Low Mass Drell Yan at ATLAS Work done by: Tayfun Ince (now at MPI) Tony Kwan R. Keeler Queen Mary University London Group DESY Group 2014-06- 04 R. Keeler U Victoria 1
Outline MoQvaQon and Theory Analysis Results QCD fits Discussion Measurement of the low- mass Drell- Yan differenqal cross secqon at S = 7 TeV using the ATLAS Detector arxiv:1404.1212, submi\ed to JHEP 2014-06- 04 R. Keeler U Victoria 2
Leading Order DescripQon Drell- Yan Process A quark from one proton annihilates with an anqquark from the other proton 2014-06- 04 R. Keeler U Victoria 3
Electroweak ProducQon of a Lepton Pair The annihilaqon of the quark- anqquark to a photon or Z boson produces the well known Z resonance. This measurement is below the Z peak. The do\ed & dashed lines show that photon exchange dominates off of the peak 2014-06- 04 R. Keeler U Victoria 4
ComplicaQons at NLO and NNLO some hand waving theory A perturbaqve expansion of a gauge theory should be renormalizable if a gauge invariant set of diagrams are summed at each order. Order α S diagrams Problem: colliding quarks are inside a hadron so some external lines are not really external Some order α S 2 diagrams Plus diagrams with 2 external lines 2014-06- 04 R. Keeler U Victoria 5
QCD and Parton DistribuQon FuncQons (PDF s) Inside the proton has 3 valance quarks A sea of quarks and anqquarks including heavy quarks (charm and beauty) Gluons Quarks are charged and therefore the proton has: Photons PDF plot: A.D. MarQn, arxiv:0901.0002 PDFs give the probability to find a parton at a fracqon x of the proton momentum 2014-06- 04 R. Keeler U Victoria 6
Drell- Yan cross secqon σ = C ab dx a dx b ˆ a,b σ f a/a (x a,q 2 ) f b/b (x b,q 2 ) + f a/b (x b,q 2 ) f b/a (x a,q 2 ) C qq = C qq = 1 9, C qg = 1 24, C gg = 1 64 2014-06- 04 R. Keeler U Victoria 7
The x range of this measurement complements the DIS measurements made at HERA. y lepton < 2.4 m ll > 12 GeV The sensiqvity to up- type and down- type quarks is different on and off the Z pole. KinemaQcs 2014-06- 04 R. Keeler U Victoria 8
Electroweak CorrecQons Over this range of masses electroweak correcqons also have to be included. Use the G µ electroweak parameter scheme Uses the mass of the Z boson and the Fermi constant G F determined from muon decay 2014-06- 04 R. Keeler U Victoria 9
Measurements S = 7 TeV Three analyses: Nominal Extended 26 < M ll < 66 GeV 1.6 fb 1 12 < M ll < 66 GeV 35 pb 1 Electrons: trigger 2 electrons P T > 12GeV η < 2.4 excluding 1.37 < η < 1.52 E e1 T > 12 and E e2 T > 15 Jet isolation requirement Muons: trigger 2 muons P T > 10GeV η < 2.4 P e1 T > 12 and P e2 T > 15 Jet isolation requirement Extended low mass muons: trigger 2 muons P T > 4GeV η < 2.4 P e1 T > 6 and P e2 T > 9 Jet isolation requirement 2014-06- 04 R. Keeler U Victoria 10
Signal + background for electrons The dominant background is mulq- jets from QCD generated processes. Entries / 5 GeV There are two sources: Real electron pairs from heavy quark decay Hadrons misidenqfied as electrons MulQ- jet background was esqmated by looking at electron- muon same- sign and opposite sign pairs Data / MC 6 10 ATLAS 5 10 10 4 3 10 2 10 10 1.2 1 0.8-1 L dt = 1.6 fb s=7 TeV Data Z/γ* ee Multijet Z/γ* ττ tt WW,WZ,ZZ 30 35 40 45 50 55 60 65 30 35 40 45 50 55 60 65 [GeV] m 2014-06- 04 R. Keeler U Victoria ee 11
Signal and Background for muons Dominant backgrounds are tau- tau. EsQmated by a template fit. 2014-06- 04 R. Keeler U Victoria 12
Extended Muons At lower invariant mass dimuon pairs, mulqjet background dominates. 2014-06- 04 R. Keeler U Victoria 13
Combine nominal electron & muon analyses The red dots are the combined electron and muon measurements The combinaqon is done by a chi square minimizaqon that takes into account correlated errors. Uncertainty: StaQsQcal ~1% Total 3.0% - 16% 2014-06- 04 R. Keeler U Victoria 14
Results of extended analysis In order to compare the extended mass range analysis with the nominal, the cross secqons have to be corrected to full acceptance. The agreement is good. 2014-06- 04 R. Keeler U Victoria 15
Comparison to theory Nominal Extended
QCD Fit by DESY Group Slide by Elena Yatsenko 2014-06- 04 R. Keeler U Victoria 17
QCD Fit by DESY Group Slide by Elena Yatsenko ß Nominal à Extended
Discussion We have made a new measurement of the Drell Yan cross secqon dσ / dm ll below the Z mass using data at S = 7 TeV taken by ATLAS at the LHC in 2011. A QCD fit was done with the HERA PDF parameterizaqon. Conclusion: NNLO QCD calculaqons are required to fit the data. 2014-06- 04 R. Keeler U Victoria 19
QCD Fit 2014-06- 04 R. Keeler U Victoria 20