Electroweak measurements at HERA

Transcription

1 Electroweak measurements at HERA Alex Tapper DESY forum 1 th & 13 th September 006 Precision electroweak measurements: What can HERA contribute?

2 Outline Introduction High Q physics at HERA Review of recent results Future prospects Summary e ± p 7.5 GeV 90 GeV Page

3 Introduction LEP/SLD M Z, Γ Z, σ 0 h, R l, A FB, P τ, R b, R c, A FBb, A FBc, A b, A c, A l, Q FB.. e e p p Tevatron M W, Γ W, m t.. HERA t-channel exchange of W ± and Z 0 High Q at HERA Q ~electroweak scale Need high luminosity Longitudinal polarisation of lepton beam Cross sections are convolution with proton structure functions Need to take care of this in EW measurements "(ep) = $ "(eq) # q(x,q ) % EW # QCD q HERA Page 3

4 Introduction Two deep inelastic scattering processes: Neutral current: exchange of γ or Z 0 Charged current: exchange of W ± Q = "q = "(k " k #) Q is the probing power x is the Bjorken scaling variable y is the inelasticity x = Q y = p"q s = (p + k) p"q p"k Q = x " y " s Page 4

5 Charged current DIS at HERA CC e + p cross section: Sensitive to density of d quark d " CC (e + p ) dxdq = G F # ( M W ) u + c + (1$ y) (d + s) M W +Q [ ] CC e - p cross section: d " CC (e # p ) dxdq = G F $ ( M W ) u + c + (1# y) (d + s ) M W +Q [ ] Sensitive to density of u quark Electroweak couplings and propagators the same but electron/positron-proton collisions probe different quark content of proton Big difference in cross section magnitude u-quark density larger than d-quark d-quark contribution suppressed by helicity factor (1-y) Page 5

6 Neutral current DIS cross section d " NC ( e ± p) dxd Q = # $ 4 Y + [ F % x Q y Y + Y F L m % Y + = 1± xf 3] Y ± (1! y) Dominant contribution Sizeable only at high y Contribution only important at high Q F = F em + Q Q +M Z F "Z + [ Q ] Q F Z +M # $ (q + q ) Z q= u...b xf 3 = F em + Q Q +M Z xf 3 "Z + [ Q ] Q xf Z +M 3 # %(q $ q ) Z q= u...b Page 6

7 Recent results: Combined fits Fit only HERA data Neutral current DIS cross sections Charged current DIS cross sections Inclusive jet cross sections in NC DIS Di-jet cross sections in photoproduction Fit for BOTH the PDFs at NLO in QCD and electroweak parameters Fits for PDFs follow previous publications H1 PDF Eur. Phys. J. C30 (003) 1. ZEUS-JETS - Eur. Phys. J. C4 (005) 1. Page 7

8 Reminder of QCD fits for PDFs F dominates cross section Directly sensitive to sum of quarks and antiquarks Gluon density via scaling violations at low x (and jet data is ZEUS fit) Valence quark distributions from high Q CC and NC cross sections and sum rules Idea that low Q data dominate the PDFs and high Q the EW parameters First H1 fit, then ZEUS. Page 8

9 Combined fit: M W Phys. Lett. B636 (000) 1 Look at the EW part of CC DIS cross section in more detail d " CC (e ± p ) dxdq ( ) $ % ± (x,q ) = G F $ M W # Q +M W Simplest fit: M W & PDF parameters free (α s fixed) G F fixed to value from muon decay NC EW parameters (α, M Z, G F ) fixed to PDG values Sensitivity comes solely from shape of cross section as a function of Q Page 9

10 Combined fit: M W Fit for M W and PDFs simultaneously yields Phys. Lett. B636 (000) 1 M W = 8.87 ±1.8(exp) "0.16 (mod) GeV Model uncertainties include α s, Q 0, Q min etc. χ/ndf= PDG M W =80.403±0.09 GeV Small correlation between PDFs and M W Model independent measurement of mass of whatever mediates CC DIS reaction at HERA Page 10

11 Combined fit: M W Phys. Lett. B636 (000) 1 Replace G F with SM expression in the on mass shell scheme d " CC (e ± p ) dxdq ( ) $ % ± (x,q ) = G F $ M W # Q +M W d " CC (e ± p ) #$ = dxdq & 4 4M W ( ' 1% M W M Z ) + *, 1 1%-r, ( M W ) Q,. ± (x,q ) +M W Not a measurement but determination of a parameter within the SM framework But what about Δr? Page 11

12 Combined fit: M W Need to calculate Δr (α, M Z, M W, M H, m t ) W propagator self energy Use EPRC by H. Spiesberger Phys. Lett. B636 (000) 1 "r = "# $ cos % W sin % W "& + "r rem "# lep "# had "# $ m t "r rem #ln M H Photon vacuum polarisation: lep is computable had from e + e - data (combined 0.059) Large mass difference between top and b-quarks (combined 0.03) Log dependence on Higgs mass, m t and higher order corrections (0.01) Page 1

13 Combined fit: M W So what did we gain? B. Portheault DIS 05 Now the normalisation of the CC DIS cross section also contributes Increase in sensitivity (also do a similar thing with NC cross section to gain some sensitivity from there too) Page 13

14 Combined fit: M W Use m t =178 GeV, M H =10 GeV χ/ndf=0.87 Phys. Lett. B636 (000) 1 M W = ± 0.05(exp) "0.09 (mod) ± 0.05(m t ) "0.084(M H ) ± 0.033(#r) GeV Use world average M Z to get sin " W =1# M W M Z sin " W = 0.151± (exp) # (th) Can also turn this around and using world average M W estimate m t or even M H from HERA data! Page 14

15 Neutral current DIS cross section Z 0 q q ig cos" W # µ v q $a q # 5 Axial coupling: a q =T 3 L (=+1/ for u, -1/ for d) Vector coupling: v q =T 3 L-e q sin θ W F = F em + Q Q +M Z F "Z + [ Q ] Q F Z +M # $ (q + q ) Z q= u...b xf 3 = F em + Q Q +M Z xf 3 "Z + [ Q ] Q xf Z +M 3 # %(q $ q ) Z q= u...b Page 15

16 Combined fit: a u,d and v u,d Phys. Lett. B636 (000) 1 F = $ ( e q " e q v q v e P Z + (v e + a e )(v q + a q )P ) Z # x(q + q ) q= u...b $ q= u...b xf 3 = ("e q a q a e P Z + 4a q v q v e a e P Z ) # x(q " q ) Remember that P Z >>P Z and v e ~0.04 xf 3 γ-z 0 interference term is largest P Z = 1 sin " W Q Q +M Z Expect axial coupling of u-quark to be best constrained Fix G F and M W in CC and α, M Z and M W in NC and fit for all four couplings a u,v u,a d,v d Page 16

17 Combined fit: a u,d and v u,d Phys. Lett. B636 (000) 1 First HERA measurements More sensitive to u-quark as we expected Page 17

18 Combined fit: Isospin Phys. Lett. B636 (000) 1 Test sensitivity to righthanded weak isospin 3 v q = T q,l 3 a q = T q,l 3 " T q,r 3 + T q,r " e q sin # W Fix T 3 q,l and sin ϑ W to SM values and fit gives right handed values consistent with zero Page 18

19 Polarised charged current DIS Charged current is left-handed in Standard Model Polarisation is asymmetry of helicity states W ± v q ig " µ 1#" 5 Can use polarised beams to directly test chiral structure of the Standard Model CC cross section modified by P e : e " ± p e CC (P e ) = (1± P e ) #" ± p CC (P e = 0) P e = N R "N L N R +N L Polarisation scales P e =0 cross section linearly - clear and large effect at HERA Standard Model predicts zero cross section for P e =+1(-1) in e -(+) p scattering Page 19

20 Dependence on P e Submitted to ICHEP 06 Clearly demonstrate linear dependence on P e Consistent with left-handed weak interaction in SM " e + p (P e = #1) = #3.9 ±.3(stat.) ± 0.7(syst.) ± 0.8( pol.) " e + p (P e = #1) = 7.4 ± 3.9(stat.) ±1.(syst.) " e # p (P e = +1) = #0.9 ±.9(stat.) ±1.9(syst.) ±.9( pol.) " e # p (P e = +1) = 0.8 ± 3.1(stat.) ± 5.0(syst.) Best constraint so far M W,R >08 GeV Page 0

21 Combined fit: M W Submitted to ICHEP 06 M W (GeV) ZEUS (HERA I 60 pb -1 ) H1 (HERA I 10 pb -1 ) ZEUS (HERA-II 40 pb -1 (prel.)) 78.9 ±.0 (stat.) ± 1.8 (syst.) ±.0 (PDF) 8.87 ± 1.8 (stat.) ± 0.5 (syst.) 79.1 ± 0.77 (stat.) ± 0.99 (syst.) Improvements in precision from: Extra luminosity Remember e - p has much higher CC DIS cross section and HERA II data is e - p Combined fit Reduction in systematic error by fitting PDFs too Page 1

22 Polarised NC DIS cross sections NC cross section modified by P: d " (e ± p) = #$ [ H ± ± dxdq xq PH ] P P = N R "N L N R +N L Unpolarised contribution Polarised contribution - only includes Z and γz terms F P = $ ( e q a e v q P Z " a e v e (v q + a q )P ) Z # x(q + q ) q= u...b $ q= u...b xf 3 P = (e q a q v e P Z " a q v q (v e + a e )P Z ) # x(q " q ) Expect vector couplings to improve with polarised data Page

23 Combined fit: a u,d and v u,d Submitted to ICHEP 06 Polarised lepton beam gives substantial improvement - particularly in v u,d as expected - fit is for v u,d (a u,d ) while fixing a u,d (v u,d ) Page 3

24 Combined fit: a u,d and v u,d Submitted to ICHEP 06 HERA measurements competitive Page 4

25 Combined fit: Isospin Test sensitivity to righthanded weak isospin Submitted to ICHEP 06 3 v q = T q,l 3 a q = T q,l 3 " T q,r 3 + T q,r " e q sin # W Fix T 3 q,l to SM values and fit gives right-handed values consistent with zero Page 5

26 Other ideas Variety of ratios and asymmetries of cross sections possible, for example R ± = " NC (e ± p ) " CC (e ± p ) DESY-THESIS A ± = " NC (e R ± p )#" NC (e L ± p ) Next slide " NC (e ± R p )+" NC (e ± L p ) sin " W = 0.7 ± # B ± = " NC (e ± R p )#" NC (e m L ) " NC (e ± R p )+" NC (e m L p ) C L,R = " NC # (e L, R # " NC (e L, R p )#" NC + (e L, R p )+" NC + (e L, R p ) p ) Idea that experimental systematics and PDF dependency reduced by cancellation WWγ coupling from radiative charged current, real W and Z production. sensitivity modest. Page 6

27 Neutral current P e asymmetry Form the polarisation asymmetry: Submitted to ICHEP 06 A ± = P R "P L # ± (P R )"# ± (P L ) # ± (P R )+# ± (P L ) to a good approximation A ± " mka e F #Z F Q 4 sin " W cos " W Q +M Z k = 1 which is quite insensitive to the PDFs and proportional to a e v q and therefore a direct measure of parity violation More accurate determination with more data (this with 0.48 fb -1 ) Page 7

28 Future prospects: Luminosity H1 fit (~10 pb -1 ) a u v u a d v d v u -a u -v d -a d -PDF 0.56± ± ± ±0.37 v u -a u -PDF 0.57± ±0.13 v d -a d -PDF -0.80± ±0.33 SM ZEUS fit (~40 pb -1 ) a u v u a d v d v u -a u -PDF 0.5±0.04± ±0.06±0.06 v d -a d -PDF -0.49±0.14± ±0.14±0.16 a d -a u -PDF 0.48±0.06± ±0.10±0.1 v d -v u -PDF 0.1±0.10± ±0.15±0.19 u-quark better constrained (e + p data will help d-quark) precision better than 0% for u-quark but ~50% for d-quark Basically measurements scale with luminosity Page 8

29 Future prospects: Polarisation HERA workshop 95 M. Kataoka M WR (GeV) Stat only Stat Syst % 4% Results a strong function of polarisation - higher polarisation would offer improvement P Page 9

30 Summary First simultaneous determinations of the PDFs and EW parameters Couplings of u and d quarks to Z 0 competitive with determinations from LEP and Tevatron experiments Can expect improvements in precision with increasing data sets (and polarisation) Need to combine H1 and ZEUS data Sensitivity to other EW parameters at a level which is complementary to other experiments, so we should stress the differences and attack the Standard Model from different angles Page 30