Search for New Physics at HERA

Transcription

1 Search for New Physics at HERA Yongdok Ri (KEK) on behalf of the H and ZEUS collaborations Introduction of HERA Model dependent search Model independent search Summary LES RENCONTRES DE PHYSIQUE DE LA VALLEE D AOSTE March 7, 009, La Thuile, Aosta Valley, Italy

2 The world only ep collider HERA DESY, Hamburg ~6.3km circumference e ± p s 7.5GeV (up to) 90GeV 38GeV Two collider experiments : H and ZEUS

3 HERA operation Integrated Luminosity (pb ) HERA delivered HERA II (polarized e ± ) ~0.4fb / exp HERA I ~0.fb / exp Started on summer 99 and ceased in June 007 Successful 6 years Upgrade in 000/00 HERA I + HERA II ~0.5 fb / experiment days of running H + ZEUS ~fb 3

4 Neutral or Charged Current (NC or CC) p Deep Inelastic Scattering at HERA HERA = e- collider e,ν (k ) e(k) γ,z W Low-Q : measure PDFs QCD evolution High-Q : Investigate fundamental e- scattering (pb/gev dσ/dq 4-mom transfer suared, Q =(k-k ), = spatial resolving power Simultaneous NC/CC investigation up to Q ~40,000GeV 0.00 proton size HERA I & II ) EM+Weak + H e p CC (prel.) - H e p CC 005 (prel.) + ZEUS e p CC ZEUS e p CC SM e p CC (HERAPDF0) - SM e p CC (HERAPDF0) y < 0.9 P e = H e p NC (prel.) - H e p NC (prel.) + ZEUS e p NC (prel.) - ZEUS e p NC SM e p NC (HERAPDF0) - SM e p NC (HERAPDF0) Pure-weak 4 Q (GeV ) New physics would appear at short distance. Let s search! 4

5 Model dependent search. HERA is a giant electron-microscope Quark structure e γ,z,w e, ν. If new interaction between e- scattering Contact Interaction p 3. If fermion is composite Excited fermion (e,ν,) 5

6 N/N CTEQ5D Are uarks really point-like? Remember Hofstadter s form-factor measurement Decrease from σ ep elastic at large scattering angle (high Q ) Discovery of Proton substructure = not point-like At HERA, analogous search for uark down to ~ -3 fm If charge distribution of uark (R ) is finite, cross section deviates from SM (sees small coupling) Quark Radius Limits (prel.) ZEUS σ=σ SM (-<R > Q /6) ZEUS (prel.) e ± p R = (0.6 6 cm) R = -(0.8 6 cm) No deviation set Q (GeV ) assuming electron is point-like limit on R (95% CL) ZEUS : R <0.6* -3 fm H : R <0.74* -3 fm 6

7 Contact Interactions (CI) Physics at much higher energy scale is described by four-fermion CI at low energy limit, remember early days of weak int., i.e. G F ~e /sin θ W M W. Various new physics, e.g. new gauge bosons, large extra dimensions, composite fermions, heavy leptouarks, would add new term SM New interaction e e e e e LEP e σnc γ + Z + HERA? Tevatron Physics at energy scale larger than modifies σ via virtual effects s Search at HERA is complementary 7

8 Effective Laglangian for vector ee CIs : L CI u, d = = i, j= L, R Coupling : η µ ij ( eiγ ei )( jγ µ η = ±4π / Λ Λ : new physics scale Different η sign makes different SM-CI interference Each model assumes different chiral structure, e e given by η a set of 8 coefficients? e:l or R :L or R :up or down j ) Considered CI models 8

9 N/N CTEQ5D N/N CTEQ5D Contact Interactions Limits (prel.) ZEUS Contact Interactions Limits (prel.) 3 4 CI search results ZEUS e + p VV Λ = 8.0 TeV VV Λ + = 6.3 TeV e + p VV ZEUS (prel.) e - p AA Λ = 6.7 TeV AA Λ + = 6. TeV e - p AA Q (GeV ) Q (GeV ) ZEUS (94-06) : Λ>.0-8.0TeV H (HERA I): Λ>.6-5.5TeV at 95% CL VV AA VA X X X3 X4 X5 X6 U U U3 U4 U5 U6 LL LR RL RR e ± p /Λ best fit value allowed ±/Λ range ZEUS (prel.) e ± p +/Λ best fit value Λ (TeV) Λ + (TeV) ±/Λ (TeV9 - )

10 Excited fermion Possible origin of three-family structure and mass hierarchy of fermions Compositeness model of fermions Excited leptons and uarks could radiate gauge bosons (Analogy : Composite matter has excited states which become stable with emission, e.g. excited atom emits X-ray) Search mass resonance in fermion+boson pair Example diagram of e* production e, ν e e* γ, Z, W γ, Z e* eγ, ez, νw ν* νγ, νz, ew * γ, Z, W with subseuent hadronic or leptonic decays of W and Z are considered.

11 Events Events Mass distribution of fermion+boson pair Search for e* at HERA (475 pb ) H H Data All SM e* signal e* e γ (ela.) e* Mass [GeV] Search for ν* at HERA (e p, 84 pb H H Data SM ν* ν γ ) ν* Mass [GeV] Events 8 6 ± Search for * HERA I+II (e p, 475 pb ) H Preliminary H Data (prelim.) All SM * signal M=40 GeV * γ e* 4 * ν* * Mass [GeV] Only fγ decay mode is shown M f* =40GeV (arbitrary norm.) No excess set limits Effective Laglangian for f-f* interaction a a * µν τ a ' ' Y λ a Lint. = FRσ gf Wµν + g f Bµν + gsfs Gµν FL + h.c. Λ SU() U() SU(3) Unknown parameters for Excited fermion f,f,f s : coupling associated to 3 gauge bosons Λ : Energy scale of compositeness For given M f* and relation f, f and f s, σ only depends on f/λ

12 Mass dependent exclusion limits on f/λ ± Search for e* at HERA (475 pb ) Search for * HERA I+II (e p, 475 pb ) f = + f f = + f, f = 0 H s at 95% CL ] f / Λ [GeV ] f / Λ [GeV f / Λ = / Me* Tevatron (0 pb ) LEP (direct) LEP (indirect) H e* Mass [GeV] Search for ν* at HERA (e p, 84 pb ) H DELPHI ν* Mass [GeV] ] f / Λ [GeV e*:7gev - f = + f H f / Λ = / M ν* ν*:96gev DELPHI BR(* γ)= H HERA I (37 pb ) *:59GeV H HERA I+II (preliminary) f / Λ = / M* H Preliminary * Mass [GeV] Assumption f=+f (f=-f also derived for ν*, not shown) HERA s uniue sensitivity e* : M e* >~00GeV & f/λ<~3* -3 ν* : M ν* >~00GeV * : M * >~80GeV, and f s =0 only tested at HERA Complementary to LEP and Tevatron

13 Model independent search Signature based search by looking for deviations from SM in various topologies. Don t rely on any a priori definition of new physics Following type of event is interesting : Rare event Accurately calculable process Having experimentally clean signature Presented are :. Multi-lepton events. Isolated lepton + missing p T event 3. General search 3

14 Multi-lepton events Example diagram of lepton-pair production e e l Dominant process is γ-γ, i.e. QED process accurately calculable Production of high p T lepton is rare and clean signature New particle could be produced l Search for multi-lepton events at high invariant mass p Scattered electron escapes the detector for almost all cases, thus : two leptons (+ e) 4

15 Events 3 - Multi-Leptons at HERA (463 pb ) H H Data All SM Pair Prod. ee ee M ee [GeV] 0.46fb 0.48fb eµ Various e and µ channels Topologies with e or µ : ee, eµ, µµ, eee, eµµ Events Multi-Leptons at HERA (463 pb ) H H Data All SM Pair Prod. µµ µµ Both experiments finished their analysis using full data M µµ [GeV] eee Events All SM Multi-Leptons at HERA (463 pb ) H H Data Pair Prod. eµµ eµµ Hottest ZEUS results! Released very recently M [GeV] 5

16 Combined results for e and µ channels Events H Data 3 All SM Pair Prod. - Multi-Leptons at HERA (e + p and e p, 463 pb ) H +3 leptons Σ P T [GeV] Consistent results with the SM Combination of H and ZEUS results is ongoing 6

17 τ-pair production Hadronic channel is important, i.e. BR(τ ν τ +hadrons)>60%, but challenging Discriminate from QCD-induced jets low mass low multiplicity pencil-like M ττ visible=4gev Data SM Good agreement with SM 7

18 Isolated lepton + missing p T event For NC or CC DIS, either isolated lepton or missing p T Events having both topologies are very rare In the SM, main source is W production Example diagram of W production X e γ,z,w e,ν W (X) ν l µ Run Event 7593 Z R High hadronic p TX > 5GeV l ν l X µ X Y 8

19 H and ZEUS results using full data 0.50fb e-channel Events H 0.47fb H Data All SM Signal e+µ-channel H also performed τ-channel and agreement with SM is confirmed : Events H H Data All SM Signal µ-channel X [GeV] P T X [GeV] P T No significant deviation from SM Measure W production cross section H.4 ± 0.5(stat.) ± 0.4(syst.)pb ZEUS SM.3 ± 0.pb (stat.) ± 0.(syst.)pb 9

20 H+ZEUS combined result H and ZEUS combined in common phase space (note: Shown is based on preliminary results.) Events e, µ + P miss T H+ZEUS Data (prelim.) All SM Signal events at HERA I+II (e ± p, 0.97 fb ) N Data N SM = 87 = 9.7 ± X (GeV) Combination using final results is ongoing 0 P T HERA Exotics Working Group

21 + H General Search at HERA (e p, 85 pb ) j-j e-j µ-j ν-j e-ν e-e e- µ µ- µ γ-j γ-e γ-µ γ-ν γ-γ j-j-j e-j-j µ-j-j ν-j-j e-e-j e-e-ν e-e-e µ -µ-j e- µ- µ µ-µ- ν e-µ-j e-ν-j µ-ν-j e-µ-ν γ-j-j γ-e-j γ-ν-j e-j-j-j ν-j-j-j γ-ν-j-j e-ν-j-j γ-e-j-j e-e-ν-j e-µ-ν-j j-j-j-j e-j-j-j-j ν-j-j-j-j j-j-j-j-j - Using e + p data e - p data also analyzed as well 3 H H Data SM 4 5 Events General search Investigate final states with high-p T (>0GeV) objects : e, γ, µ, ν, jet At least one event is found in 7 topologies for e ± p data Deviation from SM is searched for in distributions of scalar sum of p T and invariant mass M all H Data SM Region of largest deviation e - e Σp T High-p T SM phenomena at HERA are well understood e - e M all - e

22 Summary The only ep collider, HERA, finished 6 years of successful data taking, and ~fb data were taken by H+ZEUS. HERA provides the uniue and complementary sensitivity to new physics. In this talk, recent results of : uark form factor, contact interactions, excited fermions, multi-leptons, isolated lepton+p T miss, general search were presented. Generally good agreement with the SM even at new kinematic domain explored by HERA Searches at HERA are being finalized for each H and ZEUS, and to gain higher sensitivity, combination of results by two experiments is ongoing.