tt + (-3)Jets: tt -> bb + lν lν tt -> bb + lν qq tt -> bb + qq qq W + Jets: W + (-5) Jets L (fb-1):.- per sample - per sample L (fb-1): 1-1 per sample -18 per sample L (fb-1): per sample not included yet 11..4: Multijets (-5 Jets) bb + (1-3)Jets Dijets, Pythia, Pythia in pt-bins (private production, with 1..4) 1..6: sliced Alpgen Multijets L (fb-1): 3-8 per sample not included yet Maria Laach 7 8

9 Cut Variables entries/ 5GeV/ 1fb-1 pt,miss SU1 SU3 Z+Jets W+Jets tt+jets bb+jets MultiJets Jn pair production of SUSY-particles, long decay-chains to LSP phenomenology: spheric events, many jets, large missing Energy ST: transverse sphericity (normalised) All plots with preselection-cut: Zendler pt,misscarolin >8GeV, and at 1 fb-1 pt of hardest jet entries/ 5GeV/ 1fb-1 entries/ 5GeV/ 1fb -1 ST pt of 3rd -hardest Jet pt[gev] Maria Laach 7 9 pt[gev]

10 Cut Flow SU3 3 1fb SU1 1 1 preselection (pt,miss>8gev) + τ's (8) pt,miss>3gev 3 + pt(4th)>3gev 4 + pt(3rd)>5gev 5 + pt(1st)>gev 1 red: significance s= (83) (95) (97) (1) purity = OS (OS=opposite sign) 7 + Rττ< R = efficiency = SUSY BG SUSY BG SUSY SUSY after cuts all SUSY events Maria Laach 7 1 preselection (pt,miss>8gev) + τ's (5) pt,miss>3gev 3 + pt(3rd)>5gev 4 + ST>.5 () (3) (4) OS (OS=opposite sign) 6 + Rττ< R = 1

11 Invariant Mass after Cuts events/5gev/1fb-1 OS-SS 98 events/5gev/1fb-1 τ OS SU3 Z+Jets W+Jets tt+jets M [ GeV ] OS = opposite sign SS = same sign OS-SS events/5gev/1fb-1 events/5gev/1fb-1 τ OS M [GeV ] Maria Laach 7 76 SU1 Z+Jets W+Jets tt+jets M [GeV ] large percentage SS due to bad τ-id for soft τs! -> need improved τ-id for τ's with low pt! M [ GeV ] 11

12 11..4 Sum OS-SS events/4gev/1fb-1 SU3 Endpoint Determination endpoint from linear fit very susceptible to fit range bad approximation of shape at the edge M [GeV ] Maria Laach 7 1

13 11..4 New approach: approximate shape extract endpoint from other trait measure inflection point -> more stable to change of fitting range or binning -> need calibration for endpoint: -> change involved masses m, m 1, m 1 -> measure inflection point as function of known endpoint Fit function*: p x ln x p1 exp p inflection point: x IP =exp error: 1 4 p 3 1 p1 p x p1 s =s * modified adoption from: CMS NOTE 6/96 Maria Laach 7 x x sp p1 p cov p1, p x p1 x p 13

14 events/5gev/1fb Calibration: example of variation of 1-mass (SU3: 15 GeV) for fixed m, m 1 m 1 =14GeV m 1 =13GeV m 1 =16GeV M [ GeV ] m m 1 =GeV m 1 =19GeV m(τ1) [GeV] endpoint (theoret.) [GeV] max = m m 1 m 1 m 1 m 1 m 1 =1 GeV Inflection point [GeV] Maria Laach 7 14

15 11..4 calibration line: y=.71±.9 x 13±9 GeV SU3 + BG.71±.9 x 13±9 -> measured endpoint: (97 ±9stat ± 6syst) GeV theoretical: 98 GeV Maria Laach 7 15

16 1..6 calibration line: y=.7±.6 x 7±5 GeV 11..4: y=.71±.9 x 13±9 GeV SU3 + BG ±.9 x 13±9 -> measured endpoint: (97 ± 6stat ) GeV 11..4: (97 ± 9stat ± 6syst) GeV theoretical: 98 GeV Maria Laach 7 16

17 Summary and Conclusions: Study of SUSY signals with τ-leptons Cut based selection delivers clear signal over BG in both SU1 (coannihilation region) and SU3 (bulk region) Kinematic endpoint of χ -> τ±τ χ1 measurable in SU3 Inflection point method is applicable for endpoint determination Endpoint can be measured in SU3 with 1 fb-1 (15% precision) Previous results (11..4) could be confirmed with new Atlfast TauID (1..6) Maria Laach 7 17

18 backup Maria Laach 7 18

19 11..4 variation of -mass (SU3: 15 GeV) for fixed m, m 1 m 1 =77.9 GeV m 1 =97.9 GeV m(χ1) [GeV] endpoint [GeV] m 1 =17.9 GeV Infl. point [GeV] m 1 =137.9 GeV Maria Laach 7 19

20 11..4 variation of -mass (SU3: 15 GeV) for fixedm, m 1 m =178.6 GeV m =198.6 GeV m(χ) [GeV] endpoint [GeV] m =38.6 GeV Infl. point [GeV] m =58.6 GeV Maria Laach 7

21 events/3gev/1fb Calibration: example of variation of 1-mass (SU3: 15 GeV) for fixed m, m 1 m 1 =14GeV m 1 =13GeV m 1 =16GeV M [ GeV ] m m 1 =GeV m 1 =19GeV m(τ1) [GeV] endpoint (theoret.) [GeV] max = m m 1 m 1 m 1 m 1 m 1 =1 GeV Inflection point [GeV] Maria Laach 7 1

22 1..6 variation of -mass (SU3: 15 GeV) for fixed m, m 1 m 1 =77.9 GeV m 1 =97.9 GeV m(χ1) [GeV] endpoint [GeV] m 1 =17.9 GeV Infl. point [GeV] m 1 =137.9 GeV Maria Laach 7

23 1..6 variation of -mass (SU3: 15 GeV) for fixedm, m 1 m =178.6 GeV m =198.6 GeV m(χ) [GeV] endpoint [GeV] m =38.6 GeV Infl. point [GeV] m =58.6 GeV Maria Laach 7 3