- Email: [email protected]
Search for contact interactions and leptoquarks at HERA
ELSEVIER
PROCEEDINGS SUPPLEMENTS
Nuclear Physics B (Proc. Suppl.) 79 (1999) 152-154
www.elsevier.nl/locate/npe
Search for Contact Interactions and Leptoquarks at HERA J. Scheins a aI. Physikalisches Institut, R W T H Aachen, D-52056 Aachen, G e r m a n y (on behalf of the H1 collaboration) The measured single differential cross section da/dQ 2 is compared to the Standard Model expectation and analyzed to search for (ge)(~q) contact interactions. No significant deviation is observed and limits on leptonquark compositeness scales and virtual leptoquark exchange are presented. H1 preliminary
1. I N T R O D U C T I O N A
New currents or heavy bosons m a y produce indirect effects through the interference of a virtual particle exchange with the 3, and Z fields of the Standard Model. For particle masses well above the available production energy, such indirect signatures m a y be investigated by searching for a 4-fermion pointlike (~e)(Ctq) contact interaction. The Contact Interaction Lagrangian [1] including only vector t e r m s contains 8 coupling q where the indices i , f = { L , R } coefficients ~?if denote the left-handed and r i g h t - h a n d e d fermion helicities and q = (u, d}. The general ansatz [1] can be applied to any new phenomena, e. g. (e q) compositeness, leptoquarks or new gauge bosons, by an appropriate choice of the coupling coefficients. Possible deviations of the measured cross section from the S t a n d a r d Model are analyzed in t e r m s of contact interactions. Figure 1 shows the measured single differential cross section d a / d Q 2 [2], which is well described over six orders of magnitude by the S t a n d a r d Model expectation. Choosing different p a t t o n distributions ( C T E Q a D , MRST, GRV 94 ) [3] for the cross section calculation, does not change the shape of the Q2 spectrum significantly, but rather the normalisation by up to 1.2%. 2. C O M P O S I T E N E S S
SCALES
A possible fermion compositeness or substructure can be expressed by the coupling coefficients 7?if - ± g 2 // A ±if 2, where the signs indicate positive and negative interference with the Standard
10 2
'
' '''"'1
' <"
~
.......
I
e*p date
H1
Standard Model pdf
-,
CTEQID
\
-1 10
"\
1 0 -2
\ I1\,
\
10 4
1 0 -4
\ 1 0 -s
........
I
,
, ,,,,,,I
,
3 10
,
4 10
Q=(GeV2)
Figure 1. Differential NC cross sections d a / d Q 2. H I d a t a (e) are compared with the Standard Model expectation ( - - ) using C T E Q 4 D p a t t o n distributions.
Model currents. The coupling strength is conventionally chosen as g2 _ 4~r and A denotes the compositeness scale. Both u and d quarks are assumed to contribute with the same scale. The results of the X2 fits for different chiral structures are compiled in Table 1. The dominant correlated systematic errors, i. e. the experimental uncertainty of the lepton energy scale, the scattering angle and the theoretical uncertainty due
0920-5632/99/$ - see front matter © 1999 ElsevierScience B.V. All rights reserved. PII S0920-5632(99)00660-X
153
J Scheins/Nuclear Physics B (Proc. Suppl.) 79 (1999) 152-154 to the uncertainty of (~s, are taken into account in the fit procedure. The pure chiral couplings prefer negative interference resp. negative values of ~ as a consequence of the trend of the data being slightly low compared to the Standard Model prediction around Q2 _ 5,000 GeV 2 followed by an upward fluctuation at higher Q2 (fig. 2). The LR model gives a best description of the data, however the couplings r}/4~r for all investigated chiral structures are compatible with the Standard Model within two standard deviations.
H1
~o 4 D '10
o
preliminary
........
I
'
' ' r''''[
H1 e+p data
• - -
be41t f;1 f o r
.....
A" : 1.5 T e V 9 5 % C L
..........
A +
'O
1
.$ T e V
~-) int. (-) i n t
ffi 3.2 TeV 9 5 % C L (÷)
i :
t~[.
CTEQ4D pdf
i
Table 1 95% cl lower limits on compositeness scale parameters A ± i
H1 P r e l i m i n a r y coupling
A + [TeV]
A - [TeY]
,
,
,,iJ,iF
,
10
LL
2.3
1.2
LR
3.0
1.5
RL
3.0
1.5
RR
2.3
1.2
VV
5.0
1.9
AA
2.0
3.4
VA
2.6
2.6
LL+RR
3.1
1.3
LR+RL
4.1
1.8
Limits on the compositeness scale parameters are derived by fitting A+ and A - with the appropriate interference signs and taking the corresponding changes in X2 with respect to the fit to the Standard Model, which provides a good description of the data. The most conservative values of A+ obtained from a variation of the parton distributions are quoted in table 1. The limits on compositeness scale parameters vary between 1.2 TeV and 5.0 TeV depending on the chiral structure of the model. The data are particularly sensitive to the VV, AA and VA models, where all chiral structures contribute with different relative signs. In general the limits of A+ are more restrictive compared to A - due to the preference of negative interference.
3
,
,L,,,,I
j
10
4
Q= (GeV2)
Figure 2. Cross section do'/dQ 2 normalised to the Standard Model expectation (CTEQ4D); HI data (.) compared to the best fit (--) resp. limits A + and A - of the LR model
3. L E P T O Q U A R K S Leptoquarks are colour triplet bosons of spin 0 or 1, carrying lepton (L) and baryon (B) number and fractional electric charge. The leptoquark mass MLQ and its coupling A are related to the contact interaction coefficients ~iq/ ---cq/()~/MLQ) 2. The notation [1] and the coupling coefficients ~ i / f o r leptoquarks of the BuchmiillerRiickl-Wyler Model [4] are given in Table 2. The differential cross section analysis shows no evidence for a virtual leptoquark signal. The resulting lower limits on M L Q / ~ , which is the only unknown parameter, are summarized in Table 2. They include all experimental systematics and represent the most conservative values obtained from a variation of different patton densities. In general vector leptoquarks which couple to u quarks with positive interference provide the most restrictive limits of MLQ/)~ ~ O(1.0 WeV).
3:. Scheins/Nuclear Physics B (Proc. Suppl.) 79 (1999) 152-154
154
Table 2 Coupling coefficients ~qf, fermion number F and 95% cl lower limits on MLQ/)~ for scalar (S) and vector (V) leptoquarks of the Buchmiiller-Rfickl-Wyler Model. The used notation indicates the lepton chirality L, R and weak isospin I -- 0, 1/2, 1. S and V differ by two units of hypercharge from S and V. (Sign errors of the coupling coefficients for the F = 2 scalar leptoquarks and the F = 0 vector leptoquarks in [1] are corrected [5].) H1 P r e l i m i n a r y leptoquark
coupling to u quark [GeV -2]
coupling to d quark [GeV -2]
F = 3B + L
MLQ/~
598 554
SoL
77~L = +½ ()~/MLQ) 2
2
SoR
T]URR= +31 (~/MLQ)2
2
DoR
d : +-~1 ()~/MLQ)Z zlnn
sLI2
7]uLn ---- --31 ()~/MLQ)2
S~2
'7~L ~- _½ (~/MLQ)2
D~I2 Sf
?~L = +½ (;~/MLQ)2
vL
0
d = - - 3 , (A/MLQ) 2 r]RL ns.n
0 2
~?~LL= - - 1 (A/MLQ) 2 ~nn = - 1 (A/MLQ) 2
0 0
(1oR v~ 2 V~2
~uR R = - 1
(A/MLQ) 2
~ n -----+1 (A/MLQ) 2
d R = +1 ()~/MLQ) 2 7]5 ~dRL = +1 (~/MLQ) 2
V~2 VL
~ n = +1 ()~/MLQ) 2 77~L : - 2 ()~/MLQ) 2
~dLL -: - 1 (A/MLQ) 2
It should be emphasized that upper bounds on the coupling strength A can only be set for leptoquark masses exceeding the accessible center of mass energy of HERA. Masses around 300 GeV are excluded for almost all types of leptoquarks with a coupling of X ~ 1. Complementary limits coming from direct search for leptoquarks at HERA can be found elsewhere [6].
REFERENCES 1. P. Haberl, F. Schrempp and H.-U. Martyn, Proc. Workshop 'Physics at HERA', eds. W. Buchmfiller and G. Ingelman, DESY, Hamburg (1991), vol. 2, p. 1133. 2. H1 collaboration, 'Measurement of Inclusive Cross Sections for Neutral and Charged Current Interactions at High Q2,, paper # 533
2 0
0
2 2 2 0
[GeV]
202 294 3O8 416 385
655 547 299 345 859 952 400
submitted to ICHEP 98, Vancouver. 3. PDFLIB, CERN program library W5051; A.D. Martin, R.G. Roberts, W.J. Stirling and R.S. Thorne, hep-ph/9803445. 4. W. Buchmiiller, R. Riickl, D. Wyler, Phys. Lett. B191(1987)442. 5. J. Kalinowski, R. Riickl, H. Spiessberger, P. M. Zerwas, Z. Phys. C74(1997)595-603 6. H1 collaboration, T. Ahmed et al., Phys. Lett. B369 (1996) 173; H1 collaboration, 'A Search for Leptoquark Bosons in DIS at High Q2 at H E R A ', paper # 579 submitted to ICHEP 98.
PROCEEDINGS SUPPLEMENTS
Nuclear Physics B (Proc. Suppl.) 79 (1999) 152-154
www.elsevier.nl/locate/npe
Search for Contact Interactions and Leptoquarks at HERA J. Scheins a aI. Physikalisches Institut, R W T H Aachen, D-52056 Aachen, G e r m a n y (on behalf of the H1 collaboration) The measured single differential cross section da/dQ 2 is compared to the Standard Model expectation and analyzed to search for (ge)(~q) contact interactions. No significant deviation is observed and limits on leptonquark compositeness scales and virtual leptoquark exchange are presented. H1 preliminary
1. I N T R O D U C T I O N A
New currents or heavy bosons m a y produce indirect effects through the interference of a virtual particle exchange with the 3, and Z fields of the Standard Model. For particle masses well above the available production energy, such indirect signatures m a y be investigated by searching for a 4-fermion pointlike (~e)(Ctq) contact interaction. The Contact Interaction Lagrangian [1] including only vector t e r m s contains 8 coupling q where the indices i , f = { L , R } coefficients ~?if denote the left-handed and r i g h t - h a n d e d fermion helicities and q = (u, d}. The general ansatz [1] can be applied to any new phenomena, e. g. (e q) compositeness, leptoquarks or new gauge bosons, by an appropriate choice of the coupling coefficients. Possible deviations of the measured cross section from the S t a n d a r d Model are analyzed in t e r m s of contact interactions. Figure 1 shows the measured single differential cross section d a / d Q 2 [2], which is well described over six orders of magnitude by the S t a n d a r d Model expectation. Choosing different p a t t o n distributions ( C T E Q a D , MRST, GRV 94 ) [3] for the cross section calculation, does not change the shape of the Q2 spectrum significantly, but rather the normalisation by up to 1.2%. 2. C O M P O S I T E N E S S
SCALES
A possible fermion compositeness or substructure can be expressed by the coupling coefficients 7?if - ± g 2 // A ±if 2, where the signs indicate positive and negative interference with the Standard
10 2
'
' '''"'1
' <"
~
.......
I
e*p date
H1
Standard Model pdf
-,
CTEQID
\
-1 10
"\
1 0 -2
\ I1\,
\
10 4
1 0 -4
\ 1 0 -s
........
I
,
, ,,,,,,I
,
3 10
,
4 10
Q=(GeV2)
Figure 1. Differential NC cross sections d a / d Q 2. H I d a t a (e) are compared with the Standard Model expectation ( - - ) using C T E Q 4 D p a t t o n distributions.
Model currents. The coupling strength is conventionally chosen as g2 _ 4~r and A denotes the compositeness scale. Both u and d quarks are assumed to contribute with the same scale. The results of the X2 fits for different chiral structures are compiled in Table 1. The dominant correlated systematic errors, i. e. the experimental uncertainty of the lepton energy scale, the scattering angle and the theoretical uncertainty due
0920-5632/99/$ - see front matter © 1999 ElsevierScience B.V. All rights reserved. PII S0920-5632(99)00660-X
153
J Scheins/Nuclear Physics B (Proc. Suppl.) 79 (1999) 152-154 to the uncertainty of (~s, are taken into account in the fit procedure. The pure chiral couplings prefer negative interference resp. negative values of ~ as a consequence of the trend of the data being slightly low compared to the Standard Model prediction around Q2 _ 5,000 GeV 2 followed by an upward fluctuation at higher Q2 (fig. 2). The LR model gives a best description of the data, however the couplings r}/4~r for all investigated chiral structures are compatible with the Standard Model within two standard deviations.
H1
~o 4 D '10
o
preliminary
........
I
'
' ' r''''[
H1 e+p data
• - -
be41t f;1 f o r
.....
A" : 1.5 T e V 9 5 % C L
..........
A +
'O
1
.$ T e V
~-) int. (-) i n t
ffi 3.2 TeV 9 5 % C L (÷)
i :
t~[.
CTEQ4D pdf
i
Table 1 95% cl lower limits on compositeness scale parameters A ± i
H1 P r e l i m i n a r y coupling
A + [TeV]
A - [TeY]
,
,
,,iJ,iF
,
10
LL
2.3
1.2
LR
3.0
1.5
RL
3.0
1.5
RR
2.3
1.2
VV
5.0
1.9
AA
2.0
3.4
VA
2.6
2.6
LL+RR
3.1
1.3
LR+RL
4.1
1.8
Limits on the compositeness scale parameters are derived by fitting A+ and A - with the appropriate interference signs and taking the corresponding changes in X2 with respect to the fit to the Standard Model, which provides a good description of the data. The most conservative values of A+ obtained from a variation of the parton distributions are quoted in table 1. The limits on compositeness scale parameters vary between 1.2 TeV and 5.0 TeV depending on the chiral structure of the model. The data are particularly sensitive to the VV, AA and VA models, where all chiral structures contribute with different relative signs. In general the limits of A+ are more restrictive compared to A - due to the preference of negative interference.
3
,
,L,,,,I
j
10
4
Q= (GeV2)
Figure 2. Cross section do'/dQ 2 normalised to the Standard Model expectation (CTEQ4D); HI data (.) compared to the best fit (--) resp. limits A + and A - of the LR model
3. L E P T O Q U A R K S Leptoquarks are colour triplet bosons of spin 0 or 1, carrying lepton (L) and baryon (B) number and fractional electric charge. The leptoquark mass MLQ and its coupling A are related to the contact interaction coefficients ~iq/ ---cq/()~/MLQ) 2. The notation [1] and the coupling coefficients ~ i / f o r leptoquarks of the BuchmiillerRiickl-Wyler Model [4] are given in Table 2. The differential cross section analysis shows no evidence for a virtual leptoquark signal. The resulting lower limits on M L Q / ~ , which is the only unknown parameter, are summarized in Table 2. They include all experimental systematics and represent the most conservative values obtained from a variation of different patton densities. In general vector leptoquarks which couple to u quarks with positive interference provide the most restrictive limits of MLQ/)~ ~ O(1.0 WeV).
3:. Scheins/Nuclear Physics B (Proc. Suppl.) 79 (1999) 152-154
154
Table 2 Coupling coefficients ~qf, fermion number F and 95% cl lower limits on MLQ/)~ for scalar (S) and vector (V) leptoquarks of the Buchmiiller-Rfickl-Wyler Model. The used notation indicates the lepton chirality L, R and weak isospin I -- 0, 1/2, 1. S and V differ by two units of hypercharge from S and V. (Sign errors of the coupling coefficients for the F = 2 scalar leptoquarks and the F = 0 vector leptoquarks in [1] are corrected [5].) H1 P r e l i m i n a r y leptoquark
coupling to u quark [GeV -2]
coupling to d quark [GeV -2]
F = 3B + L
MLQ/~
598 554
SoL
77~L = +½ ()~/MLQ) 2
2
SoR
T]URR= +31 (~/MLQ)2
2
DoR
d : +-~1 ()~/MLQ)Z zlnn
sLI2
7]uLn ---- --31 ()~/MLQ)2
S~2
'7~L ~- _½ (~/MLQ)2
D~I2 Sf
?~L = +½ (;~/MLQ)2
vL
0
d = - - 3 , (A/MLQ) 2 r]RL ns.n
0 2
~?~LL= - - 1 (A/MLQ) 2 ~nn = - 1 (A/MLQ) 2
0 0
(1oR v~ 2 V~2
~uR R = - 1
(A/MLQ) 2
~ n -----+1 (A/MLQ) 2
d R = +1 ()~/MLQ) 2 7]5 ~dRL = +1 (~/MLQ) 2
V~2 VL
~ n = +1 ()~/MLQ) 2 77~L : - 2 ()~/MLQ) 2
~dLL -: - 1 (A/MLQ) 2
It should be emphasized that upper bounds on the coupling strength A can only be set for leptoquark masses exceeding the accessible center of mass energy of HERA. Masses around 300 GeV are excluded for almost all types of leptoquarks with a coupling of X ~ 1. Complementary limits coming from direct search for leptoquarks at HERA can be found elsewhere [6].
REFERENCES 1. P. Haberl, F. Schrempp and H.-U. Martyn, Proc. Workshop 'Physics at HERA', eds. W. Buchmfiller and G. Ingelman, DESY, Hamburg (1991), vol. 2, p. 1133. 2. H1 collaboration, 'Measurement of Inclusive Cross Sections for Neutral and Charged Current Interactions at High Q2,, paper # 533
2 0
0
2 2 2 0
[GeV]
202 294 3O8 416 385
655 547 299 345 859 952 400
submitted to ICHEP 98, Vancouver. 3. PDFLIB, CERN program library W5051; A.D. Martin, R.G. Roberts, W.J. Stirling and R.S. Thorne, hep-ph/9803445. 4. W. Buchmiiller, R. Riickl, D. Wyler, Phys. Lett. B191(1987)442. 5. J. Kalinowski, R. Riickl, H. Spiessberger, P. M. Zerwas, Z. Phys. C74(1997)595-603 6. H1 collaboration, T. Ahmed et al., Phys. Lett. B369 (1996) 173; H1 collaboration, 'A Search for Leptoquark Bosons in DIS at High Q2 at H E R A ', paper # 579 submitted to ICHEP 98.